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An osmium(II) methane complex: Elucidation of the methane coordination mode.

ABSTRACT: The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the ¹J_CH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better C─H functionalization catalysts.

SUBMITTER: Sempsrott PJ

PROVIDER: S-EPMC10256148 | biostudies-literature | 2023 Jun

REPOSITORIES: biostudies-literature

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An osmium(II) methane complex: Elucidation of the methane coordination mode.

Sempsrott Peter J PJ Trinh Brian B BB Lovitt Charity Flener CF Capra Nicolas E NE Girolami Gregory S GS

Science advances 20230609 23

The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the 1JCH coupling constan ...[more]

PMID: 37294762

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Project description:Synthesis and X-ray diffraction structures of cis and trans isomers of ruthenium and osmium metal complexes of general formulas (nBu4N)[cis-MCl4(NO)(Hind)], where M = Ru (1) and Os (3), and (nBu4N)[trans-MCl4(NO)(Hind)], where M = Ru (2) and Os (4) and Hind = 1H-indazole are reported. Interconversion between cis and trans isomers at high temperatures (80-130 °C) has been observed and studied by NMR spectroscopy. Kinetic data indicate that isomerizations correspond to reversible first order reactions. The rates of isomerization reactions even at 110 °C are very low with rate constants of 10(-5) s(-1) and 10(-6) s(-1) for ruthenium and osmium complexes, respectively, and the estimated rate constants of isomerization at room temperature are of ca. 10(-10) s(-1). The activation parameters, which have been obtained from fitting the reaction rates at different temperatures to the Eyring equation for ruthenium [?H(cis-trans)‡ = 122.8 ± 1.3; ?H(trans-cis)‡ = 138.8 ± 1.0 kJ/mol; ?S(cis-trans)‡ = -18.7 ± 3.6; ?S(trans-cis)‡ = 31.8 ± 2.7 J/(mol·K)] and osmium [?H(cis-trans)‡ = 200.7 ± 0.7; ?H(trans-cis)‡ = 168.2 ± 0.6 kJ/mol; ?S(cis-trans)‡ = 142.7 ± 8.9; ?S(trans-cis)‡ = 85.9 ± 3.9 J/(mol·K)] reflect the inertness of these systems. The entropy of activation for the osmium complexes is highly positive and suggests the dissociative mechanism of isomerization. In the case of ruthenium, the activation entropy for the cis to trans isomerization is negative [-18.6 J/(mol·K)], while being positive [31.0 J/(mol·K)] for the trans to cis conversion. The thermodynamic parameters for cis to trans isomerization of [RuCl4(NO)(Hind)]-, viz. ?H° = 13.5 ± 1.5 kJ/mol and ?S° = -5.2 ± 3.4 J/(mol·K) indicate the low difference between the energies of cis and trans isomers. The theoretical calculation has been carried out on isomerization of ruthenium complexes with DFT methods. The dissociative, associative, and intramolecular twist isomerization mechanisms have been considered. The value for the activation energy found for the dissociative mechanism is in good agreement with experimental activation enthalpy. Electrochemical investigation provides further evidence for higher reactivity of ruthenium complexes compared to that of osmium counterparts and shows that intramolecular electron transfer reactions do not affect the isomerization process. A dissociative mechanism of cis?trans isomerization has been proposed for both ruthenium and osmium complexes.

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An osmium(II) methane complex: Elucidation of the methane coordination mode.

Publications

An osmium(II) methane complex: Elucidation of the methane coordination mode.

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