Crystal and molecular structure of the {2,6-bis[1-(2-imidazol-4-ylethylimino)ethyl]pyridine}-zinc(II) and -copper(II) cations: five-co-ordinate relatives of a copper(I) oxygen carrier

Abstract

The crystal and molecular structures of the two title compounds are reported and discussed as they relate to the analogous copper(I) derivative which reacts reversibly with O₂ in solution under ambient conditions. The five-co-ordinate [Cu^II(imep)][CIO₄]₂ and [Zn^II(imep)][CIO₄]₂ complexes are isomorphous and isostructural, being derived from a single quinquedentate ligand {imep = 2,6-bis[1-(2-imidazol-4-ylethylimino)ethyl]pyridine} containing one pyridine nitrogen atom, two imine nitrogen atoms, and two imidazole nitrogen-donor atoms to give MN₅ primary co-ordination spheres. The co-ordination about Cu^II and Zn^II is best described as intermediate between an idealized trigonal bipyramid and a square pyramid, with a potential sixth site lying trans to the pyridine nitrogen-donor atom (and between the imidazole rings) where a small molecule like O₂ is most likely to bind in the copper(I) species. Crystal data for [Cu^II(imep)][ClO₄]₂: space group C2/c, Z= 8, a= 19.065(9), b= 11.370(4), c= 23.802(12)Å, and R= 0.043 for 1 939 reflections. Crystal data for [Zn^II(imep)][CIO₄]₂: space group C2/c, Z= 8, a= 19.414(8), b= 11.313(4), c= 24.012(6)Å, and R= 0.051 for 2 551 reflections.