Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

Article abstract of DOI:10.1039/d0sc01820a

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer. This journal is

Full text of DOI:10.1039/d0sc01820a

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Photo-triggered hydrogen atom transfer from an
iridium hydride complex to unactivated olefins†
Cite this: DOI: 10.1039/d0sc01820a
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have been paid for by the Royal Society
of Chemistry
Mirjam R. Schreier, Bjorn Pfund, Xingwei Guo ‡ and Oliver S. Wenger
¨
Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but
hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely
used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation
at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic
hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium
hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily
hydrogenated, and structure–reactivity relationships established with 12 different olefins are in line with
typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical
clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight
and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom
transfer (photo-HAT). The catalytically active species is identified as an Ir(II) hydride with an IrII–H bond
dissociation free energy around 44 kcal molꢀ1, which is formed after reductive 3MLCT excited-state
quenching of the corresponding Ir(III) hydride, i.e. the actual HAT step occurs on the ground-state
potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel
approach to photocatalysis with metal complexes, which is fundamentally different from the many prior
studies relying on photoinduced electron transfer.
Received 30th March 2020
Accepted 27th July 2020
DOI: 10.1039/d0sc01820a
rsc.li/chemical-science
polar functional groups that can form hydrogen bonds.23
Similarly, the interaction of Lewis acids with polar functional
1 Introduction
groups can help activate substrates that would be difficult to
engage in pure PET chemistry.24–27
Photoinduced electron transfer (PET) can readily be initiated
with many photoactive metal complexes, but photo-triggered
hydrogen atom transfer (photo-HAT) reactivity of coordination
compounds is comparatively rare.1 Much progress has been
made in recent years regarding the development of new
photosensitizers, particularly including complexes of Earth-
abundant metals2–6 and purely organic (metal-free) variants,7
but they all exhibit the well-established PET reactivity. Conse-
quently, the vast majority of current photochemical studies,
including most of the work by the photoredox community,8 still
operate on the basis of PET. Even though increasingly ther-
modynamically challenging PET reactions have been accom-
plished lately,9–18 sometimes in combination with an applied
electrochemical potential,19–21 such reactions are limited by the
redox properties of the photosensitizer and the substrates. To
overcome thermodynamic limitations imposed by PET, proton-
coupled electron transfer (PCET)22 has been exploited success-
fully in photoredox catalysis for substrates with sufficiently
We hypothesized that the photochemical hydrogenation of
olens would be an ideal test ground for exploring the photo-
HAT reactivity of a metal complex. Whilst alkene hydrogena-
tion is of course an extremely well-developed eld, photo-
chemical methods are very scarce and largely limited to
substrates with strongly electron-withdrawing substituents to
permit typical PET chemistry,28 or to the reduction with solvated
electrons.29,30 For the present work, we chose olens that do not
contain electron-withdrawing groups and which therefore
cannot be activated by PET due to their very negative reduction
potentials (Ered < ꢀ2.7 V vs. SCE31). From the perspective of PET,
these olens are unactivated.
HAT is a key process in many photochemical reactions,32,33
but usually requires auxiliary reagents such as thiols or ascor-
bate which act as H-atom donors in a secondary, light-
independent step that does not involve the photosensi-
tizer.34–37 Photoreactions, in which a metal complex donates or
accepts an H-atom in an electronically excited state, are
rare.33,38–43 For the conceptually related proton-coupled electron
transfer (PCET) reactions, there exist numerous compounds
which exhibit PCET reactivity from electronically excited states,
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel,
Switzerland. E-mail: xingwei_guo@mail.tsinghua.edu.cn; oliver.wenger@unibas.ch
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0sc01820a
‡ Current address: Center of Basic Molecular Science (CBMS), Department of
Chemistry, Tsinghua University, Beijing 100084, China.
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but these systems typically rely on polar O–H or N–H groups reports of photo-triggered HAT from metal complexes to unac-
which can form hydrogen bonds.44–47
tivated olens. The reductive excited-state quenching step
The HAT reactions considered herein do not proceed directly involved in our photo-triggered HAT gives access to a particu-
from an electronically excited state, but instead occur aer an larly reactive metal hydride with a very low M–H bond dissoci-
initial reductive excited-state quenching process with a tertiary ation free energy (BDFE), and this enables uncommon
amine. The sequence of reductive excited-state quenching fol- photochemistry that goes beyond the well-established single
lowed by single-electron transfer to the substrate is rather electron transfer (SET) reactivity. From the synthetic viewpoint,
common in organic photoredox catalysis as well as energy- our approach offers the possibility to form C–H bonds, com-
related photochemical research, but here, the situation is plementing recent work on photo-triggered PCET that focused
conceptually different: reductive quenching leads to an H-atom on the cleavage of C–H bonds.52,53
donor, which can then undergo HAT with olens. Thermal HAT
Metal hydride complexes play important roles for proton and
from metal complexes in their electronic ground state has CO2 reduction,54,55 and recently tungsten hydrides provided
attracted signicant attention48,49 and homolytic splitting of fundamental insight into multi-site PCET reactions.56–58 To
metal-hydride bonds upon excitation with visible light is known some extent their reactivity can be regarded as HAT-like, but
for a few selected examples,50,51 but we are unaware of prior with coupled proton and electron transfer reactions occurring
Fig. 1 Different photo-reactivities: iridium hydrides can undergo excited-state proton transfer60 or photodriven dihydrogen formation.61,65,67
Herein we demonstrate that photo-HAT represents an additional reaction pathway for iridium hydride complexes, complementing recent work
demonstrating sequential hydride and HAT transfer.64
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to separate base and oxidant molecules.22 Iridium cyclo- (MHAT)48,68 is typically performed with metal-hydrides that have
pentadienyl hydride complexes are a particularly well investi- BDFEs ranging from z50 to 60 kcal molꢀ1
.
