Pt(tpy)-centered reductions near -1.0 and -1.5 V vs. Ag/AgCl. The scan rate dependences of the peak potentials are consistent with large structural reorganization accompanying electron transfer, as expected for interconversion between Pt(II) and Pt(IV). Variations in the Z- and R-substituents cause the Pt(IV/II) couple (E°') to vary over a range of 200 mV.

These data establish that oxidation of the metal center becomes more difficult as the electron withdrawing character of the substituents increases. There is an approximately linear relationship between E°' and an effective Hammet parameter, and the slope is similar to that observed for the Ru(III/II) couple of ruthenium bipyridyl complexes.

3 1 $ - $ -tpy)](BF4)2.1/4Et2Oand

[Pt(tpy)(CH3COO)3](PF6) were structurally characterized. The former is the first example of a crystallographically characterized d8- ! " $1- and

$3-terpyridyl ligands. The latter is the first example of a platinum(IV) terpyridyl complex.

III IV Acknowledgements

There are many people I have to acknowledge, so that the age range is from above

65 years to minus 10 days. I can neither name all of them nor state the reasons why I am so grateful. So the fallowing is an abridged version that is expressed in limited wording.

I would like to start with Dr. Bill Connick, who has been a great mentor as well as a wonderful advisor. He has helped me grow personally and academically. His enthusiasm for the field, encouragement and thoughtful advice have made me more devoted to my studies and motivated me through rough times. I appreciate that he was always considerate of my different background and respectful to my preferences and life- style. Also, I would like to thank Dr. Baldwin and Dr. Seliskar for their expertise and guidance through this process and Dr. Stalcup for her last minute-save by attending my public defense as a substitute. In addition, I am fortunate to have had the opportunity to interact with many helpful members on the faculty and staff in the chemistry department.

In particular, I am grateful to Jeanette for helping me with crystallography, Dr. Sallans and Dr. Macha for always having time to go through less than obvious mass spectra, Dr.

Brooks and Dr. Ding for helping me utilize different functions of NMR instruments. I feel like it has been an honor and a privilege to have been a member of the Connick group. I would like to thank them all for the friendly and helpful environment they created.

In closing, I am eternally grateful to my parents and brothers. They have endured long separations and once-in-a-year visits for several years and kept me in their prayers.

Finally, I thank my dear husband for his continuous encouragement, optimism and support …and the about minus ten days old for bearing with me until the end.

V TABLE OF CONTENTS

Table of Contents I

List of Figures IV

List of Tables IX

List of Schemes XI

List of Abbreviations and Symbols XII

CHAPTER 1: Introduction. Synthesis, Characterization and Redox Studies of 1

Palladium and Platinum Complexes with mer-Coordinating

Ligands.

References 11

CHAPTER 2: Synthesis, Structures and Spectroscopic Properties of

Palladium(II) Complexes with Tridentate Piperidyl-Containing

Pincer Ligands

Introduction 15

Experimental 16

X-ray Crystallography 20

Results and Discussion 22

Synthesis 22

1H NMR Spectroscopy 22

Crystal Structures 27

Electronic Spectroscopy 35

Conclusions 41

References 42

I CHAPTER 3: Palladium(II) Halide Complexes with a Pyridyl Pincer Ligand Introduction 49

Experimental 50

X-ray Crystallography 54

Results and Discussion 55

+ Synthesis of Pd(pip2NNN)X Salts 55

Crystal Structures 62

Electronic Spectroscopy 73

1H NMR Spectroscopy 77

Anion Dependence of the Chemical Shift 80

Conclusions 94

References 96

CHAPTER 4: Palladium(II) Complexes with Two Potentially Tridentate

Pyridyl Ligands

Introduction 103

Experimental 105

X-ray Crystallography 107

Results and Discussion 108

Synthesis 108

Crystal Structures 111

1H NMR Spectroscopy 118

Electronic Spectroscopy 123

Cyclic Voltammetry 128

Conclusions 134

II References 135

CHAPTER 5: Synthesis, Characterization and Redox Tuning of Platinum (II)

Pincer Complexes with Terpyridine

Introduction 141

Experimental 143

X-ray Crystallography 150

Results and Discussion 152

Synthesis 152

Crystal Structures 155

1H NMR Spectroscopy 167

Electronic Spectroscopy 181

Cyclic Voltammetry 187

Conclusions 200

References 202

III LIST OF FIGURES

1 Figure 2.1 H NMR spectra of Pd(pip2NCN)Cl, Pd(pip2NCN)Br, 26

Pd(pip2NCN)I, [Pd(pip2NNN)Cl]Cl and [Pd(pip2NNN)Cl]Cl.

Figure 2.2 ORTEP diagrams of Pd(pip2NCN)Br. 30

Figure 2.3 ORTEP diagrams of [Pd(pip2NCN)(phpy)](BF4). 31

Figure 2.4 ORTEP diagrams of [Pd(pip2NNN)Cl]Cl 32

Figure 2.5 UV-visible absorption spectra of [Pd(pip2NNN)Cl]Cl, 36

[Pd(pip2NCN)(py)](BF4), Pd(NCN)Cl, Pd(NCN)Br and

Pd(NCN)I.

Figure 2.6 UV-visible absorption spectra of [Pd(NCN)(phpy)](BF4) and 37

[Pd(NCN)(py)](BF4).

2- Figure 3.1 UV-visible absorption spectra of solutions containing PdnX2n+2 57

+ (n=1, 2); the tan salt of Pd(pip2NNN)Cl , the orange-brown salt

+ + of Pd(pip2NNN)Br and the burgundy salt of Pd(pip2NNN)I .

Figure 3.2 UV-visible absorption spectra of the [Pd(pip2NNN)I]2[PdnI2n+2] 61

products, assuming [Pd(pip2NNN)I]2[Pd2I6] and

[Pd(pip2NNN)I]2[Pd2I8]0.5 formulations.

Figure 3.3 ORTEP diagrams of [Pd(pip2NNN)Br](BF4). 65

Figure 3.4 ORTEP diagrams of molecule A of [Pd(pip2NNN)Br]Br.H2O 66-67

Ball-and-stick representations for molecules A and B of

[Pd(pip2NNN)Br]Br.H2O.

Figure 3.5 ORTEP diagrams and ball-and-stick representation of 68

[Pd(pip2NNN)I]I.

IV Figure 3.6 ORTEP diagrams and ball-and-stick representation of 69

[Pd(pip2NNN)I]2[Pd2I6].

Figure 3.7 UV-visible absorption spectra of [Pd(pip2NNN)Cl]Cl, 75

[Pd(pip2NNN)Br]Br and [Pd(pip2NNN)I]I.

1 Figure 3.8 H NMR spectra of [Pd(pip2NNN)Cl]Cl, [Pd(pip2NNN)Br]Br 79

and [Pd(pip2NNN)I]I.

1 Figure 3.9 H NMR spectra of [Pd(pip2NNN)Cl][BF4], 82

[Pd(pip2NNN)Cl]Cl, [Pd(pip2NNN)Br]Br,1:1

[Pd(pip2NNN)Cl][BF4] / TBABr, 1:1 [Pd(pip2NNN)Cl][BF4] /

[Pd(pip2NNN)Br]Br and 1:1 [Pd(pip2NNN)Br]Br /

[Pd(pip2NNN)Cl]Cl.

1 Figure 3.10 H NMR spectra of [Pd(pip2NNN)Cl][BF4], 87

[Pd(pip2NNN)Br]Br and a 1:1 mixture of

[Pd(pip2NNN)Cl][BF4] / [Pd(pip2NNN)Br]Br at -15 °C, 0 °C,

22 °C and 60 °C.

1 Figure 3.11 H NMR spectra of [Pd(pip2NNN)Cl]Cl, [Pd(pip2NNN)Br]Br 88

and a 1:1 mixture of [Pd(pip2NNN)Cl]Cl / [Pd(pip2NNN)Br]Br

at -15 °C, 0 °C, 20 °C, 40 °C and 60 °C.

Figure 3.12 The chemical shifts of meta CH protons vs. concentration of 90

[Pd(pip2NNN)Cl]Cl and the best-fit curves.

Figure 4.1 ORTEP diagrams of the cation of 113

t [Pd(pip2NNN)( Bu3tpy)]2(BF4).2(ClCH2CH2Cl).

Figure 4.2 ORTEP diagrams of $1- $3- 114

tpy)]2(BF4)2.1/4Et2O.

V 1 Figure 4.3 H NMR spectrum of [Pd(pip2NNN)(tpy)](BF4)2. 119

Figure 4.4 Aromatic region of the COSY NMR spectrum of 120

[Pd(pip2NNN)(tpy)](BF4)2.

1 t Figure 4.5 H NMR spectrum of [Pd(pip2NNN)( Bu3tpy)](BF4)2. 121

Figure 4.6 Aromatic region of the COSY 1H NMR spectrum of 122

t [Pd(pip2NNN)( Bu3tpy)](BF4)2.

Figure 4.7 UV-visible absorption spectra of [Pd(pip2NNN)(tpy)](BF4)2, 126

t [Pd(pip2NNN)( Bu3tpy)](BF4)2 and [Pd(tpy)Cl](BF4).

Figure 4.8 Cyclic voltammograms of [Pd(pip2NNN)(tpy)](BF4)2 and 131

t [Pd(pip2NNN)( Bu3tpy)](BF4)2.

Figure 4.9 Cyclic voltammograms of [Pd(pip2NNN)(tpy)](BF4)2 at positive 132

and negative sweeps with different ranges.

Figure 4.10 Cyclic voltammograms of pip2NNN, TBAPF6/CH3CN 133

electrolyte solution and Pd(pip2NNN)Cl]Cl.

Figure 5.1 ORTEP diagram of the cation of 161

t [Pt((ph)( Bu3tpy)](BF4).1/2CH2Cl2.

t Figure 5.2 ORTEP diagram of the cation of [Pt(pip2NCN)( Bu3tpy)](BF4). 162

Figure 5.3 ORTEP diagram of the cation of 163

t [Pt(pip2NCNH2)( Bu3tpy)](PF6)3.

Figure 5.4 ORTEP diagram of the cation of [Pt(pip2NCN)(toltpy)](BF4). 164

Figure 5.5 ORTEP diagram for molecule A of the cation of 165

[Pt(NO2pip2NCN)(tpy)](BF4).1/8C4H10O.

Figure 5.6 ORTEP diagram of the cation of [Pt(tpy)(CH3CO2)3](PF6). 166

1 Figure 5.7 H NMR spectra of MeO-pip2NCNBr, pip2NCNBr and NO2- 171

VI pip2NCNBr.

1 Figure 5.8 H NMR spectra Pt(pip2NCN)Br, Pt(MeO-pip2NCN)Br and 172

Pt(NO2-pip2NCN)Br.

1 t Figure 5.9 H NMR spectrum of [Pt(Ph)( Bu3tpy)](BF4). 173

1 t Figure 5.10 H NMR spectrum of [Pt(mes)( Bu3tpy)](BF4). 174

1 t Figure 5.11 H NMR spectrum of [Pt(pip2NCN)( Bu3tpy)](BF4). 175

1 t Figure 5.12 H NMR spectrum of [Pt(pip2NCNH2)( Bu3tpy)](PF6)3. 176

1 Figure 5.13 H NMR spectrum of [Pt(pip2NCN)(toltpy)](BF4). 177

1 Figure 5.14 H NMR spectrum of [Pt(NO2-pip2NCN)(tpy)](BF4). 178

Figure 5.15 Aromatic region of the COSY spectrum of [Pt(NO2- 179

pip2NCN)(tpy)](BF4).

1 Figure 5.16 H NMR spectrum of [Pt(MeO-pip2NCN)(tpy)](OTf). 180

t Figure 5.17 UV-visible absorption spectra of [Pt(ph)( Bu3tpy)](BF4), 184

t [Pt(pip2NCN)( Bu3tpy)](BF4), [Pt(pip2NCN)(toltpy)](BF4) and

[Pt(NO2-pip2NCN)(tpy)](BF4).

t Figure 5.18 UV-visible absorption spectra of [Pt(pip2NCN)( Bu3tpy)](BF4) 186

titration with trifloroacetic acid and emission spectra of

t [Pt(pip2NCN)( Bu3tpy)](BF4) and

t [Pt(pip2NCNH2)( Bu3tpy)](PF6)3.

Figure 5.19 Cyclic voltammograms of [Pt(pip2NCN)(toltpy)](BF4), 191

t t [Pt(pip2NCN)( Bu3tpy)](BF4) and [Pt(ph)( Bu3tpy)](BF4).

Figure 5.20 Cyclic voltammograms of [Pt(NO2-pip2NCN)(tpy)](BF4), 192

[Pt(MeO-pip2NCN)(tpy)](OTf) and [Pt(pip2NCN)(tpy)](BF4).

Figure 5.21 Dependence of anodic peak current for oxidation process and 195

VII cathodic peak current for reduction process on the square root

t of the scan rate for [Pt(pip2NCN)( Bu3tpy)](BF4).

Figure 5.22 C ! # + for 199

+ Pt(Z-pip2NCN)(R-tpy) (Z=NO2, MeO, H; R=H, t-butyl,

2+ phenyl) and Ru(Z,Z'-bpy)x(bpy)3-x (Z= NO2, H, Me, MeO;x=

0,1,2,3) complexes.

VIII LIST OF TABLES

Table 2.1. Crystallographic data for Pd(pip2NCN)Br, 21

[Pd(pip2NCN)(phpy)]BF4 and [Pd(pip2NNN)Cl]Cl..

Table 2.2 Selected distances and angles for Pd(pip2NCN)Br and 33

[Pd(pip2NCN)(phpy)]BF4.

Table 2.3 Selected distances and angles for [Pd(pip2NNNCl]Cl. 34

- Table 2.4 UV-visible absorption data for palladium pip2NCN and 38

pip2NNN complexes.

Table 3.1 Crystallographic data for [Pd(pip2NNN)Br](BF4), 63

[Pd(pip2NNN)Br]Br.H2O, [Pd(pip2NNN)I]I and

[Pd(pip2NNN)I]2[Pd2I6].

Table 3.2 Selected distances and angles for [Pd(pip2NNN)Br](BF4), 64

[Pd(pip2NNN)Br]Br.H2O, [Pd(pip2NNN)I]I and

[Pd(pip2NNN)I]2[Pd2I6].

Table 3.3 UV-visible absorption data for [Pd(pip2NNNCl]Cl, 74

[Pd(pip2NNN)Br]Br, [Pd(pip2NNN)I]I and

[Pd(pip2NNN)I]2[Pd2I6].

Table 4.1 Crystallographic data for 115

t 3 [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl) and [Pd( -

1 tpy)( -tpy)](BF4)2.1/4Et2O.

Table 4.2 Selected distances (Å) and angles (°) for 116

t 3 [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl) and [Pd( -

1 tpy)( -tpy)](BF4)2.1/4Et2O.

IX Table 4.3 UV-visible absorption data for Pd(II) complexes with tpy.

t Table 5.1 Crystallographic data for [Pt(Ph)( Bu3tpy)](BF4).1/2CH2Cl2, 159

t t [Pt(pip2NCN)( Bu3tpy)](BF4), Pt(pip2NCNH2)( Bu3tpy)]

(PF6)3, Pt(pip2NCN)(toltpy)](BF4), [Pt(NO2-

pip2NCN)(tpy)](BF4).1/8C4H10O, [Pt(tpy)(CH3COO)3](PF6).

Table 5.2 Selected bond lengths and angles for 160

t [Pt(Ph)( Bu3tpy)](BF4).1/2CH2Cl2,

t t [Pt(pip2NCN)( Bu3tpy)](BF4), Pt(pip2NCNH2)( Bu3tpy)]

(PF6)3, Pt(pip2NCN)(toltpy)](BF4), [Pt(NO2-

pip2NCN)(tpy)](BF4).1/8C4H10O, [Pt(tpy)(CH3COO)3](PF6).

t + Table 5.3 UV-visible absorption data for [Pt(Ph)( Bu3tpy)] , Pt(Z- 185

+ t 3+ pip2NCN)(R-tpy)] and Pt(pip2NCNH2)( Bu3tpy)] .

t + Table 5.4 Cyclic voltammetry data for [Pt(Ph)( Bu3tpy)] and Pt(Z- 193

+ pip2NCN)(R-tpy)]

X LIST OF SCHEMES

Scheme 1.1 Potentially tridentate pincer-type ligands. 4

Scheme 1.2 Schematic representation of a reversible two-electron platinum 4

reagent.

Scheme 2.1 Synthesis of Pd(pip2NCN)X (X=Cl, Br, I) and 23

[Pd(pip2NCN)L][BF4] (L=pyridine, 4-phenyl pyridine).

Scheme 2.2 Synthesis of [Pd(pip2NNN)Cl]Cl. 24

+ Scheme 3.1 Synthesis of Pd(pip2NNN)X (X=Cl, Br, I). 56

+ + Scheme 3.2 Reactions of [Pd(pip2NNN)Cl] , [Pd(pip2NNN)Br] and 60

+ [Pd(pip2NNN)I] with iodide.

Scheme 3.3 Halide exchange. 92

Scheme 3.4 Proposed reaction profile for the halide substitution. 93

Scheme 4.1 Potentially tridentate ligands. 104

Scheme 4.2 Schematic representation of a reversible two-electron platinum 104

reagent.

Scheme 4.3 Synthesis of [Pd(pip2NNN)(tpy)](BF4)2 and 109

t [Pd(pip2NNN)( Bu3tpy)](BF4)2.

Scheme 5.1 Schematic representation of a reversible two-electron platinum 141

reagent.

Scheme 5.2 Potential tridentate ligands. 142

Scheme 5.3 Synthesis of Z-pip2NCNBr and Pt(Z-pip2NCN)Br (Z=NO2, 154

MeO).

Scheme 5.4 Synthesis of platinum(II) complexes with two potentially 155

meridional-coordinating tridentate ligands.

XI LIST OF ABBREVIATIONS AND SYMBOLS

A Electrode area bpy 4,4'-bipyridine

BuLi N-butyllithium C concentration

CD3CN deuterated acetonitrile

CDCl3 deuterated chloroform

CHCl3 chloroform COD 1,5-cyclooctadiene COSY correlation spectroscopy CV cyclic voltammogram D diffusion coefficient DMF dimethylformamide DMSO dimethylsulfoxide

E°' (Epc+Epa)/2

Eox oxidation peak potential

Epa anodic peak potential

Epc cathodic peak potential

Ered reduction peak potential

ESI electrospray ionization

EtO ethoxy

Et2O diethyl ether EtOH ethanol

FcH/FcH+ ferrocene/ferrocenium couple iox oxidation peak current ipc cathodic peak current

XII ipc / ipa ratio of cathodic peak current/anodic peak current ired reduction peak current

LF ligand field LMCT ligand-to-metal charge transfer m/z mass/charge Me methyl

- Me4 NCN 2,6-bis(dimethylaminomethyl)benzenze

MeO methoxy MeOH methanol mer meridional mes mesitlyene

MLCT metal-to-ligand charge transfer

MS mass spectrometry mV milivolts N electron stoichiometry

NBS N-bromosuccinimide

NHE normal hydrogen electrode

NMR nuclear magnetic resonance NR not resolved

ORTEP Oak Ridge thermal ellipsoid program Ph phenyl phpy 4-phenylpyridine pipNCNBr 2,6-bis(piperdylmethyl)bromobenzene pip2NCNH 1,3-bis(piperdylmethyl)benzene

XIII pip2NNN 1,3-bis(piperdylmethyl)pyridine pip2NNNBr 2,6-bis(piperdylmethyl)bromopyridine

PPh3 triphenylphosphine ppm parts per million R Alkyl groups SCE saturated calomel electrode

TBAPF6 tetrabutylammonium hexafluorophosphate

THF tetrahydrofuran

TMS tetramethylsilane tpy 2,2':6',2"-terpyridine t Bu3tpy 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine tolyl 4'-toyl-2,2':6',2''-terpyridine tacn 1,4,7-triazacyclononane ttcn 1,4,7-trithiacyclononane vs. versus

V volts

Ep peak-to-peak separation

chemical shift molar absorbptivity

( wavelength

* scan rate

XIV CHAPTER 1:

Introduction. Synthesis, Characterization and Redox Studies of Palladium and

Platinum Complexes with mer-Coordinating Ligands

The increase in the world's energy needs along with the limited supply and

environmental consequences of fossil fuels are motivating efforts to develop efficient

catalysts for energetically or kinetically unfavorable reactions. Many of the most

important substrates require activation using multiple redox equivalents such as N2

reduction in fertilizer production and splitting water with sunlight to form H2 fuel and

1 O2. One-electron strategies for carrying out reactions that require multi-electron redox

equivalents for activation risk damaging radical chemistry arising from odd-electron

intermediates and forfeit the energetic benefits of multi-electron steps. For example, the

one-electron oxidation of water to form hydroxyl radical intermediate is strongly

disfavored compared to the overall four-electron oxidation of water to dioxygen, as

indicated by the formal potentials for reduction of OH. (E°'=2.31 V vs. NHE; pH=7) and

2 O2 (E°'=0.815 V) to water.

-0.815 V

-2.31 V -0.38 V -0.89 V - +0.33 V 2H2O H2O+HO HOOH O2 O2

Previous studies3,4 suggest that square planar d8-electron and octahedral d6-

electron 2nd and 3rd row transition metal complexes are promising candidates for multi- redox catalysts. For instance, Wilkinson’s catalyst, Rh(PPh3)3Cl, a square planar 16-

1 electron rhodium complex with three large triphenylphosphine groups, catalyzes olefin

hydrogenation. The mechanism involves the initial dissociation of one of

triphenylphosphine ligands to give a 14-electron complex followed by oxidative addition

of H2 to the metal, forming a 16-e !) *"+ 4-addition of an alkene

generates an 18-electron complex, which undergoes intramolecular hydride transfer

followed by alkane extrusion to regenerate the catalytically active 14-electron rhodium

complex. At the core of this reactivity is cooperative two-electron transfer events

coupled to bond-making and bond-breaking steps. The cooperativity arises from the

stability of the d6-and d8-electron configurations with respect to the intermediate d7-

electron configuration, which can be expected to manifest as an inversion of the one-

7 8 6 7 electron reduction potentials: E1°'(d /d )>E2°'(d /d ). We have suggested that fine control

over these redox potentials and electron-transfer kinetics would be enormously valuable

to efforts aimed at improving catalysts and rationally designing new catalysts. For example, one could envision matching the inverted one-electron redox potentials of a catalyst to those of a substrate in order to facilitate multi-electron transformations.

However, at present, relatively little is known of about the electron-transfer chemistry of

2nd and 3rd row d6/d8-elecron systems. Attempts to investigate the thermodynamics and

kinetics of these electron-transfer reactions are severely hampered by the fact that these

reactions are often irreversible because of the accompanying drastic changes in the metal

coordination sphere. This lack of reversibility can prevent regeneration of a two-electron

reagent and completion of the catalytic cycle. Presently it is not clear what factors allow

for facile release of redox equivalents and reversible two-electron interconversion

between oxidation states. This gap in the knowledge base is a critical problem because it

prevents the development of effective strategies for modulating the thermodynamics and

2 kinetics of cooperative two-electron transfer reactions, which is ultimately necessary for

rationally coupling this reactivity to substrate bond-making and breaking steps.

To address this challenge, a major focus of our research is on designing metal

complexes that undergo outer-sphere cooperative two-electron transfer reactions.

Investigations of these reactions are a first step toward determining the factors that

govern multi-electron transfer reactions. Our working hypothesis is that cooperative two-

electron transfer reactions are favored for complexes with ligand architectures capable of

interconverting between square planar 4-coordinate and 6-coordinate geometries with

minimal reorganization. This hypothesis was formulated based on earlier studies of

redox reactions of 2nd and 3rd row transition metal complexes. For example,

investigations of the outer-sphere two-electron transfer reactions of d6/d8-electron Ru,5-9

Ir,10,11 and Rh12,13 arene have shown that the arene ligand can readily switch between 4-

and 6-coordination in order to stabilize the d8-and d6-electron configurations,

respectively. However, extension of this strategy to traditional square-planar d8-electron

and 6-coordinate d6-electron systems has met with difficulties. In the case of fully

encapsulating architectures based on Sargeson’s sepulchrate-type ligands, outer-sphere

electron-transfer results in ligand decomposition.14-16 Similarly, Schröder and Grant have

found that four-coordinate Pt(II) and Pd(II) complexes with one potentially facially

2+ coordinating ligand (e.g., Pt(PPh3)2(ttcn) ; ttcn=1,4,7-trithiacyclononane) often exhibit

irreversible electrochemistry.17-20 More recently, however, Geiger has shown that

- - [Rh(CO)(L)Tp'] complexes (L=phospine ligand; Tp' =HB(pz)3 , pz=pyrazoyl) undergo

one-electron oxidation to form five-coordinate d7-electron adducts.21,22 Similarly, d8-

2+ electron complexes with two potentially facially coordinating ligands, such as Pd(tacn)2

2+ 7 and Pt(ttcn)2 (tacn=1,4,7-triazacyclononane), show selective stabilization of the d -

3 23-29 electron adduct, consistent with E1°'

- by designing platinum(II) complexes with two pincer-type ligands (pip2NCN and tpy,

Scheme 1.1) that are capable of stabilizing 4- and 6-coordinate metal geometries.

N N

- pip2NCN tpy

Scheme 1.1 Potentially tridentate pincer-type ligands.

In the d6 octahedral Pt(IV) case, both ligands are expected to be tridentate occupying all

six binding sites around the metal center. In the d8 square planar Pt(II) case, tpy remains

- tridentate, whereas pip2NCN coordinates in a monodentate fashion through the central

phenyl group (Scheme 1.2).

Scheme 1.2 Schematic representation of a reversible two-electron platinum reagent.

+ Electrochemical studies have established that Pt(pip2NCN)(tpy) undergoes two

one-electron reductions characteristic of Pt-terpyridyl complexes and a two-electron 4 oxidation at 0.4 V vs. Ag/AgCl.30 The stoichiometry of the two-electron process was confirmed using the Randles-Šev6- + -centered reduction

as an internal one-electron standard. The separation between the anodic and cathodic

peaks of the oxidation wave (Ep) decreases with scan rate (150-43 mV, 20.5-0.01 V/s), as expected for structural reorganization resulting in slow reaction kinetics. Upon removal or protonation of the piperidyl groups the oxidation process is no longer observed. The accumulated data are consistent with two-electron transfer accompanied by interconversion between 4-coordinate and 6-coordinate structures.

n+ The work of Takagi on M(ttcn)2 (M=Au, Pd, Pt) revealed trends in reaction

volumes and one-electron self-exchange rates, but stopped short of determining the

reason for selective stabilization of the d7-electron adduct.31,32 Likewise, Haines’ and

Sargeson’s efforts to design reversible two-electron platinum reagents were frustrated by

chemical irreversibility. 14,33 The lack of understanding of d6/d8-electron redox chemistry

has recently been highlighted by Bercaw & Labinger as a serious problem in

characterizing and tailoring the reactivity of d6/d8 catalytic systems. 34-36 To better

understand the role of the metal and the ligands in controlling d6/d8 electron-transfer

reactions, in this dissertation we extend these studies to palladium and platinum

complexes with substituted pincer-type ligands.

In order to investigate the role of metal acidity on the reduction potentials and

electron-transfer kinetics, we have targeted complexes with electron-releasing and/or

- withdrawing groups at the para-positions of the pivoting NCN ligand (Z=MeO, H, NO2)

and the para-positions of the pyridyl groups of the tpy ligand (Rn=t-butyl, H, tolyl). At

the outset, from comparisons to one-electron ruthenium(II/III) systems, we hypothesized

that these substituents would tune the apparent two-electron reduction couple by ~400

5 mV.37,38 Similarly, we expected that changing from Pt to Pd would increase the potential by ~500 mV.39 Chapters 2-4 of this dissertation describe the development of palladium chemistry with pincer-type ligands. Chapter 5 describes an investigation of the influence

of the Z and Rn ligand substituents on the redox chemistry of analogs of

+ Pt(pip2NCN)(tpy) . In the course of these studies, we have encountered new and

surprising chemistry, especially in the case of palladium. The remainder of this chapter

summarizes the main results from this work.

In Chapter 2, a series of square planar palladium(II) complexes with pincer-type

- tridentate ligands pip2NCN and pip2NNN (2,6-bis(piperidyl-methyl)benzene and 2,6-

bis(piperidyl-methyl)pyridine) are described. X-ray crystal structures of Pd(pip2NCN)Br,

[Pd(pip2NCN)(4-phenylpyridine)]BF4, and [Pd(pip2NNN)Cl]Cl are reported.

1 Interestingly, the H NMR spectra of Pd(pip2NCN)X (X=Cl, Br, I) are consistent with

- deshielding of the pip2NCN ligand resonances along the Cl

the relative halogen electronegativities, due to decreasing filled/filled repulsions between

the d orbitals of the metal center and the lone pair orbitals of the halide ligands along

this series. Electronic absorption spectra support the notion that ligand-to-metal charge-

transfer (LMCT) states are stabilized in these palladium(II) complexes relative to their

platinum(II) analogues.

In Chapter 3, the synthesis and properties of a series of square planar

1 palladium(II) halide complexes with pip2NNN ligand is described. H NMR, mass and

UV-visible absorption spectroscopies and the physical characteristics show that

2+ [Pd(pip2NNN)X][PdnX2n+2] salts (n=1, 2) form from the same reaction as that used to

prepare the simple halide salts, [Pd(pip2NNN)X]X (X=Cl, Br, I). The X-ray crystal

structures of [Pd(pip2NNN)Br]Br, [Pd(pip2NNN)Br]BF4, [Pd(pip2NNN)I]I, and

6 [Pd(pip2NNN)I]2[Pd2I6] are mostly similar to those reported for palladium(II) and

platinum(II) complexes with pincer ligands with the surprising observation that the

conformation of the piperidyl groups and positions of the benzylic carbons in relation to

the metal center differ from structure to structure. Electronic absorption spectra of the

halide salts exhibit a long wavelength band that shifts to the red along the halide series

- Cl

1 + Pd(pip2NCN)X complexes described in Chapter 2, H NMR spectra of [Pd(pip2NNN)X]

show small downfield shifts along the halide series, Cl

show varying sensitivities to substitution of the halide ligand and counter-anion. The

influence of the anion is strongest for the meta-pyridyl and benzylic resonances whereas

variations in the halide ligand have the strongest influence on the furthest downfield - piperidyl resonance. Conductivity and 1H NMR spectroscopic measurements support the

- - - notion that there is an interaction between the exogenous anion (Cl , Br , BF4 ) and the

+ palladium cation (Pd(pip2NNN)X , X= Cl, Br). The solvent and anion dependence of

this interaction suggests the possibility of dramatic solvent and anion sensitivity of multi-

electron transfer reactions that rely on exogenous anionic ligands.

In Chapter 4, two novel Pd(II) complexes with pip2NNN and terpyridyl ligands

+ 1 are described. As in the case of Pt(pip2NCN)(tpy) , H NMR spectroscopy and X-ray crystallography show that the pip2NNN ligand is bonded monodentate whereas the

t terpyridyl ligand (tpy or Bu3tpy) is bonded tridentate. Attempts to synthesize palladium

- t + t complexes with pip2NCN were unsuccessful. For Pd(pip2NNN)( Bu3tpy) ( Bu3tpy

=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine), the piperidyl groups are located above and

below the palladium center, resulting in a short Pd...N(piperidyl) distance of 3.33 ppm.

This coordination geometry suggests that the amine groups are available to stabilize the

7 metal center upon oxidation. Interestingly, the visible absorption spectra exhibit weak,

long-wavelength absorption bands, which may result from interaction between the

+ dangling amine groups and the metal center, as noted for Pt(pip2NCN)(terpyridyl)

complexes. The observed irreversible oxidation processes around 1.5 V (vs. Ag/AgCl)

are tentatively attributed to oxidation of the piperidyl groups. Ligand modification that

will result in an increase in the electron density on the palladium center and/or a raise in

the oxidation potential of the dangling axial donor groups is suggested for lowering the

Pd(IV/II) redox potentials and realizing reversible two-electron transfer.

In Chapter 5, substituted pip2NCN ligands, their platinum(II) halide complexes

+ t and novel Pt(Z-pip2NCN)(Rn-tpy)] complexes (Z= MeO, H, NO2 and Rn= butyl, 4-tolyl) are described. For the Z-substituted ligands and their corresponding platinum(II) complexes, 1H NMR spectroscopy shows that the substituent at the para position

influences the electron density of the nearby atoms as well as the platinum center. The

t t X-ray crystal structures of [Pd(Ph)( Bu3tpy)](BF4), [Pd(pip2NCN)( Bu3tpy)](BF4),

t [Pd(pip2NCNH2)( Bu3tpy)](PF6)3,[Pt(pip2NCN)(toltpy)](BF4), [Pd(NO2-pip2NCN)(tpy)]

(BF4) and [Pt(tpy)(CH3COO)3](PF6) are reported. The piperidyl groups are located

+ above and below the platinum center, and for the Pt(Z-pip2NCN)(R-tpy)] type

structures, this results in at least one short Pt...N(piperidyl) distance of 3.44 ± 0.25 ppm.

+ As in the case of Pt(pip2NCN)(tpy) , the visible absorption spectra of Pt(Z-pip2NCN)(R-

tpy)]+ exhibit a band at ~550 nm assigned to -N)4 -transfer- transition, resulting from the weak interaction of a piperidyl group with the metal center.

+ Pt(Z-pip2NCN)(R-tpy) complexes undergo an apparent two-electron platinum-centered

oxidation near 0.4V and two Pt(tpy) centered reductions near -1.0 V and -1.5V,

respectively. Variations in the Z- and R-substituents cause the oxidation process to vary

8 over a range of 200 mV, whereas the potentials for the reduction processes vary by 160

and 280 mV, respectively. The scan rate dependence of the peak currents of the oxidation

t + and the first reduction processes for Pt(pip2NCN)( Bu3tpy) is consistent with the notion

that the oxidation process involves transfer of two electrons per platinum center.

Overall, this work investigates the role of the ligands and metal in determining the

reduction potentials and outer-sphere electron-transfer chemistry of platinum and

palladium complexes with two potentially meridional coordinating ligands. This contribution is expected to help fill the knowledge gap needed to develop strategies for

tailoring the redox properties of d6/d8-electron systems. The results of this research imply

that the cooperative two-electron chemistry is dependent on the properties of the metal

center (e.g., acidity), as well as the ligand donor and conformational properties. These

factors contribute to the tendency of the dangling nucleophiles to interact with the d8-

electron metal center at the axial sites of the square planar complex, thereby pre-

organizing the complex for electron transfer. Preorganization is expected to lower the

electron-transfer reorganization energy and improve the electron-transfer kinetics. As

illustrated in Chapter 4, there is potential to extend the two-electron chemistry to other

metals, such as palladium. However, the electron donor properties of the ligands should

be tailored to maximize preorganization in the d8-electron complex and ensure that

ligand-centered oxidation is not favored over metal-centered oxidation. Additionally, the

strong tendency of an exogenous anion to interact with a palladium(II) cation, as

described in Chapter 3, suggests the possibility of utilizing solvent-available nucleophiles

in the design of outer-sphere multielectron transfer reagents.

9 References

(1) Whitesides, G. M.; Crabtree, G. W. Science 2007, 315.

(2) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New York, 1991.

(3) Young, J. F.; Osborn, J. A.; Jardine, F. H.; Wilkinson, G. Chem. Comm. 1965,

131-132.

(4) Shilov, A. E.; Shul'pin, G. B. Russ. Chem. Rev. 1987, 56, 442-464.

(5) Finke, R. G.; Voegeli, R. H.; Laganis, E. D.; Boekelheide, V. Organometallics

1983, 2, 347-350.

(6) Voegeli, R. H.; Kang, H. C.; Finke, R. G.; Boekelheide, V. J. Am. Chem. Soc.

1986, 108, 7010-7016.

(7) Plitzko, K. D.; Rapko, B.; Gollas, B.; Wehrle, G.; Weakley, T.; Pierce, D. T.;

Geiger, W. E., Jr.; Haddon, R. C.; Boekelheide, V. J. Am. Chem. Soc. 1990, 112,

6545-6556.

(8) Plitzko, K. D.; Wehrle, G.; Gollas, B.; Rapko, B.; Dannheim, J.; Boekelheide, V.

J. Am. Chem. Soc. 1990, 112, 6556-6564.

(9) Bowyer, W. J.; Merkert, J. W.; Geiger, W. E.; Rheingold, A. L. Organometallics

1989, 8, 191-198.

(10) Bowyer, W. J.; Geiger, W. E. J. Am. Chem. Soc. 1985, 197, 5657-5663.

(11) Bowyer, W. J.; Geiger, W. E. 1988, 239, 253-271.

(12) Merkert, J.; Nielson, R. M.; Weaver, M. J.; Geiger, W. E. J. Am. Chem. Soc.

1989, 111, 7084-7087.

(13) Edwin, J.; Geiger, W. E. J. Am. Chem. Soc. 1990, 112, 7104-7112.

10 (14) Brown, K. N.; Geue, R. J.; Hambley, T. W.; Hockless, D. C. R.; Rae, A. D.;

Sargeson, A. M. Org. Biomol. Chem. 2003, 1, 1598-1608.

(15) Boucher, H. A.; Lawrance, G. A.; Lay, P. A.; Sargeson, A. M.; Bond, A. M.;

Sangster, D. F.; Sullivan, J. C. J. Am. Chem. Soc. 1983, 105, 4652-4661.

(16) Brown, K. N.; Hockless, D. C. R.; Sargeson, A. M. J. Chem. Soc., Dalton Trans.

1999, 2171-2176.

(17) Blake, A. J.; Roberts, Y. V.; Schröder, M. J. Chem. Soc., Dalton Trans. 1996, 9,

1885-1895.

(18) Grant, G. J.; Carter, S. M.; LeBron, R. A.; Poullaos, I. M.; VanDerveer, D. G. J.

Organomet. Chem. 2001, 637-639, 683-690.

(19) Grant, G. J.; Poullaos, I. M.; Gallas, D. F.; VanDerveer, D. G.; Zubkowski, J. D.;

Valente, E. J. Inorg. Chem. 2001, 40, 564-567.

(20) Grant, G. J.; Patel, K. N.; Helm, M. L.; Mehne, L. F.; Klinger, D. W.;

VanDerveer, D. G. Polyhedron 2004, 23, 1361-1369.

(21) Connelly, N. G.; Emslie, D. J. H.; Geiger, W. E.; Hayward, O. D.; Linehan, E. B.;

Orpen, A. G.; Quayle, M. J.; Rieger, P. H. J. Chem. Soc., Dalton Trans. 2001,

670-683.

(22) Geiger, W. E.; Ohrenberg, N. C.; Yeomans, B.; Connelly, N. G.; Emslie, D. J. H.

J. Am. Chem. Soc. 2003, 125, 8680-8688.

(23) Blake, A. J.; Gould, R. O.; Holder, A. J.; Hyde, T. I.; Lavery, A. J.; Odulate, M.

O.; Scrhöder, M. J. Chem. Soc., Dalton Trans. 1987, 118-120.

(24) Rawle, S. C.; Yagbasan, R.; Prout, K.; Cooper, S. R. J. Am. Chem. Soc. 1987,

109, 6181-6182.

11 (25) Blake, A. J.; Gould, R. O.; Holder, A. J.; Hyde, T. I.; Schröder, M. J. Chem. Soc.,

Dalton Trans. 1988, 1861-1865.

(26) McAuley, A.; Whitcombe, T. W. Inorg. Chem. 1988, 27, 3090-3099.

(27) Cooper, S. R.; Rawle, S. C.; Yagbasan, R.; Watkin, D. J. J. Am. Chem. Soc. 1991,

113, 1600-1604.

(28) Blake, A. J.; Crofts, R. D.; Scrhöder, M. J. Chem. Soc., Dalton Trans. 1993,

2259-2260.

(29) Grant, G. J.; Spangler, N. J.; Setzer, W. N.; VanDerveer, D. G.; Mehne, L. F.

Inorg. Chim. Acta 1996, 246, 31-40.

(30) Jude, H.; Krause Bauer, J. A.; Connick, W. B. J. Am. Chem. Soc. 2003, 125,

3446-3447.

(31) Mitsuru, M.; Itoh, M.; Funahashi, S.; Takagi, H. D. Can. J. Chem. 1999, 77,

1638-1647.

