C−CN Bond Activation of Aromatic Nitriles and Fluxionality of the η²-Arene Intermediates: Experimental and Theoretical Investigations

[Ni(dippe)H]2 has been reacted with a variety of aromatic nitriles. Both experimental and DFT calculation results have shown that an η2-arene complex with nickel coordinated to the CC double bond next to the cyano substituent is the crucial intermediate leading to C−CN bond activation. Furthermore, the fluxional processes of the η2-arene species were investigated by low-temperature experiments as well as computational methods. In the case of dicyanobenzenes, a mechanism similar to that found for PhCN was found with the Ni(dippe) fragment rotating as it migrated around the phenyl ring through a series of η3-allyl-like transition states. For polycyclic aromatic nitriles, only certain η2-arenes were stable enough to contribute to the fluxional process, and nickel migrates via an η4-coordinated transition state. The transition states connecting the η2-nitrile complex to the η2-arene intermediate and the η2-arene intermediate to the C−CN bond activation products are at much higher energies compared to those for migration around the ring. In the reaction of 9-cyanoanthracene, the instability of the η2-arene precursor and the high-energy activation barrier resulted in the absence of the C−CN oxidative addition product. The complex with 9-cyanophenanthrene undergoes only C−CN cleavage upon photolysis.