A New Robust 3D Cd(II) Coordination Polymer Engineered with Hexamethylenetetramine: Structure, Computational Insights, and Optical Properties

This work reports a novel cadmium(II)–hexamethylenetetramine (HMTA) hybrid complex, [Cd2Cl4(H2O)3(C6H12N4)]·2H2O, whose synthesis, crystal structure, and structure–property relationships are thoroughly investigated. Single-crystal X-ray diffraction analysis reveals a three-dimensional coordination polymer stabilized by both coordinate covalent Cd–N bonds and an extensive network of noncovalent interactions, quantitatively mapped via Hirshfeld surface analysis. Advanced quantum chemical topology analyses (QTAIM, NCI) confirm the presence of significant electron density at bond critical points, while natural bond orbital (NBO) calculations highlight dominant n(N) → σ*(N–C) hyperconjugative interactions that underpin the electronic delocalization. Density functional theory (DFT) computations accurately reproduce the experimental geometry and, together with time-dependent DFT (TD-DFT), identify a metal-to-ligand charge transfer (MLCT) as the origin of the lowest-energy excitation. A notable exciton binding energy of ∼0.39 eV is quantified from the disparity between the experimental optical gap (2.85 eV) and the TD-DFT-derived fundamental gap (3.24 eV). The compound exhibits a high refractive index (n ≈ 4.67), a substantial first-order hyperpolarizability (β0 = 2459 × 10–30 esu), and significant electronic polarizability, collectively positioning it as a highly promising candidate for advanced ultraviolet photonic and nonlinear optical applications.