Reaction of copper(II) with water soluble porphyrins (H₂porph) in the presence of ethylenediamine shows an unusual and so far unreported kinetic phenomenon that can be explained in terms of supramolecular assembly formation apparently involving all solute species as confirmed via light scattering and conductivity measurements

González-Vílchez, F.

Pasternack, Robert F.

Vilaplana, R. A.

Works

Swarthmore College 
Works 
Chemistry & Biochemistry Faculty Works Chemistry & Biochemistry 
5-15-1991 
Evidence For An Unusual Kinetic Phenomenon In The Metallation 
Of Porphyrins 
F. González-Vílchez 
R. A. Vilaplana 
Robert F. Pasternack 
Swarthmore College, rpaster1@swarthmore.edu 
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 Part of the Inorganic Chemistry Commons 
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Recommended Citation 
F. González-Vílchez, R. A. Vilaplana, and Robert F. Pasternack. (1991). "Evidence For An Unusual Kinetic 
Phenomenon In The Metallation Of Porphyrins". Journal Of The Chemical Society: Chemical 
Communications. Issue 10. 731-732. DOI: 10.1039/c39910000731 
https://works.swarthmore.edu/fac-chemistry/117 
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J .  CHEM. SOC., CHEM. COMMUN., 1991 73 1 
Evidence for an Unusual Kinetic Phenomenon in the Metallation of Porphyrins 
Francisco Gonzalez-Vilchez,a Rosario A. Vilaplana a and Robert F. Pasternack" b 
a Departamento de Quimica Inorganica, Facultad de Quimica, Universidad, 4 1071 Sevilla, Spain 
b Chemistry Department, Swarthmore College, Swarthmore, PA 19081, USA 
Reaction of copper(ii) with water soluble porphyrins (H2porph) in the presence of ethylenediamine shows an unusual 
and so far unreported kinetic phenomenon that can be explained in terms of supramolecular assembly formation 
apparently involving all solute species as confirmed via light scattering and conductivity measurements. 
In this contribution we present an anomalous and so far 
unreported kinetic phenomenon that occurs when copper(I1)- 
ethylenediamine complexes react with water soluble synthetic 
porphyrins (H2porph). t Other reports of porphyrin metalla- 
tion reactions in aqueous solutions,1-6 generally conducted at 
low pH to avoid hydrolysis of metal ions and precipitation of 
hydroxy species, are consistent with a rate law of the form R = 
k [M2+][H2porph]. Pseudo-first-order conditions are usually 
maintained so that R = ke[H2porph] with k, = k [M2+]. In 
contrast we observe in our studies at neutral pH in the 
presence of ethylenediamine(en), a discontinuity in the k, vs 
[CuII] profile. The experimental, first-order rate constant$ 
shows a very high-order dependence on [CuJI] over a limited 
concentration (Fig. 1) reaching a maximum value, kemaX at 
some critical copper(r1) concentration, [MIc . Values of kemaX 
and [MIc depend on solution conditions as described below but 
the profile shown in Fig. 1 persists for a variety of electrolytes, 
buffers and porphyrins. At concentrations of Cu2+ beyond 
[MI,, the rate constant k, decreases but more gradually than 
its rise. 
The Cu"-ethylenediamine system was selected because (i) 
the rate of copper(r1) insertion into porphyrins is in a 
convenient range for study, (ii) the free ligand shows no 
reactivity with the water soluble porphyrins investigated here7 
and (iii) under the conditions of the experiment {i.e., pH 7.0; 
p = 0.15 mol dm-3; T = 298 K; (en)o = 3 [CuII],}, one copper 
species, C ~ ( e n ) ~ 2 + ,  predominates over the entire concentra- 
tion range considered. 9 We calculated the concentration 
profiles of the various minor copper(I1)-containing species 
[i.e., Cu2+, Cu(OH)+, Cu(en)2+, C ~ ( e n ) ~ 2 + ]  and found that 
none of them resembles that of the reactivity profile shown in 
Fig. 1. 
