Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen

Kiyoshi Fujisawa; Taisei Kataoka; Kohei Terashima; Haruka Kurihara; Felipe de Santis Gonçalves; Nicolai Lehnert

doi:10.3390/molecules28176206

Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen

Molecules. 2023 Aug 23;28(17):6206. doi: 10.3390/molecules28176206.

Authors

Kiyoshi Fujisawa¹, Taisei Kataoka¹, Kohei Terashima¹, Haruka Kurihara¹, Felipe de Santis Gonçalves², Nicolai Lehnert²

Affiliations

¹ Department of Chemistry, Ibaraki University, Mito 310-8512, Ibaraki, Japan.
² Department of Chemistry and Department of Biophysics, University of Michigan, 930 N. University Avenue, Ann Arbor, MI 48109, USA.

Abstract

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a "noninnocent" or "redox active" ligand to transition metal ions to give metal-NO (M-NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M-NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni-NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3^- denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated ³NO^-. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni-NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound ³NO^-, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O₂ yielded [Ni(κ²-O₂N)(L1″)₂](I₃), with the oxidation of both ³NO^- and the I^- ion to yield NO₂^- and I₃^-. Both complexes were characterized by X-ray crystallography, IR, and UV-Vis spectroscopy and theoretical calculations.

Keywords: N2 ligand; crystal structure; nickel; nitrosyl; noninnocent ligand; oxidation reaction.

Grants and funding

This research was funded by an Ibaraki University Priority Research Grant and the Joint Usage/Research Center for Catalysis (Proposal 22DS0143 and 23DS0198 to KF). We further acknowledge support from the National Science Foundation (CHE-2002885 to NL). This study was funded in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001.