Absolute Estimates of PdII(η2‑Arene) C–H Acidity

Thermodynamic acidity is one of the most widely used quantities for characterizing proton transfer reactions. Measurement of these values for catalytically relevant species can be challenging, often requiring direct observation of equilibria. The C–H bonds of aromatic substrates are proposed to become substantially polarized during electrophilic activation, but quantifying the absolute acidity of the intermediate M­(η2-arene) complexes is highly challenging. Using a system that intercepts nascent protons at electrophilic PdII arene complexes, a combined experimental and computational study has demonstrated these C–H bonds to be far more acidic (pKaCH3CN = 3–6) than many “nonbasic” substrates and additives that are present in electrophilic C–H activation catalysis, and the catalytic roles of these species may need to be reassessed.