Ruthenium(II) Arene complexes with asymmetrical guanidinate ligands: Synthesis, characterization, and application in the base-free catalytic isomerization of allylic alcohols

Article abstract of DOI:10.1021/om3009124

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-C ₆H₂Me₃ (1d), 2,6-C₆H ₃ⁱPr₂ (1e)) in toluene at room temperature to generate the mononuclear complexes [RuCl{κ²N,N'-C(NR)(N ⁱPr)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)₂NH ⁱPr}(η⁶-p-cymene)] (2a) and [RuCl{κ ²N,N'-C(N-4-C₆H₄^tBu)(N ⁱPr)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)(N ⁱPr)-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.