A highly efficient Ir-catalyst for the solventless dehydrogenation of formic acid: The key role of an N-heterocyclic olefin

Article abstract of DOI:10.1039/c8gc02794c

A sturdy iridium complex that features a PCP ligand based on an N-heterocyclic olefin shows remarkable activity for the solventless dehydrogenation of formic acid. Reactivity studies highlight the importance of NHO in the activity of the catalyst. A plausible intermediate has been isolated and the mechanism was substantiated by DFT calculations.

Full text of DOI:10.1039/c8gc02794c

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A highly efficient Ir-catalyst for the solventless
dehydrogenation of formic acid: the key role of
an N-heterocyclic olefin†
Cite this: Green Chem., 2018, 20,
4875
Received 3rd September 2018,
Accepted 3rd October 2018
b
Amaia Iturmendi, a Manuel Iglesias, *a Julen Munarriz, b Victor Polo,
a
Vincenzo Passarelli, c Jesús J. Pérez-Torrente a and Luis A. Oro
*
DOI: 10.1039/c8gc02794c
rsc.li/greenchem
A sturdy iridium complex that features a PCP ligand based on an boranes or methanol.5 The ratio between the amount of H2
N-heterocyclic olefin shows remarkable activity for the solventless released and the volume of the reaction mixture used in FA dehy-
dehydrogenation of formic acid. Reactivity studies highlight the drogenation is crucial to the implementation of FA as a hydrogen
importance of NHO in the activity of the catalyst. A plausible inter- carrier. The solventless dehydrogenation of FA reduces drastically
mediate has been isolated and the mechanism was substantiated the volume of the reaction mixture compared to processes that
by DFT calculations.
require a solvent, which results in a higher energy density.
Moreover, the presence of volatile organic solvents may damage
the fuel cell electrode materials. Many outstanding catalysts for
FA dehydrogenation have been described since the seminal work
by Beller6 and Laurenczy,7 the best results being usually obtained
in solution.8 However, examples of homogeneous catalysts that
operate efficiently in neat HCOOH are scarce.9 The iridium
complex recently reported by Williams and co-workers, namely
[Ir(COD)(tBu2PCH2(2-py))]CF3SO3, is the most efficient hitherto
reported for the solventless dehydrogenation of FA. Namely, a
TOF value of 13 320 h−1 (3.7 s−1) and a TON of 12 530 after 13 h
were reported for this catalyst in the presence of catalytic
amounts of HCOONa and water (10 v%).10
Herein we describe a proficient Ir-catalyst for the solvent-
less dehydrogenation of FA, which features a PCP ligand based
on an NHO scaffold (NHO = N-heterocyclic olefin). The catalyst
[Ir(PCP)(COD)]BF4,11 (PCP = 1,3-bis(2-(diphenylphosphanyl)
ethyl)-2-methyleneimidazoline and cod = 1,5-cyclooctadiene)
brings about excellent activities in neat FA, which can be
improved in the presence of water.
Initial catalytic tests aimed at exploring the activity of
Ir-NHO complexes (1 and 2)11a in the dehydrogenation of neat
FA (Fig. 1). Under unoptimized reaction conditions,
0.016 mol% of the pre-catalyst and 5 mol% of HCOONa at
80 °C, 1 proved to be the most active pre-catalyst. NHO-Ir(III)
complex 2 performed significantly worse than 1, probably due
to the fact that one of the coordination positions is blocked by
a strongly coordinating carbonyl ligand. Ir(I) complexes related
to 1, namely 3 and 4,12 which present a chelating bis-phos-
phine without an NHO moiety, are noticeably less active than
their NHO-containing counterpart.
