This critical review summarizes progress of the rapidly developing and very active field of chemistry. The first part of the review deals with general synthetic approaches used to synthesize different silatranes. The most interesting feature of silatranes, i.e., variation of Si–N bond length on the basis of the axial substituent of , and other structural features, are described in the second part with special emphasis on crystallographic and theoretical studies. It is followed by a discussion on the reactivity of various silatranes. Silatranes have now gained acceptance for a wide variety of applications which are summarized in the last section of review. Some of them have extensive interest due to their medical use to heal wounds or stimulate hair- (pilotropic activity), biological properties, pharmacological properties e.g. antitumor, anticancer, antibacterial, anti-inflammatory, fungicidal activity, stimulating effect in animal production and effects. The review focuses on the extended potential of silatranes in sol–gel processes, mesoporous zeotypes, , commercial products such as adhesion promoters, formation and rubber compositions. This critical review will be helpful for general researchers, experts, advanced undergraduates and newcomers working on chemistry as this review presents greater emphasis on synthesis and characterization, structural properties, reactivity and applications of silatranes in the field of biology, material science, sol–gel chemistry, pharmaceutics, agriculture and medicine (311 references).
The C and N contents (%C = 4.46 and %N = 1.66) in SiNH2 are a sign of successful aminopropylation reaction. The increase in %C and %N contents in SiNTh-Py (%C = 11.34 and %N = 3.72) indicates that the (Z)-3-hydroxy-3-(pyridine-2-yl)-1-(thiophen-2-yl)prop-2-en-1-on is attached to SiNH2.
The FT-IR of SiNH2 (Figure 3A) exhibits νC–H and νNH2 bands at 2941 cm−1 and 1560 cm−1, respectively, from silylating 3-aminopropyltremethoxysilane, which are absent in the spectrum of unmodified silica gel. In the FT-IR spectrum of SiNTh-Py, obtained after reaction with β-keto-enol, the bands νC=C and νC=N at 1466 cm−1 and 1535 cm−1, respectively, demonstrate the successful immobilization of (Z)-3-hydroxy-3-(pyridine-2-yl)-1-(thiophen-2-yl)prop-2-en-1-on on SiNH2.
The Thermogravimetric curves TGA curve recorded for the starting silica shows only one mass change in the range of 25 °C–110 °C. This mass loss corresponds to the loss of the remaining absorbed water. The TGA curve of free silica, SiNH2, and SiNTh-Py are represented in Figure 3B. SiNTh-Py shows two stages of weight loss, the first one is similar to that of pure silica with 3.04% weight loss, and is followed by 10.00% weight loss around 110 °C–800 °C corresponding to the loss of the organic groups. This observation shows that the organic part is immobilized on the silica.
The solid-state 13C-NMR spectrum is shown in Figure 4. The signals observed for 3-aminopropyl-silica SiNH2 at δ= 9.02, 24.79 and 42.62 ppm have been assigned to the propyl carbon Si–CH2, –CH2–, and N–CH2, respectively. The signal at 50.62 ppm is assigned to methoxy group –OCH3 not substituted as confirmed by microanalysis. Other signals at 16.32, 48.04, 56.77, 121.71, 126.23, 137.04, and 148.01 ppm correspond to specific carbons atoms in (Z)-3-hydroxy-3-(pyridine-2-yl)-1-(thiophen-2-yl)prop-2-en-1-on moiety.
The specific surface areas (Figure 5) of free silica, SiNH2, and SiNTh-Py are 305.21, 283.08, and 229.59 m2/g respectively, and the pore volumes of these materials are 0.77, 0.69, and 0.59 cm3/g, respectively. Therefore, a decrease in the specific surface areas and pore volumes are due to the functionalization of silica.
2.3.3. Solid-Liquid Adsorption of Metal Ions
Effect of pH and Stirring Time
The effect of solution pH on the removal of Cu(II), Zn(II), Cd(II), and Pb(II) by SiNTh-Py is shown in Figure 6A. Metal ion removal by the adsorbent is increased when there is an increase in the pH of the solution. The maximal removal of Cu(II) was obtained at pH = 5, but it occurs at pH = 6 for Zn(II), Cd(II) and Pb(II).
The contact time (Figure 6B) reveals that the equilibrium is reached after only 25 min. This result indicates that the SiNTh-Py adsorbent has rapid adsorption kinetics. Therefore, it is suitable for an application in flow system as used in the preconcentration of trace metal ions.
Furthermore, the adsorbent presents higher adsorption capacity toward Cu(II) compared to the other metals under study. This is mainly dependent on several factors such as the nature, the charge, and the size of metal ions, and the affinity of donor atoms towards metals. This affinity towards Cu(II) allowing the extraction of 104 mg/g must be underlined, whereas the adsorption capacity of the SiO2 matrix was only 1 mg/g .
In order to investigate the mechanism of adsorption, kinetic parameters were evaluated using pseudo-first order  and pseudo-second order  models (Table 2). It is evident from Table 2 that, for all metals under study, values of the regression coefficient are much higher from pseudo-second order model than from pseudo-first order kinetic model. Furthermore, theoretical and experimental values of qe are close for pseudo-second order kinetics; this indicates the pseudo-second order model fits well with the experimental adsorption data.
The experimental data have been tested within two isotherm models. The first one is the Langmuir isotherm model  that describes the monolayer coverage adsorption and homogeneous surface. The second model is the Freundlich isotherm model  adapted to the description of the multilayer sorption and heterogeneous surface.
The Langmuir and Freundlich isotherm parameters for adsorption of Cu(II), Zn(II), Cd(II), and Pb(II) are given in Table 3. Comparison of the R2 values shows that the experimental data are quite well-fitted using the Langmuir isotherm model.
Energetic changes associated with the removal of Cu(II), Zn(II), Cd(II), and Pb(II) onto SiNTh-Py can be evaluated with the help of thermodynamic parameters (ΔG°, ΔH° and ΔS°) [32,33,34]. The results are given in Table 4. The negative values of ΔG° indicate the feasible and spontaneous nature of adsorption. The positive values of enthalpy ΔH° reveal that adsorption is endothermic. The positive values of ΔS° suggest a more random organization at the solid/solution interface.
The competitive adsorption experiment was carried out for Cu(II), Zn(II), Pb(II), and Cd(II) quaternary systems using an aqueous solution containing 140 mg/L of each metal ion. Figure 7 shows the adsorption capacity of metal ions in the quaternary systems. It is obvious that SiNTh-Py displays an excellent adsorption for Cu(II). However, the extraction seems to decrease with regard to the value obtained in the individual adsorption experiments, indicating a competitive complexation with other ions.
Thus, the SiNTh-Py shows promising potential to be a good adsorbent, particularly for the removal of Cu(II) from aqueous solutions containing competing ions.
Comparison with Alternative Adsorbents
Compared to several sorbents recently described in the literature (Table 5), the adsorbent prepared in the present work exhibits a higher adsorption capacity, especially for Cu(II). This efficiency is mainly due to the affinity of the ligand donor atoms towards this metal.