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Issue 36, 2016
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Half-sandwich complexes of rhodium containing cysteine-derived ligands

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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.

Graphical abstract: Half-sandwich complexes of rhodium containing cysteine-derived ligands

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The article was received on 16 Jun 2016, accepted on 01 Aug 2016 and first published on 01 Aug 2016

Article type: Paper
DOI: 10.1039/C6DT02411D
Citation: Dalton Trans., 2016,45, 14203-14215

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    Half-sandwich complexes of rhodium containing cysteine-derived ligands

    M. Carmona, R. Rodríguez, F. J. Lahoz, P. García-Orduña, I. Osante, C. Cativiela, J. A. López and D. Carmona, Dalton Trans., 2016, 45, 14203
    DOI: 10.1039/C6DT02411D

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