Theoretical and experimental study of triphenylphosphonium Schiff base of 5-hydroxy-3-methyl-1-phenyl-4-formylpyrazole

Article abstract of DOI:10.1080/10426507.2018.1424159

The Schiff base (HLBr), containing a chelating unit and triphenylphosphonium moiety has been synthesized in the reaction of 4-aminobenzyl(triphenyl)phosphonium bromide with 5-hydroxy-3-methyl-1-phenyl-4-formylpyrazole. The composition and structure of HLBr have been determined by elemental analysis, IR, 1D and 2D NMR, electronic spectroscopy and mass spectrometry. Density functional theory (DFT) calculations (6-311G(d,p) level of theory) have been carried out to investigate tautomeric forms of HL⁺ and the reaction mechanism of its formation and spectral properties. The most stable form in the solid state and in DMSO solution is pyrazolone (keto-amine) tautomeric form.

Full text of DOI:10.1080/10426507.2018.1424159

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Theoretical and experimental study of
triphenylphosphonium Schiff base of 5-hydroxy-3-
methyl-1-phenyl-4-formylpyrazole
Sergey A. Borodkin, Leonid D. Popov, Arshak A. Tsaturyan, Milica R.
Milenković, Igor N. Shcherbakov & Vladimir V. Lukov
To cite this article: Sergey A. Borodkin, Leonid D. Popov, Arshak A. Tsaturyan, Milica R.
Milenković, Igor N. Shcherbakov & Vladimir V. Lukov (2018): Theoretical and experimental study of
triphenylphosphonium Schiff base of 5-hydroxy-3-methyl-1-phenyl-4-formylpyrazole, Phosphorus,
Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2018.1424159
To link to this article: https://doi.org/10.1080/10426507.2018.1424159
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Theoretical and experimental study of triphenylphosphonium Schiff base of 5-hydroxy-3-
methyl-1-phenyl-4-formylpyrazole
Sergey A. Borodkina, Leonid D. Popova, Arshak A. Tsaturyana, Milica R. Milenkovićb, Igor N.
Shcherbakova, Vladimir V. Lukova
a
Chemistry Department Southern Federal University, 344090, Rostov-on-Don, Russia, Zorge
St., 7
b Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, Belgrade, Serbia
Email: caturyan@sfedu.ru
Abstract
The Schiff base (HLBr), containing a chelating unit and triphenylphosphonium moiety has been
synthesized in the reaction of 4-aminobenzyl(triphenyl)phosphonium bromide with 5-hydroxy-3-
methyl-1-phenyl-4-formylpyrazole. The composition and structure of HLBr have been
determined by elemental analysis, IR, 1D and 2D NMR, electronic spectroscopy and mass
spectrometry. Density functional theory (DFT) calculations (6-311G(d,p) level of theory) have
been carried out to investigate tautomeric forms of HL+ and the reaction mechanism of its
formation and spectral properties. The most stable form in the solid state and in DMSO solution
is pyrazolone (keto-amine) tautomeric form.
Keywords: Schiff base, tautomerism, phosphonium salts, quantum-chemical calculations.
Graphical Abstract
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1. Introduction
Quaternary phosphonium salts have been of interest due to their various application as reagents,
green solvents and new materials [1–3]. Also, cationic lipophilic salts containing a quaternary
phosphonium group can be used as carriers for mitochondria-targeted drugs with a high
preference towards tumor cells and cardiac muscle cells [4–6]. Phosphonium salts are easily
accessible and frequently used intermediates in organic synthesis, long known phase-transfer
catalysts and potential Lewis acid organocatalysts [7]. Pyrazolone derivatives are intensively
investigated, because of their interesting structural properties and applications in diverse areas
[8–13]. Among pyrazol-5-one derivatives, 4-acyl pyrazolones, have attracted considerable
attention due to their biological activities, photochromic properties and applications in catalysis
and analytical chemistry. Also, this class of compounds and their corresponding Schiff bases
possess interesting coordination properties [14–21]. The combination of positively charged
phosphonium group with chelatofore pyrazolone moiety can result in a promising ligand system
leading to coordination compounds with enhanced solubility, tunable redox properties and
biological activity. This work is devoted to the study of a cationic chelator (HLBr) synthesized
in the condensation reaction between 4-aminobenzyl(triphenyl)phosphonium bromide and 5-
hydroxy-3-methyl-1-phenyl-4-formylpyrazole.
