Electro- and photo-chemistry of rhenium and rhodium ... · ide from carbon dioxide, rhenium...

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Nanomaterials and EnergyVolume 2 Issue NME3

Electro- and photo-chemistry of rhenium and rhodium complexes for carbon dioxide and proton reduction: a mini reviewPortenkirchner, Oppelt, Egbe, Knör and Sariçiftçi

Pages 134–147 http://dx.doi.org/10.1680/nme.13.00004Themed Issue Review PaperReceived 12/02/2013 Accepted 05/04/2013Published online 06/04/2013Keywords: catalyst/energy conversion/photocatalysts/solar fuel generation

ICE Publishing: All rights reserved

1. IntroductionHydrocarbons are currently considered as the most versatile energy carriers available for the growing fuel demand of mankind. Due to several advantageous properties such as high-energy density, trans-portation and storage properties, this situation will certainly also continue for the next decades.1 However, the ongoing combustion of fossil fuel resources leads to many environmental impacts including the problem of global warming.2–4 Moreover, the con-ventional hydrocarbon-based fuel production chain gradually faces difficulties to keep pace with rising demand and prices.5

As an alternative approach, a sustainable production chain for fuels should be established, where combustible chemical compounds are directly generated by utilizing sunlight as the primary source of

energy. In order to obtain synthetic hydrocarbons as solar fuels, the reduction of carbon dioxide and the supply of reducing equivalents based on water as an electron donor supplier are highly desirable starting points.6–9 In this context, mixtures of carbon dioxide and hydrogen are very attractive, since they can be used in various reactors that are designed similar to the conversion of syngas (i.e. a mixture of carbon monoxide and hydrogen) into carbon-based fuels following the long-established Fischer-Tropsch process.10–12

Besides hydrocarbons, the solar-driven production of methanol starting from carbon dioxide and water as abundant resources could provide a viable solution for solving the problem of diminishing fos-sil fuels.13 An interesting achievement in this direction was reported recently by Delacourt et al.,14 who described syngas conversion into

Electro- and photo-chemistry of rhenium and rhodium complexes for carbon dioxide and proton reduction: a mini review

Engelbert Portenkirchner DI (FH)*Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Linz, Austria

Kerstin Oppelt DIInstitute of Inorganic Chemistry, Center for Nanobionics and Photochemical Sciences (CNPS), Johannes Kepler University Linz, Linz, Austria

Daniel A. M. Egbe PD Dr.rer.nat. habil.Professor, Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Linz, Austria

Günther Knör Univ.-Prof. Dr.Professor, Institute of Inorganic Chemistry, Center for Nanobionics and Photochemical Sciences (CNPS), Johannes Kepler University Linz, Linz, Austria

Niyazi Serdar Sariçiftçi o.Univ. Prof. Mag. Dr. DDr. h.c.Professor, Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Linz, Austria

Rhenium and rhodium complexes with bipyridyl ligands have been proven to be efficient homogeneous catalysts

in the field of carbon dioxide and proton reduction. In this work, the authors provide several examples of these

compounds with modified ligand structures and discuss their electro- and photo-catalytic capabilities toward carbon

dioxide reduction and NAD+ cofactor regeneration. The electrocatalysis is studied by cyclic voltammetry and controlled

potential electrolysis for determining the over potentials, Faradaic efficiencies and reaction rate constants. In addition,

the photophysics of these compounds is discussed based on UV-visible absorption, photoluminescence and infrared

absorption spectroscopy. Results on comparing two different rhenium catalysts for homogeneous photocatalytic

carbon dioxide reduction using a sacrificial electron donor are reported.

1

2

3

4

5

*Corresponding author e-mail address: [email protected]

1 2 3 4 5

mailto:[email protected]

Offprint provided courtesy of www.icevirtuallibrary.comAuthor copy for personal use, not for distribution



135

methanol using an electrochemical cell similar to proton-exchange membrane fuel cells. This group achieved 45% energy efficiency at 10 mA/cm2, but unfortunately, the efficiency was decreasing with higher current densities (30% at 100 mA/cm2).

The homogeneous catalysis of carbon dioxide assimilating fuel generation based on water splitting represents another interesting approach in solar fuel research.15,16 These artificial photosynthetic systems are inspired by the overall function of the natural enzymatic reactions, where the industrially important role of hydrogen gas as a two-electron reductant for carbon dioxide fixation is replaced by organic redox cofactors, such as nicotinamide-adenine-dinucleotide (NADH), acting as hydride-transfer reagents. In all kinds of techni-cal scenarios following this direction, this leads to the fundamental problem of catalytic cofactor regeneration, that is, a reversible NAD+ to NADH conversion to supply electrons and protons for a multistep carbon dioxide conversion into energy-rich compounds.17–20

Despite of many current activities in this direction, the most challenging basic research need is to design new efficient and robust multielectron- and multiproton-transfer catalysts for the desired reactions. To lower the actual reduction potential of the carbon dioxide reduction process, suitable redox mediators are required.21–24 Concerning the generation of carbon monox-ide from carbon dioxide, rhenium complexes with bipyridine ligands are among the most frequently investigated candidates for homogeneous catalysis in terms of activities and lifetimes.23,25 Rhodium-based systems, on the other hand, have been success-fully established for catalyzing the recycling of redox cofactors such as nicotine amides. In this context, the combination of both rhenium and rhodium complexes for the catalytic production of syngas equivalents (carbon monoxide and hydrogen

or NADH)

powered by solar energy is an attractive new approach that we have started to follow recently.26,27 These energy-storing primary reactions could then be utilized to form hydrocarbons such as methane or methanol as more convenient types of solar fuels via the established processes.13

2. Catalyst materialsAt present most of the best studied catalysts are metal complexes with bipyridine ligands, where the catalyst center consists of transi-tion metals based on rhenium, rhodium or ruthenium. Despite their high-current efficiencies and high selectivity, problems in the field of artificial solar fuel production by these catalysts are manifold. Although these molecular catalyst compounds can be used to sta-bilize intermediate steps of the carbon dioxide reduction process and thus lower the required overpotential, achieving a simultaneous multiple electron and proton transfer is kinetically extremely diffi-cult to realize and over potentials of most reported catalyst systems are still significantly high. Furthermore, systems based on these catalyst materials, as reported up to now, suffer from low stability and low turnover frequency.

