N-Heterocyclic Carbene-Supported Metaphosphoric Acids as Tunable Anionic Oxygen-Donor Ligands

Anionic oxygen-donor ligands play a central role in coordination chemistry, yet the development of new, modular ligand families remains limited. Herein, we report a series of N-heterocyclic carbene (NHC)-supported metaphosphoric acids (NHC→PO3H) spanning three distinct carbene structures. These compounds are prepared on gram scale via NHC phosphorylation followed by ester cleavage, affording air- and water-stable metaphosphate esters and oxoacids. Single-crystal X-ray diffraction and density functional theory reveal closely related structures across the series, featuring conserved C–P bonding. Protonation affords well-defined (NHC→PO3H2)+ species; characterization of a representative example reveals contraction of the C–P bond, consistent with enhanced carbene–phosphorus interaction. In aqueous solution, these metaphosphoric acids behave as monoprotic oxoacids with tunable Brønsted acidity spanning more than three pKa units (pKa = 4.07–7.14). The chemical behavior of these ligands is further demonstrated by carbene-dependent reactivity in phosphoanhydride formation and by coordination to Cu(I), yielding discrete dicopper and tetracopper complexes supported by μ2-η1:η1 and μ3-η1:η1:η1 binding modes, respectively. Two corresponding dicopper(I) complexes are competent precatalysts for aerobic oxidation of benzyl alcohol to benzaldehyde. Together, these results establish NHC–metaphosphates as an accessible and tunable class of anionic oxygen-donor ligands with adaptable structural, electronic, and coordination properties.