1
THERMAL REARRANGEMENTS OF ORGANOPLATINUM(II)
COMPOUNDS
By
DAVID C. GRIFFITHS, B.Sc., A.R.C.S.
A Thesis Submitted for the Degree of Doctor of Philosophy
of the U n iv e rsity of London and for the
Diploma of Imperial College
Department of Chemistry
Imperial College of Science & Technology
London SW7
U.K. October 1985 ABSTRACT
The preparation of a series of bis-(neophyl)platinum(II) complexes
L2Pt(CH2CMe2Ph)2 [L = EtgP, PhgP; L2 = cod, bipy, bipym, phen, Ph2phen,
Me^phen, dppe] is described. Toluene solutions of L2PtR2 when heated
(0-100°C) under inert atmospheres rearrange to yield 1-platinaindans
L2Pt(2-C^H^CMe2CH2) and tert-butylbenzene in a reaction involving an intramolecular aromatic 6-C—H activation. The compounds have been 1 13 31 extensively characterised by NMR ( H, C, P), IR and electronic spectroscopy.
Detailed studies of the metallacyclisation of L2Pt(CH2CMe2C^R^)2
[L = Et^P; L2 = bipy, bipym, Ph2phen; R = H,D] by NMR show fir s t - o r d e r kinetics for the rearrangement with both the rate and mechanism, depending critically on the ancillary ligand. Activation parameters and k in e t ic isotope effects for the reactions are presented and mechanisms proposed.
c is-(EtoP)2Pt(CH2CMe2Ph)2 also undergoes a linkage isomerisation in polar coordinating solvents to yield c is-(EtpP)2Pt(CHpCMe^h)(2-C^H^CMeo), which has been structurally characterised by an X-ray diffraction study.
Thermolysis of the deuterated analogues c is -(EtoP)2Pt(CH2CMe2Ph)(2-C^
D^H^^CMe^) and c i s - (E to P )2Pt(CH2CMe2C^Dr) (2-C^H^CMe^) in benzene-d^ establishes that aliphatic <5-C—H activation is competitive in this system, but that aromatic 6-C—H cleavage is favoured. cis-(EtoP)2Pt(CH2CMe2Ph)-
(2-C^H^CMeg) shows unusual thermal behaviour in the presence of free Et^P and undergoes further isomerisations producing trans-(Et^P)pPt(CH2CMe2Ph ^- (2-C^H^CMe^) and t ra n s - (E t qP) pPt( 2-C^H^CMeo)pas w ell as 1-platinaindan and tert-butylbenzene. None of these rearrangements participate in m e ta lla c y c lisa tio n s of c i s - CEtoP^PtCCHoCMe^PhOo in toluene.
Evidence for restricted Pt-C bond rotation in the complexes
L 2Pt(CH2CMe2Ph)(2-C6H4CMe2R) [L = E t 3P, L 2 = cod, dppe; R = H, Me] in solution is presented.
The cationic mononeophylplatinum(II) derivative [(terpy)Pt(CH2CMe2Ph)]+BF also isomerises when heated in acetonitrile-d^ to yield
[ (te rp y )P t(2 -C 6H4CMe3 ) ] +BF~. 4
ACKNOWLEDGEMENTS
I am particularly grateful to Dr G. Brent Young for constant motivation, support and friendship during this project.
The expert advice and assistance given to me by Dr Andrzej Skapski,
Colin Robinson, Roger Lincoln and particularly Dick Sheppard and Sue
Johnson, has been greatly appreciated.
I would like to thank friends in the Chemistry Department, members of the GBY group past and p resent, p a rt ic u la r ly Sue, N ia ll, Viv, Debbie and Louise, the "Inorganic Tea Room", footballers, cricketers and flat mates, especially Andy, Dave, Nick and J ill, for making the last three years so enjoyable.
Thanks are due to Miss Sandra Hoskins for typing this thesis so c h e e rfu lly and efficiently.
I am also indebted to my parents for their constant encouragement and a s s is ta n c e .
Finally, the award of a Science and Engineering Research Council
Studentship is gratefully acknowledged. 5
CONTENTS
PAGE NO.
T it le Page 1
A b stract 2
Acknowledgements 4
Contents 5
List of Figures 10
List of Tables 13
Abbreviations 15
CHAPTER 1 : INTRODUCTION 16
CHAPTER 2 : SYNTHESIS AND SPECTROSCOPIC STUDIES OF NEOPHYLPLATINUM(II) AND RELATED COMPLEXES 27
2.1 Synthesis and Spectroscopy of Bis(neophyl)platinum(II) 28 and 1-P latinaind an Complexes.
2.1a Preparation of Bis(neophyl)platinum(II) and 1-Platirtaindan Complexes with Diene and Nitrogen 28 Donor Ligands 2.1b Preparation of Bis(neophyl)platinum(II) and 1-Platinaindan Complexes with Tertiary Phosphine 33 Ligands 2.1c Spectroscopic Studies of Bis(neophyl)platinum(II) and 1-Platinaindan Complexes with Diene and Nitrogen 35 Donor Ligands
2.1c(i) Nomenclature and Numbering 35 2. lc (ii) 1H-NMR 37 6
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2.lc ( iii) 13C-NMR 39 2.1c(iv) Infrared 42 2.1c(v) Electronic Spectroscopy 44
2 .Id Spectroscopic Studies of Bis(neophyl)platinura(II) and 1-Platinaindan Complexes with Tertiary Phosphine Ligands 45
2. ld(i) ^-NMR 45 2.ld (ii) 31P-NMR 47 2.ld (iii) 13C-NMR 49 2.1d(iv) Infrared Spectroscopy 49
2.1e Synthesis and Spectroscopy of (y-bipym)- [P t-(2 -C 6H4CMe2CH2) ] 2 53
2.2 Synthesis and Spectroscopy of Asymmetric Neophyl(hydrocarbyl)- platinum(II) and Neophyl(halo)platinum(II) Complexes with Diene and Tertiary Phosphine Ligands 55
2.2a Preparation of Asymmetric Neophyl(hydrocarbyl)platinum(II) and Neophyl(halo)platinum(II) Complexes with Diene and Tertiary Phosphine Ligands 55 2.2b Spectroscopic Studies of Asymmetric Neophyl(hydrocarbyl)- and Neophyl(chloro)platinum(II) Complexes with Diene and Tertiary Phosphine Ligands 62
2.2b(i) General 62 2.2b(ii) 1H-NMR Stud ies of L 2Pt(CH2CMe2Ph)(2-C 6H4CMe2R) [L 2 = cod, L = Et^P, R = H,Me]; Evidence for Restricted Pt-C Bond Rotation in Solution 62
2.3 Synthesis and Spectroscopy of [(terpy)Pt(CH2CMe2Ph)]+BF4 and Attempts to Prepare Tertiary Phosphine Analogues 70
2.3a Preparation and '*‘H-NMR of [ (terpy)Pt(CH2CMe2Ph) ]+BF- 70 2.3b Attempts to Prepare Tris-(tertiaryphosphine)neophyl- platinum(II) Complexes 72 7
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2.4 Attempts to Prepare C a tio n ic Mononeophyl Compounds with Chelating Tertiary Phosphine Ligands
2.5 Experimental 80
2.5.1 General and Instrum ental 80 2.5.2 Preparation of Neophylplatinum(II) Compounds 81 2.5.3 Preparation of Grignard Precursors 99
CHAPTER 3 : THERMAL REARRANGEMENTS OF NEOPHYLPLATINUM(II) COMPLEXES WITH NITROGEN DONOR LIGANDS; KINETIC AND MECHANISTIC STUDIES 103
3.1 Introduction 104
3.2 Kinetic and Mechanistic Studies of the Metallacyclisation
of L 2Pt(CH2CMe2Ph)2 [L 2 = bipy, bipym, Pl^phen, Me^phen] via Aromatic 6-C—H Activation 106
3.2a Kinetics of Thermal Rearrangement of L 2Pt(CH2CMe2Ph)2 [L 2 = bipy, bipym, Pl^phen, Me^phen] 106
3.2b Kinetics of Thermal Rearrangement of L 2Pt(CH2CMe2C^D^)2 [L 2 = bipy, PJ^phen] 112 3.2c Effects of Added Ligands on Rates of Thermal Cyclisation
of L2Pt(CH2CMe2Ph)2 113 3.2d Mechanisms of Thermal Cyclisation of Bis-(neophyl)- p la tin u m (II) Complexes with Nitrogen Donor Ligands 116
3.2e Thermal Rearrangement of (bipy)Pt(CH2CMe2Ph)2 in the Presence of Dioxygen 121
3.3 Thermal Rearrangement of [(terpy)Pt(CH2CMe2P h )]+BF^ 122
3.4 Experimental 124
3.4a General 124
3.4b Kinetic Experiments on Cyclisation of L 2Pt(CH2CMe2Ph)2 124 3.4c Thermal Rearrangement of [(terpy)Pt(CH2CMe2P h )]+BF^ in Acetonitrile-dg 125 8
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CHAPTER 4 : THERMAL REARRANGEMENTS OF NEOPHYLPLATINUM(II) 126 COMPLEXES WITH TERTIARY PHOSPHINE LIGANDS; KINETIC AND MECHANISTIC STUDIES
4.1 Introduction 127
4.2 R e su lts and D iscu ssio n 129
4.2a Thermal Rearrangement of c is-(Et3P)2Pt(CH2CMe2Ph)2 in Hydrocarbon Solvents 129
4.2a(i) Kinetic Studies of Metallacyclisaston of cis-(Et^P)oPt(CH^CMe^Ph)^ in Toluene 129 4.2a(ii) Mechanistic Implications 139
4.2b Effect of Variation of Tertiary Phosphine Ligands on Rate of Neophylplatinum(II) Cyclisation 142
4.2b(i) Thermal Decomposition of cis-CPh^P^Pt- (CH2CMe2Ph)2 142 4.2b(ii) Thermal Decomposition of (dppe)Pt- (CH2CMe2Ph)2 143
4.2c Thermal Rearrangement of c is-(EtpP)2Pt(CH2CMe2Ph)2 in Polar Solvents;Isomerisation to cis-(Et3P)2Pt- 144 (CH2CMe2Ph)(2-C6H4CMe3)
4.2d Thermal Rearrangement of c is-(EtoP)2Pt(CH2CMe2C^Dc)- (2-C6H4CMe3) and c i s - ( E t 3P )2Pt(CH2CMe2P h)(2-C GD H4 CMe3) in Hydrocarbon Solvents; Evidence for Aliphatic C-H Activation 154
4.2e Thermal Rearrangement of cis-(Et3P)2Pt(CH2CMe2Ph)- (2-C^H4CMe3) in the Presence of Et3P; Isomerisation to trans-alkyl(aryl)platinum(I I ) and trans-bis- (aryl)platinum(II) Complexes 161 4.3 Experimental 173 4.3a General 175 4.3b Kinetic Studies 176 9
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CHAPTER 5 : CONCLUSIONS 176
5.1 The E ffe c t of the A n c illa ry Ligand on the Mechanism of 177
Metallacyclisation of Bis-(neophyl)platinum(II)
2 5.2 Intermediacy of rj -Arene-Metal Complexes in Aromatic 179 C-H A ctivatio n
5.3 Aromatic vs. Aliphatic and Intramolecular vs. Inter- 181 molecular C-H Activation
REFERENCES AND NOTES 182 LIST OF FIGURES
PAGE NO.
1. 3-H Elimination 18 2. Rearrangements of Transition Metal Neopentyl Complexes 20 3. Mechanisms of Intram olecular C-H A ctivatio n s 22 4. Aromatic C-H A ctiv atio n in an Irid iu m (I) Complex 24 5. Decomposition of Neophyl and 2-tert-butylphenyl- 25 platinum(II) Complexes 6. Synthesis of B is-(n e o p h yl)p latin u m (II) Complexes 29 7. Metallacyclisation of Bis-(neophyl)platinum(II) Complexes 32 8. Nomenclature and Numbering Systems Employed for Neophyl- 36 platinum(II) and Related Complexes
9. Typical P2Pt(CH2CMe2Ph)2 Methylene Resonance 45 10. ^C-NMR Spectrum of cijs-CEt^P^Pt^-C^H^^^C^) in CDCl^ 50, 51
11. Isomers of (y-bipym) [Pt^-C^H^^^C^) ^ 54
12. Thermal Decomposition of (cod)Pt(CH3)(CH2CMe2Ph) in 56 Toluene 13. Synthesis of 2-bromo- t e r t - butylbenzene 58 14. Proposed Structure of Minor Product from the Reaction of 61
(cod)Pt(CH2CMe2P h )I and dppe
15. 1H-NMR (250 MHz) Spectrum of (cod)Pt(CH2CMe2Ph)(2-C6H4CMe3) 64 in CDCl^
16. ^H-NMR Spectrum (Alkyl Region) of cis-(EtpP)2Pt(CH2CMe2Ph)- 66
(2-C,H.CMeQ) in C,D, at 250 MHz o 4 o o b 17. Possible Isomerisation of Cationic Mononeophylplatinum(II) 70 Complexes 18. Preparation of [(terpy)Pt(CH2CMe2Ph)]+BF4 71 19. Intermediacy of n^-arene Complexes in Aromatic C-H Activation 74 11
PAGE NO
20. Preparation of (2,2-dimethyl-4-penten-l-yl)platinum(II) 75 and Proposed Route to an Analogous Neophyl-Derived Complex 21. Possible Product From Reaction of (dppe^tCCI^C^^Ph)! 78 with AgBF^
22. ^ P {^H}-NMR Spectrum from Reaction of (dmpe^tCd^CN^Ph)- 79 Cl with AgBF^ in CDCl^
23. "Roll-over” C-3 Metallation in (bipy)Pt(aryl)2 Complexes 105 24. Effect of Temperature on the Rate of Metallacyclisation of 109 ( b i p y ^ t C C ^ C l ^ P h ^ in Toluene
25. Arrhenius Plot for Thermal Cyclisation of L^PtCCI^CN^Ph^ 110 in Toluene 26. Effect of Neophyl Ring Deuteration on Rates of Metallacycli- 114 sation of L^PtCC^Cl^C^R^^ [L^ = bipy, Pl^phen; R = H,D]
27. Proposed Mechanism of Cyclisation of (bipy)Pt(CH2CMe2Ph)2 117 2 28. Intermediacy of r| -Arene Metal Complexes in Reductive Elimination 120 of Arene from Aryl-Hydride Metals 29. Thermal Isomerisation of [(terpy)Pt(CH2CMe2Ph)]+BF^- in \22 Acetonitrile-d^
30. Effect of Temperature on Rate of Cyclisation of cis-CEt^P^- 132 Pt(CH2CMe2Ph)2 in the Presence of 0.037M Et^P
31. Arrhenius Plots for Cyclisation of cis-(EtpP)2Pt(CH2CMe2Ph)2 133 (i) in the Presence of 0.037M Et^P (ii) in the Absence of Et3P
32. Kinetics of Cyclisation of cis-(EtqP)^Pt(CH2CMepC^R^)^ 135 [R = H,D] at 65°C in Toluene Containing Et^P (0.07M)
33. Effect of Concentration of Added Et^P on the Cyclisation 136 Rate of cis-CEt^P^PtCCHpCMepPh^ at 80°C
34. Proposed Mechanism of Cyclisation of cis-CEtpP^Pt- 139 (CH2CMe2Ph)2 12
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35. Possible Transition States Formed During Liberation of 141 (a) tert-butylbenzene and (b) Neopentane 36. Molecular Structure of cis-(Et3P)2Pt(CH2CMe2Ph)(2-C^-H4CMe3) 145
37. Mechanism of Formation of cis-(EtoP)oPt(CHoCMeoPh)- 148 (2-C6H4CMe3)
38. Equilibria and Equations for Dissociation of Et^P from 151 L2Pt(CH2CMe2Ph)2 and L2Pt(CH2CMe2Ph)(2-C6H4CMe3 )
3,9. Thermal Rearrangements of cis_-(Et3P)2Pt(CH2CMe2Ph)2 and 152 cis-(Et3P)2Pt(CH2CMe2Ph)(2-C6H4CMe2R) [R = Me, H] in Toluene Solutions 40. Possible Mechanisms of Thermal Metallacyclisation of 156 cis-(Et3P)2Pt(CH2CMe2C6D5)(2-C6H4CMe3)
41. Thermolysis Products of cis_-(Et3P)2Pt(CH2CMe2Ph)(2-C^D - 159 H, CMe~) 4-n 3 42. ^H-NMR Spectrum (low field region) of cis-(EtoP)2Pt- 161 (CH2CMe2Ph)(2-C^DnH4_nCMe3) in C^D^ showing 68% Protiation at H-3 Position 43. Formation and Disappearance of XI and X2 during Thermolysis 163 of cis-(Et3P)2Pt(CH2CMe2C6R5)(2-C6H4CMe3) [R = H,D] in the Presence of Et3P at 65°C
44. Thermal Rearrangement of cis-(EtoP)2Pt(CH2CMe2Ph)(2-C^H4CMe3) 167 in the Presence of Excess Et3P.
45. Kinetics of cis - trans Isomerisation of cis-(Et3P)2Pt- 168 (CH2CMe2Ph)(2-C^H4CMe3) at 65°C in the Presence of 2M Et3P
46. Methylene Region of ^H-NMR Spectrum of (i) cis-(Et3P)2Pt- 170 (CH2CMe2Ph)(2-C6H4CMe3) and (ii) cis-(Et3P)2Pt(CH2CMe2Ph)- (2-C6H4CMe3) + XI in C6D6
47. Mechanism of cis - trans Isomerisation of cis-C 172 (CH2CMe2Ph) (2-05^ 0163) in the Presence of Et3P 13
LIST OF TABLES
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1. Analytical Data for Dineophylplatinum(II) and Platinaindan Complexes 30
2. NMR Characteristics of Dineophylplatinum(II) and Platinaindans with Diene and Nitrogen Donor Ligands 38 13 3. C NMR Characteristics of Dineophyl and Platinaindans with Diene and Nitrogen Donor Ligands 40 13 4. C NMR Characteristics of Nitrogen Donor Ligands 41
5. Electronic Spectra of Bis(neophyl)platinum(II) and Platinaindans with Nitrogen Donor Ligands 43
6. ^H-NMR Characteristics of Dineophylplatinum(II) and Platinaindans with Tertiary Phosphine Ligands 46 31 7. P NMR Characteristics of Neophylplatinum(II) and Related Complexes with Tertiary Phosphine Ligands 48 13 8. C NMR Characteristics for some 1-platinaindan Tertiary Phosphine Complexes 52
9. NMR Characteristics of Asymmetric Neophyl(hydrocarbyl)- and Neophyl(halo)platinum(II) Complexes 63
10. Effect of Temperature on First Order Rate Constant k for Metallacyclisation of L2Pt(CH2CMe2Ph)2 [L = bipy, bipym, 108 Pl^phen, Me^phen]
11. Activation Parameters for Metallacyclisation of l^Pt- (CH2CMe2Ph)2 in Toluene 111
12. Kinetic Isotope Effects on Thermal Cyclisation of L^Pt:- (CH2CMe2C^R^)2 [L2 = bipy, Pl^phen; R = H,D] in Toluene 113 14
PAGE NO.
13. Rate Constants for Thermal Cyclisation of cis-CEtpP^Pt- (CI^Cl^C^Rc^ Under Various Conditions 130
14. Activation Parameters for Cyclisation of cis-(EtpP)^Pt- (C^CI^^Ph^ in the Presence and Absence of Added Et^P 131
15. Selected Bond Lengths in cis-C E t ^ P ^ P t C C ^ ^ ^ ^ ) ABBREVIATIONS
adme 1-adamantylmethyl bipy 2,2’-bipyridyl bipym 2,2’-bipyrimidyl cod cycloocta-1 ,5-diene
Cp* pentamethylcyclopentadienyl dmpe bis-(1 ,2-dimethylphosphino)ethane dppe bis-(1 ,2-diphenylphosphino)ethane dppm bis-(1 ,2-diphenylphosphino)methane
Triethylphosphine Et3P HPLC High Pressure Liquid Chromatography
Me^phen 3,4,7,8-tetramethyl-l,10-phenanthroline
MO Molecular Orbital phen 1 ,10-phenanthroline
Ph^phen 4,7-diphenyl-l,10-phenanthroline ph3p Triphenylphosphine terpy 2,2'-6',2"-terpyridyl
THF Tetrahydrofuran CHAPTER 1
INTRODUCTION 17
The work described in this thesis comprises kinetic and mechanistic studies of thermal rearrangements of organoplatinum compounds in solution, and encompasses examples of both aromatic and aliphatic intramolecular carbon-hydrogen bond cleavage reactions. The major part is concerned with neophylplatinum(II)* complexes. It is appropriate, therefore, to review briefly intramolecular C— H activations in metal alkyl and aryl complexes up until October 1982, when this investigation started.
Subsequent chapters describing the thermal behaviour of the neophyl- platinum (II) and related complexes will be prefaced by a specifically relevant introduction.
(Neophyl = 2-methyl-2-phenylpropyl) 18
Many synthetically important industrial processes involving hydro
carbons are catalysed by transition metals, often heterogeneously. It
is generally accepted that these reactions involve rearrangements of the
organic groups while they are attached to the metal. One approach to
elucidating the mechanisms of these rearrangements has involved the study
of accessible transition metal alkyl complexes which are believed to model closely intermediates formed at the metal surfaces and are much more readily studied. Recent theoretical MO calculations on the cleavage
of methane C— H bonds at metal surfaces and by soluble transition metal
fragments have reinforced the similarity between the two reactions!
Early attempts to prepare transition metal to carbon Q-bonds were not encouraging and it was thought that metal-carbon bonds had low bond
energies. It was soon realised that this instability was kinetic, not 2 thermodynamic in origin. A common decomposition pathway available to many alkyl compounds involves 3-hydrogen transfer and is sometimes called 2,3 "3-hydride elimination" (see Fig.l). In this reaction the metal abstracts a hydrogen atom from the 3-carbon of the alkyl chain to form
Fig. 1 3-H Elimination
2 H\ e / R' H r
2 an olefin-hydride complex, from which hydrogen and the olefin may be lost to complete the decomposition of the organometallic compound. The reverse reaction, insertion of olefins into metal-hydrogen bonds to form a metal 19
4 alkyl is also well known.
A variety of strategies have been successfully adopted to inhibit the ready decomposition of metal-alkyls by this route and enabled other rearrangements to be observed. Since the co-ordination number of the metal is increased during the reaction, 3-hydride elimination is generally suppressed in co-ordinatively saturated and sterically crowded metal complexes. Similarly, metals ligated by alkyl groups containing no
3-hydrogen atoms are denied this decomposition pathway. The most widely studied of this latter class of compounds have been those of the neopentyl
(-CI^CMe^) group.
Restriction of 3-H transfer in this way in complexes of early transition elements (Ta, Nb, W) leads instead to migration of the a-hydrogen atom from the neopentyl group to produce one equivalent of.neopentane and 5 an alkylidene complex (Fig.2, route (i)). On the other hand, Whitesides and co-workers have studied the thermal rearrangements of cis-(EtoP)^Pt(CHoCMe,-,in hydrocarbon solvents and discovered that, in this instance, y-hydrogen abstraction from the methyl group of the neopentyl moiety occurs, to yield a metallacyclic product and neopentane 6 (Fig.2, route (ii)). The mechanism of this latter reaction has been studied in detail and is found to proceed by initial ligand dissociation of Et^P, followed by insertion of the metal into the y-C— H bond to give a hydrido(alkyl)platinum(IV) intermediate (Fig.3 (i)). Reductive elimination from this intermediate yields the observed products. A significant deuterium isotope effect and positive entropy of activation indicated that this last step is most probably rate-limiting (see Chapter 4). Fig. 2 Rearrangements of Transition Metal Neopentyl Complexes
C M e 4
nro 21
y-Hydrogen transfers have been inferred in neopentyl and related trimethylsilylmethyl (-d^SiMe^) complexes of ruthenium(II)7 and rhodium 8 (III) by isolation of the corresponding metallacyclobutanes. Similar
g reactivity has also been observed in bis-(neopentyl)thorium compounds.
In this case the thoracyclobutane produced was found to activate the C— H bonds of benzene to produce a bis-(phenyl)thorium complex.
Support for Whitesides’ proposal for the intermediacy of hydrido(alkyl)platinum(IV) species during these metallacyclisations was provided by the isolation of an iridium(III) hydride complex from the I + - 10 reaction of [Ir (PMe^)^] Cl and neopentyllithium (Fig.2, route (iii)).
This complex shows a remarkable propensity towards intramolecular C— H activations with a variety of hydrocarbyl ligands, including both aliphatic and aromatic C— H cleavage, to produce the corresponding metallacyclic iridium(III) hydrides. The only intermediate iridium(I) alkyl isolated is the trimethylsilylmethyl complex which also cyclises when heated.
At this point it is worth noting that all examples of intra-, or indeed, intermolecular C— H activations do not necessarily involve transfer of hydrogen to the metal. The a-hydride migration mentioned earlier occurs from a complex of tantalum in its highest oxidation state,
Ta(V)d°, and a mechanism involving oxidative addition of the C— H bond to the metal centre is improbable. A more plausible alternative is a concerted mechanism via a four-centre transition state which involves no change in the oxidation state or co-ordination number of the metal
(Fig.3(ii)). The same type of mechanism has been postulated in ortho- metallation reactions of a ruthenium(II)-triphenylphosphine complex, and to 23
observed in aliphatic C— H activations at d metal centres. These two examples (Fig.3 (i) and 3 (ii)) represent extreme mechanisms and it seems likely that processes intermediate between the two may also occur.
Although Y-C— H abstractions are the most common reactions studied in detail so far, aliphatic C— H activations at even more remote sites in the hydrocarbyl ligand have been observed. The 2-methylbenzyl moiety
^-CH^C^H^C^-) undergoes a 6-elimination reaction in phosphine complexes 13 14 of ruthenium(II) and platinum(II), while replacement of one of the
3- methyl groups in neopentyl by higher alkyls, such as ethyl and n-propyl, has lead to isolation of 5- and 6-membered platinacycles formed by 6- and e-hydrogen transfer reactions respectively, in complexes of the type 15 c is - (EtpP)oPt(CHpCMeolOo• The results of this latter study suggest that whilst formation of a 5-membered metallacycle is slightly favoured over the 4-membered ring and both are more favoured than the 6-membered product, the energy differences are small compared with ring strain energies in carbon homocycles.
By contrast, examples of aromatic C— H activation in hydrocarbyl groups are relatively scarce, particularly in view of the numerous reports 16 of intermolecular H— D exchange catalysed by transition metal complexes, and of intramolecular ortho-metallation of phenyl groups in phosphorus 17 and nitrogen donor ligands. The first example of a remote C— H elimination in a transition metal hydrocarbyl complex did, however, feature the aromatic cyclisation of a naphthyl(methyl)platinum(II) complex in 18 refluxing toluene with concomitant liberation of methane. 24
Apart from this the only other well characterised examples of
intramolecular remote aromatic C— H activations in hydrocarbyl groups at
the outset of this project were those involving reaction of the iridium(I) 10 complex mentioned above with benzyl and neophyl alkylating agents. The
products were formed by metallation at the ortho- C— H bond of the phenyl
rings (Fig.4).
