Ligand substitution equilibrium in the macrocyclic molybdenum(V) complex

Article abstract of DOI:10.1134/S0036023607030175

The thermodynamics of the reaction of tetraphenylporphyrin(oxo)(hydroxo) molybdenum with imidazole (Im) in toluene was studied spectrophotometrically in a polythermal mode. It was established that the reaction of Im with O=Mo(OH)TPP includes several stages, two of which can be studied by spectrophotometrical titration. The first equilibrium stage is Im coordination to the eighth coordination site (K ₁ = 1.85 × 10³ l/mol), the second stage is the coordination of the second Im molecule with the displacement of OH^- into the outer sphere (K ₂ = 4.80 × 10 ² l/mol). The enthalpy and entropy contributions to the stability constant of tetrapheny lporphyrin molybdenum(V) complexes with Im were determined. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607030175

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 3, pp. 394–397. © Pleiades Publishing, Inc., 2007.
Original Russian Text © M.Yu. Tipugina, E.V. Motorina, T.N. Lomova, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 3, pp. 444–447.
COORDINATION COMPOUNDS
Ligand Substitution Equilibrium in the Macrocyclic
Molybdenum(V) Complex
M. Yu. Tipugina, E. V. Motorina, and T. N. Lomova
Institute of Solution Chemistry, Russian Academy of Sciences,
Akademicheskaya ul. 1, Ivanovo, 153045 Russia
Received February 26, 2006
Abstract—The thermodynamics of the reaction of tetraphenylporphyrin(oxo)(hydroxo)molybdenum
with imidazole (Im) in toluene was studied spectrophotometrically in a polythermal mode. It was estab-
lished that the reaction of Im with O=åÓ(OH)TPP includes several stages, two of which can be studied
by spectrophotometrical titration. The first equilibrium stage is Im coordination to the eighth coordina-
tion site (K1 = 1.85 × 103 l/mol), the second stage is the coordination of the second Im molecule with the
displacement of OH– into the outer sphere (K2 = 4.80 × 102 l/mol). The enthalpy and entropy contributions
to the stability constant of tetraphenylporphyrin molybdenum(V) complexes with Im were determined.
DOI: 10.1134/S0036023607030175
Metalloporphyrins possess a variety of useful prop- using the procedure from work [6]: 0.1 g (0.16 mmol) of
erties owing to great possibilities of modifying their H2TPP was boiled with 0.076 g (0.53 mmol) of åÓé3
structure and the nature of central atoms. These proper-
ties are due to the reactivity of metalloporphyrins
toward bases with a simpler structure than that of mac-
rocyclic porphyrin ligand. The nature of the central
atom in metalloporphyrins and the degree of its coordi-
nation saturation are significant factors [1]. The study
of coordination of molecular ligands by coordination-
ally unsaturated metalloporphyrins is one line of in the
chemistry of porphyrins. Extra coordination for mixed-
ligand acido porphyrin complexes of metal cations in
higher oxidation states, in particular for molybde-
num(V) complexes [2, 3], is less studied. The
O=åÓ(OH)TPP complex studied in this work contains
acido ligands in the coordination sphere in addition to
the porphyrin macrocycle. In this complex acido
ligands O2– and éç– are in the trans position to the
plane of coordinated nitrogen atoms [4] and are tightly
held in the coordination sphere [5]. The Mo–N bonds in
O=åÓ(OH)TPP are even more stable: the M–N bonds
in this complex do not break even in superacids [5].
Therefore, the reactions with molecular ligands pro-
ceed for O=åÓ(OH)TPP as the substitution reactions
of acido ligands in the first coordination sphere along
with extra coordination. This work deals with a quanti-
tative study of the equilibria of reactions of tetraphe-
nylporphyrin(oxo)(hydroxo)molybdenum with imida-
zole (Im) in toluene using electronic absorption spec-
troscopy and spectorophotometric titration.
in 0.8 g of phenol at 454 K for 4 h. The synthesis was
finished, when the absorption bands of H2TPP (λmax
,
nm: 648.0, 592.0, 551.0, 516.0, 485.0, 420.0) disap-
peared from the electronic absorption spectrum of the
reaction mixture. The complex was isolated in solid
state by vacuum distillation of phenol. Then, a saturated
solution of the complex in chloroform was prepared
and purified to spectral purity by two-stage chromatog-
raphy on Al2O3 using chloroform as solvent and eluent.
