Employment of 2,6-diacetylpyridine dioxime as a new route to high nuclearity metal clusters: Mn6 and Mn8 complexes

The employment of the anion of 2,6-diacetylpyridine dioxime (dapdoH2) as a pentadentate chelate in transition metal cluster chemistry is reported. The syntheses, crystal structures, and magnetochemical characterization are described for Mn6O2(OMe)2(dapdo)2(dapdoH)4](ClO4)2 (1), Mn6O2(OMe)2(dapdo)2(dapdoH)4]Ca(NO3)4] (2), and Mn8O4(OH)4(OMe)2(N3)2(dapdo)2(dapdoH)2(H2O)2] (3). The reaction of Mn3O(O2CMe)6(py)3](ClO4) with 3 equiv of dapdoH2 (with or without 2 equiv of NEt3) in MeOH gave 1. The same cation, but with a Ca(NO3)4]2- anion, was found in complex 2, which was obtained from the reaction in MeOH between Mn(NO3)2, Ca(NO3)2, and dapdoH2 in the presence of NEt3. In contrast, addition of NaN3 to several reactions comprising MnCl2, dapdoH2, and NEt3 in MeOH gave the octanuclear complex 3. Complexes 1-3 all possess rare topologies and are mixed-valence: 2MnII, 4MnIII for 1 and 2, and 2MnII, 6MnIII for 3. The core of the cation of 1 and 2 consists of two edge-sharing Mn4 tetrahedra at the center of each of which is a micro4-O2- ion. Peripheral ligation is provided by two micro-OMe-, four micro-dapdoH-, and two micro3-dapdo2- groups. The core of 3 consists of two MnIIMnIII3(micro3-O)2]7+ "butterfly" units linked together by one of the micro3-O2- ions, which thus becomes micro4. Peripheral ligation is provided by four micro-OMe-, two micro-OH-, two micro-dapdoH-, and two micro4-dapdo2- groups. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-3 in the 5.0-300 K range; the data for 1 and 2 are identical. Fitting of the obtained magnetization versus field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting, D) established that 1 possesses an S=5 ground state with D=-0.24 cm(-1). For 3, low-lying excited states precluded obtaining a good fit from the magnetization data, and the ground state was instead determined from the ac data, which indicated an S=1 ground state for 3. The combined work demonstrates the ligating flexibility of pyridyl-dioxime chelates and their usefulness in the synthesis of new polynuclear Mnx clusters without requiring the co-presence of carboxylate ligands.