Synthesis, structure, and catalytic activities of new Cu(i) thiocarboxylate complexes

Article abstract of DOI:10.1039/c4ra05290k

Thiobenzoate complexes of Cu(i), [Cu(SCOPh)]_n(1), [Cu(bpy)(μ-SCOPh)]₂(2), [Cu(dppf)(SCOPh)] (3), [where bpy = 2,2′-bipyridyl; dppf = [1,1′-bis(diphenylphosphino)ferrocene] were synthesized and structurally characterized by X-ray crystallography. NBO calculations were performed to understand the nature of the Cu-Cu (2.706 ?) interaction in 2. All of the complexes were found to catalyze azide-alkyne cycloaddition for the regioselective synthesis of glycoconjugate triazoles under click reaction.

Full text of DOI:10.1039/c4ra05290k

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Synthesis, structure, and catalytic activities of new
Cu(I) thiocarboxylate complexes†
Cite this: RSC Adv., 2014, 4, 39790
Deepak Kumar Joshi, Kunj Bihari Mishra, Vinod Kumar Tiwari
*
and Subrato Bhattacharya
Thiobenzoate complexes of Cu(I), [Cu(SCOPh)]n(1), [Cu(bpy)(m-SCOPh)]2 (2), [Cu(dppf)(SCOPh)] (3), [where
bpy ¼ 2,20-bipyridyl; dppf ¼ [1,10-bis(diphenylphosphino)ferrocene] were synthesized and structurally
characterized by X-ray crystallography. NBO calculations were performed to understand the nature of
the Cu–Cu (2.706 A˚) interaction in 2. All of the complexes were found to catalyze azide–alkyne
cycloaddition for the regioselective synthesis of glycoconjugate triazoles under click reaction.
Received 3rd June 2014
Accepted 11th August 2014
DOI: 10.1039/c4ra05290k
www.rsc.org/advances
copper center in particulate methane monooxygenase.13 Copper
plays an important role in click chemistry,14 the concept rst
Introduction
introduced by Kolb, Finn, and Sharpless whereby chemicals can
be quickly and reliably synthesized by joining together small
modular units. Meldal15 and Sharpless16 independently discov-
ered Cu(I)-catalysed azide–alkyne cycloaddition, which was then
studied intensively, and many areas of research are benetted
by its application.17,18 In this context, we present the synthesis of
three Cu(I) thiobenzoate complexes, their structural study, and
their catalytic activity in azide–alkyne cycloaddition reactions
for the regioselective synthesis of glycoconjugate triazoles
under click reaction.
The sulfur chemistry of transition metals represents a chal-
lenging and exciting research topic.1 Sulfur species possessing
high polarizability, large negative charge, and coordination
versatility are excellent ligands and exhibit highly adaptable
chemical, redox, and electronic behaviour.2 A number of tran-
sition metal sulfur-based compounds exhibit interesting prop-
erties such as luminescence, photoconductivity, chemical
sensing, catalysis, magnetic resonance imaging, and energy
storage capability.3
Among such sulfur-based ligands, thiocarboxylates are a very
interesting class of ligands, as they possess both so (S) and
hard (O) donor sites. Accordingly, they bind to respective hard
or so metal centers and also show chelating and bridging
(through the S atom) coordination modes. Furthermore, the
replacement of oxygen from carboxylates by a sulfur atom in
thiocarboxylates initiates many structural changes in the
respective complexes. Metal thiocarboxylates undergo facile
thiocarboxylic anhydride elimination,4 and hence they can be
used as single molecular precursors for metal sulde materials.5
Quite a few metal suldes, both binary and ternary, have already
been prepared using the corresponding metal thiocarboxylate
complexes.6
Experimental
Materials and methods
Standard methods were used for the purication of solvents.
Thiobenzoic acid, bis(diphenylphosphino)ferrocene, cuprous
chloride, triphenylphosphine (Sigma-Aldrich), 2,20-bipyridine
(s. d. ne chemicals) were used as such without further puri-
cation. The precursor [Cu(dppf)m-Cl]2 was synthesized using a
method given in the literature.19 Thin layer chromatography
(TLC) was performed using silica gel 60 F-254 plates with I2
vapor as a detection agent followed by spraying with meth-
anolic-H2SO4 solution and Dragendorff reagent.