(32) Matsumoto, M.; Funahashi, S.; Takagi, H. D. Z. Naturforsch 1999, 54B, 1138-

1146.

(33) Haines, R. I.; Hutchings, D. R.; McCormack, T. M. J. Inorg. Biochem. 2001, 85,

1-7.

(34) Rostovtsev, V. V.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem.

2002, 41, 3608-3619.

(35) Scollard, J. D.; Day, M.; Labinger, J. A.; Bercaw, J. E. Helv. Chim. Acta 2001, 84,

3247-3268.

(36) Weinberg, D. R.; Labinger, J. A.; Bercaw, J. E. Organometallics 2007, 26, 167-

172.

12 (37) Fallahpour, R.-A. Eur. J. Inorg. Chem. 1998, 9, 1205-1207.

(38) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Thompson, A. M. W. C.

Inorg. Chem. 1995, 34, 2759-2767.

(39) Latimer, W. M. Oxidation Potentials, 2. Edition 1953, 53.

(40) Latimer, W. M. Oxidation Potentials, 2. Edition 1953, 202-209.

13 CHAPTER 2:

Synthesis, Structures and Spectroscopic Properties of Palladium(II) Complexes with

Tridentate Piperidyl-Containing Pincer Ligands1

Introduction

The inspiring work of van Koten and coworkers2-13 has drawn attention to

platinum(II) and palladium(II) pincer ligand complexes and their potential utility in a

wide range of applications, including gas sensing devices,2-4 catalysis,5-8 and the preparation of organometallic supramolecular structures and materials.9-13 Building on

this rich chemistry, we have recently explored the use of the cyclometallating

- aryldiamine ligand pip2NCN in the preparation of outer-sphere two-electron platinum

reagents.14

N N

- pip2NCN pip2NNN

To better understand the electronic structures of this system, we have examined the

UV-visible absorption and emission spectroscopic properties of a series of Pt(pip2NCN)L

derivatives and discovered that variation of the monodentate ligand (L) provides

remarkable control over the electronic structures of this class of compounds. Most

1 The contents of Chapter 2 appeared in a recent journal article: Tastan, S.; Krause, J. A.; Connick, W. B. Inorg. Chim. Acta, 2006, 359, 1889-1898.

14 notably, the luminescence originates from a lowest-energy excited- state having metal-to-

ligand charge-transfer (MLCT), ligand-centered or ligand field character, depending on

the nature of L. In this chapter, we extend these investigations to a series of palladium(II)

analogues, including a complex with the novel pip2NNN ligand.

Experimental

15 Materials. Pd(COD)Br2 (COD=1,5-cyclooctadiene), Pd(COD)Cl2, and

16 pip2NCNBr (2,6-bis(piperidyl-methyl)-1-bromobenzene) were prepared according to literature procedures. Deuterated acetonitrile and chloroform (0.03% tetramethylsilane

(TMS) (v/v)) were purchased from Cambridge Isotope Laboratories. All other reagents were purchased from commercial sources (Acros and Aldrich) and used as received.

Benzene and acetonitrile were distilled from CaH2 under argon, and tetrahydrofuran was distilled from Na(s) and benzophenone under argon.

Pd(pip2NCN)Br. N-butyllithium (0.92 mL, 1.6 M in hexanes) was added to a

stirred solution of pip2NCNBr (0.56 g, 1.6 mmol ) in 30 mL of THF under argon at

-70°C. After 45 min, the contents of the flask was cannula transferred into a suspension

of Pd(COD)Br2 (0.50 g, 1.34 mmol) in 50 mL THF at -70°C. The mixture was stirred for

1 h and subsequently allowed to warm to room temperature. After stirring overnight, the

black residue was removed by filtration, and the filtrate was rotoevaporated to dryness.

Water was added to the solid, and the product was extracted with dichloromethane. The

organic layer was dried over MgSO4 and reduced in volume to 5 mL. Addition of

hexanes afforded an off-white solid. Yield: 0.43 g, ~70%. Anal. Calc for C18H27N2PdBr:

1 C, 47.23; H, 5.95; N, 6.12. Found: C, 47.55; H, 5.99; N, 6 .12. H NMR (CDCl3)

15 (ppm): 1.40-1.78 (12H, m, CH2), 3.27 (4H, dd, CH2), 4.02 (4H, dd, CH2), 4.26 (4H, s,

1 benzylic CH2), 6.75 (2H, d, CH), 6.94 (1H, t, CH). H NMR (CD3CN) (ppm): 1.38-

1.82 (12H, m, CH2), 3.18 (4H, dd, CH2), 3.92 (4H, dd, CH2), 4.29 (4H, s, benzylic CH2),

6.75 (2H, d, CH), 6.95 (1H, t, CH).

Pd(pip2NCN)Cl. Method 1. Excess NaCl was added to Pd(pip2NCN)Br (0.20 g,

0.44 mmol) dissolved in 20 mL acetone, and the mixture was stirred for an hour. After

removal of the solvent, the solid was washed several times with water and ether. An off-

white solid was collected. Yield: 0.08 g, 45%.

Method 2. In the dark, AgBF4 (0.096 g, 0.5 mmol) was added to Pd(pip2NCN)Br

(0.20 g, 0.44 mmol) dissolved in 20 mL acetone. The mixture was stirred for 30 min and

then filtered through Celite. After stirring the filtrate with excess NaCl for 30 min, the

solvent was removed, and the resulting solid was washed with water and ether. Yield:

0.14 g, 76%.

Method 3. The same procedure as for Pd(pip2NCN)Br substituting Pd(COD)Cl2

(0.38g, 1.34 mmol) for Pd(COD)Br2. Yield: 0.39 g, 70%. Anal. Calc for C18H27N2PdCl.

1 0.5 CH2Cl2: C, 48.70; H, 6.14; N, 6.14. Found: C, 48.36; H, 5.99; N, 6.20. H NMR

(CDCl3) (ppm): 1.40-1.78 (12H, m, CH2), 3.28 (4H, dd, CH2), 3.94 (4H, dd, CH2), 4.26

1 (4H, s, benzylic CH2), 6.75 (2H, d, CH), 6.95 (1H, t, CH). H NMR (CD3CN) (ppm):

1.38-1.82 (12H, m, CH2), 3.14 (4H, dd, CH2), 3.81 (4H, dd, CH2), 4.27 (4H, s, benzylic

CH2), 6.75 (2H, d, CH), 6.94 (1H, t, CH).

Pd(pip2NCN)I. Excess NaI was added to a 25 mL solution of Pd(pip2NCN)Br

(0.20 g, 0.44 mmol) in acetone and the mixture was stirred for an hour. After removal of

the solvent, the solid was washed with water and ether. A light brown solid was

16 collected. Yield: 0.19 g, 85%. Crystallization from acetone gave pale yellow crystals.

Anal. Calc for C18H27N2PdI: C, 42.83; H, 5.39; N, 5.55. Found: C, 43.02; H, 5.41; N,

1 5.47. H NMR (CDCl3) (ppm): 1.36-1.80 (12H, m, CH2), 3.30 (4H, d, CH2), 4.23 (4H,

1 t, CH2), 4.28 (4H, s, benzylic CH2), 6.75 (2H, d, CH), 7.00 (1H, t, CH). H NMR

(CD3CN) (ppm): 1.36-1.90 (12H, m, CH2), 3.21 (4H, d, CH2), 3.97 (4H, t, CH2), 4.31

(4H, s, benzylic CH2), 6.77 (2H, d, CH), 6.99 (1H, t, CH).

[Pd(pip2NCN)(py)]BF4. To 25 mL of an acetone solution of Pd(pip2NCN)Br

(0.10 g, 0.22 mmol) in the dark was added AgBF4 (0.048 g, 0.25 mmol). After stirring

for 30 min, the mixture was filtered through Celite. The filtrate was stirred with pyridine

(py, 0.0174 g, 0.22 mmol) for 30 min, before removal of the solvent by rotary

evaporation. The residue was redissolved in a minimum amount of dichloromethane.

Addition of hexanes afforded an off-white solid. Yield: 0.096 g, 80%. Anal. Calc for

1 C23H32N3PdBF4: C, 50.80; H, 5.93; N, 7.73. Found: C, 50.87; H, 5.97; N, 7.58. H

NMR (CD3CN) (ppm): 1.45-1.86 (12H, m, CH2), 3.14 (4H, m, CH2), 3.35 (4H, m,

CH2), 4.29 (4H, s, benzylic CH2), 6.80 (2H, d, CH), 7.01 (1H, t, CH), 7.38 (2H, dd, CH),

7.80 (1H, dd, CH), 8.60 (2H, d, CH).

[Pd(pip2NCN)(phpy)]BF4. The same procedure as for [Pd(pip2NCN)(py)]BF4, except 4-phenylpyridine (phpy, 0.040 g, 0.22 mmol) was substituted for pyridine. Yield:

0.13 g, 92%. Anal. Calc for C29H37N3PdBF4: C, 56.10; H, 6.00; N, 6.77. Found: C,

1 55.65; H, 5.90; N, 6.67. H NMR (CD3CN) (ppm): 1.45-1.85 (12H, m, CH2), 3.14

(4H,d, CH2), 3.37 (4H, t, CH2), 4.29 (4H, s, benzylic CH2), 6.81 (2H, d, CH), 7.01 (1H, t,

CH), 7.53 (3H, m, CH), 7.63 (2H, d, CH), 7.74 (2H, d, CH), 8.63 (2H, d, CH).

17 pip2NNN. Piperidine (11.3 mL, 0.11 mol) was added to 20 mL of benzene under

argon. After cannula transfer of a benzene solution of 2,6-bis(chloromethyl)pyridine (2.0

g, 0.0114 mol), the mixture was stirred for 2 h at room temperature. The white

suspension was filtered, and the yellow filtrate was reduced in volume to give a yellow

1 oil that was used in subsequent steps. Yield: 2.8 g, 90%. H NMR (CD3CN) (ppm):

1.46 (4H, m, CH2), 1.60 (8H, m, CH2), 3.62 (4H, s, pyridyl-CH2), 7.28 (2H, d, CH), 7.60

(1H, t, CH).

[Pd(pip2NNN)Cl]Cl. Method 1. To 20 mL of an acetonitrile solution of

Pd(COD)Cl2 (0.15 g , 0.53 mmol) under argon was added 5 mL of an acetonitrile solution

of pip2NNN (0.16 g, 0.6 mmol) to give an tan precipitate. The mixture was stirred for 3 h

at room temperature before filtering, and the filtrate was reduced to dryness by rotary

evaporation. The yellow solid was dissolved in dichloromethane, and hexanes were

added to induce precipitation. Yield: 0.19 g, 80%.

Method 2. To a solution of PdCl2 (0.18 g, 1mmol) in 15 mL of acetonitrile at 40

°C was added a solution of pip2NNN (0.032 g, 1.1 mmol) in 5 mL of acetonitrile, and the

mixture was stirred for 30 min at room temperature. The tan solid was removed by

filtration, and the filtrate was evaporated to dryness. The resulting yellow solid was

dissolved in dichloromethane and precipitated with hexanes. Yield: 0.32 g, 72%. Anal.

Calc for C17H27N3PdCl2.1.5 H2O: C, 42.74; H, 6.33; N, 8.79. Found: C, 42.71; H,

6.65; N, 8.78. 1H NMR (CD3CN) (ppm): 1.42-1.90 (12H, m, CH2), 3.29 (4H, m,

CH2), 3.62 (4H, m, CH2), 4.64 (4H, s, benzylic CH2), 7.49 (2H, d, CH), 8.06 (1H, t,

CH). 1H NMR (CDCl3) (ppm): 1.52-1.95 (12H, m, CH2), 3.38 (4H, m, CH2), 3.74

(4H, m, CH2), 4.88 (4H, s, benzylic CH2), 7.85 (2H, d, CH), 8.08 (1H, t, CH).

18 Measurements. 1H NMR spectra were recorded using a Bruker AC 400 MHz

Spectrometer. UV-visible absorption spectra were recorded using a HP8453 UV-visible

spectrometer.

X-ray Crystallography. Colorless intergrown plates of Pd(pip2NCN)Br were

grown by slow evaporation of an acetone-hexanes solution. Pale-yellow blocks of

[Pd(pip2NCN)(phpy)]BF4 were prepared by slow evaporation of a dichloromethane-ether

solution. Yellow plates of [Pd(pip2NNN)Cl]Cl were obtained by slow evaporation of a

acetonitrile-ether solution. Diffraction data were collected at 150 K using a standard

Bruker SMART6000 CCD diffractometer with graphite- . /

(01)23145 Å. The data frames were processed using the program SAINT. 17

The data were corrected for decay, Lorentz and polarization effects. Absorption and

beam corrections based on the multi-scan technique were applied using SADABS.18 The structures were solved by a combination of direct methods SHELXTL 19 and the

difference Fourier technique. The models were refined by full-matrix least squares on F2.

For all compounds, non-hydrogen atoms were refined with anisotropic displacement

parameters with the exception of the disordered F atoms of [Pd(pip2NCN)(phpy)]BF4.

- The BF4 counterion is disordered for atoms F2-F4; the disorder was modeled using a

three component model with relative populations of 0.5:0.3:0.2. H atoms for all three

structures were calculated based on geometric criteria and treated with a riding model.

Crystal and refinement data are collected in Table 2.1.

19 Table 2.1. Crystal and structure refinement data for Pd(pip2NCN)Br and

[Pd(pip2NCN)(phpy)]BF4 and [Pd(pip2NNN)Cl]Cl..

Pd(pip2NCN)Br [Pd(pip2NCN)(phpy)]BF4 Pd(pip2NNN)Cl]Cl

Formula C18H27N2BrPd [C29H36N3Pd]BF4 [C17H27N3ClPd]Cl Fw (g/mol) 457.73 619.82 450.72 Crystal system Triclinic Orthorhombic Triclinic Space group P-1 Pbca P-1 a (Å) 9.8776(3) 10.8038(2) 6.23160(1) b (Å) 10.2385(3) 19.5002(4) 10.8100(2) c (Å) 19.1904(3) 25.7324(6) 14.3097(3) °) 75.369(1) 90 101.549(1)

°) 76.144(1) 90 91.598(1) °) 71.215(1) 90 106.117(1)

V (Å3), Z 1751.11(9), 4 5421.2(2), 8 903.74(3), 2 T (K) 150(2) 150(2) 150(2)

-3 Dcalc (g cm ) 1.736 1.519 1.656 (mm-1) 11.189 5.959 11.009 F (000) 920 2544 460

range °) 2.42-65.08 3.43-68.02 3.16-67.83 Refls. Coll. 14018 32427 7609 Ind. Refls. 5753/397 4859/352 3102/235

Rint 0.0307 0.0293 0.0163 Data/Parameters 5220/397 4859/352 3102/235 GOF on F2 1.033 1.039 1.073

R1/wR2 [I>2 (I)] 0.0283/0.0738 0.0286/0.0732 0.0229/0.0603

R1/wR2 (all data) 0.0315/0.0760 0.0295/0.0739 0.0234/0.0606

20 Results and Discussion

Synthesis. Drawing on the work of van Koten and coworkers,20-23 we have

- prepared a series of palladium(II) complexes with the pip2NCN ligand (Scheme 2.1).

The chloro and bromo adducts, Pd(pip2NCN)X (X=Cl, Br), were synthesized by allowing

Pd(COD)X2 to react with the lithiated tridentate ligand. The bromo derivative can be

readily converted to the chloro or iodo products by stirring with excess NaCl or NaI,

respectively. In analogy to previously reported platinum(II) complexes,16, 24 the py and

phpy compounds were prepared by reaction of the bromo adduct with one equivalent of

AgBF4, followed by treatment with the appropriate pyridyl ligand. The nearly colorless

products are soluble in chloroform, dichloromethane, acetonitrile and acetone.

The coordination properties of the new tridentate pip2NNN ligand also were

investigated (Scheme 2.2). Stirring palladium dichloride or Pd(COD)Cl2 with one

equivalent of pip2NNN yields Pd(pip2NNN)Cl. The yellow product is soluble in polar solvents, including acetonitrile, ethanol and water. A minor product with limited solubility in acetonitrile also was obtained.

1H NMR Spectroscopy. A general labeling scheme for nonequivalent protons is

+ 1 - shown in Scheme 2.1 for Pd(pip2NCN)(phpy) . The H NMR spectra of the pip2NCN

complexes exhibit patterns consistent with C2 symmetry and are qualitatively similar to

those of their platinum analogues.16, 24, 25 A triplet and a doublet due to the para and meta protons of the phenyl ring (A, B) occur between 6.7 and 7.0 ppm (Figure 2.1). The benzylic proton resonance (C) appears as a singlet near 4.2-4.3 ppm. Two multiplets between 3.1 and 4.0 ppm are attributable to the diastereotopic -protons (D', D") of piperidyl rings; the gap between these resonances (0.2-0.9 ppm) decreases along the

21 N

Pd I

(iv) +

N N N (i) (v) Br Pd Br Pd N

N N N

(ii) (iii) (vi) F E + D N C N B G H I J Pd Cl A Pd N

N N

Scheme 2.1 Synthesis of Pd(pip2NCN)X (X=Cl, Br, I) and [Pd(pip2NCN)L](BF4)

(L=pyridine, 4-phenyl pyridine). (i) THF, BuLi, -70°C; Pd(COD)Br2. (ii)

THF, BuLi, -70°C; Pd(COD)Cl2. (iii) Excess NaCl, acetone. (iv) Excess

NaI, acetone. (v) AgBF4, pyridine, acetone. (vi) AgBF4, 4-

phenylpyridine, acetone.

22 + N N Pd(COD)Cl2 or PdCl2 N N Pd Cl acetontrile, 40°C N N

Scheme 2.2 Synthesis of [Pd(pip2NNN)Cl]Cl

series I > Br > Cl > phpy ~ py and is ~0.1 ppm smaller than observed for analogous Pt(II) complexes. The remaining aliphatic proton resonances (E, F) appear as complex multiplets further upfield (1.4-1.9 ppm). For the two py and phpy complexes, the pyridyl resonances are shifted downfield of the corresponding free ligand resonances, confirming interaction with the palladium center. However, addition of a 3-fold excess of pyridine to

+ a solution of Pd(pip2NCN)(py) resulted in a single set of pyridyl resonances, indicating

that ligand exchange is fast on the NMR timescale. This behavior contrasts sharply with that noted for related platinum complexes;16, 22 though partial ligand dissociation occurs

in CD3CN, the exchange is comparatively slow and distinct sets of resonances for

coordinated and free pyridyl ligand are observed.

It is noteworthy that, for the Pd(pip2NCN)X complexes (X=Cl, Br, I), the

resonances undergo a slight downfield shift along the Cl

observed for analogous platinum(II) complexes.16 The deshielding effect going from the

chloro to iodo complex is greatest (~0.16 ppm) for the furthest downfield of the piperidyl

-proton resonances (D'). The trend opposes the relative electronegativities of the

23 halogen groups, as well as patterns in 195Pt NMR experimental and computational

results.26 However, this behavior has been noted for related compounds27 and is consistent with structural, spectroscopic and reactivity patterns of many transition metal complexes.28 Antipin, Grushin and coworkers27 have argued that similar trends in

crystallographic and NMR data for trans-Pd(PPh3)2(Ph)X (X=F, Cl, Br, I) can be

understood in terms of filled/filled repulsions between the d orbitals of the metal center

and the lone pair orbitals of the halide ligands. Infrared studies of five-coordinate

RuHX(CO)(Pt (CMe3)2Me)2 (X=F, Cl, Br, I) have established that the carbonyl stretching frequency increases along the F

29 - decrease along this series. The deshielding of the pip2NCN ligand resonances along

the Cl

electron releasing properties of the Pd-X unit.

1 + The pattern of resonances in the H NMR spectrum of Pd(pip2NNN)Cl is qualitatively similar to that of Pd(pip2NCN)Cl, though the pip2NNN resonances are

generally shifted downfield, in keeping with observations of complexes with Me4NNN

- (2,6-bis(dimethylaminomethyl)pyridine) and Me4NCN ligands (Me4NCNH=1,3-bis

30-32 (dimethylaminomethyl)benzene). For example, in CD3CN the phenyl resonances associated with protons A and B are shifted downfield by 1.1 and 0.7 ppm, respectively,

- from those of the corresponding pip2NCN complex. The shifts are smaller for the benzylic protons (C, 0.4 ppm) and the furthest upfield of the diastereotopic piperidyl - protons (D"), 0.01 ppm). In contrast the remaining -proton resonance (D') is shifted upfield, resulting in a considerably smaller gap (0.3 ppm) between the -proton resonances than observed for Pd(pip2NCN)Cl (0.7 ppm).

24 *

d 8 7 6 55 44 PPMPPM

c 8 7 6 55 44 PPMPPM

b 8 7 6 55 44 PPMPPM

a 8 7 6 55 44 PPMPPM

88577466 55 44 PPMPPM (ppm)

1 Figure 2.1 H NMR spectra of (a) Pd(pip2NCN)Cl, (b) Pd(pip2NCN)Br, (c) Pd(pip2NCN)I, (d) [Pd(pip2NNN)Cl]Cl in CD3CN and

(e) [Pd(pip2NNN)Cl]Cl in CDCl3. * indicates solvent (CHCl3).

25 In CD3CN, the difference (0.6 ppm) between the chemical shifts (=para-meta)

of the para and meta protons of the pip2NNN pyridyl ring is significantly larger than

- found for complexes with the pip2NCN pincer ligand (~0.2 ppm). Interestingly, is substantially reduced in CDCl3 (0.2 ppm), mostly as a consequence of a 0.36 ppm

- downfield shift of meta. In contrast, values of para and meta for the pip2NCN complexes

are only weakly dependent on solvent. Chemical shift correlations suggest that para is

33-37 more strongly influenced by resonance effects than meta. Therefore, the solvent

1 + sensitivity of the of the H NMR spectrum of Pd(pip2NNN)Cl is consistent with unequal perturbation of the resonance and field effects by specific solvent-solute interactions. It is reasonable to anticipate that Pd-solvent interactions are enhanced for the more acidic

+ metal center of Pd(pip2NNN)Cl , as compared to Pd(pip2NCN)Cl.

Crystal Structures. In order to investigate the coordination geometries of these

complexes, the structures of Pd(pip2NCN)Br, [Pd(pip2NCN)(phpy)]BF4 and

[Pd(pip2NNN)Cl]Cl were determined by X-ray crystallography. ORTEP diagrams are

shown in Figures 2.2, 2.3 and 2.4 and relevant data are summarized in Tables 2.1, 2.2 and

2.3. For the two salts, the anions and cations pack as discrete units, and for all three

structures, there are no unusually short intermolecular contacts.

- For each complex, the piperidyl ligand (pip2NCN or pip2NNN) is tridentate,

bonded to an approximately square planar palladium center with the central phenyl or

pyridyl ring positioned trans to the monodentate ligand (Br-, phpy or Cl-). The

- geometries of the pip2NCN ligands are similar to those found for platinum(II) complexes. Each piperidyl ring adopts a chair conformation with the metal center at an equatorial position on the N(piperidyl) atom. As expected for an equatorial substituent,

26 the angle formed by the normal to the plane defined by the - and -carbon atoms of a

piperidyl group (C8, C9, C11, C12; C14, C15, C17, C18) and the corresponding Pd-

N(piperidyl) vector approaches 90° (Br-, 86.2, 80.3, 81.6, 83.1°; phpy, 79.7, 82.8°),

whereas the normal is nearly parallel to the C(benzylic)-N(piperidyl) vector (Br-, 13.7,

+ 8.7, 10.3, 11.0°; phpy, 7.8, 10.7°). One of the piperidyl groups of Pd(pip2NNN)Cl

adopts a similar orientation with the plane normal forming an 85.2° angle with the Pd-N2

vector. Surprisingly, the remaining piperidyl group is flipped toward the Cl- ligand,

placing the palladium center at an axial position on N3. The normal to the plane defined

by the -and -carbon atoms of the piperidyl group (C14, C15, C17, C18) forms a 11.8°

angle with the Pd-N3 vector and a 77.3° angle with the C13-N3 vector. This unusual coordination geometry has not previously been encountered for palladium and platinum

16, 24, 25, 38 complexes with pip2NNN or related ligands.

The bond lengths and angles for the three compounds are in good agreement with

those observed for palladium(II) complexes with NCN- ligands. For example, though the

- Pd-C(phenyl) distances (Br, 1.927(3), 1.922(3); phpy, 1.922(2) Å) for the pip2NCN

complexes are significantly shorter than found for complexes with a phenyl group

39 positioned trans to a halide ligand (e.g., trans-Pd(Ph)(PPh3)2Cl, 2.005(3) Å), they fall within the 1.90-1.94 Å range typically observed for complexes with NCN- ligands.13, 21-23,

40-45 The Pd-N(piperidyl) distances for the Br- and phpy complexes (2.12-2.14 Å) also are similar to those reported for related compounds.13, 21, 23, 41-46 The Pd-N(pyridyl) bond

+ length for Pd(pip2NNN)Cl (Pd-N1, 1.956(2) Å) is in reasonable agreement with distances reported for complexes with similar pincer ligands, such as

47, 48 [Pd(Me4NNN)Me](CF3SO3) (1.996(8) Å). The corresponding Pd-N2 bond for

27 38, 47-49 [Pd(pip2NNN)Cl]Cl (2.109(2)) is slightly shorter, as expected. Interestingly, the

Pd-N3 distance for the flipped piperidyl group is comparatively long (2.133(2) Å),

reflecting the steric consequences of positioning the metal center at an axial site of the N3

- atom. For the monodentate ligands of the pip2NCN complexes, the metal ligand bonds

(Pd-Br; Pd-N(phpy)) are long, as a result of the strong trans influence of the phenyl

13, 21, 40, 42, 50, 51 + ligand. Similarly, the Pd-Cl distance for Pd(pip2NNN)Cl (2.3059(6) Å) is

consistent with bond distances reported for related palladium compounds with bidentate

and tridentate pyridyl donor ligands.39, 47, 48

The N(piperidyl)-Pt-N(piperidyl) bond angles are somewhat less than 180° (Br-,

163.02(10), 163.20(10); phpy, 163.46(8)°), falling within the 161-165° range observed

16, 24, 25, 38 - for related platinum(II) complexes. In the case of Me4NCN ligands, this deviation has been ascribed to the geometric preferences of the two five-membered

chelate rings.16, 24, 52, 53 These rings are slightly puckered, resulting in displacement of the benzylic carbons above and below (Br-, C7/C13, 0.54/0.49; 0.43/0.55 Å; phpy, 0.51/0.51)

the metal coordination plane, as defined by the metal and the four directly bonded atoms.

+ For Pd(pip2NNN)Cl , the displacements are on the same side of the plane and somewhat smaller (0.28/0.42 Å). To accommodate this puckering, the planar phenyl rings of the tridentate ligands are slightly rotated about the Pd-C(phenyl) bonds, forming small dihedral angles with the metal coordination plane (Pd(pip2NCN)Br, 8.43(6), 9.19(10);

+ Pd(pip2NCN)(phpy) , 8.5(1)°). These angles fall within the rather broad range (6-16°)

observed for other palladium(II) NCN and NNN complexes.13, 21, 23, 38, 40-45, 47-49 On the

+ other hand, the pyridyl group of Pd(pip2NCN)(phpy) is nearly perpendicular to the

coordination plane, forming a dihedral angle of 77.9(1)°. As noted for platinum(II)

28 analogues, this orientation is consistent with steric, as well as electronic effects, since the metal-pyridine -interactions do not strongly compete with the metal-phenyl - interactions.24, 54, 55

Figure 2.2 ORTEP diagrams of Pd(pip2NCN)Br with 50% probability ellipsoids.

Anion and H-atoms omitted for clarity.

29 Figure 2.3 ORTEP diagrams of [Pd(pip2NCN)(phpy)][BF4] with 50% probability ellipsoids. Anion and H-atoms omitted for clarity.

30 Figure 2.4 ORTEP diagrams of [Pd(pip2NNN)Cl]Cl with 50% probability ellipsoids.

Anion and H-atoms omitted for clarity.

31 Table 2.2 Selected distances (Å) and angles (°) for Pd(pip2NCN)Br and

[Pd(pip2NCN)(phpy)]BF4. * L= Br for Pd(pip2NCN)Br and N3 of 4-phpy

for [Pd(pip2NCN)(phpy)]BF4

Pd(pip2NCN)Br [Pd(pip2NCN)(phpy)]BF4 Pd(1)-C(1) 1.927(3), 1.922(3) 1.922(2) Pd(1)-N(1) 2.135(2), 2.137(2) 2.1206(18) Pd(1)-N(2) 2.128(2), 2.126(3) 2.1274(18) Pd(1)-L* 2.5468(4), 2.5612(4) 2.1650(19) N(1)-C(7) 1.513(4), 1.513(4) 1.514(3) N(2)-C(13) 1.517(4), 1.511(4) 1.514(3) C(2)-C(7) 1.501(4), 1.503(4) 1.499(3) C(6)-C(13) 1.508(4), 1.499(4) 1.503(3)

C(1)-Pd(1)-N(1) 81.56(11), 81.63(11) 81.59(9) C(1)-Pd(1)-N(2) 81.57(11), 81.57(11) 82.06(8) C(1)-Pd(1)-L* 178.54(9), 176.28(9) 178.36(9) N(1)-Pd(1)-N(2) 163.02(10), 163.20(10) 163.46(8)

N(1)-Pd(1)-L* 97.82(7), 96.64(7) 99.46(7) N(2)-Pd(1)-L* 99.10(7), 100.11(7) 96.94(7) C(7)-N(1)-Pd(1) 107.51(18), 108.68(18) 108.03(14) C(13)-N(2)-Pd(1) 108.44(17), 107.90(18) 107.89(13) C(2)-C(7)-N(1) 109.6(2), 109.6(2) 109.36(18) C(6)-C(13)-N(2) 109.6(2), 109.8(2) 109.84(18)

32 Table 2.3 Selected distances (Å) and angles (°) for [Pd(pip2NNN)Cl]Cl.

[Pd(pip2NNN)Cl]Cl Pd(1)-N(1) 1.956(2)

Pd(1)-N(2) 2.109(2) Pd(1)-N(3) 2.133(2) Pd(1)-Cl(1) 2.3059(6) N(2)-C(7) 1.505(3) N(3)-C(13) 1.497(3) C(2)-C(7) 1.498(4) C(6)-C(13) 1.498(4)

N(1)-Pd(1)-N(2) 82.31(8) N(1)-Pd(1)-N(3) 81.44(8) N(1)-Pd(1)-Cl 175.04(6) N(2)-Pd(1)-N(3) 157.70(8) N(2)-Pd(1)-Cl(1) 94.55(6) N(3)-Pd(1)-Cl(1) 102.58(6)

C(7)-N(2)-Pd(1) 106.83(14) C(13)-N(3)-Pd(1) 104.83(14) C(2)-C(7)-N(2) 111.7(2) C(6)-C(13)-N(3) 111.6(2)

33 - Electronic Spectroscopy. The pip2NCN complexes dissolve to give nearly

colorless solutions. To better understand the electronic structures of these complexes,

their absorption spectra were recorded in acetonitrile solution (Table 2.4; Figures 2.4,

2.5).

The UV-visible absorption spectra of the Pd(pip2NCN)X (X=Cl, Br, I) are

dominated by an intense absorption band near 240 nm (9,000-21,000 cm-1M-1),

tentatively assigned as having MLCT character. The transition is likely shifted to shorter

+ wavelengths in the spectrum of Pd(pip2NCN)(py) , since the metal center is expected to

be comparatively less electron rich. However, this complex also exhibits an absorption

maximum in this region (243 nm, 9450 cm-1M-1), in part, because of a pyridyl-centered

-* transition that also appears in the spectra of the free pyridine (252 nm, 2000

cm-1M-1) and protonated pyridine (256 nm, 5360 cm-1M-1),56 as well as other pyridyl

2+ -1 -1 57-61 complexes (e.g., Ru(NH3)5(py) , 244 nm, 4600 cm M ). At longer wavelengths,

the absorption intensity for the halide and py derivatives is considerably weaker.

Notably, in the region from 265 to 285 nm, the molar absorptivity is <3000 cm-1M-1 for

the Cl-, Br- and py derivatives and <4000 cm-1M-1 for the iodo complex. In contrast, the spectra of the corresponding platinum(II) complexes exhibit two relatively intense bands in this region (8000-10000 cm-1M-1).16, 24 We previously proposed that the transitions in

the spectra of the platinum(II) derivatives have significant MLCT character involving the

- pip2NCN ligand. The apparent blue shift of these bands in the spectra of these

palladium(II) complexes is in accord with this assignment. It is noteworthy that, as

24 + observed for the platinum(II) analogue, Pd(pip2NCN)(phpy) absorbs strongly from 250

34 to 283 nm, exhibiting a very broad absorption manifold with >13000 cm-1M-1. Free 4-

phpy exhibits

40 (a)

) x10 -1 cm

-1 0

(M 40 (b)

20 x10

200 250 300 350 400 450 Wavelength (nm)

Figure 2.5 UV-visible absorption spectra of (a) [Pd(pip2666. 7. 2

[Pd(pip2NCN)(py)]BF4 (.-.-9 6.6. 2 6.6: ....) and

Pd(NCN)I (----) in acetonitrile.

35 Figure 2.6 UV-visible absorption spectra of [Pd(NCN)(phpy)]BF4 2

[Pd(NCN)(py)]BF4 (----) in acetonitrile.

36 - Table 2.4 UV-visible Absorption Data for palladium pip2NCN and pip2NNN

complexes.

-1 -1 Compound (max, nm ( , cm M )

Pd(pip2NCN)Cl 247sh (8800), 279 (2600), 286sh (2300), 304 (1650)

Pd(pip2NCN)Br 205 (27100), 216sh (24700), 239 (15800), 274 (2300), 286sh (2150), 308 (1600)

Pd(pip2NCN)I 205 (24900), 216sh (23100), 243 (20700), 274sh (3300), 281sh (3000), 300sh (1900)

[Pd(pip2NCN)(py)]BF4 201(49000), 243 (9450), 262sh (3500), 273sh (2250), 281sh (2000), 300sh (1050)

[Pd(pip2NCN)(phpy)]BF4 202(66200), 271 (14200), 295sh (11600)

[Pd(pip2NNN)Cl]Cl 209 (34200), 252 (13150), 277sh (4300), 362 (1000)

37 a broad -* absorption near 255 nm (16600 M-1cm-1) that shifts to 285 nm (~17,000

M-1cm-1) on protonation.62 Similar ligand-centered transitions have been identified in the

+ -1 -1 24 2+ spectra of Pt(pip2NCN)(phpy) (280 nm, 32100 cm M ) Ru(NH3)5(4-phpy) (289 nm,

-1 -1 63 -1 -1 59 18600 M cm ) fac-Re(CO)3(Cl)(4-phpy)2 (268 nm, 36500 M cm ) and

64 Cu4Cl4(4-phpy)4 (286 nm), and a significant fraction of the intensity observed for

+ Pd(pip2NCN)(phpy) in this region is reasonably attributed to phpy-centered transitions.

+ Pd(pip2NCN)X (X=Cl, Br, I) and Pd(pip2NCN)(py) also exhibit a broad band between 300 and 310 nm (1000-2000 M-1cm-1). The intensity is somewhat greater than

expected for a d-d transition (<500 M-1cm-1),65-67 and Pd(II)-centered d -

transfer transitions are expected to occur at considerably shorter wavelengths (<200

nm).68 Similarly, it seems unlikely that this band has substantial MLCT character, since

it is clearly shifted to longer wavelengths than any comparably intense bands in the

spectra of platinum(II) analogues.16, 24, 25 The accumulated data support the notion that

this transition has significant ligand-to-metal charge-transfer (LMCT) character involving

- the pip2NCN ligand. In keeping with this interpretation, the band is relatively insensitive

to the nature of the monodentate ligand. At longer wavelengths, the spectra of the

- pip2NCN complexes exhibit long tailing absorption profiles that are attributable to ligand

field transitions.

+ The absorption spectrum of Pt(pip2NNN)Cl provides additional insight into the

electronic structures of these complexes. The UV spectrum is dominated by intense

absorptions between 250 and 300 nm (252 nm, 13150 cm-1M-1; 277sh nm, 4300 cm-1M-1).

These bands most likely have some contribution from pip2NNN ligand-centered transitions, since the free ligand absorbs moderately in this region (265 nm, 3800

38 -1 -1 - cm M ). As for the pip2NCN complexes, the spectrum exhibits a tailing profile at

longer wavelengths (<1000 cm-1M-1) due to ligand field transitions. It is noteworthy that,

- in contrast to our observations for the pip2NCN complexes, there is no evidence of a

charge-transfer transition near 300 nm in this spectrum. On the other hand, a broad

charge-transfer feature with comparable intensity occurs at considerably longer

wavelengths (362 nm, 1000 cm-1M-1, FWHM=4400 cm-1). A comparison to related

complexes confirms that this band is unlikely to have MLCT character. For example, the

lowest spin-allowed metal-to-ligand(pyridyl) charge-transfer band of Pt(2,6-

bis(aminomethyl)pyridine)(OH)+ in aqueous solution is shifted to the blue of 320 nm,69

and the corresponding transition for the palladium(II) analogue is expected to occur at

even shorter wavelengths. Similarly, the lowest spin-allowed metal-to-ligand(bpy)

charge-transfer transition of Pd(bpy)Cl2 (bpy=2,2'-bipyridine) occurs near 320 nm in

70 + aqueous solution; the MLCT transition of Pd(pip2NNN)Cl is expected to occur at shorter wavelengths because the stabilization of the unoccupied *(bpy) level relative to the *(pip2NNN) level. On the other hand, there are a number of examples of

palladium(II) complexes that exhibit LMCT transitions in the 300-400 nm range,71-73

-1 -1 -1 including trans-Pd(PPh3)2Cl2 (CH2Cl2: 345 nm, 20135 cm M , FWHM~3000 cm ) and

Pd(TPA)Cl+ (TPA=tris(pyridylmethyl)amine) (DMSO: 338 nm, 485 cm-1M-1; 380 nm,

-1 -1 74-76 + 416 cm M ). We suggest that the 362 nm band of Pd(pip2NNN)Cl also has

- significant X dx2-y2(Pd) charge-transfer character. The corresponding band for

Pd(pip2NCN)X (X=Cl, Br, I) is evidently shifted to shorter wavelengths, in keeping with

our expectation that the LMCT state will be destabilized by the strong sigma donor

39 - pip2NCN ligand, which increases electron density on the metal center and destabilizes the dx2-y2 level.

Conclusions. A new series of square planar palladium(II) complexes with pincer

- ligands, pip2NCN and pip2NNN, has been prepared: Pd(pip2NCN)X (X=Cl, Br, I),

[Pd(pip2NCN)(L)](BF4) (L=pyridine, 4-phenylpyridine), and [Pd(pip2NNN)Cl]Cl. For

- the complexes with pip2NCN , it has been shown that the halide ligand can be exchanged

for another halide by reacting with a sodium halide salt. Alternatively, ligand metathesis

with halide or pyridyl ligands can be accomplished by using a silver salt to scavenge the

halide ligand of Pt(pip2NCN)X. The X-ray crystal structures of Pd(pip2NCN)Br,

[Pd(pip2NCN)(4-phenylpyridine)]BF4, and [Pd(pip2NNN)Cl]Cl confirm the tridentate

- coordination geometries of the pincer ligands. For the pip2NCN complexes, each

piperidyl ring adopts a chair conformation with the metal center at an equatorial position

+ on the N(piperidyl) atom. However, one of the piperidyl groups of Pd(pip2NNN)Cl

adopts a previously unobserved coordination geometry, effectively placing the metal

center at an axial position on the N(piperidyl) atom.

1H NMR and UV-visible absorption measurements provide additional insight into

1 the electronic structures of these complexes. The H NMR spectra of Pd(pip2NCN)X

- (X=Cl, Br, I) are consistent with deshielding of the pip2NCN ligand resonances along the

that this trend is consistent with decreasing filled/filled repulsions between the d orbitals

of the metal center and the lone pair orbitals of the halide ligands along this series.

Electronic absorption spectra support the notion that ligand-to-metal charge-transfer

40 states are stabilized in these palladium(II) complexes relative to their platinum(II) analogues.

