Trioxatriangulenium (TOTA+) as a robust carbon-based Lewis acid in frustrated Lewis pair chemistry

Article abstract of DOI:10.1039/d0sc05893a

We report the reactivity between the water stable Lewis acidic trioxatriangulenium ion (TOTA⁺) and a series of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid-base interaction was analyzedviavariable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe₃, led to the formation of a Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P(^tBu)₃. The TOTA⁺-P(^tBu)₃FLP was characterized as an encounter complex, and found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of the C-Br bonds, and interception of Staudinger reaction intermediates. Moreover, TOTA⁺and NHC were found to first undergo single-electron transfer (SET) to form [TOTA]·[NHC]˙⁺, which was confirmedviaelectron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA-NHC]⁺adduct or a mixture of products depending the reaction conditions used.

Full text of DOI:10.1039/d0sc05893a

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Trioxatriangulenium (TOTA+) as a robust carbon-
based Lewis acid in frustrated Lewis pair chemistry†
Cite this: Chem. Sci., 2021, 12, 4841
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Aslam C. Shaikh,
Jose M. Veleta,
Jules Moutet
and Thomas L. Gianetti
´
We report the reactivity between the water stable Lewis acidic trioxatriangulenium ion (TOTA+) and a series
of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid–base
interaction was analyzed via variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction,
UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such
as PMe3, led to the formation of a Lewis acid–base adduct, frustrated Lewis pairs (FLPs) were observed
for sterically hindered bases such as P(tBu)3. The TOTA+–P(tBu)3 FLP was characterized as an encounter
complex, and found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation,
dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of the C–Br bonds, and interception of
Staudinger reaction intermediates. Moreover, TOTA+ and NHC were found to first undergo single-
electron transfer (SET) to form [TOTA]$[NHC]c+, which was confirmed via electron paramagnetic
resonance (EPR) spectroscopy, and subsequently form a [TOTA–NHC]+ adduct or a mixture of products
depending the reaction conditions used.
Received 26th October 2020
Accepted 7th February 2021
DOI: 10.1039/d0sc05893a
rsc.li/chemical-science
underdeveloped.5 Similarly, examples of Lewis acidic carbon
centers in FLP chemistry are scarce,6 and limited to trityl4,7 and
substituted trityl cations,8 electron-decient allenes,9 N-meth-
Introduction
Frustrated Lewis pairs (FLPs) arise from the combination of
Lewis acids (LAs) and bases (LBs) with un-neutralized reactivity
due to the steric repulsion between these units.1 Since their
discovery, FLPs have received considerable attention for their
potential to activate small molecules and applications in
catalysis in the absence of transition-metals.2 While signicant
exploration of the tunability of FLPs has been done, it is inter-
esting to note that the main advances are focused on the nature
of the LBs. Common bases for FLPs found in the literature
include a wide range of phosphines, amines, pyridines, N-
heterocyclic carbenes (NHCs), or carbodiphosphoranes, which
show compatibility with various solvents and functional groups
during transformations. In contrast, the LA counterparts in
FLPs are less versatile and mostly limited to peruorinated
alkyl/aryl boranes and alanes.3 Group 13 molecules are oen
highly oxophilic which limits their use and stability in wet
solvents, tolerance to several functional groups, and reaction
conditions when applied in catalysis.4 As a result, the develop-
ment of versatile and stable LAs for FLP chemistry is still highly
desirable.
ylacridinium salts,10 cationic h6-arene-Ru complex11 and C60
-
fullerenes12 (Fig. 1a). However, similar to borane systems, most
of these carbenium ions are air-sensitive, highly oxophilic, and
experience side reactivity, which limit their applications. For
example, trityl cation is an attractive LA due to its facile
+
synthesis, but its negative pKR of ꢀ6.7 makes it more oxophilic
than most boron-based FLPs.13 However, triaryl-carbenium ions
are susceptible to a nucleophilic attack at the para-aryl, or meta-
aryl positions, resulting in the formation of either cyclo-
hexadienyl or aryl-phosphonium cations, which hampers the
reactivity.14 Heterocyclic fused carbenium analogues, such as
acridinium, helicenium, and angulenium ions, are an attractive
alternative.15 Their more planar scaffold and reduced steric
hindrance facilitates interaction at the central carbon.
