Transferring photocatalytic CO2reduction mediated by Cu(N^N)(P^P)+complexes from organic solvents into ionic liquid media

Article abstract of DOI:10.1039/d0gc01627f

Photocatalytic carbon dioxide reduction utilizing metal complexes based on the earth-abundant transition metals iron and copper was transferred from organic solvents into ionic liquids with high selectivity and moderate turn-over numbers. Different classes of ionic liquids were evaluated to determinate the CO/H2 selectivity of the process. High-purity conditions were utilized to validate that CO was a result of CO2 reduction and not resulting from other carbon sources. In addition, a novel copper-based photosensitizer was synthesized and its photo- and electrochemical properties were investigated. This journal is

Full text of DOI:10.1039/d0gc01627f

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DOI: 10.1039/D0GC01627F
ARTICLE
Transferring photocatalytic CO2 reduction mediated by
Cu(N^N)(P^P)+ complexes from organic solvents into ionic liquid
media
Received 00th January 20xx,
Accepted 00th January 20xx
Paola A. Forero-Cortés,a,b,c Maximilian Marx,a Nikolaos Moustakas,a Fabian Brunner,c Catherine E.
DOI: 10.1039/x0xx00000x
Housecroft,c Edwin C. Constable,c Henrik Junge,a Matthias Bellera and Jennifer Strunka*
Photocatalytic carbon dioxide reduction utilizing metal complexes based on the earth-abundant transition metals iron and
copper was transferred from organic solvents into ionic liquids with high selectivity and moderate turn-over numbers.
Different classes of ionic liquids were evaluated to determinate the CO/H2 selectivity of the process. High-purity conditions
were utilized to validate that CO was a result of CO2 reduction and not resulting from other carbon sources. In addition, a
novel copper-based photosensitizer was synthesized and its photo- and electrochemical properties were investigated.
formation from water splitting, a process that usually requires
less negative potentials, and therefore is difficult to avoid.21, 22
Nevertheless, several successful examples of dual catalytic
Introduction
Carbon dioxide constitutes 81% of the total amount of
Greenhouse gas (GHGs) emissions.1 Utilization of CO2 captured
from emissions is, therefore, mandatory to mitigate its
negative effects on the atmosphere envisioning it as a carbon
source replacing fossil fuels.2, 3 The reduction of CO2 to CO,
generating an energy-rich commodity chemical, is one of the
strategies for CO2 valorization.3-6
systems typically involving ruthenium or iridium complexes as
photosensitizers (PS) in combination with a transition metal
catalyst (cat) have been developed.23 More recently, examples
based on earth-abundant metals as stated in recent reviews6,
24, such as Mn,24-26 Ni,27 Cu,28, 29 Co,30-32 and Fe30, 31, 33 were
reported, too. In this regard, some of us developed an efficient
system for the reduction of CO2 to CO based on
(cyclopentadienone) iron–tricarbonyl complexes as catalysts
(Scheme 1). Beneficial for this reaction is the ability of such
complexes not only to possess a hydride-donor site via the
metal center, but also a proton-donor site (via the ligand).34
The chosen photosensitizer was [Ir(dF(CF3)ppy)2-(dtbbpy)]PF6
Moreover, energy utilization plays a crucial role in the
development of sustainable catalytic processes using solar
power as energy source.7 Low-cost storage methodologies
covering variable atmospheric conditions are required in order
to use solar energy as the primary energy supply.8 A feasible
approach in this direction is the development and optimization
of thermodynamically uphill photocatalytic processes that
enable chemical transformations by the use of photonic
energy as the driving force.9
(IrPS) (where dF(CF3)ppy
=
cyclometalated 2-(2,4-
difluorophenyl)-5-trifluoromethyl-pyridine and dtbbpy = 4,4'-
di-tert-butyl-2-2'-bipyridine).35 Triethanolamine (TEOA) as
sacrificial electron donor and 1-methylpyrollidin-2-one (NMP)
as solvent were used, irradiating the reaction mixture using a
2.5 W Hg-lamp (emission range: 400–700 nm) for 5 h at room
