Complex formation of tetra(3,5-di-tert-butylphenyl)porphine with copper(II) and zinc(II) acetates in organic solvents

Article abstract of DOI:10.1134/S1070328409050042

The effect of accumulation of the tert-butyl groups in the phenyl rings of tetraphenylporphine on the complex formation rate of tetra(3,5-di-tert- butylphenyl)porphine (I) with copper(II), zinc(II), and cadmium(II) acetates in acetic acid and in a mixed e

Full text of DOI:10.1134/S1070328409050042

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2009, Vol. 35, No. 5, pp. 335–340. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © M.B. Berezin, B.D. Berezin, S.A. Syrbu, 2009, published in Koordinatsionnaya Khimiya, 2009, Vol. 35, No. 5, pp. 341–346.
Complex Formation of Tetra(3,5-di-tert-butylphenyl)porphine
with Copper(II) and Zinc(II) Acetates in Organic Solvents
M. B. Berezina, B. D. Berezina,b, and S. A. Syrbub
a Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
b Ivanovo State University of Chemical Technology, Ivanovo, Russia
E-mail: mbb@isc-ras.ru
Received June 17, 2008
Abstract—The effect of accumulation of the tert-butyl groups in the phenyl rings of tetraphenylporphine on
the complex formation rate of tetra(3,5-di-tert-butylphenyl)porphine (I) with copper(II), zinc(II), and cad-
mium(II) acetates in acetic acid and in a mixed ethanol–benzene solvent was studied by the chemical kinetics
method with spectrophotometric monitoring. The activation characteristics of the complex formation were cal-
culated. The complex with zinc was found to form three times more slowly that the copper complex. Benzene
additives to the organic solvent decreased the reaction rate because of steric hindrance created by the benzene
molecules that formed stable solvates with compound I.
DOI: 10.1134/S1070328409050042
INTRODUCTION
Tetra(3,5-di-tert-butylphenyl)porphine (ç2ê, I),
unlike tetraphenylporphine (ç2TPhP, II), has a
branched system of hydrophobic (ë4ç9) substituents in
the 3,5-positions of the phenyl rings. That is why, com-
pound I is well soluble in nonpolar organic solvents
(benzene, tetrachloromethane, cyclohexane, etc.) and is
almost insoluble in polar solvents (alcohols, ketones,
dimethylformamide, and others). Therefore, the com-
plex formation of compound I was studied in glacial
acetic acid (ç2ê is soluble in ëç3ëééç due to the for-
mation of hydrogen bonds –ëééç···N≤ and the proto-
nated form of the ligand ç4ê2+, whose UV spectrum
exhibits an absorption band at 655 nm, Fig. 1) and in an
ethanol–benzene (1 : 1) mixed solvent.
Research in the area of the chemistry of porphyrins
is still urgent and becomes more required due to the
extension of their practical application. The synthesis
of porphyrins with unusual physicochemical properties
is widely used. The tetraphenylporphine derivatives
remain to be most accessible among synthetic porphy-
rins. The metal complexes of these derivatives are
applied as active components of new materials used in
catalysis, biomonitoring and chemical monitoring,
extraction, qualitative and quantitative analyses, non-
linear optics, and in diverse fields of molecular design
and medicine [1–3]. Therefore, the study of the reactiv-
ity of new porphyrins remains urgent.
R
R
R
R
R
R
R
R
R
R
N
N
N
M
N
NH
HN
N
N
R
R
R
R
R
R
I: R = C(CH3)3
II: R = H
Metalloporphyrin (MP)
335

336
BEREZIN et al.
A
2.0
A
1.6
(‡)
(b)
1.5
1.2
0.8
1.0
0.5
0
0.4
0
350
450
550
650
400
500
600
700
λ, nm
λ, nm
Fig. 1. UV spectrum of H P in (a) benzene and (b) acetic acid.
