[Fe(OCH3)2(dbm)]12: Synthesis, Solid-state Characterization and Reactivity of a New Molecular Ferric Wheel

A novel Fe(III) mol. alkoxide, [Fe(OCH3)2(dbm)]12 (1), was synthesized by the reaction of ferric chloride with dibenzoylmethane (Hdbm) and an excess of either K or Cs methylate in anhyd. MeOH. The 12 octahedrally coordinated Fe(III) ions are linked by double OCH3 bridges in a nonplanar twisted-ring structure, to give the largest cyclic ferric cluster so far reported, contg. chem. equiv. bridging units. The quant. conversion of 1 into hexairon(III) rings [MFe6(OCH3)12(dbm)6]+ (M = Li, Na) by reaction with excess alkali-metal tetraphenylborates in CH2Cl2/MeOH 3:1 was studied by 1H NMR and UV-visible 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°. The magnetic susceptibility of 1 in the solid state is consistent with antiferromagnetic exchange interactions between the high-spin Fe(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 temps. the magnetic behavior is dominated by the energy gap between the ground S = 0 and the 1st excited S = 1 state, which is a consequence of finite-size effects.