engleski

An unusual self-assembled trimer consisting of two cis- and one trans(1-aminocyclopropane-1- carboxylato)copper(II) molecules. {;;[cis- bis(acc)2Cu][trans-bis(acc)2(H2O)2Cu][cis- bis(acc)2Cu](H2O)2

1-Aminocyclopropane-1-carboxylic acid (acc) is naturally occurring amino acid. It is well known as a precursor of the plant hormone ethylene (ethene), and is also interesting from the stereochemical point of view (modification of the peptide conformation by its insertion, study of stereoselective syntheses etc.). Despite crystal structures of the complexes of its few derivatives are known, the structure of its copper(II) complex(es) was not yet solved, albeit the acc complex with copper(II) was prepared and studied by IR spectroscopy. Stereochemical resemblance of acc to glycine (which copper(II) chelate shows cis-trans transition in crystal state), motivated us to search for the same effect in the copper(II) complex of acc. (Although the kinetic stability of cis and trans isomers of bis(N, O- aminoacidato)copper(II) in aqueous solutions is virtually equal, the crystalization process leads preferably to a single isomer.) The single crystals of copper complex of acc were obtained from water/ethanol solution. Crystal and molecular structures determined by X-ray diffraction showed that solid phase is built up of unusual trimeric (trinuclear) molecular assemblies - {; ; [cis bis(acc)2Cu][trans bis(acc)2(H2O)2Cu][cis bis(acc)2Cu]}; ; (H2O)2 - (I). Thermal properties of the compound were determined by TGA and IR (spectral data were recorded in the range 4000-200 cm− 1). Crystal data. C24H44O16N6Cu3, Mr = 863.334, Monoclinic, P 21/c (No. 14), a = 10.1556(9) Å ; , b = 17.1039(14) Å ; , c = 20.4373(17) Å ; , b = 101.340(7)°, V = 3480, 66(50) Å ; 3, Z = 4, Dc = 1.65 g cm-3, l(MoKa) = 0.71073 Å ; , R = 0.0394, wR = 0.1049 for 5079 [Fo > 4s(Fo)] out of the 6444 reflections and 462 variable parameters. The blue crystals of (I) are built up of trimers consisting of two cis and one trans monomer units. Trans molecule completes fifth coordination sites of the two pyramidal cis molecules through its carboxylate oxygen atoms. Copper(II) ion of the central trans molecule is six-coordinated - two N, O-bidentate acc ligands forms the base of a distorted octahedron, and two water molecules are situated at its apices. Molecular-mechanics calculations show that the close contacts of the chelate rings of trans and cis moieties produce considerable steric influence on the overall conformation of the trimer. Trimer molecules are interlinked by a number of O-H...O and N-H...O hydrogen bonds. Molecules of crystalization water fill the space between the trimer molecules. Each water interlinks three trimers by three O-H...O and one weak N-H...O hydrogen bonds. Molecules of crystalizational water are also bonded to the coordinated water molecules via hydrogen bonds. Each trimer molecule may be considered polar with respect to the mutual position of the amino and carboxylic groups - the amine HB-donor sites and carboxylate HB-acceptors are grouped on opposite sides of the molecule. In this way the maximum number of intermolecular hydrogen bonds is achieved. Conformation of the trimer molecule is additionally stabilized by two intramolecular N-H...O bridges.

amino acids ; copper complexes

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