Selective Formic Acid Dehydrogenation Catalyzed by Fe-PNP Pincer Complexes Based on the 2,6-Diaminopyridine Scaffold

Article abstract of DOI:10.1021/acs.organomet.6b00551

Fe(II) hydrido carbonyl complexes supported by PNP pincer ligands based on the 2,6-diaminopyridine scaffold were studied as homogeneous, non-precious-metal-based catalysts for selective formic acid dehydrogenation to hydrogen and carbon dioxide, reaching

Full text of DOI:10.1021/acs.organomet.6b00551

Article
pubs.acs.org/Organometallics
Selective Formic Acid Dehydrogenation Catalyzed by Fe-PNP Pincer
Complexes Based on the 2,6-Diaminopyridine Scaffold
†
‡
†
†
,‡
Irene Mellone, Nikolaus Gorgas, Federica Bertini, Maurizio Peruzzini, Karl Kirchner,*
,†
and Luca Gonsalvi*
†Consiglio Nazionale delle Ricerche (CNR), Istituto di Chimica dei Composti Organometallici (ICCOM), Via Madonna del Piano
0, 50019 Sesto Fiorentino (Firenze), Italy
1
‡
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060 Wien, Austria
S Supporting Information
*
ABSTRACT: Fe(II) hydrido carbonyl complexes supported
by PNP pincer ligands based on the 2,6-diaminopyridine
scaffold were studied as homogeneous, non-precious-metal-
based catalysts for selective formic acid dehydrogenation to
hydrogen and carbon dioxide, reaching quantitative yields and
high TONs under mild reaction conditions.
INTRODUCTION
obtained from Fe(BF ) ·6H O and PP ligand (PP = tris[2-
4 2 2 3 3
■
(
diphenylphosphino)ethyl]phosphine) or the well-defined
A global issue that scientists worldwide are called to answer is
to provide solutions for sustainable energy production, by
cleaner and renewable alternatives to fossil fuels. Hydrogen has
been identified as an imporant energy vector, as its chemical
bond energy can be converted into electricity using mature fuel
2
2
complexes [FeH(PP )]BF , [FeH(η -H )(PP )]BF , [FeH(η -
3
4
2
3
4
H )(PP )]BPh , and [FeCl(PP )]BF in propylene carbonate
2
3
4
3
4
(
PC) as solvent, without the need for an additional base. Except
for [FeCl(PP )]BF , excellent activities were observed for all
3
4
−1
1
these systems, with a maximum TOF of 1942 h after 3 h at 40
C using Fe(BF ) ·6H O/PP . Remarkably, this system showed
a good performance in continuous hydrogen production at 80
cell technology. Some of the major limitations to the
°
widespread use of hydrogen for energy applications remain
its efficient handling and storage, overcoming its safety issues,
and improving its cost effectiveness. As a possible answer, a
great deal of research has been carried out to identify suitable
hydrogen-rich molecules from which hydrogen can be extracted
reversibly under mild conditions of temperature and pressure.
4 2
2
3
1
1
2
,3
−1
°C with TON = 92000 and TOF = 9425 h . Lately, some of us
reported hydrogen generation from formic acid catalyzed by
iron complexes bearing the linear tetraphosphine 1,1,4,7,10,10-
hexaphenyl-1,4,7,10-tetraphosphadecane (tetraphos-1), under
4
−6
12
Among several candidates,
liquid organic hydrogen
mild reaction conditions with good activities. Laurenczy and
7
carriers (LOHCs), from which hydrogen can be released on
demand by catalytic dehydrogenation, are receiving increasing
attention. Among these, formic acid (FA), a liquid under
ambient conditions having 4.4% in weight of hydrogen, can be
safely handled, stored, and transported easily. Formic acid can
be dehydrogenated under mild conditions in the presence of a
suitable catalyst to afford fuel cell grade H and CO as the sole
co-workers described the first Fe-based catalyst for the formic
acid dehydrogenation in aqueous solution, using Fe(II) salts
together with the water-soluble meta-trisulfonated analogue of
13
PP , namely PP TS. Recently, Milstein and co-workers
3
3
described the iron dihydride pincer complex trans-[Fe-
tBuPNP)(H) (CO)] (tBuPNP = 2,6-bis(di-tert-
(
2
2
2
butylphosphinomethyl)pyridine), which showed an outstanding
byproduct. In principle, CO2 can be rehydrogenated to
activity and selectivity in formic acid dehydrogenation at 40 °C
HCOOH, so a zero-carbon-emission energy storage cycle can
14
in the presence of trialkylamines, with TONs up to 100000.
