Rare azido and hydroxido bridged tetranuclear Co(ii) complexes of a polynucleating Mannich base ligand with a defect dicubane core: structures, magnetism and phenoxazinone synthase like activity

Abstract

Three novel tetranuclear cobalt(II) complexes [Co₄L₂(μ_1,1,1-N₃)₂(N₃)₂] (1), [Co₄L₂(μ₃-OH)₂(NCS)₂]·4CH₃CN (2) and [Co₄L₂(μ₃-OH)₂(NCSe)₂]·2CH₃CN (3) have been synthesized on reacting a Mannich base ligand, N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine (H₂L), with cobalt(II) acetate tetrahydrate in the presence of azide, thiocyanate and selenocynate salts, respectively. Single crystal X-ray structural analyses showed that these discrete tetranuclear Co(II) complexes possess defect dicubane cores with μ_1,1,1-N₃ (for 1) and μ₃-OH bridges (for 2 and 3). Magnetic susceptibility measurements showed prominent intramolecular ferromagnetic interactions for 1 (J₁/k_B = −19 K, J₂/k_B = 38 K) and dominant antiferromagnetic interactions for 2 and 3 (J₁/k_B = 4.7 K, J₂/k_B = −34 K for 2 and J₁/k_B = 4.4 K, J₂/k_B = −12 K for 3). All three complexes (1–3) exhibited phenoxazinone synthase-like catalytic activity towards the aerobic oxidation of o-aminophenol. The turn over numbers (K_cat) for the aerobic oxidation of o-aminophenol were calculated to be 500.4, 508.9 and 511.2 h⁻¹ for complexes 1–3, respectively. The mechanism of phenoxazinone synthase-like catalytic activity has also been proposed for these tetranuclear Co(II) catalysts with the help of mass spectral analysis.