Electrocatalytic reduction of CO2 using Mn complexes with unconventional coordination environments

Article abstract of DOI:10.1039/c6cc03827a

New complexes, Mn{κ³-[2,6-{Ph₂PNMe}₂(NC₅H₃)]}(CO)₃⁺Br^- (1⁺Br^-) and MnBr{κ²-(Ph₂P)NMe(NC₅H₄)}(CO)

Full text of DOI:10.1039/c6cc03827a

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Electrocatalytic Reduction of CO2 using Mn Complexes with
Unconventional Coordination Environments
Received 00th January 20xx,
Accepted 00th January 20xx
Gyandshwar Kumar Rao, Wendy Pell, Ilia Korobkov, Darrin Richeson*
DOI: 10.1039/x0xx00000x
www.rsc.org/
New complexes, Mn{κ3-[2,6-{Ph2PNMe}2(NC5H3)]}(CO)3+Br- (1+Br-) selectivity of Mn complexes in electrocatalytic CO2 reduction
and MnBr{κ2-(Ph2P)NMe(NC5H4)}(CO)3 (2), are reported and motivated our efforts. Furthermore, our success encouraged
present new ligand environments for CO2 electrocatalytic expanding this investigation to other PN ligated Mn complexes
reduction to CO. Compound 1+ presents a unique metal geometry and thus demonstrate the variety of coordination geometries
for CO production (96%) in the absence of added water while 2 and ligation that are possible for active CO2 reducing
required addition of water and generated both CO and H2 electrocatalysts.
products.
Reaction
of
soluble
N,N’-bis(diphenylphosphino)-2,6-
di(methylamino)pyridine38–42 with MnBr(CO)5 produced
a
The catalytic reduction of CO2 to yield C1 products presents a
fundamental challenge in chemistry. Significant incentives for
targeting this undertaking are the environmental concerns
over release of CO2 into the atmosphere combined with the
potential to recycle this ubiquitous combustion byproduct into
complex displaying NMR spectra for a symmetrical ligand
environment and a high resolution mass spectrum that was
consistent with formation of a pincer complex Mn{κ3-[2,6-
+
+
{Ph2PNMe}2(NC5H3)]}(CO)3 Br-
(
1+Br-, MnPN3P(CO)3 Br-) that
was formed via release of two equivalents of CO and
autoionization of the starting material. Single crystal X-ray
analysis provided definitive structural connectivity and the
results are shown in Figure 1.
energy containing products. Since the appearance of ReX(
diimine)(CO)3 as a single component photocatalyst1,2 and
α-
MnX(bipy)(CO)3 (bipy
= 2,2’-bipyridine) as a competent
electrocatalyst for CO2 reduction,3 group 7 metal complexes
have held a central position in this area.4–10 While a variety of
metal centers (e.g. Fe,11 Co,12–15 Ni,16–19 Ru,20–23 Pd,24–27 Rh28,29
)
have been explored in this regard, there has been more limited
variation of the ligand architecture. The majority of
homogeneous catalysts for the electrochemical reduction of
CO2 are supported by nitrogen-based macrocycles, bipyridine
or polyphosphine ligands. Specifically in the case of group 7
electrocatalysts, these species are almost exclusively limited to
+
Br-
bidentate α-diimine ligands and facial-M(CO)3 metal
coordination geometries.30–37
1+Br-
2
One of our goals is to develop distinctive metal coordination
geometries which we anticipate may lead to novel reactivity
including reduction catalysis. To this end we now report on
applying a planar coordinating tridentate PNP (neutral pincer)
ligand in MnI chemistry and the potential of the resultant
Figure 1. Reaction scheme and structural representations of 1+Br- and 2. For clarity,
hydrogen atoms and the ligand carbon atoms are omitted for 1+ and 2. Full structural
information can be found in the Supporting Information.
+
The MnPN3P(CO)3 cation component (
1
+) displayed a planar-
unprecedented
metal
coordination
geometry
in
coordinated tridentate PNP ligand and three meridional CO
ligands yielding a pseudo-octahedral coordination geometry.
