Organometallics 2011, 30, 17–19 DOI: 10.1021/om1009439
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An Organometallic Future in Green and Energy Chemistry?† Robert H. Crabtree Chemistry Department, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States Received September 30, 2010
The title topic is reviewed with selected examples taken from recent work, such as: the ’hydrogen borrowing’ amine alkylation by alcohols; the dehydrogenative coupling of amine and alcohol to give amide; Ru complexes as solar cell photosensitizers; Ir organometallics as water oxidation catalyst precursors and as OLED emitters; as well as recent hydrogen storage strategies involving catalytic dehydrogenation of ammonia-borane and of organic heterocycles.
Increasing concern about climate change, resource depletion, and environmental degradation has posed new challenges for the scientific community. Work aimed at resolving problems of energy and the environment is likely to show strong growth in the next decade, not just in the organometallic arena but in science as a whole. It is already clear that organometallic chemistry has a big role in the development of green chemistry, one of the tenets of which is the importance of catalysis.1 Indeed, organometallic catalysis had already been a mainstay of industrial chemistry for many decades before green chemistry was defined as distinct field. The increased attention being given to green chemistry by alert industries is illustrated by the recent foundation of the ACS Green Chemistry Institute Pharmaceutical Roundtable, an association of pharmaceutical companies that aims to promote green methods in pharmaceutical production. The Warner Babcock Institute for Green Chemistry, founded in 2007, has brought together researchers from a wide variety of disciplines, including many organometallic chemists. Among academic institutions, McGill, Nottingham, Monash, and York, among others, have significant programs in the area with strong participation by organometallic chemists. Many recent advances in organometallic catalysis have green implications. To take just one example, hydrogen-borrowing activation of alcohols (eq 1) permits alkylation without the need for alkylating agents, the example shown being amine alkylation.2-4 Instead of activating the alcohol RCH2OH by conversion to a halide, such as RCH2Br, an organometallic catalyst dehydrogenates the alcohol to form an aldehyde that is readily attacked by the amine, followed by hydrogenation of the resulting imine to the corresponding amine. This third step uses the hydrogen removed from alcohol in the first step. Thus, an overall redox-neutral reaction is catalyzed via a series of steps involving both oxidation and reduction. This route avoids both the toxicity issues associated with alkyl bromides and waste formation in the shape of a bromide salt derived from the † Part of the special issue Future of Organometallic Chemistry. (1) Anastas, P.; Eghbali, N. Chem. Soc. Rev. 2010, 39, 301. (2) Fujita, K.; Tanino, N.; Yamaguchi, R. Org. Lett. 2007, 9, 109. (3) Watson, A. J. A.; Williams, J. M. J. Science 2010, 329, 635. (4) Prades, A.; Corberan, R.; Poyatos, M.; Peris, E. Chem. Eur. J. 2008, 14, 11474.
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base needed in the traditional synthesis. The reaction also has the advantage of favoring the production of secondary amine because a primary amine can more easily react with the aldehyde. In contrast, traditional alkylation tends to give overalkylation because the secondary amine is more reactive than the primary with the alkyl bromide. Strict FDA regulations for pharmaceutical products have tended to discourage the use of bromides and similar alkylating agents in their synthesis.
