Oxine based unsymmetrical (O?, N, S/Se) pincer ligands and their palladium(ii) complexes: synthesis, structural aspects and applications as a catalyst in amine and copper-free Sonogashira coupling

Article abstract of DOI:10.1039/c7nj00067g

Unsymmetrical pincer ligands having an 8-hydroxyquinoline (oxine) core viz. 2-(phenylthio/selenomethyl) quinolin-8-ol (L1/L2), 2-(N,N-dimethylthiocarbamoyl) quinolin-8-ol (L3) and 2-(pyrrolidin-1-ylthiocarbamoyl) quinolin-8-ol (L4) were synthesized. 2-Met

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The role of atropisomers on the photo-reactivity and fatigue of
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ARTICLE TYPE
Oxine based unsymmetrical (O−, N, S/Se) pincer ligands and their
palladium(II) complexes: synthesis, structural aspects and applications
as catalyst in amine and copperꢀfree Sonogashira coupling
Satyendra Kumar,a Fariha Saleem,a Manish Kumar Mishra,b and Ajai K. Singha*
5
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The unsymmetrical pincer ligands having 8ꢀhydroxyquinoline (oxine) core viz. 2ꢀ(phenylthio/
selenomethyl) quinolinꢀ8ꢀol (L1/ L2), 2ꢀ(N,Nꢀdimethylthiocarbamoyl) quinolinꢀ8ꢀol (L3) and 2ꢀ
(pyrrolidinꢀ1ꢀylthiocarbamoyl) quinolinꢀ8ꢀol (L4) were synthesized. 2ꢀMethylquinolinꢀ8ꢀol was
10 converted to 2ꢀbromomethylquninolinꢀ8ꢀol, which reacted with PhENa (E= S or Se) to give L1 and L2,
WillgerodtꢀKindler reaction on an appropriate aldehyde derivative of quinoline gave L3 and L4. On
reaction with Na2PdCl4/[Pd(CH3CN)2Cl2], L1−L4 coordinated as a (O‾, N, E) donor (E = S/Se) resulting
in complexes [Pd(L−H)Cl] (1–4; L= L1−L4). Molecular structures of L1, 1 and 2 were established with
single crystal Xꢀray diffraction. Palladium in 1 and 2 has nearly square planar geometry. The Pd−S bond
15 distance in 1 is 2.2648(14) Å and in 2, the Pd–Se bond distance is 2.3641(7) Å. Somewhat rare week
interactions (viz. C−H···Pd and Se···Cl) were noticed in the crystals of 1 and 2 respectively. Complexes 1
and 2 were found efficient to catalyze Sonogashira coupling under amine and copper free conditions. The
catalyst loading of 0.5−1.0 mol% was found promising for conversion of several aryl halides to their
coupled products. Yields were lower for ArCl in comparison to ArBr/ArI. The catalytic activity of 1 was
20 marginally lower than that of 2. DFT calculations support the catalytic activity order and bond lengths
and angles of 1 and 2.
pincers and separation protocols often reduce their yield.8d,9ꢀ11
Introduction
The carbon is a common central atom in pincer frameworks,
45 resulting M−C bond on complexation. Pd complexes of pincers
having PdꢀC bond are similar to palladacycles, popular as preꢀ
catalysts for crossꢀcoupling reactions. Its replacement with
another donor atom e.g. N, P or Si may lead to substantial
variation in the catalytic activity. It improves many fold in some
50 cases.6 The unsymmetrical backbone skeleton may reduce the
strength of M–D bond (depending on the nature of D), which in
conjunction with two E/E’–M bonds in side arms can release
active Pd(0) species faster if the combination of E and E’ results
in a hemilabile ligand system (Fig. 1).10,11 Thus unsymmetrical
55 pincer ligands based on Nꢀheterocycle framework (e.g. indole or
quinoline) may result in efficient catalysts and are worth
exploring.12ꢀ19 The coordination chemistry of quinoline
derivatives is well established.15ꢀ19 However, quinoline as a core
unit for designing pincer ligand has been scantly explored. The
60 only examples in our knowledge is that of quinolineꢀbased
(P,N,F)/(O,N,P) pincer investigated by Vigalok and coꢀ
workers15,19a for designing Pdꢀcomplexes. The complexes of
Pd(II) with pincer ligands (symmetrical and unsymmetrical) with
chalcogen donor atoms (S/Se), have emerged as a family of
65 efficient catalysts for various CꢀC coupling reactions6,9,20ꢀ25
including Sonogashira coupling. They are more stable under
ambient conditions in comparison to their organophosphorus
analogues and often easy to synthesize.23ꢀ25 Organochalcogen
ligands generally contain combination of hard and soft donor
Metal complexes of pincer ligands are of current interest1ꢀ7
and acquire stability due to binding of the pincers with metal ions
25 in a tridentate mode.6 A wide range of donor groups (such as NR2,
PR2, OR, SR, SeR , AsR2, halogen, etc.)8 are present in the two
arms of these ligands, which generally form five or sixꢀmembered
chelate rings with the metal ion on complexation.4 The pincer
ligands considered important for designing of transition metal
30 based catalysts6 are symmetrical as well as unsymmetrical.
Fig. 1 Representation of metal-pincer complexes
The two arms of an unsymmetrical pincer ligand have different
35 donor atom (Fig. 1) and/or length. Thus with unsymmetrical
pincers the advantages of two electronically different donor
atoms and / or chelate rings of different sizes can be afforded.
Both these things enhance the possibility of hemilability
favourable for catalytic activity. The number of catalysts based
40 on unsymmetrical pincer ligands,4,6 is less known than those of
their symmetrical counterparts. This is because less convenient
multiꢀstep reactions are required to prepare unsymmetrical
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Schemes 1 and 2. The signal in 77Se{1H} NMR spectrum of 2
was found deshielded by ~33.9 ppm, with respect to that of free
L2, which appears at 372.1 ppm, supporting coordination of L2
atoms, which result in hemilabile character, considered important
in catalytic activation and sensing.5,7
Palladium(II) complexes/palladacycles of several phosphorus,
oxime, NHCs, imine, thiocarboxamide/semicarbazone, and 1,2,3ꢀ
triazole based ligands are known to activate Sonogashira
coupling.26ꢀ35 The in situ formation of Pd(0) species (discrete or
nanoꢀsized) has also been reported in the catalytic process.35b,36ꢀ
1
65 with Pd via Se donor site. In H NMR of 1 and 2, a broad singlet
5
of H10 (−SCH2/−SeCH2) appears at 5.33 and 5.14 ppm
respectively, shielded by ~0.94 and 0.86 ppm with respect to
those of free ligands L1 and L2 respectively (See ESI; Figs. S2
and S4).
38,39
Among metal complexes of pincer ligands known as a
10 catalyst for Sonogashira coupling, the majority has symmetrical
pincer e.g. (P,C−,P),40 (N,C−,N),41 (S,C−,S),42a (C,N,C),42b (P, N,
P)42c and (E,C−,E) where E = SiII or GeII.39 The catalytic
activation with them generally requires CuI as a coꢀcatalyst.39,41,42
The use of unsymmetrical pincer ligand based metal complex as a
15 catalyst for this coupling reaction is rare. The examples in our
knowledge are Pd(II)ꢀcomplexes of (P,N,F) and (N,N,C)
pincer.15,43 They have shown good catalytic activity for
Sonogashira coupling under Cuꢀfree condition. The metal
complexes of pincer ligands bearing combination of different
20 donor groups15,39ꢀ43 are, therefore, worth exploring as they may
result in exciting applications in catalysis of CꢀC coupling
including Sonogashira. Palladium complex of a (O, N, E) pincer
having a combination of ‘hardꢀsoft’ donor sites may have the
advantages of hemilabile feature, which facilitates oxidative
25 addition of substrate to the metal centre. When quinoline is
central back bone of unsymmetrical pincer, (contributing N as a
central donor atom) its metal complex can dynamically dissociate
to generate coordinatively unsaturated active species.39,43 Thus
quinoline based unsymmetrical pincers are worth exploring to
30 design catalyst of high stability and reactivity.
