Multiporphyrin arrays on cyclotriphosphazene scaffolds

Article abstract of DOI:10.1021/ic501569e

We report the synthesis of first examples of hexaporphyrin and dodecaporphyrin assemblies on cyclotriphosphazene scaffold by adopting two different approaches based on Ru-pyridyl "N" coordination in decent yields. The multiporphyrin assemblies were confir

Full text of DOI:10.1021/ic501569e

Article
pubs.acs.org/IC
Multiporphyrin Arrays on Cyclotriphosphazene Scaffolds
Tejinder Kaur, Malakalapalli Rajeswararao, and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
S
* Supporting Information
ABSTRACT: We report the synthesis of first examples of
hexaporphyrin and dodecaporphyrin assemblies on cyclotriphos-
phazene scaffold by adopting two different approaches based on
Ru-pyridyl “N” coordination in decent yields. The multi-
1
1
porphyrin assemblies were confirmed by 31P, 13C, H, H−1H
COSY, and NOESY NMR spectroscopic studies. The absorption
studies showed 2-fold intensity enhancement with negligible
changes in peak maxima compared to porphyrin monomers. The
redox potentials of multiporphyrin assemblies showed the redox
features of the constituted porphyrin monomers and supported
weak interactions among the porphyrin units in noncovalent
hexaporphyrin and dodecaporphyrin arrays.
the same synthetic strategy to prepare covalent cyclo-
triphosphazenes appended with 12 porphyrin units, it resulted
in a mixture of products that requires laborious chromato-
graphic purifications. During these investigations, we realized
that cyclophosphazenes appended with pyrazoles and pyridines
have been exploited for the synthesis of metal complexes.8−11
Hence, based on porphyrin literature, we thought that
cyclotriphosphazenes appended with pyridines may bind
metalloporphyrins as an axial ligand as well as cyclo-
triphosphazene ring appended with metalloporphyrins, which
has a free axial site, may bind meso-pyridyl porphyrins to
construct multiporphyrin assemblies on cyclotriphosphazene
scaffolds. Here, we applied the synthetic strategy based on
metal-pyridine interaction to synthesize cyclotriphophazenes
appended with 6 as well as 12 porphyrin units under simple
reaction conditions in good yields. The compounds are stable,
freely soluble in toluene, CH2Cl2, CHCl3, etc. and exhibited
interesting spectral and electrochemical properties.
INTRODUCTION
■
Multiporphyrin arrays1 have attracted attention to address
aspects of photosynthesis, host−guest complexation, catalysis,
electronic device applications, etc. Porphyrins and metal-
loporphyrins can be assembled into arrays either by covalent
strategies or by adopting noncovalent strategies. Several
covalent strategies for the synthesis of multiporphyrin arrays
have evolved over the years,2 but these strategies involve many
sequential steps, separation of statistical mixtures, and arduous
chromatographic purifications, resulting in low product yield.
Noncovalent strategies3 such as hydrogen bonding, metal-
mediated self-assemblies, etc. have emerged as a viable
alternative to covalent synthesis in the construction of large
and sophisticated multiporphyrin architectures. The mixed
meso-substituted pyridyl/phenyl porphyrins have been used
extensively to construct such multiporphyrin architectures
based on self-assembly approaches using metalloporphyrin-
pyridine interaction.4 Pyridine acts as a versatile ligand toward a
variety of metal ions, and its well-known coordinating
properties can be easily adopted to construct self-assembled
systems. Pyridine can be functionalized easily; hence, it can be
readily built into the porphyrin meso-position(s). Thus, the
pyridyl containing compound such as meso-pyridyl porphyrin
can act as an axial ligand to the metal atom in metalloporphyrin,
provided that the metal atom inside the porphyrin core has at
least one axial site available for co-ordination. Metalloporphyr-
ins such as Zn(II), Mg(II), Ru(II), Os(II), Rh(III) porphyrins
have been used to construct multiporphyrin assemblies.4
Recently, we showed5−7 that cyclotriphosphazene ring can be
used as scaffold to synthesize covalent hexaporphyrin
assemblies6 (Chart 1) under very mild reaction conditions.
The hexaporphyrin assemblies on cyclotriphosphazene ring are
quite stable and freely soluble in common organic solvents.
These compounds can readily undergo metalation to synthesize
hexametalloporphyrin assemblies. However, when we extended
RESULTS AND DISCUSSION
■
Hexaporphyrin Assembly on Cyclotriphosphazene
Scaffolds. The hexaporphyrin assembly on a cyclotriphospha-
zene scaffold (2) was synthesized by using the readily available
cyclotriphosphazene containing 4-pyridyloxy pendant groups
(4). (See Scheme 1.) Compound 4 was synthesized by
refluxing 1 equiv of P3N3Cl6 with 7 equiv of 4-hydroxypyridine
in acetone in the presence of a base, as reported in the
literature.12 The noncovalent hexaporphyrin assembly on the
cyclotriphosphazene ring 2 was synthesized by treating 1 equiv
of compound 4 with 7 equiv of RuTTP(CO)(EtOH)13 in
toluene at refluxing temperature for 10−12 h. The progress of
the reaction was followed by TLC analysis, which clearly
Received: July 2, 2014
Published: October 3, 2014
© 2014 American Chemical Society
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Chart 1. Structures of Cyclotriphosphazenes Decorated with Six Porphyrins and Their Metal(II) Derivatives
Scheme 1. Synthesis of Noncovalent Hexaporphyrin Assembly 2
Figure 1. Comparison of (a) 1H NMR and (b) 31P NMR spectra of compounds 2 and 4; (c) 1H−1H-COSY NMR spectrum of 2 recorded in CDCl3.
indicated the spots corresponding to the formation of
intermediate compounds at the beginning but as the reaction
progress, those spots disappeared with an appearance of one
single spot corresponding to the required compound. The
crude compound obtained after the removal of solvent was
subjected to alumina column chromatographic purification and
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obtained pure compound 2 as a purple solid in 69% yield.
