Effect of Incremental Preorganization in Bis-triazole-substituted Phenanthroline Ligands on Eu(III) Coordination: Spectroscopy, X-ray Crystallography, and DFT Studies

Bis-triazole-substituted polypyridyl ligands have attracted considerable attention in coordination chemistry due to their excellent performance. The preorganization of the ligand backbone and side chains plays a crucial role in coordination behavior, with rigid phenanthroline backbones offering distinct structural advantages. However, the coordination behavior of bis-triazole-substituted 1,10-phenanthroline ligands with different preorganization levels toward f-block elements remains ambiguous. In this study, three ligands with incremental preorganization levels─2,9-bis(1-(2-ethylhexyl)-1H-1,2,3-triazol-4-yl)-1,10-phenanthroline (<b>L1</b>), 3-(2-ethylhexyl)-11-(1-(2-ethylhexyl)-1H-1,2,3-triazol-4-yl)-4,5-dihydro-3H-[1,2,3]triazolo[4″,5':5,6]-benzo[1,2-<i>b</i>][1,10]phenanthroline (<b>L2</b>), and 3,12-bis(2-ethylhexyl)-3,4,5,10,11,12-hexahydro-[1,2,3]triazolo[4″,5':5,6]benzo[1,2-<i>b</i>][1,2,3]triazolo[5″,4″:3,4]benzo[1,2-j][1,10]phenanthroline (<b>L3</b>)─were synthesized <i>via</i> the Friedländer reaction and systematically characterized using ¹H NMR titration, ESI-MS, UV-vis spectroscopy titration, and DFT calculations. Complexation studies revealed that <b>L1</b> and <b>L2</b> formed 1:1 and 2:1 complexes with Eu(III), and <b>L3</b> formed a 1:1 complex. Stability constants for the first stage (log β₁) followed the order: <b>L2</b> > <b>L3</b> > <b>L1</b>, indicating that preorganization enhanced the ligand's affinity for Eu(III). However, the ligand with unilateral preorganization exhibited the strongest affinity for Eu(III), rather than with bilateral preorganization. Single-crystal X-ray diffraction analysis of benzene-substituted derivatives (<b>L1-Ben</b>, <b>L2-Ben</b>, and <b>L3-Ben</b>) highlighted the role of triazole conformational rotation and torsional strain in complex stability. The DFT calculations offered deeper insight into the experimental findings. These results provide valuable insights for designing highly preorganized ligands in coordination chemistry.