Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

Article abstract of DOI:10.1021/acs.organomet.1c00053

The Co(I) complex, [Co(N2)(CyPNP)] (CyPNP = anion of 2,5-bis-(dicyclohexylphosphinomethyl)pyrrole), is active toward the catalytic hydrogenation of terminal alkenes and the semi-hydrogenation of internal alkynes under 2 bar of H2 (g) at room temperature. The products of alkyne semi-hydrogenation are a mixture of E- and Z-alkenes. By contrast, use of the related cobalt(I) precatalyst, [Co(PMe3)(CyPNP)], results in formation of exclusively Z-alkenes. A semi-stable Co(II) species, [CoH(CyPNP)], can also be generated by treatment of degassed solutions of [Co(N2)(CyPNP)] with H2. The CoII-hydride displays activity toward both alkene hydrogenation and isomerization, but its instability hampers implementation as a catalyst. Several species relevant to potential catalytic intermediates have been isolated and detected in solution. These compounds include alkene and alkyne adducts of Co(I) as well as a Co(III) dihydride species. Catalytic results with the compounds examined are most consistent with a process involving shuttling between Co(I) and Co(III) states. However, generation of small quantities of Co(II) during catalytic turnover appears to be responsible for the isomerization observed for alkyne semi-hydrogenation. The interplay of cobalt oxidation states within the same catalyst system is discussed in the context of mechanistic scenarios for catalytic hydrogenation.

Full text of DOI:10.1021/acs.organomet.1c00053

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Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer
Complex: Evidence of Roles for Both Co(I) and Co(II)
Hussah Alawisi, Hadi D. Arman, and Zachary J. Tonzetich*
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ABSTRACT: The Co(I) complex, [Co(N2)(CyPNP)] (CyPNP = anion of 2,5-bis-
(dicyclohexylphosphinomethyl)pyrrole), is active toward the catalytic hydrogenation
of terminal alkenes and the semi-hydrogenation of internal alkynes under 2 bar of H2
(g) at room temperature. The products of alkyne semi-hydrogenation are a mixture of
E- and Z-alkenes. By contrast, use of the related cobalt(I) precatalyst, [Co(PMe3)-
(CyPNP)], results in formation of exclusively Z-alkenes. A semi-stable Co(II) species,
[CoH(CyPNP)], can also be generated by treatment of degassed solutions of
[Co(N2)(CyPNP)] with H2. The CoII-hydride displays activity toward both alkene
hydrogenation and isomerization, but its instability hampers implementation as a
catalyst. Several species relevant to potential catalytic intermediates have been isolated
and detected in solution. These compounds include alkene and alkyne adducts of
Co(I) as well as a Co(III) dihydride species. Catalytic results with the compounds examined are most consistent with a process
involving shuttling between Co(I) and Co(III) states. However, generation of small quantities of Co(II) during catalytic turnover
appears to be responsible for the isomerization observed for alkyne semi-hydrogenation. The interplay of cobalt oxidation states
within the same catalyst system is discussed in the context of mechanistic scenarios for catalytic hydrogenation.
primary mechanistic cycles proposed for hydrogenation of
alkenes and alkynes by Co-based catalysts (Scheme 1). The
key difference in these two mechanisms stems from the
oxidation states of the cobalt centers and the nature of H2
activation. In the first mechanism (A), a Co(I) species
activates molecular hydrogen by oxidation addition to give a
Co(III) dihydride, often via a dihydrogen complex.23,32 Olefin
or alkyne then undergoes migratory insertion into the Co−H
bond to generate an intermediate cobalt-hydrido-hydrocarbyl
species. Facile reductive elimination from this Co(III) species
affords the reduced product and closes the cycle. In the second
mechanism (B), a Co(II)-hydride species is operative, which is
also capable of undergoing migratory insertion with olefin or
alkyne to form the corresponding hydrocarbyl compound.33
Hydrogenolysis of the Co(II)-hydrocarbyl, most likely through
a sigma-bond metathesis process, produces the reduced
product and regenerates the active hydride. Both of these
mechanisms have been put forward for different systems,
demonstrating the important role of the supporting ligand in
dictating the catalytic chemistry.
INTRODUCTION
■
The catalytic hydrogenation of unsaturated compounds
encompasses one of the foremost applications of organo-
metallic chemistry to organic synthesis. For decades, precious
metals have been employed for the hydrogenation reaction in
both heterogeneous and homogeneous catalyst systems.