temperature. Under these reaction conditions, a turnover
number (TON) of 596 was obtained using 3.75 × 10-8 mol of
catalyst, outperforming the structurally related [Fe(CO)3(bpy)]
that reached a TON of 129.36
Consequently, a combination of renewable energy and CO2
utilization towards sustainable catalytic processes such as
photocatalytic systems that convert carbon dioxide to more
valuable chemicals is an area of great interest. In this respect,
transition metal complexes have been studied as
photocatalysts where rhenium 10-13, iridium 14-16 and ruthenium
17-19
are known for being selective towards different reduction
The replacement of the iridium photosensitizer by the more
economical copper (CuPS) resulted in a slightly lower TON of
487.37 In this case 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-
benzo[d]imidazole (BIH) was used as sacrificial electron donor
products. In general, selective processes towards the
production of CO from CO2 are challenging not only because
they are thermodynamically uphill, but also numerous by-
products may be generated.20 In aqueous systems the
reduction of CO2 is also in competition with hydrogen
and again
a reductive quenching mechanism operating
between CuPS* and BIH was proposed.
a
b
c
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DOI: 10.1039/D0GC01627F
Scheme 1: Fe-based CO2 reduction catalysts and photosensitizers reported by Beller and co-workers 34, 37 and by the Sakai group.
38
More recently, another catalytic system based on the same lower viscosity and a stronger interaction between CO2 and
CuPS in combination with
a
Fe(II) catalyst bearing
a
these ILs. In addition, an elongation of the alkyl chain in 1-
tetradentate macrocyclic nitrogen-donor ligand (Scheme 1) position resulted in a significant decrease of catalytic activity.
was reported by Yamauchi, Sakai and co-workers.38 Under
Furthermore, a report by Grills described the photocatalytic
optimized conditions, a TON(CO) of 565 and a CO selectivity of
CO2 reduction in a biphasic IL/scCO2 system using the catalyst
84%
within
8 h
were
observed
in
N,N-
fac-ReCl(bpy)(CO)3
and
1-butyl-1-methylpyrrolidinium
dimethylformamide/TEOA (4:1, v:v) utilizing BIH as sacrificial
tetracyanoborate ([Bmpyrr][TCB]) as ionic liquid and TEOA as
electron donor. After 3 hours of irradiation at 400±25 nm, the
reductant and visible light (>400 nm, 179 mW/cm2).
Notably, all the aforementioned photocatalytic systems activity was optimized by the use of a high-pressure biphasic
function using organic solvents, but in the perspective of a system with TONs increasing from 0.7 to 5.43
Since CO2 is a thermodynamically highly stable molecule,44 it is
more environmentally-friendly process, non-recyclable
solvents should be replaced. In this respect, ionic liquids (ILs)
expected that many other carbon-containing molecules, i.e.
have been explored as a promising alternative to conventional
impurities incidentally present in the reactor, will react faster
organic solvents because they possess advantages including
negligible vapor pressure, high thermal stability, wide
under the selected experimental conditions. This has been
demonstrated
previously
in
numerous
reports.45-48
electrochemical
stability
window,
tunable
polarity,
Consequently, it is very important for CO2 reduction
experiments to verify that neither hydrocarbons nor CO are
produced from the catalyst, reaction media or any other C-
containing impurities that could be present in the reaction
chamber. If such outcomes are not analyzed carefully, the
photocatalytic efficiency of the tested sample can be
hydrophobicity and solvent miscibility behavior through
variation of the cations and anions.39 These properties make
them the ideal reaction media for many applications, and
replacing volatile solvents by ILs reduces solvent losses to the
atmosphere.40, 41
Indeed, Wang and co-workers reported the capture and overestimated leading to invalid conclusions.46, 49 Compared to
activation of CO2 to produce CO by ILs coupled with photocatalytic measurements of purely inorganic systems (i.e.