IR spectra were recorded using Perkin-Elmer RX-1, FTIR and
Varian-3100 FTIR instruments, and 1H and 13C NMR spectra
were obtained using a JEOL AL300 FT NMR spectrometer.
Electronic spectra were recorded on a Shimadzu UV-1700
PharmaSpec spectrophotometer using freshly prepared solu-
tions of compounds in chloroform or DMSO. Single crystal X-ray
data were collected on an Xcalibur Oxford EOS diffractometer
The chemistry of copper is extremely rich because it can
easily access Cu0, CuI, CuII, and CuIII oxidation states. Copper is
known to catalyse a variety of reactions such as coupling reac-
tions (C–C,7 C–N,8 C–O,9 C–S10). In biological systems, copper is
an important constituent in systems such as cytochrome c
oxidase,11 nitrous oxide reductase,12 and the proposed catalytic
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India. E-mail:
s_bhatt@bhu.ac.in; Tel: +91 542 6702451
† Electronic supplementary information (ESI) available: Table S-1, 1H NMR and
13C NMR data for compounds 6a–6e are provided as supplementary material.
CCDC 984646–48 contain. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ra05290k
˚
using graphite monochromated Mo Ka radiation (l ¼ 0.7107 A).
Data collections were carried out at room temperature (293 K).
The structures were solved and rened using the SHELX set
of programs20 on the WinGX (ver. 1.80.05)21 platform.
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Non-hydrogen atoms were rened anisotropically while the stirring further for 5 min, the brick red precipitate formed was
hydrogen atoms were placed at the calculated positions using ltered, washed with 10 mL of methanol, and vacuum-dried.
SHELX default parameters. Disordered atoms were modelled by Reddish brown crystals were obtained by slow evaporation of
rening them at two different sites (without xing sof). a dichloromethane solution by keeping it in a small vial inside
Restraints such as DFIX and DELU were used wherever an empty desiccator. Yield: 0.278 g (78%). M.P. 155 ꢀC (dec).
essential. NBO calculations22 were performed using the atomic Anal. calcd for C34H26Cu2N4O2S2: C 57.21; H 3.67; N 7.85%. Found:
coordinates obtained from X-ray crystallographic results. A C 57.07; H 3.84; N 7.92%. IR(KBr pellet) 1598,1563 n(C]O), 1195
summary of crystallographic data and structural renements n(Ph–C), 910 n(C–S), 696 d(SCO). 13C NMR (75 MHz, CDCl3, d ppm):
are given in Table 1. DFT calculations were performed at 125.95–143.03 (Ph, bpy rings), 187.56 (COS).
B3LYP23 level using 6-31G** as basis set for all the atoms. All the
calculations were carried out using the Gaussian 09 program
package.24
Synthesis of [Cu(dppf)(SCOPh)] (3)
Na metal (0.023 g, 1.0 mmol) and thiobenzoic acid (117 mL) were
added to 10 mL of a methanol solution while keeping it in an ice
bath. The yellow solution was stirred for a few minutes, followed
Synthesis of [CuSCOPh]n (1)
Sodium metal (0.023 g, 1 mmol) was added to methanol (10 mL) by the addition of [Cu(dppf)(m-Cl)]2 (0.653 g, 0.5 mmol). The
followed by a methanolic solution (10 mL) of thiobenzoic acid reaction mixture was stirred for 1 h, the deep yellow precipitate
(117 mL, 1 mmol). The reaction mixture was cooled in an ice formed was ltered, washed with methanol (10 mL), and
bath, and CuCl (0.099 g, 1 mmol) was added in small portions, dried in vacuum. The product was recrystallized from
causing an orange precipitate to appear immediately. The dichloromethane/diethyl ether. Yield: 0.521 g (69%). M.P.
