Introduction to the Special Issue on Organometallic Electrochemistry

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Introduction to the Special Issue on Organometallic Electrochemistry transformations, with a good chemical sense for interpretation of the reaction pathway. It is evident that these two communities are complementary: For electrochemists it is very helpful to obtain the new compounds with some comments concerning the possible application; very valuable for them are discussions related to chemical interpretation of observed phenomena. Organometallic chemists are interested in kinetics and mechanisms and value techniques that can provide this information. Many synthetic organometallic chemists appreciate the ability of electrochemistry to provide an alternative approach to the investigation of redox reactivity, but do not have sufficient formal training in the techniques to take full advantage of the information that is available. Electrochemistry can provide unique thermodynamic as well as kinetic data bearing much additional information about the problem studied originally by classical “thermal” chemistry. There are several profitable features in the electrochemical approach. The electrode serves in fact as an inert and clean oxidizing agent (when positively charged) or reducing agent (when negatively charged), with a continuous variability of oxidizing/reducing strength that is given by the applied potential. Due to this fact, the experimentalist can choose the most appropriate working potential assuring reproducibility and specificity of the process. The potential can be varied freely in real time and is thus able to initiate, accelerate, decelerate, stop, or even reverse the studied redox reaction, following all the time its course, extent, and rate by means of measurements of passing current. In addition to this, the electrochemically generated intermediates or products are relatively clean and ready to be further investigated by HPLC, UV−vis, NIR, and IR spectra, combined with MS and NMR. Last year, 2013, a specialized conference on Molecular Electrochemistry in Organometallic Science (46th Heyrovský Discussion) was organized in Trest Castle (Czech Republic), where scientists from both communities were invited. The motivation for this international meeting was to bring closer together the experts in organometallic chemistry using electrochemistry in their research and electrochemists studying organometallic and coordination compounds. The presented papers inspired us to put together a special issue of Organometallics devoted to this theme. Every day a number of new molecules are synthesized as promising catalysts, pharmaceuticals, supramolecular units, dyes, photosensitizers, redox switches, NLO materials, etc. Their expected application is based on their oxidizability or reducibility, on the stability of their radical intermediates, on their electron delocalization connected with structural and redox changes, on their electrochromic effects and electrogenerated luminescence abilities, etc. Prior to any application, a fundamental and detailed electrochemical approach on the molecular level is necessary. Therefore, the combination of

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he special issue in front of your eyes has a subtitle “Organometallic Electrochemistry”. Nevertheless, it can be equally called “Electrochemistry in Organometallic Chemistry”. What is the difference? In the first case electrochemists are studying organometallic compounds; in the second one, organometallic chemists are using electrochemical approaches for their investigations. It seems to be the same matter, so why should we differentiate? The problem is that electrochemists and organometallic chemists have been two largely separated scientific communities. Each has had its own spectrum of preferred journals, its own series of conferences, and its own network of collaborations (and friends, of course). As a result, they do not always meet each other. The basis of this distinction is even deeper: original education. Electrochemists are traditionally educated in faculties and departments of physical chemistry and analytical chemistry. They like mathematics, physics, and modeling; they like to calculate potentials, coefficients, equilibria, rate constants, transport phenomena, enthalpies, etc. For confirmation of theories and rules they rather tend to use clear and straightforward chemical systems with a minimum of disturbing properties and side effects possibly preventing a good fit of experimental and calculated values. Electrochemists like to combine voltammetry or electrolysis with other physicochemical methods (mainly UV−vis and EPR spectra), in the best case in situ and online. They like experiments, but mainly with sophisticated instruments and under assistance of a computer. Simultaneously, they often use quantum chemical calculations. Their chemistry is usually limited to several milliliters, milligrams, or millimoles. Their working place in the laboratory for doing chemistry often does not exceed one square foot. On the other hand, synthetic organometallic chemists are traditionally educated in areas of organic and inorganic chemistry. They love to design and to create completely new molecules. They like characterization by NMR, mass spectra, crystallography, and a variety of techniques, but mainly as support of the laboratory work with real compounds that have interesting chemical and physical properties. They have a good “chemical nose” or “flair” to select and to identify hot topics, and they have immediately a vision of an application. Behind any observed effect they are seeing the formulas and chemical © 2014 American Chemical Society

