N,N,C Heteroscorpionates as a Platform for Monovalent Ni Chemistry

We report herein the synthesis and characterization of a number of monovalent Ni complexes bound by a class of N,N,C heteroscorpionate ligands and their derivatives. One-electron reduction of Ni²⁺ complexes supported by less bulky heteroscorpionates led to a thermally unstable Ni⁺ species that could be trapped as the corresponding PPh₃ adduct. In situ reaction of the aforementioned Ni⁺ complex with a bulky aryl azide affords a transient Ni³⁺ imide, for which attack of the C donor at the N_imido atom, followed by rapid isomerization, furnishes a three-coordinate Ni⁺ complex ligated by a new type of “expanded” N,N,C chelate. An N,N,C heteroscorpionate ligand with large Ph₂(CH₃)C– substituents was able to support a thermally stable, isolable Ni⁺ complex, in which a phenyl group of one Ph₂(CH₃)C– unit binds Ni in an η² fashion. This complex proved to be relatively unreactive toward organic azides. The labile C donor in the former Ni⁺ complex could be trapped via the addition of B(C₆F₅)₃, affording a zwitterionic Ni⁺ complex. All Ni⁺ species were characterized by electron paramagnetic resonance (EPR) spectroscopy, which was corroborated by density functional theory (DFT) calculations. Our results are expected to guide future efforts by ourselves and others in the pursuit of low-valent Ni complexes and Ni imides in general.