Pd(II) porphyrins: Synthesis, singlet oxygen generation and photoassisted oxidation of aldehydes to carboxilic acids

Article abstract of DOI:10.1016/j.ica.2019.119339

The synthesis and spectral studies of A₃B and A₂B₂ type porphyrins and their Pd(II) complexes are reported. The meso-positions on porphyrin macrocycle are substituted with pentafluorophenyl and N-butylcarbazole or triphenylamine groups. Pd(II) porphyrins displayed decent phosphorescence ～670 nm and are able to produce singlet oxygen by type II pathway, after photoirradiation. The calculated singlet oxygen quantum yields for Pd(II) porphyrins are (Φ_Δ = 30%–63%). The catalytic application of Pd(II) porphyrins towards photoassisted aerobic oxidation of aromatic aldehydes to carboxylic acids is demonstrated.

Full text of DOI:10.1016/j.ica.2019.119339

Journal Pre-proofs
Research paper
Pd(II) porphyrins: Synthesis, singlet oxygen generation and photoassisted oxi-
dation of aldehydes to carboxilic acids
Vijayalakshmi Pandey, Drishti Jain, Nitesh Pareekh, Iti Gupta
PII:
S0020-1693(19)31211-3
https://doi.org/10.1016/j.ica.2019.119339
ICA 119339
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
16 August 2019
29 November 2019
2 December 2019
Please cite this article as: V. Pandey, D. Jain, N. Pareekh, I. Gupta, Pd(II) porphyrins: Synthesis, singlet oxygen
generation and photoassisted oxidation of aldehydes to carboxilic acids, Inorganica Chimica Acta (2019), doi:
https://doi.org/10.1016/j.ica.2019.119339
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover
page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will
undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing
this version to give early visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
©
2019 Published by Elsevier B.V.

Pd(II) porphyrins: Synthesis, singlet oxygen generation and
photoassisted oxidation of aldehydes to carboxilic acids
a
a
a
a
Vijayalakshmi Pandey, Drishti Jain, Nitesh Pareekh and Iti Gupta*
Indian Institute of Technology Gandhinagar, Village Palaj, Simkheda, Gandhinagar,
Gujarat-382355, India. *Corresponding author, e-mail: iti@iitgn.ac.in
Graphical Abstract
The synthesis and spectral studies of A B and A B type porphyrins and their Pd(II) complexes
3
2
2
are reported. The meso-positions on porphyrin macrocycle are substituted with
pentafluorophenyl and N-butylcarbazole or triphenylamine groups. Pd(II) porphyrins displayed
decent phosphorescence ~670 nm and are able to produce singlet oxygen by type II pathway,
after photoirradiation. The calculated singlet oxygen quantum yields for Pd(II) porphyrins
are (  30% to 63%). The catalytic application of Pd(II) porphyrins towards photoassisted

aerobic oxidation of aromatic aldehydes to carboxylic acids is demonstrated.
Keywords: Singlet oxygen, Photosensitizer, Palladium porphyrins, Catalyst, Oxidation

Introduction
Porphyrins are a class of naturally occurring pigments, found in biomolecules like:
haemoglobin, chlorophyll and cytochrome P450 [1,2]. Metalloporphyrins, are well known
prosthetic groups present in the various proteins and enzymes; and they are responsible for the
catalytic activities of these biomolecules [3]. In recent years, metalloporphyrins have been
developed as important synthetic tools for organic transformations. Ruthenium, iron,
manganese complexes of porphyrins are used as efficient catalyst for C-H oxidations with high
selectivity and good product yields [4,5]. Cobalt(II) porphyrin is used for the industrial scale
bioinspired oxidation of cyclohexane to cyclohexanone [6]. Iron(III) and manganese (III)
porphyrins are effectively used as catalysts for the epoxidation of alkenes by oxidizing reagents
[7,8]. The dendritic ruthenim porphyrin was employed as catalyst for the epoxidation of chiral
cholesteryl ester with high selectivity [9]. Transformation of primary alcohols to aldehydes is
important for organic synthesis and catalytic oxidation with molecular oxygen was possible
with the help of ruthenium (III) porphyrin catalyst [10]. The oxidation of hydrocarbons using
molecular oxygen under mild conditions is still a challenging issue, as O does not react with
2
organic compounds due to spin rule [11]. The selective oxidations with oxygen can be achieved
in the presence of free base porphyrins or metallporphyrins; because they are effective
photosensitizers for singlet oxygen generation [12,13]. In this regard, porphyrin derivatives are
noticeable photosensitizers owing to their high absorption coefficient, triplet excited states and
high photostability. In fact, meso-tetraphenylporphyrin (H TPP) is one of the most effective
2
photosensitizer for singlet oxygen generation [14]. Meso-aryl porphyrins and their zinc(II),
iron(III), manganese(III), cobalt(II) complexes have been used as catalysts for the oxidation of
cyclohexane and aromatic aldehydes by molecular oxygen in the presence of sunlight/light
source [15-19]. The choice of central metal ion and the meso-aryl groups on porphyrin
macrocycle can have significant role in their catalytic activity; and electron fluorinated aryl
2