reaction mixture was stirred for 10 min. The precipitate was 195 ꢀC (dec). Anal. calcd for C41H33Cu1Fe1O1P2S1: C 65.21; H
ltered, washed with methanol, re-dissolved in hot acetonitrile, 4.40. Found: C 65.07; H 4.44%. IR(KBr pellet) 1613 n(C]O),
and then crystallized. Aer two days, bright orange ne crystals 1196 n(Ph–C), 912 n(C–S), 693 d(SCO). 1H NMR (300 MHz, CDCl3,
were obtained. Yield: 0.132 g (66%). M.P. 195 ꢀC (dec). Anal. d ppm): 7.25–8.15 (Ph rings), 4.33, 4.22 (Cp rings). 13C NMR
calcd for C7H5Cu1O1S1: C 41.89; H 2.51. Found: C 42.07; (75 MHz, CDCl3, d ppm): 71.75–73.97 (Cp rings), 127.40–142.62
H 2.48%. IR(KBr pellet) 1583, 1557 n(C]O), 1206 n(Ph–C), 906 (Ph rings).
n(C–S), 686 d(SCO). 13C NMR (75 MHz, DMSO-d6, d ppm):
127.83–158.64 (Ph ring), 185.45 (COS).
Synthesis of 4-propyl-1-(2,3,4,6-tetra-O-acetyl-b-D-
galactopyranosyl)-1H-1,2,3-triazole (6a)
Synthesis of [Cu(bpy)(m-SCOPh)]2 (2)
2,3,4,6-Tetra-O-acetyl-b-D-galactopyranosyl azide 5a (0.1 g, 0.26
To a solution of NaOMe (0.054 g, 1.0 mmol in 10 mL methanol) mmol) was treated with 1-pentyne 4a (0.031 mL, 0.33 mmol) in
was added solid CuCl (0.098 g, 1.0 mmol) followed by a solution dichloromethane in the presence of DIPEA (0.045 mL, 0.26
of 2,20-bipyridine (0.156 g, 1 mmol) in 10 mL methanol. The mmol) and complex 1 as catalyst and afforded 6a (0.082 g, 70%)
solution turned reddish brown immediately. Aer stirring the as yellowish liquid (Completion of the reaction was detected by
reaction mixture for 10 min. in an ice bath, a methanolic TLC.) 1H NMR (300 MHz, CDCl3): d 7.78 (s, 1H, triazole-H),
solution of thiobenzoic acid (117 mL, 1.0 mmol) was added. Aer 5.84 (d, J ¼ 9.3 Hz, 1H, H-1), 5.50 (m, 2H, H-2, H-4), 5.30–5.25
Table 1 Crystallographic data and refinement parameters for 1–3
1
2
3
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C7H5Cu1O1S1
200.73
293
Orthorhombic
Pbca
5.7290(11)
7.0037(10)
31.751(5)
C68H52Cu4N8O4S4
1427.65
293
Monoclinic
Cc
20.7022(19)
18.1635(12)
17.000(17)
105.635(8)
6152.6(9)
4
C41H33Cu1Fe1O1P2S1
755.08
293
Monoclinic
P21/c
11.211(5)
8.560(5)
36.116(5)
98.857(6)
3425(3)
4
˚
A (A)
˚
b (A)
˚
c (A)
b (ꢀ)
˚
V (A3)
1274.0(4)
8
Z
F(000)
800
2908
1552
Datas/restraints/parameters
GOF on F2
1.012
0.804
0.728
Final R indices [I>2sI]
R1 ¼ 0.0548
R1 ¼ 0.0997
R1 ¼ 0.0472
wR2 ¼ 0.0948
wR2 ¼ 0.2576
wR2¼0.00731
Largest differenceꢁ3
˚
peak and hole (eA
)
0.611 & ꢁ0.923
02.175 & ꢁ0.656
0.490 & ꢁ0.472
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(m, 1H, H-3), 4.25–4.17 (m, 3H, H-5, Ha-6, Hb-6), 2.22–1.88 (m,
Synthesis of 4-(1,2 : 5,6-di-O-isopropylidene-a-D-
glucofuranos)-1-(methyl-2,3,4-tri-O-benzyl-6-deoxy-a-D-
glucopyranosyl)-1H-1,2,3-triazole (6e)
16H, 3 ꢂ CH3 of OAc, aliphatic-4 H), 0.97 (t, J ¼ 7.2 Hz, 3H). 13
C
NMR (70 MHz, CDCl3): d 170.3, 169.9, 169.7, 169.1, 169.0, 151.9,
119.0, 86.1, 73.9, 70.6, 68.1, 66.9, 61.1, 30.0, 20.5, 20.4, 20.1, 9.3
ppm.