It is noteworthy that Pd(pip2NCN)Br can be oxidized with Br2 to

- Pd(pip2NCN)Br3, since this indicates that the pip2NCN ligand can stabilize a Pd(IV) center. The possibility of developing outer-sphere two-electron palladium reagents using

- the pip2NCN and pip2NNN ligands is discussed in Chapter 4.

References

(1) Tastan, S.; Krause, J. A.; Connick, W. B. Inorganica Chimica Acta 2006, 359,

1889-1898.

(2) Albrecht, M.; Gossage, R. A.; van Koten, G.; Spek, A. L. Chem. Commun. 1998,

1, 1003-1004.

(3) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000, 406, 970-974.

(4) Albrecht, M.; Hovestad, N. J.; Boersma, J.; van Koten, G. Chem. Eur. J. 2001, 7,

1289-1294.

(5) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem. Eur. J. 1998, 4, 759-762.

(6) Suijkerbuijk, B. M. J. M.; Shu, L.; Gebbink, R. J. M. K.; Schlueter, A. D.; van

Koten, G. Organometallics 2003, 22, 4175-4177.

(7) Slagt, M. Q.; Jastrzebski, J. T. B. H.; Gebbink, R. J. M. K.; van Ramesdonk, H. J.;

Verhoeven, J. W.; Ellis, D. D.; Spek, A. L.; van Koten, G. Eur. J. of Org. Chem.

2003, 1692-1703.

(8) Slagt, M. Q.; van Zwieten, D. A. P.; Moerkerk, A. J. C. M.; Gebbink, R. J. M. K.;

van Koten, G. Coord. Chem. Rev. 2004, 248, 2275-2282.

41 (9) Davies, P. J.; Veldman, N.; Grove, D. M.; Spek, A. L.; Lutz, B. T. G.; van Koten,

G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1959-1961.

(10) James, S. L.; Verspui, G.; Spek, A. L.; van Koten, G. Chem. Commun. 1996, 11,

1309-1310.

(11) Albrecht, M.; van Koten, G. Angew. Chem., Int. Edit. 2001, 40, 3750-3781.

(12) Rodriguez, G.; Albrecht, M.; Schoenmaker, J.; Ford, A.; Lutz, M.; Spek, A. L.;

van Koten, G. J. Am. Chem. Soc. 2002, 124, 5127-5138.

(13) Rodriguez, G.; Lutz, M.; Spek, A. L.; Van Koten, G. Chem. Eur.J. 2002, 8, 45-57.

(14) Jude, H.; Bauer, J. A. K.; Connick, W. B. Journal of the American Chemical

Society 2003, 125, 3446-3447.

(15) Stepnicka, P.; Cisarova, I. Collect. Czech. Chem. Commun. 1996, 61, 1335-1341.

(16) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2002, 41, 2275-2281.

(17) Bruker APEX2 v1.0-27 and SAINT v27.06A programs were used for data

collection and data processing, respectively. Bruker Analytical X-Ray

Instruments, Inc., Madison WI.

(18) SADABS v2.10 was used for the application of semi-empirical absorption and

beam corrections. G. M. Sheldrick, University of Göettingen, Germany.

(19) SHELXTL v6.14 was used for the structure solution and generation of figures and

tables. Neutral-atom scattering factors were used as stored in this package. G. M.

Sheldrick, University of Göttingen, Germany and Bruker Analytical X-ray

Solutions, Inc., Madison, WI.

(20) Grove, D. M.; van Koten, G.; Louwen, J. N.; Noltes, J. G.; Spek, A. L.; Ubbels,

H. J. C. J. Am. Chem. Soc. 1982, 104, 6609-6616.

42 (21) Dijkstra, H. P.; Slagt, M. Q.; McDonald, A.; Kruithof, C. A.; Kreiter, R.; Mills, A.

M.; Lutz, M.; Spek, A. L.; Klopper, W.; van Klink, G. P. M.; van Koten, G. Eur.

J. Inorg. Chem. 2003, 5, 830-838.

(22) Dijkstra, H. P.; Chuchuryukin, A.; Suijkerbuijk, B. M. J. M.; van Klink, G. P. M.;

Mills, A. M.; Spek, A. L.; van Koten, G. Adv. Synth. Catal. 2002, 344, 771-780.

(23) van den Broeke, J.; Heeringa, J. J. H.; Chuchuryukin, A. V.; Kooijman, H.; Mills,

A. M.; Spek, A. L.; van Lenthe, J. H.; Ruttink, P. J. A.; Deelman, B.-J.; van

Koten, G. Organometallics 2004, 23, 2287-2294.

(24) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004, 43, 725-733.

(25) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2005, 44, 1211-1220.

(26) Gilbert, T. M.; Ziegler, T. J. Phys. Chem. A 1999, 103, 7535-7543.

(27) Flemming, J. P.; Pilon, M. C.; Borbulevitch, O. Y.; Antipin, M. Y.; Grushin, V.

V. Inorg. Chim. Acta 1998, 280, 87-98.

(28) Caulton, K. G. New J.Chem. 1994, 18, 25-41.

(29) Poulton, J. T.; Sigalas, M. P.; Folting, K.; Streib, W. E.; Eisenstein, O.; Caulton,

K. G. Inorg. Chem. 1994, 33, 1476-1785.

(30) Sutter, J.-P.; James, S. L.; Steenwinkel, P.; Karlen, T.; Grove, D. M.; Veldman,

N.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics 1996, 15, 941-

948.

(31) Abbenhuis, R. A. T. M.; del Rio, I.; Bergshoef, M. M.; Boersma, J.; Veldman, N.;

Spek, A. L.; van Koten, G. Inorg. Chem. 1998, 37, 1749-1758.

(32) Dani, P.; Toorneman, M. A. M.; van Klink, G. P. M.; van Koten, G.

Organometallics 2000, 19, 5287-5296.

43 (33) Wells, P. R.; Ehrenson, S.; Taft, R. W. Progr. Phys. Org. Chem. 1968, 6, 147-

322.

(34) Parshall, G. W. J. Am. Chem. Soc. 1974, 96, 2360-2366.

(35) Parshall, G. W. J. Am. Chem. Soc. 1966, 88, 704-708.

(36) Coulson, D. R. J. Am. Chem. Soc. 1976, 98, 3111-3119.

(37) Ewing, D. F. In Correlation Analysis in Chemistry; Shorter, J., Ed.; Plenum Press:

New York, 1978.

(38) Li, Y.; Lai, Y.-H.; Mok, K. F.; Drew, M. G. B. Inorg. Chim. Acta 1998, 279, 221-

225.

(39) Mentes, A.; Kemmitt, R. D. W.; Fawcett, J.; Russeli, D. R. Polyhedron 1999, 18,

1141-1145.

(40) Tsubomura, T.; Tanihata, T.; Yamakawa, T.; Ohmi, R.; Tamane, T.; Higuchi, A.;

Katoh, A.; Sakai, K. Organometallics 2001, 20, 3833-3835.

(41) Alsters, P. L.; Baesjou, P. J.; Janssen, M. D.; Kooijman, H.; Sicherer-Roetman,

A.; Spek, A. L.; van Koten, G. Organometallics 1992, 11, 4124-4135.

(42) Slagt, M. Q.; Dijkstra, H. P.; McDonald, A.; Klein Gebbink, R. J. M.; Lutz, M.;

Ellis, D. D.; Mills, A. M.; Spek, A. L.; van Koten, G. Organometallics 2003, 22,

27-29.

(43) Dijkstra, H. P.; Meijer, M. D.; Patel, J.; Kreiter, R.; van Klink, G. P. M.; Lutz, M.;

Spek, A. L.; Canty, A. J.; van Koten, G. Organometallics 2001, 20, 3159-3168.

(44) Mills, A. M.; van Beek, J. A. M.; van Koten, G.; Spek, A. L. Acta Cryst., Sec. C:

Cryst. Str. Comm. 2002, C58, m304-m306.

44 (45) Slagt, M. Q.; Rodriguez, G.; Grutters, M. M. P.; Gebbink, R. J. M. K.; Klopper,

W.; Jenneskens, L. W.; Lutz, M.; Spek, A. L.; van Koten, G. Chem. Eur. J. 2004,

10, 1331-1344.

(46) Terheijden, J.; van Koten, G.; Grove, D. M.; Vrieze, K.; Spek, A. L. J. Chem.

Soc., Dalton Trans. 1987, 6, 1359-1366.

(47) Markies, B. A.; Wijkens, P.; Boersma, J.; Kooijman, H.; Veldman, N.; Spek, A.

L.; van Koten, G. Organometallics 1994, 13, 3244-3258.

(48) Arnaiz, A.; Cuevas, J. V.; Garcia-Herbosa, G.; Carbayo, A.; Casares, J. A.;

Gutierrez-Puebla, E. J. Chem. Soc., Dalton Trans. 2002, 12, 2581-2586.

(49) Markies, B. A.; Wijkens, P.; Dedieu, A.; Boersma, J.; Spek, A. L.; van Koten, G.

Organometallics 1995, 14, 5628-5641.

(50) Vila, J. M.; Pereira, M. T.; Ortigueira, J. M.; Torres, M. L.; Castineiras, A.; Lata,

D.; Fernandez, J. J.; Fernandez, A. J. Organometallic Chem. 1998, 556, 31-39.

(51) Pfeffer, M.; Sutter-Beydoun, N.; De Cian, A.; Fischer, J. J. Organometallic

Chem. 1993, 453, 139-146.

(52) Schmuelling, M.; Grove, D. M.; van Koten, G.; van Eldik, R.; Veldman, N.; Spek,

A. L. Organometallics 1996, 15, 1384-1391.

(53) Donkervoort, J. G.; Vicario, J. L.; Jastrzebski, J. T. B. H.; Smeets, W. J. J.; Spek,

A. L.; van Koten, G. J. Organometallic Chem. 1998, 551, 1-7.

(54) Grove, D. M.; van Koten, G.; Ubbels, H. J. C.; Vrieze, K.; Niemann, L. C.; Stam,

C. H. J. Chem. Soc., Dalton Trans. 1986, 4, 717-724.

(55) Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc.

2000, 122, 11822-11833.

45 (56) Swain, M. L.; Eisner, A.; Woodward, C. F.; Brice, B. A. J. Am. Chem. Soc. 1949,

71, 1341-1345.

(57) Jørgensen, C. K. Acta Chem. Scand. 1957, 11, 151-165.

(58) Ford, P. C.; Rudd, D. P.; Gaunder, R.; Taube, H. J. Am. Chem. Soc. 1968, 90,

1187-1194.

(59) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 2888-2897.

(60) Martin, M.; Krogh-Jespersen, M.-B.; Hsu, M.; Tewksbury, J.; Laurent, M.;

Viswanath, K.; Patterson, H. Inorg. Chem. 1983, 22, 647-652.

(61) Reimers, J. R.; Hush, N. S. Inorg. Chem. 1990, 29, 3686-3697.

(62) Favini, G. Gazz. Chim. Ital. 1963, 93, 635-648.

(63) Lavallee, D. K.; Fleischer, E. B. J. Am. Chem. Soc. 1972, 94, 2583-2599.

(64) Ryu, C. K.; Kyle, K. R.; Ford, P. C. Inorg. Chem. 1991, 30, 3982-3986.

(65) Rush, R. M.; Martin, D. S., Jr.; LeGrand, R. G. Inorg. Chem. 1975, 14, 2543-

2550.

(66) Martin, D. S.; Robbins, G. A.; Rush, R. M. Inorg. Chem. 1980, 19, 1705-1709.

(67) Vanquickenborne, L. G.; Ceulemans, A. Inorg. Chem. 1981, 20, 796-800.

(68) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2 ed.; Elsevier: Amsterdam,

1986.

(69) Hofmann, A.; Jaganyi, D.; Munro, O. Q.; Liehr, G.; van Eldik, R. Inorg. Chem.

2003, 42, 1688-1700.

(70) Gidney, P. M.; Gillard, R. D.; Heaton, B. T. J. Chem. Soc., Dalton Trans. 1973,

132-134.

46 (71) Lewis, F. D.; Salvi, G. D.; Kanis, D. R.; Ratner, M. A. Inorg. Chem. 1993, 32,

1251-1258.

(72) Zhang, W.; Bensimon, C.; Crutchley, R. J. Inorg. Chem. 1993, 32, 5808-5812.

(73) Kunkely, H.; Vogler, A. J. Organomet. Chem. 1998, 559, 223-225.

(74) Choi, C. L.; Phillips, D. L. Molec. Phys. 1998, 94, 547-554.

(75) Leung, K. H.; Szulbinski, W.; Phillips, D. L. Molec. Phys. 2000, 98, 1323-1330.

(76) Zhang, Z. H.; Bu, X. H.; Zhu, Z. A.; Chen, Y. T. Polyhedron 1996, 15, 2787-

2792.

47 CHAPTER 3:

Palladium(II) Halide Complexes with a Pyridyl Pincer Ligand

Introduction

Platinum(II) and palladium(II) complexes with tridentate cyclometallating

aryldiamine ligands have attracted increasing attention due to their potential utility in

catalytic systems,1-4 gas sensing devices5-7 and preparation of organometallic supramolecular structures and materials.8-12 Our group has been investigating the

- electronic structures and redox chemistry of platinum(II) complexes with pip2NCN

ligands, and we have examined the UV-visible absorption and emission spectroscopic

properties of a series of Pt(pip2NCN)L derivatives. These studies have shown that

variation of the monodentate ligand (L) provides remarkable control over the electronic

structures of this class of compounds.13-17

N N

- pip2NCN pip2NNN

We have recently extended these investigations to a series of palladium(II) analogs, as described in Chapter 2. It was found that ligand-to-metal charge-transfer states are stabilized in Pd(pip2NCN)X (X=Cl, Br, I) complexes relative to their platinum(II)

1 - analogs. In addition, investigation of H NMR spectra revealed that pip2NCN ligand

48 resonances are deshielded along the Cl

electronegativities. This result was attributed to decreasing filled/filled repulsions

between the d orbitals of the metal center and the lone pair orbitals of the halide ligands

along this series.18 In this chapter, we describe efforts to extend these investigations to palladium(II) complexes with the pip2NNN ligand. The first in this series of complexes,

[Pd(pip2NNN)Cl]Cl, was described in Chapter 2.

Experimental

All reagents were purchased from Pressure Chemical, Aldrich or Acros. Pd(COD)Br2

(COD=1,5-cyclooctadiene), Pd(COD)Cl2,pip2NNN and [Pd(pip2NNN)Cl]Cl and

18,19 [Pd(pip2NNN)Cl](BF4) were prepared according to published procedures.

1 Acetonitrile was distilled from CaH2 under argon. H NMR spectra were recorded using a Bruker AC 400 MHz Spectrometer. Deuterated acetonitrile and chloroform (0.03% tetramethylsilane (TMS) (v/v)) were purchased from Cambridge Isotope Laboratories.

Spectra are referenced in ppm with respect to a TMS standard. Variable temperature spectra were processed by line-shape analysis in WinNuts and Excel. The rate constants

for halide ligand exchange (k, s-1) were determined from chemical shifts and widths of

the resonances of interest. Thermodynamic parameters were calculated from the slope

and intercept of a ln(k/T) vs. 1/T plot. UV-visible absorption spectra were recorded using

a HP8453 UV-visible spectrometer. Elemental analyses were performed by Atlantic

Microlabs (Norcross, GA). Mass spectra were recorded using a Micromass Q-TOF-2

hybrid quadruple time of flight mass spectrometer (Water Corporation) with electrospray

ionization. The samples were dissolved and further diluted in acetonitrile (Burdick &

Jackson) before injection into the mass spectrometer using a syringe pump. The

49 instrument was optimized and calibrated in positive ion mode using poly-alanine

(Sigma). For negative ion mode, the instrument was re-calibrated in negative ion mode using sodium iodide (Fisher Scientific). The theoretical masses were calculated and compared to the experimentally obtained values using Mass Lynx 4.0 software.

+ Pd(pip2NNN)Cl Salts. PdCl2 (0.18 g, 1.1 mmol) and pip2NNN (0.032 g, 1.1 mmol) were stirred in 20 mL of acetonitrile at 40°C for 15 min. The solution was allowed to cool and was stirred for 30 min at room temperature. The resulting tan

+ precipitate was removed by filtration, and the yellow chloride salt of Pd(pip2NNN)Cl was isolated from the filtrate, as previously described.18 The previously unidentified tan solid was sonicated in acetonitrile, filtered and dried (Yield 0.35 g). Elemental analyses

2- - 2- are consistent with a mixture of the Pd2Cl6 / Cl salts (1:1 anion ratio) and/or the PdCl4

2- / Pd2Cl6 salts (2:1 anion ratio). Anal. Calc for C17H27N3Pd1.64Cl3.29: C, 36.16; H, 4.88;

N, 7.31. Found: C, 36.16; H, 4.88; N, 7.31. MS-ESI+ (m/z): 416.1. MS-ESI- (m/z):

- - - 213.7 ([PdCl3] ), 391 ([Pd2Cl5] ), 663.7 ([Pd(pip2NNN)Cl(PdCl4)] ), 839.43

- 1 ([Pd(pip2NNN)Cl(Pd2Cl6)] ). H NMR (CD3CN) (ppm): 1.49 - 1.85(12H, m, CH2),

3.32 (4H, m, CH2), 3.63 (4H, m, CH2), 4.59 (4H, s, benzylic CH2), 7.47 (2H, d, CH), 8.05

(1H, t, CH).

+ Pd(pip2NNN)Br Salts. To 20 mL of a room-temperature acetonitrile solution of

Pd(COD)Br2 (0.2 g, 0.53 mmol) under argon was added 5 mL of an acetonitrile solution of pip2NNN (0.16 g, 0.6 mmol). The mixture was stirred for 3h and filtered in air to give a yellow filtrate and an orange-brown precipitate. The filtrate was reduced to dryness by rotary evaporation. The orange-yellow solid was dissolved in acetonitrile, and ether was added to induce precipitation. Yield: 0.27 g, 94.4%. Anal. Calc for C17H27N3PdBr2. H2O:

50 C, 36.61; H, 5.24; N, 7.54. Found: C, 36.64; H, 5.30; N, 7.52. MS-ESI+ (m/z): 460.0

+ - +1 ([Pd(pip2NNN)Br] ). MS-ESI- (m/z): 78.9 (Br ). Pd(pip2NCN)Br H NMR (CD3CN)

(ppm): 1.39-1.88 (12H, m, CH2), 3.30 (4H, m, CH2), 3.78 (4H, m, CH2), 4.66 (4H, s,

1 benzylic CH2), 7.50 (2H, d, CH), 8.08 (1H, t, CH). H NMR spectrum of

Pd(pip2NCN)Br]Br (CDCl3) (ppm): 1.49 - 1.98 (12H, m, CH2), 3.46 (4H, m, CH2), 3.93

(4H, m, CH2), 4.89 (4H, s, benzylic CH2), 7.84 (2H, d, CH), 8.08 (1H, t, CH). The

orange-brown product was sonicated in acetonitrile, filtered and dried. Elemental

2- analyses are consistent with [Pd(pip2NNN)Br]2[PdBr4] and/or a mixture of the Pd2Br6 /

- Br salts (1:2 anion ratio). Anal. Calc for C17H27N3Pd1.5Br3: C, 30.49; H, 4.06; N, 6.27.

+ Found: C, 30.32; H, 4.05; N, 6.35. MS-ESI+ (m/z): 460.0 ([Pd(pip2NNN)Br] ). MS-

- - - ESI- (m/z): 346.6 ([PdBr3] ), 612.4 ([Pd2Br5] ), 886.6 ([Pd(pip2NNN)BrPdBr4] ),

- 1 1151.3 ([Pd(pip2NNN)BrPd2Br6] ). H NMR (CD3CN) (ppm): 1.43-1.89 (12H, m,

CH2), 3.33 (4H, m, CH2), 3.79 (4H, m, CH2), 4.60 (4H, s, benzylic CH2), 7.47 (2H, d,

CH), 8.07 (1H, t, CH).

+ Pd(pip2NNN)I Salts. Method 1: To a 20 mL burgundy acetonitrile solution of

PdI2 (0.2 g, 0.56 mmol) at ~50°C under argon was added 5 mL of an acetonitrile solution of pip2NNN (0.17 g, 0.62 mmol). The mixture was stirred for 3h at room temperature.

Pd black was removed by filtration in air and the filtrate was evaporated to dryness to

give an orange solid. The product was dissolved in acetonitrile, and the solution was

allowed to slowly evaporate, yielding orange crystals of the iodide salt. Yield: 0.24 g,

68%. Anal. Calc for C17H27N3PdI2 : C, 32.22; H, 4.30; N, 6.63 Found: C, 32.66; H,

+ - 1 4.37; N, 6.65. MS-ESI+ (m/z): 506.0 (Pd(pip2NNN)I ). MS-ESI- (m/z): 126.9 (I ). H

51 NMR (CD3CN) (ppm): 1.36-1.88 (12H, m, CH2), 3.35 (4H, m, CH2), 4.02 (4H, m,

CH2), 4.63 (4H, s, benzylic CH2), 7.49 (2H, d, CH), 8.09 (1H, t, CH).

Method 2: PdI2 (0.1 g, 0.28 mmol) was suspended in 20 mL acetonitrile in a

Schlenk flask in an ice-water bath. After stirring under argon for 15 minutes, pip2NNN

(0.084g, 0.31 mmol) in 5 mL acetonitrile was injected into the flask. The mixture was

stirred at 0°C for 2h and then allowed to warm gradually to room temperature. The

intense burgundy solid was collected by filtration in air, washed with acetonitrile and

sonicated in hexanes for 2h. Yield: 0.086 g. Elemental analyses are consistent with

2- - [Pd(pip2NNN)I]2[Pd2I6] and a small amount of the PdI4 and/or I salts. Anal. Calc for

C17H27N3Pd1.93I3.85: C, 21.12; H, 2.81; N, 4.35. Found: C, 21.10; H, 2.70; N, 4.35. MS

+ - - 1 ES+ (m/z): 506 (Pd(pip2NNN)I ). MS ES- (m/z): 486 (PdI3 ), 848 (Pd2I5 ). H NMR

(CD3CN) (ppm): 1.36-1.88 (12H, m, CH2), 3.35 (4H, m, CH2), 4.02 (4H, m, CH2), 4.62

(4H, s, benzylic CH2), 7.47(2H, d, CH), 8.09 (1H, t, CH).

Method 3: To 150 mL of ethanol-acetonitrile (90%-10%) solution of

[Pd(pip2NNN)I]I (0.14 g, 0.22 mmol) was added excess KI (~30 eq.) dissolved in an ethanol-water mixture. The solution was stirred for 24h at room temperature. The organic solvents were removed by rotary evaporation, and more water was added to

dissolve the remaining KI. The resulting burgundy precipitate was collected over a frit,

and the solid was sonicated in ~30 mL of acetonitrile for 30 min, filtered and dried in a

vacuum oven. Elemental analyses are consistent with [Pd(pip2NNN)I]2[PdI4] and/or a

2- - mixture of the Pd2I6 / I salts (1:2 anion ratio). Yield (Pd): 0.045 g, 38%. Anal. Calc for

C17H27N3Pd1.5I3: C, 25.09; H, 3.34; N, 5.16. Found: C, 25.47; H, 3.38; N, 5.21. MS ES+

+ - - 1 (m/z): 506 (Pd(pip2NNN)I ). MS ES- (m/z): 486 (PdI3 ), 848 (Pd2I5 ). H NMR (CD3CN)

52 (ppm): 1.36-1.88 (12H, m, CH2), 3.34 (4H, m, CH2), 4.02 (4H, m, CH2), 4.62 (4H, s, benzylic CH2), 7.45(2H, d, CH), 8.10 (1H, t, CH).

X-ray Crystallography. Yellow plates of [Pd(pip2NNN)Br](BF4) were obtained

from an acetonitrile-ether solution. Yellow blocks of [Pd(pip2NNN)Br]Br.H2O were

obtained by diffusion of ether into an acetonitrile solution of the complex. Dark red rods

of [Pd(pip2NNN)I]I were grown by slow evaporation of an ethanol solution. Dark purple

plates of [Pd(pip2NNN)I]2[Pd2I6] were prepared by slow evaporation of an acetonitrile

solution. Diffraction data were collected at 173 K for the bromide and iodide salts, and

- 2- 193 K for BF4 and the Pd2I6 salts, using a standard Bruker SMART6000 CCD diffractometer with graphite- . / (01)23145 Å for the chloride salt and a Bruker Platinum200 CCD detector at Beamline 11.3.1 at the

Advanced Light Source (Lawrence Berkeley National Laboratory) with synchrotron

(0;)442;; Å for the others. Data frames were collected using the program

APEX2 and processed using the program SAINT within APEX2.20 Absorption and beam corrections based on the multi-scan technique were applied using SADABS.21 The structures were solved by a combination of direct methods SHELXTL22 and the

difference Fourier technique. The models were refined by full-matrix least squares on F2.

For all compounds, non-hydrogen atoms were refined with anisotropic displacement

parameters. In the case of [Pd(pip2NNN)Br]Br.H2O, the H-atom positions of the solvent

waters were located directly and held fixed in subsequent refinements. The remaining H

atoms for all four structures were calculated and treated with a riding model. The isotropic displacement parameters for the H atoms were defined as a times Ueq of the

53 - adjacent atom where a = 1.5 for OH and 1.2 for all others. The BF4 anion shows typical

disorder.

Results and Discussion

+ + Synthesis of Pd(pip2NNN)X Salts. Salts of Pd(pip2NNN)X (X=Cl, Br, I),

+ hereafter referred to as [X] , are readily prepared by stirring either PdX2 or Pd(COD)X2

- in acetonitrile solution of pip2NNN. Depending on the reaction conditions, salts with X ,

2- 2- PdX4 and/or Pd2X6 counteranions are isolated (Scheme 3.1). The resulting products have nearly identical 1H NMR spectra in acetonitrile, but are readily distinguished by their colors, solubilities, negative anion mass spectra, electronic absorption spectra, and

elemental analyses. The Cl-, Br- and I- salts are obtained as pure compounds, whereas

2- 2- samples containing PdX4 or Pd2X6 are usually contaminated with one or both of the

other two anions. The halide salts form yellow to orange solutions, whereas salts

2- containing PdnX2n+2 anions (n=1, 2) are only weakly soluble in acetonitrile and give

yellow solutions in the case of Cl- and Br- and red-brown solutions in the case of I-.

Reaction of PdBr2 with one equivalent of pip2NNN resulted in a dark yellow solution with an orange-brown precipitate. 1H NMR and mass spectra, as well as

elemental analyses, confirm that the yellow major product isolated from the yellow

filtrate is the bromide salt of [Br]+. In contrast, analyses for the orange-brown minor

2- product are consistent with [Pd(pip2NNN)Br]2][PdBr4] and/or a mixture of the Pd2Br6 /

Br- salts (1:2 anion ratio). The electronic absorption spectrum verifies the presence of

2- + PdnBr2n+2 (n=1,2) whose absorption bands >300 nm effectively mask those of [Br] ;

2- 2- 23 however, because Pd2Br6 and PdBr4 salts have nearly identical maxima, electronic

54 F E D C N B + A N Pd Cl

(i) (iv)

N N (iii) + N N Pd I (v)

N N

(iv) (ii)

N + N Pd Br

+ Scheme 3.1 Synthesis of Pd(pip2NNN)X salts. (i) Pd(COD)Cl2 or PdCl2, acetonitrile,

- 40°C, Cl . (ii) Pd(COD)Br2 or PdBr2, acetonitrile, 40°C. (iii) PdI2,

acetonitrile. (iv) Excess NaI, acetonitrile-EtOH, room temperature. (v)

Excess NaBr, acetonitrile-EtOH, room temperature.

55 3

x 5

Absorbance 1

0 200 300 400 500 600 700 Wavelength (nm)

2- Figure 3.1 UV-visible absorption spectra of solutions containing PdnX2n+2 (n=1,2):

+ acetonitrile solution of the tan salt of Pd(pip2NNN)Cl (----), the orange-

+ brown salt of Pd(pip2NNN)Br 2 the burgundy salt of

+ Pd(pip2NNN)I (....) prepared at 0°C from the reaction of PdI2 with

pip2NNN.

spectroscopy was not a reliable means of distinguishing between these anions (Figure

3.1). The positive ion mass spectrum is similar to that of the bromide salt, exhibiting a

major peak corresponding to [Br]+. However, the negative ion mass spectra are distinctly

different. The spectrum of the bromide salt is dominated by Br- (100%) with weaker

- mass peaks associated with PdBr3 (15% intensity of most abundant ion),

- - Pd(pip2NNN)Br3 (8%) and [Br][PdBr4] (5%). By contrast, the most intense peak in the

56 - spectrum of the orange-brown minor product is for PdBr3 with additional peaks

- - - - corresponding to [Br][Pd2Br6] (35%), [Br][PdBr4] (22%), [Pd2Br5] (20%), and Br

2- (2%). Taken together, the results are consistent with the presence of both PdBr4 and

2- Pd2Br6 in the minor product.

+ The reaction of PdCl2 with pip2NNN yields the yellow chloride salt of [Cl] as the

major product.18 Elemental analysis of the pink-brown minor product from a typical

2- - preparation is consistent with a mixture of the Pd2Cl6 / Cl salts (1:1 anion ratio) and/or

2- 2- the PdCl4 / Pd2Cl6 salts (2:1 anion ratio). While the electronic absorption spectra

2- support the presence of PdnCl2n+2 (n=1,2) (Figure 3.1), the spectral similarities of the

2- PdnCl2n+2 anions, as well as overlap of their absorption bands with those of the cation,

2- 2- 23 prevented accurate estimation of the relative ratios of PdCl4 / Pd2Cl6 . The positive

ion mass spectra of both the major and minor products exhibit the expected [Cl]+ peak.

Although this peak has the greatest intensity in spectra of the chloride salt, several peaks

consistent with fragments of the cation are more intense in the spectrum of the minor

product. The negative ion mass spectra also are distinctly different. For both products,

- the most intense peak corresponds to PdCl3 . Under similar conditions, the spectrum of

- Na2[PdCl4] shows exclusively the PdCl3 mass peak. Therefore, the appearance of this

2- + peak in the spectrum of the chloride salt suggests that PdCl4 can form from [Cl] and

chloride anion, at least under the conditions of the ESI-MS experiment. The spectrum of

the chloride salt also shows intense peaks that correspond to ions with multiple chlorine

- - atoms, such as [Pd(pip2NNN)Cl3] (38%) and [(Pd(pip2NNN)Cl)2Cl3] (48%). The fact

that these peaks do not appear in the negative ion mass spectrum of

[Pd(pip2NNN)Cl(BF4) confirms the role of free chloride anion in their formation. In

57 addition, these peaks are weak (<3%) or absent from the spectrum of the minor product,

suggesting that there is little or no free chloride ion in that sample. Interestingly, a mass

- peak corresponding to [Cl][PdCl4] ion is moderately intense in the spectra of both the major (15%) and the minor products (25%). However, the mass spectrum of the minor

- product also exhibits an intense peak corresponding to [Pd(pip2NNN)Cl][PdCl6] (70%),

which is entirely absent from the spectrum of the chloride salt. Taken together, the

accumulated data are consistent with the minor product containing a mixture of the

2- 2- PdCl4 and Pd2Cl6 salts.

Stirring PdI2 in acetonitrile gives a solution whose burgundy color is

2- characteristic of the presence of PdnI2n+2 (n=1,2). Addition of one equivalent of pip2NNN at ~50°C under argon yielded a black mixture, from which the orange iodide

+ salt of [Pd(pip2NNN)I] was isolated. When carried out at 0°C (i.e., Method 2), the reaction yielded a burgundy product. In some cases, the elevated temperature reaction yielded some burgundy material, whereas on occasion, some orange product could be isolated from lower temperature reactions; these observations suggest that the product distributions are highly sensitive to the reaction conditions. The elemental analysis of a sample of the orange product is consistent with [I]I, whereas the analysis of a burgundy

+ sample from a typical preparation is consistent with ~90% of Pd(pip2NNN)I in the form

2- - 2- 1 of the Pd2I6 salt and remainder in the form of the I and/or PdI4 salt(s). H NMR

spectra and positive ion mass spectra confirm that both products contain

+ [Pd(pip2NNN)I] . The UV-visible absorption spectrum shows a long wavelength

-1 -1 absorption band near 540 nm (~7200 M cm assuming [I]2[Pd2I6]), which is consistent

2- with the [I]2[Pd2I6] formulation since Pd2I6 also absorbs strongly in this region (528 nm,

58 7200 M-1cm-1) (Figure 3.2).24,25 Interestingly, yellow acetonitrile solutions of the iodide

+ salt of [Pd(pip2NNN)I] are stable, slowly evaporating to give orange crystals. However,

if diethyl ether is added to the yellow solution, a burgundy solid gradually precipitates;

2- the composition of this solid is not known, but it most likely contains PdnI2n+2 .

+ + [X] + I- [I] + X- (1)

+ - (n+2)[I] + excess I [I]2[PdnI2n+2] + (n)pip2NNN (2)

Scheme 3.2 Reactions of [Cl]+, [Br]+ and [I]+ with iodide.

Overall, our isolated yields and qualitative observations are consistent with an

increasing tendency to form bridged Pd(II)-containing anions along the Cl

26,27 as also noted for several platinum(II) systems. . When PdX2 is used as a reagent

2- (X=Cl, Br, I), lower reaction temperature increases the yield of the PdnCl2n+2 salts. In

contrast, when Pd(COD)Cl2 or Pd(COD)Br2 are used as reagents, higher reaction

2- temperature increases the yield of the PdnCl2n+2 salts; reactions with Pd(COD)I2 were

not investigated. These observations suggest that kinetic and thermodynamic factors

1 + influence the product distribution. H NMR spectroscopy shows that Pd(pip2NNN)Cl is rapidly and nearly completely converted to the bromo adduct by simply stirring a solution

- 2- of the Cl or PdnCl2n+2 salt with excess NaBr in acetonitrile (Scheme 3.1). Similarly,

+ stirring alcohol solutions of Pd(pip2NNN)X (X= Cl, Br) salts with one or two

+ equivalents of NaI yielded Pd(pip2NNN)I (Scheme 3.2). These results indicate that the

+ - - - stability of Pd(pip2NNN)X in polar solvents increases along the series Cl < Br < I . On

59 15 )

-1 10 cm -1 (M -4

x 10 5

0 200 300 400 500 600 700 Wavelength (nm) Figure 3.2 UV-visible absorption spectra of the [I]2[PdnI2n+2] products in acetonitrile

prepared by Method 2 (---, assumes [I]2[Pd2I6] formulation) and Method 3

2, assumes [I]2[Pd2I8]0.5 formulation).

the other hand, stirring [Pd(pip2NNN)X]X (X= Cl, Br, I) with excess NaI resulted in a

+ burgundy salt of Pd(pip2NNN)I (i.e., Method 3). Elemental analysis data for the

2- burgundy product are consistent with formation of the PdI4 salt and/or a mixture of the

- 2- I / Pd2I6 salts in a 2:1 anion ratio. We are unable to distinguish between these

possibilities by UV-visible absorption spectroscopy or mass spectrometry. For example,

the UV-visible absorption spectrum shows a long wavelength absorption band near 540

60 -1 -1 nm whose intensity (3200 M cm for [I]2[Pd2I8]0.5) is approximately consistent with either of these formulations (Figure 3.2).28 The intensity of the 251 nm band (~82,000

-1 -1 M cm for [I]2[Pd2I8]0.5 is in good agreement with that predicted from the molar

+ 2- 2- -1 -1 28 absorptivities of [I] (~30,000) and the anions, PdI4 or 1/2(Pd2I6 ) (~20,000 M cm ).

2- It is noteworthy that, under similar conditions, PdnX2n+2 (n=1,2) is not formed from the

- reaction of I with Pd(pip2NCN)I, presumably because of the kinetic stability of the Pd-C

bond.

Crystal Structures. The compositions of [Br](BF4), [Br]Br, [Br]I and [I]2[Pd2I6]

were confirmed by single-crystal X-ray diffraction studies. The structure of [Cl]Cl has

been reported previously.18 ORTEP diagrams are shown in Figures 3.3-3.6, and relevant

data are summarized in Tables 3.1 and 3.2. In each case, the anions, cations and solvent

pack as discrete units, and there are no unusually short intermolecular contacts.

However, it should be noted that [Br]Br.H2O crystallizes with two unique molecules, A and B, in the asymmetric unit cell, and one of the water molecules forms hydrogen bonds

with the bromide counterions (O1-H1...Br2 = 3.319(3) Å, O1-H2...Br4 = 3.328(3) Å).

For each complex, the pip2NNN ligand is tridentate, bonded to an approximately

square planar palladium center with the central pyridyl ring positioned trans to the

monodentate ligand (Cl, Br or I). The geometries of the pip2NNN ligands are similar to

- those found for platinum(II) and palladium(II)complexes with pip2NCN and a fourth

13-15,18 ligand. For [Br]Br, [I]I and [I][Pd2I6], each piperidyl ring adopts a chair

61 Table 3.1. Crystallographic Data for [Br](BF4), [Br]Br.H2O, [I]I and [I]2[Pd2I6].

. [Br]BF4 [Br]Br H2O[I]I [I]Pd2I6

[C17H27N3BrPd]Br Formula [C17H27N3BrPd]BF4 . [C17H27N3IPd]I [C17H27N3IPd]2[Pd2I6] H2O Fw (g/mol) 546.54 557.65 633.62 1987.63

Crystal system Monoclinic Triclinic Monoclinic Triclinic

Space group P21/n P-1 P21/c P-1

a (Å) 11.6663(10) 12.5030(8) 13.9401(12) 7.6198(8)

b (Å) 7.8135(7) 12.8834(8) 12.4349(11) 11.6845(12)

c (Å) 22.4998(19) 14.7501(15) 12.8822(11) 14.1861(16)

°) 90 104.811(3) 90 80.234(3)

°) 95.093(2) 100.060(2) 115.133(2) 86.577(3)

°) 90 111.725(2) 90 76.471(3)

V (Å3), Z 2042.9(3), 4 2035.7(3), 4 2021.6(3), 4 1209.9(2), 1

T (K) 193(2) 173(2) 193(2) 173(2)

-3 Dcalc (g cm ) 1.777 1.820 2.082 2.728

(mm-1) 3.594 5.988 4.954 8.224

F (000) 1088 1104 1208 908

range °) 2.07-31.10 1.63-31.12 1.76-28.97 1.59-31.13

Refls. Coll. 24223 24870 19188 23948

Ind. Refls. 5050 9835 4122 5864

Rint 0.1127 0.0629 0.0924 0.0558

Data/Parameters 5050/262 9835/433 4122/208 5864/235

GOF on F2 1.067 1.030 1.037 1.017

R1/wR2 [I>2 (I)] 0.0556/0.1505 0.0450/0.1222 0.0505/0.1326 0.0268/0.0691

R1/wR2 (all data) 0.0590/0.1549 0.0477/0.1246 0.0537/0.1361 0.0297/0.0711 a -1 2 2 2 2 2 w 0 (Fo )+(aP) +bP] where P=0.33333Fo +0.66667Fc and a,b are refined quantities.

