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Organometallics 2011, 30, 28–31 DOI: 10.1021/om1010758
Reflections on Future Directions in Organometallic Electrochemistry† William E. Geiger* Department of Chemistry, University of Vermont, Burlington, Vermont 05445, United States Received November 15, 2010
Electrochemistry is employed both as an analytical instrumental technique and as a method of inducing changes in the structure and reactivity of organometallic complexes. It is the primary implement for expanding the applications of organometallic electron-transfer (ET) chemistry and is at its most powerful when blended with the use of spectroscopy, DFT calculations of molecular structure, and chemical redox agents. Examples are given of how electrochemistry has traditionally been employed to probe changes in the structure and reactivity of organometallic complexes. Continued advances are expected for all areas in which organometallic ET processes play a role, including catalysis of small-molecule reactions, energy storage and conversion, bio-organometallic chemistry, redox polymers, and analytical and molecular devices. Particular opportunities for redox-active “hybrid” complexes in which organometallic moieties are fused with noninnocent chelate ligands or tagged onto molecular targets are discussed.
Electrochemistry for both Analysis and Synthesis Two of the main tracts of the broad field of electrochemistry are those of electroanalysis and molecular electrochemistry. The latter distinguishes electrochemistry from more specifically analytical instrumental methods such as magnetic resonance and infrared spectroscopy by actively influencing electronic structure and chemical reactivity. Thus, an electrontransfer (ET) process may be viewed both as a fundamental property of a molecule and as an approach to systematically altering its structure and reactions. Organometallic chemists have increasingly embraced electrochemical methodologies, either independently or collaboratively, in order to take advantage of ET-induced changes in molecular properties. Representative examples have been described recently in this journal,1 and the present comments are intended to build on the themes highlighted in that article.
Electrochemistry as a Mature Instrumental Method Electrochemistry is a mature instrumental technique, in the sense that the fundamentals of the method are available in a number of textbooks2 and the instruments and materials necessary to carry out studies in molecular electrochemistry are readily obtained. The commercially available essentials include excellent potentiostats with software packages that offer the most advantageous waveforms (e.g., cyclic voltammetry as well as pulsed and electrolytic techniques), variably † Part of the special issue Future of Organometallic Chemistry. *E-mail:
[email protected]. (1) Geiger, W. E. Organometallics 2007, 26, 5738. (2) Among these are: (a) Kissinger, P. T., Heineman, W. R., Eds. Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Marcel Dekker: New York, 1996. (b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd Ed.; Wiley: New York, 2001. (c) Saveant, J.-M., Elements of Molecular and Biomolecular Electrochemistry; Wiley-Interscience: New York, 2006. (d) Compton, R. G.; Banks, C. E. Understanding Voltammetry; World Scientific: London, 2007. (e) Lund, H., Hammerich, O., Eds. Organic Electrochemistry; Marcel Dekker: New York, 2001.
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sized working electrodes of different compositions, and digital simulation systems. Mechanistically valuable spectroelectrochemistry methods are also being increasingly employed.3 Future creative elements in organometallic electrochemistry will be directed toward problems and applications viewed from two distinct points of origin. A chemist who begins from the organometallic side may ask how electrochemistry can be used to advantage in the synthesis, characterization, or reactions (including catalysis) of a class of organometallic complexes. One who begins from an alternate specialization may ask whether an organometallic redox system might prove valuable in addressing a particular application in that specialty. Intersection of these two different intellectual starting points is manifest in the examples given below.
