The ruthenium-catalyzed homogeneous hydrogenation of carbonyl groups has established itself as a mature synthetic method on an academic and industrial scale [1] for the selective generation of stereogenic centers as an alternative to highenergy, pyrophoric hydride reagents. The development of such catalytic systems has gone hand in hand with that of chiral bidentate diphosphine ligands, [2] which, however, are often tedious and expensive to prepare because their multistep synthesis requires air-and moisture-free conditions. In contrast, phosphine-free chiral catalysts that operate under the industrially preferred and atom-economical hydrogenation with H 2 (HY) are still rare, [3][4][5] the most successful examples being h 5 -cyclopentadienyl [3] and h 6 -arene [4] diamine Ru II complexes. Most of these catalysts, as with those operating under transfer hydrogenation (TRHY) conditions, [6] fail to match the efficiency and selectivity requirements of industrial application. Following the work carried out at Firmenich [7] on the HY of ketones with [RuCl 2 (PNNP)] catalysts, [8] where PNNP is a chiral tetradentate ligand with a P 2 N 2 donor, [9] we developed a family of ligands in which the phosphines are replaced by thioethers (SNNS). [10] Such chiral ligands are cheap, air-and moisture-stable, and easy to prepare. This is a preliminary report of ruthenium/SNNS complexes that catalyze the asymmetric HY of ketones and aldehydes with good chemo-and enantioselectivity. [10][11][12] Ligands 1 a-f (Scheme 1) were conveniently obtained in two quantitative steps without exclusion of air by nucleophilic aromatic substitution of 2-nitro-or 2-bromobenzyl aldehydes with the appropriate thiol, followed by condensation with the chiral 1,2-cyclohexanediamine. Ligand 1 g was obtained by NaBH 4 reduction of 1 a in quantitative yield. The ruthenium complexes [RuCl 2 (SNNS)] (2 a-e,g) were prepared from the reaction of [RuCl 2 (PPh 3 ) 3 ] with the appropriate tetradentate ligand (SNNS = 1 a-e,g) and were fully characterized, whereas ligand 1 f was used in situ (see below), as its complexation failed with a number of precursors. The dichloro complexes 2 a-e and 2 g are stable for several hours in CHCl 3 , toluene, and alcohol solutions when exposed to air.The crystal structure of (R,R)-[RuCl 2 (1 a)] (R,R)-(2 a) shows a weakly distorted octahedral coordination sphere around ruthenium with a trans Cl-Ru-Cl unit, and the R configuration at both Ru-thioether moieties (Figure 1).The "stepped" conformation of the tetradentate SNNS ligand is reminiscent of the achiral diimino complex trans-[RuCl 2 -(SNNS)] (SNNS = N,N'-bis(2-tert-butylthiobenzylidene)-1,3propanediamine) [11a] and of other chiral PNNP [8,13] and salen [14] analogues.

The preparation and coordination chemistry of 5,17-bis(3-methyl-1-imidazol-2-ylidene)-25,26,27,28-tetrapropoxycalix[4]arene (1) is described. Starting from the bis(imidazolium) pro-ligand 1·2HI, the free carbene 1 was readily generated in solution through deprotonation using K[O(t)Bu] and its reactivity with rhodium(i) dimers [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) and [Rh(CO)2Cl]2 investigated. Dinuclear complexes were isolated in both cases, where the calix[4]arene-based NHC ligand adopts a bridging μ(2)-coordination mode, and in one case characterised in the solid-state by X-ray diffraction. Using instead an isolated and well-defined (mononuclear) silver transfer agent, generated by reaction of 1·2HI with Ag2O in the presence of a halide extractor, reactions with [Rh(COD)Cl]2 and [Rh(CO)2Cl]2 produced cationic dinuclear complexes bearing μ(2)-1 and μ(2)-Cl bridging ligands. The structural formulation of the novel dinuclear adducts of 1 was aided through spectroscopic congruence with model complexes, containing monodentate 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (I(i)Pr2Me2).

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh···K = 3.715(1) Å; Ir···K = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.

The ruthenium-catalyzed homogeneous hydrogenation of carbonyl groups has established itself as a mature synthetic method on an academic and industrial scale [1] for the selective generation of stereogenic centers as an alternative to highenergy, pyrophoric hydride reagents. The development of such catalytic systems has gone hand in hand with that of chiral bidentate diphosphine ligands, [2] which, however, are often tedious and expensive to prepare because their multistep synthesis requires air-and moisture-free conditions. In contrast, phosphine-free chiral catalysts that operate under the industrially preferred and atom-economical hydrogenation with H 2 (HY) are still rare, [3][4][5] the most successful examples being h 5 -cyclopentadienyl [3] and h 6 -arene [4] diamine Ru II complexes. Most of these catalysts, as with those operating under transfer hydrogenation (TRHY) conditions, [6] fail to match the efficiency and selectivity requirements of industrial application. Following the work carried out at Firmenich [7] on the HY of ketones with [RuCl 2 (PNNP)] catalysts, [8] where PNNP is a chiral tetradentate ligand with a P 2 N 2 donor, [9] we developed a family of ligands in which the phosphines are replaced by thioethers (SNNS). [10] Such chiral ligands are cheap, air-and moisture-stable, and easy to prepare. This is a preliminary report of ruthenium/SNNS complexes that catalyze the asymmetric HY of ketones and aldehydes with good chemo-and enantioselectivity. [10][11][12] Ligands 1 a-f (Scheme 1) were conveniently obtained in two quantitative steps without exclusion of air by nucleophilic aromatic substitution of 2-nitro-or 2-bromobenzyl aldehydes with the appropriate thiol, followed by condensation with the chiral 1,2-cyclohexanediamine. Ligand 1 g was obtained by NaBH 4 reduction of 1 a in quantitative yield. The ruthenium complexes [RuCl 2 (SNNS)] (2 a-e,g) were prepared from the reaction of [RuCl 2 (PPh 3 ) 3 ] with the appropriate tetradentate ligand (SNNS = 1 a-e,g) and were fully characterized, whereas ligand 1 f was used in situ (see below), as its complexation failed with a number of precursors. The dichloro complexes 2 a-e and 2 g are stable for several hours in CHCl 3 , toluene, and alcohol solutions when exposed to air.The crystal structure of (R,R)-[RuCl 2 (1 a)] (R,R)-(2 a) shows a weakly distorted octahedral coordination sphere around ruthenium with a trans Cl-Ru-Cl unit, and the R configuration at both Ru-thioether moieties (Figure 1).The "stepped" conformation of the tetradentate SNNS ligand is reminiscent of the achiral diimino complex trans-[RuCl 2 -(SNNS)] (SNNS = N,N'-bis(2-tert-butylthiobenzylidene)-1,3propanediamine) [11a] and of other chiral PNNP [8,13] and salen [14] analogues.