Chemists have unexpectedly made two differently colored crystals -- one orange, the other blue -- from one chemical in the same flask¹（烧瓶） while studying a special kind of molecular² connection called an agostic bond. The discovery, reported in Angewandte Chemie International Edition on July 29, is providing new insights into important industrial chemical reactions such as those that occur while making plastics and fuels. "We were studying agostic bonds in a project to make liquid fuels like methanol（甲醇） from carbon dioxide to replace fuels we get from oil," said chemist Morris Bullock at the Department of Energy's Pacific Northwest National Laboratory. "We knew the molecule³ we were making would have an agostic bond, but we had no idea there'd be two flavors of these metal complexes."

 

While chemists have studied these bonds in chemicals in liquid form, no one had crystallized one chemical with multiple forms of its agostic bonds. And no one expected different forms to give rise to different colors.

 

Bonds come in many varieties in molecules⁴. They string atoms together, sometimes forming a trunk and branches of atoms like a tree. But the trunk and branches of chemicals often fold up into a more compact shape, requiring additional weaker bonds to hold the shape in place. An agostic bond is one of these additional bonds, a shape-holder. They occur between a metal and a distant carbon-hydrogen bond along some chain, folding the chain back to the metal and pinning it there.

 

First discovered in the 1980s, agostic bonds frequently occur in catalysts⁵ because catalysts usually contain metals. This work will help researchers get a better handle on some catalytic reactions found in common industrial processes such as making plastic or fuels.

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