Catalytic Formic Acid Dehydrogenation and CO2 Hydrogenation Using Iron PNRP Pincer Complexes with Isonitrile Ligands

Article abstract of DOI:10.1021/acs.organomet.8b00534

It has previously been demonstrated that complexes of the form (^iPrPNP)Fe(H)(C≡NR) (^iPrPNP = N(CH₂CH₂P(ⁱPr)₂)₂^-, R = 2,6-dimethylphenyl or 4-methoxyphenyl), which contain a pincer ligand capable of metal-ligand cooperation (MLC), are active for CO₂ hydrogenation. Herein, the synthesis and catalytic activity of a second-generation of precatalysts containing a tertiary amine ligand, which cannot participate in MLC, are presented. Specifically, the complexes (^iPrPN^MeP)Fe(H)(HBH₃)(C≡NR) (^iPrPN^MeP = MeN(CH₂CH₂P(ⁱPr)₂)₂, R = 2,6-dimethylphenyl (2a), tert-butyl (2b), or adamantyl (2c)) have been prepared and crystallographically characterized. These complexes are precatalysts for both formic acid dehydrogenation and CO₂ hydrogenation to formate, and give improved activity compared to first-generation systems with isonitrile ligands. The second-generation systems 2a-c, however, give inferior activity compared to the related carbonyl complexes (^iPrPNP)Fe(H)(CO) and (^iPrPN^MeP)Fe(H)(HBH₃)(CO), which have been previously reported. This study demonstrates that a ligand which can participate in MLC is not universally advantageous for promoting the hydrogenation and dehydrogenation reactions studied in this work and provides guidance for the rational design of improved catalysts for reactions relevant to energy storage.

Full text of DOI:10.1021/acs.organomet.8b00534

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Catalytic Formic Acid Dehydrogenation and CO Hydrogenation
2
R
Using Iron PN P Pincer Complexes with Isonitrile Ligands
Julia B. Curley, Nicholas E. Smith, Wesley H. Bernskoetter,* Nilay Hazari,*
†
†
,‡
,†
and Brandon Q. Mercado†
†
The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
The Department of Chemistry, The University of Missouri, Columbia, Missouri 65211, United States
‡
*
S Supporting Information
ABSTRACT: It has previously been demonstrated that complexes of the form
iPr
iPr
i
−
(
PNP)Fe(H)(CNR) ( PNP = N(CH CH P( Pr) ) , R = 2,6-dimethyl-
2
2
2 2
phenyl or 4-methoxyphenyl), which contain a pincer ligand capable of metal−
ligand cooperation (MLC), are active for CO hydrogenation. Herein, the syn-
2
thesis and catalytic activity of a second-generation of precatalysts containing a
tertiary amine ligand, which cannot participate in MLC, are presented. Specif-
iPr
Me
iPr
Me
ically, the complexes ( PN P)Fe(H)(HBH )(CNR) ( PN P = MeN-
3
i
(
CH CH P( Pr) ) , R = 2,6-dimethylphenyl (2a), tert-butyl (2b), or adamantyl
2
2
2 2
(
2c)) have been prepared and crystallographically characterized. These com-
plexes are precatalysts for both formic acid dehydrogenation and CO hydro-
2
genation to formate, and give improved activity compared to first-generation
systems with isonitrile ligands. The second-generation systems 2a−c, however,
iPr
give inferior activity compared to the related carbonyl complexes ( PNP)Fe-
iPr
Me
(
H)(CO) and ( PN P)Fe(H)(HBH )(CO), which have been previously
3
reported. This study demonstrates that a ligand which can participate in MLC is not universally advantageous for promoting the
hydrogenation and dehydrogenation reactions studied in this work and provides guidance for the rational design of improved
catalysts for reactions relevant to energy storage.
INTRODUCTION
of formic acid for CHS requires improved catalysts for both
formic acid dehydrogenation and CO2 hydrogenation to
formic acid.
■
Decreased fossil fuel reserves, paired with rising concerns
about greenhouse gas emissions, have led to significant interest
in the utilization of sustainable energy sources. Unfortunately,
many renewable energy sources such as wind and solar are inher-
ently intermittent and their widespread use will require the
development of sustainable methods for energy storage. H is
one such clean energy carrier that can be either directly com-
busted or electrochemically oxidized in a fuel cell. Gaseous
H , however, has a low volumetric energy density, hindering its
1
Many previously developed homogeneous and heteroge-
neous catalysts for CO hydrogenation to formate or formic
2
acid dehydrogenation have utilized precious metals such as
3b,4,5
ruthenium and iridium.
Significant efforts are being made
2
to replace these costly, rare, and in some cases toxic metals
2
with more abundant and inexpensive first-row transition metals
7
such as iron. A number of homogeneous iron catalysts have
2
now been reported for both CO hydrogenation to formate
2
attractiveness as an energy storage medium. Chemical H storage
CHS) based on the reversible (de)hydrogenation of small mole-
cules could promote the use of H as an energy storage vector,
2
and formic acid dehydrogenation, but in general, these systems
give inferior activity compared to precious metal systems and
(
2
8
are mechanistically less well understood. We, along with
and circumvent more costly H compression and liquefaction
2
3
several other groups, have been investigating iron complexes
techniques. Formic acid (4.4 wt % H ), which can be
2
R
H
i
supported by the ligand HN(CH CH PR ) ( PN P, R = Pr
2
2
2 2
dehydrogenated to form H and CO , is a potential liquid CHS
2
2
or Cy), which can participate in metal−ligand cooperation
material that is currently primarily derived from nonrenewable
9
4
(MLC) to activate substrates when it is in its deprotonated
carbon sources. Renewable formic acid synthesis via the direct
i
−
R
i
8g,k,10
amido form N(CH CH P Pr ) ( PNP, R = Pr or Cy).
