CO2 Activation and Redox Activity of NN

To ancillary and beyond: CO 2 activation and redox activity of NN chelate actor ligands

Trevor William Janes

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto

To ancillary and beyond: CO 2 activation and redox activity of NN chelate actor ligands

Trevor William Janes

Doctor of Philosophy

Department of Chemistry University of Toronto

2017 Abstract

This thesis examines the fundamental coordination chemistry and subsequent reactivity of actor and redox-active ligands. In this context, the two NN chelate ligand classes that are explored are

4,5-diazafluorene and o-phenylenediamine.

In Chapter 2, actor diazafluorenyl ligands were explored. New Ru and Zn diazafluorenyl complexes undergo C–H borylation of the diazafluorenyl ligand to form the corresponding diazafluorenylboronic ester complexes, which can insert CO 2 into their C–B bonds to form boryl ester functionalities. The relevance of this new reactivity to catalytic CO 2 reduction was also explored.

In Chapter 3, the initial coordination chemistry of the redox-active N,N’ -bis(2,6-

2- diisopropylphenyl)-o-phenylenediamido ligand (L ) was investigated. Li 2L(THF)3 reacts with iron(II) sources in toluene to yield LFe(η6-toluene). DFT, NMR, and X-ray data indicate that L2-

1- 6 oxidized to L in this transformation. [FeL(η -toluene)] reacts with CO to generate [FeL(CO) 3],

- 0 with concomitant oxidation of L to L . Two equivalents of Li 2L react with both MoCl 4(THF)2 and VCl 2(tmeda) 2 yielding trimetallic sandwich compounds.

In Chapter 4, reactivity of Pb(II) and Sn(II) complexes of L 2- was investigated. [PbL] and [SnL] react with mesityl azide in analogous C-H activations in which L 2- behaves as an actor ligand.

2- [PbL] forms a Lewis pair with ONMe 3, demonstrating the behaviour of L as an ancillary ligand.

[SnL] reacts with AgOTf to yield [SnL(OTf)], demonstrating the ligand’s redox activity as L 2- is oxidized to L -. [SnL(OTf)] is a paramagnetic stannylene, a type of compound which has been previously observed in situ , but hitherto has not been isolated.

In Chapter 5, the coordination of different o-phenylenediamido sources towards [TaMe 3Cl 2] was explored. The opda framework acted as a reducing agent, as a sponsor of heterodinuclear complexes, or as a robust ancillary ligand. The use of K 2L provided access to [TaLMe 3]. The reactivity of its methyl ligands was studied; notably, UV irradiation causes formal loss of methyl radical to yield dimeric [LTaMe 2]2.

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Acknowledgments

Thanks to my PhD supervisor, Professor Datong Song, for giving me cool projects to work on and for letting me pursue what I was interested in. His guidance helped me immeasurably. I am truly thankful for his ongoing support.

Thanks to the various agencies that gave me money to do research, including the government of Ontario, University of Toronto, and the Graduate Student Endowment Fund.

Thanks to my committee members Professors Bob Morris and Ulrich Fekl, and exam committee members Professors Sophie Rousseaux and Frédéric-Georges Fontaine.

Thanks to Prof. Yang Li for hosting me in a research stint at Chongqing University; my time in his laboratory was one of the highlights of my PhD. Thanks to Prof. Parisa Mehrkhodavandi, Prof. Laurel Schafer, and Insun Yu for mentoring me during my undergraduate experience at UBC, and for helping me get “it” together.

Thanks to all the departmental staff that assisted me, especially Anna Liza Villavelez, Ken Greaves, John Ford, Jack O’Donnell, Tim Burrows, Darcy Burns, Jack Sheng, Dmitry Pichugin, Sergiy Nokhrin, Rose Balazs, Jack Jackiewicz, and Alan Lough.

Thanks to Song group members past and present for being great to work with, especially Runyu Tan, Yu Li, Tao Bai, Shaolong Gong, Xiaofei Li, Kim Osten, Rhys Batcup, Tara Cho, Celia Gendron-Herndon, Adam Pantaleo, Yanxin Yang, Charlie Kivi, Walter Liang, Fred Chiu, Ellen Yan, and Daniel Dalessandro.

Thanks to Yasser Hassan, a class act, a tenacious, enterprising scientist. It was a privilege to collaborate with you.

To the undergraduates I mentored . . .you’re welcome! Maotong Xu, Xhoana Gjergji, Pavel Zatsepin, William Kim, Menandro Cruz, and Ben Tsui. Thanks for working with me, I know I benefitted immensely from our time together.

Thanks to my parents Doug and Karen Janes. Thanks for your love and support, for the opportunities you provided, and for the best upbringing I can imagine. Thanks to my brother

Andrew Janes; you’re the nuttiest. Thanks to my aunt Sandra Robertson for being an impossibly great listener, and my uncle Al Robertson for making me explain my research when I came home for holidays. Thanks to my grandma Shirley Matheson for applying the pressure I needed to wrap up this degree.

Thanks to my special lady, my love, my sweetie, Laura Brown.

Table of Contents

Acknowledgments...... iv

Table of Contents ...... vi

List of Tables ...... xi

List of Figures ...... xii

List of Schemes ...... xvii

List of Abbreviations and Symbols...... xx

1 Introduction ...... 1

1.1 CO 2 activation ...... 1

1.1.1 Metal-free CO 2 activation ...... 1

1.1.2 Metal-Ligand Cooperation for CO 2 activation ...... 3

1.1.3 Other stoichiometric reactivity of CO 2 ...... 5

1.1.4 Insertions of CO 2...... 6

1.2 Redox-active ligands ...... 7

1.2.1 Redox-active vs. noninnocent ...... 8

1.2.2 Pyridine diimine ligands ...... 8

1.2.3 Multielectron redox chemistry of d 0 tantalum complexes ...... 9

1.2.4 Negishi-type coupling using redox active ligands on Co ...... 10

1.3 N2 activation...... 11

1.4 NN chelate actor ligands ...... 13

1.4.1 4,5-diazafluorene ...... 14

1.4.2 o-phenylenediamine ...... 16

1.5 Scope and objectives ...... 17

1.6 References ...... 20

2 Chapter 2 Insertion of CO 2 into the carbon—boron bond of a boronic ester ligand ...... 23

2.1 Abstract ...... 23

2.2 Introduction ...... 23

2.3 Results and Discussion ...... 25

2.4 Conclusion ...... 47

2.5 Experimental ...... 48

2.5.1 General procedures...... 48

2.5.2 Synthesis of 1a ...... 49

2.5.3 Synthesis of 2a ...... 49

2.5.4 Synthesis of 3a ...... 50

2.5.5 Synthesis of EtZn(Mes 2nacnac) ...... 51

2.5.6 Synthesis of 1b ...... 51

2.5.7 Synthesis of 2b ...... 52

2.5.8 Synthesis of 3b ...... 53

2.5.9 Regeneration of 2a : ...... 54

2.5.10 Catalytic experiments...... 54

2.5.11 Synthesis of 5 ...... 54

2.5.12 Further reaction of 5 with HBpin ...... 55

2.5.13 Synthesis of 6 ...... 55

2.5.14 X-ray Crystallography ...... 56

2.5.15 Computations ...... 58

2.6 References ...... 58

3 Chapter 3 Synthesis and structures of Li, Fe, Mo, and V derivatives of N,N’ -bis(2,6- diisopropylphenyl)-o-phenylenediamine ...... 60

3.1 Abstract ...... 60

3.2 Introduction ...... 60

3.3 Results and Discussion ...... 61

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3.3.1 Synthesis of Li 2L(THF) 3 (3.1 ) ...... 61

3.3.2 Fe coordination chemistry of 3.1 ...... 63

3.3.3 Mo coordination chemistry of 3.1 ...... 67

3.3.4 EPR data for 3.5 ...... 72

3.3.5 V coordination chemistry of 3.1 ...... 74

3.4 Conclusion ...... 75

3.5 Experimental ...... 76

3.5.1 General considerations ...... 76

3.5.2 Synthesis of Li 2L(THF) 3 (3.1 ) ...... 76

3.5.3 Synthesis of LFe(η 6-toluene) ( 3.2 )...... 77

3.5.4 Synthesis of LFe(CO) 3 (3.3 ) ...... 78

3.5.5 Synthesis of (LiL) 2MoCl 2(THF) 4 (3.4 ) ...... 78

3.5.6 Synthesis of LiL(Et 2O) ( 3.5 ) ...... 79

3.5.7 Synthesis of (LiL) 2V(OEt 2)2 ...... 79

3.6 X-ray crystallography ...... 79

3.7 References ...... 81

4 Chapter 4 Reactivity of heavy carbene analogues towards oxidants: redox active ligand- enabled isolation of a paramagnetic stannylene ...... 83

4.1 Abstract ...... 83

4.2 Introduction ...... 83

4.3 Results and Discussion ...... 84

4.4 Conclusion ...... 98

4.5 Experimental ...... 98

4.5.1 General considerations ...... 98

4.5.2 Cyclic Voltammetry ...... 99

4.5.3 Synthesis of N,N′-bis(2,6-diisopropylphenyl)-benzimidazolin-2-plumbylene (4.1a ) ...... 99 viii

4.5.4 Synthesis of 4.2a ...... 100

4.5.5 Synthesis of 4.2b ...... 100

4.5.6 Synthesis of 4.3 ...... 101

4.5.7 Synthesis of 4.4 ...... 101

4.5.8 X-ray crystallography ...... 101

4.6 References ...... 103

5 Chapter 5 Synthesis and reactivity of Li and TaMe 3 complexes supported by N,N’ -bis(2,6- diisopropylphenyl)-o-phenylenediamido ligands...... 106

5.1 Abstract ...... 106

5.2 Introduction ...... 106

5.3 Results and Discussion ...... 107

5.3.1 Synthesis and structures of heterodinuclear compounds 5.1 and 5.2 (solvent) n ...107

5.3.2 Coordination chemistry of L’...... 114

5.3.3 Synthesis of [TaLMe 3] (5.5) ...... 118

5.3.4 Reactivity of [TaLMe 3] ...... 121

5.4 Conclusion ...... 125

5.5 Experimental ...... 126

5.5.1 General considerations ...... 126

5.5.2 Synthesis of [Li(Et 2O)(THF)LTaMe 3Cl] ( 5.1 ) ...... 126

5.5.3 Synthesis of [Li(Et 2O)(THF)LTaMe 4] ( 5.2 (Et 2O)(THF)) ...... 127

5.5.4 Synthesis of [Li 2L’(Et 2O) 2] ( 5.3 ) ...... 128

5.5.5 Synthesis of [LiL’(Et 2O)] ( 5.4 ) ...... 128

5.5.6 Synthesis of [TaLMe 3] (5.5) ...... 129

5.5.7 Synthesis of K 2L(dme) 4: ...... 129

5.5.8 Synthesis of DCC insertion product ( 5.6 ) ...... 130

5.5.9 Synthesis of MesN 3 insertion product ( 5.7 ) ...... 130

5.5.10 Synthesis of [TaLMe 2]2 (5.8 ) ...... 130

5.5.11 X-ray crystallography ...... 131

5.6 References ...... 133

6 Chapter 6 Summary and future directions ...... 135

6.1 Introduction ...... 135

6.2 Chapter 2 ...... 136

6.3 Chapter 3 ...... 137

6.4 Chapter 4 ...... 138

6.5 Chapter 5 ...... 138

About the Author ...... 140

List of Tables

a Table 2.1 Results for the hydroboration of CO 2 by HBCat and HBpin ...... 40

Table 2.2 Selected crystallographic data ...... 56

Table 3.1 Crystallographic data for compounds 3.1-3.5 ...... 80

Table 4.1 Crystallographic data for compounds 4.1a , 4.2a , 4.2b , 4.3 , and 4.4 ...... 102

Table 5.1 Selected bond lengths in 5.3 and 5.4 ...... 116

Table 5.2 Selected analogous bond lengths in 5.6 , 5.7 , 5.8 ...... 123

Table 5.3 Crystallographic data for 5.1-5.8 and K2L(dme) 4...... 131

List of Figures

Figure 1.1 Structure of the FeMo cofactor...... 11

1 31 1 Figure 2.1 H NMR spectrum (500 MHz, 25 °C) of 1a in C 6D6. Inset: P{ H} NMR spectrum . 26

13 1 Figure 2.2 C{ H} NMR spectrum (126 MHz, 25 °C) of 1a in C 6D6...... 26

Figure 2.3 FT-IR Spectrum of 1a as a nujol mull...... 27

1 Figure 2.4 H NMR Spectrum (500 MHz, C 6D6, 25 °C) of complex 1b ...... 27

13 1 Figure 2.5 C{ H} NMR Spectrum (126 MHz, C 6D6, 25 °C) of 1b ...... 28

Figure 2.6 Molecular structure of 1b ...... 29

1 Figure 2.7 H NMR Spectrum (500 MHz, 25 °C) of 2a in C 6D6 ...... 30

13 1 Figure 2.8 C{ H} NMR Spectrum (126 MHz, 25 °C) of 2a in C 6D6 ...... 31

31 1 11 1 Figure 2.9 Left: P{ H} NMR Spectrum (202 MHz, 25 °C) of 2a in C 6D6. Right: B{ H} NMR

Spectrum (128 MHz, 25 °C) of 2a in C 6D6...... 31

Figure 2.10 FT-IR spectrum of 2a as a nujol mull...... 32

Figure 2.11 Molecular structures of 2a and 3a ...... 32

1 Figure 2.12 H NMR Spectrum (600 MHz, 25 °C) of 3a in C 6D6 ...... 34

13 1 Figure 2.13 C{ H} NMR Spectrum (126 MHz, 25 °C) of 3a in CD 2Cl 2 ...... 35

31 1 11 1 Figure 2.14 Left: P{ H} NMR Spectrum (243 MHz, 25 °C) of 3a in C 6D6.Right: B{ H} NMR

Spectrum (128 MHz, 25 °C) of 3a in C 6D6...... 35

Figure 2.15 FT-IR spectrum of 3a as a nujol mull...... 36

Figure 2.16 Molecular structures of 2b and 3b ...... 37

Figure 2.17 Computed transition state structure of the CO 2 insertion reaction from 2a to 3a ..... 38

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Figure 2.18 Thermodynamic data for the transformation of 2a to 3a ...... 39

1 Figure 2.19 H NMR Spectrum (600 MHz, 25 °C) of 5 in C 6D6. Inset: hydride region...... 42

13 1 Figure 2.20 C{ H} NMR Spectrum (151 MHz, 25 °C) of 5 in C 6D6. Inset: expanded aryl region...... 43

31 1 11 1 Figure 2.21 Left: P{ H} NMR Spectrum (243 MHz, 25 °C) of 5 in C 6D6 Right: B{ H} NMR

Spectrum (128 MHz, 25 °C) of 5 in C 6D6...... 43

1 - Figure 2.22 H NMR Spectrum (500 MHz, 25 °C) of 6•(toluene) in C 6D6. Inset: H region ...... 44

13 1 Figure 2.23 C{ H} NMR Spectrum (126 MHz, 25 °C) of 6•(toluene) in C 6D6. Inset: expanded aryl region...... 45

31 1 Figure 2.24 Left: P{ H} NMR Spectrum (162 MHz, 25 °C) of 6•(toluene) in C 6D6 Right: 11 1 B{ H} NMR Spectrum (128 MHz, 25 °C) of 6•(toluene) in C 6D6...... 45

Figure 2.25 Molecular structures of 5 and 6...... 46

Figure 3.1 Molecular structure of 3.1 ...... 63

Figure 3.2 Molecular structure of 3.2 ...... 65

Figure 3.3 Molecular structure of 3.3 ...... 67

Figure 3.4 Molecular structure of 3.4 ...... 69

Figure 3.5 A: Labelling of inner (C i) and outer (C o) butadiene carbons; B: Two extreme resonance forms of a generic diene complex ...... 71

Figure 3.6 Molecular structure of 3.5 ...... 71

Figure 3.7 EPR spectra of 3.5 in the solid state (top) and in toluene solution (bottom) at ambient temperature...... 72

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Figure 3.8 Singly occupied molecular orbital (left) for model compound [C 6H4(NPh) 2LiOMe 2] (B3LYP/6-311G**) and estimated (right, top) and computed (B3LYP/6-31G*) spin density (right bottom) for 3.5 ...... 73

Figure 3.9 Crystal structure of 3.6 ...... 75

1 Figure 4.1 H NMR Spectrum (500 MHz, 25 °C) of 4.1a in C 6D6. Inset: expanded aryl region. 85

13 1 207 1 Figure 4.2 C{ H} NMR Spectrum (151 MHz, 25 °C) of 4.1a in C 6D6. Inset: Pb{ H} NMR

Spectrum (105 MHz, 25 °C) of 4.1a in C 6D6...... 86

Figure 4.3 Molecular structure of 4.1a ...... 86

Figure 4.4 Cyclic voltammogram of 4.1a (0.5 mM in THF, 0.1M NBu 4PF 6)...... 87

1 Figure 4.5 H NMR Spectrum (400 MHz, 25 °C) of 4.2a in C 6D6...... 88

13 1 207 1 Figure 4.6 C{ H} NMR Spectrum (126 MHz, 25 °C) of 4.2a in C 6D6. Inset: Pb{ H} NMR

Spectrum (105 MHz, 25 °C) of 4.2a in C 6D6...... 88

Figure 4.7 Molecular structure (left) and crystal packing of 4.2a (right)...... 89

1 Figure 4.8 H NMR Spectrum (600 MHz, 25 °C) of 4.2b in C 6D6...... 91

13 1 119 1 Figure 4.9 C{ H} NMR Spectrum (151 MHz, 25 °C) of 4.2b in C 6D6. Inset: Sn{ H} NMR

Spectrum (224 MHz, 25 °C) of 4.2b in C 6D6...... 91

1 Figure 4.10 H NMR Spectrum (500 MHz, 25 °C) of 4.3 in C 6D6. Inset: expanded aryl region. 93

13 1 207 1 Figure 4.11 C{ H} NMR Spectrum (126 MHz, 25 °C) of 4.3 in C 6D6. Inset: Pb{ H} NMR

Spectrum (126 MHz, 25 °C) of 4.3 in C 6D6...... 93

Figure 4.12 Molecular structures of 4.2b (left) and 4.3 (right)...... 94

Figure 4.13 Molecular structure of 4.4 ...... 96

Figure 4.14 EPR spectrum (simulated on top, experimental on bottom) of 4.4 ...... 97

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7 1 1 Figure 5.1 Inset: Li{ H} NMR Spectrum (233 MHz, 25 °C) of 5.1 in C 6D6. H NMR Spectrum

(600 MHz, 25 °C) of 5.1 in C 6D6 ...... 109

13 1 Figure 5.2 C{ H} NMR Spectrum (151 MHz, 25 °C) of 5.1 in C 6D6 ...... 109

7 1 1 Figure 5.3 Inset: Li{ H} NMR Spectrum (194 MHz, 25 °C) of 5.2 (THF) in C 6D6. H NMR

Spectrum (500 MHz, 25 °C) of 5.2 (THF) in C 6D6...... 110

13 1 Figure 5.4 C{ H} NMR Spectrum (126 MHz, 25 °C) of 5.2 (THF) in C 6D6 ...... 111

1 Figure 5.5 H NMR Spectrum (600 MHz, -80 °C) of 5.2 in C 6D5CD 3 ...... 111

Figure 5.6 Expansion of the alkyl region of 1H NMR Spectrum (600 MHz, 25 °C) of 5.2 in

C6D5CD 3 ...... 112

Figure 5.7 Molecular structures of 5.1 (left) and 5.2 (THF) 2 (right)...... 112

7 1 1 Figure 5.8 Inset: Li{ H} NMR Spectrum (194 MHz, 25 °C) of 5.3 in C 6D6. H NMR Spectrum

(300 MHz, 25 °C) of 5.3 in C 6D6 ...... 115

13 1 Figure 5.9 C{ H} NMR Spectrum (101 MHz, 25 °C) of 5.3 in C 6D6 ...... 115

Figure 5.10 Molecular structures of [Li 2L’(Et 2O) 2], 5.3 (top), and [LiL’(Et 2O)], 5.4 (bottom) . 116

Figure 5.11 Experimental (top) and simulated (bottom) EPR spectrum of 5.4 ...... 117

1 13 1 Figure 5.12 H NMR Spectrum (600 MHz, 25 °C) of 5.5 in C 6D6 Inset: C{ H} NMR Spectrum

(151 MHz, 25 °C) of 5.5 in C 6D6 ...... 119

Figure 5.13 Molecular structure of K 2L(dme) 4...... 120

Figure 5.14 Molecular structure of [TaLMe 3], 5.5 ...... 120

Figure 5.15 Molecular structures of 5.6 (top left), 5.7 (top right), and 5.8 (bottom right). Inset: two views of the coordination sphere of 5.6 ...... 122

1 13 1 Figure 5.16 H NMR Spectrum (400 MHz, 25 °C) of 5.8 (pentane) 2/3 in C 6D6. Inset: C{ H}

NMR Spectrum (500 MHz, 25 °C) of 5.8 in C 6D6 ...... 125

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List of Schemes

Scheme 1.1 Three broad reactivity patterns of CO 2 ...... 1

Scheme 1.2 Hydrogenation of CO 2 to methanol...... 1

Scheme 1.3 Stephan and Erker’s FLP activation of CO2 ...... 2

Scheme 1.4 Fontaine’s highly active metal free CO 2 hydroboration catalyst ...... 2

Scheme 1.5 Cantat’s metal free CO 2 reduction catalysts...... 3

Scheme 1.6 Fryzuk’s heterolytic splitting of dihydrogen by MLC ...... 4

Scheme 1.7 Milstein’s heterolytic splitting of dihydrogen by MLC ...... 4

Scheme 1.8 Milstein and Sanford’s investigation of CO 2 activation by MLC ...... 4

Scheme 1.9 Hazari’s CO 2 activation by MLC and reduction to formate ...... 5

Scheme 1.10 Dielmann’s electron rich phosphines bind CO 2 ...... 5

Scheme 1.11 Schulz’s deoxygenation of CO 2 using a P 2N2 ring ...... 6

Scheme 1.12 Kemp and Waterman’s unexpected insertion of CO 2 into C-Si bonds ...... 6

Scheme 1.13 CO 2 arylation/deoxygenation by borinium...... 7

Scheme 1.14 Piers’ reduced boroles ring expand with CO 2 ...... 7

Scheme 1.15 Chirik’s bis(imino)pyridine Fe(II) complexes...... 9

Scheme 1.16 Heyduk’s 1 e - and 2 e - reductions using a d 0 metal complex 53 ...... 10

Scheme 1.17 Soper’s Negishi-type C-C coupling 55 ...... 10

Scheme 1.18 Synthesis of the first N 2 complex ...... 11

Scheme 1.19 Binding and silylation of dinitrogen on Fryzuk’s [NPNTa] platform ...... 12

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Scheme 1.20 Holland’s synthesis and hydrogenation of a bis-nitride complex ...... 13

Scheme 1.21 Numbering scheme/deprotonation of diazafluorene ...... 14

Scheme 1.22 Small molecule activation using Ru(II)diazafluorenyl species...... 15

Scheme 1.23 Three redox forms of o-phenylenediamine ...... 16

0 Scheme 1.24 Oxidative addition of X 2 to a d Zr complex ...... 16

Scheme 1.25 Smith’s [CpCr(opda)] system for imido formation, MLC, and catalytic cyclization ...... 17

Scheme 2.1 Synthesis of 1a and 1b ...... 25

Scheme 2.2 Synthesis of 2a/b and 3a/b ...... 30

Scheme 2.3 Synthesis of 5 and 6 ...... 41

Scheme 3.1 Three redox forms of opda ...... 61

Scheme 3.2 Synthesis of dilithium complex 3.1 ...... 62

Scheme 3.3 Two syntheses of 3.2 ...... 64

Scheme 3.4 Synthesis of 3.3 ...... 66

Scheme 3.5 Reactivity of 3.1 towards MoCl 4(THF) 2 ...... 68

Scheme 3.6 Synthesis of 3.6 and ligand transfer to Pb ...... 74

Scheme 4.1 Synthesis of 4.2a/b ...... 84

Scheme 4.2 Synthesis of Lewis pair 4.3...... 92

Scheme 4.3 Synthesis of paramagnetic stannylene 4.4 ...... 95

Scheme 5.1 Synthesis of Ta-Li heterodinuclear complexes 5.1 and 5.2 (Et 2O)(THF)...... 108

Scheme 5.2 Syntheses of Li complexes 5.3 and 5.4 ...... 114

xviii

Scheme 5.3 Synthesis of [TaLMe 3] ...... 119

Scheme 5.4 Reactivity of [TaLMe 3]: Synthesis of 5.6 , 5.7 , and 5.8 ...... 122

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List of Abbreviations and Symbols

- - [Mes 2nacnac] [(MesNC(CH 3)) 2CH] dme 1,2-dimethoxyethane daf 4,5-diazafluorenyl dafH 4,5-diazafluorene dipp 2,6-diisopropylphenyl H2L N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamine H2L’ N,N’ -bis(2,6-diisopropylphenyl)-4,5-dimethyl-o- phenylenediamine OAc acetate α/β/γ alpha/beta/gamma (crystallographic unit cell lengths) Å Angstrom, 10 -10 m (aq) aqueous Ar aryl group atm atmosphere Bn benzyl bpy bipyridine br broad Anal. Calcd calculated elemental analysis based on a formula δ chemical shift - Cp cyclopentadienyl (C 5H5 ) ° degree DFT density functional theory dpph 2,2-diphenyl-1-picrylhydrazyl DCM dichloromethane Et 2O diethyl ether DMSO dimethylsulfoxide tmeda N,N,N’,N’ -tetramethylethylenediamine eq equivalent η eta, prefix for a coordinating C atom, (hapticity) Et ethyl FT-IR Fourier transform-infrared ν frequency, stretching frequency (IR) FLP frustrated Lewis pair GC gas chromatography sept septet (NMR) hmds hexamethyldisilazane HOMO highest occupied molecular orbital iPr iso -propyl κ kappa, prefix for a coordinating heteroatom kcal kilocalorie SOMO Singly occupied molecular orbital LUMO lowest unoccupied molecular orbital MS mass spectrometry Mes mesityl (2,4,6-trimethylphenyl) Me methyl μ mu, prefix for a bridging ligand tmeda N,N,N’, N’-tetramethylethylenediamine IPr N,N’-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene DMF N,N-dimethylformamide xx

NHC N-heterocyclic carbene NMR nuclear magnetic resonance J nuclear spin-spin coupling constant ppm parts per million - Cp* pentamethylcyclopentadienyl (C 5(CH 3)5 ) Ph phenyl π pi orbital HBpin pinacolborane HBcat catecholborane h Planck’s constant Py pyridine RT room temperature σ sigma orbital s/d/t/m singlet/doublet/triplet/multiplet (NMR) tBu tert -butyl THF tetrahydrofuran OTf trifluoromethanesulfonate (triflate) UV ultraviolet VT variable temperature λ wavelength

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1 Introduction

1.1 CO 2 activation

Discovery of new fundamental reactivity of CO 2 is of interest because it may lead to new 1 ways to sequester CO 2 and utilize CO 2 as a C 1 feedstock for synthesis. This task is challenging due to the considerable thermodynamic stability of the CO 2 molecule. Known reactivity patterns 2 of CO 2 include coordination to metal centres, insertion into M–X bonds (where M is a metal 3-5 center and X is an element, most commonly H or C) and adduct formation with a Lewis base, with or without the assistance of a Lewis acid (Scheme 1.1). 6-8 These fundamental modes of reaction have resulted in the catalytic conversion of CO 2 into a variety of reduced products, which is a topic that has been reviewed 5, 7, 9-13ab Many of the examples discussed in the following section involve the transformation of CO 2 into a derivative of methanol. When H 2 is used as reductant, this reaction is exothermic and is favoured at mild temperatures 10b (Scheme 1.2). However, more reactive hydride sources such as boranes or silanes are often used to provide a kinetic pathway for the reduction of CO 2.

Scheme 1.1 Three broad reactivity patterns of CO 2

Scheme 1.2 Hydrogenation of CO 2 to methanol.

1.1.1 Metal-free CO 2 activation

Before the discovery of hydrogen splitting by bulky Lewis acids and bases 14 led to the articulation of Frustrated Lewis Pairs (FLPs), Piers reported what is now classified as an FLP- type hydrosilylation of carbonyl compounds. 15 When Stephan and Erker demonstrated that both 16 intra and intermolecular FLPs were capable of activating CO 2 (Scheme 1.3) chemists flocked to

1 2

apply the concerted action of Lewis acids and bases to activate and transform CO 2 without using metals.

Of course, the formation of a stable CO 2 activation product by an FLP is not necessary in order to catalytically transform CO 2, as demonstrated by Fontaine’s o-phenylene-bridged 17 phosphinoborane 1-Ph 2P-2-Bcat-C6H4 (Scheme 1.4). 1-Ph 2P-2-Bcat-C6H4 is an extremely fast catalyst for the hydroboration of CO 2: HBpin, HBcat, and BH 3(SMe 2) are all suitable reductants, and turnover frequencies of up to 853 h -1 were achieved. Subsequent mechanistic work indicated that weak binding of CO 2 by 1-Ph 2P-2-Bcat-C6H4 was advantageous from an energetics point of view, 18 and that the phosphinoborane precatalyst binds tightly to intermediately formed formaldehyde. The formaldehyde adduct serves as the active catalyst. 19 Another notable feature of this system is the use of the weakly Lewis acidic Bcat moiety as opposed to the strongly electron withdrawing pentafluorophenyl groups that often feature in FLP systems.

Scheme 1.3 Stephan and Erker’s FLP activation of CO2

Scheme 1.4 Fontaine’s highly active metal free CO 2 hydroboration catalyst

Hydroboration and hydrosilylation of CO 2 have been interesting to study. Results of this study have provided proof of the concept of catalytic reduction of CO 2, and have revealed the power of cooperative reactivity of Lewis acids and bases. However, given the high cost of the borane and

3 silane reductants, these processes are impractical. A promising alternative is the use of molecular hydrogen as reductant, which would be both more atom-economical and less expensive.

However, so far only two metal free systems are capable of hydrogenating CO 2, namely the 20 21 combination of 2,2,6,6-tetramethylpiperidine/B(C 6F5)3 and 1-BMes 2-2-NMe 2-C6H4.

In addition to FLPs, strong Lewis bases have been used in CO 2 reduction. Ying employed 22 N-heterocyclic carbene IMes in the hydrosilylation of CO 2 with diphenylsilane, which is the first example of metal free CO 2 reduction catalysis; solvent optimization revealed DMF to be most suitable for this reaction. Interestingly, hydrosilylation of CO 2 in DMF was found to proceed in the absence of catalyst. 23 Cantat also employed NHCs 24 as well as common nitrogen 25 bases (Scheme 1.5) to the hydrosilylation of CO 2 in the presence of amines. The amine functions as a trap for reduced CO 2, and the net result of the reaction is the N-formylation of amines. Switching to boranes and employing phosphatranes (PA) as catalysts allowed access to the fully reduced N-methyl amines, representing the first metal free catalysts for the methylation 26 of amines using CO 2.

Scheme 1.5 Cantat’s metal free CO2 reduction catalysts.

1.1.2 Metal-Ligand Cooperation for CO 2 activation

Organometallic chemists have been interested in metal-ligand cooperation (MLC), i.e . reactivity in which metal and ligand both participate in a bond forming/breaking reaction with an incoming substrate. An early example of MLC is the reaction of Fryzuk’s PNP Ir(III) complex with dihydrogen (Scheme 1.6). 27 Dihydrogen is heterolytically split across the Ir-N bond, forming Ir-H and N-H moieties. Here the Ir (metal) and amido (ligand) “cooperate” in splitting

H2. Years later, Milstein and coworkers discovered another example of reversible H 2 activation 28 via MLC (Scheme 1.7). In their PNN pincer-Ru complex, H 2 is split between the basic benzylic

4 carbon and the coordinatively unsaturated Ru centre. A key feature of this system is the ability of the pincer ligand’s central N-donor to exist in deprotonated enamido and neutral pyridine forms.

Scheme 1.6 Fryzuk’s heterolytic splitting of dihydrogen by MLC

H H H PtBu t 2 P Bu 2 H H2 H N Ru N Ru CO CO H NEt 2 NEt 2

Scheme 1.7 Milstein’s heterolytic splitting of dihydrogen by MLC

Years later, Huff and Sanford used the same Ru(PNN) platform to demonstrate reversible 29 CO 2 activation by MLC (Scheme 1.8). The benzylic carbon alpha to phosphorus attacks the electrophilic CO 2 carbon, forming a carboxylate which binds the Ru centre at its vacant site.

Similar to the H 2 activation using this platform (Scheme 1.7), the pyridine concomitantly rearomatizes. Heating the CO 2 adduct causes isomerisation to the thermodynamic product in which the benzylic carbon alpha to nitrogen is carboxylated. Milstein also observed reversible 30 CO 2 activation by MLC using a related Ru(PNP) system.

Scheme 1.8 Milstein and Sanford’s investigation of CO 2 activation by MLC

A prominent example of CO 2 activation by MLC applied in a catalytic fashion is in the 31 hydrogenation of CO 2 to formate reported by Hazari (Scheme 1.9). In this work, an Ir(III) trihydride inserts CO 2 into the Ir-H bond ( trans to the other hydride) with assistance from the secondary amine N-H proton. Notably, the CO 2 insertion product precatalyst is air and moisture stable. Moreover, the catalysis occurs in aqueous solution, and turnover numbers as high as 348,000 were achieved.

