Bio-Inspired Mn(I) Complexes for the Hydrogenation of CO2 to Formate and Formamide

Article abstract of DOI:10.1021/acscatal.7b00943

Developing new, efficient catalysts that contain Earth-abundant metals and simple, robust ligands for CO₂ hydrogenation is important to create cost-effective processes of CO₂ utilization. Inspired by nature, which utilizes an ortho-OH-substituted pyridine motif in Fe-containing hydrogenases, we developed a Mn complex with a simple N-donor ligand, 6,6′-dihydroxy-2,2′-bipyridine, that acts as an efficient catalyst for CO₂ hydrogenation. Turnover numbers of 6250 for hydrogenation of CO₂ to formate in the presence of DBU were achieved. Moreover, hydrogenation of CO₂ to formamide was achieved in the presence of a secondary amine.

Full text of DOI:10.1021/acscatal.7b00943

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Letter
Bio-Inspired Mn(I) Complexes for Hydrogenation
2
of CO to Formate and Formamide
Abhishek Dubey, Luca Nencini, Robert R. Fayzullin, Carlo Nervi, and Julia R Khusnutdinova
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Bio-Inspired Mn(I) Complexes for Hydrogenation of CO2 to Formate
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Abhishek Dubey†, Luca Nencini‡, Robert R. Fayzullin§, Carlo Nervi*,‡, and Julia R.
Khusnutdinova*,†
†Coordination Chemistry and Catalysis Unit, Okinawa Institute of Science and Technology, 1919-1 Tancha, Onna-
son, Okinawa, Japan
‡Department of Chemistry, University of Turin, Via P. Giuria 7, 10125, Turin, Italy
§A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences,
Arbuzov Street 8, Kazan 420088, Russian Federation
ABSTRACT: Developing new, efficient catalysts that contain earth-abundant metals and simple, robust ligands for CO2
hydrogenation is important to create cost effective processes of CO2 utilization. Inspired by nature, which utilizes an or-
tho-OH substituted pyridine motif in Fe-containing hydrogenases, we developed a Mn complex with a simple N-donor
ligand, 6,6’-dihydroxy-2,2’-bipyridine, that acts as an efficient catalyst for CO2 hydrogenation. Turnover numbers of 6250
for hydrogenation of CO2 to formate in the presence of DBU were achieved. Moreover, hydrogenation of CO2 to forma-
mide was achieved in the presence of a secondary amine. This is the first example of a homogeneous Mn complex for CO2
hydrogenation.
KEYWORDS: Manganese, Carbon Dioxide Hydrogenation, Homogeneous Catalysis, Second Coordination Sphere, Non-
Phosphine Ligand, Formate, Formamide
Catalytic hydrogenation of CO2 to value-added prod-
ucts is of practical interest for the chemical industry and
may find utilization as an energy storage technology since
CO2 provides an abundant and inexpensive carbon
source.1 However, in order to create a cost-efficient and
environmentally benign process, the use of non-toxic,
earth-abundant metals and robust, simple ligands is re-
quired.1a Until recently, the majority of efficient catalysts
for CO2 hydrogenation were based on precious metal
complexes such as Ir,2 Rh,2a, 3 Ru,4 and other metals.1a, 2a
Although some of these catalysts show excellent activity,
their utilization requires efficient recycling and separa-
tion process and poses a problem of possible dissipation
of the toxic metal catalyst in the environment.1a In con-
trast, natural systems have evolved to successfully utilize
earth-abundant metals (Fe, Ni) in hydrogenases under
mild conditions.5 Another remarkable feature of hydro-
genases that is particularly difficult to replicate is the uti-
lization of simple, naturally occurring N- and S-donor
ligands.6 Thus, creating an efficient catalyst based on
earth-abundant, non-toxic metals and an inexpensive
non-phosphine ligand that mimics hydrogenases’ reactivi-
ty would be important both for practical and for envi-
ronmental reasons.
