Reactions of alkynes with metallaboranes: Novel reactivities lead to new structure types
The reactions of alkynes with nido-dimetallaboranes containing monocyclopentadienyl metal fragments are highly dependent on metal identity. Typical metals from groups 6 and 7 do not react under mild conditions, Rh gives exclusively alkyne cyclotrimerization via a cluster-catalyzed reaction, and Ru reacts predominantly via insertion to give new ruthenacarborane clusters. The facile reaction observed for Ru permits the characterization of intermediates exhibiting novel structural features, e.g., external cluster-bridging monoborane or alkylidene fragments. Despite the large differences in reactivity, circumstantial evidence suggests the alkyne adds to and opens the metallaborane to an arachno-structure. The metal present determines the character of the metallaborane, which in turn determines whether further reaction occurs. For Ru, it is the hydrogen-rich character fostered by the metal fragment that leads to the copious chemistry observed.