On the Mechanisms of Degenerate Ligand Exchange in [M(CH3)]+/CH4 Couples (M=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as Explored by Mass Spectrometric and Computational Studies: Oxidative Addition/Reductive Elimination versus σ-Complex-Assisted Metathesis

Ligand effect on Ru-centered species toward methane activation

Ligands have been known to profoundly affect the chemical transformations of methane, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we demonstrate that the conversion of methane can be regulated by Ru centered cations with a series of ligands (C, CH, CNH, CHCNH). Gas-phase experiments complemented by theoretical dynamic analysis were performed to explore the essences and principles governing the ligand effect. In contrast to the inert Ru+, [RuC]+, and [RuCNH]+ toward CH4, the dehydrogenation dominates the reaction of ligand-regulated systems [RuCH]+/CH4 and [RuCHCNH]+/CH4. In active cases, CH acts as active sites, and regulates the activation of CH4 assisted by the "seemingly inert" CNH ligand.

Metathesis by Partner Interchange in σ-Bond Ligands: Expanding Applications of the σ-CAM Mechanism

In 2007 two of us defined the σ‐Complex Assisted Metathesis mechanism (Perutz and Sabo‐Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578–2592), that is, the σ‐CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal–element bonds through partner‐interchange of σ‐bond complexes. The key concept that defines a σ‐CAM process is a single transition state for metathesis that is connected by two intermediates that are σ‐bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ‐bond complexes have been isolated or computed as well‐defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ‐bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ‐CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ‐CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional σ‐bond ligands, agostic complexes, sp²‐carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal–element bonds.

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].

Selective Conversion of Methane by Rh1-Doped Aluminum Oxide Cluster Anions RhAl2O4-: A Comparison with the Reactivity of PtAl2O4. J Phys Chem A 2018;122:3950-3955. [PMID: 29578712 DOI: 10.1021/acs.jpca.8b02483]

Studying the elementary reactions of single-noble-metal-atom-doped species can give theoretical guidance for the design of related single-atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl₂O₄^- with the most stable alkane molecule CH₄ under thermal conditions has been studied. The methane tends to be converted into syngas (free H₂ and adsorbed CO) with activation of four C-H bonds. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl₂O₄^- with CH₄. Density functional theory calculations show that the difference in reactivity between RhAl₂O₄^- and PtAl₂O₄^- is found to be due to a higher energy barrier of the third C-H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al₂O₄^-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.

Thermal activation of methane by vanadium boride cluster cations VBn+ (n = 3–6)

A new type of active species, transition metal boride cluster cations [VB_n⁺ (n = 3–6)], has been experimentally identified to dehydrogenate methane under thermal collision conditions. The B₃ unit in VB₃⁺ cluster is polarized by the V⁺ cation to activate CH₄.

Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O^•). Whenever the radical is delocalized, e.g., in [MgO]_n^•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga₂O₃) to [MgO]₂^•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al₂O₂]^•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH₃ moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]₂^•+ by Al₂O₃ enables HAT and PCET to compete. Similarly, [ZnO]^•+ activates methane by PCET generating many products. Adding a CH₃CN ligand to form [(CH₃CN)ZnO]^•+ leads to a single HAT product. The CH₃CN dipole acts as an OEF that switches off PCET. [MC]⁺ cations (M = Au, Cu) act by different mechanisms, dictated by the M⁺-C bond covalence. For example, Cu⁺, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu⁺.

On the Electronic Origin of Remarkable Ligand Effects on the Reactivities of [NiL]+ Complexes (L=C6 H5 , C5 H4 N, CN) towards Methane

The gas-phase reactions of [NiL]⁺ (L=C₆ H₅ , C₅ H₄ N, CN) with methane have been explored by using electrospray-ionization mass spectrometry (ESI-MS) complemented by quantum chemical calculations. Though the phenyl Ni complex [Ni(C₆ H₅ )]⁺ exclusively abstracts one hydrogen atom from methane at ambient conditions, the cyano Ni complex [Ni(CN)]⁺ brings about both H-atom abstraction and ligand exchange to generate [Ni(CH₃ )]⁺ . In contrast, the complex 2-pyridinyl Ni [Ni(C₅ H₄ N)]⁺ is inert towards this substrate. The presence of the empty 4s(Ni) orbital dominates the proton-coupled electron transfer (PCET) processes for the investigated systems.

