OF the TRANSITIONAL METALS a Thesis Submitted by WALTER

TRINETHYLSILYLMETHYL COMPLEXES OF THE TRANSITIONAL METALS A thesis submitted by WALTER MOWAT, B.Sc., for the degree of Doctor of Philosophy of the University of London Royal College of Science, Imperial College of Science and Technology, London, S.W.7. October 1972. 2 ACKTIOWLEDGEMENTS I would like to express my gratitude to Professor G. Wilkinson F.R.S. for his guidance and encouragement during the supervision of this work and to the Science Research Council for support. I would like to thank my friends and colleagues in the depart- ment for their valuable help and advice, in particular Dr. Guido Yagupski, Dr. Matteo Giongo, Dr. Nick Hill, Tony Shortland and Barbara Wozniak. Also Ray Shadwick, John Clay, Sue Johnson and the late Harold Smith for technical assistance. Finally I would like to thank Brenda for her companionship over the last three years. 3 CONTENTS Abstract. 4 Introduction. 5 Chapter I. Trimethylsilylmethyl complexes. 13 Chapter II. Neopentyl complexes. 29 Chapter III. Neopentyl and related alkyls of chromium(IV). 36 Conclusion. 53 Experimental. 55 Abbreviations. 66 References. 67 • 4 ABSTRACT The factors influencing the stability of the transition metal-carbon bond are discussed. The importance of kinetic stability is emphasised, with particular reference to the 0-elimination/hydride transfer reaction. Thermally stable binary trimethylsilylmethyl (R) complexes of vanadium, VR4 and VOR3, niobium and tantalum, (11-R)2M2R4, and molybdenum and tungsten, M 2R6, have been prepared by the interaction of the respective metal halide with the Grignard or lithium reagent derived from chloromethyltrimethylsilane, and have been characterised by their spectroscopic properties. Similar neopentyl (R') complexes have been isolated for titanium and zirconium, MR'S , tantalum, R13 TaC12, and molybdenum, Mo 2RI 6• Binary chromium(IV) alkyls, CrR4 (R = neopentyl, neophyl, tritylmethyl and methyl) have been prepared and characterised in the same way. 5 INTRODUCTION The first stable, isolable compound with a transition metal to carbon o-bond was the tetrameric trimethylplatinum iodide [(CH3)3PtI]4, preDa'red by Pope and Peachy in 1907.1 Prior to this, there had been many attempts to establish carbon linkages with 2 transition metals, e.g. Ti, Zr, Fe and the ready availability of Grignard and lithium reagents as alkylating agents increased the scope of potential alkyls but the results were disappointing. The reason for this lack of experimental success was attributed3 to the "impossibility of the existence of this type of compound". This 2 idea became firmly established and Cotton states that transition metal alkyls and aryls are "very much less stable and accessible than those of non transition elements". Bearing all this in mind one can realise why there was little useful activity in this field during the first half of the century. \ During the early 1950's, a few series of c-bonded alkyl and aryl compounds in which other ligands were present began to emerge. These were compounds of the type 4 C5H5)2Ti(C6H5 )2, h5- C5H5Fe(C0)2 CH3,5 CH3Mn(C0)56 etc. When these compounds were shown not only to be readily isolable but in most cases to have remarkably high stability, it was assumed that the phosphines, carbonyls or other ligands present were stabilizing the complexes and that the absence of such ligands would imply loss of stability of the metal-carbon bond. Parshall and Mrowca sum up the feeling of the time "in contrast to simple alkyls some metal complexes bearing other ligands in addition to alkyl or aryl groups are strikingly stable". The first report of an isolable binary alkyl is an unsub- 2a stantiated report of trimethylrhenium, but the first proven alkyl 8 was tetramethyltitanium, prepared by Berthold and Groh in 1963. 6 TiMe!, is an unstable, yellow crystalline compound decomposing at temperatures greater than -70° and this behaviour seemed to conform with the prevailing theories of alkyl instability. Other binary alkyls are the detonatively explosive dimethylmanganese,9 bis(trity1)- 10 11 nickel and a few other titanium alkyls, Ti(CH2Ph)4, Ti(Ph)412 12 and Ti(Ph)2. Alkyl complexes which may be classified alongside 13 these include alkyl metal halides e.g. MenTC14-n and Me3 MC12 (M = Nb and Ta),14 ionic salts, e.g. Li41/Ph6,15 Li2NbPh7,16 17 18 Li3CrMe6 and Li2 WPh6 and etherates, e.g.