Copper(II) complexes of piperazine-derived tetradentate ligands and their chiral diazabicyclic analogues for catalytic phenol oxidative C–C coupling

Reaction of the chiral ligands (1S,4S)-2,5-bis(6-methylpyridyl)-diazabicyclo[2.2.1]heptane (L1) and (1S,4S)-2,5-bis(1-methyl-2-methylbenzimidazolyl)-diazabicyclo[2.2.1]heptane (L2) with CuCl2 results in the hydroxo-bridged dicopper complexes [(L1)Cu2(μ-OH)(H2O)Cl3] (3), and [(L2)Cu2(μ-OH)(H2O)Cl3] (4). Both chiral complexes were characterized spectroscopically, and 3 in the solid state by X-ray crystallography, confirming they are structurally related to their previously reported copper acetate analogues (1 and 2) due to their hydroxo-bridged bimetallic core. The achiral ligand analogues N,N′-bis(2-picolyl)piperazine (L3) and N,N′-bis(1-methyl-2-methylbenzimidazolyl)piperazine (L4) were employed to obtain the corresponding complexes with CuCl2, affording the chloro-bridged [(L3)(CuCl)2(μ-Cl)2]n (5) and [(L4)(CuCl)2(μ-Cl)2] (6), neither of which features a bridging hydroxo ligand; instead, complex 5 was structurally characterized as a coordination polymer. The acetate-derived complexes 1 and 2 are active in oxidative C–C coupling of 2,4-di-tert-butylphenol, while 3 and 4 have low activity; the achiral complexes 5 and 6, lacking a bridging hydroxo ligand, are inactive in this reaction.