2
2
2 2
hydrogenation of CO using H generated from renewable
2
2
We demonstrated that for formic acid dehydrogenation the
sources could greatly enhance the value of formic acid as an
energy storage medium.
iPr
H
3
b,4,5
iron formate complex ( PN P)Fe(H){OC(O)H}(CO)
achieves almost 1 000 000 turnovers (TON) with the addition
Although the hydrogenation of
CO to formic acid is slightly thermodynamically uphill in
2
8g
of a Lewis acid (LA) cocatalyst. The related precatalyst
organic solvents, typically in academic settings a base is used to
generate a formate salt and drive the reaction forward. On a
6
larger scale, engineering strategies exist to recycle the base and
produce formic acid. Nevertheless, at this stage, the utilization
Received: July 27, 2018
4b
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00534
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 1. (a) Formic acid dehydrogenation, (b) CO hydrogenation to formate, (c) previous PNP iron catalysts studied by our groups, and (d) com-
2
plexes studied in this work.
Me
Scheme 1. Synthesis of PN P Iron Isonitrile Complexes 2a−c
(iPrPNP)Fe(H)(CO) gives a TON of almost 9000 for CO2
hydrogenation to formate in the presence of a LA cocatalyst
both formic acid dehydrogenation and CO hydrogenation.
2
Our study provides fundamental information about the effect
of changing the nature of the π-acid ligand in iron complexes
featuring PNP ligands, which may be useful for designing even
more active and productive catalysts for a range of hydro-
genation and dehydrogenation reactions.
8k
and base (Figure 1a−c). In both cases, these are some of the
highest TONs observed for first-row transition metal systems.
Subsequently, we developed second-generation systems in
H
which the PN P ligand was replaced with a PNP ligand contain-
ing a tertiary amine, which cannot activate substrates via MLC.
R
Me
RESULTS AND DISCUSSION
Complexes of the form ( PN P)Fe(H)(HBH )(CO)
3
■
R
Me
i
Me
(
PN P = CH N(CH CH PR ) , R = Pr or Cy) are an
Synthesis of PN P Iron Isonitrile Precatalysts. The
3
2
2
2 2
2
iPr
Me
2
order of magnitude more productive for CO hydrogenation to
κ -borohydride precursor complex ( PN P)Fe(H)(κ -H BH )
2
2
2
8
k
formate than their first-generation analogues, affording almost
(1) was prepared according to literature procedures. Previously,
we reported that addition of 1 atm of carbon monoxide to 1
generates the second-generation carbonyl complex ( PN P)-
6
0 000 turnovers in the presence of a LA cocatalyst and base
Figure 1b,c). However, in both the first- and second-generation
systems, catalyst decomposition is a major issue, which needs
iPr
Me
(
8k
Fe(H)(HBH )(CO) (A, Figure 1c). In order to obtain the
3
8g,k
iPr
Me
to be addressed in order to develop improved catalysts.
related isonitrile complexes ( PN P)Fe(H)(HBH )(CNR)
3
One strategy to prevent catalyst decomposition is to modify
the ancillary ligands around the iron. In particular, the replace-
ment of a carbonyl ligand with an isonitrile ligand provides
potential benefits because of the tunable steric and electronic
properties of isonitriles. In addition, isonitriles could replace
the use of carbon monoxide at a late stage of the catalyst
synthesis, obviating the need for new method development.
Recently, we reported the synthesis of complexes of the
(R = 2,6-dimethylphenyl (2a), tert-butyl (2b), or adamantyl
(2c)), the procedure was modified, and 1 equiv of the appro-
priate isonitrile was added to 1 (Scheme 1). Complexes 2a−c
were obtained in good yields after purification by recrystalliza-
tion, provided exactly 1 equiv of isonitrile was added. Addition
of any excess isonitrile resulted in the formation of cationic
iPr
Me
bis(isonitrile) iron species of the form [( PN P)Fe(H)(C
NR) ][BH ] (R = 2,6-dimethylphenyl (3a), tert-butyl (3b), or
2
4
iPr
form ( PNP)Fe(H)(CNR) (R = 2,6-dimethylphenyl (B),
adamantyl (3c)) (Scheme 2), which were difficult to separate
from the desired mono(isonitrile) iron products. Complexes
3a−c were not isolated but characterized on the basis of their
4
-methoxyphenyl (C)) (Figure 1c), in which the carbonyl
ligand of our first-generation catalysts has been replaced with
an isonitrile ligand. These complexes were productive cata-
11
1
H NMR spectra, which clearly indicated that two isonitrile
lysts for CO hydrogenation to formate but showed decreased
activity compared to the first-generation catalysts. Herein, we
ligands were present (see the Supporting Information). A sim-
ilar problem involving coordination of two isonitrile ligands
was observed in the synthesis of first-generation isonitrile
complexes. In fact, the successful isolation of alkyl isonitrile
complexes 2b and 2c is noteworthy, since in the case of
2
describe the replacement of the carbonyl ligand with isonitrile
R
Me
11
ligands in our second-generation catalysts, featuring PN P
ligands (Figure 1d). This has resulted in active catalysts for
B
DOI: 10.1021/acs.organomet.8b00534
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Article
Scheme 2. Synthesis of Cationic Bis(isonitrile) Iron Species
of the isomers and that the isomers interconvert slowly on the
NMR time scale. The hydride regions of the room-temperature
3
a−3c Characterized in Situ
1
H NMR spectra of 2a−c in C D were also consistent with
6
6
the presence of two isomers. Apparent triplet resonances
were observed at −21.5 and −21.8 ppm for 2a, at −22.1 and
−
22.3 ppm for 2b, and at −22.1 and −22.3 ppm for 2c, in the
31
same ratio as the peaks in the P NMR spectra. At room
temperature, all three complexes exhibit a broad borohy-
1
dride signal around −3 ppm that is consistent with κ -BH coor-
4
dination and suggests rapid interchange of the bound and free
10h,12
B−H protons on the NMR time scale.
