Green, homogeneous oxidation of alcohols by dimeric copper(II) complexes

Journal of Coordination Chemistry

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Green, homogeneous oxidation of alcohols by

dimeric copper(II) complexes

Abhishek Maurya & Chanchal Haldar

To cite this article: Abhishek Maurya & Chanchal Haldar (2020): Green, homogeneous

oxidation of alcohols by dimeric copper(II) complexes, Journal of Coordination Chemistry, DOI:

10.1080/00958972.2020.1857747

To link to this article: https://doi.org/10.1080/00958972.2020.1857747

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JOURNAL OF COORDINATION CHEMISTRY

https://doi.org/10.1080/00958972.2020.1857747

Green, homogeneous oxidation of alcohols by dimeric

copper(II) complexes

Abhishek Maurya

and Chanchal Haldar

Department of Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad,

Jharkhand, India

ABSTRACT

ARTICLE HISTORY

Received 30 June 2020

Accepted 12 November 2020

Three pyrazole derivatives, 3,5-dimethyl-1H-pyrazole (DMPz) (I),

3-methyl-5-phenyl-1H-pyrazole (MPPz) (II), and 3,5-diphenyl-1H-pyra-

zole (DPPz) (III), were prepared via reacting semicarbazide hydro-

chloride with the acetylacetone, 1-phenylbutane-1,3-dione, and

1,3-diphenylpropane-1,3-dione, respectively. Complexes 1–3 were

isolated by reacting CuCl₂ꢀ2H₂O with I–III, respectively, and character-

KEYWORDS

Homogeneous catalysis;

copper complexes; pyrazole

derivatives; EPR;

1

ized by CHNS elemental analyses, FT-IR, UV-Vis, H and ¹³C NMR, EPR

alcohol oxidation

spectra, and TGA/DTA. Molecular structures of the pyrazole deriva-

tives I–III and copper(II) complexes 2 and 3 were studied through

single-crystal XRD analysis to confirm their molecular structures.

Overlapping of hyperfine splitting in the EPR spectra of the dimeric

copper(II) complexes 1–3 indicates that both copper centers do not

possess the same electronic environment in solution. The copper(II)

complexes are dimeric in solid state as well as in solution and cata-

lyze the oxidation of various primary and secondary alcohols select-

ively. Catalysts 1–3 show more than 92% product selectivity toward

ketones during the oxidation of secondary alcohols. Surprisingly pri-

mary alcohols, which are relatively difficult to oxidize, produce car-

boxylic acid as a major product (48%–90% selectivity) irrespective of

catalytic systems. The selectivity for carboxylic acid rises with decreas-

ing the carbon chain length of the alcohols. An eco-friendly and

affordable catalytic system for oxidation of alcohols is developed by

the utilization of H₂O₂, a green oxidant, and water, a clean and

greener solvent, which is a notable aspect of the study.

CONTACT Chanchal Haldar

chanchal@iitism.ac.in

Department of Chemistry, Indian Institute of Technology

(Indian School of Mines), Dhanbad-826004, Jharkhand, India

Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1857747.

ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2

A. MAURYA C. HALDAR

1. Introduction

Aldehydes, ketones, and carboxylic acids represent “the class of valuable chemicals” in

current industrial and synthetic chemistry, and demands for these chemicals are grow-

ing. These compounds are used as solvents, perfumes, and flavoring agents or critical

intermediates for the synthesis of other important compounds [1]. To meet the

demands, catalytic oxidation of alcohols to carbonyl compounds is the most common

pathway. A variety of oxidation processes like Dess–Martin oxidation [2], Swern oxida-

tion [3], Jones oxidation [4], Oppenauer oxidation [5], or traditional stoichiometric oxi-

dants, such as chromium oxide, have been reported. Despite the use of toxic

chemicals, the generation of a large number of by-products, and inherent safety

issues, large-scale manufacturing of carbonyl compounds from alcohols are still using

these methods [6]. In order to find out an inexpensive, uncomplicated, waste-free, and

green method for transformation of alcohols into carbonyl compounds, transition of

metal-catalyzed alcohol oxidation is continuously evolving [7]. Three aspects need to

be addressed to make the selective catalytic oxidation of alcohol economical and

green: a straightforward and facile synthesis of catalysts with inexpensive raw materi-

als; the use of greener and safer oxidizing agents; use of clean and green solvent.

