Kinetics and mechanism of substitution reactions of complexes—XII

J.inorg. nucLChem., 1969,Vol.31, pp. 1459to 1465. PergamonPress. Printedin GreatBritain

KINETICS AND MECHANISM OF SUBSTITUTION REACTIONS OF COMPLEXES-XII

S O L V O L Y T I C A Q U A T I O N OF SOME C I S - C H L O R O - A M I N O B 1S-ETHY LEN ED I AM I N O-Co(l I I)-COM PLEXES J. Z S A K 6 , CS. V,~.RHELYI and D. D O B O C A N Faculty of Chemistry, "Babes-Bolyai" University, Cluj, Rumania (Received 6 August 1968) Abstract-The first order rate constants for the solvolytic aquation of the cations: cis-[Co(en)zCl pyridine] 2+, cis-[Co(en)2Cl-/~-picoline]~+ and cis-[Co(en)2C1 y-picoline] 2+ have been determined. The influence of mineral acids and the minimum acidity which can hinder the base hydrolysis was determined. By measuring the rate constants at constant ionic strength and in the presence of l0 -2 M perchloric or nitric acid, at 50, 60, 65 and 70°C, the Arrhenius parameters have been calculated. Meanwhile seventeen new salts of the above cations have been described with [Cr(NCS)4(amine)~]- type anions.

THE NUCLEOPHILIC substitution reaction of trans-[Co(en)2C12]C1 with organic amines has been studied from the preparative point of view first by Meisenheimer and Kiderlen[1], later by Bailar and Clapp[2]. The last authors[2] showed the complexity of this reaction and the impossibility to find a relationship between the basicity (pK) of the amines and the nature of the products obtained. With very weak bases only a trans-cis transformation takes place, or a chloro-aquo-complex: [ C o ( e n ) 2 C 1 H 2 0 ] C 1 2 is f o r m e d .

The action of stronger aliphatic, aromatic or heterocyclic mono- and diamines leads to the formation of [Co(en)2Cl amine]C12 type complexes, or of [Co(en)3]Cla in the pure state or in a mixture [3]. The aquation kinetics of the complex cations of the type [Co(en)2C1 amine] 2+ was studied by many authors. A single temperature study was made by Basolo et a/.[4] They investigated the acid hydrolysis rate of the pyridine,/3- and - y picoline, and of the y-methoxy-pyridine derivatives at 50°C. Tobe has studied the ammonia derivative [5], Panasynk et al. the ethylamine [61, aniline, p-toluidine [71, benzylamine and pyridine [8] derivatives. In the last years also Chan and Leh have made a thorough study of this field. The solvolytic aquation of the methylamine, ethylamine [9], propyl- and isopropylamine [ 10], allylamine, propargylamine [ 1 1], I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

H. Meisenheimer and E. Kiderlen, Justus Liebigs Annln Chem. 438, 217 (1924). J. Bailar and L. B. Clapp,J.Am. chem. Soc. 67, 171 (1945). Cs. V~irhelyi, F. Mfinok and J. Mostis, Studia Univ. Vitor Babes-Bolyai. 11, (1)93 (1966). F. Basolo, J. G. Bergmann, R. E. Meeker and R. G. Pearson, J. Am. chem. Soc., 78, 2676 (1956). M. L. Tobe, J. chem. Soc. 3776 (1959). V. D. Panasyuk and L. G. Reiter, Zh. neorg. Khim. 8, 1131 (1963); 11,607 (1966). V. D. Panasyuk, L. G. Reiter and N. T. Maiboroda, Zh. neorg. Khim. 12, 402 (1967). V. D. Panasyuk and L. G. Reiter, Zh. neorg. Khim, 12, 2434 (1967). S. C. Chan and F. Leh, J. chem. Soc. (A), 1966, 129, S. C. Chart and F. Leh,J. chem. Soc. (A), 1966, 137, S. C. Chan and F. Leh, J. chem. Soc. (A), 1966, 138. 1459

