Different phenylboronic acid azaester formation modes in a substituted zinc phthalocyanine and its precursor

Article abstract of DOI:10.1016/j.poly.2012.11.044

With the aim of preparing a novel Zn phthalocyanine bearing four phenylboronic azaester substituents at peripheral positions, 4-[2-(bis(2-hydroxyethyl)amino)ethoxy]phthalonitrile (2) was synthesized by the aromatic nucleophilic substitution reaction of 4-nitrophthalonitrile with triethanolamine. 2,9(10),16(17),23(24)-Tetrakis-[2-(bis(hydroxyethyl)amino) ethoxy]-phthalocyaninatozinc (II) (4) was prepared from the cyclotetramerization of dinitrile 2 in the presence of anhydrous zinc(II) acetate by microwave irradiation. Alternatively, treatment of 2 with phenylboronic acid yielded the phenylboronic azaester (3), namely 4-(2-(2-phenyl-1,3,6,2-dioxazaborocan-6-yl) ethoxy)phthalonitrile. Treatment of phthalocyanine 4 with phenylboronic acid in a mixture of DMSO/toluene (20/1, v/v) afforded the desired phthalocyanine 5. The novel compounds were characterized by elemental analyses, IR, UV-Vis, ¹H, ¹³C and ¹¹B NMR spectra. The ¹¹B NMR data indicated that the boron atom of phthalonitrile 3 is in the tetracoordinated state with the formation of a coordinative NB bond (closed form), while in the case of phthalocyanine 5, the open conformer possessing a tricoordinated B atom is favoured.

Full text of DOI:10.1016/j.poly.2012.11.044

Accepted Manuscript
Different Phenylboronic Acid Azaester Formation Modes in a Substituted Zinc
Phthalocyanine and its Precursor
Şennur Özçelik, Ahmet Gül
PII:
S0277-5387(12)00882-0
DOI:
http://dx.doi.org/10.1016/j.poly.2012.11.044
POLY 9799
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
11 October 2012
16 November 2012
Please cite this article as: Ş. Özçelik, A. Gül, Different Phenylboronic Acid Azaester Formation Modes in a
Substituted Zinc Phthalocyanine and its Precursor, Polyhedron (2012), doi: http://dx.doi.org/10.1016/j.poly.
2012.11.044
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Different Phenylboronic Acid Azaester Formation Modes in a Substituted
Zinc Phthalocyanine and its Precursor
Şennur Özçelik, Ahmet Gül*
Department of Chemistry, Technical University of Istanbul, Maslak, TR-34469 Istanbul,
Turkey
Abstract
With the aim of preparing a novel Zn phtahlocyanine bearing four phenylboronic azaester
substituents at peripheral positions, 4-[2-(bis(2-hydroxyethyl)amino)ethoxy]phthalonitrile (2)
was synthesized by the aromatic nucleophilic substitution reaction of 4-nitrophthalonitrile
with triethanolamine. 2,9(10),16(17),23(24)-Tetrakis-[2-(bis(hydroxyethyl)amino)ethoxy]-
phthalocyaninatozinc (II) (4) was prepared from the cyclotetramerization of dinitrile 2 in the
presence of anhydrous zinc(II) acetate by microwave irradiation. Alternatively, treatment of 2
with phenylboronic acid yielded the phenylboronic azaester (3), namely 4-(2-(2-phenyl-
1,3,6,2-dioxazaborocan-6-yl)ethoxy)phthalonitrile. Treatment of phthalocyanine 4 with
phenylboronic acid in a mixture of DMSO/toluene (20/1, v/v) afforded the desired
phthalocyanine 5. The novel compounds were characterized by elemental analyses, IR, UV-
1
Vis, H, 13C and 11B NMR spectra. The 11B NMR data indicated that the boron atom of
phthalonitrile 3 is in the tetracoordinated state with the formation of a coordinative N-B bond
(closed form), while in the case of phthalocyanine 5, the open conformer possessing a
tricoordinated B atom is favoured.
Keywords: Phthalocyanine; Phenylboronic azaester; Zinc; Boronate complex; Phenylboronic
acid; Dioxazaborocane
*Corresponding authors Tel.: +90 212 285 68 27; Fax: +90 212 285 63 86
E-mail address: ahmetg@itu.edu.tr (A. Gül)
1