Our results
70,71
gated class of compounds,59 which have been shown to presented below indicate that the key-step in our photoinduced
function as photoacids60 (Fig. 1). Furthermore, they are able to hydrogenation of olens (Fig. 2) is indeed an initial HAT from
form H2 upon irradiation with visible light61,65 or when the [IrII–H] species to the substrate. Aer successful activation
used in photo-electrocatalytic settings in water or CH3CN.66 of the olen starting material (SM) via photo-triggered HAT,
“Self-quenching” reactions, in which an electron is a radical intermediate (RIc) is formed. The latter can undergo
transferred from an excited [Cp*Ir(a-diimine)H]+ complex (Cp* two different onward reactions: (i) either RIc can accept a second
¼ pentamethylcyclopentadienyl) to an iridium hydride in the H-atom to form the hydrogenation product H, or (ii) an H-atom
ground state, are responsible for photochemical H2 formation can be abstracted from a different position of radical interme-
in water and CH3CN.67 By contrast, in neat CH2Cl2 the “self- diate RIc to give the rearranged products (E)-R or (Z)-R.69,72 In
quenching” products (comprised of one oxidized iridium this work, we attempted to maximize the hydrogenation and to
hydride complex and one reduced iridium hydride complex) minimize the rearrangement yield, but the main point of the
undergo sequential hydride transfer and HAT, enabling the study was to establish a new type of photo-reactivity of metal
hydrodechlorination of two equivalents of CH2Cl2 to CH3Cl.64
complexes, as noted above.
In the present work, [Cp*Ir(phen)(H)]+ (phen ¼ 1,10-phe-
A very recent study reported on the photoenzymatic hydro-
nanthroline) was formed in situ via photo-irradiation of genation of vinyl pyridines, using “ene” reductases in combi-
[Cp*Ir(phen)Cl]+ in CH3CN in presence of excess triethylamine nation with Ru(bpy)3Cl2 as a catalytic system (with some modest
(TEA), which served as a combined electron and proton source. product yields also observable in absence of the enzyme).73
Continued irradiation then promoted [Cp*Ir(phen)(H)]+ to its Mechanistically, this formal hydrogenation proceeded via
(relatively long-lived) 3MLCT state. In presence of 250 mM TEA, a sequence of reduction, protonation and HAT steps. Such
reductive excited-state quenching to form [Cp*Ir(phen)(H)]0 combined enzyme-photoredox catalysis approaches hold much
rather than “self-quenching” is the dominant reaction pathway. promise for enantioselective conversions, but they are not
For the closely related [Cp*Ir(bpy)(H)]0 complex (bpy ¼ 2,20- straightforward to implement.74–76
bipyridine), an IrII–H bond dissociation free energy (BDFE) of
Our iridium complex serves both as photosensitizer and as
catalyst of HAT. Due to this dual role, no ne tuning of different
43.9 kcal molꢀ1 was reported previously.67
We anticipated that such a low IrII–H BDFE would make catalytic cycles is needed,77 as is sometimes necessary when
[IrII–H] complexes suitable for olen hydrogenation via photo- merging photoredox chemistry with traditional transition metal
triggered HAT. Activation of olens by thermal (i.e., not light- catalysis or with enzyme catalysts. The hydride complex is
driven) metal-hydride catalyzed hydrogen atom transfer formed in situ from a robust Ir(III) precursor complex, and HAT
Fig. 2 Visible-light driven catalytic hydrogenation of electron-neutral olefins based on photo-triggered hydrogen atom transfer (photo-HAT)
from an iridium(II) hydride (Cp* ¼ pentamethylcyclopentadienyl; phen ¼ 1,10-phenanthroline), see text for details.
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occurs to stable olen substrates and does not require highly
reactive radicals as reaction partners. Thus, alongside the
abovementioned very recent study by Miller, Dempsey, and
coworkers,64 our work opens a conceptually new avenue to
photoredox chemistry, exploiting visible-light excitation of
a simple metal complex for inducing H-atom rather than elec-
tron transfer.
