Hydrogen Sulfide Induced Carbon Dioxide Activation by Metal-Free Dual Catalysis

Article abstract of DOI:10.1002/chem.201504953

The role of metal free dual catalysis in the hydrogen sulfide (H₂S)-induced activation of carbon dioxide (CO₂) and subsequent decomposition of resulting monothiolcarbonic acid in the gas phase has been explored. The results suggest that substituted amines and monocarboxylic type organic or inorganic acids via dual activation mechanisms promote both activation and decomposition reactions, implying that the judicious selection of a dual catalyst is crucial to the efficient C-S bond formation via CO₂ activation. Considering that our results also suggest a new mechanism for the formation of carbonyl sulfide from CO₂ and H₂S, these new insights may help in better understanding the coupling between the carbon and sulfur cycles in the atmospheres of Earth and Venus. It's a gas, gas, gas: The role of metal-free dual catalysis in the hydrogen sulfide-induced activation of carbon dioxide has been explored by means of quantum chemical calculations. These results suggest a new mechanism for the formation of carbonyl sulphide in the atmospheres of Earth and Venus.

Full text of DOI:10.1002/chem.201504953

DOI: 10.1002/chem.201504953
Communication
&
Organocatalysis
Hydrogen Sulfide Induced Carbon Dioxide Activation by Metal-
Free Dual Catalysis
Manoj Kumar and Joseph S. Francisco*[a]
ported the usefulness of phosphine–borane FLPs in CO2 activa-
Abstract: The role of metal free dual catalysis in the hy-
tion.[9a] Piers and co-workers have used B(C6H5)3 and amines to
drogen sulfide (H2S)-induced activation of carbon dioxide
convert CO2 into methane with silanes.[10] Fontaine and
(CO2) and subsequent decomposition of resulting mono-
co-workers have established the catalytic effect of
thiolcarbonic acid in the gas phase has been explored.
R2PC6H4B(O2C6H4) in the hydroboration of CO2 en route to
The results suggest that substituted amines and monocar-
methanol formation.[11] Zhu and An have recently examined
boxylic type organic or inorganic acids via dual activation
the CO2 sequestration with a series of amidophosphoranes at
mechanisms promote both activation and decomposition
the DFT level and suggested the role of the interplay of the
reactions, implying that the judicious selection of a dual
ring strain and the trans influence in determining the reactivity,
catalyst is crucial to the efficient CÀS bond formation via
which provides a new platform for the design of FLPs for the
CO2 activation. Considering that our results also suggest
CO2 capture.[12] In a subsequent study, An and Zhu performed
a new mechanism for the formation of carbonyl sulfide
a comparative analysis of the sequestration of CO2 and CS2
from CO2 and H2S, these new insights may help in better
using the P/N-based FLPs and provided useful mechanistic in-
understanding the coupling between the carbon and
sights into the factors controlling the sequestration.[13]
sulfur cycles in the atmospheres of Earth and Venus.
Herein we report the role of metal-free dual organocatalysis
in the CO2 insertion into the SÀH bond of hydrogen sulfide
(H2S) in the gas phase that leads to the unprecedented CÀS
Carbon dioxide (CO2) emissions into the atmosphere are in-
creasing at an alarming rate with every passing year.[1] Accord-
ing to a recent report,[2] CO2 levels in the atmosphere have al-
ready crossed the 400 ppm mark, and continue to rise relent-
lessly. Because of its involvement in global warming, there is
an immediate need to better understand the chemistry of CO2
and identify sequestration or other processes for its removal.
