O, O ′-diester methylenediphosphonotetrathioate: Synthesis, characterization, and potential applications

A new transformation of methylene-bis(phosphonic dichloride) into tetrathiobisphosphonate derivatives is reported. The reaction of methylene-bis(phosphonic dichloride) with 1,2-ethanedithiol in bromoform in the presence of AlCl₃ formed methylene-bis(1,3,2-dithiaphospholane-2- sulfide), which gave rise to O,O′-diester-methylenediphosphonotetrathioate analogues 1a-k upon reaction with phenols and alkyl alcohols in the presence of DBU. Reaction mechanisms are proposed, and all products were characterized by ³¹P, ¹³C, and ¹H NMR. An X-ray crystal structure was obtained for intermediate 2. The potential of the novel scaffold for selective coordination of metal-ions was examined by coordination of Hg(II) and Pb(II) by 1f, as determined by FT-IR, and chelation of Zn(II), but not Ca(II), by 1b, as determined by ³¹P/¹H NMR. UV-vis measurements of 1g-Ni(II) mixture revealed a 2:1 ligand:metal complex. These derivatives are potential antioxidants, and their ability to inhibit ·OH formation in Fenton reactions was quantified by ESR measurements. Analogue 1g proved to be a most potent antioxidant (IC₅₀ 53 μM), inhibiting the Cu(I)-catalyzed Fenton reaction at lower concentrations than GSH, ascorbic acid, and EDTA. Analogue 1c inhibited the Fe(II)-catalyzed Fenton reaction at about the same concentrations as ascorbic acid (IC₅₀ 83 vs 93 μM). In summary, the novel compounds, 1a-k, proved to chelate various borderline/soft Lewis acid metal-ions, and may be useful as antioxidants and metal extractors.