Gas-Phase Synthesis and Reactivity of the Organomagnesates [CH3MgL2]- (L = Cl and O2CCH3): From Ligand Effects to Catalysis

Ion Mobility Mass Spectrometry for Large Synthetic Molecules: Expanding the Analytical Toolbox

Understanding the composition, structure and stability of larger synthetic molecules is crucial for their design, yet currently the analytical tools commonly used do not always provide this information. In this perspective, we show how ion mobility mass spectrometry (IM-MS), in combination with tandem mass spectrometry, complementary techniques and computational methods, can be used to structurally characterize synthetic molecules, make and predict new complexes, monitor disassembly processes and determine stability. Using IM-MS, we present an experimental and computational framework for the analysis and design of complex molecular architectures such as (metallo)supramolecular cages, nanoclusters, interlocked molecules, rotaxanes, dendrimers, polymers and host-guest complexes.

Organolanthanide Complexes Containing Ln-CH3 σ-bonds: Unexpectedly Similar Hydrolysis Rates for Trivalent and Tetravalent Organocerium

We report the gas-phase preparation, isolation, and reactivity of a series of organolanthanides featuring the Ln-CH3 bond. The complexes are formed by decarboxylating anionic lanthanide acetates to form trivalent [LnIII(CH3)(CH3CO2)3]- (Ln = La, Ce, Pr, Nd, Sm, Tb, Tm, Yb, Lu), divalent [EuII(CH3)(CH3CO2)2]-, and the first examples of tetravalent organocerium complexes featuring CeIV-Calkyl σ-bonds: [CeIV(O)(CH3)(CH3CO2)2]- and [CeIV(O)(CH3)(NO3)2]-. Attempts to isolate PrIV-CH3 and TbIV-CH3 were unsuccessful; however, fragmentation patterns reveal that the oxidation of LnIII to a LnIV-oxo-acetate complex is more favorable for Ln = Pr than for Ln = Tb. The rate of Ln-CH3 hydrolysis is a measure of bond stability, and it decreases from LaIII-CH3 to LuIII-CH3, with increasing steric crowding for smaller Ln stabilizing the harder Ln-CH3 bond against hydrolysis. [EuII(CH3)(CH3CO2)2]- engages in a much faster hydrolysis versus LnIII-CH3. The surprising observation of similar hydrolysis rates for CeIV-CH3 and CeIII-CH3 is discussed with respect to sterics, the oxo ligand, and bond covalency in σ-bonded organolanthanides.

Gas-phase formation of Grignard-type organolanthanide (III) ions RLnCl3 - : The influences of lanthanide center and hydrocarbyl group

RATIONALE

Compared with organomagnesium compounds (Grignard reagents), the Grignard-type organolanthanides (III) exhibit several utilizable differences in reactivity. However, the fundamental understanding of Grignard-type organolanthanides (III) is still in its infancy. Decarboxylation of metal carboxylate ions is an effective method to obtain organometallic ions that are well suited for gas-phase investigation using electrospray ionization (ESI) mass spectrometry in combination with density functional theory (DFT) calculations.

METHODS

The (RCO₂ )LnCl₃ ^- (R = CH₃ , Ln = La-Lu except Pm; Ln = La, R = CH₃ CH₂ , CH₂ CH, HCC, C₆ H₅ , and C₆ H₁₁ ) precursor ions were produced in the gas phase via ESI of LnCl₃ and RCO₂ H or RCO₂ Na mixtures in methanol. Collision-induced dissociation (CID) was employed to examine whether the Grignard-type organolanthanide (III) ions RLnCl₃ ^- can be obtained via decarboxylation of lanthanide chloride carboxylate ions (RCO₂ )LnCl₃ ^- . DFT calculations can be used to determine the influences of lanthanide center and hydrocarbyl group on the formation of RLnCl₃ ^- .

