Brønsted Acid/Base Driven Chemistry with Rhodathiaboranes: A Labile {SB₉H₉}–Thiadecaborane Fragment System

Reversible H₂ cleavage promoted by closo to nido transformations of [1,1-(PPh₃)₂-3-(NC₅H₅)-<i>closo</i>-1,2-RhSB₉H₈] (<b>2</b>)/[8,8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>1</b>) is a cooperative action with application in catalysis; the treatment of <b>2</b> and [1,1-(PPh₃)(CO)-3-(NC₅H₅)-<i>closo</i>-RhSB₉H₈] (<b>3</b>) with either HCl or HOTf in CH₂Cl₂ affords the 11-vertex <i>nido</i>-rhodathiaboranes [8,8-(PPh₃)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>4</b>) and [8,8,8-(PPh₃)(CO)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>5</b>), respectively. In contrast, the reaction of <b>1</b> with triflic acid yields the salt [8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-RhSB₉H₁₀][OTf] (<b>6</b>). These results illustrate the bifunctional nature of the clusters and their nido to closo redox flexibility, which open new routes for the tuning of the reactivity of these polyhedral compounds and widen their potential applications

Brønsted Acid/Base Driven Chemistry with Rhodathiaboranes: A Labile {SB₉H₉}–Thiadecaborane Fragment System

Reversible H₂ cleavage promoted by closo to nido transformations of [1,1-(PPh₃)₂-3-(NC₅H₅)-<i>closo</i>-1,2-RhSB₉H₈] (<b>2</b>)/[8,8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>1</b>) is a cooperative action with application in catalysis; the treatment of <b>2</b> and [1,1-(PPh₃)(CO)-3-(NC₅H₅)-<i>closo</i>-RhSB₉H₈] (<b>3</b>) with either HCl or HOTf in CH₂Cl₂ affords the 11-vertex <i>nido</i>-rhodathiaboranes [8,8-(PPh₃)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>4</b>) and [8,8,8-(PPh₃)(CO)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>5</b>), respectively. In contrast, the reaction of <b>1</b> with triflic acid yields the salt [8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-RhSB₉H₁₀][OTf] (<b>6</b>). These results illustrate the bifunctional nature of the clusters and their nido to closo redox flexibility, which open new routes for the tuning of the reactivity of these polyhedral compounds and widen their potential applications

Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient and Versatile Transfer Hydrogenation Catalysts

A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]⁺ (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH₃)(cod)(MeImR)]⁺ (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h^–1. The complexes [Ir(NCCH₃)(cod)(MeImR)]⁺ (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(O<i>i</i>Pr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity