Electronic effects on the catalytic disproportionation of formic acid to methanol by [Cp?IrIII(R-bpy)Cl]Cl complexes

Article abstract of DOI:10.1039/c5dt04606h

A series of [Cp?Ir^III(R-bpy)Cl]Cl (R-bpy = 4,4′-di-R-2,2′-bipyridine; R = CF₃, H, Me, tBu, OMe) complexes was prepared and studied for catalytic formic acid disproportionation. The relationship between the electron donating strength of the bipyridine substituents and methanol production of the corresponding complexes was analyzed; the unsubstituted (R = H) complex was the most selective for methanol formation.

Full text of DOI:10.1039/c5dt04606h

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Electronic effects on the catalytic
disproportionation of formic acid to methanol by
Cite this: Dalton Trans., 2016, 45,
III
2436
[
Cp*Ir (R-bpy)Cl]Cl complexes†
Received 24th November 2015,
Accepted 8th January 2016
A. F. Sasayama, C. E. Moore and C. P. Kubiak*
DOI: 10.1039/c5dt04606h
www.rsc.org/dalton
III
A series of [Cp*Ir (R-bpy)Cl]Cl (R-bpy = 4,4’-di-R-2,2’-bipyridine; recently in the catalytic hydrogenation of carbon dioxide to
5
R = CF
3
, H, Me, tBu, OMe) complexes was prepared and studied for methanol via formic acid and methyl formate.
The reductive hydrogenation of carboxylic acids has tra-
catalytic formic acid disproportionation. The relationship between
the electron donating strength of the bipyridine substituents and ditionally required stoichiometric quantities of strong redu-
methanol production of the corresponding complexes was ana- cing agents. Recently, there have been numerous
lyzed; the unsubstituted (R = H) complex was the most selective advancements in the catalytic hydrogenation of carboxylic
6
,7
for methanol formation.
acids.
Amongst these advancements was a breakthrough
report of the catalytic disproportionation of formic acid to
methanol and carbon dioxide by a known transfer hydrogen-
Most of the world’s energy is currently derived from the
burning of fossil fuels, which emits carbon dioxide into the
5
ation catalyst [(η -pentamethylcyclopentadienyl)iridium(2,2′-
1
atmosphere and contributes to global climate change. In
2+
2+ 8
bipyridine)] ([Cp*Ir(bpy)] ). This reaction is formally a
order to continue global modernization in a responsible
manner, society is faced with the challenge of further develop-
ing clean and sustainable energy technologies. One method of
addressing this challenge is to generate fuels from the catalytic
reduction of carbon dioxide, thus creating a sustainable
transfer hydrogenation with one molecule of formic acid
acting as the H donor, releasing CO , and another acting as
2 2
the acceptor, producing formaldehyde. This process is
repeated on formaldehyde to generate methanol (eqn (1)–(3)).
At the time of its publication, this catalyst was the only
example of methanol production directly from formic acid and
performed with a selectivity of 12% under optimized con-
ditions. The bipyridine ligand of this catalyst provides a con-
venient handle for modification by facile substitution at the
2
carbon–neutral energy cycle. An attractive immediate target
product in this endeavor is methanol. Not only is methanol a
combustible fuel in itself, it can also be used for the pro-
duction of electricity by methanol fuel cells and as a feedstock
for industrial chemical synthesis, which may one day also rely
4
and 4′ positions. The reactivity of this catalyst for ketone
3
on renewable carbon sources. However, catalysts capable of
transfer hydrogenation and for carbon dioxide and carboxylic
acid hydrogenation has been tuned and improved by this
transforming carbon dioxide to methanol are rare, and most
carbon dioxide reduction or hydrogenation catalysts stop at
the level of the two-electron reduction products carbon mono-
method, finding a positive correlation between electron donat-
7,9,10
ing strength of the substituent and catalytic activity.