RESULTS

When R = CH₃ , CID of (CH₃ CO₂ )LnCl₃ ^- (Ln = La-Lu except Pm) yielded decarboxylation products (CH₃ )LnCl₃ ^- and reduction products LnCl₃ ·^- with a variation in the relative intensity ratio of (CH₃ )LnCl₃ ^- /LnCl₃ ·^- . The trend is as follows: (CH₃ )EuCl₃ ^- /EuCl₃ ·^-  < (CH₃ )YbCl₃ ^- /YbCl₃ ·^-  ≈ (CH₃ )SmCl₃ ^- /SmCl₃ ·^-  < other (CH₃ )LnCl₃ ^- /LnCl₃ ·^- , which complies with the trend of Ln (III)/Ln (II) reduction potentials in general. When Ln = La and hydrocarbyl groups were varied as CH₃ CH₂ , CH₂ CH, HCC, C₆ H₅ , and C₆ H₁₁ , the fragmentation behaviors of these (RCO₂ )LaCl₃ ^- precursor ions were diverse. Except for (C₆ H₁₁ CO₂ )LaCl₃ ^- , the four remaining (RCO₂ )LaCl₃ ^- (R = CH₃ CH₂ , CH₂ CH, HCC, and C₆ H₅ ) ions all underwent decarboxylation to yield RLaCl₃ ^- . (CH₂ CH)LaCl₃ ^- and especially (CH₃ CH₂ )LaCl₃ ^- are prone to undergo β-hydride transfer to form LaHCl₃ ^- , whereas (HCC)LaCl₃ ^- and (C₆ H₅ )LaCl₃ ^- are not. A minor reduction product, LaCl₃ ·^- , was formed via C₆ H₅ radical loss of (C₆ H₅ )LaCl₃ ^- . The relative intensities of RLaCl₃ ^- compared to (RCO₂ )LaCl₃ ^- decrease as follows: HCC > CH₂ CH > C₆ H₅  > CH₃  > CH₃ CH₂  >> C₆ H₁₁ (not visible).

CONCLUSION

A series of Grignard-type organolanthanide (III) ions RLnCl₃ ^- (R = CH₃ , Ln = La-Lu except Pm; Ln = La, R = CH₃ CH₂ , CH₂ CH, HCC, and C₆ H₅ ) were produced from (RCO₂ )LnCl₃ ^- via CO₂ loss, whereas (C₆ H₁₁ )LaCl₃ ^- did not. The experimental and theoretical results suggest that the reduction potentials of Ln (III)/Ln (II) couples as well as the bulkiness and hybridization of hydrocarbyl groups play important roles in promoting or limiting the formation of RLnCl₃ ^- via decarboxylation of (RCO₂ )LnCl₃ ^- .

Ion-pairs as a gateway to transmetalation: aryl transfer from boron to nickel and magnesium

Gas-phase ion-ion reactions between tris-1,10-phenantholine metal dications, [(phen)₃M]²⁺ (where M = Ni and Mg), and the tetraphenylborate anion yield the ion-pairs {[(phen)₃M]²⁺[BPh₄]^-}⁺. The ion-pairs undergo transmetalation upon loss of a phen ligand to give the organometallic complexes [(phen)₂M(Ph)]⁺. DFT calculations, used to determine the energy barriers for the transmetalation reactions and the hydrolysis reactions, are entirely consistent with the experimental results.

Gas Phase Protolysis of Trisarylzincate Anions

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 Ar^XZnPh₂^- (Ar^X = 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.

ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold-Platinum Complexes-A Gas-Phase Mimetic Approach

The PPh₃ ligands in the heterodinuclear AuPt complex [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] (BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) exhibit a high fluxionality on the AuPt core. Fast intramolecular and slow intermolecular processes for the reversible exchange of the PPh₃ ligands have been identified. When [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] is heated in solution, the formation of benzene is observed, and a trinuclear, cationic AuPt₂ complex is generated. This process is preceded by reversible phenyl-group exchange between the PPh₃ ligands present in the reaction mixture as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding reversible phenyl-group exchange have originally been observed in mass-spectrometry-based CID experiments (CID = Collision-Induced Dissociation). While CID of mass-selected [Au,Pt,(PPh₃)₄]⁺ results exclusively in the loss of PPh₃, the resulting cation [Au,Pt,(PPh₃)₃]⁺ selectively eliminates C₆H₆. Thus, the dissociation of a PPh₃ ligand from [Au,Pt,(PPh₃)₃]⁺ is energetically not able to compete with processes which result in C-H- and C-P-bond cleavage. In both media, the heterobimetallic nature of the employed complexes is the key for the observed reactivity. Only the intimate interplay of the gas-phase investigations, studies in solution, and thorough DFT computations allowed for the elucidation of the mechanistic details of the reactivity of [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F].

