María José Artigas

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.

Reactions of 11-vertex rhodathiaboranes with HCl: synthesis and reactivity of new Cl-ligated clusters.

Reactions of [8,8,8-(H)(PPh(3))(2)-9-(Py)-nido-8,7-RhSB(9)H(9)] (1), [1,1-(PPh(3))(2)-3-(Py)-closo-1,2-RhSB(9)H(8)] (2), and [1,1-(CO)(PPh(3))-3-(Py)-closo-1,2-RhSB(9)H(8)] (4), where Py = Pyridine, with HCl to give the Cl-ligated clusters, [8,8-(Cl)(PPh(3))-9-(Py)-nido-8,7-RhSB(9)H(9)] (3) and [8,8,8-(Cl)(CO)(PPh(3))-9-(Py)-nido-8,7-RhSB(9)H(8)] (5), have recently demonstrated the remarkable nido-to-closo redox flexibility and bifunctional character of this class of 11-vertex rhodathiaboranes. To get a sense of the scope of this chemistry, we report here the reactions of PR(3)-ligated analogues, [8,8,8-(H)(PR(3))(2)-9-(Py)-nido-8,7-RhSB(9)H(9)], where PR(3) = PMePh(2) (6), or PPh(3) and PMe(3) (7); and [1,1-(PR(3))(2)-3-(Py)-closo-1,2-RhSB(9)H(8)], where PR(3) = PPh(3) and PMe(3) (8), PMe(3) (9) or PMe(2)Ph (10), with HCl to give Cl-ligated clusters. The results demonstrate that in contrast to the PPh(3)-ligated compounds, 1, 2, and 3, the reactions with 6-10 are less selective, giving rise to the formation of mixtures that contain monophosphine species, [8,8-(Cl)(PR(3))-9-(Py)-nido-8,7-RhSB(9)H(9)], where PR(3) = PMe(3) (11), PMe(2)Ph (12), or PMePh(2) (15), and bis-ligated derivatives, [8,8,8-(Cl)(PR(3))(2)-9-(Py)-nido-8,7-RhSB(9)H(9)], where PR(3) = PMe(3) (13) or PMe(2)Ph (14). The {RhCl(PR(3))}-containing compounds, 3, 11, 12, and 15, are formally unsaturated 12 skeletal electron pair (sep) clusters with nido-structures. Density functional theory (DFT) calculations demonstrate that the nido-structure is more stable than the predicted closo-isomers. In addition, studies have been carried out that involve the reactivity of 3 with Lewis bases. Thus, it is reported that 3 interacts with MeCN in solution, and it reacts with CO and pyridine to give the corresponding Rh-L adducts, [8,8,8-(Cl)(L)(PPh(3))-9-(Py)-nido-8,7-RhSB(9)H(9)], where L = CO (5) or Py (20). On the other hand, the treatment of 3 and 5 with Proton Sponge (PS) promotes the abstraction of HCl, as [PSH]Cl, from the nido-clusters, and the regeneration of the parent closo-species, completing two new stoichiometric cycles that are driven by Brønsted acid/base chemistry.

Mechanistic insight into the pyridine enhanced α-selectivity in alkyne hydrothiolation catalysed by quinolinolate–rhodium(I)–N-heterocyclic carbene complexes

RhI–NHC–olefin complexes bearing a N,O-quinolinolate bidentate ligand have been prepared from [Rh(μ-OH)(NHC)(η2-olefin)]2 precursors (olefin = cyclooctene, ethylene). The disposition of the chelate ligand with regard to the carbene in the square planar derivatives is strongly influenced by the steric hindrance exerted by the coordinated olefin. These complexes efficiently catalyzed the addition of thiophenol to phenylacetylene with good selectivity to α-vinyl sulfides, which can be increased up to 97% by addition of pyridine. Several key intermediates have been detected including the η1-alkenyl species resulting from alkyne insertion into a Rh–H bond. DFT calculations on the mechanism support a hydrometallation pathway that entails the oxidative addition of thiol, 2,1-insertion of the alkyne into the Rh–H bond, and reductive elimination as the rate-determining step. Remarkably, coordination of pyridine to the β-alkenyl intermediate but not to the α-alkenyl, which results in a net stabilization, is the key for the Markovnikov selectivity.