Tetranuclear NiII-Mannich base complex with oxygenase, water splitting and ferromagnetic and antiferromagnetic coupling properties

Abstract

A new Mannich base (2-(4-(2-hydroxy-3-methoxy-5-methylbenzyl)-piperazin-1-yl)methyl)-6-methoxy-4-ethylphenol (H₂L) and its tetranuclear Ni^II complex [Ni₄L₂(μ_1,1-Cl)₂(H₂O)₄]Cl₂ (compound 1) are characterised using single-crystal X-ray diffraction measurements. Compound 1 contains four different Ni^II centres in a rhombus-like structure. Two Ni atoms (Ni1 and Ni2) have a NiN₂O₄ coordination sphere, while the other two (Ni3 and Ni4) have a NiO₄Cl₂ coordination environment and Ni–Cl–Ni bridges connect them. Ni1 and Ni2 are linked to Ni3 and Ni4 by phenoxide bridges. Variable temperature magnetic susceptibility measurements of 1 indicate the presence of alternating antiferromagnetic coupling (J₁ = −6.6(1) cm⁻¹) through the phenoxide bridge along the sides of the rhombus and ferromagnetic coupling (J₂ = 8.4(1) cm⁻¹) through the double Cl bridge along the short diagonal of the rhombus with a zero-field splitting of |D| = 2.7(1) cm⁻¹. Compound 1 shows oxidase (catecholase-like and phenoxazinone synthase-like) activity. The oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) gives 3,5-di-tert-butylquinone (3,5-DTBQ) and H₂O₂ and the oxidation of o-aminophenol (OAP) produces 2-aminophenoxazin-3-one (APX) and H₂O with turnover numbers of 28.32 and 17.52 h⁻¹, respectively, under aerobic conditions. A mechanism for the oxidase activity catalysed by compound 1 is proposed in line with ESI-mass spectrometry, EPR spectroscopy, and electrochemical data. The reaction involves the cleavage of the tetranuclear Ni₄-core to form a mononuclear Ni^II complex in the presence of the substrate (3,5-DTBC/OAP). This Ni^II complex is reduced to Ni^I with the concomitant oxidation of the substrate (3,5-DTBQ/APX). Formation of a radical intermediate is confirmed using EPR. In the catecholase-like activity, O₂ is reduced to H₂O₂ while in the phenoxazinone synthase-like activity O₂ produces H₂O. Compound 1 participated in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in a strongly basic medium with an onset potential of 418 mV and a Tafel slope of 121 mV dec⁻¹ for OER and an onset potential of 477 mV and Tafel slope of 146 mV dec⁻¹ for HER.