Aqueous Synthesis of 14-15-Membered Crown Ethers with Mixed O, N and S Heteroatoms: Experimental and Theoretical Binding Studies with Platinum-Group Metals

A series of 14-15-membered O, N, and S-containing crown ethers (CEs) was synthesized by cyclization of bis-epoxides with aryl-N or S dinucleophiles using triethylamine as a catalyst and LiCl as a metal template in water. The catalyst dosage, and metal template type and dosage were critical in achieving yields of 56-93 %. Liquid-liquid extraction (LLE) was performed to evaluate the CE complexation with Pd2+ and Pt2+. Among the CEs, a dioxa-dithia dibenzo CE exhibited the highest Pd2+ selectivity even in the presence of other platinum-group metals (PGMs). Complementary DFT studies reveal that this CE has the most compatible cavity dimension (o(CE)=1.58 angstrom) with Pd2+ (o(Pd)(2+)=1.56 angstrom) forming a square-planar S-4 geometry. Binding-energy calculations showed the Pd2+ complex has the least energy requirement for structural reorientation during complexation. Overall results highlight the importance of CE cavity dimension and presence of S heteroatoms for the structural design of CEs selective towards PGMs such as Pd2+.