Intermolecular
Proton-Coupled
Electron
Transfer
Reactivity
from
a
Persistent
Charge-Transfer
State
for
Reductive
Photoelectrocatalysis
Pablo
Garrido-Barros,
‡
Catherine
G. Romero,
‡
Jay R. Winkler,
*
and Jonas
C. Peters
*
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J. Am. Chem.
Soc.
2024,
146, 12750−12757
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Supporting
Information
ABSTRACT:
Interest
in applying
proton-coupled
electron
transfer
(PCET)
reagents
in reductive
electro-
and photocatalysis
requires
strategies
that mitigate
the competing
hydrogen
evolution
reaction.
Photoexcitation
of a PCET
donor
to a charge-separated
state
(CSS)
can produce
a powerful
H-atom
donor
capable
of being
electrochemically
recycled
at a comparatively
anodic
potential
corresponding
to its ground
state.
However,
the challenge
is
designing
a mediator
with a sufficiently
long-lived
excited
state for
bimolecular
reactivity.
Here,
we describe
a powerful
ferrocene-
derived
photoelectrochemical
PCET
mediator
exhibiting
an
unusually
long-lived
CSS (
τ
∼
0.9
μ
s). In addition
to detailed
photophysical
studies,
proof-of-concept
stoichiometric
and catalytic
proton-coupled
reductive
transformations
are presented,
which
illustrate
the promise
of this approach.
1.
INTRODUCTION
Proton/electron
transfers
to chemical
substrates
permeate
metabolic
and synthetic
reactions.
1
−
7
Proton-coupled
electron
transfer
(PCET)
offers
a means
to bypass
high energy
pathways
associated
with stepwise
electron/proton
transfer
(ET/PT)
steps.
4
Yet, such reactions
pose a considerable
selectivity
challenge
when
strongly
reducing
conditions
are
required
to generate
reactive
intermediates
featuring
weak X
−
H bonds
(bond
dissociation
free energies,
BDFE
X
−
H
< 50 kcal
mol
−
1
; Figure
1A, left). Under
such conditions,
the hydrogen
evolution
reaction
(HER)
is thermodynamically
(Figure
1A,
middle)
and often also kinetically
favored
(BDFE
H
−
H
= 104
kcal mol
−
1
). This challenge
calls for approaches
that disfavor
competing
HER.
8
−
12
The widespread
use of stoichiometric
SmI
2
/ROH
reagents
in chemical
synthesis
underscores
this
idea.
13
−
15
Concomitant
with growing
interest
in electrocatalysis
for
synthetic
organic
16
as well as solar fuels
17,18
applications,
the
challenge
of competing
HER becomes
paramount.
Electrode-
mediated
HER, for example
at a commonly
used glassy carbon
(GC)
electrode,
19
can kinetically
dominate
at an applied
potential
(
E
app
) sufficiently
negative
to drive a turnover-
limiting
ET step, irrespective
of the inherent
selectivity
of a
soluble
chemical
catalyst
system.
To mitigate
this issue, our lab
recently
introduced
a strategy
wherein
a PCET
mediator
composed
of a cobaltocene
redox
site and an appended
dimethylaniline
Brønsted
base site (Figure
1A, right (inner
path),
and Figure
1B) colocalizes
a highly
reactive
but
kinetically
trapped
H
+
/e
−
equivalent
(BDFE
N
−
H
∼
37 kcal
mol
−
1
). The mediator
facilitates
single-
and multielectron
substrate
reductions
fixed to the
E
app
of its redox center.
This
E
app
is anodic
of the electrode-mediated
HER window
for acids
tuned
to the mediator’s
p
K
a
(
∼
8.6
in MeCN),
enabling
substrate
reductions
both in the absence
20
−
22
and presence
23,24
of a tandem
cocatalyst;
these reductions
are not feasible
in the
absence
of the PCET
mediator
at the same
E
app
.
12
The linear relationship
between
the driving
force (
Δ
BDFE)
and the formal
potential
required
to regenerate
the reactive
form of the PCET
mediator
can limit its scope.
This can be
illustrated
using
a Bordwell
analysis,
4,25
which
shows
that
strong
PCET
donors
(BDFE
X
−
H
< 52 kcal mol
−
1
) will have
formal
potentials
more negative
than the formal
potential
for
HER (
E
°
(HA
/ 1/2 H
2
)) (Figure
1A, middle).
