Mechanistic basis for tuning iridium hydride photochemistry from H2evolution to hydride transfer hydrodechlorination

Article abstract of DOI:10.1039/d0sc00422g

The photochemistry of metal hydride complexes is dominated by H2 evolution, limiting access to reductive transformations based on photochemical hydride transfer. In this article, the innate H2 evolution photochemistry of the iridium hydride complexes [Cp?Ir(bpy-OMe)H]+ (1, bpy-OMe = 4,4'-dimethoxy-2,2′-bipyridine) and [Cp?Ir(bpy)H]+ (2, bpy = 2,2'-bipyridine) is diverted towards photochemical hydrodechlorination. Net hydride transfer from 1 and 2 to dichloromethane produces chloromethane with high selectivity and exceptional photochemical quantum yield (Φ ≤ 1.3). Thermodynamic and kinetic mechanistic studies are consistent with a non-radical-chain reaction sequence initiated by "self-quenching"electron transfer between excited state and ground state hydride complexes, followed by proton-coupled electron transfer (PCET) hydrodechlorination that outcompetes H-H coupling. This unique photochemical mechanism provides a new hope for the development of light-driven hydride transfer reactions.

Full text of DOI:10.1039/d0sc00422g

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Stratakes, M. Chambers, D. A. Kurtz, C. L. Pitman, J. L. Dempsey and A. J. M. Miller, Chem. Sci., 2020,
DOI: 10.1039/D0SC00422G.
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DOI: 10.1039/D0SC00422G
EDGE ARTICLE
Mechanistic Basis for Tuning Iridium Hydride Photochemistry
from H Evolution to Hydride Transfer Hydrodechlorination
2
a,b
a
a,c
a
Seth M. Barrett, Bethany M. Stratakes, Matthew B. Chambers, Daniel A. Kurtz, Catherine L.
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a,
Pitman, Jillian L. Dempsey, and Alexander J. M. Miller *
DOI: 10.1039/x0xx00000x
2
The photochemistry of metal hydride complexes is dominated by H evolution, limiting access to reductive transformations
based on photochemical hydride transfer. In this article, the innate H
2
evolution photochemistry of the iridium hydride
+
+
complexes [Cp*Ir(bpy-OMe)H] (1, bpy-OMe = 4,4ʹ-dimethoxy-2,2’-bipyridine)and [Cp*Ir(bpy)H] (2, bpy = 2,2ʹ-bipyridine) is
diverted towards photochemical hydrodechlorination. Net hydride transfer from 1 and 2 to dicholoromethane produces
chloromethane with high selectivity and exceptional photochemical quantum yield ( ≤ 1.3). Thermodynamic and kinetic
mechanistic studies are consistent with a non-radical-chain reaction sequence initiated by “self-quenching” electron transfer
between excited state and ground state hydride complexes, followed by proton-coupled electron transfer (PCET)
hydrodechlorination that outcompetes H–H coupling. This unique photochemical mechanism provides a new hope for the
development of light-driven hydride transfer reactions.
include
generation);
coupling reactions);
photoredox cross-coupling catalysis).
metal–halide
complexes
(e.g.
dihalogen
3
,4,22
Introduction
metal–amide complexes (e.g. C–N cross-
2
3–29
and metal–aryl complexes (e.g. dual
The photochemistry of transition metal complexes
underpins a range of applications, including organic light-
1
0,30–32
Complexes with metal–hydride bonds would appear to be
ideal candidates for photocatalytic reductions involving net
hydride transfer to an organic substrate, but examples of
photochemical organic transformations utilizing metal hydride
complexes are scarce. The lack of development is likely due to
a challenge that is unique to metal–hydride photocatalysts:
there is an inherent competition between hydride transfer and
1
,2
emitting diode (OLED) development,
solar energy
The photocatalysts
conversion,3 and photoredox catalysis.
–6
7–11
in these systems usually operate via pathways that involve
excited state single-electron transfer (SET), and careful
photophysical and photochemical studies have established
factors that give rise to photocatalysts with long excited-state
3
3
1
2–15
lifetimes and good long-term stability.