groups at the meso-position can have positive impact on catalytic photooxidation [15].
Keeping this in mind, we have prepared a series of A B and A B type porphyrins bearing two
3
2
2
or three C F groups at the meso-positions and one or two electron donating bulky aromatic
6
5
heterocycles (N-butylcarbazole or triphenylamine). Their palladium (II) complexes were
prepared to test their potential as photosensitizers and catalysts for photooxidation of aromatic
aldehydes.
In this work, synthesis and optical studies of A B and A B type porphyrins and their
3
2
2
Pd(II) complexes are reported. The meso-positions on porphyrin macrocycle are substituted
with pentafluorophenyl and N-butylcarbazole or triphenylamine groups. Phosphorescence
emission and singlet oxygen generation studies of Pd(II) porphyrins is also reported. The TD-
DFT calculations are performed to understand the absorption properties of these molecules.
Metalloporphyrins were also tested as catalysts for the photoassisted aerobic oxidation of
aromatic aldehydes to carboxylic acids.
Results and Discussion
Synthesis and characterization
The synthetic strategy to make free base porphyrins and their Pd(II) complexes is shown in
Scheme 1. In the first step, bulky heterocyclic aldehydes (9-butyl-9H-crbazole-3-carbaldehyde
and/or 4-(diphenylamino)benzaldehyde)
were condensed with 5-pentafluoro-phenyl-
dipyrromethane in the presence of catalytic amount of TFA (trifluoroacetic acid). In the second
step, crude mixture was oxidized by DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) to
produce the target porphyrins 1-4. The desired A B -porphyrins (1 and 3) were isolated in
2
2
decent yields (15–16%) after the neutral alumina column chromatography [20]. The
scrambling of dipyrromethane also resulted in the formation of corresponding A B type
3
porphyrins (2 and 4). Again purification of the crude reaction mixture by neutral alumina
column afforded 2 and 4 in reasonable yields (10-11%). Then, the synthesis of palladium
3

complexes of porphyrins (Scheme 1) was performed by reacting the free base porphyrins 1-4
in DCM (dichloromethane) with PdCl dissolved in methanol. The longer reaction time (~24
2
h) was required for the formation of the Pd(II) porphyrins because of the poor solubility of
palladium salts. The crude products were subjected to neutral alumina column chromatography
and pure metalloporphyrins 1Pd-4Pd were obtained in 79-86% yields as orange solid.
F
F
F
F
NH
NH
CHO
F
N
CHO
N
a, b
a, b
F
F
F
F
F
F
F
F
F
F
N
N
N
N
N
N
N
M
N
M
N
N
N
N
F
F
F
F
F
F
F
M = H 3 (15%)
M = Pd 3Pd (86%)
2
M = H 1 (16%)
M = Pd 1Pd (80%)
2
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
N
N
N
N
N
N
N
M
F
N
M
F
N
N
F
F
F
F
F
F
F
F
F
M = H 2 (10%)
M = Pd 2Pd (79%)
2
M = H 4 (11%)
M = Pd 4Pd (84%)
2
F
F
Scheme 1: Synthesis of free base porphyrins (1-4) and their Pd(II) complexes (1Pd-4Pd); a: DCM, TFA,4 h
followed by DDQ, 2h, RT; b: DCM, Pd(Cl) , RT, 24h.
2
4

1
1
1
3
Figure 1. Partial H NMR and H- H COSY spectra of porphyrin 2 recorded in CDCl .
5

1
All the porphyrins were characterized by standard spectroscopic techniques viz. IR, H-
1
3
NMR, C-NMR and MALDI-TOF mass spectrometry (cf. supplementary information). The
porphyrins 1, 3 and their corresponding metal complexes 1Pd and 3Pd have been reported
1
previously [20]. The representative H and HOMO-COSY spectra of porphyrin 2 (aromatic
region) are shown in Figure 1. With the help of COSY spectrum, the structure of porphyrin 2
was identified; the eight β pyrrole protons showed up as three set of doublets between 8.95 and
8
.71 ppm. The seven aryl protons of the meso-N-butylcarbazole group appeared between 8.22
to 7.16 ppm. The upfield signal at –2.81 ppm is characteristic of the two inner N-H protons of
the aromatic porphyrin core (cf. SI). The signals at 4.50, 2.03, 1.56 and 0.81 ppm appeared for
the butyl chain of N-butylcarbazole groups. In the cases of metalloporphyrins 1Pd-4Pd, the
disappearance of the characteristic upfield signal of inner N-H protons suggests the formation
of palladium complexes. Furthermore, the corresponding molecular ion peak were also
observed in the MALDI-mass spectra of the new porphyrins (2, 4, 2Pd and 4Pd) confirming
their formation (cf. SI).
Absorption and Emission Studies
The UV-Vis absorption spectra of porphyrins and palladium porphyrins were recorded in
toluene at room temperature; the absorption and emission data are presented in Table 1. All the
porphyrins exhibited well defined, narrow absorption bands and the absorption data (Table 1)
are consistent as per the literature reports [21,22]. The free base porphyrins 2 and 4 exhibited
a strong Soret band at 419 nm range (Figure 2a), which is slightly broader than that of the
H TPP [23]. The absorption pattern of four Q-bands is similar to H TPP, with negligible shifts
2
2
in their absorption values; however, the Q3 band of both the porphyrins showed 12 nm red
shifts as compared to that of the H TPP (548 nm).
2
-6
Table 1. Photophysical studies data of porphyrins recorded in toluene; (Concentration = 2.4 x10 ).
6

-3
-1
-1
Porphyrin
λabs (nm) ( x 10 ) (M cm )
λex (nm)
λem (nm)
f
2
4
419 (218), 516 (14), 560 (0.2), 592 (0.8), 650 (0.5)
415 (333), 518 (26), 560 (10), 595 (10), 652 (0.1)
413 (409), 526 (51), 559 (4)
515
515
413
413
654, 716
0.065
0.046
---
659, 719
2
4
Pd
Pd
561, 613, 665
559, 611, 680
410 (293), 528 (36), 559 (5)
---
Figure 2. The comparision of absorption spectra of porphyrins, (a) 2 and 4; (b) 2Pd and 4Pd recorded in
toluene.
Figure 3. The comparison of emission spectra of porphyrins, (a) 2 and 4 (ex = 515 nm); (b) 2Pd and 4Pd (ex
=
4
13 nm) recorded in toluene.
The palladium complex of porphyrins 2Pd and 4Pd exhibited three absorption bands (Figure
b); the higher energy Soret band appeared at 413 nm and two Q bands between 526-559 nm
Table 1). The absorption spectra of 2Pd and 4Pd (A B type porphyrins) were similar to the
2
(
3
previously reported A B type Pd(II) porphyrins [20]; suggesting that one or two bulky
2
2
heteroaromatic group on the meso-position have less significant effect on their electronic
7

