Adducts of rhodium(II) tetraacylates with methionine and its derivatives: 1H and 13C nuclear magnetic resonance spectroscopy and chiral recognition

Abstract

Complexation of rhodium(II) dimeric tetraacylates: tetraacetate Rh 2AcO 4, tetratrifluoroacetate Rh 2TFA 4 , and ( S)-Mosher’s acid salt Rh 2MTPA 4 with both enantiomerically pure and racemic methionine and its derivatives: hydrochloric salt of methionine, hydrochloric salt of methionine methyl ester, N-formyl methionine, N-phthaloyl methionine, N-phthaloyl methyl ester of methionine, and methyl ester of N, N-dimethylmethionine has been investigated by means of 1H and 13C nuclear magnetic resonance ( 1H and 13C NMR) and absorption electronic spectroscopy in the visible range. Complexation processes were investigated in D 2O or CDCl 3 solutions, depending on the ligands’ and rhodium salts’ solubilities. Some supporting measurements were performed in the solid phase, using 13C and 15N CPMAS NMR techniques. All ligands investigated form 1:1 and 1:2 adducts in the solution, depending on the rhodium salt to ligand molar ratios. The complexation site in the ligands (S atom) was deduced on the basis of the NMR parameter adduct formation shift (Δ δ = δ adduct − δ ligand) and calculated chemical shifts (DFT, NMR GIAO). In the cases of the Rh 2TFA 4 and Rh 2MTPA 4 adducts, decreasing the temperature within the range 220–254 K slowed down the ligand exchange and allowed us to observe the signals of all diastereoisomers in the 1H and 13C NMR spectra.