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

Diorganotin(IV) complexes of composition [Me2SnL] (1), [n-Bu2SnL] (2), and [Bz(2)SnL] (3) were synthesized by reacting the N-2,N-6-di(pyridin-2-yl)pyridine-2,6-dicarboxamide pro-ligand (H2L, where H-2 denotes the two acidic protons) with Me2SnO, n-Bu2SnO, and Bz(2)SnO, respectively, in refluxing toluene. The amide ligand leads to highly stable diorganotin(IV) complexes via the dianionic tridentate donor set (N-py, 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 H-1, C-13, and Sn-119 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 10.5C(6)H(6), 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 C-2v 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 epsilon-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.