Methane activation by gold-doped titanium oxide cluster anions with closed-shell electronic structures

The reactivity of closed-shell gas phase cluster anions AuTi₃O₇^- and AuTi₃O₈^- with methane under thermal collision conditions was studied by mass spectrometric experiments and quantum chemical calculations. Methane activation was observed with the formation of AuCH₃ in both cases, while the formation of formaldehyde was also identified in the reaction system of AuTi₃O₈^-. The cooperative effect of the separated Au⁺ and O^2- ions on the clusters induces the cleavage of the first C-H bond of methane. Further activation of the second C-H bond by a peroxide ion O₂^2- leads to the formation of formaldehyde. This study shows that closed-shell species on metal oxides can be reactive enough to facilitate thermal H-CH₃ bond cleavage and the subsequent conversion.

Chemistry with methane: concepts rather than recipes

Four seemingly simple transformations related to the chemistry of methane will be addressed from mechanistic and conceptual points of view: 1) metal-mediated dehydrogenation to form metal carbene complexes, 2) the hydrogen-atom abstraction step in the oxidative dimerization of methane, 3) the mechanisms of the CH(4)→CH(3)OH conversion, and 4) the initial bond scission (C-H vs. O-H) as well as the rate-limiting step in the selective CH(3)OH→CH(2)O oxidation. State-of-the-art gas-phase experiments, in conjunction with electronic-structure calculations, permit identification of the elementary reactions at a molecular level and thus allow us to unravel detailed mechanistic aspects. Where appropriate, these results are compared with findings from related studies in solution or on surfaces.

Quantum chemical study of the reaction between Ni+ and H2S

The reaction between the Ni(+) cation and H(2)S is studied by considering both the doublet ground state and the lowest-lying quartet state. For the doublet state the reaction is endothermic, whereas it is exothermic for the quartet state. Both CCSD(T)//B3LYP and B3LYP levels of theory, combined with the triple-zeta quality TZVP++G(3df,2p), predict that there are three spin crossings along the characterized reaction path. The first one is located after the first transition state, and the second and third ones before and after the second transition state. On the quartet potential energy surface, both transition states are close in energy to the reactants, while on the doublet surface both lie quite higher in energy. The doublet and quartet states of the HNiSH(+) four-membered intermediate lie very close in energy and their corresponding electronic configurations are connected by a single electron flip. This suggests that the -SH ligand would not prevent a facile intersystem crossing at this intermediate molecule, in contrast to the larger protection provided by the more electronegative -OH ligand.

Studies on gas-phase cyclometalations of [ArNi(PPh3)n]+ (n = 1 or 2) by electrospray ionization tandem mass spectrometry

Gas-phase cyclometalation of [ArNi(PPh(3))(n)](+) (n = 1, 2) complexes have been studied by ESI-MS/MS. The electron-donating substituents of aromatic iodides in the para position were found to inhibit the cyclometalation process of losing ArH, while the electron-withdrawing substituents in the para position were found to enhance it. These results indicate that the cyclometalation process of losing ArH is favored by electron-deficient aromatic groups. In addition, the detailed dissociation pathways of the cationic nickel complexes were studied, and among these pathways, the process of aryl-aryl interchange was also found to proceed in ESI-MS/MS.

