Single-molecule magnets: preparation and properties of mixed-carboxylate complexes

Methods are reported for the preparation of mixed-carboxylate versions of the Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] family of single-molecule magnets (SMMs). Mn(12)O(12)(O(2)CCHCl(2))(8)(O(2)CCH(2)Bu(t))(8)(H(2)O)(3)] (5) and Mn(12)O(12)(O(2)CHCl(2))(8)(O(2)CEt)(8)(H(2)O)(3)] (6) have been obtained from the 1:1 reaction of the corresponding homocarboxylate species. Complex 5.CH(2)Cl(2).H(2)O crystallizes in the triclinic space group P1 with, at -165 degrees C, a = 15.762(1), b = 16.246(1), c = 23.822(1) A, alpha = 103.92(1), beta = 104.50(1), gamma = 94.23(1) degrees, Z = 2, and V = 5674(2) A(3). Complex 6.CH(2)Cl(2) crystallizes in the triclinic space group P1 with, at -158 degrees C, a = 13.4635(3), b = 13.5162(3), c = 23.2609(5) A, alpha = 84.9796(6), beta = 89.0063(8), gamma = 86.2375(6) degrees, Z = 2, and V = 4207.3(3) A(3). Complexes 5 and 6 both contain a Mn(12)O(12)] core with the CHCl(2)CO(2-) ligands ordered in the axial positions and the RCO(2-) ligands (R = CH(2)Bu(t) (5) or Et (6)) in equatorial positions. There is, thus, a preference for the CHCl(2)CO(2-) 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 Mn(12)O(12)(O(2)CR)(16)(H(2)O)(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. (1)H NMR spectra in CD(2)Cl(2) 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.