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A chromatographic and kinetic study of the base hydrolysis of oxalatobis (ethylenediamine) chromium(III) ion
/./no~. nacL Chem., 1976, Vol. 38, pp. 2251-2256. Pergamon Press. Printed in Great Britain
A CHROMATOGRAPHIC AND KINETIC STUDY OF THE BASE HYDROLYSIS OF OXALATOBIS (ETHYLENEDIAMINE) CHROMIUM(Ill) ION MICHAEL B. DAVIES Science Department, Stockport College of Technology, Stockport SKI 3UQ, England
(Received 14April 1975) Abaraet--The system Cren2C20,+ + OH- has been studied over a variety of hydroxide ion concentrations and at four temperatures. The reaction appears to involve initially "one ended" oxalate fission, the rate of this reaction is then given by k ~ = kl +k2 [OH-]. For kl, AH~t= 74.4_+3.4 kJ tool-1 and AS~t= -59.4_+0.6 JK -1 tool-~ and for k2, AH~ = 74.2 -+7.7 kJ tool 1 and AS~ = -55.9-+ 1.3 JK-' tool-'. INTRODUCTION One of the major differences between the reactions of chromium(III) octahedral complexes and the correspondhag cobaltfIII) compounds is the considerable tendency in the former for chromium-nitrogen bond fission in preference to fission of chromium-oxygen bonds[I]. In this laboratory, we have been investigating a variety of chromium(III) oxalate complexes in order to try and find out under what conditions of pH nitrogen or oxygen bond fission is favoured. Recent work [2] has shown that under conditions of very high acid and chloride ion concentration the oxalate group in Cren2C204+ ion is removed and the ethylenediamine remains, while in dilute acid the removal of ethylenediamine is favoured. It is of interest, therefore to extend the study of the hydrolysis of Cren2C20, + ion into basic solution and this paper deals with the reaction of this ion in solutions of pH 10.5 and above.
tion of chromium concentration using the SP90 atomic absorbtion spectrophotometer. 0.02g of complex were dissolved in 10cm3 of 0.1M sodium hydroxide and thermostatted at 298 K for appropriate times. 2 cm3 of the resulting solution were put on either a cation exchange column in the sodium form or an anion exchange column in the hydroxide form. This process was repeated with the complex in contact ,with sodium hydroxide for various times. The columns were eluted with various concentrations for the HCI or NaCI. Other runs were carried out by acidifying the complex solution with 2.0cm3 of 1.0M hydrochloric acid before 2cm 3 of the resulting solution were put on a cation exchange column in the H + form.
Kinetics The spectra of Cren2CeO,÷ ion recorded in an ethylenediamine buffer at pH 10.5 show fairly good isosbestic points, Fig. 1, for at least 75% reaction. This suggests that interference from reactions other than that causing the disappearance of the original complex ion is small. The kinetics of this reaction were generally followed at 372 rim. A large optical density change occurs here and very good first order kinetics were observed for more than a half life. Some runs were also carried out at 267.5 nm and the rates at the two wavelengths were the same within experimental error.
EXPERIMENTAL
Reagents The preparation of oxalatobis (ethylenediamine) chromium(m) bromide has been adequately described elsewhere[3]. The analysis is given in Ref. [4]. Newly prepared samples were checked by the • values for their maxima in the vis~le spectrum. Carbonate free sodium hydroxide solutions were made by the method of Vogel [5] and made up to the required ionic strength using BDH AnalaR grade sodium chloride. The ethylenediamine buffer solution was made by adding 5 cm3 ethylenediamine to 50 cm3 of 1.0 M sodium chloride solution. This was brought to pH 10.5 by the addition of 24 cm~ of 1.0 M HCI. Adjustments to ionic strength and pH in these solutions were made using sodium chloride and hydrochloric acid and not sodium perchlorate and perchloric acid because of the insolubility of the perchlorate salt of Cren2C204+ ion in water.
1.2
0,8
c5 6
Chromatography
0.4
Cation exchange chromatography was carried out using columns of Zeocarb 225 8% DVB 100-200 mesh resin. Anion exchange chromatography was carried out using deacidite FF-IP 100-200 mesh in the hydroxide form. The cation exchange columns were 5 diameter x 30 or 5 mm diameter xt0mm. Anion exchange columns were 5 x 60 ram. The columns were eluted each under slight positive pressure giving flow rates of 5--10cm3 per minute. The eluate was monitored for chromium in 10 cm3 aliquots using a Unicam SP 90 atomic absorption spectrophotometer against standard solutions containing chromium in the appropriate concentration range. The extinction coefficients of all species including those eluted from columns were determined by estima-
I
350
I
t
410
I
I
i
470
I
i
~
I
5:50
I
i
I
590
Wovelength / nm
Fig. 1. Variation of the spectrum of Cren~C=O4+ ion in pH 10.5 ethylenedhmine buffer at 297.5 K.
2251
2252
MICHAEL B. DAVIES
Spectra of the Cren2C20,+ ion in solutions of higher pH, e.g. in 0.1 M sodium hydroxide solution did not show isosbestic points. In order to obtain kinetic data in this area, a shoulder in the UV spectrum of Cren2C20,÷ ion at 267.5nm was used. Fairly good Guggenheimplots could be obtained at this wavelength since, as shown by preliminary experiments[6], the absorbance of the products is fairly small and does not change a great deal during kinetic runs. At other wavelengths,e.g. 373 nm straight lines could not be obtained.
