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.

The Important Role of the Hydroxyl Group on the Conformational Adaptability in Bis(l-threoninato)copper(II) Compared to Bis(l-allo-threoninato)copper(II): Quantum Chemical Study

“…49 The B3LYP/BS0 energy difference between the most stable cis and trans G-G conformers is 43.7−43.8 kJ mol −1 for both complexes. The result that in the gas phase the G-G mode trans conformers are more stable than the cis conformers is in accord with previous DFT/B3LYP 23,[30][31][32]39 and molecular mechanics calculations 30,31,50,51 for several other Cu(aa) 2 complexes. For instance, the B3LYP/BS0 energy difference between the cis and trans minima of Cu(Gly) 2 is 56.7 kJ mol −1 .…”

Automated exploration of the low-energy chemical space with fast quantum chemical methods

“…Conformer CONF23 is not populated in the gas-phase but closely resembles the experimentally observed conformation in the crystal structure. 350 This indicates its stabilization in the solid phase due to packing (or other related) effects.…”

Conformational Analyses of Physiological Binary and Ternary Copper(II) Complexes with l‐Asparagine and l‐Histidine; Study of Tridentate Binding of Copper(II) in Aqueous Solution

“…Specifically, Δ V between the lowest‐energy tNONO and cNONO conformers dropped from 35.0 kJ mol −1 (Δ V vacuum ) to 4.4 kJ mol −1 (Δ V water ). This seems to be a common characteristic for trans and cis glycinato‐mode Cu(aa) 2 conformers, that they have similar stability in aqueous solution at room temperature while the trans conformers are more stable as isolated compounds …”