<i>exo-nido</i>-Cyclooctadienerhodacarboranes: Synthesis, Reactivity, and Catalytic Properties in Alkene Hydrogenation

Abstract

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 by a single S−Rh bond. Complexes 1−3 rearrange in solution to give [closo-3-(η3-C8H13)-1-SR-2-R‘-3,1,2-RhC2B9H9] (4: R, R‘ = Ph; 5: R = Ph, R‘ = Me; 6: R = Et, R‘ = Me). The crystal structure of 4 reveals lack of bonding between C(1)−C(2) and the cluster adopts a pseudocloso geometry. The weighted average 11B{1H} NMR chemical shift of 5 and 6 suggests that these clusters have a normal closo structure. Kinetic studies of the reactions 2 → 5 and 3 → 6 showed a common first-order rate constant for the two reactions. Complexes 2 and 3 in the presence of 1 equivalent of PPh3 present similar activity in the hydrogenation of cyclohexene to the parent diphosphine complexes [Rh(7-SR-8-R‘-7,8-C2B9H10)(PPh3)2]. The closo-cyclooctenyl complexes 4 and 5 exhibit higher activity than the parent exo-nido isomers in the hydrogenation of cyclohexene.