Synthesis, spectroscopic properties and redox behavior kinetics of rare-earth bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato metal complexes with er, lu and yb

Article abstract of DOI:10.3390/molecules26082181

Novel bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato of complexes er-bium, lutetium and ytterbium were synthesized using a template fusion method to prevent any polymerization process. The complexes were separated from the reaction mixtures and characterized by NMR, IR and electron absorption spectroscopy. The spectroscopic properties of the metal ph-thalocyaninates in chloroform, acetone and tetrahydrofuran were studied. The regular bathochromic shift in the Er–Yb–Lu series was determined. In acetone medium all the complexes obtained were found to exist in an equilibrium state between neutral and reduced forms. The linearity of Lambert-Bouger-Beer curves makes it possible to study the kinetics of redox processes in the presence of phenylhydrazine and bromine. The lutetium complex showed better reducing properties and turned fully into the reduced form, while the erbium and ytterbium ones changed only partially. Upon oxidizing all the phthalocyaninates transformed into a mixture of oxidized and neutral-radical forms. The extinction coefficients and effective redox constants were calculated.

Full text of DOI:10.3390/molecules26082181

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molecules
Article
Synthesis, Spectroscopic Properties and Redox Behavior
Kinetics of Rare-Earth Bistetrakis-4-[3-(3,4-
dicyanophenoxy)phenoxy]phthalocyaninato Metal Complexes
with Er, Lu and Yb
Dmitry A. Erzunov * , Anna A. Botnar, Natalia P. Domareva, Tatiana V. Tikhomirova and Arthur S. Vashurin
Department of Inorganic Chemistry, Ivanovo State University of Chemistry and Technology,
153000 Ivanovo, Russia; anna.filippova96@gmail.com (A.A.B.); domarevanat@gmail.com (N.P.D.);
tararjkina@mail.ru (T.V.T.); asvashurin@mail.ru (A.S.V.)
* Correspondence: demon_erz@mail.ru
Abstract: Novel bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato of complexes er-
bium, lutetium and ytterbium were synthesized using a template fusion method to prevent any
polymerization process. The complexes were separated from the reaction mixtures and characterized
by NMR, IR and electron absorption spectroscopy. The spectroscopic properties of the metal ph-
thalocyaninates in chloroform, acetone and tetrahydrofuran were studied. The regular bathochromic
shift in the Er–Yb–Lu series was determined. In acetone medium all the complexes obtained were
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
found to exist in an equilibrium state between neutral and reduced forms. The linearity of Lambert-
Bouger-Beer curves makes it possible to study the kinetics of redox processes in the presence of
phenylhydrazine and bromine. The lutetium complex showed better reducing properties and turned
fully into the reduced form, while the erbium and ytterbium ones changed only partially. Upon
oxidizing all the phthalocyaninates transformed into a mixture of oxidized and neutral-radical forms.
The extinction coefficients and effective redox constants were calculated.
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Erzunov, D.A.; Botnar,
A.A.; Domareva, N.P.; Tikhomirova,
T.V.; Vashurin, A.S. Synthesis,
Spectroscopic Properties and Redox
Behavior Kinetics of Rare-Earth
Bistetrakis-4-[3-(3,4-dicyanophenoxy)
phenoxy]phthalocyaninato Metal
Complexes with Er, Lu and Yb.
Molecules 2021, 26, 2181. https://
doi.org/10.3390/molecules26082181
Keywords: phthalocyanines; rare-earth complexes; synthesis; spectroscopy; redox activity
1. Introduction
Academic Editor: Jan Janczak
Phthalocyanines are widely used in many areas of activity, such as catalysis [
1,2], light
industry [ ], medicine [ ], linear and nonlinear optics and energy storage devices [7–9].
Such a widespread usage is associated with the high stability of these molecules, the
presence of intense light absorption in the visible range, the flexibility of the design of
3
,
4
5
,
6
Received: 26 March 2021
Accepted: 8 April 2021
Published: 10 April 2021
structures [10
–12], the possibility of introducing practically any metal atom into the central
cavity [13 15] and the possession of significant coordination properties. The scope of usage
–
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with regard to jurisdictional claims in
published maps and institutional affil-
iations.
directly depends on the structure of the molecule, namely on the nature of peripheral
and non-peripheral substituents, the presence or absence of a central metal atom. The
environment in which the connections are used is also strongly influenced. For example,
unsubstituted phthalocyanine is practically insoluble in most organic media, which greatly
limits the scope of its application.
Double-decker phthalocyaninates of rare-earth elements are of particular interest to
scientists due to the special structure of molecules, namely, the presence of an unshared
electron delocalized between two aromatic fragments of ligands, which is responsible
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distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
for the manifestation of atypical physical and chemical properties [16
–18], such as elec-
trochromism [19 20], nonlinear optical absorption of light [ 21 22], redox transitions [23].
,
9, ,
The properties of such structures strongly depend on the central metal atom, which is
responsible for the distance between two coordinated phthalocyanine ligands, as well as
the nature of substituents which also make a contribute.
Molecules 2021, 26, 2181. https://doi.org/10.3390/molecules26082181
https://www.mdpi.com/journal/molecules

