Retracted Article: Ruthenium(II) CNN Pincer Complexes as Efficient Catalysts in Oxidative Annulation of Aromatic Acids with Alkynes to Isocoumarins

Abstract

The reaction of [RuHCl(CO)(EPh3)] (E = P or As) with easily accessible N-heteroaromatic-derived hydrazone ligands (L1-2) (L1 = 2-(2-(benzothiazol-2-yl)hydrazono)-1,2-diphenylethanol (BZBT), L2 = 1,2-diphenyl-2-(2-(quinolin-2-yl)hydrazono)ethanol (BZQH)) leads to a novel series of Ru(II)-CNN pincer complexes of the type [Ru(CO)(EPh3)2(L)] (E = P or As; L = BZBT or BZQH), which structure have been confirmed by analytical and spectroscopic techniques (FT-IR, UV-Vis, NMR, and ESI-MS). Single crystal X-ray diffraction proved that there is an octahedral geometry surrounding the ruthenium(II) ion with a Schiff base bound via a tridentate C^N^N coordination mode and two trans-positioned phosphine or arsine co-ligands. These ruthenium(II) imine complexes are demonstrated to serve as efficient catalysts in an oxidative annulation reaction involving variously substituted benzoic acids and alkynes to produce a large scope of isocoumarins. With the use of 1.0 mol % of Ru(II) catalyst loading and a O2 environment, a wide variety of substituted isocoumarins have been effectively produced in good to outstanding yields. In this regard, the cycloruthenated complex 1 bearing two axial triphenylphosphine co-ligands and a benzoin hydrazone functionalised with N-coordinating benzothiazole was found to outperform its congeners containing quinoline fragment instead of benzothiazole and/or AsPh3 instead of PPh3 (complexes 2-4) as well as the widely-used Ru(II) catalyst used previously for this transformation.