The yield of the complex was 60%. The electronic
absorption spectrum in chloroform (λmax, nm ( (logε)):
620.0 (2.94), 584.0 (2.92), 456.0 (3.78)) corresponds to
that measured in work [7]. The electronic absorption
spectrum was recorded on SF-26 and Specord M-40
spectrophotometers.
The thermodynamics of the reaction of
O=åÓ(OH)TPP with Im in toluene was studied by
spectrophotometric titration in a polythermal mode.
The solutions of metalloporphyrin in toluene were pre-
pared directly before the use. Optical density measure-
ments for a series of solutions with ÒO=åÓ(OH)TPP = const
at various Im concentrations and T = 298–323K were
carried out at a working wave length close to the maxi-
mum absorption band of the starting metalloporphyrin
(465 nm). The accuracy of temperature control was
0.1 K. The aforementioned series of working solutions
was prepared by mixing calculated volumes of metal-
loporphyrin solution, Im solution in toluene with a
known concentration, and pure toluene.
EXPERIMENTAL
The equilibrium constants of O=åÓ(OH)TPP reac-
tions with Im producing new complexes
The O=åÓ(OH)TPP complex was synthesized by com-
plexing purchased porphyrin and molybdenum(VI) oxide
(Im)m(X)m åÓTPP (X is an acido ligand, m = 0–3, m1 =
1
394

LIGAND SUBSTITUTION EQUILIBRIUM IN THE MACROCYCLIC MOLYBDENUM(V)
395
462.7
4
3
2
1
4
5
6
7
A
1.0
A
0.6
A
0.6
0.5
0.4
0.3
0.8
0.6
0.4
0.2
0.5
0.4
0.3
0
2
4
6
8
0
5
10
15
4
c × 10 ,
mol/L
3
c × 10 , mol/L
(‡)
(b)
350
450
350
450
λ, nm
Fig. 1. Electronic absorption spectra of O=Mo(OH)TPP in toluene depending on Im concentration and the corresponding titration
–6
–2
curves at the first (a) and second (b) stages. c
mediate concentrations.
= 5.28 × 10 mol/L; c , mol/L: (1) 0, (7) 1.41 × 10 , and (2–6) inter-
Im
O=Mo(OH)TPP
0–2) were determined for the three-component equilib-
rium system (1) using the equation (2):
The mathematical processing of the linear depen-
dence –lnK – 1/T. (6)
was carried out using the least-squares method and
the MS Excel program.
O=åÓ(OH)TPP + mIm
(Im)m(X)m åÓTPP, (1)
1
Aeq – Ao
-------------------
A∞ – Ao
∆H 1 ∆S
---------- ------
–lnK =
–
.
(6)
1
R T
R
---------------------------- ---------------------------------------
K =
,
(2)
Aeq – Ao
1 – -------------------
A∞ – Ao
Aeq – Ao
cL – co
-------------------
MP A∞ – Ao
RESULTS AND DISCUSSION
Here, coMP and cL are, respectively, starting concen-
trations of metalloporphyrin and extra ligand in toluene
solution; and A0, Aeq, and Ä∞ are optical densities at the
working wave length of metalloporphyrin solutions,
equilibrium mixture with a given concentration of the
ligand, and a solution of the extra complex, respec-
tively. The numerical values of constants K were deter-
mined from Eq. (2) by the least-squares method using
the MS Excel program. The relative error in K determi-
nation was not higher than 7%.
The reaction of O=Mo(OH)TPP with Im in toluene
was studied spectrophotometrically in a wide range of
Im concentrations from 1.20 × 10–5 – 1.42 × 10–2mol/l
at 298–323 K. The upper limit of the Im concentration
is determined by its solubility in toluene. According to
the spectrophotometrical data and the titration curves
(Fig. 1), the reaction includes two equilibrium stages.