As a specific sample of this behaviour we considered the 
reaction of Cu"-en with the tetracationic porphyrin tetrakis 
(4-N-methylpyridyl)porphyrin, H2tmpyp. The experiments 
-I Hzporph is used to designate the free base form of the porphyrin. 
Charges are neglected for the sake of convenience. 
$ The slow kinetics were followed on either a Nicolet 9420 UV-VIS or 
a Varian 2200 recording spectrophotometer which has thermostated 
cell compartments. Faster reactions were observed using a Nortec 
stopped-flow system. The reactions were run at constant wavelength 
and most of the kinetic experiments were conducted under conditions 
such that [Cu2+] 5: 10 [H2porph]o; these experiments provided good 
first-order type kinetic profiles. However, even when the [Cu2+l0 - 
[H2porphIo such first-order profiles were obtained. Values of k ,  were 
found to be independent of whether the disappearance of the free 
base-porphyrin band (518 nm for H2tmpyp) or the appearance of the 
metalloporphyrin peak (548 nm for CuH2tmpyp) were monitored. 
The existence of an isosbestic point in the visible region ( h  535 nm) 
implies that the porphyrin reactant and metalloporphyrin product are 
the only chromophores present in non-negligible concentration other 
than Cu(en)z2+ which absorbs much further into the red of the 
spectrum. 
D The formation of the reactant complex C ~ ( e n ) ~ 2 +  was achieved by 
maintaining an en-Cu" ratio of 3.0 at pH 7.0. The components were 
added in the following sequence to avoid precipitation: Cull, en, 
NaN03, milli-Q water and collidine. A stock solution (0.1 rnol dm-3) 
of 2,4,6-collidine from Sigma Chem. Co., was prepared and used as 
buffer, The final pH was adjusted to 7.0 using NaOH or HN03 if 
needed. 
shown in Fig. l(b) were conducted at pH 7.0, [collidine] = 2.0 
x 10-2 mol dm-3, p = 0.15 mol dm-3, T = 298 K, [H2tmpyp] 
= 3.5 X 10-5 mol dm-3. As shown in Fig. 1 and Table 1, the 
rate constant k, is nearly zero at low values of [Cu"] but at 
[CuII] = 7.5 x 10-4 mol dm-3, k, suddenly rises to a value of 
9.9 x 10-3 s-1. Further increase in [CuII] leads to a decrease in 
k, until at [CuII] = 1.25 X 10-3 mol dm-3, k, is once again near 
zero. 
The other porphyrins investigated are the tetracationic 
tetrakis(N,N,N-trimethyl-4-anilinium)porphyrin, H2tap, and 
the tetraanionic tetrakis(4-sulphopheny1)porphyrin H2tpps. 
The maximum rate constants for metallation, k,max, increase 
in the sequence H2tmpyp < H2tap < Hztpps, the same order 
as for porphyrin basicity. For the last of these compounds, 
stopped-flow methods must be employed for kinetic measure- 
ments. Charge and basicity of the porphyrin therefore play a 
role in determining the value of the rate constant. The 
concentration of the porphyrin also in part determines the 
critical parameters in the process. Increasing H2tmpyp con- 
1 
0 1 2 3 
[Cu2'] x 1 O3 / rnol dm-3 
Fig. 1 Metal concentration dependence of k ,  at different concentra- 
tions of H2tmpyp: (a) :  [H2tmpyp] = 4.10 x rnol dm-3; (b):  
[H,tmpyp] = 3.50 x 10-5 mol dm-3; ( c ) :  [H2tmpyp] = 1.70 X lo-' 
mol dm-' 
Table 1 Variation of the experimental rate constant with metal 
concentration in the reaction Cu(en)Z2+ + Hztmpyp 
[cu2+l0 x 103/ 
mol dm-3 k,  x lO3/s-l 
0.25 
0.50 
0.75 
0.83 
0.90 
1 .oo 
1.10 
1.25 
2.50 
0.1 
0.2 
9.9 
8.9 
8.9 
6.2 
5.4 
0.1 
negligibIe 
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732 J. CHEM. SOC., CHEM. COMMUN., 1991 
only effect exerted by changing ionic strength is a shifting of 
[MI,, the nature of the electrolyte has a profound influence on 
k,max and even the breadth of the profile. 
