Kinetics of Oxidation of Novel Ternary Complexes of Chromium (III) Involving Alanine or Cysteineand 2-(Phenylamino)Acetohydrazide by Periodate

Kinetics of Oxidation of Novel Ternary Complexes of Chromium (III) Involving Alanine or Cysteineand 2-(Phenylamino)Acetohydrazide by Periodate

Ahmed A Abdel- Khalek*

Beni-Suef University, Faculty of Science Chemistry Department

Beni-Suef City, Egypt.

Berry Abd-El Ghani Sabrah, Yasser Abdel Rhman

Fayoum University, Faculty of Science.

Chemistry Department Fayoum City, Egypt.

Abstract Kinetics of oxidation of both ternary complexes [Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+ (HL = 2-

(phenylamino)acetohydrazide , cys = Cysteine and ala. = alanine) by periodate have been studied spectrophotometrically in aqueous solution over a variety of pH and temperature ranges. The rate of the reaction increases with the increasing of periodate, temperature and pH. The reaction is independent on the complex concentration and decreases with ionic strength. The oxidation reaction [Cr(cys)(HL)(H2O)3]2+- periodate obeys the general rate law.

4

d [CrVI] / d t =( k 4 + k 3 / [H+]) [IO -] [Cr(cys)(HL)(H2O)3]2+.

Where,

4

k obs1. = (k 4 + k 3 / [H+]) [IO -]

While the oxidation of [Cr(ala)(HL)(H2O)3]2+ by periodate, found obey the rate equation,

4

d [CrVI] / d t =( k 6 + k 5 / [H+]) [IO -] [Cr(ala)(HL)(H2O)3]2+.

Where,

4 4

k obs. = (k 6+ k 5 / [H+]) [IO -] and [IO -] is the periodate concentration

An inner-sphere process accommodated through replacement of coordinate H2O in the one species, by periodate. The enthalpy of activation H* and entropy of activation S* also are calculated.

Keywords Cysteine; Alanine; Periodate; Hydrazides.

1. INTRODUCTION

   In weakly and neutral acidic solutions Periodate is a two electron oxidant, its reactions with 1, 2-diols are often invoked in cyclic intermediates. [1] Previously, it was shown that in most of its reactions with transition metal complexes, Periodate acts as an inner-sphere oxidant. The kinetics of oxidation of a number of chromium (III) complexes by Periodate has been reported. In some of these reactions, the hydroxo form of the chromium (III) complex is the most involved in the rate determining step. In most of these reactions the hydroxo and the conjugate acid forms are the reactive species at the same time and the oxidation interpreted through the formation of intermediate precursor complex followed by an intra-molecular

   electron transfer step.[2-6] The slower step determines the reaction rate determining step. It is frequently proposed that substitution happens on the chromium (III) reactive species. This seems to be unlikely, even in the existence of the labilizing OH- ligand, since chromium (III) complexes, usually, are known to be inert. The most likely process is one in which the coordination sphere of periodate is joined via coordination of the chromium (III) complex over the hydroxo ligand or by substitution on H4IO – or H5IO6. In some oxidation reactions by Periodate the hydroxo form is not likely reactive species and the conjugate acid form are the reactive species in the pH range of study. The conjugate acid form are the reactive species in the oxidation of diaqua(nitrilotriacetato)-chromium(III) by periodate in aqueous solutions .[7]

   6

   Whole trivalent Cr is assumed to be a highly safe mineral while Hexavalent Cr is more soluble than trivalent Cr and at least five times as toxic. The genotoxicity of CrVI compounds is maybe based on an oxidative DNA damage. Chromium (V) is supposed to play a role in the genotoxic effects of chromium

   (III) complexes. The ease of oxidation of chromium (III) complexes near the physiological pH value is the reason for concern as chromium (V), and chromium (VI) may be readily formed in this pH range. [8] Chromium (V) is reported to be stabilized by some amino acids [2, 9]. Chromium (V) formation is reported in the most and almost all the oxidation reactions to be in fast steps and is not a rate determining step. [5, 6, 10].

   Hydrazones have been demonstrated to own, among other, anticonvulsant, antimicrobial, anti-inflammatory, analgesic, antitubercular, antiplatelet and antitumoral activities. [11, 12]

   In this work, the kinetics of oxidation reactions [Cr(cys)(HL)(H2O)3]2+ – periodate and [Cr(ala)(HL)(H2O)3]2+ – periodate were studied. Choice of this complexes is due to the Hydrazones biological importance [11] and its using as compounds for drug design, organocatalysis, and possible ligands for metal complexes.Cysteine and Alanine exist widely inside and outside of cells. Its a probability to form binary or ternary complex with CrIII from natural food.

2. EXPERIMENTAL

   1. Materials and methods

      All chemicals used in this study were of reagent grade (BDH, Sigma and Aldrich) Stock solutions of sodium metaperiodate were prepared by weight and wrapped with

      aluminum foil and reserved in the dark Solutions of periodate are known to undergo photochemical decomposition [13]. Buffer solutions were made from known concentration of Na2HPO4 and citric acid or hydrochloric acid standard solutions and Potassium chloride [14]. NaNO3 was used in adjusting ionic strength in the different buffered solutions. Doubly distilled H2O was used in all kinetic runs.