The solid-state IR
−
1
spectra of 2a−c showed broad C−N absorptions at 1970 cm
−
1
−1
for 2a, 1990 cm for 2b, and 2015 cm for 2c. The C−N
−
1
absorptions of the free isonitrile ligands occur at 2126 cm
first-generation isonitrile complexes it was not possible to
isolate any complexes with alkyl isonitrile ligands due to the
concomitant formation of bis(isonitrile) species.
−
1
(
2,6-dimethylphenyl isonitrile), 2137 cm (tert-butyl isoni-
−
1
13
trile), and 2150 cm (adamantyl isonitrile). Given that the
differences between the free isonitrile stretch and coordinated
isonitrile stretch are approximately the same for all three com-
plexes, the relative extent of back-donation from the iron into
the isonitrile π*-orbital is presumably similar for 2a−c.
NMR spectroscopy showed that complexes 2a−c are diamag-
netic, with two isomers present in solution. We propose, based
8k
on observations with related complexes, that the two isomers
differ in having either syn- or anti-coplanar arrangements of the
The solid-state structures of 2a−c were determined using
single-crystal X-ray diffraction (Figure 2, Table 1). The high
quality of the data allowed for the location and free refinement
of the Fe−H and B−H hydrogen atoms. The coordination
geometry about the iron centers of 2a−c is distorted
N−Me and Fe−HBH moieties, with the syn-isomer being the
3
predominant species (Scheme 1). At room temperature in
31
C D , P NMR resonances were observed at 91.4 and 95.2 ppm
6
6
in an 8:1 ratio for 2a, at 91.3 and 95.6 ppm in a 16:1 ratio for 2b,
and at 91.2 and 95.6 ppm in a 17:1 ratio for 2c. The ratio of the
isomers remains constant when solutions of 2a−c are left at room
temperature for extended periods of time (∼2−3 days) in C D .
8k
octahedral, similar to that of carbonyl analog A. As expected,
the pincer ligand occupies three meridional coordination sites,
6
6
However, when a sample of 2b was heated to 50 °C in C D the
with the isonitrile ligand trans to the nitrogen donor of the
6
6
1
ratio changed to 5:1, before returning to its original value when
the sample was cooled back down to room temperature.
This suggests that we are observing the thermodynamic ratio
pincer ligand. The hydride and κ -BH
4
ligands are trans to each
other, and all three complexes crystallize as the syn isomers.
Nevertheless, when crystals of 2a, 2b, or 2c were dissolved in
Figure 2. Solid-state structures of 2a (top left), 2b (top right), and 2c (bottom) with ellipsoids drawn at 30% probability. Hydrogen atoms, besides
those attached to iron and boron, have been omitted for clarity. All hydrides were located in the difference map and freely refined. Only one
molecule from the asymmetric unit of 2c shown.
C
DOI: 10.1021/acs.organomet.8b00534
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Article
iPr
Me
Table 1. Selected Bond Distances (Å) and Angles (deg) for ( PN P)Fe Isonitrile Complexes 2a−c and Carbonyl Complex A
complex
Fe−B
Fe−H
Fe−C
Fe−N
Fe−P
CY
P−Fe−P
CN−C
2
a
2.971(6)
1.39(5)
1.766(3)
2.126(3)
2.2118(9),
1.180(4)
169.42(4)
163.0(3)
(
Y=N)
2
.2159(9)
2.1985(8),
.2070(8)
2
b
2.895(4)
2.888(6),
1.40(3)
1.776(3)
2.132(2)
1.182(4)
Y=N)
167.76(3)
157.3(3)
(
2
a
2
c
1.57(4),
1.54(5)
1.776(4),
1.803(5)
2.119(3),
2.128(3)
2.1867(12),
2.1936(12),
1.175(5),
1.159(5)
165.87(5),
166.83(5)
160.8(4),
165.8(5)
2
.927(6)
(
Y=N)
2
2
.1885(12),
.1995(12)
8k
A
2.864(3)
1.47(2)
1.728(2)
2.136(1)
2.2031(7),
.2034(6)
1.162(2)
Y=O)
165.45(3)
(
2
a
Two independent molecules are present in the asymmetric unit.
