| 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-reversibleredox couples when examined by cyclic voltammetry in CH2Cl2: one-electron reductions are observed at 0.64 and0.30 V vs ferrocene. The reaction of complex 2 with 1 and 2 equiv of NMe4I yields the one- and two-electronreduced analogues, 3 and 4, respectively. Complexes 2·3CH2Cl2, 3·4.5CH2Cl2·1/2H2O, and 4·6C7H8 crystallize inthe triclinic P1̄, monoclinic P2/c, and monoclinic C2/c space groups, respectively. The molecular structures are allvery similar, consisting of a central [MnIVO4] cubane surrounded by a nonplanar alternating ring of eight Mn andeight μ3-O2- ions. Peripheral ligation is provided by 16 bridging C6F5CO2- and 4 H2O ligands. Bond valence sumcalculations establish that the added electrons in 3 and 4 are localized on former MnIII ions giving trapped-valenceMnIV4MnIII7MnII and MnIV4MnIII6MnII2 anions, respectively. 19F NMR spectroscopy in CD2Cl2 shows retention of thesolid-state structure upon dissolution and detrapping of the added electrons in 3 and 4 among the outer ring of Mnions on the 19F NMR time scale. DC studies on dried microcrystalline samples of 2, 3, and 4·2.5C7H8 restrainedin 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 clustersexhibit out-of-phase AC susceptibility signals (χM‘ ‘) indicative of slow magnetization relaxation in the 6−8 K rangefor 2, 4−6 K range for 3, and 2−4 K range for 4; the shift to lower temperatures reflects the decreasing magneticanisotropy 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 combinedwith rate vs T data from DC magnetization decay vs time measurements at lower temperatures to generate anArrhenius plot from which the effective barrier (Ueff) to magnetization reversal was obtained; the Ueff values are59 K for 2, 49 and 21 K for the slower- and faster-relaxing species of 3, respectively, and 25 K for 4. Hysteresisloops obtained from single-crystal magnetization vs DC field scans are typical of single-molecule magnets with thecoercivities increasing with decreasing T and increasing field sweep rate and containing steps caused by thequantum tunneling of magnetization (QTM). The step separations gave |D|/g values of 0.22 cm-1 for 2, 0.15 and0.042 cm-1 for the slower- and faster-relaxing species of 3, and 0.15 cm-1 for 4.
- Studies on three Mn12 clusters with identical peripheral ligation spanning three oxidation levels has allowed an investigation of the effects of electron reduction on the magnetic properties of Mn12 complexes. The studies indicate that the magnetic anisotropy decreases with the degree of reduction, and hysteresis loops exhibited by all three complexes establish the compounds to be single-molecule magnets.
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