Barriers of Protonation Reactions of Organometallics
by Rene Kevin Rahrt
Date of Examination:2023-08-01
Date of issue:2023-09-20
Advisor:Prof. Dr. Konrad Koszinowski
Referee:Prof. Dr. Konrad Koszinowski
Referee:Prof. Dr. Ricardo A. Mata
Referee:Prof. Dr. Manuel Alcarazo
Referee:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Martin A. Suhm
Referee:Prof. Dr. Philipp Vana
Persistent Address:
http://dx.doi.org/10.53846/goediss-10103
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Abstract
English
Protonation reactions are entirely understood for simple organic and inorganic molecules and there are elaborated concepts to describe such acid-base reactions in both the solution and gas phase. For organometallic species, however, there is no such deep understanding. Thus, this dissertation about the barriers of protonation reactions of organometallics applies the concepts and methods of physical organic chemistry to organometallic species to elucidate their intrinsic reactivity towards protonation and assess the effects at play. For this purpose, mass-selected organometallic species were subjected to ion-molecule reactions with proton donors ROH (R = CF3CH2, CF2HCH2, CFH2CH2, HCO) in a three-dimensional quadrupole-ion trap mass spectrometer at T = (310±20) K. Kinetic measurements of these reactions allowed for the determination of experimental bimolecular rate constants kexp. In addition, the proton-transfer reactions were computed quantum-chemically using DFT (ωB97X-D3/def2-TZVP) and coupled-cluster methods (DLPNO-CCSD(T)/cc-pV[T;Q]Z) and simulated kinetically by carrying out Master-equation calculations based on statistical rate theory. Thereby, the theoretical bimolecular rate constants ktheo were obtained. From the interplay of experiment and theory the intrinsic reactivity of the organometallic species towards protonation was inferred. Moreover, the experimental rate constants served as the reference data to benchmark the performance of quantum-chemical methods to predict the barrier heights associated with the protonation of organometallics. For the example of closed-shell organozincate anions R3Zn− (R = Me, Et, aryl) the prototypical reaction mechanism for the protolysis reactions of organometallic species in the gas phase was investigated. For the enthalpy at 0 K, ΔH0, a double-well potential consisting of three distinct reaction steps was found: first, the formation of the pre-reactive complex, second, the actual proton transfer from the acidic site of the proton donor to the basic site of the organometallic ion and, last, the dissociation of the product complex into the products. The first reaction step could be described fairly well with the capture theory by Su and Chesnavich. The proton transfer was modelled in accordance with classical RRKM theory. The reaction barrier of the proton-transfer step was found to depend on the reaction energy ΔrH0, i.e. an increasing exothermicity of the protonation reaction lowers its activation barrier ΔH‡0. The dissociation of the product complex occurred easily. By varying the nature of the substituents of the organozincates Et2ZnX− (X = H, Et, OH, F, Cl), it was found that such bases X−, to which Eigen referred as normal (e.g. X = OH), reacted at the collision-rate limit. The reactivity emerges from the formation of hydrogen bonds between the proton donor and acceptor which mediate the proton transfer. In marked contrast, Eigen non-normal bases such as aryl or alkyl moieties (C bases) that are typical in organometallic species react surprisingly slow in proton-transfer reactions. Despite their higher basicity, such bases feature unusually high intrinsic barriers; i.e., their kinetic behavior and thermochemical properties are opposed. The finding could help to understand why some organometallic transformations such as Negishi cross coupling are feasible within protic media. As the agreement between the measured experimental rate constants kexp and the predicted theoretical rate constants ktheo was excellent, the quantum-chemical calculations of the protonation barriers were found to achieve chemical accuracy. Moreover, the protolysis reactions of the open-shell (S = 2) organoferrate anions R3Fe− (R = Ph, Mes) were investigated within the Fe-MAN challenge which stands for “Ferrates – Microkinetic Assessment of Numerical quantum chemistry”. From the poor agreement of the experimental and theoretical rate constants, kexp and ktheo, it became evident that usual quantum-chemical methods (e.g. PNO-LCCSD(T)-F12/def2-TZVP//ωB97X-D3/def2-TZVP) are still challenged with the accurate prediction of barrier heights for such open-shell organometallic systems. Thus, more experimental reference data for future benchmarking endeavors is required.
Keywords: Chemistry; Kinetics; Benchmarking; Organometallics; Protonation; Gas Phase; Mass Spectrometry; Ion-Molecule Reaction