Fig.4 Aromatic C— H Activation in an Iridium(I) Complex
The ability of the neophyl group to undergo rearrangements involving aromatic 6-C— H activation was also inferred from a study of the organic
products from the reactions of neophyl lithium and magnesium reagents with nickel halides, but no intermediate organonickel complexes were 19 isolated • 25
The X-ray crystal structure of a dimeric manganese(II) neophyl II complex [Mn ( G ^ C I ^ ^ P h ^ ^ suggests an interaction between the metal 20 and the ortho- C— H bond of the phenyl group. When heated at 100°C under
vacuum tert-butylbenzene is formed but the organometallic product was IV not characterised. The same group have also prepared Cr (C^CI^^Ph)^ 21 22 which is very stable and a crystal structure shows no such interaction. 10 These results, together with those of Tulip and Thorn, suggested
that neophylplatinum(II) compounds might be sufficiently stable to enable
a systematic study of their thermal behaviour, hopefully giving some mechanistic insight into an aromatic C— H activation reaction. None of
the hydrocarbyl aromatic C— H bond cleavage reactions mentioned earlier
have been studied in detail but the ortho- metallation of phosphorus and
nitrogen donor ligands have been found to proceed via different mechanisms,
ranging from electrophilic attack in palladium complexes to reactions in 17 which the metal shows nucleophilic characteristics. Within each of these
last two classes further mechanisms are known in which the hydrogen may,
or may not, be transferred to the metal (cf. Fig.3).
It was also recognised that a study of neophylplatmum(II) and
isomeric 2-tert-butylphenylplatinum(II) would provide a direct comparison
of the rates of aliphatic and aromatic C— H cleavage in complexes that were
otherwise identical (see Fig.5).
Figure 5. Decomposition of neophyl- and 2-tert-butylphenyl- platinum(II) Complexes \ aromatic O-C— H v R-H aliphatic a-C — H V V Pt / o 26
Recent Developments
During the course of this investigation bis-neophyl compounds of 23 24 25 26 nickel(II), palladium (II) , manganese (II) and ruthenium(II) , stabilised by trimethylphosphine and b is-(1,2-dimethylphosphino)ethane, have been isolated, but no thermal behaviour reported. Wilkinson and 27 co-workers have also described cyclisations of benzyl groups in osmium 25 and manganese phosphine compounds.
This period has seen considerable progress in the area of inter- molecular C—H activation of both alkanes and arenes by soluble transition metal complexes. The first direct observation of the homogeneous reaction of totally unactivated carbon-hydrogen bonds, usually in solvent molecules, with unsaturated transition metal intermediates to give an observable alkyl (or aryl) hydridometal has been reported upon „ v T 28 29 T 30 photolysis of Cp*M(L)H2 (M = I r , L = PMe^, CO; M = Rh , L = PMe^).
A thorough kinetic and mechanistic study of the rhodium system by Jones and Feher has enabled comparisons of intermolecular aromatic versus 30c aliphatic C—H bond cleavage, and of inter- versus intramolecular 30d activation of the same type of C—Hbond. Their results are of relevance to some of the work described later and w ill be discussed at that point.
It is not clear why these complexes activate C—H bonds of the solvent and do not internally metallate the pentamethylcyclopentadienyl or 31.3 trim ethylphosphine lig an d s, both of which are known in other compounds.
Also noteworthy is the well-established intermolecular activation of both alkane and arene C—H bonds by a c id ic so lu tio n s of p latin u m (II) and platinum(II) salts in which a-bonded hydrocarbylplatinum inter- 33 mediates are clearly involved. CHAPTER 2
SYNTHESIS AND SPECTROSCOPIC STUDIES OF 34 NEOPHYLPLATINUM(II) AND RELATED COMPLEXES 28
2.1 Synthesis and Spectroscopy of Bis(Neophyl)Platinum(II) and 1-Platinaindan Complexes
2.la Preparation of Bis-(Neophyl)Platinum(II) and 1-Platinaindan Complexes with Diene and Nitrogen Donor Ligands
Bis-neophylplatimim(II) compounds have been successfully isolated using s im ila r methodology to that previously employed in the syn th esis of bis-(alkyl) and bis-(aryl)-platinum(II) complexes (see Fig. 6).
Thus, interaction of (cod^tC^tcod = cycloocta-1,5-diene] with excess neophylmagnesiumchloride in tetrahydrofuran for 6 hours at room temperature
produces (cod^tCCI^Cl^Ph^ in good yield (50-65%). Shorter reaction
times yield appreciable amounts of the monoalkylated product
(cod)Pt(CH2CMe2Ph)Cl, while prolongued reaction favours formation of the i ' i ortho-metallated compound (cod^t^-C^H^CN^CI^). A ll three complexes are colourless air-stable crystalline solids and may be readily
differentiated by ^H-NMR (see Table 2) spectroscopy. g When solutions of (cod^^CI^Cl^Ph^ in toluene-d are heated in
excess of 50°C production of the metallacyclic compound i1 . ■■■■■I (co d )P t(2-C^H^CMe2CH2 ) and tert-butylbenzene can be observed in the
^H-NMR spectrum (see Fig. 7). A kinetic investigation showed the
rearrangement to be first-order but gave poorly reproducible results.
This type of behaviour is probably due to traces of platinum metal,
either present in the starting material, or formed during the thermolysis,
catalysing the reaction in some way?1
The m e ta lla c y c lic compound can be iso la te d from decomposed samples
of (cod)Pt(CH2CMe2Ph)2 and has been f u lly ch a ra cte rise d . Figure 6. Synthesis of Bis-(neophyl)platinum(II) Complexes
rvj (D Table 1 Analytical Data for Dineophylplatinum and Platinaindan Complexes
Complex Colour Element % Found (Calculated) C H N (COD)Pt(CH2CMe2Ph)2 Colourless 58.80(59.01) 6.70(6.68) -
(bipy)Pt(CH2CMe2Ph)2 Red 58.36(58.33) 5.47(5.54) 4.47(4.61)
(bipym)Pt(CH2CMe2Ph)2 Maroon 54.01(54.27) 5.09(5.17) 8.91(9.05)
(phen)Pt(CH2CMe2Ph)2 Red 59.24(59.89) 5.28(5.34) 4.36(4.37)
(Me4phen)Pt(CH2CMe2Ph)2 Orange 61.88(61.96) 5.91(6.07) 4.10(4.01)
(Ph2phen)Pt(CH2CMe2Ph)2 Red 66.42(66.57) 5.17(5.33) 3.89(3.58)
(COD)Pt(2-C.H.CMe~CH0) Colourless 49.10(49.60) 5.51(5.52) - 6 4 2 2
(bipy)Pt(2-C6H4CMe2(!:H2)4/7 toluene Red 53.93(53.77) 4.58(4.62) 5.21(5.23) Maroon 44.17(44.53) 3.88(3.77) 11.37(11.54) (b ipym)Pt(2-C6H4CMe2CH2)
(phen)Pt(2-C.H,CMe0CH0) Red 52.70(52.07) 3.99(3.97) 6 4 2 2 5.37(5.52)
(Me/phen)Pt(2-C.H,CMe0CH») Orange 54:93(55.41) 4.94(5.01) 4 6 4 2 2 4.93(4.97)
(Ph2phen)Pc(2-C6H4CMe2CH2) Red 61.38(61.90) 4.25(4.28) 4.04(4.25) 31
Due to the poor reproducibility of the kinetic results with the diene ligand, and in order to study the effect of the nature of the ancillary ligand on the rate of the metallacyclisation reaction, a range of bis-(neophyl)platinum(II) complexes has been prepared by displacement of the diolefin, in the first instance, by bidentate aromatic nitrogen donor ligands [2,2'-bipyridine(bipy), 2,2’-bipyrimidine (bipym),
1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-l,10-phenanthroline
(Me^phen), 4,7-diphenyl-l,10-phenanthroline (Pl^phen)]. These substitutions are characteristically slow but the bulky neophyl groups appear to 35 facilitate the reactions, relative to bis-(methyl)platinum(II), and conversion is generally complete in 10 to 30 days in hydrocarbon solvents.
A large excess (ca. 10-50-fold) of the nitrogen donor ligand greatly accelerates the reactions, the free ligand being readily separated from the organometallic product by extraction into aqueous ferrous sulphate solution as the highly coloured [FeCL^)^]^*!-011* This procedure is inefficient with Pl^phen presumably because of its poor solubility in the aqueous phase, and is generally less attractive with the more expensive ligands. The reactions are best carried out in toluene under an inert atmosphere; the initially colourless solutions become intensely coloured as the reaction proceeds. All of the complexes Li2Pt(CH2CMe2Ph)2 are air stable, brightly coloured crystalline solids and have been characterised 1 13 by elemental analysis, H-and C-NMR, IR and electronic spectroscopy
(Tables 1-4 and Section 2.5).
Solutions of these bis-neophyl complexes decompose in the presence of dioxygen (except ( b i p y n O P ^ C i ^ O ^ P h ^ ^ v e r a few days to give unidentified products (see Section 3.2e) . Similarly, the compounds are 32
unstable with respect to halocarbon solvents which also cause decompo sition by an as yet uninvestigated mechanism*.
When toluene solutions of L^Pt(CH2CMe2Ph)2 are heated under argon to temperatures ranging from 55 to 95°C, depending on L^, elimination of tert-butylbenzene is observed concomitant with formation of new i". — i complexes identified as L^Pt^-C^H^CN^Cl^) (see Fig. 7). The metalla- cyclic compounds can be isolated by thermal decomposition of the
Fig. 7 Metallacyclisation of Bis-(neophyl)platinum(II) Complexes
bis-neophyls in toluene overnight, removal of the solvent in vacuo and subsequent crystallisation of the residue, and have also been fully characterised (Tables 1-4, Section 2.5)
* (Reaction of L^Pt(adam antylm ethyl)9 with halocarbons to vield
L^P^adam antylm ethyl)C1 is found to proceed by an electron transfer mechanism
resulting in Pt— C hom olysis1?6 c is -(Et^P)^Pt(CH^CMe^Ph)^ gives analogous
I------1 37 products via c is ^ EtoPOPt^-C^H^CMeollHo) in the presence of halocarbons) 33
Bis-(neophyl)platinum(II) complexes containing pentadeuterophenyl groups in the neophyl moiety, L2Pt(CH2CMe2C^D^)2 (L^ = bipy, Ph^phen) are obtained in analogous fashion from (cod)Pt(CH2CMe2C£D,_ ^ .
2.1b Preparation of Bis-(neophyl)platinum(II) and 1-Platinaindan Complexes with Tertiary Phosphine Ligands
Displacement of the diene from (cod)Pt(CH2CMe2ph)2 by tertiary phosphines [L = triethylphosphine (Et^P)* triphenylphosphine (Ph^P);
L2 = bis-(1,2-diphenylphosphino)ethane (dppe), bis-1,2-dimethylphosphino)- ethane (dmpe), bis-(l,2-diphenylphosphino)methane (dppm)] is more rapid than by the nitrogen donor ligands discussed in the previous section.
Reactions are generally complete in 2-48 hours, depending on the nucleophilicity of the phosphine and are carried out in hydrocarbon solvents under argon. In this way the bis-(neophyl)platinum(II) compounds
L^PtCCI^Cl^Ph^ [L = Et^P; ^2 = ^PPe l have been isolated and characterised, while the corresponding dppm and dmpe complexes have been identified in
solution by NMR.
Reaction of (cod^tCCf^Cl^Ph^ with Ph^P in benzene, even at 0°C,
leads only to isolation of cis-(PhoP)2Pt(2-C^H^CMe2CH2). The bis-neophyl complex cannot be detected by ^"H-NMR during the reaction, but has been 31 observed as a transient intermediate in the P-NMR spectrum of freshly mixed solutions of a large excess (ca. 20-fold) of Ph^P and (cod)Pt(CH2CMe2ph)2. 34
sis-(EtoP)oPt(CH^CMe^Ph)o is also thermally labile; toluene r ~ i solutions decomposing rapidly at 30-35°C to yield cis-(EtpP)^Pt(2-C^H^CMepCH^) and tert-butylbenzene. The solutions are considerably stabilised by addition of free Et^P (see Chapter 4). The bis-neophyl complex is stable at ambient temperatures in the solid-state when pure but is best stored for long periods at -25°C. The metallacycle is readily crystallised from hexane solutions. Unfortunately, crystals of both cis-(EtoP)oPt- i i (CH2CMe2Ph)2 and cis-(Et^P)oPt(2-C^ECCMe^C^) show a tendency to twinning and attempts to structurally evaluate the rearrangement by a single crystal X-ray diffraction study have so far been unsuccessful.
In contrast, bis-neophyl compounds with chelating phosphine ligands are relatively thermally inert. Toluene solutions of (dppe)Pt(CH2CMe2Ph)2 have to be heated to temperatures in excess of 100°C to induce metalla- cyclisation. Preliminary studies suggest that the dppm and dmpe analogues are similarly unreactive. The photochemistry of (dppe)Pt(CH2CMe2Ph)2 has i i also been investigated and is found to produce (dppe^t^-C^H^Q^CI^) at
-5°C, although side reactions lead to other, as yet unidentified, 38 organometallic products.
cis-CEtoP^PtCCHoCMeoC^-Dr-^ is isolated from the reaction of
(cod)Pt(CH2CMe2C^D^)2 and Et^P. 35
2.1c Spectroscopic Studies of Bis-(neophyl)platinum(II) and 1-Platinaindan Complexes with Diene and Nitrogen Donor Ligands
2.1c (i) Nomenclature and Numbering
Before describing the spectra of these compounds it is appropriate to describe the nomenclature and numbering systems employed.
The metallacycles produced by ortho-metallation of the neophyl moiety may be viewed as substituted indans with the platinum at the
1-position. Thus, the correct name for these complexes is 3,3-dimethyl-
1-platinaindan and the IUPAC numbering system is shown in Fig. 8
(Convention 1).
For the convenience of the following discussion, however, a different convention has been adopted for the numbering of the aromatic rings in neophyl- and 2-alkylphenyl-platinum(II) and platinaindan compounds. This system is more consistent; in each case the phenyl carbon atom attached
(or potentially attached) to the platinum is labelled C-2, while the alkyl-substituted carbon is designated as C-l (Fig. 8, Convention 2).
Aromatic protons are numbered similarly.
Also shown in Figure 8 are the numbering systems used for the nitrogen donor ligands which are based on those previously used in the literature.
Throughout the following discussion, peaks which show a coupling to 195 Pt(spin 34% abundant) are denoted by 6(X); where 6 is the chemical 195 shift and X is the observed coupling constant to Pt (measured between the two outer peaks of the 1:4:1 resonance). Such multiplets are termed
’triplets' ( ’t’) to distinguish them from binomial 1:2:1 triplets. 5
Fig. 8 Nomenclature and Numbering Systems Employed for Neophylplatinum(II) and Related Complexes
CO 05 2.1c (ii) ^-NMR (Table 2)
^H-NMR spectroscopy has proved a very useful means of characterising
the complexes and of following the cyclisation reactions. Particularly
diagnostic of the neophyl skeleton is the doublet of doublets at low
field (6 7.5 - 7.7) corresponding to the ortho protons of the phenyl
ring. This multiplet disappears during the metallacyclisation reactions.
The platinum-bound methylene hydrogens in the two types of complexes
give well separated signals, with the metallacyclic methylene resonances
normally shifted downfield by ca. 0.15 ppm. The kinetics of the
rearrangements have been determined by measuring the decrease/increase
in intensity of these peaks as reaction proceeds. The values of 2 J(Pt— H) for these protons in the platinaindans are ca. 10 Hz greater
than those in the parent bis-neophyl complexes.
Resonances assigned to the neutral ligands are as expected and are more complicated in the metallacyclic compounds reflecting their 3 asymmetry. The values of J(Pt— H) for the hydrogens adjacent to the nitrogen atoms in the bis-neophyl compounds are about 20 Hz. In the corresponding platinaindans the values of this coupling constant for
these, now inequivalent, protons are significantly different. Typically, 3 the lowest field resonance has J(Pt— H) of 16 Hz while the upfield resonance has a value of 24 Hz, presumably reflecting different Pt— N
bond characteristics.
The spectra of the deuterated analogues are characterised by the absence of hydrocarbyl aromatic peaks but are otherwise identical. Table 2 NMR Characteristics of Dineophylplatinum and Platinamdans With Diene and Nitrogen Donor Ligands3***
ppm S HYDROCARBYL LIGANDS (Jpt-H>[ASSIGNMENT] ]H ppm : NEUTRAL DONOR LIGANDS(JPt.H) COMPLEX (ASSIGNMENT] ch2 CH3 AROMATIC (cod)Pt(CH2CMe2Ph)2 2.23 (88) 1.53(6} 7.45 dd (Jh2-H3“8) t«2| 3.87 (38) [CH], 1.46 t (18) [CH2J 7.17 - 6.92 m [H3,H4] (cod)Pt(2-C6H4CMe2CH2) 2.29 (89) 1.44 7.26 - 7.00 m 5.08 (38), 4.64 (42) [CH], 1.82 [CH2] 7.68 dd (JH2-H3-7) [H2] 8.48 dd (21) (Jh6-h5“6) [H6], 6.28 dd (H5] (b ipy)Pt(CH2CMe2Ph)2 2.78 (86) 1.78 7.04 - 6.64 (H3, H4] obscured [H3, H4] 9.22 dd (17) (Jh6'-H5'”8, JH6,-H4,-2> IH6* ] (bipy)Pt(2-C6H4CMe2CH2) 2.91 (96) 1.81 7.31 - 6.84 m 8.73 dd (24) (JH6-H5-6, JH6-H4-2> tH61 7.70 dd (Jh3’-H4'"JH3-H4*6) [H3',H3] 6.35 dd. (H5], 6.40 dd (H5] 7.68 dd (Jh2-H3*8) [H2] 8.16 m [H4.H6] (bipym) Pt(CH2CMe2Ph)2 2.74' (87) 1.78 6.78-6.60 m [H3,H4] 5.93 dd (Jh4-H5“Jh5-H6”8) tH5] (b ipym)Pt(2-C6H CMe CH2) 2.87 (97) 1.76 7.25 - 6.98 m 9.16 dd (05) (Jh6'-H5'“6, Jh6'-H4*“2) [H6'J 4 2 8.62 dd (21)(Jh6-H5*8, Jh6_h4*2) (H6] 8.20 - 8.15 m (H4,H4’]. 6.09 dd (JH5,-H4,“Jh5'-H6*“6)(H5'], 6.03 dd (JH5-H4"JhS*H6“8) [H5] (phen)Pt(CH2CMe2Ph)2 c 2.52 (87) 1.52 7.62 dd (JH2-H3-8) (H2] 9.11 dd (20) (Jh9(2)-H7(4)-15,Jh9(2)-H8(3)‘5) 7.07 - 6.88 m (H3.H4] [H2.H9], 8.52 dd (Jh8(4)-h7(3)*8' Jh9(2)-H7(4)“I.5) [H4.H7], 7.88s [H5,H6] 7.61H3.H8]
(phen)Pt(2-C6H CMe CH2) d 2.88 (95) 1.48 7.21 - 7.03 m 9.91 dd (16) (JH2-H3-5, JH2-H4"1*5) tH2J 4 2 9.56 dd (24) (JH9-H8-5, JH9-h7’'-5) [H9] 8.63 dd (JH4_H3«4) (H4], 8.54 dd(JH7_H8-4) [HI] 7.93s (H5.H6], 8.00 - 7.75 m [H3.H8] (Ph2phen)Pt(CH2CMe2Ph)2 3.08 (88) 1.96 7.89 dd (Jh -h3“ ) [H2] 8.93 dd (20) (JH2(9)-H4(8)“5> tH2,H9J 7.33 - 6.70 2m (H3,H4]6 7.54 s [H5.H6], obscured [H3.H8], 7.14 (C6H5J (Phjphen)Pt(2-C H CMe CH2)d 1.55 9.08 dd (16) (JH2-H4’5)(H21, 8.89 dd (24) 84 2 2.95 (94) 1.52 7.20 - 7.09 m (JH9-H8*5)[H9], 8.00 s [H5.H6], 7.87 d (JH3-H2’6)(H31, 7.77 d (JH8_H9»6)(H81 7.59 [C6H5] (Me4phen)Pt(CH2CMe2Ph)2 2.87 (88) 1.85 7.89 dd (JH2-H3-8) [H2] 8.50 (20) [H2.H9], 7.37 s [H5.H6] 7.04 - 6.81 m (H3,H4] 1.75, 1.81 ICH3] (Me4phen)Pt(2-CgH4CMe 2CH 2) d 2.77 (95) 1.49 7.24 - 6.99 m 9.59 (16) [H2], 9.20 (23) [H9] 8.05 s [H5.H6], 2.64 [CH3] ------(a) Recorded in toluene -dg unless otherwise noted (b) Coupling constants in Hz where observed (c) Recorded m dichloromethane -d2 (d) Recorded in chloroform -d 39
2.1c (iii) ^C-NMR_(Table_3)
All of the bis-neophyl and metallacyclic compounds with diene and 13 1 nitrogen donor ligands have been characterised by C { H } -NMR spectroscopy [except (phen^tCCI^CN^Ph^ which is too insoluble], as freshly prepared chloroform-d solutions. Most of the spectra are very complicated due to the number and variety of aromatic carbons in the complexes. Assignments are based on comparison with the spectra of the free ligands (Table 4), on observed platinum-carbon coupling constants, and on the intensity of the peaks. The spectra were recorded proton- decoupled so carbon atoms with hydrogen(s) attached experience a nuclear
Overhauser enhancement, hence quaternary carbons appear as low intensity resonances. In this way the metallacyclic carbons C-l and C-2, which are also characterised by their downfield shifts, have been assigned
(see also 2.1d(ii)).
The numbering systems used for the compounds are shown in Fig.
8(ii) and (iii). 40
Table 3 ^cnMR Characteristics of Dineophyl and Me tall acyclic Compounds3 S13C ppm : HYDROCARBYL LIGANDS(JPC_c)[ASSIGNMENT] «I3C ppm : ANCILLARY LICANDS Pt-CH2 Pt-CH2C Pt-CH2CCH3 AROMATIC (Jpt-c) [ASSIGNMENT] 154.2[C1], 127.4, (C0D)Pt(CH2CMe2Ph)2 38.9(897) 42.6 34.0(60) 125.9[C2,C3], 124.5[C4] 99.9 (43) [CH], 29.3 [CH2] 168.8[C1], 156.9[C2] (COD)Pt(2-C6H4CMe2CH2) 50.2(775) 49.8(43) 35.0(44) 134.0(30), 126.2, 99.6 (62), 96.9 (47) [CH] 123.8(54) 30.2, 29.2 [CH2] 158.0[C1] 126.9,126.7[C2, 156.0[C2], 147.9(30)[C6], 135.2[C4] (bipy)Pt(CH2CMe2Ph)2 24.0(931) 42.9 32.8(55) C3] 124.1[C4] 125.9[C3], 121.6(10)[C5] c 134.1 150.1[C6], 149.4[C6'] 136.6[C4,C4*] (b ipy)Pc(2-C6H4CMe2CH2) 32.1 47.6 34.0(52) 123.9,123.2, 121.8 126.6[C3], 126.4[C31] 122.4[C5,C5'] 158.0[C1],127.1, 126.9 155.0[C4], 154.3(24)[C2], 153.6(26)[C6] (bipym) Pt(CH2CMe2Ph)2 24.4(958) 42.9(21) 32.7(63) [C2,C3], 124.4[C4] 122.8[C5] 168.9[C 1], 142.8(C2] 158.0[C2,C2'], 156.9[C4,C4*], 155.9(26)[C6] (b ipym) Pt (2-C6H4CMe2£H2) 31.7(833) 47.6(41) 33.9(52) 133.7(34), 124.1(66), 155.4(32)[C6'], 121.5[C5,C5'] 123.8(66), 122.2 (phen)Pt(CH2CMe2Ph)2 d 169.3[C 1], 143.9IC2] 149.9(32), 149.2(37)[C2.C9], 148.3, 148.0 (phen)Pt(2-C6H4CMe2dH2) 31.6(828) 47.9(42) 34.2(53) 134.2(37), 123.9(66) [011,0121, 135.9, 135.7[C4,C7], 130.5 [C13. 123.2, 122.1(70) C14], 127.2, 127.0[C5,C6], 125.5,125.4[C3,C8J 155.0[C1], 126.8[C2,C3] 148.9[C2,C9], 141,0[C4,C7], 134.6[C3,C8] (Me4phen)Pt(CH2CMe2Ph)2 23.5 42.9 32.9(58) 123.7(C4] 122.4[C5,C6], 18.0, 14.9[CH31 1 169.4(C1] c 133.4 150.8, 150.0[C2,C9], 142.6[C4,C7] (Me4phen)Pt(2-C6H4CMe2CH2) 31.5 47.8 34.2(52) 123.8, 123.0, 121.9 134.0[C3,C3], 122.8[C5,C6], 17.7, 15.0[CH3] 155.81C1] 155.0(34)[Cl1.C12], 147.3(25)[C2,C9] (Ph2phen)Pt(CH2CMe2Ph)2 24.1(938) 43.1(19) 33.0(55) 126.9[C2,C3], 124.5IC4] 137.4[C4,C7], 128.1(8)[013,014], 125.4(16)[C3.C8], 126.9[C5,C6], 148.0 129.1, 129.0, 124.1[CgH5] 169.4[C 1], 144.5[C2] 149.3, 148.7(27)[C2.C9], 148.9 (Ph2phen)Pt(2-CgH4CMe2CH2) 32.0 47.9(42) 34.3(53) 134.3(36), 123.9(66) 148.8[C11,C12], 136.9(C4,C7], 128.8 123.2, 122.1(66) [C13.C14], 125.9, 125.7[C 5,C6], 125.2 125.0[C3,C8] e. 129.3, 129.0[C6H5] a. All spectra recorded in CDCI3 (ref. 77.0) unless otherwise stated, coupling constants m Hz if observed b. Spectrum recorded m toluene -dg. One of the metallacyclic aromatic resonances is obscured by the solvent c. Cl and/or C2 of metallacycle not resolved due to poor solubility of complex d. Compound too insoluble e. Quaternary carbon of ligand phenyl groups obscured by C2,9,11 and 12 of ligand 13 Table 4 : C-NMR Characteristics of Nitrogen Donor Ligands
LIGAND 613 C ppm [ASSIGNMENT]
2,2'-bipyridine ^ 156.8 [C2], 149.65 [C6], 137.40 [C4], 124.15 [C3], 121.34 [C5]
2,2’-bipyrimidine ^ 161.0 [C2], 156.7 [C4, C6], 120.3 [C5]
1,10-phenanthroline 150.1 [C2, C9], 146.1 [Cll, C12], 135.9 [C4, C7], 128.5 [C13, C14], 126.4 [C6, C5], 122.9 [C3, C8]
4,7-diphenyl-l,10-phenanthroline 149.8 [C2,C9], 148.4 [Cll, C12], 138.0 [C4, C7], 126.4 [C13, C14], 128.4 [C5, C 6 ] , 123.4 [C3, C8], 146.8, 129.5, 128.5, 123.9 [ C ^ ]
a : All recorded in Chloroform-d b : From Ref.3 9 c : From Ref.40 Generally, the chemical shifts and values of J(Pt-C) of the carbon atoms in both the neophyl and platinaindan skeletons remain constant
( ±0.2 ppm, ±4 Hz) as the nitrogen donor ligand is varied, indicating the similar electronic and steric environments of the hydrocarbyl moieties in this series of compounds. 3 The values of J(Pt-C), the coupling constant from platinum to the 2 methyl carbons, are consistently higher than J(Pt-C), the two-bond coupling to the methyne carbon atom, in both the neophyl and platinaindan 41 compounds. This effect has previously been observed in J(P-C) and 13 13 42 J( C- C) couplings, and in J(Pt-C) in related platinum(II) alkyl and 14,43 metallacyclic complexes, and is probably due to anomalously low values of the two-bond coupling but theories governing coupling constants in these systems are not well developed.
