Lewis acid-assisted formic acid dehydrogenation using a pincer-supported iron catalyst

Article abstract of DOI:10.1021/ja505241x

Formic acid (FA) is an attractive compound for H₂ storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO₂ hydrogenation.

Full text of DOI:10.1021/ja505241x

Communication
pubs.acs.org/JACS
Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-
Supported Iron Catalyst
Elizabeth A. Bielinski,†,⊥ Paraskevi O. Lagaditis,‡,⊥ Yuanyuan Zhang,§ Brandon Q. Mercado,†
Christian Wurtele,‡ Wesley H. Bernskoetter,§ Nilay Hazari,*,† and Sven Schneider*,‡
̈
†Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
‡Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈
̈
̈
§Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
S
* Supporting Information
Previously, we described Ir catalysts for CO2 hydrogenation8
and Ru catalysts for ammonia borane dehydrogenation,9
supported by pincer ligands of the type HN{CH2CH2(PiPr2)}2
(iPrPNHP). We also reported the synthesis of iron complexes
with the same ligand,10 which are related to catalysts for
methanol dehydrogenation with strong base,11 ester12 and
ketone13 hydrogenation, and the hydrogenation/dehydrogen-
ation of N-heterocycles.14 Here, we present the synthesis and
characterization of a series of Fe(RPNHP) compounds and their
use as catalysts for FA dehydrogenation without additional base
or ligand. We explain instead the strong effect of Lewis acid
(LA) co-catalysts, which enable TONs of almost 1,000,000.
The five-coordinate amido complexes 1a and 1b were
synthesized through the dehydrohalogenation of (RPNHP)Fe-
ABSTRACT: Formic acid (FA) is an attractive com-
pound for H2 storage. Currently, the most active catalysts
for FA dehydrogenation use precious metals. Here, we
report a homogeneous iron catalyst that, when used with a
Lewis acid (LA) co-catalyst, gives approximately 1,000,000
turnovers for FA dehydrogenation. To date, this is the
highest turnover number reported for a first-row transition
metal catalyst. Preliminary studies suggest that the LA
assists in the decarboxylation of a key iron formate
intermediate and can also be used to enhance the reverse
process of CO2 hydrogenation.
(CO)H(Cl)10a (RPNHP = HN{CH2CH2(PR2)}2; R = Pr or
i
everal approaches are currently under investigation to
1
t
S
replace fossil fuels as energy vectors. For example, H2 gas
can be directly or electrochemically combusted within a proton-
exchange membrane fuel cell.2 However, as a result of problems
with the storage of gaseous H2 and its low volumetric energy
density, chemical hydrogen storage (CHS) based on the
reversible hydrogenation/dehydrogenation of small molecules
is an attractive alternative.3 Formic acid (FA) can be obtained
renewably from biomass oxidation or CO2 hydrogenation and
is a potential liquid CHS material.3 Several heterogeneous3,4
and homogeneous5 catalysts have been reported that can
dehydrogenate FA to a mixture of H2 and CO2, which is
directly suitable as a feed for fuel cells.6 The most active
systems are based on expensive precious metals such as Ir, Ru,
Au, Ag, and Pd. In addition, many of the best homogeneous
catalysts either require the addition of base or have a
complicated pendant base attached to the ligand framework
in order to achieve a high turnover number (TON).5a−k,m
Recently, the first homogeneous iron-based systems for FA
dehydrogenation were reported.5f,k,7 Milstein and co-workers
reported TONs of up to 100,000 using a pincer-supported
system, in the presence of 50 mol% NEt3.5k Additionally, Beller
et al. described an iron catalyst which requires an extra
equivalent of the tetradentate auxiliary ligand and gives a TON
of approximately 92,000.7 Although these results with first-row
transition metal catalysts are promising, the TONs and
turnover frequencies (TOFs) are still inferior to those achieved
with precious metal catalysts, and the role of the additives in the
catalytic reactions is unclear.