2
EXPERIMENTAL
The measurements were carried out at the wave-
length near the absorption band maximum of the ligand
or near the absorption band maximum of the metal-
loporphyrin (Fig. 2). The porphyrin-ligand concentra-
tion did not exceed (1–5) × 10–5 mol/l in all experi-
ments. The salt concentration was ten and more times
higher than the ligand concentration. Under these con-
ditions, the systems studied obey the first reaction order
with respect to the porphyrin and salt (Figs. 3, 4). In this
case, the kinetic equation takes the form
Porphyrin I was synthesized by a described proce-
dure [4]. After the major portion of the organic solvent
was distilled off, the porphyrin was precipitated with
alcohol, filtered off, washed on the filter with dimethyl-
formamide, hot water, and acetone, and dried. The dry
residue was dissolved in a minimum amount of ben-
zene and twice chromatographed on alumina (Brock-
mann activity III) using benzene as the eluent. The elu-
ate was evaporated, and the porphyrin was precipitated
with methanol, filtered off, and dried at 100°ë under
dc
H2P
1
–------------ = kappcH P
(3)
reduced pressure. H NMR (CDCl3), δ (ppm): 9.03 s
2
dt
(8ç, β-ç), 8.22 s (8ç, Ó-ç), 7.90 s (4ç, n-ç), 1.64 s
(72ç, t-Bu), –2.55 s (2H, NH) (tetramethylsilane as the
internal standard, Bruker AC 200 spectrometer). The
salts and solvents for kinetic measurements were pre-
pared by known procedures [5, 6].
or in the integral form with allowance for Eq. (2)
A0 – A∞
-- ------------------
.
1
--
τ
1
τ
0
kapp
=
ln(c /cτ) = ln
(4)
Aτ – A∞
The spectrophotometric method was used to study
the formation of the metalloporphyrins. The absor-
bance was measured with SF-26 and Cary-300 spectro-
photometers. The method of spectrophotometric moni-
toring the change in the concentrations of the colored
porphyrin-ligand and metalloporphyrin formed is
based on the linear dependence between the absorbance
of the solution and the concentration of the substance
The units of the concentration are of no importance.
Since the Ä value in Eq. (1) is proportional to Ò (mol/l),
the porphyrin-ligand concentration was also expressed
in mol/l.
The activation energy (Ö‡) was calculated from the
Arrhenius plots of the reaction rate constants of metal-
loporphyrin formation vs. temperature. The activation
entropy (∆S#) was calculated by the main equation of
the transition state theory.
A = εlc.
(1)
The following equation can be applied to the system
of two colored substances (porphyrin–metalloporphy-
rin):
RESULTS AND DISCUSSION
Tetraphenylporphine and its numerous derivatives
are among the most important porphyrins used in sci-
ence and various areas of engineering and technology
[1–3]. The fourfold phenylation of the meso-positions
A0 – A∞
c0/cτ =
,
(2)
------------------
Aτ – A∞
where Ä0, Äτ, and Ä∞ is the initial, current, and final substantially changes the properties of the porphine
absorbance, respectively; Ò is the porphyrin-ligand con- itself due to the drawing back of the electron density
centration.
from the –CH= methine bridges to the benzene ring.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 35 No. 5 2009

COMPLEX FORMATION OF TETRA(3,5-DI-TERT-BUTYLPHENYL)PORPHINE
logc0/cτ
337
A
655 nm
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1
3
2
560 nm
0 50 100150200250300350400450500550600650
τ, s
Fig. 3. Time plots of logc0/c for the reaction of ç ê with
2
τ
–3
Cu(Ac) in acetic acid at 293.15 K and Ò × 10 : (1) 0.586,
2
salt
(2) 1.263, and (3) 2.55 mol/l.
of the chromophore is accompanied by the bathochro-
mic shift of λ1.
