Half-sandwich complexes of rhodium containing cysteine-derived ligands

Abstract

The modified cysteine ligand, S-benzyl-α-methyl-L-cysteine (HL2), was prepared from L-cysteine hydrochloride methyl ester. The reaction of commercial S-benzyl-L-cysteine (HL1) or HL2 with the dimer, [{(η⁵-C₅Me₅)RhCl}₂(μ-Cl)₂], gives rise to the cationic complexes, [(η⁵-C₅Me₅)RhCl(HL)]Cl (HL = HL1 (1), HL2 (2)), in which the cysteine ligand exhibits a κ²N,S coordination mode. In a basic medium, HL1 or HL2 reacts with [{(η⁵-C₅Me₅)RhCl}₂(μ-Cl)₂] to afford mixtures of two epimers at the metal centre of the neutral complexes, [(η⁵-C₅Me₅)RhCl(κ²N,O-L)] (HL = HL1 (3), HL2 (4)), in which amino carboxylate adopts a κ²N,O mode of coordination along with variable amounts of the cationic compounds, [(η⁵-C₅Me₅)Rh(κ³N,O,S-L)]Cl (HL = HL1 (6Cl), HL2 (7Cl)), which contain κ³N,O,S coordinated cysteine-derived ligands. However, in a basic medium, the N-Boc substituted cysteine S-benzyl-N-Boc-L-cysteine (HL3) only yields the κ²O,S coordinated derivative, [(η⁵-C₅Me₅)RhCl(κ²O,S-L3)] (5), as a mixture of two diastereomers depending on the configuration of the metal centre. The bidentate chelate complexes 3–5 react with AgSbF₆ to give the hexafluoroantimonates [(η⁵-C₅Me₅)Rh(κ³N,O,S-L)][SbF₆] (HL = HL1 (6Sb), HL2 (7Sb), HL3 (8Sb)) with tridentate coordination. Compound 8Sb reacts with NaHCO₃ to give the neutral complex [(η⁵-C₅Me₅)Rh(κ³N,O,S-L3_–H)] (9), which can also be prepared by reacting the dimer [{(η⁵-C₅Me₅)RhCl}₂(μ-Cl)₂] with HL3 in the presence of two equivalents of NaHCO₃. The new compounds contain up to four stereogenic centres, namely, Rh, S, N, and C. The absolute configuration of the complexes has been established by spectroscopic and diffractometric investigations, including the crystal structure determination of [(η⁵-C₅Me₅)RhCl(κ²O,S-L3)] (5), [(η⁵-C₅Me₅)Rh(κ³N,O,S-L1)][SbF₆] (6Sb), [(η⁵-C₅Me₅)Rh(κ³N,O,S-L2)][SbF₆] (7Sb) and [(η⁵-C₅Me₅)Rh(κ³N,O,S-L3_–H)] (9). Variable temperature ¹H NMR studies reveal the existence of epimerization processes and theoretical calculations were used to discriminate their nature.