Site Isolation Leads to Stable Photocatalytic Reduction of CO2 over a Rhenium-Based Catalyst

Article abstract of DOI:10.1002/chem.201502796

A porous organic polymer incorporating [(α-diimine)Re(CO)₃Cl] moieties was produced and tested in the photocatalytic reduction of CO₂, with NEt₃ as a sacrificial donor. The catalyst generated both H₂ and CO, although the Re moiety was not required for H₂ generation. After an induction period, the Re-containing porous organic polymer produced CO at a stable rate, unless soluble [(bpy)Re(CO)₃Cl] (bpy=2,2′-bipyridine) was added. This provides the strongest evidence to date that [(α-diimine)Re(CO)₃Cl] catalysts for photocatalytic CO₂ reduction decompose through a bimetallic pathway.

Full text of DOI:10.1002/chem.201502796

DOI: 10.1002/chem.201502796
Communication
&
Photocatalysis
Site Isolation Leads to Stable Photocatalytic Reduction of CO2
over a Rhenium-Based Catalyst
Weibin Liang,[a] Tamara L. Church,[a, b] Sisi Zheng,[b] Chenlai Zhou,[b] Brian S. Haynes,[b] and
Deanna M. D’Alessandro*[a]
gand[7c] to yield a ligand-centered [(bpy)Re(CO)3X]* excited
Abstract: A porous organic polymer incorporating [(a-di-
imine)Re(CO)3Cl] moieties was produced and tested in the
state that abstracts an electron from a sacrificial donor (usually
NEt3 or N[(CH2)2OH]3) to form the ligand-centered radical anion
photocatalytic reduction of CO2, with NEt3 as a sacrificial
[(bpy)Re(CO)3X]·À.[7a,f] When synthesized independently,[7m,8] this
donor. The catalyst generated both H2 and CO, although
anion forms a ReÀRe-bonded dimer, [{(bpy)Re(CO)3}2]. For X=
the Re moiety was not required for H2 generation. After
ClÀ, dimer formation is slow on the timescale of photocataly-
an induction period, the Re-containing porous organic
sis.[8b] Dimerization is also slow, but photochemically reversible,
polymer produced CO at a stable rate, unless soluble
for the neutral radical [(dmb)Re(CO)3(thf)]· (dmb=4,4’-dimeth-
[(bpy)Re(CO)3Cl] (bpy=2,2’-bipyridine) was added. This
yl-2,2’-bipyridine).[7g] In the presence of CO2, that radical can
provides the strongest evidence to date that [(a-diimi-
form a CO2-bridged dimer, [{(dmb)Re(CO)3}2(m-CO2)], in a reac-
ne)Re(CO)3Cl] catalysts for photocatalytic CO2 reduction
tion that is slow[7g] but exothermic.[7o] The metallo-acid
decompose through a bimetallic pathway.
[(dmb)Re(CO)3(CO2H)], believed to be an intermediate in the
photocatalytic reduction of CO2 by [(dmb)Re(CO)3Cl],[7q] forms
[{(dmb)Re(CO)3}2(m-CO2)] under laboratory lighting.[7j] The close-
CO2 mitigation is currently a crucial challenge. The conversion
of CO2 into chemical feedstocks offers the opportunity both to
produce chemicals from a ‘greener’ source than fossil fuels and
to reduce the amount of CO2 in the atmosphere.[1] If solar
energy can be used to drive the reaction, the result is also
a means of storing solar energy and the attractiveness of pho-
tochemical CO2 fixation has inspired significant research into
“artificial leaf” systems.[2] Lehn and co-workers’ seminal reports
that [(bpy)Re(CO)3Cl] (bpy=2,2’-bipyridine) catalyzed the
photo-[3] and electrochemical[4] reductions of CO2 to CO in the
presence of triethanolamine (N[(CH2)2OH]3) have led to a vast
body of related work that has been discussed in several recent
reviews.[5] A particular drawback of [(a-diimine)Re(CO)3Cl] pho-
tocatalysts is that they lose activity over the course of CO2 re-
duction and, although this can be mitigated by excess halide
ion[3] or high CO2 pressure,[6] stability remains an issue.