Since the rates of complex formation with metals
and their kinetic stability under the action of proton-
donating solvents are practically significant, their
detailed study is important. The kinetics and mecha-
nisms of formation of the metalloporphyrins were ana-
lyzed [7, 8]. Their dissociation was considered [9]. It is
shown in cited works that four factors exert a strong
effect on the coordination properties of porphyrins with
d metals: the strength of the M–N chemical bond, the
macrocyclic effect [10–12], the electronic effects of
substituents, and the solvation of transition states.
500
650
λ, nm
Fig. 2. Change in the UV spectrum during the formation of
CuP in ëç ëééç.
3
The phenyl rings in compound II are σ-electronic
acceptors (–I effect) toward the π system of the porphy-
rin macrocycle. As a result, the sixteen-membered
chromophore of tetraphenylporphine is stabilized and
the N–H bonds of the coordination site are polarized.
As a consequence, the complex formation of H2TPhP
with the metal salts is accelerated
The influence of alkyl substitution in the phenyl res-
idues on the properties of tetraphenylporphine remains
almost unstudied [13]. There are few studies on
tetra(tert-butylphenyl)porphyrins [14, 15]. Unlike the
simple phenyl substitution of the meso-positions of the
porphine for the ë6ç5 group (weak –I effect), the
replacement by the ë6ç3[3,5-ë(CH3)3]2 group creates a
strong positive inductive effect (+I effect). The latter
enhances the basicity of the tertiary –N= atoms and the
electron density on the N–H bonds. One should expect
a decrease in the coordinative ability with respect to
metal ions in porphine I and an increase in its basic
properties according to the reaction
H2TPhP + [MX2(S)n – 2
]
MTPhP + 2HX + S. (5)
If M = Cu(II), S is ethanol, and X is the acetate ion,
reaction (5) is fourfold accelerated compared to that of
the porphine [7, 8]. Unlike the porphines substituted at
the β-pyrrole positions, whose double protonation is
accompanied by the hypsochromic shift of the long-
wavelength band (λ1) in the UV spectrum, tetraphe-
nylporphine and its phenyl-substituted derivatives in
the H4TPhP2+ state manifest the bathochromic shift,
which can be a result of the ëmeso–ëphenyl bond extension
ç2ê + 2ëç3ëééç
ç4ê2+ + 2ëç3ëéé–. (6)
Therefore, only ç2ê reacts when reaction (5) occurs
in an acetic acid medium. It is shown [15, 16] that ç4ê2+
is incapable of coordinating metal ions. In addition to
the electronic effects, steric effects of the substituents
(
)
*
in the H TPh P(S) singlet excited state. As a result, the
2
benzene residues are conjugated with the macrocycle,
which is retained in H4TPhP2+, but the ë6ç5 groups
manifest the +I effect, and the extension of the π system also prevent the occurrence of reaction (5) involving
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 35 No. 5 2009

338
–logkapp
BEREZIN et al.
13]. It should be kept in mind that CuÄÒ2 in glacial ace-
tic acid is dimerized [13].
2.25
2.20
2.15
2.10
2.05
2.00
1.95
1.90
1.85
1.80
1.75
1.70
1.65
1.60
3
2
In a solution of CuÄÒ2 with a concentration of
0.6 mmol/l, the rate constants for reaction (1) with
H2TPhP are 0.013 s–1 at 293 K [13], whereas for the
reaction involving ç2ê the rate constant is 0.0079 s–1.
The decrease in the rate for the reaction with ç2ê is
caused by the suppression of the acceleration of reac-
tion (5) due to the +I effect of the tert-butyl groups, pos-
sibly, by a sharp decrease in the ratio of the concentra-
tions of the reactive (H2P) and nonreactive (ç4ê2+)
forms due to protonation (6) rather than by the shield-
ing of the H2N4 reaction center with the bulk “tops” of
the C(CH3)3 groups. This conclusion is confirmed by
the UV spectral data for H2P in CH3COOH (Fig. 1).
The spectrum shows that the ç4ê2+ concentration pre-
vails over the ç2ê concentration.
