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.