Redox-inactive metal cations represent a broad class of species found in chemical systems, and they have been shown to tune and/or modulate the structure and reactivity of many redox-active metal complexes. Understanding and quantifying the roles of redox-inactive cations remains challenging due to a lack of comprehensive and quantitative data on the nature of their interactions with ligands, substrates, and other metal-containing species. This situation is especially pressing for trivalent ions like La3+, Lu3+, and others, which can strongly interact with ligands and display challenging speciation behaviors. Here, we will discuss our group’s recent efforts in development of a method relying on phosphine oxide molecules as 31P nuclear magnetic resonance (NMR) probes to quantify the apparent Lewis acidity of mono-, di-, and tri-valent metal cations. Our method is a variant of the famous Guttman-Beckett method, and is useful for quantifying and parametrizing the interactions between metal ions and ligands in nonaqueous environments, like those often used in synthetic f-element chemistry. In our work, downfield migration of the 31P NMR resonance for a given probe molecule serves to quantify the electrostatic effect of the metal on the probe and affords insight into the stoichiometry and chemical speciation/equilibria involved in probe binding. Our latest results will be discussed, including studies comparing the dynamic ranges of different phosphine oxide probes and how the identity of the probe gives new insights into the behavior of metal ions under nonaqueous conditions.

Phosphine oxides as probes of the Lewis acidity of redox-inactive metal cations

ACS Spring 2022

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