Thermodynamics versus kinetics in protodemetalation/deprotometalation reactions

• Applicant: Professor Dr. Konrad Koszinowski
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation ; Inorganic Molecular Chemistry - Synthesis and Characterisation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537682158

Protodemetalation, i.e., the protolysis of organometallics, and the reverse process, so-called deprotometalation, are of singular importance in organic chemistry. The former releases the products of numerous metal-catalyzed reactions and, thus, regenerates the active catalysts. At the same time, inadvertent protodemetalation forms the chief decomposition pathway of organometallic reagents, against which generations of synthetic chemists have been struggling. Deprotometalation, in turn, is valued as one of the most important variants of C−H activation, which accomplishes the functionalization of a wide scope of substrates. Despite the ubiquitous occurrence of protodemetalation and deprotodemetalation reactions, the fundamental principles governing these processes remain poorly understood. Indeed, most of the previous work analyzing these reactions has not even clearly distinguished between thermodynamic and kinetic aspects. As a result, the rational design of new moisture-stable organometallic reagents or more efficient C−H activation catalysts still is beyond reach. To a good deal, the problems in the mechanistic elucidation of protodemetalation and deprotometalation reactions result from the complexity of organometallics. In solution, the operation of Schlenk-type equilibria leads to the simultaneous presence of multiple organometallic species, which differ in their microscopic reactivity and thereby severely hamper mechanistic analyses. The proposed studies will overcome this challenge by not only considering protonation reactions in solution, but also probing them in the gas phase, where tandem-mass spectrometry permits the mass selection of individual organometallic ions and the examination of their microscopic reactivity toward neutral proton donors. In this way, the effect of different metal centers and different organyl groups as well as different proton donors on the measured rate constants will be determined. Likewise, the reactivity of the organometallic ions will be compared to that of amide-, alkoxide-, and halide-containing metal complexes, whose ability to form hydrogen bonds to the incoming proton donor supposedly reduces the barriers associated with their protonation, in spite of their lower basicities. For selected systems, the influence of single solvent molecules bound to the organometallic ions will be determined as well. These experiments as well as the comparison of the gas-phase results with the outcome of extensive kinetic measurements in solution will be instrumental in the assessment of solvation effects. Additional gas-phase fragmentation experiments and quantum chemical calculations will give further insight into the relevant potential energy surfaces and help to achieve a quantitative understanding of protodemetalation/deprotometalation reactions.

• DFG Programme: Research Grants