109Ag NMR Chemical Shift as a Descriptor for Brønsted Acidity in Molecules and Materials

A series of silver N-heterocyclic carbene (NHC) complexes of general structure [(NHC)AgX] are synthesized and characterized. Nuclear magnetic resonance (NMR) studies show that the 109Ag NMR chemical shift of the series correlates with the Brønsted acidity of the conjugate acid of X- (i.e., HX), thus establishing an acidity scale based on 109Ag NMR chemical shift. The methodology is then applied to the analysis of the Brønsted acidity of oxide materials relevant to heterogeneous catalysis using Dynamic Nuclear Polarization (DNP-)enhanced solid-state NMR spectroscopy, showcasing a novel methodology to evaluate Brønsted acidity of catalyst supports and materials.