Copper-based catalytic systems are economical due to their high natural abundance

and lower price. The catalytic properties of copper complexes are also unquestionable.

Among the variety of other transition metals, copper occupies a special place in con-

nection with the development of greener and economic catalysts for the transform-

ation of alcohol into carbonyl compounds. Copper can be found in the active site of

monometallic enzymes [8], bimetallic enzymes [9], multimetallic enzymes [10], or even

in heterometallic enzymes [11]. A number of catalytic systems have been reported

where molecular oxygen or hydrogen peroxide were used as greener oxidants. Using

atmospheric oxygen as an oxidant in alcohol oxidation is an excellent choice, but

hydrogen peroxide as an oxidant has its advantages over atmospheric oxygen. In

large-scale production of carbonyl compounds from alcohol, handling, transportation,

and the use of gaseous oxygen can be problematic [12]. A variety of transition metal-

catalyzed alcohol oxidations in the presence of hydrogen peroxide have been reported

[13], where organic solvent dissolves these catalysts making the recovery of the cata-

lysts difficult. Therefore, the concept of economic and green process fails.

A number of reports deal with copper-catalyzed aerobic oxidation [14]. However,

transition metal-catalyzed alcohol oxidation using hydrogen peroxide as an oxidant

started getting momentum in the last decade. A number of reports of heterogeneous

[15] copper-based catalysts using hydrogen peroxide as an oxidant are available, but

similar homogeneous systems are scarce [7(c), 16].

Here, we explore the catalytic potential of three dimeric copper(II) complexes of

pyrazole derivatives, 3,5-dimethyl-1H-pyrazole (DMPz) (I), 3-methyl-5-phenyl-1H-pyra-

zole (MPPz) (II), and 3,5-diphenyl-1H-pyrazole (DPPz) (III), for selective oxidation of

alcohols in the presence of hydrogen peroxide. Pyrazole derivatives I–III and their cor-

responding copper(II) complexes, [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂(2), and

[Cu(DPPz)₂Cl₂]₂(3), are reported separately in a number of articles [17]. Pyrazole and

its derivatives are principal components of various pharmaceutical and agrochemical

products such as anti-inflammatories, anticoagulants, and antimicrobials [18]. Pyrazole

JOURNAL OF COORDINATION CHEMISTRY

3

derivatives are synthesized by reacting 1,3-dicarbonyl compounds with hydrazine

derivatives. In 1951, Wiley and Hexner reported the synthesis of DMPz, where hydra-

zine sulfate was dissolved in aqueous sodium hydroxide and reacted with acetylace-

tone in ethanol. Compounds were extracted from the reaction mixture with ether and

recrystallized in petroleum ether with a 73% yield [19]. The reaction of acetylacetone

with hydrazine hydrate in ethanol is an alternative route for the synthesis of DMPz.

However, the reaction with hydrazine hydrate is violent. Therefore the earlier method

is more acceptable. Hydrolysis and decarboxylation of 1-carbamido- or 1-carboxami-

dine derivatives also lead to the formation of DMPz [20]. The 1-carbamido derivatives

are easily prepared by reacting semicarbazide or aminoguanidine with acetylacetone

[21], and 1-carboxamidine derivatives can be prepared from 1,2-pentadiene-4-one and

hydrazine hydrate [22]. The synthesis of DPPz was reported by Kitajima et al. in 1992.