Methyl-6-azido-2,3,4-tri-O-benzyl-6-deoxy-a-D-glucopyranose 5c
(0.082 g, 0.16 mmol) was treated with 1,2 : 5,6-di-O-iso-
propylidene-3-O-propargyl-a-D-glucofuranose 4d (0.05 g, 0.16
mmol) in dichloromethane in the presence of DIPEA and
complex 1 as catalyst (5 mg) and afforded 6e (0.027 g, 20%) as a
yellowish viscous liquid (Completion of reaction was detected
Synthesis of 4-butyl-1-(2,3,4,6-tetra-O-acetyl-b-D-
galactopyranosyl)-1H-1,2,3-triazole (6b)
2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl azide 5a (0.1 g, 0.2
mmol) was treated with 1-hexyne 4b (0.036 mL, 0.32 mmol) in
dichloromethane in the presence of DIPEA (0.045 mL, 0.2
mmol) and complex 1 as catalyst (6 mg) and afforded 6b (0.85 g,
70%) as a yellowish viscous liquid (Completion of reaction was
detected by TLC). 1H NMR (300 MHz, CDCl3): d 7.49 (s, 1H,
triazole-H), 5.77 (d, J ¼ 9.3 Hz, 1H, H-1), 5.51–5.48 (m, 2H, H-2,
H-4), 5.21–5.17 (m, 1H, H-3), 4.18–4.07 (m, 3H, H-5, Ha-6, Hb-6),
2.65 (t, J ¼ 7.2 Hz, 2H), 2.15, 1.97, 1.93, 1,80 (s, 12H, 4 ꢂ CH3 of
OAc), 1.59, (t, J ¼ 7.2 Hz, 2H), 1.34–1.29 (m, 2H), 0.86 (t, J ¼ 7.2
Hz, 3H). 13C NMR (75 MHz, CDCl3): d 170.2, 169.8, 169.6, 168.6,
148.9, 118.7, 85.9, 73.8, 70.6, 67.6, 66.7, 61.1, 31.1, 25.1, 22.0,
20.5, 20.3, 20.0, 13.6 ppm.
1
by TLC). H NMR (300 MHz, CDCl3): d 7.63 (s, 1H, triazolyl-H),
7.33–7.31 (m, 15H, Ar–H), 5.85 (d, J ¼ 1.8 Hz, 1H, H-1), 5.84–
4.46 (m, 12H, Sugar-H), 2.89 (d, J ¼ 6.3 Hz, 1H, Sugar-H), 4.12–
3.96 (m, 6H, Sugar-H), 3.42 (d, J ¼ 9.1 Hz, 1H, Sugar-H), 3.17–
3.13 (m, 4H, Sugar-H), 1.48, 1.41, 1.32, 1.29 (each s, 4 ꢂ CH3,
>(CH3)2). 13C NMR (75 MHz, CDCl3): d 144.6, 138.2, 137.8, 137.8,
128.4, 128.1, 127.9, 127.6, 123.8, 111.7, 108.9, 105.0, 98.0, 82.6,
81.7, 81.0, 79.8, 77.8, 75.7, 74.8, 73.3, 72.3, 69.1, 67.2, 64.0, 55.1,
50.6, 26.7, 26.1, 25.4 ppm.
Similar procedures were adopted for two representative
reactions for the synthesis of 6c and 6e using complexes 2 and 3
as catalysts. Control experiments using CuCl (as catalyst) and
blank experiments (without any catalyst) were also carried out
by repeating all these reactions while maintaining the other
conditions as before. The results are summarized in Table 3.
