Bi- and Trinuclear Copper(II) Complexes of a Sterically Constrained Phenol-Based Tetradentate Ligand: Syntheses, 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 N3-, the products [Cu3L2Cl2(H2O)]·1/2H2L (2)
and [Cu3L2(N3)2(CH3OH)]·4H2O (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 Cu2(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 Å, respectively, in 2. The corresponding distances in 3 are 5.590, 3.178,
and 3.485 Å, 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 JCu(1)-Cu
and JCu(2)-Cu appear to be equal (−234 cm-1), resulting in an S = 1/2 ground state at temperatures near or below
77 K.