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

Abstract

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, ρ_calc = 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 R_p = 0.116 and R_wp = 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)°.