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Metal-organic frameworks based on trivalent lanthanides (LnMOFs) are a very promising class of materials for addressing the challenges in engineering of luminescent centres. Lanthanide-bearing phosphors find numerous applications in lighting, optical communications, photonics and biomedical devices. In this critical review we discuss the potential of LnMOFs as multifunctional systems, which combine light emission with properties such as microporosity, magnetism, chirality, molecule and ion sensing, catalysis and activity as multimodal imaging contrast agents. We argue that these materials present a unique chance of observing synergy between several of these properties, such as the coupling between photoluminescence and magnetism. Moreover, an integrated approach towards the design of efficient, stable, cheap, environmentally-friendly and multifunctional luminescent LnMOFs is still missing. Although research into LnMOFs is at its early stage and much basic knowledge is still needed, the field is ripe for new ideas, which will enable sensor devices and photonic prototypes to become a commercial reality (81 references).

GW190413_052954 33.4 +12.4 −7.4 23.4 +6.7 −6.3 7.2 × 10 −2 GW190413_134308 45.4 +13.6 −9.6 30.9 +10.2 −9.6 4.4 × 10 −2 GW190421_213856 40.6 +10.4 −6.631.4 +7.5 −8.2 7.7 × 10 −4 GW190424_180648 39.5 +10.9 −6.9 31.0 +7.428.5 +7.5 −7.9 1.0 × 10 −5 GW190512_180714 23.0 +5.4 −5.7 12.5 +3.5 −2.5

Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir-Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic-inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic-inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.

Functionalization via covalent grafting of organic functions allows to tune the redox and acid-base properties, and the solubility of polyoxometalates, to enhance their stability and biological activity and to reduce their toxicity, to facilitate their implementation in extended structures and functional devices. We discuss herein the electronic and binding connections, and the various synthesis methodologies. We emphasize on organonitrogen, organosilyl and organophosphonyl derivatives with special attention to synthesis, characterization and potential applications in catalysis and materials science. We also consider the giant molybdenum oxide-based clusters especially the porous capsule-type clusters (Keplerates) which have high relevance to this context.

ContentsI. Introduction 77 II. Scope and Organization of the Review 78 III. Polyoxometalates Incorporating Halides 79 IV. Polyoxometalates Incorporating Group 16 Element-Centered Ligands 81 A. Peroxopolyoxometalates 81 B. Polyoxoalkoxometalates 82 1. Polyoxoalkoxometalates Involving Unidentate Alcohols 82 2. Polyoxoalkoxometalates Involving Chelating Triols 83 C. Heavier Group 16 Element-Centered Ligands 85 1. Thiopolyoxometalates 85 2. Organosulfur and Organoselenium Ligands 85 V. Polyoxometalates Incorporating Group 15 Element-Centered Ligands 86 A. Singly Bonded Nitrogen-Donating Ligands 86 1. Amine and Related Ligands 86 2. Amide Oximes [RC(NH 2 )NOH] and Oximes 87 B. Multiply Bonded Nitrogen Ligands 89 1. Nitrido Derivatives 89 2. Organoimido Derivatives 89 3. Hydrazido and Diazenido Derivatives of Polyoxometalates 90 4. Nitrosyl Derivatives 92 C. Organophosphorus, Organoarsenic, and Organoantimony Ligands 94 1. Organophosphonate and Organoarsonate Ligands 94 2. Organophosphinate and Organoarsinate Ligands 97 VI. Polyoxometalates Incorporating Group 14 Element-Centered Ligands 97 A. Oxocarbon Ligands 97 1. Carbonate 97 2. Carboxylates 97 3. Oxalate and Squarate 97 4. Carbonyl Derivatives 98 B. Silicon Derivatives 98 C. Germanium Derivatives 99 D. Tin and Lead Derivatives 99 VII. Organometallic Derivatives of Polyoxometalates 100 A. Cyclopentadienyl Derivatives of Polyoxometalates 100 B. Cyclopentadienyl Oxide Clusters of Groups 5 and 6 100 C. Organometallic Polyoxometalates 100 1. Polyoxometalate-incorporated Organometalic Complexes 101 2. Polyoxometalate-Supported Organometallic Complexes 101 3. Integrated Cubane-Type Clusters 105 4. Organometallic Cation Salts of Keggin-Type Anions 105 VIII. Concluding Remarks 106 IX. Acknowledgments 106 X. Abbreviations 106 XI. References 106

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