REACTIVITY OF COBALT ACETYL COMPLEXES (PR3)(CO)3COCOCH3 TOWARD MONOHYDROSILANES

Monohydrosilanes HSiR3' (SiR3' = SiMe2Ph, SiMeEt2, SiEt3, SiPh3) cleave the cobalt acetyl complexes CH3C(O)CO(CO)3(PR3) (PR3 = PPh2Me, PPh3) in C6D6 at room temperature to give the cobalt silyl compounds R3'SiCo(CO)3(PR3) and ethoxysilanes. Neither acetaldehyde nor the alpha-siloxyethyl complexes CH3CH(OSiR3')CO(CO)3(PR3) were detected by NMR spectral monitoring, irrespective of whether 1 or 2 equiv of HSiR3' was used. Six cobalt silyl compounds were isolated in 85-95% yields and were fully characterized, including by H-1, C-13, P-31, and Si-29 NMR spectroscopy. Further studies involving the reaction between CH3C(O)Co(CO)3(PPh2Me) and HSiMe2Ph afforded the following observations. (1) The presence of CO (1 atm) or free phosphine (1 equiv) inhibits this reaction. (2) Wilkinson's compound, RhCl(PPh3)3, has no effect. (3) Benzaldehyde (1 equiv) inhibits this reaction, whereas acetaldehyde undergoes hydrosilation to give EtOSiMe2Ph. The cobalt acetyl also is an effective acetaldehyde hydrosilation catalyst. (4) The cobalt silyl compound PhMe2SiCo(CO)3(PPh2Me), in contrast, neither undergoes silyl exchange (with HSiEt3) nor catalyzes hydrosilation of acetaldehyde under our standard reaction conditions. These results are compared with those of previous studies involving reactions between CH3C(O)CO(CO)3(PPh3) and HSiEt3 or HSiPh3 in THF at 50-degrees-C/1 atm CO and between the manganese acetyl CH3C(O)Mn(CO)5 and HSiR3'. Mechanistic alternatives are outlined.