Complexation of tetraphenyltetrabenzoporphine with Cu(II), Cd(II), Zn(II), and Co(II) salts in organic solvents

Article abstract of DOI:10.1023/B:RUCO.0000037437.92306.c1

The rate and activation parameters of tetraphenyltetrabenzoporphine (H ₂TPTBP) complexation with 3d-metal acetates and acetylacetonates are shown to be determined by the solvent nature. With an increase in the electron-donor properties of a solvent, the reaction rate increases due to protonation of N-H bonds and decreases as MA_m(Solv)_{n - m} salt solvates become more stable. As the result, the rate of a reaction with ZnAc₂ increases in the series: DMF < DMSO < Py < PrOH-1 < CH₃CN < C₆H₆. In inert and weakly coordinating solvents, the transition state of a reaction is supposed to be formed according to the mechanism of contraction of the salt coordination sphere. The rate of H₂TPTBP reaction with metal acetates in pyridine changes in the series: Cu(II) > Cd(II) > Zn(II) > Co(II), while the stability of the obtained complexes decreases in the series Cu(II) > Co(II) > Zn(II) > Cd(II). It is shown that the spectral criterion of the complex stability can be used in the series of metal complexes with one ligand, but it is violated if the ligand structure is changed.

Full text of DOI:10.1023/B:RUCO.0000037437.92306.c1

Russian Journal of Coordination Chemistry, Vol. 30, No. 8, 2004, pp. 573–578. Translated from Koordinatsionnaya Khimiya, Vol. 30, No. 8, 2004, pp. 610–615.
Original Russian Text Copyright © 2004 by Berezin, Toldina.
Complexation of Tetraphenyltetrabenzoporphine with Cu(II),
Cd(II), Zn(II), and Co(II) Salts in Organic Solvents
D. B. Berezin* and O. V. Toldina**
*Institute of Soluiton Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
**Ivanovo State University of Chemical Technology, Ivanovo, Russia
Received May 10, 2003
Abstract—The rate and activation parameters of tetraphenyltetrabenzoporphine (H2TPTBP) complexation with 3d-
metal acetates and acetylacetonates are shown to be determined by the solvent nature. With an increase in the electron-
donor properties of a solvent, the reaction rate increases due to protonation of N–H bonds and decreases as MAm(Solv)n –
m salt solvates become more stable.As the result, the rate of a reaction with ZnAc2 increases in the series: DMF < DMSO
< Py < PrOH-1 < CH3CN < C6H6. In inert and weakly coordinating solvents, the transition state of a reaction is supposed
to be formed according to the mechanism of contraction of the salt coordination sphere. The rate of H2TPTBP reaction
with metal acetates in pyridine changes in the series: Cu(II) > Cd(II) > Zn(II) > Co(II), while the stability of the obtained
complexes decreases in the series Cu(II) > Co(II) > Zn(II) > Cd(II). It is shown that the spectral criterion of the complex
stability can be used in the series of metal complexes with one ligand, but it is violated if the ligand structure is changed.
Tetraphenyltetrabenzoporprhine (H2TPTBP, I) exhib-
R
its the properties of a rigid and not liable to deformation
tetrabenzoporphine (H2TBP, II) and of dodeca-substituted
(i.e., substituted in all β- and meso-positions) porphyrin.
Such a combination of properties determines its specific
structure and reactivity. Indeed, like most of dodeca-sub-
stituted porphyrins (H2P), compound I occurs as a nonpla-
nar saddle-point conformation in both crystal and solution
[1]. However, due to rigidity of the initial macrocycle II,
its nonplanar nature is weaker during the meso-tetraphenyl
substitution in compound I as compared to the aromatic
octaethylporphine (H2OEtP, III). Thus, in Zn complexes
of compounds I and IV (IV is dodeca-substituted octaeth-
yltetraphenylporphine H2OEtTPP), the average deviation
of the ëβ atoms from the initial plane of a macrocycle
NH
N
R
R
N
HN
R
III: R = H
IV: R = Ph
The bathochromic shift of the bands in the elec-
tronic absorption spectra of phenylbenzoporphyrins
observed with an increase in the number of the meso-
phenyl substituents and reaching its maximum in the
case of H2TPTBP [3], as well as some other specific
photophysical properties [2, 4], are likely to be due to
both factors mentioned above.
