High-Spin Molecules: Iron(III) Incorporation into [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O₄] To Yield [Mn₈Fe₄O₁₂(O₂CMe)₁₆(H₂O)₄] and Its Influence on the S = 10 Ground State of the Former

The synthesis, single-crystal X-ray structure, and electrochemical, magnetochemical and Mössbauer properties are reported for [Mn₈Fe₄O₁₂(O₂CMe)₁₆(H₂O)₄] (3) as its 2MeCO₂H₄H₂O solvate. Complex 3 represents the partially Feni-substituted form of [Mn₁₂O₁₂(O₁₂CMe)₁₆(H₂O)₄] (1), for which the benzoate analogue, [Mn₁₂O₁₂(O₂-CPh)₁₆(H₂O)₄] (2) is also known. Treatment of Fe(O₂CMe)₂ with KMnO₄ (16.3:6.4 molar ratio) in 60% aqueous acetic acid followed by slow heating to 60 °C, cooling to room temperature and layering of the golden brown solution with acetone, leads to black crystals of 3·2MeCO₂H·H₂O in ~85% yield. The crystals are isomorphous with 1·2MeCO₂H4·H₂O, with the following unit cell parameters at −158 °C: tetragonal, 14, a = b = 17.169(4), c = 12.258(3), V = 3612.9 ų and Z = 2. The structure was solved (MULTAN) and refined employing 3211 unique reflections with F > 3σ(F) to final values of R = 0.0768 and R_w = 0.0768. The molecule consists of a central [Mn^IV₄O₄]8⁺ cubane held within a nonplanar ring of eight alternating Mn^III and Fe^III ions by eight μ₃-O^2- ions. Peripheral ligation is provided by sixteen μ-MeCO₂^- and four terminal H₂O groups, the latter being ligated one each on the four Fe^III ions. The identification of the Fe^III ions was facilitated by the absence of a Jahn— Teller axial elongation as seen for the Mn^III ions. Elemental analysis data suggest a small fraction of molecules contain Fe^III ions at the Mn^III sites; Fe:Mn analysis ratios are approximately 4.37:7.63. Electrochemical studies in MeCN solution using cyclic voltammetry reveal a quasireversible oxidation at 0.81 V vs ferrocene and a quasireversible reduction at 0.17 V, in addition to irreversible oxidation and reduction features. The reversible processes occur at essentially identical potentials as for 1 suggesting the reduction and oxidation processes are occurring at manganese centers. ⁵⁷Fe Mössbauer spectra are reported for 3·2MeCO₂H·4H₂O at 300 and 120 K. At both temperatures there are two doublets present. A fit of the 300 K spectrum shows that the area associated with the main doublet is 82.6% of the total. This doublet has a quadrupole splitting (ΔE_Q) of 0.459(4) mm/s and is attributable to the four high-spin Fe^III ions identified in the X-ray structure. The other doublet with ΔE_Q = 1.061(2) mm/s at 300 K is assignable to the excess high-spin Fe^III which is disordered throughout the Mn^III sites. For a polycrystalline sample of 3·2MeCO₂H·4H₂O embedded in parafilm in a 10.0 kG field the value of eff per molecule is 11.18 μ_B at 300.0 K and decreases gradually with decreasing temperature to 4.85 μ_B at 5.00 K. Least-squares fitting of 0.50-50.0 kG data in the 2.0-30 K range with a full matrix diagonalization approach shows that this Fe₄Mn₈ complex has a well-isolated S = 2 ground state with an axial zero-field splitting of D = −1.8 cm^-1. The origin of the change from a S = 10 ground state for the Mn^IV₄Mn^III₈ complex 1 to a 5 = 2 ground state for Mn^IV₄Mn^III₄Fe^III₄ complex 3 is discussed.