Oxidative Activation of Aryldiynyl–Iron Complexes: Regioselective Dimerization

Chemical
oxidations of the iron butadiynyl complexes Cp*­(dppe)­FeCCCCAr
(Ar = Ph, 1a; Ar = C6H4-Me-4, 1b; Cp* = η5-C5Me5,
dppe = 1,2-bis­(diphenylphosphino)­ethane) achieved with 1 equiv of
ferrocenium hexafluorophosphate salt in THF at −78 °C
provide the transient species 1a(PF6) and 1b(PF6), as demonstrated by ESR experiments. Above
−35 °C, intramolecular C–C couplings take place
regiospecifically, providing the binuclear complexes [{Cp*­(dppe)­Fe}2C8Ar2]­(PF6)2 (Ar
= Ph, 2a; Ar = C6H4-Me-4, 2b), which were isolated after purification as thermally stable
deep purple powders in 61 and 57% yields, respectively. The full characterization
of the new products, including X-ray analysis of 2a(PF6)2, electrochemical data, spectroscopic properties,
and electronic structures investigated at the DFT level of theory
are reported. NMR investigations have shown that the iron building
blocks rotate slowly around the cyclobutene ring, allowing the observation
of three rotamers in equilibrium. The cyclic voltammetry of the dimer 2b(PF6)2 shows four redox events with
the current peaks corresponding to two one-electron-oxidation and
two one-electron-reduction processes. These compounds are diamagnetic
in the solid state but ESR active in solution, indicating that the
triplet excited state is readily accessible.