Binding of polyatomic anions with protonated ureido-pyridyl ligands

Abstract

Two ureido-pyridyl ligands, 1-(4-nitro-phenyl)-3-pyridine-3-ylmethyl-urea (L¹) and 1-pentafluorophenyl-3-pyridine-4-yl-urea (L²) were synthesized in good yields, and solid-state anion binding studies of (HL¹)⁺ and (HL²)⁺ with polyatomic anions such as NO₃⁻, AcO⁻, ClO₄⁻, SO₄²⁻, and SiF₆²⁻ were carried out in detail. Protonation of the pyridyl nitrogen center of L¹ with HNO₃, HClO₄, and HF in different solvent media yielded crystals of complexes 1 (HL¹·NO₃), 2 (HL¹·ClO₄), and 3 (HL¹·0.5SiF₆) suitable for single crystal X-ray diffraction studies, respectively. Similarly, protonation of L² with HNO₃, CH₃COOH, and H₂SO₄ yielded single crystals of complexes 4 [2(HL²·NO₃)·DMF], 5 (HL²·AcO), and 6 [(HL²)₂·SO₄], respectively. X-ray crystallographic analysis of all these six complexes has been carried out and the details of anion binding with the urea functionality of ureido-pyridyl ligands were investigated. In all the complexes, the pyridyl moiety of the ligands is found to be protonated and the binding of anions are observed at the urea functionality of the ligands through the R₂²(8) hydrogen bonding motif irrespective of sizes and shapes of anions or the electron withdrawing ability of aryl substitutions (p-nitro phenyl vs pentafluoro phenyl) or the position of the pyridyl nitrogen center (protonation site) of the designed ligands.