Gas Phase Protolysis of Trisarylzincate Anions

The Fe-MAN Challenge: Ferrates-Microkinetic Assessment of Numerical Quantum Chemistry

Organometallic species, such as organoferrate ions, are prototypical nucleophiles prone to reacting with a wide range of electrophiles, including proton donors. In solution, the operation of dynamic equilibria and the simultaneous presence of several organometallic species severely complicate the analysis of these fundamentally important reactions. This can be overcome by gas-phase experiments on mass-selected ions, which allow for the determination of the microscopic reactivity of the target species. In this contribution, we focus on the reactivity of a series of trisarylferrate complexes toward 2,2,2-trifluoroethanol and 2,2-difluoroethanol. By means of mass-spectrometric measurements, we determined the experimental bimolecular rate constants kexp of the gas-phase protolysis reactions of the trisarylferrate anions FePh3- and FeMes3- with the aforementioned acids. Based on these experiments, we carried out a dual blind challenge, inviting theoretical groups to submit their best predictions for the activation barriers and/or theoretical rate constants ktheo. This provides a unique opportunity to evaluate different computational protocols under minimal bias and sets the stage for further benchmarking of quantum chemical methods and data-driven approaches in the future.

C versus O Protonation in Zincate Anions: A Simple Gas-Phase Model for the Surprising Kinetic Stability of Organometallics

For better understanding the intrinsic reactivity of organozinc reagents, we have examined the protolysis of the isolated zincate ions Et₃ Zn^- , Et₂ Zn(OH)^- , and Et₂ Zn(OH)₂ Li^- by 2,2,2-trifluoroethanol in the gas phase. The protonation of the hydroxy groups and the release of water proceed much more efficiently than the protonation of the ethyl groups and the liberation of ethane. Quantum-chemical computations and statistical-rate theory calculations fully reproduce the experimental findings and attribute the lower reactivity of the more basic ethyl moiety to higher intrinsic barriers, which override the thermodynamic preference for its protonation. Thus, our minimalistic gas-phase model provides evidence for the intrinsically low reactivity of organozinc reagents toward proton donors and helps to explain their remarkable kinetic stability against moisture and even protic media.

Anionic Dimers of Fluorinated Alcohols

Negative-ion mode electrospray ionization of solutions of ethanol (R^F0OH), 2-fluoroethanol (R^F1OH), 2,2-difluoroethanol (R^F2OH), and/or 2,2,2-trifluoroethanol (R^F3OH) produces anionic dimers of the types (R^FnO)₂H^- and (R^FnO)(R^Fn+1O)H^-. The exchange reactions of these anionic dimers with the neutral alcohols are examined in a quadrupole-ion trap to extract kinetic data, from which the reaction Gibbs energies are obtained. In all cases, the formation of anionic dimers containing the more highly fluorinated alcohols is favored. Quantum chemical calculations confirm this trend and, besides affording structural data, also determine the dissociation energies of the anionic dimers. These dissociation energies are much higher than those of the corresponding neutral dimers and increase further for the more highly fluorinated alcohols due to the stronger hydrogen-bond donor ability of the latter. The present results on the interaction of individual alkoxide anions and neutral alcohol molecules contribute to a better understanding of the association of the fluorinated alcohols in solution.