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] 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 À1 . The complexes [Ir(NCCH 3 )(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(OiPr)(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.

Reversible H 2 cleave promoted by closo-to-nido transformations of [1,1-(PPh 3 ) 2 -3-(NC 5 H 5 )-closo-1,2-RhSB 9 H 8 ] (2) / [8,8,8-(PPh 3 ) 2 (H)-9-(NC 5 H 5 )-nido-8,7-RhSB 9 H 9 ] (1) is a cooperative action with application in catalysis; the treatment of 2 and [1,1-(PPh 3 )(CO)-3-(NC 5 H 5 )-closo-RhSB 9 H 8 ] (3) with either HCl or HOTf in CH 2 Cl 2 affords the 11-vertex nido-rhodathiaboranes [8,8-(PPh 3 )(Cl)-9-(NC 5 H 5 )nido-8,7-RhSB 9 H 9 ] (4) and [8,8,8-(PPh 3 )(CO)(Cl)-9-(NC 5 H 5 )-nido-8,7-RhSB 9 H 9 ] (5), respectively; in contrast the reaction of 1 with triflic acid yields the salt [8,8-(PPh 3 ) 2 (H)-9-(NC 5 H 5 )-nido-RhSB 9 H 10 ][OTf] (6). These results illustrate the bifunctional nature of the clusters and their nido-tocloso redox flexibility, which open new routes for the tuning of the reactivity of these polyhedral compounds and widen their potential applications. MAIN TEXTComplexes in which both the metal center and the ligands are involved in chemical transformations are attracting an increasing attention as new catalyst promoters. In this regard, ligand-assisted reactivity plays an important role in reactions that involve heterolytic dihydrogen activation and/or hydrogen transfer from alcohols. The most active complexes combine a transition metal center with reversible alcohol-ketone, 1-3 reversible amine-amide functionalities, [4][5][6] or with ligands that undergo facile reversible dearomatization. 7,8