Crystal Structures and Corrosion Inhibitions of Three Cobalt Complexes of Terephthalate Anion and Bis-Nitrogen Donor Ligands

Three Co(II) complexes of terephthalate anion containing 1,10-phenanthroline (phen) or 2,2 '-bipyridine (2,2 '-bipy), are prepared. The structures of dimeric [Co-2(H2O)(4)(mu-H2O)(2)(phen)(2)](4+)<middle dot>2(C8H4O42-), 1, and the polymeric [Co-2(H2O)(2)(phen)(2)(mu-C8H4O4)(2)](n), 2, complexes were confirmed by single crystal structure determination. Both complexes have CoN2O4 core in a distorted octahedral arrangement. Extensive hydrogen bonding and pi-pi stacking interactions consolidate 3-D structures in 1 and 2. Complex 3, [Co(mu-C8H4O4)(2,2 '-bipy)](n), was previously reported. It has CoN2O4 core in a distorted trigonal prismatic environment. Complexes 1, 2 and 3 exhibited good inhibition efficiencies for corrosion of carbon steel in 0.25-M sulfuric acid solution. The order of corrosion inhibiting effect of the complexes was 2 (94.6%) > 1 (90.5%) > 3 (79.5%). The increase in R-p value and the decrease in C-dl were in parallel with the increase in the concentration of complexes. Tafel plot indicated that complex 1 behaved as a cathodic-type corrosion inhibitor, whereas complexes 2 and 3 as anodic inhibitors for C-steel in sulfuric acid medium. Weight loss measurement of steel samples in 1-M HCl showed more inhibition of corrosion by complexes 1 and 2 than by complex 3. The adsorption mechanism of inhibitors on C-steel followed Langmuir isotherm. The free energy changes indicated comprehensive physical and chemical adsorption for the three complexes on the surface of the C-steel. SEM analysis of steel samples in 1-M HCl proved the efficiency of complexes in retarding corrosion as the steel surface showed smoothness compared to the blank. Quantum chemical DFT study declared that the highest corrosion inhibition efficiency is observed for complex 2, which was strongly supported by the electrochemical anticorrosion studies. The results of the TGA analysis support the X-ray crystal structures of the complexes.