Utilizing CO2 as a renewable source of carbon is an attractive
strategy for a sustainable, low-carbon society. This has led to
significant interest in developing CO2 as a C1 feedstock for
carbon–carbon bond formation and carbon–heteroatom func-
tionalization reactions in chemical synthesis.[3] However, the
considerable thermodynamic stability of CO2 renders it relative-
ly inert as a chemical reactant.[4] Therefore, low-energy catalytic
activation of CO2 is highly desirable.
bond formation. The mechanistic beauty of dual organocataly-
sis lies in the fact that the coexistence of acidic and basic sites
in a catalyst (an organic or inorganic acid) causes the concert-
ed activation of the electrophile CO2 and nucleophile H2S, and
results in the low-energy reaction (Scheme 1). To the best of
our knowledge, CÀS bond formation in the presence of
a metal-free dual catalyst has yet to be achieved.
Several transition-metal catalysts have been employed to
transform CO2 into valuable commodity chemicals.[5] Metal-free
catalysis presents an alternative approach for CO2 capture
under ambient reaction conditions.[6] The potential of N-heter-
ocyclic carbenes,[7,8] frustrated Lewis pairs (FLPs),[9–13] and vari-
ous other main-group catalysts[14] for CO2 activation has been
recently established. FLPs contain sterically hindered Lewis
acid and Lewis base that makes them catalytically interest-
ing.[15] For example, Grimme, Stephan, and Erker et al. have re-
[a] Dr. M. Kumar, Prof. J. S. Francisco
Department of Chemistry
University of Nebraska-Lincoln, Lincoln, NE 68588 (USA)
E-mail: jfrancisco3@unl.edu
Scheme 1. Proposed mechanism for the gas-phase formation of carbonyl
sulfide by carbon dioxide insertion into the HÀS bond of hydrogen sulfide
under metal-free dual organocatalysis.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504953.
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The catalytic role of explicit water (H2O) molecules in lower-
ing the barrier of atmospherically important hydrogen atom
transfer (HAT)-based reactions is well-documented.[16] However,
there are other species that can catalyze the HAT reactions in
atmosphere. In fact, it is now increasingly becomingly clear
that organic acids, which are present in significant amounts in
the Earth’s atmosphere,[17–19] can catalyze such reactions much
more efficiently.[20–25] For example, the role of acid catalysis in
influencing the energetics of various atmospheric processes
such as sulfuric acid (H2SO4) forming hydrolysis of SO3,[20,21] hy-
drolysis of ketene,[23] and tautomerization of the Criegee inter-
mediate[24,25] has been recently reported. In the Venusian at-
mosphere, the inorganic acid, H2SO4 is expected to be present
in appreciable amounts[26] and may play a role in CO2 activa-
tion.
further refined using the coupled-cluster single and double
substitution method with a perturbative treatment of triple ex-
citations (CCSD(T))[31] and the aug-cc-pVTZ basis set. Specifical-
ly, the zero-point-corrected CCSD(T)/aug-cc-pVTZ electronic en-
ergies at the M06-2X/aug-cc-pVTZ-optimized geometries have
been used. For all of the reactions, the M06-2X/aug-cc-pVTZ
calculated vibrational frequencies were used to estimate the
zero-point correction for the reactants, products, transition
states, and intermediates. The presence of zero or single imagi-
nary frequency was used to identify all the stationary points as
minima or transition states.