property. The fluorescence spectra of porphyrins 2 and 4 were recorded in toluene and the
comparative emission spectra are shown in Figure 3a. The emission spectra of the porphyrins
2
and 4 exhibited two emission bands around 656 and 719 nm. The higher energy emission
bands of 2 and 4 were 4–9 nm red-shifted as compared to that of the H TPP. The fluorescence
2
quantum yields ( ) of 2 and 4 were estimated from the emission and absorption spectra by
f
comparative method, using standards H TPP ( = 0.11) [23]. The fluorescence quantum
2
f
yields of the porphyrins 2 and 4 (Table 1) were slightly lower than H TPP; which indicated
2
noticeable nonradiative decay processes of their excited singlet states [24, 25]. Emissive
properties of the Pd(II) porphyrins (2Pd and 4Pd) were studied in the deaerated toluene; and
the photoluminescence spectra of are shown in Figure 3b. The emission spectra showed one
phosphorescence band of medium intensity for 2Pd and 4Pd; owing to the rapid singlet-triplet
intersystem crossing in these Pd(II) porphyrins; two moderately intense fluorescence bands
around 560 and 511 nm were also observed for 2Pd and 4Pd (Table 1). Significant
phosphorescence was seen for 2Pd and 4Pd, as a result of palladium induced spin orbit
coupling which promotes radiative decay from the triplet state (T1). The observed
phosphorescence bands in 2Pd (665 nm) and 4Pd (680 nm) were red shifted as compared to
their corresponding lowest energy absorption peak (QI-band).
Computational Studies
Density Functional Theory (DFT) calculations were carried out to gain further insight about
the electronic structure of the porphyrins 2, 2Pd, 4 and 4Pd, using the CPCM-TD-B3LYP/6-
3
11+(d) level of theory [26]. For palladium porphyrins 2Pd and 4Pd, LANL2D2 was used to
generate effective core potential. The geometrical parameters and the gas phase optimized
molecular structures of porphyrins 2, 2Pd, 4 and 4Pd are provided in the supplementary
information (Figures S11, S12). The DFT studies of 1, 1Pd, 3 and 3Pd are reported in the
previous work [20].
8

Figure 4. The Khon-Sham orbital representation of the frontier molecular orbitals and their corresponding
energies of 2, 2Pd, 4 and 4Pd at the CPCM-TD-B3LYP/6-311+(d) level of theory in gas phase.
The molecular orbitals of four porphyrins were produced both in the gas phase and in toluene
with time-dependent density functional theory (TD-DFT) at B3LYP/6-31G+(d) level. The
solvent effect of toluene was taken into account by the polarizable continuum model (PCM).
9

The frontier molecular orbitals involved in the electronic transitions for porphyrins 2, 2Pd, 4
and 4Pd are presented in Figure 4. For the HOMO’s of porphyrins 2, 2Pd, 4 and 4Pd, the
electron density was mainly localized on the aromatic heterocycles (N-butylcarbazole or
triphenylamine) present on the meso-positions of the porphyrin ring (Figure 4). On the contrary,
for LUMO energy levels the electron density was more concentrated on the porphyrin core.
The role of excitation energies and frontier molecular orbitals in the absorption spectra of
porphyrins 2, 2Pd, 4 and 4Pd is clear from the data presented in table 2. For compounds 4 and
5
one major absorption band was obtained and its wavelength (λ ) was selected for
abs
comparison. The calculated λabs values for the absorption band of highest oscillatory strength,
were 18 to 24 nm blue shifted as compared to the experimental values (Table 1); which could
be due to limitation of vertical approximation. The maximum oscillatory strengths were in
between 0.31 to 1.56 for the porphyrins studied here (Table 2). For free base porphyrins 2 and
4
the dominant electronic transitions were observed from HOMO-1 to LUMO+1 and HOMO-2
to LUMO level, respectively. In the case of palladium porphyrins 2Pd and 4Pd the major
transitions were obtained from HOMO-5 to LUMO+1 and HOMO-3 to LUMO+1 level,
respectively (Table 2). The DFT calculated HOMO-LUMO gaps for free base porphyrins 2
and 4 were 2.639 and 2.666 eV, respectively. Similarly, for 2Pd and 4Pd the values of HOMO-
LUMO band were 2.775 and 2.802 eV, respectively. It is clear that, the HOMO-LUMO gaps
of metallated porphyrins 2Pd and 4Pd are slightly higher than those of the corresponding
freebase porphyrins 2 and 4, respectively.
Table 2. Calculated electronic excitation energies, oscillator strengths, and eigenvectors for TD-DFT spectra of
2
, 2Pd, 4 and 4Pd using CPCM-TD-B3LYP/6-311+(d) level of approximation.
Molecule Excited
Contribution
Coefficient  (nm)
oscillatory
strength ( f)
Experimental
state
energy
λ
abs values in
Toluene
(eV)
1
0

