H2 Production Mediated by CO2 via Initial Reduction to Formate

Article abstract of DOI:10.1021/acs.organomet.6b00595

A Ru(NH-NHC) complex with an open coordination site on the metal center adjacent to ligand N-H moieties has been synthesized and characterized. This complex exhibits unique reactivity upon reaction with either CO₂ or NaHCO₃, yielding a formate-bridged bimetallic complex via a spontaneous deoxygenation reaction and formal reduction at carbon. Dehydrogenation of the formate complex leads to a Ru-carbamate species following carbon-nitrogen bond formation between the CO₂ moiety and the NHC ligand. This reactivity opens up new pathways for CO₂ reduction and is relevant to H₂ storage.

Full text of DOI:10.1021/acs.organomet.6b00595

Communication
pubs.acs.org/Organometallics
H Production Mediated by CO via Initial Reduction to Formate
2
2
†
†
Michael R. Norris, Sarah E. Flowers, Ashley M. Mathews, and Brandi M. Cossairt*
University of Washington, Department of Chemistry, Box 351700, Bagley Hall, Seattle, Washington 98195-1700, United States
*
S Supporting Information
ABSTRACT: A Ru(NH-NHC) complex with an open
coordination site on the metal center adjacent to ligand N−
H moieties has been synthesized and characterized. This
complex exhibits unique reactivity upon reaction with either
CO2 or NaHCO , yielding a formate-bridged bimetallic
3
complex via a spontaneous deoxygenation reaction and formal
reduction at carbon. Dehydrogenation of the formate complex
leads to a Ru−carbamate species following carbon−nitrogen
bond formation between the CO moiety and the NHC ligand.
2
This reactivity opens up new pathways for CO reduction and
2
is relevant to H storage.
2
1
3
ncreased global energy demand, along with concerns of CO2
as a contributor to climate change, have spurred intense
Ru(PNN) complexes that involve cooperative activation of
I
CO between a redox-active ligand and metal center.
2
research into alternative energy sources that do not result in
greenhouse gas emissions. Hydrogen gas is a clean fuel that
Recently, we reported the synthesis of a new tridentate
bisimidazole-phosphine ligand and metalation with Ru salts.
14
does not produce CO as a product of combustion; however,
We hypothesized that a Ru complex with an accessible
coordination site next to the protic NH-NHC ligands (Figure
1) could exhibit interesting metal−ligand reactivity with CO2.
2
the main source of H on an industrial scale is the re-forming of
2
1
natural gas, which results in significant CO generation. Thus,
2
if H is to be used as an alternative fuel, its source must be
2
This led us to the synthesis of a Ru−solvent complex, [1-
water, and the need to pressurize and transport H requires
2+
2
OH ] , which indeed demonstrates unique reactivity with both
2
advances in storage. Alternatively, the CO generated from
−
2
CO and HCO ; facilitating the reduction of either substrate
2
3
burning fossil fuels could be captured and reduced back to a
to formate at room temperature or direct activation across the
M−L scaffold at elevated temperature. In the case of substrate
2
useful form. A number of electrochemical and hydrogenation
3
systems have been studied that reduce CO to CO, formate/
formic acid,
2
2
c,4
5
and methanol. The continued challenge is
finding catalysts that operate under ambient conditions for
these transformations. An interesting proposition that com-
bines the use of H as a clean-burning fuel with CO reduction
2
2
is the storage of H in formic acid as a transportable liquid,
2
6
followed by dehydrogenation to retrieve the H . This strategy
2
still requires H that is not produced by the re-forming of
2
natural gas and also only remains carbon neutral if the CO2
formed in the dehydrogenation reaction is recaptured and
7
recycled.
In order to understand and exploit CO2 chemistry,
complexes that interact with CO in unique or unconventional
2
ways and that are capable of multifaceted CO transformations
2
are necessary. In biology, for example, [NiFe] CO dehydrogen-
ase enzymes utilize a push−pull mechanism to activate CO ,
2
2
+
8
where Ni acts as a Lewis base and Fe serves as a Lewis acid.
