Abstract
N-methylbenzimidazole-2-methanol (Hmbm) and Zn(NO3)2·6H2O were reacted in acetonitrile solvothermal at 80 °C for 48 h to obtain a six-nuclear Zn(II) cluster ([ZnII6(Hmbm)2(mbm)8(NO3)4]·12H2O·2CH3CN (Zn6)). Structural analysis indicated that Zn(II) in the above Zn6 clusters showed pentacoordinates. The metal centers Zn1 and Zn2 are both in the N2O3 coordination environment, and both show a triangular bipyramid configuration. Zn3 is in a NO4 coordination environment, which is also shown as a triangular bipyramid configuration. The ion source voltage of high-resolution electrospray ionization mass spectrometry (HRESI-MS) was further adjusted to bombard the Zn6 cluster, and seven major key intermediates were identified. Furthermore, we proposed that the gradual fragmentation mechanism is Zn6 → [ZnII6(mbm)8(NO3)3]+ → [ZnII5(mbm)7(NO3)2]+ → [ZnII4(mbm)6(NO3)]+ → [ZnII3(mbm)4(NO3)]+ → [ZnII2(mbm)3]+ → [ZnII2(mbm)2(OH)(H2O)2(DMSO)]+ → [ZnII(mbm)]+. In order to understand the gradual formation of Zn6 clusters, herein, we track the changes of species in the solution in different time periods by HRESI-MS. The nine key intermediates were identified and further combined with its gradual fragmentation mechanism. We proposed the gradual assembly mechanism of [ZnII(mbm)]+ → [ZnII(mbm)(Hmbm)]+ → [ZnII2(mbm)2(NO3)]+ → [ZnII2(mbm)3]+ → [ZnII3(mbm)4(NO3)]+ → [ZnII4(mbm)5(NO3)2]+ → [ZnII4(mbm)6(NO3)]+ → [ZnII5(mbm)7(NO3)2]+ → [ZnII6(mbm)8(NO3)3]+ → Zn6. To the best of our knowledge, this is the first time that a decomposition and assembly binding strategy has been used to resolve the stepwise formation of Zn(II) clusters. Photoluminescence measurements indicate that the cluster Zn6 exhibits a strong emission peak at 300 nm and an emission shoulder at 600 nm.
Highlights
The study of the assembly mechanism of coordination molecular clusters is essential for revealing their chemical reactivity and extending their inherent properties such as potential applications [4,6,7]
HRESI-MS was used to track the changes of observed a step by step decomposition of the cluster Zn6
The above decomposition process, we proposed a step by step assembly mechanism of Zn6
Summary
An important goal in chemistry is to define the precise mechanisms of chemical reactions and to apply this knowledge to the preparation of molecules with novel structures and functions [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. A major challenge in coordination chemistry is to accurately detect and identify major component changes in complex coordination molecule cluster formation processes, track their behavior in solution, and analyze their self-assembly mechanisms [8,9,10]. Due to limited research methods, technical means, and research tools, the assembly mechanism of coordination molecular clusters in hydrothermal/solvothermal. We believe that if a certain amount of energy is applied to the coordination molecule cluster to cause stepwise fragmentation, since the order of the fragmentation of clusters is related to the strength of the coordination bond, we can judge and speculate the self-assembly process (similar to inverse synthesis analysis) by stepwise cleavage of the cluster. A photoluminescence test of Zn6 showed that it mainly showed the π–π* energy level transition of the ligand and showed blue-green light emission
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