Synthesis, characterization and C-H amination reactivity of nickel iminyl complexes.

Metalation of the deprotonated dipyrrin (L)Li with NiCl(py) afforded the divalent Ni product (L)NiCl(py) () (L: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive synthon on which to explore oxidative group transfer, we used potassium graphite to reduce , affording the monovalent Ni synthon (L)Ni(py) () and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to afforded the oxidized Ni complexes (L)Ni(NMes) () and (L)Ni(NAd) (), respectively. Both and were characterized by multinuclear NMR, EPR, magnetometry, single-crystal X-ray crystallography, theoretical calculations, and X-ray absorption spectroscopies to provide a detailed electronic structure picture of the nitrenoid adducts. X-ray absorption near edge spectroscopy (XANES) on the Ni reveals higher energy Ni 1s → 3d transitions (: 8333.2 eV; : 8333.4 eV) than Ni or unambiguous Ni analogues. N K-edge X-ray absorption spectroscopy performed on and reveals a common low-energy absorption present only for and (395.4 eV) that was assigned TDDFT as an N 1s promotion into a predominantly N-localized, singly occupied orbital, akin to metal-supported iminyl complexes reported for iron. On the continuum of imido (, NR) to iminyl (, NR) formulations, the complexes are best described as Ni-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration ( = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex readily consumes 1,4-cyclohexadiene (CHD) H-atom abstraction to afford the amide (L)Ni(NHAd) (), whereas no reaction was observed upon treatment of the mesityl variant with excess amount of CHD over 3 hours. Toluene can be functionalized by at room temperature, exclusively affording the -1-adamantyl-benzylidene (). Slow addition of the organoazide substrate (4-azidobutyl)benzene () with exclusively forms 4-phenylbutanenitrile () as opposed to an intramolecular cyclized pyrrolidine, resulting from facile β-H elimination outcompeting H-atom abstraction from the benzylic position, followed by rapid H-elimination from the intermediate Ni hydride ketimide intermediate.