The electronic absorption spectrum of O=Mo(OH)TPP
in toluene without Im (Fig. 1a, spectrum 1) is a curve
with a peak at 462.7 nm. At the first stage of the equi-
librium process at low Im concentrations, an increase
in the Im proportion brings about an insignificant
increase in optical density at the wave length of the
absorption maximum (spectra 2–4), and an isosbestic
point at λ = 459 nm is observed.
The thermodynamic parameters of reactions of
O=åÓ(OH)TPP with Im ((3)–(5)) were determined
from the temperature dependence of K:
K2 T1T2
⎛
⎞
⎠
----- ----------------
∆H = Rln
,
(3)
(4)
⎝
K1 T1 – T2
∆G = –RT lnK,
∆S = (∆H – ∆G)/T.
It was established experimentally that at low Im
concentrations (1.20 × 10–5 to 7.18 × 10–4 mol/L), the
(5) equilibrium is achieved immediately after pouring the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 3 2007

396
TIPUGINA et al.
The second equilibrium stage of the reaction of
Aeq – Ao
--------------------
A∞ – Aeq
log
O=Mo(OH)TPP with Im in toluene takes place at Im
concentrations from 7.18 × 10–4 to 1.42 × 10–2 mol/L.
The equilibrium constant is K2 = (4.80
0.23) ×
2
102 L/mol. As the Im concentration increases in this
range, we observe a decrease in the intensity of the
band at 462.7 nm and a small shift of the peak of this
band to 460.5 nm (Fig. 1b, spectra 4–7) with the con-
servation of the isosbestic point at λ = 461 nm, which
points to more significant changes in the structure of the
coordination sphere of metalloporphyrin compared to
the first stage. After the titration data were treated as
described for the first stage, one can assume in view of
the data of Fig. 1b that the reaction of O=Mo(OH)TPP
with Im at the second stage is the substitution of neutral
Im for the single-charge OH– ligand; éç– is displaced
to the outer sphere, and the cationic complex is formed:
0.5
0
1
–0.5
–1.0
–1.5
K1
O=Mo(OH)(Im)TPP + Im
(8)
[O=Mo(Im)2TPP]+ ⋅ OH–.
–5
–4
–3
–2
logc
The stoichiometry of reaction (8) is confirmed by the
L
Aeq – Ao
A∞ – Aeq
--------------------
treatment of log
– logcL (n2 = 1.00) (Fig. 2).
Aeq – Ao
---------------------
Fig. 2. Function log
– logcL for the reaction of
At Im concentrations higher than 1.42 × 10–2 mol/L,
we managed to study the spectra of the reaction mix-
tures just after their preparation. Imidazole precipitates
with time in stored solutions, and spectrophotometric
titration of the complex with Im solution becomes impos-
sible. The band with the absorption peak at 460.5 nm
hypsochromically shifts to 458.5 nm. Most likely, the
Mo=O double bond is broken, and a cationic complex
with a more intricate composition is formed.
A comparison of the equilibrium constants K1 and
K2 shows that the presence of one Im molecule in the
coordination sphere of the molybdenum(V) porphyrin
complex hinders the coordination at the second stage of
the process. The OH– displacement from the coordina-
tion sphere in complex O=Mo(OH)(Im)TPP proceeds
in time; equilibrium (8) is established within 15 min.
A∞ – Aeq
O=Mo(OH)TPP with Im in toluene at the first (1) and sec-
ond (2) stages (ρ = 0.999 and 0.998, respectively).
solutions together. The equilibrium constant K1 at 298 K is
(1.85 0.11) × 103 L/mol. During the treatment of the
Aeq – Ao
--------------------
relation in coordinates log
– logcL , the num-
A∞ – Aeq
ber of Im molecules added at the first stage was found
to be equal to one (n1 = 1.05) (Fig. 2). The stoichiome-
try O=Mo(OH)TPP : Im = 1 : 1 and the spectral param-
eters permit us to write down the equation
K1
O=Mo(OH)TPP + Im
O=Mo(OH)(Im)TPP. (7)
The table displays the equilibrium constants K1 and
K2 at various temperatures.
for the first stage of the reaction. This equation
describes the extra coordination of the Im molecule by
metalloporphyrin.
