Visible-Light-Enhanced Cobalt-Catalyzed Hydrogenation: Switchable Catalysis Enabled by Divergence between Thermal and Photochemical Pathways

Article abstract of DOI:10.1021/acscatal.0c05136

The catalytic hydrogenation activity of the readily prepared, coordinatively saturated cobalt(I) precatalyst, (R,R)-(iPrDuPhos)Co(CO)2H ((R,R)-iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene), is described. While efficient turnover was observed with a range of alkenes upon heating to 100 °C, the catalytic performance of the cobalt catalyst was markedly enhanced upon irradiation with blue light at 35 °C. This improved reactivity enabled hydrogenation of terminal, di-, and trisubstituted alkenes, alkynes, and carbonyl compounds. A combination of deuterium labeling studies, hydrogenation of alkenes containing radical clocks, and experiments probing relative rates supports a hydrogen atom transfer pathway under thermal conditions that is enabled by a relatively weak cobalt-hydrogen bond of 54 kcal/mol. In contrast, data for the photocatalytic reactions support light-induced dissociation of a carbonyl ligand followed by a coordination-insertion sequence where the product is released by combination of a cobalt alkyl intermediate with the starting hydride, (R,R)-(iPrDuPhos)Co(CO)2H. These results demonstrate the versatility of catalysis with Earth-abundant metals as pathways involving open-versus closed-shell intermediates can be switched by the energy source.

Full text of DOI:10.1021/acscatal.0c05136

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Research Article
Visible-Light-Enhanced Cobalt-Catalyzed Hydrogenation:
Switchable Catalysis Enabled by Divergence between Thermal and
Photochemical Pathways
Lauren N. Mendelsohn, Connor S. MacNeil, Lei Tian, Yoonsu Park, Gregory D. Scholes,
and Paul J. Chirik*
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ABSTRACT: The catalytic hydrogenation activity of the readily
prepared, coordinatively saturated cobalt(I) precatalyst, (R,R)-
(iPrDuPhos)Co(CO)2H ((R,R)-iPrDuPhos = (+)-1,2-bis[(2R,5R)-
2,5-diisopropylphospholano]benzene), is described. While efficient
turnover was observed with a range of alkenes upon heating to 100
°C, the catalytic performance of the cobalt catalyst was markedly
enhanced upon irradiation with blue light at 35 °C. This improved
reactivity enabled hydrogenation of terminal, di-, and trisubstituted
alkenes, alkynes, and carbonyl compounds. A combination of
deuterium labeling studies, hydrogenation of alkenes containing
radical clocks, and experiments probing relative rates supports a
hydrogen atom transfer pathway under thermal conditions that is
enabled by a relatively weak cobalt−hydrogen bond of 54 kcal/
mol. In contrast, data for the photocatalytic reactions support light-induced dissociation of a carbonyl ligand followed by a
coordination-insertion sequence where the product is released by combination of a cobalt alkyl intermediate with the starting
hydride, (R,R)-(iPrDuPhos)Co(CO)2H. These results demonstrate the versatility of catalysis with Earth-abundant metals as
pathways involving open- versus closed-shell intermediates can be switched by the energy source.
KEYWORDS: cobalt, hydrogenation, switchable mechanism, radicals, visible light, alkene
styrene to ethyl benzene was observed (Scheme 1).19 Density
functional theory (DFT) calculations established a relatively
INTRODUCTION
■
The interaction of light with transition-metal complexes opens
weak Co−H bond dissociation free energy (BDFE) of 54 kcal/
mol (wB97XD/{cc-pVDZ,def2-TZVP}), raising the possibility
of a hydrogen atom transfer (HAT) pathway during
catalysis.20−25 The straightforward synthesis and bench
stability of this complex motivated additional studies into
catalytic applications with 1 using H2 as the stoichiometric
reductant.
While the relatively weak Co−H BDFE of 1 opens
opportunities for catalytic HAT, it would be notable and
potentially advantageous to use light to switch on distinct
pathways involving closed-shell intermediates. For example, a
pioneering work by Wrighton et al., later supported by more
detailed independent spectroscopic studies by Grant et al. and
new opportunities in catalysis as applied to organic synthesis.1,2
Visible light is an attractive source of energy insofar as it is
cost-effective, sustainable, and able to selectively excite
photocatalysts over substrates in solution to enable distinct
transformations.3 Most methods have relied on iridium and
ruthenium photocatalysts to transfer energy from their excited
states to secondary catalysts and ultimately organic sub-
strates.4−6 Considerably less effort has been devoted to the
excitation of transition-metal complexes by visible light for
direct use in catalysis.7−14 One attractive feature of direct,
visible-light promoted catalysis is the potential to unlock
mechanistic pathways under irradiation that are distinct from
thermal reactivity.15−18 In this way, by changing only the
energy source, a single catalyst can follow divergent pathways
for reaction with a substrate, leading to different products or
enhancement in selectivity and scope.