2. Results and discussion
2.1. Synthesis of HLBr
The phosphonium azomethine (HLBr) was prepared by refluxing of the ethanolic solution of
equivalent amounts of 4-aminobenzyl(triphenyl)phosphonium bromide and 5-hydroxy-3-methyl-
1-phenyl-4-formylpyrazole (Scheme 1).
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Scheme 1. The synthesis of HLBr
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2.2. Mass-spectrum of HLBr
Molecular ion peak (m/z = 631) was not registered in the mass spectrum of HLBr. The two
intensive peaks with m/z=262 (100) corresponding to Ph3P+ ion and m/z=183 (99) and three
peaks with lower intensity m/z=108 (60), m/z = 77 (40) and m/z=289 (34) have been observed in
the mass spectrum of HLBr. The data allows us to suggest mechanism for HLBr fragmentation
under
the
electron
impact
ionization
which
is
shown
in
Scheme
2.
5
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Scheme 2. The proposed mechanism of HLBr fragmentation under electron impact ionization.
6
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2.3 NMR, IR and UV/Vis spectroscopy study
Chemical shifts of individual atoms were assigned (see Synthesis section for results and
13
15
31
Scheme 1 for atom numbering) according to 1D (1H, C, N and P) and 2D COSY (1H-1H),
1
NOESY(1H-1H), HSQC(1H-13C), and HMBC (1H-13C, H-15N) NMR experiments (experimental
spectra are shown in Figures S 2-S 9 of the Supplemental Materials). The complete assignment
1
of the siganls in the H NMR spectra is the following (δ, ppm): the signal of pyrazolone moiety
СН3 group is registered as singlet at 2.25 ppm, the signal of the СН2 group protons form a
2
doublet due to spin-coupling with phosphorus nucleus at 5.20 ppm with JPH = 15.4 Hz. Signals
of phenyl groups protons in ortho-, meta- and para-positions are found as three complex
multiplets in the ranges 7.66-7.71 (6Н intensity), 7.73-7.79 (6Н) and 7.88-7.93 (3Н),
correspondingly. Spin-coupled protons of N-benzyl moiety (3JHH = 8.7 Hz) appear in spectra as
two doublet signals with two-proton intensity at 7.02 (Н11) and 7.45 ppm (Н10), former is
additionally splitted due to interaction with phosphorous nucleus (4JPH = 2.54 Hz). Ortho-, meta-
and para-proton signals of N-phenyl substituent in pyrazolone moiety are found at 7.95 (d, 2Н,
3
3
3
3JHH = 8.6 Hz), 7.37 (dd, 2Н, JHH = 7.4, JHH = 8.6 Hz) and 7.11 ppm (t, 1Н, JHH = 7.4 Hz),
correspondingly. The signal of H7 proton of 1H intensity is registered as broadened singlet at
8.51 ppm, even more broadened is the singlet signal of D2O exchangeable NH proton at 11.22
ppm. Location of the proton on N8 nitrogen atom is supported by the presence of cross-peak in
1
COSY H spectrum at (8.51, 11.22) ppm due to spin coupling H7 – H8. Absence of distinct
splitting of the NH and CH7 signals in 1D 1H spectrum can be attributed to fast exchange of the
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NH proton with residual water in DMSO solvent, which is supported by presence of strong
NOESY 1H cross-peak at (3.32, 11.22) ppm.