1. Introduction
Organoboron chemistry holds a specific place in organic synthesis due to the unique reactivity
offered by the Lewis structure of neutral boron. One of the most promising class of
organoboron compounds is that of dioxazaboracanes. Dioxazaboracanes are obtained with a
large variety of functionalities by condensation of non-toxic, non-corrosive and widely
available diethanolamine (DEA) with boronic acids in high yields. They are solid derivatives
with high and well-defined melting points, indicating good thermal stability [1]. Moreover,
dioxazaborocanes are extremely stable towards air and moisture, thus they can be stored at
room temperature without any precautions. Their notably high thermal, air and chemical
stability ascribe to the formation of a B-N coordinative bond by the interaction between the
vacant p orbital of boron and the lone pair of nitrogen [2].
Since their first synthesis by Letsinger and Skoog [3], dioxazaborocanes have found
numerous applications in organic chemistry. These compounds were originally made in order
to isolate, characterise, immobilise or protect boronic acids. However, they turned out to
display a wide range of applications in organic synthesis, including allene synthesis,
allylation, propargylation and borylation of α,β-unsaturated ketones, esters and their
derivatives [4]. Furthermore, Bonin and co-workers reported that aryl dioxazaborocanes
represent good boronic acid surrogates for the Suzuki–Miyaura cross-coupling reaction [5].
Phthalocyanines (Pcs) are typically purple, blue or green macrocyclic compounds similar in
structure to tetraazaporphyrins, but having four additional fused benzo rings [6]. Pcs
constitute an important class of materials from an industrial standpoint, due to their
widespread use as colorants for paint, inks, textiles, as well as catalysts for redox and
photochemical reactions, semi-conductors, photoelectric cells, blue/green dyes and catalysts
2

[6-9]. As research work has drastically diversified into many different application fields,
realizing numerous innovative uses for phthalocyanines, the subsequent use of
phthalocyanines in numerous applications has expanded rapidly [10-12]. Some advances have
been made, e.g. in photoconductivity, non-linear optics, semi-conducting properties, cancer
therapy and photo-sensitizing [13,14]. Furthermore, the extended planar shape, four-fold
symmetry and stability also makes the phthalocyanine macrocycle an excellent building block
for use in supramolecular [15] and polymer chemistry [16].
The particular two-dimensional π-electron delocalization over these macrocycles gives rise to
a great number of unique physical properties. Thus, they are chemically and thermally stable
compounds that exhibit exceptional optical and electrical behaviour.
Furthermore,
phthalocyanines are capable of incorporating more than 70 different metallic and non-metallic
cations in their ring cavity. It is also possible to attach a wide variety of substituents at the
peripheral/non-peripheral positions of the macrocycle, which can alter the electronic structure
of the system [6]. This chemical flexibility of the phthalocyanine allows the preparation of a
large variety of related structures and, consequently, tailoring of the physical, electronic and
optical properties, as well as easy processing [17].
Although phthalocyanines have been extensively studied, not much attention has been paid to
boron containing phthalocyanine derivatives, except for sub-phthalocyanines and carborane
substituted phthalocyanines. Sub-phthalocyanines are phthalocyanine analogues having three
coupled isoindole units containing a boron atom in the center [18]. Despite being non-planar,
they exhibit aromatic behavior associated with the delocalized 14 π-electron system. They
show very attractive photophysical and electrochemical properties, and are of interest for
applications as chromophores in non-linear optics [19], OLEDs [20], photovoltaic devices
3

