Thermodynamics and kinetics of reaction of (Oxo)(hydroxo)molybdenumtetraphenylporphyrin with pyridine

Article abstract of DOI:10.1007/s11173-005-0104-4

The reaction of meso-tetraphenylporphyrin with Mo(VI) oxide in boiling phenol resulted in a stable complex O=Mo(OH)TPP. Thermodynamics and kinetics of the reaction between (oxo)(hydroxo)molybdenumtetraphenyporphyrin with pyridine in toluene were studied by spectrophotometric method. This reaction was found to occur in three equilibrium elementary stages: replacement of OH^- by Py (K ₁=9.1 × 10³ l/mol, k ₁=5.25 s^-1 mol^-1 l), the formation of dication (dipyridine)(hydroxo)molybdenumtetraphenylporphyrin as a result of cleavage of a double bond Mo=O (K ²=39.3 l/mol, k ₂=1.83 × 10^-2 s^-1 mol^-1 l), and the formation of cationic complex[Mo(Py)₃TPP]³⁺ ? 3OH^- (K ₃=1.0 l/mol, k ₃=1.19 × 10^-3 s ^-1 mol^-1 l).

Full text of DOI:10.1007/s11173-005-0104-4

Russian Journal of Coordination Chemistry, Vol. 31, No. 5, 2005, pp. 357–363. Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 5, 2005, pp. 380–386.
Original Russian Text Copyright © 2005 by Tipugina, Lomova, Motorina.
Thermodynamics and Kinetics of Reaction
of (Oxo)(hydroxo)molybdenumtetraphenylporphyrin
with Pyridine
M. Yu. Tipugina, T. N. Lomova, and E. V. Motorina
Institute of Solution Chemistry, Russian Academy of Sciences,
ul. Akademicheskaya 1, Ivanovo, 153045 Russia
Abstract—The reaction of meso-tetraphenylporphyrin with Mo(VI) oxide in boiling phenol resulted in a stable
complex O=Mo(OH)TPP. Thermodynamics and kinetics of the reaction between (oxo)(hydroxo)molybde-
numtetraphenyporphyrin with pyridine in toluene were studied by spectrophotometric method. This reaction
was found to occur in three equilibrium elementary stages: replacement of OH– by Py (K1 = 9.1 × 103 l/mol,
k1 = 5.25 s–1 mol–1 l), the formation of dication (dipyridine)(hydroxo)molybdenumtetraphenylporphyrin as a
result of cleavage of a double bond Mo=O (K2 = 39.3 l/mol, k2 = 1.83 × 10–2 s–1 mol–1 l), and the formation of
cationic complex[Mo(Py)3TPP]3+ · 3OH– (K3 = 1.0 l/mol, k3 = 1.19 × 10–3 s–1 mol–1 l).
Complexes of Mo in oxidation states +4 and +5 with ume of CHCl3 and twice chromatographed on Al2O3
porphyrins belong to mixed-ligand coordination com-
pounds. The reactions of the ligand substitution in such
compounds, mainly with metalloporphyrins (åP) of
double- and triple-charged metal cations, have been
widely studied by domestic and foreign scientists [1–
5]. However, complexes with high-charge metal cat-
ions, in particular, Mo complexes, have been investi-
gated to a lesser extent [6–10]. Nevertheless, these
reactions still evoke interest, because the filed of appli-
cation of the mixed ligand porphyrin complexes
increases [11].
Previously [12], the reaction of Mo(V) tetraphenyl-
porphyrin complex with sulfur-containing ligand
(hydrogen sulfide) was studied. Spectrophotometric
data showed that this reaction occurs in one stage with
equilibrium constant 83 l/mol, which allows one to use
it in activation of gas-separating membranes and H2S
detectors. This work is devoted to spectrophotometric
study of the rates and equilibria of the reaction between
(oxo)(hydroxo)molybdenum(V)tetraphenyporphyrin
O=åÓ(OH)TPP (I) with pyridine (Py) in toluene.
with the use of chloroform. The yield of the complex
was 60%. The electronic absorption spectrum in chlo-
roform (λmax, nm ( logε )): 620.0 (2.94), 584.0 (2.92),
456.0 (3.78).
