The Role of the Redox Non-Innocent Hydroxyl Ligand in the Activation of O2 Performed by [Ni(H)(OH)]. Chemistry 2023;29:e202203128. [PMID: 36447369 DOI: 10.1002/chem.202203128] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The cationic complex [Ni(H)(OH)]⁺ was previously found to activate dioxygen and methane in gas phase under single collision conditions. These remarkable reactivities were thought to originate from a non-classical electronic structure, where the Ni-center adopts a Ni(II), instead of the classically expected Ni(III) oxidation state by formally accepting an electron from the hydroxo ligand, which formally becomes a hydroxyl radical in the process. Such radicaloid oxygen moieties are envisioned to easily react with otherwise inert substrates, mimicking familiar reactivities of free radicals. In this study, the reductive activation of dioxygen by [Ni(H)(OH)]⁺ to afford the hydroperoxo species was investigated using coupled cluster, multireference ab initio and density functional theory calculations. Orbital and wave function analyses indicate that O₂ binding tranforms the aforementioned non-classical electronic structure to a classical Ni(III)-hydroxyl system, before O₂ reduction takes place. Remarkably, we found no evidence for a direct involvement of the radicaloid hydroxyl in the reaction with O₂ , as is often assumed. The function of the redox non-innocent character of the activator complex is to protect the reactive electronic structure until the complex engages O₂ , upon which a dramatic electronic reorganization releases internal energy and drives the chemical reaction to completion.

Revisiting Alkane Hydroxylation with m-CPBA (mChloroperbenzoic Acid) Catalyzed by Nickel(II) Complexes

Mechanistic studies are performed on the alkane hydroxylation with m -CPBA ( m -chloroperbenzoic acid) catalyzed by nickel(II) complexes, Ni II (L). In the oxidation of cycloalkanes, Ni II (TPA) acts as an efficient catalyst with a high yield and a high alcohol selectivity. In the oxidation of adamantane, the tertiary carbon is predominantly oxidized. The reaction rate shows first-order dependence on [substrate] and [Ni II (L)] but is independent on [ m CPBA]; v obs = k 2 [substrate][ Ni II (L)]. The reaction exhibited a relatively large kinetic deuterium isotope effect ( KIE ) of 6.7, demonstrating that the hydrogen atom abstraction is involved in the rate-limiting step of the catalytic cycle. Furthermore, Ni II (L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the Ni II (L) in the catalytic cycle. Based on the kinetic analysis and the significant effects of O 2 and CCl 4 on the product distribution pattern, possible contributions of (L)Ni II -O• and the acyloxyl radical as the reactive oxidants are discussed.

Catalytic C-H Functionalization of Unreactive Furan Cores in Bio-Derived Platform Chemicals

C-H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the "Holy Grail" of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C-H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass-derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C-H functionalization for their modification. New methods of catalytic functionalization of non-reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.

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 C₂H₄ 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 C₂H₄→CH₃CHO conversion. Interestingly, in the course of the reaction, the emerging CH₃CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C₂H₄O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.

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 [RuO_x ]⁺ (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 [RuO_x ]⁺ (x=1-3) with CH₄ and H₂ have been compared with the previously reported results of Group 8 analogues [OsO_x ]⁺ /CH₄ /H₂ (x=1-3) and the [FeO]⁺ /H₂ system. The electronic origins for their distinctly different reaction behaviors have been addressed.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

Cryogenic ion trap vibrational spectroscopy paired with quantum chemistry currently represents the most generally applicable approach for the structural investigation of gaseous cluster ions that are not amenable to direct absorption spectroscopy. Here, we give an overview of the most popular variants of infrared action spectroscopy and describe the advantages of using cryogenic ion traps in combination with messenger tagging and vibrational predissociation spectroscopy. We then highlight a few recent studies that apply this technique to identify highly reactive ionic intermediates and to characterize their reactive sites. We conclude by commenting on future challenges and potential developments in the field.

