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 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]

Acids and bases
Diagrammatic representation of the dissociation of acetic acid in aqueous solution to acetate and hydronium ions.
Acid types
Base types

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 x (δsample – 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 Pv and Sbv derivatives) and transition-metal compounds [7][16] (e.g. TiCl4, AN 70).

References
  1. U. Mayer, V. Gutmann, and W. Gerger, "The acceptor number – a quantitative empirical parameter for the electrophilic properties of solvents", Monatshefte fur Chemie, 1975, 106, 1235–1257. doi: 10.1007/BF00913599
  2. M.A. Beckett, G.C. Strickland, J.R. Holland, and K.S. Varma, "A convenient NMR method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity", Polymer, 1996, 37, 4629–4631. doi: 10.1016/0032-3861(96)00323-0
  3. G.C. Welch, L.Cabrera, P.A. Chase, E. Hollink, J.M. Masuda, P. Wei, and D.W. Stephan,"Tuning Lewis acidity using the reactivity of "frustrated Lewis pairs": facile formation of phosphine-boranes and cationic phosphonium-boranes", Dalton Trans., 2007, 3407–3414. doi: 10.1039/b704417h
  4. V. Gutmann, "Solvent effects on reactivities of organometallic compounds", Coord. Chem. Rev., 1976, 18, 225–255. doi: 10.1016/S0010-8545(00)82045-7
  5. Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) pp 50-51 ISBN 978-0-470-74957-9
  6. Cramer, R. E.; Bopp, T. T. (1977). "Graphical display of the enthalpies of adduct formation for Lewis acids and bases". Journal of Chemical Education. 54: 612–613. doi:10.1021/ed054p612. The plots shown in this paper used older parameters. Improved E&C parameters are listed in ECW model.
  7. M.A. Beckett, D.S. Brassington, S.J. Coles, and M.B. Hursthouse, "Lewis acidity of tris(pentafluorophenyl)borane: crystal and molecular structure of B(C6F5)3.OPEt3", Inorg. Chem. Commun., 2000, 3, 530–533. doi: 10.1016/S1387-7003(00)00129-5
  8. S.C. Binding, H. Zaher, F.M. Chadwick, and D. O'Hare, "Heterolytic activation of hydrogen using frustrated Lewis pairs containing tris(2,2',2'-perfluorobiphenyl)borane", Dalton Trans., 2012, 41, 9061–9066. doi: 10.1039/c2dt30334e
  9. I.B. Sivaev, V.L. Bregadze, "Lewis acidity of boron compounds", Coord. Chem. Rev., 2014, 270/271, 75-88. doi: 10.1016/j.ccr.2013.10.017
  10. S. Brand, J. Pahl, H. Elsen, and S. Harder, "Frustrated Lewis pair chemistry with magnesium Lewis acids", European J. Inorg. Chem., 2017, 4187-4195. doi: 10.1002/ejic.201700787
  11. J. Pahl, S. Brand, H. Elsen, and S. Harder,"Highly Lewis acidic cationic alkaline earth metal complexes", Chem. Commun., 2018, 54, 8685-8688. doi: 10.1039/C8CC04083D
  12. H. Grossekappenberg, M. Reissmann, M. Schmidtmann, and T. Mueller, "Quantitative assessment of the Lewis acidity of silylium ions", Organometallics, 2015, 34, 4952-4958. doi: 10.1021/acs.organomet.5b00556
  13. S.S. Chitnis, A.P.M. Robertson, N. Burford, B.O. Patrick, R. McDonald, and M.J. Ferguson, "Bipyridine complexes of E3+ (E = P, As, Sb, Bi): strong Lewis acids, sources of E(OTf)3 and synthons for EI and Ev cations", Chemical Sciences, 2015, 6, 6545-6555. doi: 10.1039/C5SC02423D
  14. J.M. Bayne and D.W. Stephan, "Phosphorus Lewis acids: emerging reactivity and applications in catalysis", Chem. Soc. Rev., 2015, 45, 765-774. doi:10.1039/c5cs00516g
  15. B. Pan and F. Gabbai, "[Sb(C6H5)4][B(C6F5)4]: an air stable Lewis acidic stibonium salt that activates strong element-fluorine bonds", J. Am. Chem. Soc., 2014, 136, 9564-9567. doi: 10.1021/ja505214m
  16. C.-Y. Wu, T. Horibe, C.B. Jacobsen, and D. Toste, "Stable gold(III) catalysts by oxidative addition of a carbon-carbon bond", Nature, 2015, 517, 449-454. doi: 10.1038/nature14104
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