Slow Relaxation in a One-Dimensional Rational Assembly of Antiferromagnetically Coupled [Mn_4] Single-Molecule Magnets

摘要

Four discrete Mn~(III)/Mn~(II)tetranuclear complexes with a double-cuboidal core,[Mn_4(hmp)_6(CH_3-CN)_2(H_2O)_4](ClO_4)_4centre dot 2CH_3CN(1),[Mn_4(hmp)_6(H_2O)_4](ClO_4)4 centre dot 2H_2O(2),[Mn_4(hmp)_6(H_2O)_2(NO_3)_2](ClO_4)_2centre dot 4H_2O(3),and [Mn_4(hmp)_6(Hhmp)_2](ClO_4)_4 centre dot 2CH_3CN(4),were synthesized by reaction of Hhmp(2-hydroxymethyl-pyridine)with Mn(ClO_4)_2centre dot6H_2O in the presence of tetraethylammonium hydroxide and subsequent addition of NaNO_3(3)or an excess of Hhmp(4).Direct current(dc)magnetic measurements show that both Mn~(2+)-Mn~(3+)and Mn~(3+)-Mn~(3+)magnetic interactions are ferromagnetic in 1-3 leading to an S_T = 9 ground state for the Mn_4 unit.Furthermore,these complexes are single-molecule magnets(SMMs)clearly showing both thermally activated and ground-state tunneling regimes.Slight changes in the [Mn_4] core geometry result in an S_T = 1 ground state in 4.A one-dimensional assembly of [Mn_4] units,catena-{[Mn_4(hmp)_6(N_3)_2](ClO_4)_2}(5),was obtained in the same synthetic conditions with the subsequent addition of NaN_3.Double chairlike N_3~-bridges connect identical [Mn_4] units into a chain arrangement.This material behaves as an Ising assembly of S_T= 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 DELTA_tau/k_B = 47 K and tau_0 = 7 x 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 DELTA_(zeta)and DELTA_(tau)were estimated and successfully compared to the experimental values.