Novel 2′-(pyridin-4-yl)-5′-(pyridin-2-yl)-1′-(pyridin-2- yl)methylpyrrolidinyl[60]fullerene-hydroxyoxo(5,10,15,20-tetraphenyl-21H, 23H-porphynato) molybdenum(V) dyads

Article abstract of DOI:10.1134/S1070363214050272

Interaction between hydroxyoxo(5,10,15,20-tetraphenylporphyrinato) molybdenum(V) and 2′-(pyridin-4-yl)-5′-(pyridin-2-yl)-1′- (pyridin-2-yl)methylpyrrolidinyl[60]fullerene in toluene has been studied by means of spectrophotometry. The reaction proceeds via

Full text of DOI:10.1134/S1070363214050272

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 5, pp. 946–952. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © E.V. Motorina, T.N. Lomova, P.A. Troshin, M.V. Klyuev, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84, No. 5,
pp. 855–861.
Novel 2'-(Pyridin-4-yl)-5'-(pyridin-2-yl)-1'-(pyridin-2-yl)-
methylpyrrolidinyl[60]fullerene–
Hydroxyoxo(5,10,15,20-tetraphenyl-21H,23H-
porphynato)molybdenum(V) Dyads
E. V. Motorinaa, T. N. Lomovaa, P. A. Troshinb, and M. V. Klyuevc
a Krestov Institute of Solutions Chemistry. Russian Academy of Sciences,
ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: tnl@isc-ras.ru
b Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, Russia
c Ivanovo State University, Ivanovo, Russia
Received June 17, 2013
Abstract—Interaction between hydroxyoxo(5,10,15,20-tetraphenylporphyrinato)molybdenum(V) and 2'-
(pyridin-4-yl)-5'-(pyridin-2-yl)-1'-(pyridin-2-yl)methylpyrrolidinyl[60]fullerene in toluene has been studied by
means of spectrophotometry. The reaction proceeds via two stages: rapidly established equilibrium between the
reactants and their molecular complex 1 : 1 [K = (1.97 ± 0.52) × 104 L/mol] followed by slow irreversible
displacement of hydroxy group into the second coordination sphere to form cationic outer sphere complex (k =
0.26 L s–1 mol–1). The effect of fullerene fragment on pyridine fragments coordination has been considered.
Keywords: molybdenum porphyrin complex, reaction order, dyades formation, thermodynamics, chemical kinetics
DOI: 10.1134/S1070363214050272
Unsubstituted fullerenes are known as weak π-
electron acceptors. The electron affinity of C60 in
aqueous solutions is of 2.10–2.20 eV [1]. In its
complexes with π-electron donors (molecular complex
DA, charge transfer complex Dδ+Aδ–, and ion-radical
salts Dn · +An · –), fullerene acts as unique π-electron
acceptor of spherical shape, which is important factor
of the complexes stability. This is especially so in the
case of planar aromatic systems acting as donor. The
complexes with metal porphyrinates as electron donor
are of interest, as they are capable of reducing
fullerene or forming the charge transfer complexes.
For instance, formation of supramolecular complexes
of bisporphyrin with fullerenes С60 and С70 in toluene
has been studied by spectrophotometric titration [2].
The interaction proceeds through formation of a charge
transfer transition state (KС60 2.1 × 104, KС70 6.94 ×
104 L/mol). The most popular methods to prepare
porphyrin complexes with fullerene C60 have been
reviewed in [3]. Such fullerene-containing complexes
are of certain interest for modern science and
technology. In particular, accurate, sensitive, and
selective sensor for determination of bisphenol A has
been developed [4]; fullerene-encapsulated hexagonal
porphyrin nanotubes, based on fullerene С60 or С70 and
meso-tetra(pyridin-4-yl)porphynatozinc ZnP(Py)4 have
been constructed [5]; the rod-shaped electrodes built of
such nanotubes can be used in light energy trans-
formation applications [6, 7].
