An integrated Re(i) photocatalyst/sensitizer that activates the formation of formic acid from reduction of CO2

Article abstract of DOI:10.1039/c9cc03943k

The complex cis-[Re(bpy)₂(CO)₂]⁺OTf^- (1⁺OTf^-) is an integrated photosensitizer/catalyst for the selective visible light promoted photocatalytic reduction of CO₂. The formation of formic acid is unique among this class of Re catalysts, which yield CO as the selective product. A supplemental photosensitizer, Ru(bpy)₃²⁺, considerably enhanced the performance of this catalyst.

Full text of DOI:10.1039/c9cc03943k

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An Integrated Re(I) Photocatalyst/Sensitizer that Activates the
Formation of Formic Acid from Reduction of CO2
Yasmeen Hameed, Patrick Berro, Bulat Gabidullin, Darrin Richeson*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The complex cis-[Re(bpy)2(CO)2]+OTf- (1+OTf-) is an integrated yet realization of this goal remains rather limited.17,23–25 The
photosensitizer/catalyst for the selective visible light promoted expansion of ligand scaffolds has recently revealed Re(I)
photocatalytic reduction of CO2. The formation of formic acid is complexes with N-heterocyclic carbene (NHC) ligands that
unique among this class of Re catalysts, which yield CO as the display the amalgamation into a single species of CAT and PS for
2+
selective product. A supplemental photosensitizer, Ru(bpy)3
considerably enhanced the performance of this catalyst.
,
the reduction of CO2 to CO.26,27 Ligand variation led to discovery
of a new Ru complex that functions as both PS and CAT.17
With the objectives of discovering new environments for CO2
photoreduction catalysts, revealing new integrated
photosensitizer/catalyst species and potentially uncovering
new reaction pathways for this transformation we targeted the
preparation of the bis(-diimine) Re carbonyl complexes. We
were stimulated to pursue these targets based on the known
catalytic abilities of the mono(-diimine) species (e.g.
[Re(bpy)(CO)3X]) as well as the reported observation of the
The combination of addressing issues of sustainable energy
with the environmental concerns regarding emission of
greenhouse gases provides a forceful impetus to target the
efficient chemical reduction of CO2. This fundamentally
challenging goal requires an input of energy to yield useful
products. Photocatalysis is a particularly appealing approach to
surmount the reaction barriers since it employs light, a clean
and unlimited resource, for conversion of this waste by-product
into a feedstock. Formic acid is an especially interesting two-
electron reduction product of CO2 since it is an important
related proficiency of the mono- and bis(-diimine) Ru species,
28–30
cis,trans-Ru(NˆN)(CO)2Cl2
and cis-[Ru(NˆN)2(CO)2]2+ in
catalysis.31–34 We now provide the first documentation of this
coordination environment as an entirely new photocatalyst
with integrated photosensitizer ability for CO2 reduction as well
as revealing a notable change in a switch of product selectivity
from CO to HCO2H.
commodity chemical and
a
potential carrier of H2.1,2
Identification of catalysts that can use light for this chemical
transformation is critical for success.3–5 Generally,
photocatalytic systems have three integrated core components:
a photosensitizer (PS), for collecting the photon energy; an
electron donor (ED), that provides electrons for the reduction;
and a catalyst (CAT) that is a site for the conversion of CO2.
More than 30 years ago, the pioneering discovery that fac-
Re(bpy)(CO)3X (bipy = 2,2’-bipyridine, X = halide) complexes
were photocatalysts for the selective reduction of CO2 to CO
using triethanolamine (TEOA) as an electron donor established
the ongoing exploration of related fac-[Re(- diimine)(CO)3X]
catalysts and these complexes remain as touchstones for this
field.6–12 The central role of group 7 complexes in this field
continued with the report that the manganese analogue fac-
A
literature survey revealed that a target complex,
[Re(bpy)2(CO)2]+OTf- (1+OTf-) was reported.35 The preparation
involved the unusual conditions of using a melt of 2,2’-
bipyridine as the solvent at T ≈ 275°C. This reflects the stability
of the fac-[Re(bpy)(CO)3X] and is consistent with the dominance
of the mono-ligated compounds in Re(I) chemistry. Our success
with the synthesis of 1+OTf- in 83% yield was demonstrated
through the spectroscopic features of our product and
microanalysis (eqn 1). In particular, the 1H and 13C NMR spectra
displayed the expected signals for the C2 symmetric structure.