2.2 Substrate scope
As noted in the introduction, the olen hydrogenation reaction
mainly serves as a test ground to establish a new type of metal
hydride photo-reactivity, but nevertheless it seemed useful to
explore its substrate scope in the interest of gaining a reactivity
picture that is as complete as possible. We therefore probed
a range of substrates (Fig. 3) and determined hydrogenation
and rearrangement product yields by a combination of quan-
titative NMR and GC/MS analyses in presence of an internal
standard. We focused mostly on geminal disubstituted olens,
because the lack of substituents on their terminal olenic
carbon atom permits for faster initial HAT than what would be
possible with other olens.78 Furthermore, the tertiary radical
intermediate formed aer initial HAT to this substrate class can
be rather stable, particularly when using aromatic geminal
disubstituted olens, which result in benzylic radical interme-
diates.78 Consequently, the new method can be applied to
a range of substrates of this type (1-SM–5-SM), tolerating both
electron-withdrawing and electron-donating substituents and
affording the different hydrogenation products in high yields
(88–99%) and with high chemoselectivities (H/R ratios
exceeding 12.7 : 1). Both para- and meta-substituents on the
aromatic ring are well tolerated, and no dehalogenation was
observed for both chloro-substituted substrates (4-SM and 5-
SM), as opposed to what would be expected for PET with strong
photoreductants.10,79–81
2 Results and discussion
2.1 Reaction optimization
We started our reaction screening with (3-methoxyprop-1-en-2-
yl)benzene (1-SM) as substrate, because geminal disubstituted
olens with aromatic substituents are particularly well-suited
acceptors for HAT.78 In an initial experiment, [Cp*Ir(phen)Cl]
Cl was used as iridium hydride precursor and diisopropyle-
thylamine (DIPEA) was employed as sacricial donor. 85%
conversion of 1-SM was observed when irradiating the reaction
mixture at 470 nm for 44 h (Table 1, entry 1) under inert
atmosphere. However, the chemoselectivity of the reaction was
limited, as expressed by a 1-H/1-R product ratio of 2.8 : 1. When
the sacricial donor was changed to triethylamine (TEA), both
the conversion (90%) and the chemoselectivity (1-H/1-R 3.2 : 1)
of the reaction were slightly increased (entry 2). Upon addition
of excess phenanthroline (30 mol%), the chemoselectivity of the
reaction improved drastically in favor of the hydrogenation
product 1-H (1-H/1-R 14.7 : 1, entry 3) and the conversion
further increased (94%). When the reaction was performed in
the dark (but otherwise identical conditions), no conversion of
1-SM was observed (entry 4). Moreover, no conversion of 1-SM
was detected when either the iridium catalyst (entry 5) or the
sacricial donor (entry 6) was absent.
The conversion of substrate 6-SM with its free hydroxyl-
group was considerably lower, affording the hydrogenation
product in only 40% yield. This observation suggests that the
catalytic turnover is slowed down by coordinating substituents
on the substrate, even if the substrate coordinates only weakly
and reversibly, as expected for 6-SM. When using the mesityl-
Table 1 Reaction optimization and control reactions. For simplicity, only the (E)-isomer of the rearranged products (1-R) is shown
Entrya
Catalyst
Sacricial donor
Additive (mol%)
Conversionb [%]
1-H/1-Rb
1
[Cp*Ir(phen)Cl]Cl
[Cp*Ir(phen)Cl]Cl
[Cp*Ir(phen)Cl]Cl
[Cp*Ir(phen)Cl]Cl
—
DIPEA
TEA
TEA
TEA
TEA
—
—
—
85
90
94
0
0
0
2.8 : 1
3.2 : 1
14.7 : 1
n.d.
n.d.
n.d.
2c
3c
4d
5
phen (30)
—
—
—
6
[Cp*Ir(phen)Cl]Cl
a
Reaction conditions: 1-SM (50 mM), [Cp*Ir(phen)Cl]Cl (2.5 mM), sacricial donor (250 mM) and 1,10-phenanthroline (0 or 15 mM) in 0.5 mL
degassed CD3CN. Irradiation of the sample occurred at 470 nm (7.5 W) for 44 h, while the temperature was kept at 50 ꢁC. See ESI Section 1 for
details regarding the photoreactor and ESI Section 3.1 for experimental details. Conversions and H/R product ratios were determined by 1H-
b
c
NMR spectroscopy. See ESI Section 3.1.1 for details. Product formation was conrmed by GC/MS spectrometry. See ESI Section 3.1.2 for
details. d Reaction was performed in the dark.
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Fig. 3 Substrate scope for the photo-triggered hydrogenation of olefins promoted by an iridium hydride. For simplicity, only one isomer of the
rearranged products (R) is shown. Reaction conditions: substrate (50 mM), [Cp*Ir(phen)Cl]Cl (2.5 mM), TEA (250 mM) and 1,10-phenanthroline
(15 mM) in 0.5 mL degassed CD3CN. Irradiation of the sample occurred at 470 nm (7.5 W) for 44 h, while the temperature was kept at 50 ꢁC, see
Section 1 of the ESI† for details regarding the photoreactor. Yields and conversions were determined by 1H-NMR spectroscopy and the yields of
the hydrogenation products (1-H to 5-H, 8-H, 9-H and 11-H) were likewise confirmed by GC-MS spectrometry. See ESI Section 3.2† for details. In
the case of product 8-R, R means isomerization to cis-1-phenyl-1-propene. In the case of products 6-H/6-R, 10-H/10-R, and 12-H/12-R the
product conversion could not be determined by GC-MS spectrometry. Therefore, the yields for these substrates were determined based on 1H-
NMR spectroscopy exclusively.
