Synthesis, stability, and electrocatalysis by Cu(II) and Zn(II) complexes of meso-bridged isomeric porphyrinoid tetraphenylporphycene

Article abstract of DOI:10.1134/S0036023617050035

Spectral characteristics (electronic absorption spectra, fluorescence emission spectra, ¹H NMR) of prepared Cu(II) and Zn(II) complexes of 2,7,12,17-tetraphenylporphycene {H₂(β-Ph)₄Por}, their thermal stability, and persis

Full text of DOI:10.1134/S0036023617050035

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2017, Vol. 62, No. 5, pp. 688–694. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © D.B. Berezin, Vu Thi Thao, S.S. Guseinov, O.V. Shukhto, N.M. Berezina, M.I. Bazanov, D.V. Petrova, A.S. Semeikin, 2017, published in Zhurnal
Neorganicheskoi Khimii, 2017, Vol. 62, No. 5, pp. 694–700.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Synthesis, Stability, and Electrocatalysis by Cu(II)
and Zn(II) Complexes of meso-Bridged
Isomeric Porphyrinoid Tetraphenylporphycene
D. B. Berezina, *, Vu Thi Thaoa, S. S. Guseinovb, O. V. Shukhtoa,
N. M. Berezinaa, **, M. I. Bazanova, D. V. Petrovaa, and A. S. Semeikina
a Research Institute of Macroheterocyclic Compounds, Ivanovo State University
of Chemistry and Technology, Ivanovo, 153000 Russia
b Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
*e-mail: berezin@isuct.ru
**e-mail: sky_berezina@rambler.ru
Received March 30, 2016
Abstract—Spectral characteristics (electronic absorption spectra, fluorescence emission spectra, 1H NMR)
of prepared Cu(II) and Zn(II) complexes of 2,7,12,17-tetraphenylporphycene {H2(β-Ph)4Por}, their thermal
stability, and persistence toward acids, as well as electrochemical and electrocatalytic properties have been
discussed. It has been found by cyclic voltammetry that the electrocatalytic activity of metalloporphycenes
M(β-Ph)4Por in the reaction of molecular oxygen reduction is close to that of metalloporphyrins M(ms-
Ph)4P. Thermal stability and persistence of M(β-Ph)4Por in acidic medium are considerably lower than those
of porphyrin analogs. Thus, the kinetic stability of copper(II) complex of 2,7,12,17-tetraphenylporphycene in
HOAc–H2SO4 medium as compared with Cu(ms-Ph)4P is lower by more than an order of magnitude, while
its thermal stability is almost 200°С lower, which is caused by sterically unfavorable change in the shape of
coordination center of ligand H2(β-Ph)4Por upon complexation.
DOI: 10.1134/S0036023617050035
Porphyrinoids, structural analogs of porphyrins,
are united into one group of compounds, although
they include different classes of aromatic oligopyrrole
macroheterocycles [1–3]. Each class of H2P analogs
has a number of individual structural features, which
affect the properties of these ligands and their metal
complexes as compared with porphyrins themselves
(H2P and MP). In particular, among the features of
structural or meso-bridged isomers of porphyrins, por-
phycenes (H2Por), for example compound I, one can
note a change in the shape of coordination center of
H2P molecule, for example II, from square to rectan-
gular and formation of strong intramolecular N…H–N
hydrogen bond, which assessed by quantum chemical
methods to reach 50 kcal/mol [4, 5]. These structural
changes cause low stability of acid–base forms of
H2Por ligand and its weak coordinating ability [1, 2].
At the same time, the dependence of physicochem-
ical properties and reactivity of this class of com-
pounds on the structure of macrocycle remains poorly
studied [2, 5, 9] owing to complicated synthesis [1, 2,
6, 8]. Therefore in this work, we prepared and identi-
fied by spectral methods (electronic absorption spec-
tra, fluorescence emission spectra, 1H NMR,
MALDI) copper(II) and zinc(II) complexes of
2,7,12,17-tetraphenylporphycene ({H2(β-Ph)4Por},
compound I), analyzed their spectral properties, ther-
mal stability in solid state, and persistence in acid
solutions, studied electrochemical characteristics and
ability of the complexes to catalyze electrochemical
reduction of molecular oxygen.
