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

摘要

ABSTRACT: The reaction of MnIII salen-type complexes with diandntetraanionic α-Keggin-type polyoxometalates (POMs) wasnperformed, and three types of Coulombic aggregations containingnMnIII out-of-plane dimeric units (abbreviated as [Mn2]2+) that arenpotentially single-molecule magnets (SMMs) with an ST = 4 groundnstate were synthesized: [Mn2(5-MeOsaltmen)2(acetone)2]-n[SW12O40] (1), [Mn2(salen)2(H2O)2]2[SiW12O40] (2), andn[Mn(5-Brsaltmen)(H2O)(acetone)]2[{Mn2(5-Brsaltmen)2}-n(SiW12O40)] (3), where 5-Rsaltmen2− = N,N′-(1,1,2,2-ntetramethylethylene)bis(5-R-salicylideneiminate) with R = MeOn(methoxy), Br (bromo) and salen2− = N,N′-ethylenebis-n(salicylideneiminate). Compound 1 with a dianionic POM,n[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 1Dncoordinating chain with a [−{Mn2}−POM−]2− repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-nBrsaltmen)(H2O)(acetone)]2n2+) is present as a countercation. Independent of the formula ratio of [Mn2]2+/POM, MnIII dimersnand POM units in 1−3 form respective segregated columns along a direction of the unit cell, which make an alternate packing tonseparate evenly identical species in a crystal. The nearest intermolecular Mn···Mn distance is found in the order 2 < 3 < 1. Thensegregation of the [Mn2]2+ dimer resulted in interdimer distances long enough to effectively reduce the intermolecular magneticninteraction, in particular in 1 and 3. Consequently, an intrinsic property, SMM behavior, of MnIII dimers has been characterizednin this system, even though the interdimer interactions are still crucial in the case of 2, where a long-range magnetic orderncompetitively affects slow relaxation of the magnetization at low ac frequencies.