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

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 = C₈H ₁₇) 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 ZnEt ₂, 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.