Copper(ii) coordination polymers of imdc− (H2imdc+ = the 1,3-bis(carboxymethyl)imidazolium cation): unusual sheet interpenetration and an unexpected single crystal-to-single crystal transformation

Abstract

The monoanion of 1,3-bis(carboxymethyl)imidazolium (H₂imdc⁺) combines with Cu(II) to produce an undulating 2D coordination polymer of composition [Cu₂(imdc)₂(CH₃OH)₂](BF₄)₂·(CH₃OH)(H₂O) (1) in which copper acetate-like dimers, linked by imdc⁻ ligands, act as 4-connecting centres. Cationic sheets stack on top of each other in an A, B, A, B… fashion and produce a structure that contains channels running parallel to the plane of network. Tetrafluoroborate anions are located in channels between sheets. Upon removal of coordinated and non-coordinated solvent molecules a single crystal-to-single crystal transformation occurs to yield a similar compound but with BF₄⁻ anions now coordinated. CO₂ isotherms measured at 258 and 273 K show only modest uptake of CO₂ but provide an indication that the sheets move apart at elevated pressures in order to accommodate the guest molecules. A compound of composition [Cu₃(OH)₂(imdc)₂]·SiF₆·2H₂O·2MeOH (3), which possesses a 3D network, is formed by the combination of copper(II) acetate, copper(II) hexafluorosilicate and Himdc. In this structure infinite parallel Cu₃(OH)₂ chains are linked by bridging imdc⁻ ligands to form channels that have an approximately triangular cross-section. These channels are occupied by SiF₆²⁻ anions in addition to solvent molecules. When copper(II) acetate is combined with Himdc in the appropriate ratio, a 1D coordination polymer of composition Cu(imdc)₂ (4) is formed in which pairs of imdc⁻ anions bridge Cu(II) centres. When the reaction is performed in the presence of NaBF₄ a minor crystalline product with tetragonal symmetry is isolated in addition to the 1D coordination polymer. This compound of composition Cu₂(imdc)₄NaBF₄·7H₂O (5) consists of 2D Cu(imdc)₂ networks and features an unusual mode of interpenetration.