To probe the nature of the species in solution giving rise to 
these unusual kinetic effects, we have performed light 
scattering and conductance measurements on these systems. 
We find that at Cu" concentrations both above and below 
[MI,, the amount of light scattered is small but at [CuII] = 
[MI,, the scattered light intensity increases nearly sixfold 
(Table 2). Solutions giving rise to rapid kinetics (i.e., at [CuII] 
= [MI,) have much lower conductivities than solutions on 
either side of these critical concentrations (Table 2). These 
results provide evidence for the formation of some type of 
composite in solution at the critical conditions. Based upon 
the evidence, we propose that the unusual kinetic effect found 
here is attributable to the existence of molecular assemblies of 
a supramolecular nature formed by association of the metal 
complex, porphyrin and other solute species and held together 
by intermolecular forces8 having microscopic organization and 
macroscopic characteristics.9 
The authors thank the NATO Postdoctoral Program and 
the Education Ministry of Spain for grants awarded to 
R. A. V. We are also indebted to the MECFulbright 
Program for a Fellowship to F. G.-V. who thanks the 
University of Seville for sabbatical leave. We gratefully 
acknowledge support of this research from the National 
Science Foundation (CHEM-8915264) and the Monsanto 
Corporation. 
283 298 313 328 
T/K 
Fig. 2 Variation of the critical copper(r1) concentration, [MI,, with 
temperature 
Table 2 Results of light scattering and conductivity measurementsn 
[Cu2+l0 x lo3/ Scattered light Conductivity/ 
mol drn-3 (arb. units) Q-l cm-l 
0.25 2.2 8.3 
0.75 11.8 2.3 
1.25 1.3 7.9 
Received, 10th October 1990; Corn. Ol0456l F 
a [H2tmpyp] = 3.50 x 
T = 298 K; collidine = 2.0 x 
mol dm-3; pH = 7.0; p = 0.15 mol dm-3; 
mol dm-3. In all experiments [enlo 
= 3 [CU2+](). 
centration, for example, leads to a larger kemax and lower 
values of [MIc (Fig. 1) but in all cases, the anomalous kinetic 
profile remains. 
The reaction of CdI-ethylenediamine with H,tmpyp was 
studied over a 55 "C temperature range. A kinetic profile of 
the type shown in Fig. 1 was found throughout and the value of 
kernax varied by less than 9.0%. However, [MI, increases 
linearly with temperature as shown in Fig. 2. The nature of the 
buffer (collidine tris or HEPES) influences both kemax and 
[MIc but not the general kinetic profile. Finally, although the 
References 
1 P. Hambright, Coord. Chem. Rev., 1971, 6, 247; P. Hambright, 
Ann. N.Y.  Acad. Sci., 1973, 206, 443; P. Hambright and P. B. 
Chock, J .  Am.  Chem. Soc., 1974, 96, 3123. 
2 H. Baker, P. Hambright and L. Wagner, J .  Am. Chem. SOC., 1973, 
95, 5942. 
3 R. F. Pasternack, G. C. Vogel, C. A. Skowronek, R. K. Harrisand 
J. G .  Miller, Inorg. Chem., 1981, 20, 3763. 
4 D. K. Lavallee, Coord. Chem. Rev., 1985, 61, 55. 
5 M. Tabata and M. Tanaka, Inorg. Chem., 1988,27,203. 
6 F. R. Longo, E. M. Brown, D. J. Quimby, A. D. Adler and M. 
7 A. J. McHugh, M. Gouterman and Ch. Weiss, Jr., Theor. Chim. 
8 J.-M. Lehn, Angew. Chem., Int. Ed. Eng., 1988, 27, 89. 
9 J.  H.  Fendler, Membrane Mimetic Chemistry, Wiley, N.Y., 1982. 
Meot-Ner, Ann. N . Y .  Acad. Sci., 1973, 206, 420. 