1
(∆ëβ) is 0.77 and 1.09 Å, respectively. For Zn complexes
of compounds II and III, these values are close to zero [1].
In addition to the distortion factor, the properties of
dodeca-substituted ç2ê are also affected by the polar-
ization factor. Thus, the dipole moment (≈1.2–1.9 D)
was discovered in the meso-phenyl derivatives of octa-
ethylporphine, in particular, in compound IV, at an
angle to the mean macrocycle plane [2].
As compared to meso-tetraphenylporphine and tet-
rabenzoporphine, tetraphenyltetrabenzoporphine has
the higher solubility in organic solvents, more readily
undergoes the specific solvation at the coordination
N4H2 core [5], and easily enters the redox reactions [6].
Pure H2TPTBP was isolated from a mixture of meso-
phenyl-substituted tetrabenzoporphyrins about 10
years ago [7] therefore, more early data on its physico-
chemical properties are doubtful.
R
NH
N
N
Cβ
R
R
HN
Cmeso
The aim of this work was to study the kinetics of
binding of ç2íPTBP by acetates and acetylacetonates
of Cu(II), Cd(II), Zn(II), and Co(II) in organic solvents
that have different electron-donor properties, i.e., pyri-
dine (Py), dimethyl sulfoxide, N,N-dimethylformamide
(DMF), acetonitrile (AN), propanol-1 (PrOH-1), and
benzene (C6H6).
R
I: R = Ph
II: R = H
1
The characteristic of saddle-point distortion of π-system in solid phase.
1070-3284/04/3008-0573 © 2004 åÄIä “Nauka /Interperiodica”

574
BEREZIN, TOLDINA
Table 1. Kinetics of complexation of H2TPTBP and H2TBP with metal salts in organic solvents (c
= 2.6 × 10–5 mol/l)
H2P
k2v98
l mol–1 s–1*
,
cMX × 103,
E,
∆S≠,
2
Solvent
Salt
kJ mol–1
J mol–1 K–1
mol/l
H2TPTBP
C6H6
Zn(Acac)2
Zn(Acac)2
Zn(Ac)2
0.26
0.26
0.26
1.00
0.26
2.6
Very rapidly
Very rapidly
41 15
PrOH-1
6.33 0.64
21.34 2.42
46.50 5.00
0.086 0.009*
0.276 0.029
0.10 0.001
0.305 0.013
0.463 0.035
0.028 0.004*
–101 28
–67 13
–65 16
AN
Zn(Ac)2
48
47
54
56
39
55
4
5
2
2
1
8
DMSO
Py
Zn(Ac)2
Cu(Ac)2
Zn(Ac)2
Cu(Ac)2
Cu(Acac)2
Zn(Ac)2
–75
–57
8
7
3
2.6
2.6
–143
2.6
–61 20
–55 18
–92 33
2.6
58 10
57 16
DMF
2.6
H2TBP
C6H6
Zn(Acac)2
Cu(Ac)2
Zn(Ac)2
0.26
2.6
2.6
2.6
2.6
Very slowly
DMSO
0.067 0.008*
0.0069 0.0018*
0.02 0.003*
45
51
59
49
2
1
6
5
–111
–124
6
3
Py
Cu(Ac)2
Cu(Acac)2
–85 12
–89 18
0.181 0.006
* Obtained by extrapolation.
EXPERIMENTAL
where M is the metal ion, A is the salt anion, Solv is the
molecule of a solvent, åA2(Solv)n – 2 is the solvated
complex, A is a monodentate ligand.
Compound I was prepared using procedure [7]. Sol-
vents and salts were preliminarily purified and dried as
described in [8, 9].The kinetic measurements were per-
Thus, the rate of ç2íPTBP complexation with
formed by spectrophotometric method (using SF-46 ZnAc2 increases in the series of solvents DMF <
instrument). The experimental procedures and calcula- DMSO < Py ꢀ PrOH-1 < AN < C6H6 as their ability to
tions are reported in [10]. The experimental data are form stable solvates [12] and the electron-donor prop-
given in Tables 1 and 2.
erties decrease (Table 1). An exception is DMF, whose
donor number (DN 26.6) is lower than that of DMSO
(29.8) and Py (33.1) [13].
