C–F activation of perfluorophenazine at nickel: selectivity and mechanistic investigations

Abstract

The reactivity of [Ni(cod)₂] towards perfluorophenazine in the presence of phosphines is reported. When PiPr₃ and PCy₃ are used, an initial κ-(N) coordination of the nickel centre to the nitrogen atom of the perfluorophenazine ring occurs, forming the dark blue complexes [Ni{κ-(N)-C₁₂N₂F₈}(PiPr₃)₂] (1) and [Ni{κ-(N)-C₁₂N₂F₈}(PCy₃)₂] (2). Complex 1 was structurally characterized by X-ray diffraction analysis. The complexes rearranged by regioselective C–F activation of the perfluorophenazine ring in the 2-position to yield complexes trans-[NiF(2-C₁₂N₂F₇)(PiPr₃)₂] (5) and trans-[NiF(2-C₁₂N₂F₇)(PCy₃)₂] (6). The structure of 6 was also determined by X-ray diffraction analysis. Kinetic measurements for the decrease of 1 at different temperatures reveal a first order reaction with ΔH^‡ = 19 ± 7 kcal mol⁻¹. Initially, small amounts of an intermediate, assigned as [Ni(η²-1,2-C₁₂N₂F₈)(PiPr₃)₂] (3), were observed, which exhibits a 1,2-η² coordination of the perfluorophenazine. DFT calculations on the same transformation were also computed, which suggest that both a phosphine-assisted mechanism and an oxidative addition can be operating reaction pathways. The 1,2-η² complex [Ni(η²-1,2-C₁₂N₂F₈)(PEt₃)₂] (4) was obtained when PEt₃ was used as ligand, and an unstable dark red complex trans-[NiF(2-C₁₂N₂F₇)(PEt₃)₂] (7) formed rapidly by C–F activation. The reactivity of the perfluorophenazine was compared with those of perfluorodibenzo-p-dioxin. In this case, no prior coordination was observed and the C–F activation took place in a less selective manner forming trans-[NiF(1-C₁₂O₂F₇)(PiPr₃)₂] (8) and trans-[NiF(2-C₁₂O₂F₇)(PiPr₃)₂] (9), outlining the role of the nitrogen for the selectivity of the process. Treatment of two equivalents of [Ni(cod)₂] and four equivalents of PiPr₃ with perfluorophenazine afforded a double C–F activation to give [{trans-(PiPr₃)₂NiF}₂(2,7-C₁₂N₂F₆)] (10).