Intermolecular Arene C−H Activation by Nickel(II)

Intermolecular arene C-H activation mediated by a divalent nickel species under extremely mild conditions is described. The reactions of either Ni(COD)2 with H[N(o-C6H4PR2)2] (H[R-PNP]; R = iPr, Cy) in benzene at room temperature or [R-PNP]NiCl with LiBHEt3 in THF at -35 degrees C produced the corresponding [R-PNP]NiH in high yield. Addition of 1 equiv of B(C6F5)3 to a benzene solution of [R-PNP]NiH at room temperature led to the formation of a mixture that contains [R-PNP]NiPh and [R-PNP]Ni(C6F5), both of which are proposed to evolve from zwitterionic [R-PNP]Ni(mu-H)B(C6F5)3. In contrast, the reaction of [R-PNP]NiH with AlMe3 in benzene at room temperature afforded exclusively the corresponding [R-PNP]NiPh. Similar results were also observed for intermolecular toluene and m-xylene C-H activation by [R-PNP]NiH. A parallel study involving [R-PNP]NiMe (R = Ph, iPr, Cy) on the reactivity of intermolecular arene activation reveals the significance of pi basicity of Ni(II) in these molecules. The remarkable reactivity of inexpensive Ni(II) species established in this study is attractive, particularly from an economic viewpoint, as compared to the current alternatives of 4d and 5d metals.