Boronic esters of a porphyrazine and its precursor

Article abstract of DOI:10.1142/S1088424611003677

Phenylboronic acid esters of an unsaturated precursor 1,2-dicyano-1,2- bis(2-hydroxy-ethylthio)ethylene and the magnesium porphyrazine derived from it have been prepared either by refluxing a mixture of the reagents in chloroform in the presence of molecu

Full text of DOI:10.1142/S1088424611003677

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 742–747
DOI: 10.1142/S1088424611003677
Published at http://www.worldscinet.com/jpp/
Boronic esters of a porphyrazine and its precursor
Şükran Cenikli Başeren, Şennur Özçelik and Ahmet Gül*
Technical University of Istanbul, Department of Chemistry, Maslak, Istanbul TR34469, Turkey
Dedicated to Professor Karl M. Kadish on the occasion of his 65th birthday
Received 23 May 2011
Accepted 21 June 2011
ABSTRACT: Phenylboronic acid esters of an unsaturated precursor 1,2-dicyano-1,2-bis(2-hydroxy-
ethylthio)ethylene and the magnesium porphyrazine derived from it have been prepared either by
refluxing a mixture of the reagents in chloroform in the presence of molecular sieve or by solvent-
free heating in an oven under reduced pressure. The two novel compounds have been characterized
by elemental analysis together with ultraviolet-visible, infrared spectroscopy, proton nuclear magnetic
resonance, carbon-13 nuclear magnetic resonance, boron-11 nuclear magnetic resonance, and mass
spectra.
KEYWORDS: porphyrazine, boronate complex, phenylboronic acid, maleonitrile, magnesium.
esters of a porphyrazine precursor and the magnesium
INTRODUCTION
porphyrazinate derived from it.
Porphyrazines and boronic acids are two versatile
Formation of boronic ester from difunctional boronic
classes of functional materials. The former are structural
acid with diols has been used for the construction of dis-
analogs of natural porphyrins and totally synthetic phtha-
crete cycles and cages [5, 6]. The reversible nature of
locyanines, showing strong absorptions in the UV-vis
boronic acid complexation with diols makes this type of
region (at ca. 350 and 660 nm) with desirable photochem-
interaction highly suitable for the reversible self-assem-
ical and photophysical properties [1]. Boron compounds
bly of multicomponent systems [7, 8]. With these types
exhibit interesting properties such as fluorescence, pho-
of reversible systems any errors that occur during the
toconductivity, and electroluminescence; moreover, they
assembly process may be corrected because equilibration
also find wide applications in synthesis and medici-
of the reactive species results in formation of a thermody-
nal chemistry [2]. Additionally, boron neutron capture
namically favored product [9]. Although the presence of
therapy (BNCT) is a binary approach for cancer treat-
two oxygen donor atoms are generally sufficient to com-
ment that is particularly attractive in treating high-grade
plete boronic ester formation, an additional N donor has
gliomas and metastatic brain tumors [3], and either cuta-
been also effectively used to form a further B–N bond
neous primaries or cerebral metastases of melanoma [4].
[10]. The studies showed that, in such a case, the boron
As a result, these two classes of compounds are excellent
atom is tetra-coordinated and forms a dative bond with
candidates for number of applications. Combination of
the nitrogen atom. The characteristics of the dative bond
these two functionalities in the same molecule is expected
as well as the rigidity of the tridentate ligands employed
to bring together the advantageous characteristics of indi-
gave rise to monomeric molecules and, through a self-
vidual components. We therefore initiated a study on the
assembly procedure, to specific di-, tri- and tetrameric
mixed arrays of porphyrazines and boronic esters. Here
molecules [11, 12]. For this particular construction, the
we report the preparation and characterization of boronic
oxygen atoms build up mainly five- or six-membered
rings, as well as covalent boron-oxygen bonds to achieve
the oligomeric/macrocyclic-products [13–15]. As previ-
SPP full member in good standing
ously mentioned, the structure of the product is mainly
determined by the nature of the ligand; however, in some
*Correspondence to: Ahmet Gül, email: ahmetg@itu.edu.tr,
tel: +90 212-285-6827, fax: +90 212-285-6386
Copyright © 2011 World Scientific Publishing Company

BOrOnIC eSterS Of a POrPhyrazIne anD ItS PreCurSOr
743
B
O
N
O
N
S
S
O
B
O
B
S
S
O
O
N
N
N
O
O
O
O
N
S
S
S
S
S
S
S
S
Mg
N
Mg
B
B
N
N
N
N
N
N
N
N
N
B
O
B
O
O
S
S
O
S
S
O
O
a
B
b
Scheme 1. Two possible bonding patterns for the boronic ester of Mg-porphyrazine: (a) tetracoordinated boron; (b) tricoordinated
boron
cases it can be modulated by varying the reaction con-
ditions [16]. The geometrical change between planar
trigonal to tetrahedral boron atoms due to donor-acceptor
coordination has been also related to the position of
donor atoms with respect to one another. An important
structural evidence worth mentioning is that for boron
compounds constructed from tridentate NO2 ligands, the
N–B bonds are between 1.6 and 1.7 Å [17].
