High-Spin Molecules: [Mn₁₂O₁₂(O₂CR)₁₆(H₂O)₄]

The syntheses and electrochemical and magnetochemical properties of [Mn₁₂O₁₂(O₂CPh)₁₆(H₂O)₄] (3), its solvate 3-PhCOOH·CH₂Cl₂, and [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·MeCOOH·3H₂O (4) are reported. Complex 3 can be prepared either by reaction of Mn(OAc)₂·4H₂O, benzoic acid, and NBuⁿ₄MnO₄ in pyridine or by reaction of PhCOOH with complex 4 slurried in CH₂C1₂. Complex 3 crystallizes in the triclinic space group which at −146 °C has a = 27.072(19) Å, b = 17.046(11) Å, c = 14.254(8) Å, α = 98.39(3)°, β = 98.44(4)°, γ = 89.27(4) Å, and Z = 2. The structure was refined with 4814 observed [F > 3.0σ(F)] reflections to give R = 9.54 and R_w = 10.07. [Mn₁₂O₁₂(O₂CPh)₁₆(H₂O)₄] (3) consists of a central [Mn^IV₄O₄]⁸⁺ cubane held within a nonplanar ring of eight Mn^III atoms by eight µ₃-O²⁻ ions. Peripheral ligation is provided by 16 µ₂-O₂CPh⁻ and four terminal H₂O groups, where the four H₂O ligands are located on two Mn atoms. Four redox waves are seen in the cyclic voltammogram of complex 3 in CH₂C1₂: two reversible waves [an oxidation wave at 0.79 V (vs ferrocene/ferrocenium) and a reduction wave at 0.11 V] and two irreversible waves at −0.23 and −0.77 V. Complex 4 exhibits the same four redox couples in MeCN. Variable-temperature DC magnetic susceptibility data measured at 10.0 kG are presented for polycrystalline samples of complex 3 and the solvate 3·PhCOOH·CH₂Cl₂. At 320 K, µ_eff/molecule is ∼12 µ_B and increases to a maximum of ∼20–21 µ_B at ∼ 10 K, whereupon µ_eff/molecule decreases rapidly at low temperatures. It is concluded that these complexes exhibit appreciable magnetic anisotropy. Even at fields as low as 1 kG the polycrystallites have to be restrained from torquing by embedding the polycrystalline sample in parafilm. Complexes 3 and 3·PhCOOH·CH₂Cl₂ exhibit somewhat different µ_eff/molecule versus temperature curves. Magnetization measurements at 20.0, 30.0, 40.0, and 50.0 kG in the 2–4 K range are used to determine that in these fields complexes 3 and 3·PhCOOH·CH₂Cl₂ have S=10 and S = 9 ground states, respectively. A relatively large zero-field splitting is in evidence, and this was confirmed by high-field EPR experiments with a CO₂ far-infrared laser. AC susceptibility data in zero applied field are given for complexes 3 and 4 in the 4–25 K range. It is concluded that complex 3 has a S = 9 ground state at zero field, whereas complex 4 has a S = 10 ground state at zero field. The most interesting observation for complexes 3 and 4 derives from the out-of-phase (imaginary) component of the AC susceptibility, xm″. Both of these complexes exhibit a nonzero Xm″, which when measured at various frequencies shows a maximum at different temperatures. These two complexes are the only molecular solids known to exhibit a nonzero xm″ in the paramagnetic phase. The results of theoretical calculations of the ordering of spin states in a Mn₄^IVMn₈^III complex, assuming reasonable values for the exchange parameters characterizing the different pairwise interactions, are presented to rationalize the S = 8–10 ground states.