Complementarity of halide-mediated hydrogen-bonding and alkyl substitution in the construction of two-dimensional rhombic (4,4) grids using bis(N-alkylamidino-O-alkylurea)copper(ii) halides

Abstract

A series of hydrogen-bonded networks based on bis(N-alkylamidino-O-alkylurea)copper(II) cations, [CuL₂]²⁺, bridged by either chloride or bromide have been synthesised, of which six have been structurally characterised by single crystal X-ray diffraction methods. The [CuL₂]²⁺ cation acts as a four-connecting centre and forms 2-D rhombic (4,4) grids for the majority of the compounds studied. However, the grids do not adopt a common intermolecular hydrogen-bonding pattern. Three distinct systems can be differentiated: one mediated solely by halide anions, a second mediated by protic solvent molecules as well as halide anions, and a third with no intermolecular interactions. There is evidence for complementarity in (4,4) rhombic grids mediated solely by halide anions, as grid formation only occurs when there is a match between the size of the cavity formed within the grid and the space required by the pendant alkyl groups of the N-alkylamidino-O-alkylurea ligands. Such complementarity occurs for [Cu(L^mm)₂]·2Cl (L^mm = N-methylamidino-O-methylurea) and [Cu(L^me)₂]·2Br (L^me = N-methylamidino-O-ethylurea). Chloride bridges generate a cavity which can accommodate four juxtaposed methyl groups while bromide bridges generate a cavity large enough to accommodate two methyl and two ethyl groups. Rhombic (4,4) grid formation based on simple halide bridges does not take place for [Cu(L^mm)₂]·2Br·2MeOH, [Cu(L^ee)₂]·2Cl·H₂O (L^ee = N-ethylamidino-O-ethylurea), [Cu(L^ee)₂]·2Br·MeOH or [Cu(L^2m)₂]·2Cl·2MeOH (L^2m = N-benzylamidino-O-methylurea). The cavity generated by bromide bridges is too large for the four methyl groups of [Cu(L^mm)₂]²⁺ cations. To overcome this problem and still form a (4,4) grid, the hydrogen-bonding assembly in [Cu(L^mm)₂]·2Br·2MeOH incorporates a methanol molecule. The spatial requirements of the four ethyl groups of the [Cu(L^ee)₂]²⁺ cations are greater than can be provided by either of the grids generated by chloride or bromide bridges. Consequently, 1-D chains rather than 2-D rhombic (4,4) grids are formed. The cavity required to accommodate two benzyl groups and two methyl groups of the [Cu(L^2m)₂]²⁺ cations is far larger than can be provided by chloride bridges. Hence, although the disposition of copper(II) centres in [Cu(L^2m)₂]·2Cl·2MeOH is similar to those in [Cu(L^mm)₂]·2Cl and [Cu(L^me)₂]·2Br, the molecular units are not linked by hydrogen-bonding interactions. A common feature of the halide bridges is a hydrogen-bonded supramolecular synthon in which the anion acts as acceptor to two pairs of N–H donors thus forming two R¹₂(6) motifs with a common halide anion.