B. Momeni, Nastaran Fathi, F. Rahimi
Feb 22, 2019
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Journal
Journal of Chemical Crystallography
Abstract
Abstract A keto-enol equilibrium between two tautomers of the 1,5-bis(2′-pyridyl)pentane-1,3,5-trione (trione) ( 1 ) was observed in solution based on 1 H and 13 C NMR spectroscopy. NMR data shows that the trione exists as a mixture of mono-(minor product) and bis-enolic (major product) forms in solution. However, the crystal structure of trione ( 1 ) indicates that the bis-enolic form is the predominant species in the solid state, which is probably due to the C=O⋯H–O intramolecular hydrogen bonding. The crystal structure of 1 was determined by X-ray diffraction analysis. Trione ( 1 ) is monoclinic and crystallizes in the space group P2 1 /n with a = 3.7449(3) Å, b = 29.594(3) Å, c = 12.5148(12) Å, ß = 92.701(2)°. Notably, the chloride salt of doubly protonated 4′-hydroxy-2,2′:6′,2″-terpyridinium [H 2 (tpyOH)]Cl 2 ·H 2 O (tpyOH = 4′-hydroxy-2,2′:6′,2″-terpyridine) ( 2 ) obtained during the reaction of tpyOH in the presence of CrCl 3 ·6H 2 O as a Lewis acid in methanol rather than complexation. The crystal structure of [H 2 (tpyOH)]Cl 2 ·H 2 O ( 2 ) is triclinic and crystallizes in the space group P $$\bar {1}$$ 1 ¯ with a = 5.1169(15) Å, b = 12.288(4) Å, c = 13.212(4) Å, α = 109.667(7)°, ß = 100.321(7)°, γ = 94.111(7)°. The crystal structure of 2 reveals the protonation of two nitrogen atoms of outer pyridines in tpyOH to form the [H 2 (tpyOH)] 2+ cation. The packing of [H 2 tpyOH]Cl 2 ·H 2 O features weak interactions including three N–H⋯Cl, O–H⋯Cl and O–H⋯O hydrogen bonds and extensive hydrogen bonds due to the cis–cis conformation of the doubly protonated salt of [H 2 tpyOH] 2+ cation. Graphical Abstract Crystal structures of two pyridyl compounds of 1,5-bis-(2′-pyridyl)pentane-1,3,5-trione and doubly protonated 4′-hydroxy-2,2′:6′,2″-terpyridinium are reported which reveal the presence of hydrogen bonding interactions.