N-donor-stabilized Pd(II) species supported by sulphonamide-azo ligands: Ligand architecture, solvent co-ligands, C–C coupling

Article abstract of DOI:10.1016/j.molstruc.2019.127030

In this report, a series of synthetically affordable phosphine-free ligands (L1 – L4) of the form RSO₂–NH–Ph–N[dbnd]N–Ph–NH–SO₂R were prepared and examined as organic ligands for stabilizing palladium active centers; R = methyl, tolyl or triiso-propylphenyl. Palladium complexes, which were obtained in varying coordination environments as well as with varying complementary co-ligands (water, acetonitrile or pyridine), have been subjected to Suzuki and Heck coupling experiments in order to study molecular level ligand effects on preferred catalyst settings. The appreciable coupling activities for Suzuki and Heck coupling with functional group tolerance were recorded for palladium species generated from the chelate ligands. Results show that, despite the tridentate chelation characteristics of these azo-benzene ligands, the introduction of bulky units at the sulfonyl groups enabled generation of active palladium species with high turnover frequencies; e.g. 5040 h^?1 (84% yield) within 5 min at 0.2 mol % loading of Pd.L2.py in only water as solvent. A correlation between catalytic efficiencies and the bulkiness of the coordinated co-ligand was obtained. However, while Suzuki coupling activity increased with increase in co-ligand sizes of the preformed complexes (i.e. water < acetonitrile < pyridine), the pyridyl co-ligand turned out to be very unfavourable for Heck coupling where the acetonitrile-complemented complexes possessed the higher activities. Therefore, it could be concluded that the best catalyst setting for Suzuki coupling may not be the best for Heck reaction.

Full text of DOI:10.1016/j.molstruc.2019.127030

Journal of Molecular Structure 1199 (2020) 127030
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
N-donor-stabilized Pd(II) species supported by sulphonamide-azo
ligands: Ligand architecture, solvent co-ligands, CeC coupling
,
b
a
b
Hammed Olawale Oloyede a , Joseph Anthony Orighomisan Woods , Helmar Gorls ,
€
Winfried Plass b **, Abiodun Omokehinde Eseola b
a Inorganic Chemistry Unit, Department of Chemistry, University of Ibadan, Ibadan, Nigeria
,
, c, *
b
€
Institut fü;r Anorganische und Analytische Chemie, Friedrich-Schiller-Universitat Jena, Humboldtstraße 8, 07743, Jena, Germany
c Materials Chemistry Group, Department of Chemical Sciences, Redeemer's University Ede, Osun State, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history:
In this report, a series of synthetically affordable phosphine-free ligands (L1 e L4) of the form RSO2eNH
ePheN]NePheNHeSO2R were prepared and examined as organic ligands for stabilizing palladium
active centers; R ¼ methyl, tolyl or triiso-propylphenyl. Palladium complexes, which were obtained in
varying coordination environments as well as with varying complementary co-ligands (water, acetoni-
trile or pyridine), have been subjected to Suzuki and Heck coupling experiments in order to study
molecular level ligand effects on preferred catalyst settings. The appreciable coupling activities for Suzuki
and Heck coupling with functional group tolerance were recorded for palladium species generated from
the chelate ligands. Results show that, despite the tridentate chelation characteristics of these azo-
benzene ligands, the introduction of bulky units at the sulfonyl groups enabled generation of active
palladium species with high turnover frequencies; e.g. 5040 hꢀ1 (84% yield) within 5 min at 0.2 mol %
loading of Pd.L2.py in only water as solvent. A correlation between catalytic efficiencies and the bulk-
iness of the coordinated co-ligand was obtained. However, while Suzuki coupling activity increased with
increase in co-ligand sizes of the preformed complexes (i.e. water < acetonitrile < pyridine), the pyridyl
co-ligand turned out to be very unfavourable for Heck coupling where the acetonitrile-complemented
complexes possessed the higher activities. Therefore, it could be concluded that the best catalyst
setting for Suzuki coupling may not be the best for Heck reaction.
Received 18 June 2019
Received in revised form
9 August 2019
Accepted 2 September 2019
Available online xxx
Keywords:
Catalyst design
Ligand effects
Suzuki-Miyaura
Heck-Mizoroki
Homogeneous catalysis
CeC coupling
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
Although Suzuki-Miyaura reactions could be carried out under mild
reaction conditions [25,26], Heck reactions are commonly per-
Palladium-catalyzed cross couplings of organic substrates has
become a dominant synthetic tool in many industrial and academic
fields associated with routine organic chemistry syntheses [1e16].
The joint Nobel Prize award in chemistry to Suzuki, Heck and
Negishi in 2010 highlights the importance of palladium-mediated
cross coupling catalysis. Suzuki-Miyaura and Heck-Mizoroki
coupling methods [17e20] remain the most frequently employed
methods among the various carbon-coupling techniques [21e24].
formed at high temperatures, longer reaction durations, high
catalyst loadings in the presence of promoters and, in some cases,
under inert atmosphere [27e31].
Substantial efforts have been committed to the design and
syntheses of palladium compounds as precatalysts for various
research reasons [23,32] and with diverse ligand designs explored
[33e36]. As a result, several highly active palladium pre-catalysts,
which are often supported by N-heterocyclic carbene (NHC) and
phosphine ligands [22,37e41] as well as imine-based ligands, have
already been commercialized (Scheme 1) [40,42,43]. However,
many of these NHC- and phosphine-based palladium complexes
are commonly complemented by cyclometallating C^N or allylic
organometallic co-ligands as well as halides [39]. In addition to
being expensive, these commercialized precatalysts are confronted
with air and moisture sensitivity problems [44]. Therefore,
* Corresponding author. Present Address: Institut für Anorganische und Analy-
€
tische Chemie, Friedrich-Schiller-Universitat Jena, Humboldtstr. 8, D-07743, Jena,
Germany.
** Corresponding authors. Institut für Anorganische und Analytische Chemie,
€
Friedrich-Schiller-Universitat Jena, Humboldtstr. 8, D-07743, Jena, Germany.
E-mail addresses: sekr.plass@uni-jena.de (W. Plass), biodun.eseola@uni-jena.de,
bioduneseola@hotmail.com (A.O. Eseola).
https://doi.org/10.1016/j.molstruc.2019.127030
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
2.2. Syntheses of the ligands
O
2,2′-Diaminoazobenzene (DAB): [56] o-Phenylenediamine
(15.00 g, 138.8 mmol) and KO2 chunks (15.00 g, 211.0 mmol) were
weighed into 1 L three-necked round-bottomed flask. The
N
N
a
N
N
N
N
O
Pd
Pd
mixture was dissolved in dry toluene (650 mL) and stirred under
nitrogen at room temperature. After two days, another 7.5 g of KO2
chunks were carefully added and the reaction stirred for further 2
days. Although the reaction mixture still showed traces of the
starting material, the remaining KO2 was carefully quenched by
slow addition of water (500 mL) followed by further stirring for
additional 30 min. The organic layer was separated and the aqueous
layer extracted using ethyl acetate (500 mL). The combined organic
layers were washed with water (4 x 200 mL) and dried over MgSO4.
