Computational Design of Iron Diphosphine Complexes with Pendant Amines for Hydrogenation of CO2to Methanol: A Mimic of [NiFe] Hydrogenase

Article abstract of DOI:10.1002/chem.201600764

Inspired by the active-site structure of the [NiFe] hydrogenase, we have computationally designed the iron complex [P^tBu₂N^tBu₂)Fe(CN)₂CO] by using an experimentally ready-made diphosphine ligand with pendant amines for the hydrogenation of CO₂to methanol. Density functional theory calculations indicate that the rate-determining step in the whole catalytic reaction is the direct hydride transfer from the Fe center to the carbon atom in the formic acid with a total free energy barrier of 28.4 kcal mol^?1in aqueous solution. Such a barrier indicates that the designed iron complex is a promising low-cost catalyst for the formation of methanol from CO₂and H₂under mild conditions. The key role of the diphosphine ligand with pendent amine groups in the reaction is the assistance of the cleavage of H₂by forming a Fe?H^δ????H^δ+?N dihydrogen bond in a fashion of frustrated Lewis pairs.

Full text of DOI:10.1002/chem.201600764

DOI: 10.1002/chem.201600764
Full Paper
&
Catalyst Design
Computational Design of Iron Diphosphine Complexes with
Pendant Amines for Hydrogenation of CO2 to Methanol:
A Mimic of [NiFe] Hydrogenase
Xiangyang Chen,[a, b] Yuanyuan Jing,[a] and Xinzheng Yang*[a]
Abstract: Inspired by the active-site structure of the [NiFe]
hydrogenase, we have computationally designed the iron
complex [PtBu2NtBu2)Fe(CN)2CO] by using an experimentally
ready-made diphosphine ligand with pendant amines for
the hydrogenation of CO2 to methanol. Density functional
theory calculations indicate that the rate-determining step in
the whole catalytic reaction is the direct hydride transfer
from the Fe center to the carbon atom in the formic acid
with a total free energy barrier of 28.4 kcalmolÀ1 in aqueous
solution. Such a barrier indicates that the designed iron
complex is a promising low-cost catalyst for the formation
of methanol from CO2 and H2 under mild conditions. The
key role of the diphosphine ligand with pendent amine
groups in the reaction is the assistance of the cleavage of H2
by forming a FeÀHdÀ···Hd+ÀN dihydrogen bond in a fashion
of frustrated Lewis pairs.
selbaum et al.[7] reported a Ru 1,1,1-tris((diphenylphosphino)-
methyl)ethane (triphos) catalyst, [(triphos)Ru(TMM)] (TMM=tri-
methylenemethane), for the hydrogenation of CO2 to methanol
and reached a TON of 35. As we can see, those reported cata-
lysts contain noble metals and have rather low catalytic activi-
ties even under rigorous reaction conditions. The design of
high-efficiency and low-cost base metal catalysts for the pro-
duction of methanol from CO2 and H2 is highly desirable.
