Controlling copper(I) halide framework formation using N-donor bridging ligand symmetry: use of 1,3,5-triazine to construct architectures with threefold symmetry

Abstract

The formation of co-ordination polymers between copper(I) halides and 1,3,5-triazine (tri), a potentially tridentate N-donor bridging ligand with threefold symmetry, has been studied. Complexes with both 3∶1 and 2∶1 molar ratios are formed by both CuBr and CuI. The compounds [Cu₃X₃(tri)]_∞ (X = Br or I) are structurally similar, despite crystallising in different space groups. They are composed of (CuX)_∞ columns linked by triazine molecules to generate three-dimensional constructions with non-crystallographically imposed threefold symmetry. The (CuX)_∞ columnar motif can be described as a series of perpendicularly stacked Cu₃X₃ chairs, alternately rotated by 60° and linked by Cu–X contacts. The tetrahedral co-ordination geometry of the copper centres is completed by a tridentate triazine bridge which links two copper atoms in separate columns. Thus, each (CuX)_∞ column is linked to six adjacent (CuX)_∞ columns. The structure of [Cu₂Br₂(tri)]_∞ comprises (CuBr)_∞ columns and castellated (CuBr)_∞ chains linked by triazine molecules to generate a construction with crystallographically imposed threefold symmetry. The (CuBr)_∞ columns are similar to but more regular than those found in [Cu₃Br₃(tri)]_∞. In this case, however, each column is linked to six adjacent chains. The (CuBr)_∞ castellated chain motif is very unusual. The tetrahedral copper centres are co-ordinated by two adjacent bromide anions and by two triazine molecules each of which links a second chain and a column. Consequently, each chain is linked to four neighbouring chains and two neighbouring columns. Despite a stoichiometry identical to that of [Cu₂Br₂(tri)]_∞, [Cu₂I₂(tri)]_∞ has a completely different structure. The triazine molecules act as bidentate bridging ligands to link (CuI)_∞ layers thereby giving alternating inorganic and organic layers. The tetrahedral co-ordination geometry of the copper centres in the (CuI)_∞ layers, which are effectively undulating hexagonal nets, is provided by three iodide anions from the layers and by a bridging triazine molecule.