High Nuclearity Single-Molecule Magnets: a Mixed-Valence Mn26 Cluster Containing the Di-2-pyridylketone Diolate Dianion

The employment of the dianion (dpkd2−) of the gem-diol form of di-2-pyridylketone (dpk) as a tetradentate chelate in manganese chemistry is reported, and the synthesis, crystal structure, and magnetochemical characterization of [Mn26O16(OMe)12(dpkd)12(MeOH)6](OH)6·solv (3·solv) are described. The reaction of Mn(ClO4)2·6H2O, dpk, NaOMe, and NEt3 (2:1:4:2) in MeCN/MeOH affords complex 3, which possesses a rare metal topology and is mixed-valence (4MnII, 22MnIII). The complicated [Mn26(μ4-O)10(μ3-O)6(μ3-OMe)12(μ-OR)12]18+ core of 3 consists of an internal MnIII 16 cage of adjacent Mn4 tetrahedra surrounded by an external MnII 4MnIII 6 shell. The latter is held together by the alkoxide arms of twelve η1:η2:η1:η1:μ3 dpkd2− groups. Variable-temperature, solid-state direct current (dc), and alternating current (ac) magnetization studies were carried out on 3 in the 1.8−300 K range. Complex 3 is predominantly antiferromagnetically coupled with a resulting S = 6 ground state, a conclusion confirmed by the in-phase (χ′M) ac susceptibility data. The observation of out-of-phase (χ′′M) ac susceptibility signals suggested that 3 might be a single-molecule magnet, and this was confirmed by single-crystal magnetization vs dc field sweeps that exhibited hysteresis, the diagnostic property of a magnet. Combined ac χ′′M and magnetization decay vs time data collected below 1.1 K were used to construct an Arrhenius plot; the fit of the thermally activated region above ∼0.1 K gave U eff = 30 K, where U eff is the effective relaxation barrier. At lower temperatures, the complex exhibits temperature-independent relaxation, characteristic of ground-state quantum tunneling of magnetization between the lowest-lying M s = ±6 levels. The combined work demonstrates the ligating flexibility of dipyridyl-diolate chelates and their usefulness in the synthesis of polynuclear Mn x clusters with interesting magnetic properties, without requiring the co-presence of carboxylate ligands.