High phenoxazinone synthase activity of two mononuclear cis-dichloro cobalt(ii) complexes with a rigid pyridyl scaffold

Abstract

Phenoxazinone synthase (PHS) is a pentanuclear copper enzyme studied for the last two decades, and though both manganese and copper complexes have been explored extensively, there are only a handful of reports on cobalt mononuclear moieties. In this regard, we have successfully synthesized two cis dichloro cobalt(II) mononuclear complexes Co(bpy)₂Cl₂:[1] and Co(L¹)₂Cl₂:[2] with polypyridyl ligands (where bpy = 2,2′-bipyridine, L¹ = 2,2′-(1-methyl-1H-1,2,4-triazole-3,5-diyl)dipyridine) and thoroughly characterized them by different spectroscopic and analytical techniques such as FT-IR, ¹H NMR, UV-vis spectroscopy and ESI mass spectra. The molecular structures of [1] and [2] are elucidated by single-crystal X-ray diffraction studies. Spectral properties and redox behaviour for both the complexes are examined. The complexes show phenoxazinone synthase activity under ambient conditions and thorough examination of the acquired spectroscopic data imply excellent reactivity for both. Kinetic parameters for both the complexes are evaluated and the turnover numbers (k_cat value) of these two complexes are 201.24 h⁻¹ and 249.57 h⁻¹ for [1] and [2], respectively, which are quite high compared to those of previously reported similar complexes. The possible mechanistic route is also established with the help of mass spectroscopy and titrimetric analysis is performed to detect hydrogen peroxide that is a key intermediate formed in the catalysis and thus to prove the involvement of molecular oxygen in the oxidation. This report offers an in-depth summary of metal ion involvement and the extent of the phenoxazinone synthase activity.