n4-macrocyclic metal complexes - startseite · 2013. 7. 19. · n 4-macrocyclic metal complexes...

N4-Macrocyclic Metal Complexes

N4-Macrocyclic Metal

Complexes

Edited by

JOSÉ H. ZAGALDepartamento de Quı́mica de los MaterialesFacultad de Quı́mica y Biologı́aUniversidad de Santiago de ChileSantiagoChile

FETHI BEDIOUILaboratoire de Pharmacologie Chimique et GénétiqueUMR CNRS No. 8151/U 640 INSERMEcole Nationale Supérieure de Chimie de ParisParisFrance

and

JEAN-POL DODELETINRS-Énergie, Matériaux et TélécommunicationsVarennes, QuébecCanada

ABC

Editors:

José H. ZagalDepartmento de Quimica

de los MaterialesFacultad de Quimica y

BiologiaUniversidad de Santiago de

ChileCasilla 40Correo 33, SantiagoChile

Fethi Bedioui,Laboratoire dePharmacologie Chimique et

GénétiqueUMR CNRS no. 8151/U

640 INSERMEcole Nationale Supérieure

de Chimie de Paris11 rue Pierre et Marie Curie75231 Paris cedex 05France

Jean-Pol Dodelet,INRS-ÉnergieMatériaux et

Télécommunications1650 boulevard Lionel

BouletVarennes,Québec, Canada J3X 1S2.

Library of Congress Control Number: 2005938667

ISBN 10: 0-387-28429-XISBN 13: 978-0387-28429-3

Printed on acid-free paper.

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Contents

Preface xvContributors xvii

1. Billion-Year-Old Oxygen Cathode that Actually Works:Respiratory Oxygen Reduction and Its BiomimeticAnalogsRoman Boulatov

1. The Basic Concepts of Energy Metabolism . . . . . . . . . . . . . . . . . . . . . . . 12. Biological Catalysis of Respiratory Oxygen Reduction . . . . . . . . . . . . . . 5

2.1. Heme/Cu Terminal Oxidases (HCOs) . . . . . . . . . . . . . . . . . . . . . . . 52.2. Cytochromes bd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Biomimetic Catalysis of O2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1. Simple Fe Porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2. Functional Analogs of the Heme/Cu Site . . . . . . . . . . . . . . . . . . . . 193.3. Possible Physiological Role of CuB from Biomimetic

Electrocatalytic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244. Summary and Conclusions: Lessons of Biomimetic O2 Reduction for

the Design of Fuel Cell Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2. Fundamental Aspects on the Catalytic Activity ofMetallomacrocyclics for the ElectrochemicalReduction of O2José H. Zagal, Maritza A. Páez, and J. Francisco Silva

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412. Reaction Pathways for the Reduction of Molecular Oxygen . . . . . . . . . . 423. Interaction of O2 with Active Sites and the Redox Mechanism . . . . . . . 464. Two-Electron Reduction Catalysts for the Reduction of Molecular

Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575. Four-Electron Reduction Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

vi Contents

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3. Oxygen Reduction in PEM Fuel Cell Conditions:Heat-Treated Non-Precious Metal-N4 Macrocycles andBeyondJean-Pol Dodelet

1. Why Search for a Non-Pt Based Catalyst for the Reduction of O2 inPEM Fuel Cells? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

2. Activity of Electrocatalysts based on Fe–N4 and Co–N4 Macrocyclesand Beyond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882.1. The Early Days: 1964–1989 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882.2. Molecular Structure of the Catalytic Site(s) . . . . . . . . . . . . . . . . . . 91

2.2.1. Fe- and Co Macrocycles After the 1980s . . . . . . . . . . . . . . . 912.2.2. Beyond Fe– and Co–N4 Macrocycles After 1989 . . . . . . . . 1022.2.3. Simultaneous Presence of Two Catalytic Sites in all

Fe-Based Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1072.2.4. Influence of the Carbon Support on the Surface Density of

Catalytic Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1163. Kinetic and Mechanistic Aspects of Electrochemical Oxygen Reduction 121

3.1. Kinetic Aspects of Electrochemical Oxygen Reduction . . . . . . . . 1213.2. Mechanistic Aspects of Electrochemical Oxygen Reduction . . . . 127

4. Important Factors for the Use of Non-Noble Metal Electrocatalysts inPEM Fuel Cells for Automotive Applications . . . . . . . . . . . . . . . . . . . . . 130

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

4. Biomimetic NOx Reductions by Heme Models and ProteinsEmek Blair, Filip Sulc, and Patrick J. Farmer

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1492. Native Enzymes for NOx Reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

2.1. Assimilatory Nitrite Reductases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512.2. Dissimilatory Nitrite Reductases . . . . . . . . . . . . . . . . . . . . . . . . . . . 1552.3. Nitric Oxide Reductases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

3. Electrochemical Investigations of NOx Reduction . . . . . . . . . . . . . . . . . . 1593.1. Nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

3.1.1. Small Molecule Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1593.1.2. Heme Protein Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

3.2. Nitric Oxide, NO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683.2.1. Small Molecule Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683.2.2. Heme Protein Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

3.3. Hydroxylamine, NH2OH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1773.4. Co-Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Contents vii

3.5. Nonbiomimetic substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1783.5.1. Nitrate, NO−3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1783.5.2. Nitrous Oxide, N2O; Azide; N

−3 , and Nitromethane,

CH3NO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1784. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

5. Electroreduction of CO2 Catalyzed by MetallomacrocyclicsJuan A. Costamagna, Mauricio Isaacs, Marı́a J. Aguirre,Galo Ramı́rez, and Ignacio Azocar

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1912. Electroreduction of CO2 on Metallic Cathodes . . . . . . . . . . . . . . . . . . . . 1933. Biphenantroline and Bypiridine Hexaazacyclophane Systems . . . . . . . . 1954. Cyclam and Derivative Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

4.1. Studies in Solution for Ni Cyclam Systems and ElectrochemicalReduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

4.2. Studies in Supramolecular Systems . . . . . . . . . . . . . . . . . . . . . . . . . 2154.3. Modified Electrodes with [Ni (cyclam)]2+ . . . . . . . . . . . . . . . . . . . 218

5. Phthalocyanines and Porphyrins Complexes . . . . . . . . . . . . . . . . . . . . . . . 2205.1. Phthalocyanines Complexes in Solution . . . . . . . . . . . . . . . . . . . . . 2205.2. Porphyrins Complexes in Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 2215.3. Studies in Supramolecular Systems . . . . . . . . . . . . . . . . . . . . . . . . . 2255.4. Studies in Modified Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

6. Supramolecular Porphyrins as ElectrocatalystsKoiti Araki, Henrique Eisi Toma

1. Build-up of Supramolecular Porphyrins Based on Metal–LigandCoordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2551.1. Supramolecular Porphyrin Assemblies by Axial Coordination . . . 2561.2. Supramolecular Assembly Using Peripheral Pyridyl–Substituents 260

2. Synthesis and Characterization of Tetrametallated Pyridyl Porphyrins . 2652.1. Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2672.2. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

2.2.1. Spectroscopic Characterization . . . . . . . . . . . . . . . . . . . . . . . 2692.2.2. Electrochemical Characterization . . . . . . . . . . . . . . . . . . . . . 272

3. Catalytic and Electrocatalytic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 2783.1. Hydrocarbon Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2793.2. Tetraelectronic O2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

4. Electrochemical and Photoelectrochemical Properties of PorphyrinFilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2874.1. Electrochemical Properties of M-TRP Films . . . . . . . . . . . . . . . . . 288

