[Mn2III(5-Rsaltmen)2NiII(pao)2(L)]2+ :: An ST=3 building block for a single-chain magnet that behaves as a single-molecule magnet

Mn-III-Ni-II-Mn-III linear-type trinuclear complexes bridged by oximate groups were selectively synthesized by the assembly reaction of [Mn-2(5-Rsaltmen)(2)(H2O)(2)] (ClO4)(2) (5-Rsaltmen(2-)=N,N'-(1,1,2,2-tetramethylethylene) bis(5-R-salicylideneiminate); R=Cl, Br) with [Ni(pao)(2)(phen)] (pao(-)=pyridine-2-aldoximate; phen=1,10-phenanthroline) in methanol/water: [Mn-2(5-Rsaltmen)(2)Ni(pao)(2)(phen)](ClO4)(2) (R=Cl, 1; R=Br, 2). Structural analysis revealed that the [Mn-III-ON-Ni-II-NO-Mn-III] skeleton of these trimers is in every respect similar to the repeating unit found in the previously reported series of 1D materials [Mn-2(saltmen)(2)Ni(pao)(2)(L-1)(2)](A)(2) (L-1=pyridine, 4-picoline, 4-tert-butylpyridine, N-methylimidazole; A=ClO4-, BF4-, PF6-, ReO4-). Recently, these 1D compounds have attracted a great deal of attention for their magnetic properties, since they exhibit slow relaxation of the magnetization (also called single-chain magnet (SCM) behavior). This unique magnetic behavior wag explained in the framework of Glauber's theory, generalized for chains of ferromagnetically coupled anisotropic spins. Thus, in these ID compounds, the [Mn-III-ON-Ni-II-NO-Mn-III] unit was considered as an S-T=3 anisotropic spin. Direct-current magnetic measurements on 1 and 2 confirm their S-T=3 ground state and strong uniaxial anisotropy (D/k(B)approximate to-2.4 K), in excellent agreement with the magnetic characteristic deduced in the study on the SCM series. The ac magnetic susceptibility of these trimers is strongly frequency-dependent and characteristic of single-molecule magnet (SMM) behavior. The relaxation time tau shows a thermally activated (Arrhenius) behavior with tau(0)approximate to 1x10(-7) s and Delta(eff)/k(B)approximate to 18 K. The effective energy barrier for reversal of the magnetization Delta(eff) is consistent with the theoretical value (21 K) estimated from vertical bar D vertical bar S-T(2). The present results reinforce consistently the interpretation of the SCM behavior observed in the [Mn-2(saltmen)(2)Ni(pao)(2)(L-1)(2)](A)(2) series and opens new perspectives to design single-chain magnets.