Catalytic activity of new heteroleptic [Cu(PPh3)2(β-oxodithioester)] complexes: Click derived triazolyl glycoconjugates

Article abstract of DOI:10.1039/c8nj05075a

A series of four new and two known luminescent heteroleptic Cu(i) complexes of the form [Cu(PPh₃)₂(β-oxodithioester)] (β-oxodithioester = methyl-3-hydroxy-3-(2-furyl)-2-propenedithioate L1 1, methyl-3-hydroxy-3-(2-thienyl)-2-propened

Full text of DOI:10.1039/c8nj05075a

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Catalytic activity of new heteroleptic [Cu(PPh ) -
3
2
(b-oxodithioester)] complexes: click derived
Cite this: New J. Chem., 2019,
triazolyl glycoconjugates†‡
43, 1166
a
a
a
a
Kavita Kumari,
Vinod K. Tiwari,
Anoop S. Singh, Krishna K. Manar, Chote Lal Yadav,
a
b
a
Michael G. B. Drew and Nanhai Singh
*
A series of four new and two known luminescent heteroleptic Cu(I) complexes of the form [Cu(PPh
3 2
) -
(b-oxodithioester)] (b-oxodithioester = methyl-3-hydroxy-3-(2-furyl)-2-propenedithioate L1 1, methyl-3-
hydroxy-3-(2-thienyl)-2-propenedithioate L2 2, methyl-3-hydroxy-3-(4-methoxyphenyl)-2-propene-
dithioate L3 3, methyl-3-hydroxy-3-(4-bromophenyl)-2-propenedithioate L4 4, methyl-3-hydroxy-3-
benzyl-2-propenedithioate L5 5 and methyl-3-hydroxy-3-(3-pyridyl)-2-propenedithioate, L6 6) have
1
13
1
been synthesized and characterized by elemental (C, H, N) analysis, and IR, UV-visible, H, C{ H},
31
1
and P{ H} NMR spectroscopy and their structures have been ascertained by X-ray crystallography. These
complexes were exploited as catalysts for azide–alkyne cycloaddition reactions (click chemistry) for the
synthesis of triazolyl glycoconjugates, where they displayed efficient catalytic activity at room temperature.
Optimization of the reaction conditions and formation of regioselective products in high yields in the
absence of a base/additive are concomitant with the click protocol. The substrate scope was successfully
extended by varying the types of sugar azides and alkyne scaffolds, affording the corresponding triazole
products in excellent yields at low catalyst loadings (1 mol%). Furthermore, catalyst 1 showed excellent
recyclability and was reused for five cycles with little decrease in efficiency.
Received 6th October 2018,
Accepted 14th November 2018
DOI: 10.1039/c8nj05075a
rsc.li/njc
the development of new drugs, materials science, fluorescent
Introduction
4
–7
imaging and synthetic polymers. Unlike the previously used
8
Within the realm of click chemistry discovered independently Cu(II) salts and sodium ascorbate reductant generated catalyti-
1
,2
3
by Sharpless and Meldal, copper(I) mediated cycloaddition cally active Cu(I) species in situ, the use of preformed Cu(I)
reactions of azides and alkynes have occupied an important complexes under the click approach avoids the need for redu-
position because of their wide impact in diverse areas such as cing agents in the reaction mixture. The presence of ligands in
this approach is critical because they protect the Cu(I) species
a
from oxidation, facilitate mild reaction conditions and enhance
the reactivity of the overall reaction. The majority of work
Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi-221005, India. E-mail: nsinghbhu@gmail.com, nsingh@bhu.ac.in;
Fax: +91-542-2386127
9
reported so far for this reaction has been centered around the
b
Department of Chemistry, University of Reading, Whiteknights,
Reading RG6 6AD, UK
10
utilization of ligands, N-heterocyclic carbenes (NHCs), which
are potential catalysts after complexation with the Cu(I) ion.
Several Cu(I) complexes bearing N and P containing ligands such
†
Part of the work was presented at the FCASI 2017: International Conference on
Frontiers at the Chemistry–Allied Sciences Interface (July 22–23, 2017), University
of Rajasthan, Jaipur, Rajasthan.
1
1
12
as tris(2-dioctadecylaminoethyl)amines, tristriazolylamines,
1
3
‡
Electronic supplementary information (ESI) available: Fig. S1: simulated and
tris(benzimidazolylmethyl)amines, tertiary phosphines, phos-
phinite, phosphonite and phosphoramidite have served as effi-
cient catalysts for this reaction. Cu(I) carboxylate and acetate
complexes have shown remarkable catalytic activities for azide–
experimental PXRD patterns for 1–6; Fig. S2: molecular structures of 2–6 with
ellipsoids at 30% probability showing the atom numbering scheme; Fig. S3–S6:
supramolecular structure of complexes 1, 3 and 4 sustained by HÁ Á ÁH and
C–HÁ Á ÁO interactions, respectively; Fig. S7: UV-Vis absorption spectra of ligands
1
4
L1–L6; Fig. S8: details of the orbitals and transitions for 1; Fig. S9: UV-Vis alkyne cycloaddition reactions. In fact the copper centre plays a
1
15–17
spectrum of 1 showing transition from HOMOÀ3 to LUMO; Fig. S10: H and
crucial role in these catalytic transformations.
13
C NMR spectra of triazolyl glycoconjugates (9a–h); Fig. S11: synthesis of triazolyl
Metal complexes of organosulfur ligands have been widely
employed as catalysts for a variety of reactions such as oxygen
activation, hydrosilylation, asymmetric Diels–Alder, asymmetric
glycoconjugates 9a–h; Fig. S12: characterization data of the synthesized ligands;
Fig. S13: generalized methodology for the synthesis of complexes 1–6. CCDC
1
510265, 1510264, 1510266, 1532702, 1562997 and 1869643. For ESI and crystallo-
1
8
graphic data in CIF or other electronic format, see DOI: 10.1039/c8nj05075a
epoxidation and other common reactions. The catalytic
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activities of Cu(I) complexes with O/S donor ligands have been and has more diffused d orbitals in comparison with the hard
rarely studied; however, in recent years, a few preformed Cu(I) and small O donor atom with no d orbitals. This difference in
complexes of thiodiacetate and thiobenzoate ligands have been their characteristics may significantly influence the polarity and
explored for the catalytic transformation of triazole glycoconju- stability of the Cu–O/Cu–S bonds and hence the catalytic
1
9
gates through the click approach.
