Electronic Ligand Modifications on Cobalt Complexes and Their Application toward the Semi-Hydrogenation of Alkynes and Para-Hydrogenation of Alkenes

Article abstract of DOI:10.1021/acs.organomet.9b00337

The effect of the electronic modification of a bis(carbene) pincer ligand, (^MesCCC^R), on cobalt catalysis has been investigated. The pincer ligand was modified in the para position of the aryl backbone with a tert-butyl and trifluoromethyl moiety to yield the electronic variants that were applied toward the synthesis and characterization of several cobalt complexes, (^MesCCC^R)Co. The application of the (^MesCCC^R)Co^I(N₂)PPh₃ complexes toward the semihydrogenation of alkynes revealed that while the tert-butyl group does not impact reactivity, the loss of electron density at the metal center, by the installation of the CF₃ group, does affect product ratios. Further inspection of the proposed mechanism suggested that the installation of the trifluoromethyl group slows down olefin hydrogenation. This finding was further supported in the application of the (^MesCCC^R)Co^I-py (py = pyridine) complexes toward the parahydrogenation of ethyl acrylate, which demonstrated that the electron-withdrawing ligand variant produced less polarization.

Full text of DOI:10.1021/acs.organomet.9b00337

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Electronic Ligand Modifications on Cobalt Complexes and Their
Application toward the Semi-Hydrogenation of Alkynes and Para-
Hydrogenation of Alkenes
Safiyah R. Muhammad, Joseph W. Nugent, Kenan Tokmic, Lingyang Zhu, Jumanah Mahmoud,
and Alison R. Fout*
School of Chemical Sciences, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801,
United States
S
* Supporting Information
ABSTRACT: The effect of the electronic modification of a
bis(carbene) pincer ligand, (MesCCCR), on cobalt catalysis has
been investigated. The pincer ligand was modified in the para
position of the aryl backbone with a tert-butyl and trifluoromethyl
moiety to yield the electronic variants that were applied toward the
synthesis and characterization of several cobalt complexes,
(
MesCCCR)Co. The application of the (MesCCCR)CoI(N2)PPh3
complexes toward the semihydrogenation of alkynes revealed that
while the tert-butyl group does not impact reactivity, the loss of
electron density at the metal center, by the installation of the CF3
group, does affect product ratios. Further inspection of the proposed
mechanism suggested that the installation of the trifluoromethyl
group slows down olefin hydrogenation. This finding was further supported in the application of the (MesCCCR)CoI-py (py =
pyridine) complexes toward the parahydrogenation of ethyl acrylate, which demonstrated that the electron-withdrawing ligand
variant produced less polarization.
trifluoromethyl (CF3) on their (RCCC)Ir complex and
INTRODUCTION
■
commented on its application in catalytic alkane dehydrogen-
ation.12 The analogous (MesCCC)Co, [MesCCC = bis(2,4,6-
trimethylphenyl-benzimidazol-2-ylidene)phenyl)], system re-
ported by our group has been previously employed for the
catalytic hydrogenation of alkenes, alkynes, and nitriles.13−15 In
these studies, parahydrogen (p-H2) was used as a spectro-
scopic tool to probe the mechanism of hydrogenation. The use
of p-H2 is a powerful tool in hydrogenation studies because it
allows for signal amplification of hydrogenated products by
NMR spectroscopy when certain conditions are met.16 This
allows for otherwise undetectable reaction intermediates to be
observed under standard conditions. Observing the polar-
ization of substrates allowed for the mechanism of alkene and
alkyne hydrogenation to be proposed with the (MesCCC)Co
system.
Modification of the steric and electronic properties of ligands
to control the reactivity of metal centers is a common method
employed by organometallic chemists to alter the catalytic
properties of complexes.1 Alteration of the electronic proper-
ties of ligands can largely effect the selectivity,2−4 catalytic
activity,5,6 and mechanistic7,8 pathways of these catalysts.
