Oxidation states — describing the number of electrons an atom loses or gains when it joins [IrO4]+with other atoms in chemical compounds — have fascinated Gary Schrobilgen since his graduate school days in the 1970s, when he managed to make the first bromine VII cation, [BrF6]+. Today, as a chemistry professor at McMaster University, Schrobilgen is setting the bar higher, working with an international team to achieve the first compound containing an element with a formal oxidation state of IX. This new record can be explained by relativistic effects, which were first outlined by Albert Einstein.

An international team is rewriting the limits of oxidation, inspired by Albert Einstein’s theory of relativity.

An international team is rewriting the limits of oxidation, inspired by Albert Einstein’s theory of relativity. Photo Credit: Gary Schobilgen and Sebastian Riedel

“I’m interested in developing new oxidizers with the idea in mind of using those to find other oxidizers,” Schrobilgen says. “We wanted to see just how far we can go in terms of oxidation number.”

This accomplishment, which was carried out on the extremely corrosion-resistant metal iridium, was published in Nature. Schrobilgen and his graduate student, James Goettel, collaborated with colleagues Sebastian Riedel in Germany and Mingfei Zhou in China to study [IrO4]+. Mingfei’s group generated iridium oxide cations with pulsed-laser vaporization of iridium in the presence of a mixture of helium or argon with oxygen. The reaction products were then characterized by infrared photodissociation spectroscopy and mass spectrometry. Riedel’s group computationally modelled iridium oxide cations and their expected infrared spectra and energy levels. The combined results clearly show that [IrO4]+ exists and is a record-breaking species in terms of oxidation state.

Schrobilgen’s lab at McMaster examines other aspects of this kind of chemistry, such as xenon (VIII), which had represented the highest oxidation state for a main-group element. “This field is 50 years old but we still know very little about this chemistry,” he says. “Goettel has made considerable strides in advancing Xe (VIII) chemistry.” 

As for iridium, Schrobilgen is looking forward to reaching another milestone. The use of weakly coordinating anions that are highly resistant to oxidation could stabilize a salt of [IrO4]+ in the solid state. The Nature paper outlines the initial attempt to carry out this step, but it has yet to be achieved, says Schrobilgen. “[IrO4]+ has been produced in the gas phase but the real question is, ‘can you make it in macroscopic amounts?’ ”