Carbyne–Carbyne Coupling and H‑Shifts in Reactions of the Unsaturated Methoxy- and Hydroxycarbyne Complexes [Mo₂Cp₂(μ-COR)(μ-CPh)(μ-PCy₂)]⁺ with CO and Isocyanides

The unsaturated methoxycarbyne complex [Mo₂Cp₂(μ-COMe)­(μ-CPh)­(μ-PCy₂)]­(CF₃SO₃) (Cp = η⁵-C₅H₅; Mo–Mo = 2.4707(3) Å) reacted with CO (293 K, 40 bar) or CNR (233 K, R = ^<i>t</i>Bu, Xyl) to give the corresponding methoxyalkyne-bridged derivatives [Mo₂Cp₂{μ-η²:η²-C­(OMe)­CPh}­(μ-PCy₂)­L₂]­(CF₃SO₃) following from a reductive C–C coupling between methoxycarbyne and benzylidyne ligands (L = CO, CNR). This coupling could be fully reversed for the dicarbonyl product upon photolysis in tetrahydrofuran solution. The related hydroxycarbyne complex [Mo₂Cp₂(μ-COH)­(μ-CPh)­(μ-PCy₂)]­BF₄ reacted analogously with CO (293 K, 4 bar) to give the hydroxyalkyne-bridged derivative [Mo₂Cp₂{μ-η²:η²-C­(OH)­CPh}­(μ-PCy₂)­(CO)₂]­BF₄ (Mo–Mo = 2.6572(5) Å) as a result of C–C coupling between hydroxycarbyne and benzylidyne ligands, but this process could not be reversed photochemically. The latter complex could be prepared more efficiently via protonation of the ketenyl precursor [Mo₂Cp₂{μ-C­(Ph)­CO}­(μ-PCy₂)­(CO)₂] with HBF₄·OEt₂ in dichloromethane solution. The hydroxycarbyne complex also reacted with CN^<i>t</i>Bu and CNXyl to give C–C coupled products, but different than anticipated: in both cases this reaction yielded selectively the corresponding aminoalkyne-bridged derivatives [Mo₂Cp₂{μ-η²:η²-C­(NHR)­CPh}­(μ-PCy₂)­(CNR)₂]­BF₄ (Mo–Mo = 2.6525(5) Å when R = ^<i>t</i>Bu), as a result of H-transfer from hydroxycarbyne to isocyanide ligands and subsequent C–C coupling between aminocarbyne and benzylidyne ligands