On the Use of Aqueous Metal-Aqua p<i>K</i>_a Values as a Descriptor of Lewis Acidity

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject intense interest recent years. Parametrizing the these nonaqueous conditions, commonly used field, is challenging due to lack direct measures acidity polar organic solvents. Here, we report use triphenylphosphine oxide (TPPO) as 31P nuclear magnetic resonance (NMR) probe quantify library triflate salts using Gutmann-Beckett method. Plots pKa values corresponding metal-aqua species, [M(H2O)m]n+, measured H2O vs NMR shifts TPPO presence metals deuterated acetonitrile (d3-MeCN) and dichloromethane (CD2Cl2), display tightly colinear relationships, suggesting similar for water, d3-MeCN, CD2Cl2. This colinearity reinforces utility common approach aqueous descriptor acidity, regardless solvent immediate experiments, provides an insight into usefulness this wide-ranging applications. Titration studies d3-MeCN suggest 1:1 binding with monovalent ions, greater than divalent formation multiple species highly trivalent ions. Together, data that both other single-measurement descriptors, while useful, provide only snapshot influence on chemical systems.