[15]crown-5: an unsymmetrical bifacial hydrogen-bond acceptor in crystal engineering

Reaction of [15]crown-5 with erbium nitrate and a range of nickel salts results in the formation of the hydrogen-bonded chain species [Er(NO3)3(H2O)3]. [15]crown-5 H2O (3), [Ni(H2O)6]Br2 [15]crown-5 2H2O (4), [Ni(H2O)6](ClO4)2 x [15]crown-5 2H2O (5), [Ni(H2O)6](NO3)2 [15]crown-5 2H2O (6) and [[Ni(H2O)4]2(muCl)2]Cl2 [15]crown-5 (7). Complexes 3-5 exhibit low symmetry structures with 2-4 unique crown ether molecules, whereas 6 and 7 are more symmetrical. The factors contributing to this behaviour are discussed within the wider context of [15]crown-5 structures in the Cambridge Structural Database.