Special Issue: Organometallic Electrochemistry Published: September 22, 2014 4513

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́ and coelectrochemical techniques by Mikysek, Ludvik, workers. Wong and co-workers report the preparation of ruthenium−indolizine zwitterion complexes as a new class of organometallic ligand with potential in the design of functional molecular electronic/photonic elements. Electrogenerated chemiluminescence was found from monodispersed Au144(SC2H4Ph)60 clusters by Ding, Workentin, and coworkers. Wiedner and Helm, Graham and Nocera, and Darensbourg and co-workers all report on catalysts for the electrochemical generation of dihydrogen. In a related work, Evans, Glass, Lichtenberger, and co-workers discovered an unusual homoassociative stabilization of the dianion of bis(μ-methylthiolato)diironhexacarbonyl and a chemically reversible redox cycle as revealed by cyclic voltammetry and infrared spectroelectrochemistry. Spectroelectrochemistry, particularly in the infrared (IRSEC), is an important tool for the investigation of the oxidation and reduction of organometallic species. Kubiak and co-workers provide a tutorial review including cell design as well as examples of applications. Bullock, Huang, Geiger, and coworkers used IR-SEC to good advantage in the study of CObridged versus unbridged bimetallic complexes. Spectroelectrochemistry was also applied to the study of areneruthenium and areneosmium complexes by Hartl, Kaim, and co-workers, while Hartl and co-workers made similar use of spectroelectrochemistry in the study of a manganese(I) tricarbonyl complex. Similarly Heinze and co-workers used UV/vis spectroelectrochemistry in their studies of ferrocenyl amides. One of the most interesting applications of electrochemistry to organometallic chemistry is the study of the mechanism of the oxidation or reduction of organometallic compounds. Buriez, Top, Amatore, and co-workers characterized the mechanism of the anodic oxidation of a ruthenocene-based anticancer drug and found interesting degradation of the ruthenocene substituent. Calhorda, Geiger, and co-workers investigated half-sandwich rhenium complexes by cyclic voltammetry and determined the tendency for dimerization of the electrogenerated cations. Derivatized cymantrene complexes were studied using cyclic voltammetry, with simulations, and IR-SEC by Wildgoose and co-workers. Braunstein, Routaboul, Therrien, and co-workers examined the electrochemical behavior of a dinuclear arene ruthenium complex containing a zwitterionic bridging ligand and report its assembly into a hexaruthenium complex that can bind coronene. Cyclic voltammetry was used by Halet, Hamon, Lapinte, and co-workers to characterize new iron−ruthenium complexes. Iurlo, Masiero, Cozzi, Paolucci, and co-workers prepared ferrocene−guanine conjugates that can be used in DNA analysis. The ability of cyclic voltammetry to detect subtle chemical changes is illustrated by the work of Kaim, Fiedler, and co-workers, who studied hemilabile coordination of 1,3dimethyllumazine to 1,1′-bis(diorganophosphino)ferrocenecopper(I). Bis-acetylide ruthenium complexes have been prepared and their electrochemistry characterized by Kaupp, Low, and co-workers, and bis-acetylide cobalt complexes have been prepared and characterized electrochemically by Ren and co-workers as well as the first example of an all-carbon-bridged dinuclear cobalt species. Kowalski, Winter, and co-workers used cyclic voltammetry and spectroelectrochemistry to characterize ferrocenyl Meldrum’s acid donor−acceptor systems. Kraatz and co-workers have used electrochemistry to characterize bisamino acid derivatives of 1,1′-ferrocenedicarboxylic acid.

organometallic chemistry and electrochemistry will always be useful and attractive. It is evident that the use of the electrochemical approach for fundamental characterization of new coordination and organometallic compounds and for detailed investigation of their redox abilities is nothing new: this tradition goes back to the 1960s. On the other hand, the contemporary research specialization and increasing number of journals and conferences have as a consequence a lack of opportunities for interdisciplinary communication and collaboration. This special issue should promote broader mutual information and cooperation in electrochemical and organometallic research.

