Metal Complex Acts Like Enzyme

In one of the latest developments in synthetic catalysis, Seiji Ogo of Kyushu University, in Fukuoka, Japan, and coworkers report the first catalytically active nickel-based complex with a structure that effectively replicates key aspects of the active site in hydrogenases (Science 2007, 316, 585). Hydrogenases are bacterial enzymes that serve as models for developing new catalysts that one day could be the workhorses for producing hydrogen as a fuel or for hydrogenating carbon dioxide into hydrocarbons (see page 11).

Hydrogenases catalyze cleavage of H₂ into two protons and two electrons. There are two major classes of the enzymes, one with a diiron active site and one with a nickel-iron active site. Chemists have successfully prepared catalytically active mimics of the diiron type, but a catalytically active mimic of the Ni–Fe active site—which contains the distinctive feature of a hydride ligand bridging the two metal centers—has not been reported until now.

One of the difficulties has been introducing the hydride ligand into the catalyst structure, writes Thomas B. Rauchfuss of the University of Illinois, Urbana-Champaign, in a Science commentary. "More faithful structural replicas of the Ni–Fe active site are known, but they are not catalytically active, in part because they lack the hydride ligand," he notes.

Ogo and coworkers created their successful hydrogenase mimic by combining nickel and ruthenium building blocks. The ruthenium fragment serves as a stand-in for the iron building block used by other researchers. The resulting Ni–Ru complex is unique, Rauchfuss points out, because unlike Ni–Fe complexes, it undergoes the crucial reaction with H₂ to give a bridging hydride.

"The greatest surprise from the results of Ogo and coworkers is that their compound is catalytically active: It catalyzes the hydrogenation of benzaldehyde to the corresponding alcohol," Rauchfuss writes, citing unpublished results from Ogo's lab. The next step to more fully replicate nature's work, he adds, is to prepare the active complex with iron instead of ruthenium, a goal that Rauchfuss' group is working on. The Ni–Ru catalyst shows that "such a feat should be achievable and that such advanced models can deliver new kinds of catalysts," Rauchfuss concludes.