Coulombic Aggregations
of Mn<sup>III</sup> salen-Type Complexes and Keggin-Type Polyoxometalates:
Isolation
of Mn<sub>2</sub> Single-Molecule Magnets
- Publication date
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Abstract
The reaction of Mn<sup>III</sup> salen-type complexes
with di-
and tetraanionic α-Keggin-type polyoxometalates (POMs) was performed,
and three types of Coulombic aggregations containing Mn<sup>III</sup> out-of-plane dimeric units (abbreviated as [Mn<sub>2</sub>]<sup>2+</sup>) that are potentially single-molecule magnets (SMMs) with
an <i>S</i><sub>T</sub> = 4 ground state were synthesized:
[Mn<sub>2</sub>(5-MeOsaltmen)<sub>2</sub>(acetone)<sub>2</sub>][SW<sub>12</sub>O<sub>40</sub>] (<b>1</b>), [Mn<sub>2</sub>(salen)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub>2</sub>[SiW<sub>12</sub>O<sub>40</sub>] (<b>2</b>), and [Mn(5-Brsaltmen)(H<sub>2</sub>O)(acetone)]<sub>2</sub>[{Mn<sub>2</sub>(5-Brsaltmen)<sub>2</sub>}(SiW<sub>12</sub>O<sub>40</sub>)] (<b>3</b>), where 5-Rsaltmen<sup>2–</sup> = <i>N</i>,<i>N</i>′-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate)
with R = MeO (methoxy), Br (bromo) and salen<sup>2–</sup> = <i>N</i>,<i>N</i>′-ethylenebis(salicylideneiminate).
Compound <b>1</b> with a dianionic POM, [SW<sub>12</sub>O<sub>40</sub>]<sup>2–</sup>, is composed of a 1:1 aggregating set
of [Mn<sub>2</sub>]<sup>2+</sup>/POM, and <b>2</b>, with a tetraanionic
POM, [SiW<sub>12</sub>O<sub>40</sub>]<sup>4–</sup>, is a 2:1
set. Compound <b>3</b> with [SiW<sub>12</sub>O<sub>40</sub>]<sup>4–</sup> forms a unique 1D coordinating chain with a [−{Mn<sub>2</sub>}–POM−]<sup>2–</sup> repeating unit,
for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H<sub>2</sub>O)(acetone)]<sub>2</sub><sup>2+</sup>) is present as a countercation.
Independent of the formula ratio of [Mn<sub>2</sub>]<sup>2+</sup>/POM,
Mn<sup>III</sup> dimers and POM units in <b>1</b>–<b>3</b> 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 <b>2</b> < <b>3</b> < <b>1</b>. The segregation of the [Mn<sub>2</sub>]<sup>2+</sup> dimer
resulted in interdimer distances long enough to effectively reduce
the intermolecular magnetic interaction, in particular in <b>1</b> and <b>3</b>. Consequently, an intrinsic property, SMM behavior,
of Mn<sup>III</sup> dimers has been characterized in this system,
even though the interdimer interactions are still crucial in the case
of <b>2</b>, where a long-range magnetic order competitively
affects slow relaxation of the magnetization at low ac frequencies