8
be contemplated.
Finally, Schneider, Hazari, Bernskoetter and co-workers
reported a new pincer-type iron catalyst that, without the
need for added base or free ligand, in the presence of a Lewis
acid (LA) as cocatalyst (10 mol %) at 60 °C, achieved the
highest TON (ca. 1000000) reported for formic acid
In recent years, many different heterogeneous and
homogeneous catalyst systems for the dehydrogenation of
formic acid have been studied. In the case of homogeneous
catalysts, the best results were obtained with noble-metal-based
9
10
complexes, such as Ru and Ir. At present, an important target
in organometallic catalysis is the replacement of noble-metal-
based catalysts with non-precious-metal catalysts of comparable
activity. Beller’s group reported efficient hydrogen generation
from formic acid catalyzed by either the in situ catalytic system
15
dehydrogenation using a first-row transition-metal catalyst.
Received: July 8, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00551
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Organometallics
Article
In recent times, some of us synthesized new transition-metal
complexes containing PNP pincer ligands based on the 2,6-
diaminopyridine scaffold containing NH and NR linkers
between the aromatic pyridine ring and the phosphine
RESULTS AND DISCUSSION
■
Formic Acid Dehydrogenation Tests. We have tested
complexes 1 and 2 for catalytic formic acid dehydrogenation
under isobaric conditions at atmospheric pressure in the
presence of added bases and additives and different solvents,
temperatures, and catalyst loadings. The development of gases
during the catalytic tests was measured with a manual gas buret.
Aliquots of the gas mixtures produced were analyzed off-line by
FT-IR, showing the absence of CO for all tests (see the
Experimental Section).
1
6
H
moieties. In particular, the iron complexes [Fe(PNP -
Me
iPr)(H)(CO)(Br)] (1) and [Fe(PNP -iPr)(H)(CO)(Br)]
(
2) proved to be active catalysts for ketone and aldehyde
16c,h
hydrogenation.
Very recently we used complexes 1 and 2
(
Chart 1) as catalysts for CO and NaHCO hydrogenation,
2 3
Chart 1. Fe-PNP Pincer Complexes 1−3
Initially, we checked the activity of complexes 1 and 2 using
formic acid without added base, but no activity was observed
under these conditions, in contrast to the iron phosphine based
11,12,15
systems reported in the literature.
We therefore applied the reaction conditions previously
14
described by Milstein et al. for a similar pincer complex: i.e.,
adding 50 mol % of NEt (0.5 equiv to FA) as base. To our
3
delight, complexes 1 and 2 were found to be catalytically active
under these reaction conditions. Using 0.1 mol % of the
catalysts at 60 °C, formic acid dehydrogenation took place with
obtaining good results even under very mild conditions of
17
−1
TOF s (turnover frequencies at 1 h) of 95 and 276 h and
temperature and pressure. A key role in catalysis was played
1h
Me
TONs (turnover numbers) of 200 and 653 within 3 h in the
case of 1 and 2, respectively (Table 1, entries 1 and 2).
