Hammett-Driven Secondary Ligand Effects Control Geometry and Electronic Structure of 5-Substituted Copper(II)–Cytosine Complexes

Abstract Metal-nucleobase interactions play central roles in biological chemistry and metal-based therapeutic design, yet the influence of nucleobase substitution on coordination geometry and thermodynamic stability remains underexplored. Herein, we report the systematic investigation of copper(II) complexes of 5-substituted cytosine ligands, specifically [Cu(XCyt)4](OTf)2, where X = H, F, Cl, or I, designed to isolate the effect of a C5 halogen substitution on ligand–metal interactions. Characterization was accomplished via solution-state UV–vis, Fourier transform infrared (FT-IR) spectroscopy, 1H NMR spectroscopy, fluorescence spectroscopy, elemental analysis, and single-crystal X-ray diffraction spectroscopy. Single crystal X-ray diffraction reveals that C5 substitution has minimal impact on Cu···N bond distances, but induces pronounced, substituent-dependent changes in coordination geometry. Specifically, increasing substituent σpara Hammett constants lower the Lewis basicity of the cytosine carbonyl and appear to perturb Cu···O interactions and interligand hydrogen bonding, with a concomitant increase in the planarity of the CuN4 coordination environment. These correlate with systematic changes in electronic structure, as evidenced by linear shifts in UV–visible absorption and corresponding trends in paramagnetic relaxation behavior measured by 1H NMR spectroscopy. Collectively, these results demonstrate that substitution at the C5 position modulates metal-centered properties primarily through secondary-sphere interactions, rather than direct perturbation of metal–ligand bonding, which in turn drives linear perturbations of UV–vis absorbance spectra and paramagnetic relaxation properties.