Thermodynamics of the reaction between I and Py
in toluene was studied by spectrophotometric titration.
Solutions of MP in toluene were prepared directly
before use. The optical density of a series of solutions
with cO=åÓ(OH)TPP = const at different Py concentrations
was measured at a wavelength of 456 nm and at 501 nm
for high Py concentrations. The electronic absorption
spectra were recorded on SF-26 and Specord M40
instruments. The solutions were thermostatted in a
spectrophotometer cell to an accuracy of 0.1 K. The
equilibrium constants (K) for three-component equilib-
rium mixture were found from the equation
Ap – A0
------------------
A∞ – A0
1
--------------------------- --------------------------------------
K =
·
,
(1)
Ap – A0
1 – ------------------
A∞ – A0
Ap – A0
cL – c0
------------------
MP A∞ – A0
where c0MP and cL are the initial concentrations of MP
EXPERIMENTAL
and Py, respectively, in toluene; A0, A, and A∞ are the
optical densities at the working wavelength for solu-
tions of MP, equilibrium mixture at a definite concen-
tration of Py, and of the obtained axial complex. The
numerical K values were determined from Eq. (1) by
the least-squares method using Microsoft Excel pro-
gram. The relative error in determination of K did not
exceed 14%.
Complex I was synthesized using procedure [13]:
meso-tetraphenylporphyrin (H2TPP) (0.1 g, 0.16 mmol)
was boiled with åÓé3 (0.076 g, 0.53 mmol) in 0.8 g of
phenol at 454 K for 4 h. The procedure was finished
when the electronic absorption spectrum of a sample of
the reaction mixture dissolved in chloroform did not
contain absorption band typical of H2TPP (λmax, nm:
648.0, 592.0, 551.0, 516.0, 485.0, 420.0). In order to
isolate the obtained complex, phenol was distilled off in
Kinetics of O=Mo(OH)TPP reactions with Py in tol-
vacuum, the residue was dissolved in a minimum vol- uene was studied spectrophotometrically at 298 K
1070-3284/05/3105-0357 © 2005 Pleiades Publishing, Inc.

358
TIPUGINA et al.
A
A
0.6
462.7
2
1
3
463
425
496
0.4
0.2
496.3
4
2
420
1
663
622
623
589
400
450
500
λ, nm
Fig. 2. The electronic absorption spectra of O=Mo(OH)TPP
in toluene with different amounts of Py: c
400
600
λ, nm
=
O=Mo(OH)TPP
–5
–4
–1
1.11 × 10 mol/l; c = (1) 0, (2) 8.27 × 10 , (3) 1.29 × 10 ,
Fig. 1. The electronic absorption spectra of O=Mo(OH)TPP
in (1) toluene and (2) same in the presence of 1.16 mol/l Py.
Py
(4) 1.16 mol/l.
ligand (O2–) to a lesser extent as compared to metal-
loporphyrins, whose acido ligands lie on one side of a
macrocycle plane (for instance, (Cl)2ZrTPP). As a
result of the metal drawing in a macrocycle plane,
which is followed by strengthening of the dative Mo–N
π-bonds in I, the complex does not undergo dissocia-
tion at the M–N bonds even in superacids [15], where
the axially coordinated ligands can be replaced by
another acido ligands occurring in high excess in a
solution. The reactions of the ligand replacement by Py
in I were studied in toluene, since the latter has high
solvent ability and reacts neither with the complex nor
with Py.
using the data on the optical density change at the
working wavelengths 456 and 501 nm. The rate con-
stants were calculated from formally first-order equa-
tion at Py excess with respect to MP:
A0 – A∞
-- ------------------
1
τ
keff
=
ln
,
(2)
Aτ – A∞
where A0, Aτ, and A∞ are the optical densities of MP
solutions at the working wavelength at time 0, τ and
after the reaction termination. The optimization of the
effective rate constant (keff) and the determination of
standard deviation were performed using the least-
square method with Microsoft Excel program. The rel-
ative error in determining keff was 2–10%.
