A binuclear Cu(i) complex as a novel catalyst towards the direct synthesis of N-2-aryl-substituted-1,2,3-triazoles from chalcones

Article abstract of DOI:10.1039/c5ra27907k

A binuclear Cu(i) complex containing a N′,N′-bis{(1H-indol-3-yl)methylene}oxalohydrazide (H₂bioh) ligand has been synthesized and characterized. The molecular structures of the synthesized compounds have been determined by single crystal X-ray

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A binuclear Cu(I) complex as novel
catalyst towards direct synthesis of
N‒2‒aryl‒substituted‒1,2,3‒triazoles from
chalcones
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Divya Pratap Singh, Bharat Kumar Allam, Rahul Singh, Krishna Nand
Singh and Vinod Prasad Singh*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A binuclear Cu(I) complex containing N',N'ꢀbis{(1Hꢀindolꢀ3ꢀyl)methylene}oxalohydrazide
(H2bioh) ligand has been synthesized and characterized. The molecular structures of the
synthesized compounds have been determined by single crystal X–ray diffraction. The crystal
structures are stabilized by inter‒ and intra‒molecular π−π stacking and C−Hꢁꢁꢁπ interactions.
The Cu(I)ꢀcomplex has successfully been employed as an efficient catalyst for one‒pot
operation involving the azide‒chalcone click reaction and subsequent arylation has been
developed. The complex exhibits excellent catalytic activity with significantly low catalyst
loading. The over all process is insensitive to air and moisture and can be manipulated under
ambient temperature with operational ease.
facile preparation of N‒2‒substituted triazoles with low copper
loading under mild conditions remains challenging.
1. Introduction
The Click chemistry is the most straight forward and atom
economical approach for the synthesis of 1,2,3‒triazoles.1
Owing to its fidelity and compatibility, this strategy has been
widely used in chemical biology, material science, and
medicinal chemistry.2 Most of the reported click reactions are
two‒component, employing organic azides and alkynes. In this
system, the organic azides need to be synthesized in advance,
and the potential hazards of organic azides, especially in
isolation or purification processing, can be problematic. Thus, it
is desirable to develop efficient methods that use sodium azide
for direct cycloaddition.
The synthesis and functionalization of triazole derivatives is
an attractive area of research. Recently described methods for
the catalytic direct functionalization of triazole and its
derivatives have also been found attractive but are mainly
limited to the N‒1 position.3 One important group of triazole
derivatives is the N‒2‒aryl/alkyl triazoles. In N‒2‒aryl
triazoles the two aromatic rings adopt coplanar conformation
and thus posses a typical electron density distribution, and
exhibit unique photonic properties.4 It has been well
documented that the N‒2 substitution is a major challenge for
triazoles due to the lower relative electron density at the
internal nitrogen atom (N‒2) as compared to the two terminal
nitrogen atoms (N‒1 and N‒3). The efforts have been made by
Sharpless and others to further advance this difficult N‒2
functionalization of triazoles.5 Despite of these achievements, a
The Cu(I) species necessary for the click reaction is often
generated in situ either by employing Cu(II) in conjunction
with
a
reducing agent such as ascorbate,6 or via the
comproportionation of Cu(II) and Cu(0). Additionally, 1,2,3–
triazoles and other compounds have been employed as
stabilizing ligands for Cu(I), inhibiting the oxidation and
disproportionation of the ion while retaining its catalytic
activity.7 Various genuine Cu(I) catalysts that do not require a
sacrificial reductant are also known. The advantage of ligated
Cu(I), particularly with nitrogen ligands, being the rate
accelerator, for instance with the efficient polytriazoles8 and
tris(2–aminoethyl)amine derivatives.9 The nitrogen ligands
allow the use of Cu(I) catalysts in reduced amounts (most often
of the order of 1%) compared to the original, simple, and
practical catalyst CuSO4 + sodium ascorbate that is still the
most commonly utilized catalyst but in much larger
quantities.10 Although Cu(I) may be obtained directly from the
utilization of Cu(I) salts and coordination complexes,11 the
thermodynamically unstable Cu(I) either disproportionates to
Cu(II) and Cu(0) or oxidizes to Cu(II). Moreover, when Cu(I)
halides are employed as azide–alkyne cycloaddition catalysts in
organic solvents, significant amounts of undesired side–
products are generated.12 Owing to the cytotoxicity associated
with Cu(I), several groups have endeavored to minimize the
copper concentration while maintaining high reaction rates.13
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DOI: 10.1039/C5RA27907K
Heterocyclic Schiff bases contain a number of coordination with 50 mL solution of bisꢀ(triphenylphosphine)copper(I)
sites in addition to >C=N–, forming more stable bi‒, tri‒ or nitrate (2 mmol, 1.3 g) in dichloromethane in 2:1 (M:L) molar
poly‒nuclear metal complexes. It has also been observed that ratio. The reaction mixture was continuously stirred on a
the binuclear complexes of transition metal ions show better magnetic stirrer for 24 h at room temperature. The product was
catalytic efficiency than those of corresponding mononuclear crystallized from the above solution by evaporating the solvent
ones due to the synergic effect of two metal ions.14 In view of slowly at room temperature. Yellow crystal, yield (70%). M.p.