The one-bond coupling ^J(Pt-C) for the methylene carbons are larger than those observed in related complexes, [e.g. ( c o d ^ t C C ^ C l ^ P h ^
897 Hz, (cod^tCCH^^ 775 Hz, (cod)Pt(C2H^)2 843 Hz]. Unfortunately, due to low signal intensity the value of ^J(Pt-C) for the aromatic C-2 was not observed in these metallacyclic complexes.
2.1c(iv) I^f£§red_Spectroscopy
IR data for this series of compounds are listed in Section 2.5.
Absorptions characteristic of the bis-neophyl complexes are mainly from the free phenyl ring; the peak at ca. 1600 cm ^ and a stronger one at
1495-1490 cm ^ are aryl C-H vibrations; the two strong peaks at 760-750 cm and 700-695 cm ^ are out-of-plane C-H bending modes typical of five adjacent phenyl hydrogens. In the platinaindans the strong peak at Table 5 Electronic Spectra of l^PtRR* a
A max (nm)
R=R’=CH2CMe2Ph L2 RRf=2-C,H,CMe6 4 20 CH0- 2
bipy. 477 467
bipym. 502 492
phen. 482 463
Me^phen. 454 434
Ph2phen. 487 472
(a) All spectra recorded as freshly prepared dichloromethane solutions 44
1495-1490 cm ^ and the absorption at ca_. 700 cm ^ are absent. Out-of- plane bends in the region 720-750 cm ^ are still present, however, indicative of an ortho-disubstituted benzene.
Neophyl complexes containing deuterated phenyl rings are characterised by weak peaks in the region of 2270-2150 cm ^ due to aryl C-D vibrations, the heavier deuterons causing a shift to lower energy from ca. 3050 cm ^ in the protiated analogues.
2.1 d ( v) Electronic_Spectroscopy_(Table_5_)
The electronic spectra of the compounds with nitrogen donor ligands have been recorded. The absorptions observed are due to diT-TT* metal- ligand transitions and thus reflect the relative electronic effects of 35b the ligands. The values listed in Table 5 are the lowest energy transitions and to an extent represent the energy difference betweeen the highest occupied molecular orbitals on the metal and the lowest empty tt* orbital on the ligand. The different colours of the bis-neophyl complexes (Table 1) suggest that these values vary with the ligand and this is confirmed by the electronic spectra. We were interested, if possible, to correlate these differences with the metallacyclisation rates of the compounds, particularly the nominally isosteric bipy and bipym derivatives. 2.Id Spectroscopic Studies of Bis-(neophyl)platinum(II) and Platinaindans with Tertiary Phosphine Ligands
2.Id (i) ^-NMR (Table 6)
The most interesting feature of these spectra is the multiplets observed for the methylene protons. In the bis-neophyl complexes this peak appears as an apparent quartet with platinum satellites (see Fig. 9).
ca.7Hz
Fig. 9 Typical P2Pt(CH2CMe2Ph)2 Methylene
Resonance (Pt-satellites not shown)
This is due to coupling to two magnetically inequivalent phosphorus atoms (cis and trans); the resulting AA'MX2X ’ 2 spin-system produces the 24,45 ' observed second-order spectrum. During metallacyclisation this resonance collapses and is replaced by a 1 :2:1 triplet with platinum satellites suggesting that the coupling constants to the cis and trans phosphorus atoms are equal m magnitude. This effect has been noted in a related 46 platinacycle. Table 6 ^H-NMR Characteristics of Dineophylplatinum (II) And Platinaindao Complexes With Tertiary Phosphine Ligands3*^
6 hi ppm : HYDROCARBYL LIGANDS (JPt_H) [ASSIGNMENT] hi ppm : PHOSPHINE LIGAND COMPLEX [ASSIGNMENT] CH2 CH3 AROMATIC
cis-(Et3P)oPt(CHoCMeoPh)o C,d 2.26 m (64) 1.73 7.65 dd (JH2-H3=7) [H2J 1.65 - 1.15 m [CH2] 7.38 - 7.15 m 0.78 - 0.61 [C!l3]
7.82 td (53)(1JH_H=7,2J H-H=1.5) 1.64 quintet [p1-CH2], cis-(Et3P)2Pt(2-C6H4CMe2£H2)C *d 2.18 t (56) 1.78 [H6], 7.34 - 7.03 m 1.50 quintet [P2-CH2J (3Jp _h =6) 0.92 - 0.78 app. sextet [CH3]
7.65 td (53)(1jH-tf=7,2JH-H=1.5) 1.56 quintet [pl-CH2], cis-(Et3P)2Pt(2-C6H4CMe2CH2) 2.03 t (56) 1.62 [H6], 7.23 - 6.94 m 1.50 quintet [P2-CH2] (3Jp_n=6) 0.88 - 0.73 app. sextet [CH3] cis-(PtnP) 2Pt (CH2CMe2Ph) 2 e
2.37 t (62) cis-(PhgP) 9Pt(2-C^H/|CMe2CH2) (3Jp_H=5) 1.71 f 7.48 m, 6.73 m [C6H5]
7.35 - 7.24 m, 6.07 - 6.88 m cis-(dppe)Pt(CH2CMe2Ph)2 2.37 m (76) 1.21 f [C6H5], 1.86 t (2Jp_H=7)[CH2]
2.64 t (64) 7.72 - 7.22 m, 7.00 - 6.90 cis-(dppe)Pt(2-CAH/|CMe?CH2) 1.60 f (3Jp_H=6) [C6H5], 1.94 m [CH2]
7.39 - 7.29 m, 7.04 - 6.82 m cis-(dppm)Pt(CH2CMe2Ph) 2 2.51 m (75) 1.38 f [C6H5], 3.81 t (2Jp_H)=9 [CH2]
(a) recorded in d^-toluene at 90 MHz unless otherwise noted; (b) coupling constants in Hz where observed; (c) recorded in benzene-d6; (d) recorded at 250 MHz; (e) complex too labile to be observed, see text; (f) obscured by ligand phenyl peaks.
CD*>■ 47
In contrast to the diene and nitrogen donor ligand complexes discussed 2 in the previous section, the value of J(Pt— H) to the methylene protons are ca. 10 Hz smaller in the metallacyclic compounds than in the parent dineophyls.
The spectra of the coordinated phosphines are typical.
2.Id (ii) 31P-NMR (Table 7)
31 1 The P { H } -NMR data for the bis-neophyl and metallacyclic phosphine compounds are listed in the top half of Table 7. The spectra confirm the cis-stereochemistry of the complexes, with values of ^J(Pt-P) expected for phosphines trans to alkyl and aryl groups in complexes of platinum(II).
The magnitude of this coupling constant for (dppm)Pt(CH2CMe2Ph)2 which seems anomalously low (1263 Hz) is, in fact, normal for complexes of this ligand. The size of this coupling constant appears to be inversely proportional to the bulk of the hydrocarbyl group; a value as low as 47 1236 Hz has been reported for (dppm)Pt(adme)2 [adme = 1-adamantylmethyl], yet in the less sterically demanding dimethyl analogue ^j(Pt— P) is 48 1424 Hz. The value probably reflects the amount of distortion in the four-membered ring formed by the ligand.
Conversion of the bis-neophyl complexes to the metallacycles is accompanied by the collapse of the 1:4:1 'triplet' and formation of a
'triplet' of AB quartets. The ^J(Pt-P) couplings for the two distinct phosphorus atoms in the product are generally larger in magnitude by 150-300 Hz than in the bis-neophyl compounds. This may be due to relief of steric strain on metallacyclisation (see Chapter 4). Table 7 ^1p NMR Characteristics of Neophyl Platinum (II) And Related Complexes With Tertiary Phosphine Ligands a»b
COMPLEX iSP 1 JPt-P 2 IP-P
c is-(Et 3P)oPt(CHoCMeoPh ) 7 0.7 1678 -
(E 1 3 P) 2 P't (2-C6H 4CMe 2CH 2) 13.3 , 10.6 1833 , 1857 13
cis-(PhiP)2Pt(CH2CMe2Ph>2 27.2 1668 -
Cis*(Ph3 p)2Pt(2-C6HACMe2iH2) 35.0 , 34.7 1949 , 1813 9
cis-(dppe)Pt(CH7CMe?Ph ) 7 39.3 15 84 - cis-(dppe)Pt(2-CftHACMe7CH?) 50.1 , 49.5 1753 , 1882 8
cis-(dppm)Pt(CH 7C M e 7P h ) 2 - 37.8 1263 -
cis-(dmpe)Pt(CH 2C M e 2Ph)2 22.7 1634 - cis-(EtiP)oPtfCHoCMeoPh)(2-C6H4CMe3) 0.7, - 4.5 1771 , 1718 1 1 cis-CEt^P)7Pr(CHiCMe7Ph) (2-CAH/.CHMe2) 2.8 , 0.9 1723 , 1669 1 1 cis-(EtiP)7Pt(CH2CMe7Ph)(CHa) c 7.8 , 5.5 1686 , 1921 10 cis-(dmpe)Pt(CH7CMe7Ph)Cl c 33.7 , 15.2 1540 , 4242 0 cis-(dppe)Pt(CHoCMeoPh)I c 42.8 , 40.5 4283 , 1551 0
[(dppe)Pt(CH2CMe 2Ph)I]2 c 35.4 , 30.8 1677 , 4265 4 trans-(Et7P)7PtfCH7CMe7Ph)Cl c 12.0 2992 -
trans-(PhTP)?Pt(CH 7C M e 9Ph)Cl d 25.6 3365 -
trans-(Et 3 P)?Pt(CH 7C M e 7 Ph)Br d 2975 -
(a) All spectra proton decoupled in toluene -dd unless otherwise stated, chemical shifts relative to external H 3 PO 4 (b) Coupling constants in Hz (c) Recorded in Chloroform -d (d) Recorded in Acetone -d(,. 49
2.Id (iii) 13C-NMR
r—------1 The complex cis-(EtpP)^Pt(2-C^H^CHe2CHo) is particularly characterised 13 1 by its C { H } -NMR spectrum and clearly shows platinum-bound methylene and phenyl carbon atoms. Nearly all of the peaks show coupling to both platinum and two different phosphorus atoms (the spectrum and assignments are shown in Fig. 10 and Table 8). The assignment of the methyl, methylene and methyne carbons is straightforward (see Section 2.1c(iii)).
C—1 and C-2 of the metallacycle are characterised by their downfield chemical shifts and reduced intensity, C-2 by its large coupling to platinum. C-3 is assigned to the peak at 6 137.1 which is downfield of the remaining aromatic carbons on the basis of the ^H-NMR spectrum of the compound in which the proton attached to C-3 appears at lowest field
(Table 6). The unsplit resonance at 6 122.7 is ascribed to C-5 as it is the most distant from the platinum.
The^3C {^H}-NMR spectrum of (dppe)Pt(2-C^H^CMe2CH2), although not as well resolved, shows similar characteristics for the hydrocarbyl carbons.
2.Id (iv) Infrared Spectroscopy
IR data for all of these phosphine complexes are listed in Section
2.5 and show the same characteristic absorptions as those described earlier (2.1c (iv)). Figure 10. 13C {*H } -NMR Spectrum of cis-(Et3P)2^t(2-C6H4CMe2CH2) in CDC13
(a) Aliphatic region
on O (b) Aromatic region
in is*
cn 13 3 b Table 8 : C-NMR Characteristics for Some 1-Platinaindan Tertiary Phosphine Complexes ’
6 13(: : ppm (J(Pt-H)) [ASSIGNMENT] COMPLEX Pt-CH2 Pt-CH2C Pt-CH2CCH3 Aromatic Ligand
48.3 (600) 50.0 (13) 35.1 (46) 121.8 (55) Jp_c = 3 [C4 17.1 (17) Jp_c=26 [Pa - cis-(Et3P)2Pt(2-C6H4CMe2CH2) _ 7 or C6] 122.7 [C5] £H„] 18.3 (17) 2jPt-C= 87 Pt-C ' JPt-C= 6 = 4 123.6(59) Jp_c=8 [C4 or Jp_c=23 [Pb -CH2] JPc-C= 7 Pc-C * C6] 137.1(48) Jp_c=5 8.6 (10) Jp_c=12 [P-CH2
Jp_c=5 [C3] 163.0 (906) -c h 3]
Jp_c=110, Jp_c=8 [C2]
167.0(73)Jp_c=5 [Cl]
45.8 (ca. 50.5 35.5 (41) c cis-(dppe)Pt(2-C^H^CMe2CH2) 600) 2j _7 2J - 5 141.1 (52) [C3] C 2jPt-C=87 JPt-C 1 Pt-C 2j 7 169.8 [Cl] - Pc-C 2jPc.-C= 4____
a All spectra recorded in chloroform-d b Coupling constants in Hz, if observed c Aromatic region very complex
cn to I------I 2.1e Synthesis and Spectroscopy of (y-bipym) [Pt^-C^H^CM^CI^) ^
Prolongued reaction of (cod)Pt(CH2CMe2Ph)2 with half an equivalent of bipym in toluene at ambient temperature leads to isolation of a darkly coloured microcrystalline solid. The product is too insoluble in common solvents to enable NMR characterisation, but UV, IR and elemental analysis indicate the compound to be (y-bipym) [Pt^-C^H^CI^^CI^) ^ •
Complexes in which bipym bridges between two metal centres have 49 become of interest in recent years in both bioinorganic and organometallic 35a.47 chemistry. These dimeric compounds, however, are generally insoluble and this has so far restricted the study of their organometallic behaviour.
The bridging mode of bipym in the complex prepared above is confirmed by the absence of the ring-stretching mode at 1550 cm ^ in the IR spectrum 35a. which is present in mononuclear compounds (see 2.5). IR also shows both ends of the complex to be metallacyclic moieties by the lack of the strong neophyl absorptions at 1495 cm ^ and 695 cm ^ (see Section 2.1c).
The electronic spectrum of the complex as a pale green weak solution in methylene chloride contains absorptions at ca_. 370, 392, 559 and 595 cm ^ 35 which are again typical of y-bipyrimidyl-bis-platinum(II)-alkyl complexes.
It was hoped that the neophyl groups might solubilise the dimeric product, but clearly diene displacement by the ligand occurs so slowly that competitive cyclisation of ( c o d ^ ^ C l ^ C N ^ P h ^ takes place to yield the observed bis-platinacycle. The complex has two isomeric forms but it is not possible to determine which of the two is favoured (see Fig. 11) 54
Fig. 11 Isomers of (U-bipym) [Pt^-C^H^Cf^^^) ^ 55
2.2 Synthesis and Spectroscopy of Asymmetric Neophyl (Hydrocarbyl)- Platinum(II) and Neophyl(halo)platinuni(II) Complexes with Diene and Tertiary Phosphine Ligands
2.2a Preparation of Asymmetric Neophyl(hydrocarbyl)platinum(II) and Neophyl(halo)platinum(II) Complexes with Diene and Tertiary Phosphine Ligands
In order to study the effect of the nature of the leaving group on
the rate of neophylplatinum(II) cyclisation, compounds of the type
L,2Pt(CH2CMe2Ph)R have alse been prepared. (cod)Pt(CH2CMe2Ph)(CH3) is
obtained in moderate yield from the reaction of (cod)Pt(CH3)I and
neophylmagnesiumchloride in tetrahydrofuran. This complex is much more
inert than the corresponding bis-neophyl compound but does produce
(cod^t^-C^H^CN^Cl^) with elimination of methane when heated to 100°C
in toluene (Fig. 12).
NMR scale reactions show that displacement of the diolefin by
nitrogen donor ligands (bipy, bipym) is also much slower, presumably due
to the decreased steric demand of the methyl group compared to a neophyl
group. Reaction of (cod)Pt(CH2CMe2Ph)(CH3) with Et3P in diethylether,
however, yields jcis^-(Et3P)2Pt(CH2CMe2Ph)(CH3) as colourless crystals.
The detailed thermal behaviour of this complex and the isomeric cis-C E t ^ ^ P t (2-C^lhCMe3)(CH3) is being investigated separately (see also Fig. 5). 56
Figure 12. Thermal Decomposition of (cod)Pt(CHg)(CH2CMe2Ph) in Toluene 57
During the study of the rearrangement of cis-CEtoP^PtCCH^CMeoPh^ in polar solvents (see Chapter 4) it became of interest to synthesise the linkage isomer cis-CEtoP^PtCCHoCMepPh)(2-C^H^CMeo). The most straightforward route to this complex is by reaction of (cod)Pt-
(CI^CM^PlOX [X = Cl,I] with 2-tert-butylphenylmagnesiumbromide, followed by displacement of the diene by Et^P. (cod)Pt(CH2CMe2Ph)Cl is obtained in almost quantitative yield by the low temperature protolysis of either (cod^tCCH^^^Ph^ or (cod)pT(2-C^H^CMe^CH2) with ethereal hydrogen chloride (It was hoped that the latter reaction might produce
(cod)Pt(2-C^H^CMe2)Cl, but only (cod)Pt(CH2CMe2Ph)Cl is isolated).
Replacement of the chloride by iodide is readily achieved by stirring
(cod)Pt(CH2CMe2Ph)Cl with potassium iodide in acetone overnight. 51 The synthesis of the Grignard precursor 2-bromo-tert-butylbenzene did not, however, prove straightforward (see Fig. 13) and initial attempts led only to isolation of deeply coloured oils containing several different tert-butyl groups (by ^H-NMR). Further investigation showed that the reduction of the nitro-compound 3_ by catalytic hydrogenation 52 over 5% Pd on carbon was not producing just the amine _4. An improvement on this step is provided by the tin(II)chloride/hydrochloric acid 53 reduction of _3. Subsequent diazotisation of the amine in ethanol/sulphuric acid and de-ammination with hypophosphorous acid yields the required
2-bromo-tert-butylbenzene as a colourless liquid after distillation under reduced pressure. Figure 13. Synthesis of 2-bromo-tert-butylbenzene
U1 oo 59
More direct routes to 2-halo-tert-butylbenzene and to 2-tert-
butylphenylplatinum(II) compounds such as the nickel(II) catalysed 54 cross-coupling of 1,2-dichlorobenzene and tert-butylchloride, protolysis
of ( c o d ^ t ^ - C ^ H ^ ^ C I ^ ) with ethereal hydrogen chloride, and thermolysis
of (cod)Pt(CH2CMe2Ph)Cl at 90°C, were all unsuccessful.
cis-CEtpP^PtCCl^CMeoPh)(2-C^H^CMeo) is successfully prepared by
reaction of (cod)Pt(CH2CMe2Ph)X with 2-terjt-butylphenylmagnesiumbromide,
followed by addition of Et^P. Synthesis of the homologous
isopropylphenyl complex cis-CEtpP^PtCCFUCMeoPh)(2-C^H,XHMe2) requires
reaction of the diarylmagnesium reagent Mg(2-C^H^CHMe2)2 with
(cod)Pt(CH2CMe2Ph)I.
The ^H-NMR spectroscopy of the compounds described above will be
discussed in detail in 2.2b(ii). IR and analytical data are listed in
2.5. The molecular structure of cis-CEtoP^PtCCiy^C^^^-C^H^CMeo) 55 has been determined by a single-crystal X-ray diffraction study (see
Chapter 4).
A kinetic investigation of the thermal decomposition of cis-(Et2P)2Pt(CH2CMe2Ph)(2-C^H^CMeo) established the need to differentiate
between the two phenyl rings in the complex. This has been achieved by
the synthesis of cis-(Et2P)2Pt(CELCM^C^-D^) (2-C^FhCMe^) and
cis-(Et2P)2Pt(CH2CMe2Ph)(2-C^D^H^_^CMe2)» The former is readily obtained by the reaction of (cod)Pt(CH0CMe0C^Dc)I* and Z Zoo
* (cod)Pt(CH2CMe2C^D^)I is available from protolysis of (cod)Pt(CH2CMe2C^D^)2
with dry HC1, followed by replacement of the chloride by stirring with
KI. 60
2-tert-butylphenylraagnesiumbromide followed by Et^P. The latter involves the synthesis of 2-bromo-tert-butylbenzene-d^. This is prepared by the same route as described above (Fig. 13) starting from tert-butylbenzene-d^
(obtained by Friedel-Crafts alkylation of benzene-d^ with tert-butylchloride 56 in the presence of anhydrous FeCl^).
Unfortunately, during the reduction step some H/D exchange occurs at sites adjacent to the amine group. Subsequent diazotisation/deammination and Grignard formation leads to the partly deuterated ^-C^D^H^^CMe^MgBr.
Analysis of the ^H-NMR spectrum of cis-CEtoP^P^CHoCMepPh) (2-C^D^H^ ^CMe^) prepared from this Grignard shows the complex to be totally deuterated at the 6-position, whilst the 3-position is 68% protiated*. (See Fig. 8 for numbering scheme). Since the 3-proton is well separated in the
1H-NMR spectrum (250 MHz) of both c is - (E13P)^Pt(CH2CMe2Ph)(2-C6H4CMe3) and cis-(EtoP)2Pt(2-C^H^CMe2CH3) the 32% deuterium incorporation at this position is sufficient to label the tert-butylphenyl ring in comparison with the totally protiated neophyl phenyl group (see Section 4).
The precursor complexes trans-CPhoP^P^CHoCMeoPtOCl and cis-(dmpe)Pt(CH2CMe2Ph)Cl are readily prepared by displacement of the olefin from (cod)Pt(CH2CMe2Ph)Cl by the phosphine ligands in the normal way. Reaction of (cod)Pt(CH2CMe2Ph)I with dppe, however, yields two 31 products in approximately 4:1 ratio (by P-NMR). The major product 31 1 shows two ’triplets’ in the P { H} -NMR spectrum with coupling constants
* Calculated by comparative integration with the neophyl ortho-protons
at 250 MHz (see Fig. 42) typical for phosphorus atoms trans to alkyl and iodide ligands (Table 7).
The minor product has similar values of ^J(Pt-P) but also shows a P—P coupling of 4 Hz. The major product is assigned to be (dppe)CH2CMe2Ph)I
(since (dmpe)Pt(CH2CMe2Ph)Cl exhibits no P—P coupling) with the secondary complex being the phosphine-bridged dimeric species [(dppe)Pt(CH2CMe2Ph)I]
(see F ig. 14).
v j y X ? / ' y \ > x PhMe2CH2Cr D CH^M^Ph
Figure 14. Proposed Structure of Minor Product from Reaction of (cod)Pt(CH2CMe2Ph)I and dppe. 62
2.2b Spectroscopic Studies of Asymmetric Neophyl(Hydrocarbyl)- and Neophyl(Chloro)Platinum(II) Complexes with Diene and Tertiary Phosphine Ligands
2.2b (i) General
The 1H-NMR (Table 9), 31P-NMR (Table 7) and IR (Section 2.5) of most of the complexes described in this section have been recorded*.
Apart from the ^H-NMR spectroscopy of the complexes cis-LoPtCCH^CMepPh)-
(2-C^H^CMe2R) [L^ = cod, L = Et^P» R = H, Me] which will be discussed in detail, the spectra are unremarkable with peaks characteristic of the neophyl moiety as described earlier (2.1c, 2.Id).
2.2b (ii) 1H-NMR Studies of L2Pt(CH2CMe2Ph)(2-C6H4CMe2R) [L2 = cod,
L = Et^P, R = Me,H]; Evidence for Restricted Pt— C Bond
Rotation in Solution
^H-NMR studies of (cod)Pt(CH2CMe2Ph)(2-C^H4CMe2R) [R = Me,H] do not give simple spectra but show complicated olefinic peaks and more aliphatic peaks than are expected. Analysis of the spectrum of (cod)Pt(CH2CMe2Ph)-
(2-C^H4CMe2) in chloroform-d at 250 MHz effects good resolution and shows clearly the second-order nature of the diene methylene and double-bond protons (Fig. 15). More interestingly, the spectrum contains three peaks ascribable to protons in methyl groups. Integration confirms the
* The ‘*C {XH } -NMR spectra of (cod)Pt(CH2CMe2Ph)Cl, (cod)Pt(CH2CMe2Ph)-
(2-C6H4CMe3) (cod)Pt(CH2CMe2Ph)(2-C6H4CMe2H) and cis-(Et3P)2Pt-
(CH2CMe2Ph)(CH3) have been recorded and are listed in Section 2.5 Table 8 NMR Characteristics of Asymmetric Neophyl (Halide) and Neophyl (Hydrocarbyl) Platinum (II) Complexes a*b
4'H ppn : NEOPHYL (Jpt -n) [ASSIGNMENT]
COMPLEX i'H ppa : HYDROCARBYL (Jpt -«) 4'H ppn : ANCILLARY LIGAND c h 2 c h 3 C 6 « 5 [ASSIGNMENT] (Jp t-|j) [ASSIGNMENT]
5.44 (32) [CH], 3.70 (74) [CH] (cod)Pc(CH2CMe2Ph)Cl c 2.22 (80) I.St 7.40 dd [H2], 7.2 - 7.0 a ' 1.8 - 2.2 [CH2j
(cod)Pt(CH2CMe2P h)I 2.22 (82) 1.13 7.3 dd [H2], 7.2 - 7.0 a ' 5.4 (32) [CH], 4.06 (75) [CH], 1.8 [CH2]
ci«-(dnpe)Pt(C H 2CHe2Ph)Cl « d 1 . 4 3 7.52 dd [H 2], 7.22 - 7.05 m - 2.2 - 1.5 a (Jp-H-9) lCH3 and CH2]
2.40 t (67) 7.74 - 7.36 a, 7.10 - 6.89 a [C6H5] cis-(dppe)Pt(C H 2CMe2Ph)I e 1 . 1 7 d - (J p _ h - 7 ) 1.8 - 2.3 a [CH2]
8.02 a, 7.88 - 7.65 a, 7.50 - 7.38 a, - [( dppe)Pt(CH2CMe2P h)I)2 * d 1 . 0 4 d 7.15 - 7.05 a, 6.75 - 6.60 a [C6H5] 2.8 - 2.0 a [CH2]
7.59 ddd(70) (J h 5 - H 6 _ 6 - 6 » j H4 - H 6 " 2 » 5 , 4.68 a (2H) (Ca. 41) [CH] 1 . 2 1 a 7.48 dd (JH2-H3-8, J h 2-«4“ 1-5)|H2J (cod)Pt(CH2CMe2Ph)(2-C6H4CMe3) * d lH 3-H 6-t.2)tH 6], 7.38 ddd(JH3-H4-7.1. 4.59 a (1H) (Ca. 37) [CH] I.O S t(II.S ) 7.3 - 7.1 a [H3.H4] Jh 3-H 5-2.5)[H 3], 6.89 a [H4.H5], 1.55 [CH3] 4.10 a (1H) (Ca. 37) [CH], 2.4 - 1.9 [CH2]
1 . 2 8 7 . 4 6 d d (J jj _ h - , J h 2-H4“ 2)[H 2] 7.4 - 6.9 [arom atic] 4.88 - 3.74 br.m . [CH] (cod)P t(C H 2CMe2P h)(2-C 6H4CHMe2) c d 2 38 1.59 septet (JH_H-6.6)[C H ], 1.45 d (J h _ h - 6 . 6 ) 1.66 br.a . [CH2] 1 . 2 6 d [ H 3 . H 4 ] [CH3], 1.25 (J h -H-6.6)[CH3]
7.4 - 6.8 a [arom atic] 4.9 - 3.9 br.a. [CH] 1 . 1 a 7.4 dd (JH2.H3-8)[H 2] (cod)P t(C H 2CMe2Ph)(2-C 6H4CHMe2) d 3.6 aeptet (JH_n>6.6)icH], 1.26 d (JH_H-6.6) 2.2 - 1.9 a [CH2] 1.05t(11.5) d [ H 3 . H 4 ] [CH3], 1.14 d (J h -H“6.6)[CH3]
(cod)Pt(CH2CMe2Ph)(CH3) c 2.14 (89) 1.40 7.41 dd [H 2], 7.1 -6.86 a [H3.H4] 1.07 (82) [CH3] 4.39 (40)[CH], 3.81 (41)[CH], 1.5 - 1.3 [CH 2]
7 . 7 1 d d (J h 2-H3“8>^H2-H3“* [82] 3.06 a (59)(JH6. H5-7)(H 6], 7.40 ddd(18) 1.42 - 1.23 (CH 2I c i*-(E t3P)2Pt(CH2CMe2Ph)(2-C6H4CMe3)< 2 . 1 0 m (8 S ) 1 . 6 1 , 1 . 1 8 7 . 2 6 t (J h 2 - H 3 ‘ j H3-H4*8)[H3] (J h 3-H4‘8. JH3-H5*3* Jh 3-H6“ 1-5)[H3] 0.85 - 0.71 [CH3J 7.13 - 7.00 a [H4] 7.13 - 7.00 a [H4.H5], 1.85 a [CH3]
c j*-(E t3P)2Pt(CH2CMe2P h )( C H 3 ) d 1 . 4 1 7 . 5 4 d d (J h 2 - H 3 ” 8 >j h 2-H 4*^ 1 82] 0.43 . app. quartet (67) 1.90 - 1.51 [C1l2] 7.29 - 7.03 a [H3.H4] 1.2 - 0.71 |CH3)
2.0 - 1.7 [CH2J tra n s-(E t3P)2Pt(CH2CMe2Ph)Cl d 1 . 4 8 7.45 dd [H2], 7.2 - 7.0 a - 1.1 - 0.9 [CH3]
2 . 0 7 t ( 7 8 ) - 7.99 a, 7.2 - 6.8 a |C6H5] trans-(P h3P)2Pt(CH2CMe2Ph)Cl l ( J p _ H “ 8 ) 1 . 4 3 7.55 [H2] d [H3.H4]
(a) All spectra recorded at 90 MHz in choloroform-d unless otherwise stated; (b) Coupling constants in Hz if observed; (c) Recorded in toluene-d®; (d) Obscured by ligand peaks; (e) Recorded at 250 MHz; (f) Recorded in benzene-d® Figure 15. -NMR Spectrum (250 MHz) of (cod)Pt(CH2CMe2Ph)(2-C6H4CMe3) in CDC1 3
T T T T 65
resonance at 61.55 (9H) to be the tert-butylphenyl methyl protons,
with the remaining two peaks (6 1.21 (3H), 6 1.05 (3H)) being different
neophyl methyl groups, the high field peak showing platinum satellites
( J(Pt-H) = 11.5 Hz). The origin of this coupling, whether a through-
bond or through-space interaction is uncertain. A four-bond platinum
coupling of 6 Hz to the methyl protons is observed in the spectrum of
(cod)Pt(CH2CMe2Ph)2 (Table 2 ). An attempt to determine the mechanism
of this coupling in (cod)Pt(CH2CMe2Ph)^-C^H^CMe^) by observation of 195 a nuclear Overhauser enhancement to the Pt resonance while
selectively irradiating the different neophyl methyl peaks was
unsuccessful due to the small difference in chemical shifts between the methyl resonances. This experiment would have shown whether one of the methyl groups was held close to the platinum, but it did provide a 195 value of the Pt chemical shift for the complex of - 3315 ppm
(relative to K^PtCl^, ^ = 21422780 Hz).
