Synthesis and X-ray powder diffraction characterization of (OC)2RhCl2Rh(cod) (cod = cycloocta-1,4-diene)

In order to elucidate the nature and the structure of the elusive (OC)2Rh(Ph3SiO)2Rh(COd) (cod) = cycloocta-1,5-diene) complex, an important model compound for surface catalysis, (OC)2RhCl2Rh(cod) has been synthesized, and structurally characterized by ab initio X-ray powder diffraction. Crystals of (OC)2RhCl2Rh(cod) are monoclinic, space group P21/c, a = 6.659(1), b = 12.274(1) and c = 16.096(1) Å, β= 92.176(5)°, Z = 4, ρcalc = 2.209 g cm-3. The structure has been solved, from powder diffraction data only, by Patterson and Fourier-difference methods and has been ultimately refined, by the Rietveld method, down to Rp = 0.116 and Rwp = 0.154 for 4050 data points collected in the 12-93° (2θ) range. The molecule contains two square-planar rhodium atoms, one bearing two terminal carbonyls and the other bound to the chelating cod fragment, and two chlorine atoms bridging the Rh⋯Rh vector. The Rh2Cl2 core is markedly non-planar, the dihedral angle about the Cl⋯Cl hinge being 135.4(6)°.