Gas-Phase Unimolecular Reactions of Pallada- and Nickelalactone Anions

Reactions of gas-phase uranyl formate/acetate anions: reduction of carboxylate ligands to aldehydes by intra-complex hydride attack

In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (UVIO22+) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom., 2016, 27, 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO2(O2C-CH3)2(O2C-H)]- and [UO2(O2C-CH3)(O2C-H)2]-. Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO2(H)(O2C-CH3)(O2C-H)]-. We also discovered that MSn CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO2(O)(CH3)]-, which reacts with H2O to generate [UO2(O)(OH)]-. DFT calculations support the observation that formation of [UO2(O)(OH)]- by elimination of CH4 is favored over H2O addition and rearrangement to create [UO2(OH)2(CH3)]-.

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.

Mass spectrometric and theoretical study on the formation of uranyl hydride from uranyl carboxylate

Uranyl hydride in the form of HUO₂Cl₂^- was prepared upon collision-induced dissociation of (RCO₂)UO₂Cl₂^- (R = H, CH₃CH₂, CH₃CH₂CH₂, CH₃CHCH, (CH₃)₂CH, C₅H₉, C₆H₁₁ and C₆H₅CH₂CH₂) in the gas phase. It was found that uranyl hydrides result from alkene and alkyne elimination with concomitant β-hydride transfer of uranyl alkylides RUO₂Cl₂^- following decarboxylation of the carboxylates with the exception of (HCO₂)UO₂Cl₂^-, and formation of HU^VIO₂Cl₂^- through alkene/alkyne loss is in competition with neutral ligand loss to give U^VO₂Cl₂^-. According to the calculations at the B3LYP level, loss of a neutral ligand is slightly less favorable in the cases of (CH₃CH₂)UO₂Cl₂^- and (CH₃CH₂CH₂)UO₂Cl₂^-, and the situations of (CH₃CHCH)UO₂Cl₂^-, ((CH₃)₂CH)UO₂Cl₂^-, (C₅H₉)UO₂Cl₂^-, (C₆H₁₁)UO₂Cl₂^- and (C₆H₅CH₂CH₂)UO₂Cl₂^- with β-hydrogen atoms should be similar despite the fact that the yield of uranyl hydride depends on the nature of the ligand. Although no uranyl hydride was observed when β-hydrogen is not available in the carboxylate precursor, there is no HUO₂Cl₂^- generated from (C₆H₅CO₂)UO₂Cl₂^-, (2-C₆H₄FCO₂)UO₂Cl₂^- and (CH₂CHCH₂CO₂)UO₂Cl₂^- with β-hydrogen either. This is attributed to the much more favorable formation of UO₂Cl₂^- over HUO₂Cl₂^- as revealed by the B3LYP calculations, which is similar to the absence of HUO₂Cl₂^- in the (CH₃CO₂)UO₂Cl₂^- case where highly reactive CH₂ would be formed.

Detection and Structural Investigation of Elusive Palladium Hydride Intermediates Formed from Simple Metal Salts

The Mizoroki-Heck reaction is one of the most known and best studied catalytic transformations and has provided an outstanding driving force for the development of catalysis and synthetic applications. Three out of four classical Mizoroki-Heck catalytic cycle intermediates contain Pd-C bonds and are well known and studied in detail. However, a simple palladium hydride (which is formed after the product-releasing β-H-elimination step) is a kind of elusive intermediate in the Mizoroki-Heck reaction. In the present study, we performed a combined theoretical and mass spectrometry (MS) study of palladium hydride complexes [PdX₂H]^- (X = Cl, Br, and I), which are reactive intermediates in the Mizoroki-Heck reaction. Static and molecular dynamic calculations revealed that these species have a T-shaped structure with a trans-arrangement of halogen atoms. Other isomers of [PdX₂H]^- are unstable and easily rearrange into the T-shaped form or decompose. These palladium hydride intermediates were detected by MS in precatalyst activation using NaBH₄, Et₃N, and a solvent molecule as reducing agents. Online MS monitoring allowed the detection of [PdX₂H]^- species in the course of the Mizoroki-Heck reaction.

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.

Anionic Palladium(0) and Palladium(II) Ate Complexes

Palladium ate complexes are frequently invoked as important intermediates in Heck and cross-coupling reactions, but so far have largely eluded characterization at the molecular level. Here, we use electrospray-ionization mass spectrometry, electrical conductivity measurements, and NMR spectroscopy to show that the electron-poor catalyst [L₃ Pd] (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) readily reacts with Br^- ions to afford the anionic, zero-valent ate complex [L₃ PdBr]^- . In contrast, more-electron-rich Pd catalysts display lower tendencies toward the formation of ate complexes. Combining [L₃ Pd] with LiI and an aryl iodide substrate (ArI) results in the observation of the Pd^II ate complex [L₂ Pd(Ar)I₂ ]^- .

Gas-Phase and Computational Study of Identical Nickel- and Palladium-Mediated Organic Transformations Where Mechanisms Proceeding via M(II) or M(IV) Oxidation States Are Determined by Ancillary Ligands

Gas-phase studies utilizing ion-molecule reactions, supported by computational chemistry, demonstrate that the reaction of the enolate complexes [(CH2CO2-C,O)M(CH3)](-) (M = Ni (5a), Pd (5b)) with allyl acetate proceed via oxidative addition to give M(IV) species [(CH2CO2-C,O)M(CH3)(η(1)-CH2-CH═CH2)(O2CCH3-O,O')](-) (6) that reductively eliminate 1-butene, to form [(CH2CO2-C,O)M(O2CCH3-O,O')](-) (4). The mechanism contrasts with the M(II)-mediated pathway for the analogous reaction of [(phen)M(CH3)](+) (1a,b) (phen = 1,10-phenanthroline). The different pathways demonstrate the marked effect of electron-rich metal centers in enabling higher oxidation state pathways. Due to the presence of two alkyl groups, the metal-occupied d orbitals (particularly dz(2)) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz(2) to the C═C π* orbital is the key interaction leading to oxidative addition of allyl acetate to M(II). Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for Ni(IV) and Pd(IV) in organic synthesis.

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]⁻.