Iron Complexes of 4,5-Bis(diorganophosphinomethyl)acridine Ligands

The search for an iron analog of the established ruthenium-based catalysts containing methylene-extended 4,5-bis(diorganophosphinomethyl)acridine ligands, [FeHCl(CO)(L (R))], resulted in the discovery of a bidentate coordination mode of these usually tridentate pincer ligands toward iron. The acridines nitrogen atom does not coordinate to iron, leading to the formation of iron diphos-type complexes with unusually large cis bite angles of up to 124 degrees as well as trans bite angles around 155 degrees. The iron-containing complexes [FeCl2(kappa(2)-L (R))] (R = Pr-i, Ph), [FeX2(kappa(2)-L-Cy)] (X = Cl, Br) and [Fe(CO)(3)(kappa(2)-L (R))] (R = Pr-i, Cy) have been isolated in crystalline form and characterized by spectroscopic methods and mass spectrometry. Their structures were verified unambiguously through X-ray diffraction. The stability of the iron(II) complexes decreased in the order Cy > Ph > Pr-i and Cl > Br > I, although all iron(II) complexes were found to be relatively stable enough for short-term handling in air in the solid state. Notably, no iron(0) complex of the phenyl derivative could be isolated. The iron(0) complex [Fe(CO)(3)(kappa(2)-L-Cy)] was found to be significantly more stable toward hydrolysis and oxygen compared to [Fe(CO)(3)(kappa(2)-L-iPr)] and can be stored in air for months without significant decomposition in the solid state, while [Fe(CO)(3)(kappa(2)-L-iPr)] decomposes in air within seconds. The decomposition products [FeI2(kappa(2)-O2LCy)], [{Fe(CO)(3)(kappa(2)-HL (R))}(2)] (R = Pr-i, Cy) and [FeCl2(CO)(2)(kappa(1)-L-Cy)(kappa(1)-OLCy)] were identified and characterized crystallographically. The iron(0) complex [Fe(CO)(3)(kappa(2)-L-Cy)] is oxidized by [Fe(Cp)(2)](BPh4) to give the paramagnetic, low-spin iron(I) cation [Fe(CO)(3)(kappa(2)-L-Cy)](+). The electron paramagnetic resonance spectrum of the highly sensitive cation as well as density functional theory calculations suggest a partial delocalization of the unpaired electron over the three carbonyl ligands and the acridines aromatic ring system. The catalytic activity and photophysical properties of the complexes have been preliminarily investigated.