Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation

Article abstract of DOI:10.1002/chem.202101705

Outer-sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M?H bonds that are either too weak to efficiently activate H₂ or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square-planar cobalt(II) hydride complex. Photoactivation results in Co?H bond homolysis. The three-coordinate cobalt(I) photoproduct binds H₂ to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H₂ via HAT.

Full text of DOI:10.1002/chem.202101705

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Chemistry—A European Journal
Photo-Initiated Cobalt-Catalyzed Radical Olefin
Hydrogenation
Sier Sang,[a] Tobias Unruh,[b] Serhiy Demeshko,[a] Luis I. Domenianni,[b] Nicolaas P. van Leest,[c]
Philipp Marquetand,[d] Felix Schneck,[a] Christian Würtele,[a] Felix J. de Zwart,[c] Bas de Bruin,[c]
Leticia González,[d] Peter Vöhringer,[b] and Sven Schneider*[a]
Abstract: Outer-sphere radical hydrogenation of olefins
proceeds via stepwise hydrogen atom transfer (HAT) from
transition metal hydride species to the substrate. Typical
catalysts exhibit MÀ H bonds that are either too weak to
efficiently activate H2 or too strong to reduce unactivated
olefins. This contribution evaluates an alternative approach,
that starts from a square-planar cobalt(II) hydride complex.
Photoactivation results in CoÀ H bond homolysis. The three-
coordinate cobalt(I) photoproduct binds H2 to give
a
dihydrogen complex, which is a strong hydrogen atom donor,
enabling the stepwise hydrogenation of both styrenes and
unactivated aliphatic olefins with H2 via HAT.
Introduction
Homogeneous olefin hydrogenation is a mature and versatile
synthetic methodology.[1] Many catalysts follow mechanisms
defined by H2 oxidative addition, olefin insertion, and alkane
reductive elimination or, alternatively, redox neutral routes via
hydrogenolysis of the hydrocarbyl complex intermediates. In
recent years, the sustainability of olefin hydrogenation proto-
cols was considerably advanced by the development of many
first-row transition metal pre-catalysts,[2] including several cobalt
precursors (Figure 1).[3] Experimental and computational studies
identified cobalt hydrocarbyl key intermediates within olefin
insertion-based mechanisms.[4] The current renaissance of
[a] S. Sang, Dr. S. Demeshko, Dr. F. Schneck, Dr. C. Würtele,
Prof. Dr. S. Schneider
Universität Göttingen
Institut für Anorganische Chemie
Tammannstraße 4, 37077 Göttingen (Germany)
E-mail: sven.schneider@chemie.uni-goettingen.de
[b] T. Unruh, Dr. L. I. Domenianni, Prof. Dr. P. Vöhringer
Institut für Physikalische und Theoretische Chemie
Rheinische Friedrich-Wilhelms-Universität
Figure 1. Molecular cobalt catalyst precursors for olefin hydrogenation.
Wegelerstrasse 12, 53117 Bonn (Germany)
[c] N. P. van Leest, F. J. de Zwart, Prof. Dr. B. de Bruin
Van't Hoff Institute for Molecular Sciences
University of Amsterdam
organic radical chemistry[5] also revived interest in an alternative
mechanistic approach to alkene hydrogenation via stepwise
outer-sphere hydrogen atom transfer (HAT) without MÀ C bond
formation.[6] HAT-based olefin hydrogenation (HAT-OH) can be
coupled to radical isomerization, potentially offering new
selectivities.[5d,7,8] First-row transition metal hydride catalysts
with medium to weak MÀ H bond strengths are typically
employed.[6b,9] A photoactive iridium hydride complex was
recently used in photodriven HAT-OH after reductive excited
state quenching.[10] Furthermore, Chirik and co-workers reported
a cobalt catalyst that switches from HAT-OH to an olefin
insertion mechanism upon photolytic dissociation of a CO
ligand.[3k]
Science Park 904, 1098 XH Amsterdam (The Netherlands)
[d] Dr. P. Marquetand, Prof. Dr. Dr. h.c. L. González
Institute of Theoretical Chemistry, Faculty of Chemistry
University of Vienna
Währinger Straße 17, 1090 Vienna (Austria)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101705
Part of a Special Issue on Contemporary Challenges in Catalysis.
© 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial License, which permits use, dis-
tribution and reproduction in any medium, provided the original work is
properly cited and is not used for commercial purposes.
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Shevick et al. recently discussed the scope of HAT-OH
including some limitations based on the thermochemical
framework:[9] Typical low-spin hydride catalysts with strong field
(SF) ligands exhibit MÀ H bond dissociation free energies (BDFEs)
hydride complex from A was attributed to initial H2 addition
and subsequent comproportionation with parent A. This route
suggests facile HAT reactivity of the H2 adduct. Furthermore,
the thermochemical boundary condition,
around 50–60 kcal·molÀ 1, such as [CpCrH(CO)3] (BDFECr-H
=
57 kcalmolÀ 1 in MeCN).[11] Therefore, the first HAT step to
unactivated alkenes, like propylene, is thermodynamically quite
BDFEMH2 < 0:5 � BDFEH2 < BDFEMH
*
unfavorable (BDFEC-H(CH3C HCH2-H)=33 kcalmolÀ 1; Figure 2,
seems to be fulfilled. We here report a related cobalt(II)
monohydride pincer complex, which is inactive for thermal
olefin hydrogenation, but can be photolytically activated with
visible light to catalyze photo-initiated HAT-OH of styrenes and
also of unactivated, aliphatic alkenes.
red), and overall hydrogenation is driven by the second HAT
step.[12,13,14] In consequence, HAT-OH of unactivated olefins, like
propylene, with SF-ligand based catalysts is generally kinetically
inaccessible at ambient conditions. Catalysts with weak MÀ H
bonds (BDFEM-H <35 kcalmolÀ 1) can be utilized instead,[9] but
require alternative reductants due to the high BDFE of H2
(104 kcalmolÀ 1 in toluene).[6b,15]
Results and Discussion
A potential alternative approach could be provided by the
use of dihydride catalysts, LnMH2, with distinctly lower MÀ H
bond strength than the respective monohydride LnMH
(BDFEMH2 <BDFEMH). In that case, both HAT steps would be
closer in reaction free energy (Figure 2, blue), rendering
unactivated substrates more accessible while still being able to
efficiently activate H2, if the sum (BDFEMH2 +BDFEMH) is close to
BDFEH2. However, as a possible pitfall such dihydride complexes
would be thermodynamically unstable towards bimolecular H2
evolution and formation of the catalytically inept monohydride,
if BDFEMH2 <0.5·BDFEH2.