Formation and hydrolysis of gas-phase [UO2 (R)]+ : R═CH3 , CH2 CH3 , CH═CH2 , and C6 H5

The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO₂ (R)]⁺ (eg, R═CH₃ and CH₂ CH₃ ) by decarboxylation of [UO₂ (O₂ C─R)]⁺ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas-phase H₂ O. Collision-induced dissociation (CID) of both [UO₂ (O₂ C─CH₃ )]⁺ and [UO₂ (O₂ C─CH₂ CH₃ )]⁺ causes H⁺ transfer and elimination of a ketene to leave [UO₂ (OH)]⁺ . However, CID of the alkoxides [UO₂ (OCH₂ CH₃ )]⁺ and [UO₂ (OCH₂ CH₂ CH₃ )]⁺ produced [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ , respectively. Isolation of [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ for reaction with H₂ O caused formation of [UO₂ (H₂ O)]⁺ by elimination of ·CH₃ and ·CH₂ CH₃ : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO₂ (O₂ C─CH═CH₂ )]⁺ and [UO₂ (O₂ C─C₆ H₅ )]⁺ , caused decarboxylation to leave [UO₂ (CH═CH₂ )]⁺ and [UO₂ (C₆ H₅ )]⁺ , respectively. These organometallic species do react with H₂ O to produce [UO₂ (OH)]⁺ , and loss of the respective radicals to leave [UO₂ (H₂ O)]⁺ was not detected. Density functional theory calculations suggest that formation of [UO₂ (OH)]⁺ , rather than the hydrated U^V O₂ ⁺ , cation is energetically favored regardless of the precursor ion. However, for the [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ precursors, the transition state energy for proton transfer to generate [UO₂ (OH)]⁺ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO₂ (H₂ O)]⁺ . The situation is reversed for the [UO₂ (CH═CH₂ )]⁺ and [UO₂ (C₆ H₅ )]⁺ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO₂ (H₂ O)]⁺ by elimination of CH═CH₂ or C₆ H₅ radical.

Gas-Phase Reactions of the Group 10 Organometallic Cations, [(phen)M(CH3)]+ with Acetone: Only Platinum Promotes a Catalytic Cycle via the Enolate [(phen)Pt(OC(CH2)CH3)]+

Electrospray ionisation of the ligated group 10 metal complexes [(phen)M(O2CCH3)2] (M = Ni, Pd, Pt) generates the cations [(phen)M(O2CCH3)]+, whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). A new catalytic cycle has been discovered. In step 1, decarboxylation of [(phen)M(O2CCH3)]+ under CID conditions generates the organometallic cations [(phen)M(CH3)]+, which react with acetone to generate the [(phen)M(CH3)(OC(CH3)2)]+ adducts in competition with formation of the coordinated enolate for M = Pt (step 2). For M = Ni and Pd, the adducts regenerate [(phen)M(CH3)]+ upon CID. In the case of M = Pt, loss of methane is favored over loss of acetone and results in the formation of the enolate complex, [(phen)Pt(OC(CH2)CH3)]+. Upon further CID, both methane and CO loss can be observed resulting in the formation of the ketenyl and ethyl complexes [(phen)Pt(OCCH)]+ and [(phen)Pt(CH2CH3)]+ (step 3), respectively. In step 4, CID of [(phen)Pt(CH2CH3)]+ results in a beta-hydride elimination reaction to yield the hydride complex, [(phen)Pt(H)]+, which reacts with acetic acid to regenerate the acetate complex [(phen)Pt(O2CCH3)]+ and H2 in step 5. Thus, the catalytic cycle is formally closed, which corresponds to the decomposition of acetone and acetic acid into methane, CO, CO2, ethene and H2. All except the last step of the catalytic cycle are modelled using DFT calculations with optimizations of structures at the M06/SDD 6-31G(d) level of theory.

