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.

Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO4 ].+ /C2 H4 Couple by Gas-Phase Vibrational Spectroscopy.

The gas-phase reaction of the heteronuclear oxide cluster [VPO4 ].+ with C2 H4 is studied under multiple collision conditions at 150 K using cryogenic ion-trap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrier-free thermal C2 H4 →CH3 CHO conversion. Interestingly, in the course of the reaction, the emerging CH3 CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2 H4 O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.

O-O Bond Formation and Liberation of Dioxygen Mediated by N5 -Coordinate Non-Heme Iron(IV) Complexes.

Formation of the O-O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High-valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O-O bond in solution, from non-heme, N5 -coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta-chloroperbenzoic acid is administered in excess. Oxygen-isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)-initiated free-radical pathways of the peroxides, which are typical of catalase-like reactivity, and iron-borne O-O coupling, which is unprecedented for non-heme/peroxide systems. Interpretation in terms of [FeIV (O)] and [FeV (O)] being the resting and active principles of the O-O coupling, respectively, concurs with fundamental mechanistic ideas of (electro-) chemical O-O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Thermal Activation of CH4 and H2 as Mediated by the Ruthenium Oxide Cluster Ions [RuOx ]+ (x=1-3): On the Influence of Oxidation States.

Thermal gas-phase reactions of the ruthenium-oxide clusters [RuOx ]+ (x=1-3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO]+ /CH4 couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO2 ]+ brings about the generations of [Ru,O,C,H2 ]+ /H2 O, [Ru,O,C]+ /H2 /H2 O, and [Ru,O,H2 ]+ /CH2 O, whereas [RuO3 ]+ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H2 ). Regarding the reactions with H2 , as compared to CH4 , both [RuO]+ and [RuO2 ]+ react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO3 ]+ is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H-H bond activation. Furthermore, the reactivity patterns of [RuOx ]+ (x=1-3) with CH4 and H2 have been compared with the previously reported results of Group 8 analogues [OsOx ]+ /CH4 /H2 (x=1-3) and the [FeO]+ /H2 system. The electronic origins for their distinctly different reaction behaviors have been addressed.

Tuning the Reactivities of the Heteronuclear [Aln V3-n O7-n ]+ (n=1,†2) Cluster Oxides towards Methane by Varying the Composition of the Metal Centers.

The thermal gas-phase reactions of [Al2 VO5 ]+ and [AlV2 O6 ]+ with methane have been explored by using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. Both cluster ions chemisorbed methane as the major reaction channels at room temperature. [Al2 VO5 ]+ could break only one C-H bond to liberate CH3 , whereas [AlV2 O6 ]+ exhibited higher oxidizing ability such that it brings about the selective generation of formaldehyde. Mechanistic aspects are revealed and the crucial roles of the metal centers are discussed.

On the Remarkable Role of the Nitrogen Ligand in the Gas-Phase Redox Reaction of the N2 O/CO Couple Catalyzed by [NbN].

The thermal gas-phase catalytic reduction of N2 O by CO, mediated by the transition-metal nitride cluster ion [NbN]+ , has been explored by using FT-ICR mass spectrometry and complemented by high-level quantum chemical calculations. In contrast to the [Nb]+ /[NbO]+ and [NbO]+ /[Nb(O)2 ]+ systems, in which the oxidation of [Nb]+ and [NbO]+ with N2 O is facile, but in which neither [NbO]+ nor [Nb(O)2 ]+ react with CO at room temperature, the [NbN]+ /[ONbN]+ system at ambient temperature mediates the catalytic oxidation of CO. The origins of the distinctly different reactivities upon nitrogen ligation are addressed by quantum chemical calculations.

Intermediates of N-Heterocyclic Carbene (NHC) Dimerization Probed in the Gas Phase by Ion Mobility Mass Spectrometry: C-H⋠⋠⋠:C Hydrogen Bonding Versus Covalent Dimer Formation.

N-Heterocyclic carbenes (NHCs, :C) can interact with azolium salts (C-H+ ) by either forming a hydrogen-bonded aggregate (CHC+ ) or a covalent C-C bond (CCH+ ). In this study, the intramolecular NHC-azolium salt interactions of aromatic imidazolin-2-ylidenes and saturated imidazolidin-2-ylidenes have been investigated in the gas phase by traveling wave ion mobility mass spectrometry (TW IMS) and DFT calculations. The TW IMS experiments provided evidence for the formation of these important intermediates in the gas phase, and they identified the predominant aggregation mode (hydrogen bond vs. covalent C-C) as a function of the nature of the interacting carbene-azolium pairs.

Selective Nitrogen-Atom Transfer Driven by a Highly Efficient Intersystem Crossing in the [CeON]+ /CH4 System.