Received: November 23, 2020
Revised: December 29, 2020
Published: January 13, 2021
Our laboratory has recently reported the synthesis and initial
reactivity studies of (R,R)-(iPrDuPhos)Co(CO)2H (1) in the
context of alkene hydroformylation. In the presence of 50 bar
of synthesis gas (1:1 H2/CO), exclusive hydrogenation of
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Scheme 1. Previous Work: (a) Reaction of 1 with Styrene under Thermal Hydroformylation Conditions Yields the Exclusive
Hydrogenated Product.19 (b) UV Irradiation Promoted the Hydrogenation of Olefins by Fe(CO)5.26 (c) Blue Light Allows
Switchable Hydroboration of α,β-Unsaturated Ketones by [PPh(OEt)2]4CoH.46 This Work: (d) Switchable Mechanism and
Scope of Hydrogenation by 1 Using Heat or Visible Light
Harris et al. in addition to computational support from Weitz
et al., demonstrated that irradiation of Fe(CO)5 with UV light
enabled catalytic hydrogenation of alkenes by a coordination-
insertion pathway by photoinduced CO dissociation (Scheme
1b).26−32 Subsequent work by Wegman and Brown further
explored the unique photochemistry of a variety of metal
carbonyl hydrides under UV irradiation.33,34 UV irradiation of
metal carbonyl complexes has also been reported to promote a
variety of transformations including hydrogenation, hydro-
silylation, and hydroformylation.35−45 More recently, Teskey
and co-workers have used visible light to promote the
divergent 1,2- and 1,4-selective hydroboration of α,β-
unsaturated ketones by [PPh(OEt)2]4CoH under thermal
and photochemical conditions by proposed mechanisms
involving electron transfer and phosphine dissociation,
respectively (Scheme 1c).15,46 Use of visible light is an
attractive alternative to avoid complications associated with
traditional UV photochemistry for the excitation of metal
carbonyls.
associated electronic transitions of 1. A broad absorption band
was observed with a maximum in the solvent region, extending
to around 450 nm in the toluene solution UV−vis spectrum
(Figure 1). Because blue Kessil LED lamps have a maximum
Because of its absorption features, supporting CO ligands,
and preliminary hydrogenation activity, 1 was an attractive
candidate for switchable catalysis between thermal HAT-type
chemistry and photodriven organometallic chemistry. Alkene
hydrogenation was selected as a representative transformation
given its utility in synthesis, widespread use in industrial
chemistry, the rich chemistry associated with bis(phosphine)
cobalt complexes, and demonstrated applications in metal-to-
olefin HAT (MOHAT).47−52 Here, we describe the thermal
and photochemical hydrogenation activities of 1 and
exploration of the mechanism of these transformations
(Scheme 1d). In the thermal chemistry, an HAT mechanism
is proposed while upon irradiation, the data support a pathway
involving CO dissociation followed by an alkene coordination-
insertion sequence involving closed-shell intermediates.
Figure 1. Experimental UV−vis spectrum of 1 (blue), TD-DFT
computed UV−vis spectrum of 1 (red), and normal emission
spectrum of the blue Kessil LED lamp (green).
emission at 465 nm, extending to 390 nm (Figure 1), these
were selected as the light source. Ground-state DFT
calculations (wB97XD/{cc-pVDZ,def2-TZVP}) shed light on
the orbitals most likely involved in electronic transitions
(Figure 2). Specifically, calculations showed the cobalt-
centered HOMO (highest occupied molecular orbital) and
HOMO − 1 states with electron density around the Co−CO
bonding orbitals, LUMO and LUMO + 1 states primarily
centered on the benzene ring of the [(R,R)-iPrDuPhos] ligand,
and LUMO + 2 and LUMO + 3 states with a significant
amount of electron density placed on the two equatorial CO
ligands. Time-dependent DFT (TD-DFT) calculations
(wB97XD/{cc-pVDZ,def2-TZVP}) predicted a maximum
RESULTS AND DISCUSSION
■
Thermal and Photochemical Hydrogenation Activ-
ities of 1 with Unsaturated Substrates. Our studies
commenced with the evaluation of 1 as a photocatalyst. Blue-
light irradiation was selected because of the yellow color and
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Young NMR tubes and monitoring the progress of the reaction
demonstrated a dramatic rate enhancement for the photo-
chemical reaction over the thermal one (Figure 3). After 45
Figure 3. Reaction time courses for the hydrogenation of α-methyl
styrene to isopropyl benzene under blue-light (blue) and thermal
(100 °C, red) conditions.