62 Table 3.2. Selected Bond Distances(Å) and Angles( [Br](BF4), [Br]Br.H2O, [I]I and [I]2[Pd2I6].

. [Br]BF4 [Br]Br H2O [I]I [I]Pd2I6

Pd(1)-N(1) 1.963(3) 1.952(3), 1.950(3) 1.962(5) 1.963(3)

Pd(1)-N(2) 2.111(3) 2.093(2), 2.096(2) 2.094(5) 2.100(3)

Pd(1)-N(3) 2.129(3) 2.090(2), 2.106(2) 2.099(5) 2.091(3)

Pd(1)-Br(1) 2.4295(6) 2.4369(4), 2.4467(4) 2.6213(6) 2.6057(4)

N(2)-C(7) 1.523(5) 1.510(4), 1.504(4) 1.512(8) 1.500(4)

N(3)-C(13) 1.506(5) 1.513(4), 1.499(4) 1.508(7) 1.514(4)

C(2)-C(7) 1.494(6) 1.486(4), 1.497(4) 1.491(9) 1.510(5)

C(6)-C(13) 1.506(4) 1.489(4), 1.496(4) 1.490(8) 1.488(5)

Pd(2)-I(2) / Pd(2)-I(2') 2.6185(4)/2.6230(4) Pd(2)-I(3) / Pd(2)-I(4) 2.5899(4)/ 2.5849(4)

N(1)-Pd(1)-N(2) 81.23(13) 81.36(10), 82.55(10) 82.1(2) 81.00(11)

N(1)-Pd(1)-N(3) 80.82(13) 81.45(10), 81.92(10) 82.40(19) 81.94(12)

N(1)-Pd(1)-X(1) 176.60(9) 177.36(8), 178.00(8) 176.20(14) 174.97(8)

N(2)-Pd(1)-N(3) 162.03(13) 162.77(10), 161.55(10) 163.02(19) 162.44(11)

N(2)-Pd(1)-X(1) 95.48(9) 99.08(7), 97.05(7) 98.31(14) 99.11(7)

N(3)-Pd(1)-X(1) 102.48(9) 98.14(7), 98.13(7) 96.70(13) 98.25(8)

C(7)-N(2)-Pd(1) 105.8(2) 104.44(17), 106.72(18) 106.5(4) 105.18(19)

C(13)-N(3)-Pd(1) 103.6(2) 105.25(17), 106.34(17) 104.9(3) 106.4(2)

C(2)-C(7)-N(2) 110.5(3) 110.2(2), 111.7(2) 112.2(5) 110.5(3)

C(6)-C(13)-N(3) 110.4(3) 110.8(2), 111.1(2) 111.9(5) 111.9(3)

I(2)-Pd(2)-I(2') 86.105(12) Pd(2)-I(2)-Pd(2') 93.894(12)

63 Figure 3.3 ORTEP diagrams of [Br](BF4), with 50% probability ellipsoids. H-atoms

and anion are omitted for clarity.

64 (a)

Figure 3.4 (a) shows ORTEP diagrams of molecule A of [Br]Br.H2O with 50% probability ellipsoids. H-atoms and anion are omitted for clarity.

65 (b)

(c)

Figure 3.4 (b) and (c) show ball-and-stick representations in the positions of the

benzylic carbon atoms with respect to the coordination plane for

molecules A and B of [Br]Br.H2O , respectively.

66 (a)

(b)

Figure 3.5 (a) shows ORTEP diagrams of [I]I with 50% probability ellipsoids. H-

atoms and anion omitted for clarity. (b) shows ball-and-stick

representation in the positions of the benzylic carbon atoms with respect to

the coordination plane.

67 (a)

(b)

Figure 3.6 (a) shows ORTEP diagrams for [I]2[Pd2I6] with 50% probability ellipsoids.

H-atoms omitted for clarity. (b) shows ball-and-stick representations in the

positions of the benzylic carbon atoms with respect to the coordination

plane.

68 conformation with the metal center at the equatorial position of the N(piperidyl) atom.

As expected for an equatorial substituent, the angle formed by the normal to the plane

defined by the - and -carbon atoms of a piperidyl group (C8, C9, C11, C12; C14, C15,

C17, C18) and the corresponding Pd-N(piperidyl) vector approaches 90° ([Br]Br, 87.3°,

88.4°, 85.6°, 86.4°; [I]I, 86.1°, 89.5°; [I][Pd2I6], 86.0°, 87.3°), whereas the normal is

nearly parallel to the C(benzylic)-N(piperidyl) vector ([Br]Br, 11.7°, 15.2°, 10.5°, 12.7°;

[I]I, 12.8°, 13.7°; [I]Pd2I6, 11.2°, 13.9°). The structures of the cations of [Cl]Cl and

[Br]BF4 are distinctly different from the aforementioned. For each complex, only one of

the piperidyl groups adopts a similar orientation in which the normal to the plane defined

by the -and -carbon atoms of a piperidyl group forms an angle with the Pd-N vector

that approaches 90° ([Cl]+, 85.2°; [Br]+, 86.7°). Surprisingly, the remaining piperidyl group is flipped toward the coordinating halide ligand, effectively placing the palladium center at an axial position on N3. The normal to the / -carbon plane (C14, C15, C17,

C18) forms a 11.8° and 9.2°angle with the Pd-N3 vector and a 77.3° and 77.2° angle with the C13-N3 vector for [Cl]+ and [Br]+, respectively. This unusual coordination geometry

has not previously been encountered for palladium and platinum complexes with

13-15,29 pip2NNN or related ligands, and these observations suggest a relatively small

energy difference between the two conformations.

The Pd-N(pyridyl) bond distance for each complex ([Cl]Cl, 1.956(2); [Br]BF4,

1.963(3); [Br]Br, 1.952(3), 1.950(3); [I]I, 1.962(5); [I]Pd2I6, 1.963(3) F s within the

1.93-2.1 F ">

ligands.30-37 As expected, the Pd-N(piperidyl) bond lengths are somewhat longer, and all

but two fall within a narrow range (2.101 ± 0.011 F; these distances are similar to those

69 observed for palladium(II) complexes with NCN- and NNN ligands.13-15,29-32,38 The lone

outliers are the Pd-N3 distances corresponding to the flipped piperidyl groups of [Cl]Cl

(2.133(2) Å) and [Br]BF4 (2.129(3) Å); the distances are longer, reflecting the steric consequences of positioning the metal center at an axial site of the N3 atom. The Pd-X

bonds (Pd-Cl, 2.3059(6) Å; Pd-Br, 2.4295(6), 2.4369(4), 2.4467(4); Pd-I, 2.6213(6),

2.6057(4) F ? h bond distances reported for related palladium

compounds with a pyridyl group positioned trans to the halide ligand.30,31,37,39-42

As in the case of palladium complexes with NCN- ligands, the N(piperidyl)-Pd-

N(piperidyl) bond angles are somewhat less than 180º ([Cl]Cl, 157.70(8)°; [Br]BF4,

162.03(13)°; [Br]Br, 162.77(10)º, 161.55(10)º; [I]I, 163.02(19)º; [I]Pd2I6, 162.44(11)º).

Similar deviations have been attributed to the geometric preferences of the two five-

13,14,43,44 membered chelate rings. In the case of [Br]BF4 and [I][Pd2I6], puckering of these rings results in the positioning of the benzylic carbons above and below (C7/C13,

0.61/0.69 Å and 0.67/0.52 Å, respectively) the metal coordination plane, as defined by the metal and the four directly bonded atoms. For molecule A in crystals of [Br]Br, the ligand adopts a similar conformation (0.69/0.62 Å). By contrast, molecule B and the cation in crystals of [Cl]Cl and [I]I adopt a geometry in which both benzylic carbons lie on the same side of the coordination plane and the displacements are somewhat smaller

(B[Br]Br, 0.27/0.32; [Cl]Cl, 0.28/0.42; [I]I, 0.28/0.37) (Figure 3.4). These two different geometries result in slightly different conformations for the ligand pyridyl group. In cases where the benzylic carbons are above and below the coordination plane, the planar pyridyl ring is slightly twisted about the Pd-N(pyridyl) bond, forming small dihedral angles with the metal coordination plane ([Br]Br molecule A, 12.9(1)º; [Br]BF4,

70 12.7(2)º; [I][Pd2I6], 12.9(2)º). In cases where both benzylic carbons lie on the same side of the coordination plane, the pyridyl ring is tilted in the opposite direction, forming comparatively smaller dihedral angles with the metal coordination plane ([Cl]Cl, 9.6(2)º;

[Br]Br, 10.3(1)º; [I]I, 5.0(3)º). The remarkable wide variability in the tridentate ligand geometry, as characterized by the displacement of the benzylic groups from the coordination plane and the pyridyl-coordination plane dihedral angle, suggests that the conformational energies of the complexes are not strongly influenced by variation of these parameters over modest ranges. These conclusions are supported by 1HNMR data

which indicate that the benzylic proton resonances are equivalent.

2- 45-47 Crystal structures of the Pd2I6 anion are relatively uncommon. The anion

lies on a center of inversion and is almost perfectly planar with the terminal iodides

deviating from the plane defined by the Pd-I2-I2'-Pd core by 0.0742 F 3) and 0.0213F

(I4). The terminal Pd-I distances (2.5899(4), 2.5849(4) F "

Pd-I distances (2.6185(4), 2.6230(4)F) @ " effectively pushed together while the palladiums are pulled away, resulting in an acute I2-Pd2-I2' angle

(86.105(12)º) and an obtuse Pd2-I2-Pd2' angle (93.894(12)º). Thus, the coordination

geometry around the palladium atoms is distorted square planar. The Pd-I bond

distances, I-Pd-I and Pd-I-Pd angles as well as Pd...Pd intra-anionic distances (3.830 F

2- are similar to those previously reported for Pd2I6 salts with alkylpyridinium (Pd-I

(2.582-2.603 Å), I-Pd-I (86.20- 94.27º), Pd-I-Pd (93.80º), Pd...Pd (3.802 Å)),

trialkylammonium (Pd-I, 2.593-2.611 Å; I-Pd-I, 85.11-92.10º; Pd-I-Pd, 94.89º; Pd...Pd,

3.841 Å), and phosphonium (Pd-I, 2.5830-2.6074 Å; I-Pd-I, 85.737- 93.405º; Pd-I- Pd,

94.500º; Pd...Pd, 3.804 Å) cations.45-47

71 Electronic Spectroscopy. To better understand the electronic structures of the

complexes with the pip2NNN ligand, absorption spectra of acetonitrile solutions of the

halide salts of [X]+ were recorded. The data are collected in Table 3.3 and the spectra are shown in Figure 3.7.

The UV region is dominated by intense absorptions between 200 and 280 nm

(Table 3.3). Since the free ligand absorbs moderately in this region (265 nm, 3800

cm-1M-1), these bands most likely have some contribution from ligand-centered

transitions. There is a moderately intense shoulder at 297nm (5600 cm-1M-1) in the [I]I

spectrum. Pd(pip2NCN)X (X=Cl, Br, I) complexes exhibit slightly weaker absorption

bands at shorter wavelengths (265-285 nm, 2000-4000 cm-1M-1 ) that have been assigned

- 18 as having significant MLCT character involving the pip2NCN ligand. The MLCT transitions of the pip2NNN complexes are expected to occur at longer wavelengths because the greater -acceptor of pip2NNN. Therefore, this band is tentatively assigned

as having significant MLCT character involving pip2NNN. The transition is anticipated to be shifted to shorter wavelengths in the spectra of the [Cl]Cl and [Br]Br complexes, and hence obscured by other transitions. In support of this assignment, it is noteworthy

that the lowest spin-allowed MLCT transition of Pd(4-mbpy)I2 (4-mbpy=4,4'-dimethyl-

2,2'-bipyridine) occurs near 306 nm (18400 cm-1M-1, in DMF).48

A broad charge transfer feature appears at longer wavelengths in the spectrum of each halide complex, [Cl]Cl (362nm, 1000 cm-1M-1, fwhm=2100 cm-1;[Br]Br (376 nm,

850 cm-1M-1, fwhm=2250 cm-1;) and [I]I (421sh nm, 800 cm-1M-1, fwhm= 2300 cm-1 ). A

comparison to related complexes suggests that this band is unlikely to have MLCT

character. For example, the lowest spin-allowed metal-to-ligand(pyridyl) charge-transfer

72 Table 3.3 UV-Vis Absorption Data for [Pd(pip2NNN)Cl]Cl, [Pd(pip2NNN)Br]Br,

[Pd(pip2NNN)I]I and [Pd(pip2NNN)I][Pd2I6].

-1 -1 Compound (max, nm (, cm M )

209 (34200), 252 (13150), 277sh (4300), [Pd(pip2NNN)Cl]Cl 362 (1000)

218 (29300), 266sh (10200), 277sh [Pd(pip2NNN)Br]Br (6700), 376 (850) 205 (32700), 249 (29600), 277sh [Pd(pip2NNN)I]I (11400), 297sh (5600), 421 (800)

band of Pt(2,6-bis(aminomethyl)pyridine)(OH)+ in aqueous solution is shifted to the blue

of 320 nm,49 and the corresponding transition for the palladium(II) analog is expected to occur at even shorter wavelengths. Similarly, the lowest spin-allowed metal-to-

ligand(bpy) charge-transfer transition of Pd(bpy)Cl2 (bpy=2,2'-bipyridine) occurs near

50 + 320 nm in aqueous solution; the MLCT transition of Pd(pip2NNN)X is expected to

occur at shorter wavelengths because of the stabilization of the unoccupied *(bpy) level relative to the *(pip2NNN) level. On the other hand, there are several examples of palladium(II) complexes that are believed to exhibit LMCT transitions in the >300 nm

51-53 -1 -1 region, including trans-Pd(PPh3)2Cl2 (CH2Cl2: 345 nm, 20135 cm M ,

-1 54,55 -1 -1 FWHM~3000 cm ), cis-Pd(P^P)Br2 (CH3CN: 354 nm, 14600 cm M ), cis-

-1 -1 56 + Pd(dbcpe)I2 (CH3CN: 396 nm, 6300 cm M ) and Pd(TPA)Cl (DMSO: 338 nm, 485

cm-1M-1; 380 nm, 416 cm-1M-1)57 (dbcpe= 1,2-bis[di(benzo-15-crown-

73 40

) 30 -1 cm -1

(M 20 -3 x 10 x 10

0 200 300 400 500 600 Wavelength (nm)

Figure 3.7 UV-visible absorption spectra of [Cl]Cl (----), [Br]Br 2 I]I (....)

in acetonitrile.

5)phosphino]ethane ligand; TPA=tris(2-pyridylmethyl)amine). By analogy, the long wavelength band in the spectra of [X]+ is tentatively assigned to a transition having

- significant p(X )dx2-y2(Pd) charge-transfer character. There is considerable variability in the intensities of reported long wavelength LMCT transitions of palladium(II) complexes. The comparatively low intensities of the bands in the spectra of [X]+ may be a consequence of spin-forbidden character. An alternative explanation is that these bands

74 arise from a ligand field transition. The red shift of this band along the halide series

the ~2800 cm-1 red shift from Br to I fall within the range of shifts in LMCT bands

8 -1 56 reported for square planar d complexes such as Pd(dbcpe)X2 (800, 3000 cm ), trans-

-1 58 + -1 [Pd2(P(Et)2CH2P(Et)2)2X4] (Cl-Br, 1690 cm ) and Ni(Cynp3)X (700, 3000 cm ,

59 Cynp3= tris(2-dicyclohexylphosphinoethyl)amine).

The UV-visible absorption profiles of the complexes with palladium halide anions

2- are expected to agree with the sum of the spectra for the individual cation and PdnX2n+2

anion components. Accordingly, because of the comparatively weak intensity of the

+ [Pd(pip2NNN)X] bands at wavelengths >300 nm, the long wavelength region of these

spectra is expected to be dominated by the anion absorption bands. Although quantitative

analysis was not possible, the spectra of solutions of the tan chloro, orange-brown bromo

and burgundy iodo solids provided qualitative verification of the presence of the

2- PdnX2n+2 anions.

2- The electronic spectroscopy of PdnX2n+2 anions has been extensively investigated. LMCT transitions involving occupied halide and levels of the halide

2- ligand and the d *(Pd) level are anticipated for [PdX4] , whereas an additional set of

2- transitions is expected for the bridging halides in the [Pd2X6] spectra. The p and p energy levels of the bridging ligands, which donate electron density to two metal centers,

are expected to be higher than the corresponding levels arising from the terminal ligands

which contribute electron density to a single metal center. As a result, the transitions

originating from the bridging halides occur at longer wavelengths than those originating from the terminal halides.60 Overall, the energies of all transitions are expected to

75 decrease along the halide series, Cl>Br>I. In accordance with these arguments, in the

2- spectra of [PdnCl2n+2] , the transitions in the region <320 nm is anticipated to have considerable contribution from both p (X-) d * and p(X-)d * LMCT transitions

(e.g., [(C4H9)4N]2[Pd2Cl6], 2:1 2-methyltetrahydrofuran: propionitrile, 200 nm (~70000

cm-1M-1), 244 nm (31, 200 cm-1M-1 ), 288 nm (2260 cm-1M-1)).23,24,47,60 However, for

2- 2- - * [PdnBr2n+2] and [PdnI2n+2] .spectra, contribution from mostly p (X )d LMCT

transitions are expected in this region. The p(X-)d * LMCT transitions are anticipated to contribute mostly to 330-400 nm and 340-450 nm regions of the

2- 2- [PdnBr2n+2] and [PdnI2n+2] spectra, respectively. Finally, the low energy transition is

2- anticipated to have contribution from metal centered transitions ([PdnCl2n+2] , 425 nm;

2- 2- [PdnBr2n+2] , 468 nm; [PdnI2n+2] , 546 nm).

1H NMR Spectroscopy. A general labeling scheme for inequivalent protons is

+ 1 + shown in Scheme 3.1 for [Pd(pip2NNN)Cl] . The H NMR spectra of [X] (X= Cl, Br, I)

in acetonitrile exhibit patterns consistent with C2 symmetry and are qualitatively similar

- 13-15,18 to those of their pip2NCN palladium and platinum analogs. A triplet and a doublet due to the para and meta protons of the pyridyl ring (A, B) occur between 7.4 and 8.15

ppm (Figure 3.8). The benzylic protons (C) give rise to a singlet near 4.6 ppm, in accord

with crystallographic data suggesting a relatively low barrier to motion of the benzylic

groups in and out of the coordination plane. For [Cl]+, the -piperidyl proton resonance

at 3.29 ppm (D") has the appearance of a doublet and is assigned accordingly to the

equatorial proton; the resonance at 4.02 ppm (D') has the appearance of a triplet and is assigned to the axial proton. These assignments reflect the expectation of strong coupling between the axial - and -protons.61 The remaining aliphatic proton resonances (E, F)

76 appear as complex multiplets further up field (1.4-1.9 ppm). As expected, the spectra of

- 2- the complexes with X and PdnX2n+2 counter-anions are essentially identical in CD3CN.

With the exception of the axial -piperidyl proton resonances (D') each of the

pip2NNN resonances is shifted downfield from that observed for the corresponding

Pd(pip2NCN)X complex. For example, in CD3CN the para- and meta-pyridyl proton resonances (A/B) are shifted downfield by ~1 ppm ([Cl]+, 1.12/0.74); [Br]+, 1.13/0.75;

+ [I] , 1.1/0.72 ppm) from those of Pd(pip2NCN)X. The shifts are smaller for the benzylic

protons, C ([Cl]+, 0.37 ppm; [Br]+, 0.37 ppm; [I]+, 0.36 ppm) and the equatorial -

piperidyl proton, D" ([Cl]+, 0.15 ppm; [Br]+, 0.12 ppm; [I]+, 0.12 ppm). By contrast, the axial -piperidyl proton resonance, D', is shifted upfield by 0.19 and 0.14 ppm in the

spectra of [Cl]+ and [Br]+, respectively, and downfield by 0.05 ppm in the spectrum of

[I]+. Thus, the gap between D' and D" resonances (D'-D": [Cl]Cl, 0.36; [Br]Br, 0.47; [I]I,

0.67 ppm) is smaller than observed for Pd(pip2NCN)X (X=Cl, 0.67; Br, 0.74; I, 0.76 ppm) and increases along the series Cl < Br < I (Cl, 0.33; Br, 0.48; I, 0.69 ppm). When

+ + [X] is treated with AgBF4 to give Pd(pip2NNN)(solvent) , the -piperidyl protons

appear as a singlet at 3.37 ppm, indicating that Pd-N(piperidyl) bond cleavage and ring

inversion are fast on the NMR timescale.

13,18 1 As noted for the M(pip2NCN)X (M=Pd, Pt; X=Cl, Br, I) series, the H NMR

+ resonances for Pd(pip2NNN)X undergo a slight downfield shift along the Cl < Br < I series (Figure 3.8). The deshielding effect going from the chloro to iodo complex is greatest (0.40 ppm) for the piperidyl axial -proton resonance (D'), exceeding shifts

62 observed for the analogous M(pip2NCN)X complexes (Pt, 0.30; Pd, 0.16 ppm). The

sensitivity of the axial proton resonances is consistent with crystal structure data for

77 [Cl]Cl and [Br]BF4 showing that the axial protons are 0.8-1.0 Å closer to the halide

ligand than the equatorial protons when the Pd center is at the equatorial position of the

piperidyl N atom. The sensitivity of the remaining resonances to halide ligand

substitution decreases along the A > D" > C > B series, which can be qualitatively

rationalized in terms of through-bond and through-space interactions. The trend along

the Cl < Br < I series opposes the relative electronegativities of the halogen groups, as

well as patterns in 195Pt NMR experimental and computational results.63 However, this

behavior has been noted for related compounds64 and is consistent with structural, spectroscopic and reactivity patterns of many transition metal complexes.65 Antipin,

Grushin and coworkers have argued that similar trends in crystallographic and NMR data

for trans-Pd(PPh3)2(Ph)X (X=F, Cl, Br, I) can be understood in terms of filled/filled

repulsions between the d orbitals of the metal center and the lone pair orbitals of the

halide ligands. Infrared studies of five-coordinate RuHX(CO)(P(CMe3)2Me)2 (X=F, Cl,

Br, I) have established that the carbonyl stretching frequency increases along the

A B D D" c b a 8 7 6 5 4 P 8 7 6 5 4 PPM 8 7766 5 544 PPM (ppm)

1 Figure 3.8 H NMR spectra of (a) [Pd(pip2NNN)Cl]Cl, (b) [Pd(pip2NNN)Br]Br, (c)

[Pd(pip2NNN)I]I in CD3CN.

78 F < Cl < Br < I series, indicating that filled/filled repulsions decrease along this

64,66 series. The deshielding of the pip2NNN ligand resonances along the Cl

likewise consistent with decreasing filled/filled repulsions and electron releasing

properties of the Pd-X unit.

Anion Dependence of the Chemical Shift. The 1H NMR spectra of samples of

+ [X] salts dissolved in CDCl3 are qualitatively similar to those obtained for samples

dissolved in CD3CN with the surprising distinction that the chemical shifts of the cation

are strongly dependent on the nature of the anion. The influence of the anion is strongest

for the meta-pyridyl and benzylic resonances which is the reverse of the sensitivity of

these resonances to changes in the coordinated halide ligand. As shown in Figure 3.9(a)

and 3.9(b), the spectra of [Cl]BF4 and [Cl]Cl in CDCl3 are distinctly different. The meta-

- pyridyl (B) and benzylic (C) proton resonances of the BF4 salt are shifted upfield by 0.27

and 0.23 ppm, respectively. By contrast, the chemical shifts of the remaining resonances

are nearly identical to those of the chloride salt. When slightly more than one equivalent

of tetrabutylammonium chloride, TBACl, is added to a chloroform solution of [Cl]BF4,

the benzylic and meta-pyridyl resonances shift back to where they appeared for the

[Cl]Cl complex. However, when TBABF4 is added to a sample of [Cl]BF4, no appreciable change (<0.05 ppm) is observed in the chemical shift of any resonance.

Likewise, addition of TBACl to the [Cl]Cl sample does not shift any of the [Cl]Cl

resonances significantly.67 The strong influence of one equivalent of chloride ion on the

NMR spectrum is indicative of an interaction between the cation and exogenous chloride

79 68-70 1 anion. These effects are attenuated in CD3CN. The H NMR spectra of [Cl]Cl and

[Cl]BF4 in CD3CN are very similar (<0.07 ppm difference), and addition of TBACl does

- + - not alter either spectrum. It is reasonable to expect that Cl , as well as [Cl] and BF4 , is

better solvated in the higher dielectric solvent (: CH3CN, 37.5; CHCl3, 5.5). In keeping

with this interpretation, addition of one equivalent of TBABF4 to a CDCl3 solution of the

[Cl]Cl modestly shifts the resonances toward their positions in the [Cl]BF4 spectrum; for example, the benzylic resonance C shifts upfield by 0.09 ppm. These observations are consistent with decreasing cation/halide interaction with increasing ionic strength.71

Conductivity measurements in acetonitrile confirm that [Cl]BF4 and [Cl]Cl are

2 essentially 1:1 electrolytes (0.12 mM [Cl]Cl, 138 Scm /mol; 0.12 mM [Cl]BF4, 155

Scm2/mol). Interestingly, even in dilute chloroform solution, both salts are essentially

2 2 non-electrolytes (0.12 mM [Cl]Cl, <2 Scm /mol; 0.12 mM [Cl]BF4, <2 Scm /mol). This observation and the aforementioned 1H NMR data indicate that the cation-anion

interaction for [Cl]Cl in CDCl3 is stronger than for [Cl]BF4 and places the metal complex

in a significantly different chemical environment than for [Cl]BF4. The similarity

between the spectra of [Cl]BF4 in CDCl3 and CD3CN, as well as the weak Lewis basicity

- of BF4 , are consistent with conventional ion-pairing. On the other hand, the cation-anion

interaction for [Cl]Cl in CDCl3 causes a more significant perturbation of the NMR

spectrum, possibly because the chloride counterion is engaged in an inner-sphere

interaction with the metal to give a five-coordinate complex. Such a structure must be

fluxional or symmetric such that the equivalency of the piperidyl groups is preserved on

the NMR timescale.

80 *

(f) (e) (d) * (c) (b) (a)

876 54 (ppm)

1 Figure 3.9 H NMR spectra of (a) [Cl][BF4], (b) [Cl]Cl, (c) [Br]Br, (d) 1:1 [Cl][BF4]

/ TBABr, (e) 1:1 [Cl][BF4] / [Br]Br, (f) 1:1 [Br]Br / [Cl]Cl in CDCl3.*

denotes solvent residual resonance and TBABr resonance.

81 The sensitivity of certain resonances to substitution of the halide ligand or the

counter-anion is convenient for investigations of the influence of halide anion in

mixtures. For example, when one equivalent of TBABr was added to a CDCl3 solution

of [Cl]BF4, each resonance (except D') appears at chemical shifts that are close to the

average values for pure [Cl]BF4 and [Br]Br solutions (e.g., C: 4.79 ppm

(4.65+4.89)/2=4.77 ppm; B: 7.75 ppm (7.83+7.61)/2=7.72 ppm). Although the solution is

a mixture of several species (i.e., [X]BF4 and [X]X where X=Cl, Br), the coordinated

halide has little effect on the chemical shifts and consequently they are close to the

averaged values. The D' protons give rise to two distinct resonances in a 3:1 intensity

ratio (Figure 3.9(d)). The less intense resonance (3.92 ppm) is nearly coincident with that

of the equatorial -piperidyl proton resonance of [Br]Br (3.93 ppm), whereas the more

intense resonance (3.75 ppm) is coincident with the -piperidyl proton resonance of

[Cl]Cl (3.75 ppm). Thus, only about 25% of the chloride ligand is replaced by bromide

confirming the preference for chloride over bromide discussed previously. Assuming

that the coordinated halide does not influence the chemical shifts of B and C (i.e.,

[Cl]BF4)=([Br]BF4); ([Cl]Br=([Br]Br); ([Br]Cl)=([Cl]Cl)) and that the

[Cl]X:[Br]X ratio is 3:1 (i.e., the [X]Cl:[X]Br ratio is 1:3; X=Cl, Br), we estimate from

- the observed chemical shift that in a 1:1 [Cl]BF4:TBABr mixture, the BF4 ion pair and

the halide adduct [X]X are in a 9:10 ratio. This implies that the interaction strengths of

- - BF4 and Br are similar. Furthermore, making similar assumptions about a 1:1 [Cl]BF4:

- - TBACl mixture leads to the conclusion that the BF4 ion pair and the Cl adduct [Cl]Cl

- - are in a 2:5 ratio, confirming that association with Cl is stronger than with BF4 .

82 When [Cl]BF4 is mixed with one equivalent of [Br]Br at room temperature (22

°C), the aromatic and benzylic resonances are broad and appear at average chemical

shifts of the corresponding resonances of the pure solutions (Figure 3.9(e)). The two D'

piperidyl resonances appear in a 1:1 intensity ratio and are coincident with the D' resonances in the spectra of [Cl]Cl and [Br]Br, respectively. As the [Cl]BF4 /[Br]Br

mixture is cooled, the aromatic and benzylic resonances split, revealing two sets of nearly

overlapping resonances. By contrast, the D' and other piperidyl resonances sharpen

(Figure 3.10). The coalescence temperatures (Tc) of A, B and C are ~25 °C. The two D'

resonances coalesce at ~60°C but no coalescence is observed for the diastereotopic -

piperidyl resonances, D' and D", at 0B; °C. The Eyring plot of D' resonance is slightly nonlinear, becoming more shallow at low temperature. This behavior suggests underlying complexity, such as a change in rate-limiting step. Under the assumption of

linearity, H‡ and S‡ are estimated to be approximately 11 kcal mol-1 and -0.01 kcal

mol-1K-1, respectively. These values are in good agreement with those for halide

exchange reactions of square planar Pt(II)72,73 and Pd(II)74 complexes with amine ligands

(H‡, 10 to 22 kcal/mol; S‡, -30 to -16 cal mol-1 K-1).

Because of the similarities in the spectra of [Cl]Cl and [Br]Br in CDCl3, the

changes in chemical shifts are comparatively modest when [Cl]Cl is mixed with one

equivalent of [Br]Br (Figure 3.11). As expected, [Cl]+ is favored over [Br]+, as indicated by the 3:1 intensity ratio of the D' resonances. As the solution is cooled, the aromatic and

benzylic resonances broaden but no splitting is observed at 1-15 °C. The piperidyl

resonances are sharp at <0 °C, and the two D' resonances coalesce at approximately 40

°C. At 60 °C, the diastereotopic -piperidyl resonances, D' and D", are slightly closer to

83 coalescence than in the spectrum of 1:1 [Cl]BF4 /[Br]Br. The estimation of

thermodynamic parameters for the halide ligand exchange is complicated due to the

inequality in the intensity ratio of the two D' resonances. Additionally, as the D'

resonances approach coalescence, the D' and D" resonances begin to move toward each

other. Consequently, because of the exchange process between D' and D", the system can

no longer be analyzed as two resonances coalescing. Treating the system as an unequally

populated two-site exchange system,75,76 with a coalescence temperature between 40-

45°C gives the barrier to halide ligand exchange (G‡) between 15.7-15.9 kcal/mol.

‡ ‡ Assuming the same S as the 1:1 [Cl]BF4 /[Br]Br mixture (-0.01 kcal.mol), H is

calculated to be in the 12.5-12.7 kcal/mol range. On the other hand, the barrier to

exchange for the [Cl]BF4 /[Br]Br mixture is calculated to be between 16.2-16.4 kcal/mol

and H‡ at a coalescence temperature between 55-60 °C is in the 12.9-13.1 kcal/mol range.

For a rate-determining intra-molecular rearrangement (e.g., [Br]Cl [Cl]Br), the

dynamic process responsible for the coalescence of two different D' resonances, assigned

to [Cl]+ and [Br]+, is proposed to involve a 5-coordinate transition state. In other words, the barrier to halide ligand exchange likely reflects the instability of a 5-coordinate

transition state species relative to [X]+ or the [X]+/anion adduct. Therefore, assuming that

- the BF4 is not directly involved, for both [Cl]BF4 /[Br]Br and [Cl]Cl / [Br]Br mixtures,

the transition state species is expected be essentially the same. Apart from the error

introduced by the estimation methods, the variation in the barrier to halide ligand

- exchange values can be attributed to the presence of BF4 in one of the mixtures. The

facts that the conductivity measurements suggest a strong interaction between the cation

84 - and BF4 and the mixing experiments show that [X]BF4 forms even if there is enough

halide to coordinate to all [X]+ in a solution (X= Cl, Br), indicate that displacement of

- - BF4 is required before halide ligand exchange can occur. If the displacement of BF4 is

involved in the rate determining step, the overall reaction pathway is anticipated to have a

higher activation barrier. Another possibility is that the rate determining step is

bimolecular, and the rate of exchange and the coalescence temperature depend on halide

ion concentration. Since the halide ion concentration is higher for the [Cl]Cl / [Br]Br

mixture, a lower barrier, which is qualitatively consistent with what we have observed, is

expected. It is noteworthy that the ionic strength of the mixture is not expected to have a

significant influence since according to conductivity measurements the solutions do not

contain many free ions. Although the water resonance shifts upfield as the temperature is raised in both mixtures, this does not influence the chemical shifts of the complexes’ resonances appreciably.

There is precedent for the interaction of four-coordinate palladium(II) complexes with exogenous halide anions.77,78 For example, the conductivity measurements and

NMR spectroscopy show that five coordinate Pd(N^N^N)(CH3)Cl (N^N^N= 2-(2-((2'-

Pyridylmethylene)amino)ethyl)pyridine) is favored at low temperatures in chlorinated

solvents, whereas a square planar geometry with Cl- as the counter anion is observed in

acetonitrile. At higher temperatures, the neutral species with bidentate N^N^N ligand

forms regardless of the solvent.77 Not surprisingly, the flexibility of the N^N^N ligand

stabilizes the five coordinate species at low temperatures in non-coordinating solvents. A

3 similar Pd(II) complex with a phosphorus-bis(nitrogen) ligand, [Pd( -PNN)CH3]Cl

(PNN=N-(2-(diphenylphosphino)benzylidene)(2-(2-pyridyl)ethyl)amine)) is reported to

85 be ionic in acetonitrile and molecular in chloroform as indicated by conductivity

78 measurements. Compared to the N^N^N and PNN ligands, the pip2NNN ligand is

(f) (e) (d) (c) (b) (a)

876 54 (ppm)

1 Figure 3.10 H NMR spectra of (a) [Cl][BF4 and (b) [Br]Br in CDCl3. Spectra of a

1:1 mixture of [Cl]BF4 /[Br]Br at (c) -15 °C, (d) 0 °C, (e) 22 °C and (f)

60 °C. * denotes solvent residual resonance.

86 * (g) (f) (e) (d) (c) (b) (a)

87 6 54 (ppm)

1 Figure 3.11 H NMR spectra of (a) [Cl]Cl and (b) [Br]Br in CDCl3. Spectra of 1:1

mixture of [Cl]Cl / [Br]Br at (c) -15 °C, (d) 0 °C, (e) 20 °C, (f) 40 °C, and

(g) 60 °C. * denotes solvent residual resonance.

87 more rigid, and regardless of solvent or temperature no evidence of asymmetric or bi-

dentate coordination is observed by 1H NMR spectroscopy. The preservation of symmetry in the 1H NMR spectra could be due to fast exchange, ion pairing or formation of an adduct that preserves the mirror plane symmetry. In the latter case, one possibility is to position the pip2NNN and the two halide ligands in the same coordination plane. A more likely possibility is the preservation of the C2v symmetry by positioning the halide

ligands above and below the plane defined by pip2NNN.

To assess the strength of the association between the halide counterion and the palladium cation, 1H NMR spectra were recorded of a chloroform solution of [Cl]Cl at

different concentrations. The effective association constant was estimated79 to be ~104

M-1 from variations in the meta-pyridyl resonance shifts, indicating strong binding of the counter halide (Figure 3.12). The fact that [Cl]Cl is nonconductive even at low

concentrations supports the notion that the interaction between the [Cl]+ and Cl- is strong.

The association constant calculated using the available data is merely an order of

magnitude estimate. The fitting was carried out with the assumption that there are only

two species in solution, namely one having a specific interaction between the cation and

the anion and the other fully dissociated. The species with a specific interaction is

represented by the [Cl]Cl chemical shift and the fully dissociated species, [Cl]+Cl-, is

represented by [Cl]BF4 chemical shift. The poor fit is consistent with the presence of a

third form of the complex, [Cl]+/Cl-, in which the cation and the anion are paired in a

non-specific manner. Additionally, the conductivity and 1H NMR spectroscopy are

inconsistent with the notion that the 1H NMR spectrum of the fully dissociated [Cl]+Cl-

species matches that of [Cl]BF4. On the contrary, since those data suggest an interaction

88 7.87

7.86

7.85

7.84

7.83 Chemical Shift (ppm) 7.82

7.81

0 0.01 0.02 0.03 0.04 Concentration (M)

Figure 3.12 The chemical shifts of meta CH protons vs. concentration of [Cl]Cl

(0.0006-0.04 M) in CDCl3. The red and blue curves illustrate the best-fit + - + - curves according to: = [Cl] X + (([Cl]Cl - [Cl] X )/2C)(Kd+2C- 2 2 1/2 + [(Kd+2C) -4C ] ), where C is the concentration of [Cl] . For the red 4 -1 curve, X=BF4, Ka=1/Kd,=2.4 x 10 M , the chemical shift at infinite + - dilution ([Cl] X ) is assumed to be that of a solution of [Cl]BF4 (7.618

ppm) and the chemical shift of the fully associated species ([Cl]Cl) is assumed to be that of a solution of [Cl]Cl (7.867 ppm). For the blue curve, the chemical shifts at infinite dilution and full association are optimized to -1 + - obtain the best fit, X=Cl, Ka=194 M , [Cl] X = 7.808 ppm, [Cl]Cl = 7.897 ppm.

89 + - between [Cl] and BF4 , the [Cl]BF4 spectrum may better represent the spectrum of the ion paired species. However, since we know that the interaction of Cl- with the cation is

- stronger than BF4 , the spectrum of the ion paired species is probably different than the

[Cl]BF4 spectrum. Additionally, if there are three species in solution and the fully

dissociated adduct is contributing to the chemical shift, significant variations in ionic

strength is anticipated. When the chemical shifts of the two limiting species are assumed

to be different than the ones of [Cl]Cl and [Cl]BF4 and allowed to be varied along with

Ka, a better fit is obtained (Figure 3.12, blue curve). In this case, the calculated chemical

shifts of the species at full association (7.897 ppm) and infinite dilution (7.808 ppm) are

higher than the chemical shifts of [Cl]Cl (7. 867 ppm) and [Cl]BF4 (7.618 ppm), respectively, whereas the Ka (~ 102) is two orders of magnitudes lower. Since conductometry does not support a high percentage of dissociation at low concentrations, it is probable that the chemical shift calculated for the species at infinite dilution is actually that of the ion paired species (Cl]+/Cl-). It should be added that unlike other

Pd(II) complexes,68 significant formation of charged aggregates at high concentrations is not supported by conductometry. No appreciable change in the conductivity with concentration increase was recorded when the concentration of the [Cl]Cl solution was raised to 0.01M.

An associative mechanism is favored for ligand exchange reactions of related

Pd(II) pincer ligand complexes,80,81 as in the case of insertion of CO into the Pd-C bond

+ of Pd(Me4NNN)R type cations (Me4NNN=2,6-bis(dimethylamine-methyl)-pyridine;

R=methyl, phenyl, naphthyl). In that case, the transient species were modeled as having

Pd coordinated by the Me4NNN ligand in a bidentate fashion along with the R and CO

90 groups. Interestingly, although a five-coordinate structure was not energetically favored;

the transition state was found to be stabilized by an interaction between the non-

coordinated amine and the Pd center. In general, a five-coordinate intermediate(s) and/or

a transition state(s) with either square pyramidal or trigonal bipyramidal geometries are

proposed for square planar Pd(II) complexes. As illustrated in Scheme 3.3, the

accumulated data are consistent with association of the cation and halide anion prior to

halide scrambling, which is suggested to involve a five-coordinated intermediate. There

is considerable evidence that the equilibrium lies toward the Cl- coordination. A reaction profile for the top portion of the scheme describing Cl/Br exchange is shown in Scheme

3.4. The reaction profile takes shape depending on the nature of the leaving and the entering ligands. When the leaving group, Cl-, is bonded more strongly to Pd than the

entering group, Br-, the transition state is anticipated to have more Pd---Cl bond

dissociation character. It is noteworthy that since in most cases Br- bonds more tightly to

Pd(II) than Cl-, a reaction profile in which the transition state with more Pd---Br bond

dissociation character is more commonly encountered. In the present case, the size of the

Cl- ligand and the filled/filled repulsions between the d orbitals of the metal center and the lone pair orbitals of the halide ligand are anticipated to contribute to the preference for Cl- over Br- coordination.

91 [Cl]Br [Br]Cl

- +Br -Cl- - -Br +Cl-

[Cl]+ [Br]+

+Cl- -Br- -Cl- +Br- [Cl]Cl [Br]Br

+ Scheme 3.3 Halide exchange in CDCl3 where [X]X represents [Pd(pip2NNN)(X)]

associated with X-.

Br Pd Cl Br Br Pd

Pd Cl

gy Cl Ener

- - Pd Br + Cl Pd Cl + Br

Reaction Coordinate

Scheme 3.4 Proposed reaction profile for the halide substitution reaction; [Cl]+ + Br-

[Br]+ + Cl-. The transition state has more Pd---Cl bond dissociation

character.