HOMOLUMO
HOMO-1LUMO+1
HOMO-1LUMO
HOMO-1LUMO+1
HOMOLUMO
8
16
9
6
4
2
3.11
3.12
397.95
396.45
1.56
0.31
422
415
HOMO-1LUMO+1
1
HOMO-6LUMO
HOMO-5LUMO+1
HOMO-4LUMO
HOMO-4LUMO+1
HOMO-3LUMO
HOMO-3LUMO+1
HOMO-2LUMO
HOMO-1LUMO+1
2
24
6
1
6
1
3
4
2
Pd
HOMO-3LUMO
HOMO-2LUMO
HOMO-2LUMO+1
HOMO-1LUMO
HOMO-1LUMO+1
HOMOLUMO
HOMOLUMO+1
HOMO-3LUMO
HOMO-3LUMO+1
HOMO-2LUMO
HOMO-2LUMO+1
HOMO-1LUMO
HOMO-1LUMO+1
HOMOLUMO+1
7
1
13
8
9
5
1
7
1
1
4
3.14
3.16
394.40
391.38
1.53
1.20
413
410
4
Pd
17
12
2
2
Singlet Oxygen Generation
1
The production of singlet oxygen ( O ) upon irradiation of the photosensitizers has potential
2
applications in the PDT (photo-dynamic therapy) of tumors and DNA damage [27,28].
1
Typically, O is thought to be the dominant cytotoxic species in the PDT studies of porphyrin-
2
based photosensitizers [29]. Metalloporphyrins containing heavy metals like platinum and
palladium at the core are known to exhibit considerably high inter-system crossing (ISC) due
to heavy atom effect. The augmented ISC results in the sufficiently populated triplet excited
1
states of the metalloporphyrins, and they can generate O by type II pathway; which involves
2
energy transfer from porphyrin’s triplet state to the ground state molecular oxygen (triplet)
[28]. Metalloporphyrins have also been utilized as catalysts for the photooxidation of alcohols
and alkenes, owing to their ability to generate singlet oxygen [15-17].
1
1

Figure 5. UV-vis spectral changes in (a) porphyrin 2Pd and (c) Rose Bengal in CHCl
3
on photoirradiation (λ =
-2
3
65 nm, 21 J cm ); the rate of decrease in absorption measured at 324 nm for (b) 2Pd and (d) Rose Bengal.
Table 3. Absorption and singlet oxygen generation data of Pd(II) porphyrins in different solvents.
-
4
-5
-1
-1
1O
%
2
Pd(II) Porphyrins
Solvents
Chloroform
λabs (nm) ( x 10 to 10 ) (M cm )
424 (688), 526 (98), 560 (023)
407 (709), 527 (121), 561 (042)
404 (682), 522 (95), 553 (022)
401 (725), 521 (118), 554 (041)
407 (800), 521 (122), 556 (055)
402 (803), 525 (137), 561 (054)
404 (819), 522 (110), 553 (042)
401 (802), 520 (138), 554 (043)
405 (805), 526 (125), 560 (045)
404 (781), 524 (126), 555 (042)
1
2
3
4
1
2
3
4
2
2
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
46
50
35
58
38
54
30
56
49
63
Chloroform
Chloroform
Chloroform
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Dimethylformamide
Tetrahydrofuran
The singlet oxygen quantum yields of palladium porphyrins (1Pd, 3Pd and 4Pd) were
examined in two solvents chloroform and acetonitrile; oxygen gas was purged in the UV-cell
before measurement of the absorption intensity of the compounds (cf ESI). The 1,3-
diphenylisobenzofuran (DPBF) was used as oxygen scavenger in the solution and singlet
oxygen generation by Pd(II) porphyrin was monitored by UV-vis spectrometer [30]. In this
1
2

assay Rose Bengal was used as a reference compound [31]. The significant inter-system
crossing (ISC) would lead to the energy transfer from photosensitizers (1Pd-4Pd) to triplet
ground state of oxygen molecules; which in turn get excited to singlet state and react with
DPBF. The decrease in the intensity of DPBF absorption band at 324 nm was monitored over
a time period of ~2 minutes. For photo-irradiation of the porphyrins UVA light source (λ = 365
-2
nm, 21 J cm ) was used; as the time progressed the intensity of the DPBF absorption band at
3
24 nm gradually decreased (Figure 5a). The triplet photosensitization ability of the porphyrin
2
Pd was compared with the reference photosensitizer (Rose Bengal,  = 0.56 in acetonitrile)

under similar conditions (Figure 5c). The obtained absorption data were fitted to equation 1 to
calculate the values of singlet oxygen quantum yield [32]. The plots of singlet oxygen studies
for the rest of the porphyrins are provided in the supplementary information (Figures S8-S13).
The singlet oxygen quantum yield, calculated for Pd(II) porphyrin 2Pd was 54% in acetonitrile;
and for the rest of the porphyrins (1Pd, 3Pd and 4Pd) it was in the range of 30-56% (Table 3).
Also, in chloroform the singlet oxygen quantum yields of the Pd(II) porphyrins were in the
range of 35-58 (Table 3). The studies indicated that these porphyrins can be effectively used as
catalysts for photooxidation of organic compounds.
Catalytic Photooxidation of Aldehydes
The free base porphyrins and metalloporphyrins are known for their catalytic properties;
porphyrin derivatives have been used as photocatalysts for the oxidation of aldehydes, alkene
and alcohols [4]. Typically, singlet oxygen can be generated by chemical method or by
photosensitized reaction in the presence of light. The photochemical method to generate singlet
1
oxygen is quite cost-effective and porphyrin derivatives are known to produce O by energy
2
,
1
transfer to the molecular oxygen. This O can be utilized further to oxidize the organic
2
compounds /hydrocarbons under aerobic conditions. Previous report by Safari and co-workers
have used Pt(II) complexes of meso-tetraaryl porphyrins as efficient photocatalyst for similar
1
3