Another green aspect of the sequence of eq 1 is the telescoping of three steps into one. Not only does this avoid the workup procedures that would be necessary to isolate the intermediates in a full, conventional three-step synthesis, but it also has thermodynamic implications. The initial dehydrogenation step of the sequence is thermodynamically unfavorable and would not normally proceed in high yield in the absence of the thermodynamically favorable final hydrogenation step. This third step would not take place in the absence of the reducing equivalents produced in the first step. In essence, the product-forming third step drives the first dehydrogenation step over to near-completion. This shows how interesting possibilities can arise by combining multiple steps into a single overall pathway. In this case, a single catalyst suffices for both catalyzed reactions because of the mechanistic similarity of the first and third steps. Numerous catalysts have been shown to be active for this type of reaction, one of the best being 1. Milstein and co-workers have described an interesting development of this general class of reaction in which the hemiaminal intermediate;a precursor to the imine in eq 1; loses hydrogen to form the corresponding amide (eq 2).5 (5) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790. Published on Web 01/04/2011
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Clearly the catalyst (2) has to be able to lose the hydrogen abstracted from the reactant, in contrast to the situation in eq 1, where the catalyst must retain this hydrogen in order to hydrogenate the imine. Unlike the amine alkylation case, this reaction is a net oxidation, but a rather unusual one in that it proceeds by loss of hydrogen as free H2. This pathway avoids the low atom economy typically associated with standard oxidation conditions and avoids the need for a primary oxidant that is also often toxic or costly. A comparison of eqs 1 and 2 also illustrates the power of catalysis as a versatile synthetic strategy, in that a different choice of catalyst can completely alter the product mixture obtained from the very same reactants under near-identical conditions.
Amide synthesis normally requires an expensive coupling reagent,6 such as dicyclohexylcarbodiimide, resulting in the formation of waste that is hard to recycle, contrary to the green chemistry ideal of high atom economy. Of course, the Milstein route requires that the whole synthesis be redesigned, because the traditional coupling procedure starts from carboxylic acid and amine to give amide. To permit this traditional route while avoiding coupling reagents, boronic acid catalysts have been developed that operate on the traditional reactants, carboxylic acid and amine.7 Reagents formed from main-group elements such as boron have tended in the past to be stoichiometric rather than catalytic but are likely to play an increasing catalytic role in future and complement their better-known transitionmetal partners. The strategy of activating a substrate by dehydrogenation may also have broader implications for substrates other than alcohols.8 Moving to energy problems,9 the increased current interest in means of alternative energy production that avoid carbon dioxide coproduction has led to a wide variety of suggested solutions. Organometallic chemistry has so far contributed much less to the field than it has to green chemistry, and coordination chemistry has so far taken a bigger role. Alternative energy chemistry in general is a much younger field, however, and much remains to be done. In addition, it is not yet clear exactly which initiatives are going to be viable. Some of the suggestions will no doubt prove to be impractical, while others will show more promise and move on to the next stage of development. Rather than moving to a single, one-size-fits-all alternative energy solution, we are likely to see a mixture of different strategies, each having their own niche in a future energy economy.10 Considering the vast global energy demand, many decades are likely to pass before any of the alternative (6) Valeur, E.; Bradley, M. Chem. Soc. Rev. 2009, 38, 606. (7) Charville, H.; Jackson, D.; Hodges, G.; Whiting, A. Chem. Commun. 2010, 46, 1813. (8) Crabtree, R. H.; Dobereiner, G. Chem. Rev. 2010, 110, 681. (9) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. (10) Pacala, S.; Socolow, R. Science 2004, 305, 968.
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energy schemes provide a significant fraction of our total energy demand because the research, engineering, economic, and political challenges are so formidable. The greater supply of solar energy, relative to other forms of renewables, means that photovoltaics are usually considered to provide the best hope of making a significant contribution to our energy requirements in the medium term. It is also an area where organometallic chemistry can hope to make a contribution. For example, metal organic vapor deposition11 is a technique that can help in the controlled deposition of the required semiconductors. Moving beyond classical photovoltaics, Gr€ atzel cells12 have shown promise as an inexpensive photovoltaic device, in part because they do not require ultrapure semiconductor material. Metal complexes such as 3, termed the “black dye”, have proved to be excellent sensitizers for such cells, but so far coordination compounds of ruthenium such as black dye have dominated the field. These are typically attached to high-surface-area titanium dioxide nanomaterials, the role of the metal complex being to absorb the light and inject electrons into the oxide semiconductor electrode. There is no reason organometallic compounds should not be able to do equally well and perhaps even permit the use of cheaper metals and avoid the need for ruthenium.