Furthermore, despite the increased stabilization and electron
richness provided by the heteroatoms,16 recent reports have
shown that some of these carbocations retain a certain degree of
Lewis acidity.17
Trioxatriangulenium ion (TOTA+, Fig. 1b), rst reported by
Smith et al. in 1964 (ref. 18) and with more recent applications in
bioimaging and nucleic acid interaction,19 is a planar hetero-
In recent years, several cationic or electron-decient
compounds containing heavy elements of group 14 as LAs
have been reported to show FLP reactivity, but their use remains
+
cyclic fused carbenium ion with a pKR of 9, a positive charge
delocalized across the p-system, yet the central carbon
contributing 33% of the LUMO's character, and with an
acceptor number (AN) of 23.1.20 As a result, TOTA+ is water
stable, resistant to para-aryl nucleophilic attack, and has shown
preferential reactivities at the central carbon, with a variety of
University of Arizona, Department of Chemistry and Biochemistry, Tucson, AZ, USA.
E-mail: tgianetti@arizona.edu
† Electronic supplementary information (ESI) available. CCDC 2035921–2035925.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc05893a
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TOTA+ resulted in the formation of a classical Lewis acid–base
covalent adduct. This was observed by the instantaneous
change in color upon the room temperature addition of PMe3,
changing from a dark yellow solution, characteristic of TOTA+
(1) in CH3CN, to a colorless solution. Such color change is
consistent with the loss of conjugation in the p-system, due to
a change in hybridization from sp2 to sp3 in the central carbon.
The TOTA–(PMe3)+ adduct (3a) was isolated as a white solid in
96% yield (see Scheme 1 and ESI†). The 31P NMR spectrum
indicates the exclusive formation of a phosphonium species
with a chemical shi of 39.7 ppm (Fig. S7†). The 1H NMR
spectrum of 3a shows two sets of protons at d ¼ 7.53 (td) and
7.12 (dd) ppm, consistent with the presence of a C3 rotation
axis, suggesting that the phosphorous is bound to the central
carbon atom in TOTA. Slow CH2Cl2/hexane layering afforded
colorless crystals of 3a suitable for single-crystal X-ray diffrac-
tometry, that conrmed the formation of the covalent adduct at
the central carbon (Scheme 1b and ESI†). The C1–P1 distance
˚
˚
(1.864(1) A) is comparable to typical C–P bonds (ca. 1.87 A), yet
+
shorter than the P–C bond in [trityl-PMe3] (1.887(4) A)14 and the
˚
P–B bond in tris(pentauorophenyl)borane-phenylphosphane
23
˚
(2.039(4) A). These observations suggest that the planarity of
the TOTA+ scaffold promotes the formation of shorter bond in
solid-state compared to the more sterically hindered trityl and
borane strꢀuctures. Within the asymmetric unit, the counter-
anion BF4 is far from both the carbon and phosphorous
˚
˚
˚
centers in 3a (B–C1 ¼ 5.355 A, B–P ¼ 4.821 A, or F–C1 ¼ 4.573 A
Fig. 1 (a) Previously reported carbon-based LA and (b) this work.
˚
and F–P ¼ 3.891 A respectively, see Fig. S110†).
Other phosphines with small cone angles, such as PPhMe2,
P(nPr)3, and P(nBu)3, also formed covalent Lewis adducts at
room temperature, which were isolated in excellent yields
(Scheme 1, 3a, 3b, 3c, and 3d, respectively).24 Interestingly,
when the TOTA–P(nBu)3+ adduct (3d) was warmed up to 353 K in
strong nucleophiles, such as hydride, sulfoxide, and organo-
lithium or Grignard reagents, to form a cup-like molecule with
an sp3-hybridized central carbon, known as trioxatricornan.21
While TOTA+ is signicantly less acidic than common Lewis
acids such as B(C6F5)3 (pKa of 8.4 and AN of 80.1), the reported
reactivity of TOTA+ with small and hard nucleophiles,22 led our
group to investigate the nature of the interactions between
TOTA+ and soer LBs, such as phosphines.
