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

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.

Prospects and challenges for nitrogen-atom transfer catalysis

Conversion of C-H bonds to C-N bonds via C-H amination promises to streamline the synthesis of nitrogen-containing compounds. Nitrogen-group transfer (NGT) from metal nitrenes ([M]-NR complexes) has been the focus of intense research and development. By contrast, potentially complementary nitrogen-atom transfer (NAT) chemistry, in which a terminal metal nitride (an [M]-N complex) engages with a C-H bond, is underdeveloped. Although the earliest examples of stoichiometric NAT chemistry were reported 25 years ago, catalytic protocols are only now beginning to emerge. Here, we summarize the current state of the art in NAT chemistry and discuss opportunities and challenges for its development. We highlight the synthetic complementarity of NGT and NAT and discuss critical aspects of nitride electronic structure that dictate the philicity of the metal-supported nitrogen atom. We also examine the characteristic reactivity of metal nitrides and present emerging strategies and remaining obstacles to harnessing NAT for selective, catalytic nitrogenation of unfunctionalized organic small molecules.

Room-Temperature Conversion of Methane to Methanediol by [FeO2]. J Phys Chem Lett 2023;14:1633-1640. [PMID: 36752636 DOI: 10.1021/acs.jpclett.2c03786] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Inspired by the activities of P-450 enzyme and Rieske oxygenases in nature, in which the high-valent Fe-oxo complexes play a key role for oxidation of alkanes, the oxidation process of methane by the high-valent iron oxide cation [FeO₂]⁺ has been explored by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. In contrast to the previously reported [FeO]⁺/CH₄ and [Fe(O)OH]⁺/CH₄ systems, which afford [FeOH]⁺ as the main product, the generation of Fe⁺ dominates the reaction of [FeO₂]⁺ with CH₄. Theoretical calculations suggest a novel "oxygen rebound" pathway for the liberation of methanediol. In particular, the inevitable valence increase of Fe prior to C-H activation is similar to the cytochrome P-450 mediated processes. To our best knowledge, this study provides the first example of methane activation by the high-valent Fe(V)-oxo species in the gas phase, which may thus bridge the gas-phase model and the condensed-phase biosystems.

Spectroscopic Identification of the Dinitrogen Fixation and Activation by Metal Carbide Cluster Anions PtCn- (n = 4-6)

Nitrogen fixation is confronted with great challenges in the field of chemistry. Herein, we report that single metal carbides PtC_n^- and PtC_nN₂^- (n = 4-6) are indispensable intermediates in the process of nitrogen fixation by mass spectrometry coupled with anionic photoelectron spectroscopy, quantum chemical calculations, and simulated density-of-state spectra. The most stable isomers of these cluster anions are characterized to have linear chain structures. The fixation and activation of dinitrogen are facilitated by the charge transfer from Pt and C_n to N₂. The significance of π back-donation of the 5d orbital of the Pt atom to the antibonding π orbits of N₂ for dinitrogen fixation and activation is discussed in detail. This study not only provides a theoretical basis at the molecular level for the activation of dinitrogen by mononuclear metal carbide clusters but also provides a new paradigm for dinitrogen fixation.

Nitrogen Atom Transfer Catalysis by Metallonitrene C-H Insertion: Photocatalytic Amidation of Aldehydes

C-H amination and amidation by catalytic nitrene transfer are well-established and typically proceed via electrophilic attack of nitrenoid intermediates. In contrast, the insertion of (formal) terminal nitride ligands into C-H bonds is much less developed and catalytic nitrogen atom transfer remains unknown. We here report the synthesis of a formal terminal nitride complex of palladium. Photocrystallographic, magnetic, and computational characterization support the assignment as an authentic metallonitrene (Pd-N) with a diradical nitrogen ligand that is singly bonded to PdII . Despite the subvalent nitrene character, selective C-H insertion with aldehydes follows nucleophilic selectivity. Transamidation of the benzamide product is enabled by reaction with N3 SiMe3 . Based on these results, a photocatalytic protocol for aldehyde C-H trimethylsilylamidation was developed that exhibits inverted, nucleophilic selectivity as compared to typical nitrene transfer catalysis. This first example of catalytic C-H nitrogen atom transfer offers facile access to primary amides after deprotection.

On the origin of reactivity variation upon sequential ligation: the [Re(Cl)x]+/CH4 (x = 1-3) couples

The potential of [ReCl_x]⁺ (x = 1-3) in activating methane has been explored by using a combination of gas-phase experiments and high-level quantum calculations. When the number of Cl ligands increases, the reactivity towards methane activation varies accordingly. While [ReCl_x]⁺ (x = 1-2) are able to dehydrogenate methane by a three-state reactivity scenario, [ReCl₃]⁺ shows inertness towards methane at ambient conditions. Furthermore, the product ion [ClRe(H)CH]⁺ of the [ReCl]⁺/CH₄ couple could continue to activate methane and liberate molecular dihydrogen but another product ion [Cl₂ReCH₂]⁺ is unreactive with methane. Obviously, the nature and the number of ligands make a difference to the reactivity towards methane activation. The associated reaction mechanism and the electron origins for the rather different reactivities are discussed in detail. Finally and more importantly, instructive information concerning the rational design of Re-catalysts for methane conversion is obtained.

Size-Dependent Association of Cobalt Deuteride Cluster Anions Co3Dn- (n = 0-4) with Dinitrogen

Dinitrogen (N₂) activation by metal hydride species is of fundamental interest and practical importance while the role of hydrogen in N₂ activation is not well studied. Herein, the structures of Co₃D_n^- (n = 0-4) clusters and their reactions with N₂ have been studied by using a combined experimental and computational approach. The mass spectrometry experiments identified that the Co₃D_n^- (n = 2-4) clusters could adsorb N₂ while the Co₃D_n^- (n = 0 and 1) clusters were inert. The photoelectron imaging spectroscopy indicated that the electron detachment energies of Co₃D_2-4^- are smaller than those of Co₃D_0,1^-, which characterized that it is easier to transfer electrons from Co₃D_2-4^- than from Co₃D_0,1^- to activate N₂. The density functional theory calculations generally supported the experimental observations. Further analysis revealed that the H atoms in the Co₃H_n^- (n = 2-4) clusters generally result in higher energies of the Co 3d orbitals in comparison with the Co₃H_n^- (n = 0 and 1) systems. By forming chemical bonds with H atoms, the Co atoms of Co₃H_2-4^- are less negatively charged with respect to the naked Co₃^- system, which leads to higher N₂ binding energies of Co₃H_2-4N₂^- than that of Co₃N₂^-.

Ambient-temperature oxidative coupling of methane in an electric field by a cerium phosphate nanorod catalyst

The application of an electric field to a CePO₄ nanorod catalyst enabled ambient-temperature oxidative coupling of methane to C₂ hydrocarbons.