An attractive
alternative
is photochemical
generation
of highly
reactive
PCET
donors
[Figure
1A, right (outer
path)].
26,27
When
coupled
with acidic
or basic
functional
groups,
excited
molecules
can be powerful
PCET
reagents.
28
Recent
examples
include
an anthracene
−
phenol
conjugate
(Figure
1C, left)
29
and a ruthenium
tris
−
diimine
complex.
26
In these systems,
intermolecular
PCET
reactivity
is limited
by the lifetime
of the
electronic
excited
state.
For example,
in the anthracene
−
phenol
conjugate,
the PCET
reaction
was intramolecular
and
the <10 ns singlet
lifetime
of the anthracene
component
was
hence
compatible.
For bimolecular
PCET
reactions,
however,
Received:
February
21, 2024
Revised:
April 5, 2024
Accepted:
April 10, 2024
Published:
April 26,
2024
Article
pubs.acs.org/JACS
© 2024
The Authors.
Published
by
American
Chemical
Society
12750
https://doi.org/10.1021/jacs.4c02610
J. Am. Chem.
Soc.
2024,
146, 12750
−
12757
This article is licensed under CC-BY 4.0
microsecond
survival
times are desired
for efficient
reactivity.
Herein,
we describe
a photochemical
system
(Figure
1B) that
exploits
an excitation-quench
strategy
to extend
the lifetime
of
a powerful
PCET
reagent
that can be electrochemically
regenerated
at potentials
that avoid the HER.
Visible
light excitation
(
E
00
∼
2
−
3 eV) produces
transient
species
that are far more readily
reduced
than their ground
state analogues
(eq 1).
°
*
=
°
+
E
E
E
(M
)
(M
)
0/
0 /
00
(1)
Quenching
short-lived
(exponential
decay
time
τ
< 1
μ
s)
excited
states with mild reductants
(Q) can transiently
produce
strong
PCET
donors
with highly
negative
formal
potentials
(
E
°
(M
0/
−
)). In our hybrid
pe
PCET
approach,
the quencher
is
regenerated
at an electrode
with an applied
potential
that is
only slightly
more negative
than
E
°
(Q
+/0
). To quench
high-
energy
(
E
00
> 3 eV) short-lived
singlet
excited
states of organic
molecules
(e.g.,
τ
< 20 ns), the electron
donor
can be
covalently
coupled
to the photosensitizer
to force
an
intramolecular
quenching
pathway,
increasing
the relative
quantum
yield. While
permitting
rapid excited-state
CS, this
approach
runs the risk of promoting
equally
rapid
and
nonproductive
charge
recombination
(CR).
The inverted
driving-force
regime
of Marcus
theory
30
−
32
offers a potential
solution
to this problem.
We reasoned
that if the CR reaction
has sufficient
driving
force and small enough
reorganization
energy
(Figure
1C), the
survival
time for a CSS might
be extended
into the
microsecond
regime,
thereby
allowing
ample
time for
intermolecular
ET or PCET
steps. This strategy
is the basis
of energy
storage
reactions
in photosynthetic
reaction
centers
31,32
and has been employed
to protract
the lifetimes
of CS species
in photocatalytic
ETs (Figure
1C, right).
33,34
The choice
of the hybrid
pe
PCET
mediator
was motivated
by our cobaltocene
e
PCET
reagent
and features
a ferrocene
subunit
as the reductive
quencher
and redox
mediator
appended
to an alkylamino
Brønsted
base and an anthracene
photosensitizer
(abbreviated
herein
as
{Fc
−
NH
+
−
an}
in its
iron(II)
protonated
form; Figure
1B). The synthesis
of this
complex
was originally
reported
by Farrugia
and Magri for the
development
of a Pourbaix
sensor
in logic gates.
35
Near-
ultraviolet
(390 nm) irradiation
of
{Fc
−
NH
+
−
an}
promotes
the anthracene
chromophore
to its lowest
singlet
excited
state
(
1
an
*
, S
1
). This state is quenched
by intramolecular
ET from
Fc, producing
{Fc
+
−
NH
+
−
an
•−
}
(CSS).
If sufficiently
long-
lived,
the powerfully
reducing
anthracene
radical
anion,
colocalized
with a proton
on the amine
base in the CSS,
might be primed
for intermolecular
PCET
reactivity.
Figure
1.
(A) PCET
in reductive
transformations.