In terms of electronic
H
2
evolution. Indeed, metal hydride complexes are
1
0
structure, a large ligand field splitting (or a d configuration) is
often critical. In terms of chemical structure, complexes that
successfully engage in outer-sphere SET are typically supported
by rigid chelating ligands that are completely inert towards
chemical reactivity.
3
3
quintessential H
application in the synthesis of solar fuels. Understanding how
to tune the photochemical reactivity of metal hydride
2
evolution photocatalysts, with particular
3
complexes between H
2
evolution and hydride transfer
represents a fundamental challenge.
New opportunities can be imagined for photocatalysts that
go beyond SET reactivity to tightly couple light absorption with
bond-forming and bond-breaking chemical reactions.
Photochemical ligand dissociation from transition metal
complexes, such as CO release from carbonyl complexes, has
Iridium hydride complexes of the type [Cp*Ir(bpy-R)H]+
(bpy-R is 4,4'-disubstituted-2,2'-bipyridine) are promising
candidates for exploring the photochemical activation of metal–
3
4
hydride bonds. Photoexcitation generates a weakly emissive
3
5
metal-ligand charge transfer (MLCT) excited state. The high
1
6,17
been the subject of intense study.
Photodissociation can
degree of MLCT character derives from the strong -donor
initiate thermal catalytic reactions via the generation of highly
3
6
ability of the hydride ligand. The photochemistry of these Ir
hydride complexes is dominated by H evolution (Scheme 1).
Photocatalytic H evolution was first reported in aqueous
with subsequent research demonstrating
photoelectrochemical H production and photocatalytic formic
3
,18–21
reactive coordinatively unsaturated intermediates.
Other
2
systems capable of photochemical bond-forming reactions
2
3
7–41
conditions,
a. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill,
North Carolina 27599-3290, United States
2
4
2,43
b.
acid dehydrogenation.
In acetonitrile solvent, photolysis of
Department of Chemistry, Muskingum University, New Concord, OH 43762-1118,
+
United States
[Cp*Ir(bpy)H] (either alone or with added acids) also produces
c.
College of Science, Louisiana State University, Baton Rouge, LA 70803-1804,
44
2
H in high yield.
United States
Email: ajmm@email.unc.edu
Electronic Supplementary Information (ESI) available: Experimental details,
characterization data, and thermochemical analysis. See DOI: 10.1039/x0xx00000x
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2 2
hydrogen atom transfer reduction of CH Cl . Because a single
photon absorption triggers the reactivity of two different metal
centers, the maximum theoretical quantum yield is 2;
experimental conditions were found that achieve  = 1.3. The
View Article Online
DOI: 10.1039/D0SC00422G
2
+
H2O (pH 7)
Ir
H2
+
N
R
h
60 nm
OH2
4
N
R
mechanistic insight into the factors that can suppress H
2
evolution and enable hydride transfer provides general
guidance for future catalyst structures with photocontrolled
reactivity.
+
HOAc
CH3CN
H2
+
Ir
h
N
R
Ir
OAc
+
460 nm
N
Results and discussion
R
Photochemical Hydrodechlorination of Alkyl Chlorides
Ir
N
H
N
Hydride complexes [Cp*Ir(bpy-OMe)H][OTf] (1) and
R
3
+
[
Cp*Ir(bpy)H][OTf] (2) were prepared following previously
D3C
developed approaches.44,46 Hypothesizing that high
[
Cp*Ir(bpy-R)H]+
CD3CN
R
H2 + 1/2
C
Ir
concentrations of a hydride acceptor substrate could promote
hydrodechlorination, dichloromethane was explored as both
substrate and solvent.
N
trace H2O
N
N
h
60 nm
N
N
H
4
R
R
2 2 2
Hydride 1 was dissolved in dichloromethane-d (CD Cl ) and
R
R
+
I–
monitored by NMR spectroscopy while protected from light,
N
+
with only 3% conversion to [Cp*Ir(bpy-OMe)Cl] (3) observed
+
H2N
2 2
after 2 h at 25 ºC. Illumination of 1 in CD Cl with a 460 nm LED
lamp for 10 min produced chloride complex 3 quantitatively
(Scheme 2). A diagnostic 1:2:3:2:1 pentet for dissolved
O
N
+
Ir
CD3CN
H2N
N
I
h
60 nm
N
O
H H
R
4
chloromethane-d
observed (49% yield relative to 1 by H NMR). The excitation
wavelength was chosen to match the MLCT transition (max ca.