ly related [(bpy)Re(CO)3(CO2H)], an intermediate in the electro-
catalytic reduction of CO2 by [(bpy)Re(CO)3Cl],[7i,p] has been ob-
served concurrently with [{(bpy)Re(CO)3}2] during that reac-
tion.[7b] Bimetallic complexes like these have been proposed to
contribute to CO formation (although a parallel monometallic
pathway is also necessary to explain the greater rate of cataly-
sis than dimer formation),[7g,o,8b] and to catalyst decomposi-
tion.[7m] In the light, bimetallic [{(dmb)Re(CO)3}2(m-CO2)][7g]
reacts with CO2 to release <1 equivalent of CO, but it is not
clear whether the Re-containing products of the reaction could
participate in the catalytic cycle under appropriate conditions.
Given that ReÀRe-bonded dimers are, at best, minor contrib-
utors to photocatalytic CO2 reduction and could be intermedi-
ates in catalyst decomposition, an [(a-diimine)Re(CO)3X] com-
plex in which ReÀRe interactions are inhibited could be a more
stable catalyst for CO2 reduction than [(bpy)Re(CO)3Cl]. Consis-
tent with this, the bulky complex [(4,4’-tBu2-2,2’-bpy)Re(CO)3Cl],
which does not form measurable amounts of ReÀRe-bonded
dimer under spectroelectrochemical conditions, is a more
stable electrocatalyst for CO2 reduction than [(bpy)Re(CO)3Cl],
which does.[7d] [(a-diimine)Re(CO)3Cl] units have been incorpo-
rated into a metal–organic framework (MOF) based on the
UiO-67 structure and indeed generated CO from the photoca-
talytic reduction of CO2 over a longer period than the small-
molecule catalyst [(a-diimine)Re(CO)3Cl] did.[9] The photocata-
lytic activity of the Re-containing MOF catalyst did decay even-
tually, although this may have occurred by a bimetallic decom-
position pathway, as more than 40% of the Re leached into so-
lution after 20 h under the reaction conditions. Porous organic
polymers (POPs) incorporating [(a-diimine)Re(CO)3Cl] units
have been described, but their photocatalytic activity in CO2
reduction was not reported.[10] Like MOFs,[9b] POPs can harvest
Mechanistic data regarding CO2 reduction over [(bpy)Re-
(CO)3Cl] and related catalysts have been assembled[3,7] and, al-
though more information is available regarding the electroca-
talytic reaction, some details of the photocatalytic version are
known. Upon absorbing a photon, [(bpy)Re(CO)3X] (X=Cl, Br)
undergoes charge transfer from the ReÀX bond to the bpy li-
[a] W. Liang, Dr. T. L. Church, Dr. D. M. D’Alessandro
School of Chemistry, The University of Sydney
Sydney NSW 2006 (Australia)
E-mail: deanna.dalessandro@sydney.edu.au
[b] Dr. T. L. Church, Dr. S. Zheng, Dr. C. Zhou, Prof. Dr. B. S. Haynes
School of Chemical and Biomolecular Engineering
The University of Sydney
Sydney NSW 2006 (Australia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502796.
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Communication
light,[11] and permit the migration of excitons.[12] We thus syn-
thesized an [(a-diimine)Re(CO)3Cl]-containing POP and tested it
in photocatalytic CO2 reduction.
POP 1 was prepared by Sonogashira–Hagihara coupling[13]
and metalated by heating in refluxing methanol with an
excess of [Re(CO)5Cl] to give 1-Re (Figure 1; synthesis and char-
acterization details are given in the Supporting Information).
POP
1 had a
Brunauer–Emmett–Teller[14] surface area of
660 m2 gÀ–1 (see the Supporting Information, Figure S4 and
Table S2), similar to that of a bipyridine-containing POP report-
ed by Cooper and co-workers,[10] who used a 1,3,5-trialkynyl
core rather than the core A shown in Figure 1. However, the
surface area of 1 fell by less than one fifth upon metalation,
whereas the reported polymer lost approximately half of its
surface area. Thus the larger core used here may have pro-
duced
a porous polymer better able to accommodate
Figure 2. Photocatalytic production of CO and H2 over 1-Re
(2.27”10À2 mmol Re) and 2 (2.17”10À2 mmol Re) in CO2-saturated CH3CN/
{Re(CO)3Cl} moieties. Indeed, the greatest pore volume in
1 was encapsulated in pores 17 Š in size (Table S2), compared
to approximately 10 Š in the reported polymer.
