Hetero-tri-spin systems: an alternative stairway to the Single Molecule Magnets heaven?

The search for molecule-based magnetic materials has stimulated over the years the development of an extremely rich coordination chemistry. Various combinations of spin carriers have been investigated and illustrated by a plethora of hetero-spin complexes: 3d-nd, 3d-4f, 2p-3d, 2p-4f. More recently, two other classes of hetero-spin complexes are growing rapidly: compounds containing three different paramagnetic metal ions, or one radical and two different paramagnetic metal ions (all within the same molecular entity). Such new classes of systems represent a challenge both from a synthetic and theoretical point of view. Indeed, the synthetic control and the understanding from first-principles of the spin topology is a difficult problem to be solved. The presence of different spin carriers in a single molecule makes such compounds particularly interesting because they offer the possibility to lead to new magnetic properties, different from those of the hetero-bi-spin or homo-spin systems. A critical overview taking the case of 2p-3d-4f complexes is the focus of this perspective paper. An original organic picture of the state-of-art in the field and new hints about the main directions that should be pursued to achieve hetero-tri-spin systems with large anisotropy barriers, low QTM and, possibly, large blocking temperatures are provided in this article through an analysis based on numerically revisiting already published data and a critical survey of the literature appeared so far. The reasons for the poor results obtained for the largely used 3d-2p-4f topology are given along with the ones explaining the failure for the 2p-4f-3d case. The still never synthetized linear 2p-3d-4f spin topology resulted to be the most promising one based on the results obtained for the unique closed hetero-tri-spin closed triangular system synthesized so far.