Effect of octabromo substitution on the coordination properties of manganese(III) octaphenyltetraazaporphyrin

Article abstract of DOI:10.1007/s11176-005-0355-2

Comparative studies of the kinetics of formation of manganese(III) complexes with octaphenyltetraazaporphyrin and its octabromo derivative, of their dissociation and axial ligand exchange were carried out. The effect of octabromo substitution on the coordination properties of the compounds is determined by the -I effect of bromine atoms. The different effects on the kinetic parameters of the reactions [increase in the rate and significant decrease in the activation energy upon complex formation, slowing down of dissociation, and acceleration of acido ligand exchange with nitrido(tetraphenyporphyrin)manganese(V) are caused by the involvement of different reaction centers in the limiting stages (different mechanisms of the processes).

Full text of DOI:10.1007/s11176-005-0355-2

Russian Journal of General Chemistry, Vol. 75, No. 6, 2005, pp. 975 979. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005,
pp. 1031 1035.
Original Russian Text Copyright
2005 by Klyueva, Repina, Chizhova.
Effect of Octabromo Substitution
on the Coordination Properties
of Manganese(III) Octaphenyltetraazaporphyrin
,
M. E. Klyueva* **, N. V. Repina***, and N. V. Chizhova**
* Ivanovo State University of Chemical Technology, Ivanovo, Russia
** Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
*** Ivanovo State University, Ivanovo, Russia
Received February 11, 2004
Abstract Comparative studies of the kinetics of formation of manganese(III) complexes with octaphenyl-
tetraazaporphyrin and its octabromo derivative, of their dissociation and axial ligand exchange were carried
out. The effect of octabromo substitution on the coordination properties of the compounds is determined by
the I effect of bromine atoms. The different effects on the kinetic parameters of the reactions [increase in the
rate and significant decrease in the activation energy upon complex formation, slowing down of dissociation,
and acceleration of acido ligand exchange with nitrido(tetraphenyporphyrin)manganese(V) are caused by
the involvement of different reaction centers in the limiting stages (different mechanisms of the processes).
Revealing the dependence of the reactivity of com-
plex compounds on the ligand structure is one of the
major problems of coordination chemistry [1]. In the
present work we have carried out a comparative study
of the kinetics of formation and dissociation of
manganese(III) complexes with octaphenyltetraaza-
porphyrin and its octabromo derivative, as well as the
reactions of these complexes with nitrido(tetrapheny-
porphyrin)manganese(V), resulting in axial ligand
exchange. To determine most probable mechanisms of
the processes, we measured reaction rates as functions
of temperature and initial concentrations of reagents,
which allowed full kinetic equations to be found.
allows us to record a series of spectral curves relating
to different degrees of transformation of the starting
compounds and to calculate the rate of formation of
complex III or IV from the change in optical density
near strong maxima in the electronic absorption spec-
trum. The presence of well-defined isosbestic points
in the spectra of the reaction mixture in the course of
the reaction suggests that the system contains only
two colored compounds at each time moment, i.e., as
is the case with complexes with classical porphyrins
[3], manganese(II) incorporated in a macrocyclic
The essential distinctions in the positions of band
in the electronic absorption spectra of octaphenyltetra-
azaporphyrin and their complexes with manganese in
various oxidation states (Figs. 1 3) make possible
studying reactions of such compounds in solutions by
the spectrophotometric technique. The reactions of
octaphenyltetraazaporphyrin (I) or octa(bromo)phenyl-
tetraazaporphyrin (II) with manganese(II) acetate or
chloride in DMF give rise to corresponding manga-
nese(III) complexes. The spectrum of tetraazapor-
phyrin I or II, that contains two strong long-wave
bands, passes into a spectrum with one strong band in
the region of 670 nm and additional absorption bands
at 410 and 470 nm (Fig. 1), which is typical of man-
ganese(III) octaphenyltetraazaporphyrins [2]. The
reaction with Mn(OAc)2 proceeds very fast on pouring
the solutions together, whereas the use of MnCl2
2
A
1
400
500
600
700 , nm
Fig. 1. Electronic absorption spectra of compounds:
(1) I and (2) III in DMF.
1070-3632/05/7506-0975 2005 Pleiades Publishing, Inc.

976
KLYUEVA et al.
porphyrin I or II, , time, and kap, apparent rate
constant of manganese complex formation.
R
A
R
R
Cl
N
R
R
Mn
N
N
N
N
N
N
N
R
400
600
800 , nm
R
Fig. 2. Change in the electronic absorption spectrum of
compound II in 18.1 M H SO at 318 K.
