Synthesis and characterization of new molecular complexation between free base meso-tetraarylporphyrins and nitrosonium ion as π-acceptor

Article abstract of DOI:10.1016/j.ica.2011.11.048

NOBF₄ reacts with para-substituted meso-tetraarylporphyrins, H₂t(4-Xp)p, at room temperature for formation of green molecular complexes, [H₂t(4-Xp)p(NO)]BF₄. Mole ratio for the porphyrins and nitrosonium ion in the molecular complexes was 1:1, [H ₂T(4-X)PP(NO)]BF₄. FT-IR, UV-Vis, (¹H and ¹³C) NMR spectral data, elemental analysis and molar conductivity indicated that NO⁺ (as electron acceptor) is bound to the lone electron pairs of the two pyrrolenine nitrogens in a side of the porphyrin plane. In these molecular complexes, two pyrrolic nitrogen atoms of the porphyrin core coordinate to the acceptor and two protons of the pyrrolic nitrogen atoms have been remained on the porphyrin macrocycles. Molecular complexation of meso-tetraarylporphyrins with NO⁺ produces a large downfield shift for the NH signal.

Full text of DOI:10.1016/j.ica.2011.11.048

Inorganica Chimica Acta 384 (2012) 133–136
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and characterization of new molecular complexation between free base
meso-tetraarylporphyrins and nitrosonium ion as p-acceptor
⇑
Halimeh Molaei, Hossein Dehghani
Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan 87317-51167, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2011
Received in revised form 13 November 2011
Accepted 15 November 2011
Available online 28 November 2011
NOBF4 reacts with para-substituted meso-tetraarylporphyrins, H2t(4-Xp)p, at room temperature for for-
mation of green molecular complexes, [H2t(4-Xp)p(NO)]BF4. Mole ratio for the porphyrins and nitrosoni-
um ion in the molecular complexes was 1:1, [H2T(4-X)PP(NO)]BF4. FT-IR, UV–Vis, (1H and 13C) NMR
spectral data, elemental analysis and molar conductivity indicated that NO+ (as electron acceptor) is
bound to the lone electron pairs of the two pyrrolenine nitrogens in a side of the porphyrin plane. In these
molecular complexes, two pyrrolic nitrogen atoms of the porphyrin core coordinate to the acceptor and
two protons of the pyrrolic nitrogen atoms have been remained on the porphyrin macrocycles. Molecular
complexation of meso-tetraarylporphyrins with NO+ produces a large downfield shift for the NH signal.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Porphyrin
Nitrosonium ion
Molecular complex
Donor
Acceptor
Deformation
1. Introduction
nitrosonium ion in the molecular complexes was 1:1, [H2t(4-
Xp)p(NO)]BF4. In these complexes a nitrosonium ion accepts two
Porphyrins because of their important role in photosynthesis,
oxygen transport and biological redox processes are one of the
most interest class of compounds in biochemistry [1]. It is reported
that porphyrins react with various acceptors for formation molec-
ular complexes contain tilted porphyrin core. In these molecular
complexes, pyrrolic nitrogen atoms act as electron donors to
acceptor molecule and two pyrrolic protons remained in the por-
phyrin macrocycle [2–10]. Reactions with NO+, known as nitrosa-
tion reactions, are the most well-studied. Nitrosation reactions
become biologically relevant through a process called transnitrosa-
tion. This process involves the transfer of the nitrosonium cation
between a protein centers containing sulfur or nitrogen. Altera-
tions in cellular function via transnitrosation occur when NO+ is
transferred between nucleophilic centers of proteins [11]. This pro-
cess will result in cellular damage when NO+ interacts with a crit-
ical center of a protein, resulting in modification and alteration of
protein function. This phenomenon is exemplified with the
transnitrosation of amines and sulfur to yield N-nitrosamines
and S-nitrosothiols, respectively. N-nitrosamines have been found
to be chemical modifiers of nucleic acids and therefore represent
potent mutagens and carcinogens [12]. In this work we synthe-
sized new molecular complexes from meso-tetraarylporphyrins
(Fig. 1) and nitrosonium ion. Mole ratio for the porphyrins and
lone electron pairs of pyrrolic nitrogens in one porphyrin molecule.
