Cyclobutadieneiron Tricarbonyl Annelated with Homoadamantene Frameworks

Cyclobutadieneiron tricarbonyl annelated with homoadamantene 2 was synthesized from the corresponding 3,4-dibromocyclobutene 8. As a stable cationic analogue of 2, the dicarbonyl nitrosyl complex 3+ was also prepared. X-ray crystallographic studies revealed that the homoadamantene units of both 2 and 3+ are highly bent out of the cyclobutadiene ring plane away from Fe. The cyclobutadiene units in 2 and 3+ are significantly distorted to a rectangular shape, which made the transannular carbon−carbon bonds shorter than the other two. An enhanced donor ability of the cyclobutadiene unit was suggested by X-ray crystallography, infrared spectra, and DFT calculations. In accordance with this, reversible oxidation waves were observed in the cyclic voltammogram of 2, reflecting an enhanced stability of the oxidized form.