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

摘要

New imine-benzotriazole phenol ligands, ~(RFu)IBTP-H were prepared from the condensation of 3-(2Hbenzotriazol- 2-yl)-2-hydroxy-5-alkyl-benzaldehyde (R = C1 for alkyl = CH_3; R = C8 for alkyl = C_8H_(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 ([(~(C1Fu)IBTP)_2Zn] (1), [(~(C8Fu)IBTP)_2Zn] (2)) resulted from treatment of ~(RFu)IBTP-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 ([(~(C8Fu)IBTP)_2Zn(DMAP)] (3), [(~(C8Fu)IBTP)_2Zn(MeIm)] (4)). Optical properties and catalysis of epoxide/carbon dioxide (CO_2) 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 CO_2 with propylene oxide in the presence of n-Bu_4NBr 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 cocatalyst.