Synthesis of 4-phenyl-1-(2,3,4,6-tetra-O-acetyl-b-D-
glucopyranosyl)-1H-1,2,3-triazole (6c)
2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl azide 5b (0.1 g, 0.2
mmol) and phenylacetylene 4c (0.029 mL, 0.2 mmol) were dis-
solved in dichloromethane. DIPEA (0.046 mL, 0.2 mmol) and
complex 1 as catalyst (4 mg) were added in solution. The reac-
tion mixture was stirred vigorously under argon atmosphere for
12 h. Aer completion of reaction (monitored by TLC), the
reaction mixture was in vacuo concentrated followed by silica gel
column chromatography to afford 6c (0.108 g, 85%) as a white
solid. 1H NMR (300 MHz, CDCl3): d 8.00 (s, 1H, triazole-H), 7.84–
7.82 (m, 2H, Ar–H), 7.45–7.35 (m, 3H, Ar–H), 5.94 (d, J ¼ 9.0 Hz,
1H, H-1), 5.56–5.41 (m, 2H, H-2, H-4), 5.27 (t, J ¼ 9.6 Hz, 1H, H-
3), 4.36–4.30 (m, 1H, H-5), 4.18–4.02 (m, 2H, Ha-6, Hb-6), 2.08,
2.04 (m, 9H, CH3 of OAc), 1.88 (s, 3H, CH3 of OAc); 13C NMR
(75 MHz, CDCl3): d 170.4, 169.8, 169.3, 168.9, 148.4, 129.8,
128.8, 128.5, 125.8, 117.7, 85.8, 75.1, 72.6, 70.1, 67.6, 61.5, 20.6,
20.5, 20.4, 20.2 ppm.
Results and discussion
Synthesis of complexes
All of the Cu(I) complexes were synthesized using CuCl or
[Cu(dppf)Cl]2 as the precursor. The synthetic routes for all the
three complexes are shown in Scheme 1. Complex 1 and 2 were
synthesized using methanol as the solvent whereas complex 3
was prepared in dichloromethane. The attempt to synthesize
CuII(SCOPh)2 by using CuCl2 and two equivalents of thio-
benzoate ligand failed, and the only product obtained was
complex 1. It may be noted that thiolate ions are known to
undergo oxidization by transition metal ions to corresponding
disuldes,25 and cuprous thiobenzoate has been reported26 to
decompose on prolonged heating, giving dibenzoyl disulde.
We have also observed oxidation of thiobenzoate ions catalyzed
by Ag(I) ion.4b Evidently, Cu(II) ion in the present case oxidized
Synthesis of 1-(methyl-2,3,4-tri-O-benzyl-6-deoxy-a-D-
glucopyranose-5-yl)-4-phenyl-1H-1,2,3-triazole (6d)
Methyl-6-azido-2,3,4-tri-O-benzyl-6-deoxy-a-D-glucopyranose
5c (0.05 g, 0.10 mmol) was treated with 4c (0.013 mL, 0.12
mmol) in dichloromethane in the presence of DIPEA (0.017
mL, 0.10 mmol) and complex 1 as catalyst (5 mg) and affor-
ded 6d (0.057 g, 95%) as a white solid (Completion of reaction
was detected by TLC). 1H NMR (300 MHz, CDCl3): d 7.82–7.79
(m, 3H, triazole-H, Ar–H), 7.41–7.31 (m, 18H, Ar–H), 5.00–
4.56 (m, 9H, benzylic-6H, Sugar-3 H), 4.01 (t, J ¼ 8.4 Hz, 1H,
Sugar-H), 3.43–3.18 (m, 5H, Sugar-H, OCH3). 13C NMR (75
MHz, CDCl3): d 147.6, 138.4, 137.9, 130.6, 128.7, 128.4, 128.2,
128.0, 127.8, 127.6, 125.6, 121.0, 98.0, 81.8, 79.9, 77.9, 75.7,
75.0, 73.4, 69.1, 55.3, 50.6 ppm.
Scheme 1 Synthesis of complexes 1–3.
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one equivalent of the ligand, which led to the formation of
stable Cu(I) with the remaining thiobenzoate ligand.
Complex 1 was sparingly soluble in DMSO and MeCN,
whereas complexes 2 and 3 were highly soluble in DCM.
Compounds 1 and 3 are quite stable, both in the solid state and
also in solution. Solution of compound 2, however, was found to
change its color under ambient conditions upon standing for
several days. The IR spectra of these complexes showed strong
bands due to C]O, Ph–C, and C–S stretching vibrations, which
are characteristic for thiobenzoate ligands. These IR bands are
important signals for analysing the bonding modes of the thi-
obenzoate ligands.27
Structural description of complexes
All the complexes synthesized were characterized by the single
crystal X-ray diffraction technique. A summary of crystallo-
graphic data and renement parameters are given in Table 1,
and selected bond lengths and angles are listed in Table 2.
Compound 1 is a two-dimensional coordination polymer
crystallised in the orthorhombic system with the Pbca space
group. Fig. 1 shows the asymmetric unit of complex 1, and Fig. 2
represents its polymeric structure.
