Formation of supramolecular complex between imidazole and dichloro(5,10,15,20-tetraphenylporphinato)zirconium(IV)

Article abstract of DOI:10.1134/S1070363210040274

By the reaction of 5,10,15,20-tetraphenyl-21H,23H-porphin (H ₂TPP) with zirconium tetrachloride in boiling benzonitrile a complex was obtained, dichloro(5,10,15,20-tetraphenylporphinato)zirconium(IV), (Cl) ₂ZrTPP. The equilibrium and reaction rate of stepwise reactions of (Cl)₂ZrTPP with imidazole (Im) in toluene were investigated by spectrophotometry. It was established that the three steps of the complex formation include reversible processes of coordination of Im molecule and substitution of 2Cl-by the second and third Im molecules. The products of the first and second stages of the reaction, (Cl)₂(Im)ZrTPP and [(Cl)(Im)₂ZrTPP]⁺Cl^-, respectively, are unstable and dissociate slowly at one Zr-Cl bond. By the analysis of the numerical values of the reactions parameters with accounting for the existence of definite spectral response to the presence of an organic base was shown a better prospect of application of (Cl)₂ZrTPP in sensor systems than those of metallophtalocyianines and doubly charged cation complexes with porphyrins. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070363210040274

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 4, pp. 842–848. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.V. Motorina, T.N. Lomova, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 4, pp. 681–687.
Formation of Supramolecular Complex Between Imidazole
and Dichloro(5,10,15,20-tetraphenylporphinato)zirconium(IV)
E. V. Motorina and T. N. Lomova
Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya ul. 1, Ivanovo, 153045 Russia
e-mail: tnl@isc-ras.ru
Received February 2, 2009
Abstract―By the reaction of 5,10,15,20-tetraphenyl-21H,23H-porphin (H2TPP) with zirconium tetrachloride
in boiling benzonitrile a complex was obtained, dichloro(5,10,15,20-tetraphenylporphinato)zirconium(IV),
(Cl)2ZrTPP. The equilibrium and reaction rate of stepwise reactions of (Cl)2ZrTPP with imidazole (Im) in
toluene were investigated by spectrophotometry. It was established that the three steps of the complex
formation include reversible processes of coordination of Im molecule and substitution of 2Cl– by the second
and third Im molecules. The products of the first and second stages of the reaction, (Cl)2(Im)ZrTPP and
[(Cl)(Im)2ZrTPP]+Cl–, respectively, are unstable and dissociate slowly at one Zr–Cl bond. By the analysis of
the numerical values of the reactions parameters with accounting for the existence of definite spectral response
to the presence of an organic base was shown a better prospect of application of (Cl)2ZrTPP in sensor systems
than those of metallophtalocyianines and doubly charged cation complexes with porphyrins.
DOI: 10.1134/S1070363210040274
In this paper, in continuation of research in this
direction the results are presented of quantitative
studies of equilibria and irreversible processes of
complexation and substitution of ligands in dichloro-
(5,10,15,20-tetraphenylporphinato)zirconium(IV)
[(Cl)2ZrTPP] in the reaction with organic base,
imidazole (Im).
Mixed porphyrin-containing complexes and the
phenomenon of additional coordination of ligands by
metalloporphyrins (MP) with the formation of supra-
molecules currently attracts much attention [1–4].
Using the molecular complexation of MP promises
almost unlimited possibilities of their application for
transport of drugs in biological objects, molecular
recognition, for the study of biochemical processes in
simple models, for creation of selective receptors of
ions and bases, and for the synthesis of photoactive
materials. To date, it is clear that the success is
determined by two factors: the composition of the first
coordination sphere of the mixed complex and the state
of the macrocycle, that is, by degree of its substitution,
monomeric or oligomeric structure. Mixed complexes
of highly charged metal cations are very specific in
reactions with bases primarily due to high effective
charges on the central atom. In [2–4] the reactions
were studied of acetato(5,10,15,20-tetraphenylpor-
phinato)chromium(III) and hydroxooxo(5,10,15,20-
tetraphenylporphinato)tungsten(V) with nitrogen bases
Im and Py. These reactions were shown to differ
significantly from the simple coordination of ligand by
the porphyrin complexes with doubly and some triply
charged metal cations [5].