Scheme 1.9 Hazari’s CO 2 activation by MLC and reduction to formate

1.1.3 Other stoichiometric reactivity of CO 2

Dielmann has synthesized imidazolin-2-ylidenaminophosphines (IAPs) which are 32 extremely electron rich in character, and are capable of reversibly binding CO 2 (Scheme 1.10). 33 In the case of the IAP triply substituted with electron-releasing NIPr groups, the CO 2 adduct undergoes a thermal rearrangement in which one of the C-O bonds has been cleaved and there has been a formal insertion of CO into one of the P-N bonds. Another recent development in the 34 CO 2 chemistry of phosphorus-containing compounds was reported by Schulz. Open-shell singlet biradicaloid [PNTer] 2 features phosphorus in the 2+ oxidation state, and deoxygenates

CO 2 in a reduction reaction with liberation of carbon monoxide (Scheme 1.11).

Scheme 1.10 Dielmann’s electron rich phosphines bind CO 2

Scheme 1.11 Schulz’s deoxygenation of CO 2 using a P 2N2 ring

1.1.4 Insertions of CO 2

Borohydrides are capable of inserting CO 2 into their B-H bonds, which has been shown - recently by Cummins’ demonstration that BH 4 salts can sequester three equivalents of CO 2 to - 35 yield [HB(OCHO) 3] . Okuda demonstrated that his alkali metal triphenylborohydride precatalysts for hydroboration of carbonyl compounds are also capable of CO 2 insertion into 36 their B-H bonds. Kemp and Waterman observed an unexpected reaction of CO 2 with their 37 Zn(II) complexes of dearomatized phosphinopyridine ligands. One equivalent of CO 2 inserts into the C-Si bond of each ligand; this reactivity is the first example of direct insertion into a C- Si bond (Scheme 1.12). Several intermediate species were observed by 31 P NMR, and the authors posit that the formal insertion takes place via initial carboxylation of the nucleophilic benzylic carbon in the starting material followed by rearrangement to give a net insertion process.

Scheme 1.12 Kemp and Waterman’s unexpected insertion of CO 2 into C-Si bonds

Shoji and coworkers synthesized dimesitylborinium salts which are highly Lewis acidic 38 and are reactive towards CO 2. Presumably, CO 2 coordinates to the boron centre via one of its oxygen atoms before mesityl transfer to the carbon atom of CO 2 and C-O bond cleavage

(Scheme 1.13). The products of the CO 2 arylation/deoxygenation reaction are the aroyl cation and the unstable oxoborane. Piers et al . recently activated CO 2 in ring expansion reactions using the reactive C–B bonds of isomeric doubly reduced diborole and bis-cycloborabutylidene

derivatives (Scheme 1.14). The authors posit that CO 2 catalyzes the rearrangement of the bis- cycloborabutylidene into the diborole, which reacts with CO 2 in a pair of ring expansions from five to six members, distinct from simple insertion into the reactive C-B bonds. 39

Scheme 1.13 CO 2 arylation/deoxygenation by borinium.

Scheme 1.14 Piers’ reduced boroles ring expand with CO 2

1.2 Redox-active ligands

Redox-active ligands are ones at which a well-defined redox process occurs; 40 examples have been known since Gray’s work on nickel dithiolenes. 41, 42 Study of metal -based oxidation and reduction has led to numerous discoveries that underpin the essential field of catalysis, but comparatively less investigation has focused on ligands that can adopt more than one oxidation state. Ideally, such ligands will work in concert with metals to accomplish multielectron transformations that metal complexes of redox-inactive ligands cannot.

1.2.1 Redox-active vs. noninnocent

Throughout the history of coordination chemistry, the terms “redox-active” and “noninnocent” have at times been used interchangeably. In 1966, Jørgensen said “Ligands are innocent when they allow oxidation states of the central atoms to be determined.” 43 implying that non innocent ligands do not allow the oxidation states of the central atoms to be determined. At present, technological advances have led to improvements in spectroscopy and computational methods that, in many cases, allow for a physical oxidation state to be determined. As Chirik 44 2 points out, by definition the C 2H4 ligand in Cp* 2Ti(η -C2H4) is considered noninnocent, because it is ambiguous whether Ti is in the 4+ or 2+ oxidation state. For this reason, distinction is now made between the terms “redox-active” and “noninnocent.” Some of the successes of redox-active ligands are detailed in recent reviews. 40, 45, 46 Selected examples of high relevance to this thesis will be highlighted in the following section.

1.2.2 Pyridine diimine ligands

The redox-activity of pyridine diimine (PDI) ligands has been investigated by the groups 47-49 of Chirik, Wieghardt, and DeBeer (Scheme 1.15). (PDI)FeCl 2 contains a neutral PDI ligand bound to Fe(II); reduction of (PDI)FeCl 2 by one electron yields (PDI)FeCl , which is a Fe(II) complex ligated by a monoanionic PDI ligand. Reduction of (PDI)FeCl by one more electron causes two molecules of dinitrogen to bind iron to form (PDI)Fe(N 2)2, which is best described as a π-acidic redox noninnocent PDI ligand bound to a hybrid of Fe(II) and Fe(0) forms. In solution, one N 2 ligand dissociates to yield four-coordinate (PDI)Fe(N 2), which can be assigned as an Fe(II) centre bound by a dianionic PDI ligand. Among the interesting reactivity displayed by

(PDI)Fe(N 2), a standout example is the C-C bond oxidative addition of biphenylene which occurs via metal-ligand cooperation. 50 In this two-electron transformation, one electron originates from the Fe(II) centre which becomes Fe(III), and the other electron comes from the dianionic PDI ligand, which becomes monoanionic in the product. Catalytic application of these and other PDI complexes has been reviewed. 51 Even more recently, ligand tuning of PDI scaffolds has led to a breakthrough in catalytic intermolecular cycloaddition of olefins. 52

Scheme 1.15 Chirik’s bis(imino)pyridine Fe(II) complexes.

1.2.3 Multielectron redox chemistry of d 0 tantalum complexes

Since 2005 Heyduk’s group has investigated the reactivity of d 0 metal complexes bearing redox-active ligands; much of that chemistry has been summed up in a recent review contrasting the reactivity of redox active ligands with redox inactive ligands. 53 Heyduk has employed several different redox-active ligand platforms including o-phenylenediamine-based ligands, which will be covered in a subsequent section of this thesis. In the present example, a trianionic NNN pincer ligand on Ta(V) was used to demonstrate some 1 e - and 2 e - chemistry (Scheme 1.16).

(NNN)TaCl 2 reacts with PhICl 2 to generate the open shell species (NNN)TaCl 3. EPR and X-ray data indicate that the unpaired electron resides on the NNN ligand. Also, (NNN)TaCl 2 reacts with either aryl azides or diazoalkanes to yield species of the form (NNN)Ta(NX)Cl 2 (where X =

R or N=CR 2). In these species, the NNN ligand has oxidized to its monoanionic form, and the electrons have gone into the imido ligands. Ligand tuning of this system facilitated the catalytic coupling of nitrenes to yield diazenes. 54

Scheme 1.16 Heyduk’s 1 e - and 2 e - reductions using a d 0 metal complex 53

1.2.4 Negishi-type coupling using redox active ligands on Co

In 2010, Soper et al . reported the synthesis of a square planar Co(III) complex ligated by 56 - two dianionic redox-active amidophenolate ligands. This compound, [Co(amidophenolate) 2] reacts with one of the C-Cl bonds of dichloromethane to generate the square pyramidal species

[Co(iminosemiquinonate) 2(CH 2Cl)] (Scheme 1.17). The two electrons necessary for this redox reaction come not from the Co centre, but from two one electron oxidations of each amidophenolate ligand. Structurally related compound [Co(iminosemiquinonate) 2(CH 2CH 3)] reacts with PhZnBr to liberate the cross-coupling product ethylbenzene with concomitant - regeneration of [Co(amidophenolate) 2] . Taken together, these two reactions represent the important “oxidative addition” and “reductive elimination” steps of Negishi-type cross coupling, without the metal centre needing to supply any electrons.

Scheme 1.17 Soper’s Negishi-type C-C coupling 55

1.3 N2 activation

Dinitrogen activation refers to a weakening of the N-N bond in the N 2 molecule, which is a challenging task due to its notoriously unreactive nature. N 2 is ubiquitous in chemistry laboratories: it is used as an inert gas to protect sensitive compounds from undesired reactions with O2 or H 2O. Ideally, dinitrogen activation leads to conversion to a useful product, such as 8 NH 3. The Haber-Bosch process is responsible for 10 tons of NH 3 produced each year, and 57 involves the heating of high pressures of N2 and H 2 over a Ru or Fe catalyst. Certain species of bacteria can also convert N2 to NH 3; they do so using the enzyme Nitrogenase. At the active site of this enzyme is the FeMo cofactor, which contains seven Fe atoms and one Mo atom 58 (Figure 1.1).

Figure 1.1 Structure of the FeMo cofactor.

For synthetic inorganic chemists, the field of N2 activation was born in 1965 when Allen 59 and Senoff showed that N 2 could be used as a ligand in coordination chemistry (Scheme 1.18). Their discovery of the first dinitrogen complex was characterized by IR spectroscopy and -1 displayed an absorption due to the N 2 stretch at 2129 cm . Binding, activating, and transforming 60,61 N2 using soluble metal complexes continues to be a topic of much current interest.

2+ N N NH 3 N2H4 H2O 2X- RuCl 3 H3N Ru NH 3 25 oC H O 2 H3N NH 3

Scheme 1.18 Synthesis of the first N 2 complex

An important N 2 activation system is the [(NPN)Ta] system developed by Fryzuk’s group (Scheme 1.9, NPN = diamidophosphine ligand). 62,63 In a demonstration of the “hydride route” to 64 dinitrogen complexes, [(NPN)TaMe 3] was subjected to hydrogenolysis conditions. Reaction with H 2 liberated all methyl ligands as methane, leaving four bridging hydride ligands between two Ta centres in the dimeric product of this reaction. The bridging tetrahydride then spontaneously activated one equivalent of N 2, generating an N2 complex in which the N 2 ligand is bound side-on to one Ta centre and end-on to the other Ta centre, bridging between both Ta atoms. As it turned out, this new binding mode for N 2 rendered the N atoms reactive towards silylation by BuSiH 3, yielding a bridging bis-silylimido species in which the N 2 ligand had been completely cleaved, and new N-Si bonds had been formed.

Ph Ph Ph H P Ph Me Ph N N 5 H H SiMe 2 2 Me Si 2 Me 2Si N 2 N Ta Ta Ta Me N SiMe 2 Me Si H Me 2Si Me 2 N Ph H PPh PPh Ph Bu N2 H Si Ph Ph Ph 2 Ph H Ph P P N N SiMe Ph N SiMe 2 2 BuSiH H 2 Me 2Si N Ta Ta 3 Me 2Si N Ta Ta N SiMe 2 N SiMe 2 Me 2Si Me Si N N N Ph 2 N Ph P P N SiH Ph Ph Ph 2 Ph Bu

Scheme 1.19 Binding and silylation of dinitrogen on Fryzuk’s [NPNTa] platform

One seminal finding in the field of N 2 chemistry emerged from Holland’s group in 2011. 65 After extensive investigations of the chemistry of low-coordinate Fe complexes 66 sponsored by bulky nacnac ligands, the researchers made slight modifications to their N 2- binding platform. They decreased the steric bulk of the aryl groups down from 2,6- diisopropylphenyl to 2,6-dimethylphenyl, and used a nacnac backbone with a methyl group at the beta carbon. The combination of these subtle changes had a great effect on the degree of N 2 activation of the iron complexes(Scheme 1.20). Upon reduction of the chloride-bridged dimer

with KC 8, the low-valent Fe intermediate bound N 2 and self-assembled into a [K 2Fe 4] bis-nitride complex in which dinitrogen’s N-N bond was completely cleaved. The high significance of this report derives from it being the first iron complex to completely split the N-N bond in N 2. Originally, the authors claimed that reaction of the bis-nitride with hydrogen produced appreciable yields of NH 3; however, they have since found that NH 3 is not actually produced in the reaction with H 2.

Scheme 1.20 Holland’s synthesis and hydrogenation of a bis-nitride complex

1.4 NN chelate actor ligands

In his textbook, Crabtree differentiates between actor and spectator ligands: “spectator ligands remain unchanged during chemical transformations. Actor ligands dissociate or undergo some chemical conversion.” 67 Spectator ligands influence the steric and electronic environment around the metal centre, which remotely impacts upon the metal’s reactivity. A small change to a spectator ligand can cause a large change in a chemical outcome, 68 which is why research in this area is still vibrant. A recent virtual issue of Inorganic Chemistry was devoted to spectator ligand design. 69 Actor ligands have attracted much attention due to the new possibilities that open up when ligands involve themselves in chemical transformations. 70 The two ligand classes that will be discussed in this section are 4,5-diazafluorene and o-phenylenediamine.

1.4.1 4,5-diazafluorene

4,5-Diazafluorene is a versatile ligand; the metal complexes it supports have been applied in catalysis, photochemistry, and bioinorganic chemistry. The coordination chemistry of diazafluorene derivatives is the subject of a recent review. 71 In the context of this thesis however, the characteristic of diazafluorene derivatives that will be explored is the ability to behave as actor ligands. Diazafluorene is a tricyclic molecule in which two pyridine rings are joined by a methylene unit. First reported in 1977, 72 it is synthesized in two steps from 1,10- 73 phenanthroline. The methylene protons on the C 5 ring of diazafluorene, analogous to cyclopentadiene (CpH,) which has a pKa = 18.0 in DMSO, 74 can be deprotonated to yield an anionic species (Scheme 1.21). This electron-rich molecule bears electron density in the form of two nitrogen lone pairs, and also at the Cp-like C 5 ring.

Scheme 1.21 Numbering scheme/deprotonation of diazafluorene

H H H H H

KO t-Bu H2 -N 2

N N N2 thf N N N N -H2 N2 Ph 3P Ru Cl Ph 3P Ru PPh 3 Ph 3P Ru PPh 3

Ph 3P H H N2 H H CO 2 HBcat ClBcat or -[Ru]HCl -H - CO 2 2 Bcat O OH Bcat

N N C D 6 6 Ph 3P Ru PPh 3 N N N N D -HD N2 Ph 3P Ru PPh 3 Ph P Ru PPh 3 3 D D H N2 H N2 D D

Scheme 1.22 Small molecule activation using Ru(II)diazafluorenyl species.

Previous work in the Song group exploited the acidic nature of coordinated diazafluorene 75 (dafH) to synthesize zwitterionic complexes such as [Ru(PPh 3)2(daf)(H)(N 2)] (Scheme 1.22), which has shown some interesting reactivity with respect to its actor diazafluorenyl ligand. First of all, that complex reacts with molecular hydrogen in a MLC fashion, where H 2 is heterolytically split between the anionic diazafluorenyl carbon and the cationic Ru(II) centre. The diazafluorenyl ligand acts in concert with the metal centre. This takes advantage of the labile

N2 ligand, which dissociates to facilitate the transformation. When [Ru(PPh 3)2(daf)(H)(N 2)] was placed under CO 2, insertion of CO 2 into the C-H bond of the diazafluorenyl ligand occurred, 76 yielding a carboxylic acid. This result is interesting because CO 2 did not displace the N 2 ligand or insert into the Ru-H; both modes of reactivity are more common than insertion into the C-H bond. This transformation comes with a distinction from the previous H 2 reactivity: MLC is not involved, rather the ligand acts while the metal spectates. This tandem CO 2 and C-H activation + 77 reactivity was elaborated to include complexes of Rh, Cu, and CH 3 . The discovery that metal- free diazafluorenyl compounds were capable of activating CO 2 has been extended to the catalytic 78 reduction of CO 2 with HBpin, HBcat, 9BBN, and BH 3(SMe 2). Importantly, this finding shows that neither metal nor ligand is essential for tandem CO 2 and C-H activation.

A question arose as to whether the 9-position carbon could be functionalized to change the reactive C-H bond into another carbon-element bond. Experiments performed by former MSc student Adam Pantaleo showed that reaction of either HBcat or ClBcat with

[Ru(PPh 3)2(daf)(H)(N 2)] furnished the C-B bond containing compound, but its subsequent reaction with arene solvent complicated its isolation.

1.4.2 o-phenylenediamine

Ortho -phenylenediamine (opda) derivatives are classic examples of redox active ligands; this has been known for decades. In the 1960s Holm synthesized late transition metal complexes of the parent opda (R = H, Scheme 1.23) in various oxidation states, and studied their electrochemical behavior. 79,80 N,N’ -disubstituted opda ligands have been studied with respect to electronic structure by Wieghardt’s group, 81-84 by Sarkar’s group for bond activation/catalysis using Co 85 and Ir 86, and by Hahn’s group for stabilizing heavy NHC analogues. 87-90 One particularly notable example of work with opda ligands emerged from Heyduk’s group (Scheme 1.24). 91Addition of halogens to a five coordinate Zr(IV) complex ligated by two opda ligands brought about “oxidative addition” to the d 0 zirconium centre. This seemingly counterintuitive reaction is made possible via the redox-active opda ligands. Each opda ligand gives one electron to provide the reducing power. Interestingly, the radical opda ligands in the product approach each other closely (~3Å) in a bonding-type interaction between the SOMO of each ligand.

Scheme 1.23 Three redox forms of o-phenylenediamine

0 Scheme 1.24 Oxidative addition of X 2 to a d Zr complex

More recently, Smith communicated some of his group’s work on the reactivity of a [CpCr(opda)] system towards azides. 92 Distinct from Heyduk, the metal centre of interest (Cr) and the redox-active opda ligand can adopt multiple oxidation states, which enriches the reactivity considerably (Scheme 1.25). Depending upon the nature of the R group on the opda ligand, [CpCr(opda)] reacts with one equivalent of MesN 3 to generate [CpCr(NMes)(opda], a process which does not involve oxidation of the opda ligand; alternatively, two equivalents of

MesN 3 react to yield [(opda)Cr(NMes) 2] in which the Cp ligand has fallen off and one electron oxidation of the opda ligand has occurred. Again, depending upon the nature of the R group on the opda ligand, [CpCr(opda)] shows different patterns of reactivity towards sulfonyl azide

TrisylN 3. On one hand, [CpCr(opda)] reacts with one equivalent of TrisylN 3 to yield [CpCr(opda)(NHTrisyl)] in which one electron oxidation of the opda ligand has occurred, and a hydrogen atom has been abstracted from the solvent by the imido ligand. On the other hand

[CpCr(opda)] catalyzes the intramolecular cyclization of TrisylN 3 brought about by nitrene insertion into the nearby benzylic C-H bond. Taken together, this reactivity shows opda working together with Cr to do redox chemistry, resulting in diverse reactivity.

Scheme 1.25 Smith’s [CpCr(opda)] system for imido formation, MLC, and catalytic cyclization

1.5 Scope and objectives

Broadly stated, the objective of this thesis was to activate small molecules using metal complexes supported by NN chelate actor ligands. Chapter 2 comprises the first of the thesis’ two parts. Given the increasing importance of CO 2 in our world, expanding the fundamental

reactivity of CO 2 is beneficial, as it may lead to new ways to use CO 2 as a C 1 feedstock. To this end, Chapter 2’s objective was to demonstrate new fundamental reactivity of CO 2 employing actor diazafluorenyl ligands. Efforts in this chapter built on previous work in the Song group which detailed the CO 2 chemistry of zwitterionic metal complexes with reactive C-H bonds culminating in a metal-free, tandem CO 2 and C-H activation. We sought to construct diazafluorenyl complexes containing similarly reactive C-B bonds, and to test their reactivity towards insertion of CO 2, which was an unknown mode of reactivity prior to this work. In this thesis we demonstrate the catalytic hydroboration of CO 2 using parent Zn(II) and Ru(II) diazafluorenyl complexes. Heating these parent complexes with excess HBpin brought about borylation of the carbanionic 9-position of the diazafluorenyl ligand. As expected, the newly constructed C-B bond of the borylated diazafluorenyl ligand inserts one equivalent of CO 2 in both Zn(II) and Ru(II) cases.

Chapters 3-5 comprise the second part of the thesis, in which the study of o- phenylenediamido (opda) ligands is discussed. The primary objective of this part of the thesis was to bind and activate dinitrogen at Fe, V, Mo, or Ta complexes supported by opda ligands. The ultimate goal of such research is catalytic synthesis of value-added N-containing compounds directly from N 2. Chapter 3 details the initial forays into the coordination chemistry of opda ligands towards Fe, V and Mo. A dipp-substituted opda was doubly deprotonated, and the reactivity of the resulting dilithium salt was investigated as a ligand transfer agent. When reacted with iron sources, the Fe(opda) fragment that forms is trapped by an η 6-toluene ligand which prevents N 2 from binding the Fe centre. When the dilithium salt was reacted with MoCl 4(THF) 2, oxidation of the opda ligand framework occurred to yield a Li radical complex which was characterized by EPR. When two equivalents of dilithium salt were reacted with MoCl 4(THF) 2 or 4 VCl 2(tmeda) 2, η -coordination to the metal was achieved through the o-phenylene backbones of two opda ligands, forming trimetallic [LiMLi] (M = V or Mo) sandwich complexes. The [LiVLi] complex displayed interesting reactivity towards PbCl 2, namely that the Pb atom displaces other metals and takes up residence at the NN chelate site to form the N-heterocyclic plumbylene LPb. Given the rich redox active nature of the opda framework revealed in chapter 3, we decided on a brief departure from the goal of N 2 activation which led us on an excursion through main group chemistry, which is the subject of Chapter 4.

Given the great importance of carbenes across the chemical disciplines, research into the less-studied heavier group 14 analogues is necessary because it may reveal potentially beneficial and complimentary properties. To this end, opda ligands were applied in Chapter 4 to heavy group 14 metals in the 2+ oxidation state. The reactivity of these N-heterocyclic stannylenes and plumbylenes towards oxidants was studied. We hypothesized that the electrons in the redox- active opda framework were available to do chemistry in cooperation with the main group metal centre. Our objective was to accomplish a chemical transformation made possible by our combination of redox active opda ligand and main group metal centre. Reactions of LPb and LSn with mesityl azide allowed observation of nitrene insertions into nearby benzylic C-H bonds, and trimethylamine-N-oxide forms a Lewis pair with the LPb. Reaction of LSn with silver triflate generated a paramagnetic stannylene, which is a type of compound that has never been isolated.

In Chapter 5, Ta complexes of opda were targeted as potential precursors for N 2 activation. Our primary targets were LTaX 3 complexes, where X = alkyl or halide; we endeavoured to follow Fryzuk’s strategy of hydrogenolysis of Me ligands leading to [Ta-H] species that spontaneously react with N 2. When the dilithium salt of the opda ligand was reacted 4 with TaMe 3Cl 2, similar η -binding of the metal centre through the o-phenylene backbone occurred, as well as retention of the Li cation at the NN chelate site. In an effort to sterically 4 prevent the [TaMe 3Cl] fragment from binding η to the o-phenylene backbone, a doubly 2- methylated dilithium salt Li 2L’was reacted with TaMe 3Cl 2, which led to oxidation of L and isolation of a LiL’ complex. Finally, when the dipotassium salt of L 2- was employed, the desired

NN chelates of Ta were isolated. Preliminary reactivity of the LTaMe 3 species towards insertions and abstractions was investigated, but no N 2 complexes were able to be isolated.

Chapter 6 contains a summary and recommendations for future work.

The experimental work herein was performed by the author, including the collection of X-ray crystallographic data. X-ray structures were solved by the author and Prof. Datong Song. DFT calculations in chapter 2 and 3 were performed by Prof. Datong Song. In chapter 2, compounds 1b -3b were initially synthesized and partially characterized by undergraduate student Ellen Yan, while under the supervision of MSc student Adam Pantaleo. The protocol for catalytic hydroboration was developed by Yanxin Yang, and Dr. Kimberly Osten performed some of the catalytic experiments. The EPR spectrum of 3.5 was obtained and simulated by Prof.

Jeremy Rawson of University of Windsor. The EPR spectrum of 4.4 was obtained with the assistance of Dr. Sergiy Nokhrin and simulated by the author. In chapter 5, the EPR spectrum of 5.4 was obtained with the assistance of Dr. Timothy Burrow and simulated by the author. Compounds 4.1a and 4.2b were synthesized and characterized by Pavel Zatsepin, an undergraduate supervised by the author. Compound 5.8 was synthesized and partially characterized by Maotong Xu, an undergraduate student supervised by the author.

Parts of chapters 2-5 of this thesis have been published:

Chapter 2:

• Janes, T.; Osten, K. O.; Pantaleo, A.; Yan, E.; Yang, Y.; Song, D.“Insertion of CO 2 into the carbon-boron bond of a boronic ester ligand” Chem. Commun ., 2016, 52 , 4148-4151.

Chapter 3: • Janes, T.; Rawson, J. M.; Song, D. “Syntheses and structures of Li, Fe, and Mo derivatives of N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamine” Dalton Trans ., 2013, 42 , 10640-10648.

Chapter 4: • Janes, T.; Zatsepin, P; Song, D. “Reactivity of heavy carbene analogues towards oxidants: a redox active ligand-enabled isolation of a paramagnetic stannylene” Chem. Commun., 2017, 53 , 3090-3093.

Chapter 5:

• Janes, T.; Xu, M.; Song, D. “Synthesis and reactivity of Li and TaMe 3 complexes supported by N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamido ligands” Dalton Trans ., 2016, 45 , 10672-10680.

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2 Chapter 2 Insertion of CO 2 into the carbon—boron bond of a boronic ester ligand 2.1 Abstract

In chapter 2, actor diazafluorenyl ligands are explored. New Ru and Zn diazafluorenyl complexes undergo C–H borylation of the diazafluorenyl ligand to form the corresponding diazafluorenylboronic ester complexes, which can insert CO 2 into their C–B bonds to form boryl ester functionalities. The relevance of this new reactivity to catalytic CO 2 reduction is also explored.

2.2 Introduction

Discovery of new fundamental reactivity of CO 2 is of interest because it may lead to new 1 ways to sequester CO 2 and utilize CO 2 as a C 1 feedstock for synthesis. Known reactivity of CO 2 2 includes coordination to metal centres, insertion into M–X bonds (where M is a metal center and 3-5 X is an element, most commonly H or C) and adduct formation with a Lewis base (with or without the assistance of a Lewis acid). 6, 7 These fundamental modes of reaction have resulted in the catalytic conversion of CO 2 into a variety of reduced products, which is a topic that has been 5, 7-11 12-31 reviewed and has seen a flurry of recent progress. Our group has demonstrated formal insertion of CO 2 into the C–H bond of an actor diazafluorenyl (daf) ligand supported by a 32 33 spectator metal center. We elaborated this work to include metal-free insertions, and catalytic 28 hydroboration of CO 2. Although we have not yet studied the catalytic mechanism in detail, one plausible pathway involves borylation of the diazafluorenyl moiety and subsequent insertion of

CO 2 into the newly formed C–B bond. To our knowledge, the direct insertion of CO 2 into a C–B bond is unknown prior to this work, and represents a new mode of reactivity for the thermodynamically stable CO 2 molecule.

Related yet distinct C–B bond reactivity was reported by Shoji et al . who employed salts + 34 containing the electrophilic Mes 2B in a deoxygenation-arylation of CO 2. In this reaction, one + mesityl group is transferred to the carbon of CO 2, generating MesC=O . Piers et al . recently activated CO 2 in ring expansion reactions using the reactive C–B bonds of isomeric doubly 35 reduced diborole and bis-cycloborabutylidene derivatives. Two examples of related insertions

of CO 2 have also recently emerged: Waterman et al . observed an unexpected insertion of CO 2 into reactive C–Si bonds in Zn complexes chelated by a pair of 2-(phosphinomethyl)- pyridine 36 ligands, and Knopf and Cummins demonstrated formal insertion of CO 2 into three B–H bonds - 37 in borohydride salts to form [HB(OCHO) 3] . Herein we report the reactivity of

[Ru(CO)(H)(daf)(PPh 3)2], 1a and [Zn(daf)(Mes 2nacnac)], 1b toward pinacolborane (HBpin) to generate the corresponding complexes 2a and 2b of an actor diazafluorenylboronic ester ligand.

This actor ligand inserts CO 2 into its C–B bond, which is an unprecedented mode of reactivity for the boronic ester functional group. We also report the catalytic activitity of these complexes towards CO 2 reduction with catecholborane (HBcat) and HBpin.

2.3 Results and Discussion

As shown in Scheme 2.1, 1a was synthesized in 59% yield from the reaction of Na(daf) and [RuHCl(CO)(PPh 3)3]. The spectroscopic properties of 1a (Figures 2.1-2.3) are similar to -1 38 1 those of [Ru(H)(daf)(N 2)(PPh 3)2] (ν(N 2)) = 2092 cm ). In the H NMR spectrum, one resonance exists for each of the seven diazafluorenyl protons, and the Ru-hydride resonates as a triplet at - 11.5 ppm and is coupled to two equivalent phosphorus atoms, which resonate in the 31 P{1H} NMR spectrum as one singlet at 48.6 ppm. The characteristic infrared absorption of 1a occurs at 1918 cm -1 due to stretching of the CO ligand. 1b was synthesized by protonolysis in two steps from diethylzinc with an overall 87% yield (solution NMR data shown in Figures 2.4 and 2.5). In the 1H NMR spectrum of 1b one resonance exists for each pair of pyridyl protons, which are related to each other through a mirror plane of symmetry. Protons on nacnac are similarly related, consistent with C 2v symmetry in solution.

Scheme 2.1 Synthesis of 1a and 1b

1 31 1 Figure 2.1 H NMR spectrum (500 MHz, 25 °C) of 1a in C 6D6. Inset: P{ H} NMR spectrum

13 1 Figure 2.2 C{ H} NMR spectrum (126 MHz, 25 °C) of 1a in C 6D6.

Figure 2.3 FT-IR Spectrum of 1a as a nujol mull.

1 Figure 2.4 H NMR Spectrum (500 MHz, C 6D6, 25 °C) of complex 1b .

13 1 Figure 2.5 C{ H} NMR Spectrum (126 MHz, C 6D6, 25 °C) of 1b

The solid state structure of 1b as determined by single crystal X-ray crystallography

(Figure 2.6) features a ZnN 4 core with distorted tetrahedral geometry. The diazafluorenyl ligand chelates Zn1 with an N1-Zn1-N2 bite angle of 88°, which is larger than the N3-Zn1-N4 bite angle of 100° formed by the nacnac chelate. Diazafluorenyl and the plane containing N3-Zn1-N4 are nearly perpendicular (dihedral angle = 88°). Notably, the Zn-N bonds made by diazafluorenyl have statistically different lengths: Zn1-N1 is 2.060(2) Å and Zn1-N2 is 2.103(2) Å. This lack of symmetry in the solid state (likely resulting from lattice packing effects) contrasts with the symmetric solution-state structure suggested by the NMR data. Both Zn-N-(daf) bonds are longer than the Zn-N-(nacnac) bonds of 1.959(2) Å for Zn1-N3 and 1.952(2) Å for Zn1-N4.

Figure 2.6 Molecular structure of 1b . H atoms omitted for clarity. Ellipsoids shown at 30% probability. Selected bond lengths (Å) and angles (°): Zn1-N1 2.059(2), Zn1-N2 2.106(2) Zn1- N3 1.950(2), Zn1 N4 1.958(2); N3-Zn1-N4 100.09(8) N3-Zn1-N1 120.96(8), N4-Zn1-N1 113.75(8), N3-Zn1-N2 118.95(8), N4-Zn1-N2 116.09(8), N1-Zn1-N2 88.13(8).

With 1a and 1b in hand, we investigated the stoichiometric reactivity of the diazafluorenyl complexes towards HBpin: when a toluene solution of complex 1a is heated with

HBpin, 2a is formed with concomitant formation of H 2 (Scheme 2.2). Presumably, the formation of 2a starts with the formation of an adduct between the carbanion of 1a and HBpin. This interaction brings B–H and C–H bonds into close proximity, which allows the loss of H 2. In the transformation from 1a to 2a the diazafluorenyl ligand acts and the Ru centre spectates . Multinuclear NMR and IR spectra for 2a are shown in Figures 2.7-2.10. The borylation of 1a brings about a change in the carbonyl stretching frequency from 1918 cm -1 in the starting material to 1936 cm -1 in the product. The higher C–O bond strength in 2a indicates the electron- withdrawing nature of the newly formed diazafluorenylboronic ester ligand relative to the parent daf ligand in 1a . In C 6D6 at ambient temperature, complex 2a displays a singlet at 48.5 ppm in its 31 1 2 P{ H} NMR spectrum; the hydride resonates at -11.68 ppm as a triplet ( JPH = 20 Hz) and the diazafluorenyl moiety shows one signal for each of its six protons in its 1H NMR spectrum.

Bpin O O H Bpin C

HBpin CO 2 -H N N 2 N N N N [M] [M] [M] 1a, 1b 2a, 2b 3a, 3b 63%, 71% 64%, 67% 1a-3a, [M] = RuH(CO)(PPh 3)2 1b-3b, [M] = Zn(Mes 2nacnac)

Scheme 2.2 Synthesis of 2a/b and 3a/b .