      The formation of CrVI was followed at wavelength 356 nm using an Apel PD-303S spectrophotometer to measure the reaction rate. Pseudo-first-order conditions were maintained in all runs by the presence of an excess of periodate. The reaction mixture checked by acrylonitrile adding before the addition of periodate. Polymerization of acrylonitrile was not observed. The uv-visble absorption spectra of the product of this reaction was followed spectrophotometrically for a definite period of time using the doubly beam JASCO UV-530 spectrophotometer. Potentiometric measurements were performed with a Metrohm 702 SM titrino. The titroprocessor equipped with a 728 dosimat (Switzerland-Heriau). The titroprocessor and electrode were calibrated with standard buffer solution [15]. Calculations were performed using computer program MINIQUAD-75. The solution contains 5.0 ml 0.01 mol dm-3 complex, 5.0 ml 0.20 mol dm-3 NaNO3, 5.0 ml 0.04 mol dm-3 HNO3 and 25.0 ml deionized water, was titrated with 0.1 mol dm-3 NaOH at 25 oC. High performance

   2. OXIDATION PRODUCTS.

   The products of the oxidation reactions revealed the producing of a relatively stable CrVI. It is found that at the end of the reaction CrVI was formed by the sym-diphenylcarbazide test.

   The HPLC retention time indicates how long it takes for a compound to come out of the HPLC column. Each peak in the chromatogram indicates the presence of a chemical in the sample. At the same experimental condition the retention time used as a way to determine the presence of these chemicals in other samples. Qualitative analysis involves running a standard that contains the target analytes to obtain the calibration standard retention times. After that run the samples of interest. The seen peaks present in the sample that correspond to peaks in our calibration standard determine the presence of these analytes. HPLC of the products of oxidation reactions revealed the dissociation of the Chromium ternary complexes after oxidation and the releasing of the ligands.

3. RESULTS AND DISCUSSION. 3.1.CHARACTERIZATION OF COMPLEXES.

   Elemental analysis, IR, TGA and cyclic voltammetry were

   used on characterization of complexes. C H CrN O S

   liquid chromatography (HPLC) was performed with An

   11 23

   6 12

   Agilent 1100 series (Waldborn, Germany), quatenary pump(G1311A), Degasser (G1322A), Thermostated Autosamples (G1329A), variable wave length detector (G1314A); and column: Zorbax 300SB C18 column (Agilent Technologies, USA). The pH of the reaction mixture was measured using a Chertsey Surrey, 7065 pH-meter.

   2-(phenylamino)acetohydrazide abbreviated as HL were prepared by the reported method [16, 21]. Elemental anal. (%) for C8H11N3O: Calcd: C, 58.17; H, 6.71; N, 25.44.Found: C,

   58.30; H, 6.90; N, 25.00. IR band assignments of the prepared

   ligand HL NH , 3304 cm-1; NH2, 3342 cm-1; CN , 1260 cm-1; aromatic C=C, 1601 cm-1; CONH, 1650 cm-1.The mass spectrum of HL revealed ion peak at m/e = 165.09 (88.60%).

   The preparation of [Cr(cys)(HL)(H2O)3](NO3)2, was made by adding a mixture of Cr(NO3)3.6H2O ( 0.4004 g, 1.0 mmol ) and Cysteine ( 0.0761 g, 1.0 mmol ) to 50 mL distilled water . After the complete dissolving. The resulting solution was heated under an air reflux condenser at 80 C for 3 hours and the total volume kept as 50 mL. A violet colour appeared after 20 min. and increased gradually within the time. An equal moles of 2-(phenylamino)acetohydrazide (HL)( 0.1651 g, 1.0 mmol ) were added to the CrIII[cys] complex solution and the resulting solution were heated under an air reflux condenser at 80 C for 3 hours. The violet crystals of the complex separated by adding a solution of sodium bicarbonate 0.1 mol dm-3 drop by drop until the pH value of the system reached 7.0. The filtrate was washed several times with distilled water and dried in air at ambient temperature.

   [Cr(ala.)(HL)(H2O)3](NO3)2 prepared by the same method as [Cr(cys.)(HL)(H2O)3](NO3)2 except that Alanine ( 0.0891 g,

   1.0 mmol ) was added instead of Cysteine. The pale brown

   crystals of the complex separated by adding 0.1 mol dm-3 sodium bicarbonate drop by drop until the pH value of the system reached 7.5.

   Ahmed A Abdel- Khalek.

   (Found: C, 24.77; H, 4.00; N, 15.95; S, 6.10 Calcd: C, 25.63; H, 4.50; N, 16.31; S, 6.22%) while C11H23CrN6O12 (Found: C, 27.77; H, 4.90; N, 17.00 Calcd: C, 27.33; H, 4.80; N,

   17.39%).In IR spectrum of complexes bands in the 3650- 3300 cm-1region,were attributed to OH- of the coordinated water molecule. The C=O stretch and the in plane bending of the OH- group associated with carboxylate functionality in the 1390 cm- 1 and 1206 cm-1region in cysteine IR spectrum and in the 1400 cm-1 and 1210 cm-1region in alanine IR spectrum completely disappeared and a new carboxylate band COO- appeared in the region 1380 to1390 cm-1 in the metal complexes IR spectrum, indicating that the carboxylic group of cysteine or alanine participate in the coordination with the metal ion through de- protonation.[17] The shifting of an infrared peaks which denotes the (-CONH) in HL to a lower wave number value in the spectrum of solid complexes indicating that the amide group participates in the coordination with CrIII. The absence of an infrared peak between 450 cm-1 and 490 cm-1, in [Cr(cys.)(HL)(H2O)3]2+ IR spectrum, which denotes the disulfide bond, indicates that CrIII didnt oxidize cysteine to form cystine.

