The influence of rigid cyclic phosphine ligands in cis-Rh(COD)-(phosphine)2+
on inner coordination shell dynamics and catalyzed olefin hydrogenation/isomerization
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[4.2.1.02,5]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 RhP bond rotations. These barriers are quite sensitive to individual ligand structure, with L = 1 or 3 exhibiting no conformational preference and L = 2 or 4 exhibiting a significant conformational preference within the temperature range studied. A correlation between phosphine diastereomeric meso form, RhP 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. © 1990.
Topping, RJ; Quin, LD; Crumbliss, AL
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