Cy) with 1.2 equiv of BuOK (Scheme 1). The molecular
structures of 1a and 1b in the solid state are shown in Figures
S1 and S2. In both cases the coordination of the metal ion can
best be described as pseudo-trigonal bipyramidal, with the
phosphorus atoms in the apical position. There is a Y-shape
distortion, as a consequence of strong N→Fe π-donation,
which is typical for five-coordinate d6 ions with one strongly π-
donating ligand.15 Hence, 1a and 1b are 18-valence-electron
compounds with low-spin ground states, which is a prerequisite
for the formation of stable hydrides.16
The reaction of FA with both 1a and 1b results in addition
across the iron amide bond to generate (iPrPNHP)Fe(CO)H-
(COOH) (2a) and (CyPNHP)Fe(CO)H(COOH) (2b)
1
(Scheme 1). The H NMR resonances at 9.10 (2a) and 9.20
ppm (2b) are characteristic of coordinated formate ligands,
while signals assignable to the hydride ligands are found at
−25.71 (2a) and −25.82 ppm (2b).
Both the amides (1a/1b) and the formate complexes (2a/
2b), as well as a series of previously reported RPNHP-supported
iron complexes,10 were tested for the catalytic dehydrogenation
of FA (Tables 1 and S2). The reaction conditions described by
Milstein with 50 mol% NEt3 as an additive were utilized. At a
relatively high catalyst loading (0.1 mol%), 1a, 1b, 2a, and 2b
gave activity comparable to that of the iron systems reported by
Beller5f,7 and Milstein,5k particularly at 80 °C in dioxane. In all
cases, the gas mixture produced from catalysis was analyzed by
Received: May 25, 2014
Published: July 7, 2014
© 2014 American Chemical Society
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Communication
Based on these results, a mechanism for FA dehydrogenation
is proposed (Scheme 2). The five-coordinate complexes 1a/1b
can enter the catalytic cycle through addition of FA (Scheme 1)
to give the formate complexes 2a/2b, which are part of the
cycle. Subsequent decarboxylation forms dihydrides 3a/3b, in
part the reverse of stoichiometric formation of 2a/2b from 1a/
1b with H2 and CO2. Accordingly, 3a/3b are formed in high
yield upon heating 2a/2b at 80 °C in dioxane under H2 (1 atm)
(Schemes 3 and S2). In the absence of H2, complete conversion
Scheme 1. Synthesis and Reactivity of Five-Coordinate Iron
Amido Species
i
to (RPNHP)Fe(CO)2 (R = Pr or Cy), the appropriate free
ligand, and an unidentified precipitate were observed (Scheme
S1). These products are consistent with the initial formation of
3a/3b and subsequent decomposition in the absence of H2. In
our proposed cycle, regeneration of 2a/2b and liberation of H2
occur through the reaction of 3a/3b with FA (see SI). This
reaction may involve the formation of molecular H2 complexes,
but species of this type were not observed spectroscopically.
The catalyst resting state was identified as the formate
complexes 2a/2b using in situ 31P NMR spectroscopy. This
observation is consistent with decarboxylation being the
turnover-limiting step.
a
Table 1. FA Dehydrogenation Using 1a, 1b, 2a, and 2b
cat. (0.1 mol%), 50 mol% NEt3
HCO2H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2 + CO2
THF, 40 °C
dioxane, 80 °C
b
c
b
c
cat.
TOF (h−1
)
TON (time, h)
TOF (h−1
)
TON (time, h)
1a
1b
2a
2b
173
220
238
181
346 (5.5)
270 (6)
527
572
622
739
910 (4.5)
885 (2.5)
>999 (3)
994 (2.5)
The stoichiometric studies confirm the feasibility of each step
in the absence of NEt3, yet the presence of base greatly
increases catalyst activity (Table 2, entry 1). A variety of other
bases, including pyridine and tBuOK, also promote the catalytic
reaction (Table S3). In the case of NEt3, this is presumably
463 (6)
904 (7.5)
a
Reaction conditions: FA (110 μL, 2.91 mmol), catalyst (2.91 μmol,
0.10 mol%), 50 mol% NEt3, 5 mL solvent. Turnover frequencies
(TOF) were measured after the first hour. Turnover numbers (TON)
b
+
c
through the action of HNEt3 formed under the catalytic
conditions via the reaction of NEt3 with excess FA. Accordingly,
the rate of decarboxylation of 2a/2b, the proposed turnover-
limiting step, is significantly increased in the presence of
[HNEt3]+[BF4]− (Schemes 3 and S2). Although the decarbox-
ylation is a formal β-hydride elimination reaction, the absence
of a vacant coordination site on iron requires an indirect
pathway. Recently, Milstein and co-workers suggested that
decarboxylation in a related iron pincer system proceeds via
intramolecular rearrangement to an H-bound formate followed
by CO2 elimination.5k DFT studies on 2a support this
mechanism (see Scheme 4 and SI), with rate-determining
initial rearrangement. Analysis of the transition state for
intramolecular rearrangement and the H-bound formate
intermediate indicates significant buildup of negative charge
were measured using a gas buret. All numbers are an average of two
runs.
