title: Combined experimental and computational study of cis-trans isomerism in bis(L-valinato)copper(II).
abstract: 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.
authors.affiliation: Institute for Medical Research and Occupational Health, Ksaverska cesta 2, P.O. Box 291, HR-10001 Zagreb, Croatia.
authors.forename: Marijana
authors.initials: M
authors.lastname: Marković
authors.forename: Nenad
authors.initials: N
authors.lastname: Judaš
authors.forename: Jasmina
authors.initials: J
authors.lastname: Sabolović
mesh_terms: D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism; D014633:Valine / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D003300:Copper; D014633:Valine