The heteroleptic [Cu(I) phosphines-X/1,1-dithiolates] complexes form stable six-membered chelate rings, with the metal provid-
X = Cl, I/1,1-dithiolates = dithiocarbamate, dithiocarbimates) ing greater electron delocalization over the ring; (v) the Py(N) on
activities of the complexes; (iv) the b-oxodithioester ligands
(
14c–e
continue to reveal unexpected catalytic
and luminescent the substituent may provide greater electron delocalization over
20
10
properties because in a d system, low-lying dd* states are not the ring and therefore may enhance the catalytic activity of the
present; hence, the emission bands arise from the MLCT/ILCT or complex; (vi) tertiary phosphines with strong s-donor and
admixture of the two states. Furthermore, photoluminescent Cu(I) p-acceptor characteristics are well known ligands in low-valent
complexes have gained considerable attention because of their transition metal chemistry, often producing novel complexes
possible applications in solar energy schemes, sensors, electro- which display superior activity in catalytic transformations.
2
1
chemical devices, biological imaging agents and display devices.
Furthermore the introduction of b-oxodithioester ligands together
Despite their synthetic versatility and known practical applica- with PPh as a co-ligand may tune the electronic, steric and
tions, Cu(I) complexes with a somewhat similar b-oxodithioester chemical activities of the complexes; and (vii) the metal–ligand
22
3
2
2a–c
ligand bearing phosphine ligands have been rarely studied
scaffold in these complexes may show impressive luminescence
and their luminescence characteristics and catalytic activities have characteristics. The results of these investigations are described
not been investigated so far. It is therefore critical to make in this contribution.
changes to the metal coordination sphere by using functiona-
lized b-oxodithioesters (Scheme 1) with chelating (O,S) hetero
donors and PPh
catalytic activities and luminescence characteristics.
Considering together these facts, we were inspired to study
the syntheses, crystal structures, catalytic activities and lumi-
nescent properties of four new (1, 2, 4, 6) and two known (3, 5)
functionalized heteroleptic [Cu(PPh ) (b-oxodithioester)] com-
plexes. The important reasons for undertaking this work stem
from the following considerations: (i) the click transformations
are largely dependent on the understanding of the mechanism(s)
involved as well as on the modification of the ligand backbone
together with the metal–ligand co-operativity effect; (ii) studies
on the application of Cu(I) mediated ‘azide–alkyne’ cycloaddition
3
as a co-ligand in order to map out their Results and discussion
Synthesis and characterization
The six new ligands, KL1–KL6, were synthesized adopting the
2
2d,e
2
2a previously reported method.
The auburn coloured, air- and
moisture stable photoluminescent heteroleptic Cu(I) b-oxodithio-
3
2
ester complexes bearing PPh ligands 1–6 were obtained in good
3
yields by reacting a dichloromethane solution of [Cu(PPh ) NO ]
3
2
3
with KL1–KL6 in an equimolar ratio. These complexes have been
characterized by elemental analysis and spectroscopy (IR, UV-Vis,
1
13
1
31
1
H, C{ H} and P{ H} NMR) and their solid state structures
have been determined by X-ray crystallography. A comparison of
the experimental PXRD patterns of complexes 1–6 with their
respective simulated powder patterns obtained from the single
crystal data ratify that the bulk crystalline products from the
synthesis are the same as the single crystals (Fig. S1, ESI‡). All six
complexes were examined for their catalytic activities through the
click approach.
1
9
reactions in carbohydrate chemistry are scarce in the literature;
1
0
(
iii) Cu(I) with d configuration is a soft Lewis acid and shows
strong affinity for the S atom which is distinctly soft and large
Spectroscopy
In the IR spectra of the free b-oxodithioester ligands L1–L6, the
characteristic nC–OH, nCQS and nCQC vibrations appear in the
À1
1
124–1178, 1226–1238 and 1576–1613 cm regions, respec-
tively. For complexes 1–6, the nCQO, nC–S and nCQC vibrations
À1
observed at 1572–1601, 1032–1094 and 1485–1571 cm
respectively are indicative of dithioester ligand coordination.
,
1
22a
The H NMR spectra are in accordance with the literature.
1
In the H NMR spectra of the b-oxodithioesters L1–L6, in CDCl
3
,
a single resonance at d 14.70–15.18 ppm due to the –OH proton
of the ligands is absent in complexes 1–6 due to keto–enol
tautomerization. The vinylic proton resonances observed in the
range d 6.76–6.94 ppm and d 6.47–6.98 ppm for the ligands
and their corresponding complexes, respectively, are virtually
1
unchanged. The H NMR spectra of all complexes show a single
Scheme 1 Potassium salts (KL1–KL6) of the b-oxodithioester ligands
utilized in this work.
resonance due to the –SMe group at d 2.44–2.58 ppm. In the
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1
3
1
C{ H} spectra, the C–OH carbon resonances at d 159.37–169.63 Table 2 Selected bond lengths and angles for 1–4
and d 171.18–182.05 ppm in the free ligands and complexes,
respectively, are indicative of the O,S-coordination of the
b-oxodithioester ligands. The vinylic carbon in the range d
1
2
3
4
Bond lengths (Å)
Cu1–O11
Cu1–S15
2.061(6)
2.278(2)
2.260(2)
2.260(2)
1.245(10)
1.423(12)
1.397(12)
1.692(9)
1.763(9)
2.061(2)
2.2807(8)
2.2569(8)
2.2593(8)
1.256(3)
1.413(4)
1.398(4)
1.684(3)
1.762(3)
2.062(6)
2.284(2)
2.287(2)
2.252(2)
1.256(9)
1.420(10)
1.424(11)
1.668(9)
1.773(8)
2.058(4)
2.2801(17)
2.2897(15)
2.2504(16)
1.259(7)
1.415(8)
1.406(8)
1.677(7)
1.767(6)
106.54–107.92 ppm for the ligands is slightly downfield shifted
at d 109.48–110.53 ppm in the complexes. A perceptible upfield Cu1–P1
Cu1–P2
shift in the C–S carbon at d 189.29–200.00 ppm in the complexes
O11–C12
in comparison to free ligands at d 215.92–217.32 ppm is indica-
C12–C13
3
1
tive of metal–ligand coordination. In the P NMR spectra of C13–C14
C14–S15
complexes 1–6 at room temperature appearance of a sharp
C14–S16
resonance signal at d À1.16 to À1.88 ppm is indicative of
two equivalent phosphorus atoms bonded to the Cu centre. Bond angles (1)
P1–Cu1–P2
Also, the presence of one additional sharp low field resonance
O11–Cu1–S15
113.23(9)
97.89(18)
110.20(9)
116.06(9)
105.58(19)
112.8(2)
111.99(3)
98.01(6)
109.67(3)
115.57(3)
106.82(6)
114.14(6)
129.6(3)
110.86(9)
97.63(17)
110.15(8)
118.86(9)
107.09(15)
111.43(16)
129.2(8)
110.91(6)
96.89(13)
109.93(6)
119.69(6)
108.23(11)
110.13(12)
128.9(6)
at d 29.64–29.84 ppm (1–6) is indicative of the formation of
P1–Cu1–S15
+
[
Cu(PPh
leptic Cu(I) thiocarbamates
bearing phosphine ligands have been reported to associate/dissoci-
3
)] cations via association/dissociation in solution. Hetero- P2–Cu1–S15
23,24
25
O11–Cu1–P2
O11–Cu1–P1
C14–C13–C12
and thiophene-2-thiocarboxylates
128.8(8)
ate in solution to give an equilibrium mixture of different species. S15–C14–S16
S2 (ESI‡) shows the H, C and P spectra of complexes 1–6.