substituted substrate 7-SM the reaction was completely shut terminal olens is in line with prior studies, which found that
down. The combined observations made for substrates 6-SM an additional methyl substituent at the terminal carbon of 1,1-
and 7-SM suggest that coordination to the Ir center is undesir- diphenylethylene slows down the thermal HAT from
able, but sterically very demanding substituents can impede CpCr(CO)3H to various olens by about 800 times.78 Further-
HAT completely.
more, with 8-SM the formation of the hydrogenation product
To explore whether our method can also be applied to was less favored than for 1-SM–6-SM. Signicant isomerization
internal olens, trans-1-phenyl-1-propene (8-SM) was tested as to cis-1-phenyl-1-propene was observed, whereas no isomeriza-
substrate, affording propylbenzene (8-H) in 38% yield. The tion to the thermodynamically less favored terminal olen
lower conversion of this internal olen in comparison to (allylbenzene) was detected.
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To gain insight into how the stability of the presumed radical Clꢀ, Hꢀ) do not have this much reducing power (see ESI Section
intermediates inuences the outcome of the overall reaction, 4.1† for details),67 hence PET is not a viable reaction pathway.
a series of terminal olens (9-SM–12-SM) with different non-
aromatic substituents at the disubstituted olenic carbon
atom was investigated. The hydrogenation yield decreases along
the series 1-SM > 9-SM > 11-SM > 12-SM, reecting the stability
of the different radicals formed aer initial HAT to these
substrates along the substituent series Ph > COOMe > CH3 > H
(Fig. 4A).78 Furthermore, the chemoselectivity of the reaction is
highly dependent on the stability of the formed radical inter-
mediate. While the hydrogenation product is the major product
for all investigated aromatic substrates (1-SM–6-SM), the
formation of the rearranged product becomes increasingly
important for aliphatic olens. Specically, H/R ratios decrease
from ca. 15 : 1 for 1-SM to roughly 2 : 1 for 9-SM, and nally
about 1 : 3 for 12-SM. This observation reects the fact that for
aliphatic substrates, the newly formed C–H bond in the radical
intermediate (RIc in Fig. 2) is roughly 10 kcal molꢀ1 (ref. 82 and
83) weaker compared to aromatic substrates (see ESI Section
4.1† for details). Consequently, H-atom abstraction to yield
rearranged products is faster for aliphatic (e.g., 11-SM or 12-SM)
than for aromatic substrates (e.g., 1-SM).
2.3 Monitoring the reaction progress over time
Further mechanistic insight was gained by monitoring the
reaction progress at different reaction times of photo-
irradiation (Fig. 5A), using the same conditions as for the
substrates in Fig. 3. Continuous hydrogenation of the olen
substrate (1-SM) was observed over the rst 20 h, and a nal
conversion of 94% was reached aer 44 h (Fig. 5B, blue trace).
Interestingly, the ratio of the different reaction products (1-H/1-
R) changed over the reaction course. At the beginning of the
reaction the 1-H/1-R ratio was only 3 : 1, but then it increased to
14.7 : 1 aer 44 h. Moreover, the total amount of rearranged
product ((E)-R + (Z)-R) even slightly decreased over the reaction
course (Fig. 5B, turquoise traces). This is because the rear-
ranged products still contain an olenic moiety and as such can
be hydrogenated over time (leward arrow in the bottom part of
Fig. 2).
2.4 Probing the role of iridium hydride
Next we monitored the reaction progress when using
[Cp*Ir(phen)(H)](PF6) instead of [Cp*Ir(phen)Cl]Cl as iridium
source. With [Cp*Ir(phen)Cl]Cl, conversion of 1-SM was only
observed aer an initial lag phase of approximately 1 hour,
during which essentially no hydrogenation of the substrate was
observed (yellow trace in Fig. 6A). This indicates that
[Cp*Ir(phen)Cl]+ rst must be converted to the catalytically
The very good correlation between the reactivity (and che-
moselectivity) of the different substrates and the stability of the
formed radical intermediates RIc (Fig. 4A) is fully in line with
hydrogenation via an initial photo-HAT. The investigated olen
substrates have very negative reduction potentials (below ꢀ2.7 V
vs. SCE)31 and photoexcited [Cp*Ir(a-diimine)X] species (X ¼
Fig. 4 Influence of radical stability and sterics on reactivity of the substrates and product distribution: (A) the conversion of the substrate
decreases with decreasing radical stability (reference value: methyl radical ¼ 0 kcal molꢀ1).84 The chemo-selectivity changes towards the
rearranged product when aliphatic substrates are used instead of aromatic substrates. (B) Introducing sterics on the substrates hinders the initial
HAT and lowers the reactivity of the substrate, see text for details.
