Steric Influence on the Reactivity of Silyl-o-carboranes:  Oxidative-Addition Reactions Involving Si−H and B−H Activation

The reactivity of mono(silyl)- and bis(silyl)-o-carboranes (HSiR2) n (C2B10H12 - n ) (n = 1, R = Me, 1a; n = 1, R = Et, 1b; n = 2, R = Me, 3a; n = 2, R = Et, 3b) toward six-coordinate iridium [(Cp*IrCl2)2] and nine-coordinate rhenium [ReH7(PPh3)2] complexes has been investigated. Reactions between the mono(silyl)-o-carboranes (1a,b) and (Cp*IrCl2)2 resulted in the formation of four-membered, cyclic seven-coordinate iridium complexes Cp*IrH2[η1:η1-(SiR2)BC2B9H10-Si,B] (R = Me, 2a; R = Et, 2b), where Si−H activation in the mono(silyl)-o-carborane (1) is accompanied by the concomitant B−H activation of a neighboring boron hydride. The X-ray structure of 2a reveals that the iridium center is coordinated to both silicon and boron in a four-legged piano-stool arrangement. In the reaction between the bis(silyl)-o-carboranes (3a,b) and (Cp*IrCl2)2, silylation occurs at both Si−H sites, giving rise to the complexes Cp*IrH2[η1:η1-(SiR2)2C2B10H10-Si,Si ‘] (R = Me, 4a; R = Et, 4b), in which the metal center forms part of a five-membered metallacycle (Ir−Si−C−C−Si). Interestingly, the reaction of 3a with ReH7(PPh3)2 afforded the kinetically stabilized intermediate (PPh3)2ReH5[η1-SiMe2C2B10H10(SiMe2H)-Si] (8), in which only one of the Si−H groups is coordinated, as determined by X-ray crystallography.