ABSTRACT
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.
ABSTRACT
The chemistry of alkali-metal enolates is dominated by ion pairing. To improve our understanding of the intrinsic interactions between the alkali-metal cations and the enolate anions, we have applied Cooks' kinetic method to determine relative M+ (M=Li, Na, K) affinities of the stabilized enolates derived from acetylacetone, ethyl acetoacetate, diethyl malonate, ethyl cyanoacetate, 2-cyanoacetamide, and methyl malonate monoamide in the gas phase. Quantum chemical calculations support the experimental results and moreover afford insight into the structures of the alkali-metal enolate complexes. The affinities decrease with increasing size of the alkali-metal cations, reflecting weaker electrostatic interactions and lower charge densities of the free M+ ions. For the different enolates, a comparison of their coordinating abilities is complicated by the fact that some of the free anions undergo conformational changes resulting in stabilizing intramolecular interactions. If these complicating effects are disregarded, the M+ affinities correlate with the electron density of the chelating functionalities, that is, the carbonyl and/or the nitrile groups of the enolates. A comparison with the known association constants of the corresponding alkali-metal enolates in solution points to the importance of solvation effects for these systems.
ABSTRACT
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.
ABSTRACT
Anionic coordination polymerizations proceed via highly reactive intermediates, whose in situ analysis has remained difficult. Here, we show that electrospray-ionization mass spectrometry is a promising method to obtain detailed information on the polymerization process. Focusing on polymerization reactions of 1,3-dienes initiated by CoCl2 /RLi (R=Me, nBu, tBu, Ph), we directly observe the growing polymer chains and characterize the active anionic cobalt centers by gas-phase fragmentation experiments. On the basis of these results, we suggest a plausible mechanism for the polymerization reaction. Moreover, the ESI mass spectra permit the determination of molecular weight distributions, which are in good agreement with those derived from NMR-spectroscopic as well as MALDI mass-spectrometric measurements, and afford a wealth of kinetic data.
ABSTRACT
Negative-ion mode electrospray ionization of solutions of ethanol (RF0OH), 2-fluoroethanol (RF1OH), 2,2-difluoroethanol (RF2OH), and/or 2,2,2-trifluoroethanol (RF3OH) produces anionic dimers of the types (RFnO)2H- and (RFnO)(RFn+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.
ABSTRACT
We have applied a combination of tandem-mass spectrometry, quantum-chemical calculations, and statistical rate theory computations to examine the gas phase reactions between the trisarylzincate anions ArXZnPh2- (ArX = p-X-C6H4, X = NMe2, OMe, Me, H, F, and Cl) and 2,2,2-trifluoroethanol at T = 310 ± 20 K. The observed reactions bring about the protonation of one of the aryl anions, which is then released as the corresponding arene, while the formed alkoxide binds to the zinc center. The protonation is faster for the more electron-rich aryl groups and shows a linear Hammett plot if the rate constant for X = NMe2 is discarded from the analysis. Although the reactions are highly exothermic, they proceed only with relatively low efficiencies (0.1% ≤ φ ≤ 1.3%). According to the quantum-chemical calculations, this behavior can be ascribed to the reactions proceeding through a double-well potential with a tight transition structure located at the central barrier. Based on these potential energy surfaces, the statistical rate theory computations can reproduce the measured rate constants within factors of 2 to 8. A comparison of the protolysis of the trisarylzincates with that of the corresponding free aryl anions demonstrates how the coordination to the metal center not only stabilizes the carbanions energetically but also moderates their reactivity. Thus, our gas phase study contributes to a better understanding of the fundamentals of organometallic reactivity.
ABSTRACT
Previous attempts to characterize the internal energies of ions produced by electrospray ionization (ESI) have chiefly relied upon benzylpyridinium ions, R-BnPy+, as thermometer ions. However, these systems are not well suited for this purpose because of their relatively high dissociation energies. Here, we propose benzhydrylpyridinium ions, R,R'-BhPy+, as a new class of thermometer ions. DLPNO-CCSD(T)/CBS//PBE0-D3BJ calculations for R,R'-BhPy+ (R,R' = H,H'; Me,Me'; H,OMe'; Me,OMe'; OMe,OMe'; NPh2,NPh2') predict that these ions fragment by the loss of pyridine via loose transition states. The computed threshold energies of these fragmentations, 0.70 ≤ E0 ≤ 1.74 eV, are significantly lower than those of the dissociation of the benzylpyridinium ions. The theoretical predictions agree well with results from guided ion beam experiments, which find threshold energies of 1.79 ± 0.11, 1.55 ± 0.13, and 1.37 ± 0.14 eV for the fragmentation of R,R'-BhPy+, R,R' = H,H'; Me,Me'; H,OMe', respectively. The determined thermochemistry for these systems is then used to characterize the internal energies of ions produced by ESI from dichloromethane and methanol solutions under standard conditions. Correlating the measured survival yields of five of the R,R'-BhPy+ ions with the computed threshold energies including explicit consideration of their dissociation rates, we derive energy distributions with maxima at 2.06 ± 0.13/1.88 ± 0.11 eV and widths of 0.86 ± 0.07/0.86 ± 0.06 eV (dichloromethane/methanol). These energy distributions are comparable to ion temperatures between 620 ± 20/590 ± 20 and 710 ± 20/680 ± 20 K (dichloromethane/methanol).
