[Fe(OCH3)2(dbm)]12: synthesis, solid-state characterization and reactivity of a new molecular ferric wheel

A novel iron(III) molecular alkoxide, [Fe(OCH3)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 OCH3 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 [MFe6(OCH3)12(dbm)6]+ (M=Li, Na) by reaction with excess alkali-metal tetraphenylborates in CH2Cl2/CH3OH 3:1 was studied by 1H NMR and UV–Vis spectroscopies. The reactions follow pseudo-first-order kinetics with respect to 1, with kpo(Li)=1.07(2)×10−2 and kpo(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 Σi Si·Si+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.