Novel environmental catalysts were prepared from the reaction of Cu (II), Fe(III) and Pd(II) ions with some pyranothiazole ligands (PTP, MPTP and CPTP) were synthesized. Bi-dentate pyranothiazole ligands (PTP, MPTP and CPTP) were synthesized by reaction of (Benzaldehyde, 4-methoxybenzaldehyde or 4-chlorobenzaldehyde), malononitrile, 2, 4-thiazolidinedione and piperidine. Structures of all the investigated ligands were characterized by their melting points, 1H NMR, 13C NMR spectra, elemental analyses, UV-Vis and IR spectra. Molecular structures of the tested complexes were confirmed by several tools techniques including decomposition temperatures, (CHN) elemental analyses, IR, UV-Vis spectra, molar conductance and magnetic moment measurements. Thermal gravimetric analysis (TGA) was executed to confirm the presence of water molecules as well as the mono-nuclear feature of isolated complexes and indicated high stability for all the tested complexes. Accordingly, thermo kinetic parameters were calculated for all decomposition steps and proposed the presence of lattice and/or coordinated water molecules in the tested complexes. All complexes have non-electrolytic nature due to the absence of any counter ions in their structures as molar conductance measurements in fresh solutions of DMF lies in the range 5.12- 24. 85 Ω-1 cm2 mol-1.
Moreover, the obtained analytical data regarding complexation in solution, molar ratio and continuous variation methods suggest 1M: 1L molar ratio. Also, the stability constant of tested complexes was identified in solution state and the formation constants (Kf) were evaluated by applying continuous-variation method. Stability of complexes was arranged by MPTPFe > MPTPPd > CPTPFe > PTPFe > PTPCu > MPTPCu > PTPPd > CPTPPd > CPTPCu in agreement with Kf values. The pH profile revealed that the wide range of pH stability of the tested complexes is at pH = 4-10 /11 in most of them.
Computational study was implemented to extract significant features from Gaussion09, Pharmit link for ligands and its complexes. In addition to, maps of frontier orbitals, MEP and iso-surface with array plot, were obtained over formulated chk-file reflect high nucleophilic feature with reduced outer contour on two coordinating sites (NH& NH2). Also, the best atomic distribution within structural forms was obtained by Material Studio Package via DFT method. Correlation between all analytical and physical tools used in the analysis, octahedral geometry is suggested for all the prepared Cu(II) and Fe(III) complexes and square planar geometry for Pd(II) complexes. Too, crystal surface properties in packing system, was studied using Crystal explorer program, to evaluate extent of contact between surfaces. Optical band gap, electrophilic and electronegativity features are essential parameters that reflect specific characteristics of such Pd(II) complexes, which may be promising in catalytic field.
The catalytic potential of the tested complexes have been tested within synthesis of (polyfunctionalized pyrrole derivatives, pyrazole-4-carbonitrile derivatives and Dihydrotetrazolo[1,5-a]pyrimidine derivatives) in one-pot reaction using ultrasonic irradiation.
Pd (II) complexes catalyst were selected for this application based on history of Pd (II) complexes as well as the properties expected theoretically. All reaction conditions were optimized among that variable Lewis acid and base catalyst in comparing to our new complexes. Pd(II) complexes catalysts displayed superiority in overall all trials with high yield, short time and green conditions (solvent H2O or H2O/EtOH). Also, the recovery of hetero-catalyst was succeeded and reused by the same efficiency up to five times after that the efficiency was reduced. DFT study was also applied to explain the causes behind the superiorly of Pd (II) complexes as its energy gap value. Besides, the optimization process for intermediate compounds which may be appeared in catalytic cycle was executed to differentiate between their stabilities, to support the suggested mechanism. Finally, this simple, economical and green catalytic procedure may be applied to the industry in the future.