Although some fundamental physical characterization of 1+OTf-
was known, no reactivity is reported.
2+
[Mn(bpy)(CO)3Br] was, with a Ru(bpy)3 photosensitizer,
competent for photocatalytic CO2 reduction.13–16 The fac-
Re(bpy)(CO)3X catalysts are particularly unique in their ability to
function as integrated PS and CAT, thus allowing light
absorption and catalysis with one species.6,17–22 These features
continue to drive the vigorous search for PS integrated catalysts
Fortunately, we obtained crystals of cis-[Re(bpy)2(CO)2]+OTf-
(1+OTf-) and single crystal X-ray analysis definitively confirmed
the metrical parameters of this complex ( eqn 1, Figure S1). As
proposed, the cationic Re(I) species displayed a distorted
octahedral geometry with the C2 symmetry of the cis ligand
Department of Chemistry and Biomolecular Sciences, Centre for Catalysis Research
and Innovation
University of Ottawa, 10 Marie Curie, Ottawa, ON K1N 6N5
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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array and a distal triflate counter ion. The Re-C distances were The first attempts at photocatalytic reduction of CO2
+
-
identical (1.883(5) Ǻ) and two slightly different Re-N distances performed with no added photosensitizDeOr,I:410.m10o3l9/oCf9C1CO03T9f43inK
of 2.121(3) Ǻ (Re-N1, trans to N) and 2.165(3) Ǻ (Re-N12, trans DMA using TEOA as the electron donor. Irradiation of this
to CO) were observed. The largest distortion from orthogonality reaction mixture was carried out under an atmosphere of CO2
of the angles around the Re center arise from the restricted bite with visible LED light (405nm, 1050 mW at 700mA, 3.4 x 10-8 mol
angle of the bpy with the observed N-Re-N angle of 75.15(13)°. photons/sec) for 24 hr. GC analysis of the reaction headspace
We found only one reported analogue of this complex, the revealed only substoichiometric (2-4mol) amounts of CO,
bis(1,10-phenanthroline) complex cis-[Re(phen)2(CO)2]+OTf- which we attribute as arising from loss of CO from 1+. The H
and this species displayed very similar metrical features.35
1
NMR spectrum of the reaction solution disclosed that the only
product was formic acid, which was quantified by NMR analysis
(Figure S12). These observations confirmed that the complex
O13
C13
O13’
C13’
Re
cis-Re(CO)2(bipy)2 OTf- was a photocatalyst for reduction of CO2
+
2,2'-bipyridine
MELT
(i) 2,2'-bipyridine
N1
OTf
Re(CO)5Cl
N1’
(1)
to formic acid. (Table 1, Table S2). Interestingly, 1+OTf-
displayed similar catalytic activity in DMF but no products were
observed when CH3CN was used as solvent (Table S2). Table 1
also provides a comparison of selected benchmark Re-based
catalysts along with our results for 1+OTf-. Notably, care is
required when comparing the reaction parameters. For
example, in Table 1, the turnover frequency (TOF) removes the
time dependence of the TON, allowing a more direct
comparison of catalyst activities.
