Rhodium Carbene Complexes. The Structure of RhCl(C2H5OCONCS)3(P(C6H5)3)2 and the Synthesis and Proposed Mechanism of Formation of This and Related Complexes

The reaction of RhCl(PPh3)3 with benzoyl isothiocyanate, C6H5CONCS = L1, or ethoxycarbonyl isothiocyanate, C2H5OCONCS = L2, yields new carbene rhodium(III) complexes, RhClL12(PPh3)2 (la) or RhClL23(PPh3)2 (2), respectively, which are found from three-dimensional x-ray analyses to involve planar tridentate carbene ligands formed by the condensation of two molecules of L1 or by the condensation followed by the 1,3-shift of an ethoxycarbonyl group from three molecules of L2. Two analogous complexes RhCl(p-CH3C6H4CONCS)2(PPh3)2 (1b) and RhCl[(CH3)2NCONCS]2(PPh3)2 (lc) are prepared by the reaction of RhCl(PPh3)3 with the corresponding activated isothiocyanates, and their spectral data indicate that lb and 1c have structures similar to la. Cyclohexyl isocyanide is an efficient inhibitor of the condensation of activated isothiocyanates to form carbenes; rather complexes RhClL(c-C6H11NC)(PPh3)2 (3a for L = L1 and 3b for L = L2) are formed. These involve η2 coordination of isothiocyanate ligands at the C=S bond in a manner similar to complexes PdL(PPh3)2 (4a-4d; L = C6H5CO-, p-CH3C6H4CO-, C2H5OCO-, and (CH3)2NCONCS) synthesized by the reaction of Pd(PPh3)4 with the corresponding isothiocyanates. A model complex of the possible intermediate in the condensation of three molecules of L2, RhClL22(c-C6H11NC)(diphos) (6) was prepared by the reaction of “RhCl(diphos)” with L2. The compound RhCl(C2H5O-CONCS)3(PPh3)2 (CH3)2CO crystallizes with four formula units in a triclinic cell of dimensions a = 15.214 (6), b = 27.571 (9), c = 12.858 (6) Å, α = 103.12 (1), β = 90.24 (1), γ = 100.04 (1)°, V= 5167 Å3. An isotropic model for the structure has been refined by full-matrix least-squares methods to an R index of 0.065 for 401 variables and 7375 observations. The two independent molecules in the unit cell differ somewhat in conformation, but are otherwise equivalent. The Rh atom is six-coordinate, being bound to trans axial phosphine groups, to an equatorial Cl ligand, and to a tridentate planar carbene ligand formed by the condensation of the ethoxycarbonyl isothiocyanate molecules. The inner coordination sphere, including the carbene ligand, bears a very close relationship to that found previously in the structure of RhCl(C6H5CONCS)2(PPh3)2. On the basis structural, spectroscopic, and chemical evidence a mechanism for the formation of these rhodium carbene complexes is proposed.2.