103Rh chemical shifts in complexes bearing chelating bidentate phosphine ligands

Experimental and theoretical methods have been employed to investigate the influence of the chelating phosphine ligand on the Rh-103 chemical shift in complexes containing the [(P-2)Rh] fragment (P-2 = chelating bidentate phosphine). The delta(Rh-103) values obtained by 2D(P-31,Rh-103(H-1}) NMR spectroscopy for a series of neutral rhodium complexes [{R2P(CH2)(n)-PR2}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)(2)P(CH2)(4)P(p-XC6H4)(2)}Rh(hfacac)] correlates directly with the Hammett sigma(P) 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 [(P-2)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 delta(Rh-103) 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 Rh-103 chemical shifts of cationic complexes with four P donor ligands around a Rh(+I) center and selected anionic Rh(-I) complexes [(P-2)(2)Rh](-). This analysis allows for the first time a direct corroboration of geometrical variations and their effect on Rh-103 chemical shifts, demonstrating that correlations of reactivity with Rh-103 chemical shifts can give valuable information on structure/reactivity relationships.