Purely organic magnets: How to get ferromagnetic interactions

Purely organic magnets, materials with spontaneous magnetization despite no containing magnetic ions, are very promising for technological applications due to their peculiar properties as flexibility, lightness or even biocompatibility. Most of the published purely organic magnets are free radical compounds. One of the most important difficulties in order to get spontaneous magnetization in such materials is that magnetic interactions among free radicals are usually antiferromagnetic and, in addition, ferromagnetic interactions are weaker than the antiferromagnetic ones. A possible strategy to overcome this issue is the use of triradical molecules with total spin S=1/2. In this case, an adequate packing of the triradical molecules can give place to antiferromagnetic interactions between regions with positive spin density and regions with negative spin density of two close molecules. This antiferromagnetic interaction between regions with opposite spin density would result in an overall ferromagnetic interaction between the two close triradicals. With this idea in mind we have performed an study of the spin density distribution and of the intramolecular and intermolecular magnetic interactions of the triradical compound 2-[3',5'-bis(Ntert-butylaminoxyl)phenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1-Himidazol-1-oxyl-3-oxide, containing two N-tert-butyl aminoxyls and a nitronyl nitroxide groups. Combination of experimental data from a polarized neutron diffraction experiment and ab initio calculations (DFT) has allowed us to obtain the spin density distribution. In addition, the intramolecular and intermolecular magnetic interactions have been computed by ab initio quantum chemistry methods. The values for the intramolecular interactions confirm the S=1/2 ground state of the triradical. As for the intermolecular interactions, the two strongest ones are ferromagnetic, what is in agreement with the overlapping of regions with opposite spin density of the two interacting triradicals. These results support the strategy of using triradical molecules for obtaining purely organic magnets with higher magnetic transition ordering temperatures since is easier to obtain ferromagnetic interactions between the radicals and these interactions, having an antiferromagnetic origin, can be stronger than typical ferromagnetic interactions between radicals.

The Transition-Metal Citrate Cubane as a Synthon for a Large Family of SMM's

The transition-metal citrate cubane is a symmetrical, anionic molecular fragment that possesses twelve partially negatively charged oxygen atoms around its periphery. The cobalt variants have proved to be single molecule magnets (SMM), as demonstrated in studies by Murrie and by others.[1] All of the negatively charged points on the surface of the fragment are potential linkage positions for metal atoms, and extended products of from zero- to three dimensions have been synthesized and characterized.[2] In this presentation we describe products with five different combinations of linkage points for cobalt or manganese. A one-dimensional Co-containing cubane polymer has been found to undergo reversible cross-linking in the crystal to produce a two-dimensional polymer. A second Co-containg product, a discrete molecular solid with SMM behavior, undergoes reversible reaction in the crystal to produce an unsymmetrical product, also with SMM behavior -- a switchable SMM pair. A third product, a symmetrical two-dimensional Co-containing polymer, is an SMM with two blocking processes. A one-dimensional polymer of manganese citrate cubanes has been demonstrated to conduct protons via the Grotthuss mechanism.[3] All of these products have different patterns of peripheral metal-atom linkage to the twelve surface-resident oxygen atoms of the cubane fragment. A systematic naming scheme for the citrate cubane topology is used to provide simple descriptions of the diverse linkage geometries found to date.