Frustrated Lewis pair
In chemistry, a frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLP's have been devised, and many simple substrates exhibit activation.
The discovery that some FLPs split H2 triggered a rapid growth of research into FLP's. Because of their "unquenched" reactivity, such systems are reactive toward substrates that can undergo heterolysis. For example, many FLP's split hydrogen molecule. Thus, a mixture of tricyclohexylphosphine (PCy3) and tris(pentafluorophenyl)borane reacts with hydrogen to give the respective phosphonium and borate ions:
This reactivity suggests that FLP's are potentially useful for hydrogenation reactions.
Frustrated Lewis pairs can activate many small molecules, which can undergo heterolysis or are subject to polarization. The activation (i.e. reaction of) "small molecule" is an area of interest owing to the abundance of many small molecules. There is, for example, interest in use of N2 and O2 in the direct formation of C-N and C-O bonds. It also enables the utilization of environmentally damaging CO2 and CH4.
The activation H2 using FLPs was first reported in 2006. The hydrogen adduct of the original FLP, a phosphonium-borate salt, is prepared by combining a phenylene bridged phosphinoborane and dihydrogen. The salt, which is colorless, is stable in the presence of air and moisture. It releases molecular H2 when heated above 100 °C.
Along with H2 Some FLPs react with CO2. Specifically carbon dioxide fixation was demonstrated using P(tBu)3 which attacks the electronegative carbon and the Lewis acid B(C6F5)3 which accepts an electron pair from an oxygen atom:
P(t-Bu)3 + B(C6F5)3 + CO2 → (t-Bu)3P+C(O)OB−(C6F5)3
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