Figure 2. Selected relevant DFT-computed molecular orbitals of 1
relevant to visible-light absorption.
min, the thermal reaction proceeded to just over 5% yield
while the photochemical reaction reached 66% yield,
demonstrating a remarkable acceleration despite the signifi-
cantly elevated temperature in the thermal case.
absorbance at 267 nm, with a tail feature extending to 444 nm
(Figure 1). Predicted lowest energy transitions include a
singlet transition at 354 nm, corresponding to transitions from
the HOMO and HOMO − 1 orbitals to the LUMO + 2
orbital. Three additional low-probability triplet transitions,
calculated at 393, 422, and 444 nm, correspond primarily to
transitions from the HOMO and HOMO − 1 orbitals to the
LUMO + 2 and LUMO + 3 orbitals (states). Excitation into
either the predicted 354 nm singlet excited state or the low-
probability triplet states, accessible by the emission of the blue
LED, supports the possibility of light-induced CO labilization.
Subsequently, 1 was evaluated as a precatalyst for the visible-
light-driven hydrogenation of alkenes. Initial conditions
employed a 0.1 M solution of substrate in benzene-d6 with 5
mol % 1 with 4 atm of H2. In a typical experiment, the reaction
vessel was irradiated with blue Kessil LED lamps for 18 h, with
an internal temperature of approximately 35 °C (see the
Supporting Information for details). Under these conditions,
the hydrogenation of α-methyl styrene produced >99% yield
Inspired by these findings, the scope of the hydrogenation
performance of 1 was explored. Both thermal (100 °C) and
photochemical reactions (blue Kessil LED lamp) were carried
out with a variety of alkenes, alkynes, and carbonyl compounds
(Table 1). Similar to the hydrogenation of α-methyl styrene,
the yields for the catalytic hydrogenation of most alkene
substrates varied among the three operating conditions. At
room temperature and in the absence of ambient light, the
olefin hydrogenation activity with 1 as the precatalyst was
typically poor. For example, styrene (3a) was hydrogenated in
20% yield and 3,4-dihydropyran (3 m) in 3% yield. Use of the
activated alkene dimethyl 2-methylenemalonate (3o) increased
the yield to 60%. All other substrates evaluated under these
conditions did not proceed to any appreciable yield. In this
case, the low yields appear to be a result of slow reaction
kinetics, rather than catalyst death, as 1 remained intact as
1
judged by H and 31P NMR spectroscopic analysis following
1
each reaction. Heating the hydrogenation reactions to 100 °C
under dark conditions improved the yields in some cases, but
turnover remained poor for most sterically hindered, essentially
unactivated alkenes.
by H NMR spectroscopy of isopropyl benzene, while control
reactions in the dark at room temperature reached <0.5%
conversion (Scheme 2). Heating the hydrogenation to 100 °C
for 18 h produced alkane in 37% yield. Repeating the thermal
and photochemical hydrogenations of α-methyl styrene in J-
Irradiation of the catalytic hydrogenation reactions pro-
moted by 1 with blue light resulted in remarkably improved
performance (Table 1). The photocatalytic method was
effective with a range of mono-, di-, and trisubstituted alkenes,
and in most cases, the corresponding alkane was obtained in
>99% yield. Generally, thermal reactivity was more sensitive to
the degree of substitution on the substrate than the
photocatalytic method. As the substitution on the alkene was
increased or more electron-rich substrates were used, the
photocatalytic method outperformed the thermal one. For
example, the thermal hydrogenation performance of 1 with
arylated alkenes, including styrene (3a) (>99% yield), α-
Scheme 2. Hydrogenation of α-Methyl Styrene by 1 under
Both Thermal and Photochemical Conditions
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Table 1. Scope of C=E Hydrogenation with 1 as the Precatalyst in the Absence of Light at Room Temperature, 100 °C
(Thermal), and with Irradiation with Blue Light (35 °C)
methyl styrene (3b) (37% yield), 1,1-diphenyl ethylene (3c)
(38% yield), trans-stilbene (3d) (<2% yield), and trans-α-
methyl stilbene (<0.5% yield), became progressively worse
with increased substitution on the C=C bond. By contrast, the
photocatalytic method was less sensitive to these changes as
hydrogenations of styrene, α-methyl styrene, and 1,1-diphenyl
ethylene all proceeded to >99% yield, and trans-stilbene
reached 78% yield. Similar to the dark (23 °C) case,
diminished yields in all reported thermal and photocatalytic
reactions are a result of slow reaction kinetics rather than
deactivation as 1 was observed as the exclusive cobalt product
following the catalytic reactions. A similar trend was observed
with arylated alkynes. While phenyl acetylene (3e) reached full
conversion to ethyl benzene under both conditions, the
hydrogenation of diphenylacetylene (3f) to 1,1-diphenylethane
produced 73% yield upon irradiation but only 32% under
thermal conditions. Attempts to hydrogenate trans-α-methyl
stilbene with 1 under all three conditions did not produce any
appreciable conversion, establishing a limit for the cobalt-
catalyzed method.
Aliphatic alkenes followed an analogous trend in reactivity.
