Holland's Group Uses Iron for a "Noble" Cause
Catalysis is one of the wonders of science: it is one of the ways that chemists can transform compounds into other compounds specifically and efficiently. The key to a catalytic reaction is the catalyst, a compound added in small quantities which controls the outcome, yet is not used up in the reaction. The developments of catalysts in the 20th century revolutionized chemistry and greatly improved humans' quality of life with discoveries such as plastics, fertilizers, pharmaceuticals, and many commodity materials. The catalysts are often made from "noble" metals such as rhodium, ruthenium, palladium, and platinum.
However, noble metals are problematic. First, though they make efficient catalysts, supplies of these metals are limited and they are very expensive. Also, they are toxic, and therefore using them to make pharmaceuticals is risky if they cannot be removed completely. However, if Prof. Patrick Holland and his students have their way, we will be able to avoid use of "noble" metals by using a metal that isn't noble: iron. "Iron is non-toxic, plentiful, and incredibly cheap," says Holland. "It's not just cheap as dirt: it IS dirt." (Natural soils are 1-20% iron by weight.) His research program at the University of Rochester aims to figure out how to make iron do catalytic reactions as efficiently as expensive metals, reducing pollution and improving both the economics and safety of chemical processes.
Catalysts: Chemical Machines for Making the Right Products
Catalysis has been an important part of recent achievements in chemistry, and development of catalysts led to the Chemistry Nobel Prizes in 2001, 2005, and 2007. It is amazing that a tiny additive can completely change the outcome of a reaction. How can such a small amount of catalyst have such a super-sized effect? The way it works is that the catalyst binds the starting material, facilitates its selective transformation, releases the product, and this leaves the catalyst in a form where it can bind another molecule of starting material. Since the catalyst is regenerated, it can go through this cycle many times, and each molecule of catalyst can convert many molecules of starting material. And, since the catalyst can be modified to have a different shape or binding properties, reactions can be controlled or modified.
Catalysis is a trick that is used by natural systems, and the natural catalysts are known as enzymes. These enzymes often contain single metal atoms inside them where the reaction happens. (When inorganic chemists refer to a metal, they don't mean a solid metal, instead they mean the metal atom or atoms of an element like iron within a larger compound.) Chemists later harnessed metal-containing catalysts for making polymers (e.g., plastics), where the catalyst sequentially adds additional length to a growing chain-like molecule. However, catalysis is challenging when the starting materials are unreactive, because they do not bind well to the metal in the catalyst. Overcoming this limitation is an important research challenge, because some of the most plentiful starting materials are very stable. Holland's group focuses on understanding the metal binding and reactions of two frustratingly stable starting materials: dinitrogen and hydrocarbons.
Fixing Dinitrogen Because It's Hard to Break
"Dinitrogen" is N2, the molecule that makes up about 80% of the atmosphere. There is an immense amount of dinitrogen around us, so you would think that it would be easy for plants and animals to make essential nitrogen nutrients, such as amino acids and nucleic acids, using nitrogen atoms from the air. However, N2 is very unreactive and most organisms are unable to convert it into other compounds. Bacteria can turn dinitrogen into ammonia using the enzyme nitrogenase, but this reaction is relatively slow and is not fast enough to keep up with humans' agricultural needs. So, farmers need fertilizers to grow crops because the soil becomes depleted in nitrogen. These fertilizers primarily come from the industrial "fixation" of nitrogen, where "fixation" refers to turning the N2 gas into a solid "fixed" form, usually ammonium nitrate. About 120 million tons of ammonia are currently produced per year for fertilizers, and without this gigantic output there would not be enough food for the six billion people on Earth.
In industry, ammonia is produced by an iron catalyst. However, it is not a very efficient catalyst, and must be raised to high temperatures and pressures. Further, chemists do not yet understand the atomic details of how N2 interacts with the metal atoms. Holland says, "It's really crucial to figure out how iron does nitrogen-fixing reactions, because that could help us to improve the catalyst."