4.1.1. Dip-Coated Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

viii Contents

4.1.2. Electrostatic Assembled Films (EAF) . . . . . . . . . . . . . . . . . 2894.1.3. Polymeric Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

4.2. Photocatalytic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2995. Final Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

7. Electrodes Modified with Monomeric M-N4 Catalysts forthe Detection of Environmentally Important MoleculesTebello Nyokong

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3152. Phenols, Organohalides, and Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

2.1. Porphyrin and Salen Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3182.1.1. Organohalides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3182.1.2. Phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

2.2. Phthalocyanine complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212.2.1. Organohalides and Pesticides . . . . . . . . . . . . . . . . . . . . . . . . 3212.2.2. Phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

3. Thiols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3223.1. Porphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3233.2. Phthalocyanine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

3.2.1. Adsorbed Monomer: Effects of Ring Substituents . . . . . . . 3263.2.2. Adsorbed Monomer: Effects of Central Metals . . . . . . . . . . 3263.2.3. Adsorbed Monomer: Effects of Axial Ligands . . . . . . . . . . 3273.2.4. Other Methods of Electrode Modification . . . . . . . . . . . . . . 327

4. Sulfur Dioxide and Sulfur Oxoanions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3294.1. Porphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

4.1.1. Sulfur Oxoanions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3304.1.2. Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

4.2. Phthalocyanine complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3315. Carbon Dioxide/Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

5.1. Porphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3325.1.1. CO2 Reduction: M“0”Porphyrin Catalysts . . . . . . . . . . . . . . 3325.1.2. CO2 Reduction: Effects of Lewis and Brønsted Acids . . . . 3355.1.3. CO2 Reduction: Effect of Changes in the Ring and Central

Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3365.1.4. Carbon Monoxide Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . 337

5.2. Phthalocyanine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3376. Nitrites and Nitrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

6.1. Porphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3396.2. Phthalocyanine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341

7. Cyanides/Thiocyanades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3428. Hydrazine/hydroxylamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

8.1. Porphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3438.2. Phthalocyanine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

8.2.1. Hydrazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3458.2.2. Hydroxylamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

Contents ix

9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

8. Electropolymerized MetalloporphyrinMetallophthalocyanine and Metal Schiff Base ComplexFilms: Applications to Biomimetic Electrocatalysis andBioelectroanalysisAlain Pailleret and Fethi Bedioui

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3632. The Electrochemical Polymerization Strategy . . . . . . . . . . . . . . . . . . . . . 3653. Design and Characterization of Electropolymerized Metalloporphyrin

and Metallophthalocyanine Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3653.1. Doped Electropolymerized Polypyrrole and Polythiophene Films 365

3.1.1. Significant Recent Examples of Electropolymerized FilmsDoped with Porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

3.1.2. Significant Recent Examples of Electropolymerized FilmsDoped with Phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . 369

3.2. Electropolymerization of Pyrrole or Thiophene-SubstitutedN4-Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3693.2.1. Significant Recent Examples of Electropolymerized

Pyrrole and Thiophene-Substituted Porphyrins . . . . . . . . . . 3753.2.2. Significant Examples of Electropolymerized Pyrrole-

Substituted Phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . 3773.2.3. Significant Examples of Electropolymerized Pyrrole-

or Thiophene-Substituted Schiff Bases and RelatedDerivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

3.3. Immobilization of Multicharged Porphyrins into Pre-Electropolymerized Polypyrrole Films Bearing FunctionnalGroups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

3.4. Electropolymerization of Amino-, Hydroxy- and Vinyl-Substituted N4-Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3843.4.1. Significant Examples of Electropolymerized Films from

Aminophenyl-, Hydroxyphenyl- and Vinyl-SubstitutedPorphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

3.4.2. Significant Examples of Electropolymerized Tetra-AminoPhthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

3.5. Electrodeposited Films of Nickel N4-Complexes in AlkalineSolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

3.6. Electropolymerized Films of Salen Complexes . . . . . . . . . . . . . . . 4013.7. Miscellaneous Electrodeposition Processes . . . . . . . . . . . . . . . . . . 406

4. Electrocatalytic and Electroanalytic Applications of ElectropolymerizedN4-Macrocyclic Based Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4084.1. Electroassisted Biomimetic Reduction of Molecular Oxygen:

Mechanistic and Electrochemical Approach . . . . . . . . . . . . . . . . . . 408

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4.2. Electropolymerized N4-Macrocyclic Films as ElectrochemicalSensors for Nitric Oxide in Solution . . . . . . . . . . . . . . . . . . . . . . . . 414

4.3. Miscellaneous Significant Examples . . . . . . . . . . . . . . . . . . . . . . . . 4175. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422


9. Electron Transfer Processes of β-Pyrrole BrominatedPorphyrins: Structural vs. Electronic EffectsFrancis D’Souza and Karl Kadish

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4392. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442

2.1. Synthesis and Characterization of β-Pyrrole Brominated meso-Tetraphenylporphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442

2.2. Synthesis of Water-Soluble β-Pyrrole Brominated Porphyrins . . . 4453. Effect of the Br Substituents and Solvent Interactions on the UV–Visible

Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4454. Electrochemical Behavior of β-Pyrrole Brominated Metalloporphyrins 446

4.1. Electroreduction Behavior of β-Pyrrole BrominatedMetalloporphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448

4.2. Electrochemical Oxidation Behavior of β-Pyrrole BrominatedMetalloporphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450

4.3. Electrochemical Behavior of Water-Soluble β-PyrroleBrominated Porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

5. Spectroelectrochemical Studies of β-Pyrrole BrominatedMetalloporphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

6. Effect of the Br Substituents on Axial Ligand Binding . . . . . . . . . . . . . . 4567. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460

10. Photoelectrochemical Reactions at PhthalocyanineElectrodesDerck Schlettwein

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4672. Essentials of Photoelectrochemical Reactions . . . . . . . . . . . . . . . . . . . . . 4683. Photoelectrochemical Experiments at Phthalocyanine Thin Films . . . . . 470

3.1. Preparation of Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4703.2. Semiconductor Characteristics of Solid Phthalocyanine Films . . . 4713.3. Position of Frontier Energy Levels in Phthalocyanines . . . . . . . . . 4733.4. Photocurrent Direction at Phthalocyanine Electrodes . . . . . . . . . . 4743.5. Role of Higher Excited States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4783.6. Reactant Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4813.7. Surface Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482

3.7.1 Fermi-Level Pinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4823.7.2 Photoelectrochemical Electrode Kinetics . . . . . . . . . . . . . . . 484

Contents xi

4. Sensitization of Oxide Semiconductors by Phthalocyanines . . . . . . . . . . 4944.1. Sensitization of Nanoparticulate Semiconductors . . . . . . . . . . . . . 4944.2. Sensitization of Electrodeposited Semiconductor Thin Films . . . . 496

4.2.1. Electrodes Deposited in the Presence of Phthalocyanines . 4964.2.2. Sensitization by Subsequently Adsorbed Phthalocyanines . 501

5. Technology Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

11. Organisation and Photoelectrochemical Reactivity ofWater-Soluble Metalloporphyrins at the Liquid/LiquidInterfaceDavid Fermı́n and Nicolas Eugster

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5172. The Polarisable Liquid/Liquid Interface . . . . . . . . . . . . . . . . . . . . . . . . . . 518

2.1. The Structure of the Neat Liquid/Liquid Boundary . . . . . . . . . . . . 5182.2. The Potential Distribution Across the Polarisable Liquid/Liquid

Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5203. Basic Photophysics of Metalloporphyrins and Chlorins . . . . . . . . . . . . . 523

3.1. Electronic Transitions and Lifetime of Excited States inPorphyrin-Based Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523

