Manganese(II) complexes of tridentate 3N ligands as functional models of phenoxazinone synthase: Impact of ligand steric hindrance on reactivity

Abstract

Six new manganese(II) complexes [Mn(L)Cl2] 1–6 with linear tridentate 3N donor ligands L1–L6 (where, L1 = bis(pyridin-2-ylmethyl)amine (bpma), L2 = N,N-bis(pyridin-2-ylmethyl)cyclopropanamine, L3 = N,N-bis(pyridin-2-ylmethyl)cyclobutanamine, L4 = N,N-bis(pyridin-2-ylmethyl)cyclopentanamine, L5 = N,N-bis(pyridin-2-ylmethyl)cyclohexylamine, L6 = N,N-bis(pyridin-2-ylmethyl)cycloheptanamine) have been synthesized and efficiently employed as functional mimics of phenoxazinone synthase (PHS) enzymes. All the manganese(II) complexes were characterized by various analytical methods and density functional theory studies. Single crystal structures of 2 and 6 disclose that the three nitrogens of the tridentate ligand along with two chloride ions constructed a distorted square pyramidal and intermediate coordination geometries (τ5 = 0.203 and 0.438), respectively, around manganese(II) centre. The molecular structure of L5 was also determined by XRD analysis. DFT calculations showed similar coordination mode of L1 (1) and L3–L5 (3–5) ligands with varying magnitudes of structural distortion. All the six complexes catalyse the oxidative condensation of o-aminophenol into 2-aminophenoxazine-3-one at moderate rates (Vmax: 0.86–36.30 × 10–8 M s–1), exhibiting remarkable turnover numbers (kcat (h‒1): 1, 10.14; 2, 130.74; 3, 3.09; 4, 7.33; 5, 17.12; 6, 28.33) in O2 saturated methanol. Indeed, complex 2 tagged with a highly strained cyclopropyl ring, facilitates faster product dissociation, exhibits remarkably higher PHS-like reactivity. DFT optimized geometries and metric parameters indicate the meridional coordination of L1‒L6 in 1–6, and the reaction rate of the [Mn(L)(OAP] adducts are well correlated with the trigonal index values of the optimized geometries 1B‒6B (τ5 = 0.114–0.503) than with those of 1A–6A (τ5 = 0.101–0.517) suggesting the importance of substrate orientation relative to the cycloalkyl substituents. The ESI-MS characterization of (L)Mn-OAP adducts along with the detection of H2O2 in the kinetic solution collectively suggests the participation of dioxygen in the reaction. All the above results are supported by single crystal X-ray analyses, spectroscopic and redox properties and the DFT analysis of the Mn(II) complexes and their substrate adducts. Keywords: tridentate 3N donor ligands, manganese(II) complexes, five-coordinate geometry, phenoxazinone synthase-like reactivity, O2 activation.