organic transformations [32]. The group has used Pt(II) porphyrins for the aerobic oxidation of
aldehydes in 16-87% yields (of carboxylic acids) [32]. However, synthesis of Pt(II) porphyrins
is not easy and the cost of platinum salt is also high; thus, we tested the application of Pd(II)
porphyrins as catalyst for oxidation reaction in the presence of light. The potential of palladium
porphyrins (1Pd-4Pd) as catalyst for the photooxidation of aromatic aldehydes was
investigated in a series of experiments, discussed vide infra. Four aromatic aldehydes: p-
tolualdehyde, mesitaldehdye, o-nitrobenzaldehyde and p-nitrobenzaldehyde were selected as
substrate. Initially, palladium porphyrin 2Pd was used as catalyst to optimize the solvent for
the aerobic oxidation of o-nitrobenzaldehyde in the presence of light. As per the literature,
water, dimethylformamide, and dimethylsulfoxide are singlet oxygen quenchers; thus the yield
of oxidized product can get affected [33]. It is clear from the Table 4 that, maximum yield of
the o-nitrobenzoic acid was achieved when acetonitrile was used as solvent. Owing to its polar
nature, acetonitrile can dissolve the variety of substrates and the catalysts and it is relatively
inert toward oxidation; thus, it is good solvent for oxidation reactions.
Table 4: Effect of solvent on the photooxidation of 2-nitrobenzaldehyde to corresponding acid in the presence
of catalyst 2Pd and O .
2
Solvent
Chloroform
Dimethylformamide
Tetrahyrdofuran
Acetonitrile
Yield of carboxylic acid
1O
yield
2
31%
87%
50%
98%
50%
49%
63%
56%
As per the data presented in Table 5, four palladium porphyrins (1Pd-4Pd) were used for the
aerobic oxidation of aromatic aldehydes in the presence of light source (λ = 400-700 nm, 15 J
-2
cm ) at room temperature. The conversion took 24h in the presence of A B type porphyrins
3
(
2Pd and 4Pd); whereas, photocatalytic reaction was extended to 36h in the case of A B type
2 2
porphyrins (1Pd and 3Pd). The central metal ion of porphyrin ring and the meso-substituents
have significant effect on the yield of carboxylic acid; as per the literature reports, paramagnetic
1
4

metals like Fe(II) and Mn(II) show very short triplet state life time in solution and thus
conversion was negligible [15]. Previous reports by Safari and co-workers showed that, the
Zn(II) and Ni(II) porphyrins are less effective for such oxidation reactions; further Pd(II)
porphyrins were also found to be less reactive for similar conversion [15, 32].
Table 5. Photooxidation of aldehydes to carboxylic acids in the presence of catalyst and O
% Yield of Acid)
2
in CH
3
CN.
(
Porphyrin Catalyst
Aldehyde
Carboxylic Acid
1
Pd
2Pd
3Pd
4Pd
Me
CHO
Me
COOH
2
0%
17%
18%
53%
Me
Me
Me
Me
CHO
CHO
Me
COOH
57%
61%
98%
46%
91%
71%
15%
Me
COOH
NO2
2
0%
NO2
O N
2
CHO
O2N
COOH
73%
73%
40%
34%
The Pt(II) porphyrins exhibit good fluorescence and phosphorescence, with sufficiently long
triplet state lifetimes. In the presence of sunlight, Pt(II)porphyrin were employed as catalysts
by Safari and co-workers for the conversion of o-nitrobenzaldehyde and p-nitrobenzaldehyde
into their corresponding carboxylic acids; conversion yields were 83% and 69%, respectively
[
32]. It is clear from Table 5 that, Pd(II) porphyrins (1Pd-4Pd) can be used as efficient catalyst
for the photoassisted oxidation of aldehydes. The yield of carboxylic acids were moderate 17%
71%, when electron donating groups were present on the benzene ring (p-tolualdehyde and
-
mesitaldehyde); and 4Pd gave relatively higher conversion yield. However, when electron
withdrawing groups were present on the benzene ring (o-nitrobenzaldehyde and p-
nitrobenzaldehyde) the good conversion yields (91-98%) were obtained with 2Pd and 3Pd
1
5

catalysts, under similar conditions. These observations clearly suggest that, Pd(II) porphyrins
reported in this work can be used as potential catalyst for such photooxidation of organic
compounds.
Conclusions
A series of A B and A B type porphyrins and their Pd(II) complexes are synthesized and
3
2
2
characterized. Absorption and emission spectra of the A B type porphyrins showed standard
3
pattern, similar to the parent porphyrins H TPP/ PdTPP. The Pd(II) complexes of A B type
2
3
porphyrins, exhibited moderate phosphorescence peak (~670 nm) along with fluorescence
peaks in the emission spectra. Due to heavy metal effect, Pd(II) complexes of A B type
3
porphyrins were able to generate decent singlet oxygen (30-63%); which was further used for
the oxidation of aromatic aldehydes into carboxylic acids. The conversion of o-
nitrobenzaldehyde to its acid was achieved in high yields (91-98%); suggesting that Pd(II)
porphyrins have good potential as catalyst for photoassisted oxidation reactions.
Acknowledgements
The financial support from IIT Gandhinagar is greatly acknowledged. VP is grateful to IIT
Gandhinagar for fellowship and infrastructural support.
Supporting Information
1
13
The characterization data like H and C NMR, MALDI-MS spectra of selected porphyrins
have been provided. Data for the singlet oxygen quantum yields of the selected molecules have
also been provided. Geometry optimization data obtained from DFT calculation are also
provided.
Experimental
1
6

