[1,1-(η2-dppe)-3-(NC5H5)-closo-1,2-RhSB9H8]: conformational lability and reactivity with H2 upon protonation

Abstract

Metallaheteroboranes are versatile compounds that can be conveniently modified and eventually tailored by ligand modification at either the metal centre or the boron vertices. Recently, we have discovered that protonation of some rhodathiaboranes affords cationic clusters with interesting reaction chemistry. In order to tune the reactivity of some of these polyhedral boron-based compounds, we have prepared air-stable orange [1,1-(η²-dppe)-3-(NC₅H₅)-closo-1,2-RhSB₉H₈] (2) by the treatment of the known hydridorhodathiaborane [8,8,8-(H)(PPh₃)₂-9-(NC₅H₅)-nido-8,7-RhSB₉H₉] (1) with dppe. The new 11-vertex rhodathiaborane, 2, reacts readily with triflic acid (TfOH) in CH₂Cl₂ to give orange cationic [8,8-(η²-dppe)-9-(NC₅H₅)-nido-8,7-RhSB₉H₉]⁺ (3). VT NMR experiments have allowed the characterization of a structural closo ↔ nido tautomerism, which involves hapticity changes in the ligation of the {SB₉H₉-(NC₅H₅)} moiety to the {Rh(dppe)} fragment, with the proton moving between the Rh(1)–B(3) and the B(9)–B(10) edges of the closo- and nido-isomers, respectively. The proton enhances the stereochemical non-rigidity and Lewis acidity of 3versus the neutral 2. This modification of the chemical and structural basis permits the efficient heterolytic splitting of the H–H bond, leading to the formation of new hydridorhodathiaborane isomers [8,8,8-(H)(η²-dppe)-μ-8,9-(H)-9-(NC₅H₅)-nido-8,7-RhSB₉H₁₀]⁺ (4) that are in equilibrium with the reactants, H₂ and 3.