Single-Molecule Magnets: Structural Characterization, Magnetic Properties, and 19F NMR Spectroscopy of a Mn12 Family Spanning Three Oxidation Levels

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Abstract
The syntheses, crystal structures, and magnetic properties of [Mn12O12(O2CC6F5)16(H2O)4] (2), (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (3), and (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (4) are reported. Complex 2 displays quasi-reversible redox couples when examined by cyclic voltammetry in CH2Cl2: one-electron reductions are observed at 0.64 and 0.30 V vs ferrocene. The reaction of complex 2 with 1 and 2 equiv of NMe4I yields the one- and two-electron reduced analogues, 3 and 4, respectively. Complexes 2·3CH2Cl2, 3·4.5CH2Cl2·1/2H2O, and 4·6C7H8 crystallize in the triclinic P1̄, monoclinic P2/c, and monoclinic C2/c space groups, respectively. The molecular structures are all very similar, consisting of a central [MnIVO4] cubane surrounded by a nonplanar alternating ring of eight Mn and eight μ3-O2- ions. Peripheral ligation is provided by 16 bridging C6F5CO2- and 4 H2O ligands. Bond valence sum calculations establish that the added electrons in 3 and 4 are localized on former MnIII ions giving trapped-valence MnIV4MnIII7MnII and MnIV4MnIII6MnII2 anions, respectively. 19F NMR spectroscopy in CD2Cl2 shows retention of the solid-state structure upon dissolution and detrapping of the added electrons in 3 and 4 among the outer ring of Mn ions on the 19F NMR time scale. DC studies on dried microcrystalline samples of 2, 3, and 4·2.5C7H8 restrained in eicosane in the 1.80−10.0 K and 1−70 kG ranges were fit to give S = 10, D = −0.40 cm-1, g = 1.87, |D|/g = 0.21 cm-1 for 2, S = 19/2, D = −0.34 cm-1, g = 2.04, |D|/g = 0.17 cm-1 for 3, and S = 10, D = −0.29 cm-1, g = 2.05, |D|/g = 0.14 cm-1 for 4, where D is the axial zero-field splitting parameter. The clusters exhibit out-of-phase AC susceptibility signals (χM‘ ‘) indicative of slow magnetization relaxation in the 6−8 K range for 2, 4−6 K range for 3, and 2−4 K range for 4; the shift to lower temperatures reflects the decreasing magnetic anisotropy upon successive reduction and, hence, the decreasing energy barrier to magnetization relaxation. Relaxation rate vs T data obtained from χM‘ ‘ vs AC oscillation frequency studies down to 1.8 K were combined with rate vs T data from DC magnetization decay vs time measurements at lower temperatures to generate an Arrhenius plot from which the effective barrier (Ueff) to magnetization reversal was obtained; the Ueff values are 59 K for 2, 49 and 21 K for the slower- and faster-relaxing species of 3, respectively, and 25 K for 4. Hysteresis loops obtained from single-crystal magnetization vs DC field scans are typical of single-molecule magnets with the coercivities increasing with decreasing T and increasing field sweep rate and containing steps caused by the quantum tunneling of magnetization (QTM). The step separations gave |D|/g values of 0.22 cm-1 for 2, 0.15 and 0.042 cm-1 for the slower- and faster-relaxing species of 3, and 0.15 cm-1 for 4.