Sandwiching ligands finally make iron(I) stable in air

A new sandwiched iron complex allows chemists to put iron(I) to work in catalytic reactions in the same way chemists use precious metals like palladium (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.6c01660). The researchers aim to make workhorse reactions greener and cheaper.

Over the past 50 years, expensive and endangered transition metals (PDF) like palladium and rhodium have become practically indispensable in catalyzing the industrial chemical reactions that build our medicines, pesticides, and plastics, says Oliver Townrow, a coauthor of the paper and a synthetic organometallic chemist at the Institute of Nanotechnology, Karlsruhe Institute of Technology.

Yet not all transition metals are endangered. Cheap and abundant iron actually can catalyze many of the same reactions as its precious metal cousins, especially once activated into its +1 oxidation state, Townrow says. The problem is, iron(I) is so reactive that it tends to burst into flame when it touches the oxygen in air.

Chemists can create iron(I) during a reaction by adding strong reducing agents to iron(II) or iron(III) compounds. But those reducing agents create other problems. They can react indiscriminately: while modifying the intended molecule, they may reduce off-target functional groups or a catalyst’s ligands. That high reactivity is impractical for industrial settings.

Townrow’s team set out to create a stable iron(I) complex. Iron(I) is primed to rid itself of an electron—the reason it doesn’t play nice with air. The iron(I) center in the new complex holds onto that electron a bit more tightly by sandwiching the iron between two simple aromatic ligands called durene (1,2,4,5-tetramethylbenzene). The iron(I) is then “kind of in a frustrated state,” shielded just enough to be air stable yet active enough to slip into new ligand centers as a precatalyst, Townrow says.

With this iron source, Townrow and his group found they can build iron(I) catalysts they’ve never seen before or make known ones without strong reducing agents. To show what the complex can do, the team used it to pull off a one-pot Kumada coupling, which is usually conducted with more-expensive palladium or nickel catalysts in high-throughput reactions. Townrow’s latest grant may make the iron even cheaper and more sustainable: he’s building a reactor to get his iron directly from scrap metal.