Unusual stability of reaction intermediates in ortho-metalation reactions of dicyclohexylphenylphosphane with dirhodium(II) tetraacetate

Reaction of dirhodium(II) tetraacetate with 1 molar equivalent of dicyclohexylphenylphosphane afforded the complex [Rh2(μ-O2CCH3)3{μ-(C6H4)PCy2}(CH3COOH)2] (1) in which the phosphane ligand is coordinated to the rhodium atoms in a bridging ortho-metalated mode. As a second product, [Rh2(μ-O2CCH3)3{μ-(C6H4)PCy2}(CH3COOH)(PhPCy2)] (2) was isolated from the same reaction. 2 proved to be unusually stable toward further reaction to a doubly ortho-metalated complex which has been accessible in the reaction of dirhodium(II) tetraacetate with other phosphane ligands. However, doubly ortho-metalated [Rh2(μ-O2CCH3)2{μ-(C6H4)PCy2}2(CH3COOH)2] (4) was isolated in low yield after a prolonged thermal reaction between dirhodium(II) tetraacetate and 2 equivalents of PhPCy2. Exchange of the bridging acetate ligands in 1 and 4 by trifluoroacetate ligands afforded [Rh2(μ-O2CCF3)3{μ-(C6H4)PCy2}(CF3COOH)2] (5) and [Rh2(μ-O2CCF3)2{μ-(C6H4)PCy2}2(CF3COOH)2] (7a). Complex 7a without axial ligands, [Rh2(μ-O2CCF3)2{μ-(C6H4)PCy2}2] (7b) was isolated, as well. The crystal structure of complex 5 was determined showing consequences of the bulky cyclohexyl groups on several structural parameters.