Hydrazine hydrate was added dropwise into the ethanolic solution of dibenzoylme-

thane, and after 30 min of refluxing a white product with 85% yield was collected by

filtration [23]. In 2013, Lee et al. reported the solid-phase synthesis of pyrazole deriva-

tives, including DMPz, MPPz, and DPPz, where 1,3-dicarbonyl derivatives were reacted

with solid hydrazine at 70 ^ꢁC for 2 h. The isolated yields of DMPz, MPPz, and DPPz are

99%, 98%, and 98%, respectively [24]. Here, we have adopted a simple, single-step

method for preparation of I–III. We replaced reactive hydrazine hydrate with semicar-

bazide hydrochloride, which is the major difference from the reported synthetic meth-

ods. After completion of the reaction, the three pyrazole derivatives were isolated as

single crystals. Chandrasekhar et al. first reported the synthesis of [Cu(DMPz)₂Cl₂]₂by

the complete desulfurization of bis(3,5-dimethyl pyrazolyl)methylphosphine sulfide

(MeP(S)(3,5-Me₂Pz)₂) followed by hydrolysis of P-N bonds. The reaction of MeP(S)(3,5-

Me₂Pz)₂with anhydrous CuCl₂in 1:1 ratio produces [Cu(DMPz)₂Cl₂]₂along with a tetra-

nuclear copper complex [Cu₂Cl₂(3,5-Me₂Pz)₃(MePO₃)]₂. These two complexes were sep-

arated by recrystallization from a mixture of CH₂Cl₂and hexane [17(c)]. In 2015, Giles

et al. reported the synthesis of the same compounds by reacting 3,5-dimethylpyrazole

with CuCl₂ꢀ2H₂O₂in an aqueous medium [17(e)]. However, Chandrasekhar et al. did

not mention the yield of [Cu(DMPz)₂Cl₂]₂, but Giles et al. isolated the complex with

94% yield. In 2012, Soltani and co-workers first established the synthesis and structure

of [Cu(MPPz)₂Cl₂]₂. The compound was isolated (75% yield) from the reaction of DPPz

with anhydrous CuCl₂in a mixture of acetonitrile and methanol (1:1) for 12 h [17(f)].

Mezei and co-workers reported the synthesis of [Cu(DPPz)₂Cl₂]₂by reacting DPPz with

CuCl₂ꢀ2H₂O in the presence of 1 equivalent of NaOH. Recrystallization from layering a

THF solution with hexane produced single crystals (57% yield), confirming the molecu-

lar structure [17(d)]. All three copper(II) complexes (1–3) are characterized only by FT-

IR, UV-Vis spectroscopy, and single-crystal XRD analysis [17]. No other techniques were

used to analyze the properties of 1–3. Here, we have further analyzed these com-

plexes by using EPR (electron paramagnetic resonance) spectroscopy and thermogravi-

metric analysis (TGA). There are no reports on application of these complexes. Hence,

we have utilized 1–3 for the catalytic oxidation of primary and secondary alcohols.

Copper complexes 1–3 were synthesized by a straightforward, faster, and single-

step method by reacting pyrazole derivatives DMPz (I), MPPz (II), and DPPz (III) with

CuCl₂ꢀ2H₂O in refluxing methanolic solution for 2 h. After the reaction, 2 and 3 were

4

A. MAURYA C. HALDAR

separated as single crystals from the reaction mixture, whereas 1 was isolated in pow-

der form.

We have used water, a green solvent, during the catalytic oxidation of alcohol in

the presence of 1–3. In order to make the catalytic process economical, safe, and

green, hydrogen peroxide was used as an oxidant. The scope of the alcohol oxidation

was increased by using a variety of cyclic, acyclic aliphatic alcohols, and aromatic alco-

hols. Despite the inherent difficulties associated with the oxidation of primary alcohols,

1–3 showed excellent catalytic performance in terms of conversion as well as selectiv-

ity. Secondary alcohols converted into ketones with almost 100% selectivity irrespect-

ive of catalyst, but primary alcohols produced carboxylic acids as the major product.

Interestingly, the carboxylic acid selectivity increases with decreasing the carbon chain

length of the alcohol.

