Synthesis, Properties And Metal-center Exchange Mechanism Based On Polycarboxylate Coordination Complexes

Within the scope of metal-organic frameworks (MOFs), the recently developedarena of metal-center exchange via single-crystal to single-crystal (SCSC)transformation has attracted considerable interest. Taking advantage of metal-centerexchange, some new isostructural materials which are hard to be obtained byconventional method has been synthesized.5-7Likewise, the performances of thesematerials have also been improved in the application of gas sorption,photoluminescence and catalysis. Metal-center exchange is a very complicatedprocess, and the exchange mechanism is hitherto unknown.On this other hand, coordination complexes not only has fascinating structuraland topology features, but also have widely applications in gas storage/separation,heterogeneous catalysis, magnetic properties as well as ion/molecule recognition. Sorational design and synthesis novel coordination complexes and explore theirproperties is a long-term and challenging work.Base on above discussion, this paper carried out the work from two aspects: oneis the exploration on the metal-center exchange mechanism; another is the reaearchon the complexes' design, synthesis, structures and properties. The paper contains thefollowing three parts:1. By the employment of a tetradentate acid ligand H4dcpp (H4dcpp=4,5-bis(4'-carboxylphenyl)-phthalic acid)?a novel metal-organic framework1basedon an asymmetric Zn4cluster SBU has been synthesized. By introducing a newmagnetic analysis method and a theoretical calculation model, we have explored thedetail Cu/Zn, Ni/Zn and Co/Zn exchange processes, and obtained five exchangeproducts (2a,2b,2c,3,4). We disclosed a new “stepwise-resolved” metal-centerexchanism for the first time. The replacement of the four crystallographicallyindependent Zn2+ions can be realized in a stepwise-resolved fashion. Thecoordination stabilities between the central metals and the ligands as well as thecoordination geometry are considered as dominant factors in the above mechanism. Because Cu2+ion and Zn2+ion usually exhibit similar coordination modes, thecoordination stabilities between each Zn2+ion and the ligands have been proved todominate the stepwise Cu/Zn exchange process. However, due to the confinement ofcoordination geometry, Ni2+ions and Co2+ions could only selectively exchange theoctahedral Zn2+ions.2. By adopting the strategy of constructing secondary building units, we havesuccessfully synthesized four cluster-based complexes7-10based on H4dcpp and fourN-donor ligands. Complexes7-10display diverse structural features with noncluster,dinuclear Cd cluster, tetranuclear Cd cluster and infinite botryoid-like Cd clusterchains as SBUs, respectively. Our research demonstrates that the coordination modesof polycarboxylate and the employment of auxiliary N-donor ligands can effectivelytune the kinds of SBUs as well as final structures. Moreover, complexes7-10areapplied to catalyze the reaction of photodegradation of methylene orange (MO) underhigh-pressure mercury lamp irradiation. Excitingly, they exhibit good photocatalyticproperties. The UV-vis absorption spectrum and photoluminescent emissions ofcomplexes7-10were also studied.3. Five distinctive complexes11-15have been synthesized by the combination ofH4dcpp, two kinds of N-donor ligands and three kinds of metal ions (Cd2+, Co2+,Cu2+). The result indicates that, except the N-donor ligands, the kind of metal ionscould also effectively tune the structures and properties of the complexes. We alsoinvestigate the cation-recognition behaviors of the complexes, and find that complex11is sensitive to the rare-earth ion Tb3+. In addition, the magnetic property ofcomplex13indicates that there exist strong ferromagnetic interactions between theCo2+centers, which may be attributed to the complicated Co-O belts.