It has been reported that meso-tetraarylporphyrins react with
p
-acceptors of DDQ (dichloro-dicyano-benzoquinione) and TCNE
(tetracyanoethylene) for formation of 1:2 molecular complexes
as sole product [7,8]. But in this work, nitrosonium ion (as -accep-
tor) produced 1:1 molecular complexation with the meso-tetra-
arylporphyrins in contrast to known 1:2 (donor: -acceptor)
p
p
molecular complexation [7,8]. This article presents the first
example of 1:1 molecular complexation between the meso-tetra-
arylporphyrins and p-acceptors.
2. Experimental
2.1. Material and measurements
All chemicals used in this study were purchased from Merck.
Pyrrole (Fluka) was distilled before use and was reacted with benz-
aldehyde and various para-substituted benzaldehydes (CH3, OCH3,
Cl, CH(CH3)2) (Merck) in the presence of nitrobenzene as oxidant
and propionic acid as solvent [13].
A Bruker 400 MHz spectrometer was used for 1H NMR and 13C
NMR spectra of porphyrins and those molecular complexes in
CDCl3 solvent. The residual CHCl3 in the 99.8% atom CDCl3 gave a
signal at 7.27 ppm which was used as a reference. The electronic
absorption spectra were recorded in chloroform solution on a
GBC Cintra 6 UV–Vis spectrophotometer. To record FT-IR spectra,
⇑
Corresponding author.
E-mail address: dehghani@kashanu.ac.ir (H. Dehghani).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.11.048

134
H. Molaei, H. Dehghani / Inorganica Chimica Acta 384 (2012) 133–136
meso
2.2.5. [H2t(4-Clp)p(NO)]BF4
β
Anal. Calc. for C44H26N5OBF4: C, 60.8; H, 3.0; N, 8.1. Found: C,
Ar
Ar
Ar
59.7; H, 2.7; N, 7.8%. UV–Vis (CHCl3): 448.8, 663.8. 1H NMR
(CDCl3): d À2.84 (s, 2H, NH), d 8.53–8.55 (d, 8H, o), d 8.04–8.06
(d, 8H, m), d 8.78 (s, 8H, b). IR (KBr): mNH (ꢀ3320 cmÀ1). KM (in
α
N
3
2
acetonitrile) = 133.1
X .
À1 cm2 molÀ1
1
4
NH
HN
Ar =
X
2.3. Diprotonation of the meso-tetraarylporphyrins
N
To H2t4-(Xp)p solution (in chloroform) was added excess
hydrochloric acid. Evaporation of the solvent produced
diprotonated porphyrins as green solids that those molar conduc-
tivities in methanol solutions were: [H4t(4-OCH3p)p]Cl2
Ar
X=H, CH3, CH(CH3)2, OCH3, Cl
Fig. 1. Meso-tetraarylporphyrins, H2t(Xp)p.
(
KM = 165
molÀ1), [H4tpp]Cl2
KM = 199
À1 cm2 molÀ1), [H4t(4-CH(CH3)2p)p]Cl2
À1 cm2 molÀ1).
X
À1 cm2 molÀ1), [H4t(4-CH3p)p]Cl2
KM = 213
(
KM = 204
À1 cm2 molÀ1), [H2t(4-Clp)p]Cl2
KM = 179
X
À1 cm2
(
X
(
X
X
(
a Magna 550 Nicolet instrument was employed (using KBr pellets).
The molar conductance of the molecular complex (in acetonitrile)
and diprotonated species (in methanol) of the porphyrins was
measured on a METROHM 644 conductometer.
