Binuclear Gd3+ complex of a 34-membered macrocycle with six carboxymethyl arms: X-ray structures, formation constants, NMR, EPR, and H-1 NMR relaxivities

A 34-membered macrocyclic ligand with six pendant carboxymethyl groups has been synthesized by the coupling of two diethylenetriaminepentaacetate (dtp a) and two 1,4-diaminoburane units: the macrocycle is 2,12,19,29-tetraoxo-1 ,4,7,10,13,18,21,24,27,30-decaaza-4,7,10,21,24,27-cyclotetratriacontanehexa acetic acid, abbreviated as (34dtpabn)H-6. The macrocycle was crystallized as the acetone adduct (34dtpabn)H-6. 2Me(2)CO in the monoclinic space group P2(1)/n with a = 14.566(2) Angstrom, b = 10.968(1) Angstrom, c = 16.133(2) Angstrom, beta = 105.62(1)degrees, and Z = 2. The acetone molecules have c lose contacts with some carbon atoms of the macrocycle. The protonation con stants determined by potentiometric titration (0.1 M KCl, 25 degrees C) wer e log K-1 = 9.70 (standard deviation = 0.01), log K-2 = 9.18 (0.05), log K- 3 = 5.27 (0.02), log K-4 = 4.50 (0.03), log K-5 = 3.84 (0.01), and log K-6 = 3.23 (0.01). The corresponding protonation sites were identified by the p D dependence of H-1 NMR shifts; the central amino nitrogen of each dtpa moi ety has the highest basicity. The binuclear Gd3+ complex of the macrocycle was crystallized as the 2-propanol adduct [Gd-2(34dtpabn)(H2O)(2)]. 2PrOH . 7H(2)O in the triclinic space group P (1) over bar with a = 11.880(1) Angs trom, b = 15.036(1) Angstrom, c = 17.396(1) Angstrom, alpha = 94.986(5)degr ees, beta = 103.340(5)degrees, gamma = 98.286(5)degrees, and Z = 2. Each li gand molecule coordinates two Gd3+ ions forming a binuclear metal chelate w ith the Gd-Gd distance 10.8928(7) Angstrom. The coordination geometry aroun d each metal ion is a tricapped trigonal prism consisting of three carboxyl ate oxygens, two amide oxygens, three amino nitrogens, and a water oxygen. Propanol molecules are located near the macrocyclic cavity with some close C-C contacts shorter than the sum of the van der Waals radii of methyl and methylene groups. The logarithmic overall formation constants, log beta(Gdp LHr), were determined by potentiometry (0.1 M KCl, 25 degrees C): for GdLH4 , 35.85 (estimated uncertainty = 0.21); GdLH3, 33.89 (0.36); GdLH2, 30.27 ( 0.07); GdLH, 25.59 (0.04); GdL, 16.46 (0.04); GdLH-1, 4.93 (0.19); Gd2L, 30 .36 (0.33). Dipolar interaction operative between the two Gd3+ ions in the binuclear complex collapses the hyperfine structure in the EPR spectrum. Th e H-1 NMR relaxivities are enhanced as a result of the binuclear complex fo rmation.