EXPERIMENTAL
Copper(II) (Ia) and zinc(II) (Ib) complexes of
{H2(β-Ph)4Por} were synthesized by reacting ligand I
[10] with hundredfold excess of copper and zinc ace-
tates upon refluxing reaction mixture in DMF for 3 h.
Yield was 99% and 78%, respectively.
Porphycenes are considered to be promising com-
ponents of new materials for technology and medicine
[6, 7].
688

SYNTHESIS, STABILITY, AND ELECTROCATALYSIS
689
Ph
Ph
M
Ph
Ph
Ph
Ph
Ph
Ph
Ph
7
3
5
6
3
2
2
7
8
NH
N
N
N
N
N
N
N
HN
N
N
N
N
20
19
9
10
20
18
M
Ph
Ph
Ph
Ph
10
NH
N
HN
15
12
17
12
16
13
13
17
Ph
Ph
II
Ph
Ia M = Cu
Ib M = Zn
IIa M = Cu
IIb M = Zn
I
Ia: MS (MALDI) (m/z): М+ 675.94 (calculated
676.28).
Ib: 1H NMR (CDCl3, δ, ppm): 9.92 (s, (4H, ms-H),
9.45 (s, 4H, β-H), 8.30 (m, 8H, H orthо), 7.82 (m, 8H,
Hmeta), 7.69 (m, 4H, Hpara).
HOAc– H2SO4 and C6H6–HOAc media according to
the previously described procedure [11].
M(β-Ph)4Por + 2HX → H2(β-Ph)4Por + MX2, (3)
−
where X = OAc–,
HSO4.
Preparation of active mixtures of catalyst based on
compounds I, Iа, and Ib, as well as electrochemical
and electrocatalytic studies were carried out according
to the previously described procedure [12]. The mea-
surements were conducted by cyclic voltammetry in a
YaSE-2 three-electrode electrochemical cell in aque-
ous 0.1 M KOH (reagent grade) solution. In the study
of redox processes on the surface of initial and modi-
fied with porphycene electrodes, electrolyte was first
purged for 40 min with argon (99.99%) by bubbling at
a rate of 0.14 mL s–1. Next, working electrode was
merged into electrolyte and cyclic voltammetry data
were registered in potential range from 0.5 to –1.5 V.
After completion of measurements under an argon
atmosphere, gaseous oxygen was introduced into the
electrolyte. The potentials of cathode (Ecat) and anode
(Ean) maxima for electrode processes with participa-
tion of the studied compounds were measured with
accuracy 0.01 V. Organic solvents (C6H6, CHCl3,
DMF (reagent grade), HOAc (pure grade)) and water
used in experiments were purified according to recom-
mendations reported in the work [13].
H2(β-Ph)4Por + M(OAc)2(DMF)n
(1)
→ M(β-Ph)4Por + 2HOAc + nDMF.
Electronic absorption spectra of the studied com-
pounds were recorded on a SHIMADZU UV-1800
1
spectrophotometer, H NMR spectra were registered
on a Bruker Avance 500 spectrometer operating at
500 MHz at 298 K in СDCl3 solutions, mass spectra
were obtained on a SHIMADZU MALDI TOF
AXIMA Confidence mass spectrometer (2,5-dihy-
droxybenzoic acid as a matrix).
Fluorescence emission spectra (FES) obtained
upon chromophore excitation at 450 nm were
recorded on a SM 2203 spectofluorimeter (Belarus).
Diluted solutions of macrocycles in chloroform,
whose optical density in long-wavelength band of
electronic absorption spectra was not larger 0.1, were
used as samples. The Stokes shifts of boundary bands
of electronic absorption spectra and FES of com-
pounds were calculated by the formula
ΔνSI t = 10 7 (λ1fl − λIabs) λ1flλIabs
,
(2)
f l
abs
RESULTS AND DISCUSSION
where
is wavelength in the first FES band,
is
λ 1
λI
wavelength in the first band of electronic absorption
spectrum of the compounds.
