Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO2 Hydrogenation

Article abstract of DOI:10.1021/acs.inorgchem.6b01454

The pincer ligand MeN[CH₂CH₂(PⁱPr₂)]₂ (^iPrPNP) was employed to support a series of cobalt(I) complexes, which were crystallographically characterized. A cobalt monochloride species, (ⁱ

Full text of DOI:10.1021/acs.inorgchem.6b01454

Article
pubs.acs.org/IC
Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO2
Hydrogenation
Ariana Z. Spentzos, Charles L. Barnes, and Wesley H. Bernskoetter*
Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States
*
S Supporting Information
ABSTRACT: The pincer ligand MeN[CH CH (P Pr )]
2
i
2
2
2
iPr
(
PNP) was employed to support a series of cobalt(I)
complexes, which were crystallographically characterized. A
iPr
cobalt monochloride species, ( PNP)CoCl, served as a
precursor for the preparation of several cobalt precatalysts
for CO hydrogenation, including a cationic dicarbonyl cobalt
2
iPr
+
complex, [( PNP)Co(CO) ] . When paired with the Lewis
acid lithium triflate, [( PNP)Co(CO) ] affords turnover numbers near 30 000 (at 1000 psi, 45 °C) for CO -to-formate
2
iPr
+
2
2
hydrogenation, which is a notable increase in activity from previously reported homogeneous cobalt catalysts. Though
mechanistic information regarding the function of the precatalysts remains limited, multiple experiments suggest the active
iPr
species is a molecular, homogeneous [( PNP)Co] complex.
−
1
INTRODUCTION
turnover frequencies (TOFs) near 150 000 h (49−59 atm;
■
11
1
20−220 °C). Though these numbers are impressive,
Growing realization of the deleterious effects of anthropogenic
widespread application of CHS methods would be greatly
facilitated by the utilization of earth abundant metal catalysts,
which are inexpensive and less toxic.
CO emissions has provided considerable incentive to harness
2
1
and store energy from carbon-neutral sources. Solar energy
driven H production from water is one source that has the
2
2
There have been several reports of first-row transition metal
catalysts for CO -to-formate hydrogenation since the pioneer-
potential to provide a sustainable and economical fuel. Along
with further advances in water splitting, challenges associated
with the storage and transportation of H hinder broader
application of this attractive energy vector. Chemical hydrogen
storage (CHS) based on the reversible fixation of H into a
lightweight liquid organic carrier molecule may provide a
reliable method to surmount these H utilization issues. The
2
12
ing work of Inoue in 1976. The earliest earth abundant metal
catalysts displayed activities far inferior to their precious metal
congeners (TON of 5−10) but clearly indicated that first-row
2
2
2
transition metals were competent in promoting CO reduction.
2
3
13
14
More recent developments with iron and cobalt have
produced catalysts with far improved activities, reaching TONs
in excess of 5000 and 9000 for the two metals, respectively. Our
own laboratory has participated in the development of iron
2
high gravimetric H content produced from hydrogenation of
2
CO to formic acid (4.4%) and methanol (12.6%; eq 1) make
2
this cheap and renewable carbon source an especially promising
4
CHS target.
pincer catalysts for CO -to-formate hydrogenation, such as
2
R
R
(
PNP)Fe(H)CO(BH ) ( PNP = MeN{CH CH (PR )} , R =
4 2 2 2 2
CO + 3H ⇄ HCO H + 2H ⇄ CH OH + H O (1)
i
15
2
2
2
2
3
2
Pr, Cy), which exhibited a modest TON of 2800. However,
turnover increased dramatically upon application of Lewis acid
The net hydrogenation of CO to methanol is an exergonic
2
(
LA) cocatalysts, reaching a TON near 60 000 with the
reaction (−2.1 kcal/mol) under standard state conditions, but
addition of lithium triflate (LiOTf; Figure 1). Mechanistic
experiments suggest that the primary role of the LA is to assist
the hydrogenation to formic acid is slightly unfavorable (+7.6
5
kcal/mol). This endergonic intermediate step to formic acid
has hindered many thermochemical catalysts in attempts to
produce methanol directly from CO ; thus, catalytic hydro-
2
genation to formic acid typically requires an exogenous base or
alcohol to trap the product as formate salts, formamides, or
6
esters. In a few cases, the formate species have been
hydrogenated in situ to afford methanol using ruthenium
7
catalysts. Homogeneous catalytic hydrogenations of CO -to-
2
formate have largely been the domain of precious metals such
8
9
10
Figure 1. Iron pincer and Lewis acid cocatalyzed CO
Received: June 16, 2016
hydrogenation.
2
as ruthenium, iridium, and rhodium. The pincer iridium
i
catalyst [2,6-(P Pr ) C H N]IrH , reported by Nozaki and co-
2
2
5
3
3
workers, is among the most active of these promotors,
6
achieving turnover numbers (TONs) near 3.5 × 10 and
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b01454
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Inorganic Chemistry
Article
R
Figure 2. Previously described tertiary amine [( PNP)Co] cobalt complexes.
Figure 3. Synthesis of 1 and its reaction with carbon monoxide.
the dissociation of a formate anion from the iron center,
although carboxylate extrusion is still implicated as the turnover
1
5
limiting step. These insights prompted a new investigation
into analogous cobalt catalysts, which may provide a reduced
oxophilicity and accelerated formate loss. Herein, we describe
the preparation of a family of cobalt pincer precatalysts for CO2
hydrogenation. These new cobalt species provide TONs far
surpassing the state-of-the-art for homogeneous cobalt
promotors of CO hydrogenation to formate and establish an
2
important cocatalytic role for LAs in this transformation.
RESULTS AND DISCUSSION
■
Synthesis and Coordination Chemistry of Cobalt
Precatalysts. Recent research reports have described the
coordination chemistry and catalytic activity of numerous
cobalt pincer complexes supported by formally neutral
iPr
Figure 4. Molecular structure of ( PNP)CoCl (1) at 30% ellipsoids.
