Chemists Discover New Molecules That Excel at Shedding Electrons

Anyone who’s taken chemistry might know that the element cesium is the hands-down champ when it comes to “ionizing,’ or giving up electrons.

And up until about six months ago, they’d be right.

But in recently announced work that was quickly heralded as one of the highlights in chemistry for 2002, chemists have developed a remarkable new class of stable molecules that ionize easier than anything on the periodic table, including cesium.

“The ease with which an atom gives up electrons is one of its most important and basic characteristics because that defines the atom’s chemistry and capabilities,” said University of Arizona chemistry Professor Dennis L. Lichtenberger. “Until now, we’ve been limited by the range of what atoms can do and what we can do with molecules.”

“At the simplest level, all chemistry, biochemistry and materials/optical science is based upon the movement of electrons from one molecule to another,” added Nadine E. Gruhn, director of the UA chemistry department’s unique Center for Gas-Phase Electron Spectroscopy. Given the properties of the new molecules, they have potential uses in materials science, optics, medicine, and other fields, she added.

The UA gas-phase electron spectroscopy facility, which Lichtenberger began to develop when he joined the UA in 1976, is probably the only lab in the world that could have made the measurements needed to predict how to make such molecules. The lab is unique in its ability to measure ionization energies of large, highly reactive molecules with extreme precision.

Lichtenberger and Gruhn collaborated in the discovery with distinguished Texas A & M University chemist F. Albert Cotton, who synthesized the new class of molecules last summer. They reported it in the Dec. 6 issue of Science. Other authors on the research paper include Jiande Gu of the Shanghai Institute of Materia Medica, China; UA graduate student Laura O. Van Dorn; and Texas A & M chemists Penglin Huang, Carlos Murillo and Chad Wilkinson.

Chemical and Engineering News cited the achievement as one of the year’s highlights in inorganic chemistry ten days after the research was published. Scientists in Russia, China, the United States and elsewhere sent their congratulations. Industries with proprietary interests began making discreet inquiries as to details.

“Now that we know how to make these molecules, it is not all that difficult to make them in quantities,” Lichtenberger said.

The new molecules are two metal atoms bound tightly together by four pairs of electrons, a very unusual class of molecules because they are simultaneously so stable yet 10 percent more efficient at donating electrons than is cesium.

“You can think of these metals as two atoms of a metal wire, surrounded by an organic insulator, atoms of nitrogen and carbon,” Lichtenberger said. “Normally, the organic insulator isn’t very interesting, as far as electronic properties go. But when these particular organic molecules surround these particular metals, they form both strong bonds and an additional interaction that results in electrons just waiting to jump off,” he said.

The researchers made the new molecules using chromium, molybdenum or tungsten as metals.

“The amazing thing in the beginning was finding any system that we could design that would get the electrons to stick there in the first place, when they are so ready to go anywhere else. It is something that a number of people didn’t really think was going to be possible,” Lichtenberger said.

Cotton described it as the same kind of problem chemists would have if they came up with the perfect solvent. If you could make a perfect solvent that would dissolve everything, what would you keep it in?

The new molecules are so stable “that you can have them in a bottle, sitting on your shelf, ” Lichtenberger said. “We have heated small test samples in the laboratory up to nearly 300 degrees Celsius (572 degrees Fahrenheit) and they’re still stable. It is just remarkable.”

Much of Lichtenberger’s research focuses on understanding the stability and behavior of molecules. He and Cotton have been collaborating in studies of these “dimetal” molecules since the early 1980s.

As the collaborators neared their goal ? Lichtenberger and Gruhn predicted the lowest-ever ionizing molecules last January ? Lichtenberger realized he was going to need time away from his administrative duties and concluded his eight years’ service as head of the UA chemistry department.

“With the computation tools we use, and in extrapolations from experiments we’ve already done, we can design new molecules with ever lower ionization energies,” he predicts.

Already funded by the National Science Foundation to build the next generation gas-phase electron spectrometer, the UA scientists are pursuing a new collaborative project with Cotton and others. One goal is to link the new molecules end-to-end to create molecular-scale, single atom wires that could be extremely useful conductors.

“We’ve progressed much faster than I thought we would when I started working on this,” Lichtenberger said. “It’s exciting times.”