   The thermogram of the complex [Cr(cys)(HL)(H2O)3](NO3)2 shows a weight loss of 11.27% over the temperature range of (50 200) oC corresponding to loss of three molecules of coordinated water molecules (calc. 10.48%); While the complex [Cr(ala)(HL)(H2O)3](NO3)2 thermogram shows the weight loss of 10.10%) being at 200oC corresponds to the loss of three coordinated water molecules (calc. 11.18%).

   The electrochemical behaviour of Cysteine, alanine, 2- (phenylamino)acetohydrazide (HL), CrIII(cys), CrIII(ala)

   ,[Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+ was studied to identify the complexes formation and the easier on oxidation. The cyclic voltammogram(C.V) of ( 1×10-3 mol dm-

   3) [Cr(cys)(HL)(H2O)3]2+ was obtained at 25oC and pH = 2.50 over potential range -1.0 V to 1.5 V versus SCE with scanning

   rate 0.025 V sec-1and 2 mol dm-3of NaNO3 as supporting electrolyte . A well-defined redox anodic electrochemical reaction at -0.33 V and 0.62 V revealed two oxidation reactions and four cathodic reactions at -0.73 V,-0.58 V,-0.48 V, 0.71 V corresponding to a reduction reactions. The cyclic voltammogram of [Cr(ala)(HL)(H2O)3]2+ at the same conditions mentioned above, exhibited an anodic peaks at -0.11 V and 0.77 V due to oxidation reactions and one cathodic peak at 0.42V .The cyclic voltammogram of Cysteine gave anodic peak at -0.87V and cathodic peaks at -0.82, -0.24, 0.45, – 0.09V. The cyclic voltammogram of alanine gave anodic peaks at -0.89 V and cathodic peaks at -0.83,-0.29, 0.44V. C.V of 2- (phenylamino)acetohydrazide (HL) gave anodic peaks at-0.89,- 0.21, 0.35V and cathodic peaks at -1.21V. The cyclic voltametric redox peak values of ligands are different from those of its complexes which gave an evidence of the complexes formation. The oxidation potential values shifted to the more positive upon complexes formation means that the oxidation process becomes more difficult in the complexes.

   The stability constants of the complex [Cr(cys)(HL)(H O) ]2+, [Cr(ala)(HL)(H O) ]2+ and its ligands

   Fig.2. Suggested structures of [Cr(ala.)(HL)(H2O)3]2+.

   The above method used to check the presence of alanine in the oxidation products of [Cr(ala)(HL)(H2O)3]2+ the HPLC analysis revealed the presence of two main peaks at 3.36 min and 4.29 min., Fig.3. The obtained peak at time 4.29 min is identical with the retention time 4.13 min, shown in the chromatogram of alanine standard solution, under the same conditions Fig.4.

   2 3 2 3

   o

   -3 In the second system, the products of oxidation of each

   were determined at T = 25 C and I = 0.4 mol dm by potentiometric pH plotting according the reported method of Irving and Rossotti [18].The potentiometric titration curve of 2- (phenylamino)acetohydrazide and its complexes [Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+ were obtained by plotting pH versus added volume of alkali .The association constant for [Cr(cys)(HL)(H2O)3]2+ = 9.95×10-7and the association constant for [Cr(HL)(ala)(H2O)3]2+ = 1.86×10- 5.The pK value for [Cr(cys)(HL)(H2O)3]2+ = 6.00 and for [Cr(HL)(ala)(H2O)3]2+ = 4.73.

   1. Examination of oxidation products.

      Identifying the reaction products may reflect the reaction mechanism and the oxidation products examined for this reason

      3

      . The calibration standards (alanine, IO -) and the oxidation products of complexes [Cr (cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+ with periodate were check individually by HPLC, using two different mobile phases. In the first, the product of oxidation of (1×10-3) mol.dm-3 [Cr(ala)(HL)(H2O)3]2+ with 1×10-3 mol.dm-3 periodate was analysed using an eluent of isocratic mobile phase with the ratio of mobile phase A and B as 25:75, whereas mobile phase A is 2.5 mM Potassium dihydrogen phosphate with pH = 2.85 and B mobile phase is Acetonitrile and =254 nm [19].