1
CD3CN, a broadening of the H NMR resonances are observed
(Fig. S19†), suggesting the presence of a thermodynamic equi-
librium. Using PPh2Me, a phosphine of similar cone angle as
P(nBu)3 yet with lower basicity, broad and unresolved peaks in
the 1H NMR spectrum were observed at room temperature,
suggesting that the slow exchange regime of the equilibrium is
observable on the NMR time scale at room temperature
Herein we describe the use of a trioxatriangulene (TOTA+) as
water stable LA for FLP chemistry, and its reactivity with
different phosphines as LBs. The reactivity of the obtained FLPs
was probed via the heterolytic disulde bond cleavage, xation
of formaldehyde, C(sp3)–H activation of 1,4-cyclohexadiene,
heterolytic cleavage of a C–Br bond, and interception of Stau-
dinger reaction intermediate. The interaction and reactivity of
these FLPs are best described as encounter complexes.
Furthermore, formation of Lewis acid–base adducts via single-
electron transfer was observed between TOTA+ and the N-
heterocyclic carbene 1,3-di-tert-butylimidazol-2-ylidene (ItBu).
The formation of these radical species is supported by UV-vis
and EPR spectroscopies.
1
(Scheme 1a, 3e). Variable temperature H NMR analysis, along
with the thermochromic change of the solution, conrmed the
presence of a reversible adduct in which the Lewis adduct is the
major species at low temperatures (see Fig. 2 and ESI†).
Continuing with the trend, when TOTA+ was mixed with
a weakly basic phosphine such as PPh3 (Scheme 1a, 3f), or
a sterically hindered phosphines, such as P(tBu)3 (Scheme 1a,
3g), no Lewis adduct was observed by NMR spectroscopy at
either room or low temperatures. Neither the appearance of
a new peak in 31P NMR spectroscopy nor an upeld shi of the
aromatic region in TOTA+ protons, characteristic of the trioxa-
tricornan scaffold, was observed (see ESI†). However, we noted
that, in both CD2Cl2 or CD3CN solvents, the aromatic reso-
nances of TOTA+ appear as broad signals at room temperature,
Results and discussion
A variety of alkyl, alkyl–aryl, and aryl phosphines were selected and sharpened at lower temperatures (Fig. S27–S30 and S36–
to probe the interaction between TOTA+ and phosphines S40†). Furthermore, when lowering the temperature below 230
(Scheme 1a). Reacting the less sterical phosphine, PMe3, with K, a broadening of the tert-butyl and phenyl signals were
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Scheme 1 (a) Reactivity of phosphines (2a–2g) towards TOTA+. (b) ORTEP of the X-ray structure of 3a. Hydrogen atoms and counter ion are
omitted for clarity. (c) Summary of cone angle, Lewis basicity, and DFT calculated DHadduct (enthalpy of adduct formation relative to free TOTA
and phosphine) for phosphines 2a–2g.
such aggregates (see ESI, Fig. S31–S34 and S41 and S42†).26 For
example, the diffusion coefficients (D) for 1-BArF and P(tBu3)
4
alone are 1.41 ꢁ 10ꢀ9 m2 s
and 1.88 ꢁ 10ꢀ9 m2 sꢀ1 respec-
ꢀ1
tively. However, upon mixing, diffusion coefficients of 1.80 ꢁ
10ꢀ9 m2 s
(1-BArF4) and 1.74 ꢁ 10ꢀ9 m2 sꢀ1 (P(tBu3)) were
ꢀ1
recorded.27 Finally, we noted that upon mixing TOTA+ and
P(tBu)3 a slightly darker yellow color, compared to that of
TOTA+, was observed. This was further analyzed by mixing an
equimolar amounts of each species in CH2Cl2 and analyzing the
solution via UV-vis absorption spectroscopy. We observed the
appearance of weak absorption bands at lmax of 576 and
626 nm, in addition to the main absorption bands for TOTA+
Fig. 2 (a) Reversible Lewis adduct formation between TOTA+ and
PPh2Me. (b) Variable temperature (VT) 500 MHz 1H NMR spectra of
TOTA+ (12 mg, 0.01 mmol) and PPh2Me (4.2 mg, 0.02 mmol) in CD2Cl2
indicating formation Lewis adduct at ꢀ60 ꢂC.
