Inorg. Chem. 2002, 41, 5107−5118
Single-Molecule Magnets: Novel Mn8 and Mn9 Carboxylate Clusters Containing an Unusual Pentadentate Ligand Derived from Pyridine-2,6-dimethanol Colette Boskovic,1a Wolfgang Wernsdorfer,1b Kirsten Folting,1a John C. Huffman,1a David N. Hendrickson,*,1c and George Christou*,†,1a Department of Chemistry and the Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana 47405-7102, Laboratoire Louis Ne´ el-CNRS, BP166, 25 AVenue des Martyrs, 38042 Grenoble, Cedex 9, France, and Department of Chemistry-0358, UniVersity of California at San Diego, La Jolla, California 92093-0358 Received March 20, 2002
The reactions of the MnIII3 and MnIIMnIII2 complexes [Mn3O(O2CEt)6(py)3][ClO4] and [Mn3O(O2CEt)6(py)3] with pyridine2,6-dimethanol (pdmH2) afford the mixed-valence MnII6MnIII2 octanuclear complex [Mn8O2(py)4(O2CEt)8(L)2][ClO4]2 (1) and the MnII7MnIII2 enneanuclear complex [Mn9(O2CEt)12(pdm)(pdmH)2(L)2] (2), respectively. Both compounds contain a novel pentadentate ligand, the dianion of (6-hydroxymethylpyridin-2-yl)-(6-hydroxymethylpyridin-2-ylmethoxy)methanol (LH2), which is the hemiacetal formed in situ from the Mn-assisted oxidation of pdmH2. Complex 1 crystallizes in the monoclinic space group P21/n with the following cell parameters at −160 °C: a ) 16.6942(5) Å, b ) 13.8473(4) Å, c ) 20.0766(6) Å, β ) 99.880(1)°, V ) 4572.27 Å3, and Z ) 2, R (Rw) ) 4.78 (5.25). Complex 2‚0.2MeCN crystallizes in the triclinic space group P1h with the following cell parameters at −157 °C: a ) 12.1312(4) Å, b ) 18.8481(6) Å, c ) 23.2600(7) Å, R ) 78.6887(8)°, β ) 77.9596(8)°, γ ) 82.3176(8)°, V ) 5076.45 Å3, and Z ) 2, R (Rw) ) 4.12 (4.03). Both complexes are new structural types comprising distorted-cubane units linked together, albeit in two very different ways. In addition, complex 2 features three distinct binding modes for the chelating ligands derived from deprotonated pdmH2. Complexes 1 and 2 were characterized by variabletemperature ac and dc magnetic susceptibility measurements and found to possess spin ground states of 0 and 11/2, respectively. Least-squares fitting of the reduced magnetization data gave S ) 11/2, g ) 2.0, and D ) −0.11 cm-1 for complex 2, where D is the axial zero-field splitting parameter. Direct current magnetization versus field studies on 2 at 15 kG were included, the fits were not as good. This suggests that low-energy excited states with S > 11/2 are populated, even at these relatively low temperatures. As for the S ) 0 situation mentioned above, this is also a common problem in higher nuclearity Mnx systems with S > 0, for example, [Mn30O24(OH)8(O2CCH2tBu)32(H2O)2(CH3NO2)4]39 and [Mn12O8Cl4(O2CPh)8(hmp)6],12 particularly when MnII ions are present as in these two examples, since these typically give weak exchange interactions and resulting small energy separations. The S ) 11/2 and D ) -0.11 cm-1 values suggest that 2 should have an anisotropy barrier to magnetization reversal of (S2 - 1/4)|D|, or 3.3 cm-1 (4.7 K), which could be large enough to result in 2 displaying the slow magnetization relaxation characteristic of a single-molecule magnet (SMM). Thus, to both confirm the spin ground state of complex 1 and investigate the possibility of SMM behavior, ac susceptibility measurements were performed with a 3.5 G ac field oscillating at 997 Hz (Figure 7). The ac data are essentially (39) Soler M.; Rumberger, E.; Folting, K.; Hendrickson, D. N.; Christou, G. Polyhedron 2001, 20, 1365.
5116 Inorganic Chemistry, Vol. 41, No. 20, 2002
Figure 7. Plot of χ′MT vs temperature for complex 2 in the 2.0-30 K range from ac magnetic susceptibility measurements (4) and including the dc χMT data (b) from Figure 4 for this temperature range. The ac data were measured with a 3.5 G ac field oscillating at 997 Hz, and χM is the dc magnetic susceptibilty measured in a 10 kG field.
Figure 8. Plot of magnetization (normalized to the saturation magnetization Ms) vs applied field for complex 2 in the 0.04-4.0 K range at a sweep rate of 8 mT/s.
superimposable with the dc data at temperatures above 15 K. However, as the temperature is further decreased, the value of χM′T increases slightly to give a value of 18.4 cm3 mol-1 K at 1.8 K. This is consistent with an S ) 11/2 spin ground state (17.9 cm3 mol-1 K for g ) 2). No signal was observed in the out-of-phase susceptibility (χM′′) at temperatures down to 1.8 K, suggesting that the barrier to magnetization relaxation is too small to give a nonzero χM′′ signal characteristic of a SMM. However, it was possible that 2 might still exhibit SMM behavior at even lower temperatures, and dc magnetization measurements were therefore performed at