The bromide counterion was well-separated from this complex
cation. The literature provides only two similar PNP complexes
electroreduction of CO2. The relative abundance of Mn, the
anticipated hemilability of PNP ligands, and the documented
for MnI(CO)3. The first, Mn{κ3-[2,6-{Ph2PNH}2(NC5H3)]}(CO)3
+
(A) is directly related to 1
+ by replacement of N-Me with N-H in
the neutral ligand and this complex has been structurally
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characterized.43 The second is a neutral complex, Mn{κ3-[2- majority of reported Mn-based electrocatalysts function with
P(iPr)2-4-MeC6H3]2N}(CO)3, and displays a metal environment 5% added water or with added weak Brønsted acid.3,32,31,33
44
similar to cation 1+
.
However, this complex is supported by
an anionic PNP ligand rather than a neutral scaffold as in 1+
For 1+, the symmetrical M-P distances (2.2305(7), 2.2435(7)Å)
.
0
-50
and the P(1)-Mn(1)-P(2) angle of 163.69(3)° compare favorably
1+
with the analogous parameters reported in
A (2.247Å and
1-
1
2.252Å, 163.57°). Similarly, the M-Npy distance (2.053(2)Å) and
Mn-CO distances (1.794(3), 1.846(3), and 1.852(3) Å) observed
-100
-150
-200
-250
for 1+ are comparable to
A and to related bond distances in
substituted-bipyridyl complexes of fac-MnX(bipyR2)(CO)3.32,31
There are, of course, key differences between 1+ and the
documented CO2 electrocatalysts, fac-MnX(bipyR2)(CO)3. These
include the tridentate pincer ligation, the mer-Mn(CO)3
geometry, the cationic charge and the lack of coordinated halo
ligands.
Under N
2
Under CO
2
Under CO (5% H O)
2
2
-3.0
-2.5
-2.0
-1.5
-1.0
Potential (V vs. Fc/Fc+)
The electrochemistry of 1+Br- was examined in dry acetonitrile
(1 mM 1+Br-, 100 mM (n-Bu)4NPF6) using a glassy carbon
working electrode, Pt counter electrode and Ag wire pseudo-
reference electrode. Cyclic voltammetry was carried out under
nitrogen and referenced to the ferrocene/ferrocenium (Fc/Fc+)
couple.45 The first reduction was observed at -2.01V (-1.63 vs
SCE) and a return oxidation was observed at -1.94V (-1.56 vs
SCE) indicating a reversible one electron couple (Figures S1-
S3). On scanning to more negative potential, a second
Figure 2.Cyclic voltammograms of 1.0mM [Mn(PN3P)(CO)3]Br (1+Br-) under N2 (black),
CO2 (red) and CO2/5% H2O (blue) in CH3CN with 0.1M (n-Bu)4NPF6 supporting
electrolyte at 100 mV/s. Experimental details are provided in Supporting Information.
Bulk electrolysis was carried out with 1+Br- under CO2 in order
to measure the evolved products, the current efficiency for
this reaction, and to gain some insight into catalyst stability.
The catalytic reduction of CO2 with 1+Br- in dry acetonitrile
gave exclusively CO as the gaseous product as presented in
Figure 3. In order to confirm the source of CO and to help
identify additional by-products of the reduction, a similar
electrolysis experiment was carried out with 13CO2. After 120
min of bulk electrolysis, the reaction head space was admitted
into a previously evacuated gas cell and examined by FTIR
which demonstrated the formation of 13CO (Figure S7). In
addition, at the completion of this experiment, the solvent was
collected, evaporated to dryness and the resulting residue,
reduction to form
additional oxidation event was also observed at -1.26V. The
full voltammetric curve is shown in Figure and was
1
- was observed at -2.31V (-1.93 vs SCE). An
2
reproducible for at least 1000 cycles. For comparison, the
typical cyclic voltammograms of MnX(bipyR2)(CO)3 complexes
show two one electron reduction waves whose positions
depend on the substitution on the bipy.3,32 The first reduction
observed for 1+ is similar in potential to that of the second
reduction seen for the bipy compounds. The cathodic shift in
this potential is likely due to increased electron donation
experienced in 1+ from a combination of tridentate ligation
and changing the donor from N to less electronegative P.
when examined by 13C NMR in DMSO, yielded a signal at
δ
2- 47
166.7 ppm attributed to labelled carbonate, 13CO3
.
