'Tumbling' Atoms May Help Explain Hydrogen Re-Forming Reactions

"Re-forming reactions all involve making and breaking carbon-hydrogen bonds," said U. of I. chemistry professor Gregory Girolami. "But the way hydrocarbons -- such as methane -- bond to metal centers is poorly understood. Part of the problem is that hydrocarbons are very unreactive molecules."

There are a few cases of metal centers that can selectively break a carbon-hydrogen bond, Girolami said. First, the metal "inserts" into a carbon-hydrogen bond and breaks the hydrocarbon into two pieces: one hydrogen atom and the rest of the molecule. Then the carbon atom bonds directly to the metal center.

"Currently, we don't really know how methane bonds to a metal center," Girolami said. "Is it through one hydrogen atom? Two? Three? No one has been able to stabilize a complex containing a methane molecule and investigate it in solution by spectroscopic methods that can distinguish among the various possibilities."

To study re-forming reactions in greater detail, Girolami and graduate student Christopher Gross synthesized a metal-containing complex in which one of the coordinating groups to the metal center is a hydrogen atom and another coordinating group is a methyl group. (These two groups can be viewed as a broken methane molecule.) For their metal center, the researchers chose the rare and unusual element osmium. One of the hardest metals, osmium forms extremely strong bonds.

As the reaction between the hydrogen atom and the methyl group took place, Girolami and Gross used nuclear magnetic resonance spectroscopy to identify the types of hydrogen bonds that were present.

"At -120 degrees Centigrade we saw one resonance for the hydrogen atom bound to the metal center and another resonance, three times as large, for the hydrogens bound to the carbon," Girolami said. "However, at -100 degrees Centigrade the signals broadened and began to coalesce into a single peak with four times the intensity. This indicated that the hydrogen atom was swapping places with some of the hydrogens on the methyl group."

The researchers interpret their observations to mean that a bond is re-forming between the carbon of the methyl group and the hydrogen on the osmium metal center.

"The methane structure appears to be rearranging," Girolami said. "The hydrogens are tumbling around, exchanging sites with one another. Then the molecule breaks apart again, perhaps with a different hydrogen connected to the metal center."

Girolami and Gross described their experimental results in the July 8 issue of the Journal of the American Chemical Society.