Cupric and cuprous complexes of bis(2-methylbenzimidazolyl)(2-methylthiophene)amine (L(1)), bis(2-methylbenzimidazolyl)benzylamine (L(2)), bis(2-methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)amine (L(3)), bis(1-methyl-2-methylbenzimidazolyl)benzylamine (Me(2)L(2)), and bis(1-methyl-2-methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)amine (Me(2)L(3)) have been spectroscopically, structurally, and electrochemically characterised. The thioether-containing ligands L(3) and Me(2)L(3) give rise to complexes with Cu-S bonds in solution and in the solid state, as evidenced by UV-vis spectroscopy and X-ray crystallography. The Cu(2+) complexes [L(1)CuCl(2)] (1), [L(2)CuCl(2)] (2) and [Me(2)L(3)CuCl]ClO(4) (3(Me,ClO4)) are monomeric in solution according to ESI mass spectrometry data, as well as in the solid state. Their Cu(+) analogues [L(1)Cu]ClO(4), [L(2)Cu]ClO(4), [L(3)Cu]ClO(4) (4-6), [BOC(2)L(1)Cu(NCCH(3))]ClO(4) (4(BOC)), [Me(2)L(2)Cu(NCCH(3))(2)]PF(6) (5(Me)) and [Me(2)L(3)Cu](2)(ClO(4))(2) (6(Me)) are also monomeric in acetonitrile solution, as confirmed crystallographically for 4(BOC) and 5(Me). In contrast, 6(Me) is dimeric in the solid state, with the thioether group of one of the ligands bound to a symmetry-related Cu(+) ion. Cyclic voltammetry studies revealed that the bis(2-methylbenzimidazolyl)amine-Cu(2+)/Cu(+) systems possess half-wave potentials in the range -0.16 to -0.08 V (referenced to the ferrocenium-ferrocene couple); these values are nearly 0.23 V less negative than those reported for related bis(picolyl)amine-derived ligands. Based on these observations, the N(3) or N(3)S donor set of the benzimidazole-derived ligands is analogous to previously reported chelating systems, but the electronic environment they provide is unique, and may have relevance to histidine and methionine-containing metalloenzymes. This is also reflected in the reactivity of [Me(2)L(2)Cu(NCCH(3))(2)](+) (5(Me)) and [Me(2)L(3)Cu](+) (6(Me)) towards dioxygen, which results in the production of the superoxide anion in both cases. The thioether-bound Cu(+) centre in 6(Me) appears to be more selective in the generation of O(2)˙(-) than 5(Me), lending evidence to the hypothesis of the modulating properties of thioether ligands in Cu-O(2) reactions.

Copper(II) complexes (1–4) of tri‐ or tetradentate bis(2‐methylbenzimidazolyl)amine ligands (L1–L4) have been prepared and characterized by spectroscopic methods in solution, as well as in the solid state by X‐ray crystallography. The ligands act as tridentate donors towards the cupric ions through one central amine and two benzimidazole N atoms in the solid state; a water ligand and a bridging perchlorate group define the distorted octahedral environments of 2 and 3. Complex 4 has square‐pyramidal coordination geometry, with an additional thioether donor attached to the central N atom in the axial position. Electrospray mass spectrometry characterized the complexes as monomeric in acetonitrile solution. Cyclic voltammetry studies established that amine N benzyl‐containing 3 has the highest half‐wave redox potential of all of these complexes at –0.08 V (vs. Fc+/Fc) for the Cu2+/Cu+ couple. The complexes display dose‐dependent cytotoxicity against one human and four murine cancer cell lines. Complexes 3 and 4 have good antiproliferative activity against the human chronic leukemia cell line K562. Moreover, for 3 the proliferation of all malignant cell lines decreases at concentrations lower than the IC50 for healthy bone marrow cells.