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.
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Vlugt et al. (2004) studied this question.