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.