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 (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.

Synthesis, spectroscopic, and structural characterization of mixed thioether–benzimidazole copper complexes

“…X-ray quality crystals of this complex were obtained by slow evaporation of a concentrated ethanolic solution in the trigonal space group R-3. The solid state structure is similar to that determined for the related complex [L 4-CuCl]ClO 4 [6], with a square pyramidal coordination geometry around the cupric ion (Fig. 2).…”

“…In this context, we recently reported a series of five-and six-member chelating tripodal ligands, with the six-member thioether-amine copper chelates displaying reactivity patterns indicative of an incipient sulfur radical cation character during oxidation of the cuprous complexes [5]. As an extension to this work, we herein present the electrochemical and structural characterization of the thioether rich mixed-donor N 2 S 2 and N 3 S ligands (L 1 -L 3 in Scheme 1), and compare it with the previously reported L 4 [6].…”

“…This bond length is in the short end of terminal Cu-I distances, which have been reported in the range of 2.52-2.73 Å [11,13]. Although the long Cu1-N3 distance of 2.593(5) Å to the tertiary amine N3 donor in the apical site can be considered long relative to that in related complexes (2.21-2.41 Å) [11,13b,14], it is not unusual with this type of tripodal ligands, most of which lack a Cu-N(amine) interaction [6]. The angles between the donor atoms and the Cu + center on the trigonal plane are close to 120°, as expected.…”

“…Its preparation is analogous to its arylthioether analog L 4 , which we reported recently [6]. It consists of the reaction of (2-methylthio)ethylamine with two equiv.…”

Synthesis of two nickel (II) complexes bearing pyrrolide-imine ligand and their catalytic effects on thermal decomposition of ammonium perchlorate

“…In recent years, the investigations on the synthesis of ferrocene-based heterocyclic compounds have been reported frequently due to their potential applications as electrochemical sensors [5][6][7][8][9][10][11][12], fluorescent probe [13,14], metal complexes [15] and heterogeneous catalysts [16,17]. Therefore, the integration of one or more ferrocene units into a heterocyclic compound has been recognized as an attractive way to entitle a molecule with new functions.…”