GC and found to be a 1:1 mixture of H2 and CO2 with less than
0.5% CO (see SI). This is comparable to the amount of CO
observed in Beller’s best iron system.5f
Further information about catalysis was obtained through
stoichiometric reactions. The reaction of 1a/1b with a 1:1
mixture of 1 atm H2 and CO2 at room temperature results in
clean formation of the formate compounds 2a/2b. This
reaction most likely proceeds via initial heterolytic cleavage of
H2, followed by CO2 insertion into the Fe−H bond.
Accordingly, in the absence of CO2, 1a/1b rapidly adds H2
(1 atm) to give the trans-dihydride complexes 3a/3b as the
main products (Scheme 1). This reaction is fully reversible,
which prevents isolation of 3a/3b due to facile H2 loss. The
+
on the carboxylate group. Hence, Brønsted acids, like HNEt3 ,
can presumably stabilize these species through hydrogen
bonding.
1
trans-H2 configuration is supported by the characteristic H
Recently, we demonstrated that LAs facilitate β-hydride
NMR chemical shifts (−9.57 and −9.69 ppm for 3a and −9.29
and −9.37 ppm for 3b) and the mutual coupling constant of
the hydride signals (2JHH = 9.7 Hz for 3a and 9.5 Hz for 3b).13
In the case of 1a, smaller amounts of the cis-H2 complex 3a′
(Scheme 1) are also observed, based on the hydride chemical
shifts (−8.63 and −21.13 ppm), hyperfine structure (2JHH = 15
Hz), and NOESY pattern.17 1H EXSY NMR spectroscopy
indicates intramolecular hydride exchange within 3a′ and
intermolecular exchange of 3a and 3a′. Besides these main
products, minor quantities of free iPrPNHP (7%) and the Fe(0)
elimination in nickelalactones by stabilizing negatively charged
Scheme 2. Proposed Mechanism of FA Dehydrogenation
10a
complex (iPrPNHP)Fe(CO)2 (3%) are immediately formed.
Over the course of several days, 3a and 3a′ slowly convert to
(iPrPNHP)Fe(CO)2, free iPrPNHP, and an intractable precipitate.
In contrast, presumably due to steric factors, the reaction of 1b
with H2 results in the formation of only the trans-dihydride 3b
(Scheme 1). 3b is also only moderately stable and slowly
decomposes to free CyPNHP, (CyPNHP)Fe(CO)2, and an
intractable precipitate.
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Journal of the American Chemical Society
Communication
(Table S5). A series of standard tests suggest that
homogeneous catalysis is occurring (see SI).
Scheme 3. Effect of Additive on Decarboxylation of 2a
A major drawback of NEt3 as an additive with 2a/2b is the
lower yield at low catalyst loading (Table S6). In contrast,
complete substrate conversion was obtained using 0.01 mol%
2a/10 mol% LA (NaCl, NaBF4, or LiBF4), with LiBF4 again
giving the fastest time to completion (Table 3). In fact, with
catalyst loadings as low as 0.0001 mol% 2a, a TON of 983,642
and TOF of 196,728 h−1 are obtained. These are comparable
with the highest TON5l and TOF5d,g,h,j obtained with precious
metal systems, without the need for a complicated ligand or
external base (see Table S7 for a full comparison).