1
2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3
–logcsalt
Fig. 4. Plots of –logk vs. –logc for CuP formation at
app
salt
(1) 298, (2) 293, and (3) 288 K.
The low activation energies and entropies also show
that CuP is formed via reaction (5) with ç2ê and not
with ç4ê2+. The decrease in the activation energy
(Table 1) with an increase in the copper acetate concen-
tration is due to the destabilization of
CuAc2(CH3COOH)n – 2 and the loss of a part of the sol-
vate shell (possibly, external). This is observed as a
considerable decrease in the activation entropy.
ç2ê [17, 18] due to the shielding of the H2N4 reaction
center [11].
It follows from the kinetic data (Table 1) that in an
interval of the copper acetate concentrations of (2.55–
0.586) × 10–3 mol/l the true rate constants (kn) corre-
spond to bimolecular reaction (5) with ç2ê and the sec-
ond-order kinetic equation
The apparent rate constants (kapp) for reaction (5)
with CuAc2 in glacial acetic acid with a 10% benzene
additive were measured (kapp = kνcCuAc ). They remain
dc
H2P
2
–------------ = kνcH PcMAc
.
2
(7)
2
dt
almost unchanged; i.e., the state of the reactants in reac-
tion (5) is unchanged.
In this case, the high concentration dependence of kν is
observed in the temperature interval from 288 to 298 K.
Such a dependence is usually characteristic of the Zn,
Co, and Ni salts in reaction (5) and, to a less extent, of
the Cu salts and is caused by a change in the coordina-
tion sphere of the salt of the reactant (in the case, ace-
Unlike CuAc2(CH3COOH)n – 2, the coordination
sphere of ZnAc2(CH3COOH)4 in an interval of the cho-
sen concentrations of (2.2–0.54) × 10–3 mol/l undergoes
no changes. The first order with respect to the porphy-
rin and the first order with respect to the salt in reaction
tate åÄÒ2) with an increase in the concentration [7, (8) of ZnP formation are rigidly fulfilled (Table 2)
Table 1. Kinetic parameters for the formation of CuP in CH3COOH
cCu(Ac) , mol/l
2
T; Ea; ∆S#
2.55 × 10–3
1.26 × 10–3
0.586 × 10–3
kapp, s–1
kν, l s–1 mol–1
kapp, s–1
kν, l s–1 mol–1
kapp, s–1
kν, l s–1 mol–1
288 K
0.0124 0.00112
0.0184 0.00152
0.0268*
4.85
7.20
0.0088 0.00071
0.0120 0.00093
0.0199 0.00120
58
6.94
9.50
0.0059 0.00032
0.0079 0.00088
0.0145 0.00095
64 2
10.07
13.53
24.69
293 K
298 K
10.53*
15.77
Ea, kJ/mol
55
1
1
∆S#, J mol–1 K–1
–48 10
–32
7
–10 5
* Obtained by extrapolation.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 35 No. 5 2009

COMPLEX FORMATION OF TETRA(3,5-DI-TERT-BUTYLPHENYL)PORPHINE
Table 2. Kinetic parameters for the formation of ZnP in CH3COOH
339
cZn(Ac) , mol/l
2
T; Ea; ∆S#
2.098 × 10–3
1.056 × 10–3
0.540 × 10–3
kapp, s–1 kν, l s–1 mol–1
kapp, s–1
kν, l s–1 mol–1
kapp, s–1
kν, l s–1 mol–1
288 K
0.0062 0.00075
0.0099 0.00082
0.0141 0.00120
59
2.94
4.69
6.74
0.0031 0.00035
0.0049 0.00052
0.0077 0.00042
66
2.91
0.0016 0.00012
0.0025 0.00014
0.0041 0.00039
68
2.94
4.68
7.64
293 K
4.66
298 K
7.32
Ea, kJ/mol
∆S#, J mol–1 K–1
5
1
5
1
3
–39
9
–15
–7
Table 3. Kinetic parameters (kapp, s–1; kν, l s–1 l mol–1) of the reactions of CuP, ZnP, and CdP formation in an ethanol–ben-
zene (1 : 1) mixed solvent*
298.15 K
kapp × 104
kν
7.47 0.259
308.15 K
kapp × 104
kν
318.15 K
328.15 K
338.15 K
Salt
EA,
∆S#,
(csalt × 103 mol/l)
kJ/mol J mol–1 K–1
49 1 –100 15
kapp × 104 kν kapp × 104 kν kapp × 104 kν
Cu(CH3COO)2
(2.87)
14.25 0.495
25.15 0.896
Zn(CH3COO)2
(2.59)
0.027**
2.45 0.095
0.73 0.144
4.32 0.166 7.51 0.289 50 1 –110 25
1.62 0.326 3.49 0.689 70 1 –49 11
Cd(CH3COO)2
(0.506)
0.0256**
0.0288**
Cd(CH3COO)2
(2.57)
1.90 0.0739
5.18 0.202 12.58 0.489
79 3 –16 15
Notes: * Determination accuracy is 5%.
