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Inorganic Chemistry

Reactive calcium complex captures dinitrogen

Surprise reaction suggests chemists should rethink the periodic table’s s-block

by Bethany Halford
March 11, 2021 | A version of this story appeared in Volume 99, Issue 9

A crystal structure of the calcium-nitrogen complex.
Credit: Adapted from Science
A crystal structure of the calcium-dinitrogen complex with hydrogen atoms omitted for clarity

Calcium—it’s one of those elements that chemists think they know. In its most common form, calcium(II), it builds bones. It’s also one of Earth’s most abundant elements, showing up in seashells and limestone. As calcium(0) it’s a reactive metal eager to give up 2 electrons. But calcium, it seems, can still surprise. Chemists in Germany discovered that elusive calcium(I) can react with dinitrogen, a molecule that’s typically considered inert and requires transition metal catalysts along with high temperatures or pressures to react.

While there is a report of a calcium(I) complex in the literature, Sjoerd Harder, an inorganic chemist at Friedrich Alexander University of Erlangen-Nürnberg, says he has some doubts about it. So Harder and his team, graduate students Bastian Rösch and Thomas Gentner, tried to make their own calcium(I) complex using a bulky β-diketiminate ligand to stabilize the ion. But they kept running into problems: the calcium reacted with the aromatic solvents they used—benzene, toluene, and para-xylene. So the chemists turned to less reactive alkane solvents, and when they added tetrahydrofuran or tetrahydropyran to the reaction, reddish-brown crystals immediately formed. Analysis of those crystals revealed that two of the ligand-wrapped calciums had captured the N2 that was used as an inert atmosphere for the reaction, along with the added tetrahydrofuran or tetrahydropyran (Science 2021, DOI: 10.1126/science.abf2374).

“This was a big surprise,” Harder says. That an s-block metal like calcium, that typically just gives up its electrons, can activate N2 at temperatures as low as –60 °C was something he never anticipated would be possible. Harder reached out to Gernot Frenking, a theoretician at Philipps University of Marburg, to get some insight into what was going on.

Frenking and colleagues’ calculations showed that calcium’s d-orbitals bond with N2, suggesting that calcium can use its d-orbitals just like transition metals do. “I’m still amazed about it,” Frenking says. “A door has been opened on looking on this type of compounds in a different way,” and the results suggest that calcium, along with its heavier cousins strontium and barium, should be included in the transition metal section of the periodic table, he says.

The discovery will change how chemists view earth-abundant elements on the left side of the periodic table, known as the s-block, says McGill University’s Marc-André Légaré, who works on making catalysts from s-block elements. “This is a clear demonstration that calcium—and likely the other alkaline earths like magnesium—have a lot more to offer than the more ionic chemistry that we are used to seeing,” he says in an email. “It shows that calcium can use its orbitals, and not only its charge, to accomplish chemistry that is difficult even for the d-block metals that are usually used for catalysis.”

Robert J. Gilliard Jr., who studies main group chemistry at the University of Virginia, calls the result ground breaking. “It provides insight into what is yet to come for low-valent, molecular s-block chemistry,” he says in an email. He says that with innovation in ligand design and stabilization strategies, it might be possible to develop additional novel chemistry for s-block elements.

Harder says that because this calcium chemistry required potassium metal to reduce calcium(II) to calcium(I), it’s too impractical and expensive to replace the Haber-Bosch process, which uses iron or other transition metal catalysts to nab N2 from air to produce ammonia for fertilizer and other nitrogen-containing products. But, he says, perhaps an electrochemical route could be devised to make the calcium(I) complex.

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