Aggregation control by homologous tripodal tetradentate amino acidligands in oxo-bridged diiron(iii) aquo complexes

Abstract

Isoelectronic oxo-bridged diiron(III) aquo complexes of the homologous tripodal tetradentate amino acid ligands, N,N′-bis(2-pyridylmethyl)-3-aminoacetate (bpg⁻) and N,N′-bis(2-pyridylmethyl)-3-aminopropionate (bpp⁻), containing [(H₂O)Fe^III-(μ-O)-Fe^III(H₂O)]⁴⁺ cores, oligomerise, respectively, by dehydration and deprotonation, or by dehydration only, in reversible reactions. In the solid state, [Fe₂(O)(bpp)₂(H₂O)₂](ClO₄)₂ (1(ClO₄)₂) exhibits stereochemistry identical to that of [Fe₂(O)(bpg)₂(H₂O)₂](ClO₄)₂ (2(ClO₄)₂), with the ligand carboxylate donor oxygen atoms and the water molecules located cis to the oxo bridge and the tertiary amine group trans to it. Despite their structural similarity, 1²⁺ and 2²⁺ display markedly different aggregation behaviour in solution. In the absence of significant water, 1²⁺ dehydrates and dimerises to give the tetranuclear complex, [Fe₄(O)₂(bpp)₄](ClO₄)₄ (3(ClO₄)₄), in which the carboxylate groups of the four bpp⁻ ligands act as bridging groups between two [Fe₂(O)(bpp)₂]²⁺ units. Under similar conditions, 2²⁺ dehydrates and deprotonates to form dinuclear and trinuclear oligomers, [Fe₂(O)(OH)(bpg)₂](ClO₄) (4ClO₄) and [Fe₃(O)₂(OH)(bpg)₃](ClO₄) (5(ClO₄)), related by addition of ‘Fe(O)(bpg)’ units. The trinuclear 5(ClO₄), characterised crystallographically as two solvates 5(ClO₄)·3H₂O and 5(ClO₄)·2MeOH, is based on a hexagonal [Fe₃(O)₂(OH)(bpg)₃]⁺ unit, formally containing one hydroxo and two oxo bridges. The different aggregation behaviour of 1(ClO₄)₂ and 2(ClO₄)₂ results from the difference of one methylene group in the pendant carboxylate arms of the amino acid ligands.