and has been isolated in aqueous acid solution [9] (see also part 2 of Results and Discussion). It is reasonable to assume that (b), which involves "one-ended" oxalate would revert to CremC204 + in acid solution--rapid addition and fission of one of the bonds in acid solution has been postulated in a number of mechanistic schemes [10, 11]. It is proposed, therefore, that the following reaction is the first stage in the reaction of CremC204+ ion in basic solution:
RESULTS AND DISCUSSION (1) Studies at pH 10.5 Figure 1 shows the variation of the spectrum of Cren~C20, + ion in an ethylenediamine buffer at pH 10.5. The isosbestic points are seen to be at 397 nm (e = 50), 442 um (e = 27) and 529 nm (e = 58). 10 cm 3 of this solution were acidified with I0 cm 3 5 M hydrochloric acid. The maxima and minima of the resulting spectrum are shown in Table 1, where they are compared with values obtained for the starting material immediately on solution in the buffer. The isosbestic points are not perfect and clearly other reactions are occurring to some extent. Despite this, the addition of acid after more than 75% reaction produces a compound with a spectrum almost identical with that of the starting material. Possible products of the reaction are (a) CremOHC20, ° ("one ended" en) (b) Cren2C204OH ° ("one ended" oxalate) (c) Crem(OH)~- and (d) CrenC20,(OH)~-. The spectrum after three half lives does not resemble that of either (c) or (d) which are known[7, 8]. In addition neither of these is expected to give a spectrum resembling that in Table 1, since they will give their respective aquo complexes whose spectra are known. Further, (a) is not expected to produce CremC20, + in acid solution since the "one-ended" ethylene--diamine complex CrenH(H20)C2042+ is known
~~0~~+
OH- )
••O
If this is the reaction, then this is believed to be the first time that a "one-ended" oxalatoethylenediamine chromium(IH) complex has been isolated and its spectrum obtained. A "one-ended" oxalato complex intermediate was postulated as a necessary part of the mechanism of the base hydrolysis of CrenC204(H20)~ ÷ ion, but it was not isolated[12]. Removal of the second oxalate bond would be expected to be the second reaction. The fact that the optical density during a run remains almost constant after six half fives of the first reaction at 298 K suggests that the second reaction is slow compared with the first. When a solution of CremC204Br in the ethylenediamine buffer at pH 10.5 was left at about 298 K for 5 hr to ensure completion of the first reaction and then thermostatted at 308 K, preliminary spectrophotometric studies showed that the maximum at 498 nm decreases and shifts to the red, while that at 380 nm moves only slightly, both in wavelength and in
Table 1. Spectral data for various species, figuresin parenthesis represent extinction coefficientsin 1 mol-acm ao:lLdltAed8olwld.on t w halt 1 1 ~
Initial
spoot~u
of
(~Im2C-LO~I~m ~ o t z ~ = (L~tte About 3 b ~
Zlvu
at ]~ 10.5)
S ~ t ~ m 1= 0.1 M ao4t~
h.vdA-ozlde =o~tion afte~
~mz/m
Aatn/m
Aau/m
3'74
42"1
496
(eT)
(22)
(92) 496
Y75
42e
(e~)
(2~)
(ee)
380
439
5O7
(55)
(27)
(6~)
384
442
5O8
(64)
(39)
(6e)
t ~ ~d~' ltv=e Sl~et~= aA'te~ 5 bz~ at
(47)
s~otnut ot 7~ o4= ud tm= c~=2(oe12" •qud~d oft ~÷ column ~ t h 3 n ~C1
$'Jpeotee wimha,d o f f a~ ooZtam u'J.th 2 It RCI
o , , . ~ % o ) 2. Speoles ~m,~d o~L' OH" ooZ~=mr l t b
OH ~~ °
(17.3)
(47.5)
448
5~
(51.5)
(21)
(52)
~m5
439
514
(31)
(13)
(42)
3o5
4PJ
512
(24.3)
(10.7)
(41.7)
383
439
5O9
(6s)
(15)
(63)
366
443
515
(68)
(10)
(62)
377
433
490
(o7)
(31)
(9o)
A studyof oxalatubis(ethylenediomine)chromium(llI)ion optical density. When this reaction at 308 K is followed at 510 ran, the optical density is fairly constant after about 5 hr. The band maxima after this time are shown in Table 1 along with those from the spectrum calculated for a 70% cis and 30% trans mixture of Crem(OI-I)2+[13]. The agreement is good considering that further reactions undoubtedly occur (as shown by the fact that even after six half-lives of this reaction the optical density continues to drift slowly downwards). The second reaction is then taken to be:
50
50
40
4O
3O A
mb
20
~: ao
50 c
)
~o 8'o ,2o
~o &
4O
OH-
2253
Time/rain
Tlme/min
40
]
r.....
Fairly good pseudo first order plots were obtained using the Guggenheim method for both the first and second reactions in an ethylenediamine buffer at pH 10.5. Data for three temperatures for the first reaction and one temperature for the second are shown in Table 2.
