Phenylene-bridged bis(benzimidazolium) (BBIm2+): a dicationic organic photoredox catalyst

Article abstract of DOI:10.1039/d0sc03958f

A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm2+) was designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation and dimerisation of carboxylic acids. The catalytic activity of BBIm2+ was higher than that for a monocation analogue, suggesting that the dicationic nature of BBIm2+ plays a key role in these decarboxylative reactions. The rate constant for the decay of the triplet-triplet absorption of the excited BBIm2+ increased with increasing concentration of the carboxylate anion with a saturated dependence, suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the triplet excited state of BBIm2+ and a carboxylate anion.

Full text of DOI:10.1039/d0sc03958f

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Phenylene-bridged bis(benzimidazolium) (BBIm2+):
a dicationic organic photoredox catalyst†
Cite this: DOI: 10.1039/d0sc03958f
a
bc
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Takuya Kodama,
and Mamoru Tobisu
Maiko Kubo,‡a Wataru Shinji,‡a Kei Ohkubo
a
*
A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm2+) was
designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation
and dimerisation of carboxylic acids. The catalytic activity of BBIm2+ was higher than that for
a monocation analogue, suggesting that the dicationic nature of BBIm2+ plays a key role in these
decarboxylative reactions. The rate constant for the decay of the triplet–triplet absorption of the excited
BBIm2+ increased with increasing concentration of the carboxylate anion with a saturated dependence,
suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the
triplet excited state of BBIm2+ and a carboxylate anion.
Received 20th July 2020
Accepted 7th October 2020
DOI: 10.1039/d0sc03958f
rsc.li/chemical-science
at the ortho position, except for the Ni/PPh3-catalysed decar-
boxylative olenation of alkyl carboxylic acids.9 The other
catalytic method involves the use of photoredox catalysis,1b,e,10
Introduction
Given the widespread availability of carboxylic acids from
nature and commercial sources, the decarboxylative trans-
formation of carboxylic acid derivatives represents an attractive
synthetic organic method.1 However, the direct decarboxylation
of unactivated carboxylic acids remains a challenge, and the use
of stoichiometric amounts of metal oxidants,2 or pre-activation
to acid chlorides or activated esters, such as Barton esters3 and
N-(acyloxy)phthalimides (NHPI esters),4 are still required.
Although organic electron acceptors such as acridinium,5 imi-
nium,6 phthalimide,7 and cyanoarene8 have been reported to
promote the decarboxylation of carboxylic acids under photo-
irradiation conditions, a stoichiometric amount of these
acceptors is needed for the reaction to reach completion. The
use of transition metal catalysts and photoredox catalysts for
the catalytic decarboxylative transformation of carboxylic acids
has extensively been studied (Scheme 1). In transition metal
catalysed reactions, the metal carboxylates are generated in situ
and then undergo decarboxylation to produce an aryl-metal
species (Scheme 1a).1a,c–e However, the substrates are limited
to the highly activated aromatic carboxylic acids bearing
electron-withdrawing groups, such as nitro or uorine group(s),
in which the photoinduced single electron oxidation of an alkyl
carboxylate anion with a photoexcited catalyst occurs, with the
formation of an alkyl carboxyl radical (Scheme 1b). The alkyl
carboxyl radical then spontaneously undergoes decarboxylation
to produce an alkyl radical, which can participate in subsequent
reactions. However, the application of this protocol to simple
carboxylic acids is still difficult due to their highly positive
oxidation potentials (e.g., CH3COOꢀ: E1/2(CH3COOꢀ/CH3COOc)
¼ +1.47 V vs. SCE),11 when commonly used ruthenium poly-
pyridyl complexes are employed as a photoredox catalyst (e.g.,
2+
Ru(bpy)3 ; bpy ¼ 2,20-bipyridyl: E1/2 (*RuII/RuI) ¼ +0.77 V vs.