The electronic absorption spectrum of complex I in
toluene shows significant changes on addition of an
excess (with respect to the complex) of Py (Fig. 1). In
an interval of Py concentrations 8.27 × 10–6–1.16 mol/l,
the changes in the electronic absorption spectra of com-
plex I are different (Fig. 2), but are reversible in all
cases. As the Py content increases from 8.27 × 10–6 to
8.27 × 10–4 mol/l, the optical density of a solution sub-
stantially increases, though the O=Mo(OH)TPP con-
centration remains unchanged (Fig. 2, 2).At Py concen-
Toluene was purified using standard procedure [14],
water content in a dried solvent was 8.9 × 10–3 mol/l.
IR spectra of the samples were recorded on a
Specord IR-75 spectrometer in a range of 400–4000 cm–1
(with KBr pellets).
RESULTS AND DISCUSSION
The composition of a mixed-ligand coordination trations from 8.27 × 10–4 to 1.29 × 10–1 mol/l, the inten-
sphere in Mo(V) complexes was previously determined sity of a band at 462.7 nm decreases, and this band is
in [15]. The electronic absorption spectra of complex I, lightly shifted to 457.6 nm (Fig. 2, 3). With further
obtained in this work, fully agrees with that reported in increase in Py concentration (1.29 × 10–1–1.16 mol/l), a
[15]. In this complex, acido ligands O2– and éç– are in new band appears with λmax = 496.3 nm (Fig. 2, 4); at
the trans-position with respect to the plane of a macro- higher concentrations of Py (1.16 mol/l and higher),
cyclic ligand [16]. Owing to this arrangement, the this spectrum does not undergo further changes. Thus,
metal atom is shifted from the N4 plane toward oxo spectrophotometric titration of complex I with pyri-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 5 2005

THERMODYNAMICS AND KINETICS OF REACTION
359
A
A
(a)
(b)
0.56
0.57
0.54
0.51
0.52
0.48
0
4
8
0
5
10
cL × 104, mol/l
cL × 102, mol/l
A
(c)
0.4
0.2
0
0.5
1.0
cL, mol/l
–6
–4
–4
–1
Fig. 3. Curves of O=Mo(OH)TPP titration with pyridine in toluene: c = (a) 8.27 × 10 –8.27 × 10 , (b) 8.27 × 10 –1.29 × 10 ,
and (c) 1.29 × 10 –1.16 mol/l.
Py
–1
dine revealed three stages of the reaction of complex I one can use a linear section in the kinetic curve in coor-
with Py.
dinates of Eq. (2) to measure the rate of a forward reac-
tion k1 and the detailed equilibrium principle to calcu-
late the rate of a reverse reaction k–1. The effective rate
It was found that the equilibrium at the first stage
is attained within 50 min. The number of extra
ligands, i.e., Py molecules (n1 = 1.08) added at the first
reaction stage was found from the experimental plot
constants ke1ff determined at different initial concentra-
tions of Py are listed in Table 1.
Ap – A0
A∞ – Ap
------------------
log
– logcL (fig. 4, 1). The stoichiometric ratio
The true rate constants k1 and the reaction order m1
in cPy obtained from a linear dependence (Fig. 5, 1)
of I : Py equal to 1 : 1 and the spectral pattern (Fig. 2,
1, 2) make it possible to write down the first reaction
state as the process of replacement of éç– by Py:
logke1ff = logk1 + m1 logcPy,
(4)
are, respectively, 5.25 s–1 mol–1 l and 0.96. The reverse
K1
rate constant is 5.75 × 10–4 s–1 (Table 2).