3O
j'zo o u
I0
' 0
40
g
' 0
120
;
'
I 0
180
Time/min
(2) Studies at higher pH (a) Chromatographic separation of the products. It is clear from the spectra obtained at various times for CremC2OJ ion in 0.1 M sodium hydroxide solution that the reaction proceeds by a number of steps. When such a solution was put on a sodium ion cation exchange column only the species eluted by water and by 0.5 M sodium chloride were observed for up to 1½hr. The colour, spectrum and elution pattern showed that the 0.5M sodium chloride fraction was starting material. It appears from this that hydrolysis in 0.1 M sodium hydroxide solution produces either zero or negatively charged species. Several experiments showed that the starting material disappeared at a rate such that the rate constant was 5.9 x 10-~ S-t. When the sample was acidified before being put onto the H ÷ column there were products eluted by 0.5 M HCI, 2.0 M HCI and 3 M HCI. No other fractions could be distinguished. The fraction eluted with 0.5 M HC1 was the starting material. The apparent rate measured by the amount of this fraction remaining on the column 2 x 10-' S-t, i.e. considerably slower than when the solution was not first acidified. The elution pattern on an anion exchange column also gave only two fractions. One was eluted with water and this gradually fell off as a single fraction eluted by 0.005 M sodium chloride increased. The elution patterns are shown in Fig. 2. Analysis of the products eluted from the H ÷ cation exchange column (after acidification of the reaction mixture) by 2 and 3M hydrochloric acid for chromium and oxalate showed that they had chromium oxalate ratios of Table 2. Reaction of 2.8x10 3M Cren~C20/ in an ethylenediaminebufferat pH 10.5 ~z~:~/Y
ko~ /
8 "I
R~ot~a
209.0
1,80 • 10. 4
~
~.5
5.oo
~.,t
~7.5
I . ~ 2 • 1 ~ ~1
lq.ll~t
Y~.5
1.76 • l o -4
~o~
x
I o "4
Fig. 2. Elution patterns from various ion exchange columns for the reaction Cren2C=O,++ 0.I M OH-. Disappearanceof starting material and appearance of first product. (A) cation exchange column after acidification,(B) cation exchange column, (C) anion exchangecolumn. 1.15:1 and 1.0:0 respectively. The spectra of these species are shown in Table 1. In (1) above it was suggested that the first step in the base hydrolysis of Cren,C20/was the formation of a "one-ended" oxalate species, which returned to the original complex when the solution was acidified. Two pieces of evidence confirm that a similar reaction occurs in 0.1 M sodium hydroxide. Table 1 shows the maxima and minima in the visible spectrum of the species produced when the eluate from the anion exchange column is acidified. (This is from a reaction which proceeded for 0.5 hr.) It can be seen to be almost identical to the spectrum of the starting material. If the "one-ended" oxalato species is the first product it will be zero-charged and will be eluted from the anion exchange column along with the starting material (which is positively charged). When the solution is acidified the "oneended" oxalato complex is converted to the starting material. In addition the more rapid reaction which occurs when the separation is effected on a sodium ion cation exchange column compared with the apparent rate as measured using acidification and the H + column confirms that the acidification results in the re-formation of the original complex giving an apparently slower rate. Further confirmation of this conclusion is given by the fact that after two half lives of the reaction in 0.1 M sodium hydroxide, the spectrum is similar to that observed for the "one-ended" oxalato species at pH 10.5 (Table 1). When such a mixture is quenched to pH 10.5 small changes in the wavelength positions occur, so that the 384 nm band moves to 379 um and the 508 um band moves to 504 rim. The change in spectrum in 0.1 M sodium hydroxide (whether it contains ethylenediamine or not) represents a continuous movement of the band maxima to the red. When the pH of the solution containing ethylenediamine at I~H 10.5 after 3 half lives is raised to a similar value to that of 0.1 M sodium hydroxide, this
MICHAEL B. DAVIES
2254
change proceeds in the same way as the corresponding 0.1 M solution containing ethylenediamine. The reactions in 0.1M sodium hydroxide solution beyond the formation of the monodentate oxalato species are complex. All attempts to separate the ion Crem(OI-I)2÷ failed. The spectral changes do not indicate its presence. When the wavelength of the lower wavelength band has reached 383 nm that of the higher wavelength band is nowhere near 525 nm. The fact that this species is not present explains the presence of the species eluted by 2 M acid when the acidified mixture is put on a cation exchange column. This has a spectrum which suggests that it is the monodentate ethylenediamine complex Cren enH(HzO)C20/+ which is also eluted from a cation exchange column with 2 M acid[14]. The reaction scheme is then:
gives a result in agreement with the rate obtained under similar conditions using the decrease in absorption of the shoulder at 267.5 nm. This shoulder is characteristic of many oxalato complexes [16] and obeys the Lambert-Beer law. Neither its position nor its intensity vary with ionic strength. Thus, at 267.5 nm the disappearance of the starting material i's being monitored. The reaction which is occurring is: CremC204÷+ OH- -~ CremC2040H °. This is similar to the analogous cobalt complex reaction studied by Farago and Mason[15]. The spectrophotometrically determined pseudo first order rate constants for this reaction are shown in Table 3. The concentration of sodium hydroxide is always many
Acid
Cren2C204÷ ~ o.-
, CremC2040H °--~ Crenen*C204(OI-I)2--*Cren(OH)4~ acid ! CrenenHC204(I-I20) | + C20~2+ en
*monodentate ethylenediamine
polymeric products
The presence of the last of these, Cren(OH)V is confirmed by the detection of Cren(I-I20)/÷ in acid solution. This compound has been shown[12] to produce highly charged species after prolonged contact with 0.1 M sodium hydroxide. (b) Kinetic studies. The measurement of the rate of the reaction in 0.1 M NaOH by monitoring the disappearance of the starting material using a chromatographic method
times more than the concentration of the complex ion. The ionic strength was maintained at 1.0 using sodium chloride. A plot of rate constant against concentration at four temperatures is shown in Fig. 3. At higher temperatures fairly good straight lines are obtained up to 1.0 M sodium hydroxide, but at lower temperatures there is departure from linearity at higher concentrations of sodium hydroxide (see second reaction of the cobalt
Table 3. Rate constants for reaction of Cr e n2C204 + (1.12x 10 3M) with variousconcentrationsof sodiumhydroxide ~=I,AE lq
2eo.a
29e,7
[oH'3 /x
zo~o ,u~=~th
o.1~
O.67 + 0.16
1.0
0.TPJ
o.9~ + o.~e
1.0
O.547
1.1:~ +- O.07
1.0
o.71"r
1.44 +- 0.10
1.0
1.06
2.41 +- 0.02
1.0
0.05~
1.95 +- O.oo
1.o
o.1(~
2.1~ ~ 0.0~
1.0
o.~J
2.4e *
1.0
0.~47
~.07 Z o.10
1.0
0.74~
~ . ~ Z 0.10
t.O
1.oo
6.~J Z 0.1o
1.o
1.0
o.o6
0.0~7
5.0'J + 0 . ~
0.100
%9"J *
0.100
6.22 ~ 0.00
0.I
0.27~J
7.CO ~ 0.6O
1.0
0.~47 1.Ce
*=r~,
ko~/,'1~ IO 4"
9.96 +
0.~
1.0
1.09
1.0
14.42 + 0.01
1.0
0.1(~
16.17 +- 0.18
1.0
0.547
17.4 ~ 0.1
1.0
1.08
~'7.2
1.0
+- 1.1
re];~.eem~ mz~l~o~Ll~Ltt7 of tvo oa" ne-ee mzw.
