Cobalt(ii), nickel(ii) and copper(ii) complexes of a hexadentate pyridine amide ligand. Effect of donor atom (ethervs. thioether) on coordination geometry, spin-state of cobalt and MIII–MII redox potential

Abstract

Using an acyclic hexadentate pyridine amide ligand, containing a –OCH₂CH₂O– spacer between two pyridine-2-carboxamide units (1,4-bis[o-(pyrydine-2-carboxamidophenyl)]-1,4-dioxabutane (H₂L⁹), in its deprotonated form), four new complexes, [Co^II(L⁹)] (1) and its one-electron oxidized counterpart [Co^III(L⁹)][NO₃]·2H₂O (2), [Ni^II(L⁹)] (3) and [Cu^II(L⁹)] (4), have been synthesized. Structural analyses revealed that the Co^II centre in 1 and the Ni^II centre in 3 are six-coordinate, utilizing all the available donor sites and the Cu^II centre in 4 is effectively five-coordinated (one of the ether O atoms does not participate in coordination). The structural parameters associated with the change in the metal coordination environment have been compared with corresponding complexes of thioether-containing hexadentate ligands. The μ_eff values at 298 K of 1–4 correspond to S = 3/2, S = 0, S = 1 and S = 1/2, respectively. Absorption spectra for all the complexes have been investigated. EPR spectral properties of the copper(II) complex 4 have been investigated, simulated and analyzed. Cyclic voltammetric experiments in CH₂Cl₂ reveal quasireversible Co^III–Co^II, Ni^III–Ni^II and Cu^II–Cu^I redox processes. In going from ether O to thioether S coordination, the effect of the metal coordination environment on the redox potential values of Co^III–Co^II (here the effect of spin-state as well), Ni^III–Ni^II and Cu^II–Cu^I processes have been systematically analyzed.