Reactivity of Zinc Halide Complexes Containing Camphor-Derived Guanidine Ligands with Technical rac-Lactide

Abstract

Abstract:Three new zinc complexes with monoamine-guanidine hybridligands have been prepared, characterized by X-ray crystallography and NMR spectroscopy, and tested in the solvent-free ring-opening polymerization of rac-lactide. Initially the ligands were synthesized from camphoric acid to obtain TMGca and DMEGca and then reacted with zinc(II) halides to form zinc complexes. All complexes have a distorted tetrahedral coordination. They were utilized as catalysts in the solvent-free polymerization of technical rac-lactide at 150 • C. Colorless polylactide (PLA) can be produced and after 2 h conversion up to 60% was reached. Furthermore, one zinc chlorido complex was tested with different qualities of lactide (technical and recrystallized) and with/without the addition of benzyl alcohol as a co-initiator. The kinetics were monitored by in situ FT-IR or 1 H NMR spectroscopy. All kinetic measurements show first-order behavior with respect to lactide. The influence of the chiral complexes on the stereocontrol of PLA was examined. Moreover, with MALDI-ToF measurements the end-group of the obtained polymer was determined. DFT and NBO calculations give further insight into the coordination properties. All in all, these systems are robust against impurities and water in the lactide monomer and show great catalytic activity in the ROP of lactide.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

“…323 and related bidentate systems, e.g., the guanidine–quinoline ligand 324 , were used by Herres-Pawlis and co-workers predominantly for the preparation of copper­(I) − and zinc­(II) − complexes, which were used, inter alia, as initiators for atom transfer radical polymerization (ATRP) of styrene and for the ring-opening polymerization (ROP) of lactide, respectively. For the latter application, zinc complexes with chiral bis­(guanidine) ligands 325 and 326 and with camphor-derived guanidine ligands such as 327 were recently employed. , Himmel and co-workers developed several phenylene-bridged bis­(imidazolidin-2-imine) ligands such as 328 and studied for instance their Ni­(II), Co­(II), Zn­(II), and Cu­(I)/Cu­(II) complexes. − Bis­(guanidines) with biphenyl, binaphthyl, and bipyridiyl cores, e.g., 329 were also prepared and used as proton sponges and chelate ligands, , while the guanidino-functionalized naphtyridine ligand 330 was used for the construction of copper chains in Cu 4 complexes . In addition, several electron-rich tetrakis- and even hexakis­(guanidino) benzene systems such as 331 and 332 were prepared by the Himmel group ,− and used as redox-active compounds with multistage redox behavior or as ligands for the preparation of coordination polymers.…”

“…For the latter application, zinc complexes with chiral bis(guanidine) ligands 325 and 326 and with camphor-derived guanidine ligands such as 327 were recently employed. 461,462 Himmel and co-workers developed several phenylene-bridged bis-(imidazolidin-2-imine) ligands such as 328 and studied for instance their Ni(II), Co(II), Zn(II), and Cu(I)/Cu(II) complexes. 463−470 Bis(guanidines) with biphenyl, binaphthyl, and bipyridiyl cores, e.g., 329 were also prepared and used as proton sponges and chelate ligands, 471,472 while the guanidinofunctionalized naphtyridine ligand 330 was used for the construction of copper chains in Cu 4 complexes.…”

Robust Guanidine Metal Catalysts for the Ring‐Opening Polymerization of Lactide under Industrially Relevant Conditions

“…In the following, camphor‐derived guanidine zinc complexes were presented as catalysts for lactide ROP . At 150 °C and a [M]/[I] ratio of 500 : 1, rate constants between k app =7.3×10 −5 s −1 ( 22 )−12.8×10 −5 s −1 ( 23 ) were determined.…”

Highly active tridentate amino-phenol zinc complexes for the catalytic ring-opening polymerization of ε-caprolactone

“…There were only few examples of zinc chloride complexes used for the ROP of lactones. For instance, zinc dichloride or zinc halide complexes were reported to catalyze the ROP of ε-caprolactone or rac-lactide by a coordination-insertion mechanism [55,59]. Zinc chloride complexes with various ligands were also reported to be activated by methyllithium or LiOCHMe 2 to generate in situ zinc methyl species or zinc isopropoxide derivatives, respectively, which were active for the ROP of rac-LA [60e65].…”