All hydrogen atoms were removed for clarity.
16
tridentate ligands. Catalysts derived from secondary amine
R
H
R
H
PN P ligands ( PN P = HN{CH CH (PR )} ) have been
Table 1. Selected Bond Distances (Å) for 1, 2-Cl, 3, and 4
2
2
2
2
prominent among these reports, including the discovery of
1
2-Cl
3
4
highly active cobalt promotors of borylation, N-alkylation, and
1
7
Co(1)−N(1)
Co(1)−P(1)
Co(1)−P(2)
Co(1)−C(1)
Co(1)-X
2.172(3)
2.2483(8)
2.2383(8)
2.117(2)
2.091(7)
2.162(2)
2.166(2)
1.742(9)
reversible hydrogenation and dehydrogenation reactions.
R
2.2350(4)
2.1927(5)
2.1935(5)
1.752(2)
1.749(2)
X = C(2)
1.157(2)
1.157(2)
Tertiary amine PNP cobalt species have received less
investigation, and only a handful of species have been
18
1.764(2)
1.744(2)
X = C(2)
1.152(3)
1.155(3)
characterized (Figure 2).
2.2483(8)
X = Cl(1)
Our own synthetic experiments were directed toward
obtaining RPNP cobalt(I) species bearing CO and hydride
R
C(1)−O(1)
C(2)−O(2)
1.16(1)
ligands, in analogy to the successful PNP iron CO2
15
iPr
hydrogenation catalysts (Figure 1). Initial chelation of PNP
to cobalt was achieved by ligand substitution with
was also measured for 1, which is consistent with a high-spin d8
electronic structure.
(
(
Ph P) CoCl, affording the deep blue paramagnetic complex,
PNP)CoCl (1; Figure 3). The modest yield (46%) of 1 was
3 3
iPr
primarily due to difficulties in separation from the copious
With complex 1 in hand, incorporation of CO and hydride
ligands into the cobalt coordination sphere was pursued. Initial
attempts to substitute a hydride for the cobalt−chloride bond
using various borohydride reagents afforded intractable
mixtures of products along with metallic precipitate, suggesting
that competing reduction and/or disproportionation reactions
dominated. Alternatively, treatment of an ethereal solution of 1
amounts of free PPh generated in the reaction. Complex 1 was
3
1
characterized by H NMR spectroscopy, combustion analysis,
and single-crystal X-ray diffraction. The molecular structure of
1
is presented in Figure 4 with relevant metrical parameters
noted in Table 1. The molecular geometry is best described as
19
trigonal pyramidal (τ = 0.81) with P(1), P(2), and Cl(1)
comprising the basal plane and N(1) as the apical site. This
divergence from the more common tetrahedral geometry
mirrors that observed for the secondary amine analogue,
with carbon monoxide rapidly produced a cationic complex,
iPr
[( PNP)Co(CO) ]Cl (2-Cl), as an orange precipitate (Figure
2
3). Incorporation of strong field carbon monoxide ligands
enforced a low-spin, diamagnetic cobalt(I) environment and
permitted characterization by NMR spectroscopy as well as
iPr
H
(
PN P)CoCl, which was reported by Arnold and co-
17b
workers.
A solution magnetic moment of μeff = 2.52 μB
B
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infrared and X-ray diffraction experiments. Samples of 2-Cl
products may be separated from the initial reaction mixture
through an arduous sequence of recrystallizations and a final
Pasteur sorting of the dark orange specimens of 3 from the
yellow-green crystals of 4. This method was successfully used to
obtain small quantities of analytically pure material; however,
the conversion of 3 to 4 through application of additional
carbon monoxide is the preferable method to obtain the
dicarbonyl cobalt hydride species. Conversion of 4 to 3 via
thermolysis under vacuum occurred only with concurrent
decomposition and was not synthetically useful.
exhibited limited solubility in arene and ethereal solvents, but
3
1
they dissolved well in acetonitrile. The P NMR spectrum of 2-
Cl in this solvent displayed a single resonance at 89.62 ppm,
1
and the H NMR spectrum was highlighted by a singlet at 2.45
ppm assigned to the N-methyl substituent. Solid-state IR
spectra exhibited two strong C−O stretching bands at 1913 and
−
1
−1
21
1
981 cm , which shifted appropriately to 1866 and 1940 cm
13
upon CO labeling. The molecular structure of 2-Cl was
further elucidated by X-ray diffraction (Figure 5; Table 1) and
The cobalt(I) hydride products were each characterized by a
combination of X-ray diffraction, combustion analysis, as well as
1
NMR and IR spectroscopy. The upfield region of the H NMR
spectrum of the product mixture in benzene-d exhibited two
6
triplet Co−H resonances, one at −24.82 (for 3) and one at
12.18 ppm (for 4). The corresponding 31P { H} NMR
1
−
spectrum displayed two singlet resonances, one each at 80.86
for 3) and 65.17 ppm (for 4). Infrared analyses performed on
isolated samples of 3 revealed strong bands at 1877 and 1836
(
−
1
cm , which were assigned as the ν(CoH) and ν(CO)
stretches, respectively, on the basis of 13CO labeling studies.
Similarly, infrared spectra of 4 exhibited intense ν(CO) bands
21
at 1961 and 1878 cm− and a ν(CoH) stretch at 1937 cm ,
1
−1
iPr
Figure 5. Molecular structure of [( PNP)Co(CO) ]Cl (2-Cl) at 30%
with the assignments confirmed by isotopic substitution of the
carbonyls.
2
21
ellipsoids. All hydrogen atoms and an outer-sphere chloride anion
were removed for clarity.
