Single-molecule magnets:: Preparation and properties of mixed-carboxylate complexes [Mn12O12(O2CR)8(O2CR′)8(H2O)4]

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 P (1) over bar with, at -165 degreesC, a = 15.762(1), b = 16.246(1), c = 23.822(1) Angstrom, alpha = 103.92(1), beta = 104.50(1), gamma = 94.23(1)degrees, Z = 2, and V = 5674(2) Angstrom (3). Complex 6 . CH2Cl2 crystallizes in the triclinic space group P (1) over bar with, at -158 degreesC, a = 13.4635(3), b = 13.5162(3), c = 23.2609(5) Angstrom, alpha = 84.9796(6), beta = 89.0063(8), gamma = 86.2375(6)degrees, Z = 2, and V = 4207.3(3) Angstrom (3). 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 Mn-III 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 (chi (M)") signals characteristic of the slow relaxation in the millisecond time scale of single-molecule magnets. Arrhenius plots obtained from chi (M)" versus T data gave effective barriers to relaxation (U-eff) of 71 and 72 K for 5 and 6, respectively. H-1 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.