[Mn${{{{III}\hfill \atop 2\hfill}}}$(5‐Rsaltmen)2NiII(pao)2(L)]2+: An ST=3 Building Block for a Single‐Chain Magnet That Behaves as a Single‐Molecule Magnet

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Abstract
MnIII‐NiII‐MnIII linear‐type trinuclear complexes bridged by oximate groups were selectively synthesized by the assembly reaction of [Mn2(5‐Rsaltmen)2(H2O)2](ClO4)2 (5‐Rsaltmen2−=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: [Mn2(5‐Rsaltmen)2Ni(pao)2(phen)](ClO4)2 (R=Cl, 1; R=Br, 2). Structural analysis revealed that the [MnIII‐ON‐NiII‐NO‐MnIII] skeleton of these trimers is in every respect similar to the repeating unit found in the previously reported series of 1D materials [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 (L1=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 was explained in the framework of Glauber's theory, generalized for chains of ferromagnetically coupled anisotropic spins. Thus, in these 1D compounds, the [MnIII‐ON‐NiII‐NO‐MnIII] unit was considered as an ST=3 anisotropic spin. Direct‐current magnetic measurements on 1 and 2 confirm their ST=3 ground state and strong uniaxial anisotropy (D/kB≈−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 τ shows a thermally activated (Arrhenius) behavior with τ0≈1×10−7 s and Δeff/kB≈18 K. The effective energy barrier for reversal of the magnetization Δeff is consistent with the theoretical value (21 K) estimated from |D|${S{{2\hfill \atop {\rm T}\hfill}}}$. The present results reinforce consistently the interpretation of the SCM behavior observed in the [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 series and opens new perspectives to design single‐chain magnets.