Tris(3,5-dimethylpyrazole)copper(II) nitrate: As an oxidation catalyst

Article abstract of DOI:10.1016/j.molcata.2004.11.019

Tris(3,5-dimethylpyrazole)copper(II) nitrate (A) is a catalyst for mild oxidation of various organic substrates. It acts as catalyst for conversion of benzylamine to benzaldehyde in the presence of hydrogen peroxide. Various benzylic alcohols are converted to corresponding aldehydes and acids by the catalytic amount of the complex (A) with hydrogen peroxide at room temperature. The complex also catalyses dimerisation of 2,6-dimethylphenol and thiophenol. The thermal decomposition of tris(3,5-dimethylpyrazole)copper(II) nitrate at 500°C gives CuO having the monoclinic crystal system. The cyclic voltammogram of the complex tris-(3,5-dimethylpyrazole)copper(II) nitrate shows a quasi-reversible redox cycle at -101 mV (E_1/2) versus the Ag/AgCl electrode (+ve scan; scan speed 100 mV/s). The ΔE between anodic and cathodic peaks is 110 mV (i_pa/i_pc = 1.2).

Full text of DOI:10.1016/j.molcata.2004.11.019

Journal of Molecular Catalysis A: Chemical 229 (2005) 171–176
Tris(3,5-dimethylpyrazole)copper(II) nitrate: as an oxidation catalyst
Sagar Sharma, Nilotpal Barooah, Jubaraj B. Baruah∗
Department of Chemistry, Indian Institute of Technology, Guwahati 781039, India
Received 2 July 2004; received in revised form 20 November 2004; accepted 23 November 2004
Available online 12 January 2005
Abstract
Tris(3,5-dimethylpyrazole)copper(II) nitrate (A) is a catalyst for mild oxidation of various organic substrates. It acts as catalyst for conversion
of benzylamine to benzaldehyde in the presence of hydrogen peroxide. Various benzylic alcohols are converted to corresponding aldehydes
and acids by the catalytic amount of the complex (A) with hydrogen peroxide at room temperature. The complex also catalyses dimerisation of
2,6-dimethylphenol and thiophenol. The thermal decomposition of tris(3,5-dimethylpyrazole)copper(II) nitrate at 500 ◦C gives CuO having the
monoclinic crystal system. The cyclic voltammogram of the complex tris-(3,5-dimethylpyrazole)copper(II) nitrate shows a quasi-reversible
redox cycle at −101 mV (E1/2) versus the Ag/AgCl electrode (+ve scan; scan speed 100 mV/s). The ꢀE between anodic and cathodic peaks
is 110 mV (ipa/ipc = 1.2).
© 2004 Elsevier B.V. All rights reserved.
Keywords: Tris(3,5-dimethylpyrazole)copper(II) nitrate; Oxidation; Peroxide; Benzylamine; Benzylic alcohols; Copper(II) oxide
1. Introduction
and; firstly, 3,5-dimethylpyrazole ligand has ability to form
dative bond through one of the nitrogen lone pair [16,17],
secondly, the free NH bond of the ring may be utilized to cre-
ate a supramolecular environment through hydrogen bonding
which in turn may provide extra selectivity during a product
formation through catalytic reactions.
Copper complexes catalyses various reactions such as
electron transfer [1], dioxygen transport [2,3], substrate oxy-
genation/oxidation [4,5] and reduction of nitrogen oxides
[6,7]. In these reactions mononuclear [8–10] or multinu-
clear copper complex [11,12] may be involved. There are
many studies in which pyrazole or imidazole containing bin-
uclear copper complexes are used as catalyst [13,14]. In these
studies it is clear that these pyrazole and imidazole based
catalysts are very efficient in activating dioxygen for ox-
idation of primary alcohols. Product selectivity in copper
catalysed reactions can be enhanced by variation of the ratio
of copper and ligands [15]. Since majority of these com-
plexes involve simple structural features in terms of a five
membered heterocyclic nitrogen-donor ligand, we have con-
ducted our study to explore the reactivity of mono-nuclear
copper(II) complexes having 3,5-dimethylpyrazole (Fig. 1)
as one of the ligand. There can be two advantages in us-
ing 3,5-dimethylpyrazole (abbreviated as dmpyz) as a lig-
2. Results and discussion
2.1. Synthesis and characterisation of
tris(3,5-dimethylpyrazole)copper(II) nitrate
The copper complex tris(3,5-dimethylpyrazole)copper(II)
nitrate (A) can be synthesized from the reaction of 3,5-
dimethylpyrazole with copper nitrate trihydrate in methanol
(Eq. (1a)). The ratio of the copper(II) nitrate trihy-
drate and 3,5-dimethylpyrazole in 1:2 or 1:3 leads to
only tris(3,5-dimethylpyrazole)copper(II) nitrate. On mix-
ing 1:2 molar equivalent ratio of copper(II) nitrate trihydrate
with 3,5-dimethylpyrazole we could obtain only tris(3,5-
dimethylpyrazole)copper(II) nitrate (A); we had also at-
tempted to synthesise bis(3,5-dimethylpyrazole)copper(II)
∗
Corresponding author. Tel.: +91 3612582301; fax: +91 3612690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.11.019

