Coordination of N-substituted porphyrins with simple and chelate zinc salts in DMSO

Article abstract of DOI:10.1023/A:1015565132623

The effect of peripheral substitution in the porphyrin macrocycle (H ₂OEtP, H₂TPP) of the nature of the N-substituent (X = Me, Ph) and of the anion in the salt solvate (A = Ac^-, NO ₃^-, Acac^-) on the complexation of the N-substituted porphyrins with zinc salts in DMSO is studied. The positions of the bands in the electronic absorption spectrum of the (A)M(N-X)P metal complex were found to depend only slightly on the nature of the A anion in the composition of the coordination sphere of the complex. An inhibition of coordination of the N-substituted H₂TPP derivatives by zinc nitrate was discovered that is not typical of porphyrins proper. The type of hybridization of the substituted nitrogen atom in the porphyrin molecule is discussed. A mixed type of hybridization is suggested: the sp²- and p³-reforms in the ligands and an almost complete transition to the p³-form in the N-substituted metal complexes.

Full text of DOI:10.1023/A:1015565132623

Russian Journal of Coordination Chemistry, Vol. 28, No. 5, 2002, pp. 325–332. Translated from Koordinatsionnaya Khimiya, Vol. 28, No. 5, 2002, pp. 348–355.
Original Russian Text Copyright © 2002 by Berezin, Zharnikova, Andrianov, Shatunov, Semeikin.
Coordination of N-Substituted Porphyrins with Simple
and Chelate Zinc Salts in DMSO
D. B. Berezin, N. V. Zharnikova, V. G. Andrianov, P. A. Shatunov, and A. S. Semeikin
Ivanovo State Academy of Chemical Technology, Ivanovo, Russia
Received February 22, 2001
Abstract—The effect of peripheral substitution in the porphyrin macrocycle (ç2OEtP, H2TPP) of the nature of
the N-substituent (X = Me, Ph) and of the anion in the salt solvate (A = ÄÒ–, NO3– , ÄÒ‡Ò–) on the complexation
of the N-substituted porphyrins with zinc salts in DMSO is studied. The positions of the bands in the electronic
absorption spectrum of the (A)M(N-X)P metal complex were found to depend only slightly on the nature of the
A anion in the composition of the coordination sphere of the complex. An inhibition of coordination of the N-
substituted ç2TPP derivatives by zinc nitrate was discovered that is not typical of porphyrins proper. The type
of hybridization of the substituted nitrogen atom in the porphyrin molecule is discussed. A mixed type of
hybridization is suggested: the sp2- and p3-forms in the ligands and an almost complete transition to the p3-form
in the N-substituted metal complexes.
As distinct from the porphyrins proper (I, II), those reduced electron density in the macrocycle (bacterio-
in which the proton is replaced by the hydrocarbon rad- chlorins) do not undergo transannular N-substitution
ical in the coordination site N4H2 (H(N-X)P, III–V) [1] [9, 13]. However, the transannular N-substitution in the
exhibit biological activity in the processes of reversible coordination site of the H2P molecule is already accom-
inhibition of the ferrochelatase enzyme [2], destruction panied by a change in both the electronic and steric
of the blood pigments [3], reversible migration of the N properties of the porphyrin molecule [1, 14].
substituent from nitrogen to metal, and the formation of
The molecular structure of the N-substituted por-
σ-metal complexes from porphyrins with the carbon–
phyrins has several specific features. First, the hydro-
metal bond [4]. Therefore, the range of the investigated
carbon fragment introduced directly into the molecular
porphyrins wherein the coordination site is modified by
center produces a strong polarizing effect on the por-
the N-substituents has been extended over the last two
phyrin macrocycle (the ±I- and/or ±C-effects). Second,
to three decades. Porphyrins with a different type (Alk,
the N-substitution is one of the characteristic types of
Bz, CH2=CH, Ph, R–Ph, N≡C) [1] and number (from 1
induced distortion of ç2P, since the introduction of the
to 4) of the N-substitutents [5, 6], as well as bridged
bulky N-substituent into a narrow macrocycle cavity
N,N'-compounds [1, 7], have been obtained.
results in a substantial violation of its planar structure
[6, 14, 15] and causes the electron density to be redis-
tributed a second time. Third, the molecular distortion
can lead to a change in the nitrogen atom hybridization
(from the sp2- to p3-state) [1, 16] and to a local increase
in the electron density in the N-substituted pyrrole ring.
