Single-Molecule Magnet Behavior in Heterometallic M^II−Mn^III₂−M^II Tetramers (M^II = Cu, Ni) Containing Mn^III Salen-Type Dinuclear Core

The linear-type heterometallic tetramers, [MnIII2(5-MeOsaltmen)2MII2(L)2](CF3SO3)2·2H2O (MII = Cu, 1a; Ni, 2a), where 5-MeOsaltmen2- = N,N‘-(1,1,2,2-tetramethylethylene) bis(5-methoxysalicylideneiminate), and H2L = 3-{2-[(2-hydroxy-benzylidene)-amino]-2-methyl-propylimino}-butan-2-one oxime, have been synthesized and characterized from structural and magnetic points of view. These two compounds are isostructural and crystallize in the same monoclinic P21/n space group. The structure has a [MII−NO−MnIII−(O)2−MnIII−ON−MII] skeleton, where −NO− is a linking oximato group derived from the non-symmetrical Schiff-base complex [MII(L)] and −(O)2− is a biphenolato bridge in the out-of-plane [Mn2(5-MeOsaltmen)2]2+ dimer. The solvent-free compounds, 1b and 2b, have also been prepared by drying of the parent compounds, 1a and 2a, respectively, at 100 °C under dried nitrogen. After this treatment, the crystallinity is preserved, and 1b and 2b crystallize in a monoclinic P21/c space group without significant changes in their structures in comparison to 1a and 2a. Magnetic measurements on 1a and 1b revealed antiferromagnetic MnIII···CuII interactions via the oximato group and weak ferromagnetic MnIII···MnIII interactions via the biphenolato bridge leading to an ST = 3 ground state. On the other hand, the diamagnetic nature of the square planar NiII center generates an ST = 4 ground state for 2a and 2b. At low temperature, these solvated (a) and desolvated (b) compounds display single-molecule magnet behavior modulated by their spin ground state.