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Molecular tailoring of organometallic polymers for efficient catalytic CO2 reduction: mode of formation of the active species
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
219
M o l e c u l a r tailoring of organometallic polymers for efficient catalytic CO2 reduction: m o d e of formation of the active species Raymond Ziessel Laboratoire de Chimie, d'Electronique et de Photonique Molrculaires, Ecole Europrenne Chimie, Polymrres, Matrriaux, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France We here report the first example of an electrochemical polymerization process which leads to formation of a modified electrode having the generic formula [Ru0(bpy)(CO)2C1]n, and which displays outstanding electrochemical activity towards reduction of carbon dioxide to either carbon monoxide or formate. A crucial stereochemical effect of the leaving groups on the feasibility of polymerization is demonstrated. Formation of the polymer occurs stepwise, through the formation of a dimeric or a tetrameric intermediate. 1. INTRODUCTION Carbon dioxide fixation is the basic process by which natural photosynthesis produces organic matter on earth. Appreciable effort has been devoted to the design of artifical systems capable of carbon dioxide fixation with the viewpoints of converting solar energy and/or electricity into chemical energy and mimicking biological carbon assimilation. In light of the twin problems of global warming and depletion of fossil fuels, much attention has been paid to electrochemical reduction of CO2 as a potential C 1 or C2 source for chemicals and fuels. These processes are difficult to catalyse as they typically involve not only multiple electron transfers but are often coupled to chemical steps such as protonation. In general, there can be multiple competing reaction pathways leading to a variety of reactions products. These products differ in the number of redox equivalent (2 to 8e-) and the kinetics for their formation can vary broadly and depend on factors such as proton avalaibility. Although thermodynamically these processes should take place at moderately cathodic potentials, the direct reduction of CO2 at bare metal cathodes, which act as simple outer-sphere electron donors, typically requires very large overpotentials because of the formation of high energy intermediates such as CO2-" [1]. This uncatalyzed electrochemical reduction of CO2 requires very high overpotentials in the range of 1 to 2 volts (e.g. the reduction of CO2 in DMF takes place at about - 2.0 V vs SCE [2]). Transition metal complexes used in conjunction with metal cathodes decrease the activation energy barrier and circumvent the formation of high energy intermediates. These complexes can efficiently mediate the electron transfer from the cathode to CO2. In addition to lowering the overpotential, a good catalyst could in principle, increase the selectivity of the product being produced and yield high current efficiencies for a single product. A number of transition metal complexes have been shown to be effective in the electrocatalytic reduction of CO2
[3,4]. In many of these examples, the catalyst is capable of undergoing more than one reduction and thus storing multiple redox equivalents. During the reduction process the catalyst make available at least one open coordination site where CO2 can bind and ultimately be reduced
220
(case of CO formation), or the open coordination site can protonate to form a hydride in which CO2 can be inserted (case of HCOO-formation). The use of electrodes modified with surface-immobilized transition metal complexes exhibiting electrocatalytic activity towards CO2 is especially attractive and this subject has recently been reviewed [5,6]. These surface-modified electrodes could be classified in three main categories depending on the mode of preparation: (i) by chemical derivatization of the surface via covalent bonding; (ii) by coating the electrodes by sputtering or paining, e.g. cobalt phthalocyanine immobilized in this manner has been shown to reduce CO2 to CO at 0.6 V vs SCE [7]; (iii) by electropolymerization of complexes containing polymerizable substituted ligands (e.g. vinyle, pyrrole, acetylenic groups) giving rise to the formation of redox active polymeric films which can reduce CO2 to CO electrocatalytically. These modified electrodes can mediate the electron transfer from the cathode to the substrate and it has been ascertained that the potential at which CO2 is reduced strongly depend on the nature of the metal center [8]. We discovered a new way to prepare coated electrodes by using an electro-precipitation process which allows the deposition of films onto an electrode surface. The protocol of the overall process is illustrated in Fig. 1, where the use of soluble complexes beating trans-axial leaving groups afforded during the reduction process metal-metal bonds [9]. -
f J X-M-M-M-M-X
....:--:-!i'iiii
;-i-i:-::-~i-ii" X-M-M-X :~~i! ~i0.~}'.iI
[
X-M-X
X- M- M- M-X
I
{ X-M-M-Xx_M_X
--{--M---}-
X-M-X
for
% ,. fo
X--M--Xo
M
for
/ \
Figure 1. Schematic representation of the synthetic protocol used for formation of o p e n - c h a i n clusters. The complex is stabilized by various ligands such as polypyridyl or n-acceptor cart)owl or phosphine groups.
