The dicopper(II) complex with the ligand N,N,N',N',N "-pentakis[(1-methyl-2-benzimidazolyl)methyl]dipropylenetriamine (LB5) has been synthesized and structurally characterized. The small size and the quality of the single crystal required that data be collected using synchrotron radiation at 276 K. [Cu-2(LB5)(H2O)(2)][ClO4](4) : platelet shaped, , a = 11.028 Angstrom, b = 17.915 Angstrom, c = 20.745 Angstrom, alpha = 107.44 degrees, beta = 101.56 degrees, gamma = 104.89 degrees, V = 3603.7 Angstrom(3), Z = 2; number of unique data, I greater than or equal to 2 sigma(I) = 3447; number of refined parameters = 428; R = 0.12. The ligand binds the two coppers nonsymmetrically; Cu1 is coordinated through five N donors and Cu2 through the remaining three N donors, while two water molecules complete the coordination sphere. Cu1 has distorted TBP geometry, while Cu2 has distorted SP geometry. Voltammetric experiments show quasireversible reductions at the two copper centers, with redox potential higher for the CuN3 center (0.40 V) and lower for the CuN5 center (0.17 V). The complex binds azide in the terminal mode at the CuN3 center with affinity lower than that exhibited by related dinuclear polyaminobenzimidazole complexes where this ligand is bound in the bridging mode. The catechol oxidase activity of [Cu-2(LB5)](4+) has been examined in comparison with that exhibited by [Cu-2(L-55)](4+) (L-55 = alpha,alpha'-bis{bis[(1-methyl-2-benzimidazolyl)methyl]amino}-m-xylene) and [Cu-2(L-66)](4+) (L-66 = alpha,alpha'-bis{bis[2-(1-methyl-2-benzimidazolyl)ethyl]amino}-m-xylene) by studying the catalytic oxidation of 3,5-di-tert-butylcatechol in methanol/aqueous buffer pH 5.1. Kinetic experiments show that [Cu-2(L-55)](4+) is th, most efficient catalyst (rate constant 140 M-1 s(-1)), followed by [Cu-2(LB5)](4+), (60 M-1 s(-1)), in this oxidation, while [Cu-2(L-66)](4+) undergoes an extremely fast stoichiometric phase followed by a slow and substrate-concentration-independent catalytic phase. The catalytic activity of [Cu-2(L-66)](4+), however, is strongly promoted by hydrogen peroxide, because this oxidant allows a fast reoxidation of the dicopper(I) complex during turnover. The activity of [Cu-2(LB5)](4+) is also promoted by hydrogen peroxide, while that of [Cu-2(L-55)](4+) is little affected. The phenol monooxygenase activity of [Cu-2(LB5)](2+) has been compared with that of [Cu-2(L-55)](2+) and [Cu-2(L-66)](2+) by studying the ortho hydroxylation of methyl LF-hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The LB5 ring position 2 of the catechol. complexes since its reaction produces only catechol, while the main product obtained with the other complexes is an addition product containing a phenol residue condensed at ring position 2 of the catechol.