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 (4Mn^II, 22Mn^III). The complicated [Mn₂₆(μ₄-O)₁₀(μ₃-O)₆(μ₃-OMe)₁₂(μ-OR)₁₂]¹⁸⁺ core of 3 consists of an internal Mn^III₁₆ cage of adjacent Mn₄ tetrahedra surrounded by an external Mn^II₄Mn^III₆ 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.