Acta, 1972, 24, 346. 
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View Article Online


English

Evidence For An Unusual Kinetic Phenomenon In The Metallation Of Porphyrins

Swarthmore CollegeWorksChemistry & Biochemistry Faculty Works Chemistry & Biochemistry5-15-1991Evidence For An Unusual Kinetic Phenomenon InThe Metallation Of PorphyrinsF. González-VílchezR. A. VilaplanaRobert F. PasternackSwarthmore College, rpaster1@swarthmore.eduFollow this and additional works at: http://works.swarthmore.edu/fac-chemistryPart of the Inorganic Chemistry CommonsThis Article is brought to you for free and open access by the Chemistry & Biochemistry at Works. It has been accepted for inclusion in Chemistry &Biochemistry Faculty Works by an authorized administrator of Works. For more information, please contact myworks@swarthmore.edu.Recommended CitationF. González-Vílchez, R. A. Vilaplana, and Robert F. Pasternack. (1991). "Evidence For An Unusual Kinetic Phenomenon In TheMetallation Of Porphyrins". Journal Of The Chemical Society: Chemical Communications. Issue 10. 731-732.http://works.swarthmore.edu/fac-chemistry/117J .  CHEM. SOC., CHEM. COMMUN., 1991 73 1 Evidence for an Unusual Kinetic Phenomenon in the Metallation of Porphyrins Francisco Gonzalez-Vilchez,a Rosario A. Vilaplana a and Robert F. Pasternack" b a Departamento de Quimica Inorganica, Facultad de Quimica, Universidad, 4 1071 Sevilla, Spain b Chemistry Department, Swarthmore College, Swarthmore, PA 19081, USA Reaction of copper(ii) with water soluble porphyrins (H2porph) in the presence of ethylenediamine shows an unusual and so far unreported kinetic phenomenon that can be explained in terms of supramolecular assembly formation apparently involving all solute species as confirmed via light scattering and conductivity measurements. In this contribution we present an anomalous and so far unreported kinetic phenomenon that occurs when copper(I1)- ethylenediamine complexes react with water soluble synthetic porphyrins (H2porph). t Other reports of porphyrin metalla- tion reactions in aqueous solutions,1-6 generally conducted at low pH to avoid hydrolysis of metal ions and precipitation of hydroxy species, are consistent with a rate law of the form R = k [M2+][H2porph]. Pseudo-first-order conditions are usually maintained so that R = ke[H2porph] with k, = k [M2+]. In contrast we observe in our studies at neutral pH in the presence of ethylenediamine(en), a discontinuity in the k, vs [CuII] profile. The experimental, first-order rate constant$ shows a very high-order dependence on [CuJI] over a limited concentration (Fig. 1) reaching a maximum value, kemaX at some critical copper(r1) concentration, [MIc . Values of kemaX and [MIc depend on solution conditions as described below but the profile shown in Fig. 1 persists for a variety of electrolytes, buffers and porphyrins. At concentrations of Cu2+ beyond [MI,, the rate constant k, decreases but more gradually than its rise. The Cu"-ethylenediamine system was selected because (i) the rate of copper(r1) insertion into porphyrins is in a convenient range for study, (ii) the free ligand shows no reactivity with the water soluble porphyrins investigated here7 and (iii) under the conditions of the experiment {i.e., pH 7.0; p = 0.15 mol dm-3; T = 298 K; (en)o = 3 [CuII],}, one copper species, C ~ ( e n ) ~ 2 + ,  predominates over the entire concentra- tion range considered. 