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Frustration leads to radical behaviour

Nature Reviews Chemistry volume 2, page 255 (2018)Cite this article

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That FLPs cleave hydrostannanes by heterolysis is intuitive, but it would be risky to assume a priori that a similar mechanism is at play for the non-polar substrate H2. “There are theoretical studies that suggest that PR3/BR3 pairs convert into [HPR3]+[HBR3]− by cleaving H2 in a homolytic fashion, although experimental evidence in support of this mechanism is lacking,” notes Müller. Were the reaction to proceed by this radical mechanism, it might be a good idea to begin with a radical pair in the first place. Thus, the team kept the phosphines but replaced the boranes with silylium ions [SiR3]+, which are strong Lewis acids and oxidants when used with weakly coordinating anions and solvents ([B(C6F5)4]− and 1,2-Cl2C6H4, respectively). The purple colour of certain Lewis base and acid combinations indicated the formation of radicals. At equilibrium, 27% of [Si(C6Me5)3]+/P(2,4,6-iPr3C6H2)3 exists as a [Si(C6Me5)3]˙/ [P(2,4,6-iPr3C6H2)3]+˙. FRP instead of a bona fide FLP. In contrast to the trigonal pyramidal phosphine, the phosphoniumyl radical (shown in the image) has a planar structure reminiscent of the oxidant [N(4-BrC6H4)3]+˙ and triarylamines. It turns out that SET is not limited to FLPs, and the normal Lewis adduct [(SiiPr3)P(2,4,6-Me3C6H2)3]+ also exists in part as [SiiPr3]˙/[P(2,4,6-Me3C6H2)3]+˙. Each phosphoniumyl radical was identified according to electron spin resonance signals featuring 31P hyperfine coupling, but Müller admits that co-formation of putative (and reactive) silyl radicals [SiR3]˙ is yet to be confirmed.

What may seem curious is that silylium cations [SiR3]+ oxidize triarylphosphines P(2,4,6-iPr3C6H2)3 and P(2,4,6-Me3C6H2)3 but not trialkylphosphine PtBu3. While it is true that tBu is more inductively electron-donating than the aryls, the latter groups are larger, such that the triarylphosphines part with an electron more easily because this would be accompanied by a release of steric strain associated with moving to a planar geometry. Ionization potentials calculated using density-functional theory support these data, but the team wants more thermochemical information about SET. However, the reactivity of the [SiR3]˙ products is again problematic in that it hampers electrochemical measurements, let alone any intended substrate conversions.

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  1. Nature Reviews Chemistry http://www.nature.com/natrevchem/

    David Schilter

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  1. David Schilter
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Correspondence to David Schilter.

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Schilter, D. Frustration leads to radical behaviour. Nat Rev Chem 2, 255 (2018). https://doi.org/10.1038/s41570-018-0047-1

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