70
Scheme 1 Synthesis of L1 and L2 and their Pd(II) complexes
Thus current submission is focussed on (O−, N, E)
unsymmetrical pincer ligand (E = S/Se) (L1−L4) based on
quinoline core and their Pd(II) complexes [Pd(L−H)Cl] (1–4; L=
L1−L4). The 1 and 2 were found promising for catalysis of
35 Sonogashira coupling of ArX (X = Cl, Br, I) under Cu and amine
free conditions. Other substituent presents on Ar influence the
yield of coupled product. The use of 0.5 to 1 mol% of 1 or 2,
gives up to 91% yield in a reaction time of the order 3 to 12 h.
Scheme 2 Synthesis of L3 and L4 and their Pd(II) complexes
Results and discussion
75
40 In Schemes 1 and 2 syntheses of L1−L4 along with their Pd(II)
complexes 1−4 are shown. The ligands L3 and L4 were
synthesized by WillgerodtꢀKindler reaction on their aldehyde
precursors.44a This reaction, a oneꢀpot and threeꢀcomponent
process, is among important methods known for the synthesis of
45 thioamides (Scheme 2).44b The complexes 1−4 can be stored
under ambient conditions for several months. Palladium(II)
complexes of organochalcogen ligands are known for their longer
stability than their organophosphorus counterparts.23,24 The
L1−L4 were found soluble in common organic solvents. The 1−2
50 showed moderate solubility in common organic solvents except
for DMSO and DMF in which it was very good. The solubility of
3 and 4 in DMSO/DMF was only moderate. The structures of
ligands (L1−L4) and their palladium complexes (1−4) were
authenticated with C, H and N analyses, 1H, 13C{1H} and
55 77Se{1H} NMR, FTꢀIR, highꢀresolution mass spectrometry (HRꢀ
MS) and Xꢀray diffraction on single crystals in the case of L1, 1
and 2.
Probably pyramidal inversion at S/Se causes this broadening. It
results in two interconverting degenerate isomers due to the
exchange of syn and anti configuration of the phenyl group
attached to S/Se relative to methylene protons (−SCH2/−SeCH2).
80 The signal of (−SCH2/−SeCH2) in 13C{1H} NMR spectra of L1
and L2 appears at 40.9 and 33.8 ppm respectively. On
complexation, the signals shift to 53.3 and 45.5 ppm respectively,
deshielded by ~12.4 and 11.7 ppm (See ESI; Figs. S14 and S16).
The signal of –OH in 1H NMR spectra (recorded in DMSOꢀd6) of
85 free L3 and L4 appears at 9.76 and 9.74 ppm respectively. The
disappearance of –OH signal in 3 and 4 supports the coordination
of L3 and L4 with palladium via O‾. The signal of >C=S group in
13C{1H} NMR spectra of L3 and L4 appearing at 197.3 and 193.1
ppm respectively, on complexation gets shielded (∼4.6 and 4.8
90 ppm) and appears at 192.7 and 188.3 ppm for complexes 3 and 4
respectively (See ESI; Figs. S19 and S21). This may be attributed
due to reduction in C=S bond order on coordination of L3/L4
with palladium through the S donor site.44c
The highꢀresolution mass spectra (HRꢀMS) of all ligands and
95 complexes are consistent with their simulated HRꢀMS (See ESI;
Figs. S22ꢀS29). In HRꢀMS of L1−L4 peak of [M+H]+ was
1
NMR and mass spectra. H, 13C{1H} and 77Se{1H}(for 2 and
60 L2) NMR spectra (See in ESI Figs. S2ꢀS21) of L1−L4 and 1−4
were found consistent with their molecular structures shown in
2
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observed. In the case of complexes 1 and 2, the peaks appearing
at m/z = 429.9250 and 477.8680 are due to [M+Na]+. In case of
complexes 3 and 4, peaks appear at m/z = 354.9731 and
380.9937, respectively. They may be assigned to
[(M−Cl).(H2O)]+ fragment.
5
Crystal structures. The single crystal structures of L1, 1 and 2
were solved. Their single crystals of L1, complex 1 and 2 suitable
for Xꢀray diffraction were grown by slow evaporation of their
10 solutions made in the chloroformꢀhexane mixture (1:6). However
attempts made to grow single crystals of Pd(II) complexes 3 and
4 suitable for Xꢀyay diffraction in several solvents (pure as well
as mixtures) were unsuccessful. Therefore, their structures could
not be determined. The ORTEP diagrams of L1 and 1 and 2 are
15 shown in Figs. 2ꢀ4. The Tables S1ꢀS4 in ESI contain crystal data,
refinement details and selected bond distances and angles of L1
and complexes 1 and 2. Each ligand from L1−L4 forms two fiveꢀ
membered chelate rings with palladium, resulting in complexes
1−4. The geometry of palladium in both 1 and 2 is distorted
20 squareꢀplanar. The S/Se is in a position trans to O‾ whereas
chloride and N are trans to each other. The Pd−S bond lengths of
1, 2.2648(14) Å is close to that of palladacycle of 2,3ꢀ
bis[(phenylthio)methyl]quinoxaline [2.259(2) Å],45a and Pd(II)
complex of a sulfated Schiff base of 1ꢀhydroxyꢀ2ꢀacetophenone
25 [2.2704(16) Å].45b In 2, the Pd–Se bond distance [2.3641(7) Å] is
very close to the value, 2.3654(10) Å, reported for palladacycle
of an indoleꢀbased unsymmetrical (N, C‾, Se) pincer9 and
consistent with the values reported for Pd(II) complexes of
selenoether ligands (2.385(5) Å).25b,c The Pd−N bond distances in
30 both 1 and 2 are almost same [1.958(4) and 1.962(4) Å] and
consistent with the value [2.053(2) Å] reported for Pd(II)
complex of a quinolineꢀbased (P, N, F) pincer.15 The Pd−O bond
lengths in 1 and 2 are 2.042(3) and 2.049(3) Å respectively. They
are somewhat shorter than the Pd−O bond length, 2.159(8) Å,
35 reported earlier15 but close to that of Pd(II) complex of (E)ꢀ8ꢀ
hydroxyquinolineꢀ2ꢀcarbaldehyde Oꢀbenzyl
45 Fig. 3 Molecular structure of 1. Bond lengths (Å): Pd(1)─N(1) 1.958(4);
Pd(1)─Cl(1) 2.3001(14); Pd(1)─O(1) 2.042(3); Pd(1)─S(1) 2.2648(14).
Bond angles (°): N(1)─Pd(1)─S(1) 85.23(12); N(1)─Pd(1)─O(1)
83.13(15); O(1)─Pd(1)─Cl(1) 95.10(10); S(1)─Pd(1)─Cl(1) 96.52(5).
50
Fig. 4 Molecular structure of 2. Bond lengths (Å): Pd(1)─N(1) 1.962(4);
Pd(1)─Cl(1) 2.3001(14); Pd(1)─O(1) 2.049(3) Pd(1)─Se(1) 2.3641(7).
Bond angles (°): N(1)─Pd(1)─O(1) 83.03(15); O(1)─Pd(1)─Cl(1)
95.30(11); Se(1)─Pd(1)─Cl(1) 95.38(4); Se(1)─Pd(1)─N(1) 86.41(11).