118.9(5)
118.77(16)
120.2(5)
119.2(3)
1
13
31
Crystal structures
chelate ring that shows electron delocalization over the ring.
Complexes 1–6 were structurally characterized by single crystal The isomorphous structures of 5 and 6 were disordered with
X-ray diffraction techniques. Auburn crystals of the complexes the chelate ring having two different orientations called A and
were obtained by slow evaporation of a dichloromethane/methanol B, with the oxygen and sulphur reversed. In the refinements,
solution of the crude products. A summary of the crystallographic the copper atoms and all atoms in the chelate ring were given
data and refinement parameters is given in Table 1, and the different positions, with those in A and B given occupations of x
selected bond distances and bond angles are listed in Table 2. and 1 À x, respectively, x refining to 0.533(4) and 0.457(4) in 5
Fig. 1 depicts the ORTEP diagram of a representative complex 1 and 6, respectively. In the refinements, the distances in A and B
with displacement ellipsoids at 30% probability. The remaining were fixed to be equivalent and for that reason, the dimensions
complexes 2–6 have similar molecular structures (Fig. S3, ESI‡).
around the metal are not considered here. However DFT
In all the six structures, the geometry about Cu(I) is four- calculations showed that when ordered structures were con-
coordinate tetrahedral where the Cu atom is bonded to the sidered, their dimensions were similar to those found in 1–4. As
phosphorus atoms of the two PPh
3
and the (O,S) donor atoms expected, in all four complexes, the Cu1–S15 distances of
of a chelating b-oxodithioester ligand, forming a six-membered 2.278(2)–2.284(2) Å are reasonably longer than the Cu1–O11
Table 1 Crystallographic data and refinement parameters for complexes 1–6
Compound
1
2
3
4
5
6
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
44
H
37CuO
2
P
2
S
2
C
44
H
37CuOP
2
S
3
C
47
H
41CuO
2
P
2
S
2
C
46
H
38BrCuOP
2
S
2
C
46
H
39CuOP
2
S
2
C
45 2 2
H38CuNOP S
787.33
Monoclinic
P2 /n
8.9785(6)
18.6193(11)
23.3402(14)
97.175(6)
3871.3(4)
4
803.39
Monoclinic
P2
9.0481(4)
18.9483(10)
23.0819(13)
99.327(4)
3905.0(4)
4
827.40
Orthorhombic
P2
9.0195(9)
24.130(3)
18.6511(16)
90
4059.2(7)
4
1.354
876.27
Orthorhombic
P2
8.9969(4)
18.5997(8)
24.0165(11)
90
4018.9(3)
4
797.37
Monoclinic
P2
10.8413(8)
21.3325(8)
17.2051(9)
100.112(6)
3917.2(4)
4
798.36
Monoclinic
P2 /n
1
10.8070(8)
21.1948(11)
17.1444(12)
100.228(7)
3864.6(4)
4
1
1
/n
1
2
1
2
1
1
2
1
2
1
1
/n
B (1)
3
V (Å )
Z
À3
rcalc (g cm
)
1.351
1.367
1.448
1.352
1.372
T (K)
m(Mo Ka) (mm
F(000)
150(2)
0.791
1632
150(2)
0.836
1664
150(2)
0.758
1720
150(2)
1.757
1792
150(2)
0.781
1656
150(2)
0.793
1656
À1
)
Reflections collected
22 471
8824
6162
0.1082, 0.3092
0.1493, 0.3393
1.015
21 651
8680
6137
24 900
11 419
6610
17 975
11 256
8482
0.0673, 0.1005
0.0964, 0.1103
0.989
15 262
7512
5934
19 104
6697
5739
Independent reflections
Reflections with I 4 2s(I)
Final indices [I 4 2s(I)] R
a
b
2
1
, wR
0.0484, 0.0925 0.0835, 0.1425
0.0791, 0.1046 0.1481, 0.1681
1.027
0.0831, 0.1270 0.1045, 0.1518
0.1075, 0.1348 0.1222, 0.1568
1.251
a
b
R
1
, wR
2
[all data]
GOF
0.974
1.523
Max., min. residual electron density 3.079, À0.760
0.536, À0.579
1.300, À0.815
0.872, À1.028
0.389, À0.357
0.606, À0.446
À3
(e Å
)
P||F
| À |F
||/P
|F
R
2
= {[P
w(F
À F
)/ w(F
P
) ]} , w = 1/[s (F
) + (xP) ], where P = (F
+ 2F
)/3.
a
b
2
2
2 2 1/2
o
2
2
o
2
2
o
2
c
R
1
=
o
c
o
|.
o
c
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bis-dithiolate complexes in sustaining the supramolecular
2
6
structures. They defined the structural features of this inter-
action as involving the C–H bond and the centroid CG of the
four-membered MS C ring. The parameters for the interaction
2
were defined as a, the angle between the vector perpendicular
to the chelate ring and the CGÁ Á ÁH vector; b, the CGÁ Á ÁH–C
angle and d, the CGÁ Á ÁH distance. A significant interaction was
defined as having a o 201, b in the range 110–1801 and d
between 2.4 and 3.6 Å. C–HÁ Á Áp (CuC
are found in all six complexes. For complex 1, two C–HÁ Á Áp
CuC OS, chelate) interactions involve H52 and H82 of the two
3
OS, chelate) interactions
(
3
PPh moieties each with centre of gravity (CG) of the CuC OS
3
3
ring with a 13, 101, b 147, 1591 and d 2.92, 2.71 Å (Fig. 2). Also in
, 3, and 4, C–HÁ Á Áp (CuC OS, chelate) interactions between the
same hydrogen atoms H52 and H82 of PPh moieties with
centre of gravity (CG) of the CuC OS ring are observed with a, b
and d values of 15, 91, 145, 1591, 3.07, 2.70 Å; 17, 81, 156, 1451,
.11, 2.72 Å; and 14, 171, 145, 1561, 2.72, 3.20 Å, respectively.
