Progress in · progress in inorganic chemistry volume 57. advisory board jacqueline k. barton california institute of technology, pasadena, california james p. collman stanford university,

Progress inInorganic Chemistry

Volume 57

Advisory Board

JACQUELINE K. BARTON

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

JAMES P. COLLMAN

STANFORD UNIVERSITY, STANFORD, CALIFORNIA

ALAN H. COWLEY

UNIVERSITY OF TEXAS, AUSTIN, TEXAS

RICHARD H. HOLM

HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS

EIICHI KIMURA

SHIZUOKA UNIVERSITY, SHIZUOKA, JAPAN

NATHAN S. LEWIS

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

STEPHEN J. LIPPARD

MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE,

MASSACHUSETTS

TOBIN J. MARKS

NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS

KARLWIEGHARDT

MAX-PLANCK-INSTITUT, MULHEIM, GERMANY

PROGRESS IN

INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY

JOHNS HOPKINS UNIVERSITY

BALTIMORE, MARYLAND

VOLUME 57

Copyright � 2012 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any

form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise,

except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without

either the prior written permission of the Publisher, or authorization through payment of the

appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers,

MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to

the Publisher for permission should be addressed to the Permissions Department, John Wiley &

Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at

http://www.wiley.com/go/permission.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best

efforts in preparing this book, they make no representations or warranties with respect to the

accuracy or completeness of the contents of this book and specifically disclaim any implied

warranties of merchantability or fitness for a particular purpose. No warranty may be created or

extended by sales representatives or written sales materials. The advice and strategies contained

herein may not be suitable for your situation. You should consult with a professional where

appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other

commercial damages, including but not limited to special, incidental, consequential, or other

damages.

For general information on our other products and services or for technical support, please

contact our Customer Care Department within the United States at (800) 762-2974, outside the

United States at (317) 572-3993 or fax (317) 572-4002.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print

may not be available in electronic formats. For more information about Wiley products, visit

our web site at www.wiley.com.

Library of Congress Catalog Card Number: 59-13035

ISBN 978-1-118-01063-1

Printed in the United States of America

oBook ISBN: 978-1-118-14823-5

ePDF ISBN: 978-1-118-14868-6

ePub ISBN: 978-1-118-14820-4

eMobi ISBN: 978-1-118-14867-9

10 9 8 7 6 5 4 3 2 1

http://www.copyright.com

http://www.wiley.com/go/permission

http://www.wiley.com

Contents

Chapter 1 Mechanisms of Water Oxidation Catalyzed

by Ruthenium Coordination Complexes . . . . . . . . . . . . . . . . . . . . 1

AURORA E. CLARK and JAMES K. HURST

Chapter 2 Biomimetic and Nonbiological Dinuclear Mxþ

Complex-Catalyzed Alcoholysis Reactions of

Phosphoryl Transfer Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

R. STAN BROWN

Chapter 3 Photoactivated DNA Cleavage and Anticancer

Activity of 3d Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . 119

AKHIL R. CHAKRAVARTY and MITHUN ROY

Chapter 4 Design and Evolution of Artificial Metalloenzymes:

Biomimetic Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

MARC CREUS and THOMAS R. WARD

Chapter 5 Functionalization of Fluorinated Aromatics by

Nickel-Mediated C–H and C–F Bond Oxidative

Addition: Prospects for the Synthesis of

Fluorine-Containing Pharmaceuticals . . . . . . . . . . . . . . . . . . . . . 255

SAMUEL A. JOHNSON, JILLIAN A. HATNEAN, and

MEGHAN E. DOSTER

Chapter 6 DNA Based Metal Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

JENS OELERICH and GERARD ROELFES

Chapter 7 Metallo-b-lactamases and Their Synthetic Mimics:

Structure, Function, and Catalytic Mechanism . . . . . . . . . . . . . 395

MUTHAIAH UMAYAL, A. TAMILSELVI, and

GOVINDASAMY MUGESH

v

Chapter 8 A New Class of Nanostructured Inorganic–Organic

Hybrid Semiconductors Based on II–VI

Binary Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445

JING LI and RUIBO ZHANG

Chapter 9 Oxygen Evolution Reaction Chemistry

of Oxide-Based Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505

YOGESH SURENDRANATH and DANIEL G. NOCERA

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561

Cumulative Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593

vi CONTENTS

Mechanisms of Water Oxidation Catalyzed

by Ruthenium Coordination Complexes

AURORA E. CLARK AND JAMES K. HURST

Department of Chemistry, Washington State University. Pullman, WA

CONTENTS

I. INTRODUCTION

II. OXYGEN–OXYGEN COUPLING OF COORDINATED WATER

A. The [RuII(tpy)(H2O)]2(m-bpp)3þ Ion

B. The “Tanaka Catalyst”

III. HOMOLYTIC CLEAVAGE OF O–H BONDS: THE “BLUE DIMER”

A. Structure

B. Redox States

C. Isotopically Defined Reaction Pathways

D. Theoretical Analyses

E. “Noninnocent” Involvement of Bipyridine Ligands

IV. NUCLEOPHILIC ADDITION OF WATER TO ELECTROPHILIC RUTHENYL OXO

LIGANDS

A. General Reaction Characteristics

B. [Ru(bpm)(tpy)(H2O)]2þ and Related Ions

1. Reaction Pathways

2. Alternative Theoretical Analyses

V. EXPANSION OF THE COORDINATION SPHERE

VI. MEDIUM EFFECTS

A. Ion Pairing

B. Anation

Progress in Inorganic Chemistry Volume 57, First Edition. Edited by Kenneth D. Karlin.� 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

1

C. Influence on Catalytic Rates

VII. FUTURE DIRECTIONS

A. Tuning Reactivities through Modification of Organic Ligands

B. Electrocatalysis

C. “All-Inorganic” Molecular Catalysts

1. Reaction Characteristics

2. Theoretical Studies

VIII. CONSPECTUS

ACKNOWLEDGMENTS

ABBREVIATIONS

REFERENCES

I. INTRODUCTION

Interest in water oxidation catalyzed by transition metal ions can be traced to

studies in theearly1950swhen itwas suggestedbyDwyerandGyafaras (1) that tris-

2,20-bipyridine (bpy) and1, 10-phenanthroline (phen) complexes of trivalent group

8 (VIIIB) ions formed ozone (O3) and hydrogen peroxide (H2O2) during their

alkaline decomposition to the corresponding M(II) ions and the subsequent

recognition by Creutz and Sutin (2) that this instability could form the basis for

water photolysis byvisible light using [Ru(bpy)3]2þ asphotosensitizer. Sincedirect

one-electron (1e�) reduction of H2O to HO. is thermodynamically disallowed,

considerable attention was given to characterizing the reaction dynamics with the

intention of identifying reactive intermediates.Abrief reviewof this early literature

can be found in (3). Speculations concerning the nature of these intermediates

ranged from species with chemically altered bpy ligands to ion aggregates contain-

ing stabilized HO. radical [e.g., HO.(HO�)n], and even m-oxo dinuclear bridged

ions generated in a complex sequence of reactions initiated by HO. substitution on

the metal to form seven-coordinate intermediates. This last suggestion was appar-

ently inspired by contemporaneous research from Meyer and co-workers (4, 5)

demonstrating that [Ru(bpy)2(H2O)]2O4þ was an effective catalyst for water

oxidation in acidic solutions containing strong oxidants. Careful research on the

