Development of a ruthenium–aquo complex for utilization in synthesis and catalysis for selective hydration of nitriles and alkynes

Abstract

Synthesis of a ruthenium(II)–aquo complex bearing 2,2′ : 6′,2′′-terpyridine and a 1,4-diazabutadiene ligand, and exploration of its synthetic utility and catalytic activity are described. Ag⁺-assisted displacement of the coordinated chloride from the previously reported [Ru(trpy)(L-OCH₃)Cl]ClO₄ complex [denoted as complex 1; where L-OCH₃ represents 1,4-di-(p-methoxyphenyl)azabutadiene] in aqueous ethanol affords the [Ru(trpy)(L-OCH₃)(H₂O)]²⁺ complex cation, which has been isolated as a perchlorate salt (complex 1a). Complex 1a undergoes facile substitution of the aquo ligand by neutral monodentate ligands leading to the formation of complexes of type [Ru(trpy)(L-OCH₃)(L′)]²⁺, also isolated as perchlorate salts [L′ = acetonitrile (complex 1b); L′ = dmso (complex 1c); L′ = 4-picoline (complex 1d) and L′ = PPh₃ (complex 1e)]. Complexes 1b–1e can also be synthesized directly from complex 1via the Ag⁺-assisted displacement of coordinated chloride by the respective monodentate L′ ligand. The crystal structures of complexes 1a, 1b and 1d have been determined. The complexes show intense absorptions in the visible and ultraviolet regions, the origin of which has been probed into with the help of the TDDFT method. Cyclic voltammetry of the complexes shows an irreversible Ru(II)–Ru(III) oxidation within 0.9–1.6 V vs. SCE, and two ligand (trpy and L–R) based reductions on the negative side of the SCE. The aquo-complex (1a) is found to serve as an efficient catalyst for the hydration of aryl nitriles to the corresponding amides, and aryl alkynes to aldehydes.