Synthesis of new Pro-PYE ligands as co-catalysts toward Pd-catalyzed Heck-Mizoroki cross coupling reactions

Article abstract of DOI:10.1039/c9ra07912b

The present research work describes the synthesis of five new ligands containing pyridinium amine, [H₂L¹][OTf]₂-[H₂L⁵][I]₂ from two new precursors, [P³_Et][I] and [P²_Me][CF₃SO₃]. The structure elucidations of the compounds were confirmed by multinuclear NMR (¹H, ¹³C), FT-IR and by single crystal XRD techniques. Theoretical DFT studies were carried out to get better insight into the electronic levels and structural features of all the molecules. These synthesized new Pro-PYE ligands [H₂L¹][OTf]₂-[H₂L⁵][I]₂ were found to be significantly active as co-catalysts for Pd(CH₃CO₂)₂ toward Heck-Mizoroki coupling reactions with wide substrate scope in the order of [H₂L¹][OTf]₂ ? [H₂L²][OTf]₂ > [H₂L³][OTf]₂ > [H₂L⁴][OTf]₂ > [H₂L⁵][I]_2.

Full text of DOI:10.1039/c9ra07912b

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PAPER
Synthesis of new Pro-PYE ligands as co-catalysts
toward Pd-catalyzed Heck–Mizoroki cross
coupling reactions†
Cite this: RSC Adv., 2019, 9, 37986
Naima Munir,a Sara Masood,a Faroha Liaqat,a Muhammad Nawaz Tahir,b
Sammer Yousuf,c Saima Kalsoom,d Ehsan Ullah Mughal,e Sajjad Hussain Sumrra,e
f
a
*
*
Aneela Maalik
and Muhammad Naveed Zafar
The present research work describes the synthesis of five new ligands containing pyridinium amine, [H2L1]
[OTf]2–[H2L5][I]2 from two new precursors, [P3Et][I] and [P2Me][CF3SO3]. The structure elucidations of the
compounds were confirmed by multinuclear NMR (1H, 13C), FT-IR and by single crystal XRD techniques.
Theoretical DFT studies were carried out to get better insight into the electronic levels and structural
features of all the molecules. These synthesized new Pro-PYE ligands [H2L1][OTf]2–[H2L5][I]2 were found
to be significantly active as co-catalysts for Pd(CH3CO2)2 toward Heck–Mizoroki coupling reactions with
wide substrate scope in the order of [H2L1][OTf]2 [ [H2L2][OTf]2 > [H2L3][OTf]2 > [H2L4][OTf]2 > [H2L5][I]2.
Received 29th September 2019
Accepted 7th November 2019
DOI: 10.1039/c9ra07912b
rsc.li/rsc-advances
also serve as electron reservoirs.13,14 Anionic ligand in resonance
with cationic moiety can generate this kind of ligand. For example
1. Introduction
N-(1-methylpyridin-4(1H)-ylidene) amine (PYE) is a type of neutral
nitrogen donor ligand that possesses electronic density adaptable
to the requirement of metal owing to its resonance structures with
varying charge on nitrogen from uni-negative to neutral (Fig. 1).15
The additional feature includes the exocyclic nitrogen atom of PYE
that is pointed towards metal to control its steric environment.16–18
The catalytic cycle of Heck–Mizoroki C–C cross coupling
involves both oxidative addition as well as reductive elimination
reaction. This coupling method is an appropriate synthetic
route for synthesis of substituted olens. High electronic
density around the palladium via appropriate electron rich
ligand facilitates oxidative addition (the key rate determining
step) of various aliphatic or aromatic halides in the coupling
reaction.19–21 Various palladium complexes have been used to
catalyze above reaction.22–24 However, the much easier way is to
use a mixture of suitable ligand and palladium precursor.25,26 In
this regard, ve new H-PYE ligands, [H2L1][OTf]2–[H2L5][I]2,
were synthesized (Schemes 1–3) from two ligand precursors
[P3Et][I] and [P2Me][CF3SO3]. These ligands were found to act as
co-catalysts for Pd(OAc)2 and signicantly enhanced their
catalytic activity in Mizoroki–Heck coupling reactions under
diverse reaction conditions. The molecular structures of all new
compounds were characterized by using various spectroscopic
and computational techniques.
Electron donor ligands are essential for those metals undergoing
various oxidative addition reactions. Phosphines and N-
heterocyclic carbene (NHC) ligands are prevalent in this scenario
owing to their remarkable electron donor characteristics.1–3 The
steric hindrance of a substituted R group towards the metal in
NHCs facilitates reductive elimination and product selectivity that
mark this ligand as a superior choice in homogeneous catalysis,
especially in cross-coupling chemistry.4–7 In depth study reveals
that the tuning of the electronic density of a ligand is desirable
according to the requirement of a coordinated metal to catalyze
various stages of the redox process.8,9 Therefore the non-innocent
ligands are ideally suited for this type of chemistry as they have
the ability to switch and stabilize the various oxidation states of
metals during chemical transformations.10–12 These ligands are
promising in the eld of catalysis as they are redox active spectator
ligands. They not only block few coordination sites of the metal but
aDepartment of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan.
E-mail: mnzafar@qau.edu.pk; Tel: +923314503061
bDepartment of Physics, University of Sargodha, Sargodha-40100, Pakistan
cH. E. J. Research Institute of Chemistry, International Centre for Chemical and
Biological Sciences, University of Karachi, 75270, Pakistan
dCentre for Interdisciplinary Research in Basic Sciences (CIRBS), International Islamic
University, Islamabad, Pakistan
eDepartment of Chemistry, University of Gujrat, Gujrat-50700, Pakistan
fDepartment of Chemistry, COMSATS University Islamabad, Islamabad Campus, Park
Road, Islamabad-45550, Pakistan. E-mail: aneela.maalik@comsats.edu.pk; Tel:
+923335490834
2. Experimental
2.1. Reagents
† Electronic supplementary information (ESI) available. CCDC 1955342, 1955343,
1955344, 1955349 and 1961686. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9ra07912b
Methyl triate, 2-chloropyridine, cyclohexane-1,2-diamine,
pyridine-2,6-diamine, ethane-1,2-diamine, aryl halides, ethyl
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Fig. 1 Possible resonance in PYE.
