Synthesis and molecular structure of oxydiacetate complexes of nickel(ii) and cobalt(ii). Theoretical analysis of the planar and non-planar conformations of oxydiacetate ligand and oxydiacetic acid

Abstract

A novel synthetic route to [Ni(oda)(H₂O)₃]·1.5H₂O 1 and [Co(oda)(H₂O)₂]·H₂O 2 [oda = oxydiacetate, O(CH₂CO₂⁻)₂] is presented. These complexes react with bidentate N-donor ligands to yield compounds of general formula [M(oda)(N–N)(H₂O)] (M = Ni, N–N = bipy, 3; phen, 5; tmeda, 7. M = Co, N–N = bipy, 4; phen, 6; tmeda, 8), which can be isolated as crystalline solids with different numbers of hydration water molecules. The molecular structures of the compounds 3 and 4 have been shown by X-ray analysis to crystallize with 2.5H₂O. In these compounds, the oda ligand adopts a planar, tridentate conformation (mer) in binding the octahedral metal centre. By contrast, the recent X-ray characterization of 1 has shown that the oxydiacetate ligand completes the octahedral coordination of the metal in the fac arrangement. The DFT method has been adopted to investigate theoretically the flexibility of this ligand. First, calculations have been carried out on the free oxydiacetic acid (H₂oda) and its anion. Then, a model of 1 has been studied in order to find the geometric and energetic implications for the alternative oda conformations (mer and fac) in reaching the octahedral environment about the metal. In general, the geometric parameters are in good agreement with those available from crystal structures. From the energetic viewpoint, the mer conformation is favoured by a small energy difference (2.0 kcal mol⁻¹). On the other hand, the fac conformation in 1 may be justified with the presence of the water hydration molecules (not considered by our model) which may deeply affect the energetics of the overall system by forming strong hydrogen bonds with the oxydiacetate ligand.