Structure and Reactivity of Trimethylsilylmethyl Complexes of Chromium, Including the 13-Electron Alkyl Cp*Cr(CH 2 SiMe 3 ) 2 Robert A. Heintz, Somying Leelasubcharoen, Louise M. Liable-Sands, Arnold L. Rheingold, and Klaus H. Theopold* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716 Received September 22, 1998 Alkylation of Cp*Cr(THF)Cl 2 with 1 or 2 equiv of LiCH 2 SiMe 3 yielded the paramagnetic chromium alkyls [Cp*Cr(µ-Cl)(CH 2 SiMe 3 )] 2 (1), Cp*Cr(CH 2 SiMe 3 ) 2 (2), and Cp*Cr(L)(CH 2 - SiMe 3 ) 2 (3a,L ) py; 3b,L ) THF). Compound 2 is a coordinatively unsaturated, pseudo- five-coordinate Cr III complex with a 13-electron configuration, and it catalyzes the polym- erization of ethylene. The thermal decomposition of 2 in noncoordinating solvents proceeded via an intermediate, namely the bis(µ-alkylidene) complex [Cp*Cr(µ-CHSi(CH 3 ) 3 )] 2 (4). Compound 4 suffered reductive elimination to yield the dinuclear Cr II alkyl Cp* 2 Cr 2 (µ-CH 2 - Si(CH 3 ) 2 CH 2 -µ-CHSi(CH 3 ) 3 )(5). In contrast, the decomposition of 2 in THF, i.e., 3b, yielded the metallacycles Cp*Cr(L)(CH 2 ) 2 Si(CH 3 ) 3 )(6a,L ) py; 6b,L ) THF). Compounds 1, 2, 3b, 4, and 5 have been structurally characterized by X-ray diffraction. The reactions of 2 are rationalized in terms of competing R- and γ-hydrogen elimination processes yielding terminal alkylidene and metallacyclobutane intermediates existing in equilibrium. Introduction Our exploration of the organometallic chemistry of the Cp*Cr III fragment has yielded a plethora of pseudo- octahedral complexes with 15-electron configurations. Three-legged piano stools of the types [Cp*CrR 3 ] - , Cp*Cr(L)R 2 , and [Cp*Cr(L) 2 R] + abound. 1-22 To the extent that such molecules are reactivesfor example as catalysts for the polymerization of olefinssthey exhibit a more or less pronounced tendency to dissociate a ligand, thereby generating coordinatively unsaturated 13-electron alkyls featuring five-coordinate chromium- (III). 23 While the evidence for the intervention of such reactive intermediates is compelling, we have sought to prepare and structurally characterize such a species. The preference of trivalent chromium for six-coordi- nation is illustrated by [Cp*Cr(CH 3 )(µ-CH 3 )] 2 , 6 which avoids coordinative unsaturation by means of bridging methyl groups. We reasoned that substituting larger alkyl groups for the methyl ligands might stabilize a mononuclear dialkyl of the constitution Cp*CrR 2 . Herein we report on the success of this strategy and on the ensuing chemistry of trimethylsilylmethyl ligands co- ordinated to chromium(III). Results and Discussion Syntheses and Structures. Addition of 1.0 equiv of trimethylsilylmethyllithium to blue Cp*Cr(THF)Cl 2 , formed in situ by reaction of CrCl 3 (THF) 3 with Cp*Li in THF, resulted in a rapid color change to violet. Standard workup of the reaction mixture yielded the dinuclear alkyl [Cp*Cr(µ-Cl)(CH 2 SiMe 3 )] 2 (1, see Scheme 1), a purple solid that is thermally stable in solution but decomposes upon exposure to air. The molecular structure of 1 has been determined by X-ray diffraction; (1) Richeson, D. S.; Hsu, S.-W.; Fredd, N. H.; Van Duyne, G.; Theopold, K. H. J. Am. Chem. Soc. 1986, 108, 1491. (2) Richeson, D. S.; Mitchell, J. F.; Theopold, K. H. J. Am. Chem. Soc. 1987, 109, 5868. (3) Thomas, B. J.; Theopold, K. H. J. Am. Chem. Soc. 1988, 110, 5902. (4) Thomas, B. J.; Mitchell, J. F.; Leary, J. A.; Theopold, K. H. J. 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Chem. 1976, 15,5-24. 5477 Organometallics 1998, 17, 5477-5485 10.1021/om980796z CCC: $15.00 © 1998 American Chemical Society Publication on Web 11/13/1998