Scheme 1 Synthesis of [H2L1][OTf]2 and [H2L2][OTf]2.
2.3.2. Synthesis of [P2Me][CF3SO3] (2). N-(1-Methylpyridin-
2(1H)-ylidene) amine was synthesised by treating 2-chloropyr-
idine (1 eq., 1.5 mL, 15.85 mmol) with methyl triate (1.2 eq.,
2.0 mL, 19.02 mmol) in DCM. Aer overnight stirring, the
solvent was evaporated and product was precipitated out by
addition of ether.
acrylate and styrene purchased from Sigma Aldrich were used.
Solvents were dried using standard procedures. N-Methyl-4-
chloropyridinium triate was synthesized by the reported
method.27
2.2. Apparatus and instruments used
1
ꢁ
Colorless solid, yield 95%, mp: 161 C. H NMR (300 MHz,
DMSO-d6), J (Hz), d (ppm): 9.16 (dd, 1H, J ¼ 1.5, Py-H), 8.62–8.56
(m, 1H, Py-H), 8.37 (dd, 1H, Py-H), 8.12–8.07 (m, 1H, Py-H) 4.33
(s, 3H, CH3). 13C NMR (75 MHz, DMSO-d6), J (Hz), d (ppm):
148.6, 147.4, 129.9, 126.5, 123.2, 119.0, 47.8. FT-IR (KBr, cmꢀ1):
3097, 1617, 1309, 1260, 1142, 1028.
All reactions were performed under inert environment by
Schlenk line technique. Melting point analysis was performed
on Shimadzu melting point apparatus. Thermo Scientic
spectrometer ranges from 4000–400 cmꢀ1 was used to record
FT-IR spectras. NMR spectra were recorded by using Bruker
Avance Digital 300 MHz (1H) and 75 MHz (13C) at 300 K in
DMSO-d6.
2.3.3. Synthesis of [H2L1][OTf]2–[H2L5][I]2 (3–7)
[H2L1][CF3SO3]2 (3). N-methylated-4-chloropyridinium triate
(0.5 g, 1.8 mmol, 2 eq.) and pyridine diamine (0.1 g, 0.9 mmol, 1
eq.) was subjected into two neck ask (100 mL). An argon
atmosphere was provided to reaction mixture. The reaction was
heated for about 3 hours at 230 ꢁC. Aer the completion of
reaction, solid was cooled to ambient temperature and
scratched by spatula from ask. The off white crystals were
dried in vacuo to give pure product. The crystals were grown in
mixture of solvents as methanol and acetone.
2.3. Chemistry
Syntheses of seven new compounds were shown in Schemes 1–3.
2.3.1. Synthesis of [P3Et][I] (1). 2-Iodopyridinium ethyl
iodide was prepared by treating 2-chloropyridine (4.91 mL,
53 mmol, 1 eq.) with ethyl iodide (12.78 mL, 159 mmol, 3eq.) in
three neck round bottom ask (250 mL). Dichloromethane was
also subjected to ask. The resulted mixture was subjected to
heat under water bath for 12 hours. The product was cooled and
washed by subsequent addition of acetone and pure product
was achieved.
1
ꢁ
Off white solid, yield 70%, mp: 165 C. H NMR (300 MHz,
DMSO-d6), J (Hz), d (ppm): 11.06 (s, 2H, NH), 8.48–8.46 (d, 4H, J
¼ 7.2, py- H), 7.96–7.91 (d, 1H, J ¼ 7.7, py-H), 7.84–7.82 (d, 4H, J
¼ 6.3, py-H), 6.96–6.94 (d, 2H, J ¼ 8.1, py-H), 4.07 (s, 6H,
CH3).13C NMR (75 MHz, DMSO-d6),J (Hz),d (ppm): 152.6, 151.1,
145.1, 143.2, 112.9, 109.7, 45.8. FT-IR (KBR, cmꢀ1): 3296, 3080,
1638, 1581, 1365, 1278, 1199, 1025.
Royal yellow solid, yield 56%, mp: 156 ꢁC. 1H NMR (300 MHz,
DMSO-d6), J (Hz), d (ppm): 9.28 (dd, 1H, J ¼ 1.8, Py-H), 8.65 (dd,
1H, J ¼ 1.8, Py-H), 8.19–8.07 (m, 2H, Py-H), 4.75 (q, 2H, J ¼ 7.2,
CH2), 1.49 (t, 3H, J ¼ 7.2, CH3). 13C NMR (75 MHz, DMSO-d6), J
(Hz), d (ppm): 147.5, 144.7, 141.21, 127.87, 122.60, 62.31, 15.96.
FT-IR (KBr, cmꢀ1): 3069, 2956, 1600, 1357, 1272, 1153, 1065.
[H2L2][CF3SO3]2 (4). N-methylated-4-chloropyridinium triate
(1 g, 3.6 mmol, 2 eq.) along with cyclohexane diamine (0.21 mL,
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Scheme 2 Synthesis of [H2L3][OTf]2 and [H2L4][OTf]2.
Scheme 3 Synthesis of [H2L5][I]2.
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1.8 mmol, 1 eq.) were added in two necked round bottom ask
Fawn yellow solid, yield 48%; mp: 304 ꢁC. 1H NMR (300 MHz,
(50 mL). Under argon the temperature was provided to reaction DMSO-d6), J (Hz),d (ppm): 8.28 (s, 1H, N–H), 8.19 (s, 1H, Py-H),
ꢁ
mixture for about 150 minutes at 230 C. The solid was cooled 8.04 (s, 1H, Py-H), 7.40 (d, 1H, Py-H), 7.03 (s, 1H, Py-H), 4.22 (q,
and recrystallised from methanol and acetone and dried in 2H, J ¼ 5.1, CH2), 3.68 (s, 2H, CH2), 1.31 (t, 3H, J ¼ 7.2, CH3). IR
vacuo to give pure product [H2L1][OTf]2.