The ^H-NMR spectrum of c is-CEtoP^P^C^CMepPM^-C^H^CMeo) in g toluene-d also shows three distinct methyl resonances (6 1.74, 1.51,
1.05) assigned as tert-butylphenyl and two neophyl methyls respectively.*
The upfield resonance does not, in this case, show resolvable platinum
satellites at 250 MHz but is of lower intensity than the other neophyl
methyl peak suggesting a small platinum coupling. Attempts to observe
these satellites at 90 MHz (at which frequency the chemical shift
separation of the satellites w ill be larger) are complicated by overlap
with the methyl peaks due to Et^P. On warming the so lu tio n s between 40
and 50°C no broadening, convergence or coalescence of the neophyl methyl
peaks occurs but instead the growth of resonances due to
*(See Fig.16) 66
Figure 16. 250 MHz ^H-NMR Spectrum (Alkyl Region) of cis-(Et3P)2Pt(CH2CMe2Ph)(2-C6H4CMe3) in C ^ .
[0, neophyl-CH^ (showing Pt satellites); o K i ,tert-butylphenyl-CHp; 9, neophyl-C and C _H^;
LI, triethylphosphine-CH2; L2, triethylphosphine-CH^] tert-butylbenzene and cis-(Et^P)2?t(2-C^H^CMepCH^) is observed. 57 The corresponding dppe complex, (dppe^tCCI^CN^Ph)(2-C^H^CMe^) a lso shows three methyl peaks ( 6 1.40 (9H), 6 1.20 (3H), 5 1.14 (3H)).
In this case the chemical shift of the neophyl methyls are reversed with the downfield peak (61.20) being of lower intensity suggesting some coupling to platinum.
The ^H-NMR spectrum of (cod)Pt(CH2CMe2Ph)(2-C^H^CHMe2) in toluene-d or chloroform-d exhibits further complexity. The diene olefinic resonances are virtually identical to those in the tert-butylphenyl analogue whilst two neophyl methyl and two iso-propyl methyl resonances are observed. The high f ie ld neophyl methyl group again shows a platinum coupling of 11.5 Hz. The { ‘*'H } -NMR spectra of
(cod)Pt(CH2CMe2Ph)(2-C^H^CMe2R) [R = H, Me] contain very complicated aliphatic regions due to different diene methylene carbons and the distinct methyl groups making assignment very d ifficult (See 2.5).
These spectroscopic observations are consistent with restricted
rotation about either or both of the platinum-carbon bonds in these
sterically congested complexes. Conformational rigidity about the
platinum-aryl bond only is sufficient to account for the spectra. Once
the ortho-substituted phenyl group is not free to rotate the complexes
lack a plane of symmetry and a l l protons and groups attached to
prochiral centres become diastereotopic. Hence the appearance of the
two sets of resonances for each of the isopropyl and neophyl methyl
groups and the second order nature of the neophyl methylene protons in
c is-CEtpP^PtfCHo^^^1)(2-C^H^CMep) (see Figure 16). Similarly, only 68
one peak is observed for the tert-butyl moiety in L,2Pt(CH2CMe2Ph)-
(2-CgH^CMeg) because the quaternary carbon is not prochiral.
It is possible, however, that the neophyl group is locked in position. The observation of a coupling to platinum of one of the neophyl methyl groups while there is no detectable coupling to the other methyl protons suggests a conformational lock, or at least a dominance in solution of one particular rotamer, in which one of the methyl groups is held in a position which favours an interaction with the platinum, either through-bond, through-space, or a combination of the two*.
Further evidence for restricted platinum-aryl bond rotation is provided by the ^H-NMR spectra of L^PtCCH^)(2-C^H^CHRMe) [L^ = cod,
L = Et^P, R = H, Me] which exhibit the same type of behaviour, even when the alkyl moiety is the sterically less demanding methyl group5.8 Upon warming so lu tio n s of (cod)Pt(CHg)(2-CgH^CHMe2) in toluene-dg to temperatures in excess of 50°C coalescence of the isopropyl methyl resonances is observed. An evaluation of the energy barrier to free rotation of the aryl group in this system by line shape analysis of 58 computer simulated spectra is in progress.
Similar analysis of the thermally inert (dppe)Pt(CH2CMe2Ph)(2-CgH^CMeg) and of neophyl(phenyl)platinum(II) complexes may provide an insight into
* The crystal structure of c is-CEtgP^Pt(CTUCMepPh)(2-C^-H/CMeg) confirms
that one of the methyl groups of the neophyl moiety lies close to the 55 platinum, see Section A.2c. whether one or both of the platinum-carbon bonds is locked in these asymmetric neophyl(ary1)platinum(II) compounds.
Restricted metal-carbon and metal-phosphorus bond rotations have been observed previously but are relatively rare and these appear to
59 be the first such examples involving platinum-carbon a-bonds. 70
2.3 Synthesis and Spectroscopy of [(terpy)Pt(CH2CMe2Ph)]+BF^ and Attempts
to Prepare Tertiary Phosphine Analogues
2.3a Preparation and "'‘H-NMR of [ (terpy ^^ C J ^ C l ^ P h ) ]+BF^
As described briefly in the previous section the possible linkage isomerisation of the neophyl moiety to produce a 2-tert-butylphenyl complex became of interest (see also Chapter 4). It seemed reasonable that depriving the complexes of a suitable leaving group (i.e. R ) might favour the isomerisation reaction over the metallacyclisation (see Fig.
17), and so the synthesis of cationic mononeophylplatinum(II) complexes has been attempted.
Fig. 17 Possible Isomerisation of Cationic Mononeophyl platinum(II) Complexes 71
The first compound prepared involved use of the tridentate heteroaromatic nitrogen donor ligand 2,2’-6’ ,2"-terpyridine. Thus reaction of (cod)Pt(CH2CMe2Ph)I with the ligand, followed by addition of silver tetrafluoroborate produces [(terpy)Pt(CH 2CMe2Ph)]+BF^ as bright yellow crystals in almost quantitative yield (Fig. 18).
Fig. 18 Preparation of [(terpy)Pt(CH2CMe2P h )]+BF^
The product is insoluble in hydrocarbons but dissolves in polar
solvents such as methanol, dimethylsulphoxide and acetonitrile. The elemental analysis and IR (neophyl peaks at 1495, 756, 694 cm confirm
the id e n tity of the complex. The ^H-NMR spectrum (in CD^CN at 250 MHz) 72
shows neophyl methyl ( 5 1.45) and methylene ( 6 1.66 (J(Pt-H) = 80 Hz)) protons. The aromatic region is complicated but the neophylphenyl protons
(<56.82m( 1H) [H4]6 6.95 m(2H) [H3], 6 7.51 dd(2H) [H2]) can be identified, as can H5 of the terpyridyl ligand ( 67.42 m(2H)). The remaining terpyridyl resonances overlap in the region 68.25 - 8.00.
The thermal behaviour of this complex is described in Section 3.3.
2.3b Attempts to Prepare tris-(tertiaryphosphine)neophylplatinum(II) Complexes
Tertiary phosphine analogues of the terpyridyl complex described in
2.3a should be more soluble in less polar solvents and be more readily studied, therefore the preparation of compounds of the type
[L^PtCCl^CN^Ph) ] +BF^ (L = Et^P, Ph^P) by the same route was attempted.
A colourless oil, insoluble in hydrocarbon solvents, having the
2^P {^H } -NMR characteristics expected for [ (Et 2P)gPt(CH2CMe2Ph) ] +BF^
( 6 0.48, 't't (IP), XJ(P t-P ) = 1642 Hz, 2J(P -P) = 22 Hz ; 6 2.92 ’t'd (2P),
■^J(Pt-P) = 2829 Hz, 2J(P -P ) = 22 Hz)* i s iso la te d i n i t i a l l y from the reaction of (cod)Pt(CH2CMe2Ph)I with AgBF^ in the presence of excess
Et^P, but subsequent crystallisation from acetone or methanol affords colourless prisms which give microanalysis results inconsistent with this
* This spectrum was recorded as a two-phase system, with the oil in the o bottom h a lf of the NMR tube and toluene-d at the top) 73
formulation (Found : C 20.73, H 4.19, P 14.19; calculated for
C28H58P3PtBF4 : C 43*7’ H 7'6, P 12*1)* The 31p {'Hl-N M R of these g crystals (in toluene-d at 36.2 MHz) contains a broad peak centred at
6 - 13.0 ppm which when cooled to -50°C resolves into two broad peaks of equal intensity (width at half-height ca. 80 Hz). Further spectroscopic stu d ies (^F, ^H, ^3C-NMR, IR) confirm the presence of Et^P and BF^ but are otherwise inconclusive.
Exploratory experiments on the related triphenylphosphine system again leads to isolation of colourless crystals the identity of which is yet to be determined. It seems that an X-ray diffraction study of suitable crystals w ill be the only way to identify the structures of these compounds. 74
2.4 Attempts to Prepare C atio n ic Mononeophyl Compounds with Chelating
Tertiary Phosphine Ligands
Coordination of arenes to metal centres prior to C—H cleavage reactions has often been proposed to explain the observedly enhanced susceptibility of aromatic over aliphatic C—H bonds towards this type 16 2 of intermolecular reactivity (Fig. 19). This prior f| — coordination can
H M +
2 Fig. 19 Intermediacy of n-arene Complexes in Aromatic C—H Activation
3C also be used to account for differences in similar intramolecular reactions.
The recent isolation of a chelated (2,2-dimethy1-4-penten-l-yl)platinum(II) complex suggested that a sim ilar intramolecular r) -aryl neophyl-derived compound may be accessible by a parallel strategy (Fig. 20). V
Figure 20. Preparation of (2,2-dimethyl-4-penten-l-yl)platinum(II)6°and Proposed Route i Analogous Neophyl Complex
cn-4 76
Due to thermal inertness of its bis-neophyl derivative it was decided that the dppe complex might prove to be the most stable system, whilst maintaining a cis geometry. Also, since the aim of these experiments was to create a vacant site at the metal which could be stabilised by the Tr-electron density of the phenyl ring it was reasoned that the reaction would be favoured in a non-coordinating solvent.
Acetone or tetrahydrofuran might be expected to ligate the metal competitively.
Treatment of (dppe)Pt(CH2CMe2Ph)I (or [(dppe)Pt(CH2CMe2Ph)I]2) with
AgBF^ in chloroform-d in a flask protected from light results in 31 precipitation of Agl. The P-NMR spectrum of the pale yellow super natant contains two ’triplets' with P— P coupling (^J(Pt-P^ = 1983 Hz,
^J(Pt-P2) = 5137 Hz, 2J(P-P) = 4 Hz). The 1983 Hz coupling is typical for a phosphorus Pt(II) bond trans to ligand of high trans influence such as an alkyl or aryl group, while the value of 5137 Hz is one of the largest platinum(II)-phosphorus couplings observed in phosphine compounds and suggests the ligand trans to P2 has a very weak trans-influence.
The ^H-NMR spectrum (in CDCl^) also contains several interesting features. An apparent quartet with platinum satellites at 60.89
(2J(Pt-H) = 39 Hz, J(Ptr-~S H) = 7 Hz, J(P— H) = 4 Hz) can be assigned 2 to a Pt-CH2 group, although the value of J(Pt-H) is significantly lower than in bis-neophyl compounds where values are typically 55-80 Hz
(Table 6). The chemical shift of this peak is also at higher field than generally observed in other complexes. A singlet at 6 1.16 is probably due to neophylmethyl protons. In the aromatic region, as well as resonances from the phenyl protons of the ligand are triplets at fairly 7
high field 6 6.33 (1H) (J(H-H) = 7.4 Hz) [H4] and at 6 7.07 (2H)
(J(H-H) = 7.6 Hz) [H3], and an apparent doublet of doublets at 67.69 (2H)
[H_2], assignable as the neophyl phenyl protons (assignments in square brackets). The neophyl ortho protons generally appear at 67.8-7.5 as a doublet of doublets with J(H2-H3) = 7.5 Hz, J(H2-H4) =1.5 Hz. In this compound, however, this peak is further split into a doublet with a coupling of 12 Hz which must be due to a coupling to phosphorus. Also visible are possible platinum satellites corresponding to J(Pt-H) = ca.
43 Hz.
Removal of the chloroform-d and trituration of the resultant oil with hexane produces a pale yellow semi-crystalline solid. The elemental analysis of this solid gives C, H and P in approximately the correct ratios but are all low suggesting possible contamination with AgBF^. [Found :
C 47.7%, H 4.09%, P 7.21% ; Calculated for [(dppe)Pt(CH2CMe2Ph)]+BF^ :
C 51.99%, H 4.23%, P 7.89%].
The spectroscopic evidence, although not conclusive, indicates that the compound may well be the envisaged product. The equal coupling to both phosphorus and platinum of the neophyl ortho protons can be explained by an equilibrium in solution (see Fig. 21).* Further experimental data such as a single-crystal X-ray diffraction study or well resolved 13 C-NMR spectrum are required to confirm the structure of this product.
A brief investigation of the reaction of AgBF^ with (dmpe)Pt(CH2CMe2Ph)Cl 31 in chloroform-d was inconclusive but again gave P-NMR spectra with some unusual couplings (see, for example, Fig. 22). Clearly both of these systems warrant further study .
* A similar equilibrium has recently been postulated during isomerisation of Cp*Rh(PMe2CH2C6H4)D to Cp^Rh^PM^CH^C6H3D)H3 °d). 78
Fig. 21 Possible Product from Reaction of
(dppe)Pt(CH2CMe2Ph)I with AgBF^ • J p,.p - 4838 Hz ■ Jf.P = 7Hl O Jh.fs 1896 H/
200Hi
O
O
Figure 22. ^ P { } -NMR Spectrum-From Reaction of (dmpe)Pt(CH2CMe2Ph)Cl with AgBF^ in Chloroform-d 80
2.5 Experimental
2.5.1 General_and_Instrumental
Elemental analyses were by Imperial College Microanalytical
Laboratories. NMR spectra were recorded on Bruker WM250 (^H, 250.13 MHz;
31P, 101.3 MHz; 13C, 62.9 MHz; 195Pt, 53.58 MHz), JEOL FX90Q ( V 89.55
MHz; 31P, 36.21 MHz; 13C, 22.13 MHz) and Perkin-Elmer R32 (1H, 90 MHz) spectrometers. IR data were collected on a Perkin-Elmer 683 instrument as 4% KBr dispersions, and electronic spectra recorded on a Perkin-Elmer
55 spectrometer.
All reactions were carried out under nitrogen or argon, unless otherwise noted, using standard anaerobic techniques. Solvents were distilled under nitrogen prior to use; diethyl ether and tetrahydrofuran from sodium benzophenone dianion, petroleum ether (b.p. 40-60°C) and hexane from benzophenone ketyl radical anion, toluene from sodium.
Neophyl chloride, 2-bromo-isopropylbenzene and 2,2’-bipyrimidine were used as supplied by Lancaster Synthesis. 4,7-diphenyl- and 3,4,7,8- tetramethyl-1,10-phenanthrolines were supplied by Aldrich Chem. Co.
6 1 6 2 (cod)PtCl2 and (cod^tCCH^)! were prepared by literature methods.
6 3 5 6 ^ Neophylchloride-d^ and tert-butylbenzene-d^ were prepared from benzene-d by established procedures. (See also Section 4.2(v)). 81
2.5.2 P££P^£stion_of_Neophylplatinum^II)_Compounds
Bis-neophyl(cycloocta-l,5-diene)platinum(II)
To a stirred suspension of (cod)PtCl2 (0.93 g, 2.5 m mol) in THF
(15 cm^) at -20°C was added dropwise a solution of PhN^CC^MgCl (7.5 m 3 mol) in THF (35 cm ). The mixture was allowed to reach room temperature and stirred for a further 6 hours. The solvent was removed from the clear dark yellow solution in vacuo and diethyl ether (40 cm ) added.
The resulting stirred suspension was cooled to -15°C and hydrolysed by dropwise addition of degassed saturated ammonium chloride solution (ca. 3 10 cm ). The dark coloured ethereal layer was separated, dried over magnesium sulphate, and decolourised with activated charcoal to give a 3 pale yellow solution. Concentration to ca_. 10 cm and addition of 3 methanol (ca. 2 cm ) afforded large colourless prisms of the product
(0.9 g, 63%).
(IR; 3099 w, 3084 w, 3058 w, 2958 m, 2950 m, 2810 m, 1600 m,
1495 s, 1375 m, 1280 m, 1070 m, 1030 m, 977 m, 788 m, 764 vs, 700 vs,
565 m).
Bis-neophyl-d^(cycloocta-l,5-diene)platinum(II)
As above using C^D^CH^^CX^MgCl and (cod)PtCl2.
(IR; 3095 w, 3060 w, 3005 vw, 2955-2850 m, 2810 w, 2275 mw, 2179 w,
2152 w5 1566 w, 1484 mw, 1375 mw, 1368 mw, 1355 m, 1312 w, 980 ms, 865 m,
829 mw, 763 m, 570 s, 525 m, 500 ms, 450 w). 82
(cycloocta-1,5-diene)-3,3-dimethyl-l-platinaindan
A solution of (cocOPtCC^Cl^Ph^ in toluene was heated at 60°C for 48 hours. Removal of the solvent in vacuo gave a yellowish semi solid. This was crystallised from a 1:1 diethyl ether/hexane mixture as large colourless prisms.
(IR; 3055 mw, 3000 mw, 2950 m, 2875 m, 1530 w, 1450 m, 1371 m,
1350 m, 1341 m, 1135 m, 1042 m, 968 m, 860 m, 818 m, 763 s, 746 s,
735 vs, 563 m, 450 w).
Bis-neophyl(2,2 t-bipyridyl)platinum(II)
2,2'-bipyridyl (5.46 g, 35 m mol) and (cod)Pt(CH2CMe2Ph)0 (0.4 g, 3 0.7 m mol) were dissolved in the minimum amount of toluene (oa. 30 cm ).
The solution was stirred at ambient temperature for 4 days. The bright
red solution was cannulated into a stirred solution of FeSO^ (8.5 g, ca. 56 m mol) in distilled water (50 cm ). The aqueous phase immediately
became dark red due to formation of [Fe^ipy)^]^*. The mixture was
cooled to -78°C and the toluene layer filtered off from the frozen aqueous layer. The latter, upon thawing, was washed with toluene (2 x 3 30 cm ) and the extracts combined. Removal of the toluene in vacuo gave
analytically pure bright red crystalline product (0.36 g, 83%).
(IR; 3100 w, 3080 w, 3046 w, 2950 m, 2855 m, 2790 m, 1598 s, 1492 m,
1465 m, 1440 s, 1355 m, 750 vs, 746 m, 728 s, 695 vs, 563 m). S3
Bis-neophyl-d^-(2,2’-bipyridyl)platinum(II)
As above from (cod^tCCI^CM^C^D^^.
(IR; 3105 vw, 3055 vw, 2950-2840 mw, 2268 mw, 2100 w, 1600 ms,
1492 w, 1468 s, 1442 vs, 1353 mw, 1310 w, 1260 mw, 1156 m, 1045 mw,
750 vs, 740 ms, 730 vs, 700 w, 650 w, 566 vs, 500 ms, 480 m, 427 mw).
(2,2*-bipyridyl)-3,3-dimethyl-l-platinaindan
(bipy)Pt(CH2CMe2Ph)2 was heated overnight at 80°C in toluene
solution. Filtration, concentration of the solution and cooling at -25°C
yielded the product with some toluene of crystallisation as red prisms
(Yield ca. 80%).
(IR; 3035 w, 2940 mw, 2845 w, 1600 m, 1468 m, 1442 s,' 1425 m, 1384 mw,
1157 mw, 1028 mw, 748 s, 726 m, 698 mw, 556 m).
Bis-neophyl(2,2 *-bipyrimidyl)platinum(II)
2,2’-bipyrimidyl (0.14 g, 0.886 m mol) was dissolved in the minimum
amount of toluene (ca. 35 cm^) and (cod)Pt(Cf^Cl^Ph^ (0.50 g, 0.878 m
mol) was added to the stirred solution in air. The solution immediately
became red and after 10 days at room temperature concentration of the
solution yielded fine wine-red needles (0.32 g, 59%).
(IR; 3083 w, 3058 w, 3040 w, 2970 mw, 2920 w, 2890 m, 2788 ms,
1598 w, 1574 s, 1550 s, 1495 s, 1417 s, 1403 vs, 807 ms, 758 ms, 729 m,
692 vs, 682 m, 562 s). 84
(2,2-bipyrimidyl)-3,3-dimethyl-l-platinaindan
(bipyra)Pt(CH2CMe2Ph)2 was heated at 80°C in toluene overnight.
Removal of the solvent in vacuo and extraction of the residue into diethyl ether gave a dark red solution from which the product was isolated as a red solid by concentration and cooling (-25°C).
(IR; 3053 w, 2948 mw, 2852 mw, 1573 s, 1550 ms, 1448 m, 1402 vs,
1024 w, 752 s, 736 m, 659 mw, 562 w).
Bis-neophyl(1,lO-phenanthroline)platinum(II)
A suspension of 1,10-phenanthroline monohydrate (0.8 g, 4 m mol) 3 in diethylether (50 cm ) was added to ( c o d ^ ^ C I ^ C N ^ P h ^ (0.25 g,
0.44 m mol) by cannula. The mixture was stirred vigorously overnight.
The microcrystalline orange product was filtered, washed twice with aqueous FeSO^, distilled water and finally diethylether. Yield 0.25 g
(89%).
(IR; 3080 mw, 3054 w, 3013 w, 2970 m, 2885 ms, 2790 ms, 1626 vw,
1600 w, 1495 ms, 1475 m, 1425 ms, 1395 m, 1035 m, 843 vs, 760 ms, 724 s,
699 vs, 572 m).
(1 ,10-phenanthroline)-3,3-dimethyl-l-platinaindan
A suspension of ( p h e n ^ ^ C l ^ C I ^ P h ^ (ca. 0.1 g) in toluene (65 cm^) was heated to 95°C for 48 hours. During this time the orange micro- 85
crystalline solid dissolved to give a deep red solution. The hot 3 solution was filtered, concentrated to ca. 40 cm and cooled (-25°C).
This caused the product to separate as bright red crystals in almost quantitative yield.
(IR; 3050 w, 3028 w, 2950 mw, 2935 mw, 2845 m, 2780 w, 2775 w, 1624 w,
1573 mw, 1494 w, 1445 mw, 1424 s, 1405 ms, 1339 mw, 1143 mw, 1025 m,
835 vs, 753 s, 740 s, 717 vs, 478 w, 453 w).
Bis-neophyl(3,4,7,8-tetramethyl-l,lO-phenanthroline)platinum(II)
(cod)Pt(CH2CMe2Ph)2 (0.42 g, 0.74 m mol) and Me^phen (0.21 g, 0.89 3 m mol) were dissolved in the minimum amount of toluene (ca. 100 cm ).
After 5 days stirring at ambient temperature a yellow powder had formed.
The orange supernatant was filtered off, washed with aqueous ferrous
sulphate solution and concentrated to yield more of the orange/yellow
solid (overall yield ca_. 0.4 g (57%)).
(IR; 3065 w, 3040 w, 3000 w, 2950 m, 2875 m, 2776 m, 1627 w,
1612 w, 1592 w, 1490 ms, 1420 m, 1382 m, 1353 m, 1055 m, 920 m, 822 ms,
764 ms, 730 s, 700 vs, 560 mw).
(3,4,7,8-tetramethyl-l,10-phenanthroline)-3,3-dimethyl-l-platinaindan
(Me^phen)Pt(CH2CMe2Ph)2 (ca. 0.05 g) was dissolved in toluene 3 (50 cm ) and heated at 95°C for 48 hours. On cooling the product separated
as a yellow/orange microcrystalline solid in almost quantitative yield. 86
(IR; 3043 mw, 2945 m, 2844 m, 2780 w, 1620 w, 1575 mw, 1425 s,
1384 s, 1345 w, 1026 mw, 912 w, 811 ms, 754 ms, 736 vs, 716 s, 693 w,
585 vw, 475 w).
Bis-neophyl(4,7-diphenyl-l,lO-phenanthroline)platinum(II)
(cod)Pt(CH2CMe2Ph)2 (0.20 g, 0.35 m mol) and Pt^phen (0.116 g, 3 0.35 m mol) were allowed to react in toluene (20 cm ) for 13 days. The toluene was removed in vacuo and the red residue extracted into 3 diethyl ether (10 cm ). Slight concentration of the solution and cooling
to -25°C caused formation of bright orenge/red needles of the product
(0.2 g, 71%).
