The chemistry and preparation of tantalum complexes with 2,3-dihydroxy benzoic acid: Experimental and theoretical investigation

The effect of 2,3-dihydroxybenzoic acid (2,3DHBA, pyrocatechuic acid) on the chloro-alkoxo-species [TaCl5−x(OMe)x], formed by dissolving TaCl5 in MeOH, has been studied. The coordination of 2,3DHBA-H2− on Ta (V) replacing MeO-terminal groups was monitored via NMR spectroscopy. The yellow solid 1 was isolated from the mixture of TaCl5, with neutral 2,3-DHBA, in MeOH. From this solid the elemental (C, H and Ta), the thermogravimetric analyses, the IR, NMR, ESR and electronic spectra support the formula Ta2(2,3DHBA)2(O)2Cl4(MeO)4. The ESR spectrum of solid 1, at 4.2 K, shows a half-field signal apart from a multiline signal around g = 2, supporting evidence for semiquinone and Ta (IV) presence. The occurrence of superoxide radical, in the low temperature of ESR spectrum recording, cannot be ruled out. By heating the solid 1 at 500 °C, an oxide phase showing porous character (SEM) and retaining CO2 (IR), is evident. Solid 1 heated at 900 °C, leads to the formation of β-Ta2O5 orthorhombic phase, as the XRD pattern indicates. The hydrolytic process of solid 1, in aqueous solutions, has been studied; the presence of paramagnetic species generated in situ upon addition of base and the consequent degradative process of 2,3-DHBA, under aerobic conditions is obvious. In order to gain information for the structure of solid 1, DFT calculations have been performed for some theoretical models, based on the empirical formula of solid 1. The calculated structural and spectroscopic parameters have been correlated to experimental results. The energy optimized structures may give an idea about the way of MeCl and MeOMe formation as well some possible intermediates of the hydrolytic mechanism.