The solution was concentrated under reduced pressure and the
residue was purified by chromatography using silica gel and 1 n-
hexane: 4 ethyl acetate mixture. The DAB was obtained as red
crystals. Yield (5.1 g, 34%). Selected IR data (ATR, cmꢀ1):v 3458m,
3354m, 3057w, 1599s, 1573s, 1476s, 1455s, 1304s, 1254s, 1136s,
739vs. 1H NMR (400 MHz, DMSO‑d6): dppm 7.66 (d, J ¼ 8.1 Hz, 2H),
7.13 (dd, J ¼ 11.1, 4.0 Hz, 2H), 6.85 (d, J ¼ 8.1 Hz, 2H), 6.61 (t,
Pd
O
Cl
O
CX 32
CX 12
Cl
NH2
Pd
NH
P
P
Fe
Pd
P
O
O
S
O
O
J ¼ 7.5 Hz, 2H), 6.38 (s, 4H). 13C NMR (101 MHz, DMSO‑d6):
d 145.70,
RuPhos-Pd-G2
DPPF Pd G4
137.15, 131.69, 122.33, 117.21, 116.11. MS (EI) m/z 212 (Mþ, 100%):
O
183, 169, 120, 92, 65. Anal. Calc. for C12H12N4: C, 67.90; H, 5.70; N,
Scheme 1. Examples of highly active and commercially trademarked pre-catalysts.
(i) Azo-dye amine precursor
H2N
exploring simpler and more stable N-donor pre-catalysts are
considered necessary.
NH2
KO2 / Toluene / N2
r.t. 4 days
N
N
The rigidity and bulkiness with chelation characteristics of li-
gands are themes that have been associated with catalyst effi-
ciencies in palladium-catalyzed CeC couplings [45e47].
Furthermore, the azo-benzenes containing the azo- (eN]Ne)
functional group, which are characteristically photo-switchable
between the trans- and cis-isomers [48e50], are hardly rarely
seen in palladium coordination chemistries [51]. Consequently,
alkyl/aryl-substituted sulfonyl groups have been used to function-
alize 2,2'-(diazene-1,2-diyl)dianiline in this study [52e55].
We herein present the results of the synthesis, structural char-
acterization and CeC coupling activities of palladium complexes
based on structurally and electronically varied sulfonyl-substituted
2,2'-(diazene-1,2-diyl)dianiline ligands (Scheme 2 (iii)). In situ
catalytic activity studies of the prepared ligands were also tested. In
view of desires to promote green chemistry, couplings in water
were mainly considered [19,39].
NH2
o-Phen
NH2
DAB
(ii) Amine-sulfonyl chloride condensation
O
R
S
H2N
N
H
O
O
Cl S R
O
Reflux/Pyridine
0 - 120 oC
N
N
N
+
N
H
N
O
NH2
S
R
O
(iii) Studied azo-dye amine ligands
O
O
S
S
NH N
N
NH
O
O
O
O
HN
N HN
N
2. Experimental
S
S
O
O
L2
L1
2.1. General information
All starting materials for synthesis as well as substrates for the
catalytic experiments were obtained commercially as reagent grade
and used as supplied without further purification. Ligands and
ligand intermediates were either purified on silica gel columns or
recrystallized to exclude impurities. All air sensitive reactions were
carried out under nitrogen inert atmosphere using a standard
Schleck technique. IR spectra were measured with a Bruker
Equinox FT-IR spectrometer equipped with a diamond ATR unit in
the range of 4000 ꢀ600 cmꢀ1. Elemental analyses were carried out
on Leco CHNS-932 and El Vario III elemental analyzers. Mass
spectrometry (MS) spectra were measured with a Bruker MAT SSQ
710 spectrometer. 1H and 13C NMR spectra were recorded with a
Bruker AVANCE 400 MHz spectrometer.
O
S
N
N
NH2
NH
O
N
N
O
S
HN
O
HN
O
S
O
L4
L3
Scheme 2. (i e ii) Synthetic schemes and (iii) products of synthesized chelating
ligands.

H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
3
26.40%. Found: C, 67.79; H, 5.69; N, 26.38%.
J ¼ 8.3 Hz, 2H), 7.36 (t, J ¼ 7.6 Hz, 2H), 7.17 (s, 2H), 7.12 (t, J ¼ 7.6 Hz,
2H), 4.31 (hept, J ¼ 6.7 Hz, 4H), 2.90 (dt, J ¼ 13.8, 6.9 Hz, 2H), 1.25 (d,
J ¼ 6.9 Hz, 12H), 1.21 (d, J ¼ 6.7 Hz, 24H). 13C NMR (101 MHz, d6-
(E)-N,N'-(diazene-1,2-diylbis(2,1-phenylene))dimethane-
sulfonamide (L1): Methanesulfonyl chloride (1.37 mL, 17.67 mmol)
was added dropwise to a solution of DAB (1.25 g, 5.89 mmol) in
20 mL of pyridine at 0 ꢁC while stirring in a 50 mL round-bottomed
flask. The reaction mixture was then stirred at room temperature
for 12 h and subsequently concentrated under reduced pressure.
Ethanol (5 mL) was added to the residue, filtered and washed with
chloroform (5 mL) followed by ethanol (10 mL). L1 was obtained as
a yellow powder. Yield (1.83 g, 84%). Mp ¼ 264 ꢁC. Selected IR data
(ATR cmꢀ1): v 3458m, 3360m, 3057w, 3025w, 1598s, 1570s, 1476m,
1304s, 1217s, 1137s, 739vs. 1H NMR (400 MHz, DMSO‑d6): dppm 9.83
(s, 2H), 8.00 (d, J ¼ 8.0 Hz, 2H), 7.60 (dt, J ¼ 15.2, 7.7 Hz, 4H), 7.35 (t,
CDCl3):
d 153.53, 150.66, 137.89, 135.44, 133.05, 132.28, 124.13,
123.10, 122.93, 117.68, 34.12, 29.81, 25.61, 24.74, 23.47. MS (EI) m/z
744 (Mþ, 40%): 477, 371, 267. Anal. Calc. for C42H56N4O4S2: C, 66.90;
H, 7.62; N, 7.43; S, 8.50%. Found: C, 67.16; H, 7.53; N, 7.50; S, 8.32%.
2.3. Syntheses of palladium complexes
Pd.L1.w: L1 (30 mg, 0.08 mmol) and palladium(II) acetate
(18 mg, 0.08 mmol) were added together in chloroform (2 mL). The
clear solution was then precipitated by addition of n-hexane to
obtain Pd.L1.w as brown powder. Yield (20 mg, 51%). Mp ¼ 299 ꢁC.
Selected IR data (ATR cmꢀ1): v 3380w (br), 3043w, 1590m, 1572w,
1543w, 1475m, 1292vs, 1220vs, 1162m, 1119vs, 954vs, 844vs, 761vs,
627vs. MS (EI) m/z 490 (Mþ, 7%), 494 (Mþ ꢀ H2O þ Na), (473
(Mþ ꢀ H2O, 20%), 391 (Lþ þ Na). Anal. Calc. for C14H16N4O5PdS2.