1460

J. Z S A K 0 , CS. V,/~RHELYI and D. D O B O C A N

hydroxylamine[12, 13], halo-alkylamines[14], amino-alcohols[.15, 16], 2-methoxyethylamine, 3-methoxy-ethylamine [ 17, 18] and of some other derivatives have been studied by these authors. The deuterium isotope effect has been studied also by Chan and Leh [19]. In our earlier papers the solvolytic aquation of [Co(en)zCl y-picoline]z+ has been followed in neutral solutions [20] and the aquation of [Co(en)2Cl benzylamine] z+ in acid solutions [21 ]. All these studies showed the following ligand exchange reaction to take place: [Co(en)zCl amine] z+ + HzO = [Co(en)z HzO amine] a+ + CIIn the present paper the solvolytic aquation kinetics of the cations [Co(en)2Cl pyridine] 2+, [Co(en)2Cl/3-picoline] z+ and [Co(en)zCl y-picoline]z+ was studied. These complex ions are formed from trans-[Co(en)zClz]Cl and the corresponding amine in aqueous solutions at room temperature. The majority of the salts of these cations are fairly soluble in water, but they can be easily separated from aqueous solutions by picric acid and by thiocyanate-chromium(IIl) complex anions, as [Cr(NCS)d a- and [Cr(NCS)4(amine)2]- salts which are sparingly soluble. In the present paper seventeen new compounds of the type [Co(en)zC1 (Amine)][Cr(NCS)4(amine)z]z are described, where "Amine" stands for pyridine, fl- and y-picoline, and "amine" stands for aniline, 0-, m- and p-toluidine, benzylamine and p-phenetidine. All these compounds, their formula, mol. weight, yields and analysis data are listed in Table 1. The toluidine and benzylamine derivatives are isomeric compounds, due to the position or structural isomerism of the ligands. For our kinetic study the following complex salts have been used: [Co(en)2Cl Py](NOa)z, [Co(en)zCl y-pic](NOa)z and [Co(en)zCl fl-pic](C104)z. For the assignment of geometrical configurations we used the i.r. absorption spectra of the cobalt-ethylenediamine ring systems. Baldwin [23] observed that the most consistent differences between the spectra of cis- and trans isomers of [Co(en)zXY]X appear in the CHz-rocking region (i.e. 870-900 cm-1). Here complexes with a cis-configuration show two bands, while those with trans-structure show one. The splitting of the i.r. band in this region arises from the lower symmetry of the cis-isomer. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

S. C. Chan and F. Leh, J. chem. Soc. (A), 1966, 126. S. C. Chan and F. Leh, J. chem. Soc. (A), 1967, 573. S. C. Chan and F. Leh, J. chem. Soc. (A), 1967, 288. S. C. Chan and F. Leh, J. chem. Soc. (A), 1967, 908. S. C. Chan and F. Leh,J. chem. Soc. (A), 1967, 1730. S. C. Chan and F. Leh, J. chem. Soc. (A), 1968, 1079. S. C. Chan, C. Y. Cheng and F. Leh, J. chem. Soc. (A), 1967, 1586. S. C. Chan and F. Leh, J. chem. Soc. (A), 1967, 2010. J. Zsak6, Cs. Vfirhelyi and J. Mostis, Studia Univ. Vitor Babes-Bolyai. 12, (2), 127 (1967). J. Zsak6, Cs. V~irhelyi and L. Banici, Studia Univ. Vitor Babes-Bolyai. 13, (2), 21 (1968). Cs. V~helyi and 1. Ganescu, Mh. Chem. 98, (2), 472 (1967). M. E. Baldwin,J. chem. Soc. 1960, 4369.

Kinetics and mechanism of substitution r e a c t i o n s - X i I

1461

Table I. New derivatives of the [Co(en)zC1 pyridine] 2+ and [Co(en)2C1 picolinel2+ with anions of the type [Cr(NCS)4(amine)~]-

No. I 2 3 4 5 6 7 8

Formula [Co(en)2Cl pyridine] [C r( N CS)4(aniline)2],z [Co(en)zCI/3-picolinel [C r(N C S)4(aniline)2],~ [Co(en)zClpyridine] [Cr(N CSL(p-toluidine) [Co(en)2Cl/3-picoline] [Cr(N CS)4( p-toluidine)~]~ [Co(en)~Cly-picoline] [Cr(N CS)4(p-toluidine)2]z [Co(enL~Cl pyridine] [Cr¿ N CSL(o-toluidine)z]2 [Co(en)2C1,8-picoline] [C r(N CS)4(o-toluidine)2]2 [C o(e n),_,CIy-picoline]

Mol. weight catcd.