Compound 2 is sparingly soluble in common organic solvents
Chart 2. Structures of Noncovalent Hexaporphyrin and
Dodecamer Assembly on a Cyclotriphosphazene Scaffold
and characterized by MALDI-TOF mass, H, 31P, and H−1H
COSY NMR techniques. The MALDI-TOF mass analysis did
not give the molecular ion peak corresponding to compound 2,
but it did show the mass corresponding to [M+-6RuTTP-
(CO)]. The comparison of 1H and 31P NMR spectra of
compounds 2 and 4 are presented in Figures 1a and 1b,
respectively, along with 1H−1H COSY NMR spectrum in
1
1
1
Figure 1c. In H NMR, the 3,5- and 2,6-pyridyl protons of
compound 4, which appeared as two doublets at 8.60 and 6.90
ppm, experienced a strong ring current effect upon
coordination with RuTTP(CO) in compound 2 and shifted
to 5.80 and 2.40 ppm, respectively. This is also clearly evident
in the proton−proton connectivity pattern in the 1H−1H
COSY spectrum. Interestingly, unlike our earlier reported
covalently linked hexaporphyrin assembly on cyclotriphospha-
zene ring 1 (which showed a greater number of signals for β-
pyrrole and meso-aryl protons, which were shifted upfield,
compared to monomeric porphyrin), compound 2 exhibited
one singlet for pyrrole and three sets of signals for meso-aryl
protons such as monomer RuTTP(CO)(EtOH) with no shifts
in their chemical shifts. (See Figure 2.) This indicates that the
Figure 2. Comparison of (a) absorption spectra of 2 (―) and
RuTTP(CO)(EtOH) (- - -) recorded in chloroform using concen-
tration of 5 × 10−7 M.
strong interaction between the porphyrin units, which is
present in covalent hexaporphyrin assembly 1, is absent in
noncovalent hexaporphyrin assembly 2 (see Chart 2), and
Ru(II) porphyrin assembly on cyclophosphazene ring behaves
as free monomeric Ru(II) porphyrin. However, the effect of
RuTTP(CO) coordination to pyridyloxy units via Ru−N
coordination in compound 2 was clearly reflected in the 31P
NMR spectrum. Compound 4 showed a signal at 6.97 ppm in
31P NMR, which shifted significantly upfield in compound 2
and appeared at 0.82 ppm. This is because of the orientation of
Ru(II) porphyrin, which induces the strong ring current effect
on cyclotriphosphazene. This observation is in agreement with
the significant upfield shifts of 2,6- and 3,5-pyridyl protons
upon coordination of pyridyl “N” with RuTTP(CO) units in
compound 2. The absorption spectrum of compound 2 showed
absorption features similar to monomeric RuTTP(CO)(EtOH)
with identical peak positions but the intensity of absorption
bands of 2 was increased by 2-fold, compared to RuTTP-
(CO)(EtOH) as observed earlier for covalently linked
hexaporphyrin assembly 1.
The electrochemical studies of compound 2 probed through
cyclic voltammetry using tetrabutylammonium perchlorate as
supporting electrolyte in dichloromethane showed two
reversible oxidations and one quasi-reversible reduction (see
Table 1). The peak potentials are almost in the same range as
those of RuTTP(CO)(EtOH) supporting weak interaction
among RuTTP(CO) units in compound 2.
