1

The Lability Of Water Exchange rate Of Trans- bis(oxalato) diaquo Chromate(III) Ion With Arginine By Trans-Effect Of Internal Conjugated

Base Formation M. A. Abdullah* , B. K. Aziz Department of Chemistry, University of Sulaimani. Abstract The reaction of the trans- bis(oxalato) diaquo chromate (III) ion with amino acid arginine has

been studied at different temperatures and pH 4.8-5.7 in aqueous solution. The equilibrium constant for this substitution reaction at 250C gives logKequ= 4.63. The kinetic interaction involves two parallelreactions of three different subsequent steps of different Cr(III) species that gives only cis-mono-(Arg-O,N) bis(oxalato) chromate(III) complex according to scheme below:

( ArgO is amino acid Argininato ion, water molecules are omitted and I1, I2 and I3 are intermediate species)

The ∆Ea for the internal water exchange reactions of formation I1, I2 and I3 of steps k1, k2 and k3 are 6.14, 17.2 and 8.85 Kcal/mole, and the ∆S± values are -45.3, -20.36 and –49.3 eu respectively. These data are consistent with interchange associative mechanism and fit to the kinetically trans-effect labilization of water exchange rates of outer sphere complexes of internal conjugated base formation by hydrogen bonding.

Introduction:

Slow ligand substitution rates of inert Cr(III) (d3-configuration) are well known, the rate constant of water-solvent exchange at Cr(III) is ki=0.501X10-6 sec-1at 250C with activation energy ∆Ea =26.6 Kcal/mol. (1). However, many reactions of water exchange of Cr(III) with other ligands were recorded to have higher rates and lower activation parameters than water- solvent exchange in aqueous solution. We have recently studied the equilibrium, kinetic and mechanistic of replacement two axial water molecules of trans-[ Cr (C2O4)2(H2O)2]-2 with amino acids, glycine, alanine and histidine in moderately acid solution of pHs 4.8-6.7 (2). We found that the observed rate of water exchange at conjugated complex [Cr(C2O4)2(H2O)OH]-- is much faster than that of diaquo complexes [Cr(C2O4)2(H2O)2]-. The first rate constant of conjugated base, under excess of amino acid condition was found to follow the rate law: kobs1 = k + k’ [H+]-1 and it is zero order on the amino acid concentrations. While the second

rate is also zero order dependent on amino acid ligands and independent of acid concentration. The suggested mechanism was shown to follow that of Eigen – Willkenson mechanism (3) with the distinctive outer-sphere complex derived from trans-[Cr(C2O4)2(H2O)2]- and its conjugated base, trans-[Cr(C2O4)2(H2O)OH]2-,

with incoming zweeter ion of amino acids in solution.

Several similar studies of Cr(III) reactions with amino acids were reported previously (4,5),but there is no obvious reason why the interchange rates of axial water with different amino acid are faster in Cr(III) conjugated base complexes, so we have studied the reaction of arginine, amino acid polydentate ligand which causes a kinetically trans- effect via of high tendency to from hydrogen bonding formation with reactants trans–[Cr(C2O4)2(H2O)2]2-complex and its conjugated base. This paper reports the results of this study.

(I1)

(I2)

(I3)

Trans-[Cr(ox)2(H2O)OH]2-+ArgOH Trans-Cr(ox)(H2O)OH,ArgOH Trans-[Cr(ox)2(ArgO)OH]2-K'os k1

k -1 k -4

Trans-[Cr(ox)2(H2O)2] -+ArgOH Trans-Cr(ox)(H2O)2,ArgOHKos

k2k -2

k 3

k -3

cis-[Cr(ox)2(ArgO)(H2O)]2-

Trans-[Cr(ox)2(ArgO)(H2O)]2-

k -5

k 6k -6

Kh

outer-sphere

outer-sphere

k 5

k 4

[cis-Cr(ox)2(ArgO)]2-

2

Experiment: All chemicals used in this study were all

reagents analytical grade, oxalic acid, potassium hydrogen carbonate, and potassium dichromate were obtained from Riedel De Haen, Hanover, Sodium nitrate and arginine were bought from BDH.All are used with out further purification.