Relationship
between
p
K
a
and formal
potential
for PCET
donors
with different
BDFE
values
in
MeCN
(derived
using the Bordwell
equation
in the inset).
The formal
potential
of the H
+
/H
2
couple
is indicated
by a dashed
black line. C
G
value
in MeCN
from ref 13. (B) Previously
reported
electrocatalytic
PCET
(
e
PCET)
mediator
in comparison
with this report
of a photoelectrocatalytic
PCET
(
pe
PCET)
mediator.
(C) Examples
of reported
systems
invoking
a charge-separated
state (CSS).
(CS = charge
separation;
CR = charge
recombination).
Simplified
energy
diagram
for ET from a CSS.
Journal
of
the
American
Chemical
Society
pubs.acs.org/JACS
Article
https://doi.org/10.1021/jacs.4c02610
J. Am. Chem.
Soc.
2024,
146, 12750
−
12757
12751
2.
RESULTS
AND
DISCUSSION
To guide
the following
discussion,
Figure
2A provides
our
working
model
for photoelectrochemical
catalysis
using
{Fc
−
NH
+
−
an}
and the pertinent
physical
parameters.
Figure
2B
provides
a corresponding
Jablonski
representation
of the
electronic
states of
{Fc
−
NH
+
−
an}
. Anodic
cyclic voltammetry
with
{Fc
−
N
−
an}
revealed
a reversible
Fe
III/II
redox couple
[
E
°
= 0.00 V vs ferrocenium/ferrocene
(Fc
+/0
) in acetonitrile;
Fc
+/0
reference
scale used throughout]
that shifts to
E
°
= 0.13
V upon protonation
of the amine
group.
Cathodic
voltam-
metric
sweeps
indicate
that the anthracene
components
in
{N
−
an}
(an organic
model
derivative
without
an Fc subunit;
see Figure
3A) and
{Fc
−
N
−
an}
have similar
reduction
potentials
[
E
°
(an
0/
•−
) =
−
2.44
V].
36
The absorption
and
fluorescence
spectra
of the anthracene
component
in
{Fc
−
NH
+
−
an}
indicate
that
E
00
= 3.1 eV, consistent
with a
potential
of
E
°
(
1
an
*
0/
•−
) = 0.7 V in the S
1
excited
state. An
NMR titration
in deuterated
acetonitrile
indicates
a p
K
a
≈
14.3
for
{Fc
−
NH
+
−
an}
.
Using
this p
K
a
value and
E
°
(an
0/
•−
) for
{Fc
−
NH
+
−
an}
in a Bordwell
analysis,
we estimate
a very weak
BDFE
N
−
H
of
≈
17 kcal mol
−
1
for
{Fc
+
−
NH
+
−
an
•−
}
(Figure
2A;
C
G
= 52.6 in acetonitrile
3
). This value compares
with the
ground
state BDFE
N
−
H
≈
75 kcal mol
−
1
in
{Fc
−
NH
+
−
an}
,
where
Fc is the source
of reducing
equivalents
(see Supporting
Information).
Fluorescence
from
1
an
*
(400
−
475
nm) in the ET-inactive
model
complex
{NH
+
−
an}
(Figure
3A) decays
with an
exponential
time constant
of 6.45
±
0.05 ns. Steady
state
measurements
reveal that
1
an
*
fluorescence
from
{Fc
−
NH
+
−
an}
in the presence
of excess
acid {picolinium
triflate
([PicH][OTf]),
16 mM} is heavily
quenched
(99%)
(Figure
3A), suggesting
an excited
state lifetime
<60 ps. Time-resolved
fluorescence
measurements
reveal
a multiexponential
decay
with the fastest
component
having
a time constant
of
τ
1
< 20
ps (see Supporting
Information).
This lifetime
is limited
by the
response
time of our instrument.
On the basis of the steady-
state spectra
it must represent
the dominant
decay pathway
for
{Fc
−
NH
+
−
1
an
*
}
when
excess
acid is present.
The rapid
fluorescence
decay
is consistent
with rapid ET from Fc to
1
an
*
, producing
{Fc
+
−
NH
+
−
an
•−
}
(CSS)
with a rate constant
of
k
CS
> 5
×
10
10
s
−
1
(
−Δ
G
°
= 0.6 eV). The slower
observed
decay
components
may arise from
{Fc
−
NH
+
−
1
an
*
}
con-
formations
that are not suitable
for ET, or alternatively
a very
minor
(<1% based on NMR analysis
of synthesized
material)
fluorescent
impurity.