O=Mo(OH)TPP + Py
[O=Mo(Py)TPP]+ · OH–.(3)
Thus, the experimental kinetic equation of the first
stage of the reaction is
The equilibrium constant K1 calculated from Eq. (1)
for three-component equilibrium system (3) is equal to
(9.1 1.2) × 103 mol/l; this value is rather high for reac-
tions of metallopoprhyrins with bases [1]. The replace-
ment of éç– by Py is, obviously, more preferable than
the additional coordination of Py, since in the course of
experiment, the electronic absorption spectrum of MP
does not change basically, but only the optical density
is increased at the working wavelength of 456 nm near
the O=Mo(OH)TPP maximum.
–dcMP/dτ = k1cMPcPy
(5)
which is adequately interpreted within the framework
of one-stage substitution reaction following the asso-
ciative mechanism through intermediate state with
extended coordination sphere:
O=Mo(OH)TPP + Py
[O=Mo(OH)(Py)TPP]≠
[O=Mo(Py)TPP]+ · OH–.
(6)
Realization of reaction (3) is confirmed by the
kinetic experiment. Since the equilibrium at the first
stage of reaction of I with Py is established with time,
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 5 2005

360
TIPUGINA et al.
logkeff
Ap – A0
A∞ – Ap
log
0.8
0.4
0
2
–2.8
–3.2
–3.6
–4.0
1
1
3
2
3
–0.4
–0.8
–1.2
–4
–2
0
–5
–4
–3
–2
–1
0
logcL
logcL
Ap – A0
-------------------
Fig. 4. The plot of log
vs. logcL for
Fig. 5. The plot of logkeff vs. logcL for O=Mo(OH)TPP
A∞ – Ap
O=Mo(OH)TPP reaction with Py in toluene at (1) first,
(2) second, and (3) third stage (the correlation coefficient
(ρ) is equal to 0.996, 0.995, and 0.977, respectively).
reaction with Py in toluene at 298 K at (1) first, (2) second,
and (3) third state (ρ = 0.994, 0.999, and 0.995, respec-
tively).
The second equilibrium bimolecular state of reac- stage (K2 = 39.3 5.2 l/mol). The second stage of the
tion between I and Py occurs at Py concentration
8.27 × 10–4–1.29 × 10–1 mol/l. The equilibrium is estab-
lished within 40 min. The stoichiometry of the reaction
reaction is likely to occur at bidentate oxygen:
[Mo(Py)TPP]+ ⋅ OH– + Py + H2O
(7)
K2
Ap – A0
------------------
[Mo(OH)(Py)2TPP]2+ ⋅ 2OH–.
is confirmed by the plot log
–logcL (Fig. 4, 2)
A∞ – Ap
(n2 = 1.04). The equilibrium constant decreases by more
than two orders of magnitude as compared with the first which is reflected in the equilibrium constant. The
Rupture of a double bond requires sufficient energy,
Table 1. Effective rate constants (keff) of formally first-order reaction of O=Mo(OH)TPP with Py in toluene at 298 K
(cO=Mo(OH)TPP = 1.11 × 10–5 mol/l)
Stage 1
Stage 2
Stage 3
cPy × 105,
cPy × 103,
mol/l
cPy × 10,
mol/l
(ke1ff δk1eff ) × 104, s–1
(ke2ff δk2eff ) × 104, s–1
(ke3ff δk3eff ) × 104, s–1
mol/l
1.65
4.96
8.27
11.6
20.7
1.3 0.08
4.4 0.4
7.2 0.3
9.6 0.8
14 1.2
2.48
5.79
8.27
10.7
14.9
51.7
77.6
1.2 0.1
2.6 0.3
3.4 0.4
4.7 0.2
5.4 0.2
16 1.4
21 2.1
1.29
3.88
6.47
7.44
8.27
9.92
11.6
1.4 0.04
4.3 0.1
8.7 0.3
9.3 0.2
9.6 0.3
11 0.07
13 1.3
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THERMODYNAMICS AND KINETICS OF REACTION
361
Table 2. Reaction order (m) in cPy, true rate constants (k)
absorption spectrum of complex I at the second stage of
a reaction changes more significantly: not only the
decrease in the optical density is observed, but also a
hypsochromic shift of the maximum of the band at
462.7 nm (to 457.6 nm).
