Synthesis and structure of the first homoleptic 1-Aza-1,3-diene titanium complex: A tightrope walk between ligand coordination and ligand coupling

Article abstract of DOI:10.1021/om0341956

The reaction of TiCl₄(THF)₂ or ZrCl₄(THF)₂ with Mg in the presence of 1-aza-1,3-dienes (1) has been investigated. Reduction of TiCl₄(THF)₂ with 2 equiv of Mg in the presence of 2 equiv of (R¹)N=CHC(R²)=CH(Ph) yields the first homoleptic 1-aza-1,3-diene complex, [Ti-{N(R¹)CH=C(R²)CH(Ph)}₂] (7), but only if the 1-aza-1,3-diene is substituted by a sterically demanding group at the nitrogen atom (1b: R¹ = C₆H₃-2,6-iPr₂, R² = H). X-ray crystallography indicates a σ²,π coordination of the heterodienes, which are reduced to 1-azabut-2-ene-1,4-diyl dianions to form folded 1-aza-2-titanacyclopent-4-ene rings. As a byproduct, the bicyclic complex [{N(R¹)CH=C(R²)CH(Ph)}Ti{N(R¹) CH=C(R²)CH(Ph)CH{C(R²)=CH- (Ph)}N(R¹)] (8) was isolated, which is believed to arise from C=N insertion of 1b into the Ti-C bond of the intermediate [{N(R¹)CH=C(R²)CH(Ph)} TiCl₂], followed by the coordination of a further 1-aza-1,3-diene ligand. The analogous bicyclic zirconium complex [{N(R¹)CH= C(R²)CH(Ph)}ZrN{(R¹)CH=C(R²)CH(Ph) CH{C(R²)=CH(Ph)N(R¹)] (6: R¹ = cyclo-C₆H₁₁, R² = Me) is the only available product when the reduction of ZrCl₄(THF)₂ is performed in the presence of the 1-aza-1,3-diene (cyclo-C₆H₁₁)N=CHC(Me)=CHPh (1a), which is substituted by a sterically less demanding group at the terminal nitrogen atom. Addition of 2 equiv of benzophenone to 7 affords the titanium complex [Ti{OCPh₂CH(Ph)C(R²)=CHN(R¹)}₂] (9). The solid-state structure of 9 shows the titanium in a distorted-tetrahedral environment, being the center of two folded oxazatitanacycloheptene rings.