Synthesis, Structure And Properties Of Metal Complexes Based On M-xylylene Bridged Bis (1,4,7-triazacyclononane) Ligands

Bis(tacn)(tacn=1,4,7-triazacyclonon)derivatives exhibit excellent coordination ability,which enables them to form various metal complexes with novel structure and thermodynamic stability.These complexes have broad potential applications in biomimetic enzymes,molecular recognition,medical diagnosis and electrochemical catalysis.In this thesis,four m-xylylene bridged bis(tacn)ligands,namely 1,3-bis[4,7-diisoyl-1,4,7-triazacyclononane-1-methyl]benzene(L²),1,3-bis((4,7-bis(carbamoylethyl)-1,4,7-triazacyclonon-1-yl)methyl)benzene(L³),1,3-bis((4,7-di(hydroxypropyl)-1,4,7-triazacyclonon-1-yl)methyl)benzene(H₄L⁴)and1,3-bis((4,7-bis(carboxyethyl)-1,4,7-triazacyclonon-1-yl)methyl)benzene(H₄L⁵)were obtained by introducing an alkane group or coordinating pendant arms onto the the backbone N atoms of the precursor 1,3-bis(1,4,7-triazacyclodecane-1-methyl)benzene(L¹).Nine metal complexes were constructed from these bis(tacn)ligands,whose chemical composition and structure were characterized by IR spectra,electrospray ionization mass spectrometry and single crystal X-ray diffraction.Furthermore,the enzymatic activity,electrocatalytic properties and structural transformation of some complexes were investigated systematically.The full text is divided into four chapters:The first chapter includes three points.Firstly,the research progress on the utility of the metal complexes with bis/multi(tacn)derivatives to mimic the structure and function of metalloenzymes is briefly summarized.Secondly,a brief overview on the research of the multiple factors which induce structural transformation of metal-organic frameworks was presented.Finally,the research status of porous metal-organic frameworks in the field of electrocatalysis is summarized.In the second chapter,four dinuclear Cu(II)complexes,[(Cu₂L¹)Cl₄](1),[(Cu₂L²)Cl₄](2),[Cu₂L³](NO₃)₄·2H₂O·CH₃CN(3)and[Cu₂L⁴](Cl O₄)₄·2H₂O(4),were prepared from m-xylylene bridged ditopic bis(tacn)derivatives bearing steric alkyl substituents(L²),N-carbamoylethyl(L³)or N-hydroxypropyl pendant arm(H₄L⁴)and the crystal structures of complexes(2)?(4)have been determined.Based on the results of mass spectrometry and UV-vis spectrometry,complexes(1)?(4)are capable of existing in phosphate sodium buffer(PB)solution as a dinuclear specices.The chemical nuclease and SOD activitivies of complexes(1)?(4)was studied under near physiological conditions.The results show that all four complexes could cleave p BR322 plasmid DNA by employing oxidative cleavage mechanism.Complex(2)could cleave DNA in the system without H₂O₂,while the other three complexes show significant cleavage activity only in the presence of H₂O₂.At the same time,we investigated the SOD activity of these four complexes through the indirect method by detecting the reduction rate of NBT in the presence of each complex.They all displayed excellent SOD activity,and the IC₅₀values varied in the range from 0.5 to 0.8?mol/L.Furthermore,the SOD activity of these complexes were analyzed and compared according to the results of mass spectrometry.We hypothesized that complexes(1)and(2)can form a phosphate-bridged dinuclear Cu(II)species in PB buffer,resulting in a decrease in the number of exchangeable coordination sites around the Cu(II)center for binding substrate molecule.Therefore,their SOD activity is comparable to those of complexes(3)and(4)with pendant coordinating arms.In the third chapter,a tetranuclear metal complex[Ni₂(H₂L⁵)(H₂O)₂]₂(Cl O₄)₄·10H₂O(5)was prepared by treating propionic acid-functionalized bis(tacn)ligand H₄L⁵with Ni(Cl O₄)₂·6H₂O.Electrospray ionization mass spectrometry revealed that the tetranuclear moiety of complex(5)can dissociate in aqueous solution to form a stable dinuclear species,and the reason for its formation can be explained by the bond parameters obtained from single-crystal structural analysis.Considering that the pendant carboxyl group of the bis(tacn)ligand in the dinuclear nickel species derived from(5)can be used as a bridging group,the dinuclear nickel species reveal a potential as the metalloligand.Using complex(5)as a precursor,we studied its structural transformation induced by insertion of transition or rare earth metals with different radii and coordination geometries from Ni(II).Four novel complexes were obtained by dissolution-recrystallization precedure:{[Cd₂Ni₄(L⁵)₂(H₂O)₂(OH)₂](Cl O₄)₂·7H₂O·CH₃OH}_n(6)and{[Ln Ni₄(L⁵)₂(H₂O)₈](Cl O₄)₃·6H₂O}_n[Ln=La(7),Ce(8),Pr(9)].Complex(6)is a two-dimensional metal-organic framework formed by the linkage of four dinuclear units with two introducing Cd(II)nodes.Complexes(7),(8)and(9)are isomorphic and all of them exhibit a two-dimensional(4,4)layered structure constructed in such a way that one light rare earth Ln(III)ion acting as a four-connected node to bridge four[Ni₂L⁵]dinuclear units.In the crystal packing of these metal-organic frameworks,the two-dimensional coordination layers are stacked via interlayer/intralayer hydrogen bonds between metal-bound waters and lattice water molecules as well as Cl O₄^-anions to form a three-dimensional supramolecular network.In the last chapter,the hydrogen bonding between two-dimensional coordination layers in the crystals of metal-organic frameworks(7)?(9)was destroyed by ion intercalation with the help of mechanical force,then the bulk crystal samples of three metal-organic frameworks were exfoliated into the corresponding two-dimensional metal-organic framework nanosheets:Ni-La,Ni-Ce and Ni-Pr.The electrocatalytic properties of these two-dimensional MOF nanosheets were studied.The electrocatalytic oxygen evolution properties(OER)of Ni-La nanosheet,Ni-Ce nanosheet and Ni-Pr nanosheet were tested in aqueous solution with 0.5 mol/L KHCO₃(p H=7.2)as electrolyte.The overpotential of Ni-La nanosheet,Ni-Ce nanosheet and Ni-Pr nanosheet at 2m A cm^-2was 431 m V,364 m V and 445 m V respectively,and the Tafel slope was 217 m V dec^-1,197 m V dec^-1and 221 m V dec^-1,respectively.The electrocatalytic oxygen evolution performance of Ni-Ce nanosheet is significantly better than the other two ones.The interpretation for the result is that the crystal samples of metal-organic frameworks(7)?(9)were exfoliated into two-dimensional nanosheets in the KHCO₃electrolyte due to the intercalation of potassium ions,which increases the exposure of the catalytic active sites.This indicates that the difference in redox properties of rare earth ions can be used to control the electrocatalytic properties of materials.