For photocatalysis, it is important to extend the absorption of these compounds in the visible region. One way to address this is to cova-lently bind the catalyst to a photosensitizer with high absorption in the visible region. One of the best results concerning quantum yield and turnover numbers (TON) was achieved by bridging a rhenium-based catalyst with a ruthenium-based photosensitizer reported by Ishitani and coworkers.24 Another promising approach is to combine catalysts with inorganic semiconductor materials. In a recent work, Kubiak et al. showed light-assisted cogenera-tion of carbon monoxide and hydrogen from carbon dioxide and H

2O in an acetonitrile/water mixture, exhibiting high Faradic effi-

ciency at low homogeneous catalyst concentration.28 Bocarsly and coworkers29 reported selective reduction of carbon dioxide to meth-anol by combining p-GaP semiconductor electrode and pyridine as catalyst material. This attempt of a kinetically difficult 6e− aqueous photoreduction of carbon dioxide to methanol allows astonishingly low reduction potentials below the standard reduction potential of −0.52 V versus saturated calomel electrode.

The materials investigated in this study are rhenium and rhodium diimine complexes with different ligand systems. The molecular struc-tures are shown below. Figure 1 shows the schematics of four rhenium(I) tricarbonyl chloride complexes with different diimine ligand systems, that is, (2,2ʹ-bipyridyl)Re(CO)

3Cl (1), (4,4ʹ-dicarboxyl-2,2ʹ-bipyridyl)

Re(CO)3Cl (2), (5,5ʹ-bisphenylethinyl-2,2ʹ-bipyridyl)Re(CO)

3Cl

Figure 1. Schematic chemical structures of four different rhenium

compounds (2,2ʹ-bipyridyl)Re(CO)3Cl (1), (4,4ʹ-dicarboxyl-2,2ʹ-

bipyridyl)Re(CO)3Cl (2) (5,5ʹ-bisphenylethinyl-2,2ʹ-bipyridyl)Re(CO)3Cl

(3) and [5,5ʹ-bis ((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-

bipyridyl] Re(CO)3Cl (4) for carbon dioxide reduction.

H

H

O O

O

OOO

O

O O

O OO

O

NN

N N

NN

CICI

(1)

(3)

(4)

(2)

ReRe

CIRe

OO

OO

O O

HH15C8O

H

OO

N N

CIRe




136

(3) and [5,5ʹ-bis ((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-bipyridyl]Re(CO)

3Cl (4).

The (2,2ʹ-bipyridyl) and (4,4ʹ-dicarboxyl-2,2ʹ-bipyridyl) ligands were purchased from commercial suppliers (Fluka, Aldrich). The multichromophoric diimine ligand (5,5ʹ-bisphenylethinyl-2,2ʹ-bipyridyl) was synthesized from 2,2ʹ-bipyridine according to published routes through the formation of 5,5ʹ-dibromo-2,2ʹ-bipyridine and additional palladium-catalyzed Sonogashira coupling of the dibromo compound with phenylacetylene.26,30–32

[5,5ʹ-bis((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-bipyridyl] was obtained via coupling of 5,5ʹ-dibromobipyridine with 1,5-dioctyloxy-4-ethinyl-benzaldehyde. Synthesis of the rhe-nium complexes was then achieved by further metallation with Re(CO)

5Cl in toluene as described in the literature.33–36

These compounds (1–4) have been further investigated for their capability of electro- and photo-catalytical carbon dioxide reduction.

Figure 2 shows two different rhodium compounds, namely, [Cp*(bpy)RhCl]Cl (5) and [Cp*(5,5ʹ-bis(phenylethinyl)-2,2-bpy)RhCl]Cl (6). Synthesis of the rhodium complex 6 was first done by Oppelt et al.26 via palladium-catalyzed Sonogashira coupling of 5,5ʹ-dibromobipyridine with phenyl acetylene and subsequent metallation with [Cp*RhCl

2]

2 in methanol/toluene.

Compounds 5 and 6 have been further investigated for the elec-tro- and photo-catalytic reduction of NAD+ to NADH. A more detailed description of the synthesis and characterization of these compounds (1–6) can be found in the literature.26,27

3. Electrocatalytic studiesMethods for electrochemical activation of carbon dioxide have long been studied, and significant progress has been made over the

past decade).7,23,25,37 The electrochemical reduction of carbon diox-ide usually requires a high potential of about –1.9 V versus normal hydrogen electrode (NHE) for a one-electron activation process. This high potential is partly explained by the necessary large inner-shell rearrangements upon electron transfer and the corresponding bending of the molecule upon one-electron activation resulting in an O-C-O angle of 133°.38,39 By performing the carbon dioxide reduction in a multielectron process, however, this transformation can be facilitated. As a consequence of this multielectron and atom transfer, the required reduction potentials are lowered significantly. This is reflected by the change in the equilibrium redox potentials between reactions 1 and 2 in Table 1.40 The equilibrium potential of the carbon dioxide reduction progressively shifts to more positive values as the number of electrons and protons participating in the reduction process increase, comparing different net reactions 3–8 in Table 1.