The chemistry of transition metals featuring meridional
pincer ligands is an area that has benefited from the exploration
of electronic modification to the ligand backbone. Small
electronic and steric perturbations of these frameworks can
have large effects on the reactivity of these systems. For
example, studies done by the Chirik and Nakazawa groups
have shown that electronic variation of their bis(imino)-
pyridine iron complexes leads to the disruption of catalysis9 or
fluctuation of reaction rates, as dictated by the geometry
imposed by such modifications.10 Another example, offered by
Brookhart and co-workers, showed that the electronic
substitution in the para position of their bis(phosphonite)
ligand influenced the initial turnover frequencies (TOF) of
alkane transfer dehydrogenation.11 They concluded that when
there is less electron density at the metal center, the initial rates
of the reaction are faster.
Herein, we report the synthesis and characterization of both
electron-rich and -poor analogues of (MesCCC)Co complexes.
Utilizing IR spectroscopy, the CO stretching frequencies were
used to determine the perturbation of the electron density at
the cobalt centers. These catalysts were then applied toward
the semihydrogenation of alkynes and the parahydrogenation
Chianese and co-workers developed and modified the RCCC
(RCCC = bis(aryl-benzimidazol-2-ylidene)phenyl)] pincer
ligand used in this study. They reported the installation of a
Received: May 20, 2019
© XXXX American Chemical Society
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Figure 1. (a) Synthesis of complexes 1-R (R = H, tBu, or CF3). (b) X-ray crystal structures of 1-tBu and 1-CF3 with the ellipsoids pictured at the
50% probability level (one out of two symmetrically independent molecules shown for 1-CF3). Solvent molecules and hydrogen atoms have been
omitted for clarity.
of ethyl acrylate, to investigate the effect of these perturbations
on these catalytic processes.
formation of a brown solution that, upon workup, yielded the
(
MesCCCR)Co-py complexes (py = pyridine), 2-tBu and 2-CF3
1
(Figure 2). Characterization by H and 13C NMR spectros-
copies confirmed the formation of new complexes, whose
integrations were consistent with the pyridine-bound for-
RESULTS AND DISCUSSION
■
Synthesis of Modified Ligands. To achieve electronic
modification without affecting the steric profile of the ligand
framework, substituents were placed on the aryl ring in the
position para to the CAr−Co bond formed upon metalation.
The CF3 group and tert-butyl (tBu) groups were chosen for
their electronic properties and ease of ligand synthesis. The
benzimidazolium salts, [H3 (Me sCCCtBu)]Cl2 and
[H3(MesCCCCF3)]Cl2, were prepared following our modified
literature procedure.17,18
1
mulation (Figure S13−16). The H NMR spectra display a
broadening of the bound pyridine resonances and an overall
shifting of the resonances upfield from the 1-R complexes. The
number of Mes-CH3 resonances suggests that the 2-R
complexes retain their C2-symmetry.
The stretching frequency of the carbonyl group by IR
spectroscopy was shown by Tolman to reflect the strength of
the other electron donor/acceptor ligands on the metal
center.19 Comparing the carbonyl stretching frequencies of
the substituted complexes to the unmodified complex should
Synthesis of (MesCCCR)CoCl2py (1-R, R = Bu and CF3).
t
Metalation of the benzimidazolium ligand salts,
[H3(MesCCCtBu)]Cl2 and [H3(MesCCCCF3)]Cl2, following an
established protocol17 afforded the (MesCCCR)CoCl2py
t
provide data on the electronic influence of the Bu or CF3
ligand modification on the cobalt center. Therefore, the
dicarbonyl species, (MesCCCR)Co(CO)2 (3-H, 3-tBu, and 3-
CF3) were synthesized in good yields from the reduction of
complexes 1-H, 1-tBu, or 1-CF3 with two equivalents of KC8,
followed by the addition of 1 atm of CO (Figure 2).
t
products (R = Bu (1-tBu) or CF3 (1-CF3)) as bright green
powders in good yields (76% and 88%, respectively, Figure
1a). Characterization of 1-tBu by 1H NMR spectroscopy
revealed a diamagnetic spectrum with 12 resonances,
consistent with the formation of a low-spin, C2-symmetric
CoIII species, (MesCCCtBu)CoCl2py complex (Figure S8).