[M(bpy)3]3þ alkaline decomposition reactions ultimately led to the realization that

themajor, if not sole, pathways formetal ion reduction involved irreversible ligand

oxidation accompanied by negligible formation of O2 (6, 7), and interest in these

ions as potential water oxidation catalysts waned. A decade later, however, in a

publication thatdidnot receivemuch attention, Ledney andDutta (8) reported that

2 AURORA E. CLARK AND JAMES K. HURST

[Ru(bpy)3]3þ encapsulated within Y-zeolite supercages decomposed in alkaline

solution with near-stoichiometric formation of O2. Transient species suggestive

of bpy ligand modification were detected by resonance Raman (RR), cryogenic

electron paramagnetic resonance (EPR), and diffuse reflectance spectroscopy,

prompting the researchers to propose a mechanism based upon HO. addition to

the ligand. This general type of mechanism involving “noninnocent” participa-

tion of coordinated nitrogen heterocyclic ligands had been previously explored

within a wider context of metal ion reactivity without any definitive supporting

evidence having been found (9–11), and had also been considered by the

Brookhaven group (2) as a potential mechanism for [Ru(bpy)3]3þ catalyzed

water oxidation. The dramatic change of reaction course attending zeolite

encapsulation was attributed to elimination of bimolecular reactions, among

which were presumably the ligand degradation pathways observed in homoge-

neous solution. Indeed, other research indicated that when [Ru(bpy)3]3þ was

reacted with HO. at high cage occupancies, dioxygen (O2) was not formed.

Rather, carbon dioxide (CO2) evolved in a manner that evoked the solution

reactions, indicating that extensive ligand degradation had occurred (12). None-

theless, the studymade on [Ru(bpy)3]3þ at low zeolite loadings provided the first

indication that, under suitably restrictive conditions, a coordinately saturated

single ruthenium center is capable of catalyzing water oxidation.

A second instructive point arising from the early studies was that in the presence

of certain redoxmetal ions [e.g., Co(II)] (6, 13, 14) andmetal oxides (15–18),which

functioned as cocatalysts, O2 formation by [M(bpy)3]3þ reduction could become

nearly quantitative. Indeed, these observations formed the basis for several fairly

efficient photocycles forwater oxidation by electron donors using [Ru(bpy)3]2þ as a

photosensitizer (Fig. 1). In these cases, in addition to functioning as the true catalyst,

the second metal ion most likely protected the [M(bpy)3]3þ by introducing a

competitive reduction pathway that did not involve ligand degradation.

During the 1980–1990s, the perception developed in the field that efficient

homogeneous catalysis of water oxidation required the presence of at least two

metal centers within the complex. Factors contributing to this viewpoint included the

intense focus on understanding biological water oxidation (24–26), then already

known to involve a tetranuclearMn cluster (27–30), and the repeated demonstrations

that the ruthenium“blue dimer” (cis,cis-[Ru(bpy)2(H2O)]2O4þ) andanalogousm-oxo

bridged diruthenium ions were efficient catalysts (31–35) but, in addition to

[Ru(bpy)3]3þ, monomeric complexes containing water ligands, including species

that might be considered dimer fragments (e.g., cis-[Ru(bpy)2(H2O)2]3þ) were

apparently devoid of activity ((4, 31, 36); see, however, 37). Indeed, the discovery

that only two of the four Mn centers in the oxygen-evolving complex undergo redox

cycling further heightened suspicions that dinuclear centers were somehow uniquely

associatedwith catalytic activity (38, 39). However, very recent discoveries have now

MECHANISMS OF WATER OXIDATION CATALYZED 3

made it abundantly clear that this general assumption is invalid. Examples of efficient

catalysis by mononuclear, dinuclear, and tetranuclear Ru complexes, as well as

similar complexes containing other metal centers, have now surfaced; moreover, this

body of emerging work is transformative in that one no longer seeks to unlock the

mystery of how the O–O bond could possibly form, but rather how to distinguish

among the many demonstrated and proposed pathways that are revealed in these

reactions and to understand how structural factors dictate the expression of one

pathway over another.

This chapter reviews the current state of knowledge concerningwater oxidation

as revealed by reactions involving heterocyclic Ru coordination complexes. These

ions possess spectroscopic signatures that make them particularly suited to

mechanistic studies and often accumulate intermediary species during turnover

that can provide important clues to reaction mechanisms. Moreover, advanced

computational analyses based upon density functional theory (DFT), as well as

multiconfigurational self-consistent field (MCSCF) and perturbation theories have

been utilized, which are extremely helpful in evaluating the plausibility of

proposed mechanisms. Although application of DFT and wave function based

methods is now widespread within this field (40–52), it is perhaps worthwhile to

emphasize that, although important as validatory tools, their full predictive power

has not yet been realized. As a recent report suggests (40), difficulties in reliably

RuL33+

RuL32+*RuL3

2+

S2O82– Co(NH(or 3)5Cl2+)

2SO42– Co(or 2++5NH4

++Cl-)

2H2OWOCn+4

O2 4H+ +WOCn

Net: 2S2O82– 2H+ 2O 4SO4

2– 4H+ + O+ 22hν

4Co(NH3)5Cl2+ 16H+ + 2H+ 2 4CoO 2+ 20NH+ 4+ 4Cl+ – O+ 2

4hν

2–4 cycles

hν

Figure 1. Generic scheme for [RuL3]2þ photocatalyzed water oxidation. Although the reactions

proceed by repetitive cycling of the photocatalyst, only the initial and final (i.e., water oxidizing) states

of the water oxidation catalyst (WOC) are shown. Recent studies have utilized monomeric (19),

dimeric (20, 21) and tetrameric (22) Ru containingWOCs, as described in the text. Photocatalysts have

included [Ru(bpy)3]3þ and analogues containing derivatized bpy ligands, specifically [Ru(dmb)3]

2þ,where dmb¼ 4,40-dimethyl-2,20-bipyridine (19), and [Ru(dcb)2(bpy)]

2þ, where dcb¼ 4,40-dicar-bethoxy-2,2-bipyridine (20). The strongly oxidizing sulfate radical anion [Eo(SO4

.�/2�)¼ 2.4V]

formed upon 1e� reduction of S2O82� reacts with ruthenium bipyridine complexes at near-diffusion

controlled rates (23) and participates in water oxidation by oxidizing both [RuL3]2þ and intermediary

oxidation states of the WOC.


predicting mechanisms may be due to limitations in the chemical model that is

studied rather than the computational method that is employed. Indeed, most

theoretical studies do not consider the role of extended explicit solvation of the

complex during the myriad transformations that occur along the reactive

potential energy surface, thus ignoring a key facet of the experimental reaction

conditions. A pertinent case in point is the study of water oxidation catalyzed

by [RuII(tpy)(H2O)]2(m-bpp)3þ (bpp¼ 2,6-bis(pyridyl)pyrazolate anion and

tpy¼ 2,20:60.200 terpyridine) ion, which is discussed in detail in Section II.