The
1
an
*
decay time in
{Fc
−
NH
+
−
an}
Figure
2.
(A) Thermodynamic
parameters
for the
{Fc
−
N
−
an}
system
toward
ground-state
and excited-state
PCET,
including
calculated
parameters
and measured
values.
(B) Jablonski
representa-
tion of the electronic
states of
{Fc
−
N
−
an}
,
indicating
a stabilization
of the CSS in the presence
of a salt. (
k
BPCET
= rate of back PCET;
k
IC
= rate of internal
conversion;
k
ISC
= rate of intersystem
crossing).
Figure
3.
(A) Steady-state
emission
spectra
of
{Fc
−
NH
+
−
an}
vs
{NH
+
−
an}
in the presence
of 15 mM [PicH][OTf]
following
excitation
at 355 nm. (B) Spectroelectrochemistry
data for the
reduction
of
{Fc
−
N
−
an}
to
{Fc
−
N
−
an
•−
}
(0.4 mM) in 0.7 M
TBAPF
6
(THF).
Inlay includes
the TA spectrum
of
{Fc
+
−
NH
+
−
an
•−
}
from 690 to 790 nm. For the steady-state
emission
data, [
{Fc
−
N
−
an}
]
= [
{NH
+
−
an}
]
= 0.15 mM; [PicH][OTf]
= 15 mM in
DME.
[PicH][OTf]
= 2-picolinium
triflate.
Journal
of
the
American
Chemical
Society
pubs.acs.org/JACS
Article
https://doi.org/10.1021/jacs.4c02610
J. Am. Chem.
Soc.
2024,
146, 12750
−
12757
12752
in the absence
of excess
acid does not exhibit
the fastest
(<20
ps) decay component
and is not as heavily
quenched.
Instead,
the
1
an
*
decay
is biphasic
with time constants
of 2.5
±
0.1
(46%)
and 11.5
±
0.2 ns (54%).
The faster decay component
indicates
that CS is much slower
in the absence
of excess
acid.
The slower
decay
component
may be a consequence
of
conformational
heterogeneity.
A transient
absorption
(TA) spectrum
collected
after 10 ns
laser excitation
(355 nm) of
{Fc
−
NH
+
−
an}
in the presence
of
excess
acid ([PicH][OTf],
15 mM) shows
absorbance
at 700
nm, consistent
with the spectrum
of the anthracene
radical
anion
measured
spectroelectrochemically
(Figure
3B) and
hence
assignable
to
{Fc
+
−
NH
+
−
an
•−
}
(CSS).
TA kinetics
monitored
at 700 nm further
reveal
a relatively
long-lived
species
(
τ
∼
0.9
μ
s; Figure
4A, black trace).
The signal
has
greater
amplitude
and is somewhat
longer-lived
(
∼
0.9
vs 0.6
μ
s) when excess
[PicH][OTf]
is present.
When
[TBA][OTf]
(15 mM) is employed
as an electrolyte
with isolated
{Fc
−
NH
+
−
an}
,
the same species
is observed,
and its lifetime
increases
to 1.3
μ
s, consistent
with increased
stabilization
of
the CSS in a higher
dielectric
environment
(Figure
2B). Rate
constants
for excited-state
charge
shift and thermal
back
transfer
reactions
in
{Fc
−
NH
+
−
an}
display
an inverted
driving-force
behavior.
The rapid excited-state
charge
shift
reaction
at low driving
force (0.6 eV) implies
a modest
ET
reorganization
barrier.
Back ET from
{Fc
+
−
NH
+
−
an
•−
}
to
regenerate
ground-state
{Fc
−
NH
+
−
an}
is likely disfavored
by
the Marcus
inverted
effect,
owing
to the high reaction
driving
force (2.5 eV) and closed
shell products.
37
Conformational
dynamics
and electronic
(spin)
barriers
might
also modulate
the observed
ET rate constants,
warranting
future
studies
to
probe them further.
The slower
CR at a lower driving
force is
presumably
the result of an increase
in the reorganizational
energy
in the higher
dielectric
medium.
A different
420 nm TA
feature
observed
after 10 ns excitation
of
{NH
+
−
an}
(
τ
= 27.8
μ
s) or
{Fc
−
NH
+
−
an}
(
τ
= 2.1
μ
s) is also present
and is
attributable
to the
3
an
*
state.
