Di(carbene)-Supported Nickel Systems for CO2 Reduction under Ambient Conditions

Article abstract of DOI:10.1021/acscatal.6b02101

Di(carbene)-supported nickel species 1 and 2 are efficient catalysts for the room-temperature reduction of CO₂ to methanol in the presence of sodium borohydride. The catalysts feature unusual stability, particularly for a base metal catalyst, e

Full text of DOI:10.1021/acscatal.6b02101

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Letter
Di(carbene)-Supported Nickel Systems for
CO2 Reduction Under Ambient Conditions
Zhiyao Lu, and Travis J. Williams
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02101 • Publication Date (Web): 29 Aug 2016
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Di(carbene)-Supported Nickel Systems for CO Reduction Under
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Zhiyao Lu, Travis J. Williams*
Donald P. and Katherine B. Loker Hydrocarbon Institute and Department of Chemistry, University of Southern Cal-
ifornia, Los Angeles, California, 90089-1661, United States
Supporting Information Placeholder
2
, 3
possible,
but room remains for improvement in
2
ABSTRACT: Di(carbene)-supported nickel species 1
and are efficient catalysts for the room-
renewable CO to methanol conversion.
2
temperature reduction of CO to methanol in the
Certain silanes or boranes can effect CO reduc-
tion under much milder conditions. For example,
figure 1 shows known, catalytic systems for low tem-
peratures CO to methoxide reduction. More recent-
ly, an example appeared wherein BH -THF reduces
CO to methoxide with NaBH as the catalyst. While
these catalysts differ structurally, each system in-
volves a silane or borane compound as the stoichio-
metric reductant. The less expensive and more easily
handled NaBH has been sparsely investigated for
CO reduction in the last century. In 2015 Cummins
and Knopf established that CO is reduced by NaBH
to triformatoborohydride, HB(OCHO) in anhy-
drous acetonitrile. We show here how these species
can be converted catalytically to methanol at room
temperature by nickel catalysts 1 and 2.
2
2
7
presence of sodium borohydride. The catalysts fea-
ture unusual stability, particularly for a base metal
catalyst, enabling > 1.1 million turnovers of CO2.
2
Moreover, while other systems involve more expen-
3
8
sive reducing reagents, sodium borohydride is inex-
pensive and easily handled. Further, effecting reduc-
tion in the presence of water enables direct access to
methanol. Preliminary mechanistic data collected
are most consistent with a mononuclear nickel active
species that mediates rate-determining reduction of
a boron formate.
2
4
4
9
2
2
4
-
3
KEYWORDS: carbon dioxide, methanol, nickel,
catalyst, borohydride
10
Global atmospheric CO concentration passed the
2
4
00 ppm threshold last year (May, 2015) for the first
1
time on the NOAA record, which highlights effective
CO reduction as an important goal for the catalysis
2
2
community. Most CO reduction products, such as
2
methanol, formic acid, CO, etc., are useful C1 feed-
stocks in chemical synthesis; among them methanol
has the highest volume energy density and is thus a
Figure 1. Catalytic CO Reduction with Silanes and Bo-
2
ranes. TONs are based on the number of hydrides de-
3
high value product. However, it also is a challenging
livered.
target, because CO is thermodynamically robust, so
2
its activation requires a strong thermodynamic driv-
Our group’s strategy for reduction of small mole-
cules involves dual site cooperative hydride delivery.
Particularly, we have observed modest reactivity for
CO conversion to methoxide with catalysts 3 and 4
in the presence of boranes (TON ca. 20, Figure 2),
and surmised that a more electron rich complex fea-
turing an analogous ligand scaffold could improve on
these leads. We thus designed a family of nickel
complexes supported by strongly donating
4
ing force. Also, selective reduction of CO is prob-
11
2
lematic; some known catalytic systems afford a mix-
5
ture of products. Regarding synthetic routes to
2
methanol from CO , direct hydrogenation (with H )
2
2
has been observed with a few ruthenium catalysts;
these adopt forcing conditions or a multiple catalyst
cascade, and they have limited longevity. Excellent,
6
non-renewable routes to methanol via syn gas are
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bis(NHC)borate ligands, conceptually sketched in nickel in an agostic fashion. Quite unlike compound
1
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figure 2.
1, compound 2 is free of all the boron atoms that are
introduced in its synthesis. While not designed
based on the enzymes, these multi-metallic struc-
tures are reminiscent of nickel-dependent hydrogen-
H
OTf
B
B
B
N
Cl
N
N
N
N
N
N N
N
L
L
Ru
Ru
M
N
ases that are reactive catalysts for CO reduction un-
2
N R
R
12
der ambient conditions.
