Mechanism of a Luminescent Dicopper System That Facilitates
Electrophotochemical Coupling of Benzyl Chlorides via a
Strongly Reducing Excited State
Michael D. Zott
,
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Virginia M. Canestraight
,
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Jonas C. Peters
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
Photochemical radical generation has become a modern staple in chemical synthesis and
methodology. Herein, we detail the photochemistry of a highly reducing, highly luminescent
dicopper system [Cu
2
] (
E
ox
*
≈
−2.7 V vs SCE;
τ
0
≈
10
μ
s) within the context of a model reaction:
single-electron reduction of benzyl chlorides. The dicopper system is mechanistically well
defined. As we show, it is the [Cu
2
]* excited state that serves as the outer-sphere photoreductant
of benzyl chloride substrates; the ground-state oxidized byproduct, [Cu
2
]
+
, is electrochemically
recycled, demonstrating a catalytic electrophotochemical C−C coupling process.
Graphical Abstract
Corresponding Author:
Jonas C. Peters − Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, California 91125, United States; jpeters@caltech.edu.
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.2c03215
.
Experimental procedures, spectroscopic characterization (e.g., NMR) data, luminescence data, description of the Marcus analysis
performed, kinetic data pertaining to chloride (Cl
−
) attack on [Cu
2
]
+
, and compound characterization data (
PDF
)
Crystallographic data for chloro-cubane and chloro-diamond (
CIF
)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.2c03215
The authors declare no competing financial interest.
HHS Public Access
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ACS Catal
. 2022 September 02; 12(17): 10781–10786. doi:10.1021/acscatal.2c03215.
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Keywords
electrophotochemistry; photoredox catalysis; copper catalysis; photoinduced homocoupling;
luminescent copper
Photochemistry, often in conjunction with transition-metal catalysis, is growing in
prominence in modern synthetic methodology.
1
–
4
Photochemical activation of widely
available electrophiles can afford versatile reactive intermediates, such as organic radicals,
5
,
6
which can be leveraged in a variety of transformations.
7
–
10
For instance, a recent focus of
a number of laboratories, including our own, has been to partner photochemically generated
radical intermediates (R
•
) with copper-(II)-bound N-nucleophiles in catalytic, photoinduced
N-alkylations (Scheme 1, eqs 1 and 2; N
nuc
denotes an amidenucleophile).
9
,
11
–
18
Production of R
•
from alkyl halides is integral to many modern organic transformations,
19
–
21
and hence there is considerable interest in expanding the types of alkyl halides compatible
with R
•
generation under synthetically useful conditions.
22
–
24
Alkyl chlorides, with
potentials below −2 V vs SCE, are desirable electrophiles but are challenging to reduce;
22
,
25
the limited examples of their outer-sphere photochemical activation typically feature
harsh conditions.
26
–
28
Phosphine-supported copper-amide excited states
29
–
33
can be more
reducing than those of typical ruthenium or iridium systems,
34
providing a sufficient driving
force for alkyl chloride reduction. To promote photoinduced R
•
generation via a copper
species in a generalized fashion (e.g., avoiding the subsequent C−N coupling step as in
Scheme 1, eq 2), the copper byproduct of oxidative quenching must be recycled by a suitable
reductant.
In 1987, Sauvage demonstrated an elegant solution to photocatalyst regeneration via the
electrophotochemical reduction of 4-nitrobenzyl bromide with [Cu(dap)
2
]
+
(
E
ox
*
≈
−1.4 V;
τ
0
= 0.27
μ
s; dap = 2,9-dianisyl-1,10-phenanthroline).
35
Organic photosensitizers have more
recently been used to reduce (pseudo)halides under extremely reducing electrophotoredox
conditions (
E
ox
* < −3 V).
36
–
39
The suggested lifetimes (
τ
0
≈
1 ns) and nature of the
photoreductant intermediates of these processes are still under investigation.
40
In this study, we explore a dicopper diamond core system (hereafter [Cu
2
]), previously
developed by our lab
33
and featuring a combination of terminal phosphine and bridging
amide ligands, as an attractive electrophotoredox catalyst (Scheme 1, bottom). [Cu
2
] is
an especially strong excited-state reductant (
E
ox
*
≈
−2.7 V), with a long-lived excited
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state in solution at RT (
τ
0
≈
10
μ
s). Charge delocalization by the Cu
2
(
μ
-N)
2
diamond
core, as well as steric protection from ligand isobutyl and
tert
-butyl groups, is expected to
render the one-electron-oxidized state [Cu
2
]
+
non-nucleophilic. Furthermore, [Cu
2
]
+
can be
electrochemically interconverted with [Cu
2
]; [Cu
2
]
+
has been isolated and characterized in
the solid state.