While norbornene (3g), 4-tert-butylmethylenecyclohexene
(3j), and 1-hexene (3l) reached complete conversion under
both thermal and photocatalytic conditions, yields diverged
with more substituted substrates. For example, hydrogenations
of cyclohexene (3h) (75% yield), cyclopentene (3i)(>99%
yield), and 3-methylpent-2-ene (3k) (50% yield) reached
significant conversion under photochemical conditions, but no
appreciable yield of alkane was observed under thermal
conditions.
However, the thermal reaction remained more sensitive to
steric effects as hydrogenations of 3,4-dihydropyran (3m) and
ethyl (E)-3-phenylbut-2-enoate (3p) reached just 9% yield and
26% yield, respectively, under thermal conditions, while >99%
yield was obtained upon irradiation with blue light.
Other substrates bearing carbon-element (C=E) unsaturated
groups underwent hydrogenation using 1 as the precatalyst.
Interestingly, acridine (3q) was hydrogenated in the 9,10-
positions under both thermal and photochemical conditions.
Under thermal conditions, a mixture of 9,10-dihydroacridine
(20% yield) and the corresponding dimer, octahydro-9,9′-
biacridine (22% yield), were obtained. Under photocatalytic
conditions, exclusive formation of the dimer was observed
(25% yield). Acridine was the only substrate evaluated that was
hydrogenated to a higher extent under thermal than photo-
catalytic conditions. Finally, 2-phenylpropanal (3r) was
hydrogenated in >99% yield under photocatalytic conditions
but was hydrogenated to just 11% yield under thermal
conditions.
To assess the use of blue LEDs with this cobalt complex,
parallel hydrogenations of MAA (3n) (0.014 g, 0.012 mmol)
with 1 (0.002 g, 0.004 mmol, 3.3 mol %) were performed with
both blue-light and UV irradiation. The solvent was changed
from benzene to a mixture of pentane/THF (2:1) to
accommodate the UV light. Identical solutions were irradiated
for 7 min under 4 atm of H2 by either two blue Kessil LED
lamps (465 nm) or a UV irradiation box (Rayonet) cooled
with water. Monitoring the outcomes of these reactions by 1H
NMR spectroscopy revealed that while >99% conversion to the
hydrogenated product occurred with blue light, UV irradiation
resulted in just 48% conversion to the alkane. The decrease in
performance under UV conditions is likely a result of
competing absorption by the organic starting material and
product, illustrative of the problems associated with UV
Both thermal and photocatalytic methods performed well
with substrates bearing electron-withdrawing substituents. For
example, methyl 2-acetamidoacrylate (MAA, 3n) and dimethyl
2-methylene malonate (3o), both activated olefins, reached
>99% yield under thermal and photocatalytic conditions.
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chemistry and further highlighting the advantage of visible-
light-promoted reactivity.
Scheme 4. Investigations into the Role of Radical
Intermediates in Catalytic Hydrogenations Promoted by 1.
(a) H-Atom Abstraction and Regeneration of 1 with
TEMPO and H2. (b) Catalytic Hydrogenation of 2,4,6-Tri-t-
butyl-phenoxy Radical under 4 atm of H2
To determine if the [(R,R)-iPrDuPhos] ligand was unique in
promoting photodriven cobalt-catalyzed hydrogenation, a
variety of other bis(phosphines) were also evaluated. Emphasis
was placed on cobalt complexes with chelating ligands to avoid
potential complications from phosphine dissociation with
monodentate analogs. Treatment of Co2(CO)8 with a variety
of bis(phosphines) generated [(P−P)Co(CO)(μ2-CO)]2
complexes.19,53 Derivatives with MeDuPhos, dppf, dcyp, and
dppbenz (dppf = 1,1′-ferrocenediyl-bis(diphenylphosphine);
dcyp = bis(dicyclohexylphosphino)ethane; dppbenz = 1,2-
bis(diphenylphosphino)benzene) were synthesized and pro-
duced complete conversion to ethyl benzene in the photo-
driven hydrogenation of styrene (Scheme 3a). A variety of
demonstrating the ability of 1 to undergo HAT and
subsequent reconstitution using hydrogen as the terminal
reductant. Likewise, catalytic hydrogenation of 2,4,6-tert-butyl-
phenoxy radical in the presence of 5 mol % 1 and 4 atm of H2
afforded 2,4,6-tert-butyl-phenol in >95% yield after 3 days,
demonstrating the catalytic HAT performance of the cobalt
hydride, 1 (Scheme 4b).
Scheme 3. (a) Hydrogenation of Styrene Using [(P−
P)Co(CO)(μ2-CO)]2 Precatalysts Bearing the MeDuPhos,
dppf, dcyp, and dppbenz Ligands Gives >99% Ethyl
Benzene under Standard Blue-Light Conditions. (b)
Dicobalt Octacarbonyl Gives Only Trace Conversion of
Styrene under Blue-Light Conditions
Three additional catalytic experiments were conducted to
explore the possibility of HAT-type reactivity with alkenes.
