1,4-Dimethoxybutadienediyl-Bridged Diiron Compounds in Three Oxidation States: Evaluation of Delocalization Effects

The binuclear iron complexes [Cp*­(PMe₃)­(CO)­Fe–C­(OCH₃)CH–CHC­(OCH₃)–Fe­(PMe₃)­(CO)­Cp*] (1meso and 1dl) were prepared by double deprotonation of their known parents [Cp*­(PMe₃)­(CO)­FeC­(OCH₃)­CH₂–CH₂–C­(OCH₃)Fe­(PMe₃)­(CO)­Cp*]­(PF₆)₂ (5meso and 5dl) and were isolated in good yield (90%). These complexes were characterized by ESI-mass spectrometry, IR and multinuclear NMR spectroscopy, and cyclic voltammetry. The singly and doubly oxidized forms 1meso(PF₆)_n and 1dl(PF₆)_n (n = 1, 2) were prepared by oxidation of the parent neutral complexes with 1 and 2 equiv of ferrocenium salt (93–100% yield). The related complex [Cp*­(dppe)­Fe–C­(OCH₃)CH–CHC­(OCH₃)–Fe­(dppe)­Cp*]­(PF₆) (2­(PF₆)) was obtained by reduction of the known dicationic derivative [Cp*­(dppe)­Fe–C­(OCH₃)CH–CHC­(OCH₃)–Fe­(dppe)­Cp*]­(PF₆) (2­(PF₆)₂) with 1 equiv of cobaltocene (100% yield). Multinuclear NMR spectroscopy allowed us to establish the diiron­(II) conjugated μ-bis­(carbene) structure for 1meso(PF₆)₂ and 1dl(PF₆)₂. In the case of the meso derivative, ¹H NMR revealed the presence of E and Z isomers in a 4:1 ratio, confirming the presence of a CC double bond in the middle of the bridge. The three radicals 1meso(PF₆), 1dl(PF₆), and 2­(PF₆), which are thermally stable, were analyzed by IR, Mössbauer, ESR, UV–vis, and NIR spectroscopy. Experimental data, discussed with the support of quantum chemistry calculations performed at the DFT level of theory, indicate that these radical cations exhibit characteristics of oxidation on the butadienediyl bridge rather than on the metal centers.