A Radical Mechanism for Frustrated Lewis Pair Reactivity

Article abstract of DOI:10.1016/j.chempr.2017.05.022

The frustrated Lewis pairs (FLPs) derived from tBu₃P and E(C₆F₅)₃ (E = B, Al) react with pO₂C₆Cl₄ and Ph₃SnH to give [tBu₃POC₆Cl₄OE(C₆F₅)₃] (E = B 1, Al 2), [tBu₃PSnPh₃][HB(C₆F₅)₃] 3, and [tBu₃PSnPh₃][(μ-H)(Al(C₆F₅)₃)₂] 4. These products form via the accepted two-electron process involving a transient “encounter complex.” In contrast, the corresponding reactions of Mes₃P and E(C₆F₅)₃ (E = B, Al) with pO₂C₆Cl₄ give [(Mes₃POC₆Cl₄OE(C₆F₅)₃] (E = B 8, Al 9); however, the identification of the intermediates [Mes₃P·]₂[(C₆F₅)₃EOC₆Cl₄OE(C₆F₅)₃] (E = B 5, Al 6) supports a mechanism that proceeds via a single-electron transfer process. This is further supported by the reactions of Mes₃P and E(C₆F₅)₃ (E = B, Al) with Ph₃SnH, which yielded [Mes₃PH][HB(C₆F₅)₃] 10 and [Mes₃PH][(μ-H)(Al(C₆F₅)₃)₂] 11, respectively, with the concurrent formation of Ph₃SnSnPh₃.

Full text of DOI:10.1016/j.chempr.2017.05.022

Article
A Radical Mechanism for Frustrated Lewis Pair
Reactivity
Liu (Leo) Liu, Levy L. Cao, Yue
Shao, Gabriel Me´ nard, Douglas
W. Stephan
dstephan@chem.utoronto.ca
HIGHLIGHTS
The observation of radical
intermediates in the FLP system of
Mes3P/E(C6F5)3
The reactions of Mes3P/E(C6F5)3
with p-O2C6Cl4 proceed via
radical intermediates
The FLP system of Mes3P/E(C6F5)3
homolytically splits Sn–H bonds
The frustrated Lewis pairs (FLPs) derived from Mes3P and E(C6F5)3 (E = B, Al) react
via initial single-electron transfer, generating radical cationic and anionic
intermediates in contrast to the corresponding reactions of tBu3P and E(C6F5)3.
Liu et al., Chem 3, 259–267
August 10, 2017 ª 2017 Elsevier Inc.
http://dx.doi.org/10.1016/j.chempr.2017.05.022