** Obtained by extrapolation.
from 49 to 55 kJ/mol, and the activation entropy
decreases by 52 J mol–1 K–1, which is due to the stronger
solvation of the transition state in an ethanol–benzene
mixture. The rates of ZnP and CdP formation in this
medium turned out to be tenfold lower than that of CuP
formation and by 250 times lower than the rate in
CH3COOH (Tables 1, 2). It can be assumed that in a
ë6ç6–ë2ç5éç (1 : 1) medium porphyrins, especially
ç2í (3,5-di-tert-butylphenyl)porphine), decrease their
reactivity because of the shielding of the ç2N4 reaction
center by the solvate shell of the benzene molecules
strongly bound to the π system of the ë12N4 macrocycle
due to their π–π interaction. The composition and high
H2P + [ZnAc2(CH3COOH)4]
[H2P···ZnAc2(CH3COOH) ]#
2
(8)
Transition state
ZnP + CH3COOH.
The ZnP complex is formed threefold more slowly
than CuP. This is typical of H2TPhP and ç2ê and also
of all porphyrins studied to the present time. The intro-
duction of eight tert-butyl groups in the 3,5-positions of
the phenyl fragments of tetraphenylporphine decreases
the complex formation rate by four times compared to
that of H2TPhP. This is much higher than the decrease
observed for CuP. Therefore, the conditions of ZnAc2 stability of the CuP · ë6ç6 and ZnP · ë6ç6 crystal sol-
solvate coordination with H2P are much less favorable
than those in the case of CuAc2. It is most likely that in
reaction (8) zinc acetate experiences a substantial coun-
teraction of the tert-butyl groups when the transition
state is formed.
vates were studied in several works including [15]. It
can be assumed that the introduction of eight tert-butyl
groups into H2TPhP enhances this π–π interaction with
benzene. For instance, the rates of tetraphenylporphine
coordination by zinc acetate (Ò = 2.22 × 10–3 mol/l) in an
ethanol–benzene (1 : 1) medium are documented [13].
Reaction (5) with copper, zinc, and cadmium ace-
tates in an ethanol–benzene mixture was carried out in
the temperature interval from 308 to 338 K, because at
298 K this reaction proceeds too slowly (Table 3). The
rate of CuP formation in this solvent is 40 times lower
It turned out that k2ν98 = 0.335 s–1 l mol–1, Ö‡ = 61 kJ/mol,
and ∆S# = –59 J mol–1 K–1; i.e., the coordination rate is
an order of magnitude higher than that for H2P. Thus,
than that in acetic acid. The activation energy increases reaction (8) is inhibited in an ethanol–benzene mixed
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 35 No. 5 2009

340
BEREZIN et al.
