Rhenium (1) tricarbonyl schiff base complexes: a mechanistic study

[摘要] English: Both technetium and rhenium have been studied extensively over the years, due to their ability to coordinate with mono- and bidentate ligands to form metal(I) tricarbonyl complexes, fac-[M(CO)3(X)(L,L'-Bid)] (M = Tc(I) and Re(I), L,L'-bid = bidentate ligand and X = MeOH, H2O or Br). The interest in these complexes is based on the diagnostic properties of technetium and therapeutic properties of rhenium in the study of radiopharmaceuticals. These complexes possess characteristics that can be utilized for the application in nuclear medicine. The aim of this study was based on the study of the chemistry of technetium and rhenium to gain more information about their ability to coordinate with potential ligands such as Schiff base ligands. From this, a Schiff base ligand such as 5-methyl-(2- cyclohexyliminomethyl)phenol �?5Me-Sal-Cyhex was synthesized and characterized. This was successfully coordinated to fac-[M(CO)3]+ core to form metal(I) tricarbonyl complexes. The synthesis and characterization of the N,O-bidentate ligand or Schiff base ligand and all metal complexes are reported in Chapter 3. All rhenium products obtained were characterized by UV/Vis, NMR (1H and 13C) and IR. The rhenium complexes, fac-[Re(CO)3(X)(N,O-Bid)], were synthesized with variety of monodentate ligands (X = MeOH, imidazole, pyridine and pyrazole) coordinated on the sixth position. Three crystal structures of 5-methyl-(2-cyclohexyliminomethyl)phenol �?5Me-Sal- Cyhex, fac-[Re(CO)3(MeOH)(5Me-Sal-Cyhex)] and fac-[Re(CO)3(Imid)(5Me-Sal-Cyhex)] were obtained from the characterization performed by X-ray diffraction. These complexes crystallised in the orthorhombic and monoclinic crystal systems in the respective space groups of P212121, C2/c and P21/n. Technetium(I)-99 tricarbonyl complexes, fac-[99Tc(CO)3(MeOH)(5Me-Sal-Cyhex)] and fac- [99Tc(CO)3(MeCN)(5Me-Sal-Cyhex)] were synthesized and characterized by HPLC to determine the reaction completion and chemical similarity between rhenium and radioactive technetium-99. A kinetic investigation was performed on the formation reaction between 5Me-Sal-Cyhex and [NEt4]2[Re(CO)3(Br)3]. It was observed that the formation reaction would unsuccessfully be determined due to the small absorbance changes over time. A model N,N'-bidentate ligand system (ethylenediamine) was therefore selected to study the formation reaction. The UV/Vis analysis showed two reactions that occur during the formation reaction between ethylenediamine and ReAA. Since the first fast reaction has the half-life, t1/2 = <5 seconds, the stopped flow kinetic investigation is required. The second slow formation reaction between ethylenediamine and ReAA was monitored at 14.8oC, 25.2oC, 35.0oC and 45.0oC. Rate constants and activation parameters (ΔH�?= 28 ± 0.05 kJ mol-1 and ΔS�?= -188 ± 0.17 J K-1 mol-1) of the formation reaction between ethylenediamine and [NEt4]2[Re(CO)3(Br)3] were obtained. The negative ΔS�?value is indicative of an associative-type mechanism. In addition to the above UV/Vis study, 1H NMR analysis was performed which confirmed the mechanistic and kinetic observations of the reaction between ethylenediamine and ReAA that forms in over 13 hour period. From the 1H NMR investigation, it was observed that the formation reaction between ethylenediamine and ReAA occurs via two reactions, defined as a first fast and second slow reaction.

[发布机构] University of the Free State