(IR;3080 mw, 3054 w, 3013 w, 2970 m, 2885 ms, 2790 ms,- 1626 vw,
1600 w, 1495 ms, 1475 m, 1425 ms, 1395 m, 1035 m, 843 vs, 760 ms, 724 s,
699 vs, 572 m).
Bis-neophyl-dg-(4,7-diphenyl-l,lO-phenanthroline)platinum(II)
As above from (cod)Pt(CH2CMe0C^D^)2.
(IR; 3063 vw, 2962 m, 2920 m, 2870 m, 2785 mw, 2270 w, 1625 w,
1598 w, 1564 mw, 1450 mw, 1426 m, 850 vs, 836 ms, 770 vs, 740 s, 710 vs,
637 m, 566 m, 500 m). 87
(4,7-diphenyl-l,10-phenanthroline)-3,3-dimethyl-l-platinaindan
(Ph2phen)Pt(,CH2CMe2Ph)2 (0.15 g, 0.19 m mol) was heated at 90°C for 3 two days in toluene (50 cm ). The bright red solution was filtered while still hot. The product crystallised out as a bright orange solid on cooling. Yield 0.1 g (55%).
(IR; 3035 w, 2932 w, 2837 mw, 1620 mw, 1589 mw, 1570 mw, 1552 mw,
1489 mw, 1440 m, 847 s, 832 ms, 765 ms, 756 s, 730 s, 703 vs, 560 mw,
550 w, 500 mw).
cis-bis(neophyl)-bis-(triethylphosphine)platinum(II)
To a suspension of (cod)Pt(CH2CMe2Ph)2 (0.43 g, 0.76 m mol) in hexane
(10 cm ) at room temperature was added by syringe Et^P (0.4 cm , 2.7 m mol). The (cod)Pt(CH2CMe2Ph)2 dissolved with occasional swirling and the solution was left overnight at ambient temperature. Removal of the hexane in vacuo gave a colourless oil. The oil was cooled to -25°C for
48 hours to crystallise. The colourless solid was extracted into the 3 minimum amount of hexane (ca. 5 cm ). Concentration of this solution to 3 ca. 3 cm and cooling at -25°C caused the product to form as large colourless prisms. Yield 0.44 g (83%).
Found: C 55.17, H 8.18%. Calc, for C32H56P2Pt : C 55.08, H 8.09%
(IR; 3078 mw, 3048 m, 3022 w, 2970-2870 s, 1595 s, 1490 s, 1450 s,
1430 2, 1373 s, 1353 s, 1252 m, 1174 m, 1088 m, 1030 vs, 994 m, 760 s,
710 s, 693 s, 618 m, 557 mw, 500 w, 400 mw). cis-bis(neophyl-d^)-bis(triethylphosphine)platinum(II)
As above from (cod^ tCCl^ Cl^ C^ D ^ ^ .
(IR ; 2965 s , 2930 s , 2904 s , 2269 m, 2160 w, 1563 m, 1449 s ,
1430 ms, 1400 s , 1374 s , 1253 m, 1056 m, 1031 v s, 1023 s , 754 vs,
710 vs, 618 m, 552 ms, 519 m, 466 m, 405 mw).
cis-bis(triethylphosphine)-3,3-dimethyl-l-platinaindan
c i s - CEtoP^PtCCHUCMepPlOo was c y c lise d in toluene and the solvent removed in vacuo to leave a colourless oil or semi-solid. Extraction into the minimum amount of hexane, concentration and cooling (-25°C) yielded the product almost quantitatively as large colourless prisms.
Found: C 47.06, H 7.57%. Calc, for C22H43P2Pt : C 46.88, H 7.51%.
(IR ; 3060 w, 3030 w, 2960-2870 s , 1572 w, 1405 s , 1415 s , 1373 s ,
1362 m, 1340 m, 1246 m, 1144 w, 1089 w, 1030 vs, 1018 v s, 760 v s, 745 s
739 vs, 712 v s, 625 ms, 563 w, 465 w, 413 mw).
cis-bis-(triphenylphosphine)-3,3-dimethyl-l-platinaindan
(cod)Pt(CH2CMe2Ph)2 and equivalent triphenylphosphine were reacted in chloroform or benzene for two days. Removal of the solvent in vacuo extraction into the minimum amount of diethylether concentration and cooling gave the product as a white crystalline solid.
Found: C 64.51, H 5.24%. Calc, for C^t^^Pt : C 64.86, H 4.97% 89
(IR ; 3075 w, 3050 w, 2948 m, 2920 w, 2870 w, 2790 w, 1480 s , 1433 v s,
1400 s , 1393 s , 1349 m, 1310 m, 1260 ms, 1200 s , 1180 s , 1150 ms, 1118 vs,
1089 v s, 1025 m, 996 m, 800 mw, 750 ms, 740 s , 720 v s, 693 v s, 520 vs,
450 w).
cis-bis-neophyl-r b is-(1,2-diphenylphosphino)ethane]platinum(II)
(cod)Pt(CH2CMe2Ph)2 (0.22 g, 0.39 m mol) and dppe (0.16 g, 0.4 m mol) 3 were dissolved in the minimum amount of toluene (30 cm ) and allowed to
stand at ambient temperature for 3 days. The pale yellow solution was
then heated at 60°C for two hours to ensure completion of reaction.
Concentration of the solution and cooling to -25°C gave the product as
a white powder (0.24 g, 72%). The powder may be crystallised as large
colourless prisms by re-dissolving in warm toluene and addition of
hexane followed by cooling.
Found: C 63.55, H 5.76%. Calc, for C ^ ^ ^ P t: C 64.25, H 5.86%.
(IR : 3058 mw, 2957 m, 2860 m, 2810 mw, 1598 m, 1588 mw, 1490 m, 1436 s ,
1386 m, 1375 m, 1207 mw, 1190 m, 1100 s , 1030 m, 1000 mw, 820 ms, 742 s ,
693 v s, 679 s , 560 mw, 538 v s, 490 ms, 424 mw).
cis-b is-(1,2-diphenylphosphine)ethane-3,3-dimethyl-l-platinaindan
(dppe)Pt(CH2CMe2 Ph)2 (0.06 g, 0.07 m mol) was refluxed in toluene 3 (50 cm ) for 24 hours. Removal of the solvent gave a white crystalline solid
(c a . 0.05 g, 95%). 9C
Found: C 59.33, H 5.00%. Calc, for C^H^P^t: C 59.58, H 5.00%.
(IR ; 3060 w, 3043 w, 2940 mw, 2912 mw, 2850 w, 2790 2, 1484 ms, 1432 vs,
1400 v s, 1100 v s, 1024 m, 995 m, 870 m, 813 s , 745 s , 730 s , 695 v s,
646 mw, 530 vs, 500 s , 485 s ) .
Chloro(neophyl)(cycloocta-1,5-diene)platinum(II)
To a stirred solution of (cod^tCCI^Cl^Ph^ (0.35 g, 0.61 m mol)
(or (cod)Pt( 2-C^H^CMe2tH2)) in diethyl ether (15 cm^) at -78°C was added carefully by syringe a solution of hydrogen chloride in diethyl ether 3 (1 cm of a 0.865 M solution, 0.86 m mol). The colourless solution was allowed to reach room temperature and stirred overnight. Slow removal of the diethylether in vacuo yielded the product as colourless needles
(ca. 0.27 g, 95%).
Found: C 45.98, H 5.37, Cl 7.77%. Calc, for C^H^ClPt: C 45.8,
H 5.30, Cl 7.52%.
(IR ; 3058 w, 2957 mw, 2910 m, 2870 m, 1598 mw, 1495 m, 1472 m, 1438 m,
1383 m, 1265 m, 1131 m, 1012 m, 992 m, 821 m, 800 mw, 790 mw, 760 s,
698 s , 563 m, 476 mw. 13C { 1H } -NMR (CDC13) : 152.8, 127.9, 126.0, 125.2
[neophylphenyl], 115.5 (22), 85.5 (219), [cod CH], 41.85 [Pt-CI^Cj,
39.1 [Pt-CH2] 91
iodo(neophyl)(cycloocta-1,5-diene)platinum(II)
(cod)Pt(CH2CMe2Ph)Cl (0.25 g, 0.53 m mol) and a large excess of potassium iodide (0.5 g, 3m mol) were stirred overnight in acetone 3 (15 cm ) at room temperature. The in itially colourless suspension became yellow as the reactio n proceeded. Removal of the acetone in vacuo 3 and extraction of the yellow/white residue into diethylether (50 cm ) produced a lemon yellow so lu tio n from which the product was obtained as pale yellow prisms by concentration and cooling. Yield 0.25 g (84%).
Found: C 38.37, H 4.35%. Calc, for C^ I^ IPt: C 38.37, H 4.47%.
(IR ; 3080 w, 3059 mw, 3020 w, 2950 m, 2915 m, 2880 m, 1578 w, 1495 s ,
1473 s , 1433 m, 1380 m, 1360 m, 1340 m, 1310 w, 1250 mw, 1223 m, 1160 m,
1133 m, 1030 m, 1009 s , 989 s , 926 mw, 900 w, 862 m, 820 m, 786 m,
760 vs, 700 v s, 566 s , 473 m).
neophyl(2-isopropylphenyl)(cycloocta-1,5-diene)platinum(II)
To a stirred suspension of (cod)Pt(CH2CMe2Ph)I (0.17 g, 0.3 m mol) 3 in diethylether (25 cm ) at -30°C was added dropwise bis-(2-isopropyl- 3 phenyl)magnesium (9 cm of a 0.075 M ethereal solution, 0.7 m mol) in 3 diethyl ether (20 cm ). The yellow mixture became colourless during the
addition and was allowed to reach room temperature. The solvent was
removed in vacuo and the residue extracted into petroleum-ether (b.p. 3 40-60°C) (50 cm ). Removal of the petroleum ether gave a white solid. 92
This was further extracted into diethylether (30 cm ), concentrated to 3 ca. 25 cm and cooled to -25°C. This afforded colourless crystals of the product (0.15 g, 90%).
Found: C 58.39, H 6.46%. Calc, for C0-.H0,Pt: C 58.36, H 6.53%. 27 36 (IR ; 3105 vw, 3083 w, 3030 m, 2998 w, 2950 ms, 2920 m, 2885 m, 2855 m,
2835 m, 2790 mw, 1600 m, 1574 m, 1494 s , 1464 s , 1427 s , 1374 ms, 1355 s ,
1338 ms, 1280 m, 1263 m, 1213 ms, 1085 mw, 1068 m, 1043 s , 1028 ms,
1019 s , 983 ms, 860 m, 757 v s, 748 v s, 698 v s, 649 m, 568 ms, 560 m,
490 m, 454 mw, 13C { 1H} -NMR (CDC13) : 154.6, 125.8, 124.7 [neophyl aromatic], 157.0, 152.5, 136.3 (19), 125.4 (58), 124.9 (72), 122.7 (8)
[isopropylphenyl aromatic], 104.9 (48), 103.8 (58), 100.5 (47), 99.6
(41) [cod CH], 42.8 [Pt-CH2C], 42.7 (866) [Pt-CH2], 34.8 (72)
[Pt-CH2CCH3]).
2-tertbutylphenyl(neophyl)(cycloocta.-!,5-diene)platinum(II)
To a stirred suspension of (cod)Pt(CH2CMe2Ph)Cl (0.19 g, 0.4 m mol) 3 in diethylether (25 cm ) at -30°C was added by syringe 2-tertbutylphenyl- 3 magnesiumbromide (1 .7 cm of a 0.36 M ethereal so lu tio n , 0.6 m m ol). On warming the (cod)Pt(CH2CMe2Ph)Cl d issolved and the so lu tio n darkened.
The mixture was stirred overnight at room temperature, cooled to -10°C and hydrolysed with saturated ammonium chloride solution. The dark ether lay er was separated, dried over magnesium sulphate and decolourised with activated charcoal to give a colourless solution. Removal of the solvent yielded an oil from which the product was crystallised by addition of hexane (2 cm3) and cooling to -25°C. Yield 0.15 g (65%).
Found: C 59.56, H 6.80%. Calc, for C2gH3gPt: C 59.03,H 6.68%.
(IR ; 3040 w, 3000 w, 2950 m, 2920 in, 2900 mw, 2850 mw, 2795 w, 1574 mw,
1494 m, 1470 ms, 1450 ms, 1435 ms, 1410 s , 1360 s , 1337 mw, 1230 m, 1040 ms, 1030 s , 977 m, 860 m, 837 mw, 820 mw, 760 v s, 730 v s, 699 vs, 563 m,
500 m, 450 mw. 13C { 1H } -NMR (CDC13) 154.3, 127.6, 125.8, 124.7
[neophyl aromatic], 153.9, 137.1, 133.7 (31), 128.9 (69), 126.1 [tert- butylphenyl aromatic], 101.0, 100.1, 98.5, 97.6 [cod CH], 57.2 [C/CHg)^]
42.4 [Pt-CH2C], 37.5 (888) [Pt-CHj], 34.4 [C(CH3)3]).
2-tert-butylphenyl(neophyl-dq)(cycloocta-1,5-diene)platinum(II)
As above from (cod)Pt(CH2CMe2CgD3) I and 2- t e r t - butylphenyl- magnesiumbromide.
2-tert-butylphenyl-d^-(neophyl)(cvcloocta-1,5-diene)platinum(II)
As above from (cod)Pt(CH2CMe2P h)I and 2- t e r t - butylphenyl-d^- magnesiumbromide.
cis-2-isopropylphenyl(neophyl)-bis-(triethylphosphine)platinum(II)
To a solution of (cod)Pt(CH2CMe2Ph)2-C6HACHMe2) (0.17 g, 0.3 m mol) 3 3 in diethylether (15 cm ) was added by syringe Et3P (0.2 cm , 1.4 m mol).
After standing for 12 hours the solvent was removed in vacuo to give a 94
3 colourless o il. This was extracted into methanol (3 x 10 cm ). 3 Concentration of the methanolic solution to ca. 20 cm caused long colourless prisms to separate (^a. 0.15 g, 70%).
cis-2-tert_-butylpheny 1 ( neophy 1 )-b is-( triethylphosphine ) platmum( I I )
To a suspension of (cod^tCCI^Cl^Ph)^-C^H^CMe^) (0.15 g, 0.26 m 3 3 mol) in hexane (5 cm ) was added Et^P (0.3 cm , 2 m mol) at room temperature. The (cod)Pt(CH2CMe2Ph)^-C^H^CMe^) d issolved with occasional swirling of the flask. On standing overnight the product formed as colourless needles in good yield (0.15 g, 85%).
(IR ; 3088 w, 3070 w, 3046 m, 3028 m, 2955-2860 s , 1594 m, 1570 mw, 1485 s ,
1450 v s, 1407 v s, 1367 m, 1355 m, 1255 ms, 1180 m, 1086 m, 1030 v s, 992 s ,
800 mw, 770 s , 755 v s, 730 vs, 716 v s, 699 v s, 624 s , 555 m, 407 mw).
c is-2-tert-butylphenyl-(neophyl-dr-)-bis-( triethylphosphine ) platinum( I I )
As above from (cod ^ ^ C l^ O ^ C ^ D ,-) ^-C^H^CMe^) and Et^P.
(IR ; 3050 w, 3030 w, 2955-2900 s , 2260 w, 1572 mw, 1450 v s, 1407 v s,
1360 m, 1255 ms, 1187 mw, 1030 v s, 994 m, 800 ms, 755 v s, 730 v s, 716 vs,
625 m, 558 ms, 482 m, 407 mw).
cis-2-tert-butylphenyl-dn~(neophyl)-bis-(triethylphosphine)platinum(I I )
As above from (cod)Pt(CH0CMe0Ph)(2-C^-H D/7 NCMe0) and E t 0P. 2 2 6 n (4-n) 3 3 95
(IR ; 3030 w, 2965 m, 2918 raw, 2275 vw, 1596 w, 1490 in, 1455 ms, 1400 s ,
1260 v s, 1090 v s, 1030 v s, 800 v s, 772 m, 757 ms, 718 m, 703 s , 666 mw,
626 mw, 556 w, 400 mw).
Methyl(neophyl)(cycloocta-l,5-diene)platinum(II)
To a stirred solution of (cod)Pt(CHg)I (0.5 g, 1.12 m mol) in THF 3 3 (30 cm ) at -40°C was added dropwise neophylmagnesiumchloride (10 cm of a 0.6 M THF solution, 6 m mol) in THF (20 cm^). The pale yellow solution became cloudy at -10°C and after stirring overnight at room temperature a white precipitate had formed. The mixture was hydrolysed at -10°C with distilled water, cooled to -78°C and the THF layer filtered off. Removal of the THF in vacuo yielded a brown semi-crystalline solid.
This was extracted in petroleum-ether (b,p. 40-60°C) and cooled to -25°C
to afford large crystals of the product (0.25 g, 50%).
Found: C 50.41, H 6.19%. Calc, for C-^^gPt: C 50.54, H 6.25%.
cis-methyl(neophyl)-bis-(triethylphosphine)platinum(II)
To a solution of (cod)Pt(CH2CMe2Ph)(CH^) (0.10 g, 0.22 m mol) in 3 3 diethylether (15 cm ) was added triethylphosphine (0.2 cm , 1.4 m mol).
The so lu tio n was s tirr e d overnight at room temperature. Removal of the
ether in vacuo gave an oily residue. This was crystallised by addition 3 3 of methanol (2 cm ), dissolution in the minimum amount of THF (ca. 3 cm )
and cooling to -25°C, as colourless prisms (0.09 g, 70%). 96
Found: C 47.63, H 8.07%. Calc, for C23H46P2Pt: C 47.66, H 8.00%.
(IR ; 3075 mw, 3045 w, 3005 w, 2953 s , 2910 s , 2862 m, 2810 mw, 1595 m,
1493 ms, 1450 s , 1440 m, 1413 m, 1370 s , 1355 m, 1250 mw, 1200 w,
1175 w, 1155 w, 1090 mw, 1030 v s, 1024 s , 1000 m, 762 v s , 713 v s, 699 v s,
629 m, 564 mw, 522 m, 509 mw, 420 mw. 13C { l E } -NMR (CDC13) : 157.0,
127.2, 126.1, 123.9 [neophyl aromatic], 40.9 (J(Pl-C) = 94, J(P3-C) = 8)
[Pt-CH2], 34.0 (44) [Pt-CH2C(CH3)2], 17.1, 15.7, 8.4 [Et3P]).
trans-chloro(neophyl)-bis-(triphenylphosphine)platinum(II)
(cod)Pt(CH2CMe2Ph)Cl (0.20 g, 0.42 m mol) and triphenylphosphine 3 (0.22 g, 0.84 m mol) were stirred together in acetone (15 cm ). After
a few minutes a white c r y s t a llin e product had formed. The reactio n was
continued for 30 minutes. Concentration of the acetone solution cause more of the product to separate. Yield 0.35 g (94%).
The compound is insoluble in chloroform, methylene chloride, and
dmso but sparingly soluble in benzene.
Found: C 58.27, H 4.54, Cl 3.88%. Calc, for C46H43ClP2Pt: C 62.19,
H 4.88, Cl 3.99%.
(IR ; 3075 w., 3050 w, 2960 mw, 2910 w, 2890 w, 1478 m, 1432 s , 1400 s ,
1182 mw, 1092 s , 1026 m, 995 mw, 750-745 ms, 695 vs, 543 s , 530 s , 499 s ,
460 w, 420 w ). 9
cis-chloro(neophyl)\b is-(1,2-dimethylphosphino)ethane]platinum(II) 3 bis-(l,2-dimethylphosphino)ethane (0.12 cm , 0.8 m mol) was added by syringe to a solution of (cod^tCC^Cl^PlOCl (0.35 g, 0.74 m mol) 3 in diethylether (15 cm ). This caused immediate formation of a white 3 powder which was washed with d ieth yleth er (2 x 25 cm ) and dried in vacuo.
cis-iodo(neophyl)[bis-(1,2-diphenylphosphino)ethane]platinum(II)-monomer and dimer
( cod)Pt(CH2CMe2Ph)I (0.17 g, 0.3 m mol) and dppe (0.12 g, 0.3 m mol) were dissolved in toluene (10 cm ) at room temperature. On standing the solution became more yellow and was left overnight. Removal of the toluene, followed by extraction of the solid into a 1:2 diethylether/ chloroform mixture and cooling to -25°C yielded the minor dimeric product
(see text) as colourless crystals (ca. 0.05 g). Further addition of diethylether to the filtrate afforded the major product as fine colourless needles (ca. 0.15 g).[(dppe)Pt(CH2CMe2P h ) I ^ ;
Found: C 48.24, H 4.12%. Calc, for C72H74P4I 2Pt2: C 50.65, H 4.37%.
(IR ; 3040 w, 2955 mw, 2940 mw, 2890 w, 2850 w, 1569 w, 1480 mw, 1433-s ,
1411 mw, 1383 mw, 1260 m, 1100 s , 1028 m, 880 w, 820-805 m, 747 mw,
736 m, 706 ms, 693 s , 548 s , 509 m, 498 mw, 405 w ).
(dppe)Pt(CH2CMe2Ph)I Found: C 50.71, H 4.28%. Calc, for C 36H 37P2IP t :
C 50.65, H 4.37%.
(IR ; 3070 vw, 3050 w, 2950 mw, 2905 mw, 2890 w, 2860 w, 1482 m, 1433 s,
1384 mw, 1102 ms, 1030 mw, 1000 w, 880mw, 823 m, 750 ms, 705 s , 696 s ,
550 s , 503 mw). 96
neophyl(2,2T-6',2"-terpyridyl)platinum(II)(tetrafluoroborate)
(cod)Pt(CH2CMe2Ph)I (0.10 g, 0.18 m mol) and terpyridine (0.041 g, 3 0.18 m mol) were dissolved in acetone (10 cm ) at room temperature. The in itially yellow solution became orange and was stirred for 24 hours.
Addition by cannula of a solution of silvertetrafluoroborate* (0.035 g, 3 0.18 m mol) in acetone (10 cm ) caused immediate precipitation of a fine pale yellow powder (silveriodide). The mixture was filtered to give a yellow/orange solution from which the product separated as a bright yellow crystalline solid (ca. 0 .1 g, 86%).
Found: C 46.44, H 3.65, N 6.49%. Calc, for C 25H23N3BF4Pt: C 46.31
H 3.73, N 6.48%.
(IR ; 3119 w, 3090 mw, 3055 mw, 3015 w, 2960 m, 2860 m, 2800 mw, 1606 m,
1596 m, 1495 m, 1472 s , 1447 s , 1397 m, 1360 mw, 1318 m, 1248 w, 1167 m,
1100-1020 v s, 823 m, 774 s , 756 s , 726 m, 694 s , 562 m, 529 ms, 466 mw).
U-2 , 2 1-bipyrimidyl-bis-(3,3-dimethyl-l-platinaindan)
(cod^^CI^Cl^Ph^ (0.10 g, 0.18 m mol) and bipym (0.014 g, 0.09 m mol) were dissolved in toluene and stored at room temperature for two months. During this time the solution turned in itially red and then darkened with formation of a dark microcrystalline precipitate. The precipitate was collected at intervals during the reaction and appeared
* Reaction flasks containing AgBF^ were protected from light throughout. very dark red as crystalline solid but greenish when finely divided.
Insolubility of the compound precluded further study,
Found: C 41.64, H 3,73, N 6.50%. Calc, for ^*28^32^4^2*
C 41.38, H 3.72, N 6.89%.
(IR ; 3080 vw, 3035 mw, 2930 m, 2845 m, 2778 w, 1570 m, 1446 m, 1404 s ,
1345 m, 1287 w, 1260 w, 1206 w, 1095 m, 1033 m, 794 m, 750 s , 733 ms,
722 v s, 660 mw, 570 mw, 490 mw).
2.5.3 Preparation of Grignard Precursors*
6 4 para-nitro-tert-butylbenzene-d^
3 A mixture of cone, nitric acid ( 8.8 cm ) and cone, sulphuric acid 3 3 (10.2 cm ) was added dropwise to tert-butylbenzene-d^** (15 g, 17.25 cm ,
0.124 m mol) with stirring. The temperature of the mixture was maintained at 60-70°C during the addition, and at room temperature for a further 12 hours. The pale yellow mixture was carefully poured into ice/distilled 3 3 water ( c a . 200 cm ) and extracted into d ie th y l ether (c a . 300 cm ) . The ether layer was neutralised with saturated sodium bicarbonate solution 3 (ca. 800 cm ) until no more gas evolution occurred on shaking. The
* Protiated analogues were prepared in identical manner from tert-butylbenzene 6 ** Prepared by Friedel-Crafts alkylation of benzene-d with tert-butyl chloride m the presence of FeCl^. Mass spectrum, 139 . 100
ethereal la y e r was dried over MgSO^ and the solvent removed by ro tary evaporation to yield the product as a pale yellow liquid (Yield 80-90%).
(Mass spectrum : N02C^D^C^Hg+ 183).
para-nitro-ortho-bromo-te rt-butylbenzene-dp
To a mixture of para-nitro-tert-butylbenzene-d^ (18 g, 0.1 mol), 3 3 bromine (16 g, 5.2 cm , 0.1 mol), cone, sulphuric acid (90 cm ) and 3 d is t ille d water (10 cm ) was added s ilv e r sulphate (17 g, 0.1 m ol). The flask was protected from light and stirred for 18 hours, by which time the mixture contained a lot of colourless precipitate and a pale yellow supernatant. This was carefully poured into dilute sodium metabisulphite solution which was cooled in ice during the exothermic addition. This was 3 extracted in to d ie th y l ether (500 cm ) , washed with saturated sodium bicarbonate solution and dried over MgSO^/NaHCO^. Removal of the ether by rotary evaporation yielded the crude product as a pale yellow solid
(20.5 g, 79%).
(Mass spectrum : N02CgD2C^HgBr+ 262).
5 3 para-amino-ortho-bromo-te rt-butylbenzene-d^
Tin(II)chloride dihydrate (54 g, 0.24 mol) was dissolved in cone, hydrochloric acid (S.G. 1.18) (80 cm^) and cooled to 5°C. To this solution was added para-nitro-ortho-bromo-tert-butylbenzene-dp (20.5 g,
0.079 mol) and the mixture heated to boiling for 2\ hours. During this period the nitro compound dissolved and a pale yellow precipitate formed.
The mixture was stirred for a further 12 hours at room temperature and 3 then added carefully to 40% sodium hydroxide solution (340 cm ) cooled in 3 ice. Extraction into diethylether (3 x 250 cm ) yielded an orange so lu tio n which was washed with d is t ille d water (3 x 200 m l), dried
(MgSO^) and decolourised with activated charcoal. Removal of the ether by rotary evaporation gave an orange o il from which the impure product was obtained by vacuum distillation (B.p. 80°C, ca. 5 mm Hg) as a yellow oil (11.57 g, 64%).
(Mass spectrum : Nt^C^D^C^H^Br* 217 + peaks for &2 , d^ compounds
+ peaks at higher mass (247, 260, 396). ^H-NMR also showed substantial
H incorporation in aromatic positions).
ortho-bromo-tert-butylbenzene-d
To the crude para-amino-ortho-bromo-tert-butylbenzene (11.57 g, 0.05 moles) was added dropwise with vigorous s t ir r in g 2M su lp h u ric acid (75 3 cm ) . The re su ltin g white suspension was cooled to -5°C and sodium 3 nitrite (6.0 g, 0.09 mol) in distilled water (15 cm ) added dropwise while the temperature was maintained below 0°C. This was stirred for 45 minutes at 5°C and formed an orange s o lid . Hypophosphorous acid (50 ml) was added in one portion and the mixture stirred at 10°C; evolution of dinitrogen was observed. After 12 hours at room temperature the mixture contained an orange oil and a clear solution. This was extracted into diethylether (250 cm ) , washed with saturated sodium bicarbonate so lu tio n , dried and decolourised over MgSO^, NaHCO^ and activated ch a rco al. Removal of
the solvent by rotary evaporation yielded a red oil from which the product was obtained by vacuum distillation (B.p. 40°C, ca. 5 mm Hg) as a colourless liquid (£a. 5g, 50%).