R
2
4
III, IV
R = H (III), Br (IV).
The apparent rate constants of the formation of
complex III by reaction (1) in DMF are (1.4 0.1)
A
3
3
3
10 , (4.4 0.4) 10 , and (10.9 0.4) 10 at 298,
303, and 308 K, respectively. At 288 K the reaction
proceeds very slowly, and at 318 K it occurs fast,
immediately after pouring the solutions together. The
1
apparent activation energy E is 156 15 kJ mol , and
1
the activation entropy
S
is 217 51 J mol 1 K .
With complex IV, the apparent rate constant is (0.90
3
3
3
0.07) 10 , (1.3 0.1) 10 , and (1.85 0.2 10 at
1
288, 293, and 298 K, respectively, E 54 4 kJ mol ,
1
and
S
123 13 J mol 1 K . The apparent rate
constants of formation of complexes III and IV at
298 K are close to each other, but the activation
parameters of the reactions are much different. Kinetic
equation (2) agrees with the monomolecular me-
chanism of coordination of transition metal cations
by octaphenyltetraazaporphyrins, offered in [4], ac-
cording to which formation of a covalent complex is
preceded by fast coordination of a metal salt on one of
meso-nitrogen atoms of azaporphyrin to form an
amine complex. In the limiting stage, the amine
complex transforms into the metal azaporphyrin. The
formation of the amine complex is favored by the
high electron-donor power of octaphenyltetraazapor-
phyrin and by stabilization of the amine complex as a
result of interaction of the coordinated solvate with
neighboring phenyl rings. The complex formation in
this case depends only slightly on the nature of the
salt cation, as compared with the formation of com-
plexes of classical porphyrins [4, 5]. The anionic
composition of the salt has a much stronger effect. For
example, the reaction of porphyrin I with man-
ganese(II) acetate in DMF proceeds immediately on
pouring the solutions together, whereas the rate of
manganese(II) chloride reaction can be measured by
400
500
600
700 , nm
Fig. 3. Change in the electronic absorption spectra
during reactions of compounds IV (6.56 10 M) and
V (6.32 10 M) in chloroform at 298 K.
6
6
complex is immediately oxidized by air oxygen to
manganese(III) [scheme (1)].
H2P + MnCl2
Here H2P is compound I or II.
(Cl)MnIIIP.
(1)
The rate of formation of complexes III and IV in
the reactions of compounds I and II with MnCl2 in
DMF under pseudo-first order conditions in aza-
porphyrin is independent of salt concentration. The
kinetic equation takes form (2).
tr
dcH P/d = k
c
.
(2)
app H2P
2
Here cH P is the concentration of octaphenyltetraaza-
2
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EFFECT OF OCTABROMO SUBSTITUTION
977
spectrophotometry. The results of the present work
point show that the formation of manganese(III)
complexes is strongly affected by functional substitu-
tion in the octaphenyltetraazaporphyrin ligand. The
formation of complex III is characterized by a very
high activation energy. Octabromo substitution in the
macrocyclic ligand reduces the activation energy
almost three times, probably, owing to a weaker
stabilization of the amine complex. The latter effect
is associated with weakened electron-donor power of
meso-nitrogen atoms in porphyrin II as compared to
porphyrin I and with facilitated ionization of hydro-
gen atoms in the coordination center of compound II
( I effect of bromine atoms). It is probable that the
+C effect of bromine atoms, that also would be ex-
pected, does not appear, since phenyl rings deviate
from the mean macroring plane, as follows from the
MM+-optimized geometry of molecules I IV, where
the angles between the phenyl and macroring planes
are 35 .
The bromine atoms in phenyl rings of complex IV
show the I effect. The +C effect of brome on Mn N
bonds is prevented by a significant torsion of phenyl
rings relative to the macroring plane. Such electronic
effects should render the Mn N bonds in complex IV
weaker compared to complex III. At the same time,
owing to the mutual ligand effect, strengthening of
the axial Mn Cl bond in complex IV would be ex-
pected. The strengthening of the bond between the
manganese atom and the axial chlorine atom as a
result of the octabromo substitution under considera-
tion is attested by changes in the positions of absorp-
tion maxima of complex IV compared to complex III:
the bathochromic shift of the long-wave band from
665 to 668 nm and the opposite shift of the band in
the near UV region (from 413 to 411 nm) [7]. A multi-
stage mechanism of dissociation of manganese(III)
and manganese(V) complexes with octaphenyltetra-
azaporphyrin (I) in concentrated sulfuric acid has been
proposed in [2]. According to this mechanism, the
overall reaction rate is determined by the stage of
dissociation of the acido complex into the inorganic
Complexes of azaporphyrins with d metals are
stable in solutions and decompose only under the
action of acids [4]. In acidic media, meso-nitrogen
atoms can interact with proton-donor species [2, 6].