a
Table 2. LA Screening for FA Dehydrogenation Using 2a
2a (0.1 mol%), 10 mol% LA
HCO2H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2 + CO2
80 °C, dioxane
The LA effect was examined by monitoring the rate of
decarboxylation of 2a/2b, the proposed turnover-limiting step
in catalysis, in the presence of 10 equiv of LiBF4 (Scheme 3 and
S2). A large rate increase was observed compared to that of
reaction without an additive. However, a slightly slower rate
was observed compared to when [HNEt3]+[BF4]− was present,
consistent with faster catalysis using NEt3. We propose that the
LA increases the rate by stabilizing the transition state to the H-
bound formate (Scheme 4). The utility of LA-promoted FA
dehydrogenation for other catalysts was also examined. For
example, with Beller’s best catalyst, [(PP3)FeH]+[BF4]− (PP3 =
P(CH2CH2PPh3)3),5f poor catalytic activity was obtained under
our conditions (80 °C, dioxane) in the absence of a co-catalyst
(Table S8). The addition of 10 mol% LiBF4 or NaBArF4 results
in enhancement under our and Beller’s conditions (80 °C,
propylene carbonate) and eliminates the need for excess ligand
to achieve high activity (Table S8).
b
c
entry
LA
TOF (h−1
)
TON (time, h)
d
1
no additive
B(C6F5)3
−
231
255
147
193
323
263
274
132
112
133
260
231
182 (48 h)
639 (12.5)
>999 (7.5)
449 (9)
2
e
3
HCOONa
4
NaPF6
NaBArF
f
5
994 (8)
4
6
NaBF4
NaCl
LiCl
>999 (6.5)
>999 (7.5)
>999 (7)
674 (8.5)
>999 (7)
386 (23)
7
8
9
KCl
10
11
12
13
CsCl
CaCl2
MgCl2
LiBF4
>999 (8)
>999 (4 h)
a
Reaction conditions: FA (110 μL, 2.91 mmol), 2a (2.91 μmol, 0.10
mol %), LA (0.291 mmol, 10 mol %), 5 mL dioxane, 80 °C. Turnover
frequencies (TOF) were measured after the first hour. Turnover
b
c
The utilization of FA as a CHS material requires both H2
liberation and sequestration, which prompted preliminary
examination of the LA effect on CO2 hydrogenation. Treatment
of 1b with 300 equiv of 1,8-diazabicycloundecene (DBU) and a
1:1 mixture of 69 atm H2 and CO2 at 80 °C for 12 h gave a
modest yield of formate (62%, TON 186). However, the
analogous reaction in the presence of 150 equiv of LiBF4 gave
nearly full conversion to formate (96%, TON 289) in only 4 h.
Thus, LAs enhance both directions of the potential FA
utilization cycle.
numbers (TON) were measured using a gas buret. All numbers are an
d
average of two runs. Reaction performed in the absence of LA or
e
f
−
base. 9:1 FA:HCOONa. BArF = B{3,5-(CF3)2C6H3}4 .
4
Scheme 4. Proposed Pathway for Decarboxylation of 2a/2b
in the Absence (a) and Presence (b) of a LA
In conclusion, we have shown that, in the presence of a LA
co-catalyst, a new pincer-supported iron catalyst system
achieves the highest TON reported for FA dehydrogenation
using a first-row transition metal catalyst. Stoichiometric
experiments indicate the LA co-catalyst assists in the
Table 3. Optimization of Catalytic Dehydrogenation of FA
a
Using 2a
2a, 10 mol% LA
HCO2H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2 + CO2
80 °C, dioxane
b
c
entry
LA
mol% 2a
TOF (h−1
)
TON (time, h)
carboxylate groups.18 Similarly, LA addition in catalytic
amounts (10 mol%) promotes FA dehydrogenation with
catalysts 2a/2b (Tables 2 and S4). In general, the highest
TON and TOF are achieved with alkali or alkali earth metal salt
co-catalysts. Importantly, the enhancement of activity correlates
with the chemical affinity for carboxylate, with the best results
obtained for Li+ (entries 7−12). Weakly coordinating anions,
1
2
3
4
5
6
CsCl
0.01
1 440
2 142
1 580
1 756
21 150
7 495 (15)
>9 999 (7)
>9 999 (6)
>9 999 (5.5)
93 600 (6.5)
NaCl
NaBF4
LiBF4
LiBF4
LiBF4
0.01
0.01
0.01
0.001
0.0001
d
d
196 728
983 642 (9.5)
−
such as BF4 , are preferred as anions (entries 3−7). LiBF4
a
Reaction conditions: FA (110 μL, 2.91 mmol), 2a, LA (0.291 mmol,
10 mol%), 5 mL dioxane, 80 °C. Turnover frequencies (TOF) were
measured after the first hour. Turnover numbers (TON) were
measured using a gas buret. All numbers are an average of two runs.
