Metal-free dehydrogenation of formic acid to H2 and CO2 using boron-based catalysts

Article abstract of DOI:10.1039/c5sc00394f

Formic acid is at the crossroads of novel sustainable energy strategies because it is an efficient H₂ carrier. Yet, to date, its decomposition to H₂ relies on metal-based catalysts. Herein, we describe the first metal-free catalysts able to promote the dehydrogenation of formic acid. Using dialkylborane derivatives, HCOOH is decomposed to H₂ and CO₂ in the presence of a base with high selectivity. Experimental and computational results point to the involvement of bis(formyloxy)borates as key intermediates in the C-H bond activation of a formate ligand.

Full text of DOI:10.1039/c5sc00394f

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Metal-free dehydrogenation of formic acid to H2
and CO2 using boron-based catalysts†
Cite this: Chem. Sci., 2015, 6, 2938
Clement Chauvier, Anis Tlili, Christophe Das Neves Gomes, Pierre Thuery
´
´
*
and Thibault Cantat
Formic acid is at the crossroads of novel sustainable energy strategies because it is an efficient H2 carrier.
Yet, to date, its decomposition to H2 relies on metal-based catalysts. Herein, we describe the first metal-free
catalysts able to promote the dehydrogenation of formic acid. Using dialkylborane derivatives, HCOOH is
decomposed to H2 and CO2 in the presence of a base with high selectivity. Experimental and
computational results point to the involvement of bis(formyloxy)borates as key intermediates in the C–H
bond activation of a formate ligand.
Received 2nd February 2015
Accepted 4th March 2015
DOI: 10.1039/c5sc00394f
www.rsc.org/chemicalscience
proton must be abstracted with the aid of a Brønsted base to yield
a formate anion. The C–H bond cleavage then requires a Lewis-
Introduction
acidic species, able to promote hydride abstraction despite the
presence of a strongly coordinating formate medium.
Herein, we disclose that dialkylborane derivatives are
unprecedented organocatalysts for the selective dehydrogena-
tion of FA to molecular hydrogen and CO2 in the presence of a
basic additive. Mechanistic insights are provided based on
experimental observations and DFT calculations and they
highlight the role of bis(formyloxy)borates in this catalytic
transformation.
Besides its utilization as a preservative and antibacterial agent in
the agriculture industry,1 formic acid (FA) is at the crossroads of
novel sustainable energy strategies. With a high energy density of
2104 W h Lꢀ1, FA is indeed investigated as a potential energy
vector either directly as a fuel in electrochemical fuel cells2 or as
an intermediate in the formation of methanol3 (4900 W h Lꢀ1).
Additionally, FA is an attractive hydrogen carrier because of its
relatively high hydrogen content (4.4 wt%), and “hydrogen
batteries” have been proposed,4 which rely on the reversible
hydrogenation of CO2 and the dehydrogenation of formic acid in
basic media, for H2 storage. Importantly, efficient catalysts are
required to promote the rapid and selective dehydrogenation of
FA to CO2 and molecular H2 and their design has been the focus
of many efforts within the last two decades.5 While early studies
focused on the development of ruthenium6 and iridium
complexes,7 the current state of the art now also includes efficient
iron catalysts8 and the search for noble-metal free catalysts is an
active research eld. Notably, Berben et al. have disclosed in 2014
that molecular aluminium-based complexes with a bis(imino)
pyridine ligand promote the selective dehydrogenation of formic
acid in the presence of triethylamine with TOFs of up to
5200 hꢀ1.9 More recently, Maschmeyer et al.10 have shown that
sodium germanate heterogeneously promotes H2 formation from
formic acid vapors at 300 ꢁC. So far, a metal-free catalyst able to
convert HCOOH to H2 and CO2 has yet remained elusive. The
main challenge of this endeavour lies in the necessity of a double,
antagonistic, activation of formic acid. First, the acidic O–H
Results and discussion
While the deprotonation of the O–H bond in FA is easily
promoted by a Brønsted base having a pKa > 3.7, the activation
of the C–H bond in the resulting formate anion is anticipated
to be a rate-limiting step and it necessitates a Lewis acid able
to abstract a hydride anion and release CO2. For the design of
an efficient catalyst, it is thus mandatory to seek a Lewis acid
able to interconvert a formate anion and CO2, with a low
activation energy and negligible exergonicity. In this context,
we have shown recently that guanidines are highly efficient
organocatalysts in the hydroboration of CO2 at 25 ꢁC.11 In
particular, experimental and computational results pointed to
a mechanism where adduct 1, formed from MTBD (7-methyl-
1,5,7-triazabicyclo[4.4.0]dec-5-ene) and the stoichiometric
BBN–H hydroborane, promotes the reduction of CO2 to the
formate salt [2+, HCOOꢀ] with a modest activation barrier of
23.8 kcal molꢀ1 and a nearly athermic balance (DG ¼ ꢀ3.0
kcal molꢀ1) (Scheme 1). This result suggests that intermediate
2+ could serve as a catalyst to promote the reverse trans-
formation, namely the dehydrogenation of formate anions,
and catalyze H2 generation by quenching of the formed B–H
linkage with HCOOH.