The growth of global energy consumption presents urgent chal-
lenges associated with the sustainability and environmental
impact of fossil fuels.1 The energy stored in the chemical bonds
of H2 molecules may be released by electrochemical combustion
in fuel cells affording water as the only reaction product. This
alternative technology would permit the exploitation of sustain-
able energy sources such as biomass or water,2 and the storage
of the excess electrical power that renewable energy sources
generate off-peak by water electrolysis.3 The use of hydrogen gas
as a sustainable energy vector, however, presents several draw-
backs, mainly related to its storage and transportation. The use
of liquid organic hydrogen carriers (LOHCs) has been proposed
as a viable alternative to bypass the issues that liquefaction or
high-pressure storage of H2 present.4 In this regard, formic acid
(FA) shows several advantages compared to other LOHCs: (i)
CO2 may be hydrogenated to FA to render a carbon-neutral cycle
of hydrogen storage, (ii) fuel-cell-grade hydrogen requires levels
of CO below 10 ppm, which are achievable by FA dehydrogena-
tion, (iii) the energy density of FA is higher than that of most
LOHCs and H2 and (iv) FA presents fewer toxicity hazards
compared to other hydrogen carriers such as ammonia, amine
aDepartamento Química Inorgánica – Instituto Síntesis Química y Catálisis
Homogénea (ISQCH), Universidad de Zaragoza – CSIC, C/Pedro Cerbuna 12,
50009 Zaragoza, Spain. E-mail: miglesia@unizar.es, oro@unizar.es
bDepartamento Química Física – Instituto de Biocomputación y Física de Sistemas
Complejos (BIFI), Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza,
Spain
cCentro Universitario de la Defensa, Ctra. Huesca s/n, ES-50090 Zaragoza, Spain
†Electronic supplementary information (ESI) available: Experimental data and
DFT calculation details. CCDC 1858260 and 1858261. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c8gc02794c
In order to shed light on the nature of the active species, 1
was dissolved in CH2Cl2 and heated to 50 °C in the presence of
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formation of the related complex 6 (Scheme 1), which illus-
trates the facile access to this coordination site.
Crystals of complexes 5 and 6 were obtained by slow
diffusion of pentane into a saturated dichloromethane solu-
tion of the crude mixture. The molecular structures of cations
5 and 6 (Scheme 1) show a slightly distorted octahedral geome-
try around the iridium center, in which the PCP ligand
occupies three meridional coordination sites, showing a
P1–Ir–P2 angle of 166.87(12)° and 166.67(6), respectively. The
other three coordination sites are occupied by two hydride
ligands (one trans to the pyridine ligand pointing towards the
NHO moiety and the other trans to the ylidic carbon atom) and
a py ligand in 5 (Ir–N, 2.196(9) Å) or a 3,5-Me2py in 6 (Ir–N,
2.190(5) Å).
1
Diagnostic peaks in the H NMR spectrum of 5 are the two
doublets of triplets assigned to the two inequivalent hydrides,
Fig. 1 Depiction of pre-catalysts 1–4 and reaction profiles for the
dehydrogenation of neat FA (0.016 mol% of the Ir-catalyst, 5 mol% of
HCOONa at 80 °C).
which appear at δ −15.30 and −23.64 ppm in CD2Cl2 (2JH–P
=
15.3 and 20.4 Hz, respectively; JH–H = 6.2 Hz). The 31P{1H}
NMR in CD2Cl2 shows a singlet at δ −7.3 ppm that can be
assigned to the two equivalent phosphorus atoms of the PCP
ligand.
2
4 equivalents of HCOOH. Interestingly, the reaction does not
proceed under strictly anhydrous conditions. The addition of
traces of H2O (30 ppm of H2O) triggered the formation of a
new dihydride species that decomposes upon isolation.
Addition of a large excess of pyridine (ca. 20 eq.) to the reac-
tion mixture allows the preparation of the new dihydride
complex [Ir(H)2(PCP)(py)]BF4 (5) (Scheme 1), which appears
always together with small amounts of an unidentified hydride
species. When the reaction was performed in CH3CN, in the
presence of HCOONa at 80 °C, the clean formation of the
dihydride complex 6 was observed. However, the product
decomposed upon purification (ESI†). Addition of excess 3,5-
dimethylpyridine (3,5-Me2py) to a solution of 5 resulted in the
The reaction of 3 or 4 with 4 eq. of HCOOH and 20 eq. of
pyridine in CD2Cl2 (in the presence or absence of H2O) led to
the formation of a hydride complex that immediately decom-
poses to give fine black particles. This suggests that, in the
absence of the additional stabilization provided by the NHO
moiety, the unsaturated species that likely results from the
loss of the COD ligand aggregates to give inactive Ir-particles.
In order to explore the reaction mechanism, the catalysis
was studied by NMR in DMSO-d6 (80 °C, 1 mol% of 1,
10 mol% HCOONa). The 1H NMR spectrum shows the for-
mation of free H2 and two doublets of triplets (2JH–P = 17.5 Hz,
2JH–H = 5.7 Hz) at δ −15.29 and −17.38 ppm, which correspond
to the only metallic species observed by NMR. Moreover, a
singlet at δ −16.7 ppm is observed in the 31P NMR spectrum.
These peaks are reminiscent of those present in the NMR
spectra of complexes 2 and 5. It is noteworthy that no traces of
NHO-aNHC isomerization11 were observed in the 31P NMR in
stoichiometric or catalytic experiments, which supports the
key role played by the NHO moiety.
