Iron tricarbonyl α-diimine complexes: Synthesis, characterization, and electronic structure based on X-ray crystal structures

Abstract

A series of iron tricarbonyl α-diimine complexes, Fe(ArDABMe)(CO)3, were synthesized, in which the identity of the aryl substituent on the α-diimine ligand was varied (ArDABMe = Ar-NC(Me)-(Me)CNAr; 2a Ar = C6H5; 2b Ar = 3,5-(CH3)2C6H3; 2c Ar = 2,4,6-(CH3)3C6H2; 2d Ar = 3,5-Cl2C6H3), and the complexes were characterized by 1H and 13C NMR spectroscopy. The IR spectra of compounds 2a-d exhibit three carbonyl stretches between 2040 and 1950 cm−1, and all four compounds can be reduced by two electrons at potentials between −1.8 and −2.8 V vs. Cp2Fe/Cp2Fe+. The differences in νCO and reduction potentials are related to the differences in electron donating abilities of the α-diimine aryl substituents. The structures of 2a-d were determined by single crystal X-ray diffraction experiments; compounds 2a, b, and d have distorted square pyramidal structures, whereas, compound 2c is distorted trigonal bipyramidal. In order to assess the oxidation state of the α-diimine ligand in compounds 2a-d, we synthesized and crystallographically characterized a series of compounds that contain the neutral forms of the four α-diimine ligands. In compounds 2a-d, the average CN bond lengths are 1.332(1) Å and average backbone CC bond lengths are 1.403(2) Å, compared to 1.279(1) Å and 1.520(3) Å, respectively, in the neutral ligands. The lengthening of the CN bonds and shortening of the CC bonds are indicative of reduction of the α-diimine ligands. Therefore, compounds 2a-d are best described as containing Fe(I) centers bonded to radical anionic α-diimine ligands.