Despite the efficacy and selectivity of these catalysts across a
variety of substrates, recent work, particularly in the area of
homogeneous catalysis, has seen a shift in focus to more earth-
abundant transition metal systems. Several first-row transition
metals have shown great promise as homogeneous hydro-
genation catalysts with activities rivaling their precious metal
counterparts in several instances.1−9 The ligand frameworks
used to support these systems vary, but pincers have emerged
as an especially effective class of ligands for the creation of
well-defined catalysts.10,11 Use of pincer ligands among the mid
3d metals (Mn, Fe, and Co) has permitted detailed
investigations into the mechanisms of catalytic hydrogenations,
revealing pathways unique from those of precious metal
systems.12−16
In the area of cobalt chemistry, reports from several groups
have described the catalytic hydrogenation activity of well-
defined cobalt catalysts featuring pincer ligands.17−31 These
catalysts have been applied successfully for the hydrogenation
of olefins and semi-hydrogenation of alkynes, with several
systems displaying impressive stereoselectivities. Despite this
success, no unified mechanistic picture has arisen comparable
to the better understood features of H2 activation and olefin
insertion at play with heavier Group 9 metals. There are two
Received: January 29, 2021
Published: March 15, 2021
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Scheme 1. Canonical Mechanisms for Co-Catalyzed Alkene/Alkyne Hydrogenation
Our laboratory has been investigating the reactivity of cobalt
R
complexes supported by a pyrrole-based PNP ligand, PNP
(RPNP = anion of bis(dialkylphosphinomethyl)pyrrole).34
These compounds have shown efficacy in several reductive
bond activation reactions including C-X oxidative addition and
aldehyde decarbonylation.35 To further establish the potential
of this system in reductive transformations, we turned our
attention to the hydrogenation of alkenes and alkynes. In this
contribution, we describe the catalytic behavior of the
CyPNPCo platform toward hydrogenation and the unexpected
role that both Co(I) and Co(II) can play in the catalytic cycle.
This work thereby establishes a link between the two most
prevalent mechanisms put forward for cobalt hydrogenation
catalysts.
Figure 1. Thermal ellipsoid plot (50%) of the solid-state structure of
1. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Co(1)−C(31) = 1.9998(14); Co(1)−N(1) =
1.9006(12); Co(1)−Pavg = 2.212(4); P(1)−Co(1)−P(2) =
162.477(16); N(1)−Co(1)−C(1) = 177.88(6); Co(1)−C(31)−
Si(1) = 116.99(8).
RESULTS AND DISCUSSION
Stoichiometric Reactivity. We began our investigations
■
by examining the reactivity of several well-defined cobalt(II)
and cobalt(I) species with molecular hydrogen to simulate
likely steps in catalytic cycles involving both oxidation states.
Exposure of benzene-d6 solutions of the alkyl Co(II)
complexes, [CoR(CyPNP)] (R = Me, Ph), to 1−5 bar of H2
was found to result in no discernible reactivity over the course
of several hours at ambient temperature as judged by NMR
spectroscopy. In contrast, similar treatment of the trimethylsi-
lylmethyl analogue, [Co(CH2SiMe3)(CyPNP)] (1, Figure 1),
with H2 proceeded slowly to generate Me4Si and a small
amount of a new paramagnetic species that we assign as the
putative hydride complex, [CoH(CyPNP)] (2, eq 1).
solutions of [CoX(CyPNP)] with KHBEt3 generated 2 as the
1
sole product as judged by H NMR spectroscopy (eq 2).
Likewise, addition of H2 to degassed solutions of 1 similarly
gave rise to 2. Compound 2 remains stable in solution if
excluded from N2, but rapidly reverts to 3 when a nitrogen
atmosphere is introduced. The paramagnetic 1H NMR
spectrum of 2 resembles that of other square-planar cobalt(II)
complexes of the CyPNP ligand, suggesting a similar
1
Attempts to generate 2 in larger quantities by treatment of
[CoX(CyPNP)] (X = Cl, Br) with hydride sources were
unsuccessful, resulting primarily in recovery of the cobalt(I)
species, [Co(N2)(CyPNP)] (3) (see Supporting Information).
We encountered similar issues in prior work with Co(II)
complexes of the bulkier tBuPNP ligand, ascribing the
propensity for reduction, in part, to the stable nature of the
cobalt(I) dinitrogen complex.36 We now find that solutions of
2 can be generated from either 1 or [CoX(CyPNP)], but only
in the absence of N2. Accordingly, treatment of degassed
coordination geometry and S = /2 spin state (see Supporting
Information).37
In similar fashion to the results with divalent hydrocarbyl
species, treatment of the cobalt(I)-N2 adduct, 3, with 1−5 bar
of H2 resulted in no discernible reactivity when conducted in
the presence of N2. However, when H2 was introduced to
degassed solutions of 3, a mixture of 2 and 3 was produced as
judged by NMR spectroscopy (Scheme 2). Thus, H2 appears
to provide a conduit between cobalt(I) and cobalt(II) under
appropriate conditions. At present, we can only speculate on
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Co(III) dihydride species. Addition of diphenylacetylene to 3
or 5 produced a new diamagnetic species as judged by NMR
Scheme 2. Reactivity of 3 with Hydrogen
1
spectroscopy (see Supporting Information). Both H and 31P
NMR spectra are indicative of C2v symmetry, consistent with
symmetrical coordination of the alkyne to the cobalt center (6,
Scheme 3). Despite the apparent stability of compound 6, we
have been unable to crystallize the molecule in satisfactory
fashion for further analysis.
Scheme 3. Reactivity of 3 with Alkynes and Alkenes
the mechanism underlying formation of 2 from 3 and H2, but
we favor a process involving reversible generation of a Co(I)-
H2 adduct in the absence of N2. Subsequent oxidative addition
to give a Co(III) dihydride, followed by either loss of H· or
comproportionation with unreacted 3, would then afford 2.