Single crystals of 3 and 4 suitable for X-ray diffraction were
obtained from chilled solutions of hexamethyldisiloxane, and
their molecular structures are depicted in Figure 7. Complex 3
exhibits a trigonal bipyramidal geometry similar to 2-Cl, but
with the H(1)−Co(1)−N(1) angle (175(3)°) now defining the
central axis. The most notable feature of complex 4 is the
presented an approximate trigonal bipyramidal geometry with
the chloride anion lying far outside the cobalt coordination
sphere (>6 Å). The C−O bond distances (1.152(3) and
1
.155(3) Å) for the bound carbon monoxide ligands suggest
only a modest degree of π-backbonding, consistent with the
high-frequency bands observed by infrared analysis. Mindiola
and co-workers have reported a related PNP-pincer cobalt(I)
dicarbonyl species that displays an analogous geometry and
metrical parameters. The poor solubility of 2-Cl in most
organic solvents also motivated the preparation of a borate
congener. This complex, [( PNP)Co(CO) ]BAr (2-BAr )
BAr 4 = B(3,5-(CF ) C H ) ), was obtained via a straightfor-
ward salt metathesis with Na[BAr ]. The spectroscopic
features and X-ray diffraction results for 2-BAr were
analogous to those of 2-Cl and may be found in the Supporting
Information.
The reactivity of 2-Cl toward substitution of the outer-sphere
chloride anion was explored with two borohydride reagents,
sodium triethylborohydride and sodium tri-sec-butylborohy-
dride (N-selectride; Figure 6). The use of either borohydride
reagent produced a mixture of two new cobalt(I) hydrides
3
2
change in hapticity of the chelate from κ to κ with dissociation
of the N-donor. A closely related hapticity change has been
observed by Gray and Wong in cobalt(II) dihalide species when
18c
20
employing a (PhCH )N{CH CH (PPh )} ligand. While the
2 2 2 2 2
origin of the divergent synthesis of 3 and 4 from 2-Cl is
uncertain, each species suggests that the amine and CO ligands
possess some lability in this cobalt coordination environment.
Such flexibility is likely to influence catalytic applications.
iPr
F
F
2
4
4
F
(
3 2 6 3 4
F
4
F
Cobalt Precatalyst Activity for CO Hydrogenation.
2
4
iPr
Preparation of a family of [( PNP)Co] complexes provided an
opportunity to explore the catalytic hydrogenation of CO -to-
2
formate as well as to compare their activities to related iron and
cobalt catalysts. Initial catalytic experiments were performed
using conditions previously optimized for the iron catalyst
iPr
(
PNP)Fe(H)CO(BH ), which attained a reported TON of
4
iPr
59 000. Each of the newly prepared [( PNP)Co] complexes
were screened using identical conditions with the formate
production determined by H NMR spectroscopy. The results
iPr
2 i
species, ( PNP)Co(CO)(H) (3) and (κ ‑PrPNP)Co(CO) H
4), with complex 4 favored when employing the more
2
1
(
sterically hindered hydride transfer reagent. The two cobalt
are summarized in Table 2.
Figure 6. Preparation and product ratios of 3 and 4 from hydride transfer to 2-Cl.
C
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Figure 7. Molecular structures of ( PNP)Co(CO)(H) (3) (right) and (κ2‑iPrPNP)Co(CO) H (4) (left) at 30% ellipsoids. All hydrogen atoms not
iPr
2
attached to cobalt were removed for clarity.
Table 2. CO Hydrogenation Promoted by Cobalt
Precatalysts
reaction. Several catalytic trials of varying length were
conducted using 2-Cl and the conditions described in Table
2
a
2
. After 1 h the reaction had produced a TON of 7800, roughly
three-quarters of the maximum observed conversion under
these conditions (Figure 8). Unfortunately, the prospect for
b
entry
catalyst
TON
670
conv
1
2
3
4
5
6
7
1
0.8%
13%
11%
12%
11%
0.1%
0.6%
c
2-Cl
10 000
8600
2-BArF
4
c
3
4
9400
c
8900
No [Co]
2-Cl/No LiOTf
460
a
Reaction conditions: 1000 psi of CO /H (1:1), 0.3 μmol catalyst, 24
2
2
mmol DBU, 4.8 mmol LiOTf in 5 mL of THF at 80 °C for 16 h.
b
Reported conversions are based on a DBU/formate ratio of 1:1.
c
Average TON for three or more trials.
Although the conversions under these conditions were
modest compared to iron catalysts, many of the precatalysts
demonstrated TONs on par with state-of-the-art cobalt
14
catalysts for CO -to-formate hydrogenation. The cobalt(I)
2
Figure 8. TON for formate vs time profile for CO hydrogenation.
2
monochloride complex 1 was the weakest promotor (entry 1),
showing a conversion only modestly above that observed
achieving higher conversion at longer reaction times appeared
limited, as trials conducted for 24 h afforded little additional
turnover compared to 16 h. These results indicate that
degradation of the catalytically active species is a likely cause
for the limited conversions.
without a catalyst (entry 6). The two dicarbonyl cation
complexes 2-Cl and 2-BArF exhibited comparable though
4
distinct TONs (given the ±350 TON standard deviations
observed over multiple trials of these experiments). Still, the
anion effect for the reaction was quite mild. More notably,
removal of the LiOTf cocatalyst nearly inactivated the system
Solvent effects are often significant for reactions that contain
highly polar transition structures, such as CO functionalization
2
(
entry 7), indicating that the presence of a LA is critical for this
2
2
or LA components. This influence motivated solvent
CO -to-formate process. The cobalt(I) hydride species 3 and 4
2
screening for 2-Cl/LiOTf catalyzed CO2 hydrogenation
also achieved significant TONs (entries 4 and 5). The relatively
(
Table 3). Given the need to dissolve the LiOTf cocatalyst,
small variations in TON and conversion for the carbonyl-
iPr
relatively polar solvents were selected for examination. 1,4-
dioxane and ethyl acetate both performed well as solvents for
containing [( PNP)Co] precatalysts may indicate each species
promotes CO hydrogenation through a common catalytic
2
CO hydrogenation, providing TONs comparable to those
cycle; however, greater mechanistic study will be required to
probe this possibility. Given the comparable precatalyst
activities and its relative ease of preparation, 2-Cl was selected
for further experiments to enhance and understand the
2
obtained in tetrahydrofuran (THF). The use of a polar protic
solvent, ethanol, was highly detrimental to the reaction, as was
the use of dichloromethane. Significantly, acetonitrile afforded a
more robust TON of 14 000 under these conditions. This
activity in acetonitrile was further enhanced by empirical
optimization of the LA loading to 3.2 mmol of LiOTf, and
conversion of CO -to-formate.