RESULTS AND DISCUSSION
Proceeding from the stability constants of the sol-
vated complexes [ZnAc2(çéÄÒ)3Solv] measured in
acetic acid, pyridine forms the most stable solvates with
Zn2+ ion (Ks2t98 = 1700 100); DMSO and DMF form
Because of the limited solubility of unsubstituted
H2TBP, the kinetic study of its binding with metal salts
(reaction (1) written in a general form) was performed
only for Py [11]. The nonplanar dodeca-substituted
ligand I readily dissolves in the most organic solvents,
which made it possible to establish the influence of the
solvent nature on the reaction
solvates whose stabilities are almost 10 times as low
298
(K = 204 9 and 190 40, respectively), while the
st
solvates formed by alcohols (butanol-1) are still less
298
ç2P + åA2(Solv)n – 2
stable by one order of magnitude (K = 18 0.6). The
st
≠
298
st
[ç2P···åA2(Solv)n – 2
]
(1)
complexing properties of nitriles are still weaker (K
=
åP + 2çA + (n – 2)Solv,
4) [12]. The above stability constants are rough and can
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 8 2004

COMPLEXATION OF TETRAPHENYLTETRABENZOPORPHINE
575
Table 2. Effect of 3d-metal nature on the kinetics of complexation of tetrabenzoporphyrins I, II in Py (cMAc = 2.6 × 10–3 mol/l;
2
cH P = 2.6 × 10–5 mol/l)
2
k2v98 × 102,
l mol–1 s–1*
∆S≠,
Salt
Porphyrin
E, kJ mol–1
∆λI , nm
J (mol–1 K–10
)
CuAc2
I
30.5 1.3
2.02 0.081*
10.0 0.1
0.69 0.018*
4.80 0.44
0.61 0.03
2.5 0.19*
0.095
55
59
39
51
66
94
67
73
8
6
1
1
2
2
2
3
–61 20
–85 12
–45
–38
–41
–34
–40
–31
–42
–35
II
ZnAc2
CdAc2
CoAc2
I
–143
–124
–58
3
3
6
4
5
II
I
II**
I
–21
–60
II**
–122 13
* Obtained by extrapolation.
** Data from [11]; ∆λI is the shift of I-band in the electronic absorption spectrum of the complex with respect to I-band of a ligand, nm.
change significantly depending on the nature of the
We found that at comparable concentrations of the
medium-solvent that reacts differently with the elec- salts, in PrOH-1 and Py, the reaction of ç2TPTBP com-
tron-donor ligand-solvents. The replacement of Zn2+ by plexation with Zn(Acac)2 and Cu(Acac)2 occurs at the
Co2+ equalizes the stability of complexes with the indi-
cated ligands [12, 13].
rate higher than that with ZnAc2 and CuAc2 (Table 1).
Since åAc2 is a simple salt, while å(Acac)2 is a che-
late salt that is thermodynamically and kinetically more
stable [17] (the stability constants of Zn(Acac)2,
The rates of reaction (1) for compound I with ZnAc2
in Py and DMSO are close that higher than in DMF,
since according to mechanism [14], the rate is affected
by both the stability of the metal coordination sphere
and the nature of the N–H bonds in ç2ê molecule. In
the electron-donor (basic) solvents (diethylamine, Py,
DMSO, or their mixture), the uncommon porphyrins
[15] can form states with more or less ionized (delocal-
ized) N–H bonds. In this case, their cleavage during the
formation of transition state of reaction (1) is favored in
the series of solvents (PrOH-1 < AN ꢀ DMF < DMSO ≅
Py) that is opposite to the stability series of solvated
complexes with them [12]. The weak proton-donor pro-
panol-1 is obviously prevents the activation of the N–H
bonds.
Zn(Acac)+, and ZnAc+ are 1.05 × 109, 1.2 × 105, and
0.5 × 102, respectively, i.e., they differ by more than 3
orders of magnitude [18]), it would be reasonable to
suggest more intricate mechanism of complexation
with the chelate salt and slowing down of reaction (1)
for porphyrin with acetylacetonate of Zn(II) as com-
pared to acetate. In reaction with porphyrin, the
M(Acac)2 species is regarded to be kinetically inactive
and for the classic mechanism of reaction (1) to be real-
ized, the dissociation of the complex salt at the first
stage [19] is required: å(Acac)2
å(Acac)+ + Acac–.