This complex contains two hexacoordinate Ru ions templated within a hetero-

cyclic bridging bpp (Fig. 2). The coordination environment enforces a geometry

inwhich thewater ligands are facially orientedwith anO� � �Oseparation distance

of only�2.09 A. Four-electron (4e�) oxidation to the corresponding [RuIV(tpy)(O)]2(m-bpp)3þ ion leads to O2 evolution by a unimolecular pathway (54);18O-isotopic labeling studies indicate that both O atoms are obtained from the

coordination sphere of the complex ion (53). These data strongly implicate a

mechanism involving coupling between two adjacent RuIV¼O atoms, followed

by reductive elimination of O2 and regeneration of [RuII(tpy)(H2O)]2(m-bpp)3þ,

as illustrated in Fig. 2. However, a DFT computational analysis made prior to the

definitive isotope-labeling study predicted the existence of a prohibitively high

activation energy barrier for this reaction pathway (44). In this study, it was found

that a 1,2-peroxo-bridged intermediate readily formed from [RuIV(tpy)(O)]2(m-bpp)3þ, but that decomposition of this intermediate was energetically very

demanding. Thus, by this analysis, the peroxo-bridged complex was identified

as a dead-end species. An alternative low-energy pathway was found that

involved protonation of one of the ruthenyl oxo atoms, causing electron density

to be withdrawn from the adjacent ruthenyl group. This electronic polarization

rendered the ruthenyl oxygen atom sufficiently electrophilic to undergo nucle-

ophilic attack by a solvent molecule with formation of a hydroperoxo–hydroxo

intermediate. Internal electronic rearrangement then led to release of O2 with

regeneration of the catalyst in its original form (Fig. 2). However plausible this

mechanism may be, the subsequently published 18O labeling studies clearly

show it is not operative under the reaction conditions investigated. Specifically,

this mechanism requires that one O atom be obtained from solvent and the other

from the coordination sphere of the catalyst, which is clearly not the case (53).

This set of studies constitutes an example of the subtlety of forces at play that can

determinewhich of several potential pathways for water oxidation are expressed,

as well as the extreme challenge this presents to theorists in accurately predicting

activation barriers. Correspondingly, this chapter first focuses attention upon

catalysts forwhich experimental evidence has given some indication of the actual

reaction pathways and then enumerates other catalytic systems where experi-

mental evidence on proposed reaction pathways is less definitive.


II. OXYGEN–OXYGEN COUPLING OF COORDINATED WATER

A. The [RuII(tpy)(H2O)]2(m-bpp)3þ Ion

This bis-(pyridyl)pyrazolate-bridged dimer is particularly amenable to analysis

of water oxidation because each of the oxidation steps is thermodynamically

and kinetically resolved and each of the oxidation states has a distinct optical

spectroscopic signature (45, 53). Moreover, following oxidation to the highest

accessible state ([RuIV(tpy)(O)]2(m-bpp)3þ), a transient species accumulates

whose first-order decay parallels O2 release. Consequently, this species could be

a bona fide reaction intermediate in theO2 forming cycle; its accumulation presents

(tpy)RuII-L-RuII(tpy)

OH2H2O

3+

{2,2} 4Ce3+ 4H+ +

4Ce4+

(tpy)RuIV-L-RuIV(tpy)

OO

3+

{4,4}

(tpy)RuIII-L-RuIII(tpy)3+

OO

H2O


OH2OO


OH2OO

H2O

H2O

O=O

H2O

O=O


OHHOO

pathway b

pathway a

N

N

N N

NN

NN5

{2,2}

N6

Ru2Ru1

O41.854A

O3

Figure 2. Optimized calculated structure of [RuII(tpy)2(H2O)]2(m-bpp)3þ and alternative proposed

pathways for catalyzed water oxidation. For pathway a, both O atoms are derived from the coordination

sphere, whereas for pathway b, one atom is from the coordination sphere and the other is from the

solvent (as identified by the solid circle). [Adapted from (53).]


a unique opportunity for structural characterization that is lacking in other catalytic

systems. The cyclic voltammogram (CV) of [RuII(tpy)(H2O)]2(m-bpp)3þ displays

three quasireversible (1e�) waves in acidic aqueous solutions; a fourth irreversibleoxidation can be detected at potentials approaching catalytic water oxidation.

These data indicate a regular progression in thermodynamic stabilities that follow

the order: {2,2} ! {2,3} ! {3,3} ! {3,4} ! {4,4} (where the notation given is

meant to indicate only the overall oxidation state of the complex based upon

assignment of formal charges, i.e., not the actual electronic distribution). Oxida-

tion is accompanied by release of protons, as dictated by the increasing acidities of

the higher oxidation states so that, upon complete oxidation to {4,4}, the coor-

dinated aquo ligands are completely deprotonated to give ruthenyl oxo atoms. Rate

constants for stepwise oxidation by Ce4þ progressively decrease with increasing

oxidation state, so that each of the intermediary oxidation states can be isolated and

physically characterized. Upon oxidation to {4,4}, however, spontaneous O2

evolution occurs in a reaction that is associated with first-order formation

and decay of a spectroscopically distinct reaction transient. The visible spectra

of both {4,4} and the transient species (I) have been obtained by global kinetic

analysis.

Species I is suggested to be a 1,2-m-peroxo-bridging intermediate formed by

coupling of the two juxtaposed oxo radicaloid atoms on the adjacent Ru atoms of

{4,4}. Due to the close energetic spacing of the various electronic states of I, the

theoretically predicted ground state is dependent on the exact density functional

used within DFT (43, 44). However, complete active space self-consistent field

calculations with second- order M€oller–Plesset perturbation theory (CASPT2)

generally agrees quite well with the M06-L DFT implementation, predicting that

each low-spin Ru(III) couples as a triplet with its respective O.�, with the two

triplet RuIII–O.� units coupling as a net S¼ 2 configuration; these calculations also

indicate that the low-lying S¼ 0 state lies within 4 kJmol�1. From a computational

perspective, the reaction energetics of I are somewhat sensitive to the specific

density functional used. Yet the chemical model employed to mimic both I and its

solvation environment is significant and may be more important. The direct O–O

coupling pathway (Fig. 2) is predicted by both B3LYP and M06-L functionals to

have a reasonable activation barrier for formation of the first intermediate, a cyclic

1,2-peroxo bridged Ru–O–O–Ru3þ{3,3} ion. However, discrepancies exist overthe appropriate treatment of the second transition state to form the {2,2}3þ

protocatalyst. Irrespective of whether the calculation is performed in the gas

phase or utilizing a solvent continuum model to mimic the effects of the bulk

dielectric, it is apparent that the activation barrier is much too high unless the

chemical model is expanded to include more of the explicit solvation environment

surrounding the Ru–O–O–Ru3þ{3,3} intermediate. The approach of Yang and

Baik (44) was to take into account the effects of acidity present in the experimental

solution by examining formation of {2,2}3þ fromprotonated Ru–O–O–Ru3þ{3,3}.


