Oxidative addition of silanes R3SiH to the unsaturated cluster [Os3(μ-H){μ3-Ph2PCH2PPh(C6H4)}(CO)8]: Evidence for reversible silane formation in the dynamic behaviour of [Os3(μ-H)(SiR3)(CO)9(μ-dppm)]

Abstract

Oxidative addition of the silanes R₃SiH (R₃ = Ph₃, Et₃, EtMe₂) to the unsaturated cluster [Os₃(μ-H){μ₃-Ph₂PCH₂PPh(C₆H₄)}(CO)₈] leads to the saturated clusters [Os₃(μ-H)(SiR₃)(CO)₉(μ-dppm)] (SiR₃ = SiPh₃1, SiEt₃2 and SiEtMe₂3) and the unsaturated clusters [Os₃(μ-H)₂(SiR₃){μ₃-Ph₂PCH₂PPh(C₆H₄)}(CO)₇] (SiR₃ = SiPh₃4, SiEt₃5 and SiEtMe₂6). Structures are based on spectroscopic evidence and a XRD structure of [Os₃(μ-H)(SiPh₃)(CO)₉(μ-dppm)] 1 in which all non-CO ligands are coordinated equatorially and the hydride and the silyl groups are mutually cis. From variable-temperature ¹H NMR spectra of the SiEt₃ compound 2, exchange of the P nuclei is clearly apparent. Simultaneous migrations of the SiEt₃ group and of the hydride from one Os–Os edge to another generate a time-averaged mirror plane in the molecule. VT ¹H NMR spectra of the somewhat less bulky compound [Os₃(μ-H)(SiMe₂Et)(CO)₉(μ-dppm)] 3 have been analysed. Two isomers 3a and 3b are observed with the hydride ligand located on different Os–Os edges. Synchronous migration of the hydride and SiMe₂Et groups is faster than the observed interconversion of isomers which occurs by hydride migration alone. The synchronous motion of H and SiR₃only occurs when these ligands are mutually cis as in the major isomer 3a and we propose that this process requires the formation of a transient silane complex of the type [Os₃(η²-HSiR₃)(CO)₉(μ-dppm)]. Turnstile rotation within an Os(CO)₃(η²-HSiR₃) group leads to the observed exchange within the major isomer 3a without exchange with the minor isomer. This process is not observed for the minor isomer 3b because the hydride and the silyl group are mutually trans. Protonation to give [Os₃(μ-H)₂(SiR₃)(CO)₉(μ-dppm)]⁺ totally suppresses the dynamic behaviour because there are no edge vacancies.