Anion-mediated bio-relevant catalytic activity of dinuclear nickel(ii) complexes derived from an end-off compartmental ligand

Four dinuclear nickel( II) complexes, namely [ Ni4( L) 2( H2O) 8( mu 2- H2O) 2]( NO3) 6( H2O) 6 ( 1), [ Ni2( L) Cl2( mu- Cl) ( CH3OH)] ( 2), [ Ni2( L)( OAc) 2( H2O) 2] Br ( 3) and [ Ni2( L)( H2O) 4( mu 2- OH)] ( H2O) X( I) X ( 4), have been synthesized using a template synthesis technique by adding nickel( II) salts ( nitrate/ chloride/ bromide/ iodide) to the N4O donor end- off compartmental ligand ( HL) obtained via the condensation of 2-( 2- pyridyl) ethylamine and 2,6- diformyl- 4- isopropyl phenol in methanol. All complexes were characterized with the help of typical physicochemical techniques, and their solid- state structures were assigned from single crystal X- ray analysis. The variable temperature magnetic study reveals that the two nickel centers are antiferromagnetically coupled with J values ranging from - 5 to - 15 cm- 1 in the complexes. The catecholase- like activity of complexes 1- 4 was studied using 3,5- di- tert- butylcatechol ( 3,5- DTBC) as the model substrate in N, N- dimethylformamide ( DMF) medium. Complex 1 shows the catecholase activity, while the other complexes were found to be inactive. The phosphatase- like activity of the complexes was also investigated in a 97.5% ( v/ v) DMF- water mixture using the disodium salt of 4- nitrophenylphosphate ( 4- NPP) as the model substrate and the reactivity trend was 4 > 1 > 3 > 2. The reasons behind the activity, inactivity and activity trend have been explored. It has been assumed that the anions associated with the complexes are supposed to play a crucial role in the whole event. Complex 1 showed catalytic promiscuity, whereas complexes 2, 3 and 4 should be considered only as the potential hydrolytic catalyst.