“We were inspired by the efficiency of enzymes,” explains Victor Mougel from ETH Zurich in Switzerland, who led the study. In nature, many different enzymes catalyze proton-electron transfers, the type of reactions that produce hydrides. “Specifically, an iron-iron hydrogenase that follows a concerted mechanism … transfers both a proton and an electron together … to create the metal hydride,” he says. And it works thanks to one of the most common catalytic cores in living organisms – the iron-sulfur cubic complex. Mougel’s group maintained this pattern as a mediator.
“For the first time … it is possible to electrochemically create a reactive metal hydride that tunes the properties of the iron-sulfur complex,” says Eva Nichols, an expert in bio-inspired electrocatalysis at the University of British Columbia in Canada. Metal hydrides are useful in reduction reactions. More familiar examples include sodium borohydride and lithium aluminum hydride. In this case, the iron-sulfur complex promotes the electrocatalytic formation of a manganese hydride. Nichols explains that “transition metal hydrides are particularly interesting” because researchers tune their properties by modifying the metal center, its oxidation state, or the organic substituents around it. ‘This [manganese] The hydride reduces the carbon dioxide to a valuable product: formate,” he says. The key, Mougel explains, was to mimic the mechanism. Traditionally, the production of hydrides requires the repeated reduction of the metal center and then the capture of a proton. Such stepwise redox reactions take place “through high-energy intermediates, [which need] either harsh acids or extreme negative potentials,” explains Nichols. In addition, they create very reactive metal intermediates, which bind to other products present, rather than picking up protons to produce hydrides. “The manganese complex we used, for example, reacts with carbon dioxide almost 20 times faster,” adds Mougel. This favors the formation of carbon monoxide, but prevents the production of hydrides. Among the many mediators tested, the iron-sulfur complex gave the best results in terms of efficiency and selectivity. “It’s good for a lot of reasons,” Nichols explains. It exhibits a suitable affinity – the perfect bond strength to hold the hydrogen atom and later transfer it to the manganese complex. “The iron-sulfur complex is also very rigid, which contributes to its growth [reaction] percentages,” he adds. It also reorganizes and recovers its original redox state efficiently, which enhances recyclability. “The iron-sulfur complex used as a mediator regenerates very quickly, which is probably why it has naturally evolved into enzymes,” says Mougel. “Compare to [other] catalysts, ours offers high turnover with very low potential,” he says. The process also has significant selectivity toward formic acid, “one of the most interesting chemicals derived from carbon dioxide,” according to Mougel. The paper also provides a protocol for the best selection of mediators and metal hydrides. “These rules were probably obvious to everyone in the field of electrocatalysis, but we could test them experimentally – and now they’ve been verified,” he says. Typically, electrocatalysts enable the formation of value-added products using energy from sustainable sources such as solar and wind. Additionally, ‘[this] The coordinated approach eases reliance on harsh, dangerous chemicals,” comments Nichols. “It’s an impressive inspiration… similar processes could [catalyse] many other reductive transformations in the field of electrocatalysis”.