photoredox catalysis using [Ru(bpy)3]Cl2 as PS and CoCl2∙6H2O bare TiO2 in water), proof of product formation from CO2 is
as catalyst.42 In
[EMIM][Tf2N]:
a
[EMIM][Tf2N]/H2O/TEOA (3/1/1; even more important when the catalyst itself, or the reaction
1-ethyl-3-methylimidazolium medium, contains organic components such as ligands or
bis(trifluoromethanesulfonyl)imide) mixture, reduction of CO2 solvents.
and proton reduction occurred with a total TON of 45.6 and a
The formation of products from CO2 as carbon source can be
selectivity of 76.1% towards CO. The structures of the cations
and anions of the imidazolium ILs showed considerable effects
on the activation and reduction of CO2. Better performances
were obtained with [Tf2N]– and tetrafluoroborate ([BF4]–)
evidenced by the analysis of apparent product formation in a
blank experiment in the absence of CO2 gas. Under identical
reaction conditions, CO2 is replaced by an inert gas such as He
or Ar and each collected chromatogram is analyzed in order to
identify possible catalyst decomposition, solvent degradation,
or any other impurities present in the reaction set-up.50
–
counterions compared to acetate, dicyanamide ([N(CN)2 ]), L-
lactate and trifluoromethane-sulfonate ([OTf]–) attributed to a
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Extensive baseline measurements without the presence of CO2 substituent;
ARTICLE
POP-thixantphos:
4,5-bis(2,8-dimethyl-10-
View Article Online
DOI: 10.1039/D0GC01627F
should take place to determine that the production of phenoxaphosphano)-2,7-dimethylphenoxathiin.
hydrocarbons/CO comes from the substrate and not from
other potential sources of carbon.
Electrochemical and photophysical characterization of
complexes
Complex 1 has been characterized previously7 and shows
The use of noble metal free cooperative photocatalytic
processes represents vide supra
a cost-effective and
similar absorption features in acetonitrile solution compared
to our findings. The solution absorption spectra of and
environmentally friendly alternative to traditional transition
metal photocatalysis.51, 52 Copper and iron are ideal metals of
choice as their crustal abundances are 27 ppm for the former
and 52157 ppm for the later. Precious transition metals are
significantly less abundant and therefore more expensive with
the following crustal abundances: Rhenium 0.000188 ppm,
ruthenium 0.000037 ppm and iridium 0.000037 ppm.53, 54
1
2
(Figure 1) show a broad absorption at around 395 nm arising
from metal-to-ligand charge transfer (MLCT) and is
characteristic for this type of heteroleptic [Cu(N^N)(P^P)]+
52, 56
complex.7,
Upon excitation in the MLCT band, both
compounds exhibit a broad unstructured emission (Figure 2).
Complex shows an emission maximum at 539 nm, whereas
the emission of is red-shifted by around 15 nm. This is
2
1
In the present work we show the feasibility of photocatalytic
reduction of CO2 to CO in ionic liquid media by well-defined
[Cu(N^N)(P^P)][PF6] and [Fe(cyclopentadienone)(CO)3] metal
complexes under high purity conditions. More specifically, we
showcase the performance of ionic complexes in ionic liquid
media for photocatalytic applications. Using a high-purity
photoreactor and with extensive blank batch cleaning
experiments, we were able to further establish that the origin
of CO is indeed the CO2 of the feed stream that is reduced by
our tested catalyst.
attributed to the higher steric demand of the phosphane
ligand in 2. The lower flexibility of POP-xantphos in comparison
to xantphos results in a lower degree of distortion of the
distorted tetrahedral geometry upon excitation. This leads to a
hypsochromic shift of the emission, as has been observed for
57
related [Cu(N^N)(P^P)][PF6] complexes.7,
The emission
spectra reported previously are red shifted compared to our
findings due to the different choice of solvent (580 nm in
acetonitrile,58 568 nm in THF7 vs 555 nm in CH2Cl2 (Table 1)).
