Complexation of heteroaromatic N-oxides with rhodium(II) tetracarboxylates in solution: DFT and NMR investigations

Complexation of rhodium(II) tetraacetate and rhodium(II) tetrakistrifluoroacetate with a set of heteroaromatic N-oxides containing additional functional groups was investigated by means of density functional theory (DFT) calculations, and 1H, 13C and 15N nuclear magnetic resonance (NMR) spectroscopy in CDCl3 solutions. Chemical shifts for five N-oxides and their 1:1 adducts with rhodium tetraacetate were computed at the B3PW91/[6-311++G(2d,p), Stuttgart ECP)//B3LYP/[6-31G(2d), LANL2DZ] theory level applying IEF PCM (CHCl3) solvation model and taking into account various complexation modes and conformational variety. Calculated values were used for the estimation of complexation shifts Δδ (Δδ = δadduct − δligand). The largest negative complexation shift were estimated for heteroatoms bonded to Rh, from −37 to −70 ppm (N), from −100 to −160 ppm (O in NO group), from −13 to −23 ppm (O in OCH3 group), and from −12 to −22 ppm (Cl). For the remaining heteroatoms in adducts, the corresponding Δδ values ranged from −22 to +8.2 ppm (N), from +3 to +58 ppm (O) and from +6 to +51 ppm (Cl). The Δδ(1H) usually did not exceed 1 ppm, whereas Δδ(13C) varied from ca. −1 to +7 ppm. Some trends useful for the determination of the complexation site were extracted from calculated data sets. Theoretical findings were applied to analyse experimental NMR data.