As a result of rehybridization, the substituted nitrogen
atom acquires an electron pair, which is not involved in
the conjugation, and amine properties.
R'
R
R
R
R'
R
R
R'
R
I: R = Et; R' = X = H
NX
N
II: R = H; R' = Ph; X = H
III: R = Et; R' = H; X = Me
IV: R = H; R' = Ph; X = Me
V: R = H; R' = X = Ph
N
HN
The authors of [1, 16] suggested partial rehybridiza-
tion of the substituted nitrogen atom in the coordination
site of the molecule H(N-X)P; however, the mechanism
of this process remains unclear. The aforementioned
nitrogen atom in N-substituted porphyrins can exist,
most likely, in two electronic states, namely, in the
sp2-and p3-states. Obviously, the ratio of these struc-
tures will depend on the nature of H2P. The content of
R
R
R'
(H2P)
In recent years, porphyrins with a doubly modified
structure, for example, strongly distorted N-dodecasub-
stituted porphyrins [6], N-substituted corrols [8], chlo-
rins [9], and phlorins [10], N-substituted porphyrins
with an inverted coordination site [11], and N-substi-
tuted monoazaporphyrins [12], have been synthesized
and studied. It has been shown that the rigid macrocy-
the rehybridized p3-form participating in the chemi-
cal coordination is likely to increase as the ç2P mole-
cles (tetraazaporphyrins) or the macrocycles with a cule and the salt solvate approach each other, and this
1070-3284/02/2805-0325$27.00 © 2002 åÄIä “Nauka /Interperiodica”

326
BEREZIN et al.
R
R
N
N
N
NR
N
N
N
N
N
M
N
HN
HN
X
(c)
(a)
(b)
2
3
Fig 1. The electronic structures of (a) the sp - and (b, c) p -forms of (a, b) the N-substituted porphyrins H(N-X)P and (c) their com-
plexes (A)M(N-X)P (᭹ are the p electrons in the composition of the main conjugation contour,
are the p electrons outside the
main conjugation contour).
form prevails in the N-substituted metal complex where with the metal salt solvate occurs as a synchronous
the weak pseudocoordination (X)-N M bond can donor–acceptor process through the formation of a
form only due to a free electron pair of the substituted bimolecular transition state,
nitrogen atom. In nonsubstituted porphyrins, this elec-
H(N-X)P + MA2(Solv)n – 2
tron pair forms the main conjugation contour of the
≠
macrocycle (the sp2-state) (Fig. 1).
CH3
Solv
M
A
N
The above factors determine all physicochemical
properties of the nonplanar N-substituted porphyrins in
comparison with the mainly planar non-N-substituted
H2P (I and II), including their complexing properties.
For instance, in the complexation with the N-substi-
tuted molecule, only one proton is removed from the
H(N-X)P coordination sphere rather than two protons,
as with common ç2P. In this case, the åï2(Solv)n – 2
salt solvate is attacked in the course of reaction only on
one side of the macrocycle plane, while the metal ion
charge in the complex is compensated due to the addi-
tional ligand A, namely, the salt anion:
N
–Solv
N
A
N
Solv
H
Solv
(A)M(N-X)P + HA,
(1)
with participation of the tertiary nitrogen atoms and a
molecule of the partially desolvated salt.
The other approach to the mechanism of the process
under study [23] does not take into consideration the
bulky solvate sphere of the metal ion but suggests that
the event of the chemical interaction involves two metal
ions that attack the H2P molecule on both sides of the
macrocycle. It is clear that realization of this mecha-
nism is impossible, since the coordination site of the
macrocycle is shielded on one side by the N-substitu-
ent. It thus follows that, in the case of the second mech-
anism, the coordination of the N-substituted porphyrins
should be slowed down rather than accelerated.
