Conformational characterization of square planar nickel(II) tetraaza macrocyclic complexes by proton NMR. Crystal structure of [Ni(13aneN4)]ZnCl4

Conformations in solution of several diamagnetic nickel(II) complexes of macrocyclic tetraaza ligands are elucidated using proton NMR. There are six possible configurational isomers of planar Ni(13aneN₄)]²⁺ (13aneN₄ = 1,4,7,10-tetraazacyclotridecane due to the orientation of the N---H protons above or below the plane of the macrocyle. Using NMR it is shown that in aqueous solution the Ni(13aneN₄)]²⁺ complex has the R,S,R,S or trans-II configuration. A single-crystal X-ray study demonstrates the same configuration of the nitrogen atoms in the complex Ni(13aneN₄)]ZnCl₄. In the case of the 14-membered ring macrocyle cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane), previous NMR studies revealed the presence, in aqueous solution, of the previously unobserved trans-I or R,S,R,S isomer, whose spectrum is examined in greater detail here. Solution structures of nickel(II) complexes of bicyclam (1,5,8,12-tetraazabicyclo10.2.2]hexadecane) and dachden (N, N′-bis(2-aminoethyl)-1,4-diazacycloheptane) are also reported.