The spectral properties of porphycene I and its metal
complexes Ia and Ib are rather characteristic. The elec-
tronic absorption spectrum of {H2(β-Ph)4Por} (I) in
the visible region shows three absorption bands
instead of four bands for isomeric porphyrin itself
{H2(ms-Ph)4P, II}, the long-wavelength band of I
being bathochromically shifted by 10 nm [5], which
seems to be explained by the symmetrical polarization
of porphycene chromophore by two –CH=CH–
meso bridges. The number of bands in the visible
region of electronic absorption spectrum of com-
pound I decreases upon complexation to one or two
Thermogravimetric studies were conducted on a
DSC 204 F1 differential scanning calorimeter
equipped with a NETZSCH TG 209 F1 Iris thermo-
balances (Germany). A weighed sample of crystalline
substance about 5 mg was placed into a platinum cru-
cible and heated under static argon atmosphere at a
rate of 10 K/min in temperature range 298–1223 K.
The studied samples were dried prior to experiment for
several hours in vacuum (<1 mmHg) at ambient tem-
perature.
The kinetics of complex dissociation reaction (3) (Fig. 1). The coordination of metal atom to porphy-
was registered by spectrophotometric method in cene I causes hypsochromic shift of the long-wave-
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690
BEREZIN et al.
A
(а)
(b)
A
1.8
670
1.2
683
2
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
2
1
395
658
378
1
656
627
586
400
500
600
700
800
900
λ, nm
300
400
500
600
700
800
900
λ, nm
Fig. 1. Absorption spectra (1) and fluorescence spectra (2) of 2,7,12,17-tetraphenylporphycene ligand (a) and its zinc complex (b)
in CHCl , T = 298 K.
3
evidenced by broadened 1H NMR spectrum and
length band in electronic absorption spectrum of
MPor by 19 nm for CuII(β-Ph)4Por (Ia) and does not sharp, by more than two orders of magnitude,
change its position for more labile ZnII(β-Ph)4Por (Ib)
(Table 1). The corresponding metalloporphyrins IIa
and IIb display hypsochromic shifts of 108 and 54 nm.
Thus, the shift of band I in electronic absorption spec-
trum upon complexation is expressed for MPor weaker
than for MP, which indicates weaker σ binding of
decrease of fluorescence intensity in FES.
On account of low correspondence of size and
shape of coordination center of molecule I to the
structural characteristics of metal, porphycene com-
plexes Ia and Ib are less stable toward dissociation in
acid medium than metalloporphyrins [3, 14]. Thus,
metal, however, the spectral criterion of complexation CuII(β-Ph)4Por (Ia) dissociates even in 0.6–1.4 M
strength is still fulfilled [14]: the hypsochromic shift of
electronic spectral band is larger for kinetically more
stable complex Ia.
H2SO4 in HOAc, the process completes over 1 h to
give doubly protonated ligand form (Fig. 2). Accord-
ing to the literature data [14], metalloporphyrin
CuII(ms-Ph)4P (IIa) in 1.0–1.5 M H2SO4 in propionic
acid medium undergoes dissociation 10–30 times
slower than Cu tetraphenylporphycene (Table 3). Typ-
ical change in electronic absorption spectrum during
reaction (3) consists in the disappearance of bands at
629 and 585 nm (Fig. 2) and emergence of absorption
bands at 613 and 651, which correspond to dication
H4(β-Ph)4Por2+ [5]. The Soret band of the complex
(389 nm) splits into two components at 389 and 419 nm.
The dependence of logarithm of efficient rate constant
for the dissociation of Cu(β-Ph)4Por (3, keff (s–1)) vs.
acidity function H0 in the considered medium HOAc–
Н2SO4 [14] shows that its slope is within 2.3–2.6 (Fig. 2).
Reaction order toward solvated proton close to two
indicates the resemblance of dissociation mechanisms
for MP and MPor. One can suppose that solvoproto-
lytic mechanism suggested previously for porphyrin
complexes in acidic media [14] is applicable as a whole
to metalloporphycenes.