2
The modest yields observed in initial catalytic experiments
prompted investigation into the conversion profile for the
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Table 3. Solvent Screening for 2-Cl/LiOTf Catalyzed CO
Hydrogenation
°C. This temperature dependence likely reflects the limited
lifetime of the active catalyst (Figure 8), wherein the rate of
decomposition is slowed to a greater degree than that of
catalysis. At 25 °C the balance between catalyst lifetime and
catalytic rate shifts, with the production of formate slowed
sufficiently to negate the effects of enhanced catalyst lifetime.
2
a
solvent
THF
,4-dioxane
TON
b
10 000
b
1
8600
b
acetonitrile
ethanol
14 000
<100
8300
110
The activity of 2-Cl/LiOTf catalyzed CO hydrogenation more
2
than triples the highest previously reported TON for cobalt
ethyl acetate
dichloromethane
(
(
9400) described by Linehan and co-workers for (dmpe) CoH
2
dmpe = Me PCH CH PMe ), though that system operated at
2
2
2
2
a
14c,d
Reaction conditions: 1000 psi of CO /H (1:1), 0.3 μmol catalyst, 24
2
2
ambient temperature using Verkade’s base.
Cl/LiOTf at 45 °C (5700 ± 280 h ) is an order of magnitude
lower than the 74 000 ± 7500 h TOF reported for
(
The TOF of 2-
mmol DBU, 4.8 mmol LiOTf in 5 mL of solvent at 80 °C for 16 h.
−1
b
Average TON for two or more trials.
−1
1
4d
dmpe) CoH.
Though these two TOF values were
2
yielded a TON of 18 000. A nearly complete conversion of
measured over different time scales (which can greatly effect
TOF values) it is likely that 2-Cl/LiOTf produces formate
>
98% (based on a 1,8-diazabicycloundec-7-ene (DBU)/
formate ratio of 1:1) can also be achieved by lowering the
DBU to 3 mmol (see Supporting Information).
more slowly than (dmpe) CoH, but does so for a longer time
2
period.
Having empirically optimized several reagents in the CO2
hydrogenation reaction, the optimum temperature and pressure
were investigated. Unfortunately, all attempts to lower the
pressure of CO /H below 1000 psi produced inferior TONs,
Following our efforts to empirically enhance the 2-Cl/LiOTf
catalyzed CO -to-formate reaction, some consideration was
2
given to the mechanism of catalysis in hopes of guiding more
rationale-based improvements. One possible mechanism based
2
2
and trials at greater pressure were obviated by safety concerns.
However, varying the reaction temperature afforded more
favorable results (Table 4). Raising the reaction temperature
largely on studies of related iron and cobalt catalysts is
14d,15
illustrated in Figure 9.
In this mechanism, CO reduction
2
would occur from insertion into the Co−H bond of 4 (or
indirectly from 3), followed by dissociation of the formate
anion to generate a cobalt dicarbonyl cation, 2. This insertion
could occur from either a direct Co−H nucleophilic attack by
the 18-electron complex or via transient dissociation of an
Table 4. Influence of Temperature on 2-Cl/LiOTf Catalyzed
a
CO Hydrogenation
2
TOF (h−1
)
b
temperature (°C)
TON
ancillary ligand and coordination of CO . Both insertion
2
2
4
6
8
5
5
0
0
17 000
29 000
23 000
18 000
10 000
c
c
pathways have been implicated in transition metal-mediated
5700 (±280)
23
formate generation. Subsequent coordination of dihydrogen
13d
to 2 and deprotonation by DBU would close the cycle. This
12 000 (±280)
mechanistic hypothesis is appealing, as it combines several
100
13,14
a
elementary reactions that have considerable precedent,
Reaction conditions: 1000 psi of CO /H (1:1), 0.3 μmol 2-Cl, 24
mmol DBU, 3.2 mmol LiOTf in 5 mL of MeCN for 16 h. TOF
measured after the first hour including temperature equilibration.
Average TON for three or more trials.
2
2
b
proposes two intermediates that have been well-characterized,
and would be consistent with the similar TONs observed for
precatalysts 2, 3, and 4 (Table 2). Nevertheless, additional
evidence for this mechanism has remained elusive.
c
Initial experiments to probe the proposed mechanism
focused on evaluating the interconversion of 2 and 4 though
reactions with H and CO . To this end, a J. Young NMR tube
had only deleterious effects, but cooling the reaction improved
the TON considerably, reaching a maximum of 29 000 at 45
2
2
iPr
Figure 9. Hypothetical mechanism for [( PNP)Co] catalyzed CO hydrogenation.
2
E
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was charged with 2-Cl in acetronitrile-d and treated with
added as a poison and found to reduce TONs by only a small
fraction (entry 3). This suggests that the number of active
catalyst sites is nearly equal with the 2-Cl loading, as expected
for a homogeneous catalyst. If a particulate catalyst was
operative, extensive inhibition would be expected from a sub-
stoichiometric poison, as only a small fraction of surface metal
sites would be active. In fact, significant amounts of added
3
LiOTf (3 equiv) and DBU (8 equiv) under an H atmosphere.