Electron Transfer Related to Changes in Organometallic Structure and Reactivity A longstanding precept of transition-metal chemistry is that the structures and reactions of metal complexes broadly correlate with the metal “oxidation state”.4 Organic compounds also undergo dramatic changes in structures and reactions when prompted by gain or loss of electrons.2e Thus, it is no surprise that changes in the electron count of organometallic compounds generally have dramatic effects on their structures and reactions. A recent review1 focused on these effects by citing a number of examples, two of which are reiterated here. Considering first the possibility of electron-transferinduced variations in molecular structure, we consider changes in the metal-ligand hapticity of organometallic sandwich complexes. Prototypical is the η6 f η6 f η4 sequence of metal-arene bonding that accompanies the two successive (3) Kaim, W. Spectroelectrochemistry; Royal Society of Chemistry: London, 2008. (4) The strengths and weaknesses of an “oxidation state”-type formalism are well known. For an alternative model of electron-counting in metal complexes, see: Green, M. L. H. J. Organomet. Chem. 1995, 500, 127. r 2011 American Chemical Society
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one-electron reductions of [Ru(η6-arene)2]2þ to Ru(η6-arene)(η4-arene). The 18-electron nature of the metal in the dicationic and neutral complexes was firmly established5 long before even the existence of the putative 19-electron intermediate [Ru(η6-arene)2]þ was confirmed. Electrochemistry was able not only to demonstrate the authenticity and thermodynamic aspects of the three-membered ET series
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electrochemistry is an attractive synthetic option11 for such processes, especially under conditions in which fast and efficient reactions are required (e.g., at chemically modified electrodes). Broader discussions of these and other ET-induced reactions, including migratory insertion and a number of different catalytic chain processes, are available.1,12
Electrochemistry of Hybrid Organometallic Complexes
but also to address the question of whether the η6/η4 hapticity change occurred in the first or second redox reaction. Evaluation of the electron-transfer rates of the two one-electron processes led to the conclusion that the major change in bonding occurred in the reduction of the monocation to the neutral complex.6 The radical cation [Ru(η6-arene)2]þ was thereby designated as having flat arene rings, a conclusion that was later confirmed computationally.7 This brings up an important point about future directions. The primary measurable in electrochemistry, namely the E1/2 potential of a couple, is not intrinsically sensitive to the structure of the molecule, since it is determined by the free energy differences between the oxidized and reduced forms of the redox couple. Note, for example, that the oxidations of ferrocene and iodide ion have similar E1/2 values. Until fairly recently, spectroelectrochemistry has been the major tool employed to fill the structural “gap” for reactive electrochemically generated products. Now, however, DFT calculations can be employed to probe the structures of compounds that can only be generated in situ, adding an important resource to the toolbox of organometallic redox chemistry. In terms of electron-transfer-induced changes in reactivity, an archetypical example involves carbonyl substitution reactions. In 1981, Rieger and co-workers described a phosphinefor-carbonyl substitution reaction when Co3(CO)9(μ3-CPh) is reduced in the presence of a phosphine.8 Although this cathodic process appears to have been the first published example of an electrochemically induced CO-substitution reaction, it is pedagogically preferable here to discuss conceptually similar anodically induced (oxidative) processes. One-electron oxidation of a compound containing a M-CO bond will generally weaken the metal-carbon bond by diminishing the amount of metal-to-CO back-bonding and leave the metal center poised for an associative reaction.9 The net effect is that the rates of CO substitution reactions are greatly enhanced in the radical cations of organometallic complexes, sometimes as much as 109 times faster than their 18-electron counterparts.9,10 Given the fact that ET reactions can so dramatically promote ligand substitution reactions, (5) (a) Fischer, E. O.; Elschenbroich, C. Chem. Ber. 1970, 103, 162. (b) Huttner, G.; Lange, S. Acta Crystallogr. 1972, B28, 2049. (6) Pierce, D. T.; Geiger, W. E. J. Am. Chem. Soc. 1992, 114, 6063. (7) Personal communication from Mu-Hyun Baik (Indiana University) to W.E.G. , 2010. (8) Bezems, G. J.; Rieger, P. H.; Visco, S. J. Chem. Soc., Chem. Commun. 1981, 265. (9) (a) Herrinton, T. R.; Brown, T. L. J. Am. Chem. Soc. 1985, 107, 5700. (b) Meng, Q.; Huang, Y.; Ryan, W. J.; Sweigart, D. A. Inorg. Chem. 1992, 31, 4051. (10) Therien, M. J.; Ni, C.-L.; Anson, F. C.; Osteryoung, J. G.; Trogler, W. C. J. Am. Chem. Soc. 1986, 108, 4037.
