Rhodium Complexes of Sterically Demanding Diphosphonites:  Coordination Chemistry and Catalysis

The coordination chemistry of various sterically constrained diphosphonites 1−6 with rhodium are reported. The stoichiometric reaction of ligand 5 with [Rh(μ-Cl(CO)2]2 gives rise to the sole formation of trans-[RhCl(CO)(5)] (7), as shown by NMR and IR spectroscopy and X-ray crystallography. The molecular structure reveals a weak interaction between the metal center and the oxygen from the xanthene backbone. This complex serves as a structural model for the first intermediate in the catalytic cycle of rhodium hydroformylation, [Rh(H)(CO)(P⌒P)]. A detailed study on the solution structure of the catalyst resting state under catalytic conditions is performed by high-pressure NMR and FT-IR spectroscopy. The spectroscopic data show that under hydroformylation conditions the bidentate ligands form well-defined complexes with the structure [Rh(H)(CO)2(P⌒P)]. All ligands show preference for a bis-equatorial (ee) coordination mode in the trigonal-bipyramidal Rh complexes. These novel sterically constrained diphosphonite ligands are applied in the rhodium hydroformylation of 1-octene and 2-butene, and good activities and regioselectivities are obtained.