      Fig.1. Suggested structures of [Cr(cys.)(HL)(H2O) 3]2+.

      complex was detected individually at UV, 223 nm on the diode array detector C18 column which was used for separation at a flow 0.5mL/min. the mobile phase composition was 50/50 Methanol/120 mM sodium phosphate, monobasic [20]. The above method used to check the presence of IO – because it was expected that it will be one product of oxidation. The product of oxidation of 1×10-3 mol.dm-3 [Cr (cys)(HL)(H2O)3]2+ with 1×10-3 mol.dm-3 periodate revealed the presence of three peaks with high intensity at retention time 3.27, 3.92 and 4.29 min., Fig.5. The obtained peak at time 3.92 is identical with the retention time 3.89 min, shown in the chromatogram of KIO3 standard solution, under the same conditions Fig.6. while the product of oxidation of 1×10-3 mol.dm-3 [Cr (ala)(HL)(H2O)3]2+ with 1×10-3 mol.dm-3 periodate revealed presence of two peaks with high intensity at retention time 3.04 and 3.95 min., Fig. 7.The obtained peak at time 3.95 is identical with the retention time 3.89 min, shown in the chromatogram of KIO3 standard soluion.

      3

      Fig.3. HPLC Chromatogram of complex [Cr(ala)(HL)(H2O)3]2+oxidation products (10L) injection using an eluent of isocratic mobile phase with the ratio of mobile phase A and B as 25:75, whereas mobile phase A is 2.5 mM Potassium dihydrogen phosphate with pH=2.85 , B mobile phase is Acetonitrile and =254 nm).

      Fig.4. HPLC Chromatogram for alanine (5 L) injection using an eluent of isocratic mobile phase with the ratio of mobile phase A and B as 25:75, whereas mobile phase A is 2.5 mM Potassium dihydrogen phosphate with pH=2.85 and B mobile phase is Acetonitrileand and =254 nm).

      Fig.5. HPLC Chromatogram of complex [Cr(cys)(HL)(H2O)3]2+oxidation products (20L) injection. Eluent (50/50 methanol/120 mM sodium phosphate, monobasic (pH=3.00) and =223 nm).

      3

      Fig.6. HPLC Chromatogram for IO -(10L) injection using eluent (50/50 Methanol/120 mM sodium phosphate, monobasic (pH=3.00) and =223 nm).

      Fig.7. HPLC Chromatogram of complex [Cr(ala)(HL)(H2O)3]2+oxidation products (20L) injection. Eluent (50/50 methanol/120 mM sodium phosphate, monobasic (pH=3.00) and =223 nm).

      The oxidation of [Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+ with periodate were also followed by recording the UV- visible absorption spectra of the oxidation products between 320 and 700 nm as a function of time. A single peak appeared at 365-375 nm and increased with time due to the formation of CrVI which have the same peak at the same pH this provides evidence that CrVI is one of the oxidation products. Fig.8. and Fig.9.

   2. Kinetics of oxidation of [Cr(cys.)(HL)(H2O)3]2+ and [Cr(ala.)(HL)(H2O)3]2+ with periodate reaction in aqueous solution..

   The oxidation of [Cr(cys)(HL)(H2O)3]2+has been studied under condition of ionic strength range (0.2 – 0.6) mol.dm-3, pH range (1.59 – 2.60) and temperature (20 – 40oC) over range

   of periodate and complex concentration; (1.0-4.5) ×10-2 mol.dm-3and (2.0 6.0) ×10-4 mol.dm-3 respectively and the oxidation of [Cr(ala)(HL)(H2O)3]2+ has been studied under the same conditions of ionic strength, temperature , periodate and complex concentrations but the pH range of study was (1.70- 2.78) .

   Fig.8. Change in absorbance as a function of time .curves (1 –

   3

   4

   7) were recorded at 5, 10, 15, 20,25,30,35 mint, respectively from the time of initiation [Cr(cys)(HL)(H2O) 2+] = 1.0×10-3 mol dm-3,pH = 2.80 , I = 0.4 mol dm-3 (NaNO3), [IO -]

   =2.0×10-2 mol dm-3 and T = 30oC Curve (8) spectrum of

   chromate ion (1.0 ×10-3 mol dm-3) at the same pH.

   TABLE 1. Dependence of kobs1 on the initial of [Cr(cys)(HL)(H2O) 2+] at [IO -] = 4.5 × 10-2 mol dm-3, pH =

   3 4

   2.20, I = 0.4 mol dm-3 and temperature = 35±0.1 oC.

   Fig.9.Change in absorbance as a function of time .curves (1 – 8) were recorded at 5, 10, 15, 20, 25,30,35,45 mint, respectively from the time of initiation [Cr(ala)(HL)(H O) 2+] = 1.0×10-3

   TABLE 2. Dependence of kobs2 on the initial of [Cr(ala)(HL)(H2O) 2+] at [IO -] = 2.0 × 10-2 mol dm-3, pH =

   3 4

   104[ Cr(cys.)(HL)(H2O) 2+] 3 mol m-3	104[kobs1 ±SD] (s-1)
   2.0	42.40± 1.61
   3.0	42.10± 1.06
   4.0	42.20±0.95
   5.0	42.40± 1.02
   6.0	42.10±1.16

   -3 o

   2 3

   -3 -3 –

   – 2.25, I = 0.4 mol dm

   and temperature = 35±0.1 C.

   mol dm ,pH = 2.60 , I = 0.4 mol dm (NaNO3), [IO4 ] =2.5×10

   2 mol dm-3 and T = 25oC .