A
M
N
N
N
N
Although there is a great number of publications
devoted to the chemistry of the N-substituted porphy-
rins, this field lacks systematic physicochemical stud-
ies. For example, data on the effect of the nature of the
metal on the complexing properties are available only
C
H
H
H
The fact that complexation with the N-substituted for H(N-Me)TPP, but even they can be compared very
porphyrins occurs at a significantly higher rate than that infrequently due to the experimental conditions being
with the nonsubstituted H2P [19] is, in our opinion, different [16, 19, 24]. On the other hand, the influence
more evidence that the general approach to the reaction of the composition and properties of the complex sol-
mechanism developed previously in [20–22] is correct. vate (the nature of the solvent and the salt anion), as
According to this mechanism, the H2P coordination well as the effect of the porphyrin structure (the nature
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 5 2002

COORDINATION OF N-SUBSTITUTED PORPHYRINS
327
Table 1. The rate constants of complexation of the planar
of the N- and peripheral substitution) in comparable
conditions were not almost studied. Therefore, we
decided to systematically study the effect of some of
the above factors on the kinetics of the complex forma-
tion and the spectral properties of the N-substituted por-
phyrins and their metal complexes.
porphyrins with ZnAc2 and Zn(NO3)2 in DMSO at 358 K
( cH P = 2 × 10–5 mol/l; cZnA = 2 × 10–3 mol/l)
2
2
System
kv, s–1 mol–1 l*
H2OEtP + Zn(NO3)2
H2OEtP + ZnAc2
H2TPP + Zn(NO3)2
H2TPP + ZnAc2
0.42
Very slowly
0.64
EXPERIMENTAL
We studied the kinetics of complexation of the N-
methyltetraphenylporphine (H(N-Me)TPP, IV), N-phe-
nyltetraphenylporphine (H(N-Ph)TPP, V), and N-meth-
yloctaethylporphine (H(N-Me)OEtP, III) with zinc
acetate (ZnAc2), nitrate (Zn(NO3)2), and acetylaceto-
nate (Zn(Acac)2) in DMSO in the temperature interval
298–328 K. In order to compare the reactivity, the reac-
tion of complexation with tetraphenylporphine (H2TPP,
II) and octaethylporphine (H2OEtP, I) (358 K) was also
studied.
Accurately measured volumes of the solutions (por-
phyrin and zinc salt in DMSO) with known concentra-
tion preliminarily thermostatically controlled for at
least 15 min were poured into a cell. The reaction mix-
ture was thoroughly stirred, and its optical density (Di)
was measured at equal intervals of time (τ).
0.11
* The error in determining k does not exceed 10%.
v
RESULTS AND DISCUSSION
Effect of N-substitution and of the nature of the
N-substituent. The reactions of complexation with
mainly planar porphyrins H2OEtP and H2TPP occur in
DMSO very slowly as compared to other organic sol-
vents, such as EtOH, HAc, CHCl3, etc [27]. Therefore,
the rates of their coordination by Zn(Ac)2 and
Zn(Acac)2 were only measured at 358 K (Table 1). The
low rate of the process in DMSO is explained by the
high stability of the solvate shell of the salt molecule in
this solvent [28].
The process of complex formation was monitored
spectrophotometrically (using Hitachi U-2000 and
Specord M40 spectrophotometers) at the wave length
corresponding to the maximum of the first absorption
band of the Zn complex (575, 610, and 680 nm for III,
IV, and V, respectively).
The introduction of the N-substituent into the por-
phyrin molecule sharply increases the rate of complex
formation (almost by five orders in DMSO) [19]. One
of the reasons for this phenomenon lies in violation of
the macrocyclic effect, namely, of its structural compo-
nent [14] during the N-substitution, which makes the
transannular reaction centers (RC) of the porphyrin
molecule more accessible for the RC of the reagents.