1 Figure 2.7 H NMR Spectrum (500 MHz, 25 °C) of 2a in C 6D6

13 1 Figure 2.8 C{ H} NMR Spectrum (126 MHz, 25 °C) of 2a in C 6D6

31 1 11 1 Figure 2.9 Left: P{ H} NMR Spectrum (202 MHz, 25 °C) of 2a in C 6D6. Right: B{ H} NMR

Spectrum (128 MHz, 25 °C) of 2a in C 6D6. Note: spectrum contains a peak at 21.5 due to pinBOBpin.

Figure 2.10 FT-IR spectrum of 2a as a nujol mull.

Figure 2.11 Molecular structures of 2a and 3a . Ellipsoids shown at 30% probability. Hydrogen atoms except for hydrides omitted for clarity. Only one disordered component shown. Co- crystallized molecules removed for clarity. Selected bond lengths (Å) and angles (°) for 2a : Ru1 H1 1.58(3), Ru1-C12 1.840(3), Ru1-N1 2.171(2), Ru1-N2 2.266(2), Ru1-P2 2.3449(9), Ru1-P1 2.3506(8), B1-C5 1.512(4); N1-Ru1-N2 80.58(9), O3-B1-O2 111.9(3), O3-B1-C5 124.6(3), O2-

B1-C5 123.5(3), C6-C5-C4 105.3(2), C6-C5-B1 127.9(3), C4-C5-B1 126.6(3). Selected bond lengths (Å) and angles (°) for 3a : Ru1 C12 1.811(12), Ru1-N2 2.162(9), Ru1-N1 2.257(9), Ru1- P1 2.343(3), Ru1-P2 2.356(3), C5-C51 1.417(15), O2-C51 1.225(15), O3-C51 1.414(16), O3-B1 1.414(16); N2-Ru1-N1 80.4(3), C4-C5-C51 122.8(11), C4-C5-C6 107.9(9), C51-C5-C6 129.3(11), O2-C51-O3 117.8(10), O2-C51-C5 129.6(12), O3-C51-C5 112.6(11)

As shown in Figure 2.11, the solid state structure of 2a has been confirmed with X-ray crystallography. The structure of 2a features a pseudo-octahedral Ru centre with two mutually trans phosphine ligands and the hydride and carbonyl ligands oriented in a cis fashion. The diazafluorenylboronic ester ligand chelates the Ru centre through its two nitrogen donor atoms. The Ru1–N1 bond length of 2.171(2) is shorter than the Ru1–N2 bond length of 2.266(2) due to the greater trans influence of the hydride ligand relative to CO. The sum of the bond angles around C5 is 359.9(5)1 which suggests sp 2 hybridization. The B atom is three-coordinate and the O2–B1–O3 group is nearly coplanar with diazafluorenyl (a dihedral angle of 61°) such that π- donation of the carbanion into the vacant π orbital on B is possible. The C5–B1 bond length is 1.512(4) Å, which is similar to pinacol esters of other borylated cyclopentadienyl (Cp) 39, 40 compounds (12-crown-4)LiCpBpin (1.488(8) Å) and (Cp2Ru)Bpin (1.537(12) Å). In such compounds the boratafulvene resonance form also contributes to the bonding picture, having C– 2 2 41 B bond lengths between a typical C(sp )–B(sp ) single bond (e.g., 1.58 Å in BMes 3) and a 2 2 42 formal C(sp )–B(sp ) double bond (e.g. 1.444(8) Å in simple borataalkene [Mes 2B=CH 2]). When Zn complex 1b was heated in THF in the presence of HBpin, an analogous borylation of diazafluorenyl occurred and 2b formed. Similar to 1b , the NMR data for 2b reveal a symmetric structure in solution. 2b crystallizes from toluene/pentane with two molecules in the asymmetric unit (Figure 2.16). The distorted tetrahedral ZnN 4 core persists; the metric parameters around Zn are similar to those in 1b , and the metric parameters of the diazafluorenylboronic ester ligand are similar to those in 2a .

1 Figure 2.12 H NMR Spectrum (600 MHz, 25 °C) of 3a in C 6D6

13 1 Figure 2.13 C{ H} NMR Spectrum (126 MHz, 25 °C) of 3a in CD 2Cl 2

31 1 11 1 Figure 2.14 Left: P{ H} NMR Spectrum (243 MHz, 25 °C) of 3a in C 6D6.Right: B{ H} NMR

Spectrum (128 MHz, 25 °C) of 3a in C 6D6.

Figure 2.15 FT-IR spectrum of 3a as a nujol mull.

With 2a and 2b in hand we set about investigating their reactivity towards CO 2. When

CO 2 is introduced to a solution of 2a in toluene–diethylether (1 : 1 v/v), the colour of the solution changes from pink to orange. The formation of 3a can be confirmed by NMR experiments (Figures 2.12-2.14), in which all the 1H signals have shifted slightly compared to those of 2a and an additional 13 C signal appears at 162.3 ppm corresponding to the newly formed ester group from CO 2. In the infrared spectrum of 3a (Figure 2.15), the ester carbonyl stretch appears at 1647 cm -1, and the CO ligand stretch is shifted to 1942 cm -1. X-ray crystallography confirmed the structure of 3a as shown in Figure 2.11. Diazafluorenyl is nearly coplanar with its appended carboxylate group, and the B atom is canted out of this plane, giving an O2–C51–O3–B1 dihedral angle of ~37°. When CO 2 is introduced to a C 6D5Br solution of 2b , a similar change in colour and in spectral data occurs as the boryl ester product 3b forms; we confirmed the structure by X-ray crystallography (see Figure 2.16). The insertion of CO 2 into the C–B bond of 2a and 2b is intriguing from the standpoint of providing a new type of reactivity for the thermodynamically stable and environmentally deleterious CO 2 molecule.

Figure 2.16 Molecular structures of 2b and 3b . Ellipsoids shown at 30% probability. Hydrogen atoms except for hydrides omitted for clarity. Mesityl methyl groups removed for clarity Only one disordered component shown. Co-crystallized molecules removed for clarity. Selected bond lengths (Å) and angles (°) for 2b: Zn1-N1 2.102(4), Zn1-N2 2.100(4) Zn1-N3 1.945(4), Zn1-N4 1.960(4), B1-C5 1.513(8); N3 Zn1 N4 99.28(18), N3-Zn1-N1 118.77(17), N4-Zn1-N1 116.95(17), N3-Zn1-N2 118.56(17), N4-Zn1-N2 117.27(17), N1-Zn1-N2 87.49(16), C6 C5 C4 105.8(4), C6-C5-B1 126.6(5), C4-C5-B1 127.7(5) . Selected bond lengths (Å) and angles (°) for 3b: Zn1-N3 1.933(5), Zn1-N4 1.945(5), Zn1-N1 2.095(5), Zn1-N2 2.102(5), C13-C18 1.415(10), C13-C14 1.461(10), C12 C13 1.428(9), O1-C18 1.403(10), O2-C18 1.198(10); N3 Zn1 N4 99.5(2), N3 Zn1 N1 116.6(2), N4-Zn1-N1 119.0(2), N3-Zn1-N2 113.9(2), N4-Zn1-N2 122.7(2), N1-Zn1-N2 86.4(2), C18-C13-C12 129.7(7), C18-C13-C14 123.9(7), C12-C13-C14 106.3(6). O2-C18-O1 120.0(7), O2-C18-C13 129.4(8), O1-C18-C13 110.5(8).

Figure 2.17 Computed transition state structure of the CO 2 insertion reaction from 2a to 3a Colour key: O, red; N, blue; P, orange; B, green; Ru, pink; C, gray; H, white.

Figure 2.18 Thermodynamic data for the transformation of 2a to 3a

To understand this transformation further, DFT calculations have been used to locate the transition state of the insertion reaction from 2a to 3a and to obtain the thermodynamic data. The transition state of the insertion features a C–C–O–B four-membered ring (Figure 2.17). The CO 2 moiety is off linear by 46.4° with the endocyclic C–O bond elongated by 0.09 Å, while the Bpin moiety is bent away from its original position in 2a by 32.6° (measured by the change in

Cp centroid –C–B angle) to accommodate the incoming CO 2 with the B–C bond elongated by 0.08 Å. The endocyclic B–O and C–C distances are 2.11 and 1.66 Å, respectively, indicating that the C–C bond is largely formed in the transition state, but the B–O bond formation is far from complete. This result prompted us to examine the possibility of a two-step mechanism for the insertion reaction, i.e., C–C bond formation first, followed by the migration of the Bpin moiety. All attempts to locate the C–C formation intermediate failed. Although it is still possible that such an intermediate may sit in an extremely shallow well, our computation is consistent with the -1 one-step CO 2 insertion mechanism. The ΔH of the overall reaction is -15.9 kcal mol , while the ΔHǂ is 13.1 kcal mol -1 (Figure 2.18), so it is unsurprising that the insertion reaction occurs readily at ambient temperature.

a Table 2.1 Results for the hydroboration of CO 2 by HBCat and HBpin

TON from formation of each product b b Entry Cat. Borane T (°C) Time (h) HCO 2BR 2 CH 2(OBR 2)2 CH 3OBR 2 Total TON 1 1a HBcat 90 45 29 29 2 1a HBpin 100 45 2 37 39 3 2a HBpin 100 45 5 2 54 60 4 1b HBcat 60 20 16 16 5 1b HBpin 90 20 3 0.1 45 48 6 2b HBpin 90 20 5 4 31 40 aReactions were carried out in Schlenk bombs charged with catalyst (0.01 M), borane (1 M), hexamethylbenzene (2–10 mg as an internal standard), C 6D5Br (0.6 mL) and CO 2 (~1.5 atm). bTON is based on the number of C–H bonds formed in the reduced product per molecule of catalyst, determined by integration of the 1H NMR signals against the internal standard.

R2BOBR 2 is formed in all cases in addition to the carbon-containing CO 2-derived products.

This unique reactivity led us to investigate whether borylester 3a is sufficiently reactive towards further reduction of the CO 2-derived moiety. Heating a C 6D6 solution of 3a to 110 °C in the presence of 20 equivalents of HBpin led to a 0.9 : 1 ratio of 2a : 3a in 8 hours as detected by 1 H NMR spectroscopy, accompanied by the formation of CH 3OBpin and pinBOBpin. This reactivity represents the closing of a synthetic loop: the reactive diazafluorenylboronic ester 2a inserts CO 2 to make boryl ester 3a , which upon reaction with HBpin regenerates 2a and liberates the product of CO 2 reduction. With the synthetic loop established, we tested the performance of complexes 1a and 1b in catalytic hydroboration of CO 2 with HBcat and HBpin (Table 2.1).

Before discussion of the catalytic data, we must note that no attempts were made to study the kinetics of these catalytic systems. Our only data point for the performance of each catalyst is final turnover number after an arbitrary reaction period. 1a and 1b perform similarly as catalysts: their average turnover frequencies (avg. TOF) are on the order of one per hour, and the methoxyborane derivative CH 3OBR 2 was the major CO 2 reduction product in all cases. In the hydroboration of CO 2 with HBcat, 1a and 1b are not as active as (N-methyl)diazafluorenide (avg. TOF = 16 h -1 at 25 °C). However, when HBpin is used as the reductant, 1a and 1b are capable of more turnovers compared to (N-methyl)diazafluorenide (avg. TOF = 0.28 at 100 °C in 28 CDCl 3). Interestingly, 2a and 2b showed comparable catalytic performance for CO 2 hydroboration with HBpin under the same respective conditions compared to the parent compounds 1a and 1b (Table 2.1). In the Ru case the borylated species led to a slight increase in total TON (from 39 to 60). In the Zn case the borylated version led to a slight decrease in total

TON (from 48 to 40). Unfortunately, we could not identify any metal-containing species from the catalytic reaction mixtures. Therefore, the relevance of the steps in the synthetic loop to the actual catalytic runs has yet to be determined through further mechanistic studies.

We have been interested in the chemistry of the actor diazafluorenyl (daf) ligand. One successful vehicle for daf is the zwitterionic [RuH(daf)(N 2)(PPh 3)2], complex 4, which cooperatively splits hydrogen between the Ru(II) centre and the carbanion of daf. 38 1 also 32 undergoes insertion of CO 2 into a remote C-H bond of daf. In the current work we investigated the reactivity of daf in 1 towards the B-H bond in pinacolborane (HBpin) (Scheme 2.3). A close relative of 1, [RuH(daf)(CO)(PPh 3)2] reacts at elevated temperatures to borylate the carbanionic position of the diazafluorenyl ligand.

Scheme 2.3 Synthesis of 5 and 6

When a toluene solution/suspension of 4 is stirred with excess HBpin at room temperature for 36 h, the reaction mixture changes colour from pink to purple as

[Ru(daf)(H 2Bpin)(PPh 3)2] ( 5) forms (multinuclear NMR spectra of 5 shown in Figures 2.19- -1 2.21). In the infrared spectrum of 5, the N 2 stretch (2092 cm in 4) is absent, which indicates loss of the dinitrogen ligand. The 31 P{1H} NMR characteristics of 4 and 5 are very similar: each complex gives rise to one singlet (49.6 ppm and 50.9 ppm, respectively). In the 1H NMR spectrum of 5 in C 6D6, the greater symmetry of 5 relative to 4 is evident: each diazafluorenyl resonance corresponds to one pair of symmetry related protons. The singlet due to the carbanionic proton present in 4 persists in 5, which suggests borylation of daf did not occur. The hydride resonance (-12.27 ppm), is broadened relative to 4, and corresponds to two hydrides. The

PPh 3 ligands give rise to only two very broad signals at room temperature. However, cooling a toluene-d8 solution of 5 to -40 °C resulted in decoalescence into six different signals. This

phenomenon could arise from hindered rotation of each Ph 3P ligand such that one phenyl ring from each phosphine interacts with the bulky pinacolate methyl groups.

1 Figure 2.19 H NMR Spectrum (600 MHz, 25 °C) of 5 in C6D6. Inset: hydride region. Note: resonances due to residual toluene, hexanes, and pentane marked with an asterisk.

13 1 Figure 2.20 C{ H} NMR Spectrum (151 MHz, 25 °C) of 5 in C 6D6. Inset: expanded aryl region. Note: resonances due to residual toluene, hexanes, and pentane are marked with an asterisk.

31 1 11 1 Figure 2.21 Left: P{ H} NMR Spectrum (243 MHz, 25 °C) of 5 in C 6D6 Right: B{ H} NMR

Spectrum (128 MHz, 25 °C) of 5 in C 6D6.

1 Figure 2.22 H NMR Spectrum (500 MHz, 25 °C) of 6•(toluene) in C 6D6. Inset: hydride region. Note: resonances due to residual THF, dioxane and pentane are marked with an asterisk.

13 1 Figure 2.23 C{ H} NMR Spectrum (126 MHz, 25 °C) of 6•(toluene) in C 6D6. Inset: expanded aryl region. Note: resonances due to residual THF and pentane are marked with an asterisk.

31 1 Figure 2.24 Left: P{ H} NMR Spectrum (162 MHz, 25 °C) of 6•(toluene) in C 6D6 Right: 11 1 B{ H} NMR Spectrum (128 MHz, 25 °C) of 6•(toluene) in C 6D6. Spectrum contains trace pinBOBpin at 21.7 ppm and broad borosilicate peaks.

Figure 2.25 Molecular structures of 5 and 6. Ellipsoids shown at 30% probability. Hydrogen atoms except for hydrides omitted for clarity. Only one disordered component shown. Co- crystallized molecules removed for clarity. Selected bond lengths (Å) and angles (°) for 5: Ru1- B1 2.064(2), Ru1-N2 2.173(1), Ru1-N1 2.218(1), Ru1-P1 2.3506(4), Ru1-P2 2.3548(4). B1-H1 1.49(2) B1-H2 1.61(2), Ru1-H1 1.61(2), Ru1-H2 1.55(2); N2-Ru1-N1 80.77(5). Selected bond lengths (Å) and angles (°) for 6: P1-Ru1 2.3530(8), Ru1-B2 2.075(5), Ru1-N2 2.182(3), Ru1-N1 2.200(3), Ru1-P1 2.3530(8), Ru1-H1 1.59, Ru1-H2 1.41, B2-H1 1.63, B2-H2 1.60, C11 B1 1.521(7); N2-Ru1-N1 80.9(1), C4-C11-C7 106.0(3), C4-C11-B1 123.9(4), C7-C11-B1 130.1(4).

Single crystals of 5 for X-ray analysis were grown by vapour diffusion of hexanes into a dme solution (Figure 2.25). The Ru centre adopts a pseudo-octahedral geometry. Its six - coordination slots are filled by the two nitrogen donor atoms of L , two mutually trans PPh 3 ligands, and two mutually cis hydride ligands that are shared between Ru1 and B1. B1 features a pseudo-tetrahedral geometry that is completed by the two oxygen donors of the pinacolate group. The Ru1-N1 bond (2.218(2) Å) is significantly longer than the Ru1-N2 bond (2.173(1) Å), which indicates a lack of symmetry in the solid state that is not borne out in the solution data.

To test whether the carbanionic position in 5 is sufficiently nucleophilic to undergo borylation, we heated 5 and HBpin (9.1 eq), in toluene to 110 °C. After a reaction period of 35 h, analysis of the reaction mixture by 1H NMR revealed that not only had 5 been fully converted to new species 6, but also toluene borylation occurred. The meta and para toluene borylation products formed in a 2:1 ratio for a combined 55% yield based on HBpin. Efforts to improve catalytic turnover were not fruitful. Difficulties encountered in the scaling up of the reaction led us to try a different synthetic route to 6.

Reaction of a toluene suspension of 5 cooled to -35 °C with chloropinacolborane (ClBpin) caused lightening of the reaction mixture’s colour and formation of a voluminous pink precipitate, presumably the HCl adduct of 5. This precipitate was filtered off and the red filtrate was concentrated and recrystallized, yielding 6•(toluene) (multinuclear NMR spectra are shown in Figures 2.22-2.24). In the 11 B{1H} NMR spectrum of 6, two broad resonances are present (41.8 and 27.0 ppm), which indicates that the reaction has incorporated additional boron during the transformation of 5 to 6. In the 1H NMR spectrum of 6, the singlet due to the carbanionic proton in 5 (6.07 ppm) is absent, which is consistent with the formation of a C-B bond in its place. Two singlets of twelve protons each at 1.4 and 0.7 ppm are present, which indicates two distinct pinacolate environments.

X-ray quality crystals of 6 were obtained from Et 2O/hexanes/toluene solution. The molecular structure (Figure 2.25) shows that diazafluorenyl ligand borylation has occurred. The geometry and metric parameters at Ru1 and B2 are essentially the same as in compound 5. Distinct from 5, a crystallographically imposed mirror plane is coplanar with the diazafluorenyl ligand, and both pinacolate moieties are disordered.

2.4 Conclusion

In summary, new compounds 1a and 1b undergo C–H bond borylation to yield diazafluorenylboronic ester complexes 2a and 2b , which feature the unquenched π-basicity of a carbanionic group directly bound to a boron centre. It is this structural feature that facilitates the unprecedented insertion of CO 2 into their C–B bonds to yield boryl ester products 3a and 3b . DFT calculations suggest the insertion occurs in a concerted fashion. Compound 3a can be converted to 2a when reacted with HBpin, releasing the CO 2 reduction product CH 3OBpin and closing a synthetic loop of CO 2 reduction. Furthermore, compounds 1ab and 2ab all displayed

catalytic activity toward CO 2 hydroboration with HBpin and HBcat. Further mechanistic studies of the catalytic CO 2 reduction, the insertion of other unsaturated substrates into the C–B bond in our diazafluorenylboronic ester ligand, the related reactivity of other C–E bonds, and the corresponding catalytic reactions are under investigation in our laboratory.

2.5 Experimental

2.5.1 General procedures.

43 44 45 46 38 RuHCl(CO)(PPh 3)3 , diazafluorene, Mes 2nacnacH, , ClBpin and 4 were prepared from literature methods. All operations were performed using Schlenk techniques under dinitrogen or in a dinitrogen-filled glovebox. All glassware was either flame-dried or dried overnight in a 180 °C oven prior to use except for J. Young NMR tubes which were dried overnight in a 60 °C oven. Grade 4.0 carbon dioxide was purchased from Linde. THF, Et 2O, and toluene were distilled from Na/benzophenone under dinitrogen. Pentane, hexanes, and toluene-d8 were distilled from sodium under dinitrogen. C 6D6 and C 6D5Br were either sparged with dinitrogen or degassed using two freeze-pump thaw cycles. All solvents were then stored over 3 Å molecular sieves prior to use. HBpin and HBcat were purchased from Aldrich and used as received unless otherwise stated. IR spectra were collected on a Perkin-Elmer Spectrum One FT-IR spectrometer. 1H, 31 P{1H}, 13 C{1H}, and 11 B{1H} NMR spectra were recorded on a Varian 400 MHz, Agilent DD2 500 MHz, or Agilent DD2 600 MHz spectrometer. All chemical shifts are reported in ppm relative the residual protio-solvent peaks (for C 6D5Br these are the meta -H at 7.28 ppm and the ipso-C at 122.4 ppm), 31 P{1H} NMR is referenced externally using 85% 11 1 H3PO 4, and B{ H} NMR is referenced externally using BF 3(OEt 2). Elemental analyses were performed by ANALEST at the University of Toronto.

Chart 2.1 Labeling scheme for NMR assignments of Ru compounds.

2.5.2 Synthesis of 1a

Diazafluorene (60 mg, 0.36 mmol) was added to a suspension of NaH (25 mg, 1.0 mmol) in THF (5 mL). The mixture was stirred for 90 min after which it was filtered and slowly added to a suspension of RuHCl(CO)(PPh 3)3 (352 mg, 0.37 mmol) in THF (30 mL) over a period of ca. 20 min. The purple mixture was stirred at room temperature for 12 h and the solvent was removed under reduced pressure. The residue was extracted with toluene (50 mL), filtered through Celite and volatiles were removed in vacuo . THF (15 mL) was added to dissolve the product and pentane (50 mL) was layered on top. After cooling to –25 °C; the purple precipitate that separated was collected on a fritted funnel and washed with Et 2O (3 x 2 mL). The powder on the frit was then dissolved in THF, filtered, and the THF was removed. The residue was suspended in pentane (15 mL) and collected on a fritted funnel, where it was washed with pentane (10 mL) 1 and dried in vacuo . (174 mg, 0.212 mmol, 59%). H NMR (500 MHz, Benzene-d6) δ 7.96 (d, J =

4.6 Hz, 1H, Dz-C(3) H), 7.73 (d, J = 8.0 Hz, 1H, Dz-C(1)H), 7.62 (d, J = 8.1 Hz, 1H, Dz-C(1‘)H),

7.41–7.32 (m, 12H, PPh 3-H), 7.03 (dd, J = 4.7, 1.0 Hz, 1H, Dz-C(3‘)H), 6.91–6.80 (m, 18H, PPh 3-

H), 6.69 (dd, J = 8.2, 4.6 Hz, 1H, Dz-C(2)H), 6.47 (s, 1H, Dz-C(9)H), 6.20 (dd, J = 8.1, 4.7 Hz, 13 1 1H, Dz-C(2‘)H), –11.54 (t, J = 19.7 Hz, 1H, Ru -H). C{ H} NMR (126 MHz, Benzene-d6) δ

206.7 (Ru-CO, observed only via HMBC cross peak with Ru-H) 140.3 (Dz-C(4‘)), 140.1 (Dz-

C(4)), 136.2 (Dz-C(3) H), 136.1 (Dz-C(3‘)H) 133.8 (t, J = 5.9 Hz, PPh 3), 133.6 (t, J = 21.3 Hz,

PPh 3), 129.5 (PPh 3) 127.9 (PPh 3), 127.5 (Dz-C(5‘)), 127.1 (Dz-C(5)), 125.1 (Dz-C(1)H), 124.9 (Dz- 31 1 C(1‘)H), 117.6 (Dz-C(2‘)H), 117.5 (Dz-C(2)H), 81.7 (Dz-C(9)H). P{ H} NMR (202 MHz,

Benzene-d6) δ 48.6. Anal. Calcd for C 48 H38 N2OP 2Ru: C, 70.15; H, 4.66; N, 3.41. Found: C, 70.03; H, 4.68; N, 3.27. IR (nujol mull): ν(CO) 1918 cm –1.

2.5.3 Synthesis of 2a

To 1a (100 mg, 0.12 mmol) dissolved/suspended in toluene (4 mL) was added a solution of pinacolborane (140 mg, 1.1 mmol) dissolved in toluene (4 mL), The resulting purple mixture was heated to 100 °C in a bomb sealed with a Teflon stopcock for 24 h, causing a colour change to pink. Volatiles were removed under reduced pressure. The residue was dissolved in toluene and the solution was concentrated until precipitate began to form. Pentane (5 mL) was layered on top and the mixture cooled to –25 °C for two days. The precipitate was collected on a frit and washed minimally with cold toluene and thoroughly with pentane. After drying under vacuum a pink powder was obtained (72 mg, 0.076 mmol, 63%). Single crystals for X-Ray analysis were

grown by diluting a C 6D6 solution with hexanes and cooling to –25 °C. The analytical sample 1 was recrystallized an additional time. H NMR (500 MHz, Benzene-d6) δ 8.56 (d, J = 7.9 Hz,

1H, Dz-C(1)H), 8.45 (d, J = 8.0 Hz, 1H, Dz-C(1‘)H ), 7.91 (d, J = 4.7 Hz, 1H, Dz-C(3) H), 7.36–

7.31 (m, 12H, PPh 3), 6.92 (dd, J = 4.9, 1.0 Hz, 1H, Dz-C(3‘)H), 6.86 (t, J = 7.2 Hz, 6H, PPh 3),

6.81 (t, J = 7.2 Hz, 12H, PPh 3), 6.76 (dd, J = 8.1, 4.7 Hz, 1H, Dz-C(2)H), 6.26 (dd, J = 8.1, 4.9 13 1 Hz, 1H, Dz-C(2‘)H), 1.36 (s, 12H, pin-CH 3), –11.68 (t, J = 19.8 Hz, 1H, Ru-H). C{ H} NMR

(126 MHz, Benzene-d6) δ 206.3 (Ru-CO, observed only via HMBC cross peak with Ru-H) 145.0

(Dz-C(4‘)), 144.7 (Dz-C(4)), 138.3 (Dz-C(3) H), 138.1 (Dz-C(3‘)H), 135.3 (Dz-C(5‘)) , 134.7 (Dz-

C(5)), 133.7 (t, J = 5.9 Hz, PPh 3), 133.5 (t, J = 21.3 Hz, PPh 3), 129.5 (PPh 3), 128.5 (Dz-C(1)H),

128.3 (overlapped with solvent signal, Dz-C(1‘)H), 128.0 (t, J = 4.7 Hz, PPh 3), 119.1 (Dz-C(2‘)H),

118.9 (Dz-C(2)H), 81.3 (pin C-O), 25.6 (pin CH 3). We didn’t observe a signal due to Dz-C(9) due to broadening caused by the bound quadrupolar 11 B nucleus. 4 31 P{1H} NMR (202 MHz, Benzene- 11 1 d6) δ 48.5. B{ H} NMR (128 MHz, Benzene-d6, 25 °C) δ 31.3. Anal. Calcd for

C54 H49 N2O3BP 2·0.5(C 7H8): Note: the ratio of 2 to toluene was determined by integration of the 1H NMR spectrum C, 69.49; H, 5.38; N, 2.82. Found: C, 69.63; H, 5.29; N, 2.79. IR (nujol mull): ν(CO) 1936 cm –1.

2.5.4 Synthesis of 3a

2a (60 mg, 0.060 mmol) was dissolved in toluene (6 mL) and Et 2O (6 mL). The solution was transferred to a Schlenk flask capped with a rubber septum. CO 2 gas (4.5 mL, 0.18 mmol) was syringed into the flask above the solution. The flask was sealed with tape and left in the glovebox freezer (–25 °C) for 16h, which brought about a colour change from pink to orange. Removal of volatiles yielded a brown oil which solidified upon standing. Pentane (3 mL) was added and the mixture was triturated. The pentane was decanted off and the orange solid was washed again with pentane. Drying in vacuo yielded 3a (40 mg, 0.040 mmol, 64%). Single crystals were 1 grown by layering hexanes over a toluene/Et 2O solution. H NMR (600 MHz, Benzene-d6) δ 8.61

(d, J = 7.9 Hz, 1H, Dz-C(1)H), 8.51 (d, J = 8.0 Hz, 1H, Dz-C(1‘)H), 7.91 (d, J = 4.8 Hz, 1H, Dz-

C(3)H), 7.33 –7.26 (m, 12H, PPh 3), 6.96 (d, J = 4.9 Hz, 1H, Dz-C(3‘)H), 6.85 (t, J = 7.2 Hz, 6H,

PPh 3), 6.79 (t, J = 7.5 Hz, 12H, PPh 3), 6.67 (dd, J = 8.1, 4.8 Hz, 1H, Dz-C(2)H), 6.17 (dd, J = 8.0, 13 1 5.0 Hz, 1H, Dz-C(2‘)H), 1.19 (s, 12H, pin CH 3), –11.79 (t, J = 19.3 Hz, 1H, Ru-H). C{ H} NMR

(126 MHz, CD 2Cl 2) δ 205.8 (Ru-CO), 162.3 (C=O ), 145.7 (Dz-C(4‘)), 145.4 (Dz-C(4)), 141.2

(overlapped Dz-C(3) H and C(3‘) H) ,133.5 (t, J = 6.0 Hz, PPh 3), 132.8 (t, J = 21.6 Hz, PPh 3), 131.2

(Dz-C(5‘)), 130.6 (Dz-C(5)H), 130.0 (PPh 3), 128.2 (t, J = 4.7 Hz, PPh 3), 127.7 (Dz-C(1)H), 127.5

(Dz-C(1‘)H), 121.7 (Dz-C(2‘)H), 121.3 (Dz-C(2)H), 88.0 (Dz-C(9)), 83.4 (pin-CO), 24.9 (pin-CH 3) . 31 1 11 1 P{ H} NMR (243 MHz, Benzene-d6) δ 48.2. B{ H} NMR (128 MHz, Benzene-d6) δ 23.1.

Anal. Calcd for C 55 H49 BN 2O5P2Ru: C, 66.60; H, 4.98; N, 2.82. Found: C, 66.21; H, 5.09; N, 2.85. IR (nujol mull): ν(CO) 1942 cm –1, 1647 cm –1.

Chart 2.2 Labeling scheme for NMR assignments of Zn compounds.

2.5.5 Synthesis of EtZn(Mes 2nacnac)

A solution of ZnEt 2 (1.0 M in hexanes, 0.36 mL, 0.36 mmol) was dissolved in toluene (1 mL) and added dropwise to a stirring solution of Mes 2nacnacH (0.0986 g, 0.295 mmol) in toluene (1.5 mL) over 1 minute. The solution was allowed to stir for at least 3 hours, until the reaction had reached completion as determined by 1H NMR spectroscopy, after which the solvent was removed in vacuo to yield EtZn(Mes 2nacnac) as an off-white solid in quantitative yield (0.129 g). The crude product was used without further purification, however the compound can be purified by recrystallization from hexanes in the freezer if desired. 1H NMR (400 MHz,

Benzene-d6, 25 °C) δ 6.82 (s, 4H, m-Mes CH), 4.98 (s, 1H, L-CH), 2.15 (s, 6H, p-Mes CH3),

2.14 (s, 12H, o-Mes CH3), 1.64 (s, 6H, L-CH3), 1.03 (t, J = 8.1 Hz, 3H, Zn-CH 2CH3), 0.30 (q, J 13 1 = 8.1 Hz, 2H, Zn-CH2CH 3). C{ H} NMR (101 MHz, Benzene-d6, 25 °C) δ 167.0 (L-C), 145.7

(i-Mes C), 133.8 (p-Mes C), 130.9 (o-Mes C), 129.5 (m-Mes CH ), 95.7 (L-CH ), 22.5 (L-CCH 3),

21.0 (p-Mes CH 3), 18.7 (o-Mes CH 3), 12.2 (Zn-CH 2CH 3), -2.8 (Zn-CH 2CH 3). Anal. Calcd for

C25 H34 N2Zn: C, 70.2; H, 8.0; N, 6.6. Found: C, 70.16; H, 7.74; N, 6.68.

2.5.6 Synthesis of 1b

A solution of EtZn(Mes 2nacnac) (0.2103 g, 0.4914 mmol) in toluene (4 mL) was added dropwise to a stirring solution of 4,5-diazafluorene (0.0773 g, 0.460 mmol) in toluene (2 mL) over 2

52 minutes. The solution was stirred at room temperature for 3 days after which the solvent was removed in vacuo to obtain the crude complex 1b as a purple solid. The crude solid was dissolved in a minimum amount of hexanes (2-4 mL) and the solution was cooled in the freezer (-35 °C) overnight causing purple crystals to form. The crystalline product was collected by vacuum filtration and washed with hexanes, then dried under vacuum yielding pure complex 1b as a dark purple crystalline solid (0.242 g, 0.428 mmol, 87% yield). Single crystals suitable for X-ray crystallography were obtained by slow evaporation at -35 °C of a saturated solution of the complex in hexanes with toluene added (10:1 hexanes to toluene). 1H NMR (500 MHz, Benzene- d6, 25 °C) δ 7.88 (d, J = 4.6 Hz, 2H, Dz-C(3) H), 7.82 (d, J = 8.3 Hz, 2H, Dz-C(1) H), 6.90 (dd, J =

8.3, 4.6 Hz, Dz-C(2) H), 6.40 (s, 4H, m-Mes CH ), 6.27 (s, 1H, Dz-C(9) H), 5.00 (s, 1H, L-CH), 2.22 13 1 (s, 12H, o-Mes CH3), 1.77 (s, 6H, p-Mes CH3), 1.70 (s, 6H, L-CH3). C{ H} NMR (126 MHz,

Benzene-d6, 25 °C) δ 168.8 (L-C) , 144.2 (i-Mes C), 135.5 (Dz-N-C), 133.8 (p-Mes C), 131.0

(Dz-C(3) H), 130.8 (o-Mes C), 129.3 (m-Mes CH ), 129.0 (Dz-C(9) -C), 128.4 (Dz-C(1) H), 118.1

(Dz-C(2) H), 94.5 (L-CH ), 81.5 (Dz-C(9) H), 23.0 (L-CH 3), 20.6 (p-Mes CH 3), 18.5 (o-Mes CH 3).