(Mass spectrum : BrC^D^nG^H^"1" 217 + peaks for d 2 > d-^ products). 103
CHAPTER 3
THERMAL REARRANGEMENTS OF NEOPHYLPLATINUM(II) COMPLEXES
WITH NITROGEN DONOR LIGANDS; KINETIC AND MECHANISTIC STUDIES 3.1 Introduction
This chapter describes the results of a kinetic investigation into the thermal rearrangement of the series of complexes L,2Pt(CH2CMe2Ph)2
[I<2 = bipy, bipym, PL^phen, Me^phen] to form L 2^tX2-C^H^CMe^H2 ) and tert-butylbenzene via an intramolecular aromatic C-H activation (see
F ig . 7 ).
The bidentate aromatic polyimine ligands bipy and phen form complexes with metals in a range of oxidation states and are known to stabilise transition metal-carbon O-bonds. This is attributed to their ability to accept electron density from the filled metal d orbitals into the empt} tt * orbitals of the aromatic system. Hence alkyl and aryl metal complexes containing these Ligands have been known for several years.
Similar organometallic derivatives of bipym, however, have only been
3 5 a 4 7 reported recently. Bipym is nominally isosteric with bipy but the two should confer different electronic environments on the metal centre. The former is also deni ed from the "roll-over" C^-metallation decomposition 66 pathway re ce n tly observed in b is - (a ry l)p la tin u m (II) complexes of bipy
(see Fig. 23).
The phenanthroline based ligands are more conformationally rigid about the central "2-2'" bond than bipy and bipym, the methyl and phenyl substituted complexes being more soluble than (phen)Pt(CH2CMe2Ph)2 and thus more readily studied.
Previous mechanistic investigations of thermal rearrangements of transition metal hydrocarbyl complexes have focussed on compounds stabilised by tertiary phosphine ligands. The role of these nitrogen 105
polymeric products
Figure 23. "Roll-over" C-3 Metallation in (bipy)Pt(aryl)2 Complexes66
chelate ligands in such systems has received scant attention. Some evidence has been presented for radical intermediates in reactions of
67 L.2PtMe2 (L>2 = bipy, phen), while the same group has suggested that complete dissociation of bipy occurs during the thermal decomposition of (bipy)Ptiv(C3H6 )Cl28.
As part of a study of the behaviour of these ligands in organo- platinum(II) complexes the thermal behaviour of the bis-neophyl derivatives has been investigated. 106
3.2 Kinetic and Mechanistic Studies of the Metallacyclisation of L.2Pt(CH2CMe2Ph)2 [L^ = bipy, bipym, Ph^phen, Me^phen] via Aromatic
6-C— H Activation
3.2a Kinetics of Thermal Rearrangement of L^PtCCI^Cl^Ph^ [L^ = bipy,
bipym, Ph2phen, Me^phen]
As described briefly in Section 2.1a the bis-(neophyl)platinum(II) complexes produce tert-butylbenzene and platinacycles, L2^tT2-C^H^CMe^H2), when toluene solutions are thermolysed under an inert atmosphere (see
Fig. 7). The kinetics of the rearrangement are readily determined by carrying out the reaction in the probe of an NMR spectrometer which is held at a pre-set temperature and recording the ^H-NMR spectrum at regular intervals. Comparative integration of the resonances due to the methylene protons in the bis-neophyls and product metallacycles enables calculation of the relative concentrations of the two species at a given time. Attempts to follow the reactions by HPLC were unsuccessful due to ineffective separation of the organometals. Kinetic determinations were carried out on ca. 0.02M solutions of L.2Pt(CH2CMe2Ph)2, although no special precautions have been taken to keep this concentration constant for each run. Reactions with more concentrated solutions are complicated by precipitation of the less soluble metallacycles. The results quoted in Table 10 are for runs during which the solutions remained homogeneous throughout.
Plots of ln([L,2PtR2]Q/[L2PtR2]t) against time produce straight lines of gradient k (s showing that the metallacyclisations conform to 107
first-order kinetics for at least three half-lives. In contrast to the inconsistent results found for (cod^tCCI^CN^Ph^ the bis-neophyl complexes with nitrogen donor ligands exhibit reproducible kinetics.
The platinacycles and tert-butylbenzene are the only products detected by ^H-NMR when the reactions are carried out under argon or nitrogen.
There was no detectable incidence of bineophyl (PhMe2CH2CH2Me2Ph), potentially formed by reductive C-C coupling, linkage isomeric
2-tert-butylphenylplatinum(II) complexes (see Chapter 4) or benzyldimethyl-
69 carbinyl products derived from neophyl free radicals.
An investigation of the effect of temperature on the first order rate constants k for L^PtCCH^Cl^Ph^ [L^ = bipy, bipym, Ph^phen,
Me^phen] has been carried out (see Table 10). The results for
( b i p y ^ ^ C l ^ C l ^ P h ^ are represented graphically in Figure'24. These data may also be shown on an Arrhenius plot where a graph of Ink against reciprocal temperature reflects linear correlations for the three complexes studied (Fig. 25) as predicted by the equation;
Ink = InA - Ea/RT
These results show clearly that the rearrangements of the bipy and bipym complexes, despite having different electronic characteristics at the platinum (from electronic spectroscopy, see Section 2.1d(v)) are governed by activation parameters which are undistinguishable within the limits of experimental uncertainty. The Pt^phen and Me^phen complexes decompose at rates comparable to those of bipy and bipym analogues only when heated to temperatures higher by 25°C/‘' The reason for this is not
* The poor solubility of the Me^phen complex makes accurate rate determination difficult but the preliminary studies indicate no significant difference between its cyclisation rates and those of the more readily studied (Ph2phen)Pt(CH2CMe2Ph)2. 108
Table 10 : Effect of Temperature on First Order Rate Constant k for Metallacyclisation of L ^ P t C C^Cl^Ph^ [L = bipy, bipym, Pt^phen, Me^phen]
CORRELATION L2 TEMPERATURE /°C 105 k , /s 1 obs COEFFICIENT, r
73.0 8.22 0.999 70.0 6.02 0.999 L?=bipy 67.0 4.07 0.999 (i) z 62.0 2.03 0.998 60.0 1.62 0.998 58.0 1.26 0.997
70.0 6.07 0.999 65.0 3.25 0.997 (ii) L0=bipym 60.0 1.53 0.995 59.1 1.32 0.998 58.0 1.10 0.997
92.0 9.09 0.997 90.0 7.57 - 0.999 88.5 6.04 0.998 87.0 4.52 0.999 ) L0=Ph0phen 85.0 3.64 0.996 (iii Z Z 82.9 2.82 0.998 82.0 2.23 0.999 78.0 1.64 0.998 75.0 1.06 0.998
87.0 4.48 0.997 (iv) L^Me^phen 82.0 2.81 0.994 109
Figure 24. Effect of Temperature on Rate of Metallacyclisation of (bipy^tCCl^Cl^Ph^ in Toluene Figure 25. Arrhenius Plot for Thermal Cyclisation of L.2Pt(CH2CMe2Ph)2 in Toluene 111
entirely clear. Calculation of AST and AHT for the bipy/bipym and
Pl^phen systems using the following equations;
AS^ = 19.155 (log1Q A - 13.23) J mol" 1 K" 1
AH^ = E - RT kJ mol" 1 a
gives very similar entropies of activation although the activation enthalpy for the Pl^phen complex is ca_ 5 kJ mol 1 higher (see Table 11).
Correlation AS^(Jmol \ 1) E (kJmol ^ L2 a AH^298(kJmo1 *) Coefficient, r
Ph^phen + 44.8 ± 30 137.0 ±10 134.5 ±10 0.995 bipy/bipym + 42.9 ± 20 129.3 ± 5 126.8 ± 5 0.998
Table 11 : Activation Parameters for Metallacyclisation of L^PtCd^Cl^Ph^ in Toluene
Thus these values suggest a common mechanism for the reaction with bipy, bipym, Pt^phen and Me^phen as the ancillary ligand, the positive entropy of activation suggesting either a dissociative or a "product-like” rate determining step (vide infra). 3.2b Kinetics of Thermal Rearrangement of L2Pt(CH2CMe2C^D^)2
[L2 = bipy, Pl^phen]
The cyclisation reactions described above clearly involve cleavage of the ortho-C— H bond of the neophyl phenyl group followed by reductive elimination of the remaining neophyl moiety with the hydride from the presumed intermediate to produce tert-butylbenzene (see Fig. 3(i) for a similar mechanism). Thus both C-H bond scission and formation occur during the reaction. An insight into the relative importance of these processes in the mechanism, i.e. whether they occur prior to or subsequent to the rate-determining step, can be gained by substitution of the hydrogen atom involved by deuterium. The heavier deuterium atom,although not affecting the force constant of the C-H(D) bond, does decrease its vibrational frequency and this is reflected in differing kinetic behaviour.
With this in mind, the deuterated phenyl neophyl complexes L2Pt(CH2CMe2C^D^)
[L2 = bipy, Ph^phen] have been prepared and their thermal behaviour investigated.
Kinetic studies of (bipy^tCCI^CN^C^D,-^ at 65 and 70°C yield values of k^/k^ of ca_. 1.2 (see Table 12 and Fig. 26), showing only a very small kinetic isotope effect on the metallacyclisation reaction. 11
105k ( Correlation Coefficient Temperature (°C) s X) kH/kD R=H R=D for R = D, r
92.0 9.09 2.53 0.999 3.6 L2=Ph2phen 87.0 4.52 1.47 0.994 3.1
70.0 6.02 4.77 0.998 1.26 L2-bi" 65.0 3.25 2.6 0.998 1.25
Table 12 : Kinetic Isotope Effects on Thermal Cyclisation of L^PtCCI^Cf^C^R,^ [L.2 - Ph2phen, bipy; R = H,D]
in Toluene
Similar experiments on (Ph2phen)Pt(CH2CMe2C£D,-) in toluene solutions, however, exhibit a large isotope effect with R^/R^ being greater than 3 at 87 and 92°C, suggesting that the rearrangements proceed with different mechanisms for the bipy and Ph2phen neophyl complexes. [The deuterated bipyrimidyl complex has not been prepared but the similarity of the bipym and bipy systems suggests that the two decompose by the same mechanism ].
3.2c Effeets_of_Added_Ligands_on_Rates_of_Thermal_Cyclisation_of L2Pt(CH2CMe2Ph)2
The thermally or photochemically induced dissociation of neutral ligands from transition metal complexes to create a vacant site prior to Figure 26. Effect of Neophyl Ring Deuteration on Rates of Metallacyclisation of L^PtCCI^O^C^R,-^ [L^ = bipy, Pt^phen; R = H,D]
C-H activation reactions is often observed, or indeed required, particularly
in compounds containing monodentate phosphines (see Chapters 1 and 4).
As mentioned earlier, complete dissociation of bipy is thought to occur iv 68 during thermal decomposition of metallacyclic bipyPt If this
step occurs prior to, or is the rate-determining process in the mechanism 115
then a term reciprocal in the free ligand appears in the rate equation
(see, for example, Section 4.2a). Thus addition of excess ligand can inhibit the reaction, or if associative mechanisms are involved may have an accelerating effect. In order to gain further insight into the mechanism of neophyl C-H activation the following experiments have been carried out.
Cyclisation of (bipy^tCCI^CN^Ph^ in the presence of bipy(0.06M) in toluene-d0 at 65°C indicates that the rate of reaction (k = 2.6 x o 10 ^ s is virtually identical to that in the absence of free ligand
(k = ca_. 3.1 x 10 ^ s ^ from Fig. 25). Similarly, addition of Ph^phen
(ca. 0.04M) does not alter the rate of metallacyclisation of
(Pl^phen^tCCf^Cl^C^Di-^; values of k being 2.74 x 10 ^ s ^ at 92°C and
2.53 x 105 s ^ at 91.9°C in the presence and absence of added ligand respectively. These results demonstrate that complete dissociation of the neutral ligands does not occur in these reactions.
The addition of pyridine to solutions of (bipy)Pt(aryl)2 is found to alter the products of the thermal decomposition and to intercept
6 5 coordinatively unsaturated intermediates. But cyclisation of (Pt^phen)-
Pt(CH2CMe2Ph)2 (ca. 0.013M) at 90°C in d^-toluene containing pyridine
(ca. 0.04M) shows only a small effect (k = 9.02 x 10 s \ c.f. 7.57 x
10 s ^ in the absence of pyridine) with the metallacycle and tert- 1 butylbenzene being the only products detected by H-NMR.
The presence of diethylether, which often occurs as solvent of crystallisation of (Ph^phen^^Ct^Cl^Ph^ (see Section 3.4a),does inhibit the cyclisation of the Ph^phen complex. Thermolysis of 116
(Ph2phen)Pt(CH2CMe2C^D^)2 at 87°C in toluene solution containing half
an equivalent of diethyl ether (measured by ^H-NMR) proceeds with a
first order rate constant, k of 1.09 x l O ^ s ^ c . f . k = 1.47 x 10 ^ s ^
in ether-free solution. The same effect is also observed in the
protiated Pl^phen complex but has not been investigated for the bipy and bipym analogues. The reason for these observations is not apparent
at the moment; if the oxygen lone pair of the ether was competing for
coordination sites then pyridine should have a more pronounced retarding
effect on the rate, rather than the slight enhancement which is observed.
On the other hand, the low concentration of ether (ca_. 0.01M) in the
toluene solutions makes it hard to justify the effect in terms of increased
solvent polarity.
3.2d Mechanisms of Thermal Cyclisation of_Bis-(neophyl)platinum(II)
The results described in Sections 3.2a-c show clearly that the
phenanthroline-based compounds decompose by a different mechanism to
the bipy and bipym bis-neophyl complexes. The most direct evidence for
this is the large kinetic isotope effect on the cyclisation reactions observed for the former compared with a small effect in the bipy/bipym
systems. The value of k^/k^ of 1.2 found for (bipy)Pt(CH2CMe2Ph)2 suggests
that neither C-H bond cleavage nor formation is rate-limiting in the
thermal rearrangement of this complex. The positive entropy suggests a dissociative process is involved. These observations are most consistent with a mechanism m which scission of one of the platinum-nitrogen bonds 117
Figure 27. Proposed Mechanism For Thermal Cyclisation Of (bipy)Pt(CH2CMe2Ph)2 in Toluene 118
to the ligand is the slowest step (see Fig. 27). The three coordinate
intermediate then oxidatively adds to the ortho-C— H bond of the phenyl
ring and reductively eliminates tert-butylbenzene in faster steps. The
lack of any rate decrease in the presence of excess bipy confirms that
only one of the Pt-N bonds is cleaved and total dissociation of the
neutral ligand does not occur. The thermal decomposition of (bipy)Pt-
(4-CgH^CMe3)2 via "roll-over" C-3 metallation (see Fig. 23) is also
found to proceed by rate-limiting Pt-N bond scission. AST for this
reaction is + 33.7 ± 13 J mol ^ which is of similar magnitude to the
value found for metallacyclisation of (bipy)Pt(CH2CMe2Ph)2* 4 A S T = + 43 - 20 J mol -1 K —1 . The activation energy for the bis-neophyl
complex being ca_. 20 kJ mol ^ less than that found for the bis-(aryl)-
platinum(II) compound. The small kinetic isotope effect on the rate of
cyclisation can be accounted for if there is some C-H bond cleavage
during the Pt-N scission step (Fig. 27). It seems probable that
(bipym)Pt(CH2CMe2Ph)2 decomposes by the same mechanism.
The substituted phenanthroline bis-neophyl complexes, however,
thermally rearrange by a process in which C-H cleavage or formation is
rate limiting. The large kinetic isotope effect, k^/k^ = 3.1-3.6 is
of the same magnitude to that observed in cyclisation reactions of
cis-(Et3P)2Pt(CH2CMe3)2 (kH/kD = 3.0) and cis-(Et3P)2Pt(CH2CMe2Ph)2
(kn/kp = 3.4) (see Chapter 4) in which reductive elimination of neopentane
and tert-butylbenzene respectively is proposed to be the rate-determining
step. It is clear, therefore, that Pt-N bond scission is not rate-
limiting and indeed probably does not occur in this system. The three 119
coordinate intermediate so produced should be very similar to that formed during the rearrangement of (bipy)Pt(CH2CMe2Ph)2 (Fig- 27) and should show the same sort of reactivity towards C-H activation/reductive elimination, especially at the higher temperatures involved. Thus the neophyl cyclisation of L ^ P ^ Q ^ C l ^ P h ^ [L^ = Ph^phen, Me^phen] appears to occur without prior ligand dissociation at a four-coordinate platinum(II) centre. The positive entropy of activation AS^ = + 44
J mol ^ indicates that it is probably the reductive elimination of tert-butylbenzene which is the slowest step since oxidative addition would produce a more ordered transition state. Reductive elimination can also proceed via an ordered transition state and formation of biaryl from (R^P^PtCaryl^ proceeds with a negative entropy (AS^ = - 99 J mol ^ -1 71 K ). Reductive formation of aryl C-H bonds from rhodium(III)-aryl(hydride) complexes, however, have ’surprisingly’large positive entropies of activation of similar magnitude to that found in the present example
(e.g. Cp-(PMe3)Rh(C6H5)H — »[Cp-Rh(PMe3)] + C6H6 ; AS^ = + 62 ± 10 Jmol-1 -1 1 _i 30= K , AHf = 127 ± 3 kJ mol ). 2 These positive entropies can be explained by involvement of a 0 -arene complex after C-H reductive elimination (see Fig. 28). The initial C-H formation step (i) proceeds via an ordered transition state and would be expected to have a negative entropy. If dissociation of arene from an 2 intermediate r) -arene metal complex (step (iii)) is rate limiting however, this would yield a more positive entropy since the transition state resembles the products of the reaction, and it is possible that this 12C
2 Figure 28. Intermediacy of n -Arene Metal Complexes in Reductive Elimination of Arene from Aryl-Hydride Metals
is the case in the Ph^phen platinum complex described above. It must be noted, however, that the tert-butylbenzene is produced by reductive 3 2 2 elimination of sp -C-H not sp C-H, and that the formation of a n -arene complex is therefore less likely
The reason why the mechanism of the bis-(neophyl )platmum( II)
rearrangement alters on changing the ancillary ligand from bipy to Pf^phen
must be related to the conformational rigidity of the latter around the
central "2,2,H bond. The two ligands confer similar electronic and
steric environments at the metal and in the neophyl group as shown by 13 1 i electronic C { H } and H NMR spectroscopy. Inspection of Fig. 27
shows that during the Pt-N bond scission step it is advantageous if one
of the ligand pyndyl units can swing away out ol the plane of the complex.
In the phenanthrolme system the whole ligand must move in this manner
and is energetically less favourable, raising the energy barrier for
this reaction above that required for C-H activation at a ^-coordinate
platinum centre. 12
3.2e Thermal Rearrangement of (bipy)Pt(CH2CMe2Ph)2 in the Presence
Of Dioxygen
Thermolysis of (bipy)Pt(CH2CMe2Ph)2 in toluene at 60°C under an atmosphere of dioxygen results in the bright red solution becoming pale orange and precipitation of a brown-orange solid. Analysis of the
'*'H-NMR spectrum shows no metallacyclic product, although tert-butylbenzene is formed. A new methylene peak at 6 3.41 and a singlet at 6 9.1 indicate the formation of other organic products. This low field singlet is also produced when (cod)Pt(CH2CMe2Ph)R [R = CH^Cl^Ph, Me] are thermolysed in the presence of air. Other singlets at 6 1.19 and 61.10 are also observed. It seems probable that these products are formed by a radical mechanism; the singlet at 69.1 being due to an oxygen containing
69 organic compound possibly derived from a benzyldimethylcarbinyl radical, although none of the products has been identified and this reaction has not been investigated further. 3.3 Thermal Rearrangement of [(terpy)Pt(CH2CMe2Ph)]+BF^
The thermal behaviour of mono-neophyl compounds, in which there is no second R group to be reductively eliminated from the hydridoplatinum(IV)
Figure 29. Thermal Isomerisation of [(t e r p y ] BF^ in Acetonitrile-dg.
product of neophyl cyclisation (see Fig. 17), has been investigated briefly.
[ (terpy^tCCl^Cl^Ph) ]+BF^ is insoluble in toluene and remains unchanged after several days reflux in this solvent. When dissolved in acetonitrile-d^ and heated to 85-90°C, however, several changes in the
^H-NMR are observed. The resonance due to the methylene protons diminishes to zero over 10 days at these temperatures, while the neophyl methyl protons increase in intensity with time and changes occur in the appearance of the aromatic region. These results are consistent with the linkage isomerisation of the title compound to the corresponding
2-tert-butylphenyl cation [ (terpy^t^-C^M^CMe^) ]+BF^ (Fig. 29). 12
Isolation of the crude thermolysis product as a yellow/brown semi-crystalline solid provides further evidence for this reaction. The
IR spectrum (Section 3.Ac) shows several characteristic terpyridyl peaks but no neophyl absorptions at 1495, 755, 695 and 562 cm ^ or absorptions suggesting incorporation of the acetonitrile-d^ solvent. The spectrum does contain new peaks at 740 and 730 cm ^ typical of an ortho-disubstituted benzene ring. Elemental analysis of the crude solid gives C, H and N in approximately correct ratios.
As shown in Fig. 29, the isomerisation may, in principle, be a reversible reaction involving either aromatic or aliphatic 6-C— H activation with the former generally being the lower energy process (vide infra) and therefore predominating. This system could, however, provide a means of comparing the two processes by a study of the deuterated neophyl analogue [(terpy)Pt(CH2CMe2C^D^)]+BF^. The rate of C-D activation can be determined by following the decay of the resonance due to neophyl methylene protons in the ^H-NMR spectrum. Aliphatic C-H activation provides a mechanism for incorporation of protons into the neophyl ortho-position and hence into the 6-position of the 2-tert-butylphenyl group. Thus, although the kinetic equations will be complicated and assumption of the isotope effect is required, the relative rates of the two processes should be accessible by monitoring the appropriate peaks in the ^H-NMR spectrum.
Rearrangement of neophylnickel(II) to a tert-butylphenylnickel(II) species has been inferred previously but no organometallic intermediates 19 were isolated. A related isomerisation of c i s ^ EtpP^Pt(CHpCfy^Ph^ is also observed in polar co-ordinating solvents to yield cis-(Et2P2)Pt-
(CH2CMe2Ph)(2-C^H^CMe2) (see Section 4.2c). 3.4 Experimental
3.4a General
Toluene-dg for use in kinetic experiments was distilled from sodium and stored over size 4A molecular sieves under argon. Organometallic complexes were all recrystallised before use. (Ph^phen^tCCI^CN^Ph^ crystallised as a diethyl ether solvate so samples were dissolved in toluene to free the ether and flash-evaporated in vacuo prior to kinetic determinations. NMR tubes (5 mm diameter) were cleaned in aqua regia, distilled water and AnalaR acetone and dried at 180°C. All manipulations were carried out using standard air-sensitive techniques.
3.4b Kinetic Experiments on Cyclisation of L,2Pt(CH2CMe2Ph)2
All experiments were carried out on a JEOL FX90Q NMR spectrometer fitted with a variable temperature probe heater. A solution of
L2Pt(CH2CMe2Ph)2 (ca. 0.02M) in degassed toluene-dg (ca. 0.5 cm^) was prepared and cannulated into an NMR tube fitted with a rubber septum.
Kinetics were determined by heating the spectrometer probe to a pre-set temperature and recording the spectrum at regular intervals ( ±1 sec).
Calibration showed the accuracy of the machine to be ± 1°C, with the precision being±0.1°C. Data accumulation times of ca_. 5 minutes and pulse-delays of 2 seconds were employed. Kinetics were determined by integration of the resonances due to the methylene protons in L2Pt(CH2CMe2Ph) r— ■■ — i and L^Pt^-CgH^Q^CI^) ; values of k being obtained from standard first-order plots of ln([L2PtR2] Q/[L2PtR2] ) against time. 125
3.4c Thermal Rearrangement of [ (terpy)Pt(CH2CMe2Ph) ]+BF^ in Acetonitrile-d^
[ (terp^OPtCCI^CN^Ph) ]+BF^ was thermolysed in acetonitrile-d^ at
85-90°C in an NMR tube for ten days while monitoring the ^H-NMR spectrum
at regular intervals. On completion of the reaction the solvent was
removed in vacuo, the residue being extracted into acetone to give a yellow
solution. Removal of the acetone left a yellow/brown semi-crystalline
solid.
Found : C 44.2, H 3.63, N 7.02% Calc, for C25H23N3BF4Pt : C 46.31,
H 3.73, N 6.48%.
(IR; 3070 mw, 3030 mw, 2950 m, 2855 w, 1600 ms, 1569 m, 1473 ms,
1449 s, 1398 mw, 1317 mw, 1150-950 br.vs, 779 s, 740 m, 729 mw, 705 w,
549 w, 536 m, 473 w, 448 w). 126
CHAPTER 4
THERMAL REARRANGEMENTS OF NEOPHYLPLATINUM(II) COMPLEXES
WITH TERTIARY PHOSPHINE LIGANDS; KINETIC AND MECHANISTIC STUDIES 127
4.1 Introduction
The majority of previous studies of C-H activation processes have involved transition metal complexes containing tertiary phosphine ligands, most of which have been mentioned in Chapter 1. Tertiary phosphines,
PR^ (R = alkyl or aryl), have the capacity to act as strong Lewis bases
(i.e. form strong O-bonds) and to participate in 7T-back bonding to low valent metal centres, accepting electron density from the filled metal orbitals into empty, low-lying d-orbitals on the phosphorus atom. Hence many transition metal phosphine compounds, in a variety of oxidation states, have been isolated, particularly with alkyl and hydride groups.
The extent of O- and 7T-bonding depends on the metal and on the nature of the alkyl or aryl substituents at the phosphorus atom. Changes in the R group can cause differences in the electronic environment at the metal centre and hence affect the reactivity of the complex. Similarly, the steric bulk of the phosphine is often responsible for controlling the direction of reactions involving these complexes, and is particularly 72 important in many catalytic processes, while chelating phosphines,
R2P(CH2)nPR2 also modify reactivity. Thus, variation of the phosphine ligand can provide much mechanistic information about a reaction.
This chapter comprises mainly of kinetic and mechanistic studies of the thermal rearrangement of cis-CEtoP^Pt(CHoCMeoPh^ by aromatic
6-C— H activation. Triethylphosphine has been chosen as the ligand primarily to enable direct comparison of these results with those of
Whitesides’ group on the metallacyclisation of cis-CEtoP^PtCCTUCMeQo to form a platinacyclobutane via aliphatic y-hydride abstraction (see 6 Chapter 1, Figs. 2(ii) and 3(i)). The effects of variation of the phosphine and of chelation on the rate of bis-(neophyl)platinum(II) decomposition have been briefly investigated. 4.2 Results and Discussion
4.2a Thermal Rearrangement of cis-CEtoP^PtCCHoCMepPlOo in
Hydrocarbon Solvents
4.2a(i) Kinetic Studies of Metallacyclisation of cis-CEtoP^PtCCHoCMepPhOo
As described briefly in Section 2.1b, solutions of cis-(EtpP)^-
Pt(CH2CMe2Ph)2 in benzene or toluene decompose when heated above 25°C to produce tert-butylbenzene and c i s ^ E t ^ P ^ P t ^ - C ^ H ^ ^ ^ C ^ ) . These 1 3 1 1 are the only two products detected by H- and P { H } -NMR spectroscopy.