the potential catalytic applications offered by the binuclear 185 ºC. Anal. Calc. for C94H84Cu2N8O10P4 (1736.67): Cu, 7.31;
metal complexes of acyldihydrazone Schiff bases, and as a part C, 64.95; H, 4.84; N, 6.45. Found: Cu, 7.28; C, 64.76; H, 4.85;
of our recent investigations on the catalytic application of N, 6.41%. IR (ν cm–1, KBr): ν (NH) 3258b; ν (C=O) 1634m; ν
1
transition metals15 and their complexes;16 we have synthesized (C=N) 1553m; ν (N−N) 1020w. H NMR (DMSO‒d6;
δ ppm):
and characterized a novel binuclear copper(I) complex with 11.95 (s, NH); 11.67 (s, NH, indole moiety); 8.76 (s, CH);
H2bioh ligand. The catalytic efficiency of the above complex 8.24–7.14 (m, ArꢀH). 13C NMR (DMSO‒d6;
ppm): 155.97
has successfully been explored for N‒2‒aryl‒1,2,3‒triazole (C=O); 147.90 (C=N); 137.05–111.34 (aromatic carbons); 31P
δ
synthesis.
NMR (DMSO‒d6; δ ppm): 10.71.
.
2. Experimental
2.3 General procedure for the synthesis of Nꢀ
2ꢀarylꢀsubstitutedꢀ1,2,3ꢀtriazoles
2.1 Synthesis of ligand
The chalcones (1 mmol), sodium azide (1 mmol), Cu(I)
The
hydrazide (H2bioh) was synthesized by reacting 50 mL aqueous round bottom flask equipped with a stirrer. The reaction
solution of oxalic acid dihydrazide (10 mmol, 1.18 g) with 50 mixture was stirred at 50 C for 10 h, and then 2,4ꢀ
ligand,
N',N'‒bis{(1H‒indol‒3‒yl)methylene}oxalo complex (1 mol %) and 3 mL DMF were taken in a 25 mL
̊
mL methanolic solution of indole‒3‒carboxaldehyde (20 dinitrochlorobenzene/4ꢀfluoronitro benzene (1 mmol) was
mmol, 2.90 g) in 1:2 molar ratio in a round bottom flask. A added to the mixture. The reaction was continued further for 12
light yellow colored product was obtained by stirring the h at 100 ̊C. After completion of the reaction, brine water (10
reaction mixture for 5 h at room temperature. The product was mL) was added and extracted with ethyl acetate (3 × 10 mL).
filtered on a Buckner funnel and washed several times with The combined organic phases were dried over anhydrous
water followed by methanol, and dried in a desiccator over Na2SO4 and concentrated under reduced pressure in a rotary
anhydrous calcium chloride at room temperature. Single crystal evaporator. The crude product thus obtained, was purified using
of the ligand was obtained by slow evaporation of its solution column chromatography (ethylacetate/hexane). All the products
in a mixture of dichloromethane and DMSO solvents at room have been characterized based upon reported literature
temperature. Yield (85%). M.p. >300 ºC. Anal. Calc. for characteristics.
C20H16N6O2 (372): C, 64.51; H, 4.33; N, 22.57. Found: C,
64.35; H, 4.35; N, 22.45%. IR (ν cm–1, KBr): ν(NH) 3263b; 3. Results and discussion
ν(C=O) 1667s; ν(C=N) 1578m; ν(N−N) 989w. 1H NMR
(DMSOꢀd6;
moiety); 8.73 (s, CH); 8.25–7.13 (ArꢀH). 13C NMR (DMSOꢀd6; precursor bis‒(triphenylphosphine)copper(I) nitrate and ligand
ppm): 166.62 (>C=O); 158.33 (>C=N); 156.12–111.57 H2bioh occurs in 2:1 (M:L) molar ratio to give a binuclear
δ ppm): 11.90 (s, NH); 11.65 (s, NH, indole It appears from the analytical data that reaction between the
δ
(aromatic carbons).
