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 eta(4) -bound alpha,beta-unsaturated enone ligands [Fe-(CO3(eta(4)-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 H-1 and C-13 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 pi-acid. The structures of six complexes of this type have been determined by single-crystal X-ray diffraction.