Proton-assisted hydrogen activation on polyhedral cations.

The treatment of [1,1-(PR3 )2 -3-(Py)-closo-1,2-RhSB9 H8 ] (PR3 =PMe3 (2) or PPh3 and PMe3 (3); Py=pyridine) with triflic acid (TfOH) affords [1,3-μ-(H)-1,1-(PR3 )2 -3-(Py)-1,2-RhSB9 H8 ](+) (PR3 =PMe3 (4) or PMe3 and PPh3 (5)). These products result from the protonation of the 11-vertex closo-cages along the Rh(1)B(3) edge. These unusual cationic rhodathiaboranes are stable in solution and in the solid state and they have been fully characterized by multinuclear NMR spectroscopy. In addition, compound 5 was characterized by single-crystal X-ray diffraction. One remarkable feature in these structures is the presence of three {Rh(PPh3 )(PMe3 )}-to-{η(n) -SB9 H8 (Py)} (n=4 or 5) conformers in the unit cell, thus giving an uncommon case of conformational isomerism. [1,1-(PPh3 )2 -3-(Py)-closo-1,2-RhSB9 H8 ] (1), that is, the bis-PPh3 -ligated analogue of compounds 2 and 3, is also protonated by TfOH, but, in marked contrast, the resulting cation, [1,3-μ-(H)-1,1-(PPh3 )2 -3-(Py)-1,2-RhSB9 H8 ](+) (6), is attacked by a triflate anion with the release of a PPh3 ligand and the formation of [8,8-(OTf)(PPh3 )-9-(Py)-nido-8,7-RhSB9 H9 ] (9). The result is an equilibrium that involves cationic species 6, neutral OTf-ligated compound 9, and [HPPh3 ](+) , which is formed upon protonation of the released PPh3 ligand. The resulting ionic system reacts readily with H2 to give cationic species [8,8,8-(H)(PPh3 )2 -9-(Py)-nido-8,7-RhSB9 H9 ](+) (7). This reactivity is markedly higher than that previously found for compound 1 and it introduces a new example of proton-assisted H2 activation that occurs on a polyhedral boron-containing compound.