H2O.½CH3COOH: C, 33.43; H, 3.74; N, 10.40; S, 11.90%. Found: C,
33.26; H, 3.34; N, 10.17; S, 11.64%.
Pd.L1.acn: L1 (30 mg, 0.08 mmol) and palladium(II) acetate
(18 mg, 0.08 mmol) were added together and dissolved in aceto-
nitrile (1 mL) layered with diethylether (1 mL). The clear solution
was allowed to stand for 1 day. The microcrystalline product was
filtered, washed with acetonitrile and air-dried to give Pd.L1.acn as
a purple solid. Yield (0.03 g, 74%), Mp ¼ 293e295 ꢁC.Selected IR
data (ATR cmꢀ1):v 2347w, 2309w, 1593m, 1570m, 1453m, 1292vs,
1118vs, 952vs, 837vs,746vs, 626vs. MS (EI) m/z 473 (Mþ eCH3CN,
20%): 392, 314, 288, 212, 176, 105, 79, 41, 28. Anal. Calc. for
J ¼ 7.2 Hz, 2H), 3.08 (s, 6H). 13C NMR (101 MHz, DMSO‑d6):
d 143.97,
137.27, 133.22, 125.57, 124.19, 118.02, 55.36. MS (EI) m/z 368 (Mþ,
45%): 289, 210, 198, 181, 154, 108, 91, 79, 65, 51. Anal. Calc. for
C
14H16N4O4S2: C, 45.64; H, 4.38; N, 15.21; S, 17.41%. Found: C, 45.58;
H, 4.42; N, 14.96; S, 17.76%.
(E)-N,N'-(diazene-1,2-diylbis(2,1-phenylene))bis(4-
methylbenzenesulfonamide) (L2): DAB (1.25 g, 5.89 mmol) and p-
toluenesulfonyl chloride (3.37 g, 17.67 mmol) were mixed in 20 mL
of pyridine inside a 50 mL round-bottom flask while stirring at
room temperature. The reaction mixture was then refluxed at
100 ꢁC for 12 h. The product was concentrated under reduced
pressure. Ethanol (5 mL) was added to the residue which was
filtered and then washed with chloroform (5 mL) and ethanol
(10 mL) to give L2 as an orange powder. Yield (2.82 g, 92%).
Mp ¼ 272 ꢁC. Selected IR data (ATR cmꢀ1): v 3265m, 3251m, 3085w,
1594s, 1481s, 1386s, 1332s, 1161vs, 1089vs, 904vs, 654vs. 1H NMR
(400 MHz, DMSO‑d6): dppm 10.17 (s, 2H), 7.67e7.54 (m, 6H), 7.51 (d,
J ¼ 3.9 Hz, 4H), 7.26 (dt, J ¼ 8.3, 4.2 Hz, 2H), 7.14 (d, J ¼ 8.0 Hz, 4H),
2.18 (s, 6H).13C NMR (101 MHz, DMSO‑d6): dppm 144.19, 143.59,
137.44, 136.57, 132.85, 129.86, 127.06, 125.72, 125.09, 117.75, 21.28.
MS (EI) m/z 520 (Mþ, 98%): 365, 347, 210. Anal. Calc. for
C16H17N5O4PdS2C, 37.40; H, 3.33; N, 13.63; S, 12.48%. Found: C,
37.79; H, 3.27; N, 13.60; S, 12.52%.
Pd.L1.py: L1 (30 mg, 0.08 mmol), palladium(II) acetate (18 mg,
0.08 mmol) and pyridine (0.1 mL) were added together in aceto-
nitrile (2 mL). Pd.L1.py was obtained as purple powder the
following day. Yield (0.04 g, 90%). Mp ¼ 220 ꢁC. Selected IR data
(ATR cmꢀ1):v 2945w, 2877w, 1590m, 1451s, 1295vs, 1239m, 1125vs,
945vs, 856vs, 753vs, 695vs. MS (EI) m/z 474 (Mþ -Py, 20%), 368 (Lþ,
15%), 289, 210, 108, 79 (Pyþ), 52, 28. Anal. Calc. for
C
26H24N4O4S2 C, 59.98; H, 4.65; N, 10.76; S, 12.32%. Found: C, 60.13;
H, 4.66; N, 10.79; S, 12.45%.
(E)-N-(2-((2-aminophenyl)diazenyl)phenyl)-2,4,6-
triisopropylbenzenesulfonamide (L3) and (E)-N,N'-(diazene-1,2-
diylbis(2,1-phenylene))bis(2,4,6-
C19H19N5O4PdS2.1.3CH3CN: C, 42.86; H, 3.81; N, 14.58; S, 10.59%.
triisopropylbenzenesulfonamide) (L4): 2, 4, 6-triiso-pro-
pylbenzenesulfonyl chloride, (3.57 g, 11.78 mmol) was added to a
pyridine (20 mL) solution of DAB (1.25 g, 5.89 mmol) in a 50 mL
round-bottom flask. The reaction mixture was refluxed at 120 ꢁC for
12 h. The mixture was concentrated under reduced pressure to
obtain the crude product. Purification was carried out on column
chromatography with silica gel, n-hexane/THF (7:3) and later
chloroform/methanol (1:1). L3 was obtained as a red powder while
yellow powder of L4 was eluted as the second product.
Found: C, 43.27; H, 3.65; N, 14.21; S, 10.45%.
Pd.L2: L2 (30 mg, 0.06 mmol) and palladium(II) acetate (13 mg,
0.06 mmol) were added together and dissolved in chloroform
(1 mL). The clear solution was layered with n-hexane to give Pd.L2
as purple crystals. Single crystal suitable for X-ray measurement
was obtained from the mother liquor. Yield (0.02 g, 54%).
Mp ¼ 325e327 ꢁC.Selected IR data (ATR cmꢀ1): v 1690m, 1590m,
L3: Yield (1.20 g, 28%). Mp ¼ 249 ꢁC. Selected IR data (ATR cmꢀ1):
v 3449m, 3290w, 2959s, 2867w, 1617s, 1599s, 1556s, 1482s, 1307s,
1226s, 1149vs, 1039vs, 908s, 753 vs, 651vs.1H NMR (400 MHz,
DMSO‑d6): dppm 10.28 (s, 1H), 7.82e7.71 (m, 1H), 7.62e7.47 (m, 1H),
7.34 (t, J ¼ 7.1 Hz, 1H), 7.30e7.23 (m, 2H), 7.19 (s, 3H), 6.89 (s, 2H),
6.85 (d, J ¼ 8.1 Hz, 1H), 6.58 (t, J ¼ 7.2 Hz, 1H), 4.06 (dt, J ¼ 13.3,
6.6 Hz, 2H), 2.99e2.75 (m, 1H), 1.17 (d, J ¼ 6.9 Hz, 6H), 1.08 (d,
J ¼ 6.7 Hz, 12H). 13C NMR (101 MHz, DMSO‑d6):
d 152.90, 151.45,
150.28, 146.11, 145.24, 137.58, 136.53, 134.47, 134.14, 133.38, 132.48,
130.74, 126.07, 125.19, 124.27, 124.18, 118.97, 117.41, 115.77, 79.64,
33.72, 29.65, 25.03, 23.80. MS (EI) m/z 478 (Mþ, 60%): 371, 357, 293,
211, 183, 120, 91. Anal. Calc. for C27H34N4O2S2: C, 67.75; H, 7.16; N,
11.71; S, 6.70%. Found: C, 67.79; H, 7.11; N, 11.72; S, 6.34%.
