Electron-Rich Piano-Stool Iron σ-Acetylides. Electronic Structures of Arylalkynyl Iron(III) Radical Cations^†

The present study reports the isolation and the structural (X-ray), UV−vis−near-IR, and ESR characterization of a series of Fe(III) complexes of formula [(η2-dppe)(η5-C5Me5)Fe(C⋮CC6H4X)][PF6] (1a[PF6]−1k[PF6]; with X = NO2, CN, CF3, Br, F, H, Me, tBu, OMe, NH2, NMe2). The electronic substituent effect of the remote X group on the electronic structure is experimentally evidenced by means of correlations with electronic substituent parameters (ESPs) and is theoretically investigated. The spectroscopic and ESR data are discussed in connection with the DFT computations. A consistent picture of the electronic structure of these Fe(III) radical cations is obtained. Notably, the near-IR absorption observed for 1a[PF6]−1k[PF6] is assigned to a SOMO-2/SOMO electronic transition.