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Catalyst: Sustainable Catalysis

2017; Elsevier BV; Volume: 2; Issue: 3 Linguagem: Inglês

10.1016/j.chempr.2017.02.014

2451-9308

Jacob R. Ludwig, Corinna S. Schindler,

Nanomaterials for catalytic reactions

Jacob R. Ludwig received his BSc in chemistry in 2014 from Michigan State University, where he performed research in the laboratory of Jetze Tepe. After graduation, he joined the Schindler lab at the University of Michigan to pursue his PhD degree.Corinna S. Schindler received her diploma in chemistry from the Technical University of Munich. After a research stay with K.C. Nicolaou at the Scripps Research Institute, she joined the group of Erick Carreira at ETH Zürich for her graduate studies. She then returned to the US to conduct postdoctoral studies with Eric Jacobsen at Harvard before starting her independent career at the University of Michigan in 2013. Jacob R. Ludwig received his BSc in chemistry in 2014 from Michigan State University, where he performed research in the laboratory of Jetze Tepe. After graduation, he joined the Schindler lab at the University of Michigan to pursue his PhD degree. Corinna S. Schindler received her diploma in chemistry from the Technical University of Munich. After a research stay with K.C. Nicolaou at the Scripps Research Institute, she joined the group of Erick Carreira at ETH Zürich for her graduate studies. She then returned to the US to conduct postdoctoral studies with Eric Jacobsen at Harvard before starting her independent career at the University of Michigan in 2013. The concept of catalysis was introduced more than 180 years ago by the Swedish chemist Jöns Jacob Berzelius and defines a catalyst as any substance that increases the rate of a chemical reaction without getting consumed. The observed increase in rate is often significant enough that, from a practical perspective, many chemical reactions do not proceed in the absence of a catalyst. With additional advantages such as decreased energy input, diminished environmental impact, and overall financial benefit, it does not come as a surprise that catalysis has become ubiquitous in our everyday lives. More than 90% of the current chemical processes and thus the majority of all commodities produced involve catalytic transformations.1Zhou Q.-L. Angew. Chem. Int. Ed. 2016; 55: 5352-5353Crossref PubMed Scopus (78) Google Scholar There are many types of catalysts, including enzymes and small molecules, but currently the most studied and most commonly used are those that are derived from transition metals. The dominance of transition-metal catalysts largely results from a combination of their efficiency, distinct modes of reactivity, and the predictable control of both activity and selectivity upon ligand modification. Recent landmark achievements in transition-metal catalysis include the development of ruthenium- and osmium-mediated asymmetric, catalytic hydrogenations and oxidations, ruthenium- and molybdenum-catalyzed olefin metathesis, and palladium-catalyzed cross-coupling reactions. Each of these discoveries was awarded the Nobel Prize in Chemistry (in 2001, 2005, and 2010, respectively), and each displays a distinct unifying feature: relying on a transition-metal catalyst classified as a precious metal. A precious metal is as a chemical element of high economic value whose abundance in the Earth’s crust is limited. These metals are characterized by their general resistance to oxidation and corrosion and consequently are also referred to as noble metals. Precious metals include the elements found in the second and third rows of the periodic table; those commonly used as transition-metal catalysts include rhodium, palladium, platinum, ruthenium, iridium, gold, silver, and osmium.2Fürstner A. ACS Cent. Sci. 2016; 2: 778-789Crossref PubMed Scopus (438) Google Scholar Precious metals have played an essential role in the development of both homogeneous catalysis and chemical synthesis, and their popularity is a result of several unique characteristics.•Stability: Resistance to corrosion or oxidation is a hallmark of a precious metal. Many low-valent, precious-metal complexes are stable enough to oxygen to endure a variety of important catalytic transformations under ambient conditions without the need for a rigorously air-free environment, greatly facilitating their ease of operation. Additionally, the low ligand lability of precious-metal complexes is a contributing factor to their overall stability.3Holland P.L. Acc. Chem. Res. 2015; 48: 1696-1702Crossref PubMed Scopus (125) Google Scholar•Oxidation-state changes: Many important metal-mediated transformations proceed via oxidative addition and reductive elimination steps that require two-electron oxidation-state changes at the metal center. As such, the ability of precious metals to readily undergo two-electron oxidation-state changes translates to their enhanced utility in catalytic reactions.