Defining the oxidation site in Cu(ii) and Ni(ii) complexes of ligands combining amido, phenolato and pyridine donors.

Abstract

Three ligands H₃L₁, H₂L₂ and H₂L₃ and their Cu(II) and Ni(II) complexes were prepared. All feature one pyridine and one phenolato donor, one phenylenediamide bridge and differ by the connecting function: Bis(amidate) (1_M⁻), amidophenolate/iminopyridine (2_M), salicylidene/amidopyridine (3_M), where M = Cu, Ni. The metal ion is essentially square planar, while the coordination bond distances are marginally affected by the linker. In contrast, the redox potential of the oxidative processes (1^st reversible wave) are spanning over a 0.62 V window and follow the order: 1_M⁻ < 2_M < 3_M. The oxidation process is mostly ligand-centered, affording X-band EPR-silent Cu(II)-radical, bis(Ni(II)-radicals), or (S = ½) systems (Ni(II)-phenoxyl or diiminosemiquinone). For the monomers 2_Ni⁺ and 3_Ni⁺ the g_iso of 2.009 and 2.012 deviates from the value of free radicals, due to significant Mulliken spin polulation on the Ni atom (0.12, 0.39). 3_Ni⁺ has a very high propensity to bind exogeneous ligands to form penta or hexa-coordinated Ni(III) species due to the large Mulliken spin population on the metal centre, which can be regarded as a natural predisposition to exist as high valent metal. In contrast to the other nickel complexes, the oxidation of 1_Ni⁻ results in EPR-silent species, which might be explained by dimer formation. Finally, the copper complexes exhibits an activity towards benzyl alcohol oxidation: A chemioselectivity is achieved, whereby 1_Cu⁻ mostly affords benzoate in contrast to 3_Cu, which mostly produced benzaldehyde. Complex 2_Cu exhibits an intermediate behaviour.