3.2. Ultrafast Relaxation in Porphyrin Ion Pairs . . . . . . . . . . . . . . . . . . . 5263.3. Interfacial Photoelectrochemistry vs. Time-Resolved

Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5294. Organisation of Water-Soluble Porphyrins at the Liquid/Liquid

Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5324.1. Excess Charge Associated with the Specific Adsorption of Ionic

Porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5344.2. Electrocapillary Curves of the Liquid/Liquid Interface . . . . . . . . . 5364.3. Interfacial Molecular Orientation and Lateral Porphyrin

Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5404.3.1. SHG Studies of Metalloporphyrins Adsorption at the

Water/DCE Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5404.3.2. Molecular Orientation Studied by Polarisation Angle

Photocurrent Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5425. Photoelectrochemical Reactivity at Porphyrin-Sensitised Liquid/Liquid

Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5455.1. The Origin of the Photocurrent Responses . . . . . . . . . . . . . . . . . . . 5455.2. Photocurrent Responses as a Function of the Galvani Potential

Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5505.2.1. Correlation between Photocurrent and the Gibbs Free

Energy of Electron Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . 5525.2.2. The Electron Transfer Activation Energy and Solvent

Reorganisation Term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5575.2.3. Comparison Between Porphyrin Photoreactivity in Bulk

Solutions and at the ITIES . . . . . . . . . . . . . . . . . . . . . . . . . . . 559

xii Contents

5.3. Dynamics of Photocurrent Relaxation . . . . . . . . . . . . . . . . . . . . . . . 5606. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566


12. Theoretical Insights on the Chemical Reactivity ofMetalloporphyrins Using Density Functional TheoryIlaria Ciofini, Laurent Joubert, Michele Pavone, Vincenzo Barone andCarlo Adamo

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5752. Computational Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5813. The Chemical Effects Tuning the Reactivity of M(II)-N4 Complexes . . 582

3.1. Influence of the Metal Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5823.2. Influence of the N4 Functionalisation . . . . . . . . . . . . . . . . . . . . . . . 5853.3. Influence of the Environment: Solvent Effects . . . . . . . . . . . . . . . . 5913.4. Influence of the Environment: Effect of the Absorption on the

Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5934. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597


13. Organized Multiporphyrinic Assemblies forPhotoconduction and ElectroconductionJean Weiss and Jennifer Wytko

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6032. The Porphyrinic Chromophore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605

2.1. Absorption of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6052.1.1. Transition Moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6062.1.2. Excited States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6102.1.3. Nature of the Excitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616

2.2. Linking Chromophores: General Considerations . . . . . . . . . . . . . . 6172.3. Energy and Electron Transfer in Photosynthesis . . . . . . . . . . . . . . . 6182.4. Design of Synthetic Systems: The Molecular Electronics

Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6223. Scope and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6234. Covalent Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623

4.1. Covalent Dimers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6244.1.1. Choice of the Chromophore . . . . . . . . . . . . . . . . . . . . . . . . . . 6244.1.2. Electronic Coupling in Covalently Linked Dimers . . . . . . . 625

4.2. Covalent Multiporphyrin Arrays Combined with NonporphyrinicAcceptors or Donors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6404.2.1. Energy Transfer in Linear Species . . . . . . . . . . . . . . . . . . . . 6414.2.2. Photoinduced Charge Separation in Linear Arrays . . . . . . . 6444.2.3. Energy Collection in Nonlinear Structures . . . . . . . . . . . . . 649

5. Noncovalent Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6545.1. Hydrophobic Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654

Contents xiii

5.1.1. Concave Hosts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6545.1.2. J-Aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657

5.2. Axial Bond Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6595.3. Exocyclic Coordination Complexes as Linkers . . . . . . . . . . . . . . . . 6675.4. Membranes, Vesicles, and Micelles: Templated Assembling . . . . 6745.5. Noncovalent Multiporphyrin Assembling with Nonporphyrinic

Electron Acceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6796. Cofacial Arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6847. Multiporphyrin Surface Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 696

7.1. Covalent Attachment of Multiporphyrins . . . . . . . . . . . . . . . . . . . . 6977.2. Langmuir–Blodgett Multilayers of Mono- and Bisporphyrin

Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6997.3. Porphyrin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712

14. Vibrational Spectra and Surface-Enhanced VibrationalSpectra of AzamacrocyclesMarcelo M. Campos Vallete

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7252. Surface Phenomena and Applications of Azamacrocycles . . . . . . . . . . . 7253. Surface Vibrational Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726

3.1. Surface-Enhanced Raman Scattering (SERS) . . . . . . . . . . . . . . . . . 7283.2. Surface-Enhanced Infrared Absorption (SEIRA) . . . . . . . . . . . . . . 7283.3. Infrared Reflection–Absorption Spectroscopy (IRRAS) . . . . . . . . 7283.4. Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7293.5. Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729

4. Infrared, Raman, and Resonant Raman Spectra Analysis ofAzamacrocycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7314.1. Hexaazacyclophane and Its Cu(II) Complex . . . . . . . . . . . . . . . . . . 7314.2. Phthalocyanines and Naphthalocyanines and Their Metal

Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7334.3. Vanadylphthalocyanine and Vanadylnaphthalocyanine . . . . . . . . . 7354.4. Vanadyl naphthalocyanine and Vanadyl Porphine Phenyl

Substituted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7354.5. Azabipiridyl and its Ni(II), Cu(II), and Zn(II) Complexes . . . . . . 7374.6. Bis(phenylhydrazine)-1,10-Phenanthroline and its Co(II), Ni(II),

Cu(II), and Zn(II) Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7414.7. Cyclam and Cyclamdione and Their Cu(II) Complexes . . . . . . . . 7434.8. Dinaphthalenic Ni(II) and Cu(II) Azamacrocycle Complexes

Methyl and Phenyl Substituted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7445. Normal Coordinate Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746

5.1. Hexaazacyclophane and Its Cu(II) Complex . . . . . . . . . . . . . . . . . . 7475.2. Azabipiridyl Cu(II) Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7495.3. Bis(phenylhydrazine)-1,10-Phenanthroline and Its Cu(II)

Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750

xiv Contents

5.4. Ironphthalocyanine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7526. Surface-Enhanced Vibrational and RAIRS Studies . . . . . . . . . . . . . . . . . 752

6.1. Cyclam and Cyclamdione and Their Cu(II) Complexes . . . . . . . . 7536.2. Bis(phenylhydrazine)-1,10-phenanthroline and its Cu(II)

Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7546.3. Azabipiridyl and Its Co(II), Ni(II), and Cu(II) Complexes . . . . . . 756

6.3.1 Raman Spectra and SERS of the Ligand and Its Complexes 7566.3.2. SEIRA of the Ligand Azabipiridyl and Its Complexes . . . . 761

6.4. Naphthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7626.5. Phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7686.6. Porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7706.7. Miscellaneous Macrocycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773

7. Adsorbate–Substrate Interaction Vibrations . . . . . . . . . . . . . . . . . . . . . . . 7807.1. Azabipiridyl Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7817.2. Phthalocyanine Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . 7817.3. Naphthalocyanine Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . 7817.4. Dinaphthalenic Ni(II) and Cu(II) Azamacrocycle Complexes

Methyl and Phenyl Substituted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7827.5. Molecular Model and Theoretical Data of the Adsorbate-Substrate

Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7827.6. Azabipiridyl Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7837.7. Naphthalocyanine and Its Cu(II) Complex . . . . . . . . . . . . . . . . . . . 7857.8. Dinaphthalenic Ni(II) and Cu(II) Azamacrocycle Complexes

Methyl and Phenyl Substituted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7888. New Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 789