Cu(I) complex (Scheme 1).
The complex is highly soluble in water, dichloromethane,
ethanol, methanol, chloroform, chloroform, DMF and DMSO.
It melts with decomposition at 185 ºC and shows molar
2.2 Synthesis of the Cu(I) complex
The Cu(I) complex of N',N'ꢀbis{(1Hꢀindolꢀ3ꢀyl)methylene} conductance value of 59.35 ꢂ–1 mol–1 cm2 at room temperature
oxalohydrazide (H2bioh) was synthesized by reacting 50 mL in the range of 1:1 electrolyte for 10–3 M solution in DMSO.
methanolic solution of the ligand H2bioh (1 mmol, 0.334 g)
Scheme 1. Preparation of [Cu2(H2bioh)(PPh3)4](NO3)2.2CH3OH
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Fig. 1 ORTEP diagram of H2bioh showing atomic numbering scheme (ellipsoids of
30% probability).
3.1 Spectral characterization
The IR spectral bands observed at 3263, 1667, 1578 and 989
cm–1 in the free ligand are assigned to ν(N–H), ν(C=O),
ν(C=N) and ν(N–N), respectively.16 In the complex, ν(C=O)
and ν(C=N) occur at significantly lower wave numbers than the
free ligand, indicating coordination of the >C=O and >C=N
groups to metal.17 A considerable shift of ν(N–N) to higher
wave number in the complex indicates the involvement of one
of the nitrogen atoms of >N–N< moiety in bonding.18 The
presence of ν(N–H) nearly at the same position in the complex
as in the ligand shows non–participation of N–H group in
bonding.
The molecular structure of the ligand is stabilized by
inter‒molecular H–bonding as well as inter‒molecular C−Hꢁꢁꢁ
π
interactions. The inter‒molecular H–bonding interactions
N(1)−H(1A)ꢁꢁꢁO(1) (2.10 Å), N(3)−H(3A)ꢁꢁꢁO(1) (2.09 Å) and
N(3)−H(3A)ꢁꢁꢁO(1)
architectures on axis ‘a’ and ‘c’ (Fig. S5). The inter‒molecular
C−Hꢁꢁꢁ interactions occur between the centroid of
(2.34
Å)
form
supra‒molecular
π
indole/phenyl ring and phenyl hydrogen with the contact
distances of 2.974 and 3.398 Å (Fig. 2).16
The 1H NMR spectrum of H2bioh in DMSO exhibits
signals at 11.90 and 11.65 ppm due to >NH protons of −CONH
and indole–NH, respectively (Fig. S1). In the Cu(I) complex,
these protons occur almost at the same positions as in the
ligand, suggesting non–participation of >NH group in bonding
(Fig. S2). The −CH=N proton observed at 8.73 ppm in the
ligand, shows downfield shift in the complex due to
participation of −CH=N− group in bonding. The signals due to
aromatic ring protons are observed in the region 8.24–7.14 ppm
as multiplets in the ligand and complex.19 The 13C NMR
spectrum exhibits a cluster of peaks between 156.13–111.57
ppm in the ligand and 137.05–111.34 ppm in the complex
corresponding to carbons of the aromatic rings (Fig. S3). The
signals observed at 158.33 and 166.62 ppm in the ligand are
assigned to >C=N– and >C=O carbons, respectively. These
signals show a considerable high field shift in the Cu(I)
complex, suggesting the bonding of >C=N– and >C=O groups
with metal (Fig. S4). The 31P NMR spectrum of the complex
shows a peak at 10.71 ppm due to PPh3.
Fig. 2 Diagram showing intermolecular hydrogen bonding and intra‒molecular
C−H∙∙∙π interactions along ‘c’ axis in H2bioh.
3.3 Crystal structure of Cu(I) complex
Fig. 3 shows ORTEP diagram of the Cu(I) complex with
atomic numbering scheme. In the binuclear complex, each
metal centre is coordinated with NOP2 core by using
azomethine‒nitrogen and a carbonyl‒oxygen of H2bioh ligand,
and two phosphorous atoms of triphenylphosphine ligands. The
two metal atoms in the complex are arranged in a trans manner
to the coordinating sites of ligands. The average bond lengths in
the complex are: C(9)–N(2) = 1.294(3), C(10)–O(1) = 1.228 (3)
and N(2)–N(3) = 1.398(3) Å, respectively, which are longer or
shorter than those of the corresponding distances in the ligand.