Both 1 and 2 exhibit high reactivity as catalysts for
room-temperature reduction of CO by NaBH . For
3
4
CO2 reduction with
NH3BH3
CO2 reduction with
NaBH4
Proposed scaffold
for CO2 reduction
2
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example, in two weeks, 1 and 2 can deliver more than
Figure 2. Dual Center Catalytic Scaffolds for CO Re-
duction.
72000 or 143000 turnovers based on methanol prod-
uct (TOFs are 6.7 min and 3.7 min ), respectively,
without apparent loss of reactivity (Figure 3). In a
2
-
1
-1
Accordingly, we prepared bis(imidazolium)borate
cations 5 and 6 (Scheme 1), featuring diverse steric
environments. These can be doubly deprotanated to
form bis(imidazolium carbene)borate anions, re-
spectively 7 and 8, which can be treated with
longevity experiment, catalyst 2 reached to a CO2
TON of 1.1 million over 2 months (3.3 M TONs of
hydride), and was still reactive when the reaction
mixture was quenched. This TON is over three orders
of magnitude more than the highest of CO reduc-
2
Ni(acac) to form structurally novel nickel complexes
2
tion by metal catalysts and a boron hydride in litera-
1
and 2 in preparatively useful yields over 2 steps.
7
ture (Figure 1). Also noteworthy in this reaction is
Formation of 1’s nickel iodide bond results from per-
sistence of the iodide counterion that accompanies 5.
that > 90% of the total hydride groups in NaBH4
were converted to C—H bonds, which is superior to a
typical NaBH reduction of a carbonyl group. The
4
fate of boron containing species is sodium borate,
which we can quantitatively crystalize from the mix-
ture.
100
catalyst 1
catalyst 2
TON = 143000
80
60
40
20
0
BH -SMe
3
2
TON = 72000
no catalyst
0
100
200
300
400
Time (h)
Figure 3. Kinetic Profile of CO Reduction by NaBH4
2
Catalyzed by 1 and 2 in 2 Weeks. TONs are based on
CO . Y-axis is yield of methanol based on NaBH . Load-
Scheme 1. Syntheses and Structures of Nickel Com-
pounds 1 and 2. Ellipsoids are drawn at the 50%
probability level. Yields of 1 and 2 are based on the
ligands.
2
4
ings of Ni catalysts 1 and 2 and BH ·SMe are 1.9 mol,
3
2
1
.3 mol, and 20 mol, respectively.
Nickel catalysts derived from 1 and 2 are excep-
tionally robust: they work in air and they have high
tolerance for water. We take advantage of this fact
Nickel(II) complexes 1 and 2 are bimetallic and
trimetallic compounds, respectively (Scheme 1). In
the solid-state structure of complex 1, one of three
bidentate borate ligands bridges the two nickels. An-
other structural characteristic of 1 is that a B—H
bond from one nickel’s ligand interacts with another
and reduce CO in presence of a small amount of wa-
2
ter to directly synthesize methanol. In a representa-
tive NMR experiment, in the presence of 1 vol% H O,
2
the reaction yielded ca. 0.1 mmol methanol, instead
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-
1
of boron methoxides (TON 160, TOF 1.2 min , see
supporting information).
entry catalyst
conversionTON
1
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9
1
1
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1
1
2
61%
64%
< 5%
99
104
N/A
N/A
Both nickel and the ligand are important to the
mechanism (Table 1), because none of Ni(acac) , 5,
or a mixture of Ni(acac) with 5 or 6 or imidazole
effect reduction. When 7 is used to affect the same
transformation, we observe stoichiometric conver-
sion of formate to methoxide. We suspect this reac-
tivity is enabled by nucleophilic activation of boro-
2
3
4
2
Ni(acac)2
Ni(acac) + methylimidaz- < 5%
2
2
ole
5
Ni(acac) + 5 or 6
< 5%
< 5%
6.8%
27%
N/A
N/A
1
2
6
5
7
9
a,b
c
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13
7
hydride by the NHC.
d
8
20
Kinetic studies were conducted on nickel complex
1. We find that the CO pressure has little impact on
2
Conditions: formates are generated from the reac-
the reduction: in parallel runs under different CO2
pressures ranging from 15 to 45 psi, the same yield of
methoxides is observed at different time points be-
fore completion. Complexes 1 and 2 are also effective
tion between NaBH (8.0 mg, 210 μmol) and HCO H
4
2
(
8.0 µL, 210 μmol) in CD CN (0.6 mL) in a J. Young
3
tube. For the conversion of formates, catalyst (1.3
µmol) is added. Reaction progress is monitored by
catalysts for reduction of HCOOH by NaBH to
1
a
4
disappearance of formate peaks in H NMR. CO is
2
methoxides. Sequential protonations of borohydride
with HCOOH give the same formates as those gener-
ated from CO : dissolving HCOOH and NaBH in
b
used instead of FA to avoid protonation of 7. The
initial [formate] is 0.30 M. TON is based on two
c
2
4
d
carbenes per molecule of 7.