Dodecaporphyrin Assembly on a Cyclotriphospha-
zene Scaffold. The cyclotriphosphazene ring appended with
12 porphyrins was synthesized using Ru1, as shown in Scheme
2. Compound 15 was treated with Ru3(CO)12 in toluene at
reflux temperature overnight, followed by 2 h of reflux in
CH2Cl2/EtOH. The progress of the reaction was followed by
absorption spectroscopy. The crude compound was passed
through Celite pad and afforded pure compound Ru1 as
reddish orange solid in 85% yield. Ru1 is freely soluble in
solvents such as CH2Cl2, CHCl3, toluene, etc., and compound
Ru1 was confirmed by the molecular ion peak at 4760.7,
corresponding to [M+-6CO] in MALDI-TOF mass spectrum
(see the Supporting Information). 31P, 13C NMR, and 1D and
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Table 1. Absorption and Electrochemical Data of All of the Compounds
UV-Vis Data
Potential V vs SCE
compound
Soret band (λ nm log ε)
Q bands (λ nm log ε)
oxidation
1.46
reduction
2
411 (6.12)
530 (5.04)
562(sh)
1.04
0.81
−1.45
Ru(CO)TTP
412 (5.48)
419 (4.71)
530 (4.70)
564(sh)
1.43
1.24
−1.58
−1.30
5
516 (4.52)
551 (4.33)
591 (4.22)
648 (4.15)
1.34
−0.97
Ru1
3
412 (5.48)
419 (6.56)
530 (4.70)
564(sh)
0.97
1.06
1.37
1.39
−1.39
−1.34
−1.65
−1.60
522 (5.28)
551 (5.00)
591 (4.71)
648 (4.57)
1.24
−0.99
Scheme 2. Synthesis of 3
1
2D NMR were used to characterize compound Ru1. In the 31
P
the Ru(II) ion. Furthermore, the signals in H NMR of Ru1
experienced slight upfield shifts, compared to 1. Thus, it is not
NMR spectrum, Ru1 showed only one signal at 9.88 ppm,
which indicates that the Ru1 is symmetrical and all three P
1
possible to identify all the signals in the H NMR spectrum of
Ru1 based on the NMR of 1. Hence, we carried out detailed
COSY and NOESY NMR analysis of Ru1 to identify and assign
all signals in H NMR spectrum. The relevant portions of
COSY and NOESY NMR spectra of Ru1 are presented in
Figure 3 with detailed assignments. The H NMR spectrum of
Ru1 showed few slightly broadened resonances, which we
tentatively attribute to the presence of conformers in the
solution. The H NMR spectrum of Ru1 showed two singlets
1
atoms displayed the same chemical shift. In the H NMR
spectrum of Ru1, the signal corresponding to inner NH
protons, which appears at the upfield region in free base
porphyrin assembly 1 was absent, supporting that all free base
porphyrin units on cyclotriphosphazene ring were metalated
with Ru(II) ions. Recently, we assigned the NMR spectrum of
cyclotriphosphazene appended with six free base porphyrins5 1,
using 1D and 2D NMR spectroscopy. However, the
1
1
1
1
comparison of H NMR spectra of Ru1 with 1 indicated that
for methyl protons at 2.67 and 2.43 ppm, corresponding to 18
and 36 protons, respectively. The signal corresponding to 18
protons at 2.67 ppm was assigned to Type I methyl protons
the number of signals in Ru1 is greater compared to 1, because
of the unsymmetric nature of porphyrin upon introduction of
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1
Figure 3. (a,b) Partial H−1H COSY NMR and (c) NOESY NMR spectra of compounds Ru1 recorded in CDCl3.
that are opposite to the phenoxo group, and the singlet at 2.43
ppm corresponding to 36 protons was assigned to Type II
methyl protons that are trans to each other and reside in the
same chemical environment. In the NOESY spectrum, the
Type I methyl protons at 2.67 ppm showed NOE correlation
with a multiplet at 7.46 ppm, which we identified as Type I
protons of meso-aryl group. The Type I protons at 7.46 ppm
show proton−proton correlation with two doublets at 7.85 and
7.96 ppm, which we assigned arbitrarily as Type k and Type j
protons, respectively, of meso-aryl group opposite to phenoxo
group. The Type k and Type j protons showed NOE
correlation with a doublet at 8.54 ppm, which we assigned as
Type c pyrrole protons. The doublet at 8.43 ppm was identified
as being due to Type b pyrrole protons, as it showed proton−
proton correlation with Type c pyrrole protons at 8.54 ppm.
The Type II methyl protons at 2.43 ppm showed NOE
correlation with two doublets at 7.09 and 7.18 ppm, which we
assigned arbitrarily to type e and type h protons of meso-aryl
group, respectively. The Type e protons at 7.09 ppm showed
proton−proton correlation with a doublet at 7.36 ppm, which
we identified as being due to Type f protons. Similarly, the
Type h protons at 7.18 ppm showed NOE correlation with a
doublet at 7.70 ppm, which we assigned as being due to Type g
protons of the meso-aryl group. The Type f and Type g protons
showed NOE correlation with a doublet at 8.30 ppm, which we
identified as Type a pyrrole protons. The doublet at 8.70 ppm
was identified as Type d pyrrole protons, since this doublet
showed proton−proton correlation with Type a protons at 8.30
ppm. The phenoxo group also showed four doublets, which
were identified similarly, following NOESY and COSY
correlations. The Type d pyrrole protons at 8.70 ppm showed
NOE correlation with two doublets at 8.20 and 8.37 ppm,
which we arbitrarily assigned as Type n and Type o protons of
the phenoxo group. The Type o proton at 8.37 ppm showed
proton−proton correlation with a doublet at 8.09 ppm, which
we identified as Type m protons of the meso-phenoxo group.
The Type n protons at 8.37 ppm showed proton−proton
correlation with a doublet at 8.03 ppm, which we assigned as
Type l protons of meso-phenoxo group. Thus, using 1D and 2D
NMR spectroscopy, we attempted to identify all the resonances
1
observed in the H NMR spectrum of Ru1. Furthermore, the
signal at 180 ppm in 13C NMR spectrum (see the Supporting
Information) confirms the presence of CO groups in
compound Ru1. The absorption spectrum of Ru1 showed
one strong Soret band and two less-intense Q-bands. The peak
positions closely matched with those of its corresponding
Ru(II)porphyrin monomer RuTTP(CO)(EtOH). However, as
observed for compound 1, the absorption bands of compound
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1
Figure 4. (a,b) Partial H−1H COSY NMR and (c) NOESY NMR spectra of compound 2 recorded in CDCl3.
