Unimolecular Reactivity of [Cu(R)(CF3 )3 ]- Complexes (R=Organyl): Stepwise versus Concerted Mechanism in Copper-Mediated Trifluoromethylation.

The cuprate complexes [Cu(R)(CF3 )3 ]- (R=organyl) offer an efficient synthetic access to valuable trifluoromethylation products RCF3 . Here, electrospray-ionization mass spectrometry is used to analyze the formation of these intermediates in solution and probe their fragmentation pathways in the gas phase. Furthermore, the potential energy surfaces of these systems are explored by quantum chemical calculations. Upon collisional activation, the [Cu(R)(CF3 )3 ]- complexes (R=Me, Et, Bu, s Bu, allyl) afford the product ions [Cu(CF3 )3 ]⋅- and [Cu(CF3 )2 ]- . The former obviously results from an R⋅ loss, whereas the latter originates either from the stepwise release of R⋅ and CF3 ⋅ radicals or a concerted reductive elimination of RCF3 . The gas-phase fragmentation experiments as well as the quantum chemical calculations indicate that the preference for the stepwise reaction toward [Cu(CF3 )2 ]- increases with the stability of the formed organyl radical R⋅. This finding suggests that the recombination of R⋅ and CF3 ⋅ radicals may possibly contribute to the formation of RCF3 from [Cu(R)(CF3 )3 ]- in synthetic applications. In contrast, the [Cu(R)(CF3 )3 ]- complexes (R=aryl) only yield [Cu(CF3 )2 ]- when subjected to collision-induced dissociation. These species exclusively undergo a concerted reductive elimination because the competing stepwise pathway is disfavored by the low stability of aryl radicals.

Origin of the different reactivity of the high-valent coinage-metal complexes [RCuiii Me3 ]- and [RAgiii Me3 ]- (R=allyl).

High-valent tetraalkylcuprates(iii) and -argentates(iii) are key intermediates of copper- and silver-mediated C-C coupling reactions. Here, we investigate the previously reported contrasting reactivity of [RMiii Me3 ]- complexes (M=Cu, Ag and R=allyl) with energy-dependent collision-induced dissociation experiments, advanced quantum-chemical calculations and kinetic computations. The gas-phase fragmentation experiments confirmed the preferred formation of the [RCuMe]- anion upon collisional activation of the cuprate(iii) species, consistent with a homo-coupling reaction, whereas the silver analogue primarily yielded [AgMe2 ]- , consistent with a cross-coupling reaction. For both complexes, density functional theory calculations identified one mechanism for homo coupling and four different ones for cross coupling. Of these pathways, an unprecedented concerted outer-sphere cross coupling is of particular interest, because it can explain the formation of [AgMe2 ]- from the argentate(iii) species. Remarkably, the different C-C coupling propensities of the two [RMiii Me3 ]- complexes become only apparent when properly accounting for the multi-configurational character of the wave function for the key transition state of [RAgMe3 ]- . Backed by the obtained detailed mechanistic insight for the gas-phase reactions, we propose that the previously observed cross-coupling reaction of the silver complex in solution proceeds via the outer-sphere mechanism.

Modular Ion Mobility Calibrants for Organometallic Anions Based on Tetraorganylborate Salts.

Organometallics are widely used in catalysis and synthesis. Their analysis relies heavily on mass spectrometric methods, among which traveling-wave ion mobility spectrometry (TWIMS) has gained increasing importance. Collision cross sections (CCS) obtainable by TWIMS significantly aid the structural characterization of ions in the gas phase, but for organometallics, their accuracy has been limited by the lack of appropriate calibrants. Here, we propose tetraorganylborates and their alkali-metal bound oligomers [Mn-1(BR4)n]- (M = Li, Na, K, Rb, Cs; R = aryl, Et; n = 1-6) as calibrants for electrospray ionization (ESI) TWIMS. These species chemically resemble typical organometallics and readily form upon negative-ion mode ESI of solutions of alkali-metal tetraorganylborates. By combining different tetraorganylborate salts, we have generated a large number of anions in a modular manner and determined their CCS values by drift-tube ion mobility spectrometry (DTIMS) (DTCCSHe = 81-585, DTCCSN2 = 130-704 Å2). In proof-of-concept experiments, we then applied these DTCCS values to the calibration of a TWIMS instrument and analyzed phenylcuprate and argentate anions, [Lin-1MnPh2n]- and [MnPhn+1]- (M = Cu, Ag), as prototypical reactive organometallics. The TWCCSN2 values derived from TWIMS measurements are in excellent agreement with those determined by DTIMS (<2% relative difference), demonstrating the effectiveness of the proposed calibration scheme. Moreover, we used theoretical methods to predict the structures and CCS values of the anions considered. These predictions are in good agreement with the experimental results and give further insight into the trends governing the assembly of tetraorganylborate, cuprate, and argentate oligomers.

Hexanuclear Copper(I) Hydride from the Reduction-Induced Decarboxylation of a Dicopper(II) Formate.

