Unprecedented cyclic-salamo-based compound and binuclear Zn(II) salamo-based complex originated from a double-armed salamo-based ligand: Experimental and theoretical studies

In this work, a double-armed salamo-based ligand H₂L¹ modified by pyridine ring was synthesized. It was found that the cyclic salamo-based compound H₂L² and binuclear Zn(II) complex [Zn₂(L³)(H₂O)₂](NO₃)₂ are formed on the basis of X-ray single crystal diffraction. They are obtained separately by the reactions of H₂L¹ with Sr(OAc)₂ and Zn(NO₃)₂⋅6H₂O in different organic solutions. Interestingly, the compound H₂L² is formed by the cleavage and recombination of the ligand H₂L¹ catalyzed by Sr(OAc)₂, and is the first cyclic salamo-based compound. It was worth noting, in the process of forming Zn(II) complex, the ligand H₂L¹ is hydrolyzed to H₂L³, the formation of H₂L³ is due to the hydrolysis and fracture of C=N double bonds, which reduces the original formyl groups and forms a new ligand H₂L³ containing two exposed formyl groups. The specific coordination mode of the Zn(II) complex is that Zn1 atom is located in O₄ cavity, Zn2 atom is located in N₂O₂ cavity of the ligand (L³)²⁻ unit, and two oxygen atoms on formyl groups reduced by the fracture of connecting arm also participate in coordination, and two phenoxy groups on the (L³)²⁻ unit are bridged with Zn1 and Zn2 atoms at the same time. In addition, two water molecules on Zn1 and Zn2 participate in the coordination, and the Zn(II) atoms form hexacoordinated octahedral configurations. Nitrate ions do not participate in coordination, and their main function are to maintain the charge balance of the complex. UV–Vis titration experiment showed that the binding ratio between the ligand and Zn(II) atoms is 1:2, which is in accordance with the result of X-ray single crystal structure. This shows that the coordination ratio in solution state and solid state is the same. The cyclic-salamo compound H₂L² and Zn(II) complex were analyzed by DFT and Hirshfeld surface analysis. Through the analysis of fluorescence titration spectra, it was found that compared with the ligand H₂L¹, the fluorescence intensity increases obviously after adding Sr²⁺ and Zn²⁺ ions respectively, and gradually increases with the increasement of ion concentration.