In the first step, the nucleophilic addition of H2S across the
C=O bond in the CO2 electrophile occurs. The reaction is initi-
ated with the formation of a weak van der Waals association
complex, Int1 between H2S and CO2, which has a nominal bind-
ing energy of 0.9 kcalmolÀ1 (Figure 1). Int1 then transforms into
trans,cis-monothiolcarbonic acid (HCO2SH) via 4-center transi-
tion state, TSH2S. The reaction has an effective barrier of
48.7 kcalmolÀ1, measured relative to Int1, and is endothermic
by 15.8 kcalmolÀ1. Though the hydrolysis of CO2 has been ex-
tensively explored,[32–35] the reaction of CO2 with H2S has not
received any attention until recently when Baltrusaitis et al. ex-
amined the gas-phase reaction between CO2 and reduced
sulfur compounds, H2S and CH3SH, using ground- and excited-
state density functional theory and coupled-cluster methods.[36]
Our calculated barrier of 48.7 kcalmolÀ1 and an endothermicity
of 15.8 kcalmolÀ1 are in reasonable agreement with their CR-
CC(2,3)/6-311+G(2df,2p)//CAM-B3LYP/6-311 +G(2df,2p) calcu-
lated values of 49.6 and 16.6 kcalmolÀ1, respectively. Additional
calculations suggest that the barrier to nucleophile addition
across the C=O bond of CO2 is tunable depending upon the
nature of a nucleophile, that is, the amine addition is more fa-
vorable than the H2O or methanol addition, which, in turn, is
preferred over the H2S addition (Supporting Information, Fig-
ure S1). Significant barrier tuning for a nucleophilic addition re-
action between XO2 (X=C, N, or S) and H2S is also achievable
by judiciously changing the central atom in XO2; sulfur dioxide,
SO2, shows more reactivity than nitric oxide, NO2, which, in
turn, does better than CO2 (Supporting Information, Figure S2).
The gas-phase CO2 activation in the presence of various cat-
alysts with single activation site is also examined here. The
effect of catalysis on the reaction is significant; for HF-, HCl-,
and H2O-catalyzed reactions, Int1 has a binding energy of 3.7–
4.5 kcalmolÀ1, and the effective barrier is lowered to 34.0-
35.4 kcalmolÀ1. Int2 is a postreaction complex in which trans,-
cis-HCO2SH and the catalyst are H-bonded and now mediates
the reaction. Int2 is 2.6–6.9 kcalmolÀ1 more stable than the sep-
arated products. The activation effect of N-based reagents,
NH3, CH3NH2, and (CH3)2NH is even higher. The Int1s and Int2s
are 4.3–6.3 kcalmolÀ1 and 10.8–15.3 kcalmolÀ1 more stable
than separated reactants and products, respectively. The rela-
tively larger stability of Int2 implies that the endothermic for-
mation of trans,cis-HCO2SH can be achieved by judicious selec-
tion of catalysis. Interestingly, the calculated barrier for the CO2
insertion correlates with the nature of amine; the barrier for
the unsubstituted amine, NH3, is 25.2 kcalmolÀ1, which is low-
ered to 19.8 kcalmolÀ1 upon single methyl substitution
H2S is an important sulfur compound that is emitted into
the atmosphere by natural and anthropogenic activities,
with fossil fuel burning as its main source.[27] The insertion of
CO2 into H2S provides a new mechanism for the formation
of carbonyl sulfide (O=C=S) in the troposphere (Scheme 1),
where it is one of the most abundant sulfur compounds.[27]
This reaction also holds immense significance for the atmos-
phere of Venus, where CO2 is the largest constituent (96.5%)
and the formation mechanism of OCS continues to remain un-
clear.[26]
Herein, we propose a new mechanism for the OCS formation
that takes advantage of H2S-induced CO2 activation by metal-
free dual catalysis (Scheme 1), and support its feasibility with
gas-phase electronic structure calculations. All calculations
were performed using the Gaussian 09[28] software package.
