10.1002/chem.202001494
Chemistry - A European Journal
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Finally, we were curious if we also could design the one-
electron oxidation of amines via a thermal SET, simply by using
the trityl cation as the Lewis acid in the dark. Although the energy
gap between the levels of the EDA pairs formed by combining the
arylamines NPh3 or NpTol3 with [Ph3C][B(C6F5)4] and
corresponding radical pairs (G (E) = 14.4 (16.3) and 8.9 (10.0)
kcal·mol-1, respectively; Figure 5) are close or exceeding the
threshold to be detectable by EPR spectroscopy, Gomberg
dimerisation drives the equilibrium to the right allowing for
observation of the radical species (Figure 6). As the trityl radical
is mostly present as Gomberg’s dimer, the EPR spectrum is
dominated by the amine radical cations (Figure S32-33).
between two donor-acceptor species by simple analysis of their
ionization potential and electron affinity. In addition, this work
grants important insight for understanding and controlling the
generation of highly reactive radical pairs by photoinduced or
thermal single-electron transfer, which we are currently applying
to the design of new radical ion pairs with photophysical
properties tuned for exploiting radical reactivity. We envision such
systems could be highly beneficial for designing FLPs suitable for
the activation of inert substrates as well as the development of
main-group photo-redox catalysis.35
In summary, we have shown that the encounter complexes
in FLP chemistry can also be described as EDA complexes, which
are susceptible to undergo photo-induced single-electron transfer
to form the corresponding radical pairs. This knowledge has
resulted, for the first time, in the controlled generation and
detection of the radical ion pairs of the archetypal FLPs
PMes3/BCF and PtBu3/BCF via visible light induced single-
electron transfer. This study allows us to directly probe any FLP
type reaction facilitated by these systems, and determine whether
it proceeds via radical pair formation. Furthermore, we
demonstrated that the energy gap between EDA complex and the
corresponding radical pair can be readily tuned to proceed
thermally by changing the Lewis acid and base components. It is
therefore possible to predict the nature of single-electron-transfer
Acknowledgements
This work was supported by the Council for Chemical Sciences of
The Netherlands Organization for Scientific Research (NWO/CW)
by a VIDI grant (J.C.S.), NWA Idea Generator grant (J.C.S.) and
a VENI grant (A.R.J.).
Conflict of interest
The authors declare no conflict of interest.
Keywords: frustrated Lewis pair • radicals • single-electron
transfer • charge transfer complex • photoinduced
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