Synthesis and spectral properties of iron(III) tetra-tert-butylphthalocyanine complexes

Article abstract of DOI:10.1016/j.mencom.2017.09.012

Two tetra-tert-butylphthalocyanine complexes of iron(III) were synthesized in high yields from the phthalocyanine ligand and iron(III) salts; the oxidation state of iron was confirmed by M?ssbauer and EPR spectroscopy. The existence of an acid–base equilibrium during spectrophotometric titration was revealed. The ^ButPcFeCl complex catalyzed chlorination of benzene.

Full text of DOI:10.1016/j.mencom.2017.09.012

Mendeleev
Communications
Mendeleev Commun., 2017, 27, 466–469
Synthesis and spectral properties of iron(iii)
tetra-tert-butylphthalocyanine complexes
Ivan D. Burtsev, Tatiana V. Dubinina,* Yana B. Platonova,a
a
a,b
a
a
a,b
Anton D. Kosov, Denis A. Pankratov and Larisa G. Tomilova
a
b
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation.
Fax: +7 495 939 0290; e-mail: dubinina.t.vid@gmail.com
Institute of Physiologically Active Compounds, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 496 524 9508; e-mail: tom@org.chem.msu.ru
DOI: 10.1016/j.mencom.2017.09.012
But
Two tetra-tert-butylphthalocyanine complexes of iron(iii)
were synthesized in high yields from the phthalocyanine
ligand and iron(iii) salts; the oxidation state of iron
was confirmed by Mössbauer and EPR spectroscopy.
But
L
N
N
N
N
N
Fe
N
L = acac, Cl
N
The existence of an acid–base equilibrium during spectro-
t
photometric titration was revealed. The Bu PcFeCl complex
N
But
But
catalyzed chlorination of benzene.
Phthalocyanines attract attention due to broad possibilities for
have been studied less thoroughly, reliable proof of their structure
and properties being unavailable in the literature until recently.
1–7
30
structure modification: variation of substituents, expansion o
f
the p-system,8 introduction of various metals,
–17
18–22
synthesis of
We chose bulky tert-butyl groups as the peripheral substi-
sandwich complexes,2
3–25
and replacement of the axial ligand.
26
tuents as they essentially improve the solubility and decrease
3
,4,32
These modifications considerably affect the spectral, semi-
conductor and catalytic properties of phthalocyanine com-
aggregation.
By analogy with the preparation procedure of
phthalocyanine complexes of lanthanides developed in our
9
,23,27,28
33
plexes,
which opens broad prospects for compounds
laboratory, we synthesized the target compounds by metallation
possessing required properties.
of tert-butyl substituted phthalocyanine ligand 1 with iron(iii)
salts (Scheme 1). The reaction was carried out under reflux
in 1,2-dichlorobenzene (o-DCB) in the presence of 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU) as a base. Two iron sources
†
A substituted iron(iii) phthalocyanine was chosen as the object
of this study. The choice of iron as the central ion was caused by
its ability to change the oxidation state and by the possibility of
introduction of various axial ligands. Iron complexes find applica-
tion as organic semiconductors, catalysts and photosensitive
†
Immediately before the synthesis, Fe(acac) and FeCl salts were kept
3
3
2
9–31
materials.
It is important to note that, as compared to iron(ii)
in a drying cabinet at 90°C under reduced pressure. Phthalocyanine 1
was synthesized as reported previously.32
complexes, phthalocyanine and porphyrin complexes of iron(iii)
Iron(iii) 2(3),9(10),16(17),23(24)-tetra-tert-butylphthalocyaninate acetyl-
acetonate 2a. Ligand 1 (0.24 g, 0.33 mmol), Fe(acac) (0.18 g, 0.50 mmol)
and DBU (35.00 mg, 0.23 mmol) were refluxed in o-DCB (10 ml) for
3
But
But
1.5 h until the starting ligand was consumed. The reaction was monitored
N
using UV-VIS spectroscopy and TLC (Al O , toluene as the eluent). The
2
3
NH
N
N
reaction mixture was cooled to room temperature, and CHCl (25 ml)
i or ii
3
N
N
was added. The insoluble admixtures were filtered off. The solvent was
removed from the filtrate. The resulting dry residue was treated with 80%
methanol (3×50 ml). The solid remained was dried in a drying cabinet
under reduced pressure at 90°C. The yield of complex 2a was 172 mg
HN
N
But
But
But
+·
+·
1
(72%). MS (MALDI-TOF), m/z: 792 [M–acac] , 809 [M–acac+OH] .
UV-VIS [toluene, lmax/nm (I/Imax)]: 456 (0.31), 597 (0.24), 661 (1.00).
But
L
Iron(iii) 2(3),9(10),16(17),23(24)-tetra-tert-butylphthalocyaninate chloride
N
2
b. Ligand 1 (0.21 g, 0.28 mmol), FeCl (64.90 mg, 0.40 mmol) and DBU
3
N
N
(35.00 mg, 0.23 mmol) were refluxed in o-DCB (10 ml) for 1.5–2 h until
the starting ligand was consumed. The reaction was monitored using
N
Fe
N
UV-VIS spectroscopy and TLC (Al O , toluene as the eluent). The mixture
N
N
2 3
was cooled to room temperature, and CHCl (25 ml) was added. The
3
N
insoluble admixtures were filtered off. The solvent was removed from
the filtrate. The resulting dry residue was treated with 80% methanol
But
But
2
2
a L = acac (72%)
b L = Cl (81%)
(3×30 ml). The solid remainder was dried in a drying cabinet under
reduced pressure at 90°C. The yield of complex 2b was 188 mg (81%).
+
+
MS (MALDI-TOF), m/z: 780 [M–Cl–CH2 +H]
·, 792 [M–Cl] ·, 809
Scheme 1 Reagents and conditions: i, Fe(acac) , DBU, o-DCB, D; ii, FeCl ,
3
3
+
DBU, o-DCB, D.