In order to enhance the electron affinity of fulle-
rene, it can be chemically modified. Recently, a novel
possibility to prepare the porphyrin–fullerene com-
plexes has been recognized, via coordination binding
of the electron-donor site of substituted fullerene with
metal cation of porphyrin complex. In particular, axial
coordination of the substituted fullerene on low-basic
nitrogen atom of pyrrolidine ring on (5,10,15,20-tetra-
phenylporphyrinato)zinc (ZnTPP) has been confirmed
by X-ray diffraction studies [7–9]. Such coordinated
supramolecules form photoinduced stable (up to
hundreds of μs [10]) charge transfer states, similar to
946

NOVEL 2'-(PYRIDIN-4-YL)-5'-(PYRIDIN-2-YL)-1'- ...
947
Scheme 1.
N
O
N
N
N
Mo
N
N
N
OH
N
I
II
the photosynthetic antenna. Potential application of the
ZnTPP–fullerenopyrrolidine complex to develop
photoactive layers of two-layered solar cells has been
demonstrated [10].
A
Stability of such coordinated complexes has been
scarcely studied so far. We have recently published
quantitative data on supramolecular complex forma-
tion between meso-tetraphenylporphin complexes of
highly charged metal cations and free pyridine [11, 12]
in comparison with these of pyridinyl derivatives of
pyrrodinyl[60]fullerene [13, 14]. Furthermore, a new
type of free-radical complexes between molybdenum
and substituted fullerene has been prepared [15–17].
In this work we used molybdenum cation
surrounded by a readily polarized macrocyclic com-
pound. In particular, we studied equilibrium state and
kinetics of interaction between hydroxyoxo(5,10,15,20-
tetraphenylporphyrinato)molybdenum(V) [I, O=Mo(OH)·
TPP] and 2'-(pyridin-4-yl)-5'-(pyridin-2-yl)-1'-(pyridin-
log [(As – A0)/(A∞ – As)]
log cII
λ, nm
Fig. 1. UV-vis spectra of the complex I (cI 1.08 × 10–5 mol/L)
in toluene (1) and of the same solution with addition of
1.38 × 10–5–2.02 × 10–4 mol/L of the substituted fullerene
II.
Fig. 2. log [(As – A0)/(A∞ – As)] as function of log cII for
reaction of compounds I and II in toluene; сII 1.38 × 10–5–
2.02 × 10–4 mol/L. R2 = 0.918.
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948
MOTORINA et al.
2-yl)methylpyrrolidinyl[60]fullerene (II, Py3F). Toluene
was used as solvent; it has been proved experimentally
that fullerene С60 does not aggregate toluene [18]
(Scheme 1).
I, of the equilibrium mixture at certain concentration
of the fullerene II, and of the reaction product; n,
number of the bounded molecules of the substituted
fullerene II.
Figure 1 displays UV-vis spectra (Soret band range)
of compound I in toluene upon addition of varied
amounts of compound II. In the studied range of
concentrations, сII of 1.38 × 10–5–2.02 × 10–4 mol/L), the
system attained equilibrium state immediately after
mixing. With increasing concentration of II, absorp-
tion at 462 nm increased, and isosbestic point was
clearly revealed at 478 nm. Processing the experi-
mental data using the log (As – A0)/(A∞ – As) – log cII
function (Fig. 2) confirmed that the equilibrium was
established with participation of a single molecule of
the substituted fullerene (tan α ≈ 1). Therefore, the
equilibrium constant K was calculated using Eq. (1),
derived for the case of three-component equilibrated
system (2) taking advantage of the detailed balance
and the Beer–Lambert–Bouguer law for mixture of
two colored compounds. The fullerene spectrum was
taken as baseline. The equilibrium constant was
relatively high, (1.97 ± 0.59) × 104 L/mol, compared
with those of other metal porphyrin complexes with
bases.