2
The sample was monitored by NMR spectroscopy at ambient
temperature over 1 d, where 2-Cl persists as the major
diamagnetic species, along with some free ligand (vide infra)
3
1
and other minor P NMR resonances. Notably, a set of
1
paramagnetic peaks was also observed in the H NMR
spectrum, though no evidence of 4 was found. Heating the
sample at 50 °C for several days did afford trace amounts of 4,
but this occurred along with general sample decomposition.
PMe were required to effect a dramatic reduction in TON
3
(
entry 4).
The nature of the active catalytic cobalt species was further
Likewise, the reduction of CO by 4 was investigated by
2
probed with additional comparative catalysis. One such
experiment employed an alternate cobalt source, Co CO
treating an acetronitrile-d solution of 4 with LiOTf (3 equiv)
3
2
8
and a CO atmosphere. However, no evidence of formate
2
(
entry 5), which is a common precursor for the synthesis of
formation was observed by NMR spectroscopy even after
extended heating. This inability to identify a high-fidelity route
for interconversion between 2 and 4 obviates more mean-
ingfully mechanistic conclusions at this time. The proposed
mechanism in Figure 9 cannot be discounted with the
observations in hand, but clearly more extensive studies,
including high-pressure experiments to better mimic catalytic
conditions, will be required to further elucidate the catalytic
pathway. These investigations are the focus of ongoing work in
26
various cobalt nanoparticles. It is also possible that loss of the
chelating ligand from a precatalyst would generate a soluble
H Co(CO)y active species, similar to species invoked in
x
27
catalytic hydroformylation. While Co CO produced some
2
8
turnover for the CO -to-formate reaction, the TON is far
2
inferior to that observed for 2-Cl and suggests the two systems
28
do not share a common active species. Perhaps the strongest
iPr
evidence that catalysis occurs via a molecular [( PNP)Co]
iPr
complex came from doping 2-Cl with extra PNP ligand
our laboratory.
iPr
Observation of nontrivial amounts of free iPrPNP ligand
(entry 6). Supplementing the reaction with 1 equiv of PNP
ligand increased activity, achieving a TON of 21 000. A similar
enhancement has been described by Beller and co-workers for
related formate dehydrogenation catalysis and presumably
during our mechanistic experiments using 2-Cl and 4 brought
immediate questions regarding the speciation of the active
cobalt catalyst. In particular, it raised the possibility that the
2
9
iPr
originates from stabilization of the chelate−metal complex.
[
( PNP)Co] precatalysts simply serve as a source of metallic
While the precise mechanism of 2-Cl/LiOTf catalyzed CO
cobalt for a heterogeneous catalytic species instead of a
homogeneous molecular catalyst. Distinguishing between
homo- or heterogeneous catalytic speciation is rarely a
2
hydrogenation to formate remains the object of ongoing
investigation, the experiments described above strongly suggest
24
iPr
that the cobalt precatalysts give rise to a molecular [( PNP)-
straightforward process. Nonetheless, several widely used
experiments were conducted to test for evidence of
heterogeneity. These experiments were conducted at 80 °C
with the expectation that should decomposition to a
heterogeneous catalyst occur, it would be more pronounced
at higher temperature and thus easier to detect. The results of
these experiments are summarized in Table 5. Initial tests
Co] active species.
CONCLUDING REMARKS
■
The significant activity of 2-Cl and the related cobalt
precatalysts provides a substantial step forward in cobalt-
promoted CO hydrogenation. Just as significant, these catalytic
2
systems demonstrate yet another example of the remarkable
enhancement possible from LA cocatalysts in reversible CO2
hydrogenation catalysis and are the first such example using
Table 5. Tests for Speciation of Cobalt Catalyzed CO
Hydrogenation
2
a
15,30
cobalt.
In comparison to prior cobalt catalysis with
(
dmpe) CoH, 2-Cl/LiOTf provides a marked increase in
2
TON (29 000) using a more attractive base (DBU) in contrast
to a maximum TON of 9400 using Verkade’s base. It should,
however, be noted that the 2-Cl/LiOTf system requires more
forcing temperature and pressures conditions (1000 psi; 45 °C)
when compared to the mild 290 psi; 21 °C conditions
entry
catalyst
adulterant
TON
1
2
3
4
5
6
2-Cl
18 000
16 000
16 000
7000
2-Cl
∼1500 equiv of Hg
0.2 equiv of PMe3
20 equiv of PMe3
employed for (dmpe) CoH. In comparison to the ∼60 000
2-Cl
2
iPr
2-Cl
TON obtained with [( PNP)Fe] catalyst congeners, 2-Cl/
Co CO
2
410
LiOTf is clearly less productive. Yet the 2-Cl/LiOTf catalysis
8
2-Cl
1 equiv of iPrPNP
21 000
results do suggest great potential for cobalt in CO hydro-
2
genation. The TOF of 5700 (±280) h− (45 °C) observed after
the first hour of catalysis and the improved performance at
lower temperature suggests the native activity of 2-Cl/LiOTf is
quite high, but the system chiefly suffers from poor stability,
which limits its overall production. Additionally, further
elucidation of the catalytic mechanism could permit improved
targeting of precatalysts that activate with higher efficiency and
1
a
Reaction conditions: 1000 psi of CO /H (1:1), 0.3 μmol catalyst, 24
mmol DBU, 3.2 mmol LiOTf in 5 mL of MeCN at 80 °C for 16 h.
2
2
focused on poisoning experiments using Hg and PMe . The
3
addition of excess Hg (∼1500 equiv) failed to produce a
significant inhibition in catalysis (entry 2) compared to the
control reaction (entry 1). This is often consistent with a
homogeneous catalyst, though it cannot be taken as irrefutable
given the low solubility of cobalt in liquid mercury. To
augment this test, substoichiometric amounts of PMe were
provide enhanced CO hydrogenation. Efforts to achieve these
2
25
R
mechanistic insights and further develop [( PNP)Co] catalysts
are the focus of current work in our laboratory.