(KBr, cmꢀ1): 3205, 3016, 2970, 1642, 1536, 1364, 1217, 1173,
Brown crystalline solid, yield 78%, mp: 180 ꢁC.1H NMR (300 1063.
MHz, DMSO-d6), J (Hz),d (ppm): 9.11 (s, 2H, NH), 8.18–8.15 (d,
2.3.4. General method for the heck-coupling reaction. A
2H, J ¼ 6.9, py-H), 7.93–7.91 (d, 2H, J ¼ 6.9, py-H), 7.21–7.19 (m, dried evacuated tube (20 mL) tted with Teon tap was charged
2H, py-H), 6.75–6.73 (m, 2H,py-H), 3.81 (s, 6H, CH3), 3.71–3.69 with Pd(OAc)2 (4.0 mmol, 0.01), ligand (4.0 mmol, 0.01), sodium
(d, 2H, J ¼ 6, cy-H), 1.75–1.73 (d, 2H, J ¼ 7.8, cy-H), 1.53–1.49 (d, acetate (1.1 eq., 440 mmol, 36 mg), magnetic stirrer under inert
2H, J ¼ 9.3, cy-H), 1.45–1.37 (m, 4H, cy-H).13C NMR (75 MHz, environment. Styrene (1.4 eq., 64 mL, 560 mmol), halobenzene
DMSO-d6), J (Hz),d (ppm): 156.5, 144.3, 142.5, 110.4, 106.3, 56.2, (1.0 eq., 42 mL, 400 mmol) and dry DMA (2 mL) were added to
44.4, 31.5, 24.3. IR (KBR, cmꢀ1): 3360, 3053, 2943, 1646, 1588, the ask which was heated rst at (130 ꢁC) for 3 hours and then
1375, 1296, 1169, 1058.
cooled to room temperature before use. On completion, the
[H2L3][CF3SO3]2 (5). 2-Chloropyridinium methyl triate (0.5 g, reaction ask was cooled at ambient temperature. Aer cooling,
1.8 mmol, 2eq.) was added in a vacuum dried two neck round the mixture was extracted with ethyl acetate/n-hexane (1 : 5)
bottom ask (50 mL). Cyclohexane diamine (0.1 mL, 0.9 mmol 1 solution. Column chromatography was employed to purify the
eq.) was introduced by glass syringe (100 mL) under argon into product using ethyl acetate/n-hexane (20 : 80). Multinuclear
the ask. Then reaction mixture was heated for about 3 hours. NMR spectroscopy (1H NMR) conrmed the formation of
Thermometer was used to monitor the temperature. Aer the product.
completion of reaction, product was cooled to room tempera-
ture and scratched from the ask carefully via spatula. Desic-
cator was used to dry the solid. Good quality crystals can be
grown in mixture of acetone and methanol.
3. Results and discussion
3.1. Synthesis and characterization
Brown solid, yield 60%, mp: 232 ꢁC. 1H NMR (300 MHz, The N-alkylation of 2-chloropyridine was carried out with ethyl
DMSO-d6), J (Hz),d (ppm): 8.26 (t, 2H, J ¼ 6.6, NH), 8.24–8.18 (dd, iodide and methyl triate to synthesize ligand precursors. Ethyl
2H, J ¼ 6.9, py-H), 7.99–7.37 (m, 2H, py-H), 7.03–7.00 (d, 1H, J ¼ iodide being weak alkylating agent than methyl triate required
7.5, py-H), 6.53–6.45 (dd, 1H, J ¼ 8.4, py-H), 3.99 (s, CH3, 6H), more vigorous conditions to yield [P3Et][I] as compared to [P2
]
Me
3.66 (s, 1H, cy-H), 3.54–3.50 (d, 1H, J ¼ 3.6, cy-H), 2.49–2.25 (m, [CF3SO3]. During the synthesis of [P3Et][I], chloro attached at
2H, cy-H), 1.88–1.86 (d, 2H, J ¼ 6 cy-H), 1.66–1.65 (d, 2H, J ¼ 3, alpha carbon was substituted by iodo group and also appeared
cy-H), 1.19–1.17 (d, 2H, J ¼ 5.1, cy-H). 13C NMR (75 MHz, DMSO- as counter anion, upon heating in water bath for 12 hours.28,29
d6), J (Hz), d (ppm): 159.1, 153.3, 147.7, 143.2, 112.7, 43.2, 32.8, The ethylation on pyridine was suggested by a quartet and
24.5.
triplet peaks at 4.75 and 1.49 ppm in proton NMR spectrum.
[H2L4][CF3SO3]2 (6). Ethylene diamine (240.1 mL, 3.60 mmol) The other ligand precursor, [P2Me][CF3SO3] was synthesized by
and N-methylated-2-chloropyridinium triate (2 g, 7.2 mmol) just overnight stirring of 2-chloropyridine with methyl triate.
were added in a 50 mL two neck round bottom ask under inert The methyl peak appeared at 4.33, 47.8 ppm in 1H and 13C NMR
atmosphere and ask was heated for 2 hours at 180 ꢁC. During spectra respectively. The IR spectra of precursors [P3Et][I]–[P2
]
Me
this time, the solid reactant melts and undergoes melt reaction. [CF3SO3] showed aromatic stretch n(C–H) at 3069–3085,
Aer that time the melt was cooled to room temperature and the aliphatic n(C–H) at 2956, n(C]N) at 1562–1588 cmꢀ1, n(C]C) at
solid scratched from the ask. The solid was recrystallized from 1600–1646 cmꢀ1, n(C–N) at 1357–1303 cmꢀ1, n(C–I) at 445 cmꢀ1
,
methanol to give pure dark brown crystalline product.
