Two new N-substituted derivatives of the 1,4,7-triazacyclononane (tacn) macrocycle, 1-benzyl-4,7-dimethyl-1,4,7-triazacyclononane(L-2) and 1,4,7-tris(3-cyanobenzyl)-1,4,7-triazacyclononane (L-3), have been prepared and, together with 1,4-dimethyl-1,4,7-triazacyclononane (L-1), have been used to synthesize the corresponding hydroxo-bridged binuclear copper (11) complexes, [Cu-2(mu-OH)(2)L-2](ClO4)(2)(.)xH(2)O (1 L = L-1, x = 0; 2 L = L-2, x = 1; 3 L = L-3, x = 2). The X-ray crystal structures of all three complexes reveal the presence Of (Cu-2(mu-OH)(2)](2+) cores capped by pairs of facially coordinating tacn ligands so that the Cu(II) centers reside in distorted square pyramidal coordination environments. Variable-temperature magnetic susceptibility measurements indicate weak antiferromagnetic coupling (J = -36.4 cm(-1)) between the Cu(II) centers in 1, while the centers in 2 and 3 have been shown to interact ferromagnetically (J = 11.2 and 49.3 cm(-1), respectively). The variation in the strength and sign of these interactions has been rationalized in terms of the differing geometries of the [Cu-2(mu-OH)(2)](2+) cores. The ability of the Cu(II) complexes to cleave phosphate ester bonds has been probed using the model phosphate ester bis(4-nitrophenyl)phosphate (BNPP) at pH 7.4 and a temperature of 50 degrees C. The measured rate constant for 3 (3 x 10(-4) s(-1)) is significantly greater than those previously reported for the Cu(II) complexes of the fully alkylated tacn ligands, Me(3)tacn and (l)Pr(3)tacn, which until now have been rated as the most effective tacn-based phosphate ester cleavage agents.