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).