55 oxime [1.986(3)].16 The Pd−Cl bond lengths in 1 and 2 are
2.3001(14) and 2.3001(14) Å respectively and normal.25
The packing and nonꢀcovalent interactions in the crystals of
L1, 1 and 2 are interesting. Only weak and rare intermolecular
interactions hold the molecules together and stabilize the crystal
60 structure. One molecule of L1 is connected to six other molecules
of L1 through weak C−H···π, O−H···π and C−H···S
intermolecular interactions (See ESI; Fig. S1ꢀa). The π···π
interactions between the two molecules (See ESI; Fig. S1ꢀb)
further contribute to the stabilization of crystal structure.
65
One molecule of the complex 1 is surrounded by six other
similar molecules connected through nonꢀcovalent and weak
intermolecular C−H···O and C−H···Cl interactions (Fig. 5a). In
contrast to the crystal of L1, no C−H···π interaction is present in
the crystal structure of 1 but the molecules of 1 stacked together
70 have π···π interactions (Fig. 5b). A rare weak C−H···Pd
intermolecular interaction is also noticed in this crystal structure
(Fig. 5c). It is worth to note here that the anagostic C−H···Pd
intermolecular interaction is important in the constructions of the
supramolecular network present in the crystal.46a However, the
75 C−H···Pd interaction has also been considered important to
understand the catalytic reactions.46b
Fig. 2 Molecular structure of L1 Bond lengths (Å): S(1)―C(11) 1.757(3);
40 S(1)―C(10) 1.795(3); O(1)―C(1) 1.357(3); N(1)―C(9) 1.315(3);
N(1)―C(6) 1.363(3); Bond angles(º): C(11) ―S(1) ―C(10) 104.78(13);
C(9)―N(1)―C(6)
118.5(2);
O(1)―C(1)―C(6)
119.2(2);
C(12)―C(11)―S(1) 116.0(2).
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applications in the synthesis of optical/electronic materials, liquid
20 crystals, drugs (antibiotics, antimycotics etc.) and polymers,33a,42b
was explored with complexes 1ꢀ4. The 3 and 4 have poor
solubility in organic solvent and this turned out a limitation for
their applications to Sonogashira coupling. In attempts to catalyze
this coupling in several solvents / bases with them, the reaction
25 mixture turned black immediately or within 15 ꢀ 30 min and no
coupled product was observed. The situation did not change even
in the presence of copper/amine. However performance of 1 and
2 as catalysts for Sonogashira coupling was found promising.
Therefore, coupling of aryl bromide with phenylacetylene
30 catalyzed with 2 was first optimized. For this purpose, 4ꢀ
bromobenzaldehyde was used as a substrate and in its coupling
with phenylacetylene, complex 2 was employed as a catalyst,
under amine and Cuꢀfree conditions. The conversion into coupled
(a)
1
product was monitored with H NMR. The results for various
35 organic solvents and bases are given in Table 1. The N2
atmosphere was used in the catalysis as in the presence of air,
conversion reduced significantly (Table 1; entry 3). For best
results, the reaction mixture was degassed with N2 and thereafter
solution of catalyst made in DMF was added slowly. In the
40 presence of Cu(I) as a coꢀcatalyst in situ formation of Cu(I)
acetylide, and its oxidative dimerization to diphenyldiacetylene
(Glaser coupling)32b,34c,d may occur to some extent and its
removal from the coupled product may require good separation
strategy. Therefore the reaction in the absence of Cu as a coꢀ
45 catalyst is desirable.
(b)
(c)
Fig. 5 Secondary interactions in the crystal of complex 1
Table 1 Optimization of reaction condition for Sonogashira coupling
reaction catalyzed with 2a
One molecule of complex 2 is connected to five other similar
molecules through weak C−H···O, C−H···π and C−H···Cl
intermolecular interactions (Fig. 6a). Interestingly, a weak nonꢀ
covalent Se···Cl interaction is present between two molecules of
2. Such interactions are considered important in the synthesis of
various organic compounds.46c The selenium···halogen
interactions are useful in the designing of several types of
crystalline organoselenium compounds and also for seeing other
10 types of nonꢀcovalent interactions, in which a halogen atom is
taken as an electron donating group. Similar to L1 and complex
1, the molecules of complex 2 in the crystal are engaged in π ···π
interactions (Fig. 6b).
4ꢀBr−C6H4−CHO + PhCCH
→
4ꢀCHO−C6H4−CCPh
Entry No.
Solvent
DMA
DMF
DMF
DMSO
DMF
DMF
Toluene
THF
THF
Toluene
Base
Yieldc (%)
5
1
2
K2CO3
K2CO3
K2CO3
KOH
86
89
52
ꢀꢀ
60
<10
38
ꢀꢀ
3b
4
5
6
7
8
9
10
KtOBu
KOH
K2CO3
K2CO3
KtOBu
KtOBu
ꢀꢀ
47
aReaction conditions: 0.5 mmol of 4ꢀbromobenzaldehyde, 0.6 mmol of
50 phenylacetylene, 1.0 mmol of base, 0.5 mol % complex 2, 3 mL of dry
solvent, N2 atmosphere, temperature of bath 90 °C, reaction time 6 h,
breaction was performed in air for 12 h, cIsolated yield in %.
The best results were obtained with K2CO3 and DMF (Table 1;
entry 2). In the coupling of electronꢀdeficient aryl bromides,
55 impurity with coupled product was found insignificant. In the
case of deactivated (electronꢀrich) aryl halides, small impurities
were noticed and removed from the coupled product with column
chromatography. The crossed coupled product was not obtained
in the absence of 1 or 2, and reactant ArBr recovered, ruling out
60 the possibility of palladiumꢀfree coupling. The present
Sonogashira coupling protocol being amine free, may be labelled
as somewhat environmentally friendly.34c The isolated yields of
the crossꢀcoupled products for various substrates are given in
Table 2. For 4ꢀbromobenzaldehyde the yield of the coupled
65 product was ∼90% when loading of 1 or 2 was 1 mol% (Table 2;
entry 1). The yield reduced slightly on reducing the catalyst
(a)
Fig. 6 Interactions in the crystal of complex 2.
(b)
15
Catalysis of Sonogashira coupling
The catalysis of Sonogashira coupling (Csp2−Csp) having many
|Journal Name, [year], [vol], 00–00
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Rꢀ4ꢀC6H4ꢀX + R’CCH → 4ꢀR−C6H4−CCR’ (X= Br, I)
loading to half but increasing the reaction time (Table 2; entry 2).
The yield of coupled product for 4ꢀbromoacetophenone was 58
and 66% with 1.0ꢀ0.5 mol% loading of 2 and 1 respectively
(Table 2, entry 4). The loading of 1.0ꢀ0.5 mol% of 1/2 as a
Entry
No.
1
2
3
4
5
6
Aryl halide
Alkyne
1
2
Yieldb Yieldc
Iodobenzene
Bromobenzene
4ꢀIodoacetophenone
4ꢀBromoacetophenone
Iodobenzene
Bromobenzene
4ꢀIodoacetophenone
4ꢀIodoanisole
1ꢀEthynylꢀ4ꢀ
(trifluoromethyl)benzene
55
<10
43
27
61
50
ꢀꢀ
56
41
51
47
70
65
15
14
5
Table 2 Sonogashira coupling reaction catalyzed with complexes 1 and 2a
3ꢀEthynylꢀthiophene
4ꢀR−C6H4−X + PhCCH → 4ꢀR−C6H4−CCPh (X= Cl, Br, I)
Entry
No.