CEA, IRAMIS, NIMBE, CNRS UMR 3685, 91191 Gif-sur-Yvette, France. E-mail:
thibault.cantat@cea.fr; Fax: +33 1 6908 6640
† Electronic supplementary information (ESI) available: Experimental and
computational procedures and physical properties of compounds. CCDC
1035083 and 1035084. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5sc00394f
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ꢀ
shi is similar to that found in the borenium salts [3+, NTf2
]
(+59.2 ppm) and [4+, NTf2 ] (+56.9 ppm).12 The description of 2+
as a borenium cation stabilized by a p-donor nitrogen ligand
was further conrmed by X-ray diffraction (Fig. 1a). In the solid
state, the boron atom adopts a trigonal planar geometry with a
ꢀ
˚
N–B bond length of 1.428(2) A, close to the N–B distance found
in aminoboranes, suggesting p-donation from the nitrogen
atom of the guanidine.
Using 5 mol% [2+, Iꢀ] as the catalyst, the dehydrogenation of
13C-labelled FA was undertaken in d8-THF at 130 ꢁC. Monitoring
the reaction mixture by 1H and 13C NMR demonstrated the
complete consumption of FA and the clean formation of H2
(4.55 ppm) and 13CO2 (126 ppm) as the only 13C-enriched
product aer 4 days. In contrast, a blank experiment, carried
out in the absence of the catalyst, showed no decomposition of
FA under identical conditions. Nonetheless, a more careful
insight into the proceedings of the catalytic reaction at shorter
times revealed a lengthy initiation period, hence a lack of
reproducibility (vide infra).
Scheme 1 Computed pathway (M05-2X/6-31+G*) in THF for the
rate-determining step in the MTBD-catalyzed hydroboration of CO2 to
formate.
In order to increase both the rate and the reproducibility of
the reaction, the dehydrogenation of FA was investigated in
the presence of triethylamine (TEA) as an external base. The
positive inuence of bases on the kinetics of FA conversion to
H2 and CO2 is indeed well established with metal catalysts.6b,9
To our delight, the combination of 5 mol% [2+, Iꢀ] with trie-
thylammonium formate (5FA/2TEA) in THF leads reproduc-
ibly to the clean formation of H2 and CO2 in ca. 26% yield
(ꢂ5%) aer 19 h (entry 1, Table 1) and >99% yield aer 45 h,
while no H2 evolution was observed in the absence of the
catalyst (see ESI†). To the best of our knowledge, this result
represents the rst example of an organocatalytic dehydroge-
nation of FA.
(1)
Interestingly, in the presence of TEA, commercially available
BBN–I proves to be an efficient catalyst and within 19 h at 130 ꢁC
it affords H2 and CO2 in 48% yield from FA (entry 2, Table 1). In
comparison, as noted above, only 26% yield is observed with
5.0 mol% [2+, Iꢀ], suggesting that the Lewis basicity of MTBD
might be detrimental. Importantly, the solvent has a dramatic
effect on the H2 generation with BBN–I in the presence of TEA.
Whereas no reaction occurs in non-polar solvents such as
benzene or toluene (entries 3 and 4, Table 1), 84% FA is dehy-
drogenated aer 19 h at 130 ꢁC in MeCN with 5.0 mol% BBN–I
(entry 6, Table 1). Nonetheless, the catalytic system is inactive in
the presence of polar and protic solvents such as methanol,
presumably because of the competitive complexation of meth-
anol to boron (entry 5, Table 1). The reaction temperature also
plays an important role in the outcome of the reaction and the
yield plummets from 84 to 18% aer 19 h, when the reaction
temperature is loweredꢁ from 130 to 120 ꢁC. Decreasing the
temperature below 120 C results in the complete deactivation
of the catalytic system, even aer prolonged time (ca. 30 h).
