Fluorescent Lewis Adducts: A Practical Guide to Relative Lewis Acidity

Abstract

Experimentally determining the strength of a Lewis acid is a highly desirable and important task that has implications across the chemical sciences. Recently, we developed a new fluorescence-based method for evaluating the relative acidity of a small series of Lewis acids across the p- and d-blocks of the periodic table with great precision against a series of Lewis basic fluorescent dithienophosphole oxide probes. In this report, we considerably expand the scope of the fluorescent Lewis adduct method by systematically investigating the apparent acidities of more than 50 Lewis acids in toluene. Notably, a number of the investigated Lewis acids have never been experimentally measured before. Our refined guide, which now also alleviates the uncertainties that we identified with our original method, is simple and reliable. It shows extreme sensitivity to small structural or electronic perturbations and can account for coordinative flexibility or aggregation events that occur in solution, providing an alternative method for Lewis acidity determination that is complementary to the established nuclear magnetic resonance-based methods.

Group 13 Lewis Acid Catalyzed Synthesis of Cu2O Nanocrystals via Hydroxide Transmetallation

“…Among the group 13 metals, Al 3+ is the strongest Lewis acid and would be expected to be the best catalysts for esterification. 28 However, here we observe it as the worst catalyst, barely better than the copper precursor. We believe this is due to incomplete exchange of the acetylacetonate ligands during preparation of the catalyst, which would result in catalytically inactive Al 3+ cations.…”

The Synthesis Properties and Reactivity of Lewis Acidic Aminoboranes

“…We attempted to experimentally assess the Lewis acidity of 1 -6 using both the Gutmann-Beckett and the Fluorescent Lewis Adduct method. 20,[22][23][24] Unfortunately both of these methods provided inconclusive results, which we attribute to the steric bulk around the boron as well as the pseudo double bond character of the aminoborane. Therefore, fluoride ion affinity (FIA) calculations were performed to better elucidate the impacts that each amino-substituent has on the Lewis acidity of the boron centre.…”

“…We began by evaluating amines that could provide a comprehensive scope of both steric and electronic properties. As noted above, we previously reported the synthesis of the phenothiazyl aminoborane (1) and we further synthesized aminoboranes containing phenoxazine (2), diphenylamine (3), 20 trimethylsilyl (4), 21 carbazole (5), and acridane (6) motifs (Figure 2). These amines provide a platform to understand the impact that fused ring systems have on the Lewis acidity.…”

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

“…The FLA method utilizes multiple probes whose emission chromaticities span the commission international de l'éclairage (CIE) diagram (Figure 1). 20 In the context of this study, four probes (1, 2, 7, and 8) were used to measure their respective emission profiles in varying polar solvents. [21][22][23] The selected solvents ranged in polarity from non-polar to polar based on the Dimroth-Reichardt ET(30) parameter; toluene (Tol, 33.9 kcal/mol), diethyl ether (Et2O, 34.6 kcal/mol), chlorobenzene (PhCl, 37.5 kcal/mol), dichloromethane (DCM, 41.1 kcal/mol), and acetonitrile (MeCN, 46.0 kcal/mol).…”

“…[13][14][15][16][17][18] Thus, we recently established a new methodology based on fluorescence spectroscopy to enumerate Lewis acidity, termed Fluorescent Lewis Adduct (FLA). 19,20 This method derives a basis for Lewis acidity by utilizing a series of fluorescent dithieno[3,2-b:2',3'-d]phosphole oxide Lewis base probes [21][22][23] that, when bound to a Lewis acid in solution, undergo a bathochromic shift with their optical properties (Scheme 1). 19 Scheme 1.…”