Reversible oxidative addition of H2 to cobalt(I) has been
observed in related pincer systems reported by the groups of
Chirik and Fout.23,38 In these cases, however, cobalt(III)
dihydride species can be identified as the products of H2
addition. Moreover, stabilization of the dihydride species is
found to require coordination of an additional ligand. We
therefore examined the reaction of compound 3 with H2 in the
presence of PMe3 in order to determine if H2 activation occurs
in similar fashion to that observed with degassed solutions of 3.
Similar reaction of 3 with styrene derivatives gave rise to a
mixture of the unreacted starting material and a new
diamagnetic compound displaying NMR resonances indicating
1
loss of C2v symmetry. In the case of 4-vinylanisole, the H
NMR spectrum exhibits broadened resonances for both the
vinylic H atoms as well as the methylene arms of the ligand. In
addition, the 31P NMR spectrum shows an AB pattern for the
two phosphorus nuclei (see Supporting Information). These
spectroscopic features are all consistent with a complex
featuring a π-coordinated olefin in which there is some degree
of fluxional motion (7, Scheme 3). Isolation of compound 7
proved difficult owing to equilibrium dissociation of the olefin,
but crystallization from solutions of excess 4-vinylansiole
permitted isolation of a few single crystals suitable for X-ray
diffraction. The solid-state structure of 7 is displayed in Figure
2. The molecule exhibits the expected face-on binding of the
olefin to the cobalt center with an overall coordination
geometry approximating square-planar. The olefin is bound
vertically with respect to the square plane of the molecule
within the cleft created by the flanking phosphine substituents.
The Co−C contacts of 2.022(2) and 2.067(2) Å are
inequivalent, most likely as a result of stronger steric
interactions between the anisyl group and the cyclohexyl rings.
The ability of π systems to coordinate to the CyPNPCoI
fragment next prompted us to examine the reactivity of the
alkyne adduct 6 with molecular hydrogen. Given our
observations with 3, we anticipated that the alkyne ligand
might activate the cobalt center toward H2 oxidative addition.
Gratifyingly, addition of 1 bar of H2 (substoichiometric) to a
benzene-d6 solution of compound 6 was observed to give rise
to a mixture of unreacted 6, compound 3, and cis-stilbene as
1
Surprisingly, the H NMR spectrum of the resulting reaction
mixture demonstrated the formation of a new diamagnetic
cobalt species with a broad resonance apparent in the hydride
region near −18 ppm. In addition, the 31P NMR spectrum
displayed two new peaks at 88.5 and 3.5 ppm. These
spectroscopic features are consistent with formation of a new
1
diamagnetic cobalt(III) complex (4, Scheme 2). H NMR
spectra acquired at 257 K in toluene-d8 further supported the
composition of 4, showing two broadened hydride resonances
at −14.1 and −21.6 ppm. Solutions of compound 4 are not
stable in the absence of H2 and quickly revert to the cobalt(I)-
PMe3 adduct, [Co(PMe3)(CyPNP)] (5). Likewise, attempts to
isolate 4 resulted only in recovery of 5. Compound 5 could be
prepared directly by treatment of 3 with PMe3 (Scheme 2)
The solid-state structure of 5 can be found in the Supporting
Information and resembles that of 3. In agreement with the
three-component reaction described above, direct treatment of
5 with H2 also generated dihydride 4 as judged by 1H and 31
P
NMR spectroscopy.
The reactivity of 3 and 5 with molecular hydrogen
establishes the viability of both cobalt(II) and cobalt(III)
oxidation states as products of H2 activation by cobalt(I). It
remains possible, however, that the direct reaction of H2 with 3
may be less favorable then association of substrates such as
alkenes and alkynes in the context of hydrogenation chemistry.
We therefore turned our attention to the reactivity of 3 with
unsaturated hydrocarbons, reasoning that adduct formation
with either substrate may represent a resting state in the
catalytic cycle and obviate the need for direct H2 reaction with
3. Moreover, as observed with 5, coordination of additional
ligands to 3 can direct oxidative addition chemistry in favor of
1
judged by H NMR spectroscopy. This result demonstrates
that compound 6, and by extension compound 3, are viable
candidates for alkyne hydrogenation precatalysts.
As a final aspect of the stoichiometric chemistry of 3, we
sought to probe its reactivity with terminal alkynes. As
substrates, terminal alkynes have proved challenging in
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Figure 3. Thermal ellipsoid (50%) drawing of the solid-state structure
of 8. Hydrogen atoms have been omitted for clarity. Selected bond
distances (Å) and angles (deg): Co(1)−C(31) = 1.8693(19);
Co(1)−N(1) = 1.8699(15); Co(1)−Pavg = 2.1987(5); C(31)−
C(32) = 1.226(3); P(1)−Co(1)−P(1) = 164.09(2); N(1)−Co(1)−
C(31) = 171.99(7); C(31)−C(32)−C(33) = 177.7(2).
Figure 2. Thermal ellipsoid (50%) drawing of the solid-state structure
of compound 7. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): Co(1)−C(31) =
2.022(2); Co(1)−C(32) = 2.0672(19); Co(1)−N(1) = 1.9068(16);
Co(1)−Pavg = 2.2282(5); C(31)−C(32) = 1.404(3); P(1)−Co(1)−
P(2) = 161.80(2); N(1)−Co(1)−CCcentr = 177.28(8).