1
Characterization by H NMR spectroscopy revealed diamag-
netic spectra, all consistent with the formation of a C2-
symmetric complex, with integration supporting the predicted
1
Similarly, 1-CF3 was characterized by H NMR spectrosco-
(
MesCCCR)Co(CO)2 formulation (Figures S17, S19, and S20).
py, with a diamagnetic spectrum containing 11 resonances and
The 13C NMR spectra of 3-R were also consistent with this
formulation, with the appearance of a new downfield resonance
assigned to the ligated CO (Figures S18, S20, and S22).
Yellow crystals of 3-H, suitable for single-crystal X-ray
diffraction studies, were grown from a concentrated solution of
THF. The crystal data revealed a five-coordinate cobalt
complex with two carbonyl ligands on the metal center in a
distorted trigonal bipyramidal geometry (τ = 0.55),20 as shown
in Figure 2. The M−Caryl bond is shorter than the length in
analogous (POCOP)Co(I) dicarbonyl (POCOP = κ3-C6H3-
1,3-[OPR2]2, R = alkyl, aryl) pincer complexes reported by
Heinekey,21 and the (PCP)Fe(II) dicarbonyl (PCP = κ3-C6H3-
1,3-[XPR2]2, X = O, NH) complexes reported by Sortais and
co-workers.22 We attribute this, potentially, to the increased
rigidity of the CCC ligand compared with the PCP ligands.
The Co−CCO bond length, 1.7776(19) Å, falls within the
range of other reported Co(I) dicarbonyl complexes,
1.735(3)−1.800(1) Å.23−27
a
19F NMR spectrum containing a single resonance at −61.04
ppm, consistent with the formation of (MesCCCCF3)CoCl2py
(Figures S10 and S12).
Crystals suitable for X-ray diffraction were grown from slow
evaporation of a benzene (1-tBu) or chloroform (1-CF3)
solution at room temperature. Refinement of the structural
data revealed the desired octahedral-CoIII complexes (Figure
1b and Table S3). The Co−CNHC and Co−CAr bond lengths
for 1-tBu and 1-CF3 were comparable to those of the
previously reported (MesCCC)CoCl2py complex, 1-H,17 as
were the Co−Npy and Co−Cl distances for both complexes. In
addition, the CAr−Co−Npy and CAr−Co−Cl bond angles of
179.63(12) and 85.80(10)/86.90(10)° for 1-tBu and
179.51(22) and 85.56(17)/86.31(17)° for 1-CF3 supported
the predicted octahedral geometry. As expected, these results
suggest that the electronic modification had no significant
impact on the solid-state structure of the Co(III) complex.
Synthesis of Co(I) Complexes (2-R, 3-R, and 4-R).
Given the stability of complexes 1-R in air and water and the
reactivity of previously isolated Co(I) complexes, we sought to
synthesize the Co(I) analogues of 1-tBu and 1-CF3. Treatment
of 1-tBu or 1-CF3 with 2.5 equivalents of KC8 resulted in the
Characterization of 3-H by infrared spectroscopy revealed
two intense stretches at 1918 and 1973 cm−1 (Figure S23),
corresponding to the two stretching modes expected for the
dicarbonyl complex in a trigonal pyramidal geometry. The IR
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Figure 2. Synthesis of complexes 2-R, 3-R, 4-R, 5-R, and 6-R (R = H, tBu, or CF3). Crystal structures of 3-H and 4-CF3 with the ellipsoids pictured
at the 50% probability level. Solvent molecules and hydrogen atoms have been omitted for clarity.