Jenway 66405 UV-Visible spectrophotometer with locally modified thermostat cell holder was used for measuring absorbance and recording electronic absorption spectra A Buchi chromatographic pump is used to circulate the sample solution from thermostatted reaction vessel to the flow cell. A Philips pH/mv meter PW 9414 type was used for pH measurements. A circulating thermostat bath (LKB Bromma 2209 multitemp.) was used to control temperature to ± 0.1oC. Trans-K[Cr(C2O4)2(H2O)2].3H2O complex has been prepared by following the Dawson procedure (6). The obtained crystals have been washed several time with cold water, alcohol and dried, then dried sample was analyzed for C,H,N,K and Cr before using.

Kinetic and thermodynamic studied were performed by mixing thermostatted solutions of arginine and Cr(III), and adjusting the pH to the required valve pHs with NaOH ( or HNO3). The thermostatted mixture was circulated through the flow cell in the thermostatted block of spectrophotometer at the same temperature then the intensity absorption change (Aobs) was recorded with time.

Result and discussion: Preliminary spectroscopic studies of

exchange of water molecules of trans-[Cr(C2O4)2(H2O)2] -1 with incoming arginine(- N,O ) ligand, show the intensity of absorption band increasing at λmax 550nm(Єmax) and λmax

405nm(Єmax). This increasing in absorption (Єmax) with slightly change in band position of the recanting trans complex [λmax=565nm (Єmax=32) and λmax=416nm (Єmax=34.4)] was anticipated to NO5 chromophore formation (1,7). The log (Aobs –Ao)/(A∞-Aobs) correspond to log [product]/{[CrT] – [product]} versus log of total arginine concentrations shows a straight line, as shown in fig–1 with slope very close to unity. This result sustains also 1:1 ligation complex has occur. The intercept of Fig –1 gives conditional equilibrium constant (log Kcon). So the values of Kequ, for reaction 3 in the form of equation 4, were calculated at different temperatures 25, 30, 40, and 500C to be log Kequ=4.63, 4.69, 4.76, and 4.87 respectively. These values of Kequ are consistent with those were recoded previously for complexion of [Cr(C2O4)2 (H2O)2]- with bidentates glycine , alanine, histidine and oxalate ions (2,8).

Also the rate of complexion was followed with time, under various conditions of temperatures and pHs 4.8 – 6.7 (µ = 0.4 M NaNO3). First order plots of log (Aobs – Ao) vs. time give two crossover good straight lines. This is fit to two consecutive parallel complex reactions with two observed rate constants kobs1 and kobs2 of the two different starting Cr(III) species {[Cr(C2O4)2(H2O)2]- and [Cr(C2O4)2 (H2O) OH]2-} plus ArgOH ligand in solution. The most striking results are the reaction zero order depend on the arginine concentration and one of the rates constant (kobs1) is an acid depended while the other rate constant (kobs2) is acid independent.

The acid dependent rate was analyzed and found to obey the rate low kobs = k’[H+]-1+k. A typical plot of kobs1 vs [H+]-1 was shown in Fig-2,that gives two rate constants k1 and k2 (corresponding to k’ and k respectively) from slope and intercept. So overall there are three kinetically importent reaction pathways with rate determine steps k1, k2 and k3 (corresponded to pseudo first order kobs2) .

ArgOH 2

trans-[Cr(C2O4)2(H2O)2]-

ArgOH + H+ ArgO- + H+....(2)

(log Kh=7.2)

Trans-[Cr(C2O4)2(H2O)2]-+ArgO- cis-[Cr(C2O4)2(ArgO)]2- .....(3)

(log Ka1 = 2.17 and log Ka2 = 9.4)

Kequ =[cis-Cr(C2O4)2(ArgO)]2-

........(4)(trans-Cr(C2O4)2(H2O)2][ArgO-]n

(n=1)

trans [Cr(C2O4)2(H2O)OH]2- ...... (1)

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Fig. –1 A typical plot of log (Abos –Ao)/(A∞-Abos) versus total log[arginine] at temperatures 30oC (■), 40 oC(ڤ) and 50oC(▲) (pH=5.6).

