Molecules and materials can reduce the energy required to perform a chemical reaction; this is known as catalysis. Many catalysts use metals to stabilize key intermediates during the catalytic cycle. To tune the reactivity of a metal catalyst, chemists usually modify the organic environment of the active metal. The organic “ligand” can change the electronic properties of the metal and therefore tune the catalytic properties. Taking an unconventional approach, scientists at the Inorganometallic Catalyst Design Center (ICDC) have increased the effectiveness of a nitrogen-functionalizing catalyst by using a second metal center to tune the catalytic properties of the catalyst.
The scientists synthesized a novel bimetallic catalyst containing a cobalt-cobalt bond that shows an increased efficiency in catalytic silyation of nitrogen, the formation of a silicon-nitrogen bond, when compared to its monocobalt cousins. In addition, they used quantum chemical calculations to characterize its electronic structure and related this information to the mechanism by which it activates dinitrogen. Capable of turning over 200 times in one catalytic cycle (320 in two cycles), this bimetallic catalyst is one of the most active nitrogen silyation catalysts reported to date. These researchers have shown both experimentally and computationally that the metal-metal bond not only affects the catalyst’s reactivity but is crucial for its success.
Reduced nitrogen compounds are highly valued for their agricultural applications, but nitrogen is thermodynamically and kinetically inert, making its conversion to ammonia energetically demanding. Synthetic nitrogen activation was first established by Haber and Bosch during World War I. Their process of using immense pressure and high temperatures to convert nitrogen to ammonia is still used today. The process is expensive, consuming roughly 1 to 2 percent of the world’s energy usage. A catalyst that can efficiently reduce nitrogen to useful products at mild conditions is therefore highly sought after.
The catalyst works by first binding nitrogen; this facilitates an attack by a highly active silicon molecule. Following three rounds of silicon-nitrogen bond formation, the functionalized nitrogen is released and the catalyst regenerated. The cycle yields a nitrogen silicon compound that can easily be converted to ammonium in the presence of acid. The cobalt-cobalt interaction is a key functional feature of this unconventional, bimetallic catalyst.
Quantum chemical calculations of the catalytic process show the effects of catalysis on the cobalt-cobalt interaction. First, upon nitrogen binding to the catalyst, the cobalt-cobalt bond weakens. Following silyation, it breaks. Finally, as the activated nitrogen compound is released, it reforms. The fluctuations of the cobalt-cobalt bond suggest the importance of the cobalt-cobalt interaction for catalytic function.
Tuning catalysts via the supporting metal can be a game changer. "Creative ligand designs will continue to advance our understanding of metal-metal bonded complexes and innovate their uses in catalysis," said Connie Lu, the co-principal investigator of the research.
The group at ICDC has also recently published research demonstrating the effects of using different metals to tune the catalyst. Future work will investigate the possibility of carbon dioxide reduction and the continued improvement of nitrogen activation. These metal-metal catalysts are still in their infancy but have great potential. Along with dramatically changing the way we think about catalysis, their application may be key to a green, carbon neutral economy.
