Modification of [8,8,8-(H)(PPh3)2-9-(Py)- Nido -8,7-RhSB9H9], Py = NC5H5, with monodentate phosphines: Reactivity and mechanistic insights

Article abstract of DOI:10.1021/om2009205

The [8,8,8-(H)(PPh₃)₂-9-(Py)-nido-8,7-RhSB ₉H₉] (2)/[1,1-(PPh₃)₂-3-(Py)-closo- 1,2-RhSB₉H₈] (3) pair catalyzes the hydrogenation of olefins through nido-to-closo transformations. Substitution of the phosphine ligands can lead to an improvement of the catalytic activity of this system. Therefore, the substitutional chemistry of 2 with PMePh₂, PMe ₂Ph, and PMe₃ has been studied, leading to the formation of [8,8,8-(H)(PPh₃)(PR₃)-9-(Py)-nido-8,7-RhSB ₉H₉], where R₃ = Me₂Ph (5) or Me₃ (6), and [8,8,8-(H)(PR₃)₂-9-(Py)-nido-8,7- RhSB₉H₉], where R₃ = MePh₂ (4) or Me₂Ph (7). Kinetic studies on the reaction of PMe₂Ph with 2 indicate that the substitutions follow a dissociative mechanism. The thermal dehydrogenation of 5-7 affords the corresponding closo-derivatives [1,1-(L)(PPh₃)-3-(Py)-closo-1,2-RhSB₉H₈], where L = PMe₂Ph (9) or PMe₃ (10), and [1,1-(L) ₂-3-(Py)-closo-1,2-RhSB₉H₈], where L = PMe ₂Ph (11) or PMe₃ (12). The substitution of PPh₃ by the more basic, less bulky phosphines facilitates hydrogen loss and consequent nido-to-closo transformations. The reaction of 5 and 6 with C ₂H₄ promotes a nido-to-closo cluster change and the consequent formation of 10 and 11 together with small amounts of C ₂H₄-ligated [1,1-(η²-C₂H ₄)(L)-3-(Py)-closo-1,2-RhSB₉H₈], where L = PPh₃ (13) or PMe₃ (15), characterized in situ by ¹H NMR spectroscopy. In the reactions with ethylene, ethane is detected in situ, indicating that the olefin is hydrogenated. The reactions of 5 and 6 with CO afford the CO-ligated [1,1-(CO)(L)-3-(Py)-closo-1,2-RhSB ₉H₈], where L = PMe₂Ph (16) or PMe₃ (17). The reactivity of the new PR₃-ligated nido-clusters versus H₂, C₂H₄, and CO is not improved with the phosphines studied in this work; however, the changes found in the chemical beahavior of this system are dramatic, confirming the tailorability of these 11-vertex rhodathiaboranes and the potential optimization of their catalytic activity by the judicious choice of the exo-polyhedral ligands.