[M–Cl+OH] ·. UV-VIS [toluene, lmax/nm (I/Imax)]: 704 (1.00).
©
2017 Mendeleev Communications. Published by ELSEVIER B.V.
–
466 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2017, 27, 466–469
7
92
7
92
100
80
2.0
(
a)
(b)
7
92
6
5
4
3
2
1
6
5
4
3
2
60
1.5
40
2
0
0
1.0
0.5
0
792
793
794
795
792 793 794 795
8
09
m/z
m/z
1
0
7
00
800
900
1000 1100 1200 1300 1400 1500
0
.0
300
m/z
400
500
600
700
800
900
Figure 1 Mass spectrum of ButPcFe(acac). The insets show (a) the peak of
l/nm
[M–acac] and (b) the theoretically calculated isotope distribution for this peak.
Figure 3 Spectrophotometric titration of a ButPcFe(acac) solution in DMF
with acetic acid (the solid lines indicate the starting and final forms; dotted
lines indicate intermediate forms).
were tested to study the effect of the counter-ion nature on the
yield of the target compounds, namely, iron(iii) acetylacetonate
and iron(iii) chloride. In the case of iron chloride, the yield of
compound 2b was higher probably due to smaller steric hindrance
in comparison with acetylacetonate.
(DBU). This phenomenon and the existence of isobestic points
indicate an equilibrium process.
But
The Mössbauer spectrum of PcFe(acac) 2a at room tem-
‡
Complexes 2a,b were characterized by MALDI-TOF mass
perature [Figure 4(a)] is a slightly asymmetric doublet that is
satisfactorily approximated by a model involving two symmetric
doublets. The isomeric shifts of both doublets (Table 1) do not
allow the oxidation state of iron to be determined unambiguously
due to a high degree of covalency of bonds between iron and
nitrogen atoms of the phthalocyanine ligand. The values obtained
spectrometry. Figure 1 demonstrates the mass spectrum of the
But
PcFe(acac) complex 2a as an example. The mass spectrum
contains a fragmentation peak with abstraction of the axial
ligand. Its isotopic pattern agrees with the theoretically calculated
one. Furthermore, a peak corresponding to the elimination of the
axial ligand and addition of a hydroxy group is also observed.
This is the evidence of a high mobility of the axial ligand in the
complexes 2a,b.
3
+
can correspond both to high-spin iron(iii) (S = 5/2) in Fe N
4
35
3
4
2+
moieties and to low-spin iron(ii) (S = 0) in Fe N moieties.
4
On the other hand, doublets with a similar isomeric shift were
3
6
The absorption spectra of complexes 2a,b were recorded in
assigned to iron(iii) compounds elsewhere. Furthermore, low
t
Bu
–1
toluene. Figure 2 demonstrates the spectrum of PcFe(acac) 2a
as an example. Interestingly, a number of charge transfer bands
are observed for complex 2a, the most prominent line being at
quadrupole splitting (0.6–0.8 mm s ) is also typical of high-
3
7
spin iron(iii) compounds.
The Mössbauer spectra for the same sample cooled to liquid
nitrogen temperature (78 K) [Figure 4(b)] considerably differ
3
0
4
56 nm. This phenomenon is typical of iron(iii) complexes.
The possibility of protonation of phthalocyanine complexes
t
allowed us to perform the acid–base spectrophotometric titration
Table 1 Parameters of Mössbauer spectra of Bu PcFe(acac) 2a.a
t
Bu
of PcFe(acac) 2a (Figure 3). The spectrum changes upon
Sub-
spectrum mm s
d (±0.01)/ D (±0.01)/ G (±0.03)/ I (±0.5) S (±5)
exp
gradual addition of acetic acid: the Q-band shifts from 668 to
80 nm, the broadened bands appear at 550 and 800 nm, while
the shoulder at 700 nm disappears. The original spectrum shape
is recovered completely upon addition of a few drops of a base
T/K
95
–1
–1
–1
mm s
mm s
(%)
(%)
6
2
1
2
0.30
0.47
0.72
0.76
0.60
0.32
6.3
3.3
78
22
7
8
1
2
3
4
0.04
0.23
0.42
0.51
0.95
2.65
0.44
0.80
0.29
0.78
0.19
0.48
6.3
0.8
3.2
17
6
6
1
1
.5
.0
.5
6
61
15.6
71
ad is isomeric shift, D is quadrupole splitting, Gexp and I are the resonance
line width and intensity, S is relative area of the subspectrum.
0
0
.0
3
(
a)
00
400
500
600
l/nm
700
800
0
5
0
2
Figure 2 UV-VIS spectrum of ButPcFe(acac) in toluene.
2
95 K
–3
1
†
UV-VIS spectra were recorded in quartz cells (1×1 cm) using a
1
Helios-a spectrophotometer. TLC was performed on Merck Aluminium
Oxide F254 neutral plates. MALDI-TOF mass spectra were recorded on
a VISION-2000 instrument.
–
4
–2
–1
0
1
2
3
4
Velocity/mm s–1
(
b)
Mössbauer absorption spectra were obtained on an MC1104EM express
Mössbauer spectrometer manufactured by ‘Kordon’ (Rostov-on-Don).
0
5
2
3
1
57
Co in a metallic rhodium matrix with 1 mC activity from ‘RITVERTs’
(
St. Petersburg) was used as the g-radiation source. To record the spectra,
10
4
7
8 K
–3
ground powdered samples in a plastic cell were placed into a vacuum
cryostat. Spectra were recorded both at room temperature and at liquid
nitrogen temperature. The temperature of the samples was controlled to
within ±2°. Mathematical processing of experimental Mössbauer spectra
was performed for high resolution spectra using the Univem MS 9.08
program. The spectra were described by a combination of symmetric
doublets. Chemical shifts are reported relative to a-Fe.
1
2
5
0
–4
–2
–1
0
1
2
3
4
Velocity/mm s–1
Figure 4 Mössbauer spectra of complex ButPcFe(acac) 2a at (a) 295 and
(b) 78 K and models for their description (the numbers indicate the numbering
EPR spectra were recorded in the X range using a Bruker EMX plus
spectrometer.
of the subspectra in Table 1).
–
467 –