Fig. 2 Polymeric structure of complex 1.
Copper is at the center of a distorted tetrahedron constituted
by three sulfur and one oxygen atoms of four different thio-
benzoate moieties. Each sulfur atom is coordinated to three
different copper atoms, while each oxygen binds to one Cu atom
only. The Cu–S and Cu–O bond lengths are comparable to the
respective covalent bonds observed in other cases.28 Two adja-
cent Cu atoms are held together by a bidentate (S, O) thio-
benzoate ligand from one side and a sulfur atom of another
thiobenzoate group from the other side forming a ve
membered ring. Each Cu is placed at the center of four six-
membered fused rings, which are in a boat (or twisted boat)
conformation (Fig. 3).
Table 2 Selected bond lengths and angles for 1–3
˚
Complex
Bond length (A)
Bond angle (ꢀ)
1
Cu1-S1_04
Cu1-O1
Cu1-S1_03
Cu1-S1
Cu1-Cu2
Cu1-S1
Cu1-S2
Cu2-S1
Cu2-S2
Cu1-N1
Cu1-N2
Cu2-N3
Cu2-N4
Cu1-S1
Cu1-P2
Cu1-P1
2.277(14)
S1_04-Cu1-O1
O1-Cu1-S1
S1-Cu1-S1_03
S1-Cu1-S1_04
S1-Cu1-N2
N2-Cu1-N1
N1-Cu1-S2
S2-Cu1-S1
S1-Cu2-N4
N4-Cu2-N3
N3-Cu2-S2
S2-Cu2-S1
102.45(10)
105.92(10)
113.48(4)
133.75(4)
127.5(6)
85.7(7)
105.4(5)
106.6(2)
117.3(4)
79.6(6)
2.130(3)
2.494(15)
2.304(13)
2.706(3)
2.221(6)
2.413(4)
2.629(6)
2.195(4)
2.03(2)
1.86(2)
2.04(2)
2.09(2)
2.210(12)
2.260(11)
2.248(13)
˚
The Cu–Cu distance is 3.173 A, which is quite longer than the
2
sum of their van der Waals radii. It may be noted that Cu(I)
carboxylates are structurally quite different from 1. The struc-
ture of the Cu(OAc) complex has been described as distorted
square planar around the Cu(I) atom.29 The oxygen atoms are
present at three coordination sites while the fourth site is
132.6(5)
109.4(32)
3
P1-Cu1-S1
S1-Cu1-P2
P2-Cu1-P1
121.41(4)
125.99(5)
112.23(4)
Fig. 1 Thermal ellipsoid (at 30% probability level) of complex 1. For
clarity, hydrogen atoms are omitted.
Fig. 3 Twisted boat-shaped six-membered ring for complex 1.
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thiobenzoate
complex
[(PPh3)Cu(m-SCOPh)2Cu(PPh3)2]
reported earlier.31 In the latter complex, the bonding mode of
thiobenzoate ligands is similar to that in 2; however, one of
the coppers is in a four-coordinate environment while the
other one undergoes a triphenylphosphine elimination and
acquires trigonal geometry. The Cu2S2 core in 2 is essentially
non-planar, in sharp contrast to the planar core in
[(PPh3)Cu(m-SCOPh)2Cu(PPh3)2]. The benzoyl groups on the
two S atoms have syn stereochemistry in the case of 2, unlike
the anti arrangement observed in the reported complex. The
˚
Cu–Cu distance in 2 (2.706 A) is slightly longer than the
corresponding distance in the abovementioned phosphine
complex. It may be noted that the ideal Cu–Cu covalent bond
˚
distance is 2.64 A while the corresponding van der Waals'
˚
bond distance is 2.80 A. The N–Cu–N, N–Cu–S, and S–Cu–S
angles in 2 do not suggest a ve-coordinate geometry around
Cu, yet the Cu–Cu distance indicates some interaction
between the two centres. We have carried out some DFT
(density functional theory) calculations to understand the
nature of the interaction, which are described below.
Complex 3 crystallized in a monoclinic system with a P21/c
space group. The molecular structure for complex 3 is
depicted in Fig. 5. The geometry around the copper atom is
essentially trigonal planar. Two phosphorus atoms of the
dppf ligand occupy two coordination sites while the third one
is engaged by the sulfur atom of the thiocarboxylate ligand.
