Pergamon

INTRAMOLECULAR REARRANGEMENT OF ZINC, CADMIUM AND MERCURY COMPLEXES OF 1,3-

PROPYLENEDIAMINETETRAACETIC ACID IN SOLVENT MIXTURES : NMR STUDY

FE1 LI,” RUIFANG SONG, QUN LIU, YOUGANG MAO and SONGYAN QIN

Key Laboratory for Molecular Spectra and Structure, Institute of Theoretical Chemistry, Jilin University, 130023 Changchun, P.R. China

(Received 30 Nooember 1995 ; accepted 9 Fcbruar~~ 1996)

Abstract-The intramolecular acetate rearrangements of the Zn”, Cd” and Hg” complexes of 1.3-propylenediaminetetraacetic acid were studied by proton NMR spectra in the binary solvents D,O/DMSO-d,, D,O/methanol-d, and DzOidioxane, respectively. An increase in the rate of the nitrogen inversion process was observed with increasing amount of the organic component present in the solvent mixture. In the range of temperatures from 25’ C to 8O’C, the activation energy and the activation entropy of the acetate rearrangement process were found to decrease with increasing concentration of the organic component. The interaction of organic component with the complex is suggested. The rate constant of the intramolecular process is related to the relative permittivity of the solvent mixture by the Born equation. Copyright 0 1996 Elsevier Science Ltd

A wide range of solvents has been used to study chemical processes and it has been realized that the effect of changes in solvent can often be profound on chemical systems. For example, the equilibrium constant for the simple disproportionation of cuprous ions changes twelve orders of magnitude between using pure water and the aqueous ace- tonitrile mixture with an acetonitrile mole fraction of 0.05.’ The investigation of some metal complexes in water/dioxane mixtures has also demonstrated that their equilibrium constants depend upon the relative amount of cosolvent.‘.3 The dielectric prop- erty of a solvent has been taken into account for the solvation of ions or dipolar particles and the Kirkwood and Born methods working with relative permittivity of mixed solvents have been widely used to evaluate effects of mixed solvent on chemi- cal processes involving ions or dipolar particles.4 On the other hand, Waghorne considered the domi- nance of interactions of solute with near neighbor solvent molecules in determining the chemical chan-

*Author to whom correspondence should be addressed.

ges resulting from changes in solvents.’ Simply, the solute was preferentially solvated by the component with which it interacted more strongly. The pref- erential solvation was analogous to complexation, with the better solvent taking the role of the ligand. This coordination model attributes the changes in the thermodynamics of solvation simply to the changes in the composition of the solute’s coor- dination, or inner solvation. sphere. Strehlow’s studies showed the existence of preferential solv- ation in mixed solvent systems” and Bekrirek sug- gested a procedure to estimate the composition of solvation spheres around dipolar molecules or tran- sition state in binary mixtures by studying effects of solvent mixtures on spectra and reaction rates.’

The acetate intramolecular rearrangement pro- cesses of the aminopolycarboxylate metal com- plexes have been studied in aqueous solution by proton NMR.X ” They may occur either as a A. A conversion or as a nitrogen atom inversion. The former may occur when the Mu- -O(metal- oxygen) bonds are labile, while the latter is possible only if the M-0 and M-N(metal nitrogen) bonds are all labile. By proton NMR spectra one could deter-

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3830 FE1 LI etal.

mine which process occurs on the NMR time scale. The intramolecular rearrangement processes of metal aminopolycarboxylates have also been stud- ied in water/organic molecule cosolvents. The results showed that the rate constant of the nitrogen inversion increases with increasing concentration of organic molecules. “.lh In this paper, we select the Zn”, Cd” and Hg” complexes of 1,3PDTA to study the effect of solvent on their isomerization with the addition of DMSO-d,, methanol-d, and dioxane in D,O solution.

EXPERIMENTAL

1,3PDTA was synthesized according to the Weyh and Hamm method.17 The 1,3PDTA complexes of zinc, cadmium and mercury ions were prepared by mixing 1,3PDTA ligand with the metal nitrate by 1 : 1 mole ratio in an appropriate amount of distilled water and adjusting the pH of the solution to ca 7.0 by the dilute solution of HCl and NaOH, then removing water by heating. The products were dried in vacua at 70°C.

