posted on 2005-12-14, 00:00authored byLollita Lecren, Olivier Roubeau, Claude Coulon, Yang-Guang Li, Xavier F. Le Goff, Wolfgang Wernsdorfer, Hitoshi Miyasaka, Rodolphe Clérac
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.