Effect of Substituents on the Crystal Structures, Optical Properties, and Catalytic Activity of Homoleptic Zn(II) and Cd(II) β‑oxodithioester Complexes

Five novel zinc­(II) and cadmium­(II) β-oxodithioester complexes, [Zn­(L1)₂] (1), [Zn­(L2)₂]_n (2), [Zn­(L3)₂]_n (3) [Cd­(L1)₂]_n (4), [Cd­(L2)₂]_n (5), with β-oxodithioester ligands, where L1 = 3-(methylthio)-1-(thiophen-2-yl)-3-thioxoprop-1-en-1-olate, L2 = 3-(methylthio)-1-(pyridin-3-yl)-3-thioxoprop-1-en-1-olate, and L3 = 3-(methylthio)-1-(pyridin-4-yl)-3-thioxoprop-1-en-1-olate, were synthesized and characterized by elemental analysis, IR, UV–vis, and NMR spectroscopy (¹H and ¹³C­{¹H}). The solid-state structures of all complexes were ascertained by single-crystal X-ray crystallography. The β-oxodithioester ligands are bonded to Zn­(II)/Cd­(II) metal ions in an O^∧S and N chelating/chelating–bridging fashion leading to the formation of 1D (in 2–4) and 2D (in 5) coordination polymeric structures, but complex 1 was obtained as a discrete tetrahedral molecule. Complex 4 crystallizes in the C2 chiral space group and has been studied using circular dichroism (CD) spectroscopy. The multidimensional assemblies in these complexes are stabilized by many important noncovalent C–H···π (ZnOSC₃, chelate), π···π, C–H···π, and H···H interactions. The catalytic activities of 1–5 in reactions involving C–C and C–O bond formation have been studied, and the results indicated that complex 3 can be efficiently utilized as a heterogeneous bifunctional catalyst for the Knoevenagel condensation and multicomponent reactions to develop biologically important organic molecules. The luminescent properties of complexes were also studied. Interestingly, zinc complexes 1–3 showed strong lumniscent emission in the solid state, whereas cadmium complexes 4 and 5 exhibited bright luminescent emission in the solution phase. The semiconducting behavior of the complexes was studied by solid-state diffuse reflectance spectra (DRS), which showed optical band gaps in the range of 2.49–2.62 eV.