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S. Sharma et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 171–176
spectrum of the complex shows an absorbance at 700 nm
(ε = 42.1 M−1 cm−1).
The molar conductance of the complex in water showed
another important aspect of the complex. In water its molar
Fig. 1. 3,5-Dimethylepyrazole.
conductance was 232 S mol−1 cm2. This corresponds to the
presence of ionic nitrato groups in solution as shown in Eq.
(2)
nitrate (B) from a preformed 1:2 dimeric adduct by ligand dis-
placement reaction. Thus, the reaction of bis[chloro-bis-(3,5-
dimethylpyrazole)copper(II)] (C) with AgNO3 was per-
formed in an anticipation of a simple ligand displace-
ment of chloride by nitrate to give bis(3,5-dimethylpyra-
zole)copper(II) nitrate (Eq. (1b)) and AgCl. However,
−
[Cu(dmpyz)3(NO3)2] → [Cu(dmpyz)3]2+ + 2NO3
(2)
Since water is a good O-donor, it can replace the weakly
co-ordinating nitrato group. Therefore the conductance
it was not successful in this regard; in contrast, this reac-
tion also gave tris(3,5-dimethylpyrazole)copper(II) nitrate
and AgCl as isolable products.
The complex tris(3,5-dimethylpyrazole)copper(II) nitrate
is further characterized by its IR, UV–vis spectra, mag-
neticmoment, thermogravimetry, conductancemeasurement.
The IR spectra of the complex shows a strong absorption
measurements were made in acetonitrile and the molar con-
ductance was found to be 93 S mol−1cm2. This shows that
the nature of the solvent is responsible for ionisation and ace-
tonitrile being a weaker ligand for copper(II) does not cause
ionisation of one of the more tightly bound nitrato group. The
nitrato complexes having similar five-coordinated copper has
generally one of the nitrato group more tightly held due to
a chelating mode of interaction [13]. The experimental mag-
neticmomentofthecomplexwasfoundtobe1.71B.M. which
matches well with an expected value of 1.73B.M. These ob-
servations suggest that the complex has the structure A as
shown in Eq. (1a).
at 1383 cm−1 which corresponds to the symmetric
stretching of the NO3 group and the other at 1578 cm−1 corre-
sponding to the antisymmetric stretching of the same group.
Another strong absorption at 3242 cm−1 may be attributed
to the N H stretching frequency of the ligand. The visible