Although the equilibrium reduction potentials for a proton coupled multielectron carbon dioxide reduction appear to be tolerably low; in reality however, the actual reduction potentials on bare metal electrodes are much higher than the Nernst potential due to bar-rier induced over potentials. In order to decrease the actual redox potential needed for the carbon dioxide reduction process, suitable catalysts are required.

Several desirable products such as methanol and others are cur-rently industrially synthesized from syngas, a mixture of carbon monoxide and hydrogen. The electro- and photo-catalytic for-mation of syngas is therefore a desired path for the efficient and selective synthesis of various hydrocarbon products.45,46 In addi-tion, carbon dioxide reduction by a cascade of enzymatic reactions where the cofactor NADH is consumed, has been applied to obtain methanol, while at the same time, NADH is oxidized to NAD+.47

Figure 2. Schematic chemical structures of two different rhodium

compounds [Cp*(bpy)RhCl]Cl (5) and [Cp*(5,5ʹ-bis(phenylethinyl)-

2,2ʹ-bpy)RhCl]Cl (6) for the electro- and photo-catalytic reduction of

NAD+ to NADH.

(5)

(6)

N NCI-

CIRh+

N NCI-

CIRh+

Redox reaction E°/V

1 CO2 + e– → CO–

2−1.90

2 2CO2 + e– → CO + CO2

3

–−0.65

3 CO2 + 2H+ + 2e– → HCOOH −0.61

4 CO2 + 2H+ + 2e– → CO + H2O −0.53

5 CO2 + 4H+ + 4e– → C + 2H2O −0.20

6 CO2 + 4H+ + 4e– → HCHO + H2O −0.48

7 CO2 + 6H+ + 6e– → CH3OH + H2O −0.38

8 CO2 + 8H+ + 8e– → CH4 + 2H2O −0.24

*Values are a collection from Refs. 41–44.

Table 1. Redox potentials versus normal hydrogen electrode for one-

and multi-electron reduction of carbon dioxide including proton-

dependent reduction pathways in aqueous solution at pH 7.




137

NAD+ regeneration to NADH according to reaction 9 requires a potential of about −0.32 V versus NHE at pH 7.48

9. NAD H 2e NADH+ + −+ + →

10. 2H O 2NAD 2NADH 2H O2 + → + ++ +2

When compared with hydrogen production via water splitting, thermodynamically NADH regeneration is slightly favorable. According to reaction 10 a potential of −1.14 V versus NHE (at pH 7, 25°C and an activity of NAD+ of 1) is required to convert NAD+ back to NADH by electrolysis in aqueous solu-tion. This potential is slightly less compared with water elec-trolysis with a standard redox potential of −1.23 V versus NHE. Practically, however, for both processes, high overpotentials are necessary.49–51

4. Cyclic voltammetryCyclic voltammetry is a useful technique to determine the redox properties of the soluble compounds 1–6. It allows indirect studies about the catalytic activity for carbon dioxide reduction to carbon monoxide and NADH regeneration. In the presented studies, a one-compartment cell was used for cyclic voltammetry experiments, either with a platinum or glassy carbon working electrode, a plati-num counter electrode and a Ag/AgCl quasi reference electrode calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal ref-erence. For carbon dioxide reduction experiments, the cyclic vol-tammogram of the metal complexes under nitrogen saturation was compared with to the spectrum in the presence of carbon dioxide. Electrochemical experiments for carbon dioxide reduction were performed in acetonitrile with 0.1-M tetrabutylammonium hex-afluorophosphate (TBAPF

6) as supporting electrolyte. It was found

that purging of the system with nitrogen or carbon dioxide respec-tively, for about 15 min under stirring, is sufficient to achieve gas saturation of the electrolyte solution.

Figure 3 shows the cyclic voltammograms of compound 1 meas-ured in nitrogen- and carbon dioxide–saturated acetonitrile solu-tion, respectively. When the solution was saturated with nitrogen, compound 1 shows a one-electron quasi-reversible reduction wave with its maximum around –1200 mV versus NHE followed by a one-electron irreversible reduction wave at more negative potential around –1530 mV versus NHE. These characteristics for rhenium-bipyridine-based catalysts are well known and were first reported in this contents by Lehn et al. in 1984.52 The peak at −1200 mV of compound 1 can be attributed to a ligand-based reduction and is reversible with its reoxidation at around −1100 mV. The second reduction wave at −1530 mV can be assigned to a reduction at the metal centre and is not reversible. The other compounds presented in this article show a similar behavior.

In carbon dioxide–saturated solution (Figure 3, red curve), com-pound 1 shows a strong enhancement in current density at the sec-ond irreversible reduction wave at about −1750 mV (vs. NHE). This enhancement in current is known to be the catalytic reduction of carbon dioxide to carbon monoxide and is assumed to proceed in aprotic solvents according to reaction 2 in Table 1. The first quasi-reversible reduction wave at around −1200 mV does not show a significant increase in current density and is hence not participating in the catalytic reaction directly.

Although well studied in the past, the catalytic cycle for elec-trochemical carbon dioxide reduction of compound 1 (and simi-lar compounds presented herein) is still not fully understood. However, it is known that the catalytic mechanism needs an empty coordination site for carbon dioxide binding, which is identified to proceed via the loss of the halide (Cl−). Some detailed studies on this mechanism have been carried out initially by Sullivan et al.53 and Lehn et al.54 in 1985 and 1986, respectively. A more recent study on this was carried out by Johnson et al. in 1996 reviewing several proposed mechanisms.55

Figure 4 shows the cyclic voltammograms of compound 2 meas-ured in nitrogen- and carbon dioxide–saturated acetonitrile solu-tion, respectively. In comparison with compound 1, compound 2 does not show a clear and pronounced reversible one-electron quasi-reversible reduction wave around −1200 mV (vs. NHE),

Figure 3. Cyclic voltammograms of Re(2,2ʹbipyridyl)(CO)3Cl (1) in

nitrogen (black solid line) and carbon dioxide (red solid line) saturated

electrolyte solution. Scan with carbon dioxide saturation shows a

large current enhancement due to a catalytic reduction of carbon

dioxide to CO. Measurements are taken at a scan rate of 100 mVs−1 in

acetonitrile with 0.1-M TBAPF6, platinum working electrode, platinum

counter electrode and a catalyst concentration of 1 mM. A scan with

no catalyst present under carbon dioxide (blue dotted line) shows

negligible reductive current.

