Synthesis, Structure, and Properties of Group 4 and 5 Metal Pyrazolato Chloride Complexes

Treatment of titanium tetrachloride with 3,5-di-tert-butylpyrazole (2 equiv) and triethylamine (2 equiv) in toluene
afforded dichlorobis(3,5-di-tert-butylpyrazolato)titanium(IV) (93%). A similar reaction with 3 equiv of 3,5-di-tert-butylpyrazole and triethylamine gave chlorotris(3,5-di-tert-butylpyrazolato)titanium(IV) (91%). Trichloro(3,5-di-tert-butylpyrazolato)titanium(IV) was prepared in 93% yield through reaction of titanium tetrachloride
with 1-trimethylsilyl-3,5-di-tert-butylpyrazole (1 equiv) in toluene. Treatment of zirconium and hafnium
tetrachlorides with 3,5-di-tert-butylpyrazolatopotassium (4 equiv) in toluene afforded the homoleptic pyrazolato
complexes tetrakis(3,5-di-tert-butylpyrazolato)zirconium(IV) (90%) and tetrakis(3,5-di-tert-butylpyrazolato)hafnium(IV) (68%). Analogous reaction of 3,5-di-tert-butylpyrazolatopotassium (≥3 equiv) with niobium and tantalum
pentachlorides gave dichlorotris(3,5-di-tert-butylpyrazolato)niobium(V) (98%) and dichlorotris(3,5-di-tert-butylpyrazolato)tantalum(V) (83%). Reaction of the complexes tetrakis(3,5-di-tert-butylpyrazolato)metal(IV) (metal
= Zr, Hf) and dichlorotris(3,5-di-tert-butylpyrazolato)metal(V) (metal = Nb, Ta) with the metal tetrachlorides
and metal pentachlorides, respectively, in dichloromethane afforded the complexes chlorotris(3,5-di-tert-butylpyrazolato)metal(IV) (metal = Zr, 89%; Hf, 97%) and trichlorobis(3,5-di-tert-butylpyrazolato)metal(V) (metal
= Nb, 90%; Ta, 84%). Crystal structures of trichloro(3,5-di-tert-butylpyrazolato)titanium(IV), chlorotris(3,5-di-tert-butylpyrazolato)hafnium(IV), and dichlorotris(3,5-di-tert-butylpyrazolato)niobium(V) were determined. Trichloro(3,5-di-tert-butylpyrazolato)titanium(IV) crystallizes in the space group C2/c with a = 22.6542(8) Å, b = 9.5147(3)
Å, c = 16.5329(6) Å, β = 113.5780(10)°, and Z = 8. Chlorotris(3,5-di-tert-butylpyrazolato)hafnium(IV) crystallizes
in the space group P21/c with a = 10.4529(7) Å, b = 10.1437(6) Å, c = 36.986(3) Å, β = 93.3080(10)°, and Z
= 4. Dichlorotris(3,5-di-tert-butylpyrazolato)niobium(V) crystallizes in the space group P21/c with a = 10.2051(3) Å, b = 38.3650(12) Å, c = 9.7585(3) Å, β = 93.3280(10)°, and Z = 4. Kinetic analysis of a dynamic process
observed for dichlorotris(3,5-di-tert-butylpyrazolato)niobium(V) is described. The results of this study suggest
that η2-pyrazolato donors may be useful ancillary ligands in the development of new reactive early transition
metal complexes.