[21] and in multicomponent donor-acceptor systems [22]. Moreover, they are used in the
selective synthesis of unsymmetrically substituted phthalocyanines. The geometrically
constrained subphthalocyanine core is readily cleaved in the presence of a diiminoisoindoline
unit, which is then incorporated into its framework so as to form a phthalocyanine derivative
of the type A3B [23]. Carborane substituted phthalocyanines are attractive dual sensitizers for
photodynamic therapy (PDT) and boron neutron capture therapy (BNCT) of tumors, due to
their strong absorptions in the near-IR where light penetration through the tissue of interest is
substantially increased compared with visible light [24-26], their ability for generating singlet
oxygen with high quantum yields and their tendency for selective accumulation within tumor
tissues [27,28].
Herein, based on our ongoing research interest in boronated tetrapyrrole derivatives and their
precursors [29-31], we report on the synthesis of a phthalonitrile derivative possessing a N-
alkyldiethanol amine unit and the zinc pthalocyanine derived from it, as well as their
corresponding phenylboronic azaesters. The novel compounds were fully characterized by
elemental analyses, FT-IR, 1H, 13C and 11B NMR, and UV–Vis spectral data.
2. Experimental
2.1 Chemicals and instrumentation
All reagents and solvents were of reagent grade quality, obtained from commercial suppliers.
The solvents were stored over molecular sieves (4 Å). 4-Nitrophthalonitrile (1) was
synthesized as reported in the literature [32]. Triethanolamine and phenylboronic acid were
used as supplied commercially. The reaction progress was monitored by TLC (SiO2). IR
4

spectra were recorded on a Perkin Elmer Spectrum One FTIR (ATR sampling accessory)
spectrophotometer, electronic spectra in the UV-Vis region were recorded with an Scinco S-
1
3100 single beam UV/Vis PDA spectrophotometer. H, 13C and 11B NMR spectra were
recorded on an Agilent VNMRS 500 MHz spectrophotometer. TMS was used as an internal
reference. Boron trifluoride diethyl etherate was used as an external standard in the 11B NMR
spectra. Elemental analyses were performed on a Thermo Flash EA 1112. A CEM
DISCOVER SP max 300 Watt single mode microwave oven was used for the synthesis of
phthalocyanine.
2.2 Synthesis
2.2.1 Synthesis of 4-[2-(bis(2-hydroxyethyl)amino)ethoxy]phthalonitrile (2):
4-Nitrophthalonitrile (1.00 g, 5.78 mmol) was dissolved in a mixture of triethanolamine (4.60
mL, 34.7 mmol) and anhydrous DMF (20 mL) under nitrogen. After dissolution, finely
ground anhydrous potassium carbonate (7.18 g, 52.0 mmol) was added portionwise over 2 h.
The reaction mixture was stirred vigorously at room temperature under nitrogen for 48 h, then
the mixture was poured into a stirred mixture of ice water (400 mL). The aqueous phase was
extracted with ethyl acetate (3 x 30 mL). The combined extracts were dried over MgSO4 and
then the solvent was removed at reduced pressure. The green coloured viscous product was
treated with diethyl ether and dried in vacuo. Yield: 0.92 g (58%). FTIR ʋ max/cm-1: 3368
(broad, O-H), 3114-3044 (Ar-H), 2949-2879 (Aliph-H), 2230 (C≡N), 1595 (C=C), 1561,
1
1427, 1319, 1292, 1252 (C-O-C), 1022, 875, 837, 729; H NMR (d6-DMSO) δ, ppm: 8.02-
8.00 (d, 1H, Ar-H), 7.74 (s, 1H, Ar-H), 7.43-7.41 (d, 1H, Ar-H), 4.34 (s, OH), 4.18-4.16 (t,
2H, O-CH2-C), 3.42-3.40 (t, 4H, C-CH2-OH), 2.92-2.89 (t, 2H, C-CH2-N), 2.62-2.60 (t, 4H,
5

