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]

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 degreesC: a = 21.280(5), b = 34.430(8), c = 33.023(8) Angstrom, beta = 104.61(1)degrees, V = 23413 Angstrom, 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. H-1 NMR spectroscopy in CD2Cl2 and CDCl3 shows retention of the solid-state structure on dissolution in, these solvents. DC magnetic susceptibility (chi (M)) and magnetization (M) studies have been carried out in the 2.00-300 K and 1.0-7.0 T ranges. Fits Of MINUB versus HIT 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 (chi (M)") 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 (U-eff) of 50.0 cm(-1) (72.0 K) and a pre-exponential (1/tau (0)) of 1.9 x 10(8) 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 M, states of the S = 10 ground state. Fitting of the data gave S = 10, g = 1.99, D = -0.46 cm(-1), and B-4(0) = -2.0 x 10(-5), where B-4(0) 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 Mn-12 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.