Instrumentation and Reagents
Unless otherwise mentioned, all the reagents and solvents were purchased from Aldrich, or
Acros Organics and used without further purification. Silica gel (100-140 mesh) from Merck
was used for column chromatography. The NMR spectra of compounds were recorded with
Bruker Avance III 500 MHz NMR spectrometer. MALDI mass spectra of compounds were
UltrafleXtreme MALDI TOF/TOF (Bruker Daltonics). IR spectra of samples were recorded
on Perkin Elmer L1600300 Spectrum TWO LITA. Absorption spectra were recorded with
UV-1700 (Shimadzu, Japan). Fluorescence emission measurements were recorded with
Horiba-Jobin Yvon Fluorlog-3 spectrometer (France). The synthesis of porphyrins 1, 3, 1Pd
and 3Pd were reported in our previous work [20].
Computational Analysis
Density functional theory (DFT) and time-dependent density functional theory (TD-DFT)
calculations of the four porphryins (2, 2Pd, 4 and 4Pd) were carried out the at CPCM-TD-
B3LYP/6-311+(d) level [26]. The gas phase optimisation of all the free base porphyrins was
carried out using 6-31G(d) basis sets. In the case of metalloporphyrins, 6-31G(d) basis was
employed for non-metals and SDD21 basis set was used for zinc and palladium metals.
Absorption spectra of all these dyads were simulated with TD-DFT using B3LYP functional
2
2
and the same basis sets. Gaussian 09 was used for all calculations. Frequency analysis
confirmed the absence of imaginary frequencies. Theoretically, the intensity of the band is
expressed regarding the oscillator strengths (ƒ).
DFT calculations
The ground state geometry optimization of the molecules 2, 2Pd, 4 and 4Pd were performed
with density functional theory (DFT) at CPCM-TD-B3LYP/6-311+(d) level, using Gaussian
1
7

0
9 program package [21]. The vertical excitation energies were calculated employing time
dependent density functional theory (TD-DFT) approach at the same functional and basis set.
Singlet Oxygen Quantum Yield
-5
DPBF with a fixed concentration of (10 M) was prepared in dark and was covered with
aluminium foil to avoid any kind of light exposure. By making effective concentrations of
-6
Rose Bengal (10 M) and all other compounds were recorded for absorption. Readings for
Rose Bengal along with all other compound were recorded and the absorption decay was
plotted at 410 nm to get the slopes. Following equation was used to calculate singlet oxygen
quantum yield: ΔS) = ΔS0) x {Slope(S) / Slope (S0)}; where S is the slope obtained from
compounds and S is slope obtained from the standard compound.
O
General synthesis of Free base porphyrins
Aldehyde (100 mmol) and dipyrromethane (200 mmol) were dissolved in DCM (40 mL) and
stirred for 2 mins followed by the addition of trifluoroacetic acid (5 μL). The reaction mixture
was stirred under inert atmosphere for 4h, at room temperature in dark. After that, 2,3-dichloro-
5
,6-dicyano-1,4-benzoquinone (DDQ, 300 mmol) was added and reaction mixture was stirred
for another 3h under air. Formation of the desired porphyrin was identified by observation of
maroon colour spot on TLC (in 30% DCM/hexane). The crude reaction mixture was subjected
to silica gel column to filter off excess DDQ and other oligomeric products using 100% DCM.
Further purification was carried out with a neutral alumina column chromatography and the
desired porphyrins were eluted with 20–30% DCM/hexane.
O
Porphyrin 2: Purple crystal, M.P >195 C. Yield 10% (30 mg). mp. R = 0.30 (DCM/ Hexane
f
+
1
: 1.2), MALDI-MS for C H F N [M+H] calcd 1030.2027 observed m/z 1030.32. IR (neat,
5
4
27 15
5
−
1
1
cm ): 3319, 2950, 2928, 2859, 1594. H NMR (500 MHz, Chloroform-d): δ (in ppm) –2.80
s, 2H), 1.01 (t, J = 7.4 Hz, 3H), 1.53 (d, J = 7.4 Hz, 2H), 1.53 (d, J = 7.4 Hz, 2H), 4.47 (t, J =
.2 Hz, 2H), 7.22 (t, J = 7Hz, 1H), 7.51 (s, 2H), 7.68 (d, J = 5 Hz, 1H), 8.12 (d, J = 7.5 Hz,
(
7
1
8

1
4
2
1
H), 8.22 (d, J = 1.5 Hz, 1H), 8.71 (d, J = 4.5 Hz, 2H), 8.81 (m, 4H), 8.47 (s, 1H), 8.95 (d, J =
1
3
.5 Hz, 2H); C-NMR (125 MHz, Chloroform-d): δ (in ppm) 12.98, 13.08, 19.76, 21.66, 28.34,
8.67, 29.18, 30.37, 30.90, 30.31, 42.31, 100.47, 101.90, 105.86, 108.12, 118.39, 119.37,
20.72, 121.62, 123.70, 125.30, 125.86, 128.67, 130.45, 131.67, 139.33, 140.41, 144.57.
O
O
Porphyrin 4: Purple crystal, M.P >195 C .Yield 15% (25 mg). M.P 195-205 C. R = 0.33
f
+
(
DCM/ Hexane 1: 1.2), MALDI-MS for C H F N [M+H] calcd 1052.1870 observed m/z
5
6
25 15
5
−
1
1
1
052.58. IR (neat, cm ): 3309, 2959, 2916, 2848, 1593. H-NMR (500 MHz, Chloroform-d):
δ (in ppm) –2.86 (s, 2H), 7.07 (m, 4H), 7.38 (d, J = 8.4 Hz, 2H), 7.96 (d, J = 8.4 Hz, 2H), 8.74
1
3
(
m, 4H), 9.02 (d, J = 4.7 Hz, 2H); C-NMR (125 MHz, Chloroform-d): δ (in ppm) 100.55,
1
1
01.90, 114.92, 115.08, 119.74, 122.57, 122.57, 124.23, 128.58, 133.11, 134.67, 135.51,
37.51, 142.17
General synthesis of Pd(II) porphyrins
Porphyrin (0.01 mmol) was dissolved in DCM (20 mL) and palladium(II) chloride (0.04 g)
dissolved in methanol (25 mL) was added to it. The reaction mixture was stirred for 24 hrs
under inert condition. The formation of the desired product was identified by observation of
orange colour spot on TLC (80% DCM/hexane), where the Pd(II) porphyrin appear more polar
as compared to the starting porphyrin. The desired palladium porphyrins were further purified
by neutral alumina column chromatography, using 90% DCM/hexane as solvent.
O
Porphyrin 2Pd: Orange solid, M.P. > 250 C. Yield 79% (16 mg). R = 0.26 (DCM/ Hexane
f
+
1
: 1.2), MALDI-MS for C H F N Pd [M+H] calcd 1134.0905 observed m/z 1134.38. IR
5
4
25 15
5
−
1
1
(
neat, cm ): 2949, 2930, 2862, 1580. H NMR (500 MHz, Chloroform-d): δ (in ppm) 1.01 (t,
J = 7.4 Hz, 3H), 1.55 (d, J = 7.4 Hz, 2H), 2.03 (t, J = 8 Hz, 2H), 4.50 (t, J = 7.2 Hz, 2H), 7.23
(t, J = 7Hz, 1H), 7.53 (t, J = 8.5 Hz, 2H), 7.67 (d, J = 5 Hz, 1H), 8.10 (d, J = 7.5 Hz, 1H), 8.18
(d, J = 1.5 Hz, 1H), 8.68 (d, J = 4.5 Hz, 2H), 8.78 (m, 4H), 8.80 (s, 1H), 8.93 (d, J = 4.5 Hz,
1
3
2
3
H); C-NMR (125 MHz, Chloroform-d): δ (in ppm) 13.09, 19.78, 21.67, 26.07, 26.68, 29.18,
0.91, 31.74, 36.09, 42.33, 118.38, 119.69, 125.34, 128.37, 129.51, 129.85, 132.92, 140.48.
1
9