Synthesis and characterization of [CoIIH(PNP)] (1;
PNP=N(CHCHPtBu2)2)
Starting from the previosly reported chloride complex [CoCl
(PNP)],[17] the cobalt(II) hydride [CoH(PNP)] (1) can be prepared
in over 70% isolated yield by salt metathesis with LiAlH4
(Scheme 2). The analogous deuteride complex [CoD(PNP)] (1-D)
was obtained with LiAlD4 or, alternatively, by rapid H/D
exchange of 1 under an atmosphere of D2 in benzene
(Scheme 2). The molecular structure of 1 in the crystal (Figure 3)
resembles the metrical bonding parameters of the parent
chloride complex.[17] Slightly distorted square-planar coodina-
To evaluate these considerations, a cobalt pincer platform
like that reported by Choi and Lee (Scheme 1) provides an
interesting starting point.[16] The formation of a cobalt(II)
°
tion arises from the pincer bite angle (170.528(16) ). Compared
with the chloride precursor (dCo-N =1.893(2) Å), the CoÀ N bond
length of 1 (1.9369(12) Å) reflects the trans-influence of the
hydride ligand, which could be located on the electron density
map.
The CoÀ H stretching frequency of 1 (ν=1756 cmÀ 1; ΔνH/D
=
487 cmÀ 1), ranges at the lower end when compared with other
neutral cobalt(II) pincer hydride complexes (~1820–
2030 cmÀ 1),[3I,18] reflecting a large trans-influence of the divinyla-
mide ligand.[19] The three paramagnetically shifted and broad-
1
ened solution H NMR signals of the pincer ligand support C2v
Figure 2. Thermochemical considerations for HAT-OH via mono- (red) and
dihydride (blue) catalysts, respectively.
Scheme 1. Hydrogen activation by the cobalt pincer platform reported by
Choi and Lee.[16]
Scheme 2. Synthesis of the cobalt complexes 1–4.
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CASSCF(11,14) computations of a PMe2-truncated model of
complex 1 indicated that the ground state is dominated by the
(3dxy)2(3dyz)2(3dz2)1(3dxz)2 doublet configuration (88%), where the
3dx2-y2 orbital contributes to form the CoÀ H σ and σ* orbitals
with an atomic contribution of ca. 60% in both molecular
orbitals. Calculations with this PMe2-truncated model already
reproduce the large g-anisotropy (g=4.80, 1.50, 1.25) satisfac-
torily.
°
The UV/vis spectrum of 1 recorded in THF at À 95 C
(Figure 3) exhibits intense bands at 285 nm (ɛ=
1.2·104 MÀ 1 ·cmÀ 1) and 337 nm (ɛ=1.2·104 MÀ 1 ·cmÀ 1). These
absorptions are qualitatively reproduced by the SC-NEVPT2/
CASSCF(11,14) computations that included the low-lying 4pz
orbital in the active space. Accordingly, we assign the bright
state at 337 nm to the D5 doublet state (Figure 3 and Table S3),
mainly composed of the (3dxy)2(3dyz)2(3dz2)1(3dxz)1(4pz)1 config-
uration (62%) with the (dipole allowed) excitation to the 4pz
orbital, another (12%) configuration with an excitation to the
4pz orbital and a smaller contribution (8%) of an excitation to
the
CoÀ H
σ*
antibonding
orbital
with
(3dxy)2(3dyz)2(3dz2)2(3dxz)0(σ*)1 configuration. The bright state at
294 nm is also a mixture of doublet configurations: 36% and
32% of configurations involving 3d!4pz excitations and 11%
of a 3d!σ* configuration (see Table S3). In all of them, the 3dx2-
orbital participates in the CoÀ H σ orbital and remains doubly
y2
occupied. Besides these strong absorptions, a broad band at
522 nm with low intensity (ɛ=124 MÀ 1 ·cmÀ 1) was found in the
UV-vis spectrum in benzene (Figure S2). The SC-NEVPT2
computations predict a quartet state (Q3 in Table S3) in this
region, which is expected to borrow intensity via spin-orbit
coupling.
Figure 3. Top left: Molecular structure of 1 in the crystal from X-ray
diffraction with the anisotropic displacement parameters drawn at the 50%
probability level. tBu hydrogen atoms are omitted for clarity. Selected bond
°
lengths (Å) and angles ( ): Co1-H11=1.45(2); Co1-N1=1.9369(12); Co1-
P1=2.2062(3); N1-Co1-H=180.000(10); N1-Co1-P1=85.264(8); P1-Co1-
P1=170.528(16). Top right: Experimental and simulated EPR spectra of 1
measured in toluene glass at 20 K (MW freq.=9.6427, MW
power=0.6325 mW, Mod. amp.=4 G). Middle: Overlays of the experimental
EPR spectra of 1 and 1-D (g-value scaled, normalized). Bottom: Comparison
of the experimental electronic absorption spectrum of 1 in THF (blue,
Interestingly, the 1H NMR spectrum of the deuteride
isotopologue 1-D exhibit a noticable isotopic shift of up to
1 ppm for one set of methyne protons in the pincer backbone.
Pronounced NMR isotope effects on remote nuclei, sometimes
called paramagnetic isotope effect on chemical shift (PIECS),[22f]
were previously reported for several paramagnetic molecules,[22]
including some multinuclear, hydride bridged complexes.[22f–i] In
that case, PIECS was attributed to the slightly shorter MÀ D
bond lengths, resulting in increased exchange coupling of the
open-shell metal ions. However, in the current, mononuclear
case, the isotope effect of the hyperfine shift must be
associated with a perturbation of the ligand field upon CoÀ H/D
exchange. In fact, EPR characterization of 1-D (Figure 3)
supports a ligand field effect as origin of PIECS, as expressed by
the slightly smaller g-anisotropy (g=4.29, 1.62, 1.29) and 59Co
HFI in the low-field component (A1 =1570 MHz). The non-equal
PIECS contributions for the two pincer backbone 1H NMR signals
suggest a contribution from dipolar coupling, which is in
agreement with the large anisotropy of the g-tensor.[23] We
tentatively associate the isotope effect with the distinct multi-
reference character of the ground state, as a shorter CoÀ D bond
is expected to perturb contributions from configurations with
the filled dx2-y2 orbital.
°
À 95 C) with the computed one (red, SC-NEVPT2/CASSCF(11,14)/def2-TZVP)
for the PMe2 truncated model, shifted by À 50 nm.
symmetry on the NMR timescale. A 1H NMR signal for the
hydride ligand was not found.
The room temperature solution magnetic moment in C6D6
(μeff =1.97�0.27 μB) and the temperature dependent suscepti-
bility measurements of a powdered sample (see Supporting
Information) are both in agreement with an electronic low-spin
configuration (S=1/2) with a sizable orbital momentum (giso
=
2.84 for powdered sample). This is further supported by X-band
EPR spectroscopy in toluene at 20 K (Figure 3), which could be
satisfactorily fitted with a rhombic, highly anisotropic g-tensor
(g=4.35, 1.56, 1.24; hgi=2.76) and large 59Co hyperfine
interaction (HFI) for the low-field component (A1 =1585 MHz).
