Structural, spectroscopic, and electrochemical properties of tri- and tetradentate N3 and N3S copper complexes with mixed benzimidazole/thioether donors

Abstract

Cupric and cuprous complexes of bis(2-methylbenzimidazolyl)(2-methylthiophene)amine (L¹), bis(2-methylbenzimidazolyl)benzylamine (L²), bis(2-methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)amine (L³), bis(1-methyl-2-methylbenzimidazolyl)benzylamine (Me₂L²), and bis(1-methyl-2-methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)amine (Me₂L³) have been spectroscopically, structurally, and electrochemically characterised. The thioether-containing ligands L³ and Me₂L³ 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 Cu²⁺ complexes [L¹CuCl₂] (1), [L²CuCl₂] (2) and [Me₂L³CuCl]ClO₄ (3^Me,ClO4) are monomeric in solution according to ESI mass spectrometry data, as well as in the solid state. Their Cu⁺ analogues [L¹Cu]ClO₄, [L²Cu]ClO₄, [L³Cu]ClO₄ (4–6), [BOC₂L¹Cu(NCCH₃)]ClO₄ (4^BOC), [Me₂L²Cu(NCCH₃)₂]PF₆ (5^Me) and [Me₂L³Cu]₂(ClO₄)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-Cu²⁺/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 N₃ or N₃S 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 [Me₂L²Cu(NCCH₃)₂]⁺ (5^Me) and [Me₂L³Cu]⁺ (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 O₂˙⁻ than 5^Me, lending evidence to the hypothesis of the modulating properties of thioether ligands in Cu–O₂ reactions.