Titanium-Based Tetrakis-2,3-[5,6-di(Substituted)pyrazino]porphyrazine: Synthesis and Characterization

Article abstract of DOI:10.1002/ejic.202000193

Tetrapyrazinoporphyrazine (TPysPz) ligands and metal complexes find, generally, application as electronic materials and catalysts. Considering the limited application of Titanium (Ti), we prepared and characterized a family of ligands and Ti-based complexes of tetrakis-2,3-[5,6-di-R₈-pyrazino]porphyrazine (R = H, 2-Py, Ph). UV/Vis measurements in different solvents confirm molecular aggregation, which results more pronounced in the presence of 2-pyridil and phenyl substituents on the macrocycle edge. Because of low solubility, solid state NMR was applied for structure characterization. Additional IR and MALDI-TOF were carried out to complete the characterization. Cyclic voltammetry in DMSO/Bu₄NBF₄ 0.1 m unveiled that our Ti complexes can take part in up to five redox events. The first two quasi-reversible reductions involve Ti(IV), whereas the further to or three occur at the expense of the TPysPz macrocycle. To test the applicability of our compounds as catalytic materials, we performed a preliminary cyclic voltammetry investigation in the solid-state, which showed typical peaks of hydrogen redox reactions.

Full text of DOI:10.1002/ejic.202000193

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European Journal of Inorganic Chemistry
Accepted Article
Title: Titanium-based tetrakis-2,3-[5,6-
di(substituted)pyrazino]porphyrazine: Synthesis and
Characterization
Authors: Polyssena Renzi, Lucia Mazzapioda, Francesca Nardelli,
Francesca Martini, Marco Geppi, Carmine Mancone, Maria
Assunta Navarra, Francesca D'Acunzo, and Patrizia Gentili
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000193
Link to VoR: https://doi.org/10.1002/ejic.202000193
01/2020

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10.1002/ejic.202000193
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Titanium-based tetrakis-2,3-[5,6-
di(substituted)pyrazino]porphyrazine: Synthesis and
Characterization
Polyssena Renzi,[a]* Lucia Mazzapioda,[a] Francesca Nardelli,[b] Francesca Martini,[b] Marco Geppi,[b]
Carmine Mancone,[c] Maria Assunta Navarra,[a] Francesca D’Acunzo,[d] Patrizia Gentili,[a]
[a]
Dr P. Renzi, Dr L. Mazzapioda, Prof. P. Gentili, Dr M. A. Navarra
Department of Chemistry
Sapienza Università di Roma
P.le A. Moro 5, 00185 Rome, Italy
E-mail: polyssena.renzi@uniroma1.it
Homepage URL: https://www.chemistry.unito.it/do/docenti.pl/Show?_id=prenzi#tab-profilo
Dr F. Nardelli, Dr F. Martini, Prof. M. Geppi
Department of Chemistry and Industrial Chemistry
University of Pisa
Via G. Moruzzi 13, 56124, Pisa, Italy
Prof. C. Mancone
Department of Molecular Medicine
Sapienza University of Rome
[b]
[c]
[d]
Viale Regina Elena 291, 00161 Rome, Italy
Dr. F. D’Acunzo
Consiglio Nazionale delle Ricerche
Istituto per I Sistemi Biologici
Sezione Meccansimi di Reazione
c/o Department of Chemistry
Sapienza Università di Roma
P.le A. Moro 5, 00185 Rome, Italy
Supporting information for this article is given via a link at the end of the document.
