Exploring catecholase activity in dinuclear Mn(ii) and Cu(ii) complexes: an experimental and theoretical approach

Abstract

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 (H₂tdga), is less explored and requires attention. In this study, two dinuclear complexes, namely [Mn₂(tdga)₂(phen)₂]·2CH₃OH (1) and [Cu₂(tdga)₂(phen)₂]·H₂tdga (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⁺ (d⁵) with ground-state term ⁶S having a six-coordinate distorted octahedral geometry, while 2 shows two resonances, namely, g_‖ = 2.21 and g_⊥ = 2.11 due to the Jahn–Teller distortion of Cu²⁺ (d⁹) ions. The anisotropic g values are in the order of g_‖ > g_⊥ > 2.0, which is characteristic of an axially elongated tetragonal distortion with a d_x²–y² 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.