Palladium(II) complexes of (t-butyl salicylidene) diphenyl disulfide diamine: synthesis, structure, spectral characterization and catalytic properties

Article abstract of DOI:10.1007/s11243-021-00471-7

The hexadentate N₂S₂O₂ donor ligand N,N’-bis(3,5-tert-butylsalicylidene) diphenyl disulfide-2,2’-diamine was synthesised by the condensation of 2-aminophenyl disulfide and 3,5-di-tert-butyl-2-hydroxybenzaldehyde and its mo

Full text of DOI:10.1007/s11243-021-00471-7

Transition Metal Chemistry
https://doi.org/10.1007/s11243-021-00471-7
Palladium(II) complexes of (t‑butyl salicylidene) diphenyl disulfde
diamine: synthesis, structure, spectral characterization and catalytic
properties
Minu G. Bhowon1 · Sabina Jhaumeer Laulloo1 · Eric C. Hosten2 · Sumayya Elaheebacus1 · Mounisha R. Dowlut1
Received: 10 May 2021 / Accepted: 13 July 2021
© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2021
Abstract
The hexadentate N2S2O2 donor ligand N,N’-bis(3,5-tert-butylsalicylidene) diphenyl disulfde-2,2’-diamine was synthesised
by the condensation of 2-aminophenyl disulfde and 3,5-di-tert-butyl-2-hydroxybenzaldehyde and its molecular structure
was confrmed by X-ray studies. One of the tert-butyl groups in the Schif base has rotational disorder around the C–C
bond with ratio 0.56:0.44. The palladium complexes were prepared by the direct reaction of PdCl2(CH3CN)2 and Schif
base ligands N,N’-bis (5-tert-butylsalicylidene) diphenyl disulfde-2,2’-diamine and N,N’-bis(3,5-tert-butylsalicylidene)
diphenyl disulfde-2,2’-diamine, respectively. The structure of the metal complexes was characterized by physico-chemical
and spectroscopic methods. Palladium is in square-planar geometry bonded to imine nitrogen and phenolic O in both the
complexes. The catalytic efciency of the palladium complexes was evaluated in the cross-coupling reactions; Heck-Mizoroki
reaction of iodobenzene and methyl acrylate and the Suzuki-Miyaura reaction of phenylboronic acid and iodobenzene, which
gave low to moderate yields. Higher conversions were obtained for 2a as catalyst due to the increase in the number of bulky
tertiary butyl groups in the structure.
Introduction
preferred choice for C–C bond formation as they show many
advantages [13]. However, the major drawbacks include the
cost of the phosphine ligand itself, air and moisture-labilitye
as well as being readily oxidized [14]. Schif bases and their
metal complexes continue to be of current interest for the
design of catalysts. Schif bases possess many advantages
such as facile approach, relative tolerance, readily adjusted
ancillary ligands, and tuneable steric and electronic coor-
dination environments on the metal centre. Moreover, they
can be synthesized both simply and cheaply in bulk amount,
and their high thermal and moisture stabilities make them
valuable catalysts in reactions carried out at high tempera-
ture. The interest with Schif base ligands with N, O and S
donating sites containing disulphide moiety has increased
due to their capabilities of forming stable complexes of four,
fve or six co-ordinated metal complexes [15, 16]. In lit-
erature, there are not many disulfde-containing palladium
Schif base complexes displaying catalytic properties. Our
interest in the synthesis of these types of disulphide metal
complexes is due to contributions for materials chemistry in
terms of molecular architectures, and development of new
catalyst. Recently, we have reported the palladium disulfde
complexes containing NS donor sites in the Mizoroki-Heck
reactions, which have shown promising results [17]. The
Palladium complexes have been used as catalysts in many
cross-coupling organic reactions mainly for C–C bond for-
mation such as Mizoroki-Heck and Suzuki-Miyaura due
to their versatile nature [1–5]. These reactions have been
extensively applied for the synthesis of natural products,
bioactive compounds, pharmaceutical intermediates and
industrial applications [6]. The palladium complexes bear-
ing diferent ligand combinations such as phosphines [7,
8], imines [9], carbenes [10, 11], and thiols [12] have been
reported to show moderate to good catalytic activity with
various substrates. Phosphine-based ligands have been the
* Minu G. Bhowon
mbhowon@uom.ac.mu
* Sabina Jhaumeer Laulloo
sabina@uom.ac.mu
1
Department of Chemistry, Faculty of Science, University
of Mauritius, Reduit, Mauritius
2
Department of Chemistry, Nelson Mandela Metropolitan
University, Summerstrand Campus South, Port Elizabeth,
South Africa
Vol.:(0123456789)
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Transition Metal Chemistry
presence of NOS donor sites in the studied ligand can infu-
ence the properties of complexes in terms of electronic and
steric factors. Therefore, such systems are worth exploring
as new catalyst for cross-coupling reactions.