Experimental and computational studies on the iridium activation of aliphatic and aromatic C-H bonds of alkyl aryl ethers and related molecules

Reaction of the Ir(III) complex [(Tp(Me2))Ir(C(6)H(5))(2)(N(2))] (1N(2)) with ortho-cresol (2-methylphenol) occurs with cleavage of the O-H and two C(sp(3))-H bonds of the phenol and formation of the electrophilic hydride alkylidene derivative [(Tp(Me2))Ir(H){=C(H)C(6)H(4)-o-O}] (2). The analogous reaction of 2-ethylphenol gives a related product 3. Both 2 and 3 have been shown to be identical to the minor, unidentified products of the already reported reactions of 1 with anisole and phenetole, respectively. Thus, in addition to the route that leads to the known heteroatom-stabilized hydride carbene [(Tp(Me2))Ir(H){=C(H)OC(6)H(4)-o-}] (B), anisole can react with 1 with cleavage of the O-CH(3) bond and formation of a new carbon-carbon bond. In contrast, only C-H bond-activation products with structures akin to B result from 1N(2) and 3,5-dimethylanisole (complex 8) or 4-fluoroanisole (9). Using anisole as a model, a computational study of the triple C-H bond activation (two aliphatic C-H bonds plus an ortho-metalation reaction) that is responsible for the formation of these heteroatom-stabilized hydride carbenes has been undertaken.

Pd/X group interchange in the [Pd(Br)(PH3)(C6H5)(C6H5X)] system — Theoretical insights from the isolobal analogy perspective

DFT-B3LYP calculations have been carried out to assess the fate of the Pd/X group intermolecular exchange in the [PdBr(PH3)(C6H5)(C6H5X)] system, where X is either H, an electropositive atom, or a group such as Li, Na, BH2, AlH2, BeH or BeCH3, and an electronegative atom, or a group such as F, Cl, Br, CH3, OH, and SH. The transfer of H is best viewed as involving the migration of a proton between the two phenyls. At variance with this result, the transfer of the more electropositive entities such as X = Li, Na, BH2, AlH2, BeH, or BeCH3 is not complete. It stops halfway to yield a stable structure in which X can experience interactions with the two phenyl groups that are quite ionic. These stable structures are rationalized through isolobal analogy arguments. In the case of beryllium, the correspondence has been made also with the experimentally known cyclopentadienylberyllium borohydride system, CpBeH4. The results of the DFT geometry optimization call for a re-examination of the gas-phase electron-diffraction structure determination, especially for the bond distances and angles that pertain to the two bridging hydrogens. For the halogen series X = F, Cl, or Br and for the electronegative groups CH3, OH, or SH, the transfer between the two phenyls takes place via a two-step Pd(II)/Pd(IV) oxidative addition/reductive elimination mechanism. The associated energy barriers are nevertheless quite high, except for Br and SH for which the process might be feasible. The dimerization of the PdBr(PH3)(C6H5) system is also analyzed within the isolobal analogy framework.

"Roll-over" cyclometalation of 2,2'-bipyridine platinum(II) complexes in the gas phase: a combined experimental and computational study

In a combined experimental/computational investigation, the gas-phase behavior of cationic [Pt(bipy)(CH(3))((CH(3))(2)S)](+) (1) (bipy=2,2'-bipyridine) has been explored. Losses of CH(4) and (CH(3))(2)S from 1 result in the formation of a cyclometalated 2,2'-bipyrid-3-yl species [Pt(bipy-H)](+) (2). As to the mechanisms of ligand evaporation, detailed labeling experiments complemented by DFT-based computations reveal that the reaction follows the mechanistically intriguing "roll-over" cyclometalation path in the course of which a hydrogen atom from the C(3)-position is combined with the Pt-bound methyl group to produce CH(4). Activation of a C-H-bond of the (CH(3))(2)S ligand occurs as well, but is less favored (35 % versus 65 %) as compared to the C(3)-H bond activation of bipy. In addition, the thermal ion/molecule reactions of [Pt(bipy-H)](+) with (CH(3))(2)S have been examined, and for the major pathway, that is, the dehydrogenative coupling of the two methyl groups to form C(2)H(4), a mechanism is suggested that is compatible with the experimental and computational findings. A hallmark of the gas-phase chemistry of [Pt(bipy-H)](+) with the incoming (CH(3))(2)S ligand is the exchange of one (and only one) hydrogen atom of the bipy fragment with the C-H bonds of dimethylsulfide in a reversible "roll-over" cyclometalation reaction. The Pt(II)-mediated conversion of (CH(3))(2)S to C(2)H(4) may serve as a model to obtain mechanistic insight in the dehydrosulfurization of sulfur-containing hydrocarbons.