4,7,10,23-Tetramethyl-17-oxa-1,4,7,10,13,23-hexaazabicyclo[11.7.5]pentacosane (L), a two-binding-site ligand for the assembly of the [Zn2(mu-OH)2]2+ cluster

The synthesis of the new macrobicyclic ligand 4,7,10,23-tetramethyl-17-oxa-1,4,7,10,13,23-hexaazabicyclo[11.7.5]-pentacosane (L) is described. Its basicity and Zn2+ complexation have been studied by means of potentiometric titrations in 0.15 mol dm(-3) NaClO4 aqueous solution at 298.1 +/- 0.1 K. L behaves as tetraprotic base for pH > 2.5. The crystal structure of [H(3)L](ClO4)(3) (P ($) over bar 1, a = 10.358(3) Angstrom, b = 11.383(4) Angstrom, c = 14.858(3) Angstrom, alpha = 105.24(2)degrees, beta = 95.26(2)degrees, gamma = 95.08 degrees, Z = 2, R = 0.0813, wR(2) = 0.1997) evidences the formation of intramolecular H-bond networks, which stabilize the protonated form. Mononuclear Zn2+ complexes are formed in aqueous solution, while both mono- and binuclear species are present in methanol. The dibridged binuclear {[Zn-2(mu-OH)(2)]L}(2+) species is almost the unique complex formed by L in presence of 2 equiv of the metal ion. The crystal structure of {[Zn-2(mu-OH)(2)]L}(ClO4)(2) . H2O . 0.5CH(3)OH (P2(1)/c, a = 8.552(2) Angstrom, b = 39.23(1) Angstrom, c = 11.419(3) Angstrom, beta = 111.90(2)degrees, Z = 4, R = 0.087, R(W) = 0.091) has been determined. Reaction of {[Zn-2(mu-OH)(2)]-L}(2+) with various substrate molecules has been investigated: the binuclear complex recognizes chelating agents such as 2-hydroxypyridine and cytosine. The crystal structure of {[Zn-2(mu-OH)(2-hydroxypyridinate)]L}(ClO4)(2) (P2(1)/m, a = 11.796(3) Angstrom, b = 13.035(6) Angstrom, c = 14.842(3) Angstrom, beta = 107.75(2)degrees, Z = 2, R = 0.085, R(W) = 0.083) reveals that the chelating ligand binds a single metal center. The binuclear structure is retained in reactions with substrate molecules, suggesting the use of the {[Zn-2(mu-OH)(2)]L}(2+) species as a model for zinc-containing enzymes.