spectra of 3-tBu and 3-CF3 also contained two stretches for the
ligated CO (Figures S24 and S25), suggesting these complexes
were isostructural to 3-H. For 3-CF3, there is a blue shift in the
C−O stretching frequencies, to 1951 and 2004 cm−1,
consistent with electron density being removed from the
metal center. This suggests a weakening of the extent of π-
backbonding which would strengthen the C−O bond in the
carbonyl moieties. However, the values for 3-tBu do not shift
from 3-H as expected, despite the presence of a more electron-
donating R-group, as they also blue shift to 1939 and 1990
Crystallographic characterization of 4-CF3 confirmed the
expected distorted square pyramidal geometry at the cobalt
center (τ = 0.14), with a bound dinitrogen ligand trans to the
Co−CAr bond (Figure 2). We proposed that if there is indeed
less π back-bonding, there would be a strengthening of the
Co−CAr bond, while the Co−N bond would weaken in the
structure of 4-CF3 when compared to 4-H. The Co−CAr bond
was found to be 1.8677(21) Å, slightly shortened from the
bond length of 1.8750(13) Å in 4-H. The Co−N bond
distance of 1.8412(19) Å is elongated from that of 1.8270(12)
Å observed in 4-H. The N−N bond distance of 1.0873(26) Å
is consistent with a relatively unactivated N2 ligand (free N2 at
1.0975(1) Å).30
t
cm−1. We hypothesize that because Bu is a relatively weak
electron donor, its effect may be more delocalized over the aryl
backbone as opposed to that seen with the strongly electron-
withdrawing CF3 group.28
Reactivity of (MesCCCR)Co(N2)PPh3 (4-R) with H2 and
D2. In our previous reports of the hydrogenation of olefins and
alkynes, the active catalyst was proposed to be the cobalt(I)-
dihydrogen complex, (MesCCC)Co(H2)PPh3 (5-H). There-
fore, to understand how electronic modifications might affect
the formation of the dihydrogen complex, we sought to
synthesize this complex with the modified ligands. Exposure of
a benzene-d6 solution of 4-tBu to 1 atm of H2(g) at 77 K
resulted in an immediate color change of the solution from red
to orange (Figure 2). 1H NMR spectroscopy revealed a
diamagnetic spectrum with features distinct from the starting
complex. A resonance at 4.47 ppm was assigned to free H2(g)
and a broad resonance at −5.39 ppm, integrating to 2H, was
assigned as the bound H2 molecule in the proposed
Previously, we reported the complex (MesCCC)Co(N2)PPh3
(4-H) (νNN = 2112 cm−1) as a precatalyst for a variety of
hydrogenation studies. We sought to isolate the analogous
complexes with the modified ligands to then compare the
reactivity of each. Addition of one equivalent of PPh3 to 2-R
produced a red solution that upon workup yielded the
corresponding (MesCCCR)Co(N2)PPh3 complexes, 4-R (Fig-
1
ure 2). Characterization of 4-tBu by H NMR and 13C NMR
spectroscopies revealed spectra analogous to those reported for
4-H (Figures S26 and S27).17 Characterization by IR
spectroscopy revealed a stretch at 2112 cm−1 that was assigned
as the vibrational mode of a largely unactivated dinitrogen
ligand (free dinitrogen νNN = 2331 cm−1)29 and was consistent
with the designation of this complex as (MesCCCtBu)Co(N2)-
PPh3 (Figure S31). Because of the lipophilicity of 4-tBu,
crystals suitable for X-ray diffraction were not able to be
obtained.
(
MesCCCtBu)Co(H2)PPh3 complex, 5-tBu (Figure S33).
1
Similar treatment of 4-CF3 with H2(g) yielded a H NMR
spectrum with a broad resonance located at −5.26 ppm,
integrating for 2H, which too was assigned as bound
dihydrogen (Figure S34). All other resonances were consistent
with the formation of the (MesCCCCF3)Co(H2)PPh3 complex,
5-CF3. It was previously reported that exposure of 5-H to a
dinitrogen atmosphere resulted in the displacement of the
bound H2 by N2 to regenerate 4-H.13 Similarly, when 5-tBu
and 5-CF3 were re-exposed to a dinitrogen atmosphere, an
immediate color change from orange to red was noted,
indicating reformation of the dinitrogen complexes 4-tBu and
1
Similarly, the H and 13C NMR spectra of 4-CF3 is minorly
perturbed from 4-H and 4-tBu, while the dinitrogen vibrational
mode at 2123 cm−1 is consistent with the suggested
formulation of (MesCCCCF3)Co(N2)PPh3 (Figures S28, S29,
S32). The blue shift of the ligated N2 in 4-CF3 as compared
with 4-H and 4-tBu further suggests that the installation of the
electron-withdrawing group leads to less π back-donation into
the dinitrogen antibonding orbitals, strengthening the N−N
bond.