SCE).12 The use of iridium complexes containing pyridine-based
ligands bearing electron-withdrawing groups has recently
emerged as viable photocatalysts for promoting the oxidative
decarboxylation of simple aliphatic carboxylic acids.13,14
However, these transition-metal complexes are expensive and
potentially toxic.15 In the light of the above situation, the
development of a simple and robust catalyst system that
aDepartment of Applied Chemistry, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan. E-mail: tobisu@chem.eng.osaka-u.ac.jp
bInstitute for Advanced Co-Creation Studies, Osaka University, Suita, Osaka 565-0871,
Japan. E-mail: ohkubo@irdd.osaka-u.ac.jp
cInstitute for Open and Transdisciplinary Research Initiatives, Osaka University, Suita,
Osaka 565-0871, Japan
† Electronic supplementary information (ESI) available. CCDC 1992124. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc03958f
Scheme 1 Direct catalytic transformation of carboxylic acids.
‡ These authors contributed equally.
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enables oxidation of unactivated carboxylic acids without the
use of harmful reagents would be highly desirable. Organic
photoredox catalysts have the potential to address these
issues.16–18 An organic acridinium dye could be used in photo-
catalytic decarboxylative reactions of alkyl carboxylic acids
including hydrodecarboxylation and decarboxylative tri-
uoromethylthiolation.16,17 Yoshimi and co-workers reported on
decarboxylative transformations of a wide variety of alkyl
carboxylic acids using an arene/electron acceptor system by
photoinduced electron transfer.18,19 Although these examples
demonstrate the potential utility of organic photoredox cata-
lysts for use in oxidative decarboxylative transformation of
unactivated organic acids, new classes of organic catalysts20
continue to be developed to further expand the scope of these
reactions.
We envisioned that dicationic organic salts could function as
photoredox catalysts with high oxidation ability because their
electron decient nature allows the HOMO energy level to be
lowered, thereby making the reduction potential in the excited
state to be sufficiently high to allow carboxylates to be oxidised.
To date, organic dicationic molecules have not been applied to
photoredox reactions, except for 2,7-diazapyrenium,21 trisami-
nocyclopropenium22 and bisphosphonium.23 Herein, we report
on the design, synthesis and properties of a new dicationic
organic photocatalyst based on
a bis(benzimidazolium)
(BBIm2+) framework, and its application to photocatalytic
decarboxylative reactions is also demonstrated.
Results and discussion
As cationic moieties, we focused on imidazolium scaffolds,
which have found various applications in modern synthetic and
material chemistry, for example, as precursors of N-heterocyclic
carbenes24 and components of ionic liquids.25 Thermal
stability,26 redox properties,27 and ease of structural modica-
tion would be desirable properties of such a catalyst. We
designed
a new phenylene-bridged bis(benzimidazolium)
(BBIm2+), with the expectation that two cationic imidazolium
moieties should result in sufficiently low HOMO energies that
would permit unactivated carboxylic acids to be oxidised upon
photoexcitation (Scheme 2a). Density functional theory calcu-
lations at the uB97XD/6-31G(d,p) level of theory were performed
to investigate the structural and electronic properties of the
BBIm2+. Structural optimisation revealed that the benzimida-
zolium rings of BBIm2+ are twisted relative to the central phenyl
ring by 47ꢁ with a C2h symmetry. The electrostatic potential
indicated that the majority of the positive charges in BBIm2+ are
distributed over the imidazolium rings (Scheme 2c). BBIm2+
possesses a lower HOMO (j161 ¼ ꢀ13.48 eV) than that of
monocation analogue 1+ (j91 ¼ ꢀ11.60 eV, Scheme 2d, cf.
Fig. S5 in the ESI† for details), possibly because of the more
positive nature of the molecule. These results indicate that
BBIm2+ would be reduced more easily than 1+, thereby serving
as a stronger oxidant than 1+ as we initially envisioned. Indeed,
the calculated HOMO energy of BBIm2+ was lower than that of 9-
mesityl-10-methylacridinium (Acr-Mes+, j83 ¼ ꢀ11.24 eV, cf.
Fig. S5 in the ESI†), which can oxidize alkyl carboxylic acids in
Scheme 2 Molecular structures of (a) BBIm2+ and (b) monocation 1+;
(c) SCF potential (red positive and green negative) and (d) frontier
orbitals (HOMO, LUMO) of BBIm2+ generated by DFT calculations at
the uB97XD/6-31G(d,p) level of theory along with their energy levels.
the photo excited state.16,17 The HOMO of BBIm2+ is distributed
across the benzimidazolium and central phenylene rings,
whereas the LUMO is largely located at the N]C–N moiety of
the benzimidazolium ring and the phenylene moiety.