As found by spectrophotometry [2], in strongly acidic
solutions, complex III is protonated by two meso-
nitrogen atoms and dissociates at a measurable rate in
concentrated sulfuric acid at room temperature. Dis-
sociation of the complex is accompanied by destruc-
tion of the macrocycle. The electronic absorption
spectrum of bromine derivative IV in concentrated
sulfuric acid at 306 328 K changes similarly to the
spectrum of its bromine-free analog III (Fig. 2). The
dissociation rate of complexes III and IV in 17.4
18.6 M H2SO4 is independent of acid concentration.
The kinetic equation takes form (3).
n
anion X and the octaphenyltetraazaporphyrinman-
ganese cation. After the slow acido ligand dissociation
stage fast dissociation of the complex by Mn N bonds
and destruction of the macroring occur. The enhanced
stability of bromine-substituted complex IV than un-
substituted complex III provides evidence for this
mechanism.
Reactions of axial coordination of metal porphyrins
are of special importance and in many respects
determine properties of macrocyclic complexes [8].
Manganese porphyrins have a unique property:
manganese(II) and manganese(III) complexes can
enter reactions with nitridomanganese(V) porphyrins,
involving complete intermolecular transfer of the
nitride nitrogen atom uncomplicated by side reactions
[9 11]. The transfer of the nitrogen atom from nitrido-
(tetraphenylporphyrin)manganese(V) (V) to man-
ganese(III) octaphenyltetraazaporphyrins is irrever-
sible [12, 13], which is caused by stronger -acceptor
properties of the tetraazaporphyrin macroring com-
pared to the porphyrin ring. Changes in the electronic
absorption spectra in the course of the reaction of
complexes IV and V [reaction (4)] in chloroform are
shown in Fig. 3.
dis
dc(Cl)MnP/d = k c(Cl)MnP
.
(3)
ap
Here c(Cl)MnP is the concentration of complex III or
IV, , time, and kadpis, apparent dissociation rate of the
complex.
dis
According to [2], the average k
values for
ap
4
complex III are (1.14 0.05) 10 , (2.9
0.2)
4
4
1
10 , and (7.6 0.2) 10
s
at 288, 298, and 308 K,
respectively. The apparent activation energy E is 70
1
2 kJ mol , and the activation entropy S is 83.5
1
dis
ap
6 J mol 1 K . The k
values for complex IV are
(N)MnVTPP + (Cl)MnVP
(Cl)MnIIITPP. (4)
4
4
(3.1 0.2) 10 , (5.9 0.4) 10 , and (12.6 0.9)
10 at 308, 318, and 328 K. The k at 298 K extra-
4
dis
ap
dis
Here TPP is tetraphenylporphyrin and P, dianion of
octaphenyltetraazaporphyrin or of its octabromo
derivative.
polated from the dependence log k 1/T is (1.4
ap
4
1
1
0.1) 10 s ; E and
S
are 59 7 kJ mol and
128 22 J mol 1 K , respectively. Thus, octabromo
substitution slightly slows down destruction of
manganese(III) octaphenyltetraazaporphyrin.
1
The rate constants of reaction (4) determined by a
second-order kinetic equation (5) are much higher for
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 6 2005

978
KLYUEVA et al.
bromine-substituted complex IV than for complex III
at practically identical activation characteristics. Thus,
the rate constants k of the reaction of complexes V
and III in chloroform are 23 3, 44 5, and 70
of acido ligand exchange with complex V] are as-
sociated with different reaction centers. The I effect
of bromine atoms in phenyl rings attenuates the elec-
trondonor power of meso-nitrogen atoms in octa-
(bromophenyl)tetraazaporphyrin (II) compared to
octaphenyltetraazaporphyrin (I), strengthens the axial
Mn Cl bond, and enhances the electrophilic power of
the manganese(III) central atom in bromine derivative
IV compared to complex III. Research into the effect
of functional substitution in macrocyclic compounds
on their coordination properties provides an important
information on reaction mechanisms.
