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

Article abstract of DOI:10.1021/acs.organomet.5b00432

We introduce ENENES, a new family of air-stable and low-cost NNS ligands bearing NH functionalities of the general formula E(CH₂)_mNH(CH₂)_nSR, 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 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[N,N'] or κ²[N',S]) or tridentate (κ³[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 κ⁴[N,N',S,C]-tetradentate core-structure complexes of the type [Ir^IIIHCl{κ⁴(N,N',S,C)-ligand}], resulting from ortho-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 trans-[Ru^IICl₂{κ³[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 (35-40 °C, 25 bar H₂) with reasonable turnover numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes [Ir^IIIHCl{κ⁴(N,N',S,C)-ligand}] exhibit excellent catalytic activities and high chemoselectivity for the same reaction, reaching turnover numbers of >10 000.