Having estimated theoretically that BBIm2+ has a sufficient
oxidation ability to oxidise carboxylates by photoinduced elec-
tron transfer, we commenced our synthetic studies of BBIm2+
.
BBIm2+ was synthesised by a commonly used condensation
reaction between a diamine and an aldehyde (Scheme 3).28
Thus, the treatment of diamine 2 with terephthalaldehyde and
a catalytic amount of p-toluenesulfonic acid afforded the double
cyclised product 3 in 83% yield. Subsequent oxidation of 3 with
N-bromoacetamide provided BBIm2+$2[Brꢀ]. Because of the
poor solubility of BBIm2+$2[Brꢀ] in common organic solvents,
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Scheme 3 Synthesis of BBIm2+$2[PF6ꢀ]. aReaction conditions: (a)
terephthalaldehyde, p-TSA, MgSO4, toluene, RT, 2 h, 83%. (b) N-Bro-
moacetamide, DMF, toluene, RT, 1 h. (c) AgPF6, MeCN, RT, 1 h, 89% (2
steps). p-TSA ¼ p-toluenesulfonic acid, DMF ¼ N,N-dimethylforma-
mide, RT ¼ room temperature.
the counter anion was changed to PF6ꢀ by treatment with AgPF6
in acetonitrile to give BBIm2+$2[PF6ꢀ] in 89% yield (2 steps from
3), which is soluble in acetonitrile, DMF, and
hexauoroisopropanol.
Fig. 2 (a) UV-vis absorption (red) and emission (blue) (lex ¼ 290 nm)
spectra of BBIm2+$2[PF6ꢀ] (1.0 ꢃ 10ꢀ5 M in acetonitrile, room
temperature); (b) cyclic voltammogram of BBIm2+$2[PF6ꢀ] (5.0 ꢃ
10ꢀ4 M in acetonitrile containing nBu4NPF6, room temperature, scan
rate ¼ 0.5 V sꢀ1).
Recrystallization from DMSO/methanol gave colorless crys-
tals of BBIm2+$2[PF6ꢀ], which were suitable for a single X-ray
crystallographic analysis (Fig. 1a, see Fig. S1 in ESI†). The
dihedral angle for the central phenyl ring and benzimidazolium
moieties was found to be 59ꢁ (Fig. 1b), which was larger than the
calculated value. This difference can be attributed to the exis-
tence of counter anions and/or a packing force. The counter
anions were located close to the imidazolium moieties in the
crystal, in good agreement with the sites predicted based on the
calculated electrostatic potential (Scheme 1b).
a weak broad tail in the longer wavelength region (ꢂ500 nm),
while a single emission peak with lmax at 428 nm was observed.
A large Stokes shi (ꢂ140 nm) of BBIm2+$2[PF6ꢀ] suggested that
BBIm2+$2[PF6ꢀ] undergoes a signicant structural change in
the excited state. According to the time-dependent (TD)
uB97XD/6-31G(d,p) calculation of BBIm2+, the absorption band
at around 290 nm of BBIm2+ is assigned to HOMO (j161) /
LUMO (j162) transitions (l ¼ 288 nm and f ¼ 0.99).
The cyclic voltammogram (CV) of BBIm2+$2[PF6ꢀ] in aceto-
nitrile exhibited a reversible redox wave (E1/2 ¼ ꢀ1.02 V vs. SCE)
(Fig. 2b).29 The corresponding monocation 1+$[Brꢀ] displayed
an irreversible wave (Ep/2 ¼ ꢀ1.47 V vs. SCE, Fig. S6 in ESI†).