Results and discussion
Synthesis and characterization of complexes
Synthesis
Initially, copper(I) complexes 155 and
according to known protocols (Schemes 1 & 2). A more rigid
and sterically demanding ligand was used to form complex as
the related complex bearing POP-thixantphos ( ) has been
reported to give remarkable performance in photocatalytic
2 were obtained
2
3
55
applications.7,
reaction of 4,5-bis(2,8-dimethyl-10-phenoxaphosphano)-9,9-
dimethylxanthene (POP-xantphos) with
Complex 2 was successfully synthesized by
Figure 1: Solution absorption spectra of complexes
1 and 2
(CH2Cl2, 2.5 x 10-5 mol dm-3).
tetrakis(acetonitrile)copper(I) hexafluorophosphate in CH2Cl2,
followed by the addition of the 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (bathocuproine) ligand.
Scheme 2. Heteroleptic complexes
[Cu(bcp)(POP-xantphos)][PF6].
2
and
Complex
3
. Complex
2
3
:
:
Figure 2: Normalized emission spectra of copper(I) complexes
and
(λexc = 400 nm, degassed CH2Cl2, 2.5 x 10-5 mol dm-3).
Complexes and show high photoluminescence quantum
[Cu(bcp)(thixantphos)][PF6] (bcp: 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (bathocuproine); POP–xantphos: 4,5-
bis(2,8-dimethyl-10-phenoxaphosphano)-9,9-
1
2
1
2
yields (PLQY) in deaerated solution (40 and 38% respectively)
dimethylxanthene); POP: 2,8-dimethyl-10-phenoxaphosphano
and excited state lifetimes of 9.42 and 7.24 µsb, respectively.
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The here reported solution quantum yield and excited state Table 3: Excited state redox potentials. E00VievwaAlrutieclse Onalirnee
lifetime of
in CH2Cl2 are significantly higher than those estimated from the solution emissionDOonI:s1e0t.10(359%/D0eGmC0is1s6i2o7nF
1
reported in a previous study with a quantum yield of 8% and intensity) at room temperature.
an excited state lifetime of 6.4 µs in deaerated THF.7 Note that
the higher excited state lifetime is not a result of a decreased
concentration, which has previously been shown to impact the
value due to self-quenching (Table 1).59
Complex
E*ox/V
–1.77
–1.84
E*red/V
0.52
E00/eV
2.63
1
2
0.57
2.67
Table 1: Photophysical properties of the complexes in CH2Cl2
solution at a concentration of 2.5 x 10-5 mol dm-3 (λexc
=
365 nm).
X-ray crystallographic structure of 2
Crystals of were obtained by slow diffusion of n-hexane in a
saturated solution of the complex in CH2Cl2. The single crystal
structure of
has previously been reported.61 The
coordination geometry of in the solid state is similar to that
of complex (Figure 3, Table 4) and can be best described as
τ1/2 µsc
2
Complex
UV-Vis
MLCT
λabs/nm
λem/nm
PLQY %
1
2
1
2
393
398
555
539
1.1a/38b
1.3a/40b
0.338a/7.24b
0.446a/9.42b
1
62
distorted tetrahedral with a τtet-sq index value of 0.98 (with
1.00 being the value for an ideal tetrahedral geometry).
aNon-degassed solution bDegassed solution by bubbling an
argon stream through the solution for 20 min. Values ±15%
c
instrumental error.
The redox behaviour of complexes
1 and 2 was investigated
using cyclic voltammetry and data are summarized in Table 2.
Each compound exhibits a partially reversible oxidation at
+0.86 or 0.83 V (vs. Fc/Fc+) which is assigned to a copper
oxidation. Additionally,
reduction processes at –2.11 or –2.10 V, respectively. The
oxidation and reduction potentials for in CH2Cl2 are slightly
1 and 2 both show partially reversible
1
lower than previously observed values in acetonitrile and are
discussed in depth in literature.7 It is noteworthy that the
literature cyclic voltammogram in acetonitrile shows
a
reversible ligand reduction and irreversible copper oxidation,
which is contrary to our findings in CH2Cl2.
