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

The reaction of Mn-III salen-type complexes with di- and tetraanionic alpha-Keggin-type polyoxometalates (POMs) was performed, and three types of Coulombic aggregations containing Mn-III out-of-plane dimeric units (abbreviated as [Mn-2](2+)) that are potentially single-molecule magnets (SMMs) with an S-T = 4 ground state were synthesized: [Mn-2(5-MeOsaltmen)(2)(acetone)(2)]-[SW12O40] (1), [Mn-2(salen)(2)(H2O)(2)](2)[SiW12O40] (2), and [Mn(5-Brsaltmen)(H2O)(acetone)](2)[{Mn-2(5-Brsaltmen)(2)}-(SiW12O40)] (3), where 5-Rsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate) with R = MeO (methoxy), Br (bromo) and salen(2-) = N,N'-ethylenebis(salicylideneiminate). Compound 1 with a dianionic POM, [SW12O40](2-), is composed of a 1:1 aggregating set of [Mn-2](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 [-{Mn-2}-POM-](2-) repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H2O)(acetone)](2)(2+)) is present as a countercation. Independent of the formula ratio of [Mn-2](2+)/POM, Mn-III 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 center dot center dot center dot Mn distance is found in the order 2 < 3 < 1. The segregation of the [Mn-2](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 Mn-III 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.