The dried complexes were dissolved in DzO with a concentration of 0.2 M. The organic solvents DMSO-d,, methanol-d, and dioxane were added respectively into the D,O solutions of the complexes by a minute-amount injector. The pH of the D,O/dioxane solution of the Zn-1,3PDTA complex was 7.0. The pH of the D20/DMSO-d,, D,O/ methanol-d, and D,O/dioxane solutions of the Cd- 1,3PDTA complex were 6.5, 3.5 and 3.5, respec- tively. The pH of the D,O/DMSO-d, and DzO/ methanol-d, solutions of the Hg-1,3PDTA complex were 7.0.

The ‘H NMR measurements were performed using a Varian FT-80A NMR spectrometer at a resonance frequency of 79.542 MHz. The chemical shifts were measured relative to DSS(sodium 3- (trimethylsilyl)-1-propanesulfonate). The sample temperature was controlled by a Model 1906790- 11 variable-temperature controller calibrated by a coppercontantan thermocouple to the precision of kO.5”C.

RESULTS AND DISCUSSION

The ‘H NMR spectra of the Zn-, Cd and Hg- 1,3PDTA complexes have been reported previously. “Go’ The ‘H NMR spectrum of the ace- tate methylenic protons of the Zn-1,3PDTA com- plex in D,O at room temperature is an AB pattern. However the acetate methylenic protons of the Cd- 1,3PDTA give rise to a multiplet composed of the AB part of an ABX pattern superimposed on an AB pattern. The AB part of an ABX pattern is due

to the 1,3PDTA complexes of “‘Cd and ‘13Cd which have a natural abundance of 25% and a nuclear spin of l/2, while the AB pattern arises from the 1,3PDTA complexes of the other isotopes of cadmium. The ‘H NMR spectrum of the acetate methylenic protons of the Hg-1,3PDTA in D,O at room temperature is also an AB pattern super- imposed on the AB part of an ABX pattern due to the 1,3PDTA complex of “‘Hg which has a natural abundance of 16.9% and a nuclear spin of l/2, and the AB pattern results from the 1,3PDTA com- plexes of the other isotopes of mercury. The AB patterns of the acetate methylenic protons indicate that the M-O bonds of these complexes are labile while the M-N bonds are nonlabile on the NMR time scale in D,O at room temperature, i.e., that the A, A conversion occurs at a fast rate, whereas the nitrogen inversion is a slow process under these conditions. With the increase of the temperature, the AB quartets collapse and progressively coalesce to a singlet, indicative of a progressive increase in the rate of the exchange among the acetate groups to give the nitrogen inversion which averages the environments of the acetate methylenic protons and as a consequence results in the coalescence of the AB quartets.

When an appropriate amount of the organic sol- vents, in our case, dioxane, methanol-d, and DMSO-d,, was added respectively into DzO solu- tions of these complexes, the broadening or collapse of the AB quartets was also observed for Zn-, Cd- and Hg-1,3PDTA complexes. With increasing mole fraction of the organic components, the broadening or collapse of the AB patterns became more remarkable. It suggests that the rate of the nitrogen inversion process increases with increasing amount of the organic component. Figure 1 shows rep- resentatively the ‘H NMR spectra of the acetate methylenic protons of the Cd-1,3PDTA complex in D,O/methanol-d, solvent mixtures with various mole fraction of methanol-d,.

The two low-intensity lines of the AB quartet are barely visible at room temperature and are not shown in Fig. 1. The two shoulder lines are the AB part of the ABX pattern arising from the 1,3PDTA complexes of the “‘Cd and “Cd isotopes. x is the mole fraction of the organic component in solvent mixture. The dependence of the rate constant k of the nitrogen inversion upon the mole fraction x of the organic component in the binary solvents of the Cd- and Hg-1,3PDTA complexes is illustrated in Fig. 2. The rate constants at various compositions of solvent mixtures were obtained by matching experimental ‘H NMR spectra with theoretical spectra by computer simulation.‘h

Figure 2 shows that the k increases with increas-

Intramolecular acetate rearrangements

i

0.127

c, 0.087

L 0.055

c, 0.0

I I I

3.20 3.10 3.00

Fig. I. The ‘H NMR spectra of the acetate methylenic protons of the Cd-1,3PDTA complexes in D,O/meth- anol-tl, solvent mixture at the temperature of 25”C,

pH = 3.5.