The EPR data support metal-centered radical character and
resemble the related cobalt(II) PNP pincer hydride complex
[CoH{NC5H3(CHPtBu2)(CH2PtBu2)}] reported by Chirik and co-
workers.[18a] Large 59Co HFI and g-anisotropy are generally found
for square-planar, low-spin cobalt(II) due to pronounced mixing
of the ground state with low-lying excited states mediated by
spin-orbit coupling.[18a,20,21] Multiconfigurational SC-NEVPT2/
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Synthesis and characterization of [Co(L)(PNP)] (L=none (2),
N2 (3), H2 (4))
Reaction of the chloride precursor with LiHBEt3 as hydride
source, primarily leads to reduction, as reported by Chirik and
co-workers for related Co pincer complexes.[24] However, the
cobalt(I) compounds [Co(L)(PNP)] (L=none (2), N2 (3)) are more
conveniently prepared by reduction with KC8 under Ar or N2,
respectively (Scheme 2). The [Co(PNP)] platform exhibits ex-
tremely high nitrogen affinity and 3 is obtained in the presence
of only traces of dinitrogen. N2-binding is reversible and 2 can
be obtained from 3 by sublimation at 110 C (10À 3 mbar).
°
The molecular structure of the square-planar dinitrogen
complex 3 was confirmed by X-ray diffraction (Figure S49). The
closed-shell complex exhibits the typical NMR signature (δP =
81.6 ppm) of diamagnetic, C2v symmetric complexes with this
divinylamido PNP pincer ligand. The N2 stretching vibration (νN�
N =2012 cmÀ 1) indicates weak activation. In contrast to complex
3, strongly paramagnetically shifted and broadened 1H NMR
signals support an electronic triplet ground state for 2, as was
previously reported for other T-shaped three-coordinate cobalt
(I) complexes.[3h,25] DFT computations confirm this notion,
predicting that the triplet (S=1) ground state of [Co(PNP)] is
20 kcal·molÀ 1 below the singlet (S=0, see Supporting Informa-
tion, Section 8.1). Notably, Lee’s 3-coordinate cobalt(I) PNP
pincer complex (Scheme 1, A) was reported to exhibit a low-
spin ground state.16] Unfortunately, the extremely high N2
affinity of 2 so far prevented reliable combustion analysis and
thus magnetic characterization.
Figure 4. Top: Computed BDFECoH2 of 4 and BDFECoH of 1. Middle: Mechanistic
model for the conversion of 2 to 1 with H2. Bottom: H2-pressure dependent
kinetic data for the decay of 4.
1
of 4 with H NMR spectroscopy (Figure 4, Bottom). The reaction
rate follows second order dependence in [4] and inverse
dependence of kobs on H2 pressure with a zero-intercept of the
While the dinitrogen complex 3 is thermally stable under H2
(1 atm in C6D6), the monohydride 1 is selectively formed upon
photolyzing (λexc =390 nm) this mixture (Scheme 2). In contrast,
complex 2 directly gives 1 (and minor amounts of 3 from
residual N2) with H2 (1 atm) over the course of several hours at
r.t., reflecting the work from Choi and Lee (Scheme 1).[16]
À 1
kobs vs. p(H2) plot. These findings are in agreement with a
rapid pre-equilibrium of 2 and H2 with 4 that is followed by
comproportionation via formal HAT (Figure 4, Middle and
Supporting Information, Section 3.8). Third-order rate laws (r=
k·[M]2 ·[H2]) were previously reported for the addition of H2
across two metal atoms (2 M+H2!2 MH).[29] Notably, Wayland
and Sherry associated a small kinetic isotope effect with a
°
Notably, at À 70 C in d8-toluene, a diamagnetic product (δP =
96.3 ppm) was selectively obtained. A 1H NMR signal at
À 28.5 ppm (2JHP =74.2 Hz), which integrates over two protons,
supports the initial formation of [CoH2(PNP)] (4) as a cobalt(III)
dihydride or cobalt(I) dihydrogen complex.[3i,16,26,27] Under HD
termolecular reaction via
a
linear {M…H…H…M} transition
state.[29b] However, Hoff attributed small enthalpies of activation
observed for several compounds to a stepwise reaction with
initial, enthalpically driven H2 binding to one metal ion,[30] as
represented by the mechanism of Choi and Lee (Scheme 1).[16]
Our spectroscopic and kinetic results fully support such a
stepwise pathway via an H2 binding preequilibrium.
°
(0.7 atm) at À 100 C (Figure S17), the linewidth of the hydride
NMR signal (39 Hz) did not allow for unequivocal differentiation.
Furthermore, a statistical mixture of H2/HD was observed,
indicating rapid H/D scrambling already at very low temper-
atures. Warming to room temperature resulted in selective
conversion of 4 to paramagnetic 1 (Scheme 2), which confirmed
that 4 is an intermediate but also impeded structural assign-
ment based on the T1 criterion.[3i] Computational examination of
4 by DFT supports the dihydrogen isomer with a computed H…
H distance of 0.94 Å (Figure S45). The computations also
revealed a CoÀ H bond strength (BDFECoH2 =37 kcal·molÀ 1; Fig-
ure 4 Top)[28] that is distinctly lower than that of the mono-
hydride HAT product 1 (BDFECoH =62 kcal·molÀ 1), which is in
Photochemical activation of [CoH(PNP)] (1)
The strong CoÀ H bond of monohydride 1 obviously demands
additional activation to initiate efficient HAT-based catalysis.
Photolytic MÀ H homolysis has been reported for several
transition metal hydrides,[31] motivating the examination of the
photochemistry of 1. In the dark, hydride 1 exhibits high
chemical stability. No reaction was observed over the course of
line with the decay of 4 to 1 and 0.5 equiv. H2 (0.5·BDFEHH
=
52 kcal·molÀ 1).
several days at 60 C in C6D6. In contrast, photolysis in C6D6 at
°
The mechanism of the formation of 1 was examined by
monitoring the H2 pressure dependence (2–4 bar) of the decay
r.t. near the low energy edge of the strong absorption bands
(LED, λexc =390�20 nm) under a static vacuum[32] resulted in
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slow (kobs �1.1·10À 3 hÀ 1) conversion to 1-D, as confirmed by the
Both the cobalt(II) hydride (1) and deuteride complexes (1-
D) were therefore examined by UV/mIR spectroscopy in liquid
n-hexane solutions. A selection of experimental pump-probe
spectra of 1-D recorded at various representative time delays is
depicted in Figure 6. Two relatively strong negative bands are
observed at 1235 cmÀ 1 and 1511 cmÀ 1, i.e. exactly at the
spectral positions of the CÀ H in-plane bending and the C=C
stretching modes of the PNP pincer ligand. These signals
indicate an increase of the sample’s transmission and arise from
the electronic excitation of 1-D and the concomitant depletion
of population in its electronic ground state. Each of these two
ground state bleaches (GSBs) is accompanied by a slightly
downshifted transient absorption (TA). Immediately after ab-
sorption of the pump photon, i.e. at a time delay of zero, the
optically prepared excited electronic state is the most obvious
species, which can be held accountable for the induced
absorptions. Finally, a third but much weaker GSB band is
observed that is maximal at 1285 cmÀ 1, and hence, is due to the
CoÀ D stretching vibration. Notice that there is again an
accompanying TA-band peaking at 1246 cmÀ 1 whose low-
1
PIECS-induced H NMR shift, mass spectrometry and IR spectro-
scopy (Figure 5). Approximately equimolar generation of C6D5H
confirmed H/D exchange with the solvent (Figure S19). In
addition, trace quantities of complex 2 (~5%) were rapidly
formed but did not further accumulate during photolysis. A low
quantum yield of Φ390 =0.097% was derived for the photolytic
H/D exchange reaction with d6-benzene.
