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

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 complex [NiL(μ-Cl)(HO)]Cl (compound 1) are characterised using single-crystal X-ray diffraction measurements. Compound 1 contains four different Ni centres in a rhombus-like structure. Two Ni atoms (Ni1 and Ni2) have a NiNO coordination sphere, while the other two (Ni3 and Ni4) have a NiOCl 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 ( = -6.6(1) cm) through the phenoxide bridge along the sides of the rhombus and ferromagnetic coupling ( = 8.4(1) cm) through the double Cl bridge along the short diagonal of the rhombus with a zero-field splitting of || = 2.7(1) cm. Compound 1 shows oxidase (catecholase-like and phenoxazinone synthase-like) activity. The oxidation of 3,5-di--butylcatechol (3,5-DTBC) gives 3,5-di--butylquinone (3,5-DTBQ) and HO and the oxidation of -aminophenol (OAP) produces 2-aminophenoxazin-3-one (APX) and HO with turnover numbers of 28.32 and 17.52 h, 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 Ni-core to form a mononuclear Ni complex in the presence of the substrate (3,5-DTBC/OAP). This Ni complex is reduced to Ni 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, O is reduced to HO while in the phenoxazinone synthase-like activity O produces HO. 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 for OER and an onset potential of 477 mV and Tafel slope of 146 mV dec for HER.