Two mononuclear cobalt(III) complexes exhibiting phenoxazinone synthase activity

Abstract

ClO 4 ⋅MeOH (1) (tmtp = tri(m-tolyl)phosphine) and [LCo(PPh 3 )(H 2 O)]PF 6 (2), have been prepared from a polydentate ligand, N,N′-bis(3methoxysalicylidehydene)cyclohexane-1,2-diamine (H 2 L). Standard analytical techniques such as elemental analysis and UV-visible and Fourier transform infrared spectroscopies were used to characterize both complexes. The solidstate molecular structures of both complexes were confirmed from single-crystal X-ray diffraction analysis. Structural analyses show that the Co(III) ion occupies the centre of a distorted octahedron in a complex cation: [LCo(tmtp) (H 2 O)] + and [LCo(PPh 3 )(H 2 O)] + for 1 and 2, respectively. Phenoxazinone synthase activities of both complexes were screened. Kinetic studies and other experimental observations reveal that the reaction follows rate saturation kinetics and proceeds through the formation of a catalyst (complex)-substrate adduct. The turnover number (K cat ) of complex 2 is 54.07 h −1 , exhibiting better catalytic activity compared to 1 (K cat = 45.11 h −1 ). KEYWORDS cobalt(III) complex, phenoxazinone synthase activity, Schiff base, single-crystal X-ray diffraction, turnover number | INTRODUCTIONSchiff bases cover an enormous area in coordination chemistry predominantly due to their smooth synthesis, controllability of steric and electronic environment and diversity in applications. [1] Schiff bases can easily accommodate single or multiple metal ions of various oxidation states. [2] Judicial selection of Schiff bases and metal ions often results in homo-or heteronuclear metal complexes including those of transition metals and main group elements. [3] Depending on the nature of the ligands and metal ions, these complexes exhibit structural diversities and open up applications in several areas such as biology, [4] catalysis, [5] materials science [6] and magnetism. [7] Interestingly, salen-type ligands, which are prepared from various diamines and salicylaldehyde derivatives, are most widely explored among all Schiff bases in coordination chemistry. [8] Modification of salicylaldimine ligands, for example by anchoring a methoxy group (-OMe) on the salicylaldimine moiety, increases their denticity making the ligands more prone to form polynuclear complexes. [9] Numerous examples of metal complexes of Schiff base ligands have been reported in the literature. However, metal complexes containing cobalt(II/III) ions have recently started to receive attention. [10] Cobalt(II/ III) complexes of Schiff base ligands find important applications in magnetism, [11] biochemistry [12] and catalysis. [13] These complexes have been used in mimicking important biological co-factors such as phenoxazinone synthase, [14] photosynthesis, [15] methionine aminopeptidase, [16] cobalamin, etc. Among these, phenoxazinone synthase facilitates the conversion of o-aminophenol