Synthesis and Electronic Structure of Bis(imino)pyridine Iron Metallacyclic Intermediates in Iron-Catalyzed Cyclization Reactions

The bis­(imino)­pyridine iron dinitrogen compound, (^iPr(TB)PDI)­Fe­(N₂)₂ (^iPr(TB)PDI = 2,6-(2,6-ⁱPr₂-C₆H₃-NC-(CH₂)₃)₂(C₅H₁N)) is an effective precatalyst for the [2π + 2π] cycloaddition of diallyl amines as well as the hydrogenative cyclization of N-tosylated enynes and diynes. Addition of stoichiometric quantities of amino-substituted enyne and diyne substrates to (^iPr(TB)PDI)­Fe­(N₂)₂ resulted in isolation of catalytically competent bis­(imino)­pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and Mössbauer spectroscopic and computational studies established <i>S</i> = 1 iron compounds that are best described as intermediate-spin iron­(III) (<i>S</i>_Fe = 3/2) antiferromagnetically coupled to a chelate radical anion (<i>S</i>_PDI = 1/2). Catalytically competent bis­(imino)­pyridine iron diene and metallacycles relevant to the [2π + 2π] cycloaddition were also isolated and structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe­(I)–Fe­(III) catalytic cycle where the bis­(imino)­pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active bis­(imino)­pyridine chelate for enabling catalytic cyclization chemistry with iron

Oxidative Addition of Carbon–Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex

Addition of biphenylene to the bis­(imino)­pyridine iron dinitrogen complexes, (^iPrPDI)­Fe­(N₂)₂ and [(^MePDI)­Fe­(N₂)]₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R₂C₆H₃NCMe)₂C₅H₃N; R = Me, ⁱPr), resulted in oxidative addition of a CC bond at ambient temperature to yield the corresponding iron biphenyl compounds, (^RPDI)­Fe­(biphenyl). The molecular structures of the resulting bis­(imino)­pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis­(imino)­pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis­(imino)­pyridine ligand

Oxidative Addition of Carbon–Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex

Addition of biphenylene to the bis­(imino)­pyridine iron dinitrogen complexes, (^iPrPDI)­Fe­(N₂)₂ and [(^MePDI)­Fe­(N₂)]₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R₂C₆H₃NCMe)₂C₅H₃N; R = Me, ⁱPr), resulted in oxidative addition of a CC bond at ambient temperature to yield the corresponding iron biphenyl compounds, (^RPDI)­Fe­(biphenyl). The molecular structures of the resulting bis­(imino)­pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis­(imino)­pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis­(imino)­pyridine ligand