Tyrosinase Models. Synthesis, Structure, Catechol Oxidase Activity, and Phenol Monooxygenase Activity of a Dinuclear Copper Complex Derived from a Triamino-Pentabenzimidazole Ligand

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. [Cu2(LB5)(H2O)2][ClO4]4:
platelet shaped, P1h, a ) 11.028 Å, b ) 17.915 Å, c ) 20.745 Å, R ) 107.44°, â ) 101.56°, ç ) 104.89°, V )
3603.7 Å3, Z ) 2; number of unique data, I g 2ó(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 [Cu2(LB5)]4+ has been examined in comparison with that exhibited by [Cu2(L-55)]4+
(L-55 ) R,R¢-bis{bis[(1-methyl-2-benzimidazolyl)methyl]amino}-m-xylene) and [Cu2(L-66)]4+ (L-66 ) R,R¢-
bis{bis[2-(1-methyl-2-benzimidazolyl)ethyl]amino}-m-xylene) by studying the catalytic oxidation of 3,5-di-tertbutylcatechol
in methanol/aqueous buffer pH 5.1. Kinetic experiments show that [Cu2(L-55)]4+ is the most efficient
catalyst (rate constant 140 M-1 s-1), followed by [Cu2(LB5)]4+ (60 M-1 s-1), in this oxidation, while [Cu2(L-
66)]4+ undergoes an extremely fast stoichiometric phase followed by a slow and substrate-concentration-independent
catalytic phase. The catalytic activity of [Cu2(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 [Cu2-
(LB5)]4+ is also promoted by hydrogen peroxide, while that of [Cu2(L-55)]4+ is little affected. The phenol
monooxygenase activity of [Cu2(LB5)]2+ has been compared with that of [Cu2(L-55)]2+ and [Cu2(L-66)]2+ by
studying the ortho hydroxylation of methyl 4-hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The LB5
complex is much more selective than the other 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.