Transition Met. Chem. 1t, 259-261 (1986) Reactions of diaquonitrilotriacetatonickelate(II) 259

Kinetic Studies on the Formation of Ternary Complexes in the Reaction of Diaquonitrilotriacetatonickelate(II) with Amino Acids in Aqueous Solution

Asim Das, Soma Gangopadhyay and Debabrata Banerjea*

Department of Chemistry, Inorganic Chemistry Division, University College of Science, 92 Acharya P.C. Road, Calcutta 700009, India

Summary

Kinetics of formation of ternary complexes in the reaction of Ni(NTA)(HeO)~- with several amino acids, LH +- (glycine, c~- alanine, [3-alanine, L-valine and L-phenylalanine) have been studied by a pH indicator method using stopped-flow spec- trophotometry. The results conform to 1/kob~ = 1/k + [H+]/ kKTL where K is the equilibrium constant for formation of Ni(NTA)(-L)(H20) 2-, and k is the specific rate constant for the subsequent rate-determining ring-closure step leading to Ni(NTA)(=L) 2-. For the different amino acids studied, the k values decrease in the sequence: glycine > ct-alanine > L-phenylalanine > L-valine > [3-alanine. These k values are ca. 1000 times lower than the values for compiexation of Ni(NTA)(H20)~ with NH3 and imidazole and the spread in k values is much less than the pK~ values of the amino acids. The relative rates are enthalpy controlled and the AS* values are highly negative in conformity with ring closure as the rate determining step.

Introduction

Mixed ligand complexes are biologically very important 0, 2), hence studies on the thermodynamics and kinetics of forma- tion of such complexes are 0f much significance. Although the biological role of nickel(II) is much less understood than that of several other bivalent metal ions of the first transition series, there are some advantages in using nickel(II) in studies on thermodynamics and kinetics of complex formation in view of the fact that, due to contribution of ligand field stabilization energy, nickel(II) forms quite stable complexes which are also formed at a relatively slow rate; in fact for the labile bivalent metal ions of the 8 first transition series d nickel(II) is the least labile for ligand substitution. Thermodynamics (3) and kine- tics (4) of formation of a mixed ligand (ternary) complex in the reaction of Ni(NTA)(HzO)~ with imidazole have been reported and the results compared with those of the NH3-Ni(NTA)(H20)~- system, furnishing valuable informa- tion on factors affecting the stability of ternary complexes. Kinetic studies on the formation of ternary complexes in the reaction of Ni(NTA)(H20)~ with bipyridine and o-phenan- throline have also been reported (s). Results of our studies on the formation of ternary complexes in the reaction of Ni(NTA)(H20)~ with several different amino acids are reported in this paper. The amino acids were selected for this study in view of their known biological role and these were so chosen that the influence of ligand basicity and of steric factors on the kinetics of formation of such complexes could be ob- tained.

* Author to whom all correspondence should be directed.

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Results and Discussion

Spectral observations of aqueous solutions containing Ni- (NTA)(H20)~ (0.004 M) and glycine (0.04 M) at pH 5.5 indi- cated formation of the ternary complex Ni(NTA)(Gly) 2- (Figure 1) and for such a mixture pH has no effect in the pH 4-7 range. At pH 5.5, variation of glycine concentration from 0.008 to 0.08 M also has no effect. These observations indicate that under these conditions formation of the ternary complex is quantitative and hence, for the kinetic studies, the system can be treated as non-reversible. Other amino acids studied behave similarly. These reactions involve the replacement of two c i s -aqua ligands of Ni(NTA)(H20)2 by the amino acid anion serving as a bidentate chelating ligand binding in the usual fashion. The reactions have been followed in the ca. 5.5-6.0 pH range for glycine, a-alanine, L-phenylalanine, and L-valine and in the ca. 6.3-6.9 pH range in the case of [3- alanine. In this pH range these amino acids are present essen- tially in undissociated zwitterion form (LH -+) and the overall reaction may be represented as follows:

M- + LH -+ ~ ML 2- + H + (1)

where M- = Ni(NTA)(H20)2. Since formation of the ternary (ML -) in this system is not accompanied by any complexes z

pronounced change in spectra, it was found convenient to fol- low the rate of this reaction by a conventional pH indicator method (6), i.e. by monitoring the rate of H + generation. Since the rates of these reactions are pH-dependent it was necessary to use a partially buffered system so that the total change in

0,06-

B

. / "%o 004 A "-.\.

