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

New rhodium complexes of PCNHCP have been synthesized by using the silver transfer reagent, [Ag3(PCNHCP)2Cl]Cl2 (2). In the reaction between 2 and [Rh(COD)Cl]2 in dichloromethane, the presumably formed nucleophilic RhI(PCNHCP)Cl intermediate (A), undergoes a C–Cl bond activation of CH2Cl2 giving cis,mer-RhIII(PCNHCP)(CH2Cl)Cl2 (3) as the final product. Attempts to isolate A affords the oxidative degradation product of mer-RhIII(PCNHCP)Cl3 complex (4). In contrast, the rhodium(I) center in Rh(PCNHCP)(CO)Cl (5) is stabilized by the π-back bonding of CO 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.

Graphical abstractThree new rhodium complexes of PCNHCP have been synthesized by using the silver transfer reagent, [Ag3(μ-Cl)(PCNHCP)2]Cl2. The cis,mer-RhIII(PCNHCP) (CH2Cl)Cl2 complex is formed via the oxidative addition of dichloromethane to the RhI(PCNHCP)Cl intermediate. All the complexes are catalyst precursors for efficient, chemoselective hydrosilylation of alkynes.Figure optionsDownload full-size imageDownload as PowerPoint slide