observed in the 1H NMR spectra (Fig. S27 and S37†). The
broadening and coalescences of the TOTA+ aromatic peaks and
the phosphine substituents indicate that only a weak van der
Waals adduct is formed, as observed in the case of FLPs derived
from B(C6F5)3 and bulky phosphines, and that these aggregates
have random relative orientations (see DFT calculations vide
infra).8,25 Diffusion ordered spectroscopy (DOSY) NMR experi-
ments were performed and further supported the formation of
Fig. 3 Normalized UV-vis spectra of TOTA+ (blue trace, 1 ꢁ 10ꢀ5 M in
CH2Cl2), PtBu3 (black trace, 1 ꢁ 10ꢀ5 M in CH2Cl2), and a 1 : 1 mixture
of PtBu3/TOTA+ (red trace, both substrates 1 ꢁ 10ꢀ5 M in CH2Cl2).
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(458 and 483 nm) (see Fig. 3). These data indicate the presence (Fig. 4a). With [TOTA–P(tBu)3]+, the thermodynamically favor-
of an encounter complex in solution.28 We postulate that the able conformation 3g-I has the phosphorus atom directly in
increased steric bulk of the phosphine inhibits the formation of front of central carbon atom of TOTA+, while in 3g-II the P atom
a Lewis adduct, resulting in a dynamic equilibrium between free is pointing in the opposite direction (Fig. 4b). These calcula-
TOTA+/P(tBu)3 and its encounter complex.
Density functional theory (DFT) calculations were used to supra).
tions support the NMR spectra obtained for these FLPs (vide
further understand the energetic and steric factors between
We then tested the reactivity of our Lewis pairs for the acti-
TOTA+ and the different phosphines (Scheme 1c and ESI†). As vation of non-polar covalent bonds, such as those present in
expected, the strength of the LA–LB interaction, quantied via disuldes. Using diphenyl sulde, we observed full conversion
+
both the [P–CTOTA
]
bond length and stabilization energy, in good yields for the desired S–S bond cleavage products at
depends on the phosphine's cone angle and basicity. For room temperature for the FLP encounter complexes 3f and 3g.
example, the TOTA+-adducts with phosphines of comparable LB
In the case of the reaction with 3g, the initially dark yellow
character, such as PMe3, P(nBu)3, and P(tBu)3, show a correla- solution gradually turned colorless aer 10 min at room
+
tion between increasing cone angle and increasing [P–CTOTA
]
temperature. The crude mixture was thoroughly washed with
bond length. The [TOTA–PMe3]+ adduct had the shortest bond pentane, and crystallization at ꢀ20 ꢂC of the pentane washes
t
+
˚
compared to the [TOTA–P( Bu)3] (1.88 vs. 2.06 A, Table S11†). resulted in TOTA–SPh (4a) as colorless crystals. The formation
Similarly, the stabilization energy for these adducts was of 4a was supported by the conservation of the C3 rotation axis
inversely proportional to the cone angle of the phosphine used, and two set of protons for the TOTA+ moieties in the H NMR
1
with decreasing energies of ꢀ23.4, ꢀ16.5, and ꢀ11.8 kcal molꢀ1 spectrum, as well as the presence of a new quaternary carbon
for increasing cone angles (Me < nBu < tBu, see ESI†). Looking at signal at 40.9 ppm in the 13C{1H} NMR spectrum. Furthermore,
phosphines with comparable cone angles, such as PPh2Me and the structure of 4a was unambiguously conrmed by X-ray
P(nBu)3 (cone angle of 136ꢂ), a shorter [P–CTOTA]+ bond length by diffraction crystallography which attests to the heteroleptic
˚
0.02
A
and larger stabilization energy (ꢀ22.7 vs. cleavage of the disulde bond of (SPh)2 (Scheme 2b). Notably,
ꢀ16.5 kcal molꢀ1) was observed for the more Lewis basic the C1–S1 bond of 1.902(2) A is slightly longer than the typical
˚
P(nBu)3.
