Hydrogen bonded polymers and oligomers from metal salts and 18-crown-6

Abstract

Reaction of hydrated metal salts M(ClO₄)₂ with 18-crown-6 in water results in the isolation of [M(H₂O)₆][ClO₄]₂· (18-crown-6) (M = Ni, 1a; Co, 1b; Zn, 1c) which adopt an infinite hydrogen bonded sandwich type structure. Reaction of Cu(ClO₄)₂ with 18-crown-6 under similar conditions leads to a mixture of the analogous [Cu(H₂O)₆][ClO₄]₂· (18-crown-6) 1d and a new complex [Cu(H₂O)₃(18-crown-6)]₂[ClO₄]₄·(18-crown-6) 2, containing both complexed and free crown ether rings as a discrete unit. This difference in behaviour arises from the preference of Cu(II) to adopt a Jahn–Teller distorted geometry. Reaction of NiBr₂ results in the formation of the striking complex [Ni(H₂O)₆]₃[NiBr₂(H₂O)₄]Br₆·4(18-crown-6)·2H₂O 3, closely related to complexes of type 1. With a 3∶1 electrolyte, hydrogen bonded dimers of [Al(H₂O)₆][NO₃]₃·(18-crown-6) 4 are formed, unrelated to complexes 1. Removal of equatorial aqua ligands as in the complex [Cu₂(O₂CCH₃)₄(H₂O)₂] gives [Cu₂(O₂CCH₃)₄(H₂O)₂]·(18-crown-6)·2H₂O 5 which consists of a very different linear, hydrogen bonded chain. Analogous reactions with eight-coordinate complexes led to [M(CF₃CO₂)₂(H₂O)₆][CF₃CO₂]·(18-crown-6) (M = Eu, 6a; Y, 6b) which somewhat resemble complexes 1. In general, it is found that the stability of the structure as a whole, and not one dominant set of interactions, governs crystal packing, and even molecular stoichiometry within these equilibrating systems.