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.
The X-ray crystal and molecular structure of a new polymorph
of cis-aquabis(l-isoleucinato)copper(II),
obtained
by recrystallization from an acetic acid–water mixture and
determined at 120 and 295 K, revealed triple dynamic disorder over
one isoleucinato ligand at both temperatures. The complicated multipart
disorder was resolved with the help of extensive computational crystal
structure simulations, which were undertaken for the first time to
interpret disorder in a crystal of bioinorganic compound. The new
polymorph (space group C2) is conformationally polymorphic
with the already known P212121 crystal form. To discover the conformers that can participate
in self-associations in solution, and to rationalize an interplay
of intramolecular and intermolecular interactions in the crystallization
of different conformers, conformational analyses of cis and trans isomers were performed using the same
force field in vacuo, in aqueous solution, and for selected conformers
in P212121 and C2 crystals. Three conformers identified in the disorder
were estimated to form the most favorable intermolecular interactions
in the solid state, and one of them (the most populated conformer
in the disorder) also in aqueous solution. The crystal structure reproduction
of all possible arrangements of the three conformers in C2 unit cell helped to find the most plausible crystal packing motif.
The solid‐state 13C and 2H NMR spectra of paramagnetic anhydrous trans‐bis(L‐valinato)copper(II) and cis‐aquabis(L‐valinato)copper(II) complexes have been obtained. Under the very fast MAS conditions, both the 13C and 2H MAS spectra were well enough resolved to allow the easy distinction between the trans and cis stereoisomers. The conformational disorder observed previously in the X‐ray structure of the cis isomer was also reflected in the 13C and 2H MAS spectra. Variable‐temperature 2H MAS spectra of differently deuterated ligand species, that is, the copper(II) complexes with L‐[D2]valine, L‐[D8]valine, and L‐[D10]valine, suggested the dynamic nature of this disorder in the aqua cis isomer and confirmed static ND2 deuterons in the anhydrous trans isomer. Quantum chemical DFT/B3LYP calculations of the 13C hyperfine (Fermi contact) shifts of the paramagnetic term were useful as assignment aids in the interpretation of the 13C MAS spectra.
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