Single-Molecule Magnets: Novel Mn8 and Mn9 Carboxylate Clusters Containing an Unusual Pentadentate Ligand Derived from Pyridine-2,6-dimethanol

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Abstract
The reactions of the MnIII3 and MnIIMnIII2 complexes [Mn3O(O2CEt)6(py)3][ClO4] and [Mn3O(O2CEt)6(py)3] with pyridine-2,6-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 P1̄ with the following cell parameters at −157 °C: a = 12.1312(4) Å, b = 18.8481(6) Å, c = 23.2600(7) Å, α = 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 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 Ueff = 3.1 cm-1 = 4.5 K.