H, 7.64; N, 4.11; S, 9.41. Found: C, 73.54; H, 8.02; N, 4.33;
S, 9.28. IR (ν/cm−1): 2999, 2952 (O–H), 3067 (aromatic
C-H), 2867, 2903 (CH3), 1612 (C=N), 1054 (C-O), 748
(C–S), 510 (S–S). UV–Vis (λ (nm), log ε (dm3 mol−1 cm−1)):
(In DCM): 228.4 (80 211), 285.7 (53 920), 364.8 (36 560),
(In Toluene): 285.4 (26 008), 363.3 (19 070).
Herein, we describe the synthesis and X-ray structure of
Schif base N,N′-bis(3,5-tert-butylsalicylidene) diphenyl
disulfde-2,2’-diamine, 2. The catalytic study efciency
towards the Heck-Mizoroki and the Suzuki-Miyaura reac-
tions using the synthesised palladium complexes under dif-
ferent parameters were investigated.
Synthesis of metal complexes 1a, 2a
PdCl2(CH3CN)2 was mixed with 1/2 in a 1:1 molar ratio in
acetonitrile (35 ml) and NEt3 (0.1 ml). The resulting solution
was refuxed with stirring for 6 h followed with stirring for
1 week at room temperature. The resulting precipitate was
fltered, washed successively with cold and hot acetonitrile
and dried in vacuo.
Experimental
Materials and physical measurements
1a: Colour: Orange; Yield: (56%), Mp. 331.4 °C, Elemen-
tal analysis, Calc. for C34H34N2S2O2 Pd (727.24 g mol−1): C,
56.15; H, 5.54; N, 3.85; S, 8.84. Found: C, 51.57; H, 4.22;
N, 3.53; S, 8.60. IR (ν /cm−1): 1589 (C=N), 2901–2864
(C(CH3)3), 742 (C–S), 1166 (C–O), 473 (S–S), 684 (Pd–O),
443 (Pd–N). UV-Vis (λ (nm), ε (M−1 cm−1) (In DCM): 228
(33 484), 255 (36 171), 303 (18 009), 430 (8279).
All the reagents and solvents were of analytical grade and
were used without any purifcation. Bis(2-aminophenyl)
disulphide, 3,5–di-tert-butyl-2-hydroxybenzaldehyde and
the 5–tert-butyl-2-hydroxybenzaldehyde were purchased
from Sigma-Aldrich Chemicals private Ltd. The FTIR spec-
tra were recorded at the range of 400–4000 cm−1 using an
Alpha-Bruker FTIR Diamond. CHNS elemental analyses
were carried out using a Euro Vector EA-3000 CHNS ana-
lyzer. 1H and 13C NMR, DEPT, COSY AND HMBC spec-
tra were obtained in CDCl3 by using a Bruker electro spin
250 MHz Spectrometer. The UV-vis spectra were recorded
in the range 200–800 nm using a Biochrom Libra S22 Spec-
trophotometer. The Fluorescence spectra were obtained on a
Perkin Elmer LS55 fuorimeter. Mass spectra were recorded
on a thermo Finnigan ESI-Ion trap using acetonitrile as sol-
vent. A Perkin Elmer Clarus 560S GC-MS equipped with
30 m×0.25 mm×0.25 µm BPX35 capillary column, using
the following oven temperature programme: 100 °C for
2.00 min, 150 °C/min up to 280 °C, and hold for 15.00 min.
