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(mu-Cl)(eta(6)-p-cymene)}(2)] readily reacted with 4 equiv of guanidines ((PrHN)-Pr-i)(2)C=NR (R = Pr-i (1a), 4-(C6H4Bu)-Bu-t (1b), 4-C6H4Br (1c), 2,4,6-C6H2Me3 (1d), 2,6-(C6H3Pr2)-Pr-i (1e)) in toluene at room temperature to generate the mononuclear complexes [RuCl{kappa N-2,N'-C(NR)((NPr)-Pr-i)(NHPr)-Pr-i}(eta(6)-p-cymene)] (2a-e) and the easily separable guanidinium chloride salts [((PrHN)-Pr-i)(2)C(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{kappa N-2,N'-C((NPr)-Pr-i)(2)(NHPr)-Pr-i}(eta(6)-p-cymene)] (2a) and [RuCl{kappa N-2,N'-C(N-4-(C6H4Bu)-Bu-t)((NPr)-Pr-i}(NHPr)-Pr-i}(eta(6)-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{kappa N-2,N'-C(NR)((NPr)-Pr-i)-(NHPr)-Pr-i}(eta(6)-p-cymene)] and [RuCl{kappa N-2,N'-C((NPr)-Pr-i)(2)NHR}(eta(6)-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.