Quo Vadis, Polyoxometalate Chemistry?

Initial Steps of the Acid-Catalyzed Polyoxometalate-Functionalization with Phosphonic Acid Esters

“…The surface functionalization of metal oxides with organic moieties is a key strategy to tune structure and function and has been used in areas ranging from biomedicine to electronics and energy materials. − One of the most popular approaches is the linkage of organic groups to metal oxide surfaces using phosphonic acid derivatives as they are synthetically versatile, sometimes even commercially available and form stable anchoring sites on metal oxides. − Phosphonic acids have also been employed for the site-selective functionalization of molecular metal oxides, so-called polyoxometalates (POMs). − These cluster compounds are molecular analogues of solid-state oxides and have unique features which make them interesting for catalysis, energy conversion, and other current fields of research. − While application-driven development of new POM-phosphonate hybrids is thriving, less focus has been on understanding the mechanism of phosphonate-linkage to POM surfaces. Currently, there are several routes to achieve phosphonate anchoring to POMs, which proceed either by acid-catalyzed linkage of phosphonic acids, using trimethylsilyl-activated phosphonic acid esters, or by phosphonic acid chlorides as reactive intermediates for POM or metal oxide surface functionalization, ,, see Figure . − Especially, the TMS activation and the chloride activation are very mild methods and the condensation with POMs like the α 2 -Dawson-type cluster can be achieved at room temperature; the methods are also very sensitive to water and thus need to be handled within an inert atmosphere.…”