Rhodium complexes of PCNHCP:: Oxidative addition of dichloromethane and catalytic hydrosilylation of alkynes affording (E)-alkenylsilanes

New rhodium complexes of (PCP)-P-NHC have been synthesized by using the silver transfer reagent, [Ag-3((PCP)-P-NHC)(2)Cl]Cl-2 (2). In the 1((PCP)-P-NHC)Cl intermediate (A), reaction between 2 and [Rh(COD)Cl](2) in dichloromethane, the presumably formed nucleophilic Rh undergoes a C-Cl bond activation of CH2O2 giving cismer-Rh-111((PCP)-P-NHC)(CH2Cl)Cl-2 (3) as the final product. Attempts to isolate A affords the oxidative degradation product of mer-Rh-111((PCP)-P-NHC)Cl-3 complex (4). In contrast, the rhodium(I) center in Rh((PCP)-P-NHC)(CO)Cl (5) is stabilized by the pi-back bonding of C equivalent to O ligand; a robust complex is, therefore, obtained. The solid-state structures of 2 and 3 were determined by X-ray diffraction. Complexes 3-5 are catalyst precursors for efficient, chemoselective hydrosilylation of alkynes. For the reaction between phenylacetylene and dimethylphenylsilane, a rapid hydrosilylation occurs, producing isomers of alkenylsilanes; then a slow isomerization pathway converts (Z)-alkenylsilane to its (E)-isomer. For 3, under catalytic condition, a facile reductive elimination of dichloromethane giving A is anticipated. The similarity in reactivity and selectivity between 3, 4 and 5 suggests the involvement of A as the active species in a common catalytic cycle. (c) 2005 Elsevier B.V. All rights reserved.