Slow Relaxation in a One-Dimensional Rational Assembly of Antiferromagnetically Coupled [Mn4] Single-Molecule Magnets

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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 addition of 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 state for the Mn4 unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing both thermally activated and ground-state tunneling regimes. Slight changes in the [Mn4] core geometry result in 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 chairlike N3- bridges connect identical [Mn4] units into a chain arrangement. This material behaves as an Ising assembly of ST = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Δτ/kB = 47 K and τ0 = 7 × 10-11 s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the noncompensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gaps Δξ and Δτ were estimated and successfully compared to the experimental values.