The steady state H/D exchange experiment suggests initial
photolytic CoÀ H bond cleavage. MÀ H photofragmentation was
first examined within closed-shell hydride carbonyl complexes
by matrix isolation techniques.[33] Multiconfigurational computa-
tions of these systems have attributed HAT with excited state
crossings that lead to a dissociative triplet surface.[34] However,
besides MÀ H homolysis, the photolysis of hydride complexes
can also lead to net proton or hydride transfer, respectively,[31]
sometimes even for the same compound. For example,
3
Fukuzumi and Guldi showed that the MLCT excited state of
Ziessel’s photocatalyst [Cp*IrH(bipy)][35] undergoes facile proton
transfer to MeOH,[36] while Miller and co-workers reported
photochemical H2 evolution in MeCN via a photo-driven
disproportionation mechanism.[37] In view of the rich photo-
chemistry of hydride complexes, the scarcity of transient
absorption studies is quite surprising.[3k,10,36,38,39]
Figure 5. Top: Reaction conditions for photo-induced H/D exchange of 1 and
C6D6. Middle: ATR-IR spectra of solid 1(red), 1-D(blue) and 1 after photolysis
in C6D6 for 36 h (black). Bottom: 1H NMR spectra of 1 in C6D6 before (red) and
after photolysis for 20 h (green, 53% yield) and 36 h (blue, 81% yield).
Figure 6. Top: Time resolved UV-pump mid-IR-probe spectra of 1-D in n-
hexane following 400 nm-excitation and probing the spectral regions of CÀ H
in-plane bending (left), the CoÀ D stretching (middle), and C=C stretching
(right) modes. Bottom: Comparison of kinetic traces of 1 and 1-D.
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frequency edge is likely obscured by the nearby, much stronger
GSB of the CÀ H in-plane bend. Importantly, the existence of an
induced absorption in the CoÀ D stretching region is sufficient
evidence that the species responsible for the TA-bands in
Figure 6 has an intact CoÀ D bond. At a time delay of zero, this
finding supports the notion of an optically prepared excited
state of 1 that is bound with respect to the CoÀ D stretching
coordinate. Note also that except for the isotope shift of the
deuteride stretching bands, qualitatively very similar results
were obtained for 1 (Figure S44).
The TA-bands associated with the pincer modes are
significantly broader than their corresponding GSB-bands. In
addition, they progressively shift to higher wavenumbers and
spectrally narrow as the time delay is increased. These
observations suggest that a significant fraction of the pump
photon energy is effectively converted into excess kinetic
energy residing in the system’s vibrational modes and that the
complex vibrationally relaxes on a time scale of a few
picoseconds.[40] Finally, the band integrals of the three induced
absorptions decay with increasing delay while simultaneously
the bleaching bands fade away. As the system vibrationally
cools, the population of the electronic ground state gradually
recovers within a few tens of picoseconds. Kinetic traces for
selected probe wavenumbers (Figure 6) of both, 1 (blue) and 1-
D (red) reveal a complicated temporal evolution due to the
superposition of vibrational cooling and ground state recovery.
Therefore, the traces can only be fitted in a purely phenomeno-
logical fashion using multi-exponential kinetics (solid curves in
Figure 7). Clearly, both isotopologues feature identical traces at
corresponding probe wavenumbers, which rules out any
significant kinetic isotope effect. All traces practically decay to
zero at infinite delays indicating (near) quantitative recovery of
the ground-state population without significant photochemical
conversion. The absence of a residual GSB corroborates the
results from H/D scrambling with C6D6 under static photolysis,
which proceeds with low quantum yields.
To rationalize the photochemistry, the solvent was changed
to liquid benzene-h6, now employing broadband white-light
probing of electronic transitions after the optical excitation at
400 nm. Figure 7a displays such UV/nUV-vis spectra of 1-D for
time delays that are complementary to those from Figure 6.
Following the absorption of the pump pulse, two distinct
induced transient absorption bands peaking near 540 nm and
460 nm are observed and maybe a third, slightly weaker band
around 650 nm. The ensuing spectro-temporal dynamics of the
two major bands appear to be markedly different as evidenced
by a reversal of their relative amplitudes. Whereas the 540 nm-
band is the dominant component at early delays, only the band
at shorter wavelength prevails for delays in excess of 10 ps. In
addition, both bands experience a subtle dynamic blue-shift
with increasing delay (cf. gray arrows in Figure 7a), which may
be linked to the same dynamics of vibrational relaxation already
seen in the femtosecond mIR-spectra.
Figure 7. a) Femtosecond vis-to-near-UV transient spectra of 1-D in liquid
benzene-h6 solution at room temperature. b) Experimental photoproduct
spectrum. c) Stationary UV/vis-spectrum of 1 (black curve) and 2 (red curve).
The blue spectrum is the difference spectrum, 2–1.
characterize this asymptotic absorption with a satisfactory
signal-to-noise ratio all transient spectra recorded between 70
and 150 ps were averaged into a “final” photoproduct spec-
trum, the result of which is displayed in Figure 7b (blue curve).
This spectrum contains only a single band that is centered at
444 nm. Its amplitude is a factor of 102 smaller than that of the
UV/nUV-vis-spectrum recorded at zero time delay, which is
again suggestive of a very small quantum yield of the order
of�1%. Most importantly, the experimental spectrum is
reminiscent of the stationary electronic absorption spectrum of
2, which exhibits a characteristic absorption band centered at
435 nm (Figure 7c). The shift of the pump-probe spectrum by
9 nm as well as the absorption depletion on the high-frequency
edge at 420 nm relative to the stationary spectrum can be
attributed to the onset of the GSB of 1. The latter, in turn, is
given by the inverted stationary absorption spectrum of 1. If
the only photochemical pathway is the formation of 2, the
“final” photo-product spectrum is simply given by the difference
between the red and the black spectra shown in Figure 7c. In
fact, the resultant difference spectrum (blue curve) agrees
remarkably well with the experimental spectrum of 2. Due to
the low quantum yield of 2, no unambiguous conclusions can
be drawn regarding the dynamics of its formation.
In contrast to the mIR-data, however, the UV/nUV-vis
spectra reveal a very faint residual transient absorption that
appears to be stationary from ~70 ps onwards and may
therefore be attributed to a primary photochemical product. To
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Photo-initiated olefin hydrogenation
Table 2. Photo-induced hydrogenation of styrenes and aliphatic olefins by
1.
The photoactivation of monohydride complex 1 can be coupled
to H2 activation. For example, photolysis of 1 (10.6 mM, 390 nm
LED) in benzene or toluene under a D2 atmosphere (1 bar) gives
around 3 turnovers for aromatic CÀ H deuteration over the
course of 40 h. Motivated by these results, olefin hydrogenation
was examined next, using styrene as a benchmark substrate.
