Unprecedented coordination modes for PDC (pyridine-2,5-dicarboxylate) in polymorphic 3D heterobimetallic compounds α- and β-[MNa2(PDC)2(H2O)4], with M = Ni, Co

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Abstract
Four 3D coordination polymers have been hydrothermally synthesized using PDC (PDC = pyridine-2,5-dicarboxylate), and subsequently characterised. The four compounds are heterobimetallic, their formula being [MNa2(PDC)2(H2O)4], with M = Ni, Co. Two polymorphous compounds having the formula [NiNa2(PDC)2(H2O)4] have been obtained: polymorph α is monoclinic, and β is triclinic. Similarly, two phases have been characterised for the [CoNa2(PDC)2(H2O)4] formula, which are isostructural with the nickel compounds. All the studied compounds are 3D by covalent bonding. The four of them exhibit sodium octahedral chains whose extension allows the presence of perpendicular nickel octahedral chains for the monoclinic phases (α polymorphs). However, for the triclinic phases (β polymorphs) the connections between nickel ions just extend to trimers. The interest of these compounds lies on several aspects: two new coordination modes have been detected for PDC and all compounds exhibit a novel 3D complex multi-nodal topology. Density functional calculations have been carried out to analyse the crystallographic and chemical relationships between these phases.

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