Introduction
The increasing greenhouse effect caused by the emission of
carbon dioxide (CO2) is forcing the human society to seek ef-
fective ways to reduce the concentration of CO2 in the Earth’s
atmosphere by converting CO2 into valuable chemicals and
fuel products. Among the fuel products derived from CO2,
methanol is an attractive feedstock in synthetic chemistry[1]
and a promising hydrogen-storage material with a high hydro-
gen density (12.6 wt%).[2] In 1993, Tominaga et al.[3] reported
the first direct CO2 hydrogenation into methanol by using a ho-
mogeneous [Ru3(CO)12]–KI catalytic system under harsh reac-
tion conditions (i.e., 2408C and 80 bar). With the advocacy of
the development of “methanol economy” by Olah and his col-
leges at the beginning of this century,[4] steady progresses
have been achieved in the catalytic hydrogenation of CO2 to
methanol.[5] In 2011, Sanford and co-workers[6] presented a cas-
cade homogeneous catalytic system with the combination of
three reactions, the hydrogenation of CO2 to formic acid cata-
lyzed by [(PMe3)4Ru(Cl)(OAc)], the esterification to methyl for-
mate catalyzed by Sc(OTf)3, and the further ester hydrogena-
tion to methanol catalyzed by [(PNN)Ru(CO)(H)]. However, the
turnover number (TON) of methanol is only 2.5. Recently, Wes-
One of the key steps in hydrogenation reactions is the cleav-
age of the HÀH bond. Among the catalysts for H2 activation,
hydrogenases have attracted increasing attentions in recent
years because they are efficient catalysts for the reversible oxi-
dation of H2 with only the base metals iron and nickel at their
active sites. The active site structures[8] and catalytic reaction
mechanisms of [Fe],[9] [FeFe],[10] and [NiFe] hydrogenases[11]
have been well elucidated. Inspired by the active site structure
of [Fe] hydrogenase,[12] we have computationally designed
a series of iron complexes with experimentally ready-made
acylmethylpyridinol and aliphatic PNP pincer ligands for the
catalytic hydrogenation of CO2 to formic acid.[13] Guided by the
pendant SCH2NHCH2S ligand in the active sites of [FeFe] and
[NiFe] hydrogenases, a series of base metal diphosphine com-
plexes with pendant amines, which could assists H2 cleavage
through the formation of FeÀHdÀ···Hd+ÀN dihydrogen bonds,
have been developed as electrocatalysts for hydrogen activa-
tion.[14] Recently, Bullock and co-workers[14a–h] reported a series
of Mn, Fe, Co, and Ni complexes with pendant amine groups
for the oxidation of hydrogen. Appel et al.[14i,j] reported the oxi-
dation of alcohol by nickel phosphine complexes with pendant
amines. Murray et al.[15] further revealed that a biomimetic cata-
lyst featuring a bidentate diphosphine group with an internal
nitrogen base is promising for the H2 activation and hydroge-
nation reactions through density functional theory (DFT) calcu-
lations. We recently proposed a series of iron dicarbonyl di-
[a] X. Chen, Dr. Y. Jing, Prof. Dr. X. Yang
Beijing National Laboratory for Molecular Sciences
State Key Laboratory for Structural Chemistry
of Unstable and Stable Species, Institute of Chemistry
Chinese Academy of Sciences, Beijing 100190 (P.R. China)
E-mail: xyang@iccas.ac.cn
[b] X. Chen
University of Chinese Academy of Sciences
Beijing 100049 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600764.
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phosphine complexes with pendant amines based on the
active site structure of [FeFe] hydrogenase and computational-
ly predicted their catalytic properties for the hydrogenation of
CO2 to methanol.[16] Therefore, the combination of the active
sites of hydrogenases and the diphosphine ligand with pend-
ant amine groups seems promising for the hydrogenation of
CO2.
In this paper, we report the bioinspired computational
design of an iron diphosphine catalyst incorporated with pend-
ant amine groups as a mimic of the active site structure of the
[NiFe] hydrogenase for the hydrogenation of CO2 to methanol.
A detailed reaction mechanism with three cascade catalytic
cycles, that is, the hydrogenation of CO2 to formic acid, the hy-
drogenation of formic acid to formaldehyde with the forma-
tion of water, and the hydrogenation of formaldehyde to
methanol, is predicted and analyzed through DFT calculations.
Figure 2. Optimized structures of complex 1 and its isomer 1’. tert-Butyl
groups are omitted for clarity. Bond lengths are given in [Š].
iron complexes with weak interactions between the Fe and N1
atoms. A H2 or a solvent molecule, such as water, could easily
insert into the Fe···N1 bond and bind to the Fe center for fur-
ther reactions.
