The structural dynamics of molecular catalysts can also have a profound effect on catalytic performance. Dynamics has been used as a design feature to modulate chemoselectivity and enantioselectivity, to enhance electron transfer, to determine enantiomeric excess, and even to turn the activity of the catalyst on or off. However, predicting and controlling the effects that ligand dynamic processes can have on reactivity is challenging when designing new catalytic systems, whether synthetic molecular systems or within a protein framework.

In this study, the importance of controlling structural dynamics in catalysis is demonstrated through the modulation of catalytic rates over three orders of magnitude for molecular Ni electrocatalysts for H₂ production. This rate enhancement is achieved by controlling the structural dynamics involved in key proton-transfer steps.

—Cardenas et al.

The team at PNNL has been developing a nickel-based catalyst modeled on an enzyme from nature called a hydrogenase for several years. Back in 2011, working in the Center for Molecular Electrocatalysis, a DOE Energy Frontier Research Center, they made a synthetic catalyst that was 10 times faster than the natural one. That natural one clocked in at 100,000 hydrogen molecules per second. (Earlier post.)

As they worked on the catalyst development, the scientists would test their catalysts in reactions by combining the catalyst and acids in different media. One thing they noticed was that the synthetic catalyst produced hydrogen faster in a viscous liquid as opposed to a free-flowing liquid.

We used this medium that was like pancake syrup and saw very fast rates. The catalyst has arms that move around to position the pieces of the chemical reaction. Normally they are flopping around like crazy and the pieces don’t always hit the right target. When this happens, the arms can actually get stuck in a position where the catalyst can’t put the pieces together at all. We thought that this thick syrup might be slowing down the flopping, letting the arms put the pieces together more efficiently.

—Molly O’Hagan, corresponding author

To test this hypothesis, the team designed the catalyst to have longer arms that would drag and slow down the flopping. They tested different arm lengths and found the longer the arms, the faster the catalyst produced hydrogen molecules.

They also measured how fast the arms were swinging around. The longer the arms, the slower the movement, allowing them to attribute the faster hydrogen production to the slower arm movements.

By including control of ligand structural dynamics as a design feature of molecular catalysts, the rates of H₂ production by the [Ni(P^Ph₂N^C6H4R₂)₂]²⁺ system are enhanced by three orders of magnitude without a significant loss in energy efficiency. The remarkable rate enhancements observed in the [Ni(P^Ph₂N^C6H4R₂)₂]²⁺ system as a function of controlled structural dynamics illustrate how this new design parameter can be used to obtain catalytic performance that rivals that of enzymes.

—Cardenas et al.

Some computational work for this research was performed in EMSL, the Environmental Molecular Sciences Laboratory a DOE Office of Science User Facility at PNNL. The study was supported by the Department of Energy Office of Science.

Resources

• Allan Jay P. Cardenas, Bojana Ginovska, Neeraj Kumar, Jianbo Hou, Simone Raugei, Monte L. Helm, Aaron M. Appel, R. Morris Bullock, and Molly O’Hagan (2017) “Controlling Proton Delivery through Catalyst Structural Dynamics,” Angew. Chem. Int. Ed. doi: 10.1002/anie.201607460