SYNTHESIS, STABILITY, AND CRYSTAL-STRUCTURE STUDIES OF SOME CA2+, CU2+ AND ZN2+ COMPLEXES OF MACROCYCLIC POLYAMINO CARBOXYLATES

The stability constants of the complexes of three ligands, DO3A (1,4,7 ,10-tetrazacyclododecane-1,4,7-triacetic acid), HP-DO3A yl)-1,4,7,10-t etraazacyclododecane-1,4,7-triacetic acid), and DO3MA (1R,4R,7R)-alpha ,alpha',alpha hyl-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) , with Ca2+, Cu2+, and Zn2+ were measured at 25.0 +/- 0.1 degrees C an d mu = 0.1 (TMACl). Calcium and gadolinium complexes which have five-m embered chelate rings follow linear log K-ML vs Sigma pK(a) and log K- GdL vs log K-CaL correlations. suggesting Ca2+ and Gd3+ have similar c oordination behavior. A linear correlation between log K-ML and Z/r (c harge density on the metal ion, for HP-DO3A complexes is consistent wi th the lower stability constant of the Ca2+ vs Gd3+ complex. The overa ll selectivity constants for Gd3+ over other metal ions were calculate d. The crystal structures of three hydrated forms of [Ca(HP-DO3A)](2) were determined by X-ray single-crystal analysis: a C-2 symmetric trin uclear structure. Ca(H2O)(4)[Ca(HP-DO3A)](2) . 10H(2)O, a centrosymmet ric hexanuclear structure. {Ca(H2O);[Ca(HP-DO34A)](2) . 3H(2)O}(2), an d a C-2 symmetric trinuclear structure, Ca(H2O)(4)[Ca(HP-DO3A)](2) . 2 H(2)O, which is formed through topotactic dehydration of the first str ucture at 22 degrees C. The conformations of the macrocyclic ligands, [3333], and their coordination to the embedded calcium ions are virtua lly identical in all forms. The embedded calcium is sandwiched between two nearly parallel planes of four nitrogens and oxygens. The macrocy clic ring in the crystal structure of HCu(DO3A) adopts a quadrangular [2334] conformation in which the four nitrogens and two opposite carbo xylate arms are coordinated to the Cu(II) atom.