THERMODYNAMIC, KINETIC, AND STRUCTURAL STUDY OF THE LIGATIONAL PROPERTIES OF THE MACROBICYCLIC AZA-LIGAND 4,7,10,17,23-PENTAMETHYL-1,4,7,10,13,17,23-HEPTAAZABICYCLO[11.7.5]PENTACOSANE (L1) AND OF ITS MACROCYCLIC PRECURSOR 1,4,7,13-TETRAMETHYL-1,4,7,10,13,16-HEXAAZACYCLOOCTADECANE (L2) - CRYSTAL-STRUCTURE OF [ZN(L1)(H2O)](BPH4)2

The ligational properties of the macrobicyclic ligand 4,7,10,17,23-pentamethyl-1,4,7,10,13,17,23-heptaazabicyclo[11.7.5]pentacosane (L1) and of its monocyclic precursor 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane (L2) toward transition metal and alkali metal cations are considered. The equilibrium constants of the species formed by L1 with Cu2+, Zn2+, and Cd2+ and by L2 with Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ have been determined by means of potentiometric measurements in 0.15 mol dm-3 NaClO4 at 298.1 K. The enthalpy changes for the reaction of L2 with Ni2+, Cu2+, Zn2+, and Cd2+ have been calorimetrically measured. The unmethylated analog of L2, [18]aneN6 (1,4,7,10,13,16-hexaazacyclooctadecane), is also considered for the sake of comparison. The tetramethylated ligand L2 forms complexes of stability lower than that of [18]aneN6; the decrease in stability is due to a great loss in the enthalpic contribution, while the entropic contribution is favorable to the formation of the complexes with L2. The Cu2+, Zn2+, and Cd2+ complexes of L1 are even less stable; no cryptate effect is observed. The equilibrium and spectral data are in accord for the involvement of only the four nitrogen atoms of the 14-membered tetraazamacrocyclic moiety of L1 in the coordination to these metal ions, as observed in the crystal structure of [Zn(L1)H2O](BPh4)2. Crystals of [Zn(L1)(H2O)](BPh4)2 (C71H93B2N7OZn) are monoclinic, space group C2/c, with a = 38.166(13) angstrom, b = 13.823(11) angstrom, c = 24.186(5) angstrom, beta = 98.08(2)-degrees, Z = 8, R = 0.065, and R(w) = 0.064. The coordination polyhedron around the Zn2+ ion can be described as a distorted trigonal bipyramid, with the methylated nitrogen atoms and the water oxygen in the equatorial plane and the bridgehead nitrogens in apical positions. The kinetics of dissociation of the copper(II) complex of Ll in HClO4 solutions has been studied at different temperatures (317, 324, 332, 343 K) in 0.5-1.0 mol dm-3 (I = 1 Mol dm-3) and in 3.0 mol dm-3 acid concentrations. The results showed that the dissociating species is [Cu(H3L1)]5+, the rate of dissociation (k = 1.30 X 10(-7) s-1, DELTAH* = 23.9 +/- 0.7 kcal mol-1, DELTAS double dagger = -10 +/- 2 cal K-1 mol-1 at 298 K) being independent of H+ concentration, within the considered range, and first order in the complex concentration. A reaction mechanism in which the rate-determining step is a conformational change occurring on detachment of a coordinated nitrogen is proposed.