S. Sharma et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 171–176
173
Fig. 2. Thermogram of the complex [Cu(dmpyz)3(NO3)2].
The thermogravimetry of the [Cu(dmpyz)3(NO3)2] com-
plex (Fig. 2) shows an initial loss of 60.1% of the complex in
the temperature range of 140–160 ◦C. This weight loss cor-
responds to the loss of three 3,5-dimethylpyrazole ligands.
The complex then shows a second weight loss in the range of
300–450 ◦C (a loss of 22.4%) which corresponds to the loss
of N2O5, to give a residue of CuO. The sequence of weight
losses are shown in Scheme 1.
To confirm that the final residue is CuO, we heated the
complex to 500 ◦C in a controlled manner in a separate ex-
periment. The XRD powder pattern is shown in Fig. 3. It was
Scheme 1.
found that the final residue is CuO monoclinic crystal system
[18].
Thus, the thermal degradation of [A] involves two steps,
one is the formation of anhydrous copper(II) nitrate and
Fig. 3. XRD powder pattern of the residue obtained from [Cu(dmpyz)3(NO3)2] after heating to 500 ◦C.

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S. Sharma et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 171–176
Scheme 2.
the other is the formation of copper oxide. This is impor-
tant as direct heating of copper(II) nitrate trihydrate leads
to a mixture of products and dehydration is not possible
[19]. So, this method also constitutes and a mild method
for formation of anhydrous copper(II) nitrate and copper(II)
oxide.
The cyclic voltammogram of the complex [Cu(dmpyz)3-
(NO3)2] in acetonitrile with tetrabutylammonium perchlo-
rate as supporting electrolyte shows a quasi-reversible redox
cycle at −101 mV (E1/2) versus the Ag/AgCl electrode (+ve
scan; scan speed 100 mV/s). The ꢀE between anodic and ca-
thodic peaks is 110 mV (ipa/ipc = 1.2). This is attributed to a
Cu(II)–Cu(I) quasi-reversible redox cycle. As change in the
oxidation state from +2 to +1 involves change in ligand en-
vironment of the copper center from trigonal pyramidal to
tetrahedral geometry and this slow process makes ꢀE far
greater than ideal 90 mV of the Fe2+/Fe3+ reversible couple
of ferrocene [20].
2.2. Catalytic oxidation by
tris(3,5-dimethylpyrazole)copper(II) nitrate
The complex tris(3,5-dimethylpyrazole)copper(II) nitrate
(A)catalysesvariousoxidativereactionsoforganicsubstrates
in the presence of hydrogen peroxide as an oxidant. The rep-
resentative reactions are shown in Scheme 2 and the results
of different substrates are listed in Table 1. Benzylamine re-
acted with catalytic amount (1 mol%) of copper complex (A)
in the presence of hydrogen peroxide gave benzaldehyde in
good yield (Table 1, entry 1).
The oxidation of benzylic alcohols led to the correspond-
ing aldehydes and carboxylic acids; however, the carboxylic
acids were the major products (Scheme 2c) in all the reac-
tions. Since it is possible that the benzylic alcohol first trans-
forms to aldehyde followed by its further oxidation to car-
boxylic acid, the reactions of the corresponding aldehydes
were carried out. These reactions led to the formation of the
Table 1
Results of [Cu(dmpyz)3(NO3)2] catalysed oxidation reactions
Entry no.
Substrate
Temperature (◦C)
Time (h)
Product, yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
C6H5CH2NH2
C6H5CH2OH
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
70 ◦C
RT
5
8
6
6
6
6
1
1
1.5
6
1
6
0.5
C6H5CHO (75%)
C6H5COOH (70%), C6H5CHO (30%)
p-MeO-C6H4COOH (75%); p-MeO-C6H4CHO (25%)
p-NO2-C6H4COOH (85%); p-NO2-C6H4CHO (15%)
p-Cl-C6H4COOH (80%); p-Cl-C6H4CHO (20%)
p-Me-C6H4COOH (82%); p-Me-C6H4CHO (18%)
C6H5COOH (100%)
p-MeO-C6H4COOH (100%)
p-NO2-C6H4COOH (100%)
CH3COOH (trace)
4,4ꢀ-Dihydroxy-3,5,3ꢀ,5ꢀ-tetramethylbiphenyl (98%)
C6H5COOH (12%)
p-MeO-C6H4CH2OH
p-NO2-C6H4CH2OH
p-Cl-C6H4CH2OH
p-Me-C6H4CH2OH
C6H5CHO
p-MeO-C6H4CHO
p-NO2-C6H4CHO
CH3CHO
2,6-Dimethylphenol
CH3C6H5
C6H5SH
C6H5SSC6H5 (100%)
All the reactions were carried out in acetonitrile (5 ml) with H2O2 (1 ml, 30 vol%) with substrates (5 mmol) and tris(3,5-dimethylpyrazole)copper(II) nitrate
(0.05 mmol).