Because of the large covalent radius of the central
metal atom, the porphyrin complexes of zirconium(IV)
have a specific structure with the cis-arrangement of
two acido-ligands. Thus, according to X-ray analysis
[6], in (AcO)2ZrOEP (OEP is dianion 2,3,7,8,12,13,17,18-
octaethyl-21H,23H-porphin) the zirconium atom is
deviated from the plane of the macrocycle by more
than 1.0 Å in the direction of the two cis-located
acetate ligands. The similarity of electron absorption
spectra of Zr(IV) complexes with H2OEP and
5,10,15,20-tetraphenylmacroporphin (H2TPP) [7], as
well as moderate kinetic stability of (Cl)2ZrTPP in
proton-donor environment [8 ] suggest that the atom of
Zr in (Cl)2ZrTPP is removed substantially from the N4
plane (see the scheme). The cis-configuration of the
chloride ions, as will be seen further on, becomes
apparent from the specificity of ligand substitution
reactions in the (Cl)2ZrTPP complex.
842

FORMATION OF SUPRAMOLECULAR COMPLEX
843
Cl
Cl
to determine the data for construction titration curves
(Fig. 2) indicating that the reaction of (Cl)2ZrTPP with
Im in toluene proceeds in three equilibrium stages.
Zr
N
N
Stepwise equilibrium constants calculated with
formula (1), remain satisfactorily unchanged (see ex-
N
N
For the study of the reactions of (Cl)2ZrTPP with
Im we selected toluene as a solvent, for it was inert
with respect to both reactants. The reactions were
studied in a wide range of concentrations of Im
(3.58×10–6–1.40×10–2 mol l–1) at 298 K. Electron
absorption spectrum of (Cl)2ZrTPP in toluene (Fig. 1)
consists of a line with the maximum at 418.6 nm
which is changed upon adding Im. At a gradual
increase in the concentration of Im from 3.58×10–6 to
1.19×10–5 the optical density near the operating
wavelength (417 nm) increases and there is a slight
shift of the maximum of this band to 419.3 nm (Fig. 1,
spectrum 2). Further increase in the concentration of
Im leads to a decrease in the band intensity (Fig. 1,
spectrum 3) and then the intensity increases again and
the maximum is shifted to 420.7 nm (Fig. 1, spectrum 4).
A
0.26
0.24
1
0.22
0.20
0
0.4
0.8
CL×105, mol l–1
A
At the concentrations of Im over 1.40×10–2 mol l–1
a
0.26
solution is formed from which eventually precipitates
Im. Regular changes in the electronic spectrum allow
A
0.22
0.18
2
2
1
0.3
4
3
0
2.0
4.0
CL×104, mol l–1
A
0.28
0.24
3
0.20
0.0
CL×104, mol l–1
400
425
450
λ, nm
0
0.4
0.8
Fig. 1. Electron absorption spectra of (Cl)2ZrTPP in
toluene depending on the concentration of Im.
C[(Cl)2ZrTPP] = 2.32×10–6 mol l–1, CIm, mol l–1: (1) 0,
(2) 1.19×10–5, (3) 5.97×10–4, and (4) 1.40×10–2.
Fig. 2. The curves of spectrophotometric titration of
(Cl)2ZrTPP with imidazole in toluene. CIm, mol l–1:
(1) 3.58×10–6–1.19×10–5, (2) 1.19×10–5–5.97×10–4, and
(3) 5.97×10–4–1.40×10–2.
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844
MOTORINA, LOMOVA
certain concentration of Im, and supramolecular
0.5
0.0
complex with Im; n is a number of added (Im)
2
3
molecular ligands.