Dark brown solid, yield 70%; mp: 205 ꢁC. 1H NMR (300 MHz, appeared in their respective regions.
All the ligands, [H2L1][OTf]2–[H2L5][I]2, were synthesised by
and n(C–Cl) band at 633 cmꢀ1 respectively. All remaining peaks
DMSO-d6), J (Hz),d (ppm): 8.28 (t, 2H, J ¼ 5.7, NH), 8.09–8.03 (m,
4H, Py-H), 7.42–7.39 (d, 4H, J ¼ 9, Py-H), 7.05–6.99 (m, 2H, Py-H), the melt reaction between respective ligand precursor and cor-
3.79 (d, 6H, J ¼ 4.8, CH3), 3.67 (d, 4H, J ¼ 4.5, CH2). 13C NMR (75 responding amine. In these ligands, NH peak appeared in the
MHz, DMSO-d6), J (Hz),d (ppm):153.2, 143.5, 142.4, 123.3, 118.9, range of 9–11 ppm that indicated the conversion of all primary
113.2, 111.5, 42.2, 40.7. IR (KBr, cmꢀ1): 3325, 3086, 2952, 1649, amines to secondary amines. In aliphatic region, N-alkylated
1591, 1348, 1243, 1159, 1025.
protons showed peaks for methyl and ethyl between 4.24–
[H2L5][I]2 (7). 2-Iodo-1-ethyl-pyridinium iodide (0.2 g, 3.70 ppm in all ligands. Similarly carbon NMR showed these
4.2 mmol, 2 eq.) was added in two necks round bottom ask. peaks between 62.3–47.8 ppm. The aromatic protons and their
The ask was charged with ethylene diamine (0.138 mL, carbon atoms showed the respective peaks in their standard
2.07 mmol, 1 eq.) and potassium carbonate (K2CO3) (1.7 g, region. The peaks for cyclohexyl group appeared between 1.17–
12.46 mmol, 6 eq.) under inert argon. The mixture was stirred 3.71 ppm for [H2L2][OTf]2 and [H2L3][OTf]2. In FT-IR spectra, the
and reuxed in dichloromethane (DCM) for 20 hours at 80 C. broad peaks appeared between 3296-3205 cmꢀ1 for n(N–H).
ꢁ
Aer reuxing, volatiles were removed. The product was While peaks of d(N–H) appeared at 1581–1591 cmꢀ1 in [H2L1]
recrystallized in DCM, collected and dried in vacuo to give pure [OTf]2–[H2L5][I]2. Peaks of aromatic region n(C–H) ranges 3101–
product of orange yellow color.
3016 cmꢀ1 while skeleton vibrations or peaks due to breathing
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Fig. 2 Resonance forms of [H2L2][OTf]2.
vibrations of aromatic ring were observed around the following [OTf]2 with the head pyridine ring resulting in reduction of the
frequencies; 1600, 1560, 1500, 1460 cmꢀ1 30 In aliphatic region, N1/C5 and N1/C5 p-interactions is one possible explanation for
.
the characteristic stretching band n(C–H) appeared at 2970– this different behavior. Hence rotation about the N1–C5 bond
2943 cmꢀ1 and bending vibrations associated with d(C–H) became rapid on the NMR timescale.
present in the particular range of 1436–1459 cmꢀ1. Signals
Crystals suitable for single crystal XRD analysis were
appeared for both secondary C]N and primary C–N stretch in successfully grown for both precursors. [P3Et][I] crystals were
the region of 1690–1590 cmꢀ1 and 1350–1280 cmꢀ1 respectively. grown from its acetone solution through slow evaporation
Peaks between 1350–1225 cmꢀ1 were assigned to both asym- method. The ORTEP depiction of [P3Et][I] is shown in Fig. 3. The
metric and symmetric n(S]O), while signals between 828– crystal structure of [P3Et][I] conrms the successful ethylation of
863 cmꢀ1 were designated to n(S–O). Sharp peak associated with pyridine nitrogen. It also reveals that iodide acts as counter
n(C–F) observed in the range of 1025–1063 cmꢀ1
.
anion as well as replacement of o-chloro with iodo group.
A comparison of proton and carbon NMR spectra of [H2L1] Crystals of [P2Me][CF3SO3] were prepared through slow evapo-
[OTf]2 and [H2L2][OTf]2 reveals that B type resonance structure ration process from its methanol solution. The ORTEP depic-
is predominant for [H2L2][OTf]2 unlike in [H2L1][OTf]2 as shown tion of [P2Me][CF3SO3] is shown in Fig. 4 that shows successful
in Fig. 2. The 1H and 13C NMR (DMSO-d6) spectra for [H2L2] methylation on nitrogen.
[OTf]2 indicated the chemical equivalence in solution form of
both pyridinium groups on NMR timescale. This lead to the
appearance of only one signal for the both methyl groups at
1
3.81 ppm in H NMR and 44.4 ppm in 13C NMR spectra. Simi-
1
larly, only one signal for NH appeared in H NMR spectrum at
9.11 ppm. Though the individual atoms making up each of the
pairs C2/C3 and C1/C4 (as well as their symmetry related pairs)
within each pyridinium group were found unequal and we
observed separate signals in each case. Such as, for C2/C2, C3/
C3, C1/C1 and C4/C4 in the 13C NMR spectrum, separate
singlet signals were observed at 144.35, 142.46, 110.37, and
106.31 ppm, respectively. This situation most likely result
because of the conformational changes that equilibrate the
positions of each of the pairs of atoms, C2/C3, C1/C4, C2/C3 and
1
C1/C4, were slow on the NMR timescale. Whereas the H and
13C NMR (DMSO-d6) spectra of [H2L1][OTf]2 showed the chem-
ical equivalence of both pyridinium groups and the pairs of
atoms C1/C4 and C2/C3 (and the corresponding pairs of atoms
in the second pyridinium ring) in solution on NMR timescale.