(L4): Yield (1.45 g, 33%). Mp ¼ 158 ꢁC. Selected IR data (ATR
cmꢀ1): v 3290s, 2960s, 2871m, 1591m, 1558m, 1479s, 1378s, 1281
vs, 1165vs, 1150vs, 1039s, 906vs, 825s, 665vs. 1H NMR (400 MHz,
d6-CDCl3): dppm 10.07 (s, 2H), 7.63 (d, J ¼ 7.8 Hz, 2H), 7.47 (d,
Fig. 1. Ortep plot of ligand L1 with thermal ellipsoids drawn at the 40% probability
level. Protons have been omitted for clarity.

4
H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
Solvent
Ligand
+
Pd(AcO)2
+
Co-ligand
Complex
rt
N
N
N
N
N
H
N
N
O
N
Pd
N
O
O
O
Pd
N
O
O
N
S
N
S
O
S
S
Pd
O
O
S
O
O
N
O
N
S
O
H
Pd.L1.w
Pd.L1.acn
Pd.L1.py
N
N
N
N
N
N
N
S
O
N
S
O
O
Pd
N
N
S
O
Pd
N
O
O
N
Pd
O
O
N
S
O
S
O
O
N
S
O
Pd.L2
Pd.L2.acn
Pd.L2.py
N
N
N
N
N
Pd
N
N
Pd
O
O
O
N
O
O
Pd
N
O
S O
N
O
S O
N
N
O
O
N
O
S O
N
S
S
S
H
H
Pd.L4.w
Pd.L4.acn
Pd.L4.py
Scheme 3. Synthetic route and compositions of the obtained palladium complexes.
1546m, 1421m, 1304s, 1228s, 1140s, 1079s, 939s, 658vs. MS (ESI) m/
Pd.L2.acn: L2 (30 mg, 0.06 mmol) and palladium(II) acetate
(13 mg, 0.06 mmol) were added together and dissolved in aceto-
nitrile (1 mL). The clear solution was layered with diethylether
(1 mL) and allowed to stand for 3 days. Pd.L2.acn was obtained as
z
C
625 (Mþ, 100%): 646 ([Mþ
26H22N4O4PdS2: C, 49.96; H, 3.55; N, 8.96; S, 10.26%. Found: C,
þ
Na], 100%). Anal. Calc. for
49.98; H, 3.48; N, 9.00; S, 10.37%.
Table 1
Crystal data and refinement details for the X-ray structure determinations of the ligand L1 and complexes Pd.L2 e Pd.L4.py.
Compound
L1
Pd.L2
Pd.L2.acn
Pd.L2.py
Pd.L4.acn
Pd.L4.py
formula
fw (g∙molꢀ1
ꢁC
crystal system
space group
a/Å
C14H16N4O4S2
368.43
ꢀ140(2)
orthorhombic
P b c n
21.2115(6)
7.8440(2)
9.6694(2)
90
C26H22N4O4PdS2
625.00
C28H25N5O4PdS2
666.05
C31H27N5O4PdS2
704.10
ꢀ140(2)
monoclinic
P 21/c
17.1098(4)
8.1173(2)
22.6646(5)
90
107.378(1)
90
C44H57N5O4PdS2[*]
890.46[*]
ꢀ140(2)
monoclinic
P 21/c
17.6369(4)
16.4686(4)
17.0233(4)
90
112.460(1)
90
4569.44(19)
4
C47H59N5O4PdS2[*]
928.51[*]
ꢀ140(2)
monoclinic
P 21/c
17.6300(2)
17.2122(3)
17.2556(3)
90
113.666(1)
90
4795.87(13)
4
)
ꢀ140(2)
ꢀ140(2)
Triclinic
triclinic
ꢀ
P ı
ꢀ
P ı
8.9570(2)
13.1679(3)
21.3330(3)
93.815(1)
97.185(1)
99.427(1)
2452.89(9)
4
8.7606(6)
12.2968(9)
14.1200(10)
68.308(2)
74.082(2)
78.832(2)
1351.92(17)
2
b/Å
c/Å
ꢁ
ꢁ
a
/
b
/
90
90
ꢁ
g
/
V/Å3
1608.82(7)
4
1.521
3004.10(12)
4
1.557
Z
r
(g∙cmꢀ3
(cmꢀ1
)
1.692
9.69
1.636
8.86
1.294
5.42[*]
1.286
5.2[*]
m
)
3.59
8.02
measured data
data with I > 2
unique data (Rint
wR2 (all data, on F2)a)
11602
1654
1829/0.0393
0.0784
0.0317
32585
9762
11194/0.0293
0.0766
0.0355
13659
4568
5262/0.0300
0.1307
0.0478
20467
6366
6827/0.0437
0.0906
0.0453
55905
8309
10451/0.0616
0.1292
0.0664
35477
9301
10947/0.0536
0.1065
0.0515
s
(I)
)
R1 (I > 2
s
(I))a)
Sb)
1.087
1.054
1.054
1.182
1.167
1.115
Res. dens./e∙Åꢀ3
absorpt method
absorpt corr Tmin
CCDC No.
0.332/-0.359
multi-scan
0.7048/0.7456
1880299
0.923/-0.666
multi-scan
0.7043/0.7456
1880300
0.651/-0.743
multi-scan
0.5884/0.7456
1880301
0.688/-0.835
multi-scan
0.6641/0.7456
1880302
1.018/-0.863
multi-scan
0.6798/0.7456
1880303
1.106/-0.624
multi-scan
0.7102/0.7456
1880304
/
max
*
Derived parameters do not contain the contribution of the disordered solvent.
P
P
a)
b)
Definition of the R indices: R1 ¼ ð jjFoj ꢀ jFcjjÞ= jFoj; wR2 ¼ {S[w(F2o-Fc2)2]/S[w(Fo2)2]}1/2 with wꢀ1
¼
s
2(F2o) þ (aP)2þbP; P ¼ [2Fc2 þ Max(F2O]/3.
s ¼ {S[w(F2o-Fc2)2]/(NoeNp)}1/2
.

H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
5
purple crystals from which single crystal was picked for X-ray
measurement. Yield (0.03 g, 75%). Mp ¼ 316 ꢁC. Selected IR data
(ATR cmꢀ1): v 2936w, 2325w, 1674m, 1594m, 1546m,1418m,
1309m, 1229s, 1142vs, 1081vs, 912vs, 860vs, 751vs, 657vs. MS (EI)
m/z 625 (Mþ eCH3CN, 100%): 558, 467, 406, 366, 41. Anal. Calc. for
C
28H25N5O4PdS2: C, 50.49; H, 3.78; N, 10.51; S, 9.63%. Found: C,
50.77; H, 3.70; N, 10.55; S, 9.56%.