Ru1 are 2-fold to 3-fold more intense, with large extinction
coefficients, compared to RuTTP(CO)(EtOH), as presented in
Table 1.
along with its corresponding components 5 and Ru1, is
presented in Figure 4. As clearly shown in Figure 4, the H
1
NMR spectrum of 3 is very complex and exhibit signals
corresponding to Ru(II) porphyrin, as well as free base
monopyridylporphyrin 5. However, we adopted the same
strategy that we used for assigning signals for Ru1 and deduced
the molecular structure of compound 3. Similar to Ru1, in
compound 3 also, we noted few broadened resonances in NMR
spectra that could be due to the presence of conformers in
solution. The COSY and NOESY NMR spectra of 3, along with
all proton assignments, are presented in Figure 4. In the
NOESY spectrum of 3, we noted four singlets at 1.90, 2.58,
2.60, 2.28 ppm, which we assigned as Type I−IV methyl
protons, respectively. The Type I methyl protons of Ru(II)
porphyrin units showed NOE correlation with two doublets at
6.60 and 6.83 ppm, which we arbitrarily assigned to Type i and
i′ protons, respectively, of the meso aryl group opposite to
phenoxo group. The Type i and i′ protons showed proton−
proton correlation with a multiplet at ∼7.58 and a doublet at
∼7.72 ppm, which we identified as Type k and j protons,
respectively. The Type k and j protons, in turn, showed NOE
connectivity with a doublet at 8.51 ppm, which we identified as
Type c pyrrole protons of the Ru(II) porphyrin unit. The
The cyclophosphazene appended with 12 porphyrins was
prepared by reacting Ru1 with 5,10,15-tri(tolyl)-20-(p-pyridyl)
porphyrin 5 (MPy) in toluene at reflux temperature for 24 h.
The crude compound was passed through Celite and afforded
pure compound 3 in 88% yield after recrystallization using
petroleum ether and CH2Cl2. Compound 3 is soluble in
toluene, CH2Cl2, CHCl3 and characterized by the MALDI-
TOF mass spectrum, as well as 13C 31P, and 1D and 2D NMR
spectroscopy. In compound 3, the signal at 180 ppm in 13C
NMR spectrum and a strong sharp peak at 1943 cm−1 in IR
spectrum corresponding to CO group of RuTTP(CO) is
observed. In MALDI-TOF mass spectrum, a small peak at
∼8711.8 corresponding to [M+-6CO] was observed along with
the base peak corresponding to [M+-6(CO+MPy)] (where M =
complete molecule, MPy = monopyridyl porphyrin). This type
of fragmentation is common for metal-pyridyl “N” interaction-
based porphyrin arrays.3 The 31P NMR spectrum of 3 showed
one signal, which is shifted slightly upfield, compared to Ru1,
supporting the symmetrical arrangement of substituents on
1
three P atoms. The comparison of the H NMR spectra of 3,
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1
Figure 5. Comparison of H NMR spectra of compound 3, along with corresponding monomers 5 and Ru1 (top to bottom).
doublet at 8.64 ppm was assigned as Type b pyrrole protons, as
this doublet showed proton−proton connectivity with Type c
pyrrole protons. The Type II methyl protons of Ru(II)
porphyrin unit at 2.59 ppm showed NOE connectivity with two
doublets at 7.86 and 7.96 ppm, which we arbitrarily assigned as
Type e and h protons of Ru(II) porphyrin unit, respectively.
The Type e and h protons showed proton−proton connectivity
with a multiplet at 7.37−7.43 ppm, which we assigned as Type
g and f protons. The Type g and f protons showed NOE
connectivity with Type b, as well as with a doublet at 8.53 ppm,
which we assigned as Type a pyrrole protons. The doublet at
8.64 ppm showed proton−proton connectivity with Type a
pyrrole protons, which we assigned as Type d protons.
Furthermore, the Type d protons showed NOE connectivity
with a multiplet at ∼8.50 ppm, which we assigned as Type o
protons of the phenoxo group of Ru(II) porphyrin unit. The
Type o protons, in turn, showed proton−proton connectivity
with a multiplet at ∼8.28 ppm, which we identified as being due
to the Type m proton of the meso phenoxo group of the Ru(II)
porphyrin unit.