1
The equilibrium constant in the first phase, K1, is
relatively high: (1.85±0.13)×105 l mol–1. It is found
experimentally that the equilibrium in the first stage is
reached within 5 minutes. Processing of experimental
data using the dependence log [(AS – A0)/(A∞ – AS)]–
log CL (Fig. 3, line 1) confirms the number of Im
molecules added in the first stage (tan α = 1.2). The
reaction product in the first stage, which is in
equilibrium with (Cl)2ZrTPP, is not stable and
eventually subjected to a slow and irreversible
transformation into the complex spectrally identical
with the product of the reversible reaction of MP with
Im in the second stage (Fig. 1, curve 3). Table 1 shows
the values of the effective rate constants k1eff of this
transformation found for different initial concentra-
tions of Im.
–0.5
–1.0
–1.5
5
4
3
2
log CL
Fig. 3. Dependence log [(AS – A0)/(A∞ – AS)]–log CL for the
reaction of (Cl)2ZrTPP with Im in toluene at the (1) first,
(2) second, and (3) third stages (ρ is 0.995, 0.993, and
0.997, respectively).
perimental part) for the ternary equilibrium system,
that is, at n = 1:
(AS – A0)/(A∞ – A0)
1 – [(AS – A0)/(A∞ – A0)]
K =
By analysis of the linear dependence (2) (Fig. 4,
line 1) we have determined the value of reaction order
m1 on CIm, which amounted to the value of 0.06, and
the true rate constant k1 = 1.09×10–3 s–1.
1
×
(1)
.
{CL – C0МР·[(AS – A0)/(A∞ – A0)]}n
log k1eff = log k1 + m1 log CIm.
(2)
In Eq. (1), derived in accordance with the laws of
mass action and the Bouguer–Lambert–Beer for a
mixture of two colored compounds [the complex
(Cl)2ZrTPP and the product of its supramolecular
complexation with Im], C0MP and CL are initial
concentrations of (Cl)2ZrTPP and Im, respectively, in
toluene, A0, AS, and A∞ are optical densities at the
working wavelength (see Experimental), respectively,
solutions of (Cl)2ZrTPP, equilibrium mixture at a
From the stoichiometric ratio (Cl)2ZrTPP : Im = 1:1
in the equilibrium reaction in the first stage, the
spectral picture of the transformation (Fig. 1, curves 1,
2, and 3), and the close to zero order of the reaction
with Im in an irreversible reaction in the first stage
follows the reaction equation for this stage:
K1
(Cl)2ZrTPP
(Cl)2ZrTPP + Im
Table 1. Effective rate constants keff of the formal first-order
reaction of (Cl)2ZrTPP with Im in toluene at 298 K.
C[(Cl)2ZrTPP] = 2.32×10–6 mol l–1
k1
[Zr(Cl)(Im)TPP]+Cl−.
(3)
Obviously, the first stage includes a reversible co-
ordination of Im molecule through the lone electron
pair with the formation of a supramolecule (Cl)2(Im)·
ZrTPP, and then a slow irreversible displacement of
acido-ligand Cl– to the second coordination sphere in
the formed mixed complex. Coordination of Im, as one
might expect accounting for the influence of cis-
coordinated molecular ligand Im in the supramolecule
on the Zr–Nporphyrin bonds, leads to destabilization of
the porphyrin HOMO and its growing population. In
fact, this is confirmed by hyperchromic and
bathochromic effects in a series of spectral curves of
electron spectra in the first stage.
Stage 1
(k1eff±δk1eff)×
Stage 2
(k2eff±δk2eff)×
СIm×106,
СIm×105,
mol l–1
104, s–1
4.9±0.3
5.0±0.6
5.1±0.3
5.2±0.2
5.3±0.4
mol l–1
104, s–1
6.2±0.3
6.5±0.7
6.9±0.3
7.2±0.6
7.5±0.4
8.2±0.2
3.58
5.97
7.16
9.55
11.9
2.03
3.58
5.97
8.35
11.9
35.8
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 4 2010

FORMATION OF SUPRAMOLECULAR COMPLEX
845
In each second and third stages of transformation in
log keff
the system (Cl)2ZrTPP–Im also react one at a time Im
–3.1
molecule (Fig. 3): tan α2 = 0.99, tan α3 = 0.97.
