N-Confused porphyrins: Complexation and 1H NMR studies

Article abstract of DOI:10.1039/c7nj01814b

The complexation of 2-aza-21-carba-tetraphenylporphyrin and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin with nickel and zinc acetates in organic solvents has been investigated by UV-Vis spectroscopy and ¹H NMR. It has been shown tha

Full text of DOI:10.1039/c7nj01814b

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N-Confused porphyrins: complexation and
1
H NMR studies
ab
Cite this:DOI: 10.1039/c7nj01814b
a
b
ac
Ilya A. Khodov, Olga V. Maltceva, Vladimir V. Klochkov, Oscar I. Koifman and
Nugzar Zh. Mamardashvili
a
*
The complexation of 2-aza-21-carba-tetraphenylporphyrin and 2-aza-2-methyl-5,10,15,20-tetraphenyl-
Received 25th May 2017,
Accepted 21st June 2017
2
1-carbaporphyrin with nickel and zinc acetates in organic solvents has been investigated by UV-Vis
1
spectroscopy and H NMR. It has been shown that the enhanced reactivity of these tetrapyrrolic
macrocycles is mainly determined by their ability to exist in different tautomeric forms.
DOI: 10.1039/c7nj01814b
rsc.li/njc
The acid–base properties of a series of meso-aryl substituted
N-confused porphyrins (NCPs) were examined in aqueous
Introduction
N-Confused porphyrins with one or more inverted pyrrole rings sodium dodecyl sulfate (SDS) micellar solutions by both spectro-
1
2,22–29
possess a wide range of properties which are uncharacteristic to photometric methods and theoretical methods.
1
–5
the ‘‘classical’’ porphyrins. Among them, these ligands tend
6–8
to form complexes with ‘‘carbon–metal’’ organometallic bonds.
The ability of the inverted ligands to stabilize metal ions in
9
8
10
11
unusual oxidation levels, e.g., Ni(III), Ag(III), Fe(II) or Rh(I),
12
allows them to be used in catalysis or design of heterometallic
5,8,13,14
molecular assemblies.
The inclusion of carbon atoms into
the inner conjugation loop of the molecule significantly changes
its reactivity. Besides the relatively easy formation of C–M bonds
6
,15
with metal ions in the complexes, atom C is easily methylated
1
6
and nitrated. Very characteristic UV-Vis and fluorescence
properties of the inverted porphyrin ligands allow their use as
molecular sensors for halogenide ion detection, and as photo-
The question of MꢀꢀꢀC–H interactions in N-confused porphyrin
1
7
30
sensitizers in the photodynamic therapy of tumors.
complexes is also actively discussed in the literature. The
The varied reactivity of the inverted porphyrins is concerned chemical structure of the N-confused Ni(II) metal complex was
1
with the possibility of their ligands to form different tautomeric studied based on experimental data from H NMR spectroscopy.
1
8
forms. The electronic structure of the tautomeric forms of These data, however, does not give unambiguous evidence of
N-confused porphyrin has been investigated in ref. 19 and the presence of either monomeric or dimeric forms of these
revisited in ref. 20 using density functional theory (DFT) compounds. Modern methods of two-dimensional NMR are
2
1
calculations. These studies found an energetic preference for successfully applied for the determination and confirmation
3
1–39
the NCP-3H and NCP-2H tautomers as the two most stable forms. of chemical structures of both macromolecules
and small
4
0–51
The detailed study of the solvent effect on the N-confused biologically active compounds.
In addition, approaches
porphyrin tautomeric forms is presented in ref. 22 with the using DOSY are progressing; they are widely used for the
evidence of the hydrogen bonding between the exterior N2 determination of structures of various supramolecular porphyrin
3
1,43,52–57
nitrogen and the solvent.
complexes.
The goal of this work is to perform a comprehensive analysis
of the inverted 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin
a
G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences,
(
2-NHCTTP) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carb-
aporphyrin (2-NCH CTPP) complexes formed using Ni(OAc)
and Zn(OAc) in organic solvents of different nature using
UV-Vis spectroscopy and H NMR.
