Organometallics 2011, 30, 13–16 DOI: 10.1021/om100910d
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Future Trends in Organometallic Chemistry: Organometallic Approaches to Water Splitting Warren E. Piers* Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada, T2N 1N4 Received September 21, 2010
Advances in the field of organometallic chemistry have historically been driven by the imperatives of the petrochemical industry. Catalytic manipulation of C-H and C-C bonds is now at an advanced stage of sophistication, although significant opportunities continue to exist in this aspect of organometallic chemistry. However, application of the formidable toolbox of organometallic chemistry;developed in the context of hydrocarbon chemistry;to the sustainable production of environmentally benign solar fuels is a research problem that humanity needs organometallic chemists to tackle. Organometallic approaches to water splitting driven by sunlight will constitute a growing area of focus in the coming years.
Introduction Mr. McGuire: I just want to say one word to you;just one word. Ben: Yes, sir. Mr. McGuire: Are you listening? Ben: Yes, I am. Mr. McGuire: “Plastics”. Ben: Exactly how do you mean? Mr. McGuire: There is a great future in plastics. Think about it. Will you think about it? Ben: Yes, I will.1 In one of the most famous exchanges in movie history, family friend Mr. McGuire provides advice to a rudderless young Ben Braddock in Mike Nichols’ classic 1967 film The Graduate. Regardless of the purpose of the exchange in the context of the film, from the perspective of an organometallic chemist Mr. McGuire’s “one word” was remarkably prescient; if Ben were a graduate with a chemistry degree, perhaps he would not have been so confused! Few would argue that the petrochemical industry has supplied the main driving force for the advancement of the discipline of organometallic chemistry. Petrochemicals are the primary source of energy and products for human society, and the need for efficient catalytic processes for both their production and transformation has been at the foundation of the field for decades. A prominent example is the development of highly active single-site olefin polymerization catalysts for the production of Mr. McGuire’s plastics: surely one of the success stories of organometallic chemistry.2 In addition to providing access to tailored polymers and mindboggling quantities of commodity plastics, the evolution of these catalysts has driven the development of new experimental techniques for studying the operation of catalysts at a detailed level. *E-mail:
[email protected]. (1) Willingham, C.; Henry, B. The Graduate; MGM, 1967. (2) Gladysz, J. A. Chem. Rev. 2000, 100, 1167–1168. r 2011 American Chemical Society
This subfield has also driven catalyst design principles through ligand modification and led to sophisticated structure/ activity concepts now broadly used in homogeneous catalyst development.3-5 Thus, the petrochemical industry has been the engine driving the development of organometallic chemistry to the level of sophistication it now enjoys, pushing applications into other areas of chemisty as we have learned how to control bond activation and functionalization reactions. In a 1987 Organometallics paper Parshall wrote of trends and opportunities for organometallic chemistry focusing on the perspective from industry.6 Now, nearly 25 years later, it still makes for a fascinating read. Despite the implied;and somewhat contentious;message that organometallic chemistry needed to dissolve into other disciplines in order to survive, the trends Parshall identified are certainly areas of research that have witnessed tremendous growth in the intervening decades. And while his main thesis was that, for the industrial chemist, “the future of organometallic chemistry is not a simple extrapolation of the past”, it is not inaccurate to state that the field is still dominated by the need to utilize petrochemicals for society’s energy and material needs. The search for efficient activation and functionalization reactions of C-H and C-C bonds is still at the core of the discipline and will still be required in the coming years. While Parshall explicitly acknowledged the central role of hydrocarbon chemistry in the discpline, one thing missing from the article was any kind of foreshadowing of the concern over rising atmospheric CO2 levels that is now so prominent;and controversial. Despite a vocal minority community of skeptics,7 it is widely acknowledged that rising atmospheric carbon dioxide levels are a driving force behind evident climate change phenomena8 and that levels of CO2 and other greenhouse gases (3) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–514. (4) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2000, 34, 18–29. (5) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461–1473. (6) Parshall, G. W. Organometallics 1987, 6, 687–692. (7) http://www.friendsofscience.org/index.php?id=196. (8) Hansen, J.; Johnson, D.; Lacis, A.; Lebedeff, S.; Lee, P.; Rind, D.; Russell, G. Science 1981, 213, 957–966. Published on Web 01/04/2011
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Organometallics, Vol. 30, No. 1, 2011 Scheme 1
are influenced by humanity’s growing energy needs.9 Thus, while there appear to be abundant sources of carbon-based energy (particularly coal and bitumen) to meet these needs over the next few hundred years, continued use of such resources to meet increasing global energy demand will result in levels of atmospheric CO2 that are unprecedented in human history.10 No one really knows what the exact consequences of this will be, but even conservative scenarios are not encouraging.11 It is therefore of considerable interest for humanity to adopt global energy conservation measures and increasingly exploit renewable energy sources to avoid the potentially disastrous effects of climate change. Of the options available for sources of renewable energy, the sun has the greatest potential to provide enough carbon-free energy to meet predicted demands.12 More energy in the form of sunlight strikes the Earth’s surface in one hour than is required to “power the planet” at current consumption levels for one year.13 However, the diffuse nature of sunlight, its inherent intermittency, and the low efficiency at which this resource is converted into storable energy present major challenges for the widespread usage of solar energy.14 Not surprisingly, therefore, only a very small percentage of the fuels used today are produced directly from solar irradiation. The conversion of sunlight into usable “solar fuels” is one of the most urgent scientific imperatives of the 21st century,15,16 and chemists;organometallic chemists;will play a leading role in rising to this challenge.