9 We calculated the concentration profiles of the various minor copper(I1)-containing species [i.e., Cu2+, Cu(OH)+, Cu(en)2+, C ~ ( e n ) ~ 2 + ]  and found that none of them resembles that of the reactivity profile shown in Fig. 1. As a specific sample of this behaviour we considered the reaction of Cu"-en with the tetracationic porphyrin tetrakis (4-N-methylpyridyl)porphyrin, H2tmpyp. The experiments -I Hzporph is used to designate the free base form of the porphyrin. Charges are neglected for the sake of convenience. $ The slow kinetics were followed on either a Nicolet 9420 UV-VIS or a Varian 2200 recording spectrophotometer which has thermostated cell compartments. Faster reactions were observed using a Nortec stopped-flow system. The reactions were run at constant wavelength and most of the kinetic experiments were conducted under conditions such that [Cu2+] 5: 10 [H2porph]o; these experiments provided good first-order type kinetic profiles. However, even when the [Cu2+l0 - [H2porphIo such first-order profiles were obtained. Values of k ,  were found to be independent of whether the disappearance of the free base-porphyrin band (518 nm for H2tmpyp) or the appearance of the metalloporphyrin peak (548 nm for CuH2tmpyp) were monitored. The existence of an isosbestic point in the visible region ( h  535 nm) implies that the porphyrin reactant and metalloporphyrin product are the only chromophores present in non-negligible concentration other than Cu(en)z2+ which absorbs much further into the red of the spectrum. D The formation of the reactant complex C ~ ( e n ) ~ 2 +  was achieved by maintaining an en-Cu" ratio of 3.0 at pH 7.0. The components were added in the following sequence to avoid precipitation: Cull, en, NaN03, milli-Q water and collidine. A stock solution (0.1 rnol dm-3) of 2,4,6-collidine from Sigma Chem. Co., was prepared and used as buffer, The final pH was adjusted to 7.0 using NaOH or HN03 if needed. shown in Fig. l(b) were conducted at pH 7.0, [collidine] = 2.0 x 10-2 mol dm-3, p = 0.15 mol dm-3, T = 298 K, [H2tmpyp] = 3.5 X 10-5 mol dm-3. As shown in Fig. 1 and Table 1, the rate constant k, is nearly zero at low values of [Cu"] but at [CuII] = 7.5 x 10-4 mol dm-3, k, suddenly rises to a value of 9.9 x 10-3 s-1. Further increase in [CuII] leads to a decrease in k, until at [CuII] = 1.25 X 10-3 mol dm-3, k, is once again near zero. The other porphyrins investigated are the tetracationic tetrakis(N,N,N-trimethyl-4-anilinium)porphyrin, H2tap, and the tetraanionic tetrakis(4-sulphopheny1)porphyrin H2tpps. The maximum rate constants for metallation, k,max, increase in the sequence H2tmpyp < H2tap < Hztpps, the same order as for porphyrin basicity. For the last of these compounds, stopped-flow methods must be employed for kinetic measure- ments. Charge and basicity of the porphyrin therefore play a role in determining the value of the rate constant. The concentration of the porphyrin also in part determines the critical parameters in the process. Increasing H2tmpyp con- 1 0 1 2 3 [Cu2'] x 1 O3 / rnol dm-3 Fig. 1 Metal concentration dependence of k ,  at different concentra- tions of H2tmpyp: (a) :  [H2tmpyp] = 4.10 x rnol dm-3; (b):  [H,tmpyp] = 3.50 x 10-5 mol dm-3; ( c ) :  [H2tmpyp] = 1.70 X lo-' mol dm-' Table 1 Variation of the experimental rate constant with metal concentration in the reaction Cu(en)Z2+ + Hztmpyp [cu2+l0 x 103/ mol dm-3 k,  x lO3/s-l 0.25 0.50 0.75 0.83 0.90 1 .oo 1.10 1.25 2.50 0.1 0.2 9.9 8.9 8.9 6.2 5.4 0.1 negligibIe Published on 01 January 1991. Downloaded by Swarthmore College on 30/04/2015 14:39:38. View Article Online / Journal Homepage / Table of Contents for this issue732 J. CHEM. SOC., CHEM. COMMUN., 1991 only effect exerted by changing ionic strength is a shifting