N-CH2-C). Anal. Calc. for C14H17N3O3 (275.30 g/mol): C, 61.08; H, 6.22; N, 15.26 %; Found:
C, 61.03; H, 6.28; N, 15.19 %
2.2.2
Synthesis
of
the
phenylboronic
azaester
of
4-[2-
(bis(hydroxyethyl)amino)ethoxy]phthalonitrile (3)
4-(2-(Bis(2-hydroxyethyl)amino)ethoxy)phthalonitrile (2) (100 mg, 0.36 mmol) was
dissolved in 30 mL of ethyl acetate and then poured into a solution of phenylboronic acid
(53.0 mg, 0.44 mmol) in 20 mL of ethyl acetate under nitrogen. The reaction mixture was
refluxed in a 100 mL round bottom flask charged with 3 Å molecular sieves (2.00 g) in order
to remove the water released from the esterification reaction. After refluxing for 24 h, the
solution was filtered while hot, and then the solvent was evaporated to dryness. The solid
product was washed with hexane and diethyl ether, and then dried in vacuo. Yield: 96 mg (73
%). M.p. 217 °C. FTIR ʋ max/cm-1: 3036-29950 (Ar-CH), 2920-2854 (Aliph-H), 2232 (C≡N),
1595 (C=C), 1505, 1457, 1435, 1409, 1320, 1261 (C-O-C), 1213 1195, 1084, 1066, 1007,
1
972, 950, 909, 839, 733, 708, 660; H NMR (CDCl3) δ, ppm: 7.77-7.70 (d, 2H, Ar-H), 7.65-
7.63 (d, 2H, Ar-H), 7.32-7.31 (d, 2H, Ar-H), 7.23 (s, 1H, Ar-H), 7.09-7.06 (dd, 1H, Ar-H),
4.20 (t, 4H, C-CH2-O), 4.11-4.09 (t, 2H, O-CH2-C), 3.47 (t, 2H, N-CH2-C), 3.16 (t, 2H, N-
13
CH2-C), 2.84-2.82 (t, 2H, C-CH2-N). C NMR (CDCl3) δ, ppm: 160.56 (Ar C), 135.39 (Ar
CH), 133.74 (Ar CH-meta-B), 133.33 (Ar C-O-B), 128.17 (Ar C-ortho-B), 127.69 (Ar CH-
para-B), 119.29 (Ar CH), 119.15 (Ar CH), 117.79 (Ar C-CN), 115.21(C≡N), 114.87 (C≡N),
108.58 (Ar C-CN), 65.83 (O-Aliphatic CH), 65.20 (Aliphatic CH-O), 63.37 (Aliphatic CH-
N), 57.91 (N-Aliphatic CH), 57.76 (N-Aliphatic CH). 11B NMR, (CDCl3) δ, ppm: 14 (s,
tetracoordinated B); Anal. Calc. for C20H20BN3O3 (361.20 g/mol): C, 66.50; H, 5.58; N, 11.63
%. Found: C, 66.53; H, 5.61; N, 11.59 %.
2.2.3 Synthesis of 2,9(10),16(17),23(24)-tetrakis-[2-(bis(hydroxyethyl)amino)ethoxy]-
phthalocyaninatozinc (II) (4)
6

A mixture of 0.73 mmol (0.200 g) phthalonitrile derivative 2, 0.18 mmol (0.023 g) of
anhydrous Zn(CH3COO)2 and a catalytic amount (8 μL) of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) in 2 mL of anhydrous 2-(dimethylamino)ethanol was irradiated in a microwave
oven at 135 °C for 15 minutes. The reaction mixture was cooled to room temperature and the
crude product was precipitated by addition of methanol. After filtration, the resulting green
product was first washed with water in order to remove any unreacted metal salt, and then
was washed sequentially with hot methanol, ethyl acetate, acetone and diethyl ether. The
desired compound was obtained as a green solid. Yield: 0.142 g (67%); M.p.>200 °C. FTIR
ʋ max/cm-1: 3257 (broad, OH), 2947-2851 (Aliph-H), 1603 (C=C), 1487, 1394, 1337, 1285
1
1226 (C-O-C), 1185, 1124, 1042, 954, 873, 825, 761, 744; H NMR (d6-DMSO) δ, ppm:
9.21-9.17 (d, 4H, Ar-H), 8.83-8.77 (d, 4H, Ar-H), 7.74 (s, 4H, Ar-H), 4.62 (t, 8H, O-CH2-C),
4.41 (s, OH), 3.63 (t, 16H, C-CH2-OH), 2.87 (t, 16H, N-CH2-C), 2.62 (t, 8H, C-CH2-N); UV-
Vis (DMF): λmax/nm (log ε, L mol-1 cm-1): 677 (5.15), 341 (4.85), 263 (4.79); Anal. Calc. for
C56H68N12O12Zn (1166.59 g/mol): C, 57.66; H, 5.88; N, 14.41; Found: C, 57.58; H, 5. 93; N,
14.44 %
2.2.4 Synthesis of phenylboronic azaester sbstituted 2,9(10),16(17),23(24)-ttrakis-[2-
(bis(hydroxyethyl)amino)ethoxy]-phthalocyaninatozinc (II) (5)
A
suspension of 0.086 mmol (0.100 g) 2,9(10),16(17),23(24)-tetrakis-[2-
(bis(hydroxyethyl)amino)ethoxy]phthalocyaninatozinc (II) (4), 0.043 mmol (0.052 g)
phenylboronic acid and 84 mL of a mixture of dimethylsulfoxide/toluene (1/20) were placed
into a flask equipped with a stirrer and a Dean Stark Trap. The reaction mixture was refluxed
for 24 h under stirring. The water generated in the reaction and some of the solvent were
removed using the Dean-Stark trap. The mixture was cooled to room temperature and then the
solvent was removed under reduced pressure. The dark-green solid product was washed with
hexane, ethyl acetate and diethyl ether, and dried in vacuo [33]. Yield: 0.057 g (44%); M.p. >
7