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In the literature, there are a sufficient number of examples of the synthesis and study
of the properties of peripherally and non-peripherally substituted double-decker phthalo-
cyaninates of rare-earth elements with alkyl-, aryl-, etc. functional groups [24,25]. However,
often these types of substitution lead to strong aggregation of molecules, which adversely
affects all the properties they exhibit. The introduction of aryloxy- fragments into the struc-
ture of the phthalocyanine ring followed by the formation of a sandwich structure has been
described to a limited extent; there are only a few works devoted to this topic [26]. Despite
this, as we previously shown [27,28] this type of substitution for mono-complexes provides
excellent solubility for compounds along with intense light absorption and aggregation
stability, due to the presence of aryl- fragments that provide solubilization and oxygen
bridges that impart spatial flexibility of molecules, which together can facilitate the further
use of materials based on these compounds. The introduction of cyano- groups to the
periphery leads to difficulties in the synthesis and purification of complexes; therefore,
there are practically no works on this topic in the literature. However, the presence of
terminal cyano- groups on the periphery greatly simplifies the preparation of functional
materials based on the described ones due to the emerging possibility of simple and stable
binding of molecules to any carrier.
This work describes the preparation of new double-decker bistetrakis-4-[3-(3,4-dic
yanophenoxy)phenoxy]phthalocyaninates with erbium, ytterbium and lutetium. The
complexes were obtained as a result of template fusion, leading to the suppression of the
course of polymerization processes, characterized and studied for their spectroscopic and
redox properties.
2. Results and Discussion
2.1. Synthesis of 4,4’-[1,3-Phenylenebis(oxy)]phthalodinitrile
At the first stage, 4,4’-[1,3-phenylenebis(oxy)]phthalodinitrile (
2) was obtained by
nucleophilic substitution in 4-nitrophthalonitrile ( ) in a reaction with resorcinol (Scheme 1
1
)
following a modified version of the experimental protocol reported in [27]. The reaction
proceeded in anhydrous DMSO for 8 h, and its progress was monitored by thin layer
chromatography on SiO2 using ethanol as an eluent. After the reaction, the compound
was precipitated with a 0.1 mol/L hydrochloric acid solution, filtered on a Schott filter,
and washed with distilled water, an aqueous acid solution (HCl), and water to neutral pH.
Excess solvent was removed under vacuum. The compounds were purified by recrystal-
lization from ethanol. The compound yield was 82%.
Scheme 1. Synthesis of 4,4’-[1,3-phenylenebis(oxy)]phthalodinitrile (2).
Compound
etry. In the IR spectrum, all characteristic vibrations inherent in the described structure are
observed, such as 3088–2850 cm−1 (Car-H); 2243 cm−1 (C N); 1586–1389 cm−1 (Car=Car);
1256 cm−1 (Ar–O–Ar). The degree of completion of the nucleophilic substitution reaction
was also monitored using IR spectroscopy. The spectrum of compound after purification
does not contain symmetric (1340–1350 cm−1) and asymmetric (1560–1565 cm−1) vibrations
2 was characterized by IR, NMR spectroscopy and MALDI-TOF spectrom-
≡
2
of the nitro groups of the initial 4-nitrophthalonitrile (1). Also, the appearance of a band
at 1256 cm−1 is observed, which is responsible for the vibrations of the Ar-O-Ar group
resulting from the reaction. The high resolution mass spectra represent a signal at m/z