Signals in 13C NMR spectrum are readily assigned, especially in arylphosphonium moiety due to
characteristic spin coupling with 31P nucleus resulting in doublet-shape signals. The greatest
1
coupling JPC = 85.5 Hz is observed for signals of the closest phenyl substituents carbon atoms
1
(117.7 ppm) and JPC = 46.5 Hz for methylene carbon atom (27.8 ppm). Carbon atoms of P-
phenyl substituents in ortho-, meta- and para-positions are observed at 134.0 (2JPC = 9.9 Hz),
130.1 (3JPC = 12.4 Hz) and 135.1 ppm (4JPC = 3.0 Hz), correspondingly. Doublet signals at 124.3
(2JPC = 8.8 Hz), 131.9 (3JPC = 5.6 Hz), 117.9 (4JPC = 3.3 Hz) and 138.7 (5JPC = 4.0 Hz) are
assigned to N-benzyl moiety carbon atoms C12, C11, C10 and C9, respectively. Singlet signals are
1
assigned with the help of 2D HSQC and HMBC H-13C NMR techniques. N-Phenyl substituent
carbon atom signals are assigned as following: 138.9 (i-NPh), 117.5 (o-NPh), 128.7 (m-NPh) and
123.8 ppm (p-NPh). Pyrazolone ring carbons C5 and C3 are substantially deshielded (164.7 and
149.0 ppm, correspondingly) relative to C4 one (102.2 ppm). Formyl and, methyl groups carbon
signals are found at 145.2 and 12.6 ppm, respectively.
In 15N NMR spectrum three signals are observed at -92.4 (N2), -190.9 (N1), -252.5 ppm (N8). In
1H-15N HMBC spectra the following cross-peaks were found: N2-methyl group protons, N1 -
ortho- and N1 - meta-protons of N-phenyl substituent and weak signal corresponding to N1 -
methyl group protons interaction. For nitrogen atom N8 cross-peak was registered only with C10-
H protons.
Thus, based on NMR data we can conclude that HL+ exists in DMSO solution in keto-
amine tautomeric form.
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Assignment of the absorption bands in IR spectrum was made by comparison of the
simulated (at the B3LYP/6-311G(d,p) level of theory) and experimental spectrum (see Table 1).
Table 1. The calculated and experimental IR spectroscopy data for HLBr
Type of vibration
Experiment, cm–1
Calculation
Intensity
ν, cm–1
3181
ν(NH)
3300-3450 (br)
2975-3105 (w)
2885 (w)
212.2
0.5-28.1
15.3
ν(CH) arom
ν(CH2)
3028-3132
2971
ν(CH3)
2857 (w)
2954
12.4
ν(C(O)-C=C)as
ν(C(O)-C=C)s
ν(C=C) aminobenzyl
δ(NH)
1650 (s)
1660
1036
235
1623 (m)
1639
1596 (m)
1597
444.5
320
1488 (m)
1477
ν(C=C) P+-Ph
1440 (m)
1422-1424
1275
76
ν(C-N)
ν(P+-C)
1291 (s)
1015
11.8-58.3
1113 (m, broadened)
1075-1079
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The broad band at 3300 cm–1 in the IR spectrum of azomethine HLBr can be attributed to
stretching vibrations of intramolecular H-bonded NH group. Weak absorption bands in the
region 3028-3132 cm–1 are due to numerous C-H stretching vibrations of aromatic rings, while
aliphatic C-H stretching vibrations of methylene and methyl groups are found at 2885 and 2857
cm–1, correspondingly. Delocalized antisymmetric and symmetric stretching vibrations with
major contribution of conjugated carbonyl and C=C exocyclic double bonds of pyrazolone
moiety are found at 1650 and 1623 cm–1, respectively. Medium strength absorption at 1596 cm–1
is due to the C=C para-substituted aromatic ring stretching vibration of the aminobenzyl moiety.
Deformation vibration of NH group appears as a medium strength band at 1488 cm–1. Medium
intensity bands at 1113 cm–1 and 1440 cm–1 are characteristic for triphenylphosphonium salts and
correspond to Р+–С stretching and P-phenyl rings skeleton vibrations, correspondingly. IR data
also supports the proposal that in solid sample pyrazolone tautomeric form is present, due to
satisfactory correspondence between experimental and calculated vibrational spectrum of the
isomers.
2.4. Quantum-chemical study of HL+ tautomeric forms
It is well-known that 5-hydroxy-4-formyl-pyrazol azomethine derivatives can exist in
different prototropic tautomeric forms [22]. Quantum-chemical calculations have been carried
out in order to estimate the relative stability of the tautomers and the most stable tautomeric
forms A - D according to calculations of HL+ are shown in Scheme 3.
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Scheme 3. The most stable tautomeric forms of HL+.
