A series of cocrystals involving theophylline and fluorobenzoic acids highlights the difficulty of predicting supramolecular interactions in molecular crystals.

No abstract

We report a metal-organic design that allows decoration and cross-linking of metal centres through halogen-bond-directed co-crystallisation. Halogen bonds have emerged as reliable and directional interactions, alternative to hydrogen bonds, in supramolecular engineering of organic solids. [1] Halogen bonds that form between a polarisable and electron-deficient halogen atom (the halogen-bond donor) and an electron-rich Lewis base (the halogen-bond acceptor) [2] have been utilised to construct co-crystals, [3] salts, [4] ionic inclusion frameworks [5] and liquid crystals. [6] An attractive target in halogen-bond-based materials engineering is the inclusion of transition metals, which are expected to introduce interesting magnetic, electrical and optical properties. So far, formation of halogen-bonded co-crystals with metal-organic species has proven difficult. [7,8] Although the self-assembly of metal-organic molecules with self-complementary halogen-bonding functionalities was demonstrated, [9] attempts to construct multicomponent molecular materials have not been successful.[10] This difficulty was explained by competition with other non-covalent interactions, such as hydrogen bonds [11] and halogen-halogen interactions. In our search for a suitable strategy to decorate metal-organic complexes using halogen bonds we sought to achieve the highest possible generality. Thus, we were interested in a modular [12] approach that would allow adding different halogen-bonding functionalities onto a given metal-organic fragment. We recognised planar complexes of divalent metals with dibenzoylmethanate anions (dbm À ) as suitable candidates. In particular, nickel(II) and cobalt(II) dibenzoylA C H T U N G T R E N N U N G meth- A C H T U N G T R E N N U N G anates ([NiA C H T U N G T R E N N U N G (dbm) 2 ] and [CoA C H T U N G T R E N N U N G (dbm)2], Scheme 1 a) provide a coordinatively unsaturated and equatorially protected platform onto which a molecule with a halogen-bonding functionality could be attached. We targeted the ditopic molecules morpholine (mor) and thiomorpholine (tmo) as suitable ligands [13] that would attach through Co À N (or Ni À N) bonds to form coordinatively saturated complexes:

Heating of polycrystalline cis aquabis(L-valinato)copper(II) at 90 °C resulted in a dehydrated powder. Recrystallization from aqueous solution of the obtained product yielded anhydrous trans bis(L-valinato)copper(II). The X-ray crystal and molecular structures of trans bis(L-valinato)copper(II) and cis aquabis(L-valinato)copper(II) are presented. Molecular modeling calculations were attempted to resolve factors that influenced the isomerization and crystallization of either the aqua cis- or the anhydrous trans-isomer. Conformational analyses of trans- and cis-isomers were completed in vacuo and in crystal by molecular mechanics, and in aqueous solution by molecular dynamics (MD) simulations using the same force field. Although the conformers with trans-configuration are the most stable in vacuo, those with cis-configuration form more favorable intermolecular interactions. Consequently, both cis- and trans-isomers are predicted to be present in aqueous solution. According to the crystal structure simulations and predictions, cis-isomer requires water molecules to form energetically more stable crystal packings than trans-isomer. The MD modeling of the self-assembly of 16 bis(L-valinato)copper(II) complexes in aqueous solution for the first time predicted the crystallization nucleus formation to proceed from monomers to oligomers by Cu-to-O(carboxylato) and/or N-H···O(carboxylato) weak bonds; these oligomers then bind together via water molecules until they acquire the right positions for noncovalent bonding like in the experimental crystal structures. Fifty-nanosecond MD simulations accomplished for a system consisting of equal numbers of complexes and water molecules at 298 and 370 K suggested complete cis-to-trans transformation at the higher temperature. Prevalence of either cis- or trans-conformers in water upon dissolvation may explain the crystallization results.

A copper(II) complex with 1-aminocyclopropane-1-carboxylic acid assembles by apical Cu...O bonds and hydrogen-bonding interactions into discrete trimeric units that exhibit both cis and trans binding modes.

1,5-Bis(salicylidene)carbohydrazide, prepared by a condensation reaction of salicylaldehyde and carbohydrazide, afforded a diversity of crystalline forms depending on crystallization conditions: three polymorphs (I, II, and III) and two solvates (IV and V). All solid phases comprise the same tautomer of the title compound, which bears the salicylidene residues in the enol-imino form and the carbamide fragment in the keto form, as established via X-ray crystallography, IR spectroscopy, and cross-polarization magic-angle spinning (CP MAS) NMR ( 13 C and 1 H). Conformations of 1,5-bis(salicylidene)carbohydrazide molecules and their hydrogen-bonding patterns are similar for two polymorphs (II and III), while the third one (I) in this context differs significantly. Thermal analyses revealed that all phases undergo complicated degradation upon heating, yielding salicylideneazine, CO 2 , and N 2 either directly or via intermediates. Relative phase stability was inspected mechanochemically and by solvent-mediated experiments, establishing polymorph II as the stable one at ambient conditions. NMR and UV−visible solution studies revealed that the same tautomer of 1,5bis(salicylidene)carbohydrazide present in the solid state is also the dominant one in solution.

Key indicatorsSingle-crystal X-ray study T = 293 K Mean (C-C) = 0.004 Å R factor = 0.030 wR factor = 0.088 Data-to-parameter ratio = 17.0For details of how these key indicators were automatically derived from the article, see

The mol­ecule of 2‐benzyl‐1,3‐diphenyl­propane‐1,3‐dione, C22H18O2, being a diketo tautomer, shows different stereochemistry from that of the closed‐ring cis‐ketoenol tautomer of the parent 1,3‐diphenyl­propane‐1,3‐dione. The mol­ecular packing involves C—H⋯O and π–π stacking inter­actions.