Modulated Magnetic Coupling in Alkoxoiron(III) Rings by Host-guest Interactions with Alkali Metal Cations

Two novel host-guest complexes of alkali-metal ions with cyclic iron(III) clusters, [LiFe6(OCH3)12(dbm)6]X (X = PF6-, ClO4-; Hdbm = dibenzoylmethane), were synthesized. The x-ray structure of [LiFe6(OCH3)12(dbm)6]PF6 ([1]PF6) is described and compared with that of sodium complexes, previously reported. [1]PF6.1/2CHCl3.9MeOH crystallizes in trigonal space-group R-3 with (at 298 K) a = 14.523(2) Å, α = 84.35(1), V = 3021.3(7) Å3 and Z = 1. The radius of the encapsulated alkali-metal ion has a marked influence on the geometry of the [Fe6(OCH3)12] host. Substitution of sodium with lithium decreases nearest-neighbor Fe···Fe sepns. and Fe-O-Fe angles, while it does not affect Fe-O distances. A quant. anal. of the magnetic susceptibility of solid samples revealed that weaker Fe-Fe antiferromagnetic interactions occur in the lithium complexes. The authors suggest that host-guest interactions modulate the magnetic coupling by detg. the exact geometry at the bridging methoxide ligands. 7Li and 23Na NMR expts. have beautifully illustrated the soln. behavior of the reported complexes and the selectivity of six-membered iron(II) rings for alkali-metal ions. The structural and functional analogy between iron(III) rings and crown ethers is stressed.