Organometallic Mixed-Valence Systems. Electronic Coupling through an Alkyndiyl Bridge Incorporating Methylene Groups

The binuclear complexes [Cp*(dppe)FeC⋮C−(CH2)x−C⋮CFe(dppe)Cp*] (x = 3 (8a), x = 4 (8b)) were prepared by deprotonation of the parent bis(vinylidene) derivatives [Cp*(dppe)FeCC(H)−(CH2)x−(H)CCFe(dppe)Cp*] [PF6]2 (x = 3 (7a), x = 4 (7b)). The X-ray crystal structure of 8a revealed that the metal−metal distance (8.706 Å) is much shorter that the sum of the bond distances of the Fe−Fe assembly (12.24 Å). An X-ray powder diffraction analysis of an amorphous sample shows that the Fe−Fe distance may fall into the wide range 8.50−10.75 Å. The resolved separations between the redox processes in the cyclic voltammograms evidenced the through-bridge interaction between the iron centers and enabled computations of the comproportionation constants (Kc) and the molar fractions of the different species 8an+ and 8bn+ (n = 0, 1, 2) present in solution. The dicationic complexes 8a[PF6]2 and 8b[PF6]2 were prepared in good yield by oxidation of 8a and 8b with 2 equiv of ferrocenium. The IR, Mössbauer, ESR, and UV−vis spectra provided evidence for a small through-bridge interaction between the FeIII sites. The NIR spectroscopy study of the MV complexes 8a[PF6] and 8b[PF6] clearly shows that through-bridge electron transfer occurs for both compounds, which well obey Hush's theory.