Abstract: Tetrapyrazinoporphyrazine (TPysPz) ligands and metal
complexes find, generally, application as electronic materials and
catalysts. Considering the limited application of Titanium (Ti), we
properties, activity and solubility in organic or aqueous media. As
reported in the literature, several metals (Mg, Zn, Cu, Ni, Fe, Co,
Si) can be coordinated by TPysPzs. Among them, Titanium (Ti)
has been seldom employed in the template synthesis of
tetrapyrazinoporphrazine metal complexes,[3] despite its
abundance in the Earth’s crust and its similarity in ionic radius and
coordination chemistry with iron (FeIII).[4-6] In general, in oxidizing
atmosphere, titanium exists as Ti(IV) and it can be coordinated by
tetrapyrrole ligands as a titanyl unit (TiO2+).[7-10]
In this paper, we present the synthesis and the characterization
of a series of ligands and Ti-based complexes of tetrakis-2,3-[5,6-
di-R8-pyrazino]porphyrazine (R= H, 2-Py, Ph). For all the
compounds (2a-c and 3a-c, Scheme 1) a full spectroscopic and
electrochemical characterization is presented. Aggregation
behavior was highlighted by UV-Vis spectroscopy, while cyclic
voltammetry was employed to unveil the presence of differences
in the redox behavior between Ti-based complexes 3a-c, the
corresponding ligands 2a-c and similar metal complexes of
TPysPzs previously described in literature.
prepared and characterized
a family of ligands and Ti-based
complexes of tetrakis-2,3-[5,6-di-R8-pyrazino]porphyrazine (R= H, 2-
Py, Ph). UV-Vis measurements in different solvents confirm molecular
aggregation, which results more pronounced in the presence of 2-
pyridil and phenyl substituents on the macrocycle edge. Because of
low solubility, solid state NMR was applied for structure
characterization. Additional IR and MALDI-TOF were carried out to
complete the characterization. Cyclic voltammetry in DMSO/Bu4NBF4
0.1 M unveiled that our Ti-complexes can take part in up to five redox
events. The first two involve quasi-reversible Ti(IV) reductions
followed by two or three reduction at the expense of the TPysPz
macrocycle. To test the applicability of our compounds as catalytic
materials, we performed
a
preliminary cyclic voltammetry
investigation in the solid-state, which showed typical peaks of
hydrogen redox reactions.
The interest towards compounds 3a-c arises from the potential
application of these molecules as catalysts in oxygen reduction
reaction (ORR),[11-12] as dyes in dye sensitized solar cells
(DSSC),[13-15] and in the detection of hydrogen peroxide.[16] Indeed,
the increasing energy demand connected with the replacement of
fossil fuels and environmental concerns has made necessary the
development of advanced and next-generation technologies for
energy production. Among the approaches pursued to enhance
these processes, the use of catalysts appears very promising.
Although porphyrin, corrole and phtalocyanine-based ligands can
Introduction
Azaanalogues of phthalocyanines, tetrapyrazinoporphyrazines
(TPysPzs) ligands and metal complexes have seen a rapid
development in the last three decades. Ranging from electronic
materials to photodynamic therapy and catalysis, a plethora of
applications has been documented in the literature for these
compounds.[1-2] Interestingly, the introduction of different
substituents in the periphery of the macrocycle is able to tune its
1
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be a valid choice,[11-12] their synthesis is frequently characterized
by low yields, a feature which can hamper their production on
industrial scale. While the proof of concept has been
demonstrated, the potentiality of tetrapyrazinoporphyrazines
derivatives is still to be explored.
and
crystallized
from
water/ethanol
prior
to
use.
Pyrazinoporphyrazine macrocycles 2a-c were prepared
according to the procedure reported by Ercolani, Kadish et al. by
autocyclotetramerization of 1a-c at high temperatures (from
175°C to 240°C according to 1a-c melting-point) in the presence
of DBU as catalyst (2a: 21% yield; 2b: 20% yield; 2c: 18%
yield).[18-19] While the ligand synthesis can be realized without
paying specific attention to the water content, the synthesis of the
complex needs the application of Schlenk technique and stringent
dry reaction conditions in the presence of an excess of 1a-c in
order to avoid the formation of titanium dioxide (TiO2). The Ti-
complex was obtained by melting the starting-material 1a-c and
subsequently adding titanium (IV) butoxide against a stream of
inert gas. The reaction was kept at high temperature for 5 hours,
then the greenish/bluish solids 3a-c were purified by washing with
MeOH and/or CH2Cl2 in order to remove the contaminants. The
solids were collected each time by centrifugation and dried under
vacuum to constant weight (3a: 78% yield; 3b: 65% yield; 3c: 95%
yield).
