New Coordination Modes for Modified Schiff Base Ti(IV) Complexes and Their Control over Lactone Ring-Opening Polymerization Activity

Abstract

A series of 8 new bis(alkoxy)bis(phenoxy-imine)titanium(IV) catalysts, coordinated by Schiff base ligands derived from o-vanillin (2-hydroxy-3-methoxybenzaldehyde), show good activity and control for the ring-opening polymerization (ROP) of ε-caprolactone or ω-pentadecalactone. The new complexes are easily prepared in two high-yield steps from commercial reagents. The new ligands can all adopt two different coordination modes, depending on the steric bulk on the imine: a 6-membered NO chelate and/or a 5-membered O-O chelate. The complexes show three different structures, depending on the ligand coordination mode: Type A (N-O:N-O); Type B (N-O:O-O); and Type C (O-O:O-O). in all cases, the structures are confirmed, in solution, using variable temperature NMR spectroscopy and, in the solid state, using X-ray crystallography. The complex structure influences the polymerization rate, with the catalytic activities decreasing in the order: Type C > Type B > Type A for both monomers. Overall, the work demonstrates potential to use these new ligands to access particular coordination modes which allows enhancement of catalytic activity. Scheme 1. Summary of the coordination types observed for bis(phenoxy-imine)Ti(IV) complexes, where R1 = R2 = H, R3 = Aryl, Alkyl (top), 26 compared with those observed in this work for o-vanillin derived ligands (bottom).

Zr(IV) Catalyst for the Ring-Opening Copolymerization of Anhydrides (A) with Epoxides (B), Oxetane (B), and Tetrahydrofurans (C) to Make ABB- and/or ABC-Poly(ester-alt-ethers)

“… 34 , 44 − 47 Ti(IV) complexes, coordinated by two ligands and two isopropoxide groups, show coordination-isomer-dependent activities for lactone ROP. 45 First, we tested these Ti(IV) complexes, i.e., [Ti(IV)( o -vanillin schiff base) 2 (OR) 2 ] in epoxide/anhydride ROCOP but they were inactive. Next, a Zr(IV) complex, coordinated by the Schiff base ligand, featuring 2,6-diisopropylphenyl substituents (HL Me ), was targeted.…”

Ortho-vanillin derived Al(iii) and Co(iii) catalyst systems for switchable catalysis using ε-decalactone, phthalic anhydride and cyclohexene oxide

“…It has previously been demonstrated that the presence of o-methoxy groups can lead to high activity for lactide ROP for Al(III) salen derivative catalysts. 68 The presence of both monofunctional (chloride, acetate) and bifunctional (CHD) initiating groups in bimodal PDL molar mass distributions in all cases (Fig. S20 †).…”

“…67 We have also reported bidentate o-vanillin Ti(IV) catalysts, coordinated by two chelating ligands, which showed very high rates in caprolactone ROP (CL ROP, TOF = 1800 h −1 , 0.5 mol%, 80°C). 68 Here, we target the effective tetradentate ortho-vanillin ligand for coordination with metal centres known to catalyse both lactone ROP and epoxide/anhydride ROCOP, i.e. Al(III) and Co(III), to overcome the known limitations of metal salen catalysts in switchable catalysis.…”

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…Titanium complexes of Schiff-base ligands derived from ortho-vanillin have been recently synthesized by Williams et al (Figure 4) [43]. In this species, each metal center is bound to two Schiff-base moieties and two isopropoxy groups derived from the metal precursor.…”