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Electrochemical behaviour of a heme model complex in aqueous solution
179
J. Electroanal. Chem., 235 (1987) 179-187 Elsevier Sequoia S.A., Lausanne - Prmted
ELECTROCHEMICAL AQUEOUS SOLUTION
HENRIQUE Insrrtuto
E. TOMA
in The Netherlands
BEHAVIOUR
and EDUARDO
OF A HEME
MODEL
COMPLEX
IN
STADLER
de Quimica, Unruersrdade de S. Paula, Carxa Postal ZO780, Sao Paula. SP (Brazil)
(Received
27th March
1987; m revised form 7th May 1987)
ABSTRACT
The electrochemical behaviour of the meso-5.5,7,12.12,14-hexamethyl-1,4,8.ll-tetraazacyclotetradeca1.4.8,11-tetraene iron(H) complex has been studied in aqueous solution. in the presence of the N-methyl imidazole ligand. Below pH 5, the bts(imidazole) complex (A,,,,, = 692 nm, E,,, = 0.78 V vs. SHE, I = 0.50 M NaCI) dissociates, yielding the monosubstituted complex (E,,, = 0.64 V). The cyclic voltamrnograms were analysed successfully in terms of Shain and Nicholson’s formalisms for a chemical reaction coupled with a reversible charge transfer process. The equilibnum and kinetic constants for the dissociation reactions m the tron(I1) complexes were calculated as K = k,/k, = 5.4~ 10-s Mm’; k, =1.2 s-’ and k, = 2.3X lo4 M-’ ss’. In the case of the iron(II1) complexes, the dissociation constant was evaluated as K’= kA/k; =1.2X10-* Mm’, where k: = 0.13 ss’ and k; =ll M-’ s-‘. The reduction of the uon(I1) macrocyclic complexes proceeds at about -0.6 V, leading to a red species (A,,, = 520 nm) which is believed to be a hydride product. By reversing the potential scan, the startmg Fe(I1) complex is regenerated quantitatively
INTRODUCTION
Transition metal complexes of synthetic macrocyclic ligands are of great interest because of the expected analogies with biological molecules, such as vitamin B,,, hemoglobin and cytochromes [l]. They are also good candidates for studying a great variety of chemical phenomena, including ligand reactions [2,3], oxygen [4] and CO [5] transport, and the stabilization of unusual oxidation states [6]. In this work we have investigated the electrochemistry of the meso-5,5,7,12,12,14-hexamethyl1,4,8,11-tetraazacyclotetradeca-1,4,8,ll-tetraene iron(I1) complex, here referred as Fe(mac) (Fig. 1). in the presence of the N-methyl imidazole (Nmim) ligand. This macrocyclic complex is derived from the classical ligand discovered by Curtis and Hay [7]. It exhibits very interesting kinetic and electrochemical behavior in aqueous solution, associated with the labilization of the axial ligands. This aspect is of much 0022-0728/87/$03.50
8 1987 Elsevier Sequoia
SA
180
Fig. 1. The Iron macrocychc
complex
employed
m this work.
current research interest [8], because of its relevance incorporating a metalloporphyrin moiety.
to bioinorganic
systems
EXPERIMENTAL
The [Fe(mac)(CH,CN),] (BF4)2 compound was prepared according to the procedures previously published in the literature [9]. Anal. Calc. for FeC,,N,H,,&Fs: C, 40.87; N, 14.29; H, 5.78. Found: C, 41.2; N, 13.7; H, 5.9%. The N-methyl imidazole derivatives were prepared freshly by dissolving the acetonitrile complex in an aqueous solution of the N-heterocyclic ligand (0.05 M) under an argon atmosphere. All other reagents were of high purity and were used as supplied. Cyclic voltammetry was carried out with a Princeton Applied Research instrument, consisting of a 173 potentiostat and a 175 universal programmer. A glassy carbon disc electrode was employed for the measurements, using the conventional Luggin capillary with a Ag/AgCl (1 M KCl) reference electrode. The auxiliary electrode was a platinum wire dipped into the electrolyte solution (0.5 M NaCl) in a small compartment separated from the working solution by a fine glass frit. The temperature for all experiments was 25 o C. The measured potentials were converted to the standard hydrogen scale by adding 0.222 V. The cathodic peak currents were evaluated according to the procedure previously proposed by Nicholson [lo]. Spectroelectrochemical measurements were carried out with the PARC 173 potentiostat attached to a Hewlett-Packard 8451-A diode-array spectrophotometer. A three electrode system was designed for a thin layer cell of 0.028 cm internal optical pathlength. A gold minigrid was used as transparent working electrode, in the presence of a small Ag/AgCl (1 M KCl) reference electrode and of a platinum auxiliary electrode. All the experiments were performed at 25 o C under semi-infinite diffusion conditions, as described by Kuwana and Winograd [ll]. RESULTS
AND
DISCUSSION
Equilibrium studies The electronic spectrum of the bis( N-methyl iron(I1) complex consists of a strong absorption
imidazole) (tetraazamacrocyclic) at 692 nm (e = 9.3 X lo3 M-’
181
500
600
700
hlnm
*+ (4 mM) m the presence of [Nmim] = 0.05 M, Fig. 2. Spectroelectrochemistry of [Fe(mac)(Nmim),] [NaCI] = 0.5 M, pH 8.0. 25 o C, showmg (A) the Fe(II)/(III) oxidation process, startmg from 0 V vs. SHE, and (B) the reduction of the Fe(H) complex at -0.6 V followed by the regeneration of the initial complex at 0 V (. . . .).
