Rare Earth Polyoxometalates

Longstanding and important applications make use of the chemical and physical properties of both rare earth metals and polyoxometalates of early transition metals. The catalytic, optical, and magnetic features of rare earth metal ions are well-known, as are the reversible multielectron redox and photoredox capabilities of polyoxomolybdates and polyoxotungstates. The combination of rare earth ions and polyoxometalates in discrete molecules and coordination polymers is of interest for the unique combination of chemical and physical properties that can arise. This Account surveys our efforts to synthesize and investigate compounds with rare earth ions and polyoxometalates (RE-POMs), sometimes with carboxylate-based organic coligands. Our general synthetic approach is "bottom-up", which affords well-defined nanoscale molecules, typically in crystalline form and amenable to single-crystal X-ray diffraction for structure determination. Our particular focus is on elucidation of the physical properties conferred by the different structural components with a view to ultimately being able to tune these properties chemically. For this purpose, we employ a variety of spectroscopic, magnetochemical, electrochemical, and scattering techniques in concert with theoretical modeling and computation. Studies of RE-POM single-molecule magnets (SMMs) have utilized magnetic susceptibility, inelastic neutron scattering, and ab initio calculations. These investigations have allowed characterization of the crystal field splitting of the rare earth(III) ions that is responsible for the SMM properties of slow magnetic relaxation and magnetization quantum tunneling. Such SMMs are promising for applications in quantum computing and molecular spintronics. Photophysical measurements of a family of hybrid RE-POMs with organic ligands have afforded insights into sensitization of Tb(III) and Eu(III) emission through both organic and polyoxometalate chromophores in the same molecule. Detailed variable-temperature studies have revealed the temperature dependence of the POM-based sensitization, which is relevant for potential applications in phosphor thermometry. Novel RE-POM coordination polymers demonstrate the promise of higher-dimensional materials for catalytic and sensing applications that can make use of either or both rare earth and polyoxometalate capabilities. Finally, structural, electrochemical, and density functional theory studies on a family of modular RE-POMs that incorporate molybdotungstates with amino acid coligands have revealed how closed Mo-oxo loops that are reduced preferentially can act as electron reservoirs in mixed-metal molybdotungstates. This has important implications for mixed-metal polyoxometalates in redox and photoredox catalysis. Notably, these hybrid RE-POMs are stable in solution and maintain the chirality induced by amino acid ligands. The RE-POMs surveyed in this Account provide a glimpse of possible structural features that are accessible with this family of compounds. The studies of the ensuing chemical and physical properties reveal the promise of RE-POMs for diverse and varied applications and lay an excellent foundation for the future development of this new class of functional materials.