Aryl halide
1
2
Mol% t (h) Yieldb Mol% t (h) Yieldb
7
8
Ethynyltriꢀ
isopropylsilane
1
2
3
4
5
6
7
8
4ꢀBromobenzaldehyde
4ꢀBromobenzaldehyde 0.5
4ꢀBromobenzonitrile
4ꢀBromoacetophenone 0.5
4ꢀBromonitrobenzene 0.5
1
6
8
8
12
6
6
12
8
12
12
15
86
84
91
66
98
72
61
56
35
28
36
1
0.5
1
6
10
8
12
6
6
12
6
91
89
95
58
94
79
63
72
56
31
28
ꢀꢀ
45 aReaction conditions: 1.0 mmol of aryl halide, 1.1 mmol of alkyne, 2.0
mmol of base (K2CO3), 2 mol% of catalysts 1 or 2, 3 mL dry DMF,
temperature of bath 90 °C, N2 atmosphere, bIsolated yield in %.
1
1
0.5
0.5
1
1
1
Bromobenzene
4ꢀBromotoluene
4ꢀIodoanisole
0.5
1
0.5
1
2
2
The comparison of catalytic efficiency of 1/2 for Sonoagashira
coupling with those of other Pd(II) complexes of symmentrical/
50 unsymmetrical pincers (in presence/absence of coꢀcatalyst) is
important. On using Pd(II) complex40a of a (P,C,P)ꢀpincer as a
catalyst good conversion can be achieved with its 5 mol% loading
in presence of ZnCl2 (10 mol%) as a coꢀcatalyst) at 160 °C.
Aminophosphineꢀbased (P,C,P) pincer complex catalyzes
55 Sonogashira Coupling under Cu and amine free conditions with
1ꢀ2 ppm loading of catalyst which much lower than our catalysts.
Though the catalysis is carried out in environmentally more
9
4ꢀBromoanisole
12
12
15
10 4ꢀChlorobenzaldehyde
11 4ꢀChloronitrobenzene
2
2
aReaction conditions: 1.0 mmol of aryl halide, 1.1 mmol of
phenylacetylene, 2.0 mmol of base (K2CO3), 3 mL dry DMF, temperature
of bath 90 °C, N2 atmosphere, bIsolated yield in %.
10 catalyst resulted in good conversion of 4ꢀbromobenzonitrile and
4ꢀbromonitrobenzene into coupled product (Table 2, entries 3 and
5). In the case of deactivated and neutral aryl halides (viz. 4ꢀ
bromotoluene, bromobenzene and 4ꢀbromoanisole and 4ꢀ
iodoanisole), yield of the crossꢀcoupled product was good with
15 1.0ꢀ0.5 mol% loading of 1 or 2 (Table 2, entries 6ꢀ9). The
coupling of 4ꢀbromoanisole and 4ꢀbromotoluene, with alkyne was
negligible when catalyst was 0.5 mol%. The increase in the
amount of catalyst 1/2 up to 1 mol% results in significant
coupling (Table 2; entries 7 and 9). 4ꢀIodoanisole and 4ꢀ
20 bromobenzene gave good yield of the coupled product in 6ꢀ8 h
with 0.5 mol% of 1 or 2 (Table 2; entries 6 and 8). For some
substrates (Table 2, entries 1, 3 and 6ꢀ9), the activity of 1 was
marginally lower than that of 2. The activated 4ꢀ
chlorobenzaldehyde and 4ꢀchloronitrobenzene under optimum
25 conditions for coupling with alkyne gave ∼28ꢀ36 % yield with 2
mol% loading of 1/2 in 12ꢀ15 h (Table 2; entries 10 and 11).
The scope of 1 and 2 for catalysis of Sonogashira coupling was
explored for other alkynes and results are summarized in Table 3.
For this purpose ArBr/ArI were reacted with 1ꢀethynylꢀ4ꢀ
30 (trifluoromethyl)benzene, 3ꢀethynylꢀ thiophene and ethynyltriꢀ
isopropylsilane under the optimized reaction conditions at 1
mol% of catalyst loading. The 2 having Se ligand was found
somewhat more effective with these alkynes than 1, as the
conversions were good (Table 3; entries 1ꢀ6). Complex 1 gave
35 good results with 1ꢀethynylꢀ4ꢀ(trifluoromethyl)benzene and 3ꢀ
ethynylꢀthiophene when they were reacted with iodobenzene
(Table 3; entries 1 and 5). Both 1 and 2 were found ineffective
with ethynyltriꢀisopropylsilane (Table 3; entries 7 and 8) as the
reactivity of aliphatic alkynes is significantly lower than aromatic
40 ones.
benign solvents but at higher temp (140 °C)40b
. The protocols
with 1/ 2 are much milder than those of (P, C, P) pincer and free
60 from the requirement of coꢀcatalyst. In efficiency 1/2 is better
than Pd(II) complexes of diimino (N,C,N)41a pincer and pyrazole
based (N,C,N) pincer catalyzes Sonogashira coupling of ArI
using amine as a solvent/base at moderate temperature but the
catalyst loading is found to be only 0.1 mol% of Pd.41b (S,C,S)
65 pincers42a as for good yield their 1ꢀ2 mol% loading is required
and the reaction has to be run for 12ꢀ18 h.
The
Pd(II)
complexes designed with unsymmetrical (P,N,F)15 and (N,N,C)
pincers43 give good results at their 1ꢀ2 mol% loading (Cuꢀfree)
and moderate reaction temperature (55 and 80 °C respectively).
70 BisꢀNHCs based (C,N,C) pincers form Pd(II)34b,42b, complexes
efficient for Sonoagashira coupling. The optimum loading is 0.1ꢀ
1.7 mol% and comparable with that of the present catalysts. The
catalytic activity of bis(PyꢀtzNHC)ꢀPd(II)35a complex is also
comparable with those of 1 and 2.
75
The homogeneous nature of catalyst 1/2 in Sonoagashira
coupling was supported by mercury poisoning and two phase
tests.15,34b,35,47 In the representative Hg poisoning experiment,
reaction of 4ꢀbromobenzaldehyde with alkyne was catalyzed with
2 under optimum conditions (Table 2, entry 1), in the presence of
80 a large excess of elemental Hg (Pd: Hg:: 1: 400) added at
different time to the catalytic reaction as given in Table 4. The
reaction was continued in each case with vigorous stirring after
addition of Hg up to 8 h. The conversion into coupled product
1
(monitored with H NMR), did not reduce significantly on the
85 addition of Hg at different stages and almost reached to
maximum after 8 h of reaction. The results of the poisoning test
reveal that catalytic process is not quenched in the presence by
Hg.
Table 3 Sonogashira coupling of various alkynes catalyzed with
1 and 2a
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Table 4 Mercury poisoning experiment for Sonogashira coupling with 2a
45 The density functional theory (DFT) calculations made for 1 and
2 gave a qualitative idea of the lowest energy configuration and
frontier orbitals of the complexes. For both the complexes 1 and
2, the HOMO−1 (highest occupied molecular orbital) is
contributed by Pd dꢀorbital, pꢀorbitals of S or Se, chlorine and
50 oxygen as shown in Fig. 7. The complex 2 having lower value
HOMO−LUMO energy gap48 is expected to be a better catalyst
than 1 (Fig. 7), as found experimentally in case of present
Sonogashira coupling reactions, of course marginally only.
The experimentally observed and calculated (by DFT)
55 distances for Pd−Cl, Pd−N, and Pd−O bonds are consistent. Some
variations exist between calculated and observed Pd−E (E = S/Se)
bond distances but it is not exceptional.35b The DFTꢀcalculated
and experimentally found bond angles are also close to each other
(See Tables S5 and Table S1ꢀS4 in ESI).