3
F
DOI: 10.1021/acs.inorgchem.6b01454
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
8
.52; N, 2.85%. H NMR (CD CN): δ 1.29−1.38 (m, 24H,
EXPERIMENTAL SECTION
3
■
PCH(CH ) ), 2.37−2.43 (m, 2H, PCH CH N), 2.45 (s, 3H,
3
2
2
2
General Considerations. All manipulations were performed using
standard vacuum, Schlenk, cannula, or glovebox techniques. Hydrogen,
carbon dioxide, and high-purity carbon monoxide were purchased
from Airgas and used as received. (Ph P) CoCl, MeN-
NCH ), 2.49−2.54 (m, 2H, PCH CH N), 2.49−2.54 (m, 2H,
3
2
2
PCH(CH ) ), 2.55−2.60 (m, 2H, PCH(CH ) ), 2.77−2.91 (m, 4H,
3
2
3 2
31
1
13
1
PCH CH N). P{ H} NMR (CD CN): δ 89.62 (s). C{ H}
2
2
3
3
3
(
(
(
(
1
CD CN): δ 17.59 (PCH(CH ) ), 17.83 (PCH(CH ) ), 18.36
i
3 3 2 3 2
{
CH CH (P Pr )} , and sodium tetrakis[3,5-bis(trifluoromethyl)-
2 2 2 2
31
PCH(CH ) ), 18.55 (PCH(CH ) ), 25.07 (t, PCH CH N), 27.26
t, PCH(CH ) ), 27.49 (t, PCH(CH ) ), 52.75 (NCH ), 65.36
PCH CH N), 198.28 (CO), 199.12 (CO). IR (KBr): ν = 1981,
913 cm . Selected data for [( PNP)Co( CO) ]Cl. IR (KBr): ν
3
2
3
2
2
2
phenyl]borate were prepared as previously described. All other
chemicals were purchased from Aldrich, Fisher, VWR, Strem, or
Cambridge Isotope Laboratories. Nonvolatile solids were dried under
3
2
3
2
3
2
2
CO
−1
iPr
13
2
CO
vacuum at 60 °C for 2 d. DBU was dried over CaH and twice distilled
−1
2
= 1940, 1866 cm .
Preparation of [( PNP)Co(CO) ]Cl (2-BAr ). A 20 mL scintillation
prior to use. Solvents were dried and deoxygenated using literature
iPr
F
4
32
1
13
31
2
procedures. H, C, and P NMR spectra were recorded on Bruker
00 MHz DRX, 500 MHz DRX, or 600 MHz spectrometers at
vial was charged with 37 mg (0.08 mmol) of 2-Cl and a solution of 70
3
mg (0.08 mmol) of NaBArF in 5 mL of THF. A clear orange solution
4
1
13
ambient temperature, unless otherwise noted. H and C chemical
was immediately formed and allowed to stir for 1 h at ambient
temperature. The solvent was removed in vacuo, and the residue was
triturated with pentane to remove traces of ethereal solvent, giving a
red-orange solid. The solid was washed with ∼3 mL of pentane and
shifts are referenced to residual solvent signals; 31P chemical shifts are
referenced to an external standard of H PO . Probe temperatures were
3
4
calibrated using ethylene glycol and methanol as previously
33
described. IR spectra were recorded on a Thermo-Nicolet FTIR.
X-ray crystallographic data were collected on a Bruker CCD
diffractometer with Mo Kα radiation or Cu Kα radition. Samples
were collected in inert oil and quickly transferred to a cold gas stream.
The structures were solved from direct methods and Fourier syntheses
and refined by full-matrix least-squares procedures with anisotropic
thermal parameters for all non-hydrogen atoms. Crystallographic
calculations were performed using the SHELX programs. High-
then extracted with diethyl ether. The solution was reduced to 2 mL
F
and chilled at −35 °C overnight yielding 47 mg (56%) of 2-BAr as
4
large scarlet blocks. Additional recrystallizations from the original
supernatant gave a total of 68 mg (82%). Anal. Calcd for
C H BCoNO P F : C, 47.21%; H, 3.96%; N, 1.08%. Found: C,
51
51
2 2 24
1
4
2
2
7.73%; H, 3.84%; N, 1.01% H NMR (CD Cl ): δ 1.31−1.39 (m,
2
2
4H, PCH(CH ) ), 2.24−2.28 (m, 2H, PCH CH N), 2.31−2.36 (m,
3
2
2
2
H, PCH CH N), 2.39 (s, 3H, NCH ), 2.42−2.46 (m, 2H,
2
2
3
pressure catalytic CO hydrogenation reactions were performed using
2
PCH(CH ) ), 2.50−2.56 (m, 2H, PCH(CH ) ), 2.71−2.80 (m, 4H,
3
2
3 2
F
F
a Parr 5500 series compact reactor with glass insert. Elemental analyses
PCH CH N), 7.56 (s, 4H, p−CH BAr ), 7.72 (s, 8H, o−CH BAr ).
2
2
4
4
13
1
were performed at Robertson Microlit Laboratory in Ledgewood, NJ.
