A series of manganese(II) complexes [Mn(L)Cl2] 1–5 composed of tripodal tetradentate 4N donor ligands, (L1: tris(pyridin-2-ylmethyl)amine, L2: N,N-bis(pyridin-2-ylmethyl)-2-(pyrrolidin-1-yl)ethan-1-amine, L3: 2-(piperidin-1-yl)-N,N-bis-(pyridin-2-ylmethyl)ethan-1-amine, L4: N,N-bis((6-methylpyridin-2-yl)methyl)-2-(pyrrolidin-1-yl)ethan-1-amine and L5: N,N-bis((6-methylpyridin-2-yl)methyl)-2-(piperidin-1-yl)ethan-1-amine), have been synthesized, characterized and evaluated as functional mimics of phenoxazinone synthase enzyme. The single crystal X-ray structures of the complexes [Mn(L3)Cl2] 3, [Mn(L4)Cl2] 4 and [Mn(L5)Cl2] 5 have been determined. All the complexes adopt a distorted octahedral coordination geometry in which the manganese(II) center is wrapped by an N4Cl2 chromophore. The E1/2 values of the complexes 1–5 and manganese(II)-aminophenol adducts were obtained from differential pulse voltammetry and are in the potential range + 0.368 to +0.440 V and + 0.174 to +0.283 V, respectively. The negative shift in the E1/2 values of the adducts is mainly due to the bidentate coordination of the substrate. All the complexes efficiently catalyze the oxidative dimerization of the model substrate o-aminophenol to phenoxazinone synthase-like activity in dioxygen-saturated methanol at 25 °C with impressive reaction rate (Vmax, 1.25 ± 0.01–16.2 ± 0.03 × 10−7 M s−1) and turnover numbers (kcat, 45 ± 0.36–583.20 ± 1.08 h−1). Complex 4, incorporating pyrrolidinyl and 6-methylpyridine nitrogen donor arms, exhibited the highest catalytic efficiency (kcat/Km, 16.76 ± 0.36 × M−1 h−1) within the series. DFT-optimized [Mn(L)(OAP)] adducts, axial coordination occurs via either the phenolic OH donor (1A–5A, Δd = 0.134–0.175 Å) or the amino –NH donor (1B–5B, Δd = 0.134–0.182 Å). The octahedral mean deviation (Δd) is correlated with the catalytic activity, suggesting that the steric and electronic effects modulate the manganese(II) coordination geometry and influence substrate activation.
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