NMR studies on interaction of rhodium(II) tetratrifluoroacetate with the ligands containing nitrile, isonitrile, isothiocyanate or isocyanate functional groups

The complexation in CDCl3 solution of rhodium(II) tetratrifluoroacetate Rh4TFA4 and tetraacetate Rh2AcO4 with ligands having nitrile, isonitrile, isothiocyanate and isocyanate functional groups has been investigated by means of 1H, 13C and 15N nuclear magnetic resonance (NMR) and absorption spectroscopy in the visible range.Dimeric rhodium(II) salts in CDCl3 solution form 1:1 or/and 1:2 axial adducts with nitriles (1) via nitrogen and isonitriles (2) via carbon atom. The complexation decreases NMR shielding of the atom bonded to rhodium salt (C or N atoms), by several dozen ppm. According to NMR results, DFT energy calculation and UV/vis spectroscopy the complexation via sulfur atom is preferred in case of isothiocyanates 3 and 4. In contrast, the energy calculations performed for isocyanate 5 suggest the complexation by nitrogen atom to be more stable. Spectroscopic data did not provide unambiguous answer in this case.The calculations satisfactory reproduce known from X-ray structural parameters of adducts and allow to predict experimental 13C and 15N NMR chemical shifts qualitatively.