2
gas (CHD
2
Cl δ 2.99, JHD = 1.6 Hz) was also
+
H2
1
R
Scheme 1. Photochemical reactivity of iridium hydride complexes [Cp*Ir(bpy-
R)H] (bpy-R is 4,4ʹ-disubstituted-2,2ʹ-bipyridine).
4
3
+
420 nm, Figure S23). When the same experiment was run in
CH
2
Cl
2
, chloromethane (CH
3
Cl) was produced in 69% yield
1
relative to 1 ( H NMR). Some of the volatile chloromethane
Complex [Cp*Ir(bpy)H]+ undergoes H
2
evolution via a
product is lost to the headspace over time, so the solution yields
mechanism that is thus far unique amongst molecular hydrides.
The MLCT excited state undergoes “self-quenching” electron
transfer with a ground state hydride complex, followed by rapid
1
are considered lower limits. Soluble H
2
was not detectable by H
NMR spectroscopy. The gas phase yield of H
2
was less than 4%
relative to 1, according to headspace analysis by gas
chromatography (GC).
4
5
bimolecular H–H coupling. As such, the photochemical
quantum yield for H production approaches unity as the initial
concentration of iridium complex is increased above 20 mM.
Due to the bimolecular mechanism, H is produced in good yield
2
Hydride 2 exhibits similar reactivity, producing chloride
+
complex [Cp*Ir(bpy)Cl] (4; >95% yield) and CHD
2
Cl (43% yield
Cl
Cl
Cl) was produced in 50% yield. When
2
relative to 2) after 10 min of 460 nm illumination in CD
Scheme 2). When the same experiment was run in CH
2
2
even in the absence of external proton sources.
(
2
2
,
+
While prior studies of [Cp*Ir(bpy-R)H] complexes have
chloromethane (CH
3
2
focused on photochemical H evolution, we were surprised to
protected from light, only 6% conversion of 2 was observed
after 1 h.
observe net photochemical hydride transfer to a nicotinamide
4
4
derivative in acetonitrile solvent (Scheme 1). While promising
as the first example of even a stoichiometric light-driven
hydride transfer, this reaction suffered from low yields and
competing H
added). The reaction mechanism has not yet been explored.
Nonetheless, the precedent for both H evolution and hydride
2
evolution (even though no proton source was
2
transfer in the same iridium hydride complex led us to embark
on further studies to elucidate the factors that control reactivity
between H
2
evolution and hydride transfer.
Here we report near-complete suppression of H
2
evolution
and a switch to net hydride transfer in the presence of Scheme 2. Visible light photochemical hydrodechlorination of CD
2 2
Cl .
dichloromethane or other alkyl chlorides. Mechanistic studies
of dichloromethane photoreduction elucidate a self-quenching
To probe the effect of substrate concentration on the
electron transfer pathway that generates reactive selectivity between H2 formation and hydrodechlorination
organometallic intermediates capable of hydride transfer and systematically, photolysis was carried out in various
2
| J. Name., 2012, 00, 1-3
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dichloromethane/acetonitrile mixtures. Figure 1 shows the
View Article Online
DOI: 10.1039/D0SC00422G
1
yields of dissolved CH
spectroscopy after illumination of hydride 1 in CH
mixtures for 10 min. Even when CD CN makes up the majority
of the solvent, the yield of dissolved CH Cl is higher than that of
3
Cl and H
2
determined by H NMR
2 2 3
Cl /CD CN
3
3
2
H .
Scheme 3. Photochemical hydrodechlorination of primary alkyl chlorides.
The selective hydride transfer reactivity of 1 and 2 in
dichloromethane solution draws a sharp contrast to the
efficient H
complexes in acetonitrile or water solutions.
acetonitrile solutions without any acid present, some H
evolution is observed, followed by Ir nucleophilic attack on the
2
evolution reactivity of these same hydride
4
1–44
In pure
2
4
4
solvent to form an iminoacyl complex. In the presence of
acetic acid, triethylammonium, or pyridinium, quantitative H
evolution is observed.
