Fluorescent biogenic Schiff base compounds of dimethyltin

Abstract

Herein, biologically crucial amino acids have been explored for the synthesis of fluorescent organotin compounds. One-pot reaction of dimethyltin oxide, 2-hydroxy-4-methoxybenzophenone and α-amino acids, i.e., glycine, L-isoleucine and L-methionine, afforded respective organotin compounds with biogenic Schiff base skeletons 1–3. Although all the compounds were constituted from similar ONO type Schiff base units, only compound 1 (derived from glycine) crystallized as a hexacoordinated organotin compound of the type (ONO)SnMe₂(MeOH) with a tridentate chelating Schiff base ligand. Analogous syntheses with L-isoleucine and L-methionine did not afford the related Schiff base compounds 2 and 3. Instead, some kind of precursors, namely 2′ and 3′, crystallized as solvent mediated Sn₄O₄ ladders and two Schiff base ligand moieties bound to the terminal Sn atoms via carboxylate moiety only. Interestingly, solution NMR spectroscopy (¹H, ¹³C, ¹¹⁹Sn) of 1 unambiguously complied with its solid state structure while for 2′ and 3′ the behaviour was entirely different suggesting their transformation to mononuclear compounds 2 and 3 in solution phase. The distinctive structural aspects of 2′/2 and 3′/3 in solid and solution phase were also supported by solid state ¹¹⁹Sn CP/MAS NMR studies. All the compounds were found to be fluorescent when excited at 365 nm and the solutions of compound 3′ experienced quenching in the presence of paramagnetic metal ions due to chelation enhanced quenching.