exo-nido-Cyclooctadienerhodacarboranes: Synthesis, Reactivity, and Catalytic Properties in Alkene Hydrogenation

Reaction of [Rh(acac)(cod)] with 1 equivalent of HClO4 and subsequent treatment with tetramethylammonium or cesium salts of [7-SR-8-R‘-7,8-C2B9H10]- anions affords [Rh(7-SR-8-R‘-7,8-C2B9H10)(cod)] (1:  R, R‘ = Ph; 2:  R = Ph, R‘ = Me; 3:  R = Et, R‘ = Me). The structure of 1 has been determined by crystallographic studies. The Rh(I) has a normal four-coordinated square planar geometry and the carborane ligand is bonded to the metal by means of a S−Rh and a B(11)−H−Rh bond. The cod ligand fulfills the coordination sphere of the metal with two olefin−Rh bonds. Variable-temperature 1H{11B} NMR experiments indicate that 1 undergoes B−H/B−H−Rh exchange coupled with rotation of the [Rh(cod)]+ moiety. This dynamic process appears to involve exclusively the B(11)−H group. The presence of two species that exchange at room temperature is detected for complexes 2 and 3. A structure analogous to 1 is suggested for the major species, whereas the minor species appears to involve a [Rh(cod)]+ fragment bonded to the cage...