- R3Cr(THF)3 (R = Me,Ph etc)19 and TiPh2(Et20).20 The early conceptions of intrinsic thermal instability of the transition metal-carbon bond and subsequent "Tr-stabilisation" by donor ligands can be shown to have little foundation as follows. The available thermodynamic data on transition metal-carbon bond energies (Table 1) are hardly extensive but certainly seem to be TABLE 1. Bond Energies of Transition Metal-Carbon a-bonds. _1 Bond Compound Energy in KJmol Ref. Pt - C6H5 (Et3P)2Pt(C6H5 )2 250 21 Pt - CH3 (h57C5N5 )Pt(cH3)3 164 22 Ti - CH3 (h57C5H5)Ti(CH3)2 250 23 Ti - C6H5 (h5-05H5)2 Ti(C6H5)2 350 23 Ti - C2H5 Ti(C2H5 )3C1 130 24 insufficient to support the view that the bond is weaker than those ' between carbon and non-transition metals, or between transition metals and other first row elements e.g. oxygen and nitrogen in the alkoxides and dialkylamides respectively, where the isolable compounds are 7 25 numerous. There is additional evidence in the form of force constant data derived from vibrational spectra. The carbon-fluorine force constants of CF3I and CF3Mn(C0)5 have been compared26 and the inference was that the CF3-Mn bond is strong. Similar comparisons between CH3TiC13 and the tin and silicon analogues27 show the titanium value to be of the same order of magnitude as the others. Finally, the value of 2.28 mdyne A found for the metal carbon bond in TiMe4 is only 20% lower than a Si-Pb group tetramethyl of the same mass.28 The theory of "donor ligand stabilization" of the transition metal-carbon bond can be dismissed when one takes into consideration the homogeneous hydroformylation catalysts RhCl(PPh3)3 and RhH(C0)(Pa3)3. 29The activity of these complexes is in fact completely dependent on the lability of the metal-carbon bond, and thus we can see that the presence of donor ligands provides no guarantee of stability. It should also be noted that some substitution-inert octahedral 30 metal complexes have particularly stable transition metal-carbon 2+ 2+ bonds, e.g. Cr(H20)5 Me and Ph(NH 3)5C2H5 . Other substitution inert complexes where coordination sites are completely occupied include 31 adducts of alkyl complexes, e.g. TiMe4L2 and TiMeC13L2 are considerably more stable than the parent alkyls, and chelate complexes, where chelation not only blocks the coordination site but also contributes to the resistance of the bond to homolytic fission e.g. tris[w-dimethylarsino-o-tolyl]chromium(III).52 One can also compare coordinative saturation in binary alkyls. WMe633 is more 8 stable and less reactive than TiMe4 or CrMe4,34,35 and Cr(1-nor)4 is completely air and moisture stable, but Cr(4-cam)3 decomposes instantaneously in either.36 It can be seen that "stabilizing" 8 ligands in compounds such as (h5- CO5)Fe(C0)2R etc. are occupying coordination sites, thus these compounds may also be considered substitution inert. It is clear from the above arguments that we can consider two distinct types of stability, namely thermodynamic and kinetic. At this stage it is worthwhile discussing the differences between them as current theories37-39 propose that it is largely on kinetic grounds that transition metal alkyls are stable or unstable. Cross and Braterman39 have attempted to go into this in considerable depth, but it is the view of the author that due to the lack of physical data available for the alkyls, particularly the more recent ones, their conclusions cannot be considered any more significant than those of Wilkinson.37,38 Thermodynamic stability is concerned with the strength of the metal-carbon bond relative to that of the alkyl decomposition products, i.e. M-C vs C-C. Figure 1 shows a thermodynamically unstable alkyl, and it can be seen that for the complex, represented as M-R, the energy of its final decomposition products is lower, resulting in a positive free energy of decomposition. Thus if the activation energy I Fd 1 ING STOLE IDEMPIPOSKitir4 PROOkeT5 Figure 1 •Ea for the transition state can be overcome, it is far more likely that the reactive intermediates will break up further to the final decomposition products rather than recombine to give the original 9 complex. As the end products are unreactive (alkanes, alkenes, metal etc.) the reaction is irreversible. Thus to make the compound stable, one must consider ways of increasing the activation energy, the quantity which determines the kinetic stability. The way to do this is to block the decomposition pathways. There are several mechanistic paths available for decomposition. Zeiss and co-workers44°-42 have shown that the principal organic decomposition products of the metal alkyl are the respective alkane, 1-alkene and 2-alkene. The alkenes arise mainly through the 13 elimination reaction presumably via a 4-membered transition state (equn 1), but Zeiss says that some a elimination occurs. M-CH2CH2R M-H + CH2 = CHR Equn. 1 H---:CH2 The alkane is formed by subsequent interaction of the metal hydride with an alkyl group. The presence of the hydride was demonstrated by using 0-deuterated alkyls and adding deuterium to a scavenger olefin present before the decomposition began. It is agreed41'43 that radical or ionic mechanism (see equn 2 and 3) form a relatively minor part of the decomposition, but the presence of small amounts of cyclic or coupled alkanes shows that they do occur (equn 2).

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