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

Abstract

Metalation of the deprotonated dipyrrin (^AdFL)Li with NiCl₂(py)₂ afforded the divalent Ni product (^AdFL)NiCl(py)₂ (<b>1</b>) (^AdFL: 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 <b>1</b>, affording the monovalent Ni synthon (^AdFL)Ni(py) (<b>2</b>) and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to <b>2</b> afforded the oxidized Ni complexes (^AdFL)Ni(NMes) (<b>3</b>) and (^AdFL)Ni(NAd) (<b>4</b>), respectively. Both <b>3</b> and <b>4</b> 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 (<b>3</b>: 8333.2 eV; <b>4</b>: 8333.4 eV) than Ni^I or unambiguous Ni^II analogues. N K-edge X-ray absorption spectroscopy performed on <b>3</b> and <b>4</b> reveals a common low-energy absorption present only for <b>3</b> and <b>4</b> (395.4 eV) that was assigned <i>via</i> 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 (<i>i.e.</i>, NR^2-) to iminyl (<i>i.e.</i>, ²NR^-) formulations, the complexes are best described as Ni^II-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration (<i>S</i> = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex <b>4</b> readily consumes 1,4-cyclohexadiene (CHD) <i>via</i> H-atom abstraction to afford the amide (^AdFL)Ni(NHAd) (<b>5</b>), whereas no reaction was observed upon treatment of the mesityl variant <b>3</b> with excess amount of CHD over 3 hours. Toluene can be functionalized by <b>4</b> at room temperature, exclusively affording the <i>N</i>-1-adamantyl-benzylidene (<b>6</b>). Slow addition of the organoazide substrate (4-azidobutyl)benzene (<b>7</b>) with <b>2</b> exclusively forms 4-phenylbutanenitrile (<b>8</b>) as opposed to an intramolecular cyclized pyrrolidine, resulting from facile β-H elimination outcompeting H-atom abstraction from the benzylic position, followed by rapid H₂-elimination from the intermediate Ni hydride ketimide intermediate.