Entry time
No.
% Conversion
at the time of Hg addition
% Conversionb after 8 h of
Hg addition
1
1
2
3
0 h
0.5
2 h
3 h
Nil
15
47
81
83
77
78
85
aReaction conditions: 4ꢀbromobenzaldehyde (1.0 mmol), 1.1 mmol of
phenylacetylene, 2.0 mmol of K2CO3, 3 mL DMF, temperature of bath 90
°C, N2 atmosphere, 1 mol % complex 2, After standard workup of the
b
5
whole reaction mixture).
The blackening of catalytic reaction mixture due to the formation
of colloidal Pd was also not noticed. Therefore the formation of
Pd cluster or NPs is unlikely in significant amount as indicated by
Hg poisoning test.15,34b The twoꢀphase test47a,b (also called threeꢀ
10 phase test when catalyst is in solid form) as shown in Scheme 3
was also applied (See experimental details in ESI). 4ꢀ
Bromobenzaldehyde and 4ꢀbromobenzoic acid (as amide)
immobilized on silica were treated with phenyl acetylene under
optimum reaction conditions (given
60
15
Scheme 3 Twoꢀphase test
Fig. 7 Frontier molecular orbital diagrams of complexes 1−2
in Table 2; entry 1) loading 2 mol% of complex 2 as catalyst. In
case of heterogeneous catalysis, the substrate immobilized on
20 solidꢀphase is not expected to be converted into a coupled
product. In the present case anchored substrate is converted ~81%
to the coupled product. This implies that the required
Pd(0)15,34b,23,24 is released from 1/2 and drives the catalysis
homogeneously.
Conclusions
Unsymmetrical pincer ligands (O−, N, Eꢀtype) where E=S/Se,
65 having quinoline as a core unit and their palladium complexes of
the type [Pd(L−H)Cl] (1−4) (where L = L1−L4) have been
synthesized and characterized by multinuclear NMR, IR and
mass spectrometry. Singleꢀcrystal structures of L1, complexes 1
and 2 were solved. The geometry around Pd in 1 and 2 is
70 distorted square planar. Some interesting intermolecular rare
week interactions (such as C−H···Pd and Se···Cl) were found in
1 and 2 which stabilizes their crystal structures. The 1 and 2 act
as a catalyst for amine and Cuꢀfree Sonogashira coupling. The
catalyst loading 0.5−1.0 mol% is optimum for the coupling of
75 aryl halides with terminal alkynes viz phenyl acetylene, 1ꢀ
ethynylꢀ4ꢀ(trifluoromethyl)benzene, 3ꢀethynylꢀ thiophene and
ethynyltriꢀisopropylsilane. The results of DFT calculations
support the experimental observation that Pd(II) complex 2 (Se
ligated) is marginally better for Sonogashira coupling than the
80 complex 1 (sulfur analogue). The experimentally observed and
theoretically calculated (by DFT) bond lengths and angles are
consistent.
25
In solid state, complexes have good thermal stability as
revealed by their m.p.’s. H NMR spectrum of solution of 1/2
1
made in DMSOꢀd6, on heating at 120 °C for 0.5 h, does not show
any change. This indicates that on using 1/2 as a catalyst in
Sonogashira coupling carried out at 90 °C their just thermal
30 decomposition to new catalytic species is unlikely. The stability
of complexes is also supported by thermogravimetric analysis
(TGA) plots of complexes 1−4 (See Figs. S30–S33 in ESI),
which do not have any sign of decomposition below 200 °C. Due
to skeleton of the ligand present in the complex the weight loss in
35 the temperature range 200−400 °C occurs. Thus overall good
thermal stability of complexes 1 /2 in conjunction with the results
of poisoning and twoꢀphase experiment suggests that the catalysis
with 1/2 occurs homogeneously via in situ generated
Pd(0)15,34b,35a which appears to be real catalytic species for this
40 coupling. However, it may be protected with ligands L1/L2 or
their fragments in the present case.23,24
Experimental
85
DFT calculations
Diphenyl diselenide, sodium borohydride, 2ꢀ(hydroxymethyl)ꢀ
6
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quinolinꢀ8ꢀol,
phenyl
acetylene,
1ꢀEthynylꢀ4ꢀ
Syntheses of L1 and L2: 2ꢀ(Bromomethyl)quinolinꢀ8ꢀyl acetate
60 (0.560 g, 2.0 mmol) prepared from 2ꢀ(hydroxymethyl)ꢀquinolinꢀ
8ꢀol by reported method55 was dissolved in ethanol. A solution of
PhSNa/PhSeNa (2.0 mmol) generated in situ by the reaction of
NaOH with thiophenol/NaBH4 reduction of diphenyldiselenide at
70 °C under nitrogen atmosphere was added dropwise. The
(trifluoromethyl)benzene, 3ꢀEthynylꢀthiophene and Ethynyltriꢀ
isopropylsilane were used as received from SigmaꢀAldrich
(USA). For thiophenol, sulfur powder, N,Nꢀdimethylamine
hydrochloride, pyrrolidine and aryl halides local resources were
used. Bis(acetonitrile)dichloropalladium(II) was prepared by a
5
known procedure.49 The ligands, their precursors and coupled 65 reaction mixture was heated at 70 °C for 8 h and cooled to room
products were purified by column chromatography on silica gel
(60−120 mesh). nꢀHexane and its mixtures with chloroform/ethyl
10 acetate in varying proportions were used as eluent. Glassware
dried under ambient condition was used for all reactions. Melting
temperature. Its solvent was reduced to 4ꢀ5 mL on a rotary
evaporator and mixed with 50 mL of water. The mixture was
neutralized with 10% HCl and extracted with chloroform (3 × 20
mL). The organic extracts were combined, washed with water (3
points were determined by taking the sample in a glass capillary 70 × 30 mL) and dried over anhydrous sodium sulfate. Its solvent
sealed at one end, with an apparatus equipped with electric
heating and reported as such. The commercially available
15 nitrogen gas was purified by passing it successively through traps
containing solutions of alkaline anthraquinone sodium dithionite,
alkaline pyrogallol, conc. H2SO4 and KOH pellets. A nitrogen
atmosphere was created using Schlenk techniques.
was evaporated off on a rotary evaporator resulting L1/L2 as
yellow solid. Further purification was done by column
chromatography on silica gel using hexaneꢀchloroform mixture
(95:5) as an eluent.
75
L1: 2ꢀ(phenylthiomethyl)quinolinꢀ8ꢀol, Light yellow crystalline
solid, Yield: (0.485 g, 91%); m.p. 78 °C; Anal. Found: C, 71.85;
H, 4.80; N, 5.75 %. Calcd. for [C16H13NOS]: C, 71.88; H, 4.90;
20 Physical measurements
1
N, 5.64 %. H NMR (300 MHz, CDCl3, 25 °C, TMS); δ (ppm):
Bruker Spectrospin DPX 300 NMR spectrometer was used to 80 4.39 (s, 2H, H10), 7.12−7.17 (m, 1H), 7.19−7.28 (m, 4H),
1
record H, 13C{1H} and 77Se{1H} NMR spectra at 300.13, 75.47
and 57.24 MHz respectively. The chemical shifts are reported
25 relative to internal standards. 13C DEPT NMR was used routinely
to determine the number of hydrogen atoms linked to a carbon
7.34−7.42 (m, 3H), 7.49 (d, 1H, H6, J= 8.4), 8.04 (d, 1H, H7, J=
8.7). 13C{1H} NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm):
40.9 (C10), 110.1, 117.5, 121.6, 126.6, 127.2 (C5), 127.4, 128.9
(C13), 130.1 (C12), 135.3 (C8), 136.8, 137.3, 151.9 (C9), 156.0
atom. IR spectra (4000−400 cm−1) in KBr were recorded on a 85 (Cq1). IR (KBr; cm−1): 478 (w), 570 (w), 746 [m; νC−H (bending)],
Nicolet Protége 460 FTꢀIR spectrometer. Elemental analyses
were carried out with a Perkin–Elmer 2400 Series II C, H, N
30 analyzer. Thermogravimetric analyses (TGA) (up to 700 ºC) were
carried out on a PerkinꢀElmer Pyris Diamond thermogravimetric
analyzer (N 535ꢀ0010).