complexes of manganese, another earth-abundant and
non-toxic metal, have not been reported in homogeneous
CO2 hydrogenation, and they remain largely underuti-
lized in homogeneous hydrogenation.8 There were several
recent examples on the use of Mn complexes with phos-
phine pincer ligands in hydrogenation of carbonyl
groups.9 Although homogeneous Mn complexes with N-
donor ligands such as 2,2’-bipyridines were studied in
electrocatalytic CO2 reduction, they have not been used
for hydrogenation of CO2.10
Enzymes in nature successfully employ second coordi-
nation sphere interactions in hydrogenases that utilize
earth-abundant metals such as Fe and Ni.5-6 For example,
the ortho-hydroxypyridine structural fragment present in
[Fe]-hydrogenase is proposed to play an important role in
H2 splitting through participation of the adjacent ortho-
hydroxyl group.5-6, 11 H2 heterolytic splitting in Fe-Fe hy-
drogenases and in related biomimetic catalysts that is
assisted by a pendant amine have been studied in detail.12
A similar approach led to the development of an efficient
iridium catalyst supported by hydroxy-substituted bipyri-
dine or bipyrimidine ligands.13 Computational and exper-
imental studies confirmed the important role of the or-
tho-OH group in promoting bond scission.14 Interestingly,
recent reports also suggested that a Mn complex contain-
ing an OH-functionalized ligand as a “local proton
source” shows improved catalytic activity for electro-
There are several recent reports on the use of Fe com-
plexes for CO2 hydrogenation to formate, in all cases sup-
ported by polydentate phosphines.7 On the other hand,
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chemical CO2 reduction to HCOOH via transient Mn hy-
dride intermediates that were confirmed by detailed spec-
troscopic studies.15 Therefore, the presence of proton-
responsive functional groups in the secondary coordina-
tion sphere of a complex, that resemble the ligand motif
present in [Fe]-hydrogenase, may play an important role
in improving catalytic activity for CO2 reduction.
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Inspired by these developments, we set out to investi-
gate the catalytic reactivity of a series of manganese com-
plexes supported by substituted bipyridine-based ligands
containing proton-responsive functional groups (Chart 1)
in CO2 hydrogenation. We were quickly able to identify
Mn complexes with a simple, commercially available 6,6’-
dihydroxy-2,2’-bipyridine ligand as active catalysts for
CO2 hydrogenation to formate, reaching TONs of over
6000. The complexes also catalyze formation of forma-
mide from CO2 in the presence of a secondary amine.
These are the first examples of a manganese catalysts for
homogeneous CO2 hydrogenation. During our study, we
proved that the presence of ortho-OH groups play a key
role in inducing high catalytic activity. Thus, we demon-
strate that utilization of a hydrogenase-inspired N-donor,
non-phosphine ligand gives rise to reactivity in a non-
precious metal complex competitive with some of the
currently reported phosphine-based catalysts.1a
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Chart 1. Manganese complexes used in this study.
We synthesized a series of manganese bromotricarbon-
yl complexes 1-5 with substituted 2,2’-bipyridyl ligands
(Chart 1). Afterwards, abstraction of bromide by silver
triflate AgOTf in acetonitrile led to the formation of the
cationic acetonitrile complexes 6-8. All new complexes
have been characterized by elemental analysis, NMR, FT-
IR spectroscopy, and ESI-MS (electrospray ionization
mass spectrometry). Complexes 2 and 5 were previously
reported.16 Bromide complexes 1-5 are moderately sensi-
tive to ambient light; however, they can be stored for at
least 2 days under ambient light without significant
changes. Complexes 6-8 need to be stored in the dark to
avoid decomposition, but they can be safely handled un-
der ambient light for limited periods, such as the time
required for reaction setup. The structures of 1, 3, 4, 6,
and 7 were confirmed by single crystal X-ray diffraction
analysis (Figure 1 and the Supporting Information). All
complexes feature octahedral coordination at the Mn cen-
ter with a fac-coordinating tricarbonyl motif. The H-atom
positions of the OH groups in complexes 1 and 6 and NH2
groups in complex 4 were determined from difference
electron density maps.
Figure 1. ORTEP representation (ellipsoids at 50% probabil-
ity) of 1 (a) and 6 (b). For both 1 and 6, two symmetry-
independent molecules are present.
First, we tested catalytic activity of 1 in 1,4-dioxane in
the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)
as a base under H2 : CO2 (3 MPa : 3 MPa) pressure. To our
satisfaction, heating at 80 °C for 24 h led to formate salt
formation, reaching a TON of 1224, corresponding to
~97% yield based on DBU determined by NMR spectros-
copy (Table 1, entry 1). Surprisingly, 1 was also efficient
when the reaction was carried out in the coordinating
solvent MeCN (entry 2) with TOF reaching 238 h-1 at 65 °C
(see Supp. Info, Table S3). Since NMR spectrum of 1 in
MeCN is indicative of bromide dissociation even at room
temperature (RT),17 independently prepared acetonitrile
complex 4 was also tested in MeCN, which allowed us to
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obtain similar results (entry 3). The control experiments feature required for high catalytic activity. The related
1
2
3
4
5
6
7
8
using the carbonyl precursor Mn(CO)5Br and a run with-
out any Mn catalyst did not yield any detectable amounts
of formate. ICP-MS analysis of Mn(CO)5Br as well as
complexes 1 and 6 do not show the presence of detectable
amounts of platinum group metals (see Supp. Info for
details).
complex 4, which contained amino-groups in the ortho-
positions showed very low TONs. This in turn indicates
that the pKa of the ortho-substituent needs to be carefully
adjusted to allow facile deprotonation.11
In order to investigate the positional effect of the OH
group, we have examined catalytic activity of complexes 5
and 8 in which the OH group is present in the para-
positions. Although the catalytic activity of 5 and 8 (en-
tries 11 and 12) was higher than that for bipyridine and
methoxy-substituted bipyridine complexes, it was still
significantly less when compared to the ortho-OH substi-
tuted 1 and 6.