3. Results and discussion
3.1. UV–Vis spectra
2.2. General procedure
An evidence for formation of the molecular complexes of meso-
tetraarylporphrins and NOBF4 was UV–Vis spectral data. Porphy-
rins have an intense Soret band at 400–420 nm and there are 3–
4 Q-bands at 500–650 nm because of electron transition within
the porphyrin core. Upon the addition of NOBF4 to free base
[H2t(4-Xp)p], the UV–Vis spectrum of the porphyrins is red shifted,
Fig. 2. These red shifts provide evidence for the out of plane distor-
tion of the porphyrins core, which causes a strong interaction to
0.05 mmol NOBF4 dissolved in 10 ml acetonitrile and added to
0.05 mmol of meso-tetraarylporphyrins in 15 ml chloroform and
the mixture was stirred for 10 min. With slow evaporation of the
solvent at room temperature, afford the green molecular complex,
[H2t(P-Xp)p(NO)]BF4. The completion of the reaction was deter-
mined by the disappearance of the Soret band (ꢀ420 nm) of the
porphyrins in those UV–Vis spectra. The results of elemental anal-
yses for the molecular complexes which were dried under vacuum
oven for 12 h at 60 °C were consistent with 1:1 mol ratio, [H2t(4-
Xp)p(NO)]BF4. The synthesized molecular complexes are decom-
posed to the related porphyrins at 150 °C.
occur between the aryl rings and porphyrin p-system [2–10]. The
UV–Vis spectra for the titration of the NOBF4 into [H2t(4-CH3p)p]
(0.5:1, 0.75:1, 1:1, excess:1) showed a new absorption band at
448.3 nm, and shrinking of the 418.9 nm peak, which belong to
the 1:1 molecular complex and H2t(4-CH3p)p, respectively. The
spectrum of the 0.5:1 NOBF4-H2t(4-CH3p)p reaction mixture
clearly demonstrates the superimposition of the [H2t(4-CH3p)p]
and [H2t(4-CH3p)p(NO)]BF4 spectra. The employment of an excess
of NOBF4 beyond the 1:1 mole ratio led to no detectable changes in
the spectrum of the [H2t(4-CH3p)p(NO)]BF4 complex. Conse-
quently, The UV–Vis titration results showed that mole ratio of
porphyrin to nitrosonium ion was 1:1.
2.2.1. [H2tpp(NO)]BF4
Anal. Calc. for C44H30N5OBF4: C, 72.2; H, 4.4; N, 9.6. Found: C,
71.3; H, 4.0; N, 9.2%. UV–Vis (CHCl3): 445.5, 661.5. 1H NMR
(CDCl3): d À2.80 (s, 2H, NH), d 8.64–8.65 (d, 8H, o), d 8.03–8.06
(t, 8H, m.p.), d 8.79 (s, 8H, b). IR (KBr): mNH (ꢀ3320 cmÀ1). KM (in
acetonitrile) = 134.3
X .
À1 cm2 molÀ1
3.2. 1H and 13C NMR spectra
2.2.2. [H2t(4-CH3p)p(NO)]BF4
The 1H NMR spectra of H2tpp and its molecular complex with
NOBF4 are shown in Fig. 3. When the amount of NOBF4 is less than
H2tpp, the spectra of the porphyrin and the related molecular com-
plex are superimposed. An excess amount of NOBF4 caused no
change in the spectum of the 1:1 molecular complex. As a result,
the molecular complex between H2tpp and NOBF4 had 1:1 ratio.
Anal. Calc. for C48H38N5OBF4: C, 73.2; H, 4.9; N, 8.9. Found: C,
72.1; H, 4.5; N, 8.6%. UV–Vis (CHCl3): 448.3, 669.4. 1H NMR
(CDCl3): d À2.80 (s, 2H, NH), d 2.79 (s, 12H, CH3), d 8.49–8.50 (d,
8H, o), d 7.82–7.84 (d, 8H, m), d 8.72 (s, 8H, b). IR (KBr): mNH
(ꢀ3320 cmÀ1). KM (in acetonitrile) = 132.9
X .
À1 cm2 molÀ1
2.2.3. [H2t(4-CH(CH3)2p)p(NO)]BF4
Anal. Calc. for C56H54N5OBF4: C, 74.7; H, 6.1; N, 7.8. Found: C,
73.5; H, 5.8; N, 7.4%. UV–Vis (CHCl3): 446.8, 661.9. 1H NMR
(CDCl3): d À2.90 (s, 2H, NH), d 1.59–1.61 (s, 12H, CH3), d 3.34–
3.38 (s, 4H, CH), d 8.54–8.56 (d, 8H, o), d 7.87–7.89 (d, 8H, m), d
8.73 (s, 8H, b). IR (KBr): mNH (ꢀ3320 cmÀ1). KM (in
acetonitrile) = 158.0
X .