charge transfer transition. This complex cm-‘), associated with a metal-to-diimine can be oxidized around 0.7 V versus SHE, leading to a complete decay of the charge transfer band, as shown in Fig. 2A. The process, however, was only poorly reversible due to appreciable decomposition of the oxidized products at pH 8.0, in the timescale (e.g., 5-20 min) of the spectroelectrochemical measurements. Cyclic voltammograms of the iron(I1) macrocyclic complex measured at several pHs, in the presence of the N-methyl imidazole ligand (0.05 M), are shown in Fig. 3A. Above pH 5 the voltammograms consist of a nearly reversible wave, associated with the oxidation of the bis( N-methyl imidazole) complex. The anodic peak can be fitted accurately by the theoretical equation [12] for a reversible monoelectronic charge transfer process, with El/2 - 0.78 V vs. SHE and a diffusion coefficient of 5.0 X 10e6 cm* s-‘. Below pH 5 there is a systematic decrease in the peak current for the bis( N-methyl imidazole) complex, with a concomitant growth of a new peak at 0.6 V. The process
i,(a)
i (D)
B
-
/i’
104 [rrmfml
1 1.0 E (0)
p‘/v
1.0
0.5
-
0.52
_
0.48
-
/M
10-4n
’
l-o-c-3
0.56
x
“h
\
C , 3.5
4.0
,\ 4.5
5.0
PH
E/V "5 SHE
Fig. 3. (A) Cyclic voltammograms of the iron macrocyclic complex (2 mM) in the presence of [Nrmm] = 0.05 M, [NaCI] = 0.5 M, at several pHs, 25°C. u = 400 mV s-t, showing the bts(N-methyl imidazole) complex B in eqmlibrium with the dissociated species D. (B) Plots based on eqn. (6) used in the determinatton of the dissociation constants. (C) pH dependence of the cathodic peak potentials for the dissociated species.
is completely reversible equilibrium (1).
as a function
[Fe”(mac)(Nmim),]*+
+ H,Oz
(B)
of pH, and can be ascribed
[Fe”(mac)(Nmim)(H,O)]*+
to the dissociation
+ Nmim
(1)
(D)
Protonation of the free ligand (pK, = 7.35) shifts the equilibrium to the right, favoring the dissociation of the bis(N-methyl imidazole) complex (B). However, as the pH approaches 2, dissociation of the second axial ligand starts to occur, leading to an irreversible decomposition of the complex. The anodic peak corresponding to the monodissociated species (D) is also reversible, with Ei,* = 0.64 V. If i,(B) is the anodic peak current for the initial bis(N-methyl imidazole) species (B), and i(D) is the anodic peak current for the
183
dissociated i,(B)
species (D), the following
equations
can be derived: (2)
=fa[B],
i(D) =fn[D]
(3)
[B]o = [D] + [Bl
(4)
PI = [Dl[Nkml/& b(B)/i(D) = (.fdf~>(1+ [N~ml/&)
(9 (6)
In the present
case, the contribution of species (B) in eqn. (3) is negligible. The ratio to (DB/DD)“2, where D, and D, are the diffusion coefficients of species B and D, respectively [12]. This factor is expected to be very close to unity. A typical plot based on eqn. (6) is shown in Fig. 3B. From the linear plots, the dissociation constant K, was evaluated as 5.4 X lo-’ M-l. Another interesting point shown in Fig. 3A is the systematic shift of the cathodic peaks corresponding to the dissociated species to lower potentials as a function of pH. This kind of behavior is typical of acid-base equilibria involving electroactive species. In the present case, the most strongly acidic group is the coordinated water molecule in the oxidized complex.