Mechanistic understanding of catalysis by combining mass spectrometry and computation

The combination of mass spectrometry and computational chemistry has been proven to be powerful for exploring reaction mechanisms. The former provides information of reaction intermediates, while the latter gives detailed reaction energy profiles.

Periodic Trends Manifested through Gas-Phase Generation of Anions Such as [AlH4]-, [GaH4]-, [InH4]-, [SrH3]-, [BaH3]-, [Ba(0)(η2-O2CH)1]-, [Pb(0)H]-, [Bi(I)H2]-, and Bi- from Formates

Metal-hydride anions of main group elements, such as BaH₃ ^- and InH₄ ^-, were generated by dissociating formate adducts of the respective metal formates. Upon activation, these adducts fragment by formate-ion ejection or by decarboxylation. For adducts of alkali-metal formates, the formate-ion ejection is the preferred pathway, whereas for those of alkaline-earth and group 13-15 metals, the expulsion of CO₂ is the more favorable pathway. Decarboxylation is deemed to yield a metal-hydrogen bond presumably by a hydride transfer to the metal atom. For example, the decarboxylation of Al(η-OCOH)₄ ^- and Ga(η-OCOH)₄ ^- generated AlH₄ ^- and GaH₄ ^-, respectively. The initial fragment-ion with a H-M bond formed in this way from adducts of the heavier metals of group 13 (Ga, In, and Tl) undergo a unimolecular reductive elimination, ascribable to the "inert-pair" effect, to lower the metal-ion oxidation state from +3 to +1. As group 13 is descended, the tendency for this reductive elimination process increases. PbH₃ ^-, generated from the formate adduct of lead formate, reductively eliminated H₂ to form PbH^-, in which Pb is in oxidation state zero. In the energy-minimized structure [H-Pb(η²-H₂)]^-, proposed as an intermediate for the process, a H₂ molecule is coordinated with PbH^- as a dihapto ligand. The formate adducts of strontium and barium produce monoleptic ions such as [M(0)(η²-O₂CH)₁]^-, in which the formate ion is chelated to a neutral metal atom. The bismuth formate adduct undergoes a double reductive elimination process whereby the oxidation state of Bi is reduced from +3 to +1 and then to -1. Upon activation, the initially formed [H-Bi-H]^- ion transforms to an anionic η²-H₂ complex, which eliminates dihydrogen to form the bismuthide anion (Bi^-).

C-C Bond Formation of Mg- and Zn-Activated Carbon Dioxide

Gas-phase activation of CO₂ by chloride tagged metal atoms, [ClM]^- (M=Mg, Zn), has been investigated by mass spectrometry and high-level quantum chemistry. Both metals activate CO₂ with significant bending of the CO₂ moiety to form complexes with the general formula [ClM,CO₂ ]^- . The structure of the metal-CO₂ complex depends on the method of formation, and the energy landscapes and reaction dynamics have been probed by collisional induced dissociation and thermal ion molecule reactions with isotopically labeled species. Having established these structural relationships, the gas-phase reactivity of [ClM(κ² -O₂ C)]^- with acetaldehyde (here considered a carbohydrate mimic) was then studied. Formation of lactate and enolate-pyruvate complexes are observed, showing that CO₂ fixation by C-C bond formation takes place. For M=Zn, even formation of free pyruvate ([C₃ H₃ O₃ ]^- ) is observed. Implications of the observed CO₂ reactivity for the electrochemical conversion of carbon dioxide, and to biochemical and artificial photosynthesis is briefly discussed. Detailed potential energy diagrams obtained by the quantum chemical calculations offer models consistent with experimental observation.

Unimolecular dissociation of anions derived from succinic acid (H2Su) in the gas phase: HSu- and ClMgSu-. Relationship to CO2 fixation

Electrospray ionization of mixtures of succinic acid (here denoted H₂Su) and magnesium chloride in water/methanol give rise to ions of the type ESu^- (E = H or ClMg). The unimolecular dissociation of these ions was studied by collisionally induced dissociation mass spectrometry and interpreted by quantum chemical calculations (density functional theory and the composite Gaussian-4 method) of relevant parts of the potential energy surfaces. The major dissociation pathways from HSu^- were seen to be dehydration and decarboxylation, while ClMgSu^- mainly undergoes decarboxylation. The latter reaction proceeds without barrier for the reverse reaction; addition of CO₂ to a Grignard type structure ClMg(CH₂CH₂CO₂)^-. In contrast, addition of CO₂ to the analogous H(CH₂CH₂CO₂)^- ion has a substantial barrier. Dehydration of HSu^- gives rise to deprotonated succinic anhydride via a transition state for the key intramolecular proton transfer having an entropically favorable seven-member ring structure. The succinate systems studied here are compared to the previously reported analogous maleate systems, providing further insight to the structure-reactivity relationship.