1134 (m; νC−O), 1238 (m), 1361 [m; νC−H (rocking)],1437 [m; νC−C
(aromatic)], 1505 [m; νC−H (aromatic)], 1632 [w; overtones],
2914 [s; νC−H (aliphatic)], 3051 [m; νC−H (aromatic)], 3401 [b;
νO−H]. HRꢀMS [M + H] (m/z) = 268.0790; calcd. value for
90 C16H14NOS = 268.0791 (error δ : 0.4 ppm).
Bruker AXS SMART Apex CCD diffractometer with MoꢀKα
(0.71073 Å) source was used to collect singleꢀcrystal data. The
35 software SADABS50 was used for absorption correction and
SHELXTL for space group, structure determination, and
L2: 2ꢀ(phenylselenomethyl)quinolinꢀ8ꢀol, Yellow solid, Yield:
(0.554 g, 88%); m.p. 73 °C; Anal. Found: C, 56.15; H, 4.17; N,
6.16 %. Calcd for [C16H13NOSe]: C, 56.22; H, 3.97; N, 6.04 %.
refinement.51 For hydrogen atoms included in idealized positions 95 1H NMR (300 MHz, CDCl3, 25 °C, TMS); δ (ppm): 4.28 (s, 2H,
isotropic thermal parameters were set at 1.2 times those of the
carbon atoms to which they were bonded The leastꢀsquares
40 refinement cycles on F2 were performed until the model
converged. Bruker Micro TOFꢀQ II machine using electron spray
H10), 7.03−7.06 (m, 1H), 7.15−7.20 (m, 3H), 7.26−7.34 (m, 3H),
7.40−7.43 (m, 2H), 7.96 (d, 1H, H7, J = 8.7). 13C{1H} NMR (75
MHz, CDCl3, 25 °C, TMS): δ (ppm): 33.8 (C10), 110.1, 117.5,
122.0, 127.0 (C5), 127.3, 127.8 (C13), 128.9, 131.6 (C11), 133.9
ionization (10 eV, 180 °C source temperature and sodium formate 100 (C12), 136.7, 134.4 (C8), 151.9 (C9), 157.2 (C1). 77Se{1H} NMR
as a calibrant) was used for highꢀresolution mass spectral (HRꢀ
MS) measurements on solutions made in CH3CN. HRꢀMS was
45 simulated using program developed by Scientific Instrument
Services.52 All DFT calculations were carried out at the
(57 MHz, CDCl3, 25 °C, Me2Se): δ (ppm) 372.1. IR (KBr; cm−1):
476 (w), 529 (w), 774 [m; νC−H (bending)], 1071 (m; νC−O), 1134
(w), 1333 [m; νC−H (rocking)], 1421 [m; νC−C (aromatic)], 1517
[m; νC−H (aromatic)], 1607 [w; overtones], 2923 [s; νC−H
Supercomputing Facility for Bioinformatics and Computational 105 (aliphatic)], 3043 [m; νC−H (aromatic)], 3436 [br; νO−H]. HRꢀMS
Biology, IIT Delhi, with the GAUSSIANꢀ0353 programs. The
geometries of complexes 1−2 were optimized at the B3LYP54
50 level using an SDD basis set for metal atoms and chalcogen and
6ꢀ31G* basis sets for C, N, and H. Geometry optimization was
[M + H] (m/z) = 316.025047; calcd. value for C16H14NOSe =
316.023571 (error δ: ꢀ4.7 ppm).
Syntheses of L3: 2ꢀFormylquinolinꢀ8ꢀyl acetate (4.30 g, 2.0
carried out without any symmetry restriction with Xꢀray 110 mmol) prepared from 2ꢀ(hydroxymethyl)quinolinꢀ8ꢀol as reported
coordinates of the molecule. Harmonic force constants have been
computed at the optimized geometries to characterize the
55 stationary points as minima. The molecular orbital plots are
created using the Chemcraft program package (http://www.
chemcraftprog.com).
earlier,56 was taken in round bottom flask with sulfur powder
(0.086 g, 3.0 mmol), CH3COONa (0.246 g, 3.0 mmol), and N,Nꢀ
dimethylamine hydrochloride (0.243 g, 3.0 mmol). The mixture
was heated at 100 °C in presence of 5 mL of DMF. It turned dark
115 brown after sometime under ambient conditions. The heating was
continued further for 6 h. Thereafter this mixture was cooled to
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room temperature and poured into cold water (~5 ºC) with
53.4 (C11), 53.8 (C14), 112.7, 118.0, 122.0, 128.6 (C5), 128.7,
stirring. The stirring was continued until a yellow precipitate 60 136.5 (C8), 137.0, 154.0 (C9), 156.6 (C1), 192.4 (C10). IR (KBr;
appeared. It was separated by filtering through G4 crucible. The
solid residue left in the crucible was dissolved in CHCl3 (30 mL)
and washed with 20 mL of water (only once). The organic phase,
so obtained, was dried over anhydrous sodium sulfate and its
cm−1): 465 (w), 507 (w), 754 [m; νC−H (bending)], 1109 (m; νC−O),
1240 (m), 1320 [m; νC−H (rocking)], 1395 [m; νC−C (aromatic)],
1489 [m; νC−H (aromatic)], 1561 [m; νC−H (aromatic)], 1631 [w;
νC−H overtones], 2968 [s; νC−H (aliphatic)], 3042 [m; νC−H
5
solvent was evaporated off on a rotary evaporator resulting L3 as 65 (aromatic)], 3375 [br; νO−H]. HRꢀMS [M
+ H] (m/z) =
a yellow solid. Its further purification was done by column
chromatography on silica gel using hexaneꢀethyl acetate mixture
259.089806; calcd. value for C14H15N2OS = 259.089961 (error δ :
ꢀ0.6 ppm).
10 (95:5) as an eluent.
Synthesis of Pdꢀcomplexes 1 and 2: To a solution of L1 (0.106
L3: 2ꢀ(N,Nꢀdimethylthiocarbamoyl) quinolinꢀ8ꢀol, Light yellow 70 g, 0.4 mmol)/L2 (0.126 g, 0.4 mmol) in acetone (5 mL) was
solid, Yield: (0.431 g, 93%); m.p. 122 °C; Anal. Found: C, 61.88;
H, 5.01; N, 11.98 %. Calcd. for [C12H12N2OS]: C, 62.04; H, 5.21;
15 N, 12.06 %. H NMR (300 MHz, CDCl3, 25 °C, TMS); δ (ppm):
added a solution of [Na2PdCl4] (0.120 g, 0.41 mmol) in water (2
mL). The appearance of pale yellow colour indicated the
formation of the complex. The reaction mixture was stirred for 6
h at room temperature. After completion of the reaction, the
1
3.22 (s, 3H, H12), 3.65 (s, 3H, H11) 7.16−7.19 (m, 1H), 7.30−7.33
(m, 1H), 7.43−7.48 (m, 1H), 7.69 (d, 1H, H6, J= 8.4 Hz), 75 solvent was removed under reduced pressure on a rotary
7.80−8.08 (br s, 1H, OH), 8.17 (d, 1H, H7, J= 8.7). 13C{1H} NMR
(75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 42.9 (C12), 43.7 (C11),
20 110.9, 107.7, 121.4, 127.5 (C5), 128.3, 135.9 (C8), 136.9, 152.1
(C9), 156.0 (C1), 197.3 (C10).
evaporator and the residue left in the flask was mixed with water
(20 mL). The mixture was extracted with CHCl3 (3 × 10 mL).