The interaction possibility of the N1 and N1 lone pairs in [H2L1]
Fig. 3 ORTEP depiction of [P3Et][I].
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Fig. 4 ORTEP depiction of [P2Me][CF3SO3].
Fig. 5 ORTEP depiction of [H2L3][OTf]2.
Fig. 6 (A) ORTEP depiction of [H2L4][OTf]2, (B) wire frame work of
[H2L4][OTf]2 showing hydrogen bonding.
Attempts were made to grow the single crystals of all the
ligands but [H2L3][OTf]2–[H2L5][I]2 were successfully grown and
analyzed. [H2L3][OTf]2 was crystallized out in Pbcn space group
from dichloromethane and ether (50 : 50) solution. ORTEP
diagram is shown in Fig. 4. Crystal structure analysis conrms
the attachment of two pyridinium rings to the cyclohexyl
diamine and triate is acting as counter anion. Cyclohexyl
group adopts chair conformation with two substitutions at
consecutive axial and equatorial positions show transoid
geometry. Due to steric repulsion, the two substituted arms of
cyclohexyl group shows torsion C7N1C5N2 angle of
171.8ꢁ.(Fig. 5).
presence of monoclinic system. The X-ray crystal structure
conrms that two pyridinium rings are attached to nitrogen
atoms of bidentate ethylene diamine. The two pyridinium
amine groups beside ethylene unit adopt transoid geometry due
to N1C1C1N1 torsion angle of precise 180ꢁ in both [H2L4][OTf]2
and [H2L5][I]2.
The wire frame work structures of [H2L3]+2–[H2L5]+2 are
shown in Fig. 8(A). It reveals that the bond lengths of [H2L3]
[OTf]2–[H2L5][I]2 are in accordance with the resonance form B in
[H2L4][OTf]2 was crystallized out in the space group P21/c in
methanol and acetone solution. ORTEP depiction is shown in
Fig. 6(A). Examination of crystal structure shows that there is
torsion angle of 180.0ꢁ (N1–C1–C2–N3). Fig. 6(B) presents the
3
˚
Fig. 8(B). The bond length of C5–N1 is 1.347 A in [H2L ][OTf]2.
Similarly [H2L4][OTf]2 and [H2L5][I]2 C2–N1 bond distances are
˚
˚
1.338 A and 1.331 A respectively that correspond to exocyclic
carbon double bond nitrogen.
˚
hydrogen bonding that exists with NH–O distance of 2.92 A (N1
and O2). These distances are within the normal range of NH–O
5
31
˚
bond lengths 2.81–3.04 A. . [H2L ][I]2 was crystallized out in the
3.2. Computational analysis of molecular structures
ꢀ
space group R3 in dichloromethane solvent. ORTEP depiction is
shown in Fig. 7. Crystal structure cell coordinates reveal the
Computational studies were carried out on all the compounds
using Gaussian 09 soware to gain a greater insight into the
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structural and electronic properties. The most signicant edge
to semi-empirical calculations is the prominent increase in
computational accuracy without further increase in computing
time. The geometry optimization of the structures was per-
formed using AM1 semi-empirical method and an energy
calculation was carried out for each optimized structure to nd
the energy minimum, (in the presence and absence of ligands).
The energy of the frontier molecular orbitals (HOMO–LUMO)
was also calculated (and conrmed to earlier results by
Gaussian) by MOE 2016 soware32 using semi-empirical
method with a Hamiltonian force eld, MMFF94x and 0.0001
Gradient. All the computed data available in this study shows
highly convergent values obtained from energy minimization of
compounds using Gaussian and MOE.
A number of structural parameters have been obtained from
semi-empirical studies on conformers with minimum energy.
The energy of the frontier molecular orbitals is also instru-
mental in obtaining the values of chemical hardness (h),
chemical potential (c) and electrophilicity index (u) using the
Fig. 7 ORTEP depiction of [H2L5][I]2.
Fig. 8 (A) Wire framework of [H2L3]2+–[H2L5]2+ showing important bond distances, (B) resonance forms of [H2L3][OTf]2–[H2L5][I]2.
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Table 1 Ionization energy, electron affinity and band gap values of [P3Et][I]–[H2L5][I]2
Ionization energies
(eV per molecule)
Electron affinity
(eV per molecule)
Band gap (eV
per molecule)
Ligands
Values (eV)
[P3Et][I]
HOMO ¼ ꢀ8.15, LUMO ¼ ꢀ0.47
HOMO ¼ ꢀ9.39, LUMO ¼ ꢀ2.27
HOMO ¼ ꢀ14.69, LUMO ¼ ꢀ6.77
HOMO ¼ ꢀ14.50, LUMO ¼ ꢀ6.12
HOMO ¼ ꢀ9.49, LUMO ¼ ꢀ2.55
HOMO ¼ ꢀ9.15, LUMO ¼ ꢀ2.33
HOMO ¼ ꢀ14.82, LUMO ¼ ꢀ6.75
ꢀ8.15
ꢀ9.39
ꢀ0.47
ꢀ2.27
ꢀ6.77
ꢀ6.12
ꢀ2.55
ꢀ2.33
ꢀ6.75
ꢀ7.67
ꢀ7.12
ꢀ8.22
ꢀ8.38
ꢀ6.94
ꢀ6.82
ꢀ8.07
[P2Me][CF3SO3]
[H2L1][OTf]2
[H2L2][OTf]2
[H2L3][OTf]2
[H2L4][OTf]2
[H2L5][I]2
ꢀ14.69
ꢀ14.50
ꢀ9.49
ꢀ9.15
ꢀ14.82
u ¼ c2/2h
(3)
Table 2 Chemical hardness (h), chemical potential (c) and electro-
philicity index (u) of [P3Et][I]–[H2L5][I]2
Chemical
hardness (h)
Chemical
potential (c)
Electrophilicity
index (u)
The reactivity of a compound can also be gauged from the
HOMO and LUMO orbitals energy difference (DEHꢀL); a larger
band gap indicates lower reactivity and higher chemical hard-
ness.35,36 The greater is the energy of HOMO, greater would be
ionization potential and thus more is the ability of a molecule to
donate an electron pair. While LUMO has the ability to accept
electron density via back bonding from transition metal that is
associated with electron affinity of molecule.37,38 The pictorial
representations of all the frontier molecular orbitals (HOMO–
LUMO) were acquired. HOMO–LUMO of [P3Et][I] and [H2L1]
[OTf]2 are shown in Fig. 9 and 10. The important bond lengths
of optimized structures and XRD structures of compounds
[H2L3][OTf]2–[H2L5][I]2 are compared in Tables 3–5. It shows
good agreement between the computed and experimental
values for all the compounds. Slight divergences in some bond
lengths can be attributed to the choice of computational
method, and the basis set used.35–39
Ligands
[P3Et][I]
ꢀ4.31
ꢀ5.83
4.31
5.83
ꢀ2.15
ꢀ2.91
[P2
]
Me
[CF3SO3]
[H2L1][OTf]2
[H2L2][OTf]2
[H2L3][OTf]2
[H2L4][OTf]2
[H2L5][I]2
ꢀ10.73
ꢀ10.31
ꢀ6.02
10.73
10.31
6.02
5.74
10.78
ꢀ5.36
ꢀ5.15
ꢀ3.01
ꢀ2.87
ꢀ5.39
ꢀ5.74
ꢀ10.78
eqn (1)–(3);33,34 where I and A are the ionization potential and
electron affinity of a molecule, shown in Tables 1 and 2.