Index 801

Preface

Metal complexes of N4-ligands, such as porphyrins and phthalocyanines, arewidely studied due to their numerous physico-chemical properties and the greatvariety of their applications in many fields. For example, metalloporphyrins areused as biomimetic models for studying several biological redox processes, in par-ticular for molecular oxygen transport and catalytic activation to mimic monooxy-genase enzymes of the cytochrome P450. They are also well known as efficientcatalysts for oxidative degradation of various types of pollutants (organohalides,for example) and residual wastes. The high stability of metallophthalocyaninesmakes them suitable for applications in various fields such as catalysis (for ex-ample, the MEROX process for the sweetening of oils), electrocatalysis (airbatteries and fuel cells), dye stuffs, coloring for plastics and metal surfaces, sensorapplications, chromatographic detectors, photoconducting agents, etc. Both thesefamilies of complexes are now also used for photobiology and photodynamiccancer therapy, electrochemical removal of organic wastes, display devices, elec-trochromism, electroluminescence, molecular metals, and nonlinear optical ap-plications. Additionally, the intrinsic diversity and selectivity of axial ligationof these macrocycles confer to them formidable challenging potential uses aselectrochemical-sensing devices for several fields of application in analytical,electro-, and spectrophotochemistry.

The rich and reversible redox chemistry of metalloporphyrins and metal-lophthalocyanines is the key factor that allows them to serve as mediators in manyelectron transfer reactions. Since the 1970s and more importantly in recent years,numerous reports have demonstrated that these complexes can be successfullyused as electrocatalysts for a great variety of electrochemical reactions. One cancite some examples such as the oxidation of dopamine, thiols, H2S, HS−, reducedglutathione, L-cysteine, coenzyme A, penicillin, oxalic acid, NADH, hydroxy-lamine, hydrazine, nitrite, nitric oxide, cyanide, organic peroxides, hydrogen per-oxide, propylgallate, ascorbic acid, hydroquinone, catechol, phenols, chlorophe-nols, sulfite, etc. and the reduction of molecular oxygen, hydrogen peroxide,carbon dioxide, L-cystine, disulfides, thionylchloride, etc. It is remarkable thata large number of the studied reactions involve significantly relevant biologicalcompounds and the list keeps increasing as more publications appear in the lit-

xvi Preface

erature reporting on new reactions and electrocatalytic processes. Concerning thefields of photoelectrochemistry and photocatalysis, although the list of the stud-ied reactions and processes involving photoassistance is less abundant and fewersystems have been studied, this area of investigation is experiencing intense devel-opment due to the implication of these compounds in photobiology and nanosizedsemiconductor materials.

Nowadays, the double desire to mimic enzymatic or natural systems and todevelop new complex structures that do combine a well-defined topology and amarked chemical flexibility allowing both the finetuning of the properties of theelectron transfer reactions and the expansion of the supramolecular architectures,is incontestably leading to an active area of research devoted to the concept of“design of intelligent molecular material electrodes” with predetermined reactiv-ity. To do so, highly elaborate synthesis routes are developed to design chem-ically modified metalloporphyrins and metallophthalocyanines that can then besimply strongly adsorbed on conventional materials, electropolymerized on con-ducting substrates, incorporated into hybrid organic/inorganic gel or solid matrixto produce catalytic electrodes with long-term stabilities, for expanded practicalanalytical applications.

Thus, it is clear that the numerous and varied possibilities of uses and ofapplications ensure that porphyrin and phthalocyanine compounds will remain ofvital importance for many years to come and that the related fields of investigationare expected to have significant ramifications. The publication since 1997 of TheJournal of Porphyrins and Phthalocyanines, an international journal of significantimpact factor entirely devoted to these molecular materials, is a significant indi-cator. Also the Society of Porphyrins and Phthalocyanines provides a forum forinteraction among researchers around the world.

The main objective of this monograph is to provide a general updated viewof the vast applications of these materials in electrochemistry by focusing on afew significant topics and examples. It is also aimed at offering future projectionsand opening new fields of research and investigations.

J.H. ZagalF. Bedioui

J.P. Dodelet

Contributors

Carlo Adamo, Ecole Nationale Supérieure de Chimie de Paris, Laboratoire d’Electro-chimie et Chimie Analytique, UMR 7575, 11 rue P. et M. Curie, F-75231 Paris cedex05, France

Marı́a J. Aguirre, Departamento de Quı́mica de los Materiales, Facultad de Quı́mica yBiologı́a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

Koiti Araki, Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes 748,Butantã, São Paulo 05508-900, Brazil

Ignacio Azocar, Departamento de Quı́mica de los Materiales, Facultad de Quı́mica yBiologı́a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

Vincenzo Barone, Laboratorio di Struttura e Dinamica Molecolare, Dipartimento diChimica, Complesso Universitario Monte Sant’Angelo, Via Cintia, I-80126 Napoli,Italy

Fethi Bedioui, Laboratoire de Pharmacologie Chimique et Génétique, UMR CNRS no.8151/U 640 INSERM, Ecole Nationale Supérieure de Chimie de Paris, 11 rue Pierreet Marie Curie, 75231 Paris cedex 05, France

Emek Blair, Department of Chemistry, University of California, Irvine, CA, USARoman Boulatov, Department of Chemistry, University of Illinois, 600 South Mathews

Ave, Urbana, IL, USAMarcelo M. Campos Vallette, Department of Chemistry, Faculty of Sciences, Universi-

dad de Chile, P.O. Box 653, Santiago, ChileIlaria Ciofini, Ecole Nationale Supérieure de Chimie de Paris, Laboratoire d’Electro-

chimie et Chimie Analytique, UMR 7575, 11 rue P. et M. Curie, F-75231 Paris cedex05, France

Juan A. Costamagna, Departamento de Quı́mica de los Materiales, Facultad de Quı́micay Biologı́a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

Jean-Pol Dodelet, INRS-Énergie, Matériaux et Télécommunications, 1650 boulevardLionel Boulet, Varennes, Québec, Canada J3X 1S2

Francis D’Souza, Department of Chemistry, Wichita State University, 1845 Fairmount,Wichita, KS 67260-0051, USA

Nicolas Eugster, Laboratoire d’Electrochimie Physique et Analytique, Institut de Sci-ences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, LausanneCH-1015, Switzerland

Patrick J. Farmer, Department of Chemistry, University of California, Irvine, CA, USADavid J. Fermı́n, Departement für Chemie und Biochemie, Universität Bern, Freiestrasse

3, Bern CH-3012, Switzerland

xviii Contributors

Mauricio A. Isaacs, Departamento de Quı́mica, Facultad de Ciencias, Universidad deChile, Casilla 653, Santiago, Chile

Laurent Joubert, Ecole Nationale Supérieure de Chimie de Paris, Laboratoire d’Electro-chimie et Chimie Analytique, UMR 7575, 11 rue P. et M. Curie, F-75231 Paris cedex05, France

Karl M. Kadish, Department of Chemistry, University of Houston, Houston, TX 77005-2004, USA

Tebello Nyokong, Department of Chemistry, Rhodes University, P.O. Box 94, Graham-stown, South Africa

Maritza A. Páez, Departmento de Quimica de los Materiales, Facultad de Quimica yBiologia, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

Alain Pailleret, Laboratoire Interfaces et Systèmes Electrochimiques, UPR CNRS no.15, Université Pierre et Marie Curie-Paris VI, 4 place Jussieu, 75252 Paris cedex 05,France

Michele Pavone, Laboratorio di Struttura e Dinamica Molecolare, Dipartimento di Chim-ica, Complesso Universitario Monte Sant’Angelo, Via Cintia, I-80126 Napoli, Italy

