Single-molecule magnets:: Structural characterization, magnetic properties, and 19F NMR spectroscopy of a Mn12 family spanning three oxidation levels

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 center dot 3CH(2)Cl(2), 3 center dot 4.5CH(2)Cl(2)center dot 1/2H(2)O, and 4 center dot 6C(7)H(8) crystallize in the triclinic P (1) over bar, monoclinic P2/c, and monoclinic C2/c space groups, respectively. The molecular structures are all very similar, consisting of a central [(MnO4)-O-IV] cubane surrounded by a nonplanar alternating ring of eight Mn and eight,mu(3)-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 Mn-III ions giving trapped-valence (Mn4Mn7MnII)-Mn-IV-Mn-III and (Mn4Mn6Mn2II)-Mn-IV-Mn-III anions, respectively. F-19 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 center dot 2.5C(7)H(8) 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, vertical bar D vertical bar/g = 0.21 cm(-1) for 2, S 19/2, D = -0.34 cm(-1), g = 2.04, vertical bar D vertical bar/g, = 0.17 cm(-1) for 3, and S = 10, D = -0.29 cm(-1), g = 2.05, vertical bar D vertical bar/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 (chi M '') 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 chi m '' 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 (U-eff) 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 separatons gave vertical bar D vertical bar/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.