Yield (%)

1234.6

58

1248.6

60

1290.7

75

1304-7

70

1304.7

85

1290,7

60

1304"7

55

1304.7

80

1290.7

68

1304.7

60

1304.7

86

[Cr(NCS)~(o-toluidine%]z 9 10 11 12 13 14 15 16 17

[Co(en)~Cl pyridine] ICr(N CS),(m-toluidine)2]~ [Co(en)~Cl/3-picoline] [Cr(N CS)4{m-toluidine)2]2 [Co(en)2C1 y-picoline] [Cr( N CSL(m-toluidine)2]z lCo(en)eCl pyridine] [C r(NCS)4(benzylamine)2]z [Co(en)2Cl fl-picoline] [Cr(N CS)~(benzylamine)~]2 [Co(en)2C1T-picoline] [Cr(N CS)4(benzylamine)2l~ [Co(en)sCl pyridine] [CrtN CS)4(p-phenetidine)~]~ [Co(en)2Clfl-picoline] [Cr(N CS)~(p-phenetidine)z]z [Coten)2Cl y-picoline] [Cr(NCS)4(p.phenetidine)2] ~

1290-7

78

1304.7

73

1304.7

90

1410,9

90

1424-9

86

1424.9

92

Analysis Calcd.

Found

Co + 2Cr S Co + 2Cr S Co + 2Cr S Co+2Cr S C o + 2Cr S C o + 2Cr S Co + 2Cr S C o + 2Cr S C o + 2Cr S

13.20 20.78 13,05 20.54 12-63 19.87 12.49 19-66 12.49 19.66 12.63 19.87 12-49 19.66 12.49 19.66 12.63 19.87 Co+2Cr 12-49 S 19.66 Co + 2Cr 12.49 S 19,66 C o + 2Cr 12.63 S 1%87

13.35 20.85 12-93 20.40 12-50 20-05 12.65 19-58 12-63 19.54 12.55 20.00 12-40 19-48 12.61 19-60 12.75 19-69 12-63 19-54 12-76 19.59 12.71 19.73

C o + 2Cr 12.49 S 19.66 C o + 2Cr 12.49 S 19.66 Co + 2Cr 11.55 S 18.18

12.54 19.48 12-60 19-55 t 1.68 I7-96

Co + 2Cr 11.44 S 18-00 Co + 2Cr 11,44 S 18.00

11.36 18-27 11.57 17.94

The CH~-rocking frequencies of the chelate rings in our complexes are given in Table 2, and those for some cis- and trans[Co(en)2XY]X, found by other authors, are included for comparison. We conclude that our assignment of a cis configuration on the basis of i.r. evidence, is reasonable. Our earlier studies [21] showed the solvolysis rate to depend upon the acidity of the solution. That is why we studied first the influence of mineral acids on the aquation of the complexes. The results obtained are given in Table 3. These experimental data show the same variation of reaction rate with acidity as in the case of the corresponding benzylamine derivative[21]. In neutral

1462

J. Z S A K 0 , CS. V A R H E L Y I and D. D O B O C A N Table 2. CH2-rocking frequencies (cm -1) in the infrared spectra o f some [Co(en)~XY]X type complexes

Formula

Frequency

trans-[Co(en)~(NH3)(H20)l(NOs)3 cis- [Co(en)2Cl(NHs)](NOa)2 trans-lCo(¢n)~Cl(N Ha)](CIO~)~ cis- [Co(en)2Cl(ethanolamine)]CI2 cis- [Co(en)2Cl(allylamine)l(CIO4)~ cis- [Co(en)2Ci(n-propylamine)]Cl2 [Co(enMCl~ lCo(¢n)~Cl pyridine]Cl~ lCo(en)~Cl fl-picoline] ( C104)2 [Co(en)~Cl y-picolinelCl2

