β-Hydrogen Eliminations from Organoaluminates and -Cuprates in the Gas Phase
by Wilbur Richter
Date of Examination:2025-11-04
Date of issue:2025-12-10
Advisor:Prof. Dr. Konrad Koszinowski
Referee:Prof. Dr. Konrad Koszinowski
Referee:Prof. Dr. Manuel Alcarazo
Referee:Prof. Dr. Franc Meyer
Referee:Prof. Dr. Ricardo Mata
Referee:Dr. Holm Frauendorf
Persistent Address:
http://dx.doi.org/10.53846/goediss-11677
Files in this item
Name:Dissertation Wilbur Richter - β-Hydrogen Eli...pdf
Size:2.85Mb
Format:PDF
The following license files are associated with this item:
Abstract
English
The β-hydride elimination is a fundamental process in many organometallic reaction mechanisms and decomposition pathways. This thesis investigates the β-hydride elimination of alkyl organoaluminates and alkyl organocuprates in the gas phase using electrospray ionization mass spectrometry (ESI-MS). The formation of homoleptic and heteroleptic alkyl organoaluminates and alkyl organocuprates was observed in ESI-MS measurements of a solution of the corresponding metal salts and transmetalation reagents. Alkyl aluminates could be generated using both Grignard and organolithium reagents. As expected, only the latter yielded the corresponding alkyl cuprate species. Using a single transmetalation reagent predominantly produced the homoleptic species [R4Al]−. These species, when mass-selected and subjected to CID, exclusively fragmented to [R3AlH]− with the loss of one alkene. This reaction was confirmed to be a β-hydride elimination by using a deuterium labeled (in the β position) ligand. The MSn functionality of the used mass spectrometer allowed for the investigation of the fragment ions (hydrides), revealing two possible reaction pathways: first, another β-hydride elimination to [R2AlH2]−, or second, the loss of H2, with the former being the major reaction pathway. This behavior was observed for all measured alkyl ligands (Et, nBu, Hex, C14H29, iPr, Cy), whereas unexpectedly the homoleptic complex [iPr4Al]− exhibited direct loss of H2. For systems containing a Al−H moiety, two possible reaction pathways are feasible: first the reductive elimination, if two hydrides are directly bound to the metal center, or second via the loss of one aluminum-bound hydride and a β-hydride from an alkyl ligand. To directly compare the reactivity of different alkyl ligands, heteroleptic complexes of the form [RnAlBu4-n]− were generated using two different transmetalation reagents. In all cases, at least one ligand was Bu, serving as a reference for the comparison of the reactivity of the other ligand to that of Bu. These experiments revealed differences in the reactivity that could be rationalized through a combination of steric effects, differences in double bond strength, and the difference in hydridic character of the β-hydrogens. The observed reactivity of both homoleptic and heteroleptic complexes was further rationalized by supporting DFT calculations. The same approach was applied for the investigation of alkyl cuprates, which were observed in two different oxidation states. The main species, homoleptic alkylcopper(III) complexes [R4Cu]− (R = nBu, sBu, tBu, Hex, Dec), exhibit a single reaction pathway: the reductive elimination of two alkyl ligands, forming the corresponding copper(I) species [R2Cu]−. These species, which were also observed in the MS1 spectra, react only via a β-hydride elimination forming [RCuH]−, which could further eliminate a second alkene yielding [CuH2]−. Unlike the aluminates, no loss of H2 was observed, neither from the homoleptic species nor from the hydride. Next, the reactivity of homoleptic copper(I) complexes with different butyl ligands (nBu, sBu, tBu) was compared, enabled by the same molar mass and the same number of degrees of freedom of these systems. This revealed an at first unexpected trend of reactivity sBu > nBu > tBu, which could be rationalized by steric effects in the transition states. For the heteroleptic complex [HexCuBu]−, a strong preference for the elimination of butene was experimentally observed. This selectivity could not yet be rationalized and may be caused by a mass discrimination effect of the mass spectrometer. The observed reactivity of the homoleptic complexes was further rationalized by supporting DFT calculations, which revealed a significantly lower barrier for the β-hydride elimination of homoleptic copper(I) complexes (163 kJ mol−1) compared to the analogous organoaluminate species (241 kJ mol−1). This difference can be rationalized by the reduced steric hindrance and the more easily accessible vacant coordination site in the cuprate. In addition, agostic interactions between the copper center and the β-C−H bond may further contribute to the lower barrier observed for the cuprate species.
Keywords: β-hydride elimination; ESI-MS; mass spectrometry; aluminates; cuprates; collision-induced dissociation; gas phase
