Alkyne Metathesis Reactions of Rhenium(V) Carbyne Complexes

Although alkyne metathesis reactions mediated by high-valent d0 carbyne complexes are well established, similar reactions mediated by well-defined low-valent (non-d0) carbyne complexes are rare. This work demonstrates that d2 Re­(V) carbyne complexes Re­(CR)­Cl2(PMe2Ph)3 (R = CH2(o-C6H4Br), Ph, CO2Et) can undergo stoichiometric alkyne metathesis reactions. For example, reactions of Re­{CCH2(o-C6H4Br)}­Cl2(PMe2Ph)3 with TMSCCR (R = CO2Et, CH2Ph) and PhCCPh produced carbyne complexes Re­(CR)­Cl2(PMe2Ph)3 and Re­(CPh)­Cl2(PMe2Ph)3, respectively. Theoretical studies suggest that the metathesis reactions most likely proceed through six-coordinate alkyne-carbyne intermediates Re­(CR)­Cl2(η2-alkyne)­(PMe2Ph)2 that undergo reversible cycloaddition reactions.