Ivanovo, 153045, Russia. E-mail: ngm@isc-ras.ru
b
3
2
Kazan Federal University, Kazan, 420008, Russia
c
Research Institute of Macroheterocycles, Ivanovo State University of Chemistry and
2
1
Technology, Sheremetevskiy Av. 7, Ivanovo 153000, Russian Federation
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Experimental
Bouguer–Lambert–Beer law: lg e = lg(A/(lꢀc)), where A is the
optical density, l is the length of the light-absorbing layer
Tetraphenylporphyrin H
2
TPP (97%, Aldrich), solvents (99%,
COO) (both from Acros, 99%)
were used. Compounds I–III were prepared using the procedures
(
(
cm), and c is the molar concentration of the porphyrin solution
mol L ). All measurements were performed in standard
Merck), Ni(CH
3
COO)
2
and Zn(CH
3
2
ꢂ1
1
0 ꢁ 10 mm quartz cells.
1
5,18,24,58
adopted from the earlier studies.
Electron absorption
All NMR experiments were performed on a Bruker Avance III
00 NMR spectrometer equipped with a 5 mm probe using
1
(
Table 1) and H NMR spectra of the prepared products fully
5
coincided with the reference data.
standard Bruker TOPSPIN Software. Temperature control was
performed using a Bruker variable temperature unit (BVT-2000)
in combination with a Bruker cooling unit (BCU-05) to provide
chilled air. Experiments were performed at 303 K without
Reactions of complex formation were studied by monitoring
the UV/VIS spectra of the solutions using a Cary 300 (Varian)
spectrophotometer. To this end, a solution of the studied
ꢂ5
porphyrin (c = 2.5 ꢁ 10 M) in DMF was put into a temperature-
sample spinning. Solvents used for NMR spectroscopy were
controlled cell, and the absorbance at the wavelength corresponding
to the absorbance maximum of the formed nickel(II) or zinc(II)
porphyrins was monitored.
ꢂ3
one of the following: CDCl and DMSO-d (c
E 5 ꢁ 10 M)
3
6
H2P
with TMS as the internal standard.
1
The H NMR spectra were recorded (500 MHz) using a WET
Temperature was maintained with an accuracy of ꢃ0.1 1C
59,60
(Water suppression Enhanced through T
1
effects)
pulse
throughout the experiment.
sequence using shaped selective pulses and pulsed magnetic
field gradients to suppress two solvent signals; the spectral
width was 10.59 ppm; and 512 scans were acquired.
The current concentration of the substrate was calculated
using the equation:
Two-dimensional Diffusion Ordered Spectroscopy (2D DOSY)
spectra were recorded with a PGSTE pulse sequence using
bipolar gradient pulses and the insertion of a supplementary
c = c
, and A are the solution absorbance at time zero,
at time t, and after the reaction was complete, respectively; and
0
(A
N
ꢂ A
t
)/(A
N
ꢂ A
0
),
where A
0
, A
t
N
61
delay (LED). The PGSTE sequence was used with a diffusion
c
0
is the initial concentration of the substrate.
delay of 0.1 s, a total diffusion-encoding pulse hwidth/durationi
The effective rate constant keff (related to the true rate
constant) was determined using the pseudo first-order kinetic
equation:
of 5 ms. For each of 32 gradient amplitudes, 64 transients of
16384 complex data points were acquired.
0
keff = (1/t)ln(c /c).
Results and discussion
The activation energy (E ) of the reactions was calculated from
a
the temperature dependence on the rate constant of complex
The transformation from one tautomeric form (a) to another (b)
is accompanied by proton transfer, reorganization of the molecule
chromophoric system and coordination core. In a solvent media
with weak Lewis basicity and acidity the inverted porphyrin exists
in form a, carrying on the periphery of the molecule the amine-
type pyrrolic nitrogen (–NH–) and imine-type pyrrolic nitrogen
formation using the Arrhenius equation. The activation entropy
a
(
DS ) was calculated using the fundamental equation of the
transition state theory:
ꢁ
#
kkBT
ꢀDH#
exp
ꢂ TDS
kv ¼
h
R
(–NQ) composition. Dissolution in an electronodonor solvent (e.g.,
a
where DH is the activation enthalpy, k is the transmission
dimethylformamide (DMF)) results in a migration of the one of
NH-group protons to the peripheral N-atom and formation of
form b (Fig. 1).
