Mohamed S. A. Hamza

A novel spectrophotometric method for batch and flow injection determination of cyanide in electroplating wastewater

A sensitive and highly selective spectrophotometric method is described for the determination of cyanide. It is based on a reaction of cyanide with aquacyanocobyrinic acid heptamethyl ester (ACCbs) reagent (orange color) at pH 9.5 to give dicyanocobester (DCCbs) (violet color). The increase of the absorption bands of the reaction product at 368 and 580nm and the decrease of the reagent band at 353nm are linearly proportional to the cyanide concentration. The method is used in static mode for determining cyanide over the concentration range 0.04-1.20mugml(-1) with a detection limit of 0.02mugml(-1) and for hydrodynamic analysis of 0.4-5.2mugml(-1) cyanide. Application for batch and flow injection monitoring of cyanide in electroplating wastewater samples gives results agree within+/-1.2% with those obtained by the standard potentiometry using the cyanide ion selective electrode. The method is practically free from interferences by PO(4)(3-), NO(3)(-), NO(2)(-), SO(4)(2-), F(-), Cl(-), Br(-), I(-), S(2-) and SCN(-) ions and gives results with average recoveries of 97.6-99.2%. Advantages offered by using ACCbs as a chromogen for cyanide assay are: (i) high selectivity and sensitivity of the coordination site of the reagent towards cyanide ion; (ii) fast reaction, since legation takes place at the axial position of the reagent; (iii) good solubility and stability of the reagent in aqueous solutions over a wide pH range; (iv) high stability of the reagent (ACCbs) and the colored complex product (DCCbs) and (v) possible absorbance measurements at three different wavelengths.

Kinetics and mechanism of the reactions of Pd(II) complexes with azoles and diazines. Crystal structure of [Pd(bpma)(H2O)](ClO4)2·2H2O

Substitution reactions of the complexes [Pd(bpma)(H2O)]2+, [Pd(bpma)Cl]+, [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+, where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO4 at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H2O)]2+ and [Pd(bpma)Cl]+, are more reactive than the complexes of dien, viz. [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+. Also, the aqua complexes, [Pd(bpma)(H2O)]2+ and [Pd(dien)(H2O)]2+, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (Delta S++, Delta V++) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O was determined by X-ray diffraction. Crystals are monoclinic with the space group P2(1)/c. The coordination geometry of [Pd(bpma)(H2O)]2+ is distorted square-planar. The Pd-N (central) bond distance, 1.958(5) A, is shorter than the other two Pd-N distances, 2.007(5) and 2.009(5) A. The Pd-O distance is 2.043(5) A.

Mechanistic studies on the reaction between R2N-NONOates and aquacobalamin: evidence for direct transfer of a nitroxyl group from R2N-NONOates to cobalt(III) centers.

The gaseous radical nitric oxide (•NO, NO) is a signaling molecule that plays a vital role in biology. It facilitates vasodilation and inhibits platelet aggregation in the cardio-vascular system, initiates the pro-inflammatory immune response, and regulates neurotransmission.[1,2] Impaired NO bioavailability is associated with a wide variety of vascular pathologies, including endothelial cell dysfunction.[3] Conse-quently, there is considerable interest in NO donor molecules, such as 1-(N,N-dialkylamino)diazen-1-ium-1,2-diolates (R2N-NONOates; Figure 1), which spontaneously decompose by first-order acid-catalyzed processes to release up to two NO molecules and the corresponding amine.[4–6] R2N-NONOates are widely used as NO precursors in studies of NO-dependent biological processes[5,7] and as NO prodrugs with applications in NO-releasing biomaterials, wound healing, organ protection, and chemotherapy.[5,8] R2N-NONOates can also be combined with anti-inflammatory drugs[9] and incorporated into dendrimers, nanoparticles, or microspheres to optimize their delivery.[10–12] These species are typically synthesized by reacting amines with NO(g) at high pressures.[6,13,14] O2-alkylated R2N-NONOate conjugates with considerably enhanced stability have also been developed.[15–17]

REACTIONS OF CHROMIUM AND MOLYBDENUM CARBONYLS WITH A QUADRIDENTATE SCHIFF BASE

Abstract Interaction of a Schiff base derived from 2-hydroxyacetophenone and ethylenediamine (hapenH2) with M(CO)6, M = Cr and Mo, in THF under atmospheric pressure gave the oxo derivatives M(O) (hapen) with the metal atom in +4 formal oxidation state. IR spectra of the complexes revealed the presence of the M = O bond. The dihydride complex MoH2(CO) (hapen) was also isolated from the reaction of Mo(CO)6 and happen H2 under reduced pressure; v(CO) and v(Mo-H) frequencies were clearly indicated in its IR spectrum. The three complexes were found to be paramagnetic. Magnetic susceptibility measurements suggested that the d 4 Mo atom in MoH2 (CO) (hapen) exists in a low spin configuration. Electronic spectra of the ligand and its complexes in different donor solvents displayed bands due to compound-solvent, charge-transfer complexes.