Holland and his research team have made significant progress toward this goal. In a 2001 paper, he and postdoctoral scientist Jeremy Smith reported in the Journal of the American Chemical Society the first iron compounds that weaken the N-N bond in N2. "The N-N bond in N2 is a triple bond, and our iron compounds reduced it to a double bond," as Holland says. In two additional JACS papers in 2006, they showed the special property of the iron compounds that makes them active: the small number of bonds to the iron atom. The "low-coordinate" iron atoms have only three bonds, whereas most iron compounds have five or six bonds. Since this discovery, research teams at Caltech, MIT, and Berkeley have found other iron-N2 compounds with similar properties, and research is continuing in this area.
These initial discoveries led to funding from the National Institutes of Health, which has enabled Holland and his students to extend their exploration of low-coordinate iron compounds to other compounds with Fe-S and Fe-H bonds that resemble possible reactive states in biological ammonia-producing catalysts. Holland says, "By learning the details of the chemistry of these compounds in our lab, we are helping to sort out the natural and commercial reactions of dinitrogen."
Breaking Other Strong Bonds With Iron
Hydrocarbons like petroleum have gotten a bad name because burning them emits the greenhouse gas CO2, as well as trace amounts of nitrogen- and sulfur-containing compounds that can lead to acid rain and ozone depletion. "There's another reason we shouldn't be burning them – they would be better used as starting materials for making useful compounds!" Holland says. The problem is that most petroleum hydrocarbons are quite unreactive, and there are not yet efficient catalysts to transform the most unreactive hydrocarbons. This has led to research aiming to find selective reactions of simple petroleum compounds. Some of the seminal work in this area was done in the early 1980's by Holland's colleague at Rochester, Prof. William Jones, and by his Ph.D. advisor, Prof. Robert Bergman at Berkeley.
"Those guys have been using noble metals that reduce [add electrons to] hydrocarbons, which is one very promising strategy. We've noticed that natural systems often use iron enzymes to oxidize [remove electrons from] hydrocarbons, and so we're trying a different way that might be possible with cheap metals," says Holland. Specifically, Holland's group intends to use iron compounds to catalyze the formation of C-N bonds from petroleum-based hydrocarbons, using financial support from the National Science Foundation. So far, they have isolated an interesting "iron-imido" compound with an iron-nitrogen double bond, which reacts rapidly with hydrocarbons.
Recently, graduate student Ryan Cowley used the initial results of previous Ph.D. student Nathan Eckert to demonstrate a catalytic reaction of the iron-imido compounds, which was reported in Chemical Communications in early 2009. They showed that the iron-imido compound has the right balance of binding and releasing abilities to be an effective catalyst. Interestingly, the catalyst has three-coordinate iron, similar to the low-coordinate iron atoms in the iron-N2 compounds described above.
Iron Without All Its Bonds
Why do compounds containing iron become more active when they have only three bonds to the iron atom? This question, which lies at the heart of the N2 and hydrocarbon chemistry described above, has become a central question for Holland and his research team. "We were initially inspired to make three-coordinate iron compounds when we saw the amazing structure of the nitrogenase enzyme, which has a bunch of iron atoms with very few bonds to them. It makes sense that an iron atom without enough bonds would be better at binding weak starting materials like N2 and hydrocarbons, and so we used this as a hypothesis that started our research." Along the way to figuring out the catalytic reactions mentioned above, his research group has constantly been surprised by the unusual properties of the compounds. As a result of the interesting discoveries along the way, the group has published dozens of papers on compounds that have arisen during their studies.
Ironing Out Other Questions in the Future
Since the Holland group has mastered three-coordinate iron, they hope to expand the scope of what they can do. For example, graduate student Matthew McLaughlin has figured out how to put three-coordinate iron into a water-soluble protein. "No one has ever seen three-coordinate iron in a natural environment, so this is really significant," says Holland. The Holland group recently received funding from the U.S. Department of Energy to explore other catalytic reactions of three-coordinate iron, cobalt, and nickel that are relevant to bio-renewable resources.
In addition to research, grantwriting, and teaching in the Department of Chemistry, Holland spends time traveling to describe his group's research at invited lectures around the world. "There's a lot of interest, which is gratifying after all the effort by the research team," Holland says. "I've been fortunate to have fabulous coworkers in the lab!" Holland's students have gone on to positions in chemical industry (Shepherd, ChemRoutes, Chevron), research universities (professors at New Mexico State and Iowa State University), and teaching (SUNY Brockport, St. Johns College High School in D.C.).