The fluorescence spectrum of macrocycle I and its
zinc complex Ia has one-band structure in the region
St
600–800 nm (Table 1). The Stokes shift (
nm) of
ΔνI ,
boundary band 1 of fluorescence spectrum of ligand
H2(β-Ph)4Por relative to electronic absorption spec-
trum is 14 nm (2) and increases considerably on pass-
ing to ZnII(β-Ph)4Por (25 nm, Fig. 1), which indicates
the growth of out-of-plane distortions of molecule
upon complexation. The magnitude of shift (2) for
H2(ms-Ph)4P (compound II) and ZnII(ms-Ph)4P (com-
pound IIb) is 6 and 7 nm, respectively [3] (Table 1).
The aromaticity of macrocycle I seems to be little
changed upon complexation, which is evidenced by
the minimal upfield shifts of signals of aromatic C–H
protons (0.02–0.05 ppm) in 1H NMR spectrum1
except for β-CH protons, although thermal stability of
complexes Ia and Ib decreases (Table 2), while the
Stokes shift of fluorescence spectrum of Ib is large rel-
ative to electronic absorption spectrum (Table 1).
Similarly to Cu(II) porphyrin (IIa), copper complex
of porphycene Ia has paramagnetic nature, which is
The dissociation of ZnII(β-Ph)4Por (Ib) occurs in
0.17–0.67 M HOAc in benzene solution, whereas that
of Zn porphyrin of close structure (IIb) takes place only
in glacial HOAc. The dissociation of compound Ib is fea-
tured by the fact that ligand I in this reaction (3) is
1
The values of δ for compound I are 9.95 ppm for meso,
H
9.72 ppm for β, and 7.71–8.36 ppm for phenyl protons.
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SYNTHESIS, STABILITY, AND ELECTROCATALYSIS
691
Table 1. Spectral characteristics of ligands I and II and their metal complexes in CHCl3 solutions, 298 K
Absorption spectra
Fluorescence spectra
Compound
ΔνSt, cm–1 (nm)
λ
Soret, nm (logε)
λI, nm (logε)
λ1, nm
H2(ms-Ph)4P (II) [3]
Cu(ms-Ph)4P (IIa) [3]
Zn(ms-Ph)4P (IIb) [3]
H2[(β-Ph)4Por] (I)
Cu[(β-Ph)4Por] (Ia)
Zn[(β-Ph)4Por] (Ib)
418 (5.67)
415 (5.70)
421 (5.62)
376 (5.08)
395 (5.01)
397 (4.95)
647 (3.82)
539 (4.34)
593 (3.79)
657 (4.66)
636 (4.80)
658 (4.89)
653
–
146 (6)
–
605
671
–
334 (12)
317 (14)
–
683
556 (25)
Table 2. Thermal destruction parameters of 2,7,12,17-tetraphenylporphycene (I), 5,10,15,20-tetraphenylporphin (II), and
their metal complexes under inert atmosphere (Ar)
Destruction of extra complex
Destruction of macrocycle
Compound
weight
loss, %
weight
loss, %
tonset, °C
tm, °C
tfin, °C
tonset, °C
tm, °C
tfin,°C
H2(ms-Ph)4P (II) [15]
Zn(ms-Ph)4P (IIb) [15]
H2[(β-Ph)4Por] (I)
Cu[(β-Ph)4Por] (Ia)
Zn[(β-Ph)4Por] (Ib)
–
–
–
–
–
–
–
–
470.2
497.6
448.5
254.4
264.2
486.1
537.4
466.6
310.5
324.9
498.5
577.4
486.6
355.3
357.0
51.5
41.7
45.2
50.3
21.0
–
–
–
–
67.6
99.5
99.3
133.6
126.0
151.1
5.2
4.2
evolved in unprotonated form because it has much roreduction of macrocycle (processes I, II, Table 4)
weaker (by ten orders of magnitude in MeСN– associated with the insertion of electrons into the π
СF3СООН system [5]) basic properties as compared system of H2Por ligand, which is evidenced by cyclic
with porphyrin II.
voltammetry curves (Fig. 3). The first stage of elect-
roreduction of H2(β-Ph)4Por under considered condi-
tions proceeds at more positive potentials than for por-
II
The thermal stability of H2(β-Ph)4Por (I) and its
complexes under an inert atmosphere (Ar) is also
lower than that of H2P and MP of close structure; in
contrast to MP, the complexes of MPor are consider-
ably, by 185–195°C, less stable as compared with the
ligand (Table 2). On the contrary, thermal stability
slightly increases under these conditions on passing
from porphyrin II to its zinc complex.