The reaction of complex formation between porphy-
rins and Cu(II) and Zn(II) acetylacetonates was studied
We have shown previously that due to the electronic by the authors [10, 17, 19]. Both the reaction accelera-
structure of tetrabenzoporphyrin molecules, they can tion [19] and slowing down [10] were observed. Thus,
occur, depending on the conditions, as NH-localized or as a result of the replacement of Zn acetate by acetylac-
NH-delocalized form and therefore, can be classified as etonate, the reaction of complex formation of N-substi-
uncommon H2P [15, 16]. Hence, the activation energy tuted porphyrins in DMSO slows down by 5 to 30 times
of reaction between compound I and ZnAc2 is mini- [10]. The N-substituted ligands have nonplanar unsym-
mum in Py, which activates the N–H bonds, and in metric structure [1], which should favor their coordina-
PrOH-1, in which the solvate form of Zn2+ ion is signif- tion by a metal. However, the coordination center of
icantly less stable as compared to that in Py and DMF. such a porphyrin is screened on one side of the mole-
However, DMF and DMSO form stable solvated com- cule plane by the alkyl-aryl endocyclic substituent.
plexes with ZnAc2 and therefore, the activation energy of Obviously, this prevents reaction (1) with the bulky
reaction (1) in these solvents is almost one third higher acetylacetonate, since the rate of its binding with the N-
as compared with that in Py and PrOH-1 (Table 1).
substituted porphyrin decreases as compared to the reac-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 8 2004

576
BEREZIN, TOLDINA
tion rate in the case of acetate with the increasing size of has common explanation. Reaction (1) can follow the
N-substituent [10].
mechanism other than the classic mechanism described
for the coordinating solvents [14].
At the same time, the decrease in the rate of porphy-
rin binding when going from metal acetate to acetylac-
etonate is an exception, but not a rule. For instance,
almost planar classic tetraphenylporphine (ç2TPP)
reacts with Cu(Acac)2 in DMF with the true rate constant
kv298 = 4.83 0.01 l/(mol s) at cH P = 5.04 × 10–5 mol/l and
csalt = 5.44 × 10–4 mol/l [17], while the reaction with
CuAc2 occurs at substantially lower rate (k2v98 = 0.096
In particular, in benzene, the salt molecule does not
undergo additional solvation, i.e., the solvate sheath is
absent and no energy is required for its destruction in
the transition state of reaction (1). On the other hand,
the molecules of nonpolar benzene are not capable of
polarizing the N–H bonds of ç2ê in transition state.
The reaction of porphyrin and salt in benzene is likely
to occur not because of the preliminary dissociation of
2
the salt Zn(Acac)2
Zn(Acac)+ + Acac–, but due to
0.011 l/(mol s) at cH P = 5.00 × 10–5 mol/l and csalt
5.00 × 10–4 mol/l) [20].
=
2
contraction of its coordination sphere or due to dis-
placement of coordinates of the ligands in the complex
We believe that the higher rate of reaction (1) with composition [21]. In this case, the transition state of
acetylacetonate as compared to acetate and the higher reaction of the complex formation is realized without
rate of Zn(Acac)2 interaction with compound I in ben- the preliminary partial destruction of the solvate sheath
zene as compared to the coordinating solvents (Table 1) of a metal
CH3
CH3
CH
C
C
C
O
CH
C
O
H3C
H3C
N···H
N—H
N
N
O
O
Zn
O
O
Zn
O
+
N
N—H
N
N···H
O
CH3
H3C
C
C
C
HC
C
CH
CH3
CH3
Main state
Transition state
The elimination of one acetylacetonate ligand probably tively [18]), the former is considerably more reactive in
occurs beyond the peak of potential barrier of the reac-
tion and does not affect the energy of the limiting stage
of the process. The protons of NH group are likely to be
solvated by water that is always present in low concen-
trations in organic solvent [19] and are likely to migrate
to the solution from the reaction sphere simultaneously
with elimination of acetylacetonate ligands from the
coordination sphere.
The contribution of the contraction of the chelate
salt coordination sphere to the change in the kinetic
parameters of reaction (1) is also observed in the coor-
dinating solvents in which the reaction rate is higher
than that with a simple salt. Thus, reaction (1) for
H2TPTBP with M(Acac)2 occurs at the higher rate than
reactions with porphyrins [19]. This can indicate that
no partial dissociation of the å(ÄÒ‡Ò)2 chelate com-
plex takes place at the limiting state of the complex-
ation process.