Organometallic Approaches to Water Splitting Solar fuels are transportable energy sources derived from the conversion of abundant small molecules such as water (H2O) or carbon dioxide (CO2) using the sun’s energy to drive these thermodynamically unfavorable reactions. The efficient splitting of H2O into its component elements hydrogen (H2) and oxygen (O2) would provide a clean source of H2 for use in fuel cells, but the process is more complex than meets the eye, involving multiple proton and electron transfers and a significant uphill thermodynamic barrier (Scheme 1).17 As such, electrochemical or photochemical energy is required to drive the reaction. Furthermore, the need to couple proton and electron transfers contributes to substantial kinetic barriers inherent to the process, and to achieve an acceptable rate, a catalyst is required to mediate these processes. While there have been some recent breakthroughs in water oxidation catalysis, the mechanism of this complex reaction (9) Weaver, A. Keeping Our Cool: Canada in a Warming World; Penguin Group Canada: Toronto, 2008. (10) Davis, S. J.; Caldeira, K.; Matthews, H. D. Science 2010, 329, 1330–1333. (11) Hansen, J. Storms of My Grandchildren; Bloomsbury: New York, 2009. (12) Nocera, D. G. Inorg. Chem. 2009, 48, 10001–10017. (13) World Energy Assessment Report: Energy and the Challenge of Sustainability; United Nations: New York, 2003. (14) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Gleria, M. Science 1975, 189, 852–856. (15) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. 2006, 103, 15729–15735. (16) Gray, H. B. Nat. Chem. 2009, 1, 7–7. (17) Dempsey, J. L.; Esswein, A. J.; Manke, D. R.; Rosenthal, J.; Soper, J. D.; Nocera, D. G. Inorg. Chem. 2005, 44, 6879–6892.
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remains somewhat obscure. The molecular, homogeneous transition metal catalysts employed are typically based on ruthenium,18-20 iridium,21 or platinum,22 while heterogeneous systems based on cobalt phosphates23,24 or nickel borates25 have enormous promise. Still, these important discoveries of basic science require significant effort to develop into usable technologies.12,26 A detailed understanding of the mechanism by which they mediate the water splitting reaction is crucial to the realization of optimal catalysts, not only in terms of activity, efficiency, and selectivity but also in terms of practicality and economic viability. The tools and concepts used routinely by organometallic chemists in the development of catalysts for hydrocarbon manipulations should be broadly deployed to discover and optimize water-splitting catalysts. Can we become as adept at making and breaking O-H and O-O bonds as we now are at C-H and C-C bonds? In other words, can we bring to bear the extensive toolbox of organometallic chemistry to the problem of water splitting to better understand and improve this catalytic process? An early articulation of these questions was provided by Lewis and Nocera in their provocative 2006 article in Proc. Natl. Acad. Sci. that framed the basic science needs for solar energy utilization.15 By drawing analogies to known hydrocarbyl group transformations at a transition metal center, they pointed to potential (unknown) reactions that might be feasible for the utilization of water in the production of hydrogen and oxygen. Following this train of thought, a network of potential reactions, depicted in Scheme 2, might be envisioned.27 Mechanistic proposals for the multistep transformations involved in splitting water typically invoke a variety of interconverting intermediates, ranging from water adducts (i.e., aquo complexes LxM(OH2)) to hydroxy hydrides and bis-hydroxides (LxM(OH)H and LxM(OH)2), to metal oxo and peroxo complexes (LxMdO and LxM(OOH)). While some of these transformations, such as the oxidative addition of H-O bonds to a metal center LxMn (reaction a), are not completely unknown,28-30 much remains to be studied concerning the properties of these crucial intermediates and the intimate details of the mechanisms by which they interconvert, i.e., a-l in Scheme 2. The organometallic community is already beginning to turn its attention to these crucial