38
The Jablonski
diagram
(Figure
2B) illustrates
the complex
array of radiative
and nonradiative
pathways
available
in
{Fc
−
NH
+
−
an}
.
An alternative
assignment
for the 700 nm transient
would
be
a species
resulting
from protonation
of the anthracene
radical
anion by acid ([PicH][OTf],
15 mM) to produce
the neutral
radical
instead.
Experimental
precedent
suggests
that proto-
nation
at C
9
by [PicH][OTf]
would
lead to a neutral
radical
fragment
with absorbance
below 500 nm,
38
in contrast
with the
observed
700 nm absorbance.
Moreover,
in an isotope
scrambling
experiment,
where
a DME
solution
of
{Fc
−
N
−
an}
and [PicD][OTf]
was irradiated
at 390 nm for 1 h,
2
H
NMR
spectra
show no indication
of deuterium
incorporation
into the anthracene
moiety
(see Supporting
Information).
We next tested
whether
photochemically
generated
{Fc
+
−
NH
+
−
an
•−
}
could
undergo
intermolecular
PCET
reactions,
using acetophenone
(estimated
BDFE
O
−
H
= 36 kcal mol
−
1
based on DFT calculations;
see Supporting
Information)
20
as
an initial test substrate.
{Fc
−
N
−
an}
(1 mM) in the presence
of [PicH][OTf]
(10 mM, p
K
a
(MeCN)
= 13.3)
39
does not
react with acetophenone
(10 mM) in the absence
of light
excitation,
owing
to the large unfavorable
BDFE
X
−
H
mismatch.
An analogous
experiment
with 390 nm irradiation,
however,
afforded
29% of the pinacol-coupled
product
expected
from
net H atom transfer
to acetophenone
and subsequent
coupling
of the
α
-ketyl
radical
intermediates
(Figure
5). A photo-
chemical
quantum
yield of 6% was measured
under
these
conditions
(see Supporting
Information).
Control
experiments
using
{Fc
−
NMe
+
−
an}
in the presence
of [PicH][OTf],
or
{Fc
−
N
−
an}
and a weaker
acid incapable
of protonating
the
amine
base (
p
-CF
3
-benzoic
acid), did not produce
the (<1%)
pinacol
product.
A binary
mixture
composed
of just the organic
fragment
{N
−
an}
(1 mM) and ferrocene
(10 mM) with 10
mM [PicH][OTf]
also failed to generate
the product,
likely
owing
to the short lifetime
of the anthracene
singlet
excited
state and the low energy
of the longer-lived
anthracene
triplet
excited
state [
E
°
(
3
an
*
0/
•−
)
≈ −
0.6 V].
40
Interestingly,
despite
the large driving
force toward
HER, no H
2
was detected
upon
irradiation
in the absence
of the substrate
(see Supporting
Information).
Among
the factors
that may preclude
such
reactivity
are an unfavorable
bimolecular
reaction
between
two
cationic
species,
either
{Fc
−
NH
+
−
an}
or
{Fc
+
−
NH
+
−
an
•−
}
,
and the statistical
improbability
of the needed
collision
between
two CSS molecules,
given the CSS lifetime.
In the presence
of excess
[PicH][OTf],
transient
spectros-
copy reveals
that the charge-separated
intermediate
{Fc
+
−
NH
+
−
an
•−
}
reacts rapidly
with acetophenone
(Figure
4A). A
plot of the transient
decay
rate constant
versus
quencher
concentration
is linear,
consistent
with a second-order
rate
constant
H
k
PCET
= 9.0
±
1.3
×
10
7
M
−
1
s
−
1
(Figure
4B). The
second-order
rate constant
for reaction
of
{Fc
+
−
NH
+
−
an
•−
}
with acetophenone
in the presence
of [PicD][OTf]
(15 mM)
Figure
4.
(A) Time-resolved
TA decays
for
{Fc
+
−
NH
+
−
an
•−
}
in the
presence
of excess
acid and quencher
(acetophenone,
sub
), exciting
with a 355 nm laser pulse and monitoring
the absorbance
at 700 nm
after a time-delay
of 10 ns. (B) Stern
−
Volmer
quenching
plots for the
rate of decay
of
{Fc
+
−
NH(D)
+
−
an
•−
}
in the presence
of varying
concentrations
of
sub
, relative
to the rate of decay in the absence
of
sub
, and an extracted
deuterium
kinetic
isotope
effect (KIE).
