¹⁰⁹Ag NMR chemical shift as a descriptor for Brønsted acidity from molecules to materials

Abstract

Molecular-level understanding of the acid/base properties of heterogeneous catalysts requires the development of selective spectroscopic probes to establish structure-activity relationships. In this work we show that substituting the surface protons in oxide supports by isolobal N-heterocyclic carbene (NHC) Ag cations and measuring their ¹⁰⁹Ag nuclear magnetic resonance (NMR) signatures enables to probe the speciation and to evaluate the corresponding Brønsted acidity of the substituted OH surface sites. Specifically, a series of silver N-heterocyclic carbene (NHC) Ag(i) complexes of general formula [(NHC)AgX] are synthesized and characterized, showing that the ¹⁰⁹Ag NMR chemical shift of the series correlates with the Brønsted acidity of the conjugate acid of X^- (<i>i.e.</i>, HX), thus establishing an acidity scale based on ¹⁰⁹Ag NMR chemical shift. The methodology is then used to evaluate the Brønsted acidity of the OH sites of representative oxide materials using Dynamic Nuclear Polarization (DNP-)enhanced solid-state NMR spectroscopy.