O
Porphyrin 4Pd: Orange solid, M.P. > 250 C. Yield 85% (84 mg). R = 0.30 (DCM/ Hexane
f
+
1
: 1.2), MALDI-MS for C H F N Pd [M+H] calcd 1156.0749 observed m/z 1156.83. IR
5
6
23 15
5
−
1
1
(
neat, cm ): 2954, 2912, 2835, 1580. H-NMR (500 MHz, Chloroform-d): δ (in ppm) 7.10 (m,
3
9
1
H), 7.37 (d, J = 4.2 Hz, 9H), 7.93 (d, J = 8.4 Hz, 2H), 8.72 (d, J = 4.5 Hz, 2H), 8.76 (m, 4H),
1
3
.01 (d, J = 4.7 Hz, 2H); C-NMR (125 MHz, Chloroform-d): δ (in ppm) 103.82, 119.75,
22.68, 124.22, 128.50, 128.58, 129.58, 129.91, 132.51, 132.74, 134.10, 140.11, 140.44.
General synthesis of carboxylic acids
The aldehydes (0.5 mmol) and 1 mL DCM solution of Pd(II) porphyrins (0.5 x 10 mmol)
1Pd/2pd/3Pd/4Pd) were dissolved in 14 mL of acetonitrile in a two neck round bottom flask.
Reaction flask was fixed into a cardboard box in such a way that, it could be exposed to the
-3
(
-2
light source (λ = 400-700 nm, 15 J cm ) and reaction was stirred at room temperature under
oxygen atmosphere. Reaction was monitored by TLC (50% ethyl acetate/ pet-ether), the spot
corresponding to the carboxylic acid was clearly visible on TLC under UV-lamp. The solvent
was removed on rotary evaporator under vacuum and the crude product was purified by silica
gel column chromatography in 50% ethyl acetate /pet-ether mixture. The conversion of
aldehydes into carboxylic acid was measured by IR spectroscopy.
4
2
2
4
-methylbenzoic acid: IR (neat, cm-1): 3384, 2922, 2855, 1547, 1399, 1085.
,4,6-methylbenzoic acid: IR (neat, cm-1)3442, 2924, 2854, 1688, 1607, 1458.
-nitrobenzoic acid: IR (neat, cm-1): 3450, 2920, 2855, 1695, 1526, 1456.
-nitrobenzoic acid: IR (neat, cm-1): 3411, 2915, 2848, 1705, 1606, 1539.
References
1] J. S. Lindsey, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard,
Academic Press, San Diego, 1 (2000) Chapter 2.
2] L. R. Milgrom, The Colours of Life-An Introduction to the Chemistry of Porphyrins and
Related Compounds, Oxford University Press, Inc.: New York, NY, USA, 22 (1997).
[
[
2
0