To identify the signals corresponding to the mono meso-
pyridyl porphyrin, we first identified the 2,6 and 3,5 protons of
meso pyridyl group of mono meso-pyridyl porphyrin. The 2,6
and 3,5 meso pyridyl protons of monomeric meso-pyridyl
porphyrin, which normally appears as two doublets at 9.01 and
8.18 ppm, experienced significant upfield shifts upon
coordination with Ru(II) porphyrin unit via Ru(II)-pyridyl N
interaction and appears at 1.81 and 5.88 ppm, respectively, in
compound 3. The 2,6 and 3,5 meso-pyridyl protons were
affected by both the inherent deshielding effect of the axial free
base meso-pyridyl porphyrin unit and the shielding effect of the
Ru(II)porphyrin unit, as detected by the proton connectivity
pattern in the 1H−1H COSY spectrum. The 3,5 protons at 5.88
ppm showed NOE correlation with a doublet at 7.12 ppm,
which we identified as Type p pyrrole protons of meso-pyridyl
porphyrin unit. The doublet at 8.46 ppm was assigned as Type
q pyrrole protons, since this doublet showed proton−proton
correlation with Type p protons. The Type IV methyl protons
at 2.28 ppm showed NOE correlation with a doublet at 7.12
ppm, which we identified as Type u meso-aryl protons of the
meso-pyridyl porphyrin unit. The multiplet at 7.57−7.62 ppm
was identified as Type t protons as this multiplet showed COSY
correlation with Type u protons. The Type t protons showed
NOE connectivity with Type q protons, as well as with a
doublet at 8.46 ppm, which we assigned as Type r pyrrole
protons. The Type r protons further showed proton−proton
correlation with a doublet at 8.84 ppm, which we identified as
Type s pyrrole protons of the meso-pyridyl porphyrin unit. The
Type s protons showed NOE connectivity with a multiplet at
∼7.62 ppm, which we assigned as Type v meso-aryl protons of
the meso-pyridyl porphyrin unit. The Type v protons also
showed 1H−1H COSY correlation with Type w protons at 7.54
ppm, which, in turn, showed NOE connectivity with Type III
methyl protons, which appeared as a singlet at 2.60 ppm.
Furthermore, the upfield shifts noted for various protons of
Ru(II) porphyrin, as well as meso-pyridyl porphyrin units in
compound 3, were due to porphyrin ring current effect
experienced by these protons.
To know more precisely about the homogeneity of
1
compound 3, the H diffusion-ordered spectroscopy (DOSY)
experiments were carried out on compounds 5, Ru1, and 3
separately, as well as on a mixture containing all three
compounds (5, Ru1, and 3). The DOSY experiments gives
direct measure of diffusion coefficient of different compounds
present in the mixture, and the diffusion coefficient is
dependent on various factors, including the size and shape of
the compounds. The results are displayed as a 2D spectrum in
which signals are dispersed according to chemical shift in one
dimension and diffusion coefficients (D) in the other. The
diffusion coefficients Dexp obtained for compounds 5, Ru1, and
3 are 7.08, 5.37, and 3.80 (Dexp = D/10−10 m2 s−1), respectively.
The diffusion coefficients are in agreement with the Stokes−
Einstein equation:15 monomeric compound 5 diffuses faster,
whereas dodecamer compound 3 diffuses slower, according to
their relative sizes. The diffusion coefficients obtained from the
mixture of compounds 5, Ru1, and 3 were in agreement with
the independent compounds, suggesting that compound 3 is
homogeneous. Thus, NMR studies unambiguously confirmed
the structure of the porphyrin dodecamer assembly on a
cyclotriphosphazene ring.
The comparison of absorption spectra of compound 3 along
with its constituted porphyrin monomers monopyridyl
porphyrin and RuTTP(CO)(EtOH) recorded in chloroform
using same concentration is shown in Figure 5.This figure
clearly shows that compound 3 exhibited absorption features of
both constituted porphyrin units, and the peak maxima were
almost same as those of the constituted monomers, but with a
slight increase in bandwidth. Futhermore, there is a 7-fold
intensity enhancement of absorption bands of compound 3,
compared to its constituted porphyrin monomers.
A representative cyclic voltammogram of 3, along with those
of the associated monomers, is shown in Figure 6. Generally,
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solvents used were of analytical grade and were purified and dried by
1
routine procedures immediately before use. H NMR spectra were
recorded with Bruker 400 and 500 MHz instruments, using
trimethylsilane (TMS) as an internal standard. All NMR measure-
ments were carried out at room temperature in deuterochloroform
(CDCl3). For DOSY experiment, the gradient shape was sinusoidal.
The strength of the gradient shape was increased linearly, acquiring 32
gradient levels. The time between the midpoints of the gradients (Δ)
was chosen as 40 ms. Low and high gradient strengths were set at 2%
and 95% of maximum, respectively. All DOSY experiments were
obtained with a longitudinal eddy current delay (LED) bipolar
gradient pulse pair and two spoil gradients pulse sequence ledbpgp2s
in the standard Bruker Pulse Sequence Library. The processing was
done with standard Bruker 1D and 2D DOSY software. Absorption
spectra were obtained with Varian Cary-Eclipse equipment. MALDI-
TOF mass spectra were recorded on a Bruker MALDI-TOF
spectrometer. Cyclic voltammetry (CV) studies carried out with
BAS electrochemical system utilizing the three-electrode configuration
consisting of a glassy carbon (working electrode), platinum wire
(auxiliary electrode), and saturated calomel (reference electrode)
electrodes in dry dichloromethane using 0.1 M tetrabutylammonium
perchlorate as supporting electrolyte.
Figure 6. Comparison of absorption spectra of compound 3 and a 1:6
mixture of compounds Ru1 and 5 recorded in chloroform using a
concentration of 5 × 10−7 M.
the meso-pyridyl porphyrin monomers show one ill-defined
oxidation due to the electron-deficient meso-pyridyl group but
showed two quasi-reversible or reversible reductions. In
compound 3, because the meso-pyridyl groups are involved in
binding with Ru(II), it showed three quasi-reversible oxidations
and three reversible reductions. The oxidation/reduction waves
were assigned by comparing with the redox properties of
corresponding monomers 5 and Ru1. Thus, in compound 3,
the oxidation at 1.06 and 1.39 V mainly correspond to the
Ru(II) porphyrin unit and oxidation at 1.24 V was mainly due
to the monopyridyl porphyrin unit. Similarly, the reduction at
−0.99 V corresponds to the monopyridyl porphyrin unit; the
reduction at −1.34 V was due to both the monopyridyl
porphyrin and Ru(II) porphyrin units, and the reduction at
−1.60 V was only due to Ru(II) porphyrin unit. Since the
oxidation and reduction potentials of 3 are almost in the same
range as its constituted monomers, it indicated weak interaction
among the porphyrin moieties in compound 3 as supported by
absorption spectroscopy.