Article
A Radical Mechanism
for Frustrated Lewis Pair Reactivity
Liu (Leo) Liu,1 Levy L. Cao,1 Yue Shao,1 Gabriel Me´ nard,1,2 and Douglas W. Stephan1,3,
*
SUMMARY
The Bigger Picture
For many years, it was believed
that the combination of a Lewis
base and Lewis acid resulted in
the formation of a Lewis acid-base
adduct, and consequently the
neutralization quenched the
donor-acceptor ability of Lewis
pairs. This paradigm changed in
2006 with the discovery of
The frustrated Lewis pairs (FLPs) derived from tBu3P and E(C6F5)3 (E = B, Al)
react with pO2C6Cl4 and Ph3SnH to give [tBu3POC6Cl4OE(C6F5)3] (E = B 1,
Al 2), [tBu3PSnPh3][HB(C6F5)3] 3, and [tBu3PSnPh3][(m-H)(Al(C6F5)3)2] 4. These
products form via the accepted two-electron process involving a transient
‘‘encounter complex.’’ In contrast, the corresponding reactions of Mes3P
and E(C6F5)3 (E = B, Al) with pO2C6Cl4 give [(Mes3POC6Cl4OE(C6F5)3] (E =
B
8, Al 9); however, the identification of the intermediates [Mes3P$]2
[(C6F5)3EOC6Cl4OE(C6F5)3] (E = B 5, Al 6) supports a mechanism that proceeds
via a single-electron transfer process. This is further supported by the reac-
tions of Mes3P and E(C6F5)3 (E = B, Al) with Ph3SnH, which yielded [Mes3PH]
[HB(C6F5)3] 10 and [Mes3PH][(m-H)(Al(C6F5)3)2] 11, respectively, with the con-
current formation of Ph3SnSnPh3.
frustrated Lewis pairs (FLPs). This
finding paved the way for the
rapid development of FLP
chemistry and its application
relevant to organic syntheses,
enzymatic models, and material
sciences.
INTRODUCTION
Reaction mechanisms are the basis of our understanding of chemical reactivity, and
their elucidation has been of paramount importance in the continuing evolution of
synthetic organic, organometallic, and biological chemistry.1–3 The formation of a
new two-electron bond in many reactions proceeds via electron migration of a
pair of electrons from an electron-rich to an electron-deficient center. The chemical
literature is replete with such diamagnetic processes; however, other reactions yield
two-electron bonds via successive single-electron transfer (SET) reactions thus pro-
ceeding via paramagnetic intermediates. Such reactions are increasingly prominent
in photochemical reactions, organic synthesis, catalysis, material synthesis, and
biochemistry. Mechanisms in main-group chemistry are also dominated by two-elec-
tron diamagnetic processes, and perhaps none is more common than the formation
of dative-bonded adducts from the combination of an electron pair donor and an
electron pair acceptor.
Understanding reaction
mechanisms is crucial to progress
in fundamental chemical research.
The accepted mechanism of FLP
reactivity involves the polarization
of a substrate molecule in the
pocket of an ‘‘encounter
complex,’’ leading to heterolytic
cleavage. The present work shows
differing reaction mechanisms for
FLPs derived from tBu3P/E(C6F5)3
and Mes3P/E(C6F5)3 (E = B, Al).
This finding broadens both our
understanding and the potential
reactivity of FLP systems.
Such reactions led to Gilbert Lewis’s 1923 classification of donors and acceptors as
bases and acids, respectively.4 A few anomalous Lewis acid-base combinations are
known not to form adducts,5 but the reactive potential of such systems was uncov-
ered in 2006 with the demonstration of reversible, metal-free hydrogen activation
by p(Mes2P)C6F4(B(C6F5)2)6 (Mes = mesityl). In this case, the Lewis base and acid cen-
ters are surrounded by bulky substituents that preclude dative bond formation and
allow the electron donor and acceptor to act cooperatively to split H2. Shortly there-
after, this notion was generalized to include simple combinations of Lewis bases and
Lewis acids that are either non-interacting as a result of steric demands or form weak
bonds governed by dissociative equilibria. Such systems, known as frustrated Lewis
pairs (FLPs),7 led to the development of metal-free hydrogenation catalysis8 for a
range of polar and non-polar unsaturated compounds. Over the past decade, the
Chem 3, 259–267, August 10, 2017 ª 2017 Elsevier Inc. 259

Figure 1. Plausible Mechanisms of Small-Molecule Activation by FLPs
(A) Mechanism of activation of generic substrate (X2) by FLPs via an ‘‘encounter complex.’’
(B) Proposed mechanism of activation of generic substrate by FLPs via the SET process.
chemistry of FLPs has been extended to include the activation of a variety of small
molecules, including H2, CO2, SO2, N2O, NO, olefins, and alkynes, as well as C–H
bonds.9–11 Such advances have led to synthetic applications in organic, polymer,
and radical chemistry, and the concept has also been applied to uncover new tran-
sition-metal, enzymatic, and surface chemistry.12
1Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, ON M5S 3H6,
Canada
Computational studies that have probed the mechanism of H2 activation have been
critical to our understanding of FLP chemistry.13–21 Such studies have been done for
several systems, and the precise details differ slightly depending on the system and
level of theory. In the case of tBu3P and B(C6F5)3, a two-electron diamagnetic reac-
tion mechanism is thought to proceed via an ‘‘encounter complex’’13,19 (Figure 1A)
that resides in a shallow thermodynamic well and features an approach of the Lewis
2Present address: Department of Chemistry and
Biochemistry, University of California, Santa
Barbara, Santa Barbara, CA 93106, USA
3Lead Contact
*Correspondence: dstephan@chem.utoronto.ca
http://dx.doi.org/10.1016/j.chempr.2017.05.022
260 Chem 3, 259–267, August 10, 2017