888 900 888 880 894 891 895 899 899 900

Reference [23] [23] [23] [16] [ 11 ] [10] [23] ----

893 870 876 873 879 883 888 878

Table 3. Influence o f mineral acids on aquation rate constant o f [Co(en)2 CI/3-picoline] 2+ and [Co(en)~Cl y-picoline] z+ at 60°C [Co(en)2Clfl-picoline] 2+ [HC104] mole/l

k. 10Ss-~

0 10-5 10-4 3 x 10-4 10-z 3 × 10-3 10-2

7-28 6.75 6.55 4.56 4.18 4.03 4.09

[Co(en)2Cl y-picoline] ~÷ [HNO3] mole/l, 0 I0 -5 3 x 10-5 10 -4 3 × 10-4 10-3 3 × 10-3 10-2 3 X 10-2 10-1

k. 105 s -x 7.06 6.65 6.53 5.92 5.14 4.26 4.05 3"95 3"97 4"06

solutions and in the presence of very small quantities of acid, the rate constants are larger, than in acid solutions, and at least a mineral acid concentration of l 0 -a M is necessary for total hindering of base hydrolysis. Taking into account these results all the following runs were carried out in the presence of 10-2 M mineral acid (HNO~, HCIO4). Measurements were made at a constant ionic strength o f ~ = 0.1, realized with NaNOz. Kinetic measurements have been made for various initial concentrations of complex at 50, 60, 65 and 70°C. The concentration variation of the free chloride ions was followed potentiometrically. The aquation was found to be a first order reaction, since a plot of log C/Co vs. time showed good linearity. The values obtained with three different initial concentrations fall on the same straight line, as seen from Fig. 1 which illustrates the behaviour of the [Co(en)~Clfl-picoline]2+ complex.

Kinetics and mechanism of substitution reactions - X! 1 20

0

40

1463

SO 80 I00 120 140 160 /80 200~m/n

0

50 °

0.1-

•

0

•

o

0"2

GO0

0~3\650 ~4t-

O-Co=3.15xlO-3m

0'5-

" - Co=3~4x lO-_~m x - CO= ~.73x lO-3m

% x~

0'6°~

700

I

Fig. 1. Aquation kinetics of [Co(en)2CI~-picoline] 2+. Influence of initial concentration Coand of temperature, T w o o t h e r c o m p l e x e s s h o w the s a m e relation. F r o m t h e s e e x p e r i m e n t a l d a t a the first o r d e r rate c o n s t a n t s w e r e c a l c u l a t e d for e a c h r u n , u s i n g the least s q u a r e s m e t h o d . T h e v a l u e s o b t a i n e d are g i v e n in T a b l e 4. T h e s e rate c o n s t a n t s h a v e b e e n u s e d for c a l c u l a t i o n of A r r h e n i u s p a r a m e t e r s of t h e s e r e a c t i o n s . T h e c o r r e s p o n d i n g v a l u e s are i n c l u d e d in T a b l e 5. A s s e e n f r o m T a b l e s 4 a n d 5, the s u b s t i t u t i o n o f CH3 in the p y r i d i n e n u c l e u s has p r a c t i c a l l y n o i n f l u e n c e o n the k i n e t i c p a r a m e t e r s o f the a q u a t i o n r e a c t i o n . Table 4. First order rate constants of the aquation k x 105s-~ Complex ion [Coten)2C1 pyridine]~+

[Co(en)2Cl/3-picoline]z+

[Co(en)2Cly-picoline] z+

Initial conc. (co :x: 103 mole/l.)

