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

The bis(imino)pyridine iron dinitrogen compound, ((PDI)-P-iPr(TB))Fe(N-2)(2) ((iPr(TB)) PDI = 2,6-(2,6-Pr-i(2)-C6H3- N=C center dot(CH2)(3))(2)(C5H1N)) is an effective precatalyst for the [2 pi + 2 pi] 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 ((PDI)-P-iPr(TB))Fe(N-2)(2) resulted in isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and Mossbauer 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 [2 pi + 2 pi] 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.