Results and Discussion
The hydrogenation of CO2 to formic acid
As shown in Figure 1, the active site of the [NiFe] hydrogenase
contains two metal centers ligated together by two bridging
Cys-S ligands, in which the Ni center is coordinated by two ter-
minal Cys-S ligands and the Fe center is coordinated by one
terminal CO and two terminal CN ligands. According to previ-
The hydrogenation of CO2 to formic acid is the first step in the
three catalytic cycles for the formation of methanol from CO2
and H2. Scheme 1 shows the catalytic cycle for the hydrogena-
tion of CO2 to formic acid catalyzed by complex 1. Figure 3
shows the corresponding free energy profile. The op-
timized structures of key intermediates and transition
states are displayed in Figure 4. At the beginning of
the reaction, a H2 molecule inserts into the Fe···N1
bond in compound 1 and forms the dihydrogen
complex 2. The H2 molecule in complex 2 is easily
split by the Fe and N1 atoms in a fashion of FLP for
the formation of the slightly more stable intermedi-
ate 3 with a strong FeÀHdÀ···Hd+ÀN dihydrogen
Figure 1. A) Observed crystal structure of the active site of the [NiFe] hydrogenase, B) an
iron complex with pendant amine groups synthesized by Bullock and co-workers, and
C) our newly designed iron complex.
bond. The HdÀ···Hd+ distance in compound 3 is
1.57 Š, which is very close to the observed H···H dis-
ous studies of the electrocatalysts with pendant amine
groups,[14] the diphosphine ligand with pendant amines is an
excellent assistant for the heterolytic cleavage of H2 in a fashion
of frustrated Lewis pairs (FLP). Therefore, we incorporate it into
the Fe center as a replacement of the Ni center and two bridg-
ing Cys-S ligands in the active site of the [NiFe] hydrogenase,
and build the six-coordinated iron diphosphine complex
[(PtBu2NtBu2)Fe(CN)2CO] (1).
The optimized structures of complex 1 and its isomer 1’ are
shown in Figure 2. They are neutral complexes with singlet
Figure 3. Free energy profile for the hydrogenation of CO2 to formic acid
ground states. Complex 1 with the Cs symmetry is 0.6 kcal
molÀ1 more stable than its isomer 1’. Therefore, we use com-
plex 1 as the catalyst for the hydrogenation reactions in this
study. The distances between the Fe atom and two CN ligands
(1.93 Š) and the CO ligand (1.74 Š) in compound 1 are close to
the observed FeÀCN (1.91 and 1.88 Š) and FeÀCO (1.75 Š)
bond lengths in the crystal structure of the [NiFe] hydrogena-
se.[11a] The Fe···N1 distances in compounds 1 and 1’ are 2.29
and 2.24 Š, respectively. The above-given structure parameters
indicate that compounds 1 and 1’ are stable six-coordinated
(cycle 1).
tance of 1.49 Š in a similar metalÀdihydrogen bond in the crys-
tal structure of [CpC F NFeH(PtBu2NtBu2H)]+ (Cp=cyclopentadien-
yl) reported by Liu et al.[14b]
5
4
Once the metalÀdihydrogen bond is formed, a CO2 molecule
approaches compound 3 and forms a formate anion by taking
the hydride directly from the Fe center through the transition
state TS3,4 (Figure 4) with a free energy barrier of 12.7 kcal
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Scheme 1. Predicted catalytic cycle for the hydrogenation of CO2 to formic acid catalyzed by complex 1 (cycle 1).
molÀ1 (3!TS3,4). The newly formed formate anion could easily
dissociate from compound 4 for the formation of the mono-
cation intermediate 5. Then a H2 molecule is introduced into
the vacant position to assist the proton on the N1 atom to
transfer to an oxygen atom in HCOOÀ for the formation of
formic acid through TS7,8. The release of formic acid from inter-
mediate 8 and the regeneration of compound 2 is a 7.9 kcal
molÀ1 downhill step.
In addition to the formation of compound 6, the monocat-
ion 5 could easily attract the dissociated formate anion and
form the much more stable structure 4’, which is 16.1 kcal
molÀ1 more stable than compound 4 because of the formation
of a strong FeÀO bond (2.034 Š). Therefore, the intermediate
4’ can be considered as the resting state of the catalytic reac-
tion. According to the energy span model, structures 4’ and
TS3,4 are the rate-determining states in the catalytic cycle with
a free energy difference of 22.5 kcalmolÀ1 (4’!TS3,4), which in-
dicates that the hydrogenation of CO2 to formic acid could
happen easily under a mild condition.