2
Figure 1. Yield of dissolved CH
by H NMR spectroscopy after 10 min photolysis of 1 in CH
3
Cl (filled circles) and H
2
(empty squares) measured
Cl /CD CN mixtures of
Cl and
1
2
2
3
In this context, we also examined chlorobenzene as a
chlorinated substrate that would be unlikely to undergo
varying solvent ratios (where % VolCH2Cl2 is the volume percent of CH
CH Cl ] is the concentration of CH Cl ).
2
2
[
2
2
2
2
4
7
reduction via hydride transfer. When chlorobenzene-d
evolution and (C Cl) solutions of 1 or 2 were illuminated with a 443 nm
hydride transfer, reactions were carried out in the presence of lamp, a signal for H
acid. Illumination of hydride complex 1 in CD Cl
solutions without evidence for the hydrodechlorination product benzene.
containing 1 equiv acetic acid generated the chloride complex 3 Photolysis of a 2 mM solution of 2 in C Cl for 35 min produced
in 52% yield (GC). An unidentified iridium-containing yellow
Even in the presence of acid, selective hydrodechlorination of precipitate formed in these reactions. The results in
dichloromethane is observed.
chlorobenzene underscore the innate H evolution reactivity of
5
To further probe the competition between H
2
6 5
D
1
2
was detected by H NMR spectroscopy
2
2
6 5
H
4
4
—
and not the acetate complex expected upon H
2
release.
H
2
2
The tolerance of proton donors suggested that an aqueous 1 and 2 in the absence of a suitable hydride acceptor, even
formate solution could close a catalytic cycle by regenerating when no proton donor is added.
the iridium hydride after initial hydride transfer to the
substrate. Illumination of biphasic mixtures of chloride complex hydride transfer reactivity, we carried out a series of
in CD Cl and 1 M formate in H
O indeed produced thermodynamic and kinetic studies to examine the detailed
chloromethane (TON > 2, see Section III in the SI for details).
mechanism of photochemical hydrodechlorination by iridium
To understand the origin of the selective photochemical
3
2
2
2
The reactivity can also be extended to other simple hydride complexes, focusing on the highest yielding substrate,
chlorinated organic substrates dissolved in acetonitrile. Three dichloromethane.
model substrates were selected, 2-chloroethylbenzene, 3-
chloropropiophenone, and 2-chloroethoxybenzene. As shown Overview of Mechanistic Pathways Considered
3
in Scheme 3, illumination of CD CN solutions of 1 and the
Scheme 4 shows several possible reaction pathways for
photochemical hydrodechlorination. The pathways can be
divided into two categories: in monometallic pathways, the
excited state of 1 (1*) interacts with CH Cl directly; in bimetallic
2 2
pathways, the excited state undergoes a “self-quenching”
primary alkyl chloride substrate produced the dechlorinated
1
products in approximately 20% yield relative to 1 by H NMR
2
spectroscopy (with the balance presumably producing H gas,
detected in solution in up to 66% yield; no other organic
products were detected).
process with an equivalent of 1 in the ground state before
3
1,45,48–50
2 2
interacting with CH Cl .
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J. Name., 2013, 00, 1-3 | 3
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The role of CH
photoluminescence lifetime of 1* in solutions of CH CN with
3
2
Cl
2
was probed by analyzing the
View Article Online
DOI: 10.1039/D0SC00422G
Monometallic pathways
CH Cl
SET
2
2
2
Path A
HAT
IrCl+
2 2
varying concentration of CH Cl (and a constant 1 mM
CH Cl + Cl– + IrH2+
•
CH Cl
3
2
–
concentration of 1). As shown in Figure 2, the lifetime of 1* is
essentially invariant up to 8 M CH Cl , suggesting that
monometallic pathways A and B that involve CH Cl quenching
CH Cl
2
2
2
Path B HIT
CH Cl + Cl– + Ir2+
CH3Cl
3
IrCl+
–
2
2
are not operative. While the lifetime decreases above 8 M (50
vol%) CH Cl , the lack of a linear correlation in the Stern-Volmer
CH Cl2
2
Path C
+ HCl
– IrCl+
Ir0
IrCH Cl+
2
2
CH3Cl
–
H+
– Cl–
2
1
*
analysis (Figure 2) shows that the change in lifetime is not
related to a diffusional quenching process. The change in
lifetime is instead attributed to solvation effects changing with
solvent composition, as has been observed for other charge-
IrH+*
Bimetallic pathways
+
2
IrH
IrH+
2
CH Cl2
IrH0
HAT
2
Ir+
2•CH Cl
2CH Cl
3
2
IrH2+ – H2
– 2IrCl+
Path D
4
9,54–57
transfer-based excited states.