Catalytic Reactions with Alkenes. In an initial set of
experiments, we examined the catalytic activity of compounds
1 and 3 toward the hydrogenation of styrene in benzene-d6.
We also tested the bulkier variant, [Co(N2)(tBuPNP)], to
determine if enhanced steric encumbrance about the metal
center plays a role in catalytic efficacy. At room temperature
under 2 bar of H2, both complexes 1 and 3 promoted the
catalytic hydrogenation of styrene to afford ethylbenzene at a
loading of 2 mol % as judged by NMR spectroscopy. With 3 as
a catalyst, the reaction proceeded to full conversion over 2 h;
however, the use of 1 resulted in only 25% yield of
ethylbenzene. This lower yield observed for 1 likely reflects
the difficulty in forming 2 and its attendant instability, which is
the presumed active species (vide infra). In contrast to the
results with 1 and 3, the bulkier [Co(N2)(tBuPNP)] complex
did not promote hydrogenation of styrene under the
conditions examined.
hydrogenation protocols featuring cobalt catalysts.25 This
incompatibility is not unexpected given the enhanced reactivity
of the Csp−H bond, which can engage in orthogonal chemistry,
limiting catalytic turnover. In the case of 3, treatment with
phenylacetylene led to immediate formation of the Co(II)
acetylide complex, 8 (eq 3). Compound 8 displays a
1
paramagnetic H NMR spectrum akin to that of 1 and 2,
consistent with a low-spin Co(II) center. The solid-state
structure of 8 is displayed in Figure 3 and is consistent with
other square-planar Co(II) hydrocarbyl species such as 1.
Formation of 8 mostly likely occurs via initial oxidative
addition of the alkyne C−H bond to 3 in similar fashion to
what we observed previously for carboxylic acids.35 In order to
capture the possible Co(III) intermediate responsible for this
chemistry, we examined the same reaction in the presence of
PMe3. Much like the case for 4, we anticipated that phosphine
ligation would stabilize the Co(III) species. Accordingly,
addition of phenylacetylene to 5 produced a new diamagnetic
spectrum displaying a doublet of triplets resonance for the Co-
H centered at −11.64 ppm (see Supporting Information). The
Given the preliminary results with styrene, we turned our
attention to compound 3 as a precatalyst for olefin hydro-
genation. Using p-chlorostyrene as a test substrate, we
investigated the role of solvent and catalyst loading. Reactions
performed in the absence of solvent led to low conversion,
likely due to strong coordination of the alkene to the catalyst
(vide supra compound 7). Furthermore, replacing benzene
with THF as solvent also afforded lower conversion, indicating
that noncoordinating solvents are most suitable. In terms of
catalyst loading, increasing the amount of precatalyst employed
from 0.5 to 2 mol % led to a corresponding climb in
conversion, consistent with some amount of catalyst
deactivation during turnover.
We next looked at a series of different olefinic substrates. In
all cases, formation of the corresponding alkane was observed,
although some classes of substrates required longer reactions
times (Table 1). Electronic effects, as seen with para
substituted styrene derivatives, demonstrated that electron-
withdrawing groups (EWG) provided lower yields compared
with the parent styrene and that containing a methoxy group
(Table 1, entries 1−3). We attribute this result to inhibition by
an alkene adduct, which should prove more stable for alkenes
2
values for the JHP coupling constants of 55 and 135 Hz
indicate a cis arrangement of the two P atoms of the pincer and
a trans disposition of the PMe3 ligand with respect to the
hydride. Although stable in solution for several hours, we have
been unable to isolate this hydride species. Nonetheless,
observation of this species supports a reaction pathway for
terminal alkynes featuring C-H oxidative addition. In the
absence of PMe3, the corresponding alkynyl-hydride is
unstable and rapidly leads to formation of compound 8.
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a
Surprisingly, addition of 1 equiv of PMe3 with respect to 3
under identical conditions resulted in selective formation of Z-
stilbene in 54% yield with no E-stilbene detectable by NMR
spectroscopy. Similar results were obtained by using isolated 5
as a precatalyst.
Table 1. Alkene Hydrogenation by Compound 3
The presence of PMe3 in these reactions likely serves to
deactivate the cobalt catalyst toward isomerization of the
initially produced Z-alkene. We believe this inhibition occurs
by preventing formation of 2 during catalytic turnover. As
discussed above, the addition of PMe3 to the reaction of 3 with
H2 results in cleaner formation of the cobalt(III) dihydride
species 4. Thus, by diverting the chemistry exclusively to the
Co(I)/Co(III) pathway, the presence of PMe3 is able to avoid
formation of 2, which is most likely responsible for
isomerization (vide infra). The use of a bona fide Co(II)
precatalyst, 1, under the standard conditions, afforded only
25% yield of the Z-stilbene. We attribute this low yield and lack
of isomerization to inefficient formation of 2 from 1 during
catalytic turnover (cf. eq 1).
Next, we investigated the scope of alkyne hydrogenation by
both 3 and 5. In general, both complexes provided modest to
good yields of the semi-hydrogenated products (Table 2). As
with diphenylacetylene, 3 was found to produce a mixture of E
and Z isomers in most instances, whereas 5 led to selective
formation of the Z-alkene. These results are further consistent
with the hypothesis that 2 can form in small quantities from 3
during catalytic turnover and mediate olefin isomerization.