The hydrogenation of formic acid to formaldehyde and
water
The catalytic cycle for the hydrogenation of formic acid to
formaldehyde and water (cycle 2) is shown in Scheme 2. The
corresponding free energy profile is shown in Figure 5. The op-
timized structures of the stable intermediate 9’, and the key
Figure 4. Optimized structures of compounds 3, TS3,4 (619i cmÀ1), 4’ and
TS7,8 (295i cmÀ1). tert-Butyl groups are omitted for clarity. Bond lengths are
given in [Š].
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Figure 6. Optimized structures of TS3,9 (334i cmÀ1), 9’, TS10,11 (540i cmÀ1), and
TS11,12 (219i cmÀ1). tert-Butyl groups are omitted for clarity. Bond lengths are
given in [Š].
proton far away from the Fe center in compound 10 could
transfer to the N1 atom through TS10,11 for the formation of
the intermediate 11. Once compound 11 is formed, a CH2O
molecule is easily released through the cleavage of the CÀO
bond (TS11,12). TS11,12 is only 1.4 and 12.4 kcalmolÀ1 higher than
compounds 11 and 1, respectively. The transfer of the nitrogen
proton to the hydroxyl group on the Fe center in intermediate
12 forms a water molecule. Then the water molecule could be
released from structure 13 for the regeneration of complex 1,
or replaced directly by a H2 molecule for the regeneration of
compound 2. Instead of TS10,11, the hydroxyl proton on the Fe
center in intermediate 10 could transfer back to the N1 atom
through TS10,9’ for the formation of structure 9’, which is
a 16.0 kcalmolÀ1 more stable isomer of compound 9 with
a strong FeÀO bond (2.035 Š) like intermediate 4’.
Scheme 2. Predicted catalytic cycle for the hydrogenation of formic acid to
formaldehyde and water (cycle 2).
Figure 5. Free energy profile for the hydrogenation of formic acid to formal-
dehyde and water (cycle 2).
The hydrogenation of formaldehyde to methanol
transition states for hydride transfer and the cleavage of the
CÀO bond are displayed in Figure 6. Similar to cycle 1, cycle 2
also begins with H2 cleavage and the formation of compound
3. Then a formic acid molecule attacks the intermediate 3,
takes the hydride from the Fe center to its carbon atom and
forms the anionic ligand HOCH2OÀ through transition state
TS3,9 (Figure 6), which is the rate-determining step of this cycle
with a free energy barrier of 18.6 kcalmolÀ1 (3!TS3,9). After
the formation of the unstable hydrido alkoxo intermediate 9,
the transfer of the proton from the N1 atom to HOCH2OÀ
(TS9,1) for the formation and release of HOCH2OH is fast. Then
the HOCH2OH molecule recombines to complex 1 and forms
the 3.4 kcalmolÀ1 more stable intermediate 10. The hydroxyl
The catalytic cycle for the hydrogenation of formaldehyde to
methanol (cycle 3) is shown in Scheme 3. The corresponding
free energy profile is shown in Figure 7. The optimized struc-
tures of the transition state for the metal hydride transfer to
formaldehyde and one stable intermediate are displayed in
Figure 8. Similarly, cycle 3 also begins with the formation of
compound 3 through direct H2 cleavage. When a formaldehyde
molecule approaches compound 3, the hydride on the Fe
center is transferred directly to the carbon atom in formalde-
hyde for the formation of a methoxy anion through transition
state TS3,14. Then the proton on the N1 atom can easily transfer
to CH3OÀ through TS14,15 and forms the final product CH3OH.
The following dissociation of methanol from intermediate 15
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Scheme 3. Predicted catalytic cycle for the hydrogenation of formaldehyde to methanol (cycle 3).
stable isomer of compound 14 with a strong FeÀO bond
(2.024 Š).