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The calculated total energies and relative stabilities (ΔE, kcal/mol) for the modeled
tautomeric forms of HL+ are shown in Table 2.
Table 2. The quantum-chemical calculation data for the most stable tautomeric forms A –
D of HL+ (total (E, a.u.) and relative (∆E, kcal/mol) energies, B3LYP/6-311G(d,p))
gas phase
DMSO
CHCl3
Structure
∆E,
∆E,
∆E,
kcal/mol
8.77
E, a.u.
E, a.u.
E, a.u.
kcal/mol
5.20
kcal/mol
10.12
-
-
-
A
B
C
D
1971.546742
1971.607891
1971.594127
-
0.00
11.82
18.91
-
0.00
13.98
21.35
-
0.00
13.50
20.50
1971.555029
1971.624018
1971.608098
-
-
-
1971.536195
-
1971.524898
1971.601747
-
1971.589989
1971.586577
-
1971.575427
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The results of quantum-chemical calculations showed that the most stable form of HL+ in
gaseous phase is the keto-amine (pyrazolone) form B stabilized by intramolecular H-bonding
between NH group and oxygen atom of pyrazolone moiety. The enol-imine (5-hydroxypyrazole)
form A, with OH···N intramolecular H-bonding is 5.2 kcal/mol less stable relative to form B.
The keto-imine tautomers (С and D) are 11.82–18.91 kcal/mol destabilized relative to the B
isomer. The data of quantum-chemical modeling of HL+ tautomers in solvents within PCM
approximation show the growth of the relative stability of the B tautomer both in DMSO and
CHCl3 media, especially noticeable for A isomer which is nearly twice destabilized relative to B
in the solution compared to gaseous phase. These results are in line with experimental NMR and
IR data, evidencing both in DMSO solution and in solid phase formation of the pyrazolone
tautomeric form.
2.5. Intramolecular proton transfer HL+ in gas phase
Since the compound HL+ is obtained in the condensation reaction of 4-
aminobenzyl(triphenyl)phosphonium
bromide
and
5-hydroxy-3-methyl-1-phenyl-4-
formylpyrazole, the first stage of the most probable mechanism of this reaction includes the
formation of A tautomer (kinetic product) followed by its transformation into the
thermodynamically preferred form B. The latter is taking place through 1-5 sigmatropic shift of
proton of hydroxypyrazole fragment. This rearrangement is carried out through the transition
state shown in Figure 1.
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Figure 1. Energy diagram of tautomeric proton shift leading from А to В isomer
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The calculated activation energy of this process in gas phase is only 2.68 kcal/mol so kinetic
enol-imine product A will readily convert to thermodynamically most stable keto-amine B
tautomer.
2.6. Electronic spectra
In UV/vis spectra of HLBr ethanolic solution (Figure 2, solid curve) the absorption bands
at λmax = 346 nm (lg εmax = 4.37), 198 nm (lg εmax = 4.38) and shoulder at 226 (lg εmax = 3.91) –
256 (lg εmax = 3.62) nm are registered. Changes in рН of solution do not exert essential influence
on HLBr electronic spectrum, the longwave band shifts to λmax = 350 nm (lg εmax = 4.32) after
the addition of triethylamine and to λmax = 350 nm (lg εmax = 4.13) when HCl was added. The
performed quantum chemical modelling of UV/vis spectrum of tautomeric form B within
TDDFT approximation (6-311G(d,p)) revealed that the longwave absorption band arises mainly
from the electron transition from HOMO-1 to LUMO (Table 3).
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Figure 2. Experimental (ethanol solution, solid line) and calculated (vertical lines) UV/vis
spectrum of HLBr
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In Figure 2 the calculated transition wavelengths are shown as black vertical lines. The
calculated excitation wavelength (350 nm) is in excellent agreement with experimental result.
Also close to experiment is the calculated wavelength of the transition appearing as a shoulder at
256 nm.