First, the catalytic hydrogenation of an established radical
clock, α-cyclopropyl styrene (ring opening rate constant of 3.6
× 105 s−1 at 22 °C),55 was conducted. Under thermal
conditions (100 °C) with 5 mol % 1, full conversion to a
mixture including α-cyclopropyl ethyl benzene, 2-phenyl
pentane, and 2-phenyl pent-2-ene was observed as judged by
1H NMR spectroscopy and GC−MS (Scheme 5a). Observa-
Scheme 5. Radical Probes for the Catalytic Reaction of 1
with Alkenes. (a) Hydrogenation of α-Cyclopropyl Styrene.
(b) Hydrogenation of 4-tert-Butylmethylenecyclohexene
and (c) of Methyl Acetamido Acetate (MAA)
other substrates evaluated with the dcyp variant achieved
similar yields to reactions with 1 (see the Supporting
Information for details). However, while general reactivity
was not impacted, these complexes could not be generated as
cleanly as the [(R,R)-iPrDuPhos] variant. Additionally,
complexes bearing the [(R,R)-iPrDuPhos] ligand were selected
for the added advantage of chirality as a mechanistic handle. A
control reaction with Co2(CO)8 with blue-light irradiation and
4 atm of H2 produced no ethyl benzene (Scheme 3b).
Although this cobalt precursor is red, this control experiment
was conducted for completeness and to ensure that if blue-light
irradiation induced phosphine dissociation, there was no
background hydrogenation reaction.
Mechanistic Investigations of Thermal Hydrogena-
tion with 1. The hydrogenation activity of 1 under thermal
(100 °C) and photocatalytic conditions and remarkably
improved performance upon irradiation prompted investiga-
tions into the mechanism of the reaction under both
conditions. The relatively weak Co−H BDFE of 54 kcal/
mol, previously reported for 1, raised the possibility of proton
coupled electron transfer (PCET) or HAT as a possible
mechanism for thermal hydrogenation.20,22,24,54
tion of ring-opened products supports the formation of a
benzylic radical during hydrogenation. Next, the catalytic
hydrogenation of 4-tert-butylmethylenecyclohexene (3j) was
performed. As reported by Shenvi and co-workers,56 this
alkene favors the formation of the cis-product under a classical
hydrogenation pathway (kinetic control) and the trans-
diastereomer from radical reactivity (thermodynamic control).
With 5 mol % 1 under thermal conditions, a 7.3:1 ratio
favoring the trans diastereomer of the alkane along with a
To explore potential radical reactivity, a 15 mM solution of 1
was treated with an excess (1.3 equiv.) of TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy radical) (Scheme 4a). Monitoring
1
the reaction by H and 31P NMR spectroscopy at ambient
temperature revealed complete conversion to
[(R,R)-iPrDuPhosCo(CO)(μ2-CO)]2 (2) after 10 min. Sub-
sequent exposure to 1 atm of H2 gas resulted in complete
regeneration of 1 after 10 h at room temperature,
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minor (22%) endocyclic olefin arising from isomerization
(Scheme 5b) was observed, supporting radical chemistry.
Finally, the hydrogenation of MAA at 100 °C in the presence
of 5 mol % 2 generated a racemic mixture of products (Scheme
5c). By comparison, [(R,R)-(iPrDuPhos)Co(μ2-CO)]2, a
precatalyst that differs by only one carbon monoxide ligand
from 1, hydrogenates MAA with >99% ee (enantiomeric
excess), suggesting an outer-sphere pathway with 1.53
similarities. For example, catalytic hydrogenation of 3 in the
presence of 5 mol % 1 and irradiation with blue light also
favored the trans diastereomer of the alkane product (4.6:1
trans/cis), suggesting that the pathway is under thermody-
namic control (Scheme 5b) Likewise, photocatalytic hydro-
genation of MAA in the presence of 5 mol % 1 and 4 atm of H2
gave a racemic product as judged by chiral GC analysis
(Scheme 5c).
As has been reported previously,19 the reaction of (R,R)-
(iPrDuPhos)Co(CO)2D (1-D) with 5 equiv. of styrene in
benzene at 60 °C for 3 h resulted in deuterium incorporation
into the two β-positions of the alkene as well as in the
formation of a small amount of α,β-deuterated ethyl benzene
(Scheme 6). Although exclusive β-deuteration is possible
By contrast, photochemical hydrogenation of an alkene
containing a radical clock, α-cyclopropyl styrene, in the
presence of 5 mol % 1 resulted in 98% conversion to α-
cyclopropyl ethyl benzene with less than 1% of each of the ring
opened and isomerized products (Scheme 5a). Photodriven
hydrogenation of vinyl cyclopropane, a radical clock with a fast
(estimated 7 × 106 s−1 at 22 °C)55 ring-opening rate constant,
1
Scheme 6. Reactions of 1-D with Styrene
produced only trace ring opening by H NMR spectroscopy,
suggesting that radical intermediates are not formed during
hydrogenations of 1 with blue light (see the Supporting
Information for details). Together, these results suggest that
while similarities in the overall thermal and photochemical
reaction pathways may exist, the initial step is likely different
and radical intermediates derived from the alkene are not
formed with an appreciable concentration or lifetime.
