Structurally well-characterized zinc complexes bearing imine-benzotriazole phenoxide ligands: Synthesis, photoluminescent properties and catalysis for carbon dioxide/epoxide coupling

• New imine-benzotriazole phenoxide zinc complexes are synthesized and characterized.
• Well-defined Zn complex 2 was an active catalyst for CO2/PO coupling.
• Single-component Zn complex 3 was able to effectively afford propylene carbonate.

New imine-benzotriazole phenol ligands, RFuIBTP-H were prepared from the condensation of 3-(2H-benzotriazol-2-yl)-2-hydroxy-5-alkyl-benzaldehyde (R = C1 for alkyl = CH3; R = C8 for alkyl = C8H17) with 1-(2-furyl)methylamine (1.1 equiv) in diethyl ether. Air-tolerant zinc complexes of bis-IBTP derivatives were synthesized and structurally characterized. The monomeric and homoleptic zinc complexes ([(C1FuIBTP)2Zn] (1), [(C8FuIBTP)2Zn] (2)) resulted from treatment of RFuIBTP-H (2.0 equiv) with ZnEt2, in which the metal center is tetra-coordinated with two N,O-bidentate IBTP– ligands. Further reaction of 2 with 4-dimethylaminopyridine (DMAP) or 1-methylimidazole (MeIm) in stoichiometric proportion furnished the penta-coordinated monomeric zinc complexes ([(C8FuIBTP)2Zn(DMAP)] (3), [(C8FuIBTP)2Zn(MeIm)] (4)). Optical properties and catalysis of epoxide/carbon dioxide (CO2) coupling of these IBTP-containing zinc complexes were systematically investigated. The luminescent properties showed that Zn complexes 1–4 in solution produce intense blue fluorescence with wavelength of emission maximum in a range 473–482 nm. IBTP-ligated zinc complex 2 is an active catalyst (TOF: 33.3 h−1) for the cycloaddition of CO2 with propylene oxide in the presence of n-Bu4NBr to give propylene carbonate (PC) under mild conditions. The single-component character of Zn complex 3 that is extra-coordinated with one DMAP molecule also enabled an efficient preparation of PC without a co-catalyst.

This study reports the synthesis, characterization, and photoluminescent properties of mononuclear zinc complexes based on imine-benzotriazole phenoxide ligands. Air-tolerant Zn complex 2 has been demonstrated to efficiently catalyze cycloaddition of propylene oxide and CO2 in the presence of n-Bu4NBr with moderate activity (TOF: 33.3 h−1) under mild conditions.Figure optionsDownload as PowerPoint slide