Theoretical calculations predict the dissociation of the
1,5-cyclooctadiene ligand (COD) as a pre-activation step,‡ thus
allowing FA coordination (A to B, Fig. 2). Subsequently, the
intramolecular activation of the O–H bond through TSBC leads
to the monohydride-formate species C. The formation of CO2
from the formate ligand has been often proposed to occur via
β-hydride elimination13–15 or hydride abstraction.14,16 In this
case, the β-hydride elimination pathway is more favorable due
to the presence of two accessible coordination sites. The
coordination of a molecule of FA to C yields D, which evolves
Scheme 1 Synthesis and ORTEP view of 5 and 6 (ellipsoids are drawn to E by deprotonation, thus leading to an energy barrier of
–
17.8 kcal mol−1 for TSCF. Elimination of CO2 from C yields the
at the 50% probability level). The phenyl rings and PF6 counterions
were omitted for clarity. Selected bond lengths (Å) and angles (°): 5: Ir–
C1 2.258(12), Ir–P1 2.284(3), Ir–P2 2.275(3), Ir–N36 2.196(9), C1–C2
1.434(16), P1–Ir–P2 166.87(12), Ir–C1–C2 100.2(8). 6: Ir–C1 2.247(6), Ir–
dihydride intermediate F, which, upon coordination of
formate, affords G (−9.7 kcal mol−1). This species can be
stabilized by exchange of the formate ligand by pyridine to
P1 2.2751(17), Ir–P2 2.2733(17), Ir–N36 2.190(5), C1–C2 1.430(9), P1–Ir–
P2 166.67(6), and Ir–C1–C2 100.9(4).
give H (−17.2 kcal mol−1), which agrees with the stoichio-
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Fig. 2 DFT calculated Gibbs free energy profile (in kcal mol−1) for the dehydrogenation of FA.
metric experiments that afford 5 and the in operando NMR
experiments described above. Coordination of a molecule of
FA to F yields I, which upon protonation of one of the hydride
ligands regenerates C and produces a molecule of H2.
The intermolecular protonation of a hydride ligand by FA
has been suggested in the literature.14,15,17 However, in this
case, the vacant coordination site in F permits the coordi-
nation of FA and the intramolecular protonation of the
hydride by a low energy process dictated by TSIC.
Pre-activation of the catalysts requires isomerization of the
Fig. 3 Effect of H2O addition on the FA dehydrogenation rate using
5 mol% of HCOONa and 0.016 mol% of 1.§
ligand from facial (A) to meridional (C). Throughout the cata-
lytic cycle the PCP ligand adopts a meridional coordination in
Ir(I) and Ir(III) centers. All these species feature an NHO moiety
that coordinates end-on by the terminal C atom; however, the
Ir–C–C bond angle is, in all cases, ca. 100° (similar angles are sociation of HCOOH, the stabilization of intermediate species
observed in the crystal structures of 5 and 6), far from 114° of the catalytic cycle or the fact that a significant percentage of
observed for A and 109° expected for sp3 hybridization. This the water molecules may engage in hydrogen bonding with
suggests that donation from the π orbital of the olefin is HCOOH. By DFT calculations, we were unable to model an
substantial in these species (ESI†).
alternative mechanism triggered by the presence of water that
The positive effect of the addition of catalytic amounts of would lead to a lower energy pathway. In particular, the use of
water on the formation of 5 prompted us to evaluate the a water molecule as proton shuttle does not reduce the acti-
impact of H2O concentration on the dehydrogenation of FA. To vation energy of the rate-limiting step, the β-hydride elimin-
our delight, not only was the catalyst stable to water but also ation (TSCF). However, transition state stabilization through
its activity improved. The addition of water, from 0 mol% to hydrogen bonding should not be discarded.18 Plausibly, the
150 mol% (0, 5, 10, 30, 50, 70, 100, 120 and 150 mol% values combined effect of several H2O molecules, involved in second-
were explored), resulted in a progressive improvement of the sphere hydrogen bonding, lowers the activation energy of the
activity (Fig. 3). Remarkably, H2O addition increases the reac- process.