Based
4
xide or formate. Since the balanced reduction of carbon
on those successes, we prepared and evaluated five differently
dioxide to methanol requires six protons and six electrons, it
may be difficult for a single molecular catalyst to facilitate the
entire transformation. As proposed by Benson, et al., utilizing
a panel of catalysts, each catalyzing a different step on the reac-
tion pathway from carbon dioxide to methanol (Scheme 1),
III
substituted [Cp*Ir (R-bpy)Cl]Cl complexes (Scheme 2) for
formic acid disproportionation in order to elucidate the effect
of the electron donating strength of the bipyridine substituent
on activity and selectivity in an attempt to improve methanol
formation.
2
may render the transformation more feasible. The successful
application of a similar approach has been demonstrated
Department of Chemistry & Biochemistry, University of California, San Diego, 92093,
USA. E-mail: ckubiak@ucsd.edu
III
2 2
Ir complexes 1–5 were prepared by the reaction of [Cp*IrCl ]
†
Electronic supplementary information (ESI) available. CCDC 1437713 and
437714. For ESI and crystallographic data in CIF or other electronic format see
with the corresponding bipyridine ligand in methanol accord-
ing to previously reported methods and purified as needed
1
1
1
DOI: 10.1039/c5dt04606h
2436 | Dalton Trans., 2016, 45, 2436–2439
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Scheme 1 Proposed scheme for stepwise reduction of carbon dioxide to methanol.
nol selectivity is a measure of the amount of formic acid that
undergoes disproportionation to generate methanol rather
than undergoing decomposition to produce side products,
mainly hydrogen. See the discussion below for further detail.
The Hammett parameter is dependent upon the ratio between
the ionization constant of benzoic acid and a substituted
benzoic acid and serves as an approximate measure of the rela-
1
3
tive electron donating strength of said substituent.
Of all the complexes, 4 (R = tBu) demonstrated the highest
−
1
TOF for methanol formation at 21 hours of 0.92 ± 0.17 h
Scheme 2 Preparation of iridium complexes 1–5.
(
0
Table 1, Fig. 2a). This was slightly higher than those of 2 (R = H),
.91 ± 0.04 h− , and 3 (R = Me), 0.76 ± 0.24 h , although the
1
−1
measured performances of the three complexes were equi-
valent within error. The methanol TOF of the complex substi-
tuted with the electron withdrawing trifluoromethyl group (1)
was 0.01 ± 0.02 h . This was expected due to the previously
demonstrated detrimental effect of electron withdrawing sub-
with column chromatography and recrystallization. Complexes
1
and 4 are newly reported compounds and were fully charac-
−1
1
terized by H NMR spectroscopy, X-ray crystallography, and
elemental analysis. The characterization data for complexes 2,
stituents on catalytic activity for ketone transfer hydrogenation
1
0,12
3
, and 5 matched previously reported values.
Catalytic
9,10
and carbon dioxide reduction.
The complex with the most
reactions of each iridium complex were carried out in
triplicate. J. Young tubes were charged with solutions of
electron donating substituent, 5 (R = OMe), displayed a metha-
−1
nol TOF of 0.26 ± 0.07 h , lower than 2–4. Therefore, contrary
0
.3 mM iridium complex, 3 M HCOOH, and 40 mM sodium
tosylate in H O. Each tube contained a capillary tube of D
for locking. The reactions were assessed after 21 hours at
0 °C by quantification of methanol and methyl formate,
to the cases of ketone transfer hydrogenation and carbon
dioxide transfer hydrogenation, substitution on the bipyridine
ligand with electron donating groups did not improve the TOF
for formic acid disproportionation.
For methanol selectivity, complex 2 was the highest of
those examined (1.17 ± 0.30%), with both electron donating
and electron withdrawing groups having an adverse effect on
selectivity. The complexes substituted with electron donating
alkyl groups, 3 and 4, performed more poorly than the un-
substituted complex 2 in terms of methanol selectivity, unlike
with methanol TOF, where the three complexes performed
equivalently.
2
2
O
6
which is formed from methanol and formic acid, by inte-
1
gration of H NMR peaks against a sodium tosylate standard
(Fig. 1).