With 10 mol% loading of 1 (Table 1, entry 1), quantitative
hydrogenation of styrene to ethylbenzene was observed upon
photolysis (LED, 390 nm) over 15 h at r.t. under 1 bar H2.
Notably, with 5 mol% 1 a quantum yield of Φ390 =2.29% was
obtained, which is more than an order of magnitude higher
than the H/D isotopic exchange reaction with benzene.
Addition of Hg (Table 1, entry 2) does not affect the reactivity,
supporting homogeneous catalysis. No conversion is obtained
in the absence of light (Table 1, entry 3). Lower catalyst loadings
(entry 4) led to reduced yields. In turn, increased H2 pressures
afforded higher catalytic rates, enabling quantitative hydro-
genation with 1 mol% catalyst loading (Table 1, entries 5–8).
Photo-HAT-OH catalyzed by [Cp*IrH(phen)] (5 mol%, 44 h,
Entry Substrate
Product
t
Yield (conversion)
[h] [%][a]
1
2
3
15
15
15
100
100
100
4
5
15
15
100
39 (41)
6
40
15
15
51 (53)
92 (93)
7[b]
8
°
50 C, 470 nm; reductant: NEt3) exhibits a preference for 1,1-
75 (octane)
25 (2-octene)
disubstituted arylalkenes.[10] Aliphatic substrates generally gave
low yields and considerable amounts of 1,2-isomerization to
internal alkenes. This selectivity parallels HAT rates of [CpCrH
(CO)3], which follow the thermochemical stability of the formed
alkyl radical with rate retardation for sterically encumbered
olefins.[7] For comparison, the reaction scope with catalyst 1 was
examined (Table 2). Using a standard protocol (10 mol% 1,
390 nm LED, 15 h, r.t. 1 bar H2), styrenes with both electron
withdrawing and donating para-substituents (Table 2, entries 1–
4) were quantitatively hydrogenated. 1,1-Disubstituted olefins
(Table 2, entries 5 and 6) exhibited incomplete conversion even
at increased reaction times.
9
40
15
32 (33)
37 (37)
10
62 (C6H10)
15 (C6H12)
12 (C6H6)
(90)
11
40
12
13
14
15
15
26
66 (67)
20 (22)
100
Notably, aliphatic α-olefins are rapidly consumed, contrast-
ing with the trends found for [Cp*IrH(phen)]. In fact, propene
was almost quantitatively hydrogenated, at 5 mol% catalyst
loading and 2 bar H2 (entry 7). For the longer chain α-olefins,
like 1-octene (Table 2, entry 8), significant isomerization to 2-
octene (25%) was observed. This is in line with the low
hydrogenation rates found for internal olefins, such as 2-
hexene, cyclooctene, or cyclohexadiene (Table 2, entries 9–11).
[a] Determined by 1H NMR spectroscopy. [b] Conditions: substrate (1.5 bar,
0.042 mmol), 1 (5 mol%), H2 (2 bar).
As for the aromatic substrates, we attribute this selectivity to
steric reasons, exemplified by bulky (Table 2, entry 12) and
branched (Table 2, entry 13) α-olefins, or completely inert 1,1,2-
trisubstituted substrates (Table 2, entry 13). Apparently, the
same steric considerations apply to 1,2-isomerization: The
terminal double bond of 3,7-dimethyl-1,6-octadiene to 2,6-
dimethyl-2-octene (entry 14) was quantitatively and selectively
hydrogenated under these conditions within 26 h. Importantly,
except for isomerization, selectivities were quantitative within
error in all cases.
Table 1. Photo-induced hydrogenation of styrene by 1.
Entry
Cat [mol%]
H2 Pressure [bar]
t [h]
Yield (conversion) [%][a]
1
10
10
10
5
5
5
1
1
1
1
4
8
4
8
15
15
15
15
15
15
15
15
100
100
0
68 (68)
100
100
62 (63)
100
2[b]
3[c]
4
5
6
7
8
1
1
[a] Determined by 1H NMR spectroscopy. [b] In the presence of Hg. [c]
Without irradiation.
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Mechanistic examinations
tion of [Co(CH2CH2Ph)(PNP)] (5; Scheme 4), which was prepared
by salt metathesis from the chlorido complex. 5 does not react
with H2 under thermal conditions. However, photolysis (390 nm
LED) under Ar converts 5 to 1 and styrene, while 1 and Ph(CH2-
nDn)(CH3-mDm) are obtained under D2, respectively. Importantly,
the isotopic distribution in the deuteration product (n:m�1:2)
resembles that of catalytic styrene deuteration with 1. These
observations are in line with initial photochemical styrene
extrusion from 5 (Scheme 4), disfavoring the hydrogenolysis of
the cobalt(II) hydrocarbyl intermediate as a relevant step for
catalysis.
These control experiments indicate that HAT-OH is initiated
by the formation of a photoproduct that activates H2 as first
step in catalysis. Along these lines, the considerably higher
quantum yield for styrene hydrogenation than for H/D scram-
bling with the solvent is suggestive of thermal HAT-OH, which
is catalyzed by an in situ formed photoproduct. In fact, initial
photochemical styrene hydrogenation with 1 for 1 h and
subsequent monitoring without irradiation (Figure 8) revealed
catalytic ethylbenzene formation in the dark, yet with lower
reaction rates. We therefore turned to the examination of 2,
which is the product of both steady state and time resolved
photolysis of 1 (see above). Unlike H2, styrene does not bind to
2, presumably for steric reasons. Styrene hydrogenation
(2 mol% 2, 1 bar H2, r.t.) without simultaneous photolysis
confirmed that complex 2 is, in fact, a thermal catalyst
(Figure 8). Catalytic rates gradually slow down on a timescale
that is consistent with the decay of 4 to 1, which is inactive as a
thermal catalyst. As shown by the kinetic examinations (Fig-
ure 4), the deactivation rate is second order in 4. We therefore
examined initial turn-over frequencies at different catalyst
loadings (1 bar H2, 390 nm, r.t.). With 5 mol% 1, turn-over after
3 h (TOF3 h =0.7 hÀ 1) was considerably lower than with 1 mol%
(3.4 hÀ 1), supporting that catalyst deactivation has a higher
reaction order in cobalt than competing hydrogenation.
The nature of the hydrogenation reaction was examined with
the radical-clock substrate α-cyclopropylstyrene under standard
conditions (10 mol% 1, 390 nm LED, 1 bar H2). Selective hydro-
genation to the ring-opened product 2-phenyl-2-pentene is
observed (Scheme 3). This finding supports a radical mechanism
via initial HAT and subsequent ring opening of the resulting
tertiary alkyl radical, which proceeds with a unimolecular rate
[41]
constant of k=3.6·105 sÀ 1 at 22 C. In the absence of H2, this
°
substrate is isomerized to 4-phenyl-1,3-pentadiene, yet with
slightly lower rates. Catalytic styrene reduction with D2 results
in higher deuterium incorporation at the primary site of the
product (Dα:Dβ =1:1.5), suggesting reversibility of the first HAT
step, as was observed for carbonyl hydride complexes, which
undergo HAT mediated, thermal H/D exchange with olefins.[6a,7]
The nature of the active catalyst species was examined by
stoichiometric and catalytic control experiments. Hydride 1 and
styrene do not react in the absence of light. The short excited-
state lifetime precludes direct bimolecular excited-state reac-
tivity. Furthermore, photolysis of 1 and styrene under vacuum
(Scheme 3) confirms that hydrogenation requires the presence
of H2. An olefin insertion mechanism was probed by examina-
Our results support the proposed mechanism shown in
Scheme 5. Photolysis of 1 results in CoÀ H homolysis as the
activating step. After H2 addition, the dihydrogen complex 4 is
Scheme 3. Mechanistic control experiments.