O
N
N
Mo
N
N
HO
K1
N
(2)
I + II
N
N
N
III
Chemical structure of the reaction product III
formed via a single-stage coordination of compound II
at the eighth coordination site of molybdenum was
elucidated basing on the following facts: single-step
shape of the titration curve (Fig. 3), stoichiometric
ratio of I to II of 1 : 1, and no new electronic
absorption bands appearing in the course of the
interaction. IR spectrum of the reaction product
contained new absorption bands assigned to the
(As – A0)/(A∞ – A0)
1
K = ————————–· ———————————– . (1)
1 – (As – A0)/(A∞ – A0) {cII – c0I [(As – A0)/(A∞ – A0)]}n
In Eq. (1), c0I and cII are initial concentrations of
compounds I and II, respectively; А0, Аs, А∞ are
absorbance of at time zero of the solution of compound
+
O
N
O
N
N
N
Mo
Mo
N
N
N
N
HO
k2
OH_
N
N
(3)
N
N
N
N
N
N
IV
III
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 5 2014

NOVEL 2'-(PYRIDIN-4-YL)-5'-(PYRIDIN-2-YL)-1'- ...
949
log kef
A
cII × 104, mol/L
log сII
Fig. 4. Effective rate constant as function of concentration
of the substituted fullerene II in toluene at 298 K. R2 =
0.993.
Fig. 3. Curve of spectrophotometric titration of complex I
with compound II.
coordinated fullerene II, at 1472, 1432, 1306, 1281,
1216, 1184, 1123, 1095, 574, and 527 cm–1 absent in
the spectra of initial compounds.
Hence, the rate-limiting stage (3) revealed the rate
constant of k2 = k/K1 = 0.132 × 10–4 s–1. It was the low
rate of irreversible reaction (3) as compared with
almost immediate direct reaction corresponding to
equilibrium (2) that enabled experimental determina-
tion of the equilibrium constant K1.
As the UV-vis spectra did not change upon further
increase of the fullerene II concentration up to 2.4 ×
10–4 mol/L, no other equilibriums were established in
the system. However, upon titration of metal porphyrin
with the base, some slow ireversible process was
revealed occurring after the equilibrium had been
established, in the cases of all the studied
concentrations of II. The kinetics study (see above)
confirmed the monomolecular type of transformation
of product III.
The suggested scheme of quasi-stationary state was
confirmed by studies of spectral changes in the course
of irreversible stage (Fig. 5, с0II of 4.16 × 10–5 mol/L).
The reaction was accompanied with decreasing the
absorption at 462 nm and increasing the absorption at
485 nm. The red shift of the Soret band and the slow
reaction course suggested that significant changes
occurred in the coordination sphere of the complex,
namely, displacement of the covalently bound
Effective rate constants determined at different
initial concentrations of compound II are collected in
Table 1. It is to be seen that the reaction rate obeyed
well the first-order kinetics with respect to the initial
complex I (the mean-squared deviation of the rate
constants did not exceed 10%). The reaction rate order
with respect to сII equaled to unity (Fig. 4), the
reaction rate constant was of k = 0.26 L s–1 mol–1, and
the rate equation was thus as follows [Eq. (4)]:
Table 1. Effective rate constants of the reaction between the
complex I and the substituted fullerene II in toluene at
298 K as function of concentration of compound II
cII × 104, mol/L
(kef ± δkef) × 105, s–1
0.66
1.1 ± 0.09
–dcIII/dτ = –dcI/dτ = k·cI·cII.
(4)
0.76
0.96
1.15
1.44
1.63
2.02
1.3 ± 0.15
1.7 ±0.15
2.1 ± 0.18
2.4 ± 0.23
2.8 ± 0.19
3.7 ± 0.48
The reaction rate equation written for the quasi-
equilibrated system of reactions (2) and (3) coincided
with the experimentally derived Eq. (4). The
experimental reaction rate constant k included the rate
constant k2 of the slow irreversible reaction (3) as well
as the equilibrium constant K1 of reaction (2) [Eq. (5)].
