One chemical forms two colors of crystals, sheds insight on agostic bonds important in industrial catalysis

Aug 03, 2013
One flask of chemicals gives rise to either blue or orange crystals. Credit: van der Eide/PNNL

Chemists have unexpectedly made two differently colored crystals – one orange, the other blue – from one chemical in the same flask while studying a special kind of molecular 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 like methanol 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 molecule we were making would have an agostic bond, but we had no idea there'd be two flavors of these ."

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 molecules. 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 catalysts because catalysts usually contain metals. This work will help researchers get a better handle on some catalytic reactions found in common such as making plastic or fuels.

Heart of the Matter

The metal in a is usually the reactive heart of the molecule. Bullock and postdoctoral chemist Edwin van der Eide knew an agostic bond in their catalyst would help protect the reactive metal from working at the wrong time: The carbon-hydrogen bond blocks the reactive metal until conditions were right, which in turn would help the scientists better control the . So van der Eide set about producing and crystallizing catalysts that contain a metal atom—in this case, molybdenum.

In the lab, van der Eide's flask of chemicals held a molybdenum-containing molecule that turned the solution violet. He added another liquid to coax the molybdenum complex to crystallize, just as salt crystallizes from seawater to form flakes at the seashore. Some formed at the bottom of the flask and others formed near the top of the violet solution.

Oddly, the crystals were two different colors.

Orange crystals formed at the bottom of the flask and blue above. If van der Eide dissolved either the orange or blue crystals in a fresh flask of the original solvent, the violet color returned, with the same properties as the original violet solution. These results suggested that either molecule in the two colored solids could give rise to both structures in liquid, where they easily change back and forth.

The researchers examined the differently colored crystals to determine their structures. The molecule forms a shape like a piano stool: a ringed section forms a stool seat on top of the molybdenum atom, with multiple legs connecting to the molybdenum at the bottom.

One of the legs, however, is longer than the others and contains a chain of three carbon atoms, each with at least one protruding hydrogen. The team found that the long leg was involved in the agostic bonds, with the middle carbon atom involved in the orange crystals and an end carbon involved in the blue crystals.

PNNL's Ping Yang at EMSL, DOE's Environmental Molecular Sciences Laboratory on the PNNL campus, took to EMSL's supercomputer Chinook to perform theoretical calculations on the orange and blue structures. Chemically, the two structures were almost equally likely to form, with the blue one having a slight edge. The analysis also revealed why the crystals were different colors, which is due to subtleties within the structures.

Explore further: Bioengineers develop highly elastic biomaterial for better wound healing

More information: Edwin F. van der Eide, Ping Yang, and R. Morris Bullock. Isolation of Two Agostic Isomers of an Organometallic Cation: Different Structures and Colors, Angewandte Chemie July 29, 2013, doi:10.1002/anie.201305032

Related Stories

Converting Nitrogen to a More Useful Form

Jan 09, 2007

Nitrogen-containing organic compounds are important products as well as intermediates for many pharmaceuticals, agrochemicals, and chemicals used in electronics. Air contains plenty of nitrogen, but it is in a form that cannot ...

Watching catalysts at work—at the atomic scale

Jul 25, 2013

Scientists of Helmholtz-Zentrum Berlin (HZB) and collaborators have now combined the spectroscopic method "RIXS" with so-called ab initio theory in order to describe these processes in detail for a model ...

Recommended for you

Metal encapsulation optimizes chemical reactions

Jul 01, 2015

The chemical industry consumes millions of tons of packing materials as catalytic sup- port media or adsorbents in fixed-bed reactors and heat storage systems. Fraunhofer researchers have developed a means of encapsulating ...

Fuel and chemicals from steel plant exhaust gases

Jul 01, 2015

Carbon monoxide-rich exhaust gases from steel plants are only being reclaimed to a minor extent as power or heat. Fraunhofer researchers have developed a new recycling process for this materially unused carbon resource: They ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.