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) 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)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.

Computational Insight into 103Rh Chemical Shift–Structure Correlations in Rhodium Bis(phosphine) Complexes

“…20 ppm degree −1 at 89 degrees). Overall, Rh-P bond distances emerge as the decisive parameters governing the 103 Rh chemical shifts of cationic bis(phosphine) complexes, in complete accord with previous findings for neutral acetylacetonato complexes, 18 or the Rh-N bonds in complexes with N-donor ligands. 20 The importance of the Rh-P bonds can be ascribed to inductive and resonance aspects together with polarization effects.…”

“…This particular combination of levels has performed well in previous computations of 103 Rh chemical shifts. 16,[18][19][20][21][22]44 In order to obtain the corresponding chemical shifts from the magnetic shieldings, a reference is needed. Due to the lack of a suitable compound, ζ(standard) has been evaluated as the intercepts at δ = 0 of the ζ(calc) vs. δ(expt) linear regressions involving complexes 1-7 ( Figure 1) together with several species which cover a wide range of the 103 Rh chemical shift window (see Figure S1 in Supporting Information).…”

“…The key role of distance parameters on chemical shifts has been already noted. 18 In a computational experiment, complexes 1-7 were reoptimized using M06, but keeping all Rh-P and Rh-C(sp 2 ) bond distances constrained to the PBE0 data. In all cases, the energy difference between the fully relaxed and the fixed structures was lower than 0.5 kcal mol −1 .…”

“…3600 ppm. 16,17 The scope of this methodology has been extended to species containing phosphines, 18,19 N-donor, 20 and alkene ligands, 21 as well as imidazole-based radiosensitizers. 22 Other complexes including indacenyl 23 and porphyrin 24 derivatives have also been analyzed.…”

Thermal Effects and Vibrational Corrections to Transition Metal NMR Chemical Shifts

“…Of the geometrical parameters that affect the chemical shift of a metal, the bond lengths to its ligands are usually the most important ones, [76,77] even in cases where bond angles had been initially believed to be decisive. [76] The sensitivity towards these parameters can be quantified in terms of the shielding/bond-length derivatives, @s M /@r M-L .…”