Coulombic Aggregations of MnIII salen-Type Complexes and Keggin-Type Polyoxometalates: Isolation of Mn2 Single-Molecule Magnets

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Abstract
The reaction of MnIII salen-type complexes with di- and tetraanionic α-Keggin-type polyoxometalates (POMs) was performed, and three types of Coulombic aggregations containing MnIII out-of-plane dimeric units (abbreviated as [Mn2]2+) that are potentially single-molecule magnets (SMMs) with an ST = 4 ground state were synthesized: [Mn2(5-MeOsaltmen)2(acetone)2][SW12O40] (1), [Mn2(salen)2(H2O)2]2[SiW12O40] (2), and [Mn(5-Brsaltmen)(H2O)(acetone)]2[{Mn2(5-Brsaltmen)2}(SiW12O40)] (3), where 5-Rsaltmen2– = N,N′-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate) with R = MeO (methoxy), Br (bromo) and salen2– = N,N′-ethylenebis(salicylideneiminate). Compound 1 with a dianionic POM, [SW12O40]2–, is composed of a 1:1 aggregating set of [Mn2]2+/POM, and 2, with a tetraanionic POM, [SiW12O40]4–, is a 2:1 set. Compound 3 with [SiW12O40]4– forms a unique 1D coordinating chain with a [−{Mn2}–POM−]2– repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H2O)(acetone)]22+) is present as a countercation. Independent of the formula ratio of [Mn2]2+/POM, MnIII dimers and POM units in 1–3 form respective segregated columns along a direction of the unit cell, which make an alternate packing to separate evenly identical species in a crystal. The nearest intermolecular Mn···Mn distance is found in the order 2 < 3 < 1. The segregation of the [Mn2]2+ dimer resulted in interdimer distances long enough to effectively reduce the intermolecular magnetic interaction, in particular in 1 and 3. Consequently, an intrinsic property, SMM behavior, of MnIII dimers has been characterized in this system, even though the interdimer interactions are still crucial in the case of 2, where a long-range magnetic order competitively affects slow relaxation of the magnetization at low ac frequencies.

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