Therefore, the advantage of using BBIm2+$2[PF6ꢀ] over the
corresponding monocation 1+$[Brꢀ] can be attributed to not
only its higher reduction potential as we initially envisioned,
but also to the stability of the reduced species. Optical and
electrochemical measurements led us to estimate the excited
singlet state reduction potential of BBIm2+$2[PF6ꢀ] to be
1E*red ¼ þ2:56 V vs. SCE,30 the value of which is sufficiently high
to permit a wide variety of organic compounds such as
benzene31 or carboxylic acids to be oxidised.11 The triplet state
reduction potential of BBIm2+$2[PF6ꢀ] was also estimated to be
3E*red ¼ þ0:89 V vs. SCE.30
Absorption and emission spectra of BBIm2+$2[PF6ꢀ] (Fig. 2a)
displayed an intense absorption band at around 290 nm with
Fig. 1 (a) ORTEP drawing of BBIm2+$2[PF6ꢀ] at the 50% probability
level and (b) side view of BBIm2+$2[PF6ꢀ].
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We next examined the catalytic activity of BBIm2+$2[PF6ꢀ] in
the photocatalytic decarboxylative hydroxylation of carboxylic
acids.16 Irradiation of an acetonitrile solution of carboxylic
acid 4a with a xenon lamp (>300 nm) in the presence of
BBIm2+$2[PF6ꢀ] (10 mol%) and NaOH (60 mol%) under an
atmosphere of oxygen for 5 h afforded the desired alcohol 5a in
68% yield, aer reductive work up with NaBH4 (Table 1, entꢀry
Table
2
Substrate scope for BBIm2+-catalysed decarboxylative
hydroxylationa
1).16b Control experiments revealed that both BBIm2+$2[PF6
]
and irradiation are essential for the reaction to efficiently
proceed (entries 2 and 3). The reaction proceeded more effi-
ciently under an atmosphere of oxygen compared with reactions
performed under an atmosphere of nitrogen (entry 4). In the
absence of NaOH, the yield was decreased to 32% (entry 5). The
use of the corresponding monocation 1+$[Brꢀ] (entry 6) or
Acr-Mes+$[ClO4
]
(entry 7) as a photocatalyst also gave 5a,
ꢀ
16a,32
but in lower yields of 40% and 46%, respectively. In addition, the
quantum yield for the photocatalytic hydroxylation reaction
using BBIm2+$2[PF6ꢀ] was determined to be 0.60, which is
signicantly higher than that for 1+$[Brꢀ] (0.004) or
ꢀ
Acr-Mes+$[ClO4 ] (0.23) catalysed reactions.33 The reaction also
proceeded by using 365 nm or 405 nm LED lights instead of
a xenon lamp (entries 8 and 9). It was possible to oxidise an array
of carboxylic acids using BBIm2+ under the optimised conditions
(Table 2). Benzylic carboxylic acids bearing methyl (4a), phenyl
(4b) and cyclohexyl (4c) groups at the benzylic position were
successfully decarboxylated to give the corresponding benzyl
alcohols (5a–5c) in good yields. Whereas primary benzylic (4d)
a
ꢀ
Reaction conditions: carboxylic acid (0.15 mmol), BBIm2+$2[PF6
]
(0.015 mmol), NaOH (0.090 mmol) in acetonitrile (6.0 mL), xenon
lamp (500 W) irradiation for 18 h, RT. GC yield. c NMR yield.
b
Table 1 Photoinduced BBIm2+-catalysed decarboxylative hydroxyl-
ation of 4aa
and secondary non-benzylic (4e) carboxylic acids were not good
substrates in this decarboxylative hydroxylation, tertiary alkyl
carboxylic acids 4f and 4g underwent the reaction to give prod-
ucts in moderate to good yield.
During the course of our investigation of photocatalytic
decarboxylative hydroxylation reactions using BBIm2+$2[PF6ꢀ],
we found that the tertiary benzylic carboxylic acid 6a underwent
decarboxylative dimerisation to give 7a under identical condi-
tions. Although this type of decarboxylative dimerisation reac-
tion has already been reported, including Kolbe electrolysis34a
Entry
Variation from the standard conditions
GC yields (%)
1
2
3
4
5
6
7
8
9
None
68
0
0
and reactionꢀs using a stoichiometric amount of an oxidant,
Without BBIm2+$2[PF6
]
ꢀ
34b
such as S2O8
,
and Hg(II),34c and a heterogeneous a Pt–TiO2
In the absence of light source
Under N2
photocatalyst,14b,c this is the rst example of the use of an
organic photoredox catalyst, to the best of our knowledge.
