The electronic sphere of influence in transition metals is bigger than chemists thought. On Wednesday at the ACS Fall 2022 meeting in the Division of Inorganic Chemistry, postdoctoral researcher Chris S.G. Seo of the University of Michigan described how functional groups not bonded directly to the metal center of a nickel imido complex can completely change Ni’s selectivity in a C–H activation reaction (J. Am. Chem. Soc. 2022, DOI: 10.1021/jacs.2c06662). Attaching a boron group to the imido nitrogen makes the late transition metal act like an early transition metal, and selectively attack the strongest C–H bond in an arene. This gives deeper understanding to how the reactivities of metal complexes work and is a new way to control where chemists can add new groups to carbon atoms. Such reactions are important for making drugs and many other organic compounds.
“It’s a completely unique way to tune the reactivity of one metal into another,” said organometallic chemist Nathaniel Szymczak, who leads the group at the university. Scientists have long used transition metal imido complexes to do C–H activation reactions, but the character of the complex depends on where its metal sits on the periodic table, Szymczak said. Electron-starved early transition metals, closer to the left of the periodic table, attack an arene’s strongest C–H bonds—reactivity that organic chemists find most useful. Late transition metals, bloated with electrons, reach for the easier-to-access weak C–H bonds. This led to chemists using early transition metals for some types of reactions and late metals for other types. However, early metals often react with oxygen and various functional groups on arenes, making complexes of such metals more finicky to work with.
The trick to making the late metal Ni act more like an early metal was adding a Lewis acid group that pulls electrons toward itself—not directly to the Ni atom, but to the imido bound to it. Akin to being influenced by the friend of a friend, the B group drags electron density through the N and away from the Ni center. As a result, the Ni becomes more electron-hungry and attacks the strongest C–H bond of a nearby molecule instead of the weakest C–H bond, as Ni imido complexes normally do. This means that chemists can potentially use one metal to do both types of C–H activation reactions, Szymczak said. “It helps to bridge that gap a little bit, and enables chemists to do reactions that are a little bit more functional-group compatible,” he said.
Daniel Mindiola, an organometallic chemist at the University of Pennsylvania, praised the work. Szymczak and team blended the use of an added Lewis acid with computational and mechanistic studies to gain understanding of the C–H activation step and how such transition states can be electronically tuned. This work opens the gate to figuring out catalytic reactions for directly adding amine groups to arenes using late transition metals such as Ni, he said.
Szymczak said that the work is exciting for synthetic chemistry. “Being able to traverse both types of reactions with a single system is really powerful,” he said.
This article was updated on Aug. 29, 2022, to correct the structure label. The structure is the product of the nickel imido C–H activation reaction, not the nickel imido complex itself.