50°

60°

65°

70°

3'83 4-78 5"74 3'15 3"94 4.73 3.70 4"62 5'56

1.5 1-5 1.4 1.3 1-3 1-3 1-6 1.5 1.5

4.8 4.6 5-0 3.8 4.4 4.0 4-6 4.2 5'3

7-9 8.1 7.9 7.0 5.6 8-0 9'9 8.6 9.1

13.5 14.9 13.2 13-3 11.7 12.0 12.6 13.5 13-2

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J. ZSAKO, CS. V,/~RHELYI and D. D O B O C A N Table 5. Arrhenius parameters of the aquation reactions

Ea

$298

Complex ions

(kcal/mole)

log PZ

[Co(en)~CI pyridine] z+ [Co(en)2CI/3-picoline] 2+ [Co(en)~CI y-picoline] 2÷

24-4 _-_0.2 24.4 ± 0.3 24.3 ± 0.2

11.7 11.7 11.7

(e.s.u.) - 5 - 0 ± 1.2 - 5 . 0 ± I "8 - 5 . 0 ± 1.2

This is not surprising since the basic dissociation constants of the corresponding amines are very close. Comparing our results with published data we can observe the following. Basolo et a/.[4] measured the rate constant of aquation for all these complex ions, but only at 50°C in acid solutions, in the presence of 0.0025 M perchloric acid. Their experimental values are nearly identical with ours. The acid hydrolysis of the pyridine derivative was studied at more temperatures by Panasyuk and Reiter[8]. The first order rate constants found by these authors are smaller than our experimental data, e.g. at 65°C they found 4.9 × 10-Ss-~ and we find 8 × 10-Ss-1. (A similar difference was reported by Chan et al.[9] who also found larger rate constants than Panasyuk et al. in the case of the ethylamine derivative). This considerable difference appears because of incorrect linearization of the experimental data in papers by Panasyuk et al. The plot of c[co vs. time, instead of the plot of log c/co vs. time is permissible only if c[co does not fall below 0.9. These authors use this plot in a large interval, between the c[co values from 1.0 to 0.4. Under such conditions we cannot trust the Panasyuk et al. activation energy values. They have found indeed activation energy values of 26.0 kcal/mole, while our experimental results are smaller. For the benzylamine derivative we have found 24.2 kcal/mole[21] while Panasyuk and Reiter gave a larger value, namely 26.0 kcal/mole. Since all papers by P anasyuk et al. contain the same systematic error, our experimental results cannot be compared with their data. The comparison of our data with the results obtained by Chan is difficult too. Chan et al. studied the solvolytic aquation of similar complexes in neutral solutions. But in neutral solutions the basic hydrolysis is not eliminated completely a n d the apparent activation energy can differ sensibly from the activation energy of t h e acid hydrolysis. Thus, for the 7-picoline derivative we found earlier Ea----26"1 kcal/mole in neutral solutions[20], while in acid solutions we found now Ea = 24"3 kcal/mole. Only in a few cases did Chan et al. study the aquation in acid solutions [ 13, 14]. Their results are consistent with our values. EXPERIMENTAL

Synthesis of cis-[Co(enLCl pyridine](NOzL. 28.5 g (0-1 mole) of trans-[Co(en)~Cl~]Cl is dissolved in 100ml water and 9 g (0.11 mole) of pyridine is added drop by drop. The green solution becomes gradually violet-red. After standing about 24 hr it is mixed with a concentrated aqueous solution of 100 g NaNOs. Red crystals of cis-[Co(en)~Cl pyridine](NO3)2 are filtered off, washed with a little ice water and dried in air. The complex salt forms rectangular red prisms. Yield: 50 per cent. Anal. Calcd. for [Co(C~HsN2)~CI CsH~N](NO~)2 (Mol. weight: 417.5): Co, 14.11; ND3, 29.71. Found: Co, 14.25; NOa, 29"90.

