metal-catalyzed reversible conversion between chemical and electrical energy designed towards a...

www.tcr.wiley-vch.de 169

Metal-Catalyzed Reversible Conversion between Chemical and Electrical Energy Designed Towards a Sustainable Society

KOJI TANAKAInstitute for Molecular Science Higashiyama 5-1, Myodaiji, Okazaki 444-8787, JapanE-mail: [email protected]: +81-564-59-5580; Fax: +81-564-59-5582

Received 15 October 2008Revised 12 February 2009

ABSTRACT: Proton dissociation of an aqua-Ru-quinone complex, [Ru(trpy)(q)(OH2)]2+ (trpy = 2,2′ : 6′,2″-terpyridine, q = 3,5-di-t-butylquinone) proceeded in two steps (pKa = 5.5 and ca. 10.5). The fi rst step simply produced [Ru(trpy)(q)(OH)]+, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(sq)(O−.)]0 (sq = 3,5-di-t-butylsemiquinone), owing to an intra-molecular electron transfer from the resultant O2− to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru2(btpyan)(q)2(OH)2]2+ (btpyan = 1,8-bis(2,2′-terpyridyl)anthracene), was prepared to place two Ru(trpy)(q)(OH) groups at a close distance. Deprotonation of the two hydroxy protons of [Ru2(btpyan)(q)2(OH)2]2+ generated two oxyl radical Ru-O−. groups, which worked as a precursor for O2 evolution in the oxidation of water. The [Ru2(btpyan)(q)2(OH)2](SbF6)2 modifi ed ITO electrode effectively catalyzed four-electron oxidation of water to evolve O2 (TON = 33500) under electrolysis at +1.70 V in H2O (pH 4.0). Various physical measurements and DFT calculations indicated that a radical coupling between two Ru(sq)(O−.) groups forms a (cat)Ru-O-O-Ru(sq) (cat = 3,5-di-t-butylcathechol) framework with a µ-superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of [Ru2(btpyan)(cat)(sq)(µ-O2

−)]0 assisted with an attack of two water (or OH−) to Ru centers, which causes smooth O2 evolution with regeneration of [Ru2(btpyan)(q)2(OH)2]2+. Deprotonation of an Ru-quinone-ammonia complex also gave the corresponding Ru-semiquinone-aminyl radical. The oxidized form of the latter showed a high catalytic activity towards the oxidation of methanol in the presence of base.

Three complexes, [Ru(bpy)2(CO)2]2+, [Ru(bpy)2(CO)(C(O)OH)]+, and [Ru(bpy)2(CO)(CO2)]0 exist as an equilibrium mixture in water. Treatment of [Ru(bpy)2(CO)2]2+ with BH4

− gave [Ru(bpy)2(CO)(C(O)H)]+, [Ru(bpy)2(CO)(CH2OH)]+, and [Ru(bpy)2(CO)(OH2)]2+ with genera-tion of CH3OH in aqueous conditions. Based on these results, a reasonable catalytic pathway from CO2 to CH3OH in electro- and photochemical CO2 reduction is proposed. A new pbn (pbn = 2-pyridylbenzo[b]-1,5-naphthyridine) ligand was designed as a renewable hydride donor for the six-electron reduction of CO2. A series of [Ru(bpy)3-n(pbn)n]2+ (n = 1, 2, 3) complexes undergoes photochemical two- (n = 1), four- (n = 2), and six-electron reductions (n = 3) under irradiation of visible light in the presence of N(CH2CH2OH)3. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 169–186; 2009: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.200800039

Key words: energy conversion, green chemistry, photochemistry, redox chemistry, ruthenium

The Chemical Record, Vol. 9, 169–186 (2009) © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

T H E C H E M I C A L

R E C O R D


T H E C H E M I C A L R E C O R D

170 www.tcr.wiley-vch.de

Introduction

Since the great invention of the steam engine in the middle of the 18th century, we have been burning fossil fuels to obtain almost all the energy required in our daily life. In other words, our society is completely dependent on the consumption of tremendous amounts of fossil fuels without regenerating any energy resources. Rapid expansion of economic activity throughout the world from the latter half of the 20th century causes serious problems, such as the depletion of natural resources including fossil fuels and global warming arising from the accumulation of CO2 in the air. Accordingly, we will be forced to change the present highly-developed consumer society to a sustainable one to cope with the predictable energy and resources depletion within this century.

Dihydrogen is considered as a promising energy for the future from the viewpoint of not only the suppression of the increase of CO2 in the air but also the inexhaustible supply by water reduction. Indeed, fuel cells driven by the oxidation of dihydrogen and reduction of dioxygen using Pt electrodes have already been in practical use, and much attention has been paid to H2 production by photocatalysts1 (Eq. 1). On the other hand, the energy cycle in nature is essentially maintained by photosynthesis by green plants, algae, and cyanobacteria, and metabolic reactions coupled with oxygen respiration by living organisms (Eqs. 2 and 3). The former converts solar energy into chemical energy by oxidizing water to O2 and reducing CO2 to carbohydrates such as glucose synthesis (forward reaction of Eqs. 2 and 3).2 The latter oxidizes organic molecules (backward reaction of Eq. 3) coupled with reduction of O2 (backward reaction of Eq. 2). Although arti-fi cial simulation of the energy cycle by the combination of Eqs. 2 and 3 is practically impossible under the current chemi-cal reactions, construction of catalytic systems that are able to convert natural energy into chemical energy, and then trans-form this into electrical energy would be the most reasonable pathway to realize a sustainable society. Developments of

catalysts designed for reversible conversion between CO2 and methanol through the six-electron redox reaction (Eq. 4), therefore, may play a central role in the construction of a sustainable society. Multi-electron reactions such as Eqs. 2–4 through stepwise one-electron transfer are subject to generate high-energy free-radical intermediates, which often yield undesired products. Therefore, metal complexes that exhibit multi-electron redox reactions would be reasonable candi-dates for catalysts designed to mediate energy transformation reactions.

2H 2O O2 + 2H2 (1)

2H 2O O2 + 4H+ + 4e- (2)

6CO2 + 24H + + 24e- C6H12O6 + 6H2O (3)

CO 2 + 6H+ + 6e- CH 3OH + H 2O (4)

Metal complexes with non-innocent ligands show charac-teristic redox reactions,3 and are featured by particular combi-nations of metals and ligands rather than by redox-active ligands alone. Among various metal complexes bearing non-innocent ligands, such as dioxolenes, dithiolenes, and benzoquinonediimines, ruthenium-dioxolene complexes are particularly interesting because of their close energy levels between the d-orbital and the π-orbital of the metal and the dioxolene ligand, respectively.4 As a result, there are formally six possible electronic structures for RuII/III-dioxolene com-plexes (Scheme 1).5

Our study is to develop molecular catalysts for the revers-ible conversion between chemical and electrical energy. Along this line, this paper describes water and methanol oxidation reactions mediated with the redox series of Scheme 1, photo-chemical multi-electron reduction of Ru complexes aimed at six-electron reduction of CO2, and oxalate generation by CO2 coupling on metal-sulfur clusters.

� Koji Tanaka was born in 1946. He received his PhD degree from Osaka University with a thesis on the synthesis of metal carbamato complexes under the supervision of Prof. Toshio Tanaka, Osaka University in 1975. After he graduated from a Master course at the Graduate School of Osaka University in 1971, he was employed as an Assistant Professor at Osaka University. He joined Prof. R. B. King’s research group as a visiting scholar at the University of Georgia in USA for 1 year in 1978. In 1990, he moved to the Institute for Molecular Science as a Professor. He was awarded the Chemical Society of Japan for Creative Work 1998, and Japan Society of Coordina-tion Chemistry 2008. He is now the president of the Japan Society of Coordination Chemistry. His current research focuses on metal-catalyzed reversible conversion between chemical and electri-cal energy, intended to build a sustainable society. �

M e t a l - C a t a l y z e d R e v e r s i b l e C o n v e r s i o n

www.tcr.wiley-vch.de 171The Chemical Record, Vol. 9, 169–186 (2009) © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

[RuII(bpy)(trpy)(OH2 )]2 +

[RuIII(bpy)(trpy)(OH)]2 +

[RuIV(bpy)(trpy)(=O)]2 +

-e -

-e - (5)

Some of those high valent Ru=O complexes are proven to be active in the oxidation of some organic molecules.7 However, the oxidation potentials of aqua-Ru complexes to generate high valence Ru=O ones are too positive to couple the reduction of dioxygen. It is worthwhile noting that ligand localized redox reactions of polypyridyl-Ru complexes usually take place at potentials more negative than −1.0 V vs. SCE, while those of dioxolene-Ru complexes occur over a wide potential range, and can be controlled substantially by the substituents of dioxolene ligands (Scheme 1). For example, controllable redox potentials of the quinone (q)/semiquinone (sq), and sq/cathecol (cat) couples are expected to work as elec-tron reservoirs for the conversion among aqua, hydroxy, and oxo ligands in the acid-base equilibrium reaction (Eq. 6). Accordingly, when an acid-base equilibrium of Eq. 6 is coupled with a redox reaction of aqua-Ru-dioxolene complexes ([L(q)M(OH2)]2+; L = tridentate ligand), a variety of electronic structures of the complex are generated (Scheme 2).8 For example, if one negative charge of the OH− group of [L(q)Ru(OH−)]+ formed by deprotonation of [L(q)Ru(OH2)]2+ shifts to q, a hydroxy radical complex, [L(sq)Ru(OH.)]+, will be generated. Furthermore, if deprotonation of [L(q)Ru(OH−)]+ is coupled with intramolecular one or two electron transfer to q, [L(sq)Ru(O−.)]0 or [L(cat)Ru(O..)]0 would be produced. The number of the complexes in Scheme 2 becomes doubled if the metal centered Ru(II)/Ru(III) redox reaction is involved in the acid-base redox reaction of Scheme 2.