2a: Colour Red; Yield 66%; Mp. 240 °C; Elemental
analysis, Calc. for C42H50N2O2S2Pd (785.38): C, 64.23; H,
6.37; N, 3.57; S, 8.16. Found: C, 63.89; H, 6.96; N, 3.62; S,
7.94. IR (ν/ cm−1): 1590 (C=N), 1055 (C–O), 747 (C–S),
516 (S–S), 656 (Pd–O), 447 (Pd–N). UV-Vis (λ (nm), ε
(M−1 cm−1) (DCM): 227 (54,066), 255 (40,666), 437 (9000).
FAB-mass m/z, 785.25 [Pd[C42H50N2O2S2]⌉+·, 726.60
[Pd[C38H41N2O2S2]⌉+·, 681.35[C42H50N2O2S2⌉+·.
X‑ray difraction study
X-ray difraction studies were performed at 200 K using a
Bruker Kappa Apex II difractometer with graphite mono-
chromated Mo Kα radiation (λ=0.71069 A). APEXII was
used for the data collection and SAINT for cell refnement
and data reduction [19]. Data were corrected for absorp-
tion effects using the multi-scan method in TWINABS
[19]. The structures were solved by dual methods with
SHELXT-2018/2 [20] and refned by least squares proce-
dures using SHELXL-2018/3 [21], with SHELXLE [22]
as a graphical interface. Three refections with large dif-
ferences between their observed and calculated intensities
were omitted. All non-hydrogen atoms were anisotropically
refned. Carbon-bound hydrogens were placed in calculated
positions and were included in the refnement in the riding
model approximation, with Uiso(H) set to 1.2Ueq(C). The
hydrogen atoms of the non-disordered methyl groups were
allowed to rotate with a fxed angle around the C–C bond to
best ft the experimental electron density (HFIX 137 in the
SHELXL programme [21]) with Uiso(H) set to 1.5Ueq(C).
The hydrogen atoms of the disordered methyl groups were
Synthesis
The synthesis, characterisation and X-ray crystal struc-
tures of N,N’-bis(5-tert-butylsalicylidene)diphenyl disul-
phide-2,2’-diamine 1 has been reported earlier [18]. The
NMR data for the diferent compounds are given in Table 1.
N,N′‑ Bis(3,5‑tert‑butylsalicylidene) diphenyl
disulfde‑2–2′‑diamine, 2
Bis(2-aminophenyl) disulfde (1 mmol) and 3,5-bis(tert-
butyl)-2-hydroxybenzaldehyde (2 mmol) were stirred in
absolute ethanol (15 ml) and the resulting yellow precipi-
tate formed after 4 days was fltered and dried under vacuo.
Recrystallization with acetonitrile gave X-ray-grade crystals.
2: Colour: Yellow; Yield: (61%), Mp. 209.3–211.2 °C,
Elemental analysis, Calc. (%) for C42H52N2O2S2: C, 74.10;
1 3

Transition Metal Chemistry
1 3

Transition Metal Chemistry
placed in calculated positions. The hydrogen atoms of the
hydroxyl groups were allowed to rotate with a fxed angle
around the C–O bond to best ft the experimental electron
density (HFIX 147 in the SHELXL programme [21]), with
Uiso(H) set to 1.5Ueq(O).
X-ray crystallography. The palladium complexes 1a–2a
were obtained on reacting the Schiff bases 1–2 with
PdCl2(CH3CN)2, in acetonitrile (Scheme 1). Both complexes
were found to be air stable and were soluble in DCM, chlo-
roform and DMSO. Suitable single crystals could not be
grown for the complexes. However, the obtained elemental
analyses and 2D NMR spectra were in good agreement with
the suggested structures where, one mole of the Schif bases
containing N2O2 donor atoms are co-ordinated to the pal-
ladium centre with no S–S bond cleavage.
