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.本研究系统探究了阳离子型铼(III)配合物[(C₅H₅)ReBr]+与不同杂原子取代炔烃的碳-碳偶联活性。以硫取代的双(苄硫基)乙炔为底物时，我们得到了前所未有的铼杂环戊三烯配合物。对配合物[(C₅H₅)ReBr(CSBn)₄]+开展X射线衍射（X-ray Diffraction, XRD）结构解析后发现，远端乙烯片段与铼中心配位，使得C₄(SBn)₄配体以η⁴配位模式结合，同时形成了折叠型五元金属杂环。与之相反，单炔烃配合物[(C₅H₅)ReBr(CSBn)₂]+分别与二苯基乙炔、二碘乙炔反应，均可得到混合双炔烃配合物。基于密度泛函理论（Density Functional Theory, DFT）计算结果，上述反应行为差异可归因于动力学因素。对阳离子双炔烃配合物[(C₅H₅)ReBr(CSBn)₂(C₂Ph₂)]+与[(C₅H₅)ReBr(CSBn)₂(C₂I₂)]+开展的X射线衍射与红外光谱（Infrared Spectroscopy, IR）研究表明，同一金属中心上配位的炔烃键强存在显著差异。针对偶联活性的进一步研究还得到了两类产物：一类是与水的加成产物，另一类是两分子二苯基乙炔与一分子双(苄硫基)乙炔的三聚产物；二者均为副产物，且反应过程中伴随硫键的断裂。