2
In the second stage of the reaction, similar to the
first stage, at all Im concentrations after reaching the
equilibrium a slow irreversible process occurs (Table 1),
for which by the processing the linear dependence (2)
(Fig. 4, line 2) the values are determined of the
reaction order on CIm and the rate constant.
–3.2
–3.3
–3.4
1
6.0
5.5
5.0
4.5
4.0
3.5
log CL
The product of the transformation in the third stage
does not change in time, as seen from the retained
pattern of the electron spectra of equilibrium mixtures
at CIm from 5.97×10–4 to 1.40×10–2 mol l–1.
Fig. 4. Dependence log keff – log CL for the reaction of
(Cl)2ZrTPP with Im in toluene at 298 K in the (1) first and
(2) second stage (ρ is 0.998 and 0.992, respectively).
may be related to the fact that heptacoordinated
complexes in the left and right sides of the Eq. (5)
differ only by the number of donor–acceptor and
covalent bonds, and hence by different deviations of
the central Zr atom from the plane of the macrocycle
(see the scheme at the beginning of the article).
We could not determine experimentally whether the
reaction with Im proceeds to completion in the
considered three stages, since the upper limit of the
concentration of Im is defined by its solubility in
toluene.
The formation of donor–acceptor complexes of the
metalloporphyrin with Im in the reactions (3)–(5) is
confirmed by the study of IR spectra of their mixtures
with Im in toluene with Im concentrations related to
the points of equivalence in the first–third stages
(Fig. 2). The IR spectrum of the reaction product of the
first stage, (Cl)2(Im)ZrTPP, (Fig. 5a) contains a new
absorption band of coordinated imidazole [9] at
721.3 cm–1 that is absent in the spectrum of the initial
complex. In the spectrum of the reaction product of the
second stage, [(Cl)(Im)2ZrTPP]+Cl–, the frequency
(728.9 cm–1) and intensity of this peak increase slightly
(Fig. 5b). This absorption occurs also in the IR
spectrum of reaction product of the third stage,
[(Im)3ZrTPP]2+·2Cl–, and two new bands appear, with
frequencies 622.1 and 521.4 cm–1, which also cor-
respond to the vibrations of the coordinated imidazole
[9].
Phase of reaction
1
2
m
0.06
1.09×10–3
0.10
1.84×10–3
k, s–1
According to Fig. 3 and above data, the reactions
equations of the second and third stages should be
written as follows:
K2
[(Cl)(Im)2ZrTPP]+Cl−
(Cl)2ZrTPP + Im
k2
[(Im)2ZrTPP]2+⋅2Cl−,
(4)
(5)
[(Cl)(Im)2ZrTPP]+Cl− + Im
[(Im)3ZrTPP]2+⋅2Cl−.
It is seen from the reaction Eq. (4) that in the
reversible process in the second stage the molecular
coordination of the ligand does not lead to a change in
the coordination polyhedron and requires consumption
of large energy for the replacement of covalently
bound chlorine. This is well consistent with a decrease
in the numerical value of the equilibrium constant
(K2 = 7.50 ± 0.08×103 l mol–1) and with the spectral
pattern of transformations at this stage: A reverse
transformation occurs, a decrease in the absorption
intensity, but the position of the absorption maximum
does not change (Fig. 1, lines 2, 3).
The third stage (at Im concentrations 5.97×10–4 to
1.40×10–2 mol l–1) of the reaction is characterized by
even lower constant K3, which is equal to (1.4±0.1)×
102 l mol–1. Here a substitution of Cl– by Im molecule
in an equilibrium process takes place, accompanied by
increasing intensity of the absorption and bathochromic
shift of the absorption maximum in the electronic
spectrum to 420.7 nm (Fig. 1). These spectral changes
The high value of the coordination constant of the
first Im molecule (~105), its sharp decrease at the next
stages, as well as spectral responses to the coordination
of the base suggest that it is promising to use
(Cl)2ZrTPP as a receptor of organic N-bases.
