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

The binuclear complexes [Cp*(dppe)FeC⋮C−(CH 2 ) 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)−(CH 2 ) x −(H)C C Fe(dppe)Cp*] [PF 6 ] 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 ( K c ) and the molar fractions of the different species 8a n + and 8b n + ( n = 0, 1, 2) present in solution. The dicationic complexes 8a [PF 6 ] 2 and 8b [PF 6 ] 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 Fe III sites. The NIR spectroscopy study of the MV complexes 8a [PF 6 ] and 8b [PF 6 ] clearly shows that through-bridge electron transfer occurs for both compounds, which well obey Hush's theory.