A studyof oxalatobis(ethylenediamine)chromium(Ill)ion
,oo
f
I
2255
The pH dependent part is readily explained on the basis of the reaction:
3 ,K
+
Crel~,C204 + OH- --*CremC204OH° as in the case of the corresponding cobalt complex. The pH independent part may be explained by postulating that the reaction may also be:
13.0 12.0 II0
Cren~C~O/ k, ~Cren2C204OH~÷ o.- )Cren2C2040H0 rapid
29EI.7K
"one-ended" oxalate unstable.
I0.0 9.0
~
•
8.0 7.0 6.0 5.0
•
e
@
4.0 30
/
2,0 L@" @ ~
i
/
I
~
2
8
9
K
@~"-
1.0 ~.... @... @-,.--"
0
+ H20
0.2
O.
06 [OH]/M
O~
1,0
Fig. 3. Variationof ko~,with [OH-]at varioustemperatures. complex observed by Farago and Mason[15]). Unlike the data obtained for C0en~20/the lines do not pass through the origin. This suggests that there are two pans to this reaction, i.e. a part which is independent of [OH-] and a pan which is dependent on [OH-]: ko~ = k~ + k2[OH-].
This reaction will occur in solutions of higher pH when the rapid acid-base equilibrium of the "one-ended" oxalate complex lies largely to the fight. The aquo "oneended" oxalate complex itself is expected to be unstable. The values of k2 at four temperatures can be determined from the slopes of the ko~ against [OH-] graphs and the intercepts give values of k~. These are shown in Table 4 together with the activation parameters for each reaction. The values of AH" and AS s for the pH independent and pH dependent reaction do not differ significantly. It is interesting to note that the values of AH ~ determined here are lower than the value obtained for the base hydrolysis of CrenC:~S)4(H:43)2+, which is believed to be loss of oxalate, i.e. fission of the second oxalate bond[12]. This is analogous to the situation found for the complex CoemC20/. Using data from the paper by Farago and Mason[15] it is possible to calculate values for AH ~ for the formation of the "one-ended" oxalate species and the dihydroxo species in basic solution. These are approximately 73 and 93 kJ tool-~ respectively. It has been shown that the first reaction of Coen2C20J ion with hydroxide ion occurs via breaking of the C-O bond and not the C o O bond, while the second reaction results from fission of the C o O bond. If this accounts for the difference in activation enthalpies in the cobalt complex reaction, then a similar situation might obtain for the chromium complex and the first reaction might occur via C-O rather than Cr-O bond fission.
Table 4. Variationin derivedrate constants withtemperature and activationparameters for first reaction T~[
~p*mieat z.ua~tlo, ~ paaS~ut
k~lO"4 ~'lS
280.8
1.1~ +-.0.18
2~.0
2.23 +- 0.09
~xje.7 ~.? ~eo.8
o.~a +- o.o~
~.0
1.86 + 0.02
298.T
~zl
*
Det~
-55.9~1.3
T4.4~ 3.4
~9.4~0.6
8.85 +- 0.55
maa~Lon f ~ m
~.T
74.2~ T.7
21.66+ 1.0
s~ 4,,,~,p~ml~
4,,~.,~
,~ E~,/~ IQ1"1 .~/.T['~I01 "1
4.~
+- 0 . ~
t2.50 +- 1.t
m stu~la~ dtev~Latto==
..4=f • oo=~t~ p z ~
t.me~ on eq~t~== ~ Ref. 17~ 100.
2256
MICHAEL B. DAVIES
Acknowledgements--The author would like to thank Miss O. Harper for computing, Mrs. B. Bradley for technical assistance and Dr. J. W. Lethbridge for valuable discussion. The provision of financial assistance by S.R.C. towards a Unicam SP 1800 spectrophotometer is also gratefully acknowledged. REFERENCES 1. C. S. Garner and D. A. House, Transition Metal Chemistry 6, 263 (1970). 2. M. B. Davies and J. W. Lethbridge, J. Inorg. Nucl. Chem. 37, 171 (1975). 3. G. G. Schlessinger, Inorganic Laboratory Preparations, p. 227. Chem. Pub., New York (1%2). 4. M. B. Davies, J. W. Lethbridge and Othman Nor, J. Chromatogr. 68, 231 (1972). 5. A. I. Vogel, A Textbook of Quantitative Analysis, 3rd Edn. p. 284. Longmans, London (1%1). 6. D. Merrill, L.R.I.C. Dissertation, Stockport College of Technology (1970).
7. M. B. Davies and J. W. Lethbridge, unpublished research. 8. A. W. Adamson, In Mechanisms of Inorganic Reactions, Advances in Chemistry Series, Chap. 10, No. 49, Am. Chem. Soc., Washington D.C. (1965). 9. M. B. Davies, J. W. Lethbridge, Othman Nor and Lai-Yoong Goh, J. lnorg. Nucl. Chem. 37, 175 (1975). 10. M. Casula, G. Illuminati and G. Ortaggi, Inorg. Chem. 11, 1062 (1972). 11. M. B. Davies and J. W. Lethbridge, J. lnorg. Nuel. Chem. 37, 141 (1975). 12. M. B. Davies, J. Inorg. Nucl. Chem. 36, 3789 (1974). 13. F. Woldbye, Acta Chem. $cand. 12, 1079 (1958). 14. J. W, Lethbridge, personal communication. 15. M. E. Farago and C. P. V. Mason, J. Chem. Soc. (A), 3100 (1970). 16. K. V. Krishnamurty and G. M. Harris, Chem. Rev. 61, 219 (1961). 17. A. A. Frost and R. G. Pearson, Kinetics and Mechanism, 2nd Edn. Wiley, New York (1961).