C{ H} NMR (CD Cl ): δ 17.58 (PCH(CH ) ), 17.84 (PCH-
2
2
3 2
iPr
Preparation of ( PNP)CoCl (1). A 200 mL round-bottom flask was
(
CH ) ), 18.29 (PCH(CH ) ), 18.58 (PCH(CH ) ), 24.71 (t,
3 2 3 2 3 2
charged with 1.49 g (1.69 mmol) of (Ph P) CoCl, 649 mg (2.04
3
3
PCH CH N), 27.09 (m, PCH(CH ) ), 52.51 (NCH ), 65.21
2 2 3 2 3
iPr
mmol, 1.2 equiv) of PNP, and ∼30 mL of diethyl ether. The reaction
was stirred for 1 h at ambient temperature. The resulting green-black
solution was filtered through a pad of Celite. The filtrate solution was
reduced by half and chilled to −35 °C. After it was cooled for 30 min,
an initial crop of royal blue crystals was obtained. The solid was
washed with cold pentane (ca. 5 mL) until the eluent was no longer
green. A second recrystallization from diethyl ether at −35 °C yielded
F
(
PCH CH N), 117.86 (p-CH BAr ), 124.99 (q, CF ), 129.25 (q,
2 2 4 3
F
F
m-CH BAr ), 135.19 (o-CH BAr ), 162.14 (q, C-B), CO resonances
not located. P{ H} NMR (CD Cl ): δ 88.50 (s). IR (KBr): ν =
4
4
31
1
2
2
CO
−1
1
996, 1937 cm .
iPr
Preparation of ( PNP)Co(CO)(H) (3). A 20 mL scintillation vial was
iPr
charged with 105 mg (0.22 mmol) of [( PNP)Co(CO) ]Cl and ∼8
mL of diethyl ether and then frozen in a liquid nitrogen chilled cold
well. 225 μL (0.23 mmol) of 1.0 M Na(Et) BH in THF was quickly
2
3
23 mg (46%) of analytically pure 1 as blue plates. Higher yields of
3
less pure material may be obtained from extended recrystallization
times and subsequent crops of solid. These samples typically contain
added to the frozen solution. The reaction was thawed and stirred at
ambient temperature for 30 min. The orange suspension initially
lightened to yellow-orange before darkening to a cloudy brown-green
solution. The diethyl ether was removed in vacuo leaving a brown
residue. The residue was triturated with pentane to remove traces of
ethereal solvent followed by extraction of a lime-green solution with
additional pentane. The removal of solvent in vacuo afforded a dark
green oil. Recrystallization with hexamethyldisiloxane yielded 26 mg
(29%) of 3 as small dark orange crystals, which often must be
manually separated from green crystals of 4. The crystals were quickly
washed with minimal amounts of cold hexamethyldisiloxane.
Subsequent recrystallization with additional hexamethyldisiloxane
may be required to remove all traces of 4. Anal. Calcd for
C H CoNOP : C, 53.07; H, 9.90; N, 3.44. Found: C, 52.90; H,
1
significant amounts of free PPh . H NMR (C D ): δ 64.3, 42.2, 30.4,
3
6
6
1
5.7, 12.3, 9.8, −0.24, −1.4, −11.7. Anal. Calcd for C H ClCoNP :
C, 49.34; H, 9.50; N, 3.38. Found: C, 49.37; H, 9.61; N, 3.33%.
1
7
39
2
Magnetic susceptibility (Evans method) μ = 2.52 μB.
eff
iPr
Preparation of [( PNP)Co(CO) ]Cl (2-Cl). Method A: A heavy-
2
walled glass vessel was charged with 219 mg (0.53 mmol) of
iPr
(
PNP)CoCl and ∼15 mL of diethyl ether. On a high-vacuum line,
the solution was frozen at −196 °C, and the vessel was pressurized to
4
00 torr with carbon monoxide. The solution was warmed to ambient
temperature and stirred for 1 h giving an orange precipitate with a
colorless supernatant. The vessel was degassed, and the precipitate
collected by filtration and then washed with ∼5 mL of diethyl ether.
The orange powder was dried in vacuo yielding 242 mg (98%) of 2-Cl.
Method B: A heavy-walled glass vessel was charged with 658 mg (0.75
mmol) of (Ph P) CoCl and ∼6 mL of diethyl ether. The brown-green
18
40
2
1
9.95; N, 3.35%. H NMR (C D ): δ −24.82 (t, 57.1 Hz, 1H, Co-H),
6
6
1.06−1.12 (m, 12H, PCH(CH ) ), 1.14−1.19 (m, 2H, PCH CH N),
3
2
2
2
3
3
1.25−1.32 (m, 12H, PCH(CH ) ), 1.47−1.53 (m, 2H, PCH CH N),
3
2
2
2
suspension was frozen in a liquid nitrogen chilled cold well, and a
solution of 287 mg (0.90 mmol, 1.2 equiv) of PNP in ∼3 mL of
1.61−1.67 (m, 2H, PCH(CH ) ), 1.77−1.89 (m, 4H, (PCH CH N),
3
2
2
2
iPr
13 1
2.00−2.06 (m, 2H, PCH(CH ) ), 2.42 (s, 3H, NCH ). C{ H} NMR
3
2
3
diethyl ether was slowly added, ensuring that the suspension remained
frozen. The vessel was quickly removed from the glovebox and kept
frozen at −196 °C. The vessel was evacuated and pressurized to 400
torr with carbon monoxide. The suspension was thawed and stirred at
ambient temperature for 4 h, during which time the precipitate
changed from brown-green to a vibrant orange. The suspension was
filtered through a glass frit and washed with copious amounts of
diethyl ether (ca. 12 mL). The orange solid was then washed with ∼8
mL of cold THF, or until the filtrate was no longer a dark orange, and
then finally with a small quantity of pentane to aid drying. This
method yielded 327 mg (93%) of 2-Cl. Anal. Calcd for
C H ClCoNO P : C, 48.57; H, 8.37; N, 2.98. Found: C, 48.29; H,
(C D ): δ 18.64 (PCH(CH ) ), 19.06 (t, (PCH(CH ) ), 20.15 (t,
6 6 3 2 3 2
(PCH(CH ) ), 20.32 (t, (PCH(CH ) ), 26.00 (t, PCH CH N) 26.41
(t, PCH(CH ) ) 29.93 (q, PCH(CH ) ), 55.06 (NCH ), 62.96 (t,
PCH CH N), 211.26 (CO). P{ H} NMR (C D ): δ 80.86 (s). IR
= 1877 cm , ν = 1836 cm . Selected data for
( PNP)Co( CO)H. H NMR (C
JP−H = 57.1 Hz, 1H, Co-H). IR (KBr): ν
3
2
3
2
2
2
3
2
3
2
3
31
1
2
2
6
6
−1
−1
(KBr): ν
Co−H
CO
iPr
13
1
2
D
6
): −24.82 (dt, JC−H = 13.9 Hz,
6
2
−1
= 1865 cm , ν
=
CO
Co−H
−1
1802 cm .