Ru-OH2 Ru-OH- Ru-O2--H+ -H+

+H++H+ (6)

O

ORuII

O

ORuII

O

ORuII

O

ORuIII

O

ORuIII

-e-+e-

-e-+e-

-e-+e-

RuII-quinone

RuII-semiquinone

RuII-catecholato

RuIII-semiquinone

RuIII-catecholato

-e-+e-

O

ORuIII

RuIII-quinone

Scheme 1. Redox series of RuII/III-dioxolene complex.

[L(q)M(OH2)]2+ [L(q)M-OH-]+ [L(q)M-O2-]0

[L(sq)M-OH]+ [L(sq)M-O ]0

-H+-H+

-H+

+H+ +H+

+H+

[L(cat)M-O ]0M = Ru(II); L = Tridentate Ligand

Ru-oxyl Radical Complex Aimed at Energy Transformation

A minimum requirement to create electrical energy by the combination of redox reactions of organic molecules and dioxygen is to oxidize the former at potentials more negative than the reduction of O2. Myer et al. have reported a versatile synthetic route for (polypyridyl)RuIV=O complexes through successive electron and proton losses of the corresponding (polypyridyl)RuII-OH2 complexes (Eq. 5).6

Scheme 2. Acid-base equilibrium coupled with redox reactions of dioxolen of aqua-Ru-dioxolene complex.




Among a variety of Ru-dioxolene complexes that we prepared, [Ru(trpy)(q)(H2O)]2+ (q = 3,6- or 3,5-di-t-butylbenzoquinone, Figure 1) appears to be a suitable complex to demonstrate the acid-base equilibrium coupled with redox reactions of dioxolene ligands (Scheme 2)9 from the viewpoint of stability and solubility of the complex. The electronic absorption spectrum of [Ru(trpy)(q)(H2O)]2+ shows a strong MLCT band from Ru(II) to q at 600 nm in CH2Cl2. An addition of an equimolar amount of a methanolic solution of tBuOK to the solution shifted the MLCT band from 600 nm to 576 nm (Figure 2).9 Such a small blue shift of the MLCT transition from Ru(II) to q is simply explained by the acid-base equilibrium between [Ru(trpy)(q)(OH2)]2+ and [Ru(trpy)(q)(OH)]+. The intensity of the 576 nm MLCT band of [Ru(trpy)(q)(OH)]+ decreases by an addition of more than one equiv of tBuOK to the solution, and completely dis-appeared in the presence of more than three equiv of tBuOK. Instead, a new band appeared at 870 nm assignable to an MLCT transition from Ru(II) to sq. The electronic absorption spectra remained unchanged after an addition of a large excess amount of tBuOK. The same spectral changes were

also observed in H2O/CH3OH (8 : 2 v/v), and the pKa values of [Ru(trpy)(q)(H2O)]2+ and [Ru(trpy)(q)(OH)]+ were determined as 5.5 and ca. 10.5, respectively, by pH titration at 20°C. The signifi cant red shift of the MLCT band from 576 nm to 870 nm upon the deprotonation of [Ru(trpy)(q)(OH)]+ is ascribed to the reduction of q by the intramolecular electron transfer from the resultant O2− group.

The similar spontaneous reduction of q of [Ru(trpy)(q)(OH−)]+ by the addition of base was also observed in the cyclic voltammogram (CV) (Figure 3). The CV of [Ru(trpy)(q)(H2O)]2+ in CH2Cl2 displays the reversible q/sq and sq/cat couples at E1/2 = 0.31 and −0.47 V (vs. SCE), respectively. An addition of one equiv of BuOK to the solution shifts the redox couples to E1/2 = 0.07 and −0.57 V owing to the formation of [Ru(trpy)(q)(OH)]+. The observation that the rest potentials of the [Ru(trpy)(q)(H2O)]2+ and [Ru(trpy)(q)(OH)]+ solutions are located at the positive sites of the q/sq redox couple demonstrates the RuII(q) cores of both complexes. On the other hand, an addition of more than two equiv of BuOK to the [Ru(trpy)(q)(H2O)]2+ solution shifted the rest potential of the solution across the q/sq redox couple. Based on these results, it is concluded that an electron transfer from OH− to q is not involved in the Ru-OH2 and Ru-OH equilibrium, whereas [Ru(trpy)(sq)(O−.)]0 is produced by deprotonation of [Ru(trpy)(q)(OH)]+ accompanied with one-electron transfer from the resultant O2− to q (Eq. 7).

N

NN

RuII

OH2

OO

tButBu

2+

Fig. 1. [RuII(trpy)(q)(OH2)]2+.

Fig. 2. Uv-vis spectra of [Ru(trpy)(q)(OH2)]2+ in the presence of various amounts of tBuOK in CH2Cl2.

BuOK

0 eq.

0.5 eq.

1.0 eq.

3.0 eq.

Fig. 3. CV of of [Ru(trpy)(q)(OH2)]2+ in the presence of various amounts of tBuOK in CH2Cl2.



NN

NRu

III

O

O

OH2

t -Bu

t -Bu-H+

+H+ NN

NRu

III

O

O

OH

t -Bu

t-Bu

2 + +

NN N

NN

N=

NN

NRu

III

O

O

O

t -Bu

t -BuN

N

NRuII

O

O

O

t -Bu

t -Bu

2

00

-H+

+H+

(7)

The EPR spectrum of [Ru(trpy)(sq)(O−.)]0 at 5 K displayed the triplet signals resulting from I = 1 spin state (Figure 4), and spin trap experiments evidenced the presence of one unpaired electron on the oxo ligand.10 It should be noticed that the hydroxy proton of analogous [Ru(trpy)(bpy)(OH)]+ does not dissociate under similar conditions. In addition, aqua- and hydroxy-metal complexes usually form µ-oxo dimers upon treatments with strong bases. Thus, the q/sq redox couple in the Ru(q) group takes a major role in the unprecedented conversion between Ru-O−. and Ru-OH groups (Eq. 7), which created new types of not only an energy transducer from proton gradients to elec-tricity,11 but also a renewable agent for oxidation of organic molecules.11

The [Ru(trpy)(sq)(O−.)]0/[Ru(trpy)(q)(O−.)]+ redox couple is E1/2 = 0.07 V (Figure 3). The neutral complex did not show any reactivity toward oxidation of organic molecules, but the mono-cationic form generated by the treatment of [Ru(trpy)(sq)(O−.)]0 with one equiv of Ag+ oxidized cyclic

dienes to form the corresponding aromatic compounds and [Ru(trpy)(q)(OH)]+.12 Thus, [Ru(trpy)(q)(O−.)]+ has an ability to cleave C-H bonds of some organic molecules.

2[RuII(trpy)(q)(O-.)]+

2[RuII(trpy)(q)(OH)]+

(8)

Four-electron Oxidation of Water

In photosystem II, water-oxidation takes place at the O2 evolving center (OEC) composed of a tetranuclear Mn cluster with di-µ-oxo dimeric Mn units.13 Several di- and tetranuclear metal complexes have been prepared as functional models of OEC to simulate the function, and some dinuclear metal complexes are proven to be active for the four-electron oxida-tion of water.14 The structure having two redox centers and a hydroxo (aqua, oxo) group is common in model complexes of OEC. However, the mechanism for the O2 evolution has hardly been elucidated so far.

The fi nding that [Ru(trpy)(q)(H2O)]2+ is reversibly converted to [Ru(trpy)(sq)(O−.)]0 at a pH higher than 10.5 prompted us to synthesize dimeric [RuII

2(btpyan) (3,6-t-Bu2q)2(OH)2](SbF6)2 (btpyan = 1,8-bis(2,2′-terpyridyl)anthracene) (Figure 5) to create an O-O bond through the radical coupling of two Ru-O−. groups. Furthermore, the analogous 2,2′-bipiridine dimer complex [RuII

2(btpyan)(bpy)2

(OH)2](SbF6)2 was also prepared, since comparison of the catalytic activity of these two complexes may give a clue to elucidate the role of quinone ligand as a second redox center in the oxidation of water.15

The conception of a bridging ligand, BTPYAN is to put two oxyl radicals forcibly at a short distance in a cavity.

g = 4.18

Fig. 4. Epr spectra of [Ru(trpy)(sq)(O−.)]0 at 5 K.

2,2':6',2"-terpyridine

O

O

N

RuN

OHN

HON

RuNO

ON

tBu

tButBu

tBu

=

N

RuN

OHN

HON

RuN

N

N

N N

N

(SbF6)2

N N

N

(SbF6)2

Fig. 5. [RuII2(btpyan)(q)2(OH)2](SbF6)2 and [RuII

2(btpyan)(q)2(OH)2](SbF6)2.




