Artistic and Practical Expressions in Molecular Chemistry

Sometimes the thrill and utility of scientific research experienced by the practicing chemist is not completely shared by the public. In fact, it is not unusual for an undergraduate chemistry student or even a new Ph.D. graduate to state, "What am I going to tell my parents when they ask me what I have been doing? They will never understand." Such comments, as well as negative press coverage heaped on science, suggest that many scientists have a difficult time conveying to others the central importance of their enterprise. Why is this? Well, one reason is the overuse of technical lingo that excludes all but specialists from understanding an issue -- a problem that pervades many fields. Further, some scientists have grown to enjoy a Scrooge McDuck image that intimidates others and allows scientists to pursue their idiosyncratic interests without justification. Finally, though able, some scientists just do not take the time to communicate outside their peer group. In this forum, I will attempt to strip away some of the mystery surrounding the chemist. I will also tell you a little about what my research group does at UNM and, I hope, show that the chemical research enterprise is worthy of public attention.

What do chemists really do? It turns out that the answer is very simple. They make things -- this is called synthesis, and they determine what and how much they have -- this is called analysis. After accomplishing these activities, they communicate the results usually to specialists in a paper, patent, or talk. The exercise is not that much different from creative cooking --you make it, taste it, and tell someone about it. No matter how difficult the subject of chemistry may seem to the casual observer or to the student taking his/her first chemistry class, this is all chemistry really is.

So, where do the fear and confusion arise for the public? The answer is in the activity of communication. Let's return to the word "synthesis." I suggest that you look this word up in Webster's Dictionary. Unfortunately, you may still be puzzled by the stilted description that says nothing about cooking or tasting. Sir John W. Cornforth, Nobel Laureate, 1975, also recognized this confusion, and he suggested that synthesis might best be looked at as "...the intentional construction of molecules by means of chemical reactions." The key word is construction, which should provide a clear image for everyone. The tradesman builds structures -- macroscopic objects -- from boards and nails and bricks and mortar. In a related fashion, chemists construct molecules -- microscopic objects -- from what? Well, they use supplies found in the periodic table, a construction yard full of all the known elements, where the elements serve as the boards and bricks, and the atomic electrons and orbitals serve as nails and mortar to hold the molecules together. Continuing with this analogy, the tradesman carries a toolbox with instruments that help him with his job. So also do chemists. Some tools are very simple, a glass beaker for example, while others are-very complicated, a vacuum line, drybox, or NMR spectrometer for example. The tradesman carries printed instructions and plans for his jobs. So also do chemists through fundamental, theoretical foundations and the chemical literature.

So, now we know what many chemists do; they construct microscopic structures called molecules. Next, we should ask why do chemists want to construct molecules? Perhaps before looking into the answers to this question, however, we should seek some additional perspective and recognize one synthetic chemist surrounding us every day: Nature. Indeed, chemical synthesis is an art form that has been practiced and perfected by Nature since the dawn of time. We often overlook them, but dynamic chemical factories exist all around us in our daily lives in both the living and non-living world. Simple and complex molecules are under construction all the time. Chemists have only imitated and elaborated on this larger and ancient chemical enterprise during the last two centuries.

Why do chemists build new molecules? There are many answers and justifications. Many industrial colleagues would say the primary reason to do synthesis is to derive new, useful products that provide profits for the company. In fact, the chemical industry is interrelated to many parts of our nation's economy. In apparent contrast, many academic chemists seem to pursue synthesis as a form of artistic expression. Others may synthesize molecules as models for much more complex natural systems that they are attempting to understand. Still others attack the synthesis of a specific molecule or class of new compounds for pure sport. Can they be, for example, the first to derive an unusual composition or structural form? There is nothing wrong with any of these motivations. Each, in its own way, serves to advance civilization through enhanced knowledge and improved quality of life. Whenever and wherever, chemical synthesis should be purposeful and driven by fundamental, scientific questions and practical objectives.