value for single C–S bond length (ꢃ1.83 A), but is consistent
˚
DFT calculations also provided insight on the structure of with the covalent C–S distance found in the sulde(trityl) bond
29
˚
the FLPs aggregates. For both FLPs, several conformers of close (C–S ¼ 1.90 A). Moreover, the solid residue from the washes
energy were found (see ESI† and Fig. 4), with the most ther- was dried under vacuum for 12 h, and the yellow compound was
modynamic stable aggregate, 3f-I and 3g-I, being more favor- crystallized from slow diffusion of pentane into a concentrated
able than the free components by ꢃ12 kcal molꢀ1 (see Fig. 5). solution in CH2Cl2 at ꢀ20 C. The desired thio-phosphonium
ꢂ
For [TOTA–PPh3]+, the most thermodynamically stable salt (5a) was isolated in pure form and characterized by NMR
conformer 3f-I contains a p–p stacking interaction between the spectroscopy (Scheme 2a and ESI†). We further tested the ability
planar TOTA+ and the arylphosphine, where the phosphorus of 3g to cleave the S–S bond of a variety of disuldes RS–SR (R ¼
atom is pointing away from the carbocation scaffold, while 3f-II p-tolyl, 4-Cl-C6H4, –Bn) which gave the desired products 4b–4d
shows a P–C interaction characteristic of a Lewis adduct and 5b–5d in good yields. Similar reactivity was observed when
the FLP-3f was used. The desired S–S bond cleavage products
were isolated in excellent yields (Scheme 2a and see ESI†).
It is noted that TOTA+ or phosphine alone were inert towards
the disulde bond cleavage (Scheme 2b). Excitingly, when the
disulde cleavage was performed in wet acetonitrile, the desired
products were obtained in high yields (Scheme 2b). This result
highlights the robustness and water tolerance of the system.
Interestingly, the covalent adducts 3a and 3d also resulted in
S–S bond cleavage but required temperatures of 60 ꢂC and 35 ꢂC
respectively (see Scheme 2b and ESI†). Furthermore, premixing
the PMe3 with disulde, followed by addition of TOTA+, resulted
in a mixture of the desired S–S bond cleavage product along
with TOTA–PMe3 adduct 3a. These observations suggest that
LA–LB dissociation is needed to promote disulde activation.30
The reaction coordinate for the activation of disulde bonds
was studied using DFT (Fig. 5). Grimme D3 empirical dispersion
damping forces were applied to better understand non-covalent
interactions between the TOTA+, (SPh)2, and PR3 (R ¼ Me, Ph,
tBu) fragments. Starting from the FLP or covalent adduct (step 1,
Fig. 5), a dissociation step in the presence of (SPh)2 is consid-
ered (step 2, Fig. 5). As observed experimentally, the dissocia-
tion energies for the TOTA+ adducts with PPh3 and P(tBu)3 is
Fig. 4 DFT geometry optimized structures of TOTA+ adducts with (a)
PPh3 and (b) P(tBu)3 using the CAM-B3LYP functional and 6-311G(d,p)
basis set with PCM for acetonitrile (relative energy in kcal molꢀ1).
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Fig. 5 Reaction coordinate for the activation of (SPh)2 using [TOTA–PR3]+ (R ¼ tBu, Ph, or Me) adducts. All models were calculated using the
CAM-B3LYP functional, 6-311G(d,p) basis set, SCRF PCM acetonitrile solvation model, and Grimme D3 empirical dispersion damping functions.
Transition states were found with a single imaginary frequency and as local maxima by IRC calculations.
4, Fig. 5). The overall reaction coordinate for the activation of
(SPh)2 is representative of an exothermic process, with the
formation of TOTA–SPh and [R3P–SPh]+ adducts ca.
40 kcal molꢀ1 lower in energy than the correspondent reactants
(FLP or adducts, step 5, Fig. 5).
