X-ray crystal structure of a macrocyclic copper chelate stable enough for use in living systems: Copper(II) dihydrogen 6-(p-nitrobenzyl)-1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate

With the goal of radioimmunotherapy using copper-67 attached to monoclonal antibodies, the title complex (I) has been synthesized and shown to retain the copper ion for days under physiological conditions.¹⁰ Here we report the crystal structures of I and also of the binuclear complex dicopper(II) (6-p-nitrobenzyl)-1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate (II). Crystals of I are triclinic, with cell dimensions a = 8.537 (4) Å, b = 8.684 (6) Å, c = 21.614 (15) Å, α = 99.48°, β = 98.69°, and γ = 90.51°; Z = 2, and space group is P1. The R value with anisotropic thermal parameters for all non-hydrogen atoms is 0.066 for 2934 reflections. In I, the Cu(II) is six-coordinate, with two axial nitrogen ligands (bond lengths 2.4 Å). The other two nitrogen atoms and two carboxylate oxygens form an approximately square plane (bond lengths 2.0 Å). Crystals of II are monoclinic, with cell dimensions a = 10.501 (3) Å, b = 9.216 (3) Å, c = 17.640 (4) Å, and β = 92.06°; Z = 2, and space group is Pc. The R value is 0.055 Å for 2650 reflections. In the binuclear complex the two coppers are pentacoordinate, and the structure is polymeric.