The reaction of H2S with CO2 leading to monothiolcarbonic
acid (HCO2SH), and subsequent decomposition of HCO2SH re-
sulting in OCS and H2O in the gas phase, were extensively ex-
amined by means of first-principles quantum chemical calcula-
tions. Both reactions were calculated with and without cata-
lysts. There are two possible decomposition pathways for
HCO2SH (Scheme 1). We have considered both in the present
work. The catalysts studied in the present work were H2S, H2O,
hydrogen fluoride (HF), hydrogen chloride (HCl), ammonia
(NH3), methylamine (CH3NH2), dimethylamine ((CH3)2NH),
HCO2SH, H2CO2S, HCOOH, and H2SO4. For analyzing the
HCO2SH-induced catalysis, we have considered three possible
modes of catalysis: ÀCOOH, ÀCOSH, and ÀCSOH. HCO2SH is ca-
pable of undergoing intramolecular HAT, which results in the
cis,cis conformer. We have used a different notation, H2CO2S, to
represent that cis,cis conformer, which has no hydrogen atom
directly bonded to sulfur atom. Considering that HCO2SH and
H2CO2S can exist in multiple conformers depending upon the
relative orientation of OH and/or SH moieties (Supporting In-
formation, Schemes S1 and S2), we have only explored the cat-
alysis that is due to their cis,cis and cis,trans conformations in
our calculations because the cis,cis conformer is the most
stable and the cis,trans form is involved in the sacrificial reac-
tions. The equilibrium and transition-state structures were fully
optimized using the M06-2X[29] level of density functional
theory and the augmented correlation-consistent triplet zeta
basis set, aug-cc-pVTZ.[30] The energetics of these reactions was
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Figure 1. Calculated reaction profile for the formation of carbonyl sulfide via PATH-A, which involves insertion of carbon dioxide into the HÀS bond of hydro-
gen sulfide, conformational isomerization in trans,cis-monothiolcarbonic acid (HCO2SH), and decomposition of cis,trans-HCO2SH. Inset: the dimer conforma-
tions of HCO2SH that lead to the decomposition of cis,trans-HCO2SH. The zero point-corrected electronic energies [kcalmolÀ1] are calculated at the CCSD(T)/
aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level.
(CH3NH2), and 16.3 kcalmolÀ1 upon double methyl substitution
((CH3)2NH).
necessitates a double conformational change in the trans,cis
form to produce the catalytically active cis,trans conformer.
The calculations suggest that these conformational switches in
the gas phase are moderately expensive, with barriers of 5.6
and 8.1 kcalmolÀ1 (Figure 1). Moreover, cis,trans-HCO2SH is
3.0 kcalmolÀ1 more stable than the trans,cis analogue. From
the cis,trans conformer, the OCS forming dehydration occurs,
which involves the concerted cleavage of CÀOH and SÀH
bonds. The uncatalyzed dehydration step is 5.1 kcalmolÀ1 exo-
thermic and has a barrier of 39.6 kcalmolÀ1 (Figure 1). Overall,
the gas-phase formation of OCS is 7.7 kcalmolÀ1 endothermic,
and the decomposition of cis,trans-HCO2SH is the rate-limiting
step of the reaction.
The barrier to dehydration is lowered to 23.9 kcalmolÀ1 in
the presence of a H2O molecule (Figure 1). HCOOH and H2SO4
acids further lower the dehydration barrier by about 5.0 kcal
molÀ1. Notably, Int3 and Int4 in the acid-catalyzed reactions are
significantly more stable than those in the H2O-catalyzed one,
implying that acid catalysis is capable of tuning the energetics
of the decomposition reaction. The effect of HCO2SH on its
own decomposition is also examined. The calculated barriers
for the autocatalytic reactions are 18.7 and 26.5 kcalmolÀ1 for
the ÀCOOH-based and ÀCOSH-based catalysis, respectively.
The larger barrier for the ÀCOSH-based catalysis is due to
lower polarity of SÀH bond. The barrier of 40.9 kcalmolÀ1 for
the noncatalytic reaction is significantly higher than any of the
catalytic reactions. The catalytic effect of substituted amines
on the decomposition is larger than any of acid catalysts; the
effective barrier of 13.6 kcalmolÀ1 for the (CH3)2NH-catalyzed
reaction is 5.3 kcalmolÀ1 lower than that calculated for the
HCOOH-catalyzed reaction.