1

0

–1

–2

–3

–4

–5–2000 –1500 –1000

Potential (mV) versus NHE–500 0

Cu

rren

t d

ensi

ty (

mA

cm

–2)

Compound 1, N2 purgedCompound 1, CO2 purgedTBAPF2 + CO2 purged




138

although the onset is still observable. A nonreversible wave can be observed at around –1500 mV (vs. NHE), which can be attrib-uted to the additional carboxyl groups on the bipyridyl ligand. In contrast to previously published data,56 the modified compound 2 showed some catalytic behavior toward carbon dioxide reduction when the electrolyte solution was saturated with carbon dioxide, as can be seen in Figure 4, red curve. However, carbon dioxide poten-tiostatic bulk electrolysis at –2100 mV (vs. NHE) revealed that the compound seems to be unstable and loses its catalytic activity within several minutes of electrolysis time.

Figure 5 shows the cyclic voltammograms of compound 3 recorded under N2

− and carbon dioxide–saturated acetonitrile solution. In contrast to compound 1, compound 3 does not show a clear separation between different reductive waves. In addition, the onset of the reductive current occurred at a potential around −750 mV (vs. NHE) which is about 330 mV more positive com-pared with compound 1 (Figure 3). This significant differences between compound 1 and 3 can be attributed to the addition of the bisphenylethinyl groups at the 5,5ʹ position. When compound 3 was scanned repeatedly under N2

− saturated conditions to very negative potentials (i.e. –1600 mV vs. NHE), a violet film formed on the platinum working electrode. The origin of this film forma-tion and its potential toward carbon dioxide reduction is currently under investigation.

When the acetonitrile solution was saturated with carbon dioxide, compound 3 showed a strong enhancement in the second reduction wave current density. In relative comparison, a 6.5-fold increase at the second irreversible reduction wave under carbon dioxide at −1750 mV (vs. NHE; Figure 5) was observed. Comparing this with compound 1, the relative increase in current density is higher for compound 3. A more detailed study on compound 3 has been published elsewhere.27

Different to the compound materials 1–3, compound 4 was poorly soluble in acetonitrile. Therefore, cyclic voltammograms were measured in dimethylformamide (DMF). Figure 6 shows the cor-responding cyclic voltammograms of compound 4 recorded in N2

− and carbon dioxide–saturated DMF solution. Due to the extended ligand of this compound, the redox characteristics are much more diverse compared with above presented measurements of the more simple compounds 1–3. Within the potential of 0 to −1800 mV (vs. NHE), four distinct reversible reduction peaks can be found with their half-wave potential (E

1/2) at −635, −1020, −1250 and −1500

mV (vs. NHE). Since it is generally known for rhenium compounds with bipyridine ligands that a metal-center-based reduction shows an irreversible behavior, these peaks might be attributed to a ligand-based reduction. Further studies would be necessary to fully clarify their true nature. Different to the other compounds 1–3, the cyclic voltammogram of compound 4 did not reveal catalytic current

Figure 4. Cyclic voltammograms of (4,4ʹ-dicarboxyl-2,2ʹ-bipyridyl)

Re(CO)3Cl (2) in nitrogen (black solid line) and carbon dioxide (red

solid line) saturated electrolyte solution. Scan with carbon dioxide

saturation shows a large current enhancement due to the catalytic

reduction of carbon dioxide to carbon monoxide. Measurements are

taken at a scan rate of 50 mVs-1 in acetonitrile with 0.1-M TBAPF6,

platinum working electrode, platinum counter electrode and a catalyst

concentration of 2 mM. A scan without catalyst under carbon dioxide

(blue dotted line) shows negligible reductive current.

0

–1

–2

–3

–4

–5

–6

–2000 –1500 –1000Potential (mV) versus NHE

–500 0

Cu

rren

t d

ensi

ty (

mA

cm

–2)


Figure 5. Cyclic voltammograms of Re(5,5ʹ-bisphenylethinyl-

2,2ʹbipyridyl)(CO)3Cl (3) in nitrogen (black solid line) and carbon

dioxide (red solid line) saturated electrolyte solution. The scan with

carbon dioxide saturation shows a large current enhancement due

to a catalytic reduction of carbon dioxide to carbon monoxide.

Measurements are taken at 100 mVs-1 in acetonitrile with 0.1-M

TBAPF6, platinum working electrode, platinum counter electrode

and a catalyst concentration of 1 mM. A scan without catalyst under

carbon dioxide (blue dotted line) shows negligible reductive current.

0·5

–3·0

–2·5

–2·0

–1·5

–1·0

–0·5

0·0


–500 0C

urr

ent

den

sity

(m

A c

m–2

)

Compound 3, N2 purged

Compound 3, CO2 purged

TBAPF6 + CO2 purged




139

enhancement within the measured potential range when the elec-trolyte solution was saturated with carbon dioxide. The recorded current enhancement can be attributed to the additional background current as can be seen from the scan with no catalyst present (blue dotted line).