/ . .--. \ . \ . Z4: D

< o.o2-

- - . Z - - - ~ o ~

Waveteng~'h (rim)

Figure 1. Absorption spectra: A, Ni(NTA)(H20)2, 0.004 M at pH 5.5; B, Ni(NTA)(H20)[, 0.04 M + glycine, 0.04 M at pH 5.5. A change in glycine concentration from 0.08 to 0.008 M causes no change in the spectrum; C, Ni 2+, 0.004 M + glycine, 0.04 M at pH 5.5. The observed kmax agrees with that reported for Ni(Gly) + (13); D, Same as C at pH 6.98. The observed Xm~ corresponds to that reported for Ni(Gly)2 (13).

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260 A, Das, S. Gangopadhyay and D. Banerjea

pH due to the complete reaction did not exceed 0.2 pH unit which was adequate enough to follow the progress of the reac- tion spectrophotometrically by means of a pH indicator. Under this condition little error is involved by assuming that the reaction occurred at a pH which is the average of the pH at the beginning and at the end of the reaction. Observations were made on the rate of formation of ternary complexes in each system at different concentrations of amino acids and different pH's within the aforementioned range and keeping the amino acid concentration in ten-fold excess or more com- pared to Ni(NTA)(H20)~- so that pseudo-first order conditions were maintained. The observed rate constant (kobs) was found to conform to:

kobs = kKTL/{[H +1 + KTL} (2)

which on rearrangement leads to:

1/kobs = 1/k + [H +]/kKTL, (3)

where TL stands for the total concentration of LH -+ ill the solution, K is an equilibrium constant, k is a rate constant and [H +] the hydrogen ion concentration of the reaction as obtained from the average pH value (vide supra) above and refering to an experimentally determined pH vs [H +] calibra- tion curve for the electrode system. Equations (2) and (3) are in conformity with the following scheme:

M- + LH -+ KL M- �9 LH_+ _Kk (M-LH)-

(Outer sphere complex) 11 K3

(M=L) 2- ~ (M-L) 2- + H + (4)

where (M-L) 2- is an inner-sphere complex having the amino acid anion bound in an unidentate fashion presumably through the carboxyl oxygen and (M=L) 2- is the final product having the amino acid anion bound as a bidentate ligand in the usual fashion. This leads to the relationship as given in Equations (2) and (3) with K = KaKzK3. Values of k and K were evaluated graphically by the least squares method using kob s values obtained at different concentrations of LH- and different pH. kobs values were the average of the values obtained in two to three experiments for each set of conditions (which indicated good reproducibility in kobs). From values of the rate constant k obtained at three different temperatures (35~ ~ the cor- responding activation parameters AH* and AS* were evalu- ated using the Eyring equation (7). The results are given in Table 1.

The observed AH* values are 50.5 to 56.6 kJM -1 with AS* values in the -70.9 to -54.8 JK-ZM -1 range. The highly nega- tive AS* values are in conformity with a rate-determining step involving ring closure (chelation). For the different amino acids investigated, the k values (in the temperature range studied) are in the sequence: glycine > a-alanine > L-phenyl-

Transition Met. Chem. 11, 259-261 (1986)

alanine > L-valine > ~-alanine; the relative rates are enthalpy-controlled, since the most reactive glycine has the most negative AS* value and least AH* value (see Table 1). However, both k and AH* values cover a rather narrow range compared to that of the pKa values (for the dissociation reac- tion LH + ~ L- + H +) of the amino acids. This is also in keeping with the proposed mechanism (Equation 4). The observed sequence parallels the behaviour of glycine (8), ct- alanine (9) and ~-alanine (9) towards hexaaquo-cobalt(II) and -nickel(II) ions; the lower reactivity of [3-alanine compared to that of c~-alanine is due to higher ring strain in the former. For the a-amino acids bulky substituents as in L-phenylalanine and L-valine cause reduction in rate, compared to that of glycine, due to steric effect. In this connection it is worth mentioning that formation of ternary complex in the reaction of Ni(trien)- (H20) 2+ with glycine 0~ and sarcosine (11) has also been reported to involve similar ring closure as the rate determining step.