Clearly, these results show that the orientation of one phenyl
2
3
3
3
3
Fig. 1 Molecular structure of 1 with ellipsoids at 30% probability showing ring in each PPh ligand facilitates these HÁ Á ÁCG interactions in
3
the atom numbering scheme. Complexes 2–6 show the metal in similar each structure.
coordination spheres (Fig. S3, ESI‡).
Weak HÁ Á ÁH interactions are found in 1, 3, and 4 with H72–
H86 (1 À x, 2 À y, 2 À z) distance of 2.34 Å in 1 (Fig. S4, ESI‡),
H65–H26 (Àx, 1/2 + y, 3/2 À z) distance of 2.37 Å and H64–H53
distances of 2.058(4)–2.062(6) Å, showing the asymmetrical (Àx, 1/2 + y, 3/2 À z) distance of 2.36 Å in 3 (Fig. S5, ESI‡), giving
O,S-bonding behavior of the dithioester ligands. These the supramolecular structure
a zig-zag appearance, and
The Cu1–P H75–H35 (À1 + x, y, z) distance of 2.39 Å in 4 (Fig. S6, ESI‡).
2
2a
distances are well within the literature range.
distances of 2.2504(16)–2.2897(15) Å are comparable. The dis- Complex 1 also exhibits C44–H44Á Á ÁO25 (1 À x, Ày, 1 À z)
tortion from the ideal tetrahedral geometry originates mainly interactions with distances of 2.64 Å (Fig. S7, ESI‡).
from the significant deviations in ligand bite angles S15–Cu1–O11
and P1–Cu1–P2 in the range 96.89(13)–98.08(6)1 and 110.86(6)– Absorption and emission spectra
1
13.23(9)1 respectively. The P1–Cu1–P2 angle is well within the
The UV-visible absorption spectra of the b-oxodithioester ligands
L1–L6) and complexes 1–6 were recorded in dichloromethane.
literature range but significantly shorter in comparison to the
analogous Cu(I) PPh –dithiocarbamate/xanthate complexes,
3
(
20b,22a
which may be attributed to the smaller S–Cu–S bite angles in
the latter. It is to be noted that the angles involving the P atoms
are different in each structure. Thus the P1–Cu1–S15 angle
(109.64(3)–110.20(9)1) is considerably smaller than the P2–Cu1–
S15 angle (115.56(3)–119.69(6)1) and similar differences are also
found in the P–Cu1–O11 angles. It seems likely that this disparity
may be attributed to the disposition of the phenyl rings.
For complexes 1–4, the angle between the two least-square
planes formed by Cu1, S15, O11, C14, C13 and C12 and by P1,
Cu1 and P2 is 85.09(13), 84.56(6), 86.85(11), and 87.23(8)1,
respectively, indicating that the two planes are approximately
perpendicular to each other. The root mean square (r.m.s.)
deviations from the planarity of atoms Cu1, S15, C14, C13, C12
and O11, defining the chelate ring, are 0.015, 0.031, 0.064, and
0.056 Å, respectively.
Concomitant with electron delocalization over the six-
membered chelate ring, the C–O, C–C and C–S bond lengths
Table 2) at 1.245(10)–1.259(7) Å, 1.397(12)–1.424(11) Å and
.668(9)–1.692(9) Å are intermediate between the single and
(
1
double bond lengths in 1–4.
Fig. 2 C–HÁ Á Áp (CuC
3
OS, chelate) interactions in complex 1 showing how
two phenyl rings are orientated to facilitate these interactions. Similar
Tiekink and Zukerman-Schpector have reported the signifi-
cant role of C–HÁ Á Áp (MCS
2
, chelate) interactions in metal interactions are found in all other complexes.
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Fig. 5 Emission spectra for complexes 1–6 in CH
and (b) lex = 330 nm.
2
Cl
2
at: (a) lex = 260 nm
Fig. 3 UV-Vis absorption spectra of 1–6 in CH
2
Cl
2
solution.
The ligands L1–L6 are non-emissive/weakly emissive in
solution. Upon complexation with Cu(I), when excited at 260 nm
2 2
in CH Cl solution at room temperature, they show two unstruc-
All the ligands show almost similar features (Fig. S8, ESI‡)
tured medium emission bands near 315 and 360 nm arising from
the ILCT states of the ligands (Fig. 5a). All six complexes emit
bright green colour when irradiated under UV light. When excited
at 330 nm, they show an unstructured broad emission band at
higher wavelength, B420 nm, with a Stokes shift of 90 nm
consisting of two to three absorptions in the 250–280 nm
4
À1
À1
(
1
e = 1.39–4.26 Â 10 M cm ) and 370–400 nm (e = 7.00–9.02 Â
4 À1
À1
0 M cm ) range arising from p–p* intraligand charge transfer
(ILCT) transitions. Likewise, the features of absorptions of the
heteroleptic Cu(I) complexes 1–6 (Fig. 3) are virtually alike, display-
3
emanating mainly from the PPh ligand mediated ILCT with
4
À1
À1
ing absorption bands near 260 nm (e = 2.15–4.44 Â 10 M cm ),
some admixture of MLCT states (Fig. 5b). The broad feature of
this band is assignable to the spin-allowed d–p* (MLCT)
transition mainly involving the 3d orbitals of Cu(I) and p*
orbitals of the ligand. Notably, the somewhat strong lumines-
cence characteristics of 3 as compared to the remaining com-
plexes may be ascribed to the enhanced conjugation in the
molecule by the methoxy substituent at the para position in the
aromatic ring.
4
À1
À1
3
30 nm (e = 1.05–2.18 Â 10
M
cm ) and 420 nm (e =
cm ); the first higher energy band is
assigned to ILCT transitions whereas the second higher energy
band which is not present in the spectra of ligands is most
4
À1
À1
0.78–1.23 Â 10
M
3
likely due to the coordinated PPh as a co-ligand. A broad low
energy band near 420 nm in all complexes is assignable to
metal perturbed ILCT charge transfer transitions.
