Structurally isomeric ditopic 2-mercaptobenzoxazole and 2-hydroxybenzothiazole as ligands for design of 2D...

The novel ditopic centrosymmetric soft-base ligands 3,7-dihydrobenzo[1,2-d:4,5-d']bis(oxazole)-2,6-dithione (H₂L1) and 3,7-dihydrobenzo[1,2-d:4,5-d']bis(thiazole)-2,6-dione (H₂L2) were obtained via a one-pot method. Both compounds form 2D coordination polymers (CPs) through the reaction of free ligands with sodium bis(trismethylsilyl)amide in various solvent media. H₂L1 containing products were [Na₂L1(DME)₂] (1) and [Na₂L1(DMSO)₄] (2), and for H₂L2, [Na₆L2₃(DMSO)₁₂] (3) was formed. X-ray analysis revealed that compound 1 is a 2D CP, in which deprotonated H₂L1 acts as a tetratopic linker, linking four sodium atoms via nitrogen and bridging sulfur. Unlike 1, in CP 2, deprotonated H₂L1 acts as a ditopic linker coordinating sodium atoms in an unprecedented Na₄O₄ metallocenter formed by the oxygens of DMSO molecules. Compound 3 is a 2D CP featuring rare Na₃O₆ secondary building unit (SBU), whereas H₂L2 exhibits both bridging and mixed chelating/bridging mode, linking four sodium atoms via nitrogen and oxygen atoms. Despite the fact that free ligands H₂L1 and H₂L2 are non-luminescent in solution and in the solid state, their deprotonated forms at 298 K in DME media demonstrate moderate photoluminescence (PL) in DME solutions with broad bands in the range of 360-500 nm. At 77 K, the same solutions show broadened and intense PL consisting of bands in the 360-670 nm region corresponding to ligand fluorescence and phosphorescence. CPs 1-3 in the solid state exhibit PL at 298 and 77 K. At 298 K, CPs 1 and 3 exhibit intense fluorescence in the range of 370-450 nm and moderate microsecond timescale phosphorescence in the range of 500-670 nm, while CP 2 exhibits only fluorescence. Upon cooling to 77 K, all three CPs demonstrate both fluorescence and phosphorescence. Based on the low-temperature phosphorescence spectra of 1 and 3 in DME solutions, the triplet energy levels of H₂L1 and H₂L2 were established at 21 000 and 20 800 cm^-1, respectively.