Finally, the amount of base present in the reaction medium
strongly controls the productivity and, using 5.0 mol% BBN–I,
no reaction occurs in pure FA with 0.0 or 5.0 mol% MTBD, while
49% dehydrogenation is measured aer 19 h with 10.0 mol%
MTBD (see ESI†).
To test this hypothesis, [2+, Iꢀ] was synthesized in 81% yield
by nucleophilic displacement of the iodine atom in BBN–I by
MTBD in THF (eqn (1)). The 11B{1H} NMR spectrum of 2+
displays a resonance at +57.2 ppm, consistent with the structure
of an electrophilic tricoordinate boron center. This 11B chemical
Fig. 1 (a) ORTEP view of 2+ in [2+, Iꢀ]. Displacement ellipsoids are
drawn at the 50% probability level. Counterion and hydrogen atoms
are omitted. (b) ORTEP view of compound 5ꢀ in [TBDH+, 5ꢀ].
Displacement ellipsoids are drawn at the 30% probability level. TBDH+
and hydrogen atoms on the BBN bicycle have been removed for
clarity.
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Table 1 Metal-free catalytic dehydrogenation of HCOOH/NEt3 (5 : 2)a
Entry
Catalyst (amount, mol%)
Solvent
Conv.b [%]
TON (time, h)
1
[2+, Iꢀ] (5.0)
TDF
26
5.2 (19)
2
BBN–I (5.0)
TDF
48
9.6 (19)
3
BBN–I (5.0)
C6D6
<5
—
4
5
6
7
8
9
10
11
12
13
BBN–I (5.0)
BBN–I (5.0)
BBN–I (5.0)
BBN–H (5.0)
[TBDH+, 5ꢀ] (5.0)
Cy2B–I (10.0)
Cy2B–I (5.0)
Tol-d8
CD3OD
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
<5
<5
84
52
—
—
16.8 (19)
10.4 (19)
13.4 (19)
10 (4.5)
20 (19)
79 (19), 88 (23), 100 (40)
20 (8)
67c
>99d
>99
79
Cy2B–I (1.0)
[Et3NH+, 6ꢀ] (5.0)
[Et3NH+, 6ꢀ] (1.0)
>99c
78c
78 (19), 94 (23), 100 (26)
a
Reaction conditions: 0.2 mmol FA, 0.08 mmol TEA, 0.2 mL deuterated solvent. b Conversions determined by 1H NMR using mesitylene (10 mL) as
an internal standard; mean values over at least 2 runs. c Catalyst was recovered unchanged. d Full conversion reached within 4.5 h.
The ability of [TBDH+, 5ꢀ] to promote the decarboxylation of
formate ions was further probed using this salt as a catalyst in
the dehydrogenation of FA. With 5.0 mol% [TBDH+, 5ꢀ], 67%
(2)
CO2/H2 are produced from a 5FA/2TEA mixture aer 19 h at
+
ꢀ
ꢁ
130 C and [Et3NH , 5 ] could be recovered at the end of the
reaction (entry 8, Table 1).15 In agreement with eqn (3), BBN–H
is also catalytically active (entry 7, Table 1).
Monitoring the product distribution by 1H and 13C NMR
reveals the formation of a common catalytic intermediate in
the dehydrogenation of 13C-labelled FA with 2/5 TEA when [2+,
Iꢀ] or BBN–I are used as catalysts. This intermediate is the only
boron species detected in solution at room temperature within
the reaction mixture and it can be formulated as a bis(formy-
loxy)borate anion (5ꢀ). 5ꢀ features two formate moieties
characterized by a doublet at 8.41 ppm (1JCH ¼ 203 Hz) in the
1H NMR spectrum and a 13C{1H} resonance at 167 ppm. The
11B{1H} NMR spectrum presents a single peak at 8.98 ppm in
d3-MeCN, consistent with an electron-poor tetra-coordinated
boron center. In fact, 5ꢀ is formed quantitatively when [2+, Iꢀ]
is reacted with 2 equiv. FA and 1 equiv. TEA at 25 ꢁC (eqn (2)).