Table 1. [Mn]-catalyzed CO2 hydrogenation to formate.a
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entry [Mn]
(μmol)
solvent
T (°C)
TONb yield
(%)c
These findings resemble the reactivity of iridium com-
plexes with OH-substituted bipyridine or bipyrimidine
ligands in which the presence of the OH groups as well as
their position were shown to play an important role. As
demonstrated in detailed studies of these systems by
Himeda, Fujita et al., deprotonation of OH groups in bi-
pyridine ligands leads to the altering of electronic proper-
ties in the resulting anionic ligand, leading to improved
catalytic activity.14 Concurrently with the electronic effect,
a significantly higher catalytic activity was noted for or-
tho-substituted ligands over the para-substituted ones
due to the ability of adjacent oxo-groups to participate in
H2 cleavage.14
1
1 (5 μmol)
1 (5 μmol)
6 (5 μmol)
1 (5 μmol)
1 (2 μmol)
1 (1 μmol)
2 (5 μmol)
3 (5 μmol)
7 (5 μmol)
4 (5 μmol)
5 (5 μmol)
8 (5 μmol)
dioxane 80
1224
1313
1299
946
3445
6250
17
97
96
99
58
87
98
<2
<2
7
2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
65
65
65
65
65
65
65
65
65
65
65
3
4d
5
6
7
8
9
10
11
12
18
88
As the presence of the ortho-OH group was a key factor
in inducing efficient catalysis, we attempted to investigate
possible reaction intermediates in the presence of a base.
The solution of 6 in CD3CN shows three aromatic multi-
plets centered at 7.11, 7.83, and 7.95 ppm (Figure S14 in the
SI). Upon addition of 3 equiv of DBU, pyridine proton
signals shift upfield and appear at 6.08, 6.72, and 7.18 ppm
(Figure S1). At the same time, the OH peak originally pre-
sent at 10.1 ppm disappears, and a new broad peak ap-
pears at 12.3 ppm assigned to a DBU*H+. The upfield shift
of aromatic protons suggests increase of electron density
at the deprotonated anionic ligand similar to NMR chem-
ical shift observed in dihydroxy-bipyridine Ir complexes
in basic media.14 Similarly, pyridine protons of para-OH
substituted complex 8 undergo upfield shift by ca. 1 ppm
upon addition of 3 equiv of DBU.
20
<2
10
161
164
12
aTypical conditions: [Mn] (1-5 μmol), base (~6.5 mmol),
solvent (5 mL), H2 (3 MPa), CO2 (3 MPa), 65 °C or 80 °C, 24h.
bTON = mmol formate/mmol [Mn]; determined as average of
three trials; amount of formate was determined by NMR in-
tegration relative to DMF standard added to reaction mixture
c
after completion. yield = (mmol formate*100%)/mmol DBU
(based on DBU:formate 1:1); determined as average of three
d
trials by integration of formate peak relative to DBU. H2 1
MPa, CO2 1 MPa.
DBU was selected for further studies as a model system
as it was the most efficient and widely used base for pro-
moting high conversion.4c, 18 Other amine bases such as
Et3N and DABCO (1,4-diazabicyclo[2.2.2]octane) also
showed catalytic turnover using 6, but ultimately gave
lower conversion, and reactions carried out in protic sol-
vents were less efficient (Tables S1-S2 in the SI). Even at
reduced pressure, H2 : CO2 1 MPa : 1 MPa, a TON of 946
was obtained (entry 4). Overall, further optimization of
reaction conditions led to the TON reaching 6250 when 1
was used at low catalytic loading (entry 6).
Interestingly, when a solution of complex 6 in MeCN or
THF was reacted with 3 or more equiv of DBU in a similar
way, the FT-IR spectrum features a shift of the carbonyl
peaks to lower energies by ca. 40 cm-1 (Figure 2a and Fig-
ures S7-S8 in the SI). This is also consistent with the gen-
eration of a deprotonated anionic ligand which increases
the electron density at the metal, inducing stronger π-
back donation. The absorption bands in UV-vis spectrum
of 6 at 329-343 nm attributed to MLCT are red-shifted by
ca. 60 nm upon addition of 2 or more equiv of DBU (Fig-
ure 2b and Figure S6 in the SI), similar to the MLCT band
red-shift observed in deprotonated di-hydroxy-bipyridine
iridium complexes due to the increase of the electron
density at the metal.14
Notably, when 2,2’-bipyridine complex 2 was used as a
catalyst, only a trace amount of formate was detected.
