Group 4 Metal Complexes Bearing Thioetherphenolate Ligands. Coordination Chemistry and Ring-Opening Polymerization Catalysis

Abstract

A series of group 4 metal complexes 1–4 (1 = (t-BuOS)2Ti(O-i-Pr)2; 2 = (t-BuOS)2Zr(O-t-Bu)2; 3 = (t-BuOS)2Hf(O-t-Bu)2; 4 = (CumOS)2Zr(O-t-Bu)2) supported by two phenolate bidentate ligands (t-BuOS–H = 4,6-di-tert-butyl-2-phenylsulfanylphenol and CumOS–H = 4,6-dicumyl-2-phenylsulfanylphenol) were synthesized by the reaction of appropriate metal tetra(alkoxide)s with 2 equiv of the ligands. The X-ray structure of 2 revealed that the two ligands were κ2-chelated to the metal center with two phenoxy groups in trans positions and the two thioether moieties in cis positions. VT NMR analysis revealed a fast inversion of configuration at the metal center. Complexes 1–4 promoted the ring-opening polymerization (ROP) of lactide and ε-caprolactone. Complex 4 exhibited the highest activity with a pseudo-first-order rate constant of 0.061 ± 0.003 min–1 at 100 °C, which compares favorably with those reported for the most active group 4 complexes. Polymerizations were well-controlled, giving predictable molecular weights and narrow molecular weight distributions. Essentially atactic PLA were obtained in the ROP of rac-lactide. A DFT study on ROP promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive metal center. In presence of isopropanol, lower PDI indexes and molecular weights of the PLAs proportional to the equivalents of isopropanol suggested that adequate conditions for effective “immortal” polymerizations were achieved. More interestingly, these catalysts promoted the copolymerization of l-lactide and ε-caprolactone. The microstructure disclosed by 13C NMR analysis and the thermal behavior exhibited in DSC studies indicated a random distribution of the two monomer along the polymer chain

Controlled random copolymerization of rac‐lactide and ɛ‐caprolactone by well‐designed phenoxyimine Al complexes

“…Their study suggests that the introduction of bulky i Pr 3 Si groups on ortho ‐position of phenolate rings strongly disturbs the coordination of LA to the Al center, leading to a decrease in r LA , but the bulky groups have less effect on the incorporation of ɛ ‐CL monomer. Beyond the salen‐type system, pyrrolylpyridylamido‐Al, 5(b) phenoxyimine‐amido‐Zn, 4(b) pyridonates and amidates‐Ti, thioetherphenolate‐Zr, and dinuclear‐Al complexes could also achieve the random copolymerization of LA and ɛ ‐CL. Nevertheless, copolymers of LA and ɛ ‐CL with the strict Bernoullian distribution could be only prepared by the salen‐type Al system so far 5(a),14 …”

Cyclic Polyester Formation with an [OSSO]‐Type Iron(III) Catalyst

“…[8] However,t he formation of cyclic polyesters remains a challenge. [27,28,35] Under these conditions, no product was detected by using 1 HNMR spectroscopy ( Table 1, entries 1a nd 2). Indeed,t he biodistribution of cyclic aliphatic polyesters (cAPEs)i sd ifferent from that of linear APEs, [10] which offers an opportunity to develop new drug deliverys ystems.…”

“…[27,28,35] Under these conditions, no product was detected by using 1 HNMR spectroscopy ( Table 1, entries 1a nd 2). At the beginning, we explored rac-LA polymerization in THF and toluene solutions following the standard procedures reported previously.…”

Hydroboration of Aldehydes, Ketones, and Carbodiimides Promoted by Mono(imidazolin‐2‐iminato) Hafnium Complexes

“…Complexes of hafnium have been predominantly used as Lewis acid catalysts in various organic transformations, such as the polymerization of cyclic esters, and as catalysts for the homo‐ and copolymerization of α‐olefins . Recently, Hf‐MOF compounds have been used as active catalysts for the activation of small molecules .…”