SYNTHESIS AND STUDY OF THE RUTHENIUM SILENE COMPLEXES (ETA(5)-C5ME5)(PR3)RUH(ETA(2)-CH2=SIR'2) (R = (I)PR, CY R' = ME, PH)

The silene complexes Cp*(PR3)RuH(eta2-CH2=SiR'2) (Cp* = eta5-C5Me5; 3: R = (i)Pr, R' = Me; 4: R = Cy (cyclohexyl), R' = Me; 5: R = (i)Pr, R' = Ph; 6: R = Cy, R' = Ph) have been obtained by reaction of the appropriate ClMgCH2SiHR'2 reagent with Cp*(PR3)RuCl. Chemical and physical properties for these compounds are reported, including the X-ray structure for 5, which reveals geometric parameters (Ru-Si = 1.78(2) angstrom; sum of angles about Si = 344-degrees) that are consistent with significant sp2 character at silicon. Compound 5 thermally decomposes in solution to the cyclometalated product Cp*Ru[iPr2P(CMeH)CH2](H)(SiMePh2), isolated as a 7:1 mixture of diastereomers (7a and 7b). The crystal structure of 7a was determined. Compounds 4 and 6 undergo thermolysis to the dicyclohexyl-(cyclohexenyl)phosphine complex Cp*RuH[Cy2P(eta2-C6H9)] (8), with elimination of HSiMeR2 (R = Me or Ph). A kinetic and mechanistic study of the conversion of 5 to 7 indicates that the rate-limiting step is rotation of the silene fragment, to place the silene carbon atom near the migrating hydride ligand, and that the rotation and migration steps are irreversible. Compound 5 reacts with PMe2R (R = Me, Ph) via hydride migration to the silene carbon atom and phosphine exchange to afford Cp*(PMe2R)2RuSiMePh2 (9, R = Me; 10, R = Ph). Mechanistic studies show that this reaction occurs via two pathways, involving irreversible migration of hydride to the silene carbon atom and reversible migration of hydride to silicon. The latter process appears to lead to Cp*(PMe3)RuH(eta2-CH2=SiPh2), which rearranges rapidly to Cp*(PMe3)RuSiMePh2. Thus, Cp*(PMe3)2RuCH2SiHPh2 (11) is not a kinetic product of the reaction, as demonstrated by its independent synthesis and study. The reaction of 5 with carbon monoxide proceeds in a similar manner, via both hydrogen-migration manifolds, to Cp*(PiPr3)(CO)RuCH2SiHPh2 (12) and Cp*(PiPr3)(CO)RuSiMePh2 (13). The reaction of 5 with hydrogen can also be explained by a mechanism involving two hydride-migration pathways, to give Cp*(PiPr3)RuH3 (14) and HSiMePh2 (via trapping of Cp*(PiPr3)RuCH2SiHPh2), and Cp*(PiPr3)RuH2(SiMePh2) (15, via trapping of Cp*(PiPr3)RuSiMePh2). Compound 15 may also be obtained by hydrogenation of 7. The reaction of 5 with COS leads to cleavage of the C-S bond and formation of Cp*(PiPr3)(CO)RuSSiMePh2 (16). Electrophiles Me3SnCl and HCl appear to react with 5 via initial electrophilic attack at ruthenium, but different migration processes are observed to occur. Thus, the reaction with Me3SnCl gives principally Cp*(PiPr3)RuCl and Me3SnCH2SiHPh2. The reaction with HCl produces ClSiMePh2, 7, Cp*(PiPr3)RuCl, and 14. Mechanisms for the above reactions are discussed.