Exploring “Triazole-Thiourea” Based Ligands for the Self-Assembly of Photoluminescent Hg(II) Coordination Compounds

This study represents the first explorative investigation on the supramolecular structural diversity in Hg(II) coordination chemistry with triazole-thiourea ligands leading to a variety of mononuclear, binuclear, and coordination polymers: {[Hg(L1)2(L1–)2]} (1), {[Hg2(L1)2(μ2-I)2I2]·DMSO} (2), {[Hg(L2)(μ2-I)I]·MeOH}∝ (3), {[Hg2(μ-L3–)4]}∝ (4), {[HgCl(L4–)L4]·MeOH} (5), {[Hg2(L4)2(μ2-I)2(I)2]·2MeOH} (6), {[Hg2(μ2-L5–)4]}∝ (7), {[Hg2(μ2-Cl)2(L6–)2(L6)2]} (8), {[Hg2(μ2-Br)2(L6–)2(L6)2]} (9), and {[Hg2(μ2-I)2(L6–)2(L6)2]} (10). A reaction mechanism was suggested for the unexpected ligand rearrangement occurring in {[Hg2I3(μ3-L5′)]}∝ (11). The ligands were fully characterized including by X-ray crystallography and computational means. This includes six new triazole-thiourea based ligands, namely, 1-R-3-(4H-1,2,4-triazol-4-yl)thiourea (where R = methyl (L1), ethyl (L2), propyl (L3), isopropyl (L4), and its polymorph (L4-poly), allyl (L5), ethyl acetate (L6), and its solvate (L6_MeOH)). Under UV light excitation, 7, 10, and 11 exhibit visible photoluminescence of wide origin, ranging from ligand-centered (LC) fluorescence combined with organic-ligand-to-metal charge transfer (LMCT) emissive states in 7 and 10, up to halide-to-metal charge transfer (XMCT) combined with halide-to-ligand charge transfer (XLCT) emissive states in 11. The variable emission mechanisms in the obtained coordination polymers were elucidated by experimental proofs confronted with theoretical calculations of the electronic densities of states, proving that Hg(II) halide coordination polymers involving flexible 1,2,4-triazole-based ligands form a promising class of luminescent molecular materials