X-ray Structural Characterization and Catalytic Properties of Hydridobis[4,5-bis((diphenylphosphino)methyl)-2,2-dimethyl-1,3-dioxolane]rhodium(I)

The compound HRh[(+)-diop]₂, (+)-diop = (45,5S)-bis((diphenylphosphino)methyl)-2,2-dimethyl-1,3-dioxolane, crystallizes in the orthorhombic space group P2₁2₁2₁ with cell dimensions a = 17.253 (2) Å, b = 20.976 (2) Å, c = 16.336 (3) Å, and Z = 4. Of the 5726 reflections measured by counter methods (Mo Kα radiation), 3758 > 3 were used for the structure determination. Full-matrix least-squares refinement gave a conventional R value of 0.060. The rhodium(I) atom is coordinated to two chelating diop ligands, each with 5,5 chirality, and a hydride in a distorted trigonal bipyramid, the hydride (located in a difference Fourier, Rh-H = 1.6 Å) and a phosphorus being at the trans axial sites. The ³¹P and high-field NMR data reveal a fluxional structure at room temperature. Kinetic and spectroscopic studies on catalytic hydrogenation of styrene support earlier work on prochiral substrates and are consistent with an “unsaturate” mechanism via a HRh(diop)(diop*) species containing monodentate diop*. Optical induction at the substrate coordination stage is not readily attributed (by studying models) to interaction between the substrate and chiral ligand.