Fe2 O+ Cation Mediated Propane Oxidation by Dioxygen in the Gas Phase

The mass-selected Fe₂ O⁺ cation mediated propane oxidation by O₂ was investigated by mass spectrometry and density functional theory calculations. In the reaction of Fe₂ O⁺ with C₃ H₈ , H₂ was liberated by C-H bond activation to give Fe₂ OC₃ H₆⁺ . Interestingly, when a mixture of C₃ H₈ /O₂ was introduced into the reactor, an intense signal that corresponded to the Fe₂ O₂⁺ cation was present; the experiments indicated that O₂ was activated in its reaction with Fe₂ O(C₃ H₆ )⁺ to give Fe₂ O₂⁺ and C₃ H₆ O (acetone or propanal). A Langmuir-Hinshelwood-like mechanism was adopted in the propane oxidation reaction by O₂ on gas-phase Fe₂ O⁺ cations. In comparison with the absence of Fe₂ O₂⁺ in the reaction of Fe₂ O⁺ with O₂ , the ligand effect of C₃ H₆ on Fe₂ OC₃ H₆⁺ is important in the oxygen activation reaction. The theoretical results are consistent with the experimental observations. The propane oxidation by O₂ in the presence of Fe₂ O⁺ might be applied as a model for alkane and O₂ activations over iron oxide catalysts, and the mechanisms and kinetic data are useful for understanding corresponding heterogeneous reactions.

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.

Thermal Methane Activation by the Metal-Free Cluster Cation [Si2 O4 ]

The thermal reaction of methane with the metal-free cluster cation [Si₂ O₄ ]^.+ has been examined by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In addition to generating a methyl radical via hydrogen-atom abstraction, [Si₂ O₄ ]^.+ can selectively oxidize methane to formaldehyde. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations.

Ménage-à-trois: single-atom catalysis, mass spectrometry, and computational chemistry

Genuine, single-atom catalysis can be realized in the gas phase and probed by mass spectrometry combined with computational chemistry.

Thermal Methane Activation by [Si2 O5 ].+ and [Si2 O5 H2 ].+ : Reactivity Enhancement by Hydrogenation

The thermal reactions of methane with the oxygen-rich cluster cations [Si₂ O₅ ]^⋅+ and [Si₂ O₅ H₂ ]^⋅+ have been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. In contrast to the inertness of [Si₂ O₅ ]^.+ towards methane, the hydrogenated cluster [Si₂ O₅ H₂ ]^.+ brings about hydrogen-atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed.

Spectroscopically Characterized Synthetic Mononuclear Nickel-Oxygen Species

Iron, copper, and manganese are the predominant metals found in oxygenases that perform efficient and selective hydrocarbon oxidations and for this reason, a large number of the corresponding metal-oxygen species has been described. However, in recent years nickel has been found in the active site of enzymes involved in oxidation processes, in which nickel-dioxygen species are proposed to play a key role. Owing to this biological relevance and to the existence of different catalytic protocols that involve the use of nickel catalysts in oxidation reactions, there is a growing interest in the detection and characterization of nickel-oxygen species relevant to these processes. In this Minireview the spectroscopically/structurally characterized synthetic superoxo, peroxo, and oxonickel species that have been reported to date are described. From these studies it becomes clear that nickel is a very promising metal in the field of oxidation chemistry with still unexplored possibilities.

Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a Ni(II) precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., C-H bonds, C=C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a Ni(III) -oxyl compound.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

Isomer-selective thermal activation of methane in the gas phase by [HMO]+ and [M(OH)]+ (M=Ti and V)

Methane scrabble: To have the right elements is sometimes just not sufficient, as shown by [M(OH)](+) (M=Ti, V), which do not react with methane. However, reshuffling of the "tiles" to [HMO](+) changes the reactions behavior completely, leading to the first example of C-H bond activation of methane by an early first-row transition-metal cation.

Gas-phase reactions of cationic vanadium-phosphorus oxide clusters with C2H(x) (x=4, 6): a DFT-based analysis of reactivity patterns

The reactivities of the adamantane-like heteronuclear vanadium-phosphorus oxygen cluster ions [V(x)P(4-x)O(10)](.+) (x=0, 2-4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas-phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT-based calculations; homolytic C-H bond activation constitutes the initial step, and for all systems the P-O(.) unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen-atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e(-) reduction of the cationic clusters.

Hydrogen-abstraction reactivity patterns from A to Y: the valence bond way

"Give us insight, not numbers" was Coulson's admonition to theoretical chemists. This Review shows that the valence bond (VB)-model provides insights and some good numbers for one of the fundamental reactions in nature, the hydrogen-atom transfer (HAT). The VB model is applied to over 50 reactions from the simplest H + H(2) process, to P450 hydroxylations and H-transfers among closed-shell molecules; for each system the barriers are estimated from raw data. The model creates a bridge to the Marcus equation and shows that H-atom abstraction by a closed-shell molecule requires a higher barrier owing to the additional promotion energy needed to prepare the abstractor for H-abstraction. Under certain conditions, a closed-shell abstractor can bypass this penalty through a proton-coupled electron transfer (PCET) mechanism. The VB model links the HAT and PCET mechanisms conceptually and shows the consequences that this linking has for H-abstraction reactivity.