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6.4.8: Frustrated Lewis pair chemistry uses Lewis acid and base sites within a molecule that are sterically restricted from forming an adduct with each other.

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    162913
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    In certain cases when a Lewis acid-base adduct is sterically hindered from quantitatively forming an adduct (and perhaps even when they are so able), the Lewis acid and Lewis base can act together to heterolytically cleave chemical bonds. In such cases the pair of electrons that would otherwise be donated by the base is said to be a frustrated Lewis pair (FLP) and in performing the heterolytic cleavage the Lewis acid and base are said to exhibit frustrated Lewis pair chemistry.

    The Stephan's phosphinoborane illustrates how FLP chemistry works. It reversibly cleaves dihydrogen to give a zwitterionic species containing both acidic and basic element hydride bonds.

    clipboard_ef3314963c81c03039ff4b74f4e287c59.png

    Note that the Lewis acid and base do not need to be part of the same molecule. Hydrogen is also heterolytically cleaved by mixtures of sterically encumbered triarylphosphines and triarylboranes. This time the reaction gives phosphonium and hydroborate ions:

    clipboard_e54d5b43168d9f5bb1c5cc35766253e35.png

    Given the description of FLP systems as frustrated Lewis acid-base adducts, it is natural to think about the cleavage of substrates like H2 as involving nucleophilic attack by the Lewis base while the Lewis acid acts like an electrophile in a heterolytic cleavage mechanism.

    clipboard_e2088b1fabf5b9123934152bc56250675.png

    However, only some FLP systems cleave substrates through this mechanism. In others the mechanism involves homolytic cleavage by a diradical intermediate.

    clipboard_e8374104f4b63ad692a033fe20705b0b6.png

    In either case, when substrates like \(\ce{H2}\) are cleaved by FLPs, they are generally separated into more reactive fragments (e.g., in the case of \(\ce{H2}\), \(\ce{H^{+}}\) and \(\ce{H^{-}}\)). Because of this the substrates are said to be activated to undergo further reactions. Because FLP systems can cleave and activate substrates, there is considerable interest in the development of FLP systems that can facilitate reactions of economic or environmental interest. So far FLP systems have been developed that can catalyze various hydrogenations and activate numerous substrates, including \(\ce{CO}\), \(\ce{SO2}\), and \(\ce{N2}\).

    References

    1. Stephan, D. W., Frustrated Lewis Pairs: From Concept to Catalysis. Accounts of Chemical Research 2015, 48 (2), 306-316.
    2. Fontaine, F.-G.; Stephan D. W., On the concept of frustrated Lewis pairs. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 2017, 375 (2101), 20170004.
    3. Jupp, A. R.; Stephan, D. W., New Directions for Frustrated Lewis Pair Chemistry. Trends in Chemistry 2019, 1 (1), 35-48.