C D at room temperature a thermodynamic mixture of the syn
reaction conditions B undergoes a 1,2-addition of H across Fe−
6
6
2
iPr
H
11
and anti-isomers was immediately observed in all cases, sug-
gesting that the establishment of a dynamic equilibrium is
rapid. The only major difference in the solid-state structures of
the isonitrile complexes and A is the Fe−C bond length, which
is significantly longer in 2a−c. This indicates that the isonitrile
ligands are weaker π-acceptors than carbon monoxide. Addi-
tionally, the Fe−C bond lengths for 2a−c are the same within
error, which suggests that changing the electronics of the sub-
stituent on the isonitrile ligand from aryl (2a) to alkyl (2b and
N bond to form ( PN P)Fe(H) (CNR). As a conse-
2
quence, our results with B are comparable to those obtained
with 2a−c, which we propose lose BH in the presence of DBU
3
iPr
Me
to form ( PN P)Fe(H)
(CNR). Specifically, 2,6-dime-
2
thylphenyl complex 2a achieves about an order of magnitude
higher TON for this reaction than its first-generation isonitrile
analog B (Table 2, entries 2 and 5). This agrees with the
previously observed trend that second-generation complexes
are approximately an order of magnitude more productive for
CO hydrogenation than their first-generation congeners and
2c) does not have a significant effect on their back-bonding
2
8k
suggests that this is a general phenomenon. It also indicates
that the preparation and evaluation of tertiary PN P ligands
ability. This is further supported by the similar CN−C bond
angles observed in the isonitrile ligands of all three complexes,
which is also consistent with comparable back-bonding.
R
with other substituents on the nitrogen may lead to more
productive catalysts and that ligands capable of MLC are not
required for this reaction. Although complexes 2a−c are about
an order of magnitude less productive than second-generation
carbonyl species A (Table 2, entry 1), they achieve comparable
or higher TONs than the majority of the iron catalysts
Catalytic CO Hydrogenation. The isonitrile iron com-
2
plexes were tested for their ability to catalyze CO hydro-
2
genation to formate using the optimized conditions previously
8
k
utilized for A. Complexes 2a−c were found to be effective pre-
catalysts (Table 2), with 2,6-dimethylphenyl isonitrile complex
4b
currently described in the literature. This demonstrates the
R
R′
privileged nature of PN P ligands in generating productive
Table 2. Comparison of Iron Catalysts for CO2
Hydrogenation to Formate
a
iron catalysts for CO hydrogenation.
2
Formic Acid Dehydrogenation. Isonitrile complexes 2a−c
were also investigated for their ability to dehydrogenate formic
acid to CO and H . In order to provide an accurate comparison
2
2
to the second-generation carbonyl complexes, A was also tested
in formic acid dehydrogenation (Table 3). Triethylamine (NEt3)
was used as an additive at 50 mol % loading, instead of a Lewis
,
bc
entry
catalyst
DBU/LiOTf
TON
acid, in order to remove BH from the precatalysts and
3
8
k
generate the proposed catalytically active dihydride species
1
2
3
4
5
A
7.5/1
7.5/1
7.5/1
7.5/1
7.5/1
42000
5300
1300
710
in situ. The NEt was added before the precatalysts were
2a
2b
2c
B
3
treated with formic acid to ensure activation before the NEt3
was protonated with formic acid. At 0.1 mol % iron loading,
11
complete conversion of formic acid to CO and H was
610
2
2
a
observed in 3 h using A as the precatalyst (Table 3, entry 1).
When the catalyst loading was decreased to 0.01 mol % iron,
the total TON increased to 2600 over 4 h (Table 3, entry 2).
The gas mixture produced from catalysis was analyzed by gas
chromatography, and no CO was observed. The three isonitrile
complexes 2a−c were all found to be viable precatalysts,
although their TONs were approximately an order of mag-
nitude lower than their carbonyl congener A. This follows the
Reaction conditions: 69 atm of CO /H (1:1), 0.3 μmol of catalyst
2
2
b
in 10 mL of THF (ca 0.01 M), 3.60 g of DBU at 80 °C. Formate
production quantified by H NMR spectroscopy. Reported values are
the average of two trials.
1
c
2
a achieving 5300 turnovers, the highest out of these
complexes (Table 2, entry 2). Alkyl isonitrile complexes 2b
and 2c achieved 1300 and 710 TONs, respectively (Table 2,
entries 3 and 4). All three of the second-generation iso-
nitrile precatalysts gave considerably higher TON than the
first-generation isonitrile species B. We note that under the
trend observed in CO hydrogenation: the isonitrile π-acceptor
2
ligands are less effective at stabilizing catalytic systems than
carbonyl ligands. Precatalyst 2a achieves 140 turnovers over
D
DOI: 10.1021/acs.organomet.8b00534
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Article
Table 3. Comparison of Iron Catalysts for Formic Acid
Dehydrogenation
CONCLUSIONS
■
a
Me
In this work we have prepared three novel PN P supported
iron precatalysts with isonitrile ligands that are analogs of the
related first- and second-generation PN P and PN P iron
H
Me
carbonyl complexes studied extensively in the literature. In
contrast to our findings with PN P supported iron complexes
entry
catalyst
mol % [Fe]
TOF (h−1)b
TON (time)
c
H
1
2
3
4
5
6
A
A
2a
2b
2c
B
0.1
0.01
0.1
0.1
0.1
0.1
750
2100
120
100
110
30
1000 (3 h)
2600 (4 h)
140 (4 h)
120 (4 h)
120 (4 h)
with isonitrile ancillary ligands, it was possible to synthesize
Me
PN P supported iron precatalysts with both alkyl and aryl
isonitrile ligands. The second-generation isonitrile complexes
are active for both CO hydrogenation to formate as well as
2
formic acid dehydrogenation; however, they achieve about an
order of magnitude fewer TONs for these reactions than the
second-generation carbonyl complex. While the iron isonitrile
complexes reach TONs comparable to other published iron
57 (18 h)
a
Reaction conditions: formic acid (110 μL, 2.91 mmol), [Fe] (0.1 or
b
0
.01 mol %), 50 mol % NEt , 5 mL of dioxane, 80 °C. Turnover fre-
3
c
quencies (TOF) were measured after the first hour. Turnover num-
bers (TON) were measured using a gas buret. The time indicates the
point at which no further increase in TON was observed. Reported
results are the average of two trials.