This approach did not yield significantly improved energetics and, as such, this

reaction pathway was dismissed as a viable mechanism for I. Instead, Yang and

Baik (44) proposed that an alternate pathway consisting of coupling of the

terminal oxo and water oxygen atoms (Fig. 2) would be energetically more

favorable. However, improvement in the microsolvation environment around

Ru–O–O–Ru3þ{3,3} through addition of two waters of hydration yielded a

calculated activation barrier for formation of {2,2}3þ (45) that agreed within

9 kJ mol�1 with the experimental value.

Although the experimental and theoretical results present a self-consistent

and intuitively reasonable model for catalyzed water oxidation, the reaction

itself presents some unexplained anomalies. The rate laws for oxidation of {2,2}

to {3,3} are first order in both Ce4þ and the dimer. However, the rate law

for oxidation of {3,4} shows apparent saturation of the dependence upon Ce4þ

concentration. Potential causes are discussed below in Section VI on medium

effects. More strikingly, the global kinetic analyses for reactions made at ambient

temperatures indicate that, following a single turnover, the {2,2} product under-

goes apparent sequential conversion to two new species that have markedly

altered optical absorption spectra (45). These are suggested to be anated species

that may be similar to Ru2–bpp complexes that have been isolated containing

bridging Cl�, MeCOO�, and CF3SO3� anions in place of the coordinated water

molecules (53, 54). However, the optical changes are considerably greater than

have been reported for m-oxo bridged Ru dimers, where SO42� substitution

occurs (32, 33) and where ClO4� and CF3SO3

� anation has been proposed based

upon kinetic effects (55) (Section VI.A). In those cases, the modified catalysts

exhibit optical spectra that are almost indistinguishable from the corresponding

catalytically active diaquo forms. Under conditions where Ce4þ is in large excess,

[RuII(tpy)(H2O)]2(m-bpp)3þ is reported to catalyze water oxidation through as

many as �500 cycles prior to deactivation, so it appears that either the structural

changes implied by the optical spectra occurring after a single cycle are reversible or

the chemically modified complexes are also capable of catalyzing water oxidation.

It was also reported that “exhaustive” electrochemical oxidation led to formation of

a small amount of dinuclear complex containing an oxidized bpp ligand.

B. The “Tanaka Catalyst”

A long-lived diruthenium catalyst for water oxidation containing a binucleating

anthracene-linked pair of terpyridyl groups with redox-active benzoquinone and

hydroxide ions as additional ligands (Fig. 3) was first reported in 2000 (57).

Athough this complex, isolated as [Ru2(OH)2(3,6-Bu2Q)(btpyan)](SbF6)2; struc-

ture given in (Fig. 3), is water insoluble, Tanaka and co-workers (68) were able to

demonstrate limited electrocatalytic activity by constant potential electrolysis


(CPE) in trifluoroethanol containing 10%water.When the complex was deposited

as a solid on an indium–tin oxide (ITO) electrode, remarkably efficient electro-

catalyzed water oxidation could be achieved in aqueous media, with O2 evolution

turnover numbers per catalyst molecule exceeding 33,000 being measured.

However, the catalytic rate constant was very low. Several structurally similar

complexes containing modifications within the bridging group (xanthene for

anthracene) of the templating macrocyclic ligand (59) or different substituted

quinones (46) have been prepared in efforts to improve catalytic rates within this

class of compounds. However, to date, none of these complexes have been found to

exhibit detectable electrocatalytic activity.

Figure 3. The DFT predicted mechanism for water oxidation catalyzed by [Ru2(OH)2(3,6-Bu2Q)

(btpyan)]2þ ion [3,6-Bu2Q¼ 3,6-di-tert-butyl-1,2-benzoquinone and btpyan¼ 1,8-bis(2,20:60,200-ter-pyrid-40-yl)anthracene]. Two proton-coupled electron transfer (PCET) steps on the resting form of the

catalyst (top) lead to oxidation of juxtaposed hydroxo ligands,which couple to formabridging superoxo

ion (bottom), with the additional electron being distributed over the quinone ligands. Further PCET

reoxidizes the quinones, leading to incorporation of solvent into the coordination sphere (left); at this

point, the superoxo ligand is terminally coordinated. Thefinal PCEToxidizes the superoxide and returns

the catalyst to its original form. The RIMP2 calculated geometric structure of the complex ion

containing 3,5-dimethyl-substituted quinone ligands (in place of tert-butyl substituents) is shown

within the catalytic cycle. [Adapted from (56).] (See the color version of this figure in Color Plates

section.)


The aqueous insolubility and the “noninnocent” nature of the quinone ligands

present formidable challenges to characterization of the “Tanaka complex”, as it is

now known, in its various accessible oxidation states. In particular, the complex

is representative of a large class of Ru–NIL (NIL¼ noninnocent ligand) com-

plexes whose ligand and metal orbitals are extensively mixed, giving rise to

apparent noninteger oxidation states and nearly isoenergetic electronic states with

differing spin multiplicities (56, 60), so that even ground-state configurations are

difficult to assign. Despite the challenges, mechanistic analyses of this reaction

have been carried forward with considerable success by the Tanaka and Fujita/

Muckerman groups using a combination of experimental and theoretical

approaches. These efforts have been aided by the availability of a model of the

“half-molecule”, (i.e., [Ru(H2O)(3,5-Bu2Q)(tpy)]2þ) (61). Although apparently

not capable of oxidizing water itself (46), this ion is more amenable to compu-

tational and physical analyses than the dimer. Controversies concerning

the ground-state representation of this ion, prevalent in the earlier literature

(46, 61), appear to have been recently resolved through in-depth electrochemical,

spectroscopic, and computational analyses (47, 56).

The computational studies utilized a combination ofDFT, time-dependent DFT

(TD-DFT) (using the B3LYP functional) and CASSCF (complete active space

self-consistent field) methodologies to probe the relative energies of the various

available spin states of the reaction intermediates. Despite the relative simplicity of

the monomer relative to the dimer, significant computational difficulty was

encountered. Although the authors utilized the broken-spin broken-symmetry

(BS/BS) method (62–64) to obtain open-shell singlet states, a wide variety of hS2ivalues were observed, indicating spin contamination from alternative S states with

the same Ms values. Indeed, spin contamination was even observed for the open-

shell triplet states using DFT. Interestingly, the authors avoided using the

Noodleman’s spin projection correction to the BS/BS singlet-state energy within

their calculations, perhaps due to the large amount of spin contamination observed

in the open-shell singlet states. To further test the relative energies of the various

spin states, the authors utilized TD-DFT to examine which spin states were higher

than the predicted ground state. Unfortunately, many of the excited states

encountered were charge transfer (CT) in nature, bringing into question the

reliability of the calculations, as DFT (specifically density functionals without

long-range corrections) is known to perform very poorly for CT excitations (65).