3. Koifman, O.I., Semeikin, A.S., and Berezin, B.D., Por-
firiny: struktura, svoistva, sintez (Porphyrins: Structure,
Properties, Synthesis), Enikolopyan, N.S., Ed., Moscow:
Nauka, 1985.
solvent due to the shielding of the coordination site by
the tert-butyl groups and the solvate shell with the
strongly bound benzene molecule.
It seems most probable that the benzene–porphyrin
solvate destabilizes the transition state, creating hin-
drance for the cleavage of two N–H bonds and removal
of protons to the solvent medium
4. Syrbu, S.A., Semeikin, A.S., and Lyubimova, T.V.,
Khim. Geterotsikl. Soedin., 2004, no. 10, p. 1464.
5. Karyakin, Yu.V. and Angelov, I.I., Chistye Khimicheskie
Veshchestva (Pure Chemical Substances), Moscow:
Khimiya, 1974.
#
O
6. Weissberger, A., Proskauer, E., Riddick, J., and Toops, E.,
Organic Solvents. Physical Properties and Methods of
Purification, NewYork: Wiley, 1955.
7. Berezin, V.D., Koordinatsionnye soedineniya porfirinov
i ftalotsianina (Coordination Compounds of Porphyrins
and Phthalocyanine), Moscow: Nauka, 1978.
H
H3C
N
N···H
C
H
O
H
H
H
H
R
R
O
O
H
M
X
O
N
8. Berezin, B.D., Golubchikov, O.A., and Koifman, O.I.,
C
H3C
H
Zh. Fiz. Khim., 1973, vol. 47, no. 11, p. 2817.
N···H
O
9. Berezin, B.D. and Lomova, T.N., Reaktsii dissotsiatsii
kompleksnykh soedinenii (Dissociation of Complex
Compounds), Moscow: Nauka, 2007.
Shielding region of N–H bonds
10. Mamardashvili, G.M. and Berezin, B.D., Uspekhi khimii
porfirinov (Advances in Porphyrin Chemistry), Golub-
chikov, O.A., Ed., St. Petersburg: NII khimii SPbGU,
2004, vol. 4.
The most expensive activation steps should be sur-
mounted to surpass the transition state by the reacting
system (porphyrin–salt): the removal of the N–H pro-
tons as H+···S (S is ë2ç5éç) to the solution and the
removal of two ë2ç5éç molecules from the solvate
shell of metal acetates. The leaving ligands (two
ë2ç5éç and ëç3ëéé–) leave the formed metallopor-
phyrin behind the top of the potential barrier and meet
no steric hindrance.
11. Berezin, B.D. and Berezin, M.B., Zh. Fiz. Khim., 1989,
vol. 63, no. 12, p. 3166.
12. Berezin, B.D., Berezin, M.B., and Berezin, D.B., Ross.
Khim. Zh., 1997, vol. 41, no. 3, p. 105.
13. Berezin, B.D. and Golubchikov, O.A., Koordinatsion-
naya khimiya solvatokompleksov solei perekhodnykh
metallov (Coordination Chemistry of the Solvate Com-
plexes of Transition Metal Salts), Moscow: Nauka, 1992.
14. Kuvshinova, E.M., Golubchikov, O.A., Semeikin, A.S.,
and Berezin, B.D., Izv. Vyssh. Uchebn. Zaved., Khim.
Khim. Tekhnol., 1986, vol. 29, no. 10, p. 58.
ACKNOWLEDGMENTS
15. Balantseva, E.V., Antina, E.V., Berezin, M.B, and
V’yugin, A.I., Zh. Neorg. Khim., 2005, vol. 50, no. 10,
p. 1676 [Russ. J. Inorg. Chem. (Engl. Transl.), vol. 50,
no. 10, p. 1566].
This work was supported by the Russian Foundation
for Basic Research, project no. 07-03-00818.
16. Berezin, B.D., Zh. Fiz. Khim., 1970, vol. 15, no. 8,
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