Unique mononuclear Mn^IIcomplexes of an end-off compartmental Schiff base ligand: experimental and theoretical studies on their bio-relevant catalytic promiscuity

Abstract

Three new mononuclear manganese(ii) complexes, namely [Mn(HL)2]·2ClO4 (1), [Mn(HL)(N(CN)2)(H2O)2]·ClO4 (2) and [Mn(HL)(SCN)2] (3) [LH = 4-tert-butyl-2,6-bis-[(2-pyridin-2-yl-ethylimino)-methyl]-phenol], have been synthesized and structurally characterized. An "end-off" compartmental ligand (LH) possesses two symmetrical compartments with N2O binding sites but accommodates only one manganese atom instead of two due to the protonation of the imine nitrogen of one compartment. Although all three complexes are mononuclear, complex 1 is unique as it has a 1 : 2 metal to ligand stoichiometry. The catalytic promiscuity of complexes 1-3 in terms of two different bio-relevant catalytic activities namely catecholase and phenoxazinone synthase has been thoroughly investigated. EPR and cyclic voltametric studies reveal that radical formation rather than metal centered redox participation is responsible for their catecholase-like and phenoxazinone synthase-like catalytic activity. A computational approach suggests that imine bond bound radical generation rather than phenoxo radical formation is most likely responsible for the oxidizing properties of the complexes.

Valence‐Tautomerism‐Driven Aromatic Nucleophilic Substitution in Cobalt‐Bound Tetrabromocatecholate Ligands – Influence of Positive Charge at the Ligand Backbone on Phenoxazinone Synthase Activity

“…The activity of 1 as a phenoxazinone synthase mimic was examined by UV/Vis spectrophotometry with o ‐aminophenol (OAP) as the substrate . Firstly, it was important to check whether the present complex could catalyze the oxidation of OAP under aerobic conditions.…”

New Artificial Biomimetic Enzyme Analogues Based on Iron(II/III) Schiff Base Complexes: An Effect of (Benz)imidazole Organic Moieties on Phenoxazinone Synthase and DNA Recognition ‡

“…By ensuring an excess of the substrate with regard to the complex, the initial rates method exhibits a first-order dependence on complex concentration and was treated with the Michaelis–Menten model, in which linearization affords a double reciprocal Lineweaver–Burk plot allowing analysis of the following parameters: the maximum velocity (V max ), binding constant (K M ), and rate constant (k cat ) (Figure 3b,c) and Figures S37–S46, Supplementary information). Indeed, rate saturation kinetics was observed, which confirms the validity of the applied kinetic model and also indicates that the reaction involves the formation of complex-substrate intermediate in a pre-equilibrium stage and its subsequent irreversible oxidation is associated as the rate-determining step of the catalytic cycle [25].…”

“…Schiff base ligands were recognized as very potent candidates for the construction of such libraries due to their robustness and stability, facile synthetic methodologies and rich complexation behaviour [15,16]. Specifically, iron(II/III) complexes were investigated as DNA binding agents [17,18,19] or as phenoxazinone synthase (PHS) artificial analogues [16,20,21,22,23,24,25]. Interestingly, in Nature it is the copper(II) ion that catalyses oxidation of o-aminophenols to phenoxazinones [26,27], which means that PHS-active iron systems could be used to gain additional insight into the oxidative biological mechanisms [20,28].…”

New Heteroleptic 3D Metal Complexes: Synthesis, Antimicrobial and Solubilization Parameters

“…Biomedical specialists aim to develop more potent and aggressive pharmacological alternatives due to the significant increase in drug-resistant microbial infections [ 1 ]. Researchers have embraced the potentials of natural products as effective pharmacological substitutes to combat microbial infections [ 2 ].…”

“…In coordination chemistry, nitrogen-donor ligands, with strong redox stability and quite easy access to functionalization, have been extensively explored for the formation of heteroleptic complexes, that is, metal species which have more than one type of ligand [ 11 , 12 ]. The biological potentials including cytotoxicity, genotoxicity, antineoplastic, antitumor and bactericidal attributes have been reported for the heteroleptic chelates encompassing the bidentate ligands [ 2 , 13 ]. The chelation in metal complexes plays an important role in improving their lipophilicity in comparison to their precursor ligands [ 14 ].…”