Dissociation of Mg(ii) and Zn(ii) complexes of simple 2-oxocarboxylates – relationship to CO2fixation, and the Grignard and Barbier reactions

The glyoxylate and pyruvate carboxylates have been complexed to Mg(ii) and Zn(ii) to investigate the intrinsic interactions of these important biochemical species in the gas phase.

Activation of carbon dioxide by a terminal uranium-nitrogen bond in the gas-phase: a demonstration of the principle of microscopic reversibility

Activation of CO2 is demonstrated by its spontaneous dissociative reaction with the gas-phase anion complex NUOCl2(-), which can be considered as NUO(+) coordinated by two chloride anion ligands. This reaction was previously predicted by density functional theory to occur exothermically, without barriers above the reactant energy. The present results demonstrate the validity of the prediction of microscopic reversibility, and provide a rare case of spontaneous dissociative addition of CO2 to a gas-phase complex. The activation of CO2 by NUOCl2(-) proceeds by conversion of a U[triple bond, length as m-dash]N bond to a U[double bond, length as m-dash]O bond and creation of an isocyanate ligand to yield the complex UO2(NCO)Cl2(-), in which uranyl, UO2(2+), is coordinated by one isocyanate and two chloride anion ligands. This activation of CO2 by a uranium(vi) nitride complex is distinctive from previous reports of oxidative insertion of CO2 into lower oxidation state U(iii) or U(iv) solid complexes, during which both C-O bonds remain intact. This unusual observation of spontaneous addition and activation of CO2 by NUOCl2(-) is a result of the high oxophilicity of uranium. If the computed Gibbs free energy of the reaction pathway, rather than the energy, is considered, there are barriers above the reactant asymptotes such that the observed reaction should not proceed under thermal conditions. This result provides a demonstration that energy rather than Gibbs free energy determines reactivity under low-pressure bimolecular conditions.

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

Progress on the activation and transformation of important inorganic and organic molecules by negatively charged bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others.

Reductions of oxygen, carbon dioxide, and acetonitrile by the magnesium(II)/magnesium(I) couple in aqueous media: theoretical insights from a nano-sized water droplet

Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + e(-) → Mg(+•)) confined in a nanosized water droplet ([Mg(H2O)16](•+)) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O2 requires an overall doublet spin state of the system. The reductions of CO2 and CH3CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O2(•-), CO2(•-), and CH3CN(•-), are initially formed on the cluster surface. O2(•-) and CO2(•-) can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[MgOCO](•+) core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO2)(H2O)16](•+) are -36 ± 4 kJ mol(-1) and -30 ± 9 kJ mol(-1), respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO2 and [Mg(H2O)16](•+). The values for the formation of ssip and cip of [Mg(O2)(H2O)16](•+) are estimated to be -112 ± 18 kJ mol(-1) and -128 ± 28 kJ mol(-1), respectively. CH3CN(•-) undergoes protonation spontaneously to form the hydrophobic [CH3CN, H](•). Both CH3CN and [CH3CN, H](•) cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol(-1) and ∼40 kJ mol(-1), respectively. These results provide fundamental insights into the solvation dynamics of the Mg(2+)/Mg(•+) couple on the molecular level.

