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Physcial Sciences Division
Research Highlights

October 2012

Searching for Molecular Radical's Secrets of Stability

Once thought inherently unstable, these oxidized metal complexes actually hold together

Synthesizing metalloradicals
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Synthesizing metalloradicals.

Results: For the first time, scientists synthesized and characterized a metal-containing complex long thought to be too unstable to study. This complex piqued the interest of scientists at Pacific Northwest National Laboratory because the metals inside this metalloradical are held together by a bond between two tungsten or molybdenum atoms. The lack of strings of atoms, called ligands, around the metals is unusual, and the instability of the molecules has made them impossible to observe. Until now.

"We were happy to find that the oxidized metal-metal bonded complex could be isolated.  The computational study of the molecular orbitals provides an explanation for the observation that the two metals are closer in the oxidized complex," said Dr. Morris Bullock, a PNNL scientist who worked on the study.

Why It Matters: Proteins are often based on metalloradicals. To be able to delve into the arrangement of atoms and how they behave could open doors in the development of biofuels as well as the creation of catalysts that mimic natural proteins in creating energy and fuels.

Methods: Unlike the radical molecules that scientists knew existed, but could not observe, the new version is stable enough to be isolated and characterized. The team conducted theoretical studies, including relativistic density functional theory calculations, and experimental studies on the complex. The team benefitted from a new capability that allowed them to integrate magnetic resonance measurements and electronic structure calculations to understand the structure and dynamics of complex systems.

In characterizing the complex, researchers made two key discoveries. First, the complex contains a shorter-than-expected metal-metal bond and, as desired, the metals are not supported by bridging ligands. Second, the complex's lone electron does not favor one metal over the other. Rather, it spends equal time around the two tungsten or molybdenum atoms. This research challenges the paradigm that metal-bridging ligands are vital to stabilizing dinuclear metalloradicals.

What's Next? The researchers are studying how removal of an electron (oxidation) affects the bonding and reactivity of metal hydrides and related complexes that can be used as molecular catalysts.


Sponsors: U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division

User Facility: EMSL

Research Team: Edwin F. van der Eide, Ping Yang, Eric D. Walter, Tianbiao Liu, R. Morris Bullock, PNNL

Reference: van der Eide EF, P Yang, ED Walter, T Liu, and RM Bullock. 2012. "Dinuclear Metalloradicals Featuring Unsupported Metal-Metal Bonds."Angewandte Chemie International Edition 51(33):8361-8364. DOI:10.1002/anie.201203531.

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Defining Our Dinuclear Metalloradical

Dinuclear = it contains two clusters with different types of atoms

Metallo = those clusters are metals; specifically tungsten and molybdenum

Radical = it has lost an electron and is highly reactive.

That said, the new complex is unique because those metals are held together without strands of atoms, called ligands. This complex is also cationic, meaning it has a positive charge.