[
3] M. O. Senge, S. A. MacGown, J. M. O’Brein, Chem. Commun. 51 (2015) 17031.
4] J.C. Barona-Castano, C.C. Carmona-Vargas, T.J. Brocksom, K.T. De Oliveira, M. Graca,
P.M.S. Neves, M. Amparo, F. Faustino, Molecules. 21 (2016) 310.
[
[5] M. Costas, Coord. Chem. Rev. 255 (2011) 2912.
[6] C.-C. Guo, X.-Q. Liu, Q. Liu, Y. Liu, M.-F. Chu, W.-Y. Lin, J. Porphyrins Phthalocyanines
1
3 (2009) 1250.
[
[
[
[
(
[
[
7] C.-M. Che, J.-S. Huang, Chem. Commun. 27 (2009)3996.
8] H. Srour, P. Le Maux, G. Simmonneaux, Inrog. Chem. 51 (2012) 5850.
9] J. L. Zhang, H. B. Zhou, J. S. Huang, C. M. Che, Chem. Eur. J. 8 (2002) 1554.
10] H.-B. Ji, Q.-L. Yuan, X.-T. Zhou, L.-X. Pei, L.-F. Wang, Biorg. Med. Chem. Lett. 17
2007), 6364.
11] M. Hajimohammadi, N. Safari, H. Mofakham, F. Deyhimi, Green Chem. 13 (2011) 991.
12] M. Hajimohammadi, N. Safari, H. Mofakham, A. Shaabani, Tetrahedron Lett. 51 (2010)
4
061.
13] A. Maldotti, C. Bartocci, G. Varani, A. Molinari, P. Battioni, D. Mansuy, Inorg. Chem.
5 (2002) 1126.
14] M. A. Faustino, M. G. Neves, J. A. Cavaleiro, M. Neumann, H. D. Brauer, G. Jori,
Photohem. Photobiol. 72 (2000) 217.
[
3
[
[
[
[
[
[
[
15] M. Hajimohammadi, N. Safari, J. Porphyrins Phthalocyanines 14 (2010) 639.
16] H. R. Khavasi, N. Safari, J. Mol. Catal. A. 220 (2004) 127.
17] P.E. Ellis, J. E. Lyons, Coord. Chem. Rev. 105 (1990) 181.
18] M.R.Iesce, C. Flavio, F. Temussi, Curr. Org. Chem. 9 (2005) 109.
19] G. Jiang, J. Chen., J. S. Huang, C.-M. Che, Org. Lett. 11 (2009) 4568.
20] S. Das, H.R. Bhat, N. Balsukuri, P.C. Jha, Y. Hisamune, M. Ishida, H. Furuta, S. Mori, I.
Gupta, Inorg. Chem. Front. 4 (2017) 618.
2
1

[
[
(
[
[
[
[
21]S. Das, S., I. Gupta, J. Porphyrins Phthalocyanines. 23 (2019) 367.
22] M. Gouterman, in The Porphyrins, ed. D. Dolphin, Academic Press: New York, Vol. 3
1978) 1.
23] I. Gupta, M. Ravikanth, J. Chem. Sci. 117 (2005) 161.
24] N. Balsukuri, I. Gupta, Dyes Pigments 144 (2017) 223.
25] N. Balsukuri, S. Das, I. Gupta, New J. Chem. 39 (2015) 482.
26]M. Frisch, G. T., H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. Hratchian,
A.Izmaylov, J. Bloino, G. Zheng, J. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery, Jr., J. Peralta, F. Ogliaro, M. Bearpark, J. Heyd, E. Brothers, K. Kudin, V.
Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. Burant, S.
Iyengar, J. Tomasi, M. Cossi, N. Rega, J. Millam, M. Klene, J. Knox, J. Cross, V. Bakken, C.
Adamo, J. Jaramillo, R. Gomperts, R. Stratmann, O. Yazyev, A. Austin, R. Cammi, C. Pomelli,
J. Ochterski, R. Martin, K. Morokuma, V. Zakrzewski, G. Voth, P. Salvador, J. Dannenberg,
S. Dapprich, A. Daniels, O. Farkas, J. Foresman, J. Ortiz, J. Cioslowski, and D. Fox, , 09,
Revision D. 01, Gaussian. Inc., Wallingford, CT 2009.
[27] J. Zhang, C. Jiang, J.P. Figueiró Longo, R.B. Azevedo, H. Zhang, L.A. Muehlmann, Acta
Pharm. Sin. B. 8 (2018) 137.
[28] P. R. Ogilby, Chem. Soc. Rev. 39 (2010) 3181.
[
29] S. Singh, A. Aggarwal, N.V.S.D.K. Bhupathiraju, G. Arianna, K. Tiwari, C.M. Drain,
Chem. Rev. 115 (2015) 10261.
[30] M.K. Raza, S. Gautam, A. Garai, K. Mitra, P. Kondaiah, A.R. Chakravarty, Inorg. Chem.
5
6 (2017) 11019.
[31] P. Pushpanandan, Y.K. Maurya, T. Omagari, R. Hirosawa, M. Ishida, S. Mori, Y.
2
2

Yasutake, S. Fukatsu, J. Mack, T. Nyokong, H. Furuta, Inorg. Chem. 56 (2017) 12572.
32] M. Hajimohammadi, H. Mofakham, N. Safari, A.M. Manesh, J. Porphyrin
Phthalocyanines 16 (2012) 93.
33] I. Kruk, A. E. Hassan, T. Michalaska, K. Ichszteld, K. Kubasik-Kladna, S. Olgen,
Luminescence 22 (2007) 379.
[
[
2
3

Pd(II) porphyrins: Synthesis, singlet oxygen generation and
photoassisted oxidation of aldehydes to carboxilic acids
a
a
a
a
Vijayalakshmi Pandey, Drishti Jain, Nitesh Pareekh and Iti Gupta*
Indian Institute of Technology Gandhinagar, Village Palaj, Simkheda, Gandhinagar,
Gujarat-382355, India. *Corresponding author, e-mail: iti@iitgn.ac.in
Highlights
.
.
.
.
Synthesis and optical studies of A B and A B type porphyrins and their Pd(II)
3 2 2
complexes is reported.
The meso-positions on porphyrin macrocycle are substituted with pentafluorophenyl
and N-butylcarbazole or triphenylamine groups, for better catalytic activity.
Pd(II) porphyrins displayed decent phosphorescence ~670 nm and generated singlet
oxygen by type II pathway, after photoirradiation.
The catalytic application of Pd(II) porphyrins towards photoassisted aerobic oxidation
of aromatic aldehydes to carboxylic acids is demonstrated.
2
4

Authors state that, all co-authors have seen and approved the final version of the manuscript
being submitted. Authors warrant that the article is our original work, which has not received
prior publication and is not under consideration for publication elsewhere.
Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☒
The authors declare the following financial interests/personal relationships which
may be considered as potential competing interests:
Authors declare no conflict of interest.
Authors declare no financial interest.
2
5