Noncovalent Hexaporphyrin Assembly (2). Compound 2 was
synthesized by refluxing P3N3(p-oxypyridine)6 (4) (20 mg, 0.029
mmol) and RuTTP(CO)(EtOH) (137 mg, 0.172 mmol), in dry
toluene (20 mL) for 12 h under a nitrogen atmosphere. The solvent
was evaporated under reduced pressure, and the resulted residue was
subjected to basic alumina column. The desired product was eluted
with petroleum ether/CH2Cl2 (40:60) and was recrystallized using
dichloromethane/n-hexane mixture to afford compound 1. Yield: 69%
1
(109 mg, 0.020 mmol); H NMR (400 MHz, CDCl3, δ in ppm): δ =
2.31 (d, J = 8.0 Hz, 12H; py), 5.74 (d, J = 8.7 Hz, 12H; py), 7.71−7.72
(m, 72H; Ar), 8.02 (d, J = 8.2 Hz, 24H; Ar), 8.20 (d, J = 7.1 Hz, 24H;
Ar), 8.59 (s, 48H; β-py); 31P{1H} NMR (161.8 MHz, CDCl3, δ in
ppm): 0.82 (s). MALDI-TOF MS: m/z: Calcd (M+) 5492.9; Found:
701.1 [M+-6RuTTP(CO)].
Ru1. To a solution of 1 (50 mg, 0.012 mmol) in dry toluene,
Ru3(CO)12 (77 mg, 0.120 mmol) was added and refluxed for 10 h.
The color changed from purple to reddish orange as the reaction
progressed. The reaction was monitored by TLC and UV-vis
spectroscopy. Solvent was evaporated under vacuum, and the solid
orange compound was again refluxed in dichloromethane/ethanol
(1:1, 20 mL) for 2 h. After the completion of the reaction, the solvent
was removed under vacuum and the resultant solid orange compound
was dissolved in dichloromethane and passed through Celite. The
compound was recrystallized using petroleum ether/CH2Cl2 (3:1, v/v)
and afforded the pure compound as reddish orange solid. Yield: 85%
CONCLUSIONS
■
In summary, we used cyclotriphosphazene as a scaffold to
construct two multiporphyrin assemblies such as hexaporphyrin
and dodecaporphyrin assemblies by adopting two simple
approaches based on Ru-pyridyl “N” coordination. The
hexaporphyrin assembly is sparingly soluble but the dodeca-
porphyrin assembly was freely soluble in common organic
solvents. The formation of multiporphyrin assemblies was
confirmed by 31P, 13C, and 1D and 2D NMR spectroscopy. The
absorption studies showed a 2-fold to 3-fold enhancement in
intensity with negligible shifts in peak maxima, compared to
constituted porphyrin monomers. The redox potentials of
multiporphyrin assemblies are in the same range as those of
porphyrin monomers. Thus, we demonstrated that the robust
cyclotriphosphazene ring can be used to construct multi-
porphyrin assemblies by simple approaches in decent yields.
1
(53 mg, 0.010 mmol); H NMR (400 MHz, CDCl3, δ in ppm): δ =
2.43 (s, 36H; tol), 2.66 (s, 18H; tol), 7.08 (d, J = 7.6 Hz, 12H; Ar),
7.18 (d, J = 7.2 Hz, 12H; Ar), 7.35 (d, J = 7.3 Hz, 12H; Ar), 7.47 (d, J
= 7.6 Hz, 12H; Ar), 7.69 (d, J = 7.0 Hz, 12H; Ar), 7.85 (d, J = 7.2 Hz,
6H; Ar), 7.95 (d, J = 6.4 Hz, 6H; Ar), 8.04 (d, J = 7.5 Hz, 6H; Ar),
8.09 (d, J = 8.5 Hz, 6H; Ar), 8.19 (d, J = 8.4 Hz, 6H; Ar), 8.30 (d, J =
4.8 Hz, 12H; β-py), 8.38 (d, J = 8.1 Hz, 6H; Ar), 8.43 (d, J = 4.9 Hz,
12H; β-py), 8.54 (d, J = 4.9 Hz, 12H; β-py), 8.69 (d, J = 4.8 Hz, 12H;
β-py); 31P{1H} NMR (161.8 MHz, CDCl3, δ in ppm): 9.88 (s).
MALDI-TOF MS: m/z: Calcd (M+) 4928.2; Found: 4760.7 [M+-6
(CO)].