Pd.L2.py: L2 (30 mg, 0.06 mmol), palladium(II) acetate (13 mg,
0.06 mmol) and pyridine (0.1 mL) were added together and dis-
solved in acetonitrile (1 mL). The clear solution was allowed to
stand for 1 week under slow evaporation after which crystals
suitable for X-ray measurement were obtained. Yield (0.03 g, 67%).
Mp ¼ 273 4 ꢁC. Selected IR data (ATR cmꢀ1): v 1609m, 1588m,
1466m, 1214m, 1140vs, 1082vs, 910vs, 838vs, 755vs, 663vs. MS (EI)
m/z 624 ([Mþ -Py], 7%): 646 ([Mþ ꢀ Py þ Na], 100%). Anal. Calc. for
Fig. 3. Ortep plots of palladium complexes Pd.L4.acn and Pd.L4.py with thermal el-
lipsoids drawn at the 40% probability level. Protons have been omitted for clarity.
C31H27N5O4PdS2: C, 52.88; H, 3.87; N, 9.95; S, 9.11%. Found: C,
52.67; H, 3.76; N, 10.10; S, 8.98%.
Pd.L4.w: L4 (30 mg, 0.04 mmol) and palladium(II) acetate (9 mg,
0.04 mmol) were added together and dissolved in chloroform
(2 mL). The clear solution was layered with n-hexane. After 1 day,
the crystalline product was filtered, washed with hexane and air-
dried to give Pd.L4.w as a purple solid. Yield (30 mg, 86%).
Mp ¼ 210 ꢁC. Selected IR data (ATR cmꢀ1): v 3317w (br), 2951m,
2863w, 1600m, 1538w, 1475m, 1415m, 1301m, 1214s, 1119vs,
1031vs, 942vs, 819vs745vs, 629vs. MS (EI) m/z 848 (Mþ ꢀ H2O,
25%): 837, 778, 720, 671, 614, 553, 512, 447, 386, 332, 272. Anal. Calc.
for C42H56N4O5PdS2.H2O: C, 56.97; H, 6.60; N, 6.33; S, 7.24%. Found:
C, 57.25; H, 6.45; N, 6.23; S, 7.07%.
Pd.L4.acn: L4 (30 mg, 0.04 mmol) and palladium(II) acetate
(9 mg, 0.04 mmol) were added together and dissolved in acetoni-
trile (2 mL). The clear solution was allowed to stand for 3 days
under slow evaporation to obtain purple crystals suitable for X-ray
measurement. Yield (30 g, 81%). Mp ¼ 198 ꢁC. Selected IR data (ATR
cmꢀ1): v 2956m, 2863w, 2312w,1599m, 1544m, 1420s, 1295s,
1234m, 1131 m, 1033vs, 941m, 845vs, 757vs, 649vs. MS (ESI) m/z
873 [(Mþ ꢀ CH3CN)], 100%. Anal. Calc. for C44H57N5O4PdS2: C,
59.35; H, 6.45; N, 7.86; S, 7.20%. Found: C, 59.35; H, 6.41; N, 7.79; S,
7.13%.
Pd.L4.py: L4 (30 mg, 0.04 mmol), palladium(II) acetate (9 mg,
Table 2
Selected bond lengths (Å) around the metal centers of Pd.L2, Pd.L2.acn, Pd.L2.py,
Pd.L4.acn and Pd.L4.py
Bonds lengths (Å)
Pd.L2
Pd.L2.acn Pd.L2.py Pd.L4.acn Pd.L4.py
Pd1eN1
1.960(2)
2.006(4)
1.945(4)
2.041 (4)
2.037(3) 2.041(3)
1.978(3) 1.958(4)
2.018(3) 2.026(3)
2.034(3) 2.017(4)
2.013(3)
1.957(3)
2.052(3)
2.033(3)
Pd1eN2/Pd1eN3a 1.927(2)
Pd1eN4
Pd1eN5/Pd1eO1
2.023(2)
2.257(2)b 2.022(4)
a
Fig. 2. Ortep plots of complexes Pd.L2, Pd.L2.acn and Pd.L2.py with thermal ellipsoids
drawn at the 40% probability level. Protons have been omitted for clarity.
Pertains to N-azo bonds with the palladium(II) center.
PdeO bond.
b

6
H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
Table 3
Table 4
Optimization of reaction parameters for Suzuki-Miyaura coupling
Catalytic performance of the prepared palladium complexes
Br
Br
Pd.L2.acn
0.2 mol % Pd(OAc)2
+ 0.4 mol % ligand
0.2 mol %
+
+
HO
OH
B
or
B
OH
OH
HO
0.2 mol % Pd complex
5 min reflux, in water
HO
OH
OH
Entry
Complex (solvent)
Time (min)
Base
Yield (%)a
Catalyst
Temp (oC)
Yield (%)
TOF (hꢀ1
)
1
3
6
Pd.L2.acn (EtOH/H2O)
Pd.L2.acn (EtOH/H2O)
Pd.L2.acn (H2O)
10
10
10
K3PO4
K2CO3
K2CO3
83
93
92
L1 þ Pd(OAc)2
L2 þ Pd(OAc)2
L3 þ Pd(OAc)2
L4 þ Pd(OAc)2
Pd(OAc)2 alone
Pd.L1.w
Pd.L1.acn
Pd.L1.py
Pd.L2
Pd.L2.acn
Pd.L2.py
Pd.L2.py
Pd.L2.py
Pd.L2.py
100
100
100
100
100
100
100
100
100
100
50
60
70
80
100
100
100
100
5
300
23
46
63
56
3
1380
2760
3780
3360
180
Reaction conditions: Solvent (4 mL), base (1.0 mmol), time (30 min reflux), tem-
perature of oil bath (100 ꢁC), Pd-catalyst (0.2 mol%), (4-bromophenyl)methanol
(1.0 mmol),benzeneboronic acid (1.2 mmol).
Yield was determined by 1H NMR and reported to the nearest whole number.
a
7
420
78
47
66
50
57
62
75
84
67
76
77
4680
2820
3960
3000
3420
3720
3800
5040
4020
4560
4620
0.04 mmol) and pyridine (0.2 mL) were allowed to stand in aceto-
nitrile (1 mL) to obtain Pd.L4.py as purple crystals after 2 days.
Yield (0.027 g, 73%). Mp ¼ 283e285 ꢁC. Selected IR data (ATR
cmꢀ1): v 3049w, 2945s, 2858m, 1599s, 1453s, 1360m, 1293s, 1238s,
1139vs, 1071s, 841vs, 837vs, 754vs, 649vs. MS (ESI) m/z 849 ([Mþ ꢀ
Py], 20%): 766 (Lþþ Na), 743 (Lþ, 100%). Anal. Calc. for
Pd.L2.py
Pd.L4.w
Pd.L4.acn
Pd.L4.py
C
47H59N5O4PdS2: C, 60.79; H, 6.40; N, 7.54; S, 6.91%. Found: C,
60.92; H, 6.36; N, 7.82; S, 6.53%.