•Pi bond acidity: Precious-metal complexes often exhibit a high affinity for pi bonds, which are common to many functional groups present in organic molecules. Several reactions used to generate molecular complexity proceed via pi bond activation, making precious metals well suited as catalysts for these reactions.2Fürstner A. ACS Cent. Sci. 2016; 2: 778-789Crossref PubMed Scopus (438) Google Scholar•Selectivity: Noble metals regularly display unique selectivity for a specific type of transformation. For example, the choice of a platinum-, silver-, or palladium-based catalyst for the oxidation of ethylene can provide CO2 and H2O, ethylene oxide, or acetaldehyde as products, respectively.4Freyschlag C.G. Madix R.J. Mater. Today. 2011; 14: 134-142Crossref Scopus (65) Google Scholar•Characterization: As a result of being very stable, catalytically relevant precious-metal complexes are often capable of being isolated and characterized by a variety of common and well-established techniques, including X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy. Specifically in the case of NMR, precious metals typically form diamagnetic complexes that are subject to relatively straightforward NMR analysis, which contrasts the paramagnetic nature of most first-row metal complexes.3Holland P.L. Acc. Chem. Res. 2015; 48: 1696-1702Crossref PubMed Scopus (125) Google Scholar Ultimately, the predictable reactivity characteristic of precious-metal catalysts can be attributed to their electronic structure, which culminates in highly stable, uniquely selective, and easily characterized complexes as catalysts. Despite the prevalence of precious metals in catalysis, there are problems associated with their continued use in catalytic processes. For one, the fact that precious metals are by definition scarce indicates that they lack abundance, are very expensive, and are susceptible to supply fluctuations, which fuels growing concerns about their continued availability. In 2011, the British Geological Society released a list of metals at risk of supply disruption.5Umile T.P. Catalysis for Sustainability: Goals, Challenges, and Impacts. CRC Press, 2015Google Scholar Antimony, the platinum-group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum), mercury, and tungsten are estimated to be at the highest risk. Furthermore, both the US and European countries rely on imports of the majority of these “strategic” metals. The continuous need for precious metals of low abundance not only raises economic concerns but also has significant implications for the environment. The mining and refinement of lower-grade metal ores is being pursued and often results in an increased use of fossil fuels and CO2 emissions. For example, 1 oz of pure platinum requires the extraction of 10–40 tons of raw ore that is currently being mined at depths of up to 1 mi in the Earth’s crust. Despite their high demand and scarcity, it is estimated that less than 1% of precious metals are being recycled for reuse as a result of the low economic viability of these processes. As such, the ability to transform the reliance of current industrial processes from precious metals, which represent diminishing resources, to more sustainable alternatives is at the forefront of current research. The term “sustainable development” stems from efforts initiated by the UN General Assembly in 1983 as a response to growing concerns about the deterioration of natural resources and stresses the necessary unity of both economic and social development with environmental protection.5Umile T.P. Catalysis for Sustainability: Goals, Challenges, and Impacts. CRC Press, 2015Google Scholar Specifically, sustainability is defined as “the kind of development that meets the needs of the present without compromising the ability of future generations to meet their own needs,”5Umile T.P. Catalysis for Sustainability: Goals, Challenges, and Impacts. CRC Press, 2015Google Scholar which remains the most frequently used definition of the concept of sustainability to date. A current approach for the development of more sustainable catalysis is the use of metals with high crustal abundance, which are often referred to as base metals. The first-row transition metals, including titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper, represent base metals that offer additional advantages such as low cost and global availability. In comparison with precious metals, which are often toxic, base metals are considered to be metals with minimal safety concerns. For example, 1,300 ppm of residual iron is tolerable in active pharmaceutical ingredients, whereas often less than 10 ppm is acceptable for platinum or palladium or any combination of iridium, rhodium, ruthenium, and osmium.6Bauer E.B. Iron Catalysis: Historic Overview and Current Trends.in: Topics in Organometallic Chemistry. Volume 50. Springer International Publishing, 2015: 1-18Google Scholar Furthermore, removing precious-metal catalysts from active pharmaceuticals often requires special measures that are accompanied by the production of significant amounts of waste.6Bauer E.B. Iron Catalysis: Historic Overview and Current Trends.in: Topics in Organometallic Chemistry. Volume 50. Springer International Publishing, 2015: 1-18Google Scholar The low cost, high natural abundance, balanced global distribution, and low toxicity of base metals suggest that they could serve as a suitable alternative to precious metals. However, progress in their development as catalysts has lagged far behind efforts invested in precious metals. Several significant challenges associated with the implementation of base metals as general catalysts arise from their inherent reactivity, which leads to problems with their stability, selectivity, and scope.