Variable Temperature Neutron Diffraction and X-Ray Charge Density Studies of Tetraacetylethane

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

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.

New homodinuclear tris(3-alkylpyrazolyl)borate complexes of CoII and NiII with a tetraacetylethane dianion as a bridging ligand

Polynuclear complexes and coordination polymers of 3d metals have attracted significant interest evoked by a number of their unique properties. One of the most common approaches to the directed synthesis of coordination polymers is the linking of pre-prepared discrete coordination units by polydentate ligands. The formation of polynuclear complexes is usually a spontaneous process and precise prediction of the products of such reactions is virtually impossible in most cases. Tris(pyrazolyl)borates (Tp) act as tripodal `capping' ligands which form stable complexes with 3d metal ions. In such 1:1 compounds, three metal-ion coordination sites are occupied by N atoms from a Tp anion. This limits the number of remaining coordination sites, and thus the number of additional ligands which may coordinate, and opens an attractive approach for the directed design of desirable structures by exploiting ligands with appropriate composition and topology. In the present study, Tp anions with neopentyl [Tp<sup>Np</sup>, tris(3-neopentylpyrazolyl)borate] and cyclohexyl [Tp<sup>Cy</sup>, tris(3-cyclohexylpyrazolyl)borate] substituents were used as `capping' ligands and the dianion of tetraacetylethane (3,4-diacetylhexa-2,4-diene-2,5-diolate, tae^2-) was employed as a bridge. The dinuclear complexes (μ-3,4-diacetylhexa-2,4-diene-2,5-diolato-κ⁴O²,O³:O⁴,O⁵)bis{[tris(3-cyclohexyl-1H-pyrazol-1-yl-κN²)borato]cobalt(II)} acetonitrile disolvate, [Co₂(C₂₇H₄₀BN₆)₂(C₁₀H₁₂O₄)]·2CH₃CN, (I)·2CH₃CN, and (μ-3,4-diacetylhexa-2,4-diene-2,5-diolato-κ⁴O²,O³:O⁴,O⁵)bis{[tris(3-neopentyl-1H-pyrazol-1-yl-κN²)borato]nickel(II)}, [Ni₂(C₂₄H₄₀BN₆)₂(C₁₀H₁₂O₄)], (II), were synthesized by the reaction of the mononuclear complexes Tp^CyCoCl or Tp^NpNiCl with H₂tae (3,4-diacetylhexane-2,5-dione or tetraacetylethane) in the presence of NEt₃ as base. Compounds (I) and (II) were characterized by mass spectrometry, elemental analysis, and X-ray crystallography. They possess similar molecular structures, X-ray diffraction revealing them to be dinuclear in nature and composed of discrete Tp-M units in which two metal ions are linked by a tae^2- dianion. Each metal ion possesses a five-coordinate square-pyramidal environment. The interplanar angles between the acetylacetonate fragments are significantly smaller than the near-90° values commonly observed.

Advances in understanding of chemical bonding: inputs from experimental and theoretical charge density analysis

The development of charge density analysis has undergone a major renaissance in the last two decades. In recent years, the characterization of bonding features associated with atoms in molecules and in crystals has been explored using high-resolution X-ray diffraction data (laboratory or synchrotron) complemented by high level ab initio theoretical calculations. The extraction of one electron topological properties, namely, electrostatic charges, dipole moment and higher moments, electrostatic potential, electric field gradients, in addition to evaluation of the local kinetic and potential energy densities, have contributed toward an understanding of the electron density distributions in molecular solids. New topological descriptors, namely, the source function (SF) and electron localization function (ELF) provide additional information as regards characterization of the topology of the electron density. In addition, delocalization indices have also been developed to account for bonding features pertinent to M-M bonds. The evaluation of these properties have contributed significantly toward the understanding of intra- and intermolecular bonding features in organic, inorganic, and biomolecules in the crystalline phase, with concomitant applications in the understanding of chemical reactivity and material/biological properties. In recent years, the focus has strongly shifted toward the understanding of structure-property relationships in organometallic complexes containing labile M-C bonds in the crystal structure with subsequent implications in catalysis. This perspective aims to highlight the major developments in electron density measurements in the past few years and provides pointers directed toward the potential use of this technique in future applications for an improved understanding of chemical bonding in systems that have been unexplored.

Directionality of inter- and intramolecular OHO hydrogen bonds: DFT study followed by AIM and NBO analysis

The directionality of inter- and intramolecular OHO hydrogen bonds has been compared. For intramolecular bridges it is determined by an orbital formed in the proton transfer process. For intermolecular bonds, the hydrogen-bonded proton is attached to two lone pairs of the acceptor and the OHO angle is not fixed but can change in a broad range. Depending on the OHO angle, the interaction changes continuously from electrostatic interaction to strong OHO hydrogen bond.

Methyl dynamics flattens barrier to proton transfer in crystalline tetraacetylethane

We analyze the interplay between proton transfer in the hydrogen-bond bridge, O···H···O, and lattice dynamics in the model system tetraacetylethane (TAE) (CH(3)CO)(2)CH═CH(COCH(3))(2) using density functional theory. Lattice dynamics calculations and molecular dynamics simulations are validated against neutron scattering data. Hindrance to the cooperative reorientation of neighboring methyl groups at low temperatures gives a preferred O atom for the bridging proton. The amplitude of methyl torsions becomes larger with increasing temperature, so that the free-energy minimum for the proton becomes flat over 0.2 Å. For the isolated molecule, however, we show an almost temperature-independent symmetric double-well potential persists. This difference arises from the much higher barriers to methyl torsion in the crystal that make the region of torsional phase space that is most crucial for symmetrization poorly accessible. Consequently, the proton-transfer potential remains asymmetric though flat at the base, even at room temperature in the solid.

The whole range of hydrogen bonds in one crystal structure: neutron diffraction and charge-density studies of N,N-dimethylbiguanidinium bis(hydrogensquarate)

N,N-Dimethylbiguanidinium bis(hydrogensquarate) features an impressive range of hydrogen bonds within the same crystal structure: neighbouring anions aggregate to a dianionic pair through two strong O—H...O interactions; one of these can be classified among the shortest hydrogen bonds ever studied. Cations and anions in this organic salt further interact via conventional N—H...O and nonclassical C—H...O contacts to an extended structure. As all these interactions occur in the same sample, the title compound is particularly suitable to monitor even subtle trends in hydrogen bonds. Neutron and high-resolution X-ray diffraction experiments have enabled us to determine the electron density precisely and to address its properties with an emphasis on the nature of the X—H...O interactions. Sensitive criteria such as the Laplacian of the electron density and energy densities in the bond-critical points reveal the incipient covalent character of the shortest O—H...O bond. These findings are in agreement with the precise geometry from neutron diffraction: the shortest hydrogen bond is also significantly more symmetric than the longer interactions.