Figure 3. Molecular structure of 2. Hydrogen atoms and the
[PF6]– anion are omitted for clarity. Thermal ellipsoids are set
to 30% probability level.
Table 2: Cyclic voltammetry data for copper(I) complexes
referenced to internal Fc/Fc+ = 0 V; degassed HPLC grade
CH2Cl2 solution with [nBu4N][PF6] as supporting electrolyte and
a scan rate of 100 mV s−1.
Table 4: Selected bond lengths *Ǻ+, angles *°+, and geometry
indexes for the solid state structures of complexes 161 and
2.
Complex 161
80.41(10)
102.63(8)
119.41(8)
97.34(8)
Complex
2
Complex
Eox1/2/V (Epc
Epa/mV)
–
Ered1/2/V (Epa
Epc/mV)
–
Parameter
N1-Cu1-N2
N1-Cu1-P1
N1-Cu1-P2
N2-Cu1-P1
N2-Cu1-P2
P1-Cu1-P2
N-Cu1
80.65(10)
100.78(7)
120.51(7)
99.43(7)
124.58(8)
121.80(3)
2.061(2)
2.059(3)
2.2124(8)
2.2789(8)
0.98
1
2
+0.86 (81)
+0.83 (72)
–2.11 (154)
–2.10 (127)
Excited state redox potentials are estimated according to
formulas (1) and (2) 60 and the results are summarized in Table
3 where E00 is the (non-observable) energy difference between
the lowest vibrational level of the electronic ground and
electronic excited states. Eox1/2 and Ered1/2 are the copper-based
oxidation and ligand-based reduction potentials. E00 is
estimated from the solution emission onset (5% emission
intensity) at room temperature.
128.84(8)
119.68(4)
2.065(3)
2.099(3)
P-Cu1
2.2425(10)
2.3048(10)
0.94
E*ox = Eox1/2 – E00 (1)
τtet-sq (Θ/90°)62
E*red = Ered1/2 + E00
(2)
DFT Calculations: HOMO and LUMO characteristics
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Ground state DFT calculations were carried out for the
[Cu(bcp)(xantphos)]+ and [Cu(bcp)(POP-xantphos)]+ cations in
the gas phase and the compositions of the HOMO and LUMO
of each complex are shown in Figure 4. The orbital character is
partitioned on the ligands with the LUMO being localized on
the bcp ligand (Figures 4b and 4d), while the HOMO is
centered largely on the copper in [Cu(bcp)(xantphos)]+ (Figure
4a) and on the copper and bisphosphane ligand in the POP-
xantphos containing complex (Figure 4d). Similar orbital
distributions have been observed for heteroleptic
[Cu(N^N)(P^P)]+ complexes.60, 63-65
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DOI: 10.1039/D0GC01627F
Entry Complex
Solvent
TON TON Selectivity
(CO) (H2)
[%]
137
1
2
NMP/TEOA
(5:1, v:v)
427
5
≥98
2
NMP/TEOA
(5:1, v:v)
505
23
≥93
Reaction conditions: [Fe] (1 μmol), *Cu(N^N)(P^P)+*PF6]
(5 μmol), BIH (150 mg) and TEOA (1.25 mL) were illuminated at
400-700 nm (1.50 W) under 1 atm of CO2 in 6.25 mL of NMP
for 5 h. Results presented are the averages of reactions carried
out in duplicate (single experiment results can be found in
Table S2 in the ESI), TON values calculated based on the iron
catalyst. Entry 1: TON for HCO2H are not reported in the
1
literature. Entry 2: TON of HCO2H of 10 was observed by H
NMR spectroscopy using benzene as internal standard.
The CuPS/Fe systems were then tested using different ionic
liquids as solvents (Table 6) in order to ascertain the impact of
the latter on the catalytic performances. While the study
conducted by Wang and co-workers mainly evaluated the
influence of different anions on the catalytic performance, we
were intrigued by the impact of different classes of ionic
liquids. Hence, we compared an imidazolium-, a pyrrolidinium-
, and a phosphonium-based IL under similar conditions.
