[FE(OME)(2)(O2CCH2CL)](10), A MOLECULAR FERRIS WHEEL

The synthesis of [Fe(OMe)(2)(O2CCH2Cl)](10), a molecular ferric wheel, from basic iron chloroacetate and ferric nitrate in methanol is described. Spectroscopic analysis of methanol solutions used to prepare the compound revealed a (mu-oxo)(mu-carboxylato)diiron(III) intermediate having terminal oxygen donor ligands. The structure of the crystalline ferric wheel was revealed in a single crystal X-ray diffraction investigation. The molecule has idealized D-5d symmetry and consists of a 20-membered ring comprised of 10 ferric ions linked by 20 bridging methoxide and 10 bridging chloroacetate ligands. The 10 iron atoms ace approximately coplanar and are coordinated in a distorted octahedral manner by 6 oxygen donor atoms. Quantitative analysis of the geometry indicated that the curvature arises from a combination of convex bending by 205 degrees (interior angle) across the bridging,methoxides and concave bending. by 119 degrees (interior angle) at the iron atoms. The Mossbauer spectrum of a polycrystalline sample of the ferric wheel at 4.2 K consisted of a single quadrupole split doublet with delta = 0.52 mm s(-1) and Delta E(Q) = 0.62 mm s(-1). The solid state magnetic properties of the compound were extensively investigated. The T-chi value decreased from 32.3 emu mol(-1) K at 300 K to 0.355 emu mol(-1) K at 2.5 K, consistent with antiferromagnetic exchange coupling. The temperature dependence of the susceptibility could be adequately fit by a classical linear chain treatment down to 50 K with a nearest-neighbor coupling constant of similar to 10 cm(-1), where H = JS(i).S-i+1, and g = 2.0. In order to account for the temperature dependence over the entire temperature range,the Heisenberg-Dirac-van Vleck spin Hamiltonian was applied and solved numerically for a ring of eight iron(III) ions an approach that gave J = 9.6 cm(-1) with g = 2.0, and an excellent fit to the experimental data. The low-lying states of th spin manifold were dramatically revealed by high-field DC and pulsed magnetization measurements at 0.6 K. The reduced magnetization increased in discrete steps with evenly spaced increments in the magnetic field, a result that was quantitatively accounted for by the theoretical spin manifold.