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Single-Molecule Magnets:  Site-Specific Ligand Abstraction from [Mn12O12(O2CR)16(H2O)4] and the Preparation and Properties of [Mn12O12(NO3)4(O2CCH2But)12(H2O)4]

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posted on 06.07.2001, 00:00 authored by Pau Artus, Colette Boskovic, Jae Yoo, William E. Streib, Louis-Claude Brunel, David N. Hendrickson, George Christou
Site-selective carboxylate abstraction has been achieved from [Mn12O12(O2CR)16(H2O)4] complexes by treatment with HNO3 in MeCN. The reaction of the R = Ph or CH2But complexes with 4 equiv of HNO3 gives [Mn12O12(NO3)4(O2CR)12(H2O)4] (R = CH2But (6) or Ph (7)) in analytical purity. Complex 6·MeNO2 crystallizes in monoclinic space group C2/c with the following cell parameters at −168 °C:  a = 21.280(5), b = 34.430(8), c = 33.023(8) Å, β = 104.61(1)°, V = 23413 Å, and Z = 8. The four NO3- groups are not disordered and are bound in bridging modes at axial positions formerly occupied by bridging carboxylate groups. 1H NMR spectroscopy in CD2Cl2 and CDCl3 shows retention of the solid-state structure on dissolution in these solvents. DC magnetic susceptibility (χM) and magnetization (M) studies have been carried out in the 2.00−300 K and 1.0−7.0 T ranges. Fits of M/NμB versus H/T plots gave S = 10, g = 1.92, and D = −0.40 cm-1, where D is the axial zero-field splitting parameter. AC magnetic susceptibility studies on 6 have been performed in the 1.70−10.0 K range in a 3.5 Oe field oscillating at frequencies up to 1500 Hz. Out-of-phase magnetic susceptibility (χM‘ ‘) signals were observed in the 4.00−8.00 K range which were frequency-dependent. Thus, 6 displays the slow magnetization relaxation diagnostic of a single-molecule magnet (SMM). The data were fit to the Arrhenius law, and this gave the effective barrier to relaxation (Ueff) of 50.0 cm-1 (72.0 K) and a pre-exponential (1/τ0) of 1.9 × 108 s-1. Complex 6 also shows hysteresis in magnetization versus DC field scans, and the hysteresis loops show steps at regular intervals of magnetic field, the diagnostic evidence of field-tuned quantum tunneling of magnetization. High-frequency EPR (HFEPR) spectroscopy on oriented crystals of complex 6 shows resonances assigned to transitions between zero-field split Ms states of the S = 10 ground state. Fitting of the data gave S = 10, g = 1.99, D = −0.46 cm-1, and B40 = −2.0 × 10-5, where B40 is the quartic zero-field coefficient. The combined results demonstrate that replacement of four carboxylate groups with NO3- groups leads to insignificant perturbation of the magnetic properties of the Mn12 complex. Complex 6 should now be a useful starting point for further reactivity studies, taking advantage of the good leaving group properties of the NO3- ligands.