Synthesis and Structure of the First Homoleptic 1-Aza-1,3-diene Titanium Complex: A Tightrope Walk between Ligand Coordination and Ligand Coupling^†

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)}Zr{N(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.