Figure 8. Time dependent ethylbenzene formation by catalytic hydrogena-
tion of styrene (0.048 mmol, 1 bar H2) using either 1 (black squares) with 1 h
photolysis (blue shaded area) and subsequent reaction without irradiation,
or isolated 2 (3.8 mol%, red circles) as catalysts.
Scheme 4. Styrene deuteration with complex 5.
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amend this scheme by SF dihydride (or dihydrogen) complexes
with distinctly lower BDFEMH2 than the BDFEMH of their respective
monohydride HAT product. This thermochemical framework
enables, besides facile radical hydrogenation of styrenes, the
hydrogenation of unactivated, aliphatic olefins, which is
attributed to the increased driving force for the first HAT step
giving the thermochemically unfavourable alkyl radical.
A challenge that has to be overcome arises from the relative
bond strengths BDFEMH2 <0.5·BDFEH2 <BDFEMH, which favours
the formation of the catalytically inactive monohydride. We
demonstrated that photochemical MÀ H homolysis is a possible
strategy for catalyst (re-)activation. The stationary photolysis
and transient absorption data show that excitation of 1 with
blue light (λ=400 nm) gives the cobalt(I) complex 2 as the
direct photoproduct. The observation of a CoÀ H/D stretching
absorption in the IR pump-probe spectrum at a time delay
around t=0 suggests that the electronic excitation does not
directly populate a dissociative state. Unfortunately, more
detailed analysis of the excited state dynamics was hampered
by the low quantum yields (Φ400 <1 %), which in turn may
reflect that the photon energy (71 kcal·molÀ 1) is close to the
energy required for CoÀ H homolysis (BDFECoH =62 kcal·molÀ 1).
The spectroscopic and computational data support the
formation of the low-spin cobalt(I) dihydrogen complex 4 with
low BDFECoH2 (37 kcal·molÀ 1) after addition of H2 to the three-
coordinate photoproduct 2. The decay of 4 proceeds by
comproportionation with parent 2 via formal HAT as supported
by the distinct pressure dependence. Peters and co-workers
recently associated the dihydrogen character of a trisphosphi-
noborane {Co0H2} complex with outer-sphere HAT reactivity to a
hydrogen atom acceptor.[27g] In turn, Fout and co-workers
showed for a cobalt pincer hydrogenation catalyst that the
cobalt(I) dihydrogen resting state isomerizes to a cobalt(III)
dihydride upon olefin coordination.[3g] At the current point, our
level of mechanistic understanding does not provide compre-
hensive structure/reactivity relationships for the formal HAT
reactivity of 4 and 1. However, it is likely that the bulky pincer
ligand supports outer-sphere HAT catalysis by preventing olefin
binding that leads to full H2 oxidative addition as an entry to an
insertion-based pathway. Notably, Hanson’s catalyst that carries
a PCy2 substituted, saturated analogue of our pincer ligand was
proposed to operate via an inner-sphere mechanism.4 Steric
shielding might also be instrumental to prevent the formation
of multinuclear PNP-bridged hydrides, as observed by Arnold’s
and Mindiola’s groups.[44] Besides mechanistic predictors, the
scope and activity of such photo-assisted HAT-OH dihydride
catalysts need to be advanced in future work.
Scheme 5. Proposed mechanism for photo-initiated HAT-OH with 1.
a thermal HAT-OH catalyst. The computational evaluation
confirmed distinctly different BDFEs for the two hydrogen
atoms bound to the metal. The low CoÀ H bond strength of 4
(BDFECoH2 =37 kcal·molÀ 1) is close to the CÀ H bond strength of
the α-methylbenzyl radical (BDFECH �42 kcal·mol-1 in
toluene),[13,42] as a basis for facile and reversible initial HAT of 4
to styrene, but also to aliphatic, unactivated olefins. HAT from 4
to the olefin gives parent 1 (BDFECoH =62 kcal·molÀ 1), which is
insufficient to undergo direct HAT to styrene, but to the alkyl
radical producing a strong CÀ H bond (ethylbenzene: BDFECH
�82 kcal·molÀ 1 in toluene).[13,43] Concomitant regeneration of 2
closes the proposed catalytic cycle.
Deactivation of the catalytic cycle could result from different
processes. The low BDFECoH2 of 4 renders comproportionation
with 2 to inactive 1 thermochemically feasible and was shown
as stoichiometric, pressure dependent decay path (Figure 4).
Continuous photolysis maintains a steady state concentration
of 4 that enables thermal catalysis. We attribute the higher
catalytic rates at increased H2 pressures to higher steady state
concentrations of active catalyst 4. Deactivation could also arise
by HAT from 4 to the α-methylbenzyl radical that escaped from
the solvent cage after the first HAT. However, the low primary
quantum yields for the photoactivation of 1 suggest low steady
state concentrations of 2 and 4.
Conclusions
In this proof-of-principle study, a complementary approach to
HAT-OH was evaluated. Based on thermochemical arguments,
Holland, Shenvi and co-workers distinguished between two Experimental Section
types of catalysts that define the scope and chemoselectivity:
Materials and methods: All experiments were performed under
(a) Hydride complexes with strong field (SF) ligands and low-
spin configuration (BDFEMH =50–60 kcal·molÀ 1) enable the
reduction of activated substrates with H2, vs. (b) hydrides with
weak ligand fields (WF) in higher spin states (BDFEMH
<35 kcal·molÀ 1), which allow for the hydrogenation of unac-
tivated alkenes with alternative reductants. We would like to
inert conditions using standard Schlenk and glove-box techniques
under argon atmosphere. Photolysis experiments were carried out
using a Kessil PR160-390 LED (390 nm, 40 W) while keeping the
sample at room temperature by a water bath. Solvents were
purchased in HPLC quality (Sigma Aldrich) and dried using an
MBraun Solvent Purification System. THF was additionally dried
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°
over Na/K. Deuterated solvents were obtained from Deutero GmbH
and dried over Na/K (benzene-d6, THF-d8, Tol-d8). 1-Cyclopropylvin-
yl)benzene was synthesized according to a reported procedure.[45]
All other chemicals were used as received: Styrene (TCI), 4-
fluorostyrene (TCI), 4-methylstyrene (Sigma Aldrich), 4-meth-
oxystyrene (Sigma Aldrich), 4-tert-butylstyrene (TCI), propene
(Sigma Aldrich), 1,4-cyclohexadiene (Sigma Aldrich), cyclooctene
(Sigma Aldrich), trans-2-hexene (Thermo Fischer), 1-octene (Sigma
Aldrich), (R)-(+)-limonene (Sigma Aldrich), (+)-β-citronellene (Sigma
Aldrich), 1,1-diphenylethylene (Sigma Aldrich), α-methylstyrene
(Sigma Aldrich), propylene (Sigma Aldrich), LiALH4 (Sigma Aldrich).