92 Conclusions

Following the method for the synthesis of [Pd(pip2NNN)Cl]Cl, a series of new

square planar palladium(II) complexes with the pip2NNN pincer ligand has been

1 prepared: [Pd(pip2NNN)X]X (X= Br, I). H NMR, mass and UV-visible absorption

spectroscopies and the physical properties of the complexes are consistent with the

2+ formation of [Pd(pip2NNN)X][PdnX2n+6] salts (m=1 or 2 n=4 or 6, respectively)

complexes from the same reaction set up. In case of iodine, X-ray crystallography and

2- 2– elemental analysis results are consistent with formation of the Pd2I6 and PdI4 salts. In

2- - general, low temperatures favor the formation of PdmXn complexes over X complexes

but the starting material also is important. The X-ray crystal structures of

[Pd(pip2NNN)Br]Br, [Pd(pip2NNN)Br]BF4, [Pd(pip2NNN)I]I, [Pd(pip2NNN)I]2[Pd2I6]

are similar to reported structures of Pd(II) and Pt(II) complexes with pincer ligands.

However, the conformation of the piperidyl groups and positions of the benzylic carbons

in relation to the metal center differ from structure to structure. For example, for

[Pd(pip2NNN)Br]BF4, one of the piperidyl groups adopts a chair conformation with the metal center at an equatorial and the other at an axial position on the N(piperidyl) atom and the benzylic carbons are above and below the metal coordination plane whereas for

[I]I, both piperidyl rings are at the equatorial positions and both benzylic carbons lie on

the same side of the coordination plane.

Electronic absorption spectra of the halide salts contain transitions of the

+ - [Pd(pip2NNN)X] cations. The lowest band has significant X dx2-y2(Pd) charge-transfer

character. The apparent red shift of this band along the halide series Cl

energy progression is in accord with this assignment. In addition to the cation transitions,

93 2- 2- the spectra of PdI4 and Pd2I6 salts exhibit bands that are consistent with transitions

seen in the spectra of the corresponding anions.

1 H NMR spectra of [Pd(pip2NNN)X]X show small downfield shifts for all the resonances along the halide series, Cl

has the strongest influence on the benzylic and meta-CH resonances, whereas variations in the halide ligand have the strongest influence on the furthest downfield -piperidyl resonance. Conductivity and 1H NMR measurements are consistent with association of the halide anion and the palladium complex. The importance of exogenous halide anions on the kinetics of outer-sphere two-electron transfer has been noted for 5- coordinate

palladium(II) complexes.82 The solvent and type-of-anion dependence of the interaction

- - - + between the exogenous anion (Cl , Br , BF4 ) and the palladium cation (Pd(pip2NNN)X ,

X= Cl, Br) suggests the possibility of dramatic solvent and anion sensitivity of

multielectron transfer reactions that rely on exogenous anionic ligands.

94 References

(1) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem. Eur. J. 1998, 4, 759-762.

(2) Suijkerbuijk, B. M. J. M.; Shu, L.; Gebbink, R. J. M. K.; Schlueter, A. D.; van

Koten, G. Organometallics 2003, 22, 4175-4177.

(3) Slagt, M. Q.; Jastrzebski, J. T. B. H.; Gebbink, R. J. M. K.; van Ramesdonk, H. J.;

Verhoeven, J. W.; Ellis, D. D.; Spek, A. L.; van Koten, G. Eur. J. Org. Chem.

2003, 1692-1703.

(4) Slagt, M. Q.; van Zwieten, D. A. P.; Moerkerk, A. J. C. M.; Gebbink, R. J. M. K.;

van Koten, G. Coord. Chem. Rev. 2004, 248, 2275-2282.

(5) Albrecht, M.; Gossage, R. A.; van Koten, G.; Spek, A. L. Chem. Commun. 1998,

1, 1003-1004.

(6) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000, 406, 970-974.

(7) Albrecht, M.; Hovestad, N. J.; Boersma, J.; van Koten, G. Chem. Eur. J. 2001, 7,

1289-1294.

(8) Davies, P. J.; Veldman, N.; Grove, D. M.; Spek, A. L.; Lutz, B. T. G.; van Koten,

G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1959-1961.

(9) James, S. L.; Verspui, G.; Spek, A. L.; van Koten, G. Chem. Commun. 1996, 11,

1309-1310.

(10) Albrecht, M.; van Koten, G. Angew. Chem., Int. Edit. 2001, 40, 3750-3781.

(11) Rodriguez, G.; Albrecht, M.; Schoenmaker, J.; Ford, A.; Lutz, M.; Spek, A. L.;

van Koten, G. J. Am. Chem. Soc. 2002, 124, 5127-5138.

(12) Rodriguez, G.; Lutz, M.; Spek, A. L.; Van Koten, G. Chem. Eur.J. 2002, 8, 45-57.

(13) Jude, H.; Bauer, J. A. K.; Connick, W. B. Inorg. Chem. 2002, 41, 2275-2281.

95 (14) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004, 43, 725-733.

(15) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2005, 44, 1211-1220.

(16) Jude, H.; Bauer, J. A. K.; Connick, W. B. J. Am. Chem. Soc. 2003, 125, 3446-

3447.

(17) Shi, L.-L.; Liao, Y.; Yang, G.-C.; Su, Z.-M.; Zhao, S.-S. Inorg. Chem. 2008, 47,

2347-2355.

(18) Tastan, S.; Krause, J. A.; Connick, W. B. Inorg. Chim. Acta 2006, 359, 1889-

1898.

(19) Stepnicka, P.; Cisarova, I. Collect. Czech. Chem. Comm. 1996, 61, 1335-1341.

(20) Bruker APEX2 v1.0-27 and SAINT v27.06A programs were used for data

collection and data processing, respectively. Bruker Analytical X-Ray

Instruments, Inc., Madison WI.

(21) SADABS v2.10 was used for the application of semi-empirical absorption and

beam corrections. G. M. Sheldrick, University of Göettingen, Germany.

(22) SHELXTL v6.14 was used for the structure solution and generation of figures and

tables. Neutral-atom scattering factors were used as stored in this package. G. M.

Sheldrick, University of Göttingen, Germany and Bruker Analytical X-ray

Solutions, Inc., Madison, WI.

(23) Harris, C. M.; Livingstone, S. E.; Reese, I. H. J.Chem.Soc. 1959, 1505-1511.

(24) Day, P.; Smith, M. J.; Williams, R. J. P. J. Chem. Soc. [Sec.] A: Inorg. Phys.,

Theo. 1968, 3.

(25) Jorgensen, C. K., Absorption Spectra and Chemical Bonding in Complexes,

Permagon, Oxford, 1962, p286.

96 (26) Cotton, F. A.; Wilkinson, G.; Bochmann, M.; Murillo, C. 1998.

(27) Pitteri, B.; Marangoni, G.; Cattalini, L.; Visentin, F.; Bertolasi, V.; Gilli, P.

Polyhedron 2001, 20, 869-880.

(28) Olsson, L. F. Inorg. Chem. 1986, 25, 1697-1704.

(29) Li, Y.; Lai, Y.-H.; Mok, K. F.; Drew, M. G. B. Inorg. Chim. Acta 1998, 279, 221-

225.

(30) Markies, B. A.; Wijkens, P.; Boersma, J.; Kooijman, H.; Veldman, N.; Spek, A.

L.; van Koten, G. Organometallics 1994, 13, 3244-3258.

(31) Arnaiz, A.; Cuevas, J. V.; Garcia-Herbosa, G.; Carbayo, A.; Casares, J. A.;

Gutierrez-Puebla, E. J. Chem. Soc. Dalton Trans. 2002, 12.

(32) Markies, B. A.; Wijkens, P.; Dedieu, A.; Boersma, J.; Spek, A. L.; van Koten, G.

Organometallics 1995, 14, 5628-5641.

(33) Bugarcic, Z. D.; Petrovic, B.; Zangrando, E. Inorg. Chim. Acta 2004, 357, 2650-

2656.

(34) Liu, X.; Renard, S. L.; Kilner, C. A.; Halcrow, M. A. Inorg. Chem. Commun.

2003, 6, 598-603.

(35) Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H. Inorg. Chim. Acta 2006,

359, 1855-1869.

(36) Miecznikowski, J. R.; Gruendemann, S.; Albrecht, M.; Megret, C.; Clot, E.;

Faller, J. W.; Eisenstein, O.; Crabtree, R. H. Dalton Trans. 2003, 5, 831-838.

(37) Hahn, F. E.; Jahnke, M. C.; Gomez-Benitez, V.; Morales-Morales, D.; Pape, T.

Organometallics 2005, 24, 6458-6463.

97 (38) Gosiewska, S.; Veld, M. H. i. t.; De Pater, J. J. M.; Bruijnincx, P. C. A.; Lutz, M.;

Spek, A. L.; Van Koten, G.; Klein Gebbink, R. J. M. Tetrahedron: Asymm. 2006,

17, 674-686.

(39) Yao, W. R.; Liu, Z. H.; Zhang, Q. F. Acta Crystallogr. Sec. C: Cryst. Struct.

Comm. 2003, C59, m139-m140.

(40) Vicente, J.; Abad, J.-A.; Foertsch, W.; Lopez-Saez, M.-J.; Jones, P. G.

Organometallics 2004, 23, 4414-4429.

(41) Grundemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.; Crabtree, R. H.

Organometallics 2001, 20, 5485-5488.

(42) Mentes, A.; Kemmitt, R. D. W.; Fawcett, J.; Russeli, D. R. Polyhedron 1999, 18,

1141-1145.

(43) Schmuelling, M.; Grove, D. M.; van Koten, G.; van Eldik, R.; Veldman, N.; Spek,

A. L. Organometallics 1996, 15, 1384-1391.

(44) Donkervoort, J. G.; Vicario, J. L.; Jastrzebski, J. T. B. H.; Smeets, W. J. J.; Spek,

A. L.; van Koten, G. J. Organomet. Chem. 1998, 551, 1-7.

(45) Neve, F.; Crispini, A. Cryst. Eng. Comm. 2003, 5, 265-268.

(46) Evans, J.; O'Neill, L.; Kambhampati, V. L.; Rayner, G.; Turin, S.; Genge, A.;

Dent, A. J.; Neisius, T. J. Chem. Soc. Dalton Trans. 2002, 10, 2207-2212.

(47) Tonde, S. S.; Kelkar, A. A.; Bhadbhade, M. M.; Chaudhari, R. V. J. Organomet.

Chem. 2005, 690, 1677-1681.

(48) Anbalagan, V.; Srinivasan, R.; Pallavi, K. S. Trans. Met. Chem. 2001, 26, 603-

607.

98 (49) Hofmann, A.; Jaganyi, D.; Munro, O. Q.; Liehr, G.; van Eldik, R. Inorg. Chem.

2003, 42, 1688-1700.

(50) Gidney, P. M.; Gillard, R. D.; Heaton, B. T. J. Chem. Soc., Dalton Trans. 1973,

132-134.

(51) Lewis, F. D.; Salvi, G. D.; Kanis, D. R.; Ratner, M. A. Inorg. Chem. 1993, 32,

1251-1258.

(52) Zhang, W.; Bensimon, C.; Crutchley, R. J. Inorg. Chem. 1993, 32, 5808-5812.

(53) Kunkely, H.; Vogler, A. J. Organomet. Chem. 1998, 559, 223-225.

(54) Choi, C. L.; Phillips, D. L. Molec. Phys. 1998, 94, 547-554.

(55) Leung, K. H.; Szulbinski, W.; Phillips, D. L. Molec. Phys. 2000, 98, 1323-1330.

(56) Lu, X.-X.; Cheng, E. C.-C.; Zhu, N.; Yam, V. W.-W. Dalton Trans. 2006, 14,

1803-1808.

(57) Zhi, H. Z.; Xian, H. B.; Zhi, A. Z.; Yun, T. C. Polyhedron 1996, 15, 2787-2792.

(58) Pamplin, C. B. R., Steven J.; Patrick, Brian O.; James, Brian R. Inorg. Chem.

2003, 42, 4117-4126.

(59) Stoppioni, P.; Morassi, R.; Zanobini, F. Inorg. Chim. Acta 1981, 52, 101-106.

(60) Mason III, W. R.; Gray, H. B. J. Am. Chem. Soc. 1968, 90, 5721-5729.

(61) Ansari, S. M.; Robien, W.; Schlederer, M.; Wolschann, P. Monatshefte fur

Chemie 1989, 120, 1003-1014.

(62) The chemical shifts for Pt(pip2NCN)X were determined in CDCl3.

(63) Gilbert, T. M.; Ziegler, T. J. Phys. Chem. A 1999, 103, 7535-7543.

(64) Flemming, J. P.; Pilon, M. C.; Borbulevitch, O. Y.; Antipin, M. Y.; Grushin, V.

V. Inorg. Chim. Acta 1998, 280, 87-98.

99 (65) Caulton, K. G. New J. Chem. 1994, 18, 25-41.

(66) Poulton, J. T.; Sigalas, M. P.; Folting, K.; Streib, W. E.; Eisenstein, O.; Caulton,

K. G. Inorg. Chem. 1994, 33, 1476.

(67) It should be noted that, as expected from the spectrum of TBACl, addition of

TBACl shifts the water resonance in the [Cl]Cl and [Cl]BF4 spectra.

Nevertheless, when a few equivalents of water are added to either [Cl]Cl or

[Cl]BF4, no substantial shift is observed in the [Cl]+ resonances.

(68) Crociani, B.; Bianca, F. D.; Giavenco, A.; Bosch, T. Inorg. Chim. Acta 1987, 127,

169-182.

(69) Irving, A.; Koch, K. R.; Matoetoe, M. Inorg. Chim. Acta 1993, 206, 193-199.

(70) Romeo, R.; Nastasi, N.; Scolaro, L. M.; Plutino, M. R.; Albinati, A.; Macchioni,

A. Inorg. Chem. 1998, 37, 5460-5466.

- 2- (71) The influence of PF6 and the PdnX2n+2 anions could not be studied due to the

insolubility of their complexes in CDCl23.

(72) Belluco, U.; Ettorre, R.; Basolo, F.; Pearson, R. G.; Turco, A. Inorg. Chem. 1966,

5, 591-593.

(73) Tucker, M. A.; Colvin, C. B.; Martin, D., S. Jr. Inorg. Chem. 1964, 3, 1373-1383.

(74) Roulet, R.; Gray, H. B. Inorg. Chem. 1972, 11, 2101-2104.

(75) Shanan-Atidi, H.; Bar-Eli, K. H. J. Phys. Chem. 1970, 74, 961.

(76) Egan, W. 1971, Princeton University, Dessertation Thesis.

(77) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van Leeuwen, P. W. N.

M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769-5778.

100 (78) Ruelke, R. E.; Kaasjager, V. E.; Wehman, P.; Elsevier, C. J.; van Leeuwen, P. W.

N. M.; Vrieze, K.; Fraanje, J.; Goubitz, K.; Spek, A. L. Organometallics 1996, 15,

3022-3031.

(79) Wilcox, C. S. 1991, Frontiers in Supramolecular Organic Chemistry and

Photochemistry, p.123-143.

(80) Langford, C. H.; Gray, H. B. Ligand Substitution Processes 1966, Chapter2.

(81) Cross, R. J. Adv. Inorg. Chem. 1989, 34, 219-292.

(82) Chatterjee, S., University of Cincinnati, Dissertation thesis.

101 CHAPTER 4:

Palladium(II) Complexes with Two Potentially Tridentate Pyridyl Ligands

Introduction

Late transition metal compounds typically undergo two-electron changes in oxidation state by inner-sphere mechanisms (e.g., oxidative addition and reductive elimination reactions). This behavior suggests the possibility of designing cooperative outer-sphere two-electron reagents, shuttles or reservoirs that reversibly transfer two

electrons at the same potential in multi-redox catalytic reactions or molecular devices. In the case of platinum, an obvious practical problem is that the interconversion between square planar Pt(II) and octahedral Pt(IV) by outer-sphere electron transfer is slow and characterized by irreversible electrochemistry because of the accompanying large molecular reorganization.1,2 Though several previously investigated systems are capable

of supporting both oxidation states, the electronic and steric properties of the ligands in

these complexes result in CVs (cyclic voltammograms) characterized by one-electron

waves corresponding to generation of Pt(III) at lower potentials than Pt(IV). In those

systems, Pt(III) is apparently preferentially stabilized with respect to Pt(IV).3-10

We have recently designed the first examples of platinum complexes that undergo

nearly electrochemically reversible two-electron transfer reactions.11,12 The metal center

in these complexes is bonded to two potentially meridional (mer) coordinating ligands,

- such as pip2NCN and tpy (Scheme 4.1). This ligand architecture is capable of stabilizing

both the four-coordinate Pt(II) and six-coordinate Pt(IV) geometries (Scheme 4.2).

102 N N N

N N

N N N

- pip2NCN tpy pip2NNN

Scheme 4.1 Potentially tridentate ligands.

In the d6 octahedral Pt(IV) case, both ligands are expected to be tridentate occupying all

six binding sites around the metal center. In the d8 square planar Pt(II) case, tpy remains

- tridentate, whereas pip2NCN coordinates in a monodentate fashion through the central

phenyl group.

Scheme 4.2 Schematic representation of a reversible two-electron platinum reagent.

+ It has been demonstrated that Pt(pip2NCN)(tpy) undergoes a nearly reversible two-

electron oxidation near 0.4 V vs. Ag/AgCl.11 The remarkable reversibility of the redox

+ chemistry of Pt(pip2NCN)(tpy) is suggestive of a non-covalent interaction between the

- pip2NCN amine groups and the Pt(II) metal center, which effectively preorganizes the

103 square planar complex for electron transfer. The notion of preorganization is supported

by crystal structures of a series of two-electron reagents which reveal relatively short

Pt...N(piperidyl) interactions. In addition, UV-visible absorption spectra of these

complexes exhibit long wavelength absorption features (>500 nm), which are absent from

the spectra of model complexes, such as Pt(tpy)(phenyl)+.13

Toward the goal of better understanding the role of the metal in the electron-

transfer reactivity, we have extended these studies to palladium. In this chapter, we

report the synthesis of potential two-electron palladium(II) terpyridyl complexes with the

pip2NNN ligand (Scheme 4.1).

Experimental

All reagents were purchased from Pressure Chemical, Aldrich or Acros.

14 [Pd(pip2NNN)Cl]Cl was prepared according to the published procedure. Acetonitrile

1 was distilled from CaH2 under argon. H NMR spectra were recorded at room temperature using a Bruker AC 400 MHz Spectrometer. Deuterated acetonitrile and

chloroform (0.03% tetramethylsilane (TMS) (v/v)) were purchased from Cambridge

Isotope Laboratories. Electronic spectra were recorded using a HP8453 UV-visible spectrometer, and absorption bands were modeled as Gaussian functions. Mass spectra were recorded using a Micromass Q-TOF-2 hybrid quadrapole time of flight mass

spectrometer with electrospray ionization. Elemental analyses were performed by

Atlantic Microlabs (Norcross, GA). Cyclic voltammograms (CVs) were measured using a

BAS100B/W Electrochemical Analyzer. Scans were collected in acetonitrile solution

containing 0.1 M TBAPF6. The voltammograms that were recorded using a platinum

104 wire auxiliary electrode, a Ag/AgCl reference electrode (3.0 M NaCl) either a glassy

carbon or gold working electrode. The data with the glassy electrode are presented since

it consistently provided higher quality voltammograms of the electrolyte solution (blank)

in addition to the ability to sweep larger potential windows. Between scans, the working

? ? ;);2 ) ? ? ?

using a Kimwipe.

[Pd(pip2NNN)(tpy)](BF4)2 (1). To 25 mL of an ethanol solution of

[Pd(pip2NNN)Cl]Cl (0.100 g, 0.22 mmol) in the dark was added AgBF4 (0.086 g, 0.44

mmol). After stirring for one hour, the mixture was filtered through Celite. The filtrate

was stirred with 2,2';6'2"-terpyridine (tpy, 0.22 mmol, 0.052 mg) overnight. After removal of the solvent by rotary evaporation, the orange solid was washed with hexanes and collected by vacuum filtration. Yield 0.125 g, 72%. Anal. Calcd. for

[C32H38N6Pd](B2F8) C, 48.85; H, 4.87; N, 10.68 Found: C, 48.59; H, 4.81; N, 10.59.

+ 1 MS(ESI): m/z =699.224 ((Pd(pip2NNN)(tpy).BF4) . H NMR (CD3CN, ): 0.89 (8H, m,

CH2), 1.16 (4H, m, CH2), 2.43 (8H, m, CH2), 3.97 (4H, s, CH2), 7.50 (2H, d, CH), 7.61

(2H, m, CH), 7.72 (2H, d, CH), 8.19 (1H, t, CH), 8.30-8.40 (6H, m, CH), 8.55 (1H, t,

CH).

t [Pd(pip2NNN)( Bu3tpy)](BF4)2 (2). The product was isolated as an orange solid

by following the procedure for 1 and substituting 4,4',4''-tri-tert-butyl-2,2':6',2''-

t terpyridine ( Bu3tpy, 0.22 mmol, 0.091 mg) for tpy. Yield 0.168 g, 80%. Anal. Calcd. for

C44H64N6Pd](B2F8).H2O C, 54.34; H, 6.63; N, 8.64. Found: C, 54.24; H, 6.53; N, 8.51.

1 H NMR (CD3CN, ): 0.86 (8H, m, CH2), 1.15 (4H, m, CH2), 1.41 (18H, s, CH3), 1.57

105 (9H, s, CH3), 2.42 (8H, m, CH2), 3.96 (4H, s, CH2), 7.34 (2H, d, CH), 7.53 (2H, d, CH),

7.71 (2H, d, CH), 8.18 (1H, t, CH), 8.37 (4H, m, CH).

t X-ray Crystallography. Yellow blocks of [Pd(pip2NNN)( Bu3tpy)](BF4)2.

2(ClCH2CH2Cl) were obtained by diffusion of hexanes into a dichloromethane solution

1 3 of the complex. Yellow plates of $ - $ -tpy)](BF4)2.1/4Et2O formed upon

diffusion of ether into an acetonitrile solution of [Pd(pip2NNN)(tpy)](BF4)2. Diffraction

t 1 data were collected at 193 K for [Pd(pip2NNN)( Bu3tpy)](BF4)2 and 173 K for $ -

3 $ -tpy)](BF4)2 using a Bruker Platinum200 CCD detector at Beamline 11.3.1 at the

Advanced Light Source (Lawrence Berkeley National Laboratory) with synchrotron

(0;)442;; Å. The data frames were collected using the program APEX2 and processed using the program SAINT within APEX2.15 Absorption and beam corrections

based on the multi-scan technique were applied using SADABS.16 The structures were

solved by a combination of direct methods SHELXTL17 and the difference Fourier technique. The models were refined by full-matrix least squares on F2. Non-hydrogen

atoms were refined with anisotropic displacement parameters with the exception of

t C37A, C37B, C38A and C38B of [Pd(pip2NNN)( Bu3tpy)](BF4)2.1/4Et2O. The H atoms were calculated and treated with a riding model. The isotropic displacement parameters were defined as a times Ueq of the adjacent atom where a = 1.5 for methyl and 1.2 for all

t others. For [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl), C41A and C41B anisotropic

displacement parameters were set to be equivalent to the better behaved C40A and C40B,

respectively. C42 is disordered as indicated by the enlarged anisotropic displacement

parameter; no suitable two-component disorder model was realized. C35-C37A, C35-

C37B, C35-C38A and C35-C38B distances were restrained nearer to a single bond using

106 DFIX. The refinement converged with crystallographic agreement factors of R1=4.92%,

?FG01H)3;I 2G52 ? JG F102);3I ?FG01H)22I data)

and 341 variable parameters. The largest residual density appears near the disordered

1 3 solvent of crystallization. For $ - $ -tpy)](BF4)2.1/4Et2O, the solvent is located

about a center of symmetry and exhibits large thermal motion and misbehaved bonding

parameters. The solvent contribution was subtracted from the reflection data using the

program SQUEEZE.18 The refinement converged with crystallographic agreement factors

of R1=3)BKI ?FG01;)B1I 2;2G ? JG F103)14I ?FG01;)KBI

for all data) and 451 variable parameters.

Results and Discussion

Synthesis. Drawing on strategies developed for the synthesis of related

- 11 platinum(II) complexes with the pip2NCN ligand, the tetrafluoroborate salts of

+ t + Pd(pip2NNN)(tpy) and Pd(pip2NNN)( Bu3tpy) were prepared by reaction of

[Pd(pip2NNN)Cl]Cl with two equivalents of AgBF4, followed by treatment with the

appropriate tpy ligand in ethanol (Scheme 4.3). Both compounds were isolated as orange

solids and are soluble in acetonitrile, chloroform and dichloromethane, but poorly soluble

in water.

We have previously observed that the related platinum(II) complex,

+ 3+ Pt(pip2NCN)(tpy) , can be reversibly protonated to give Pt(pip2NCNH2)(tpy) , in which the dangling piperidyl groups are protonated. However treatment of these palladium(II) adducts with stoichiometric amounts of an inorganic acid (e.g., HCl) resulted in

decomposition to Pd metal and organic ligands. As noted for analogous platinum

107 - 1 2+ complexes with the pip2NCN ligand, H NMR spectra of [Pd(pip2NNN)(tpy)] and

t 2+ [Pd(pip2NNN)( Bu3tpy)] confirm that these palladium(II) complexes gradually

2+ decompose (~7% in one hour) in acetonitrile, to give Pd(pip2NNN)(solvent) and free

2+ terpyridyl ligand. Attempts to grow crystals of [Pd(pip2NNN)(tpy)] salts sometimes

resulted in

+ N 2+ N R

1) AgBF4 / EtOH N N Pd Br N Pd N t R 2) tpy or Bu3tpy N N N R

Scheme 4.3 Synthesis of [Pd(pip2NNN)(tpy)](BF4)2 (R = H) and

t t [Pd(pip2NNN)( Bu3tpy)](BF4)2 (R = Butyl).

new products (e.g., [Pd(tpy)Br](PF6).CH3CN), suggesting that the complex is somewhat

unstable in the presence of nucleophiles. One of the most interesting products resulted

from an attempt to grow crystals of [Pd(pip2NNN)(tpy)](BF4)2 by diffusion of ether into

an acetonitrile solution. The resulting yellow plate-like crystals were found to contain

1 3 $ - $ -tpy)]2(BF4)2.1/4Et2O, in which one tpy ligand is tridentate and the other is

monodentate, bonding to palladium through a terminal pyridyl group of tpy. The tpy

108 ligand is known to bind in different modes depending on the metal, the other ligands and

the solvent.19-26 Several examples of Ni(II) complexes with two tridentate tpy ligands are

known.27-32 From stopped-flow measurements, Priimov et al. have concluded that bis- terpyridyl Ni(II) is formed when excess tpy is added to NiCl2.2H2O in a 80:20

methanol:water mixture. 19,33 Although there have been reports of Pd and Pt complexes with fluxional tpy ligands, only complexes with bidentate or tridentate tpy have been observed.22-24,34-40 To our knowledge, this is the first example of a palladium complex

with two tpy ligands and the first example of crystallographically characterized d8-

electron complex that has both monodentate and tridentate terpyridyl ligands.

+ Attempts to extend the synthetic procedures for Pt(pip2NCN)(tpy) and

+ + Pd(pip2NNN)(tpy) to the preparation of Pd(pip2NCN)(tpy) were unsuccessful.

14 Building on our previous work, Pd(pip2NCN)Br was allowed to react with AgBF4.

After removal of the AgBr precipitate, the filtrate was stirred with tpy for several hours.

Even after heating the reaction solution, the 1H NMR spectrum of the isolated product

- showed that the -piperidyl protons of pip2NCN ligand are diastereotopic, confirming that the NCN- ligand remained tridentate (Chapter 2)14 and that the tpy ligand did not

displace the piperidyl ligands under these conditions. Interestingly, the aromatic region

of the spectrum exhibited an asymmetrical pattern of resonances for terpyridine, which is

consistent with either monodentate coordination of a peripheral pyridyl group or

bidentate coordination of two adjacent pyridyl groups. Since Pd(II) pincer complexes are

known to be square planar, we believe that 1-tpy is more probable. In support of this

+ + view, our efforts to isolate Pd(pip2NCN)(bpy) and Pd(pip2NCN)(phen) (bpy=2,2'- bipyridine; phen=1,10-phenanthroline) have been thus far unsuccessful. As in the case of

109 1 - tpy, H NMR spectrum shows that the -piperidyl protons of pip2NCN ligand remain

diastereotopic. On the other hand, pyridine (py) and 4-phenylpyridine (4-phpy) react

+ + with [Pd(pip2NCN)(solvent)] to form readily characterized adducts, Pd(pip2NCN(py)

+ 14 and Pd(pip2NCN)(4-phpy) , respectively.

t Crystal Structures. The structures of [Pd(pip2NNN)( Bu3tpy)](BF4)2

1 3 .2(ClCH2CH2Cl) and $ - $ -tpy)]2(BF4).1/4Et2O were confirmed by single-

crystal X-ray diffraction studies. ORTEP diagrams are shown in Figures 4.1 and 4.2, respectively, and relevant data are summarized in Tables 4.1 and 4.2. For both structures, the cations, anions and solvent pack as discrete units, and there are no unusually short

t intermolecular contacts. For [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl), the cation

t lies on a two-fold rotational axis along the Pd-N1( Bu3tpy) bond (Figure 4.1). The

t methyl groups of the Bu3tpy t-butyl substituents are disordered and the molecule

crystallizes with two disordered 1,2-dichloroethane molecules, which are attributed to an

3 1 impurity in the methylene chloride crystallization solvent. $ - $ -tpy)](BF4)2

crystallizes with a disordered diethyl ether molecule (~0.25 occupancy).

The complexes adopt distorted square planar geometries that are qualitatively

similar to those of other palladium(II) terpyridyl complexes.34,41-47 In both structures, a

terpyridyl ligand is tridentate, whereas the other potentially tridentate ligand is bonded

monodentate at the fourth coordination site of the metal center. The Pd-N1(central

t 2+ pyridine of tridentate terpyridyl ligand) distances for Pd(pip2NNN)( Bu3tpy) (1.929(3)

Å) and Pd($3- $1-tpy)2+ (1.933(2) Å ) are similar to those reported for palladium(II)

terpyridyl complexes with N-donor ligands (Pd(tpy)(pyridine)2+ (1.932(4) Å)42,

2+ 43 + 44 t + Pd(tpy)(1-methylcytosine) (1.932(7) Å) , Pd(tpy)(OH) (1.934(3) Å , Pd( Bu3tpy)Cl

110 45 t + 34 (1.929(2) Å) , and Pd( Bu3tpy)(tosylazide) (1.935(6) Å) ). The Pd-N bonds to the two

t 2+ peripheral pyridyl groups are somewhat longer (Pd(pip2NNN)( Bu3tpy) , 2.017(2) Å;

$3- $1-tpy)2+, 2.036(2) Å), as found for structures of related palladium complexes,

such as Pd(tpy)(pyridine)2+ (2.039(4) Å, 2.038(4) Å), Pd(tpy)(1-methylcytosine)2+

+ t + (2.013(4) Å, 2.011(4) Å), Pd(tpy)(OH) (2.022(3) Å, 2.035(3) Å) Pd( Bu3tpy)Cl

t + (2.030(2) Å, 2.020(2) Å), and Pd( Bu3tpy)(tosylazide) (2.023(7) Å, 2.022(7) Å). As

expected, the resulting trans-N-Pd-N angles are substantially less than the idealized 180°

t 2+ 3 1 2+ angle (Pd(pip2NNN)( Bu3tpy) , 161.31(13)°; $ - $ -tpy) , 161.96(9)°). For both complexes, the coordinated pyridyl ring of the monodentate ligand is nearly perpendicular to the Pd coordination plane, defined by Pd and the four atoms bonded to the Pd center, forming dihedral angles of 86.6(1)° and 78.1(1)°

t 2+ 3 1 2+ for(Pd(pip2NNN)( Bu3tpy) $ - $ -tpy) , respectively. These angles are

larger than the value reported for Pd(tpy)(pyridine)2+ (61.9 (2)°) but similar to those

reported for Pd(tpy)(1-methylcytosine)2+ (84.2(2)°).42,43 A similar trend is found for

Pt(phtpy) complexes (phtpy = 4'-phenyl-2,2':6',2"-terpyridine); the dihedral angle for

+ Pt(phtpy)(pip2NCN) (85.5(1)°, 89.1(1)°) is significantly larger than that of

Pt(phtpy)(phenyl)+ 51.40(6)°.48 These results are consistent with the notion that the steric

t demands of the monodentate NNN and Bu3tpy ligands tend to increase the dihedral

angle. However, it should be noted that the Pt-N4 bond lengths

t 2+ 3 1 2+ (Pd(pip2NNN)( Bu3tpy) G);2BH9 $ - $ -tpy) , 2.051(2) Å) are similar to that

reported for Pd(tpy)(pyridine)2+ (2.038(4) Å), and therefore, there is no indication of

diminished bond strength. The Pt-N4 bond lengths are somewhat longer than the Pd-N

111 bonds to the two peripheral terpyridyl pyridine groups, reflecting the strong trans- influence of the central terpyridyl group.

Figure 4.1. ORTEP diagrams of the cation of

t [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl) with 50% probability

ellipsoids. H atoms are omitted for clarity.

112 1 3 Figure 4.2 ORTEP diagrams of the cation of [P $ - $ -tpy)](BF4)2.1/4Et2O with

50% probability ellipsoids. H atoms are omitted for clarity.

113 Table 4.1 Crystal and structure refinement data for

t 3 1 [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl) and [Pd( -tpy)( -

tpy)](BF4)2.1/4Et2O.

[Pd(pip NNN)(tBu tpy)](BF ) 2 3 4 2 3 1 [Pd( -tpy)( -tpy)](BF4)2.1/4Et2O .2(ClCH2CH2Cl)

[C44H62N6BrPd](BF4)2 Formula [C30H22N6Pd](BF4)2 .1/4Et2O .2(ClCH2CH2Cl) Fw (g/mol) 1152.92 765.09 Crystal system Monoclinic Monoclinic

Space group C2/c P21/n a (Å) 18.7379(14) 12.0862(9) b (Å) 18.3312(14) 12.7485(10) c (Å) 15.8496(12) 20.9773(16) °) 90 90 °) 95.120(1) 90.142(2) °) 90 90 V (Å3), Z 5422.4(7), 4 3232.2(4), 4 T (K) 193(2) 173(2) -3 Dcalc (g cm ) 1.412 1.572 (mm-1) 0.756 0.812 F (000) 2384 1530 range °) 1.70-28.97 2.04-27.87 Refls. Coll. 39641 27099 Ind. Refls. 5547 5868

Rint 0.0754 0.0505 Data/Parameters 5547/341 5868/451 GOF on F2 1.030 1.061

R1/wR2 [I>2 (I)] 0.0492/0.1340 0.0369/0.1061

R1/wR2 (all data) 0.0504/0.1355 0.0417/0.1096

114 Table 4.2. Selected distances (Å) and angles (°) for

t 3 1 [Pd(pip2NNN)( Bu3tpy)](BF4)2.2(ClCH2CH2Cl) and [Pd( -tpy)( -

tpy)](BF4)2.1/4Et2O.

t 3 1 [Pd(pip2NNN)( Bu3tpy)](BF4)2 [Pd( -tpy)( -tpy)](BF4)2

.2(ClCH2CH2Cl) .1/4Et2O Pd(1)-N(1) 1.929(3) 1.933(2)

Pd(1)-N(2) 2.017(2) 2.036(2) Pd(1)-N(3) - 2.036(2) Pd(1)-N(4) 2.056(3) 2.051(2) N(2)-C(7) 1.362(3) 1.367(4) N(3)-C(12) - 1.375(3) C(2)-C(12) 1.486(3) 1.468(4) C(6)-C(7) - 1.472(4) N(1)-Pd(1)-N(2) 80.66(6) 82.5(3) N(1)-Pd(1)-N(3) - 80.98(9) N(1)-Pd(1)-N(4) 180.0 173.64(9) N(2)-Pd(1)- 161.96(9) 161.31(13) N(2A)/N3 N(2)-Pd(1)-N(4) 99.34(6) 99.71(9) N(3)-Pd(1)-N(4) - 98.16(9) C(7)-N(2)-Pd(1) 113.73(17) 112.71(19) C(12)-N(3)-Pd(1) - 112.50(17) C(6)-C(12)-N(3) - 114.8(2) C(2)-C(7)-N(2) 114.2(2) 115.0(3)

115 t 2+ In the case of Pd(pip2NNN)( Bu3tpy) , the piperidyl groups of the pip2NNN

ligand are situated above and below the coordination plane. These groups are rotated

toward the metal center, resulting in a relatively short Pd...N5 distance of 3.318(23) Å.

Although significantly longer than a conventional Pd-N covalent bond, this distance is

comparable to the sum of van der Waals radii of the two atoms (~3.2-3.3 ppm).49,50 In

+ the case of platinum two-electron reagents, such as Pt(tpy)(pip2NCN) , it has been

suggested that the persistence of an apical Pt...N(pip) interaction in solution effectively

preorganizes the complex for cooperative two-electron transfer.11

$3- $1-tpy)2+ salt, the non-coordinating central and

terminal pyridyl groups of the $1-terpyridyl ligand lie above the coordination plane of the

complex. The central pyridyl group forms a dihedral angle of 41.4(1)° with the pyridyl

group bonded to the metal center. However, the terminal non-coordinating pyridyl group

is nearly coplanar with the central pyridyl group, forming a dihedral angle of 4.01(1)°.

This geometry results in the lone electron pairs of N5 and N6 being directed toward N3

(N5...N3, 3.56; N6...N3, 3.47 Å) of the tridentate tpy ligand, rather than away from the

complex. By contrast, Pruchnik et al. have reported an example of a weakly bonded

1 monodentate tpy ligand in the structure of Rh2( -OAc)4( -tpy)2 (Rh-N, 2.337(7) Å); the central pyridyl N atom is directed away from the six-coordinate metal centers.25 1H

NMR data for the rhodium dimer indicate that the tpy ligand undergoes rapid exchange

resulting in the chemical equivalency of the terminal pyridyl groups even at 213 K.25 In

$3- $1-tpy)2+, there is a rather short contact between the metal center

and the N atom of the central pyridyl ring (Pd...N5, 2.902(22) Å), whereas the Pd...N6,

distance involving the terminal pyridyl N atom is rather long (3.939(34) Å). The bonded

116 pyridyl group of the 1-tpy ligand is situated on the other side of the plane defined by the

Pd center and the N atoms of the 3-tpy ligand, resulting in a N1-Pd-N4 bond angle of

173.64(9)°. Therefore, the monodentate tpy ligand appears well-positioned to form a

second Pd-N bond, as might be expected for an associative step leading to other isomers.

1 1 + H NMR Spectroscopy. The H NMR spectra of the Pd(pip2NNN)(tpy) and

t + 1 Pd(pip2NNN)( Bu3tpy) were recorded in deuterated acetonitrile. H NMR spectra are

shown in Figures 4.3 and 4.5. Resonances were fully assigned by 2-D 1H-NMR COSY

(Figures 4.4 and 4.6) and comparison with related palladium and platinum

11,14,51 complexes. Pyridyl resonances A and B of the pip2NNN ligand appear as a coupled triplet and doublet near 8.2 ppm and 7.7 ppm, respectively. Compared to

+ Pd(pip2NNN)Br (8.08, 7.50 ppm) (Chapter 3), these resonances are shifted downfield by

~0.1 ppm, in keeping with the notion that the ligand is bonded to a more electron

+ deficient metal. For Pd(pip2NNN)(tpy) , the tpy resonances L and K of the central

pyridyl ring are shifted further downfield near 8.55 and 8.35 ppm, respectively; the

remaining tpy resonances occur at 7.50 (G), 7.61 (H) and ~8.35 ppm (I, J). Interestingly,

the electron-donating t-butyl substituents have the greatest impact on the terminal pyridyl

t resonance G, causing it to shift upfield by 0.16 ppm; the remaining Bu3tpy resonances

(H, J, K) are shifted by <0.1 ppm from those of the tpy complex. For both complexes,

the resonances for the benzylic protons, C, appear as singlets near 4.0 ppm and are shifted

+ ~0.6 ppm upfield from the corresponding resonances of Pd(pip2NNN)X (X=Cl, Br, I)

14 and 0.4 ppm downfield from that of the free pip2NNN ligand. The -piperidyl resonances,D, are broad singlets at 2.4 ppm.