The increasing number of n motive leads to the formation of insoluble polymers on the electrode surface. A relatively uniform coating could easily be obtained and the film thickness could be electrochemically controlled. The use of this technique is advantageous from a number of standpoints. The effective concentration of electroactive material can reach levels that are not accessible in homogeneous solution. The distance between adjacent metal centers are close enough that cooperativity effects are enhanced or effective. Finally, the process of film formation could be studied step-by-step starting either from chemically or electrochemically prepared dimers, tetramers... The unique stereochemical positionning of the leaving groups could be studied in details. This alternative and powerful novel method socalled "electrogeneratedopen-chain clusters" for immobilizing of CO2 redox active catalysts onto electode surfaces is the purpose of the present account.
221
2. RESULTS AND DISCUSSION Electrolysis of monomeric mono-bipyridine bis-carbonyl ruthenium(H) complexes bearing two trans leaving groups (e.g. chloride anions or solvent molecules) generate at the working electrode a strongly adherent deep-blue film (Fig. 2A). This modified electrode demonstrate outstanding catalytic activity for the reduction of CO2 to CO (Fig. 2B) and was introduced in an effort to overcome the above limitations [10]. The overpotential was decreased to about 0.8V, and selective and quantitative formation of CO was obtained in aqueous electrolyte.
(A)
I~ i A
(B)
i/ILtA 100
~
I10 p.A t
-2
JF~"I~/--S,-~-,I ~
0 E/V I
-2
E/V
-100-
//
a
-200 Fi2ure 2. (A) Cvclic voltammo~rams recorded for 3 (cf Fie 4~ in CI-hCN enntninino T R A P (3 1 M n f a t carbon electrode.( ...... ) First scan between - 0.45 and- 2.00 V and (. . . . . ) 2nd to 28 th successive scans between - 0.85 and - 2.00 V. (B) Cyclic voltammogramsrecorded in H=O solution containing LiCIO4 (0.1M) for the C/[Ru(bpy)(CO)2], modified eleclrode prepared by electrolysis of 3 at -1.65V a) argon-purged solution, b) CO2-purged solution. .
.
.
.
.
.
~
- "
,.3
.
.
.
.
.
The bulk material has been characterized and identified as an organometallic polymer consisting of [Ru~ repeating units (Fig. 3A). The polymer which morphology is shown in Fig. 3B comprises an extended Ru~ ~ backbone having a staggered arrangement. Eq. 1 summarizes the overall process (bpy for 2,2'-bipyridine). Polymerization results from the overall addition of two electrons per mole of trans-(C1)-[RuII(bpy)(CO)2C12] and is associated with the loss of both coordinated chloride ligands.
n trans-(C1)-[RuII(bpy)(CO)2C12] + 2 ne"
[Ru0(bpy)(CO)2]n + 2 nCl"
(1)
Oxidation of the resulting polymer at - 0.6V vs SCE induces breakage of the Ru~ ~ bonds and causes desorption of the film and ultimate quasi-quantitative formation of the soluble [Ru II(bpy)(CO)z(CH3CN)2 ] 2+ complex (eq. 2).
222 [Ru~
- 2 ne" ~
[RuII(bpy)(CO)2(S)2] 2+
(2)
In light of this results it could be concluded that within the polymer the basic structure of the ruthenium "Ru(bpy)(CO)2" core is maintained in the molecular film. ""
,..-'-.tit
WN
.
"
.
. ~
-,~
~
:"~
.~"
" "t"
-
"~ " ~ -
.:~d,=
"
-
q
I/~
Ru o -
Ru o _
RuO.--- Ru o
oc' l/ql o/\
il/),1% C~ L,%
co
~0 .
"..~
:
"
""
.
" - ~--
(A)
"/
e,
" ~"
=~
"d"
.
