Modification of [8,8,8-(H)(PPh3)2-9-(Py)-nido-8,7-RhSB9H9], Py = NC5H5, with Monodentate Phosphines: Reactivity and Mechanistic Insights

The [8,8,8-(H)­(PPh3)2-9-(Py)-nido-8,7-RhSB9H9] (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, PMe2Ph, and PMe3 has been studied, leading to the formation of [8,8,8-(H)­(PPh3)­(PR3)-9-(Py)-nido-8,7-RhSB9H9], 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 = PMe2Ph (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 C2H4 promotes a nido-to-closo cluster change and the consequent formation of 10 and 11 together with small amounts of C2H4-ligated [1,1-­(η2-C2H4)(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-RhSB9H8], 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.