Does H3O+ Really Act as a Ligand in the Solid State?

The evidence for the existence of metal complexes containing H₃O⁺ as a ligand in the solid state is examined. Each of the 68 examples in the Cambridge Structural Database in which H₃O⁺ is bound to a transition metal, lanthanoid, actinoid, or main group metal ion is detailed and critically appraised. It is concluded that none of the reported examples of complexes containing coordinated H₃O⁺ have been unequivocally characterized and that they result from either curation errors or misinterpretations of the crystallographic data. These conclusions are supported by computational techniques, which show that three purported H₃O⁺ complexes based on the 1,4,7,10,13,16,21,24-octa-azabicyclo(8.8.8)hexacosane azacryptand skeleton are better described as aqua complexes, with protonation occurring at the amine ligand.

Porphyrin and phthalocyanine-based metal organic frameworks beyond metal-carboxylates

Given the ubiquitous role of porphyrins in natural systems, these molecules and related derivatives such as phthalocyanines are fascinating building units to achieve functional porous materials. Porphyrin-based MOFs have been developed over the past three decades, yet chemically robust frameworks, necessary for applications, have been achieved much more recently and this field is expanding. This progress is partially driven by the development of porphyrins and phthalocyanines bearing alternative coordinating groups (phosphonate, azolates, phenolates…) that allowed moving the related MOFs beyond metal-carboxylates and achieving new topologies and properties. In this perspective article we first give a brief outline of the synthetic pathways towards simple porphyrins and phthalocyanines bearing these complexing groups. The related MOF compounds are then described; their structural and textural properties are discussed, as well as their stability and physical properties. An overview of the resulting nets and topologies is proposed, showing both the similarities with metal-carboxylate phases and the peculiarities related to the alternative coordinating groups. Eventually, the opportunities offered by this recent research topic, in terms of both synthesis pathways and modulation of pore size and shape, stability and physical properties, are discussed.