Reductive C-C elimination from the complex would produce bineophyl
(2,5-dimethyl-2,5-diphenyl-hexane) and (Et^P^Pt^ in the presence of excess Et^P, neither of which are observed in these reactions.
Thermolysis of cis-CEt^P^Pt(CTUCMeo ^ at higher temperatures does yield 0 6 detectable amounts of (Et^P^Pt and the coupling product. Decomposition of the bis-neophyl complex by a free-radical mechanism could produce
2,2-dimethyl-benzylcarbinyl-derived products arising from rearrangement 69 of the neophyl radicals, but no such compounds are observed* The solutions remain colourless throughout the decompositions.
The kinetics of the rearrangement are conveniently evaluated by 31 1 monitoring the P { H } -NMR spectra of the solutions at regular intervals
(see Section 4.3). The results in the absence of added Et^P generally follow first-order kinetics, but occasionally produce curved plots of ln[(Pt)Q/(Pt) ] against time and show poor reproducibility. Similar
* Similarly, no products arising from y-aliphatic C-H activation of the neophyl methyl groups are detected.6 13C
Table 13 : Rate Constants for Thermal Cyclisation of cis-^EtoP^PtfCHpCMep- q C^Rc)o Under Various Conditions 6 5'2
Correlation R L Temp/°C 1.2PtR2 ^ o b s d . / s - 1 Coefficient r 1 H 0.04 0 35 18 b 0.993 2 D 0.04 0 35 ca. 6.4 b »^ - 3 H 0.036 0 29.5 4.3 b 0.986 i o b y d 4 H 0.036 0 25 ca. 1.8 - / b,d 5 H 0.08 0 29.5 ca. 4 * - 6 H 0.04 0.07 65 0.59 b 0.997 7 D 0.04 0.07 65 0.17 b 0.998 8 H 0.037 0.04 60 1.31 c 0.994 9 H 0.037 0.04 62.5 2.47 c 0.997 10 H 0.037 0.04 64.9 3.84 c 0.995 11 H 0.037 0.04 70 10.5 c 0.999 12 H 0.073 0.68 80 3.73 c 0.996 13 H 0.073 0.40 80 6.4 c 0.995 14 H 0.073 0.25 80 8.44 c 0.998 15 H 0.06 0 35 0.67 c ’e 0.955 16 H 0.06 0 40 2.98 c ’ 0.994 17 H 0.037 0.04 62.5 2.97 0.998
g a : All carried out in toluene-d under Ar/^ b : Run carried out using method 2 (see Section A.3) c : Run carried out using method 1 (see Section 4.3) d : kQks calculated from first 2 or 3 points of curve
e : Run carried out in the presence of cis^EtoP^P^CHoCMepPlO^-C^H^CMeo) (see Section 4.2c) 131
problems were reported during the decomposition of cis-(EtgP)Pt(CIUCMeg)o in sealed NMR tubes and have been attributed to the presence of traces 6 of dioxygen. It seems likely that the same is true in the present case despite rigorous efforts to exclude oxygen from the NMR tubes. The majority of kinetic experiments have therefore been carried out on toluene-dg solutions of cis-CEtoP^PtCCHoCMeoPh)^ containing added EtgP.
The results of these studies are summarised in Table 13 which also includes measurements on the deuterated analogue, cis-fEtgPl^PtfCHoCMenC^Dr-lo.
Examination of Table 13 enables several conclusions to be drawn.
Entries 1,3,4 and 8-11 clearly show the large temperature dependence of the observed first-order rate constant k , both in the presence and absence of EtgP (see also Fig. 30), indicative of a large positive entropy of activation for the rearrangement. These data give a linear Arrhenius plot, the results in the presence of EtgP giving much better linearity than those for solutions containing only cis-(EtoP)oPt(CH^CMe^Ph)^ (see also Fig. 31). Although the accuracy of the latter results is not as good, the two give similar activation parameters (see Table 14).
[Added EtgP](mol dm ^) Eact *(kJ mol"1) InA AS^ (J mol-1K-1) r
0.037 196 ± 10 59 + 237 ± 20 0.999
0 174 ± 30 59 + 237 ± 50 0.989
Table 14 : Activation Parameters for Cyclisation of cis-(EtoP)oPt(CfLCMepPhlo
in the Presence and Absence of Added Et^P 132
Figure 30 : Effect of Temperature on Rate of Cyclisation of cis-CEtpP^Pt (CHoCMepPh)^ in the Presence of
0.037 Molar Et^P TEMP/°C TEM P/°C
Figure 31 : Arrhenius Plots for Cyclisation of cis-CEtoP^PtCCHpCMeoPhlo (i) in the Presence of 0.037 Molar Et^P (ii) in the Absence of Et^P 3 3 1 Comparison of runs 1 and 2, and 6 and 7 indicates that the kinetic isotope effect for the cyclisation, when the neophyl phenyl ring hydrogens are replaced by deuterium atoms, in the absence and presence of Et^P respectively, is k^/k^ - 3.4 (see Fig. 32). Again, the results in solutions containing free phosphine are more accurate but the two are qualitatively similar. Analysis of the ^H-NMR spectrum at 250 MHz of the decomposed sample of c i s - CEtoP^PtCCHoCMe^C^Dc)^ confirm s the presence of C^D^^CC^D); the -C ^ D appears as a 1:1:1 triplet with the left hand satellite obscured by the CMe2 resonance and is shifted slightly upfield of the protiated tert-butylmethyls.
Runs 3 and 5 show that the rate-constant is unaffected by the i n i t i a l concentration of c i s - CEtoP^PtCCH^CMe^Ph)^ and confirm the f i r s t - order kinetics of the rearrangement. The dependence of this rate constant on the concentration of added Et^P at 80°C is obtained from results 12-14.
A plot of the reciprocal of the rate constant against the phosphine concentration produces a straight line with a non-zero intercept (see
F ig . 33).
4.2a (ii) Mechanistic_Implications
The work described in this section has been considerably facilitated by the pioneering work of Whitesides and co-workers who have carried out a sim ilarly thorough kinetic and mechanistic study of the related thermal decomposition of c is -fEt^P^Pt (CH^CMe^,) ^ via an aliphatic y-C—H activation
They have also studied 6- and e -aliphatic C—H cleavage reactions in analogous complexes and found sim ilar activation parameters and that there 135
Figure 32. Kinetics of Cyclisation of cis-(Et2P)2Pt- i{N„/[p«]t] (CH2CMe2C6R5) 2 [R=H,D] at 65°C in Toluene Figure 33. Effect of Concentration of Added Et^P on Cyclisation Rate of cis-(Et3P)2Pt(CH2CMe2Ph)2 at 80°C
103/ k /s 137
is little difference in ring strain energies of the 4,5- and 6-membered
15 platinacycle products. Thus, one purpose of this present study has been
to compare these re s u lts with an intram olecular arom atic C—H a c tiv a tio n
in the corresponding bis-neophyl complex c is-(EtoP)oPt(CHoCMeoPh)^.
The mechanism of the neopentylplatinum (II) rearrangement i s shown
in Fig. 3(i), with the final step, i.e. reductive elimination of 0 neopentane, proposed to be rate-limiting. The results detailed in
Section 4.2a(i) are consistent with a similar mechanism for cyclisation
of the bis-neophyl complex with rate equations of the same form
describing the reaction (see Fig. 34). The rate equation found for 6 c i s - (EtoP)oPt(CH^CMe^,)^ i s shown below (see F ig . 3 ( i) for rate c o n sta n ts);
- d[L2PtR2] klk2k3^-k2^tk2^ (1) dt + k_^(k_2 + 1^3) [ Et^P ]
Thus, at constant concentrations of Et^P the equation can be written in
fir s t- o r d e r form ;
d[L2PtR2 ] kobs. [L2PtR2] (2) dt
Where k obs. k l k2k3 (3) k^k2 + k_^(k_2+k^)[Et^P]
Inversion of this equation shows that a graph of l/k0ks against
[Et^P] should be a straight line with the intercept yielding a value
of k^. 136
k-i(k_2+k3) [E t3P] 1/k = 1/k. (4) obs. kl k2k3
The mechanism shown in F ig . 34 for the neophyl c y c lis a tio n a lso 2 includes possible n -arene type intermediates both prior to and subsequent to C-H activation (vide infral. These w ill add further complexity to the rate equations but the general form of equation (4) w ill remain unchanged (Equation 5 ).
1/ k + K [Et3P] (5) obs.
So, from Fig. 33 a value of k^, the rate of dissociation of Et^P from L,2 Pt(CH2 CMe2 Ph)2 , can be estimated as cau 1.7 x 10 ^ s ^ at 80°C.
This is of the same magnitude as that found in the bis-neopentyl complex -3 -1 6 (k^ > 1.3 x 10 s at 157°C in cyclohexane) and shows clearly that phosphine dissociation occurs at significantly faster rates than the metallacyclisation and that ligand loss is not the rate-limiting step in the reaction. Whitesides1 group also investigated the exchange of
^2^5^3^ -*-nt0 t at ^0°C anc* f ° und k^lO ^ s ^ providing further evidence that these reactions occur orders of magnitude 6 faster than the metallacyclisations.
The observed kinetic isotope effect, k^/k^ = 3.4 indicates that the rate-determining step must involve a significant amount of C-H(D) bond cleavage or formation (see Fig. 32), and is comparable with the value of 6 -3.0 found for the cyclisation of cis-fEtoP^PtCCHpCMe^^• The very
140
positive entropy of the neophyl reaction (AS^ = + 237 Jmol is even larger than that observed during decomposition of cis-(Et^P)oPt(CHoCMe^)o 1 -i _i s (AST = + 160 Jmol K ) and suggests that it is the C-H formation step, reductive elimination of tert-butvlbenzene from the platinum(IV) intermediate, which is rate-limiting. Cleavage of the C-H bond would proceed via an ordered transition state and would produce a negative or near zero entropy if rate-determining. Comparison of the activation energies for the aromatic and aliphatic C-H reactions shows similar values of ca. 200 kJmol Hence the reason for the more facile aromatic
C-H activation of the neophyl moiety in cis-(EtpP)^Pt(CH^CMe^Ph)9( 35°C) compared with aliphatic C-H cleavage in cis-(EtoP)oPt(CHoCMeo)o ( 118°C) appears to be entropic in origin. Whether this reflects increased steric crowding in the bis-neophyl complex or a greater degree of dissociation in the transition state is unsure, although the very positive entropy of activation suggests that the transition state in the aromatic cleavage reaction lies closer to the products than that during aliphatic neopentyl cyclisation. It is clear, however, that the enhanced neophyl reactivity 2 is not due to formation of a r| -arene metal complex prior to C-H 16 activation, although this may well occur, but it is possible that a 2 H -arene complex after C-H formation will contribute favourably to the entropy of the reaction, relative to purely aliphatic C-H liberation. If 2 dissociation of tert-butylbenzene from a T) -arene platinum(II) complex 3 is the rate-determining step, i.e. after reductive sp -C— H bond formation*, then this transition state resembles more closely the products of the reaction (high AS^) (Fig. 35(a)) than does the corresponding transition
See Fig. 34 (a) (b)
Figure 35. Possible Transition States Formed During Liberation of (a) tert-butylbenzene and (b) neopentane. 142
state for aliphatic C-H formation in which both the C and H atoms are
still partially bound to the platinum (Fig. 35(b)). The neopentane
produced by the latter should dissociate rapidly from the co-ordination sphere. There is no direct evidence, however, for n^-arene metal complexes formed after reductive elimination of aliphatic C-H bonds and the higher AS^ for the bis-neophyl cyclisation may simply be. steric in origin (see also Section 3.2a).
4.2b Effect of Variation of Phosphine on the Rate of Bis-(neophyl)- Platinum(II) Cyclisation
4.2b(i) Thermal Decomposition of c i s ^ P h g P ^ P t C C ^ ^ ^ ^ 1^
As described in Section 2.1b, attempts to prepare cis-CPhuP^Pt-
(CH2CMe2Ph)2 Pea<^ only to isolation of cis-(Ph2P)2Pt(2-C^H^CMe2CH2), 31 with the intermediate dialkyl only being observed transiently by P-NMR spectroscopy. From this experiment the rate constant for metallacyclisation of cis-(Ph2P)2Pt(CH2CMe2Ph)2 in the presence of a 5-10-fold excess of -4 -1 Ph^P at ambient temperature is estimated to be > 4 x 10 s . Under analogous conditions the cyclisation of cis-^t^P^Pt(CH2CMe2Ph)2 is negligibly slow.
The increased bulk of Ph^P (the cone angle 0 for Ph^P is 145°C 72 compared with a value of 132° in Et^P) will affect the mechanism of the cyclisation reaction in two ways. Firstly, it will favour ligand dissociation to give the three-coordinate intermediate LPtl^ and secondly, it will contribute significantly to steric crowding m the transition state which should promote reductive elimination of tert-butylbenzene from the congested coordination sphere. Evidence from dissociation equilibria in nickel(O) and palladium(O) phosphine compounds suggests that Ph^P does dissociate more readily than Et^P but that the effect is not so dramatic as to explain the difference in reactivity of 72 the bis-(neophyl)platinum(II) complexes. Results described in the previous section show clearly that phosphine loss is not rate-limiting in the thermal cyclisation of cis-(EtoP)^Pt(CH^CMe^R) [R = Me, Ph] and would suggest that P h ^ accelerates the reaction by making the entropy of the process more favourable, due to steric crowding probably both in the starting complex and in the transition state. It should be noted, however, that even though phosphine-loss is not rate-determining, it can still speed up reaction by effectively increasing the concentration of the precursor to the rate-limiting step.
4.2b (ii) Thermal Decomposition of (dppe)Pt(CH2CMe2Ph)2
Bis-(neophyl)platinum(II) complexes with chelating tertiary phosphine ligands (dppe, dmpe, dppm) are considerably more thermally stable than the corresponding analogues containing mono-dentate phosphines, and only produce L^Pt^-C^H^^^CF^) and tert-butylbenzene when heated at ca. 110°C in toluene solution. The thermal and photochemical behaviour of (dppe)Pt(CH2CMe2Ph)2 is being studied separately?8
The thermal inertness of these complexes can be attributed to the higher energies required for Pt-P bond scission in the chelating phosphine
Indeed, preliminary kinetic studies of the dppe compound indicate that in contrast to the other bis-(neophyl)platinum complexes studied so far 144
(Chapters 3 and 4) the thermal metallacyclisation proceeds with a negative entropy of activation (AS^ = -65 kJ mol suggesting a different
38 rate-determining step with an ordered transition state. It seems likely that in this example initial ligand dissociation does not take place
(cf. (Pl^phen)(Pt(CH2CMe2Ph)2) but that rate-limiting 6 C-H cleavage at the four co-ordinate complex occurs. Presumably there is considerable steric crowding in the six-coordinate platinum(IV) hydride intermediate due to the ligand phenyl groups and subsequent reductive elimination of tert-butylbenzene occurs relatively rapidly.
Metallacyclisation of (dppe)Pt(CH2CMe2Ph)2 can be photochemically
38 induced at -5°C but is not the sole reaction.
4.2c Thermal Rearrangement of ci^-CEt^P^PtCC^^^^Ph^ in Polar Solvents;
Isomerisation to cis-(EtpP)2Pt(CH2CMe2Ph)(2-C^H^CMeo)
During attempts to grow single crystals of cis-CEtoP^PtCCHoCMepPh^ suitable for an X-ray diffraction study a variety of solvent systems have been employed. From a solution of the complex in ca. 1:1 THF/ methanol colourless crystals suitable for such a study were isolated. The 31 1 P { H} -NMR spectrum of these crystals, however, reveals a ’triplet* of AB quartets with chemical shifts and coupling constants different from 4------1 those of cis-(Et^P) ) and clearly not the spectrum expected for cis-(Et2P)2Pt(CH2CMe2Ph)2. The complex also exhibits unusual ^H-NMR characteristics (see Section 2.2b(ii)). That this compound Figure 36. Molecular Structure of cis-fEtoP^PtfCHpCMepPh)(2-C^H^CMeo).
is in fact the isomeric tert-butylpheny1 (neophyl) compound cis-(Et^P)pPt(CH^CMe^Ph)(2-C^H^CMe^) has been confirmed by an X-ray diffraction study (see Fig. 36).
[Crystal Data : PtC^H^^^^j monocli-ni-c» § . = 10*052(2), _b = 16.222(3), c = 20.593(3) X, 6 = 98.87(1), U = 3317.8 X3 (at 20°C), space group =
P21/n, Z = 4],
The structure has been refined to R = 0.039. The bond angles within the square planar coordination of platinum are shown in Figure 36, bond lengths are listed in Table 15. 146
BOND LENGTH (X)
Pt-C(neophyl) 2.133(7)
Pt-C(tert-butylphenyl) 2.080(9)
Pt-P(trans to neophyl) 2.331(2)
Pt-P(trans to tert-butylphenyl) 2.322(3)
Table 15. Selected Bond Lengths in cis-(Et3P)2Pt(CH2CMeoPh)(2-C^H^CMe3)
The crystal and molecular structures of cis-CEtpP^PtCCl^CMe^)^ ...* and the product of its thermal cyclisation cis-(Et^P)pPt(CH^CMepCH^) have been determined previously and analysed for evidence of steric crowding in the former. The Pt-P (2.322 X) and Pt-C (2.118 X) bond lengths found in cis-(Et^P)oPt(CH^CMe^)^ are of similar magnitude to those found in the present structure (Table 14). Evidence for steric congestion in cis-(Et^P)^Pt(CH^CMeoPh)(2-C^LCMeo) is provided by the P-Pt— P angle of
95.9° (cf. 94.09° in cis-(Et3P)2Pt(CI^CMe^ and 103° in cis-(Et3P)2 ------73 P^Ct^Cl^Cl^)) and by a slight tetrahedral distortion at the platinum such that the trans-P-Pt— C angles are ca. 174°.
Of relevance to the unusual spectroscopic properties in solution
(Section 2.2b(ii)) and to the energetics of aromatic and aliphatic
6-C— H activation (vide infra) found for the asymmetric complex are the non-bonded interactions between the platinum and methyl groups in the neophyl and tert-butylphenyl moieties. The structure shows clearly that 147
the two neophyl methyl groups are in distinct environments with one of them being physcially close (Nearest Pt .... H, 2.90 X), while the second neophyl methyl group is relatively distant (Pt .... H, 3.72 X). O Excepting the neophyl methylene protons (Pt .... H, 2.57 A) the nearest approach to the platinum is made by the tert-butylphenyl methyl C-H bonds (Closest Pt .... H, 2.77 X). The neophyl phenyl group, however, is orientated away from the metal.
The formation of cis-fEtoP^PtfCH^CMeoPtO^-C^H^CMeo) from cis-CEt^P^PtfCH^CMeoPh)^ clearly involves activation of an aromatic carbon-hydrogen bond of the neophyl phenyl ring, followed by reductive elimination of the hydride with a different Pt-C bond to that which forms the metallacycle and tert-butylbenzene (see Fig. 37). Toluene solutions of the asymmetric compound in the absence of added Et^P (vide infra) also undergo thermal rearrangement at ca. 35°C to yield cis-(EtoP)o-
Pt(2-C6H4CMe2tH2) and tert-butylbenzene. It is possible, therefore, that thi-s complex is an intermediate in the metallacyclisation of c is - (Et^,P)oPt(CH^CMe^Ph)^ in toluene that was trapped out by the low- temperature crystallisation procedure. cis-(EtoP)2Pt(CHoCMe2Ph)(2-C^H4CMe,-,) 31 1 is not detected, however, by P { H } -NMR spectroscopy during cyclisations of the bis-neophyl complex at temperatures ranging from 35°C to -20°C
(at the lower temperature the rate of metallacyclisation is infinitesimal).
Furthermore, cis-(EtoP)oPt(CHUCMeoPh)(2-C^H^CMep) is found to isomerise to give a complex which is less labile than both the bis-neophyl and neophyl(tert-butylphenyl) compounds when heated in toluene-d„ in the o presence of excess Et^P (see Section 4.2d), but during thermal rearrangement of cis-(Et^P)^Pt(CH^CMe^,Ph)^ in toluene solutions containing Et^P the
build-up of this isomer is never observed. Figure 37. Mechanism of Formation of cis-(Et^P)2pt(CH^CMe^Ph)(2-C^H/|CMe^) from cis (Et2p)2Pt(CH2CMe2Ph)2 148 The asymmetric compound has been prepared more directly by reaction of (cod^tCCI^CI^^Ph)! with Mg(2-C^H^C^Hg)Br followed by addition of Et^P
(Section 2.2a). The homologous 2-isopropylphenylplatinum(II) complex, cis-(Et^P)2?t(CH2CMe2Ph)(2-C^-H^CHMe2 ), has been isolated similarly.
Kinetic studies of both of these compounds provide further evidence for the non-intermediacy of cis-CEtpP^PtCCI^CMeoPh) (2-C^H^CMeg) during the toluene cyclisations of cis-fEtgP^Pt (Cl^Ct^^Ph^« A comparative experiment, in which the kinetics of the thermal rearrangement of both cis-CEtoP^PtCCHoCMeoPfrOo and cis-(EtgP)2Pt(CH2CMe2Ph)(2-C^H^CMeg) in 31 the same toluene-dg solution were determined by P-NMR at temperatures of 30, 35 and 40°C, demonstrates that the asymmetric compound ( ^ q o q =
1.4 x 10 ^ s reacts at approximately half the rate of the bis-neophyl complex (K^q oq = 3.0 x 10 ^ s ^). Carrying out the reactions in the same solution eliminates the poor reproducibility of results in the absence of added Et^P (see Section 4.2a). The complicated thermal behaviour of cis^(EtgP)2Pt(CH2CMe2Ph)(2-CgH^CMeg) in the presence of EtgP precludes a comparison under these conditions (vide infra).
