In situ complexation of rhodium(II) tetracarboxylates with some derivatives of cysteine and related ligands studied by 1H and 13C nuclear magnetic resonance spectroscopy

Abstract

The complexation of N-phthaloyl, N-formyl, and N,N-dimethyl derivatives of S-methylcysteine methyl ester (both racemic and optically pure) with three dimeric rhodium(II) salts, acetate Rh2AcO4, trifluoroacetate Rh2TFA4, and (R)-(+)-α-methoxy-α-trifluoromethylphenylacetate Rh2Mosh4 was investigated by nuclear magnetic resonance spectroscopy (NMR) at room and lower temperatures. The complexation was carried out in situ, in CDCl3 solution using titration procedure; the results were examined by the analysis of 1H and 13C NMR chemical shift change (Δδ). The complexation of free S-methyl cysteine and hydrochloride salt of its methyl ester was performed in D2O solution. For comparison, complexation of some derivatives of leucine, phenylalanine, and proline was examined.N-phthaloyl and N-formyl derivatives of cysteine formed 1 : 1 and 1 : 2 axial complexes with all dirhodium salts. Rhodium substrates were bonded via sulfur. In one case, the complexation of Rh2TFA4 by both sulfur and N-formyl oxygen was noted. Similar complexation of Rh2TFA4, via CHO group, was found for N-formyl derivatives of leucine, phenylalanine, and proline. For N,N-dimethyl derivative of cysteine, both N and S atoms were involved in bonding. At room temperature, in all cases, ligand exchange was fast on the NMR timescale.