Chemistry of the 1,3,5,7-octatetraynediyl carbon rod end-capped by two electron-rich (η5-C5Me5)(η2-dppe)Fe groups

The synthesis of the organoiron complex [(η5-C5Me5)(η2-dppe)FeCCCCCCCCFe(η2-dppe)(η5-C5Me5)] (2, dppe=1,2-bis(diphenylphosphino)ethane) is reported with its full spectroscopic characterizations (1H-, 31P-, and 13C-NMR, IR, Raman, UV–vis and 57Fe Mössbauer). The X-ray analysis of 2 shows that the molecule adopts a geometry very close to the anti conformation in the solid state. The shortening of the FeC bond distance associated with the increase in the number of carbon atoms suggests some cumulenic contribution to the description of the electronic structure of the all-carbon bridge. The 13C-NMR, 57Fe Mössbauer data and theoretical calculations confirm this trend and indicate that the cumulenic contribution is significant in the vicinity of the metal center but vanishes in the middle of the carbon rod. Vibrational spectroscopy carried out on the single crystals of [(η5-C5Me5)(η2-dppe)FeCCCCFe(η2-dppe)(η5-C5Me5)] (1) and 2 and on solutions of these compounds indicates that the CC bond stretching mode is not very sensitive to the relative orientation of the terminal endgroups. The electronic structure of the titled compound has been investigated using density functional theory. The geometrical changes occurring upon elongation of the carbon chain were nicely reproduced and interpreted. Time-dependant density functional theory calculations have been performed to rationalize the optical spectra.