From coordination to protection: 1,10-phenanthroline-based Cu(II) complexes against metal corrosion

A new one-dimensional thiocyanato bridged Cu(II) polynuclear coordination polymer containing 1,10-phenanthroline (phen), [Cu(phen)(?-SCN)(?-NCS)]n (1), has been synthesized. Complex (1) has been characterized by elemental analyses and spectroscopic techniques. The X-ray single crystal structure of (1) shows the formation of polymeric chains of thiocyanate bridges. In the polymer, the CuII ion in an octahedral environment, coordinated by one phen and four thiocyanato ligands. Two of the thiocyanato ligands are N-bonded and the other two Sbonded forming a CuN4S2 core. Whereas the N atoms have almost linear Cu?N?C angles (164.4?), the S atoms are bonded with a Cu?S?C angle of 96.80?. All thiocyanato ligands are bridging Cu ions into one-dimensional chains. The lattice of (1), shows polymers connected through C?H?S, offset-face-to-face ??? stacking interactions and S?S interactions consolidating a 3D structure. The supramolecular structures of bridged dimeric complexes bis[(?2-chloro)chloro(phen)copper(II)] (2) and bis[(?2-azido)-chloro-(phen)?copper(II)] (3) were also briefly discussed. Comparative studies with the dimeric analogues (2) and (3) reveal that polymer (1) exhibits enhanced thermal stability, delaying phenanthroline release to higher temperature. Electrochemical impedance and potentiodynamic polarization measurements show all three complexes function as mixed-type corrosion inhibitors with predominant cathodic behaviour for C-steel in 0.25 M H2SO4. At optimal dosages, (1) achieves 73.3 % inhibition at 100 ppm, while (2) and (3) reach 95.1 % and 91.2 % at 120 ppm, respectively. Weight loss experiments confirmed the decrease in corrosion rates when the complexes were added to the blank 1 M HCl. The free energies calculated from Langmuir adsorption isotherms and SEM analysis confirm the formation of protective precipitates on the steel surface. These results highlight the critical influence of molecular topology on anticorrosion performance and identify the dimeric structures as particularly promising for steel protection