Octa- and Nonamethylfluorenyl Complexes of Zr(II), Zr(IV), and Hf(IV). Investigation of Steric and Electronic Effects

The lithium salts of 1,2,3,4,5,6,7,8-octamethylfluorenyl (Flu′′) and 1,2,3,4,5,6,7,8,9-nonamethylfluorenyl (Flu*) were reacted with Zr and Hf half-sandwich complexes to generate mixed-ligand-metallocenes Cp′′Flu′′ZrCl2 (1), Cp′′Flu*ZrCl2 (2), and Cp*Flu*HfCl2 (3) (Cp′′ = 1,3-(SiMe3)2C5H3, Cp* = C5Me5). Reaction of the metallocene dichlorides with MeLi yielded the dimethyl derivatives Cp′′Flu*ZrMe2 (4) and Cp*Flu*HfMe2 (5). Reduction of the zirconocene dichlorides in the presence of CO gas yielded the dicarbonyl derivatives Cp′′Flu′′Zr(CO)2 (6) and Cp′′Flu*Zr(CO)2 (7), which represent the first examples of zirconocene dicarbonyls containing a fluorenyl ligand derivative. Structural characterization of the zirconocene dichloride 2 and the dicarbonyls 6 and 7 revealed that the methylated fluorenyl ligands adopt an η5-coordination and display a twisted tricyclic core in each case. A spectroscopic study of a small series of zirconocene dicarbonyl analogues indicates that the methylated fluorenyl ligands are strong electron donors and similar in this regard to heptamethylindenyl (Ind*) and Cp*.