Synthesis of a tetraphenyl-substituted dihydropentalene and its alkali metal hydropentalenide and pentalenide complexes

We report a high-yielding solution phase synthesis of 1,3,4,6-tetraphenyl-dihydropentalene based on a simple annulation reaction of cyclopentadiene with a chalcone. Deprotonative metalation of 1,3,4,6-tetraphenyl-dihydropentalene with alkali metal bases (Li, Na, K) of moderate basicity (pKa > 15) cleanly yields the corresponding hydropentalenide complexes, which exist as solvent-separated ion pairs in coordinating solvents such as THF, pyridine, and DMSO. A second deprotonative metalation with stronger alkali metal bases (pKa > 25) yields the double-anionic, 10 π aromatic tetraphenyl-pentalenide complexes, which are of low solubility in the case of Li2, Na2, and K2. In the solid state the two metals are bound η5 to each anionic ring in anti configuration as in the unsubstituted pentalenide Li2[C8H6]. Mixed-metal tetraphenyl-pentalenide complexes of Li/Na/K display remarkably enhanced solubility, allowing for solution phase characterization via NMR and UV–vis spectroscopy and application in transmetalation reactions.