41
As a representative study of the excited-state intermolecular photochemistry of Cu
I
−amide
systems, with an eye toward photoreductions using alkyl chlorides as R
•
precursors, we
explore herein photochemically driven, electrochemically cycled radical couplings using
[Cu
2
] and benzyl chloride substates (
E
p
up to −2.5 V vs SCE). The dicopper system
described here is mechanistically well defined, and as we show, it is the [Cu
2
]* excited
state that serves as the outer-sphere photoreductant of benzyl chloride substrates; the
ground-state oxidized byproduct, [Cu
2
]
+
, is electrochemically recycled to afford a catalytic,
electrophotochemical C−C coupling process.
We began by investigating the reactivity of 4-methylbenzyl chloride (
1
) (
E
p
= −2.5 V vs
SCE) as a model substrate. Benzyl chlorides are important substrates in modern synthesis
and methodology
42
–
44
and also provide a convenient radical termination pathway via
diffusion-limited dimerization, simplifying our mechanistic studies.
45
On exposure of
1
to blue-light irradiation (440 nm) in 1,2-dimethoxyethane (DME), no reaction is observed.
However, when [Cu
2
] is added, bibenzyl product
1-D
is formed quantitatively (Figure 1A).
Benzyl chloride photoreduction was mechanistically interrogated via Stern−Volmer (SV)
studies to establish outer-sphere electron transfer (ET) and to probe rates of ET. Time-
resolved photoluminescence spectroscopy confirmed that electronically distinct benzyl
chlorides
1
–
8
quench [Cu
2
] in a dynamic (i.e., diffusional) process. The rates of quenching,
determined from linear SV plots (Figure 1B), were in the range of ~10
8
–10
10
M
−1
s
−1
for
K
SV
/
τ
0
. These values indicate rapid quenching, reaching diffusion-limited values with
electron-poor
2
. Using benzyl chloride peak potentials obtained from cyclic voltammetry (
E
p
= −1.7 to −2.5 V; see the Supporting Information), the quenching rates could be analyzed
as a function of driving force, using
E
ox
*
≈
−2.7 V. Notably, a quadratic relationship
between log
K
SV
and driving force was observed, consistent with the behavior predicted
by Marcus theory for outer-sphere electron transfer (Figure 1C).
46
Although such outer-
sphere dynamic quenching is commonly assumed in photoredox mechanisms, this contrasts
with the behavior of some organic electrophotoredox catalysts hypothesized to involve
preassembly of the photocatalyst and substrate to compensate for short lifetimes.
38
These
photophysical measurements thus indicate a rapid dynamic oxidative quenching step in
which [Cu
2
] undergoes outer-sphere electron transfer to benzyl chloride electrophiles.
We expected oxidative quenching to produce the stable, red-brown, mono-oxidized species
[Cu
2
]
+
(Figure 2A).
41
A 440 nm irradiation of [Cu
2
] and
1
in DME produces a pale
yellow solution, the UV−vis spectrum of which is mostly featureless (Figure 2B). Thus,
the expected UV−vis features for [Cu
2
]
+
at 520, 600, and 800 nm were not observed.
Surprisingly, this suggests that the oxidative quenching reaction may involve either
degradation following quenching or chemical steps at copper.
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We hypothesized that the stability of [Cu
2
]
+
could be compromised by chloride, a byproduct
of benzyl chloride reductive C−Cl bond cleavage. Accordingly, addition of lithium chloride
to a solution of [Cu
2
]
+
in DME resulted in a loss of red-brown color over several hours,
producing a yellow solution. Off-white crystals isolated from the reaction mixture were
characterized by two
31
P NMR peaks (Figure 2C), and single-crystal XRD revealed
the presence of two independent dimers, each comprised of two CuCl (chloro-cubane)
or one CuCl (chloro-diamond) per H-PNP
t
Bu
ligand equivalent: i.e., [(H-PNP
t
Bu
)Cu
2
(
μ
-
Cl)
2
]
2
or [(H-PNP
t
Bu
)Cu(
μ
-Cl)]
2
, respectively. An independent synthesis of chloro-cubane
and chloro-diamond (see the Supporting Information), produced white solids whose
31
P
NMR resonances reproduced those of the cocrystalline material (Figure 2C), and the
characterization of chloro-diamond enabled its identification as a reaction product in the
stoichiometric reaction described in Figure 1A (SI).
