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 pr...