1
4-CF3 (confirmed by H NMR spectroscopy).
C
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To further test the effect of ligand modification on the
lability of the H2 ligand, we measured the exchange rate of
bound H2 with free H2. A spin saturation study was conducted
on each dihydrogen complex which revealed that the exchange
rates for 4-H, Bu, and CF3 were 0.25, 0.31, and 0.30,
respectively, demonstrating that ligand substitution does not
significantly impact H2 lability.
par with 4-H in terms of the ratio of desired alkene to alkane,
4-CF3 generated a significantly less amount of the alkane, 5%
compared to the 18% from 4-H. Similarly, for trimethyl-
(thiophen-3-ylethynyl)silane, 4-CF3 outperformed both the
original catalyst and 4-tBu. This theme was consistent with
((2-bromophenyl)ethynyl)trimethylsilane, as well, with both
4-tBu and 4-CF3, showing incomplete isomerization of the Z
isomer to the E isomer.
t
Our group has shown previously that upon exposure of 4-H
to HD gas, the complex generates 6-H, (MesCCC)Co(HD)-
PPh3, as well as H2 and D2(g). Similarly, upon exposure of 4-H
to a 50:50 mixture of H2 and D2(g), HD gas was generated
(Figure 2).13 Both experiments provide evidence that the
dinitrogen complex activates H2/D2 to generate transient Co-
hydrides/deuterides that reductively eliminate to form HD or
H2/D2. Because of the modified electron density at the cobalt
center, we sought to explore if the H2 activation chemistry may
have been altered.31 To this end, complexes 4-tBu and 4-CF3
were treated with 1 atm of a 50:50 mixture of H2 and D2 gas at
77 K. The 1H NMR spectra of the resulting complexes
revealed the presence of free H2 gas at 4.47 ppm, a triplet at
4.43 ppm which was assigned to free HD gas, and doublet of
triplets at −5.46 and −5.42 ppm for (MesCCCtBu)Co(HD)-
PPh3, 6-tBu, and −5.33 and −5.28 ppm for (MesCCCCF3)Co-
(HD)PPh3, 6-CF3, which were assigned to the bound HD
molecule (Figures S35 and S36). The JHD coupling constants
for the bound HD triplets were 31 and 33 Hz for 6-tBu and 6-
CF3, respectively, both comparable to that of 6-H at 33 Hz.
Utilizing the correlations established by Morris,32 we
calculated the dHH to be 0.90 and 0.87 Å for 6-tBu and 6-
CF3, respectively, which is consistent with other Co(I)-H2
complexes.33,34
To better understand whether the proposed mechanism14 of
cis-hydrogenation, followed by trans-isomerization was being
affected by the installation of the CF3 moiety, parahydrogen
induced polarization (PHIP) transfer studies were used to
track the addition of hydrogen onto the substrate (see pages
S33−36). The addition of para-hydrogen (p-H2) to unsatu-
rated substrates allows for signal amplification of the
hydrogenated product by NMR spectroscopy, making the
detection of reaction intermediates, undetectable under
standard conditions, accessible. In order to observe signal
enhancement with p-H2 some conditions must be met: the H
atoms from a single p-H2 molecule must be added in a pairwise
fashion to the same substrate in magnetically distinct
positions.16,35 Utilizing a 45° pulse and a double quantum
OPSY (Only Parahydrogen Spectroscopy) filter,36 we were
able to investigate trimethyl(thiophen-3-ylethynyl)silane under
catalytic conditions (Figure 3). With 4-tBu and 4-CF3 as the
Alkyne Semi-Hydrogenation Studies. With evidence
demonstrating similarities in the ability of the modified
complexes to interact with dinitrogen and dihydrogen, their
catalytic performance was evaluated to determine if electronic
differences would impact their catalytic performance. Our
group has previously reported the semihydrogenation of
internal alkynes bearing various functionalities with 4-H;
therefore, this was chosen as a model reaction to benchmark
the modified ligand frameworks in catalysis.14 As depicted in
Table 1, three substrates were compared and analyzed for
product distribution and ratios of the E to Z isomers formed.