117 F E D **

N 2+ C B N A N Pd N L N K C G J N HI I J K

D E B G F H L A [

8 6 4 2 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 (ppm)

1 Figure 4.3 H NMR spectrum of [Pd(pip2NNN)(tpy)](BF4)2 in CD3CN. * denotes characteristic solvent resonances (CH3CN, 1.94

[ 2+ ppm; H2O, 2.13 ppm) and denotes resonances due to the [Pd(pip2NNN)(solvent)] ligand dissociation product (3.35, 4.60 ppm).

118 Figure 4.4 Aromatic region of the COSY NMR spectrum of

[Pd(pip2NNN)(tpy)](BF4)2 in CD3CN.

119 F I E D

N 2+ C B N A N Pd N N K L * L G J N H I

J C F D * * E B G A H *

8 7 6 5 4 3 2 1 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 (ppm)

1 t Figure 4.5 H NMR spectrum of [Pd(pip2NNN)( Bu3tpy)](BF4)2 in CD3CN. * denotes solvent resonances (Et2O, 1.12, 3.42 ppm;

CH3CN, 1.94 ppm; CH3COCH3, 2.08 ppm; H2O, 2.13)

120 Figure 4.6 Aromatic region of the COSY 1H NMR spectrum of

t [Pd(pip2NNN)( Bu3tpy)](BF4)2 in CD3CN.

121 + These resonances are shifted by 0.9-1.6 ppm upfield of those of Pd(pip2NNN)X and show no evidence of the diastereotopic splitting that is characteristic of tridentate binding of the pip2NNN ligand. The -and -piperidyl resonances, E and F, appear as multiplets

between 0.9 ppm and 1.2 ppm. Taken together, these observations are consistent with

monodentate coordination of the pip2NNN ligand to the metal center through the central

t 2+ pyridyl group, as suggested by the crystal structure of the Pd(pip2NNN)( Bu3tpy) salt.

Electronic Spectroscopy. The tpy complex dissolves in polar organic solvents

t (e.g., CH3CN) to give orange solutions, whereas the Bu3tpy complex gives yellow

solutions. To better understand the electronic structures of palladium(II) complexes with

the pip2NNN ligand and potential tridentate pyridyl ligands, absorption spectra of the

complexes were recorded in acetonitrile. The UV-visible absorption data are collected in

Table 4.3, and the spectra are summarized in Figure 4.7.

Comparisons to other spectra confirm that the intense absorptions at wavelengths

<315 nm have contributions from metal perturbed ligand-centered transitions associated

with the terpyridyl and pip2NNN ligands. For example, the free ligands exhibit similarly

intense absorption bands in this region (tpy: 200 nm (28000 cm-1M-1), 236 nm (19100

-1 -1 -1 -1 t -1 -1 cm M ), 279 nm (18100 cm M ); Bu3tpy: 204 nm (49500 cm M ), 241 nm (21500

-1 -1 -1 -1 -1 -1 cm M ), 283 nm (19000 cm M ); pip2NNN: 265 nm (3800 cm M )). The presence of

comparable transitions in the spectra of other Pd(II) terpyridine complexes (Pd(tpy)Cl+,

205 nm (57500 cm-1M-1), 246 nm (25800 cm-1M-1), 279 nm (23900 cm-1M-1);41

+ -1 -1 -1 -1 52 Pd(tpy)(N3) , 242 nm (31000 cm M , 271 nm (23200 cm M )) provides further evidence for the notion that this region is dominated by ligand-centered transitions. The

122 fact that Zn(tpy)Cl2 absorbs in this region also supports these assignments (264 nm (9700

cm-1M-1), 274 nm (12000 cm-1M-1), 283 nm (14800 cm-1M-1)).53

There is a progression of three absorption features between 330-360 nm. These

bands are similar to those reported for other palladium(II) terpyridyl complexes.41,44,52

Although there has been some disagreement on the orbital character of these transitions,

Guglielmo et al. have assigned features in this region for related complexes to ligand-

centered transitions.52 In support of this assignment, we note that the approximately 1330

cm-1 spacings are consistent with vibronic structure associated with a transition involving

the terpyridyl ligand.54 In addition, the maxima are nearly coincident with those of other palladium(II) terpyridyl complexes, indicating that the absorption profile in this region is relatively insensitive to changes in the fourth ligand (Table 4.3). It also is noteworthy that similar bands are observed in the spectra of platinum(II) terpyridyl complexes (e.g.,

Pt(tpy)Cl+ in DMF, 306 nm (12630 cm-1M-1), 318 nm (12800 cm-1M-1), 334 nm (17330 cm-1M-1), 350 nm (8860 cm-1M-1)).54

+ For Pd(pip2NNN)(tpy) , a long wavelength absorbance feature occurs near 410

nm (600 cm-1M-1). A similar band is slightly more intense and blue-shifted (380 nm, 700

-1 -1 t cm M ) in the spectrum of [Pd(pip2NNN)( Bu3tpy)](BF4)2. These bands are absent from

+ + + the spectra of Pd(tpy)Cl , Pd(tpy)(OH) and Pd(tpy)(N3) (Table 4.3). A long wavelength feature has been reported in the spectrum of Pd(tpy)I+ (430nm, ~1000 cm-1M-1) and attributed to a formally spin-forbidden ligand-centered transition, enhanced by the heavy

52 atom effect. However, in the case of the pyridyl pip2NNN ligand, significant

enhancement of spin-forbidden transitions is not expected. For Pd(II) complexes with

four N-donor ligands, the longest wavelength d-d transitions typically occur at <400 nm

123 2+ -1 -1 -1 55 (e.g., [Pd(NH3)4] , 298 nm, 212 cm M , fwhmC3;;; ). Additionally, the

estimated full-with at half-maximum (fwhm) of the lowest energy bands (~5000-8000

cm-1) is significantly greater than expected for a single d-d transition. Goshe and Bosnich

have reported electronic spectra of tetrameric palladium(II) complexes in which each

metal center is bonded to a tridentate para-substituted terpyridine and monodentate 4,4'-

bipyridyl. These spectra suggest that transitions associated with the

Pd(terpyridyl)(pyridyl)2+ unit are <200 cm-1M-1 per Pd center at wavelengths <465 nm.56

In addition, since the terpyridyl ligand has an aromatic substituent at the 4'-position in

those systems, charge-transfer transitions are expected to be shifted to lower energies

than for the complexes reported here. Therefore, in this context it is plausible that the

long wavelength absorption arises from a charge-transfer transition, resulting from a

weak interaction between the palladium center and the dangling piperidyl groups of

pip2NNN. A similar long-wavelength feature (albeit red-shifted) is observed in the

- + spectra of platinum complexes with a pip2NCN ligand (Pt(tpy)(pip2NCN) , 550sh nm,

-1 -1 + -1 -1 300 cm M ; Pt(phtpy)(pip2NCN) , 563sh nm, 400 cm M ), as well as platinum(II)

diimine complexes with a potentially facially-coordinating 1,4,7-trithiacyclononane

ligand (ttcn).13,57 This transition has been tentatively assigned as a Pt/L *(terpyridyl)

charge-transfer where L is N(piperidyl) or S(ttcn) depending on the ligand.58 In the

present case, the long-wavelength band for the tpy complex is shifted by ~2000 cm-1 to

t the red of that of the Bu3tpy adduct, in qualitative agreement with the expected relative

energies of the ligand * levels. It also is noteworthy that the ~6000 cm-1 blue-shift of

the longest wavelength transition in these palladium complexes, as compared to

124 + Pt(pip2NCN)(terpyridyl) analogs is consistent with a substantial negative shift of the

Pd(II) oxidation potential from that of Pt(II).

3 ) 1 - cm 1 -

M 2 4 - (x 10 1 X 10

0 250 350 450 550 650 Wavelength (nm)

2+ Figure 4.7 UV-visible absorption spectra of Pd(pip2NNN)(tpy) (---),

t 2+ + Pd(pip2NNN)( Bu3tpy) (2) and Pd(tpy)Cl (···) in CH3CN.

125 Table 4.3 UV-visible absorption data for Pd(II) terpyridyl complexes with in

acetonitrile.

Absorption Bands Compound -1 -1 Reference (max, nm ( , cm M )

192(68000), 205(57000), 243(31700), 268(22100), Pd(pip NNN)(tpy)2+ this work 2 304(10200), 330sh(6800), 346(7600), 362 (7000), 410sh(600)

193(58800), 214(45600), 245(35900), 269(24000), Pd(pip NNN)(tBu tpy)2+ this work 2 3 304(11000), 327sh(6500), 342(6800), 358 (6000), 380sh(700)

205(57500), 246(25800), 36 Pd(tpy)Cl+ 279(23900), 328(8670), 345 (9050), 362(8180 )

242(31000), 271(23200), Pd(tpy)N + 36 3 330(10700, 345(10900), 362(9000)

126 2+ Cyclic Voltammetry. Cyclic voltammetry data for Pd(pip2NNN)(tpy) and

t 2+ Pd(pip2NNN)( Bu3tpy) in 0.1 M TBAPF6 acetonitrile are presented in Figures 4.8 and

4.9. A surface absorption process is observed in the cyclic voltammograms (CV). The CV of the electrolyte solutions in the absence of sample is shown for comparison.

Voltammograms of [Pd(pip2NNN)Cl]Cl and pip2NNN are shown in Figure 4.10.

2+ t 2+ For both Pd(pip2NNN)(tpy) and Pd(pip2NNN)( Bu3tpy) in 0.1 M TBAPF6 in

acetonitrile, a chemically irreversible oxidation occurs near 1.5 V (Epa) vs. Ag/AgCl.

Even at scan rates up to 1000 mV/sec, no evidence of a return wave is observed.

Reductions for the electrochemically generated products are either shifted to < -2.0 V or

obscured by the reduction processes of the parent Pd(II) complex. Sweeping first in the

cathodic direction, both complexes give rise to multiple irreversible reduction waves in

the 0.4 V to -1.5 V region. The free terpyridyl ligands also undergo irreversible

t processes in this region (Epc: tpy, -0.43, -0.62, -1.20 V; Bu3tpy: -0.68, -0.88, -1.20 V vs.

Ag/AgCl). For the palladium complexes, the same features are observed when initially

sweeping in either the anodic or cathodic sweep directions, indicating that these

processes are not related to the irreversible oxidation process (Figure 4.9). Under similar

conditions, [Pd(pip2NNN)Cl]Cl is reduced at -0.92 V, whereas no reduction is observed

for the free pip2NNN ligand until -2.0 V. Terpyridyl-centered reductions of palladium(II)

complexes have been reported to occur irreversibly at slightly more negative potentials

2+ + 59 (e.g., Pd(tpy)(DMF) in DMF, Epc=-1.0 V vs. Fc /Fc). Reductions of platinum(II) terpyridyl complexes are reversible and also occur at more positive potentials.11,46,60-62

On the other hand, Pd-centered reductions have been reported to occur over a wide range

3 2+ of potentials, depending on the ligands (e.g., Pd3(dppm)3() -CO) , -0.24 V and -0.50 V

127 vs. Ag/AgCl, THF (dppm=bis(diphenylphosphino)methane); Pd(R2dithiocarbamate)2

complexes, quasireversible, -1.34 V to -1.60 V vs. Ag/AgCl, acetone, (R=alkyl

63 2+ 2+ 0 substituents); Pd(diphosphine)2 complexes, irreversible, -1.22 to -1.48 V (Pd /Pd )

and -0.55 to -0.94 V (Pd2+/Pd+) and -0.95 to -1.22V (Pd+/Pd0) vs. Fc/Fc+, acetonitrile).64

The first reduction process occurs at significantly more negative potentials than the metal-centered reduction of [Pt(tpy)C]0 (i.e., the Pt(II) terpyridyl anion radial adduct) (0 -

60 65 1.3 V vs. Ag/AgCl, 0.1 M TBAPF6 in DMF) as expected for electrostatic effects and the stabilization of the dx2-y2 orbital of the palladium system. The cathodic shift of the

t + reduction processes for Pt(pip2NCN)( Bu3tpy) as compared to the tpy complex is

t consistent with the increased donor properties of the Bu3tpy ligand. For example,

t 66 reduction waves for platinum(II) Bu3tpy complexes with aryl-acetylide ligands or a

- pip2NCN ligand are shifted by ~0.1 V to more positive potentials than for tpy complexes.

The redox chemistry of these palladium complexes is strikingly different than that

+ 11 of Pt(pip2NCN)(tpy) . The latter compound undergoes a nearly reversible two-electron

oxidation at 0.4V and two reversible one-electron reductions at -0.98 V and -1.50 V vs.

Ag/AgCl in 0.1M TBAPF6 acetonitrile solution. It is noteworthy that, the availability of the amine lone electron pairs is critical to facilitating reversible two-electron oxidation and stabilizing the resulting d6-electron metal center. When the piperidyl groups are

removed, as in Pt(tpy)(ph)+, the oxidation wave is absent; when the groups are

11,13 2+ protonated, the resulting oxidation is irreversible. In the cases of Pd(pip2NNN)(tpy)

t 2+ and Pd(pip2NNN)( Bu3tpy) , the Epa values are cathodically shifted by ~1.1 V from the

+ apparent formal Pt(IV/II) potential of Pt(pip2NCN)(tpy) . Overall, the accumulated data are consistent with an irreversible ligand-centered oxidation. However, we caution that

128 the observed process is irreversible, and it is not possible to reach firm conclusions

concerning electron stoichiometry or the extent of ligand/metal participation.

Comparisons with related complexes provide some insights. For example, solutions of

[Pd(pip2NNN)Cl]Cl (Epa = 1.17 V) and pip2NNN (Epa = 1.24 V) under similar conditions

show an irreversible oxidation process at more negative potentials than either

2+ t 2+ Pd(pip2NNN)(tpy) or Pd(pip2NNN)( Bu3tpy) . In the case of [Pd(pip2NNN)Cl]Cl, the observed process is associated with Cl- ion oxidation (1.16 V vs. Ag/AgCl; 1.36V vs.

67 NHE), whereas oxidation of pip2NNN involves the amine groups. From the M(IV/II)

+ couple of Pt(pip2NCN)(tpy) , we can estimate the potential of the analogous process for

2+ Pd(pip2NCN)(tpy) . Directly measured redox potentials and those derived from free

energies67 suggest that substituting palladium for platinum in metal complexes shifts the

3+/2+ M(IV/II) redox potential by ~0.5 V. On the other hand, the Ru(bpy)3 potential (1.35

3+/2+ V) is shifted by ~1.1 V from that of Ru(bpy)2(2-phenylpyridine) (0.47 V vs. NHE), as

expected for the lower positive charge of the phenylpyridine analog and the increased

/-donor properties of phenyl versus a pyridyl group.68,69 Therefore, we anticipate the

2+ Pd(IV/II) couple for Pd(pip2NNN)(tpy) to lie near 2.0 V vs. Ag/AgCl. In support of this conclusion, palladium-centered oxidation of Pd(phenylpyridine)(bipyridine)+ occurs

+ 70 at >2.0 V vs. Ag/AgCl (>2V vs. Fc/Fc in CH2Cl2). Therefore, the observed irreversible

oxidation of these terpyridyl complexes most likely involves the dangling amine groups

of the pip2NNN ligand.

129 (a)

2 uA

(b)

2.01.0 0.0 -1.0 -2.0 Potential (V)

Figure 4.8 Cyclic voltammograms of a) 1 mM [Pd(pip2NNN)(tpy)](BF4)2 (2) and; b)

t 1 mM [Pd(pip2NNN)( Bu3tpy)](BF4)2 (2) in 0.1 M TBAPF6 CH3CN (glassy

carbon working electrode, 0.25 V/sec). The CV of the electrolyte solution is

shown for comparison (---).

130 (a)

(b) 2 uA

(c)

(d)

2.01.0 0.0 -1.0 -2.0 Potential (V)

Figure 4.9 Cyclic voltammograms of 1 mM [Pd(pip2NNN)(tpy)](BF4)2 in 0.1 M

TBAPF6 CH3CN solution (glassy carbon working electrode, 0.1 V/sec).

(a) two cycle positive sweep between 0.0 and 2.0 V, (b) three cycle

positive sweep between 0.0 and 2.0 V starting at 0.0 V, (c) two cycle

negative sweep between 0.0 and -1.5 V, (d) three cycle negative sweep

between 0.0 and -1.5 V starting at 0.0 V.

131 (a)

2 uA

(b)

2.01.0 0.0 -1.0 -2.0 Potential (V)

Figure 4.10 Cyclic voltammograms of a) pip2NNN (2) and the electrolyte solution (---)

recorded at 0.25 V/sec; b) Pd(pip2NNN)Cl]Cl recorded at 0.1 V/sec, in 0.1

M TBAPF6 CH3CN vs. Ag/AgCl, glassy carbon working electrode.

132 Conclusions

Two novel Pd(II) complexes with two potentially tridentate ligands have been

1 prepared. H NMR spectroscopy and X-ray crystallography show that the pip2NNN

t ligand is bonded monodentate whereas the terpyridyl ligand (tpy or Bu3tpy) is bonded tridentate. The approximate square planar coordination geometry around the metal center

t + in Pd(pip2NNN)( Bu3tpy) is consistent with structures of palladium(II) complexes.

Interestingly, the piperidyl groups are located above and below the palladium center, resulting in a short Pd...N(piperidyl) distance of 3.33 ppm. This coordination geometry suggests that the amine groups are available to stabilize the metal center upon oxidation.

Interestingly, the visible absorption spectra exhibit weak, long-wavelength absorption bands, which may result from interaction between the dangling amine groups and the

+ metal center, as noted for Pt(pip2NCN)(terpyridyl) complexes. Both palladium complexes undergo irreversible oxidation at potentials that are anodically shifted by ~1.1

+ V from the reversible oxidation of Pt(pip2NCN)(terpyridyl) systems. This result is rationalized in terms of the expected relative influences of the ligands

- (pip2NNN/pip2NCN ) and metals (Pd/Pt) on the metal-centered redox potentials. The observed irreversible oxidation processes are tentatively attributed to oxidation of the piperidyl groups. A suggested strategy for lowering the Pd(IV/II) redox potentials in these systems and realizing reversible two-electron transfer involves ligand modification in order to increase electron density on the palladium center and/or raise the oxidation potential of the dangling axial donor groups.

133 References

(1) Lappin, G. Redox Mechanisms in Inorganic Chemistry, Ellis Horwood: New

York, 1994.

(2) Hubbard, A. T.; Anson, F. C. Anal. Chem. 1966, 38, 1887-1893.

(3) Blake, A. J.; Gould, R. O.; Holder, A. J.; Hyde, T. I.; Lavery, A. J.; Odulate, M.

O.; Schröeder, M. J. Chem. Soc. Chem. Comm. 1987, 118-120.

(4) Blake, A. J.; Crofts, R. D.; Schröeder, M. J. Chem. Soc. Dalton Trans. 1993,

2259-2260.

(5) Grant, G. J.; Spangler, N. J.; Setzer, W. N.; VanDerveer, D. G.; Mehne, L. F.

Inorg. Chim. Acta 1996, 246, 31-40.

(6) Matsumoto, M.; Funahashi, S.; Takagi, H. D. Z. Naturforsh 1999, 54b, 1138-

1146.

(7) Matsumoto, M.; Itoh, M.; Funahashi, S.; Takagi, H. D. Can. J. Chem. 1999, 77,

1638-1647.

(8) Usón, R.; Forniés, J.; Tomás, M.; Menjón, B.; Bau, R.; Süenkel, K.; Kuwabara,

E.; Organomet. 1986, 1576-1581. Organometallics 1986, 5, 1576-1581.

(9) Haines, R. I.; Hutchings, D. R.; McCormack, T. M. J. Inorg. Biochem. 2001, 85,

1-7.

(10) Forniés, J.; Menjón, B.; Sanz-Carrillo, R. M.; Tomás, M.; Connelly, N. G.;

Crossley, J. G.; Orpen, A. G. J. Am. Chem. Soc. 1995, 117.

(11) Jude, H.; Bauer, J. A. K.; Connick, W. B. J. Am. Chem. Soc. 2003, 125, 3446-

3447.

134 (12) Jude, H.; Carroll, G. T.; Connick, W. B. ChemTracts-Inorg. Chem 2003, 16, 13-

22.

(13) Jude, H.; Krause, J. A.; Connick, W. B. manusicript in preparation.

(14) Tastan, S.; Krause, J. A.; Connick, W. B. Inorg. Chim. Acta 2006, 359, 1889-

1898.

(15) Bruker APEX2 v1.0-27 and SAINT v27.06A programs were used for data

collection and data processing, respectively. Bruker Analytical X-Ray

Instruments, Inc., Madison WI.

(16) SADABS v2.10 was used for the application of semi-empirical absorption and

beam corrections. G. M. Sheldrick, University of Göettingen, Germany.

(17) SHELXTL v6.14 were used for the structure solution and generation of figures

and tables. Neutral-atom scattering factors were used as stored in this package.

G. M. Sheldrick, University of Göttingen, Germany and Bruker Analytical X-ray

Solutions, Inc., Madison, WI.

(18) SQUEEZE as implemented in Platon. A.L. Spek, (1990), Acta. Cryst. A1946, C-

1934.

(19) Priimov, G. U.; Moore, P.; Helm, L.; Merbach, A. E. Inorg. React. Mech. (Amst.,

Nether.) 2001, 3, 1-23.

(20) Chotalia, R.; Constable, E. C.; Hannon, M. J.; Tocher, D. A. J. Chem. Soc. Dalton

Trans.: Inorg. Chem. 1995, 22, 3571-3580.

(21) Constable, E. C.; Cargill Thompson, A. M. W. Inorg. Chim. Acta 1994, 223, 177-

179.

135 (22) Barloy, L.; Ramdeehul, S.; Osborn, J. A.; Carlotti, C.; Taulelle, F.; De Cian, A.;

Fischer, J. Eur. J. Inorg. Chem. 2000, 2523-2532.

(23) Abel, E. W.; Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V. Chem. Commun.

1996, 20, 2329-2330.

(24) Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V. J. Chem. Soc. Dalton Trans.:

Inorg. Chem. 1998, 6, 937-946.

(25) Pruchnik, F. P.; Robert, F.; Jeannin, Y.; Jeannin, S. Inorg. Chem. 1996, 35, 4261-

4263.

(26) Ferretti, V.; Gilli, P.; Bertolasi, V.; Marangoni, G.; Pitteri, B.; Chessa, G. Acta

Crystallogr., Sec. C: Cryst. Struct. Comm. 1992, C48, 814-817.

(27) Arriortua, M. I.; Rojo, T.; Amigo, J. M.; Germain, G.; Declercq, J. P. Bull. Soc.

Chim. Belg. 1982, 91, 337.

(28) McMurtrie, J.; Dance, I. Cryst. Eng. Comm. 2005, 7, 230-236.

(29) Alcock, N. W.; Barker, P. R.; Haider, J. M.; Hannon, M. J.; Painting, C. L.;

Pikramenou, Z.; Plummer, E. A.; Rissanen, K.; Saarenketo, P. Dalton Trans.

2000, 9, 1447-1462.

(30) Jeitler, J. R.; Turnbull, M. M.; Wikaira, J. L. Inorg. Chim. Acta 2003, 351, 331-

344.

(31) Stroh, C.; Turek, P.; Rabu, P.; Ziessel, R. Inorg. Chem. 2001, 40, 5334-5342.

(32) Constable, E. C.; Lewis, J.; Liptrot, M. C.; Raithby, P. R. Inorg. Chim. Acta 1990,

178, 47-54.

(33) Priimov, G. U.; Moore, P. Inorg. React. Mech. (PA, US.) 2003, 5, 21-30.

136 (34) Barloy, L.; Gauvin, R. g. M.; Osborn, J. A.; Sizun, C.; Graff, R.; Kyritsakas, N.

Eur. J. Inorg. Chem. 2001, 1699-1707.

(35) Ramdeehul, S.; Barloy, L.; Osborn, J. A.; De Cian, A.; Fischer, J.

Organometallics 1996, 15, 5442-5444.

(36) Abel, E. W.; Orrell, K. G.; Osborne, A. G.; Pain, H. M.; Sik, V.; Hursthouse, M.

B.; Malik, K. M. A. J. Chem. Soc. Dalton Trans.: Inorg. Chem. 1994, 23, 3441-

3449.

(37) Rotondo, E.; Giordano, G.; Minniti, D. J. Chem. Soc. Dalton Trans.: Inorg.

Chem. 1996, 2, 253.

(38) Abel, E. W.; Orrell, K. G.; Osborne, A. G.; Pain, H. M.; Sik, V.; Hursthouse, M.

B.; Malik, K. M. A. J. Chem. Soc. Dalton Trans.: Inorg. Chem. 1996, 2, 253-254.

(39) Burmeister, J. L.; Gysling, H. J. Inorg. Chim. Acta 1967, 1, 100-104.

(40) Ainscough, E. W.; Brodie, A. M.; Burrell, A. K.; Derwahl, A.; Jameson, G. B.;

Taylor, S. K. Polyhedron 2004, 23, 1159-1168.

(41) Zhang, W.; Bensimon, C.; Crutchley, R. J. Inorg. Chem. 1993, 32, 5808-5812.

(42) Bugarcic, Z. D.; Petrovic, B.; Zangrando, E. Inorg. Chim. Acta 2004, 357, 2650-

2656.

(43) Cosar, S.; Janik, M. B. L.; Flock, M.; Freisinger, E.; Farkas, E.; Lippert, B. J.

Chem. Soc. Dalton Trans. 1999, 2329-2336.

(44) Castan, P.; Dahan, F.; Wimmer, S. J. Chem. Soc. Dalton Trans. 1990, 2679-2683.

(45) MacLean, E. J.; Robinson, R. I.; Teat, S. J.; Wilson, C.; Woodward, S. J. Chem.

Soc. Dalton Trans. 2002, 3518-3524.

137 (46) Liu, X.; McInnes, E. J. L.; Kilner, C. A.; Thornton-Pett, M.; Halcrow, M. A.

Polyhedron 2001, 20, 2889-2900.

(47) Rulke, R. E.; Han, I. M.; Elsevier, C. J.; Vrieze, K. Inorg. Chim. Acta 1990, 169,

5-8.

(48) Jude, H., University of Cincinnati, Dissertation Thesis, p288.

(49) Hambley, T. W. Inorg. Chem. 1998, 37, 3767-3774.

(50) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.

(51) Hamann, C.; Kern, J.-M.; Sauvage, J.-P. Dalton Trans. 2003, 19, 3770-3775.

(52) Guglielmo, G.; Ricevuto, V.; Giannetto, A.; Campagna, S. Gazz. Chim. Ital. 1989,

119, 457-460.

(53) Nakamoto, K. J. Phys. Chem. 1960, 64, 1420-1425.

(54) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer, W. P.; Gray,

H. B. Inorg. Chem. 1995, 34, 4591-4599.

(55) Mason III, W. R.; Gray, H. B. J. Am. Chem. Soc. 1968, 90, 5721-5729.

(56) Goshe, A. J.; Bosnich, B. Synlett 2001, 941-944.

(57) Chatterjee, S.; Krause, J. A.; Connick, W. B. manusicript in preparation.

(58) Green, T. W.; Lieberman, R.; Mitchell, N.; Bauer, J. A. K.; Connick, W. B. Inorg.

Chem. 2005, 44, 1955-1965.

(59) DuBois, D. L.; Miedaner, A.; Haltiwanger, R. C. J. Am. Chem. Soc. 1991, 113,

8753-8764.

(60) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1996, 35,

4585-4590.

(61) Hershel Jude dissertation thesis, pg. 207-208, 348-351.

138 (62) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.

Organometallics 2001, 20, 4476-4482.

(63) Van der Linden, J. G. M.; Dix, A. H. Inorg. Chim. Acta 1979, 35, 65-71.

(64) Raebiger, J. W.; Miedaner, A.; Curtis, C. J.; Miller, S. M.; Anderson, O. P.;

DuBois, D. L. J. Am. Chem. Soc. 2004, 126, 5502-5514.

(65) The influence of electrostatics on redox potentials is well studied; see for

example: Zhou, C.; Raebiger, J. W.; Segal, B. M.; Holm, R. H. Inorg. Chim. Acta

2000, 300-302, 892-902.

(66) Lam, S. C.-F.; Yam, V. W.-W.; Wong, K. M.-C.; Cheng, E. C.-C.; Zhu, N.

Organometallics 2005, 24, 4298-4305.

(67) Latimer, W. M. Oxidation Potentials, 2. Edition 1953, 53.

(68) Constable, E. C.; Holmes, J. M. J. Organomet. Chem. 1986, 301, 203-208.

(69) Reveco, P.; Cherry, W. R.; Medley, J.; Garber, A.; Gale, R. J.; Selbin, J. Inorg.

Chem. 1986, 25, 1842-1845.

(70) Pugliese, T.; Godbert, N.; Aiello, I.; Ghedini, M.; La Deda, M. Inorg. Chem.

Commun. 2006, 9, 93-95.

139 CHAPTER 5:

Synthesis, Characterization and Redox Tuning of Platinum (II) Pincer Complexes with Terpyridine

Introduction

A successful strategy for designing reversible outer-sphere two-electron transfer

reagents has been described recently.1,2 The central premise is that the two potentially

mer-coordinating ligands bonded to a platinum center are capable of stabilizing both the

Pt(II) and Pt(IV) oxidation states. In the d6 octahedral Pt(IV) case, both ligands are

expected to be tridentate occupying all six binding sites around the metal center. In the d8

square planar Pt(II) case, one ligand will remain tridentate, while the other coordinates in

a monodentate fashion through the central binding site (Scheme 5.1).

Scheme 5.1 Schematic representation of a reversible two-electron platinum reagent.

+ This strategy has been tested by preparing Pt(pip2NCN)(tpy) which has both

- 6 pip2NCN and tpy tridentate, occupying all six binding sites around platinum, in the d

- octahedral Pt(IV) case, and tpy remaining tridentate whereas pip2NCN coordinating in a monodentate fashion through the central phenyl group in the d8 square planar Pt(II) case.

+ It has been demonstrated that Pt(pip2NCN)(tpy) undergoes a nearly reversible two-

140 electron oxidation near 0.4 V vs. Ag/AgCl.1 The remarkable reversibility of the redox

+ chemistry of Pt(pip2NCN)(tpy) is suggestive of a non-covalent interaction between the

- pip2NCN amine groups and the Pt(II) metal center, which effectively preorganizes the

square planar complex for electron transfer. The notion of preorganization is supported

by crystal structures of a series of two-electron reagents which reveal relatively short

Pt...N(piperidyl) interactions. In addition, UV-visible absorption spectra of these

complexes exhibit long wavelength absorption features (>500 nm), which are absent from

the spectra of model complexes, such as Pt(tpy)(phenyl)+.3 In chapter 4, we have

extended these studies to palladium and synthesized palladium(II) terpyridyl complexes

with the pip2NNN ligand (Scheme 5.2). Similar to the analogous platinum complex,

t + Pd(pip2NNN)( Bu3tpy) has piperidyl groups located above and below the palladium center, resulting in a short Pd...N(piperidyl) distance of 3.33 ppm and the visible absorption spectra exhibit weak, long-wavelength absorption bands, which may result from

N N N

N Z R2 N

N N N

R3 - pip2NNN Z-pip2NCN R-tpy

Scheme 5.2 Potential tridentate ligands.

141 interaction between the dangling amine groups and the metal center. However, the

+ Pd(pip2NNN)(terpyridyl) complexes do not undergo metal-centered oxidation <2.0V. In

addition to suitable coordination geometry, development of ligand systems that can

optimize the electron density on the metal center is necessary to obtain reversible two-

electron transfer.

In order to investigate the role of metal acidity on the redox potentials, in chapter

5 we report the synthesis of platinum complexes with electron-releasing and withdrawing

- groups (Z=MeO, NO2) at the para-positions of pivoting NCN ligands and the 4'- substituted positions of tridentate tpy ligands (R= t-butyl (C(CH3)3), tolyl (4-

metylphenyl)) (Scheme 5.2).

Experimental

All reagents were purchased from Pressure Chemical, Aldrich or Acros.

4 5 6 7 Pt(pip2NCN)Cl, Pt(COD)(Ph)Cl, Pt(COD)(Mes)Cl, [Pt(Ph2)SEt2]2, pip2NCNBr (2,6-

bis(benzylpiperidine)bromide),4 and 4-Bromo-3,5-bis(bromomethyl)anisole8 were

prepared according to published procedures. Acetonitrile and EtOH were distilled from

1 CaH2 and NaOH/Al(s), respectively, under argon. H NMR spectra were recorded at

room temperature using a Bruker AC 400 MHz Spectrometer. Deuterated acetonitrile

and chloroform (0.03% tetramethylsilane (TMS) (v/v)) were purchased from Cambridge

Isotope Laboratories. UV-visible absorption spectra were recorded using a HP8453 UV- visible spectrometer. Emission spectra were recorded using a SPEX Fluorolog-3 fluorimeter equipped with a double emission monochromator and a single excitation monochromator. Emission spectra were corrected for instrumental response. Mass

142 spectra were recorded using a Micromass Q-TOF-2 hybrid quadrapole time of flight mass

spectrometer with electrospray ionization. Elemental analyses were performed by

Atlantic Microlabs (Norcross, GA). Cyclic voltammograms were measured using a

BAS100B/W Electrochemical Analyzer. Scans were collected in acetonitrile solution

containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6), which was recrystallized from boiling methanol and dried under vacuum prior to use.

Voltammograms were recorded using a platinum wire auxiliary electrode, a Ag/AgCl reference electrode (3.0 M NaCl) and a gold working electrode. Between scans, the

? - ? ? ;);2 ) ? ? wiped dry using a Kimwipe. Adsorption causes electrode fouling as indicated by larger

Ep values with consecutive cycles. Reported potentials are referenced against Ag/AgCl.

Peak currents (ip) estimated with respect to the extrapolated baseline current as described

4,9 elsewhere. The values of (Epc+Epa)/2, which is an approximation of the formal potential for a redox couple, are referred to as E°'.

NO2-pip2NCNBr. This compound was prepared by a modification of the

10 procedure for the preparation of NO2-Me4NCNBr. To 2g (5.7 mmol) of pip2NCNBr

0 was added 5 mL concentrated H2SO4 dropwise while stirring at 0 C. After subsequent

0 addition of 0.8 mL 16 M HNO3 while keeping the temperature below 10 C, the mixture

was left stirring at room temperature for 3 hours. The solution was poured into an ice-

water mixture (8 mL water, ~30 g ice), and the pH was brought to 7 by addition of KOH

and to 8 by addition of small amounts of K2CO3. A white solid (K2SO4) was removed by filtration, and the filtrate was extracted with CH2Cl2 (4 x 200 mL). The combined extracts were dried over MgSO4 and filtered. The filtrate was reduced to dryness by

143 rotary evaporation and the yellow solid was collected. Yield: 2.1g, 93%. 1H NMR

(CDCl3 M 1)GB 3H, m, CH2), 1.62 (8H, m, CH2), 2.47 (8H, m, CH2), 3.59 (4H, s, CH2),

8.27 (2H, s, CH).

MeO-pip2NCNBr. This compound was prepared by a modification of the

11 procedure for the preparation of MeO-Me4NCNBr. To a 10 mL benzene solution of

piperidine (1.03 mL, 1.05 mol) at 0 0C was added dropwise 15 mL of a benzene solution of 4-bromo-3, 5-bis(bromomethyl)anisole (0.78 g, 2.1 mmol). After stirring overnight, the mixture was filtered. The resulting pale yellow filtrate was rotary evaporated to give

1 a yellow oil. The product was used without further purification. H NMR (CDCl3 M

1.47 (4H, m, CH2), 1.63 (8H, m, CH2), 2.48 (8H, m, CH2), 3.55 (4H, s, CH2), 3.82 (3H, s,

CH3), 7.05 (2H, s, CH).

Pt(NO2-pip2NCN)Br. This compound was prepared by a modification of the

10 procedure for the preparation of Pt(NO2-Me4NCN)Br. To a 20 mL suspension of

[Pt(Ph2)SEt2]2 (0.15g, 0.17 mmol) in benzene at room temperature under argon was added 15 mL of a benzene solution of NO2-pip2NCNBr (0.132 g, 0.34 mmol) dropwise.

The solution was refluxed for 3h. After the yellow solution cooled to room temperature, it was reduced to dryness by rotary evaporation. The remaining yellow solid was washed with ether and collected over a frit. No impurities were detectable in the 1H NMR

spectrum, and the material was used without further purification. Yield: 0.155 g, 78%. 1H

NMR (CDCl3,): 1.48 (8H, m, CH2), 1.62 (4H, m, CH2), 1.82 (8H, m, CH2), 3.27 (4H, m,

CH2), 4.13 (4H, m, CH2), 4.33 (4H, s with Pt satellites, JPt-H = 26 Hz, CH2), 7.76 (2H, s,

CH).

144 Pt(MeO-pip2NCN)Br. This compound was prepared by a modification of the

12 procedure for the preparation of Pt(MeO-Me4NCN)Cl. To a 20 mL THF solution of

o n MeO-pip2NCNBr (0.29 g, 0.74 mmol) at -78 C under argon was added BuLi (0.93 mL,

1.6 M in hexane), causing the color of the solution to change from yellow to purple.

After allowing the solution to warm to room temperature, Pt(SEt2)2Cl2 (0.33g, 0.74

mmol) in 15 mL THF was added to the flask. The mixture was stirred at room

temperature for 12 hours. The orange-yellow solution was rotary evaporated to dryness.

The solid was sonicated in hexanes and collected by vacuum filtration. The light orange

1 product was used without further purification. Yield: 0.30 g, 70%. H NMR (CDCl3,):

1.47-1.75 (12H, m, CH2), 3.27 (4H, m, CH2), 3.76 (3H, s, CH3), 4.08 (4H, m, CH2), 4.25

(4H, s with Pt satellites, CH2), 6.47 (2H, s, CH).

t [Pt( Bu3tpy)(mes)](BF4). This compound was prepared by a modification of the

2 procedure for the preparation of [Pt(tpy)(mes)]BF4. To a 25 mL acetone solution of

Pt(COD)(Mes)Cl (0.100 g, 0.218 mmol) in the dark was added AgBF4 (0.043 g, 0.22

mmol). After stirring for 30 min, the mixture was filtered through Celite. The filtrate

t was stirred with Bu3tpy (0.218 mmol, 0.090 mg) for 4 hours before removal of the

solvent by rotary evaporation. The residue was dissolved in a minimum amount of

dichloromethane. Addition of hexanes afforded a bright yellow solid. Yield: 0.158 g,

90%. Anal. Calcd. for [C36H46N3Pt](BF4).0.5H2O: C, 54.76; H, 6.32; N, 7.02. Found: C,

t + 1 54.58; H, 6.41; N, 6.82. MS(ESI): m/z = 715.334 (Pt( Bu3tpy)(mes)) . H NMR (CDCl3,

M 1)31 15# .#3), 1.61 (9H, s, CH3), 2.33 (3H, s, CH3), 2.48 (6H, s, CH3), 6.84 (2H, s, CH), 7.39 (2H, d, CH), 8.04 (2H, d with Pt satellites, JPt-H =55 Hz, CH); 8.37 (2H, s,

CH); 8.46 (2H, s, CH).

145 t [Pt( Bu3tpy)(ph)](BF4). This product was isolated as a bright green-yellow solid

t by following the procedure for [Pt( Bu3tpy)(mes)]BF4 and substituting Pt(COD)(Ph)Cl

(0.100 g, 0.241 mmol) for Pt(COD)(mes)Cl. Yield: 0.118 g, 65%. Anal. Calcd. for

[C33H40N3Pt](BF4).3/4 CH2Cl2: C, 49.18; H, 5.07; N, 5.10. Found: C, 49.11; H, 5.11; N,

t + 1 5.22. MS(ESI): m/z = 673.287 (Pt( Bu3tpy)(ph)) . H NMR (CDCl3 M 1)34 15#

CH3), 1.60 (9H, s, CH3), 7.14 (1H, t, CH), 7.25 (2H, t, CH), 7.46 (2H, d, CH), 7.51 (2H, d

with Pt satellites, JPt-H =35 Hz, CH), 8.31 (2H, d with Pt satellites, JPt-H =55 Hz, CH),

8.37 (2H, s, CH), 8.46 (2H, s, CH).

t [Pt(pip2NCN)( Bu3tpy)](BF4). A mixture of Pt(pip2NCN)Cl (0.100 g, 0.2 mmol) and AgBF4 (0.079 g, 0.2 mmol) in 35 mL of acetone was allowed to stir for 45

minutes in the dark at room temperature. The resulting precipitate was removed by

t vacuum filtration through Celite. After addition of Bu3tpy (0.080 g, 0.20 mmol), the

filtrate was allowed to stir for 18 hours, and the solvent was removed by rotary-

evaporation.