•Stability: Low-valent base-metal complexes are often sensitive to oxidizing conditions and, as such, require an inert atmosphere for their synthesis. Alternatively, the active base-metal catalyst can be generated in situ from suitable precursors, which precludes a rigorously oxygen-free environment.•Oxidation-state changes: An additional challenge associated with first-row transition metals is their inherent preference to undergo single-electron transfer processes as opposed to two-electron redox events common to precious metals. Many important metal-mediated redox steps require two-electron changes in oxidation state at the metal, thus limiting the applicability of single-electron oxidation-state changes in a majority of commonly employed catalytic transformations. Current efforts focus on the development of distinct tactics that compel a base-metal complex to undergo favorable two-electron changes in oxidation state, as opposed to single-electron transfer, to ultimately mimic the reactivity of precious-transition-metal catalysts.•Selectivity: The single-electron transfer events that are generally observed for base metals can lead to the generation of free radicals. These reactive species are difficult to control and generate products in an unselective fashion. Ultimately, the indiscriminate behavior of base-metal complexes translates to lower functional-group tolerance than for complexes derived from precious metals and therefore limits substrate scope.•Characterization: Base-metal catalysts regularly contain a weak ligand field, which often gives rise to paramagnetic complexes. As a result, analysis of these base-metal complexes requires less traditional characterization techniques, including paramagnetic NMR and electron paramagnetic resonance spectroscopy. Furthermore, unlike precious-metal catalysts, many first-row metal complexes are difficult to isolate and characterize because of their instability. All together, the instability, preference for single-electron transfer, and characterization challenges that arise from the inherent reactivity of first-row transition metals yield a reaction manifold that is less predictable and more challenging to control than that of precious metals. Although there are many challenges associated with the use of base metals as catalysts for reactions that have historically relied on precious metals, the potential future benefits for both our society and the environment are substantial. Recent years have seen a significant increase in research efforts in the area of base-metal catalysis, and important advancements have already been reported. Successful approaches to controlling the reactivity of first-row transition metals are inspired by natural enzymes that contain heme-iron centers (Figure 1A). In nature, heme iron is composed of a porphyrin ligand bearing an extended pi system and an iron center that together enable the enzyme to catalyze specific redox events via two-electron transfer chemistry, much like precious-metal catalysts. Base-metal complexes incorporating “redox-active” ligands with stabilizing pi systems have since been developed as successful enzyme mimics.7Belin P. Le Du M.H. Fielding A. Lequin O. Jacquet M. Charbonnier J.-B. Lecoq A. Thai R. Courçon M. Masson C. et al.Proc. Natl. Acad. Sci. USA. 2009; 106: 7426-7431Crossref PubMed Scopus (153) Google Scholar A key requirement of a redox-active ligand, also referred to as a “non-innocent” ligand, is to have low-energy transitions associated with changes in its oxidation state such that reducing or oxidizing a complex leads to changes in the oxidation state of the ligand rather than the metal center. The formally low-valent metal centers of these base-metal complexes were shown to exist with the ligand, representing a formal dianion as a delocalized diradical, whereas the metal center adapts a higher oxidation state. This metal-ligand cooperativity presumably allows for the base-metal complex to undergo formal two-electron redox reactions essential for catalysis and avoids competing single-electron transfer. One of the most successful classes of redox-active ligands currently known are diiminopyridine ligands (Figure 1B), and many synthetically valuable base-metal catalysts rely on this ligand class.8Hoyt J.M. Schmidt V.A. Tondreau A.M. Chirik P.J. Science. 