Fig. 4 Thermal ellipsoid (at 30% probability level) of complex 2. For
clarity, hydrogen atoms are omitted.
˚
occupied by the neighbouring Cu atom at a distance of 2.54 A.
Each oxygen atom binds with two Cu centres forming a ribbon-
like structure. Cu(O2CPh), however, is tetrameric.30 Four Cu(I)
atoms are bridged by benzoate groups forming a parallelogram.
The O–Cu–O bond angles range between 167 and 178ꢀ,
imparting a nearly linear coordination geometry around the
metal.
˚
The Cu–O distance (3.314 A) is too long to have any bonding
Complex 2 crystallized in a monoclinic system with the
space group Cc. Fig. 4 shows the molecular structure of the
complex. The molecule is dimeric, with the two sulfur atoms
from two different thiobenzoate moieties acting as bridging
atoms. Both copper atoms possess distorted tetrahedral
geometry formed by the coordination of two nitrogens of
bipyridyl ligand and the two bridging sulfurs of thiobenzoate
groups. The observed distortion in the structure arises
possibly due to the small bite angle (approximately 79.1ꢀ) of
the bipyridyl ligand. Complex 2 may be compared with a
interaction between the two centers. The corresponding
carboxylate complex [Cu2(m-OCOCH3)2(dppf)2]32 is a dimer in
which both acetates act as bridging ligands. As a result, each
of the copper atoms possesses a four-coordinate geometry.
The monomeric nature of complex 3 can be attributed to
the poor tendency of the thiocarboxylate group to bind
bidentately.
Scheme 2 Synthesis of triazolyl saccharides via CuAAC.
Fig. 5 Thermal ellipsoid (at 30% probability level) of complex 3. For Scheme 3 The reported structures of intermediates formed during
clarity, hydrogen atoms have been omitted.
cycloaddition.18
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Electronic absorption spectra
Density functional calculations
The electronic absorption spectrum of compound 1 was DFT calculations were carried out for complex 2. The distance
recorded in DMSO while those of 2 and 3 were recorded in between the two copper atoms in this bimetallic complex is
˚
chloroform. Complex 1 gives a broad band in the UV region 2.706 A, which is slightly longer than the corresponding cova-
34
˚
centered at 315 nm. Complex 2 gives 3 bands in the UV region lent bond length (2.64 A) but much smaller than the sum of
– a sharp band at 244 nm and two broad bands at 297 and 355 their van der Waals radii. To understand the nature of the
nm. Complex 3 absorbs in the visible region giving a broad Cu–Cu interaction (if any), NBO calculations were carried out.
band at approximately 450 nm. All the absorption peaks Notably, cuprophilic interactions have been reported in several
(except the one at 450 nm in the case of 3) may be assigned as Cu(I) complexes.34 Close Cu–Cu contacts in some cases are
intra/interligand and LMCT transitions while the band at 450 stabilized by bridging ligands or by electrostatic interactions. In
nm for 3 can be assigned as an e2g / e1g transition in the other cases, such as in a Cu(I) dimer unsupported by ligands,
ferrocenyl moiety.33
the Cu/Cu s bond was held responsible for such interac-
tions.35 An NBO study in the present case actually revealed the
Table 3 Optimization of copper complexes for azide–alkyne cycloaddition reactions
Alkyne (R)
Azido sugar
Product
Time (h)
12 h
Catalyst, yielda (%)
1, 70 CuCl, 40
12 h
1, 70 CuCl, 50
12 h
1, 85 2, 82 3, 40 CuCl, 50
12 h
1, 95 CuCl, 35
12 h
1, 20 2, 88 3, 45 CuCl, 65
a
Without using a catalyst [CuCl or complexes (1–3)], the yields of the products were not detectable.
