High-Yield Syntheses and Reactivity Studies of Fe₁₀ “Ferric Wheels”: Structural, Magnetic, and Computational Characterization of a Star-Shaped Fe₈ Complex

Convenient, high-yield routes have been developed to [Fe10(OMe)20(O2CR)10] (1) “ferric wheels” involving the alcoholysis of [Fe3O(O2CR)6(H2O)3]+ salts in MeOH in the presence of NEt3. Reactivity studies have established [Fe10(OMe)20(O2CMe)10] (1a) to undergo clean carboxylate substitution with a variety of other RCO2H groups to the corresponding [Fe10(OMe)20(O2CR)10] product. In contrast, the reaction with phenol causes a nuclearity change to give a smaller [Fe8(OH)4(OPh)8(O2CR)12] (2) wheel. Similarly, reactions of [Fe10(OMe)20(O2CR)10] with the bidentate chelate ethylenediamine (en) cause a structural change to give either [Fe8O5(O2CMe)8(en)8](ClO4)6 (3) or [Fe2O(O2CBut)(en)4](NO3)3 (4), depending on conditions. Complex 3 possesses a “Christmas-star” Fe8 topology comprising a central planar [Fe4(μ4-O)]10+ square subunit edge-fused to four oxide-centered [Fe3(μ3-O)]7+ triangular units. Variable-temperature, solid-state dc and ac magnetization studies on complexes 1a−4 in the 5.0−300 K range established that all the complexes possess an S = 0 ground state. The magnetic susceptibility data for 4 were fit to the theoretical χM versus T expression derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, and this revealed an antiferromagnetic exchange parameter with a value of J = −107.7(5) cm−1. This value is consistent with that predicted by a previously published magnetostructural relationship. Theoretically computed values of the exchange constants in 3 were obtained with the ZILSH method, and the pattern of spin frustration within its core and the origin of its S = 0 ground state have been analyzed in detail.