Single-Molecule Magnets: Two-Electron Reduced Version of a Mn12 Complex and Environmental Influences on the Magnetization Relaxation of (PPh4)2[Mn12O12(O2CCHCl2)16(H2O)4]

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Abstract
The complex [Mn12O12(O2CCHCl2)16(H2O)4] (2) in MeCN exhibits three quasi-reversible one-electron reduction processes at significantly higher potentials than [Mn12O12(O2CMe)16(H2O)4] (1). This has allowed the two-electron reduced version of 2 to be generated and isolated. Reaction of 2 with one and two equivalents of PPh4I led to isolation of (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] (3) and (PPh4)2[Mn12O12(O2CCHCl2)16(H2O)4] (4), respectively. The latter represents a new isolated oxidation level of the Mn12 family of single-molecule magnets (SMMs). Crystallization from CH2Cl2/hexanes yields a mixture of two crystal forms, 4·4CH2Cl2·H2O (4a) and 4·6CH2Cl2 (4b), both of which have been structurally characterized as triclinic and monoclinic, respectively. The molecular structures are very similar, with the added electrons localized on former Mn(III) ions to give a trapped-valence 2Mn(II), 6Mn(III), 4Mn(IV) oxidation state description. Dried solid analyzed as unsolvated 4. 1H NMR spectral data in CD2Cl2 confirm that 4 retains its solid-state structure in solution. Bulk DC magnetization data for dried 4 in the 1.80−4.00 K and 10−70 kG ranges were fit to give S = 10, D = −0.275 cm-1, g = 2.00 and |D|/g = 0.14 cm-1, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4a and 4b give resolvable frequency-dependent out-of-phase (χM‘ ‘) signals in AC susceptibility studies resulting from the magnetization relaxation of SMMs. Relaxation rate vs T data to 1.8 K obtained from the χM‘ ‘ vs temperature studies were supplemented with rate vs T data measured to lower temperatures via magnetization vs time decay data, and these were fit to the Arrhenius equation to give the effective barrier to relaxation (Ueff). The Ueff values are 18.5 and 30.3 K for 4a and 4b, respectively. A similar analysis for dried 4 using AC data gave Ueff = 32 K. Magnetization vs DC field sweeps on single crystals of 4a and 4b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded |D|/g values of 0.087 and 0.14 cm-1 for 4a and 4b, respectively, suggesting that the differences in Ueff are primarily caused by changes to D. The combined work demonstrates the sensitivity of the magnetic properties of these new [Mn12]2- SMMs to subtle differences in their environment as determined by the precise packing, solvent molecules, and overall crystal symmetry (space group) and represents an important caveat to workers in the field.

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