To further experimentally probe these mechanistic differ-
ences, 5 equiv. of styrene were added to a benzene solution of
1-D and irradiated with blue light for 3 h. Deuterium
incorporation was observed in both the α- and β-positions of
the alkene in a 1:1:1 ratio as judged by 2H NMR spectroscopy,
while the ethyl benzene product was deuterated only in the β-
position (Scheme 6). Repeating this experiment and
monitoring the progress of the reaction by 2H NMR
spectroscopy over the course of 3 h at ambient temperature
established that the three styrene positions were deuterated at
the same rate and that the appearance of β-deuterated ethyl
benzene was slower.
As shown in Scheme 7, it is likely that upon irradiation with
blue light, CO dissociation occurs, opening a site for alkene
coordination and insertion. The deuterium incorporation
pattern in the recovered styrene demonstrates, for this
substrate, that both 1,2- and 2,1-insertion pathways are
under a selective 2,1-insertion mechanism, this outcome is
more consistent with an HAT pathway and when combined
with the mechanistic probes detailed above, the observed β-
deuteration likely arises from formation of a stabilized α-
carbon radical. Reaction of 1-D with 5 equiv. of styrene in
benzene under 1 atm of CO for 3 h at 75 °C established that
the reaction proceeded in the presence of excess carbon
monoxide, demonstrating that CO dissociation is not required
for interaction of the alkene with cobalt in the thermal
pathway. Holistically, these results all point to HAT as the
operative mechanism for thermal hydrogenation. Such a
pathway also accounts for the lack of thermal hydroformylation
activity observed with 1.19 Outer-sphere HAT between 1 and
the alkene suppresses subsequent and requisite insertion events
required for carbonylation; instead, capture of the alkyl and
subsequent C−H bond formation release the alkane product.
Investigations into the Mechanism of Light-Pro-
moted Hydrogenation. The enhanced alkene hydrogenation
performance upon irradiation with blue light prompted
investigations into the mechanism of the photodriven reaction
and what differences, if any, exist from the thermal process.
Using the probes applied to the thermal reaction demonstrated
Scheme 7. Proposed Pathways to Account for the
Deuteration of Styrene and Ethylbenzene in Stoichiometric
Reactions with 1-D
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operative and reversible. In the absence of H2, the observed
isotopomer of deuterated ethylbenzene demonstrates that only
the 2,1-insertion intermediate is on the pathway for alkane
formation. The formation of ethyl benzene may occur by a
bimetallic event whereby the cobalt alkyl and 1 (or 1-D)
cooperatively release the alkane by either an inner- or outer-
sphere pathway.57 If the rate of insertion and elimination is
much faster than the rate of reductive elimination or second
HAT event in the absence of H2, an abundance of Co−H
generated from insertion/elimination would account for the
observation of ethyl benzene deuterated only in the β-position.
Such a pathway also resolves the seemingly conflicting
outcomes from the radical probes outlined in the previous
section. Specifically, while initial hydrogen transfer may occur
by HAT in the thermal case and by a CO dissociation-alkene
coordination-insertion sequence under photochemical con-
ditions, formation of a common intermediate following these
steps may enable identical pathways for reductive elimination.
Guided by this hypothesis, additional experiments were
conducted to understand the source of divergent reactivity in
the initial reaction step(s). Light-induced labilization of
monodentate ligands is well-established to promote hydro-
genation and other alkene functionalization reactions by
creating a vacant site for alkene coordination.15,26,35 In
contrast to Fe(CO)5, which only undergoes exchange with
13CO gas upon irradiation, the carbonyl ligands in 1 underwent
rapid exchange with 13CO gas under dark, room-temperature
conditions and at a variety of pressures.58 Even at relatively low
pressures (0.2 atm), isotopic exchange occurred too rapidly to
be studied quantitatively by 13C NMR spectroscopy. While
these results demonstrate that CO is labile under both dark
and light conditions, they do not establish where the
equilibrium lies between the five-coordinate complex, 1, and
the putative four-coordinate, mono(carbonyl) cobalt complex,
(R,R)-(iPrDuPhos)Co(CO)H, or its lifetime in solution.
Because CO exchange is a thermoneutral process, when
13CO is in excess, any amount of four-coordinate (R,R)-
(iPrDuPhos)Co(CO)H formation results in exchange. On the
other hand, CO for alkene exchange is an unfavorable reaction,
and thus, light-induced destabilization of 1 and formation of a
greater population of (R,R)-(iPrDuPhos)Co(CO)H is likely the
origin of the observed rate enhancement in catalytic hydro-
genation reactions.
