A unified treatment of the relationship between ligand substituents and spin state in a family of iron(II) complexes

Laurence J. Kershaw Cook, Rafal Kulmaczewski, Rufeida Mohammed, Stephen Dudley, Simon A. Barrett, Marc A. Little, Robert J. Deeth, Malcolm A. Halcrow

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86 Citations (Scopus)

Abstract

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base-metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2]2+ (bpp=2,6-di{pyrazol-1-yl}pyridine) can be high-spin, low-spin, or spin-crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T1/2) in solution vs. the relevant Hammett parameter show that the low-spin state of the complex is stabilized by electron-withdrawing pyridyl ("X") substituents, but also by electron-donating pyrazolyl ("Y") substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L σ and π bonding. Highs and lows: The low-spin state of the complex shown is stabilized by electron-withdrawing pyridyl "X" substituents, but also by electron-donating pyrazolyl "Y" substituents. DFT calculations reproduce these results, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L σ and π bonding.

Original languageEnglish
Pages (from-to)4327-4331
Number of pages5
JournalAngewandte Chemie-International Edition
Volume55
Issue number13
Early online date17 Mar 2016
DOIs
Publication statusPublished - 18 Mar 2016

Keywords

  • density functional calculations
  • iron
  • N ligands
  • spin state
  • substituent effects

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