Multiple silicon-hydrogen bond activations at adjacent rhodium and iridium centers

The reaction of 1 equiv of primary silanes, SiH(3)R (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-SiPh(2))(dppm)(2)], and the subsequent addition of silane in the presence of CO yields the silyl/silylene product RhIr(H)(SiPh(2)H)(CO)(3)(κ(1)-dppm)(μ-SiPh(2))(dppm)]. The reaction of RhIr(CO)(3)(dppm)(2)] with 2 equiv of SiH(2)Me(2) yields the analogous product RhIr(H)(SiMe(2)H)(CO)(3)(κ(1)-dppm)(μ-SiMe(2))(dppm)]. Low-temperature NMR spectroscopic observation of some key intermediates, such as RhIr(H)(SiH(2)Ph)(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(CH(3))(CO)(2)(dppm)(2)]CF(3)SO(3)] 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(CH(3))(CO)(2)(κ(1):η(2)-SiHPh(2))(dppm)(2)]CF(3)SO(3)], contains an agostic Si-H interaction with Rh. In solution, at ambient temperature, this complex converts to two species, RhIr(H)(COCH(3))(CO)(μ-H)(μ-SiPh(2))(dppm)(2)]CF(3)SO(3)] and RhIr(CO)(2)(μ-H)(μ-SiPh(2))(dppm)(2)] CF(3)SO(3)], 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.