Interestingly, the activity and selectivity of the system was
highly dependent on the ionic liquid used. A significant
increase in selectivity was obtained using 1-butyl-1-
methylpyrrolidinium dicyanamide ([Bmpyrr][N(CN)2]) (Table 6,
entries 1 and 2) compared to 1-butyl-3-methylimidazolium
triflate ([Bmim][OTf]) (Table 6, entries 4 and 5), independently
from the CuPS used. In particular, almost equal TON(CO) and
Figure 4. Orbital characteristics of the (a) HOMO and (b) LUMO
of [Cu(bcp)(xantphos)]+ and of the (c) HOMO and (d) LUMO of
[Cu(bcp)(POP-xantphos)]+.
Photocatalysis
Our photocatalytic investigations started using the well-
TON(H2) were observed for
6, entry 4) and a significant overproduction of hydrogen was
observed when in [Bmim][OTf] was used (Table 6, entry 5).
This undesired selectivity towards hydrogen formation was
2 in [Bmim][OTf] as solvent (Table
defined photosensitizers
1 and 2 in combination with the iron
catalyst and N-methyl-2-pyrrolidone (NMP) as molecular
I
1
solvent (Table 5). Under these reaction conditions a TON (CO)
of 427 was obtained with high selectivity toward CO (≥ 98%)
when using the well-defined PS
TON(CO) was observed when using
decrease in selectivity (Table 5, entry 2).
Table Iron photocatalyzed CO2 reduction with
1
(Table 5, entry 1).37 A higher
even more pronounced when using
(tributylmethylphosphonium
1 in [Tbmp][NTf2]
2
, despite a marginal
bis(trifluoromethylsulfonyl)imide) (Table 6, entry 6).
Interestingly, similar selectivity towards H2 production was
found when [Bmpyrr][NTf2] was utilized (Table 6, entry 3). This
result contrasts with the high CO selectivity obtained when
utilizing the dicyanamide analogue of [Bmpyrr]. While we
cannot fully exclude an impact of the potentially coordinating
5
.
[Cu(N^N)(P^P)][PF6] complexes as PS in molecular solvent
systems.
−
N(CN)2 on the catalysts, the difference in selectivity could
result from the biphasic reaction system resulting from non-
−
miscibility of the NTf2 -based ILs and TEOA. In summary,
[Bmpyrr][N(CN)2] was found to be the most suitable IL to
selectively produce CO and
1 was identified as the best
photosensitizer both in terms of activity (TON(CO) = 77) and
selectivity (≥92%, Table 6, entry 2). Importantly, the in-situ
formation of the CuPS is ineffective when using ILs as solvents
(Table 6, entry 7), in contrast to the outcome noticed in
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organic solvents. Finally, the production of CO varies only single experiments can be found in Table S2 in the ESI). TONs
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marginally by addition of a second equivalent of CuPS 1
, which of HCO2H: Entry 3 = 5, entry 4 = 6, entry D5O=I:1101.1a0n3d9/De0nGtrCy0616=279F
shows that the catalyst deactivates over longer reaction times were observed by 1H NMR spectroscopy using benzene as
a
as it has been stated previously37 (Table 6, entry 8).
internal standard. Note: The IL was not miscible with TEOA
resulting in a biphasic reaction mixture. bResult of a single
reaction. cAn in situ CuPS system consisting of
[Cu(CH3CN)4][PF6] (5.0 μmol), bathocuproine (5.1 μmol), and
Catalyst deactivation impacts the long term stability and
recyclability of our reaction system. To achieve a fully
recyclable system the stabilities of catalyst and CuPS require
further enhancement. For Cu(N^N)(P^P)+, better interlocking
caused by ligand-ligand interactions and/or supported
complexes could enhance the stability of the catalytic system
in ILs. Deactivation, specifically of the CuPS remains a
d
xantphos (15 μmol) has been utilized. A second equivalent of
CuPS
1 (5.0 μmol) in *Bmpyrr+*N(CN)2] (1.0 mL) was added
after 5.5 h and the reaction illuminated for a total 23 h.