CH Cl2
HIT
+ Cl–
HAT
– IrCl+
2
IrH+
CH2Cl2
IrH0
Ir+
IrH2+
•CH Cl
2
CH Cl
–
IrH2+
3
IrH2+ – Cl
IrCl+
Path E
–
–
CH Cl
3
Scheme 4. Photochemical hydrodechlorination pathways considered.
Monometallic pathways could proceed via Path A, single
electron transfer (SET) followed by hydrogen atom transfer
(
HAT); via Path B, hydride ion transfer (HIT); or via Path C,
+
51
photoacid generation of H and a nucleophilic Ir(I) center.
Complex 2 catalyzes thermal hydrodehalogenation of alkyl
bromides (but not alkyl chlorides) via a HIT mechanism.
5
2
Bimetallic pathways could proceed via initial self-quenching
electron transfer to form a reactive pair of Ir(II) and Ir(IV)
hydrides,45 with Path D proceeding by a radical chain reaction;
or the PCET Path E proceeding by stoichiometric HIT and HAT
steps. Radical chain hydrodehalogenation is observed for
reagents such as Bu
3
SnH, which requires a radical initiator to
Figure 2. Evaluation of excited state quenching of [Cp*Ir(bpy-OMe)H][PF
Cl by time-resolved photoluminescence spectroscopy. Observed lifetimes (,
light gray left arrows) and Stern-Volmer analysis ( = 80 ns, dark gray right arrows)
in CH Cl /CH CN mixtures at a constant 1 mM concentration of 1. Excitation at 445
nm, luminescence decay monitored at 650 nm. In the axes, % VolCH2Cl2 is the
volume percent of CH Cl and [CH Cl ] is the concentration of CH Cl
6
] (1) by
5
3
generate •CH
hydride could undergo photochemical H
Ir(II) species that could undergo Cl-atom abstraction from
CH Cl to generate •CH Cl, which in turn could react with an Ir
hydride to furnish CH Cl and initiate a new chain (Path D). The
alternative bimetallic pathway would involve hydride transfer
to CH Cl from the neutral hydride complex Cp*Ir(bpy-R)H,
followed by Cl• abstraction from a second CH Cl and finally
2
Cl. In the present case, a small amount of Ir CH
2
2
release, generating an
o
2
2
2
3
2
2
2
2
2
2
2
2
2
.
3
The other monometallic pathway, Path C, involves initial
2
2
+
photochemical H release to form Cp*Ir(bpy) (5) followed by Ir
2
2
nucleophilic attack on CH
intermediate [Cp*Ir(bpy)(CH
protonolysis to release CH
2
Cl
2
to form a chloromethyliridium
Cl)] (6) that could undergo
2
+
HAT from the oxidized hydride [Cp*Ir(bpy-R)H] (Path E).
Thermochemical analyses of the pathways shown in Scheme
+
2
3
Cl. This pathway could not be ruled
4
were carried out, as detailed in Section V in the SI. Each
out by the Stern-Volmer quenching experiment, but was
discounted based on independent reactivity of complexes 5 and
pathway appears to be thermodynamically viable. Some steps
are close to ergoneutral, however, and other steps are likely to
face significant kinetic challenges. To distinguish between these
viable pathways, a kinetic analysis of the photochemical
hydrodechlorination reaction was undertaken.
6
.
Dissolving
5
in CH
2
Cl
2
leads to formation of
chloromethyliridium complex 6 within 20 min at room
temperature (Scheme 5). Chloromethyl complex 6 is quite
stable, however. Treatment with various acids does not
generate significant amounts of CH
3
Cl, even under illumination
Photoluminescence Quenching Kinetic Studies
(Scheme 5). The chloromethyl complex 6 is there not kinetically
The kinetics of excited state quenching, probed by time- competent to carry out the observed reactivity.
resolved photoluminescence measurements, can differentiate
amongst the pathways considered in Scheme 4. In
monometallic pathways, the luminescence lifetime should
2 2
shorten with increasing concentration of CH Cl . In bimetallic
pathways, the luminescence lifetime should shorten with
increasing concentration of iridium hydride.
