An Azide Adduct of Ti(II) with a Sterically Encumbering Trityl Azide and Independent Synthesis of a Bulky Ti(IV) Imido and Its Isomer

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We report here the synthesis and characterization of a bulky titanium(IV) imido via the bimolecular deazotation of trityl azide by the Ti(II) precursor [(Tp 3-tBu,5-Me )TiCl] ( 1, Tp 3-tBu,5-Me = hydrido(tris(3-methyl-5- tert -butylpyrazolyl)borate)). The sterically encumbering trityl substituent on an azide group discourages the unimolecular deazotation pathway and allows the isolation and full characterization of a rare organic azide adduct of Ti, namely, the diazenylimide complex [(Tp 3-tBu,5-Me )Ti(N 3 CPh 3 )Cl] ( 2 ). We show how free Ti(II) is a necessity to promote deazotation, thus allowing for a bimolecular conversion of 2 to a sterically crowded triphenylimido complex, [(Tp 3-tBu,5-Me )Ti(NCPh 3 )Cl] ( 3 ). Complex 3 is the kinetic species formed from deazotation of 2, and mild thermolysis results in the formation of its isomer [{HB(pyz tBu,Me ) 2 (pyz Me,tBu )}Ti{NCPh 3 }Cl] ( 4 ), {HB(pyz tBu,Me ) 2 (pyz Me,tBu )} = hydrido(bis(3-methyl-5- tert -butylpyrazolyl)(5-methyl-3- tert -butylpyrazolyl)borate) via a 1,2-borotropic shift. This study demonstrates that unimolecular deazotation of 2 to 3 is significantly disfavored, and attributed to the steric bulk of the trityl substituent. By congesting the imido fragment, we also show mechanistically how conversion of 3 takes place to alleviate congestion by the CPh 3 group to form 4 . This process involves a rate-limiting 1,2-borotropic shift with activation parameters E a = 30(1) kcal/mol, Δ H ‡ = 30(1) kcal/mol, and Δ S ‡ = 15(1) cal/mol·K as measured by VT- 1 H NMR spectroscopy.