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

Abstract

Reactions of methyl (S)‐2‐[(2‐hydroxybenzylidene)amino]‐3‐(1H‐imidazol‐4‐yl)propanoate (S‐H2LMe) and various transition‐metal(II) salts (MX2: M = Mn, Fe, Co, and Ni; X = ClO4, Cl, OAc, etc.) are examined. During the reactions with MnII and CoII salts, the MII ion is oxidized by air to form a cationic complex with two mono‐deprotonated ligands, [MIII(S‐HLMe)2]X [M, X = Mn, ClO4 (S‐1ClO4); Co, ClO4 (S‐3ClO4); and Co, Cl (S‐3Cl)], where (S‐HLMe)– is coordinated to a MIII center in a tridentate κ3O,N,N′ mode with a mer configuration. In contrast, the NiII ion is not oxidized; only one of the ligands is deprotonated to form a similar monocationic complex, [NiII(S‐HLMe)(S‐H2LMe)]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 NiII complexes of [Ni(HLMe)(H2LMe)](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(H2LLMe)]Cl (7Cl, H4LLMe = 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).