The effect of catalysts with dual activation site, HCOOH and
H2SO4 on the gas-phase CO2 insertion is investigated. The pres-
ence of carbonyl- and hydroxyl-type functionalities in these
catalysts allows them to cause the concerted activation of CO2
and H2S by forming two point H-bonding interactions in a ster-
eoeffective manner (Scheme 1). The effective barriers for the
HCOOH and H2SO4 reactions are 27.0 and 27.9 kcalmolÀ1, 21.7
and 20.8 kcalmolÀ1 lower than that for the uncatalyzed reac-
tion. These acid catalysts outperform HF, HCl, and H2O, but
turn out to be less effective than NH3, CH3NH2, and (CH3)2NH.
The prospect of the CO2 insertion being an autocatalytic
process is explored by calculating the reaction profiles for the
H2S- and cis,cis-HCO2SH-catalyzed gas-phase reactions. The H2S
reaction has an effective barrier of 42.8 kcalmolÀ1, which is
5.9 kcalmolÀ1 lower than that for the uncatalyzed reaction, but
at least 7.4 kcalmolÀ1 higher than that calculated with any
monofunctional catalyst. On the other hand, the catalytic
effect of cis,cis-HCO2SH is appreciably higher; the effective bar-
rier is 7.4 kcalmolÀ1 lower than that for the H2S reaction. It is
interesting to note that though the catalysis mechanism of the
cis,cis-HCO2SH reaction is similar to that for the HCOOH and
H2SO4 reactions, the effect of cis,cis-HCO2SH on the reaction
barrier is less pronounced. This is attributed to the lesser polar-
ity of the SÀH bond in cis,cis-HCO2SH compared to the OÀH
bond in HCOOH and H2SO4.
There are two mechanistic choices for converting trans,cis-
HCO2SH into OCS: 1) PATH-A, which involves direct decomposi-
tion, and 2) PATH-B, which involves intramolecular HAT fol-
lowed by subsequent decomposition (Scheme 1). We calculat-
ed and compared both pathways to determine the most effi-
cient route for the OCS formation. It is important to mention
that HCO2SH is produced in a trans,cis form, which is an inac-
tive conformation for the direct decomposition (Figure 1). This
The PATH-B for the OCS formation from trans,cis-HCO2SH ini-
tiates with the intramolecular HAT from thiol sulfur to carbonyl
oxygen. This HAT reaction leads to the cis,cis-H2CO2S. The unca-
talyzed HAT in the gas phase involves a significant barrier of
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Figure 2. Calculated reaction profile for the formation of carbonyl sulfide via PATH-B, which involves insertion of carbon dioxide into the HÀS bond of hydro-
gen sulfide, intramolecular hydrogen atom transfer (HAT) in trans,cis-HCO2SH, conformational isomerization in cis,cis-monothiolcarbonic acid (H2CO2S) and de-
composition of cis,trans-H2CO2S. Insets: the dimer conformations of HCO2SH and H2CO2S that assist in the intramolecular HAT and the decomposition of cis,-
trans-H2CO2S. The zero point-corrected electronic energies [kcalmolÀ1] are calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level.
31.2 kcalmolÀ1 and is 3.9 kcalmolÀ1 endothermic. Baltrusaitis
et al. have recently reported a similar barrier of 31.4 kcalmolÀ1
and an endothermicity of 3.4 kcalmolÀ1 for this intramolecular
HAT reaction in the gas phase.[36] Our calculations reveal that
the reaction becomes appreciably favorable under H2O or acid
catalysis. Interestingly, the HAT barrier is tuned to the extent
that the formation of cis,cis-H2CO2S from trans,cis-HCO2SH is
found to be barrierless under acid catalysis. Just like acids,
cis,cis-HCO2SH also makes its intramolecular HAT barrierless
when the catalysis is ÀCOOH-based (Figure 2). The ÀCOSH-
based autocatalytic HAT reaction is relatively less effective, as it
the effective barrier for the (CH3)2NH-catalyzed reaction is
0.9 kcalmolÀ1 lower than that calculated for the HCOOH-cata-
lyzed reaction.