NEt3 (180:20 v/v). CO2 (35 mLminÀ1) was bubbled for 1 h prior to irradiation.
To act as a photosensitizer and catalyst, an excited [(a-diimi-
ne)Re(CO)3Cl] complex must react with an electron donor,[7e]
and the Stern–Volmer kinetics[15] for the reaction of NEt3 with
photoexcited 1-Re or [(bpy)Re(CO)3Cl] (2) were therefore mea-
sured in N2-purged CH3CN (see the Supporting Information,
Figure S8a). The Stern–Volmer slope KS–V measured for 2 was
porous polymers (CMPs) with optical band gaps in the range
of 1.94–2.95 eV. These materials acted as photocatalysts for hy-
drogen evolution in the presence of a sacrificial electron
donor.[17] Similarly, 1-Re also produced H2, and its rate of pro-
duction mirrored that of CO. As no efforts were made to dry
the CH3CN solvent or NEt3 donor, the photocatalytic reduction
of adventitious H2O was the likely source of H2 (no H2 was pro-
duced in the absence of catalyst). Consistent with this, CO2
was not required for H2 generation; rather, H2 was produced at
a slightly greater rate upon irradiation of an argon-saturated
solution of 1-Re (see the Supporting Information, Figure S9).
However, if the CH3CN solvent and NEt3 donor were both dried
over CaH2 prior to photocatalysis, the reaction yielded no CO,
and only a trace of H2 (see the Supporting Information, Fig-
ure S11). This not only supports the hypothesis that H2 is
formed from adventitious H2O in the photocatalytic reduction
of CO2 by 1-Re, but also reveals that H2O is required for CO for-
mation in this system. The molecular catalyst 2 can photocata-
lyze H2O reduction in the absence of CO2, but is highly selec-
tive for CO2 when the latter is available.[7f] DFT calculations on
the reaction of an [(a-diimine)Re(CO)3]À anion have rationalized
this selectivity in terms of the formation of a s and a p interac-
tion with CO2.[7i] The latter involved the delocalized HOMO of
the anion, which had contributions from both ligand and
metal orbitals.[7i,l] As computations of the radical [Re(bpy)(CO)3]·
have indicated a similarly delocalized structure for its SOMO,[18]
the high selectivity of [(a-diimine)Re(CO)3Cl] complexes for CO2
rather than H2O reduction is justified regardless of whether the
photocatalytic reaction occurs from a singly reduced radical or
doubly reduced anion, which led us to doubt that the forma-
tion of H2 during the photoreduction of CO2 over 1-Re oc-
curred at the Re centers.
2.3mÀ1, in the same range as values reported for the reaction
[7a,e,f]
of [(bpy)Re(CO)3X] with N[(CH2)2OH]3
or NEt3.[16] For meta-
lated POP 1-Re, KS–V =1.7mÀ1, albeit with a much poorer linear
fit. Nevertheless, the rates of reaction of photoexcited 2 and
photoexcited 1-Re with NEt3 were comparable in order.
The photocatalytic (l>300 nm; lmax =535 nm) activity of
1-Re toward CO2 was tested in CO2-saturated CH3CN/NEt3
(9:1 v/v) and compared to the same reaction catalyzed by 2
(Figure 2). Catalyst 1-Re formed CO slowly at first, but reached
a stable rate of CO generation (that was still slower than CO
formation from 2) after approximately 100 min. Recently,
Cooper and co-workers reported a series of conjugated micro-
In the presence of CO2 and H2O, mobile electrons photogen-
erated in semiconductors can participate in H2 generation,
whereas a separate transition-metal-based catalyst produces
CO. Kubiak and co-workers described the light-assisted co-gen-
eration of CO and H2 over p-Si and [(4,4’-tBu2-2,2’-bpy)Re-
Figure 1. Structures, scanning electron micrographs, and digital images of
1 (left) and 1-Re (right).
