The investigation of complexation of uranium with biological active ligands is vital for understanding uranium speciation in biosystems. A number of studies have been undertaken for investigating the complexation of uranium in its (VI) oxidation states but similar investigations pertaining to the interaction of uranium, in lower oxidation states, with biological ligands is scarce. The aim of the work is to bridge this gap and studies have been carried out to determine the coordination pattern of pyridine-3-carboxylic acid with uranium(IV). Semi-micro analysis, spectro-analytical techniques, magnetic susceptibility and cyclic voltammetry have been employed for the characterization of the synthesized complex.
The presence of uranium in the crust of the earth is about 2 parts per million and among the elements, ranks about 48th in natural abundance in crust’s rocks. It is also present in seawater, though in very small concentrations of about 3.3 ng/mL, but distributed uniformly all over the world [1,2]. We cannot ignore the importance of research on behavior of this metal in biosystems. A score of publications have been surfaced on the complexation tendencies of uranium(VI) with different biological ligands [3, 4]. The aim of this work is to examine the interaction of uranium, in its (IV) oxidation state, with a ligand of biological significance i.e. pyridine-3-carboxylic acid (
Uranium(VI) stock solution, prepared by dissolving ura-
nyl nitrate hexahydrate, (B.D.H) (1.255 g, 2.5 mmol) in 0.2 M HCl (25 ml), was reduced to uranium(IV) by catalytic hydrogenation, in the presence of platinized alumina (Aldrich) as a catalyst [
mol.wt. 552.22 g/mol. Found: C26.09; H, 2.48; N, 5.55; U, 42.99; Anal. Calcd: C, 26.07; H, 2.53; N, 5.07; U, 43.09, Yield (75.39%).
The complex was analyzed for its uranium(IV), C, H, N and S contents. uranium(IV) analysis was performed by means of a volumetric titration with K2Cr2O7, using barium salt of diphenylamine sulphonic acid as an indicator [
The bathochromic shift for the bands at 644.0 nm and 631.0 nm is almost equal but the increase in the intesity of 644.0 nm band is stronger than that for 631.0 nm. This
indicates that in the complex the largest decrease in transition energy occurs for the transition between the ground level 3H4 and 1G4. The bathochromic shift and an increase in the absorption intensity indicate a more uniform distribution of electronic charge over the whole compound and that around the central ion [7,8].
The infrared spectrum of uranium(IV) complex with pyridine-3-carboxylic acid is shown in
uency shifts (
The results of magnetic susceptibility measurements are summarized in
The cyclic voltammograms of uranium(IV)-pyri-dine-3- carboxylic acid complex (5e−3 M) in aqueous media (2 M HClO4) at different scan rates are shown in
The complex exhibits a pair of cathodic and anodic waves. The anodic peak shifts somewhat towards positive side and cathodic peak moves towards negative side with the raise in scan rate (
The ratio of cathodic to the anodic peak current IpC/IpA
is less than 1 and the ratio decreases with scan
aμso is spin-only magnetic moment. bμeff is an experimental effective magnetic moment calculated assuming curie behavior. cThe number of unpair electrons per uranium ion calculated from the values of μeff using μ = [n(n + 2)]1/2.
rate (Figures 8 and 9). These observations lead to an assumption of an ECE mechanism. The U(V) species formed by the oxidation of U(IV) is unstable and disproportionates into U(IV) and U(VI). No reduction current due to U(VI) species was observed [
The diffusion coefficient of the synthesized complex was calculated using Randles-Sevcik equation [
where Ip is the peak current (in amperes), n is the number of electrons transfer in the reaction, F, R and T have their usual meanings, A is the surface area of the electrode (0.0314 cm2 in this case), C is the concentration (in mole/ cm3), Do is the diffusion coefficient (in cm2/s) and is the scan rate (in V/s).
The system seems not to be reversible but quasi-reversible. The above equation is verified in
The heterogeneous electron transfer rate constant “ks” was calculated by using the following equation [
Where β is the dimensionless parameter known as electron transfer coefficient, Do the diffusion coefficient of the oxidized species in cm2/s, n the number of electrons transferred, the scan rate, the peak separation and F, R and T have their usual meanings. Assuming the value of 0.5 for β the value of heterogeneous electron transfer rate constant “ks”, at 50 mV/s, was calculated to be 1.25e−5 cm∙s−1.
Evaluation of thermodynamic parameters were made by calculating heterogeneous rate constants at various temperatures (
Above equation can be rewrite as
As
Hence, the expression can be modified as
where is the collision number for the heterogeneous electron transfer process and by using the equation
, its value can be determined at a particular temperature, where M is the molecular weight of the reacting species, R is the general gas constant and T is the temperature in Kelvin.
The value of, an apparent free energy of activation, at 298 K were calculated using the equation.
The value of ∆H* and ∆S* for each of the synthesized complex were evaluated from the slope and intercept of the plots of versus 1/T (
The values of
and for the synthesized complex are found to 16.97, −60.71 and 35.06 respectively. The oxidation of uranium(IV) is a non-spontaneous and endothermic process, as specified by positive values of and [18-20].
The study of uranium(IV) complex with pyridine-3-carboxylic acid shows that the ligand coordinates only via its pyridine ring nitrogen with metal to ligand ratio 1:2. Ahuja et al. also observed complexation of the same pattern while complexing this ligand with uranyl ion [
The oxidation of the synthesized complex of uranium (IV) leads to the formation of uranium(V) species. Following the oxidation process, disproportionation of uranium (V) takes place, representing an ECE mechanism. Moreover, the oxidation of uranium(IV) at platinum working electrode is an endothermic and non-spontaneous process.