A crystallographic and mechanistic investigation of manganese(i) tricarbonyl complexes
Manganese has largely been used as a catalyst in epoxidation of olefins and the selective oxidation of unactivated C-H bonds in alkanes. Another major use of manganese is in water oxidizing catalysis, where manganese compounds are used in the production of hydrogen by water splitting. By looking at the manganese triad, a lot of radiopharmaceutical studies have been performed with Technetium. 99mTc is the radionuclide of choice since it has the ideal properties necessary for potential radiopharmaceuticals. Rhenium has been used as model for technetium radiopharmaceuticals and 188Re and 186Re have proved their use in bone metastases. Manganese has not been used in radiopharmaceuticals as it is somewhat toxic to the body. Some studies have been performed on tumor suppression using MnSOD, a mitochondrial enzyme. Very little work has been done on manganese coordination chemistry especially in terms of tricarbonyl aqua complexes. So far about 5 manganese tricarbonyl aqua complexes have been reported on the CSD. In this study, a comparison of manganese (I) and rhenium (I) tricarbonyl complexes is made. Rhenium (I) tricarbonyl aqua complexes have been synthesized by Kemp and Schutte et al. using N,N’-, N,O- and O,O’- bidentate ligands. The chosen N;N’- bidentate ligands are 2,2’-bipyridyl (Bipy) and 1,10-phenanthroline (Phen), N,Obidentate ligands are quinoline-2,4-dicarboxylic acid (2,4-Quin) and picolinic acid (Pico) and finally the O,O’-bidentate ligands chosen were 3-hydroxyflvone (Flav) and tropolone (Trop). In this study, the same N,N’- and N,O-bidentate ligand systems were used as well as 3-hydroxyflavone as O,O’-bidentate ligand in order to successfully compare these two metal cores. The synthesis of all the complexes has been reported in Chapter 3 and these were characterized by UV/vis, 13C NMR, 1H NMR and IR. Only two of the complexes were characterized by X-ray diffraction; fac-[Mn(CO)3(Bipy)(H2O)][CF3SO3] and fac- [Mn(CO)3(Phen)(H2O)][CF3SO3]. These monoclinic complexes both crystallized in the P21/c space group. The Mn-N,N’ bond distances were 2.051(2) Å and 2.058(19) Å for fac-[Mn(CO)3(Phen)(H2O)][CF3SO3], and 2.042(4) Å and 2.040(4) Å for the fac- [Mn(CO)3(Bipy)(H2O)][CF3SO3] crystal structure. A kinetic study was performed for the reaction between fac-[Mn(CO)3(Phen)(MeOH)]+, fac-[Mn(CO)3(Bipy)(MeOH)]+, fac-[Mn(CO)3(Pico)(MeOH)] and fac-[Mn(CO)3(2,4- Quin)(MeOH)] and different monodentate entering ligands pyridine (Py), thiourea (TU) and bromide ions (Br-). For all the reactions, the first order rate constants followed the following trend: k1 Br- > k1 TU > k1 Py. It was also observed that the N,O-bidentate complexes overall reacted faster than the N,N’-bidentate ligands. Both these trends were observed for the Re (I) complexes as well. Slightly negative ΔS† values, with large esd’s, were observed for all of the reactions which indicates towards an associative interchange mechanism. Overall, it was confirmed that the manganese complexes reacted much faster than the corresponding rhenium complexes. For the Phen and Bipy complexes, an increase in k1 of ~10 and ~7 for the reaction with bromide ions and ~40 and ~30 for the reaction with pyridine was found respectively. The Pico complexes showed an increase in k1 of a factor ~40 for the reaction with bromide ions and a factor ~590 for the reaction with pyridine. The complexes with the quinoline-2,4-dicarboxylic acid as bidentate ligand showed an tremendous increase of ~1150 for the reaction with pyridine from the rhenium complex to the manganese complex. This indicates the influence quinoline- 2,4-dicarboxylic acid has on the reactivity of the manganese (I) centre.