   Kinetic measurements were carried out under pseudo first order conditions with periodate concentration 10 [Cr (III)]. Plots of ln(A – At) versus time, where A and At are absorbance values at infinity and time t respectively, are linear up to 85 % of reaction. Values of the pseudo-first-order rate constants, Kobs, at fixed periodate concentration, ionic strength pH, and temperature, are independent of the initial complex concentration as shown in Table 1,2 indicating first order dependence on complexes concentration, eq.(1,2).

   3

   Rate1 = Kobs1[Cr(cys)(HL)(H2O) 2+] (1)

   2+

   104[ Cr(ala)(HL)(H2O) 2+] 3 mol m-3	104[kobs2 ±SD] 1)	(s-
   2.0	16.80 ± 0.33
   3.0	16.80 ± 0.45
   4.0	16.80 ± 0.57
   5.0	16.80 ± 0.71
   6.0	16.80 ± 0.82

   20C

   70 25C

   30C

   35C

   Rate2 = Kobs2[Cr(ala)(HL)(H2O)3 ] (2)

   40C

   60

   104 k (s-1)

   obs

   4

   50

   The variation of kobs with [IO -] at various temperatures and pH values were found to be linear without intercept

   4

   according to Eq. (3, 4). Plot of Kobs1, Kobs2 against [IO -] 40

   (Fig.10, 11).

   4

   Kobs1 = k1 [IO -] (3)

   Kobs2 = k2 [IO4-] (4)

   The dependence of the reaction rate on pH has been investigated over the above mentioned pH ranges, complex

   [Cr(cys)(HL)(H2O) 2+] or [Cr(ala)(HL)(H O) 2+]

   30

   20

   10

   5 10 15 20 25 30 35 40 45

   103[IO-4](mol dm-3)

   4

   Fig.10.Variation of Kobs1 with [IO -] at pH = 2.60, I = 0.4 mol

   3 2 3

   concentration, temperature and periodate concentration. Variation of (k1 or k2) with pH at different temperatures is listed in Tables (3, 4) .From which the reaction rate increased gradually with increasing pH values. Plotting of the slopes (k1 or k2) versus (1 / [H+]) is found to be linear with an intercept as shown in (Fig.12, 13). This behaviour can be described by Eq.

   (5, 6). For complex [Cr(cys)(HL)(H O) 2+], Eq.5

   dm-3 and [Cr(cys)(HL)(H2O) 2+]= 4.0×10-4 mol dm-3.

   3

   60

   55

   50

   45

   40

   104 k (s-1)

   obs

   2 3

   35

   k1= k4+ k3 (1/[H+]) (5)

   3

   And for complex [Cr(ala)(HL)(H2O) 2+] , Eq.6

   k2= k6+k5 (1/[H+]) (6)

   Where, k1, k2, k3, k4 k5, k6 are constants. From equation (5,

   6) and substitution in equation (1, 2) the following equation can obtain the general rate low equations (7, 8);

   Rate1 = (k4 + k3 / [H+])[IO4-] [Cr(cys)(HL)(H2O)32+] (7)

   Rate2 = (k + k / [H+])[IO -] [Cr(ala)(HL)(H O) 2+] (8)

   30

   25

   20

   15

   10

   5

   0

   10 20 30 40 50

   103[IO-4](mol dm-3)

   3

   Fig.11.Variation of Kobs2 with [IO4-] at pH = 2.25, I = 0.4 mol

   6 5 4

   2 3 dm-3 and [Cr(ala)(HL)(H2O) 2+] = 4.0×10-4 mol dm-3.

   Where;

   3.0

   T= 20C T= 25C T= 30C

   K = (k + k / [H+]) [IO -] (9)

   2.5

   T= 35C

   obs1 4 3 4

   T= 40C

   4

   Kobs2= (k 6 + k5 / [H+]) [IO -] (10)

   The rate of complexes [Cr(cys)(HL)(H2O)3]2+ or [Cr(ala)(HL)(H2O)3]2+ oxidation reactions by periodate were decreasing by increasing the ionic strength of the solution as shown in Table 5, 6 and this behaviour can attributed to the fact that the reactions takes place between different charged species.

   2.0

   k (mol-1 dm3 s-1)

   2

   1.5

   1.0

   0.5

   0 100 200 300 400 500 600

   (1 / [H+] ) mol dm-3

   Fig.13. Variation of k2 with pH at different temperature.

   TABLE 3. Variation of k1 with pH of [Cr(cys)(HL)(H2O)3]2+at different temperatures.

   4.20±0.10

   pH	1/ [H+] (mol-1dm3)	101 [k		1±SD] ( mol-1 dm3	s1	)
   		T = 20C	T = 25C	T = 30C	T = 35C	T = 40C
   1.59	38.90	3.58±0.18	6.16±0.18	8.28±0.21	9.39±0.38
   1.85	70.79	3.61±0.12	4.66±0.17	6.72±0.19	8.97±0.35	10.56±0.59
   2.20	158.49	3.94±0.12	5.21±0.29	7.54±0.23	10.47±0.30	12.42±0.37
   2.60	398.11	5.14±0.23	6.86±0.25	9.91±0.31	13.11±0.43	16.15±0.29

   TABLE 4. Variation of k2 with pH of [Cr(ala)(HL)(H2O)3]2+at different temperatures.

