Synthesis and structural characterization of sterically crowded hydridotris(pyrazolyl)borato complexes: Unusual double 1,2-borotropic shift at a titanium centre

The potassium and thallium(I) derivatives of the hydrido-tris(3-Me2Bz,5-Me-pyrazol-1-yl)borato ligand, K[TpMe2Bz,Me] (1) and Tl[TpMe2Bz,Me] (2), are reported. Their reactions with TiCl4 under mild conditions afforded, after rearrangement of the ligand, recognized as a double 1,2-borotropic shift, the related Ti(IV) complex {hydrido-[bis(3-Me,5-Me2Bz)(3-Me2Bz,5-Me)-pyrazol-1-yl]borato}trichlorotitanium(IV), [TpMe2Bz,Me∗∗] TiCl3 (3). The 1H- and 13C NMR spectra of the new compounds are discussed, the molecular structure of 3 has been determined by X-ray diffraction methods, the titanium centre has a pseudo-octahedral coordination, with the nitrogen and chloride ligands in a fac geometry. A possible mechanism of formation of 3 is proposed.