B
coefficient (taken equal to unity), k is the Boltzmann constant,
and h is the Planck constant.
The molar extinction coefficient (e) for bands in the UV-Vis
spectra of porphyrin ligands was calculated by applying the
Form a is more aromatic and, consequently, more thermo-
dynamically stable. For example, the ring current effect is
stronger in this tautomer than in form b, as is indicated by
1
the values of the chemical shifts of signals in the H NMR
Table 1 Characteristics of the UV-Vis spectra of compounds I–III in
organic solvents
6
2–64
spectra.
For form a: dNH = ꢂ2.41 and d = ꢂ4.99 ppm (in
CH
CDCl ). For form b: dNH = +2.27 and dCH = +0.76 V ppm (in DMF-d ).
3
7
l
Soret
,
l
IV
,
l
III
,
II
l ,
I
l ,
Also, while form a in solution is stable and can be stored in the
dark for several months, form b in DMF solution undergoes
degradation in several days.
The inverted porphyrin analogues in their UV-Vis spectra
belong to the group of tetrapyrrole macrocycles, possessing the
unusual properties not only in structure but also in spectral
Solvents nm (lg e) nm (lg e) nm (lg e) nm (lg e) nm (lg e)
I
CHCl
DMF
Py
3
3
3
418 (5.67) 514 (4.29) 549 (3.87) 590 (3.71) 646 (3.55)
416 (5.62) 513 (4.28) 548 (3.91) 590 (3.73) 646 (3.68)
418 (5.58) 514 (4.33) 548 (3.96) 591 (3.81) 647 (3.68)
II CHCl
DMF
437 (5.38) 505 (3.95) 541 (4.32) 584 (4.44) 729 (4.41)
442 (5.35) 545 (3.96) 594 (4.08) 644 (4.24) 698 (4.34)
441 (5.31) 547 (3.95) 590 (4.14) 647 (4.08) 703 (4.41) properties towards tetraphenylporphyrin (H TPP) and other
Py
2
6
5
classical porphyrins.
The electronic absorption spectra of the tautomers differ in
III CHCl
DMF
441 (5.35)
441 (5.38)
447 (5.18)
—
—
—
613 (4.08) 660 (4.34) 715 (4.44)
605 (4.11) 652 (4.34) 706 (4.45)
603 (4.27) 652 (4.31) 710 (4.32) position and intensity of the Soret band and the absorption
Py
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m = 2.37 D (but with high DN = 33.1 and AN = 14.2), a mixture of
tautomers a and b is observed. This means that the tauto-
merization strongly depends on the polarity of the solvent as a
medium. Apparently, the medium should significantly affect
the rate of complexation reaction of the inverted porphyrin.
Unfortunately, quantitative data on the reactivity of the
inverted porphyrin in the complexation reactions cannot be
found in the literature. In order to fill this gap in the chemistry
of inverted porphyrins, we investigated the complexation of the
inverted porphyrins in solvents with different donor–acceptor
nature with nickel(II) and zinc(II) cations. The reaction of
complexation of compounds I–III with nickel(II) and zinc(II)
acetates in DMF is investigated (Fig. 3 and 4). Table 2 presents
the kinetic data of the complexation of I–III with Ni(OAc)
Zn(OAc) in DMF.
The coordination properties of II in the reaction of complexation
with Ni(OAc) in solvents of different nature (N-dimethylformamide
DMF), pyridine (Py) and in the chloroform–methanol (1 : 1)
2
and
2
2
(
Fig. 1 The chemical structures of porphyrins I–III.
medium) have been investigated.
