Single-Molecule Magnets: Preparation and Properties of Mixed-Carboxylate Complexes [Mn12O12(O2CR)8(O2CR‘)8(H2O)4]

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Abstract
Methods are reported for the preparation of mixed-carboxylate versions of the [Mn12O12(O2CR)16(H2O)4] family of single-molecule magnets (SMMs). [Mn12O12(O2CCHCl2)8(O2CCH2But)8(H2O)3] (5) and [Mn12O12(O2CHCl2)8(O2CEt)8(H2O)3] (6) have been obtained from the 1:1 reaction of the corresponding homocarboxylate species. Complex 5·CH2Cl2·H2O crystallizes in the triclinic space group P1̄ with, at −165 °C, a = 15.762(1), b = 16.246(1), c = 23.822(1) Å, α = 103.92(1), β = 104.50(1), γ = 94.23(1)°, Z = 2, and V = 5674(2) Å3. Complex 6·CH2Cl2 crystallizes in the triclinic space group P1̄ with, at −158 °C, a = 13.4635(3), b = 13.5162(3), c = 23.2609(5) Å, α = 84.9796(6), β = 89.0063(8), γ = 86.2375(6)°, Z = 2, and V = 4207.3(3) Å3. Complexes 5 and 6 both contain a [Mn12O12] core with the CHCl2CO2- ligands ordered in the axial positions and the RCO2- ligands (R = CH2But (5) or Et (6)) in equatorial positions. There is, thus, a preference for the CHCl2CO2- to occupy the sites lying on the MnIII Jahn−Teller axes, and this is rationalized on the basis of the relative basicities of the carboxylate groups. Direct current magnetic susceptibility studies in a 10.0 kG field in the 2.00−300 K range indicate a large ground-state spin, and fitting of magnetization data collected in the 10.0−70.0 kG field and 1.80−4.00 K temperature range gave S = 10, g = 1.89, and D = −0.65 K for 5, and S = 10, g = 1.83, and D = −0.60 K for 6. These values are typical of [Mn12O12(O2CR)16(H2O)4] complexes. Alternating current susceptibility studies show the out-of-phase susceptibility (χM‘ ‘) signals characteristic of the slow relaxation in the millisecond time scale of single-molecule magnets. Arrhenius plots obtained from χM‘ ‘ versus T data gave effective barriers to relaxation (Ueff) of 71 and 72 K for 5 and 6, respectively. 1H NMR spectra in CD2Cl2 show that 5 and 6 are the main species present on dissolution, but there is evidence for some ligand distribution between axial and equatorial sites, by intra- and/or intermolecular exchange processes.

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