Further optimization for this dimerisation was performed using
6a as a substrate (Table 3). The yield of 7a was increased when
the reaction was carried out under an atmosphere of air rather
than oxygen (entries 1 and 2). Finally, shortening the reaction
time to 3 h resulted in a better yield, because the undesired
8
Without base
32
40
46
67
35
1+$[Brꢀ] instead of BBIm2+$2[PF6
]
ꢀ
Acr-Mes+$[ClO4ꢀ] instead of BBIm2+$2[PF6
]
ꢀ
365 nm LED lamp (60 W) instead of Xe lamp
405 nm LED lamp (60 W) instead of Xe lamp
decomposition of 7a could be suppressed (57%, entryꢀ3).35 The
use of the monocation 1+$[Brꢀ] or Acr-Mes+$[ClO4
]
as
16a,32
a catalyst resulted in lower yields of 4% and 33%, respectively
(entries 4 and 5). These results again underscore the prominent
activity of this dicationic catalyst for the oxidation of unac-
tivated organic acids.
a
Reaction conditions: (a) 4a (0.15 mmol), BBIm2+$2[PF6ꢀ] (0.015
mmol), NaOH (0.090 mmol), O2, in acetonitrile (6.0 mL), xenon lamp
(500 W) irradiation for 5 h, RT; (b) NaBH4, MeOH.
A range of tertiary benzylic carboxylic acids (6a–ꢀ6h) could be
oxidatively
dimerised
by
BBIm2+$2[PF6
]
under
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Table 3 Photocatalytic decarboxylative dimerisation of 6aa
acids bearing a spiro ring system at the benzylic position, such
as 6e and 6f, gave the corresponding dimers 7e and 7f. p-
Extended derivatives, as represented by 6g and 6h, also served
as good substrates for decarboxylative dimerisation. Aliphatic
and secondary benzylic carboxylic acids failed to give any
decarboxylative dimerisation products under these conditions,
but alcohols, as in Table 2, were formed as products instead.
To elucidate the mechanism for this photoinduced decar-
boxylation reaction, quenching measurements36 were per-
formed by uorescence and triplet–triplet (T–T) absorption
spectroscopies. Electron transfer from tBuCOOꢀ to the singlet
excited state of BBIm2+ (1BBIm2+*) is energetically favorable
NMR
yields
(%)
Entry
Cat.
Atmosphere
Time (h)
7a
6a
because the free energy change is signicantly negative: DGet
¼
1b
2b
3
4
5
BBIm2+$2[PF6ꢀ
BBIm2+$2[PF6ꢀ
BBIm2+$2[PF6
1+$[Brꢀ]
]
]
]
O2
18
18
3
3
3
23
47
57
4
0
1
9
0
0
ꢀ
1
Eox ꢀ Ered ꢀ E* ¼ 1.02 ꢀ (ꢀ0.50) ꢀ 3.58 ¼ ꢀ2.06 eV. However,
no uorescence quenching was observed when tBuCOOꢀ was
added to an acetonitrile solution containing BBIm2+. The uo-
rescence lifetime of BBIm2+ was determined to be 0.4 ns by
time-resolved uorescence measurements, which is too short to
quench the low concentration of tBuCOOꢀ (0–0.2 mM) under
the present experimental conditions.37 It therefore appears that
the reactive species may be the triplet excited state of BBIm2+
(3BBIm2+*) rather than the short-lived singlet excited state. The
free energy change for the electron transfer from tBuCOOꢀ to
Air
Air
Air
Air
Acr-Mes+$[ClO4
]
33
ꢀ
a
Reaction conditions: 6a (0.15 mmol), cat. (0.015 mmol), NaOH (0.090
mmol), in acetonitrile/H2O (6.0 mL), xenon lamp (500 W) irradiation,
RT. b The reaction was conducted in CH3CN/H2O (1 : 1).