Thermal destruction curves for metalloporphy-
cenes, except for the stage of decomposition of macro-
heterocycle itself (250–360°C), also show low-tem-
perature stages of weight loss in the region 65–150°C
(Table 2) corresponding to the destruction of extra
complex MPor with electron donating solvent, proba-
bly DMF. Onset temperature for this process tonset for
copper complex Ia is 30°C lower than that for com-
pound Ib due to larger propensity of zinc atom to addi-
tional coordination of O-donating ligands.
phyrin macrocycle II. Potential
of the second
E
cat
process of porphycene ligand reduction is shifted by 60 mV
to the negative region as compared with the closet
structural analog II, i.e., the insertion of the second
electron into π-electron system proceeds more com-
plex. On the contrary, complex ZnII(β-Ph)4Por (Ib) is
reduced at the second process stage more easily as
compared with porphycene ligand and copper(II)
complex. Additional transition Cu2+ ↔ Cu1+ with
Ered/ox = –0.26 V localized at the metal atom was
revealed for CuII(β-Ph)4Por (Ia) (Fig. 3).
To analyze the electrocatalytic activity of com-
pounds I, Ia, and Ib in the reaction of electroreduction
of molecular oxygen, we obtained I–E curves (Fig. 3,
curve 2) corresponding to the ultimate saturation of
We established by cyclic voltammetry under an electrolyte with oxygen. The growth of catalytic activ-
argon atmosphere that ligand I and its complexes Ia ity of the studied compounds leads to depolarization
and Ib have two general sequential processes of elect- effect that appears as the shift of electroreduction wave
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692
BEREZIN et al.
(а)
(b)
A
0.9
–logkeff
3.5
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
y = –2.6086x + 8.8417
3.0
2.5
2.0
1.5
1.0
651
y = –2.5431x + 8.4637
389
419
y = –2.3054x + 8.3018
629
613
585
350
450
550
650
750
λ, nm
2.2
2.3
2.4
2.5
2.6
2.7
–H0
Fig. 2. Change in electronic absorption spectra during dissociation of Cu(ms-Ph) Por (Ia) in 0.71 M H SO in HOAc (298 K)
4
2
4
(a) and dependence of logarithm of efficient rate constant logk of reaction (3) of the complex vs. acidity function H in HOAc–
eff
0
H SO system (b).
2
4
O2 + 2e + H2O ꢀ HO2− + OH−,
O2 + 2e + 2H+ ꢀ H2O2,
of molecular oxygen E1/2(O2) to the region of positive
values as compared with system without catalyst,
which indicates the participation of the considered
macrocycles in electrode processes (4а)–(4с). The cata-
lytic activity of the studied compounds in the electrore-
duction of molecular oxygen increases in the series:
Cu(β-Ph)4Por (Ia) < H2(ms-Ph)4P (II) < H2(β-Ph)4Por
(4a)
(4b)
(4c)
O2 + 4e + 4H2O ꢀ4OH−.
The obtained results show that compounds I and Ib
exhibit more expressed electrocatalytic effect in the
reaction of oxygen electroreduction as compared with
macroheterocycle II and their activity comes close to
that of complex Co(ms-Ph)4P (Table 4).
(I) < ZnII(β-Ph)4Por (Ib) ≈ CoII(ms-Ph)4P.
I, mA
–0.6
Thus, the complexes of isomeric porphyrin ana-
logs, in contrast to the previously studied N-substi-
tuted metalloporphyrinoids [15] and metallocorroles
[11, 12], in the context of electrocatalytic activity in
the reaction of molecular oxygen reduction, are only
close to metalloporphyrins themselves and showed
relatively low thermal stability and persistence toward
acids.
