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 of<i>rac</i>-Lactide

Abstract

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 [tBu(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 [tBu(OCO)H3]Cl (1) with ClTi(OiPr)3 or Ti(OiPr)4 to afford the corresponding NHC-Ti complexes [tBu(OCO)]TiCl2 (2) and [tBu(OCO)]TiCl(OiPr) (3), respectively, when the reaction is carried out in noncoordinative solvents such CH2Cl2 and toluene. When these reactions are performed in THF, the corresponding Ti-THF adducts [tBu(OCO)]TiCl2(THF) (2-THF) and [tBu(OCO)]TiCl(OiPr)(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 tBu(OCO)2− pincer to Ti. While the alcohol elimination pathway appears to be most suited to access Ti complexes of the type [tBu(OCO)]TiX2 (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(NMe2)4 in THF afforded the corresponding NHC-Ti amido complex [tBu(OCO)]TiCl(NMe2)(THF) (5) in a modest yield. The direct reaction of the salt species 1 with 0.5 or 1 equiv of TiCl4(THF)2 in the presence of NEt3 afforded the homoleptic bis-adduct Ti complex [tBu(OCO)]2Ti (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 tBu(OCO)2− ligand as well as some level of steric protection provided by the tBu groups may rationalize the excellent stability of the Ti−NHC bond in the present systems.