Novel reactive nickel species sparks new aromatic chemistry
Transient nickel-nitrido complex is capable of C-H activation of arenes
Chemists at the Van 't Hoff Institute for Molecular Sciences (HIMS) have been able to generate a transient nickel nitrido species and show that this fleeting intermediate can activate C-H bonds or aromatic substrates. Their result, published earlier this month in Angewandte Chemie International Edition may lead to new chemistry related to the production of amines, which are relevant building blocks in organic synthesis. It also sheds light on new dinitrogen (N2) functionalization chemistry, which is of great biological and industrial importance.
HIMS PhD student Vincent Vreeken and his supervisor Dr Jarl Ivar van der Vlugt of the Homogeneous, Supramolecular and Bio-inspired Catalysis research group (part of the UvA Research Priority Area Sustainable Chemistry) are the first to report on aromatic C-H bond activation by a trapped transient nitrido species of a late transition metal.
In the periodic table of the elements the transition metals span from scandium to mercury. Although grouped together in the so-called 'd-block', there are strong differences between the individual metals. Most notably, there is a distinct change in reactivity between the early transition metals (group 4-8, such as titanium, chromium and iron) and late transition metals (group 9-12 such as cobalt, nickel and copper).
Transition metals are capable of forming multiple bonds with elements such as nitrogen, but the stability of these species decreases rapidly for metals beyond group 8. However, the anticipated chemistry of such species, which could ultimately even be useful for catalytic transformations, sparks much interest to study their preparation and reactivity in detail. The current research, which focussed on nickel as a group 10 metal and a redox-active ligand scaffold to stabilize the metal center, shows that generation of a plausible nickel-nitrogen triply bonded species results in formation of a highly reactive complex that undergoes hitherto unobserved follow-up reaction with a range of aromatic C-H bonds from external substrates. This finding holds promise for the establishment of new routes to activate and functionalize aromatic rings, which are omnipresent in modern-day chemistry.
This research was funded by the European Union through the ERC Starting Grant EuReCat awarded to Jarl Ivar van der Vlugt, aimed at developing novel ways to activate and functionalize small molecules using innovative ligand designs. The team has collaborated with crystallography experts from Baltimore and Utrecht University.
The UvA-HIMS chemists observed the new reactivity upon UV-irradiation of a well-characterized and bench-stable nickel-azido complex, making use of a combination of experimental, spectroscopic and theoretical techniques. This photo-triggered cascade of events initially generated a putative transient nitrido fragment, which is selectively trapped by the ligand scaffold. Subsequent reaction with the aromatic solvent resulted in selective C-H activation and provided a species that could be structurally characterized using single crystal X-ray diffraction. The proposed mechanism of this reaction sequence was investigated using DFT calculations. The thus-formed Ni complex can be further derivatized. The team is currently expanding its efforts in this direction as well as exploring the possibilities to selectively functionalize particular arene C-H bonds.
Vincent Vreeken, Maxime A. Siegler, Bas de Bruin, Joost N. H. Reek, Martin Lutz, and Jarl Ivar van der Vlugt: C-H Activation of Benzene by a Photoactivated NiII(azide): Formation of a Transient Nickel Nitrido Complex Angew. Chem. Int. Ed. 2015, 54, 7055–7059 DOI: 10.1002/anie.201501437