Several studies have utilized multimetallic complexes or metal−
Figure 1. Ru(NH-NHC) complex with an open coordination site [1-
2+
OH ] , the formate-bridged bimetallic complex 2, and the carbamate
ligand cooperation to achieve new and unique reactivity with
2
complex 3.
CO2, including studies of bifunctional Co(salen-R)M com-
9
plexes, Zr/Co heterobimetallic complexes where CO
2
10
I
oxidatively adds across the two metal centers, a Cu dimer/
tetramer that reduces CO to oxalate, and Ru(PNP) and
Received: July 26, 2016
11
12
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00595
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Organometallics
Communication
+
reduction, the formate complex (2) undergoes spontaneous
Cl] , although the N−H protons cannot be located in CD OD,
3
dehydrogenation in solution to yield H and the product of
likely due to H/D exchange.
2
We studied the reaction of [1-OH2]2+ and CO with the goal
direct M−L activation, a Ru−carbamate complex (3) where the
2
CO remains activated and added to the ligand scaffold.
of exploring possible CO reduction chemistry. Addition of
2
2
2+
Synthesis of the phosphine ligand PhP(Etbim) was carried
CO to solutions of [1-OH ] in CH Cl yielded orange
2
2 2 2 2
out using a modified procedure with benzimidazole as the
crystals after standing at room temperature for several days.
These crystals were analyzed by X-ray diffraction and were
found to be a formate-bridged bimetallic complex (2, Figure 2).
Complex 2 was also independently prepared by the addition of
1
4
NHC precursor. Protonation of the benzimidazole moieties
in PhP(bim) followed by reaction with [Ru(Bz)(bpy)(OTf)]-
2
(
OTf) (Bz = benzene, bpy = 2,2′-bipyridine, OTf =
2+
trifluoromethanesulfonate) leads to a precursor where only
the P atom binds to the Ru center (Figure S1 in the Supporting
Information). This complex is then cleanly converted to [1-
sodium formate to a solution of [1-OH ] in CH Cl . The O−
2
2
2
C−O bond angle in 2 is 125.3° with C−O bond lengths of
1
.246 Å, consistent with reported formate-bridged metal
2+
15
OH2]
at 175 °C in ethylene glycol (Figure S2 in the
complexes. The IR spectrum of 2 displays a strong stretch
at 1548 cm (Figure S7 in the Supporting Information) and a
peak in the C NMR at 170 ppm, both suggestive of a metal
formate species. It should be noted that while the Ru−
formate complex crystallizes in the bimetallic form in CH Cl , a
solvent in which it is sparingly soluble, we believe that it readily
converts to the monometallic form and 1 equiv of Ru−solvento
complex in a coordinating solvent (Figures S8 and S9 in the
Supporting Information). We hypothesized that the mechanism
1 31
Supporting Information). Interestingly, the H NMR and
NMR spectra of [1-OH2] in CD Cl or DMSO-d show
P
−1
2
+
13
2
2
6
multiple products with broad resonances (Figure S3 in the
15
−
Supporting Information). However, addition of a Cl source to
2
+
2
2
a solution of [1-OH2] in either solvent leads immediately to
1
31
13
clean and interpretable H, P, and C NMR spectra (Figures
S4 and S5 in the Supporting Information). Crystals suitable for
+
X-ray diffraction were grown of [1-Cl] by slow diffusion of
−
Et O into an acetone solution (Figure 2). The Cl acts to lock
2
for reduction of CO to formate proceeds via initial formation
2
of a Ru−bicarbonate intermediate, as CO can insert into M−
2
16
OH and M−OH bonds to equilibrate with bicarbonate. This
2
−
provides a plausible route for formation of a Ru−HCO
3
species which could undergo deoxygenation to form the
reduced Ru−formate. Direct addition of NaHCO to solutions
3
2+
of [1-OH ] in CH Cl also leads to complex 2 in low yield,
consistent with the intermediacy of a Ru−HCO species.
Although other examples exist of reduction of HCO to
2
2
2
−
3
−
3
formate, they require added energy by initial formation of a M−
1
7
H bond, the use of high pressures of H , or the use of an
2
18
applied electrochemical potential.