The speed of the carbon dioxide reduction reaction is an impor-tant measure for the capability of a homogeneous catalyst. Comparison between various compounds is generally given by the rate constant k. To define k of the catalyst, usually rotating disk experiments are applied to determine the diffusion coeffi-cient of the complex.56 For this one can use the Levich-Koutecky equation to calculate the diffusion coefficient according to Equation 1.57

1. i n CL = −( . )0 620 2 3 2 3 1 6FAD ω υ

where iL is the Levich current from the rotating disk experiment,

n is the number of electrons, F is Faraday’s constant, A is the electrode area, D is the diffusion coefficient, ω is the rotation rate, υ is the kinematic viscosity of the solution and C is the con-centration of the analyte in solution. The catalytic rate constant (k) can then be extracted from Equation 2, which holds true for a reversible electron-transfer process followed by a fast catalytic reaction.58

2. i n D yc F (cat) k(Q)= A

Where ic is the catalytic current, (cat) is the catalyst concentration, (Q) is the concentration of the substrate (in this case carbon diox-ide), y is the order of the substrate in the reaction in question and the other parameters are the same as in Equation 1.

However, since rotating disk experiments are usually difficult to obtain, a combination of Equation 2, for the catalytic current, and Equation 3, for determining the peak current ip of a compound with a reversible electron transfer and without any following reaction, can be elegantly used to obtain a good estimate for k from cyclic voltammetry measurements.

3. i n A DFRTp = ⋅0 446

32F cat[ ] ν

where ν is the applied scan rate and the other abbreviations have their described meaning.59

The ratio of ic to i

p will then yield Equation 4, where the diffusion

coefficient cancels out and hence k can be estimated knowing ic

and ip.60

4.

ii

RTn

k Qc

p

y

= 10 446.

[ ]F ν

The units of k depend on the order of reaction. The homogeneous carbon dioxide reduction to carbon monoxide with the compounds 1–4 proceed via a second-order type of reaction resulting in a rate constant k with the units of M−1 s−1 or l mol−1 s−1.61

Given the Equation 4, detailed analysis of the cyclic voltammogram data as presented in the Figures 3–6 allows to estimate the second-order rate constant for the carbon dioxide reduction. Using, for example, the value of 2.1 for the ratio of i

c to i

p measured for com-

pound 1 in Equation 4 yields in a rate constant of about 60 M−1 s−1, which is in good agreement with the reported literature values.56 Applying the same method to the other compounds 2 and 3 gives a rate constant of about 170 M−1 s−1 for compound 2 and a rate constant of about 220 M−1 s−1 for compound 3.

Another important quantity is the TON. It is a dimensionless num-ber and defined as the total number of turnovers the catalyst can achieve until its total decay, independent of the time involved.62 As such it is an important measure to evaluate the catalyst lifetime and robustness. Experimentally, it can be found by the ratio between the amounts of products formed, if there is no limit in substrate (car-bon dioxide) divided by the amount of catalyst material in the sys-tem (mol reduced product of carbon dioxide/mol catalyst). Typical TON for Re(bipy)(CO)3X (X=Cl, Br) compounds are in the order of 300.52,56,63

Figure 6. Cyclic voltammograms of (5,5ʹ-bis ((2,6-bis-octyloxy-4-

formyl)phenylethinyl)-2,2ʹ-bipyridyl] Re(CO)3Cl (4) in nitrogen (black

solid line) and in carbon dioxide (red solid line) saturated electrolyte

solution. The scan with carbon dioxide saturation shows some current

enhancement due to an additional background current as recorded at

the scan with no catalyst present under carbon dioxide (blue dotted

line). Measurements are taken at 100 mVs−1 in DMF with 0.1-M

TBAPF6, platinum working electrode, platinum counter electrode and

a catalyst concentration of 0.5 mM. DMF, dimethylformamide.

–1·2

–0·8

–0·4

0·0


–500 0 500

Cu

rren

t d

ensi

ty (

mA

cm

–2)





140

Figure 7 shows a comparison of cyclic voltammograms and UV-vis absorption spectra between compounds 1 and 4. These measure-ments indicate the strong dependence of these compounds on the ligand system. For compound 4 the extended conjugated ligand results in a clear shift for the first reduction wave to a more posi-tive potential. As a consequence, also the UV-vis absorption maxi-mum (a metal-to-ligand charge-transfer [MLCT] band) is shifted to a longer wavelength as expected. This behavior is important to tune the properties of these metal-organic compounds to improve their capability for electro- and photo-catalytic carbon dioxide reduction.27,42,56

Figure 8 shows the comparison between cyclic voltammogram measurements of [Cp*(5,5ʹ-bis(phenylethinyl)-2,2ʹ-bpy)RhCl]Cl (6) with (red solid line) and without (black solid line) NAD+ pre-sent. When the systems contains NAD+, a large reductive current enhancement can be observed, starting at about –200 mV versus NHE with its peak maximum at about –350 mV versus NHE. This increase in current density is attributed to the catalytic reduction of NAD+ to NADH by the catalyst compound 6. The reduction of NAD+ is assumed to proceed according to reaction 9, which would require a potential of about –0.32 V versus NHE at pH 7. This is considerably close to the observed potential peak of about –350 mV versus NHE.48

5. Controlled potential electrolysisControlled potential electrolysis experiments were generally performed in a gas-tight one-compartment or H-cell with a platinum

working electrode, a platinum counter electrode and a Ag/AgCl quasi reference electrode. Although a one-compartment cell is easier to handle in terms of setup building and sealing, it has the big disadvan-tage that reduction products formed at the working electrode might

Figure 8. Cyclic voltammetry of Rh compound 6 with (black solid line)

and without (red solid line) NAD+ present.The scan with NAD+ in the

system shows a large current enhancement attributed to the catalytic

reduction of NAD+ to NADH. Voltammograms are taken at 50 mVs−1

in 50-mM potassium phosphate buffer at pH 7, glassy carbon working

electrode, Ag/AgCl/3-M NaCl reference electrode and a platinum wire

as counter electrode.