The quantum yield for the catalytic hydrogenation of styrene
with 1 was measured by using an iron(oxalate) photochemical
standard.59 With 0.8 mmol of styrene and 0.2 mol % 1 in 1 mL
of benzene-d6, an 11% yield of ethyl benzene was obtained in 4
min following irradiation by a blue Kessil LED lamp (440 nm)
fitted with a 420 nm long pass filter, corresponding to a
quantum yield of 0.17. Performing an “on−off” experiment for
the photodriven hydrogenation of α-methyl styrene demon-
strated that light was required for catalytic turnover (Scheme
8b). Monitoring the hydrogenation by NMR spectroscopy
demonstrated that 1 was the only cobalt complex observed
during the course of the reaction, both in the light and in the
dark. Transient absorption spectroscopic measurements
conducted at 420 nm established an excited state lifetime of
37 ps, and no appreciable excited state quenching was
observed in the presence of styrene, cyclopentene, or CO
(see the Supporting Information for details). These results
demonstrate that 1 does not react directly with the substrate
but is rapidly converted to another intermediate upon
irradiation. After 54 ps, a slightly positive TA signal was
Scheme 8. Additional Studies into the Photochemical
Reactions of 1. (a) 13CO Exchange Reactions; (b) Catalytic
On−Off Reaction with α-Methyl Styrene; (c) Conversion of
1 to 2 upon Irradiation with Blue Light
observed in the absence of the quencher or in the presence of
CO, while a slightly negative signal was obtained in the
presence of cyclopentene or styrene. While these results
suggest partial photodegradation of 1 by alternative pathways,
the newly formed species cannot be assigned from TA data
alone. Additional experiments were performed to understand
the fate of 1 following photolysis.
While heating a benzene solution of 1 at 120 °C for 7 days
in the absence of alkene produced no conversion of the metal
complex, irradiation with blue light at 35 °C rapidly produced
a color change from yellow to orange. Monitoring the
photochemical reaction by 31P NMR spectroscopy established
16% conversion of 1 to 2 after 2 h with concomitant formation
of H2 gas (Scheme 8c). Repeating the photolysis under H2 or
CO gas fully suppressed this reactivity.
These results are analogous to those reported by Wegman
and Brown in the study of HCo(CO)4 and other related metal
carbonyl hydrides.33,34 The data support a radical pathway
initiated by photoinduced cleavage of Co2(CO)8 to form
Co(CO)4· that in turn initiates a chain reaction resulting in
conversion of HCo(CO)4 to Co2(CO)8 and H2. Invoking this
precedent, the photoconversion of 1 to 2 and H2 is likely
initiated by the photolysis of 2, a red compound, to generate
(R,R)-(iPrDuPhos)Co(CO)2·. However, no reaction was
observed upon exposure of a benzene-d6 solution of 1 to
green LEDs (525 nm), which emit light within the absorption
range of 2 but not 1, suggesting that the photoinitiation step is
not homolytic cleavage of 2. While additional exposure to a
blue LED fitted with a 475 nm long pass filter for 1 h produced
trace (<2%) conversion of 1 to 2, exposure to a blue LED
fitted with a 420 nm long pass filter resulted in a 9%
conversion from 1 to 2. These results support the conclusion
that visible-light transitions of 1 are responsible for the
photodegradation activation mode. Because of the sterically
demanding phosphines, it is more likely that blue light assists
in the photodissociation of a carbonyl ligand from 1 to
generate (R,R)-(iPrDuPhos)Co(CO)H. This intermediate then
likely reacts with 1 to form a bimetallic intermediate that
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Research Article
releases H2 and generates 2. This conclusion is consistent with
previously reported experiments from our group, whereby
treatment of (R,R)-(iPrDuPhos)Co(CO)Cl with NaHBEt3
rapidly produced [(R,R)-(iPrDuPhos)Co(μ2-CO)]2, demon-
strating the instability of the proposed (R,R)-(iPrDuPhos)Co-
(CO)H intermediate.19
both the thermal and photochemical pathways may reach the
same cobalt-alkyl intermediate. This intermediate is then
intercepted with another equivalent of 1 to cooperatively
release the saturated substrate and 2, which can homolytically
cleave H2 to restart the catalytic cycle.
To demonstrate the impact of switchable catalysis, 1 was
used to hydrogenate 4-vinylcyclohex-1-ene, a substrate
containing both an endocyclic and a terminal alkene, under
both thermal and photocatalytic conditions (Scheme 9). The
CONCLUSIONS
■
The coordinatively saturated, air-stable cobalt hydride, (R,R)-
(iPrDuPhos)Co(CO)2H, is active for the catalytic hydro-
genation of a host of alkenes under both thermal and
photochemical conditions. Competition, deuterium labeling,
and radical clock experiments established two distinct
pathways from each of the catalytic conditions. In the thermal
reactions, the data are most consistent with a hydrogen atom
transfer pathway, consistent with the relatively weak cobalt−
hydrogen bond of 54 kcal/mol. Notably, the cobalt catalyst can
be regenerated using H2 following HAT to the substrate.
