Ferromagnetic Exchange Coupling in a Family of Mn^III Salen-Type Schiff-Base Out-of-Plane Dimers

A series of Mn^III saltmen dimers, [Mn₂(5-Rsaltmen)₂(X)₂](A)_2n (saltmen^2- = N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminate); R = H, Cl, Br, MeO, Me; X = H₂O, ReO₄^-, NO₃^-, N₃^-, NCS^-, A^- = ClO₄^-, PF₆^-, CF₃SO₃^- for X = H₂O) were synthesized and structurally and magnetically investigated to understand the correlation between their intradimer ferromagnetic (FM) interaction and single-molecule magnet (SMM) behavior. All complexes had a similar di-μ-phenolate-bridged out-of-plane dimer structure but displayed different bridging Mn-O_ph∗ distances depending on the R substituents of the saltmen ligand and axial X ligand. Magnetic susceptibility studies revealed intradimer FM coupling (J_Mn-Mn∗), resulting in an S_T = 4 ground state for all dimers. However, the magnitude of FM coupling strongly depended on R and X. J_Mn-Mn∗ increased with decreasing Mn-O_ph∗ distance but decreased with decreasing Mn-X distance with a relation of H₂O ≈ ReO₄^- > NO₃^- > N₃^- ≈ NCS^- with a linear trend for R = H, Cl, Me but not for R = Br, MeO. Theoretical investigations revealed that a larger orbital overlap stabilized a FM spin configuration through competition between the orbital degeneracy and on-site Coulomb repulsion of out-of-phase and in-phase orbitals. Most dimers showed typical SMM behavior. The dimers with larger J_Mn-Mn∗ tended to have higher blocking temperatures.