S. Sharma et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 171–176
175
corresponding carboxylic acids (Table 1, entries 7–9). The
reaction time required was much less than the corresponding
time required for aldehyde formation from benzylic alcohol
(cf. Table 1 entries 2–4), suggesting that the conversion of
aldehyde to carboxylic acid by the catalyst is rapid and that
once aldehyde is formed from benzylic alcohol it gets con-
verted to the corresponding acid under the reaction condition.
The reaction of cinnamyl alcohol, at 60 ◦C for 12 h re-
sulted in C C bond cleavage to give benzoic acid (25% with
1 mol% of catalyst) and several side products which could
not be characterised. The exact nature of the reaction pro-
cess involved in this reaction is not clear. We obtained only
cinnamic acid from oxidation of cinnamdehyde. Moreover
it was also found that cinnamic acid did not undergo de-
carboxylation under similar reaction condition. This led us
to assume that in the case of cinnamyl alcohol oxidation
there may be intermediacy of ␤-ketocarboxylic acid, which
underwent further oxidation. Further study on the mecha-
nistic part of this reaction is underway. A phenolic com-
pound having substituents at 2- and 6-positions of the ring,
namely 2,6-dimethylphenol undergoes dimerisation to give
4,4ꢀ-dihydroxy-3,3ꢀ,5,5ꢀ-tetramethylbiphenyl in high yield.
The catalytic C H activation reactions of side chain car-
bon adjacent to an aromatic substrates was also carried out
with the complex. The C H activation of toluene gave sev-
eral products; only benzoic acid could be characterized in a
pure form. The reaction of diphenylmethane gave benzophe-
none (4% isolated) as one of the products in the reaction with
catalytic amount of A (1 mol%) in the presence of hydrogen
peroxide at 60 ◦C for 8 h.
Control experiments suggest that none of 3,5-
dimethylpyrazole, hydrogen peroxide alone and a mixture
of the two together causes these transformations under
identical reaction conditions; thus, copper(II) complex
(A) is the essential catalyst in these transformation. We
have also tested the catalytic activity of copper(II) nitrate
trihydrate with hydrogen peroxide in acetonitrile for similar
oxidation reactions; this combination did not lead to the
formation of desired products under identical condition,
either. However, the oxidation reactions with a lower yield
can be effected from a mixture of copper(II) nitrate trihydrate
and 3,5-dimethylpyrazole along with hydrogen peroxide
in very low yield. Since, during the synthesis of tris(3,5-
dimethylpyrazole)copper(II) nitrate irrespective of the ratio
of 3,5-dimethylpyrazole and copper(II) nitrate trihydrate
only the tris(3,5-dimethylpyrazole)copper(II) nitrate is
formed; the active catalyst is believed to be derived from the
ionised species of tris(3,5-dimethylpyrazole)copper(II), as
shown in Scheme 3, which anchors a peroxide unit similar to
the five-coordinated intermediates described in the literature
[21]. The driving force for such intermediate formation is
reflected in the quasi-reversible couple present in the copper
complex which facilitates oxidation of hydrogen peroxide
to give a copper(I) species and followed by formation of the
reactive intermediate as shown in Scheme 3.
In conclusion we have demonstrated that tris(3,5-
dimethylpyrazole)copper(II) nitrate is a good catalyst for var-
ious organic transformation in the presence of hydrogen per-
oxide and it also serves as a precursor for synthesis of CuO
monoclinic crystal system.
Scheme 3.

