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07.01.2016

 




Four-electron deoxygenative reductive coupling of carbon monoxide at a single metal site





Journal name:

Nature

Volume:

529,

Pages:

72–75

Date published:


DOI:

doi:10.1038/nature16154



Received


Accepted


Published online









Carbon dioxide is the ultimate source of the fossil fuels that are both central to modern life and problematic: their use increases atmospheric levels of greenhouse gases, and their availability is geopolitically constrained1. Using carbon dioxide as a feedstock to produce synthetic fuels might, in principle, alleviate these concerns. Although many homogeneous and heterogeneous catalysts convert carbon dioxide to carbon monoxide2, further deoxygenative coupling of carbon monoxide to generate useful multicarbon products is challenging3. Molybdenum and vanadium nitrogenases are capable of converting carbon monoxide into hydrocarbons under mild conditions, using discrete electron and proton sources4. Electrocatalytic reduction of carbon monoxide on copper catalysts5 also uses a combination of electrons and protons, while the industrial Fischer–Tropsch process uses dihydrogen as a combined source of electrons and electrophiles for carbon monoxide coupling at high temperatures and pressures6. However, these enzymatic and heterogeneous systems are difficult to probe mechanistically. Molecular catalysts have been studied extensively6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 to investigate the elementary steps by which carbon monoxide is deoxygenated and coupled, but a single metal site that can efficiently induce the required scission of carbon–oxygen bonds and generate carbon–carbon bonds has not yet been documented. Here we describe a molybdenum compound, supported by a terphenyl–diphosphine ligand, that activates and cleaves the strong carbon–oxygen bond of carbon monoxide, enacts carbon–carbon coupling, and spontaneously dissociates the resulting fragment. This complex four-electron transformation is enabled by the terphenyl–diphosphine ligand24, 25, which acts as an electron reservoir and exhibits the coordinative flexibility needed to stabilize the different intermediates involved in the overall reaction sequence. We anticipate that these design elements might help in the development of efficient catalysts for converting carbon monoxide to chemical fuels, and should prove useful in the broader context of performing complex multi-electron transformations at a single metal site.





At a glance




Figures







left


  1. Deoxygenative coupling of CO to produce a C2O1 fragment.
    Figure 1: Deoxygenative coupling of CO to produce a C2O1 fragment.

    The overall reaction (shown at the top) involves the transformation of two Mo-bound carbonyls, with the addition of four reducing equivalents (e) and four equivalents of electrophile (E+), to generate a metal-free C2O1 product. A detailed scheme follows. Starting with compound 1, successive electron loading (to 2, then 3, then 4)—using KC8 and facilitated by electron storage in the pendant arene—leads to substantial CO activation. The addition of the silyl electrophile Me3SiCl to 3 results in C–O cleavage and the formation of silylcarbyne 7, proposed to proceed via a terminal molybdenum carbide, 8. From 7, two electrons are required for the formation of C2 products (6b and 6c), which are spontaneously displaced by N2, providing compound 5. Addition of bulkier silyl electrophiles (i-Pr3SiCl) to 3 or the more-reduced 4 results directly in the generation of compound 5 and a C2 organic fragment (6a). Synthesis of these C2O1 products (6a, 6b and 6c) from two CO ligands represents an overall four-electron transformation. E+, electrophile; e, electron; Mo, molybdenum; OTf, trifluoromethanesulfonate.




  2. X-ray crystal structures of compounds 2, 3, 4 and 7.
    Figure 2: X-ray crystal structures of compounds 2, 3, 4 and 7.

    Structures displaying a full molybdenum terphenyl–diphosphine unit are shown at the top; truncated enlargements of the molybdenum–arene cores of complexes 2 and 3 are shown in the inset. Reduction of compound 2 to generate 3 and 4 leads to deplanarization of the arene, consistent with a partial cyclohexyldienyldianion character. In the more-oxidized compound 7, no metal–arene interaction is observed. The molecular structures are displayed with anisotropic displacement ellipsoids shown at the 50% probability level. Co-crystallized solvent molecules, potassium-bound tetrahydrofuran molecules, and hydrogen atoms are omitted for clarity. A single molybdenum core is represented for the polynuclear clusters 3 and 4.




  3. NMR spectroscopic data.
    Figure 3: NMR spectroscopic data.

    These 13C{1H} NMR spectroscopic data (126 MHz, 25 °C) are for a solution of compound 8, bearing 13C-labels on the CO-derived carbon atoms, dissolved in tetrahydrofuran/benzene-d6. The coupling pattern (2J(C, C) = 3.46 Hz; 2J(P, C) = 3.26 Hz; 2J(P, CO) = 12.55 Hz) and the chemical shifts of the isotopically enriched carbon atoms are consistent with the coordination of carbide (546.20 p.p.m.) and CO ligands (233.16 p.p.m.) to the same metal centre.






right










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