Bi- and trinuclear copper(II) complexes of a sterically constrained phenol-based tetradentate ligand: Synthesis, structures, and magnetic studies

Copper(II) complexes (1-3) of a sterically constrained phenol-based tetradentate N2O2 ligand 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)piperazine (H2L) have been reported. The associated anions of the copper(II) ion precursors have profound influence on the stoichiometry of the products. Thus, with perchlorate ion, the product is a binuclear compound [Cu2L2] (1), while with coordinating anions viz. Cl- and N-3(-), the products [Cu3L2Cl2(H2O)].1/2H(2)L (2) and [Cu3L2(N-3)(2)(CH3OH)].4H(2)O (3) have triangulo trinuclear composition. The syntheses, X-ray structures, and spectroscopic and magnetic properties of these complexes are described. Compound 1 has a noncentrosymmetric structure with a rectangular Cu-2(OPh)(2) core. It appears to be a rare example of a phenolato-bridged Cu(II) dimer exhibiting ferromagnetic interactions (J = 0.93 cm(-1)), a behavior in agreement with the theoretical predictions but seldom observed experimentally. In compounds 2 and 3, the copper centers are triangularly disposed, and the molecules have a shape much like that of a butterfly. The terminal copper centers Cu(1) and Cu(2) in 2 and 3 have distorted square pyramidal geometry, connected to each other by a bridging chloro- (in 2) or azido ligand (in 3) in "end to end" fashion. The central copper center (Cu(3) in 2 and Cu in 3) in both the compounds has distorted square planar geometry. The separations between the metal centers, viz. Cu(1)...Cu(2), Cu(2)...Cu(3), and Cu(3)...Cu(1), are 4.826, 3.214, and 3.244 Angstrom, respectively, in 2. The corresponding distances in 3 are 5.590, 3.178, and 3.485 Angstrom, respectively. The overall magnetic behaviors in 2 and 3 are consistent with antiferromagnetic interactions between the spin centers. In 3, the exchange couplings between the terminal and central copper centers J(Cu(1)-Cu) and J(Cu(2)-Cu) appear to be equal (-234 cm(-1)), resulting in an S = 1/2 ground state at temperatures near or below 77 K.