The equilibrium constants K1 and K2 are linear func-
tions of temperature (ρ is 0.994 and 0.997, respec-
tively), which allows one to determine the thermody-
namic parameters of reaction of O=Mo(OH)TPP with
Im for both stages. The thermodynamic parameters are in
accordance with the suggested scheme of stepwise reac-
tions (7) and (8). At the first stage, the reaction is an exo-
Stepwise equilibrium constants K1 and K2 of the reaction of
O=Mo(OH)TPP with Im in toluene at various temperatures
(working wave length, 465 nm)
thermic process (∆H01 = –27.6 1.5 kJ/mol) with a low
negative value of ∆S0 (∆S01 = –29.8 5.0 J/(mol K)). The
second stage is endothermic (∆H02 = 8.9 0.4 kJ/mol)
T, K
K1 × 10–2, L/mol
K2 × 10–2, L/mol
298
303
308
313
318
323
18.5 1.4
16.3 0.6
14.1 0.5
11.6 1.0
9.2 0.6
8.1 0.2
4.8 0.2
5.0 0.3
5.3 0.2
5.6 0.2
5.9 0.3
6.3 0.3
and has a greater positive value of ∆S0 (∆S20 = 81.0
1.2 J/(mol K)). The endothermic character of the second
stage (Eq. (8)) points to the energy-consuming bond rup-
ture in the reacting molecule O=Mo(OH)(Im)TPP, and
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LIGAND SUBSTITUTION EQUILIBRIUM IN THE MACROCYCLIC MOLYBDENUM(V)
397
the greater positive value of ∆S0 points to the formation
of less solvated species than the starting compounds.
Indeed, the ionic associate [O=Mo(Im)2TPP]+ · OH– is
a self-solvated species and is poorly solvated by the sol-
vent molecules, while the reagents for reaction (8) must
be desolvated. The combination of the two factors leads
to the disordering of the system and the increase in its
entropy. The first stage of the reaction produces a neu-
tral polar species, as the starting complex. Therefore,
the slight decrease in the entropy of the system in the
course of reaction (7) can be explained by a mere
decrease of the number of species.
A comparative analysis of the data for the reactions
of O=Mo(OH)TPP with various bases shows the fol-
lowing. The equilibrium of the reactions
O=Mo(OH)TPP with Py, Im, and H2S is characterized
by a decreasing equilibrium constant, which is 9100 [8],
480, and 83 [9] L/mol, respectively. The products of the
reaction with Py or H2S, ([O=Mo(Py)TPP]+ or
O=Mo(SH)TPP) are characterized by the same coordi-
nation number, which corresponds to the increase in the
protonation energy Ö from 438 [10] to 520 kJ/mol (cal-
culated by the PM3 method by S.V.Zaitseva). The pro-
tonation energy means the energy amount absorbed
upon the addition of one proton to the molecule of an
organic base in vacuum. From this, the stability of axial
ligands in complexes changes in accordance with the
increase in the strength of the molybdenum–donor
atom σ bond of the extra ligand. Among the studied
bases, Im has the minimum E value (398 kJ/mol [10]).
The equilibrium constant of the substitution of Im for
OH– does not correspond to the general tendency,
which can be explained by the presence of an extra Im
molecule in the complex [O=Mo(Im)2TPP]+. This Im
molecule is coordinated at the previous stage.
complexes are absent: mixed-ligand Im molybdenum
complexes were obtained only in solution in contrast to
the pyridine complexes studied in works [8, 12–14]; the
latter can be easily prepared in solid state from metal-
loporphyrin solutions in 100% pyridine through pyri-
dine evaporation.
ACKNOWLEDGMENTS
This study was supported by the Russian Founda-
tion for Basic Research (project no. 06-03-96343-r) and
Program no. 8 of the Russian Academy of Sciences
“Synthesis of Compounds with Designed Properties
and Design of Functional Materials.”
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dinated
complexes
O=Mo(OH)(Im)TPP
and
[O=Mo(Im)2TPP]+ with comparatively high stability
constants for the imidazole ligand. These constants dif-
fer fourfold. The presence of stable stereochemical
configurations can be expected for the complexes
whose rigidity is due to the presence of an aromatic
macrocycle in their composition. Unfortunately, other
data on the geometry of the coordination spheres in the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 3 2007