Bis(imino)pyridine iron dinitrogen compounds revisited: Differences in electronic structure between four- and five-coordinate derivatives.

S. Chantal E. Stieber, Carsten Milsmann, Jordan M. Hoyt, Zoë R. Turner, Kenneth D. Finkelstein, Karl Wieghardt, Serena Debeer, Paul J. Chirik

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Abstract

The electronic structures of the four- and five-coordinate aryl-substituted bis(imino)pyridine iron dinitrogen complexes, ( iPrPDI)FeN 2 and ( iPrPDI)Fe(N 2) 2 ( iPrPDI = 2,6-(2,6- iPr 2-C 6H 3-N=CMe) 2C 5H 3N), have been investigated by a combination of spectroscopic techniques (NMR, Mössbauer, X-ray Absorption, and X-ray Emission) and DFT calculations. Homologation of the imine methyl backbone to ethyl or isopropyl groups resulted in the preparation of the new bis(imino)pyridine iron dinitrogen complexes, ( iPrRPDI) FeN 2 ( iPrRPDI = 2,6-(2,6- iPr 2-C 6H 3-N=CR) 2C 5H 3N; R = Et, iPr), that are exclusively four coordinate both in the solid state and in solution. The spectroscopic and computational data establish that the ( iPrRPDI)FeN 2 compounds are intermediate spin ferrous derivatives (S Fe = 1) antiferromagnetically coupled to bis(imino)pyridine triplet diradical dianions (S PDI = 1). While this ground state description is identical to that previously reported for ( iPrPDI)Fe(DMAP) (DMAP = 4-N,N-dimethylaminopyridine) and other four-coordinate iron compounds with principally σ-donating ligands, the d-orbital energetics determine the degree of coupling of the metal-chelate magnetic orbitals resulting in different NMR spectroscopic behavior. For ( iPrRPDI)Fe(DMAP) and related compounds, this coupling is strong and results in temperature independent paramagnetism where a triplet excited state mixes with the singlet ground state via spin orbit coupling. In the ( iPrRPDI)FeN 2 family, one of the iron singly occupied molecular orbitals (SOMOs) is essentially dz 2 in character resulting in poor overlap with the magnetic orbitals of the chelate, leading to thermal population of the triplet state and hence temperature dependent NMR behavior. The electronic structures of ( iPrRPDI)FeN 2 and ( iPrPDI)Fe(DMAP) differ from ( iPrPDI)Fe(N 2) 2, a highly covalent molecule with a redox noninnocent chelate that is best described as a resonance hybrid between iron(0) and iron(II) canonical forms as originally proposed in 2004.

Original languageEnglish (US)
Pages (from-to)3770-3785
Number of pages16
JournalInorganic Chemistry
Volume51
Issue number6
DOIs
StatePublished - Mar 19 2012

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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