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° as well as trans bite angles around 155°. The iron-containing complexes [FeCl₂(κ²-L^R)] (R = Pr, Ph), [FeX₂(κ²-L^Cy)] (X = Cl, Br) and [Fe(CO)₃(κ²-L^R)] (R = Pr, 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 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(κ²-L^Cy)] was found to be significantly more stable toward hydrolysis and oxygen compared to [Fe(CO)₃(κ²-L)] and can be stored in air for months without significant decomposition in the solid state, while [Fe(CO)₃(κ²-L)] decomposes in air within seconds. The decomposition products [FeI₂(κ²-O₂L^Cy)], [{Fe(CO)₃(κ²-HL^R)}₂] (R = Pr, Cy) and [FeCl₂(CO)₂(κ¹-L^Cy)(κ¹-OL^Cy)] were identified and characterized crystallographically. The iron(0) complex [Fe(CO)₃(κ²-L^Cy)] is oxidized by [Fe(Cp)₂](BPh₄) to give the paramagnetic, low-spin iron(I) cation [Fe(CO)₃(κ²-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.