Galo Ramı́rez, Departamento de Quı́mica de los Materiales, Facultad de Quı́mica yBiologı́a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

Derck Schlettwein, Institute of Applied Physics, Justus-Liebig-University Gießen,Heinrich-Buff-Ring 16, D-35392 Gießen, Germany

J. Francisco Silva, Departmento de Quimica de los Materiales, Facultad de Quimica yBiologia, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

Filip Sulc, Department of Chemistry, University of California, Irvine, CA, USAHenrique E. Toma, Institute of Chemistry, University of São Paulo, Av. Prof. Lineu

Prestes 748, Butantã, São Paulo 05508-900, BrazilJean Weiss, Laboratoire de Chimie des Ligands à Architecture Contrôlée, Institut de

Chimie, Université Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, FranceJennifer Wytko, Laboratoire de Chimie des Ligands ‘a Architecture Contrôlée, Institut

de Chimie, Universit’e Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, FranceJosé H. Zagal, Departmento de Quimica de los Materiales, Facultad de Quimica y

Biologia, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile

1Billion-Year-Old Oxygen Cathode thatActually Works: Respiratory OxygenReduction and its BiomimeticAnalogs

Roman Boulatov

1. The Basic Concepts of Energy Metabolism

Life exists only through a constant dissipation of energy. This energy isextracted from the environment, either as sunlight (photosynthesis) or as food (re-duced organic matter, such as glucose or H2). Two nonphotosynthetic forms ofenergy metabolism are fermentation and respiration1. In fermentation complexorganic molecules, such as glucose, are broken down exergonically to simplerproducts (reactions 1.1), such as lactate (in muscle cells) or ethanol (in baker’syeast). Although fermentation proceeds by a series of electron transfer steps, itdoes not consume any external oxidants. The organic compounds of the food un-dergo disproportionation. In contrast, respiration involves the oxidation of food byan environmental oxidant (reaction 1.2). Mechanistically, food is converted intoreduced respiratory electron carriers, such as NADH and FADH2 (reaction 1.3),which enter the respiration cycle (reaction 1.4). Respiration proceeds by enzyme-catalyzed electron transfer between respiratory electron carriers, starting with thestrongest reductants, NADH and FADH2 and ending with the weakest reductants,quinols and cytochrome c2. Oxidation of the latter carriers in the final redox stepof respiration requires an environmental (or terminal) oxidant.

C6H12O6 � 2CH3CH(OH)CO2H �G = −6 kcal mol−1 (1.1a)C6H12O6 � 2CO2 + 2CH3CH2OH �G = −54 kcal mol−1 (1.1b)

}fermentation

Roman Boulatov • Department of Chemistry, University of Illinois, 600 South Mathews Ave.,Urbana, IL.

N4-Macrocyclic Metal Complexes, edited by José H. Zagal, Fethi Bedioui and Jean-Pol Dodelet.Springer Science+Business Media, Inc., New York, 2006.

1

2 Roman Boulatov

C6H12O6 + 6O2 � 6CO2 + 6H2O�G = −600 kcal mol−1 (1.2) aerobic respiration

C6H12O6 + 6H2O + 10NAD+2FAD−→ 6CO2 + 10H+ + 10NADH + 2FADH2 (1.3) glycolysis

2NADH + 2H+ + O2 −→ 2NAD+ + 2H2O (1.4) respirationThe free energy available from photosynthesis, fermentation, or respiration

is captured in the form of transmembrane electrochemical gradients, generallyof protons3. The gradient-generating system is embedded in a closed, relativelyproton-impermeable membrane. The membrane-enclosed space (such as mito-chondrial matrix, chloroplast’s thylakoid space, or prokaryotic cytoplasm) ismaintained at a pH and electrostatic potential different from that of the outsideenvironment by membrane-embedded enzymes of the respiratory (or photosyn-thetic) cycle. Some of these enzymes use the free energy of exergonic chemicalreactions they catalyze to actively translocate protons from the less acidic com-partment to the more acidic one (proton pumps). Others create the transmembranegradient by carrying out proton-consuming and proton-releasing reactions at theopposite sites of the membrane because of the appropriate location of the corre-sponding catalytic domains. The membrane-embedded ATP synthase converts theexergonic movement of protons down this electrochemical gradient into the freeenergy of the pyrophosphate bond in ATP (Figure 1.1)2–5.

Depending on the environmental niche occupied by an organism, its respira-tion can be based on molecular oxygen, nitrate, fumarate, sulfate, carbon dioxide,etc. as the terminal oxidant6. Among these bioavailable oxidants, O2 is by farthe strongest, enabling an aerobe to extract the largest amount of energy froma given amount of food (Figure 1.2). Other advantages of O2 as the terminaloxidant include its high abundance on the modern Earth, high permeation rateacross biological membranes,∗ and the nontoxic reduction product, H2O†. As aresult, all multicellular organisms are obligatory aerobes, whereas fermentationand anaerobic respiration are limited to prokaryotes‡ and certain single-celledeukaryotes1, 6.

Although O2 is a powerful four-electron/four-proton (4e/4H+) oxidant, itis kinetically inert under ambient conditions. The inertness of O2 arises from it

∗ A lipid membrane provides no barrier for O2 diffusion, its permeability coefficient being∼40 cm s−1, which is ∼20% higher than the O2 permeability coefficient of an H2O layer ofthe same thickness7. Inside cells, most of O2 flux occurs not by passive diffusion of O2, whichis insufficiently soluble in H2O to provide required fluxes, but by diffusion of oxymyoglobin

8,which unloads O2 at the point of O2 consumption (e.g., at the outer mitochondrial membrane).

† However, partially reduced oxygen species, such as O−2 , HO2, H2O2, and •OH, are toxic.‡ Prokaryotes are single-celled organisms whose cell lacks a well-defined, membrane-enclosed

nucleus and other organelles. Eukaryotes are cells with a distinct nucleus and cytoplasm.

Respiratory Oxygen Reduction and Its Biomimetic Analogs 3

Figure 1.1. (A) A cross-sectional view of the mitochondrion indicating its four distinctcomponents. (B) A schematic view of the mammalian respiratory electron transfer chainembedded in the inner mitochondrial membrane. Exergonic electron flow (narrow arrows)from NADH to quinone (Q), from quinol to ferricytochrome c, and finally, from ferro-cytochrome c to O2 are catalyzed by NADH dehydrogenase, cytochrome bc1, andcytochrome c oxidase, respectively. Although thermodynamically favorable, electrontransfer between the electron carriers bypassing the respiratory enzymes is too slow. Therespiratory enzymes utilize a fraction of the free energy of these redox processes for ender-gonic translocation of H+ from the basic, negatively charged matrix, to the more acidic,positively charged intermembrane space (gray block arrows)2, 3. Protons spontaneouslyflow from the intermembrane space to the matrix, down the electrochemical H+ gradient,through ATP synthase, powering the synthesis of ATP from ADP and inorganic phosphate(Pi). White and gray arrows depict exergonic and endergonic processes, respectively.

having a low affinity for H+ (reaction 1.5), other electrophiles and H atoms10(reaction 1.6), being a poor 1e oxidant particularly in neutral aqueous media(reaction 1.7)§, and having a triplet electronic ground state. The latter precludes

§ Superoxide is a moderately strong base (pKa of the conjugate acid, HO2, is ∼4.2) so that at pH< 4.5 the formal potential of 1e− reduction of O2 to O−2 depends on the concentration of H+ as:Eo′ = Eo + RT/F ln(1+[H+]/K (HO2)).