200 °C. FTIR ʋ max/cm-1: 3070 (Ar-H), 2874 (Aliph-H), 1603 (C=C), 1487, 1439, 1336, 1284,
1
1224 (C-O-C), 1087, 1043, 949, 832, 745, 726, 703; H NMR (d6-DMSO) δ, ppm: 9.17 (br,
2H, Ar-H), 8.78 (br, 4H, Ar-H), 8.00 (s, 2H, Ar-H), 7.77-7.66 (m, 8H, Ar-H), 7.37-7.21 (m,
8H, Ar-H), 6.51 (s, 8H, Ar-H), 4.60 (t, 8H, O-CH2-C), 4.09 (t, 16H, C-CH2-O), 3.62 (t, 16H,
N-CH2-C), 2.92-2.85 (t, 8H, C-CH2-N); 11B NMR, (d6-DMSO) δ ppm: 20.1 (s, tricoordinated
B); UV-Vis (DMF): λ max/nm (log ε, L mol-1 cm-1): 678 (5.13), 346 (4.83), 243 (4.98); Anal.
Calc. for C80H80B4N12O12Zn (1510.19 g/mol): C, 63.62; H, 5.34; N, 11.13; Found: C, 63.68;
H, 5.39; N, 11.09 %.
3. Results and Discussion
Scheme 1 shows the selected synthetic routes for compounds 2-5. Characterization of the
1
products involved a combination of methods, including elemental analysis, H and 13C NMR,
UV-Vis, FT-IR and also 11B NMR for boronated compounds. Spectral data on the newly
synthesized compounds were consistent with the proposed structures.
Although the synthesis and characterization of small dioxazaboracane molecules have been
studied extensively, arylboronic ester substituted phthalocyanine derivatives have not been
reported yet. We have therefore sought a phthalonitrile derivative in which the free hydroxy
groups and the nitrogen electron pair are capable of linking to the boron atom of
phenylboronic acid to give a phenylboronic azaester bearing a phthalocyanine precursor. For
this purpose, 4-[2-(bis(2-hydroxyethyl)amino)ethoxy]phthalonitrile (2) was obtained via an
aromatic substitution reaction of 4-nitrophthalonitrile [32] with triethanolamine in the
presence of K2CO3. The reaction was carried out in dry dimethylformamide under a N2
atmosphere at room temperature [Scheme 1]. Compound 2 is highly soluble in common
organic solvents, including methanol, CHCl3, THF, ethyl acetate, acetone, DMF and DMSO.
8

The IR spectrum of compound 2 clearly indicated the formation of the proposed structure by
the broad -OH stretching band at around 3368 cm-1, an intense new absorption band at 1252
cm-1 attributable to aromatic C–O–C [34] and the disappearance of the aromatic –NO2 band at
1548 cm-1. In addition, the characteristic C≡N stretching vibrations appeared as a single
intense peak at the expected frequency (2230 cm-1) [35]. The aromatic CH and the aliphatic
CH stretching bands appeared at 3114-3044 and 2949-2879 cm-1, respectively.
1
The H NMR spectrum of compound 2 recorded in d6-DMSO also confirmed the proposed
structure by a broad peak at 4.34 ppm assignable to the protons of -OH groups. The aromatic
protons appeared at 8.02-8.00, 7.74 and 7.43-7.41 ppm as doublet, singlet and doublet peaks,
respectively. The aliphatic protons neighbouring the O atom of the phthalonitrile group
appeared at 4.18-4.16 ppm as a triplet while those neighboring –OH appeared at 3.42-3.40
ppm as a triplet. The aliphatic protons of CH2 groups attached to the N atom were at 2.92-2.89
ppm and those neighbouring -OH appeared at 2.62-2.60 ppm as triplets.
The complexation reaction of compound 2 and phenylboronic acid in ethyl acetate under
reflux conditions yielded the desired dioxazaborocane 3 in a yield of 76%. In order to remove
the by-product of water efficiently, the reaction was performed in the presence of 3 Å
molecular sieves. Phenylboronic azaester 3 exhibited high solubility in CHCl3, acetone, DMF
and DMSO. Although we could not manage to grow single crystals of sufficient quality of 3
to probe the structural properties of the complex, spectral data revealed formation of the
proposed sp3-hybridized boronate ester structure [36].
9