Mendeleev Commun., 2017, 27, 466–469
Cl
from the spectra obtained at room temperature. First, we should
Cl
Cl
Cl
note the twofold increase in the overall resonance absorption
effect which is characteristic of iron complexes with large organic
i
ii
+
PhCl
C6H6
PhCl
3
8,39
ligands.
The spectrum itself can be satisfactorily described
Cl
by a model involving four symmetric doublets with strongly
differing parameters. In this model, minor components are
described by doublets with maximum values of quadrupole
splitting and line widths (subspectrum 2) and with minimum
values of similar parameters (subspectrum 3). On the one hand,
these subspectra can be components of a sextet that is indicative
of weak intermolecular interactions of iron atoms and is not
resolved due to relaxation phenomena. On the other hand, these
subspectra may correspond to independent iron atoms in strongly
differing environments. In such a case, the doublet with a large
line width (line 2) is attributed to iron atoms in an unordered
environment (amorphous phase), whereas, conversely, the doublet
with a very small width (line 3) corresponds to atoms in a highly
ordered environment (crystalline phase).
Cl
But
Scheme 2 Reagents and conditions: i, Cl ,
PcFeCl 2b, in the dark.
PcFeCl 2b, hv; ii, Cl2,
2
t
Bu
The second product was chlorobenzene, whose mass spectrum
contained peaks: 77 (Ph ), 112 (PhCl).
+
If the reaction was carried out for 30 min in the dark, it
proceeded selectively to give chlorobenzene (see Scheme 2).
But
The catalyst ( PcFeCl) underwent partial oxidation without
destruction of phthalocyanine macrocycle.
A more detailed study of chlorination processes will be
performed separately.
In conclusion, we obtained hitherto unreported tetra-tert-butyl-
phthalocyanine complexes of iron(iii) in high yields. The struc-
ture and oxidation state of iron were confirmed by UV-VIS
spectroscopy, mass spectrometry, Mössbauer and EPR spectro-
scopy. The target complex was successfully used as the catalyst
in benzene chlorination, both radical and electrophilic.
The doublet with the smallest isomer shift (subspectrum 1)
can be ascribed to iron(ii) or iron(iii) atoms in the low-spin state
(S = 0 or 1/2, respectively). The presence of considerable
quadrupole splitting indicates that this doublet belongs to
iron(iii) atoms. The appearance of this doublet at low
temperatures indicates that there is a thermal crossover transition
for a fraction of the complexes corresponding to this substance.
However, assuming that the transition was complete, one may
suppose that the doublet in question matches the doublet in
subspectrum 2 at room temperature. In such a case, the doublet
in subspectrum 4 at low temperature corresponds to the doublet
in subspectrum 1 at room temperature. The absence of a strong
temperature dependence of quadrupole splitting for this doublet
also indirectly supports that it belongs to an iron(iii) complex.
This study was supported by the Russian Science Foundation
(grant no. 17-13-01197).
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Received: 30th December 2016; Com. 16/5142
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