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nature of this interaction. The Wiberg bond index was found to which the yield is only 20%. Interestingly, 2 showed excellent
be 0.12, and NBO calculations revealed that the d orbital of Cu catalytic activity during the same reaction.
atoms participates in bonding. The natural electronic congu-
The role of ligands in the catalytic effect of Cu(I) in triazole
ration of Cu(I) is 4 s0.353d9.484p0.67 instead of 4s03d10. The formation is primarily to protect Cu(I) from oxidation in the
hybrid orbital of Cu(1), for example, which formed a Cu–S presence of adventurous oxygen. Furthermore, the number and
covalent bond, is the result of mixing 27.42% 4s, 4.14% 3d and nature of ligands may affect the mechanism of the catalytic
68.41% 4p orbitals. The details of orbital occupancies of Cu(I) process in several ways.18 Non-labile ligands occupying all four
are listed in Table S-1 (ESI†).
coordination sites of Cu(I) would reduce its catalytic activity.37
The Cu(1) dx2ꢁy2 orbital's occupancy is 1.81, enabling it to Thiobenzoate ligands in complexes 1–3 are expected to serve
overlap (to a limited extent only) with the analogous orbital of both purposes: on the one hand, they will not allow oxidation of
Cu(2), as shown in Fig. 6.
the metal ion, and on the other hand, their labile nature is
expected to provide a free coordination site at Cu(I) for the
bonding of alkyne. In view of the accepted structures of inter-
mediates A or B (Scheme 3),18 increased catalytic activity of 2 is
expected. Although bipyridine ligands are known to adversely
affect the kinetics in these reactions,37 the good performance of
2 in the synthesis of 6c and 6e makes it clear that longer reac-
tion times allow for completion of the reactions, and the overall
yield is not affected. Other factors that depend on the nature of
the ligands as well as the substrates also play a signicant role
in the kinetics and mechanism of these reactions.
Due to the complexity of ligand interaction with Cu(I) and the
nature of alkyne complexation, it is not possible with the
present state of knowledge to unambiguously explain the
structural aspects of the transition state responsible for the
selectivity and rate enhancement in the Cu(I)-catalyzed cyclo-
addition reactions.18
Catalytic activity of developed Cu-complexes in click
chemistry: synthesis of 1,2,3-triazolyl glycoconjugates
To investigate the catalytic activity, these complexes were
employed as independent catalysts in azide–alkyne cycloaddi-
tion reactions36 of different alkynes and sugar azides to achieve
regioselective 1,4-disubstituted triazolyl glycoconjugates. A
group of experiments were designed for cycloaddition reaction
between simple alkynes (4a, 4b, 4c) with 2,3,4,6-tetra-O-acetyl-b-
D-glucopyranosyl azide (5a)/(5b) or methyl-6-azido-2,3,4-tri-O-
benzyl-6-deoxy-a-D-glucopyranose (5c) for the synthesis of tri-
azolyl monosaccharides. Similarly, a reaction of 1,2 : 5,6-di-
O-isopropylidene-3-O-propargyl-a-D-glucofuranose (4d) and
methyl-6-azido-2,3,4-tri-O-benzyl-6-deoxy-a-D-glucopyranose
(5c) for the synthesis of triazolyl disaccharide was also carried
out. Reaction without DIPEA does not proceed well, and there-
fore, we have used DIPEA as a base and dry dichloromethane as
a solvent in our reactions. Complex 1 was screened as a catalyst
for azide–alkyne click reactions. Two representative reactions
were also carried out by using 2 or 3 as catalyst (For a proper
comparison, each reaction has been attempted using CuCl as a
catalyst and also without using any catalyst). The details of the
results are summarized in Scheme 2 and Table 3.
Conclusions
Three Cu(I) thiobenzoate complexes, [Cu(SCOPh)]n (1), [Cu(b-
py)(m-SCOPh)]2 (2), and [Cu(dppf)(SCOPh)] (3), were synthe-
sized. Complex
1 was polymeric, adopting a distorted
tetrahedral geometry. Complex 2 was dinuclear, in which both
copper atoms assumed a distorted tetrahedral geometry. NBO
studies on 2 revealed the existence of orbital overlap between
the two Cu centers, which is responsible for the short adjacent
Cu–Cu distances. The geometry around the Cu atom in 3 was
found to be trigonal planar. The complexes were found to
catalyze azide–alkyne cycloaddition for the regioselective
synthesis of glycoconjugate triazoles under click reaction.
From Table 3, it is evident that 1 acts as a very good catalyst
in the cyclization process except during the formation of 6e, in
Acknowledgements
Financial assistance from the Council of Scientic and Indus-
trial Research, New Delhi [Scheme number 01(2680)/12/EMR-II]
is gratefully acknowledged.
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