The IR spectrum of compound 3 is in agreement with the proposed phenylboronic ester
structure. The aromatic and aliphatic CH peaks were observed at 3036-2950 and 2920-2854
cm-1, respectively, and also an intense C≡N peak was observed at 2232 cm-1. The IR spectra
for compound 3 showed the disappearance of the bands corresponding to the OH groups,
which are present in the dinitrile compound 2 at around 3360 cm-1; as another important
diagnostic parameter, B-N and B-O peaks appeared at 1457-1435 and 1320 cm-1, respectively
[37]. Furthermore, the stretching vibrations of C-O-C (1261 cm-1) and mono substituted
benzene (708 cm-1) appeared at the expected frequencies.
The 1H and 13C NMR spectra of compound 3 were recorded in CDCl3. The 1H NMR spectrum
of the boron complex 3 indicated that the hydroxy groups of the diethanol amine moiety took
part in the complexation with boron since those hydroxy protons completely disappeared.
Furthermore, additional chemical shifts arising from the phenyl protons of the boronic ester
group in compound 3 appeared in the aromatic region. The aliphatic protons of the OCH2
groups neighbouring the O-B bond appeared at 4.20 ppm as a triplet and those of OCH2
attached to the CH2N group appeared at 4.10 ppm as a triplet. Furthermore, the protons of
NCH2 groups forming the heterobicycle ester split into diastereotopic signals at 3.47 and 3.16
1
ppm. The existence of these diastereotopic signals in the H NMR spectrum evidenced the
presence of the intramolecular N→B coordination bond [38-40]. The protons of the C-CH2N
group were observed at 2.84-2.82 ppm as a triplet. Moreover, the aromatic protons were
observed between 7.77 and 7.06 ppm. The signals in the 13C NMR spectrum for the phenolic
carbon atoms of the phenylboronic acid moeity at 133.74, 133.33, 128.17 and 127.69 ppm
also confirmed the binding of phenylboronic acid with compound 3.
10

11
As B NMR spectroscopy is a well-established method for probing the structure of boronic
acids and boronate ester complexes, the hybridization around the boron atom was also
11
11
investigated by its B NMR spectrum [36]. The B-NMR spectrum of 3 in CDCl3 revealed
one signal at 14 ppm attributed to a tetrahedral boron atom connected to one carbon, two
oxygen and one nitrogen atom [41,42].
The next target compound, zinc phthalocyanine bearing peripheral 2-(bis(2-
hydroxyethyl)amino)ethoxy substituents (4), was prepared in 67% overall yield from the
dinitrile derivative 2 and anhydrous zinc(II) acetate in the presence of a few drops of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) as a strong base. Cyclotetramerization of 2 was
accomplished by microwave irradiation in 2-(dimethylamino)ethanol at 135 °C for 15 min
[43]. Phthalocyanine 4 is only soluble in pyridine, DMF and DMSO. The lack of solubility in
common organic solvents is probabily due to the aggregation behaviour of the phthalocyanine
molecules in solution and the existence of intra- and inter-molecular hydrogen bondings. On
the other hand, phthalocyanine 4 exhibited high solubility in 1N HCl solution and when the
acidic solution of compound 4 was treated with 1N NaOH solution it precipitated as a green
solid. These solubility features of 4 indicate the presence of diethanolamine moities on the
phthalocyanine molecule.
The IR spectra of phthalocyanine 4 and its precursor 2 are very similar, with a broad -OH
stretching band at around 3257 cm-1, an intense etheric band at 1226 cm-1 and aliphatic CH
stretching bands at 2947-2851 cm-1. The diagnostic difference was the disappearence of the
C≡N stretching vibration in the IR spectrum of 4 indicating the formation of a phthalocyanine
structure.
11