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Fig. 5 Reaction progress as a function of time: (A) 1H-NMR spectra of the reaction mixture after different time intervals following irradiation with
a 7.5 W 470 nm LED (NMR acquisition occurred in the dark, using 8 aliquots of the same stock solution irradiated in parallel for different amounts
of time). The characteristic signals of the starting material (1-SM, pink background) disappear, while formation of the hydrogenation product (1-H,
blue background) and the rearranged products (1-(E)-R and 1-(Z)-R, turquoise backgrounds) are observable. (B) Conversion of the substrate (1-
SM, pink trace) and 1H-NMR yields of the different products (1-H, blue trace; 1-(E)-R and 1-(Z)-R, turquoise traces) over the reaction course.
Fig. 6 Role of iridium hydride in the reaction mixture. (A) Comparison of the different reaction time profiles when using [Cp*Ir(phen)Cl]+ (yellow
trace) or [Cp*Ir(phen)(H)]+ (green trace). (B) Olefin hydrogenation with [Cp*Ir(phen)(H)](PF6), yields determined by 1H-NMR spectroscopy. The
yellow trace in panel A is the same data set as the blue trace in Fig. 5B.
active species. When instead using [Cp*Ir(phen)(H)](PF6) (see Cl]+ in CH3CN in presence of 1.0 mM TEA with an LED at
ESI Sections 2.1 and 4.2† for details), no lag phase is observed 455 nm, the UV-Vis absorption spectrum of [Cp*Ir(phen)Cl]+
(green trace in Fig. 6A), and the hydrogenation product 1-H converts slowly but steadily to the spectral signature of
begins to form without any signicant delay. This suggests that [Cp*Ir(phen)(H)]+. The series of spectra shown in Fig. S33† were
the initial lag phase when using [Cp*Ir(phen)Cl]+ is due to the recorded aer different irradiation times and have an isosbestic
light-driven
[Cp*Ir(phen)(H)]+ in presence of TEA, and furthermore that spectra of neat [Cp*Ir(phen)Cl]+ and [Cp*Ir(phen)(H)]+.
[Cp*Ir(phen)(H)]+ is a catalytically relevant species. However,
To probe this conversion under conditions resembling more
conversion
of
the
chloro-complex
to point at 375 nm, in line with expectation based on the UV-Vis
even with that iridium(III) hydride, no substrate conversion closely those of the reactions in Section 2.2, 3.5 mM
occurred when the reaction mixture was stirred in the dark [Cp*Ir(phen)Cl]+ in CD3CN containing 350 mM TEA (but no
(entry 2 in Fig. 6B), indicating that [Cp*Ir(phen)(H)]+ must be olen substrate) was irradiated at 470 nm. The 1H-NMR spectra
activated by visible light in order for the HAT to occur. recorded (in the dark) aer photo-irradiation for 2 hours
Evidently, the [Cp*Ir(phen)(H)]+ complex in its electronic provide clear evidence for the formation of [Cp*Ir(phen)(H)]+
ground state is not a sufficiently good H-atom donor for the (ESI, Section 4.2†). This set of experiments furthermore
olen substrates considered here (see ESI Section 4.1† for demonstrates that prolonged photo-irradiation leads to disso-
further details).
ciation of the phen ligand, which explains why the photo-
The electrochemical and chemical conversion of [Cp*Ir(bpy) chemical hydrogenation proceeds better in the presence of
Cl]+ to [Cp*Ir(bpy)(H)]+ is well established,66,85 but so far does excess phen (Table 1).
not seem to have been studied in photochemical settings. For
Presumably, this photoreaction proceeds via an initial
related rhodium complexes, the photochemical conversion of reductive dissociation of the chloro-ligand, thereby leading to
chloro- to hydride complexes has been explored to some [Cp*Ir(phen)]0. Protonation of that species (with TEAc+ as
extent.86–88 When irradiating a 80 mM solution of [Cp*Ir(phen) proton source) can then in principle lead directly to the
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formation of [Cp*Ir(phen)(H)]+, but this is speculative and our (TAA-OMe) instead of TEA provides clear evidence for the tri-
data does not provide insight into the individual elementary arylamine radical cation (absorption band at 720 nm in Fig. 7B/
steps involved in this conversion. Whilst TEA is principally an D) superimposed with the spectroscopic signature of the iri-
electron donor, its one-electron oxidized form TEAc+ is compe- dium(I) species. Thus, it is clear that both TEA and the triaryl-
tent to act either as acid or H-atom donor.89
amine act as electron donors to the 3MLCT-excited
Having established that [Cp*Ir(phen)(H)]+ forms from [Cp*Ir(phen)(H)]+ complex.