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Communication
(CO)3Cl] in wet acetonitrile;[19] H2 was formed directly at the
surface of the semiconducting Si even when no Re complex
was present. Maeda and co-workers reported a porous semi-
conducting C3N4 and a Ru complex that combined to photoca-
talyze the reduction of CO2 to H2, CO, and HCO2H in CH3CN/
N((CH2)2OH)3.[20] In the case of 1-Re, ligand 1 was a likely
source of mobile electrons that did not reside (at least in part)
on Re, as the ligand alone absorbed UV and visible light (see
the Supporting Information, Figure S5). Luminescence from
1 was most intense upon irradiation at 270 nm (see the Sup-
porting Information, Figure S6); however, 1 was somewhat lu-
minescent when irradiated with near-UV light. Thus, when sus-
pensions of 1 in CO2- and Ar-saturated CH3CN/NEt3 were irradi-
ated (lꢀ300 nm), H2 (but not CO) was generated (see the Sup-
porting Information, Figure S10). Thus, although adventitious
H2O is the likely source of H2, it could apparently not react di-
rectly with photoexcited 1-Re, as no H2 was formed when 1-Re
was irradiated in CO2-saturated CH3CN without NEt3 (see the
Supporting Information, Figure S9). Similarly, the reported
C3N4–Ru system did not produce H2 in the absence of an
amine.[20]
Figure 3. Effect of adding 2 during the photocatalytic reduction of CO2 over
1-Re in NEt3/CH3CN.
was due to soluble [(a-diimine)Re(CO)3Cl] analogues. Consis-
tent with this, the induction period was observed even for the
formation of H2 upon irradiation of Re-free 1 in CO2-saturated
NEt3/CH3CN.
Overall, 1-Re was a photocatalyst for the generation of
syngas (H2/CO=1.4–2.2) under the reaction conditions. Both
H2 and CO were generated slowly at first, with the rates of
their generation increasing over time. Whereas the rate of CO
formation over 1-Re was highly stable after 100 min [d(CO)/
To test our hypothesis that the porous structure of 1-Re con-
ferred stability by preventing interactions between Re centers,
we performed an additional test (Figure 3). The photocatalytic
reduction of CO2 over 1-Re was repeated as before for around
2 h. Then, 20 mg of 2 was added. Initially, both CO and H2 pro-
duction spiked, but both declined quickly. Importantly, the rate
of CO production fell below the level seen before 2 was
added. In fact, after a total of 325 min (206 min after 2 was
added), CO production ceased completely. Thus the introduc-
tion of a soluble [(a-diimine)Re(CO)3Cl] catalyst for the photo-
catalytic reduction of CO2 actually inactivated the immobilized
catalyst 1-Re. H2 formation, in contrast, stabilized again at
a rate similar to that observed before 2 was added, demon-
strating that both the photoexcitation of the unmetalated con-
jugated polymer 1 and electron abstraction from the sacrificial
donor NEt3 continued unabated. Only the Re catalyst, which
coordinates and reduces the CO2 molecule, was inactivated,
suggesting that the deactivation usually observed in [(a-diimi-
ne)Re(CO)3Cl]-type catalysts is frustrated in 1-Re until a soluble
small-molecule analogue is added. This is, to our knowledge,
the most unambiguous demonstration to date that [(a-diimi-
ne)Re(CO)3Cl] catalysts for photocatalytic CO2 reduction de-
compose via a bimetallic pathway. Thus, although 1-Re shows
lower initial activity for the photocatalytic reduction of CO2 in
the presence of a sacrificial donor than the soluble catalyst 2
does, the isolation of the Re active sites within the POP yielded
a more stable catalyst. Furthermore, POP 1 itself was a photoca-
talyst for H2 generation (presumably from H2O reduction), so
1-Re catalyzed the photocatalytic generation of H2 and CO. Ad-
ditional experiments will be required in order to understand
(and potentially control) the relative rates of H2 and CO forma-
tion.