Table 2. Alkyne Semi-hydrogenation by Compounds 3 and
a
5
a
All reactions performed with 2 mol % of 3 under 2 bar of H2 in 1 mL
of benzene-d6; yields were determined by 1H NMR spectroscopy with
b
respect to an internal standard of 1,3,5-trimethylbenzene. ‡ trans-
Stilbene detected in 41% yield.
bearing EWG substituents. In contrast to styrene derivatives,
our system failed to hydrogenate internal alkenes, as shown in
entries 4−7 of Table 1. Notably, attempted hydrogenation of
cis-stilbene did not give the corresponding alkane, but instead
led to formation of the isomerization product, trans-stilbene, in
41% yield (Table 1, entry 8).
Unlike the results from catalytic trials with 3, stoichiometric
reactions of both cyclohexene and cis-stilbene with H2 in the
presence of in situ-generated 2 resulted in formation of the
corresponding alkanes. These results demonstrate that 2 has
the capacity to serve as an efficient catalyst for alkene
hydrogenation but that catalyst deactivation, most likely via
formation of 3, renders the hydride inactive after a few
turnovers.
Catalytic Reactions with Alkynes. The results observed
with internal alkenes demonstrate that 3 is incapable of
hydrogenating such substrates under the conditions employed.
We therefore reasoned that the catalyst system would be
suitable for the semi-hydrogenation of internal alkynes. Indeed,
3 proved effective for such reactions. In a representative
experiment, hydrogenation of diphenylacetylene was found to
proceed to full conversion over 4 h when using 2 mol % of 3 as
a catalyst in benzene-d6 under 2 bar of H2. The products of
hydrogenation were found to be a mixture of E- and Z-stilbene
isomers (ca. 9:1, respectively) in an overall yield of 90%.
a
All reactions performed with 2 mol % of 3 under 2 bar of H2 in 1 mL
of benzene-d6 for 4 h; yields were determined by 1H NMR
spectroscopy with respect to an internal standard of 1,3,5-
trimethylbenzene.
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Scheme 4. Proposed Mechanism for Alkyne Semi-hydrogenation by Compound 3
Moreover, dropping the catalyst loading to 1 mol % was found
to lead to higher Z:E ratios in the case of diphenylacetylene.
Such a result is logical since reducing the catalyst loading
further attenuates the ability of 2 to form from 3, thereby
decreasing the likelihood of isomerization. As a final substrate
class, terminal alkynes were investigated in hydrogenation
reactions with 3 but proved unsuccessful. We attribute this lack
of productive reactivity to the facile activation of the C−H
bond leading to formation of compounds such as 8.
Alkene Isomerization. The differential quantities of trans
olefin observed during alkyne semi-hydrogenation with
precatalysts 3 and 5 implies that a species other than a
Co(III)-hydride is responsible for isomerization. Furthermore,
the isomerization event most likely occurs subsequent to
hydrogenation of the alkyne as observed in several other
systems.24,39 To verify this possibility, we examined the
hydrogenation of diphenylacetylene by 3 in the presence of
1 equiv of cis-stilbene. The reaction was found to proceed, but
trans-stilbene was not detected until all the PhCCPh was
consumed, as judged by NMR, confirming that isomerization
takes place after hydrogenation. We also directly compared the
isomerization activity of 3 and 5 under 2 bar of H2 employing
cis-stilbene. Compound 3 demonstrated good activity,
producing a mixture of stilbenes within 5 min. By contrast, 5
required 12 h to produce detectable quantities of trans-
stilbene.
On the basis of these results, a logical proposal for the
observed isomerization encountered during alkyne hydro-
genation implicates compound 2. As discussed above, 2 is
observed to form from 3 and H2, especially in the absence of
additional ligands. In the presence of alkyne, compound 6 is
formed from either precatalyst 3 or 5 and hinders formation of
small quantities of 2. As alkyne is hydrogenated to alkene,
compound 3 becomes the major cobalt containing species and
is subject to reaction with H2 to form 2. With 5 as a
precatalyst, however, the presence of PMe3 further prevents
formation of 2 even when all the alkyne is consumed. In those
reactions where the concentration of 2 can accumulate (i.e.,
those employing 3), isomerization of the new alkene can take
place.
As a final test of our proposed mechanism, we investigated if
in situ generated 2 was in fact active toward alkene
isomerization. Gratifyingly, treatment of 2 with cis-stilbene
caused immediate formation of the trans isomer in the absence
of added H2. We therefore favor a scenario for cobalt-catalyzed
hydrogenation as shown in Scheme 4. The CoI/III couple
appears to be responsible for the hydrogenation of alkynes and
terminal alkenes as observed in other systems, but isomer-
ization and/or hydrogenation of substituted alkenes must
proceed through CoII. Suppression of cobalt(II) species by
addition of PMe3 (precatalyst 5) results in more selective semi-
hydrogenation of internal alkynes.