Experimental

Materials and reagents

The amino acids (A.R. grade) were recrystallised and weighed amounts were dissolved in H20 to prepare the stock solutions. Standardised nickel(II) nitrate solution (0.004 M) and a weighed amount of recrystallised nitritotriacetic acid (0.004 M) were mixed and adjusted to the desired pH. The buffer solutions were prepared from recrystallised NaOAc, KH2PO4 and Na2HPO4 and standardised solution of HOAc. The indicator stock solutions (0.002 M) were prepared in 50% EtOH. All other chemicals used were either of reagent grade or purified by known methods. Doubly distilled H20 was used to prepare all the solutions. The pH meter with its electrode system was calibrated using standard buffer solutions of known pH and solutions of acids and alkalies of known [H+].

Apparatus and procedure

The spectra of the complexes were recorded using a Carl- Zeiss spectrophotometer (VSU-2 P; Jena, East Germany), pH measurements were made with a digital pH meter (Model No. 335; Systronics, India) and the kinetic runs were monitored in a stopped-flow spectrophotometer (SF-3A; Hi-Tech, UK) coupled to an oscilloscope (OS 1000, Advance Instruments, UK) and a transient recorder (DL 905; Data Lab, UK). The observed trace of the absorbance versus time as displayed on the oscilloscope was finally recorded in a XY-t recorder (PL-3; J. J. Lloyds, U.K.). The flow module of the SF-3A had arrangements for thermostating (_+ 0.05 ~ the reacting solu- tions and the observation cell of the flow module. For follow- ing the reactions by the pH indicator method, methyl red

Table 1. Values of kinetic and thermodynamic parameters for the formation of ternary complexes between Ni(NTA)(H20)~-amino acids (LH+): Ni(NTA)(H20)2, 0.002 M; LH -+, 0.02-0.05 M; I, 0.1 M (KNO3).

LH • pK~ ~) 105 K k (s -1) AH* AS§ 35 ~ 40 ~ 45 ~ 35 ~ 40 ~ 45 ~ (kJmo1-1) (JK-lmo1-1)

Glycine 9 . 6 1 3.98+0.01 6.00+0.02 8.88+0.06 3.96+0.06 5.42+0.05 7,60+0.04 50.55+0.36 -70.92+1.15 ct-Alanine 9 . 7 0 2.40_+0.01 3.70+0.01 5.50+0.03 3.54+0.04 5.10_+0.03 7.00+0.03 52.65+0.17 -65.40+0.58 L-Phenylalanine 9 . 1 0 5.10-+0.02 7.08+0.05 9.80+0.05 3.36+0.01 4.80+0.02 6.66+0.04 53.55+0.77 -62.64+2.47 L-Valine 9.50 2.40 + 0.01 3.68_+ 0.01 5.52 + 0.04 3.28 + 0.02 4.72 + 0.02 6.56 + 0.03 53.90 + 1.22 -63.44 + 3.80 [3-Alanine 10.15 1.40+_0.01 2.20+0.01 3.75+0.05 2.60+_0.01 3.70+0.01 5.33+0.02 56.56+1.54 -54.84+4.80

a) Values at 25~ I, 0.1 M, corresponding to the equilibrium: LH • ~ L- + H §

Transition Met. Chem. 11, 261-264 (1986)

(pKa, 4.80) (12) was used for glycine, c~-alanine, L-phenyl- alanine and L-valine, whereas bromothymol blue (pKa, 7.0) (12) was used for ~-alanine. Observations were made at 550 nm in systems having methyl red indicator and 620 nm in those hav- ing bromothymol blue indicator. The indicator concentration was 2 �9 10 .5 M in all the cases. One of the two driving syringes of the flow module of the stopped flow system was filled with an aqueous solution of Ni(NTA)(H20)f (0.004 M) adjusted to the desired pH (5.5-6.9) and ionic strength (I) of 0.1 M with KNO3, and the other was filled with the amino acid solution (I, 0.1 M with KNO3) at the same pH buffered with HOAc (8 • 10 .4 M) and requisite amount of NaOAc systems using methyl red and with phosphate buffer ([H2PO4], 1.2 x 10 .3 M) and the requisite amount of HPO 2- in the sys- tem using bromothymol blue indicator. Data from the absorb- ance versus time traces were analysed graphically by the method of least-squares using the relationship for pseudo-first- order reactions, since pseudo-first-order conditions were maintained in the experimental solutions.