ing s and hence it is expected that the activation energy of the nitrogen inversion process would decrease with increasing x. The thermodynamic parameters of the nitrogen inversion process for the Zn-. Cd- and Hg-1,3PDTA complexes in the solvent mixtures with various I are listed in Table 1. All the parameters were obtained using the Arrhenius equation and Eyring’s absolute rate theory.” As we expected, the activation energies decrease with increasing X. The same results were also obtained for Cd-EGTA in the mixed so1vents.‘6 A possible explanation of this is the interaction of organic molecules with the complex. The organic molecules could interact with metal center to facilitate the

(a)

(b)

20.00 - 0.00 0.05 0.10 0.15 0.20 0.25

X Fig. 2. Dependences of the rate constants k of the nitro- gen inversion upon the mole fraction .Y of the organic components in D,O/DMSO-d, (+), DzO/methanol-ci, (A) and D,O/dioxane 0 for (a) Cd-1.3PDTA and (b)

Hg-1.3PDTA.

dissociation of the nitrogen. The CH? releasing elec- tron groups on DMSO and CH,OH may play a significant role in facilitating the dissociation of the nitrogen. For dioxane, the two oxygen atoms on it could enhance its possibility of encountering the metal ion.

In addition, the negative value observed for ASI in aqueous solution strongly suggests a net increase in the bonding interactions on approaching the transition state. a situation which would occur in the existence of extensive solvent participation. which has been explained by the water-dis- placement mechanism.“.” In the solvent mixtures the more negative values for ASI were observed, which would suggest the net increase in the bonding interactions of organic molecules with the complex on approaching the transition state.

The interactions of organic molecules to the Mm N bond would be supported by the variation of the chemical shift of the acetate methylenic protons in

3832 FE1 LI et al.

Table 1. The thermodynamic parameters of the intramolecular processes for Zn-, Cd- and Hg-1,3PDTA in the solvent mixtures with various x

.x

0.0 0.0025 0.0154

EuikJ mall’

52.7 kO.4“ 46.4kO.4 35.4+0.5

AH$/kJ mall’ ASS/J mall’ K-’

50+4 43.6f0.4 32.6kO.5

-86k6” -104*1 -141+1

AG$/kJ mol-’

76k4” s1.2*0.2 83.4kO.3

-68f7 69.0f0.2 -71*4 68.6kO.2

-123k4 67.8 f0.2 -144+14 67.OkO.2 - 176k 10 66.7kO.2

-68+1 66.8f0.2 -89+5 65.9kO.2 -97$4 65.OkO.2

-108k12 68.7 + 0.2 - 132k4 69.7 kO.2 - 139+7 70.3 kO.2 -141+9 70.0$_0.2

Zn- 1,3PDTA D,O/dioxane pH = 7.0

Cd- 1,3PDTA D,O/DMSO-d, pH = 6.5

0.0 50+2 48+2 0.025 49*1 47*1 0.118 33fl 30+1 0.186 26+5 24+5 0.262 16f3 13+3

Cd-l ,3PDTA 0.0 D,O/dioxane 0.01 pH = 3.5 0.03

53*1 50.4 _t 0.4 47k2 44k2 44&l 41+ 1

Hg-1,3PDTA D,O/DMSO-d, pH = 7.0

0.0 39*4 36*4 0.025 31+1 28+1 0.046 28k3 26f3 0.084 28k2 25+3

’ These data are quoted from ref. 20.

the solvent mixtures. When DMSO-d, methanol-d, and dioxane were added respectively into the D,O solution of the Zn-, Cd- and Hg-1,3PDTA complexes, the chemical shifts 6* and SB moved upfield and the chemical shift at the higher field moved faster than that at the lower field. The inter- actions of the solvent with metal ion weakens the M-N bond and as a result increases the proton shielding. Moreover, the chemical shift observed in the experiment is an average of the complex moi- eties bound and unbound by the solvent molecules. With increasing organic components the ratio of

the complex moieties bound by the organic solvents increases and the upfield movement of the chemical shifts becomes larger.