(C)
Demonstration
of the zero-order
dependence
of CR and PCET
on
[H
+
]. For the TA data, [
{Fc
−
N
−
an}
]
= 0.15 mM; [[PicH(D)]-
[OTf]]
= 15 mM; [
sub
] = 18 mM in DME.
For the acid-dependence
study,
[[PicH][OTf]]
+ [[PicMe][OTf]]
= 32 mM.
Journal
of
the
American
Chemical
Society
pubs.acs.org/JACS
Article
https://doi.org/10.1021/jacs.4c02610
J. Am. Chem.
Soc.
2024,
146, 12750
−
12757
12753
is
D
k
PCET
= 3.2
±
0.5
×
10
7
M
−
1
s
−
1
(Figure
4B); the
H
k
PCET
/
D
k
PCET
KIE = 2.8
±
0.4.
Because
an H-bonding
interaction
has been proposed
between
acetophenone
substrates
and phosphoric
acids of
similar
p
K
a
to [PicH][OTf]
by Knowles,
42
we explored
a
similar
mechanism
in our studies,
where
[PicH][OTf]
would
form an H-bonded
complex
with acetophenone
followed
by
ET from
{Fc
+
−
NH
+
−
an
•−
}
. We postulated
that if this
mechanism
was operative
in the present
system,
we should
see a dependence
of the rate of PCET
on both [H
+
] and
[acetophenone].
To keep the dielectric
of the medium
relatively
constant
while varying
[[PicH][OTf]],
we employed
the methylated
salt [PicMe][OTf]
to maintain
the total
concentration
of ions throughout;
[PicMe][OTf]
similarly
stabilizes
the CSS (see Supporting
Information).
While
holding
[acetophenone]
constant,
varying
[[PicH][OTf]]
shows
zero-order
dependence
(Figure
4C), suggesting
that
PCET
from
{Fc
+
−
NH
+
−
an
•−
}
to acetophenone
does not
proceed
via H-bonded
preassociation
with [PicH][OTf].
The
KIE value of 2.8 and the lack of dependence
on [PicH
+
] is
consistent
with concerted
PT and ET to acetophenone
from
{Fc
+
−
NH
+
−
an
•−
}
.
20
We also explored
the reactivity
of
{Fc
−
NH
+
−
an}
with
N
-phenylbenzylimine
and diphenylfumarate
(Figure
5). Reaction
with
N
-phenylbenzylimine,
featuring
a
C
�
N
π
-bond
and an associated
BDFE
N
−
H
for its correspond-
ing iminyl
radical
calculated
to be 50 kcal mol
−
1
(Figure
5),
afforded
65% of the aza-pinacol
coupling
product.
Using
diphenylfumarate
as the substrate
(C
�
C
π
-bond;
BDFE
C
−
H
for the succinyl
radical
calculated
to be 45 kcal mol
−
1
) afforded
42% of the fully reduced
succinate
product.
41
To test the photochemical
PCET
mediator
on an inorganic
substrate
we turned
to the hydrazido
complex
[(TfO)
W
(NNH
2
)][OTf]
[
W
= (dppe)
2
W; OTf = triflate];
we had
recently
reported
its thermal
reactivity
with a cobalt
PCET
mediator
toward
N
−
N cleavage.
23
Gratifyingly,
irradiation
of
[(TfO)
W
(NNH
2
)][OTf]
in the presence
of [PicH][OTf]
and
{Fc
−
N
−
an}
afforded
∼
70%
yield of the [(TfO)
W
(NH)]-
[OTf]
imido
product
(evidenced
by
31
P NMR
spectroscopy;
eq 2). When
the
15
N-labeled
complex
[(TfO)
W
(
15
N
15
NH
2
)]-
[OTf]
was used instead,
15
NH
4
OTf was detected
via
1
H
−
15
N
heteronuclear
multiple
bond
correlation
(HMBC)
NMR
spectroscopy.
Observation
of
15
NH
4
OTf and also [(TfO)
W
(
15
NH)][OTf]
(via
31
P NMR)
are consistent
with photo-
induced
PCET
concomitant
with N
−
N
bond cleavage
(see
Figure
5.
Scope
of organic
substrates
amenable
to photochemical
reduction
via
p
PCET
using
{Fc
−
NH
+
−
an}
.
