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-C6H4tBu (1b), 4-C6H4Br
(1c), 2,4,6-C6H2Me3 (1d), 2,6-C6H3iPr2 (1e)) in toluene at room temperature to generate the
mononuclear complexes [RuCl­{κ2N,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{κ2N,N′-C­(NiPr)2NHiPr}­(η6-p-cymene)] (2a) and [RuCl­{κ2N,N′-C­(N-4-C6H4tBu)­(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{κ2N,N′-C­(NR)­(NiPr)-NHiPr}­(η6-p-cymene)] and [RuCl­{κ2N,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.