Importantly, the formation of 2 from CO and HCO
−
2
3
represents a formal reduction of the added carbon species. A
plausible explanation is that HCO3− bound to the Ru center is
deoxygenated by another species in solution, leading to the
reduction of HCO3− to formate (Scheme 1). Deoxygenation of
a bound carbonate has at least one precedent in the literature
where an O atom was transferred to a phosphine with
III
I
concomitant production of CO mediated by a Rh −Rh
+
2
Figure 2. Molecular structures of [1-Cl] , 2, and 3 with thermal
19
cycle. Addition of excess triphenylphosphine or trioctylphos-
+
ellipsoids shown at 50% probability. [1-Cl] displays H-bonding
interactions between N−H groups and bound Cl with one Cl
counterion omitted for clarity. Complex 2 is a formate-bridged
bimetallic Ru complex with three PF6 counterions removed for
clarity. The same structure was determined for addition of CO ,
HCO , and HCO to 1. The structure of 3 shows a carbamate with
the O atom bound to the Ru center with one PF6 counterion
2
+
−
phine, in the presence or absence of a nitrile, to [1-OH ] and
2
−
CO or HCO3 also did not lead to the buildup of phosphine
oxides, as evidenced by P NMR spectroscopy. In reactions of
complex [1-OH ] with CO and HCO , an oxidized product
has not been identified; the fate of the O atom is the subject of
continuing investigation.
To probe the deoxygenation reaction pathway further,
,1,3,3-tetramethyldisiloxane (TMDS) was added to a solution
of [1-OH2] and C-labeled NaHCO in DMSO-d . Since an
2
31
−
2
+
−
2
2 2 3
−
−
3
2
−
removed for clarity. Selected interatomic distances (Å) and angles
+
(
deg): for [1-Cl] , Ru1−Cl1 2.534(2), Ru1−C21 2.011(6), Ru1−C30
1
2
.024(6), H3−Cl1 2.475, H5−Cl1 2.509; for 2, Ru1−O1 2.181, C35−
2+
13
3
6
O1 1.246, O1−C1−O1′ 125.3; for 3, Ru1−O1 2.219, O1−C35 1.301,
−
oxygen atom must be removed from HCO3 in order to make
formate, it was hypothesized that the added silane reagent
would offer an attractive acceptor for the O atom. After reacting
C35−O2 1.2229, C35−N1 1.457, H3A−O1 2.532, O1−C35−O2
1
27.29.
13
at 50 °C overnight, the only observable species by C NMR is
a new 13C-containing species, 3 ( C 155 ppm), in near 100%
13
the N−H protons into a symmetric geometry due to H-
bonding interactions, which may also explain why the H NMR
spectrum of [1-OH2] shows multiple resonances when H O
is bound if a variety of H-bonding interactions cause multiple
1
1
yield as determined by H NMR spectroscopy (Figures S10−
S13 in the Supporting Information). Running the same reaction
with 10 equiv of NaH CO in the absence of TMDS, however,
also leads to clean conversion to 3 (Figure S14 in the
Supporting Information), albeit in lower (64%) yield. These
experiments taken together suggest that the new species 3 may
2
+
2
13
3
1
conformers to exist in solution. The H NMR spectrum of [1-
OH2]2 is also sharp and well-defined in CD OD (Figure S6 in
+
3
the Supporting Information), similar to the spectrum of [1-
B
DOI: 10.1021/acs.organomet.6b00595
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Organometallics
Communication
2 with two C−O stretches at 1634 and 1602 cm− (Figure S16
in the Supporting Information). These data suggest that the
formate C atom remains bound to the Ru complex in some
1
Scheme 1. Proposed Overall Transformation of Reduction of
H O to H2
2
13
fashion and is not released as CO gas ( C NMR 124 ppm).
Reaction of [1-OH ] with NaH CO in acetone also leads to
, with a C signal at 155 ppm and release of H , as confirmed
2
2+
13
2
2
13
3
2
by GC analysis (Figure S17 in the Supporting Information).
