Origin of the Bite Angle Effect on Rhodium Diphosphine Catalyzed Hydroformylation

The bite angle effect on the rhodium diphosphine catalyzed hydroformylation was investigated in detail. A series of xantphos-type ligands with natural bite angles ranging from 102° to 121° was synthesized, and the effect of the natural bite angle on coordination chemistry and catalytic performance was studied. X-ray crystal structure determinations of the complexes (nixantphos)Rh(CO)H(PPh3) and (benzoxantphos)Rh(CO)H(PPh3) were obtained. In contrast to the natural bite angle calculations, approximately the same diphosphine bite angles were observed in both crystal structures. The solution structures of the (diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)2H complexes were studied by IR and NMR spectroscopy. The spectroscopic studies showed that all (diphosphine)Rh(CO)2H complexes exhibit dynamic equilibria between diequatorial (ee) and equatorial−apical (ea) isomers. The equilibrium compositions could not be correlated with the calculated natural bite angles. In the hydroformylation of 1-octene an increase in selectivity for linear aldehyde formation and activity was observed with increasing natural bite angle. For styrene the same trend in selectivity for the linear aldehyde was found. For the first time CO dissociation rates of (diphosphine)Rh(CO)2H complexes were determined using 13CO labeling in rapid-scan high-pressure (HP) IR experiments. The observed CO dissociation rates for three complexes proved to be orders of magnitude higher than the hydroformylation rates and, contrary to the hydroformylation activity, did not reveal a correlation with the natural bite angle. These findings indicate that the bite angle effect on hydroformylation activity is dominated by the rates of reaction of the reactive, unsaturated (diphosphine)Rh(CO)H intermediates with CO and alkene. The bite angle affects the selectivity in the steps of alkene coordination and hydride migration; the structure of the saturated (diphosphine)Rh(CO)2H complex has only some circumstantial relevance to the selectivity.