Metal Complexes with Macrocyclic Ligands. Part XXXVIII . Steric effects in the copper(II) and nickel(II) complexes with tetra‐ N ‐alkylated 1,4,8,11‐tetraazacyclotetradecanes

A series of tetra‐ N ‐alkylated 1,4,8,11‐tetraazacyclotetradecanes have been synthesized and their complexation potential towards Ni 2+ and Cu 2+ studied. In the case of sterically demanding alkyl substituents, such as i‐Pr, PhCH 2 , or 2‐MeC 6 H 4 CH 2 , no metal complexes are formed, whereas for substituents such as Me, Et, and Pr, the metal ion is incorporated into the macrocycle. The spectroscopic properties of the Ni 2+ and Cu 2+ complexes in aqueous solution indicate that, depending on the sterical hindrance of the N ‐substituents, the complexes are either square planar or pentacoordinated. All these Ni 2+ and Cu 2+ complexes react with N   3 −to give ternary species, the stability of which have been determined by spectrophotometric titrations. The tendency to bind N   3 −decreases with increasing steric hindrance of the alkyl substituents. The X‐ray studies of the Ni 2+ complex with the macrocycle 11 , substituted by two Me and two Pr groups, and that of the Cu 2+ complex with the tetraethyl derivative 8 show that in the solid state, the metal ions exhibit square planar coordination with a small distortion towards tetrahedral geometry.