eSelectivity towards CO.
significant challenge in this research area. Recently, Lin and co- The only system described in the literature that reduces CO2 to
workers have tackled the stability drawbacks of copper(I) CO in pure ionic liquid is based on fac-[ReCl(bpy)(CO)3]and 1-
photosensitizers by integration of the Cu(N^N)(P^P)+ complex butyl-1-methylpyrrolidinium
tetracyanoborate
into a metal-organic framework.66 Further investigations on ([Bmpyrr][TCB]), using supercritical scCO2 where CO2 acts both
the applicability of these Cu-modified MOFs might eventually as solvent and reactant in order to avoid substrate solubility
enable
reduction reactions.
Table Iron photocatalyzed CO2 reduction with
[Cu(N^N)(P^P)][PF6] complexes as PS in different ionic liquids.
a
recyclable IL-based system for photocatalytic problems.43 This system showed dramatically lower TON´s
between 0.7 and 5 when compared with our respective values.
Moreover, the use of an ionic liquid in combination with an
organic solvent was reported to enhance the intrinsic activity
of a photocatalytic system based on [Ru(bpy)3Cl2] as the
photosensitizer and CoCl2·6H2O as the catalyst. The chosen
6
.
ionic
liquid
was
1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([Emim][NTf2]), obtaining a
TON(CO) of 34.7 and a selectivity of 76%, using water in the
reaction mixture.42 Both values are lower than the ones
achieved with our system. Unfortunately, the authors did not
report the activity in pure ionic liquid. Nevertheless, besides
the higher catalytic activity of the system reported herein, the
sole utilization of earth-abundant copper and iron render this
system interesting. Notably, these results indicate the
possibility of transferring established photocatalytic CO2
reduction protocols from organic solvents into ionic liquids.
Entry Complex
Solvent
TON TON
(CO) (H2)
Sel.e
[%]
1
2
2
1
1
2
1
1
1
1
[Bmpyrr][N(CN)2]
/TEOA(4:1, v:v)
[Bmpyrr][N(CN)2]
/TEOA(4:1, v:v)
[Bmpyrr][NTf2]
/TEOA (1:1, v:v)
[Bmim][OTf]
29
77
60
≤3
≥82
≥92
15
≤3
Control experiments were performed in order to confirm the
necessity of all the components used in the aforementioned
protocol (Table 7). First, irradiating the ionic liquid in
combination with the sacrificial reductant and the base (BIH
and TEOA) in the presence of CO2 produced only traces of CO
and H2 (Table 7, entries 1 and 2). Similar results were obtained
3a
4
338
171 171
49
using either only I (Table 7, entry 3) or CuPS (Table 7, entry 4).
/TEOA (4:1, v:v)
[Bmim][OTf]
Therefore, all the components need to be present in order to
effectively convert CO2, which does not proceed in the
absence of light, confirming its photochemical nature (Table 7,
entry 5). Finally, replacing CO2 with argon atmosphere did not
yield CO at the end of the reaction (Table 7, entry 6)
confirming that this methodology forms CO from CO2 and not
from decomposition of the catalyst or other components of
the reaction system.
5b
6a
7b,c
8d
60
60
1
124
274
≤3
30
/TEOA (4:1, v:v)
[Tbmp][NTf2]
17
/TEOA (4:1, v:v)
[Bmpyrr][N(CN)2]
/TEOA (4:1, v:v)
[Bmpyrr][N(CN)2]
/TEOA (4:1, v:v)
14
Table 7. Blank reactions for evaluation of the necessity of the
components of the catalytic system.