–dcIII/dτ = –dcI/dτ = k2·cIII = k2·K1cIcII.
(5)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 5 2014

950
MOTORINA et al.
A
λ, nm
Fig. 5. Electronic absorption spectra of complex I in toluene, at concentration of substituted fullerene II of 4.16 × 10–5 mol/L,
immediately after mixing (1) and after 24 h (2). Other curves correspond to the intermediate states.
hydroxyl to the second coordination sphere to form
cationic outer sphere complex IV.
some special features. Three reversible stages of the
interaction were observed. In contrast to the case of
substituted fullerene II, those equilibriums were estab-
lished relatively slowly, thus enabling determination of
the rates of the corresponding direct and back reactions
(Table 2) (see the procedure and raw experimental
results in [12]). Product of the first stage of the
interaction was similar to that obtained in the system
containing the fullerene II: outer sphere cationic
complex [O=Mo(Py)TPP]+·OH–, the equilibrium
constant K1 (Table 2) being only two times lower than
that in the case of fullerene II. At the second and the
third stages, successive substitution of the bidentate
oxo group with two pyridine molecules occurred.
Thus, the mixture of metal porphyrinate I and the
fullerene II could be considered a self-assembling
system. A stable coordination complex formed via
binding of electron-donor atom of the pyridine groups
with the central metal ion existed in the solution. In the
supramolecular complex, π-electronic donor (porphyrin
macrocycle) and acceptor (fullerene molecule) co-
existed. We elucidated that the coordination complex
was formed via the only pyridinic nitrogen atoms of
the three. Likely, that was the most sterically available
nitrogen atom of the pyridin-4-yl fragment in the
position 2' of pyrrolidine. Such complexes can form
promising materials due to formation of stable charge
separation systems upon photoexcitation.
Evidently, the higher equilibrium constant in the
case of reaction between compounds I and II was due
to the presence of two additional pyridine and one
additional pyrrolidone nitrogen atoms in molecule II,
making it more basic. That the second and the third
stages of interaction were not observed in the case of
II, was due to steric hindrance of bulky base molecule.
A reaction similar to the above-discussed one
(substitution of the covalently bound hydroxyl group
with a molecular ligand) occurs in the mixture of I
with unsubstituted pyridine (Py), revealing, however,
Stability of coordination complexes of metal
porphyrins with pyridine and pyridinyl derivatives of
fullerenes (PyF) decreases in the following series.
Table 2. Constants of stepwise equilibriums in the complex
I–pyridine system (Kp), the order of direct reactions with
respect to cPy (m), and rate constants of direct (k) and reverse
(k–) reactions in toluene at 298 K
(Cl)(PyF)InTPP (K1 6.92 × 104 L/mol, in CHCl3)
→ O=Mo(Py3F)(OH)TPP (1.97 × 104, in toluene)
→ (PyF')ZnTPP (1.6 × 104, in cyclohexanone [19])
→ (Py2F)ZnTPP (1.2 × 104, in cyclohexane [19])
→ [O=Mo(Py)TPP]+·OH– (9.1 × 103, in toluene [12])
→ (Cl)(Py3F')InTPP (2.6 × 105 L4/mol4,
Reaction
stage
m
k, L s–1 mol–1
k–, s–1
5.75 × 10–4 9.1 × 103
Kp, L/mol
1
2
3
0.96
0.83
1.03
5.25
4 isomers, in CHCl3).
(6)
1.83 × 10–2 4.65 × 10–4
1.19 × 10–3 1.20 × 10–3
39.3
1.0
In the series (6), PyF, PyF', Py2F, and Py3F'
represent, respectively, the following derivatives of
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NOVEL 2'-(PYRIDIN-4-YL)-5'-(PYRIDIN-2-YL)-1'- ...