Over the last thirty years at UNM, my research group has had a number of different synthetic objectives. We have, for example, demonstrated that some compounds with metal-fluorine bonds undergo unexpectedly reversible reactions at very low temperatures (10°K). We have discovered that metal-phosphorus double bonds exist and display unique reactivity.  We have found that main-group (or non-metallic) elements can be systematically assembled and arranged into 3-D cage-like structures.  We have also observed that some organic molecules can bond with metal atoms in unexpected ways. Most recently, we have become involved in several projects that, like those above, have both great theoretical chemical interest and considerable practical implications. I will briefly outline these projects in order to illustrate the synergy that can exist between fundamental and applied chemistry.  Some additional detail with literature references appear in two subsequent sections.

Boron nitride, BN, is a very simple diatomic molecule that is isoelectronic (i.e., it has the same number of electrons as carbon). This simple molecule is obtained from high-temperature reactions between readily available and cheap starting materials such as boric acid and urea. The molecule exists in two solid forms at room temperature: one like graphite and one like diamond (the primary forms of carbon). The graphite form, in fact, looks like stacked layers of chicken wire. The molecule is stable at very high temperatures (3000°C), so it is called a ceramic compound. Boron nitride has many useful properties (e.g., thermal conductor, electrical insulator) that make it useful in industry, and huge quantities are manufactured in the U.S. and abroad. The industrial applications are as diverse as crucibles used to melt silicon and other refractory materials and formulations for many fine cosmetics. As prepared and used in industry, BN is obtained as a soft white powder that can be fabricated into commercial articles by using technology not too different from that employed in the production of artistic and practical pottery. Although useful applications exist for BN powders, they can not be employed to manufacture products that require fibers, coatings, and porous structural forms for their construction. As a result, there is interest in developing new syntheses for BN that would yield these forms.

Our group saw this problem and posed the question: Can entirely new physical forms of BN be obtained from polymers that contain mostly boron and nitrogen atoms? Similar approaches are used in carbon chemistry, even by Nature. For example, trees produce a natural polymer called pitch, which can be drawn into a fiber and carefully burned to make carbon fibers used in aircraft brakes and nose cones. Similarly, peach pits can be burned to give porous carbon used for gas separations. Unfortunately, before we could answer this question, we had to synthesize new polymers because Nature has not provided boron-nitrogen polymers for us.

Our group at UNM was successful in achieving this objective. Starting with simple monomers (e.g., the six-membered borazine rings), we succeeded in forming polymers that repeat the composition and structure of the monomer many times over. The polymers have adjustable properties that allow them to be formed into fibers, coatings, porous bodies with large holes (macroporous) or with very small holes (microporous), and composite materials. When the polymeric forms are heated, they produce boron nitride ceramics or composites with these same physical forms. The new materials have a variety of potential applications including selected high-strength construction materials, thermal management materials, and gas separations.  As detailed in a following section our current activities in ceramics chemistry have moved on to new methods for synthesis of these novel forms.

In another area, our group has designed special molecules that selectively attach to a single type of metal ion in solution. These molecules are often called "chelators," which means "crab-like." Pictorially, we can imagine synthesizing molecules with chemical "claws" or "hooks" that could be made by electronic or spatial/geometrical demands to bind, "pinch" or "snag" to only one metal in a solution containing many different types of metals. We have been successful here also. By posing specific fundamental questions regarding how molecules interact with metals, we have learned how to "design" chelators with unique, targeted functions and properties. Just as an engineer changes the shape of an airplane wing design based on wind tunnel behavior, we modify the electronic and geometrical features of new chelator molecules in order to achieve selective metal ion binding. With these properties, the chelators become useful for the separation of specific and, in some cases, toxic metals from waste solutions.

With these two examples, I hope I have communicated clearly the utility of chemical design and synthesis. Still, you might rightly ask: Why are we so interested in synthesis? The answer is found in all of the general reasons mentioned earlier:  for the sport or fun of being first to make something that produces a new concept or insight in our field; for artistic expression or for the joy of assembling beautiful new structures with atomic-scale tinker toys, or for the satisfaction gained in generating materials with useful properties and potential profitability. Most importantly though, for me, our projects have been pursued as training experiences for new, young scientists at the undergraduate, graduate, and postdoctoral levels. Through these projects, more than eighty students have learned the scientific method, suffered through failures, experienced the elation of successes, recognized the responsibility of their activities, and realized the importance of communicating their contributions to society. Their growth and development has been a continuous source of enjoyment for their mentor, and I wish to thank them for their dedication over the years.

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