A Rationale for the Linear Correlation of Aryl Substituent Effects in Iron(0) Tricarbonyl Complexes Containing α,β-Unsaturated Enone (Chalcone) Ligands

A library of iron(0) tricarbonyl complexes containing η4-bound α,β-unsaturated enone ligands [Fe(CO)3(η4-RCHCH−C{Ph}O)] has been prepared to facilitate comprehensive correlation of the electronic withdrawing/donating properties of the substituent, R, with the strength of the metal−ligand interaction. The IR and NMR spectroscopic data proved invaluable in aiding a comprehensive correlation and global understanding of the aryl substituent effects. The frequency of the M−CO bands in the infrared spectra of these species exhibits a linear correlation with the Hammett parameters for the substituents. The coordination shifts in both the 1H and 13C NMR spectra for the ligands exhibit a similar linear relationship. The largest coordination shifts are observed when more electron-withdrawing groups are present, implying that the organic ligand is primarily acting as a π-acid. The structures of six complexes of this type have been determined by single-crystal X-ray diffraction.