.
g
.t~
,=
." t = =
(B)
(A) Schematic representation of the [Ru(bpy)(CO)2].polymer; (B) Scanning electron micrograph of the polymeric film formed electrochemicallyon an ITO electrode; the length of the bar corresponds to 10 l~m.
Figure
3:
An important endeavour in the understanding of polymer formation arise from the discovery that the chemically prepared trans-(C1)-[Ru(bpy)(CO)2C1]2dimer 4 (Fig. 4) allows the formation of the same polymeric material by continous cycling of the potential between - 0.9 and - 2.0V (Fig. 5A). It is worth noting that if the potential range is limited to - 1.5V, growth of the film is highly inefficient. Exhaustive electrolysis at Ep = - 1.40V after consumption of one-electron per mole of 4, produces a red-brown solution due to the formation of the trans-(Cl)-[{Ru(bpy)(CO)2}4C12]tetramer 5 (Fig. 4). This complex exhibits one irreversible reduction peak at Epc = - 1.60V and growth of the [Ru0(bpy)(CO)2]n film is ensured by continuous cycling of the potential between - 0.9 V and - 2.0 V (Fig. 5B).
CI F~ = -1.46 v E~ = -1.60 V CI
CI
Figure 4: Schematic representation of the different steps effective during formation of the suitable polymer Moreover, exhaustive electrolysis at- 1.60 V of a solution of 4 or 5 leads to deposition of the deep-blue, strongly adherent [Ru0(bpy)(CO)2]n film on the working electrode, after exchange of two electrons per mole of complex. The reaction results in quantitative conversion into a polymeric film upon exhaustive electrolyses.
223
14HA
11
)
(B)
(c)
Figure 5 9Cyclic voltammograms in DMSO solution containing 0.1 M TBAP under an argon atmosphere at a Pt electrode showing in each case (..... ) initial sweep and ( ) 2nd to 30th successive scans between - 0.9 and - 2.0 V of a) solution of 3 ; b) from 4 ; c) from compound 5.
trans-(C1)-[RuI(bpy)(CO)2Cl]2
These results clearly show that polymerization occurs directly upon reduction of 3 by an electrochemical propagation process (eqs. 5-7 and Figure 4). This is a consequence of the easier or similar reducibility of dimer 4 and parent oligomer 5. In terms of mechanism it means that the polymerization proceeds via the initial formation of a Ru I species (eqs. 5-6), which dimerizes into compound 5 after the release of one chloride ion, rather than through a direct two electron reduction of 3 into a Ru ~ species followed by an aggregation process [ 11 ]. [RuII(bpy)(CO)2C12] + e" [RuII(bpy-')(CO)2C12] [RuI(bpy)(CO)2C1]
=___ [RuII(bpy")(CO)2C12]" =
~ 4
[RuI(bpy)(CO)2C1] + el= 5
=-- polymer
(5) (6) (7)
These modified electrode having the generic formula [Ru0(bpyRR)(CO)2C1]n, display outstanding electrochemical activity towards the reduction of carbon dioxide to either: (i) carbon monoxide, 100 % faradic yield in water at -1.2 V vs SCE, bpy = 2,2'-bipyridine, R = H; (ii)or formate, 95 % faradic yield in aqueous electrolyte at -1.2 V vs Ag/Ag +, R = isopropylesters groups. 3. C O N C L U S I O N The use of these surface-immobilized electrocatalysts allows for the easy removal of the catalysts from the reaction vessel, and the use of much lower quantities of catalyst which is here highly concentrated in the reaction layer. In many cases the immobilization of the catalyst on the electron source provides its stabilization and allows an marked increase of the turnover frequency compared to the numbers found in related homogeneous systems. Taking
224 into account the low yield of formiate which is produced we suggest that the major pathway for CO2 reduction involves interaction of a [Ru(bpy")(CO)]" fragment with CO2 to form a metallo-carboxylate intermediate which after protonolysis and reduction regenerates the starting [Ru(bpy)(CO)2] fragment which then perpetuates the catalytic cycle (Fig. 6). The [Ru(bpy")(CO)]" fragment is formed in dimer 2 or in the polymer by the reduction of the bpy ligand (electron reservoir) followed by the release of a carbonyl ligand. o...,,,''c~ ]~
~CO
I-L J" /
Ri "~CO
N,~ .CO l~l R0 ..... I "~*CO CO
Ru
CO]" CO2,H+