À1 cm2 molÀ1
2.2.4. [H2t(4-OCH3p)p(NO)]BF4
Anal. Calc. for C48H38N5O5BF4: C, 67.7; H, 4.5; N, 8.2. Found: C,
66.5; H, 4.2; N, 8.0%. UV–Vis (CHCl3): 453.0, 686.8. 1H NMR
(CDCl3): d À2.76 (s, 2H, NH), d 4.17 (s, 12H, OCH3), d 8.52–8.54
(d, 8H, o), d 7.54–7.56 (d, 8H, m), d 8.58 (s, 8H, b). IR (KBr): mNH
Fig. 2. UV–Vis spectra of (a) H2tpp (b) [H2tpp(NO)]BF4 molecular complex in
chloroform solution.
(ꢀ3320 cmÀ1). KM (in acetonitrile) = 149.7
X
À1 cm2 molÀ1
.

H. Molaei, H. Dehghani / Inorganica Chimica Acta 384 (2012) 133–136
135
3.4. FT-IR spectra
Nitrosonium ion (NO+) shows a band around 2200 cmÀ1 that is
assigned to N„O stretching vibration [16]. In the molecular com-
plexes, [H2t(4-CH3p)p(NO)]BF4, it has been seen that stretching
vibration of the NO+ shifted to lower frequencies and were ob-
served around 1285–1295 cmÀ1. This band is related to stretching
vibration of N–O single bond. Nitrosonium ion has an empty p⁄
orbital and is accepted two lone electron-pairs from two pyrrolic
nitrogens of the meso-tetraarylporphyrins. On the other hand,
meso-tetraarylporphyrins show NH stretching band around
3320 cmÀ1 [17], that upon the molecular complexation with NOBF4
is lost. This may be related to hydrogen bonding between hydrogen
atoms of N–H and one F atom of BF4À in the molecular complexes.
3.5. Proposition of the structure
The UV–Vis, 1H NMR and 13C NMR correspondences between
spectral data of the molecular complexes of meso-tetraarylporphy-
Fig. 3. 1H NMR spectra of (a) H2tpp (b) [H2tpp(NO)]BF4 molecular complex in CDCl3.
rins with
r- or p-acceptors or diprotonated species and [H2t(4-
CH3p)p(NO)]BF4 suggests a similar structure for the porphyrin core
in all of those species, with non-coplanar pyrrole rings tilted alter-
natively up and down. This conformation causes the lone pairs of
two pyrrolic nitrogens to act as the electron donors to one NO+ cat-
ion, which is located above or below the mean plane of the porphy-
rin, and so two hydrogen atoms of pyrroles (N–H) are located on
the other side of the porphyrin plane (Fig. 4). In the proposed struc-
ture, the bond between oxygen and nitrogen atoms is single and a
negative charge locates on the oxygen atom of nitrosonium ion.
Nitrosonium ion is a strong electrophile and can be add to aro-
matic rings, but some evidences showed that nitrosonium ion
interacts to pyrrolic nitrogens and nitrosation of aryl rings was
not done in our reaction system. Firstly, the molecular complexes
decomposed to the related poprhyrins by adding water to solution
of those in chloroform. Secondly, the 1H NMR spectra of the molec-
ular complexes were quite symmetry (a singlet for b-hydrogens,
two doublets for ortho- and meta-hydrogens) whereas 1H NMR
spectra of porphyrins contain NO group on the aryl rings are
unsymmetry. Consequently, in our reaction system is only oc-
curred the molecular complexation with the pyrrolic nitrogens.