Preparation of (κ2 PNP)Co(CO) H (4). A 20 mL scintillation vial
was charged with 70 mg (0.15 mmol) of [( PNP)Co(CO) ]Cl and
∼5 mL of diethyl ether and then frozen in a liquid nitrogen chilled
cold well. 149 μL (0.15 mmol) of 1.0 M in Na(sec-butyl) BH in THF
‑iPr
2
iPr
2
19
39
2
2
3
G
DOI: 10.1021/acs.inorgchem.6b01454
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
was quickly added to the frozen solution and then allowed to warm to
ambient temperature over 20 min. The reaction initially formed an
orange-brown suspension, cleared to a dark orange solution, and then
finally became a green-brown solution with a white precipitate. The
diethyl ether was removed in vacuo, affording a brown residue. The
residue was triturated with pentane (ca. 3 mL) to remove traces of
ethereal solvent followed by extraction of a light-green solution with
additional pentane. Removal of solvent in vacuo afforded a dark green
oil, which was dissolved in ∼3 mL of diethyl ether and transferred to
heavy-walled glass vessel. The green solution was frozen at −196 °C,
and the vessel was pressurized with 130 torr of carbon monoxide. The
reaction was allowed to thaw and stirred for 2 min, turning a pale
orange. The carbon monoxide and diethyl ether were immediately
removed in vacuo, and the residue was extracted with pentane to give a
yellow solid. Recrystallization with hexamethyldisiloxane at −35 °C
overnight initially afforded 31 mg (41%) of 4 as yellow-green crystals
of analytically pure material. Prolonged recrystallizations gave a total of
ACKNOWLEDGMENTS
■
This article is based upon work supported by the National
Science Foundation (CHE-1350047) and the Curators of the
Univ. of Missouri. W.H.B is a fellow of the Alfred P. Sloan
Foundation.
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3
8 mg (58%) with slightly lower purity. Anal. Calcd for
C H CoNO P : C, 52.41; H, 9.26; N, 3.22. Found: C, 52.53; H,
1
9
40
2 2
1
9
1
(
.09; N, 3.17%. H NMR (C D ): δ −12.18 (t, 41.7 Hz, 1H, Co-H),
6
6
.06 (q, 12H, PCH(CH ) ), 1.12 (q, 12H, PCH(CH ) ), 1.30−1.34
3
2
3 2
m, 4H, (PCH CH N), 1.76−1.82 (m, 4H, PCH(CH ) ), 1.88 (s, 3H,
2
2
3 2
13 1
NCH ), 2.26−2.30 (m, 4H, (PCH CH N). C{ H} NMR (C D ): δ
3
2
2
6
6
1
4
(
1
(
(
8.52 (PCH(CH ) , 23.54 (t, PCH CH N), 28.31 (t, PCH(CH ) ),
3 2 2 2 3 2
31
1
2.73 (NCH ), 51.47 (PCH CH N), 214.60 (CO) . P{ H} NMR
C D ): δ 65.17 (s). IR (KBr): ν
878 cm . Selected data for ( PN P)Co( CO) H. H NMR
C D ): 12.17 (tt, J
3
2
2
2
−
1
−1
= 1937 cm , ν = 1961 cm ,
6
6
Co−H CO
−1
iPr
M
13
1
2
2
2
= 3.7 Hz, J
= 41.6 Hz, 1H, Co-H). IR
P−H
6
6
C−H
−
1
−1
KBr): νCo−H = 1932 cm , ν = 1912, 1842 cm .
CO
(
1
General Methods for Catalytic CO Hydrogenation Studies.
2
In a drybox, a 50 mL glass reactor liner was charged with catalyst as a
stock solution in acetonitrile (ca. 0.0017 M), a corresponding amount
of DBU, LA, and 5 mL of solvent. The cylinder liner was placed into
the Parr reactor, and the vessel was sealed. The reactor was removed
2
015, 6, 693. (d) Rezayee, N. M.; Huff, C. A.; Sanford, M. S. T. J. Am.
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from the drybox and pressurized with 500 psi of CO followed by an
2
additional 500 psi of H at ambient temperature. The reactor was then
2
heated to the indicated temperature and stirred for the indicated time.
The reaction was stirred at an estimated 1000 rmp, based on the
manufacturer’s indicated maximum stir rate for the vessel. The
reaction was stopped by placing the reactor in an ice bath and venting
7
(
5
78.
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2
D O to ensure a freely flowing suspension (ca. 3−5 mL) and 100 μL
2
of dimethylformamide were added as an internal standard for
quantification of the formate product by 1H NMR spectroscopy.
The NMR sample was prepared from an aliquot of the cloudy
suspension, which was diluted with D O until fully dissolved.
2
ASSOCIATED CONTENT
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■
1
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AUTHOR INFORMATION
■
*
(
k) Onishi, N.; Xu, S.; Manaka, Y.; Suna, Y.; Wang, W.-H.;
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Corresponding Author
E-mail: bernskoetterwh@missouri.edu.
5
Notes
(
The authors declare no competing financial interest.
1465. (b) Leitner, W.; Dinjus, E.; Gassner, F. In Aqueous-Phase
H
DOI: 10.1021/acs.inorgchem.6b01454
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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DOI: 10.1021/acs.inorgchem.6b01454
Inorg. Chem. XXXX, XXX, XXX−XXX