Selectivity controlling factors in the reductive elimination of coinage metal complexes
by Bastian Zimmer
Date of Examination:2024-06-11
Date of issue:2024-07-12
Advisor:Prof. Dr. Konrad Koszinowski
Referee:Prof. Dr. Lutz Ackermann
Referee:Prof. Dr. Franc Meyer
Persistent Address:
http://dx.doi.org/10.53846/goediss-10609
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Abstract
English
In this thesis, reductive elimination reactions and their selectivity-determining factors were investigated using ESI and tandem-MS. To this matter, square-planar coordinated coinage- metalate complexes bearing trifluoromethyl ligands were chosen as the model system. The anionic complexes were transferred into the gas phase using ESI and studied through tandem- MS. Fragmentation experiments were used to determine the unimolecular reactivity of these complexes and their fragmentation products. Quantum chemical computations were applied to understand potential energy surfaces of various reaction pathways. The electronic structures of high-valent coinage metal complexes were analyzed, revealing an inverted ligand field and quasi-d10 configuration. Negative-ion mode ESI mass spectrometry of [M(CF3)4][NBu4] in THF provided an efficient method to generate gas-phase metalate complexes. The addition of organyl magnesium halides or organolithium reagents to [M(CF3)4][NBu4] allowed the exchange of one or more of the trifluoromethyl groups. The generated coinage-metal complexes [M(R)n(CF3)4-n]− (n = 0-4) were investigated through gas-phase fragmentation experiments regarding the intrinsic reactivity, and the underlying properties of substituents and coinage metals governing the selectivity were determined. It was found that with the stronger metal-carbon bonds of the heavier homologues, increasing internal energies are necessary for collision-induced dissociation experiments. This contributed to higher reaction barriers, which were calculated with quantum chemical methods. The calculations revealed that cuprate complexes preferably undergo concerted coupling reactions, which renders them superior for synthetic purposes, while argentates exhibited a more pronounced preference for homolytic bond cleavages. The highest bond strengths were found in aurate complexes, which facilitated the CF2-extrusion and β-hydrogen competing pathways. Moving forward with the analysis of the substituents, I examined the influence of the coupled moieties on the reactivity. It was found that alkyl substituents with increasing radical stabilization energies promoted the homolytic bond cleavage pathway. On the other hand, aryl moieties showed a strong preference for concerted coupling through their hybridization, which enhanced the orbital overlap in the transition state structures to improve the coupling process. In the end, a systematic analysis of the reactivity of the [Cu(Me)n(CF3)4−n]− complexes was conducted with advanced quantum chemical methods. It was found that the properties of the methyl groups supported their participation in the concerted coupling reaction. In contrast, the trifluoromethyl groups, though their nucleophilic character, remained preferably at the cuprate anion and facilitated the elimination. Therefore, this thesis deepened the understanding of product selectivities during reductive elimination, enhancing mechanistic insight for optimizing practical applications.
Keywords: Mass spectrometry; reductive elimination; coinage metal