˚
acid and base that is sterically limited to ca 4 A. Addition of H2 into the polarized
pocket of the encounter complex prompts interactions of the H–H s bond with
the Lewis acid and concurrent donation of the Lewis base to the s* orbital of H2.
The result is protonation of the base and hydride delivery to the Lewis acid. More
recently, detailed diffusion and temperature-dependent studies22 provided experi-
mental support for the association between tBu3P or Mes3P and B(C6F5)3 in aromatic
solvents and were consistent with a slightly endergonic association via weak disper-
sive interactions. More recently, molecular dynamics simulations of the tBu3P/
B(C6F5)3 pair in toluene probed the degree of intermolecular association and sug-
gested that the concentration of reactive, pre-organized states (0.5%) is comparable
with levels typical of reactive intermediates.23–26 This mechanism is thought to be
operative for a range of main-group and transition-metal FLPs. Moreover, it is
directly analogous to that exhibited by bifunctional Noyori-type transition-metal
complexes.27 Conceptually, this is also related to the roles of an acceptor and
back-bonding orbitals that allow the activation of H2 at a single atom in transition-
metal complexes28 and by singlet carbenes29,30 and analogs.31,32
RESULTS AND DISCUSSION
The notion of SET mechanisms in reactions of Lewis acids and bases are rare, and
there are only a few examples of SET from transition-metal complexes to strong
Lewis acids.33,34 For example, Wang et al.35 have recently reported the irreversible
one-electron oxidation of triarylamine by an equimolar portion of B(C6F5)3 gener-
ating the highly reactive radical anion [$B(C6F5)3]À,36 which was quenched by subse-
quent reaction with solvent. These data prompted questions of the possible role of
SET reactions in the mechanism of FLP chemistry (Figure 1B).
To begin to address this question, we examined the quintessential FLPs derived
from the combination of tBu3P and one of E(C6F5)3 (E = B or Al) in toluene. The spec-
tral data showed little reaction, with only traces of by-products. In the case of the
borane combination, the minor by-product was identified as the para-substitution
zwitterion [tBu3PC6F4BF(C6F5)2].37 In a separate control experiment, combination
of B(C6F5)3 with tetrachloro-1,4-benzoquinone (p-O2C6Cl4) showed no reaction
even on cooling to À78ꢀC, whereas Al(C6F5)3 forms a Lewis acid-base adduct with
quinone at room temperature (Figure S47). Regardless, treatment of the FLP system
consisting of tBu3P/E(C6F5)3 with an equimolar amount of pO2C6Cl4 in chloroben-
zene immediately afforded light-yellow solutions. 1H, 13C, 31P, 19F, 11B, and 27Al
NMR spectroscopic data were consistent with the formation of compounds derived
from a 1:1:1 combination of the reagents, suggesting the formulation of these prod-
ucts as [tBu3POC6Cl4OE(C6F5)3] (E = B, 1; Al, 2) (Figures 2 and S1–S9). Crystallo-
graphic characterization of 1 (Figure 2; Table S1) confirmed its zwitterionic nature
˚
and revealed the P–O and B–O bond distances to be 1.607(5) and 1.503(9) A.
In an analogous fashion, E(C6F5)3 (E = B or Al) in reactions with Ph3SnH showed ev-
idence of only weak dative interactions (Figures S48–S50), however, mixtures of
tBu3P and E(C6F5)3 (E = B, Al) react readily with Ph3SnH, providing the (stannyl)phos-
phonium salts [tBu3PSnPh3][HB(C6F5)3] 3 and [tBu3PSnPh3][(m-H)(Al(C6F5)3)2] 4 (Fig-
ures 2 and S10–S19). The formation of the P–Sn bond was evident from 119Sn satel-
lites on the 31P NMR resonance at 66.5 parts per million (ppm) and the doublet
structure of the 119Sn NMR resonance at À128.5 ppm (Figures S12 and S15). Both
of these signals exhibit Sn–P coupling of 185 Hz. In the case of 4, the solid-state
structure was unambiguously confirmed by single-crystal X-ray diffraction analysis
(Figure 2; Table S1).
Chem 3, 259–267, August 10, 2017 261

