Tuning N2 Lability in Ni(II) N2 Complexes Supported by Strongly Donating C,C,N Pincer Ligands

We report herein the N₂ chemistry of Ni bound by a new class of strongly σ-donating pincer ligands. Chan–Lam coupling between a bulky pyrazole and a 3,5-di-tBu or diadamantyl phenylboronic acid, followed by N-directed ortho-lithiation and iodination, furnishes the N-(2-iodophenyl)-pyrazoles utilized as proligands for this work. These iodoarenes underwent oxidative addition with Ni(COD)₂ to afford square planar, diamagnetic Ni²⁺–I complexes bound by the pyrazole and C_aryl donors, with an agostic interaction between Ni and the adjacent tBu/adamantyl substituent. Reaction of these two Ni species with the strong, hindered base NaN(Si(CH₃)₃)₂ resulted in deprotonation of the agostic C–H group with loss of NaI to afford the corresponding (C,C,N)Ni²⁺–N₂ complexes. The complex with tBu groups undergoes facile, reversible loss of N₂ under reduced N₂ pressure to afford a binuclear complex in which the Ni sites are bridged through the H₂C^– donor groups. For the adamantyl-substituted Ni²⁺–N₂ complex, in contrast, formation of such a binuclear species is repressed. These divergent outcomes have been corroborated by DFT calculations.