The results for the “half-molecule” most relevant to the catalytic activity of

the binuclear ion are that the best description of the formal oxidation state of

the aquo complex is [RuII(H2O)(Q)(tpy)]2þ, rather than the initially proposed

[RuIII(H2O)(SQ.�)(tpy)]2þ (SQ.�¼ 3,5-di-tert-butylbenzosemiquinone) (61),

and that sequential deprotonation leads to [RuII(OH)(Q)(tpy)]þ and [RuII(O.�)(SQ.�)(tpy)]0. The doubly deprotonated molecule is unique in possessing an oxyl

radical ligand, formed by internal transfer of an electron to the quinone.


This radical is expected to be highly reactive and, in experimental systems, appears

to abstract a hydrogen atom (from unspecified sources) to give [RuII(OH)(SQ.�)(tpy)]0 as the final product. The calculated electronic spin states for these three

protonation states are difficult to assign using DFT, as spin contamination is

observed for the varying states. As such, the hS2i values were interpreted in terms

of a simple generalized valence bond configuration interaction (GVB-CI) within a

(2,2)CAS typemodel as in stretchedH2. This interpretation suggests that low-lying

singlet, open-shell singlet, and triplet spin multiplicities can exist that contain Ru

in formal oxidation states ranging from Ru(II) to Ru(IV) (47).

Thewater-oxidizing capacity of the dinuclear catalyst is attributed to formation

of intermediates similar to [RuII(O.�)(SQ.�)(tpy)]0, in which the templating

btpyan ligand juxtaposes the coordinated oxyl groups to direct O–O bond

formation via radical coupling (Fig. 3). These researchers originally proposed a

mechanism based upon DFT computational results in which sequential deproto-

nation of the resting form of the catalyst ([(RuII)2(OH)2(Q)2(btpyan)]2þ) led to an

intermediate containing a bridging superoxide anion with electron density shifting

to the quinone ligands (i.e., best described as [(RuII)2(O2�)(Q�1.5)2(btpyan)]

0),

following which net 4e� oxidation led to release of O2 with regeneration of the

resting form of the catalyst (46). More recently, this mechanism has been modified

so that the overall cycle contains a series of four PCET steps (Fig. 3) (56). Here, the

resting form of the catalyst is indicated as an asymmetrically hydrogen-bonded

pair of coordinated hydroxo ligands. The intermediate formed following the first

PCET step contains an oxyl anion that is stabilized by hydrogen-bonding to the

adjacent hydroxyl ligand. Loss of this proton in the second oxidation step then

allows O–O bond formation, in which the 1,2-bridging O2 group is formulated as

superoxo anionwith the additional electron density shifting to the quinone ligands.

Subsequent PCET then leads to formation of a terminally coordinated superoxo

anion via addition of solvent and, in the final step, oxidation of the coordinated

O2.� releases O2, closing the catalytic cycle.

One remarkable feature of this reaction as written is that the Ru ions do not

change their formal oxidation states throughout the cycle. Instead, redox changes

occur primarily through complementary changes in electron density in orbitals that

are centered in the oxo and quinone ligands and reflect the highly delocalized

character of the frontier orbitals in this coordination complex. Nonetheless, the

complex nature of the wave functions observed here and elsewhere, as well as the

small energy differences between spin states, call for more thorough computa-

tional studies. In particular, note that few benchmarking calculations have been

performed on Ru catalysts so as to understand more broadly the performance of

various density functionals and how that performance changes with varying

systems. While it is becoming more commonplace for CASSCF and CASPT2

methods to be used in conjunction with DFT, this needs to become standard

practice and researchers must ensure that the size of the active space in which the


electronic excitations are allowed to occur is sufficiently large to capture the

essential aspects of the wave function. Moreover, in both DFT and wave function

based methods, benchmarking of the basis sets used to describe the metal and

ligands must be performed. To our knowledge, no studies have examined the basis

set dependence of the reaction energetics and spin state distributions, nor have any

attempts beenmade to extrapolate any type of basis set orKohn–Sham limit for any

methodology employed. Similarly, no examples exist that have benchmarked

the performance of varying continuum approximations and their effects upon the

reaction energetics.

III. HOMOLYTIC CLEAVAGE OF O–H BONDS:

THE “BLUE DIMER”

A. Structure

The water oxidizing capacity of the m-oxo bridged cis,cis-[RuIII(bpy)2(H2O)]2O

4þ “blue dimer” (hereafter identified as {3,3}) was originally reported

by Meyer’s group (4) in 1982. For the ensuing �20 years, this ion and structural

analogues bearing substituted bipyridine ligands were the only known homoge-

neous catalysts for water oxidation whose reactivity could be reproducibly

demonstrated (5, 31–35). Correspondingly, they are the ions whose physical

properties and reactivities have been most extensively investigated. X-ray

crystallographic analyses of {3,3} (5) and {3,4} (as the dihydroxy-ligated

[Ru(bpy)2(OH)]2O3þ ion) (66) reveal a nearly linear oxo-bridge and torsional

dislocation about the Ru–O–Ru bond that places the O atoms of the adjacently

coordinated H2O or OH ligands at a distance of �4.5 A. The DFT calculations

indicate that this samegeneral orientation ismaintained in the chemically unstable,

catalytically relevant higher oxidation states of the complex (Fig. 4) (67), and the

near-linear bridging character of the Ru–O–Ru bond over the entire range of

accessible oxidation states ({3,3} to {5,5}) has been experimentally confirmed by

resonance Raman (RR) measurements of the 18O isotope-dependent frequency

shifts occurring in the ns(Ru–O–Ru) symmetric stretching vibrational modes (68).

CASSCF methods have characterized the electronic ground state of {3,3} as

a weakly antiferromagnetically coupled singlet (43). In the computed structures,

progressive oxidation of the metal centers leads primarily to modest shortening

of the metal–ligand bonds throughout the complex accompanied by an increase

in the torsional angle between the adjacently coordinated terminal oxo ligands,

the net effect being that their critical O� � �O distances do not change appreciably

upon oxidation (67). Consequently, although compositionally similar to

the bis(pyridyl)pyrazolate-bridged diruthenium complex recently described by


Llobet and associates (53, 54), the conformational differences suggest that a

significant activation barrier to intramolecular coupling of oxo atoms may exist

in the “blue dimer” arising from the molecular distortions required to bring these

groups into close contact. Indeed, the 18O isotope labeling studies described below

reveal that these two dimers catalyze water oxidation by distinct mechanisms.

B. Redox States

Extensive mechanistic investigations have been undertaken by two groups, who

have generally used alternative approaches of analysis (67, 69). Although this has

led to somewhat different viewpoints, particularly concerning the nature of reaction

intermediates, the groups concur that the oxygen-evolving form of the catalyst is

{5,5}, an oxidation state in which the coordinated water molecules have been fully

deprotonated to generate ruthenyl oxo atoms, (i.e., [RuV(bpy)2(O)]2O4þ). The

identity of this species was first inferred by Meyer and co-workers (5) using

electrochemical analyses and later confirmed by redox titrations in our laboratory,

whichmadeuseof a columnarflow-through carbonfiber electrode for fastCPE (70).

