Pentamethylcyclopentadienyl Zirconium and Hafnium Polyhydride Complexes:  Synthesis, Structure, and Reactivity

The half-sandwich zirconium and hafnium N,N-dimethylaminopropyl complexes Cp*M[(CH2)3NMe2]Cl2 (Cp* = η5-C5Me5, M = Zr, 1; Hf, 2) and Cp*M[(CH2)3NMe2]2Cl (M = Zr, 3; Hf, 4) were synthesized by mono- or dialkylation of Cp*MCl3 with the corresponding alkyl-lithium and Grignard reagents. Hydrogenolysis of the monoalkyl species resulted in the formation of the polyhydride complexes Cp*3M3(μ-H)4(μ-Cl)2Cl3 (M = Zr, 5; Hf, 6) and Cp*MCl3. A crystal structure determination of Cp*3Hf3H4Cl5 (6) revealed a fully asymmetric trinuclear structure with three widely differing Hf···Hf distances. Reaction of Cp*3M3H4Cl5 with PMe3 resulted in fragmentation of the cluster and ligand redistribution to give Cp*MCl3(PMe3) and the dimeric hydride complexes Cp*2M2(μ-H)3Cl3(PMe3) (M = Zr, 7; Hf, 8), structurally characterized for M = Zr. The trinuclear polyhydride Cp*3Hf3H4Cl5 reacts with 2,6-xylylisocyanide to give three distinct products, a μ-enediamide complex, [Cp*HfCl2]2[μ-xyNCHCHNxy] (11, xy = 2,6-dimethylphenyl), which was structurally characterized, an imido complex, [Cp*Hf(μ-Nxy)Cl]2 (12), and an azaallyl species, Cp*Hf(η3-CH2CHNxy)Cl2 (13). The reactivity of 6 can be interpreted as proceeding through initial cleavage of the trinuclear complex into the fragments “Cp*2Hf2(μ-H)3Cl3” and “Cp*HfHCl2”, followed by the separate reactivity of these fragments.