Anal. Calcd for C 34 H36 N4Zn: C, 72.14; H, 6.41; N, 9.90. Found: C, 72.36; H, 6.88; N, 9.98.

2.5.7 Synthesis of 2b

Complex 1b (0.290 g, 0.512 mmol) and pinacolborane (0.290 g, 2.27 mmol) were transferred to a Teflon-sealed bomb with THF (5 mL). The sealed bomb was heated to 100 °C for 36 h, which brought about a subtle colour change from dark purple to pink. Volatiles were removed in vacuo ; pentane (5 mL) and toluene (0.5 mL) were added, the mixture was filtered through Celite and cooled to -25 °C for 6 h. The pink crystals that formed were collected on a frit, and were washed with several drops each of cold toluene and cold ether, after which they were washed thoroughly with cold pentane and dried under vacuum, which yielded analytically pure

2b •(toluene) 1/2 (pentane) 1/3 (0.156 g, 0.205 mmol, 46% yield). Anal. Calcd for

C40 H47 N4O2BZn·1/2(C 7H8)·1/3(C 5H12 ) C, 71.18; H, 7.27; N, 7.35. Found C, 71.09, H, 7.60, N, 7.33 (note that the ratio of 2b to toluene and pentane was determined by integration of the 1H NMR spectrum of the sample used for EA). Concentration and recrystallization of the filtrate yielded a second crop of crystals (0.121 g, 0.159 mmol, total yield 71%). 1H NMR (600 MHz,

Benzene-d6, 25 °C) δ 8.68 (dd, J = 8.4, 1.2, 2H, Dz-C(1) H), 7.86 (dd, J = 4.8, 1.2, 2H, Dz-C(3) H),

6.93 (dd, J = 8.4, 4.8, 2H, Dz-C(2) H), 6.44 (s, 4H, m-Mes CH ), 4.98 (s, 1H, L-CH ), 2.17 (s, 12H, 1 o-Mes CH 3), 1.80 (s, 6H, p-Mes CH 3), 1.68 (s, 6H, L-CH 3), 1.26 (s, 12H, Pin-CH 3). H NMR

(500 MHz, Bromobenzene-d5, 25 °C) δ 8.42 (d, J = 8.5, 2H, Dz-C(1) H), 7.94 (d, J = 4.5, 2H, Dz-

C(3) H), 6.99 (dd, J = 8.5, 4.5, 2H, Dz-C(2) H), 6.40 (s, 4H, m-Mes CH ), 4.98 (s, 1H, L-CH ), 2.15

(s, 12H, o-Mes CH 3), 1.75 (s, 6H, p-Mes CH 3), 1.68 (s, 6H, L-CH 3), 1.21 (s, 12H, Pin-CH 3). 13 1 C{ H} NMR (151 MHz, Benzene-d6, 25 °C) δ 168.8 (L-C), 144.1 (i-Mes C), 139.5 (Dz-N-C),

136.7 (Dz-C(9) C), 133.9 (p-Mes C), 133.0 (Dz-C(3) H), 131.9 (Dz-C(1) H), 130.8 (o-Mes C), 129.3

(m-Mes CH ), 119.5 (Dz-C(2) H), 94.5 (L-CH ), 81.4 (Pin-C), 25.4 (Pin-CH 3), 23.0 (L-CH 3), 20.7

(p-Mes CH 3), 18.4 (o-Mes CH 3). We didn’t observe a signal due to Dz-C(9) due to broadening 11 47 13 1 caused by the bound quadrupolar B nucleus. C{ H} NMR (126 MHz, Bromobenzene-d5, 25

°C) δ 168.6 (L-C), 143.9 (i-Mes C), 139.0 (Dz-N-C), 136.0 (Dz-C(9) C), 133.5 (p-Mes C), 133.0

(Dz-C(3) H), 131.4 (Dz-C(1) H), 130.6 (o-Mes C), 129.0 (m-Mes CH ), 119.3 (Dz-C(2) H), 94.5 (L- 11 1 CH ), 81.2 (Pin-C), 25.3 (Pin-CH 3), 23.1 (L-CH 3), 20.7 (p-Mes CH 3), 18.4 (o-Mes CH 3). B{ H}

NMR (128 MHz, Benzene-d6, 25 °C) δ 31.4

2.5.8 Synthesis of 3b

2b (45.1 mg, 0.0652 mmol) was dissolved in bromobenzene (3 mL) in a 50 mL bomb sealed with a Teflon screw cap. The solution was frozen in liquid nitrogen and the headspace was evacuated. The frozen solution was thawed and allowed to warm to room temperature at which time 1 atm CO 2 was admitted to the flask. The pink solution gradually turned orange and was kept under CO 2 for 3 h. The orange solution was filtered through Celite and volatiles were removed in vacuo leaving 3b ·(PhBr) as a waxy orange solid (39.1 mg, 0.0438 mmol, 67 %).

Anal. Calcd for C 41 H47 N4O4BZn·(C 6H5Br) C, 63.21; H, 5.87; N, 6.27. Found C, 63.21; H, 5.74; N, 6.02 (note that the ratio of 3b to bromobenzene was determined by integration of the 1H NMR spectrum of the sample used for EA). X-ray quality crystals of 3b were grown from a toluene/pentane solution stored at -25 °C. IR (nujol mull): ν(CO) 1626 cm -1. 1H NMR (400 MHz,

Bromobenzene-d5, 25 °C) δ 8.45 (d, J = 8.1, 2H, Dz-C(1) H), 7.95 (d, J = 4.8, 2H, Dz-C(3) H), 7.01

(dd, J = 8.4, 4.6, 2H, Dz-C(2) H), 6.44 (s, 4H, m-Mes CH ), 4.98 (s, 1H, L-CH ), 2.11 (s, 12H, o- 13 1 Mes CH 3), 1.78 (s, 6H, p-Mes CH 3), 1.67 (s, 6H, L-CH 3), 1.16 (s, 12H, Pin-CH 3). C{ H} NMR

(101 MHz, Bromobenzene-d5, 25 °C) δ 168.9 (L-C), 161.5 (C (9) -CO 2-B), 143.7 (i-Mes C), 139.7

(Dz-N-C), 135.3 (Dz-C(3) H), 133.8 (p-Mes C), 131.9 (Dz-C(9) C), 131.2 (Dz-C(1) H), 130.5 (o-Mes

C), 129.1 (m-Mes CH ), 121.6 (Dz-C(2) H), 94.6 (L-CH ), 87.7 (Dz-C(9)), 82.8 (Pin-C), 24.8 (Pin- 11 1 CH 3), 23.1 (L-CH 3), 20.7 (p-Mes CH 3), 18.4 (o-Mes CH 3) B{ H} NMR (193 MHz, C 6D5Br, 25 °C) δ 22.5.

2.5.9 Regeneration of 2a :

3a (3.4 mg, 3.4 x 10 -3 mmol, 1 eq) and pinacolborane (8.8 mg, 6.9 x 10 -2 mmol, 20 eq) were dissolved in C 6D6 in a thick-walled J. Young NMR tube which was heated in an oil bath preheated to 110 °C. After 8 h of heating, the ratio of 2a : 3a was 0.9 : 1.

2.5.10 Catalytic experiments

Into a vial was weighed the desired catalyst (0.0065 mmol), borane (0.65 mmol) and hexamethylbenzene (2-10 mg). The reagents were transferred to a Teflon sealed 50 mL glass bomb using bromobenzene-d5 (0.6 mL). The solution was degassed through one freeze-pump- thaw cycle using liquid nitrogen, then it was refrozen in a -70 °C dry ice/isopropanol bath. The bomb was opened a flow of CO 2 for 10 minutes, then it was sealed and allowed to warm to room temperature before heating at the desired catalysis temperature. The bomb was cooled after the desired amount of time and a portion of the reaction mixture was transferred to a J. Young NMR tube for analysis by 1H NMR spectroscopy (delay time extended to 10 sec for quantitative integration). The hexamethylbenzene standard peak (2.05 ppm) was used to quantify the amounts of each reduction product present in the sample (for HBPin: CH 3OBPin at 3.48 ppm,

CH 2(OBPin) 2 at 5.34 ppm and HCO 2BPin at 8.35 ppm; for HBCat: CH 3OBCat at 3.6 ppm).

2.5.11 Synthesis of 5

4 (44 mg, 0.054 mmol), pinacolborane (44 mg, 0.34 mmol), and toluene (4 mL) were combined in a 20 mL scintillation vial in the glovebox and the resulting mixture was stirred for 36 h. Volatiles were removed in vacuo , and purple crystals were grown from the residue by vapor diffusion of THF/hexanes. The supernatant was decanted and the purple crystals were washed 1 with Et 2O (2 x 1mL) and dried in vacuo (32 mg, 0.035 mmol, 65 %). H NMR (600 MHz, C6D6,

25 °C) δ 8.34 (d, J = 4.6 Hz, 2H, C 3-H), 7.44 (d, J = 8.0 Hz, 2H, C 1-H), 7.39 (br, 12H, PPh 3),

6.90 (br, 18H, PPh 3), 6.65 (dd, J = 8.1, 4.7 Hz, 2H, C 2-H), 6.07 (s, 1H, C 9-H), 0.72 (s, 12H, pin- 1 CH 3), -12.27 (br, 2H, RuH 2). H NMR (400 MHz, tol-d8, -40 °C) δ 8.43 (d, J = 4.6 Hz, 2H, C 3-

H), 8.17 (br, 8H, PPh 3), 7.49 (d, J = 8.1 Hz, 2H, C 1-H), 7.18 (br, overlapped with tol-d8 signal,

8H, PPh 3), 7.09 (br, overlapped with tol-d8 signal, 4H, PPh 3), 6.68 (dd, J = 8.1, 4.6 Hz, 2H, C 2-

H), 6.51 (br, 2H, PPh 3), 6.20 (br, 4H, PPh 3), 6.16 (s, 1H, C 9-H), 5.73, (br, 4H, PPh 3) 0.71 (s, 12H, 11 1 31 1 pin-CH 3), -12.17 (t, J = 11.0 Hz, 2H, RuH 2) B{ H} NMR (128 MHz, C6D6) δ 42.3. P{ H} 13 1 NMR (243 Hz, C6D6) δ 50.9. C{ H} NMR (151 MHz, C6D6) δ 141.9 (C 4), 135.4 (C 3), 134.0

(br, PPh 3), 128.9 (br, PPh 3), 127.2 (C 5), 126.9 (br, PPh 3), 123.6 (C 1), 117.6 (C 2), 82.6 (H 2Bpin- -1 C), 82.3 (C 9) 24.1 (pin-CH 3). IR (neat solid): ν(Ru-H): 1988 cm Anal. Calcd for

C53 H51 N2O2BP 2Ru: C, 69.06; H, 5.58; N, 3.04. Found C, 69.00; H, 5.43; N, 3.05.

2.5.12 Further reaction of 5 with HBpin

5 (10.8 mg, 0.0117 mmol, 1 eq), toluene (5 mL), and pinacolborane (13.7 mg, 0.107 mmol, 9.1 eq) were combined in a 50 mL flask sealed with a Teflon stopcock. The flask was heated to 110 °C for 35 h with stirring. Volatiles were removed in vacuo , 1,2-dimethoxyethane was added as 1 11 1 internal standard and the residue was dissolved in C 6D6 for analysis by H and B{ H} NMR. The 1H NMR spectrum revealed full conversion of 5 to 6. Tol-Bpin (meta:para ratio ca . 2:1) formed in 55% yield, and despite care taken to exclude moisture, pinBOBpin also formed in 19% yield.

2.5.13 Synthesis of 6

To a mixture of 5 (76.3 mg, 0.0828 mmol,) and toluene (5 mL) cooled to -35 °C in a glovebox freezer was added dropwise a similarly cooled heptane solution of ClBpin (79.7 mg, 17 wt.%, 0.085 mmol) diluted with toluene (1 mL). The mixture was allowed to warm to room temperature with stirring for 30 min during which time the reaction mixture lightened from purple to red and a pink precipitate formed. After filtration, solvents and excess ClBpin were removed in vacuo . The pink residue was recrystallized by layering hexanes (3 mL) onto a saturated toluene (1.5 mL) solution at –35 °C. Decanting the supernatant followed by rinsing the crystals with pentane (2 x 1 mL) and drying in vacuo yielded 6•(toluene) (34.5 mg, 0.0302 1 mmol, 36%). H NMR (500 MHz, C6D6, 25 °C) δ 8.32 (d, J = 4.7 Hz, 2H, C3-H), 8.16 (d, J = 8.0

Hz, 2H, C 1-H), 7.36 (br, 12H, PPh 3), 6.91 (br, 6H, PPh 3) 6.84 (br, 12 H, PPh 3), 6.70 (dd, J = 8.1,

4.8 Hz, 2H, C2-H), 1.37 (s, C9-Bpin-CH 3, 12H), 0.71 (s, 12H, H 2Bpin-CH 3), -12.36 (br, 2H, Ru - 11 1 31 1 H2). B{ H} NMR (128 MHz, C 6D6) δ 41.76, 26.99. P{ H} NMR (202 MHz, C 6D6) δ 50.8. 13 1 C{ H} NMR (126 MHz, C 6D6) δ 146.6 (C 4), 137.5 (C 3), 135.0 (C 5), 133.7 (br, PPh 3), 129.0 (br,

PPh 3), 127.1 (C 1), 127.0 (br, PPh 3), 118.9 (C 2), 82.6 (H 2Bpin-C), 81.1 (C 9-Bpin-C), 25.4 (C 9-

Bpin-CH3) 24.1 (H 2Bpin-CH 3). We did not observe a signal due to C9 due to broadening caused by the bound quadrupolar 11 B nucleus.47 IR (neat solid): ν(Ru-H) 1998 cm -1 (weak) Anal. Calcd for C 59H62 N2O4B2P2Ru·(C 7H8): C, 69.54; H, 6.19; N, 2.46. Found: C, 69.79; H, 5.83; N, 2.14. Note: the ratio of 3 to toluene was determined by integration of the 1H NMR spectrum.

2.5.14 X-ray Crystallography

The X-ray diffraction data were collected on a Bruker Kappa Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K controlled by an Oxford Cryostream 700 series low-temperature system and processed with the Bruker Apex 2 software package. 48 The structures were solved by direct methods and refined using SHELXL-2013 and SHELXL-2014. 49, 50 All non-hydrogen atoms (except for those involved in disordered BPin moieties) were refined anisotropically, except for a few atoms involved in the disordered portion. The hydrides were located directly from the difference Fourier map, while all other hydrogen atoms were calculated using the riding model. The diffuse residual electron density from disordered solvents in the lattices of 2a, 2b , 3b, and 6 was removed with the SQUEEZE function of PLATON, 51 and these removed solvent molecules were not included in the formula or the refinement. Selected crystallographic data are listed in Table 2.2.

Table 2.2 Selected crystallographic data 2a 3a 1b 2b 3b

Formula C63 H66 BN 2O3P2Ru C55 H49 BN 2O5P2Ru C34 H36 N4Zn C45 H50 BN 4O2Zn C49 H53 BN 4O2Zn FW 1072.99 991.78 566.04 691.99 736.00 T (K) 150(2) 150(2) 150(2) 150(2) 150(2)

space group P-1 P 4 1 C2/c P-1 Pbca a (Å) 12.4687(13) 10.4711(4) 21.3090(19) 14.4273(7) 15.0418(7) b (Å) 15.7310(16) 10.4711(4) 10.0652(9) 14.5211(10) 21.3952(11) c (Å) 17.5343(19) 44.073(4) 29.168(3) 21.5927(15) 28.2235(14)

a (deg) 67.020(4) 90 90 100.551(2) 90 b (deg) 80.866(5) 90 108.052(4) 91.968(2) 90 g (deg) 70.636(4) 90 90 102.418(2) 90 V (Å 3) 2985.5(6) 4832.4(5) 5947.9(10) 4330.3(5) 9083.0(8) Z 2 4 8 4 8

−3 Dc (g∙cm ) 1.194 1.363 1.264 1.061 1.076

m (mm −1 ) 0.360 0.441 0.854 0.600 0.579 no. of refln 25438 64351 34255 collected 48173 33106

57 no. of indept 6770 19613 10394 13690 10940 refln GOF on F2 0.953 1.079 1.002 0.883 0.984

a) R [I > 2s (I)] R1 = 0.0506 R1 = 0.0783 R1 = 0.0434 R1 = 0.0554 R1 = 0.0737

b) wR2 = 0.1013 wR2 = 0.1663 wR 2 = 0.0882 wR 2 = 0.1088 wR 2 = 0.1949

R (all data) R1 = 0.1001 R1 = 0.1090 R1 = 0.0821 R1 = 0.1285 R1 = 0.1450

wR2 = 0.1129 wR2 = 0.1790 wR 2 = 0.0990 wR 2 = 0.1250 wR 2 = 0.2156

5 6

Formula C53 H51 B1N2O2P2Ru C59 H62 B2N2O4P2Ru

F.W. 921.78 1047.73

T (K) 150(2) 150(2)

Space group Pī Pnma

a (Å) 12.9101(6) 17.3235(11)

b (Å) 13.2983(7) 14.6930(11)

c (Å) 14.7717(8) 22.2123(17)

α (°) 98.591(2) 90

β (°) 99.662(2) 90

γ (°) 111.512(2) 90

V (Å 3) 2263.6(2) 5653.8(7)

Z 2 4

-3 Dc (g·cm ) 1.352 1.231

μ (mm -1) 0.461 0.379

no. reflns 46321 51142 collcd

no. indept 14992 6753 reflns

GOF on F 2 1.004 1.072

R [I > 2σ R1 = 0.0388, R1 = 0.0700, (I)] wR 2 = 0.0738 wR 2 = 0.1382

R (all data) R1 = 0.0631, R1 = 0.0509,

wR 2 = 0.0817 wR 2 = 0.1476

2.5.15 Computations

All calculations were performed using Gaussian 09 program 52 at the B3LYP 53, 54 level. The SDD basis set and effective core potential were used on Ru, while 6-31G* basis set was used on all other elements. All structures were optimized in the gas phase first and the solvation correction was performed using self-consistent reaction field by means of the PCM method using toluene as the solvent. Vibrational frequency analyses were performed on all optimized structures to obtain thermodynamic data and to confirm that the optimized structures of 2a and 3a have no imaginary frequency, while the transition state structure has one imaginary frequency.

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3 Chapter 3 Synthesis and structures of Li, Fe, Mo, and V derivatives of N,N’ -bis(2,6-diisopropylphenyl)-o- phenylenediamine 3.1 Abstract

Double deprotonation of N,N’-bis(2,6-diisopropylphenyl)-o-phenylenediamine (H 2L) with n-BuLi in THF yields the dilithium complex Li 2L(THF)3 (3.1 ) The reaction of Li 2L(THF)3 6 with FeBr 2(THF)2 and subsequent replacement of the solvent with toluene yield LFe(η -toluene)

(3.2 ), which can also be synthesized from Fe(hmds) 2(THF) and H 2L in toluene. The NMR data, bond lengths obtained from an X-ray diffraction study, and DFT calculations indicate that the diamide ligand L 2- undergoes oxidization to a radical ligand L -. Reaction of 3.2 with 1 atm CO yields the tricarbonyl complex LFe(CO) 3 (3.3 ). MoCl 4(THF)2 reacts with two equivalents of

Li 2L(THF)3 to yield (LiL) 2MoCl 2(THF)4 (3.4 ) in which the phenylene backbone of L has been dearomatized. Similarly, VCl 2(tmeda) 2 reacts with two equivalents of Li 2L(THF)3 to yield analogous compound (LiL) 2V(OEt 2)2 (3.6 ). One-electron oxidation of Li 2L(THF)3by

EuCl 3(dme) 2 yields the open-shell species LiL(OEt 2) (3.5 ), which was characterized by X-ray crystallography and EPR spectroscopy.

3.2 Introduction

The study of redox-active ligands has led to clearer understanding of fundamental coordination chemistry 1 as well as exciting new examples of catalysis including the use of abundant first row metals. 2,3 o-Phenylenediamine (opda) derivatives are classic examples of redox-active ligands. Scheme 3.1 depicts the three possible redox forms of this ligand series with o-phenylenediamide (opda 2−) as its most reduced form, and o-diiminosemiquinonate (opda 1−) and o-benzoquinonediimine (opda 0) as intermediate and most oxidized forms, respectively (Scheme 3.1).

R R R N N N - e- - e- + e- + e- N N N R R R

Scheme 3.1 Three redox forms of opda

Early reports from Holm emerged in the 1960s on the synthesis and electrochemical behaviour of late-transition metal complexes of the various redox forms of pda. 4,5 To date the unsubstituted, 4-11 N-monosubstituted, 12-16 and N,N′-disubstituted variants of this family of ligands have been synthesized and employed as ancillary ligands for main group 17,18 and transition metals. 19-25 The 2,6-diisopropylphenyl (dipp) group has found use in chelating ligand design because it can provide unique sterics around the metal centre, and can stabilize coordinatively unsaturated metal centres. Despite the popularity of the dipp group in ligand design and the ease of preparation of the dipp-substituted opda, 26 the coordination chemistry of a dipp-substituted opda ligand is underexplored compared to other opda derivatives. Only recently the dipp- substituted pda 2− ligand has found use in the isolation of boryl anions. 27 Our group is interested in exploring the coordination chemistry of this ligand toward transition metals. Herein we report the synthesis and characterization of the dilithium salt of this ligand, its coordination behaviour towards Fe, Mo, and V, as well as the synthesis and characterization of the monolithium salt of the corresponding dipp-substituted opda 1− ligand.

3.3 Results and Discussion

3.3.1 Synthesis of Li 2L(THF)3 (3.1 )

As shown in Scheme 3.2, H2L can be doubly deprotonated at −70 °C with two equiv. of n-butyl lithium in THF; the dilithium complex 3.1 forms as a precipitate. The solid state structure of 3.1 has been confirmed by X-ray crystallography. As shown in Figure 3.1, Li1 adopts a highly distorted tetrahedral coordination geometry with two amido nitrogen donor atoms, one oxygen donor atom from a terminal THF ligand, and another oxygen donor atom from a bridging THF ligand occupying the four coordination sites. The coordination geometry around Li2 is better described as trigonal pyramidal with N1 and N2 from the chelating ligand and O3 from the terminal THF ligand occupying the trigonal base and O2 from the bridging THF ligand occupying the apical position. Within the trigonal base, the N1–Li2–O3, N1–Li2–N2, and N2–

Li2–O3 angles are 154.4(2), 82.7(1), and 121.9(2)°, respectively. The dihedral angle between the phenylenediamide plane and the Li1–N1– N2 plane is ~74°, while that between the phenylenediamide plane and the Li2–N1–N2 plane is ~24°. The analogous monomeric dilithium complexes of N,N′-disilyl-o-phenylenediamide derivatives reported by Lappert and coworkers 28 have both a three-coordinate and a four-coordinate lithium centre, with one and two terminal THF ligands, respectively. The solution 1H NMR spectrum of 3.1 reveals a symmetrical molecule in solution, i.e ., the protons of the dipp groups on both sides of the phenylenediamide plane show only one set of 1H resonances. The three THF ligands only show one set of resonances at 1.17 and 3.25 ppm, similar to those reported by Lappert. The single peak in the solution 7Li{1H} NMR spectrum at 2.6 ppm also suggests that the two lithium centres are equivalent in solution. The N2–C30 and N1–C25 bond lengths are 1.396(2) and 1.395(2) Å, respectively, consistent with the typical C–N single bond in phenylenediamide complexes. The bridging coordination mode and the distorted tetrahedral local geometry of the two nitrogen atoms also suggest that both nitrogen atoms are sp 3 hybridized amido nitrogen with two lone pairs on each.

Scheme 3.2 Synthesis of dilithium complex 3.1

Figure 3.1 Molecular structure of 3.1 (50% probability thermal ellipsoids). All hydrogen atoms and the iso-propyl groups are omitted and only one orientation of the disordered portion is shown for clarity. Selected bond lengths (Å): C30–N2 1.396(2), C25–N1 1.395(2), C30–C25 1.439(2), C25–C26 1.399(3), C26–C27 1.400(3), C27–C28 1.384(3), C28–C29 1.397(3), C29–C30 1.394(3); selected bond angles (°): O1–Li1–O2 114.09(16), O1–Li1–N2 122.92(17), O2–Li1–N2 106.25(15), O1–Li1–N1 128.91(17), O2–Li1–N1 99.46(15), N1–Li1–N2 78.65(12), O3–Li2–N2 121.89(18), O3–Li2–N1 154.4(2), N1–Li2–N2 82.72(13), O3–Li2–O2 95.62(15), N2–Li2–O2 94.64(15), N1–Li2–O2 88.57(13).

3.3.2 Fe coordination chemistry of 3.1

3.1 reacts with FeBr 2(THF)2 in THF at −60 °C to afford a navy blue solution. Our efforts to determine the identity of the blue species have been unsuccessful. However, the addition of toluene to the reaction mixture after the removal of solvents yields compound 3.2 (Scheme 3.3), the formation of which is accompanied by a rapid colour change to purple. Compound 3.2 can

also be synthesized by reacting 3.1 with [Fe(HMDS) 2(THF)] in toluene at elevated temperatures

Scheme 3.3 Two syntheses of 3.2

In the 1H NMR spectrum, three resonances at 5.41, 5.19, and 4.91 ppm (triplet, doublet, triplet) suggest the presence of highly shielded arene protons, consistent with an η 6-toluene adduct of iron. 29,30 An X-ray diffraction study revealed the two-legged ‘piano stool’ structure of 3.2 (Figure 3.2). The C30–N2 and C25–N1 bond lengths are 1.361(2) and 1.366(2) Å, respectively, shorter than those in 3.1 , but longer than a typical C–N double bond. The C25–C30 bond length (1.419(2) Å) in 3.2 is shorter than that in 3.1 (1.439(2) Å). The bond lengths in the phenylene backbone reveal subtle dearomatization: the average C β–Cγ bond length in 3.2 is

1.377(2) Å, and the average C α–Cβ bond length is 1.411(2) Å. The metric parameters suggest that the N,N -chelating ligand may contain radical character with the unpaired electron antiferromagnetically coupled with that of the low spin iron(I) centre. Wieghardt and coworkers have demonstrated that when ambiguous, the oxidation states of metal and bis-o-substituted N or O-donor ligands may be assigned based on high-quality single crystal X-ray diffraction data. 31 The metric parameters of the chelating ligand in 3.2 compare well with literature examples of o- diiminosemiquinonate complexes characterized crystallographically. 15,19,21-24,32,33

Figure 3.2 Molecular structure of 3.2 (50% probability thermal ellipsoids). All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe–N1 1.880(1), Fe1–N2 1.882(1), C25–N1 1.366(2), C30–N2 1.361(2), C25–C26 1.409(2), C26–C27 1.379(2), C27–C28 1.407(2), C28–C29 1.374(2), C29–C30 1.413(2), C30–C25 1.419(2), C32–C33 1.419(2), C33– C34 1.409(3), C34–C35 1.414(3), C35–C36 1.415(2), C36–C37 1.409(3), C37–C32 1.415(2); N2–C30– C25 113.2(1), N1–C25–C30 112.8(1), N2–Fe1–N1 82.68(5).

To obtain further information on the electronic structure of compound 3.2 , we carried out DFT calculations. Our calculations showed that the model with antiferromagnetically coupled Fe(I) and open-shell ligand is more stable than the closed shell model with Fe(0) and a neutral diimine ligand. The spin density on the Fe centre is 0.734 in the antiferromagnetically coupled model, smaller than 1.00, which is reasonable due to covalent bonding and orbital mixing between the metal and the ligand. 20 The simplified orbital spin density analysis (i.e., if the spin density is lower than 0.5, the orbital is considered empty; if the spin density is greater than 0.5, the orbital is considered occupied) showed the following electronic configuration on Fe: empty 2 2 2 dyz , singly occupied d xy , and doubly occupied d xz , d x −y , and d z orbitals. Although the actual electronic structure is somewhere in between the two extremes, i.e., Fe(0) with a neutral diimine ligand and Fe(I) with a monoanionic radical ligand, such an approximation in orbital spin density

66 analysis indicates that the actual structure is closer to the latter extreme in the continuum. Overall, the transformation from 3.1 to 3.2 involves reduction of Fe(II) to Fe(I) by the electron- rich dilithium diamide complex, accompanied by coordination of the oxidized ligand. Chirik et al . have characterized a series of complexes with a general formula [ArN=C(Me)C(Me)=NAr]Fe(η 6-arene), via the reduction of dichloro [ArN=C(Me)C(Me)=NAr]Fe(II) complexes with sodium amalgam in the presence of various arenes. 30 When the Ar group in Chirik’s complexes is 2,6-diisopropylphenyl and the η 6-arene is toluene, the average C-N bond length is 1.359(3) Å, which is similar to that in 3.2 . The average C–C bond length of the η 6-toluene in 3.2 is 1.414(3) Å, nearly identical to that in Chirik’s complex (1.412(5) Å). When a THF solution of 3.2 is stirred under an atmosphere of CO, the toluene ligand can be displaced to yield LFe(CO) 3 (3.3 ), in which three CO ligands are coordinated to the iron centre (Scheme 3.4). The diagnostic NMR data for 3.3 are the absence of the shielded η 6-toluene peaks from the 1H NMR spectrum and the presence of a highly deshielded carbonyl peak at 210 ppm in the 13 C{1H} NMR spectrum. The crystal structure of 3.3 is shown in Figure 3.3. The C25–N1 and C30–N2 bond lengths are 1.354(3) and 1.353(3) Å, respectively, and are marginally shorter than those in 3.2 but not statistically different.

CO OC CO Fe I Fe 0 1 atm CO dipp N N dipp dipp N N dipp

3.2 3.3

Scheme 3.4 Synthesis of 3.3

Figure 3.3 Molecular structure of 3.3 (50% probability thermal ellipsoids). All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe1– N2 1.914(2), Fe1–N1 1.911(2), Fe1–C31 1.799(2), C31–O1 1.132(3), C30–N2 1.353(3), C25–N1 1.354(3), C25–C26 1.414(3), C26–C27 1.367(4), C27–C28 1.416(3), C28–C29 1.369(4), C29–C30 1.414(3), C25– C30 1.425(3); N2–C30– C25 113.2(2), N1–C25–C30 113.1(2), N1–Fe–N2 81.15(8).

To elucidate the electronic structure of 3.3 , we performed DFT calculations. The spin- unrestricted and restricted models from DFT are similar in energy. In the spin-unrestricted model, the spin density on the Fe centre is 0.391. The simplified orbital spin density analysis on such a structure showed the following electronic configuration on Fe: empty d yz , and doubly 2 2 2 occupied d xz , d xy , d x −y , and d z orbitals, consistent with Fe(0). Such an approximation indicates that the actual structure is closer to the Fe(0) with a neutral diimine ligand extreme in the continuum. Presumably the better π-acceptor ligand, CO induced further electron transfer from the chelating ligand to the metal centre.

3.3.3 Mo coordination chemistry of 3.1

The reaction of 3.1 with 0.5 equiv. of MoCl 4(THF)2 afforded emerald green

(LiL) 2MoCl 2(THF)4 (3.4) (Scheme 3.5), which is diamagnetic as evidenced by its sharp NMR spectra within the normal chemical shift range. The 7Li{1H} NMR spectrum reveals the presence

68 of lithium in the product: one singlet at 1.37 ppm, which is upfield shifted compared to the singlet at 2.62 ppm present in the starting material 3.1 . In the 1H NMR spectrum, the conversion of 3.1 to 3.4 is accompanied by the disappearance of multiplets at 6.57 and 6.33 ppm corresponding to the phenylene protons of the starting material, and the appearance of resonances at 5.30, 4.88, 2.69, and 2.30 ppm hinting at the vanished aromaticity of the phenylene ring. The solid state structure of 3.4 was confirmed by a single crystal X-ray diffraction study.

As shown in Figure 3.4, the molecule is C 2 symmetric with a crystallographically imposed C 2 axis bisecting the Cl–Mo–Cl′ angle, relating a pair of chloride ligands and a pair of organic ligands, respectively. Interestingly, instead of coordinating to the molybdenum centre, the two nitrogen donor atoms of each organic ligand are coordinating to a lithium centre, which are each further ligated by two THF molecules via the oxygen donor atoms. The C25–N1 and C30– N2 bond lengths are 1.314(7) and 1.309(7) Å, respectively, which are more similar to C–N bond lengths found in typical diimine complexes (~1.28 Å)34 than those found in typical diamide complexes (~1.38–1.42 Å). 35 The short C–N and long C–C bond (C25–C30 1.498(6) Å) lengths are consistent with a neutral diimine ligand. The molybdenum centre is sandwiched between two central C 6 rings of two diimine ligands. Both C 6 rings coordinate to the molybdenum centre in an η4 fashion.

