Syntheses And Catalytic Performances Of Tetracarboxyl-functionalized Metallosalen Based MOFs

Metal-organic frameworks(MOFs)are an interesting class of porous materials that can be formed by coordination of self-assembly processes of metal ions or metal clusters with organic ligands,it is a network framework with architectural aesthetics.Due to its extremely high surface area,adjustable pore volume and shape,variable composition(organic connection frames or metal clusters)and operability of bottom-up assembly methods,MOFs are widely used in many fields,such as:gas adsorption and separation,chemical sensing,biomedical and heterogeneous catalysis.Among them,catalysis is the earliest application field,and with the rapid development of more than 20 years,it has become one of the most promising applications of MOFs materials.From the structure point of view,MOFs are similar to their homogeneous transition metal complex,and thus,they can be regarded as basic building blocks in the infinite structural of MOFs;the periodic structure enables the catalytic sites of MOFs to distribute uniformly throughout the framework;porosities contribute to the accessibility of active catalytic sites and the transport of catalytic substrates/products in the catalytic process.On the other hand,as a heterogeneous catalyst,MOFs can be easily separated and recovered in the catalytic process.Therefore,MOFs catalyst has the dual advantages of high activity homogeneous catalyst and recyclable heterogeneous catalyst and it can be widely used in many organic transformations.The special structure of MOFs enables metal salen centers to connect with each other through metal ions at frame joints,thus avoiding dimerization and improving stability and catalytic activity.It is a further development of homogeneous catalyst heterogeneous methods.This paper designed a new type of salen ligand(R,R)-N,N’-bis(3-tert-butyl-5-(3,5-dicarboxybenzyl)salicylidene)-1,2-diphenylethylenediame(H6salen),several salen-based MOFs with different functions have been constructed by coordination with different metals.Their functional structures and catalytic properties have been discussed and the research results are as follows:1、A three-dimensional porous framework SMOF-1 was constructed with L1(R,R)-N,N’-bis(3-tert-butyl-5-(3,5-dicarboxybenzyl)salicylidene)-1,2-Diphenylethylenediame nickel(Ⅱ)ligand.Single crystal X-ray diffraction indicate that the basic asymmetric unit of SMOF-1 consists of a Ni(salen)ligand,two Cd2+and three DMF molecules.In the junction nodes of binuclear Cd2 cluster[Cd2(COO)4(DMF)3],Cdl coordinates with six oxygen atoms from four carboxyl groups on L1 ligand to form a distorted octahedron structure.Cd2 coordinates with eight oxygen atoms and three DMF molecules from three carboxyl groups of L1 ligand to form a quasi-octahedron geometry.SMOF-1 has an open pore structure along a-axis.The pore size is about 8.0 x 9.0 A2 and occupied by DMF and water molecules.In addition,SMOF-1 can be used as an efficient catalyst for cycloaddition of CO2 with epoxides and its catalytic activity is much higher than its corresponding homogeneous catalyst.XRD powder diffraction showed that SMOF-1 has good recyclability,and it can be used 5 times without loss of activity.2、A three-dimensional porous framework SMOF-2 with isomorphism SMOF-1 was constructed using L2(R,R)-N,N’-bis(3-tert-butyl-5-(3,5-dicarboxybenzyl)salicylidene)-1,2-diphenylethylenedia me copper(II)as ligand.The secondary building unit in SMOF-2 consists of two Cd2+,one tridentate chelate bridging ligand,two bidentate bridging ligands,one carboxyl group from salen ligand and three DMF molecules.SMOF-2 has an open channel of 8.0 x 9.0 A2 along a axis.The postsynthetic reduction modification was used for the first time,SMOF-2 was reduced to SMOF-2R by the reduction with NaBH4,produced a more flexible chiral induction environment and a stronger alkaline sites.SMOF-2R is shown to be an efficient and recyclable heterogeneous catalyst for henry reaction of aldehydes with various substituents with up to 98%conversion and 98%ee.The application of MOF catalyst in asymmetric henry reaction was realized for the first time.