Design, synthesis, structural, spectral, and redox properties and phenoxazinone synthase activity of tripodal pentacoordinate Mn() complexes with impressive turnover numbers

• Published in: Dalton transactions : an international journal of inorganic chemistry 2021-11, Vol.5 (45), p.1661-16612
• Main Authors: Kumbhakar, Sadananda, Giri, Bishnubasu, Muley, Arabinda, Karumban, Kalai Selvan, Maji, Somnath
• Format: Article
• Language: English
• Subjects: Aminophenol; Catalytic oxidation; Catechol; Coordination Complexes - chemistry; Coordination compounds; Crystallography, X-Ray - methods; Electron Spin Resonance Spectroscopy - methods; Hydrogen peroxide; Infrared spectroscopy; Kinetics; Manganese; Manganese Compounds - chemistry; Molecular Structure; NMR; Nuclear magnetic resonance; Oxidation; Oxidation-Reduction; Oxidoreductases - chemistry; Oxidoreductases - metabolism; Oxygen; Quinones; Single crystals; Spectra; Spectroscopic analysis; Spectrum analysis; Substrates; Synthesis; Transition metal compounds
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d1dt01925b

Catechol oxidase (CO) and phenoxazinone synthase (PHS) are two enzymes of immense significance due to their capability to oxidize catechols and o -aminophenols to o -quinones and phenoxazinones, respectively. In this connection two mononuclear manganese complexes with the molecular framework [Mn II ( L n )Cl]Cl { L 1 : tris((1 H -benzo[ d ]imidazol-2-yl)methyl)amine; n = 1 and L 2 : tris( N -methylbenzimidazol-2-ylmethyl)amine; n = 2} have been designed to be potential catalysts for OAPH ( o -aminophenol) oxidation. Both the ligands and their corresponding metal complexes have been successfully synthesized and thoroughly characterized by different spectroscopic and analytical techniques such as FT-IR, 1 H NMR, UV-vis spectroscopy, EPR spectroscopy and ESI mass spectroscopy. The molecular structures of [Mn II ( L 1 )Cl]Cl ( 1 ) and [Mn II ( L 2 )Cl]Cl ( 2 ) have been revealed by a single-crystal X-ray diffraction study. The spectral properties and redox behaviour of both the complexes were examined. Under ambient conditions, 1 and 2 show excellent phenoxazinone synthase activity as both are very susceptible to oxidize o -aminophenol to phenoxazinone. The kinetic parameters for both complexes have been determined by analyzing the experimental spectroscopic data. The turnover numbers ( k cat value) of these two complexes are extremely high, 440 h −1 and 234 h −1 for 1 and 2 , respectively. The present report offers a thorough overview of information involving the role of the metal ions and their extent of phenoxazinone synthase mimicking activity. The oxidation of o -aminophenol to 2-amino-3 H -phenoxazine-3-one (APX) by catalytic oxidation of oxygen (O 2 ) by the reaction with transition metal complexes has been an important study for the last few decades. The current study evidently showed better performance of our synthesized Mn( ii ) complexes than all the predecessors. The plausible mechanism has been reiterated based on the experimental data via ESI-MS spectra and considering the concepts from the previously reported mechanisms involved in the formation of hydrogen peroxide (H 2 O 2 ) as an intermediate substrate is fairly indicating the involvement of molecular oxygen in the catalytic cycle. Two mononuclear pentacoordinate manganese complexes with the molecular framework [Mn II ( L n )Cl]Cl have been synthesized and thoroughly characterized. Under ambient conditions, both the complexes show excellent phenoxazinone synthase activity.