Síntese e caracterização do cristal L-valinato de cobre (II) monohidratado

Abstract

The study of bioinorganic molecules draws more and more attention from the scientific community regarding its industrial and biotechnological applications. Organic crystals of amino acids have interesting biological properties. However, odd features of these crystals can be potentiated when complexed with transition metals. Thus, this work seeks to synthesize and characterize L-Valine hybrid crystals complexed with Copper (II) in order to verify its structural, vibrational and thermal properties. For the synthesis of the cis-LVCu crystal, the technique of slow evaporation of the solvent in an equimolar ratio at pH = 8 was used. The crystal was characterized using X-ray diffraction together with the Rietveld method, FTIR and Raman spectroscopy, thermal analysis (TG/DTA and DSC), as well as X-ray and Raman diffraction as a function of temperature. The obtained crystal is crystallized in monoclinic structure with four molecules per unit cell presenting network parameters a = 21,371(4), b = 9,565(2), c = 7,423(1) Å and angle β = 108,752(3)° stabilized by four hydrogen bonds. FTIR analysis suggests the complexation of the amino acid to the metal since the bands referring to the NH3+ group disappeared and there is a shift of carbonyl vibration (C=O) to larger wave numbers. Raman spectra show the vibrations related to the metal bound to the amine and carboxyl groups of the amino acid, highlighting the band shifts to higher wave numbers of the vibrations referring to the amino acid suggesting a compression of the unit cell. Raman analysis also exhibits characteristic vibrations of the amine group (NH2) and water molecule (H2O). The thermal curves obtained in TG / DTA and DSC show that the crystal loses water of coordinates between 40-70 °C and decomposes after 180 °C. The X-ray diffraction pattern as a function of temperature shows that the crystal undergoes phase transformation after 70 °C, where the complete transformation occurs after temperature of 110 ºC to form trans-LVCu. This new triclinic structure has network parameters a = 4,907(4), b = 5,245(2), c = 12,231(1) Å and angle α = 79,416(6), β = 108,752(3) e γ = 79,367(3)°, without phase reversibility. The diffractogram shows partial defects of the material after 80 °C. The Raman measurement as a function of temperature agrees with the DXR analysis as a function of temperature due to the widening of bands due to partial defects, in addition to showing offsets of the external modes between 67-87 ºC suggesting phase transformation by the loss of water. According characteristics observed, the cis-LVCu crystal has low thermal stability, transiting to a structural form trans-LVCu that is more stable at room temperature and angle.