[Fe(OCH 3) 2(dbm)] 12: synthesis, solid-state characterization and reactivity of a new molecular ferric wheel

• Published in: Inorganica Chimica Acta 2000, Vol.297 (1), p.291-300
• Main Authors: Abbati, G.L., Caneschi, A., Cornia, A., Fabretti, A.C., Gatteschi, D.
• Format: Article
• Language: English
• Subjects: Cluster complexes; Iron complexes; Magnetic properties; Supramolecular chemistry
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(99)00316-3

A novel iron(III) molecular alkoxide, [Fe(OCH 3) 2(dbm)] 12 ( 1), was synthesized by the reaction of ferric chloride with dibenzoylmethane (Hdbm) and an excess of either potassium or cesium methylate in anhydrous methanol. The 12 octahedrally coordinated iron(III) ions are linked by double OCH 3 bridges in a nonplanar twisted-ring structure, to give the largest cyclic ferric cluster so far reported, containing chemically equivalent bridging units. The quantitative conversion of 1 into hexairon(III) rings [MFe 6(OCH 3) 12(dbm) 6] + (M=Li, Na) by reaction with excess alkali-metal tetraphenylborates in CH 2Cl 2/CH 3OH 3:1 was studied by 1H NMR and UV–Vis spectroscopies. The reactions follow pseudo-first-order kinetics with respect to 1, with k po(Li)=1.07(2)×10 −2 and k po(Na)=1.63(3)×10 −2 s −1 at 25°C. The magnetic susceptibility of 1 in the solid state is consistent with antiferromagnetic exchange interactions between the high-spin iron(III) ions and closely follows the behavior predicted for a Heisenberg s=5/2 quantum-spin chain down to 50 K ( J=22.2 cm −1 for H = J Σ i S i · S i+1 and g=2.00). At low temperatures the magnetic behavior is dominated by the energy gap between the ground S=0 and the first excited S=1 state, which is a consequence of finite-size effects.