With significant salt excess (cZnA = 2 × 10–3, cH P
=
2
2
4 × 10–5 mol/l), the reaction was always the first-order
reaction in porphyrin (linear dependence ( c0H P / cH P ) =
X-ray diffraction data [1, 15, 16] point to a substan-
tial distortion of the N-substituted porphyrins in the
crystal, which appears, first of all, as a change in the
bond length and angles of the substituted pyrrole ring
and its deviation from the mean plane of the nonsubsti-
tuted macrocycle (28° for H(N-Me)T(4-Br)PP [15] and
29° for H(N-CN)TPP) [1]. The N-susbtituent also
extends from the plane of the substituted pyrrole ring
and makes an angle of 120° and 150° with it for H(N-
Me)T(4-Br)PP and H(N-CN)TPP, respectively.
2
2
f(τ)), which indicates that the general stoichiometric
mechanism of the reaction of complex formation sug-
gested for H2P in [20] (reaction (1)) can also be applied
to the N-substituted porphyrins. In connection with
this, the salt concentration in the reaction course was
regarded to be unchanged and the effective constant
(keff) was calculated as
0
H2P
c
D∞ – D0
-- -------------------
1
--
τ
1
τ
Distortion of the planar structure of the molecule is
likely to also occur in the complexes with N-substituted
porhyrins. According to X-ray diffraction data [29], the
(Cl)Zn(N-Me)TPP complex has the shape of a distorted
pyramid. Zinc in the complex forms four bonds with
the nitrogen atoms and one bond with the Cl– anion, the
bond between zinc atom and the methylated nitrogen
keff
=
ln-------- = ln
.
(2)
D∞ – Dτ
cMP
The values of the true rate constant of complex for-
mation (kv) were found from the formula
kv = keff/cZnA
.
(3)
2
Porphyrins were prepared according to procedures atom being the least stable (2.530 Å) and that between
described in [1, 12, 25, 26]. The reagent grade DMSO the nitrogen atom in the trans-position with respect to
was purified from impurities and water by vacuum dis- the methylated nitrogen atom and Zn being the most
tillation, but it was kept previously over BaO for a day. stable (2.018 Å). This fact indicates that the hybridiza-
The reagent grade zinc acetate and nitrate were addi- tion of the N-substituted nitrogen atom of the complex
tionally purified by recrystallization. Zinc acetylaceto- changes to p3. A similar structure is also typical of other
nate (reagent grade) was recrystallized from benzene.
N-substituted metalloporphyrins [29].
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 5 2002

328
BEREZIN et al.
Drel
3.0
Drel
a
b
d
0.30
c
b
2.5
2.0
1.5
1.0
0.5
0.25
0.20
0.15
0.10
0.05
a
c
d
0
380
430
470
520
570
620
670
λ, nm
Fig. 2. The electronic absorption spectra of (a) H TPP, (b) ZnTPP, (c) H(N-Me)TPP, and (d) (Ac)Zn(N-Me)TPP in DMSO.
2
Other evidence of rehybridization is the change in
As compared with porphyrins proper, the activation
the chromophore properties of these compounds. The energy in the reaction of complex formation with the N-
general pattern of the electronic absorption spectrum of substituted H2P decreases almost by half [31]. The acti-
porphyrin remains almost unchanged in the course of vation entropy also decreases, which suggests an
the N-substitution (four bands in the visible region and increase in the solvation of the transition state [20].
the Soret band (see Fig. 2, spectra a, c)), whereas in the During complex formation, the H(N-X)P reactivity
visible region of the spectrum of the N-substituted increases still more (by 3–35 times, depending on the
metal complex, one can observe three more diffuse nature of the salt) when going from porphyrins with
bands with lower intensity (see Fig. 2, spectrum d) X = Me to derivatives with X = Ph (Table 3) due to both
instead of the two-band pattern typical of MP (symmetry the higher distortion of the macrocycle (the bulky sub-
D4h, Fig. 2, spectrum b). This can be explained by the stituent) and the electron-acceptor properties of the N-
reduction in the symmetry of the coordinated porphyrin Ph group.
and by the change in conjugation contour in the mole-
According to X-ray diffraction data, H(N-Ph)TPP
cule. The change in the contour consists in the greater
(V) is only slightly more nonplanar than H(N-Me)TPP
involvement of the π-electrons of the β-carbon atoms of
(IV). For example, the N-substituted pyrrole ring devi-
the substituted pyrrole ring into the main chromophore
ates from the mean plane of the macrocycle by 42° and
(see Fig. 1).