The crystal structure of BTPYAN (Figure 6) shows π-π stacking of two terpyridyl groups, and the distance between the central nitrogen atoms of terpyridyl (4.22 Å) is shorter than that of 1-C and 8-C of anthracene (4.88 Å). The reac-tion of [Ru2Cl3(btpyan)] with 3,6-di(tert-butyl)cathecol in the presence of ammonium acetate gave the acetato com-plex. Hydrolysis of the acetato complex gave the complex [RuII

2(btpyan)(3,6-tBu2q)2(OH)2]2+, which showed a strong band at 576 nm assignable to the MLCT transition from Ru(II) to quinone in MeOH. Based on the btpyan structure, the two Ru(II)-hydroxo groups of [RuII

2(btpyan)(3,6-tBu2q)2(OH)2]2+ are considered to be located face-to-face inside the cavity formed by the two terpyridyl and two bulky 3,6-di(tert-butyl)-1,2-benzoquinone groups. Similarly, an ana-logous bipyridine complex [RuII

2(btpyan)(bpy)2(OH)2](SbF6)2 also seems to have the similar structure.

Figure 7 shows the electronic absorption spectra of [Ru2(btpyan)(q)2(OH)2]2+ in the presence of various amounts of tBuOK in CH3OH. Similar to [Ru(trpy)(q)(OH)]+, [Ru2(btpyan)(q)2(OH)2]2+ displays the MLCT band from Ru(II) to q at 576 nm. An addition of tBuOK to the solution results in a decrease of the 576 nm band, and a new band appears at 850 nm assignable to the MLCT from Ru(II) to sq. The 576 nm band completely disappeared by the addition of 2 equiv of BuOK. The addition of 2 equiv of HClO4 to the solution completely recovered the original electronic spectrum of [Ru2(btpyan)(q)2(OH)2]2+. Based on the

stoichiometry, the change of electronic absorption spectra of [Ru2(btpyan)(q)2(OH)2]2+ is expressed by Eq. 9. On the other hand, no spectral change was observed in the treatment of [RuII

2(btpyan)(bpy)2(OH)2]2+ with an excess amount of tBuOK in CH3OH. Thus, neither monomeric [Ru(trpy)(bpy)(OH)]+ nor dimeric [Ru2(btpyan)(bpy)2(OH)2]2+ dissociates the hydroxy protons, even under very basic conditions.

QRuII RuIIQOH HO

QRuII RuIISQOH O

2+

+

tBuOK (1.0 eq.)

HClO4 (1.0 eq.)

tBuOK (1.0 eq.)

HClO4 (1.0 eq.)

SQRuII RuIISQO O0

(9)

The CV of [Ru2(btpyan)(q)2(OH)2]2+ in CH3OH shows two reversible q/sq and two sq/cat redox couples at E1/2 = (0.43 V and 0.35 V) (vs. Ag/AgCl) and (−0.47 V and −0.56 V), respec-tively (Eq. 10). The rest potential of the solution at 0.49 V supports the electronic structure of the RuII(q) cores of the complex. An addition of two equiv of BuOK to the solu-tion shifted the rest potential of the solution to −0.12 V across the q/sq redox couples, indicating the formation of [Ru2(btpyan)(sq)2(O−.)2]0 in the solution. At the same time, the redox potentials of the (q/sq) and (sq/cat) couples shifted cathodically and appeared at E1/2 = (0.40 V and 0.30 V) and (−0.43 V and −0.68 V), respectively (Eq. 11).

Fig. 6. Molecular structure of BTPYAN.

0.0

0.2

0.4

0.6

0.8

1.0

400 600 800 1000 1200

Abs

orba

nce

Wavelength (nm)

576 nm

850 nm

Fig. 7. Uv-visible spectra of [RuII2(btpyan)(q)2(OH)2]2+ in the presence of

various amounts of tBuOK in CH3OH.



[Ru2(btpyan)(q)2(OH)2]2+

[Ru2(btpyan)(q)(sq)(OH)2]+

[Ru2(btpyan)(sq)2(OH)2]0

E1/2 =0.43 V

E1/2 =0.35 V

[Ru2(btpyan)(sq)(cat)(OH)2]+

E1/2 =-0.47 V

E1/2 =-0.56V

[Ru2(btpyan)(cat)2(OH)2]+

+ e-

+ e-

+ e-

+ e- (10)

[Ru2(btpyan)(q)2(O-.)2]0

[Ru2(btpyan)(q)(sq)(O-.)2]+

[Ru2(btpyan)(sq)2(O-.)2]0

E1/2 =0.40 V

E1/2 =0.30 V

[Ru2(btpyan)(sq)(cat)(O-.)2]+

E1/2 =-0.43 V

E1/2 = -0.68V

[Ru2(btpyan)(cat)2(O-.)2]+

+ e-

+ e-

+ e-

+ e-

(11)

The CV of [Ru2(btpyan)(q)2(OH)2]2+ was also measured in CF3CH2OH/diethylether to avoid irreversible oxidation of CH3OH at potentials more positive than +1.0 V. Besides the reversible [RuII(btpyan)(q)2(OH)2]2+/[RuII(btpyan)(sq)(q)(OH)2]+ and [RuII(btpyan)(sq)(q)(OH)2]+/[RuII(btpyan)(sq)2(OH)2]0 redox couples, an irreversible anodic wave at

Ep = +1.34 V (vs. Ag/AgCl), possibly assignable to a process of electron and proton losses of [RuII(btpyan)(q)2(OH)2]2+, was also observed (Figure 8). Addition of H2O to the solution (10%) caused catalytic currents of water oxidation at potentials more positive than +1.3 V, since 0.69 ml of dioxygen evolved in the controlled-potential electrolysis of [Ru2(btpyan)(q)2

(OH)2](SbF6)2 (1.5 mmol) at +1.7 V in trifl uoroethnol/water (9 : 1 v/v) with a current effi ciency of 91% (TON = 21). The complex, however, gradually lost the catalytic ability to oxidize water because the redox-coupled proton-loss of [Ru2(btpyan)(q)2(OH)2]2+ (Eq. 9) is less favorable under more acidic conditions owing to the accumulation of protons in the progress of O2 evolution (Eq. 12).

2H2O O2 + 4H+ + 4e-[Ru2(btpyan)(q)2(OH)2]2+

+1.70 V in CF3CH2OH/ H2O (9:1 v/v) (12)

The bipyridine analog [RuII2(btpyan)(bpy)2(OH)2](SbF6)2

displayed a nearly reversible redox couple at E1/2 = +0.89 V (vs. Ag/AgCl) assignable to the [RuII

2(btpyan)(bpy)2(OH)2]2+/[RuIII

2(btpyan)(bpy)2(OH)2]4+ couple and an irreversible anodic wave Ep = +1.43 V. Based on the redox behavior of [Ru(bpy)(trpy)(OH2)]2+, the redox is ascribed to the process of electron and proton losses of the [RuIII

2(btpyan)(bpy)2(OH)2]4+/[RuIV

2(btpyan)(bpy)2(O)2]4+ couple (Eq. 13). In fact, an addi-tion of H2O (10 %) into the solution resulted in cathodic shift of +1.43 V to +1.31 V owing to the enhancement of the deprotonation of the hydroxy group. However, catalytic cur-rents arising from the oxidation of water hardly fl owed, and O2 evolution was not confi rmed in the controlled potential electrolysis of [RuII

2(btpyan)(bpy)2(OH)2](SbF6)2 at 1.7 V in the same medium. Thus, the redox reaction of dioxolene

Fig. 8. CV of [Ru2(btpyan)(q)2(OH)2]2+ (a) and [Ru2(btpyan)(bpy)2(OH2)]2+ (b) in the absence (black) and the presence of 10% water (blue) in CF3CH2OH/Ether (1 : 1 v/v).




ligands of [RuII2(btpyan)(q)2(OH)2]2+ also plays the key role in

the water-oxidation.

-2 e-

[RuIV2(btpyan)(bpy)2(O)2]4+

+1.43 V

[RuII2(btpyan)(bpy)2(OH)2]

2+-2e-

+0.89 V

[RuIII2(btpyan)(bpy)2(OH)2]4+

+ 2H+

(13)

Water-oxidation by [RuII2(btpyan)(q)2(OH)2]

(SbF6)2 and [RuII2(btpyan)(bpy)2(OH)2](SbF6)2

Deposited on ITO

Despite the insolubility of [RuII2(btpyan)(q)2(OH)2](SbF6)2 in

water, the complex deposited on a glass plate showed pH dependent electronic absorption spectra. When a [RuII

2(btpyan)(q)2(OH)2](SbF6)2 deposited glass plate was immersed in H2O at pH 2.0, it displayed a strong MLCT band at 578 nm. A new band emerged at 850 nm at the expense of the peak intensity of the 578 nm band with increasing pH. The spectral change almost stopped at pH 3 and became almost constant above pH 3.0. It is worthwhile noting that the solid state of [RuII

2(btpyan)(q)2(OH)2](SbF6)2 undergoes an acid-base equilibrium reaction when it contacts with an aqueous phase, whereas the complex existing in the inner solid is not affected by the change of pH in the aqueous phase. Accordingly, the solid state of [RuII

2(btpyan)(q)2(OH)2](SbF6)2 immersed in neutral and alkaline water showed both 578 nm and 850 nm bands. The CV of [RuII

2(btpyan)(q)2(OH)2](SbF6)2 modifi ed on an ITO electrode (1.2 × 10−8 mol/2.0 cm2) in water (pH 4.0) exhibited a broad redox couple centered at +0.40 V (vs. Ag/AgCl), an irreversible anodic wave at +1.20 V, and a strong anodic current at potential more positive than +1.5 V. Taking into account that both [RuII