–0·2

–0·1

0·0

0·2


–200 0–400 200

0·1

Cu

rren

t d

ensi

ty (

mA

cm

–2)

Rh complex 6

Rh complex 6 + NAD+

Blank

Figure 7. Comparison of Re(2,2ʹbipyridyl)(CO)3Cl (1) and (5,5ʹ-bis

((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-bipyridyl] Re(CO)3Cl

(4) in (a) cyclic voltammograms and (b) UV-vis absorption spectra.

Voltammograms are taken at 100 mVs-1 in dimethylformamide

with 0.1-M TBAPF6, platinum working electrode, platinum counter

electrode and a catalyst concentration of 0.5 mM. UV-vis absorption

spectra are recorded in methanol.

–0·4

–0·3

–0·2

–0·1

0·0

0·1

0·2


–500 0 500

Cu

rren

t d

ensi

ty (

mA

cm

–2)

Compound 1, N2 purged(a) (b)

Compound 4, N2 purged

300 400 500Wavelength (nm)

600 700A

bso

rpti

on

(ar

bit

rary

un

it) Compound 1

Compound 4




141

be reoxidized on the counter electrode. This problem can be avoided using an H-cell. However, since the expected product with this type of catalysts is carbon monoxide and the solubility of carbon monoxide in a given solvent is typically in the order off 100 times less than for carbon dioxide, it is expected that little to no carbon monoxide will remain in solution. In fact, these experiments did not show any differ-ence concerning carbon monoxide yield between the one-compart-ment cell electrolysis experiment and the H-cell experiment.

Figure 9 shows a typical current density versus time plot for carbon dioxide electrolysis experiment with compound 3 as performed in a one-compartment cell. It can be seen that the reductive cur-rent initially dropped significantly and afterwards stayed constant with a small decay reflecting the declining concentration of carbon dioxide substrate in solution over the electrolysis time. In addition, a plot of current density versus 1/time1/2, as shown in Figure 9b, can give useful information on the kinetics of the process. For exam-ple, Fick’s second law of diffusion shows that a linearity in the current versus 1/time1/2 plot suggests both, a fast electron-transfer rate and a time-independent surface concentration of the reactant (in this case carbon dioxide) within the electrolysis time, compare Equation 5.59

5.

j n D c c

t

s

=

−F≠

12 0

12

Where j is the current density, n is the number of electrons, F is the Faraday constant, D is the diffusion coefficient, c0 is the bulk concentration, c

s is the electrode surface concentration and t is the

measurement time.

The slope of the line also allows calculating the diffusion coefficient D, if the bulk and electrode surface concentrations are known. The equation cannot be applied for very short time scales, since the initial concentration gradient is very high and the current as such is not limited by diffusion, but by the electron-transfer process, compare Figure 9(a).43

As a direct proof of the catalytic carbon dioxide reduction capability of the rhenium catalysts, headspace gas samples are taken and analyzed with regard to the carbon monoxide concentra-tion using gas chromatography (GC) and fourier transform infrared spectroscopy (FTIR) measurements. The application of transmis-sion IR gas measurements, compared with standard GC analysis, has several advantages. IR measurement has a very short measure-ment time, high reproducibility, works at ambient temperatures and pressures, shows no vulnerability to interact with a mobile or sta-tionary phase, and the gas sample will not come in contact with the detector system. Details to this method used for carbon monoxide measurements have been reported in a previous articles.27

The Faradaic efficiency (ηF) can then be calculated according to

Equation 6.

6. ηF

COgas COsol=× +( )2 n n

ne

Where nCO gas

is the number of carbon monoxide molecules in the gas phase, n

CO sol is the number of carbon monoxide molecules dis-

solved in solution and ne is the number of electrons put into the

system during the electrolysis experiment.

The number of molecules of carbon monoxide in the gas phase was obtained by GC and FTIR analysis, while the number of molecules

Figure 9. Typical current density versus time plot for potential static

carbon dioxide electrolysis experiment of Re(5,5ʹ-bisphenylethinyl-

2,2ʹbipyridyl)(CO)3Cl (3) at constant −1950 mV versus NHE, performed

in acetonitrile solution saturated with carbon dioxide and an

electrolysis time of 3000 s (a) and same data plotted as current density

versus 1/time1/2 (b). NHE, normal hydrogen electrode.

–0·4

–0·0

0·0 0·1

(b)

(a)

0·21/t1/2

0·40·3 0·5

–0·3–0·2–0·1

–0·5

Cu

rren

t d

ensi

ty (

mA

cm

–2)

–0·4

–0·0

0 500 1000Time (s)

20001500 2500 3000

–0·3–0·2–0·1

–0·5

Figure 10. Schematic energy diagram of the lowest-lying excited

states of complexes 1–4. Data taken from Ref. 66.

AbsorptionPhosphorescence

1MLCT*3MLCT*

IC

ICISC

1ππ*

3ππ*

G.S.




142

of carbon monoxide dissolved in the electrolyte solution was esti-mated using Henry’s law following Equation 7.

7. p = ⋅k cH

The Henry constant kH is taken with 2507 atm mol solvent per

mol carbon monoxide, derived from data of Castillo et al.,64 p is the partial pressure of the solute carbon monoxide and c is the concentration of carbon monoxide in solution. The number of electrons consumed in the carbon dioxide electrolysis was deter-mined by integration of the current–time curve of the electrolysis experiment.

With this approach, a Faradaic efficiency for the reduction of car-bon dioxide to carbon monoxide by the compound 3 of about 43% was calculated and ranks equally with the efficiencies that have been measured for compound 1.27 Literature values of reported Faradaic efficiencies of similar or modified compounds reach val-ues up to 100%.44,56,65 A control experiment with a pure platinum plate electrode under otherwise identical conditions did not yield detectable amounts of carbon monoxide.

