Comparison\nof Inverse and Regular 2‑Pyridyl-1,2,3-triazole\n“Click” Complexes: Structures, Stability, Electrochemical,\nand Photophysical Properties

Abstract

Two inverse 2-pyridyl-1,2,3-triazole\n“click” ligands, 2-(4-phenyl-1<i>H</i>-1,2,3-triazol-1-yl)­pyridine\nand 2-(4-benzyl-1<i>H</i>-1,2,3-triazol-1-yl)­pyridine, and\ntheir palladium­(II), platinum­(II), rhenium­(I), and ruthenium­(II) complexes\nhave been synthesized in good to excellent yields. The properties\nof these inverse “click” complexes have been compared\nto the isomeric regular compounds using a variety of techniques. X-ray\ncrystallographic analysis shows that the regular and inverse complexes\nare structurally very similar. However, the chemical and physical\nproperties of the isomers are quite different. Ligand exchange studies\nand density functional theory (DFT) calculations indicate that metal\ncomplexes of the regular 2-(1-<b>R</b>-1<i>H</i>-1,2,3-triazol-4-yl)­pyridine\n(<b>R</b> = phenyl, benzyl) ligands are more stable than those\nformed with the inverse 2-(4-<b>R</b>-1<i>H</i>-1,2,3-triazol-1-yl)­pyridine\n(<b>R</b> = phenyl, benzyl) “click” chelators.\nAdditionally, the <i>bis</i>-2,2′-bipyridine (bpy)\nruthenium­(II) complexes of the “click” chelators have\nbeen shown to have short excited state lifetimes, which in the inverse\ntriazole case, resulted in ejection of the 2-pyridyl-1,2,3-triazole\nligand from the complex. Under identical conditions, the isomeric\nregular 2-pyridyl-1,2,3-triazole ruthenium­(II) bpy complexes are photochemically\ninert. The absorption spectra of the inverse rhenium­(I) and platinum­(II)\ncomplexes are red-shifted compared to the regular compounds. It is\nshown that conjugation between the substituent group <b>R</b> and triazolyl unit has a negligible effect on the photophysical\nproperties of the complexes. The inverse rhenium­(I) complexes have\nlarge Stokes shifts, long metal-to-ligand charge transfer (MLCT) excited\nstate lifetimes, and respectable quantum yields which are relatively\nsolvent insensitive.