The influence of selected transition metal ions on the structure, thermal and microbiological properties of pyrazine-2-carboxylic acid

Abstract Pyrazine-2-carboxylic acid (2-PC) and 3d transition metal pyrazine-2-carboxylates (2-PCA) were characterized by means of X-ray, spectroscopic (Raman, FTIR) and thermogravimetric analysis. X-ray diffraction data were collected at 100 K and the crystal structures were solved and refined. The general formula of the copper(II), manganese(II), nickel(II), zinc(II) and cobalt(II) pyrazine-2-carboxylates was [M(2-PCA)2(H2O)2] (where M is a metal). The cobalt(II), nickel(II), copper(II) and zinc(II) complexes crystallize in the space group P21/c, in the monoclinic crystal system The manganese(II) complex crystallizes in the space group F2dd in the orthorhombic crystal system. The coordination sphere around the metal cations can be described as octahedral, however in the case of the Cu(II) complex, the octahedral geometry is distorted in the axial direction, which is caused by the Jahn–Teller effect. The products of dehydration and decomposition processes of the studied compounds were determined from the TG (thermogravimetric) and DSC (differential scanning calorimetry) curves. The theoretical structures and the IR spectra of pyrazine-2-carboxylic acid and the manganese(II) complex were calculated at the B3LYP/6-311++G(d,p) level of theory. The aromaticity indices (HOMA, I6) were calculated for 2-PC and [M(2-PCA)2(H2O)2]2 on the basis of the theoretical bond lengths in the aromatic ring. The influence of the 3d-metals on the aromatic system of pyrazine-2-carboxylic acid is discussed. 2-PC acid complexes with transition 3d-metals show a very weak antimicrobial effect, as no inhibition of bacterial and fungal growth was noted up to 10 mg/mL.