Figure 6: Schematicrepresentationof the proposedcatalyticcycle. One fragmentof the polymer is shown. The application of such systems to the multi-electron reduction of carbon dioxide to formate, methanol or methane is now under active investigation in our laboratory. REFERENCES 1. C. Amatore and J.-M. Sav6ant, J. Am. Chem. Soc., 103 (1981) 5021. 2. R. Kostecki and J. Augustynski, Ber. Bunsen-Ges Phys. Chem., 98 (1994) 1510. 3. J.-P. Collin and J.-P. Sauvage, Coord. Chem. Rev., 93 (1989) 245. 4. R. Ziessel, In "Photosensitization and Photocatalysis using Inorganic and Organometallic Compounds" K. Kalyanasundaram and M. Gratzel (eds) Kluwer Academic Publishers, 1993, pp 217-240; B. P. Sullivan, K. Krist and H. E. Guard (eds) in "Electrochemical and Electrocatalytic Reactions of Carbon Dioxide", Elsevier, Amsterdam, 1993. 5. H. D. Abruna, Coord. Chem. Rev., 86 (1988) 135. 6. A. Deronzier and J.-C. Moutet, Coord. Chem. Rev., 147 (1996) 339. 7. P. A. Christensen, A. Hammett and A. V. G. Muir, J. Electroanal. Chem., 241 (1988) 361. 8. J. A. Ramos Sende, C. R. Arana, L. Hernandez, K. T. Potts, M. Keshevarz-K and H. D. Abruna, Inorg. Chem., 34 (1995) 3339. 9. M.-N. Collomb-Dunand-Sauthier, A. Deronzier et R. Ziessel, J. Chem. Soc., Chem. Comm., 1994, 189. 10. M.-N. Collomb-Dunand-Sauthier, A. Deronzier et R. Ziessel, Inorg. Chem., 1994, 33, 2961. 11. S. Chardon-Noblat, A. Deronzier, D. Zlodos, R. Ziessel, M. Haukka, T. Pakkanen and T. Ven~il~iinen, J. Chem. Soc., Dalton Trans., (1996) 2581.
219
M o l e c u l a r tailoring of organometallic polymers for efficient catalytic CO2 reduction: m o d e of formation of the active species Raymond Ziessel Laboratoire de Chimie, d'Electronique et de Photonique Molrculaires, Ecole Europrenne Chimie, Polymrres, Matrriaux, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France We here report the first example of an electrochemical polymerization process which leads to formation of a modified electrode having the generic formula [Ru0(bpy)(CO)2C1]n, and which displays outstanding electrochemical activity towards reduction of carbon dioxide to either carbon monoxide or formate. A crucial stereochemical effect of the leaving groups on the feasibility of polymerization is demonstrated. Formation of the polymer occurs stepwise, through the formation of a dimeric or a tetrameric intermediate. 1. INTRODUCTION Carbon dioxide fixation is the basic process by which natural photosynthesis produces organic matter on earth. Appreciable effort has been devoted to the design of artifical systems capable of carbon dioxide fixation with the viewpoints of converting solar energy and/or electricity into chemical energy and mimicking biological carbon assimilation. In light of the twin problems of global warming and depletion of fossil fuels, much attention has been paid to electrochemical reduction of CO2 as a potential C 1 or C2 source for chemicals and fuels. These processes are difficult to catalyse as they typically involve not only multiple electron transfers but are often coupled to chemical steps such as protonation. In general, there can be multiple competing reaction pathways leading to a variety of reactions products. These products differ in the number of redox equivalent (2 to 8e-) and the kinetics for their formation can vary broadly and depend on factors such as proton avalaibility. Although thermodynamically these processes should take place at moderately cathodic potentials, the direct reduction of CO2 at bare metal cathodes, which act as simple outer-sphere electron donors, typically requires very large overpotentials because of the formation of high energy intermediates such as CO2-" [1]. This uncatalyzed electrochemical reduction of CO2 requires very high overpotentials in the range of 1 to 2 volts (e.g. the reduction of CO2 in DMF takes place at about - 2.0 V vs SCE [2]). Transition metal complexes used in conjunction with metal cathodes decrease the activation energy barrier and circumvent the formation of high energy intermediates. These complexes can efficiently mediate the electron transfer from the cathode to CO2. In addition to lowering the overpotential, a good catalyst could in principle, increase the selectivity of the product being produced and yield high current efficiencies for a single product. A number of transition metal complexes have been shown to be effective in the electrocatalytic reduction of CO2
