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)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
[IrIIIHCl­{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-[RuIICl2{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
[IrIIIHCl­{κ4(N,N′,S,C)–ligand}] exhibit excellent catalytic activities
and high chemoselectivity for the same reaction, reaching turnover
numbers of >10 000.