fB/fD in eqn. (6) corresponds
[ Fe”‘(mac)(Nmim)(H,O)]
3+ + [ Fe”‘(mac)(Nmim)(OH)]
*+ + H+(aq)
(7)
The plots of the cathodic peak potentials against pH shown in Fig. 3C can be fitted by the Nernst equation, with a slope of about 60 mV/pH. The pK, of the coordinated water molecule was evaluated from the intercept [13], as 4.1 f 0.1. This value is close to the pK, = 2.7 [14] of the [Feui(H,0),]3+ ion; however, in comparison to the pK, = 8.4 [15] of the [Fen’(CN),H,0]3complex, one can see that the conjugated macrocyclic ligand does not inhibit the Lewis acidity of the ferric ion. Kinetics of dissociation In order to evaluate the rates of dissociation of the bis(N-methyl imidazole) complex, the cyclic voltammograms were repeated at several potential scan rates. Typical results are shown in Fig. 4A. Below pH 5, one can observe an inversion of the ratio of the anodic peak currents for the species B and D, as a function of the potential scan rates. At high scan rates, e.g. 300-500 mV/s, the ratios of these peak currents are practically constant, reflecting the actual equilibrium concentration of species B and D. However, at low scan rates, the peak currents of the dissociated species D increase with respect to those of species B. This means that the electrochemical process is coupled with the dissociation reaction. as in the scheme: B2D+Nmim ki -eI D+
(8)
184
d/s
200
-
A .___._-
f
20
il.9
R
100
20
10 mV/s 20 60 100 140 200
P/
300
D
v
0
,
E/V
“5
SHE
I
1.0
0.5
V/S
1.0
VF, SHE
0.6
0.2
-0.2
-0.6
Ftg. 4. Cyclic voltammograms of the iron macrocyclic complex (2 mM) recorded as a function of the potential scan rates, in the presence of [Nmim] = 0.05 M, [NaCl] = 0.5 M, at 25 o C, and (A) pH 4.206, or (B) pH 6.00.
The kinetic constants involved in this scheme can be evaluated from eqn. (9), where i,(D) is the diffusion controlled current for species D in the absence of u = potential scan rate, and the other reaction, i.e. [D], = [B] + [D]; a = nFu/RT, terms have the conventional meaning [12]. i,(D)/i(D)
= 1/[1.02
+ 0.47 ~“~(kr+
kd)“*
K-‘]
(9)
In eqn. (9), K is given by K = k,/k,
= K,/[Nmim]
(IO)
Typical values of (k, + kd) obtained at pH 4.20, and scan rates of 200, 180, 160, 140, 120, 100, 80, and 60 mV/s were 2.36, 2.30, 2.32, 2.36, 2.37, 2.35, 2.69 and 2.44
185
-l, respectively, with an average of 2.36 s-‘. By substituting this value into eqn. ;IO). k d and k, were calculated as 1.24 and 1.11 s-l, respectively. The kinetic constant k, refers to the substitution of the N-methyl irnidazole ligand in the [Fe”(mac)(Nmim),]2’ complex by a solvent molecule. Since the mechanisms of substitution in iron(I1) macrocyclic complexes are mainly dissociative [16], a high value of k, implies a strong labilization effect on the axial ligands. In the case of the bis(imidazole)bis(dimethylglyoximate)iron(II) complex [17], kd has been measured in aqueous solution as 0.36 s-i, which is very close to that observed in this work. However, in comparison to the imidazolepentacyanoferrate(I1) complex [18] (kd = 1.33 X lop3 s-i), the macrocyclic systems are more labile by three orders of magnitude. The second-order rate constant for the substitution reaction in [Fe(mac)(Nmim) (H,O)l 2+complex can be calculated as k2 = k,/[Nmim] = 2.3 X lo4 M-’ s-l. The corresponding values for the pentacyanoferrate(I1) and bis(dimethy1 glyoximate) iron(I1) complexes are 2.4 x 10’ and 1.7 X lo2 M-r s-l, respectively [17,18]. The difference of about two orders of magnitude in k, seems to reflect a characteristic property of the macrocyclic system employed in this work. According to molecular models, the macrocyclic system can adopt a boat conformation in the complex in order to minimize the steric hindrance raised [Fe(mac)(Nmim)(H20)]‘+ by the methyl groups. This would lead to a structural distortion, with the iron(I1) ion slightly displaced from the plane of the conjugated ring, as in the hemoglobin molecule. A hydrophobic pocket would also