The presence of a base appeared to be mandatory for the
reaction to occur. Initially, we tested the effect of different
by the in situ formed complex trans-[Fe(PNP -iPr)-
(
H) (CO)] (3). Encouraged by these results, we decided to
2
explore the possible application of these complexes as catalysts
for formic acid dehydrogenation. Hereby we present a series of
experimental results including detailed screening of reaction
conditions and mechanistic considerations based on stoichio-
metric NMR reactions, which allowed for the description of a
proposed catalytic cycle for these systems.
amounts of NEt as base on the catalytic activity (Table 1). For
3
complex 2, lowering the amount of NEt to 25 mol % led to a
3
significant decrease in the catalytic activity (TON = 204, entry
3). On the other hand, better performances were obtained in
Table 1. Formic Acid Dehydrogenation using Fe-PNP complexes 1−3 Screening FA/Base Ratios, FA Concentrations, Nature of
a
Base, Solvent, Temperature, and Catalyst Concentration Effects
d
entry
catalyst
[FA] (mol/L)
solvent
THF
base (mol %)
T (°C)
TOF1h (h−1)c
TON
conversn (%)
1
2
3
4
5
6
7
8
9
1
2
2
2
2
1
2
2
1
2
2
2
1
2
2
2
2
2
2
2
2
2
3
3
2.5
2.5
2.5
2.5
2.5
2.5
5.0
10.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
10.0
10.0
5.0
5.0
5.0
NEt (50)
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
60
80
60
80
80
60
60
95
200 (3)
653 (3)
204 (3)
816 (3)
827 (3)
369 (3)
1000 (2.5)
1000 (2)
1000 (2)
980 (3)
980 (3)
571 (3)
76 (3)
20
3
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
PC
NEt (50)
276
102
398
418
174
612
770
716
593
673
459
51
65
3
NEt (25)
20
3
NEt (100)
82
3
NEt (200)
83
3
NEt (100)
37
3
NEt (100)
100
100
100
98
3
NEt (100)
3
NEt (100)
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
NEt (50)
3
DMOA (50)
DBU (50)
98
57
DMOA (50)
2
NEt (100)
500
378
165
79
1000 (3)
878 (3)
650 (3)
180 (3)
1000 (3)
1000 (0.6)
2245 (6)
10000 (6)
6286 (6)
0 (3)
100
88
3
1,4-dioxane
EtOH
THF
PC
NEt (100)
3
NEt (100)
65
3
NEt (100)
18
3
NEt (100)
500
1800
918
2635
1714
0
100
100
22
3
e
PC
NEt (100)
3
b
b
b
THF
PC
NEt (100)
3
NEt (100)
100
63
3
PC
NEt (100)
3
THF
THF
-
0
NEt (100)
633
1000 (2)
100
3
a
b
Reaction conditions unless specified otherwise: 10.0 μmol of catalyst; 10.0 mmol of FA, specified amount of base, specified solvent. Reaction
conditions: 5.0 μmol of catalyst; 50.0 mmol of FA, specified amount of base, specified solvent. Gas evolution was measured with a manual gas buret.
c
d
Defined as mmolH2 produced/(mmolcatalyst h), calculated after 1 h. Defined as mmolH2 produced/mmolcatalyst. Run times (h) are given in parentheses.
e
TOF calculated after 20 min due to fast reaction. All tests were repeated at least twice to check for reproducibility (error ±10%).
B
DOI: 10.1021/acs.organomet.6b00551
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the presence of 100 mol % (1 equiv to FA) of NEt (TON =
On the basis of these results, the effect of different amines
3
816, entry 4). It is worth noting that under these conditions the
and solvents on the catalytic activity was examined (Table 1,
activity shown by 2 was comparable to that of Milstein’s
entries 10 to 12), showing that replacement of NEt with other
3
1
4
14,18
catalyst. A further increase of amine content to 200 mol % did
not lead to a significant improvement (TON = 827, entry 5).
The catalytic activity of complex 1 also increased using 100 mol
bases did not lead to any remarkable improvement.
Using
complex 2, with 50 mol % of dimethyloctylamine (DMOA), the
TON was unchanged in comparison to that with NEt ,
3
although TOF1h slightly increased from 593 to 673 h− (entry
1
%
of NEt (entry 6 vs 1), although also under these conditions
3
catalyst 1 performed less efficiently than 2 (TON = 369, entry
).
