FULL PAPER Cationic Iridium Complexes with Chiral Dithioether Ligands: Synthesis, Characterisation and Reactivity under Hydrogenation Conditions Leticia Flores-Santos, [a] Erika Martin,* [a] Ali Aghmiz, [b] Montserrat Diéguez, [b] Carmen Claver, [b] Anna M. Masdeu-Bultó,* [b] and Miguel Ángel Muñoz-Hernández [c] Keywords: Chiral auxiliaries / S ligands / Iridium / Hydrides / Asymmetric catalysis A series of cationic Ir I complexes containing chiral dithio- ether ligands have been prepared in order to study the in- fluence of the sulfur substituents and the metallacycle size on the acetamidoacrylate hydrogenation reaction. In the case of complexes 6, 7 and 10, a mixture of diastereomers is ob- served in solution due to the sulfur inversion processes. In contrast, this fluxional behaviour is efficiently controlled by using bicyclic ligands which inhibit the S-inversion in com- plexes 8 and 9. The solid-state structure of complex 10b shows only one diastereomer with the sulfur substituents in a relative anti disposition and in an overall configuration of S C S C S S S S at the coordinated dithioether ligand. Iridium com- Introduction The study of chiral sulfur transition-metal complexes as asymmetric catalysts has notably increased during the last decades. [1,2] In particular, complexes containing thioether mixed-donor ligands such as the P,S-, [3–8] N,S- [9–17] and O,S- [18–22] donors have been successfully applied in asym- metric hydrogenation, [5,6,23] allylic substitutions [7–10,18] and enantioselective hydrosilylation of ketones. [4,23] In contrast, catalysts with chiral bidentate thioether ligands have been scarcely investigated, [24–32] although some have shown promising catalytic properties. In fact, we have recently shown that metal complexes with chiral dithioether ligands afford up to 81% ee in palladium-catalysed asymmetric al- lylic substitutions [24] and 68 % ee in iridium-catalysed asym- metric hydrogenation reactions. [25,26] Upon coordination to a metal, thioether ligands generate a stereogenic centre at the sulfur atom, whose close proxim- [a] Departamento de Química Inorgánica, Facultad de Química, Universidad Nacional Autónoma de México, Cd. Universitaria, 04510 México DF, México Fax: +52-55-5622-3720 E-mail: erikam@servidor.unam.mx [b] Departament de Química Física i Inorgànica, Universitat Ro- vira i Virgili, c/Marcel·lí Domingo s/n, 43005 Tarragona, Spain Fax: +34 977-559563 E-mail: masdeu@quimica.urv.es [c] Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001 Col. Chamilpa, Cuernavaca, México Fax: +52-77-7329-7998 E-mail: mamund2@ciq.uaem.mx Eur. J. Inorg. Chem. 2005, 2315–2323 DOI: 10.1002/ejic.200400828 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2315 plexes containing seven- and six-membered metallacycles (6b–d, 7b,c, 10a,b) react with the substrate through S-ligand substitution, and the rate of this substitution is related to the position of the fluorine atom on the aromatic ring. On the contrary, complexes containing a bismetallacycle (8 and 9) are not displaced by the substrate. The catalytic hydrogena- tion activity of complexes 8 and 9 is analysed in terms of the high stability of the corresponding dihydride complexes (13 and 14). In both cases, only two of the four possible dia- stereomeric dihydride species are formed in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) ity to the metal coordination sphere may be advantageous for enantioinduction; however, these centres are not config- urationally stable, since sulfur inversion has a low energy barrier. In the case of chiral dithioether complexes, the pres- ence of two stereogenic centres at sulfur atoms, in addition to the carbon stereogenic centres at the ligand backbone, can lead to mixtures of diastereomers. The presence of mix- tures of diasteromeric species and the difficulty to control their interconversion in solution have been regarded as a problem for asymmetric induction in catalytic reactions. In spite of this behaviour, enantioinductions have been im- proved when the configuration at the sulfur atom is con- trolled by steric effects. [24,31,32] Unfortunately, having few examples of S,S-homodonor ligands, which also differ con- siderably in their skeleton size and rigidity, makes it difficult to elucidate the structure-performance relationship. In order to understand the influence of structural and electronic factors on the reactivity of chiral dithioether-con- taining catalysts, in the present work we prepared a series of cationic iridium complexes with chiral dithioether li- gands 1–5 (Figure 1) derived from -(+)-diethyltartrate [33] and (2R,4R)-2,4-pentanediol. [34] These ligands were selected in order to study electronic and ring size effects and sulfur inversion control, by the following strategies: (i) introduc- tion of fluorine atoms at ortho- meta- or para- positions of a thioether aromatic ring in order to change the donor/ acceptor electronic properties of sulfur atoms (ligands 1a–c and 2a–c); (ii) evaluation of two ring sizes to tune the rigid- ity of the molecular backbone (ligands 1a–d and 2a–c vs.