Table 3. The vertical singlet excitations characteristics for HL+: number of the excited states,
transition composition, excitation energy (eV), wavelength (nm), oscillator strength (TDDFT,
6-311G(d,p))
Excited State Transition (contrib. %)
∆E, eV
3.55
λ, nm f (oscillator
strength)
2
HOMO-1 → LUMO (77%)
HOMO → LUMO (8%)
350
256
1.2335
17
HOMO-4 → LUMO (11%)
HOMO → LUMO+8 (49%)
4.85
0.2607
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HOMO-1, HOMO and LUMO are of π-type so the longwave transitions are of π-π*
character (for the shapes of the frontier molecular orbitals see Figure S1 in Supplemental
Materials). Large value of oscillator strength of the transition at 350 nm is due to localization of
HOMO-1 and LUMO on different moieties of π-conjugated bond system.
A theoretical study of vertical excitation energies for deprotonated form of the
compound, L, revealed an insignificant long-wave shift of the adsorption band coinciding well
with the experimental observations.
3. Experimental
3.1. Reagents
Triphenylphosphine and 1-phenyl-3-methyl-2-pyrazolin-5-one were purchased from Aldrich. All
solvents were purified with standard methods before usage.
3.2. Instruments
Mass spectra of HLBr were measured with Finnigan MAT INCOS 50 instrument (electron
13
31
1
1
1
impact, 70 eV). 1Н, C, 15N, P NMR, the 2D spectra of H-1H COSY, H-1H NOESY, H-13C
1
1
1
HSQC, H-13C, H-15N, H-31P HMBC spectra were measured on a Bruker Avance-600
spectrometer at 600 (1H), 151 (13C), 60 (15N) or 243 MHz (31P), using DMSO-d6 as a solvent. 1H
and 13C chemical shift were referred to TMS internal standard. The 15N spectra were referenced
to nitromethane, the 31P spectra to trimethylphosphate. IR spectra were measured on a Vertex 70
(Bruker) spectrometer using a diamond attachment of total internal reflection Platinum ATP.
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UV/vis spectra were recorded on a Varian Cary 5000 UV-Vis-NIR spectrophotometer.
Microanalysis on C, H and N was performed on a Perkin-Elmer 240C Analyzer.
3.3. Synthesis
4-Aminobenzyl(triphenyl)phosphonium bromide has been obtained by a method previously
described in the literature [23]. 5-hydroxy-3-methyl-1-phenyl-4-formylpyrazole has been
synthesized by the Vilsmeier–Haack reaction between 1-phenyl-3-methyl-2-pyrazolin-5-one,
DMF and POCl3 [24].
(4-(((3-methyl-5-oxo-1-phenyl-1,5-dihydro-4H-pyrazol-4-ylidene)methyl)amino)benzyl)-
triphenylphosphonium bromide (HLBr)
The HLBr was prepared by refluxing an ethanolic solution (10 ml) of 4-
aminobenzyl(triphenyl)phosphonium bromide (0.5 g, 12 mmol) and 5-hydroxy-3-methyl-1-
phenyl-4-formylpyrazole (0.25 g, 12 mmol) for 4 h. The solution was evaporated to a fifth of the
initial volume followed by the addition of tenfold excess of ethyl acetate. The precipitate was
separated and recrystallized from a mixture of chloroform/ethyl acetate (1 : 3). Melting point 255
оС. Found C, 68.70; H, 5.10; N, 6.50 %. С36Н31BrN3OP Calculated C, 68.36; H, 4.94; N, 6.64 %.
Yield 85 %. IR spectrum (cm-1): 3300-3450 (br), 2975-3105 (w), 2885 (w), 2857 (w), 1650 (s),
1623 (m), 1596 (m), 1488 (m), 1440 (m), 1291 (s), 1113 (m, broadened). Atom numbering used
in NMR description is shown on Scheme 1. For simplicity, the protons of phenyl groups are not
numbered and designated as position and fragment name, for example, o-NPh means ortho-
protons of N-phenyl group in pyrazolone moiety, p-PPh – para protons of the phenyl
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1
substituents in triphenylphosphonium group. H NMR: (600 MHz, DMSO-d6, δ/ppm) 2.25 (s,
3Н, Н6), 5.20 (d, 2Н, Н13, 2JPH = 15.4 Hz), 7.02 (dd, 2Н, Н11, 3JHH = 8.7, 4JPH = 2.54 Hz), 7.11 (t,
1Н, p-NPh, 3J = 7.4 Hz), 7.37 (dd, 2Н, m-NPh, 3JHH = 7.4, 3JHH = 8.6 Hz), 7.45 (d, 2Н, Н10, 3JHH
= 8.7 Hz), 7.71-7.66 (m, 6Н, o-PPh), 7.79-7.73 (m, 6Н, m-PPh), 7.93-7.88 (m, 3Н, p-PPh), 7.95
(d, 2Н, o-NPh, 3JHH = 8.6 Hz), 8.51 (bs, 1H, H7), 11.22 (bs, NН).