NMR spectra were recorded on Bruker Avance III 300 or Avance III
400 MHz spectrometers and the spectra were calibrated to the
residual solvent signals (benzene-d6: δH =7.16 ppm, THF-d8: δH =
1.74 and 3.58 ppm, toluene-d8: δH =2.08 ppm). Magnetic moments
in solution (benzene-d6) were determined at room temperature by
Evans’ method as modified by Sur and corrected for diamagnetic
contribution.[46] LIFDI (Linden CMS) mass spectra were measured by
the Zentrale Massenabteilung, Fakultät für Chemie, Georg-August-
Universität. Elemental analyses were obtained from the Analy-
tisches Labor, Georg-August-Universität using an Elementar Vario
EL 3 analyzer. IR spectra were obtained in Nujol on a Thermo
Science Nicolet iZ10 and using a Bruker ALPHA FTIR spectrometer
with Platinum ATR module. Electronic absorption spectra were
recorded with a Varian Cary 300 Scan spectrophotometer and an
Agilent Cary 60 equipped with an Unisoko Cryostat (CoolSpek)
using J-Young quartz cuvettes. EPR measurements were performed
in air-tight J-Young quartz tubes in an atmosphere of purified
argon. Frozen solution EPR spectra were recorded on a Bruker EMX-
plus CW X-band spectrometer equipped with a Bruker ER 4112HV-
CF100 helium cryostat. The spectra were obtained on freshly
prepared solutions of 1–10 mM compound and simulated using
EasySpin[47] via the cwEPR GUI.[48] Temperature-dependent magnetic
susceptibility measurement was carried out with a Quantum-Design
MPMS3 SQUID magnetometer equipped with a 7 Tesla magnet in
the range from 210 to 2.0 K at a magnetic field of 0.5 T.
in pentanes and recrystallized at À 36 C overnight. The solution
was decanted and the dark purple crystalline material (yield: 44 mg,
90%) was dried in vacuo. Anal. Found (Calcd) for C20H41CoNP2: C,
54.32 (54.17); H, 9.39 (9.09); N, 9.12(9.48). 31P NMR (C6D6, 300 MHz,
ppm): 81.6. 1H NMR (C6D6, 300 MHz, ppm): 1.47 (36 H, CH3,
3
3
A18BCXX’A’18B’C’, JAX =12.97 Hz), 4.01 (2H, PCH, JBC =5.23 Hz), 6.65
3
3
3
~
(2H, NCH, N= j JCX + JCX’ j =37.67 Hz, JBC =5.23 Hz). ATR-IR: nNN
=
2012 cmÀ 1
.
Photolysis of 1in C6D6: 1 (5.0 mg, 0.012 mmol) in 0.4 mL C6D6 in a
J-Young NMR tube was degassed by two freeze-pump-thaw cycles.
The solution was photolyzed by a 390 nm LED lamp in a water bath
at room temperature. The same procedure was carried out for the
photolysis in different deuterated solvents.
Synthesis of [Co(CH2CH2Ph)(PNP)] (5): PhCH2CH2MgCl (1 M,
0.15 mL, in THF) was dropwise added to the solution of [CoCl(PNP)]
(15.0 mg, 0.033 mmol) in THF (4 mL). The reaction mixture was
stirred at room temperature for 1 h and the solvent subsequently
removed in vacuo. The residue was extracted with pentanes. After
filtration, the solution was dried in vacuo. The residue was dissolved
°
in pentanes and recrystallized at À 36 C overnight. The solution
was decanted and the dark red crystalline product dried in vacuo
(yield: 7.8 mg, 45%). Anal. Found (Calcd) for C28H49CoNP2: C, 64.25
1
(64.60); H, 9.46 (9.49); N, 2.64(2.69). H NMR (C6D6, 300 MHz, ppm):
13.98 (tBu), 3.85, 0.24, À 6.56, À 23.01, À 58.69, À 71.92.
General procedure for photo-induced olefin hydrogenation: In a
typical experiment, complex
1 (2 mg, 4.8 μmol, 1 eq.) and
hexamethylbenzene (2 mg, 12.3 μmol, 2.5 eq.) as internal standard
were dissolved in C6D6 (0.45 mL) in a J-Young NMR tube. After
addition of the substrate (48 μmol, 10 eq.), the NMR tube was
degassed by one freeze-pump-thaw cycle and 1 bar of hydrogen
gas was added after warming to room temperature. The resulting
solution was then photolyzed at 390 nm (LED), while the temper-
ature was kept at room temperature using a water bath. The
conversion and yield were determined by 1H NMR spectroscopy.
Crystallographic details: Single crystals of 1 and 3 were selected
from the mother liquor under an inert gas atmosphere and
transferred in protective perfluoro polyether oil on a microscope
slide. The selected and mounted crystals were transferred to the
cold gas stream on the diffractometer. The diffraction data were
obtained at 100 K on a Bruker D8 three-circle diffractometer,
equipped with a PHOTON 100 CMOS detector and an INCOATEC
microfocus source with Quazar mirror optics (Mo-Kα radiation, λ=
0.71073 Å). The data obtained were integrated with SAINT and a
semi-empirical absorption correction from equivalents with SADABS
was applied. The structure was solved and refined using the Bruker
SHELX 2014 software package.[49] All non-hydrogen atoms were
refined with anisotropic displacement parameters. All CÀ H hydro-
gen atoms were refined isotropically on calculated positions by
using a riding model with their Uiso values constrained to 1.5Ueq of
their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all
other atoms. The CoÀ H hydrogen atom in 1 was found from the
residual density map and isotropically refined.
Synthesis of [CoH(PNP)] (1): A vial was charged with [CoCl(PNP)]
(50.0 mg, 0.111 mmol), LiAlH4 (2.1 mg, 0.055 mmol) and THF
(10 mL). The reaction mixture was stirred at room temperature for
1 min. The solvent of the orange solution was removed in vacuo.
The residue was extracted with pentanes and the resulting solution
was filtered over Celite, and dried in vacuo. The residue was
°
dissolved in pentanes and recrystallized at À 36 C overnight. The
solution was decanted and the orange crystalline material (yield:
35 mg, 76%) was dried in vacuo. Anal. Found (Calcd) for
C20H41CoNP2: C, 57.28 (57.68); H, 9.89 (9.92); N, 3.36 (3.36). μeff
=
1.97�0.27 μB. 1H NMR (C6D6, 300 MHz, ppm): 9.85 (tBu), À 26.28
~
(CH2), À 48.28 (CH2), the hydride signal was not found; ATR-IR: nCoH
=
1756 cmÀ 1. The isotopologue [CoD(PNP)] (2-D) was analogously
prepared from [CoCl(PNP)] (40.0 mg, 0.088 mmol) and LiAlD4
(1.9 mg, 0.044 mmol) and obtained in as orange crystalline product
(yield: 25 mg, 69%). 1H NMR (C6D6, 300 MHz, ppm): 9.63 (tBu),
~
À 26.53 (CH2), À 46.97 (CH2); ATR-IR: nCoD =1269 cmÀ 1
.
