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

Abstract

We introduce ENENES, a new family of airstable and low-cost NNS ligands bearing NH functionalities of the general formula E(CH 2 ) m NH(CH 2 ) n SR, where E is selected from −NC 4 H 8 O, −NC 4 H 8 , or −N(CH 3 ) 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 (κ 2 [N,N′] or κ 2 [N′,S]) or tridentate (κ 3 [N,N′,S]) binding modes, depending on the basicity of the sulfur atom, CH 2 chain length (m or n parameter), or identity of the transition metal. In the case of Ir, ligands bearing benzyl substituents lead to unprecedented κ 4 [N,N′,S,C]-tetradentate corestructure complexes of the type [Ir III HCl{κ 4 (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 II Cl 2 {κ 3 [N,N′,S]−ligand}(L)] (L = PPh 3 , PCy 3 , 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 H 2 ) with reasonable turnover numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes [Ir III HCl{κ 4 (N,N′,S,C)−ligand}] exhibit excellent catalytic activities and high chemoselectivity for the same reaction, reaching turnover numbers of >10 000.

Ruthenium (II) complexes bearing thioether‐appended α‐iminopyridine ligands: Arene precursors permit access to κ2‐N,N and κ3‐N,N,S complexes

“…While numerous ligands have been developed featuring P and N donors, less attention has been paid to other main group donors. Inclusion of a soft thioether donor has been observed to generate ligands with greater air and moisture tolerance when compared to their phosphine counterparts, with properties that are highly tunable based on substitution of the thioether, or connectivity between the sulfur moiety and the amine donors . The presence of a third donor may also encourage the formation of more robust κ 3 complexes, relative to bidentate counterparts.…”

“…Inclusion of a soft thioether donor has been observed to generate ligands with greater air and moisture tolerance when compared to their phosphine counterparts, with properties that are highly tunable based on substitution of the thioether, or connectivity between the sulfur moiety and the amine donors. [32][33][34][35][36][37] The presence of a third donor may also encourage the formation of more robust κ 3 complexes, relative to bidentate counterparts. Pd(II) and Ag(I) complexes bearing thioether-functionalized Nheterocyclic carbenes have been shown to demonstrate hemilability in the presence of moderate to strong donors such as phosphines.…”

Ruthenium(II) Complexes Bearing Thioether-Appended α- Iminopyridine Ligands: Arene Precursors Permit Access to K2-N,N and K3-N,N,S Complexes

“…[36][37][38] Inclusion of a soft thioether donor has been observed to generate ligands with greater air and moisture tolerance when compared to their phosphine counterparts, with properties that are highly tunable based on substitution of the thioether, or connectivity between the sulfur moiety and the amine donors. 39,40 A recent report by Phillips and co-workers describes the preparation and catalytic performance of Ru(II) arenes featuring b-thioketoiminates, a bidentate N,S analogue of the ubiquitous b-diketiminate (or "nacnac") ligands. 41 Ru(II) arene complexes featuring sulfur donors have also been shown to exhibit anticancer activity, wherein that activity is strongly influenced by the binding mode and oxidation state of the thiolate moiety.…”

Inexpensive Ruthenium NNS‐Complexes as Efficient Ester Hydrogenation Catalysts with High C=O vs. C=C Selectivities

“…Although manganese and iron are more earth‐abundant transition metals than ruthenium, these catalysts have the drawback that, most of the existing catalysts rely on non‐symmetrical phosphine ligands, which can make the ligand more expensive than the metal employed –,,,. This cost aspect was recently addressed by the development of sulfur containing SNS‐ and NNS‐pincer ligands. Although thus far these ligands have proven effective only with ruthenium and iridium, the resulting complexes are air stable and the ligands easily obtained by simple nucleophilic substitution or condensation reactions.…”