Increased dimensionalities of zinc–diphenic acid coordination polymers by simultaneous or subsequent addition of neutral bridging ligands

Abstract

Three coordination polymers containing zinc and diphenic acid (H₂dpa) were synthesised by solvothermal reaction. Zn(dpa)(H₂O) is a one-dimensional coordination polymer that consists of parallel ladder-like chains. One carboxylate group of the diphenic acid coordinates two zinc atoms forming a dinuclear unit which composes the steps of the ladder. The other carboxylate connects to a zinc atom in the next step of the ladder. The fourth coordination site at the zinc atom is occupied by water. Attempts to crosslink the chains by replacing the water molecule with the neutral ligands triethylenediamine (dabco) or 4,4′-bipyridyl lead to the compounds Zn₂(dpa)₂(dabco) and Zn(dpa)(4,4′-bpy). Their structures can be rationalised as being derived from action of the neutral ligand on Zn(dpa)(H₂O), and while they are most conveniently prepared in a one-pot synthesis, it is also possible to obtain them by exposing Zn(dpa)(H₂O) to the respective neutral ligand. Zn₂(dpa)₂(dabco) is a layered two-dimensional coordination polymer in which dinculear zinc carboxylate paddle wheel units and the dabco ligand form infinite linear chains. The chains are interconnected by the dpa unit. The structure of Zn(dpa)(4,4′-bpy) consists of two identical interpenetrating three-dimensional networks. In the network, helical Zn(dpa) chains are interconnected by the rigid 4,4′-bipyridine ligand. Thermogravimetric analysis indicates a high thermal stability of this coordination polymer with decomposition occurring in the range 350–450 °C. This is complemented by X-ray thermodiffractometry that indicates a phase transition at 337 °C and the final loss of crystallinity at 427 °C. The room temperature phase expands drastically along one axis and contracts along the other two axes on heating.