Kinetics and mechanism of substitution reactions - XI 1

1465

Synthesis of cis-[Co(en)2Cl y-picoline](NOz)2. The synthesis is analogous to that described above for the pyridine derivative. The crystalline product formes red plates. Yield: 65 per cent. Anal. Caicd. for [Co(C2HaN2)~CI CsHTN](NOa)2 (MoL weight: 431.5): Co, 13.60; NO3, 28.73. Found: Co, 13-53; NOz, 28.86. Synthesis of cis-[Co(en)2Cl[3-picoline](Cl04)~. Since the nitrate of the complex cation is very readily soluble in water and does not precipitate even in the presence of large quantities of NaNO3 or NH4NO3, the corresponding perchlorate was prepared by means of a double exchange reaction, using 80 ml of 70% perchloric acid solution. The salt forms red hexagonal plates (aggregates). Yield: 40 per cent. Anal. Calcd. for [Co(C2HsN2)CI CrHrN](CIO4)2 (Mol, weight: 506.45): Co, 11-64; C104, 39.27. Found: Co, I 1-75; CIO4, 39.04. Synthesis of complexes of the type [Co(en)2CI pyridine][Cr(NCS)4(amine)2]2 and [Co(enhCl picoline][Cr(NCS)4(amine)2]2. 5 m-moles of [Co(en)2Cl-pyridine](NOa)2 or [Co(en)zC1 picoline] (CIO4)2 are dissolved in 500 ml water and a solution of 10 m-moles of amine. H[Cr(NCS)4(amine)2] in 50 ml ethanol is added to the red solution, drop by drop, with continuous stirring. The microcrystalline reddish-violet product precipitates immediately. The precipitate is filtered off and washed several times with destilled water and dried in air, at room temperature for 3-4 days. In Table 1 are given the synthesis and analysis data for seventeen new complexes of this type. In this table the products numbered 3, 6, 9, 12 and similarily the products numbered 4, 5, 7, 8, 10, 11, 13 and 14 are isomers, because of the position or structural isomerism of the ligands. The isomeric compounds are very similar, one to another in their colour, solubility and some other properties.

Synthesis of the complexes of the type amine H[Cr(NCS)4(amine)2], used in the above double exohange reactions. 6 g (10 m-moles) of anhydrous K3[Cr(NCS)d are mixed thoroughly in a mortar with 50-60m-moles of the aromatic amine ( p K - 1 0 - 1 2 ) , i.e. with 5 g aniline, 5.5g, o-, m-, or ptoluidine, 5-5 g benzylamine or 6.0 g p-phenetidine (p-ethoxy-aniline) respectively. The mixture is heated on water bath in a corked 25 ml round bottom flask, for 3-4hr[22]. The colour of the mixture turns slowly from violet to dark red. After completion of the reaction an excess of 30-40% solution of acetic acid is added in order to remove the unreacted Ks[Cr(NCS)d, KCNS and the excessive amine. The product is filtered, washed with water and dissolved in 50 ml alcohol. The red solution is used for the double exchange reactions. Solubility of the complexes of the type: [Co( en )2CIAmine][ Cr( NCS )4(amine )2]2. These compounds are readily soluble in such polar organic solvents as acetone, methyl-ethyl-ketone, acetyl-acetone, ethylacetate, dimethylformamide and pyridine. In methanol and ethanol they are sparingly soluble and in water practically unsoluble. They are insoluble in nonpolar solvents such as benzene, toluene, chloroform, carbon tetrachloride. Analysis. Cobaltous was determined complexometrically-after destruction of the samples with sulfuric acid ( 9 8 % ) - in the presence of murexide indicator. The sum of the metals was determined by weighing the CoaO4 + Cr20.~, obtained by a 2 hr calcination of the complex at 900-920 °. Sulphur was determined gravimetrically as BaSO4. The anions NO3- and CIO4- were determined volumetrically. A sample of 0,400-0.600 g of the complex salt was dissolved in 50 ml water. The solution was passed through an ion exchange column (with Amberlite IR 120, R - H form) and the HNO.~ o r I - t C I O 4 in the eluant titrated with a 0.05 M N a O H solution. Kinetic measurements. The weighed samples of the complex were dissolved at the desired temperature in distilled water, and the acidity and ionic strength adjusted with HNOz, HCIO4, and NaNO3 solutions (all preheated to the temperature of the experiment). The liberated CI- ions were determined during the runs by potentiometric titrations. I 0 ml samples were cooled quickly to 0°C, 20 ml 0.1 N HNO~ was added (cooled to 0°C too) and the chloride ion was titrated with 0"01 N AgNO~ solution, using a silver wire as indicator electrode.