The organic layers were combined together, washed with water
(20 mL) and dried over anhydrous sodium sulfate. Its volume was
1H NMR (300 MHz, DMSOꢀd6, 25 °C, TMS); δ (ppm): 3.21 (s, 80 reduced (~1 mL) with a rotary evaporator and hexane was added
3H, H12), 3.58 (s, 3H, H11), 7.12 (d, 1H, J= 6.9 Hz), 7.39−7.47
(m, 2H), 7.61 (d, 1H, H6, J= 9 Hz ), 8.35 (d, 1H, H7, J= 8.4 Hz),
25 9.76 (s, 1H, OH). IR (KBr; cm−1): 508 (w), 549 (w), 749 [m; νC−H
(bending)], 1146 (m; νC−O), 1239 (m), 1318 [m; νC−H (rocking)],
till precipitation of the complex as a yellow solid was complete.
The precipitate was filtered and dried in vacuo.
Complex 1; Yellow solid, Yield: (0.138 g, 85%); m.p. 178 °C
1453 [m; νC−C (aromatic)], 1535 [m; νC−H (aromatic)], 1628 [w; 85 (d); Anal. Found: C, 29.91; H, 1.96; N, 2.02 %. Calcd for
1
overtones], 2928 [s; νC−H (aliphatic)], 3053 [m; νC−H (aromatic)],
3438 [br; νO−H]. HRꢀMS [M + H] (m/z) = 233.074558; calcd.
30 value for C12H13N2OS = 233.074310 (error δ : 1.1 ppm).
[C16H12ClNOPdS]: C, 30.07; H, 2.17; N, 2.30 %. H NMR (300
MHz, CDCl3, 25 °C, TMS); δ (ppm): 5.04−5.63 (br m, 2H, H10),
6.80 (d, 1H, J= 8.1 Hz), 7.10 (d, 1H, J= 7.8 Hz), 7.40−7.45 (m,
1H), 7.49−7.58 (m, 3H), 7.61 (d, 1H, H6, J= 8.7 Hz), 7.91−7.94
Syntheses of L4: In a round bottom flask 2ꢀformylquinolinꢀ8ꢀyl 90 (m, 2H), 8.49 (d, 1H, H7, J= 8.7 Hz). 13C{1H} NMR (75 MHz,
acetate (4.30 g, 2.0 mmol), sulfur powder (0.086 g, 3.0 mmol),
pyrrolidine (0.221 g, 3.0 mmol) and 5 mL DMF were taken. The
35 mixture was heated for 30 min at 100 °C with protection from
moisture. The temperature was maintained for 6 h by heating it
CDCl3, 25 °C, TMS): δ (ppm): 53.3 (C10), 111.7, 114.5, 119.3,
128.8 (C5), 129.1 (C11), 130.0 (C6), 130.5, 130.7, 130.9 138.5,
142.3 (C8), 157.2 (C9), 170.8 (C1). IR (KBr; cm−1): 457 (w), 508
(w), 756 [m; νC−H (bending)], 1018 (m; νC−O), 1197 (m; νC−C),
further. On completion of the reaction, the resulting dark brown 95 1373 [m; νC−H (rocking)],1439 [m; νC−C (aromatic)], 1587 [m;
solution was cooled to room temperature. Cold water (40 mL)
was poured into the reaction mixture with stirring resulting in a
40 dark yellow precipitate. After a work up similar to the one used
for L3, dark yellow solid (L4) was obtained.
νC−H (aromatic)], 1650 [w; νC−H; overtones], 2922 [s; νC−H
(aliphatic)], 3056 [m; νC−H (aromatic)], 3432 [br; νO−H]. HRꢀMS
[M + Na] (m/z) = 429.9250; calcd. value for C16H12ClNNaOPdS
= 429.9258 (error δ : 1.8 ppm).
100
L4: 2ꢀ(pyrrolidinꢀ1ꢀylthiocarbamoyl) quinolinꢀ8ꢀol Dark yellow
solid, Yield: (0.474 g, 92%); m.p. 114 °C; Anal. Found: C, 64.85;
45 H, 5.32; N, 10.66 %. Calcd for [C14H14N2OS]: C, 65.09; H, 5.46;
Complex 2: Yellow solid, Yield: (0.148 g, 82%); m.p. 172 °C
(d); Anal. Found: C, 38.81; H, 2.61; N, 2.58 %. Calcd for
1
[C16H12ClNOPdSe]: C, 38.91; H, 2.66; N, 2.60 %. H NMR (300
1
N, 10.84 %. H NMR (300 MHz, CDCl3, 25 °C, TMS); δ (ppm):
MHz, CDCl3, 25 °C, TMS); δ (ppm): 4.84−5.44 (br m, 2H, H10),
2.01−2.14 (m, 4H, H12ꢀ13), 4.04 (t, 2H, H11, J= 6.3 Hz), 3.68 (t, 105 6.79 (d, 1H, J= 8.1 Hz), 7.03 (d, 1H, J= 7.8 Hz), 7.39−7.42 (m,
2H, H14, J= 6.0 Hz), 7.17−7.19 (d, 1H, H4, J= 6.9 Hz), 7.33 (d,
1H, H2, J= 8.1 Hz), 7.44−7.49 (m, 1H, H3), 7.82 (d, 1H, H6, J=
50 8.4 Hz), 8.17 (d, 1H, H7, J= 8.4). 13C{1H} NMR (75 MHz,
CDCl3, 25 °C, TMS): δ (ppm): 24.2 (C13), 26.4 (C12), 53.4
1H), 7.48−7.56 (m, 4H), 8.01−8.03 (m, 2H), 8.41 (d, 1H, H7, J=
8.7 Hz). 13C{1H} NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm):
45.5 (C10), 112.1, 114.9, 120.8, 128.6 (C5), 129.6 (C11), 130.6
(C13), 130.7, 131.0, 132.6 (C12), 138.6, 143.5 (C8), 158.9 (C9),
(C14), 53.5 (C11), 110.8, 117.8, 121.8, 127.8 (C5), 128.5, 135.8, 110 171.1 (C1). 77Se{1H} NMR (57 MHz, CDCl3, 25 °C, Me2Se): δ
136.8 (C8), 152.2 (C9), 156.2 (C1), 193.1(C10).