As is depicted in Table 3, the dependence of the reaction rate
of the tautomer b from the donor ability and polarity of the
medium is explained by the fact that high DN of the solvent
characterizes the bond strength of the donor center with the
outer NH-proton. And a high m promotes tautomerization due
to better stabilization of the ionic particles by the solvent.
However, one should note the influence of the steric effect on
the reaction rate of tautomer b complexation.
To unravel the necessary information, we applied an
approach in NMR spectroscopy which employs double-band
WET solvent signal suppression using a series of selective variable-
angle pulses with field dephasing by field gradient pulses.
Fig. 2 UV-Vis of II (c = 2.5 ꢁ 10ꢂ5 M) in organic solvents: a – CHCl
3
;
b – DMF; and c – Py.
bands in the visible region. In DMF the Soret band is at 442 nm
and has a shoulder at 450 nm. The UV-Vis spectra are red
shifted by 4 nm, as compared with the spectra in CH
2 2
Cl
(Table 1 and Fig. 2). Absorption band I at 698 nm is blue
shifted by 27 nm, and the intensity of band II of form b, having
the absorption maximum at 644 nm is much higher than that
of form a. When the peripheral nitrogen atom in the molecule
is methylated a fixed porphyrin(III) molecule gains the marked
stepped structure of the bands in the visible region in UV-Vis
spectra with the most intense I-absorption band, shifting to the
shorter wavelength region, while the Soret band is red shifted.
UV-Vis spectra suggest the presence of either only one of the
forms or their mixture in solution.
Thus, according to ref. 54 and 66, the ‘‘pure’’ form a exists in
non-polar solvent media with weak electron-donor properties,
such as chloroform. Usually, the dielectric constant (e) of such
solvents is less than 5, and donor numbers (DN) do not exceed
Fig. 3 The changing of the UV-Vis spectrum during the complexation of
in DMF (Csalt = 2.5 ꢁ 10ꢂ3 M, cH2P = 2.5 ꢁ 10ꢂ5 M, T = 298 K).
II with Ni(OAc)
2
15. The b tautomer is formed in polar electron-donor solvents
such as DMF (e = 36.7; m = 3.8 D; DN = 26.6; AN = 16). At the
same time, the spectrum of the individual form b is observed
only in the case when the dielectric constant (e), dipole moment
(m), and donor number (DN) of the solvent are sufficiently high,
and the acceptor number (AN) does not exceed the donor
number (e Z 30; m > 3.5 D; DN > 25; DN Z AN). Thus, for
0
Fig. 4 The kinetic curve and the dependence of log(C /C) on the inter-
2
example, in pyridine (Py), which is a solvent with e = 12.3 and action time of II with Ni(OAc) in DMF at T = 298 K.
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Table 2 Kinetic data of reaction complex formation between I–III, and
Ni(OAc)
2
and Zn(OAc)
2
in DMF (csalt = 2.5 ꢁ 10ꢂ3 M, cH2P = 2.5 ꢁ 10ꢂ5 M)
2
98 b
a
k
eff
ꢁ
k
v
,
E
a
,
DS ,
l,
nm
3
a
ꢂ1
ꢂ1 ꢂ1
ꢂ1
ꢂ1
T, K 10 , s
L mol
s
kJ mol
J (mol K)
Ni(OAc)
2
I
358 Very slow
II 318 0.467
0.641
64.2 ꢃ 5.8 ꢂ42 ꢃ 16 696
69.0 ꢃ 6.3 ꢂ30 ꢃ 19 706
3
3
28 0.904
38 1.954
III 338 0.701
0.375
0.226
3
3
48 1.430
58 2.760
Zn(OAc)
2
I
318 0.079
89.0 ꢃ 3.0 32 ꢃ 10 645
1
Fig. 5 The informative fragment of the H NMR WET spectrum of the
2-NCH CTPP)Ni in CDCl
3
3
28 0.213
38 0.576
(
3
3
.
II 298 Instantly
III 298 Instantly
a
b
Error does not exceed 5%. Obtained by extrapolation.