photoirradiation to be converted to the corresponding dimers
(Table 4). Benzylic carboxylic acids bearing a uoro group on
the phenyl ring, such as 6b and 6c, effectively underwent
dimerisation. The introduction of a triuoromethyl group, as in
6d, decreased the yield of the dimerised product. Phenylacetic
3
3BBIm2+* was estimated to be DGet ¼ Eox ꢀ Ered ꢀ E* ¼ 1.02 ꢀ
(ꢀ0.50) ꢀ 1.91 ¼ ꢀ0.39 eV, where 3E* was determined from the
phosphorescence spectrum (lmax ¼ 641 nm) observed in an
ethanol glass at 77 K. The negative DGet value indicates that the
electron transfer from tBuCOOꢀ to 3BBIm2+* is energetically
feasible. To observe the electron transfer process between
tBuCOOꢀ and 3BBIm2+*, the T–T absorption measurements
were carried out by nanosecond laser ash spectroscopy
(Fig. 3a). A typical T–T absorption at lmax ¼ 580 nm was detected
taken at 0.8 ms aer laser excitation at 355 nm due to the
absorption of BBIm2+. The T–T absorption decays obeying rst-
Table 4 Scope for photocatalytic decarboxylative dimerisation of
tertiary-alkyl carboxylic acidsa
order kinetics with the rate constant (kT) of 1.9 ꢃ 102 sꢀ1
.
tBuCOOꢀ was added to an acetonitrile solution of BBIm2+$2
[PF6ꢀ] and the decay time prole was monitored at 580 nm. The
observed decay rate constant (kobs) increased with increasing
concentration of tBuCOOꢀ, as shown in Fig. 3b. Interestingly,
a saturated dependence was observed, rather than the typical
linear relationship observed for
a
simple quenching
Fig. 3 (a) Transient absorption spectra of BBIm2+ taken at 0.8 ms
(black), 5 ms (red), 20 ms (green) after laser excitation at 355 nm in
deaerated MeCN. (b) Dependence of decay rate constant of triplet–
triplet absorption of 3BBIm2+*on the concentration of pivalate.
a
ꢀ
Reaction conditions: carboxylic acid (0.15 mmol), BBIm2+$2[PF6
]
(0.015 mmol), NaOH (0.090 mmol) in acetonitrile (4.0 mL)/H2O (2.0
mL), xenon lamp (500 W) irradiation for 3 h, RT. Run for 18 h.
b
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Table 5 Sumꢀmaary of the electrochemical and optical properties of
mechanism. The saturated behavior revealed that photoin-
duced electron transfer occurred with the formation of an ion
pair complex composed of the 3BBIm2+* and tBuCOOꢀ, in
competition with bimolecular electron transfer from tBuCOOꢀ
BBIm2+$2[PF6
]
Ered/V 1E*/eV Er*ed=V 3E*/eV Er*ed=V lmax/nm lem/nm sS/ns
1
3
3
3
to BBIm2+* and triplet decay of BBIm2+* to the ground state
ꢀ1.02 +3.58
+2.56
+1.91
+0.86
290
428
0.4
without electron transfer (Scheme 4).38 The association constant
All potentials in V vs. SCE. Ered ¼ ground state reduction potential, 1E*
a
(Kassoc
)
for the formation of an encounter complex
1
¼ singlet excitation energy, E*red ¼ excited singlet state reduction
3BBIm2+*$[tBuCOOꢀ][PF6ꢀ] and the rate constant (kET) for intra-
complex photoinduced electron transfer from tBuCOOꢀ to
potential, 3E* ¼ triplet excitation energy, Er*ed ¼ excited triplet state
3
reduction potential. lmax ¼ absorption peak, lem ¼ emission peak
excited at 290 nm. sS ¼ uorescence lifetime.
3BBIm2+* were estimated to be 5.8 ꢃ 105 Mꢀ1 and 3.4 ꢃ 102 sꢀ1
respectively, by curve tting using eqn (1).
,
kobs ꢀ kT ¼ kETKassoc[tBuCOOꢀ]/[1 + Kassoc[tBuCOOꢀ]] (1)
the carboxyl radical B and the radical cation BBImc+. The carboxyl
radical B rapidly releases CO2 to produce the alkyl radical C.