Resonance Raman spectroscopy clearly identified Ru¼O stretching vibrational

modes in the {5,5} ion at�800 cm�1 (Fig. 5) (70, 71); furthermore, {5,5} underwent

first-order decay with a rate constant that was equal to the rate constant for O2

evolution measured under steady-state catalytic conditions (70, 72, 73).

Undermost experimental conditions,CVs of the “blue dimer” inwater exhibit two

well-defined oxidation waves above {3,3} whose relative amplitudes indicate that

they are {3,4} and {4,5}, as well as an additional wave that just precedes the onset of

Figure 4. The B3LYP/6-31G�/LANL2DZ high-spin ferromagnetically coupled optimized confor-

mation of the “blue dimer” in its catalytically active {5,5} ([Ru(bpy)2(O)]2O4þ) oxidation state.


solvent breakdown (5, 23). Based upon this behavior, one can assign the following

sequence of accumulating redox states: {3,3} ! {3,4} ! {4,5} ! {5,5}. Thesepotentials are pH dependent, reflecting the different states of protonation of the

coordinated aquo ligands under varying medium conditions. Below pH 2, the two

more anodic waves coalesce, so that the voltammograms appear as two waves with

relative amplitudes of 1:3, indicating that the higher oxidation step appears as the

three-electron (3e�) process: {3,3} ! {3,4} ! {5,5} (5). However, an intermediate

species can still be detected when more sensitive methods are used. For example,

redox spectrometric titrations utilizing the flow CPE cell described above with RR

detection clearly demonstrate the accumulation of an intermediary oxidation state at

potentials slightly lower than those required to oxidize the complex to {5,5} (Fig. 5);

furthermore, decay of flow CPE prepared {5,5} is biphasic, with the first step

proceeding to an intermediary species that only slowly converts to {3,4}, the highest

stable oxidation state (70). The identity of this intermediate has been controversial.

Based primarily upon titrimetric and transient kinetic studies using Ce4þ as oxidant

Figure 5. Resonance Raman spectroelectrochemical titration of the “blue dimer” {3,4} ion in 0.5M

CF3SO3H. The inset shows the low-frequency spectra of the various detectable oxidation states. Bands

highlighted in light gray are the Ru–O–Ru symmetric stretching frequency and its first overtone; the

band highlighted in dark gray (lowest trace) is the stretching frequency of the terminal Ru¼O bond.

[Adapted from (70).]


and employing global kinetic analysis for spectral deconvolution, Meyer and co-

workers assigned this oxidation state as {4,5} (55, 74); their kinetic analyses identified

{4,4} as an unstable transient species whose concentration levels were vanishingly

small. However, several different titrimetric measurements made in our laboratory

using flow CPE prepared solutions in various oxidation states (70), as well as direct

titrationwith Ce4þ (71) indicate that the accumulating intermediary oxidation state is

actually {4,4}. Recent RR and optical spectroscopic measurements have confirmed

this assignment. Specifically, as anticipated from the CVanalyses (5), {4,5} contains

ruthenyl bonds, which are readily detected in the RR spectrum by their isotope-

sensitive Ru¼O stretching modes (23). These bands are not observed in the

intermediate that accumulates in acidic solutions, however (Fig. 5) (70). Furthermore,

the optical spectrum of {4,4} determined in neutral solutions by pulse radiolysis is

unlike that of {4,5}, but identical to the spectrum of the accumulating intermediate in

acidic solutions (23). Assignment as {4,4} is also supported by pH jump experiments

in which solutions of {4,5} are rapidly acidified. One observes by X-band EPR

spectroscopy the immediate formation of {5,5}, but no {3,4}, the inference being that

the other accumulating oxidation state is {4,4}, which is EPR silent. Upon standing,

theEPRsignal of {3,4} slowly appears as the signal associatedwith {5,5}disappears at

a rate characteristic of water oxidation; that is, the following reaction sequence:

2{4,5} ! {4,4} þ {5,5} ! ! (redox decay to {3,4} and O2) (Fig. 6) (23).

Collectively, this body of evidence forms overwhelming support for the reaction

sequence (Scheme 1), in which {4,4} is the accumulating intermediary state in acidic

solutions, but {4,5} is the accumulating state under more alkaline conditions:

4000360032002800

1.50 V

Magnetic field (G)

1.35 V

1.30 V{3,4}

{5,5}

380360340

Magnetic field (mT)

(a) (b)

Figure 6. The X-band cryogenic EPR spectra of paramagnetic “blue dimer” oxidation states in 0.5M

CF3SO3H. Panel a: spectra of {3,4} and {5,5} formed by flowCPE at the indicated potentials (vs NHE);

panel b: spectral changes accompanying a pH jump of {4,5} from pH 7 to 0.3. Formation of {5,5} is

immediate and its subsequent decay is accompanied by slow accumulation of {3,4}, consistent with the

reaction sequence: 2{4,5} ! {4,4} þ {5,5} ! ! ! {3,4}. [Adapted from (70 and 23).]


A similar pH dependent cross over of relative stabilities of {4,4} and {4,5} hasbeen reported for the analogous [(bpy)2Os

III(H2O)]2O4þ ion; as discussed by

Meyer and co-workers (75), this unusual behavior reflects the influence of protic

equilibria involving the aquo/hydroxo/oxo ligands upon the reduction potentials of

the various redox states. The [RuIII(tpy)2(H2O)2]O4þ ion exhibits a somewhat

different pattern, in which {3,3}, {3,4}, and {4,4} are stable oxidation states

throughout the pH range, but {4,5} becomes unstable to disproportionation to

{4,4} and {5,5} below pH �2 (76).

In addition to {5,5}, the complex in its {4,5} oxidation state has the

thermodynamic potential to oxidize water, in this case according to the reaction:

2{4,5} þ 2H2O ! 2{3,4} þ O2. At pH 7, the thermodynamic driving force for

this reaction (DG�) is �1.0V (5). Involvement of two dimer molecules in the

overall reaction necessitates a multistep reaction mechanism, however. At pH 7,

{4,5} decays by a complex rate law without generating any O2 (23, 77). At pH

5–6, Meyer’s group has reported O2 formation, but we have been unable to

confirm this, and detect no O2 accumulation during decay under these condi-

tions (23). The rates of {4,5} decay increase with increasing solution alkalinity,

which is a feature commonly shared with the group 8 (VIII B) [M(bpy)3]3þ ions

noted above, as well as other highly oxidizing monomeric Ru species, such as

[Ru(tpy)(bpy)O]2þ and several Os analogues, including [(bpy)2OsV(O)]2O

4þ

(75). These reactions have been attributed to oxidative degradation of the polyimine

ligands. In acidic solutions, {4,4} is also thermodynamically capable of oxidizing

water in a reaction requiring twodimers. In this case,DG� ¼�0.4Vfor the reaction: 2

{4,4} þ 2H2O ! 2{3,3} þ O2.Thedriving force for this reaction is pH independent

in the acid range because the protons released upon oxidation of water are consumed

in conversion of the dihydroxy-ligated {4,4} to the diaquo-ligated {3,3}. Experimen-

tally, one observes that electrochemically or chemically prepared {5,5} decays by

first-order kinetics to a redox-equilibrated solution containing primarily {4,4}, which

then undergoes considerably slower reduction by a complex reaction mechanism to

the stable {3,4} ion (70). As noted above, the rate constant for the first step, (i.e., {5,5}

{3,3} {3,4}

{4,4}

{4,5}

{5,5}(pH < 2)

(pH > 2)

slow(t1/2 min)~

–e- –2e-

–e-–2e-

–e-

O2

Scheme 1. Thermodynamically accessible redox states of the “blue dimer”.


reduction) parallels turnover rate constants for O2 evolution measured under steady-

state conditions (70, 73), identifying it as the catalytically active species. The capacity

for {4,4} to oxidize water has not been determined, although it is apparent that if this

reaction occurs at all, it is considerably slower than the reaction catalyzed by {5,5}.

