Proton disorder in a short intramolecular hydrogen bond investigated by single-crystal neutron diffraction at 2.5 and 170 K

Abstract

The substituted acetylacetone 3-[2-(pyridin-4-yl)ethyl]pentane-2,4-dione, C12H15NO2, (1), with an ethylene bridge between the acetylacetone moiety and the heteroaromatic ring, represents an attractive linker for mixed-metal coordination polymers. In the crystal, (1) adopts an antiperiplanar conformation with respect to the C—C bond in the central ethylene group and almost coplanar acetylacetone and pyridyl groups. The ditopic molecule exists as the enol tautomer, with proton disorder in the short intramolecular hydrogen bond. Single-crystal neutron diffraction at 2.5 K indicated site occupancies of 0.602 (17) and 0.398 (17). The geometry of the acetylacetone moiety is in agreement with such a site preference of the bridging hydrogen: the O atom associated with the preferred H-atom site subtends the longer [1.305 (2) Å] and the more carbonyl-like O atom the shorter [1.288 (2) Å] C—O bond. Based on structure-factor calculations for the alternative H-atom sites, reflections particularly sensitive for proton distribution were identified and measured in a second neutron data collection at 170 K. At this temperature, 546 independent neutron intensities were used to refine positional and isotropic displacement parameters for a structure model in which parameters for C and O atoms were constrained to those obtained by single-crystal X-ray diffraction at the same temperature. The site occupancies for the disordered proton do not significantly differ from those at 2.5 K.

Argentophilic Interactions in Two AgI Complexes of 3-(2-(Pyridin-4-yl)ethyl)pentane-2,4-dione, a Promising Ditopic Ligand

“…HacacPyen was synthesized by Domasevitch and coworkers [14] and used as O,O' chelating ligand for Be II ; the original authors also communicated the structure of a bimetallic chain polymer obtained by N coordination of the pyridyl rings in the Be II complex to CdCl 2 . We recently investigated the crystal structure of HacacPyen by temperature-dependent neutron diffraction and confirmed that the uncoordinated organic molecule exists as the enol tautomer [35]. In this study, we report the syntheses and structural characterizations of two bis adducts of HacacPyen to Ag I and put the resulting hexafluorophosphate and tetrafluoroborate salts in the scientific context of argentophilic interactions.…”

“…These ligands adopt slightly different conformations: the torsion angles between the central C of the acetylacetone moieties, the C atoms of the ethylene bridge and the para C in the pyridine ring amount to 155.3(4) • (C3-C6-C7-C8) and −173.6(4) • (C15-C18-C19-C20). In contrast to uncoordinated HacacPyen [35] both ligands exist as diketo tautomers, with short C --O double and long C -C single bonds (Table 1) We note that the enol tautomer is more popular for acetylacetones with N donor substitutents in 3-position ( [34] and references cited therein.). In addition to the almost linear coordination by the two symmetrically independent pyridine N donor atoms, each Ag I cation is engaged in short contacts to the keto oxygen atoms of a neighboring HacacPyen and to a Ag I cation of an inversion-related bis(HacacPyen) silver complex ( Figure 2).…”

Coordination of a Potentially Tritopic Ligand via Phosphorus, Nitrogen and Oxygen: From Ligand Design to Heterobimetallic Metal–Organic Frameworks and Supramolecules

“…This characteristic of β-diketo moieties in the solid state has been described previously. 53 This delocalization also allows a conjugation with the two adjacent phenylene rings; thus, a coplanar arrangement should be the electronically favored state. In reality the phenylene rings are slightly tilted with respect to the β-diketo moiety, as reflected in the respective torsion angles ω.…”

Combined X-ray and neutron single-crystal diffraction in diamond anvil cells

“…X-ray and neutron diffraction are complementary experimental techniques for detailed studies of crystalline materials. Neutron diffraction is indispensable when X-ray diffraction fails to probe, for instance, magnetic (dis)order (Devi et al, 2019), compounds containing light elements (Truong et al, 2018) or (dis)ordering of elements over crystallographic sites (Hering et al, 2015). Because of the complementarity of X-ray and neutron data, their joint use allows the study of compounds in which magnetic order has a direct influence on the underlying crystal structure or vice versa, e.g.…”

“…It provides precise information on crystal and magnetic structures, including the reliable and accurate characterization of anisotropic displacement parameters in materials with highly absorbing elements. Typical investigations at HEiDi focus on (i) ionic conductors relevant for energy applications and data storage (Ceretti et al, 2018), (ii) superconductors (Jin et al, 2016), (iii) multiferroic materials (Regnat et al, 2018), (iv) small-molecule structures, in which hydrogen bridges play a key role as a structure building element (Truong et al, 2017(Truong et al, , 2018, or (v) complex zeolitic crystal structures (Gatta et al, 2018).…”