The yellow solid was dissolved in dichloromethane, and hexanes were added to induce precipitation. The product was washed with hexanes and dried under vacuum. Yield 0.15 g, 78%. Anal. Calcd. for [C45H62N5Pt](BF4).1/2CH2Cl2: C, 54.76; H, 6.32; N, 7.02.

Found: C, 54.58; H, 6.41; N, 6.82. MS(ESI): m/z = 955.476 ((Pt(pip2NCNH)( t + t + 1 Bu3tpy).BF4) ), m/z =867.485 (Pt(pip2NCN)( Bu3tpy) ). H NMR (CDCl3, M 1)1H 5#

m, CH2), 1.21 (4H, m, CH2), 1.46 (18H, s, CH3), 1.63 (9H, s, CH3), 2.30 (8H, m, CH2),

3.71 (4H, s, CH2), 7.15 (3H, m, CH), 7.31 (2H, d, CH), 7.97 (2H, d with Pt satellites, JPt-H

1 = 42 Hz, CH), 8.32 (2H, s, CH), 8.45 (2H, s, CH). H NMR (CD3C6 M 1);H 5#

CH2), 1.16 (4H, m, CH2), 1.40 (18H, s, CH3), 1.57 (9H, s, CH3), 2.28 (8H, m, CH2), 3.68

146 (4H, s, CH2), 7.10 (3H, m, CH), 7.49 (2H, d, CH), 7.93 (2H, d with Pt satellites, JPt-H = 42

Hz, CH), 8.29 (2H, s, CH), 8.40 (2H, s, CH).

t [Pt(pip2NCNH2)( Bu3tpy)](PF6)3. 0.100 g (0.105 mmol) of

t [Pt(pip2NCN)( Bu3tpy)]BF4 was dissolved in ~5 mL acetone, and HNO3 (1 M) was added dropwise until the red solution turned bright yellow. Water (20 mL) was added and the volume was reduced to ~20 mL by rotary evaporation. The mixture was filtered and 3 mL of concentrated aqueous NH4PF6 was added to precipitate the product. The yellow solid was collected by vacuum filtration, washed with ether and dried under vacuum. Yield

0.123 g, 90%. The product was recrystallized by diffusing diethyl ether into an acetone solution of the complex. Anal. Calcd. for [C45H64N5Pt](PF6): C, 41.42; H, 4.94; N, 5.37.

t 3+ Found: C, 41.15; H, 5.04; N, 5.33. MS(ESI): m/z = 434.235 (Pt(pipNCNH2)( Bu3tpy) ),

t + 1 m/z =1159.400 ((Pt(pip2NCNH2)( Bu3tpy).2PF6) ). H NMR (CD3CN, ): 1.20-170

(12H, m, CH2), 1.46 (18H, s, CH3), 1.63 (9H, s, CH3), 2.83 (4H, m, CH2 ), 3.33 (4H, m,

CH2), 4.68 (4H, d, CH2 ), 7.07 (2H, broad, NH), 7.53 (3H, m, CH), 7.66 (2H, d, CH),

7.83 (2H, d with Pt satellites, JPt-H = not resolved, CH), 8.40 (2H, s, CH), 8.49 (2H, s,

CH).

[Pt(pip2NCN)(toltpy)](BF4). This product was isolated as a red solid by

t following the procedure for [Pt(pip2NCN)( Bu3tpy)]BF4 and substituting toltpy (0.065 g,

t 0.20 mmol) for Bu3tpy. Yield: 0.12 g, 68%. Anal. Calcd. for [C40H44N5Pt](BF4).3/4

CH2Cl2: C, 52.05; H, 4.88; N, 7.45. Found: C, 52.20; H, 4.84; N, 7.26. MS(ESI): m/z =

+ + 1 877.339 ((Pt(pip2NCNH)(toltpy).BF4) , m/z =789.3 (Pt(pip2NCN)(toltpy) ). H NMR

(CDCl3 M 1)12-1.26( 12H, m, CH2), 2.18 (3H, s, CH3), 2.32 (8H, m, CH2), 3.76 (4H, s,

147 CH2), 7.17-7.26 (5H, m, CH), 7.42 (2H, t, CH), 7.90(2H, d, CH) , 8.14 (2H, d with Pt

satellites, JPt-H = not resolved, CH), 8.33 (2H, t, CH), 8.53 (2H, s, CH), 8.63 (2H, d, CH).

[Pt(NO2-pip2NCN)(tpy)](BF4). This product was isolated as an orange solid by

t following the procedure for [Pt(pip2NCN)( Bu3tpy)]BF4 and substituting

Pt(NO2pip2NCN)Br for Pt(pip2NCN)Br (0.075 g, 0.13 mmol) and tpy (0.03 g, 0.13

t mmol) for Bu3tpy. Yield: 0.09 g, 86%. Anal. Calc. for [C33H33N6O2Pt](BF4)1.5 CH2Cl2:

C, 44.70; H, 4.35; N, 9.07. Found: C, 44.52; H, 4.40; N, 9.06. MS(ESI): m/z = 832.281

+ + 1 ((Pt(NO2-pip2NCNH)(tpy).BF4) ), m/z =744.267 (Pt(NO2-pip2NCN)(tpy) ). H NMR

(CDCl3,): 1.10 (8H, m, CH2), 1.23 (4H, m, CH2), 2.32 (8H, m, CH2), 3.75 (4H, s, CH2),

7.45 (2H, t, CH), 7.96 (2H, d with Pt satellites, JPt-H =not resolved, CH), 8.02 (2H, s, CH),

8.31 (2H, t, CH), 8.52 (2H, d, CH), 8.64 (3H, m, CH).

[Pt(MeO-pip2NCN)(tpy)]OTf. To a 15 mL 9:1 acetone-water solution of

Pt(MeO-pip2NCN)Br (0.05 g, 0.052 mmol) was added AgOTf (0.013 g, 0.052 mmol),

and the mixture was stirred for 30 minutes in the dark. After filtering through Celite, tpy

(0.02 mg, 0.085 mmol) was added to the filtrate, and the solution was stirred for 4 hours.

The red solution was reduced to dryness by rotary evaporation. The residue was

dissolved in dichloromethane, and hexanes were added. The orange-red solid was

collected by vacuum filtration and dried. Yield: 0.03 g, 40%. MS(ESI): m/z = 879.25

+ + 1 ((Pt(MeO-pip2NCNH)(tpy).OTf) ), m/z =729.24 (Pt(MeO-pip2NCN)(tpy) ). H NMR

(CDCl3,): 1.16-1.23 (12H, m, CH2), 2.36 (8H, m, CH2), 3.75 (4H, s, CH2), 3.89 (3H, s,

CH3), 6.92 (2H, s, CH), 7.44 (2H, t, CH), 8.16 (2H, d with Pt satellites, JPt-H = not

resolved, CH), 8.28 (2H, t, CH), 8.55 (2H, m, CH), 8.66 (3H, m, CH).

148 t X-ray Crystallography. Yellow plates of [Pt(pip2NCN)( Bu3tpy)](BF4)

t (hexanes/diethyl ether), [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 (diethyl ether/acetone) and

t [Pt(Ph)( Bu3tpy)](BF4).0.5CH2Cl2 (diethyl ether/ dichloromethane-ethanol) were obtained

by vapor diffusion into a solution of each complex. Red blocks of

[Pt(pip2NCN)(toltpy)](BF4) and orange crystals of [Pt(NO2-pip2NCN)(tpy)](BF4).

1/8C4H10O were obtained by vapor diffusion of ether into a dichloromethane solution of the complex. Pale pink crystals of [Pt(tpy)(CH3COO)3](PF6) were obtained from an acetonitrile-diethyl ether solution. Diffraction data were collected for

t t [Pt(pip2NCN)( Bu3tpy)](BF4), [Pt(Ph)( Bu3tpy)](BF4).0.5CH2Cl2, [Pt(pip2NCN)(toltpy)]

t (BF4),[Pt(tpy)(CH3COO)3](PF6), [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 and [Pt(NO2-pip2NCN)

(tpy)](BF4).1/8C4H10O using a Bruker Platinum200 CCD detector at Beamline 11.3.1 at the Advanced Light Source (Lawrence Berkeley National Laboratory) with synchrotron

(0;)442;; Å/ (0;)443K; Å or a Bruker SMART6000 CCD detector with graphite- . / (01)23145 Å. The data frames were collected using the program APEX2 and processed using the program SAINT within APEX2.13

Absorption and beam corrections based on the multi-scan technique were applied using

SADABS.14 The structures were solved by a combination of direct methods

SHELXTL15 and the difference Fourier technique. The models were refined by full- matrix least squares on F2. Non-hydrogen atoms were refined with anisotropic

displacement parameters with the exception of the disordered F atoms in

t t [Pt(pip2NCN)( Bu3tpy)](BF4), [Pt(pip2NCNH2)( Bu3tpy)](PF6)3, [Pt(NO2 -pip2NCN)

(tpy)](BF4). 1/8C4H10O and [Pt(tpy)(CH3COO)3](PF6), as well as the disordered C atoms

t of [Pt(pip2NCN)( Bu3tpy)](BF4) (C35A-C38A, C35B-C38B, C41A-C42A, C41B, C42B,

149 t C45A-C46A, C45B-C46B), [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 (C38B) and [Pt(NO2-

pip2NCN)(tpy)](BF4).1/8C4H10O (C68). H atoms were either located directly from the

difference map or calculated and treated with a riding model. The isotropic displacement

parameters were defined as a times Ueq of the adjacent atom where a =1.5 for methyl and

t 1.2 for all others. For [Pt(pip2NCN)( Bu3tpy)](BF4), a high degree of disorder is

observed for the t-butyl groups and F-atoms. Bond distance restraints were applied to

C4-C35B, C35A-C38A, C35B-C36B, C35B-C37B, C39-C41B, C39-C42A, C43-C44,

C43-C45B, C43-C46B, B1-F2A, B1-F3A and B1-F3B. The largest residual densities

appear near a disordered t-butyl group and the Pt atom. For

t [Pt(pip2NCNH2)( Bu3tpy)](PF6)3, a high degree of disorder is observed for the t-butyl groups and the F-atoms bonded to P3. Bond distance restraints were applied to C43-

C44A, C43-C45A, P3-F15B, P3-F16B, P3-F17b and P3-F18A. For

t [Pt(Ph)( Bu3tpy)](BF4).1/2 CH2Cl2, the contribution of a misbehaved half molecule of

CH2Cl2 in the lattice was removed from the reflection data using the program

16 - SQUEEZE. For [Pt(pip2NCN)(toltpy)](BF4), several carbon atoms in the pip2NCN

ligand show disorder. However, suitable components for these individual atoms were not

resolved. For [Pt(NO2-pip2NCN)(tpy)](BF4). 1/8C4H10O, disorder is observed in a

number of atoms in the piperidyl groups. However a reasonable multi-component was not

obtained. A partial (1/8 occupancy) diethyl ether solvent of crystallization is located at a

center of symmetry (O5 resides on a special position at 0,1/2,0). The disordered carbon

atom (C68) was resolved and described with a two-component model.

150 Results and Discussion

Synthesis. A series of platinum(II) complexes with two potentially mer-

coordinating ligands has been prepared. One ligand is an NCN- pincer ligand with a substituent Z (H, NO2 or MeO) at the para-position; the other is a terpyridyl ligand (tpy,

t Bu3tpy or toltpy) with substituents labeled Rn. The synthesis of the Z-pip2NCNBr ligand

precursors and their corresponding Pt(Z-pip2NCN)Br complexes is shown in Scheme 5.1.

The brominated ligand precursors were prepared by procedures that are similar to

8,10 published methods. The NO2 group was introduced by aromatic nitration of pip2NCNBr with H2SO4/HNO3 and subsequent quenching with KOH/K2CO3. Because

8 direct methoxylation of Me4NCNBr was shown to be unsuccessful, the synthesis of the

MeO-pip2NCNBr was accomplished by adding piperidine to 4-bromo-3,5-

10 bis(bromomethyl)anisole. Drawing on the work of van Koten et al., Pt(NO2-

pip2NCN)Br was prepared by refluxing the ligand precursor with [Pt(Ph)2SEt]2 in

benzene, whereas Pt(MeO-pip2NCN)Br is prepared by stirring the lithiated ligand with

Pt(SEt2)2Cl2 in THF. The latter product was typically slightly contaminated with

Pt(MeO-pip2NCN)Cl as identified by an additional -piperidyl proton resonance at 3.94

ppm in the 1H NMR spectrum. The synthesis of complexes with dangling nucleophilic

groups at the axial sites of a square planar metal center poses a significant challenge

because of difficulties in forcing the metal to bond to one donor ligand over another. The

synthetic route to two-electron platinum reagents with NCN- and terpyridyl ligands involves treatment of an acetone solution of Pt(Z-pip2NCN)Br with a suitable silver salt

(e.g., AgBF4), followed by filtration. Within 15 minutes of adding one equivalent of tpy,

t Bu3tpy or toltpy to the filtrate, the pale yellow solution turned orange or red, indicating

151 displacement of the piperidyl groups by the terpyridyl ligand. The identities of the

products were verified by 1H NMR spectroscopy, mass spectrometry and X-ray

crystallography. Interestingly, though solutions of the four complexes are red, their

t colors vary in the solid state from yellow ([Pt(pip2NCN)( Bu3tpy)](BF4) and [Pt(NO2-

pip2NCN)(tpy)](BF4) to orange-red ([Pt(MeO-pip2NCN)(tpy)]OTf and [Pt(pip2NCN)

(toltpy)](BF4).

+ We have previously observed that Pt(pip2NCN)(tpy) is unstable in acetonitrile

+ solution, gradually undergoing tpy ligand dissociation to form a Pt(pip2NCN)(solvent)

t + adduct. The half-life of that complex is ~2 hours. Pt(pip2NCN)( Bu3tpy) is considerably more stable in acetonitrile. The loss in the intensity of the 420 nm electronic absorption band is consistent with 3% decomposition in one hour. The increased stability of the

t Bu3tpy complex is attributable to the comparatively greater electron-donor properties of

t the Bu3tpy ligand, which are anticipated to decrease the electrophilicity of the metal

center and thereby increase the reaction barrier of an associative mechanism. This

observation implicates modulation of ligand electron-donor properties as an effective

strategy for tuning the relative stabilities of two-electron platinum reagents.

t + Each piperidyl group of Pt(pip2NCN)( Bu3tpy) was readily protonated by dropwise addition of 1 M HNO3 to the deep red acetone solution of the complex until the

solution turned bright yellow. However, attempts to protonate the other complexes with

substituted pincer ligands did not yield similar diprotonated compounds. Surprisingly, in

t + the case of Pt(MeO-pip2NCN)( Bu3tpy) , attempts to grow crystals of the product obtained from treatment with acetic acid gave a platinum(IV) salt,

[Pt(tpy)(CH3COO)3](PF6). The positive ion mass spectrum of the product confirms the

152 crystallographic data, which shows a tridentate meridional tpy ligand and three monodentate acetate groups. To our knowledge, this is the first example of platinum(IV)

+ complexes with a tridentate terpyridine. In the case of [Pt(NO2-pip2NCN)(tpy) , treatment with acid caused dissociation of the terpyridine ligand, as determined by 1H

NMR spectroscopy.

N N N (i) (ii) Br O2N Br O2N Pt Br

N N N

Br N N (iii) (iv) O Br O Br O Pt Br

Br N N

Scheme 5.3 Synthesis of Z-pip2NCNBr (Z=NO2, MeO) ligand precursors and the

Pt(Z-pip2NCN)Br complexes. (i) H2SO4 / HNO3, KOH / K2O3 (ii)

o [Pt(Ph2)SEt2]2, benzene (iii) piperidine, benzene, 0 C (iv) Pt(SEt2)2Cl2,

nBuLi,THF,-78oC.

153 N +

N R1 (i) N Z Pt Br Z Pt N R2 N N R N 1

Scheme 5.4 Synthesis of platinum(II) complexes (Z=H, NO2, MeO; R1=H, t-butyl; R2=

H, t-butyl, tolyl) with two potentially meridional-coordinating tridentate

t ligands: (i) AgBF4 or AgOTf; tpy, Bu3tpy or toltpy; acetone.

t Crystal Structures. The structures of [Pt(Ph)( Bu3tpy)](BF4).1/2CH2Cl2,

t t [Pt(pip2NCN)( Bu3tpy)](BF4), [Pt(pip2NCNH2)( Bu3tpy)](PF6)3, [Pt(pip2NCN)(toltpy)]

(BF4), [Pt(NO2pip2NCN)(tpy)](BF4).1/8C4H10O and [Pt(tpy)(CH3COO)3](PF6)were

confirmed by single-crystal X-ray diffraction studies. ORTEP diagrams are shown in

Figures 5.1-5.6, respectively, and relevant data are summarized in Tables 5.1-5.2. For

each structure, the cations, anions and solvent pack as discrete units, and there are no

unusually short intermolecular contacts.

In each case, the platinum(II) complex adopts a distorted square planar geometry

that is qualitatively similar to those of other platinum(II) terpyridyl complexes.2,17-26 In

- all cases, a terpyridyl ligand is tridentate and the pip2NCN ligand is monodentate,

bonded through the phenyl group at the fourth coordination site of the metal center. The

154 geometries of the Pt(tpy) units and Pt-C(phenyl) distances are in excellent agreement

with structures of other complexes.2,17-26 The Pt-N1(central pyridine of tpy) distances

(1.987 0.007 Å) are consistent with having a strong trans directing group, as observed

+ 17 + 2 for Pt(pip2NCNH2)(tpy) (2.005(6) Å) and Pt(4-phenyl-tpy)(mes) (1.998(4) Å).

When N(tpy) is trans to a weaker trans directing group, this distance is reported to be

t + 22 significantly shorter such as in Pt( Bu3tpy)Cl (1.935(6) Å). The presence of substituents on either the NCN- or terpyridyl ligands does not have a significant influence

+ on the bond distances and angles of the complexes. For Pt(NO2-pip2NCN)(tpy) , the NO2

substituent at the para position of the pincer ligand is rotated slightly out of the plane of

the phenyl group (A, -10.75°, -11.51°; B, 3.93°, -3.34°) and bond lengths and angles are

similar to the previously reported values.27

The dihedral angles formed by the coordination plane, defined by Pt and the four

bonded atoms, and the phenyl group are in the 62-89° range. Similar angles are observed

for related platinum complexes such as Pt(Ph)(phtpy)+ (51.40(6)°) and

+ 2 Pt(pip2NCN)(phtpy) (85.5(1)°/ 89.1(1)°). One noticeable trend in our data and these earlier results is that the dihedral angle is smaller when there are no piperidyl groups or when the complex is protonated. It would appear that interactions of the meta piperidyl substituents of the NCN- ligand with the remainder of the complex favor a larger dihedral

angle. A contributing factor is that for each complex there is at least one close intra-

molecular contact in the 3.2-3.7 Å range involving the N(piperidyl) and Pt atoms

t + + (Pt(pip2NCN)( Bu3tpy) , N4, 3.474 Å; N5, 3.694 Å; Pt(pip2NCN)(toltpy) , N4, 4.570(3)

+ Å; N5, 3.188(5) Å; Pt(NO2-pip2NCN)(tpy) , A, N10, 3.624(7) Å; N11, 4.691(6)Å; B,

t + N4, 3.437(8) Å; N5, 3.515(8) Å). For Pt(pip2NCN)( Bu3tpy) and molecule B of Pt(NO2-

+ pip2NCN)(tpy) , it appears that the lone electron pairs of both piperidyl N atoms are

155 directed approximately toward the platinum metal center, consistent with a Lewis acid-

+ + base interaction. However, for Pt(pip2NCN)(toltpy) and A of Pt(NO2-pip2NCN)(tpy) , one of the piperidyl groups is rotated away from the metal center, and there is essentially

no interaction with the metal center. The observed short Pt...N(piperidyl) distances are comparable to those in crystals of [Pt(pip2NCN)(p-phenyl-tpy)](BF4)(A: 3.512(5); B:

3.195(5) Å), and they reflect the tendency of the acidic metal center to accommodate

interactions with Lewis bases at the axial sites of the complex. The accumulated

Pt...N(piperidyl) distances do not follow an obvious pattern based on the electronic

properties of the substituents on the Z-pip2NCN- and terpyridyl ligands, as might be

expected if packing forces also influence the resulting distances .

The structure of [Pt(tpy)(OCOCH)3](PF6) affords the opportunity to assess the

structural changes within the platinum(II) terpyridyl unit that accompany the outer-sphere

+ two-electron transfer reactions of complexes such as Pt(pip2NCN)(tpy) . Structures of

platinum(IV) carboxylate complexes with bidentate amine ligands have been reported

previously.28 For example, Canty and coworkers determined the structure of Pt(2,6-

29 dimethylaminomethyl)-phenyl)(OCOPh)3. In addition, several crystal structures of metal complexes with bidentate terpyridine have been reported. 30-35 However, to our

knowledge, [Pt(tpy)(OCOCH)3](PF6) is the first example of a Pt(IV) complex with a

tridentate terpyridine ligand. The complex adopts a distorted octahedral geometry. The

tpy ligand adopts a conventional meridional coordination geometry with the three

remaining meridionally disposed coordination sites occupied by monodentate acetate

ligands. The Pt-O bond distances (2.01-2.03 Å) are in good agreement with those of

Pt(II) and Pt(IV) complexes with carboxylate ligands (R=alkyl, phenyl).28,29,36 The bond

lengths and angles of the Pt(tpy) unit are remarkably similar to those of other platinum(II)

156 terpyridyl complexes.18,37-41 The latter observation suggests that changes in the tpy ligand geometry during the outer-sphere two-electron transfer reactions of [Pt(Z-

3+/+ pip2NCN)(tpy)] complexes are not major contributors to the Marcus reorganization energy.42 Interestingly, the carbonyl groups of both axial acetate ligands are directed toward the tpy ligand, resulting in short intramolecular contacts (O4...C6, 2.802 Å;

O4...N1, 2.837 Å; O6...C6, 2.712 Å; O6...N1, 2.877 Å) Though short O...N distances between carbonyl and amine groups in Pt(IV) complexes have been attributed to intramolecular hydrogen bonding,28,43 in the present case it is evident that neither group the carbonyl nor tpy group is acting as a H-bond donor. The carbonyl group of the equatorial acetate is directed toward the metal-bonded oxygen of one of the axial acetate groups resulting in an O2...O3 distance (2.715 Å) that is shorter than the sum of the van der Waals radii of the atoms (3.04 Å).

157 Table 5.1 Crystallographic Data t t t [Pt(Ph)( Bu3tpy)] [Pt(pip2NCN)( Bu3tpy)] Pt(pip2NCNH2)( Bu3tpy)] Pt(pip2NCN)(toltpy)] [Pt(NO2-pip2NCN)(tpy)] [Pt(tpy)(CH3COO)3] (BF4) (PF6)3 (BF4) (BF4).1/8C4H10O (PF6) (BF4).1/2CH2Cl2

[C33H40N3Pt]BF4 [C40H44N5Pt]BF4 [C33H37N6O2Pt]BF4 [C21H20N3O6Pt]PF6 [C H N Pt]BF [C H N Pt](PF ) Formula .1/2CH2Cl2 45 62 5 4 45 64 5 6 3 .1/8C4H10O Fw (g/mol) 803.04 954.90 1305.01 876.70 841.61 750.46 Crystal system Monoclinic Triclinic Monoclinic Monoclinic Triclinic Triclinic

Space group P21/c P-1 P21/n C2/c P-1 P-1 A (Å) 13.4477(2) 11.2202(3) 12.8887(12) 22.7583(4) 13.6438(14) 8.8302(9) B (Å) 13.4350(3) 11.6000(3) 19.8107(17) 18.8972(3) 14.1501(14) 11.9064(12) C (Å) 18.7424(3) 17.5631(4) 20.5427(18) 18.3860(3) 19.1665(19) 12.4991(13) °) 90 91.732(1) 90 90 77.129(2) 77.092(2) °) 103.542(1) 92.489(1) 93.892(2) 107.427(1) 82.697(2) 82.650(2) °) 90 109.146(1) 90 90 76.858(2) 69.892(2) V (Å3), Z 3292.05(10), 4 2155.04(9), 2 5233.2(8), 4 7544.3(2), 8 3501.4(6), 4 1200.9(2), 1 T (K) 150(2) 150(2) 193(2) 150(2) 183(2) 150(2) -3 Dcalc (g cm ) 1.620 1.472 1.656 1.544 1.597 2.075 (mm-1) 9.141 6.532 3.565 7.415 5.039 7.436 Refls. Coll. 26634 17787 28844 30775 39100 15702 Ind. Refls. 5872 7343 10733 6762 11843 4884

Rint 0.0279 0.0332 0.0423 0.0271 0.0379 0.0389 Data/Parameters 5872/379 7343/486 10733/700 6762/460 11843/864 4884/340 GOF on F2 1.066 1.040 1.044 1.214 1.053 1.082

R1/wR2 [I>2 (I)] 0.0237/0.0615 0.0429/0.1088 0.0354/0.0950 0.0333/0.0821 0.0476/0.1366 0.0324/0.0872

R1/wR2 (all data) 0.0252/0.0625 0.0524/0.1147 0.0403/0.0986 0.0352/0.0829 0.0533/0.1429 0.0329/0.0876

158 Table 5.2 Selected bond lengths (Å) and angles (°).

t t t [Pt(Ph)( Bu3tpy)] [Pt(pip2NCN)( Bu3tpy)] Pt(pip2NCNH2)( Bu3tpy)] Pt(pip2NCN)(toltpy)] [Pt(NO2-pip2NCN)(tpy)] [Pt(tpy)(CH3COO)3] (BF4).1/2CH2Cl2 (BF4) (PF6)3 (BF4) (BF4). 1/8C4H10O (PF6)

Pt(1)-C(17) 2.039(3) 2.036(6) 2.037(3) 2.045(4) 2.012(6), 2.009(7) Pt(1)-N(1) 1.994(2) 1.990(5) 1.983(3) 1.980(3) 1.986(5), 1.990(6) 1.961(4) Pt(1)-N(2) 2.039(2) 2.023(5) 2.021(3) 2.030(3) 2.018(5), 2.018(5) 2.012(4) Pt(1)-N(3) 2.035(2) 2.038(5) 2.020(3) 2.039(3) 2.014(5), 2.025(5) 2.027(4) C(17)-Pt-N(1) 178.98(11) 176.1(2) 179.06(13) 174.58(15) 175.9(2), 176.3(2) C(17)-Pt-N(2) 100.28(10) 100.3(2) 100.20(13) 100.98(14) 98.5(2), 97.7 (2) C(17)-Pt-N(3) 100.01(10) 99.5(2) 99.45(13) 99.37(14) 101.1(2), 102.(2) N(1)-Pt-N(2) 79.75(9) 80.4(2) 80.35(12) 79.81(13) 80.17(19), 80.3(2) 81.04(13) N(1)-Pt-N(3) 79.93(9) 79.8(2) 79.97(12) 79.87(13) 80.5(2), 79.9(2) 80.98(13) N(2)-Pt-N(3) 159.61(9) 160.2(2) 160.22(12) 159.64(13) 160.4(2), 160.2(2) 162.02(13)

159 t Figure 5.1 ORTEP diagram of the cation of [Pt((ph)( Bu3tpy)](BF4).1/2CH2Cl2 with

50% probability ellipsoids. H atoms are omitted for clarity.

160 t Figure 5.2 ORTEP diagram of the cation of [Pt(pip2NCN)( Bu3tpy)](BF4) with 50%

probability ellipsoids. H atoms are omitted for clarity.

161 t Figure 5.3 ORTEP diagram of the cation of [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 with 50%

probability ellipsoids. H atoms are omitted for clarity.

162 Figure 5.4 ORTEP diagram of the cation of [Pt(pip2NCN)(toltpy)](BF4) with 50%

probability ellipsoids. H atoms are omitted for clarity.

163 Figure 5.5 ORTEP diagram for molecule A of the cation of

[Pt(NO2pip2NCN)(tpy)](BF4).1/8C4H10O with 50% probability

ellipsoids. H atoms are omitted for clarity.

164 Figure 5.6 ORTEP diagram of the cation of [Pt(tpy)(CH3CO2)3](PF6) with 50%

probability ellipsoids. H atoms are omitted for clarity.

165 1H NMR Spectroscopy. The 1H NMR spectra of the pincer ligands, their platinum halide complexes and each the terpyridyl complexes (excepting

t [Pt(pip2NCNH2)( Bu3tpy)](PF6)3) were recorded in CDCl3. For reasons of solubility, the

1 t H NMR spectrum of [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 was recorded in CD3CN (Figure

5.12). Drawings showing the proton labeling scheme (A-L) also are shown with each spectrum in Figures 5.7-5.16. Resonances were assigned by comparison with those of

related complexes, analysis of splitting patterns, COSY spectroscopy in the case of

+ 195 Pt(NO2-pip2NCN)(tpy) and the presence of Pt satellites associated with resonances C

and G.

For the Z-pip2NCNBr ligand precursors and the Pt(Z-pip2NCN)Br complexes, the

1 H NMR spectra exhibit patterns consistent with effective C2v symmetry and are

qualitatively similar to those of pip2NCNBr, pip2NNN and their

4,44-46 palladium(II)/platinum(II) analogs. As expected, for the Z-pip2NCNBr (Z=NO2, H,

MeO) series, the meta phenyl proton resonances shift upfield with increasing electron

donation by the para-substituent. The resonances for the Pt(Z-pip2NCN)Br (Z=NO2,

7.76; Z=H, 6.80; Z=MeO, 6.47 ppm) analogs follow a similar trend, albeit each is shifted

upfield by 0.5-0.6 ppm from those of the ligand precursors. Comparable chemical shifts

- are observed for platinum halide complexes with the Z-Me4NCN ligand (Z=NO2, 7.74

ppm; Z=MeO, 6.45 ppm).12,47 Coordination to the metal causes the methoxy methyl

proton resonance to shift upfield by 0.06 ppm, whereas the benzylic (C) and -piperidyl

resonances (D) of each complex are shifted downfield by ~0.7 ppm and ~0.8-1.7 ppm,

respectively. These shifts suggest that upon coordination, the electron density decreases

near the amine coordination site. The para-phenyl substituent has little influence on the

166 benzylic proton resonances (C) near 4.3 ppm for the ligand precursors (±0.01 ppm) and

3.55 ppm for the platinum complexes( ±0.08 ppm). For the complexes, the benzylic resonances appear with distinct Pt satellites (JPt-H: Z=NO2, 45; Z=H, 47; Z=MeO, 43 Hz),

confirming coordination of the piperidyl groups. Unlike the ligand precursors, the

-piperidyl protons are diastereotopic, giving rise to two resonances (D' and D"). As

expected for strong coupling between the axial - and -protons,48 the axial proton resonance (D') has the appearance of a triplet, whereas the equatorial proton resonance

(D") has the appearance of a doublet. Interestingly, the MeO and NO2 substituents do not

shift the D" resonance, but cause D' to shift downfield by 0.02 ppm and 0.08 ppm,

respectively. As in the case of the ligands, the chemical shifts (1.4-1.8 ppm) of the -

and - piperidyl proton resonances (E and F) are relatively insensitive the para-

substituent.

The coordination geometry illustrated in Scheme 5.2, in which the tpy ligand is

- tridentate and the Z-pip2NCN ligand is monodentate, is confirmed by (1) the presence of

195Pt satellites associated with the -tpy proton (G) resonance, (2) the absence of 195Pt satellites on the benzylic proton (C) resonance, and (3) the appearance of a single -

- piperidyl proton (D) resonance. The substituents on pip2NCN (Z) and tpy (Rn) ligands exert the greatest influence on nearest neighbor protons. For instance, substituting MeO for NO2 causes the meta-phenyl proton resonance to shift upfield by 1.1 ppm. Similarly,

substitution of t-butyl groups on tpy, shifts the nearby terpyridyl J and K proton

resonances upfield by 0.26 ppm. Changes in the Z and Rn substituents have little

influence on the chemical shifts of the benzylic and piperidyl resonances. For example,

resonances C (3.73 ± 0.02 ppm) and D (2.33 ± 0.03 ppm) each fall within a very narrow

167 range for the five two-electron reagents. In addition, substitution of t-butyl or tolyl groups

on the terpyridyl ligand has little impact on the chemical shifts of the overlapping para-

and meta-phenyl resonances, A and B (7.15± 0.02 ppm). On the other hand, changes in

the para-phenyl substituents causes changes in the the terpyridyl proton chemical shifts,

suggesting significant perturbation of the metal electronic properties. Interestingly, the

- pip2NCN NO2 and terpyridyl t-butyl substituents shift the G proton resonances upfield

- by 0.17 ppm, whereas the pip2NCN MeO shifts G downfield by 0.15 ppm and terpyridyl tolyl substituent does not shift it at all. The influence of the t-butyl group is consistent

with its comparative electron-releasing properties. However, the origin of the effect of

the NO2 and MeO substituents is more surprising since their electron

withdrawing/donating properties are expected to have an opposite influence (decrease

and increase, respectively) on the electron density of the platinum center and terpyridyl

ligand through inductive effects. The MeO and NO2 substituents shift the remaining terpyridyl resonances upfield and downfield, respectively, by 0;);4 )

Mediation of electron donor properties through a metal has been investigated by different researchers. For a series of Ru(X-tpy)(Y-tpy)2+ complexes (X,Y=substituents on the para position of the central pyridine, i.e., NO2, NH2, Cl) in acetone, Constable et al.

reported that the chemical shifts of each tpy ligand are independent of the substituent on

the other ligand. 49 However, Fallahpour et al. observed that the substituent on one of the

tpy ligands influences certain resonances on the other tpy for a series of similar Ru(II)

and Fe(II) complexes in acetonitrile. 50 Interestingly, they report that the resonance that is

most sensitive to substituent effects changes depending on the metal. For instance,

2+ replacing one of the NH2-tpy ligands with NO2-tpy in Ru(NH2-tpy)2 -pyridyl

168 resonance (G) of the other NH2-tpy ligand downfield by 0.2 ppm, but the effect of the

2+ same replacement in Fe(NH2-tpy)2 is a downfield shift of only 0.02 ppm. From these observations, we suggest that the mediation of electron donor properties of a substituent on a ligand to another ligand is influenced by the metal, solvent, type of ligand and coordination geometry.

As in the case of the deprotonated complex, the phenyl proton resonances of

t 3+ Pt(pip2NCNH2)( Bu3tpy) overlap and appear as a multiplet at 7.53 ppm. The 0.43 ppm

t + downfield shift from the corresponding resonances for [Pt(pip2NCN)( Bu3tpy)] (7.10 ppm) reflects the electron-withdrawing of the piperidinium groups. The resonance for the

- 195 pip2NCN benzylic protons (C) appears as a doublet (4.1 Hz) without Pt satellites. The

-piperidyl protons (D) are diastereotopic (2.83, 3.33 ppm) and appear as multiplets shifted ~0.4 ppm upfield from those of Pt(pip2NCN)Br. Resonances arising from protons

E and F overlap with the t-butyl resonances further upfield. Interestingly, all terpyridine resonances except G are shifted downfield by 0.1-0.2 ppm from the corresponding

t + resonances of [Pt(pip2NCN)( Bu3tpy)] . The t-butyl resonances, I and L, appear as

singlets upfield (1.63, 1.46 ppm, respectively). For reasons that are not fully understood,

t 3+ the resonances for Pt(pip2NCNH2)( Bu3tpy) in the spectrum in Figure 5.12 are

3+ significantly broader than those in previously reported spectra of Pt(pip2NCNH2)(tpy)

3+ 2 and Pt(pip2NCNH2)(phtpy) .

169 F E D C B N A Z Br

B C D E, F (c) B A C D E, F (b) * B MeO C D E, F (a) * * 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm)

1 Figure 5.7 H NMR spectra of Z-pip2NCNBr ligand precursors in CDCl3. (a) MeO-pip2NCNBr, (b) pip2NCNBr, (c) NO2-

pip2NCNBr. * denotes TMS (0.0 ppm) and CHCl3 (7.26 ppm), (b) has some impurities around 3.0 ppm and 1.8 ppm.

170 F * E D C B N * A Z Pt Br *

B N C DE,FD

7 6 5 4 3 2 1 0 PP 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm) 1 Figure 5.8 H NMR spectra of Pt(Z-pip2NCN)Br complexes in CDCl3 (a) Pt(pip2NCN)Br, (b) Pt(MeO-pip2NCN)Br, (c)

Pt(NO2-pip2NCN)Br. * denotes CHCl3 (7.26 ppm), water (1.55 ppm) and TMS (0.0 ppm). 171 I

+ L* B C N * A Pt N L N K G J H I * * K J G H B C A

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm) 1 t Figure 5.9 H NMR spectrum of [Pt(Ph)( Bu3tpy)](BF4) in CDCl3. * denotes CHCl3 (7.26 ppm), acetone (2.17 ppm), water (1.55 ppm) and TMS (0.0 ppm).

172 + C B N A Pt N * * * I N K L J G K H I B J G H L

8. 8. 8. 7. 7. 7. 7. 7. PP 8.0 7.0 C A

8 7 6 5 4 3 2 1 PP 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm)

1 t Figure 5.10 H NMR spectrum of [Pt(mes)( Bu3tpy)](BF4) in CDCl3. * denotes CHCl3 (7.26 ppm), water (1.55 ppm) and TMS (0.0 ppm).

173 F E D

N + I * C B N A K Pt N N K L G J J A, B H L G H N I

8. 8. 8. 8. 7. 7. 7. 7. PP 8.0 7.0

* C D E, F *

8 7 6 5 4 3 2 1 0 PP 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm)

1 t Figure 5.11 H NMR spectrum of [Pt(pip2NCN)( Bu3tpy)](BF4) in CDCl3. * denotes CHCl3 (7.26 ppm) and TMS (0.0 ppm).

174 3+ E D H F N I C B N K A Pt N J A,B N K L G H G J L H N(pip2NCN)-H N I H

8. 8. 8. 8. 7. 7. 7. 7. 7. PP 8.0 7.0 * CD E,F

88 7 66 5 44 3 22 PP 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 (ppm)

1 t Figure 5.12 H NMR spectrum of [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 in CD3CN. * denotes CH3CN (1.97 ppm).

175 F E D

N + C B N * A Pt N N K L M N G J H I N *

K C N E,F A,B D G *

8 7 6 5 4 3 2 1 PP 8.07.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm) 1 Figure 5.13 H NMR spectrum of [Pt(pip2NCN)(toltpy)](BF4) in CDCl3. * denotes CHCl3 (7.26 ppm), water (1.66 ppm) and TMS

(0.0 ppm). 176 F E D

N + C B N O2N Pt N L * N K * G J H I N C E,F D B K,L J I G H

8 6 4 2 0 PP 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 (ppm)

1 Figure 5.14 H NMR spectrum of [Pt(NO2-pip2NCN)(tpy)](BF4) in CDCl3. * denotes CHCl3 (7.26 ppm) and TMS (0.0 ppm).

177 Figure 5.15 Aromatic region of the COSY spectrum of [Pt(NO2-pip2NCN)(tpy)](BF4) in CDCl3.

178 F E D

N + C B A N MeO Pt N L N K G J A * H I N *

E,F K,L B C J I D G H • • •• • *

8 6 4 2 0 PP 9.0 8.08 7.0 6.06 5.0 4.04 3.0 2.02 1.0 0.00 PP (ppm)

1 Figure 5.16 H NMR spectrum of [Pt(MeO-pip2NCN)(tpy)]OTf in CDCl3. * denotes CHCl3 (7.26 ppm), acetone (2.17 ppm) and

TMS (0.0 ppm). denotes free tpy impurities (8.72-8.62 ppm, 8.46 ppm, 7.97 ppm, 7.88 ppm and 7.35 ppm).