Octakis(2-hydroxyethylsulfanyl)porphyrazine has been
an important precursor for our ongoing work on tetrapy-
rrole derivatives during the last decade to prepare por-
phyrazines with desired functionalities such as ferrocene
groups, crown ethers, triphenylphosphonium moieties,
etc. by ester formation with suitable carboxylic acid
derivatives. Enhanced solubility in organic solvents/
water, formation of multinuclear complexes have been
the main outcome of these products [18–22].
The aim of the present work has been to prepare
boronic ester of octakis(2-hydroxyethylsulfanyl)porphy-
razine and its precursor unsaturated dinitrile derivative.
As described above, the critical point here is whether
meso-N-atoms of porphyrazine core will take part in
binding to boron group as shown in Scheme 1a or only
O-donors will form the ester structure in Scheme 1b.
reaction of the precursor, namely 1,2-dicyano-1,2-bis(2-
hydroxyethylthio)ethyleneandtwodifferentmethodshave
been applied. In the first method; the esterification reaction
is carried out at reflux temperature and dichloromethane
was preferred as the solvent (Scheme 2). 3 Å molecular
sieves is present for removing the water released from
this esterification reaction. The second method involves
heating a homogenous mixture of phenylboronic acid
and 1,2-dicyano-1,2-bis(2-hydroxyethylthio)ethylene in
a vacuum oven at 70 °C under 0.5 atm pressure for 24 h
[23]. Products obtained through both routes are identical.
The stoichiometry of the product corresponds to
1:1 ratio of 1,2-dicyano-1,2-bis(2-hydroxyethylthio)
ethylene/phenylboronic acid. GC/MS spectrum of the
compound exhibited a molecular ion peak at m/z = 316
confirming the formation of the desired compound. FTIR
spectrum of the product clearly indicates that both of the
OH groups of maleonitrile derivative has been esterified
by the disappearance of the broad OH band at 3345 cm-1
present in the precursor. In the 1H NMR spectrum of the
product, protons assignable to the phenyl group appear
as a multiplet at 7.68–7.32 ppm and aliphatic CH groups
of O–CH2–CH2–S moiety appears at 4.27–3.67 ppm and
3.35 ppm as triplet together. The chemical shifts assign-
able to OCH2 groups come out at 4.27–4.16 and 3.80–
3.67 ppm as triplet of doublet (multiplet) suggesting the
presence of two isomers for the compound. The isomer-
ism arises from the existence of the rigid C=C bond in
the structure. The vibrations of S–CH2 are restricted by
RESULTS AND DISCUSSION
As a model compound for octakis(2-hydroxyethylsul-
fanyl)porphyrazine, we have started with the esterification
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744
S¸.C. BaS¸eren et al.
of boronic acid and OH-groups on precursor
porphyrazine has been accomplished repro-
ducibly only when these two reactants are
brought together in solution (Scheme 3).
Here, THF was the solvent applicable due
to solubility reasons and molecular sieves as
the dehydrating agent. Enhanced solubility of
HO
S
S
O
O
N
N
C
C
S
S
OH
OH
N
N
C
C
B
B
HO
1
Scheme 2. Synthetic route of 1,2-dicyano-1,2-bis(2-hydroxyethylthio)eth-
ylene boronic (i) CH2Cl2, 62 °C; (ii) 0.5 atm, 70 °C
boronic ester in less polar solvents when we compare it
with that of the octakis(hydroxyethylthio) derivative is
a clear indication of the conversion to phenylboronate
product.
In the case of octakis(hydroxyethylthio)porphyrazine,
esterified product is essentially made up of four phenylbo-
ronic acid unit to each porphyrazine derivative. In the mass
spectrum, the fragment ion corresponding to [M + K]+
at 1328.885 is a clear evidence for the formation of
the ester in 1:4 porphyrazine/phenylboronic acid ratio.
