On the Use of Aqueous Metal-Aqua pKa Values as a Descriptor of Lewis Acidity

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in nonaqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31 P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the p K a values of the corresponding metal-aqua species, [ M (H 2 O) m ] n+ , measured in H 2 O vs the 31 P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile ( d 3 -MeCN) and deuterated dichloromethane (CD 2 Cl 2 ), display tightly colinear relationships, suggesting similar behavior for these ions in water, d 3 -MeCN, and CD 2 Cl 2 . This colinearity reinforces the utility of the common approach of using the aqueous p K a values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d 3 -MeCN suggest a 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Together, these data suggest that both aqueous p K a values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.