(Cl)2ZrTPP reacts similarly [like the reactions (3)–(5)]
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 4 2010

846
MOTORINA, LOMOVA
(a)
(b)
(c)
3500
3000
2500
2000
1500
1000
500
ν, cm–1
Fig. 5. IR spectra of the reaction products of (Cl)2ZrTPP with Im: (a) (Cl)(Im)ZrTPP, CIm = 1.1×10–5 mol l–1, (b) [(Cl)(Im)2ZrTPP]+ Cl–,
CIm = 2.0×10–4 mol l–1, and (c) [(Im)3ZrTPP]2+·2Cl–, CIm = 6.1×10–3 mol l–1.
also with the other N-base pyridine (Py). The
numerical values of the constants for the reaction of Py
are the following: K1 = 0.365 ×105, K2 = 0.16×103 and
K3 = 0.45 l mol–1 [10], which are 5, 47 and 313 times,
respectively, lower than similar constants for the
reaction with Im. The values of equilibrium constants
of formation of heptacoordinated complexes (K1)
correlate with the basicity of Im and Py molecules
receptor of bases. In addition, the dependence of the
reaction parameters on the nature of the incoming
ligand at the substitution of Cl– in (Cl)2ZrTPP by the
base points to associative mechanism of quasi-
equilibrium reaction (3) (Fig. 6).
Compared to the complexes of H2TPP with other
highly charged metal cations, (Cl)2ZrTPP is one of the
best reversible acceptor of Im molecules (Table 2).
This complex is less than the complexes of Hf and W
only in the magnitude of K1, and less than the Hf
complex in the magnitude of K2. If we consider all
MPs shown in Table 2, we have to point out one
additional practically important property: a significant
(pKBH 6.65 and 5.15, respectively [11]). This allows,
+
on the one hand, to conclude that the Nbase → Zr bond
in the supramolecular complexes (Cl)2ZrTPP is of σ-
type, and, on the other hand, indicates the high
selectivity of the reactions with bases that is important
for the practical applications of (Cl)2ZrTPP as a
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 4 2010

FORMATION OF SUPRAMOLECULAR COMPLEX
847
difference in the stepwise equilibrium constants of
E
Im
Cl
−Zr−
supramolecular complex formation of the same MP.
This property makes it possible to use these complexes
not only for detection of bases, but also for the
quantitative determination them by consideration of
direction and nature of the response in electron
spectrum. Finally, the rapid reaching equilibrium and
its displacement to the opposite direction at diluting
the solution by the base provides another advantage of
these complexes, now compared to the phthalocyanine
complexes, at their application in sensor devices where
the main obstacle is the long time (hours) of return
[15].
Cl
Im
Cl
−Zr−
Cl
Cl
−Zr−
Cl
Im
−Zr+−
Cl
Im
Cl
−Zr−
Cl
Reaction coordinate
Fig. 6. Energy profile of reaction of (Cl)2ZrTPP with Im in
first stage of transformation.
EXPERIMENTAL
Dichloro (5,10,15,20-tetraphenylporphinato)-
zirconium(IV) (Cl)2ZrTPP is obtained by the method
[8], by the reaction of coordination of 5,10,15,20-
tetraphenylporphin (H2TPP) with zirconium(IV)
chloride. 0.25 g (0.41 mmol) of H2TPP was boiled
with 0.5 g (2.64 mmol) of ZrCl4 in 15 ml of
benzonitrile at reflux for 1 h. The synthesis was
stopped when in the electron absorption spectra of the
reaction mixture disappeared the absorption bands of
H2TPP (λmax, nm: 648.0, 592.0, 551.0, 516.0, 485.0,
420.0). The complex was isolated in the solid form
after distilling benzonitrile off in a vacuum. Then the
saturated solution of the complex in chloroform was
twice subjected to chromatography on Al2O3 of
Brockmann activity II using CHCl3 as elluent. After
chromatography the solution of the zirconium complex
was treated additionally with gaseous HCl. Yield 60%.
Electronic spectrum in chloroform [λmax, nm (log ε)]:
563.0 (sh), 535.0 (4.16), 497.0 (3.67), 461.0 (3.91),
414.0 (4.64). The electronic spectra were recorded on
instruments SF-26 and Specord M-40.