38.5° for V and IV, respectively. The effect of the
peripheral substituents (R and R') in the molecule on its
The bathochromic shift of bands in the electronic
distortion in the crystal was found to be also negligible
absorption spectra of the metal complexes with N-sub-
[33, 34].
stituted porphyrins, as compared with their positions in
The above data allow one to suggest that the sub-
stantial differences in the reactivity of H2P with differ-
ent peripheral and N-substituents are due to electronic
factors rather than structural factors. For instance, the
reduction of the N-methylporphyrins always affords a
respective chlorin [9], whereas the reduction of the N-
phenyl derivative (with the electron density in the
meso-positions reduced owing to the two above-men-
tioned reasons) results in the destruction of the conju-
gation contour and the formation of an unusually stable
phlorin structure [10]. Therefore, the electronic factor
is the most important in the change of the properties of
the N-substituted porphyrins. As was previously shown
the spectra of the respective complexes with nonsubsti-
tuted H2P, also indicates that the metal complexes
AM(N-X)P also contain a distorted macrocycle [6, 14].
Thus, in the electronic absorption spectrum of ZnTPP
in DMSO, the first absorption band is observed at
599 nm, while in the spectrum of (Ac)Zn(N-Me)TPP, it
lies at 653 nm (Table 2). In this case, ∆λI = λHI P – λMI P
2
for the first complex is equal to –46 nm, while for the
second complex, it is equal to –22 nm. The more signif-
icant shift of the first band in the electronic absorption
spectrum of the Zn complexes with nondistorted por-
phyrins (H2OEtP, H2TPP) (the spectral criterion of the
complex stability [20]) agrees with their higher kinetic for other nonplanar porphyrins (in particularly, for
stabilities in proton–donor media as compared with the dodeca-substituted porphyrins) [14], the transfer of the
corresponding complexes of the N-substituted porphy- electronic effects does not weaken upon distortion,
rins [30].
while the distortion of the porphyrin molecule even
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 5 2002

COORDINATION OF N-SUBSTITUTED PORPHYRINS
329
Table 2. The electronic absorption spectra of porphyrins and their metal complexes in DMSO
λ, nm (Drel)
Porphyrin
(metalloporphyrin)
∆λI, nm
Soret
IV
III
II
I
H2OEtP
394.5(21.2)
495.5(2.1)
528.5(1.6)
564.5(1.3)
537.5(1.0)
530.0(1.1)
558.0(1.0)
559.0(1.5)
558.5(1.3)
583.5(1.2)
618.0(1.0)
574.0(1.0)
572.5(1.0)
645.0(1.0)
599.0(1.0)
599.0(1.0)
640.5(1.0)
613.5(0.68)
616.5(1.0)
ZnOEtP (from Zn(NO3)2) 408.5(15.9)
–44.0
–45.5
ZnOEtP (from ZnAc2)
H2TPP
408.0(4.4)
416.0(35.4)
426.0(31.7)
426.0(13.7)
407.5(20.5)
513.0(2.0)
503.0(2.6)
548.0(1.2)
ZnTPP (from Zn(NO3)2)
ZnTPP (from ZnAc2)
H(N-Me)OEtP
–46.0
–46.0
533.5(1.4)
513.0(2.0)
(Ac)Zn(N-Me)OEtP
416.0(23.1)
424.0(20.0)
413.0(33.9)
412.5(30.5)
425.0(21.4)
438.0(30.9)
447.5(15.4)
457.0(13.6)
445.0(15.1)
455.0(13.0)
445.0(22.2)
454.0(19.0)
429.0(34.0)
438.0(16.1)
446.0(14.3)
435.0(22.5)
444.0(18.91)
434.5(26.6)
444.0(21.8)
581.0(2.5)
–24.0
(NO3)Zn(N-Me)OEtP
(Acac)Zn(N-Me)OEtP
533.5(2.2)
533.5(2.2)
582.0(2.8)
583.0(2.5)
615.0(1.0)
615.0(1.0)
–25.5
–25.5
H(N-Ph)TPP
557sh(1.4)
596.5(3.1)
567.5(1.0)
634sh(1.4)
628.0(1.3)
704.0(1.0)
681.5(1.0)
(Ac)Zn(N-Ph)TPP
–22.5
–26.0
–23.0
(NO3)Zn(N-Ph)TPP
(Acac)Zn(N-Ph)TPP