5ꢀ can be isolated as a well-dened salt via the dehydrogena-
tion of hydroborane BBN–H with 2 equiv. FA and a stoichio-
metric amount of a base, namely 1,5,7-triazabicyclo[4.4.0]dec-
5-ene (TBD), MTBD or TEA (eqn (3)). While [Et3NH+, 5ꢀ] is a
colorless oil, [TBDH+, 5ꢀ] was isolated as a crystalline solid
enabling its structure determination by X-ray diffraction
(Fig. 1b). To date, [TBDH+, 5ꢀ] is the rst example of a bis-
(formyloxy)borate characterized and it displays B–O and C–O
(3)
(4)
Overall, the experimental ndings gathered with BBN–I as a
catalyst reveal the importance of bis(formyloxy)borate 5ꢀ as a
catalytically active species. R2B–X (X ¼ Cl, Br, I, OMe, OTf)
precursors were thus screened to determine the electronic and/
or steric inuence of the R group on the catalytic activity.
Catechol borane and pinacol borane derivatives (catB–Cl,
catB–Br or pinB–OMe) are completely inactive in the dehydro-
genation of FA, even in the presence of TEA (entries 25, 26 and
27, Table S1†). In contrast, dicyclohexylborane derivatives
exhibit a three-fold increase in activity, compared to the BBN
system; and the quantitative dehydrogenation of 5FA/2TEA is
achieved within only 4.5 h at 130 ꢁC with 10 mol% Cy2B–I (entry
9, Table 1). Interestingly, the reaction still proceeds smoothly
˚
bond lengths (B–O: 1.542(3) and 1.528(3) A, C–O: 1.279(3) and
˚
˚
1.293(3) A, C]O: 1.217(3) and 1.218(3) A) similar to those
encountered in anionic mono(formyloxy)borates derived from
B(C6F5)3 (ref. 13) or HBAr2 (ref. 14) (Ar ¼ 2,4,6-tris(tri-
uoromethyl)phenyl).
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with 1.0 mol% Cy2B–I and 79% conversion is observed aer 19 h
Formed by the decarboxylation of 7, the hydridoborate
(TON19h ¼ 79, TOF ¼ 4.1 hꢀ1) and 88% aer 23 h (TON23h ¼ 88, intermediate 9ꢀ lies 12.9 kcal molꢀ1 above the starting mate-
TOF ¼ 3.8 hꢀ1). A TON of 100 requires a longer reaction time of rials, accounting for our unsuccessful attempts to observe a B–H
43 h. Importantly, with either Cy2B- or BBN-based catalysts only functionality.16 9ꢀ is prone to protonolysis and two different
<0.2% CO was detected by GC (see ESI†). Similar to BBN–I, pathways have been computationally unveiled for the dehy-
Cy2B–I is readily converted in the reaction medium to a cata- drogenation of FA with 9ꢀ. H2 release by quenching of the B–H
lytically active bis(formyloxy)borate anion (6ꢀ) in the presence group with an external FA molecule proceeds via TS4, with an
of FA and TEA (eqn (4) and entries 12 and 13, Table 1).
activation barrier of 15.9 kcal molꢀ1 from 9ꢀ. Interestingly, a
The pivotal role of the bis(formyloxy)borates 5ꢀ and 6ꢀ was pathway involving the displacement of the formate ligand in 9ꢀ
further probed using DFT techniques (at the M06-2X/6- by HCOOH and subsequent intramolecular dehydrogenation
311+G(d,p) level of theory), so as to account for their observation (via TS3) cannot be ruled out, as it involves a low activation
throughout the catalysis (Scheme 2). In agreement with the barrier of 18.7 kcal molꢀ1 from 9ꢀ. The dehydrogenation step
experimental observations, 5ꢀ is found as the lowest energy reforms 7, which is ultimately converted to the more stable
species on the potential energy surface. No direct transition state borate 5ꢀ. Overall, the decomposition of FA involves two steps,
(TS) could be localized to directly connect the saturated bis(for- namely the decarboxylation of a trigonal boron formate inter-
myloxy)borate 5ꢀ with CO2 and the hydridoborate intermediate mediate (via TS1, 26.6 kcal molꢀ1) and the subsequent depro-
9ꢀ, suggesting that the decarboxylation of 5ꢀ requires the tonation of FA by the reactive hydridoborate intermediate (via
decoordination of one formate ligand to restore a vacant orbital TS3 or TS4, 31.6 and 28.8 kcal molꢀ1, respectively) (Scheme 2).