Under an O2 atmosphere, the generated radical C is captured by
oxygen39 to generate peroxyl radical D [Ered (tBuO2c/tBuO2ꢀ) ¼
+0.50 V vs. SCE],16a,38 which is subsequently reduced by the radical
cation BBImc+ (Ered ¼ ꢀ1.02 V vs. SCE) to form the peroxyl anion E
with the regeneration of BBIm2+. Anion E is prone to undergo
disproportionation to generate a mixture of an alcohol and
a ketone, which was isolated as a single alcohol product by
treatment with NaBH4 in this study. When the reaction is per-
formed under an atmosphere of air, in which the oxygen
concentration is relatively low, two alkyl radicals C undergo
radical–radical coupling to form dimer G before it is captured by
oxygen.40 In this cꢀase, SET from the radical cation BBImc+ to
oxygen [Ered(O2/O2c ) ¼ ꢀ0.86 V vs. SCE]41 regenerates the catalyst
BBIm2+ with the concomitant formation of superoxide (O2cꢀ),
which is eventually reduced to hydrogen peroxide.33
The electrochemical and optical properties of BBIm2+$2
[PF6ꢀ] are summarised in Table 5 and an energy diagram of
BBIm2+ is shown in Scheme 4. The photoirradiation initially
1
generates BBIm2+*, which is not quenched with tBuCOOꢀ by
electron transfer under the experimental conditions. Inter-
system crossing, which is in competition with the uorescence
decay bound for the ground state, results in the formation of
3BBIm2+*. An intimate encounter complex between 3BBIm2+*
and the anionic tBuCOOꢀ substrate is formed, and electron
transfer in the complex then occurs to produce a radical pair
(BBImc+/tBuCOOc). Based on the saturated behavior shown in
Scheme 3b, an outer sphere electron transfer process from
3
tBuCOOꢀ to BBIm2+* is presumably signicantly slower than
the intra-complex electron transfer. No back electron transfer
from BBImc+ to tBuCOOc occurs due to the rapid spontaneous
decarboxylation of tBuCOOc leading to the products.
A plausible reaction mechanism for decarboxylative hydrox-
ylation and dimerisation is shown in Scheme 5. Photoredox
catalyst BBIm2+ is initially excited by photoirradiation and
subsequent intersystem crossing generates 3BBIm2+*, which oxi-
dises the carboxylate A via single electron transfer (SET) to give
The key to the successful application of BBIm2+ in photo-
catalytic decarboxylative transformation of benzylic carboxylic
acids is its dicationic nature, which serves to reduce the activation
barrier of the SET from carboxylate by lowering the reorganiza-
tion energy of the polar solvents.31b,c When an electronically
neutral photocatalyst (PC) oxidises a monoanionic substrate (Sꢀ),
a radical anion of PC (PCcꢀ) and a neutral radical (Sc) are gener-
ated (Scheme 6a). In this process, the charges of the both
components changes (PC: neutral / negative; S: negative /
neutral), which requires the solvent molecules around PC and S
Scheme 4 Energy diagram of BBIm2+. R ¼ tBu, sS ¼ fluorescence
lifetime, sT ¼ phosphorescence lifetime, kISC ¼ rate constant for
intersystem crossing, ket ¼ rate constant for electron transfer from
tBuCOOꢀ to 3BBIm2+*, Kassoc ¼ association constant for the formation
of an encounter complex 3BBIm2+*$[tBuCOOꢀ][PF6ꢀ], kET ¼ rate
constant for intra-complex photoinduced electron transfer from Scheme 5 Proposed mechanism for the photocatalytic hydroxylation
tBuCOOꢀ to 3BBIm2+*. Numbers in parentheses refer to energy levels under O2 (blue) and dimerisation of carboxylic acids under an atmo-
of the species relative to BBIm2+ in the ground state.
sphere of air (red). SET: single electron transfer.
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University) and Prof. Asahara (Osaka University) for helpful
discussion and Prof. Minakata, Prof. Takeda, Prof. Kida, and
Prof. Mori (Osaka University) for their assistance with the
uorescence lifetime measurements.
Notes and references
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Scheme 6 Comparison of the reorganization of polar solvents during
single electron transfer processes from monoanionic substrate (Sꢀ) to
(a) neutral, (b) monocationic, and (c) dicationic photocatalysts (PCs).
