Exploring catecholase activity in dinuclear Mn(II) and Cu(II) complexes: An experimental and theoretical approach

While a plenty of work has been done on metal complexes incorporating N- and O-functionalised dicarboxylic acids (iminodiacetic acid and oxydiacetic acid, respectively), S-functionalised dicarboxylic acid, i.e. thiodiglycolic acid (H(2)tdga), is less explored and requires attention. In this study, two dinuclear complexes, namely [Mn-2(tdga)(2)(phen)(2)]center dot 2CH(3)OH (1) and [Cu-2(tdga)(2)(phen)(2)]center dot H(2)tdga (2), were characterised employing spectral, single-crystal X-ray diffraction and DFT/TD-DFT studies. Time-dependent density functional theory (TD-DFT) reveals the types of electronic transition between metal-to-ligand charge transfer (MLCT) and intra-ligand charge transfer (ILCT). The crystal structure reveals the presence of various non-covalent interactions, which are further corroborated from Hirshfeld surface analysis. In EPR spectral analysis, 1 shows an isotropic signal at g = 1.99 due to Mn2+ (d(5)) with ground-state term S-6 having a six-coordinate distorted octahedral geometry, while 2 shows two resonances, namely, g(||) = 2.21 and g(perpendicular to) = 2.11 due to the Jahn-Teller distortion of Cu2+ (d(9)) ions. The anisotropic g values are in the order of g(||) > g(perpendicular to) > 2.0, which is characteristic of an axially elongated tetragonal distortion with a dx(2)-y(2) ground-state energy level. Assessment of catecholase-like activity provides K-cat = 962.56 (h(-1)) for 2, indicating enhanced enzymatic activity of 2 towards the oxidation of catechol (3,5-DTBC) to a quinone derivative (3,5-DTBQ). The catecholase activity of 2 is also corroborated by density functional theory (DFT) analysis.