¹⁰³Rh Chemical Shifts in Complexes Bearing Chelating Bidentate Phosphine Ligands

Abstract

Experimental and theoretical methods have been employed to investigate the influence of the chelating phosphine ligand on the 103Rh chemical shift in complexes containing the [(P2)Rh] fragment (P2 = chelating bidentate phosphine). The δ(103Rh) values obtained by 2D(31P,103Rh{1H}) NMR spectroscopy for a series of neutral rhodium complexes [{R2P(CH2)nPR2}Rh(hfacac)] (R = Ar, Ph, Cy, Me, n = 1−4, hfacac = hexafluoroacetylacetonate) have been compared. Systematic variation of the phosphine ligand has allowed separation of electronic and geometrical effects. The purely electronic influence of para substituents in complexes [{(p-XC6H4)2P(CH2)4P(p-XC6H4)2}Rh(hfacac)] correlates directly with the Hammett σP constants of X, but leads to variations in the chemical shift of less than 80 ppm between X = CF3 and X = OMe. In contrast, geometrical changes in complexes [(P2)Rh(hfacac)] lead to variations in the chemical shift over a range of approximately 800 ppm. The individual contributions of various structural parameters on the δ(103Rh) values have been assessed by density-functional-based calculations for suitable model compounds. The same approach has been extended to the rationalization of the trends in 103Rh chemical shifts of cationic complexes with four P donor ligands around a Rh(+I) center and selected anionic Rh(−I) complexes [(P2)2Rh]-. This analysis allows for the first time a direct corroboration of geometrical variations and their effect on 103Rh chemical shifts, demonstrating that correlations of reactivity with 103Rh chemical shifts can give valuable information on structure/reactivity relationships.