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

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(N₃CPh₃)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(NCPh₃)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{NCPh₃}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 CPh₃ group to form 4. This process involves a rate-limiting 1,2-borotropic shift with activation parameters E_a = 30(1) kcal/mol, ΔH^‡ = 30(1) kcal/mol, and ΔS^‡ = 15(1) cal/mol·K as measured by VT-¹H NMR spectroscopy.