Redox equilibrated solutions at this level of oxidation necessarily contain small

amounts of {5,5} and {3,4} (70), as governed by the equilibrium: 3{4,4} $2{3,4} þ {5,5}. Consequently, it might prove difficult to distinguish between O2

generated by residual {5,5} and {4,4}under these conditions. In any event, {4,4} is not

the O2 evolving species under normally measured catalytic conditions where strong

oxidants are in considerable excess.

C. Isotopically Defined Reaction Pathways

Oxygen isotopic labeling studies have been particularly informative in deter-

mining the reaction dynamics of the “blue dimer”. Early studies from the

Meyer (78) and Hurst (68) laboratories using [18O]–H2O labeled complex

suggested that several pathways may exist for O2 formation. These studies were

made using different oxidants (Ce4þ andCo3þ, respectively) and different reactantstoichiometries (slight excess of {3,3} and Co3þ, respectively); both studies gaveO2 isotopomer distributions that identified two major pathways, one in which

one O atom was derived from the coordination sphere and the other from the

solvent (pathway a), and a second in which both O atoms were derived from

solvent (pathway b). A minor pathway comprising �10% of the total reaction in

which both O atoms were obtained from the complex coordination sphere

(pathway c) was suggested from the Ce4þ oxidation study, but this pathway was

not detected in the study using Co3þ as oxidant. The basis for the quantitative

differences in these two studies is uncertain. One possible explanation is based

upon differing reactant compositions; in the study using Ce4þ as oxidant, the

predominant oxidation statewas {4,4}, whereas in the studywith Co3þit was {5,5}.If {4,4} were contributing to O2 formation in the Ce4þ experiments, a likely

pathway would involve bimolecular reaction between these ions, which could

account for the product derived from two coordinated oxo atoms observed in this

study (78).

More recently, we developed methods that allow real-time mass spectrometric

determination of evolved O2 during catalytic turnover (72). This procedure has the

advantages that it provides a temporal record that canbe used to test reaction kinetic

schemes and allows measured isotopomer ratios to be extrapolated to zero time to

correct for isotopicdilutionof the 18O label in thecoordinationsphereas thereaction

proceeds. This approach has been used to probe reaction pathways for catalysis of

water oxidation by the “blue dimer” and several congeners whose bpy ligands

contain electron-donating or -withdrawing substituents. Typical results are illus-

trated inpanelaofFig.7;under theexperimental conditions, theoxidant (Ce4þ) is in


N

N

N

N

L2RuV-O-LRuV L2RuV-O-LRuIV

L2RuIV-O-LRuIV

N

N

N

N

L2RuIII-O-LRuIII

{5,5} {4,5-ΟΗ.}

{3,3}

O

4+ 4+

O

OH

{4,4}

4+

4+

OH2

O OHH

H2O

OHH

OH2

HO

H

L2RuIII-O-LRuIII

N

NOH2

{3,3}

4+

OH2

OH

O2

OO

HO

HO H

Scheme 3. A mechanism for pathway b involving HO addition to bpy ligands. Solvent-derived O

atoms are shown as solid circles with solvent-derived H atoms in bold type.

Ru O Ru(bpy)2

O OHO

H

(bpy)2

δ+

δ−

{5,5} 4+

Ru O Ru(bpy)2

O OH

(bpy)2

4+

OH

{4,4}

Ru O Ru(bpy)2

OH2 OH2

(bpy)2

4+{3,3}

H2O

O

Scheme 2. A mechanism for pathway a involving concerted addition of H2O across the two ruthenyl

groups. Solvent-derived O atoms are shown as solid circles with solvent-derived H atoms in bold type.


25–100-fold excess, so that {5,5} remains theO2evolving species over the course of

the measurements. Only traces of CO2 are detected, indicating that ligand decom-

position is negligible. The results displayed in Fig. 8 confirm the absence of

pathways involving intramolecular or bimolecular reductive elimination of coor-

dinatedoxoatoms,asdetermined in theearlierworkusingCo3þasoxidant (68).Thetwo pathways expressed correspond to the major ones identified in the earlier

work (68, 78); their distributiondependsmodestly on the electronic character of the

bpy ligand substituent groups, for which electronwithdrawal increases the relative

contribution of the pathway involving O2 formation from two water molecules.

In principle, pathway b, involving O2 formation from two solvent water

molecules, could be artifactual if isotopic scrambling occurred on the time scale

of the O2 measurements. For example, a mechanism is illustrated in Scheme 4 that

involves competitive water exchange on one of the intermediary oxidation states

leading to the appearance of a second pathway involving two water molecules

derived from solvent. Direct measurement of water exchange on the stable {3,3}

and {3,4} ions was made using RR spectroscopy by a technique that involved

incubation of labeled mixtures of these oxidation states, followed by oxidation to

{5,5} and determination of the relative intensities of the well-separated Ru¼16O

andRu¼18O stretching vibrations (71). These experiments indicated that, whereas

water exchange at the cis-aquo positions was relatively rapid on {3,3} (t1/2� 100 s

at 23 �C), no exchange occurred within 24 h when the complex was oxidized to

{3,4}. Kinetic modeling of O2 evolution profiles for the various isotopomers

provided indirect evidence that water exchange also did not occur from interme-

diary oxidation states, such as {4,4}; specifically, introducing water-exchange

steps into the model introduced severe distortion within the elution profiles for the

various isotopomers [cf., Fig. 7c], whereas a model based upon two independent

1501005000

20

40

60

80

100

36O2

34O2

32O2

36O2

34O2

32O2

36O2

34O2

32O2 (c)(b)(a)

Time (s)

MS

Ion

Cur

rent

(a.

u.)

Time (s)150100500

0

20

40

60

80

100

Sim

ulat

ed I

on C

urre

nt (

a.u.

)

Sim

ulat

ed I

on C

urre

nt (

a.u.

)Time (s)

1501005000

40

80

120

160

Figure 7. Kinetic traces for evolution of O2 isotopes from �90% H218O enriched [Ru

(bpy)2(H2O)]2O4þ in�8% enriched solvent during water oxidation by Ce4þ ion. Panel a: experimental

data (72); panel b: kinetic simulation based upon concurrent water addition pathways described in

Schemes 2 and 3; panel c: kinetic simulation based upon water exchange (Scheme 4). Solid,

dash–dotted, and dotted lines show the time course evolution of 32O2,34O2 , and

36O2 isotopomers,

respectively. [Adapted from (79).]