y = 2.02E-07x + 0.0518R2 = 0.9884

00.020.040.060.080.1

0.120.140.160.18

0 100000 200000 300000 400000 500000 600000 700000[H+]-1

k obs

( min

-1)

kobs1

kobs2

Fig.2 The effect of hydregon ion concentration on the observed rate constants kobs1 and kobs2 of the reaction of Trans- bis(oxalato) diaquo Chromate(III) with arginine

The above result suggests that the reaction of trans- bis(oxalato) diaquo chromate with arginine is very similar to that of our previous studies of amino acids glycine, alanine and histidine complexation (2) and follow the same mechanism, but with the slight difference results from hydrogen bond tendency of arginine of free NH2 and other amino groups of non leaving H2O (or OH) on Cr(III) center. This interaction may result a relatively conjugated bases (OICB1 and OIVB2), which are depicted below.

The pectoral illustration shows the probability that both the hydrogen bonding and the trans – effect may contribute and have strong kinetically effect on interchange rate of water substitution in both trans-bis(oxalato)diaquo Cr(III) and its conjugated species. This was virtually observed in both the rates and activation parameters of step k1 and k3 [see scheme below and table –2]. Therefore, using the same assumption as in our pervious studies (2), in which k1, k2 and k3 are rate determine steps of intermediates I1, I2 and I3 formation respectively and these intermediates are followed by rapid rates of internal ring closer reaction to final product(P) (steady state condition) the rate laws corresponding to the scheme below are expressed in equation 5 and 6 and the observed rate constants in eq. 7 and 8 :

N CHC

CH2

CH2

CH2

NH

C

N

NH

H

H

H

H

Cr(III)

H2O

O

O

O O

O

HH

H O

ON CH

C

CH2

CH2

CH2

NH

C

N

NH

H

H

H

H

Cr(III)

H2O

O

O

O O

O

H

H O

O

Outersphere Internal Conjugated Base via Hydrogen Bonding(OICB2)

Outersphere Internal Conjugated Base viaHydrogen Bonding(OICB1)

-0.4-0.2

00.20.40.60.8

1

-1.8 -1.6 -1.4 -1.2 -1 -0.8

log[Arginine]T

log

obs- A

o)/( A

&- A

obs)

4

From the reactions 9 and10 of the scheme below the rates of product formation are: d[p]/dt = k-4[I1]-k4[p]+k2[I2]-k-5[p] ……(5) d[p]/dt=k6[I3]-k-6[p] ……….(6) k1 Kh K’os . 1 kobs1= + k2 ...(7) Kos [H+]

kobs2= k3 ……..(8) According to the above equations 7,8 the

values of k1 K’os/Kos, k2 and k3 are calculated and tabulated in table –2, at different temperatures with activation parameters of each rate determinant step in the reaction. Table –3 gives some related complexation of cis-and trans[Cr(C2O4)2(H2O)2]- .

In comparison, we can see the values of the rate constants in arginine reactions are higher than that for water-solvent exchange at Cr(III) , but very similar to those values were recorded for glycine, alanine and histidine in our previous study (2).

Scheme of the consecutive parallel reactions of [Cr(C2O4)2(H2O)2]- and [Cr(C2O4)2 (H2O)OH]2- (Kos and K’os are outer-sphere associative constants)

Cr

H2O

H2O

O

O O

OArgOH

k2[-H2O]

k -2

Cr

ArgO

H2O

O

O O

O

k5[-H2O]

k -5Cr

O

O

O

O N

O

Cr

OH

H2O

O

O O

OArgOH

k1[-H2O]

k -1

Cr

O

O

O

O H2O

ArgO

k3[-H2O]

k -3Cr

H2O

H2O

O

O O

O,ArgOH

Cr

OH

H2O

O

O O

O

,ArgOHCr

OH

ArgO

O

O O

O

Cr

O

O

O

O N

O

Cr

H2O

H2O

O

O O

O,ArgOH

[-H2O]

(I1)

