Tetrasubstituted Selenophenes from the Stepwise Assembly of Molecular Fragments on a Diiron Frame and Final Cleavage of a Bridging Alkylidene

A series of 2,3-dicarboxylato-5-acetyl-4-aminoselenophenes, 5a-j, was obtained via the uncommon assembly of building blocks on a diiron platform, starting from commercial [Fe2Cp2(CO)(4)] through the stepwise formation of diiron complexes [2a-d]CF3SO3, 3a-d, and 4a-j. The selenophene-substituted bridging alkylidene ligand in 4a-j is removed from coordination upon treatment with water in air under mild conditions (ambient temperature in most cases), affording 5a-j in good to excellent yields. This process is highly selective and is accompanied by the disruption of the organometallic scaffold: cyclopentadiene (CpH) and lepidocrocite (gamma-FeO(OH)) were identified by NMR and Raman analyses at the end of one representative reaction. The straightforward cleavage of the linkage between a bridging Fischer alkylidene and two (or more) metal centers, as observed here, is an unprecedented reaction in organometallic chemistry: in the present case, the carbene function is converted to a ketone which is incorporated into the organic product. DFT calculations and electrochemical experiments were carried out to give insight into the release of the selenophene-alkylidene ligand. Compounds 5a-j were fully characterized by elemental analysis, mass spectrometry, IR, and multinuclear NMR spectroscopy and by X-ray diffraction and cyclic voltammetry in one case.