Photoelectrocatalytic
reduction
of
organic
substrates
by
{Fc
−
NH
+
−
an}
in the presence
of [PicH][OTf]
under irradiation
at 390 nm and an
E
app
of
−
0.1 V vs Fc
+/0
using a carbon
cloth cathode
and either
a Zn or GC anode.
All reported
yields
are NMR
yields
measured
against
an internal
standard
(see Supporting
Information).
Only trace H
2
(<2%
Faradaic
efficiency)
was detected
when
sub
= acetophenone.
Calculated
BDFE
values
of key organic
intermediates
are reported
as well.
41
The inlayed
plot is measured
current
response
in the presence
and absence
of irradiation
for the
photoelectrocatalytic
reduction
of acetophenone
by
{Fc
−
NH
+
−
an}
.
(SM = starting
material).
Journal
of
the
American
Chemical
Society
pubs.acs.org/JACS
Article
https://doi.org/10.1021/jacs.4c02610
J. Am. Chem.
Soc.
2024,
146, 12750
−
12757
12754
Supporting
Information,
Section
S14). No reaction
occurred
in
the absence
of irradiation
or in the absence
of
{Fc
−
N
−
an}
.
However,
when
{N
−
an}
and ferrocene
were used in place of
{Fc
−
N
−
an}
,
appreciable
[(TfO)
W
(NH)][OTf]
was gener-
ated (35% yield),
in contrast
to a related
control
experiment
for the photochemical
reduction
of acetophenone,
where
none
of the reduced
product
is detected
(see Supporting
Information).
We suspect
that this result
reflects
reactivity
between
the triplet
excited
state of
{NH
+
-an}
and [(TfO)
W
(NNH
2
)][OTf].
+
[
][
]
+
[
][
] +
+
{
}
{
}
+
+
x
W
W
(TfO)
NNH
s
PicH
OTf
(TfO)
NH
NH
OTf
Pic
hv
2
Fc
N
an
Fc
N
an
4
(2)
We then tested
the efficacy
of
{Fc
−
NH
+
−
an}
under
photoelectrocatalytic
conditions
using
a high surface
area
carbon
cloth cathode
held at a constant
E
app
relative
to the
{Fc
+/0
−
N
−
an}
wave. With acetophenone
as a test substrate
(50 mM),
a controlled
potential
electrolysis,
using a divided
cell with a DME solution
containing
0.15 M TBAPF
6
, 100 mM
[PicH][OTf]
and 1 mM
{Fc
−
N
−
an}
,
afforded
36
±
4%
(TON
= 18
±
2) of the pinacol
coupled
product
after 24 h at
an
E
app
of
−
0.1 V under
390 nm irradiation
(Figure
5).
Accounting
for the remaining
starting
material
(SM) provided
a mass balance
of 87%. For comparison,
electrocatalytic
turnover
to the same pinacol
product
using the previously
reported
Co(II,
NH)
+
mediator
required
an
E
app
of
−
1.3 V.
20
The photoactive
appendage
in the ferrocene-derived
mediator
enables
an
E
app
that is positively
shifted
by more than 1 V. The
effect
of photoirradiation
on the electroreduction
can be
gleaned
via light on/off
cycles
(Figure
5); on removing
light,
the reductive
current
decreases
as
{Fc
+
−
NH
+
−
an}
is depleted
from the surface
of the electrode.
When
irradiation
resumes,
the current
increases
as
{Fc
+
−
NH
+
−
an}
is regenerated
via
PCET
to acetophenone.
Photoelectrochemical
reduction
of
N
-phenylbenzylimine
(Figure
5) parallels
these
results,
with the corresponding
coupling
product
obtained
in 30% yield (TON
= 15), and an
85% mass balance
accounting
for the remaining
SM. In the
case of diphenylfumarate,
the fully hydrogenated
succinate
product
is obtained
in only 9% yield (TON
= 9), with a mass
balance
of 84%. By comparison,
diphenylfumarate
reduction
is
more favorably
mediated
by the cobaltocene-derived
mediator
under
electrochemical
conditions.
21
In the latter
case,
reduction
of the succinyl
radical
intermediate
by the electrode
(
E
app
=
−
1.3 V) enabled
the net two-electron
process.
For
pe
PCET
with
{Fc
−
N
−
an}
,
reduction
of the radical
intermediate
(
E
red
=
−
0.7 V) is not feasible.
Interestingly,
since some reduction
still occurs
for the case of
{Fc
−
N
−
an}
,
a
multiproton/electron
process
is apparently
feasible
via
pe
PCET.
