Bimetallic cleavage of aromatic C-H bonds by rare-earth-metal complexes

Recent progress in rare-earth metal-catalyzed sp2 and sp3 C–H functionalization to construct C–C and C–heteroelement bonds

The review presents rare-earth metal-catalyzed C(sp2/sp3)–H functionalization accessing C–C/C–heteroatom bonds and olefin (co)polymerization, highlighting substrate scope, mechanistic realization, and origin of site-, enantio-/diastereo-selectivity.

Quadruple C-H Bond Activations of Methane by Dinuclear Rhodium Carbide Cation [Rh2C3]. JACS Au 2021;1:1631-1638. [PMID: 34723266 PMCID: PMC8549038 DOI: 10.1021/jacsau.1c00265]

The structure of the [Rh₂C₃]⁺ ion and its reaction with CH₄ in the gas phase have been studied by infrared photodissociation spectroscopy and mass spectrometry in conjunction with quantum chemical calculations. The [Rh₂C₃]⁺ ion is characterized to have an unsymmetrical linear [Rh-C-C-C-Rh]⁺ structure existing in two nearly isoenergetic spin states. The [Rh₂C₃]⁺ ion reacts with CH₄ at room temperature to form [Rh₂C]⁺ + C₃H₄ and [Rh₂C₂H₂]⁺ + C₂H₂ as the major products. In addition to the [Rh₂C]⁺ ion, the [Rh₂ ¹³C]⁺ ion is formed at about one-half of the [Rh₂C]⁺ intensity when the isotopic-labeled ¹³CH₄ sample is used. The production of [Rh₂ ¹³C]⁺ indicates that the linear C₃ moiety of [Rh₂C₃]⁺ can be replaced by the bare carbon atom of methane with all four C-H bonds being activated. The calculations suggest that the overall reactions are thermodynamically exothermic, and that the two Rh centers are the reactive sites for C-H bond activation and hydrogen atom transfer reactions.

Scandium and lanthanum hydride complexes stabilized by super-bulky penta-arylcyclopentadienyl ligands

Hydrogenolysis of the half-sandwich penta-arylcycopentadienyl-supported rare-earth metal dibenzyl complexes [(Cp^Ar5)Ln(p-CH₂-C₆H₄-Me)₂(THF)] (Cp^Ar5 = C₅Ar₅, Ar = 3,5-ⁱPr₂-C₆H₃; Ln = Sc, La) afforded a bimetallic scandium complex [(Cp^Ar5)Sc(H)(μ-OC₄H₉)]₂ (2) with two terminal hydrido ligands, and a double-sandwich bimetallic lanthanum hydride complex [(Cp^Ar5)La(μ-H)]₂ (4) bearing the reduced Cp^Ar5 ligand. DFT calculations were conducted to elucidate the reaction profiles.

Ni(0)-promoted activation of Csp2 -H and Csp2 -O bonds

A dinickel(0)-N2 complex, stabilized with a rigid acridane-based PNP pincer ligand, was studied for its ability to activate C(sp2)-H and C(sp2)-O bonds. Stabilized by a Ni-μ-N2-Na+ interaction, it activates C-H bonds of unfunctionalized arenes, affording nickel-aryl and nickel-hydride products. Concomitantly, two sodium cations get reduced to Na(0), which was identified and quantified by several methods. Our experimental results, including product analysis and kinetic measurements, strongly suggest that this C(sp2)-H activation does not follow the typical oxidative addition mechanism occurring at a low-valent single metal centre. Instead, via a bimolecular pathway, two powerfully reducing nickel ions cooperatively activate an arene C-H bond and concomitantly reduce two Lewis acidic alkali metals under ambient conditions. As a novel synthetic protocol, nickel(ii)-aryl species were directly synthesized from nickel(ii) precursors in benzene or toluene with excess Na under ambient conditions. Furthermore, when the dinickel(0)-N2 complex is accessed via reduction of the nickel(ii)-phenyl species, the resulting phenyl anion deprotonates a C-H bond of glyme or 15-crown-5 leading to C-O bond cleavage, which produces vinyl ether. The dinickel(0)-N2 species then cleaves the C(sp2)-O bond of vinyl ether to produce a nickel(ii)-vinyl complex. These results may provide a new strategy for the activation of C-H and C-O bonds mediated by a low valent nickel ion supported by a structurally rigidified ligand scaffold.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Phosphido complexes derived from 1,1'-ferrocenediyl-bridged secondary diphosphines