2(btpyan)(q)2(OH)2]2+ and [RuII2

(btpyan)(sq)2(O)2]0 exist in the solid state immersed in H2O at pH 4, the broad couple at +0.40 V results from the mixture of the [RuII

2(btpyan)(q)2(OH)2]2+/[RuII2(btpyan)(sq)2(OH)2]0

and [RuII2(btpyan)(q)2(O)2]2+/[RuII

2(btpyan)(sq)2(O)2]0 redox couples (Eqs. 10 and 11). The irreversible anodic wave at +1.20 V is associated with two-electron oxidations of [RuII

2

(btpyan)(q)2(OH)2]2+ and [RuII2(btpyan)(q)2(O)2]2+. A strong

anodic current at potential more positive than +1.2 V is appar-ently coursed by the oxidation of water to dioxygen (Figure 9). Controlled-potential electrolysis of the complex modifi ed on an ITO electrode at 1.70 V (vs. Ag/AgCl) in water (pH 4.0) evolved O2, but the anodic current gradually decreased with a decrease of pH in the aqueous phase and almost stopped at pH 1.2. The current density of the electrode recovered after the adjustment of pH at 4.0 by an addition of aqueous KOH

to the aqueous phase. The oxidation of water by a [RuII2

(btpyan)(q)2(OH)2](SbF6)2 modifi ed ITO fi nally evolved 15.2 ml of O2 (turnover 33500) in 40 h. Finally, the complexes fell off the surface of the ITO and the reaction completely stopped.

Under the same conditions, CV measurements were carried out by using the ITO electrode modifi ed with the bipyridine complex [[RuII

2(btpyan)(bpy)2(OH)2](SbF6)2. The complex was not deprotonated at pH 4, and displayed the nearly reversible redox couple at E1/2 = 0.79 V assigned to the [RuII

2(btpyan)(bpy)2(OH)2]2+/[RuIII2(btpyan)(bpy)2

(OH)2]4+ redox couple. Although an increase of anodic currents was observed in the CV of the complex at potentials more positive than 1.5 V, the amount of dioxygen evolved in the controlled-potential electrolysis of [RuII

2(btpyan)(bpy)2(OH)2](SbF6)2 deposited on ITO at 1.7 V was too small to be con-fi rmed by GC.

Recently, J. Muckerman et al. reported DFT calculations of the catalytic cycle of water oxidation.16 According to the calculations, the most stable form of the double deprotonation of [Ru2(btpyan)(q)2(OH)2]2+ is [Ru2(btpyan)(sq)(cat)(µ-O2

−)]0 with a superoxide RuII-O-O-RuII bond rather than [Ru2(btpyan)(sq)2(O−.)2]0 bearing two oxyl radicals. Accord-ingly, four negative charges generated by dissociation of two hydroxyl protons of [Ru2(btpyan)(q)2(OH)2]2+ are expected to be stored in sq, cat, and µ-O2

− ligands of [Ru2(btpyan)(sq)(cat)(µ-O2

−)]0. Removal of two electrons from dioxolene ligands gives [Ru2(btpyan)(sq)(q)(µ-O2

−)]2+. The fi rst attack of OH− (or H2O) takes place on the RuII(q) site of the (sq)Ru-µ-O2

−-Ru(q) framework, which breaks the Ru-O-O-Ru bridge to form the RuII(sq)O2

− and RuII(q)(OH) groups.17 Oxidation of sq of the RuII(sq)O2

− site followed by electron fl ow from RuII-O2

− to q produces [Ru2(btpyan)(q)(sq)(O2)(OH)]+. The oxidation of Ru(sq)(O2) site induces the second attack of

-0.5 0.0 0.5 1.0 1.5 2.0

2 x 10 -4 A

V vs. Ag /A gC l

Fig. 9. CV of [Ru2(btpyan)(q)2(OH)2](SbF6)2 deposited on ITO in H2O at pH 4.0.



OH− (or H2O) at the resultant RuII(q)(O2) one, which evolves O2 with the regeneration of [Ru2(btpyan)(q)2(OH)2]2+ (Scheme 3). The key process in the catalytic O2 evolution by [Ru2(btpyan)(q)2(OH)2]2+ is the accumulation of four elec-trons in the (cat)RuII-µ-O2

−-RuII(sq) framework without forming high valence Ru=O bonds.

Four-electron oxidation of [RuII2(btpyan)(bpy)2(OH)2]2+

at +1.40 V (vs. Ag/AgCl) would give [(RuIV=O)2(btpyan)(bpy)2]4+ based on the redox behavior of monomeric [Ru(bpy)(trpy)(OH2)]2+. However, the electrolysis of [RuII

2

(btpyan)(bpy)2(OH)2](SbF6)2 modifi ed on an ITO in H2O did not evolve a detectable amount of O2 under similar conditions. The distinct difference in the water oxidation by [RuII

2(btpyan)(q)2(OH)2]2+ and [RuII

2(btpyan)(bpy)2(OH)2]2+ would be asso-ciated with the ease of the O-O bond formation by the cou-pling reaction of two (sq)RuO−. groups in the former compared with that of two RuIV=O groups in the latter. The dinuclear complex, [Ru2(btpyan)(q)2(OH)2]2+, is now called the Tanaka Catalyst because of the unusual stability and high turnover of O2 evolution it offers as a molecular catalyst.

Methanol Oxidation by Ru-Amino Radical Complex

The CV of [Ru(trpy)(q)(NH3)]2+ showed the cat/sq and sq/q redox couples at −0.47 V and +0.34 V (vs. SCE), respectively, in CH3OH (Figure 10). An addition of one equiv of tBuOK to the solution causes the appearance of a new anodic wave at Ep = 0.50 V. The new anodic wave shifts to negative potentials with increasing amounts of tBuOK added in the solution. The redox behavior is explained by an electron and a proton loss of [RuII(trpy)(q)(NH3)]2+. An addition of more than 10 equiv of tBuOK to the solution caused catalytic currents owing to

the oxidation of CH3OH. The controlled potential electrolysis of [Ru(trpy)(q)(NH3)]2+ at 0.35 V (vs. SCE), in the presence of a large excess of BuOK, smoothly proceeded in the initial state. However, the reaction completely stopped after 73 F mol−1 of electricity passed in the electrolysis. ESI-mass spectra of the reaction mixtures of the fi nal electrolyte solution revealed the complete conversion from [Ru(trpy)(q)(NH3)]2+ to [Ru(trpy)(q)(OCH3)]+ arising from the substitution reac-tion of Ru-NH3 by CH3OH during the electrolysis. Indeed, [Ru(trpy)(q)(OCH3)]+ has no ability to catalyze the oxidation of MeOH under similar electrolysis conditions.18

The reaction mechanism for the oxidation of MeOH is proposed in Scheme 4. In the absence of BuOK,

RuII RuIIQ Q

HO

OHRuII RuIIQ SQ

-.O

OH

Ru

II RuII

Cat

SQO

O

2+ + 0

-H+-H+

+H++H+

RuII RuIIQ SQO

O

2+

-2e-

RuII RuIIQ SQ

2+

RuII RuIIQ SQO2

+

OH-

OH-

RuII RuIIQ QO2

2+

OH-

-e-OH-O2

O2

-e-, +OH-

Scheme 3. Catalytic cycle of water oxidation proposed by DFT calculations.

Fig. 10. CV of [Ru(trpy)(q)(NH3)]2+ in the presence of various amounts of tBuOK in CH3OH.




[RuII(trpy)(q)(NH3)]2+ is reversibly oxidized to [RuIII(trpy)(q)(NH3)]3+. The peak current of the anodic-wave shifts to negative potentials with increasing amounts of base in the solution. The threshold potential of the catalytic current of the oxidation of CH3OH in the presence of more than 10 equiv of BuOK is consistent with that of the oxidation of [RuII(trpy)(q)(NH3)]2+. As mentioned above, deprotonation of [RuII(trpy)(q)(OH)]+ is followed by the electron transfer from O2− to q to give [RuII(trpy)(sq)(O−.)]0. In addition, the redox potential of the RuII/RuIII couple of [RuII(trpy)(q)(NH3)]2+ becomes quite close to that of the sq/q couple in the presence of excess amounts of base. These results indicate that deprot-onation of [RuIII(trpy)(q)(NH3)]3+ will cause electron fl ow from NH2

− to RuIII or q. As a result, the actual electronic structure of the oxidation product of [RuII(trpy)(q)(NH3)]2+ under basic conditions would be expressed by the resonance of Eq. 14. Among the three species in the resonance equation of Eq. 14, [RuIII(trpy)(sq)(NH2

.)]2+ or [RuII(trpy)(q)(NH2.)]2+

formed by electron transfer from NH2− to q or RuIII of

[RuIII(trpy)(q)(NH2−)]2+. is considered to be the active species

for the oxidation of CH3OH. If NH2. and RuIII of [RuIII(trpy)

(sq)(NH2.)]2+ or q of [RuII(trpy)(q)(NH2

.)]2+ work as hydrogen and electron acceptors, respectively, the oxidation of methanol by these species gives H2CO without involving a radical inter-mediate (Scheme 4).19

[RuIII(trpy)(q)(NH2-)]2+

[RuII(trpy)(q)(NH2.)]2+[RuIII(trpy)(sq)(NH2

.)]2+ (14)