The scope for the self-assisted reaction is assessed by explor-
ing the gas-phase decomposition from three dimer conforma-
tions. As far as autocatalysis is concerned, the HCO2SH-mediat-
ed reaction has a 3.0 kcalmolÀ1 lower barrier than that for the
H2CO2S-mediated reaction and produces an acid-like catalytic
effect on the reaction. The dehydration barrier for the noncata-
lytic reaction is larger than any of the autocatalytic reactions
owing to the additional cleavage of a CÀOH bond. Interesting-
ly, the effect of catalysis on the decomposition of the cis,trans-
H2CO2S is marginally greater than that on the cis,trans-HCO2SH
decomposition. This is due to the fact that cis,trans-H2CO2S has
larger dipole moment (m=3.2 D) than that of cis,trans-HCO2SH
(m=2.3 D), which makes the catalytic cleavage of OÀH bond in
H2CO2S easier compared to that of SÀH bond in HCO2SH. The
comparative analysis of PATH-A and PATH-B implies that the
PATH-A would be favored in the absence of any catalysis due
to relatively lower barrier for the rate-limiting decomposition
of cis,trans-HCO2SH. However, under acid or amine catalysis in-
fluence, both pathways might become equally likely because
the intramolecular HAT in trans,cis-HCO2SH becomes barrierless
and the subsequent conformational isomerization of cis,cis-
H2CO2S benefits from its coupling to the exothermic decompo-
sition of the cis,trans-H2CO2S under catalysis.
involves a noticeable barrier of 11.1 kcalmolÀ1
.
The cis,cis-H2CO2S must be converted into the cis,trans form
to allow the subsequent decomposition into OCS and H2O
(Figure 2). This conformational change in the gas phase in-
volves a moderate barrier of 10.3 kcalmolÀ1 and could be ach-
ieved under atmospheric conditions because the cis,cis-H2CO2S
in the acid-catalyzed HAT reaction is produced as a H-bonded
complex, Int4, which has at least 14.0 kcalmolÀ1 binding energy
except in the ÀCOSH-based autocatalytic reaction. This extra
energy stored in Int4 may drive the formation of cis,trans-
H2CO2S.
The cis,trans-H2CO2S then dehydrates into OCS and H2O. The
gas-phase dehydration is 14.5 kcalmolÀ1 exothermic and has
a barrier of 40.4 kcalmolÀ1. Though the barrier to the dehydra-
tion of the cis,trans-H2CO2S is 0.8 kcalmolÀ1 higher than that
for the cis,trans-HCO2SH decomposition, the reaction is 9.4 kcal
molÀ1 more exothermic, which should have a positive effect on
the preceding conformational isomerization in the cis,cis-
H2CO2S. The effect of H2O or acid catalysis on the decomposi-
tion of cis,trans-H2CO2S is enormous. H2O lowers the barrier for
the H2CO2S dehydration by 22.5 kcalmolÀ1. Acids HCOOH and
H2SO4 produce greater catalysis, causing an additional barrier
lowering of about 9.0 kcalmolÀ1. Substituted amines produce
optimum catalysis in the decomposition of cis,trans-H2CO2S;
The main conclusions are that substituted amines or organic
and inorganic acids exert significant catalysis on both the gas-
phase CO2 activation step and the subsequent rate-limiting de-
composition of cis,trans-HCO2SH or cis,trans-H2CO2S. This im-
plies that the insertion of H2S into CO2 can lead to OCS and
H2O for little energy cost within an effective dual organocataly-
sis scheme. The ability of these reagents to lower the energet-
ics for the CO2 +H2S reaction by inducing a stereoeffective
proton wire makes them potentially interesting catalysts for
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Keywords: atmospheric chemistry · CCSD(T) theory · CO2
activation · organocatalysis · potential energy profiles
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Received: December 9, 2015
Published online on February 16, 2016
Chem. Eur. J. 2016, 22, 4359 – 4363
www.chemeurj.org
4363
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