Magnetic Ordering of MnII CuII Bimetallic Systems: Design of Molecular Ferromagnets

Abstract

For a few years, we have participated to the efforts to design molecular systems ordering ferromagnetically. Molecular in this context means that we use the synthesis methods of the molecular chemistry -we work in solution with mild conditions of temperature and pressure- and that we attempt to build the tridimensional lattice by assembling molecular bricks in a controlable fashion. Our first step along this line has been to find a strategy leading to ferromagnetic interactions between nearest neighbor magnetic centers. This strategy is that of the orthogonality of the magnetic orbitals. When in an AB pair, the magnetic orbitals around A are all orthogonal to the magnetic orbitals around B, the interaction between A and B is purely ferromagnetic and the ground state of the pair has the highest spin multiplicity. We have synthesized several compounds of this kind involving pairs of metal ions1,2, or a metal ion and an organic radical3. This strategy to achieve a ferromagnetic interaction is very efficient in the sense that we have not found any counterexample so far. When the strict orthogonality of all the magnetic orbitals is realized, then the ground state has the highest spin multiplicity. However, the symmetry requirements allowing such an orthogonality are often difficult to obtain. A small distortion with regard to the ideal symmetry generally destroys this orthogonality, so that the interaction becomes antiferromagnetic with a stabilization of the state of lowest spin multiplicity.