Results and Discussion
Preparative Aspects.
As reported in the Experimental section, ligands 2a-c and
complexes 3a-c can be prepared from the common starting-
materials 1a-c (Scheme 1). The synthesis of 1a-c was
accomplished by modifying the procedure reported by Mørkved
employing diaminomaleonitrile and an α-diketone as starting-
material in glacial acetic as solvent (1a: 76% yield; 1b: 73% yield;
1c: 85% yield).[17] The solids obtained were purified by extraction
and washing with water, brine and saturated solutions of NaHCO3,
Complexes
Ligands
R
R
R
R
N
R
N
R
N
N
R
R
N
N
N
N
N
N
N
O
Ti
Ti(OBu)4
Neat
N
R
R
N
CN
CN
HN
HN
N
DBU
N
N
N
N
Neat
N
N
N
N
Δ, under Ar
N
N
Δ
N
N
N
1a-c
N
R
R
N
R
N
N
R
N
R= H, 2-Py, Ph
R
R
R
3a-c
2a-c
R
N
N
N
6
7
6
5
4
5
4
N
N
3
N
N
N
7
3
N
3
N
N
N
N
N
N
N
8
N
N
N
N
2
2
O
Ti
N
N
O
Ti
2
N
O
Ti
N
N
N
1
1
1
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
3b
3a
3c
N
N
Scheme 1. Reaction scheme for the synthesis of ligands 2a-c and Ti-based complexes of tetrakis-2,3-[5,6-di-R8-pyrazino]porphyrazine (R= H, 2-Py, Ph) 3a-c.
Numbers in blue will be useful in the assignment of the 13C-SSNMR spectra reported in Figure 1.
Solid State Studies:
Solid State NMR Studies.
ligands 2a-c. It should be noted that in CP spectra the relative
intensities of the different peaks are not quantitative, since signals
form carbon-13 nuclei in closest proximity to protons are
enhanced by the cross-polarization process.
For complex 3a, a single peak centered at 148.9 ppm is observed
in the aromatic region, similarly to what already reported in
literature, [3] which is ascribable to carbons 1-3 (Figure 1a). In the
Due to the poor solubility of complexes 3a-c in common organic
solvents, NMR characterization was performed by Solid State
Nuclear Magnetic Resonance (SSNMR). 1H-13C Cross
Polarization-Magic Angle Spinning (CP-MAS) spectra were
acquired and compared with the spectra of the corresponding
2
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case of complex 3c, three main peaks can be identified, which are
centered at 128.4 ppm (carbons 5, 6, 7), 137.7 ppm (carbon 4)
and at about 151.0 ppm (carbons 1, 2, 3) (Figure 1b). The
spectrum of complex 3b presents four main signals in the
aromatic region centered at 123.7 ppm (carbons 5 and 7), 136.3
ppm (carbon 6), 147.9 ppm (carbon 8) and 155.6 ppm (carbons 1,
2, 3, 4) (Figure 1c).
The spectra of each ligand present carbon resonances similar to
the ones observed for the corresponding complex; only a small
change of their chemical shifts can be observed (see Supporting
Information) indicating that the coordination with the titanyl group
IR Spectra.
Pyrazinoporphyrazine macrocycles object of this study exhibit
quite similar IR spectral patterns. In general, signals of Ti-
complexes are sharper and more intense than those of the
corresponding ligands, especially between 900 and 1340 cm-1.
