Coordination Chemistry and Diphenylacetylene Hydrogenation Catalysis of Planar Chiral Ferrocenylphosphane‐Thioether Ligands with Cyclooctadieneiridium(I)

The reaction of the P,S ligands CpFe{1,2-C5H3(PPh2)(CH2SR)},(1-R; R = Et, Ph, tBu), with 0.5 equivalents of [Ir(COD)Cl]2 (metal/ligand = 1:1) furnishes the stable mononuclear adducts [Ir(COD)Cl(1-R)], 2-R, in high yields. The molecular structures of the three complexes 2-R have been determined by single-crystal X-ray diffraction. Compounds 2-Et and 2-Ph have similar five-coordinate molecular structures, with an intermediate coordination geometry between a distorted trigonal-bipyramid and a distorted square-pyramid. In the solid state, the sulfur atom chirality (R group exo, lone pair endo and parallel to the Cp-Fe axis) as well as the iridium chirality (Cl atom exo relative to the ferrocene group) is controlled by the ligand planar chirality. In solution, only one diastereoisomer is observed in both cases, indicating that the geometries observed in the solid state are maintained in solution, or that the different species are in rapid exchange on the NMR time scale. Complex 2-tBu, on the other hand, shows a four-coordinate square-planar molecular structure with a dangling thioether moiety, probably for steric reasons. For metal/ligandratios greater than 1, the compounds [Ir(COD){κ2-P:S-(1-R)}]+[Ir(COD)Cl2]– (3-R) are obtained, resulting from the chloride abstraction by the excess iridium metal on complex 2-R. The structure of 3-Ph was confirmed by single-crystal X-ray diffraction. A rapid equilibration between 2-R and 3-R gives rise to only one set of NMR signals for each metal/ligand ratio. For the ligand 1-tBu, a similar equilibration between 2-tBu and 3-tBu is again observed, in addition to a slower equilibration with a third product, [Ir(COD)Cl](μ-1-R)[Ir(COD)Cl] (4-tBu), which is proposed to contain a bridging P,S ligand spanning two Ir centres. Complex 2-Ph reacts with [Rh(COD)Cl]2 to give only the heterometallic ionic pair [Ir(COD){κ2-P:S-(1-Ph)}][Rh(COD)Cl2] without transfer of ligand 1-Ph from Ir to Rh. Furthermore, the rhodium complex RhCl(COD)(1-Ph) reacts quantitatively with [Ir(COD)Cl]2 at room temp. to yield 2-Ph and [Rh(COD)Cl]2, thereby illustrating the much stronger affinity of ligands 1-Ph for iridium compared to rhodium. Finally, the complexes 2-tBu and 2-Ph were found to be efficient catalysts for the diphenylacetylene hydrogenation, affording a mixture of monohydrogenation [both (E)- and (Z)-stilbene] and dihydrogenation (1,2-diphenylethane) products. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)