toluene, 
(ii) AgOTf/CH2Cl2
N2
N2’
1+OTf-
Before applying 1+OTf- as a photoreduction catalyst, some
fundamental physical chemistry characterizations using cyclic
voltammetry, DFT optimization and UV-vis electronic spectral
characterization were performed. Under reducing potentials we
confirmed that, in acetonitrile, 1+OTf- exhibited reversible one
electron reductions at E1/2 = -1.67 V and -1.90 V vs Fc/Fc+
(Figures S2, S6-S8).35 Both reduction peaks showed a linear
dependence of peak current on the square root of the scan rate
(ν1/2) indicating freely diffusing species (Figures S3—S5). An
irreversible reduction at a more negative potential of -2.63 V
was also observed. Furthermore, the two reduction potentials
clearly shift to less negative potentials as the solvent is changed
to dimethylacetamide (DMA) and DMF. Importantly, the two
reduction potentials further shifted to E1/2 values of -1.46 V and
-1.70 V when TEOA is added to DMF solutions (Figure S8). This
Three key points are notable. First, complex 1+OTf- contributes
a wholly new geometry to photocatalysis with Re. Second, this
compound displayed activity in the absence of an added
photosensitizer which was similar to reported mononuclear -
diimine Re(I) catalysts under visible light. Finally, in contrast to
reported Re catalysts, which produce CO as the reduction
product, CO2 reduction with complex 1+OTf- led selectively to
formic acid.11,36
By adding the photosensitizer [Ru(bpy)3]2+ to this active catalyst
system the plan was to improve the catalytic behaviour of
1+OTf- by taking advantage of the longer lived excited state
(1100 ns37) for [Ru(bpy)3]2+ and increasing the light absorption
of the catalyst system in the visible region (see Figure S10). A
similar strategy was employed to increase the activity of
[Re(dmb)(CO)3Cl] by adding an equimolar amount of
[Ru(dmb)3]2+ as a supporting photosensitizer (dmb = 4,4-
dimethyl-2,2-bipyridine) (Table 1).38 As anticipated, this
resulted in a significant increase in production of HCO2H as
measured by TON, TOF and  and improvement of catalytic
ability was expanded across all of the three solvents, DMA, DMF
and CH3CN that were used in this investigation (Tables 1 and
S2).
is significant since TEOA is
a common ED species in
photocatalysis. DFT optimization (B3LYP, def2TZVP) of the
cationic complex 1+ yielded results consistent with the X-ray
analysis (Figure S9, Table S1). A TD-DFT analysis using
the integral equation formalism variant of the PCM model with
CH2Cl2 as solvent confirmed MLCT (d-*) absorptions at visible
wavelengths of 419, 439, 501, 503, and 544nm. Consistent with
these calculations is the appearance of the visible portion
(350nm-800nm) of the electronic spectrum of 1+OTf- in CH2Cl2
(Figure S10).
2 | J. Name., 2012, 00, 1-3
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Table 1. Comparison of the performance parameters for selected Re photocatalysts.
CAT [conc.]
Solvent/ED
PS [conc.]
 (nm) Product TON (time)
 TOF (hr-1)
[Re(bpy)2(CO)2]+ [1mM]
[Re(bpy)2(CO)2]+ [0.1mM]
Re(bpy)(CO)3Cl [0.87mM]6,18
[0.5mM]19
DMA/TEOA
DMA/TEOA
DMF/TEOA
-
405
405
365
HCO2H
HCO2H
CO
CO
CO
10 (24h)
428 (24h)
27 (4h)
15 (25h)
15 (25h)
101 (16h)
32 (4h)
1.4
5.8
8.7
16
-
0.43
17.8
6.75
0.60
0.60
6.3
Ru(bpy)32+ [1mM]
-
-
-
Re(dmb)(CO)3Cl38,(a) [0.05mM]
DMF/TEOA/BNAH
MeCN/TEA/BIH
DMF/TEOA/BIH
DMF/TEOA
365
480
Ru(dmb)32+ [0.05mM]
CO
6.2
26,(b) [0.1mM]
Re(N,S-NHC)27,(b) [0.5mM]
[Re(2-PN)(CO)3Br] 39 (c) [0.1mM]
-
>300
CO
8.0
12.8
6.8
10.2
14.3
Re(Py-NHC-PhCF3)
Ir(ppy)3
51 (4h)
-
-
≥400
≥ 480
405
CO
CO
HCO2H
102 (15h)
153 (15h)
343 (24h)
,
Ru(bpy)32+ [1mM]
4.7
(a) dmb = 4,4-dimethyl-2,2-bipyridine. (b) structures shown in Fig. S11, (c) PN = (Ph2P)NH(NC5H4)
Adding either water or phenol to the reaction mixture led to an The time profile for the photocatalytic reduction of CO2 with
increase of H2 production and decrease in formic acid (Table S4). 1+OTf- is presented in Figure 2 and display behaviour typical of
Under these conditions, with added PS, it was determined that related photocatalytic systems.29,30,38 There is a decrease in
changing the ED from TEOA to N-benzyl- 1,4- selectivity over the time (Table S3) and this may be associated
dihydridonicotinamide (BNAH), ascorbic acid, sodium ascorbate with a change in pH during the experiment.
