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

Abstract

We introduce ENENES, a new family of air-stable and low-cost NNS ligands bearing NH functionalities of the general formula E­(CH₂)_<i>m</i>NH­(CH₂)_<i>n</i>SR, where E is selected from −NC₄H₈O, −NC₄H₈, or −N­(CH₃)₂, <i>m</i> and <i>n</i> = 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 metal–ligand 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 (κ²[<i>N,N</i>′] or κ²[<i>N</i>′<i>,S</i>]) or tridentate (κ³[<i>N,N</i>′<i>,S</i>]) binding modes, depending on the basicity of the sulfur atom, CH₂ chain length (<i>m</i> or <i>n</i> parameter), or identity of the transition metal. In the case of Ir, ligands bearing benzyl substituents lead to unprecedented κ⁴[<i>N,N</i>′<i>,S,C</i>]-tetradentate core-structure complexes of the type [Ir^IIIHCl­{κ⁴(<i>N,N</i>′<i>,S,C</i>)–ligand}], resulting from <i>ortho</i>-metalation via C–H oxidative addition. Fourteen of these Ru and Ir complexes have been crystallographically characterized. Air- and moisture-stable complexes of the type <i>trans</i>-[Ru^IICl₂{κ³[<i>N,N</i>′<i>,S</i>]–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 (35–40 °C, 25 bar H₂) with reasonable turnover numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes [Ir^IIIHCl­{κ⁴(<i>N,N</i>′<i>,S,C</i>)–ligand}] exhibit excellent catalytic activities and high chemoselectivity for the same reaction, reaching turnover numbers of >10 000