[Cp*Ir(phen)Cl]+ in the course of continuous photo-irradiation
In the presence of the olen substrates, we expect that
in presence of excess TEA, the next logical step is to elucidate [Cp*Ir(phen)(H)]0 acts as H-atom donor, leading to the
possible onward photochemical reactions of that iridium(III) [Cp*Ir(phen)]0 complex and the radical intermediates (RIc in
hydride species. [Cp*Ir(phen)(H)]+ exhibits 3MLCT photo- Fig. 2). In the transient absorption experiments of Fig. 7, where
luminescence with a lifetime (s) of 148 ns in deaerated CH3CN there is no olen substrate present, [Cp*Ir(phen)]0 is expected
at a [Cp*Ir(phen)(H)]+ concentration of 0.2 mM with 0.6 mM to form via H2 evolution from two equivalents of
NBu4PF6 at room temperature (Fig. S36†). TEA induces reduc- [Cp*Ir(phen)(H)]0 as illustrated in Fig. S37.† This reaction is
tive 3MLCT quenching with a rate constant of 4.5 ꢂ 108 Mꢀ1 sꢀ1 exergonic by ca. 16 kcal molꢀ1
according to Stern–Volmer luminescence quenching experi-
.
67
ments (inset of Fig. S36†), thereby leading to the respective
iridium(II) hydride, [Cp*Ir(phen)(H)]0.
2.5 Radical clock experiment
The reactivity patterns identied above (Fig. 4) are fully compatible
with a HAT mechanism. To gain further mechanistic insight, an
experiment with (1-(2-phenylcyclopropyl)vinyl)benzene (13-SM),
a radical clock-type substrate, was performed (Fig. 8). The exact
rate constant for the ring-opening of the radical formed aer HAT
to this particular substrate (13-RI1c) is not known, but based on
structurally closely related compounds (ESI Section 4.5†) it seems
plausible that the rate constant (kRO) for the opening of the
cyclopropane-ring in 13-RI1c (leading to 13-RI2c) occurs with a rate
That iridium(II) hydride species cannot be observed directly
by transient absorption spectroscopy, because this is a highly
reactive species that does not accumulate in detectable
concentrations. However, UV-Vis transient absorption spec-
troscopy provides unambiguous evidence for a follow-up tran-
sient species, namely the iridium(I) complex [Cp*Ir(phen)]0,
which forms from the iridium(II) hydride [Cp*Ir(phen)(H)]0
aer H-atom release. This iridium(I) species exhibits diagnostic
absorption bands near 600 and 640 nm (Fig. 7C, vertical black
lines), which are observable aer pulsed 445 nm excitation of
[Cp*Ir(phen)(H)]+ in presence of excess TEA in CH3CN (Fig. 7A).
An analogous experiment with a triarylamine electron donor
on the order of 108 sꢀ1 90,91 Under our standard reaction conditions
.
(Fig. 8, top le) using [Cp*Ir(phen)Cl]Cl, the olen substrate 13-SM
converts exclusively to the ring-opened product 13-RO (71% yield
aer 44 h). No formation of the ring-retention hydrogenation
product (13-RR) is observed. The observation of the ring-opened
product 13-RO is consistent with a radical mechanism, and
furthermore indicates that the intramolecular ring-opening reac-
tion is faster than secondary HAT. Because 13-RO still contains an
olenic functional group, it reacts onwards to hydrogenation and
rearranged products, analogously to substrates 1-SM–12-SM (ESI
Section 4.5†).
2.6 Experiment with a deuterated substrate probing the
reversibility of primary HAT
The prior section conrmed the radical nature of the reaction
and provided already some insight into the kinetics of
secondary HAT to the radical intermediate. By exploring the
visible-light induced hydrogenation of deuterated substrate 1-
SM-d4 we sought to probe the initial HAT step in more detail.
Aer an initial HAT to 1-SM-d4 (kHAT1 in Fig. 9), the formed
Fig. 7 (A) Transient absorption spectrum of a solution containing radical intermediate (1-RIc-d4) can in principle directly react
[Cp*Ir(phen)(H)](PF6) (0.5 mM) and TEA (10 mM). The spectrum was
measured at a time delay of 60 ns after excitation at 445 nm (9 mJ) and
was time-integrated over 100 ns. (B) Transient absorption spectrum of
a solution containing [Cp*Ir(phen)(H)](PF6) (0.5 mM) and TAA-OMe (10
onwards to the hydrogenation (1-H-d4) or the rearranged (1-R-
d3) products. On the other hand also a reverse HAT (H-atom
abstraction from 1-RIc-d4 to afford 1-SM-d3, k
in Fig. 9)
ꢀHAT1
could occur prior to product formation. Thus, the deuterated
mM) in deaerated CH3CN. The spectrum was measured at a time delay
of 10 ns after excitation at 445 nm (9 mJ) and was time-integrated over substrate is useful to probe the reversibility of initial HAT: if
100 ns. See ESI Section 2.6† for the molecular structure of TAA-OMe.
reverse HAT from 1-RIc-d4 were kinetically competitive with
(C) Absorption spectrum of [Cp*Ir(phen)]0 in deaerated CH3CN at
onward reaction to 1-H-d4 or 1-R-d3, there should be proton
room temperature obtained after deprotonation of [Cp*Ir(phen)(H)]+
incorporation into the originally fully deuterated double bond
(bond labeled “a” in Fig. 9) and some 1-SM-d3 should be
formed.
with KOtBu (1.3 eq.). (D) Absorption spectrum of TAA-OMec+ in dea-
erated CH3CN after chemical oxidation of a TAA-OMe solution (31 mM)
with NOBF4 (3.0 eq.).
