Structural Features of Two Pyridyl Compounds of 1,5-Bis-(2′-pyridyl)pentane-1,3,5-trione and a New Salt of Doubly Protonated Hydroxyterpyridinium

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 ¹H and ¹³C 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 P2₁/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 [H₂(tpyOH)]Cl₂·H₂O (tpyOH = 4′-hydroxy-2,2′:6′,2″-terpyridine) (2) obtained during the reaction of tpyOH in the presence of CrCl₃·6H₂O as a Lewis acid in methanol rather than complexation. The crystal structure of [H₂(tpyOH)]Cl₂·H₂O (2) is triclinic and crystallizes in the space group P$\bar {1}$ 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 [H₂(tpyOH)]²⁺ cation. The packing of [H₂tpyOH]Cl₂·H₂O 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 [H₂tpyOH]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.