Synthesis and structure of the hafnium alkylidene complex [P2Cp]Hf=CHPh(Cl) ([P2Cp] = (eta(5)-C5H3-1,3-(SiMe2CH2PPr2i)(2)))

The synthesis, structural characterization, and solution behavior of hafnium complexes stabilized by the potentially tridentate ancillary ligand [P2Cp] ([P2Cp] = (eta (5)-C5H3-1,3-(SiMe2CH2PPr2i)(2))) are described. The reaction of [P2Cp]Li with HfCl4(THT)(2) produces the hafnium trichloride complex [P2Cp]HfCl3 (1), the structure of which was determined by X-ray crystallography. Trichloride 1 is isostructural with the analogous zirconium complex [P2Cp]ZrCl3 (2) in the solid state, but in solution 1 exists as an equilibrium mixture of two isomers that interconvert by fluxional phosphine coordination. Treatment of 1 with 2 equiv of KCH2Ph, followed by thermolysis, yields the first structurally characterized hafnium alkylidene complex, [P2Cp]Hf=CHPh(Cl) (3). A crystal structure determination obtained for 3 shows this complex to be isostructural with the zirconium analogue [P2Cp]Zr=CHR(Cl) (4). The primary difference between the Hf systems presented here and the previously studied Zr analogues is that metal-ligand bonding is stronger in the former, which accounts for shorter bond distances, a greater degree of chemically inertness, and the divergent solution behaviors observed for the trichloride derivatives 1 and 2.