Detailed kinetic and thermodynamic studies on the cyanation of alkylcobalamins. A generalized mechanistic description

Ligand substitution equilibria of different cobalamins (XCbl, X = Ado, CF3CH2, n-Pr, NCCH2 and CN−) with cyanide have been studied. It was found that CN− substitutes the 5,6-dimethylbenzimidazole (DMBz) moiety in the α-position in all cases. A reinvestigation of the reactions of coenzyme B12 (X = Ado) and CF3CH2Cbl with CN− and an investigation of the same reaction for X = n-Pr, demonstrate that the unfavorable formation constants in these cases require very high cyanide concentrations to produce the 1 ∶ 1 complex, which causes the kinetics of the displacement of DMBz by cyanide to be too fast to follow. The kinetics of the displacement of DMBz by CN− could be followed for X = β-NCCH2 and CN− to form NCCH2(CN)Cbl and (CN)2Cbl, respectively. Both reactions show saturation kinetics at high cyanide concentration and the limiting rate constants are characterized by the activation parameters: X = NCCH2, ΔH≠ = 85 ± 2 kJ mol−1, ΔS≠ = +97 ± 6 J K−1 mol−1, ΔV≠ = +12.7 ± 0.5 cm3 mol−1; X = CN−, ΔH≠ = 105 ± 2 kJ mol−1, ΔS≠ = +81 ± 6 J K−1 mol−1 and ΔV≠ = +13.1 ± 0.3 cm3 mol−1. These parameters are interpreted in terms of a limiting D mechanism. A complete analysis of the trans effect order of the substituent X is presented. The results enable the formulation of a general mechanism that can account for the substitution behavior of all the investigated alkylcobalamins.

Kinetic and thermodynamic studies on ligand substitution reactions and base-on/base-off equilibria of cyanoimidazolylcobamide, a vitamin B12 analog with an imidazole axial nucleoside

Ligand substitution reactions of the vitamin B12 analog cyanoimidazolylcobamide, CN(Im)Cbl, with cyanide were studied. Cyanide substitutes imidazole (Im) in the alpha-position more slowly than it substitutes dimethylbenzimidazole in cyanocobalamin (vitamin B12). The kinetics of the displacement of Im by CN- showed saturation behaviour at high cyanide concentration; the limiting rate constant was found to be 0.0264 s(-1) at 25 degrees C and is characterized by the activation parameters: DeltaH(not =) = 111 +/- 2 kJ mol(-1), DeltaS(not =) = +97 +/- 6 J K(-1) mol(-1), and DeltaV(not =) = +9.3 +/- 0.3 cm3 mol(-1). These parameters are interpreted in terms of an I(d) mechanism. The equilibrium constant for the reaction of CN(Im)Cbl with CN- was found to be 861 +/- 75 M(-1), which is significantly less than that obtained for the reaction of cyanocobalamin with CN- (viz. 10(4) M(-1)). pKbase-off for the base-on/base-off equilibrium was determined spectrophotometrically and found to be 0.99 +/- 0.05, which is about 0.9 pH units higher than that obtained previously in the case of cyanocobalamin. In addition, the kinetics of the base-on/base-off reaction was studied using a pH-jump technique and the data obtained revealed evidence for an acid catalyzed reaction path. The results obtained in this study are discussed in reference to those reported previously for cyanocobalamin.

The chemistry of vitamin B12. Part 30. Co-ordination of amines, pyridines and diazines by aquacyanocobinamide; nitrogen-containing bases as probes for basicity and other effects in metal–ligand bonding

Equilibrium constants K have been determined by UV/VIS spectrophotometry for the substitution of co-ordinated H2O in the cobalt(III) corrinoid aquacyanocobinamide by NH3, four primary alkylamines (pK values 5.3–8.5), NH2NH2, NH2OH, pyridine, four 4-substituted pyridines (pK values 1.9–9.8) and three diazines in aqueous solution with I ca. 0.1 mol dm–3 at 25 °C. The results demonstrate that both the amines and the six-membered heterocycles obey the linear free energy relationship log K=a·pK+b, where a= 0.58 and b=–2.6 for the amines and a= 0.47 and b=+0.2 for the azines, and that NH2NH2, NH2OH and the 1,2-diazine pyridazine all show enhanced values of log K which can be ascribed to operation of the α effect. Present and published data have been summarised to show that bases belonging to the three families (amines, six-membered heterocycles end five-membered heterocycles) can routinely be used to probe the role of at least four factors (basicity, the ‘group-specific’ factors which distinguish the three families, the α effect and steric effects) in metal–ligand bonding.