Gas phase studies of the Pesci decarboxylation reaction: synthesis, structure, and unimolecular and bimolecular reactivity of organometallic ions

CONSPECTUS: Decarboxylation chemistry has a rich history, and in more recent times, it has been recruited in the quest to develop cheaper, cleaner, and more efficient bond-coupling reactions. Thus, over the past two decades, there has been intense investigation into new metal-catalyzed reactions of carboxylic substrates. Understanding the elementary steps of metal-mediated transformations is at the heart of inventing new reactions and improving the performance of existing ones. Fortunately, during the same time period, there has been a convergence in mass spectrometry (MS) techniques, which allows these catalytic processes to be examined efficiently in the gas phase. Thus, electrospray ionization (ESI) sources have been combined with ion-trap mass spectrometers, which in turn have been modified to either accept radiation from tunable OPO lasers for spectroscopy based structural assignment of ions or to allow the study of ion-molecule reactions (IMR). The resultant "complete" gas-phase chemical laboratories provide a platform to study the elementary steps of metal-catalyzed decarboxylation reactions in exquisite detail. In this Account, we illustrate how the powerful combination of ion trap mass spectrometry experiments and DFT calculations can be systematically used to examine the formation of organometallic ions and their chemical transformations. Specifically, ESI-MS allows the transfer of inorganic carboxylate complexes, [RCO2M(L)n](x), (x = charge) from the condensed to the gas phase. These mass selected ions serve as precursors to organometallic ions [RM(L)n](x) via neutral extrusion of CO2, accessible by slow heating in the ion trap using collision induced dissociation (CID). This approach provides access to an array of organometallic ions with well-defined stoichiometry. In terms of understanding the decarboxylation process, we highlight the role of the metal center (M), the organic group (R), and the auxiliary ligand (L), along with cluster nuclearity, in promoting the formation of the organometallic ion. Where isomeric organometallic ions are generated and normal MS approaches cannot distinguish them, we describe approaches to elucidate the decarboxylation mechanism via determination of their structure. These "unmasked" organometallic ions, [RM(L)n](x), can also be structurally interrogated spectroscopically or via CID. We have thus compared the gas-phase structures and decomposition of several highly reactive and synthetically important organometallic ions for the first time. Perhaps the most significant aspect of this work is the study of bimolecular reactions, which provides experimental information on mechanistically obscure bond-formation and cross-coupling steps and the intrinsic reactivity of ions. We have sought to understand transformations of substrates including acid-base and hydrolysis reactions, along with reactions resulting in C-C bond formation. Our studies also allow a direct comparison of the performance of different metal catalysts in the individual elementary steps associated with protodecarboxylation and decarboxylative alkylation cycles. Electronic structure (DFT and ab initio) and dynamics (RRKM) calculations provide further mechanistic insights into these reactions. The broad implications of this research are that new reactions can be discovered and that the performance of metal catalysts can be evaluated in terms of each of their elementary steps. This has been particularly useful for the study of metal-mediated decarboxylation reactions.

Gas-phase reactions of the rhenium oxide anions, [ReOx]- (x = 2 - 4) with the neutral organic substrates methane, ethene, methanol and acetic acid

The ion-molecule reactions of the rhenium oxide anions, [ReOx](-) (x = 2 - 4) with the organic substrates methane, ethene, methanol and acetic acid have been examined in a linear ion trap mass spectrometer. The only reactivity observed was between [ReO(2)](-) and acetic acid. Isotope labelled experiments and high-resolution mass spectrometry measurements were used to assign the formulas of the ionic products. Collision-induced dissociation and ion-molecule reactions with acetic acid were used to probe the structures of the mass-selected primary product ions. Density functional theory calculations [PBE0/LanL2DZ6-311+G(d)] were used to suggest possible structures. The three primary product channels observed are likely to arise from the formation of: the metallalactone [ReO(2)(CH(2)CO(2))](-) (m/z 277) and H(2); [CH(3)ReO(2)(OH)](-) (m/z 251) and CO; and [ReO(3)](-) (m/z 235), H(2) and CH(2)CO.

Copper mediated decyano decarboxylative coupling of cyanoacetate ligands: Pesci versus Lewis acid mechanism

A combination of gas-phase ion trap multistage mass spectrometry (MSⁿ) experiments and density functional theory (DFT) calculations have been used to examine the mechanisms of the sequential decomposition reactions of copper cyanoacetate anions, [(NCCH₂CO₂)₂Cu]⁻.