Reaction conditions: H2O (4 mL), k2CO3 (1.0 mmol), time (5 min reflux), oil bath
temperature (100 ꢁC), (4-bromophenyl)methanol (1.0 mmol), benzeneboronic acid
(1.2 mmol), Catalysts system: in situ, ‘Pd(OAc)2 (0.2 mol% þ ligand (0.4 mol %)’ or
precatalyst (0.2 mol %). Yield was determined by 1H NMR and reported to the
nearest whole number.
2.4. Suzuki-Miyaura coupling experiment
In a typical catalytic reaction, (4-bromophenyl)methanol (0.19 g,
1.0 mmol), benzeneboronic acid (0.15 g, 1.2 mmol), K3CO3 (0.14 g,
1.0 mmol) and preformed complex (0.2 mol % relative to (4-
bromophenyl)methanol) were added together in a 10 mL round-
bottom flask equipped with magnetic stirrer bar. In the case of in
situ, 0.2 mol % Pd(AcO)2 and 0.4 mol % ligand were used relative to
(4-bromophenyl)methanol. Water (4 mL) was added and the
mixture refluxed on an oil bath set at 100 ꢁC for the desired reaction
time. An aliquot of the reaction mixture was taken into a 10 mL
conical flask which was dried under vacuum to exclude the sol-
vents. The residue was subjected to 1H NMR measurement using
deuterated DMSO. The reactivity of the catalyst was evaluated by
comparison of the NMR signals for the methylene (eCH2e) protons
methylene signals [57].
3. Results and discussion
3.1. Syntheses and characterization of ligands and complexes
The ligand series L1 e L4 were obtained by beginning with
oxidation of o-phenylenediamine into 2,20-diaminoazobenzene
100 oC
50 oC
of (4-bromophenyl)methanol at
responding biphenyl product at
d
z 4.4 ppm with that of the cor-
z 4.6 ppm. The yields were
d
100
80
60
40
20
0
estimated by determining the integral of the product peak as a
percentage of the sum of all observed methylene signals [57].
2.5. Heck coupling catalysis experiment
For typical Heck reaction, (4-bromophenyl)methanol (0.19 g,
1.0 mmol), styrene (0.17 mL,1.5 mmol), K2CO3 (0.17 g,1.5 mmol) and
preformed complex (0.2 mol % relative to (4-bromophenyl)meth-
anol) were added together in a 10 mL round-bottom flask equipped
with magnetic stirrer bar. DMF (3 mL) and water (1 mL) were added
and the mixture refluxed on an oil bath set at 140 ꢁC for the desired
reaction time. An aliquot of the reaction mixture was taken directly
into NMR tube with deuterated DMSO and subjected to 1H NMR.
The reactivity of the catalyst was evaluated by comparison of the
NMR signals observed for the methylene (eCH2e) protons of (4-
0
10
20
30
40
50
60
Time (Minutes)
bromophenyl)methanol
d
and
z 4.4 ppm with that of the corresponding Heck product at
z 4.6 ppm. The yields were estimated by determining the integral
(4-vinylphenyl)methanol
at
Fig. 4. Catalytic time profile of precatalyst Pd.L2.py at low and high temperatures
(Reaction conditions: H2O (4 mL), K2CO3 (1.0 mmol), time (5e60 min reflux), oil bath
temperature (50 or 100 ꢁC), (4-bromophenyl)methanol (1.0 mmol), benzeneboronic
acid (1.2 mmol), Pd.L2.py (0.2 mol %)).
d
of the product peak as a percentage of the sum of all observed

H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
7
(DAB), which was then condensed with various sulfonyl chlorides
using pyridine as base as well as solvent (Scheme 2). Elemental
analyses and spectroscopic data gave good agreement with the
identity of the ligands. Presence of solvent adducts were not found
in the compounds as revealed by characterization analysis and
molecular structure of compound L1 (Fig. 1). Compound L3 with
only one sulfonyl group was isolated in addition to the targeted
compound L4.
with ligand L3 were unsuccessful. Various neutral monodentate
donor (D) species such as water, acetonitrile and pyridine occupied
the fourth coordination position except for Pd.L2, where ligand L2
provided the fourth coordination donor from the sulfonyl oxygen
accompanied by significant distortion around the square plane of
the palladium center. All the complexes were isolated as brown or
purple powders/crystals.
The palladium(II) complexes were generally obtained in good
yields by stirring palladium(II) acetate and the ligand in suitable
solvent at room temperature or by allowing the salt and ligand to
stand in given solvents (Scheme 3). Defined coordination products
were obtained for ligands L1, L2 and L4, which generally stabilized
palladium(II) as tridentate supports. Complexation experiments
3.2. X-ray characterization
Single crystals of L1 and Pd.L2.py were obtained by slow
evaporation of their acetonitrile solutions while suitable crystals of
Pd.L2.acn and Pd.L4.acn were grown by slow diffusion of dieth-
ylether into their acetonitrile solution. Pd.L2 was also crystalized
Table 5
Yields from various substrates with Pd.L2.py as precatalyst. Reaction conditions: H2O (4 mL), K2CO3 (1.0 mmol), time (5 min reflux), oil bath temperature (100 ꢁC), (4-
bromophenyl)methanol (1.0 mmol), (benzeneboronic acid (1.2 mmol), preformed complex (0.2 mol %). Yield was determined by 1H NMR and reported to the nearest
whole number. *Yield recorded after 12 h.
Substrates
Yield TOF
(%) (h-1)
61
55
88
75
3660
3300
5280
4500
HO
O
Br
B
HO
HO
HO
HO
HO
Br
Br
B
O
F
HO
HO
B
HO
HO
Br
Br
B
HO
HO
13
26
32
780
HO
O2N
B
HO
OH
NO2
1560
1920
HO
Br
COOH
Br
O
B
HO
OH
HO
Ph
B
HO
OH
trace*
36*
H
O
Cl
Cl
B
H
O
H2N
HO
HO
B
Cl
HO
OH
trace*
31*
HO
N
N
B
HO
OH
HO
B
Cl
O2N
HO
OH
19*
HO
B
Cl
NaO3S
HO
OH

8
H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
Table 6
the palladium may play important roles in the ease of stable active
site generation.