Average of four runs.
b
(entry 13) results in the most rapid completion of the reaction;
however, full conversion is also obtained with much cheaper
additives, such as NaCl. The optimal loading of LA is 10 mol%,
with a decrease in efficiency at both lower and higher loadings
c
d
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Journal of the American Chemical Society
Communication
Technol. 2014, 4, 34. (n) Myers, T. W.; Berben, L. A. Chem. Sci. 2014,
5, 4771.
(6) Boddien, A.; Loges, B.; Junge, H.; Beller, M. ChemSusChem 2008,
1, 751.
decarboxylation of an iron formate intermediate, obviating the
need for base or excess ligand. Preliminary experiments indicate
that this LA promotion is general for other catalysts for FA
dehydrogenation, as well as the reverse process of CO2
hydrogenation.
(7) Boddien, A.; Loges, B.; Gartner, F.; Torborg, C.; Fumino, K.;
̈
Junge, H.; Ludwig, R.; Beller, M. J. Am. Chem. Soc. 2010, 132, 8924.
(8) Schmeier, T. J.; Dobereiner, G. E.; Crabtree, R. H.; Hazari, N. J.
Am. Chem. Soc. 2011, 133, 9274.
ASSOCIATED CONTENT
* Supporting Information
■
S
(9) (a) Kass, M.; Friedrich, A.; Drees, M.; Schneider, S. Angew.
̈
Chem., Int. Ed. 2009, 48, 905. (b) Friedrich, A.; Drees, M.; Schneider,
S. Chem.Eur. J. 2009, 15, 10339. (c) Staubitz, A.; Sloan, M. E.;
Robertson, A. P. M.; Friedrich, A.; Schneider, S.; Gates, P. J.; auf der
Experimental procedures, X-ray information for 1a and 1b, and
details about DFT calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
Gunne, J. S.; Manners, I. J. Am. Chem. Soc. 2010, 132, 13332.
̈
(d) Marziale, A. N.; Friedrich, A.; Klopsch, I.; Drees, M.; Celinski, V.
AUTHOR INFORMATION
Corresponding Authors
nilay.hazari@yale.edu
■
R.; auf der Gunne, J. S.; Schneider, S. J. Am. Chem. Soc. 2013, 135,
̈
13342.
(10) (a) Koehne, I.; Schmeier, T. J.; Bielinski, E. A.; Pan, C. J.;
Lagaditis, P. O.; Bernskoetter, W. H.; Takase, M. K.; Wurtele, C.;
̈
sven.schneider@chemie.uni-goettingen.de
Hazari, N.; Schneider, S. Inorg. Chem. 2014, 53, 2133. (b) Fillman, K.
L.; Bielinski, E. A.; Schmeier, T. J.; Nesvet, J. C.; Woodruff, T. M.; Pan,
C. J.; Takase, M. K.; Hazari, N.; Neidig, M. L. Inorg. Chem. 2014, 53,
6066.
Author Contributions
⊥E.A.B. and P.O.L. made equal contributions.
Notes
(11) Alberico, E.; Sponholz, P.; Cordes, C.; Nielsen, M.; Drexler, H.-
J.; Baumann, W.; Junge, H.; Beller, M. Angew. Chem., Int. Ed. 2013, 52,
14162.
(12) (a) Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N.
T.; Gibson, M. S.; Krause, J. A.; Guan, H. J. Am. Chem. Soc. 2014, 136,
7869. (b) Werkmeister, S.; Junge, K.; Wendt, B.; Alberico, E.; Jiao, H.;
Baumann, W.; Junge, H.; Gallou, F.; Beller, M. Angew. Chem., Int. Ed.
2014, DOI: 10.1002/anie.201402542..
(13) Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Wan, K. Y.;
Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136, 1367.
(14) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem.
Soc. 2014, 136, 8564.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support from the National Science
Foundation through the “Center for the Capture and
Conversion of CO2” (CHE-1240020) and from the Deutsche
Forschungsgemeinschaft (SCHN 950/4-1). We thank Daniel
DeCiccio and Dr. Christoph Rose-Petruck for help with GC.
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