Interestingly, the experiment in the same solution yields rate constants for metallacyclisations of both complexes which are less than those found when the compounds are decomposed separately. The value,
^35°C = 0*7 x 10 ^ s ^ found for c i j ^ E t ^ P ^ P t C C P ^ ^ ^ P h ^ in the mixed experiment is substantially less than that found when only the bis-neophyl is present, kg,_o0 = 18 x 10 ^ s ^ (see Table 12). Decomposition of the asymmetric compound is retarded also, but to a lesser extent; k^ o ^ =
1.5 x 10 s on its own, ^350^ = ca. 0.3 x 10 ^ s ^ in the presence of 150
cis-(EtoP)oPt(CHoCMenPh)^. These observations can be rationalised by considering the equilibria involved in dissociation of Et^P from the two complexes (Fig. 38). The slower rate of metallacyclisation of the asymmetric compound and its unusual rearrangements (vide infra) suggest
that this complex is more steadily congested and will dissociate more
Et^P, i.e. K Thus both will adjust the respective equilibria to give less of the reactive three co-ordinate species LPtl^ and LPtRR’ then would be present if the compounds were in separate solutions; the bis-neophyl complex being affected to a greater extent, since most of the free phosphine originates from the asymmetric compound. The lower concentrations of'the reactive intermediates give rise to the observed decrease in cyclisation rates. Thermal decomposition of cis-(EtoP)^Pt(CH^CMeoPh)(2-C^H^CHMe^) in toluene solutions produces cis-(EtoP)oPt(2-C^H^CMeoCH^) and isopropylbenzene as the major products* at rates much slower than both the bis-(neophyl) and neophyl(tert-butylphenyl) compounds; k ^ o Q - 1.5 x 10 6 s ^, k^QOQ = 12 x 10 ^ s ^ (see Fig. 39). Hence it is established that cis-(EtoP)^Pt(CH^CMe^Ph)(2-C^HX^H^) is not an intermediate in the thermal rearrangement of * Aliphatic C-H activation would yield cis-(EtqP)^Pt(2-C^H^CMeHCHo) and tert-butylbenzene. kx [LPt(CH2Me2Ph)2][L] _ — = -1 [L2Pt(CH2CMe2Ph)2] k'x [LPt(CH2CMe2Ph)(2-C ^ C ^ )][L] K' - — 1 = -1 [L2Pt(CH2CMe2Ph)(2-C6H4C4H9)] Figure 38. Equilibria and Equations for Dissociation of Et^P from L2Pt(CH2CMe2Ph)2 and L2Pt(CH2CMe2Ph)(2-C^H4C4H9) Solvent- toluene Figure 39. Thermal Rearrangements of c is -CEt^P^Pt^IIoCMe^Ph)^ and c is -(E t )^Pt(CH^CMe^Ph)(2-C^H^CMe^R) [R = Me, H] in Toluene Solutions. * Recorded in the same solution 152 c i s - CEtoP^PtCCHpCMe^Ph)^ in hydrocarbon so lv e n ts, although the complex r i does form cis-CEtoP^Pt^-C^H^CMeoCfh,) by a reaction involving both aromatic and aliphatic 6-C—H activation (vide infra) at a rate slower than the dineophyl compound. The difference in metallacyclisation ra te s between c i s - (EtoP)^Pt(CHoCMeoPh)(2-C^H^CMeo) and c i s - (Et^P)oPt- (CH2CMe2Ph)2 under the same conditions (i.e. ^1:2) reflects the number of a v a ila b le ortho-arom atic C-H bonds in the two compounds and suggests similar energetics for the reactions. Since the asymmetric complex i s not formed from the bis-neophyl complex in toluene, i t s formation from c ia - ( E t 2P)2Pt(CH2CMe2Ph)2 in 31 1 1:1 THF/methanol-d^ (to permit NMR lock) has been investigated by P { H } NMR spectroscopy at ca_. 10°C. This shows that c is^ EtpP^Pt(CHpCMepPh)- ^-C^H^CMe^) is indeed produced (ca. 1%) in addition to the metallacycle (99%). Presumably at lower temperatures (i.e . -25°C) the asymmetric product i s formed p re fe re n tia lly to the m e ta lla cy cle . The iso la te d y ie ld of the original X-ray quality crystals was ca_. 0.04 g from 0.4 g of the bis-neophyl, i.e . 10%. The reasons for the isomerisation under these conditions are not entirely clear, but a polar co-ordinating solvent system may stabilise certain isomeric forms of the intermediate hydrido- platinum (IV) complex from which reductive C-H elim in atio n y ie ld in g c ia - ( E t 2P)2Pt(CH2CMe2Ph)^-C^H^CMe^) is favourable. 4.2d Thermal Rearrangement of c i s - CEtoP^PtCCHpCMeoC^D^)(2-C^H^CMep) and c i s - (EtpP) ^Pt( CH^CMe^Ph) ( 2-C^D CMe^) in Hydrocarbon 55 Solvents; Evidence for Aliphatic C-H Activation The thermal decomposition studies of c is -(EtoP)oPt(CHoCMeoPh)- (2-C^H^CMe^) outlined in the previous section established the need to differentiate between the products of aliphatic and aromatic <5-C—H activation in this complex, since both reactions yield i------1 c i s - (EtgP)^Pt(2-C^H^CMe,-,CTL) and tert-butylbenzene. Thus the behaviour of c i s - (EtoP)^Pt(CH^CMeoC^D^-)( 2-C^H^CMe^) has been in v e stig a te d . Thermolysis of this complex in benzene-dg at 54°C produces s ig n ific a n t amounts (73%) of c i s - CEtoP^^t^-C^H^CMepCHo) and presumably 27% of c is-(Et^P)pPt(2-C^D^CMe^CHo). The amount of protiation is determined by comparative integration of the resonance due to the aromatic H(D)-3 and the methylene protons (100%H) of the metallacyclic product in the ^H-NMR spectrum at 250 MHz. This precludes reactions in toluene-dg since the peak due to residual protons in the methyl group of the g toluene-d ( 6 2.03) overlaps with the methylene resonance, hence benzene-dg i s employed as the therm olysis solvent for these experim ents. The decomposition at 54°C also appeared,at first, to produce considerable amounts of (Q^D)(CHg^CgH^D^). As shown in F ig . 40, however, both aromatic C-D and aliphatic C-H activation produce only deuterium incorporation into the ring of tert-butylbenzene. The production of this tert-butyl-d^-benzene and protiated metallacycle can be accounted for by a mechanism which in vo lves i n i t i a l interm olecular C-D a c tiv a tio n 155 of the benzene solvent by LPtRR', followed by reductive elimination of tert-butyl-d-^-benzene-dr. and subsequent aliphatic C-H activation (Fig. 40). This seems an attractive proposition, particularly in the light of the recent results of Jones and Feher who suggested that the 30d energetics of intra- and intermolecular C-H activation are very similar. Examination of the ^H-NMR spectrum, however, shows no increase in the intensity of the C^D^H peak which should be produced by this mechanism. Further evidence that solvent activation does not occur in this, and,presumably,in related systems is provided by carrying out the thermolysis in C^H^ at 54°C. This produces the same amount of c is -CEtoP^Pt^-C^H^CMe^CHo) and tert-butyl-d-^-benzene and establishes that the source of these deuterons is not the solvent. The amount of t e r t - butyl-d-^-benzene formed has been traced to deuterium incorporation into the methyl groups of the deuterated neophylchloride during its preparation by reaction of benzene-d^, m e th y la lly lch lo rid e and c a t a ly t ic cone. H2S0^. Th is reactio n proceeds by in itia l protiation of the methylallylchloride double bond which then undergoes an electrophilic substitution reaction with the benzene-d^, and results in formation of neophylchloride-d,- and expulsion of D+. If only a catalytic amount of sulphuric acid is used the concentration of D+ soon exceeds that of H+ and thus the D+ becomes significantly incorporated into the methyl groups of the product by the same mechanism. Hence, by using a large excess of cone. this problem can be avoided. Aromatic 5-C— D Activation Figure 40. P ossib le Mechanisms of Thermal M e ta lla cy clisa tio n of c i s - (E t qP) ^Pt (CH^CMepC^D^) ( 2-C^ H^CMe,-,) 156 157 It is clear, therefore, that the amount of protiated metallacycle produced on therm olysis of c i s - CEt^P^PtCCH^CMe^C^Dr) (2-C^H^CMe,-,) a ris e s solely from an aliphatic 6-C—H activation of one of the tert-butylphenyl methyl groups. Thus, given a choice between the aliphatic C-H bond and an aromatic C-D bond to form the same 5-membered p la tin a c y c le from the same starting complex, the platinum inserts to a large extent into the aliphatic C-H bond. This experiment allows calculation of the energetic preference for aromatic C-D over a lip h a t ic C-H a c tiv a tio n (AAG^) from the following equation; aromatic AAG+ RT In a lip h a tic Since the cyclometallation is observedly an irreversible reaction, the ratio of the products reflects the kinetic selectivity of the inter mediate and is equal to the ratio of the rate constants for the two processes. A statistical correction factor must also be applied since there are 9 tert-butyl methyl protons but only 2 ortho-phenyl neophyl C-H bonds. Thus; AAG+ = P T 1 f 9 27 ' RT ln (1.66) RT ln [2 73 -ti ll > > o 1.4 kJ mol"1 at 327K (54°C) T h is shows that arom atic C-D cleavage i s s lig h t ly favoured e n e rg e tic a lly over aliphatic C-H activation. A direct comparison of the kinetic selectivity for aromatic versus aliphatic C-H activation can be calculated by assuming a kinetic isotope effect of ^3 (cis-(EtoP)^Pt(CH^CMeoPh)^ has 15E k^/k^ = 3.4) for the aromatic C-H s c is s io n in c i s - CEtoP^PtCCHpCMeoC^Dc) - (2-C6H4CMe3). AAG^ = RT ln ( 3 x l.6 6 ) = 4.4 kJ mol ^ at 327K This value is of similar magnitude to that found for the kinetic preference of [(Cp*)Rh PMe^)] towards intermolecular aromatic benzene activation over aliphatic cyclopentane activation of 0.8 kcal mol ^ - l 3 0 c (3.3 kJ mol ). Qualitatively similar preferences have been observed 2 8 . 2 9 in related systems which intermolecularly activate C-H bonds. The compound c is - CEtpP^PtCCH^CMe^Ph)(2-C^H^CMe^) is unique in that it yields the same products by either aliphatic or aromatic C-H scission. Although it is recognised that intermediates along the two reaction paths are not necessarily identical, any energetic differences between the two should provide information about the mechanisms and selectivities involved, and thus warrants a detailed study. The experiment on the deuterated neophyl compound c is -(Et3P)pPt- (C^Cl^C^D^^-C^H^CMe^) shows that aliphatic C-H cleavage does occur but requires assumption of the deuterium isotope effect for the aromatic C-H(D) activation during calculation of the energetics. By study of the p a r t ia lly deuterated complex c i s - CEt^P^PtCCH^CMe^Ph) ( 2,-C^D^H^ ^CMe^) , however, this problem is avoided, assuming that there are no secondary isotope effects arising from deuteration on the tert-butylphenyl ring, and the two C-H cleavage reactions can be evaluated directly (see Fig. 41). E t 0P. Figure 41. Therm olysis Products of c i s - (Et^P)^,Pt(CH^CMeoPh)(2-C^DnH^ ^CMe^) ( i ) Aromatic C-H A ctivatio n ( i i ) A lip h a tic C-H A ctivatio n 16 C The H-NMR spectrum of the starting material indicates that the 3-position of the tert-butylphenyl ring is 32% deuterated (see Fig. 42). This is sufficient to label the metallacyclic product formed by aliphatic C-H activation (Fig. 41) and the relative extents of aliphatic and aromatic C-H cleavage are readily determined by examination of the ^H-NMR (250 MHz) spectrum after thermolysis of the complex in benzene-d^. Comparative integration of the resonance due to the 3-proton and the peak arising from the methylene protons in the metallacyclic product indicates the amount of deuterium incorporation at the 3 -p osition and hence the r e la tiv e amounts of c i s - CEtoP)^- P t( 2-C^DnH^_nCMe2tH2) and c i s - (EtoP)oPt(2-C^H^CMe^CH^) present. Thus, thermal decomposition of cis-(E tr,P)nPt(CHnCMenPh)(2-C^D H. CMe~) ------3 2 2 2 bn 4-n 3 in benzene-d^ at 80°C affords only protiated metallacycle indicating exclusive aromatic C-H activation. When the reaction is carried out at 65°C, however, the product metallacycle shows 9% deuterium incorporation at the 3-position, implying 28% aliphatic C-H activation at this temperature. Calculation, as above, produces a value of AAG^ of 7 kJ mol ^ at 338K (65°C) in favour of aromatic C-H cleavage. Further experiments at other temperatures w ill reveal the temperature dependence of AAG^ and allow calculation of AAH^ and AAS^ for the two processes. These preliminary results suggest that as the temperature is increased, the aromatic C-H activation becomes more favourable, implying a more positive entropy of reaction for this process compared with that for the aliphatic activation. This is consistent with the value of AS^ for aromatic C-H cleavage in c is-(EtoP)oPt(CH2CMe2Ph)^ being larger than that for the corresponding 161 Figure 42. ^H-NMR Spectrum (low field region) of £i£~(EtgP)2Pt(CP2(“'^e 2^ 1)- (2-CLD H. CMe0) in C.D , Showing 68% P ro tia tio n at H-3 v 6 n 4-n 3 66 P o sitio n 162 aliphatic reaction in cis-(Et3P)2Pt(CH2CMe3)2» 4.2e Thermal Rearrangement of c is - CEtoP^PtCCH^CMe^Ph)(2-C^H^CMe^) in the Presence of Et^P; Isomerisation to trans-alkyl (aryl) platinum( I I ) and trans-bis-(aryl)platinum (II) Complexes During a comparison of the rates of metallacyclisation of cis-(E t3P)2Pt(CH2CMe2Ph)2 and cis-(E t3P)2Pt(CH2CMe2Ph)(2-C^CMeo) (and in a parallel experiment the neophyl-d^ analogues) at 65°C in the presence of Et^P (0.07M) in toluene-dg the formation of two new complexes XI and X2, as well as c is-(Et2P)2Pt(2-C^H^CMeoCEU), was observed by 3 1 1 P { H} -NMR spectroscopy. Subsequent experiments revealed that both XI and X2 were formed when ju s t c i s - ( E t 2P )2Pt(CH2CMe2Ph)(2-C^H^CMeg) was heated in toluene solutions containing EtgP. These new compounds have platinum-phosphorus coupling constants which suggest a trans-geometry (XI; 6P = -2.7, (Pt-P) = 3148 Hz. X2; 6P = -4.1, XJ (Pt-P) = 3075 Hz. c f . trans-(Et3P)2Pt(C6H5)2 ; 6P = -8.0, (Pt-P) = 2824 Hz). A kinetic analysis of the formation of XI and X2 from c i s - ( E t 3P )2Pt(CH2CMe2Ph)(2-CgH^CMeg) and c is - ( E t 3P )2P t- (CH2CMe2CgD3 )(2-CgH^CMe3) i s very inform ative (F ig . 4 3 ). In both cases XI i s formed i n i t i a l l y to the same r e la t iv e extent (c a . 25% of the original amount of the asymmetric complex present). XI then decays with O =X1 A =X2 filled symbols R=H empty symbols R= D TIME/HOURS Figure 43. Formation and Disappearance of XI and X2 during Therm olysis of c i s - CEtoP^PtCCH^CMepC^R^)(2-C^H^CMe^) [R = H,D] in the Presence of Et^P at 65°C 163 time while the concentration of X2 increases. Significantly the rates of decrease of XI and increase of X2 are both slower in the deuterated neophyl compound, suggesting that the step which converts XI to X2 involves the making and/or breaking of a C-H(D) bond. After 122 hours at 65°C in the presence of Et^P (0.07M) in toluene-dg c is - CEtgPloPtCCHoCMeoPh)(2-C^H^CMeo) is converted almost completely into cis-(EtoP)oPt(2-C^H^CMe2CHo) and X2 in a 3 to 1 ratio, plus tert-butyl- benzene and a trace of XI. Analysis of the ^H-NMR spectrum of this solution at 250 MHz and comparison with the spectra of the metallacycle, tert-butylbenzene and the starting material enables peaks due to X2 to be assigned. Particularly characteristic of X2 is a doublet of doublets with platinum satellites at 6 8.05 (J(Pt-H) = 37 Hz, 3J(H-H) = 4 7.5 Hz, J(H-H) = 2 Hz), a doublet of doublets also with platinum coupling at 67.39 (J(Pt-H) = ca. 9 Hz, 3J(H-H) = 8 Hz, 4J(H-H) = 2 Hz), and a singlet at 61.72. This latter peak is the same intensity as the resonance due to the metallacycle methyl groups at 6 1.62 (6H). Since the i— i ratio of X2 to cis-(EtgP)oPt(2-C^TkCMepCH^) in the mixture is 1:3 (from 31 P NMR) the singlet at 61.72 corresponds to 18 equivalent methyl protons. 1 31 On the basis of this H and P-NMR evidence, X2 is proposed to be trans-CEtoPloPt^-C^H^CMeo^ with the peaks at 6 8.-05 and 6 7.39 being 3 H3 and H6 of the phenyl rings. The value of J(Pt-H) for the H3 proton of 37 Hz is low, cf. 59 Hz in cis-CEtgP^Pt(2-C^H^CMe2CH2), consistent with the platinum-carbon aryl bond being trans to a group of high trans in flu e n ce , such as an a lk y l or a r y l, as compared with a phosphine lig an d . 165 In the same manner, analysis of the H-NMR spectrum of c is - (EtoP)2Pt(CHoCMe2Ph)(2-C^H^CMeo) plus Et^P (ca. 0.07M) in benzene-d^ after 18 hours at 65°C enables peaks due to XI to be assigned. At this stage in the reaction the parent complex, XI, metallacycle and X2 are present in the ratio of 56.3 : 23.2 : 14.6 : 5.9 respectively. Peaks ascribable to XI are a doublet of doublets at 67.78 (^J(H-H) = 7.3 Hz, ^J(H-H) = 1.6 Hz) and a similar multiplet at 67.45 (^J(H-H) = 7.8 Hz, 4 J(H-H) =1.5 Hz). In the high field region two singlets at 6 1.77 and 6 1.70 are also due to XI. The two low field resonances indicate a coordinated tert-butylphenyl group and the two singlets are assigned as tert-butyl and neophyl methyl protons. Thus, XI is proposed to be tra n s - (EtpP) ^Pt(CHpCMepPh) ( 2-C^H^CMe2 ). There are no well characterised examples of trans-bis(alkyl)-bis- (phosphine)platinum(II) complexes. (Ph^P^PtCadme^ originally proposed 1 47 to be trans on the basis of its large J(Pt-P) coupling constant has 75 re ce n tly been shown to be a c is compound. The reportedly larg e coupling arose from reaction of c is-CPhoP^PtCadme)^ with chloroform-d to give 36 trans-(PhoP)2Pt(adme)Cl. There are, however, a few examples of trans- (R^P^PtCCHg)(Ar) [Ar = fluoroaryl group] which have been characterised by NMR spectroscopy!6 Hence, XI, trans-CEt^P^PtfC^CN^Ph)^-C^H^CMe^). represents one of very few bis-(phosphine)alkyl(hydrocarbyl)platinum(II) complexes in which a trans configuration is observed. Square planar platinum(II) complexes normally favour geometries in which ligands of strong trans influence (e.g. R ) are not mutually trans but are trans to groups of lower influence (e.g. L, X~). 166 The assignment of XI and X2 as tra n s - CEtoP^PtCCH^CMepPh)(2-C^-H^CMeo) and tra n s - (EtpP) ^Pt( 2-C^H^CMe^,)^ respectively on the basis of and 31 P-NMR data i s supported by k in e tic s tu d ie s. The proposed mechanism of the isomerisation of c is -CEtoP^PtCCHpCMeoPlQ^-C^H^CMe,-,), including m etallacyclisations, is shown in Fig. 44. The asymmetric compound _1_ is lost from the reaction by two processes; firstly, the metallacyclisation reaction to give _2 mainly by aromatic C-H activation accompanied by some aliphatic cleavage (see Section 4.2b) and also by isomerisation to produce XI in a reaction apparently catalysed by free Et^P. The relative amount of XI formed i s notably in s e n s itiv e to the concentration of added Et^P (only a trace of phosphine is required to induce isomerisation) and is not affected by deuteration on the neophyl phenyl ring (see Fig. 43). Conversion of XI to X2, however, does involve aromatic C-H activation and formation and is clearly retarded by neophyl ring deuteration. XI may also form metallacycle _2 by aromatic C-H activation, whereas X2 can only do so (or isomerise back to XI) by the higher energy aliphatic C-H cleavage. Inspection of the reaction scheme (Fig. 44) reveals that a ll processes except the isomerisation _1 to XI are inhibited by Et^P. Thus, thermolysis of l (ca. 0.01 g, 0.01 m mol) in benzene-d^ at 65°C in the presence of 3 Et^P (0.15 cm , 1 m mol, 100-fold excess) yields only _1_ and XI after 12 hours. A kinetic investigation of the isomerisation under these conditions (under which a ll C-H activation processes are effectively suppressed) shows that an equilibrium is established after about 8 hours (see Fig. 45), the mixture co n sistin g of 1_ (70%) and XI (30%) (by ^ P NMR). Th is allow s 167 L = Et/jP ; (i) aromatic C -H activation ; (ii) aliphatic C -H activation Figure 44. Thermal Rearrangements of cis-(Et2P)2Pt(^ 2 ^ e2^^^~^6^4^e3^ in the Presence of Excess Et^P. Figure 45. Kinetics of cis - trans Isomerisation of cis-( E t )^Pt(CH^CMepPh)(2-C^H^CMe^) at 65°C in the Presence of 2M Et^P. %X1 169 calculation of the equilibrium constant as follows Fi- P 1 XI K 30 0.43 70 AG° = - RT In K eq. AG° = + 2.4 kJ mol-1 at 338 K (65°C) The cis complex _1_ is therefore thermodynamically favoured as expected for diorganoplatinum(H). The fact that increasing temperature favours XI formation (no isomerisation of _1 is observed at room temperature in the presence of Et^P) suggests a positive entropy for the reaction, presumably due to relief of the conformational restriction present in 1_ (see Section 2.2 b (ii)),although cis to trans isomerisations are usually 7 7 entropy favoured. This experiment provides further evidence in favour of the structures proposed for XI and X2, and the mechanism suggested to account for the re a c tio n s. The ^H-NMR (250 MHz) of this mixture is much simpler due to the absence of X2 and metallacycle and confirms the previous assignment of peaks due to XI. Unfortunately, the resolution is not good enough to observe platinum satellites of the low field aromatic multiplets. Especially significant is the methylene region of the spectrum which shows clearly new resonances indicating that XI does contain a platinum- bound CH2 group (see F ig . 46). 170 (i) I lI ! I I I •V “\ , A A ' /l i 9 Figure 46. Methylene Region of ^H-NMR Spectrum of ( i ) c i s - (Et^,P)oPt(CH^CMe^Ph)(2-C^H^CMe^) and (ii) cis-(Et3P)2Pt(CH2CMe2Ph)(2-C6H4CMe3) + XI in C Dg. 171 The formation of XI from JL_ is therefore a cis - trans isomerisation catalysed by free Et^P. The most plausible mechanism for the isomerisation is an associative pseudo-rotation involving formation of trigonal bipyramidal five-coordinate intermediates(see Fig. 47). Alternative processes such as a consecutive displacement mechanism are precluded since it is very unlikely that the alkyl or aryl groups will dissociate as X . cis - trans isomerisations in square planar complexes have been 77 reviewed recently and this appears to be the first example of such a reaction in a bis-(organo)platinum complex and provides direct evidence for the pseudo-rotation mechanism. X2 is most plausibly produced from XI by aromatic neophyl C-H activation followed by reductive elimination of the hydride with the 3 sp -carbon to yield the trans-bis(aryl) complex. Further experiments including attempted syntheses of trans-(EtoP^Pt- (CH2CMe2Ph)^-C^H^CMe^) and trans-CEtoP^PtC^-C^H^CMeQo* kinetic studies of the isomerisation and an investigation of the corresponding isopropylphenyl system are being undertaken to confirm these conclusions 78 and to determine the mechanisms involved. Figure 47. Mechanism of cis - trans Isomerisation of cis-CEtoP^PtCCH^CMe^Ph)(2-C^H^CMe^) in the Presence of Et^P L R' R X > < > < ? R 173 4.3 Experimental 4.3a General Toluene-dg and benzene-dg were distilled from sodium and stored over 4A molecular sieves under nitrogen prior to use. Organometallic compounds were recrystallised before use in kinetic experiments. NMR tubes (5 mm diameter) were cleaned with cone. HNOg/conc. HC1, distilled water and AnalaR acetone, and dried at 180°C. All manipulations were carried out under argon or nitrogen using standard anaerobic techniques. 4.3b Kine tic_Studi.es These were carried out using one of the following two methods METHOD 1 ; ^22EEi222_i2_!:!22_EEE2EEEE of_EtgP A stock solution of the organoplatinum complex of known concentration was prepared in toluene-dg and EtgP added by microsyringe in a grease-free Teflon stoppered flask. This solution was stored at -25°C for use in kinetic runs at different temperatures as described below. ?E2EEi2E2_iE_E!}£_^k2ence EtgP These were performed in individual NMR tubes (5 mm) fitted with rubber septa and solutions were generally prepared immediately prior to the kinetic run. 174 52l:2E5iE2!:i2PJ3LJ^inetics These were carried out on a JEOL FX90Q NMR spectrometer fitted with a variable temperature probe heater. The heater maintained the probe at the required temperature (±0.1°C) throughout the run and was accurate 31 1 ( ± 1°C). Kinetics were determined by recording the P { H} - NMR spectrum of the sample at regular intervals during the experiment ( ± 1 second). Concentrations of the solution were arranged to give short accumulation times relative to this time interval, e.g. 5 minutes in 30 minutes, and results were more accurate with strong solutions. The pulse-delay (p.d.) was set at 2 seconds for all experiments; variation of the p.d. between 1 and 9 seconds showed no change in the relative integrals of resonances due to cis-(EtoP)oPt(CHoCMe^Ph)^ and cis-(EtpP)pPt(2-C^H^CMe^CH^) indicating that the phosphorus atoms in both compounds (and presumably cis-(EtoP)^Pt(CKUCMeoPh)(2-C^H^CMe2R) [R = H, Me]) have similar relaxation times and experience similar nuclear Overhauser enhancements from proton-decoupling. Uncoordinated Et^P, however, experiences less than half the enhancement of the coordinated 79 31 1 ligand and P { H } -NMR cannot be used to accurately determine the concentration of the free phosphine, this is best done volumetrically. No internal standards were used; the original amount of organo- platinum(II) complex [Pt]g being determined by summation of the amount of the complex at time t [Pt] and the amount of metallacycle present at that time, since the metallacycle is the only organometallic product of the cyclisations. 175 Thus, [PtR2]0 [PtR2]t + [pQt]t In ------In l[PtR2]t ) [PtR2]t METHOD 2 These were carried out on solutions of known concentration in grease-free reaction flasks which were immersed in a Gallenkamp isothermal water bath set at the required temperature ( ±0.2°C). Kinetics were determined by cannulation of aliquots into NMR tubes (5 mm) at regular intervals. In reactions involving no added Et^P the NMR tubes contained a solution of the phosphine in toluene to quench the cyclisation. Samples 31 1 were stored at -25°C prior to P { H } -NMR analysis and kinetic evaluation as described above. CHAPTER 5 CONCLUSIONS 17' 5.1 The Effect of the Ancillary Ligand on the Mechanism of Metallacyclisation of Bis-(neophyl)platinum(II) The results described in Chapters 3 and 4 illustrate how the energetics of the cyclisation of L ^ P t C C I ^ O ^ P h ^ can be altered by changes in the ancillary ligand such that, dependent upon L (or L^) nearly every step in the mechanism can be rate-limiting. Thus, the mechanism of lowest energy occurs when L is a monodentate tertiary phosphine (L = Et^P, Ph^P) and is similar to that found 6 previously for the metallacyclisation of cis-CEt^P^PtCfflUCMe^)^. Prior dissociation of one of the phosphine ligands produces a three- coordinate intermediate, LPtl^, which undergoes thermal cyclisation (0-35°C), with liberation of tert-butylbenzene being the slowest step (large positive AS^). Bis-(neophyl)platinum(II) complexes containing the chelating but conformationally flexible nitrogen donor ligands (bipy/bipym) also rearrange via cleavage of one of the platinum-ligand bonds to give the "LPtl^" type intermediate, but total dissociation of the ligand does not occur. This step is rate-determining; subsequent C-H cleavage, formation and dissociation processes are faster at these higher temperatures (55-70°C). Decreasing the conformational flexibility of the nitrogen donor ligand about the "2,2™ bond apparently raises the activation energy for Pt-N bond scission above that required for C-H activation at four coordinate platinum, L^Ptl^. Thus, bis-(neophyl)platinum(II) compounds containing substituted-phenanthroline ligands (Me^phen, Pl^phen) decompose at higher temperatures (80-95°C) without Pt-N cleavage; liberation of tert-butylbenzene again being rate-limiting (positive AS^). At even higher temperatures ( > 100°C) bis-(neophyl)platinum(II) complexes of the chelating, sterically crowded tertiary phosphine ligand, dppe, also rearrange without prior ligand dissociation at four coordinate L^Ptl^ with either C-H oxidative addition or reductive 1 38 elmination being the slowest step (negative AS'). It is clear, although not unexpected, that the neutral ligand plays a crucial part in controlling the energetics of the intramolecular C-H activation of the neophyl moiety and subsequent reductive elimination of tert-butylbenzene. This study has quantified these differences and demonstrated that the nitrogen donor ligands are intermediate between monodentate and chelating tertiary phosphines in their effect on the behaviour of such systems, and that even slight adjustments of the nitrogen donor ligands can alter the mechanism of a reaction. 2 5.2 Intermediacy of r| -Arene-Metal Complexes in Aromatic C-H Activation 2 The apparently more facile activation of sp -hybridised C-H bonds, 3 especially in arenes, relative to sp -C-H bonds, despite the higher bond energies of the former, has been in part ascribed to prior coordination of the metal to the ir-electrons in the aromatic ring, fol- 16 lowed by the oxidative addition step (see Fig. 19). Recent observations by Jones and Feher have provided strong support for the existence of such -arene complexes during both inter- and intramolecular activation of 30 arene C-H bonds by [Cp^RhCPMe^)]. The results described in Section 2.4 show that an intramolecular neophyl-derived complex of this type may well be produced by abstraction of iodide from (dppe^tCC^Cl^Ph)! by silver tetrafluoroborate in chloroform. Bergman and Stoutland, however, 2 have recently reported that intermolecular activation of an sp -C-H bond in ethylene by [Cp*Ir(PMe^)] occurs concurrent with, but not prior to, 80 formation of a 7r-complex. These authors make the valid point that the 2 observation of f| -arene complexes does not necessarily mean that they are formed along the reaction pathway that leads to aromatic C-H activation the two may proceed via different intermediates. Kinetic studies of the metallacyclisation of I ^ P t C C ^ C N ^ P h ^ [L = Et^P, I>2 = Pf^phen] reveal that the rate-determining steps in the two processes have positive entropies of activation and significant kinetic deuterium isotope effects, even though in the Pl^phen system ligand dissociation does not apparently occur. These suggest a dissociative, product-like, transition state and may possibly be due to liberation of tert-butylbenzene from a r) -arene-platinum complex. It should be noted, however, that cyclisation of cis-CEt^P)pPt (CH^CMeQp also proceeds with a very positive entropy (although not as large as that found for the corresponding neophyl cyclisation), and that formation 3 of tert-butylbenzene in the bis-neophyl complexes involves sp -C-H 2 formation and makes t| -terfebutylbenzene coordination less likely than during, for example, aromatic C-H activation in cis-(Et^,P)^Pt(CH^CMepPh)- 2 ^-C^H^CMe^) in which sp -C-H bond making occurs. 2 Recent experiments by Jones suggest that H -arene complexes are indeed true intermediates in the intermolecular aromatic C-H activations by [Cp*Rh(PMe3)].81 181 5.3 Aromatic vs. Aliphatic and Intramolecular vs. Intermolecular C-H Activation Kinetic studies of cis-CEtoP^PtCC^CMeoPEQ^-C^H^CMeo) and specifically deuterated analogues have illustrated that, although activation of aromatic C-H bonds is favoured in this system, the energy difference between this and aliphatic C-H activation is small. At higher temperatures the aliphatic activation becomes less significant, suggesting a more positive entropy of activation for the aromatic 2 reaction which can be attributed to the involvement of r| -arene complexes (vide supra) in the latter. The small energy difference is in apparent contrast to the numerous examples of intermolecular C-H activations in arenes compared with alkanes, but is in agreement with the selectivities of [Cp*M(L)] [M = Ir, L = CO, PMe^; M = Rh, L = PMe^] which show only a 2 small kinetic preference for sp -hybridised C-H bonds. It is noteworthy, however, that no aliphatic Y-C— H abstraction from the methyl groups is detected during thermal rearrangements of the neophyl moiety (cf. Ref.6). Despite the relatively mild conditions under which both aromatic and aliphatic C-H bonds are cleaved intramolecularly in cis-(EtoP)oPt- (CH2CMe2Ph)(2-C^H^CMe2), no intermolecular C-H activation, even of benzene, is observed. 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