tion rate until it reaches a plateau at 30 mol%. From 50 mol-%
In view of the good performance of 1 in the presence of
onwards a sharper rise of activity occurs. Above 100 mol% no water, its activity in an aqueous solution of FA was also tested
significant increase was observed upon H2O addition. This in order to broaden its scope. The use of 1 mol% of 1 and
intricate behavior could be ascribed to a combination of 10 mol% of HCOONa at 80 °C in water led to a TOF§ value of
different effects: the role that H2O molecules play in the dis- 9840 h−1. We were able to recycle the catalyst 14 times,
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although activity loss was observed throughout the recycling 11 760 h−1 reported by Williams et al.10 and Gelman et al.,9b
experiments (ESI†). respectively. At variance with the two excellent catalysts
Subsequently, we studied the performance of the catalyst at reported by Williams’ and Gelman’s groups, 1 was proved to
different formate loadings and, consequently, the effects of work also using water as solvent. Therefore, to the best of our
the pH of the reaction media on the TOF of the reaction. In knowledge, this is the only catalyst so far reported to work
this regard, a number of reports have proved that the activity both in water and under solventless conditions.
of the catalyst depends on the pH of the solution, with
optimum values being usually observed between 3 and 4.15,19
The reaction rates measured at temperatures between 60
and 90 °C allowed the experimental estimation of the acti-
Fig. 4 shows the TOF values for HCOONa loadings that vation energy by the Arrhenius plot (Ea = 19.9 0.8 kcal mol−1
range from 5 mol% to 100 mol%, with the optimum base con- at 100 mol% of H2O, 30 mol% of HCOONa and 0.016 mol%
centration being 30 mol% independently of the H2O concen- of 1). The activation energy in the absence of H2O was also
tration (5 mol% or 100% of H2O). The temperature was opti- calculated under the same experimental conditions (to give a
mized using 100 mol% of H2O and 30 mol% of sodium value of 24.5 1.8 kcal mol−1). These Ea values are consider-
formate, displaying the best performance at 90 °C (Fig. 5).
ably lower than those reported for FA decomposition by means
The TOF values§ calculated at 60, 70, 80 and 90 °C were 410, of a non-catalyzed process.21
1390, 2580 and 5170 h−1, respectively. The positive effect of the
Taking into consideration the limitations inherent to DFT
increasing temperature on the catalytic performance suggests calculations for the quantitative prediction of activation ener-
that the catalyst is thermally stable in this range of tempera- gies for these types of systems, the theoretical activation
tures.20 In this regard, addition of a drop of mercury to the reac- energy (17.8 kcal mol−1) agrees well with the experimental
tion mixture shows no influence on the reaction rate, which sup- energy barrier calculated from the Arrhenius plots (ESI†).
ports the homogeneous nature of the active species.
Remarkably, a TOF§ value of 11 590 h−1 and a TON of 8030
(after 5 h) were obtained upon reduction of the catalyst (1) Conclusions
loading to 0.001 mol%. Under these conditions, the formation
In conclusion, we have demonstrated that 1 is an exceptionally
of carbon monoxide was below the detection limit of the infra-
active catalyst for the solventless dehydrogenation of FA. The
red spectrum (3 ppm).3a The TOF value of 11 590 h−1 obtained
fact that 1 performs markedly better than complexes 3 and 4
by us compares well with the highest TOFs so far reported for
emphasizes the importance of the NHO moiety, which is key
the solventless dehydrogenation of FA, namely 13 320 h−1 and
to the success of the reaction. The DFT calculations presented
here agree with the stoichiometric reactions that lead to com-
pounds 5 and 6, which are closely related to intermediate G.
Moreover, the low activity of 2 agrees with the postulated
mechanism, which requires a vacant coordination site at the
position that the carbonyl ligand occupies. Remarkably, the
addition of water to the reaction mixture significantly
increases the reaction rates. In fact, 1 also works in an
aqueous solution of FA, this being as far as we know the only
example of a catalyst that is active under solventless conditions
and in water. The effect of water may be related to a decrease
of the activation energy by second-sphere hydrogen bonding;
Fig. 4 Effect of HCOONa addition on the FA dehydrogenation rate with
100 mol% and 5 mol% of H2O, using 0.016 mol% of 1.§
however, further investigation is required to understand the
nature of these interactions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Spanish Ministry of Education,
Culture and Sports (MECD) (project CTQ2016-75884-P,
“Ramón y Cajal” program RYC2016-20864 (M. I.) and FPU14/
06003 (J. M.)) and the DGA/E42_17R. J. M. and V. P. gratefully
Fig. 5 Reaction profiles for the dehydrogenation of FA in the tempera-
acknowledge the resources from the supercomputer “memento”
and technical assistance provided by BIFI-ZCAM.
ture range 60–90 °C (100 mol% of H2O, 30 mol% of HCOONa and
0.016 mol% of 1).
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