4
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A self-quenching mechanism should give rise to strongly
concentration-dependent photochemistry. Figure 4 presents
+
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DOI: 10.1039/D0SC00422G
CH2Cl2
20 min
acids
the quantum yields for consumption of 1 (based on UV-vis
spectroscopic monitoring of the initial rate, see SI for details)
upon photolysis with 443 nm light in CH Cl , revealing a
2 2
Ir
Ir
trace
CHCl3
N
Cl
C
h
60 nm
N
N
H2
N
4
dramatic dependence on iridium concentration that reaches a
maximum exceeding unity,  = 1.3. The linear increase in
quantum yield with increasing iridium concentration up to 8
mM 1 is further evidence of a bimetallic mechanism. The
maximum quantum yield exceeding unity suggests either a
photochemically triggered radical chain reaction (Path D) or a
bimetallic self-quenching pathway that generates two
equivalents of chloromethane per photon absorbed (Path E).
5
6
Scheme 5. Chloromethyl complex 6 was synthesized and shown to be kinetically
incompetent for CH Cl reduction.
2
2
To assess the possibility of a bimetallic self-quenching
pathway in which excited state 1* is quenched by hydride 1, the
luminescence lifetime of 1* in CH
concentration of 1 was increased from 0.094 mM to 31 mM
Figure 3). The luminescence lifetime decreases with increasing
concentration of 1, from which the intrinsic lifetime,  = 50 ns,
can be obtained by extrapolation to infinite dilution (Figure 3).
The intrinsic lifetime of 1 in CH Cl is shorter than the intrinsic
lifetime of 2 in CH CN with 0.1 M [nBu
2 2
Cl was evaluated as the
(
o
2
2
4
5
3
4 6 o
][PF ] ( = 98 ns),
consistent with the radiative decay rates being affected by the
solvent composition.
Figure 4. Quantum yield of photochemical CH
2 2
Cl hydrodechlorination (443 nm
illumination) by [Cp*Ir(bpy-OMe)H][PF ] (1) as a function of concentration of 1.
6
Distinguishing Between Bimetallic Pathways
Of the mechanisms considered in Scheme 4, only Paths D and E
can explain the bimetallic reactivity with a quantum yield
greater than 1. In each pathway, photoexcitation of 1 is
followed by self-quenching electron transfer to generate a pair
6
Figure 3. Evaluation of excited state self-quenching in [Cp*Ir(bpy-OMe)H][PF ] (1)
by time-resolved photoluminescence spectroscopy. Observed lifetimes (, light of Ir(IV) and Ir(II) hydrides. In the radical chain mechanism of
gray left arrows) and Stern-Volmer analysis (
= 54 ns, dark gray right arrows) in Path D, photochemical H release would generate an Ir(II)
CH Cl . Excitation at 445 nm, luminescence decay monitored at 650 nm. The linear
fit corresponds to a k
o
2
2
2
intermediate capable of abstracting a chlorine atom from
CH Cl , which would initiate radical hydrodehalogenation
reminiscent of Bu The quantum yield for this
pathway could be much higher than unity, depending on the
rates of chain propagation and termination. In the
stoichiometric mechanism of Path E, the Ir(II) hydride formed
8
–1 –1
q
= 6.6 × 10 M ·s .
2
2
4
7,53
3
SnH reactivity.
A linear correlation in the Stern-Volmer analysis varying the
5
8
concentration of 1 (Figure 3) is consistent with a bimetallic self-
8
–1 –1
quenching pathway, with k
quenching in CH Cl is notably slower than in CH
0 M ·s ), perhaps due to the relatively low polarity of
CH Cl disfavoring the reaction of two cationic metal complexes.