[3,4]. In many of these examples, the catalyst is capable of undergoing more than one reduction and thus storing multiple redox equivalents. During the reduction process the catalyst make available at least one open coordination site where CO2 can bind and ultimately be reduced
220
(case of CO formation), or the open coordination site can protonate to form a hydride in which CO2 can be inserted (case of HCOO-formation). The use of electrodes modified with surface-immobilized transition metal complexes exhibiting electrocatalytic activity towards CO2 is especially attractive and this subject has recently been reviewed [5,6]. These surface-modified electrodes could be classified in three main categories depending on the mode of preparation: (i) by chemical derivatization of the surface via covalent bonding; (ii) by coating the electrodes by sputtering or paining, e.g. cobalt phthalocyanine immobilized in this manner has been shown to reduce CO2 to CO at 0.6 V vs SCE [7]; (iii) by electropolymerization of complexes containing polymerizable substituted ligands (e.g. vinyle, pyrrole, acetylenic groups) giving rise to the formation of redox active polymeric films which can reduce CO2 to CO electrocatalytically. These modified electrodes can mediate the electron transfer from the cathode to the substrate and it has been ascertained that the potential at which CO2 is reduced strongly depend on the nature of the metal center [8]. We discovered a new way to prepare coated electrodes by using an electro-precipitation process which allows the deposition of films onto an electrode surface. The protocol of the overall process is illustrated in Fig. 1, where the use of soluble complexes beating trans-axial leaving groups afforded during the reduction process metal-metal bonds [9]. -
f J X-M-M-M-M-X
....:--:-!i'iiii
;-i-i:-::-~i-ii" X-M-M-X :~~i! ~i0.~}'.iI
[
X-M-X
X- M- M- M-X
I
{ X-M-M-Xx_M_X
--{--M---}-
X-M-X
for
% ,. fo
X--M--Xo
M
for
/ \
Figure 1. Schematic representation of the synthetic protocol used for formation of o p e n - c h a i n clusters. The complex is stabilized by various ligands such as polypyridyl or n-acceptor cart)owl or phosphine groups.
The increasing number of n motive leads to the formation of insoluble polymers on the electrode surface. A relatively uniform coating could easily be obtained and the film thickness could be electrochemically controlled. The use of this technique is advantageous from a number of standpoints. The effective concentration of electroactive material can reach levels that are not accessible in homogeneous solution. The distance between adjacent metal centers are close enough that cooperativity effects are enhanced or effective. Finally, the process of film formation could be studied step-by-step starting either from chemically or electrochemically prepared dimers, tetramers... The unique stereochemical positionning of the leaving groups could be studied in details. This alternative and powerful novel method socalled "electrogeneratedopen-chain clusters" for immobilizing of CO2 redox active catalysts onto electode surfaces is the purpose of the present account.
221
2. RESULTS AND DISCUSSION Electrolysis of monomeric mono-bipyridine bis-carbonyl ruthenium(H) complexes bearing two trans leaving groups (e.g. chloride anions or solvent molecules) generate at the working electrode a strongly adherent deep-blue film (Fig. 2A). This modified electrode demonstrate outstanding catalytic activity for the reduction of CO2 to CO (Fig. 2B) and was introduced in an effort to overcome the above limitations [10]. The overpotential was decreased to about 0.8V, and selective and quantitative formation of CO was obtained in aqueous electrolyte.
(A)
I~ i A
(B)
i/ILtA 100
~
I10 p.A t
-2
JF~"I~/--S,-~-,I ~
0 E/V I
-2
E/V
-100-
//
a
-200 Fi2ure 2. (A) Cvclic voltammo~rams recorded for 3 (cf Fie 4~ in CI-hCN enntninino T R A P (3 1 M n f a t carbon electrode.( ...... ) First scan between - 0.45 and- 2.00 V and (. . . . . ) 2nd to 28 th successive scans between - 0.85 and - 2.00 V. (B) Cyclic voltammogramsrecorded in H=O solution containing LiCIO4 (0.1M) for the C/[Ru(bpy)(CO)2], modified eleclrode prepared by electrolysis of 3 at -1.65V a) argon-purged solution, b) CO2-purged solution. .