Testing of different reaction parameters on Heck C e C coupling
HO
3.3. Determining suitable Suzuki-Miyaura coupling reaction
settings
0.2 mol %
Pd.L2.Py
+
Using Pd.L2.acn as precatalyst (0.2 mol%), 10 min reflux dura-
tion and thermostated oil bath temperature of 100 ꢁC, the yields of
coupling (4-bromophenyl)methanol (1.0 mmol) and benzenebor-
onic acid (1.2 mmol) were compared in the presence of 1 mmol
each of K2CO3 and K3PO4 as well as in the presence of 4 mL each of
water and ethanol/water mixture (3:1). Table 3 shows that the
phosphate salt did not offer advantage over the carbonate and that
using only water or a mixture of water/ethanol mixture yielded no
tangible difference. Therefore, further comparison of catalyst sys-
tems via in situ ‘ligand þ Pd(OAc)2’ or the comparison of preformed
complexes were carried out using K2CO3 (1 mmol) in the eco-
friendly water (4 mL). Since very high yields were already recor-
ded at 10 min, 5 min reflux was utilized in further experiments. The
Br
HO
Solvent
Base
Temp. (oC)a
Time (hr)
Yield (%)
CH3CN/H2O (1:1)
CH3CN/H2O (1:1)
CH3CN/H2O (1:1)
CH3CN/H2O (1:1)
CH3CN/H2O (1:1)
CH3CN/H2O (1:1)
DMF/H2O (3:1)
DMF/H2O (3:1)
EtOH
K2CO3
K3PO4
Na2CO3
NaOH
(NH4)2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
100
100
100
100
100
100
140
140
80
100
120
100
80
1
1
1
1
1
1
1
6
1
1
1
"
28
13
17
8
6
Trace
40
74
12
8
10
16
12
EtOH/H2O (3:1)
Toluene
other parameters remained unchanged: 0.2 mol
% palladium
loading, 1.0 mmol aryl bromide, 1.2 mmol boronic acid and oil bath
Water
temperature of 100 ꢁC.
Acetonitrile
"
a
Temperatures presented are thermostated oil bath temperatures. Reaction
conditions: Solvent: (4 mL), base (1.5 mmol), (4-bromophenyl)methanol
(1.0 mmol), styrene (1.5 mmol), preformed complex (0.2 mol %). Yield was deter-
mined by 1H NMR and reported to the nearest whole number.
3.4. Effect of coordination environments on Suzuki coupling (by in
situ or precatalysts)
Using water (4 mL), K2CO3 (1.0 mmol), ‘0.2 mol
%
Pd(OAc)2 þ 0.4 mol % ligand’, oil bath temperature of 100 ꢁC and
5 min' reflux duration, the results of in situ catalysis, which is
presented in Table 4, generally showed that increasing bulkiness of
the sulfonyl-substituents correlates with increasing catalyst per-
formance (i.e. yields followed the trend L1 (5%) < L2 (23%) < L3 (5
46) < L4 (63%)). Therefore, it could be concluded that the ability of
the ligands to form strong and stable chelate decreases from ligand
L1 to L4. In fact, ligands L1, L2 and L3 appear to chelate via in situ in
such a manner that limits the availability of palladium active cen-
ters when their yields are compared with the yield for coupling in
the presence of Pd(OAc)2 alone.
by slow diffusion of n-hexane layered over its chloroform solution
while suitable crystals of Pd.L4.py were obtained by slow evapo-
ration of its recrystallization filtrate in acetonitrile. Structural
refinement data and processing parameters were collected in
Table 1 and their Ortep plots are presented in Figs. 1e3.
Crystal structures of the complexes revealed tridentate N^N^N
ligand chelation to the palladium center through one N-azo and
two sulfonamide-substituted amine N-atoms such that one five-
membered and one six-membered chelate rings occur in each of
the complexes. Each complex exhibits a distorted square planar
geometry around the palladium(II) metal center and bear a neutral
co-ligand N- or O-donor (D) in the fourth coordination position
except for Pd.L2 (Figs. 2 and 3). Bond lengths around the coordi-
nation environment for the structures of Pd.L2, Pd.L2.acn,
Pd.L2.py, Pd.L4.acn and Pd.L4.py, which are summarized in
Table 2, are within expected values [58] while the azo (eN]Ne)
bond length of 1.264 Å in L1 also fall within the normal range
[55,59]. The shorter bond lengths between all the N-azo donors and
the palladium(II) centers observed in all analyzed crystals (i.e.
Table 2; PdeN3 for Pd.L2, Pd.L2.acn and Pd.L4.py; Pd1eN2 for
Pd.L2.py and Pd.L4.acn) indicate that the azo nitrogen donors
possess stronger donor strengths than the sulfonyl-substituted
nitrogen donors. This observation is attributable to the electron-
withdrawing influence of the sulfonyl-substituents.
It could be observed that the bond lengths between the flanking
sulfonamide-substituted N-donors and the metal center is gener-
ally not sensitive to steric difference between acetonitrile and
pyridine co-ligands. Adding the two bonds for Pd.L2.acn gives
z4.047 while same addition for Pd.L2.py gives z4.055. On the
other hand, adding the two bonds for Pd.L4.acn gives z4.067
while same addition for Pd.L4.py gives z4.055, which indicates
that the N-donors of the sulfonamides are slightly closer to palla-
dium in the pyridine substituted analogue than when acetonitrile is
coordinated (Table 2). Therefore, difference between acetonitrile-
and pyridyl-coordinated counterparts could be more electronic in
nature and such functional difference in the coordination sphere of
The precatalysts generally performed better than the corre-
sponding in situ approach. However, a similar trend of ligand
Table 7
Catalytic performance of prepared complexes on Heck C e C coupling
HO
HO
0.2 mol %
Pd complex
+
DMF/H2O (3:1)
Br
Catalyst
Yield (%)
TOF (hꢀ1
)
Pd.L1.w
27
41
29
24
84
48
55
72
20
45
68
48
40
139
80
92
120
33
Pd.L1.acn
Pd.L1.py
Pd.L2
Pd.L2.acn
Pd.L2.py
Pd.L4.w
Pd.L4.acn
Pd.L4.py
Reaction conditions: DMF/H2O (3 mL: 1 mL), K2CO3 (1.5 mmol), time (3 h reflux)
oil bath temperature (140 ꢁC), (1.5 mmol), (4-bromophenyl)methanol (1.0 mmol),
styrene (1.5 mmol), preformed complex (0.2 mol %). Yield was determined by 1H
NMR and reported to the nearest whole number.

H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
9
influence was observed for the precatalysts as in the in situ catalyst
generation. The complexes bearing the methyl-substituted L1 dis-
played poorer catalyst efficiency then complexes bearing the
bulkier aromatic substituents (Table 4).
the palladium coordination sphere by the studied ligand back-
bones, the more favoured the palladium catalysts is. The difference
observed between Pd.L2 (yield ¼ 47%) without co-ligand and
Pd.L2.acn (yield ¼ 66%) in which acetonitrile is present as co-ligand
also supports the importance of bulkier co-ligands towards catalyst
improvement. Such steric demands probably create an easy route
for the decomposition of the catalyst towards the formation of the
active specie. [60].
