A REVERSIBLE SILICON-CARBON BOND-CLEAVAGE PROCESS - DYNAMICS AND REACTIVITY OF CP-ASTERISK-2RU2(MU-CH2)(SIME3)(MU-CL)

The methylene/silyl complex C(p)*2Ru2(mu-CH2)(SiMe3)(mu-Cl), where Cp* = C5Me5, has been prepared by alkylation of [Cp*RuCl]4 With 1 equiv of Mg(CH2SiMe3)2; the product is formed via the unusual cleavage of the alpha-C-Si bond of the alkyl group. The presence of a bridging methylene ligand and a terminal trimethylsilyl ligand has been demonstrated by H-1 and C-13 NMR spectroscopy and confirmed by an X-ray single crystals structure determination. Variable-temperature NMR spectroscopy shows that this molecule undergoes two dynamic processes: the lower temperature process (DELTAH(double dagger) = 9.0 +/- 0.2 kcal mol-1, DELTAS(double dagger) = 0.5 +/- 0.8 cal mol-1 K-1) involves hopping of the silyl ligand between the two ruthenium centers, whereas the higher temperature process (DELTAH(double dagger) = 12.0 +/- 0.3 kcal mol-1, DELTAS(double dagger) = -7 +/- 1 cal mol-1 K-1) involves the reversible reformation of the C-Si bond between the silyl and methylene ligands. Further evidence that the C-Si bond can be re-formed rests on the observation that treatment Of CP*2RU2(mu-CH2)-(SiMe3)(mu-Cl) with PMe3 or CO instantly gives the mononuclear Ru(II) products Cp*Ru(CH2-SiMe3)L2 (2) and Cp*RuCIL2 (3), in essentially quantitative yield. Protonation Of CP*2RU2(mu-CH2)(SiMe3)(mu-Cl) with HO2CCF3 at low temperature gives the new alkylidene complex CP*2RU2(mu-CH2)(mu-O2CCF3)(mu-Cl) (4), while protonation of 1 with HO2CCF3 at room temperature gives a mixture of 4 and the different alkylidene complex CP*2RU2(mu-CHSiMe3)(mu-O2CCF3)-(mu-Cl) (5), which evidently results from C-Si bond re-formation in 1. Deuterium labeling studies show that protonation of 1 occurs at a ruthenium center and not at the methylene carbon. Treatment of 4 and 5 with PMe3 yields the cationic alkylidene complexes [Cp*2RU2(mu-CH2)(PMe3)(mu-CI)][O2CCF3] (6) and [CP*2RU2(mu-CHSiMe3)(PMe3)(mu-CI)][O2CCF3] (7), respectively. Crystal data for 1: monoclinic, space group P2(1)/c, with a = 10.373(3) angstrom, b = 14.896(4) angstrom, c = 17.508(9) angstrom, beta = 104.36(4)-degrees, V = 2621(3) angstrom3, Z = 4, R(F) = 0.044, and R(wF) = 0.049 for 254 variables and 2707 data with I > 2.58sigma(I).