Substrate concentration effects were then studied (Table 1,
10 vs 11). With DBU as base, the catalytic performance
dropped with a TON of 571 and TOF of 459 h (entry 12).
1h
Complex 1 showed no activity with DBU and was almost
inactive with DMOA (entry 13).
The results of solvent screening showed that the highest
catalytic activity was achieved in aprotic solvents such as THF
−1
6
entries 4−9). For catalyst 2, increasing the FA concentration
from 2.5 to 10.0 mol/L resulted in an increase of TOF from
1h
1
3
98 to 770 h− (entry 4 vs 8), and full conversions were
−
1
achieved with FA concentrations of 5.0 and 10.0 mol/L (entries
and 8). Interestingly, catalyst 2 achieved complete conversion
(TOF = 612 h , Table 1, entry 7), propylene carbonate (PC,
1h
−
1
7
TOF = 500 h , entry 14), and 1,4-dioxane (TOF = 378
1h 1h
−1
using a FA concentration of 5.0 mol/L, showing in this case a1
h , entry 15), whereas the use of a protic solvent such as
EtOH resulted in significantly lower reaction rates (TOF
165 h , entry 16). The same order THF > PC > 1,4-dioxane >
EtOH was observed for TONs and FA conversions at 3 h
reaction time.
faster initial rate in comparison to 1 with a TOF1h of 716 h−
=
1h
−1
(
entry 9). A comparison of reaction profiles at various NEt and
3
FA concentrations is shown in Figure 1.
The effect of Lewis acids as cocatalysts was then tested. As
recently reported by Hazari et al. for other Fe pincer based
15
systems, such additives can accelerate FA dehydrogenation
dramatically. However, this was not the case for our systems, as
no FA conversion was observed under standard reaction
conditions in the presence of LiBF (10 mol %) instead of bases
4
using complexes 1−3.
The effect of temperature was then evaluated for 2 (Table 1,
entries 17−19). TON = 180 and TOF1 = 79 h− were
1
h
obtained at 40 °C using a 2:FA ratio of 1:1000 (entry 17) after
3
h. To test higher temperature conditions, PC was used as a
solvent. In this case, complete conversion (TON = 1000) was
achieved at 80 °C after only 30 min, with a high TOF = 1000
1
h
−1
h
(entry 19).
The effect of catalyst loading was studied in the case of
Figure 1. Reaction profiles of selected FA dehydrogenation tests run at
reactions catalyzed by 2.
When a catalyst to substrate ratio of 1:10000 was used at 60
6
0 °C with 2 (0.1 mol %) with increasing amounts of NEt (50 mol %,
3
Table 1, entry 2; 100 mol %, entry 4; 200 mol %, entry 5), with
increasing FA amounts (5.0 mol/L, entry 7; 10.0 mol/L, entry 8), and
at a different temperature (40 °C, 5.0 mol/L FA, 100 mol % NEt3,
entry 17). Other details are given in the footnotes of Table 1.
°
2
C with a FA concentration of 10.0 mol/L in THF, TON =
245 was achieved after 6 h with a 22% conversion (Table 1,
entry 20). When the test was run in PC at 80 °C, full
conversion was reached within the same period, giving a
rewarding TON of ca. 10000 (entry 21). Decreasing the FA
Scheme 1. Proposed Simplified Catalytic Cycle for FA Dehydrogenation Catalyzed by 1 and 2 (R = iPr; R′ = H, Me)
C
DOI: 10.1021/acs.organomet.6b00551
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Organometallics
Scheme 2. Effect of Excess FA and Added Base on the Shift of the H NMR Hydride Resonance of 4
Article
1
concentration to 5.0 mol/L led to a significant decrease in the
catalyst activity with a TON of 6286 and a conversion of 63% at
envisage that the latter proceeds via a very similar but reverse
reaction pathway. The precatalysts (1 and 2) are activated by
bromide abstraction, giving the coordinatively unsaturated
cationic intermediate [Fe(PNP-iPr)(H)(CO)]+ (4′). Subse-
quently, the formate ion may coordinate the iron metal center
on the vacant site via the O atom, resulting in neutral
80 °C (entry 22).
