Copper(I) Chelation by Tetradentate NSSN Donor Ligands: Reversible Oxidation but no Oxygenation

Abstract

The copper(I) complexes [Cu(L4)](ClO4), L4 = 1,6‐bis(N‐methyl‐1H‐benzimidazol‐2‐yl)‐2,5‐dithiahexane (bmbdh) or 1,7‐bis(N‐methyl‐1H‐benzimidazol‐2‐yl)‐2,6‐dithiaheptane (bmdhp) were obtained and investigated through structure analysis (bmdhp compounds), cyclic voltammetry, UV/Vis/NIR and EPR spectroelectrochemistry. Structure comparison of the complexes [Cu(bmdhp)](ClO4) (1) and [Cu(bmdhp)](SO3CF3) (2) with the previously reported [CuI(bdhp)(H2O)](PF6)0.66(BF4)0.34 (4), bdhp = 1,7‐bis(1H‐benzimidazol‐2‐yl)‐2,6‐dithiaheptane, with the related [CuI(mmb)2](BF4) (5), mmb = 1‐methyl‐2‐methylthiomethyl‐1H‐benzimidazole, with the oxidised [CuII(bmdhp)(H2O)](ClO4)2 (7), and with the zinc(II) species [Zn(bmbdh)(H2O)(SO3CF3)](SO3CF3) (8) and {[Zn(μ‐bmdhp)(H2O)(SO3CF3)]2}(SO3CF3) (9) reveal a variety of coordination numbers, L4 ligand coordination modes, M–N bond lengths and especially N–M–N angles, affecting the accessibility of the central metal ions. Electronic consequences as evident from oxidation potentials and from EPR and absorption spectra of the copper(II) forms are discussed. The lack of reactivity of the copper(I) compounds towards O2 is attributed to steric shielding and to electronic effects from two π accepting thioether donors.

From Cyclen to 12‐Crown‐4 Copper(II) Complexes: Exchange of Donor Atoms Improves DNA Cleavage Activity

“…However, differences in the electron‐transfer process have to be considered: while the electrochemical reduction is due to an outer‐sphere process, the reduction of oxygen by the Cu I species that leads to ROS is due to an inner‐sphere electron‐transfer process. This means that activity in DNA cleavage does not necessarily have to correlate with reduction potentials 41. Nevertheless, a slight correlation between the number of O atoms, cleavage activities and redox potentials between complexes 1 – 4 and 6 cannot be denied.…”

Plasticity in the copper–thioether bond: Manifestation in blue Cu proteins and in synthetic analogs

“…Soon after the discovery of the relevance of Cu-thioether bonds in blue copper proteins, coordination chemists [27][28][29][30][31][32][33][34][35][36], including our laboratory, started to design compounds where the thioether group was allowed to bind to Cu(II). For easy synthesis, initially benzimidazole-based ligands were used, later followed by synthetically less-easily accessible imidazole [35,37].…”

Straightforward approach to efficient oxidative DNA cleaving agents based on Cu(ii) complexes of heterosubstituted cyclens

“…Electrochemistry, however, displays an outersphere electron transfer process, whereas the reduction of O 2 generating ROS must be attributed to inner sphere electron transfer. 23 Therefore, activity in DNA cleavage does not necessarily correlate with redox potentials. The attenuated cleavage activity of 1 might be explained by hydroxo-bridged dimer formation 16 which blocks the coordination sites of Cu(II) leaving no space for coordination of reducing agents, thus inhibiting the reduction process and the release of Cu(I).…”