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 S = 1 iron compounds that are best described as intermediate-spin iron(III) (S_Fe = 3/2) antiferromagnetically coupled to a chelate radical anion (S_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.