–0.4
–0.2
0
2
IIcat
Icat
Cu2+|Cu+
1
0.2
0.4
0.6
ACKNOWLEDGMENTS
This work was performed in the Research Institute
of Macroheterocyclic Compounds with financial sup-
port by the Russian Science Foundation (project
no. 14–23–00204) (in the part related to the synthe-
sis, thermogravimetric analysis, and persistence
toward acids) and the Council for Grants of the Presi-
dent of the Russian Federation for Support of Young
Scientists (grant no. MK-249.2017.3) (in the part
related to electrochemical studies).
IIan
Cu+|Cu2+
Ian
0.4 0.2
0
–0.2 –0.4 –0.6 –0.8 –1.0 –1.2 –1.4
Е, V
Fig. 3. I–E curves for electrode coated with Cu(β-Ph) Por
4
–1
in 0.1 M KOH (V = 0.02 V s ). (1) Under Ar atmosphere;
(2) complete saturation of electrolyte with molecular oxygen.
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SYNTHESIS, STABILITY, AND ELECTROCATALYSIS
693
Table 3. Kinetic parameters of dissociation reaction (3) for copper and zinc complexes with ligands I and II in acid solution
E,
kJ mol–1
ΔS,
keff × 103, s–1
CHX, mol/L
–H0
Complex
T, K
J mol–1 K–1
0.60*
2.25
298
308
318
0.88 0.08
1.20 0.11
2.18 0.16
36.0 8.6
31.4 5.1
48.3 3.9
41.9 2.1
–191 29
–199 17
–135 13
0.86*
1.20*
1.43*
2.51
2.60
2.67
298
308
318
2.16 0.25
3.59 0.22
4.82 0.32
Cu(β-Ph)4Por (Ia)
298
308
318
4.76 0.31
9.67 0.48
16.3 0.96
298
308
318
9.32 0.82
15.5 1.19
27.00 2.63
–152
7
1.00**
1.50**
0.17***
298
298
0.11 0.01
0.99 0.01
76.1
79.0
–75
–46
Cu(ms-Ph)4P (IIa)
298
308
318
0.22 0.01
0.33 0.02
0.71 0.04
46.5 10.9
42.8 2.3
51.0 6.5
–168 35
0.34***
0.51***
298
308
318
0.73 0.03
1.34 0.08
2.17 0.13
–169
7
Zn(β-Ph)4Por (Ib)
Zn(ms-Ph)4P (IIb)
298
308
318
1.28 0.06
2.20 0.12
4.65 0.21
–138 21
0.67***
298
308
318
1.80 0.07
3.37 0.19
5.48 0.29
43.7 2.8
46.1 2.0
–159
–155
9
8
17.50****
298
0.26 0.02
In system: *HOAc–H SO ; **C H COOH–H SO [14]; ***C H –HOAc; **** in HOAc [3].
2
4
2
5
2
4
6
6
Table 4. Redox potentials (vs Ag/AgCl, in V) for electrodes coated with 2,7,12,17-tetraphenylporphycene (I) and its Cu(II)
and Zn(II) complexes (V = 0.02 V s–1
)
M2+ ↔ M1+
Process I
Process II
E1/2(O2)
Compound
II
red/ox
ErIed/ox
E
II
cat
II
an
EcIat
EaIn
E
E
Ecat
Ean
Ered/ox
H2(ms-Ph)4P (II) [12]
H2[(β-Ph)4Por] (I)
Cu[(β-Ph)4Por] (Ia)
Zn[(β-Ph)4Por] (Ib)
Co(ms-Ph)4P [12]
–
–
–
–0.66 –0.58 –0.62 –1.08 –0.97 –1.03 –0.28
–0.56 –0.59 –0.57 –1.14 –0.25
–
–
–0.38 –0.13 –0.26 –0.68 –0.57 –0.63 –1.17 –0.87 –1.02 –0.31
–0.60 –0.59 –0.60 –1.02 –0.81 –0.92 –0.23
0.13*
0.26* 0.20* –0.58 –0.48 –0.53 –1.20 –1.00 –1.11
–0.23
2+
3+
* In process M ↔ M
.
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BEREZIN et al.
8. W. Brennera, J. Maliga, C. Oelsner, et al., J. Porphyrins
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No. 5
2017