951
[60]fullerene: 2'-(pyridin-4-yl)-1'-methylpyrrolidinyl-,
cis-2'-(pyridin-2-yl)-5'-(4-imidazolylphenyl)pyrroli-
dino[3',4':1,2]-, cis-2',5'-di(pyridin-2-yl)pyrrolidino-
[3',4':1,2]-, and 2'-(pyridin-4-yl)-5'-(pyridin-2-yl)-1'-
(pyridin-3-yl)methylpyrrolidinyl-.
1215 (С–N); 1432, 1184, 574, 527. UV-vis spectrum
(toluene): λmax, nm 285, 315, 333 sh, 432.
Equilibrium and kinetics of the reaction between
compounds I and II was studied by means of spectro-
photometry (the molar ratios and the excessive con-
centrations methods). Freshly prepared solutions of I
and II in toluene were used for experiments, to avoid
peroxides formation. Absorbance at 462 nm was me-
asured for a series of solutions with сI of 1.08 ×
10–5 mol/L and cII of 1.38 × 10–5 to 2.02 × 10–4 mol/L
immediately after the preparation, and then was
followed in time. UV-vis spectra of the molecular
complexes were registered in the subtraction mode,
using solution of compound II of the corresponding
concentration as reference. Solutions temperature was
maintained at 298 ± 0.1 K during the measurement.
All the pyridine derivatives of the fullerene coor-
dinated readily at the central atom of the corresponding
metal porphyrin; the complex III had the second high
stability constant, right after the sterically not hindered
(Cl)(PyF)InTPP.
To conclude, ability of the pyridinyl-substituted
pyrrolodinofullerene to axial coordination was higher
in the case of complexes of highly charged InIII and
МоV. Significantly high charge at the central ion of
metal porphyrin and the presence of additional
nucleophilic sites favored the complex formation. Such
complexes, in particular that of the fullerene II with
the molybdenum compound I, can be promising for
application as photoactive layer in organic solar cells.
Reaction rate constants as function of cII were
determined following the first-order Eq. (7) under
condition of excess of the base II with respect to
compound I:
EXPERIMENTAL
kef = 1/τln [(A0 – A∞)/(Aτ – A∞)],
(7)
UV-vis spectra were registered with the UV-Vis
Agilent 8453 spectrophotometer; IR spec-tra were
recorded with the VERTEX 80v instrument.
where А0, Аτ, А∞ being, respectively, initial, current
(time τ), and final absorbance at 462 nm.
Equilibrium constant K and rate constant kef were
determined via the least squared method (Microsoft
Excel).
Hydroxyoxo(5,10,15,20-tetraphenylporphyrinato)-
molybdenum(V) [I, O=Mo(OH)TPP] was prepared
following the procedure published in [20]. 0.1 g
(0.16 mmol) of H2TPP was refluxed with 0.076 g
(0.53 mmol) of МоО3 in 0.8 g of phenol at 454 K
during 4 h. The solid complex was isolated after
phenol distillation in vacuum, dissolved in CHCl3, and
twice purified via column chromatography on Al2O3
(CHCl3). The two successive zones were unreacted
porphyrin (pink) and the complex (green). Yield 60%.
IR spectrum (KBr), ν, cm–1: 701, 752 [γ(C–H)]; 1070,
1177 [δ(C–H)]; 1484, 1596 (C=C); 800 [γ(C–H)];
1018 [C3–C4, C–N, δ(C–H)]; 1336 (C–N); 1441
(C=N); 441 (Мо–N), 659 (Мо–O), 928 (Мо=O). UV-
vis spectrum (CHCl3), λmax, nm (log ε): 620.0 (2.94),
584.0 (2.92), 456.0 (3.78).
ACKNOWLEDGMENTS
This work was financially supported by the N. 8
Program of Fundamental Research of Russian
Academy of Sciences and by Russian Foundation for
Basic
Research
(12-03-00967,
12-03-33031
mol_a_ved, 13-03-01170).
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