+

Trans monaquo mono hydroxy bis(oxalato) chromate (III) ion

Kh [-H+]

trans -mono(ArgO) monohydroxy bix(oxalato)chromate(III) ion

Trans-diaquo bis(oxalato)chromate(III) ion

+

Outer sphere complex(OICB2)

Outer sphere complex(OICB1)

Kos

K'os

k-4 k4

Kos

Trans-monoaquo mono(ArgO)bis(oxalato) chromate(III) ion

Cis-mono (ArgO) bis(oxalato) chromate(III) ion

Cis-mono (ArgO) bis(oxalato) chromate(III) ion

Cis-monoaquo mono (ArgO)bis(oxalato) chromate(II) ion

k -6

k6[-H2O]

outer spher complex

(I3)

....(9)

.....(10)

5

Table –2 The derived rate constant values for k1 K’os/Kos,k2,and k3 and calculated activation parameters for

amino acid arginine with trans- bis(oxalato)diaquo chromate(III) ion.

Temp. 0C

k1 (K’os/Kos)x102 sec1

k2 x104sec-1

k3 x104sec1

35 9.76 3.58 1.65 40 12 6.98 1.8 45 12.82 12 2.63 50 15.35 15.7 2.9 55 8.56

∆Ea(Kcal/mol)= 6.14 ∆H≠(Kcal/mol)= 5.51 ∆S≠(Cal/mol)= -45.3

20.1

∆Ea(Kcal/mol)= 17.2 ∆H≠( Kcal/mol)= 16.55 ∆S≠ (Cal/mol)= -20.36

3.92

∆Ea(Kcal/mol)= 8.85 ∆H≠ (Kcal/mol)=8.22 ∆S≠ (Cal/mol)= -49.3

Table – 3 Kinetic Activation parameters for related complexation reactions

with bis(oxalato)diaquo chromate(III).

Complex ∆H≠ Kcal/ mol ∆S≠ e.u trans cis isomerization of [Cr(C2O4)2(H2O)]- 17.5a -15.3 trans cis isomerization [Cr(C2O4)2 (H2O) (OH)]2- 14.6a -23.0 Aquation of [Cr( C2O4)2(H2O)O2CCH3]2- 16.3b -18.8 Aquation of [Cr (C2O4)2(O2CCH3) OH]-3 8.4 b -34.4 Anation of cis-[Cr(C2O4)2(H2O)2)]- with C2O4

= 19.8 -4.8 [Cr(C2O4)2 (H2O) +glyO Trans-,Cis- intermediates 19.80 , 15.26c -10.9 , -22.5 = + alanO- formation respectively 15.15 , 12.75 -27.0 , -33.5 = + HistO-1 15.50, 12.10 -18.33,-33.94 [Cr(C2O4)2(H2O)OH + glyO- 9.35 (Ea 9.97)c -34.47 = + glanO- 8.60 (Ea 9.24) -37.80 = + HistO- 6.67 (Ea 7.33) -46,45 H2Oexchange of [Cr(H2O)6]+3 26d +12

[Cr(H2O)6]+3 + glyO- 18e - 23 (a) M. Casual, G. illauminati and G.OP. Taggi, Inorg. Chemisry; II (5), 1062, (1972). (b) Thomas W. Kallen and Randall, E. Hamm; Inorg. Chem. 18(8), 2151 (1979). (c) M.Abdullah and B. Kamal submitted paper for publication (2002) . (d) R. Plane and H. Tube; J. Phys. Chem, 56,533 (1952). Illuminati and G. Ortaggi; Inorg. Chem. 11, 1052

(1972). (e) M.A.Abdullah, J. Barret, and P.O Brien;J.Chem. Dalton Trans., 1647 (1984) Therefore, this close similarly leads to the

same mechanism as we have mentioned in previous study (2), in addition to that the reaction took place by hydrogen bond formation and trans-effect of OICB1 and OICB2. These may have general great effect on interchange water molecule with incoming ArgO- in the both rate determinant steps and the followed ring closure steps. The values of k1 (K’os/Kos ) are observed larger than that of other amino acids glycine, alanine, arginine and monodentate SCN- substitution (2,9), k1(K’os/Kos) = 12 x 10-2 sec–1 at 400C, ∆Ea = 6.14 Kcal/mole for arginine, while for glycine, alamine and histidine, are 5.18x 10-2, 2.67 x 10-2 and 1.01 x 10-3 sec–1 at 400C, respectively with average ∆Ea = 8.8 kcal/mole. But the values of k3 are observed in the same range to that for glycine, alanine and histidine at