Performing the catalytic hydrogenations with blue light
significantly enhanced reactivity and expanded the scope of the
cobalt-catalyzed method to include sterically hindered,
unactivated alkenes and carbonyl compounds. Mechanistic
experiments support a pathway that involves initial photo-
dissociation of a carbonyl ligand followed by coordination-
insertion of the substrate into the metal−hydrogen bond. This
work opens new opportunities for catalysis where Earth-
abundant elements containing weak metal−hydrogen bonds
under thermal conditions can be applied to outer-sphere
reactivity while exposure to visible light can be used to open
coordination sites and enhance performance.
Scheme 9. Divergent Outcomes from the Hydrogenation of
4-Vinylcyclohex-1-ene from Thermal versus Photochemical
Conditions
thermal procedure resulted in the exclusive hydrogenation of
the terminal alkene, forming 4-ethylcyclohex-1-ene in 45%
yield and leaving the endocyclic double bond untouched. By
contrast, the photochemical reaction resulted in the hydro-
genation of both the endocyclic and terminal alkenes to give
ethylcyclohexane in 85% yield. These divergent outcomes,
obtained from simply changing from heat to light stimulus,
demonstrate the ability to alter the chemoselectivity of the
reaction using an Earth-abundant metal catalyst.
The combined experimental data support the pathway
presented in Scheme 10. While experimental results strongly
Scheme 10. Proposed Catalytic Cycle for Thermal and
Photodriven Hydrogenation with 1
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05136.
Experimental details, compound characterization data
and preparation, characterization data for hydrogenation
products, and spectroscopic and transient absorption
data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Paul J. Chirik − Department of Chemistry, Frick Chemistry
Laboratory, Princeton University, Princeton, New Jersey
08544, United States; orcid.org/0000-0001-8473-2898;
Email: pchirik@princeton.edu
Authors
Lauren N. Mendelsohn − Department of Chemistry, Frick
Chemistry Laboratory, Princeton University, Princeton, New
Jersey 08544, United States; orcid.org/0000-0002-0596-
6838
Connor S. MacNeil − Department of Chemistry, Frick
Chemistry Laboratory, Princeton University, Princeton, New
Jersey 08544, United States; orcid.org/0000-0001-6434-
8397
Lei Tian − Department of Chemistry, Frick Chemistry
Laboratory, Princeton University, Princeton, New Jersey
08544, United States
Yoonsu Park − Department of Chemistry, Frick Chemistry
Laboratory, Princeton University, Princeton, New Jersey
08544, United States; orcid.org/0000-0003-1511-0635
suggest that the thermal reaction occurs by initial hydrogen
atom transfer, the enhanced reactivity of 1 under light is likely
the distinct result of photoinduced CO dissociation followed
by substrate coordination and migratory insertion into the
cobalt−hydride bond. By opening this pathway, light enables
the hydrogenation of a variety of substrates that are unreactive
toward HAT, which relies on the stability of the incipiently
formed radical. While caged-radical pairs for the thermal
reaction and H2 oxidative addition in the photochemical
reaction have not been ruled out, similarities in experimental
results regarding the final hydrogenation step suggest that
following either radical recombination or migratory insertion,
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(17) Peiris, E.; Sarina, S.; Waclawik, E. R.; Ayoko, G. A.; Han, P.; Jia,
J.; Zhu, H. Y. Plasmonic Switching of the Reaction Pathway: Visible-
Light Irradiation Varies the Reactant Concentration at the Solid−
Solution Interface of a Gold−Cobalt Catalyst. Angew. Chem., Int. Ed.
2019, 58, 12032−12036.
(18) Fumagalli, G.; Rabet, P. T. G.; Boyd, S.; Greaney, M. F. Three-
Component Azidation of Styrene-Type Double Bonds: Light-
Switchable Behavior of a Copper Photoredox Catalyst. Am. Ethnol.
2015, 127, 11643−11646.
(19) MacNeil, C. S.; Mendelsohn, L. N.; Zhong, H.; Pabst, T. P.;
Chirik, P. J. Synthesis and Reactivity of Organometallic Intermediates
Relevant to Cobalt-Catalyzed Hydroformylation. Angew. Chem., Int.
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functionalization of Alkenes by Hydrogen-Atom Transfer. In Organic
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383−468.
Gregory D. Scholes − Department of Chemistry, Frick
Chemistry Laboratory, Princeton University, Princeton, New
Jersey 08544, United States; orcid.org/0000-0003-3336-
7960
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05136
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
Catalysis Science program, under Award DE-SC0006498.
(21) Hu, Y.; Shaw, A. P.; Estes, D. P.; Norton, J. R. Transition-Metal
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Cobaloximes with Hydrogen: Products and Thermodynamics. J. Am.
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