The thermal gas-phase reactions of [CeON]+ with methane have been explored by FT-ICR mass spectrometry and high-level quantum-chemical calculations. Nitrogen-atom transfer from the cluster ion to methane was observed as the only reaction channel. Based on computational work, the neutral molecule formed corresponds to either CH2 NH2 or CH3 NH. In addition to a rather weak OCe+ -N bond, this reaction benefits from a highly efficient intersystem crossing. Mechanistic aspects and the associated electronic origins are discussed, and a detailed comparison of [CeON]+ , [CeO]+ , [CeN]+ , [CeO2 ]+ , and atomic N in their reactions with CH4 is given.

Oriented external electric fields as mimics for probing the role of metal ions and ligands in the thermal gas-phase activation of methane.

In the course of combined computational and mass spectrometry-based mechanistic studies, recently we came across rather unusual, if not unprecedented, effects of transition-metal ions and ligands when simple metal oxides or carbides are subjected to thermal gas-phase reactions with methane. Interestingly, "Gedankenexperiments" demonstrate how these effects can be modeled using oriented external electric fields (OEEFs), thus expanding their predicted role as "smart reagents" (Shaik et al., Nat. Chem., 2016, 8, 1091), and further suggesting that the OEEFs may be used in controlling the adsorption/desorption behavior of methane as well as serving as a tool to explore mechanistic features.

Selective C-O Coupling Hidden in the Thermal Reaction of [Al2 CuO5 ]+ with Methane.

The thermal gas-phase reaction of [Al2 CuO5 ]+ with methane has been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. The generation of atomic [Cu]+ from the [Al2 CuO5 ]+ /CH4 couple corresponds to the only reaction channel. Labeling experiments and computational studies strongly suggest that methane activation is indeed involved in the production of [Cu]+ , and generation of CH2 O prevails. Mechanistic aspects and the associated doping effects are discussed.

The Electric Field as a "Smart" Ligand in Controlling the Thermal Activation of Methane and Molecular Hydrogen.

While pristine [TaO2 ]+ is rather inert in its thermal reactions with both methane and molecular hydrogen, the cluster oxide becomes reactive upon ligation with the closed-shell ligand CO2 . While the presence of CO2 increases the intrinsic barriers of the C-H and H-H bond activation steps, the efficiency of the overall reaction to generate the insertion product [Ta(R)(OH)O]+ (R=CH3 , H) is greatly enhanced by the superb leaving-group properties of CO2 . The ligand effect of CO2 has been well mimicked by the electric field of a negative point charge, and mechanistic aspects are addressed by high-level quantum chemical calculations.

On the Origin of the Distinctly Different Reactivity of Ruthenium in [MO]+ /CH4 Systems (M=Fe, Ru, Os).

The thermal gas-phase reactions of [RuO]+ with methane have been explored by FT-ICR mass spectrometry and high-level quantum-chemical calculations. In contrast to the previously studied [FeO]+ /CH4 and [OsO]+ /CH4 couples, which undergo oxygen/hydrogen atom transfers and dehydrogenation, respectively, the [RuO]+ /CH4 system produces selectively [Ru(CH)2 ]+ and H2 O, albeit with much lower efficiency. Various mechanistic scenarios were uncovered, and the associated electronic origins were revealed by high-level quantum-chemical calculations. The reactivity differences observed for the [MO]+ /CH4 couples (M=Fe, Ru, Os) are due to the subtle interplay of the spin-orbit coupling efficiency, orbital overlap, and relativistic effects.

Direct Room-Temperature Conversion of Methane into Protonated Formaldehyde: The Gas-Phase Chemistry of Mercury among the Zinc Triad Oxide Cations.

In thermal reactions of methane with diatomic metal oxides [MO].+ of the zinc triad (M=Zn, Cd, Hg), protonated formaldehyde [CH2 OH]+ is generated as the major product only for the [HgO].+ /CH4 couple. Mechanistic insight is provided by high-level quantum-chemical calculations, and relativistic effects are suggested to be the root cause for the unexpected thermal production of [CH2 OH]+ from [HgO].+ /CH4 .

Spin-Selective, Competitive Hydrogen-Atom Transfer versus CH2 O-Generation from the CH4 /[ReO4 ]+ Couple at Ambient Conditions.

The thermal gas-phase reactions of [ReO4 ]+ with methane have been explored by using Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. Upon reacting with methane, this cluster oxide, having an even-number of valence electrons, brings about both hydrogen-atom abstraction (HAT) to generate [ReO4 H].+ and the formation of formaldehyde. Mechanistically, HAT occurs on the ground-state triplet surface, while for the generation of formaldehyde a two-state reactivity scenario prevails. The branching ratio of these competing processes is affected by the rather inefficient spin-orbit coupling to bring about the required triplet-singlet intersystem crossing.