Scheme 3.5 Reactivity of 3.1 towards MoCl 4(THF) 2

Figure 3.4 Molecular structure of 3.4 (50% probability thermal ellipsoids). All hydrogen atoms and the isopropyl groups are omitted and only one orientation of the disordered portion is shown for clarity. Selected bond lengths (Å) and angles (°): Mo1–Cl1 2.522(1), Mo1–C26 2.342(5), Mo1–C27 2.214(5), Mo1–C28 2.207(6), Mo1–C29 2.363(6), C25–N1 1.314(7), C30–N2 1.309(7), N1–Li1 2.032(8), N2–Li1 2.043(9), C25–C26 1.425(6), C26–C27 1.437(7), C27–C28 1.405(6), C28–C29 1.444(7), C29–C30 1.428(9), C25–C30 1.498(6); Cl1–Mo–Cl1’ 90.60(4).

Such a coordination mode for an opda-type ligand is complementary to one mode 4 reported for a MoRu 2 trimetallic complex by Mealli et al ., in which Mo is coordinated η to the opda ligand through both amide nitrogens and both α-carbons. 36 A more similar coordination 2− mode has been observed in Mo–Al bimetallic complexes of the [ o-C6H4-(NSiMe 3)2] ligand 37 4 reported by Boncella and coworkers. Distinct from this literature example, the η -C6 ring in compound 3.4 is far from planar; the ring folds about the C26–C29 vector with a dihedral angle of ~30° into a boat conformation, suggesting the diminishing conjugation between the coordinating portion (C26, C27, C28 and C29) and the remaining portion of the C 6 ring. Such folding is similar to that observed in the Mo-(η 4-phenazine) type of complexes reported by

Parkin and coworkers. 38 The coordination environment around the molybdenum centre in 3.4 is 4 39 reminiscent of the pseudo-C2 symmetric (Me 3P) 2Mo(η -butadiene) 2, in that both compounds bear two syn diene ligands and two mutually cis monodentate ligands, and have an overall 4 coordination number of six. The P–Mo–P angle in (Me3P) 2Mo(η -butadiene) 2 is 98°, slightly larger than the 91° Cl–Mo–Cl angle in compound 3.4 , likely due to sterics. An additional structural parameter that has been used to describe butadiene ligand geometry is Φ, the dihedral angle between the butadiene plane and the plane containing both outer carbons and the metal 40 centre (C o–M–C′o, see Figure 3.5A). In this respect, 3.4 (~78°) closely resembles 4 (Me 3P) 2Mo(η -butadiene) 2 (~83°). Butadiene complexes are most often compared to two canonical resonance forms, namely the neutral diene bound to a metal and the metallacyclopentene form in which the metal is formally oxidized by two electrons (see Figure 41 3.5B). Metallacyclopentenes feature a negative Δd, where Δd = avg. M–Co bond length – avg.

M–Ci bond length. Another characteristic feature of metallacyclopentenes is that C i–Ci bonds are shorter than C i–Co bonds. In “neutral” butadiene complexes however, Δd is positive and all 41 butadiene C–C bond lengths are similar. The Δd parameter for 3.4 is 0.142(6) Å and the C i–Ci 4 (1.405(6) Å) and C i–Co (avg. 1.440(7) Å) bond lengths in 3.4 are statistically similar. Similar η - butadiene C–C bond lengths and the positive Δd are indicative of a neutral butadiene complex. 4 Similarly, in [(Me 3P) 2Mo(η -butadiene) 2] the Δd (0.065 Å average of both ligands) is positive, and the average C i–Ci (1.398(5) Å) and C i–Co (1.413(5) Å) distances are statistically indistinguishable. Although the actual structure of the η 4-ligand in 3.4 still lies between the two extreme resonance forms, all the metric parameters from the crystal structure of 3.4 suggest that the neutral diolefin resonance form of the η 4 -ligand contributes more to the overall structure. In 4 the resonance structure with an L 2 type η -ligand, the molybdenum centre has a formal oxidation state of zero. The reaction between 3.1 and MoCl 4(THF)2 could then be rationalized as the reduction of Mo(IV) to Mo(0) by 2 equiv. of 3.1 where the diamido ligand is oxidized into diimine.

Figure 3.5 A: Labelling of inner (C i) and outer (C o) butadiene carbons; B: Two extreme resonance forms of a generic diene complex

Figure 3.6 Molecular structure of 3.5 (30% probability thermal ellipsoids). Hydrogen atoms are omitted and only one orientation of the disordered portions is shown for clarity. Selected bond lengths (Å) and angles (°): Li1–O1 1.882(7), Li1–N1 1.974(6), N1–C13 1.341(3), C13–C13’ 1.478(5), C13–C14 1.422(4), C14–C15 1.364(4), C15–C15’ 1.400(6); O1–Li1–N1 136.7(2), N1– Li1–N1’ 86.7(3), C13–N1– C7 117.8(2), C13–N1–Li1 109.4(2), C7–N1–Li1 132.9(2).

When the reaction between 3.1 and MoCl 4(THF)2 is quenched within 2 h, an emerald green paramagnetic species 3.5 can be isolated by a rapid crystallization. Alternatively, 3.5 can be synthesized by reacting 3.1 with EuCl 3(dme) 2. The crystal structure of 3.5 is shown in Figure

3.6. The molecule of 3.5 has a crystallographically imposed C 2 symmetry along the Li1–O1 direction. The lithium centre adopts a trigonal planar coordination geometry with two nitrogen donor atoms from the chelating ligand and an oxygen donor atom from an ether ligand occupying the three coordination sites. The N,N-chelate ligand has a net charge of −1, i.e., a radical bound to a relatively innocent Li ion. Interestingly, the N1–C13 and C14–C15 bond lengths (1.341(3) and 1.364(4) Å, respectively) are shorter than their counterparts in 3.2, while the C13–C14 and C13–C13′ bond lengths (1.429(3) and 1.471(5) Å, respectively) are longer than their counterparts in 3.2. The C15–C15′ bond length (1.400(5) Å) is similar to its counterpart in 3.2.

3.3.4 EPR data for 3.5

Figure 3.7 EPR spectra of 3.5 in the solid state (top) and in toluene solution (bottom) at ambient temperature.

The solid state EPR spectrum of compound 3.5 (Figure 3.7) revealed a sharp singlet (ΔH = 2.8 G, with a Lorentzian profile) at g = 2.0030, close to the free electron value of 2.0036 consistent with an organic radical with little anisotropy and where spin– orbit coupling is negligible. The deep green solution of 3.5 in toluene at room temperature revealed a multiplet pattern consistent with delocalisation over the phenylenediamine framework as well as the Li atom. The observation of hyperfine coupling to 7Li is consistent with ion-pairing in solution and retention of the molecular structure determined from X-ray diffraction. In contrast, the Zn complex of the open-shell neopentyl- substituted phenylenediamide derivative showed a broad EPR signal at room temperature in a THF solution with unresolved hyperfine coupling. 33 The spin density distribution based on coupling to two chemically equivalent 14 N nuclei and two pairs of chemically distinct 1H nuclei (along with 7Li coupling) is consistent with a π-delocalised radical based on the ortho-phenylene framework and in good agreement with the computationally determined spin distribution. An estimate of the spin density distribution at C 42 can be made based on McConnell’s empirical relationship (aH = QCH ρC where QCH = 22.5 G). 43 Whilst similar relationships are proposed to exist for N (a N = QNρN and QN = 26.9 G), the behaviour for N appears more complex.43 Nevertheless the EPR data are consistent with a ligand-based π-radical (Figure 3.8).

3.3.5 V coordination chemistry of 3.1

3.1 reacts with VCl 2(tmeda) 2 to yield dark blue crystals of (LiL) 2V(OEt 2)2 (3.6 ) (Scheme 3.6) which is structurally analogous to Mo compound 3.4 (Figure 3.9) in that V is sandwiched 4 between two o-phenylene C 6 rings, engaging in two η -butadiene-type interactions. Distinct from the Mo species 3.4, 3.6 is homoleptic; no chloride ligands remain. Additionally, the Li atoms in 3.6 are three coordinate instead of four-coordinate. We were interested in the reactivity of 3.6 because of its electron rich nature. Given that 3.6 contains two redox active opda ligands and a redox active V centre we attempted reaction with PbCl 2, which is known to oxidize early metal 44 complexes. Reaction of 3.6 with PbCl 2 in THF caused the solution to change colour from dark blue to reddish purple. Analysis of the crude product by 1H NMR revealed a diamagnetic species containing the opda ligand. Analysis of single crystals of the diamagnetic product revealed its identity to be mononuclear lead complex LPb ( 4.1a). No oxidation occurred; instead the opda ligand transferred to Pb, presumably accompanied by the loss of LiCl and VCl 2. Synthesis and reactivity of LPb will be discussed in chapter 4.

Scheme 3.6 Synthesis of 3.6 and ligand transfer to Pb

Figure 3.9 Crystal structure of 3.6 . Hydrogen atoms, isopropyl groups, and cocrystallized solvent molecules omitted for clarity.

3.4 Conclusion

A dilithium complex of N,N′-bis(2,6-diisopropylphenyl)-o-phenylenediamide (3.1) was synthesized and its coordination behaviour towards halides of iron and molybdenum was investigated. Rather than forming mid-valent transition metal complexes with π-donating diamido ligands, the dilithium diamide complex 3.1 reduced both iron and molybdenum. In the case of Fe, the addition of external π-acceptor ligands, toluene and CO, facilitated the formation and isolation of low valent Fe complexes 3.2 and 3.3, respectively. In the case of Mo, the C 6 ring of the resulting pda 0 ligand lost its planarity and acted as a π-acceptor ligand that coordinated to the resulting low-valent Mo centre in an η 4 fashion to form 3.4 . The 1e − oxidation of 3.1 by Mo(IV) or Eu(III) yielded complex 3.5 featuring an open-shell ligand supported by a lithium cation. Complex 3.5 showed typical ligand-based radical character with delocalized spin density. The coordination chemistry of 3.1 towards other metals is underway in our laboratory.

3.5 Experimental

3.5.1 General considerations

All operations were performed using standard Schlenk techniques under a nitrogen atmosphere, 45 46 or in a nitrogen-filled MBraun glovebox. FeBr 2(THF )2, [Fe(HMDS) 2(THF)], 47 48 26 MoCl 4(THF)2, EuCl 3(dme) 2, and H 2L were prepared according to literature procedures. Glassware was either flame-dried or dried overnight in a 160 °C oven prior to use except for NMR tubes which were dried overnight in a 60 °C oven. Carbon monoxide was purchased from Linde and used without further purification, n-BuLi (1.6 M in hexanes) was purchased from

Aldrich. THF, toluene, ether, and benzene-d6 were dried over Na/benzophenone, distilled under nitrogen, and stored over activated molecular sieves. Room temperature solid state and toluene solution EPR spectra of 6 were recorded on a Bruker EMXplus X-band EPR spectrometer and spectra simulated using Winsim. 49 The 1H, 7Li{1H}, and 13 C{1H} NMR spectra were recorded on either a Varian 400 MHz, an Agilent 500 MHz, or an Agilent 600 MHz NMR spectrometer. All chemical shifts are reported in ppm relative to the residual protio-solvent peaks; 7Li{1H} NMR spectra are referenced externally using 9.7 M LiCl in D 2O. FT-IR spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer. Elemental analysis was performed by ANALEST at the University of Toronto.

3.5.2 Synthesis of Li 2L(THF)3 (3.1 )

N,N ′-bis(2,6-diisopropylphenyl)-o-phenylenediamine (0.50 g, 1.2 mmol) was dissolved in THF (5 mL) and cooled to −70 °C. A hexanes solution of n-butyl lithium (1.5 mL, 1.6 M) was added dropwise. The reaction mixture was removed from the cooling bath and stirred for 90 min, during which a precipitate formed. Hexane (10 mL) was added and the mixture cooled to −25 °C. The precipitate was collected on a frit and was washed with cold hexanes. Drying in vacuo left a light yellow powder (0.528 g, 67%), which was stored at −25 °C. Not only is this material extremely air-sensitive, but it also decomposes under prolonged vacuum at room temperature. Consequently no satisfactory analytical data were obtained. Single crystals were obtained from a 1 C6D6 solution sitting at room temperature for days. H NMR (400 MHz, 25 °C, C6D6) δ 7.39 (d, J = 7.6 Hz, 4H), 7.23 (t, J = 7.6 Hz, 2H), 6.57 (m, 2H), 6.33 (m, 2H), 3.43 (sept, J = 7.1 Hz, 4H), 3.25 (m, 12H), 1.38 (d, J = 6.8 Hz, 12H), 1.29 (d, J = 7 Hz, 12H), 1.17 (m, 12H). 13 C{ 1H}NMR

(100.58 MHz, C6D6, 25 °C) δ 153.6, 148.7, 144.4, 121.2, 115.0, 112.1, 68.9, 29.9, 25.8, 25.7, 7 1 25.2. Li{ H} NMR (233 MHz, 25 °C, C6D6): δ 2.6.

3.5.3 Synthesis of LFe(η 6-toluene) ( 3.2 )

Method A

Li 2L(THF)3 (0.10 g, 0.15 mmol) was dissolved in THF (2 mL) and cooled to an internal temperature of −60 °C. The resulting yellow solution was added smoothly to a −60 °C suspension of FeBr 2(THF)2 (0.055 g, 0.15 mmol) in THF (1 mL). The resulting mixture was allowed to warm to room temperature with stirring for 24 h. Volatiles were removed under vacuum, leaving a navy blue oil. The addition of toluene (3 mL) resulted in a purple solution which was stirred for 1 h. The mixture was filtered through Celite and dried under vacuum. The residue was dissolved in hexanes (1 mL) and cooled to −25 °C, resulting in the precipitation of 3.2 . The supernatant was decanted, leaving a purple solid (0.054 g, 61%) which was dried in vacuo. Trace LiBr could be removed by recrystallization from hexanes. NMR data for 3.2 obtained this way are identical to those obtained by method B.

Method B

H2L (0.47 g, 1.1 mmol) was added to a solution of Fe (HMDS) 2(THF ) (0.45 g, 1.0 mmol) in toluene (15 mL). The mixture quickly turned purple and was refluxed for 16 h. After cooling to room temperature, volatiles were removed under vacuum at 50 °C, leaving a purple solid which was dissolved in hexanes and filtered through Celite. The filtrate was concentrated and cooled to −25 °C, which caused formation of purple crystals which were collected on a frit and washed with cold hexanes, yielding 3.2 (0.29 g, 0.50 mmol, 50%). Single-crystals suitable for XRD studies were grown by slow evaporation of a hexanes solution at room temperature. Anal. Calcd 1 for C 37 H46 N2Fe: C, 77.34; H, 8.07; N, 4.88. Found: C, 77.09; H, 7.94; N, 4.88. H NMR (500

MHz, 25 °C, C6D6): δ 7.49 (t, J = 7.5 Hz, 2H), 7.41 (d, J = 7.5 Hz, 4H), 6.60 (m, 2H), 6.44 (m, 2H), 5.41 (t, J = 5.7 Hz, 1H), 5.19 (d, J = 6.1 Hz, 2H), 4.91 (t, J = 6 Hz, 2H), 3.28 (sept, J = 6.9 Hz, 4H), 2.10 (s, 3H), 1.46 (d, J = 6.9 Hz, 12H), 0.88 (d, J = 6.9 Hz, 12H). 13 C{ 1H} NMR

(125.72 MHz, 25 °C, C6D6): δ 153.6, 152.0, 143.8, 126.1, 124.2, 119.1, 115.2, 97.6, 83.4, 82.8, 81.4, 28.7, 26.5, 24.7, 19.8.

3.5.4 Synthesis of LFe(CO) 3 (3.3 )

3.2 (70 mg, 0.12 mmol) was dissolved in THF (2 mL) in a Schlenk bomb. The solution was degassed before one atmosphere of carbon monoxide was introduced. The solution was stirred for 16 h at room temperature. The solvent was removed under vacuum and the purple residue was extracted into hexanes and filtered through Celite. Concentration of the filtrate and storage at −25 °C yielded 3.3 as purple crystals. The supernatant was decanted and the solid dried in vacuo. (22 mg, 32%). Single crystals of 3.3 were grown by slow evaporation of a room temperature hexanes solution. Anal. Calcd for C 33 H38 N2O3Fe: C, 69.96; H, 6.76; N, 4.94. Found: −1 1 C, 70.01; H, 6.97, N, 4.91. IR (nujol, cm ) ν CO : 2049, 1985, 1977. H NMR (600 MHz, 25 °C,

C6D6): δ 7.29 (t, J = 7.9 Hz, 2H), 7.22 (d, J = 7.7 Hz, 4H), 6.62 (m, 4H), 2.86 (sept, J = 7.1 Hz, 13 1 4H), 1.34 (d, J = 7 Hz, 12H), 0.94 (d, J = 6.7 Hz, 12H). C{ H} NMR (150.9 MHz, C6D6, 25 °C): δ 210.0, 155.0, 150.7, 142.4, 128.0, 124.4, 123.3, 117.2, 28.6, 25.6, 24.7.

3.5.5 Synthesis of (LiL) 2MoCl 2(THF)4 (3.4)

To a suspension of MoCl 4(THF)2 (36 mg, 0.095 mmol) in THF (1 mL) which was precooled to

−60 °C was slowly added a solution of Li 2L(THF)3 (0.125 g, 0.19 mmol) in THF (2 mL) which was also precooled to −60 °C. The mixture immediately turned emerald green and was removed from the cold well and stirred for 18 h at ambient temperature. Volatiles were removed in vacuo and toluene (4 mL) was added. The solution was filtered through Celite and dried under vacuum. The residue was recrystallized from a mixture of THF and hexanes at −25 °C to yield green crystals, which were collected by vacuum filtration and dried under vacuum yielding 3.4 as a green powder (61 mg, 56%). Single crystals for XRD studies were grown by cooling a THF 1 solution to −25 °C. H NMR (400 MHz, 25 °C, C6D6) δ 7.33 (dd, J = 7.6 Hz, J = 1.4 Hz, 2H), 7.24 (dd J = 5.6 Hz, J = 1.4 Hz, 2H), 7.22 (dd, J = 5.6 Hz J = 1.6 Hz, 2H), 7.15 (dd overlapped with solvent peak, J = 1.8 Hz, 2H), 7.10 (td, J = 7.6 Hz J = 2.9 Hz, 4H), 5.30 (td, J = 6.8 Hz, J = 1.4 Hz, 2H), 4.88 (dd, J = 6.8 Hz, J = 2.4 Hz, 2H), 4.20 (sept, J = 6.8 Hz, 2H), 3.40 (m, 6H), 3.31 (m, 16H), 2.69 (dt, J = 7.4 Hz, J = 2.6 Hz, 2H), 2.30 (dd, J = 6.6 Hz, J = 1.0 Hz, 2H), 1.90 (d, J = 6.7 Hz, 6H), 1.51 (d, J = 6.8 Hz. 6H), 1.43 (d, J = 6.8 Hz, 6H), 1.30 (m, 18H), 1.17 (d, J = 6.8 13 1 Hz, 6H), 1.04 (d, J = 6.8 Hz, 6H). C{ H} NMR (100.58 MHz, C6D6, 25 °C): 167.9, 164.9, 147.0, 146.6, 142.3, 141.9, 140.2, 139.8, 125.0, 124.7, 124.1, 123.9, 123.7, 123.6, 86.8, 86.4, 76.6, 68.6, 61.1, 28.9, 28.8, 28.2, 27.8, 26.2, 25.7, 25.1, 24.8, 24.8, 24.7, 24.6, 24.1, 23.1, 14.7.

7 1 14.6. Li{ H} NMR (233 MHz, 25 °C, C6D6): δ 1.4. Anal. Calcd for C 76 H108 N4O4MoLi 2Cl 2: C, 69.02; H, 8.23; N, 4.23. Found: C, 68.78; H, 8.00; N, 4.41.

3.5.6 Synthesis of LiL(Et 2O) ( 3.5 )

LiL(Et 2O) ( 3.5 ) was first isolated as a minor product in the reaction of MoCl 4(THF )2 with two equivalents of Li 2L(THF)3 when the reaction time was shortened from 18 h to 2 h. However, 3.5 could be synthesized more reliably and in better yield from the reaction of Li 2L(THF)3 with

EuCl 3(dme) 2. In a glove box cold well, a solution of Li 2L(THF)3 (69 mg, 0.11 mmol) in THF (2 mL) was cooled to an internal temperature of −70 °C. This solution was smoothly added to a similarly cooled suspension of EuCl 3(dme) 2 (46 mg, 0.11 mmol) in THF (2 mL). The mixture turned emerald green instantly, was allowed to warm to 28 °C, and was then stirred for 18 h. Volatiles were removed in vacuo, ether (5 mL) was added, and the mixture was filtered. After removal of ether and recrystallization from cold pentane, green crystals of LiL(Et 2O) (18 mg,

34%) were isolated, and stored at −25 °C. Anal. Calcd for C 34 H48 N2OLi: C, 80.43; H, 9.53; N, 5.52. Found: C, 79.78; H, 9.46; N, 5.49.

3.5.7 Synthesis of (LiL) 2V(OEt 2)2

A solution of Li 2L(THF)3 (110 mg, 0.17 mmol) in THF (3 mL) was cooled using a glove box cold well to -70 °C. This solution was added smoothly to a similarly cooled suspension of

VCl 2(tmeda) 2 (30 mg, 0.085 mmol) in THF (2 mL). The mixture was allowed to come to room temperature and stirred for 16 h, leaving a green solution. Volatiles were removed in vacuo and toluene (3 mL) was added, yielding a deep blue solution, which was filtered. Solvents were again removed and crystals were grown from a diethyl ether solution. The supernatant was removed, the crystals were washed with cold diethyl ether and then dried in vacuo . Yield: 58 mg, 66% Despite multiple attempts to purify this material, we were not able to achieve satisfactory combustion analysis. Anal. Calcd for C 68 H96 N4Li 2O2V: C, 76.59; H, 9.07; N, 5.25. Found: C, 74.96; H, 9.10; N, 5.20.

3.6 X-ray crystallography

All crystals were mounted on the tip of a MiTeGen MicroMount and the single-crystal X-ray diffraction data were collected on a Bruker Kappa Apex II diffractometer. All data were collected with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K controlled by

an Oxford Cryostream 700 series low temperature system. The diffraction data were processed with the Bruker Apex 2 software package. 49 Raw data were processed and multiscan absorption corrections were applied using the Bruker Apex 2 software package. 50 All structures were solved by the direct methods and refined using SHELXTL V6.14. 51 Compounds 3.1 , 3.2 , 3.3 , 3.4

crystallized in the monoclinic space groups P21, P21/n, P21/c, and C2/ c, respectively, while 3.5

crystallized in the tetragonal space group P43212. The disordered THF ligand in 3.4 and the ether ligand and isopropyl groups in 3.5 were modeled successfully. The residual diffuse electron density from disordered, unidentified solvent molecules in the lattice of 3.4 was removed with the SQUEEZE function of the PLATON program 52 and their contributions were not included in the formula. All non-hydrogen atoms were refined anisotropically, except for the disordered portions. In all structures hydrogen atoms bonded to carbon atoms were included in calculated positions and treated as riding atoms. The crystallographic data are summarized in Table 3.1. All DFT calculations were performed with B3LYP functional 53, 54 using the Gaussian 09 software package 55 and NBO 56 analysis was performed on the optimized structures. The 6-311G** basis set was used for calculations of 3.2 and 3.3 . All geometry optimizations were performed starting from X-ray crystal structures. Both the 6-311G** and 6-31G* basis sets were used for the model complex simplified from compound 3.5 for spin density distribution. The coordinates of the model compound were derived from the crystal structure of 3.5 without geometry optimization.

Table 3.1 Crystallographic data for compounds 3.1-3.5 .

3.1 3.2 3.3 3.4 ●C 6H14 3.5

Formula C42 H62 Li 2N2O3 C37 H46 FeN 2 C33 H38 FeN 2O3 C76 H108 Cl 2Li 2MoN 4O4●2C 6H14 C34 H48 LiN 2O

F.W. 656.82 574.61 566.50 1494.81 507.68

T (K) 150(2) 150(2) 150(2) 150(2) 150(2)

Space group P21 P21/n P21/c C2/c P43212 a (Å) 10.9983(4) 10.5211(6) 16.3433(8) 21.8154(18) 10.4120(5) b (Å) 17.1078(7) 17.3327(11) 11.3928(6) 25.039(3) 10.4120(5) c (Å) 11.1127(4) 17.7438(17) 16.7688(8) 16.3773(13) 29.2952(12)

 (deg) 90 90 90 90 90

 (deg) 110.887(2) 91.243(4) 106.838(2) 118.185(4) 90

(deg) 90 90 90 90 90

V (Å 3) 1953.52(13) 3235.0(4) 2988.4(3) 7885.3(14) 3175.9(3)

Z 2 4 4 4 4

-3 Dc (g·cm ) 1.117 1.180 1.259 1.114 1.062

 (mm-1) 0.068 0.493 0.539 0.280 0.062 no. reflns collcd 11742 28548 60497 23017 28934 no. indept reflns 3461 7423 9186 6890 2193

GOF on F 2 1.027 1.020 1.005 1.061 1.016

R1=0.0296 R1 = 0.0350 R1 = 0.0570 R1 = 0.0820 R1 = 0.0587

R [I > 2 (I)] wR 2=0.0779 wR 2 = 0.0855 wR 2 = 0.1111 wR 2 = 0.1700 wR 2 = 0.1633

R1=0.0310 R1 = 0.0493 R1 = 0.1224 R1 = 0.1277 R1 = 0.0823

R (all data) wR 2=0.0791 wR 2 = 0.0927 wR 2 = 0.1328 wR 2 = 0.1829 wR 2 = 0.1855

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4 Chapter 4 Reactivity of heavy carbene analogues towards oxidants: redox active ligand-enabled isolation of a paramagnetic stannylene 4.1 Abstract

A new o-phenylenediamido Pb(II) complex LPb ( 4.1a ) was synthesized from H 2L and

Pb(hmds) 2. LPb and its tin relative LSn ( 4.1b ) react with mesityl azide in analogous C-H activation reactions: nitrene insertion into a benzylic C-H bond of a nearby dipp group occurs to yield three coordinate species L(NMes)Pb ( 4.2a ) and L(NMes)Sn ( 4.2b ). LPb reacts with common oxidant ONMe 3 to form Lewis pair LPb(ONMe 3) ( 4.3 ). LSn reacts with AgOTf yielding paramagnetic stannylene LSn(OTf) ( 4.4 ); such compounds have been previously observed in situ , but hitherto have not been isolated.

4.2 Introduction

Since the discoveries of Bertrand 1 and Arduengo, 2 the field of carbene research has blossomed 3 to span the fields of synthesis, catalysis, materials, medicine 4, 5 and surface chemistry. 6 Among the group 14 carbene analogues,7 Pb and Sn derivatives (plumbylenes and stannylenes) were first to be synthesized, by Lappert in 1973. 8 This initial report piqued interest in the chemistry of plumbylenes and stannylenes. 9-17 Numerous recent publications have expanded upon known reactivity patterns of two-coordinate plumbylenes and stannylenes: alkyl, aryl, boryl, and amido substituents have been employed by the groups of Li and Kira, 18, 19 Power, 20-22 Aldridge, 23, 24 and Hahn, 25, 26 respectively. In addition, Jones reported hydrido stannylenes 27, 28 capable of effecting catalytic hydroboration of carbonyl compounds, and Saito has employed group 14 dilithiometalloles 29-31 as precursors for zero-valent tin compounds 32 and as ditopic ligands for transition metals. 33,34 Wesemann, 35-37 Goicoechea, 38-40 and others 41-44 have also made recent contributions.

Redox active ligands have captured the imagination of the chemistry community. 45-47 In 2004, Cherkasov et al. employed redox active 1,4-diazabutadiene (DAB) ligands to observe the first examples of paramagnetic stannylenes. These compounds were generated in situ from 48 mixtures of radical anion DAB ligands and SnCl 2. These paramagnetic stannylenes decompose

84 in solution to tin metal even at -20 °C and could not be isolated. In subsequent reports employing DAB 49 and o-phenylene-linked OO and NO ligands, 50 paramagnetic stannylenes were observed by EPR, but in all cases isolation was unsuccessful. Herein we report reactivity of o- phenylenediamido (opda) Pb(II) and Sn(II) complexes, 4.1a and 4.1b , towards a variety of oxidants culminating in the synthesis and isolation, for the first time, of a paramagnetic stannylene.

4.3 Results and Discussion

4.1a was synthesized by heating N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamine 25 (H2L) with Pb(hmds) 2 in toluene by a procedure similar to Hahn’s preparation of 4.1b . The formation of 4.1a is accompanied by a drastic colour change from orange to reddish purple. Multinuclear NMR spectra of 4.1a are shown in Figures 4.1 and 4.2. A characteristic feature of 1 3 the H NMR spectrum of 4.1a in C 6D6 is a heptet centred at 2.97 ppm ( JCH = 6.9 Hz) arising from the four equivalent methine protons on the 2,6-diisopropylphenyl (dipp) groups. In the 207 Pb{1H} NMR spectrum of 4.1a , a broad peak was located at 3279 ppm, which is similar to the only other shift reported for a two-coordinate NHPb. 51 This chemical shift agrees well with the shift predicted using Wrackmeyer’s empirical correlation (3049 ppm) of δ 207 Pb with δ 117 Sn for pairs of structurally analogous Sn(II) and Pb(II) compounds. 52

Scheme 4.1 Synthesis of 4.2a/b

Single crystals of 4.1a were grown from cold THF/pentane; the X-ray molecular structure is shown in Figure 4.3. The asymmetric unit contains half of one 4.1a molecule: a crystallographic mirror plane bisects the o-phenylene plane and relates one dipp group to the other. The two- coordinate lead atom adopts a bent geometry; the two nitrogens of L chelate Pb with a N1-Pb1-

N1’ bite angle of 77.6(2)°, and the five membered C2N2Pb chelate ring is essentially planar. Akin to the structure of tin analogue 4.1b 25 , 4.1a features intermolecular Pb1-C3(C3’) interactions 3.585 Å in length.

1 Figure 4.1 H NMR Spectrum (500 MHz, 25 °C) of 4.1a in C 6D6. Inset: expanded aryl region.

13 1 207 1 Figure 4.2 C{ H} NMR Spectrum (151 MHz, 25 °C) of 4.1a in C 6D6. Inset: Pb{ H} NMR

Spectrum (105 MHz, 25 °C) of 4.1a in C 6D6.

Figure 4.3 Molecular structure of 4.1a with 30% probability ellipsoids (left), and crystal packing of 4.1a with 50% probability ellipsoids (right). Hydrogen atoms omitted for clarity. Only one orientation of disordered isopropyl groups shown at left; isopropyl groups are omitted for clarity at right. Selected bond lengths (Å) and angles (°): Pb1-N1 2.134(3), C1-N1 1.330(4), C1-C1’

1.422(7), C1-C2 1.353(5), C2-C3 1.337(5), C3-C3’ 1.362(8); intermolecular distance Pb1--- C3(C3’) 3.585; N1-Pb1-N1’ 77.60(16).

Figure 4.4 Cyclic voltammogram of 4.1a (0.5 mM in THF, 0.1M NBu 4PF 6). The first and second oxidation peaks denoted by the * occur at -92 mV and 143 mV vs Fc/Fc +, respectively.

With 4.1a in hand, we obtained a cyclic voltammogram in THF solution. Scanning in the anodic direction revealed two oxidation peaks (Figure 4.4), which indicated that electrons are available for reactivity towards chemical oxidants. We sought to use the electrons within the opda framework of L 2- to accomplish a chemical reduction, so we reacted 4.1a with mesityl azide (MesN 3), which is potentially a two-electron oxidant. Heating the reaction to 110 °C over three days caused a subtle colour change from purple to burgundy accompanied by conversion of 1 4.1a to a new species, 4.2a (Scheme 4.1) . The H NMR spectrum of 4.2a in C 6D6 (Figure 4.5) contains three new singlets (6.65, 2.08 and 1.85 ppm) in a 2:3:6 ratio indicating incorporation of the mesityl group. Also, the number of resonances associated with the opda ligand increased, demonstrating a loss of mirror plane symmetry. For example, the methine isopropyl resonance appears as a multiplet due to the overlapping of inequivalent protons. Furthermore, this multiplet corresponds to three protons, suggesting the reaction had occurred at one of these methine isopropyl C-H bonds. In the 207 Pb{1H} NMR spectrum (Figure 4.6 inset), the lone resonance

88 shifted upfield from 3279 ppm to 2657 ppm, which indicates a more shielded 207 Pb nucleus consistent with an increase in coordination number.52

1 Figure 4.5 H NMR Spectrum (400 MHz, 25 °C) of 4.2a in C 6D6. Note: spectrum contains resonances due to trace THF, pentane, and grease marked with an asterisk. Inset: expanded aryl region.

13 1 207 1 Figure 4.6 C{ H} NMR Spectrum (126 MHz, 25 °C) of 4.2a in C 6D6. Inset: Pb{ H} NMR

Spectrum (105 MHz, 25 °C) of 4.2a in C 6D6.