A CHROMATOGRAPHIC AND KINETIC STUDY OF THE BASE HYDROLYSIS OF OXALATOBIS (ETHYLENEDIAMINE) CHROMIUM(Ill) ION MICHAEL B. DAVIES Science Department, Stockport College of Technology, Stockport SKI 3UQ, England
(Received 14April 1975) Abaraet--The system Cren2C20,+ + OH- has been studied over a variety of hydroxide ion concentrations and at four temperatures. The reaction appears to involve initially "one ended" oxalate fission, the rate of this reaction is then given by k ~ = kl +k2 [OH-]. For kl, AH~t= 74.4_+3.4 kJ tool-1 and AS~t= -59.4_+0.6 JK -1 tool-~ and for k2, AH~ = 74.2 -+7.7 kJ tool 1 and AS~ = -55.9-+ 1.3 JK-' tool-'. INTRODUCTION One of the major differences between the reactions of chromium(III) octahedral complexes and the correspondhag cobaltfIII) compounds is the considerable tendency in the former for chromium-nitrogen bond fission in preference to fission of chromium-oxygen bonds[I]. In this laboratory, we have been investigating a variety of chromium(III) oxalate complexes in order to try and find out under what conditions of pH nitrogen or oxygen bond fission is favoured. Recent work [2] has shown that under conditions of very high acid and chloride ion concentration the oxalate group in Cren2C204+ ion is removed and the ethylenediamine remains, while in dilute acid the removal of ethylenediamine is favoured. It is of interest, therefore to extend the study of the hydrolysis of Cren2C20, + ion into basic solution and this paper deals with the reaction of this ion in solutions of pH 10.5 and above.
tion of chromium concentration using the SP90 atomic absorbtion spectrophotometer. 0.02g of complex were dissolved in 10cm3 of 0.1M sodium hydroxide and thermostatted at 298 K for appropriate times. 2 cm3 of the resulting solution were put on either a cation exchange column in the sodium form or an anion exchange column in the hydroxide form. This process was repeated with the complex in contact ,with sodium hydroxide for various times. The columns were eluted with various concentrations for the HCI or NaCI. Other runs were carried out by acidifying the complex solution with 2.0cm3 of 1.0M hydrochloric acid before 2cm 3 of the resulting solution were put on a cation exchange column in the H + form.
Kinetics The spectra of Cren2CeO,÷ ion recorded in an ethylenediamine buffer at pH 10.5 show fairly good isosbestic points, Fig. 1, for at least 75% reaction. This suggests that interference from reactions other than that causing the disappearance of the original complex ion is small. The kinetics of this reaction were generally followed at 372 rim. A large optical density change occurs here and very good first order kinetics were observed for more than a half life. Some runs were also carried out at 267.5 nm and the rates at the two wavelengths were the same within experimental error.
EXPERIMENTAL
Reagents The preparation of oxalatobis (ethylenediamine) chromium(m) bromide has been adequately described elsewhere[3]. The analysis is given in Ref. [4]. Newly prepared samples were checked by the • values for their maxima in the vis~le spectrum. Carbonate free sodium hydroxide solutions were made by the method of Vogel [5] and made up to the required ionic strength using BDH AnalaR grade sodium chloride. The ethylenediamine buffer solution was made by adding 5 cm3 ethylenediamine to 50 cm3 of 1.0 M sodium chloride solution. This was brought to pH 10.5 by the addition of 24 cm~ of 1.0 M HCI. Adjustments to ionic strength and pH in these solutions were made using sodium chloride and hydrochloric acid and not sodium perchlorate and perchloric acid because of the insolubility of the perchlorate salt of Cren2C204+ ion in water.
1.2
0,8
c5 6
Chromatography
0.4
Cation exchange chromatography was carried out using columns of Zeocarb 225 8% DVB 100-200 mesh resin. Anion exchange chromatography was carried out using deacidite FF-IP 100-200 mesh in the hydroxide form. The cation exchange columns were 5 diameter x 30 or 5 mm diameter xt0mm. Anion exchange columns were 5 x 60 ram. The columns were eluted each under slight positive pressure giving flow rates of 5--10cm3 per minute. The eluate was monitored for chromium in 10 cm3 aliquots using a Unicam SP 90 atomic absorption spectrophotometer against standard solutions containing chromium in the appropriate concentration range. The extinction coefficients of all species including those eluted from columns were determined by estima-
I
350
I
t
410
I
I
i
470
I
i
~
I
5:50
I
i
I
590
Wovelength / nm
Fig. 1. Variation of the spectrum of Cren~C=O4+ ion in pH 10.5 ethylenedhmine buffer at 297.5 K.
2251
2252
MICHAEL B. DAVIES
Spectra of the Cren2C20,+ ion in solutions of higher pH, e.g. in 0.1 M sodium hydroxide solution did not show isosbestic points. In order to obtain kinetic data in this area, a shoulder in the UV spectrum of Cren2C20,÷ ion at 267.5nm was used. Fairly good Guggenheimplots could be obtained at this wavelength since, as shown by preliminary experiments[6], the absorbance of the products is fairly small and does not change a great deal during kinetic runs. At other wavelengths,e.g. 373 nm straight lines could not be obtained.
and has been isolated in aqueous acid solution [9] (see also part 2 of Results and Discussion). It is reasonable to assume that (b), which involves "one-ended" oxalate would revert to CremC204 + in acid solution--rapid addition and fission of one of the bonds in acid solution has been postulated in a number of mechanistic schemes [10, 11]. It is proposed, therefore, that the following reaction is the first stage in the reaction of CremC204+ ion in basic solution:
RESULTS AND DISCUSSION (1) Studies at pH 10.5 Figure 1 shows the variation of the spectrum of Cren~C20, + ion in an ethylenediamine buffer at pH 10.5. The isosbestic points are seen to be at 397 nm (e = 50), 442 um (e = 27) and 529 nm (e = 58). 10 cm 3 of this solution were acidified with I0 cm 3 5 M hydrochloric acid. The maxima and minima of the resulting spectrum are shown in Table 1, where they are compared with values obtained for the starting material immediately on solution in the buffer. The isosbestic points are not perfect and clearly other reactions are occurring to some extent. Despite this, the addition of acid after more than 75% reaction produces a compound with a spectrum almost identical with that of the starting material. Possible products of the reaction are (a) CremOHC20, ° ("one ended" en) (b) Cren2C204OH ° ("one ended" oxalate) (c) Crem(OH)~- and (d) CrenC20,(OH)~-. The spectrum after three half lives does not resemble that of either (c) or (d) which are known[7, 8]. In addition neither of these is expected to give a spectrum resembling that in Table 1, since they will give their respective aquo complexes whose spectra are known. Further, (a) is not expected to produce CremC20, + in acid solution since the "one-ended" ethylene--diamine complex CrenH(H20)C2042+ is known
~~0~~+
OH- )
••O
If this is the reaction, then this is believed to be the first time that a "one-ended" oxalatoethylenediamine chromium(IH) complex has been isolated and its spectrum obtained. A "one-ended" oxalato complex intermediate was postulated as a necessary part of the mechanism of the base hydrolysis of CrenC204(H20)~ ÷ ion, but it was not isolated[12]. Removal of the second oxalate bond would be expected to be the second reaction. The fact that the optical density during a run remains almost constant after six half fives of the first reaction at 298 K suggests that the second reaction is slow compared with the first. When a solution of CremC204Br in the ethylenediamine buffer at pH 10.5 was left at about 298 K for 5 hr to ensure completion of the first reaction and then thermostatted at 308 K, preliminary spectrophotometric studies showed that the maximum at 498 nm decreases and shifts to the red, while that at 380 nm moves only slightly, both in wavelength and in
Table 1. Spectral data for various species, figuresin parenthesis represent extinction coefficientsin 1 mol-acm ao:lLdltAed8olwld.on t w halt 1 1 ~
Initial
spoot~u
of
(~Im2C-LO~I~m ~ o t z ~ = (L~tte About 3 b ~
Zlvu
at ]~ 10.5)
S ~ t ~ m 1= 0.1 M ao4t~
h.vdA-ozlde =o~tion afte~
~mz/m
Aatn/m
Aau/m
3'74
42"1
496
(eT)
(22)
(92) 496
Y75
42e
(e~)
(2~)
(ee)
380
439
5O7
(55)
(27)
(6~)
384
442
5O8
(64)
(39)
(6e)
t ~ ~d~' ltv=e Sl~et~= aA'te~ 5 bz~ at
(47)
s~otnut ot 7~ o4= ud tm= c~=2(oe12" •qud~d oft ~÷ column ~ t h 3 n ~C1
$'Jpeotee wimha,d o f f a~ ooZtam u'J.th 2 It RCI
o , , . ~ % o ) 2. Speoles ~m,~d o~L' OH" ooZ~=mr l t b
OH ~~ °
(17.3)
(47.5)
448
5~
(51.5)
(21)
(52)
~m5
439
514
(31)
(13)
(42)
3o5
4PJ
512
(24.3)
(10.7)
(41.7)
383
439
5O9
(6s)
(15)
(63)
366
443
515
(68)
(10)
(62)
377
433
490
(o7)
(31)
(9o)
A studyof oxalatubis(ethylenediomine)chromium(llI)ion optical density. When this reaction at 308 K is followed at 510 ran, the optical density is fairly constant after about 5 hr. The band maxima after this time are shown in Table 1 along with those from the spectrum calculated for a 70% cis and 30% trans mixture of Crem(OI-I)2+[13]. The agreement is good considering that further reactions undoubtedly occur (as shown by the fact that even after six half-lives of this reaction the optical density continues to drift slowly downwards). The second reaction is then taken to be:
50
50
40
4O
3O A
mb
20
~: ao
50 c
)
~o 8'o ,2o
~o &
4O
OH-
2253
Time/rain
Tlme/min
40
]
r.....
Fairly good pseudo first order plots were obtained using the Guggenheim method for both the first and second reactions in an ethylenediamine buffer at pH 10.5. Data for three temperatures for the first reaction and one temperature for the second are shown in Table 2.
3O
j'zo o u
I0
' 0
40
g
' 0
120
;
'
I 0
180
Time/min
(2) Studies at higher pH (a) Chromatographic separation of the products. It is clear from the spectra obtained at various times for CremC2OJ ion in 0.1 M sodium hydroxide solution that the reaction proceeds by a number of steps. When such a solution was put on a sodium ion cation exchange column only the species eluted by water and by 0.5 M sodium chloride were observed for up to 1½hr. The colour, spectrum and elution pattern showed that the 0.5M sodium chloride fraction was starting material. It appears from this that hydrolysis in 0.1 M sodium hydroxide solution produces either zero or negatively charged species. Several experiments showed that the starting material disappeared at a rate such that the rate constant was 5.9 x 10-~ S-t. When the sample was acidified before being put onto the H ÷ column there were products eluted by 0.5 M HCI, 2.0 M HCI and 3 M HCI. No other fractions could be distinguished. The fraction eluted with 0.5 M HC1 was the starting material. The apparent rate measured by the amount of this fraction remaining on the column 2 x 10-' S-t, i.e. considerably slower than when the solution was not first acidified. The elution pattern on an anion exchange column also gave only two fractions. One was eluted with water and this gradually fell off as a single fraction eluted by 0.005 M sodium chloride increased. The elution patterns are shown in Fig. 2. Analysis of the products eluted from the H ÷ cation exchange column (after acidification of the reaction mixture) by 2 and 3M hydrochloric acid for chromium and oxalate showed that they had chromium oxalate ratios of Table 2. Reaction of 2.8x10 3M Cren~C20/ in an ethylenediaminebufferat pH 10.5 ~z~:~/Y
ko~ /
8 "I
R~ot~a
209.0
1,80 • 10. 4
~
~.5
5.oo
~.,t
~7.5
I . ~ 2 • 1 ~ ~1
lq.ll~t
Y~.5
1.76 • l o -4
~o~
x
I o "4
Fig. 2. Elution patterns from various ion exchange columns for the reaction Cren2C=O,++ 0.I M OH-. Disappearanceof starting material and appearance of first product. (A) cation exchange column after acidification,(B) cation exchange column, (C) anion exchangecolumn. 1.15:1 and 1.0:0 respectively. The spectra of these species are shown in Table 1. In (1) above it was suggested that the first step in the base hydrolysis of Cren,C20/was the formation of a "one-ended" oxalate species, which returned to the original complex when the solution was acidified. Two pieces of evidence confirm that a similar reaction occurs in 0.1 M sodium hydroxide. Table 1 shows the maxima and minima in the visible spectrum of the species produced when the eluate from the anion exchange column is acidified. (This is from a reaction which proceeded for 0.5 hr.) It can be seen to be almost identical to the spectrum of the starting material. If the "one-ended" oxalato species is the first product it will be zero-charged and will be eluted from the anion exchange column along with the starting material (which is positively charged). When the solution is acidified the "oneended" oxalato complex is converted to the starting material. In addition the more rapid reaction which occurs when the separation is effected on a sodium ion cation exchange column compared with the apparent rate as measured using acidification and the H + column confirms that the acidification results in the re-formation of the original complex giving an apparently slower rate. Further confirmation of this conclusion is given by the fact that after two half lives of the reaction in 0.1 M sodium hydroxide, the spectrum is similar to that observed for the "one-ended" oxalato species at pH 10.5 (Table 1). When such a mixture is quenched to pH 10.5 small changes in the wavelength positions occur, so that the 384 nm band moves to 379 um and the 508 um band moves to 504 rim. The change in spectrum in 0.1 M sodium hydroxide (whether it contains ethylenediamine or not) represents a continuous movement of the band maxima to the red. When the pH of the solution containing ethylenediamine at I~H 10.5 after 3 half lives is raised to a similar value to that of 0.1 M sodium hydroxide, this
MICHAEL B. DAVIES
2254
change proceeds in the same way as the corresponding 0.1 M solution containing ethylenediamine. The reactions in 0.1M sodium hydroxide solution beyond the formation of the monodentate oxalato species are complex. All attempts to separate the ion Crem(OI-I)2÷ failed. The spectral changes do not indicate its presence. When the wavelength of the lower wavelength band has reached 383 nm that of the higher wavelength band is nowhere near 525 nm. The fact that this species is not present explains the presence of the species eluted by 2 M acid when the acidified mixture is put on a cation exchange column. This has a spectrum which suggests that it is the monodentate ethylenediamine complex Cren enH(HzO)C20/+ which is also eluted from a cation exchange column with 2 M acid[14]. The reaction scheme is then:
gives a result in agreement with the rate obtained under similar conditions using the decrease in absorption of the shoulder at 267.5 nm. This shoulder is characteristic of many oxalato complexes [16] and obeys the Lambert-Beer law. Neither its position nor its intensity vary with ionic strength. Thus, at 267.5 nm the disappearance of the starting material i's being monitored. The reaction which is occurring is: CremC204÷+ OH- -~ CremC2040H °. This is similar to the analogous cobalt complex reaction studied by Farago and Mason[15]. The spectrophotometrically determined pseudo first order rate constants for this reaction are shown in Table 3. The concentration of sodium hydroxide is always many
Acid
Cren2C204÷ ~ o.-
, CremC2040H °--~ Crenen*C204(OI-I)2--*Cren(OH)4~ acid ! CrenenHC204(I-I20) | + C20~2+ en
*monodentate ethylenediamine
polymeric products
The presence of the last of these, Cren(OH)V is confirmed by the detection of Cren(I-I20)/÷ in acid solution. This compound has been shown[12] to produce highly charged species after prolonged contact with 0.1 M sodium hydroxide. (b) Kinetic studies. The measurement of the rate of the reaction in 0.1 M NaOH by monitoring the disappearance of the starting material using a chromatographic method
times more than the concentration of the complex ion. The ionic strength was maintained at 1.0 using sodium chloride. A plot of rate constant against concentration at four temperatures is shown in Fig. 3. At higher temperatures fairly good straight lines are obtained up to 1.0 M sodium hydroxide, but at lower temperatures there is departure from linearity at higher concentrations of sodium hydroxide (see second reaction of the cobalt
Table 3. Rate constants for reaction of Cr e n2C204 + (1.12x 10 3M) with variousconcentrationsof sodiumhydroxide ~=I,AE lq
2eo.a
29e,7
[oH'3 /x
zo~o ,u~=~th
o.1~
O.67 + 0.16
1.0
0.TPJ
o.9~ + o.~e
1.0
O.547
1.1:~ +- O.07
1.0
o.71"r
1.44 +- 0.10
1.0
1.06
2.41 +- 0.02
1.0
0.05~
1.95 +- O.oo
1.o
o.1(~
2.1~ ~ 0.0~
1.0
o.~J
2.4e *
1.0
0.~47
~.07 Z o.10
1.0
0.74~
~ . ~ Z 0.10
t.O
1.oo
6.~J Z 0.1o
1.o
1.0
o.o6
0.0~7
5.0'J + 0 . ~
0.100
%9"J *
0.100
6.22 ~ 0.00
0.I
0.27~J
7.CO ~ 0.6O
1.0
0.~47 1.Ce
*=r~,
ko~/,'1~ IO 4"
9.96 +
0.~
1.0
1.09
1.0
14.42 + 0.01
1.0
0.1(~
16.17 +- 0.18
1.0
0.547
17.4 ~ 0.1
1.0
1.08
~'7.2
1.0
+- 1.1
re];~.eem~ mz~l~o~Ll~Ltt7 of tvo oa" ne-ee mzw.
A studyof oxalatobis(ethylenediamine)chromium(Ill)ion
,oo
f
I
2255
The pH dependent part is readily explained on the basis of the reaction:
3 ,K
+
Crel~,C204 + OH- --*CremC204OH° as in the case of the corresponding cobalt complex. The pH independent part may be explained by postulating that the reaction may also be:
13.0 12.0 II0
Cren~C~O/ k, ~Cren2C204OH~÷ o.- )Cren2C2040H0 rapid
29EI.7K
"one-ended" oxalate unstable.
I0.0 9.0
~
•
8.0 7.0 6.0 5.0
•
e
@
4.0 30
/
2,0 L@" @ ~
i
/
I
~
2
8
9
K
@~"-
1.0 ~.... @... @-,.--"
0
+ H20
0.2
O.
06 [OH]/M
O~
1,0
Fig. 3. Variationof ko~,with [OH-]at varioustemperatures. complex observed by Farago and Mason[15]). Unlike the data obtained for C0en~20/the lines do not pass through the origin. This suggests that there are two pans to this reaction, i.e. a part which is independent of [OH-] and a pan which is dependent on [OH-]: ko~ = k~ + k2[OH-].
This reaction will occur in solutions of higher pH when the rapid acid-base equilibrium of the "one-ended" oxalate complex lies largely to the fight. The aquo "oneended" oxalate complex itself is expected to be unstable. The values of k2 at four temperatures can be determined from the slopes of the ko~ against [OH-] graphs and the intercepts give values of k~. These are shown in Table 4 together with the activation parameters for each reaction. The values of AH" and AS s for the pH independent and pH dependent reaction do not differ significantly. It is interesting to note that the values of AH ~ determined here are lower than the value obtained for the base hydrolysis of CrenC:~S)4(H:43)2+, which is believed to be loss of oxalate, i.e. fission of the second oxalate bond[12]. This is analogous to the situation found for the complex CoemC20/. Using data from the paper by Farago and Mason[15] it is possible to calculate values for AH ~ for the formation of the "one-ended" oxalate species and the dihydroxo species in basic solution. These are approximately 73 and 93 kJ tool-~ respectively. It has been shown that the first reaction of Coen2C20J ion with hydroxide ion occurs via breaking of the C-O bond and not the C o O bond, while the second reaction results from fission of the C o O bond. If this accounts for the difference in activation enthalpies in the cobalt complex reaction, then a similar situation might obtain for the chromium complex and the first reaction might occur via C-O rather than Cr-O bond fission.
Table 4. Variationin derivedrate constants withtemperature and activationparameters for first reaction T~[
~p*mieat z.ua~tlo, ~ paaS~ut
k~lO"4 ~'lS
280.8
1.1~ +-.0.18
2~.0
2.23 +- 0.09
~xje.7 ~.? ~eo.8
o.~a +- o.o~
~.0
1.86 + 0.02
298.T
~zl
*
Det~
-55.9~1.3
T4.4~ 3.4
~9.4~0.6
8.85 +- 0.55
maa~Lon f ~ m
~.T
74.2~ T.7
21.66+ 1.0
s~ 4,,,~,p~ml~
4,,~.,~
,~ E~,/~ IQ1"1 .~/.T['~I01 "1
4.~
+- 0 . ~
t2.50 +- 1.t
m stu~la~ dtev~Latto==
..4=f • oo=~t~ p z ~
t.me~ on eq~t~== ~ Ref. 17~ 100.
2256
MICHAEL B. DAVIES
Acknowledgements--The author would like to thank Miss O. Harper for computing, Mrs. B. Bradley for technical assistance and Dr. J. W. Lethbridge for valuable discussion. The provision of financial assistance by S.R.C. towards a Unicam SP 1800 spectrophotometer is also gratefully acknowledged. REFERENCES 1. C. S. Garner and D. A. House, Transition Metal Chemistry 6, 263 (1970). 2. M. B. Davies and J. W. Lethbridge, J. Inorg. Nucl. Chem. 37, 171 (1975). 3. G. G. Schlessinger, Inorganic Laboratory Preparations, p. 227. Chem. Pub., New York (1%2). 4. M. B. Davies, J. W. Lethbridge and Othman Nor, J. Chromatogr. 68, 231 (1972). 5. A. I. Vogel, A Textbook of Quantitative Analysis, 3rd Edn. p. 284. Longmans, London (1%1). 6. D. Merrill, L.R.I.C. Dissertation, Stockport College of Technology (1970).
7. M. B. Davies and J. W. Lethbridge, unpublished research. 8. A. W. Adamson, In Mechanisms of Inorganic Reactions, Advances in Chemistry Series, Chap. 10, No. 49, Am. Chem. Soc., Washington D.C. (1965). 9. M. B. Davies, J. W. Lethbridge, Othman Nor and Lai-Yoong Goh, J. lnorg. Nucl. Chem. 37, 175 (1975). 10. M. Casula, G. Illuminati and G. Ortaggi, Inorg. Chem. 11, 1062 (1972). 11. M. B. Davies and J. W. Lethbridge, J. lnorg. Nuel. Chem. 37, 141 (1975). 12. M. B. Davies, J. Inorg. Nucl. Chem. 36, 3789 (1974). 13. F. Woldbye, Acta Chem. $cand. 12, 1079 (1958). 14. J. W, Lethbridge, personal communication. 15. M. E. Farago and C. P. V. Mason, J. Chem. Soc. (A), 3100 (1970). 16. K. V. Krishnamurty and G. M. Harris, Chem. Rev. 61, 219 (1961). 17. A. A. Frost and R. G. Pearson, Kinetics and Mechanism, 2nd Edn. Wiley, New York (1961).