Oxidative activation of aryldiynyl-iron complexes: Regioselective dimerization

Abstract

Chemical oxidations of the iron butadiynyl complexes Cp*(dppe) FeCî -CCî -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}2C 8Ar2](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. © 2013 American Chemical Society.