catalysts for CO hydrogenation, their decrease in activity rela-
2
tive to the second-generation carbonyl system is most likely
related to the instability of their catalytic intermediates, which
we were unable to cleanly generate. Nevertheless, the catalytic
data presented here provides valuable insight into the design of
4
h (Table 3, entry 3), and 2b and 2c both reach 120 turnovers
improved iron catalysts for CO hydrogenation and formic acid
2
over 4 h (Table 3, entries 4 and 5) at 0.1 mol % catalyst
loading. In addition, the rate of formation of gaseous products
from catalysis using 2a was monitored (Figure S11) to better
understand the activity and decomposition of isonitrile
complexes 2a−c. In the first 30 min of catalysis, 2a reaches
an average of 96 out of 140 total TON, or 69% of its total
productivity. In contrast, carbonyl precatalyst A reaches
dehydrogenation. It indicates that for CO hydrogenation
2
Me
PN P supported iron catalysts are more productive than the
H
corresponding systems containing a PN P ligand and that
further modification of the identity of the substituent on the
nitrogen donor could lead to systems that give higher TON.
It also demonstrates that a ligand that can participate in MLC
is not required for formic acid dehydrogenation, which will
also impact ligand design. Finally, our results suggest that if the
carbonyl ancillary ligand is to be replaced in first- and second-
5
00 TONs after 30 min, which is only 50% of its total TON.
In general, isonitrile complexes 2a−c exhibit high initial
activities but deactivate much more quickly than their carbonyl
congener A. A comparison was also performed between the
first- and second-generation iron isonitrile complexes for formic
acid dehydrogenation. Complex B reaches only 57 TON over
H
Me
generation PN P and PN P supported iron catalysts, stronger
π-acceptor ligands, such as a nitrosyl ligand, should be pursued
rather than weaker π-acceptor ligands. Future work in our
laboratories will focus on using these guidelines to modify the
primary ligand environment around iron to design more active
and productive catalysts.
1
8 h (Table 3, entry 6). A more direct comparison between
2
a−c and the analogous first-generation isonitrile precatalyst
iPr
H
11
(
PN P)Fe(H)(HBH )(2,6-dimethylphenylisonitrile) indi-
3
cated that the first-generation system was completely inac-
tive for catalysis, although this may be related to the insta-
bility of this species at elevated temperatures. Nevertheless,
these results demonstrate that second-generation isonitrile com-
plexes 2a−c are more productive than their first-generation
congener B.
EXPERIMENTAL PROCEDURES
■
General Methods. Experiments were performed under a
dinitrogen atmosphere in an M-Braun glovebox or using standard
Schlenk techniques unless otherwise noted. Under typical operating
conditions, the glovebox was not purged between use of benzene,
diethyl ether, pentane, THF, dioxane, or toluene. As a consequence,
each solvent should be assumed to contain trace amounts of the
others. All moisture- and air-sensitive liquids were either transferred
inside the glovebox or using a stainless-steel cannula on a Schlenk line.
Solvents were dried by passage through a column of activated alumina
and stored under dinitrogen unless otherwise noted. Ethanol (200 proof)
was purchased from Decon Laboratories, Inc. and degassed and
stored under dinitrogen prior to use. All commercial chemicals were
In order to further probe the reason for the relatively poor
catalytic performance and quick deactivation of 2a−c,
we attempted to synthesize and isolate either the dihydride
iPr
Me
species ( PN P)Fe(H) (CNR) or the formate species
2
iPr
Me
(
PN P)Fe(H){OC(O)H}(CNR) (R = 2,6-dimethyl-
phenyl, tert-butyl, adamantyl), which are proposed to be
catalytic intermediates in both formic acid dehydrogenation
8g,k
used as received except where noted. Anhydrous FeCl was purchased
2
and CO hydrogenation. However, none of these complexes
2
from Alfa Aesar. Sodium borohydride was purchased from Acros
Organics. 2,6-Dimethylphenyl isonitrile, tert-butyl isonitrile, and
adamantyl isonitrile were used as received from Santa Cruz Biotech
or Sigma-Aldrich. Formic acid was purchased from EMD Millipore
Inc. and degassed prior to use. Triethylamine was purchased from
Fisher Scientific, degassed, and dried over calcium hydride prior to
use. Deuterated solvents were obtained from Cambridge Isotope
Laboratories. C D was dried by passage through a plug of activated
were stable to isolation, and none could be generated cleanly
in situ. The instability of these important catalytic intermediates
is a likely cause for the decreased catalytic activity of the second-
generation isonitrile complexes compared to that of their
carbonyl analog A. Overall, the catalytic activity of A and 2a−c
for formic acid dehydrogenation highlights the general
principle that a ligand which can cooperate with the metal is
not a prerequisite to promote this type of dehydrogenation
6
6
alumina. NMR spectra were recorded on Bruker AMX-400, AMX-
500, or AMX-600, spectrometers at ambient probe temperatures,
unless otherwise noted. Chemical shifts are reported in ppm with
1
4
reaction. This is notable for the design of improved catalytic
systems, as the potential to incorporate a substituent other
than hydrogen on the nitrogen donor of the pincer ligands
expands our ability to tune the properties of these ligands for
catalysis.