Figure 2. Time profile for the catalytic production of HCO2H (red) and H2 (blue) using
1mol cis-Re(CO)2(bipy)2+OTf-, 1mol PS, TEOA in DMF under a CO2 atmosphere.
or triethylamine gave little or no product.Definitive
confirmation that the formic acid originated from
photocatalytic CO2 reduction was given through a 13C tracer
Based on literature precedent and the experimental
experiment that was carried out for the photoreaction with
observations, a proposed catalytic mechanism for HCO2H
1+OTf-, Ru(bpy)32+, and TEOA in DMA under a 13CO2 atmosphere.
production from the CO2 photoreduction using complexes 1+ is
HRMS analysis of the reaction headspace clearly documented
presented in Figure 3. Photoreduction catalysis couples an
selective formation of H13CO2H with less than 3% H12CO2H
electron production cycle with a catalysis cycle.28,30,40 The
(Figure S11). Furthermore, the NMR data shown in Figure 1
electron production cycle provides the electrons to the catalyst.
(Figures S12, S13) provided conclusive evidence that the formic
In the case of an integrated catalyst/photosensitizer, electrons
acid observed in these experiments arose from 13CO2, further
for the catalytic cycle can come directly from the ED through
highlighting the remarkable switch in product selectivity with
electron transfer to photoexcited 1+ (i.e. reductive quenching)
the reported production of CO from all other Re - diimine
(Figure S20). When photosensitizer is added to the reaction, the
species. 11,36
electrons for the catalytic cycle are likely produced from a
reduced photosensitizer (PS-). In addition to the two electron
reduction of CO2 this mechanism relies on a source of protons
H13COOH
that is provided by the decay reactions of oxidized TEOA (Figure
S20). The catalytic cycle represents the steps where the Re
complex, 1+, catalytically reduces CO2 using these supplied
electrons/protons. Generally, successful transformation of CO2
to formate is envisioned to proceed through a metal hydride
intermediate (ReH) that can insert CO2 leading to a formato
complex.41,42 To help guide the proposed reactions, all of the Re
compounds in the mechanism were computationally optimized
using DFT with the B3LYP functional and def2TZVP basis set
(Figures S6 and S15-S19). A likely entry into the catalysis cycle
comes from the reduction of 1+ and the two photoreduction
routes to complex A are presented in Figure S20 and
summarized in Figure 3.
HCOOH
HCOOH
Figure 1. 1H NMR spectra for the formyl proton of formic acid obtained from
photocatalytic reduction of CO2 using cis-Re(CO)2(bipy)2+OTf- as the catalyst. The bottom
spectrum is reaction with unlabeled CO2 and the top spectrum is with 13CO2.
300
200
100
0
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provides an alternate catalytic cycle, bypasses formic acid
production and rationalizes the experimental observation of H2
formation. This proposed mechanism is reminiscent of the
current mechanism invoked for conversion of CO2 using the
isoelectronic ruthenium (II) complex [Ru(bpy)2(CO)2]2+.30The
insertion of CO2 into the Re-H complex would yield the formate
complex Re(form). Closing the cycle with a proton transfer to
liberate formic acid and trapping of the Re complex by CO (or
solvent) regenerates a 1+ equivalent.
15
16
17
18
19
20
21
22
23
This report has revealed
a
functionally integrated
photosensitizer/catalyst, cis-[Re(bpy)2(CO)2]+OTf- (1+OTf-), for
the visible light photocatalytic reduction of CO2. Not only does
this complex possess a structure that is unprecedented among
Re(I) photoreduction catalysts, the selective formation of
formic acid is unique among this class of catalysts, which
uniformly yield CO as the reduction product. We further
established substantial improvements in performance
parameters and more broad application of the photocatalyst
when a PS is added. Our active goals are to expand on this class
of catalysts and interrogate the mechanism of this
transformation.
24
25
26
27
28
29
30
31
Conflicts of interest
“There are no conflicts to declare”.
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cis-[Re(bpy)2(CO)2]+OTf- is a new integrated photosensitizer/catalyst for the selective visible light
promoted photocatalytic reduction of CO2 to yield formic acid.
CO2 /2e-
/2H+
+
HCO2H
Re
h