Synthesis and structural characterization of diorganotin(IV) complexes with a tridentate N‐heterocyclic podand ligand and investigation of their catalytic activity in Baeyer–Villiger oxidation of cyclic ketones to lactones

Abstract

Diorganotin(IV) complexes of composition [Me2SnL] (1), [n‐Bu2SnL] (2), and [Bz2SnL] (3) were synthesized by reacting the N2,N6‐di(pyridin‐2‐yl)pyridine‐2,6‐dicarboxamide pro‐ligand (H2L, where H2 denotes the two acidic protons) with Me2SnO, n‐Bu2SnO, and Bz2SnO, respectively, in refluxing toluene. The amide ligand leads to highly stable diorganotin(IV) complexes via the dianionic tridentate donor set (Npy, N−, N−) with pendent pyridine arms, and the coordination domain is completed by the respective two alkyl ligands. Solution‐state structures of the compounds 1–3 were studied by 1H, 13C, and 119Sn nuclear magnetic resonance (NMR) spectroscopy, revealing that the complexes adopt a five‐coordinate structure in non‐coordinating solvent. Additionally, homogenity and purity of the bulk materials of 1–3 were judged using the high‐resolution mass spectrometry in acetonitrile solution. The molecular and crystal structures of 1·0.5C6H6, 2, and 3 in the solid phase were established by single‐crystal X‐ray diffraction analysis. For all three compounds, the core defined by the rigid tridentate pyridine dicarboxamide ligand and the Sn‐coordinating C atoms adopts nearly C2v symmetry and thus matches none of the most popular geometries for five‐coordinated species, namely square‐pyramidal or trigonal‐bipyramidal coordination. Thermogravimetric analysis (TGA) was performed to probe the thermal stabilities of 1–3. The results serve as a basis for designing highly efficient diorganotin(IV) mimics for catalytic applications. Catalytic studies of ε‐caprolactone formation via Baeyer–Villiger oxidation of cyclohexanone with aqueous H2O2 were performed under microwave (MW) irradiation. The consequences of several reaction parameters like catalyst quantity, time, temperature, and solvent were studied.