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, [Mn^III₂(5-MeOsaltmen)₂M^II₂(L)₂](CF₃SO₃)₂·2H₂O (M^II = Cu, 1a; Ni, 2a), where 5-MeOsaltmen^2- = 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 [M^II−NO−Mn^III−(O)₂−Mn^III−ON−M^II] skeleton, where −NO− is a linking oximato group derived from the non-symmetrical Schiff-base complex [M^II(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 Mn^III···Cu^II interactions via the oximato group and weak ferromagnetic Mn^III···Mn^III interactions via the biphenolato bridge leading to an S_T = 3 ground state. On the other hand, the diamagnetic nature of the square planar Ni^II center generates an S_T = 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.