General procedure for Mizoroki‑Heck reaction
Aryl halide (1 mmol), octadecane (1 ml, 50 mg/ml), methyl
acrylate (2 mmol), base (2.4 mmol), 1a/2a (1 mmol %)
and DMF (5 ml) were heated to 120 ºC in an oil bath in a
closed boiling tube for 18 h. 1 ml of the reaction mixture was
diluted to 5 ml with DCM and was analyzed by GC.
Crystal structure of 2
The molecular structure of 2 with atom labelling is given
in Fig. 1. Crystallographic data, data collection param-
eters and details of the refnements are given in Table S1.
The crystal studied was refned as a two-component twin
(BASF= 0.282(2)). One of the tert-butyl groups has rota-
tional disorder around the C–C bond with ratio 0.56:0.44.
The disulfde single bond length was found to be 2.025(2) Å
(S1–S2) with bond S2–S1–C22 and S1–S2–C32 angles of
104.0(2)° and 104.8(2)°, respectively. The C=N bond dis-
tance of 2 is 1.265(7)–1.271(7) Å which is in accordance
to the reported C=N bond length in salicylaldiminato [23].
The torsion angles of C22–S1–S2–C32, S2–S1–C22–C21,
S2–S1–C22–C23, S1–S2–C32–C31 and S1–S2–C32–C33
were found to be 80.8(3)°, 173.4(4)°, −4.4(5)°, −175.7(4)°
and 2.6(5)°, respectively. One of the salicylideneaniline
groups is planar with a least square plane rms deviation
of 0.0177 through the non-hydrogen atoms. In contrast,
the phenyl ring of the other salicylideneaniline is rotated
by 54.9(2) ° at N2. Strong intra-molecular bonding of
General procedure for Suzuki–Miyaura reaction
Iodobenzene (1.0 mmol), phenyl boronic acid (1.5 mmol),
K2CO3 (2.0 mmol), 1a/2a (1 and 2 mmol%), octadecane as
internal standard (0.3 mmol) in DMF (5 mL) were heated in
a sealed tube in an oil bath of 120 °C for 18 h. The reaction
mixture was diluted with DCM to 2 ml and analyzed using
GC-MS.
Results and discussion
Synthesis
The suitable single crystals of N,N’-bis(3,5-tert-butyl-
salicylidene) diphenyl disulfide-2-2’-diamine, 2 was
obtained by slow evaporation of acetonitrile solution over
a period of one week and its structure was determined by
R1
4
R2
3
5
H
C
13
2
6
R1
8
12
N
CH
Pd
O
1
R2
N
7
OH
S
11
O
S
9
S
HO
10
R2
H
C
S
N
N
PdCl2(CH3CN)2
R1
C
H
R2
R1
1a = R1 = t-butyl; R2 = H
2a = R1 = R2 = t-butyl
Scheme 1 Formation of the palladium complexes 1a & 2a
1 3

Transition Metal Chemistry
Fig. 1 Asymmetric unit of 2. Ellipsoids drawn at 50% probability and minor disordered components are not shown
O1–H1A…N1 and O2–H2A…N2 was observed with HN
distance of 1.88 Å and angles of 148° and 145°, respec-
tively. Inter-molecular interactions include two π…π ring
interactions between the rings C11–C16 and C21-C26 of
length 3.903(4) Å and slippage of 1.814 and 1.727 Å. Two
long inter-molecular C–H…π ring interactions of 2.88 and
2.96 Å occur. Selected bond lengths, bond angles, torsion
angles and hydrogen-bonding data are presented in Tables
S2 and S3 respectively.
the complexes. The tertiary butyl groups in the metal com-
plexes shifted upfeld due to the anisotropic efect triggered
by close proximity to the aromatic rings [26].