2.6 µmol catalyst is
1
acetonitrile affords identical H-NMR spectra (see
supporting information, Figure S21). This enables
formic acid as a convenient liquid surrogate for CO2
for kinetic studies. The formate compounds (Figures
S20, S21) are stable at room temperature and do not
undergo further reduction until nickel catalyst 1 or 2
is introduced. Thereafter, methoxides form until all
formates are consumed. We find this formate reduc-
tion to have first order dependence on [formate] and
first order dependence on [1].
added.
When 2 is treated with a stoichiometric amount
-
of NaBH , we see rapid conversion of BH to a new
4
4
11
borane species ( B-NMR, Figure S26) and a new,
1
1
broad hydride peak in H-NMR at δ( H) = - 13.8 ppm,
suggesting the formation of Ni-H species, which is
consistent with a hydrogen bridging Ni and B (Figure
S25). This Ni—H species, if charged with 1 atm CO ,
yields formate and methoxide peaks in H-NMR
7
c
2
1
(
Figure S27). Similarly, in an isotope labelling exper-
Table 1. Formate Conversion to Methoxides.
iment, sodium formate-d (DCOONa) was clearly
1
reduced by this Ni—H system to a methoxide-d1
product (Figure S30). These data show us that our
conditions result in the formation of a nickel hydride
intermediate that is capable of reduction of both CO2
and our formate species. While we don’t know that it
is a resting state, we propose that this is part of our
catalytic cycle.
We have isolated two nickel(II) species from the
working reaction conditions, tetra(carbene) species
9
and 10 (Figure 4). Complex 9 shows only modest
reactivity in formate reduction: its reaction rate is ca.
5 times slower than 1, and we have only observed a
modest TON (72) in 24 h with it. Compound 10 is
not long-lived: while we were fortunate enough to
obtain crystallography data; it was not sufficiently
robust to test in catalysis. We speculate that the ac-
tive catalyst is a monomeric nickel carbene complex
with a reactive nickel hydride in its reducing form.
We base this on the observation of first order kinetic
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dependence on [1], the isolations of 9 and 10, and the of the system’s catalytic mechanism, preliminary da-
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observation of a hydride in stoichiometric model
reactions, although these data are not conclusive of a
monomeric active catalyst. We suspect that dimer 1
cleaves to make species 9 and a bis(carbene)nickel
species, e.g. 11, which is reduced to give an active hy-
dride (12, Scheme 2). Although we do not observe
free ligands in the catalytic solution, we expect that 9
can slowly convert to 11, which then goes on to re-
duce formate. While our mechanistic proposal remains
speculative at this point, it is ironic to see that, while this
design concept of a bifunctional transition metal borate
ta accommodate a single, probably mononuclear cat-
alyst that enables rate-determining reduction of a
boron formate. Further development of this system
will involve the identification of a co-catalyst that
will enable its turnover based on H itself.
2
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of
charge on the ACS Publications website at DOI:
0
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(figure 2) led us to our initial design, our mechanistic
Experimental procedures, graphical and tabular
data effectively eliminate this boron atom’s participation
characterization information, and CIF files. CDCC
11d
in the mechanism, a situation we find frequently.
#
1455844 (1), #1455845 (2), #1455846 (6), #1455847
(
9), and #1455848 (10) contain supplementary crys-
tallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
2+
2 I-
AUTHOR INFORMATION
N
NH
Corresponding Author
N
NH
Ni
* travisw@usc.edu
HN
N
HN
Notes
N
The authors declare no competing financial interests.
10
ACKNOWLEDGMENT
Figure 4. Nickel Complexes 9, 10 and their Crystal
Structures. ORTEP ellipsoids drawn at the 50%
probability level.
This work was sponsored by the National Science
Foundation (CHE-1054910, CHE-1566167) and the Hy-
drocarbon Research Foundation. We thank the NSF
(DBI-0821671, CHE-0840366, CHE-1048807) and NIH (1
S10 RR25432) for sponsorship of research instrumenta-
tion. Fellowship assistance from The Sonosky Founda-
tion of the USC Wrigley Institute (ZL) is gratefully
acknowledged. We thank Rasha Hamze and Prof. G. K.
Surya Prakash for valuable discussions and Prof. Ralf
Haiges for help with X-ray crystallography.
9 or 10
CO2
2+
BH4-
O
O
O
N
N
N
N
H
N
N
Ni
H
O
BX3
Ni
1
H
BH3
BH4-
N
N
11
12
CH3OBX3
BH4- X = H, OMe, O2CH
Scheme 2. Mechanistic Speculation.
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can catalyze CO2 reduction to methoxides with
NaBH under ambient conditions. The catalysts fea-
ture unprecedented stability, enabling a stunning > 1
4
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