CONCLUSIONS
■
In this contribution, we have demonstrated the catalytic
hydrogenation activity of a well-defined cobalt system
employing a PNP pincer ligand. The cobalt catalyst, typified
by the N2-adduct 3, was active for the hydrogenation of
terminal alkenes as well as the semi-hydrogenation of internal
alkynes through a mechanism most consistent with a CoI/III
cycle (Figure 1, panel A). Use of a related cobalt(II)-hydride
species, 2, permitted hydrogenation of internal alkenes, but the
instability and difficulty in generating this species precluded its
use as catalyst. In the case of alkyne semi-hydrogenation, we
could influence the formation of E- or Z-alkenes through
judicious choice of precatalyst. The propensity for isomer-
ization was found to correlate with the formation of compound
2, which is suppressed in the case of added phosphine (Scheme
4). As a whole, these results demonstrate that interplay
between Co(I) and Co(II), mediated by H2, can play an
important role during catalytic hydrogenation. Such behavior is
unique to earth-abundant transition metal systems owing to
their propensity to engage in one-electron chemistry. The
generality of this phenomenon is likely to extend beyond the
present cobalt system to other catalysts based on 3d metals.
EXPERIMENTAL SECTION
■
General Comments. All manipulations were performed under an
atmosphere of purified nitrogen gas using a Vacuum Atmospheres
glovebox. Tetrahydrofuran, diethyl ether, pentane, and toluene were
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purified by sparging with argon and passage through two columns
packed with 4 Å molecular sieves and/or activated alumina (THF).
1H NMR spectra were recorded on a Bruker spectrometer operating
at 500 MHz (1H) in benzene-d6 unless otherwise noted and
referenced to the residual protium resonance of the solvent (δ 7.16
ppm). 31P NMR spectra were recorded at 202 MHz and referenced
(app t, 4 Cy), 1.77 (app d, 4 Cy), 1.72 (app d, 4 Cy), 1.62 (m, 8 Cy),
1.43 (m, 8 Cy), 1.27 (m, 8 Cy), 1.22 (m, 4 Cy) 1.11 (app t, 9 PMe3);
31P{1H} δ 66.0 (PCy2), −1.1 (PMe3). Anal. Calcd for C33H59CoNP3:
C, 63.76; H, 9.57; N, 2.25. Found: C, 60.03; H, 8.95; N, 1.93.
Repeated analyses returned low values for C.
[Co(η2-PhCCPh)(CyPNP)] (6). A round-bottom flask was charged
with 100 mg (0.174 mmol) of 3 and 10 mL of toluene. To the brown
solution was added 34 mg (0.19 mmol) of PhCCPh. The resulting
solution was allowed to stir at ambient temperature for 12 h, during
which time the color became more red. All volatiles were removed in
vacuo to afford 98 mg (78%) of the crude product as a dark red
powder. Further attempts to purify the material by recrystallization
were unsuccessful. NMR: 1H δ 8.30 (d, 4 o-Ph), 7.29 (t, 4 m-Ph), 7.13
(t, 2 p-Ph), 6.45 (s, 2 pyr-CH), 2.74 (app t, 4 CH2P), 2.27 (app d, 4
Cy), 1.65 (app t, 4 Cy), 1.54 (app d, 12 Cy), 1.46 (app t, 8 Cy), 1.12
(app q, 4 Cy), 1.01 (app q, 4 Cy), 0.89 (m, 8 Cy); 13C{1H} δ 139.09
(t), 130.67, 130.21, 125.88, 105.25 (t), 88.20 (t), 33.87 (t), 29.98,
28.02, 27.82 (t), 27.55 (t), 26.71, 22.66 (t); 31P{1H} δ 46.70.
[Co(η2-H2CCHArOMe)(CyPNP)] (7). In an NMR tube, compound 3
(10 mg, 17 μmol) was dissolved in 0.6 mL of benzene-d6. To the
solution was added 4.8 μL (36 μmol, 2 equiv) of 4-vinylanisole. The
solution was then subjected to NMR analysis which displayed a 2:1
ratio of 7:3 along with free 4-vinylanisole. Small quantities of single
crystalline material suitable for X-ray diffraction were obtained by
slow cooling of a heptane solution of 3 in the presence of excess 4-
vinylanisole at −30 °C. NMR: 1H δ 7.68 (d, 2 o/m-Ph), 6.76 (d, 2 o/
m-Ph), 6.41 (s, 2 pyr-CH), 3.86 (m, 1 vinyl-CH), 3.38 (s, 3 OMe), 2.8
(br s, 2 CH2P), 2.6 (br s, 2 CH2P), 2.50 (m, 1 vinyl-CH), 2.32 (app d,
2 Cy), 1.98 (m, 4 Cy), 1.8−1.0 (multiple overlapping Cy), 0.89 (m, 1
vinyl-CH); 31P{1H} 46.6 (AB pattern).
2
automatically using the H lock frequency. Elemental analyses were
performed by the CENTC facility at the University of Rochester. In
each case, recrystallized material was used for combustion analysis.
Materials. [Co(N2)(CyPNP)] (3), [CoX(CyPNP)] (X = Cl, Br),
and [CoN2(tBuPNP)] were prepared according to published
procedures.35,36 Hydrogen gas was obtained from Airgas and
delivered to NMR samples or reaction mixtures via a needle/septum.
For larger pressures of H2 (2−5 bar), a medium-pressure NMR tube
was employed with a direct connection to the gas regulator. All other
reagents were purchased from commercial suppliers and used as
received.
