Ligand dechelation effect on a [Co(tpy)2]2+ scaffold towards electro-catalytic proton and water reduction

Abstract

This study presents the synthesis of the 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)quinoline (4Ql-tpy) ligand and its corresponding cobalt complexes, i.e. [Co(4Ql-tpy)₂]Cl₂ and [Co(4Ql-tpy)₂](PF₆)₃. The [Co(4Ql-tpy)₂]Cl₂ complex has been employed for electro-catalytic reduction of protons in DMF/H₂O (95 : 5 v/v) as well as in phosphate buffer (pH 4.0–7.0). The spectroscopic investigations reveal the dissociation of one of the 4Ql-tpy ligands from the [Co(4Ql-tpy)₂]Cl₂ complex in the presence of excess AcOH in organic medium. However, in phosphate buffer it is evident that the dechelation of one of the peripheral pyridine rings followed by the protonation of pyridine-N occurs with the subsequent coordination of one water molecule. The aqueous electrochemical studies reveal that the Co(I) species is dependent on the proton concentration and the gradient for potential vs. pH indicates that a PCET (1H⁺/1e⁻) is involved between [Co^II(4Ql-tpy)(²4Ql-tpy)(OH₂)]²⁺ and [Co^I(4Ql-tpy)(²4Ql-tpyH)(OH₂)]²⁺ in the pH range 2.8 to 8.0. The plausible mechanism has been described based on the spectroscopic and electrochemical investigations.