h ¼ (I ꢀ A)/2
(1)
(2)
c ¼ ꢀ(I + A)/2
Fig. 9 Electron density distribution of HOMO and LUMO of [P3Et][I].
Fig. 10 Electron density distribution of HOMO and LUMO of [H2L1][OTf]2.
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Table 3 Experimental and theoretical bond lengths [A˚] of [H2L3][OTf]2
Concerned
atoms
Experimental
Calculated
Concerned atoms
Experimental
Calculated
C1–C2
C2–C3
C3–C4
C4–C5
C5–N2
N2–C6
1.35
1.39
1.36
1.41
1.36
1.47
1.38
1.38
1.39
1.41
1.37
1.48
N2–C1
C5–N1
N1–C7
C7–C8
C8–C9
C7–C7
1.36
1.35
1.46
1.53
1.53
1.54
1.35
1.39
1.46
1.54
1.53
1.55
Table 4 Experimental and theoretical bond lengths [A˚] of [H2L4][OTf]2
Atom no
Experimental
Calculated
Atom no
Experimental
Calculated
C1–N1
N1–C2
C2–C3
C3–C4
C4–C5
1.46
1.34
1.40
1.36
1.39
1.48
1.34
1.41
1.39
1.38
C5–C6
C6–N2
N2–C7
N2–C2
C1–C1
1.34
1.36
1.46
1.36
1.51
1.38
1.35
1.48
1.38
1.52
Table 5 Experimental and theoretical bond lengths [A˚] of [H2L5][I]2
Concerned
atoms
Experimental
Calculated
Concerned atoms
Experimental
Calculated
C1–N1
N1–C2
C2–C3
C3–C4
C4–C5
1.46
1.33
1.39
1.36
1.37
1.45
1.39
1.41
1.39
1.38
C5–C6
C6–N2
N2–C7
C7–C8
C5–C6
1.32
1.36
1.49
1.46
1.32
1.38
1.36
1.49
1.52
1.38
that binds with palladium acetate. This conjugation in ligand
can make amido nitrogen electron rich that in turn increases
the electronic density on palladium catalyst that eventually
speeds up the rate of oxidative addition of bromobenzene,
thus activating the catalyst towards coupling reaction.
Alkyl substituted ligands, [H2L2][OTf]2–[H2L5][I]2, were
found less active comparatively. Nevertheless, they showed
coupled product in the range of 77 to 83% owing to the
inductive effect of ethyl and hexyl groups. The minor varia-
tions in product yield can be related to the steric of N-alkylated
group of varying degree around the donor amido nitrogen.
This catalytic activity order of co-catalysts can also be related
with their EHOMO values i.e., greater is the energy of HOMO,
greater would be the electron donation from ligand to Pd(II)
thus more would be the stability of in situ generated active
catalyst.
Substrates with a different range of electron withdrawing to
electron donating substituent were coupled under optimum
conditions to test the substrate scope of best catalyst system
([H2L1][OTf]2/Palladium acetate). As expected, more activated
aryl iodides couple more effectively than aryl bromides and
aryl chloride (Table 8, entry 1–8). The high reactivity of aryl
iodides are due to small dissociation energy of C–I bond and
ease of activation towards Pd(0) catalyst in oxidative addition
3.3. Evaluation of Mizoroki–Heck cross-coupling reaction
The estimation of potential efficiency of [H2L1][OTf]2–[H2L5][I]2
as a co-catalysts toward Heck coupling reaction were evaluated
in the presence of palladium acetates catalyst. The results ob-
tained are tabulated in Table 6. [H2L1][OTf]2 was used for the
optimization of various parameters such as amount of catalyst
loading (Table 6, entry 1–5), solvent system (Table 6, entry 5–9),
base (Table 6, entry 10–15), temperature (Table 6, entry 15–19)
and molar ratio of palladium acetate to ligand (Table 6, entry
19–20, 21, 22), with pure Pd (OAc)2 catalyst and without any
ligand reaction time 4 hours resulted in lowest 6% yield (Table
6, entry 20). Initially, bromobenzene with styrene reaction was
selected for optimization of reaction conditions. A reaction with
only palladium acetate or ligand yield trace amount of product
(Table 6, entry 1 or 20). The optimum reaction conditions were
obtained for (entry 19, Table 6); 0.01 mol% palladium acetate,
DMF solvent, NaOAc 1.1 mmol, 130 ꢁC and 1 : 1 palladium
acetate to [H2L1][OTf]2.