In the area of artificial photosynthesis, catalysis of water splitting to hydrogen and oxygen is a central element. Many of these catalysts are coordination compounds, but some organometallic species have recently been shown to be active precatalysts. Organometallic chemistry has traditionally been associated more with reduction than with oxidation, but certain organometallic structures may be able to survive even harsh oxidizing conditions. Bernhard and co-workers described a cyclometalated iridium complex as a precatalyst for water oxidation.13 Somewhat higher activity was achieved with a series of Cp*Ir complexes, such as 4.14 The most unusual catalytic property of these complexes, however, proved to be CH bond hydroxylation with retention of configuration at carbon during the reaction.15 A number of similar organometallic catalysts have also been described for the reduction of water to hydrogen.16 Caution is needed in the interpretation of these experiments, since it is always very hard to exclude a heterogeneous contribution to the catalysis, particularly since iridium dioxide is a known water oxidation catalyst. Consistent with genuine homogeneous catalysis, kinetic isotope effect experiments at low Ce concentrations show a distinct difference between the water oxidation behavior of the organometallic complex and authentic iridium dioxide, however: the former
(11) Liu, X.; Aspnes, D. E. Appl. Phys. Lett. 2008, 93, 203104. (12) Gr€atzel, M. Nature 2001, 414, 338. (13) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J. Am. Chem. Soc. 2008, 130, 210. (14) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. J. Am. Chem. Soc. 2009, 131, 8730.
Review
shows an inverse isotope effect, while the latter shows a normal isotope effect.14
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trolled, but several catalysts have now been described that produce selective partial dehydrogenation: e.g., eq 3. It has to be admitted that the hope of easily and efficiently regenerating the initial storage material by hydrogenation of the dehydrogenated form is so far unfulfilled.19 In the related area of virtual hydrogen storage, organometallic catalysts have been invoked for directly and reversibly storing electrons and protons in liquid organic storage materials, bypassing the need for free hydrogen.20
To better conserve energy, more efficient sources of lighting are eagerly sought;organic light emitting diodes (OLEDs) are thus currently attracting attention on this account. Cyclometalated Ir compounds such as 5 have proved exceptionally useful as green and blue emitters.17
Energy storage is an important part of alternative energy schemes, because most such resources, for example both solar and wind energy, are intermittent. Although many schemes have been proposed for the storage of energy in chemical form, hydrogen storage has attracted the most attention in an organometallic context. Borane-ammonia has been suggested as a hydrogen storage material because of its very high hydrogen content.18 The uncatalyzed thermal release of hydrogen from this material is not easily con(15) Zhou, M.; Schley, N. D.; Crabtree, R. H. J. Am. Chem. Soc. 2010, 132, 12550. (16) Cline, E. D.; Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47, 10378. (17) Adamovich, V. I.; Cordero, S. R.; Djurovich, P. I.; Tamayo, A.; Thompson, M. E.; D’Andrade, B. W.; Forrest, S. R. Org. Electron. 2003, 4, 77. (18) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613.
Government also has a role to play in maintaining conditions that favor development of these fields. The funding by the Department of Energy Basic Energy Sciences division of a series of Energy Frontier Research Centers, again with strong participation by organometallic chemists, is an example of the type of initiative whose implications may well be felt for years to come. While both energy and green chemistry fields are still under development, we are beginning to see the emergence of concepts and strategies that may prove to have lasting value. Organometallic chemists are certainly not ignoring the opportunities that are offered by the rise of these new and important challenges.
Acknowledgment. I thank the YINQE and the U.S. Department of Energy, Office of Basic Energy Sciences, for their generous support of our work in this area under Grants DE-FG02-84ER13297 and Energy Frontier Research Center Grants DE-PS02-08ER15944, DE-FG0207ER15909, and DE-SC0001298 and by the ACS-GCI Pharmaceutical Round Table. (19) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048. (20) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134.