Bromide and acetonitrile complexes 3 and 7, respectively,
bearing donating methoxy-substituents in the ortho-
positions, were significantly less active. These results sug-
gest that the presence of the adjacent OH group that can
be easily deprotonated under basic conditions is a key
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however, it has not been reported for Mn catalysts. Using
1
2
3
4
5
6
7
8
diethylamine and CO2 : H2 (2MPa : 5MPa), we obtained a
TON of ca. 588 after 24h at 80 °C (Scheme 1). A series of
control experiments: in the absence of Mn catalyst, in the
presence of Mn(CO)5Br, or in the presence of complex 2,
all indicated that either no reaction occurs, or only trace
product is formed under the reaction conditions (see
Supp. Info). While the conditions and the scope of sub-
strates require further optimization, this result shows the
versatility of 6,6’-dihydroxy-2,2’-bipyridyl based Mn cata-
lysts in various processes involving CO2 hydrogenation.
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Scheme 1. Hydrogenation of CO2 to formamide catalyzed
by 1.
In conclusion, complexes 1 and 6 are first examples of
manganese catalysts that show high activity for CO2 hy-
drogenation to formate and formamide at low tempera-
ture. In contrast to other first row transition metal hydro-
genation catalysts, these complexes do not contain air
sensitive phosphine ligands. The complex 1, for example,
can be handled under air without decomposition and can
be synthesized without strict exclusion of oxygen. A stoi-
chiometric 1 : 1 mixture of CO2 : H2 could be used effi-
ciently to produce formate, without the need to resort to
excess H2 pressure. In the future, we plan to further inves-
tigate the reactivity of Mn complexes 1 and 6 in hydro-
genation, transfer hydrogenation and hydrogen-
borrowing catalytic processes while investigating the role
of adjacent OH groups in enabling such reactivity.
Figure 2. (a) FT-IR spectra changes of complex 6 in MeCN
solution upon addition of 3-6 equiv of DBU; (b) UV-vis spec-
tra changes of complex 6 in MeCN solution upon addition of
DBU.
When a CD3CN solution of 6 in the presence of DBU
was exposed to H2 (5 bar), no hydride signal could be de-
tected by NMR spectroscopy at RT or at -30 °C (Figure
S2). However, when the analogous solution was filled
with a 1 : 1 mixture of H2 : CO2 (5 bar), catalytic formate
formation is observed, leading to a TON of 12 after 16h at
50 °C (Figure S4).
Interestingly, when the analogous experiment was per-
formed using a 2,2’-bipyridine complex 2 in CD3CN solu-
tion in the presence of DBU, pressurizing with H2 : CO2 1 :
1 mixture led to formation of a dark-colored solution,
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is
available free of charge via the Internet at
http://pubs.acs.org.
General information, experimental details, and characteriza-
tion data.
1
which features broadened signals in H NMR spectrum.
Analysis of the resulting solution by UV-vis spectroscopy
shows absorption bands at 810, 634 and 390 nm, con-
sistent with reduced Mn(0) dimer (bipy)2Mn2(CO)6 re-
ported in the literature.16a This observation suggests that
the formation of inactive dimer from 2 could be one of
the reasons for its lack of catalytic activity. Consistent
with this experiment, a previous report suggests that the
highly unstable hydride Mn(bipy)(CO)3(H) decomposes
to form the dimer [Mn2(bipy)2(CO)6].19 Recent computa-
tional studies suggested that Mn(bipy)(CO)3(H) may form
through the reaction of 6 with H2 and Me2NH as a base.20
AUTHOR INFORMATION
Corresponding Author
*Email: (J.R.K.) juliak@oist.jp; (C.N.) carlo.nervi@unito.it
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors would like to thank Mr. Sebastien Lapointe for
performing X-ray diffraction analysis. We thank Dr. Yoshit-
eru Iinuma and Dr. Michael C. Roy from the Instrumental
Analysis Section for ICP-MS support and HR-MS support,
respectively. The authors acknowledge OIST for full funding
of the project.
Having observed efficient conversion of CO2 to formate,
we set out to expand the scope of Mn-catalyzed CO2 hy-
drogenation to formamide formation in the presence of a
secondary amine. CO2 hydrogenation to formamide was
reported previously, catalyzed by precious metal-
complexes as well as by polydentate phosphine-supported
Fe and Co complexes.1a, 7c To the best of our knowledge
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