4 Roman Boulatov

NADH NAD++ H++ 2e-

-300

0

300

600

900 O2+4H++4e- 2H2O

NO3-+2H++2e- NO2

-+H2O

CO2H

HO2C

CO2H

HO2C

+2H++2e-

Aer

obic

res

pira

tion

Nitr

ate

resp

iratio

n

Fumarate respiration

Potential at pH 7vs. NHE, mV

Figure 1.2. Comparison of the free energy available in three common forms of respiration:aerobic, based on nitrate, and on fumarate.

direct reaction between O2 and most organic matter, which is in a singlet groundstate. This inertness of O2 allows for the existence of reduced organic matter inthe highly oxidizing atmosphere of the modern Earth. However, this inertness alsomeans that reduction of O2 to H2O at a rate and electrochemical potentials thatare useful for energy metabolism requires catalysis.

O2 + H+ � HO+2 �G0 ∼ 95 kcal mol−1, K ∼ 10−70(est.)∗∗ (1.5)O2 + 1/2H2 � HO2 �G0 = 3 kcal mol−1, K = 6 × 10−3 (1.6)O2 + e− � O−2 �G0 = 8 kcal mol−1, K = 3 × 10−6,

�E0′ = −0.33 V(vs.NHE)†† (1.7)

The appearance of photosynthesis and the subsequent increase in the con-centration of O2 in the Earth’s atmosphere ∼2 billion years ago11 provided theenvironmental pressure for the evolution of enzymes to catalyze the 4e/4H+reduction of O2 to H2O as an energy source. All known enzymes that per-form this task, called terminal oxidases12–22, are membrane-bound proteins.Two distinct classes of terminal oxidases are known. The better-studied super-family of heme/Cu terminal oxidases12–21 are distributed throughout all threedomains of life (Bacteria, Archaea, and Eukarya) and contain a binuclear por-phyrinatoiron(heme)–Cu site at which O2 is reduced. The much less studiedand smaller group of cytochrome bd terminal oxidases22 is widely distributed inGram-negative heterotrophs only‡‡. Cytochromes bd contain a binuclear heme–

∗∗ The thermodynamic values are referenced to the standard states of 1 atm partial pressure or 1 Maqueous solution at 298 K for gases, and H+, HO+2 , and HO2, respectively.

†† The values are for neutral (pH 7) solution in equilibrium with 1 atm partial pressure of O2.‡‡ Gram-negative bacteria (e.g., E. coli) are microorganisms that have two cell membranes, separated

by a periplasmic space; Gram-positive bacteria (e.g., Streptococci) have only one membrane. Het-erotrophic organisms require complex organic compounds for metabolism as opposed to beingable to utilize CO2 as the only source of carbon.


N

N N

N

CO2H CO2H

R

R�

Fe

R

CHO

OH

CH3

OH

CH3

Heme a

Heme b

Heme o

R�

Figure 1.3. Chemical structures of some common hemes.

heme O2 reduction site. Chemical structures of biologically significant hemes areillustrated in Figure 1.3.

2. Biological Catalysis of Respiratory OxygenReduction

2.1. Heme/Cu Terminal Oxidases (HCOs)

The superfamily comprises three classes: cytochrome c oxidases(CcOs)12–20, quinol oxidases, and cytochrome cbb3 oxidases20, 21. CcOs are theonly HCOs present in eukaryotes. Enzymes of all classes contain a binuclearheme–Cu catalytic site and all act as proton pumps. The water-soluble elec-tron carrier, ferrocytochrome c, is the physiological electron donor for CcOs andcytochrome cbb3 oxidases. Certain CcOs can also oxidize specific high-potentialiron–sulfur proteins (HiPIPs)23. Quinol oxidases catalyze a 2e/2H+ oxidation ofvarious quinols to quinones as the source of electrons for O2 reduction.

There is substantial structural and functional homology among CcOs fromdifferent organisms and between CcOs and quinol oxidases (Figure 1.4)24, 25. Thevast majority of HCOs have at least two subunits: subunit I, containing the cat-alytic domain, and subunit II. These are the only subunits required for O2 re-duction and proton pumping. Mammalian CcOs contain as many as 11 additionalsubunits, of uncertain function15, 18. HCOs reduce O2 at a bimetallic heme/Cu site(Figure 1.5)26, whose Fe–Cu distance varies around ∼5 Å, depending on the ex-ogenous ligation of the metal ions. The ∼5-Å distance is appropriate for a bridg-ing peroxide ligand, but whether such an intermediate ever forms at the heme/Cusite is uncertain. Most known CcOs and quinol oxidases appear to undergo apost-translational modification, linking one of the Cu-ligating imidazoles to a phe-nol residue of a tyrosine (Figure 1.5). The existence of this link is often taken toindicate that a phenoxyl radical is formed during initial enzyme turnovers27.

All HCOs also have a six-coordinate heme in the same subunit as thecatalytic site. In addition all cytochrome c oxidases have a binuclear CuA siteand some also have a six-coordinate heme in the noncatalytic subunit (subunitII)20. These centers are absent in quinol oxidases. The six-coordinate heme(s)and CuA sites are responsible for electron-relay between the external electron

6 Roman Boulatov

Inte

rmem

bran

e sp

ace

(neu

tral

)

Lipi

dm

embr

ane

Mat

rix (

basi

c, n

egat

ivel

y ch

arge

d)Ubi

quin

ol o

xida

seC

cO (

bact

eria

l)

Hem

e a 3

/Cu B

site

(O2

redu

ctio

n)H

eme

o 3/C

u B(O

2 re

duct

ion)

Fig

ure

1.4.

Posi

tion

ofth

ere

dox

activ

em

etal

site

sre

lativ

eto

the

lipid

mem

bran

ein

two-

subu

nit

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eria

lC

cO(P

.de

nitr

ifica

ns)2

4an

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iqui

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ase

(E.c

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25fr

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X-r

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atO

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ese

site

s.


proximal side

Distal side

Tyr244

His240His291

His290

~5 Å

CuB

His376

Heme a3

Figure 1.5. The catalytic O2 reduction site of CcO26. The numbering of the residues is

from the crystal structure of bovine heart CcO.

Ferrocytochrome cIntermembrane spac

(neutral)

Matrix(basic, negatively charged)

O2-reductionsite

CuB

CuA

Mg

Heme a3

Heme a

AlternativeET path

e−

e−

e−

Figure 1.6. Electron transfer paths in mammalian CcO26. The six-coordinate heme aand the catalytic heme–Cu site are located at approximately the same depth within thedielectric.

donors (ferrocytochrome c, reduced HiPIP, or quinols) and the catalytic heme/Cusite (Figure 1.6)26. They may also be involved in the storage of external reduc-ing equivalents during enzymatic turnover and/or controlling the redox poten-tials of the heme/Cu site (see below). The presence of additional electron-relaysites in CcOs relative to quinol oxidases is due to the different physicochemicalproperties of the electron donors utilized by these two classes of HCOs. CcOs ac-cept electrons from water-soluble ferrocytochrome c. The corresponding dockingsite is located outside of the membrane and relatively far from the six-coordinateheme a (or the heme/Cu site). Hence, the need for an additional electron-relaysite. In contrast, quinols are localized in the lipid bilayer and dock much closer tothe six-coordinate heme b site28.