1
13
1
respect to residual internal protio solvent for H and C{ H} NMR
31
1
1
spectra. P{ H} NMR spectra are referenced via the H resonances
15
based on the relative gyromagnetic ratios. All J coupling constant
values are given in Hertz. Elemental analyses were performed by
E
DOI: 10.1021/acs.organomet.8b00534
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Robertson Microlit Laboratories, Inc. Infrared data were obtained on
a Bruker ALPHA FTIR spectrometer with a platinum ATR attach-
(m, 6H, CHCH ). The following peaks were distinct in the major and
3
minor isomers: Major isomer: −22.1 (t, 1H, Fe−H, J = 54.3 Hz).
1
3
1
ment inside a N -filled glovebox. All samples were taken of the neat
Minor isomer: −22.3 (t, 1H, Fe−H, J = 52.7 Hz). C{ H} (151 MHz,
2
solid. UV−vis data were collected on a Hewlett-Packard 8453 diode
C D ): 65.4, 50.6, 31.3, 31.1 (t, J = 6.9 Hz), 28.4 (t, J = 6.1 Hz), 26.1
6
6
3
1
1
array spectrophotometer. Literature procedures were used to syn-
(t, J = 11.5 Hz), 21.3, 19.4, 18.5. P{ H} NMR (202 MHz, C D ):
Major isomer: 91.3. Minor isomer: 95.7. IR (cm ): 1990 (υCN, br).
UV−vis (THF, (λmax, ε)): (438 nm, 989 M cm ).
iPr Me
6 6
8k
8k
11
−1
thesize 1, A, and B.
−1
−1
X-ray Crystallography. Crystal samples were mounted on
polyimide loops with immersion oil. Low-temperature diffraction
data (ω-scans) were collected on a Rigaku MicroMax-007HF
diffractometer coupled to a Saturn994+ CCD detector with Cu Kα
( PN P)Fe(H)(HBH )(adamantyl isonitrile) (2c). A Schlenk flask
3
Me 2
iPr
was charged with ( PN P)Fe(H)(κ -H BH ) 1 (25 mg, 0.064 mmol),
2 2
adamantyl isonitrile (10 mg, 0.064 mmol), and pentane (5 mL). The
mixture was stirred for 30 min at room temperature to afford a
heterogeneous yellow mixture with a yellow precipitate. The volatiles
were removed in vacuo. The crude product was extracted in 3 × 2 mL
of THF and concentrated. Yellow crystals were grown from the slow
diffusion of 5 mL of pentane into the THF solution at −35 °C and
afforded clean 2c (30 mg, 0.054 mmol, 85%). Crystals suitable for
X-ray diffraction were also grown from slow diffusion of pentane into
a concentrated THF solution at −35 °C.
(λ = 1.54178 Å) for the structure of 2c. The diffraction images for 2c
were processed and scaled using Rigaku Oxford Diffraction software.
A Rigaku SCX Mini diffractometer coupled to a Rigaku Mercury275R
CCD with Mo Kα radiation (λ = 0.71073 Å) was used for 2a and 2b.
The diffraction images for 2a and 2b were processed and scaled using
16
the Rigaku CrystalClear software. The structures were solved with
2
SHELXT and refined against F on all data by full-matrix least-squares
17
with SHELXL. Details of the crystal and refinement data for 2a−c
are described in the Supporting Information.
Anal. found (calcd) for C28
H
59
P
2
N
2
BFe: C, 61.11 (60.88); H,
10.69 (10.77); N, 5.33 (5.07). H NMR (500 MHz, C ): The fol-
lowing peaks were overlapping in both the major and minor isomers:
3.11 (m, 2H, CH ), 2.31 (m, 2H, CH ), 2.02 (s, 3H, N−CH ), 1.73−
1.95 (m, 15H, adamantyl), 1.70 (br, 4H, CH), 1.34 (m, 18H,
CHCH ), 1.11 (m, 6H, CHCH ). The following peaks were distinct
1
Gas Chromatography. Gas chromatography was performed on a
ThermoFisher Trace 1300 GC apparatus equipped with a thermal
conductivity detector and a Supelco fused silica capillary column (5 Å
D
6 6
2
2
3
molecular sieves, 30 m × 0.53 mm). The system uses N as a carrier
2
gas and allows for determination of H at concentrations greater than
3
3
2
1
ppm and CO at concentrations greater than 100 ppm.
in the major and minor isomers: Major isomer: −22.1 (t, 1H, Fe−H,
J = 54.4 Hz). Minor isomer: −22.3 (t, 1H, Fe−H, J = 51.6 Hz).
