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.