Noncovalent Dodecaporphyrin Assembly (3). To a solution of
1 (30 mg, 0.006 mmol) in dry toluene, 5,10,15-tri(tolyl)-20-(p-
pyridyl) porphyrin (5) (23 mg, 0.035 mmol) was added and refluxed
for 18 h. The reaction was monitored by TLC and UV-vis
spectroscopy. After the completion of the reaction, the solvent was
evaporated under vacuum and the resulted solid compound was
dissolved in CH2Cl2 and passed through Celite. The compound was
recrystallized using petroleum ether/CH2Cl2 (3:1, v/v) and afforded
the pure compound as a purple solid. Yield: 88% (45 mg, 0.005
EXPERIMENTAL SECTION
■
All general chemicals and solvents were procured from SD Fine
Chemicals, India. The known compounds such as P3N3(p-oxy-
pyridine)6 (4),12 covalently linked hexaporphyrin assembly on
cyclotriphosphazene ring (1),5 and 5,10,15-tri(tolyl)-20-(p-pyridyl)
porphyrin (MPy, 5)14 were synthesized using literature methods.
Column chromatography was performed using silica gel and basic
alumina obtained from Sisco Research Laboratories, India. All of the
1
mmol); H NMR (500 MHz, CDCl3, δ in ppm): δ = −3.31 (s, 12H;
−NH); 1.81 (d, J = 6.0 Hz, 12H, pyr), 1.90 (s, 18H; Tol), 2.28 (s,
36H; Tol), 2.59 (s, 36H; Tol), 2.66 (s, 18H; Tol), 5.87 (d, J = 6.7 Hz,
11058
dx.doi.org/10.1021/ic501569e | Inorg. Chem. 2014, 53, 11051−11059

Inorganic Chemistry
Article
12H, pyr), 6.60 (d, J = 7.8 Hz, 6H, Ar), 6.83 (d, J = 7.7 Hz, 6H, Ar),
7.04 (d, J = 7.8 Hz, 24H, Ar), 7.12 (d, J = 4.4 Hz, 12H, β-py), 7.37−
7.43 (m, 24H; Ar), 7.54 (d, J = 8.0 Hz, 12H, Ar), 7.57−7.62 (m, 30H;
Ar), 7.72 (d, J = 7.9 Hz, 6H; Ar), 7.86 (d, J = 8.0 Hz, 12H; Ar), 7.96
(d, J = 7.7 Hz, 12H; Ar), 8.14−8.19 (m, 24H; Ar), 8.28 (d, J = 4.8 Hz,
12H; β-py), 8.46 (d, J = 4.8 Hz, 12H; β-py), 8.47−8.50 (m, 12H; Ar),
8.51 (d, J = 4.7 Hz, 12H; β-py), 8.56 (d, J = 4.8 Hz, 24H; β-py), 8.84
(d, J = 4.7 Hz, 12H; β-py); 31P{1H} NMR (161.8 MHz, CDCl3, δ in
ppm): 9.01 (s). 13C NMR (400 MHz CDCl3): 181.0, 151.2, 144.1,
140.1, 139.4, 136.8, 135.7, 134.3, 133.6, 131.7, 127.3, 127.1, 121.9,
120.3, 32.1, 29.9, 29.5, 22.8, 21.6, 21.5, 14.3; MALDI-TOF MS: m/z:
Calcd (M+) 8881.2; Found: 8711.8 [M+-(6CO)].
Bargellitti, F.; Cola, L. D.; Flamigni, L.; Balzani, V. Inorg. Chem. 1991,
30, 4230. (o) Harriman, A.; Odobel, F.; Sauvage, J. P. J. Am. Chem. Soc.
1994, 116, 5481. (p) Collin, J. P.; Harriman, A.; Heitz, V.; Odobel, F.;
Sauvage, J. P. J. Am. Chem. Soc. 1994, 116, 5679. (q) Amabillino, D. B.;
Sauvage, J. P. New. J. Chem. 1998, 395. (r) Collin, J. P.; Gavina, P.;
Heitz, V.; Sauvage, J. P. Eur. J. Inorg. Chem. 1998, 1, 14. (s) Flamigni,
L.; Barigelletti, F.; Armaroli, N.; Ventura, B.; Collin, J. P.; Sauvage, J.
P.; Williams, J. A. G. Inorg. Chem. 1999, 38, 661. (t) Noblat, S.;
Dietrich-Buchecker, C. O.; Sauvage, J. P. Tetrahedron Lett. 1987, 28,
5829. (u) Solladie, N.; Chambron, J.-C.; Dietrich-Buchecker, C. O.;
Sauvage, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 906.
(4) (a) Iengo, E.; Zangrando, E.; Alessio, E. Acc. Chem. Res. 2006, 39,
841. (b) Iengo, E.; Zangrando, E.; Alessio, E. Eur. J. Inorg. Chem. 2003,
13, 2371.
ASSOCIATED CONTENT
■
(5) Rajeswara Rao, M.; Gayatri, G.; Kumar, A.; Sastry, G. N.;
Ravikanth, M. Chem.Eur. J. 2009, 15, 3488.
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
(6) Rajeswara Rao, M.; Ghosh, A.; Ravikanth, M. Inorg. Chim. Acta
2011, 372, 436.
(7) Pareek, Y.; Ravikanth, M. Chem.Eur. J. 2012, 18, 8835.
Rajeswara Rao, M.; Ghosh, A.; Ravikanth, M. Inorg. Chim. Acta 2011,
372, 436.
(8) Ainscough, E. W.; Brodie, A. M.; Depree, C. V. J. Chem. Soc.,
Dalton Trans. 1999, 4123.