different temperatures, with lower values ∆Ea = 8.8 Kcal/mole.This value is very similar to that obtained in case of CrOH reaction with the above mentioned amino acids (2) and aquation energy of hydroxyl species, [Cr(C2O4)2(O2CCH3)OH]3- (10). Nevertheless, the values of k2 are also very similar to those observed previously (2) in cis- bis(oxalato)monoamino acid monoaquo chromate(III) intermediate formation(I2), with ∆E = 17.2 Kcal/mole. Also this activation energy is observed in the same range of trans

cis isomerization of [Cr(C2O4)2(H2O)2]-(5),, anation of NCS- of cis-[Cr(C2O4)2(H2O)2]-(9),, aquation energy of Cr(C2O4)3 and [Cr(C2O4)2 (O2CCH2)(H2O)]3- (12,10), in these cases the values of activation energies were recorded in the range 13 – 18 Kcal/mole(see table-3).

6

The low values of activation energy (or ∆H≠) in arginine complexation reaction of k1 and k3 may be explained on the basis of probability hydrogen bond formation of OICB1 and OICB2 which react faster after hydrogen bond formation to the intermediate I1 and I3. However, the presence of OH and formation conjugated base as in OICB1 and OICB2 cause great trans effect on water exchange with low values of activations energies.

Consequently, the data of rate constants and activation parameters specially –ve values of (∆S≠) (table –2) for all internal exchange water reactions in this study are consistent

with interchange associative mechanism (Ia) at Cr(III) center and sustain to the existence of outer- sphere conjugated base formation OICB1 and OICB2, which are later reacting much faster in water exchange by trans-effect labilization (11,12). The lability of OH- has been reported before (13,15), as to decrease the value of activation energy or (∆H≠) by 2-6 kcal/mole. However the OH- group and that results of outer sphere conjugated base via hydrogen bonding are able to donate more electron density to Cr(III) center and hence labilize it to react more faster in water interchange reaction.

References:

1- Thomasw. Kallen and Randall E. Hamm; Inorg. Chem (1979), 18(8),2151. 2- M. A. Abdullah and B. K.Aziz, paper submitted for publication 2002. 3- D. F. Sheriver, P. W. Atkins and C. H. Langford, “Inorganic chemistry”, Oxford university

(1992), ch. 15, p-477. 4- K.R. Ashely, J.8. Leipoldt and V.K.Josghi; Inorg Chem (1980), 19, 1608-1612. 5- a-M. Casula, G.Illumunati and G. Ortaggi, Inorg. Chem., 115(5), 1062(1972).

b-K. R. Ashley and I. Trent, Iorg. Chim. Acta, 163, 159(1989). 6- B.E.Dawson;”Practical Inorganic Chemistry” Methuen and Co.Ltd. London,2nd

ed.(1967).214. 7- Kotra V. Krishuamurty and Goron M. Harris; “ The chemistry of the metal oxalato

complex” a chpter of chemical reviews, Vol.6(1), (1961), published by A. Chem. Soc. 8- Return to ref. 238. 9- N. V. Duffy and J. E. Ericy; J. Am. Chem. Soc. 89(1967), 272. 10- A- K. R. Ashely and S. Kulprathipanja, Inorg. Chem. 11, 444(1980).

B - Thomas W. Kallen and Randall, E. Hamm; Inorg. Chem. 18(8), 2151 (1979). 11- J. Benjamin Coe and S.J. Glen Wright; “Trans-effects in octahedral transition metal

complex’, coordination chemistry reviews, Vol. 203 (1), 5 – 80 (2000). 12- Ryuni Nakata, Kimio Isa and Hisaya Oki; Analytical Science, 17,453-456 (2001). 13- H. Taube Chem. Rev. 50, 69 (1952).