All the other synthesized ligands, [H2L2][OTf]2–[H2L5][I]2,
were compared with [H2L1][OTf]2 and the results are shown
(Table 7). [H2L1][OTf]2 presented maximum coupled product
comparatively. The reason can be associated with the extensive
delocalization of electrons in the in situ generated L1 (Fig. 11)
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Table 6 Optimization of catalytic Mizoroki–Heck cross coupling reactiona
Pd(OAc)2
(mol)
Temperature
(ꢁC)
Entry
Base
Pd(OAc)2 : L
Solvent
Yield (%)
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
NEt3
Pyridine
CH3COONa
CH3COONa
CH3COONa
CH3COONa
CH3COONa
CH3COONa
CH3COONa
CH3COONa
—
0.1
0 : 1 without catalyst
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
100
100
100
100
100
80
110
100
75
100
100
100
100
100
100
25
DMA
DMA
DMA
DMA
DMA
Hexane
Toluene
H2O
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
No reaction
83
80
78
50
43
47
56
68
84
80
78
41
46
86
53
63
72
93
6
0.05
0.01
0.001
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1 : 1
1 : 1
1 : 1
50
75
130
130
130
130
1 : 0 without ligand
1 : 2
2 : 1
54
56
a
Conditions: 3 h except entry 20, styrene (1.4 eq.) versus halobenzene. Isolated yields were based on aryl halide.
Table 7 Comparison of [H2L1][OTf]2–[H2L5][I]2 for Mizoroki–Heck cross coupling reactionsa
Entry
Ligands
%Yield
1
2
3
4
5
[H2L1][TfO]2
[H2L2][TfO]2
[H2L3][TfO]2
[H2L4][TfO]2
[H2L5][I]2
93
83
81
79
77
a
Conditions: 3 h, 130 ꢁC, styrene (1.4 eq.) versus halobenzene, Pd(OAc)2 : L ¼ 1 : 1, DMF (2 mL), ligand loading (0.01 mol), Pd(OAc)2 loading (0.01
mol) and NaOAc (4.3 mmol, 3.1 eq.). Isolated yield based on aryl halide.
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containing electron withdrawing substituents such as nitro,
aldo and keto groups (Table 8, entry 1–13). The more electron
decient quaternary carbon of aryl halide better would be the
rate of oxidative addition reaction. Similar trend was shown by
ethyl acrylate for the above mentioned aryl halides activated to
different degrees. However the yield of all coupled products
with ethyl acrylate was comparatively more than styrene (Table
8, entry 14–18). The reason can be attributed to the greater
polarization of the alkene with more electron-withdrawing
adjacent carbonyl group in ethyl acrylate. This polarization
enhances the insertion of ethyl acrylate into aryl–palladium
bond during catalytic cycle. Moreover, as explained earlier, out
of 3.1 eq. sodium acetate, 1.1 eq. of sodium acetate was used for
catalytic reaction however remaining 2 eq. of sodium acetate
was used to in situ deprotonate the ligand leading to the
formation of an effective complex with ligand.
Fig. 11 Proposed structure of L1.
Efforts to synthesize pure palladium complexes with all the
ligands, [H2L1][OTf]2–[H2L5][I]2 were proved to be futile owing to
strong trans inuence by in situ generated deprotonated
ligands. The resulted mixtures were attempted to be puried by
various analytical techniques but was proved in vain. To full ll
the urge of identifying the actual nature of catalyst, standard
mercury test was performed. 150 to 300 equivalent of Hg(0)
step.40 Generally coupling of aryl chlorides demand high
catalyst loading with low yield of coupled product.41
Likewise, those aryl halides with electron donating substit-
uents like methoxy, methyl or naphthyl and electron neutral
group such as –H showed less reactivity than aryl halide
Table 8 Synthesis of aryl halides catalyzed by Pd(OAc)2/L through Mizoroki–Heck cross-coupling reactiona
Entry
RC6H4X
R'CHCH2
Yield (%)
Time (h)
1
2
3
4
5
6
7
8
ClC6H5
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
C6H5CHCH2
67
65
72
70
98
95
93
89
87
96
92
94
85
96
94
93
98
97
98
92
6
7
6
6
4
5
4
6
4-ClC6H4CH3
4-ClC6H4NO2
4-ClC6H4COCH3
I–C6H5
4-IC6H4CH3
BrC6H5
4-BrC6H4CH3
4-BrC6H4OCH3
4-BrC6H4NO2
4-BrC6H4CHO
4-BrC6H4COCH3
1-BrC10H7
9
6
4.5
4
10
11
12
13
14
15
16
17
18
19
20
4
C6H5CHCH2
16
3.5
4
4
3
2
2
12
BrC6H5
CH2CHCO2C2H5
CH2CHCO2C2H5
CH2CHCO2C2H5
CH2CHCO2C2H5
CH2CHCO2C2H5
CH2CHCO2C2H5
CH2CHCO2C2H5
4-BrC6H4CH3
4-BrC6H4OCH3
4-BrC6H4NO2
4-BrC6H4CHO
4-BrC6H4COCH3
1-BrC10H7
a
Conditions: 130 ꢁC, styrene (1.4 eq.) versus halobenzene, Pd(OAc)2 : L ¼ 1 : 1, NaOAc (4.3 mmol, 3.1 eq.), DMF (2 mL), ligand loading 0.01,
Pd(OAc)2 loading (0.01 mol%). Isolated yield based on aryl halide.