In the 13C NMR spectra of complexes, the azomethine
carbons shifted upfeld compared to the free ligands, while
the tertiary phenolic carbon shifted downfeld.
The complete assignment of diferent protons and car-
bons in 1a and 2a is based on 2D NMR data (Table 1). The
chemical shift values of diferent carbons are based on the
HSQC spectra. The chemical shift value of carbons 2 and
6 is established via the HMBC correlations from H4 while
that of C3 and C7 via the correlation with H5. In addition,
the imine proton is found to correlate with chemical shift
values of C9, C13, C2 via 3C–H and weak interactions with
C8 via 2C–H. The COSY, HSQC and HMBC spectra of 1a
are illustrated in Fig. 2. (i, ii, iii).
Spectral data
In the IR spectra of the free ligands (1, 2), the phe-
nolic OH appeared as medium peak at 2978–2999 and
2952–2959 cm−1 which suggested strong inter-molecular
hydrogen bonding between OH and the nitrogen of the
azomethine group [18]. The symmetric and asymmetric
stretching vibration bands of tert-butyl appeared in the
region of 2867–2904 cm−1. The characteristic stretching
frequencies at 1607, 738 and 1612 cm−1, 748 cm−1 for the
azomethine υ(C=N) and υ(C–S) in 1 and 2, shifted to 1589,
1590 cm−1 [24] and 742, 747 cm−1 in the complexes 1a and
2a due to coordination of nitrogen to the metal centre. The
peaks at 684, 635 and 443, 424 cm−1 were due to Pd–O and
Pd–N respectively [25].
The molecular ion peak of 2a at m/z 787.2582 (785.3593)
corresponded to the molecular formula Pd[C42H50N2O2S2].
The fragment ion at m/z 681.3579 (680.9376) corresponded
to the free ligand (2) indicating no cleavage of the S–S bond.
The Schif bases 1 and 2 exhibited three intense absorp-
tion bands at 230 and 228, 269 and 286 nm, 352 and 364 nm
due to intraligand π→π* and n→π* charge transfer of the
aromatic rings and imine groups, respectively, in the elec-
tronic spectroscopy. Unlike, many other aryl salicylaldimine
derivatives there was no absorption above 400 nm suggest-
ing that 1 and 2 both exist only in enol-imine tautomer form
at room temperature [18, 27]. The three absorption bands
shifted to 227, 255 and 303 nm in the palladium complexes.
The disappearance of the peaks at δ 12.80, 13.17 ppm
in the 1H NMR spectra of 1a and 2a indicated deprotona-
tion and coordination of oxygen atom to palladium. The
azomethine protons at 8.61, 8.64 ppm shifted upfeld in
1 3

Transition Metal Chemistry
Fig. 2 (i) COSY spectra of 1a;
(ii) HSQC spectra of 1a; (iii)
HMBC spectra of 1a
ppm
6.2
(i)
6.4
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2
ppm
ppm
110
115
120
125
130
135
140
145
150
155
(ii)
160
8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2 6.1
ppm
ppm
(iii)
115
120
125
130
135
140
145
150
155
160
165
8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2
ppm
1 3

Transition Metal Chemistry
The metal to ligand charge transfer occurred at ~ 430 nm,
which was indicative of a square-planar geometry [27].
from 120 to 170 °C (Table 2). On using polar solvents such
as water, ethanol, no conversion was observed.
Higher conversions for the product were obtained for 2a
compared to 1a as catalyst due to the increase in the num-
ber of bulky tertiary butyl groups in the structure which are
presumed to enhance the oxidative and reductive steps [28].
1a and 2a were not found to be efcient catalyst for the
coupling of aryl iodide/4-bromobenzaldehyde and phe-
nylboronic acid (Suzuki reaction) and the maximum yield
obtained was only 44% (Table 3).