566.5(0.9)
567.0(0.9)
624.0(1.4)
627.5(1.4)
678.0(1.0)
681.0(1.0)
H(N-Me)TPP
531.5(1.5)
573.0(2.0)
558.5(1.2)
612.0(0.9)
607.5(1.3)
675.0(1.0)
653.0(1.0)
(Ac)Zn(N-Me)TPP
–22.0
–23.0
–20.5
(NO3)Zn(N-Me)TPP
(Acac)Zn(N-Me)TPP
558.0(1.1)
562.5(1.3)
606.5(1.5)
609.5(1.5)
652.0(1.0)
654.5(1.0)
favors its internal polarization in a number of cases that with H2OEtP (Table 1). Similarly, H(N-Me)TPP
reacts with ZnAc2 almost 10 times as fast as H(N-
Me)OEtP (Table 3). Eight ethyl groups in the β-posi-
tions of the H2P molecule produce the donor effect (+I)
on the macrocycle and depolarize the N–H bond, thus
hampering its dissociation in the transition state.
[35]. The type of transfer of the electronic effects is
monitored in this case by the degree of macrocycle dis-
tortion. Thus, as in the nonsubstituted H2P, the meso-
phenyl rings in the N-substituted H2P act as electron
density buffers and compensate its excess or shortage in
the macrocycle in the course of chemical conversions
(acid–basic equilibria [5]).
Although involved in its nature, the mechanism of
porphyrin complexation is a synchronous process
occurring in one stage; this is why the limiting stage
cannot be distinguished. However, it is known [20] that,
during the formation of the transition state, the maxi-
mum energy is required for the elimination of the NH
protons from the coordination site of the macrocyclic
ligand, especially, in electron-donor solvents (DMSO,
Py) [22].
Effect of substituents R and R' in the periphery
of the H2P molecule. The kinetic data in Tables 1 and
3 make it possible to conclude that the change in the
rate of complexation in DMSO for both planar and non-
planar porphyrins distorted by the N-substitution agrees
with the concepts of the electronic effects of the periph-
eral substituents in them, their transfer to the coordina-
tion site of the molecule, and with the concepts on the
mechanism of the process under study [20]. Thus, the
The meso-phenyl substitution produces the opposite
electronic effect, and, therefore, the rate of complex-
complexation with H2TPP in DMSO occurs faster than ation in this case increases. The kinetic (Table 3) and
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 5 2002

330
BEREZIN et al.
Table 3. Kinetic parameters of complexation of the N-substituted porphyrins in DMSO (cporph = 4 × 10–5 mol/l; cZnA
2 × 10–3 mol/l)
=
2
kv , l s–1 mol–1
*
Porphyrin
Salt
ZnAc2
λexp, nm
T, K
Ea, kJ mol–1
∆S#, J mol–1 K–1
H(N-Me)TPP
610
298
308
318
298
308
318
298
308
318
328
293
298
303
298
308
318
298
308
318
328
298
308
318
298
308
318
298
318
328
338
6.14
10.90
14.59
0.14
33.8
–125.0
Zn(NO3)2
Zn(Acac)2
63.6
48.8
–56.0
0.30
0.72
0.20**
0.38
–103.0
0.64
1.20
H(N-Ph)TPP
ZnAc2
680
20.18
29.67
33.00
5.25
34.2
41.0
56.0
–110.0
–102.0
–69.0
Zn(NO3)2
Zn(Acac)2
8.89
14.88
0.65**
1.26
2.68
4.78
H(N-Me)OEtP
ZnAc2
565
0.55
62.4
57.9
64.3
–48.0
–51.0
–56.0
1.29
2.74
Zn(NO3)2
Zn(Acac)2
2.69
5.29
15.27
0.11**
0.55
1.00
2.29
* The error in determining k does not exceed 10%.
v
** Calculated from the Arrhenius equation.
calorimetric [18] data show that the electronic effects of transition state which, in the case of H(N-Me)OEtP, is
less polarized than in the case of H(N-Me)TPP.
the peripheral substituents are also effectively trans-
ferred to the reaction center in the course of the porphy-
rin molecule distortion by the N-substituent.
