Halogen Bonding in the Decoration of Secondary Coordination Sphere of Zinc(II) and Cadmium(II) Complexes: Catalytic Application in Cycloaddition Reaction of CO2 with Epoxides

Three new zinc(II) complexes [Zn(H2L3)(2)(H2O)(3)] (Zn-2), [Zn(H3L2a)(H2O)3]n (Zn3) (H3L2a = 2,4-diiodo-5-(2-(2,4,6trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl)isophthalate) and [Zn(HL4)(DMF)(H2O)]n (Zn-4) were synthesized by the reaction of Zn(II) salts with 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl) isophthalic acid (H3L3), 2,4,6-triiodo-5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl) isophthalic acid (H5L2) (in the presence of NH2OHHCl) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)-2,4,6-triiodoisophthalic acid (H3L4), respectively. According to the X-ray structural analysis, the intramolecular resonance-assisted hydrogen bond ring remains intact, with NO distances of 2.562(5) and 2.573(5) & Aring; in Zn2, 2.603(6) & Aring; in Zn3, and 2.563(8) & Aring; in Zn4. In the crystal packing of Zn3, the cooperation of IO and II types of halogen bonds between tectons leads to a one-dimensional supramolecular polymer, while IO interactions aggregate 1D chains of coordination polymer Zn4. These new complexes (Zn2, Zn3, and Zn4) and known [Zn(H3L1)(H2O)(2)](n) (Zn1) (H3L1 = 5-(2-(2,4,6-trioxotetrahydropyrimidin5(2H)-ylidene) hydrazineyl)isophthalate), {[Zn(H3L1)(3H(2)O)(2)](n) (Zn5), [Cd(H3L1)(H2O)(2)](n) (Cd1), {[Cd(HL3)-(H2O)(2)(DMF)]H2O}n (Cd2), [Cd(H3L3)]n (Cd-3), {[Cd2(mu-H2O)(2)(mu-H2L4)(2)(H2L4)(2)]2H2O}n (Cd4), and {[Cd(H3L1)-(H2O)(3)]4H(2)O}n (Cd5) were tested as catalysts in the cycloaddition reaction of CO(2 )with epoxides in the presence of tetrabutylammonium halides as the cocatalyst. The halogen-bonded catalyst Zn4 is the most efficient one in the presence of tetrabutylammonium bromide by affording a high yield (85-99%) of cyclic carbonates under solvent-free conditions after 48 h at 40 bar and 80 degrees C.

Automated summary

In this study, three new zinc(II) complexes were synthesized to further investigate their molecular structures and crystal packing. X-ray structural analysis revealed that the intramolecular resonance-assisted hydrogen bond ring remained intact in the zinc(II) complexes, and the presence of halogen bonds resulted in one-dimensional supramolecular polymers in the crystal packing. In the catalytic experiments, it was found that the zinc(II) complex Zn4 exhibited high catalytic activity and could efficiently catalyze the cycloaddition reaction to produce cyclic carbonates under solvent-free conditions.