Premium
Pseudo‐Polymorphism in a Conformationally Rigid Copper Helicate – Structural Studies of a Shape‐Awkward System
Author(s) -
Martinez Belmonte Marta,
Price Daniel J.
Publication year - 2012
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201200676
Subject(s) - chemistry , crystallography , protonation , tetrahydrate , denticity , counterion , hydrogen bond , copper , ligand (biochemistry) , crystal structure , ion , double salt , crystallization , stereochemistry , molecule , inorganic chemistry , biochemistry , receptor , organic chemistry
The coordination of copper(II) by the acyclic polydentate ligand ethylene‐1,2‐bisoxamate [(oeo) 4– ] results in the formation of an anionic dinuclear double‐stranded helicate, [Cu 2 (oeo) 2 ] 4– . The potassium salt of this large, chiral, approximately spherical, D 2 symmetric and highly charged complex molecular anion is found to crystallise in five different hydrated forms; a trihydrate 6a , a tetrahydrate 6b , a tetrahemihydrate 6c , a pentahydrate 6d and an approximate heptahydrate 6e . All of these forms crystallise from solution under similar conditions. Together this large anion and the relatively small K + cation provide something of a shape‐awkward system, as evidenced by the high degree of pseudo‐polymorphism. X‐ray structural analysis of 6a – e reveals much similarity in the order, symmetry and packing of the anions, but great irregularity and disorder in the packing of the K + counterions and water of crystallisation. The structures are further complicated by a Cu nonstoichiometry. Although 6e has a 100 % Cu site occupancy, 6a and 6c have site occupancies less than 100 % and contain both [Cu 2 (oeo) 2 ] 4– and [Cu(Hoeo) 2 ] 4– complex ions as a solid solution. In 6b and 6d , where the Cu site occupancy is below 50 %, the compound comprises [Cu(Hoeo) 2 ] 4– and [(H 2 oeo) 2 ] 4– units distributed over the same site. It is a remarkable feature of this system that hydrogen bond interactions between pairs of the protonated acyclic ligands result in a dimeric structure, which has the same shape, charge and structure as that of the copper complex of the fully deprotonated ligand. So similar are the [Cu 2 (oeo) 2 ] 4– , [Cu(Hoeo) 2 ] 4– and [(H 2 oeo) 2 ] 4– components that they are undifferentiated in the crystal structures, and the solid solutions that they form can be viewed as molecular alloys.