The Cu–O(1) and Cu–N(2) bond distances are 2.197 (18) Å and
2.076 (2) Å, which fall in the range reported for Cu(I)
tetrahedral complexes.16 The C(9)–N(2) distance (1.294 Å) in
the complex is significantly longer than the free ligand (1.285
Å) due to coordination of azomethine–N to the metal ion.
3.2 Crystal structure of H2bioh ligand
The crystallographic data, structural refinement details, selected
bond lengths, bond angles and hydrogen bonding parameters of
H2bioh and its Cu(I) complex are given in Table S1, S2 and S3,
respectively. Fig. 1 shows ORTEP diagram of the ligand with
atomic numbering scheme. The molecule displays an E
configuration about the >C=N bond.16 The C(10)–O(1) and
C(9)–N(2) display bond distances of 1.236 (2) and 1.285 (3) Å ,
respectively as reported for double bonds.16 The N(2)–N(3)
bond distance is 1.390 Å, which is slightly shorter than the
single bond distance (1.411 Å), indicating some double bond
character.20 All the C–C bond length (1.390 Å) and C–C–C
bond angle (116.62º) of the indole ring as well as the C–N bond
length (1.361 Å) and C–N–C angle (109.48º) are in accordance
with a typical indole moiety.21
Fig. 3 ORTEP diagram of [Cu2(H2bioh)(PPh3)4](NO3)2·2CH3OH showing atomic
numbering scheme with ellipsoids of 30% probability (phenyl rings, methanol
and nitrate ions are omitted for clarity).
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Fig. 4 Diagram showing intra‒molecular C−H∙∙∙π interactions along ‘c’ axis in the
Cu(I)‒ complex.
The molecular structure of Cu(I)‒complex is stabilized by a
number of inter‒molecular hydrogen bonding and
intra‒molecular C−Hꢁꢁꢁ interactions (Fig. 4). The
inter‒molecular hydrogen bonding interactions form
π
3.4 Catalytic properties
a
supra‒molecular architecture on axis ‘c’ (Fig. S6). The
intra‒molecular C−Hꢁꢁꢁπ interactions occur between a phenyl
proton and the centroid of the phenyl ring with contact
distances of 3.168, 3.289 and 3.386 Å. The C−Hꢁꢁꢁπ interactions
also take place when phenyl protons approach the centroid of
the chelate rings with contact distances of 2.968 and 3.183 Å
(Fig. 4).16
The synthesized Cu(I)‒complex has successfully been applied
as a catalyst for the synthesis of N‒2‒aryl‒1,2,3‒triazoles
through azide‒chalcone oxidative cycloaddition and
post‒triazole arylation. The initial experiment was performed
by using a model reaction between chalcone and sodium azide
using 0.5 mol% of Cu(I)‒complex under air for 10 h first, then
2,4‒(NO2)2‒C6H3Cl was added to the mixture. The reaction was
continued further for 12 h to deliver the desired product 4a with
67% yield (Table 1, entry 1). Further optimization of the
reaction conditions resulted 84% product yield when 1 mol%
Cu(I) complex was used as a catalyst (entry 2). Various
solvents were also probed for their effect on the reaction yield
and found DMF as the most suitable solvent (entries 4 and 5).
The Cu(I)‒precursor was also tested for its efficiency and found
lower yield (entry 6). The ligand H2bioh was failed to promote
the reaction (entry 7).
Table 1 Effect of various parameters on reaction conditions
O2N
NO2
N
N
O
N
O2N
NO2
Cl
1 mol% Cu(I) complex
DMF
O
NaN3
2
50-100 °C
22 h
1
3
4
Entry
Catalyst mol (%)
Cu(I) complex (0.5)
Cu(I) complex (1)
Cu(I) complex (2)
Cu(I) complex (1)
Cu(I) complex (1)
[Cu(PPh3)2]NO3 (5)
H2bioh (5)
Solvent
Yield (%)
1
2
3
4
5
6
7
DMF
DMF
DMF
CH3CN
H2O
67
84
84
69
n.r
45
n.r
DMF
DMF
aUsing chalcone (1 mmol), Sodium Azide (1 mmol, 65 mg), DMF (4 mL), Cu(I) complex (1 mol%), DMF (4 mL), at 50 °C under air for 10 h first, then
b
2,4‒(NO2)2‒C6H3Cl/4‒NO2C6H4F (1 mmol) and DBU (1 equiv.) was added to the mixture, and the reaction continued further for 12 h at 100 °C . Isolated
yields.
