High nuclearity single-molecule magnets: A mixed-valence Mn26 cluster containing the di-2-pyridylketone diolate dianion

Abstract

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)2·6H 2O, dpk, NaOMe, and NEt3 (2:1:4:2) in MeCN/MeOH affords complex 3, which possesses a rare metal topology and is mixed-valence (4Mn II, 22MnIII). The complicated [Mn26(μ 4-O)10(μ3-O)6(μ3- OMe)12(μ-OR)12]18+ core of 3 consists of an internal MnIII16 cage of adjacent Mn4 tetrahedra surrounded by an external MnIIMnIII6 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 (χM) 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 Ueff = 30 K, where Ueff 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 Ms = ±6 levels. The combined work demonstrates the ligating flexibility of dipyridyl-diolate chelates and their usefulness in the synthesis of polynuclear Mnx clusters with interesting magnetic properties, without requiring the co-presence of carboxylate ligands. © 2008 American Chemical Society.