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Fig. 12 The catalytic cycle showing the role of reagents, ligands and etc., under the best reaction conditions.
compared to catalytic system was added in the catalytic reactor product in just 85 minutes at 120 ꢁC but with much high
during coupling reaction. The yield of coupled product was not catalyst amount i.e. 10 mol%. The best one seems to be in
affected in this test reaction. This result clearly conrmed the in entry 4; utilizes less catalyst at the cost of 20 ꢁC more
situ generation of homogeneous palladium catalysts and temperature comparatively. The catalyst in entry 5 is useless
heterogeneous Pd(0) catalysts were not obtained by any oppo- for chlorobenzenes. Whereas with respect to catalyst shown in
nent way.42,43
The plausible catalytic cycle of one of the reactions between chlorobenzenes.
entry 4, our catalysts showed more substrate scope for
iodobenzene and styrene under the best reaction conditions
We observed a probable limitation of our new catalytic
(mentioned in Table 8) shown in Fig. 12 involved pre- system entry 12 as it was not found to be the best for the Heck
activation of catalyst followed by oxidative addition of palla- coupling of iodobenzenes with styrene. The published literature
45c,h
dium in benzene to iodide bond of iodobenzene, Pd(II) p-
(entry 10 and 11) proved it inferior to them with respect to
complex formation, migratory insertion of styrene in palla- catalyst loading but it still looks reasonable when compared to
dium and carbon bond, relief of torsional strain with the entries 7–9 (ref. 45a,f,g) of the above table.
elimination of beta hydride, another p-complex formation
The comparison of our result with literature with regard to
with stilbene and generation of Pd(0) catalyst with reductive the Heck coupling of bromobenzene with alkyl acrylate
45f,i,j
elimination. Sodium acetate plays a vital role in regeneration (entries 13–15)
suggested that the catalyst system gener-
of catalyst.44
ated in this work (entry 19) proved to be an excellent attempt
Herein, we performed the critical comparison (mentioned specially with respect to very less time utilization (3.5 hours).
in Table 9) of the catalytic activity (entry 1 to 5)45a–e of this work Likhar et al.,45f got 92% of coupled product with 0.01 mol% but
in entry 6 related to Heck coupling of chlorobenzene and in 20 h and at 145 ꢁC. In 2017, Abdol R. Hajipour45i synthesized
styrene with literature showed that this catalyst is potentially highly active catalyst and got 95% yield at only 50 ꢁC but using
the trademark for the coupling of chlorobenzene with styrene. more time than our catalytic system i.e. 5 hours. Wang et al., 45j
It gave 67% yield in 6 hours at 130 ꢁC with 1 mol%. The used 0.005 mol% of catalyst but with only 4% yield. The other
problem with catalysts in entries 1 and 2 catalysts is utilization remarkable feature of our catalyst (entry 16) was the use of
of too much time for catalytic conversion of chlorobenzene to 400 mmol of halobenzene transformed into 96% of coupled
coupled product. The catalyst in entry 3 yields 70% coupled product compared to literature (1 mmol) (Table 9).
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Table 9 Comparison of developed catalytic system (entries 6, 14, 16) with the literature reported systems (entries 1–5, 7–11, 13–15) showing
different ligands and conditionsa
Halo-benzene
(mmol)
Solvent (ml),
base
Entry Reactions
Ligands (L), Pd(OAc)2 mol%
Time (h) T (ꢁC) Y (%) Ref.
1
2
C6H5Cl + styrene
1
DMF, K2CO3
12
24
60
80
62
99
45a
45b
C6H5Cl + styrene
0.5
Pd(OAc)2: Dave-Phos (2 : 6) 2
1,4- dioxane,
TBAE
3
C6H5Cl + styrene
1
K2CO3, DMF
85 min
120
70
45c
4
C6H5Cl + styrene
1.5
NMP, Ca(OH)2
6
150
62
45d
5
6
C6H5Cl + styrene
C6H5Cl + styrene
1
NMP, K2CO3
DMF, NaOAc
2.5
6
120
130
Trace 45e
400
Pd (OAc)2 : [H2L1][TfO]2 (1 : 1)
67
98
This
work
7
8
C6H5I + styrene
1
1
DMF, K2CO3
12
60
45a
C6H5I + styrene
DMF, Na2CO3 20
145
95
45f
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Table 9 (Contd.)
Halo-benzene
(mmol)
Solvent (ml),
base
Entry Reactions
Ligands (L), Pd(OAc)2 mol%
Time (h) T (ꢁC) Y (%) Ref.
9
C6H5I + styrene
1
1
DMF, K2CO3
20
130
120
90
97
45g
45h
10
C6H5I + styrene
DMF, Et3N
40 min
11
12
C6H5I + styrene
C6H5I + styrene
1
NMP, K2CO3
DMF, NaOAc
2.5
4
120
130
96
98
45c
400
Pd (OAc)2 : [H2L1][TfO]2 (1 : 1)
This work
13
C6H5Br + alkyl acrylate
1
DMF, Na2CO3 20
145
92
45f
14
C6H5Br + alkyl acrylate
1
H2O, K2CO3
DMF, NaOAc
5
50
95
96
45i
15
C6H5Br + alkyl acrylate 400
Pd (OAc)2 : [H2L1][TfO]2 (1 : 1)
3.5
130
This work
a
TBAI ¼ tetrabutylammonium iodide, TEA ¼ triethylamine, NMP ¼ N-methyl-2-pyrrolidone.
4. Conclusion
Conflicts of interest
In conclusion, ve new ligands, [H2L1][OTf]2–[H2L5][I]2, were There are no conicts of interest among authors to declare.
synthesized from two new precursors, [P3Et][I] and [P2
]
Me
[CF3SO3]. These ligands were successfully synthesized by the
melt reaction between precursor and corresponding diamine.
All the synthesized compounds were successfully characterized
by various analytical techniques such as 1H and 13C NMR, FTIR,
single crystal XRD and DFT studies. These ligands acted very
efficiently as co-catalysts for palladium acetate in Heck–Mizor-
oki cross coupling reactions. These ligand systems are useful
addition in the eld of electron donor ligands such as NHC and
phosphines that can stabilize palladium in different oxidation
states during C–C coupling reactions.
Acknowledgements
The authors gratefully acknowledged Higher Education
Commission (HEC) Pakistan for research funding.
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