Gutmann–Beckett method

In chemistry, the Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide (Et₃PO, TEPO) is used as a probe molecule and systems are evaluated by ³¹P-NMR spectroscopy. In 1975, Viktor Gutmann [de] used ³¹P-NMR spectroscopy to parameterize Lewis acidity of solvents by acceptor numbers (AN).^[1] In 1996, Michael A. Beckett recognised its more generally utility and adapted the procedure so that it could be easily applied to molecular species, when dissolved in weakly Lewis acidic solvents.^[2] The term Gutmann–Beckett method was first used in chemical literature in 2007.^[3]

Background

The ³¹P chemical shift (δ) of Et₃PO is sensitive to chemical environment but can usually be found between +40 and +100 ppm. The O atom in Et₃PO is a Lewis base, and its interaction with Lewis acid sites causes deshielding of the adjacent P atom. Gutmann, a chemist renowned for his work on non-aqueous solvents, described an acceptor-number scale for solvent Lewis acidity ^[4] with two reference points relating to the ³¹P NMR chemical shift of Et₃PO in the weakly Lewis acidic solvent hexane (δ = 41.0 ppm, AN 0) and in the strongly Lewis acidic solvent SbCl₅ (δ = 86.1 ppm, AN 100). Acceptor numbers can be calculated from AN = 2.21 × (δ_sample − 41.0) and higher AN values indicate greater Lewis acidity. It is generally known that there is no one universal order of Lewis acid strengths (or Lewis base strengths) and that two parameters (or two properties) are needed (see HSAB theory and ECW model) to define acid and base strengths^[5][6] and that single parameter or property scales are limited to a smaller range of acids (or bases). The Gutmann–Beckett method is based on a single parameter NMR chemical shift scale but is in commonly used due to its experimental convenience.

Application to boranes

Interaction of triethylphosphine oxide with a Lewis acid

Boron trihalides are archetypal Lewis acids and have AN values between 89 (BF₃) and 115 (BI₃).^[2] The Gutmann–Beckett method has been applied to fluoroarylboranes ^{[7] [8]} such as B(C₆F₅)₃ (AN 82), and borenium cations, and its application to these and various other boron compounds has been reviewed.^[9]

Application to other compounds

The Gutmann–Beckett method has been successfully applied to alkaline earth metal complexes,^[10][11] p-block main group compounds ^{[7][12][13][14][15]} (e.g. AlCl₃, AN 87; silylium cations; [E(bipy)₂]³⁺ (E = P, As, Sb, Bi) cations; cationic 4 coordinate P^v and Sb^v derivatives) and transition-metal compounds ^[7][16] (e.g. TiCl₄, AN 70).