   pH	1/ [H+] (mol-1dm3)	101 [k 2±SD] ( mol-1 dm3 s1 )
   		T = 20C	T = 25C	T = 30C	T = 35C	T = 40C
   1.70	50.12	2.99±0.06	3.84±0.12	4.68±0.10	6.46±0.21	8.89±0.42
   2.25	177.83	4.42±0.11	5.82±0.28	8.32±0.27	10.04±0.47	13.07±0.47
   2.52	331.13	6.10±0.14	8.13±0.28	10.78±0.22	16.72±0.96	18.35±0.79
   2.78	602.56	9.10±0.34	13.09±0.60	17.54±0.41	24.72±0.71	31.65±2.55

   3

   1.7 TABLE 5. Dependence of kobs1on ionic strength at pH = 1.59,

   I	(mol dm-3)	104[kobs1 ±SD] (s-1)
   0.2		13.20±0.27
   0.3		12.60±0.15
   0.4		12.20±0.23
   0.5		12.00±0.19
   0.6		11.70±0.17

   1.6

   T=20oC

   periodate = 4.0× 10-2 mol dm-3 [Cr(cys)(HL)(H2O)

   2+] = 4.0

   1.5

   1.4

   1.3

   k (mol-1 dm3 s-1)

   1

   1.2

   1.1

   1.0

   0.9

   0.8

   0.7

   0.6

   0.5

   0.4

   0.3

   T=25oC T=30oC T=35oC T=40oC

   0 50 100 150 200 250 300 350 400 450

   (1 / [H+] ) mol dm-3

   ×10-4

   mol dm-3

   and temp. = 20 ºC.

   Fig.12. Variation of k1 with pH at different temperature.

   TABLE 6. Dependence of kobs2 on ionic strength at pH = 1.70, periodate = 2.5× 10-2 mol dm-3 [Cr(ala.)(HL)(H2O) 32+] = 4.0

   ×10-4 mol dm-3, and temp. = 40 ºC.

4. DISCUSSION.

Periodate ion is well-known to be involved in complexes equilibrium in aqueous solution as shown in Eq. (11, 12, 13) [22].Under the reaction conditions, the most likely periodate species are IO4-, H4IO6- and H5IO6 .

6

4

I	(mol dm-3)	104[kobs2 ±SD] (s-1)
0.2		38.30±0.51
0.3		20.80±0.12
0.4		19.50±0.44
0.5		14.90± 0.06
0.6		13.40±0.11

H4IO – IO – + 2H2O Kd= 40 (11)

6

H4IO – + H+ H5IO6 Kd1 = 5×102 (12)

6

6

H4IO – H3IO 2- Kd2= 4.3×10-9 (13)

From transition state theory equation, the thermodynamic activation parameters including enthalpy, H*, and entropy,

, 2

S* associated with constant K are composite values and can be obtained by plotting ln Ki / T against 1 / T respectively as shown in (Fig.14, 15). The enthalpy of activation of

,

[Cr(cys)(HL)(H2O) 3]2+, H* have been calculated as 49.31 kJ

The [Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+complexes also undergoes deprotonationprotonation equilibrium as shown in Eqs. (14, 15).

[Cr(cys)(HL)(H2O)3]2+ [Cr(cys)(HL)(H2O)2OH] + +

H+ K7 (14) [Cr(ala)(HL)(H2O)3]2+ [Cr(ala)(HL)(H2O)2OH] + +

H+ K8 (15)

The reported values of K7 and K8 are 9.95×10-7 and

1.86×10-5.mol dm-3, respectively, at 25.0 ºC and I = 0.4 mol

dm-3. The oxidation reactions of [Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H O) ]2+ by Periodate are not likely proceed via

mol-1. The corresponding entropy of activation, S* is equal to 2 3

-153.12JK-1 -1

, the hydroxo form reactive species and the conjugate acid form

,

mol and The enthalpy of activation of [Cr(ala.)(HL)(H2O)3 ] 2+, H* have been calculated as 48.38 kJ

mol-1. The corresponding entropy of activation, S* is equal to

are most likely reactive species in the pH range of study .The complexes deprotonated species is not involved in the rate

-148.94 JK-1 mol-1.

-12.0

-12.2

-12.4

ln( K /T)

i

-12.6

-12.8

-13.0

-13.2

-13.4

, determining step. Also the most likely periodate species

6

present in the pH range covered in this study are H4IO – and H5IO6. The mechanism of oxidation of [Cr(cys)(HL)(H2O)3]2+ by periodate may be represented by the reaction sequence in Eq. (16, 17, 18, 19,20,21) While the mechanism of oxidation of [Cr(ala)(HL)(H2O)3]2+ by periodate may be represented by the reaction sequence in Eq. (22, 23, 24, 25,26,27).

The reaction may go on via one or two electron transfer, giving chromium (IV) or chromium (V), respectively, in the rate determining step leading to chromium (VI). Two electron transfer is the most likely pathway. This seems to be confirmed by the absence of polymerization with acrylonitrile.

[Cr(cys)(HL)(H2O)3]2++ H4IO6-

3.20 3.25 3.30 3.35 3.40 3.45

103 ( 1/T ) K-1

Fig.14. Plot of (ln Ki / T) against (1/ T) for complex [Cr(cys)(HL)(H2O) 3]2+ over different temperatures.