Although the net photochemistry is starkly different in CH CN
evolution) and CH Cl (hydrodechlorination), the Ir hydrides
q
= 6.6 × 10 M ·s . The rate of self-
2
2
3
CN (k = 3.8 ×
q
after self-quenching would hydrodechlorinate CH
hydride transfer. The resulting Ir(II) intermediate would
abstract a chlorine atom from CH Cl , followed by H atom
2 2
Cl by
9
–1 –1 45
1
2
2
2
2
3
transfer from the Ir(IV) hydride. This mechanism would have a
theoretical quantum yield of 2.
(
H
2
2
2
undergo the same initial self-quenching reaction in each
solvent. The self-quenching step is estimated to be slightly
thermodynamically unfavorable (by about 5 kcal/mol on the
basis of excited state redox potentials, Figure S19). Other
examples of excited state self-quenching are also close to
ergoneutral, including a recent nickel example that is
To probe the viability of chlorine atom abstraction from
CH Cl by an Ir(II) intermediate, cyclic voltammograms of
2 2
[
[
3
Cp*Ir(bpy)(NCCH )][PF
]
6 2
were obtained in CH
2
Cl
2
(0.25 M
n
4 6
Bu N][PF ]). At scan rates above 1 V/s, a single irreversible
reduction feature was observed. At slower scan rates, however,
a feature attributed to the reduction of chloride complex 4 was
observed (Figure S24), consistent with formation of 4 upon
3
1,45,48–50
substantially endergonic.
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Page 6 of 8
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Chemical Science
reduction of [Cp*Ir(bpy)(NCCH
confirmed as 4 based on H NMR spectroscopic analysis of the photochemical hydrodechlorination via Path E. Scheme 7
3
)][PF
6
]
2
. The product was
The collected mechanistic data is most consistent with a
View Article Online
DOI: 10.1039/D0SC00422G
1
products produced during controlled potential electrolysis of depicts the sequence. Photoexcitation is followed by
[
Cp*Ir(bpy)(NCCH
3
)][PF
6
]
2
in CH
2
Cl
2
(Figure S25). These results bimolecular self-quenching electron transfer and then rapid,
+
indicate that the reduced Ir species (Ir in Scheme 4, above) can sequential hydride ion transfer and hydrogen atom transfer to
2 2 2 2
abstract a chlorine atom from CH Cl . A mechanism for this CH Cl . Key observations consistent with this mechanism are (a)
reaction is proposed in Scheme 6. The formally Ir(II) the quantum yield that approaches (but does not exceed) the
intermediate in Scheme 6 has been proposed previously in theoretical limit of 2; (b) the bimolecular self-quenching of the
CH
3
CN solution, where further reduction or disproportionation excited state indicated in iridium-dependent Stern-Volmer
5
9,60
steps produce the formally Ir(I) complex 5.
quenching studies; and (c) the highly selective reactivity without
evident hallmarks of freely propagating radical chains. We
2
+
+
speculate that the H–H bond formation reaction in the H
evolution pathway occurs within the solvent cage, based on the
highly exergonic and efficient H release step that follows self-
2
e–
CH2Cl2
Ir
[Cp*Ir(bpy)]+
Ir
N
N
N
2
Cl
–
NCCH3
+
C
Ir
N
CH3
N
quenching. By working in a substrate-rich reaction medium,
part or all of the solvent cage itself becomes a reactive
substrate, enabling PCET reactions between the reactive pair of
e– CH CN
3
Cp*Ir(bpy)
4
5
2 2
metal hydrides and the surrounding CH Cl .
Scheme 6. Proposed mechanism of electrochemically triggered formation of 4.
A key difference between Paths D and E is that the radical Conclusions
2
chain pathway (D) requires a freely diffusing •CH Cl radical,
Tuning the reaction medium and conditions provides a method
for changing the reactivity of an iridium hydride complex from
whereas the HIT/HAT pathway (E) does not. Two key pieces of
evidence are inconsistent with Path D. First, the reaction is not
inhibited by hydroquinone (Figure S29), which has been
reported to strongly inhibit radical chain hydrodehalogenation
H
2
evolution towards hydride transfer. Photochemical
reduction of dichloromethane to chloromethane is achieved
with high quantum yield, even exceeding unity as the
concentration of the hydride complex is increased. In neat
5
3,61,62
mediated by tin hydrides.
