New complexation behaviour of the potentially bis-terdentate triazole based ligands PMAT and PMPT: FeIII4 oxo-bridged metallomacrocycles

Abstract

The reaction of the potentially bis-terdentate ligand PMPT with [FeII(py)4(NCS)2] in methanol under an inert atmosphere gives [FeIIn(PMPT)n(NCS)2n] (1), where PMPT = 4-pyrrolyl-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole. In contrast, conducting this reaction in dimethylformamide (DMF), or recrystallizing 1 from DMF, leads to the formation of the tetranuclear FeIII metallomacrocycle [FeIII4(PMPT)2(O)2(NCS)8(DMF)2] (2). This is the first structurally characterised complex of a ligand of this type that does not bind in a bis-terdentate manner with a 1:1 metal:ligand stoichiometry and N1N2 bridging of the bound metal ions. Instead, each ligand strand binds in an asymmetric fashion, providing two well separated (no triazole bridge) bidentate and terdentate binding pockets, with the additional binding sites on the iron(III) centres occupied by N-bound thiocyanate anions (NB. previous complexations have involved non-coordinating anions), oxide bridges, and/or DMF. Analogous reactions of [FeII(py)4(NCS)2] and the closely-related ligand PMAT appear to result in ligand decomposition, although a few crystals of the complex [FeIII4(PMAT)2(O)2(NCS)8(DMF)2]·solvents, (3·solvents) were obtained (PMAT = 4-amino-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole).