- Email: [email protected]
Catalysis of complex formation reactions
$.lJ~3flg. l q l d . ~
1960, VOI. I$. pp. 250 IO2.f,4. ~
PgeB Ltd.
CATALYSIS OF COMPLEX FORMATION REACTIONS* M. T. BEcK
Institute of Inorganic and Analytical Chemistry,The University, Szeged, Hungary (Received 15 September 1959; in revisedform 21 December 1959) Abstract--The catalysed complex formation reactions can be classifiedinto (1) co-ordination catalysis, (2) electron transfer catalysis, (3) induced complex formation, (4) heterogeneous catalysis. The general features of these types are outlined and the considerations are illustrated by characteristic examples.
THE catalytic phenomena of complex chemistry can be classified into two groups: (1) catalysis of formation of complexes and (2) catalysis by complex compounds. Since these phenomena are extremely various, it is impossible to present a unified treatment. As far as we know this is the first attempt to systematize the catalysis of complex formatiofi reactions. Dealing with the complex formation reactions it has to be considered that every formation reaction really is an exchange process. In the reactions which are termed commonly as formation reactions the displacement of solvent molecules bound in the co-ordination sphere occurs. The phenomena of catalysis of formation of complex compounds may be divided into three groups: (i) co-ordination catalysis, (ii) catalysis connected with redox reactions, (iii) heterogeneous catalysis. (i) Co-ordination catalysis In the case of co-ordination catalysis the formation of the complex is preceded by a temporary formation of another complex. (a) Catalysis by ligand. If the Me central ion reacts slowly with ligand A and rapidly with the B ligand--which forms a thermodynamically less stable complex-and the MeB complex reacts also rapidly with ligand A, then ligand B catalyses the formation of the MeA complex. Schematically: Me + A ~
MeA kl
Me + B ,( • MeB k¢'
kI
MeB + A ~
MeBA,
• MeA + B
* Part of the paper presented at the International Conference on Co-ordination Chemistry, London, April, 1959. 250
Catalysis of complex formation reactions
251
A well known example of co-ordination catalysis is the catalytic effect of hydroxide ions on the formation of different complexes, c1'2) A specific catalytic phenomenon was observed by us in connection with the formation of Cr(III)-thiocyanate complex. The reaction was followed by the determination of formed neutral complex [Cr(SCN)s]: aliquot parts of the reaction mixture were periodically shaken with isoamylalcohol, and the organic layer containing the neutral complex was examined spectrophotometrically.
0.4
.(>2
o
I
2
tt
3
hr
FIo. l . - - T h e change of al~sorbancy of aliquots at 4 2 0 m ~ : Ccr = 0"0277 mole/I. CscN = 0.2710 mole/1. Upper curve: pH adjusted to 4.2 by N a H C O 8 1 = 1.03. Lower curve: pH adjusted to 4.2 by Ba(OH)~ I = 1-08 t = 25°C.
Fig. 1 shows that the rate of reaction greatly varies depending on whether the pH of solutions was adjusted with barium hydroxide or sodium hydrogen carbonate. The pH of solutions being equal and the difference between the ionic strength negligible, the cause of the difference in reaction velocity is the different carbonate and hydrogencarbonate content of solutions. Gasometric experiments showed that the amount of dissolved carbonic acid is considerable. So it is reasonable to suppose that the temporary formation of a Cr(III)-carbonate or Cr(III)-hydrogen-carbonate complex is responsible for the increased velocity. A similar catalytic effect of carbonate ion was not observed in formation of other Cr(III) complexes. (b) Catalysis by cation.* The acceleration effect of certain metal ions on the equation of different halogen complexes (s-6) also may be termed as co-ordination catalysis. In this case the accelerating effect can be ascribed to the temporary formation of a binuclear complex; ~'s) which may be formed either as the transition state or, in some cases have a more permanent existence. MeXo__,, HI + M --~ Xo_,~MeH1M -,- MeX6_,~ H20 + MHI The catalysis of aquation of monobromo-ethylenediaminetetraacetato--cobalt(III) * This problem was not discussed in the lecture. (1~ R. E. H ^ ~ , J. Amer. Chem. Soc. 75, 5670 (1953). (2~ C. POSTMUS and E. L. KING,.I. Phys. Chem. 59, 1216 (1955). (s) j. N. BR6NSTED and R. LIVINOSTON,J. Amer. Chem. Soc. 49, 435 (1927). t4) p. j. ELWNO and B. ZEM~L, J. Amer. Chem. Soc. 79, 5855 (1957). (~) W. C. E. HXOOINSON and M. P. HILL, J. Chem. Soc. 1620 (1959). (s) M. T. BECK, Thesis, Szeged (1956).
252
M. T~BgcI:
complex by lead ion (s) may serve as an example of this type of reaction. Mention must be made that in this case we can speak about catalysis only with some reservation, the complex formed (PbBr +) in this way being rather stable. Nevertheless it is beyond doubt that the reason of accelerating effect is the formation of a binuclear complex and not the combination of Pb s+ with dissociated bromide ions. Namely, according to our experiments, Co(NHa)6(NO0s has no effect on the rate of aquation, although it forms an "outer sphere" type complex with bromide ion the stability of which equals to that of the PbBr +(~) ion. Naturally in this case there is'no possibility that a binuclear complex is formed. (ii) Catalysis connected with redox reactions The following two examples of catalysis connected with redox reactions represent two different types, namely electron transfer catalysis and induced complex formation. (a) If the following displacement reaction MeA + B = M e B + A where Me ion is in a higher oxidation state is accelerated by the Me ion in a lower oxidation state, the reason of catalysis is an electron transfer reaction. An example of this type, the reaction of iron(III) xylenolorange complex with aminopolycarboxylic acids which is greatly catalysed by ferrous ion. (s) The reaction could be followed by spectrophotometric determination of concentration of iron(III)xylenolorange complex. On the basis of our kinetic investigation of the reaction with 1,2 diaminocyclohexametetraacetic acid (Q) the mechanism of catalysis is as follows: Fe(III)XO + Fe(II)Q = Fe(III)Q + Fe(II)XO Fe0I)XO ----Fe(II) + XO Fe(II)Q is supplied by the following equilibrium Fe(II) + Q = Fe(II)Q which is shifted toward the dissociation at the given acidic medium. Considering h e possible concentration of the Fe(II)Q complex, the ratio of the velocity constants of the catalysed and non-catalysed reactions is about 10~. (b) If the formation of a complex is accelerated by a simultaneous redox reaction t h e phenomenon may be termed as induced complex formation. The oxidation of certain Co(II) amino acid complexes takes place in two steps. (9'1°) First, an oxygen carrying Co(II) complex forms by a reversible oxygen uptake and then--in an irreversible reactionmthe Co(III) complex forms. Both reactions are rather slow. We observed(m that the Co(II) glycylglycinecomplex catalyses the autoxidation of ascorbic acid and the ascorbic acid accelerates the oxygenation of the Co(II)glycylglycine complex, i.e. the formation of oxygen carrying complex. This latter phenomenon is shown in Fig. 2. where the change of absorbancy at about 520 m/~ (filter S~) is plotted as a function of time, in the presence and absence of ascorbic (7) j. BJERRUM, G. SCHWAg.ZENBACHand L. J. S[LL~N, Stability Constants Part II. The Chemical Society, London (1959). (s) R. PRxelL and J. K6RaL, Private communication (1957). (') C. TANFOaD, D. C. KInK and M. K. CtlANI"OON[,J. Amer. Chem. Soc. 76, 5325 (1954). (is) j. Z. HEXaON and D. BuaK: J. Nat. Cancer Inst. 9, 337 (1949). (11) M. T. BECK and S. G6R6o, Acta~Phys. Chem. Szeged4, S.62 (1958).
Catalysis of complex formation reactions
253
0-8
(>4
0
I
20
I
40 t,
60
min
FIG. 2 . - - T h e change of absorbancy during the oxygenation of Co(II)-glycylglycine complex in presence (upper curve) and absence (lower curve) of ascorbic acid: Coo = 10"-a mole/l; C u e . = 10-z mole/l; p H = 8; cuvette 3, 5 cm, filter S52.
I
O.4
Co: Oscorbic ocid i,25
\
r
/./'/"1't"~
/ 0
I"'\
/
/ I
I
I
20 t,
I
40
I
I
60
min
FIG. 3.--Effect of ascorbic acid on the formation of intermediate. Coo = 10"-' mole/l; p H = 7.5, cuvette 5 cm, wavelength 350 m/t. The molar absorbancy of oxygen carrying complex is 8000.
acid. In the visible region the absorbancy of oxygen carrying and the Co(III) complexes are the same, but in the near ultra-violet the absorbancy of the oxygen carrying complex is much higher than that of the Co(III) glycylglycinecomplex. So the change of concentration of the oxygen carrying complex with time can be observed by measuring the absorbancy at 350 m/~. Fig. 3 shows the effect of ascorbic acid on the formation of oxygen carrying Co(II) complex. The induced complex formation and the catalysis of the autoxidation of ascorbic 4
254
M.T. 1 1 ~
acid can be explained by the step by step activation of molecular oxygen. In the first step the ascorbic acid forms a labile adduct with oxygen: A + OI = AOi the collision of which with a Co0I)-glycylglycine complex results in the oxygen carrying complex reacting with a greater velocity than the reaction of oxygen with the Co(II)-glycylglycine complex:
Co(ll)gg2 + 02 ~'~ Co(II)gg~O2 Co(II)ggs -t- AO2 ~'~ Co(II)ggsO~ -F A k2 > kl On the other hand the oxygen carrying complex oxidizes the ascorbic acid with greater velocity than the molecular oxygen does. Beside these reactions, an irreversible oxidation of the Co(II) complex takes place which results in the change of the concentration of the oxygen carrying complex m time according to a maximum curve. (iii)
Heterogeneous catalysis
The heterogeneous catalysis of displacement reactions of complexes is well known and frequently used for preparative purposes. ~lz~ Nevertheless till now the mechanism o f heterogeneous catalysis was not investigated. We studied the reactions of different Co(lII) amines with ethylenediamintetraacetic acid catalysed by active carbon. ~m~ On the basis of the comparative study of different complexes the following reasonable mechanism is suggested*: The co-ordination sphere of the adsorbed complex ion--due to the adsorption~becomes less firm. The loosening of the co-ordination sphere results in an increased reactivity; the collision of the adsorbed complex ion with a ligand forming a thermodynamically more stable complex is more effective than a collision in the bulk of solution. Naturally, the loosening of the co-ordination sphere is inversely proportional to the symmetry of the complexes. The experimental findings are in accordance with this view: the hexammine-Co(III) reacts much slower than the aquopentammine-Co(III) or the chloropentammine-Co(III)complexes. The loosening of the co-ordination sphere on the effect of active carbon is exhibited in the acceleration of the reduction of complexes by ascorbic acid. The reduction of chloropentammine-Co(III) is very slow in acidic medium while in the presence of active charcoal the reduction takes place rapidly, but with a messurable rate, whilst at pH 5 the reduction is immediate. Under these circumstances the hexammine-Co(III) cannot be reduced. Acknowled~,ements--The author wishes to express his gratitude to Prof. Z. G. SZA~6 for his interest and the discussions. Thanks are due to Dr. S. G6nOo, I. BXRDXand K. TOn-I for their help in the experimental work and for the discussions. * This refers only to exchange reactions catalyscd by active carbon. u~ F. B^SOLOand R. G. PEARSON,Mechanism of Inorganic Reactions p. 355. John Wiley, New York (1958). ua~ M. T. B~CKand I. B£Rol, Acta Phys. Chem. Szeged, 4, S.74 (1958).
1960, VOI. I$. pp. 250 IO2.f,4. ~
PgeB Ltd.
CATALYSIS OF COMPLEX FORMATION REACTIONS* M. T. BEcK
Institute of Inorganic and Analytical Chemistry,The University, Szeged, Hungary (Received 15 September 1959; in revisedform 21 December 1959) Abstract--The catalysed complex formation reactions can be classifiedinto (1) co-ordination catalysis, (2) electron transfer catalysis, (3) induced complex formation, (4) heterogeneous catalysis. The general features of these types are outlined and the considerations are illustrated by characteristic examples.
THE catalytic phenomena of complex chemistry can be classified into two groups: (1) catalysis of formation of complexes and (2) catalysis by complex compounds. Since these phenomena are extremely various, it is impossible to present a unified treatment. As far as we know this is the first attempt to systematize the catalysis of complex formatiofi reactions. Dealing with the complex formation reactions it has to be considered that every formation reaction really is an exchange process. In the reactions which are termed commonly as formation reactions the displacement of solvent molecules bound in the co-ordination sphere occurs. The phenomena of catalysis of formation of complex compounds may be divided into three groups: (i) co-ordination catalysis, (ii) catalysis connected with redox reactions, (iii) heterogeneous catalysis. (i) Co-ordination catalysis In the case of co-ordination catalysis the formation of the complex is preceded by a temporary formation of another complex. (a) Catalysis by ligand. If the Me central ion reacts slowly with ligand A and rapidly with the B ligand--which forms a thermodynamically less stable complex-and the MeB complex reacts also rapidly with ligand A, then ligand B catalyses the formation of the MeA complex. Schematically: Me + A ~
MeA kl
Me + B ,( • MeB k¢'
kI
MeB + A ~
MeBA,
• MeA + B
* Part of the paper presented at the International Conference on Co-ordination Chemistry, London, April, 1959. 250
Catalysis of complex formation reactions
251
A well known example of co-ordination catalysis is the catalytic effect of hydroxide ions on the formation of different complexes, c1'2) A specific catalytic phenomenon was observed by us in connection with the formation of Cr(III)-thiocyanate complex. The reaction was followed by the determination of formed neutral complex [Cr(SCN)s]: aliquot parts of the reaction mixture were periodically shaken with isoamylalcohol, and the organic layer containing the neutral complex was examined spectrophotometrically.
0.4
.(>2
o
I
2
tt
3
hr
FIo. l . - - T h e change of al~sorbancy of aliquots at 4 2 0 m ~ : Ccr = 0"0277 mole/I. CscN = 0.2710 mole/1. Upper curve: pH adjusted to 4.2 by N a H C O 8 1 = 1.03. Lower curve: pH adjusted to 4.2 by Ba(OH)~ I = 1-08 t = 25°C.
Fig. 1 shows that the rate of reaction greatly varies depending on whether the pH of solutions was adjusted with barium hydroxide or sodium hydrogen carbonate. The pH of solutions being equal and the difference between the ionic strength negligible, the cause of the difference in reaction velocity is the different carbonate and hydrogencarbonate content of solutions. Gasometric experiments showed that the amount of dissolved carbonic acid is considerable. So it is reasonable to suppose that the temporary formation of a Cr(III)-carbonate or Cr(III)-hydrogen-carbonate complex is responsible for the increased velocity. A similar catalytic effect of carbonate ion was not observed in formation of other Cr(III) complexes. (b) Catalysis by cation.* The acceleration effect of certain metal ions on the equation of different halogen complexes (s-6) also may be termed as co-ordination catalysis. In this case the accelerating effect can be ascribed to the temporary formation of a binuclear complex; ~'s) which may be formed either as the transition state or, in some cases have a more permanent existence. MeXo__,, HI + M --~ Xo_,~MeH1M -,- MeX6_,~ H20 + MHI The catalysis of aquation of monobromo-ethylenediaminetetraacetato--cobalt(III) * This problem was not discussed in the lecture. (1~ R. E. H ^ ~ , J. Amer. Chem. Soc. 75, 5670 (1953). (2~ C. POSTMUS and E. L. KING,.I. Phys. Chem. 59, 1216 (1955). (s) j. N. BR6NSTED and R. LIVINOSTON,J. Amer. Chem. Soc. 49, 435 (1927). t4) p. j. ELWNO and B. ZEM~L, J. Amer. Chem. Soc. 79, 5855 (1957). (~) W. C. E. HXOOINSON and M. P. HILL, J. Chem. Soc. 1620 (1959). (s) M. T. BECK, Thesis, Szeged (1956).
252
M. T~BgcI:
complex by lead ion (s) may serve as an example of this type of reaction. Mention must be made that in this case we can speak about catalysis only with some reservation, the complex formed (PbBr +) in this way being rather stable. Nevertheless it is beyond doubt that the reason of accelerating effect is the formation of a binuclear complex and not the combination of Pb s+ with dissociated bromide ions. Namely, according to our experiments, Co(NHa)6(NO0s has no effect on the rate of aquation, although it forms an "outer sphere" type complex with bromide ion the stability of which equals to that of the PbBr +(~) ion. Naturally in this case there is'no possibility that a binuclear complex is formed. (ii) Catalysis connected with redox reactions The following two examples of catalysis connected with redox reactions represent two different types, namely electron transfer catalysis and induced complex formation. (a) If the following displacement reaction MeA + B = M e B + A where Me ion is in a higher oxidation state is accelerated by the Me ion in a lower oxidation state, the reason of catalysis is an electron transfer reaction. An example of this type, the reaction of iron(III) xylenolorange complex with aminopolycarboxylic acids which is greatly catalysed by ferrous ion. (s) The reaction could be followed by spectrophotometric determination of concentration of iron(III)xylenolorange complex. On the basis of our kinetic investigation of the reaction with 1,2 diaminocyclohexametetraacetic acid (Q) the mechanism of catalysis is as follows: Fe(III)XO + Fe(II)Q = Fe(III)Q + Fe(II)XO Fe0I)XO ----Fe(II) + XO Fe(II)Q is supplied by the following equilibrium Fe(II) + Q = Fe(II)Q which is shifted toward the dissociation at the given acidic medium. Considering h e possible concentration of the Fe(II)Q complex, the ratio of the velocity constants of the catalysed and non-catalysed reactions is about 10~. (b) If the formation of a complex is accelerated by a simultaneous redox reaction t h e phenomenon may be termed as induced complex formation. The oxidation of certain Co(II) amino acid complexes takes place in two steps. (9'1°) First, an oxygen carrying Co(II) complex forms by a reversible oxygen uptake and then--in an irreversible reactionmthe Co(III) complex forms. Both reactions are rather slow. We observed(m that the Co(II) glycylglycinecomplex catalyses the autoxidation of ascorbic acid and the ascorbic acid accelerates the oxygenation of the Co(II)glycylglycine complex, i.e. the formation of oxygen carrying complex. This latter phenomenon is shown in Fig. 2. where the change of absorbancy at about 520 m/~ (filter S~) is plotted as a function of time, in the presence and absence of ascorbic (7) j. BJERRUM, G. SCHWAg.ZENBACHand L. J. S[LL~N, Stability Constants Part II. The Chemical Society, London (1959). (s) R. PRxelL and J. K6RaL, Private communication (1957). (') C. TANFOaD, D. C. KInK and M. K. CtlANI"OON[,J. Amer. Chem. Soc. 76, 5325 (1954). (is) j. Z. HEXaON and D. BuaK: J. Nat. Cancer Inst. 9, 337 (1949). (11) M. T. BECK and S. G6R6o, Acta~Phys. Chem. Szeged4, S.62 (1958).
Catalysis of complex formation reactions
253
0-8
(>4
0
I
20
I
40 t,
60
min
FIG. 2 . - - T h e change of absorbancy during the oxygenation of Co(II)-glycylglycine complex in presence (upper curve) and absence (lower curve) of ascorbic acid: Coo = 10"-a mole/l; C u e . = 10-z mole/l; p H = 8; cuvette 3, 5 cm, filter S52.
I
O.4
Co: Oscorbic ocid i,25
\
r
/./'/"1't"~
/ 0
I"'\
/
/ I
I
I
20 t,
I
40
I
I
60
min
FIG. 3.--Effect of ascorbic acid on the formation of intermediate. Coo = 10"-' mole/l; p H = 7.5, cuvette 5 cm, wavelength 350 m/t. The molar absorbancy of oxygen carrying complex is 8000.
acid. In the visible region the absorbancy of oxygen carrying and the Co(III) complexes are the same, but in the near ultra-violet the absorbancy of the oxygen carrying complex is much higher than that of the Co(III) glycylglycinecomplex. So the change of concentration of the oxygen carrying complex with time can be observed by measuring the absorbancy at 350 m/~. Fig. 3 shows the effect of ascorbic acid on the formation of oxygen carrying Co(II) complex. The induced complex formation and the catalysis of the autoxidation of ascorbic 4
254
M.T. 1 1 ~
acid can be explained by the step by step activation of molecular oxygen. In the first step the ascorbic acid forms a labile adduct with oxygen: A + OI = AOi the collision of which with a Co0I)-glycylglycine complex results in the oxygen carrying complex reacting with a greater velocity than the reaction of oxygen with the Co(II)-glycylglycine complex:
Co(ll)gg2 + 02 ~'~ Co(II)gg~O2 Co(II)ggs -t- AO2 ~'~ Co(II)ggsO~ -F A k2 > kl On the other hand the oxygen carrying complex oxidizes the ascorbic acid with greater velocity than the molecular oxygen does. Beside these reactions, an irreversible oxidation of the Co(II) complex takes place which results in the change of the concentration of the oxygen carrying complex m time according to a maximum curve. (iii)
Heterogeneous catalysis
The heterogeneous catalysis of displacement reactions of complexes is well known and frequently used for preparative purposes. ~lz~ Nevertheless till now the mechanism o f heterogeneous catalysis was not investigated. We studied the reactions of different Co(lII) amines with ethylenediamintetraacetic acid catalysed by active carbon. ~m~ On the basis of the comparative study of different complexes the following reasonable mechanism is suggested*: The co-ordination sphere of the adsorbed complex ion--due to the adsorption~becomes less firm. The loosening of the co-ordination sphere results in an increased reactivity; the collision of the adsorbed complex ion with a ligand forming a thermodynamically more stable complex is more effective than a collision in the bulk of solution. Naturally, the loosening of the co-ordination sphere is inversely proportional to the symmetry of the complexes. The experimental findings are in accordance with this view: the hexammine-Co(III) reacts much slower than the aquopentammine-Co(III) or the chloropentammine-Co(III)complexes. The loosening of the co-ordination sphere on the effect of active carbon is exhibited in the acceleration of the reduction of complexes by ascorbic acid. The reduction of chloropentammine-Co(III) is very slow in acidic medium while in the presence of active charcoal the reduction takes place rapidly, but with a messurable rate, whilst at pH 5 the reduction is immediate. Under these circumstances the hexammine-Co(III) cannot be reduced. Acknowled~,ements--The author wishes to express his gratitude to Prof. Z. G. SZA~6 for his interest and the discussions. Thanks are due to Dr. S. G6nOo, I. BXRDXand K. TOn-I for their help in the experimental work and for the discussions. * This refers only to exchange reactions catalyscd by active carbon. u~ F. B^SOLOand R. G. PEARSON,Mechanism of Inorganic Reactions p. 355. John Wiley, New York (1958). ua~ M. T. B~CKand I. B£Rol, Acta Phys. Chem. Szeged, 4, S.74 (1958).