The presence of a singlet for b-protons in the 1H NMR spectrum
of [H2tpp(NO)]BF4 molecular complex is in contrast to unsymmet-
rical pyrrole rings in the proposed structure (Fig. 3b). Because the
Upon formation of the molecular complex, b-protons shift to up-
field and the aryl protons shift to downfield. With the molecular
complexation of meso-tetraarylporphyrins and nitrosonium ion,
the NH protons signal small shifted to upfield and are observed
at À2.76 to À2.90 ppm. The shift to upfield is similar to the molec-
ular complexation of meso-tetraarylporphyrins with electron
acceptor of BF3 [10]. Weak hydrogen bonding between the F atoms
of BF3 and the hydrogen atoms of NH is suggested as main factor in
the upfield shifts of NH signals [10]. However, it may more reason-
able that high electronegativity of F atoms causes an increase in
the shielding of the NH protons and in result the upfield chemical
shifts. 1H NMR spectral shifts upon molecular complexation of dif-
ferent meso-tetraarylporphyrins with NOBF4 indicate that p-reso-
nance effect is predominantly transmitted from aryl substituents
to the porphyrin core in the molecular complexes [2–10].
The 13C NMR spectrum of H2t(4-CH3p)p shows six signals in the
aromatic region: one broad signal for b-carbons (131.4 ppm) and
five sharp signals (C1 = 139.7, C2 = 134.9, C3 = 127.8, C4 = 137.7
and Cmeso = 120.5) [7,8]. The
a-carbons peak at about 145 ppm is
weak to be seen. 13C NMR spectrum of the molecular complex,
[H2t(4-CH3p)p(NO)]BF4, had seven signals in the aromatic region:
124.2 (Cmeso), 130.1 (Cb), 130.5 (C3), 138.3 (C1), 139.9 (C2),
141.9(C4), 147.3 (C ). Molecular complexation of H2t(4-CH3p)p
a
_
with NOBF4 sharpens the
a-carbon signal. Further, it causes an up-
O
field shift in Cb and C1 signals and a downfield shift in the lines of
C
meso, C2, C3, C4. Molecular complexation of H2t(4-CH3p)p with var-
ious acceptors causes the similar changes in the 13C NMR spectra
[2,3,7,8].
N
+
+
N
N
3.3. Molar conductivity measurements
The UV–Vis, 1H NMR (except for the N–H chemical shift) and 13
C
NMR spectra of the molecular complexes are similar to the corre-
spondence spectra of the their diprotonated porphyrin species
[14], but the molar conductivities of [H2t(4-Xp)p(NO)]BF4 com-
plexes (in acetonitrile) and [H4t(4-Xp)p]Cl2 (in methanol) were
completely different. The measurements of molar conductivity
showed that the molecular complexes were 1:1 electrolytes
whereas the diprotonated porphyrins were 1:2 electrolytes [15].
The remarkable spectral correspondence between the molecular
complexes and their diprotonated species is related to analogous
tilted porphyrin core structures in two species.
N
H
N
H
F
_
BF
3
Fig. 4. Only the tetrahedrally tilted pyrrolenine nitrogens of a porphyrin core are
shown. Two pyrrolenine nitrogen atoms act as electron donors to a nitrogen atom of
À
nitrosonium ion and BF4 is as counter ion.

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H. Molaei, H. Dehghani / Inorganica Chimica Acta 384 (2012) 133–136
ring inversion of the tilted core conformation of the porphyrin is
fast on the NMR time scale [2,3,10].
References
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4. Conclusion
The reaction of NOBF4 with meso-tetraarylporphyrins (H2t(p-
Xp)p) produced green molecular complexes, [H2t(4-Xp)p(NO)]BF4.
The results of UV–Vis, 1H NMR (b- and aryl protons) and 13C NMR
spectra showed tilted porphyrin core of the produced compounds
was similar to porphyrin diacids [14] and the molecular complexes
of meso-tetraarylporphyrins with various acceptors [2–10]. Conse-
quently, two nitrogen atoms of pyrrolic in porphyrin act as electron
donors to a nitrosonium ion.
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Acknowledgment
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Compounds, fifth ed., John Wiley & Sons, New York, 1997, Part B, p. 309.
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This work was partly supported by University of Kashan Re-
search Council.