Additionally, FTIR spectrum provides further evidence
to support the ester formation by disappearance of OH
stretches in precursors after completion of the reaction.
the presence of double bond next to the S atom while the
CH2–O bond is more free to rotate because of its distance
from the double bond. In boron containing compounds,
11B NMR spectrum is extremely useful in deciding on the
coordination number of boron atom. It is a general rule
that 4-coordinate boron compounds give a signal around
10 ppm, while in the case of 3-coordinate ones it comes
out around 30 ppm [24–26]. Here in the present example
of boronic ester, a single peak at 29.11 ppm is a proof of
tricoordinated structure precisely.
When similar synthesis procedures are applied to
parent porphyrazine derivative, condensation reaction
OH
OH
S
S
N
N
HO
HO
OH
OH
N
S
S
S
S
HO
Mg
4
N
N
B
HO
N
N
N
S
S
OH
OH
B
O
O
N
S
S
S
S
N
O
O
O
O
N
S
S
S
S
B
Mg
N
B
N
N
N
N
O
O
B
2
Scheme 3. Synthetic route of 2,3,7,8,12,13,17,18-octakis[2′-(phenylboronate)ethylthio]-porphyrazinato N21 N22 N23 N24 magnesium
in THF at 60 °C
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BOrOnIC eSterS Of a POrPhyrazIne anD ItS PreCurSOr
745
Fig. 1. Aggregation behavior of 2 in THF at different concentrations: 1 × 10-5 (––––), 8 × 10-6 (·······), 6 × 10-6 (– – –), 4 × 10-6
(–– · ––), 2 × 10-6 (–– ––) and 1 × 10-6 (− ··)
The peaks around 1390–1312 cm-1 arising from B–O
stretches confirmed the formation of proposed boronic
B-band shows a bigger absorption coefficient. In most
tetrapyrrole derivatives, absorption coefficient of Q-band
is bigger than that of the B-band although both of them
correspond to π → π* transition [29–31]. This contro-
versy arises from the combination of the absorption due
to π → π* transition of benzene groups with the absorp-
tion of porphyrazine core.
To assess the aggregation property as a consequence
of enhanced solubility after ester formation in less polar
solvents, UV-vis spectra of porphyrazine were mea-
sured at different concentration in THF (Fig. 1). The
intensity of absorption of Q-band increased in parallel
with the increase in concentration and no new band was
monitorated in the spectrum. The Beer-Lambert law was
obeyed for the porphyrazine for concentrations ranging
from 10-6–10-5 M. In the light of these results, we might
conclude that phenylboronic ester groups are sufficiently
bulky to hinder aggregation in less polar solvents in the
studied concentration range.
1
ester structure. The H, 13C and 11B NMR spectra are
all consistent with the proposed structure. Especially
11B NMR of the product is illustrative and simple with
a single chemical shift at 28.18 ppm as a consequence
of the tricoordinated boron atom present in the struc-
ture. These data provide the critical proof to decide on
the structure where boron atoms are esterified with two
hydroxyethylthio-units without any interaction with the
aza functionalities of porphyrazine core. In addition to
these data, those obtained from the phenylboronic acid
ester of 1,2-dicyano-1,2-bis(2-hydroxyethylthio)ethylene,
a model compound for octakis(2-hydroxyethylsulfanyl)-
porphyrazine, show that only O-donors form the ester
structure. Therefore, the structure given in Scheme 1a
can be totally eliminated.
As in the case of all tetrapyrrole derivatives, porphyra-
zines are characterized by their strong absorptions in both
high and low energy regions of UV-vis spectrum [27, 28].
The absorption spectrum of the porphyrazine synthesized
in the present work showed these typical B- and Q-bands
of symmetrical substituted derivatives. The single band
at 666 nm of porphyrazine is the characteristic Q-band
of tetrapyrrolic macrocycle with D4h symmetry and is
related to π → π* transition [18–22]. The Q-band position
of boronated porphyrazine 2 does not show appreciable
shift revealing that the boronic ester groups do not sig-
nificantly affect the Q-band position of this porphyrazine.
The affect of eight S-substituents on the periphery of the
porphyrazine core was a shift in these intense Q-bands to
longer wavelengths when compared with those of unsub-
stituted or alkyl substituted ones. Another single band
of high intensity appeared at 380 nm as B-band. When
we compare the absorbance values in these two bands,
EXPERIMENTAL
IR spectra were recorded on a Perkin-Elmer Spectrum
One FT-IR spectrometer, electronic spectra on a Unicam
UV2 spectrophotometer. Elemental analyses were per-
formed by the Instrumental Analysis Laboratory of the
TUBITAK Marmara Research Centre. A Bruker Ultra
Shield Plus 400 MHz spectrometer was used to record
1
1H NMR, 11B NMR and 13C NMR spectra. H NMR
and 13C NMR spectra were recorded using tetrameth-
ylsilane as internal reference. Boron trifluoride diethyl
etherate was used as an external standard in 11B NMR
spectra. Mass spectra were performed on Perkin Elmer
Clarus 500 GCMS and Bruker microflex LT MALDI-
TOF MS. All reagents and solvents were of reagent
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746
S¸.C. BaS¸eren et al.
