Host–guest chemistry of a chiral Schiff basecopper(ii) complex: can chiral information be transferred to the guest cation?

Abstract

The in situ formation of [Cu{(R,R)-1}] through the reaction of copper(II) acetate with 1,6-bis(3-ethoxy-2-hydroxyphenyl)-(3R,4R)-(−)-cyclohexane-1,2-diyl-2,5-diazahexa-1,5-diene, (R,R)-H₂1, followed by reaction with NH₄PF₆, NaClO₄, KBr, KSCN, CsCl, Ca(NO₃)₂·4H₂O or BaI₂ leads to the isolation of crystalline products, which illustrate the ability of the vacant O₄-donor cavity in [Cu{(R,R)-1}] to host ammonium or s-block metal cations in a variety of manners. The smallest cations (Na⁺ and Ca²⁺) are bound in the plane of the O₄-donor set, with axial sites being occupied by methanol or coordinating anions. The bridging mode adopted by [NO₃]⁻ anions results in the assembly of a chiral, one-dimensional coordination polymer. Larger cations ([NH₄]⁺, K⁺, Cs⁺, Ba²⁺) are sited above the plane of the O₄-donor cavity and give rise to either sandwich-type complexes (both transoid and cisoid) or, in the case of Ba²⁺, to a 1 : 2 complex in which the [Cu{(R,R)-1}] ligands tend towards a mutually orthogonal arrangement. The 9-coordinate Ba²⁺ coordination environment is completed by a methanol ligand. In all but the latter complex, a common feature of the molecular packing is the ability of the O₄-domain to host a cyclohexane-1,2-diyl unit through C–H⋯O non-classical hydrogen bonds. The O₄-donor set appears to be too distant from the chiral auxiliary for its stereochemical information to be recognized by the cation hosted within the O₄-cavity.