1
The H NMR spectrum of zinc phthalocyanine 4 in d6-DMSO provided the characteristic
chemical shifts for the structure proposed. As tetra substituted phthalocyanines obtained from
1
4-substituted phthalonitriles are naturally a mixture of isomers, the H NMR spectrum of
phthalocyanine 4 is somewhat broader than the corresponding signals in the dinitrile
1
derivative 2 [44]. In the H NMR spectrum of compound 4, OH protons were observed as a
singlet at δ 4.41 ppm. The aromatic protons appeared at 9.21-9.17, 8.83-8.77 and 7.74 ppm as
doublet, doublet, singlet signals, respectively. The aliphatic protons of OCH2 attached to the
phthalocyanine core were observed at 4.62 ppm and those neighbouring the –OH group
appeared at 3.63 ppm as triplets. The aliphatic protons of CH2 groups attached to the N atom
were observed at 2.62 ppm and those neighbouring the -OH group appeared at 2.87 ppm as
triplets.
Electronic spectra with two strong absorption bands known as Q and B bands are especially
useful to identify the structure of phthalocyanines [45]. The B band in the UV region at about
300-400 nm arises from deeper level π – π* transitions and the Q band in the visible part of
the spectrum, around 600-700 nm, is attributed to π - π* transitions from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the Pc−2
ring [46-48]. The absorption spectrum of phthalocyanine 4 in DMF shows the expected
absorption Q and B bands at 677 and 341 nm, respectively [Fig. 1].
Treatment of phthalocyanine 4 with phenylboronic acid in a boiling DMSO/toluene (1/20,
v/v) mixture, in order to improve solubility of the starting materials [33], for 24 hours
afforded the expected dioxazaborocane substituted zinc phthalocyanine 5. A Dean-Stark
apparatus was used for the azeotropic removal of water. This condensation reaction led to the
desired phthalocyanine 5 in only 44% isolated yield; this can be attributed to steric and
solubility restrictions originating from the phthalocyanine macromolecule 4.
12

The IR spectrum of compound 5 clearly indicates the formation of the target ester structure by
the disappearance of the broad -OH stretching band which was observed at around 3282 cm-1
in the IR spectrum of phthalocyanine 4. The appearance of a new band at 1336 cm-1
confirmed the formation of a B–O bond [30,37]. In addition to the aromatic and aliphatic CH
stretching bands at 3070 and 2874 cm-1 ppm respectively, the stretching vibrations of C-O-C
(1224cm-1) and mono substituted benzene (703 cm-1) were at the expected frequencies.
1
The most significant difference between the H NMR spectra of compounds 4 and 5 is the
disappearance of the OH chemical shifts in the latter compound. The aromatic protons
1
appeared between 9.17 and 6.51 ppm in the H NMR spectrum of 5 taken in d6-DMSO.
Aromatic ortho protons of the phenyl boronic acid moiety have been significantly upfield
shifted by the anisotropic shielding effect resulting from the conjugated π-electrons of the
phthalocyanine moieties and this shift can be taken as a particularly important aid for
confirming the target phenylboronic azaester structure [49]. Moreover, the aliphatic protons
of the OCH2 group appeared at 4.60 ppm and those on the heterocycle appeared at 4.09 ppm
as broad peaks. The aliphatic protons of NCH2 groups forming the heterocycle were at 3.62
ppm and those neighbouring the OCH2 group were observed at 2.92-2.85 ppm as broad peaks.
A UV-Vis spectrum of phthalocyanine 5 was recorded in DMF [Fig. 1]. The Q-band
absorbtion peak of phthalocyanine 5 appeared at 678 nm, suggesting that the introduction of
boronated substituents on peripheral positions has no influence on the position of the Q band
in UV-Vis spectrum of compound 5 compared to phthalocyanine 4. Additional higher energy
bands, namely N and L bands, due to other π- π* transitions within the delocalized
phthalocyanine ring system, were at wavelengths shorter than 300 nm (at 263 nm for complex
4). Because of their relatively lower intensities, these N and L bands are generally ignored in
13

