engleski

Why does anhydrous bis(L-valinato)copper(II) crystallise as a trans-isomer? Molecular mechanics calculations in simulated crystal lattice

Bis(amino acidato)copper(II) complexes are biologically important compounds. They take part in transporting copper to copper enzymes (such as superoxide dismutase, cytochrome c oxidase and nitrous-oxide reductase) and electron-transfer copper proteins (e.g. plastocyanin, azurin). The X-ray crystal structures are the most common source of experimental information on the structural properties of this class of compounds. Bis(L-valinato)copper(II), Cu(L-Val)2, after being heated in a dryer for 24 hours at 80 oC crystallised from aqueous solution as anhydrous trans-isomer (space group P1) as determined by the X-ray diffraction measurements [1]. If not heated, it crystallised as a cis-isomer with one water molecule in the asymmetric unit [1]. The coordination geometry of the copper(II) in the anhydrous modification is a distorted planar. The crystal structure contains polymeric chains that are made up of complex dimers that are formed by interlinkage of adjacent molecules via axial copper(II)-to-carbonyl oxygen contacts, and are further stabilised by N– H• • • O hydrogen bonds. Molecular mechanics force field FFW [2], developed for studying the properties of anhydrous and aqua bis(amino acidato)copper(II) complexes with either cis- or trans-N2O2 copper(II) coordination geometry in the solid state and in vacuo, was used for conformational analysis of the title compound. Each chelate ring of Cu(L-Val)2 can have 6 conformations, with C in 3 axial and 3 equatorial positions, and therefore the molecule can have 21 trans and 21 cis conformations. Conformational analysis in vacuo, without the influence of the intermolecular interactions, showed that the trans conformers were more stable than the cis ones (by ≈ 21.3 kcal mol-1). To account the crystal lattice effects, the geometry optimisations of all possible conformers were attained by taking the experimental molecule orientation, unit cell lengths and angles, as well as the P1 space group symmetry operations as the starting input data. During the energy minimisation of a crystal all degrees of freedom were allowed to vary. The calculations of the unit cell packings and intermolecular interactions for the series of conformers suggest the reasons why experimentally obtained conformer occurs in the real crystal structure. [1] Marković, M. ; Judaš, N. ; Sabolović, J. Humboldt Conference on Noncovalent Interactions, Vršac, Serbia 2007 ; Book of abstracts, p. 58 [2.] Sabolović, J. ; Tautermann, C. S. ; Loerting, T. ; Liedl, K. R. Inorg. Chem. 47 (2003) 2268-2279.

copper; amino acids; crystal structure; MM crystal simulation

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