Multiple Silicon−Hydrogen Bond Activations at Adjacent Rhodium and Iridium Centers

• Published in: Inorganic chemistry 2010-12, Vol.49 (24), p.11556-11572
• Main Authors: Mobarok, Md Hosnay, Oke, Okemona, Ferguson, Michael J, McDonald, Robert, Cowie, Martin
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic1017196

The reaction of 1 equiv of primary silanes, SiH3R (R = Ph, Mes), with [RhIr(CO)3(dppm)2] yields mono(silylene)-bridged complexes of the type [RhIr(H)2(CO)2(μ-SiHR)(dppm)2] (R = Ph or Mes), while for R = Ph the addition of 2 equiv yields the bis(silylene)-bridged complexes, [RhIr(CO)2(μ-SiHPh)2(dppm)2]. The kinetic isomer of this bis(silylene)-bridged product has the phenyl substituent axial on one silylene unit and equatorial on the other, and in the presence of excess silane this rearranges to the thermodynamically preferred “axial−axial” isomer, in which the phenyl substituents on each bridging silylene unit are axial and parallel to one another. The reaction of 1 equiv of diphenylsilane with [RhIr(CO)3(dppm)2] produces the mono(silylene)-bridged product, [RhIr(H)2(CO)2(μ-SiPh2)(dppm)2], and the subsequent addition of silane in the presence of CO yields the silyl/silylene product [RhIr(H)(SiPh2H)(CO)3(κ1-dppm)(μ-SiPh2)(dppm)]. The reaction of [RhIr(CO)3(dppm)2] with 2 equiv of SiH2Me2 yields the analogous product [RhIr(H)(SiMe2H)(CO)3(κ1-dppm)(μ-SiMe2)(dppm)]. Low-temperature NMR spectroscopic observation of some key intermediates, such as [RhIr(H)(SiH2Ph)(CO)2(μ-CO)(dppm)2], formed during the formation of the mono(silylene)-bridged species provides evidence for a mechanism involving initial Si−H bond activation at Rh, followed by the subsequent Si−H bond activation at Ir. The Si−H bond activation of a second equivalent of silane seems to be initiated by dissociation of the Rh-bound end of one diphosphine. The reaction of diphenylsilane with the cationic complex [RhIr(CH3)(CO)2(dppm)2][CF3SO3] gives rise to a different reactivity pattern in which Si−H bond activation is initiated at Ir. In this case, the cationic silyl-bridged species, [RhIr(CH3)(CO)2(κ1:η2-SiHPh2)(dppm)2][CF3SO3], contains an agostic Si−H interaction with Rh. In solution, at ambient temperature, this complex converts to two species, [RhIr(H)(COCH3)(CO)(μ-H)(μ-SiPh2)(dppm)2][CF3SO3] and [RhIr(CO)2(μ-H)(μ-SiPh2)(dppm)2] [CF3SO3], formed by the competing methyl migration to CO and reductive elimination of methane, respectively. In the diphenylsilylene dihydride product, a weak interaction between the bridging silicon and the terminal Ir-bound hydride is proposed on the basis of NMR evidence.