Studies were conducted using previously optimized con-
ditions for catalyst 4-H with 4 atm of H2 gas at 30 °C in THF
for 17 h. Diphenylacetylene showed complete conversion of
the starting alkyne to a ratio of the E-alkene and alkane when
4-H was the catalyst. Interestingly, while 4-tBu performed on
Figure 3. Parahydrogenation of trimethyl(thiophen-3-ylethynyl)silane
to the cis-isomer by 4-tBu and 4-CF3.
catalysts, the only hyperpolarized intermediate observed was
3
that of the cis-hydrogenated product, with a JHH coupling
constants of 13.8 and 14.0 Hz for 4-tBu and 14.3 and 14.3 Hz
3
for 4-CF3 (the trans-hydrogenated product has JHH coupling
constants of 18.8 Hz).37 This suggests that the reaction still
proceeds through cis-hydrogenation followed by trans-isomer-
ization to yield the products observed by GC-MS and NMR
spectroscopy.
a
Table 1. Alkyne Semihydrogenation Data with 4-R
We propose that the modification of the electronic
properties of the catalyst does not interfere with the proposed
mechanism (cis-hydrogenation followed by trans-isomeriza-
tion) (Figure S56); however, the installation of the electron
withdrawing group does influence alkene hydrogenation, as a
decreased production of the overhydrogenated product
(alkane) was observed. With the use of 4-CF3, olefin activation
may be the new rate-determining step since neither the binding
of alkyne nor its activation is being hindered on the same time
scale (alkynes were completely hydrogenated). In addition,
H2/D2 exchange experiments and calculated bond lengths of
H2 support that the activation of H2 is not affected and
therefore not responsible for the differing product ratios.
a
The first value is the alkene yield, the value in parentheses is the
alkane yield, and the third value below is the E:Z isomer ratio. Each is
given as an average of duplicate runs.
D
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In order to probe olefin activation, 4-R was reacted with 4-
vinylcyclohexene and H2(g) and the reaction was monitored
by 1H NMR spectroscopy (Figures S47 and S48). Catalyst 4-H
converts 4-vinylcyclohexene to 4-ethylcyclohexene in 2 h,
while at the same time point, only 52% of the 4-
ethylcyclohexene product is formed when 4-CF3 is used as
the catalyst. This result supports that alkene hydrogenation is
impacted by the ligand modification in 4-CF3.
Parahydrogenation of Ethyl Acrylate. Lastly, we
wanted to study how the installation of these modifying
groups would effect the signal enhancement observed with the
use of para-hydrogen. Recently, we have shown the use of 2-H
as a PHIP catalyst in the hydrogenation of ethyl acrylate to
ethyl propionate.38 The hyperpolarization of ethyl propionate
in the 1H spectrum was observed, and the transfer of
polarization to neighboring 13C nuclei, notably to the carbonyl
moiety, was observed in the 13C spectrum. Given the
differences observed in the alkyne and alkene hydrogenation
studies, especially for 4-CF3, we were curious how the ligand
modification would impact the overall polarization of ethyl
propionate.
hydrogenation, there was no buildup of polarization (see SI for
requirements for observance of polarization).
Although the spontaneous transfer of polarization is not
thought to be dependent on the catalyst, but only on the
magnetic field, the presence of catalyst can increase relaxation
of the polarized product, thus decreasing the height of the
polarized signal. Monitoring the catalysis by 13C spectroscopy
revealed that both 2-tBu and 2-CF3 are able to catalyze the
polarization of α, β, and carbonyl carbons, albeit with lower
signal enhancement than that observed with 2-H (Figures S52
and S54).