(ppm) 406.0. IR (KBr; cm−1): 440 (w), 521 (w), 741 [m; νC−H
(bending)], 1020 (m; νC−O), 1154 (m; νC−C), 1276 (w), 1361 [m;
νC−H (rocking)], 1444 [m; νC−C (aromatic)], 1564 [m; νC−H
(aromatic)], 1632 [w; νC−H; overtones], 2919 [s; νC−H (aliphatic)],
1H NMR (300 MHz, DMSOꢀd6, 25 °C, TMS); δ (ppm):
55 1.94−2.02 (m, 4H, H12ꢀ13), 3.70−3.72 (m, 2H, H14), 3.84−3.86 (m,
2H, H11), 7.10ꢀ7.12 (m, 1H), 7.40−7.49 (m, 2H), 7.71−7.73 (m,
1H), 8.31−8.35 (m, 2H), 9.74 (s, 1H, ꢀOH). 13C{1H} NMR (75 115 3053 [m; νC−H (aromatic)], 3436 [br; νO−H]. HRꢀMS [M + Na]
MHz, DMSOꢀd6, 25 °C, TMS): δ (ppm): 24.3 (C13), 26.5 (C12),
(m/z) = 477.8680; calcd. value for C16H12ClNNaOPdS =
8
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477.8706 (error δ : 5.4 ppm).
protect it from moisture. Thereafter solution of palladium
60 complex 1 or 2 (0.5–1 mol%) (0.5ꢀ1 mol%) made in DMF was
added and the mixture heated at 90° C for an optimum time under
nitrogen atmosphere. The progress of reaction was monitored by
1H NMR. When maximum conversion of ArX into coupled
product occurred, the reaction mixture was cooled to room
Syntheses of palladium complexes 3 and 4: To a solution of L3
(0.092 g, 0.4 mmol)/L4 (0.103 g, 0.4 mmol) in acetonitrile (5
5
mL)
was
added
a
solution
of
bis(acetonitrile)
dichloropalladium(II) (0.103 g, 0.4 mmol). The appearance of
dark yellowꢀgreen colour indicated the formation of the complex. 65 temperature and extracted with ethylacetate (2×10 mL) and
The reaction mixture was stirred for 4 h at room temperature.
After completion of the reaction, the solvent was removed under
10 reduced pressure and the residue left in the flask was mixed with
water (20 mL). The complex was extracted with CHCl3 (3 × 10
washed with water (2×15 mL). After drying over anhydrous
Na2SO4, the solvent of organic phase was evaporated off with
rotary evaporator. The resulting residue was purified by column
chromatography on silica gel (60−120 mesh) using nꢀhexane and
mL) and extracts combined together. The combined extract was 70 its mixtures with chloroform/ethyl acetate in varying proportions
washed with water (2 × 20 mL) and dried over anhydrous sodium
sulfate. Its volume was reduced to ~1 mL with a rotary
15 evaporator and mixed with nꢀhexane to complete precipitation of
the complex. The Pd(II) complex precipitated as yellow solid was
filtered and dried vacuo.
as eluent. The yields are reported in Table 2. The authentication
1
by matching H and 13C{1H} NMR with literature data15,35 was
carried out for each coupled product. The spectra are given in
ESI.
75 Acknowledgements
Complex 3: Yellow solid, Yield: (0.128 g, 79%); m.p. 187 °C
Council of Scientific and Industrial Research, Department of
Atomic Energy (BRNS), and Nanomission, Department of
Science and Technology, India supported the work through the
award of projects. SK, and FS thank UGC (India).
20 (d); Anal. Found: C, 43.32; H, 2.58; N, 5.31 %. calcd for
[C12H11ClN2OPdS]: C, 43.45; H, 2.60; N, 5.40 %. H NMR (300
1
MHz, DMSOꢀd6, 25 °C, TMS); δ (ppm): 3.71 (s, 3H, H12), 3.97
(s, 3H, H11), 6.67 (d, 1H, H4, J= 7.8 Hz), 6.98 (d, 1H, H2, J= 6.9
Hz), 7.50ꢀ7.52ꢀ (m, 1H, H3), 7.98 (d, 1H, H6, J= 9.6 Hz), 8.49 (d,
25 1H, H7, J= 9.0 Hz). 13C{1H} NMR (75 MHz, DMSOꢀd6, 25 °C,
TMS): δ (ppm): 47.7 (C12), 48.9 (C11), 111.3, 115.6, 122.7,
132.7, 136.2 (C5), 138.5, 145.5 (C8), 147.2 (C9), 173.4 (C1),
192.7 (C10). IR (KBr; cm−1): 515 (w), 548 (w), 762 [m; νC−H
(bending)], 1098 (m; νC−O), 1339 (w), 1378 [m; νC−H (rocking)],
30 1499 [m; νC−C (aromatic)], 1541 [m; νC−H (aromatic)], 1621 [w;
νC−H overtones], 2960 [s; νC−H (aliphatic)], 3048 [m; νC−H
(aromatic)]. HRꢀMS [(M – Cl) .H2O] (m/z) = 354.9744; calcd.
value for C12H13N2O2PdS = 354.9731 (error δ : ꢀ3.7 ppm).
80 Notes and References
aDepartment of Chemistry, Indian Institute of Technology Delhi, New
Delhi–110016,
India.
E–mail:
aksingh@chemistry.iitd.ac.in,
ajai57@hotmail.com; FAX: +91 11 26581102; Tel: +91 11 26591379;
bDepartment of Chemistry, McGill University, Montreal, QC H3A 0B8
85 Canada.
† Electronic Supplementary Information (ESI) available: Spectral data of
L1–L4 and 1–4; single crystal data of L1 and complexes 1 and 2 (CCDC
1523404–1523406), See DOI: 10.1039/b000000x/
1
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90
95
35 Complex 4: Yellow solid, Yield: (0.134 g, 75%); m.p. 181 °C
(d); Anal. Found: C, 41.88; H, 3.25; N, 6.98 %. Calcd for
2
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[C14H13ClN2OPdS]: C, 42.12; H, 3.28; N, 7.02 %. H NMR (300
3
4
MHz, DMSOꢀd6, 25 °C, TMS); δ (ppm): 2.09−2.16 (m, 4H, H12ꢀ
13), 4.02−4.07 (m, 2H, H14), 4.42−4.47 (m, 2H, H11), 6.65 (d, 1H,
40 H4, J = 7.8 Hz), 6.95 (d, 1H, H2, J = 7.5 Hz), 7.51 (m, 1H, H3, J =
7.8 Hz), 8.01 (d, 1H, H6, J = 9 Hz), 8.45 (d, 1H, H7, J = 9.3 Hz).
13C{1H} NMR (75 MHz, DMSOꢀd6, 25 °C, TMS): δ (ppm): 23.7
(C13), 26.7 (C12), 57.5 (C14), 57.6 (C11), 111.4, 115.5, 121.6,
132.6 (C5), 136.0, 138.4, 145.2 (C8), 146.8 (C9), 173.1 (C1),
45 188.3 (C10). IR (KBr; cm−1): 536 (w), 589 (w), 762 [m; νC−H
(bending)], 1011 (m; νC−O), 1228 (m), 1339 [m; νC−H (rocking)],
1441 [m; νC−C (aromatic)], 1548 [m; νC−H (aromatic)], 1657 [w;
νC−H overtones], 2863 [s; νC−H (aliphatic)], 3023 [m; νC−H
(aromatic)], 3416 [br; νO−H]. HRꢀMS [(M – Cl) .H2O] (m/z) =
50 380.9937; calcd. value for C14H15N2O2PdS = 380.9889 (error δ : ꢀ
12.7 ppm).
100
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Oxine based unsymmetrical (O−, N, S/Se) pincer ligands and their
palladium(II) complexes: synthesis, structural aspects and applications as
catalyst in amine and copperꢀfree Sonogashira coupling
SatyendraKumar,aFarihaSaleem,a Manish Kumar Mishrab and Ajai K. Singha*
aDepartment of Chemistry, Indian Institute of Technology Delhi, New Delhi, India.
bDepartment of Chemistry, McGill University, Montreal, QC H3A 0B8 Canada
Newly synthesized and characterized (single crystal structure),[Pd(O−, N, S/Se)Cl],
efficiently catalyse Sonogashira coupling of ArX at 0.5ꢀ1 mol%.