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S. Sharma et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 171–176
3. Experimental
3.1. Materials
chlorobenzyl alcohol was added and the reaction mixture was
stirred to form a homogeneous solution. Hydrogen peroxide
(4 equiv., 2.3 ml) was then added and stirring was continued
at room temperature. After 1 h, the reaction was stopped and
solvent was evaporated. The crude mixture was then treated
with NaHCO3 solution and extracted with dichloromethane.
The aqueous layer was neutralized with 2N HCl and extracted
with ether to give the corresponding acid (0.556 g, 78%). The
identity of the product was ascertained by recording its IR and
NMR spectra.
All the reactions were carried out with analytical or lab-
oratory grade chemicals. The solvents used in the reaction
and for column chromatography were distilled by literature
procedure. All the oxidized products were purified and char-
acterized by comparing their NMR, IR and GC data with
those of authentic samples.
Bis-[chloro-bis-(3,5-dimethylpyrazole)copper(II)] was
prepared by literature procedure [22].
Acknowledgement
3.2. Preparation of the [Cu(dmpyz)3(NO3)2]
The authors thank Department of Science and Technology,
New Delhi for financial support.
To a solution of Cu(NO3)·3H2O (4 mmol, 0.967 g) in 5 ml
methanol in a round bottom flask, 5 ml methanolic solu-
tion of 3,5-dimethylpyrazole (12 mmol, 1.154 g) was added.
The colour of the solution changed to dark blue. The so-
lution was stirred for one and half hour at room tempera-
ture. Solvent was removed under pressure to give a deep blue
coloured solid. The solid was repeatedly washed with diethyl
ether to remove any unreacted 3,5-dimethylpyrazole. The
crude product was redissolved in dichloromethane (15 ml)
and kept at 10 ◦C to yield regular small needle like crys-
tals after 5 days. Yield: 1.74 g (91%). Elemental analysis:
Calcd. for C15H24N8O6Cu; C, 37.81; H, 5.04; N, 23.53;
found: C, 37.63; H, 5.06; N, 23.46. IR (KBr cm−1): 3242(w),
2925(vw), 1578(s), 1470(s), 1383(vs), 1265(m), 1055(m)
UV–vis (H2O): λmax = 700 nm, ε = 43.3 M−1 cm−1; mag-
netic moment (RT): 1.71 B.M; molar conductance (water):
231.66 S mol−1 cm2.
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3.3. Procedure for oxidation
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Since all oxidation reactions involves similar procedures,
two typical oxidation procedures are given below.
3.3.1. Oxidation of benzylamine
To a solution of [Cu(dmpyz)3(NO3)2] (4 mmol, 0.0952 g)
in 5 ml of acetonitrile in a round bottom flask, benzylamine
(4.5 mmol, 0.4823 g) was added and reaction mixture was
stirred to form a homogeneous solution. Then hydrogen per-
oxide (4 equiv., 2.0 ml) was added dropwise to this solution
and stirring was continued at room temperature. The reaction
was monitored by TLC time to time. After 4 h the reaction
was stopped, solvent was evaporated in a rotary flash evap-
orator, the catalyst was washed with water and finally the
product was isolated by extracting with ether. The product
benzaldehyde was obtained (0.334 g, 75%) by column chro-
matography (silica gel, hexane/ethylacetate).
3.3.2. Oxidation of p-chlorobenzyl alcohol
To a solution of [Cu(dmpyz)3(NO3)2] (0.05 mmol,
0.0237 g) in 5 ml of acetonitrile, 5 mmol, 0.7130 g of p-