Unimolecular dissociation of anions derived from maleic acid (MaH2) in the gas phase: MaH- and MaMgCl--- relationship to Grignard chemistry and reductive CO2 fixation

We have conducted collision induced dissociation experiments on the hydrogen maleate anion (MaH(-), m/z = 115) and the anionic maleate MgCl complex (MaMgC(-), m/z = 173). In addition, we have computationally investigated the observed fragmentation reactions. We find that both anions readily undergo two consecutive decarboxylations resulting in product ions at m/z = 71 and 27 for MaH(-), and at m/z = 129 and 85 for MaMgCl(-). The first decarboxylation is more facile for MaMgCl(-) than for MaH(-), while loss of CO(2) from Ma(-CO(2))H(-) is more facile than for Ma(-CO(2))MgCl(-). We also find that MaH(-) loses water, and we propose a mechanism for this loss. No first-generation fragmentation product other than Ma(-CO(2))MgCl(-) is seen for MaMgCl(-). Based on the observed unimolecular chemistry, we discuss some of its implications on reductive CO(2)-fixation and Grignard chemistry.

Molecular salt effects in the gas phase: tuning the kinetic basicity of [HCCLiCl]⁻ and [HCCMgCl₂]⁻ by LiCl and MgCl₂

A combination of gas-phase ion-molecule reaction experiments and theoretical kinetic modeling is used to examine how a salt can influence the kinetic basicity of organometallates reacting with water. [HC≡CLiCl](-) reacts with water more rapidly than [HC≡CMgCl2](-), consistent with the higher reactivity of organolithium versus organomagnesium reagents. Addition of LiCl to [HC≡CLiCl](-) or [HC≡CMgCl2](-) enhances their reactivity towards water by a factor of about 2, while addition of MgCl2 to [HC≡CMgCl2](-) enhances its reactivity by a factor of about 4. Ab initio calculations coupled with master equation/RRKM theory kinetic modeling show that these reactions proceed via a mechanism involving formation of a water adduct followed by rearrangement, proton transfer, and acetylene elimination as either discrete or concerted steps. Both the energy and entropy requirements for these elementary steps need to be considered in order to explain the observed kinetics.

Negative charge induced dissociation: fragmentation of deprotonated N-benzylidene-2-hydroxylanilines in electrospray ionization mass spectrometry

Unimolecular reactivities of different N-benzylidene-2-hydroxylaniline anions were investigated in gas phase by electrospray ionization tandem mass spectrometry. All the collision-induced dissociation spectra of N-benzylidene-2-hydroxylaniline anions show similar ions at phenyl anions, neutral loss of benzonitrile and benzoxazole anions, respectively. The possible fragmentation pathway was probed through deuterium labeling and various group substituents experiments. Computational results were applied to shed light on the mechanism of fragmentation patterns. The proton in the CH=N is reactive in the formation of the concerned ions. Its direct transfer to the oxygen results in 2-hydroxyphenyl anion. Proton abstraction between benzoxazole and phenyl anion leads to the formation of benzene and benzoxazole anion.

Radical migration-addition of N-tert-butanesulfinyl imines with organozinc reagents

A novel migration-addition sequence was discovered for the reaction of enantioenriched N-tert-butanesulfinyl iminoacetate 1a with functionalized benzylzinc bromide reagents, producing tert-leucine derivatives in excellent diastereoselectivity (dr 98:2). The absolute configurations of two new chiral centers were unambiguously assigned by chemical transformations and X-ray crystallography. In addition, the regio- and diastereoselectivities of this novel reaction were both explained through the key N-sulfinamine intermediate M6 generated by the tert-butyl radical attack on the imine. Computational analysis of this reaction process, which was performed at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31G*-LANL2DZ level, also supported our proposed two-stage mechanism.

Formation and characterization of gaseous adducts of carbon dioxide to magnesium, (CO2)MgX- (X=OH, Cl, Br)

A good fix: the structure and chemical reactivity of a reduced form of CO(2) bonded to magnesium, XMg(η(2)-O(2)C)(-), is reported. Upon reaction with water it loses CO, while it adds CH(3) upon reaction with alkyl halides, thereby signifying nucleophilicity of the carbon atom in XMg(η(2)-O(2)C)(-) in S(N)2 reactions.