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362.11, corresponding to the structure of phthalodinitrile
2
. The structure was confirmed
by the position, shifts, and signal multiplicity in the 1H- and 13C-NMR spectra.
2.2. Synthesis of f-Metal Phthalocyaninates
There are several methods for obtaining metal complexes of phthalocyanines. One of
the most commonly used is condensation in a high boiling solvent. For these purposes,
4,4’-[1,3-phenylenebis(oxy)]phthalodinitrile (2) was mixed with anhydrous erbium(III)
acetate in a molar ratio of 8:1 in dry iso-amyl alcohol medium in the presence of DBU
at refluxing (Scheme 2). The reaction progress control was carried out using thin layer
chromatography (SiO2, chloroform as eluent). Unfortunately, even after 15 min after the
start of the reaction, the target compound was not detected, at the same time, a high degree
of polymerization of the reaction mass was observed and the final product was black
crystals, insoluble in commonly used organic solvents. Varying the reaction temperature,
solvent and component ratio did not give any result. Thus, a different approach was
chosen, consisting in cyclic tetramerization in the absence of a solvent (Scheme 2). These
conditions are more stringent, which has a detrimental effect on the yields of complexes;
however, they open up the possibility of direct preparation of compounds, as we have
shown earlier [29]. In a ceramic crucible 4,4’-[1,3-phenylenebis(oxy)]phthalodinitrile (2) and
anhydrous lanthanide (III) acetate in a molar ratio of 8:1 were placed, then urea was added
◦
to facilitate the transfer of the mixture into the alloy. The reaction proceeded at 190 C
for about 20–30 min (depending on the metal), after which the alloy was cooled to room
temperature and dissolved in chloroform, followed by filtration in order to separate it from
the impurities of the polymer composition and salt residues. The mass-spectrum of the
filtrate showed the presence of three main compounds in the mixture, namely, metal-free
tetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyanine, lantanide tetrakis-4-[3-(3,4-
dicyanophenoxy)phenoxy]phthalocyaninato monocomplex and target sandwich lantanide
bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato complex (Figure 1).
Figure 1. HR MALDI-TOF mass spectrum of reaction mixture after template fusion reaction with
lutetium(III) acetate (CHCA as matrix).
To isolate the desired compound the solvent was distilled off under vacuum and the
mixture was preliminarily purified by column chromatography on silica gel M60, eluting
with a gradient of chloroform with ethanol (from 0 to 15 vol.%). Final purification was
performed by gel permeation chromatography on Bio-Beads S-X1 gel with a mixture of
ethanol (2.5%) in chloroform.

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Scheme 2.
Synthetic routes of erbium
(3
), lutetium
(4)
and ytterbium
(5
)
bistetrakis-4-[3-(3,4-
dicyanophenoxy)phenoxy]phthalocyaninato complexes obtaining. i: template fusion in ceramic crucible in absence of
◦
solvent, 190 C, 20–30 min; ii: tetramerization in high-boiling solvent (iso-amyl alcohol), reflux, 24 h.
Compounds 3–5 were identified using IR, NMR spectroscopy, MALDI-TOF mass
spectrometry data. The mass-spectrum of the compound after the two-step purification
no longer contained the signals of the monocomplex and metal-free phthalocyanine, as
previously shown (Figures S1 and S2). There is only an intense band corresponding to the
target complex (Figure 2). This partly proves successful cleaning of the compound.
Figure 2. HR MALDI-TOF mass-spectrum of bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalo
cyaninato lutetium (4) (CHCA as matrix).
The IR spectra of complexes 3–5 contain the entire set of characteristic bands (Figure 3).
Besides, a phthalocyanine metal complex obtained from corresponding dinitrile has more
intensive band responding to vibrations of C≡N group (~2232 cm−1).
1H-NMR spectra were also recorded for all complexes (Figure 4, Figures S3 and S4).
For paramagnetic erbium and ytterbium, a broadening of all manifested signals and shifts
of protons of macrocyclic fragments to the region of a strong field were observed under the
influence of proximity to a metal atom. An interesting fact is that the signals of the protons
of the peripheral substituents were not shifted and appeared in the aromatic region, which
is probably due to their relative remoteness from the complexing atom; however, the effect
of the paramagnetic metal broadens the described signals as well (Figure 4).