Facile Synthesis of Well-Defined Titanium Alkoxides Based on Benzotriazole Phenoxide Ligands: Efficient Catalysts for Ring-Opening Polymerization of Cyclic Esters

New titanium alkoxide complexes incorporated by mono-
or bis-BTP ligands (BTP = N,O-bidentate benzotriazole phenoxide) were
synthesized and structurally characterized. The reaction of 2-(2H-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)­phenol
(C8BTP-H) with Ti­(OiPr)4 (1.0 mol equiv) in hexane produced the monoadduct complex
[(μ-C8BTP)­Ti­(OiPr)3]2 (1), but treatment of Ti­(OiPr)4 with 2.0 mol equiv of C8BTP-H at 50 °C gave the bis-adduct titanium complex [(C8BTP)2Ti­(OiPr)2] (2). The monomeric titanium analogue
([(t‑BuBTP)2Ti­(OiPr)2] (3), [(TMClBTP)2Ti­(OiPr)2] (4)) was prepared by employing sterically bulky 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol (t‑BuBTP-H) or 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol
(TMClBTP-H) as the ligand according to
the same synthetic route with a ligand to metal precursor ratio of
2:1. X-ray diffraction of single crystals indicates that 1 displays a dimeric Ti­(IV) complex containing a Ti2O2 core bridging through one of the aryloxy oxygen atoms and
the geometry around each Ti atom is distorted from an octahedral environment.
Catalysis for lactone polymerizations of BTP-containing Ti complexes
was investigated. Experimental results demonstrated that complex 1 is an active catalyst for the ring-opening polymerization
of ε-caprolactone (ε-CL) and l-lactide (l-LA) with “living” and “immortal” characters.
Kinetic studies of ε-CL polymerization initiated by 1 were investigated using in situ 1H NMR spectroscopy;
the polymerization exhibited first-order dependence on both complex 1 and ε-CL concentrations.