Although the reaction to produce CO/CO32- has been discussed
in the literature, these are not the products from conventional
Mn electrocatalyts.35,48 Importantly, this signal was not
observed in analogous experiments under natural abundance
CO2.
Carrying out this same measurement with a solution saturated
with and under an atmosphere of CO2 produced an 8x
enhancement of the reduction current indicating
electrocatalytic CO2 reduction as shown in Figure 2. Control
measurements were carried out under the same conditions
both in the absence of 1+Br- and with only free ligand. In these
cases, there was no observed current change in the presence
or absence of CO2. Compound 1+Br-, therefore, represents a
unique metal complex environment for electrocatalytic CO2
reduction. The catalytic current enhancement observed for 1+
occurs at a similar voltage (-2.0V vs. Fc+/Fc) that has been
The direct correlation between the production of CO and the
transfer of electrons corresponded to an average coulombic
efficiency of 96% over the first 125 minutes of electrolysis. The
rate of CO production remained relatively constant at about 6
micromoles CO/hr for an additional 200 minutes.
The addition of 5% H2O to this system revealed that water
was, in fact, an inhibitor for the catalysis with 1+. Figure 2
shows that in order to achieve an equivalent current, a
significantly more negative potential is required than with dry
solvent and bulk electrolysis (Figure 3) revealed that reduction
not only converted CO2 to CO but also resulted in production
of H2.23,49 Over the course of this bulk electrolysis (100 min),
evolved H2 was, on average, 41% of the gas produced meaning
reported
MnX(bipyR2)(CO)3
one half of the bipy ligand has been replaced with an N-
for
catalytic
reduction
with
fac-
3,32,31
and for manganese complexes where
heterocyclic carbene to give
species, fac-MnBr(NᴖC)(CO)3.33,37
a pyridyl/NHC coordinated
Perhaps the most intriguing aspect of the electrocatalytic
behavior of 1+Br- is the fact that the reaction occurred in dry
solvent. Other than the recent report employing the local
protons provided by hydroxyl substitutents on the ligand,46 the
that
a significant percentage of the reducing Coulomb
equivalents go toward H2 production. Again, these results
contrast with reported Mn catalytic systems which operate
without hydrogen production.37 Clearly, 1+ is not only a unique
2 | J. Name., 2012, 00, 1-3
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structure for this electrocatalysis but also appears to follow a mixed pyridyl/NHC ligated complex.33,37 The similarity of
different pathway for CO2 electroreduction than more conditions between these reported Mn complexes and is
appropriate given that these complexes fall into the same
2
conventional catalysts.
broad class of bidentate coordinated, fac-MnBrL2(CO)3 species.
60
This general similarity further accentuates the uniqueness of
1+Br-
CO dry acetonitrile
.
hydrogen 5%water/acetonitrile
CO 5%water/acetonitrle
50
40
30
20
10
0
As with 1+Br-, bulk electrolysis was carried out with
2 under
CO2 in 5%H2O/CH3CN in order analyze the products, examine
the coulombic efficiency and stability for CO2 reduction with
this species. The data shown in Figure 4 (200 minutes of
reaction time) documents an evolution in the identity of the
reduction products in this system. During the first 50 minutes
the transferred electrons selectively generate CO. After this
point, H2 production begins and increases during the
experiment with concomitant decrease in CO production. The
direct correlation between the production of CO and H2
corresponded to an excellent coulombic efficiency (>96%).
0
10
20
30
40
½ micromoles electrons consumed
50
60
Figure 3.Controlled potential electrocatalytic production of CO from CO2 by 1.0mM
[Mn(PN3P)(CO)3]Br (1+Br-) over 325 min in dry acetonitrile at −2.30 V (blue). For
measurements in 5% H2O/acetonitrile at -2.30V the products are CO (green) and H2
(red). All potentials are vs. Fc+/Fc. A black line indicating 100% efficiency is shown to
guide the eye.