As described above, substitution of the NH3 of [RuII(trpy)(q)(NH3)]2+ with CH3OH loses the catalytic

activity for the oxidation of CH3OH. To depress the substitu-tion of NH3, [RuII(NH2CH2CH2bpa)(q)]2+, having a tetra-dentate ligand, was newly prepared. The oxidized form of [RuII(NH2CH2CH2bpa)(q)]2+ showed an ability to oxidize CH3OH to produce H2C=O in the presence of a stoichio-metric amount of base. However, β-hydrogen abstraction of the benzyl hydrogen of [Ru(NH2CH2CH2bpa)(q)]2+ took place in the presence of excess amounts of base to afford [RuII(NH=CHCH2bpa)(sq)]+ (Eq. 15). The catalytic ability of [Ru(NH=CHCH2bpa)(q)]2+ toward the oxidation of MeOH under the controlled potential electrolysis is much lower than that by [Ru(NH2CH2CH2bpa)(q)]2+.20

NH2

RuIINN

O

O

N

t-But-Bu

2+

excess t-BuOK

N

RuIINN

O

O

N

t-But-Bu

+

(15)

To avoid β-hydrogen elimination of [RuII(NH2CH2CH2

bpa)(q)]2+, the deprotonation behavior of [Ru(NH2Ph-bpa)(q)]2+, having no CH bond at the βposition of the NH2 group, was examined.21 The amino proton of [Ru(NH2Ph-bpa)(q)]2+ reversibly dissociates in the presence of base to gave [Ru(NHPh-bpa)(q)]+, and both complexes are diamagnetic. On the other hand, the treatment of [Ru(NHPh-bpa)(q)]+ with one equiv of tBuOK produced a triplet state of [Ru(−.NPh-bpa)(sq)]0 (s = 1), which undergoes the sq localized redox reaction at −1.5 V (vs. SCE) to produce [Ru(−.NPh-bpa)(cat)]− (s = 1/2). The EPR spectra of the latter displayed a rhombic pattern signal with g1 = 2.175, g2 = 2.105, and

NN

NRuII

NH3

O

O

t-Bu

t-Bu

NN

NRuII

NH3

O

O

t-Bu

t-Bu

-e-, -H+

CH3OH

CH2O

NN

NRuII

NH2

O

O

t-Bu

t-Bu

+

2+

+e-

-e-

Active Species

NN

NRuIII

NH2-

O

O

t-Bu

t-Bu

2+

2+

+

H+

Scheme 4. Oxidation of MeOH catalyzed by [Ru(trpy)(q)(NH3)]2+.



g3 = 1.950. The g3 component is split into three signals as a result of hyperfi ne coupling with the N nucleus (I = 1) and an odd electron with a hyperfi ne coupling constant (A(N)) of 8.2 mT (Figure 11). Thus, deprotonation of [Ru(NHPh-bpa)(q)]+ causes the intramolecular electron transfer from nitrogen to quinone to induce the generation of an anilino radical and a semiquinone radical. However, an anilino radical stabilized by a phenyl group essentially loses the ability to oxidize CH3OH.21

Reversible Conversion Between CO2 and CO on a Ru Complex

In 1983, the author found the equilibrium among [Ru(bpy)2(CO)2]2+, [Ru(bpy)2(CO)(C(O)OH)]+, and [Ru(bpy)2(CO)(CO2)]0 in H2O (Figure 12).21 The population of each species is simply regulated by the pH of the aqueous

solutions.22 The X-ray analysis of single crystals of [Ru(bpy)2(CO)(η1-CO2)]0.3H2O showed the existence of hydrogen bonding between three solvated water molecules and the oxygen atoms of the CO2 group.23 A small non-equivalency in the two C-O bond distances of the Ru-CO2 group prob-ably results from the unsymmetrical hydrogen bonding with three solvated water molecules. Such hydrogen bonding would decrease the electron density of the Ru-CO2 group and greatly contribute to the stability of the complex, since [Ru(bpy)2(CO)(η1-CO2)]0 shows an unusual stability to air and temperature in contrast to that of metal-η1-CO2 com-plexes reported so far.24 The most prominent feature of the molecular structure of [Ru(bpy)2(CO)(η1-CO2)]0.3H2O is the O-C-O angle of 120.9°, indicating the sp2 hybrid orbital of the central carbon. Protonation of oxygen of the Ru-η1-CO2 group produces [Ru(bpy)2(CO)(η1-C(O)OH)]+. Further pro-tonation of the hydroxy group generates [Ru(bpy)2(CO)2]2+. Gradual decrease of the bond distance between Ru and carbon in the order of CO2 > C(O)OH > CO (Figure 13) is appar-ently associated with the increase of p-acceptor and the decrease of s-donor ability in the series. The Ru-N bond length also shortens in response to the change of the Ru-CO2, -C(O)OH, and -CO lengths. Thus, bpy, having σ-donor and π-acceptor character, works as a buffer to absorb the drastic changes of the electronic structures of the Ru-X bond in the equilibrium reaction of [Ru(bpy)2(CO)X]n+ (X = CO2 (n = 0), C(O)OH (n = 1), and CO (n = 2)) depending on pH (Figure 12).

The reaction of [Ru(bpy)2(CO)2](PF6)2 with the same amount of aqueous NaBH4 in CH3OH/H2O (1 : 1 v/v) at −20 °C selectively produced [Ru(bpy)2(CO)(CHO)](PF6) as a yellow precipitate. When the same reaction was conducted in CH3CN/H2O, [Ru(bpy)2(CO)(CHO)]+ was further Fig. 11. Epr spectra of [Ru(−.NPh-bpa)(cat)]− at 20 K.

Fig. 12. Equilibrium among [Ru(bpy)2(CO)2]2+, [Ru(bpy)2(CO)(C(O)OH)]+, and [Ru(bpy)2(CO)(CO2)]0 in H2O at 25 C.




reduced to [Ru(bpy)2(CO)(CH2OH)]+, since the former is soluble in the medium. Furthermore, the treatment of [Ru(bpy)2(CO)(CH2OH)]+ with acidic water gave CH3OH and [Ru(bpy)2(CO)(OH2)]2+ in an almost quantitative yield. Thus, the CO2 group of [Ru(bpy)2(CO)(η1-CO2)]0 is succes-sively reduced to CH3OH, and the structural changes from Ru-η1-CO2 to Ru-CH2OH have been determined by X-ray analysis of each intermediate (Scheme 5).

As described above, [Ru(bpy)2(CO)(CO2)]0, [Ru(bpy)2

(CO)(C(O)OH)]]+, and [Ru(bpy)2(CO)2]2+ exist as an equi-librium mixture in aqueous solutions. A CO ligand of the latter is stoichiometrically reduced to CH3OH in the presence of BH4

−. However, BH4− is not a suitable reducing agent for

the construction of a catalytic six-electron reduction of CO2, because it directly reacts with CO2 to produce HCOOH. Electro- and photochemical reduction of CO2 catalyzed by metal complexes must be a feasible process to realize catalytic six-electron reduction of CO2. From this point of view, ligand localized redox reactions of polypyridyl-Ru complexes provide

suitable methodologies for the construction of an electron reservoir required in the reduction of CO2. For example, bpy localized two-electron reduction of [Ru(bpy)2(CO)2]2+ takes place around −1.20 V (vs. SCE), which cleaves a Ru-CO bond (CO evolution) owing to electron fl ow from the π* orbital of bpy to the σ* of the Ru-CO bond. In CO2-saturated aqueous solutions, an electrophilic attack of CO2 to the resultant penta-coordinate [Ru(bpy)2(CO)]0 affords [Ru(bpy)2(CO)(CO2)]2+, which is smoothly converted to [Ru(bpy)2(CO)2]2+ according to the proton concentrations of the solution (Scheme 6). Schemes 5 and 6 clearly reveal the turning point between the Ru-CO bond cleavage (CO evolution) and successive reduc-tion to Ru-CH2OH through Ru-CHO. There have been reported a large number of studies on CO evolution by the reduction of CO2 catalyzed by metal complexes. Taking into account that once Ru-η1-CO2 is formed, it is quantitatively converted to Ru-CO (Scheme 5), the key process for the cata-lytic six-electron reduction of CO2 is to reduce metal-CO without evolving CO. There would be three types of approach

N Ru C

O

O

120.9°

1.25

1.282.062.204

C

O

1.15

1.81

N Ru C

O

OH

114.9°

1.242

1.3452.0032.146

C

O

1.15

1.81N Ru C O

176°

1,872.073

C

O

1.12

1.91

1.15

Fig. 13. Crystal structures around the Ru center of [Ru(bpy)2(CO)(CO2)].3H2O, [Ru(bpy)2(CO)(C(O)OH)](CF3SO3)(H2O), and [Ru(bpy)2(CO)2](PF6)2.