CONCLUSIONS
■
In conclusion, a series of cobalt complexes bearing the
electronically modified monoanionic bis(carbene) (MesCCCR)
ligand, with a trifluoromethyl group as the electron-with-
drawing moiety and the tert-butyl group for the electron-
donating moiety were synthesized. The dicarbonyl complexes
were synthesized to quantify the electron density at cobalt by
IR spectroscopy, revealing a blue shift in the C−O frequencies
for the CF3 functionalized ligand, consistent with less π-
backbonding. The tert-butyl group is thought to impart a weak
effect on the cobalt center, which unexpectedly also leads to a
slight blue shift in the carbonyl stretching frequencies.
For direct comparisons, the catalytic conditions reported for
2-H were used in this study. The catalyst (2-tBu or 2-CF3 at
9% loading) was reacted with ethyl acrylate and 4 atm of p-H2
1
to produce ethyl propionate. The reaction was studied by H
The CoI(H2) complexes, 5-tBu and 5-CF3, demonstrated H2
activation chemistry analogous to the unmodified complex,
leading us to examine complexes 4-tBu and 4-CF3 as catalysts
in the semihydrogenation of internal alkynes. Results showed
that 4-tBu performs comparably to the unmodified (MesCCC)-
Co(N2)PPh3 complex; however, 4-CF3 resulted in less
overhydrogenation of the alkene, suggesting that it may be a
slower catalyst. Closer examination of the olefin hydrogenation
process revealed that the activation of olefin is hampered by
the installation of the CF3 group, explaining the lower amounts
of alkane product observed. Application of the (MesCCCR)Co-
py complexes, 2-tBu and 2-CF3, toward the parahydrogenation
of ethyl acrylate demonstrated their ability to facilitate signal
enhancement and polarization transfer to neighboring nuclei,
albeit at lower yields for 2-CF3. Overall, the tert-butyl
functionalization had little effect on the reactivity of the
resulting Co complexes; however, the electron-withdrawing
trifluoromethyl group altered the selectivity of the semi-
hydrogenation of alkynes. This altered selectivity stems from
the diminished reactivity toward alkene hydrogenation, which
was suggested by the hydrogenation of 4-vinylhexene and
corroborated by parahydrogenation of ethyl acrylate.
and 13C NMR spectroscopies at low magnetic field (∼0.5
gauss). As depicted in Figure 4, both 2-tBu and 2-CF3 were
ASSOCIATED CONTENT
Figure 4. Parahydrogenation of ethyl acrylate by 2-R, 1H NMR
spectra.
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00337.
capable of hyperpolarizing the ethyl propionate product, with
the H NMR spectra showing the emissive and absorptive
1
Experimental data (PDF)
resonances expected for a reaction taking place at a low
magnetic field (Figure 4, see SI for more information).39
Accession Codes
1
The extent of polarization in the H NMR spectrum for
CCDC 1917121−1917124 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2-tBu was comparable to that of 2-H; however, the polarization
by 2-CF3 was significantly reduced in comparison. This is
consistent with the trifluoromethyl installation resulting in a
slower olefin hydrogenation. Since there was not a significant
concentration of hydrogenated product due to slow olefin
E
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(15) Tokmic, K.; Jackson, B. J.; Salazar, A.; Woods, T. J.; Fout, A. R.
Cobalt-Catalyzed and Lewis Acid-Assisted Nitrile Hydrogenation to
Primary Amines: A Combined Effort. J. Am. Chem. Soc. 2017, 139,
13554−13561.
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.R.F.: fout@illinois.edu.
ORCID
Alison R. Fout: 0000-0002-4669-5835
Notes
The authors declare no competing financial interest.
■
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ACKNOWLEDGMENTS
■
The authors thank the NSF for financial support with a
CAREER award (1351961) to ARF. SRM thanks the Ford
Foundation for a predoctoral graduate fellowship. Lastly, the
authors also thank Dr. Toby Woods and Dr. Danielle Gray for
assistance with crystallography.
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