Abstract
The chemistry of technetium (99Tc) remains relatively underdeveloped in comparison to its nearest neighbors, mainly due to its radioactive nature. This knowledge-gap is particularly evident in research fields that comprise reactive complexes with small, labile ligands like gases. The research state of its metastable 99mTc isotope is much more researched as it constitutes a pivotal part for imaging in nuclear medicine. Outside this applied domain, investigations with technetium typically focus on very fundamental aspects, and rhenium is thus often taken as its surrogate, especially at low oxidation states. Despite these inherent challenges, the central position of technetium in the periodic table suggests vastly unexplored reactivity potentials in comparison to its neighbors. In this context, the interactions of small molecules, such as N2, CO2 and H2, with Tc complexes has received sparse attention but may be very promising. This thesis is dedicated to exploring reactivities of Tc pincer-type complexes towards a range of small molecules with reoccurring comparisons to homologous rhenium compounds whenever possible. The readily accessible TcIII precursor [TcCl3(MeCN)(PPh3)2] was employed, among other precursors, to assess the reactivities towards a variety of pincer-type ligands. These encompassed selected, tridentate binding motifs, including PNP, PCP, SNS, CNC and NNN, occupying the three meridional coordination sites at the metal center. In general, the PCP-, SNS- and CNC-type ligands exhibited incompatibility under the applied conditions and did not coordinate. Facile phosphine and/or chloride substitutions from the precursor complex were found with PNP- and NNN-type ligands. The products were mostly structurally characterized. Most suitable for the study of their interactions with small molecules were the penta-coordinate [TcIII(PNPtBu)Cl2] and octahedral [TcIII(PyrPNPtBu)Cl3] due to their superior characteristics over the NNN-type complexes (e.g., solubility) and well-defined coordination geometries. The study of small molecules in interaction with {Tc(PNP)}-frameworks followed the general strategy of reducing the precursor complexes under a defined atmosphere (e.g., N2, H2 or CO). In this line, the reductions of meridional TcIII pincer complexes [TcIII(PNP)Clx] under an N2 atmosphere gave products that comprise the small diatomic molecule as ligands. These TcI complexes expanded the small list of known N2 complexes with the radioactive element. One of the most notable differences to the reactivities of the homologous Re compounds is the lack of N2 splitting into nitrido-complexes ([ReN(PNPtBu)Cl]) or formation of NH3 in the presence of proton sources and reduction equivalents. This divergence is an example of contrasting reactivities between the two elements, which are often (and sometimes wrongly) considered to be identical. These results highlight how even small kinetic and/or thermodynamic differences between Re and Tc lead to substantially diverging reactivities. The well-defined coordination geometries of the TcPNP compounds were further explored towards their interaction with reactive H2. While the structural characterization of Tc complexes comprising hydride or H2 ligands proved challenging, some indirect indications from spectroscopic analyses were obtained. For instance, the paramagnetic NMR signature of [TcIII(PyrPNPtBu)Cl(H)2] matched the presence of two hydride ligands after oxidative addition of H2 to the formal, electron-deficient [TcI(PyrPNPtBu)Cl]. This intermediate is speculated to form transiently after two-electron reduction of [TcIII(PyrPNPtBu)Cl3]; however, no spectroscopic evidence was found, and its existence remains a hypothetical formality. The complex [TcIII(PyrPNPtBu)Cl(H)2] displayed catalytic activity in preliminary hydrogenation experiments with the strained olefin norbornene. Albeit of only preliminary character, this investigation provides the groundwork for a future, more sophisticated approach towards a catalytic application of a technetium complex. In contrast, the analogous reactions with Re resulted in the spectroscopic identification of the polyhydridic species [Re(PyrPNPtBu)Cl(H)4] and [Re(PyrPNPtBu)Cl(H)6]. Based on the analytical data, the interactions of H2 with the [MI(PyrPNPtBu)Cl]-frameworks differ and clearly depend on the type of metal center, Tc or Re. For CO, contrasting reactivities were found depending on the nature of the PNP ligand. The reduction of [Tc(PNPtBu)Cl2] with three different reducing agents in separate reactions, gave three distinct products. While all comprised two CO ligands, they varied in the properties of the central nitrogen donor atom of the PNP ligand. Deprotonation at the adjacent methylene position yielded [Tc(PNPtBu)(CO)2Cl] with a neutral imine moiety ([Co(C5Me5)2] as reducing agent) and protonation, when employing SmI2 as reductant, gave the cationic [Tc(PNHPtBu)(CO)2]+ with a neutral amine moiety. Solely the reduction with KC8 led to [Tc(PNPtBu)(CO)2], comprising the amide-based form of the PNP ligand, retained from the starting material. In the case of [Tc(PyrPNPtBu)Cl3], the reduction cleanly yielded trans-[Tc(PyrPNPtBu)(CO)2Cl]. In turn, this TcI complex marked the starting point for a study of metal-ligand cooperativity with technetium. The dearomatized [Tc(PyrPNPtBu*)(CO)2] showed analogous reactivities as its Re congener with H2 and CO2. An elaborate investigation enabled access to the bond activation in a multitude of substrates with weakly acidic protons, and yielded fully characterized complexes of the cis-[Tc(PyrPNPtBu)(CO)2R]-type (R = anionic ligand). The CO2-bound compound cis-[Tc(PyrPNPtBu–COO)(CO)2] was dearomatized by deprotonation and the resulting cis-[Tc(PyrPNPtBu*–COO)(CO)2]+ assessed for CO2-hydrogenation to produce formate. The strong preference of the PyrPNPtBu ligand to bind transition metals in a meridional geometry was exploited to investigate the fac–mer isomerizations of the fac-{M(CO)3}+-cores with Re and 99(m)Tc. The reactions of the Re and 99Tc precursors, fac-[MX3(CO)3]2–, with PyrPNPtBu in the presence of a halide scavenger gave the mer-[M(PyrPNPtBu)(CO)3]+ complexes via intramolecular rearrangement. The reactivity transfer to the aqueous, saline chemistry of 99mTc was successful under anaerobic conditions. The mer-[99mTc(PyrPNPtBu)(CO)3]+ compound was synthesized in either a one- or two-step pathway. This marks the first example of a complex comprising the mer-{99mTc(CO)3}+-moiety. While the macromolecular reactions of Re and Tc with terpyridine did not lead to the same isomerization, the mechanistic considerations implied that the inherently different 99mTc chemistry could enable the synthesis of further meridional tricarbonyl complexes, such as with terpy.
Besmer, Manuel