Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me₂^MesCNC)Fe(N₂)₂ (3,5-Me₂^MesCNC = 2,6-(2,4,6-Me-C₆H₂-imidazol-2-ylidene)₂-3,5-Me₂-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-Me₂^MesCNC)Fe(Ph)(N₂)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d₂ upon oxidative addition to (3,5-Me₂^MesCNC)Fe(N₂)₂ are consistent with radical intermediates during C(sp³)-X bond cleavage. Importantly, while C(sp²)-X and C(sp³)-X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.