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Fig. 8 Radical clock experiment with a cyclopropane substituted olefin confirms that the reaction proceeds via a radical intermediate. For
simplicity, only the (E)-isomer of the ring-opened product 13-RO is shown.
When the deuterated substrate (1-SM-d4) was subjected to hydrogenation product 1-H-d4 is likely due to the presence of
the standard conditions (see Fig. 3) and the reaction mixture TEA oxidation products such as TEAc+ or TEAc.89,92 These highly
was irradiated for 44 h, both hydrogenation (1-H-d4, 87%) and reactive species presumably serve as the H-atom sources for
isomerization (1-R-d3, 6%) products were observed. No forma- a secondary HAT to yield the nal hydrogenation product.
tion of 1-SM-d3 was detectable throughout the irradiation
As noted previously for thermal MHAT, the chemoselectivity
process (ESI Section 4.6†). Thus, H-atom abstraction from the of the overall reaction strongly depends on the concentration of
radical intermediate (1-RIc-d4) to form 1-SM-d3 can be neglec- the H-atom donor.72 When the concentration of the H-atom
ted. Evidently, secondary HAT (kHAT2) yielding the nal hydro- donor is relatively high, such as for
a system where
genation product 1-H-d4 as well as isomerization (kR) leading to CpCr(CO)3H was readily regenerated under H2 pressure,78
the rearranged product 1-R-d3 are both faster than reverse HAT hydrogenation products are formed preferably. With lower Hc
(kꢀHAT1). The fast onward reaction to give preferably the concentrations, however, which was the case in a previously
Fig. 9 Experiment with deuterated substrate 1-SM-d4: no proton incorporation into the terminal double bond (bond labeled “a”) was observed,
indicating that H-atom abstraction from 1-RIc-d4 to afford 1-SM-d3 is negligible (crossed gray leftward arrow labeled kꢀHAT1). For simplicity, only
the (E)-isomer of the rearranged product 1-R-d3 is shown.
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investigated Co–H/H2 system,72 the rearranged products (1-R-d3) too high for formation of the radical intermediates RIc (ESI
are formed preferably. The preferential formation of hydroge- Section 4.1†).
nation products in our study therefore indicates an elevated
Photoexcitation of [Cp*Ir(phen)(H)]+ populates a long-lived
(local) H-atom donor concentration, presumably due to the 3MLCT excited state (s of 148 ns at a [Cp*Ir(phen)(H)]+
presence of iridium hydrides and TEA oxidation products in concentration of 0.2 mM),59,67 which is quenched reductively by
sufficiently close proximity.
TEA with a rate constant of kq ¼ 4.5 ꢂ 108 Mꢀ1 sꢀ1 to form
[Cp*Ir(phen)(H)]0 (ESI Section 4.3†). This iridium(II) hydride is
likely the key catalytic intermediate, acting as H-atom donor vis-
2.7 Proposed reaction mechanism
`
a-vis the olen substrates. At the relevant TEA concentrations
From the various studies in the prior sections, the mechanistic
picture in Fig. 10 emerges. Before the actual catalytic cycle
initiates, the catalyst precursor [Cp*Ir(phen)Cl]+ rst needs to
be converted into [Cp*Ir(phen)(H)]+ (Section 2.4). The
[Cp*Ir(phen)(H)]+ complex with the metal in its +III oxidation
state is not yet reactive enough to catalyze the HAT-step to the
olen substrate (SM), as demonstrated by the lack of conversion
when the reaction mixture is stirred in the dark (Fig. 6B, entry
for our olen hydrogenations (250 mM), the rate for reductive
3MLCT excited state quenching exceeds the rate for “self-
quenching” by at least one order of magnitude.
Direct detection of [Cp*Ir(phen)(H)]0 by transient absorption
spectroscopy is not possible, because this is a highly reactive
and short-lived species, which does not accumulate in detect-
able concentrations. Instead, [Cp*Ir(phen)]0, which is formed
from [Cp*Ir(phen)(H)]0 aer H-atom release, is readily detected
(Section 2.4).
2). The IrIII–H BDFE of [Cp*Ir(phen)(H)]+ is not known, but for
a related complex the IrIII–H BDFE is 69 kcal molꢀ1
, which is
93
Fig. 10 Proposed reaction mechanism. For simplicity, only the (E)-isomer of the rearranged product is shown.
