Tandem Deacetalization–Knoevenagel\nCondensation\nReactions for the Synthesis of Benzylidene Malononitrile Using Ruthenium(II)\nCymene Complexes

In this study, we report the synthesis and full characterization\nof five novel ruthenium(II) cymene complexes with the general formula\n[Ru(cym)(L’)Cl], featuring N,O- and N,N-coordinating pyrazolone-based\nhydrazone ligands. We have characterized these complexes using single\nX-ray crystallography, Fourier-transform infrared spectroscopy (FT-IR),\nNuclear magnetic resonance (NMR), elemental analysis, and Electrospray\nIonization Mass Spectroscopy (ESI-MS). Crystallographic analysis confirmed\nthat all of the complexes have a similar type of half-sandwich, pseudo-octahedral\n“three-legged piano-stool” geometry where the cymene\nmoiety displays the typical η<sup>6</sup>-coordination mode\nand the hydrazone ligands coordinate to the Ru(II) center in a bidentate\nfashion. These complexes, with multiple catalytic sites, demonstrated\nhigh efficiency in catalyzing one-pot cascade deacetalization–Knoevenagel\ncondensation reactions under mild conditions, achieving up to 92%\nof final product yield at 75 °C after 4 h of reaction time under\nsolvent-free condition. Additionally, DFT calculations provided insight\ninto the catalytic mechanism, suggesting a pathway driven by metal–ligand\ncooperation, assisted by the basic oxygen site of the pyrazolone ring\nand by the weakly acidic character of the NNH proton of the hydrazone\ngroup.