Hydroxide-bridged dicopper complexes: the influence of secondary coordination sphere on structure and catecholase activity

Amide-based ligands (H2L1-6) with assorted functional groups appended to them have been used for the synthesis of dicopper(II) complexes 1-6 having a Cu(mu-OH) Cu core. The crystal structures of 1-6 show that while every Cu(II) ion is ligated within the N-3 pincer cavity of a potentially multidentate ligand, two Cu(II) centers are bridged by a hydroxide group. Notably, the Cu(mu-OH) Cu core is encased within the secondary coordination sphere intricately created by the appended groups. While complexes 1 and 2 exhibit the presence of an H-bond acceptor in the proximity of the Cu(mu-OH) Cu core, complexes 3 and 4 display the occurrence of both the H-bond donor as well as H-bond acceptor groups in the vicinity of the Cu(mu-OH) Cu core. In contrast, complexes 5 and 6 present modified secondary coordination spheres around the Cu(mu-OH) Cu core with limited H-bonding interacting groups in 5 and no such groups in 6. We show that the extent of H-bonding by the appended groups modulates not only the Cu-OH bond distance, Cu(mu-OH) Cu angle and Cu-Cu separation but also the Cu-2 broken vertical bar/Cu-broken vertical bar redox potential. All six complexes were utilized for their ability to oxidize 3,5-di-tert-butylcatechol, and the catecholase activity results have been correlated to the secondary coordination sphere created by the appended groups in all six complexes.