Figure 4.7 Molecular structure (left) and crystal packing of 4.2a (right). Selected bond lengths (Å) and angles (°): Pb1-N1 2.207(2), Pb1-N2 2.178(2), Pb1-N3 2.839(3), C1-N1 1.389(3), C2- N2 1.379(3), C1-C2 1.425(4), C2-C3 1.403(4), C3-C4 1.386(4), C4-C5 1.388(4), C5-C6 1.384(4) C1-C6 1.404(4); intermolecular distances Pb1-C9 3.658 Pb1-C10 3.675(3); N1-Pb-N2 75.06(8), N1-Pb1-N3 106.03(8), N2-Pb2-N3 71.10(8). At left, non-hydrogen atoms are shown as 30% probability ellipsoids, the hydrogen atom H3a on N3 is shown as a sphere of arbitrary radius; all other hydrogen atoms are omitted for clarity. At right, all hydrogen atoms and most alkyl carbons are omitted for clarity.

X-ray crystallography revealed 4.2a to be the product of insertion of mesityl nitrene into the benzylic C-H bond of one of the isopropyl groups of 4.1a (Figure 4.7). The newly formed secondary amine forms a dative interaction with the Pb centre (a Pb1-N3 bond length of 2.839(3) Å), raising the coordination number of Pb to three. The lead centre resides at the apex of a highly distorted trigonal pyramid (the sum of the angles at Pb is 252.2(1)°), indicating the presence of a stereochemically active lone pair of electrons at the Pb(II) centre. Ideally, a Lewis base would donate into the plumbylene’s vacant p orbital to make an angle of 90° with the C 2N2Pb chelate plane. Distinct from this ideal case, the actual angle (at which the plane defined by N3-Pb1-N2

intersects the plane of the C 2N2Pb chelate ring), is ca. 67°, which can be explained by restriction imposed by the three-carbon tether between nitrogen donors. The long C phenylene -N bonds (C1-N1 1.389(3), C2-N2 1.379(3)) confirm that L retains its 2- oxidation state and that oxidation of an isopropyl carbon occurred instead of oxidation of the opda ligand framework. 4.2a crystallizes as pairs of enantiomers in which each molecule’s Pb centre makes a close contact with the other’s N-dipp group.

Ours is the first report of reactivity of an NHPb with an azide; more broadly, reports of plumbylene reactivity with azides are scarce: Klinkhammer found that adamantyl azide (AdN 3) inserts into the Pb-Si bond of a disilylplumbylene.53 Recently, Wesemann et al. reacted a two- coordinate plumbylene bearing a pendant phosphine with AdN 3, which used its distal N to form a 36 bridge between the Lewis acidic Pb centre and the Lewis basic P atom. In both reports, no N 2 is expelled and all three azide nitrogens are incorporated into the product.

We also subjected the NHSn analogue 4.1b to these reaction conditions. Instead of a 3 day reaction period, the reaction was complete after 24 h. Analogous insertion into the benzylic C-H bond of the dipp group occurred to generate 4.2b . Similar changes to the 1H NMR spectrum 119 1 25 occurred (Figure 4.8). The Sn{ H} NMR peak in C 6D6 shifted upfield from 216.1 ppm to 69.1 ppm (Figure 4.9 inset). The molecular structure of 4.2b is very similar to that of 4.2a except that the intermolecular close contacts between metal centre and dipp group are absent. Additionally, the dative bond between the newly formed secondary amine and the Sn centre is much shorter (2.555(5) Å), reflecting the smaller size of tin compared to lead.

Stannylene reactivity towards azides has been investigated before:36, 42, 54-62 common reactivity modes include stannaimine formation (R 2Sn=NR’) and coupling of stannaimine and 57 azide to make tetraazastannolines (R 2Sn(R’N-N=N-NR’)). Although Meller and Weidenbruch 58 have observed C-H activation in azide reactions with diamino and diaryl stannylenes respectively, formation of 4.2b is unique in that it involves formation of a new C-N bond and maintenance of the 2+ oxidation state for tin.

1 Figure 4.8 H NMR Spectrum (600 MHz, 25 °C) of 4.2b in C 6D6.Note: residual pentane resonance marked with an asterisk. Left inset: expanded aryl region. Right inset: expanded alkyl region.

13 1 119 1 Figure 4.9 C{ H} NMR Spectrum (151 MHz, 25 °C) of 4.2b in C 6D6. Inset: Sn{ H} NMR

Spectrum (224 MHz, 25 °C) of 4.2b in C 6D6.

2- We turned next to Me 3NO with the hope of reducing it using electrons from L . Reacting a toluene solution of 4.1a with ONMe 3 caused a gradual colour change from purple to blood-red

1 as 4.3 formed (Scheme 4.2). The H NMR spectrum in C 6D6 (Figure 4.10) indicates maintenance of mirror plane symmetry; it also showed slight downfield shifting of all resonances compared to the starting material. For example the heptet associated with the four equivalent isopropyl methine protons shifted from 2.97 ppm to 3.57 ppm. One new resonance appeared, namely a singlet corresponding to nine protons at 2.20 ppm, suggesting the presence of a NMe 3 moiety. 207 1 The Pb{ H} NMR spectrum in C 6D6 (Figure 4.11 inset) contains one broad resonance at 2233 ppm, which is shifted upfield compared to starting material 4.1a (3279 ppm), and is consistent with increased coordination number at the lead centre. Single crystals for X-ray analysis were grown by slow evaporation at -25 °C of a pentane/Et2O solution. The molecular structure of 4.3

(Figure 4.12) indicates that no redox process took place; instead, intact ONMe 3 forms a Lewis pair with NHPb 4.1a. We are not aware of any previous reports on the behaviour of plumbylenes towards N-oxides. Compared to their stannylene relatives, simple adduct formation with ONMe 3 is a departure from established reactivity, which is that ONMe 3 is known to oxidize stannylenes to stannoxane. 63,64 The asymmetric unit of 4.3 contains two similar independent molecules. One of them is shown in Figure 4.12. Like 4.1a , the C 2N2Pb chelate ring is essentially planar. The lead atom of 4.3 adopts a pyramidal geometry (the sum of the angles at Pb is 261.4(5)°), indicating presence of a lone pair. Without the tether present in 4.2a , the ONMe 3 ligand donates into the p orbital on Pb making an angle of 85° with the C 2N2Pb chelate plane, which is much closer to the ideal value of 90°.

Scheme 4.2 Synthesis of Lewis pair 4.3.

1 Figure 4.10 H NMR Spectrum (500 MHz, 25 °C) of 4.3 in C 6D6. Inset: expanded aryl region.

13 1 207 1 Figure 4.11 C{ H} NMR Spectrum (126 MHz, 25 °C) of 4.3 in C 6D6. Inset: Pb{ H} NMR

Spectrum (126 MHz, 25 °C) of 4.3 in C 6D6.

Figure 4.12 Molecular structures of 4.2b (left) and 4.3 (right) with most hydrogen atoms omitted for clarity. H3 is shown as a sphere of arbitrary radius; all other atoms are drawn as 30% probability ellipsoids. Selected bond lengths(Å) and angles (°) for 4.2b : Sn1-N1 2.093(5), Sn1- N2 2.094(5), Sn1-N3 2.555(5), C1-N1 1.409(7) C2-N2 1.438(6), C1-C2 1.371(7), C2-C3 1.387(7), C3-C4 1.394(8), C4-C5 1.376(8), C5-C6 1.380(8), C1-C6 1.405(8); N1-Sn1-N2 78.11(18), N1-Sn1-N3 110.49(17), N2-Sn1-N3 78.15(17). No intermolecular interactions akin to those found in 4.1a and 4.2a were present. Of the two similar crystallographically independent molecules in the asymmetric unit of 4.3 , only one is shown, and only one orientation of the disordered isopropyl groups is shown. Selected bond lengths (Å) and angles (°) for 4.3 : Pb1-N1 2.213(8), Pb1-N2 2.203(8), Pb1-O1 2.374(7), C1-N1 1.384(12), C2-N2 1.395(12), C1-C2 1.438(13), C2-C3 1.385(13), C3-C4 1.380(14), C4-C5 1.403(15), C5-C6 1.377(15), C1-C6 1.386(14); N1-Pb1-N2 75.8(3), N1-Pb1-O1 94.8(3), N2-Pb1-O1 90.8(3).

Scheme 4.3 Synthesis of paramagnetic stannylene 4.4 .

During our investigations of the reactivity of 4.1a and 4.1b with MesN 3 and ONMe 3 we were unable to observe redox activity in the opda framework, so we turned to common one electron oxidant silver(I) triflate (AgOTf). Reactions of 4.1a with AgOTf led to a complex mixture of diamagnetic products as observed by 1H NMR, but use of the NHSn 4.1b yielded a different outcome. Addition of AgOTf to a toluene solution of 4.1b immediately caused the yellow/orange solution to turn dark brown as 4.4 formed (Scheme 4.3). After removal of metallic silver, analytically pure single crystals of 4.4 were grown in 53% yield by layering hexanes over a toluene solution cooled to -35 °C. The solid-state molecular structure of 4.4 (Figure 4.13) indicates that the three-coordinate tin centre adopts a distorted trigonal pyramidal geometry. The coordination sphere of tin is comprised of two nitrogen atoms from the chelating opda ligand and one oxygen atom from a terminal triflate ligand. The sum of the angles at Sn is 254.9(2)°, and the pyramidalization demonstrates that 4.4 retains its stannylene character during the reaction.

The C 2N2Sn chelate ring is essentially planar, and it intersects with the plane defined by O1-Sn1- N1 at an angle of ca. 87°. Information about the oxidation state of the opda ligand (at 150 K in the solid state) can be gleaned by analyzing the C-C and C-N bond lengths of the o-phenylene framework. The C1-N1 and C2-N2 bond lengths of 1.344(4) and 1.341(3) Å, are shortened compared their counterparts in 4.1b (1.384(2) Å). 25 The C3-C4 and C5-C6 bond lengths of 1.364(4) and 1.349(4) Å are also shortened compared to their counterparts in 4.1b (1.385(2) Å), while the C1-C6 and C2-C3 bond lengths of 1.425(4) and 1.421(4) Å are lengthened compared to their counterparts in 4.1b (1.398(2) Å). These bond length changes demonstrate bond localization in the C 6 ring and an increase in C-N bond order. Both pieces of evidence support our assignment of the opda ligand in 4.4 as being in the monoanionic diiminobenzoquinonate radical form.

Figure 4.13 Molecular structure of 4.4 . Non-hydrogen atoms are shown as 30% probability ellipsoids. Only one orientation of disordered CF 3 and isopropyl groups is shown. Selected bond lengths (Å) and angles (°) for 4.4 . : Sn1-N1 2.163(2), Sn1-N2 2.160(2), Sn1-O1 2.222(2), C1-N1 1.344(4), C2-N2 1.341(3), C1-C2 1.450(4), C2-C3 1.421(4), C3-C4 1.364(4), C4-C5 1.419(4), C5-C6 1.349(4), C1-C6 1.425(4), N1-Sn1-N2 75.72(9),N1-Sn1-O1 86.95(8),N2-Sn1-O1 92.22(9).

Figure 4.14 EPR spectrum (simulated on top, experimental on bottom) of 4.4 , obtained as a toluene solution at 298 K. X-band microwave frequency: 9.346 GHz. Parameters used in 117 simulation: 97% Gaussian lineshape, g = 1.992, a(N) = 4.5 G, a(H) = 3.0 G, a( Sn) = 109.6 G, a(119 Sn) = 114.8 G.

Sn(III) 65, 66 and Sn(IV) 67, 68 complexes are known. However, isolable paramagnetic Sn(II) complexes (stannylenes) are unknown. Paramagnetic stannylenes ligated by OO, ON, and NN donors have been studied in situ by EPR, 48-50 but none of these compounds could be isolated. To our knowledge 4.4 is the first isolable paramagnetic stannylene. The room temperature toluene solution EPR spectrum of 4.4 is in agreement with this assignment (Figure 4.14). The spectrum exhibits a central seven-line multiplet centred at g = 1.992 flanked by two Sn satellites with 117 119 hyperfine coupling constants a( Sn) = 109.6 G, and a( Sn) = 114.8 G. The isotropic g-factor

98 is lower than the free electron value, indicative of spin-orbit coupling introduced by the tin atom’s contribution to the molecular orbital of the unpaired electron. Simulation of the central pattern took into account hyperfine splitting of the EPR signal by one pair of equivalent I = 1 14 N nuclei and one pair of equivalent I = ½ protons. The satellite peaks are accounted for by additional splitting of the signal by I = ½ 117 Sn and 119 Sn nuclei (of natural abundance 7.61% and 8.58%, respectively). Hyperfine coupling to the protons on the beta carbons is not resolved, 69 which is distinct from the case of LiL(OEt 2), in which coupling to all four o-phenylene protons was observed. The EPR spectral parameters of in situ generated paramagnetic stannylenes 48-50 are comparable with those of 4.4 . The g values tend to be low (ranging from 1.996 to 2.003), and the 117/119 Sn hyperfine coupling constants range from 94.3 to 215.0 G.

4.4 Conclusion

In summary, new NHPb 4.1a was synthesized. 4.1a and its Sn analogue 4.1b both react with mesityl azide, where nitrene inserts into the benzylic C-H bond of a nearby dipp group. The analogous products of these insertions, 4.2a /4.2b , feature three-coordinate divalent group 14 centres bound by a newly formed diamidoamine ligand. When 4.1a was reacted with ONMe 3, no oxidation of the Pb centre occurred; instead, Lewis pair 4.3 formed. NHSn 4.1b reacts with AgOTf to generate three-coordinate tin(II) complex 4.4 , featuring a monoanionic diiminosemiquinonate ligand. Compound 4.4 is the first isolated paramagnetic stannylene.

4.5 Experimental

4.5.1 General considerations

70 71 25 72 H2L , Pb(hmds) 2, 4.1b , and MesN 3, were prepared according to the literature. Silver triflate was purchased from Sigma Aldrich. Trimethylamine-N-oxide dihydrate was purchased from Alfa Aesar and was dehydrated according to the literature.73 All operations were performed using Schlenk techniques under dinitrogen or in a dinitrogen-filled glovebox. All glassware was either flame-dried or dried overnight in a 160 °C oven prior to use except for NMR tubes which were dried overnight in a 60 °C oven. THF, Et 2O, toluene, and C 6D6 were distilled from

Na/benzophenone under N 2. Pentane and hexanes were distilled from sodium under dinitrogen. All solvents were then stored over 3 Å molecular sieves prior to use. 1H, 13 C{1H}, 119 Sn{1H}, and 207 Pb{1H} NMR spectra were recorded on a Varian 400 MHz, Agilent DD2 500 MHz, or

Agilent DD2 600 MHz spectrometer. For 1H and 13 C{1H} NMR spectra, chemical shifts are reported in ppm relative the residual protio-solvent peaks. 119 Sn and 207 Pb chemical shifts are reported in ppm relative to SnMe 4 and PbMe 4, respectively. Electron paramagnetic resonance (EPR) spectra were obtained at 298 K in toluene solution using a Bruker ECS-EMX X-band EPR spectrometer equipped with an ER4119HS cavity. The g values were determined using 2,2- diphenyl-1-picrylhydrazyl (dpph) as the reference (g = 2.0037). Simulation was carried out using PEST WinSIM software. 74 Elemental analyses were performed by ANALEST at the University of Toronto.

4.5.2 Cyclic Voltammetry

CV scans were run using a BASi Epsilon Electrochemical Workstation in a nitrogen-filled glovebox. A 1.6 mm diameter platinum disk, a platinum wire, and a silver wire were used as working, counter, and pseudoreference electrodes, respectively. Experiments were conducted in dry, deoxygenated tetrahydrofuran; 0.1 M NBu 4PF 6 (recrystallized from EtOH) was used as supporting electrolyte. CV scans were obtained without compensation for internal resistance. Potentials were referenced to ferrocene (Fc) and are reported vs. the Fc/Fc + redox couple. Under our experimental conditions (0.5 mM Fc, 100 mV/s) the peak separation of the Fc oxidation and reduction waves was 219 mV. A cyclic voltammogram of a 0.5 mM solution of 4.1a recorded at a scan rate of 100 mV/s is shown in Figure 4.4

4.5.3 Synthesis of N,N′-bis(2,6-diisopropylphenyl)-benzimidazolin-2- plumbylene (4.1a )

A solution of H 2L (150 mg, 0.350 mmol, 1 eq) in toluene (1mL) was added to a solution of

Pb(hmds) 2 (183 mg, 0.347 mmol) in toluene (1 mL). The mixture was diluted with toluene (1mL) before being heated in a Pyrex bomb to 105 °C for 15 h, which caused the mixture to turn purple. Volatiles were removed in vacuo , and the residue was recrystallized from toluene/hexanes (1:1 v/v) at -25 °C. Two crops of purple crystals were collected (117 mg, 0.185 1 mmol, 53%). H NMR (500 MHz, C 6D6) δ 7.36 (d, J = 7.7 Hz, 4H), 7.18 (t, J = 7.7 Hz, 2H), 6.44 (m, 2H), 6.21 (m, 2H), 2.97 (hept, J = 6.9 Hz, 4H), 1.15 (d, J = 7.0 Hz, 12H), 1.02 (d, J = 6.9 Hz, 13 1 12H). C{ H} NMR (151 MHz, C 6D6) δ 154.0, 147.2, 142.3, 127.0, 123.9, 118.9, 116.7, 27.5, 207 1 26.8, 24.4. Pb{ H} NMR (105 MHz, C6D6, 25 °C) δ 3279. Anal. Calcd for C 30 H38 N2Pb: C,

100

56.85; H, 6.04; N, 4.42 Found: C, 57.68; H, 5.82; N, 4.43. Single crystals for X-ray were grown from a cold THF/pentane solution.

4.5.4 Synthesis of 4.2a

4.1a (300 mg, 0.484 mmol) and MesN 3 (93.9 mg, 0.582 mmol) were dissolved in toluene (8 mL), transferred to a Pyrex bomb, and heated to 110 °C for 72 hours. Volatiles were removed in vacuo , and the burgundy residue was recrystallized from cold pentane/toluene of ratio ca . 4:1. The crystals were washed with cold pentane (3 x 1 mL), and dried under vacuum (139.8 mg, 1 0.1858 mmol, 38%). H NMR (400 MHz, C 6D6) δ 7.48 (dd, J = 7.7, 1.3 Hz, 1H), 7.43 (d, J = 8.0 Hz, 2H), 7.26 (t, J = 7.7 Hz, 1H), 7.23 (dd, J = 8.0, 1.4 Hz, 1H), 7.03 (t, J = 7.8 Hz, 1H), 6.65 (s, 2H), 6.62 (m-overlapped, 2H), 6.34 (m, 1H), 6.27 (m, 1H), 3.46 (s, 1H), 3.40 – 3.19 (m, 3H), 2.08 (s, 3H), 1.85 (s, 6H), 1.46 (s, 3H), 1.28 (d, J = 7.0 Hz, 3H), 1.25 (d, J = 6.9 Hz, 5H), 1.17 (two overlapping doublets, 6H), 1.16 (s, overlapped, 3H), 1.13 (d, J = 6.9 Hz, 3H). 13 C{1H}

NMR (126 MHz, C 6D6) δ 153.5, 152.6, 150.2, 147.0, 146.4, 146.3, 144.4, 142.3, 139.9, 133.3, 131.8, 130.5, 126.0, 126.0, 125.3, 124.5, 123.9, 123.6, 117.5, 117.3, 116.0, 115.0, 60.3, 33.2, 28.7, 28.0, 27.7, 27.3, 27.3, 27.0, 26.8, 25.0, 24.4, 22.1, 20.6, 20.4. 207 Pb{1H} NMR (105 MHz,

C6D6, 25 °C) δ 2657. Anal. Calcd for C 39 H49 N3Pb: C, 61.07; H, 6.44; N, 5.48 Found: C, 61.32; H, 6.42; N, 5.46. Single crystals for X-ray were grown from a cold pentane/hexamethyldisiloxane solution.

4.5.5 Synthesis of 4.2b

4.1b (35.3 mg, 0.0647 mmol) and MesN 3 (20.2 mg, 0.125 mmol) were dissolved in toluene, transferred to a Pyrex bomb, and heated to 110 °C for 24 h. Volatiles were removed in vacuo , and the orange residue was recrystallized from pentane at -35 °C. The orange crystals of 2b were 1 dried under vacuum (15.0 mg, 0.0226 mmol, 34%). H NMR (600 MHz, C 6D6) δ 7.42 (m, 1H), 7.32 (m, 3H), 7.12 (m, 2H), ), 6.81 (t, J = 7.1 Hz, 1H), 6.76 (t, J = 8.5 Hz, 1H), 6.54 (d, J = 7.7 Hz, 1H), 6.48 (d, J = 7.8 Hz, 1H) 3.63 (s, 1H), 3.44 (hept, J = 7.0 Hz, 1H), 3.33 (hept, J = 6.9 Hz, 1H), 3.21 (hept, J = 6.9 Hz, 1H), 2.05 (s, 3H), 1.88 (s, 6H), 1.40 (s, 3H), 1.26 (d, J = 7.4 Hz, 6H), 1.23 (d, J = 7.0 Hz, 6H), 1.20 (d, J = 6.9 Hz, 3H), 1.14 (s, 3H), 1.11 (d, J = 6.9 Hz, 3H). 13 1 C{ H} NMR (151 MHz, C 6D6) δ 149.8, 146.4, 146.1, 145.8, 145.3, 145.0, 142.3, 142.0, 139.1, 133.7, 131.9, 130.6, 126.3, 126.2, 125.4, 124.8, 124.2, 123.7, 117.6, 117.2, 112.4, 111.4, 60.6, 33.3, 28.9, 28.6, 28.3, 27.8, 26.9, 26.7, 26.4, 24.9, 24.2, 22.1, 20.7, 20.5. 119 Sn{1H} NMR (224

101

MHz, C 6D6) δ 69.1. Anal. Calcd for C 39 H49 N3Sn: C, 69.03; H, 7.28; N, 6.19 Found: C, 69.36; H, 7.24; N, 5.96. Single crystals for X-ray were grown from a cold pentane solution.

4.5.6 Synthesis of 4.3

4.1a (87.5 mg, 0.141 mmol) was dissolved in toluene (4 mL) and added to a suspension of trimethylamine-N-oxide (10.6 mg, 0.141 mmol) in toluene (1 mL). After stirring at room temperature for 10 h the mixture changed colour from purple to blood-red. Volatile components were removed under reduced pressure. The residue was extracted into toluene (2 mL) and filtered through Celite; pentane (5 mL) was layered on top. Cooling to -25 °C overnight brought about the formation of orange microcrystals. The supernatant was decanted away and the crystals were washed with cold 30% v/v toluene/pentane (2 x 1 mL) and pentane (4 x 1 mL). Drying in 1 vacuo yielded analytically pure 4.3 (69.7 mg, 0.0983 mmol, 70%). H NMR (500 MHz, C 6D6) δ 7.44 (d, J = 7.7 Hz, 4H), 7.28 (t, J = 7.6 Hz, 2H), 6.55 (m, 2H), 6.23 (m, 2H), 3.57 (hept, J = 6.9 Hz, 4H), 2.20 (s, 9H) 1.31 (d, J = 7.0 Hz, 12H), 1.20 (d, J = 6.8 Hz, 12H). 13 C{1H} NMR (126 207 1 MHz, C 6D6) δ 153.9, 147.8, 146.2, 125.6, 123.9, 116.5, 116.3, 60.6, 27.6, 26.5, 25.4. Pb{ H}

NMR (126 MHz, C6D6, 25 °C) δ 2233 Anal. Calcd for C 33 H47 N3OPb: C, 55.91; H, 6.68; N, 5.93 Found: C, 56.33; H, 6.61; N, 5.81. Single crystals for X-ray were grown by slow evaporation of a cold pentane/Et 2O solution.

4.5.7 Synthesis of 4.4

Toluene (1.5 mL) was added to silver triflate (32.7 mg, 0.127 mmol), and the mixture was added dropwise to an orange solution of LSn (67.5 mg, 0.127 mmol) in toluene (1.5 mL). The reaction mixture turned brown and was stirred for 90 min. The reaction mixture was filtered through Celite and the filtrate was concentrated to ca . 1.5 mL. Hexanes (1.5 mL) was layered on top and the mixture was cooled to -35 °C overnight causing formation of brown X-ray quality crystals. The supernatant was decanted and the crystals were washed with cold pentane (3 x 1 mL). Drying in vacuo yielded analytically pure 4.4 (46.5 mg, 0.670 mmol, 53%). Anal. Calcd for

C31 H38 N2O3F3SSn: C, 53.61; H, 5.52; N, 4.03. Found: C, 53.44; H, 5.57; N, 4.06.

4.5.8 X-ray crystallography

General considerations: The X-ray diffraction data were collected on a Bruker Kappa Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K

102 controlled by an Oxford Cryostream 700 series low-temperature system and processed with the Bruker Apex 2 software package. 75 The structures were solved by direct methods and refined using SHELXL-2013 and SHELXL-2014. 76 N-H atoms in 4.2a and 4.2b were located directly from the difference Fourier map, while all other H atoms were calculated with the riding model. Non-hydrogen atoms were refined anisotropically, except for atoms involved in the disordered isopropyl moieties in 4.1a , 4.3 , and 4.4 ; the disordered ONMe 3 ligand in 4.3 ; and the disordered

CF 3 group in 4.4 . The diffuse residual electron density from disordered solvent molecules in the lattice of 4.3 was removed with the SQUEEZE function of PLATON,77 and were not included in the formula or refinement. Selected crystallographic data are listed in Table 4.1.

Table 4.1 Crystallographic data for compounds 4.1a , 4.2a , 4.2b , 4.3 , and 4.4

4.1a 4.2a 4.2b 4.3 4.4

Formula C30 H38 N2Pb C39 H49 N3Pb C39 H49 N3Sn C33 H47 N3OPb C31 H38 F3N2O3SSn

F.W. 633.81 767.00 678.50 708.92 694.38

T (K) 150(2) 150(2) 150(2) 150(2) 150(2)

Space Pnma P21/n Cc P21/c C2/c group a (Å) 10.9016(7) 11.2497(8) 10.2651(16) 14.8076(12) 30.2756(12) b (Å) 21.6336(17) 22.1876(18) 22.732(3) 26.177(2) 11.6808(5) c (Å) 11.6083(8) 13.7655(10) 15.116(2) 20.3861(17) 17.9498(7)

α (°) 90 90 90 90 90

β (°) 90 93.600(3) 100.724(6) 92.810(4) 94.9769(18)

γ (°) 90 90 90 90 90

V (Å 3) 2737.7(3) 3429.1(4) 3465.8(9) 7892.7(11) 6323.9(4)

103

Z 4 4 4 8 8

-3 Dc (g·cm ) 1.538 1.486 1.300 1.193 1.459

μ (mm -1) 6.181 4.950 0.767 4.298 0.925 no. reflns 24612 30953 16085 70266 28694 collcd no. indept 3219 7823 7845 18031 7288 reflns

GOF on F 2 1.019 1.016 0.977 0.868 1.029

R [I > 2σ R1 = 0.0275 R1 = 0.0263 R1 = 0.0426 R1 = 0.0655 R1 = 0.0371 (I)] wR 2 = wR 2 = 0.0498 wR 2 = wR 2 = 0.1304 wR 2 = 0.0776 0.0571 0.0672

R (all data) R1 = 0.0429 R1 = 0.0430 R1 = 0.0601 R1 = 0.1555 R1 = 0.0547

wR 2 = wR 2 = 0.0535 wR 2 = wR 2 = 0.1530 wR 2 = 0.0850 0.0614 0.0719

4.6 References

1 A. Igau, H. Grutzmacher, A. Baceiredo and G. Bertrand, J. Am. Chem. Soc., 1988, 110 , 6463-6466. 2 A. J. Arduengo, R. L. Harlow and M. Kline, J. Am. Chem. Soc., 1991, 113 , 361-363.

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5 Chapter 5 Synthesis and reactivity of Li and TaMe 3 complexes supported by N,N’ -bis(2,6-diisopropylphenyl)-o- phenylenediamido ligands 5.1 Abstract

The dilithium complex of N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamide,

[Li 2L(THF)3], reacts with TaMe 3Cl 2 in THF/Et 2O to yield [Li(Et 2O)(THF)LTaMe 3Cl] in which the phenylene backbone of L 2- is bound η 4 to the Ta centre. This dinuclear species reacts with

MeLi to yield the tetramethyltantalum complex [Li(Et 2O)(THF)LTaMe 4]. Double deprotonation of N,N’ -bis(2,6-diisopropylphenyl)(4,5-dimethyl)-o-phenylenediamine (H 2L’) in Et 2O yielded 2- the dilithium complex [Li 2L’(OEt 2)2]. The two additional methyl groups on L’ change the observed reactivity towards TaMe 3Cl 2: rather than bridging between Ta and Li, oxidation to mononuclear [LiL’(OEt 2)] occurs. This monolithium radical species, which was characterized by

EPR spectroscopy, can also be synthesized using the more conventional oxidant AgBF 4. Double deprotonation of H2L with KCH 2Ph in toluene followed by reaction with TaMe 3Cl 2 furnished

[TaLMe 3]. Preliminary reactivity studies show [TaLMe 3] reacts with unsaturated substrates N,N’ -dicyclohexylcarbodiimide and mesityl azide to undergo migratory insertion into one of the Ta-C bonds: the corresponding amidinate and triazenido ligands are generated. When subjected to UV irradiation, [TaLMe 3] undergoes reduction accompanied by loss of a methyl group to yield the dimeric species [TaLMe 2]2.

5.2 Introduction

1 Since Juvinall’s 1964 preparation of the first tantalum methyl complex, TaMe 3Cl 2, organometallic chemists have been interested in the synthesis and reactivity of Ta-Me species. Members of the research community have continued to design and invent Ta-Me complexes supported by diverse ancillary ligand sets which have facilitated fascinating reactivity. Schrock’s 2 3 syntheses of the homoleptic TaMe 5 which violently decomposes via alpha abstraction, and the

4 first transition metal methylidene complex, [Cp 2Ta(CH 3)(CH 2)] are seminal examples. Fryzuk 2- and coworkers discovered that a TaMe 3 fragment chelated by their (PhP(CH 2SiMe 2NPh) 2) ligand undergoes hydrogenolysis to yield a dinuclear tetrahydride species, 5 a lynchpin in the field

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of N 2 activation. More recently, phosphoramidate ligands have been used to sponsor TaMe 3Cl precatalysts for room temperature hydroaminoalkylation of olefins. 6 Ta methyls have also been grafted onto silica supports for use as well-defined heterogeneous precatalysts for ethylene trimerization 7 and alkane metathesis. 8

Multiple reports have emerged on the synthesis and reactivity of tantalum complexes of diamido ligands with pendant neutral donor functionalities. Fryzuk’s work with diamidophosphine 5, 9-19 and diamidodiphosphine 20-22 ligand families is very well-represented. Other prominent examples include Heyduk’s redox active NNN-pincer ligand, 23-25 diamidoamines 26-31 and diamidoaminopyridines. 32, 33 Tridentate diamido ligands in which the additional donor is a pyridine, 27, 34, 35 carbene, 36 arsine, 37 and thioether 38 are also known. Compared to this abundance of examples of Ta complexes of decorated diamido ligands, simple diamido ligands have not been as well-studied. Tantalum complexes of ligands based on 1,8- diamidonaphthalene, 39-41 1,3-diamidopropane, 42 ,43 1,4-diaza-1,3-butadiene, 44-49 and o- phenylenediamide (opda) are known. Of the opda complexes, most employ N,N ’-disilyl groups, 41, 50-53 with one report on neopentyl groups. 54 To our knowledge, tantalum complexes of N,N ’-diaryl substituted opda ligands are heretofore unknown.

We have been investigating the coordination chemistry of N,N’ -bis(2,6- diisopropylphenyl)-o-phenylenediamide, 55 L2-, and sought to prepare Ta complexes of this simple bulky ligand, which may engender new and complementary reactivity to related ligands mentioned above. Herein we report our efforts to coordinate dipp-substituted opda ligands to Ta.

5.3 Results and Discussion

5.3.1 Synthesis and structures of heterodinuclear compounds 5.1 and 5.2 (solvent) n

Addition of an Et 2O solution of TaMe 3Cl 2 to a THF solution of Li 2L(THF)3 at -70 °C caused the reaction mixture to gradually turn orange as compound 2 formed (Scheme 5.1). After removal of LiCl, The 7Li{1H} NMR spectrum of 2 (Figure 5.1 inset) still featured a singlet at 2.0 ppm, distinct from the 7Li resonance of 1 (2.6 ppm), which indicated incomplete transfer of the diamido ligand from Li. The 1H NMR resonances of the two sets of equivalent o-phenylene protons shifted upfield from 6.57 and 6.33 ppm in starting material 1 to 5.64 and 4.02 ppm in

108 product 2, which is consistent with coordination of the phenylene backbone to Ta (Figure 5.1). The nine Ta-bound methyl protons resonate as a broad singlet at 0.91 ppm at room temperature.

Single crystals were grown by cooling an Et 2O solution of 2 to -25 °C. X-ray crystallography revealed the heterodinuclear nature of 2 (Figure 5.7). The two opda nitrogen donor atoms of L2- chelate the Li atom; its distorted tetrahedral coordination geometry is completed by the oxygen of one disordered Et 2O molecule and the oxygen of one ligand modeled as a 50:50 mixture of 2- 4 THF/Et 2O. L forms a bridge to Ta via an η interaction with its phenylene backbone. The coordination geometry at Ta is akin to a distorted trigonal bipyramid in which one apical ligand has been replaced with a π-bound diene. Cl - occupies the other apical position and three equatorial methyl groups complete the coordination sphere of Ta. The solid state structure suggests two different environments for Ta-Me protons, but cooling a toluene-d8 solution of 2 to -80 °C did not resolve the 1H NMR signal of the methyl groups, which suggests conformational fluxionality with a low barrier.

