Synthesis, crystal structures, and ring size effects on thermodynamic stability of divalent metal complexes of [ 14]aneN 3O

Abstract

The 14-membered macrocyclic ligand containing the N 30 donor set, 1-oxa-4,8,12-triazacyclotetradecane([14]aneN 3O), has been synthesized and characterized. Synthetic procedures for the larger macrocycles, 1,13-dioxa-4,7,10,16,20,24-hexaazacyclohexacosane ([26]aneN 6O 2) and 1,15-dioxa-4,8,12,18,22,26-hexaazacyclooctacosane ([28]aneN 6O 2) are also described. The crystal structures of the macrocycle and its copper(II) complex have been determined by single crystal X-ray diffraction. [14]aneN 3O·3HBr crystallizes in the triclinic space group P 1 with a = 7.806(2) A ̊ , β = 14.977(4) A ̊ , c = 15.399(5) A ̊ , α = 94.61(2)°, β = 100.04(2)°, γ = 101.69(2)°, Z = 4 and V = 1723.4(8) A ̊ 3 . The copper(II) complex [Cu(II)([14]aneN 3O)Br]Br·0.5H 2O crystallizes in the monoclinic space group C2/ c with a = 21.381(4) A ̊ , b = 11.455(2) A ̊ , c = 14.301(3) A ̊ , β = 117.61(1)°, Z = 8, and V = 3103.7(10) A ̊ 3 . The copper ato pentacoordinated by three nitrogens, one oxygen, and one bromide ion in a slightly distorted square-pyramidal coordination environment. The comparison of crystal structures with analogous [Cu(II)(13aneN 3O)Br]Br indicates that the copper(II) ion can fit into the macrocyclic ring of [14]aneN 3O better than into that of [13]aneN 3O. The former also has higher stability in aqueous solution than the latter. The smaller metal ion, Cu 2+, coordinates more strongly to the larger macrocycle [14]aneN 3O than to the smaller macrocyclic ligands [13]aneN 3O and [12]aneN 3O. The reverse is true of larger metal ions, such as Cd 2+. The protonation constants of [14]aneN 3O and the formation constants of its metal complexes with Cu(II), Ni(II), Zn(II), Co(II) and Cd(II) are determined potentiometrically at 25.0°C and ionic strength 0.10 M (KC1), with the stability order Cu(II) > Ni(II) > Zn(II) > Co(II) > Cd(II). The stabilities of various metal complexes are discussed in relation to the sizes of the metal ions and of the macrocyclic cavities.