1
6 mol 1 l s at 288, 298, and 308 K, respectively, E
1
1
41 5 kJ mol , and S 83 18 J mol 1 K . For the
reaction of complexes V and IV at the same tempera-
1
tures, k 151 14, 233 24, and 438 37 mol 1 l s ,
1
1
E 40 9 kJ mol , and
The transfer of the nitride nitrogen atom from
complex V can proceed through formation of a
S
74 30 J mol 1 K .
-
nitrido-bridged intermediate in two ways [11, 12].
Kinetic studies showed that reactions between man-
ganese(III) and manganese(V) complexes of classical
porphyrins (tetraphenylporphyrin and octaphenylpor-
phyrin) begin with equilibrium Mn Cl dissociation to
form a cationic manganese(III) complex. Then the
highly electrophilic cationic complex reacts with the
nitridomanganese(V) complex at the limiting stage
[11]. An alternative possibility consists in nucleo-
philic attack of manganese(III) directly in acido com-
plex III or IV from the side opposite to the chloride
ligand. These mechanisms can be competing, and
their relative contribution should depend on reaction
conditions. Analysis of the kinetic parameters of the
process as a function of the structure of the reagents
allows the most probable reaction mechanism to be
determined. When reaction (4) follows the first me-
chanism, the strengthening of the axial Mn Cl bond
due to the I effect of eight bromine atoms in com-
plex IV should decelerate its reaction compared to
complex III. As the opposite effect is observed ex-
perimentally, namely, the reaction rate increases 5
6 times, we should accept that the second mechanism
is more probable. The enhanced electrophilicity of
manganese(III) in bromine-substituted complex IV
compared to complex III facilitates reaction with the
nucleophilic nitride nitrogen atom even in the un-
ionized form of the complex [Eq. (5)].
EXPERIMENTAL
The electronic absorption spectra of compounds
were recorded on Hitachi U-2000 and SF-26 spectro-
photometers.
Compound I was synthesized by the procedure in
[14]. Compound II was prepared by bromination of
octaphenyltetraazaporphyrin (I) with molecular
bromine in trifluoroacetic acid at 293 298 K within
8 days [15]. Complex III was obtained by the reaction
of compound I with MnCl2 4H2O in boiling DMF.
After cooling, the reaction mixture was poured into
water, the precipitate was filtered off, dried, and twice
purified by column chromatography on alumina,
eluent chloroform. Complexes IV and V were syn-
thesized by the procedures in [16] and [12], respec-
tively. The electronic absorption spectra of com-
pounds I V were consistent with published data [12,
14 16].
Manganese(II) acetate was recrystallized from
acetic acid and dehydrated by boiling with acetic
anhydride within 3 h. Manganese(II) chloride was
dried at 493 K to constant weight. DMF was purified
by distillation with benzene according to the proce-
dure in [17].
The concentration of H2SO4 was determined by
titration with an error of less than 0.15%. Solutions
of complex IV in sulfuric acid were filtered through
a Schott filter no. 40 before kinetic measurements.
Chloroform (medical grade) was dried with CaCl2 and
distilled with a Vigreux column directly before use.
V
III
dc(N)Mn TPP/d
=
dc(Cl)Mn P/d
V
III
= kc(N)Mn TPP (Cl)Mn
c
.
(5)
P
V
Here c(N)Mn TPP is the concentration of complex V,
c(Cl)Mn , concentration of complex III or IV, and
k, rate constant of reaction (4).
III
P
Thus, the effect of octabromo substitution in
phenyl rings on the coordination properties of octa-
phenyltetraazaporphyrin and its complex with man-
ganese is determined by the I effect of bromine
atoms. The different effects on the kinetic parameters
of the reactions [increase in the rate and significant
decrease in the activation energy of complex forma-
tion, slowing down of dissociation, and acceleration
Kinetic measurements were performed by spectro-
photometry. The temperature during experiments was
maintained with an accuracy of 0.1 K. The reaction
rate constants were calculated from the changes in the
optical density of the solutions near the strongest
absorption bands. The apparent rate constants of the
formation and dissociation of complexes III and IV
were optimized from ln(c0/c ) dependences using
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EFFECT OF OCTABROMO SUBSTITUTION
979
the Microsoft Excel program. The rate constants of
reaction (4) were calculated as described in [12].
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optimization of the linear dependence logk 1/T. The
activation entropies S were determined by the basic
equation of the transition state theory, transformed to
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= 19.1logkT + E/T
19.1logT
205. (6)
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The geometries of compounds I IV were opti-
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1
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