It has been shown that under these conditions (ACN:TBAPF6) also

small amounts of formate and oxalate can be formed, however, with typical Faradaic efficiencies below 1%.44

6. Photophysics and photocatalysisThe photocatalytic efficiency of different diimino rhenium carbonyl complexes for the reduction of carbon dioxide to carbon monoxide is strongly dependent on the electronic structure and the redox properties of these compounds. As stated by Takeda et al.66 the low-est electronic excited states of bipyridine-based rhenium diimime

carbonyl complexes, mainly of 3MLCT* and 3ππ* character, are frequently quite close in energy and may also be mixed with each other. Focusing on photochemical carbon dioxide reduction, almost all applicable catalysts have in common, that their lowest excited state is of the 3MLCT* character with a relatively long lifetime in the 10–100 ns range.66 This state can be quenched reductively by suitable sacrificial electron donors such as triethanolamine (TEOA or TEA) to generate a one-electron-reduced (OER) species. These OER molecules can form adducts with carbon dioxide, the struc-ture of which is not yet fully clear. There are various propositions for this intermediate by different research groups.53,67,68 In most of the suggested catalytic pathways, one of the OER-carbon dioxide adducts reacts with a second OER radical in order to obtain carbon monoxide as a reaction product and retrieve the initial catalyst.66

The nature of the bipyridyl ligand influences the energetic levels of the possible electronic transitions upon excitation with UV and visible light. A simplified scheme of the most common electronic structure for similar rhenium carbonyl complexes is shown in Figure 10.66 The prototype compound Re(bpy)(CO)

3Cl (1) is taken

as a basic template for the following descriptions. Its UV-visible absorbance spectrum in toluene is shown in Figure 12 (black trace). It shows two distinct maxima at 299 and 400 nm, and the room temperature photoluminescence (red trace) at an excitation wave-length of 380 nm shows a broad emission from 450 to 750 nm with a maximum at 550 nm.

The absorbance spectra of the rhenium bipydridyl complexes 1–4 exhibit strong electronic 1ππ* intraligand transitions of the diimine ligands in the higher energetic region, usually at wavelengths shorter than 330 nm and MLCT signatures at lower energies.26,69,70 Additional weaker UV bands of IL origin can be observed for com-pounds 3 and 4 in the 300–320-nm spectral region. These bands

Compound SolventAbsorbance

λmax (nm)Emission λmax (nm)

1 Re(2,2ʹ-bpy)(CO)3Cl Toluene 299, 403 550

1 Re(2,2ʹ-bpy)(CO)3Cl Methanol 369

2 Re(4,4ʹ-dicarboxy-2,2ʹ-bpy)(CO)3Cl Methanol 394

3 Re[5,5ʹ-bis(phenylethinyl)-2,2ʹ-bpy](CO)3Cl Dichloromethane 245, 290, 390

442, 689

3 Re[5,5ʹ-bis(phenylethinyl)-2,2ʹ-bpy](CO)3Cl Acetonitrile 360

4 Re[5,5ʹ-bis((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-bpy](CO)3Cl Toluene 430 428, 695

4 Re[5,5ʹ-bis((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-bpy](CO)3Cl Dichloromethane 427

4 Re[5,5ʹ-bis((2,6-bis-octyloxy-4-formyl)phenylethinyl)-2,2ʹ-bpy](CO)3Cl Methanol 411

ACN, acetonitrile; DCM, dichloromethane; MeOH, methanol.

Table 2. Summary of photophysical data of rhenium tetracarbonyldiimino complexes 1–4.




143

suggest energetic splitting of approximately 2200 cm−1 due to coupling to phenylethinyl C^C vibrational modes.26 The absorb-ance and emission data in various solvents for compounds 1–4 are summarized in Table 2.

Besides substituent effects on the spectra, what is most remarkable and also well known about the absorbance of the described com-pounds is the fact that the 1MLCT transition is strongly dependent on solvent effects;33,34,36 this can also be seen in Figure 11 compar-ing the spectra of compound 3 in dichloromethane to the spectrum recorded in acetonitrile. In the more polar solvents such as metha-nol or acetonitrile, the lowest lying-1MLCT absorbance is shifted to

the red by 30 nm compared with less polar solvents such as toluene or dichloromethane.

Furthermore, the carboxyl substituted compound 2 shows a strong red shift of the MLCT of about 2770 cm−1 as compared with rhe-nium complex 1, due to the electron withdrawing effect of the carboxyl groups.

The photophysics of the rhenium complex 3 is dominated by the presence of the reducing rhenium(I) tricarbonyl chloride donor fragment, which leads to a luminescent lowest-energy triplet excited state of the MLCT type.26

Figure 11. UV-visible absorption spectra of the rheniumcarbonyl-

complexes 1–4 in different solvents. In more polar solvents, such as

methanol or ACN, the lowest-lying 1MLCT absorbance is shifted to the

red by 30 nm compared with less polar solvents such as toluene or

DCM. ACN, acetonitrile; DCM, dichloromethane.

300 400 500 600

Ab

sorp

tio

n (

arb

itia

ry u

nit

)

Wavelength (nm)

Compound 4, in MeOHCompound 3, in ACN

Compound 1, in MeOHCompound 2, in MeOHCompound 3, in DCM

Figure 12. UV-visible absorption(black) with two maxima at 299 and

403 nm and photoluminescence (red) spectra of rhenium carbonyl

complex 1 in toluene at room temperature and λexc. = 380 nm.


700600

Ab

sorp

tio

n (

arb

itia

ry u

nit

)

Lum

ines

cen

ce (

arb

itia

ry u

nit

)

Figure 13. UV-visible absorption (black) and emission spectra

at λexc. = 388 nm (red) of rhenium carbonyl complex 3 in

dichloromethane at room temperature. The 3MLCT emission spectra

are indicated by a maximum at approximately 650 nm and additional

shoulders at 590 and 700 nm.


