Schiff Base Ligands Derived from l-Histidine Methyl Ester: Characterization, Racemization, and Dimerization of Their Transition-Metal Complexes

Reactions of methyl (S)-2-[(2-hydroxybenzylidene)amino]-3-(1H-imidazol-4-yl)propanoate (S-H₂L^Me) and various transition-metal(II) salts (MX₂: M = Mn, Fe, Co, and Ni; X = ClO₄, Cl, OAc, etc.) are examined. During the reactions with Mn^II and Co^II salts, the M^II ion is oxidized by air to form a cationic complex with two mono-deprotonated ligands, [M^III(S-HL^Me)₂]X [M, X = Mn, ClO₄ (S-1ClO₄); Co, ClO₄ (S-3ClO₄); and Co, Cl (S-3Cl)], where (S-HL^Me)^– is coordinated to a M^III center in a tridentate κ³O,N,N′ mode with a mer configuration. In contrast, the Ni^II ion is not oxidized; only one of the ligands is deprotonated to form a similar monocationic complex, [Ni^II(S-HL^Me)(S-H₂L^Me)]PF₆ (S-6PF₆), which forms a dimer structure by double hydrogen bonds between the coordinating phenoxide and phenol groups in the crystal. Interestingly, the above complexes show slow racemization of the ligand, as confirmed by circular dichroism spectroscopy. Racemic Ni^II complexes of [Ni(HL^Me)(H₂L^Me)](PF₆ or Cl) (rac-6PF₆ or rac-6Cl) are isolated and their crystal structures are confirmed by X-ray diffraction analysis. In the case where Co(OAc)₂ is used as the metal(II) salt, a new ligand dimerization reaction takes place to afford a complex of [Co(H₂LL^Me)]Cl (7Cl, H₄LL^Me = methyl (E)-3-{[3-(1H-imidazol-4-yl)-1-methoxy-1-oxopropano-2-yl]amino}-2-[(1H-imidazol-4-yl)methyl]-2-[(2-hydroxybenzylidene)amino]-3-(2-hydroxyphenyl)propanoate).