We further examined the utility of the FLP 3g towards bond
activation using 1,4 cyclohexadiene,7a and tbutyl bromide
(Scheme 3). Addition of 1,4-cyclohexadiene to a solution of 3g in
acetonitrile resulted in the dehydrogenation of hexadiene to
form benzene, the formation of the phosphonium salt 7 in 86%
yield and trioxatricornan TOTA–H (6) in 64% yield. The phos-
phonium salt shows a characteristic P–H signal at 6.25 ppm in
1
the H NMR spectrum and a doublet at 50.52 ppm in the 31P
NMR spectrum, with a 1JP–H coupling constant of 469.8 Hz (see
ESI†). The reactivity of 3g towards tbutyl bromide resulted in the
formation of phosphonium salt 7 and [TOTA][Br] in quantitative
yields, with the expected isobutylene side-product as reported
by Wass et al.31 Importantly, no reaction was observed upon
exposure of 3g to H2 gas, at either atmospheric pressure or 4
bar, with or without saturated substrates such as alkyne.
Scheme 2 (a) FLP-mediated activation of disulfide bonds and crystal
Next, we investigated the reactivity of FLP 3g toward unsat-
structures of 4a (thermal ellipsoids shown at 50% probability, hydrogen
urated substrates such as formaldehyde and benzyl azide
atoms and counter ions are omitted for clarity. C grey, O red, S yellow).
(Scheme 3). When a 1.0 molar equivalent of paraformaldehyde
(b) Control experiments, Lewis adduct mediated S–S bond cleavage
was added to a solution of 3g–BF4 in CH3CN, we observed the
formation of a new product by NMR spectroscopy, assigned to
a formaldehyde adduct. However, the new species slowly
returns to the starting material suggesting that the strongly
coordinating acetonitrile was not suited to isolate the adduct
formed. To improve the stability of the adduct formed we
turned to the non-coordinating solvent CH2Cl2. Since [TOTA]
with 3a and 3d, as well as 3g in wet solvent.
thermodynamically more accessible than for PMe3, with
a dissociation barrier ducts ca. 10 kcal molꢀ1 more accessible
for PPh3 and P(tBu)3 than for the PMe3 adduct. In the following
step, a TOTA–SPh intermediate stabilized by p-stacking inter-
actions between the aryl groups in TOTA and the disulde
substrate is formed (step 3, Fig. 5). The p-complex is then
attacked by the phosphine via a concerted transition state (step
[BF4] is sparingly soluble in this solvent, we synthesized the
ꢀ
more soluble [TOTA][BArF4] analog ([BArF
]
¼ [{3,5-(CF3)2-
4
C6H3}4B]ꢀ) following previously reported literature.17a An
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Finally, an organic azide was employed which led to the
isolation of a Staudinger-type reaction intermediate from the
reaction between P(tBu)3, TOTA+, and benzyl azide. The
combination of 3g and benzyl azide in CH2Cl2 afforded adduct 9
in 85% yield aer 12 h at room temperature (Scheme 3). NMR
and crystallographic analysis of 9 revealed it to be the addition
product of BnN3 between TOTA+ and P(tBu)3 (Scheme 3). Suit-
able crystals for X-ray diffraction studies of 9 were obtained by
slow diffusion of pentane into a concentrated CH2Cl2 solution,
resulting in pale-yellow plates. The trapping of Staudinger-type
intermediate by 3g is characterized by the creation of a single
C1–N1 bond (benzyl nitrogen from the substrate) length of
˚
˚
1.524(4) A longer than the classic C–N bond (ꢃ1.47 A). At the
same time, an N3–P1 bond is formed with the terminal nitrogen
t
˚
(1.680(3) A) and the P( Bu)3 having a typical length of a phos-
27
˚
phorus–azide bond (1.658(3) A). It was further noted that the
distances N1–N2 and N2–N3 (respectively 1.310(3) and 1.299(3)
˚
A) retain a double bond character while the length of the N1–
˚
C20 bond (1.467(3) A) of the benzyl group is also unaffected.