Scheme 5.1 Synthesis of Ta-Li heterodinuclear complexes 5.1 and 5.2 (Et 2O)(THF).

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7 1 1 Figure 5.1 Inset: Li{ H} NMR Spectrum (233 MHz, 25 °C) of 5.1 in C 6D6. H NMR Spectrum

(600 MHz, 25 °C) of 5.1 in C 6D6

13 1 Figure 5.2 C{ H} NMR Spectrum (151 MHz, 25 °C) of 5.1 in C 6D6

The apical chloride ligand of 5.1 can be replaced by a methyl group by treatment with

MeLi. Addition of MeLi in Et 2O to a solution of 5.1 at -35 °C yielded the tetramethyltantalum complex [Li(Et 2O)(THF)LTaMe 4], 5.2 (Et 2O)(THF). Multinuclear NMR data at room temperature are shown in Figures 5.3 and 5.4. The transformation from 5.1 to 5.2 (Et 2O)(THF) causes a subtle change in the 7Li{1H} NMR spectrum: the singlet shifts from 2.0 ppm to 2.3 1 ppm. Similarly, subtle peak shifts are observed in the H NMR spectrum of 5.2 (Et 2O)(THF). Distinct from 5.1 , the Ta-Me proton resonance at 0.89 ppm is even more dramatically broadened at room temperature. Single crystals for X-ray analysis were obtained by cooling a pentane solution of 3(Et 2O)(THF) to -25 °C (Figure 5.7). The solid state data confirmed the replacement of the chloride by a methyl ligand and that the η4 butadiene-type interaction is conserved in the

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methylation. The solid state structure contains two THF ligands on Li rather than one Et 2O and one THF, which is expected based on 1H NMR data. This phenomenon can be explained by ligand exchange on Li and preferential crystallization of 5.2 (THF)2. When 5.2 (Et 2O)(THF) is recrystallized from a mixture of pentane and Et 2O, resonances from coordinated THF disappear, and only one Et 2O ligand is present. This species, 5.2 (Et 2O), possesses a simpler alkyl region of 1 the H NMR spectrum; upon cooling a toluene-d8 solution to -80 °C, the broad Ta-Me peak splits into three distinct singlets (Figures 5.5 and 5.6) consistent with the solid state structure. Two of the peaks integrate to three protons each (2.35 and 1.00 ppm) for the two methyl ligands sitting on the mirror plane that bisects the L 2- and the third peak integrates to six protons (0.42 ppm) for the two methyl ligands that are related by this mirror plane.

7 1 1 Figure 5.3 Inset: Li{ H} NMR Spectrum (194 MHz, 25 °C) of 5.2 (THF) in C 6D6. H NMR

Spectrum (500 MHz, 25 °C) of 5.2 (THF) in C 6D6. Note: spectrum contains dichloromethane at 4.28 ppm

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13 1 Figure 5.4 C{ H} NMR Spectrum (126 MHz, 25 °C) of 5.2 (THF) in C 6D6

1 Figure 5.5 H NMR Spectrum (600 MHz, -80 °C) of 5.2 in C 6D5CD 3

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Figure 5.6 Expansion of the alkyl region of 1H NMR Spectrum (600 MHz, 25 °C) of 5.2 in

C6D5CD 3

Figure 5.7 Molecular structures of 5.1 (left) and 5.2 (THF)2 (right). Non-hydrogen atoms are shown as 30% probability ellipsoids except for the disordered portion of Li-coordinated THF. Ta-Me H atoms are shown as spheres of arbitrary radius, the rest of the H-atoms along with the isopropyl groups on L are omitted for clarity. Selected bond angles (°) for 5.1 : N1-Li1-N2 82.1(2), C7-Ta1-C8 118.7(1), C8-Ta1-C9 120.2(1), C7-Ta1-C9 111.8(1), Cl1-Ta1-C7 80.25(9),

Cl1-Ta1-C8 80.01(9), Cl1-Ta1-C9 78.96(9). Selected bond angles (°) for 5.2 (THF)2 : N1-Li1-N2 82.6(3), C7-Ta1-C8 116.5(2), C8-Ta1-C9 118.3(2), C7-Ta1-C9 114.7(2). C7-Ta1-C10 78.1(2), C8-Ta1-C10 80.6(2), C9-Ta1-C10 78.3(2)

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Chart 5.1 Two resonance forms for L 2-

The solid state molecular structures of 5.1 and 5.2 (THF)2 are very similar. In both, the 2- lithium atom is chelated by the two nitrogen donor atoms of the L . The C phenylene -N bond lengths (1.295(4) Å in 5.1 and 1.306(5) and 1.307(5) Å in 5.2 (THF)2) are significantly shortened 55 compared to the dilithium complex 3.1 (C phenylene -N bond lengths of 1.396(2) and 1.395(2) Å). This compression of C-N bonds suggests the diamido ligand in 3.1 has become a diimine ligand 4 in 5.1 and 5.2 (THF)2. The tantalum atom in 5.1 and 5.2 (THF)2 is bound η - to the o-phenylene 4 ring of the opda ligand in a butadiene-type interaction. In these complexes, the η -C6 ring is folded along the C3-C6 vector at angles of 31° and 25°, respectively which are very similar to 55 the corresponding angle in the previously reported trimetallic [(Li(THF)2L) 2MoCl 2] (30°). On

56 the continuum between the Chatt-Dewar diene (L 2) and metallocyclopentene (LX 2) extremes, we formulated this MoLi 2 species as more of an L 2 butadiene-type complex. However, the metric parameters of the η4 ligand used to make this assignment are different from Ta complexes 5.1 and 5.2 (THF)2. The Δ d parameter (where Δ d = avg. M-Couter bond length – avg. M-Cinner bond length; C outer refers to C3 and C6 and C inner refers to C4 and C5) is 0.034(3) Å for 5.1 and

0.060(4) Å for 5.2 (THF)2, compared to 0.142(6) Å in the MoLi 2 species. In the MoLi 2 species, the C inner -Cinner and avg. C inner -Couter bonds are statistically similar, but in 5.1 and 5.2 (THF)2, the

Cinner -Cinner bonds are 1.377(4) and 1.368(6) Å, respectively. These bonds are significantly shorter than the avg. C inner -Couter in 5.1 and 5.2 (THF)2, which are 1.431(4) and 1.427(4) Å. 5.1 and 5.2 (THF)2 more closely resemble the metallocyclopentene resonance form, which is consistent with the greater electropositivity of Ta relative to Mo. Taken together, the metric 2- parameters of both 5.1 and 5.2 (THF)2 suggest contribution of L resonance form B (Chart 5.1) is significant.

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5.3.2 Coordination chemistry of L’

In hopes of disfavouring the phenylene carbons relative to the diamido nitrogens as coordination site for Ta we increased the steric bulk at the phenylene backbone. According to the 57 method of Wenderski et al. , we synthesized the doubly methylated diamine proligand H2L’. Double deprotonation was achieved by addition of two equivalents of n-BuLi to a -70 °C diethylether solution of H2L’ (Scheme 5.2). After removal of volatiles and precipitation with cold pentane, [Li 2L’(Et 2O) 2], 5.3 , was isolated as a white powder. Multinuclear NMR spectra of 1 5.3 are shown in Figures 5.8 and 5.9. In the H NMR spectrum of 5.3 in C 6D6, the N-H resonance present in the starting material is absent, replaced by a quartet at 2.92 ppm and a triplet at 0.74 ppm corresponding to the ethyl groups on two coordinated Et 2O molecules. In the 7Li{1H} NMR spectrum, one signal is observed at 0.8 ppm which indicates that the two Li atoms are equivalent in solution. Single crystals for X-ray were obtained by cooling a pentane/Et 2O solution to -25 °C.

dipp dipp dipp NH 2 n-BuLi N TaMe Cl Li OEt 2 3 2 N -70 °C - rt or AgBF Li OEt Li 4 2 NH OEt 2 Et 2O N N dipp dipp dipp H2L' 5.3 63%5.4 63%

Scheme 5.2 Syntheses of Li complexes 5.3 and 5.4

Different from the solution data, the solid state structure of 5.3 (Figure 5.10) reveals that the two Li atoms occupy distinct coordination environments. Li1 resides in a distorted trigonal planar geometry: two amido nitrogen donor atoms and one oxygen from a terminal diethylether ligand occupy the three coordination sites. Li2 is tetracoordinate, engaging in an η 4- interaction with the two diamido nitrogen donor atoms and the two N-bound o-phenylene carbons. The coordination sphere of Li2 is completed by a terminal diethylether ligand. The NN chelate forms longer N-Li bonds with Li2 (2.056(4) and 2.064(4) Å) than with Li1 ((1.961(3) and 1.968(4) Å). Li1 sits on one side of the C1-C2-N1-N2 plane with a dihedral angle between this plane and N1-

Li1-N2 plane of 37°; Li2 sits on the other side of the C1-C2-N1-N2 plane, with a dihedral angle between this plane and N1-Li2-N2 plane of 80°. Among related dilithium complexes of o- phenylenediamido ligands such as 1, the N,N’ -disilyl versions by Lappert, 68 or the N,N’ -2,4,6-

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69 triisopropylphenyl version by Liddle, 5 is structurally unique in that Et 2O is used as a co-ligand instead of THF and that among these related complexes, the ipso carbons in 5 are bound the closest to Li (C1-Li2, 2.291(4) Å; C2-Li2, 2.324(4) Å).

7 1 1 Figure 5.8 Inset: Li{ H} NMR Spectrum (194 MHz, 25 °C) of 5.3 in C 6D6. H NMR Spectrum

(300 MHz, 25 °C) of 5.3 in C 6D6

13 1 Figure 5.9 C{ H} NMR Spectrum (101 MHz, 25 °C) of 5.3 in C 6D6

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Figure 5.10 Molecular structures of [Li 2L’(Et 2O) 2], 5.3 (top), and [LiL’(Et 2O)], 5.4 (bottom) with

30% probability ellipsoids. Only one disordered component of Et 2O molecules is shown. Isopropyl groups and hydrogen atoms are omitted for clarity. Selected bond angles (°) for 5.3 : N1-Li1-N2 85.4(1), O1-Li1-N1 129.4(2), O1-Li1-N2 141.6(2), N1-Li2-N2 80.6(1), O2-Li2-N1 134.5(2), O2-Li2-N2 144.9(2). Selected bond angles (°) for 5.4 : O1-Li1-N1 118.7(2), O1-Li1-N2 153.8(3), N1-Li1-N2 87.0(1)

Table 5.1 Selected bond lengths in 5.3 and 5.4 Bond Length in 5.3 (Å) Length in 5.4 (Å) Bond Length in 5.3 Length in 5.4 (Å) (Å) C1-C2 1.442(2) 1.464(2) Li1-N1 1.961(3) 1.950(3) C2-C3 1.404(3) 1.426(2) Li1-N2 1.968(4) 1.960(3) C3-C4 1.405(3) 1.374(2) Li1-O1 1.871(3) 1.901(6) C4-C5 1.404(2) 1.426(2) Li2-N1 2.056(4) C5-C6 1.405(3) 1.374(2) Li2-C1 2.291(4) C1-C6 1.394(2) 1.427(2) Li2-C2 2.324(4) C1-N1 1.417(2) 1.337(2) Li2-N2 2.064(4) C2-N2 1.407(2) 1.339(2) Li2-O2 1.997(4)

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Reaction of dilithium compound 5.3 with TaMe 3Cl 2 causes the solution to darken in colour. After replacing the solvent with pentane and removing insoluble material by filtration, dark green single crystals of 5.4 grew from the cooled concentrated pentane extract. 2- Unexpectedly, TaMe 3Cl 2 acted as an oxidant towards the dilithium species 5.3 and the L’ complex has lost an electron and a Li ion to become a monolithium diiminosemiquinonate complex. Efforts to structurally characterize the Ta-containing species generated in this reaction have so far not been fruitful. Notably, 5.4 can also be prepared by oxidation of 5.3 with a source of Ag +.

Figure 5.11 Experimental (top) and simulated (bottom) EPR spectrum of 5.4 (THF solution, room temperature) X-band microwave frequency 9.856 GHz. Parameters used in simulation:

100% Lorentzian lineshape, g = 2.0075, ɑN = 4.58 G, ɑH = 3.39 and 1.53 G, ɑLi = 0.85 G.

The room temperature EPR spectrum of a THF solution of 5.4 is shown in Figure 5.11. The g factor of 5.4 is 2.0075; it is close to the free electron value of 2.0036, which is expected for an organic radical with negligible spin-orbit coupling. The multiplet pattern was simulated taking into account hyperfine coupling to a pair of equivalent 14 N nuclei, a pair of equivalent o- phenylene 1H nuclei, the six equivalent 4,5-dimethyl 1H nuclei, and one 7Li nucleus. The multiplet pattern is consistent with delocalization of the unpaired electron throughout the o- 55 phenylene framework, which has been observed in EPR spectra of [LiL(OEt 2)].

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The solid-state molecular structure of 5.4 (Figure 5.10) reveals that Li1 is chelated by the L’ - ligand and is bound to a terminal disordered solvent molecule modelled as a 70/30 ratio of

Et 2O/THF. The sum of the angles around Li is 359.5(4)°, consistent with a distorted trigonal planar geometry. One electron oxidation of L’ 2- led to a pronounced decrease in the respective C1-N1 and C2-N2 bond lengths to 1.337(2) and 1.339(2) Å relative to the counterparts in 5.3 7 (1.417(2) and 1.407(2) Å). Akin to the structure of [LiL(OEt 2)], elongation of the C1-C6 and C2-C3 bonds to 1.427(2) and 1.426(2) Å and compression of the C5-C6 and C3-C4 bonds each to 1.374(2) Å is also observed. In the starting diamido complex 5.3 these bond lengths are statistically similar.

5.3.3 Synthesis of [TaLMe 3] (5.5)

In further efforts to synthesize an NN chelate complex of Ta with dipp-substituted opda 2- ligands we employed dilithium salts of L free of coordinated Et 2O/THF ligands; we also attempted protonolysis by reacting H 2L with TaMe 3Cl 2 or Ta(NMe 2)5, but none of these attempts succeeded. However, the generation of the dipotassium salt of L2- by double deprotonation of

H2L with benzylpotassium (for its molecular structure with complexed 1,2-dimethoxyethane ligands see Figure 5.13) followed by reaction with TaMe 3Cl 2 in toluene yielded the desired 1 [TaLMe 3], 5.5 (Scheme 5.3). In its H NMR spectrum in C 6D6 at room temperature (Figure

5.12), the three methyl ligands resonate as a single peak at 1.03 ppm. Cooling a toluene-d8 solution to -80 °C did not lead to any decoalescence of this signal. X-ray quality single crystals were grown by cooling a saturated pentane solution to -25 °C. The molecular structure of 5.5 (Figure 5.14) reveals L2- chelating a five-coordinate Ta centre, which is bound to three terminal methyl ligands. Ta occupies a distorted square pyramidal geometry, with N1, N2, C7 and C8 forming the square base and C9 in the apical position. The diamido ligand binds Ta with Ta1-N1 and Ta1-N2 bond lengths of 2.055(3) and 2.052(3) Å, respectively. The diamido’s N1-Ta1-N2 bite angle is 75.5(1)°, and the five-membered TaN 2C2 chelate ring is essentially planar such that the electron pairs on the nitrogen atoms are oriented with correct symmetry to engage in π- bonding with Ta. All o-phenylene C-C bond lengths are statistically similar and both C phenylene -N bond lengths are 1.413(5) Å, statistically similar to the C phenylene -N bond lengths in 3.1 . These data suggest the potentially redox-active L2- maintains its dianionic charge, with resonance form A in Chart 5.1 as the major contributor.

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dipp dipp NH N Me 2 KCH 2Ph TaMe 3Cl 2 K2L Ta Me toluene -70 °C - rt NH 86% N Me toluene dipp dipp H2L 5.5 37%

Scheme 5.3 Synthesis of [TaLMe 3]

1 Figure 5.12 H NMR Spectrum (600 MHz, 25 °C) of 5.5 in C 6D6 spectrum contains pentane at 13 1 1.26 and 0.87 Inset: C{ H} NMR Spectrum (151 MHz, 25 °C) of 5.5 in C 6D6 Note: spectrum contains pentane at 34.5, 22.7, and 14.3.

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Figure 5.13 Molecular structure of K 2L(dme) 4 with 30% probability ellipsoids. Isopropyl groups and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°): C1-C2 1.462(3), C2-C3 1.412(3), C3-C4 1.410(3), C4-C5 1.371(4), C5-C6 1.413(3), C1-N1 1.362(3), C2-N2 1.362(3), K1-N1 2.719(2), K1-N2 2.713(2), K1-O5 2.974(2), K1-O6 2.787(2), K1-O7 2.879(3), K1-O8 2.850(2), K2-C1 3.077(2), K2-C2 3.231(2), K2-C3 3.204(2), K2-C4 3.168(2), K2-C5 3.101(2), K2-C6 3.021(2), K2-O1 2.743(2), K2-O2 2.880(2), K2-O3 2.858(2), K2-O4 2.670(2), N1-K1-N2 61.24(6), O5-K1-O6 57.66(6), O7-K1-O8 61.68(7), O1-K2-O2 60.04(6), O3-K2-O4 59.84(6).

Figure 5.14 Molecular structure of [TaLMe 3], 5.5 . Non-hydrogen atoms are shown as 30% probability ellipsoids. Hydrogen atoms on Ta-Me groups are shown as spheres of arbitrary

121 radius, and the rest of the H-atoms are omitted for clarity. Only one orientation of disordered isopropyl group is shown. Selected bond lengths (Å) and angles (°) for 5.5 : Ta1-C7 2.177(4), Ta1-C8 2.137(6), Ta1-C9 2.137(5), Ta1-N1 2.055(3), Ta1-N2 2.052(3), C1-C2 1.390(6), C2-C3 1.391(6), C3-C4 1.390(6), C4-C5 1.394(6), 1.387(6), C6-C1 1.400(6), C1-N1 1.413(5), C2-N2 1.413(5), N1-Ta1-N2 75.5(1), N1-Ta1-C9 112.3(2), N1-Ta1-C8 138.6(2), N2-Ta1-C7 152.9(1), N2-Ta1-C8 92.3(2), N2-Ta1-C9 105.3(2), C7-Ta1-C8 89.9(2), C7-Ta1-C9 99.5(2), C8-Ta1-C9 109.1(2).

5.5 could also be generated by thermal loss of LiCl from 5.1. Heating a solution of 5.1 in 1 C6D6 to 80 °C for 3 h caused consumption of 5.1 ; analysis of the H NMR spectrum indicated formation of 5.5 in a mixture that also included [TaLMe 2Cl] and 5.2 (respective ratio of 1 : 1.5 : 2). This result can be explained by the exchange of Me and Cl ligands at Ta which has been known for some time. 58

5.3.4 Reactivity of [TaLMe 3]

Insertion of carbodiimides into Ta-CMe bonds has been known since Wilkins’ report in 1974 59 . To test the ability of L to sponsor this reaction we subjected 5.5 to one equivalent of N,N’-dicyclohexylcarbodiimide (DCC) (Scheme 5.4); the reaction mixture turned cherry red as insertion of DCC into one of the Ta-C bonds occurred. Diagnostic features of the 13 C{1H} NMR spectrum of 5.6 are the amidinate NCN and CH 3 resonances at 180.2 ppm and 15.1 ppm, 1 respectively. In the H NMR spectrum, the amidinate and Ta-bound CH 3 protons resonate as singlets integrating to three and six protons at 1.63 ppm and 1.03 ppm, respectively. All other alkyl protons give rise to broadened, overlapped resonances. X-ray analysis of single crystals revealed the molecular structure of 5.6 (Figure 5.16) which includes a newly formed amidinate ligand. The geometry at Ta can be described as distorted trigonal prismatic, with N1-N2-C8 forming one triangular face and N3-N4-C7 forming the other. The amidinate coordination bond lengths are significantly different: N4-Ta1 (2.136(4) Å) is shorter than N3-Ta1 (2.221(4) Å). Its C-N bonds (1.325(6) and 1.344(7) Å) are equal within experimental error. The amidinate ligand chelates the tantalum centre with an N3-Ta1-N4 bite angle of 60.4(1)°, and the four-membered

CN 2Ta ring is planar. Unlike 5.5 , the five-membered C 2N2Ta ring exists in an envelope conformation, with the Ta atom out of plane. The dihedral angle between the o-phenylene plane and the plane defined by N1-Ta1-N2 is ca. 25°.

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N TaMe 3 5.5 N CyN=C=NCy toluene MesN 3 hnn nn

Me Me CyN N N Me Me N N N N NCy N Ta Ta Me 1/2 Ta Ta N Me N N N Me Me Me Me 5.6 93% 5.7 63% 19% 5.8

Scheme 5.4 Reactivity of [TaLMe 3]: Synthesis of 5.6 , 5.7 , and 5.8 .

Figure 5.15 Molecular structures of 5.6 (top left), 5.7 (top right), and 5.8 (bottom right). Inset: two views of the coordination sphere of 5.6 . Non-hydrogen atoms are shown as 30% probability ellipsoids. Hydrogen atoms on the Ta-Me groups of 5.8 are shown as spheres of arbitrary radius. The rest of the H-atoms and isopropyl groups on L are omitted for clarity. Selected bond lengths (Å) and angles (°) for 5.6 : Ta1-N3 2.221(4), Ta1-N4 2.136(4), C33-N3 1.325(6), C33-N4 1.344(7). N1-Ta1-N2 77.0(1), N1-Ta1-N3 129.5(2), N1-Ta1-N4 86.3(1), N1-Ta1-C7 136.3(2), N1-Ta1-C8 95.1(2), N2-Ta1-N3 152.4(2), N2-Ta1-N4 139.5(1), N2-Ta1-C7 79.9(2), N2-Ta1-C8 88.2(2), N3-Ta1-N4 60.4(1), N3-Ta1-C7 82.9(2), N3-Ta1-C8 82.2(2), N4-Ta1-C7 87.8(2), N4-

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Ta1-C8 130.3(2), C7-Ta1-C8 120.9(2). Selected bond lengths (Å) and angles (°) for 5.7 : Ta1-N3 2.049(2), Ta1-N4 2.221(2) N4-N5 1.263(3), Ta1-N3-N4 79.0(1), N1-Ta1-N2 75.57(8), N1-Ta1- N3 83.71(8), N1-Ta1-N4 117.33(8), N1-Ta1-C7 137.13(8), N1-Ta1-C8 106.60(8), N2-Ta1-N3 152.50(8), N2-Ta1-N4 166.45(7), N2-Ta1-C7 85.78(8), N2-Ta1-C8 95.11(8), N3-Ta1-N4 36.16(8), N3-Ta1-C7 97.76(9), N3-Ta1-C8 108.17(9), N4-Ta1-C7 81.63(8), N4-Ta1-C8 85.35(8), C7-Ta1-C8 113.34(9). Selected distances (Å) and angles (°) for 5.8 : Ta1-Ta2 2.7120(5) Ta1-H31a 1.99, Ta1-H34a 1.98, Ta1-C31 2.618(5), Ta1-C34 2.607(4), Ta2-H32a 2.00, Ta2- H33a 1.99, Ta2-C32 2.627(4), Ta2-C33 2.614(4), Ta2-N3 2.033(3), Ta2-N4 2.032(3), C35-N3 1.425(5), C36-N4 1.432(5), C35-36 1.409(6) , C36-C37 1.376(6), C37-C38 1.379(7), C38-C39 1.371(6), C39-C40 1.394(6), C35-C40 1.385(6). N2-Ta1-N1 82.1(1), N1-Ta1-H34a 72.2, H34a- Ta1-Ta2 67.5, Ta2-Ta1-H31a 67.7, H31a-Ta1-N2 70.6, C33-Ta1-C32 127.1(2), N3-Ta2-N4 81.9(1), N4-Ta2-H32a 71.8, H32a-Ta2-Ta1 67.7, Ta1-Ta2-H33a, H33a-Ta2-N3 71.3, C31-Ta2- C34 126.7(2)

Table 5.2 Selected analogous bond lengths in 5.6 , 5.7 , 5.8 .

Bond Length in 5.6 Length in 5.7 (Å) Length in 5.8 Bond Length in 5.6 Length in 5.7 (Å) Length in 5.8 (Å) (Å) (Å) (Å) Ta1-N1 2.067(4) 2.046(2) 2.026(3) C1-C2 1.416(7) 1.404(4) 1.408(6) Ta1-N2 2.080(4) 2.074(2) 2.031(3) C2-C3 1.392(7) 1.396(4) 1.404(6)

Ta1-Cmethyl 2.208(5) 2.188(2) 2.187(4) C3-C4 1.399(8) 1.388(3) 1.388(9) 2.175(5) 2.146(2) 2.186(5) C4-C5 1.375(9) 1.386(4) 1.352(9) C1-N1 1.412(7) 1.408(3) 1.437(5) C5-C6 1.381(8) 1.386(4) 1.415(7) C2-N2 1.408(7) 1.405(3) 1.426(5) C1-C6 1.387(8) 1.391(3) 1.371(6)

When mesityl azide was added to a solution of 5.5 , yellow precipitate formed. The 1H NMR resonance associated with the Ta-bound Me groups shifted upfield to 0.67 ppm and its integration was reduced from nine to six protons, which indicated that the azide had reacted with one of the Me ligands on tantalum. Aside from resonances associated with the new mesityl group, an additional methyl singlet appeared at 2.53 ppm. X-ray crystallography on single crystals of 5.7 revealed that insertion of the azide into the tantalum-carbon bond had taken place, 2 forming a κ -N2 triazenido ligand. The Ta centre adopts a distorted trigonal bipyramidal geometry, with the equatorial positions occupied by the two Ta-Me groups and N1 of L2-. N2 of 2- 2 L and the κ -N2 interaction occupy the axial positions. The N3-Ta1 bond (2.049(2) Å) is significantly shorter than the N4-Ta1 bond (2.221(2) Å) and the N4-N5 bond (1.263(3) Å) is shorter than the N4-N3 bond (1.335(3) Å), which informs our formulation of 5.7 shown in

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Scheme 5.4. The sum of the angles at N3 (359.8(3)°) indicates its planarity such that its lone pair be oriented perpendicular to this plane with correct symmetry to engage in π-bonding with Ta. The plane of the o-phenylene ring and the plane defined by the three nitrogen atoms of the triazenido ligand meet with a dihedral angle of 32°. For the diamido moiety, intraligand metric parameters are the same as in 5.5 ; the diamido behaves as a spectator ligand during the transformation from 5.5 to 5.7 .

The reactivity most relevant to this transformation is the single and double insertion of aryl azides into the Ta-C bonds of [(ONO)TaMe 2] (ONO = bis(phenoxy)amide ligand) to form mono and bis triazenido complexes. 60 Distinct from these compounds, the triazenido ligands in 5.7 are κ 2N1,2 -bound to Ta. Although this binding mode is well known for f-elements, 61-65 to our knowledge this is the first crystallographically characterized example of a triazenido ligand κ2N1,2 -bound to Ta.

When a toluene solution of 5.5 was subjected to UV light, the colour of the solution 1 darkened. In the H NMR spectrum in C 6D6, the Ta-Me resonance shifted upfield from 1.03 to 0.08 ppm, and there are six Ta-Me protons per diamido ligand, indicating loss of one of the Ta- Me groups (Figure 5.16). In the 13 C NMR spectrum the carbon peak attributable to the Ta-Me 1 1 groups shifted from 83.5 ( JC-H = 118 Hz) to 49.2 ppm ( JC-H = 116 Hz). Single crystals of 5.8 were obtained from cold pentane; the molecular structure (Figure 5.16, bottom) revealed a dimeric formally Ta(IV) species possessing a Ta-Ta bond length of 2.7120(5) Å. This bond is intermediate between the Ta-Ta bonds found in (μ-H)4 ditantalum complexes of diamidophosphine (2.830(4) Å) 9 and diamidodiphosphine (2.6165(5) Å)21 complexes. Each Ta centre is bound to two methyl groups, and each methyl group makes a close contact with its neighbouring Ta centre two bonds away; these distances range from 2.607(4) Å for Ta1-C34 to 2.627(4) Å for Ta1-C32. Although the methyl hydrogens were not located on the Fourier difference map, the proximity of the methyl carbons to both Ta centres and the marked shift of the methyl protons to high field in the 1H NMR spectrum lend support for the presence of agostic interactions between one methyl C-H bond and its opposite Ta centre. Taking these interactions into account, the geometry at each seven-coordinate Ta centre is best understood as distorted pentagonal bipyramidal. On each Ta centre, L2-, the other Ta atom, and two agostic interactions reside in equatorial positions (sum of the bond angles: 360.1 at Ta1 and at Ta2). The two-centre two-electron bound methyl carbons occupy axial positions. This reactivity of [TaLMe 3], which

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contains a simple diamido ligand, is distinct from that of Fryzuk’s [(P2N2)TaMe 3] (P2N2 = diamidodiphosphine ligand), which undergoes loss of methane to generate a monomeric methyl methylidene complex. 20 The authors proposed that the first step of the reaction was the photoinduced Ta-C bond homolysis, generating a methyl radical. Presumably the transformation of 5.5 to 5.8 could share the same initial step, where the methyl radical could be scavenged by toluene solvent and the resulting [TaLMe 2] species dimerizes to form 5.8 .

1 13 1 Figure 5.16 H NMR Spectrum (400 MHz, 25 °C) of 5.8 (pentane) 2/3 in C 6D6. Inset: C{ H}

NMR Spectrum (500 MHz, 25 °C) of 5.8 in C 6D6 Note: volatile impurities in the NMR solvent

(THF, Et 2O, pentane) are marked with an asterisk

5.4 Conclusion

Attempts were made to install simple, bulky opda ligands on Ta via salt metathesis with

TaMe 3Cl 2. Use of dilithium complex 3.1 led to the isolation of heterodinuclear 5.1 in which the NN chelate site of the diamide ligand was occupied by Li +, and the o-phenylene backbone was engaged in a η 4-interaction with Ta. This species underwent clean methylation with MeLi to generate tetramethyl species 5.2 (Et 2O)(THF) in which heterodinuclearity is preserved. A new dilithium complex with increased steric bulk at the o-phenylene backbone ( 5.3 ) was synthesized.

When reacted with TaMe 3Cl 2 it underwent oxidation to open-shell monolithium complex 5.4 , which could be synthesized in higher yield and purity using AgBF 4. By using a dipotassium derivative of L 2-, the desired trimethyltantalum complex ( 5.5 ) could be synthesized. Preliminary

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reactivity studies show 5.5 undergoes insertion reactions with DCC and MesN 3 to generate compounds 5.6 and 5.7 . In 5.7 the newly formed triazenido ligand is bound κ2N1,2 , a bonding mode which is unusual for Ta. Irradiation of a toluene solution of 5.5 leads to isolation of dimeric Ta(IV) species 5.8 , which illustrates the complementary reactivity engendered by the simple bulky opda ligand. Our initial attempts to generate isolable Ta-H species via hydrogenolysis of 5.5 have not succeeded, but investigation of the reactivity of this and other o- phenylenediamido Ta complexes towards H 2 is ongoing in our laboratory.

5.5 Experimental

5.5.1 General considerations

55 57 4 66 67 Compounds 3.1 , H2L, H 2L’, TaMe 3Cl 2, KCH 2Ph, and MesN 3 were prepared from literature methods. Methyl lithium (1.6 M in ether), n-butyl lithium (1.6 M in hexanes), AgBF 4, and N,N’-dicyclohexylcarbodiimide were purchased from commercial sources. All operations were performed using Schlenk techniques under dinitrogen or in a dinitrogen-filled glovebox. All glassware was either flame-dried or dried overnight in a 180 °C oven prior to use except for

NMR tubes which were dried overnight in a 60 °C oven. THF, Et 2O, toluene, and C 6D6 were distilled from Na/benzophenone under N2. Pentane, hexanes, and toluene-d8 were distilled from sodium under dinitrogen. All solvents were then stored over 3 Å molecular sieves prior to use. 1H, 31 P{1H}, 13 C{1H}, and 11 B{1H} NMR spectra were recorded on a Varian 400 MHz, Agilent DD2 500 MHz, or Agilent DD2 600 MHz spectrometer. Electron paramagnetic resonance (EPR) spectra were obtained at 298 K in THF solution using a Bruker ECS-EMX X-band EPR spectrometer equipped with an ER4119HS cavity. Simulation was carried out using PEST WinSIM Software. All chemical shifts are reported in ppm relative the residual protio-solvent 7 peaks Li NMR is referenced externally using 9.7 M LiCl in D 2O. Elemental analyses were performed by ANALEST at the University of Toronto.

