Crystal structure of compound 1
X-ray crystallography analysis indicates that compound 1 features a 3D heterometallic framework that crystallizing in tetragonal I41/acd space group. Each asymmetric unit of 1 is composed of one forth Cd(II) ion, half K(I) ion, half L2− ligand, as well as one coordinated water molecule. The Cd1 ion bonds to eight oxygen atoms from four carboxylate groups of four independent L2− ligands, affording a distorted tetragonal antiprism, and the K1 ion bonds to one terminal water ligand and three carboxylate oxygen atoms from three different L2− ligands, generating a distorted tetrahedron (Fig. 1). The Cd-O bond distances range from 2.336(4) to 2.517(5) Å, and the K-O bond lengths are in the range of 2.692(5)-2.708(5) Å. Each L2− ligand adopts a (µ2-µ2)-(µ2-µ2)-µ6 coordination mode linking with two Cd(II) ions and four K(I) ions (Fig. S1). In the structure of 1, each {CdO8} polyhedron shares four edges with four adjacent {KO4} tetrahedrons, and each {KO4} tetrahedron shares two edges with two adjacent {CdO8} polyhedrons, these connectivities further link the polyhedrons into a 1D chain extending along crystallographical b direction (Fig. 2a). Finally, these 1D infinite 1D chains are connected by the L2− ligands, giving rise to a 3D heterometallic framework (Fig. 2b). Calculated by PLATON, it can be found that the framework of 1 contains 25.5% solvent accessible volume (1391.8 Å3 total accessible volume/5538.8 Å3 per unit cell volume).
Powder X-ray diffraction pattern (PXRD) and thermogravimetric analysis (TGA)
The experimental and simulated PXRD patterns are shown in Fig. S2a. It is obvious that the experimental pattern matches well with the simulated one generated from the single crystal diffraction data, indicating that the crystal structure of 1 is truly representative of the obtained bulk samples.
The TGA curve of 1 was shown in Fig. S2b. It displays two-step weight loss process. The first weight loss is 12.31% occurring in the range of 72–117℃ that can be assigned to the removal of the coordinated water molecules (calcd: 12.35%), and the second occurred ranging from 300 to 410℃ with a weight loss of 49.48% that can be attributed to the decomposition of the organic ligand (calcd: 49.5%).
Photoluminescent sensing properties of 1
The solid-state luminescent spectra of 1 and free H2L were measured at room temperature (Fig. S3). Compound 1 shows intense luminescence with emission band centered at 456 nm (λex = 350 nm), and the free H2L has an emission band at 436 nm (λex = 340 nm) that was caused by intraligand π*→π/n electron migration transitions. Based on the reported literature [20], the luminescence origin of 1 can be tentatively assigned to intraligand charge transfer. Compared to that of the free H2L ligand, the obvious red-shift of 20 nm for 1 may be closely related to the coordination of L2− ligand to central metal ions.
The intense emission and good thermal stability of 1 further impelled us to investigate its sensing properties for different solvent molecules. The luminescence intensities of 1 dispersed in different organic solvent were measured at room temperature. As shown in Fig. 3a, the solvents of H2O, DMF, DMA, MeOH, and EtOH showed negligible effect on the luminescence intensity of 1. In the case of THF, CH3CN, acetone and EG, the luminescence intensity of 1 decreased with quenching efficiencies ranging from 27.5–51.6%. Interestingly, we found that the luminescent intensity of 1 was almost completely extinguished when dispersed in CS2. These results imply that compound 1 has highly selective detection ability toward CS2. Then, the sensing sensitivity of 1 toward CS2 was further investigated via the titration experiment by addition of 10 ppm CS2 each time. As shown in Fig. 4b, the luminescence intensities of 1 decreased gradually upon the addition of CS2, and when the CS2 concentration is up to 110 ppm, the luminescence of 1 was almost completely quenched with a high quenching efficiency of 95.92%. Moreover, the detection limit of 1 toward CS2 was calculated with 3σ/k (k: slope, σ: standard) via a linear fitting in the concentration range of 0–60 ppm, and the value of detection limit for CS2 is calculated to be about 0.113 ppm (Fig. S4), indicating that compound 1 exhibits high detection sensitivity toward CS2. After luminescence sensing of CS2, the structural skeleton of 1 remains intact that was demonstrated by the PXRD experiment (Fig. S2a). Thus, compound 1 can be served as a structurally stable sensing material for the detection of CS2 with high selectivity and sensitivity.
Compound significantly reduced the body blood glucose levels of dose and time dependently
For the treatment of the children diabetes, the compound with new structure was synthesized in this research, and its inhibitory activity on the increased blood glucose levels was determined with blood glucose meter. As the results showed in Fig. 4, we can see that the there was a significantly increased level of blood glucose in the model group than the control group, with P < 0.005. After the treatment of the new compound with 1, 2 and 5 mg/kg, the levels of body blood glucose in the diabetes mice was reduced in a dose and time dependent manner. This result showed that the compound has excellent application values on the diabetes treatment.
Compound obviously inhibited the relative expression of the insulin receptor on liver cells
As introduced above, the new compound could obviously reduce the body blood glucose in a dose and time dependent manner. As the insulin receptor on the liver cells could significantly regulate the body's blood sugar. Thus, the real time RT-PCR was conducted and the content of the insulin receptor on the liver cells was measured. The results in Fig. 5 showed that the relative expression of the insulin receptor on the liver cells of the model group was much lower than that of the control group. After the treatment of the new compound, the content of the insulin receptor on the liver cells was significantly increased. This promotion of the new compound exhibited a dose and time dependent relationship.