When approaching the transition state from the ground state in the isomerization of the metal com- plexes, water or organic molecules play a role both as solvent and as reactant. Therefore, it is reason- able to analogize the intramolecular process with the reaction between complex ion and solvent mol- ecule. For this type of reaction, the dependence of the rate constant upon the relative permittivity of the solvent can be discussed by the classical electro-

(a) 3.60, I lb)

3.20 -

2.60 -

2.40 -

3.60 -

3.20 -

2.00 L I I 3.001 ' ' ' ' 0 ' 0 1 0.010 0.016 0.020 0.026 0.012 0.014 0.016 0.016 0.020

l/&r l/&r Fig. 3. Plots of In k us l/a, in D,O/DMSO-d, (+), D,O/methanol-d, (A) and D,O/dioxane (0) for

(a) Cd-1,3PDTA and (b) Hg-1,3PDTA.

Intramolecular acetate rearrangements

static theory involving the Born equation. By apply- ing the Born energy to the transition-state theory, the following equation is obtained

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3. 4.

plerrs-Purr B. IUPAC Chemical Data Series No. 22. Pergamon Press, Oxford (1979). H. Sigel, Chem. Sot. Rw. 1993. 22. 255. J. W. Moore and R. G. Pearson, Kinetics und nwh- anism, A Stucfv cf Homogeneous Chemical Rcwtions. 3rd edn. Wiley-Interscience, New York (1981). W. E. Waghorne, Chem. Sot. Rw. 1993. 22, 285. H. Schneider and H. Strehlow, Z. Ph.lx. C’hcvn. 1966. 49. 44.

Ink = Ink,, + ,~f~~~~, (1)

where k represents the rate constant in a medium of infinite dielectric constant. L is the Avogadro constant. Ze is the charge on the ions. E, stands for the relative permittivity of a medium. r and Y: represents the radius of the reactant ion and the activated complex. respectively. Since r: is larger than I’, the rate should be somewhat greater in a medium of lower relative permittivity. Equation (1) predicts a linear relation between In k and 1 /e, with a positive slope. Figure 3 exhibits the plots of In k vs 1 it:, for the Cd- and Hg-1,3PDTA complexes in various solvent mixtures. Apparently, the exper- imental results are in good agreement with the theoretical prediction for larger E, (or smaller x). For the smaller E,. the k values are smaller than those predicted by eq. (1).

Ackno,~l~~9~ment--We would like to thank National Key Laboratory of Computational Theoretical Chem- istry of Jilin University for financial support.

REFERENCES

I. B. G. Cox, A. J. Parker and W. E. Waghorne. J. PI7J*.s. Chrm. 1974, 78, I73 I.

2. D. D. Perrin. Stahilitj, Constants @‘Mrtul-Ion Com-

5. 6.

7.

8.

9.

IO.

II.

12.

13.

14.

15. 16.

17.

18.

19.

20.

21.

V. Bekarek. C’olkct. Czech. C’/wm. Cot71777u77. 1989. 54. 3162. M. C. Gennaro. P. Mirti and C. Casalino. Po/>,hcrlvon 1983.2, 13. P. Mirti. M. C. Gennaro and M. Vallinotto. Twns.

Met. Chm7-77. 1982, 7. 2. P. Mirti and M. C. Gennaro. J. /7707:9. :Vvuc!. C’hrn7. 1981.43, 3221. R. F. Song, F. Li and L. Li. .Mu,q77. Rcson. Cha77. 1992, 30. 367. Y. Ba. R. F. Song and Z. W. Qiu, Muyr7. Rcson. c‘l7em. 1989. 27. 9 16. F. Li and R. F. Song, Mqn. Rcson. C’hrt71. I99 I. 29. 735. R. F. Song. F. Li and L. Li, Spc,r,troc,hinJica Ac,ta 1993.49A, 285. P. Mirti. Tra77.v. Met. Chem. 1984, 9. 357. R. F. Song, F. Li and C. Z. Zhang. .I. c’/lc~r77. Sock.. Dalton Trams. 1992, 709. J. A. Weyh and R. E. Hamm, Inoq C/7(,777. 1968, 7, 2431. D. L. Rabenstein and B. J. Fuhr. Inory. C‘hc~771. 1972, 11.2430. D. L. Rabenstein, G. Blakney and B. .I. Fuhr, Cu77. J. C’hern. 1975. 53. 787. R. F. Song, L. Li and F. Li. Chin. .I. M77,ytyrl. Rrson. 1992, 9, I 13. J. Sandstrom. Dpamic NMR Spec,tro.~c,o/Jl,. Aca- demic Press. London (1982).