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Based on previously determined electrochemical potentials CH2Cl2 proceeds via excited-state “self-quenching” producing a pair
for analogous bpy-based (instead of phen-based) iridium of [IrIV–H]2+ and [IrII–H]0 species, which then undergoes (sequen-
3
+
0
0
complexes, neither the MLCT-excited [Cp*Ir(phen)(H)] (E (IV/ tial) hydride transfer and HAT to form two equivalents of CHCl3
III*)
z
ꢀ1.3 vs. SCE,67 ESI Section 4.1†) nor the from two molecules of CH2Cl2 solvent. Here, the [IrII–H]0 H-atom
[Cp*Ir(phen)(H)]0 complex (E (III/II) z ꢀ1.4 V vs. SCE,67 ESI donor is formed via reductive MLCT-excited state quenching of
Section 4.1†) have the reducing power to permit electron the [IrIII–H]+ species, which itself is formed from the robust [IrIII–
transfer to the olen substrates (Ered < ꢀ2.7 V vs. SCE).31 Cl]+ precursor in photochemical fashion. In our case, the overall
Consequently, olen hydrogenation cannot occur via a PET HAT reaction is catalytic and occurs to olen substrates rather than
0
3
0
pathway (ESI Section 4.1†).
carbon-centered radicals derived from CH2Cl2 solvent. Contrary to
However, [Cp*Ir(phen)(H)]0 exhibits an IrII–H BDFE of previously
explored
polyoxometallate-based
photo-HAT
approximately 44 kcal molꢀ1
,
considerably lower than the systems,38–43 the iridium hydride explored herein operates under
67
M–H BDFEs of common MHAT-catalysts, which are typically in visible-light irradiation.
the range of 50–60 kcal molꢀ1
.
This very weak IrII–H bond
We focused on the photoinduced hydrogenation of relatively
70,71
permits an initial HAT to the olen substrates (ESI Section 4.1†). simple olens as a test ground to explore the reactivity patterns
The achievable hydrogenation yields for the various substrates of photo-HAT, but prior studies on thermal (i.e., light-
1-SM–12-SM reect the relative stabilities of the involved independent) reactions established that metal-catalyzed HAT
(charge-neutral) radical intermediates RIc (Fig. 4), as expected has numerous applications in organic synthetic chemistry.68
for HAT. Furthermore, the radical clock experiment (Fig. 8) Compared to thermal HAT, the photo-HAT with
underpins the radical pathway and speaks against a mechanism [Cp*Ir(phen)(H)]+ has the advantage that even lower M–H
involving olen insertion into the Ir–H bond.
BDFEs (ca. 44 kcal molꢀ1 for the reductive quenching product
No conversion at all is observed with the sterically [Cp*Ir(phen)(H)]0) are reachable, which stands in contrast to
demanding mesityl-substituted olen 7-SM (Fig. 3), suggesting the traditional thermal manner, where most M–H BDFEs are
that the catalyst and 7-SM cannot approach sufficiently closely above 50 kcal molꢀ1 70 This gain in driving force of around DDG
.
to undergo HAT. The formation of sufficiently intimate z 6 kcal molꢀ1 can in principle translate to an approximately
contacts, different however from actual substrate ligation to the 104 times faster reaction rate. Very low M–H BDFEs are typically
metal center or olen insertion into the M–H bond, seems to be unstable to bimolecular H2 evolution,65–67 but in the presence of
a key requirement for successful reaction.
For substrates undergoing successful photo-HAT, the well and exhibited the typical reactivity patterns of HAT.
formed radical intermediate (RIc) can either react via secondary In our reactions, even aliphatic substrates yielding highly
50 mM olen substrate our hydrogenation reactions proceeded
HAT to yield the hydrogenation product (H, top right in Fig. 10) reactive radical intermediates were activated successfully. The
or rearrange to the isomerization product (R, top le in Fig. 10). investigated olen substrates have strongly negative reduction
TEA oxidation products such as TEAc+ or TEAc likely serve as H- potentials (<ꢀ2.7 V vs. SCE)31 and therefore cannot be activated
atom sources for secondary HAT. Whilst the hydrogenation easily by the traditional path of photoinduced electron transfer.
products (H) are inert and do not react onwards, the rearranged
products (R) can be activated via HAT from [Cp*Ir(phen)(H)]0 to
re-form radical intermediate (RIc). Thus, irradiation over
Conflicts of interest
extended time periods drives the overall reaction towards the There are no conicts to declare.
hydrogenation product (Fig. 5B). Excess phen ligand (30 mol%)
in the reaction mixture enhances that H/R chemoselectivity by
decelerating catalyst degradation and enabling photo-reactions
Acknowledgements
over longer irradiation periods (Section 2.4).
This work was supported by the Swiss National Science Foun-
dation through the NCCR Molecular Systems Engineering. Felix
Glaser is thanked for skillful experimental support.
3 Conclusions
This work illustrates a new type of photo-reactivity for an archetypal
metal hydride complex, complementing prior studies on photoacid
and “self-quenching” behavior of this well-known class of
compounds.59–67 From a photochemical perspective, the discovery
of photo-HAT from the [Cp*Ir(phen)(H)]+ complex could represent
an important breakthrough, because the vast majority of previously
explored metal-based photosensitizers operate on the basis of
photoinduced electron transfer. Consequently, photoredox catal-
ysis until now largely relied on single electron transfer, but the
possibility of photo-HAT could open completely new perspectives.
Our new insights complement recent photophysical and -chemical
studies of iridium hydride59–67 and related complexes.94,95 In
particular, recent work established that the hydrodechlorination of
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