The porphyrazine macrocycle, which is characterized by the
presence of conjugated CC and CN double bonds, gives typical
absorptions in the range 1000 – 1600 cm-1 due to skeletal
vibrations. Interestingly, the introduction of the Ti=O group in
complexes 3a-c causes the appearance of peaks located near
700 and 900 cm-1 (3a: 744 cm-1, 906 cm-1; 3b: 704 cm-1, 743
cm-1, 993 cm-1; 3c: 762 cm-1, 938 cm-1).[10, 28-29] Because of the
presence of the NH stretching, broad absorptions above 3000 cm-
induces only
a slight modification of the symmetry and
conformational properties of the tetrapyrazinotetraazaporphyrine
macrocycle. Moreover, the spectra of all complexes and ligands
present low intensity signals in the aliphatic region between 0 ppm
and 70 ppm (Figure 1d-f, see Supporting Information), probably
due to residual solvent molecules (such as acetone) co-
crystallized with the complexes and/or to reaction by-products.
1
are observed for ligands 2b-c (2b: 3292 cm-1, 2c: 3053 cm-1),
while for compound 2a this broad peak is shifted to 2958 cm-1.
Complex 3c also shows a broad absorption around 3056 cm-1,
which can be attributed to the stretching of aromatic CH (see
Supporting Information for full IR spectra).
Solution Studies: UV-Vis Spectra.
The UV-vis spectral behavior of Titanium-based complexes 3a-c
was studied in dimethylformamide (DMF), a polar aprotic solvent,
and in methylenchloride (CH2Cl2), a nondonor solvent. A greater
solubility was obtained in DMF. A summary of 3a-c absorptions,
together with a comparison with the corresponding ligands 2a-c
is presented in Table 1. The UV-vis spectra of compounds 3a-c
strictly parallel the ones of the corresponding un-metallated
macrocycles, with the exception in the B band region (Table 1).
Table 1. UV-Vis spectral data for ligands 2a-c and complexes 3a-c in
different solvents.
Compound
Solvent
B band region
[nm]
Q band region [nm]
2a
3a
3a
2b
3b
3b
2c
2c
3c
3c
DMF
DMF
404 435 576
326 587
338 585
355 421 581
315
636
647
CH2Cl2
CH2Cl2
DMF
646
617 634 668
603 667
634 668
657
Figure 1. Aromatic (left panel) and aliphatic (right panel) regions of the 1H-13
CP-MAS spectra of complexes 3a (a, d), 3c (b, e) and 3b (c, f). Spinning side
bands are marked with asterisks.
C
CH2Cl2
DMF
320 577
336
MALDI-TOF Analysis.
Ligands and complexes were analyzed by MALDI-TOF analysis
employing 2,5-dihydroxybenzoic acid (DHB) as matrix. DHB was
preferred because it undergoes less fragmentation with respect to
other reference compounds under laser shot. To this end,
samples were prepared according to the procedures reported in
literature.[30-32] In general, for ligands 2a-c, the precursor ion
results from the addition of a proton to form the positively charged
molecular ion [M+H]+. Moreover, sodium [M+H+Na]+ and
potassium [M+H+K]+ adducts are also identified. Similarly,
complexes 3a-c are detected as precursor ion [M+H]+ as well as
in cation adduct clusters. MALDI spectra and a summary of all the
species detected are displayed in Supplemental Information.
CH2Cl2
DMF
342
638 667
605 665
607 637 666
375
CH2Cl2
362 441
Ligands are, generally, characterized by an envelope in the 300-
400 nm region, whereas the corresponding metallated compound
presented more defined and red-shifted absorptions (see
Supporting Information). The UV-vis spectra of complexes in DMF
are characterized by intense and sharp peaks in the Q-region
respectively at 647 (3a), 667 (3b) and 665 (3c) nm (Figure 2 and
Table 1). Generally, a negligible solvent effect is observed in the
Q band. On the contrary, changing the solvent from DMF to
3
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