Improved syntheses, structures, spectral and electrochemical properties of [Mn2III(µ-O)(µ-O2CMe)2L2]2+ and [Mn2IV(µ-O)3L2]2+ complexes. Two homologous series derived from eight N-substituted 1,4,7-triazacyclononanes

Abstract

A series of eight N-substituted 1,4,7-triazacyclononanes L has been prepared having combinations of hydrogen, methyl, ethyl or propyl substituents on the three nitrogens. From the monoprotonated macrocycles dinuclear manganese complexes [Mn₂^III(µ-O)(µ-O₂CMe)₂L₂]X₂(X = ClO₄ or PF₆) were prepared under anhydrous conditions in high yield. A relationship between the absorption bands and the electrochemical properties of the complexes containing variously methyl-substituted L has been observed. From the Me_xEt_3–x- and Pr₃-substituted macrocycles triple oxygen-bridged dinuclear manganese complexes [Mn₂^IV(µ-O)₃L₂]²⁺ were prepared starting from L, a manganese(II) salt and counter ion, subsequently treated with alkaline hydrogen peroxide. The yields of this reaction depend on the sizes of substituents at N on the macrocycle. The crystal structure of [Mn₂^IV(µ-O)₃L₂][PF₆]₂·0.5KPF₆(L = 1,4,7-trimethyl-1,4,7-triazacyclononane) has been determined. This revealed the core to be almost identical to that of known triethyl analogue, with the methyl groups of the ethyl substituents pointing outwards to minimize steric interaction. The Mn–Mn distance of 2.295 Å is likewise exceptionally short and the value for the Mn–O–Mn bond angle of 77.9° is also very low. The NMR and UV/VIS absorption spectra and electrochemical measurements indicate very similar structures for these triple oxygen-bridged dinuclear manganese complexes.