Chromium-Nitrogen Complex Wins Bonding Triple Crown

In 1978, David N. Clark and Richard R. Schrock of Massachusetts Institute of Technology reported an unusual tungsten alkyl-alkylidene-alkylidyne complex. It was notable for being the first transition-metal compound containing single, double, and triple metal-ligand bonds in the same molecule. The “yl-ene-yne” complex was one of a string of complexes Schrock and his colleagues made that contributed to Schrock’s Nobel Prize-winning olefin metathesis research.

Fast-forward to 38 years later, and Evan P. Beaumier and Aaron L. Odom of Michigan State University and their colleagues are now reporting the first nitrogen analog of the yl-ene-yne complex, a chromium amido-imido-nitrido complex containing single, double, and triple metal-nitrogen bonds in the same molecule (Chem. Sci. 2016, DOI: 10.1039/c5sc04608d).

“Iconic molecules in metal-ligand multiple-bond chemistry come along infrequently, and with this chromium system, the Odom group gives us an important new milestone,” comments MIT inorganic chemist Christopher C. Cummins. Such linkages are important in metal complexes to enable diverse reactions, including olefin metathesis, C–H bond activation, and dinitrogen cleavage, Cummins says.

The team made the complex by using the strong base potassium hydride to deprotonate one amido linkage in a nitrido tris(amido) chromium complex, forming the imido ligand. Besides the unusual bonding, the new complex displays unusual reactivity, with electrophiles attacking both the imido and nitrido nitrogen atoms.

The chromium complex is part of a larger project by the Odom group to use nuclear magnetic resonance spectroscopy to define ligand-donor parameters for high-valent metals. Odom explains that these parameters can be used to develop new catalysts for olefin poly­merizations and other processes. But the research took an unplanned side trip when the team realized it had an opportunity to make the amido-imido-nitrido complex.

“I think scientists should be allowed to go exploring every now and then to see what they find, even if they can’t tell you why they decided to wander that direction,” Odom says. “In this case we were in the neighborhood, so we dropped by.”