Bis(imino)aryl NCN Pincer Aluminum and Zinc Complexes: Synthesis, Characterization, and Catalysis on l-Lactide Polymerization

Abstract

Reactions of 2,6-(ArNdCH) 2 C 6 H 3 Li with AlEt 2 Cl afford a number of NCN pincer aluminum complexes (2, 7)) and monoligated NCN pincer zinc complex (2,6-(ArNdCH) 2 C 6 H 3 )ZnEt (Ar=2,6-i Pr 2 C 6 H 3 (8)). All complexes were characterized by 1 H and 13 C NMR spectroscopy, and the molecular structures of complexes 3, 4, 6, 7, and 8 were determined by X-ray crystallography. The X-ray diffraction analysis reveals that both complexes 3 and 4 adopt a distorted trigonal-bipyramidal geometry around the aluminum central metal with three carbon atoms in the equator and the two imine nitrogen atoms in the apical positions. Complexes 6 and 7 adopt a distorted tetrahedral geometry around their zinc metal centers, while complex 8 adopts a square-planar geometry around its metal center. All these Al and Zn complexes are efficient initiators for L-lactide ring-opening polymerization in the presence of benzyl alcohol, and the polymerization reaction takes place in an immortal manner. The productivity of the Zn complexes is generally higher than that of the Al complexes under similar conditions.

Synthesis, structure, and catalytic activity of trinuclear zinc complexes with new chiral biaryl-based NO2 ligands

“…Both complexes 8 and 9 allow the complete conversion of 500 equiv of lactide within 24 h at 70 C in toluene at [rac-LA] ¼ 0.5 mol L À1 , and the molecular weight distribution is narrow (down to 1.13) over the entire monomer-to-initiator ratio range (Table 3, entries 2 and 3), in agreement with a single-site catalyst system [9,38]. While polymerizations with 7 produces a much broader molecular weight distribution (2.54) ( Table 3, entry 1), due to its two different mediated sites, or consistent with its C 2v quasi-symmetric core EtZnO 2 ZnN 2 (OPh) 2 ZnEt.…”

“…Although 7e9 can initiate the polymerization of lactide in toluene and THF solutions, no polymerization occurs in CH 2 Cl 2 solution even when heated at 40 C for 72 h. The resulting polylactides are all heterotactic-rich under the conditions examined, and the molecular weights of the polymers range from 28.6 to 36.1 kg mol À1 . Our results show that the catalytic activities of 7e9 resemble that of zinc complex [2,6-(2,6-i Pr 2 C 6 H 3 N ¼ CH) 2 C 6 H 3 }ZnEt [38], while the microstructure of the resulting polylactides is similar to those initiated by [{(S)-PhCH(Me)(N¼CMe)} 2 CH]Zn(OPr i ) [9].…”

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

“…The only examples of homoleptic metal complexes bearing two tridentate pincer ligands were recently reported. [40,41] The crystal structure of 3 is shown in Figure 1. Zinc displays a nearly trigonal-planar coordination geometry (the sum of the bond angles about the metal center is close to 3608) [32,34,42,43] with three phosphorus atoms (P2, P3, and P1) belonging to two different PPP molecules acting as bidentate and monodentate ligands, respectively.…”

Zinc complexes incorporating with symmetrical and asymmetrical polydentate nitrogen-donor pyrrolyl ligands: Synthesis, characterization, and ring-opening polymerization

“…In order to distinguish the two methylene protons of the CH 2 NH t Bu and CH 2 NMe 2 , homo-nuclear decoupling and 2D-NMR spectra of 4 are considered leaving corresponding assignments (see Supporting Information). The 1 H NMR spectra of 5 only show one t Bu resonance at d 0.92 indicating a fast exchange between coordinated and free N t Bu fragments of the pyrrolyl ligand to Zn atom [55]. Variable temperature 1 H NMR spectra of 5 show no pronounced change on resonance sequences even at À60 C, an indication involving a fast fluxionality among the geometries of 5 in solution as shown in Scheme 6.…”