(9) Ainscough, E. W.; Brodie, A. M.; Depree, C. V.; Moubaraki, B.;
Murray, K. S.; Otter, C. A. Dalton Trans. 2005, 3337.
(10) Ainscough, E. W.; Brodie, A. M.; Depree, C. V.; Jameson, G. B.;
Otter, C. A. Inorg. Chem. 2005, 44, 7325.
(11) Ainscough, E. W.; Brodie, A. M.; Depree, C. V.; Otter, C. A.
Polyhedron 2006, 25, 2341.
(12) Carriedo, G. A.; Elipe, P. G.; Alonso, F. J. G.; Catuxo, L. F.;
Diaz, M. R.; Granda, S. G. J. Organomet. Chem. 1995, 498, 207.
(13) Collman, J. P.; Barnes, C. E.; Brothers, P. J.; Collins, T. J.;
Ozawa, T.; Gallucci, J. C.; Ibers, J. A. J. Am. Chem. Soc. 1984, 106,
5151.
(14) Shetti, V. S.; Ravikanth, M. Inorg. Chem. 2011, 5, 1713.
(15) Holz, M.; Mao, X.-a.; Seiferling, D.; Sacco, A. J. Chem. Phys.
1996, 104, 669.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ravikanth@chem.iitb.ac.in.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.R. and T.K. thanks Department of Science & Technology,
Government of India for funding the project. We thank Central
Facility, MALDI-TOF/TOF, IIT−Bombay, for recording the
MALDI-TOF mass spectra.
REFERENCES
■
(1) (a) Nakamura, Y.; Aratani, N.; Osuka, A. Chem. Soc. Rev. 2007,
36, 831. (b) Shinokubo, H.; Osuka, A. Chem. Commun. 2009, 1011.
(c) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem.
Rev. 2001, 101, 2751. (d) Aratani, N.; Tsuda, A.; Osuka, A. Synlett
2001, 11, 1663. (e) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735.
(f) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35,
57.
(2) (a) Wagner, R. W.; Seth, J.; Yang, S.-I.; Kim, D.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Org. Chem. 1998, 63, 5042. (b) Ambroise,
J.; Li, A.; Yang, S.-I.; Diers, J. R.; Seth, J.; Wack, C. R.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1999, 121, 8927. (c) Song,
H.-E.; Kirmaier, C.; Schwartz, J. K.; Hindin, E.; Yu, L.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2006, 110, 19131.
(d) Sugiura, K.; Fujimoto, Y.; Sakata, Y. Chem. Commun. 2000, 1105.
(e) Cho, H. S.; Rhee, H.; Song, J. K.; Min, C.-K.; Takase, M.; Aratani,
N.; Cho, S.; Osuka, A.; Joo, T.; Kim, D. J. Am. Chem. Soc. 2003, 125,
5849. (f) Shoji, O.; Okada, S.; Satake, A.; Kobuke, Y. J. Am. Chem. Soc.
2005, 127, 2201. (g) Drain, C. M.; Varotto, A.; Radivojevic, I. Chem.
Rev. 2009, 109, 1630. (h) Beletskaya, I.; Tyurin, V. S.; Tsivadze, A. Y.;
Guilard, R.; Stern, C. Chem. Rev. 2009, 109, 1659−1713.
(3) (a) Shinmori, H.; Kajiwara, T.; Osuka, A. Tetrahedron Lett. 2001,
42, 3617−3620. (b) Hunter, C. A.; Sarson, L. D. Angew. Chem., Int. Ed.
Engl. 1994, 33, 2313. (c) Sessler, J. L.; Wang, B.; Harriman, A. J. Am.
Chem. Soc. 1995, 117, 704. (d) Karl, V.; Springs, S. L.; Sessler, J. L. J.
Am. Chem. Soc. 1995, 117, 8881. (e) Aoyama, Y.; Tanaka, Y.; Toi, H.;
Ogoshi, H. J. Am. Chem. Soc. 1988, 110, 4076. (f) Anderson, H. L.;
Sanders, J. K. M. J. Chem. Soc. Chem. Commun. 1993, 456. (g) Chi, X.;
Guerin, A. J.; Haycock, R. A.; Hunter, C. A.; Sarso, L. D. J. Chem. Soc.
Chem. Commun. 1995, 2567. (h) Anderson, H. L.; Sanders, J. K. M. J.
Chem. Soc. Chem. Commun. 1989, 1714. (i) Bonar-Law, R. P.; Mackay,
L. G.; Sanders, J. K. M. J. Chem. Soc. Chem. Commun. 1993, 456.
(j) Drain, C. M.; Lehn, J. M. J. Chem. Soc. Chem. Commun. 1994, 2313.
(k) Karia, N.; Imamura, T.; Sasaki, Y. Inorg. Chem. 1998, 37, 1658.
(l) Anderson, H. L.; Hunter, C. A.; Neah, M. N.; Sanders, J. K. M. J.
Am. Chem. Soc. 1990, 112, 5780. (m) Harriman, A.; Sauvage, J. P.
Chem. Soc. Rev. 1996, 41. (n) Collin, J. P.; Guillerez, S.; Sauvage, J. P.;
11059
dx.doi.org/10.1021/ic501569e | Inorg. Chem. 2014, 53, 11051−11059