79
≤3
≥92
Entry
[Fe]
*μmol+
[CuPS]
*μmol+
TON
(CO)
TON
(H2)
TON
(HCO2H)
Reaction conditions: [Fe] (1 μmol), *Cu(N^N)(P^P)+*PF6]
(5.0 μmol), BIH (150 mg) and TEOA (1.5 mL) were illuminated
at 400-700 nm (1.50 W) under 1 atm of CO2 in 6.0 mL of
solvent for 5 h. Unless stated otherwise results presented are
the averages of reactions carried out in duplicate (results of
1a
2
-
-
-
-
≤1
≤1
≤3
≤3
≤3
≤3
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3
4
5b
6c
1
-
-
≤1
≤1
≤1
≤1
≤3
≤3
≤3
≤3
≤3
Conclusions
View Article Online
DOI: 10.1039/D0GC01627F
5
5
5
≤3
≤3
≤3
Ionic liquids proved to be a suitable reaction media for the
photocatalytic CO2 reduction to CO using
a copper(I)
1
1
photosensitizer and an iron catalyst. High selectivity for CO
and moderate turnover numbers were achieved. Overall our
results show that the presented system is competitive
compared to previously reported ones with rhenium43 or
ruthenium and cobalt42 in ionic liquid media for the reduction
of CO2 to CO. Moreover, this work indicates the possibility of
transferring photocatalytic processes mediated by earth-
abundant metals from an organic solvent to an ionic liquid.
Using high-purity conditions, the origin of produced CO was
proven to be directly related to the presence of CO2 in the
reaction mixture. In reference measurements under visible
light irradiation in inert gas, the components of the reaction
mixture appeared to be stable, and no carbon-containing
compounds were detected in the gas phase. In addition, a new
Reaction conditions: [Fe] and
the table, BIH (150 mg) and TEOA (1.5 mL) were illuminated at
400-700 nm (1.50 W) in 6.0 mL of 1-butyl-1-
methylpyrrolidinium dicyanamide for 5 h. Results presented
were obtained from single experiments. a1-Butyl-3-
methylimidazolium triflate (6.0 mL) was applied as the solvent.
bReaction conducted in the absence of light. cReaction
performed under an atmosphere of argon.
2 [Cu(N^N)(P^P)]PF6 as stated in
To further confirm that the CO produced originated from CO2,
the sample, containing Fe complex
I (0.13 mM), CuPS 1
(0.72 mM), BIH (30 mg), and TEOA (0.30 mL) in 0.90 mL of 1-
butyl-1-methylpyrrolidinium dicyanamide, was introduced to a
high-purity photocatalytic reactor and tested for CO2 reduction
in batch mode. This setup is predominantly used to test
samples in powder form (immobilized or not), but for the first
time a liquid sample was successfully tested in such reactor. To
ensure that the sample remains intact under the selected
reaction conditions and that no products are formed without
the presence of CO2 (i.e., from potential degradation of the
catalyst system), reference measurements took place under
pure (6.0) helium atmosphere (Figure 5).
CuPS
photophysical properties were examined. Albeit showing
promising properties, the well-known CuPS proved to be
more selective for the given process. Further investigations
2 was synthesized and its electrochemical and
1
regarding the applicability of CuPS
research in our laboratories.
2 are part of on-going
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
P.A.F.-C. would like to thank the Marie Skłodowska-Curie
Actions for funding; Prof. Paul C. J. Kamer for funding and
valuable discussions. M.M. is grateful to the Fonds der
Chemischen Industrie for a Kekulé fellowship. Part of this work
was funded by the German Federal Ministry of Education and
Research (BMBF, Grant No. 033RC003A, PROPHECY). P.A.F.-C.,
M.M, N.G.M., F.B., C.H., E.C., H.J., M.B. and J.S. thank Dr. Anke
Spannenberg for X-ray analysis of compound
2 and Hilario D.
Huerta for preparing one batch of complex I.
Figure
5
:
Iron photocatalyzed CO2 reduction with
as PS: Production of CO over 6
[Cu(N^N)(P^P)][PF6] complex
1
Notes and references
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