Quantitative Ligand Affinity Scales for Metal Triflate Salts: Application to Isomer Differentiation

Abstract

Owing to the importance of metal triflates in catalysis, the affinity of the cationic center for a selection of organic ligands was explored for InIII and ZnII triflates. The organic Lewis bases include a variety of carbonyls (amides, unsaturated ketones, a lactone) and cyclic 1,2‐diols. The relative affinity of the ligands for the cationic center in triflates was quantitatively determined on the basis of relative ion concentrations determined by electrospray‐ionization mass spectrometry. The affinity scales were discussed with reference to gas‐phase proton basicity and Lewis basicity scales. Structural isomers and stereoisomers display significant affinity differences in several cases. In the case of isomer mixtures, a model describing the relative peak intensities in the mass spectra was developed. On this basis, an isomer titration method was set up. Remarkably, this MS‐based method overcame the blindness of mass spectrometry to isomers without the need for isotope labeling or MS/MS experiments. This model may prove to have applications in analytical chemistry and catalysis.

A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence

“…Instead, the Lewis acidity of these compounds is often based on binding constants developed from known reactions. 44,45 Notably, as mentioned above, our chromaticity trends are indeed a visual indicator fundamentally related to the binding constant of the FLAs, which further solidifies our universal-scale approach. By using our method, the strength of any given LA can be measured with only three chromaticity measurements, one from each FLA with three phosphole oxide probes.…”

Probing the Impact of Solvent on Lewis Acid Catalysis via Fluorescent Lewis Adducts

“…To further expand the scope on the variation of our methodology with respect to solvent effects, the following five representative Lewis acids were chosen based on their utility in catalysis: AlCl3, B(2,4,6-C6F3H2)3, In(OTf)3, Sc(OTf)3 and Zn(OTf)2. 19,[32][33][34][35][36][37] The LAU values of these five Lewis acids were again determined using our previously reported procedure 20 and we observed the same trends as seen with B(C6F5)3 in the different solvents. LAU values were generally found to proportionally increase relative to the polarity of the respective solvent, as shown in Figure 2 and Table S3, whereas lower LAU values were observed with stronger donating solvents.…”

Probing the Impact of Solvent on Lewis Acid Catalysis via Fluorescent Lewis Adducts

“…Sc(OTf)3 and Zn(OTf)2. 19,[32][33][34][35][36][37] The LAU values of these five Lewis acids were again determined using our previously reported procedure 20 and we observed the same trends as seen with B(C6F5)3 in the different solvents. LAU values were generally found to proportionally increase relative to the polarity of the respective solvent, as shown in Figure 2 and Table S1, whereas lower LAU values were observed with stronger donating solvents.…”