Synthesis, structural determination, theoretical studies and catalytic activity of Mn(II) complex of N-isonicotinyl phosphoric triamide ligand

Abstract

A new N-isonicotinyl phosphoric triamide ligand with the formula 4-NC5H4C(O)NHP(O)(NC4H8)2 (L) was synthesized and characterized by 1H, 13C, 31P NMR and IR spectroscopies. The reaction of MnCl2·4H2O with L led to the formation of a new 3D system {MnL2Cl2}n·34CH3OH (C1). X-ray crystallographic data revealed that the ligand binds to the neighboring manganese ions through the nitrogen atom of pyridine (Npy) and the oxygen atom of phosphoryl (OP) in a bidentate manner. The Mn(II) centers showed a distorted cis(Npy, Npy) cis(OP, OP) cis(Cl, Cl) octahedral configuration. In order to compare the relative stability of C1 (with all-cis configuration) and its possible all-trans isomer, C′1, density functional theory (DFT) calculations were performed and the results showed the preference of C1 over C′1 from energy point of view. Quantum theory of atoms in molecules (QTAIM) analysis was applied to elucidate the nature of interactions, and the results suggested an ionic character (closed-shell interaction) for Mn–OP bond, and a partial covalent contribution for Mn–Npy interaction. Natural bond orbital (NBO) analysis was also used to calculate the charge distribution on atoms in the complex. The complex C1 showed efficient catalytic activity in the oxidation reaction of some alkenes to their corresponding epoxides. In addition, the effect of temperature and reaction time on the conversion and selectivity of cyclohexene epoxidation reaction in the presence of C1 as catalyst was investigated.