Dense Dithiolene Units on Metal–Organic Frameworks for Mercury Removal and Superprotonic Conduction

Abstract

With 2-COOH and 4-SH donors all packed onto the benzene ring, tetrasulfanyl terephthalic acid (TST) is a simple yet fully equipped ligand to move the field of metal-coordination materials─it is now accomplished. The hard-soft carboxyl-thiol synergy is leveraged here in selectively bonding the carboxyl units to Zr(IV) ions to form the same cubic net of UiO-66 (this being based on the terephthalic linker)─with the free-standing dithiolene units equipping the grid of ZrTST. The 3D network of ZrTST averages about 7.6 connections [as in Zr₆O₄(OH)₄(C₈H₄O₄S₄)_3.8], with the other 4.4 sealed by acetate ions. The ZrTST solid is stable in boiling water (it is formed in water/acetic acid/ethane dithiol) and remains ordered even above 300 °C. The thiol-enabled ZrTST (powder) takes up mercury from water with a high distribution coefficient <i>K</i>_d (e.g., 1.2 × 10⁶ mL·g^-1); it also shows proton conductivity (1.9 × 10^-3 S·cm^-1 at 90 °C and 90% relative humidity), which, most notably, increases to a highest value of 3.7 × 10^-1 S·cm^-1 after oxidizing the -SH into the -SO₃H groups.