Scheme 5. Stepwise Reduction of CO2 to CH3OH on [Ru(bpy)2(CO)X]n+ (X = CO2, C(O)OH, CO, C(O)H, CH2OH), and their molecular structure.



to realize electrochemical reductions of Ru-CO without the accompanying bond cleavage: the fi rst one is to reduce Ru-CO complexes at low temperature,25 the second is to form a metal-lacyclic ring including the Ru-CO bond,26 and the third is to develop an electrochemically renewable hydride donor in place of BH4

−.27

A mono-carbonyl complex, [Ry(bpy)(trpy)(CO)]2+, undergoes a reversible one-electron reduction at E1/2 = −1.35 V (vs. Ag/AgNO3 (0.1 M)) and an almost irreversible reduc-tion at Epc = −1.69 V at room temperature. Electrochemical reduction of [Ry(bpy)(trpy)(CO)]2+ at −1.60 V in CO2-saturated CH3CN produced CO as the main product at room temperature. On the other hand, the irreversible cathodic wave (Epc = −1.69 V) of [Ry(bpy)(trpy−.)(CO)]+ became a reversible one at −20 °C. This result implies a possibility of the reduction (hydrogenation) of the Ru-CO bond with the two electrons stored in [Ru(bpy−.)(trpy−.)(CO)]0 at −20 °C in protic condi-tions. Indeed, [Ru(bpy)(trpy)(CHO)]+ was confi rmed in the electrolysis of [Ry(bpy)(trpy)(CO)]2+ at −20°C in EtOH/H2O. Furthermore, electrochemical reduction of [Ry(bpy)(trpy)(CO)]2+ in CO2-saturated EtOH/H2O at −1.60 V (vs. Ag/

AgNO3 (0.1 M)) produced not only C1 but also C2 products at −20°C. The formation of those products are explained by the existence of the [Ru(bpy)(trpy)(CHO)]+ intermediate (Scheme 7). However, the rate of the electrochemical reduction of CO2 at −20°C was too slow for practical use.25

It is well known that 1,8-naphthyridine (napy) coordi-nates to metal centers through η1- and η2-modes.26 Some η1-napy ligated to metals exhibit the bidentate character in solutions owing to the fast site exchange isomerization between two nitrogens of napy. Electrochemical reduction of [Ru(bpy)2(1,8-napy-kN)(CO)]2+ at −1.10 V (vs. SCE) greatly enhances the basicity of the free nitrogen of the napy ligand. As a result, a fi ve-membered metallacycle is formed arising from an attack of the nitrogen atom on the carbonyl carbon of the Ru-CO bond (Eq. 16). Electrochemical reduction of [Ru(bpy)2(1,8-napy-kN)(CO)]2+ at −1.40 V in H2O with the expectation of the reduction of CO in the fi ve-membered ring without the Ru-CO bond cleavage, however, resulted in selec-tive hydrogenation of the 5-position of napy, and the CO group in the metallacycle remained unchanged (Eq. 17). The electrochemical oxidation of the metallacyclic complex at 0.30 V (vs. SCE) quantitatively regenerated the original [Ru(bpy)2(1,8-napy-kN)(CO)]2+ in H2O (Eq. 17).

Ru CNN

NN

N N

O + e-

2+

Ru CNN

NN

N N

O

+

-1.40 V in CH3CN

0 V in CH3CN

(16)

Ru CNN

NN

N N

O + 2e-

2+

Ru CNN

NN

N N

O

+ H+

HH +

-1.4 V in H2O

+0.3 V in H2O

(17)

[RuL2L'(CO)]2+

[RuL2L'(CO2)]0

[RuL2L']0[RuL2L'(C(O)OH)]+

2e-

CO

CO2

H+

H+

H2O

Scheme 6. Two electron reduction of CO2 catalyzed by [Ru(bpy)2(CO)2]2+.

2e-,2H+2e-,H+[Ru-CO2]0 [Ru-C(O)OH]+ [Ru-CO]2+ [Ru-CHO]+ [Ru-CH2OH]+

[Ru]0

[Ru] = [Ru(bpy)(trpy)]

HCOOH COHOOCCHO

HCHOCO2

HOOCCH2OH

CH3OHCO2

CO2HCOO-

CO2

H+ H+H+

Scheme 7. Multi-electron reduction of CO2 catalyzed by [Ru(bpy)(trpy)(CO)]2+.




Ru CNN

NN

N N

O

HH +

-1.80 V in H2O

Ru

C

NN

NN

O

H

Ru CNN

NN

N N

O

HH +

+

+

HH

HH

(18)

The CO group in the metallacyclic complex still remained unchanged even in the electrochemical reduction of the metal-lacyclic complex at −1.80 V (vs. SCE) in H2O. Instead, 6- and 7-positions of the napy group were hydrogenated, and [Ru(bpy)2(CO)H]+ generated by the dissociation of the napy ligand was formed as a minor product. Although the reaction of Eq. 18 was irreversible and the reduction product formed by hydrogenation at the 5-, 6-, and 7-positions was extremely labile, hydrogenation of the 5-position of the napy group driven by the two-electron and one-proton transfer (Eq. 17) was a completely reversible process, and the stoichiometry is essentially the same as the enzymatic NAD/NADH redox couple.27

Nicotinamide adenine dinucleotide (NAD+) is one of the most important coenzymes that mediate various biological redox reactions. NAD+ is reduced with two electrons and one proton, and the resultant NADH releases hydride to various biological substrates while regenerating NAD+.28 The charac-teristic NAD+/NADH redox couple has been drawing much attention, since chemical reactions without any accompanying byproducts using renewable catalysts are highly desired in order to realize a sustainable society. Large numbers of photo-chemical reactions mediated by NADH model compounds, such as reduction of alkyl halides, olefi ns, ketones, and photo-induced electron transfer29 as well as thermal reactions

mediated by NADH analogs have been extensively studied. Reversible conversions between NAD+ and NADH analogs, however, have not been achieved so far owing to the coupling reactions of radical intermediates formed by one electron reduction of the oxidized form (Scheme 8). As a result, reac-tions mediated with NADH models have been limited to stoichiometric ones.

To suppress the coupling reaction of Scheme 8, a new type of BNA model ligand, 2-(2-pyridyl)benzo[b]-1,5-naphthyridine (pbn), was prepared by a coupling reaction of 3-aminoquinaldehyde and 2-acetylpyridine (Eq. 19).30

N NH2

CHO+

N C(O)Me

N

NN (19)

Treatment of Ru(bpy)2Cl2 with two equiv of AgPF6, fol-lowed by the reaction with pbn, afforded [Ru(bpy)2(pbn)](PF6)2 (pbn = 2-pyridylbenzo[b]-1,5-naphthyridine) as a red-purple solid. Electrochemical reduction of [Ru(bpy)2(pbn)]2+ at −1.14 V (vs. Fc/Fc+) in a 1 : 1 mixture of CH3CN and 0.1 M AcOH/AcONa buffer consumed two electrons per molecule, and selectively produced [Ru(bpy)2(pbnH2)]2+ (Eq. 20). Fur-thermore, the [Ru(bpy)2]2+ framework in [Ru(bpy)2(pbn)]2+ functioned as a photosensitizer, since irradiation of visible light (λ > 420 nm) to [Ru(bpy)2(pbn)]2+ in the presence of N(CH2CH2OH)3 as a sacrifi cial electron donor smoothly produced [Ru(bpy)2(pbnH2)]2+ with a high quantum number (32 %) in similarity to the electrochemical reduction of [Ru(bpy)2(pbn)]2+ under protic conditions (Eq. 20).31 A similar visible light irradiation of [Ru(bpy)2(pbn)]2+ in the presence of triethylamine in dry CH3CN selectively afforded [Ru(bpy)2

(pbn−.)]+ as the one-electron reduced form of [Ru(bpy)2(pbn)]2+. Thus, N(CH2CH2OH)3 (pKa = 7.7) acts as electron and proton donors in the photochemical two-electron reduction of [Ru(bpy)2(pbn)]2+ (Eq. 21). The photochemical two electron reduction of [Ru(bpy)2(pbn)]2+ was followed by means of pulse radiolysis using CO2

−.; [Ru(bpy)2(pbn)]2+ is rapidly reduced by CO2

−. (k = 4.6 × 109 M−1s−1). The resultant [Ru(bpy)2(pbn−.)]+

CONH2

N+

R

HCONH2

N

R

H

CONH2

N

R

HH

H2NCO

N RH

CONH2

NRH

H+, e-

12

coupling

e-

Scheme 8. Radical coupling of one-electron reduced species of BNA+.



undergoes protonation on the free nitrogen of pbn within a diffusion time to give [Ru(bpy)2(pbnH.)]2+. The rate of the decrease of the protonated one-electron reduced form is found to be second order with respect to the concentration of the metal complexes, indicating the occurrence of a dimerization through π−π interaction of the two neutral pbnH. ligand. Intramolecular electron and proton transfer, or hydrogen transfer from one pbnH. to another inside the dimer, affords a 1 : 1 mixture of [Ru(bpy)2(pbnH2)]2+ and [Ru(bpy)2(pbn)]2+ (Scheme 9).32 Scheme 9 shows the fi rst example to demonstrate photochemical two-electron reduction of a metal complex driven by one-photon irradiation.

N

NN H H

2+2+N

NN

RuL2

H

+ 2e- + 2H+

L2RuL = bpy

(20)

N

NN H H

2+2+N

NN

RuL2

H

L2RuL = bpy

hn > 420 nm

N(CH2CH2OH)3

(21)

The fi nding that two-electron reduced complexes [Ru(bpy)2(pbnH2)]2+ can be generated through the dimeriza-tion of a protonated one-electron reduced complex, {[Ru(bpy)2(pbnH.)]2+}2, followed by a disproportionation reaction opens a new era for the photochemical multi-electron reduction of metal complexes, since irradiation of [Ru(bpy)(pbn)2]2+ and [Ru(pbn)3]2+ with visible light in the presence of N(CH2CH2OH)3 produced [Ru(bpy)(pbnH2)2]2+

and [Ru(pbnH2)3]2+, respectively, as four- and six-electron reduction products (Eqs. 22, 23).33 Thus, Ru-pbn complexes allow photochemical 2-, 4-, and 6-electron reductions under visible-light irradiation, which provides a great contribution in the development of the conversion of light energy to chemi-cal energy.