Acknowledgements

The investigations were carried out with financial assistance from Department of Science & Technology, Government of India, New Delhi. This and the award of a senior research

[Cu([18]aneN49)] 2+ and [Ni([17]aneN48)] z+ dissociation kinetics 261

fellowship (S.G.) and a junior research fellowship (A.D.) are gratefully acknowledged.

References

(1) R. P. Martin in H. Sigel (Ed.), Metal ions in Biological @stems, Vol. 2, New York, 1973, p. 1. - (2) H. Sigel, Angew. Chem. Int. Edit. Engl., 14, 394 (1975); H. Sigel in D. Banerjea (Ed.), Coordination Chemistry-20, Pergamon Press, Oxford, 1980, p. 27. - (3)D. Banerjea, T. A. Kaden and H. Sigel, Inorg. Chem., 20, 2586 (1981). - (4) D. Banerjea, T. A. Kaden and H. Sigel, lnorg. Chim. Acta, 56, L53 (1981). - (5) j. C. Cassatt, W. A. Johnson, L. M. Smith and R. G. Wilkins, J. Am. Chem. Soc., 94, 8399 (1972). - <6) j. C. Cassatt and R. G. Wilkins, J. Am. Chem. Soc., 90, 6045 (1968). - (7)W. F. K. Wynne-Jones and H. Eyring, J. Chem. Phys., 3, 492 (1935). - (8) G. Davies, K. Kustin and R. F. Pasternack, Inorg. Chem., 8, 1535 (1969). _ (9) K. Kustin, R. F. Pasternack and E. M. Weinstock, J. Am. Chem. Soc., 88, 4610 (1966). - (a0~ R. K. Steinhaus and B. J. Lee, Inorg. Chem., 21, 1829 (1982).

(11) R. K. Steinhaus and L. H. Kolopajlo, Inorg. Chem., 24, 1839 (1985). - (12) M. Windholz (Ed.), The Merk Index, 9th Edn., Merk & Co., N.J., USA (1976), p. 1448 and p. 5989. - (13) D. L. Leussing and D. C. Shultz, J. Am. Chem. Soc., 86, 4846 (1964).

(Received February 12th, 1986) TMC 1504

Acid Dissociation Kinetics of the Copper(II) Complex of 1,5,8,12- Tetra-azacyclo-octadecane ([18]aneN4) and the Nickel(II) Complex of 1,5,8,12-Tetra-azacycloheptadecane ([ 17]aneN4)

Robert W. Hay* and Mahesh P. Pujari

Chemistry Department, University of Stirling, Stirling, FK94LA, U.K.

Ramesh Bembi

Chemistry Department, University of Roorkee, Roorkee - 247667, U.P., India

Summary

The kinetics of the dissociation of [Cu([18]aneN49)] 2+ and [Ni([17]aneN48)] 2+ i n acidic solution have been studied in detail. The dissociation of [Cu([18]aneN48)] 2+ displays satura- tion kinetics beyond 0.4 tool dm -3 HC104 with values of kobs becoming independent of [HC104]. The kinetic behaviour can be rationalised in terms of the scheme,

CuL 2 + + H + ~ CuLH 3+

CuLH3+ k CuZ+ > + protonated ligand

with K = 64 dm3mo1-1 and k = 0.625 s -1 at 25 ~ Saturation kinetics are not observed in the dissociation of

* Author to whom all correspondence should be directed.

�9 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1986

[Ni([17]aneN48)] 2+ and significant amounts of the protonated complex do not occur even in 0.5 tool dm -3 HC104. In this case protonation of the complex may be the rate-determining step. Dissociation of [Ni([17]aneN48)] z+ is 101~ faster than that of [Ni(cyclam)] 2+ at 25 ~

Introduction

The dissociation of copper(II) and nickel(II) from 14-mem- bered tetra-azamacrocycles such as cyclam (1,4,8,11-tetra- azacyclotetradecane) is extremely slow. Thus the half-life of [Ni(cyclam)] 2+ in 1 tool dm -3 HC104 is estimated to be 30

o ( l ) 2+ years at 25 C and the complex [Cu(cyclam)] dissociates extremely slowly even in 6 mol dm -3 HC1 (2). However, com- plexes of the larger 17- and 18-membered tetra-aza-macrocy- cles dissociate quite readily in acidic solution (3' 4). The present paper discusses the kinetics of the acid dissociation of the cop-

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