Synthesis, characterization, crystal structure, electrochemical properties and electrocatalytic activity of an unexpected nickel(II) Schiff base complex derived from bis(acetylacetonato)nickel(II), acetone and ethylenediamine

Abstract

The synthesis, crystal structure and electrochemical properties of a Ni(II) Schiff base complex, [Ni(L)]PF₆ (where L is 2,4,9,11,11-pentamethyl-2,3,4 triaza-1-one-4-amine) are reported herein. The complex has been characterized by its electrochemical behavior, X-ray crystallographic structural analysis, physio-chemical methods and spectroscopic techniques. Electrospray mass spectroscopic analysis gives a dominant ion peak with m/z = 296 which corresponds to the {[Ni(L)]PF₆–HPF₆}⁺ fragment. Cyclic voltammograms for [Ni(L)]PF₆, obtained in DMF (0.1 M Bu₄NPF₆) at a glassy carbon electrode with a scan rate of 100 mV s⁻¹, exhibit reversible ([Ni^II(L)]⁺/[Ni^I(L)]) reduction and chemically irreversible ([Ni^II(L)]⁺/[Ni^III(L)]²⁺→ electroactive product) oxidation processes at −2.05 and 0.62 V, respectively. The diffusion coefficient, calculated using the Randles–Sevcik relationship, is 9.7 × 10⁻⁶ cm² s⁻¹. Electrochemical studies reveal that the Ni^I reduced form of the complex is capable of catalyzing CO₂ reduction at a potential that is thermodynamically more favorable than for the reduced [Ni(N,N′-ethylenebis(acetylacetoneiminato)]complex. Spectroelectrochemical analyses following bulk electrolysis of [Ni(L)]PF₆ under CO₂ revealed the formation of oxalate and bicarbonate.