Iron Complexes\nof 4,5-Bis(diorganophosphinomethyl)acridine\nLigands

The search for an iron analog of the established ruthenium-based\ncatalysts containing methylene-extended 4,5-bis(diorganophosphinomethyl)acridine\nligands, [FeHCl(CO)(L<sup>R</sup>)], resulted in the discovery of\na bidentate coordination mode of these usually tridentate pincer ligands\ntoward iron. The acridines nitrogen atom does not coordinate to iron,\nleading to the formation of iron diphos-type complexes with unusually\nlarge <i>cis</i> bite angles of up to 124° as well\nas <i>trans</i> bite angles around 155°. The iron-containing\ncomplexes [FeCl<sub>2</sub>(κ<sup>2</sup>-L<sup>R</sup>)] (R\n= <sup><i>i</i></sup>Pr, Ph), [FeX<sub>2</sub>(κ<sup>2</sup>-L<sup>Cy</sup>)] (X = Cl, Br) and [Fe(CO)<sub>3</sub>(κ<sup>2</sup>-L<sup>R</sup>)] (R = <sup><i>i</i></sup>Pr, Cy)\nhave been isolated in crystalline form and characterized by spectroscopic\nmethods and mass spectrometry. Their structures were verified unambiguously\nthrough X-ray diffraction. The stability of the iron(II) complexes\ndecreased in the order Cy > Ph > <sup><i>i</i></sup>Pr and\nCl > Br > I, although all iron(II) complexes were found to be\nrelatively\nstable enough for short-term handling in air in the solid state. Notably,\nno iron(0) complex of the phenyl derivative could be isolated. The\niron(0) complex [Fe(CO)<sub>3</sub>(κ<sup>2</sup>-L<sup>Cy</sup>)] was found to be significantly more stable toward hydrolysis and\noxygen compared to [Fe(CO)<sub>3</sub>(κ<sup>2</sup>-L<sup><i>i</i>Pr</sup>)] and can be stored in air for months without\nsignificant decomposition in the solid state, while [Fe(CO)<sub>3</sub>(κ<sup>2</sup>-L<sup><i>i</i>Pr</sup>)] decomposes\nin air within seconds. The decomposition products [FeI<sub>2</sub>(κ<sup>2</sup>-O<sub>2</sub>L<sup>Cy</sup>)], [{Fe(CO)<sub>3</sub>(κ<sup>2</sup>-HL<sup>R</sup>)}<sub>2</sub>] (R = <sup><i>i</i></sup>Pr, Cy) and [FeCl<sub>2</sub>(CO)<sub>2</sub>(κ<sup>1</sup>-L<sup>Cy</sup>)(κ<sup>1</sup>-OL<sup>Cy</sup>)] were identified and characterized crystallographically. The iron(0)\ncomplex [Fe(CO)<sub>3</sub>(κ<sup>2</sup>-L<sup>Cy</sup>)] is\noxidized by [Fe(Cp)<sub>2</sub>](BPh<sub>4</sub>) to give the paramagnetic,\nlow-spin iron(I) cation [Fe(CO)<sub>3</sub>(κ<sup>2</sup>-L<sup>Cy</sup>)]<sup>+</sup>. The electron paramagnetic resonance spectrum\nof the highly sensitive cation as well as density functional theory\ncalculations suggest a partial delocalization of the unpaired electron\nover the three carbonyl ligands and the acridines aromatic ring system.\nThe catalytic activity and photophysical properties of the complexes\nhave been preliminarily investigated.