2 ∶ 2 Fe(iii) ∶ ligand and “adamantane core” 4 ∶ 2 Fe(iii) ∶ ligand (hydr)oxo complexes of an acyclic ditopic ligand

Abstract

A bis-hydroxo-bridged diiron(III) complex and a bis-μ-oxo-bis-μ-hydroxo-bridged tetrairon(III) complex are isolated from the reaction of 2,6-bis((N,N′-bis-(2-picolyl)amino)methyl)-4-tert-butylphenol (Hbpbp) with iron perchlorate in acidic and neutral solutions respectively. The X-ray structure of the dinuclear complex [{(Hbpbp)Fe(μ-OH)}₂](ClO₄)₄·2C₃H₆O (1·2C₃H₆O) shows that only one of the metal-binding cavities of each ligand is occupied by an iron(III) atom and two [Fe(Hbpbp)]³⁺ units are linked together by two hydroxo bridging groups to form a [Fe(III)–(μ-OH)]₂ rhomb structure with Fe⋯Fe = 3.109(1) Å. The non-coordinated tertiary amine of Hbpbp is protonated. Magnetic susceptibility measurements show a well-behaved weak antiferromagnetic coupling between the two Fe(III) atoms, J = −8 cm⁻¹. The tetranuclear complex [(bpbp)₂Fe₄(μ-O)₂(μ-OH)₂](ClO₄)₄ (2) was isolated as two different solvates 2·4CH₃OH and 2·6H₂O with markedly different crystal morphologies at pH ca. 6. Complex 2·4CH₃OH forms red cubic crystals and 2·6H₂O forms green crystalline platelets. The Fe₄O₆ core of 2 shows an adamantane-like structure: The six bridging oxygen atoms are provided by the two phenolato groups of the two bpbp⁻ ligands, two bridging oxo groups and two bridging hydroxo groups. The hydroxo and oxo ligands could be distinguished on the basis of Fe–O bond lengths of the two different octahedral iron sites: Fe–μ-OH, 1.953(5), 2.013(5) Å and Fe–μ-O, 1.803(5), 1.802(5) Å. The difference in ligand environment is too small for allowing Mössbauer spectroscopy to distinguish between the two crystallographically independent Fe sites. The best fit to the magnetic susceptibility of 2·4CH₃OH was achieved by using three coupling constants J_{(Fe–OPh–Fe)} = 2.6 cm⁻¹, J_{(Fe–OH–Fe)} = −0.9 cm⁻¹, J_{(Fe–O–Fe)} = −101 cm⁻¹ and iron(III) single ion ZFS (|D| = 0.15 cm⁻¹).