Arrows represent the dipole moments of the solvents.
to be reorganised to form polarity-matched clusters. Similarly,
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(Scheme 6c).31c
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In conclusion, we report on the development of a novel dicationic
photoredox catalyst BBIm2+, the photophysical and electrochemical
properties of which indicate that it has a strong oxidation ability in
the excited state. In fact, BBIm2+ was demonstrated to be a viable
organic photocatalyst for promoting the decarboxylative hydroxyl-
ation and dimerisation of unactivated carboxylic acids. The striking
feature of BBIm2+ is its dicationic nature, which allows it to form an
intimate ion pair with an anionic substrate, resulting in an ener-
getically favorable SET process. Further development of new
cationic organic photocatalysts and their application to otherwise
difficult oxidative transformations is now ongoing in our
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work was supported by Scientic Research on Innovative
Area “Hybrid Catalysis” (20H04818, 20H04819) and Grant-in-
Aid for Early-Career Scientists (19K15564) from MEXT, Japan.
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MT wishes to thank the Hoansha Foundation for their nancial 10 Selected reviews: (a) J. M. R. Narayanama and
support. We also thank Prof. Ikeda (Osaka Prefecture
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a reference and converted to vs. SCE using Fc/Fc+ ¼ +0.38
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1
7250–7255; related reactions using cerium salts (c)
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10ꢀ7)] ꢃ 1/2 ¼ 3.58 eV. labs ¼ 290 nm, lem ¼ 428 nm for
BBIm2+. Er*ed ¼ 1E* + Ered ¼ +3.58 + (ꢀ1.02) ¼ +2.56 V vs.
1
6688–6698; (d) V. R. Yatham, P. Bellotti and B. Konig,
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¼
3E* + Ered ¼ +1.91 + (ꢀ1.02) ¼ +0.89 V vs.
3
¨
3
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19 The photocatalytic decarboxylative transformation of 33 The quantum yields for the decarboxylative hydroxylation
aromatic carboxylic acids was recently achieved:
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reaction of 4a with photocatalysts BBIm2+$2[PF6ꢀ], 1+$[Brꢀ]
and Acr-Mes+$[ClO4ꢀ] were determined using a 365 LED
lamp (60 W) as the light source under the same
absorbance condition [abs. at 365 nm ¼ 1.0]. The quantum
yield for the photocatalytic oxygenation of p-xylene with
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Acr-Mes+$[ClO4ꢀ] was used as a reference value: K. Ohkubo,
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irradiation time proles are shown in Fig. S4 and the
quantum yields were summarized in Table S2 in ESI.†
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39 18O labeling experiments were performed to conrm the
origin of oxygen atom in 5a. When the photocatalytic
hydroxylation reaction was conducted in 18O2-saturated
acetonitrile using 4a as a substrate, [18O]5a was detected
by HRMS (Fig. S3a in ESI†). Similarly, when the reaction
was performed in non-labeled O2-saturated acetonitrile,
only [16O]5a was detected and no [18O]5a was observed
(Fig. S3b in ESI†). This result indicates that the oxygen in
the 5a product is derived from O2.
40 The use of water decreased the oxygen concentration in the
system, which increased the yield of the dimerisation
product. Indeed, when the reaction was conducted in
acetonitrile using 6a as a substrate, the yield of the dimer
decreased to 24% and hydroxylation proceeded in 41%
yield. There is a difference in the solubility of oxygen
between water (1.3 mM) and acetonitrile (13 mM), see (a)
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37 Notes: (a) The intensity of the uorescence of BBIm2+
apparently increased with increasing amount of added
sodium pivalate, which is likely due to a signicant
structural change by complexation between BBIm2+ and
pivalate. We were able to isolate BBIm2+$[tBuCOOꢀ][PF6ꢀ],
ꢀ
which was immediately formed by mixing BBIm2+$2[PF6
]
and sodium pivalate in CH3CN; (b) The yield of
photocatalytic decarboxylative dimerisation was not
signicantly affected by the concentration of BBIm2+
,
suggesting that the short-lived 1BBIm2+* is not an active
species for the reaction; (c) In addition to the short life
time, another possible reason for the absence of
uorescence quenching by tBuCOOꢀ in spite of the highly 41 D. T. Sawyer, T. S. Calderwood, K. Yamaguchi and
negative driving force is that the process is located in the
C. T. Angelis, Inorg. Chem., 1982, 22, 2577–2583.
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Chem. Sci.