Single-Molecule Magnets:  Preparation and Properties of Mixed-Carboxylate Complexes [Mn 12 O 12 (O 2 CR) 8 (O 2 CR‘) 8 (H 2 O) 4 ] (original) (raw)

2001, Inorganic Chemistry

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 h with, at -165°C, a ) 15.762(1), b ) 16.246-(1), c ) 23.822(1) Å, R ) 103.92(1), ) 104.50(1), γ ) 94.23(1)°, Z ) 2, and V ) 5674(2) Å 3 . Complex 6‚CH 2 Cl 2 crystallizes in the triclinic space group P1 h with, at -158°C, a ) 13.4635(3), b ) 13.5162(3), c ) 23.2609(5) Å, R ) 84.9796(6), ) 89.0063(8), γ ) 86.2375(6)°, Z ) 2, and V ) 4207.3(3) Å 3 . Complexes 5 and 6 both contain a [Mn 12 O 12 ] core with the CHCl 2 CO 2ligands ordered in the axial positions and the RCO 2ligands (R ) CH 2 Bu t (5) or Et ) in equatorial positions. There is, thus, a preference for the CHCl 2 CO 2to 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 ( M ′′) signals characteristic of the slow relaxation in the millisecond time scale of single-molecule magnets. Arrhenius plots obtained from 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. (6) (a) Aubin, S. M. J.; Sun, Z.; Guzei, I. A.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Chem. Commun. 1997, 2239. (b) Sun, Z.; Ruiz, D.; Rumberger, E.; Incarvito, C. D.; Folting, K.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Inorg. Chem. 1998, 37, 4758. (7) (a) Brechin, E. K.; Yoo, J.; Nakano, M.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Chem. Commun. 1999, 783. (b) Yoo, J.; Brechin, E. K.; Yamaguchi, A.; Nakano, M.; Huffman, J. C.; Maniero, A. L.; Brunel, L.-C.; Awaga, K.; Ishimoto, H.; Christou, G.; Hendrickson, D. N. Caneschi, A.; Cornia, A.; Fabrizi de Biani, F.; Gatteschi, D.; Sangregorio, C.; Sessoli, R.; Sorace, L.