With its four redox centers (CuA, heme a, heme a3 and CuB) mammalianCcO can exist in one of four redox states: fully oxidized and singly reduced,

8 Roman Boulatov

which are aerobically stable; and mixed-valence (2e-reduced), 3e−, and fully re-duced, all of which reduce O2 to the redox level of water within


OO

FeIII

NIm

O

O

FeIV

NIm

NIm

FeIII

NIm

FeIII

FeIIIc

FeIIc

NIm

FeII

FeIII

NIm

NIm

OOH

TyrOH

TyrOH TyrOH

Oxoferrylcompound F (PR)

TyrOH

Oxyheme(compound A)

Hydroperoxo(postulated by DFT)

Compound PM

TyrO

Compound H

H+, FeIIc

FeIIIc

FeIIIcH+, FeIIc

TyrOH

O2

H2O, FeIIIc

H2O

CuBII CuB

IICuBII

CuBII

CuBII

CuBI CuB

I

Compound E

TyrOH

Compound R

FeIIIc

(CuIICuII)A FeIIIa (CuIICuII)A FeIIIa (CuIICuII)A FeIIIa

(CuIICuIII)A FeIIIa

(CuIICuIII)A FeIIIa (CuIICuIII)A FeIIa

FeIV

Figure 1.7. A plausible sequence of steps for O2 reduction by mixed-valence CcO basedon single-turnover spectroscopic studies of CcO and DFT calculations. Structures in thesquare frame depict the catalytic site (Figure 1.5); imidazole ligation of CuB is omitted forclarity; exogenous ligation of CuB is uncertain and is not shown. The rectangular dashedframe signifies the other redox-cofactors. FeIIc and Fe

IIIc are ferro- and ferricytochrome c,

respectively. Compound H is an “activated” analog of the resting-state CcO (compoundO)19. The external reducing equivalent in singly reduced CcO (compound E) may belocalized mainly on CuB

19 The heme a3/O2 adduct (oxyheme or compound A) is oftenconsidered as ferriheme/superoxide, FeIIIa3–(O

−2 ), complex.

The dual physiological role of HCOs determines the energetics of O2 reduc-tion by these enzymes (reactions 1.8–1.13, Figure 1.8). By clearing the respira-tory electron transfer chain from low-potential (weakly reducing) electron CcOsallow a continuous electron flow from NADH to quinones to ferricytochromec. The redox potential of the last electron carrier, cytochrome c (∼250 mV atpH 7), determines the overall potential drop (∼550 mV) available for the NADHdehydrogenase and cytochrome bc components of the respiratory chain. The dif-ference between the redox potentials of ferri-/ferrocytochrome c (∼250 mV, re-action 1.8) and O2/H2O (∼800 mV at pH 7, reaction 1.12, Figure 1.8) is utilizedby CcO to increase the electrochemical potential gradient across the inner mito-chondrial membrane (the protonmotive force, Figure 1.1).

This increase is brought about by two mechanisms. First, the enzyme drawsfour protons for the reduction of O2 from the basic, negatively charged site of themembrane (N-side or matrix). Four electrons come from the opposite site (P-sideor intermembrane space, Figure 1.1). The annihilation of these opposite chargesat the O2 reduction site (reactions 1.9 and 1.10) is equivalent to the transloca-tion of four charges across the membrane against the electrostatic potential. Thisconsumes ∼220 mV (the value of the transmembrane gradient) of the free en-ergy of reaction 1.1219. CcO expends another ∼220 mV by physically moving

10 Roman Boulatov

Table 1.1. Summary of Properties of Mammalian CcO

Catalyzed reactiona O2 + 4(FeIIc)IMS + 8H+matrix� 2H2O + 4(FeIIIc)IMS + 4H+IMS

Driving force 100 mVTurnover frequencyb 10–100 s−1 19Power output 0.4–4 MW mol−1 or 2–20 W g−1 cEnergy-tranducing efficiency 80%Lifetime Days13

Selectivityd >99%Potential of the reductant 250 mVPotential of the catalytic site > 350 mV12, 19

in compound Re

Rates of intramolecular 2 × 104 s−1 (CuA/heme a); > 3 × 104 s−1redox equilibration (heme a/heme a3)

f 12, 18, 70

O2 affinity, Kd 0.3 mM (ferroheme a3); 8 mM (CuIB)

18

Inhibitors Strong: CN−, N−3 ; weak: CO12,83

a IMS: intermembrane space (Figure 1.1), Fec: cytochrome c.b At physiologically relevant potentials; with Ru(NH3)

2+6 in air-saturated buffers,

turnover frequency of up to 500 s−1 was observed13.c Molecular weight of CcOs is up to 200 kDa.d Fraction of the O2 flux converted to H2O (vis-à-vis partially reduced oxygenspecies).e The heme/Cu potential depends both on the site’s protonation state70 and on theredox states of heme a and CuA

12; cited is “the best guess” potential in the mixed-valence intermediate ready to bind O2.f During turnover, the reduction rate of the heme/Cu site is limited by H+ flow70.

four protons from the N-side (matrix) to the P-side (intermembrane space) of themitochondrion, by an unknown mechanism14, 16–19. As a result, out of ∼550 mVof the potential difference between the electron donor (ferrocytochrome c) andthe electron acceptor (O2), CcO captures ∼450 mV (>80%) in a form that canbe directly utilized by the cell to satisfy its energy-dissipating requirements.Quinol oxidases utilize a stronger reductant (e.g., ubiquinol, ∼50 mV vs. ferro-cytochrome c, ∼250 mV)13 and operate against a lower electrochemical gradient(∼180 mV)1 than CcOs do, so that their energy-transducing efficiency is corre-spondingly lower (∼45%).

Cytochrome cbb3 oxidases comprise the third class of enzymes belong-ing to the HCO superfamily20, 21. Like other HCOs, cytochromes cbb3 catalyzereduction of O2 at a bimetallic heme/Cu site, contain an additional six-coordinateheme in the catalytic subunit, and act as proton pumps. Although both CcOs andcytochromes cbb3 utilize cytochrome c as the physiological reductant, structurallyand functionally they differ more than CcOs and quinol oxidases do. UnlikeCcOs, which are distributed throughout all three domains of life, cytochromescbb3 are found almost exclusively in Proteobacteria (a group of Gram-negative


(FeIIc)IMS

H+matrix

E = −250 mV(FeIIIc)IMS + e−IMS

H+heme/Cu

e−IMS e−heme/Cu

H+matrix H+

IMS

(O2 + 4e− + 4H+)heme/Cu

2H2O + 4(FeIII

c)IMS + 4H+

IMS

E = −60 mV

E = 800 mV

E = −140 mV

2H2O

O2 + 4(FeIIc)IMS + 8H

+matrix

E = −220 mV

E = 100 mV

Proton pump

(1.8)

(1.9)

(1.10)

(1.11)

(1.12)

(1.13)

Figure 1.8. Energetics of O2 reduction by mammalian CcO. Subscripts signify the loca-tion of charged species in the transmembrane electrochemical gradient: the intermembranespace (IMS); the heme/Cu site or the matrix (Figure 1.1). The potentials are approximateand referenced to the normal hydrogen electrode at pH 7.

bacteria, see footnote ‡‡ on page 4)21. Cytochromes cbb3 appear to have evolvedfor respiration under microaerobic conditions. This is suggested by the very highO2 affinity of their heme/Cu site (KM ∼ 7 nM20 vs. 0.1–1 µM for CcOs andubiquinol oxidases18; compare to the O2 affinity of mammalian O2 carrier, myo-globin: Kd ∼ 0.5–2 µM32). In the absence of crystallographic data, the struc-tural origin of this high affinity remains unknown. Analysis of the primary aminoacid sequences suggests the absence of the histidine–tyrosine21 linkage observedin catalytic sites of most CcOs and quinol oxidases (Figure 1.5). What moietyprovides the fourth electron for O2 reduction by the mixed-valence (2e-reduced)state of cytochromes cbb3 is not known. Other data, such as gas binding andrecombination studies, also suggest a unique organization of the heme/Cu site incytochromes cbb3. Cytochromes cbb3 also lack the electron-relay CuA site, whichis replaced by 3 six-coordinate hemes. The enzymes’ energy-transducing effi-ciency is lower than that of CcOs. There is evidence suggesting that physiologicalroles of cytochromes cbb3 may also include signal transduction and O2 scaveng-ing21. Cytochrome cbb3 oxidases are the only type of terminal oxidases expressedin a number of pathogenic organisms, including H. pylori, N. gonorrhoeae, andN. meningitidis. Expression of cytochrome cbb3 oxidases was suggested to be re-quired for colonization of anoxic tissues and may be an important determinant ofpathogenicity21.