2. Experimental

2.1. Materials

Acetyl acetone (Merck, India), benzoyl acetone (Alfa-Aesar, India), 1,3-diphenyl-1,3-pro-

panedione (Sigma-Aldrich, USA), semicarbazide hydrochloride (Avra, India), CuCl₂ꢀ2H₂O

(Loba-Chemie, India), benzyl alcohol (Merck, India), 1-heptanol (TCI, Japan), 1-hexanol

(Alfa-Aesar, India), 1-pentanol (TCI, Japan), cyclohexanol (Alfa-Aesar, India), 1-butanol

(TCI, Japan), 1-propanol (TCI, Japan), 2-propanol (Merck, India), 2-butanol (TCI, Japan),

30% H₂O₂(Merck, India), TBHP (Alfa-Aesar, India), deionized water, and AR-grade solv-

ent (Merck & Rankem, India) were used as received. HPLC-grade methanol and ethyl

acetate (Spectrochem, India) were used for GC analysis.

2.2. Physical methods and analysis

FT-IR spectra of the ligands and their copper complexes were recorded on an Agilent

Cary 600 Series FT-IR spectrometer by the ATR (attenuated total reflection) method.

Electronic spectra of the ligands and Cu(II) complexes were recorded with a

1

SHIMADZU UV-1800 spectrophotometer using methanol/DMF as solvent. The H and

¹³C NMR spectra of the pyrazole derivatives were recorded with a Bruker AC-400 NMR

spectrometer using DMSO-d₆/CDCl₃as a solvent with standard parameter settings.

Waters Q-Tof Micromass was used for ESI^þ-MS of metal complexes. TGA and differen-

tial thermal analysis (DTA) were performed with a PerkinElmer, Diamond TG/DTA

instrument. X-band EPR spectra of the copper(II) complexes were recorded in a JES-

FA200 ESR spectrometer at 77 K. The single-crystal data of ligands and their respective

copper complexes were collected by the Rigaku Oxford Diffraction system equipped

with a state of the art CCD Eos S2 detector using Mo Ka radiation (wavelength

0.71073 Å) at room temperature. Catalytic oxidations of various alcohols and alkenes

ware monitored by an Agilent gas-chromatograph (7890B) fitted with an HP-5 capillary

column (30 m ꢂ 0.32 mm ꢂ 0.25 mm) and an FID detector. Catalytic oxidation prod-

ucts of alcohols were identified by comparing with commercially available standards

in GC. The confirmation of prepared pyrazoles I–III and oxidation products was done

JOURNAL OF COORDINATION CHEMISTRY

5

Scheme 1. Proposed synthetic route for synthesis of 3,5-dimethyl-1H-pyrazole (DMPz) (I), 3-methyl-

5-phenyl-1H-pyrazole (MPPz) (II), and 3,5-diphenyl-1H-pyrazole (DPPz) (III) and their corresponding

copper complexes [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂(2), and [Cu(DPPz)₂Cl₂]₂(3).

by GC-MS (Trace 1300 ISQ QD) with a TG-5MS capillary column (30 m ꢂ 0.25 mm ꢂ

0.25 mm) and an EI^þ-Mass detector.

2.3. Synthesis

2.3.1. Synthesis of pyrazole derivatives: 3,5-dimethyl-1H-pyrazole (DMPz) (I), 3-

methyl-5-phenyl-1H-pyrazole (MPPz) (II) and 3,5-diphenyl-1H-pyrazole (DPPz) (III)

Synthesis of DMPz (I) has been known for a long time. A number of reports can be

found in the literature describing the synthesis of I–III [19, 24, 25]. Solution phase syn-

thesis of I–III was done by simplifying the method described in the literature [19]. A

mixture of 20 mL methanolic solution of semicarbazide hydrochloride (1.11 g, 10 mmol)

and 20 mL methanolic solution of acetylacetone (1.00 g, 10 mmol) was refluxed for

ꢃ20 h (Scheme 1) with constant stirring. The volume of the final solution was reduced

to ꢃ10 mL and kept in a refrigerator overnight. White crystals suitable for single-crystal

X-ray analysis separated. Crystals were filtered, washed with cold methanol, and dried

in vacuum over silica gel.