[Cr(cys)(HL)(H2O)2H4IO6]++ H2O K9 (16)

[Cr(cys)(HL)(H2O)3]2++ H5IO6

3

[Cr(cys)(HL)(H2O)2H5IO6]+2+ H2O K10 (17) [Cr(cys)(HL)(H2O)2H4IO6]+

-11.2

CrV+ HL + IO – + Cys. k 11

3 12

[Cr(cys)(HL)(H2O)2H5IO6]+2

(18)

-11.4

ln( K /T)

i

-11.6

-11.8

-12.0

-12.2

-12.4

CrV+ HL + IO – + Cys. k (19) CrV + IVII fast CrVI + IVI (20)

2IVI fast IVII + IV (21)

6

[Cr(ala)(HL)(H2O)3]2++ H4IO –

[Cr(ala)(HL)(H2O)2H4IO6]++ H2O K13 (22) [Cr(ala)(HL)(H2O)3]2++ H5IO6

3

[Cr(ala)(HL)(H2O)2H5IO6]+2+ H2O K14 (23) [Cr(ala)(HL)(H2O)2H4IO6]+

-12.6

3.20 3.25 3.30 3.35 3.40 3.45

CrV+ HL + IO – + ala. k 15

[Cr(ala)(HL)(H2O)2H5IO6]+2

(24)

V –

103 ( 1/T ) K-1

Fig.15. Plot of (ln Ki / T) against (1/ T) for complex [Cr(ala)(HL)(H2O) 3]2+ over different temperatures.

Cr + HL + IO3 + ala. k 16 (25) CrV + IVII fast CrVI + IVI (26)

2IVI fast IVII + IV (27)

3

IO – and Alanine in products confirmed using HPLC which support the dissociation of the complex [Cr(cys.)(HL)(H2O) 3]2+ or [Cr(ala.)(HL)(H2O) 3]2+ after oxidation.

From the above mechanisms, the rate of the reactions can

be described by the following equations:

6

6

6

6

d [CrVI] /dt = k12 [Cr(cys.)(HL)(H2O)2 H5IO 2+] + k11 [Cr(cys.)(HL)(H2O) 2H4IO + ]. (28) d [CrVI] /dt = k16 [Cr(ala)(HL)(H2O)2 H5IO 2+] + k15 [Cr(ala)(HL)(H2O) 2H4IO + ]. (29)

Using a steady state approximation for

6

6

[Cr(cys.)(HL)(H2O)2 H5IO 2+],[Cr(cys.)(HL)(H2O) 2H4IO + ],

1. Yousif Sulfab, Nadir A. Al-Jallal, Inner-sphere oxidation of cis- diaquabis(oxalato)chromate(III) by periodate in aueous weakly acidic solutions, Transition Metal Chemistry 29: 216220, 2004.

2. Ahmed A. Abdel-Khalek, El-Said M. Sayyah and Hassan A. Ewais, Kinetics and mechanism of oxidation of the chromium(III)-DL- valine complex/periodate reaction. Evidence for iron(II) catalysis, Transition Met. Chem., 22, 375±380 (1997).

3. Ahmed A. Abdel-Khalek, El-Said M. Sayyah and Hassan A. Ewais, Kinetics and mechanism of oxidation of chromium(III)-L-arginine complex by periodate, Transition Met. Chem., 22, 557±560 (1997).

4. Ahmed A. AbdellKhalek, Mahmoud M. Elsemongy, Kinetics of the oxidation of diaqua(nitrilotriacetato)- chromium(III) by periodate in aqueous solutions, Transition Met. Chem., 14, 206-208 (1989).

   [Cr(ala)(HL)(H2O)2 H5IO 2+] and [Cr(ala)(HL)(H2O) 2H4IO +

5. A. Pechova, L. Pavlata, Chromium as an essential nutrient: a

   6

   ], and substitution in Equation (28,29) gives:

   6

   review, Veterinarni Medicina, 52, 2007 (1): 118.

6. NasmaD.Eljack,YousifSulfab, Kinetics of the biphasic oxidation of

   3 5 6 12

   d [CrVI] /dt = k12k10{[Cr(cys.)(HL)(H2O) 2+][H IO ] }/(k +

   3 6

   k-10 ) +k11k9 {[Cr(cys.)(HL)(H2O) 2+][H4IO -]} /( k11+ k-9) (30)

   3

   d [CrVI] /dt = k16k14 { [Cr(ala.)(HL)(H2O) 2+] [H5IO6] }/( k16

   3 6

   + k-14 ) +k15k13 { [Cr(ala)(HL)(H2O) 2+] [H4IO -]} /( k15+ k-13)

   (31)

   Substitution in equation (30, 31) from equation 12 gives:

   3

   d[CrVI]/dt = k12k10 {[Cr(cys.)(HL)(H2O) 2+][H5IO6] }/(k12 + k-

   3

   10 )+Kd1 k11k9 {[Cr(cys.)(HL)(H2O) 2+][H5IO6]}/(k11+k-9) [H+]

   (32)

   3

   3 5 6 15 –

   d [CrVI] /dt = k16k14 { [Cr(ala)(HL)(H2O) 2+] [H5IO6] }/( k16 + k-14 ) +Kd1 k15k13 { [Cr(ala)(HL)(H2O) 2+] [H IO ] } /( k + k 13)[H+] (33)