2.2. Cytochromes bd

Cytochrome bd oxidases are aerobic terminal oxidases unrelated to HCOs.They function as quinol oxidases and are widely distributed in Gram-negativebacteria (see footnote ‡‡ on page 4) and possibly some Archaea§§ 22. Like cy-tochromes cbb3, cytochromes bd are also suggested to be essential for microaer-obiosis and may protect anaerobic processes from O2. Cytochromes bd are notknown to operate as proton pumps. The enzymes generate a transmembrane elec-trochemical potential of ∼180 mV relying solely on substrate protons, e.g., by

§§ Archaea comprise one of the two major divisions of prokaryotes.

12 Roman Boulatov

releasing 4H+ in quinol oxidation at the acidic P-side of the membrane and up-taking 4H+ from the N-side of the membrane for O2 reduction. It was reported,however, that cytochromes cd may translocate Na+ 22.

Cytochromes bd are two-subunit enzymes, containing one heme d and twoheme b groups, but not Cu or nonheme Fe. Whereas one heme b is six-coordinateand is thought to be the initial site of electron uptake from quinol, the other heme band the heme d center are five-coordinate, although the nature of the axial ligandsis not known. The two hemes share a binding pocket. In this pocket, however, onlyheme d is a high-affinity ligand binding site, which is also the O2 reduction site.The Feheme b–Feheme d distance within this pocket is too long for diatomic mole-cules to bridge the two metals and there is little if any bimetallic cooperativity inligand binding (unlike that in CcOs, where O2 first binds to CuB and is subse-quently transferred to heme a318). As expected for a terminal oxidase optimizedfor microaerobiosis, O2 affinity of heme d is high (Kd ∼ 25 nM).

The redox potentials of the hemes in cytochrome bd oxidases are ∼100 mVmore reducing than the potentials of the redox cofactors in HCOs. Up to three ex-ternal reducing equivalents can be stored in these hemes. The primary O2-bindingredox form in vivo was hypothesized to be a complex between singly reduced cy-tochromes cd and the quinol, or the fully reduced enzyme, both of which wouldhave a total of three external reducing equivalents. Only two oxygen-containingintermediates have been observed spectroscopically in the enzyme under turnover:a heme d/O2 adduct, where O2 does not appear to interact strongly with any othermoieties in the binding pocket, and heme d oxoferryl. As in HCOs, no peroxo-level intermediates have convincingly been detected in cytochromes bd.

Cytochromes cbb3 and particularly cytochromes bd catalyze O2 reductionby mechanism(s) that are likely quite different from that observed in CcOs andquinol oxidases. Obtaining crystallographic structures of these enzymes wouldsignificantly contribute to our understanding of the diversity of the biochemicalstrategies of aerobic respiration.

3. Biomimetic Catalysis of O2 Reduction

The significance of 4e/4H+ reduction of O2 for the existence of the ter-restrial biosphere and the importance of heme centers in catalyzing this reactionstimulated a significant effort aimed at replicating this reactivity in artificial sys-tems containing a heme/Cu unit. No biomimetic studies of cytochromes bd havebeen carried out due to the narrower distribution of these enzymes in the biosphereand the lack of crystallographic data on their catalytic site. Biomimetic studies ofHCOs currently develop along two major directions. First includes work on elec-trocatalytic O2 reduction using electrode-confined Fe porphyrins (one so-called“functional” heme/Cu analogs). Second, O2 reactivity of synthetic heme/Cuanalogs is studied under stoichiometric conditions. A clever and promising ap-proach, which is yet to be fully recognized, involves engineering the heme/Cusite into a simpler enzyme, such as myoglobin33. Synthetic systems designed toreproduce only the spectroscopic properties of the heme/Cu site, mainly of CcO,are of historic interest28, 34. This review deals only with heme/Cu analogs whose


electrocatalytic behavior has been characterized. Studies of the stoichiometric O2reactivity of biomimetic analogs and spectroscopic heme/Cu mimics have beenextensively reviewed elsewhere28, 34, 35. The recent developments are described inrefs [36–38].

Biomimetic studies usually have one of three objectives: (a) to reproduce ina synthetic system the reactivity pattern theretofore observed only in an enzyme,(b) to design useful synthetic catalysts based on the principles learned from study-ing the corresponding enzyme(s), and (c) to better understand the mechanism andstructure/activity relationship of an enzymatic catalytic site by studying a properlydesigned biomimetic analog.

Initial work in biomimetic O2 reduction was aimed almost exclusively atdesigning Fe porphyrin-based compounds that, when immobilized on an elec-trode, would catalyze electrochemical O2 reduction mainly to H2O (as opposedto H2O2) regardless of the electrochemical potential or the lifetime of the cat-alyst28, 32. Such an objective is problematic in several aspects, as discussed be-low. It has evolved into studying the catalysts under conditions that would moreclosely resemble those under which HCOs operate in vivo (potentials, electronflow vs. O2 flux, hydrophobicity of the microenvironment). More biologicallyrelevant conclusions, particularly regarding the structure/activity relationship atthe enzymatic catalytic site (objective (c) above) can be drawn from such experi-ments. It remains to be seen if a useful catalyst (e.g., for O2 reduction in fuel cells)can be designed based on the knowledge acquired in biomimetic studies of HCOs.

Biomimetic electrocatalytic O2 reduction is studied almost exclusively byrotating disk or rotating ring–disk voltammetry39 (Figure 1.9). Usually, a water-insoluble catalyst is deposited on the graphite disk as a film of poorly definedmorphology either by spontaneous adsorption from a solution of the catalyst inan organic solvent or by evaporation of an aliquot of such a solution onto theelectrode. It is impossible to know the amount of catalyst immobilized on theelectrode by spontaneous adsorption (although the amount of electroactive cata-lyst can be determined coulommetrically). Nor is it generally possible to vary theamount of the deposited catalyst. Spin casting is free of such limitations but it mayproduce more morphologically heterogeneous catalytic films due to nonuniformprecipitation of the catalyst during the rapid evaporation of the solvent.

The modified electrode is immersed in a buffered air- (or O−2 ) saturatedaqueous solution and the electrode potential is scanned. The selectivity of thecatalysis, often represented as nav , the average number of electrons delivered toan O2 molecule (reaction 1.14), is determined either from the ratio of the ringand disk currents or from Koutecky–Levich plots (plots of inverse catalytic cur-rents vs. inverse square–root of the rotational frequency of the electrode). Thering–disk ratio quantifies selectivity at both the potential-dependent and plateauparts of a linear-sweep voltammogram (Figure 1.9). However, for practical rea-sons only limiting currents can be acquired with adequate reproducibility to beused in Koutecky–Levich plots. Koutecky–Levich plots also afford informationabout catalytic turnover frequency at potentials corresponding to the plateau ofthe catalytic waves, provided that substrate and/or charge transfer within the cat-alytic film are not turnover limiting.

n4-macrocyclic metal complexes - startseite · 2013. 7. 19. · n 4-macrocyclic metal complexes...

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