Selective Construction and Structural Transformation of Homogeneous Linear Metalla[4]catenane and Metalla[2]catenane Assemblies.

Although the synthesis of mechanically interlocked molecules has been extensively researched, selectively constructing homogeneous linear [4]catenanes remains a formidable challenge. Here, we selectively constructed a homogeneous linear metalla[4]catenane in a one-step process through the coordination-driven self-assembly of a bidentate benzothiadiazole derivative ligand and a binuclear half-sandwich rhodium precursor. The formation of metalla[4]catenanes was facilitated by cooperative interactions between strong sandwich-type π-π stacking and non-classical hydrogen bonds between the components. Moreover, by modulating the aromatic substituents on the binuclear precursor, two homogeneous metalla[2]catenanes were obtained. The molecular structures of these metallacatenanes were unambiguously characterized by single-crystal X-ray diffraction analysis. Additionally, reversible structural transformation between metal-catenanes and the corresponding metallarectangles could be achieved by altering their concentration, as confirmed by mass spectrometry and NMR spectroscopy studies.

[Variation Characteristics of Surface Water Quality in China in Recent Years].

Based on data from 839 comparable sections (sites) of the national surface water environmental quality monitoring network from 2012 to 2020, the variation tendency of surface water environmental quality over the past nine years was analyzed. The results showed that the environmental quality of surface water in China has continuously improved, the proportion of Grade Ⅰ-Ⅲ water quality increased steadily, and the proportion of inferior Grade Ⅴ water quality decreased in succession. The annual average concentration of ammonia nitrogen, total phosphorus, and permanganate index all showed a decreasing trend annually; compared with those in 2012, the three indicator concentrations respectively declined 75.9%, 48.2%, and 17.5% by 2020. In Guangxi, Hainan, and Ningxia, the proportion of Grade Ⅰ-Ⅲ water quality sections was maintained at 100%, whereas Hubei and Jiangxi showed a consecutive decreasing trend, and the other provinces showed a consecutive increasing trend. In Guangxi, Hainan, Ningxia, Hunan, and Fujian, the proportion of inferior Grade Ⅴ water quality sections remained at 0, and the other provinces showed a decreasing trend yearly. The annual average concentration of total phosphorus in Guangxi and Jiangxi and the permanganate index in Hubei, Hainan, and Liaoning increased annually, whereas that in the other provinces decreased to varying degrees. The proportion of Grade Ⅰ-Ⅲ water quality sections in Ten Major basins showed a fluctuating upward trend. The proportion of inferior Grade Ⅴ water quality in the Zhejiang and Fujian Slice Rivers was maintained at 0, and the other river basins showed a fluctuating and declining trend. The annual average of the main pollution indicators all decreased to varying degrees. In 2020, 53% of 32 important lakes were eutrophic, which increased 12% compared to that in 2012. On the whole, surface water quality has generally improved in China during the past nine years; especially since the 13th Five-Year Plan period, remarkable achievements have been made in the prevention and control of water pollution. However, there are differences among various provinces and basins, the improvement in water environmental quality is unbalanced and uncoordinated, water resources are distributed unevenly, and the eutrophication trend of lakes and reservoirs is not optimistic. In the future, water resources, water environment, and water ecology should be overall managed, and great attention should be focused on precise pollution control and ecological restoration of surface water.