Polynuclear oxomolybdenum(VI) complexes of dihydroxybenzoic acids: Synthesis, spectroscopic and structure characterization of a tetranuclear catecholato-type coordinated 2,3-dihydroxybenzoate and a novel tridentate salicylato-type coordinated 2,5-dihydroxybenzoate trinuclear complex

Theoretical (in B3LYP/6-3111++G** level), spectroscopic (FT-IR, FT-Raman) and thermogravimetric studies of gentisic acid and sodium, copper(II) and cadmium(II) gentisates

The DFT calculations (B3LYP method with 6-311++G(d,p) mixed with LanL2DZ for transition metals basis sets) for different conformers of 2,5-dihydroxybenzoic acid (gentisic acid), sodium 2,5-dihydroxybenzoate (gentisate) and copper(II) and cadmium(II) gentisates were done. The proposed hydrated structures of transition metal complexes were based on the results of experimental findings. The theoretical geometrical parameters and atomic charge distribution were discussed. Moreover Na, Cu(II) and Cd(II) gentisates were synthesized and the composition of obtained compounds was revealed by means of elemental and thermogravimetric analyses. The FT-IR and FT-Raman spectra of gentisic acid and gentisates were registered and the effect of metals on the electronic charge distribution of ligand was discussed.

Theoretical and experimental investigation of the oxidized and oxygenated forms of pyrocatechuic acid (2,3-dihydroxybenzoic acid)

The catecholic derivative 2,3-dihydroxybenzoic acid (2,3-DHBA or pyrocatechuic acid) represents a diversity of actions in enzymatic processes. In the present study DFT calculations (at the B3LYP/TZVP level of theory) have been performed for neutral 2,3DHBA and its dimer (models 1-1a), several semiquinone forms of 2,3-DHBA, namely the neutral (models 2-4), the monoionized (models 5-7), the di-ionized (models 8) and the dimer 7a. The more stable species in each case are those with the carboxyl group protonated. Oxygenated adducts were also constructed (models 10-15) in which the dioxygen is either H-bonded to the catecholic or carboxylic hydrogen or it is concerned to act on the intradiol or extradiol carbon atoms. The side-on placement of O(2) on C(2) facilitates the intradiol C-C cleavage. Protonation of the oxygenated on C(1) adduct leads to decarboxylation of 2,3-DHBA. Isolation in the solid state and characterization by ESMS, IR, NMR, electronic spectra of the blue-green oxidized product of 2,3-DHBA (solid 1) supports the possibility of the existence of the semiquinone form or the hydroperoxide-adduct. Experimental spectroscopic data are correlated to the calculated spectroscopic parameters. In the ESMS the decarboxylation and degradation products as well a peroxo-adduct have been detected. Oxygenated species may also account for the plethora of redox signals in the cyclic voltammograms of solid 1 (in DMSO solutions).

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 [TaCl(5-x)(OMe)(x)], formed by dissolving TaCl(5) in MeOH, has been studied. The coordination of 2,3DHBA-H(2)(-) on Ta (V) replacing MeO-terminal groups was monitored via NMR spectroscopy. The yellow solid 1 was isolated from the mixture of TaCl(5), 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 Ta(2)(2,3DHBA)(2)(O)(2)Cl(4)(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 CO(2) (IR), is evident. Solid 1 heated at 900°C, leads to the formation of β-Ta(2)O(5) 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.

Diverse Structures and Dimensionalities in Hybrid Frameworks of Strontium and Lanthanum with Isomeric Dihydroxybenzoates

Investigations of Sr and La derivatives of 2,3-, 2,4-, 3,5-, and 2,6-dihydroxybenzoic acids have revealed that it is possible to obtain chain and layered structures of these materials. [Sr2(2,3-DHB)4(H2O)4].3H2O, I, and [Sr(2,4-DHB)2(H2O)4].H2O, II, are both one-dimensional coordination polymers based on dimers; both the carboxylate and phenolic oxygens bond to the metal in I and only the carboxylate oxygens bond to the metal in II. [Sr(3,5-DHB)2(H2O)2].4H2O, III, has a one-dimensional chain structure with large channels and involves extended Sr-O-Sr connectivity. [Sr(CH3CO2)(2,4-DHB)(H2O)3].H2O, IV, also has a chain structure with extended Sr-O-Sr connectivity, but the chain structure is formed by bridging acetate units, unlike in III where the Dihydroxybenzoate (DHB) is the bridging unit. [Sr(2,6-DHB)2(H2O)2].0.5H2O, V, is a two-dimensional coordination polymer where both carboxylate and phenolic oxygens coordinate to the metal. A lanthanum compound with the composition [La(CH3CO2)2(2,4-DHB)H2O].0.18H2O, VI, with a one-dimensional chain structure with extended La-O-La connectivity, has been prepared. Besides these chain and layered structures, zero-dimensional dimers of rare-earth DHBs with the general formula [La(CH3CO2)2(DHB)(H2O)2], with 3,5- (VII), 2,6- (VIII), and 2,3-dihydroxybenzoic acids (IX) have been synthesized. The diverse structures of the strontium and lanthanum DHBs are described. The study demonstrates that it is possible to obtain interesting structures of metal DHBs with different dimensionalities. The structures appear to be controlled largely by geometrical rather than electronic factors.