To test the versatility of the optimized reaction conditions, the nitrophenyl)propꢀ2ꢀenꢀ1ꢀone (3e) and aryl fluorides/chlorides
reactions of a variety of chalcones were studied with sodium having electronꢀwithdrawing groups were participated well in
azide and 2,4‒(NO2)2‒C6H3Cl/4‒NO2C6H4F (Table 2). these reactions. The presence of an electronꢀwithdrawing group
Chalcones having various substituents viz. (
phenylpropꢀ2ꢀenꢀ1ꢀone (3a), )ꢀ3ꢀ(4ꢀchlorophenyl)ꢀ1ꢀ successful post arylation. Functional groups appended to
)ꢀ3ꢀ(4ꢀchlorophenyl)ꢀ1ꢀ(4ꢀ chalcones and aryl halides were also well tolerated in these
(3c), )ꢀ1ꢀphenylꢀ3ꢀ(4ꢀ reactions.
)ꢀ1ꢀ(4ꢀmethoxyphenyl)ꢀ3ꢀ(4ꢀ
E)ꢀ3ꢀ(phenyl)ꢀ1ꢀ on the aryl halide counterpart is necessary driving force for a
(
E
phenylpropꢀ2ꢀenꢀ1ꢀone (3b),
methoxyphenyl)propꢀ2ꢀenꢀ1ꢀone
methoxy)propꢀ2ꢀenꢀ1ꢀone (3d), (
(E
(
E
E
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Table 2 Substrate scope for the copper catalyzed N‒2‒aryl‒1,2,3‒triazole synthesisa
Yield
(%)
Entry
1.
Aryl halide
Chalcone
Product
84
81
1a
1b
3a
3a
4a
4b
2.
3.
4.
75
73
1a
1b
3b
3b
4c
NO2
Cl
N
N
N
4d
O
5.
6.
82
79
1a
1b
3c
3c
4e
4f
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7.
8.
74
72
1a
1b
3d
3d
4g
4h
9.
82
1a
3e
4i
10.
1b
3e
4j
85
aReaction conditions: Using chalcone (1 mmol), Sodium Azide (1 mmol, 65 mg), Cu(I) complex (1 mol%), DMF (4 mL), at 50 °C under air for 10 h first, then
b
2,4‒(NO2)2‒C6H3Cl/4‒NO2C6H4F (1 mmol) and DBU (1 equiv.) was added to the mixture, and the reaction continued further for 12 h at 100 °C. Isolated
yields.
Notes and references
4. Conclusions
Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi 221005, India; Tel: +919450145060, Email:
singvp@yahoo.co.in
The present work describes the synthesis of
a ligand
N',N'‒bis{(1H‒indol‒3‒yl)methylene}oxalohydrazide (H2bioh)
and its Cu(I) complex and their characterization by various
spectroscopic techniques. The molecular structures of the
ligand and its Cu(I) complex have been determined by X‒ray
†
Electronic Supplementary Information (ESI) available:
Experimental details and spectral data for ligand. CCDC
984536 and 984537 contain the supporting information
crystallographic data for the ligand H2bioh and Cu(I) complex,
respectively. These data can be obtained free of charge via the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223ꢀ336ꢀ033; or eꢀmail:
deposit@ccdc.cam.ac.uk.
crystallography. The intermolecular π−π and C−Hꢁꢁꢁ
π
interactions are observed in the ligand as well as in its Cu(I)
complex. The complex has a binuclear structure with a
4‒coordinate tetrahedral geometry around each metal ion. The
catalytic activity of the complex was successfully explored for
the N‒2‒aryl‒substituted‒1,2,3‒triazoles synthesis.
Acknowledgements
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The authors D.P. Singh and B.K. Allam are thankful to CSIR,
New Delhi for the award of Senior Research Fellowships
Espeel
,
C. J. Hawker, T. Junkers, H. Schlaad and W. Van Camp,
, 60−62.
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RSC Advances
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DOI: 10.1039/C5RA27907K
Graphical Abstract
The binuclear Cu(I) complex as novel catalyst towards direct
synthesis of N‒2‒aryl‒substituted‒1,2,3‒triazoles from chalcones
Divya Pratap Singh, Bharat Kumar Allam, Rahul Singh, Krishna Nand Singh and Vinod Prasad Singh*
Department of Chemistry, Centre of Advanced Study, Institute of Science, Banaras Hindu University,
Varanasi, India 221005
A binuclear Cu(I)‒complex with N',N'−bis{(1Hꢀindolꢀ3ꢀyl)methylene}oxalohydrazide is
synthesized and characterized. The complex has been employed as a catalyst for one‒pot
operation involving the azide‒chalcone click reaction and subsequent arylation.