Synthetic Procedures and Characterizing Data for New
iPr
Me
13
1
Compounds. ( PN P)Fe(H)(HBH )(2,6-dimethylphenyl isonitrile)
C{ H} (151 MHz, C D ): 65.5 (t, J = 5.6 Hz), 56.0, 50.6, 44.7, 36.0,
6 6
3
iPr
Me
2
(2a). A Schlenk flask was charged with ( PN P)Fe(H)(κ -H BH )
31.2 (t, J = 6.9 Hz), 29.5, 28.5 (t, J = 6.2 Hz), 26.2 (t, J = 12.4 Hz),
2
2
3
1
1
(
0
1) (25 mg, 0.064 mmol), 2,6-dimethylphenyl isonitrile (8.4 mg,
.064 mmol), and pentane (5 mL). The mixture was stirred for 30 min
21.5 (m), 19.5, 18.6 (m), 14.3. P{ H} NMR (202 MHz, C D ):
6 6
−
1
Major isomer: 91.2. Minor isomer: 95.6. IR (cm ): 2015 (υCN, br).
UV−vis (THF, (λmax, ε)): (437 nm, 901 M cm ).
−1
−1
at room temperature to afford a heterogeneous yellow mixture with a
yellow precipitate. The volatiles were removed in vacuo. The crude
product was extracted in 3 × 2 mL of THF and concentrated. Yellow
crystals were grown from the slow diffusion of 5 mL of pentane into
the THF solution at −35 °C and afforded clean 2a (20 mg, 0.039 mmol,
General Methods for Catalytic CO2 Hydrogenation. In a
glovebox, a 50 mL glass reactor liner was charged with catalyst as a
stock solution in THF (ca. 0.02 M, 0.3 μmol catalyst), LiOTf
(492 mg, 3.15 mmol), DBU (3.6 g, 24 mmol), and 10 mL of THF.
The cylinder liner was placed into a 50 mL Parr reactor and the vessel
sealed. The reactor was removed from the glovebox and pressurized
sequentially with 34 atm of CO and then 34 atm of H via a Y-value
61%). Crystals suitable for X-ray diffraction were also grown
from slow diffusion of pentane into a concentrated THF solution
at −35 °C.
2
2
Anal. found (calcd) for C H P N BFe: C, 60.07 (59.79); H,
inlet at ambient temperature. The reactor was then heated to 80 °C, a
process which occurred in approximately 10 min, and mechanically
stirred for 24 h. The reaction was stopped by removal of the heat
source, cooling in an ice water bath, and venting of the vessel’s
atmosphere. The reaction solution, which contained some suspended
solid, was then transferred to a 100 mL round-bottomed flask, using
2
6
53
2
2
1
1
0.40 (10.23); N, 5.08 (5.36). H NMR (500 MHz, C D ): The
6 6
following peaks were overlapping in both the major and minor
isomers: 6.80 (d, 2H, CHAr J = 7.5 Hz), 6.70 (m, 1H, CH ), 3.13 (m,
Ar
2
H, CH ), 2.18 (m, 2H, CH ), 2.06 (s, 3H, N−CH ), 1.87 (m, 4H,
2
2
3
CH ), 1.75 (m, 4H, CH), 1.64 (m, 6H, CHCH ), 1.22 (m, 12H,
2
3
CHCH ), 1.06 (m, 6H, CHCH ), −2.64 (br, 4H, HBH ). The
D O to dissolve the solid products. All volatiles were removed
3
3
3
2
following peaks were distinct in the major and minor isomers: Major
in vacuo. The residue was then dissolved in D O and DMF was added
2
1
isomer: 2.56 (s, 6H, C CH ), −21.48 (t, 1H, Fe−H, J = 53.5 Hz).
as an internal standard for quantification of the formate product by H
Ar
3
Minor isomer: 2.49 (s, 6H, C CH ), −21.83 (t, 1H, Fe−H, J = 52.2 Hz).
NMR spectroscopy. Delay times of 60 s were used between scans to
ensure accurate integrations.
Ar
3
13
1
C{ H} (151 MHz, C D ): 133.7, 124.1, 65.6, 50.5, 29.8 (t, J = 7.6 Hz),
6
6
2
1
7.4 (t, J = 6.1 Hz), 26.0 (t, 12.9), 21.1 (m), 20.3 (m), 19.8, 19.6,
General Methods for Catalytic Formic Acid Dehydrogen-
ation. In a glovebox, a 25 mL Schlenk flask was charged with catalyst
as a stock solution in dioxane (10.0 mM, 2.91 μmol catalyst), and
dioxane (4.7 mL). The Schlenk flask was sealed with a septum,
removed from the glovebox, and attached to a gas buret setup (see the
Supporting Information). The gas buret and tubing were subjected to
8.3 (m). 31P{ H} NMR (202 MHz, C D ): Major isomer: 91.4.
1
6
6
−
1
Minor isomer: 95.2. IR (cm ): 1970 (υCN, br). UV−vis (THF, (λmax
ε)): (440 nm, 913 M cm ).
,
−
1
−1
iPr
Me
(
PN P)Fe(H)(HBH )(tert-butyl isonitrile) (2b). A Schlenk flask
3
iPr
Me
2
was charged with ( PN P)Fe(H)(κ -H BH ) (1) (25 mg,
2
2
three vacuum/N purge cycles. Triethylamine (202 μL, 1.46 mmol)
0.064 mmol), tert-butyl isonitrile (7.2 μL, 0.064 mmol), and pentane
2
was added to the reaction flask via syringe. The Schlenk flask was
lowered into an oil bath preheated to 80 °C and allowed to equilibrate
for 5 min. Formic acid (110 μL, 2.91 mmol) was then injected via
syringe and the change in water level in the gas buret was used to
determine the TOF and TON.