This FLP addition reaction is consistent and can be compared
to literature examples for a range of aryl azides and trityl-boron
or phosphorous–boron FLPs.7a,27,32
To test the compatibility of TOTA+ with other Lewis bases, we
used an NHC, 1,3-di-tert-butylimidazol-2-ylidene (ItBu, 10), as
a possible LB (see Scheme 4 and Table S2†). Upon addition of 10
to a solution of [TOTA][BArF4] in toluene at room temperature,
an immediate light green solution was observed which quickly
turned to pale yellow. Analysis via 1H NMR spectroscopy of the
crude reaction mixture shows the formation of C4-adduct (11,
79%), and C2-adduct (110, 10%), and a minor byproduct
assigned to the imidazole salt (12, 10%). When the reaction was
performed at ꢀ78 ꢂC, the C4/C2 selectivity was reversed, and the
Scheme
3 FLPs mediated activation of small molecules. Unless
otherwise noted all reactions were carried out on a 0.05 mmol scale
using [1+], 0.05 mmol P(tBu)3, 4 ml solvent, at room temperature.
Isolated yields are given in parentheses (BArF ¼ tetrakis(3,5-bis(tri-
4
fluoromethyl)phenyl)borate). Crystal structures of 8 and 9; (thermal
ellipsoids shown at 50% probability, hydrogen atoms and counter ion
are omitted for clarity. P orange, N blue, C grey, O red).
equimolar solution of 3g–BArF and paraformaldehyde were
4
stirred for 24 h at room temperature in CH2Cl2. The resultant
colorless turbid solution was dried under vacuum, and the
residue was washed with pentane affording the formaldehyde
adduct 8. Slow diffusion of pentane into a CH2Cl2 concentrated
solution at ꢀ20 ꢂC led to the formation of colorless single
crystals of 8. Analysis via 31P and 1H NMR spectroscopies
revealed the presence of resonances that support the complex-
31
ation of formaldehyde in between the FLP 3g (d ¼ 44.2 ppm,
P
13
1H
d
¼ 50.27 (d, J ¼ 55.5 Hz), and d ¼ 4.63 ppm (d, J ¼ 5.4 Hz,
C
CH2)). The structure of 8 was unambiguously conrmed by
single crystal X-ray diffraction (Scheme 3), showing the forma-
tion of a single bond between the carbon center of TOTA and
the oxygen of paraformaldehyde with a C1–O4 of 1.513(4),
˚
longer than a classic C–O bond (ca. 1.43 A). The C20–P1 bond
˚
length of 1.825(4) A is consistent with a single carbon–phos-
phorus bond. We also noted that the elongated C20–O4 bond
˚
˚
(1.410(5) A vs. 1.2 A in classic C]O bond) support a single bond
and the reduction of the paraformaldehyde to an alcoholate (see
ESI†).
Scheme 4 Reactivity of TOTA+ with NHC carbene. aUnless otherwise
noted all reactions were carried out on a 0.05 mmol scale using [1+],
0.05 mmol ItBu (10), 4 ml solvent, rt. aSee ESI† for dimer structures.
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predominant formation of the C2 adduct 110 was observed, (C4–NHC adduct); being lower in energy than its conforma-
along with a trace of imidazole salt. The two isomers are easily tional isomer 110 by 37.8 kcal molꢀ1. The difference in energy is
differentiated by 1H NMR spectroscopy, specically with the also reected in the elongation of the C(TOTA)–C(NHC) bond
t
˚
signals corresponding to the Bu groups. In the C4-adduct 11, distances, with a 0.06 A difference between the C4 and C2
t
the Bu groups are non-equivalent (1.36 and 1.34 ppm), while adduct. These differences in energy and bond distances are
the C2-adduct 110 conserves a C2 rotation axis rendering both attributed to the increased steric hindrance of the C2 adduct,
tBu groups equivalent (1.53 ppm). Furthermore, crystals of 11, where both tBu groups are in close proximity to TOTA,
obtained by slow diffusion of pentane into CH2Cl2 at ꢀ20 C, compared to the C4 adduct, where one of the Bu groups is
were subjected to X-ray diffraction analysis which conrmed its pointing away of TOTA.
