Date of Award
Chemistry and Biochemistry
Douglas W. Stephan
Pure sciences, Frustrated Lewis pairs, Hydrogen, Lewis acid-base adducts
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The concept of 'frustrated' Lewis pairs involves donor and acceptor sites in which steric congestion precludes Lewis acid–base adduct formation. In the case of sterically demanding phosphines and some boranes, this lack of active site-quenching prompts nucleophilic attack by P at a carbon para to B of B(C6F5)3 followed by fluoride transfer, which affords zwitterionic phosphonium borates of the form [R3P(C6F4)BF(C6F5) 2] and [R2PH(C6F4)BF(C6F 5)2], where R = aryl, alkyl. Additionally, a series of tertiary and secondary phosphine-B(C6F5)3 adducts are shown to undergo facile, thermal-induced rearrangement to give analogous zwitterionic species of the form [R3P(C6F4)BF(C 6F5)2] and [R2PH(C6F 4)BF(C6F5)2], respectively, where R = aryl, alkyl.
These species can be easily transformed into anionic phosphine-borates [R2P(C6F4)BF(C6F5) 2]-, cationic phosphonium-boranes [R3P(C 6F4)B(C6F5)2]+ and [R2PH(C6F4)B(C6F5) 2]+ or the charge neutral phosphino-boranes [R2 P(C6F4)BF(C6F5)2]. This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic boron center remain constant and yet the variation in substitution at phosphorus provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann–Beckett and Childs methods for determining Lewis acid strength, it was demonstrated that the cationic boranes are more Lewis acidic than B(C6F5) 3, while the acidity of the phosphino-boranes is diminished.
Sterically demanding tertiary and secondary phosphines, as well as secondary phosphides, have been shown to react with (THF)B(C6F5) 3 (THF = tetrahydrofuran) to give the THF ring-opened compounds [R 3P(C6H8O)B(C6F5)3], [R2PH(C4H8O)B(C6F5) 3] and [R2P(C4H8O)B(C6F 5)3Li(THF)2] (R = aryl, alkyl). With appropriate stoichiometry, these reactions also occur consecutively to give the double THE ring-opened compounds [Mes2P(C4H8OB(C 6F5)3)2] [Li(THF)4] and rBu2P(C4H8OB(C6F5)3) 2][Li].
Finally, it has been reported that the compounds [R2P(C 6F4)BH(C6F5)2]2 (R = aryl or alkyl), cleanly liberate H2 at temperatures above 100 °C to give the dehydrogenated products R2P(C6F4)B(C 6F5)2, which are stable and react with 1 atmosphere of H2 at 25°C to reform the starting complex. Combinations of sterically demanding phosphines R3P and B(C6F 5)3 also uptake H2 at ambient temperature and pressure. H2 liberation from the series of compounds can be facilitated using a weak Lewis base. Preliminary kinetic and deuterium labelling experiments indicate that the reversible activation of H2 follows an intermolecular mechanism.
Welch, Gregory Charles, "‘Frustrated’ Lewis pairs: From Lewis acid -base adducts to the reversible, metal-free activation of hydrogen" (2008). Electronic Theses and Dissertations. 7911.