TD calculations were carried out on the optimized struc-
ture of complex 1. By far the largest peak at 352.87 nm (see
Fig. S10, ESI‡) was obtained involving the transition from
HOMOÀ3 to LUMO. The orbitals are shown in Fig. 4. The
HOMOÀ3 involves the metal and one phosphorus atom
while the LUMO has contributions from the majority of atoms
in ligand L. It is interesting to note that the four other
unoccupied frontier orbitals involve the metal and parts of
the PPh3 ligands. By contrast, the five occupied frontier
orbitals involve the metal and ligand and not the PPh3
ligands. Further details of the orbitals and transitions are
given in the ESI,‡ S9.
Catalytic activity of the developed Cu(I) complexes in click
chemistry: synthesis of 1,4-disubstituted 1,2,3-triazolyl
glycoconjugates
Because of the diverse applications of carbohydrate-containing
1,4-disubstituted triazoles in various fields ranging from chemical
biology to catalysis and medicinal chemistry to materials sciences,
27
the development of a variety of glyco-based architectures with
the aid of Cu(I) azide alkyne cycloaddition (CuAAC) reactions has
been receiving increasing attention over the last one and a half
decades. In comparison to the uncatalyzed alkyne–azide cyclo-
addition reaction resulting in a mixture of 1,4- and 1,5-regioisomers
of triazoles at higher temperatures and long reaction times, we
used the click-inspired CuAAC process that combined organic
azides and terminal alkynes in a regioselective way, affording the
corresponding 1,4-disubstituted 1,2,3-triazoles exclusively, with an
exceedingly high rate of reaction compared with the reaction
2,3
carried out in the absence of a catalyst. Cu(I) complexes 1–6 were
employed as independent catalysts to evaluate their catalytic activity
in azide–alkyne cycloaddition reactions of different alkynes and
sugar azides to produce the corresponding 1,4-disubstituted
4
a
1
,2,3 triazolyl glycoconjugates. A prototype reaction was set
up using 2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl azide 7a
100 mg, 0.268 mmol) and phenylacetylene 8a (35 ml, 0.321 mmol)
(
in the presence of all the individual Cu-catalysts (1–6) (0.05 equiv.)
in dry DCM (5 mL). The reaction mixtures were stirred for 10 h at
Fig. 4 The orbitals involved in the most intense transition for 1 namely
HOMOÀ3 and LUMO.
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Table 3 Optimization of reaction conditions for click catalysis
NJC
a
Furthermore, the optimization of complex 6 under the same
reaction conditions was done, and the best result was obtained
when the reaction was carried out with 1 mol% of catalyst 6 in
dry DCM for 3 h at room temperature (Table 3, entry 19), giving
97% yield.
Taking into account the percentage yield, complexes 1 (96%
mol% Temp. (1C) Time (h) Yield (%) yield in 5 h) and 6 (97% yield in 3 h) showed the highest activity
b
Entry Catalyst
for the Huisgen cycloaddition reaction and therefore exploration
of the reaction variables was performed using these catalysts.
With the optimized conditions in hand, substrate generality
of CuAAC for catalysts 1 and 6 was surveyed by varying the type
of sugar azides and alkyne scaffolds, affording the corresponding
triazole products (9a–h) in excellent yields (Scheme 2). We found
no drastic change in the yields of the final products upon varying
the functional groups on the alkyne. The catalytic product was
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
1
2
3
4
5
6
1
1
1
1
1
1
1
1
1
1
5
5
5
5
5
5
2
1
0.5
0
1
1
1
1
1
1
1
1
1
1
1
1
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
10
10
10
10
10
10
10
10
10
10
1
2
3
4
5
5
5
5
3
96
81
89
77
85
97
93
96
74
Trace
21
75
86
90
96
94
89
96
97
74
95
38
0
1
2
3
4
5
6
7
8
9
0
1
2
1
13
confirmed by H and C NMR studies (see S11, ESI‡). For the
synthesis of the triazolyl glycoconjugates, see S12 (ESI‡).
It is worth mentioning that despite the efficient catalytic
14e
activity of [Cu(PPh ) NO ] (used as the starting material for
c
3 2
3
d
the synthesis of complexes 1–6) at room temperature under
neat conditions for the cycloaddition of azides and alkynes, in
the present study for clicking of 7a and 8a utilizing 1 mol% of
[Cu(PPh ) NO ], the yield of the product formed was rather
3 2 3
poor (Table 3, entry 22). Thus the catalytic activity of complexes
1
6
6
6
1
3
5
c
6
[Cu(PPh
3
)
2
NO
3
]
a
b
1–6 is dramatically enhanced after complexation with functio-
Molar ratio: 7a (1 equiv.), 8a (1.2 equiv.), DCM (1 mL). Yields reported
c
after purification by column chromatography. Reaction with base. nalized b-oxodithioester ligands thereby modifying the steric,
d
Reaction under neat conditions with 3 equiv. of phenylacetylene.
electronic and catalytic activities of the heteroleptic [Cu(PPh ) -
3
2
(b-oxodithioester)] complexes.
room temperature. On comparing the catalytic reactivity towards
The recyclability of the catalyst is a very important property,
the CuAAC reaction (Table 3, entries 1–6), it was observed that especially for commercial applications. Therefore, the recycling
complexes 1–6 afforded triazole 9a with yields in the 77–97% efficiency of the Cu-catalyst 1 was investigated for clicking azide
range. We found that complexes 1 and 6 remarkably gave triazole 7a and alkyne 8a. Catalyst 1 showed catalytic transformation of
9a in an excellent yield of about 97%, implying their higher 2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl azide 7a and phenyl-
activity; thus we found them to be the best for the exploration acetylene 8a to 1-(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl)-4-
of the click reaction in our study. Firstly we optimized the amount phenyl-[1,2,3]-triazole under the given experimental conditions
of catalyst 1 used in a prototype reaction, as shown in Table 3 in CDCl . A fresh lot of azide 7a and alkyne 8a was added to the
3
(
entries 7–10). Triazole 9a was formed in an excellent yield of 96% reaction vessel after completion of a cycle of the click reaction
in 10 h at room temperature at a very low catalyst loading of (see the Experimental section). The Cu-catalyst 1 demonstrates
mol%. At 0.5 mol%, the product was isolated in 74% yield good catalytic activity up to the fourth cycle. After four cycles,
Table 3, entry 9). Therefore, using 1 mol% as the optimum only a minor conversion decrease from 98% to 92% was
1
(
catalyst concentration, we further optimized the time-period at observed, which further decreased to 81% after the fifth cycle,
room temperature and found that the optimization reaction indicating that 1 can be employed as an efficient recyclable
needs at least 5 h for complete conversion of the starting material
(Table 3, entries 11–15). The time profile (Fig. 6) of the click
reaction between azide 7a and alkyne 8a with catalyst 1 reveals an
increase in the percentage conversion of the reactant to product
from 21 to 96, when the reaction time is increased from 1 to 5 h.