structure as a C4–NHC adduct (Scheme 4). As observed in NMR,
t
ꢂ
the formation of the thermodynamically favorable compound
(“C4” adduct) is characterized by the formation of the single
Conclusions
We have demonstrated that trioxatrianguleniums (TOTA+) and
˚
bond C1–C20 (1.565(3) A) corresponds to the homolytic
coupling product following the SET process. This bond,
sterically hindered phosphines, such as P(tBu)3, can act as
˚
although slightly longer than a standard C–C ((ꢃ1.54) A, +0.02
a frustrated Lewis pair. The VT-NMR experiments, single crystal
X-ray diffraction crystallography, and UV-visible spectroscopic
˚
A) has a covalent character that is only weakened by the steric
t
hindrance imposed by the Bu groups. The presence of a C]C
analysis supports the existence of a FLP encounter complex.
double bond is also noted between the C20–C21 atoms (1.361(3)
These newly identied FLPs mimic the reactivity of main group
1
˚
A) as postulated by H NMR spectroscopy.
FLPs in reactions such as heterolytic S–S bond cleavage, dehy-
drogenation of 1,4-cyclohexadiene, xation of formaldehyde, C–
Br bond cleavage in tert-butyl bromide, and interception of
Interestingly, when TOTA+ and ItBu were mixed in CH3CN at
room temperature a deep-green solution was formed, which
immediately turned to brown with a purple precipitate. From
a Staudinger reaction intermediate. DFT calculations have
the reaction mixture, the imidazolium salt (12), TOTA–H (7),
demonstrated the possible reactivity mode of the FLP system for
TOTA-dimers were isolated, with only traces of the C4-adduct
disulde cleavage. Furthermore, the reactivity of TOTA+ with
(11) (see Scheme 4 and ESI†). The TOTA-dimers formation can
other Lewis bases such as NHC was investigated. Control
experiments and EPR studies revealed that this pair undergoes
the SET step for the formation Lewis acid–base adduct. It is
noteworthy that this carbon-based Lewis acid is signicantly
less oxophilic than boron, aluminum, and silicon-based Lewis
acids, making it both air and moisture stable. Considering the
efficiency of this FLPs system, it could be useful for other
be rationalized by the dimerization of TOTAc radicals.33
However, consistent with previous report of such radical,34
monitoring the CH3CN reaction mixture by UV-Vis spectroscopy
and EPR spectroscopy did not allow the observation of TOTAc
radical (see Fig. S107†).35 Trapping experiments with TEMPO
and benzoyl peroxide were also unsuccessful (see ESI†). These
observations suggest that a single electron transfer (SET)36
organic transformation in the future. Extending this family of
between TOTA+ and the NHC occurred, and then followed by
carbon Lewis acids by developing FLPs containing other triox-
radical recombination to form 11 or 110 in non-polar solvent
atrianguleniums salts with and different Lewis bases are
such as toluene, or a mixture of products in dissociative solvent
ongoing in our lab.
such as acetonitrile. The calculated electron affinity and ioni-
zation potential of TOTA+ (ꢀ7.41 eV) and ItBu (+5.59 eV) suggest
that SET process in the ground state is accessible (see Table Conflicts of interest
S15†).37
There are no conicts to declare.
The TOTA–NHC adducts 11 and 110 were studied using
computational methods (Fig. 6). As observed experimentally,
the ground state conguration of the adduct corresponds to 11
Acknowledgements
We are grateful for nancial support from the University of
Arizona for this work. We thank Dr Andrei Astashkin (EPR and
X-Ray facility of the University of Arizona) for performing the
EPR measurements and helping with the analysis of the EPR
spectra, as well as providing help for the XRD data collection
and analysis. All NMR data were collected in the NMR facility of
the Department of Chemistry and Biochemistry at the Univer-
sity of Arizona, and we thank Dr Jixun Dai for his help with 2D-
NMR analysis. The purchase of the Bruker NEO 500 MHz
spectrometer were supported by the National Science Founda-
tion under Grant Number 1920234, and the University of Ari-
zona. J. M. V. thanks the diversity and inclusion efforts from
individuals, groups, and institutions towards a more united
science community.
Fig. 6 Relative energy and bond TOTA–NHC bond distances from the
calculated DFT models of 11 and 110.
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