Metal-organic Coordination Polymers Based On N-heterocyclic Ligands: Design, Syntheses And Catalytic Research

The construction of metal-organic frameworks (MOFs) based on metal-ligand coordinative covalent bonding is of great interest, mostly motivated by their intriguing structural features and enormous range of applications in magnetism, catalysis, adsorbent and molecular recognition. Compared with the formation of inorganic porous materials such as zeolites and metal phosphates, MOFs can be designed and constructed to generate cavities or channels of various sizes and shapes through the control of architecture and functionalization of organic linkers. However, to date, the precise prediction of MOFs is still a daunting task, because the formation of framework is governed by many factors, such as possible noncovalent intermolecular forces, solvent, reaction composition, etc. On the other hand, the systematic correlation between the crystal structure and properties of metal-organic polymers remains elusive. In fact, it is an urgent problem because it becomes a main roadblock to design materials with anticipatory properties from metal-organic polymers. Thus, many work need to be done.To explore the structure-properties relationships of metal-organic polymers, in this paper, we chose five structurally related mufti-haptoes ligands 1,1'-(1,3-propylene)-bis-1H-benzotriazole (pbbt), 1,1'-(1,4-butanediyl)bis-1H-benzotriazole (bbbt), 1,4-bis(triazol-l-ylmethyl)benzene) (btx), 1,1'-(1,4-butanediyl)bis-1H-benzimidazole) (bbbm) and 1,1'-(1,5-pentanediyl)bis-1H-benzimidazole (pbbm). By the assembling of the above ligands with metal salts, sixteen metal-organic polymers with different metal centers and various structures have been obtained at room temperature or under hydrothermal conditions. We investigated their structures, catalytic properties and fluorescence properties, and deduced some useful regulations. Furthermore, the density functional theory (DFT) calculations have also been employed to ligand and the investigated polymers based on LanL2MB basis set in order to better understand the nature of the observed fluorescence emissions. The research work about the paper mainly includes three parts:Firstly, eight coordination polymers (1-8) contained Cu(â…¡),Co(â…¡) and Mn(â…¡) ions are designed and synthesized in order to investigate the catalysis of coordination polymers. Single crystal X-ray diffraction shows that 1 exhibits discrete binuclear structure; 2 displays infinitely extended two-dimensional reticulation grid structure; 3 assumes two-dimensional rhombohedral grid network structure; 4 and 5 are one-dimensional zigzag structures; 6 and 8 feature 1-D double-chain frameworks; 7 possesses double helix chain structure. Their catalytic abilities are investigated in detail on the oxidative couplings of 2,6-dimethylphenol (DMP), 2,6-di-t-butylphenol (2,6-DBP) and 2,4-di-t-butylphenol (2,4-DBP) and the oxidation reaction of phenol by using H₂O₂ (30 mass%) as an oxidant. The results indicate that they are highly efficient catalysts. For example, under the optimal conditions, the selectivity to PPE and TBDPQ are almost up to 95% and 90%, respectively, when polymers 2 and 3 displaying 2-D structures are employed as catalysts in the oxidation couplings of DMP and 2,6-DBP, comparable to those observed for highly active catalyst systems in the literature. Further comparison of catalytic performances of the investigated polymers with those of the corresponding copper salts together with organic ligands, copper salts alone and free ligands reveals that the coordination of ligands to center metal ions plays a key role in generating the superior reactivities of polymers. Meanwhile, it is found that the nature of substrate, the structure and metal nature of polymers may be responsible for their catalytic behaviors. In addition, for the same dimensional polymers, the catalysis of the polymer containing coordinatively unsaturated metal sites is detected to be stronger.Secondly, a comparison of homogeneous vs. heterogeneous catalytic activity for the oxidative couplings of 2,6-DBP and 2,4-DBP by the aforementioned polymers is performed by using H₂O₂ (30 mass%) as an oxidant in methanol-toluene mixed solvent at room temperature or in aqueous medium at elevated temperature. The results indicate that the polymers as heterogeneous catalysts record the higher conversion but with the depressed selectivity. We presume the reasons for this. Thirdly, eight coordination polymers contained Zn(â…¡), Cd(â…¡), Hg(â…¡) and Pb(â…¡) ions (9-16) from pbbm ligand are designed and synthesized in order to explore the structural consequences of metal ions, anions, solvents, etc. on materials of metal-organic polymers. These polymers have same metal ions, anions and different structures, or different metal ions, anions and similar structures. The influences of metal ions, anions, solvents, et al. on structures and fluorescences of coordination polymers are researched and some rules are observed. Furthermore, to better understand the nature of the observed fluorescence emissions, we conducted theoretical computation on the title polymers and the free ligand. The experimental parameters obtained from fluorescent emission are consistent with the computed results.