Crystallography. Crystals suitable for X-ray diffraction were
mounted, using Paratone oil, onto a nylon loop. All data were
collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted
with Mo Kα radiation (λ = 0.71075 Å). Low-temperature data
collection was accomplished with a nitrogen cold stream maintained
by an X-Stream low-temperature apparatus. Data collection and unit
cell refinement were performed using CrystalClear software.40 Data
processing and absorption correction, giving minimum and maximum
transmission factors, were accomplished with CrysAlisPro41 and
SCALE3 ABSPACK,42 respectively. The structure, using Olex2,43 was
solved with the ShelXT44 structure solution program using direct
methods and refined (on F2) with the ShelXL refinement package
using the full-matrix, least-squares techniques.45 All nonhydrogen
atoms were refined with anisotropic displacement parameters. All
hydrogen atom positions were determined by geometry and refined
by a riding model.
[Co(CCPh)(CyPNP)] (8). A flask was charged with 100 mg (0.17
mmol) of 3 and 10 mL of toluene. To the solution was added 20 μL
(0.18 mmol) of phenylacetylene. The resulting mixture was allowed to
stir at ambient temperature for 12 h. All volatiles were removed in
vacuo, and the remaining solid was triturated with cold pentane and
isolated by filtration to afford 87 mg (77%) of a red powder. Crystals
suitable for X-ray diffraction were grown from a concentrated heptane
solution at 23 °C. 1H NMR: δ 40.6, 35.3, 21.2, 12.0, 9.5, 7.4, 7.1, 3.7,
−0.3, −2.0, −8.3, −16.6, −45.8. Anal. Calcd for C38H55CoNP2: C,
70.57; H, 8.57; N, 2.17. Found: C, 74.85; H, 9.15; N, 1.16. The high
values for C and H and low values for N are consistent with retention
of small quantities of heptane and toluene.
Hydrogenation Experiments. In the glovebox, a 50 mL Schlenk
flask was charged with alkene or alkyne, 1,3,5-trimethylbenzene
(internal standard), and compound 3 or 5 (0.5−2 mol %). The
reactants were dissolved in 1 mL of benzene-d6. The flask was sealed
with a rubber septum and removed from the glovebox. A balloon
containing ca. 2 bar of H2 gas was fitted to the flask via a needle
through the septum. The reaction was then permitted to stir at room
temperature for the allotted time. Conversion of starting material and
yield of product were assayed by NMR spectroscopy.
[Co(CH2SiMe3)(CyPNP)] (1). In a round-bottom flask, 200 mg
(0.344 mmol) of [CoCl(CyPNP)] was dissolved in 10 mL of THF. To
this solution was added dropwise 36 μL (0.36 mmol) of a 1.0 M
solution of Me3SiCH2MgCl in THF. The reaction mixture was
allowed to stir at room temperature for 30 min, during which time the
color remained brown. The mixture was filtered through Celite, and
all volatiles were removed in vacuo. The remaining residue was
dissolved in toluene and filtered a second time through Celite to
ensure removal of all Mg salts. The toluene solution was evaporated
to dryness, producing a solid precipitate that was collected by
filtration, washed with pentane, and dried in vacuo to afford 176 mg
(80%) of a brown powder. Crystals suitable for X-ray diffraction were
obtained from a concentrated 1,4-dioxane solution at 23 °C. 1H
NMR: δ 24.2 (4 H), 20.7 (4 H), 14.3 (4 H), 8.7 (4 H), 7.6 (4 H), 5.8
(4 H), 3.1 (4 H), 1.5 (8 H), 0.9 (4 H), −12.8 (9 Me3Si), −41.9 (2
pyr-CH). Anal. Calcd for C34H61CoNP2Si: C, 64.53; H, 9.72; N, 2.21.
Found: C, 63.54; H, 9.58; N, 2.00.
[Co(H)(CyPNP)] (2). The species was observed in solution by
treating [CoBr(CyPNP)] with 1 equiv of KHBEt3 in THF or by
exposure of degassed solutions of 1 and 3 to 1−2 bar of H2. NMR: 1H
δ 46.8, 40.1, 22.3, 12.2, 9.3, 8.0, 4.1, −1.1, −1.4, −3.6, −38.5 (pyr-
CH).
ASSOCIATED CONTENT
■
sı
* Supporting Information
cis-[Co(H)2(PMe3)(CyPNP)] (4). This species was observed in
solution by treating 3 with 1−2 bar of H2 in the presence of 1 equiv of
PMe3. NMR (toluene-d8, 257 K): 1H δ 6.44 (s, 2 pyr-CH), 3.15 (dt, 2
CH2P), 2.90 (dt, 2 CH2P), 2.32 (app d, 2 Cy), 1.99 (m, 4 Cy), 1.88−
1.58 (20 Cy), 1.38 (m, 10 Cy), 1.16 (m, 8 Cy), 0.94 (app d, 9 PMe3),
−14.1 (br s, CoH), −21.6 (br s, CoH); 31P{1H} δ 89.0 (PCy2), 4.3
(PMe3).
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00053.
