Synthesis and electronic structure determination of N-alkyl-substituted bis(imino)pyridine iron imides exhibiting spin crossover behavior

Amanda C. Bowman, Carsten Milsmann, Eckhard Bill, Zoë R. Turner, Emil Lobkovsky, Serena Debeer, Karl Wieghardt, Paul J. Chirik

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Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, ( iPrPDI)FeNR ( iPrPDI = 2,6-(2,6- iPr 2-C 6H 3-N=CMe) 2C 5H 3N; R = 1-adamantyl ( 1Ad), cyclooctyl ( CyOct), and 2-adamantyl ( 2Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, ( iPrPDI)Fe(N 2) 2. SQUID magnetic measurements on the isomeric iron imides, ( iPrPDI)FeN 1Ad and ( iPrPDI)FeN 2Ad, established spin crossover behavior with the latter example having a more complete spin transition in the experimentally accessible temperature range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compounds with relatively short Fe-N imide bond lengths and two-electron reduction of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mössbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For ( iPrPDI)FeN 2Ad, Mössbauer spectroscopy also supports spin crossover behavior and allowed extraction of thermodynamic parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (S imide = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (S PDI = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (S Fe = 3/2) accounts for the S = 1 state observed at higher temperatures. Other possibilities based on the computational and experimental data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.

Original languageEnglish (US)
Pages (from-to)17353-17369
Number of pages17
JournalJournal of the American Chemical Society
Issue number43
StatePublished - Nov 2 2011

All Science Journal Classification (ASJC) codes

  • General Chemistry
  • Biochemistry
  • Catalysis
  • Colloid and Surface Chemistry


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