Second, a freely diffusing
•
CH Cl radical would be expected to react with
2
dichloromethane solvent, H
pathway in water and acetonitrile, even in the absence of acids
is completely suppressed. The high selectivity for hydride
transfer is significant because most other transition metal
hydride complexes produce only H
2
evolution — the dominant
dichloromethane or acetonitrile solvent, but none of the
products expected from radical reactivity — chloroform,
isotopologues of dichloromethane and chloromethane, or
—
chloromethyl complex 6 — are observed. For comparison, •CH
3
2
.
free radical formation by photolysis of the iridium methyl
Mechanistic studies of hydrodehalogenation are consistent
with photochemical access to a triplet metal-ligand charge
transfer excited state that undergoes self-quenching in a
bimolecular reaction. The resulting reactive pair of oxidized and
reduced organometallic hydrides is proposed to undergo
sequential hydride ion transfer (HIT) and hydrogen atom
transfer (HAT) PCET reactions with two dichloromethane
molecules. Therefore, the quantum yield would have a
theoretical maximum of two, because two equivalents of
chloromethane could be produced per absorbed photon.
The highly efficient reduction is noteworthy because alkyl
+
complex [Cp*Ir(bpy)(CH
3
)] gave ethane via C–C radical coupling
6
3
as well as H and D atom abstraction from solvents.
H evolution path
2
CH Cl
CH Cl
2
2
2
2
IrH+
IrH0
IrH2+
Ir0
Ir2+
IrH+*
+
H2
SET
self-
quench
CH Cl
CH Cl
2
2
2
2
–
Cl– HIT
CH Cl
3
chlorides are traditionally challenging substrates in photoredox
IrH+
Ir+
IrH2+
7,8,64–66
catalysis reactions that rely on SET pathways.
The
Hydro-
dechlorination
path
unusual reactivity with unactivated alkyl chlorides is attributed
to the incorporation of a reactive metal–hydride bond in the
photocatalyst, which engenders a long-lived charge-transfer
excited state and enables a self-quenching pathway that
effectively couples light absorption and bond-forming PCET
CH Cl
2
2
CH Cl
CH Cl
IrCl+
IrCl+
HAT
3
3
+ Cl–
IrCl+
IrH2+
36,45
steps.
•
CH Cl
CH3Cl
Although the net reaction is a hydride transfer from iridium
2
2 2
to CH Cl , two different C–H bond-forming pathways are
Scheme 7. Proposed mechanisms of competing photochemical
hydrodechlorination and H evolution.
2
involved in the transformation. The possibility of direct excited
state hydride transfer was considered, but despite the
extremely potent thermodynamic hydricity predicted for the
4
4
excited state of 1 (∆GºH– = 14 kcal/mol in MeCN), the kinetic
6
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Chemical Science
ARTICLE
study shows that self-quenching electron transfer occurs
preferentially. Then, hydride transfer occurs from the reduced
hydride intermediate, Cp*Ir(bpy)H, with estimated ∆GºH– = 45
kcal/mol in MeCN (see SI Section V). Although not as potent as
the excited state hydride, neutral complex Cp*Ir(bpy)H is
predicted to be almost 20 kcal/mol more hydridic than complex
566.
View Article Online
DOI: 10.1039/D0SC00422G
7
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4
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(∆GºH– = 62 kcal/mol in MeCN). The second C–H bond-
forming pathway involves HAT, rather than hydride transfer.
The present mechanistic study offers insight into future
designs. The presence of reactive metal–hydride bonds is a key 11
feature of this system, enabling rapid PCET reactivity
immediately after excited state electron transfer. The step that 12
10
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H–H coupling can occur. Alternatively, the H–H coupling step
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Acknowledgements
This work was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Award
No. DE-SC0014255. S.M.B. acknowledges support from a NSF
Graduate Research Fellowship (DGE-1144081). C.L.P.
acknowledges the support of the Royster Society of Fellows.
D.A.K. acknowledges the support of the Kenan Graduate
Fellowship. Hydrogen gas detection was performed using a GC
in the AMPED EFRC Instrumentation Facility established by the
Alliance for Molecular PhotoElectrode Design for Solar Fuels,
an Energy Frontier Research Center (EFRC) funded by the U.S.
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