Considering the influence of the co-ligand for complexes of L1,
L2 and L4, it is noteworthy that the catalytic productivity follows
the trend pyridyl > acetonitrile > water. The pyridyl-complemented
complexes all performed better than their corresponding analogues
and even its presence in Pd.L1.py made a tremendous difference to
transform yields from 3% for Pd.L1.w and 7% for Pd.L1.acn to 78%
for Pd.L1.py. The high turnover frequency (TOF) values of 4560 hꢀ1
for Pd.L1.py, 5040 hꢀ1 for Pd.L2.py and 4620 hꢀ1 for Pd.L4.py (420
Using Pd.L2.py, substantial activity was recorded at 50 ꢁC, which
increased with increasing temperature (Fig. 4, Table 4). Further-
more, increasing time at 50 ꢁC and 100 ꢁC also showed a very high
initial rate as well as living catalyst behaviour.
h
ꢀ1) highlight the importance of the bulkier pyridyl as co-ligand. In
general, it could be concluded that, the greater steric crowding of
Table 8
Other substrates in the Heck coupling. Reaction conditions: DMF/water (3:1), K2CO3 (1.5 mmol), time (3 h reflux), oil bath temperature (140 ꢁC), aryl halide (1.0 mmol), olefin
(1.5 mmol), preformed complex (0.2 mol %). Yield was determined by 1H NMR and reported to the nearest whole number. *Yield recorded after 12 h.
Substrates
HO
Yield (%)
30
Br
Br
Br
+
+
+
35
66
22
O
HO
HO
Cl
N
Br
+
HO
O
47
Br
+
Cl
53
54
Br
+
Cl
Br
+
Cl
Cl
Cl
44
N
Br
+
42
O2N
Br
Cl
+
41*
O
HO
Cl
+
Cl
43*
42*
+
Cl
H2N
HO
+
Cl
Cl
44*
+
O2N
Cl
Cl

10
H.O. Oloyede et al. / Journal of Molecular Structure 1199 (2020) 127030
3.5. Study of substrate scope using Pd.L2.py as precatalyst
4. Conclusion
The precatalyst Pd.L2.py was deployed towards sterically and
electronically varied substrates under the developed reaction
conditions and the results are presented in Table 5. The catalytic
results showed that pre-catalyst generally displayed functional
group tolerance. These results are generally attractive consid-
ering the short reflux time of 5 min in the ecofriendly solvent
(water). Selected aryl chlorides were also tested and products in
lower yields were obtained especially for activated aryl chloride.
Ligands L1 e L4 and palladium complex pre-catalysts of these
ligands were successfully prepared with different co-ligands in the
fourth coordination position. Identities of the synthesized com-
pounds were confirmed by single crystal structures of representa-
tive ligand and complex analogues. Within 5 min’ reflux in water
during in situ catalyst generation, higher TOF values were obtained
for Suzuki couplings in the presence of palladium species bearing
the bulkier triiso-propylphenyl-substituted ligand L4 while the less
bulky methyl-substituted ligand L1 behaved as poison to palla-
dium. Similarly, the results of Suzuki couplings for the preformed
complexes showed higher catalyst activities with functional group
tolerance for precatalysts stabilized by ligands possessing bulkier
sulfonyl substituents. High yields of 84% and 77% were obtained for
the tolyl-substituted Pd.L2.py and 2,4,6-triiso-propyhenyl-
substituted Pd.L4.py respectively. The complexes also displayed
appreciable coupling activities for Heck coupling with functional
group tolerance in varying substrates.
An interesting trend observed in this study is that, while Suzuki
coupling activity increased with increase in co-ligand sizes of the
preformed complexes (i.e. water < acetonitrile < pyridine), the
pyridyl co-ligand turned out to be very unfavourable for Heck
coupling in which the best catalyst performances resulted from the
acetonitrile-complemented complexes. Therefore, it could be
concluded that the best catalyst setting for Suzuki coupling may not
be the best for Heck reaction. Considering the fact that these Suzuki
couplings were carried out within 5 min’ reflux with low catalyst
loading (0.2 mol %) in water, it could also be concluded that these
catalysis results are generally attractive since it is often taken for
granted that a strongly chelating tridentate ligands would not
support active site generation.
3.6. Determining suitable Heck coupling reaction settings
Complex Pd.L2.py (0.2 mol %) was used to test for suitable
reaction time, solvent mixture/ratio, reaction temperature and
base additive in the coupling of (4-bromophenyl)methanol and
styrene as a typical Heck coupling reaction. Due to volatility,
1.5 mmol equivalent of styrene relative to (4-bromophenyl)
methanol was used and reactions were typically refluxed for 1 h.
The results, which are summarized in Table 6, shows that K2CO3
supports the Heck coupling better than K3PO4, Na2CO3, NaOH,
(NH4)2CO3 and Cs2CO3. Furthermore, the test of solvent media
revealed that dimethylformamide/water mixture (3:1), which
enabled access to the higher temperature of 140 ꢁC, is preferable
for higher yield. Further reflux until 6 h in the dimethylforma-
mide/water mixture produced 74% yield, which shows that the
generated palladium active species is stable and living. Conse-
quently, it was decided to conduct further reactions in dime-
thylformamide/water (3:1) at 140 ꢁC reflux. Increasing the
amount of K2CO3 and styrene to 2.0 mmol did not produce any
significant improvement over use of 1.5 mmol of both reaction
components. Therefore, 1.5 mmol of K2CO3 and styrene were used
for further reactions; i.e. DMF/H2O (3 mL:1 mL), K2CO3
(1.5 mmol), aryl bromide (1.0 mmol) and olefin (1.5 mmol) at oil
bath temperature of 140 ꢁC for 3 h’ reflux time.
Acknowledgements
HOO is grateful to the Government of the Federal Republic of
Nigeria for financial support through TET fund research grants (TET
fund AST&D year 2012 intervention) and to Adeyemi College of
Education, Ondo, Nigeria for granting study leave. AOE is thankful
to Alexander von Humboldt Foundation for granting post-doctoral
scholarship. The DFG grants PL 155/11, PL 155/12 and PL155/13 are
thankfully acknowledged.
3.7. Effect of precatalysts coordination environments on Heck
coupling
Table 7 presents the Heck coupling performance for the palla-
dium precatalysts according to the selected reaction conditions. It
could also be established that the bulkiness of the o-sulfonamide-
azo ligands correlated with catalytic performance of the complexes.
Similar to the observation in Suzuki coupling, the better catalytic
results were obtained from complexes with tolyl- and 2,4,6-triiso-
propylphenyl-substituted ligands relative to their less bulky methyl
analogues (Table 7, complexes of L2 and L4 compared to L1).
However, contrary to the trend of co-ligand size observed in Suzuki
coupling, pyridyl complemented coordination environments
consistently yielded clearly poorer catalyst performances. Rather,
the acetonitrile complemented complexes produced the highest
yields among the various complexes of each ligand (see Table 7,
Pd.L1.acn, Pd.L2.acn and Pd.L4.acn). In general, achieving a yield
of 84% for complex Pd.L2.acn within 3 h of reflux under normal
atmospheric condition is very encouraging because such catalytic
performance is very rare within shorter reaction time. Further-
more, it could be concluded that influence of co-ligand sizes may
differ for Suzuki and Heck reaction. Yields for Heck coupling of
some selected aryl halide and olefin substrates catalyzed by
Pd.L2.acn (0.2 mol %) according to the established catalytic settings
above are presented in Table 8. The results generally show substrate
functional group tolerance.
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