grade quality obtained from commercial suppliers. 1,2-
dicyano-1,2-bis(2-hydroxyethylthio)ethylene and (2,3,-
7,8,12,13,17,18-octakis(2′-hydroxyethylthio porphyrazi-
nato-N21 N22 N23 N24)magnesiumwerepreparedaccording
to a reported procedure [19].
d ppm 7.81–7.79 (m, 8H, Ar–H), 7.43–7.32 (m, 12H,
Ar–H), 5.05–5.02 (m, 8H, O–CH2), 4.21–4.18 (m, 8H,
O–CH2), 3.89–3.62 (m, 16H, S–CH2). IR: νmax, cm-1
3062 (Ar–H), 2955 (aliph. C–H), 1602 (Ar C=C), 1390–
1312 (B–O), 1230, 1024, 860, 768, 698 (B–Ar C). MS:
m/z 1328.885 [M + K]+, 1131.267 [M - 2(C6H5)]+. 13C
NMR (400 MHz; DMSO-d6): C, ppm 134.04 (B–Ar),
133.66, 129, 127.8, 61.06 (O–C), 34.37 (S–C). 11B NMR
(400 MHz; DMSO-d6; BF3·OEt2): B, ppm 28.18.
Phenylboronic acid ester of 1,2-dicyano-1,2-bis-
(2-hydroxyethylthio)ethylene (1). This compound was
synthesized by two different methods. Method (i): 100 mg
(0.43 mmol) of 1,2-dicyano-1,2-bis(2-hydroxyethylthio)-
ethylene [19] was dissolved in 10 mL dichloromethane
and then poured into the solution of phenylboronic acid
(53 mg, 0.43 mmol) in 10 mL dichloromethane. The
reaction solution was refluxed in 100 mL round bottom
flask charged with 3 Å molecular sieve in order to remove
the water released from the esterification reaction. After
24 h reflux, the mixture was cooled to room temperature
and then was filtered to remove the molecular sieves. The
solvent was evaporated under reduced pressure yielding
yellow colored solid product. The product is soluble in
chloroform, diethyl ether, THF and is insoluble in hexane
and benzene. Yield: 0.068 g (50%). Anal. calcd. for
C14H13BN2O2S2: C, 53.18; H, 4.14; B, 3.42; N, 8.86; S,
CONCLUSION
In conclusion, phenylboronic acid ester of octakis[2′-
(phenylboronate)ethylthio]-porphyrazinato N21 N22 N23
N24 magnesium 2 was obtained for the first time by the
esterification reaction of OH groups on the porphyra-
zine with phenylboronic acid. In this molecule boron
atoms are three-coordinated and no interaction between
borons and meso-nitrogens of porphyrazine core is
present.
Acknowledgements
1
20.28%. Found: C, 53.42; H, 3.98; N, 8.75%. H NMR
This work was supported by the Research Fund of
Istanbul Technical University and TUBITAK (project
number: 110T833).
(400 MHz; CDCl3; Me4Si): H, d ppm 7.68–7.61 (m,
2H, Ar–H), 7.39–7.32 (m, 3H, Ar–H), 4.27–4.16 (m,
2H, O–CH2), 3.80–3.67 (m, 2H, O–CH2), 3.35–3.10 (m,
4H, S–CH2). IR: νmax, cm-1 3053 (Ar–H), 2941–2890
(alkyl-H), 2212 (C≡N), 1599 (Ar C=C), 1300 (B–O),
1170, 1025, 734, 699 (B–Ar). GC/MS: m/z 316. 13C NMR
(400 MHz; CDCl3): C, d ppm 133.54 (B–Ar), 131.12,
128.03, 122.02, 112.34, 61.17 (O–C), 36.97 (S–C). 11B
NMR (400 MHz; CDCl3; BF3·OEt2): B, d ppm 29.11.
Method (ii): 100 mg (0.43 mmol) of 1,2-dicyano-1,2-
bis(2-hydroxyethylthio)ethylene [19] and 53 mg (0.43
mmol) of phenylboronic acid were mixed homogeneously
by grinding in a glass vessel. The mixture was put in the
vacuum oven and kept 24 h at 70 °C under 0.5 atm. The
product was obtained as a yellow solid. Yield: 0.0655 g
(48%). The spectral and analytical results for this com-
pound are similar to the values given above.
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