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 rac-Lactide.
Fiche publication
Date publication
mars 2010
Auteurs
Membres identifiés du Cancéropôle Est :
Dr BELLEMIN-LAPONNAZ Stéphane
Tous les auteurs :
Romain C, Brelot L, Bellemin-Laponnaz S, Dagorne S
Lien Pubmed
Résumé
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)H(3)]Cl (1) with ClTi(O(i)Pr)(3) or Ti(O(i)Pr)(4) to afford the corresponding NHC-Ti complexes [(tBu)(OCO)]TiCl(2) (2) and [(tBu)(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 [(tBu)(OCO)]TiCl(2)(THF) (2-THF) and [(tBu)(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 (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)]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 [(tBu)(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 [(tBu)(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 (tBu)(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.
Référence
Organometallics. 2010 Mar 8;29(5):1191-8.