NN

N

N

N

NN

NRu

NN

N

N

N

NN

NRu

hn(l > 420 nm)

H

H H

HH

H

2+ 2+

TEOA

(22)

NN

N

N

N

NNNN

RuN

N

N

N

N

NNNN

Ru

hnl ( > 420 nm)

H

H H

HH

HH

HH

2+ 2+

TEOA

(23)

Dimerization of CO2

Electrochemical oxalate formation also attracts interest from the viewpoint of the utilization of carbon dioxide. Outer-sphere electron transfer from an electrode to CO2 molecules, however, is a high energy consumption process, since direct electrochemical reduction of CO2 at potentials more negative than −2.0 V (vs. SCE), under dry conditions, predominantly produce oxalates by the C-C coupling of CO2

−. (Eq. 24).34 It is, therefore, highly desired to develop catalytic oxalate generation under mild reaction conditions. It should be noted

NN

N+

Ru(bpy)2N

N

N2+

Ru(bpy)2

H

H

NN

N2+

Ru(bpy)2

H

H HN

N

N

Ru(bpy)2

H

H

NN

N

(bpy)2Ru

H

H

4+

H+

–H+

pKa = 11.0

k = 2.2× 108 M–1s–1

CO2•–N

N

N2+

Ru(bpy)2

k = 4.6× 109 M–1s–1

1/2N

N

N2+

Ru(bpy)2

+ 1/2

Scheme 9. Photochemical two electron reduction of [Ru(bpy)2(pbn)]2+.




that the conversion from metal-CO2 to metal-CO is achieved either by dehydration in protic media (Eq. 25) or oxide transfer to CO2 under aprotic conditions (Eq. 26), and the process consumes two electrons at the metal center. In other words, the CO2 group of M-CO2 adducts will remain unchanged unless two electrons are provided from the central metals. Taking into account that two electrons and two CO2 molecules are required in oxalate formation, a 1 : 2 adduct between a two-electron reduced metal complex and CO2 may work as a possible precursor to oxalate without CO evolution. A series of [(MCp′)3(µ3-S)2]2+ (M = Co, Rh, Ir; Cp′ = pentamethyl- or methylcyclopentadiene) are pentagonal bypiramidal structures with three coordinatively saturated metal ions (48 electron species). The LUMO of the cluster is located in anti-bonding orbitals of metal-metal bonds. Dahl et al. demonstrated that two electron reduction of [(CoCp′)3(µ3-S)2]2+ results in a cleav-age of one of the Co-Co bonds of the 50 electron species (Scheme 10).35 This result indicates that two-electron reduc-tion of the M3(µ3-S)2 core creates two coordinatively unsatu-rated metal centers as possible binding sites of CO2.

2CO2 + 2e- -OOC-COO- (24)

[M-CO2]n+ + [M-CO](n+2)+ + H2O2H+ (25)

[M-CO2]n+ + CO2 [M-CO](n+2)+ + CO32- (26)

The reaction of two electron reduced forms of [(MCp′)3

(µ3-S)2]2+ with CO2 in CH3CN gave C2O42− (ca. 40% yield).

In accordance with this, the electrochemical reduction of [(CoCp′)3(µ3-S)2]2+ at −0.70 V (vs. Ag/AgCl) in CO2-saturated CH3CN catalytically produced oxalate with a current effi -ciency of 80%.36 The reaction of [(IrCp*)3(µ3-S)2]2+ (Cp* = pentamethylcyclodiene) with CO2 in CH3CN also produced C2O4

2− with the unexpected generation of [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)(µ3-S)2]+, in which NCCH2

− is attached to one of the Cp* rings of [(IrCp*)3(µ3-S)2]2+. The CH3CN adduct showed very high reactivity toward catalytic C2O4

2− produc-tion, which could be followed by the IR spectra under the

electrolysis of [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]+ in CO2-saturated CH3CN. Three Ir atoms of the CH3CN adduct are coordinatively saturated. In addition, the bulky Cp* ligands seemed to block an attack of CO2 to Ir. Despite these facts, the IR spectra of the solution clearly displayed a ν(CO2) band at 1682 cm−1 arising from adduct formation between [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]+ and CO2 at room temperature. Furthermore, when the CO2 adduct was reduced by electrolysis at −1.50 V (vs. Ag/AgCl) in CO2-saturated CH3CN, another ν(CO2) band appeared at 1603 cm−1 prior to the appearance of the ν(CO2) band at 1633 cm−1 arising from the catalytic formation of oxalate. The molecular struc-ture of [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]+ determined by X-ray analysis revealed that the space on µ3-S seems to allow an attack of CO2 on µ3-S (Figure 14). So, the fi rst attack of CO2 on [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]+ must take place on µ3-S. One electron reduction of [(Ir-η5-Cp*)2

(Ir-η4-Cp*CH2CN)](µ3-S)2]+ will cleave one of the Ir-Ir bonds, which provides the coordination site for an attack of CO2 on Ir. Further one electron reduction of the resultant [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2](CO2)2 causes the coupling reaction of CO2 with regeneration of [(Ir-η5-Cp*)2

(Ir-η4-Cp*CH2CN)](µ3-S)2]+ and C2O42− (Scheme 11).37

S

S

Co CoCo

2.12

2.52

[(CoCp')3S2]2+

48 e

S

S

Co CoCo

2.15

2.49

[(CoCp')3S2]+

49 e

2.87

S

S

Co CoCo

2.17

2.48

[(CoCp')3S2]+

50 e

3.19

+e-

+0.35 V

e-

-0.37 V

Scheme 10. Structural change of the Co3S2 core upon one- and two-electron reduction of [(CoCp′)3(µ3-S)2]2+.

Fig. 14. Crystal structures of [(Ir-η5-Cp*)(Ir-η4-C5H4CH2CN)(µ3-S)2]+.



[(MCp')3(3-S)µ 2]0

C2O42-

Me4NBF4 in CH3CN2CO2

(M = Co, Rh, Ir: Cp' = MeCp, Me5Cp)

(27)

Free carbon dioxide is a linear molecule. The O-C-O angle of [Ru(bpy)2(CO)(η1-CO2)]0 is 120°, indicating that the central carbon is linked to Ru with sp2 hybrid orbital. On the other hand, that of [Co(η1-CO2)(salen)]+ is 135°,24 which is consistent with that of the one-electron-reduced form of CO2. It is, therefore, reasonably assumed that an O-C-O angle of the metal-η1-CO2 complex refl ects the amount of electron fl ow from the central metal to the CO2 group. The O-C-O angles of M-η1-CO2 is evaluated by the comparison of ν(CO2) band of the 12CO2 and 13CO2 adduct (Eq. 28).38 The two CO2 molecules bonded on [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]0 were calculated as 152° and 135° based on compari-son of the νasym(CO2) band of the 1 : 2 adduct with 12CO2 and 13CO2. The CO2 molecules linked on the Ir3 cluster with the O-C-O angle of 152° is assigned to µ3-S-η1-CO2, since three Ir atoms of [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]+ have no coordination to bind CO2 (Figure 14). One-electron reduction of [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]+ will result in a cleavage of one of the Ir-Ir bonds, which will provide the coordination site of CO2. The second CO2 bonded to [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]0 with a O-C-O angle of 135°, therefore, is associated with the Ir-η1-CO2 adduct. Thus, the CO2 group linked to Ir is considered as the one-electron-reduced form based on the OCO angle. The coupling reaction of two CO2 molecules activated on S and Ir of [(Ir-η5-Cp*)2(Ir-η4-Cp*CH2CN)](µ3-S)2]0 is explained from the view that the electron density of both CO2 molecules are enough to form oxalate, but not enough to undergo the oxide

transfer reaction (Eq. 26), since the CO2 group linked to Ir is considered as a one-electron reduced form based on the OCO angle.

12C + 2 x 16O sin2a

13C + 2 x 16O sin2a=13w

12w

2a*

O

O

CX

12C

13C (28)

Summary

Both Ru-quinone-aqua and -amino complexes are proposed as possible candidates for energy converters of chemical energy to electrical energy. Acid-base equilibrium reactions of the aqua and amino complexes smoothly produce Ru-oxyl radical and -amino radical complexes that have an ability to catalyze elec-trochemical oxidation of CH3OH under very mild conditions. Furthermore, a Ru2 dimer having two oxyl radicals at a short distance showed a remarkable activity for O2 evolution by four-electron oxidation of water (TON = 35000). The most distinctive feature is that four-electron oxidation of water was achieved only by the redox reactions of two dioxolene ligands without changing the Ru(II) valence state in the catalytic cycle.

