I fell in love with copper as an assistant professor. How did it happen? By happenstance flavored with a soupçon of sloth, and after an offhanded introduction.
While a graduate student at California Institute of Technology, I worked with organolithium reagents (RLi). As the time to graduate approached, I needed my own problem to work on. At that time, as little was known about the properties of s-bonded transition-metal alkyls as about the mating habits of kraken, and for the same reason--few of either had been reliably observed. There was a radical idea that such organometallic compounds might somehow be involved in catalysis, and their descriptive chemistry was essentially unexplored. The area was appealing, but the question was, "Where to start?"
Several organometallic compounds of gold were known, and to my unsophisticated eye, "RAu" looked vaguely and comfortingly like "RLi." I skimmed the literature of organogold chemistry briefly (very briefly, since there was almost none--a great advantage, I thought, since libraries were not my thing) and decided that gold would be a great place to begin exploring transition-metal alkyls. Then, one day, my research director, Jack Roberts, ambled by and asked what I was going to start on at Massachusetts Institute of Technology. I replied, "Gold organometallic chemistry." He considered this idea for 30 seconds, then mumbled "No. Don't do that. Work on copper--it's more interesting--and cheaper," and wandered off. And that was that--the die was cast.
He was right, of course. As I discovered over the following years, copper was a kind of pixie dust for organic chemistry. It turned fireworks and flames green; it made Grignard reagents do conjugate additions; it was essential for the Sandmeyer and Ullman reactions; it coupled acetylenes; it was a component of exotic heterogeneous catalysts; it promoted apparent nucleophilic substitution reactions on aromatic halides; it catalyzed autoxidation reactions. None of these reactions were understood mechanistically, but all were probably different. That kind of mechanistic chaos was, of course, candy to an aspiring mechanistic organometallic chemist.
At MIT, after considerable technical difficulty (largely due to my experimental incompetence), I finally found a copper(I) salt [(CH3)2SCuI] that was soluble in cold ether and added CH3MgI to it. A stunningly beautiful (to me), bright yellow, fluffy precipitate immediately formed, which, when warmed, turned black. What else could it be but CH3Cu? And copper metal on decomposition? Such pleasure!
Organic reactions involving copper are interesting, but they are a tiny part of the long and complicated relationship between this element and our species. We are, of course, ourselves part copper: Copper-containing enzymes are essential catalysts for a number of redox reactions. We use copper--especially copper metal--in many ways. Copper salts are easily reduced to copper metal, and copper is one of the few metals that occur in large quantities in nature. (The occasional availability of large lumps of native copper contributed to the charming sociopathology of potlatch, highly refined among the Kwakiutl of the Pacific Northwest.) Pure copper is suitable for working into ornamental objects but too soft to use for serious mischief. That deficiency was fixed by alloying copper with tin to give bronze, an excellent alloy for spear tips and swords, and an early example of the willingness of our species to exercise high levels of technological creativity in the service of weaponry. (Of course, bronze was also useful for kettles and spoons, but a spear in the room fixes the attention.)
Silver is valuable but too soft to make durable coins: Alloying it with a little copper gives sterling silver. Copper sheeting makes excellent roofs, splendid in their red-gold color when new, and soothing in the soft, mottled green of verdigris when they age. And copper wire! The world (at least the electrical world) is, in a sense, built of it. Copper is an excellent and ductile electrical conductor, easily drawn into the wire used in countless circuits--from the wiring of houses to the armatures of motors. Electroplating and vapor deposition have now also made it a part of microprocessors. It has appeared yet again in high-Tc superconductors. Who could measure the aggregated magnetic fields generated by electrons streaming through copper wire?
In the plays acted by the elements, copper is a bit player, but one with indispensable roles--as a component of materials, reagents, and catalysts; as a metal; and as a part of life.
And I still love it, but now I've gone back to gold.
|COPPER AT A GLANCE|
STUNNING Crystals of copper sulfate (grown by Elizabeth Tran), composed on copper foil, with a section of copper wire.
George M. Whitesides is Mallinckrodt Professor of Chemistry at Harvard University. He was a member of the MIT chemistry faculty from 1963 to 1982 and joined Harvard in 1982. At the beginning of his career, he worked on organocopper chemistry; now, among other things, he works on self-assembled monolayers (SAMs) on gold.