The preceding section focused on how electrochemistry can be employed to probe the structures and reactions of organometallics when the electron-transfer process is significantly metal-based. Although there are well-known examples of more highly ligand-based organometallic ET processes,13 the efficacy of ligand-based organometallic redox chemistry needs to be more broadly studied. A promising approach to the more systematic development of ligandbased organometallic redox processes is the expanding work on “hybrid” organometallic complexes. In this strategy, an organometallic fragment is coordinated and electronically linked to a redox-active chelating ligand which is usually of the “non-innocent”,14a π-donating14b type. Reactions at the organometallic fragment are varied by addition or subtraction of electrons from orbitals that are primarily chelatebased. Considering one innovative example,15 it has been shown that ET at the chelating ligand in pentamethylcyclopentadienyl Ir complexes having the hybrid structure 1 modulates the reaction of the metal center with Lewis bases without introducing undesirable radical processes at the metal. Another interesting recent example of the hybrid approach involves ET-related changes in polymerization catalysis of (imino)pyridine iron alkyl complexes.16
The hybrid redox concept can also be used in an opposite sense, wherein a seemingly metal-based redox process promotes a reaction at an otherwise inactive ligand. A recent entry into this redox category is the report by Maurer et al. on ruthenium vinyl complexes, for which the one-electron oxidation can be either metal-vinyl delocalized or vinyl dominated, depending on subtleties in the molecular design.17 Comparison of the (11) Ohrenberg, N. C.; Paradee, L. M.; Dewitte, R. J., III; Chong, D.; Geiger, W. E. Organometallics 2010, 29, 3179. (12) (a) Reference 1, pp 5756-5759. (b) Zeng, X.; Li, Z.; Liu, X. Electrochim. Acta 2010, 55, 2179. (c) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; pp 373-375. (d) Chanon, M. Acc. Chem. Res. 1987, 20, 214. (e) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Lemos, M. A. Coord. Chem. Rev. 2001, 219-221, 53. (13) Braden, D. A.; Tyler, D. R. Organometallics 2000, 19, 3762 and references therein. (14) (a) Ward, M. D.; McCleverty, J. A. Dalton Trans. 2002, 275. (b) Hartl, F.; Rosa, P.; Ricard, L.; Le Floch, P.; Zalis, S. Coord. Chem. Rev. 2007, 251, 557. (15) Ringenberg, M. R.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R. Organometallics 2010, 29, 1956. (16) Tondreau, A. M.; Milsmann, C.; Patrick, A. D.; Hoyt, H. M.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 15046. (17) Maurer, J.; Linseis, M.; Sarkar, B.; Schwederski, B.; Niemeyer, M.; Kaim, W.; Zalis, S.; Anson, C.; Zabel, M.; Winter, R. F. J. Am. Chem. Soc. 2008, 130, 259.
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reaction pathways of metal-linked organic radicals such as reported by these authors with those of comparable purely organic radicals will be necessary if advantages of the organometallic redox systems are to be evaluated. An offshoot of the organometallic hybrid approach is to employ polydentate ligands related to those common to coordination chemistry but having one or more coordinative carbon centers in place of the traditional coordinating atoms. For example, cyclometalation of 218 may furnish NCN0 -coordinated complexes similar to those of the classic NN0 N00 -coordinating terpyridine ligand but differing in their charge (the cyclometalated ligand is formally negatively charged), the strength of the metal-ligand bonding, and their electrochemical and photochemical properties.19 A similar approach employs carbene ligands such as 3, which promise the variations inherent to M-C vs M-N bonding, but retain the same charge as the terpyridine ligand.20 Interest in these ligands is predominantly applications-driven, with emphasis on the photoelectrochemical and magnetic properties of the complexes. More broadly, one expects interesting results to come from future work involving the electrochemistry of metal carbene complexes.
Organometallic Redox Tags The covalent attachment to a targeted molecule of an organometallic redox “tag” is related to the hybrid approach discussed above, but conversely so in the sense that the redox process is largely metal-based and carried out at “arm’s length” to the molecular target. Tagging strategy makes use of a redox reaction at the organometallic group to alter the chemical and physical properties of the target. In some cases, the redox tag might be used for purely analytical purposes, such as electrochemical or luminescent monitoring of the molecular target.21 In other cases, it is the electrostatic effect brought about by the organometallic redox reaction that alters the reactivity of the target or influences its physical properties, such as its solubility and separation science.22 The notion of controlling the chemistry at the molecular target by turning on and off the charge at the redox site is (18) Beley, M.; Collin, J.-P.; Louis, R.; Metz, B.; Sauvage, J.-P. J. Am. Chem. Soc. 1991, 113, 8521. (19) (a) Wadman, S. H.; Lutz, M.; Tooke, D. M.; Spek, A. L.; Hartl, F.; Havenith, R. W. A.; van Klink, G. P. M.; van Koten, G. Inorg. Chem. 2009, 48, 1887. (b) Bomben, P. G.; Robson, K. C. D.; Sedach, P. A.; Berlinguette, C. P. Inorg. Chem. 2009, 48, 9631. (c) Schmittel, M.; Lin, H. Inorg. Chem. 2007, 46, 9139. (20) Park, H.-J.; Kim, K. H.; Choi, S. Y.; Kim, H.-M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K. Inorg. Chem. 2010, 49, 7340. (21) (a) Zhang, R.; Wang, Z.; Wu, Y.; Fu, H.; Yao, J. Org. Lett. 2008, 10, 3065. (b) Rochford, J.; Rooney, A. D.; Pryce, M. T. Inorg. Chem. 2007, 46, 7247. (c) Martinez, R.; Ratera, I.; Tarraga, A.; Molina, P.; Veciana, J. Chem. Commun. 2006, 3809. (22) S€ ussner, M.; Plenio, H. Angew. Chem., Int. Ed. 2005, 44, 6885. (23) A sampling of examples involving ferrocenyl redox tags may be found in: Cuffe, L.; Hudson, R. D. A.; Gallagher, J. F.; Jennings, S.; McAdam, C. J.; Connelly, R. B. T.; Manning, A. R.; Robinson, B. H.; Simpson, J. Organometallics 2005, 24, 2051. Note also that dendrimers which are end-tagged with ferrocenyl groups may also be considered in this category. For an introduction, see Astruc, D.; Ornelas, C.; Ruiz, J. Acc. Chem. Res. 2008, 41, 841.