One-electron oxidation of [RuII(trpy)(q)(NH3)]2+ as an analog of [RuII(trpy)(q)(OH2)]2+ under basic conditions is accompanied with proton dissociation to produce [RuII(trpy)(q)(NH2

.)]2+, in which q and NH2. probably play

the roles of electron and hydrogen acceptor, respectively, for the oxidation of CH3OH. The catalytic activity of [RuII(trpy)(q)(NH3)]2+ was gradually lost owing to the substi-tution reaction of the Ru-NH3 group with CH3OH. Treat-ments of [Ru(NH2Ph-bpa)(q)]2+ with one and two equiv of tBuOK afforded diamagnetic [Ru(NHPh-bpa)(q)]+ (s = 0) and paramagnetic [Ru(−.NPh-bpa)(sq)]0 (S = 1), respectively. Fur-thermore, electrochemical one-electron reduction of the latter produced [Ru(−.NPh-bpa)(cat)]− which clearly showed a hyper-fi ne coupling with the N nucleus (I = 1) of the anilino radical and an odd electron with a coupling constant, A(N) = 8.2 mT. Thus, deprotonation of [Ru(NHPh-bpa)(q)]+ is followed by the intramolecular electron transfer from Ru-N2− to q, which generates [Ru(−.NPh-bpa)(sq)]0 with anilino radical and semiquinone radicals.

A series of complexes, [Ru(bpy)2(CO)(X)]n+ (X = CO2 (n = 0), C(O)OH (n = 1), CO (n = 2), C(O)H (n = 1), and CH2OH (n = 1)), were prepared and their molecular structures were determined by X-ray analysis. Based on the structural changes of the complexes, a reasonable pathway from CO2 to CH3OH by electro- and photochemical CO2 reduction was proposed. Along this line, a new ligand having a NAD/NADH

Ir *Cp

Cp*Ir

Cp*Ir

Cp*Ir

Cp*Ir

Ir *Cp

+

0

Cp*Ir

Cp*Ir

Ir *Cp

+

C2O42-

S

S

S

S

S

S

CO2

CO2

CO2

e-, CO2

e-

CO2

Scheme 11. Proposed mechanism for catalytic generation of oxalate mediated eith tri-Ir cluster.




function was synthesized with a desire for the catalytic generation of CH3OH in the reduction of CO2. A series of [Ru(bpy)3−n(pbn)n]2+ (n = 1, 2, 3) undergoes photochemical two- (n = 1), four- (n = 2), and six-electron reduction (n = 3), respectively, under irradiation of visible light in the presence of N(CH2CH2OH)3. The photochemical multi-electron reduction would open new era for the energy conversion from light energy to chemical energy.

REFERENCES

[1] Fujishima, A., Honda, K. Nature 1972, 238, 37; McEvoy, J. P., Brudvig, G. W. Chem. Rev. 2006, 106, 4455.

[2] Bassham, J. A. Photosynth. Es. 2003, 76, 35. [3] Haga, M.; Dodsworth, E. S., Lever, A. B. P. Inorg. Chem. 1986,

25, 447. [4] Bhattacharya, S., Boone, S. R., Fox, G. A., Pierpont, C. G.

J. Am. Chem. Soc. 1990, 112, 1088.; Wada, T., Tanaka, K. Eur. J. Inorg. Chem. 2005, 3832.

[5] Wada, T., Yamanaka, M., Fujihara, T., Miyazato, Y., Tanaka, K. Inorg. Chem. 2006, 45, 8887.

[6] Moyar, B. A., Meyer, T. J. Inorg. Chem. 1981, 20, 436; Takeuchi, K. J., Thompson, M. S., Pioes, D. W., Meyer, T. J. Inorg. Chem. 1984, 23, 1845.

[7] Lebeau, E. L., Meyer, T. J. Inorg. Chem., 1999, 38, 2174; Rodriguez, M., Romero, I., Llobet, A., Deronzier, A., Biner, M., Parella. T., Stoeckli-Evans, H. Inorg. Chem. 2001, 40, 4150.

[8] Dioxolene and polypyridyl are redox active ligands. The fi rst electrochemical reduction of [Ru(trpy)(q)(OH2)]2+ takes place at a quinone localized orbital around 0 V vs. SCE, while that of [Ru(trpy)(bpy)(OH2)]2+ occurs at a trpy localized one at potentials more negative than −1.2 V. The functionality and reactivity of the Ru-dioxolene-polypyridyl complexes presented in the manuscript are generated by the redox reaction of diox-olene but not by polypyridyl. So, Ru-dioxolene-polypyridyl complexes are abbreviated as Ru-dioxolene complexes even though dioxolene is involved in the complexes.

[9] Tsuge, K., Kurihara, M., Daniel, S., Tanaka, K. Inorg. Biochem. 1997, 67, 440; Kurihara, M., Daniele, S., Tsuge, K., Sugi-moto, H., Tanaka, K. Bull. Chem. Soc. Jap., 1998, 71, 867.

[10] Kobayashi, K., Ohtsu, H., Wada, T., Kato, T., Tanaka, K. J. Am. Chem. Soc. 2003, 125, 6729.

[11] Tsuge, T., Kurihara, K., Tanaka, K. Bull. Chem. Soc. Jpn. 2000, 73, 607.

[12] Wada, T., Tsuge, K., Tanaka, K. Chem. Lett., 2000, 910.[13] Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J.,

Iwata, S. Science, 2004, 303, 1831; Loll, B., Kern, J., Saenger, W., Zouni, A., Biesiadka, J. Nature, 2005, 438, 1040.

[14] Meyer, T. J., Hang, M., Huynh, V., Thorp, H. H. Angew. Chem., Int. Ed. 2007, 46, 5284.

[15] Wada, T., Tsuge, K., Tanaka, K. Inorg. Chem. 2001, 40, 329; Wada, T., Tsuge, K., Tanaka, K. Angew. Chem. Int. Ed. 2000, 39, 1479.

[16] Muckerman, J., Polyansky, D., Wada, T., Tanaka, K., Fujita, E. Inorg. Chem. 2008, 47, 1787.

[17] To be published.[18] Hino, T., Wada, T., Fujihara, T., Tanaka, K. Chem. Lett. 2004,

1596.[19] To be published.[20] Miyazato, Y., Wada, T., Tanaka, K. Bull. Chem. Soc. Jpn. 2006,

79, 745.[21] Miyazato, Y., Wada, T., Muckerman, J. T., Fujita, E., Tanaka,

K. Angew. Chem., Int. Ed., 2007, 46, 5728.[22] Tanaka, K., Morimoto, M., Tanaka, T. Chem. Lett., 1983, 901;

Ishida, H., Tanaka, K., Morimoto, M., Tanaka, T. Organome-tallics, 1986, 5, 724.

[23] Tanaka, H., Nagao, H., Peng, S-M., Tanaka, K. Organometal-lics, 1992, 11, 1450; Tanaka, H., Tzeng, B-C., Nagao, H., Peng, S-M., Tanaka, K. Inorg. Chem. 1993, 32, 1508; Toyohara, K., Tsuge, K., Tanaka, K. Organometallics, 1995, 14, 5099; Tanaka, K., Ooyama. D. Coordination Chemistry Reviews, 2002, 226, 211.

[24] Calabrese, J. C., Herskovitz, T., Kinney, J. B. J. Am. Chem. Soc. 1983, 105, 5914; Gambarotta, S., Arena, F., Floriani, C., Zanazzi, P. F. J. Am. Chem. Soc. 1982, 104, 5082.

[25] Nagao, H., Mizukawa, T., Tanaka, K. Inorg. Chem. 1994, 33, 3415.

[26] Nakajima, H., Nagao, H., Tanaka, K. J. Chem. Soc., Dalton Trans. 1996, 1405.

[27] Tomon, T., Koizumi, T., Tanaka, K. Angew. Chem., Int. Ed. Engl. 2005, 44, 2229.

[28] Walsh, C. Acc. Chem. Res. 1980, 13, 148; Stout, D. M., Meyers, A. I. Chem. Rev. 1982, 82, 223.

[29] Fukuzumi, S. Advances in Electron-Transfer Chemistry, (Ed. Mariano, P. S), JAI Inc., Greenwich, CT, 1992, pp67–175; Gebicki, J., Marcinek, A., Zielonka, J. Acc. Chem. Res. 2004, 37, 379.

[30] Koizumi, T., Tanaka, K. Angew. Chem., Int. Ed. 2005, 44, 5891.

[31] Polyansky, D., Cabelli, D., Muckerman, J. T., Fujita, E., Koizumi, T., Fukushima, T., Wada, T., Tanaka, K. Angew. Chem., Int. Ed. 2007, 46, 4169.

[32] Polyansky, D., Cabelli, D., J. Muckerman, J. T., Fukushima, T., Tanaka, K., Fujita, E. Inorg. Chem. 2008, 47, 3958.

[33] To be published.[34] Amatore, C., Saveant, J.-M. J. Am. Chem. Soc. 1981, 103,

5021.[35] Pulliam, C. R., Thoden, J. B., Stacy, A. M., Englert, M. H.,

Dahl, L. F. J. Am. Chem. Soc. 1991, 113, 7398.[36] Kushi, Y., Nagao, H., Nishioka, T., Isobe, K., Tanaka, K.

J. Chem. Soc., Chem. Commun. 1995, 1223; Kushi, Y., Nagao, H., Nishioka, T., Isobe, K., Tanaka, K. Chem. Lett. 1994, 2175.

[37] Tanaka, K., Kushi, Y., Tsuge, K., Toyohara, K., Nishioka, K., Isobe, K. Inorg. Chem. 1998, 37, 120.

[38] Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley-Interscience Publication, 1986.

metal-catalyzed reversible conversion between chemical and electrical energy designed towards a...

Documents