À1
dt=4.6Æ0.5 mmolminÀ1 mmolRe over t=110–1200 min], that
of H2 actually increased slightly, causing the ratio of H2 to CO
in the gas phase to increase over time (Figure 2, inset). Al-
though the source of the observed induction period for photo-
catalyst 1-Re is still being investigated, the similar profiles of
H2 and CO formation suggest that a single phenomenon ac-
counts for both. Especially interesting, however, was the very
stable rate of CO generation by 1-Re. This was in contrast to
the case of 2, over which the rate of CO production quickly
reached a peak before declining significantly. After 2 h, cata-
lysts 1-Re and 2 were producing CO at similar rates, but CO
generation over catalyst 2 had nearly ceased after 4 h. Thus it
was clear that 1-Re was a more stable photocatalyst for CO2 re-
duction than 2. Over 20 h, catalyst 1-Re yielded 5.0 mol CO
and 8.9 mol H2 per mol Re (i.e. TONCO =5, TONH2 =8.8; Fig-
ure S13). Therefore, the slower but more stable 1-Re produced
more CO than 2 had before losing activity (TONCO over 2 was
3.5).
The reaction mixture from CO2 reduction over 1-Re was
sampled after 60 and 120 min and the aliquots were analyzed
by inductively coupled plasma (ICP) and optical emission spec-
troscopy detection. Each contained around 7 ppm Re, which
represents roughly a third of the Re originally present in 1-Re.
However, this Re was not photocatalytically active; when the
catalyst was used for 120 min, then filtered from solution, the
filtrate was not an active photocatalyst for CO2 reduction, al-
though it produced a small amount of H2 upon irradiation (see
the Supporting Information, Figure S14). Therefore, CO produc-
tion over 1-Re was stable despite the loss of some Re to solu-
tion. Moreover, the slow build-up of catalyst activity in 1-Re
could not be explained by assuming that all catalytic activity
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Experimental Section
POP 1 was prepared by Sonogashira–Hagihara cross-coupling.[13]
1,3,5-Tri-(4-ethynylphenyl)benzene (378.5 mg, 1.0 mmol), 5,5’-dibro-
mo-2,2’-bipyridine (314 mg, 1.0 mmol), and CuI (10 mg) were dis-
solved in DMF (4 mL) and NEt3 (4 mL). The mixture was stirred and
bubbled with N2 for 30 min. Thereafter, tetrakis(triphenylphosphi-
ne)palladium(0) (20 mg) was added, and the reaction mixture was
heated under N2 at 908C for 72 h. The mixture was cooled to room
temperature and the insoluble polymer was removed by filtration
and washed with CHCl3 (3”5 mL), H2O (3”5 mL), MeOH (3”5 mL),
and acetone (3”5 mL), then washed with MeOH (5 mL) in a Soxhlet
apparatus for 24 h. The product was dried under vacuum and iso-
lated as fine yellow powder in a 70% yield.
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POP 1 was metalated through treatment with an excess of
[Re(CO)5Cl] in refluxing MeOH (Figure 1).[10] 1 (200 mg) was added
to a solution of [Re(CO)5Cl] (134 mg) in MeOH (100 mL), and the
suspension was heated at reflux for 24 h. The mixture was allowed
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5 mL), and acetone (3”5 mL), then washed with CH2Cl2 in a Soxhlet
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A laboratory photochemical reactor (Heraeus) with a TQ 150 Z2
lamp (150 W medium-pressure mercury lamp, lꢀ300 nm centered
at 535 nm) and Duran 50 immersion tube (cuts off light at l<
300 nm) was used for all photochemical experiments. For each
test, CCH3N (180 mL) was mixed with NEt3 (20 mL) before the cata-
lyst (10 mg 2, 23 mg 1, or 30 mg 1-Re) was added. CO2 or Ar
(35 mLminÀ1) was introduced in the absence of light and bubbled
for 1 h to saturate the system before irradiation commenced. The
gas flow (35 mLminÀ1 CO2 or Ar) and vigorous magnetic stirring
were maintained while the reaction was monitored for 20 h,
during which the gaseous products were analyzed by GC approxi-
mately every 30 min to determine the amount of CO and H2. De-
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Acknowledgements
The authors are grateful to Dorothy Yu of the University of
New South Wales Mark Wainwright Analytical Centre for ele-
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Science and Industry Endowment Fund.
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Keywords: carbon dioxide · photocatalysis · porous organic
polymers · reduction · rhenium
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Published online on November 5, 2015
Chem. Eur. J. 2015, 21, 18576 – 18579
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