13C NMR (151 MHz, DMSO-d6, δ/ppm) 164.7 (s, C5), 149.0 (s, C3), 145.2 (s, C7), 138.9 (s, i-
4
2
NPh), 138.7 (d, C9, 5JPC = 4.0 Hz), 135.1 (d, p-PPh, JPC = 3.0 Hz), 134.0 (d, o-PPh, JPC = 9.9
Hz), 131.9 (d, C11, 3JPC = 5.6 Hz), 130.1 (d, m-PPh, 3JPC = 12.4 Hz), 128.7 (s, m-NPh), 124.3 (d,
C12, 2JPC = 8.8 Hz), 123.8 (s, p-NPh), 117.9 (d, C10, 4JPC = 3.3 Hz), 117.7 (d, i-PPh, 1JPC = 85.5
1
15
Hz), 117.5 (s, o-NPh), 102.2 (s, C4), 27.8 (d, C13, JPC = 46.5 Hz), 12.6 (s, C6). N NMR (60
31
MHz, DMSO-d6, δ/ppm): -92.4 (s, N2), -190.9 (s, N1), -252.5 (s, N8). P NMR (243 MHz,
DMSO-d6, δ/ppm): 44.2 (s).
3.4. Quantum-chemical calculations
The calculations of electronic and spatial structure of tautomers were carried out within density
functional theory (DFT). The hybrid exchange-correlation B3LYP functional [25] with an
exchange part in the form suggested by Becke [26] and Lee-Yang-Parr on the correlation part
[27] were utilized. The standard extended 6-311G(d,p) split-valence basis set was employed. The
structure of the molecules under consideration was optimized without any symmetry restrictions.
All stationary points on the potential energy surface were characterized by normal vibration
analysis. The Gaussian’03 program [28] was used for calculations. ChemCraft program [29] was
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used for the data preparation and visualization of the results. 40 singlet–singlet electronic
excitations were simulated for modelling of the UV/vis spectra within the TDDFT method [30,
31]. The influence of the media (ethanolic solution) was taken into account using the polarizable
continuum model (PCM) [32, 33] with the standard parameters as implemented in Gaussian’03
program. The IR spectrum was modelled within harmonic oscillator approximation at the same
level of theory (B3LYP/6-311G(d,p)), calculated frequencies of fundamental vibrations were
scaled by 0.967 as recommended in [34].
4. Conclusions
The phosphonium salt (HLBr) was obtained in the reaction of 4-
aminobenzyl(triphenyl)phosphonium
bromide
and
5-hydroxy-3-methyl-1-phenyl-4-
formylpyrazole. The existence of pyrazolone isomeric form of HL+ in DMSO solution and solid
phase observed from its NMR and IR spectra was further supported by DFT calculations. The
results of quantum chemical modelling showed that the activation energy of 5-hydroxypyrazole
form of HL+ conversion to pyrazolone-5 one through 1-5 proton sigmatropic shift is 2.68
kcal/mol. Bands in experimental UV/vis spectra for molecular and deprotonated forms of HL+
are in a satisfactory agreement with the results of quantum-chemical calculations. This
compound will be further used for synthesis of d-metal coordination compounds.
Acknowledgments
This work was supported by the RA MES SCS, within the frames of the «RA MES SCS
– YSU-RF SFEDU» joint project No. VnGr-07/2017-35, Ministry of Education, Science and
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Technological development of the Republic of Serbia (Grant OI 172055) and taking advantage
of the facilities of Resource Centers of SFEDU (“Molecular spectroscopy”, “High-productive
calculations” and “Nanoscale structure of matter”).
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