Reactivity of CpRh 16e Half-Sandwich Complexes Containing a Chelating 1,2-Dicarba-closo-dodecaborane-1,2-dichalcogenolato Ligand. Intermediates in the Catalyzed Trimerization of Methyl Acetylene Carboxylate and Phenylacetylene

Reactions of the 16-electron (16e) half-sandwich complexes Cp*Rh[E2C2(B10H10)] [E = S (1S), Se (1Se)] with methyl acetylene carboxylate and phenylacetylene were studied in order to obtain information on both the primary addition products and the potential catalyzed oligomerization of the alkynes as a function of reaction conditions. Both rhodium complexes 1S and 1Se reacted with HC⋮C−CO2Me under mild conditions to give in low yield the dimers 2S and 2Se, of which 2Se could be characterized by X-ray structural analysis. Both 1S and 1Se act as catalysts for the cyclotrimerization of HC⋮C−CO2Me and HC⋮C−Ph to give the respective 1,3,5- and 1,2,4-trisubstituted benzenes in a 1:1 ratio, if the reaction is carried out at 70 °C in toluene. In boiling chloroform or at room temperature in dichloromethane, trimerization is of minor importance. In the case of 1Se and methyl acetylene carboxylate, an intermediate 3Se, formed in the course of the trimerization process, could be characterized by X-ray structural analysis. The dimer 2Se catalyzes trimerization in the same way as 1Se. In the absence of HC⋮C−CO2Me, 2Se rearranges upon heating in toluene at 70 °C completely into the known carborane-substituted complex 8Se. One of the final products of the reaction of 1S with HC⋮C−CO2Me, the 16e complex 4S, also catalyzes the trimerization of HC⋮CCO2Me.