Heterometallic 3d-4f polyoxometalates: An emerging field with structural diversity to multiple applications

The research on heterometallic 3d-4f polyoxometalates has made immense progress over the last decade with a larger number of structures being reported every year until the present day. The formation of 3d-4f POMs has been considered a challenge as both the 3d and 4f ions compete for coordination to the lacunary POMs ligands. It was seen that mostly amorphous precipitate was obtained instead of crystalline material. Hence, as a result, the synthetic parameters such as pH, temperature, solvent, time of reaction has to be precisely controlled in order to isolate the desired product. This review focuses on the heterometallic 3d-4f polyoxometalates reported in the last decade mainly after 2011, when the field of 3d-4f heterometallic polyoxometalates was considered an incipient field. The structural variety exhibited by this kind is diverse with a large number of structures including dimers, trimers, tetramers which can be divided mainly into four types of polyanions. These are pure 3d-4f polyanions, 3d substituted anions with 4f linkers, 4f substituted anions with 3d linkers, and POMs with both 3d and 4f as linkers. The reason for the desire to create such complexes is mainly due to the interesting magnetic properties exhibited by them. Introduction of 4f ions into the paramagnetic 3d cluster enhances the already present magnetic properties and allows the formation of single-molecule/ion magnets (SMMs or SIMs). Many of the complexes show slow magnetic relaxation which is ideal for SMMs/SIMs. A few of the complexes displaying such properties are discussed in the review. Apart from that, the catalytic properties exhibited by such 3d-4f complexes have also been discussed.