   Equation (32) is identical to the experimental rate law at

   Equation (7) and, equation (33) is identical to the experimental rate law at equation (8) therefore:

   k 4= k12k10 /( k12 + k-10 ) (34)

   k 3 = Kd1 k11k9/( k11+ k-9) (35)

   k 6= k16k14 /( k16 + k-14 ) (36)

   k 5 = Kd1 k15k13/( k15+ k-13) (37)

   In conclusion it may be stated that the oxidation of both [Cr(cys)(HL)(H2O)3]2+ and [Cr(ala)(HL)(H2O)3]2+ by Periodate are likely proceed via the conjugate acid form and the hydroxo form reactive species are not the reactive species in the pH range of study. The oxidation of [Cr(ala)(HL)(H2O)3]2+ by periodate is faster than that of [Cr(cys)(HL)(H2O)3]2+ as obvious from the values of H* and voltammogram oxidation potentials. The complex formation between CrIII and HL gave an attention to use the ligand in CrIII carcinogenic inhibition.

   REFERENCES

   1. Stewart R (1969) Oxidation mechanisms, application to organic chemistry. Benjamin, Benjamin .

   2. Ismat H. Ali ,Yousif Sulfab, Kinetics and mechanism of oxidation of cis-diaquabis(glycinato)chromium(III) by periodate ion in aqueous solutions, Transition Met Chem, DOI 10.1007/s11243-012-9663-7, publisshed online: 08 November2012.

   3. A.A. Abdel-Khalek, Kinetic studies on the oxidation of [N- phenylethylethylenediaminetriacetate] chromium(III) by periodate, Transition Met. Chem., 15, 112-115 (1990).

   aqua ethylenediaminetetraacetatochromate(III) and cis- diaquabis(oxalato)chromate(III)complexes by N-bromosuccinimide in aqueous solutions:Formation of long- livedchromium(IV)complexes, Polyhedron 44 (2012) 2833.

7. Ahmed A. Abdel-Khalek, Said M. Sayyah and Fadia F. AbdeI- Hameed, Kinetics and mechanism of oxidationof chrornium(III)- tetra-oxalylurea complex by periodate, Transition Metal Chemistry, 19 (1994) 108-110

8. S.Rollas, N.Gülerman, H.Erdeniz, Synthesis and antimicrobial activity of some new hydrazones of 4-fluorobenzoic acid hydrazide and 3-acetyl-2,5-disubstituted-1,3,4-oxadiazolines, Farmaco 2002, 57, 171-174.

9. .G. Küçükgüzel, S. Rollas, .Küçükgüzel, M. Kiraz, Synthesis and Antimycobacterial activity of some coupling products from 4- aminobenzoic acid hydrazones, Eur. J. Med. Chem. 1999, 34, 1093- 1100.

10. Frank S. H. Head, H. A. Standing, Reactions in aqueous solutions of sodium metaperiodate exposed to artificial light, J. Chem. Soc. 1952, 1457-1460.

11. G.Svehla,Vogels text book of macro and semimicro qualitative inorganic analysis , 5 th ed.,Longman group London, 1979,pp.51-53.

12. R. G. Bates,Determination of pH, Wiley-Interscience, New- York (1973).

13. S. M. Emam, F. A. El-Saied, S. A. Abou El-Enein, H. A. El-Shater, Cobalt(II), nickel(II), copper(II), zinc(II) and hafnium(IV) complexes of N- (furan-3-ylmethylene)-2-(4- methoxyphenylamino)acetohydrazide, Spectrochimica Acta Part A 72 (2009) 291297.

14. K.M. Dokken , J. G. Parsons a, . McClure , Jorge L. Gardea- Torresdey,Synthesis and structural analysis of copper(II) cysteine complexes, Inorganica Chimica Acta 362 (2009) 395401.

15. H. M. Irving, H. S. Rossotti ,The calculation of formation curves of metal complexes from pH titration curves in mixed solvents, J. Chem. Soc., 1954, 2904-2910.

16. K.D. Golden , O.J. Williams, Amino Acid, Fatty Acid, and Carbohydrate Content of Artocarpus altilis (Breadfruit), Journal of Chromatographic Science, Vol. 39, June 2001.

17. Dionex Corporation. Acclaim Mixed-Mode WAX-1 Product Manual, Determination of Iodide and Iodate in Seawater and Iodized Table Salt by HPLC with UV Detection ,2009, LPN 2312 PDF 09/09, Sunnyvale, CA.

18. ShashidharKerur , Kallangouda Alagawadi , DHailiang zhu and Fakkirappa Manvi, Synthesis, Characterization And Molecular Docking Simulation Of Disubstituted 1,3,4-Oxadiazole Derivatives For Antibacterial Studies, World Journal Of Pharmacy And Pharmaceutical Sciences, Volume 3, Issue 10, 573-585.

19. Headlam, H., Weeks, C., Turner, P., Hambley, T., Lay, P,Dinuclear chromium(V) amino acid complexes from the reduction of chromium(VI) in the presence of amino acid ligands: XAFS characterization of a chromium(V) amino acid complex. , Inorganic Chemistry ,2001, 40:5097.