most papers on phthalocyanines. However, the boronated phthalocyanine 5 exhibited an
additional band at 243 nm with a higher absorbance value than that of phthalocyanine 4 at 263
nm. This increase in the absorbance value indicates the formation of boronic acid ester as it
can be attributed to the π- π* transitions of the phenyl groups attached to phthalocyanine 5 by
the condensation reaction of phenyl boronic acid and phthalocyanine 4.
11
The B NMR spectrum of 5, showing a single peak at 20 ppm, is a powerful indication of
the presence of only sp2 boron – open form - in solution [36]. As it is well documented in the
literature, due to the nature of the B-N bond, azaesters of boronic acids can exist in one of the
extreme conformations, i.e. either a closed or an open form [Scheme 2] [50-52].
In the present work we have encountered both conformers in two molecules, although the
binding groups to phenylboronic acid are the same, both in the case of phthalonitrile 2 and
phthalocyanine 4. It appears that the nature of the attached group to the dioxaazaboracane
moiety plays an important role for which conformer is present. If there is no steric constraint,
the closed conformer with a four coordinate boron ion is the preferred one, as in the case of
compound 3. However, when the phenyl group in he phthalonitrile is converted into a totally
planar phthalocyanine moiety, the π-π interaction tendency of these aromatic units sterically
hinders the voluminous closed conformer formation with an sp3 boron; instead, the open
conformer is preferably formed with an sp2 boron, as in compound 5.
4. Conclusion
4-[2-(Bis(2-hydroxyethyl)amino)ethoxy]phthalonitrile
dioxazaborocan-6-yl)ethoxy)phthalonitril (3),
(2),
4-(2-(2-phenyl-1,3,6,2-
2,9(10),16(17),23(24)-tetrakis-[2-
14

(bis(hydroxyethyl)amino)ethoxy]phthalocyaninatozinc (II) (4) and Zn phthalocyanine bearing
four phenylboronic azaester substituents on peripheral positions (5) were prepared for the
1
first time. The novel compounds were fully characterized by elemental analyses, FT-IR, H,
11
11
13C and B NMR, and UV–Vis spectral data. B NMR data indicated that the boron atom
attached to phthalonitrile 3 is in a bulky tetracoordinated state, with the formation of a
coordinative N-B bond (closed form). Completely planar phthalocyanine moieties with high
aggregation tendencies at the concentrations used in NMR measurements lead the boronic
ester side of the molecule into a planar tricoordinated form.
Acknowledgements
This work was supported by the Research Fund of Istanbul Technical University, TUBITAK
(Project number: 110T833) and Turkish Academy of Sciences (TUBA)
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18

Figure and Scheme Captions
Figure 1.
Absorption spectra of compound 4 (2 x 10-6 M) and 5 (5 x 10-6 M) in DMF .
Scheme 1.
Synthetic routes for compounds 2-5. (i) K2CO3, DMF, room temperature (ii)
Phenylboronic acid, ethyl acetate, reflux (iii) Zinc(II) acetate, 2-
(dimethylamino)ethanol, microwave irradiation (iv) Phenylboronic acid,
DMSO:toluene: (1/20 v/v), Dean-Stark, reflux
Scheme 2. Two possible forms of arylboronic azaesters: (a) closed form with a dative B–N
bond and (b) open conformer with non-interacting B and N atoms

Figures
Figure 1.

Scheme
Scheme 1.

Scheme 2.

Graphical abstract pictogram

Graphical abstract Synopsis
Different Phenylboronic Acid Azaester Formation Modes in a Substituted
Zinc Phthalocyanine and its Precursor
Şennur Özçelik, Ahmet Gül *
Department of Chemistry, Technical University of Istanbul, Maslak, Istanbul, 34469, Turkey
Graphical Abstract
A novel zinc phthalocyanine and its precursor with bis(2-hydroxyethyl)amino)ethoxy groups
were synthesized. Treatment of these compounds with phenylboronic acid afforded
phenylboronic azaesters. The boron atom in the phthalonitrile derivative is tetra-coordinated
with the formation of a coordinative N-B bond (closed form), while in the case of
phthalocyanine 5, the open conformer possessing a tricoordinated B atom is favoured due to
steric hindrance of the planar macrocycles.