As a control,
in the absence
of
{Fc
−
N
−
an}
or at an applied
potential
anodic
of the
{Fc
+/0
−
N
−
an}
couple,
<1% or TON
<1 was observed
for all substrates.
Furthermore,
these various
reductions
could not be achieved
via direct electrolysis
at an
E
app
set to that of the reduced
anthracene
moiety;
at such a
reducing
potential,
the background
HER
dominates,
as
evidenced
by CV (see Supporting
Information).
Taken
together,
these data show that
pe
PCET
from a mediator
that
operates
through
a long-lived
CSS is a promising
strategy
to
achieve
electrocatalytic
proton-coupled
reductions
at modest
applied
potentials
using light as the primary
driving
force.
3.
CONCLUSIONS
Strategies
that harness
light for selective
PCET
to a substrate
offer an attractive
approach
for solar-to-chemicals
conversions.
In our
pe
PCET
system
using a 3-electrode
potentiostat,
the
ultimate
source
of reducing
equivalents
was the reaction
at the
counter
electrode.
In most cases, this involved
oxidation
of a
zinc electrode,
although
we also examined
a bulk photo-
electrochemical
reduction
of acetophenone
using Fc in the
counter-electrode
compartment
(as a sacrificial
donor
to
recycle
the ferrocene-derived
pe
PCET
mediator).
The reaction
with Fc proceeded
similarly
(Figure
5), and Fc
+
was generated
in the counter-electrode
compartment
as expected.
This
approach
offers attractive
opportunities
for photoelectrochem-
ical catalysis
using a two-electrode
electrochemical
cell with a
counter
electrode
based
on the HA / 1/2 H
2
redox couple.
Because
E
°
(Fc
+
/Fc) is anodic
of
E
°
(HA
/ 1/2 H
2
) for acids
with p
K
a
> 0 (Figure
1B), an applied
potential
would
be
necessary
only to drive higher
currents,
allowing
H
2
to be both
the ultimate
source
of electrons
and protons
for PCET
reactivity.
For acetophenone,
we calculate
the addition
of H
2
to
be endergonic
(H
2
+ 2 Ph(Me)CO
→
Ph(Me)(HO)C
−
C(OH)(Me)Ph;
Δ
G
0
= +14 kcal mol
−
1
in MeCN
at RT),
implying
the possibility
of photochemical
energy
storage.
Future
studies
will be needed
to explore
engineering
half
−
cell
reactions
that work in synergy
such that H
2
oxidation
can be
partnered
with reductive
pe
PCET
as a means
of using light to
generate
energy-rich
chemical
products.
The design
of second
generation
pe
PCET
mediators
with even longer
CSS lifetimes
correlated
to higher
quantum
efficiency
would
complement
such efforts.
■
ASSOCIATED
CONTENT
Data
Availability
Statement
All data are available
in the main text or the Supporting
Information.
*
sı
Supporting
Information
The Supporting
Information
is available
free of charge
at
https://pubs.acs.org/doi/10.1021/jacs.4c02610.
Experimental
methods;
synthetic
details;
electrochemical
data; UV-vis
data; p
K
a
calculation;
data from stoichio-
metric
and catalysis
experiments;
fluorescence
data;
transient
absorption
data;
reaction
quantum
yield
determination;
spectroelectrochemistry
of
{Fc
−
N
−
an}
; isotope
scrambling
experiment;
H
2
quantification
for CPE; and DFT calculations
(PDF)
■
AUTHOR
INFORMATION
Corresponding
Authors
Jay R. Winkler
−
Division
of Chemistry
and Chemical
Engineering,
California
Institute
of Technology
(Caltech),
Pasadena,
California
91125,
United
States;
orcid.org/
0000-0002-4453-9716;
Email:
winklerj@caltech.edu
Jonas
C. Peters
−
Division
of Chemistry
and Chemical
Engineering,
California
Institute
of Technology
(Caltech),
Pasadena,
California
91125,
United
States;
orcid.org/
0000-0002-6610-4414;
Email:
jpeters@caltech.edu
Journal
of
the
American
Chemical
Society
pubs.acs.org/JACS
Article
https://doi.org/10.1021/jacs.4c02610
J. Am. Chem.
Soc.
2024,
146, 12750
−
12757
12755