Copper(I) hydride complexes represent a promising entry into formic acid dehydrogenation catalysis. Herein we present the spontaneous decarboxylation of a µ1,3-formate-bridged dicopper(II) complex (1H) to a hexacopper(I) hydride cluster (2H) upon reduction. Isotopic labeling studies revealed that both the H- and CO2 originate from the bound µ1,3-formate in 1H, which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1H to 2H is established. The structure of 2H features two Cu3 triangles, each capped by a hydride ligand. Typical hydride reactivity of 2H is demonstrated by the addition of phenylacetylene, leading to the replacement of the hydrides by alkynide ligands -C≡CPh (3) while retaining the hexacopper(I) core. Temperature-dependent dynamic behavior in solution on the NMR time scale was observed for both 2H and 3, reflecting the rich structural landscape of the bis(pyrazolate)-bridged hexacopper(I) core (four isomers each for 2H and 3) predicted by DFT calculations.

Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation.

In contrast to the extensive knowledge of lithium cation affinities and basicities, the thermochemistry of microsolvated lithium cations is much less explored. Here, we determine the relative stabilities of Li(A,B)n+ complexes, n = 2 and 3, by monitoring their gas-phase reactions with A and B substrate molecules, A/B = Me2O, Et2O, tetrahydrofuran, and MeCN, in a three-dimensional quadrupole-ion trap mass spectrometer. Kinetic analysis of the observed ligand displacement reactions affords equilibrium constants, which are then converted into Gibbs reaction energies. In addition, we use high-level quantum chemical calculations to predict the structures and sequential ligand dissociation energies of the homoleptic Li(A)n+ complexes, n = 1-3. As expected, the ligands dissociate more easily from complexes in higher coordination states. However, the very nature of the ligand also matters. Ligands with different steric demands can, thus, invert their relative Li+ affinities depending on the coordination state of the metal center. This finding shows that microsolvation of Li+ can result in specific effects, which are not recognized if the analysis takes into account only simple lithium cation affinities and basicities.

Benzhydrylpyridinium Ions: A New Class of Thermometer Ions for the Characterization of Electrospray-Ionization Mass Spectrometers.

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).

Argentate(i) and (iii) complexes as intermediates in silver-mediated cross-coupling reactions.

Despite the potential of silver to mediate synthetically valuable cross-coupling reactions, the operating mechanisms have remained unknown. Here, we use a combination of rapid-injection NMR spectroscopy, electrospray-ionization mass spectrometry, and quantum chemical calculations to demonstrate that these transformations involve argentate(i) and (iii) complexes as key intermediates.

Imaging quantum stereodynamics through Fraunhofer scattering of NO radicals with rare-gas atoms.

Stereodynamics describes how the vector properties of molecules, such as the directions in which they move and the axes about which they rotate, affect the probabilities (or cross-sections) of specific processes or transitions that occur on collision. The main aspects of stereodynamics in inelastic atom-molecule collisions can often be understood from classical considerations, in which the particles are represented by billiard-ball-like hard objects. In a quantum picture, however, the collision is described in terms of matter waves, which can also scatter into the region of the geometrical shadow of the object and reveal detailed information on the pure quantum-mechanical contribution to the stereodynamics. Here we present measurements of irregular diffraction patterns for NO radicals colliding with rare-gas atoms that can be explained by the analytical Fraunhofer model. They reveal a hitherto overlooked dependence on (or 'propensity rule' for) the magnetic quantum number m of the molecules, and a previously unrecognized type of quantum stereodynamics that has no classical analogue or interpretation.

Association and Dissociation of Grignard Reagents RMgCl and Their Turbo Variant RMgCl⋠LiCl.

Grignard reagents RMgCl and their so-called turbo variant, the highly reactive RMgCl⋅LiCl, are of exceptional synthetic utility. Nevertheless, it is still not fully understood which species these compounds form in solution and, in particular, in which way LiCl exerts its reactivity-enhancing effect. A combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, NMR spectroscopy (including diffusion-ordered spectroscopy), and quantum chemical calculations is used to analyze solutions of RMgCl (R=Me, Et, Bu, Hex, Oct, Dec, iPr, tBu, Ph) in tetrahydrofuran and other ethereal solvents in the absence and presence of stoichiometric amounts of LiCl. In tetrahydrofuran, RMgCl forms mononuclear species, which are converted into trinuclear anions as a result of the concentration increase experienced during the electrospray process. These trinuclear anions are theoretically predicted to adopt open cubic geometries, which remarkably resemble structural motifs previously found in the solid state. The molecular constituents of RMgCl and RMgCl⋅LiCl are interrelated via Schlenk equilibria and fast intermolecular exchange processes. A small portion of the Grignard reagent also forms anionic ate complexes in solution. The abundance of these more electron-rich and hence supposedly more nucleophilic ate complexes strongly increases upon the addition of LiCl, thus rationalizing its beneficial effect on the reactivity of Grignard reagents.