Turnover frequency (TOF) and methanol selectivity were cal-
8
culated according to formulas detailed by Miller, et al. Metha-
Analyzing the results in terms of formic acid conversion
8
(
Fig. 3), also detailed by Miller, et al., it is evident that there is
a correlation between the electron donating power of the bi-
Table 1 TOF, MeOH selectivity, and HCOOH conversion of 1–5
Hammett
parameter TOF
MeOH
selectivity
(%)
HCOOH
conversion
(%)
R
−
1
Complex group (σ
p
)
(h )
1
2
3
4
5
CF
H
Me
tBu
OMe
3
0.540
0
−0.170
−0.197
−0.268
0.01 ± 0.02 0.04 ± 0.06 20.0 ± 7.0
0.91 ± 0.04 1.17 ± 0.30 33.9 ± 5.2
0.76 ± 0.24 0.50 ± 0.17 96.7 ± 3.5
0.92 ± 0.17 0.77 ± 0.16 75.2 ± 8.6
0.26 ± 0.07 0.16 ± 0.05 99.9 ± 0.0
Fig. 1 Thermal ellipsoid plot of the crystal structure of 1 shown at the
50% probability level. Hydrogen atoms, uncoordinated counterions, and
solvent molecules are omitted for clarity.
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Fig. 2 Catalytic activity for MeOH formation vs. Hammett parameter for reactions of 0.3 mM iridium complexes with 3 M HCOOHaq at 60 °C for
1 h. (a) TOF at 21 h. (b) Methanol selectivity.
2
pyridine substituent and the quantity of formic acid consumed the decomposition of formic acid into hydrogen and carbon
in the reaction. This is consistent with the observation that 3 dioxide to proceed more quickly, thus outcompeting the for-
and 4 had decidedly lower selectivities than 2 even though mation of methanol. This would be consistent with the reac-
8
they had roughly the same TOF. In effect, while the complexes tion scheme proposed by Miller, et al., where reaction of the
with the more electron donating substituents consume formic iridium complex with one equivalent of formic acid forms the
+
acid more rapidly, they do not necessarily transform it into catalytically active hydrido species, [Cp*Ir(bpy)H] , which then
methanol more efficiently. The most likely explanation for this reacts, presumably by outer sphere hydride transfer, with
observation is that the extra electron donation simply drives further equivalents of formic acid, possibly protonated, to
afford methanol or with a proton to form hydrogen. In this
scenario, a more strongly donating ligand would not be pre-
dicted to make the catalyst favor reaction with formic acid to
form methanol over hydrogen even if the resulting hydride is
stronger. Additionally, if the key reaction is indeed between
+
[
Cp*Ir(bpy)H] and protonated formic acid, the formation of
protonated formic acid may be involved in the rate limiting
step and thus, additional hydride strength would not necess-
arily affect the rate of methanol formation, and would accele-
rate the formation of hydrogen.
While increasing electron donation from the bipyridine
substituent increased rates of ketone transfer hydrogenation
with this catalyst, catalysis of formic acid disproportionation
9
did not follow the same trend. One plausible explanation for
this is that the yields and rates of ketone transfer hydrogen-
ation reactions do not account for the total quantity of formic
acid consumed. Therefore, a low selectivity could go un-
noticed. An additional advantage in the transfer hydrogenation
of ketones over that of formic acid, along with the inherently
higher reactivity of ketones in general, is that ketone transfer
hydrogenation can operate at higher pH values, thus disfavor-
ing hydrogen production, whereas high concentrations of
formic acid are preferred for formic acid disproportionation
Fig. 3 Formic acid conversion vs. Hammett parameter for reactions of
.3 mM iridium complexes with 3 M HCOOHaq at 60 °C for 21 h.
8
0
and no increase in selectivity was seen at higher pH values.
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Conclusions
References
III
In summary, a series of five [Cp*Ir (R-bpy)Cl]Cl complexes
were prepared and compared for formic acid disproportiona-
tion catalysis. The most proficient complex in terms of metha-
nol selectivity was the unsubstituted bipyridine complex 2. In
terms of rate of methanol formation, 2 and 4 were the most
active complexes, equivalent to each other within error.
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Research through the MURI program under AFOSR Award No. 15 S. Savourey, G. Lefevre, J. C. Berthet, P. Thuery, C. Genre
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