Spontaneous Formation of an η⁴‑Ethylene Bis(carbene) Ligand by Alkyne Coupling at Rhenium(III)

The C/C coupling activity of the cationic Re­(III) species [(C5H5)­ReBr]+ with different heteroatom-substituted alkynes was investigated. By using the sulfide-substituted bis­(benzylsulfanyl)­acetylene an unprecedented rhenacyclopentatriene complex was obtained. An XRD structure determination of [(C5H5)­ReBr­(CSBn)4]+ uncovered the coordination of the remote ethylene moiety at Re leading to an η4-coordination mode of the C4(SBn)4 ligand and a folded five-membered metallacycle. In contrast, reaction of the monoalkyne complex [(C5H5)­ReBr­(CSBn)2]+ with diphenylacetylene as well as diiodoacetylene led to the mixed bis­(alkyne) complexes. The different behavior is attributed to kinetic reasons according to DFT calculations. XRD as well as IR studies with the cationic bis­(alkyne) complexes [(C5H5)­ReBr­(CSBn)2(C2Ph2)]+ and [(C5H5)­ReBr­(CSBn)2(C2I2)]+ revealed clear differences in the bond strengths of the coordinated alkynes at the same metal center. Further investigations on the coupling activity resulted in addition products with H2O and a trimerization of two diphenylacetylene molecules and one bis­(benzylsulfanyl)­acetylene, both as byproducts and involved in a sulfide bond cleavage.