problems.28 Several studies relevant (18) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4029–4030. (19) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 6802–6827. (20) Wasylenko, D. J.; Ganesamoorthy, C.; Koivisto, B. D.; Henderson, M. A.; Berlinguette, C. P. Inorg. Chem. 2010, 49, 2202–2209. (21) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J. Am. Chem. Soc. 2008, 130, 210–217. (22) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2006, 128, 7726–7727. (23) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072–1075. (24) Kanan, M. W.; Surendranath, Y.; Nocera, D. G. Chem. Soc. Rev. 2009, 38, 109–114. (25) Dinca, M.; Surendranath, Y.; Nocera, D. G. Proc. Natl. Acad. Sci. 2010, 107, 10337–10341. (26) Eisenberg, R.; Gray, H. B. Inorg. Chem. 2008, 47, 1697–1699. (27) Scheme 2 does not mean to imply that all of these reactions will be operative in one system; rather it depicts plausible transformations that might be mediated by a homogeneous metal complex. Furthermore, note that (for clarity) reactions h and g in Scheme 2 are a reversible pair and the bis-hydroxide species on either side of the terminal oxo complex is the same. (28) Ozerov, O. V. Chem. Soc. Rev. 2009, 38, 83–88. (29) Dorta, R.; Togni, A. Organometallics 1998, 17, 3423–3428. (30) Ladipo, F. T.; Kooti, M.; Merola, J. S. Inorg. Chem. 1993, 32, 1681–1688.
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Scheme 2
Scheme 3
to the chemistry of Scheme 2 have emerged recently,31-35 but two key reports by the Milstein group illustrate the feasibility of some of these proposed transformations and challenge some long-held beliefs concerning the viability of some of the intermediates and transformations depicted in Scheme 2. The essential elements of these studies, published in Nature36 and Science,37 are summarized in Scheme 3 and serve as state of the art examples of the organometallic approach to water splitting; the bold letter tags connect the reactions depicted in Scheme 2 to the demonstrated examples uncovered by Milstein et al. (31) Melnick, J. G.; Radosevich, A. T.; Villagran, D.; Nocera, D. G. Chem. Commun. 2010, 46, 79–81. (32) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.; Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318–10319. (33) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508–2509. (34) Karunadasa, H. I.; Chang, C. J.; Long, J. R. Nature 2010, 464, 1329–1333. (35) Millard, M. D.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. J. Am. Chem. Soc. 2010, 132, 8921–8923. (36) Poverenov, E. E., I.; Frenkel, A. I.; Ben-David, Y.; Shimon, L. J. W.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Nature 2008, 455, 1093–1096. (37) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74–77. (38) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759–1792.
Both systems utilize pincer ligands38 as the “Lx” ligand environment. These versatile tridentate ligands are widely used and can support four-, five-, and six-coordinate environments about a metal center. Previous studies show that metal complexes utilizing such ligands are capable of activating the N-H bonds of ammonia,39,40 suggesting promise for similar reactivity with the O-H bonds of water. One of the most attractive features of these ligands is the tunability of the donor array; the elements directly bonding to the metal center in the pincer array can be modified extensively, allowing for a wide variety of specific ligand types with a range of electronic properties. In the chemistry shown in Scheme 3A, the supporting ligand is an “NCP” pincer, while an “NNP” array is enlisted in Scheme 3B. The platinum chemistry of Scheme 3A is significant because it has been suggested that metal-oxo compounds with greater than six d-electrons at the metal would be destabilized by π-π repulsions between these d-electrons and the π pairs on the oxo ligand. Milstein’s study, which includes the isolation of the d6-configured Pt-2, breaks down this so-called “oxo wall”41 (39) Morgan, E.; MacLean, D. F.; McDonald, R.; Turculet, L. J. Am. Chem. Soc. 2009, 131, 14234–14236. (40) Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080–1082. (41) Limberg, C. Angew. Chem., Int. Ed. 2009, 48, 2270–2273.
