A NOVEL HEXACHELATING AMINO THIOL LIGAND AND ITS COMPLEX WITH GALLIUM(III)

The ligand 1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclononane, TS-TACNH3 (1), was synthesized from the parent amine, TACN, by reaction with ethylene sulfide in benzene. TS-TACNH3 undergoes reaction with Ga(NO3)3-9H2O in ethanol to give the neutral complex Ga(TS-TACN) (2). The complex may be recrystallized from dichloromethane as pale pink needles of the disolvate, in monoclinic space group P21/n (No. 14), with a = 9.313 (3) Å, b = 20.108 (7) Å, c = 11.921 (3) Å, and β = 96.71 (1)°. There are four molecules of complex and two molecules of solvation (CH2C12) present per unit cell. The gallium is fully chelated in a slightly distorted octahedral environment by the three amine nitrogens and the three thiolate sulfurs. For comparison, the Ga3+ complex of the similarly hexachelating ligand 1,4,7-triazacyclononane-1,4,7-triacetic acid, NOTA, is also presented. The complex Ga(NOTA) (3), crystallizes from boiling water also as pale pink needles, in monoclinic space group P21/n (No. 14), with a = 8.835 (3) Å,b= 13.456 (2) Å, c = 11.914 (5) Å, β = 105.57 (2)°, and Ζ = 4. Again, the metal center is fully chelated in a slightly distorted octahedral environment. The degree of distortion may be expressed as trigonal twist, ϕ, where ϕ = 0° for a true octahedron and ϕ = 60° for a trigonal prism. While both Ga(TS-TACN) and Ga(NOTA) are potentially isostructural, in regard to coordination geometry, with trigonal-prismatic Fe(NOTA) (ϕ = 34.8°), the gallium(III) complexes are only slightly distorted from a regular octahedral coordination sphere (Ga(NOTA) ϕ = 12.4° and Ga(TS-TACN) ϕ = 10.4°). Preliminary results indicate 68Ga(TS-TACN) to be stable in vivo versus the blood protein transferrin, indicating a relatively high stability constant for this hexachelating ligand. © 1990, American Chemical Society. All rights reserved.