Ferromagnetic Exchange Coupling in a Family of MnIII Salen-Type Schiff-Base Out-of-Plane Dimers

A series of Mn-III saltmen dimers, [Mn-2(5Rsaltmen)(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-, 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-mu-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 ST = 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 approximate to ReO4- > NO3- > N-3(-) approximate to 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 dinners with larger J(Mn-Mn)* tended to have higher blocking temperatures.