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 = tBu, 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 CNtBu 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 = tBu), as a result of H-transfer from hydroxycarbyne to isocyanide ligands and subsequent C–C coupling between aminocarbyne and benzylidyne ligands.