0

10

Me Me

Me Me

N N

N

N N

N NCOOHHOOC

N

20

30

40

50

60

70

80

90pH 7.0

% I

soto

pe-d

efin

ed P

athw

ay

(c) (c) (c)(c)

(b)

(b) (b)(b)

(a)(a)

(a)

(b)

(a)

(a)

(c)

Figure 8. Distribution of isotope-defined O2 catalysis pathways for the “blue dimer” and analogues

containing substituted bpy ligands. Dark gray bars: pathway a (e.g., Scheme 2); light gray bars:

pathway b (e.g., Scheme 3); white bars (where visible); pathways c. For most reactions, the oxidant

was Ce4þ in 0.5M CF3SO3H; entries within the dashed rectangle compare results obtained for the

“blue dimer” under these conditions to that obtained in 50 mM phosphate, pH 7.0, using a

[RuII(dcb)2(bpy)]2þ–S2O8

2� photocatalytic system (Fig. 1). [Adapted from (79).]

Ru–O–Ru

OH OH

Ru–O–Ru

OH OH

H2O H2O

Ru–O–Ru

OH O

Ru–O–Ru

O O

Ru–O–Ru

OO

H2O

H2O

(34O2)

(32O2)

{5,5}{4,4}{3,4}

O

Scheme 4. Water exchange mechanism for isotope scrambling by pathway a. Solid circles indicate

solvent-derived O atoms.


reaction pathways (a and b) running concurrently easily fits the experimental

kinetic profiles [Fig. 7(b) 79]. Since {3,3} is in vanishingly small concentrations

under turnover conditions and the higher oxidation states are substitution inert,

water exchange on the complex ion (e.g., Scheme 4) cannot account for the results.

The alternative possibility that accumulating O2 might undergo exchange with

solvent by an unrecognized catalyzed mechanism was explored by mass spec-

trometry (MS) by running the reaction using components with natural isotopic

abundances of oxygen in an atmosphere of 36O2. A scrambling mechanism of this

type could then be detected by the appearance of 34O2 in the product gases, (i.e., the

reaction): 32O2 þ 36O2 $ 2 34O2. No mixed-isotope 34O2 was observed in the

product–gas mixture, however, indicating the absence of any scrambling mech-

anism of this type as well (79).

Another possibility is that the oxidant participates directly in the dioxygen-

forming reaction. In this case, the reaction could occur through a single reactive

intermediate, but with two decay channels that lead to isotopically distinguish-

able products. A hypothetical example is given in Scheme 5, wherein one

pathway involves dissociation of O2 from the coordination sphere of a hydro-

peroxo-hydroxo intermediate formed by reaction of aRu¼OcenterwithH2O and

the other from oxidative reaction of the hydroperoxide with Ce4þ, which carriesout O atom transfer to the hydroperoxide, forming an O2 product with both

O atoms originating in solvent. Experimental data suggest that this second

pathway is unimportant in reactions catalyzed by {5,5}, however. Specifically,

the isotopomer ratio is insensitive to the Ce4þ concentration over a wide

range (72, 78), whereas the suggested mechanism corresponding to Scheme 5

would predict a strong dependence; moreover, the same ratio is obtained with

various oxidants, including Ce4þ and Co3þ ions (68, 72) and a photochemical

system (Fig. 8) that uses [Ru(bpy)3]3þ and SO4

.� under entirely different

medium conditions (79). Similar mechanisms involving direct addition of water

to the hydroperoxide to form a terminally coordinated HO3� intermediate or

Ru O RuL2

O OHO

H

L2

{5,5} 4+

Ru O RuL2

O OH

4+

OH

{4,4}

Ce

OH

O=O

L2Ru O RuL2

O OH

4+

OH

{4,4}

L2

O=O

Scheme 5. Competing pathways for decay of a common hydroperoxo/hydroxo intermediate leading

to different O2 isotopomers. Solid circles indicate solvent-derived O atoms.


collapse of the hydroperoxo-hydroxo intermediate to give a binuclear ion

containing a 1,3-m-bridging ozonide (O2�3 ) ligand, for example,

½RuðbpyÞ2ðOOHÞORuðOHÞðbpyÞ2�4þ!½ðRuðbpyÞ2Þ2ðm�OÞðm�1;3�O3Þ�2þþ2Hþ

ð1Þ

followed by H2O attack at the central O atom (80) could also give an O2molecule

whose atoms are both obtained from solvent. These suggestions are plausible in

the sense that dihydrogen trioxide (H2O3) and higher polyoxo analogues of water

are known chemical entities that decompose spontaneously to give O2 and other

oxidized water species (81–84). However, rough thermodynamic estimates

based upon a recent determination of the enthalpy of formation of H2O3 (85),

suggest that formation of coordinated O 2�3 entities in reaction (1) or similar

reactions is energetically highly unfavorable, (i.e., with DH� 1.0 V) (72). This

conclusion is reinforced by DFT calculations made by Yang and Baik (42), who

examined various possible scenarios for water addition to form terminally

coordinatedHO3� or bridgingO 2�

3 complexes fromother reaction intermediates

and concluded that these reactions would be energetically uphill by at least

36 kcal mol�1. Overall, it appears that conceptually reasonable mechanisms

involving formation of a hydroperoxo-hydroxo intermediate as a common

precursor to two different isotopically distinguishable products are improbable

on energetic grounds.

Very large H/D kinetic isotope effects are often found for oxidation of O–H and

C–H bonds in peroxides, hydroquinones, alcohols, and arenes by monomeric

ruthenyl polypyridyl compounds (e.g., [RuIV(bpy)2(py)O]2þ (py¼pyridine))

suggesting that these reactions occur by hydrogen-atom abstraction mechanisms

(86–89). Assuming similar processes occur in water oxidation by the diruthenyl

{5,5} ion, one can write self-consistent mechanisms that rationalize the existence

of two distinct pathways (Schemes 2 and 3). Formation of free HO. by H atom

abstraction from H2O is energetically prohibitive, but can be avoided by

concerted addition of the nascent HO fragment to an adjacent atom. In Scheme 2,

reaction at the adjacent ruthenyl oxo atom leads directly to the hydroperoxo-

hydroxo intermediate, which then undergoes internal electronic rearrangement,

leading to release of O2. In this pathway, one O atom is obtained from the

coordination sphere and the other from solvent. Hydrogen bonding of the reactive

water molecule to the bridging m-oxo atom is inferred from RRmeasurements of

Ru–O–Ru stretching frequencies that, at least in their stable oxidation states,

undergo small shifts to lower energies in D2O (72). Resonance Raman

spectroscopy also has been used to probe whether or not the bridging atom

undergoes exchange during catalysis. The Ru–O–Ru bond is slightly nonlin-

ear, so that the stretching frequency is weakly dependent on the mass of the


Documents

Progress in · progress in inorganic chemistry volume 57. advisory board jacqueline k. barton california institute of technology, pasadena, california james p. collman stanford university,