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INTRODUCTION TO THE CONTENT OF THE SPECIAL ISSUE Electronic interaction between two or more parts of a molecule is of high current interest. Barriére discusses medium effects on the standard potentials for the two oxidation steps of dinickel bisfulvalene, a case with strong electronic interaction. Chen and co-workers, Haga and co-workers, and Halet, Bruce, Lapinte, and co-workers have studied various linkages between two identical or different metal centers. Ferrocenyl groups have been used as detectors of interaction by Lang and co-workers and by Speiser and co-workers, whereas Rigaut, Winter, and coworkers have developed spacers that act as molecular wires. Norel, Rigaut, and co-workers used electronic communication to give NIR luminescence with redox switching. Metal biscarbene spacers were used to link porphyrins by Ruppert and co-workers. Significant electronic interaction between porphyrins was noted. Winter has reviewed the use of differences in standard potentials for two-step oxidation or reduction to infer electronic coupling. Štěpnička and co-workers prepared Fe−Ru heterobimetallic complexes and characterized their oxidation and reduction by electrochemistry. Holze has reviewed the relation between optical and electrochemical band gap measurements. Pryce and co-workers studied new redox switches incorporating cobalt carbonyl moieties. In a study of new materials for nonlinear optics application, Mendes and co-workers used electrochemistry to help characterize the molecules. New triazaborine chromophores have been prepared and characterized by a variety of 4514

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Notes

Finally, Rabeah, Brückner, and co-workers have characterized cage complexes involving vanadium and arsenic for possible application to new materials. Zhang and Popov provide a review of endohedral metallofullerenes and note that the cage can be considered as an innocent ligand. On the other hand, dithiolate ligands in certain nickel, copper, and cobalt complexes are definitely noninnocent, as demonstrated by Rapta and co-workers. Sarkar and co-workers prepared new quinonoid-bridged d8 metal complexes and studied their electrochemistry. Electrochemical synthesis shows promise in organometallic chemistry. Moeller provides a methodological discussion of electrochemically generated organometallic reagents on a microelectrode array. Michman and co-workers provide examples of electrolytic oxidation, via mediators, and also of chlorination through oxidation of the substrate. A review of the electrochemical synthesis of organonickel σ-complexes has been provided by Yakhvarov and co-workers, while Magdesieva and co-workers have investigated the use of electrochemically deprotonated nickel(II) glycinate and the production of new chiral nickel(II) binuclear complexes. Finally, Sun and Mellah report on the electrosynthesis of SmCl2, SmBr2, and Sm(OTf)2. Kvapilová, Zális, and co-workers reported an analysis of the efficacy of DFT calculations with solvation accounted for by the PCM model for both the reduction and oxidation of chromium aminocarbene complexes. DFT was also used to gain insight into the mechanism of rhodium-catalyzed O−H insertion reactions, in a study by Xie, Verpoort, Fang, and co-workers. Beley and Gros have reviewed their work on ruthenium polypyridine complexes bearing pyrroles and π-extended analogues, while Barnard, Hogan, and co-workers have investigated five heteroleptic Ir(III) complexes in connection with the search for efficient electrogenerated chemiluminescence. Fabre, Lorcy, and co-workers report on the assembly of platinum diimine dithiolaate complexes on hydrogen-terminated silicon. A review of molecular recognition and sensing based on ferrocene systems has been provided by Wang and coworkers. These examples illustrate a wide range of mutual benefits from the cross-interaction between organometallic chemistry and electrochemistry. We hope this issue is enlightening to both synthetic organometallic chemists and electrochemists and serves as a springboard for future developments and applications of organometallic electrochemistry.

Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. Biographies

After studies of inorganic and coordination chemistry at the Charles University, Prague, Jiřı ́ Ludvı ́k joined J. Heyrovský Institute of Physical Chemistry and Electrochemistry, where he received his doctoral degree (CSc) in 1985. His further work in the field of organic and organometallic electrochemistry was focused on interception and characterization of short-lived radical intermediates (spectroelectrochemistry, electrochemically generated luminescence). His postdoctoral stay was at the University of Padova (Italy) in 1986. After 1990 he was a visiting scientist in Sweden and the USA. Since 1990 he has been a professor of physical chemistry and head of the group of molecular electrochemistry in the J. Heyrovský Institute and a visiting lecturer at the Prague Institute of Chemical Technology, where he later became Professor of Physical Chemistry. His current research interests include intramolecular electronic interaction between two redox-active centers, electrochemical studies of electron delocalization and aromaticity, electrochemical investigation of new ligands and complexes, and electrochemically generated precursors and/or catalysts for new ways of special organic synthesis or degradation.

Jiří Ludvík,* Guest Editor

J. Heyrovský Institute of Physical Chemistry ASCR, v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic

Dennis H. Evans,* Guest Editor Department of Chemistry, Purdue University, West Lafayette, United States

Dennis L. Lichtenberger,* Associate Editor

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Dennis Evans obtained his B.S. degree in 1960 from Ottawa University (Kansas) and A.M. (1961) and Ph.D. (1964) degrees from Harvard, studying analytical chemistry under J. J. Lingane. After two years as an Instructor in Chemistry at Harvard he moved to the University of Wisconsin, Madison, where, over a twenty-year span, he rose through the faculty ranks to become the Meloche-Bascom Professor of Chemistry. From 1986 to 2003 he was Professor of Chemistry at the University of Delaware. He moved to The University of Arizona in 2004 and retired in 2009 followed by a northeastern migration to Purdue University, where he is now an Adjunct Research Professor in

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona, United States

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.L.: [email protected]. *E-mail for D.H.E.: [email protected]. *E-mail for D.L.L.: [email protected]. 4515

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the Department of Chemistry. Throughout his career Professor Evans has engaged in the study of the mechanisms of the electrochemical reactions of organic and organometallic compounds. A central theme has been structural changes (conformational change; isomerization) that precede or follow electron transfer at the electrode. These structural changes in turn are related to the relative values of the individual standard potentials in multielectron overall reactions. Still another related area involves protonation and deprotonation steps in electrode reactions and hydrogen-bonding between electrogenerated anions and hydroxylic additives. Dimerization of electrogenerated anion radicals and cation radicals has also been studied by Professor Evans and his co-workers, and in recent years, in collaboration with Richard Glass and Dennis Lichtenberger (The University of Arizona), catalysis of electrochemical generation of hydrogen using mimics of the active site of hydrogenase enzymes has been thoroughly investigated.

Dennis Lichtenberger is Professor of Chemistry and Biochemistry at The University of Arizona, where he served as Head of the Department from 1994 to 2002. He has also served as Chair of the Organometallic Subdivision of the American Chemical Society and is currently an Associate Editor of the ACS journal Organometallics. He obtained his Ph.D. degree from the University of Wisconsin under the direction of Professor Richard F. Fenske in 1974 and was a postdoctoral research associate with Professor Theodore L. Brown at the University of Illinois before joining the faculty at The University of Arizona. A central theme of Dr. Lichtenberger’s research has been the development of both instrumentation and theory to probe and understand the nature of matter at the level of electronic structure. Much of his research has focused on the electron energies and properties of organometallic molecules and multiple metal−metal bonds by the technique of gas-phase photoelectron spectroscopy, and the extension of these gas-phase properties to solution electrochemistry. The principles that have emerged from his research are fundamental to broad areas of industrial catalysis, electronic materials, biological functions, and most recently solar energy conversion to clean and sustainable fuels. Scientists worldwide have utilized the instrumentation capabilities developed in his laboratory.

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