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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.

Three new complexes namely [Cu(L1)H2O]·2H2O (1), [Cu2(L4)2(DMF)2(H2O)2](NO3)2 (4) and [{Cd2(L4)2Cl2(DMF)2}] (5), and two coordination polymers namely [Co(μ-L2)(DMF)2]·2H2O (2) and [Co(μ-L3)(DMF)2]·H2O (3) have been synthesized from a series of pyrazole-carboxylate ligands namely 1,1′-(ethane-1,2-diyl)bis(5-methyl-1H-pyrazole-3-carboxylic acid) (H2L1), 1,1′-(oxybis(ethane-2,1-diyl))bis(5-methyl-1H-pyrazole-3-carboxylic acid) (H2L2), 1,1′-(butane-1,4-diyl)bis(5-methyl-1H-pyrazole-3-carboxylic acid) (H2L3), and 2-(1′,5,5′-trimethyl-1H,1′H-[3,3′-bipyrazol]-1-yl)acetic acid (H2L4), and CuII/CdII/CoII ions. Single crystal structures of these compounds are discussed in the context of the effect of conformation of the ligands and supramolecular interactions observed within these edifices. Temperature-dependent magnetic data of 4 could be fitted best using two non-interacting CuII ions (S = 1/2) per molecule with giso = 2.28 and a temperature-independent paramagnetism, TIP = 1.75 × 10−5 cm3 mol−1.

The 1D chain [Fe(Etutrz)3](ClO4)2 ⋅ 1.5H2O (2) (Etutrz = 1-ethyl-3-(4H-1,2,4-triazol-4-yl) thiourea), displays pronounced thermochromism with a purple color at 77 K while the sample is white at 300 K. Investigation of magnetic properties reveal an abrupt spin transition around 227 K. Differential scanning calorimetry studies on cooling display a first order phase transition at around 200 K with an entropy variation of ΔS = 61.3 J mol− 1 K− 1. 57Fe Mossbauer spectroscopy of 2 confirms a complete spin transition with a 100% high-spin population at 300 K (isomer shift δHS = 1.04(1) mm/s, quadrupole splitting ΔEQ = 2.86(2) mm/s). The Fe(II) ions convert to the low-spin state at 78K (δLS = 0.53(2) mm/s). The quadrupole splitting, ΔEQ = 0.29(2) mm/s, confirms the presence of distorted octahedra within the 1D chain. The 1D chain [Fe(Etutrz)3](BF4)2 ⋅2MeOH (1) exhibits a different magnetic behavior with a gradual spin conversion at T1/2 = 221 K, whereas thermochromic properties are maintained.