According to the data in Table 2, the rate of
ç2íPTBP reaction with Zn acetate increases as com-
pared to ç2íBP. When planarity of molecule II is dis-
torted during its meso-phenyl substitution (dodeca-sub-
stitution), its reaction N4H2 site becomes more avail-
able for the reagent attack. Thus, compound I reacts
with Zn(Acac)2 in benzene almost instantaneously,
while the reaction with the rigid ç2íBP does not occur
for several hours at 318 K. At the same time, ç2íBP
with åAc2 in both propanol-1 and pyridine, but the dif- reacts with ZnAc2 in Py at the rate that is only 17 times
ferences in the rates of reactions in PrOH-1 (that is a
weaker ligand) are more significant (1.5–2 times as
high) as compared to the difference in Py (Table 1).
as low as that with dodeca-substituted ç2íPTBP [3].
The nonplanarity of compound I is not very high,
since it is a derivative of the highly aromatic compound
II, whereas octaethylporphine III (which is the less
rigid macrocycle) and its dodeca-substituted derivative
IV are distorted to considerably greater extent. As the
result, ç2éÖtíPP reacts with ZnAc2 in Py at the rate
The mechanism of the coordination sphere contrac-
tion is also confirmed by the fact that even though
Cu(Acac)2 is significantly more stable as compared to
298
Zn(Acac)2 (K
1.45 × 1015 and 1.05 × 109, respec-
st
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 8 2004

COMPLEXATION OF TETRAPHENYLTETRABENZOPORPHINE
577
that is almost 6 × 105 times as high as that with ç2éEtP
[22].
In order to estimate the effect of d metal on com-
ACKNOWLEDGMENTS
We are grateful to E. Kudrik (assistant professor,
Ivanovo State University of Chemical Technology) for
experimental performing of the synthesis and purifica-
tion of reagents and to Foundation for Assistance to
Domestic Science for financial support.
plexation of compound I, we studied the kinetics of its
reactions with Cu(II), Cd(II), Zn(II), and Co(II) ace-
tates in pyridine (Table 2). It was previously mentioned
that ç2íBP was more sensitive to the metal nature as
compared to the other porphyrins, which was explained
by its rigidity, i.e., by significantly less possibilities of
macrocyclic deformations in the course of coordination
[11]. Indeed, data from Table 2 show that when going
from a rigid compound II to the less planar compound
I, the dependence of the rates of complex formation on
the metal nature decreases. The ratio of
kv(CuAc2)/kv(CoAc2) for ç2íBP in Py is more than 20,
whereas for ç2íPTBP, it is only 12. Obviously, such
leveling of the coordination rates should be observed
also in the case of the other nonplanar porphyrins with
the coordination sites that are not screened spatially.
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2
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The analysis of the literature data [23] revealed that
the spectral criterion of stability is only applicable to
the series of metals that form analogous complexes
with the same ç2ê ligand, but it is not applicable to the
series of compounds with the same metal but with por-
phyrin ligands having different structures. Thus, the
spectral criterion of stability is valid in the series of
metal complexes with ç2íBP, where hypsochromic
shift of I-band increases (∆λI, nm): Cd (–31) ≈ Mg
(−26) < Zn (–34) < Co (–35) < Cu (–38) < Pd (–55)
[11], but does not hold when going from the planar aro-
matic H2íBP, which forms more stable complexes, to
its dodeca-substituted analog ç2íPTBP. In line with a
decrease in stability of complexes formed by com-
pound I as compared to compound II [23], the values of
∆λI should also be lower for MTPTBP; however, the sit-
uation is reverse in reality. In Py, ZnTBP gives ∆λI =
−34 nm, while ZnTPTBP shows the characteristic shift
∆λI = –41 nm. One of the reasons for the lower ∆λI val-
ues in the planar, as compared to the distorted porphy-
rins, consists in that in the latter case, complexation is
accompanied by substantial conformation rearrange-
ments of a ligand, which is also manifested in the elec-
tronic absorption spectra and sometime, even exceeds
the contribution of the chemical factors (the electronic
effect of coordination [14]).
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