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.

Metal-Free, Room-Temperature Oxygen-Atom Transfer in the N2 O/CO Redox Couple as Catalyzed by [Si2 Ox ].+ (x=2-5).

The thermal reduction of N2 O by CO mediated by the metal-free cluster cations [Si2 Ox ].+ (x=2-5) has been examined in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with quantum chemical calculations. Three successive oxidation/reduction steps occur starting from [Si2 O2 ].+ and N2 O to form eventually [Si2 O5 ].+ ; the latter as well as the intermediate oxide cluster ions react sequentially with CO molecules to regenerate [Si2 O2 ].+ . Thus, full catalytic cycles occur at ambient conditions in the gas phase. Mechanistic aspects of these sequential redox processes have been addressed to reveal the electronic origins of these unparalleled reactions.

Electronic Origin of the Competitive Mechanisms in the Thermal Activation of Methane by the Heteronuclear Cluster Oxide [Al2 ZnO4 ].

The thermal gas-phase reactions of [Al2 ZnO4 ].+ with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Two competitive mechanisms, that is, hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) are operative. Interestingly, while the HAT process is influenced by the polarity of the transition structure, both the ionic nature of the metal-oxygen bond and the structural rigidity of the cluster oxide affect the PCET pathway. As compared to the previously reported homonuclear [Al2 O3 ].+ and [ZnO].+ , the heteronuclear oxide [Al2 ZnO4 ].+ exhibits a much higher chemoselectivity towards methane. The electronic origins of the doping effect have been explored.

Metal-Dependent Strengthening and Weakening of M-H and M-C Bonds by an Oxo Ligand: Thermal Gas-Phase Activation of Methane by [OMH]+ and [MH]+ (M=Mo, Ti).

The thermal gas-phase reactions of methane with [OMoH]+ and [MoH]+ were investigated by using electrospray-ionization mass spectrometry (ESI-MS) complemented by quantum-chemical calculations. In contrast to the inertness of [MoH]+ towards methane, [OMoH]+ activates the C-H bond to form the ionic product [OMo(CH3 )]+ concomitantly with the liberation of H2 . The origin of the varying reactivities is traced back to a different influence of the oxo ligand on the Mo-C and Mo-H bonds. While the presence of this ligand weakens both the Ti-H and the Ti-CH3 bonds, both the Mo-H and Mo-CH3 bonds are strengthened. The more pronounced strengthening of the Mo-CH3 bond compared to the Mo-H bond favors the exothermicity of the reaction of [OMoH]+ with CH4 .

Control of Product Distribution and Mechanism by Ligation and Electric Field in the Thermal Activation of Methane.

An unexpected mechanistic switch as well as a change of the product distribution in the thermal gas-phase activation of methane have been identified when diatomic [ZnO].+ is ligated with acetonitrile. Theoretical studies suggest that a strong metal-carbon attraction in the pristine [ZnO].+ species plays an important role in the rebound of the incipient CH3. radical to the metal center, thus permitting the competitive generation of CH3. , OH. , and CH3 OH. This interaction is drastically weakened by a single CH3 CN ligand. As a result, upon ligation the proton-coupled single electron transfer that prevails for [ZnO].+ /CH4 switches to the classical hydrogen-atom-transfer process, thus giving rise to the exclusive expulsion of CH3. . This ligand effect can be modeled quite well by an oriented external electric field of a negative point charge.

Sequential Gas-Phase Activation of Carbon Dioxide and Methane by [Re(CO)2]+: The Sequence of Events Matters!

The potential of carbonyl rhenium complexes in activating and coupling carbon dioxide and methane has been explored by using a combination of gas-phase experiments (FT-ICR mass spectrometry) and high-level quantum chemical calculations. While the complexes [Re(CO)x]+ (x = 0, 1, 3) are thermally unreactive toward CO2, [Re(CO)2]+ abstracts one oxygen atom from this substrate spontaneously at ambient conditions. Based on 13C and 18O labeling experiments, the newly generated CO ligand is preferentially eliminated, and two mechanistic scenarios are considered to account for this unexpected finding. The oxo complex [ORe(CO)2]+ reacts further with CH4 to produce the dihydridomethylene complex [ORe(CO)(CH2)(H)2]+. However, coupling of the CO and CH2 ligands to form CH2═C═O does not take place. Further, the complexes [Re(CO)x(CH2)]+ (x = 1, 2), generated in the thermal reaction of [Re(CO)x]+ (x = 1, 2) with CH4, are inert toward CO2. Mechanistic insight on the origin of this remarkable reactivity pattern has been derived from detailed quantum chemical calculations.