be formed, containing a weakly bound water molecule. On the other hand, the species is also expected to be near the spin-crossover, analogous to the porphyrin systems. Both effects would increase the lability of the coordinated water molecule in the dissociated complex. The effects of the potential scan rate on the cyclic voltammograms were also very informative. Above pH 5, the [Fe(mac)(Nmim),]‘+ complex is the predominant species in solution. As the pH increases the voltammograms become more irreversible. There is a systematic decrease of the cathodic peak at 0.75 V, and an enhancement of the broad cathodic peak corresponding to the [Fetn(mac)(Nmim) (OH)12+ species, as one can see in Fig. 4B. The observed behavior as a function of the pH and of the potential scan rate is consistent with the dissociation of a N-methyl imidazole ligand after the oxidation of the Fe(I1) complex, as in the scheme: [Fe”(mac)(Nmim),]2’
+ [Fe”‘(mac)(Nmim)2]3t + e- Nmim .J, k,(III) [Fem(mac)(Nmim)(H,0)]3+ +OH-
Jr rapid, pK, = 4.1
[Fe”f(mac)(Nmim)(OH)]2+ Based on the variation of the ratio of the cathodic and anodic peak currents as a function of the potential scan rate [10,12] the dissociation rate constant, k,(III) was calculated as 0.13 + 0.02 s-l. The equilibrium constant, K,(II) and the electrochemical potentials obtained in
186
this work, were employed
in the following
K,(II)=5.4x10-5
+
[Fe’t(mac)(Nmim),]*’ E,=0.78
VJ
+e-
thermodynamic
cycle:
AK
[Fe”(mac)(Nmim)(H,O)1Zf +e- 11E,=0.64
+ Nmim
V
K,UIU
[Fen’(mac)(Nmim)2]3’ In this evaluated calculated imidazole)
+
[Fe’n(mac)(Nmim)(H,0)]3’
+ Nmim
way, the dissociation constant of the oxidized complex, K,(III) was as 1.2 x lo-2 M-‘. Considering that K,(III) = k,(III)/k,(III), the second-order rate constant for the formation of the ferric bis(N-methyl complex was 11 M-’ s-i.
Reduction of the iron macrocyclic complex The iron(I1) macrocyclic complex can be reduced electrochemically, leading to a broad cathodic peak at about - 0.6 V vs. SHE (pH 6) as one can see in Fig. 4B. The resulting species are oxidized above 0 V, giving rise to a sharp anodic peak of low intensity. At the end of a complete cycle, the starting [Fe(mac)(Nmim),]*’ is regenerated quantitatively. The oxidation-reduction cycle can be repeated several times with only minor decomposition of the macrocyclic complex. Spectroelectrochemical measurements were carried out at several applied potentials, as shown in Fig. 2B, in order to characterize the reduced species. The product exhibits a deep red color, with an absorption band at 520 nm (E = 7700 M-’ cm-‘), and is extremely unstable. Its rapid decay in aqueous solution probably contributes to the decrease in the intensities of the anodic waves at about 0 V, in the cyclic voltammograms shown in Fig. 4B. By reversing the applied potentials, the starting iron(I1) complex is regenerated quantitatively, in agreement with the cyclic voltammetry. The red species does not correspond to the triene species, which might result from the reduction of the tetraene ligand. The iron(I1) triene complex absorbs at 614 nm, and exhibits a contrasting electrochemical behavior [3]. The alternative hypothesis is the reduction of the iron(I1) ion, yielding initially a d7 low spin iron(I) complex. Analogous to the vitamin B t2 systems [19], this species would behave as a radical ion, forming iron-hydride complexes in aqueous solution. Such a species would also exhibit charge transfer bands characteristic of iron(diimine chromophores [20], as we have observed in the spectroelectrochemical measurements. The hypothesis of hydride formation requires further investigation; however, it should be mentioned that hydride species have already been isolated from the reduction of an iron(I1) complex containing a saturated macrocyclic ligand, in non-aqueous solvent [6]. ACKNOWLEDGEMENTS
We thank CNPq, FAPESP and FINEP for financial fellowship and support from CAPES and UFSC.
support.
