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 CH2Cl2. Complex 3 crystallizes in the triclinic space group P1BAR, which at -146-degrees-C has a = 27.072(19) angstrom, b = 17.046(11) angstrom, c = 14.254(8) angstrom, alpha = 98.39(3)degrees, beta = 98.44(4)degrees, gamma = 89.27(4) angstrom, and Z = 2. The structure was refined with 4814 observed [F > 3.0sigma(F)] reflections to give R = 9.54 and R(w) = 10.07. [Mn12O12(O2CPh)16(H2O)4] (3) consists of a central [MnIV4O4]8+ cubane held within a nonplanar ring of eight Mn(III) atoms by eight mu3-O2- ions. Peripheral ligation is provided by 16 mu2-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 CH2Cl2: 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, mu(eff)/molecule is approximately 12 mu(B) and increases to a maximum of approximately 20-21 mu(B) at approximately 10 K, whereupon mu(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 mu(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, chi(M)''. Both of these complexes exhibit a nonzero chi(M)'', 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 chi(M)'' in the paramagnetic phase. The results of theoretical calculations of the ordering of spin states in a Mn4IVMn8III complex, assuming reasonable values for the exchange parameters characterizing the different pairwise interactions, are presented to rationalize the S = 8-10 ground states.