Terminal Mo Carbide and Carbyne Reactivity: H₂ Cleavage, B–C Bond Activation, and C–C Coupling

Abstract

Transition-metal carbides have been posited as intermediates in the upgrading of C 1 feedstocks, including the Fischer− Tropsch synthesis of higher olefins from CO and H 2 . Still, molecular examples remain rare, and their reactivity is poorly understood. In a molecular platform supported by a flexible terphenyl diphosphine ligand, the important C−C coupling step was previously demonstrated on the reaction of Mo carbide and carbene species with CO. Methylidyne and methylidene complexes were accessible on sequential treatment of the carbide with sources of H + and H − , surrogates for heterolyzed H 2 . Herein, we demonstrate that the terminal carbide complex is also capable of directly activating H 2 , yielding a P−C-bonded ylide complex. The carbide moiety also inserts into the B−C bond of triphenylborane, yielding an unusual example of a borylcarbene with a direct Mo−B contact. C−C coupling from the terminal methylidyne, a potential intermediate formed on heterolytic H 2 cleavage and H + transfer to the carbide, yields a Mo ketenyl complex, thus giving information for the first time on the relative rates of coupling from carbide, carbyne, and carbene species in the same molecular platform.

Synthesis of the Sterically Shielded Rhodium(I) Arene Complex by Cycloaddition of the Phosphorous‐Substituted Alkyne

“…Biaryl phosphine ligands (Buchwald‐type ligands) are widely used in palladium and gold chemistry, due to stabilizing effect of the metal‐arene π‐interaction. Similar ligands with two phosphorous atoms have also attracted some attention because such chelating systems allow for a variable metal‐arene coordination while maintaining the stability of the complex through the strong M−P bonds [1–4] . To the best of our knowledge, almost all of these ligands have been synthesized by reactions of the corresponding lithium or magnesium aromatic derivatives with R 2 PCl.…”

“…The dissolution in CDCl 3 results in slow decomposition of 1 and therefore the spectra were recorded in CD 2 Cl 2 solution. 1 H NMR (400 MHz, CD 2 Cl 2 ): δ = 7.65 (dd, J = 7.6, 3.6 Hz, 1H), 7.52-7.22 (m, 17H), 6.92 (dd, J = 7.7, 3.7 Hz, 1H). 31 P NMR (162 MHz, CD 2 Cl 2 ): δ = À 8.88.…”

“…A single crystal suitable for X-ray diffraction was obtained by cooling the solution of the complex in a mixture of CH 2 Cl 2 and Et 2 O (2 : 1) at À 15 °C for several days. 1 H NMR (400 MHz, CDCl 3 ): δ = 7.72-7.63 (m, 4H), 7.43-6.66 (m, 17H), 6.61 (dd, J = 7.7, 3.1 Hz, 1H), 6.43 (t, J = 7.6 Hz, 1H), 6.39-6.32 (m, 1H), 5.73-5.67 (m, 1H), 5.63 (d, J = 7.6 Hz, 1H). 31 P NMR (162 MHz, CDCl 3 ): δ = 38.73 (ddd, J = 380.0, 115.5, 24.0 Hz), 25.51 (ddd, J = 380.0, 110.5, 28.8 Hz), 18.33 (ddd, J = 92.7, 28.8, 24.0 Hz).…”

“…Similar ligands with two phosphorous atoms have also attracted some attention because such chelating systems allow for a variable metal-arene coordination while maintaining the stability of the complex through the strong MÀ P bonds. [1][2][3][4] To the best of our knowledge, almost all of these ligands have been synthesized by reactions of the corresponding lithium or magnesium aromatic derivatives with R 2 PCl. However, long time ago, Dr. Winter has reported an intriguing alternative approach based on the metal-promoted 2 + 2 + 2-cycloaddition of the phosphoroussubstituted alkyne 1 (Scheme 1).…”

Transition Metal Carbide Complexes

“…the protonation-hydride uptake sequence in Figure ). However, this reaction suffers from low selectivity and release of the diphosphine ligand. In searching for a Lewis-acid that would render the reaction selective toward protonation of the terminal carbide ligand, it was found that [(2,2′′- i Pr 2 P-terph)­(OC)­MoC] reacts with BPh 3 to generate a spectroscopically observable carbide-borane adduct, [(2,2′′- i Pr 2 P-terph)­(OC)­MoC–BPh 3 ].…”