Figure 2. Reactivity of tBu3P/E(C6F5)3 toward pO2C6Cl4 and Ph3SnH
Reactions of tBu3P, E(C6F5)3 (E = B or Al) with p-O2C6Cl4 and Ph3SnH (see also Figures S1–S19). POV-ray depictions of the molecular structures of 1 and 4
are shown (see also Table S1). Colors are as follows: C, black; F, pink; B, yellow-green; Al, light blue; O, red; Sn, green-blue; P, orange; Cl, aquamarine; H,
gray. All hydrogen atoms except the bridging hydride of the anion of 4 are omitted for clarity.
These observations are consistent with the accepted FLP mechanism involving
concerted action of the Lewis acid and base on the substrate, whereby the Lewis
base donates an electron pair to the substrate and the Lewis acid accepts an electron
pair from the substrate. In these cases, this reactivity results in FLP addition to the
quinone or heterolytic cleavage of the Sn–H bond.
In contrast, mixing Mes3P and E(C6F5)3 (E = B, Al) in an equimolar ratio in dry
toluene or chlorobenzene at room temperature gave rise to a very pale purple
solution in the case of the borane and a much more distinctly purple solution for
the Al species. Although these observations suggest that chemical reactions
have occurred, both solutions exhibit NMR spectroscopic features showing signals
attributable to the initial reagents. In addition, evidence for the formation of
trace amounts of by-products are demonstrated by peaks in 19F NMR spectra after
additional scans (e.g., 300 scans for Mes3P/Al(C6F5)3). In the case of the borane
reaction, the electron paramagnetic resonance (EPR) spectrum showed only a
weak signal. However, the solution generated with Mes3P/Al(C6F5)3 revealed a
room-temperature EPR spectrum consisting of a doublet resonance with a g value
of 2.0089 and a hyperfine coupling constant of 238 G. This signal is attributable
to the coupling of an unpaired electron to a P center and is consistent with the
formation of the known radical cation [Mes3P$]+ (Figure 3).38,39 This observa-
tion suggests that this radical is formed via SET and generates the correspond-
ing radical anion [$Al(C6F5)3]À. This radical anion was not detected by EPR spec-
troscopy, which is consistent with the short lifetimes of such species.40–42
Collectively, these observations of both diamagnetic and paramagnetic species
support the notion of SET equilibria between the FLP and [Mes3P$]+ and
[$Al(C6F5)3]À.
Quinones are well known to be electron acceptors with the formation of charge-
transfer complexes.43,44 Control experiments revealed that no reaction was
observed on treatment of Mes3P with pO2C6Cl4 in C6H5Cl. Moreover, the UV-visible
262 Chem 3, 259–267, August 10, 2017

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Figure 3. Reactivity of Mes3P/E(C6F5)3 toward pO2C6Cl4 and Ph3SnH
Reactions of Mes3P and E(C6F5)3 (E = B or Al) with pO2C6Cl4 and Ph3SnH (see also Figures S20–S46).
Insert: picture of a solution of Mes3P and Al(C6F5)3 in chlorobenzene and the experimental and
Chem 3, 259–267, August 10, 2017 263

Figure 3. Continued
simulated X-band EPR spectra derived from the Mes3P/Al(C6F5)3 solution (observed, black;
simulated, red); POV-ray depictions of the molecular structures of 7 and 8 are shown (see also Table
S1). Colors are as follows: C, black; F, pink; B, yellow-green; Cl, aquamarine; P, orange; O, red; H,
gray. All hydrogen atoms except the phosphonium proton of 7 are omitted for clarity.
spectrum showed no absorption bands from 450 to 900 nm (Figure S51). However,
slow addition of half an equivalent of pO2C6Cl4 to the combinations of Mes3P and
E(C6F5)3 (E = B, Al) in toluene at À78ꢀC gave rise to deep purple solutions that
showed a visible absorption maximum at 573 nm, consistent with that of the known
radical [Mes3P$]+.38,39 31P-NMR spectra for these solutions showed no signals, and
19F NMR data documented the complete consumption of E(C6F5)3 within 30 min.
The stoichiometry and the spectral data are consistent with the formation of the
radical cation salts [Mes3P$]2[(C6F5)3EOC6Cl4OE(C6F5)3] (E = B 5, Al 6). Attempts
to crystallize these salts were unsuccessful. However, Ph3SnH was added to these so-
lutions by a strategy previously used in the literature to intercept radical salts,45,46
generating the phosphonium salts [Mes3PH]2[(C6F5)3BOC6Cl4OB(C6F5)3] 7 (Figures
S20–S24), as demonstrated by the observation of the expected resonance in the
31P NMR spectrum. Further confirmation of the formulation was obtained by a
crystallographic study that established the connectivity unambiguously (Figure 3;
Table S1).
In a similar fashion, altering the stoichiometry to a 1:1:1 ratio of E(C6F5)3 (E = B, Al):
Mes3P:pO2C6Cl4 gave solutions that immediately became deep purple in color with
an absorption maximum at 573 nm. Upon stirring at room temperature, this color
faded to a light yellow over the course of 3 min for B and 20 min for Al (Figures
S34 and S35). The 31P NMR spectra of the resulting solutions displayed singlets at
70.8 and 71.1 ppm for the B(C6F5)3 and Al(C6F5)3 reactions (Figures S27 and S32),
respectively. The spectroscopically similar products of these reactions, isolated as
pale yellow solids, were formulated as [Mes3POC6Cl4OE(C6F5)3] (E = B 8, Al 9) (Fig-
ures S25–S33). This formulation was confirmed via a single-crystal X-ray diffraction
˚
study of 8, which revealed P–O and O–B bond lengths of 1.606(3) and 1.500(6) A,
respectively (Figure 3; Table S1).
The initially intense purple coloration of the solutions suggests the intermediate for-
mation of 5 and 6 en route to 8 and 9. Addition of quinone to 5 or 6 is thought to
generate the transient radical anions [$OC6Cl4OE(C6F5)3]À, which are ultimately
quenched by the radical cation to give the zwitterionic products. This view is sup-
ported by independent reaction of [Mes3P$][(m-HO)(Al(C6F5)3)2]38 with pO2C6Cl4,
where no reaction was observed in 1 hr. The ultimate products of these reactions us-
ing Mes3P (8 and 9) are analogous to those obtained with tBu3P (1 and 2), but the
above data demonstrate that an alternative mechanism initiated by SET between
the phosphine and the Lewis acid is operative.
To further support this hypothesis, addition of Ph3SnH to toluene solutions of
Mes3P and one of E(C6F5)3 (E = B, Al) were shown to result in an immediate color
change. 19F, 11B, 31P, and 1H NMR spectroscopic data confirmed the formation of
the known salts [Mes3PH][HB(C6F5)3] 1047 and [Mes3PH][(m-H)(Al(C6F5)3)2] 11 (Fig-
ures S36–S46),48 respectively. Concurrent with this was the quantitative formation
of the di-stannane Ph3SnSnPh3 as demonstrated by the observation of a singlet in
the 119Sn NMR spectra (À138.9 ppm [CH2Cl2] and À136.8 ppm [C6H5Br] for reac-
tions generating 10 and 11 [Figures S41 and S46], respectively).49 The formation of
these products is consistent with radical abstraction of H$ from the Sn species. This
evidence for homolytic cleavage of the Sn–H bond stands in marked contrast to
264 Chem 3, 259–267, August 10, 2017

