Synthesis and Structural Characterization of a Novel Family of Titanium Complexes Bearing a Tridentate Bis-phenolate-N-heterocyclic Carbene Dianionic Ligand and Their Use in the Controlled ROP ofrac-Lactide

The present contribution describes the synthesis and structural characterization of a novel family of robust titanium complexes, supported by a tridentate pincer ligand of the type bis-phenolate-N-heterocyclic carbene [ t Bu (OCO) 2− ]. For the most part, these complexes were found to be accessible in high yields via an alcohol elimination route involving the reaction of the imidazolinium salt [ t Bu (OCO)H 3 ]Cl ( 1 ) with ClTi(O i Pr) 3 or Ti(O i Pr) 4 to afford the corresponding NHC-Ti complexes [ t Bu (OCO)]TiCl 2 ( 2 ) and [ t Bu (OCO)]TiCl(O i Pr) ( 3 ), respectively, when the reaction is carried out in noncoordinative solvents such CH 2 Cl 2 and toluene. When these reactions are performed in THF, the corresponding Ti-THF adducts [ t Bu (OCO)]TiCl 2 (THF) ( 2-THF ) and [ t Bu (OCO)]TiCl(O i Pr)(THF) ( 3-THF ) are isolated in quantitative yields. The molecular structures of complexes 2, 2-THF, and 3-THF were determined by X-ray crystallographic studies, establishing the effective coordination of the t Bu (OCO) 2− pincer to Ti. While the alcohol elimination pathway appears to be most suited to access Ti complexes of the type [ t Bu (OCO)]TiX 2 (X = halide, alkoxide), the amine elimination was also found to be effective, albeit in lower yield. Thus, the reaction of salt species 1 with Ti(NMe 2 ) 4 in THF afforded the corresponding NHC-Ti amido complex [ t Bu (OCO)]TiCl(NMe 2 )(THF) ( 5 ) in a modest yield. The direct reaction of the salt species 1 with 0.5 or 1 equiv of TiCl 4 (THF) 2 in the presence of NEt 3 afforded the homoleptic bis-adduct Ti complex [ t Bu (OCO)] 2 Ti ( 6 ), whose molecular structure was confirmed by X-ray crystallographic analysis. As for the potential of such complexes in catalysis, the Ti isopropoxide chloro complex 3-THF was found to readily initiate the ring-opening polymerization of rac -lactide in a controlled manner and, interestingly, without apparent involvement of the NHC moiety in the catalytic process. The tridentate nature of the t Bu (OCO) 2− ligand as well as some level of steric protection provided by the t Bu groups may rationalize the excellent stability of the Ti−NHC bond in the present systems.