Selective coordination of multidentate ligands in manganese(II) complexes: Syntheses, structures and phenoxazinone synthase mimicking activity

The present report describes the syntheses, structures and catalytic activity mimicking the function of phenoxazinone synthase of four new mononuclear manganese(II) complexes, [Mn(L1)Cl2] (1), [Mn(L2)Cl2] (2), [Mn(L3)Cl2] (3) and [Mn(L4)Cl2]·H2O (4), derived from the multidentate ligands L1–L4, which are the 1:2 condensation products of triamines (or tetraamine) and 2-pyridinecarboxaldehyde (or 6-methyl-2-pyridinecarboxaldehyde). X-ray crystallography reveals the preferential coordination ability of all the ligands (cyclic isomers over the acyclic analogues) to a common metal center (Mn2+) and in one case (2) methanol addition across the –CN– bond. All the complexes show moderate phenoxazinone synthase activity and the experimental data suggest that a correlation can be drawn between E1/2 and Vmax of the complexes, namely the lower the E1/2 value the higher the catalytic activity. The effect of methyl substitution on the pyridine ring (in 3) on the matrix parameters and on the phenoxazinone synthase activity is also noticeable. The ESI-MS positive spectrum of a mixture of a representative complex (1 or 4) and o-aminophenol shows a peak corresponding to [Mn(L)(OAP)]+ (OAPH = o-aminophenol), suggesting that the catalytic activity proceeds through the formation of a stable complex–substrate intermediate.

The preferential complexation ability (cyclic isomers over acyclic Schiff base form) of multidentate ligands in manganese(II) coordination chemistry has been described. All the complexes exhibit catalytic activity mimicking the function of phenoxazinone synthase, and their relative catalytic activity is nicely related to the electrochemical behavior as well as the structural flexibility of the complexes.Figure optionsDownload as PowerPoint slide