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

The employment of the dianion (dpkd(2-)) 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) center dot Solv (3 . solv) are described. The reaction of Mn(ClO4)(2) center dot 6H(2)O, dpk, NaMe, 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 [Mn-26(mu(4)-O)(10)(mu(3)-O)(6)(mu(3)-OMe)(12)(mu-OR)(12)](18+) core of 3 consists of an internal Mn-16(III) cage of adjacent Mn-4 tetrahedra surrounded by an external (Mn4Mn6III)-Mn-II shell. The latter is held together by the alkoxide arms of twelve eta(1):eta(2):eta(1):eta(1):mu(3) dpkd(2-) 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 (chi'm) ac susceptibility data. The observation of out-of-phase (chi '' 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 chi '' 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 similar to 0.1 K gave U-eff = 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 M-s = +/- 6 levels. The combined work demonstrates the ligating flexibility of dipyridyl-diolate chelates and their usefulness in the synthesis of polynuclear Mn. clusters with interesting magnetic properties, without requiring the co-presence of carboxylate ligands.