E.S. is grateful
for a
187 REFERENCES 1 V.L. Goedken in G.A. Melson (Ed.), Coordination Chemistry of Macrocyclic Compounds. Plenum Press, New York, 1979. p. 603. 2 V. Katovic, L. Taylor and D.H. Busch, J. Am. Chem. Sot., 91 (1969) 2122. 3 H.E. Toma and E. Stadler, Inorg. Cmm. Acta, 119 (1986) 49. 4 R.D. Jones, D.A. Summervtlle and F. Basolo, Chem. Rev., 79 (1979) 139. 5 CA. Koval, R.D. Noble, J.D. Way, B. Louie, E. Reyes. B.R. Bateman, G.M. Horn and D.L. Reed, Inorg. Chem., 24 (1985) 1148. 6 D.C. Olson and J. Vasilevskis, Inorg. Chem., 11 (1972) 980. 7 N.F. Curtis and R.W. Hay, J. Chem. Sot.. Chem. Commun., (1966) 524. 8 N.K. Kildahl, G. Antonopoulos, N.E. Fortier and W.D Hobey. Inorg. Chem.. 24 (1985) 429. 9 A.M. Tait and D.H. Busch, Inorg. Synth., 18 (1978) 2 10 R S. Nicholson. Anal. Chem., 38 (1966) 1406. 11 T Kuwana and N. Winograd m A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 7. Marcel Dekker, New York, 1974. p 1. 12 R.S. Nicholson and I. Sham, Anal. Chem., 36 (1964) 706. 13 S. Wawzonek m A. Weissberger and B.W. Rossiter (Eds.), Physical Methods of Chemistry, Part IIA, Wiley-Interscience, New York, 1971, Ch. 1. 14 J. Burgess. Metal Ions in Solution, Halsted Press, New York, 1978, p 147. 15 J.H. Espenson and S.G. Wolenuk, Jr., Inorg. Chem., 11 (1972) 2034. 16 N.K. Kildahl, T.J. Lewis and G. Antonopoulos. Inorg. Chem.. 20 (1981) 3952. 17 H E. Toma and A.C.C. Silva, Can. J. Chem.. 64 (1986) 1280. 18 H.E. Toma, J.M. Martins and E. Giesbrecht, J. Chem. Sot. Dalton Trans., (1978) 1610. 19 D. Lexa and J.M. Savtant. Act. Chem. Res.. 16 (1983) 235. 20 P. Krumholz, Struct. Bondmg. 9 (1971) 139.
J. Electroanal. Chem., 235 (1987) 179-187 Elsevier Sequoia S.A., Lausanne - Prmted
ELECTROCHEMICAL AQUEOUS SOLUTION
HENRIQUE Insrrtuto
E. TOMA
in The Netherlands
BEHAVIOUR
and EDUARDO
OF A HEME
MODEL
COMPLEX
IN
STADLER
de Quimica, Unruersrdade de S. Paula, Carxa Postal ZO780, Sao Paula. SP (Brazil)
(Received
27th March
1987; m revised form 7th May 1987)
ABSTRACT
The electrochemical behaviour of the meso-5.5,7,12.12,14-hexamethyl-1,4,8.ll-tetraazacyclotetradeca1.4.8,11-tetraene iron(H) complex has been studied in aqueous solution. in the presence of the N-methyl imidazole ligand. Below pH 5, the bts(imidazole) complex (A,,,,, = 692 nm, E,,, = 0.78 V vs. SHE, I = 0.50 M NaCI) dissociates, yielding the monosubstituted complex (E,,, = 0.64 V). The cyclic voltamrnograms were analysed successfully in terms of Shain and Nicholson’s formalisms for a chemical reaction coupled with a reversible charge transfer process. The equilibnum and kinetic constants for the dissociation reactions m the tron(I1) complexes were calculated as K = k,/k, = 5.4~ 10-s Mm’; k, =1.2 s-’ and k, = 2.3X lo4 M-’ ss’. In the case of the iron(II1) complexes, the dissociation constant was evaluated as K’= kA/k; =1.2X10-* Mm’, where k: = 0.13 ss’ and k; =ll M-’ s-‘. The reduction of the uon(I1) macrocyclic complexes proceeds at about -0.6 V, leading to a red species (A,,, = 520 nm) which is believed to be a hydride product. By reversing the potential scan, the startmg Fe(I1) complex is regenerated quantitatively
INTRODUCTION
Transition metal complexes of synthetic macrocyclic ligands are of great interest because of the expected analogies with biological molecules, such as vitamin B,,, hemoglobin and cytochromes [l]. They are also good candidates for studying a great variety of chemical phenomena, including ligand reactions [2,3], oxygen [4] and CO [5] transport, and the stabilization of unusual oxidation states [6]. In this work we have investigated the electrochemistry of the meso-5,5,7,12,12,14-hexamethyl1,4,8,11-tetraazacyclotetradeca-1,4,8,ll-tetraene iron(I1) complex, here referred as Fe(mac) (Fig. 1). in the presence of the N-methyl imidazole (Nmim) ligand. This macrocyclic complex is derived from the classical ligand discovered by Curtis and Hay [7]. It exhibits very interesting kinetic and electrochemical behavior in aqueous solution, associated with the labilization of the axial ligands. This aspect is of much 0022-0728/87/$03.50
8 1987 Elsevier Sequoia
SA
180
Fig. 1. The Iron macrocychc
complex
employed
m this work.
current research interest [8], because of its relevance incorporating a metalloporphyrin moiety.