This paper focuses on ferrocene-based secondary diphosphines of the type [Fe{η⁵-C₅H₄(PHR)}₂] with P-substituents of distinctly different steric and electronic properties, namely methyl, neopentyl (Np), tert-butyl, phenyl and 3,5-bis(trifluoromethyl)phenyl (XyF). The reaction of [Fe{η⁵-C₅H₄(PHPh)}₂] (H₂1a) and [Fe{η⁵-C₅H₄(PHt-Bu)}₂] (H₂1b) with n-BuLi in the presence of TMEDA afforded lithium diphosphides of the type [Li₂(μ-1)(TMEDA)₂], which contain a cyclic non-planar Li₂P₂ core. The analogous reactions of [Fe{η⁵-C₅H₄(PHMe)}₂] (H₂1c) and [Fe{η⁵-C₅H₄(PHNp)}₂] (H₂1d) furnished dimeric aggregates exhibiting a ladder-type Li₄P₄ motif, viz. [Li₄(μ-1c)₂(TMEDA)₃] and [Li₂(μ-1d)(TMEDA)]₂. H₂1e (R = XyF) did not afford a stable lithium diphosphide. A Brønsted metathesis with Zr(NMe₂)₄ was possible with the aryl-substituted compounds H₂1a and H₂1e, leading to products of the type [{Zr(NMe₂)₃}₂(μ-1)]. In contrast, the alkyl-substituted congeners H₂1b-H₂1d were inert towards Zr(NMe₂)₄. The reaction of [Fe{η⁵-C₅H₄(PHR)}₂] with nickelocene afforded intractable mixtures of numerous products in the case of H₂1c and H₂1e. In the other three cases, compounds of the type [(NiCp)₂(μ-1)] were isolated. For H₂1b and H₂1d a two-stepped reaction via a phosphino-phosphido intermediate of the type [NiCp(H1)] was observed, which could be isolated and fully characterised in the case of [NiCp(H1b)].

Methane Activation by Tantalum Carbide Cluster Anions Ta2C4. J Phys Chem Lett 2017;8:605-610. [PMID: 28088857 DOI: 10.1021/acs.jpclett.6b02568]

Methane activation by transition metals is of fundamental interest and practical importance, as this process is extensively involved in the natural gas conversion to fuels and value-added chemicals. While single-metal centers have been well recognized as active sites for methane activation, the active center composed of two or more metal atoms is rarely addressed and the detailed reaction mechanism remains unclear. Here, by using state-of-the-art time-of-flight mass spectrometry, cryogenic anion photoelectron imaging spectroscopy, and quantum-chemical calculations, the cooperation of the two Ta atoms in a dinuclear carbide cluster Ta₂C₄^- for methane activation has been identified. The C-H bond activation takes place predominantly around one Ta atom in the initial stage of the reaction and the second Ta atom accepts the delivered H atom from the C-H bond cleavage. The well-resolved vibrational spectra of the cryogenically cooled anions agree well with theoretical simulations, allowing the clear characterization of the structure of Ta₂C₄^- cluster. The reactivity comparison between Ta₂C₄^- cluster and the carbon-less analogues (Ta₂C₃^- and Ta₂C₂^-) demonstrated that the cooperative effect of the two metal atoms can be well tuned by the carbon ligands in terms of methane activation and transformation.

Rare-earth metal π-complexes of reduced arenes, alkenes, and alkynes: bonding, electronic structure, and comparison with actinides and other electropositive metals

Rare-earth metal complexes of reduced π ligands are reviewed with an emphasis on their electronic structure and bonding interactions.