These electrochemical observations suggest that complex
2
follows a similar CO2 reduction mechanism as with the
published Mn complexes. Furthermore, a DFT optimization
carried on
Mn{κ2-(PN)}(CO)3 and free bromide anion (Figure S13). This is
analogous to the first step in the currently accepted catalytic
2 reduced by two electrons yielded five-coordinate
-
Our observation of the first P-ligated MnI catalyst for CO2
reduction coupled with the reported competency for mixed
ligand
pyridyl/NHC
33,37
coordinated
species,
fac-
pathway of MnBr(bipyR2)(CO)3, and supports a proposal that
2
MnBr(NᴖC)(CO)3
prompted the consideration of
a
follows a similar reduction route to reported
complexes.
α-diimine Mn
bidentate PN ligand scaffold in the design of new catalyst for
this reaction. As shown in Fig. 1, Mn(CO)5Br was allowed to
react with N-(diphenylphosphino)-2-methylaminopyridine to
50
yield a new complex,
data
indicating the anticipated formulation MnBr{κ2
(Ph2P)NMe(NC5H4)}(CO)3. Single crystal X-ray analysis of
2, with spectroscopic data and HR-MS
sum
H2
CO
-
40
30
20
10
0
2
confirmed the structural details of the fac-tricarbonyl species
which was supported by a bidentate PN ligand. The sixth
coordination site of the pseudo-octahedral geometry was
completed by
a
bromo ligand. Coordination of 2-
and 2-
(dipheny1phosphinoamino)pyridine
(diphenylphosphinoamino)-6-methylpyridine to the MnBr(CO)3
fragment has been reported but no NMR or structural data
was provided for these species.50 In the structure of
2, the Mn-
0
10
20
30
40
50
½ micromoles of electrons consumed
Npy and Mn-P distances (2.075(2) and 2.2753(9) Å respectively)
and the Npy-Mn-P angle (80.73(8)°) were similar to the
analogous parameters observed in 1+. Furthermore, the bond
Figure 4. Controlled potential electrocatalytic reduction using 1.0mM
2 in
5%H2O/CH3CN at -2.50V vs. Fc/Fc+. The products are CO (green) and H2 (red). A
black line indicating 100% efficiency is shown to guide the eye.
parameters of
substituted-bipyridyl complexes of MnX(bipyR2)(CO)3.32,31
The cyclic votammetry of was examined in dry acetonitrile
2 are comparable to related bond distances in
The drive to uncover new catalysts for electrocatalytic
reduction of CO2 is a significant and active endeavor and our
efforts to bring novel metal coordination environments to bear
on this issue led to synthesis of the pincer complex Mn{κ3-[2,6-
2
under an N2 atmosphere and the cathodic scan gave a broad
reduction current arising from two poorly resolved irreversible
reductions at -2.43V and -2.69V versus Fc/Fc+ (-2.05 and -2.31
V vs SCE respectively, see Figure S8). On the return anodic
scan, a small oxidation was observed at -1.56V. An additional
+
{Ph2PNMe}2(NC5H3)]}(CO)3 Br- (1+Br-). In contrast to reported
Mn electrocatalysts, this species does not possess
a
coordinated halide, has a mer-tricarbonyl geometry and a PN3P
neutral pincer ligand. Like other MnI catalysts, 1+ is selective,
yielding only CO under electrocatalytic conditions. However, in
contrast to the majority of reported Mn catalysts, 1+ has
excellent efficiency in the absence of added water. Credence
for the ability of PN ligation to support active electrocatalytic
reduction was further provided by the activity observed for
400 mV potential is require to reduce
2
as compared to 1+Br-
or to reported MnX(bipyR2)(CO)3 complexes. Repeating this
experiment under a CO2 atmosphere showed no enhancement
of current during the reduction of 2. However, addition of 5%
H2O to these solutions led to an increase in cathodic current of
>1.7x (Figure S8). These observations are in line with the
3,32,31
MnBr{κ2-(Ph2P)NMe(NC5H4)}(CO)3
(
2). While this compound
observations for fac-MnX(bipyR2)(CO)3
and with the
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