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 Caryl 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.