Single-molecule magnets:: Novel Mn8 and Mn9 carboxylate clusters containing an unusual pentadentate ligand derived from pyridine-2,6-dimethanol

The reactions of the Mn-3(III) and (MnMn2III)-Mn-II complexes [Mn3O(O2CEt)(6)(py)(3)][ClO4] and [Mn3O(O2CEt)(6)(PY)(3)] with pyridine-2,6-dimethanol (pdmH(2)) afford the mixed-valence (Mn6Mn2III)-Mn-II octanuclear complex [Mn8O2(py)(4)(O2CEt)(8)(L)(2)][ClO4](2) (1) and the (Mn7Mn2III)-Mn-II enneanuclear complex [Mn-9(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 pdmH(2). Complex 1 crystallizes in the monoclinic space group P2(1)/n with the following cell parameters at -160 degreesC: a = 16.6942(5) Angstrom, b = 13.8473(4) Angstrom, c = 20.0766(6) Angstrom, beta = 99.880(1)degrees, V = 4572.27 Angstrom(3), and Z = 2, R (R-w) = 4.78 (5.25). Complex 2-0.2MeCN crystallizes in the triclinic space group P (1) over bar with the following cell parameters at -157 degreesC: a = 12,1312(4) Angstrom, b = 18.8481(6) Angstrom, c = 23.2600(7) Angstrom, alpha = 78.6887(8)degrees, beta = 77.9596(8)degrees, gamma = 82.3176(8)degrees, V = 5076.45 Angstrom(3), and Z = 2, R (R-w) = 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 pdmH(2). Complexes 1 and 2 were characterized by variable-temperature 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 <1 K show hysteresis behavior at <0.3 K, establishing 2 as a new single-molecule magnet. Magnetization decay measurements gave an effective barrier to magnetization relaxation of U-eff = 3.1 cm(-1) = 4.5 K.