Synthesis of [Co(PNP)] (2): A vial was charged with [CoCl(PNP)]
(50.0 mg, 0.111 mmol), KC8 (18 mg, 0.133 mmol) and Na/K dried
THF (10 mL). The reaction mixture was stirred under static vacuum
at room temperature overnight. The solvent of the solution was
removed in vacuo. 1H NMR (THF-d8, 300 MHz, ppm): 27.35 (tBu),
86.84 (CH), À 90.62 (CH).
Deposition Numbers 2078520 (for 1) and 2078519 (for 3) contain
the supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge Crystallographic
Data Centre and Fachinformationszentrum Karlsruhe Access Struc-
tures service www.ccdc.cam.ac.uk/structures.
Computational details: Density functional theory (DFT) calculations
of Gibbs free energies were carried out with the ORCA program
package.[50] Due to charge transfer between the sterically close-
lying parts of the tert-butyl groups to the central cobalt atom in 1,
the range-separated ωB97XÀ D3BJ functional[51] was employed
together with the zeroth-order regular approximation (ZORA) for
Synthesis of [CoN2(PNP)] (3): A vial was charged with [CoCl(PNP)]
(50.0 mg, 0.111 mmol), KC8 (18 mg, 0.133 mmol) and THF (10 mL).
The reaction mixture was stirred under N2 atmosphere at room
temperature overnight. The solvent of the solution was removed in
vacuo. The residue was extracted with pentanes and the resulting
solution was filtered, and dried in vacuo. The residue was dissolved
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scalar relativistic effects[52] and the corresponding ZORA-def2-TZVP
basis set.[53] The resolution of identity and chain of spheres
exchange (RIJCOSX) algorithm[54] with automatically selected auxil-
iary basis sets[55] was invoked to speed up the calculations. The
Conductor-like Polarizable Continuum Model (CPCM)[56] was used to
model the solvent benzene. In order to avoid numerical instabilities,
a Gaussian smearing[57] was employed for the point charges. The
state characters and the electronic absorption spectrum were
additionally modelled in ORCA with SC-NEVPT2/CASSCF (strongly
contracted N-electron valence state perturbation theory on top of
complete active space self-consistent field) calculations on a
reduced model of 1, where the tert-butyl groups were replaced
with methyl groups, employing an active space of 11 electrons in
14 orbitals, a def2-TZVP basis set,[53] the RI-JK (resolution of identity
for Coulomb and exchange integrals) approximation,[58] and
perturbative spin-orbit couplings for 20 quartet and 30 doublet
states. EPR spectra of the complex 1 were calculated both with DFT
as a single-reference method and SC-NEVPT2 as a multiconfigura-
tional method. The DFT calculations were carried out with the ADF
program[59] using the ωB97XÀ D functional,[51] a TZ2P basis set,[60]
ZORA including spin-orbit effects, and toluene as implicit solvent.
The SC-NEVPT2 calculations were done as described above, using
only 2 quartet and 3 doublet states to save computational time.
VO593/8-1). Open access funding enabled and organized by
Projekt DEAL.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: cobalt · hydrogenation · hydrogen atom transfer ·
photochemistry · transient absorption spectroscopy
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Transient spectroscopy: Femtosecond UV-pump/mid-infrared-
probe (UV/mIR) and UV-pump/near-UV-to-visible probe (UV/nUV-
vis) spectroscopy was carried out with a setup previously described
elsewhere.[61] In brief, 60 fs-duration pulses with a center wave-
length of 800 nm were provided by a commercial Ti:sapphire
oscillator/regenerative amplifier front-end system (Newport Spectra
Physics, Solstice Ace) at a repetition rate of 1 kHz. In both
experiments, pump pulses centered at 400 nm were generated by
frequency doubling the front-end output in a type-I BBO crystal.
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properly tuned home-built optical parametric amplifier (OPA) in a
type-I AgGaS2 crystal. Vis-to-near-UV white light continuum probe
pulses with a spectrum covering the range from 380 nm to 950 nm
were generated by focusing a small fraction of the signal pulses of
a
commercial OPA (tuned to 1240 nm, TOPAS prime, Light
Conversion) into a CaF2-substrate. UV/nUV-vis spectra were re-
corded with a commercial transient absorption spectrometer (TAS,
Newport/Spectra Physics). Solutions of 1 and 1-D in n-hexane,
benzene-h6, as well as benzene-d6 were prepared in a glovebox and
measured in a sealed stationary sample cell to minimize decom-
position upon contact with moisture and air. For the UV/mIR
experiments, the sample cell was equipped with two CaF2 windows
that were held apart by a lead spacer at a distance of 100 μm.
Probing in the mid-IR with benzene solutions was not possible due
strong solvent background absorptions in the spectral regions of
interest. For the UV/nUV-vis experiment, a commercial cell (Hellma,
QS) with an optical pathlength of 1 mm optical path was used.
Each measurement was repeated several times with fresh solutions
in thoroughly cleaned cuvettes. A slow degradation of the sample
over a period of several hours was observed and the dinitrogen
complex 3 was found to accumulate in the small sample volume of
about 350 μL.
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Acknowledgements
This work was supported by Deutsche Forschungsgemeinschaft
(DFG) within Priority Program SPP 2102 “Light-controlled
reactivity of metal complexes” (GO1059/8-1, SCHN950/6-1,
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www.chemeurj.org
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© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
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These are not the final page numbers!

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doi.org/10.1002/chem.202101705
Chemistry—A European Journal
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Manuscript received: May 13, 2021
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www.chemeurj.org
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© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
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These are not the final page numbers!

FULL PAPER
S. Sang, T. Unruh, Dr. S. Demeshko,
Dr. L. I. Domenianni, N. P. van Leest,
Dr. P. Marquetand, Dr. F. Schneck, Dr. C.
Würtele, F. J. de Zwart, Prof. Dr. B.
de Bruin, Prof. Dr. Dr. h.c. L. González,
Prof. Dr. P. Vöhringer, Prof. Dr. S.
Schneider*
Radical hydrogenation of olefins via
stepwise hydrogen atom transfer
(HAT-OH) is a complementary
approach to alkene reduction that has
attracted considerable interest in
recent years. Blue light photolysis of a
cobalt(II) monohydride pincer
complex enabled olefin hydrogena-
tion via photo-initiated HAT-OH. Fem-
tosecond pump-probe spectroscopy
confirmed Co–H homolysis to a three-
coordinate cobalt(I) complex, which
binds H2 and catalyzes thermal HAT-
OH even of unactivated alkenes.
1 – 13
Photo-Initiated Cobalt-Catalyzed
Radical Olefin Hydrogenation