the corresponding reactions of tBu3P in which heterolytic cleavage of the Sn–H
bond is observed.
The possible, and previously unappreciated, role of SET processes in the reactivity of
FLPs has interesting implications. For example, the activation of H2 by the FLPs used
here is known to give the salts [R3PH][HB(C6F5)3] and [R3PH][(m-H)(Al(C6F5)3)2]
(R = tBu, Mes). Although the FLPs based on the two phosphines provide similar prod-
ucts, the data here suggest differing reaction mechanisms for the systems based on
tBu3P and Mes3P. The former is thought to operate via two-electron processes af-
fording heterolytic cleavage of H2; however, the present study suggests the possibil-
ity of homolytic H2 cleavage via one-electron processes.
In conclusion, more than a decade after FLP reactivity was uncovered, the divergent
reactivity observed for combinations of tBu3P/E(C6F5)3 or Mes3P/E(C6F5)3 (E = B, Al)
with pO2C6Cl4 and Ph3SnH support differing reaction mechanisms. The tBu3P sys-
tems react via the conventional diamagnetic two-electron mechanism, but the reac-
tions of Mes3P proceed via intermediate radical salts generated by SET from the
Lewis base to the Lewis acid. The detection of this SET radical mechanism further
broadens the potential reactivity of FLP systems, which is currently the subject of
ongoing study.
EXPERIMENTAL PROCEDURES
Full experimental procedures are provided in the Supplemental Information.
ACCESSION NUMBERS
Crystallographic data have been deposited in the Cambridge Crystallographic Data
Centre (CCDC) under accession numbers CCDC: 1539089–1539093. These data can
be obtained free of charge from the CCDC at http://www.ccdc.cam.ac.uk/
data_request/cif.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 51
figures, 1 table, and 1 data file can be found with this article online at http://dx.
doi.org/10.1016/j.chempr.2017.05.022.
AUTHOR CONTRIBUTIONS
L.L., L.L.C., and Y.S. performed the synthetic studies. L.L. and L.L.C. carried out sin-
gle-crystal X-ray diffraction studies. L.L.C. and G.M. performed the EPR experi-
ments. D.W.S. and L.L. designed the experiments. D.W.S. directed the project
and wrote the manuscript with input from all authors. All authors analyzed the results
and commented on the manuscript.
ACKNOWLEDGMENTS
D.W.S. is grateful for the financial support of the Natural Sciences and Engineering
Research Council of Canada and the award of a Canada Research Chair and to the
Einstein Foundation for the award of an Einstein Fellowship. L.L.C. is grateful for
the award of an Ontario Graduate Scholarship.
Received: April 1, 2017
Revised: May 2, 2017
Accepted: May 31, 2017
Published: August 10, 2017
Chem 3, 259–267, August 10, 2017 265

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