2015; 349: 960-963Crossref PubMed Scopus (138) Google Scholar Another approach centered on metal-ligand cooperativity relies on non-innocent ligands, whose dynamic nature enables complementary redox processes (Figure 1C). Specifically, the aromatized form of the ligand provides a reducing complex, whereas the dearomatized form gives an oxidizing complex.9Casey C.P. Guan H. J. Am. Chem. Soc. 2007; 129: 5816-5817Crossref PubMed Scopus (441) Google Scholar In addition to metal-ligand cooperativity, metal-metal cooperativity is a similar strategy for controlling the reactivity of base metals and supporting multi-electron transfer processes. Nature uses this concept in [NiFe] hydrogenases containing a central Ni–Fe bond to enable energy storage in the form of hydrogen. Taking a lead from nature, synthetic base-metal complexes incorporating metal-metal bonds have been developed to offer reactivity, selectivity, and activity complementary to those of their mononuclear counterparts (Figure 1D).10Pal S. Uyeda C. J. Am. Chem. Soc. 2015; 137: 8042-8045Crossref PubMed Scopus (79) Google Scholar The aforementioned strategies represent successful approaches to altering the fundamental reactivity of base metals. However, endorsing the intrinsic reactivity of base-metal complexes can provide the unique opportunity to discover new and important transformations otherwise not accessible with precious metals. Specifically, the preference for base-metal complexes to undergo single-electron transfer allows for novel catalytic processes that open the door to unexplored chemical space (Figure 1E).11Hennessy E.T. Betley T.A. Science. 2013; 340: 591-595Crossref PubMed Scopus (436) Google Scholar Another unique characteristic of first-row transition metals is their superior ability to function as Lewis acids in comparison with precious metals (Figure 1F). The Lewis acidities of certain base metals can vary strongly with oxidation state and range from moderate to very high. The reactivity of base metals as Lewis acids can be tuned and controlled by the choice of ligand and access to specific oxidation states, a property that should be exploited given that many important bond-forming processes are mediated by Lewis acids.2Fürstner A. ACS Cent. Sci. 2016; 2: 778-789Crossref PubMed Scopus (438) Google Scholar, 12Ludwig J.R. Zimmerman P.M. Gianino J.B. Schindler C.S. Nature. 2016; 533: 374-379Crossref PubMed Scopus (141) Google Scholar Efforts focused on the development of base-metal precursors that are more convenient to handle are expected to further advance base-metal catalysis, analogous to how precious-metal precatalysts have greatly contributed to the current preeminence of second- and third-row transition metals. It is to be expected that increased research efforts in the area of base-metal catalysis to efficiently manage two-electron redox events on the basis of metal-ligand and metal-metal cooperativity will lead to the development of more advanced and sustainable alternatives to precious-metal complexes. Further embracing the inherent reactivity of first-row transition metals as single-electron transfer systems and endorsing their distinct reactivity as Lewis acids will advance sustainable catalysis and uncover new and important reactivity complementary to that of precious metals. Reaction: Earth-Abundant Metal Catalysts for Energy ConversionsR. Morris BullockChemApril 13, 2017In BriefMorris Bullock is a laboratory fellow at Pacific Northwest National Laboratory and director of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the Department of Energy. He is a fellow of the AAAS, American Chemical Society, and Royal Society of Chemistry. He received his PhD while working for Prof. Charles Casey at the University of Wisconsin-Madison and worked as a postdoc for Prof. Jack Norton at Colorado State University. He edited the book Catalysis Without Precious Metals (Wiley-VCH, 2010). He has been a cheapskate for a long time and is glad to have an excuse to practice that habit in his research. Full-Text PDF Open ArchiveReaction: Sustainable Catalysis without MetalsDavid J.C. ConstableChemApril 13, 2017In BriefDavid J.C. Constable is the science director of the American Chemical Society’s Green Chemistry Institute. In this role, he works to catalyze and enable the implementation of sustainable and green chemistry and engineering throughout the global chemistry enterprise. Full-Text PDF Open ArchiveReaction: Opportunities for Sustainable CatalystsPatrick L. HollandChemApril 13, 2017In BriefPatrick Holland pursued doctoral training at the University of California at Berkeley with Robert Bergman and Richard Andersen and postdoctoral training with William Tolman at the University of Minnesota. As a faculty member at the University of Rochester, he began his independent research in 2000 by focusing on the study of three-coordinate Fe-Co complexes. Since then, his research group has addressed Fe-N2 chemistry, reactive metal-ligand multiple bonds, engineered metalloproteins, redox-active ligands, solar H2 production, and the mechanisms of organometallic transformations at base-metal complexes. In 2013, he moved to Yale University, where he is a professor of chemistry. Full-Text PDF Open Archive