.
.
.
.
.
~
- "
,.3
.
.
.
.
.
The bulk material has been characterized and identified as an organometallic polymer consisting of [Ru~ repeating units (Fig. 3A). The polymer which morphology is shown in Fig. 3B comprises an extended Ru~ ~ backbone having a staggered arrangement. Eq. 1 summarizes the overall process (bpy for 2,2'-bipyridine). Polymerization results from the overall addition of two electrons per mole of trans-(C1)-[RuII(bpy)(CO)2C12] and is associated with the loss of both coordinated chloride ligands.
n trans-(C1)-[RuII(bpy)(CO)2C12] + 2 ne"
[Ru0(bpy)(CO)2]n + 2 nCl"
(1)
Oxidation of the resulting polymer at - 0.6V vs SCE induces breakage of the Ru~ ~ bonds and causes desorption of the film and ultimate quasi-quantitative formation of the soluble [Ru II(bpy)(CO)z(CH3CN)2 ] 2+ complex (eq. 2).
222 [Ru~
- 2 ne" ~
[RuII(bpy)(CO)2(S)2] 2+
(2)
In light of this results it could be concluded that within the polymer the basic structure of the ruthenium "Ru(bpy)(CO)2" core is maintained in the molecular film. ""
,..-'-.tit
WN
.
"
.
. ~
-,~
~
:"~
.~"
" "t"
-
"~ " ~ -
.:~d,=
"
-
q
I/~
Ru o -
Ru o _
RuO.--- Ru o
oc' l/ql o/\
il/),1% C~ L,%
co
~0 .
"..~
:
"
""
.
" - ~--
(A)
"/
e,
" ~"
=~
"d"
.
.
g
.t~
,=
." t = =
(B)
(A) Schematic representation of the [Ru(bpy)(CO)2].polymer; (B) Scanning electron micrograph of the polymeric film formed electrochemicallyon an ITO electrode; the length of the bar corresponds to 10 l~m.
Figure
3:
An important endeavour in the understanding of polymer formation arise from the discovery that the chemically prepared trans-(C1)-[Ru(bpy)(CO)2C1]2dimer 4 (Fig. 4) allows the formation of the same polymeric material by continous cycling of the potential between - 0.9 and - 2.0V (Fig. 5A). It is worth noting that if the potential range is limited to - 1.5V, growth of the film is highly inefficient. Exhaustive electrolysis at Ep = - 1.40V after consumption of one-electron per mole of 4, produces a red-brown solution due to the formation of the trans-(Cl)-[{Ru(bpy)(CO)2}4C12]tetramer 5 (Fig. 4). This complex exhibits one irreversible reduction peak at Epc = - 1.60V and growth of the [Ru0(bpy)(CO)2]n film is ensured by continuous cycling of the potential between - 0.9 V and - 2.0 V (Fig. 5B).
CI F~ = -1.46 v E~ = -1.60 V CI
CI
Figure 4: Schematic representation of the different steps effective during formation of the suitable polymer Moreover, exhaustive electrolysis at- 1.60 V of a solution of 4 or 5 leads to deposition of the deep-blue, strongly adherent [Ru0(bpy)(CO)2]n film on the working electrode, after exchange of two electrons per mole of complex. The reaction results in quantitative conversion into a polymeric film upon exhaustive electrolyses.
223
14HA
11
)
(B)
(c)
Figure 5 9Cyclic voltammograms in DMSO solution containing 0.1 M TBAP under an argon atmosphere at a Pt electrode showing in each case (..... ) initial sweep and ( ) 2nd to 30th successive scans between - 0.9 and - 2.0 V of a) solution of 3 ; b) from 4 ; c) from compound 5.