700600

Ab

sorp

tio

n (

arb

itia

ry u

nit

)

Lum

ines

cen

ce (

arb

itia

ry u

nit

)

Figure 14. UV-visible absorption and photoluminescence spectra

upon excitation at 380 nm of rhenium carbonyl complex 4 in toluene

at room temperature. The 1MLCT absorbance lies at a wavelength of

430 nm; the emission spectrum shows two peaks at 428 nm and a

broad 3MLCT emission from 600 nm on with a maximum at 695 nm.


700

Ab

sorp

tio

n (

arb

itia

ry u

nit

)

Lum

ines

cen

ce (

arb

itia

ry u

nit

)




144

In some cases, such as with compounds 3 and 4, the emission quan-tum yield is very low (Φ < 0.001). The broad 3MLCT emission of 3 in dichloromethane solution is also covering a typical wide spectral region including orange and red light exhibiting a maxi-mum at around 650 nm and showing additional shoulders at 590 and 700 nm (Figure 13). Interestingly, upon UV-light excitation at the absorption peak maximum of 3, the compound also shows an additional structured blue green intraligand emission with max-ima at 443 and 465 nm, which is not completely quenched by the lower-lying MLCT states (Figure 13).26 This is not the case for the nonsubstituted parent compound (2,2ʹ-bipyridyl)Re(CO)

3Cl (1)

(Figure 12) and was found to be caused by the additional pheny-lethinyl groups of the diimine ligand of 3.

Comparable dual luminescence behavior has also been reported for similar multichromophore systems investigated by Schanze and coworkers.70 In addition, compound 4, the absorbance and lumines-cence spectra of which are shown in Figure 14, exhibits similar spec-troscopic properties as catalyst 3. This is not surprising, considering that 3 and 4 are structurally related. The IL luminescent transition of 4 lies in the range between 400 and 500 nm in this case, whereas the 3MLCT emission is observed in the region higher than 600 nm.

Comparing now photocatalytic carbon dioxide reduction experi-ments applying compounds 1 and 3, it was found that the reduc-tion in DMF/TEOA(5:1/v:v) similar to previous works of Koike and coworkers67 shows big differences in reaction efficiency. As has been reported by other groups before, compound 1 showed a good photocatalytic activity with a quantum yield in the range of of φ = 0.1.67 In later experiments with the new catalyst 3, a significantly lower performance in the generation of carbon monoxide was reported under similar conditions.27 These findings can be inter-preted by analyzing the above described spectrophotometric meas-urements,27 considering the assumption that a long lived 3MLCT* state is a necessary requirement for the successful application of the catalyst in a photochemical system for the reduction on carbon dioxide to carbon dioxide.

7. Conclusion and outlookIn the present work, several tricarbonylchlororhenium(I)pyridyl complexes (1–4) were reviewed regarding to their potential as cata-lysts for homogeneous electrochemical and photochemical reduc-tion of carbon dioxide to carbon monoxide. In addition, rhodium catalysts (5,6) for electrochemical and photochemical NAD+ cofac-tor regeneration and proton reduction were investigated.

It was found that the carbon dioxide reduction potential, determined by cyclic voltammetry, can be positively influenced by a modified ligand system. A comparison between the catalyst material 3 with added phenylethinyl groups at the 5,5ʹ position to the nonmodi-fied catalyst 1 showed a shift in the onset carbon dioxide reduction potential by about 300 mV (vs. NHE) to more positive values. Bulk

carbon dioxide electrolysis experiments showed Faraday efficien-cies around 45–50% for the formation of carbon monoxide. The best estimated rate constants according to cyclic voltammetry data are in the order of 200 M−1 s−1.

NAD+ cofactor regeneration using organometallic catalysts was also investigated by cyclic voltammetry measurements. First experiments show a large reductive current enhancement attributed to NAD+ reduction to NADH at potentials of about −350 mV (vs. NHE).

Furthermore, the compounds with an extended ligand system showed significantly higher optical absorption in the visible range, which is basically a clear benefit for photocatalytic applications. However, photocatalytic experiments of compounds 1 and 3 indi-cate that the a modified ligand system can lead to an inversion of the lowest-lying excited state properties of such compounds from a MLCT character present in 1 to an intraligand situation present in 3. Following this argumentation it can be concluded that certain ligand-based modifications are to be avoided since it is generally assumed that the long lived 3MLCT state is necessary for the suc-cessful application of the catalyst in a photochemical system for the direct reduction of carbon dioxide to carbon monoxide.

As a next step new and improved electro- and photo-catalysts for carbon dioxide reduction and NADH regeneration have to be devel-oped and characterized. The modification of the attached ligand system is a suitable way for systematic tuning of the excited state properties of such materials and hence their electro- and photo-catalytic abilities. A clear demonstration of this was shown by the comparison of compounds 1 and 3. Following this idea a modifica-tion where the phenylethinyl groups are attached to the 4,4ʹ posi-tion might be highly interesting, assuming a better conjugation to the metal center of the complex.

Although further catalyst improvements are crucial, finally, the two systems capable of catalytic carbon dioxide reduction and H

2O

splitting could be combined in a photo- or photoelectron-chemical cell. Such a system would be capable of solar-powered production of syngas and its equivalents (carbon monoxide and hydrogen or NADH).

AcknowledgementsFinancial support by the Austrian Science Foundation (FWF) within the Wittgenstein Prize as well as the projects P21045: “Bio-inspired Multielectron Transfer Photosensitizers” and P25038: “Functional Light-Responsive Carbonyl Systems” is gratefully acknowledged.

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