Synthesis, Structure, and Properties of Zirconium and Hafnium Complexes Containing η2‐Pyrazolato Ligands

Abstract

AbstractTreatment of tetrakis(dimethylamido)zirconium with four equiv. of 3,5‐dimethylpyrazole, 3,5‐di‐tert‐butylpyrazole, or 3,5‐diphenylpyrazole in refluxing toluene afforded tetrakis(η2‐3,5‐dimethylpyrazolato)zirconium (86 %), tetrakis(η2‐3,5‐di‐tert‐butylpyrazolato)zirconium (88 %), and tetrakis(η2‐3,5‐diphenylpyrazolato)zirconium (85 %), respectively, as colorless crystalline solids. The analogous hafnium complexes were prepared through treatment of hafnium tetrachloride with four equiv. of the potassium salts of 3,5‐dimethylpyrazolate, 3,5‐di‐tert‐butylpyrazolate, or 3,5‐diphenylpyrazolate in tetrahydrofuran to afford tetrakis(η2‐3,5‐dimethylpyrazolato)hafnium (75 %), tetrakis(η2‐3,5‐di‐tert‐butylpyrazolato)hafnium (58 %), and tetrakis(η2‐3,5‐diphenylpyrazolato)hafnium·toluene (38 %), respectively, as colorless crystalline solids. X‐ray crystal structures of representative members of these complexes revealed monomeric species containing four η2‐pyrazolato ligands and approximate dodecahedral geometry about the metal centers. Treatment of tetrakis(η2‐3,5‐dimethylpyrazolato)hafnium with 3,5‐dimethylpyrazole in a 1:1 molar ratio afforded tris(η2‐3,5‐dimethylpyrazolato)(η1‐3,5‐dimethylpyrazolato)(η1‐3,5‐dimethylpyrazole)hafnium as colorless crystals (84 %). Tris(η2‐3,5‐dimethylpyrazolato)(η1‐3,5‐dimethylpyrazolato)(η1‐3,5‐dimethylpyrazole)hafnium contains three η2‐3,5‐dimethylpyrazolato, one η1‐3,5‐dimethylpyrazolato, and one η1‐3,5‐dimethylpyrazole ligands. The η1‐3,5‐dimethylpyrazolato and η1‐3,5‐dimethylpyrazole ligands are connected through a N–H···N hydrogen bond. Tetrakis(η2‐3,5‐dimethylpyrazolato)hafnium did not form an adduct with pyridine or tetrahydrofuran, suggesting that the formation of a hydrogen bond is important to the stability of adducts. Attempted sublimation of all of the new pyrazolato complexes led to some decomposition, as evidenced by the formation of the free pyrazoles in the sublimates and significant amounts of nonvolatile residues. This work greatly expands the number of zirconium and hafnium complexes that contain η2‐pyrazolato ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)