Binuclear Copper(II) Complexes of Xylyl-Bridged Bis(1,4,7-triazacyclononane) Ligands

Copper(II) complexes of three bis(tacn) ligands, [Cu₂(T₂-o-X)Cl₄] (1), [Cu₂(T₂-m-X)(H₂O)₄](ClO₄)₄·H₂O·NaClO₄ (2), and [Cu₂(T₂-p-X)Cl₄] (3), were prepared by reacting a Cu(II) salt and L·6HCl (2:1 ratio) in neutral aqueous solution [T₂-o-X = 1,2-bis(1,4,7-triazacyclonon-1-ylmethyl)benzene; T₂-m-X = 1,3-bis(1,4,7-triazacyclonon-1-ylmethyl)benzene; T₂-p-X = 1,4-bis(1,4,7-triazacyclonon-1-ylmethyl)benzene]. Crystals of [Cu₂(T₂-m-X)(NPP)(μ-OH)](ClO₄)·H₂O (4) formed at pH = 7.4 in a solution containing 2 and disodium 4-nitrophenyl phosphate (Na₂NPP). The binuclear complexes [Cu₂(T₂-o-XAc₂)(H₂O)₂](ClO₄)₂·4H₂O (5) and [Cu₂(T₂-m-XAc₂)(H₂O)₂](ClO₄)₂·4H₂O (6) were obtained on addition of Cu(ClO₄)₂·6H₂O to aqueous solutions of the bis(tetradentate) ligands T₂-o-XAc₂ (1,2-bis((4-(carboxymethyl)-1,4,7-triazacyclonon-1-yl)methyl)benzene and T₂-m-XAc₂ (1,3-bis((4-(carboxymethyl)-1,4,7-triazacyclonon-1-yl)methyl)benzene), respectively. In the binuclear complex, 3, three N donors from one macrocycle and two chlorides occupy the distorted square pyramidal Cu(II) coordination sphere. The complex features a long Cu···Cu separation (11.81 Å) and intermolecular interactions that give rise to weak intermolecular antiferromagnetic coupling between Cu(II) centers. Complex 4 contains binuclear cations with a single hydroxo and p-nitrophenyl phosphate bridging two Cu(II) centers (Cu···Cu = 3.565(2) Å). Magnetic susceptibility studies indicated the presence of strong antiferromagnetic interactions between the metal centers (J = −275 cm^-1). Measurements of the rate of BNPP (bis(p-nitrophenyl) phosphate) hydrolysis by a number of these metal complexes revealed the greatest rate of cleavage for [Cu₂(T₂-o-X)(OH₂)₄]⁴⁺ (k = 5 × 10^-6 s^-1 at pH = 7.4 and T = 50 °C). Notably, the mononuclear [Cu(Me₃tacn)(OH₂)₂]²⁺ complex induces a much faster rate of cleavage (k = 6 × 10^-5 s^-1 under the same conditions).