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

The reaction of (Cp*Rh) 2 B 2 H 6 ( 1 ) with BH 3 THF under mild conditions leads to the intermediate (Cp*Rh) 2 B 3 H 7, which is shown to exist in the two isomeric forms 1,2-(Cp*Rh) 2 B 3 H 7 ( 2a ) 1,2-(Cp*Rh) 2 (μ-H)B 3 H 6 ( 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) 3 B 4 H 4 ( 4 ), which has been characterized by a solid-state structure determination. 2 converts on heating to 2,3-(Cp*Rh) 2 B 3 H 7 ( 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 PPh 3 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.