Synthesis, structure and phenoxazinone synthase-like activity of three unprecedented alternating CoII–CoIII 1D chains

Abstract

Three new one-dimensional mixed valence coordination polymers {[Co^III(L¹)₂][Co^II(bpy)₂]·ClO₄·H₂O}_∞ (1), {[Co^III(L¹)₂][Co^II(phen)₂]·ClO₄·H₂O}_∞ (2) and {[Co^III(L²)₂][Co^II(bpy)(CH₃OH)₂]·ClO₄·H₂O}_∞ (3) (H₂L¹ = N-salicylidene-L-alanine, H₂L² = N-salicylidene-L-phenylalanine, bpy = 2,2′-bipyridine and phen = 1,10-phenanthroline) have been synthesized. Structural analysis reveals that the hexa-coordinated Co^III unit, [Co^III(L¹)₂]⁻, formed by two binegative tridentate Schiff base ligands contains uncoordinated oxygen atoms in the pendent carboxylate arm which further link the neighboring [Co^II(bpy)₂]²⁺ units to produce the μ_1,3syn–anti carboxylato bridged one-dimensional (1D) coordination polymer in complex 1. Complexes 2 and 3 feature similar syn–anti carboxylato bridged one dimensional coordination polymers formed by joining of alternating Co^II and Co^III units. Using o-aminophenol (OAPH) as the substrate, the phenoxazinone synthase-like activity of the complexes has been studied and the turnover numbers (k_cat) have been calculated to be 1.2, 11.5, and 2.7 h⁻¹ for 1–3, respectively. The mechanism of phenoxazinone synthase-like activity is proposed on the basis of mass spectral analysis, suggesting the cooperativity of Co^II and Co^III centres, i.e., the Co^II centre binds to the incoming substrate molecule, whereas the Co^III centre is involved in electron transfer from the substrate to molecular oxygen.