Tetranuclear iron(III) complexes with amino acids involving a planar (µ-oxo)(µ-hydroxo)bis(µ-alkoxo)bis(µ-carboxylato)tetrairon core†

Abstract

Reactions of Fe^III(NO₃)₃·9H₂O with 0.5 equivalent of 2-hydroxypropane-1,3-diamine-N,N,N′,N′-tetraacetic acid (H₅dhpta) and an excess of amino acid [glycine (Gly) or L-alanine (L-Ala)] in water with the pH adjusted to ca. 5 by a solution of NaOH gave pale green crystals formulated as Na[Fe₄(dhpta)₂(µ-O)(µ-OH)(O₂CCHRNH₃)₂] (R = H 1 or CH₃ 2) in moderate yields. The tetrairon(III) complexes were characterized by elemental analyses, IR and ⁵⁷Fe Mössbauer spectra, variable-temperature magnetic susceptibility, and X-ray crystallographic and absorption analyses. The structure of 2·6H₂O was determined by X-ray crystallography to comprise a (µ-oxo)(µ-hydroxo)bis(µ-alkoxo)bis(µ-carboxylato)tetrairon(III) cluster core bridged by two dhpta and two amino acid ligands. Two identical dinuclear iron(III) units [Fe(1)Fe(2) and Fe(3)Fe(4)] are each co-ordinated by a pentadentate dhpta ligand. The Fe(1) and Fe(4) are then bridged by the carboxylate of an amino acid and an oxo group, with the Fe(2) and Fe(3) similarly linked. The central two oxo groups are protonated to form a strongly hydrogen bonded (O–H–O)^3– bridge [O· · ·O 2.426(4) Å]. The average Fe· · ·Fe distance for the µ-alkoxo dinuclear units is 3.692 Å (intradimer) and that for the bis(µ-carboxylato)(µ-oxo/hydroxo)diiron units is 3.463 Å (interdimer), both being in accord with the corresponding values from EXAFS analyses. The two L-alanine moieties are in zwitterionic form and act as interdimer bridging ligands with the carboxylate groups. The Na⁺ counter cation is well packed between the tetrairon(III) complex anions, resulting in an infinite chain aggregation. The ⁵⁷Fe Mössbauer spectrum for 2·6H₂O clearly demonstrated that the four iron sites are equivalent, being in a high-spin octahedral iron(III) state. The variable-temperature magnetic susceptibility measurement for 2·6H₂O showed modest antiferromagnetism with an interdimer coupling constant (J) of –42.3 cm^–1 and an intradimer coupling constant (J′) of –20.8 cm^–1.