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.