179 Electronic Spectroscopy. To assess the influence of the dangling piperidyl

groups on the electronic structures of these complexes, UV-visible absorption spectra

t were recorded of dichloromethane solutions of [Pt(Ph)( Bu3tpy)](BF4),

t [Pt(pip2NCN)( Bu3tpy)](BF4), [Pt(pip2NCN)(toltpy)](BF4) and [Pt(NO2-pip2NCN)(tpy)]

t t (BF4) (Figure 5.17, Table 5.3). [Pt(Ph)( Bu3tpy)](BF4) and [Pt(pip2NCN)( Bu3tpy)](BF4)

are yellow solids whereas [Pt(pip2NCN)(toltpy)](BF4) and [Pt(NO2-pip2NCN)(tpy)](BF4)

t are orange solids. [Pt(Ph)( Bu3tpy)](BF4) dissolves to give yellow solutions, whereas the

two-electron reagents dissolve to give red solutions. The UV-visible absorption spectra

of the terpyridyl complexes are qualitatively similar for wavelengths <500 nm, and this

region of the spectrum is largely consistent with previous studies.18,23,36,38,51-53 For

example, each complex exhibits intense absorption bands between 200 nm and 300 nm

-1 -1 t + (>30000 M cm ). Platinum(II) terpyridyl complexes, such as Pt( Bu3tpy)Cl (215 nm,

46000 M-1cm-1; 256 nm, 45100 M-1cm-1; 282 nm, 33700 M-1cm-1; in acetonitrile)22 and Pt

-1 -1 4 pincer complexes such as Pt(pip2NCN)Cl (275 nm, 9050 M cm ; in dichloromethane)

give rise to intense bands in this region. The moderately intense bands observed between

300-365 nm and the low intensity band observed near 425 nm are similar to those

- observed for other Pt(II) pip2NCN or phenyl complexes with terpyridyl ligands. These

features have been assig 44 ?

allowed 5d(Pt)4 (tpy) MLCT transitions, respectively.2 The red shift in the transitions

near 425 nm from where similar absorption features occur in Pt(II) terpyridine complexes

t -1 -1 -1 -1 22 such as Pt( Bu3tpy)Cl (373 nm, 3990 M cm ; 386 nm, 3530 M cm in acetonitrile) and

Pt(tpy)Cl (372 nm, 1300 M-1cm-1; 398 nm, 1800 M-1cm-1 in acetonitrile)38 is consistent with having the electron donating aryl group. For all complexes, a weak shoulder is

180 observed near 500 nm in the tail of the 1MLCT transition. As expected from a corresponding 3MLCT transition, this feature is shifted 2000 to 3500 cm-1 to the red of the 1MLCT maximum.22,54,55 The apparent singlet-triplet MLCT splitting is in agreement

t -1 22 with that observed for Pt( Bu3tpy)Cl (4800 cm ) and Pt(6-phenyl-2,2'-bipyridine)(4-

aminopyridine)+ (2000 cm-1).54

t + + In the spectra of Pt(pip2NCN)( Bu3tpy) , Pt(pip2NCN)(toltpy) and Pt(NO2-

+ pip2NCN)(tpy) , there is an additional long wavelength absorption feature appearing near

550sh nm (~300 M-1cm-1). This weak band is absent from the spectra of model

t + complexes such as Pt(Ph)( Bu3tpy) . In fact, this band has been observed for only the

+ [Pt(pip2NCN)(R-tpy)] complexes and proposed to be the result of a weak interaction between the N(piperidyl) groups and the platinum center. This conclusion is supported by the fact that this feature is not observed if the piperidyl groups are placed further away

+ 2 from the platinum center, as in [Pt(3,5-pip2NCN)(tpy)] , or when the piperidyl groups

t 3+ 3 are protected by protonation, as in Pt(pip2NCNH2)( Bu3tpy) . The sp hybridized

N(piperidyl) lone-pair orbital is expected to combine with the 6pz(Pt) orbital to give rise to a filled -N) orbital and an empty -N)* level. These axial interactions also are

2 expected to destabilize the metal d orbitals, especially the dz (Pt) level, resulting in a red-

shift in MLCT transitions, as previously noted for other platinum(II) complexes with

dangling nucleophiles.56 The presence of a new long-wavelength absorption band is

> -N)4 (tpy) charge-transfer transition. The influence of piperidyl

and terpyridyl substituents on the energy of this transition is consistent with this

- assignment. Substitution of NO2 for H on the pip2NCN ligand and substitution of t-butyl

groups for H on the terpydiyl ligand cause the band to blue shift 7600 cm-1 and 5100

181 cm-1, respectively. By contrast, substitution of a tolyl group for H on the terpyridyl ligand causes the band to red shift by 3900 cm-1. The influence of the Z substituents is

? -N) level, whereas the influence of the Rn " 4 > ) These

results also give deeper insight into the variation in colors of salts of these two-electron

reagents. For the 5 reported crystal structures of two-electron reagents, only crystals of

t [Pt(pip2NCN)( Bu3tpy)](BF4) are yellow, whereas the others are orange or red in color.

The variation in color is related to differences in Pt...N(piperidyl) distances. In crystals

t of [Pt(pip2NCN)( Bu3tpy)](BF4), the shortest Pt...N(piperidyl) distance (3.474 Å) is

longer than that in any of the other crystallized complexes (0H)33 O) P >r, the t-

" " 4 > ?

destabilizes the charge-transfer transition.

t In keeping with this description, we note that [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 is a

yellow solid that dissolves to give yellow solutions (Table 5.3; Figure 5.18). The

spectrum is identical to that obtained when two equivalents of TFA are added to an

t acetonitrile solution of [Pt(pip2NCN)( Bu3tpy)](BF4). During the addition of acid, the

intensities of the low-energy absorption bands (>400 nm) decrease, whereas a band at

382 nm emerges. Additionally, small shifts observed at wavelengths <350 nm, and the

t overall spectrum becomes qualitatively similar to that of [Pt(ph)( Bu3tpy)](BF4). This

observation is consistent with protonation of the N(piperidyl) groups, which prevents

interaction with the metal center. The intensity increase near 380 nm is likely a

consequence of the long-wavelength MLCT band shifting slightly to shorter wavelengths

due to the reduced donor properties of the phenyl group.

182 4 ) -1 cm

-1 3 M -4 10 (

1 X 25

300 400 500 600 700 Wavelength (nm)

t Figure 5.17 UV-visible absorption spectra of [Pt(ph)( Bu3tpy)](BF4) (2),

t [Pt(pip2NCN)( Bu3tpy)](BF4) (2), [Pt(pip2NCN)(toltpy)](BF4) (2)

and [Pt(NO2-pip2NCN)(tpy)](BF4) (2) in dichloromethane.

183 t + + Table 5.3 UV-visible absorption data for [Pt(Ph)( Bu3tpy)] and Pt(Z-pip2NCN)(R-tpy)]

t 3+ complexes in dichloromethane and Pt(pip2NCNH2)( Bu3tpy)] in acetonitrile.

Absorption Bands Compound -1 -1 (max, nm ( , cm M )

251(29600), 274(32700), 285(27500), 307(13000), t + [Pt(Ph)( Bu3tpy)] 321(16600), 342(8900), 360 (2400), 392(2500), 425(2900), 471sh(600)

261(45800), 273(45900), 283(44000), 319(22800), t + [Pt(pip2NCN)( Bu3tpy)] 365(4200), 390(3200), 420(2700), 458 (1800), 490(800), 535(300)

244(29600), 274(27300), 315(20800), 339 (18200), + [Pt(NO2-pip2NCN)(tpy)] 361(9700 ), 393(3700), 431 (1500), 481(400 ), 528(300 )

241(29800), 260(32500, 270(35000), 285(34800), + [Pt(pip2NCN)(toltpy)] 317(20700), 339(15400), 362(7000), 401(3200), 426(2500), 458(1100), 500(550), 562(280)

218(64800), 252(41300), 271(38600), 283(33700), t 3+ [Pt(pip2NCNH2)( Bu3tpy)] 311(19200), 329(17100), 346 (8700), 362(2700), 382(2900), 403(1500)

184 Figure 5.18 UV->" " 1G; )P

t acetonitrile solution of [Pt(pip2NCN)( Bu3tpy)](BF4) (2) with 2

equivalents of TFA (trifloroacetic acid, 0.3 M in acetonitrile) in 0.2

equivalents increments. Relative emission spectra of

t t [Pt(pip2NCN)( Bu3tpy)](BF4) (2) and [Pt(pip2NCNH2)( Bu3tpy)]

(PF6)3 (2) in acetonitrile.

185 The room-temperature fluid-solution emission spectra of

t t [Pt(pip2NCN)( Bu3tpy)](BF4) and [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 are shown in Figure

t + t 3+ 5.23. [Pt(pip2NCN)( Bu3tpy)] is non-emissive, whereas [Pt(pip2NCNH2)( Bu3tpy)] is weakly emissive or non-emissive. The emission from the protonated adduct is characteristically structured (max= 470, 502, 535sh, 585sh nm). The significantly diminished intensity of emission from fluid solution samples of

t [Pt(pip2NCN)( Bu3tpy)](BF4) is consistent with the notion that the lowest excited states of platinum(II) terpyridyl complexes are susceptible to quenching by interactions of the metal center with nucleophiles.57 The deprotonated piperidyl groups are expected to be

+ strong nucleophiles, and previously studied [Pt(pip2NCN)(tpy)] and

+ [Pt(pip2NCN)(phtpy)] are also nonemissive. The structured emission profile of

t 3+ [Pt(pip2NCNH2)( Bu3tpy)] is similar to those observed for previously reported platinum(II) terpyridyl complexes.22,38,51-53 The emissions are assigned to a predominantly spin-forbidden 34-4* terpyridyl ligand-centered lowest excited state. The

vibronic spacings (1200-1500 cm-1) 4-4 -centered

lowest excited state.18 The origin of the emission observed for

t -1 [Pt(pip2NCNH2)( Bu3tpy)](PF6)3 is only shifted by approximately 2000 cm from the

phosphorescence observed for free terpyridine.58 In addition, the bandshapes and Franck-

59 Condon factors, as indicated by the Huang-Rhys ratios (I1,0 /I0,0 ~0.85), are similar to

3 * that of the free ligand (I1,0 /I0,0 ~0.85). Both facts are consistent with the 4-4 assignment.

t Cyclic Voltammetry. Cyclic voltammograms (CVs) of [Pt(ph)( Bu3tpy)](BF4),

t [Pt(pip2NCN)( Bu3tpy)](BF4), [Pt(pip2NCN)(toltpy)](BF4), [Pt(NO2-pip2NCN)(tpy)]

(BF4) , and Pt(MeO-pip2NCN)(tpy)](BF4) in 0.1M TBAPF6 acetonitrile are shown in

186 Figures 5.19-5.20. The CV of [Pt(pip2NCN)(tpy)](BF4) in 0.1 M TBAPF6 acetonitrile is

shown in Figure 5.20 for comparison. Cyclic voltammetry data are collected in Table 5.4.

The CVs of each complex in acetonitrile solution (0.1 M TBAPF6, 0.1 V/s) exhibit two reversible one-electron reduction waves near -1.0 V(E°') and -1.5 V (E°'), with peak-to-- Ep) of 62±8 and 61±8 mV, respectively. Assignment of

the observed redox processes can be inferred from comparison to the electrochemical

behavior of a series of related compounds. Under identical conditions, neither free tpy,

pip2NCNBr nor Pt(pip2NCN)Cl is reduced at potentials larger than -2.10 V, suggesting

that the one-electron reduction processes are associated with the Pt(tpy) unit. Pt(tpy)Cl+

has been reported to undergo reversible one-electron reductions in DMF (0.1 M TBAPF6)

at E°'=-0.74 and E°'=-1.30 V vs. Ag/AgCl whereas Zn(tpy)Cl2 undergoes reversible one- electron reduction at E°'=-1.36 V.60 The cathodic shift of the ligand-centered couples in

platinum(II) complexes is attributed to stabilization of the reduced tpy ligand as a result

* 60 of coupling between the empty 6pz 4 (tpy) orbitals. As observed for [Pt(Z-

+ + pip2NCN)(R-tpy)] complexes, the fact that [Pt(ph)(tpy)] and similar complexes without

+ the piperidyl groups such as [Pt(2,6-dimethylphenyl)(tpy)] (E°'=-;)KB R Ep =60 mV,

2 ipc/ipa =0.91; E°'=-1)3K R Ep=64 mV, ipc/ipa =0.90) in acetonitrile solutions also undergo two reversible one-electron reductions support this assignment.

The Z and Rn substituents influence the apparent potentials of both cathodic processes. The NO2 substituent that anodically shifts the reductions, waves by 0.1 V and

0.23V, respectively. The tolyl terpyridyl substituent shifts only the second reduction

- anodically by 0.02 V. On the other hand, the pip2NCN MeO and t-butyl terpyridyl

substituents act as electron donating groups and shift both reductions cathodically by 0.02

187 + V and 0.13V from those of Pt(pip2NCN)(tpy) , respectively. Similar shifts (-0.11 V and

-0.14 V, respectively) have been observed for the introduction of t-butyl substituent in

red t Pt(R-tpy)–C.–phenyl–C.–Re(N^N)(CO)3 (tpy, E1/2 : -0.90 V, -1.37 V; Bu3tpy,

red E1/2 : -1.01V, -1.51 V , in 0.1M TBAPF6 acetonitrile solution, vs. SCE, glassy carbon

61 + working electrode). However, for Pt(tpy)(C. -C6H5-Z) complexes, introduction of either NO2 or MeO substituents shift the first reduction process to more negative

potentials (by -0.21V and -0.08V, respectively) while not influencing the second

+ red reduction process (Pt(tpy)(C.-C6H5) , E1/2 = -0.97 V, -1.46V, in 0.1M TBAH acetonitrile, vs. SCE, glassy carbon working electrode).62 Apparently, the nature of the fourth ligand carrying the substituent influences the way the substituent effect on Pt(tpy) reductions.

t + Pt(ph)( Bu3tpy) is not oxidized at <1.2 V vs. Ag/AgCl. By contrast, each of the

- platinum(II) complexes with both a Z-pip2NCN and a terpyridyl ligand undergo a two-

electron oxidation process in the 0.4-0.6 V range (E°'), as previously noted for

+ 17 t + [Pt(pip2NCN)(tpy)] (E°'=0.40 V, Ep=74 mV, 0.25 V/s), [Pt(pip2NCN)( Bu3tpy)] and

+ [Pt(pip2NCN)(toltpy)] undergo a chemically reversible and nearly electrochemically

reversible two-electron oxidation processes at E°'=0.36 V (ipc / ipa 01);G Ep=68 mV,

0.1 V/s) and E°'=0.37 V (ipc / ipa 01)1 Ep=94 mV, 0.25 V/s), respectively. For

+ + [Pt(NO2-pip2NCN)(tpy)] and Pt(MeO-pip2NCN)(tpy)] , the oxidation process occurs at

E°'= 0.62 V (ipc / ipa 01)HH Ep=89 mV, 0.1 V/s) and E°'=0.37 V (ipc / ipa 0;)K4 Ep=102

mV, 0.1 V/s), respectively. No splitting of any of the waves was observed over the range of investigated scan rates. As assessed by the anodic-cathodic peak-to-peak separation

(Ep) at a given sweep rate, the electrochemical reversibility varies among the five two-

188 electron reagents (Ep, 50-100 mV, 0.1 V/s). The reversibility does not vary with

electron-donor properties of the substituents in an obvious manner; however it appears

- that the presence of the Z substituents on the pip2NCN ligand tends to slow the electron-

transfer kinetics. The reversibility of the Ru(II)/Ru(III) process for a library of ruthenium

terpyridyl complexes also varies somewhat irratically.49,63 Nazeeruddin et al. have

reported diminished reversibility for ruthenium bipyridyl complexes with multiple strong

64 electron donor substituents such as NMe2. In the present case, it appears that Ep tracks

with the severity of electrode passivation problems as indicated by the shift in Epa and loss of current with consecutive sweeps. For all five complexes, the ratios of the peak anodic current of the oxidation process to the peak cathodic current of the first reduction wave are between 2.0 and 2.3. Although the ipa/ipc are less than the predicted value for a

Nernstian two-electron process (2.8 (=23/2)), the ratios clearly exceed the expected value

of 1.0 for a one-electron process. The Ep for a diffusion controlled two-electron

Nernstian process is expected to be 29.5 mV.65 In fact, at 0.1 V/s the couples are clearly

>" " > > Ep, and

therefore the values of ipa/ipc cannot be expected to approach the Nernstian limits. Since the oxidations clearly involve transfer of considerably more charge than observed for the reduction, the processes are attributed to a net two-electron oxidation of the complexes.

The apparent two-electron oxidation waves observed for [Pt(Z-pip2NCN)(R-

tpy)]+complexes are absent in cyclic voltammograms of related compounds. For example,

2+ neither Pt(tpy)(dmph)+, pip2NCNBr nor pip2NCNBrH2 is oxidized at potentials <1.2

17 4 V, and Pt(pip2NCN)Cl undergoes irreversible metal-centered oxidation near 0.8 V.

- Taken together, these data indicate that both the pip2NCN and terpyridyl ligands play

189 (a)

5 uA (b)

(c)

1.00.5 0.0 -0.5 -1.0-1.5 -2.0 Potential (V)

Figure 5.19 Cyclic voltammograms of a) [Pt(pip2NCN)(toltpy)](BF4) (2); b)

t t [Pt(pip2NCN)( Bu3tpy)](BF4) (2) and c) [Pt(ph)( Bu3tpy)](BF4) (2) in

acetonitrile (0.1 M TBAPF6, gold working electrode, 0.25 V/sec).

190 (a)

(b) 10 uA

(c)

0.80.4 0.0 -0.4 -0.8 -1.2 -1.4 Potential (V)

Figure 5.20 Cyclic voltammograms of (a) [Pt(NO2-pip2NCN)(tpy)](BF4)(2), (b)

[Pt(MeO-pip2NCN)(tpy)]OTf (2) and (c) [Pt(pip2NCN)(tpy)](BF4)(2)

in 0.1 M TBAPF6 CH3CN (gold working electrode, 0.1 V/sec).

191 t + + Table 5.4 Cyclic voltammetry data for [Pt(Ph)( Bu3tpy)] and Pt(Z-pip2NCN)(R-tpy)]

complexes in acetonitrile (0.1 M TBAPF6, gold working electrode, 0.1 V/s).

Compound Eox°' R Ep (mV)) Ered°' R Ep (mV))

t + [Pt(Ph)( Bu3tpy)] - -1.06(62) , -1.57(69)

t + [Pt(pip2NCN)( Bu3tpy)] 0.36(68) -1.11(54), -1.62(58)

+ [Pt(pip2NCN)(toltpy)] 0.37(94) -0.98(67), -1.48(67)

+ [Pt(NO2-pip2NCN)(tpy)] 0.62(89) -0.88(61), -1.33(53)

+ [Pt(MeO-pip2NCN)(tpy)] 0.37(102) -1.00(70), >-1.5

+ [Pt(pip2NCN)(tpy)] 0.40(54) -0.98(65), -1.50

192 + important roles in the unusual redox chemistry of Pt(Z-pip2NCN)(R-tpy) complexes.

The availability of the amine lone electron pairs is critical to facilitating reversible two-

electron oxidation and stabilizing the resulting Pt(IV) center. For example, protonation of

3+ 2 the piperidyl groups (e.g., Pt(pip2NCNH2)(tpy) ), results in irreversible oxidation near

0.4 V accompanied by electrode fouling.

In order to further characterize the electrochemical behavior of these systems,

t + CVs of Pt(pip2NCN)( Bu3tpy) were recorded for the first reduction process (-0.8 to -1.3

V) and the oxidation process (0.2 to 0.6 V) over a range of scan rates from 0.01 to 25 V/s.

Ep of the first reduction (E°'=-1.11 V) is essentially invariant (59±6 mV) for scan rates

ranging from 0.01 to 25 V/s. The cathodic peak current (ipc) exhibits an approximately

+ *1/2), as predicted by the Randles-

>6- + 6 M66-68

5 3/2 1/2 1/2 ip= 2.69x10 n AD .* (1)

where n=electron stoichiometry, A=electrode area, D=diffusion coefficient, and

.0 ) U ! Ep increases continuously from 43 to 103

mV as the scan rate is increased from 0.01 to 1.5 V/s (Figure 5.21). With decreasing scan

65,69 Ep approaches the two-electron Nernstian limit of 29.5 with ipc / ipa =1.08 at

0.01 V/s. The behavior of the other complexes is qualitatively simila Ep increases with

) @ Ep on scan rate is consistent with a large structural reorganization resulting in slow reaction kinetics.

In order to verify the electron stoichiometry of the oxidation process for

t 1/2 [Pt(pip2NCN)( Bu3tpy)](BF4), the anodic peak current (ipa * in

Figure5.21. Though the process clearly exhibits non-Nernstian behavior, the data are

193 10

5 )

A 0

(

-5 Current

-10

-15 0 1020304050 1/2 (mV/s)1/2

Figure 5.21 Dependence of anodic peak current (ipa) for 0.36 V oxidation process (J )

and cathodic peak current (ipc) for -1.11 V reduction process () on the square root of the

1/2 t scan rate ( ) for 1 mM [Pt(pip2NCN)( Bu3tpy)](BF4) in acetonitrile (0.1 M TBAPF6).

Scans recorded from 0.2 to 0.6 V and from -1.2 to -1.7 V. Lines represent linear fits of all

oxidation data and reversible reduction data.

194 remarkably linear over the entire range of scan rates (0.01 to 25 V/s) as predicted by

equation (1). The ratio of the slope of the best fit line to that obtained for the first

3/2 reduction process is used to derive an estimate of (nox / nred) . The resulting estimate of

t 1.8 is consistent with the notion that oxidation of [Pt(pip2NCN)( Bu3tpy)](BF4) involves

transfer of two electrons per Pt center.

- The substituents on the pip2NCN and tpy ligands significantly influence the apparent Pt(IV/II) redox couple, E°', the influence of the Rn substituents is comparatively small. For example, E°' is cathodically shifted by 0.04 and 0.03 volts in

t + + [Pt(pip2NCN)( Bu3tpy)] and [Pt(pip2NCN)(toltpy)] , respectively. A comparable

cathodic shift (0.05 V) has been observed for the irreversible Pt(II)/Pt(III) process when

t tpy is replaced by Bu3tpy in Pt(R-tpy)–C.–phenyl–C.–Re(N^N)(CO)3 complexes

t (N^N= 4,4'-bis-t-butylbipyridine; R=H (tpy), Epa= 1.41 V; R= t-butyl ( Bu3tpy), Epa= 1.36

61 V; in 0.1M TBAPF6 acetonitrile solution, vs. SCE, glassy carbon working electrode).

2+ Similarly, replacing tpy ligands with phenyl-tpy in Ru(tpy)2 causes a cathodic shift of

2+ 2+ 0.02 volts (Ru(tpy)2 , E°' = 0.92 V; Ru(ph-tpy)2 , E°'= 0.90 V; in acetonitrile, vs.

Fc/Fc+).63

The Z substituents have a more substantial influence on the apparent redox

+ potential. For Pt(NO2-pip2NCN)(tpy) , E°' (0.62 V) is anodically shifted by 0.22 V from

+ the two-electron oxidation process observed for Pt(pip2NCN)(tpy) (E°'=0.40 V), as expected for a more electron-poor metal center. The involvement of the metal-center is confirmed by comparison to the redox chemistry of ruthenium(III/II) polypyridyl complexes. Previously, Nazeeruddin et al. have shown that there exists an approximate linear correlation between the ruthenium(III/II) redox potentials of

195 2+ Ru(Z,Z'-bpy)x(bpy)3-x complexes and the Hammett parameters of the bipyridyl

substituents.64 Figure 5.22 shows the dependence of E°' on the effective Hammett

+ + p ? > p values for each of the substituents

+ ! ) @ p is rationalized on grounds that it is a more

suitable descriptor for a reaction that involves increasing positive charge on the metal

+ center. We have applied this analysis to the five Pt(Z-pip2NCN)(tpy) two-electron

+ ? p values are summed for the three substituents of the terpyridyl ligand

- and the single substituent of the pip2NCN ligand. The analysis reveals a similar

+ ! " > ? p (Figure 5.22).

For both Pt(II) and Ru(II) complexes, the observed E°' reveal, as expected, that the oxidation of the metal center becomes more difficult as the electron withdrawing

+ character of the substituents increases. The relative slopes of best-fit lines E X > p

(Ru, 0.13 V; Pt, 0.11 V) reflect the fact that replacement of a MeO group with NO2

causes a slightly greater anodic shift in E°' for the two-electron platinum reagents (0.25

' 2+ V) than for the Ru(4,4-R2bpy)(bpy)2 (0.18 V per MeO/NO2 substitution). Potentials for

2+ Ru(R-tpy)2 also would seem to suggest a more shallow dependence on EtO/NO2

substitution (0.19 V),50,63 however, for this limited data set, we regard the slopes of the

best-fit lines in Figure 5.22 as essentially identical within the scatter of the data. The

general agreement between these data sets confirms that the two-electron process is

metal-centered and suggests that, over this narrow range of potentials, the d6/d7-electron

and d7/d8-electron couples are comparably affected by changes in ligand substituents.

2+ ’ 2+ Interestingly, in the cases of Ru(R-tpy)2 and Ru(4,4 -R2bpy)(bpy)2 complexes, the

nitro and ethoxy substituents each strongly shift the redox couple from that of the

196 2+ 2+ 2+ unsubstituted complex (Ru(NO2-tpy)2 , 1.114 V; Ru(tpy)2 , 0.92V; Ru(OEt-tpy)2 ,

+ 50,63 ’ 2+ 2+ 0.74V, in acetonitrile vs. Fc/Fc ; Ru(4,4 -(NO2)2bpy)(bpy)2 , 1.48V; Ru(bpy)3 ,

' 2+ 64 1.26V; Ru(4,4-(MeO)2bpy)(bpy)2 , 1.05V, in acetonitrile vs. SCE). The potential shifts per substituent upon by replacing H with MeO or with NO2 are comparable (Ru(R-

2+ 50,63 ’ 2+ tpy)2 , R=NO2, 0.97 V; R=EtO, -0.09 V; Ru(4,4 -R2bpy)(bpy)2 , R=NO2, 0.098 V;

64,70 + R=MeO, -0.075 V). By contrast in the Pt(Z-pip2NCN)(tpy)] series, the nitro group has a much stronger influence (+0.22 V) on the apparent redox couple than the methoxy

+ substituent (-0.03 V). A similar effect is observed for Pt(tpy)(C. -C6H5-Z) (Z=NO2,

MeO, H) complexes where the NO2 substituent is proposed to cause the irreversible one

electron Pt(II)/Pt(III) oxidation potential to shift anodically by 0.24 volts from that of

+ Pt(tpy)(C.-C6H5) (Epa= 1.22V, in 0.1M TBAH acetonitrile solution, vs. SCE, glassy

carbon working electrode); however, in that case the influence of the MeO substituent

also is substantial (-0.22V), and it is not clear that these shifts represent thermodynamic

potentials.62 In the case of the Ru(III/II) systems, the redox process involves a d orbital,

whereas the oxidation of platinum(II) formally involves a dz2 orbital. On the other hand, the Hammett inductive ( I) and mesomeric ( m) constants for nitro (0.7, 0) and methoxy

(0.31, -0.41) groups suggest significant differences in the communication of substituent

electron-donor properties. Therefore, the comparative insensitivity of the platinum

system to the methoxy substituent is consistent with the influence of resonance on the dz2

level being significantly reduced as compared to its influence on the d levels of

ruthenium(III/II) polypyridyl complexes. Similarly, the increased sensitivity to the nitro

substituent suggests that the influence of induction on the dz2 level is significantly increased.

197 1.5

) 1 V ( l a i t n

ote 0.5 P

-6 -4 -2 0 2 4 + p

Figure 5.22 Correlation of oxidation potential (E°' vs. SCE) to Hammett parameter

+ + . L represents Pt(Z-pip2NCN)(R-tpy) complexes where Z=NO2,

+ MeO, H and R=H, t-butyl, phenyl, E X0 ;)1Hp + 0.41 (R=0.952).

2+ 64 represents Ru(Z,Z'-bpy)x(bpy)3-x complexes where Z= NO2, H, Me,

+ + MeO, (x= 0,1,2,3), E X0 ;)1;p + 1.25 (R=0.995). p is calculated by

+ p values for each substituent; a 6 coordinate Ru(Z,Z'-

2+ bpy)x(bpy)3-x complex has six different substituents, whereas a

+ Pt(Z-pip2NCN)(R-tpy) complex has four substituents.

198 Conclusions

- Substituted pip2NCN ligands, their Pt(II) halide complexes and novel

+ Pt(Z-pip2NCN)(Rn-tpy)] complexes where Z and Rn are different substituents have been

prepared. For the Z-ligands and their corresponding Pt(II) halide complexes, 1H NMR

spectroscopy show that the substituent at the para-position influences the electron density

+ 1 on the nearby atoms, as well as the platinum center. For Pt(Z-pip2NCN)(R-tpy)] , H

- NMR spectroscopy and X-ray crystallography show that the pip2NCN ligand is bonded

t monodentate whereas the terpyridyl ligand (tpy or Bu3tpy) is bonded tridentate. The approximate square planar coordination geometry around the metal center is consistent with structures of platinum(II) complexes. The piperidyl groups are located above and below the platinum center, resulting in at least one short Pt...N(piperidyl) distance of 3.44

± 0.25 ppm. This coordination geometry suggests that the amine groups are available to

+ stabilize the metal center upon oxidation. As in the case of Pt(pip2NCN)(tpy) , the

+ electronic absorption spectra of Pt(Z-pip2NCN)(R-tpy)] exhibit a band at ~550 nm

-N)4 -transfer transition, resulting from the weak

interaction of a piperidyl group with the metal center. The fact that this band is missing

t + t 3+ from the spectra of Pt(ph)( Bu3tpy) and Pt(pip2NCNH2)( Bu3tpy) , supports this

+ conclusion. Pt(Z-pip2NCN)(R-tpy) complexes undergo an apparent two-electron

platinum centered oxidation near 0.4V and two Pt(tpy) centered reductions near -1.0 V

and -1.5V. Variation in the Z and Rn substituents allows for tuning of the oxidation

process over a 200 mV range and the reduction processes over 160 mV and 280 mV

ranges, respectively. As anticipated, the electron-withdrawing substituents (e.g., NO2)

anodically shift the two-electron couple, whereas electron-donating groups (e.g., MeO

199 cathodically shift the redox potential. The scan rate dependence of the peak currents of

t + the oxidation and the first reduction processes for Pt(pip2NCN)( Bu3tpy) supports the conclusion that the oxidation process involves transfer of two electrons per platinum center.

200 References

(1) Jude, H.; Bauer, J. A. K.; Connick, W. B. J. Am. Chem. Soc. 2003, 125, 3446-

3447.

(2) Jude, H., University of Cincinnati, Dissertation Thesis.

(3) Jude, H.; Krause Bauer, J. A.; Connick, W. B., manuscript in preparation.

(4) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2002, 41, 2275-2281.

(5) Eaborn, C.; Odell, K. J.; Pidcock, A. J. Chem. Soc. Dalton Trans. 1978, 357-368.

(6) Hackett, M.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 1449-1462.

(7) Steele, B. R.; Vrieze, K. Transition Metal Chemistry 1977, 2, 140-144.

(8) van de Kuil, L. A.; Luitjes, H.; Grove, D. M.; Zwikker, J. W.; van der Linden, J.

G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth, W.; van Koten, G.

Organometallics 1994, 13, 468-477.

(9) Kissinger, P. T.; Heineman, W. R. J. Chem. Ed. 1983, 60, 702-706.

(10) Slagt, M. Q.; Klein Gebbink, R. J. M.; Lutz, M.; Spek, A. L.; van Koten, G. J.

Chem. Soc. Dalton Trans. 2002, 13, 2591-2592.

(11) van de Kuil, L. A.; Luitjes, H.; Grove, D. M.; Zwikker, J. W.; van der Linden, J.

G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth, W.; van Koten, G.

Organometallics 1994, 13, 468-477.

(12) Albrecht, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2001, 123, 7233-

7246.

(13) Bruker APEX2 v1.0-27 and SAINT v27.06A programs were used for data

collection and data processing, respectively. Bruker Analytical X-Ray

Instruments, Inc., Madison WI.

201 (14) SADABS v2.10 was used for the application of semi-empirical absorption and

beam corrections. G. M. Sheldrick, University of Göettingen, Germany.

(15) SHELXTL v6.14 were used for the structure solution and generation of figures

and tables. Neutral-atom scattering factors were used as stored in this package.

G. M. Sheldrick, University of Göttingen, Germany and Bruker Analytical X-ray

Solutions, Inc., Madison, WI.

(16) SQUEEZE as implemented in Platon. A.L. Spek, (1990), Acta. Cryst. A1946, C-

1934.

(17) Jude, H.; Krause Bauer, J. A.; Connick, W. B. J. Am. Chem. Soc. 2003, 125,

3446-3447.

(18) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer, W. P.; Gray,

H. B. Inorg. Chem. 1995, 34, 4591-4599.

(19) Chemega, A.; Droz, A. S.; Vilaivan, T.; Weaver, G. W.; Lowe, G. J. Chem. Res

1996, 402, 2457.

(20) Roszak, A. W.; Clement, O.; Buncel, E. Acta Crystallogr., Sect C: Cryst. Struct.

Commun. 1996, 52, 1645-1648.

(21) Buchner, R.; Cunningham, C. T.; Field, J. S.; Haines, R. J.; Mcmillin, D. R.;

Summerton, G. C. J. Chem. Soc, Dalton Trans. 1999, 711-717.

(22) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg. Chem. 1999, 38,

4262-4267.

(23) Romeo, R.; Scolaro, L. M.; Plutino, M. R.; Albinati, A. J. Organomet. Chem.

2000, 593-594, 403-408.

202 (24) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Cheung, K. K. Organometallics

2001, 20, 4476-4482.

(25) Field, J. S.; Haines, R. J.; Mcmillin, D. R.; Summerton, G. C. J. Chem.Soc.

Dalton Trans. 2002, 1369-1376.

(26) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002, 124, 6506-

6507.

(27) Slagt, M. Q.; Dijkstra, H. P.; McDonald, A.; Gebbink, R. J. M. K.; Lutz, M.; Ellis,

D. D.; Mills, A. M.; Spek, A. L.; van Koten, G. Organometallics 2003, 22, 27-29.

(28) Kim, K. M.; Lee, Y.-A.; Lee, S. S.; Sohn, Y. S. Inorg. Chim. Acta 1999, 292, 52-

56.

(29) Canty, A. J.; Denney, M. C.; van Koten, G.; Skelton, B. W.; White, A. H.

Organometallics 2004, 23, 5432-5439.

(30) Deacon, G. B.; Patrick, J. M.; Skelton, B. W.; Thomas, N. C.; White, A. H. Aust.

J. Chem. 1984, 37, 929-945.

(31) Constable, E. C.; Hannon, M. J.; Thompson, A. M. W. C.; Tocher, D. A.; Walker,

J. V. Supramol. Chem. 1993, 2, 243-246.

(32) Abel, E. W.; Dimitrov, V. S.; Long, N. J.; Orrell, K. G.; Osborne, A. G.; M., P.

H.; Sik, V.; Hursthouse, M. B.; Mazid, M. A. J. Chem. Soc, Dalton Trans. 1993,

597-603.

(33) Sugimoto, H.; Tsuge, K.; Tanaka, K. J. Chem. Soc, Dalton Trans. 2001, 57-63.

(34) Abel, E. W.; Dimitrov, V. S.; Long, N. J.; Orrell, K. G.; Osborne, A. G.; Sik, V.;

Hursthouse, M. B.; Mazid, M. A. J. Chem. Soc. Dalton Trans: Inorg. Chem.

1993, 2, 291-298.

203 (35) Abel, E. W.; Orrell, K. G.; Osborne, A. G.; Pain, H. M.; Sik, V.; Hursthouse, M.

B.; Malik, A. K. M. J. Chem. Soc, Dalton Trans. 1994, 3441-3449.

(36) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994, 33, 722-727.

(37) Wong, Y.-S.; Lippard, S. J. Chem. Commun. 1977, 824-825.

(38) Yip, H.-K.; Cheng, L.-K.; Cheung, K. K.; Che, C. M. J. Chem. Soc. Dalton Trans.

1993, 2933-2938.

(39) Cini, R.; Donati, A.; Giannettoni, R. Inorg. Chim. Acta 2001, 315, 73-80.

(40) Tzeng, B.-C.; Lee, G.-H.; Peng, S.-M. Inorg. Chem. Comm. 2003, 6, 1341-1343.

(41) Angle, C. S.; DiPasquale, A. G.; Rheingold, A. L.; Doerer, L. H. Acta Cryst., Sect

C: Cryst. Struct. Commun. 2006, C62, m340-,342.

(42) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.

(43) Neidle, S.; Murrer, B. A.; Barnard, C. F. Bioorg. Med. Chem. Lett. 1996, 6, 421-

426.

(44) Tastan, S.; Krause, J. A.; Connick, W. B. Inorg. Chim. Acta 2006, 359, 1889-

1898.

(45) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004, 43, 725-733.

(46) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2005, 44, 1211-1220.

(47) Slagt, M. Q.; Rodriguez, G.; Grutters, M. M. P.; Gebbink, R. J. M. K.; Klopper,

W.; Jenneskens, L. W.; Lutz, M.; Spek, A. L.; Van Koten, G. Chemistry-A

Europian Journal 2004, 10, 1331-1344.

(48) Ansari, S. M.; Robien, W.; Schlederer, M.; Wolschann, P. Monatshefte fur

Chemie 1989, 120, 1003-1014.

204 (49) Constable, E. C.; Thompson, A. M. W. C.; Tocher, D. A.; Daniels, M. A. M. New

J. Chem. 1992, 16, 855-867.

(50) Fallahpour, R.-A. Eur. J. Inorg. Chem. 1998, 9, 1205-1207.

(51) Field, J. S.; Haines, R. J.; McMillin, D. R.; Summerton, G. C. J. Chem. Soc.,

Dalton Trans. 2002, 1369-1376.

(52) Arena, G.; Calogero, G.; Campagna, S.; Scolaro, L. M.; Ricevuto, V.; Romeo, R.

Inorg. Chem. 1998, 2763-2769.

(53) Hobert, S. E.; Carney, J. T.; Cummings, S. D. Inorg. Chim. Acta 2001, 318, 89-

96.

(54) Yip, J. H. K.; Suwarno; Vittal, J. J. Inorg. Chem. 2000, 39, 3537-3543.

(55) Connick, W. B.; Miskowski, V. M.; Houlding, V. H.; Gray, H. B. Inorg. Chem.

2000, 39, 2585-2592.

(56) Green, T. W.; Lieberman, R.; Mitchell, N.; Krause Bauer, J. A.; Connick, W. B.

Inorg. Chem. 2005, 44, 1955-1965.

(57) Crites-Tears, D. K.; McMillin, D. R. Coord. Chem. Rev. 2001, 211, 195-205.

(58) Sarkar, A.; Chakravorti, S. J. Lumin. 1995, 63, 143-148.

(59) Huang, K.; Rhys, A. Proc. Roy. Soc. (London) 1950, 204A, 406-423.

(60) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1996, 35,

4585-4590.

(61) Lam, S. C.-F.; Yam, V. W.-W.; Wong, K. M.-C.; Cheng, E. C.-C.; Zhu, N.

Organometallics 2005, 24, 4298-4305.

(62) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.

Organometallics 2001, 20, 4476-4482.

205 (63) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Thompson, A. M. W. C.

Inorg. Chem. 1995, 34, 2759-2767.

(64) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Kalyanasundaram, K. J. Phys. Chem.

1993, 97, 9607-9612.

(65) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.

(66) Randles, J. E. B. Trans. Faraday Soc. 1948, 44, 327-338.

(67) Sevcik, A. Coll. Czech. Chem. Commun. 1948, 13, 349-377.

(68) van Benschoten, J. J.; Lewis, J. Y.; Heineman, W. R.; Roston, D. A.; Kissinger, P.

T. J. Chem. Ed. 1983, 60, 772-776.

(69) Myers, R. L.; Shain, I. Anal. Chem. 1969, 41, 980.

(70) Balzani, V.; Juris, A.; Barigelletti, F.; Belser, P.; Von Zelewsky, A. Sci. Papers of

the Inst. of Phys. and Chem. Res. 1984, 78, 78-85.

206