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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 non-aqueous 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 31P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library
of metal triflate salts using the Gutmann-Beckett method. A plot of the
pKa values of the corresponding metal-aqua species, [M(H2O)m]n+,
measured in H2O, vs. the 31P NMR shifts of TPPO in the presence of
these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD2Cl2) displays a tightly co-linear relationship, suggesting similar behavior for these ions in water, d3-MeCN, and CD2Cl2.
This collinearity reinforces the utility of the common approach of using
the aqueous pKa 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 d3-MeCNsuggest 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 pKa values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis
acidity on multimetallic chemical systems.