Local Intracoronary Eptifibatide versus Mechanical Aspiration in Patients with Acute ST-Elevation Myocardial Infarction.

Objectives. We compared local delivery of intracoronary eptifibatide via perfusion catheter to thrombus aspiration in primary PCI. Background. Perfusion catheter increases local concentration of the drugs at the culprit site and prolongs their residency time. Methods. 75 patients with acute STEMI were randomized to three groups: 25 received local intracoronary eptifibatide and verapamil via perfusion catheter; 25 patients were managed by Diver CE thrombectomy device and 25 patients by primary PCI without thrombus aspiration. Primary end point was assessment of postprocedural TIMI flow, MPG, and corrected TIMI frame count (cTFC) in the culprit vessel. Results. Perfusion catheter was superior to thrombus aspiration and conventional PCI as regards MBG (68% versus 36% in Diver CE and 20% in the control arm; P value = 0.002), with shorter cTFC rates than thrombectomy and control groups (20.76 ± 4.44 versus 26.68 ± 8.40 and 28.16 ± 5.96, resp.; P = 0.001). TIMI flow was not different between the 3 groups. Eptifibatide led to less time to peak CK (13.12 hours versus 16.5 and 19.5 hours, respectively, P value = 0.001). Conclusion. Local intracoronary eptifibatide by perfusion catheter reduces thrombus burden with better results in microvascular perfusion assessed by cTFC and MBG compared to aspiration device or conventional PCI.

Substitution behaviour of an unusual seven-coordinate iron(III) complex in different solvents

Abstract In order to clarify the solution chemistry of 3d metal hepta-coordinate complexes, a detailed mechanistic study of the two-step substitution reaction between the “tweezer-like” iron(III) complex, [Fe(dapsox)(H 2 O) 2 ] + (H 2 dapsox=2,6-diacetylpyridine-bis-(semioxamazide)), and SCN − was performed in water and methanol as solvents. Based on the observed rate law and reported activation parameters ( ΔH # , ΔS # and ΔV # ) for both reaction steps in aqueous solution, the operation of an I a mechanism for the diaqua, an I d mechanism for the aquathiocyanato, and an I d or D mechanism for the aquahydroxo forms of the Fe(III) complex, is proposed. The operation of a limiting D mechanism is proposed for both reaction steps in methanol as solvent.

Effect of ester side chains and solvent polarity on the electronic spectra and coordination chemistry of aquacyanocobyrinic acid heptamethylester

Abstract Aquacyanocobyrinic acid heptamethyl ester (Aquacyanocobester) has been prepared from vitamin B 12 after converting the seven amide side chains into methyl esters. The solvent dependence of the absorption spectrum of aquacyanocobester has shown that the wavelengths of the γ-band varied from 362.5 nm in non-polar solvent (CCl 4 ) through 359.5 nm in a more polar solvent (CH 3 CN) to 353 nm in protic solvents (H 2 O, CH 3 OH). The equilibrium constants K L for the coordination of some selected nitrogen and sulfur containing ligands by aquacyanocobester have been determined spectrophotometrically at 25 ∘ C in order to assess the effect of the side chain (COOCH 3 ) on the values of K L . The values of log K L for the coordination of imidazole, 4-aminopyridine, pyridine, 4-cyanopyridine, bromoethylamine, N 3 − and SO 3 2− were 3.95, 4.3, 2.19, 0.75, 2.25, 2.33 and 4.25, respectively. These values are less than those obtained in the case of aquacyanocobinamide where the side chains are CONH 2 . The LFER (log K L =a p K a +b ) has been investigated in water and methanol. The values of a and b of the LFER are 0.49 ± 0.03 and −0.25 ± 0.16 for the coordination of aquacyanocobester by pys in aqueous solutions and 0.50 ± 0.02 and −0.45 ± 0.13 in CH 3 OH. The value of ( a ) obtained in the case of H 2 O agrees with that previously obtained in the case of aquacyanocobinamide but the value of ( b ) is more negative.