Synthesis and electronic structure of bis(imino)pyridine iron metallacyclic intermediates in iron-catalyzed cyclization reactions

Jordan M. Hoyt, Kevin T. Sylvester, Scott P. Semproni, Paul J. Chirik

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Abstract

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) (SFe = 3/2) antiferromagnetically coupled to a chelate radical anion (SPDI = 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.

Original languageEnglish (US)
Pages (from-to)4862-4877
Number of pages16
JournalJournal of the American Chemical Society
Volume135
Issue number12
DOIs
StatePublished - Mar 27 2013

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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