It is noteworthy that all these reactions have been carried
out under base free conditions and afforded the desired triazole
as the sole product. For comparison, the reaction was also
performed with a base (DIPEA) under identical reaction condi-
tions. A complete conversion of reactants into the product with
94% yield was obtained (Table 3, entry 16). Thus carrying out
the reaction with a base did not cause any significant change in
the final yield. When the reaction was carried out under neat
conditions, the yield obtained was 89% (Table 3, entry 17). Fig. 6 Time profile of the click reactions of azide 7a and alkyne 8a.
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(
SPECTROCHEM), were used as received. The starting com-
2
8
3 2 3
pound [Cu(PPh ) NO ] was prepared by the literature method.
The solvents were distilled and purified by standard methods
and dried before use wherever required. The experimental
details pertaining to the elemental analysis (C, H, N) and
2
0b,22d
recording of IR (KBr) are the same as described earlier.
UV-Visible absorption and emission spectra were collected at
room temperature using Shimadzu UV-1800 and PerkinElmer
LS-55 fluorescence spectrophotometers, respectively. X-ray powder
diffraction data were collected using a Br u¨ ker D8 diffracto-
meter with Cu Ka radiation (l = 1.541836 Å) in the 2y range
À1
5–501, with a scan speed and a step size of 11 and 0.021 min
,
respectively. Thin layer chromatography (TLC) was performed on
Merck 60 F254 silica gel pre-coated on aluminium plates and
revealed with either a UV lamp (lmax = 254 nm) or by spraying
with methanolic–H
heating at 100 1C. NMR ( H, C{ H}, P{ H}) spectra were
obtained on a JEOL AL500 FT-NMR spectrometer in CDCl
2 4
SO solution and subsequently charring by
1
13
1
31
1
3
.
Chemical shifts are given in ppm downfield from internal TMS
and J values are in Hz. Column chromatography was carried out
on a silica gel 234–400 mesh, E-Merck, using n-hexane and ethyl
acetate as the eluent.
Synthesis of ligands (L1–L6)
Scheme 2 Substrate scope studies for synthesis of glycoconjugates.
Molar ratio: Path-a: sugar azide (1 equiv.), alkyne (1.2 equiv.), catalyst 1 The ligands methyl-3-hydroxy-3-(furyl)-2-propenedithioate L1,
(
1 mol%), DCM (1 mL), 5 h. Yields reported after purification by flash column
chromatography (SiO ). Path-b: sugar azide (1 equiv.), alkyne (1.2 equiv.),
catalyst 6 (1 mol%), DCM (1 mL), 3 h. Yields reported after purification by
flash column chromatography (SiO ).
methyl-3-hydroxy-3-(thienyl)-2-propenedithioate L2, methyl-3-
hydroxy-3-( p-methoxyphenyl)-2-propenedithioate L3, methyl-3-
hydroxy-3-( p-bromophenyl)-2-propenedithioate L4, methyl-3-
hydroxy-3-benzyl-2-propenedithioate L5 and methyl-3-hydroxy-
2
2
3-(3-pyridine)-2-propenedithioate L6 and their corresponding
potassium salts were synthesized according to a previously
2
2d
reported procedure (Scheme 3). To a solution of NaH (0.6 g,
5 mmol) dissolved in a DMF : n-hexane mixture (4 : 1; 20 mL)
was added gradually 2-acetylfuron (1.10 g, 10.0 mmol) (for L1),
-acetylthiophene (1.26 g, 10.0 mmol) (for L2), 4-bromo-
2
2
acetophenone (2.0 g, 10.0 mmol) (for L3), 4-methoxyaceto-
phenone (1.5 g, 10.0 mmol) (for L4), acetophenone (1.20 g,
10.0 mmol) (for L5), and 3-acetylpyridine (1.20 g, 10.0 mmol)
Fig. 7 Recycling efficiency of catalyst 1 for the click reaction.
catalyst for the click reaction furnishing the triazole 9a. The
results of the recycling experiments with Cu-catalyst 1, with
respective yields after each run, are depicted in Fig. 7.
Experimental
Materials and methods
All the experiments were performed in air at ambient tempera-
ture and pressure. The reagent grade chemicals, 2-acetylfuran,
2-acetylthiophene (Sigma-Aldrich), p-methoxy acetophenone,
Scheme 3 Synthetic methodology for the ligands L1–L6 and their corres-
p-bromoacetophenone, acetophenone and 3-acetylpyridine ponding potassium salts KL1–KL6.
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NJC
3
(for L6) in DMF (20 mL) in each case. The reaction mixture was (–SCH ), 109.59 (–CHQC–), 127.50, 127.68, 149.02, 164.12
allowed to stir for 1 h in an ice bath under a N atmosphere and (–C H S), 128.43–133.93 (C-PPh ), 174.85 (–CQO), 189.29 (Q C–
2
4 3 3
3
1
1
a solution of dimethyltrithiocarbonate (TTC) (1.38 g, 10.0 mmol) S–). P{ H} NMR (202.46 MHz, CDCl , ppm): d À1.38 (–PPh ) ,
3
3 2
+
À1
À1
was added slowly and the mixture was stirred for 10 h at room 29.81 [Cu(PPh )] . UV-Vis (CH Cl , l
(nm), e (M cm )): 266
3
2
2
max
4
4
4
temperature. Excess NaH was neutralized by the addition of (2.38 Â 10 ), 334 (1.06 Â 10 ), 415 (1.95 Â 10 ).
.1 M HCl (50 mL) and the product was extracted three times [Cu(PPh (L3)] 3: (0.71 g, 86%). M.pt. 146–150 1C. Anal.
with 50 mL portions of dichloromethane, washed with brine calcd for C47
0
3 2
)
H
41CuO
2
P
2
S
2
(827.40): C 68.22, H 4.99%. Found: C
À1
solution and water, concentrated and dried over anhydrous 67.88, H 5.12%. IR (KBr, cm ): 1601 (nCQO), 1530 (nCQC), 1032
1
Na
100–200 mesh) chromatography using n-hexane as the eluent to –SCH
collect crystalline yellow solids and characterized by IR and NMR (m, 34H, Ar-H). C{ H} NMR (125.76 MHz, CDCl , ppm): d
2
SO
4
. The product thus obtained was purified by silica gel (nC–S). H NMR (500.15 MHz, CDCl
3
, ppm): d 2.57 (s, 3H,
(
3
), 3.84 (s, 3H, –OCH ), 6.85 (1H, s, –CHQC–), 6.87–7.81
3
1
3
1
3
spectroscopy (ligand characterization data, see in S13, ESI‡).