of forward and reverse reactions of O=Mo(OH)TPP with Py
Reaction
stage
k, s–1 mol–1
l
k, s–1
m
(forward reaction) (reverse reaction)
The kinetic parameters of reaction (7), i.e., the
1
2
3
0.96
0.83
1.03
5.25
5.75 × 10–4
4.65 × 10–4
1.20 × 10–3
effective rate constants ke2ff , the true rate constants k2,
1.83 × 10–2
1.19 × 10–3
and the reaction order m2 in cPy, found in the same way
as for reaction (3) (Fig. 5, 2) are given in Tables 1, 2.
The rate constant of reverse equilibrium reaction (7) is
4.65 × 10–4 s–1. The first orders of the reaction in MP
and Py concentration indicate bimolecular process,
which follows associative mechanism, and indirectly
confirm the nature of equilibrium (7) with participation
of one Py molecule (Fig. 4, 2).
later (Table 3). The spectrum of the initial complex I
contains the absorption band at 625 cm–1 corresponding
to the Mo=O bond and disappearing from the spectrum
of the complex [Mo(Py)3TPP]3+ · 3OH–. IR spectrum of
the complex isolated at the third stage from a solution
in Py contains new absorption bands due to vibrations
of Py bonds, namely, at 412, 590, 700, 990, 1040, 1384,
1425, 1570 cm–1 [14]. The bands at 1384, 1425, and
1570 cm–1, which correspond to vibrations of the C–C
and C–N bonds in pyridine cycle, are shifted toward
low-frequency region, probably, due to coordination of
pyridine in the complex.
At Py concentration 1.29 × 10–1–1.16 mol/l, the
third equilibrium stage of the reaction of I with Py is
observed. The equilibrium is attained in 50 min. At this
state, the electronic absorption spectrum of the reaction
mixture shows a decrease in the optical density in the
region of the band at 456 nm. When Py concentration is
7.44 × 10–1 mol/l and higher, the band at 457.6 nm
almost disappears, while a new band appears with
λmax = 496.3 nm. The thermodynamic study was per-
formed at wavelength 501 nm, when the optical density
changes are the most pronounced. The equilibrium con-
stant at the third stage K3 = 1.0 0.1 mol/l, the number
of interacting Py molecules n3 = 1.26 (Fig. 4, 3). The
absorption spectrum of extra ligand formed at the third
stage (Fig. 1, 2) is similar to that of O=Mo(OH)TPP
treated with an excess gaseous HCl, i.e., to the spec-
trum of (Cl)3MoTPP [9]. Extra complex, formed in the
course of the reaction with Py, is likely to be Mo(V)
compound. The authors of [10] reported somewhat dif-
ferent (in the number and positions of the bands) elec-
tronic absorption spectrum of O=åÓIVTPP in toluene
(λmax, nm): 645, 590, 453, 397.
The ratio of the stage equilibrium constants for anal-
ogous reaction with participation of W complex
O=W(OH)TPP (1.3 × 104, 8.4 × 103, and 89 l/mol) pre-
viously studied in [17] shows that the reaction of W
complex with Py occurs at bidentate oxygen O2– only at
the third stage. This fact agrees well with the suggested
scheme of transformations for Mo complex. It is known
[15, 18] that the W=O bond is noticeably stronger than
the Mo=O bond in the metal coordination compounds.
Therefore, the reactivity of the latter bond is also higher
in the case of complex I: this bond is the reaction center
already at the second stage of the reaction with Py.
