Hypervalent Organotin(IV) Compounds as Efficient Catalysts for the Knoevenagel Condensation of Aldehydes.

We report the synthesis of six mononuclear organotin(IV) compounds, [R2Sn(L1)] [R = Ph (1), t-Bu (2), and Me (3)] and [R2Sn(L2)] [R = Ph (4), t-Bu (5), and Me (6)] supported by two polydentate pro-ligands, (E)-2,4-dichloro-6-(((2-hydroxy-5-nitrophenyl)imino)methyl)phenol (H2L1) and (E)-2,4-dichloro-6-(((2-hydroxyphenyl)imino)methyl)phenol (H2L2). HR-MS, multinuclear (1H, 13C, and 119Sn) NMR spectroscopy, and FT-IR spectroscopy were used to characterize all compounds (1-6). Structural investigation via single-crystal X-ray diffraction confirmed a mononuclear geometry for all compounds, each featuring a five-coordinate tin(IV) center. The Lewis acidity of the compounds (1-6) was examined through the Gutmann-Beckett method. The effectiveness of all Sn(IV) compounds (1-6) as catalysts for the Knoevenagel condensation of various aldehydes with malononitrile was assessed. In addition, a plausible mechanism is proposed based on the mechanistic studies and theoretical calculations.