Syntheses and Structures of Cationic and Neutral, Homo- and Heteroleptic <i>tert</i>-Butoxides of the Group 4 Metals

The utility of tri-tert-butoxystannate as a chelating tridentate ligand for group 4 metals was investigated. The highly Lewis acidic metals degraded the stannate ion in a series of tert-butoxide abstraction steps to produce a variety of group 4 tert-butoxides. A total of 1 equiv of NaSn(OtBu)3 reacted with cis-MCl4(THF)2 [M = Zr (1), Hf (2)] in THF solutions to furnish the salts fac-{[M(OtBu)3(THF)3](SnCl3)}, which are separated ion pairs featuring weakly coordinating trichlorostannate ions. Neutral complexes, namely, [M(OtBu)2Cl2(THF)2] [M = Zr (3), Hf (4)], were isolated when 2/3 equiv of sodium stannate was used in these reactions. Titanium tetrachloride formed analogues neither of 1 and 2 nor of 3 and 4, but Ti(OtBu)3Cl reacted with silver triflate to give [Ti(OtBu)2(OTf)2(THF)2] (5). Anion exchange of triflate for trichlorostannate transformed 1 to the contact ion pair fac-[Zr(OtBu)3OTf(THF)2] (6). A total of 2 equiv of NaSn(OtBu)3 reacted with cis-MCl4(THF)2 to give the complexes fac-[Sn(μ-OtBu)3M(OtBu)3] [M = Zr (7), Hf (8)]. Tri-tert-butoxystannate may be used as a selective alkoxylating agent for group 4 metals, and it can be transferred to these metals intact if their Lewis acidity is appropriately attenuated as in fac-{[M(OtBu)3(THF)3](SnCl3)}. Single-crystal X-ray studies revealed distorted octahedral coordination geometries for all compounds (1−8), with 1, 2, 7, and 8 being crystallographically C3 symmetric.