to bioinorganic
systems
EXPERIMENTAL
The [Fe(mac)(CH,CN),] (BF4)2 compound was prepared according to the procedures previously published in the literature [9]. Anal. Calc. for FeC,,N,H,,&Fs: C, 40.87; N, 14.29; H, 5.78. Found: C, 41.2; N, 13.7; H, 5.9%. The N-methyl imidazole derivatives were prepared freshly by dissolving the acetonitrile complex in an aqueous solution of the N-heterocyclic ligand (0.05 M) under an argon atmosphere. All other reagents were of high purity and were used as supplied. Cyclic voltammetry was carried out with a Princeton Applied Research instrument, consisting of a 173 potentiostat and a 175 universal programmer. A glassy carbon disc electrode was employed for the measurements, using the conventional Luggin capillary with a Ag/AgCl (1 M KCl) reference electrode. The auxiliary electrode was a platinum wire dipped into the electrolyte solution (0.5 M NaCl) in a small compartment separated from the working solution by a fine glass frit. The temperature for all experiments was 25 o C. The measured potentials were converted to the standard hydrogen scale by adding 0.222 V. The cathodic peak currents were evaluated according to the procedure previously proposed by Nicholson [lo]. Spectroelectrochemical measurements were carried out with the PARC 173 potentiostat attached to a Hewlett-Packard 8451-A diode-array spectrophotometer. A three electrode system was designed for a thin layer cell of 0.028 cm internal optical pathlength. A gold minigrid was used as transparent working electrode, in the presence of a small Ag/AgCl (1 M KCl) reference electrode and of a platinum auxiliary electrode. All the experiments were performed at 25 o C under semi-infinite diffusion conditions, as described by Kuwana and Winograd [ll]. RESULTS
AND
DISCUSSION
Equilibrium studies The electronic spectrum of the bis( N-methyl iron(I1) complex consists of a strong absorption
imidazole) (tetraazamacrocyclic) at 692 nm (e = 9.3 X lo3 M-’
181
500
600
700
hlnm
*+ (4 mM) m the presence of [Nmim] = 0.05 M, Fig. 2. Spectroelectrochemistry of [Fe(mac)(Nmim),] [NaCI] = 0.5 M, pH 8.0. 25 o C, showmg (A) the Fe(II)/(III) oxidation process, startmg from 0 V vs. SHE, and (B) the reduction of the Fe(H) complex at -0.6 V followed by the regeneration of the initial complex at 0 V (. . . .).
charge transfer transition. This complex cm-‘), associated with a metal-to-diimine can be oxidized around 0.7 V versus SHE, leading to a complete decay of the charge transfer band, as shown in Fig. 2A. The process, however, was only poorly reversible due to appreciable decomposition of the oxidized products at pH 8.0, in the timescale (e.g., 5-20 min) of the spectroelectrochemical measurements. Cyclic voltammograms of the iron(I1) macrocyclic complex measured at several pHs, in the presence of the N-methyl imidazole ligand (0.05 M), are shown in Fig. 3A. Above pH 5 the voltammograms consist of a nearly reversible wave, associated with the oxidation of the bis( N-methyl imidazole) complex. The anodic peak can be fitted accurately by the theoretical equation [12] for a reversible monoelectronic charge transfer process, with El/2 - 0.78 V vs. SHE and a diffusion coefficient of 5.0 X 10e6 cm* s-‘. Below pH 5 there is a systematic decrease in the peak current for the bis( N-methyl imidazole) complex, with a concomitant growth of a new peak at 0.6 V. The process
i,(a)
i (D)
B
-
/i’
104 [rrmfml
1 1.0 E (0)
p‘/v
1.0
0.5
-
0.52
_
0.48
-
/M
10-4n
’
l-o-c-3
0.56
x
“h
\
C , 3.5
4.0
,\ 4.5
5.0
PH
E/V "5 SHE
Fig. 3. (A) Cyclic voltammograms of the iron macrocyclic complex (2 mM) in the presence of [Nrmm] = 0.05 M, [NaCI] = 0.5 M, at several pHs, 25°C. u = 400 mV s-t, showing the bts(N-methyl imidazole) complex B in eqmlibrium with the dissociated species D. (B) Plots based on eqn. (6) used in the determinatton of the dissociation constants. (C) pH dependence of the cathodic peak potentials for the dissociated species.
is completely reversible equilibrium (1).