Complex 3 can also be formed via deprotonation of a N−H
2
+
13
moiety of [1-OH ] , followed by reaction with CO , as
2
2
1
13
13
confirmed by H and C NMR ( C 155 ppm, Figures S18 and
S19 in the Supporting Information). We hypothesized 3 to be a
carbamate that formed through reaction of the C atom of CO2
with the deprotonated N atom of the benzimidazole ligand
backbone. This would be consistent with both the deprotona-
tion followed by reaction with CO and the dehydrogenation of
2
formate where a proton from an N−H and a hydride from
formate could form H with simultaneous formation of the N−
2
C bond to make a carbamate (Scheme 1). Additionally, the
carbamate may form directly from reaction with bicarbonate via
acid/base chemistry and formation of a N−C bond (Scheme
2
). This pathway is consistent with observation of the facile
2+
+
formation of 3 when [1-OH2]1 or [1-Cl] was heated in
3
DMSO in the presence of NaH CO .
3
Scheme 2. Alternative Mechanism of Formation of 3 from
2
+
[1-OH2]
form independently of the formate-bridged bimetallic complex
2
.
To determine if 2 was relevant to the formation of 3, the
+
13
conversion of [1-Cl] in the presence of 1 equiv of NaH CO
in DMSO-d6 was monitored by P, C, and H NMR
spectroscopy. Over the course of days at 68 °C, [1-Cl] cleanly
converts to the formate species, which goes on to give 3 with
concomitant formation of H (4.61 ppm in the H NMR,
Figure 3). The new species 3 shows a shift in the P NMR
from 52 ppm in 2 to 47 ppm and a shift in the C NMR from
2
3
1
13
1
+
1
2
3
1
1
3
1
70 ppm for 2 to 155 ppm for 3 (Figure S15 in the Supporting
Information). The IR spectrum of 3 is also distinct from that of
The 13C NMR chemical shift of 3 and IR stretches are also
consistent with those of other reported carbamates with an O
20
atom bound to an adjacent metal center. Crystals suitable for
X-ray diffraction were grown from slow evaporation of a
solution of 3 in CH Cl (Figure 2). The structure displays a
2
2
carbamate moiety where the O atom is bound to the Ru center,
which also removes a plane of symmetry in the molecule,
making the benzimidazole arms of the PhP(Etbim) ligand
2
1
13
chemically distinct. This is reflected in the H and C NMR
spectra, where the N−H (12.2 ppm) integrates for one proton,
the diastereotopic − CH − protons of the PhP(Etbim) split
2
2
into distinct resonances (Figure S20 in the Supporting
Information), and the 13C resonances for the C1 carbon
atoms of the carbene ligands show up as distinct with chemical
2
shifts of 202.9 ppm (s) and 193.6 (d, J = 15 Hz; Figure S21
CC
in the Supporting Information).
In conclusion, we have synthesized and characterized a new
1
Figure 3. H NMR (500 MHz, DMSO-d ) spectra over time of the
6
+
13
Ru complex that interacts with CO via a unique pathway
reaction of [1-Cl] with NaH CO in DMSO at 60 °C, showing
conversion to 2 followed by formation of 3 along with release of H2,
evidenced by the singlet at 4.6 ppm.
2
2
−
where HCO3 is initially formed and then deoxygenated to give
−
+
the 2e /2H reduced formate. Dehydrogenation of formate,
C
DOI: 10.1021/acs.organomet.6b00595
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Organometallics
Communication
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00595.
2
(
Experimental details, NMR spectra, IR spectra, and
crystal structure data. (PDF)
Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
E-mail for B.M.C.: cossairt@chem.washington.edu.
Author Contributions
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
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ACKNOWLEDGMENTS
We would like to thank the ACS Petroleum Research Fund
DNI) and the David and Lucile Packard Foundation for
support of this work. Analytical data were obtained from the
CENTC Elemental Analysis Facility at the University of
Rochester, funded by NSF CHE-0650456. The authors thank
Prof. Werner Kaminski for assistance with solving crystal
structures, Benjamin Glassy for assistance in obtaining IR
spectra, Jennifer Stein for fluorescence spectra, and Dr. Jared
Silvia for helpful discussions.
Okuda, J.; Cummins, C. C. Angew. Chem., Int. Ed. 2015, 54, 9115−
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