We sought to detect and track the fate of [Cu
2
]
+
in the presence of chloride via a UV−vis
time course analysis, photolyzing [Cu
2
] and
1
under 440 nm irradiation (Figure 3A). Bands
characteristic of [Cu
2
]
+
grow in throughout 15–30 s, after which the 520 nm absorbance
rapidly decreases. This accounts for our failure to observe the presence of [Cu
2
]
+
in Figure
2B. Knowing that chloride in the form of lithium chloride slowly degrades [Cu
2
]
+
over a
period of several hours, we investigated whether lithium salts could sequester chloride via
tight ion pairing to mitigate degradation of [Cu
2
]
+
.
47
When [Cu
2
] and
1
were irradiated in the presence of 0.2 M LiNTf
2
, bands for [Cu
2
]
+
became persistent, decreasing in intensity by only ~20% after 20 min (Figure 3B). This is
consistent with kinetic measurements which indicate a rate of ~3 × 10
−2
M
−1
s
−1
for the
reaction between [Cu
2
]
+
and tetrabutylammonium chloride in the presence of 0.2 M LiNTf
2
;
without LiNTf
2
, the reaction is almost instantaneous (Supporting Information). An analysis
of [Cu
2
] photolyzed in the presence of
1
and 0.2 M LiNTf
2
by EPR provided orthogonal
support for assigning the product as [Cu
2
]
+
(Figure 3C).
48
Thus, these analyses indicate
[Cu
2
]
+
to be the oxidative quenching product and corroborate its degradation by chloride.
Stabilizing [Cu
2
]
+
enables the prospect of electrochemically regenerating [Cu
2
]. [Cu
2
]
+
was photochemically generated from [Cu
2
] and
1
in DME, with LiNTf
2
serving as both
a chloride sequestrant and the electrolyte, and then transferred into a two-compartment
electrochemical cell. Applying
E
app
= −0.15 V for 5 min using a carbon cloth working
electrode, cathodic of
E
ox
= 0 V for [Cu
2
]
0/+
, 0.76 e
−
equivalents of current was passed
(Figure 3D). One electron is required to fully reduce [Cu
2
]
+
to its photoactive neutral state;
thus, up to 76% could be reduced. The UV−vis spectrum of this solution showed recovery
of the 440 nm peak characteristic for [Cu
2
], albeit with ~60% of its original intensity,
indicating successful, albeit incomplete, regeneration.
49
An electrochemical analysis of
chloro-cubane and chloro-diamond indicated no electron transfer pathway for recovering
[Cu
2
] at our operating potential, highlighting the importance of stabilizing [Cu
2
]
+
and
rationalizing the incomplete regeneration of [Cu
2
].
The described reactivity of the [Cu
2
] system constitutes the requirements for an
electrophotoredox cycle (Scheme 1); thus, we turned to catalytic investigations under
controlled-potential conditions (Table 1). Indeed, [Cu
2
] is a competent electrophotoredox
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catalyst, generating
1-D
from
1
in 89% yield using 3 mol % [Cu
2
] (entry 1). Reactions
of additional substrates
2
–
8
proceeded in 68–91% yield (entries 12−18). No reaction was
observed in the absence of either [Cu
2
] or light (entries 2 and 3). In the absence of an
applied potential, only the expected stoichiometric amount of
1-D
relative to [Cu
2
] was
produced (entry 4).
The intermediacy of benzyl radicals during catalysis is supported by several pieces of
circumstantial evidence. The production of
1-D
in the presence of added water (Table
1, entry 5) and the dimerization of tertiary and ester-substituted benzyl chlorides are
inconsistent with the intermediacy of benzyl anions. Although the reaction is highly
sensitive to air due to quenching of [Cu
2
]* (entry 6), 4-methylbenzaldehyde becomes the
major product (7% yield). Benzaldehydes are known products of the reaction between
benzyl radicals and oxygen.
35
,
50
Attempts to trap benzyl radicals with the radical trap
TEMPO were unsuccessful, as TEMPO quenches [Cu
2
].
51
The catalytic reaction is very sensitive to factors that alter chloride binding to [Cu
2
]
+
. Li
+
from LiNTf
2
likely interacts with chloride through ion pairing as a Lewis acid; electrolytes
expected to exhibit weaker ion pairing with chloride, such as tetrabutylammonium salts,
performed notably worse (Table 1, entries 7–9). The poorer performance of LiClO
4
(entry
7) is attributed to the fact that in DME ClO
4
−
is more tightly associated with Li
+
than
is NTf
2
−
,
52
possibly limiting the sequestration of Cl
−
. Isolated chloro-cubane and chloro-
diamond (Figure 2D) were catalytically inactive under the conditions (entries 10 and 11).