as a function
[Fe”(mac)(Nmim),]*+
+ H,Oz
(B)
of pH, and can be ascribed
[Fe”(mac)(Nmim)(H,O)]*+
to the dissociation
+ Nmim
(1)
(D)
Protonation of the free ligand (pK, = 7.35) shifts the equilibrium to the right, favoring the dissociation of the bis(N-methyl imidazole) complex (B). However, as the pH approaches 2, dissociation of the second axial ligand starts to occur, leading to an irreversible decomposition of the complex. The anodic peak corresponding to the monodissociated species (D) is also reversible, with Ei,* = 0.64 V. If i,(B) is the anodic peak current for the initial bis(N-methyl imidazole) species (B), and i(D) is the anodic peak current for the
183
dissociated i,(B)
species (D), the following
equations
can be derived: (2)
=fa[B],
i(D) =fn[D]
(3)
[B]o = [D] + [Bl
(4)
PI = [Dl[Nkml/& b(B)/i(D) = (.fdf~>(1+ [N~ml/&)
(9 (6)
In the present
case, the contribution of species (B) in eqn. (3) is negligible. The ratio to (DB/DD)“2, where D, and D, are the diffusion coefficients of species B and D, respectively [12]. This factor is expected to be very close to unity. A typical plot based on eqn. (6) is shown in Fig. 3B. From the linear plots, the dissociation constant K, was evaluated as 5.4 X lo-’ M-l. Another interesting point shown in Fig. 3A is the systematic shift of the cathodic peaks corresponding to the dissociated species to lower potentials as a function of pH. This kind of behavior is typical of acid-base equilibria involving electroactive species. In the present case, the most strongly acidic group is the coordinated water molecule in the oxidized complex.
fB/fD in eqn. (6) corresponds
[ Fe”‘(mac)(Nmim)(H,O)]
3+ + [ Fe”‘(mac)(Nmim)(OH)]
*+ + H+(aq)
(7)
The plots of the cathodic peak potentials against pH shown in Fig. 3C can be fitted by the Nernst equation, with a slope of about 60 mV/pH. The pK, of the coordinated water molecule was evaluated from the intercept [13], as 4.1 f 0.1. This value is close to the pK, = 2.7 [14] of the [Feui(H,0),]3+ ion; however, in comparison to the pK, = 8.4 [15] of the [Fen’(CN),H,0]3complex, one can see that the conjugated macrocyclic ligand does not inhibit the Lewis acidity of the ferric ion. Kinetics of dissociation In order to evaluate the rates of dissociation of the bis(N-methyl imidazole) complex, the cyclic voltammograms were repeated at several potential scan rates. Typical results are shown in Fig. 4A. Below pH 5, one can observe an inversion of the ratio of the anodic peak currents for the species B and D, as a function of the potential scan rates. At high scan rates, e.g. 300-500 mV/s, the ratios of these peak currents are practically constant, reflecting the actual equilibrium concentration of species B and D. However, at low scan rates, the peak currents of the dissociated species D increase with respect to those of species B. This means that the electrochemical process is coupled with the dissociation reaction. as in the scheme: B2D+Nmim ki -eI D+
(8)
184
d/s
200
-
A .___._-
f
20
il.9
R
100
20
10 mV/s 20 60 100 140 200
P/
300
D
v
0
,
E/V
“5
SHE
I
1.0
0.5
V/S
1.0
VF, SHE
0.6
0.2
-0.2
-0.6
Ftg. 4. Cyclic voltammograms of the iron macrocyclic complex (2 mM) recorded as a function of the potential scan rates, in the presence of [Nmim] = 0.05 M, [NaCl] = 0.5 M, at 25 o C, and (A) pH 4.206, or (B) pH 6.00.
The kinetic constants involved in this scheme can be evaluated from eqn. (9), where i,(D) is the diffusion controlled current for species D in the absence of u = potential scan rate, and the other reaction, i.e. [D], = [B] + [D]; a = nFu/RT, terms have the conventional meaning [12]. i,(D)/i(D)
= 1/[1.02
+ 0.47 ~“~(kr+
kd)“*
K-‘]
(9)
In eqn. (9), K is given by K = k,/k,
= K,/[Nmim]
(IO)
Typical values of (k, + kd) obtained at pH 4.20, and scan rates of 200, 180, 160, 140, 120, 100, 80, and 60 mV/s were 2.36, 2.30, 2.32, 2.36, 2.37, 2.35, 2.69 and 2.44
185
-l, respectively, with an average of 2.36 s-‘. By substituting this value into eqn. ;IO). k d and k, were calculated as 1.24 and 1.11 s-l, respectively. The kinetic constant k, refers to the substitution of the N-methyl irnidazole ligand in the [Fe”(mac)(Nmim),]2’ complex by a solvent molecule. Since the mechanisms of substitution in iron(I1) macrocyclic complexes are mainly dissociative [16], a high value of k, implies a strong labilization effect on the axial ligands. In the case of the bis(imidazole)bis(dimethylglyoximate)iron(II) complex [17], kd has been measured in aqueous solution as 0.36 s-i, which is very close to that observed in this work. However, in comparison to the imidazolepentacyanoferrate(I1) complex [18] (kd = 1.33 X lop3 s-i), the macrocyclic systems are more labile by three orders of magnitude. The second-order rate constant for the substitution reaction in [Fe(mac)(Nmim) (H,O)l 