New 1,3-Diene Complexes of Zirconium and Hafnium: One-Step Conversion of the Butadiene Fragment with Carbon Monoxide to Cyclopentadienes under Mild Conditions

Substituted cis-diene complexes Cp*M(diene)Cl (Cp* = η-C5(CH3)5; M = Zr, Hf) were obtained by reduction of Cp*MCl3 with Na/Hg in the presence of trans-1,3-dienes. Exceptionally low 13C-1H coupling constants (138-141 Hz) of the terminal diene carbon atoms indicate a metallacyclo-3-pentene type of bonding for the diene-metal fragment. Reaction with carbon monoxide (1 atm, 25 °C) results in complete reduction of CO, splitting of the carbon oxygen bond, and formation of cyclopentadienes in which the carbon atom of the CO molecule is incorporated in the ring.