(
5 mL). The mixture was stirred for 30 min at room temperature to
afford a yellow solution. The volatiles were removed in vacuo. The
crude product was extracted in 3 × 2 mL of THF and concentrated.
Yellow crystals were grown from the slow diffusion of 5 mL of pen-
tane into the THF solution at −35 °C and afforded clean 2b (18 mg,
0
.038 mmol, 60%). Crystals suitable for X-ray diffraction were also
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.8b00534.
grown from slow diffusion of pentane into a concentrated THF
solution at −35 °C.
■
*
S
Anal. found (calcd) for C H P N BFe: C, 55.65 (55.71); H,
2
2
53
2
2
1
1
1.11 (11.26); N, 5.94 (5.91). H NMR (500 MHz, C D ): The fol-
6 6
lowing peaks were overlapping in both the major and minor isomers:
3
(
(
Additional information about selected experiments,
NMR spectra, and details about the X-ray structures of
2a−c (PDF)
.07 (m, 2H, CH ), 2.27 (m, 2H, CH ), 2.00 (s, 3H, N−CH ), 1.84
2
2
3
m, 4H, CH ), 1.77 (m, 6H, CHCH ), 1.68 (m, 4H, CH), 1.31
2
3
m, 6H, CHCH ), 1.25 (m, 6H, CHCH ), 1.14 (s, 9H, CH ), 1.08
3
3
3
F
DOI: 10.1021/acs.organomet.8b00534
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Accession Codes
(8) (a) Boddien, A.; Loges, B.; Gar
Junge, H.; Ludwig, R.; Beller, M. Iron-Catalyzed Hydrogen
̈
tner, F.; Torborg, C.; Fumino, K.;
CCDC 1858636−1858638 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.
Production from Formic Acid. J. Am. Chem. Soc. 2010, 132, 8924−
934. (b) Federsel, C.; Boddien, A.; Jackstell, R.; Jennerjahn, R.;
Dyson, P. J.; Scopelliti, R.; Laurenczy, G.; Beller, M. A Well-Defined
Iron Catalyst for the Reduction of Bicarbonates and Carbon Dioxide
to Formates, Alkyl Formates, and Formamides. Angew. Chem., Int. Ed.
8
2
010, 49, 9777−9780. (c) Boddien, A.; Mellmann, D.; Gartner, F.;
̈
Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller,
M. Efficient Dehydrogenation of Formic Acid Using an Iron Catalyst.
Science 2011, 333, 1733−1736. (d) Langer, R.; Diskin-Posner, Y.;
Leitus, G.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Low-Pressure
Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer
Complex Exhibiting Noble Metal Activity. Angew. Chem., Int. Ed.
AUTHOR INFORMATION
■
Corresponding Authors
E-mail: bernskoetterwh@missouri.edu.
E-mail: nilay.hazari@yale.edu.
*
*
2011, 50, 9948−9952. (e) Ziebart, C.; Federsel, C.; Anbarasan, P.;
ORCID
Jackstell, R.; Baumann, W.; Spannenberg, A.; Beller, M. Well-Defined
Iron Catalyst for Improved Hydrogenation of Carbon Dioxide and
Bicarbonate. J. Am. Chem. Soc. 2012, 134, 20701−20704. (f) Zell, T.;
Butschke, B.; Ben-David, Y.; Milstein, D. Efficient Hydrogen
Liberation from Formic Acid Catalyzed by a Well-Defined Iron
Pincer Complex under Mild Conditions. Chem. - Eur. J. 2013, 19,
Wesley H. Bernskoetter: 0000-0003-0738-5946
Nilay Hazari: 0000-0001-8337-198X
Notes
The authors declare no competing financial interest.
8
068−8072. (g) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado,
ACKNOWLEDGMENTS
■
B. Q.; Wurtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S.
Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-
̈
N.H. and W.H.B. acknowledge support from the U.S. Depart-
ment of Energy, Office of Science, Basic Energy Sciences,
Catalysis Science Program, under Award DE-SC0018222.
J.B.C. thanks the NSF for support as a NSF Graduate Research
Fellow. We thank Upul Jayarathne and Matthew Mills for
assistance in evaluating the activity of isonitrile containing
Supported Iron Catalyst. J. Am. Chem. Soc. 2014, 136, 10234−10237.
(h) Bertini, F.; Mellone, I.; Ienco, A.; Peruzzini, M.; Gonsalvi, L.
Iron(II) Complexes of the Linear rac-Tetraphos-1 Ligand as Efficient
Homogeneous Catalysts for Sodium Bicarbonate Hydrogenation and
Formic Acid Dehydrogenation. ACS Catal. 2015, 5, 1254−1265.
(
i) Fong, H.; Peters, J. C. Hydricity of an Fe−H Species and Catalytic
species for CO hydrogenation.
2
CO Hydrogenation. Inorg. Chem. 2015, 54, 5124−5135. (j) Rivada-
2
Wheelaghan, O.; Dauth, A.; Leitus, G.; Diskin-Posner, Y.; Milstein, D.
Synthesis and Reactivity of Iron Complexes with a New Pyrazine-
Based Pincer Ligand, and Application in Catalytic Low-Pressure
Hydrogenation of Carbon Dioxide. Inorg. Chem. 2015, 54, 4526−
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DOI: 10.1021/acs.organomet.8b00534
Organometallics XXXX, XXX, XXX−XXX