Ruthenium(II) Arene Complexes with Asymmetrical Guanidinate Ligands: Synthesis, Characterization, and Application in the Base-Free Catalytic Isomerization of Allylic Alcohols

The ruthenium(II) arene dimer [{RuCl(μ-Cl)(η6-p-cymene)}2] readily reacted with 4 equiv of guanidines (iPrHN)2CNR (R = iPr (1a), 4-C6H4 tBu (1b), 4-C6H4Br (1c), 2,4,6-C6H2Me3 (1d), 2,6-C6H3 iPr2 (1e)) in toluene at room temperature to generate the mononuclear complexes [RuCl{κ2 N,N′-C(NR)(NiPr)NHiPr}(η6-p-cymene)] (2a–e) and the easily separable guanidinium chloride salts [(iPrHN)2C(NHR)][Cl] (3a–e). Compounds 2a–e and 3a–e were fully characterized by elemental analysis and IR and NMR spectroscopy. The structures of [RuCl{κ2 N,N′-C(NiPr)2NHiPr}(η6-p-cymene)] (2a) and [RuCl{κ2 N,N′-C(N-4-C6H4 tBu)(NiPr)NHiPr}(η6-p-cymene)] (2b) were also determined by X-ray diffraction analysis. Regardless of the steric requirements of the aromatic substituents, a nonsymmetric coordination of the guanidinate anions in 2b–e was observed, in complete accord with theoretical calculations (DFT) on the corresponding [RuCl{κ2 N,N′-C(NR)(NiPr)-NHiPr}(η6-p-cymene)] and [RuCl{κ2 N,N′-C(NiPr)2NHR}(η6-p-cymene)] models. Remarkably, complexes 2a–e were active catalysts for the redox isomerization of allylic alcohols in the absence of base, which represents the first catalytic application known for ruthenium guanidinate species.