Complex 3 was inactive in FA dehydrogenation in the
14
absence of amine (entry 23), similarly to Milstein’s catalyst,
whereas it gave activity comparable to that of 1 and 2 in the
Me
1
presence of NEt under the same test conditions (Table 1,
[Fe(PNP -iPr)(H)(CO)(η -OCOH)] (4). Then, the formate
3
1
1
entry 24 vs 7 and 9).
ligand switches from η -O to η -H coordination to Fe. Facile
carbon dioxide elimination occurs, yielding 3, which upon
Then, a series of experiments with 2 were carried out to test
catalyst deactivation vs product inhibition, by adding neat
HCOOH aliquots (0.47 mL each) after the first run had
reached 50% substrate conversion (see the Experimental
Section). Using this procedure, an overall TON = 12170 was
reached after ca. 8.5 h with an initial catalyst to substrate ratio
of 1:5000 on running the test in PC at 80 °C. A decrese in
activity was observed after the fourth addition. At an initial
catalyst to substrate ratio of 1:1000, a higher number of
consecutive cycles (11) was possible, reaching however a lower
overall TON = 5574 after 4.5 h (see Table S7 and Figure S7 in
the Supporting Information). Complex 3 (1:5000 catalyst to
substrate ratio) gave results comparable to those for 2 under
otherwise identical conditions (overall TON = 12300 after ca. 9
h).
hydride protonation releases H to give back 4′.
2
To understand the role of the base in the mechanism, we
performed a series of stoichiometric NMR experiments on the
reactivity of precatalyst 2 with FA. No reaction could be
observed after the addition of 10 equiv of neat FA to a solution
of 2 in THF (with 20% C D for deuterium lock), even upon
6
6
heating the NMR tube to 60 °C for 1 h. On the other hand,
when the experiment was repeated adding also 10 equiv of
NEt under otherwise identical conditions, the spectra revealed
3
partial formation of complex 4 (ca. 25% on the basis of
integration) and conversion of the substrate, as demonstrated
by a decrease in the signals due to free formate. These
observations confirm that a base is needed to activate the
precatalyst, facilitating bromide dissociation and freeing a
coordination site on the metal center.
It was observed experimentally (see above) that another role
of amine is to promote catalytic turnover. This was confirmed
by NMR experiments showing that addition of FA (1 equiv) to
Mechanistic Studies. A plausible mechanism for the
catalytic dehydrogenation of formic acid with our complexes
17
is outlined in Scheme 1. On the basis of our recent studies
related to carbon dioxide hydrogenation, i.e. the reverse
reaction of FA dehydrogenation, using catalysts 1 and 2, we
a solution of 3 in THF/C D6 (20%) caused immediate
6
D
DOI: 10.1021/acs.organomet.6b00551
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Article
1
formation of 4, as demonstrated by the disappearence of the H
started. The gas evolution was monitored throughout the experiment
by reading the values of liquid displacement reached on the burets.
The gas mixtures were analyzed off-line by FTIR spectroscopy using a
NMR triplet at −8.76 ppm (J = 42.9 Hz) due to 3 and the
PH
appearance of a new triplet due to 4 at −24.4 ppm. Under these
conditions, 4 proved to be stable in solution without evolving
further. In a separate NMR experiment, addition of a known
excess of FA (100 equiv) led to a slight shift of the hydride
resonance of 4 in the H NMR spectrum (−25.1 ppm), along
with a significant color change of the respective solution from
1
0 cm gas cell (KBr windows) to check for CO formation (detection
20
limit 0.02%). Each test was repeated at least twice for reproducibility.
Typical Procedure for Slow Substrate Feed Experiments. In
a typical experiment carried out with the experimental setup described
above, using either 2 or 3 (0.005 mmol), FA (initial amount 50
1
mmol), and NEt (50 mmol) at a set temperature of 80 °C in PC as
3
orange to bright yellow (Scheme 2). When NEt (1 equiv to
solvent, once 50% of the initial amount of FA had converted, neat FA
(0.47 mL, 12.5 mmol) was added by syringe to the reaction vessel.