16.75 (–SCH ), 55.36 (–OCH ), 110.37 (–CHQC–), 128.43–133.93
3 3
3
1
1
(
Ar-C), 181.42 (–CQO), 200.11 (Q C–S–). P{ H} NMR (202.46
Synthesis of complexes 1–6
+
MHz, CDCl
3
, ppm): d À1.44 (–PPh
3 2 3
) , 29.78 [Cu(PPh )] . UV-Vis
cm )): 263 (2.79 Â 10 ), 310
À1
À1
4
For the preparation of the target complexes, potassium salts of (CH
2
Cl , lmax (nm), e (M
2
4
4
the ligands KL1–KL6 were obtained according to the following (1.81 Â 10 ), 400 (2.83 Â 10 ).
procedure. To a stirred 10 mL acetone solution of the ligand L1
[Cu(PPh
3 2
) (L4)] 4: Yield: (0.68 g, 78%). M.pt. 154–158 1C.
(
(
0.20 g, 1 mmol), L2 (0.22 g, 1 mmol), L3 (0.24 g, 1 mmol), L4 Anal. calcd for C46
0.29 g, 1 mmol), L5 (0.21 g, 1 mmol) and L6 (0.21 g, 1 mmol) Found: C 62.75, H 4.45%. IR (KBr, cm ): 1584 (nCQO), 1571
H
38BrCuOP
2
S
2
(876.27): C 63.05, H 4.37%.
À1
1
was added solid K CO (0.21 g, 1.5 mmol) separately and in (nCQC), 1094 (nC–S). H NMR (500.15 MHz, CDCl , ppm): d 2.44
2
3
3
each case, the reaction mixture was stirred further for 4 h under (s, 3H, –SCH ), 6.75 (1H, s, –CHQC–), 7.11–7.43 (m, 34H, Ar-H).
3
1
3
1
reflux conditions. The solution was cooled, filtered and dried
3 3
C{ H} NMR (125.76 MHz, CDCl , ppm) d 16.78 (–SCH ),
on a vacuum evaporator to yield yellow to orange solids 109.98 (–CHQC–), 124.54–134.17 (Ar-C), 180.33 (–CQO),
3
1
1
of KL1–KL6.
191.51 (Q C–S–). P{ H} NMR (202.46 MHz, CDCl
3
, ppm): d
+
The heteroleptic Cu(I) b-oxodithioester complexes 1–6 bear- À1.88 (–PPh
) , 29.78 [Cu(PPh )] . UV-Vis (CH Cl , lmax (nm), e
3 2 3 2 2
À1
À1
4
4
4
ing PPh
general procedure. To a stirred (10 mL) dichloromethane
solution of [Cu(PPh ) NO ] (0.65 g, 1 mmol) was added slowly Anal. calcd for C H CuOP S (797.37): C 69.28, H 4.93%.
3
ligands were synthesized according to the following (M cm )): 260 (2.92 Â 10 ), 335 (1.24 Â 10 ), 400 (2.11 Â 10 ).
[Cu(PPh ) (L5)] 5: Yield: (0.68 g, 85%). M.pt. 166–170 1C.
3
2
3
2
3
46 39
2 2
À1
a (15 mL) methanol solution of KL1 (0.24 g, 1 mmol), KL2 Found: C 69.05, H 4.95%. IR (KBr, cm ): 1585 (nCQO), 1532
1
(
0.25 g, 1 mmol), KL3 (0.28 g, 1 mmol), KL4 (0.33 g, 1 mmol), (nCQC), 1068 (nC–S). H NMR (500.15 MHz, CDCl
KL5 (0.25 g, 1 mmol) or KL6 (0.25 g, 1 mmol) separately at room (s, 3H, –SCH ), 6.98 (1H, s, –CHQC–), 7.21–7.72 (m, 35H, Ar-H).
C{ H} NMR (125.76 MHz, CDCl , ppm) d 16.85 (–SCH ),
short while and the reaction mixture was stirred for an addi- 110.53 (–CHQC–), 127.28–133.98 (Ar–C), 182.05 (–CQO),
3
, ppm): d 2.54
3
1
3
1
temperature. An orange brown precipitate was formed within a
3
3
3
1
1
tional 4 h (Scheme S14, ESI‡). The dark red solid thus formed 190.08 (Q C–S–). P{ H} NMR (202.46 MHz, CDCl
3
, ppm): d
(nm), e
max
+
was filtered, washed with methanol followed by diethyl ether, À1.71 (–PPh ) , 29.84 [Cu(PPh )] . UV-Vis (CH Cl , l
3
2
3
2
2
À1
À1
4
4
4
dried in the open and dissolved in dichloromethane to yield (M cm )): 262 (2.15 Â 10 ), 330 (1.04 Â 10 ), 397 (1.59 Â 10 ).
auburn crystals within 3 weeks.
[Cu(PPh ) (L6)] 6: Yield: (0.66 g, 83%). M.pt. 170–174 1C.
3 2
Anal. calcd for C45
H
38CuONP
2
S
2
(798.42): C 67.69, H 4.80, N
Characterization
À1
1.75%. Found: C 67.45, H 4.94, N 1.62%. IR (KBr, cm ): 1576
1
[Cu(PPh
3 2 3
) (L1)] 1: (0.69 g, 88%). M.pt. 172–176 1C. Anal. calcd (nCQO), 1525 (nCQC), 1094 (nC–S). H NMR (500.15 MHz, CDCl ,
for C44
H
37CuO
2
P
2
S
2
(787.34): C 67.12, H 4.74%. Found: C 66.84, ppm): d 2.54 (s, 3H, –SCH
3
), 6.86 (1H, s, –CHQC–), 7.21–7.37
À1
13
1
H 4.79%. IR (KBr, cm ): 1572 (nCQO), 1534 (nCQC), 1094 (nC–S). (m, 34H, Ar-H), 7.89–8.88 (m, 4H, –C
5
H
4
N). C{ H} NMR
1
H NMR (500.15 MHz, CDCl , ppm): d 2.58 (s, 3H, –SCH ), 6.47 (125.76 MHz, CDCl , ppm) d 16.79 (–SCH ), 110.09 (–CHQC–),
3
3
3
3
3
1
1
(s, 1H, –CHQC–), 7.02–7.49 (m, 3H, –C H O), 7.11–7.42 128.38–137.51 (Ar-C), 179.14 (–CQO), 193.07 (Q C–S–). P{ H}
4 3
1
3
1
(
1
m, 30H, Ar-H). C{ H} NMR (125.76 MHz, CDCl , ppm): d NMR (202.46 MHz, CDCl , ppm): d À1.16 (–PPh ) , 29.78
3
3
3 2
À1
+
À1
6.92 (–SCH
3
), 109.48 (–CHQC–), 112.31, 114.18, 144.51, 155.48 [Cu(PPh
3
)] . UV-Vis (CH
2
Cl
2
4
, lmax (nm), e (M
cm )): 263
4
4
(
–C
4
H
3
1
O), 128.43–133.93 (C-PPh
3
), 171.18 (–CQO), 190.43 (Q C– (4.44 Â 10 ), 335 (2.18 Â 10 ), 400 (1.08 Â 10 ).
