Bis(alkynyl)borane: A New Class of Acyclic Boron-Containing π Ligands in η5-Coordination Mode.

Reactions between Rosenthal's zirconocene synthon, [Cp2Zr(py)Me3SiC≡CSiMe3] (py = pyridine), and two different types of bis(alkynyl) boranes, (Me3Si)2NB(C≡CSiMe3)2 and MesB(C≡CSiMe3)2 (Mes = 2,4,6-trimethylphenyl), both resulting in the formation of η5-coordinated bis(alkynyl)borane zirconocene complexes. X-ray diffraction analysis revealed their six-membered cyclic structures featuring a boat conformation with considerable Zr-B interactions. The Zr-B bonding strength and conformational lability of the ZrC4B ring are dependent on the π basicity of the exocyclic B substituent. These compounds represent the first examples of η5-coordinated acyclic boron-containing π ligands.