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

Kumar, Amit; Blakemore, James D.

doi:10.1021/acs.inorgchem.0c03239

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

Journal Article · Wed Jan 06 00:00:00 EST 2021 · Inorganic Chemistry

DOI:https://doi.org/10.1021/acs.inorgchem.0c03239· OSTI ID:1777182

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Univ. of Kansas, Lawrence, KS (United States); University of Kansas
Univ. of Kansas, Lawrence, KS (United States)

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. Plots of the pK_a 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) display tightly co-linear relationships, 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 pK_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 d₃-MeCN suggest 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. Altogether, these data suggest that both aqueous pK_a values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.

View Accepted Manuscript (DOE)

Research Organization:: Univ. of Kansas, Lawrence, KS (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: SC0019169

OSTI ID:: 1777182

Journal Information:: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 2 Vol. 60; ISSN 0020-1669

Publisher:: American Chemical Society (ACS)Copyright Statement

Country of Publication:: United States

Language:: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
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On the Use of Aqueous Metal-Aqua pKa Values as a Descriptor of Lewis Acidity

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