With regard to the equilibrium constant of the reaction
of displacement of a single-charged acido ligand to the
second coordination sphere, the complexes (AcO)CrTPP
[19], O=W(OH)TPP [17], and O=Mo(OH)TPP can be
arranged in the following series:
Table 1 presents effective rate constants ke3ff deter-
mined for different initial Py concentrations, while
Table 2 contains the values of the true rate constants k3
and of the reaction order m3 in cPy determined from the
data in Fig. 5. The rate constant of a reverse reaction is
1.20 × 10–3 s–1 (Table 2). The experimental kinetic equa-
tion is identical to those for stage one and stage two.
The above data show that the third stage of the reac-
tion between molybdenum(V)porphyrin with Py is in
fact displacement of the third OH– group to the second
coordination sphere stable in organic solvent toluene:
O=Mo(OH)TPP (K = 9100) ≈ O=W(OH)TPP (8400)
> (AcO)CrTPP (2.8).
This series coincides with the series of the kinetic sta-
bility of the complexes at the M–Nporphyrin bonds [20].
One can see that an increase in the formal charge of the
metal cation is attended by an increase in the affinity of
metalloporphyrin to both Py and a macrocycle.
[Mo(OH)(Py)2TPP]2+ ⋅ 2-– + Py
The comparison of reactions of O=Mo(OH)TPP
with different bases revealed the following conclusions.
The equilibrium of the reaction of I with Py is charac-
terized by a higher rate constant (K = 9100 l/mol) as
(8)
K3
[Mo(Py)3TPP]3+ ⋅ 3OH–.
The structure of the product of reaction (8) is con- compared to analogous parameter in the case of imid-
firmed by IR spectra of the initial complex azole (Im) (480 l/mol, unpublished data of the authors)
O=Mo(OH)TPP and of the complex isolated at the third and hydrogen sulfide (83 l/mol [12]). This corresponds
stage from a solution in Py by vacuum distillation of the to the growing energy of protonation when going from Py
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 5 2005

362
TIPUGINA et al.
Table 3. Frequencies of the bands (cm–1) in IR spectra of molybdenum(V)tetraphenylporphyrin before and after treatment
with pyridine
No. of band
O=Mo(OH)TPP [Mo(Py)3TPP]3+ · 3OH–
412
Band assignment
ν(Mo–NPy)
ν(Mo–N)
ν(Mo–O)
1
2
447
475
447
480 (w.ab.)
590
Vibrations of Py bonds
5
625
ν(Mo=O)
700
708
758
805
γ(C–H) of pyridine
γ(C–H) of benzene ring
7
9
708
758
805
930
The same
10
14
γ(C–H) of pyrrole ring
ν(Mo–O)
990
1025
1040
1072
1170
1325
1384
1425
1445
1470
1540
1570
1600
2852
2870
2922
2960
δ(C–H) of pyridine
δ(C–H) of pyrrole ring, ν(C3–C4), ν(C–N)
ν(C–C), ν(C–N) of pyridine
δ(C–H) of benzene ring
The same
16
1020
17
19
23
1075
1180
1335
ν(C–N)
ν(C=C) of pyridine
ν(C=N) of pyridine
ν(C=N)
ν(C=C) of benzene ring
Vibratioans of pyrrole ring
ν(C=C) of pyridine
ν(C=C) of benzene ring
ν(C–H) of pyrrole ring
The same
25
26
27
1445
1480
1540
29
1610
Unstudied region
ν(C–H) of benzene ring
The same
1
Properties and Development of Materials on Their
Basis” (project no. 34/03 I).
to H2S (438 [21] and 520 kJ/mol , respectively). Hence,
the stability of extra complexes [O=Mo(Py)TPP]+ and
O=Mo(SH)TPP changes in accordance with an
increase in stability of the bond molybdenum–donor
atom of the extra ligand. Among the investigated bases,
Im has the minimum value of E (398 kJ/mol [21]). The
equilibrium constant of the reaction of OH– replace-
ment by Im does not comply with this tendency, which
can be explained by the presence in extra complex
[O=Mo(Im)2TPP]+ of an additional Im molecule coor-
dinated at the previous stage.
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