Geiger
often referred to as “redox switching”. As one might expect, the ferrocenyl group has been employed almost exclusively as the organometallic tag (see 4),21-23 owing to its relatively benign redox behavior. However, it is easy to see opportunities for growth in this area. Ferrocene does not have particularly attractive spectroscopic properties and cannot be easily removed or structurally modified after attachment. Owing to its iron center, it also introduces analytical limitations in sampling of biological targets. It would be beneficial to develop alternative organometallic tags having an assortment of metals, differences in redox-induced charges (including two-electron processes), and enhanced spectroscopic options.24
A Medley of Future Applications Further to the question of expected growth areas in organometallic electrochemistry, one needs only to make a partial list of the applications in which there is now intense activity involving ET-based chemistry. These include the crucial areas of ET-oriented approaches to catalysis of small-molecule reactions,25,26 energy storage and conversion,26,27 single-site olefin polymerization,28,29 bio-organometallic chemistry (see the antitumor agent 5),30 redox polymers,31 optic and magnetic materials,32 sensors,33 and molecular devices (see 6).34,35 Although beyond the scope of this article, any of these areas would warrant its separate review and prognosis.
(24) Laws, D. R.; Chong, D.; Nash, K.; Rheingold, A. L.; Geiger, W. E. J. Am. Chem. Soc. 2008, 130, 9859. (25) Nann, T.; Ibrahim, S. K.; Woi, P.-M.; Xu, S.; Ziegler, J.; Pickett, C. J. Angew. Chem., Int. Ed. 2010, 49, 1574 and references therein. (26) Dubois, M. R.; Dubois, D. L. Acc. Chem. Res. 2009, 42, 1974. (27) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Science 2010, 110, 6474. (28) Chirik, P. J.; Weighardt, K. Science 2010, 327, 794. (29) Gregson, C. K. A.; Gibson, V. C.; Long, N. J.; Marshall, E. L.; Oxford, P. J.; White, A. J. P. J. Am. Chem. Soc. 2006, 128, 7410. (30) Hillard, E.; Vessieres, A.; Thouin, L.; Jaouen, G.; Amatore, C. Angew. Chem., Int. Ed. 2006, 45, 285. See also: Top, S.; Kaloun, E. B.; Jaouen, G. J. Am. Chem. Soc. 2000, 122, 736. (31) Byrne, P. D.; Lee, D.; M€ uller, P.; Swager, T. M. Synth. Met. 2006, 156, 784. (32) (a) Lu, J.; Chamberlin, D.; Rider, D. A.; Liu, M.; Manners, I. Nanotechnology 2006, 17, 5792. (b) Ren, T. Organometallics 2005, 24, 4854. (33) For an introduction to multitasking sensors see: de Silva, A. P. Nature 2007, 445, 718. (34) Low, P. J. Dalton Trans. 2005, 2821. (35) Semenov, S. N.; Taghipourian, S. F.; Blacque, O.; Fox, T.; Venkatesan, K.; Berke, H. J. Am. Chem. Soc. 2010, 132, 7584.
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Concluding Remarks The examples in this brief essay represent only a tiny fraction of the interesting and innovative ways in which chemists are now employing organometallic redox processes in applications and in discovery. Owing to the prime place of electron-transfer reactions in the energy “triumvirate” of heat, light, and electricity, ET-based applications will undoubtedly grow in importance as organometallic systems are increasingly employed in research and development
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and as the chemistry of odd-electron species continues to expand. As the primary method for effecting and studying ET reactions, electrochemistry is certain to play an important future role in the development of organometallic chemistry.
Acknowledgment. I am grateful to the National Science Foundation for support of our work on organometallic electrochemistry, and I thank reviewers for helpful suggestions.
