Metallaborane Reaction Chemistry. <i>nido</i>-Dirhodapentaborane Isomer Structures and Stabilities and Utilization of Dirhodaboranes as Catalysts for Alkyne Cyclotrimerization

The reaction of (Cp*Rh)2B2H6 (1) with BH3THF under mild conditions leads to the intermediate (Cp*Rh)2B3H7, which is shown to exist in the two isomeric forms 1,2-(Cp*Rh)2B3H7 (2a) 1,2-(Cp*Rh)2(μ-H)B3H6 (2b) by a solid-state structure of 2a and low-temperature NMR data of the mixture. 1 slowly decomposes at room temperature to yield a number of products including pileo-(Cp*Rh)3B4H4 (4), which has been characterized by a solid-state structure determination. 2 converts on heating to 2,3-(Cp*Rh)2B3H7 (3), of known structure. 1−3 catalyze the cyclotrimerization of both terminal and internal alkynes with differing activities. The rate of catalysis is inhibited by PPh3 and pyridine and competition experiments show evidence for a multistep mechanism with concentration-dependent selectivity. The pathway proposed suggests the borane fragment acts in an ancillary role by providing a cluster environment for the dimetal fragment. Metal polyhedral edge opening on alkyne (or base) addition is proposed as the novel activation step thereby making rhodaboranes hybrid cluster catalysts.