Catalytic activity
Palladium-catalyzed C–C bond-forming reactions are impor-
tant for the synthesis of biphenyls, which are important
intermediates for many organic molecules. Hence, 1a and 2a
were explored as catalyst in the Mizoroki-Heck and Suzuki-
Miyaura reactions. 2a was employed to explore the various
parameters such as catalyst loading, time, temperature, base
system and solvent.
The phenyl acrylates were obtained in moderate yields
in Mizoroki-Heck reactions using methyl acrylate and aryl
iodide/4-bromobenzaldehyde. When the reaction was carried
out using 0.1–2 mmol %, the yield of the product increases
from 55 to 75% (entry 2–4). No signifcant diference was
observed on increasing the catalyst loading from 1 to 2 mmol
%; hence, further reactions were carried out using 1 mmol %
catalyst loading. The optimum reaction time for the reaction
was 18 h and Na2CO3 as base. When the reaction was carried
out at 50 and 70 °C, no product was formed. On varying the
temperature, the yield increases up to 70% but no signifcant
increase was observed on further increasing the temperature
Conclusion
In this work, hexadentate Schif bases and their palladium
complexes were synthesised and characterized fully using
2D NMR spectra. Palladium(II) complexes have square-
planar geometry co-ordinated to nitrogen and oxygen with-
out S–S bond cleavage-bearing bulky substituents on the
aromatic ring and act as catalyst for Mizoroki-Heck and
Suzuki-Miyaura reactions. Good conversions were obtained
in Mizoroki-Heck reaction when carried out at 120 °C in
Na2CO3 as base. However, both the complexes were not that
efective for the conversion in Suzuki-Miyaura reaction. 2a
Table 2 Catalytic activity of palladium complexes in Mizoroki-Heck reaction
COO2Me
R
X
COO2Me
R
R=H, CHO; X = I, Br
Entry
Aryl halide
Catalyst
Catalyst loading Time (h)
(mmol %)
Temp(°C)
Base
Solvent
Yielda (%)
TONa,b
1
2
3
4
5
6
7
8
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
4-Bromobenzaldehyde
4-Bromobenzaldehyde
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
1a
0.1
1
2
1
1
1
1
1
1
1
1
1
1
18
18
18
24
18
24
18
18
18
18
18
18
18
120
120
120
120
120
120
100
120
140
170
120
120
120
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaOH
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
55
70
75
68
77
48
36
71
69
54
45
67
58
443
62
33
62
86
65
28
65
55
47
45
60
30
9
10
11
12
13
K2CO3
Na2CO3
Reaction conditions: Aryl halide (1.0 mmol), methyl acrylate (2.0 mmol), base (2.4 mmol), solvent (5 mL)
aThe reported values are the average of those of two samples
bTON is given as moles of product per mole catalyst
1 3

Transition Metal Chemistry
Table 3 Catalytic activity of complexes in Suzuki-Miyaura reaction
B(OH)2
+
X
R
R
X= I ,Br
R=H, CHO
Entry
Aryl halide
Catalyst
Catalyst loading Time (h)
Temp (°C)
Base
Solvent
Yielda (%)
TONa,b
(mmol %)
1
2
3
4
5
6
7
8
9
10
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
4-Bromobenzaldehyde
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
2a
2a
2a
2a
2a
2a
2a
2a
1a
1a
0.1
1
1
1
1
1
1
1
1
18
18
5.5
24
18
18
18
18
18
18
120
120
120
120
120
100
120
120
120
120
K2CO3
K2CO3
Na2CO3
Na2CO3
K2CO3
K2CO3
NaOH
Na2CO3
K2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
34
44
18
41
1
39
3
40
28
30
3
43
18
41
1
40
3
41
28
15
2
Reaction conditions: Aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), base (2 mmol), solvent (5 mL)
aThe reported values are the average of those of two samples
bTON is given as moles of product per mole catalyst
Hernández-Garrido JC, Boronat M, Leyva-Pérez A, Corma A
(2019) J Am Chem Soc 141(5):1928–1940
was a more efcient catalyst compared to 1a for both reac-
tions due to the presence of two tert-butyl groups.
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lographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
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