on the boron center. The trigonal formyloxyborane 7 is indeed Both steps thus have similar energy demands and no rate-
engaged in an unfavorable equilibrium with 5ꢀ (DG ¼ +11.1 determining step (RDS) can be unequivocally proposed. Never-
kcal molꢀ1). Formation of 9ꢀ from 7 can then proceed via a theless, the consistency of the latter barriers with regard to the
b-hydride elimination (TS2), similar to ruthenium catalysts.6 In applied reaction conditions was further established using the
addition, Berben et al. proposed that aluminium complexes of energetic span model.17 With an energetic span of dE ¼ 28.8
the bis(imino)pyridine ligand can promote the decarboxylation of kcal molꢀ1, the calculated TOF at 130 ꢁC (403 K) is 1.0 hꢀ1 which
a formate ligand via a b-hydride abstraction assisted by the is in the same order of magnitude as the experimental TOF
ancillary nitrogen ligand.9 Nonetheless, the corresponding acti- (0.6–0.9 hꢀ1 from Table 1, entries 6–8).
vation energy of 51.9 kcal molꢀ1 is incompatible with the applied
The inuence of replacing the BBN scaffold with Cy2B- or
reaction conditions. In addition, a lower energy pathway was catB-backbones has been investigated computationally
identied that corresponds to the direct hydride abstraction of an (Schemes S2 and S3†). TS1, TS3 and TS4 are very close in energy
external formate anion by the neutral boron electrophile 7 via TS1 for the catalytically active BBN and Cy2B systems and the
(26.6 kcal molꢀ1). This result conrms that, with a base/boron mechanistic model does not account for the greater catalytic
catalyst molar ratio of 1, no free formate anion is available and activity of Cy2B–I. Nonetheless, the energy surface is signi-
the decarboxylation of 7 can only proceed via TS2, therefore cantly modied when catechol borane derivatives are used as
inhibiting the catalytic dehydrogenation of pure FA with [2+, Iꢀ]. catalysts (Scheme S3†). Indeed, the coordination of the p-donor
Scheme 2 Computed pathways in acetonitrile (at the M06-2X/6-311+G(d,p) level of theory) and proposed catalytic cycle for the dehydroge-
nation of formic acid catalyzed by BBN–H derivatives.
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catecholate ligand to the boron atom lowers the electrophilicity
of the catalyst and hampers the activation of the C–H bond by
hydride abstraction, which necessitates an activation energy of
38.8 kcal molꢀ1. Finally, the mechanism depicted in Scheme 2
also stresses that a bis(formyloxy)borate species (such as 5ꢀ or
6ꢀ) is the resting state of the catalyst and must dissociate to a
catalytically active trigonal formyloxyborane intermediate. As a
consequence, a highly electrophilic borane derivative might
prevent the generation of the active species, and, experimen-
tally, the potent electrophile B(C6F5)3 was found inactive in the
dehydrogenation of 5FA/2TEA, even aer 48 h at 130 ꢁC
(entry 28, Table S1†). This result suggests that the catalyst
should bear at least two formate ions to balance the electro-
philicity of the boron catalyst. In that respect, BCl3 enables the
slow dehydrogenation of forꢁmic acid with a TON of 9 and a TOF
of 0.5 hꢀ1 aer 19 h at 130 C (entry 31, Table S1†).
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Conclusions
In conclusion, the rst organocatalysts able to promote the
dehydrogenation of formic acid have been unveiled with TONs
of up to 100. Using dialkylborane derivatives and their corre-
sponding bis(formyloxy)borates, HCOOH is decomposed to H2
and CO2 in the presence of a base with high selectivity. Capi-
talizing on the mechanistic insights, current efforts are devoted
to increasing the catalytic activity of these new boron catalysts
and exploring the reduction chemistry of bis(formyloxy)borates.
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Acknowledgements
For nancial support of this work, we acknowledge CEA, CNRS,
Labex CHARMMMAT, CINES and ERC (Starting Grant Agree-
ment n.336467). T.C. thanks the Foundation Louis D. – Institut
´
de France for its support. The authors thank Dr Cedric Tard
´
(Universite Paris Diderot, France) for the assistance with GC
analyses.
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15 Contrary to trialkyboranes, 5ꢀ was found insensitive to
acidic protonolysis during the catalysis and the cleavage of
one of the Csp3–B bonds within the BBN-backbone was
never observed
16 Although there is no experimental evidence for the
formation of 9ꢀ, isolation of the related [HC(O)OB(H)Ar2]ꢀ
anion has been reported by Wang et al., see ref. 14.
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