Tandem Deacetalization–Knoevenagel Condensation Reactions for the Synthesis of Benzylidene Malononitrile Using Ruthenium(II) Cymene Complexes

In this study, we report the synthesis and full characterization of five novel ruthenium(II) cymene complexes with the general formula [Ru(cym)(L')Cl], featuring N,O- and N,N-coordinating pyrazolone-based hydrazone ligands. We have characterized these complexes using single X-ray crystallography, Fourier-transform infrared spectroscopy (FT-IR), Nuclear magnetic resonance (NMR), elemental analysis, and Electrospray Ionization Mass Spectroscopy (ESI-MS). Crystallographic analysis confirmed that all of the complexes have a similar type of half-sandwich, pseudo-octahedral "three-legged piano-stool" geometry where the cymene moiety displays the typical η⁶-coordination mode and the hydrazone ligands coordinate to the Ru(II) center in a bidentate fashion. These complexes, with multiple catalytic sites, demonstrated high efficiency in catalyzing one-pot cascade deacetalization-Knoevenagel condensation reactions under mild conditions, achieving up to 92% of final product yield at 75 °C after 4 h of reaction time under solvent-free condition. Additionally, DFT calculations provided insight into the catalytic mechanism, suggesting a pathway driven by metal-ligand cooperation, assisted by the basic oxygen site of the pyrazolone ring and by the weakly acidic character of the NNH proton of the hydrazone group.