The influence of rigid cyclic phosphine ligands in cis-Rh(COD)(phosphine)2+ on inner coordination shell dynamics and catalyzed olefin hydrogenation/isomerization

Article abstract of DOI:10.1016/0022-328X(90)87152-4

Conformational barriers in cis-phosphine-rhodium(I) complexes with two pairs of isomeric ligands (Rh(COD)(L)2⁺, L = anti- or syn-9-phenylphosphabicyclo<4.2.1>nona-2,4,7-triene (1 or 2) or L = anti- or syn-9-phenylphosphatricyclo<4.2.1.0^2,5>nona-7-ene (3 or 4) were studied by variable temperature 13C, 31P and 1H NMR spectroscopy.Conformational barriers result from interligand steric interactions encountered during Rh-P bond rotations.These barriers are quite sensitive to individual ligand structure, with L = 1 or 3 exhibiting no conformational preference and L = 2or 4 exhibiting a significant conformational preference within the temperature range studied.A correlation between phosphine diastereomeric meso form, Rh-P rotational barrier and catalytic activity/selectivity has been found.The hydrogenation and isomerization of 1-hexene homogeneously catalyzed by Rh(COD)L2⁺ (L = PPh3, 3, 4) were investigated in acetone solvent in order to assess the influence of inner coordination shell steric crowding on catalytic reactivity and selectivity.The relative initial rates of catalyzed 1-hexene hydrogenation are 1/7.5/13 for L = PPh3, 3, and 4, respectively.The rate of isomerization of 1-hexene is faster for a Rh(COD)(3)2⁺ catalyzed reaction than for a Rh(COD)(4)2⁺ catalyzed reaction.The difference in hydrogenation and isomerization rates for Rh(COD)(3)2⁺ and Rh(COD)(4)2⁺ catalyzed reactions are discussed in conjunction with the variable temperature NMR data and found to be due to the greater steric crowding imposed by 4 on the inner coordination shell of the catalyst.