Investigation of four Cd(ii) sulfonate complexes: crystal structure, Hirshfeld surface analysis, thermogravimetric and spectroscopic properties

Abstract

Four cadmium sulfonate complexes [Cd(L₁)₂(THU)₂]_n (HL₁ = p-toluenesulfonic acid; THU = thiourea) (1), [Cd(NO₃)(L₂)(4,4′-bipy)(DMF)]_n (HL₂ = 8-aminonaphthalene sulfonic acid) (2), {[Cd(Ac)(L₃)(4,4′-bipy)(H₂O)]·H₂O}_n (3), {[Cd(4,4′-bipy)_2.5(H₂O)₃](NO₃)·(L₃)·H₂O}_n (4) (HL₃ = p-aminobenzenesulfonic acid) were synthesized by solvent diffusion at room temperature using a water/methanol solvent system. X-ray crystal structure analysis shows that the adjacent Cd(II) octahedra in 1 are linked via the sulfonate group of the L₁⁻ monoanion in bis-monodentate mode to form one-dimensional chains. The Cd(II) pentagonal bipyramids in 2 and 3 are linked via the 4,4′-bipy to form one-dimensional chains, where the two Cd(II) geometries in 3 are further linked by acetate to produce a one-dimensional molecular ladder, the sulfonate groups of the L₂⁻ and L₃⁻ monoanions act as terminal coordinate groups and arrange along the sides of the chains. The Cd(II) octahedra in 4 are linked by 4,4′-bipy to form a two-dimensional layered structure, with the L₃⁻ monoanions free in the layers. The C–H⋯O, O–H⋯O hydrogen bonding interactions stabilize the crystal packing for the four complexes. Multiple interactions of the four complexes were explained using Hirshfeld surface analysis, and 2D fingerprinting was used to accurately quantify intermolecular interactions. Thermal stability showed 1 and 2 were thermally stable at 270 °C and 230 °C, respectively, while 3 and 4 begin to decompose at relatively low temperatures due to the presence of water molecules. The band gaps of 1–4 were estimated from the UV/vis/NIR spectra as 4.40, 2.92, 2.94, and 2.88 eV, indicating that the four complexes' band gaps decrease in the order of S > O > N. According to the luminescence spectra, 1 and 2 emitted at wavelengths of 473 and 484 nm and showed a visualized blue light, the positions of the emission peaks for 3 and 4 are both located at 342 nm and based on the HL₃ ligand.