The experimental parameters were optimized in order to obtain
high-resolution spectra of ((2-NHCTTP)Ni) and ((2-NCH CTPP)Ni)
3
complexes using the WET approach. This approach and
optimization of spectral parameters allowed us to obtain well
1
resolved signals in H NMR spectra of the studied macrocyclic
nickel complexes (2-NCH
chloroform (Fig. 5 and 6). Assignments of NMR spectra of these
types of compounds are desirable from the viewpoint of structural Fig. 6 The informative fragment of the H NMR WET spectrum of
3
CTPP)Ni and (2-NHCTPP)Ni in
1
(
3
2-NHCTPP)Ni in CDCl .
elucidation. Two-dimensional NMR approaches, 2D DOSY and 2D
COSY, have been applied to achieve this goal. The homonuclear
two-dimensional NMR spectroscopy (2D COSY) revealed the
correlation of proton signals from the 2-NH and 3-CH groups
for the (2-NHCTPP)Ni complex in chloroform (Fig. 7 and 8).
Furthermore, solvent signals were unambiguously separated
from signals of the nickel–porphyrin complex based on the 2D
DOSY experiment. Comparing our results obtained from 2D
COSY and 2D DOSY spectra of (2-NHCTPP)Ni with those
5
7
Fig. 7 2D COSY (left) and 2D DOSY (right) NMR spectra of (2-NHCTPP)Ni
in CDCl
presented in the literature, we made sure that the obtained
.
1
3
H NMR spectra were correctly assigned to the chemical structure
of the considered compound. However, some discrepancies were
found while comparing experimental data on H NMR of different solvents.
1
15,26
All NMR spectra have been recorded in
(2-NCH
3
CTPP)Ni with the literature data, which were due to CDCl
3
because it is the most common solvent. The proton
Table 3 Kinetic data of the reaction complex formation between II, and Ni(OAc)
c
2
and Zn(OAc)
2
in organic solvents (csalt = 2.5 ꢁ 10ꢂ3 M,
H2P = 2.5 ꢁ 10ꢂ5 M)
3
a
ꢂ1
298 b
ꢂ1 ꢂ1
ꢂ1
a
ꢂ1
T, K
k
eff ꢁ10 , s
k
v
, L mol
s
a
E , kJ mol
DS , J (mol K)
l, nm
Ni(OAc)
2
DMF
Py
318
0.467
0.904
1.954
32.051
56.043
98.060
17.500
30.992
59.705
0.641
64.2 ꢃ 5.8
22 ꢃ 1
ꢂ42 ꢃ 16
ꢂ249 ꢃ 3
ꢂ123 ꢃ 27
696
3
3
28
38
298
12.820
7.002
692
745
3
3
08
18
CHCl
3
–CH
3
3
OH (1 : 1)
298
49 ꢃ 8
3
3
08
18
Zn(OAc)
2
DMF
Py
CHCl
298
Instantly
3
–CH
OH (1 : 1)
a
b
Error does not exceed 5%. Obtained by extrapolation.
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1
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H. Nanami and T. Ogawa, Inorg. Chem., 2000, 39, 5424–5425.
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1
1
1
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CTPP)Ni
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1
5,26
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2
Cl
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3
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involves not only modern modification of 1D NMR experiments
V. N. Nemykin, Inorg. Chem., 2011, 50, 6902–6909.
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2015, 80, 11031–11038.
and H NMR a comprehensive analysis of the complexation 23 R. Sakashita, M. Ishida and H. Furuta, J. Phys. Chem. A,
of inverted 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin and
2015, 119, 1013–1022.
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116, 767–768.
has been performed. It has been shown that the enhanced 25 A. Ghosh, T. Wondimagegn and H. J. Nilsen, J. Phys. Chem.
Phys. Chem. Chem. Phys., 2015, 17, 5290–5297.
(
1
2
2
2
reactivity of these tetrapyrrolic macrocycles is mainly determined
by their ability to exist in different tautomeric forms.
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Acknowledgements
This work was supported by the Russian Scientific Foundation,
project No 14-23-00204-P.
0 E. Pacholska-Dudziak and L. Latos-Gra ˙z y n´ ski, Eur. J. Inorg.
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