Air-Stable NNS (ENENES) Ligands and Their Well-Defined Ruthenium and Iridium Complexes for Molecular Catalysis

We introduce ENENES, a new family of air-stable and low-cost NNS ligands bearing NH functionalities of the general formula E(CH2)(m)NH(CH2)(n)SR, where E is selected from -NC4H8O, -NC4H8, or -N(CH3)(2), m and n = 2 and/or 3, and R = Ph, Bn, Me, or SR (part of a thiophenyl fragment). The preparation and characterization of more than 15 examples of well-defined Ru and Ir complexes supported by these ligands that are relevant to bifunctional metalligand M/NH molecular catalysis are reported. Reactions of NNS ligands with suitable Ru or Ir precursors afford rich and diverse solid-state and solution chemistries, producing monometallic molecules as well as bimetallics in which the ligand coordinates to the metal via either bidentate (kappa(2)[N,N'] or kappa(2)[N',S]) or tridentate (kappa(3)[N,N',S]) binding modes, depending on the basicity of the sulfur atom, CH2 chain length (m or n parameter), or identity of the transition metal. In the case of Ir, ligands bearing benzyl substituents lead to unprecedented kappa(4)[N,N',S,C]-tetradentate core-structure complexes of the type [(IrHCl)-H-III{kappa(4)(N,N',S,C)ligand}], resulting from ortho-metalation via CH oxidative addition. Fourteen of these Ru and Ir complexes have been crystallographically characterized. Air- and moisture-stable complexes of the type trans-[(RuCl2)-Cl-II{kappa(3)[N,N',S]ligand}(L)] (L = PPh3, PCy3, DMSO), and others, effect the selective hydrogenation of methyl trifluoroacetate into the important synthon trifluoroacetaldehyde methyl hemiacetal in basic methanol under relatively mild conditions (3540 degrees C, 25 bar H-2) with reasonable turnover numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes [(IrHCl)-H-III{kappa(4)(N,N',S,C)ligand}] exhibit excellent catalytic activities and high chemoselectivity for the same reaction, reaching turnover numbers of >10 000.