The effect of the salt anion A. When the NO3–
anion is used in complexation with nondistorted por-
phyrins (H2TPP, H2OEtP), the reaction proceeds at a
higher rate than in the case of A = Ac [20, 36]. This is
explained by the structure of the complex solvate
involved in reaction (1) and by the strength of the M–A
and M–Solv bonds in this complex [28]. The Zn–Ac
The polarization and depolarization of the N–H
bond by the peripheral substituents are also manifested
in the kinetic parameters of the complexation process.
The complexation with H(N-Me)OEtP requires a
higher energy (62 kJ/mol for ZnAc2) than that with
H(N-Me)TPP (34 kJ/mol for ZnAc2). The high activa- bond is known to be more covalent than the Zn–NO3
tion energy is specified by the lower solvation of the bond, as a result of which more reactive solvates, i.e.,
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 5 2002

COORDINATION OF N-SUBSTITUTED PORPHYRINS
331
[Zn(DMSO)5Ac]+Ac– or [Zn(DMSO)6]2+Ac2– , cannot
form in the DMSO solution. On the contrary, since the
Zn–NO3 bond is weaker, Zn(NO3)2 more readily forms
coordination sphere of the N-substituted metal com-
plex.
ACKNOWLEDGMENTS
outer-sphere
complex
solvates
such
as
[Zn(DMSO)6]2+(NO3)–2 [28].
This work was supported by the RF Ministry of
Education (project no. E00-5.0-129).
We have discovered that the nitrate anion inhibits
the reaction of complex formation. For instance, the
rate of the reaction of H(N-Me)TPP (IV) with
Zn(NO3)2 is 38 times, while that of H(N-Ph)TPP (V)
with Zn(NO3)2 is 5.5 times, as low as that with ZnAc2.
At the same time, the complexation with H(N-
Me)OEtP (III) exhibits no abnormalities and the rate of
reaction with Zn(NO3)2 is 5 times as high as that with
ZnAc2. One should note that reaction (1) was always
conducted under the same conditions; i.e., the structure
of the Zn(NO3)2 complex solvate remained unchanged.
This fact indicates that the mechanism of complexation
of the N-substituted porphyrins strongly depends not
only on the nature of the metal ion, salt anions, the
structure of the complex sovate, and on the nature of the
solvent, but also on the nature of the substituents on the
periphery of the molecule (the sensitivity of the salt sol-
vate to the nature of H2P). In addition, the types of dis-
tortion of molecules III and IV (their most stable con-
formations) are likely to differ significantly, while the
reaction centers of the Zn(NO3)2 complex solvate read-
just to the reaction centers of porphyrin more easily in
the case of III than in the case of IV.
When a simple Zn2+ salt is replaced by a chelate salt,
for example, Zn(Acac)2, the mechanism of complex-
ation becomes more involved [31] and, hence, the pro-
cess slows down. The reaction of complexation of
Zn(Acac)2 or other chelate salts (dithizonate, diphenyl-
carbazonate) with the planar porphyrins in DMSO does
not almost occur [31]. All of the three investigated N-
substituted porphyrins (III–V) react with Zn(Acac)2 at
rates which are 5–30 times lower than those with ZnAc2
and 8–25 times lower than with Zn(NO3)2 except for
(III).
The authors are grateful to P.A. Stuzhin (the associ-
ate professor of the Organic Chemistry Chair in the
Ivanovo State Academy of Chemical Technology) for
the provision of instrument time (Hitachi U-2000).
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