3
1
S–). P{ H} NMR (202.46 MHz, CDCl
3
, ppm): d À1.44 (–PPh
3 2
) ,
+
À1
À1
General procedure for the synthesis of 1,2,3-triazolyl
glycoconjugates 9a–h
29.64 [Cu(PPh
3
)] . UV-Vis (CH
2
Cl
2
, lmax (nm), e (M cm )): 261
4
4
4
(
2.52 Â 10 ), 334 (1.27 Â 10 ), 411 (2.42 Â 10 ).
[
Cu(PPh ) (L2)] 2: (0.63 g, 79%). M.pt. 158–162 1C. Anal. A solution of the azide (1 equiv.) and alkyne (1.2 equiv.) were
3 2
calcd for C H CuOP S (803.40): C 65.77, H 4.64%. Found: C taken in dry CH Cl (1 mL) in the presence of the Cu-catalysts 1
4
4
37
2
3
2
2
À1
À1
6
5.45, H 4.76%. IR (KBr, cm ): 1582 (nCQO), 1485 (nCQC), 1060 (0.02 equiv. or 1 mol%, 2 g L ) and 6 (0.02 equiv. or 1 mol%,
C–S). H NMR (500.15 MHz, CDCl
.88 (s, 1H, –CHQC–), 7.00–7.51 (m, 3H, –C H
0H, Ar-H). C{ H} NMR (125.76 MHz, CDCl
1
À1
(n
3
, ppm): d 2.54 (s, 3H, –SCH
S), 7.01–7.41 (m, and 3 h, respectively. The reaction was monitored by TLC. On
, ppm): d 16.76 the completion of the reaction, the solvent was removed under
3
), 2 g L ) and the mixture was stirred at room temperature for 5 h
6
3
4 3
1
3
1
3
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reduced pressure to obtain the crude product. It was purified pair of electrons on the Py-N at the 3-position, showed a signifi-
by flash chromatography (ethyl acetate : n-hexane) to afford a cantly higher activity with 97% yield in reduced time. More
triazolyl glycoconjugate derivative (complete description of pro- importantly, these catalysts can be readily prepared in high yield
cedure for the synthesis of triazolyl gycoconjugates is provided in utilizing cost effective materials. This study widens the scope of
the ESI,‡ S12).
heteroleptic Cu(I) dithioester complexes bearing phosphine
ligands for their possible applications as click catalysts. Overall,
a variety of triazolyl glycoconjugates 9 were isolated in excellent
yield at a low catalyst loading of 1 mol%. Thus this represents an
efficient and economical protocol for the synthesis of triazolyl
glycoconjugates at room temperature with a broad substrate scope.
Moreover, catalyst 1 could be reused for five consecutive cycles
without significant loss of catalytic activity. These advantages make
the process highly valuable from the synthetic point of view. All six
complexes show bright green luminescent emission near 420 nm,
arising from the admixture of MLCT-ILCT states.
General procedure for reusability test
2,3,4,6-Tetra-O-acetyl-b-D-glucopyranosyl azide (100 mg, 0.268 mmol)
and phenylacetylene (35 ml, 0.321 mmol) were taken in CDCl (4 ml)
in the presence of the Cu-catalyst 1 (2 mg, 2.7 mmol) and the mixture
was stirred at room temperature for 5 h. An aliquot (100 to 200 ml)
3
1
was taken for analysis by H NMR spectroscopy, and a new batch of
2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl azide (100 mg) and phenyl-
acetylene (35 ml) was added. The reaction mixture was stirred
for another 5 h. The aliquot analysis by H NMR and fresh
1
addition of azide 7a and alkyne 8a to the reaction mixture was
repeated five times.
Conflicts of interest
X-ray crystal structure determination
There are no conflicts to declare.
Single crystals of 1–6 were obtained by the slow evaporation of
2 2
solutions of the compound in CH Cl . Single crystal X-ray
diffraction data were collected on an Oxford X-calibur CCD
Acknowledgements
diffractometer at 150 K using MoKa radiation. Data reduction
We gratefully acknowledge financial support from the Science
and Engineering Research Board (SERB) for the project (SB/SI/
IC-15/2014) and the UGC, New Delhi for the award of BSR
Faculty Fellow (ref No. F.18-1(BSR) 30 Dec 2016) to NS and UGC-
JRF fellowship to KK and UGC-SRF fellowship to CLY. VKT
sincerely thanks the Science and Engineering Research Board
2
9
was carried out using the CrysAlis program. The structures
3
0
were solved by direct methods using SHELXS-97 and refined
2
31
on F by full matrix least-squares method using SHELXL2016-6.
Nonhydrogen atoms were refined anisotropically and hydrogen
atoms were geometrically fixed with thermal parameters equi-
valent to 1.2 times that of the atom to which they were bonded.
(SERB), Department of Science & Technology, New Delhi (Grant
3
2
The diagrams of the complexes were prepared using ORTEP,
No. EMR/2016/001123) for the funding. We thank the Depart-
ment of Chemistry, Institute of Science, Banaras Hindu Uni-
versity, UGC CAS-II, for infrastructural facilities. We also thank
the University of Reading and the EPSRC (UK) for funds for the
diffractometer used for the data collections of structures 1–6.
3
3
34
Mercury and GaussView software. In 2, the thiophene ring is
disordered with the sulfur atom in two possible positions. In 5
and 6, the chelate ring is disordered over two overlapping
orientations so that donor atoms are O/S and S/O. Crystal data
for 1, 2, 3, 4, 5 and 6 have been deposited at the Cambridge
Crystallographic Data Centre with reference numbers CCDC
1510265, 1510264, 1510266, 1532702, 1562997 and 1869643.‡
Notes and references
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