Olefin epoxidation catalyzed by [Ru(TDL)(tmeda)H2O] complexes (TDL = tridentate Schiff-base ligand; tmeda = tetramethylethylenediamine)

Mixed-chelate complexes of ruthenium have been synthesized using tridentate Schiff-base ligands (TDLs) derived from condensation of 2-aminophenol or 2-aminobenzoic acid with aldehydes (salicyldehyde, 2-pyridinecarboxaldehyde), and tmeda (tetramethylethylenediamine). [Ru^III(hpsd)(tmeda)(H₂O)]⁺ (1), [Ru^III(hppc)(tmeda)(H₂O)]²⁺ (2), [Ru^III(cpsd)(tmeda)(H₂O)]⁺ (3) and [Ru^III(cppc)(tmeda)(H₂O)]²⁺ (4) complexes (where hpsd²⁻ = N-(hydroxyphenyl)salicylaldiminato); hppc⁻ = N-(2-hydroxyphenylpyridine-2-carboxaldiminato); cpsd²⁻ = (N-(2-carboxyphenyl)salicylaldiminato); cppc⁻ = N-2-carboxyphenylpyridine-2-carboxaldiminato) were characterized by microanalysis, spectral (IR and UV–vis), conductance, magnetic moment and electrochemical studies. Complexes 1–4 catalyzed the epoxidation of cyclohexene, styrene, 4-chlorostyrene, 4-methylstyrene, 4-methoxystyrene, 4-nitrostyrene, cis- and trans-stilbenes effectively at ambient temperature using tert-butylhydroperoxide (t-BuOOH) as terminal oxidant. On the basis of Hammett correlation (log k_rel vs. σ⁺) and product analysis, a mechanism involving intermediacy of a [Ru–O–OBu^t] radicaloid species is proposed for the catalytic epoxidation process.