Pyridyl-Thiourea Ruthenium and Osmium Complexes: Coordination of Ligand and Application as FLP Hydrogenation Catalysts

Pyridyl-thiourea complexes of formula [(Cym)MCl(κ²<i>N</i>_py,<i>S</i>-<b>H₂NNS</b>)][SbF₆] (Cym = <i>η</i>⁶-<i>p</i>-MeC₆H₄<i>i</i>Pr; <b>H₂NNS</b> = <i>N</i>-(<i>p</i>-tolyl)-<i>N'</i>-(2-pyridylmethyl)thiourea); M = Ru (<b>1</b>), Os (<b>2</b>)) were synthesized by reacting the corresponding metal dimers [{(Cym)MCl}₂(<i>μ</i>-Cl)₂] with <b>H₂NNS</b> in the presence of NaSbF₆. Subsequent chloride abstraction with AgSbF₆, followed by NH deprotonation using NaHCO₃, afforded the cationic complexes [(Cym)M(κ³<i>N</i>_py,<i>N</i>_amide,<i>S</i><b>-HNNS</b>)][SbF₆] (M = Ru (<b>5a</b>), (<b>5c</b>); M = Os (<b>6a, 6c</b>)) and [(Cym)M(κ²<i>N</i>_amide,<i>S</i><b>-HNNS</b>)][SbF₆] (M = Ru (<b>5b</b>); M = Os (<b>6b</b>)). The proposed structures for the prepared compounds are based on NMR data. Complexes <b>5a</b>, <b>5b,</b> and <b>6a</b>, <b>6b</b> evolve to the thermodynamically more stable species <b>5c</b> and <b>6c</b>, respectively, in which the deprotonated ligand <b>HNNS</b> adopts a κ³<i>N</i>_py,<i>N</i>_amide,<i>S</i> coordination mode. Complexes <b>5c</b> and <b>6c</b> activate H₂, behaving as frustrated Lewis pair (FLP) species, and catalyze (<b>5c</b> and/or <b>6c</b>) the hydrogenation of polar multiple bonds, including the C=N bonds of <i>N</i>-benzylideneaniline and quinoline, the C=C bond of methyl acrylate, and the C=O bond of 2,2,2-trifluoroacetophenone.