The accuracy of the temperature control of solutions in
the spectrophotometer chamber at 298 K was ±0.1 K.
The numerical values of equilibrium constants Kn were
determined using Eq. (1) by the least-squares method
with Microsoft Excel program. The relative error did
not exceed 11% (Table 3).
Reaction kinetics was studied at 298 K by the
spectrophotometric method of excess concentrations
using the data on changes in optical density at the
working wavelength (417 nm). The rate constants of
reactions (Cl)2ZrTPP with Im were calculated with the
equation:
keff = 1/τln [(A0 – A∞)/(Aτ – A∞)].
(6)
Here A0, Aτ, and A∞ are optical densities at the working
wavelength of solutions of MP at the time 0, τ, and at
the end of reaction. Optimization of the keff value and
the determination of the mean-square deviations were
performed using the least-squares method and
Microsoft Excel program. The relative error in deter-
mining keff was 2–10%.
IR spectra of intermediate products in toluene were
registered as the subtraction spectra on a VERTEX 80v
instrument. As the zero line was taken the spectrum of
a mixture toluene–imidazole with the same concentra-
tion of Im as in the tested solution.
Table 2. The equilibrium constants of reactions of MP with
imidazole at 298 K (Kn)
Kn, l mol–1
Complex
K1
K2
7500
14000
750
K3
140
27.4
–
Equilibrium of reaction (Cl)2ZrTPP with Im in
toluene was studied by spectrophotometry by the
molar ratio method. Solutions of MP in toluene were
prepared just prior to use. Measurements of optical
density for a series of solutions with C[(Cl)2ZrTPP] =
const (2.32×10–6 mol l–1) at different concentrations of
Im were carried out at the working wavelength near
the maximum of the MP absorption band (417 nm).
(Cl)2ZrTPP
185000
234000
210000
1850
(Cl)2HfTPP [12]
O=W(OH)TPP [13]
О=Мо(ОН)TPP [14]
(AcO)CrTPP [3]
480
–
2300
820
–
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 4 2010

848
MOTORINA, LOMOVA
Table 3. Examples of calculation of the equilibrium constants and reaction rate (Cl)2ZrTPP with idimazole in the second
stage
СIm = 3.58×10–4 mol l–1
СIm×105,
K2298
,
(K2298±δK2)×10–3,
AS
mol l–1
l mol–1
l mol–1
298
τ, min
0
transmission
k
×104, s–1 (k22e9f8f±δk2eff)×104, s–1
2eff
1.19
2.03
3.58
5.97
8.35
11.9
35.8
59.7
0.2581
0.2480
0.2395
0.2358
0.2300
0.2187
0.2012
0.1791
7.50±0.08
0.638
0.644
0.656
0.6605
0.662
0.664
0.667
0.6672
8.2±0.2
7328
8735
6660
6677
8443
7225
5
7.83
8.11
8.29
8.32
8.29
8.15
20
30
35
45
105
∞
7. Buchler, J.W., Eikelmann, G., Puppe, G., Rohbock, K,
Schneehage, H.H., and Weck, D., Liebigs Ann. Chem.,
1971, vol. 745, p. 135.
ACKNOWLEDGMENTS
This work was partially supported by grants from
the Presidium of Russian Academy of Sciences for
Basic Research Program no. 18 and the Departmental
target program of the Ministry of Education and
Science, “Development of Scientific Potential of
Higher Schools” (project no. 2.2.1.1/2820).
8. Berezin, B. D. and Lomova, T.N., Zh. Neorg. Khim.,
1981, vol. 26, no.12, p. 203.
9. Elkin, M. D., Ziganshina, O.D., Berezin, K.V., and
Nechaev, V.V., Zh. Strukt. Khim., 2004, vol. 45, no. 6,
p. 1132.
10. Lomova, T.N., Motorina, E.V., Ovchenkova, E.N., and
Klyueva, M.E., Izv. Akad. Nauk, Ser. Khim., 2007,
no. 4, p. 636.
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