NMR spectra, additional structural diagrams, and tables
of crystallographic data and refinement parameters
(PDF)
[Co(PMe3)(CyPNP)] (5). A flask was charged with 150 mg (0.258
mmol) of 3 and 68 μL (2.5 equiv) of PMe3. The mixture was
dissolved in 10 mL of toluene, and the resulting purple solution was
allowed to stir at room temperature for 12 h. The toluene was
removed under reduced pressure to furnish 95 mg (88%) of a dark
purple solid. Crystals suitable for X-ray diffraction were grown from a
Accession Codes
CCDC 2059458−2059461 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.
1
concentrated heptane solution of the complex at 23 °C. NMR: H δ
6.55 (s, 2 pyr-CH), 2.96 (app t, 4 CH2P), 2.33 (app d, 4 Cy), 1.82
1068
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Organometallics 2021, 40, 1062−1070

Organometallics
pubs.acs.org/Organometallics
Article
Amine−Borane Dehydrogenation by a Versatile Boryl-Ligand-Based
Cobalt Catalyst. ACS Catal. 2015, 5, 2754−2769.
(14) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K.
Understanding the mechanisms of cobalt-catalyzed hydrogenation
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AUTHOR INFORMATION
■
Corresponding Author
Zachary J. Tonzetich − Department of Chemistry, University
of Texas at San Antonio (UTSA), San Antonio, Texas
78249, United States; orcid.org/0000-0001-7010-8007;
Email: zachary.tonzetich@utsa.edu
(15) Gorgas, N.; Brunig, J.; Stöger, B.; Vanicek, S.; Tilset, M.;
̈
Veiros, L. F.; Kirchner, K. Efficient Z-Selective Semihydrogenation of
Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes.
J. Am. Chem. Soc. 2019, 141, 17452−17458.
Authors
Hussah Alawisi − Department of Chemistry, University of
Texas at San Antonio (UTSA), San Antonio, Texas 78249,
United States; Department of Chemistry, King Faisal
University, Al Hofuf, Kingdom of Saudi Arabia
Hadi D. Arman − Department of Chemistry, University of
Texas at San Antonio (UTSA), San Antonio, Texas 78249,
United States
(16) Wang, Y.; Zhu, L.; Shao, Z.; Li, G.; Lan, Y.; Liu, Q. Unmasking
the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mecha-
nistic Insight and Catalytic Application. J. Am. Chem. Soc. 2019, 141,
17337−17349.
(17) Ritz, M. D.; Parsons, A. M.; Palermo, P. N.; Jones, W. D.
Bisoxazoline-pincer ligated cobalt-catalyzed hydrogenation of alkenes.
Polyhedron 2020, 180, 114416.
(18) Zuo, Z.; Xu, S.; Zhang, L.; Gan, L.; Fang, H.; Liu, G.; Huang, Z.
Cobalt-Catalyzed Asymmetric Hydrogenation of Vinylsilanes with a
Phosphine-Pyridine-Oxazoline Ligand: Synthesis of Optically Active
Organosilanes and Silacycles. Organometallics 2019, 38, 3906−3911.
(19) Muhammad, S. R.; Nugent, J. W.; Tokmic, K.; Zhu, L.;
Mahmoud, J.; Fout, A. R. Electronic Ligand Modifications on Cobalt
Complexes and Their Application toward the Semi-Hydrogenation of
Alkynes and Para-Hydrogenation of Alkenes. Organometallics 2019,
38, 3132−3138.
(20) Merz, L. S.; Blasius, C. K.; Wadepohl, H.; Gade, L. H. Square
Planar Cobalt(II) Hydride versus T-Shaped Cobalt(I): Structural
Characterization and Dihydrogen Activation with PNP-Cobalt Pincer
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00053
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Welch Foundation (AX-1772) for
financial support of this work. NMR and X-ray crystallographic
facilities at UTSA are supported by grants from the National
Science Foundation (CHE-1625963 and CHE-1920057).
(21) Landge, V. G.; Pitchaimani, J.; Midya, S. P.; Subaramanian, M.;
Madhu, V.; Balaraman, E. Phosphine-free cobalt pincer complex
catalyzed Z-selective semi-hydrogenation of unbiased alkynes. Catal.
Sci. Technol. 2018, 8, 428−433.
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L.; Rueping, M.; El-Sepelgy, O. Highly Chemo- and Stereoselective
Transfer Semihydrogenation of Alkynes Catalyzed by a Stable, Well-
Defined Manganese(II) Complex. ACS Catal. 2018, 8, 4103−4109.
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Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/
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Defined Nonclassical Co−H2 Catalyst: A H2 Spin on Isomerization
and E-Selectivity. J. Am. Chem. Soc. 2016, 138, 13700−13705.
(25) Fu, S.; Chen, N.-Y.; Liu, X.; Shao, Z.; Luo, S.-P.; Liu, Q.
Ligand-Controlled Cobalt-Catalyzed Transfer Hydrogenation of
Alkynes: Stereodivergent Synthesis of Z- and E-Alkenes. J. Am.
Chem. Soc. 2016, 138, 8588−8594.
(26) Chen, J.; Chen, C.; Ji, C.; Lu, Z. Cobalt-Catalyzed Asymmetric
Hydrogenation of 1,1-Diarylethenes. Org. Lett. 2016, 18, 1594−1597.
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