trans-(C1)-[RuI(bpy)(CO)2Cl]2
These results clearly show that polymerization occurs directly upon reduction of 3 by an electrochemical propagation process (eqs. 5-7 and Figure 4). This is a consequence of the easier or similar reducibility of dimer 4 and parent oligomer 5. In terms of mechanism it means that the polymerization proceeds via the initial formation of a Ru I species (eqs. 5-6), which dimerizes into compound 5 after the release of one chloride ion, rather than through a direct two electron reduction of 3 into a Ru ~ species followed by an aggregation process [ 11 ]. [RuII(bpy)(CO)2C12] + e" [RuII(bpy-')(CO)2C12] [RuI(bpy)(CO)2C1]
=___ [RuII(bpy")(CO)2C12]" =
~ 4
[RuI(bpy)(CO)2C1] + el= 5
=-- polymer
(5) (6) (7)
These modified electrode having the generic formula [Ru0(bpyRR)(CO)2C1]n, display outstanding electrochemical activity towards the reduction of carbon dioxide to either: (i) carbon monoxide, 100 % faradic yield in water at -1.2 V vs SCE, bpy = 2,2'-bipyridine, R = H; (ii)or formate, 95 % faradic yield in aqueous electrolyte at -1.2 V vs Ag/Ag +, R = isopropylesters groups. 3. C O N C L U S I O N The use of these surface-immobilized electrocatalysts allows for the easy removal of the catalysts from the reaction vessel, and the use of much lower quantities of catalyst which is here highly concentrated in the reaction layer. In many cases the immobilization of the catalyst on the electron source provides its stabilization and allows an marked increase of the turnover frequency compared to the numbers found in related homogeneous systems. Taking
224 into account the low yield of formiate which is produced we suggest that the major pathway for CO2 reduction involves interaction of a [Ru(bpy")(CO)]" fragment with CO2 to form a metallo-carboxylate intermediate which after protonolysis and reduction regenerates the starting [Ru(bpy)(CO)2] fragment which then perpetuates the catalytic cycle (Fig. 6). The [Ru(bpy")(CO)]" fragment is formed in dimer 2 or in the polymer by the reduction of the bpy ligand (electron reservoir) followed by the release of a carbonyl ligand. o...,,,''c~ ]~
~CO
I-L J" /
Ri "~CO
N,~ .CO l~l R0 ..... I "~*CO CO
Ru
CO]" CO2,H+
Figure 6: Schematicrepresentationof the proposedcatalyticcycle. One fragmentof the polymer is shown. The application of such systems to the multi-electron reduction of carbon dioxide to formate, methanol or methane is now under active investigation in our laboratory. REFERENCES 1. C. Amatore and J.-M. Sav6ant, J. Am. Chem. Soc., 103 (1981) 5021. 2. R. Kostecki and J. Augustynski, Ber. Bunsen-Ges Phys. Chem., 98 (1994) 1510. 3. J.-P. Collin and J.-P. Sauvage, Coord. Chem. Rev., 93 (1989) 245. 4. R. Ziessel, In "Photosensitization and Photocatalysis using Inorganic and Organometallic Compounds" K. Kalyanasundaram and M. Gratzel (eds) Kluwer Academic Publishers, 1993, pp 217-240; B. P. Sullivan, K. Krist and H. E. Guard (eds) in "Electrochemical and Electrocatalytic Reactions of Carbon Dioxide", Elsevier, Amsterdam, 1993. 5. H. D. Abruna, Coord. Chem. Rev., 86 (1988) 135. 6. A. Deronzier and J.-C. Moutet, Coord. Chem. Rev., 147 (1996) 339. 7. P. A. Christensen, A. Hammett and A. V. G. Muir, J. Electroanal. Chem., 241 (1988) 361. 8. J. A. Ramos Sende, C. R. Arana, L. Hernandez, K. T. Potts, M. Keshevarz-K and H. D. Abruna, Inorg. Chem., 34 (1995) 3339. 9. M.-N. Collomb-Dunand-Sauthier, A. Deronzier et R. Ziessel, J. Chem. Soc., Chem. Comm., 1994, 189. 10. M.-N. Collomb-Dunand-Sauthier, A. Deronzier et R. Ziessel, Inorg. Chem., 1994, 33, 2961. 11. S. Chardon-Noblat, A. Deronzier, D. Zlodos, R. Ziessel, M. Haukka, T. Pakkanen and T. Ven~il~iinen, J. Chem. Soc., Dalton Trans., (1996) 2581.