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

A series of MnIII saltmen dimers, [Mn2(5-Rsaltmen)2(X)2]­(A)2n (saltmen2– = 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–Oph* distances depending on the R substituents of the saltmen ligand and axial X ligand. Magnetic susceptibility studies revealed intradimer FM coupling (JMn–Mn*), resulting in an ST = 4 ground state for all dimers. However, the magnitude of FM coupling strongly depended on R and X. JMn–Mn* increased with decreasing Mn–Oph* 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 JMn–Mn* tended to have higher blocking temperatures.