2+complex can be calculated as k2 = k,/[Nmim] = 2.3 X lo4 M-’ s-l. The corresponding values for the pentacyanoferrate(I1) and bis(dimethy1 glyoximate) iron(I1) complexes are 2.4 x 10’ and 1.7 X lo2 M-r s-l, respectively [17,18]. The difference of about two orders of magnitude in k, seems to reflect a characteristic property of the macrocyclic system employed in this work. According to molecular models, the macrocyclic system can adopt a boat conformation in the complex in order to minimize the steric hindrance raised [Fe(mac)(Nmim)(H20)]‘+ by the methyl groups. This would lead to a structural distortion, with the iron(I1) ion slightly displaced from the plane of the conjugated ring, as in the hemoglobin molecule. A hydrophobic pocket would also be formed, containing a weakly bound water molecule. On the other hand, the species is also expected to be near the spin-crossover, analogous to the porphyrin systems. Both effects would increase the lability of the coordinated water molecule in the dissociated complex. The effects of the potential scan rate on the cyclic voltammograms were also very informative. Above pH 5, the [Fe(mac)(Nmim),]‘+ complex is the predominant species in solution. As the pH increases the voltammograms become more irreversible. There is a systematic decrease of the cathodic peak at 0.75 V, and an enhancement of the broad cathodic peak corresponding to the [Fetn(mac)(Nmim) (OH)12+ species, as one can see in Fig. 4B. The observed behavior as a function of the pH and of the potential scan rate is consistent with the dissociation of a N-methyl imidazole ligand after the oxidation of the Fe(I1) complex, as in the scheme: [Fe”(mac)(Nmim),]2’
+ [Fe”‘(mac)(Nmim)2]3t + e- Nmim .J, k,(III) [Fem(mac)(Nmim)(H,0)]3+ +OH-
Jr rapid, pK, = 4.1
[Fe”f(mac)(Nmim)(OH)]2+ Based on the variation of the ratio of the cathodic and anodic peak currents as a function of the potential scan rate [10,12] the dissociation rate constant, k,(III) was calculated as 0.13 + 0.02 s-l. The equilibrium constant, K,(II) and the electrochemical potentials obtained in
186
this work, were employed
in the following
K,(II)=5.4x10-5
+
[Fe’t(mac)(Nmim),]*’ E,=0.78
VJ
+e-
thermodynamic
cycle:
AK
[Fe”(mac)(Nmim)(H,O)1Zf +e- 11E,=0.64
+ Nmim
V
K,UIU
[Fen’(mac)(Nmim)2]3’ In this evaluated calculated imidazole)
+
[Fe’n(mac)(Nmim)(H,0)]3’
+ Nmim
way, the dissociation constant of the oxidized complex, K,(III) was as 1.2 x lo-2 M-‘. Considering that K,(III) = k,(III)/k,(III), the second-order rate constant for the formation of the ferric bis(N-methyl complex was 11 M-’ s-i.
Reduction of the iron macrocyclic complex The iron(I1) macrocyclic complex can be reduced electrochemically, leading to a broad cathodic peak at about - 0.6 V vs. SHE (pH 6) as one can see in Fig. 4B. The resulting species are oxidized above 0 V, giving rise to a sharp anodic peak of low intensity. At the end of a complete cycle, the starting [Fe(mac)(Nmim),]*’ is regenerated quantitatively. The oxidation-reduction cycle can be repeated several times with only minor decomposition of the macrocyclic complex. Spectroelectrochemical measurements were carried out at several applied potentials, as shown in Fig. 2B, in order to characterize the reduced species. The product exhibits a deep red color, with an absorption band at 520 nm (E = 7700 M-’ cm-‘), and is extremely unstable. Its rapid decay in aqueous solution probably contributes to the decrease in the intensities of the anodic waves at about 0 V, in the cyclic voltammograms shown in Fig. 4B. By reversing the applied potentials, the starting iron(I1) complex is regenerated quantitatively, in agreement with the cyclic voltammetry. The red species does not correspond to the triene species, which might result from the reduction of the tetraene ligand. The iron(I1) triene complex absorbs at 614 nm, and exhibits a contrasting electrochemical behavior [3]. The alternative hypothesis is the reduction of the iron(I1) ion, yielding initially a d7 low spin iron(I) complex. Analogous to the vitamin B t2 systems [19], this species would behave as a radical ion, forming iron-hydride complexes in aqueous solution. Such a species would also exhibit charge transfer bands characteristic of iron(diimine chromophores [20], as we have observed in the spectroelectrochemical measurements. The hypothesis of hydride formation requires further investigation; however, it should be mentioned that hydride species have already been isolated from the reduction of an iron(I1) complex containing a saturated macrocyclic ligand, in non-aqueous solvent [6]. ACKNOWLEDGEMENTS
We thank CNPq, FAPESP and FINEP for financial fellowship and support from CAPES and UFSC.
support.
E.S. is grateful
for a
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