Effect of ligand substituents on nickel and copper [N4] complexes: electronic and redox behavior, and reactivity towards protons

Abstract

This paper describes the synthesis, and electronic and redox properties of three nickel(II) and two copper(II) complexes, namely, [Ni(L¹)(Cl)₂]·2H₂O (1), [Ni(L²)(Cl)₂]·H₂O (2), [Cu(L²)(ClO₄)]ClO₄ (3), [Ni(L³)(Cl)₂]·H₂O (4), and [Cu(L³)(ClO₄)]ClO₄·2H₂O (5). Ligands L¹, L² and L³ each contain a 1,2-bis((pyridine-2-ylmethyl)-azaneyl)ethane framework decorated with electron donating methyl and methyl/methoxy substituents. We observe a correlation between the electron donating capability of the ligands (L¹ < L² < L³) and the redox potential for the M^II/M^I couple (1 > 2 > 4 and 3 > 5). After thorough identification of the spectroscopic and redox properties of 1–5, the reactivity of the complexes was investigated towards proton and water reduction by means of electro and photocatalysis. These species seemingly support H₂ evolution and display tunable catalytic properties achieved through the presence of differentiated electron donicity. Evidence of degradation with nanoparticle conversion was found to depend on the nature of both ancillary ligand and metal center.