Therefore, the detection of chloro-diamond by
31
P NMR at the end of the standard reaction
(entry 1) suggests one pathway by which catalysis ceases.
To close, we have described the electrophotochemical reactivity of [Cu
2
] in the presence
of benzyl chloride substrates. Our mechanistic studies enable the assignment of facile
electron transfer from the excited state [Cu
2
]* with substrate to liberate [Cu
2
]
+
, Cl
−
, and
a benzyl radical that undergoes homocoupling to produce bibenzyl. By tracking down
off-path copper-cubane and -diamond chloride sinks, and devising a means of sequestering
the chloride produced, we were able to demonstrate the electrophotocatalytic chemistry of
interest. Our study complements other recent reports employing organo-photocatalysts for
R(Ar)−X electrophotochemical couplings where the nature of the photoreductants are still
being studied.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
ACKNOWLEDGMENTS
The authors are grateful to the National Institutes of Health (NIGMS: R01-109194) for support of this research.
M.D.Z. acknowledges the Resnick Sustainability Institute at Caltech and the National Science Foundation
(DGE-1745301) for support via Graduate Fellowships. V.M.C. acknowledges the John Stauffer Charitable Trust
and Caltech’s Summer Undergraduate Research Fellowships program for summer research funding. The Beckman
Institute Laser Resource Center and Jay R. Winkler are acknowledged for providing support with steady-state and
time-resolved luminescence experiments. We acknowledge Dr. Joseph Derosa for insightful discussions.
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(48). EPR parameters from this measurement were identical with those of isolated [Cu
2
]
+
(49). The remaining current is likely due to capacitive (charging) current; due to the low catalyst
loading, the total current amounted to only 0.16 C.
(50). Salta Z; Kosmas AM; Segovia ME; Kieninger M; Tasinato N; Barone V; Ventura ON
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(51). The reduction potential of TEMPO is −1 V vs SCE, and therefore electron transfer is
substantially exergonic. Reduction potential taken from:Ryan MC; Whitmire LD; McCann
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(52). Brouillette D; Perron G; Desnoyers JE Apparent Molar Volume, Heat Capacity, and Conductance
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Chem 1998, 27, 151–182.
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Figure 1.
Photoreduction of benzyl chlorides: (A) reaction performed for 2 h with yield analyzed by
1
H NMR versus CH
2
Br
2
internal standard; (B) Stern−Volmer quenching; (C) Marcus theory
analysis in the presence of various benzyl chloride quenchers.
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Figure 2.
Influence of chloride on oxidized copper products. UV−vis spectra in DME of (A) [Cu
2
]
+
and (B) a mixture of [Cu
2
] and 4-methylbenzyl chloride irradiated (440 nm) for 5 min. (C)
31
P NMR spectra of chloride-bound copper products and (D) their structures.
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Figure 3.
Stability and regeneration of [Cu
2
]
+
. Time course studies for a mixture of [Cu
2
] and
1
under
440 nm irradiation. (A) UV−vis spectra and (B) 520 nm absorbance vs time in the presence
and absence of 0.2 M LiNTf
2
. (C) 77 K EPR spectrum recorded after 15 s of irradiation in
the presence of LiNTf
2
. (D) UV−vis spectra pre- and postirradiation, as well as after 5 min
of −0.15 V applied potential in the dark.
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Scheme 1.
Electrophotochemical Organohalide Reduction
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Table 1.
Electrophotocatalytic Benzyl Chloride Reduction
a
Entry
Variation
Yield
b
1
none
90
c
2
no [Cu
2
]
0
3
no light
0
4
no applied potential
2
5
2 equiv. H
2
O
62
6
5 mL air
4 {7}
d
7
LiCIO
4
instead of LiNTf
2
30
8
TBAPF
6
instead of LiNTf
2
10
9
TBANTf
2
instead of LiNTf
2
11
10
chloro-cubane instead of [Cu
2
]
0
11
chloro-diamond instead of [Cu
2
]
0
12
2
→
2-D
77
c
13
3
→
3-D
77
c
14
4
→
4-D
68
c
15
5
→
5-D
81
c
16
6
→
6-D
77
c
17
7
→
7-D
91
c
18
8
→
8-D
75
c
a
Performed for 1.5−3 h with 0.15 mmol of benzyl chloride.
b
Yields of known products determined by
1
H NMR versus CH
2
Br
2
internal standard.
c
Average of two runs.
d
Value for 4-methylbenzaldehyde in braces.
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