MECHANICAL FORCE ACTIVATES CATALYST CATALYSIS: Ultrasound turns on silver- and ruthenium-based catalysts
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ECHANICAL FORCE now joins light, heat, and
chemicals on the short list of ways to activate a homogeneous catalyst. Chemists are intrigued by the possibilities of the new mechanical approach, which was developed by chemistry professor Rint P. Sijbesma and coworkers at Eindhoven University of Technology, in the Netherlands. “Coupling catalyst activity to a mechanical force is in principle completely orthogonal to the use of light, heat, or chemical activation,” says Stephen L. Craig, a chemist at Duke University. One can imagine systems of multiple catalysts where one catalyst can be controlled specifically by force while others are controlled by more conventional stimuli, he says. Sijbesma and postdocs Alessio Piermattei and Karthikeyan Sivasubramanian report two different metal-carbene catalysts that can be activated by mechanical force. They demonstrate that ultrasound-generated force triggers a silver-based catalyst to perform transesterification and a ruthenium-based catalyst to facilitate olefin metathesis (Nature Chem., DOI: 10.1038/nchem.167). “We have shown in two distinct systems that a catalyst could be ‘switched on’ when one of the carbene ligands was pulled off from an inactive precursor metal complex,” Sijbesma says. “In each case, the mechanical force was transferred to the metal complex through poly(tetrahydrofuran) polymer chains attached to the ligands,” he adds. They note that without polymers on the ligand, no ultrasound-induced activity is observed.
They also find that catalytic PERKED UP BY FORCE activity stops when ultrasound Silver- and ruthenium-based ceases because the catalyst decatalysts respond to mechanical grades over time, and dissociated force. ligands do not rebind to regenerate a latent catalyst. N N N N Ar R The mechanical force comes R Cl from the collapse of cavitation Ag+ Ru bubbles that form in solution Cl Ph during exposure to ultrasound. R Ar R N N Applying ultrasound to accelerN N ate reactions is not new, and researchers at the University of IlR = poly(tetrahydrofuran) Ar = aryl linois, Urbana-Champaign, have Ph = phenyl shown that mechanical energy can trigger stoichiometric ringopening reactions (C&EN, March 26, 2007, page 9). Although the yields of both of the small-molecule transformations achieved with these polymeric catalysts are no match for their respective nonmechanochemically activated parent catalysts, the team’s mechanochemically activated version of the ringopening methathesis polymerization (ROMP) reaction is promising, write ACTIVATED Jitendra S. Rathore and Alshakim Mechanical force breaks a Nelson of IBM Almaden Research metal-carbene bond in a latent Center, in San Jose, Calif., in a recatalyst, allowing it to catalyze a lated commentary. “ROMP reactions polymerization reaction. by a dormant catalyst hold significance in light of the recent interest in self-healing materials,” they add. Although such applications have a way to go to become reality, the possibilities are exciting, Craig says. “Chemists are just beginning to think about using mechanical forces as productive tools for bond-forming chemistry,” he says.—RACHEL PETKEWICH
CLIMATE CHANGE First round of UN negotiations on new accord end quietly The first round of United Nations negotiations on a new climate-change treaty that ended last week gave the international community its first opportunity to feel out the Obama Administration’s positions. The March 29–April 8 meeting in Bonn was the first of three negotiating sessions scheduled for this year. It served as a venue for governments around the world to toss about ideas ahead of tougher talks to come later this year and a December meeting to complete a new accord. The meeting yielded no spectacular
breakthroughs, but none were expected at this stage of the talks, Rosário Bento Pais, the European Commission’s deputy head of climate strategy, informed reporters. Jonathan Pershing, U.S. deputy special envoy for climate change, explained that the Obama Administration wants to blend science-based urgency to address mounting levels of greenhouse gases with a politically pragmatic approach. U.S. negotiators want the new treaty to be one that Congress will implement, he told reporters. Specifically, the U.S. representatives do not want to repeat what happened
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with the 1997 Kyoto protocol. The Senate rejected that agreement, which calls for industrialized countries to reduce their collective emissions to 5.2% below 1990 levels between 2008 and 2012. Later this month, governments will submit their ideas for the new treaty. These will be merged into a draft version of the accord, which will be the focus of the next round of talks on June 1–12 in Bonn. Negotiators are also scheduled to meet in Bangkok in September before culminating their work on the agreement in Copenhagen in December.— CHERYL HOGUE
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