| Abstract
| - Four discrete MnIII/MnII tetranuclear complexes with a double-cuboidal core, [Mn4(hmp)6(CH3CN)2(H2O)4](ClO4)4·2CH3CN (1), [Mn4(hmp)6(H2O)4](ClO4)4·2H2O (2), [Mn4(hmp)6(H2O)2(NO3)2](ClO4)2·4H2O(3), and [Mn4(hmp)6(Hhmp)2](ClO4)4·2CH3CN (4), were synthesized by reaction of Hhmp (2-hydroxymethylpyridine) with Mn(ClO4)2·6H2O in the presence of tetraethylammonium hydroxide and subsequent additionof NaNO3 (3) or an excess of Hhmp (4). Direct current (dc) magnetic measurements show that both Mn2+−Mn3+ and Mn3+−Mn3+ magnetic interactions are ferromagnetic in 1−3 leading to an ST = 9 ground statefor the Mn4 unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing boththermally activated and ground-state tunneling regimes. Slight changes in the [Mn4] core geometry resultin an ST = 1 ground state in 4. A one-dimensional assembly of [Mn4] units, catena-{[Mn4(hmp)6(N3)2](ClO4)2}(5), was obtained in the same synthetic conditions with the subsequent addition of NaN3. Double chairlikeN3- bridges connect identical [Mn4] units into a chain arrangement. This material behaves as an Isingassembly of ST = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetizationis observed at low temperature for the first time in an antiferromagnetic chain, following an activated behaviorwith Δτ/kB = 47 K and τ0 = 7 × 10-11 s. The observation of this original thermally activated relaxationprocess is induced by finite-size effects and in particular by the noncompensation of spins in segments ofodd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segmentsof antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gapsΔξ and Δτ were estimated and successfully compared to the experimental values.
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