of [MI,, the nature of the electrolyte has a profound influence on k,max and even the breadth of the profile. To probe the nature of the species in solution giving rise to these unusual kinetic effects, we have performed light scattering and conductance measurements on these systems. We find that at Cu" concentrations both above and below [MI,, the amount of light scattered is small but at [CuII] = [MI,, the scattered light intensity increases nearly sixfold (Table 2). Solutions giving rise to rapid kinetics (i.e., at [CuII] = [MI,) have much lower conductivities than solutions on either side of these critical concentrations (Table 2). These results provide evidence for the formation of some type of composite in solution at the critical conditions. Based upon the evidence, we propose that the unusual kinetic effect found here is attributable to the existence of molecular assemblies of a supramolecular nature formed by association of the metal complex, porphyrin and other solute species and held together by intermolecular forces8 having microscopic organization and macroscopic characteristics.9 The authors thank the NATO Postdoctoral Program and the Education Ministry of Spain for grants awarded to R. A. V. We are also indebted to the MECFulbright Program for a Fellowship to F. G.-V. who thanks the University of Seville for sabbatical leave. We gratefully acknowledge support of this research from the National Science Foundation (CHEM-8915264) and the Monsanto Corporation. 283 298 313 328 T/K Fig. 2 Variation of the critical copper(r1) concentration, [MI,, with temperature Table 2 Results of light scattering and conductivity measurementsn [Cu2+l0 x lo3/ Scattered light Conductivity/ mol drn-3 (arb. units) Q-l cm-l 0.25 2.2 8.3 0.75 11.8 2.3 1.25 1.3 7.9 Received, 10th October 1990; Corn. Ol0456l F a [H2tmpyp] = 3.50 x T = 298 K; collidine = 2.0 x mol dm-3; pH = 7.0; p = 0.15 mol dm-3; mol dm-3. In all experiments [enlo = 3 [CU2+](). centration, for example, leads to a larger kemax and lower values of [MIc (Fig. 1) but in all cases, the anomalous kinetic profile remains. The reaction of CdI-ethylenediamine with H,tmpyp was studied over a 55 "C temperature range. A kinetic profile of the type shown in Fig. 1 was found throughout and the value of kernax varied by less than 9.0%. However, [MI, increases linearly with temperature as shown in Fig. 2. The nature of the buffer (collidine tris or HEPES) influences both kemax and [MIc but not the general kinetic profile. Finally, although the References 1 P. Hambright, Coord. Chem. Rev., 1971, 6, 247; P. Hambright, Ann. N.Y.  Acad. Sci., 1973, 206, 443; P. Hambright and P. B. Chock, J .  Am.  Chem. Soc., 1974, 96, 3123. 2 H. Baker, P. Hambright and L. Wagner, J .  Am. Chem. SOC., 1973, 95, 5942. 3 R. F. Pasternack, G. C. Vogel, C. A. Skowronek, R. K. Harrisand J. G .  Miller, Inorg. Chem., 1981, 20, 3763. 4 D. K. Lavallee, Coord. Chem. Rev., 1985, 61, 55. 5 M. Tabata and M. Tanaka, Inorg. Chem., 1988,27,203. 6 F. R. Longo, E. M. Brown, D. J. Quimby, A. D. Adler and M. 7 A. J. McHugh, M. Gouterman and Ch. Weiss, Jr., Theor. Chim. 8 J.-M. Lehn, Angew. Chem., Int. Ed. Eng., 1988, 27, 89. 9 J.  H.  Fendler, Membrane Mimetic Chemistry, Wiley, N.Y., 1982. Meot-Ner, Ann. N . Y .  Acad. Sci., 1973, 206, 420. Acta, 1972, 24, 346. Published on 01 January 1991. Downloaded by Swarthmore College on 30/04/2015 14:39:38. View Article Online

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Evidence For An Unusual Kinetic Phenomenon In The Metallation Of Porphyrins

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