Synthesis And Properties Of MOFs Based On Chiral Metallosalen Ligands

Objective:New chiral metallosalen ligands were synthesized and used as the basic linkers to assemble with metal ions or metal clusters to prepare metal-organic frameworks materials,and its properties were investigated.Methods:1.Synthesis and characterization of ligandsA series of chemical reactions,such as Duff reaction,imine condensation and metal coordination,were carried out to synthesize the chiral Ni（II）-Salen complex with carboxyl groups from 4-hydroxy-3-methylbenzoic acid.A series of chemical reactions,such as Suzuki-Miyaura reaction,imine condensation and metal coordination,were carried out to synthesize pyridine type chiral Cu（II）-Salen complex from 5-bromo-2-hydroxyacetophenone.The structures of the target compounds and intermediates were characterized by ¹H NMR,¹³C NMR and HRMS,respectively.2.Synthesis and characterization of MOFsMOFs were synthesized from Ni（II）-Salen or Cu（II）-Salen and transition metal by solvothermal reaction,by the means of adjusting the solvent,temperature,pH,reaction time and the ratio of metal ion-carboxylic acid ligand.And their structures were confirmed by single crystal X-ray diffraction,elemental analysis and IR.The BET and pore size of MOFs were calculated by N₂ adsorption/desorption experiments.The thermal stability and purity of the crystal were investigated by TGA and PXRD,respectively.3.Catalytic performance of MOFsMOF 1 acts as a heterogeneous catalyst for the ring expansion of CO₂ with epoxide and the selective oxidation of terminal olefins.MOF 2 acts as a heterogeneous catalyst for selective oxidation of benzyl alcohol,and the adsorption of CO₂ and N₂ was tested.The products were characterized by ¹H NMR and ¹³C NMR.Results:1.The chiral Ni（II）-Salen ligand was synthesized and self-assembled with Zn²⁺to obtain a 3D MOF 1 with double interpenetration.Its thermal stability reaches 400°C.2.A pyridine type chiral Cu（II）-Salen ligand was synthesized.The mixed ligand of the ligand and 4,4’-biphenyldicarboxylic acid was assembled with Zn²⁺under solvothermal conditions to obtain a 3D MOF 2 with double interpenetration.Its thermal stability reaches350°C.3.MOF 1 can catalyze the conversion of epoxides and CO₂ to cyclic carbonates with a maximum yield of 97%uner 1 atm and 50°C.The activity of the catalyst is almost unchanged after five cycles.4.Using hydrogen peroxide as oxidant,MOF 1 can catalyze the selective oxidation of terminal olefins to aldehydes in aqueous phase with a maximum yield of 96%.5.The BET surface area of MOF 2 is 752 m²/g,and the adsorption capacity of CO₂ and N₂ at 1 atm is 3.47 mmol/g and 0.57 mmol/g,respectively.6.Using TEMPO as the additive,MOF 2 can catalyze the selective oxidation of benzyl alcohol with a maximum yield of 99%in the aqueous phase.The catalyst has been reused five times without activity loss.Conclusion:MOF 1 and MOF 2 based on Ni（II）-Salen and Cu（II）-Salen ligands respectively have high thermal stability and catalytic activity.MOF 1 can catalyze the epoxide and CO₂ to form cyclic carbonate,and selective oxidation of terminal olefin in water phase.MOF 2can catalyze the selective oxidation of benzyl alcohol in water phase.The catalyst can be used repeatedly,and its activity is basically unchanged.Based on the preparation and catalytic properties of metallosalen ligand MOFs,this paper provides a possibility for the preparation and application of heterogeneous catalysts.