Synthesis, crystal structures, magnetic properties and catecholase activity of double phenoxido-bridged penta-coordinated dinuclear nickel (II) complexes derived from reduced Schiff-base ligands: mechanistic inference of catecholase activity

Three double phenoxido-bridged dinuclear nickel (II) complexes, namely [Ni₂(L¹)₂(NCS) ₂] (1), [Ni₂(L²)₂(NCS) ₂] (2) and [Ni₂(L³)₂(NCS) ₂] (3) have been synthesized using the reduced tridentate Schiff-base ligands 2-[1-(3-methylamino-propylamino)-ethyl]-phenol (HL¹), 2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol (HL²) and 2-[1-(3-dimethylamino-propylamino)-ethyl]-phenol (HL³), respectively. The coordination compounds have been characterized by X-ray structural analyses, magnetic-susceptibility measurements and various spectroscopic methods. In all complexes, the nickel (II) ions are penta-coordinated in a square-pyramidal environment, which is severely distorted in the case of 1 (Addison parameter τ = 0.47) and 3 (τ = 0.29), while it is almost perfect for 2 (τ = 0.03). This arrangement leads to relatively strong antiferromagnetic interactions between the Ni (II) (S = 1) metal centers as mediated by double phenoxido bridges (with J values of −23.32 (1), −35.45 (2) and −34.02 (3) cm³ K mol^–1, in the convention H = −2JS₁S₂). The catalytic activity of these Ni compounds has been investigated for the aerial oxidation of 3,5-di-tert-butylcatechol. Kinetic data analysis following Michaelis–Menten treatment reveals that the catecholase activity of the complexes is influenced by the flexibility of the ligand and also by the geometry around the metal ion. Electrospray ionization mass spectroscopy (ESI-MS) studies (in the positive mode) have been performed for all the coordination compounds in the presence of 3,5-DTBC to characterize potential complex–substrate intermediates. The mass-spectrometry data, corroborated by Electron Paramagnetic Resonance (EPR) measurements, suggest that the metal centers are involved in the catecholase activity exhibited by the complexes