Anion Coordination Effect on the Nuclearity of CoII, NiII, CuII, and ZnII Complexes with a Benzimidazole Pendant‐Armed Crown

Abstract

AbstractThe study presented in this paper provides a rare example of a macrocyclic receptor allowing the formation of mono‐ or binuclear complexes on the same first‐row transition‐metal guest cation depending on the nature of the counterion present. Reaction of N,N′‐bis(benzimidazol‐2‐ylmethyl)‐4,13‐diaza‐18‐crown‐6 (L4) with CoII, NiII, CuII, or ZnII perchlorates leads to the formation of the expected mononuclear complexes with the metal ion showing a distorted octahedral coordination environment. In these complexes, the metal ion is placed at one end of the macrocyclic cavity, and two of the oxygen atoms of the crown ether moiety remain uncoordinated. 1H and 13C NMR spectroscopic studies on the diamagnetic [Zn(L4)]2+ complex indicate that the asymmetric coordination of the metal ion inside the macrocyclic cavity is maintained in acetonitrile solution, but translocation of the ZnII ion from one end of the macrobicyclic cavity to the second one occurs. However, reaction of L4 with CoII, NiII, or CuII perchlorates in the presence of chloride anions results in the formation of binuclear complexes where the metal ion is either five‐coordinate in a distorted trigonal bipyramidal coordination environment (Co or Cu) or six‐coordinate in a distorted octahedral geometry (Ni), and a chloride anion coordinates to each of the two metal ions in all cases. The ZnII ion behaves in a different way, and addition of chloride anions to the ZnII perchlorate in the presence of L4 does not lead to the formation of a binuclear complex. The formation of these binuclear systems was followed by spectrophotometric titrations of the mononuclear [M(L4)](ClO4)2 complexes with [nBu4N]Cl in acetonitrile solution indicating the formation of a [M2(L4)(Cl)]2+ intermediate species, followed by the formation of the binuclear complex [M2(L4)(Cl)2]2+ (M = Co, Ni, or Cu). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)