5.5.2 Synthesis of [Li(Et 2O)(THF)LTaMe 3Cl] ( 5.1 )

To [Li 2L(THF)3] (115 mg, 0.175 mmol) dissolved in THF (3 mL) and cooled to -70 °C was added a similarly cooled solution of TaMe 3Cl 2 (48.1 mg, 0.162 mmol) in Et 2O (2 mL). The reaction mixture was allowed to warm to room temperature and was stirred for 2 h. Volatiles were removed under reduced pressure. The orange residue was extracted with Et 2O (5 mL),

127 filtered, concentrated to ca. 2 mL and cooled to -25 °C overnight. The supernatant was decanted off the crystals that had formed, which were then washed with cold Et 2O (3 x 0.5 mL). Residual solvent was removed by briefly applying vacuum to the yellow crystals of 1 [Li(Et 2O)(THF)LTaMe 3Cl], 5.1 (63.9 mg, 0.760 mmol, 47%). H NMR (600 MHz, Benzene-d6) δ 7.21 (dd, J = 7.0, 2.1 Hz, 2H), 7.15 – 7.10 (m, 4H), 5.66 – 5.62 (m, 2H), 4.04 – 4.01 (m, 2H), 3.19 (sept, J = 6.9 Hz, 2H), 3.12 – 3.08 (m, 4H), 3.06 (sept, J = 6.2 Hz, 2H), 1.34 (d, J = 6.9 Hz, 6H), 1.22 (d, J = 6.9 Hz, 6H), 1.20 (d, J = 6.9 Hz, 6H), 1.10 (d, J = 6.8 Hz, 10H) 1.10 13 1 (overlapped, 4H), 0.97 – 0.94 (m, 6H), 0.91 (br, 9H) C{ H} NMR (151 MHz, Benzene-d6) δ 168.4, 145.4, 142.0, 140.1, 124.8, 123.9, 123.8, 109.9, 86.7, 68.3, 66.0, 28.7, 27.9, 25.3, 24.7, 24.7, 24.5, 24.1, 15.2. We did not observe a resonance attributable to the Ta-Me carbons, 7 1 presumably due to exchange broadening. Li{ H} NMR (233 MHz, Benzene-d6) δ 2.0. Anal.

Calcd for C 41 H65N2O2LiClTa: C, 58.53; H, 7.79; N, 3.33. Found: C, 58.64; H, 7.60; N, 3.47.

Single crystals for XRD were obtained by cooling an Et 2O solution to -25 °C.

5.5.3 Synthesis of [Li(Et 2O)(THF)LTaMe 4] ( 5.2 (Et 2O)(THF))

[Li(Et 2O)(THF)LTaMe 3Cl] (31.5 mg, 0.0374 mmol), was dissolved in Et 2O (5 mL) and cooled to -35 °C. Methyl lithium ( ca. 0.16 M in diethylether, 0.22 mL) was added dropwise. The reaction mixture was allowed to warm to room temperature protected from light, and was stirred for 60 min, causing the colour of the solution to lighten. Filtration and removal of volatiles yielded yellow microcrystalline [Li(Et 2O)(THF)LTaMe 4], 5.2 (Et 2O)(THF) (30.3 mg, 0.0369 mmol, 99%). Anal. Calcd for C 42 H68 N2O2LiTa: C, 61.45; H, 8.35; N, 3.41. Found: C, 61.72; H,

8.09; N, 3.41. Single crystals of 3(THF)2 for XRD were obtained by cooling a pentane solution 1 to -25 °C. H NMR (500 MHz, Benzene-d6) δ 7.25 (dd, J = 7.2, 1.8 Hz, 2H), 7.19 – 7.11 (m, 4H), 5.54 – 5.45 (m, 2H), 4.26 – 4.18 (m, 2H), 3.28 (sept, J = 7.0 Hz, 2H), 3.15 (br, 4H), 3.07 (sept, J = 6.9 Hz, 2H), 3.04 (q, J = 7.0 Hz, 4H), 1.34 (d, J = 6.9 Hz, 6H), 1.25 (d, J = 6.9 Hz, 6H), 1.23 (d, J = 6.8 Hz, 6H), 1.16 (d, J = 6.8 Hz, 4H), 1.04 (br, 4H), 0.90 (t, J = 7.0 Hz, 6H), 13 1 0.89 (br, 12H) C{ H} NMR (126 MHz, Benzene-d6) δ 164.3, 146.5, 142.5, 140.9, 128.1, 124.3, 123.8, 123.7, 109.3, 89.7, 68.4, 66.0, 28.6, 27.8, 25.2, 24.8, 24.8, 24.6, 24.3, 15.0. We did not observe a resonance attributable to the Ta-Me carbons, presumably due to exchange broadening. 7 1 Li{ H} NMR (194 MHz, Benzene-d6) δ 2.3. Anal. Calcd for C 42 H68 N2O2LiTa: C, 61.45; H,

8.35; N, 3.41. Found: C, 61.72; H, 8.09; N, 3.41. Single crystals of 5.2 (THF)2 for XRD were obtained by cooling a pentane solution to -25 °C.

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5.5.4 Synthesis of [Li 2L’(Et 2O) 2] ( 5.3 )

A hexanes solution of nBuLi (1.6 M, 0.29 mL, 0.46 mmol) was added dropwise to a solution of

H2L’(102 mg, 0.223 mmol) in Et 2O cooled to -70 °C using a glove box cold well. The reaction was allowed to warm to room temperature and was stirred for 2 h at which point volatiles were removed in vacuo . Pentane (2 mL) was added and the mixture was cooled to -25 °C overnight. The supernatant was decanted off the precipitate which was washed with cold pentane (2 mL) and dried under reduced pressure, leaving fluffy white powder (86 mg, 0.14 mmol, 63%). Due to the highly sensitive nature of this dilithium complex, we could not obtain satisfactory elemental 1 analysis. H NMR (300 MHz, Benzene-d6) δ 7.37 (d, J = 7.6 Hz, 4H), 7.22 (dd, J = 8.0, 7.1 Hz, 2H), 6.16 (s, 2H), 3.40 (sept, J = 6.8 Hz, 4H), 2.92 (q, J = 7.1 Hz, 4H), 2.03 (s, 6H), 1.36 (d, J = 6.8 Hz, 12H), 1.29 (d, J = 7.0 Hz, 12H), 0.74 (t, J = 7.1 Hz, 12H). 13 C{1H} NMR (101 MHz,

Benzene-d6) δ 152.7, 144.8, 143.8, 123.8, 121.3, 120.9, 115.1, 66.2, 28.4, 25.3, 25.2, 19.3, 14.1. 7 1 Li{ H} NMR (194 MHz, Benzene-d6) δ 0.8. Single crystals for XRD were obtained by cooling a pentane/Et 2O solution to -25 °C.

5.5.5 Synthesis of [LiL’(Et 2O)] (5.4 )

Method A

[Li 2L’(Et 2O) 2] (0.213 g, 0.353 mmol) was dissolved in THF (8 mL) and cooled to -70 °C using a glove box cold well. A similarly cooled solution of TaMe 3Cl 2 (0.104 g, 0.351 mmol) in Et 2O (4 mL) was added dropwise, and the mixture was allowed to warm to room temperature and stirred for 16 h. Volatiles were removed in vacuo and pentane (10 mL) was added to the mixture. Insoluble material was filtered off and the filtrate was concentrated and cooled to -25 °C. Crystals formed and were isolated on a fritted funnel; washing with cold pentane (2 x 1 mL) and drying in vacuo yielded dark green crystals (0.104 g, 0.194 mmol, 55%). This material was characterized by EPR and XRD, but even multiple recrystallizations did not lead to satisfactory combustion analysis.

Method B

[Li 2L’(Et 2O) 2] (160.8 mg, 0.2607 mmol) was dissolved in toluene (6 mL) and cooled to -25 °C.

Under subdued lighting, AgBF 4 (50.6 mg, 0.260 mmol) was added as a solid and the mixture was allowed to warm to room temperature with stirring for 2.5 h. At the end of the reaction time, the

129 dark green mixture was filtered through Celite and stripped. Recrystallization of the residue from

5 mL of ca . 5% Et 2O in pentane cooled to -25 °C yielded dark green crystals, which were washed with cold pentane and dried in vacuo (87.4 mg, 0.163 mmol, 63%). Anal. Calcd for

C36 H52 N2OLi: C, 80.71; H, 9.78; N, 5.22. Found: C, 80.22; H, 9.77; N, 5.05. Single crystals for

XRD were obtained by cooling a pentane/Et 2O solution to -25 °C.

5.5.6 Synthesis of [TaLMe 3] (5.5)

K2L: To a solution of H 2L (0.94 g, 2.3 mmol) in toluene (15 mL) cooled to -25 °C was added benzylpotassium (0.61 g, 4.7 mmol)), in one portion. The reaction mixture was allowed to warm to room temperature, and was stirred for 16 h. Pentane (60 mL) was added and the mixture was cooled to -25 °C. The pale green suspension was decanted from any unreacted orange benzylpotassium onto a frit, which left a pale green powder that was then washed with cold toluene (2 x 5 mL) and pentane (2 x 10 mL), and then dried in vacuo leaving a pale green pyrophoric powder 0.97 g, 86%.

TaMe 3Cl 2 (174 mg, 0.586 mmol) was dissolved in toluene (10 mL) and cooled to -70 °C using a glove box cold well. The vial was removed from the cold well and finely ground K 2L (330 mg, 0.653 mmol) was added as a solid, in portions over five minutes. The reaction turned gradually turned brownish red as it warmed to room temperature. After 5 h, the mixture was filtered through Celite and the toluene was removed under reduced pressure. Recrystallization of the residue from pentane afforded two crops of crystals (143 mg, 0.219 mmol, 37%). 1H NMR (600

MHz, Benzene-d6) δ 7.22-7.21 (m, 6H), 6.52-6.50 (m, 2H), 6.00-5.97 (m, 2H), 3.52 (sept, J = 6.9 Hz, 4H), 1.30 (d, J = 6.9 Hz, 12H), 1.06 (d, J = 6.8 Hz, 12H), 1.03(s, 9H). 13 C{1H} NMR (151

MHz, C 6D6) δ 147.8, 146.1, 145.0, 127.6, 124.7, 120.8, 114.8, 83.5, 28.7, 26.3, 24.3. Anal. Calcd for C 33 H47 N2Ta: C, 60.73; H, 7.26; N, 4.29. Found: C, 60.58; H, 7.19; N, 4.21.

5.5.7 Synthesis of K 2L(dme) 4:

K2L (130 mg, 0.258 mmol) was recrystallized from cold 1,2-dimethoxyethane (dme, 5 mL). The supernatant was decanted from the yellow crystals, which were washed with cold dme (2 x 2 mL) and pentane (3 x 2 mL). Drying in vacuo yielded yellow K 2L(dme) 4 (110 mg, 0.127 mmol,

49%), which is insoluble in C 6D6.

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5.5.8 Synthesis of DCC insertion product (5.6 )

[TaLMe 3] (66 mg, 0.10 mmol) was dissolved in pentane and cooled to -25 °C. A solution of N,N’-Dicyclohexylcarbodiimide (21 mg, 0.10 mmol) in pentane (2 mL) at room temperature was added dropwise. The solution immediately turned cherry red, and was allowed to warm to room temperature with stirring for 16 h. Volatiles were removed under reduced pressure, leaving the insertion product (81 mg, 93%). The analytical sample was washed with pentane. 1H NMR (600

MHz, Benzene-d6) δ 7.31 (d, J = 7.5 Hz, 4H), 7.25 (t, 2H), 6.55 – 6.51 (m, 2H), 6.06 – 6.02 (m, 2H), 3.71 (br, 2H), 3.63 (br, 2H), 3.40 (br, 2H), 1.63 (s, 3H), 1.55–1.05 (broad, overlapped, 46 13 1 H), 1.03 (s, 6H). C{ H} NMR (151 MHz, C 6D6) δ 180.2, 149.1, 146.2, 145.5, 126.8, 125.2 (br), 124.0 (br), 120.4, 115.9, 75.3, 65.9, 59.4, 35.4, 32.8, 28.6, 28.1, 26.5, 25.8, 24.9, 24.3, 15.1.

Anal. Calcd for C 46 H69 N4Ta: C, 64.32; H, 8.10; N, 6.52. Found: C, 64.23; H, 8.06; N, 6.69. Single crystals for XRD were obtained by allowing a pentane solution to slowly evaporate at room temperature.

5.5.9 Synthesis of MesN 3 insertion product ( 5.7 )

Pentane was added to [TaLMe 3] (51.5 mg, 0.0789 mmol) and the brown mixture was cooled to - 25 °C. A solution of mesityl azide (13.4 mg, 0.0831 mmol) in pentane (0.5 mL) was added dropwise. The mixture was allowed to warm to room temperature and was stirred for 2 h, causing a precipitate to form. Volatiles were removed under reduced pressure and the residue was washed with cold pentane (3 x 0.5 mL). Drying in vacuo left orange powder (40.7 mg, 0.0500 mmol, 63%). The analytical sample was recrystallized from toluene. 1H NMR (600 MHz,

Benzene-d6) δ 7.24 – 7.21 (m, 6H), 6.70 (s, 2H), 6.67 – 6.61 (m, 2H), 6.29 – 6.24 (m, 2H), 3.35 (sept, J = 6.9 Hz, 4H), 2.53 (s, 3H), 2.09 (s, 6H), 2.06 (s, 3H), 1.10 (d, J = 6.8 Hz, 12H), 1.09 (d, 13 1 J = 6.8 Hz, 12H), 0.67 (s, 6H). C{ H} NMR (151 MHz, Benzene-d6) δ 147.9, 146.0, 145.5, 145.3, 135.6, 129.6, 129.4, 127.2, 124.8, 120.5, 114.9, 63.1, 35.0, 28.2, 25.4, 24.8, 20.8, 18.6.

Anal. Calcd for C 42 H58 N5Ta: C, 61.98; H, 7.18; N, 8.60. Found: C, 61.76; H, 6.98; N, 8.76.

Single crystals for XRD were obtained by cooling an Et 2O solution to -25 °C.

5.5.10 Synthesis of [TaLMe 2]2 (5.8 )

A pyrex reaction vessel was charged with a solution of [TaLMe 3] (46.7 mg, 0.0716 mmol) in toluene (30 mL) and was sealed with a Teflon screw cap. The bomb was irradiated with a 450 W Ace Glass medium-pressure mercury lamp inside a photochemical reaction cabinet for 3 h. After

131 removal of volatile components, the residue was extracted with pentane and filtered through Celite. Concentration of the filtrate and cooling to -25 °C caused formation of red-orange crystals. The supernatant was decanted and the crystals were washed with cold pentane (3 x 1 mL). Drying in vacuo yielded analytically pure [TaLMe 2]2·(pentane) 2/3 (9.0 mg, 0.0068 mmol, 1 19%). H NMR (500 MHz, Benzene-d6) δ 7.21 – 7.18 (m, 8H), 7.13 (dd, J = 8.5, 6.7 Hz, 4H), 6.30 – 6.25 (m, 4H), 5.81 – 5.75 (m, 4H), 3.66 (sept, J = 6.8 Hz, 8H), 1.19 (d, J = 6.9 Hz, 24H), 13 1 1.03 (d, J = 6.7 Hz, 24H), 0.08 (s, 12H). C{ H} NMR (126 MHz, C 6D6) δ 150.1, 145.4, 145.2,

127.5, 124.8, 121.3, 114.7, 49.2, 28.0, 26.2, 24.7. Anal. Calcd for C 64 H88 N4Ta 2(C 5H12 )2/3 : C, 61.11; H, 7.31; N, 4.23. Found: C, 61.17; H, 7.29; N, 3.96. Note: the ratio of 5.8 to pentane was determined by integration of the 1H NMR spectrum. Single crystals for XRD were obtained by cooling a pentane solution to -25 °C.

5.5.11 X-ray crystallography

The X-ray diffraction data were collected on a Bruker Kappa Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K controlled by an Oxford Cryostream 700 series low-temperature system and processed with the Bruker Apex 2 software package. 70 The structures were solved by direct methods and refined using SHELXL-2013 and SHELXL-2014. 71,72 All non-hydrogen atoms were refined anisotropically, except for a few atoms involved in the disordered portions. All hydrogen atoms were calculated using the riding model. The diffuse residual electron density from a disordered cocrytallized pentane molecule in the lattice of 5.8 was removed with the SQUEEZE function of PLATON, 73 and was not included in the formula or the refinement. Selected crystallographic data are listed in Table 5.3

Table 5.3 Crystallographic data for 5.1-5.8 and K2L(dme) 4.

5.1 5.2(THF)2 5.3 5.4 5.5

Formula C41 H66 ClLiN 2O2Ta C42 H66 LiN 2O2Ta C40 H62 Li 2N2O2 C36 H51.4 LiN 2O C33H47 N2Ta FW 842.29 818.85 616.79 535.13 652.67

T (K) 150(2) 150(2) 150(2) 150(2) 150(2)

space group P2 1/n Pcba P2 1/c P2 1/n P2 1/n

a (Å) 11.0302(4) 21.1920(16) 11.4110(15) 8.9697(4) 10.6170(19)

b (Å) 23.2106(8) 18.2269(12) 19.573(3) 21.8449(8) 17.966(3)

132

c (Å) 17.0382(7) 21.3088(16) 17.749(2) 17.0042(7) 16.185(3)

a (deg) 90 90 90 90 90

b (deg) 108.6064(15) 90 93.323(6) 94.395(2) 98.038(7)

g (deg) 90 90 90 90 90

V (Å 3) 4134.1(3) 8230.8(10) 3957.6(9) 3322.0(2) 3056.8(9)

Z 4 8 4 4 4

−3 Dc (g∙cm ) 1.353 1.322 1.035 1.070 1.418

m (mm −1 ) 2.757 2.705 0.061 0.063 3.618

no. of refln 39363 70581 19128 28427 24798 collected no. of indept 9494 9454 8876 7647 7023 refln GOF on F2 1.183 1.150 1.010 1.040 1.128

a) R [I > 2s (I)] R1 = 0.0233 R1 = 0.0318 R1 = 0.0579 R1 = 0.0623 R1 = 0.0290

b) wR2 = 0.0577 wR2 = 0.0797 wR2 = 0.1525 wR2 = 0.1718 wR2 = 0.0441

R (all data) R1 = 0.0339 R1 = 0.0647 R1 = 0.1088 R1 = 0.0865 R1 = 0.0683

wR2 = 0.0799 wR2 = 0.1160 wR2 = 0.1884 wR2 = 0.1896 wR2 = 0.0889

5.6 5.7 5.8 K2L(dme) 4

Formula C46H69N4Ta C42 H58 N5Ta C64 H88 N4Ta 2 C46 H78 N2O8K2 FW 859.00 813.88 1275.28 865.30

T (K) 150(2) 150(2) 150(2) 149(2) space group P2 1/n P-1 P2 1/c P2 1/n a (Å) 14.1372(15) 10.4591(8) 15.9017(5) 10.8034(7) b (Å) 15.1014(15) 12.8533(10) 19.5480(4) 27.6688(19) c (Å) 20.108(2) 16.2426(13) 22.0555(5) 17.1421(12) a (deg) 90 69.066(3) 90 90 b (deg) 96.647(4) 82.852(4) 92.2620(11) 98.503(2) g (deg) 90 76.536(3) 90 90

V (Å 3) 4264.1(8) 1981.3(3) 6850.5(3) 5067.7(6)

Z 4 2 4 4

−3 Dc (g∙cm ) 1.338 1.364 1.236 1.070 m (mm −1 ) 2.613 2.808 3.227 0.235 no. of refln 37634 23806 65508 33778 collected

133 no. of indept 9755 9050 15746 8874 refln GOF on F2 0.956 1.040 0.933 1.039

a) R [I > 2s (I)] R1 = 0.0407 R1 = 0.0209 R1 = 0.0356 R1 = 0.0519

b) wR2= 0.0839 wR2 = 0.0482 wR2 = 0.0791 wR2 = 0.1425

R (all data) R1 = 0.0893 R1 = 0.0257 R1 = 0.0595 R1 = 0.0715

wR2 = 0.0990 wR2 = 0.0499 wR2 = 0.0853 wR2 = 0.1600

a) b) 2 2 2 2 2 1/2 R1 = S (Fo − Fc)/ S Fo wR2 = [ S [w(Fo – Fc ) ]/ S w(Fo ) ]

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6 Chapter 6 Summary and future directions 6.1 Introduction

This thesis detailed the coordination chemistry of actor NN chelate ligands. The first part of the thesis is comprised of chapter 2, the objective of which was to discover a new fundamental mode of reactivity for the CO 2 molecule using actor diazafluorenyl ligands. To this end, we carried out borylation of the diazafluorenyl ligand via template synthesis, which resulted in the construction of a reactive C-B bond. Even at this preliminary step, I benefitted greatly from the prior efforts of Adam Pantaleo and Ellen Yan. Adam studied the reactivity of

[Ru(H)(daf)(N 2)(PPh 3)2] towards HBcat and obtained inseparable mixtures of products, and Ellen demonstrated smooth borylation and carboxylation of the [(Mes 2nacnac)Zn(daf)] system involving Bpin derivatives. When I took over this project, I found that substituting the N 2 ligand for a CO ligand on the starting Ru complex suppressed some of the complicating reactivity. I also found that reactivity involving HBcat was highly moisture sensitive, and that protonation of - the carbanionic daf ligand occurred with concomitant formation of Bcat 2 ions. Switching to HBpin allowed synthesis of the product bearing the reactive C-B bond of interest. Reaction of these compounds with CO 2 was challenging because of their sensitivity to moisture due to an extremely favourable protodeborylation pathway. Eventually, my labmate Yanxin Yang suggested I bring CO 2 into the glovebox and carry out the reaction there in order to protect the carboxylation reaction from moisture. Syringing a slight excess of CO 2 gas above the solution of

[Ru(H)(dafBpin)(CO)(PPh 3)2] in the -25 °C freezer allowed us to observe and crystallize the product of CO 2 insertion into the C-B bond. During the time we were demonstrating that these diazafluorenyl derivatives were capable of catalytic hydroboration of CO 2, the literature filled up with many reports of catalytic hydroboration/hydrosilylation of CO 2. Although it was exciting to work in a fast-paced area of science with such an active community, the vast number of compounds capable of catalyzing such transformations suggested that our hydroboration/hydrosilylation of CO 2 was not so special. Given the high cost of silanes and boranes, research efforts in CO 2 reduction are probably best directed towards reactions that are more useful, such as CO 2 hydrogenation.

136

In part 2 of this thesis, which is comprised of the work on redox-active opda ligands detailed in chapters 3-5, the stated objective was not achieved. No N 2 complex sponsored by an opda ligand was able to be synthesized, so there is much still to be done. However, several important lessons from these chapters can be gleaned and would be beneficial to an incoming

Song group researcher if she were to continue to attempt N 2 chemistry sponsored by opda ligands. One such lesson focuses on the synthesis of metal complex precursors, i.e. compounds which are one synthetic step away from binding N2. In order to synthesize a precursor, the opda ligand must be installed in the desired coordination mode on the desired metal in the desired oxidation state. Before the research detailed in this thesis, the only ligand oxidation state 2- available was L in the form of LH 2. We newly synthesized Li 2L, which is highly reducing and consequently sometimes ended up engaging in undesired electron transfer with the metal. The discovery that stable monolithium radical complexes (LiL and LiL’) could be synthesized using Ag + to oxidize their parent dilithium complexes. These sources of monoanionic opda ligand are less reducing and their coordination chemistry towards N 2 activation-relevant metals is likely to be much different and begs to be explored.

Another lesson learned was that Li + has quite a high affinity for the NN chelate site of opda; consequently, salt metathesis chemistry was often incomplete, and equivalents of LiCl were not eliminated. In a related matter, the opda ligand often bound metals through its o- phenylene carbon backbone, which enabled the synthesis of lithium-containing transition metal complexes. When K + was employed as counterion for L 2- instead of Li +, no K + analogues formed; instead, the desired salt metathesis reactions went to completion, happily eliminating KCl and binding the transition metal at the NN chelate site.

6.2 Chapter 2

This chapter deals with the reactivity of metal complexes’ diazafluorenyl (daf) ligands towards borylation and carboxylation. Seeking to build upon our group’s demonstration that diazafluorenyl zwitterions can effect tandem CO 2 and C-H activations, we prepared diazafluorenyl complexes amenable to borylation at the nuceophilic carbon so that analogous reactivity of C-B bonds could be investigated. We synthesized [Ru(H)(daf)(CO)(PPh 3)2], which reacts with HBpin to liberate H 2 and undergo borylation at the nucleophilic backbone carbon of

137

the diazafluorenyl ligand, yielding [Ru(H)(dafBpin)(CO)(PPh 3)2]. When reacted with CO 2, this compound inserts CO 2 into the C-B bond of the diazafluorenyl ligand to yield [Ru(H)(daf(CO 2)

Bpin)(CO)(PPh 3)2]. DFT calculations suggest that the insertion is concerted.

[Ru(H)(daf)(CO)(PPh 3)2] and [Ru(H)(dafBpin)(CO)(PPh 3)2] are capable precatalysts for the hydroboration of CO 2 with HBpin, yielding H 3COBpin and pinBOBpin. The CO 2 insertion product [Ru(H)(daf(CO 2)Bpin )(CO)(PPh 3)2] reacts with HBpin at elevated temperatures to regenerate [Ru(H)(dafBpin)(CO)(PPh 3)2]. This work was extended to a new platform, namely (nacnac)Zn(daf), which possesses no potentially reactive metal hydride functionality. Analogous borylation, carboxylation, and catalysis by that platform was achieved.

We also investigated the reactivity of one of the storied compounds in our group,

[Ru(H)(daf)(N 2)(PPh 3)2], towards HBpin. The first equivalent of HBpin displaces the labile N 2 ligand at room temperature to yield the symmetric borohydride species

[Ru(H 2Bpin)(daf)(PPh 3)2]. Heating [Ru(H 2Bpin)(daf)(PPh 3)2] in the presence of toluene and excess HBpin causes borylation of the diazafluorenyl ligand to yield

[Ru(H 2Bpin)(dafBpin)(PPh 3)2] as well as toluene borylation products.

[Ru(H 2Bpin)(dafBpin)(PPh 3)2] can also be generated by reacting [Ru(H)(daf)(N 2)(PPh 3)2] with ClBpin; although this method requires that half of the Ru starting material be used as an internal Bronsted base, this method was more amenable to being scaled up. Future directions include investigating the reactivity of these two compounds towards CO 2 to investigate the effect of changing the ligand set and therefore the electronics of the spectator metal centre. Also possible is further mechanistic work to gain insight into the catalytic hydroboration of CO 2.

6.3 Chapter 3

Chapter 3 contains our initial foray into the coordination chemistry of the N, N’-bis(2,6- 2- diisopropylphenyl)-o-phenylenediamide ligand (L ). Reaction of Li 2L(THF)3 with halides of V, Fe, Mo, and Eu gave insight into the reducing nature of L 2-. Notably, complexes of L 2-, L -, and 0 L were isolated and characterized. In future studies, the radical species LiL(OEt 2) will be used in coordination chemistry. LiL(OEt 2) is less reducing, and may give rise to a coordination chemistry less complicated by reduction of the metal centres. Especially in the case of iron, if π- accepting co-ligands such as toluene and CO can be avoided, the potential for small molecule activation is great.

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6.4 Chapter 4

Synthesis of the compounds contained in chapter 4 came about through the unexpected discovery that [(LiL) 2V(OEt 2)2] reacts with PbCl 2 not to oxidize the V centre or the opda ligand, but to completely transfer L 2- on to Pb to yield N-heterocyclic plumbylene LPb. To explore for redox activity of L 2-, we examined the reactivity of the accidentally formed LPb and its Sn analogue towards typical oxidants such as MesN 3, ONMe 3, and AgOTf. At elevated temperatures mesityl nitrene inserts into the benzylic C-H bond of the dipp group of LPb and LSn to yield new three-coordinate tetrylenes. This reactivity is unique in that compared to the literature of reactivity of tetrylenes reacting with azides, the nonbonding electron pair on LPb or LSn is much less basic, which allows it to remain unchanged in these reactions, and allows the thermally formed nitrene to react at the benzylic C-H position.

This lack of basicity of the nonbonding electron pair on LPb was borne out again in is reactivity with ONMe 3: simple formation of a Lewis pair occurs as ONMe 3 donates into lead’s empty p orbital. We took advantage of this lack of basicity by oxidizing LSn with AgOTf. The oxidant ignores the lone pair and instead removes an electron from the o-phenylenediamido framework, producing a diiminosemiquinonate ligand L - bound to Sn. The product of this reaction (LSn(OTf)) belongs to the class of paramagnetic stannylenes and is the first of its kind to be isolated. Future work on this project may include synthesis of stannylenes without benzylic C-H bonds in the ligand framework, such that suitably selected organic azides might undergo catalytic C-H bond amination.

6.5 Chapter 5

2- In Chapter 5, Ta complexes of L of the form LTaX 3 were targeted. For X = halide, reduction of LTaX 3 would be attempted in order to generate low-valent Ta species capable of binding and transforming dinitrogen. Alternatively, for X = alkyl, hydrogenolysis of the alkyl ligands would be pursued in hopes of generating Ta hydrides which would spontaneously evolve

H2 and then bind and transform dinitrogen.

In practice, Li 2L(THF)3 turned out to be unsuitable for generating LTaX 3 due to the high 2- + affinity of L for Li . However, use of K 2L was effective, and allowed for the successful synthesis of LTaMe 3. The reaction of LTaMe 3 in different solvents with different pressures of

139 hydrogen is still under study and will be the subject of future work. Apart from hydrogenolysis, some different reactivity of the Ta-Me ligands was explored, namely insertion of unsaturated substrates (allowing for the generation of triazenido and amidinato ligands by template synthesis) and photochemical demethylation. The reactivity of these species towards H 2 will also be explored.

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About the Author

Biography:

Trevor Janes was born in Vancouver, BC in August of 1989. He initiated post-secondary studies at Langara College where he graduated with an ASc Science (general) in 2009. Trevor then moved several kilometers west to the University of British Columbia where he received his BSc (Honours Chemistry) in 2011. While at UBC, he was pleased to do research in the group of Prof. Parisa Mehrkhodavandi. Trevor obtained his PhD in 2017 advised by Prof. Datong Song at University of Toronto, where he was delighted to grow the first of many X-ray quality single crystals. Trevor loves watching baseball and chilling at the beach.

Publications: 1. Janes, T.; Zatsepin, P; Song, D. “Reactivity of heavy carbene analogues towards oxidants: redox active ligand-enabled isolation of a paramagnetic stannylene” Chem. Commun., 2017, 53 , 3090-3093. 2. Liang, Q.; Janes, T.; Gjergji, X.; Song, D. “Iron complexes of a bidentate picolyl-NHC ligand: synthesis, structure and reactivity” Dalton Trans., 2016, 45 , 13872-13880. 3. Hassan, Y.; Janes, T.; Pensack, R. D.; Rafiq, S.; Brodersen, P. M.; Winnik, M. A.; Song, D.; Scholes, G. D. “Direct synthesis of CdSe nanocrystals with electro-active ligands” Chem. Mater., 2016, 28 (14), 4953-4961. 4. Janes, T.; Xu, M.; Song, D. “Synthesis and reactivity of Li and TaMe 3 complexes supported by N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamido ligands” Dalton Trans ., 2016, 45 , 10672-10680. 5. Janes, T.; Osten, K. O.; Pantaleo, A.; Yan, E.; Yang, Y.; Song, D.“Insertion of CO 2 into the carbon-boron bond of a boronic ester ligand” Chem. Commun ., 2016, 52 , 4148-4151. 6. Janes, T.; Rawson, J. M.; Song, D. “Syntheses and structures of Li, Fe, and Mo derivatives of N,N’ -bis(2,6-diisopropylphenyl)-o-phenylenediamine” Dalton Trans ., 2013, 42 , 10640-10648.

Conference Presentations:

1. Janes, T.; Osten, K. O.; Yang, Y.; Song, D. “Insertion of CO 2 into the C-B bond of a borylated Ru(II) complex” Poster presentation, 15 th Boron in the Americas, Kingston, ON, June 25-28, 2016. 2. Janes, T.; Zatsepin, P.; Song, D. “Nitrene transfer mediated by Pb and Sn complexes of a bulky o-phenylenediamido ligand” Oral presentation, 99 th Canadian Chemistry Conference and Exhibition, Halifax NS, June 5-9, 2016. 3. Janes, T.; Xu, M.; Song, D. “Synthesis and reactivity of TaMe 3 complexes of bulky o- phenylenediamido ligands” Poster presentation, 99 th Canadian Chemistry Conference and Exhibition, Halifax NS, June 5-9, 2016

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4. Janes, T.; Zatsepin, P.; Song, D. “Reactivity of iron complexes of a radical o- phenylenediamine-based ligand.” Poster presentation, 48 th Inorganic Discussion Weekend, Kingston, ON, November 6-8, 2015. 5. Janes, T.; Pantaleo, A.; Yan, E.; Yang, Y.; “Formal insertion of CO 2 into a C-B bond and application in CO 2 reduction catalysis by borylated 4,5-diazafluorene derivatives” Oral presentation, 98 th Canadian Chemistry Conference and Exhibition, Ottawa, ON, June 13- 17, 2015. 6. Janes, T.; Pantaleo, A.; Yan, E.; Yang, Y.; Song, D. “Formal insertion of CO 2 into a C-B bond and application in CO 2 reduction catalysis by borylated 4,5-diazafluorene derivatives” Oral presentation, Inaugural Pharmaron/DFG Symposium, Toronto, ON, March 20, 2015. 7. Janes, T. and Song, D “Synthesis and reactivity of tantalum complexes of a bulky o- phenylenediamide ligand” Poster presentation, 46 th Inorganic Discussion Weekend, Toronto, ON, November 8-10, 2013. 8. Janes, T. and Song, D. “Redox activity and dearomatization behaviour of a bulky diamide ligand.” Oral presentation, 45 th Inorganic Discussion Weekend, Ottawa, ON, November 2-4, 2012.