Hybrid Tuning in NHC Ligands: Synergistic Effects of BIAN π-Conjugation and Aryl σ-Modulation in Gold(I) Complexes

In this study, we explore extended design of metal-NHC complexes that relies on dual strategies for tuning ligand properties: (i) electronic modulation through π-extended frameworks such as bis(imino)acenaphthene (BIAN), and (ii) systematic aryl substitution to influence σ-donor strength and steric environment. We report the first integrated exploration of these two strategies on a unified ligand platform. A series of Au(I)/BIAN-NHC_X complexes were synthesized by introducing electron-withdrawing substituents (X = F, Cl, Br, CF₃) at the ortho-, meta-, and para-positions of the N-aryl groups. This hybrid tuning approach allowed us to probe the synergistic and antagonistic effects arising from π-σ interplay across multiple dimensions. Each complex was thoroughly characterized with respect to synthetic yield, structural features (including X-ray structure and dynamics in solution), photophysical properties (UV-Vis and luminescence), catalytic performance in hydroamination of phenylacetylene, and cytotoxicity in HEK293T cells. Structure-property correlations revealed distinct cooperative and counteractive regimes depending on the substituent type and position. These findings demonstrate that combining orthogonal electronic control elements enables a more predictive and fine-grained design of NHC-based metal complexes, with future implications for catalysis, materials science, and biomedical applications.