Stability constants of the zinc(II), cadmium(II) and mercury(II) complexes of the mono-aza coronands 1,4,7,10-tetraoxa-13- azacyclopentadecane (L 1), 1,4,7,10,13-pentaoxa-16-azacyclooctadecane (L 2) and 1,4,7-trioxa-10-azacyclododecane (L 3). Crystal and molecular structures of the cadmium complexes of L 1 and L 2

Abstract

The stability constants for the 1:1 complexes of zinc, cadmium and mercury with 1,4,7,10-tetraoxa-13-azacyclo- pentadecane (L 1), 1,4,7,10,13-pentaoxa-16-azacyclooctadecane (L 2) and 1,4,7-trioxa-10-azacyclododecane (L 3) have been determined potentiometrically (0.1 M NEt 4ClO 4, 95% methanol). Reaction between L 1 or L 2 and cadmium nitrate tetrahydrate in methanol results in the isolation of colourless crystalline solids. Crystals of [Cd(L 1)(NO 3) 2] are monoclinic, space group P2 1/ c with a=7.614(2), b=28.566(2), c=7.707(1) Å, β=107.364(9) o; R=0.025, R w=0.027 for 2195 ( I> 3σ( I)) reflections. The structure of [Cd(L 1)(NO 3) 2] consists of an almost planar macrocyclic ligand enclosing a cadmium ion in its cavity, with a monodentate and a bidentate nitrate ligand coordinating the cadmium ion on opposite sides of the macrocycle. Crystals of [Cd(L 2)(NO 3) 2] are orthorhombic, space group Pca2 1 with a=8.738(11), b=14.140(9), c=15.176(8) Å; R=0.019, R w=0.022 for 3058 ( I> 2.5σ( I)) reflections. The structure of [Cd(L 2)(NO 3) 2] consists of a cadmium atom enclosed in the cavity of the macrocycle, with a monodentate and a bidentate nitrate ligand on opposite sides of the macrocycle. Two of the oxygen atoms of L 2 in [Cd(L 2)(NO 3) 2] do not coordinate the cadmium ion, but the remaining donor atoms of the macrocycle form an almost planar assembly around the cadmium.