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and shows that such compounds are indeed viable and highly reactive.42 The addition of an O-H bond of water across this reactive PtdO bond and the nucleophilic attack of the oxo group by hydride are two model reactions for critical steps in water activation chemistry. In Scheme 3B, the oxygen-oxygen bond-forming reductive elimination step (photochemically driven) converting Ru-3 to Ru-4 is unprecedented43 and a highly significant observation for the goals of water splitting catalysis. Furthermore, the ingenious ligand design in the NNP pincer framework provides a means for dealing with the problem of proton-coupled electron transfers in an elegant way, via the protonation/deprotonation, aromatization/dearomatization sequences depicted. The capabilities imparted to the ligand by this design feature will likely figure prominently in effective water splitting catalysts. Enzymes often utilize secondary coordination features in the complex protein-ligand environment to manage protons and electrons; this is a strategy that chemists have not come close to mastering. In water chemistry, such ligand features will aid in the stabilization of reactive intermediates and the enhancement of reaction rates because of the modulating electronic properties of the ligand as the cycle proceeds. Other examples of ligands with such functionality built into their design are beginning to appear,44-48 but significant challenges remain in delineating and controlling the principles of ligand cooperativity. A further challenge lies in the fact that most of the chemistry in this area reported to date has involved second- or third-row transition metals; clearly a move to more economical and abundant firstrow metals must be made, not only here but also in other areas of catalysis. Readers might question whether these types of investigations constitute organometallic chemistry because the focus has shifted away from the chemistry of metal-carbon bonds. The fundamental approach put forward here is certainly (42) Verat, A. Y. F., H.; Pink, M.; Chen, Y.-S.; Caulton, K. G. Chem.;Eur. J. 2008, 14, 7680–7686. (43) Eisenberg, R. Science 2009, 324, 44–45. (44) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40–73. (45) MacBeth, C. E.; Golombek, A. P.; Young, V. G. J.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938– 941. (46) Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; Rakowski DuBois, M.; DuBois, D. L. J. Am. Chem. Soc. 2005, 128, 358–366. (47) Hesp, K. D.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. J. Am. Chem. Soc. 2008, 130, 16394–16406. (48) Zweifel, T.; Naubron, J. V.; Gr€ utzmacher, H. Angew. Chem., Int. Ed. 2009, 48, 559–563.
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informed by that developed in the discipline, but, by definition, the lack of reactive hydrocarbyl ligands perhaps pushes it out of the realm of the field. To accommodate the chemistry of metal to p-block element bonds (other than carbon), the term “inorganometallic chemistry” has been proposed,49 and while this term has not enjoyed widespread adoption, the organometallic approaches to water splitting outlined here fall under its domain.
Conclusions “Prediction is very difficult, especially when it’s about the future.” To this quote, attributed to Nobel Prize winning physicist Niels Bohr, one could add that prediction is not only difficult but also risky;and perhaps foolish! In predicting the future of any field, it is appropriate to ask what that discipline has to offer society given the daunting technological problems currently faced by humanity. Collectively, the global community of organometallic chemists represents a staggering intellectual resource for humankind, and the formidable analytical and computational sophistication available makes advancement at unprecedented rates possible. How should these resources be directed? What challenges should take priority? Concerned organometallic chemists who seriously consider these questions are growing in number, and this will naturally influence the future of the field. While there is no doubt that organometallic chemists will continue to make significant and important contributions to traditional areas of inquiry in the activation and transformation of hydrocarbons, there are tremendous opportunities for parallel developments in the utilization of small molecules other than hydrocarbons as potential fuel sources. Organometallic chemists, equipped with the battery of synthetic, spectroscopic, kinetic, and computational techniques largely developed studying the activation of hydrocarbons, have much to offer in solving the problem of catalytic water activation. To finish with one last quote, this time attributed to computer scientist Alan Kay, “the best way to predict the future is to invent it!”
Acknowledgment. The author would like to thank all current members of the Piers group for helpful comments. This work was funded by NSERC of Canada through a Discovery Grant to W.E.P. (49) Fehlner, T. P. Inorganometallic Chemistry; Plenum Press: New York, 1992.