The procedure was repetead until no further gas evolution was
observed.
3
FA) was placed in the NMR tube, the hydride resonance shifted
back to its initial value and also the color of the reaction
solution turned back to orange.
We attribute this upfield shift of the hydride resonance to the
change from an anionic (formate) to a neutral (FA) oxygen
ligand coordinated trans to it. An excess of FA might thus lead
to substitution/reprotonation of the formate ligand, resulting in
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00551.
■
*
S
Me
1
the cationic complex [Fe(PNP -iPr)(H)(CO)(η -
HCOOH)](HCO ) (5), which in turn gives back 4 in the
2
presence of added base.
Additional tables for catalytic tests for screening of
various effects and corresponding reaction profiles and
computational details for 4′ and 5 (PDF)
The trend of the hydride resonances to shift toward more
16c,h,19
negative values is known for similar systems.
In our case,
DFT calculations confirmed the chemical shift trend (see
Scheme S1 in the Supporting Information). Thus, the role of
the base in this step is to deprotonate the formic acid ligand in
Cartesian coordinates for the calculated structure of 4′
(XYZ)
Cartesian coordinates for the calculated structure of 5
5
3
to give back 4, which in turn eliminates CO and regenerates
by β-hydride elimination, closing the catalytic cycle.
2
(XYZ)
CONCLUSIONS
AUTHOR INFORMATION
Corresponding Authors
*E-mail for K.K.: karl.kirchner@tuwien.ac.at.
*E-mail for L.G.: l.gonsalvi@iccom.cnr.it.
Author Contributions
■
■
In summary, we have shown that Fe(PNP) pincer-type
complexes bearing the easily accessible and tunable 2,6-
diaminopyridine scaffold are efficient catalysts for selective
formic acid dehydrogenation, in the presence of added base,
under mild reaction conditions. Studies are in progress to fine-
tune the structure of the complexes in order to obtain more
robust catalysts, allowing for improved long-term stability and
more efficient recycling.
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
EXPERIMENTAL SECTION
General Methods and Materials. Complexes 1−3 were prepared
■
ACKNOWLEDGMENTS
■
16c
Financial contributions by the ECRF through projects
HYDROLAB-2.0 and ENERGYLAB are gratefully acknowl-
edged. This work was also supported by COST Action
CM1205 CARISMA (Catalytic Routines for Small Molecule
Activation) through an STSM for N.G. to the CNR. N.G. and
K.K. gratefully acknowledge financial support by the Austrian
Science Fund (FWF) (Project No. P28866−N34).
according to recently reported procedures. Formic acid, triethyl-
amine, dimethyloctylamine, and DBU were purchased from
commercial suppliers and degassed under nitrogen prior to use. All
manipulations were carried out using standard Schlenk and glovebox
techniques. Solvents were freshly distilled over appropriate drying
agents, collected over Linde type 3 or 4 Å molecular sieves under
nitrogen, and degassed with nitrogen or argon gas. Deuterated solvents
for NMR measurements were purchased from commercial suppliers
and stored onto activated 4 Å molecular sieves under Ar before use.
1
13
1
31
1
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Typical Procedure for FA Dehydrogenation Tests. In a typical
experiment, a solution of catalyst (typically 0.010 mmol) in THF (2.0
mL) was placed under a nitrogen atmosphere in a magnetically stirred
glass reaction vessel thermostated by external liquid circulation and
connected to a reflux condenser and gas buret (2 mL scale). After the
solution was heated to the desired temperature, NEt (1.38 mL, 0.01
mol) and FA (0.38 mL, 0.01 mol) were added and the experiment was
̈
̈
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E
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Organometallics
Article
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DOI: 10.1021/acs.organomet.6b00551
Organometallics XXXX, XXX, XXX−XXX