Carbon monoxide and tertiary phosphines as ligands in cyclopentadienyl derivatives of Group 4 elements in high oxidation state

The titanocene dicarbonyl dication [TiCp2(CO)2][BPh4]2, (1) has been obtained in toluene under carbon monoxide by double protonation of TiCp4 with [NHnBu3][BPh4] or by two-electron oxidation of TiCp2(CO)2 with [FeCp2][BPh4]. Protonation reactions on ZrCp4 and HfCp4 proceed with elimination of 1 mol of cyclopentadiene independently on the ammonium salt/MCp4 molar ratio used. By this route the high electrophilic, solvent-free [ZrCp3]+ cation has been isolated and characterized by IR, 1H-NMR and elemental analysis. In the case of hafnium, the isolation has not been possible; nevertheless, in the presence of CO a rare example of a carbonyl derivative of hafnium(IV), [HfCp3(CO)][BPh4], has been isolated and characterized.