By comparing all relative energies in the above-described
three catalytic cycles, we can conclude that the direct hydride
transfer from the Fe center to the carbon atom in formic acid
for the formation of the anionic ligand HOCH2OÀ is the rate-de-
termining step with a total free energy barrier of 28.4 kcal
molÀ1 (4’!TS3,9) for the hydrogenation of CO2 to methanol
catalyzed by complex 1.
Conclusion
Figure 7. Free energy profile for the hydrogenation of formaldehyde to
methanol (cycle 3).
In summary, inspired by the active-site structure of the [NiFe]
hydrogenase, we have computationally designed the iron com-
plex [PtBu2NtBu2)Fe(CN)2CO] by using an experimentally ready-
made diphosphine ligand with pendant amine groups. DFT cal-
culations indicate that the newly designed iron complex is
promising to catalyze the formation of methanol from CO2 and
H2 through three cascade hydrogenation reactions with a total
free energy barrier of 28.4 kcalmolÀ1 in aqueous solution. The
hydrogenation of formic acid for the formation of HOCH2OÀ is
the rate-determining step in the whole reaction. As a replace-
ment of Ni and the pendant SCH2NHCH2S in the active site of
the [NiFe] hydrogenase, the diphosphine ligand with pendant
amine groups plays an essential role in the reaction by assist-
ing the cleavage of H2 and proton transfer through the forma-
tion of a FeÀHdÀ···Hd+ÀN dihydrogen bond in a fashion of FLP.
Our findings not only provide a promising base metal catalyst
for the production of methanol from CO2 and H2, but also
reveal the importance of the formation of metal dihydrogen
bonds in hydrogenation reactions and enlighten ideas for the
rational design of base metal catalysts based on the active-site
structures and mechanisms of metalloenzymes. Further com-
putational design and evaluation of more non-noble-metal
complexes with pendant amine groups for catalytic hydroge-
nation and dehydrogenation reactions are underway.
Figure 8. Optimized structures of TS3,14 (315i cmÀ1) and 14’. tert-Butyl groups
are omitted for clarity. Bond lengths are given in [Š].
for the regeneration of complex 1 is very quick. The rate-deter-
mining step in this catalytic cycle is the hydride transfer from
the Fe atom to CH2O (3!TS3,14) with a total free energy barrier
of only 8.7 kcalmolÀ1. Such a low barrier indicates that the hy-
drogenation of formaldehyde for the formation of methanol is
very fast. Once CH3OH is formed, it can approach complex
1 and form intermediate 14’, which is a 21.3 kcalmolÀ1 more
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Computational Details
All DFT calculations were performed in the Gaussian 09 suite of
programs[17] by using the M06 functional[18] and an ultrafine inte-
gration grid (99,590) in conjugation with the all-electron 6-31G(d,p)
basis set[19] for carbon and hydrogen atoms and the 6-31+G(d)
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vent by using the integral equation formalism polarizable continu-
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structure terms in the SMD solvation model[22] for water. The
ground states of the intermediates and transition states were con-
firmed as singlets through comparison with the optimized high-
spin analogues. Thermal corrections were calculated within the
harmonic potential approximation on optimized structures under
T=298.15 K and 1 atm pressure. Unless otherwise noted, the rela-
tive energies reported in the text are Gibbs free energies with the
solvent effect corrections. The calculated structures were verified
to have no imaginary frequency (IF) for all intermediates and only
one IF for each transition state. All transition states were also con-
firmed to connect reactants and products by intrinsic reaction co-
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This work is supported by the 100-Talent Program of the Chi-
nese Academy of Sciences (CAS) and the National Natural Sci-
ence Foundation of China (21373228).
Keywords: carbon dioxide · density functional calculations ·
hydrogenases · hydrogenation · methanol
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Published online on May 25, 2016
Chem. Eur. J. 2016, 22, 8897 – 8902
www.chemeurj.org
8902
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