Issue 5, 2020

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


Metalation of the deprotonated dipyrrin (AdFL)Li with NiCl2(py)2 afforded the divalent Ni product (AdFL)NiCl(py)2 (1) (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 1, affording the monovalent Ni synthon (AdFL)Ni(py) (2) and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to 2 afforded the oxidized Ni complexes (AdFL)Ni(NMes) (3) and (AdFL)Ni(NAd) (4), respectively. Both 3 and 4 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 (3: 8333.2 eV; 4: 8333.4 eV) than NiI or unambiguous NiII analogues. N K-edge X-ray absorption spectroscopy performed on 3 and 4 reveals a common low-energy absorption present only for 3 and 4 (395.4 eV) that was assigned via 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.e., NR2−) to iminyl (i.e., 2NR) formulations, the complexes are best described as NiII-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration (S = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex 4 readily consumes 1,4-cyclohexadiene (CHD) via H-atom abstraction to afford the amide (AdFL)Ni(NHAd) (5), whereas no reaction was observed upon treatment of the mesityl variant 3 with excess amount of CHD over 3 hours. Toluene can be functionalized by 4 at room temperature, exclusively affording the N-1-adamantyl-benzylidene (6). Slow addition of the organoazide substrate (4-azidobutyl)benzene (7) with 2 exclusively forms 4-phenylbutanenitrile (8) 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.

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

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Article information

Article type
Edge Article
26 Sep 2019
06 Dec 2019
First published
11 Dec 2019
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2020,11, 1260-1268

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

Y. Dong, J. T. Lukens, R. M. Clarke, S. Zheng, K. M. Lancaster and T. A. Betley, Chem. Sci., 2020, 11, 1260 DOI: 10.1039/C9SC04879K

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