Research Of Distance Within Molecular Vises And Impact On Reactivity In Metal-Organic Frameworks(MOFs)

Metal-Organic Frameworks(MOFs)are new types of porous crystalline materials constructed by inorganic components and organic linkers.The topology and pore environment of MOFs could be precisely tuned and controlled through initial design of inorganic components(metals or metal clusters)and geometry and functionalities of organic linkers.Taking the advantages of excellent characteristics of both structural building blocks,combined with its permanent porosity,MOFs were widely applied in the field of gas storage/separation/conversion,drugs delivery,heterogeneous catalysis,sensors and optoelectronic materials.Coordination bond is one of the fundamental chemical bonds in the creation and synthesis of new substances.Though the inner water sphere and outer water sphere around the metal center have been well known by researchers in the 1950s,most of the complexes are designed in the primary coordination sphere,and rarely involve the secondary coordination sphere due to the limitation of coordination mode of metal ions.It inspired us to study the steric and electronic effects at a further distance from the metal center to precisely regulate its catalytic performance,and even reveal new laws.Actually,the distance between ligands and metal centers is difficult to control for the ligands are usually directly coordinated with metal center in traditional coordination complexes,hence it is hard to pull the ligands far away from the metal center.In this thesis,we reported a method to construct molecular vise by using the rigid backbone of MOFs which can provide fixed distance and geometry between functional groups,and to study the impact of distance on catalytic performance.The main contents and innovations are as follows:1.Synthesis of linkers with different connectivities,including 4',4''',4''''',4'''''''-methanetetrayltetrakis([1,1'-biphenyl]-4-carboxylic acid)(MTBC),4',4''',4'''''-nitrilotris([1,1'-biphenyl]-4-carboxylic acid)(TBPA)and 4',4''',4'''''-phospha-netriyltris([1,1'-biphenyl]-4-carboxylic acid)(PBCA).Typically,a new synthetic route,involving a p-toluenesulfonyl phosphine precursor,was developed to obtain PBCA with high reproducibility,purity and yield.2.Design and characterization of series of MV-MOFs with flu topology.Molecular vise(MV)was constructed by pairing a tritopic linker and a monotopic linker in opposite positions within a metal-organic framework.This avoided direct contact between the functional group in the monotopic linker and the metal coordinated to the tritopic linker,thus,the distances at metal-binding site within MVs can be precisely tuned by varying the functional groups of monotopic linker(N/P-MV-PCN-521-R;R=formic acid,FA;benzoic acid,BA;and 4-nitrobenzoic acid,NBA)without altering the geometric angle of the linkers.In addition,a series of characterizations were used to confirmed the the structures of MV-MOFs,including single crystal X-ray diffraction(SCXRD),powder X-ray diffraction(PXRD),nitrogen adsorption test,thermogravimetric analysis(TGA),and laser scanning confocal microscopy(LSCM).3.The distance within molecular vise,for the first time,is accurately measured by ¹H-³¹P solid-state nuclear magnetic resonance(SSNMR).Two monotopic linkers,dichloroacetic acid(DCA)and p-toluic acid(PTA),with different characteristic ¹H,were used to construct MVs,P-MV-PCN-521-DCA and-PTA,respectively.By comparison between the spectra obtained with(S)and without(S₀)¹H-³¹P REDOR dipolar dephasing,the distances between ¹H from monotopic linkers(DCA and PTA)and ³¹P of PBCA within MVs could be obtained,indicating their proximity in space.The accurate measurement of these distances further confirmed the geometry of the linker pairs,and led to the discovery of distance impact on catalytic performance beyond the primary coordination sphere of metal center.4.Study the impact of distance within MVs on reactivity.The metal-binding sites in MVs were coordinated with Au(III)to construct catalysts for a classic reaction,halogenation of aromatics.PBCA linker with P elments can provide higher polarizability hence stronger coordination to late-transition metals than N.Indeed,Au-P-MVs exhibited much better performance than the Au-N-MVs counterparts with identical monotopic linker.In the test reaction,halogenation of aromatics,using Au(III)as the metal center,too close position of functional group 3.3?(NBA)and3.9?(PTA)resulted in the blockage of substrates,while sufficient distance 10.2?(FA)and 8.5?(DCA)brought in excellent conversion efficiency for all kinds of substrates with reasonable recyclability.It is worth noting that,when the functional group(BA with distance 4.7?)was arranged around the secondary coordination sphere of the metal,selectively towards different substrates was observed,demonstrating the uniqueness of MV approach.