By following the same synthetic procedure as described above, we isolated MPPz

(II) and DPPz (III) by replacing acetylacetone with 1-phenylbutane-1,3-dione and 1,3-

diphenylpropane-1,3-dione, respectively. All three pyrazoles were isolated as single

crystals from the reaction mixture. The straightforward and single-step process with

easy product separation (as single crystals) makes the current synthetic route prefer-

able among the other synthetic methods reported.

Data for DMPz (I): Yield: 0.50 g (53%); Anal. Calcd for C₅H₈N₂(MW 96.13); C,

62.47%; H, 8.39%; N, 29.14%. Found: C, 61.98%; H, 8.21%; N, 29.55%. FT-IR (ATR, cm^ꢄ1):

3194 and 3119 (m_N-H), 1596 (m_C¼N), 1520 (m_C¼C); UV-Vis [k_max(nm), e(Lmol^ꢄ1cm^ꢄ1)]: 219

1

(2.98 ꢂ 10⁴); H NMR (DMSO-d₆, d in ppm): 2.2 (s, 6H), 5.9 (s, 1H), 13.1 (s, 1H); ¹³C NMR

(DMSO-d₆, d in ppm): 10.6, 106.0, 144.4; EI^þ-MS of I: m/z experimental ¼ 96.16,

observed ¼ 96.13 ([DMPz]^þ).

Data for MPPz (II): Yield: 0.92 g (58%); Anal. Calcd for C₁₀H₁₀N₂(MW 158.20); C,

75.92%; H, 6.37%; N, 17.71%. Found: C, 75.98%; H, 7.09%; N, 18.11%. FT-IR (ATR, cm^ꢄ1):

3176 and 3127 (m_N-H), 1606 (m_C¼N), 1533 (m_C¼C); UV-Vis [k_max(nm), e(Lmol^ꢄ1cm^ꢄ1)]: 255

1

(2.95 ꢂ 10⁴); H NMR (CDCl₃, d in ppm): 2.3 (s, 3H), 6.8 (s, 1H), 7.3–7.4 (m, 3H), 7.8–7.9

(m, 2H), 14.1 (s, 1H); ¹³C NMR (DMSO-d₆, d in ppm): 11.4, 103.4, 126.9, 127.0, 129.3,

6

A. MAURYA C. HALDAR

130.4, 145.2, 147.2; EI^þ-MS of II: m/z experimental

¼

158.18, observed

¼

158.20 ([MPPz]^þ).

Data for DPPz (III): Yield: 1.06 g (48%); Anal. Calcd for C₁₅H₁₂N₂(MW 220.27); C,

81.79%; H, 5.49%; N, 12.72%. Found: C, 81.58%; H, 5.21%; N, 12.55%. FT-IR (ATR, cm^ꢄ1):

3201 and 3120 (m_N-H), 1602 (m_C¼N), 1531 (m_C¼C); UV-Vis [k_max(nm), e(Lmol^ꢄ1cm^ꢄ1)]: 251

(1.71 ꢂ 10⁴); ¹H NMR (DMSO-d₆, d in ppm): 7.0 (s, 1H), 9.7 (s, 1H), 7.2–7.3 (m, 2H),

7.4–7.4 (m, 4H), 7.8–7.8 (s, 4H); ¹³C NMR (DMSO-d₆, d in ppm): 100.7, 125.9, 129.0,

129.2, 130.0, 147.4, 158.0; EI^þ-MS of III: m/z experimental ¼ 220.20, observed ¼

220.27 ([DPPz]^þ).

2.3.2. Synthesis of [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂(2) and [Cu(DPPz)₂Cl₂]₂(3)

Complexes 1–3 were prepared by following a simple synthetic route (presented in

Scheme 1). CuCl₂ꢀ2H₂O (1.70 g, 10 mmol) was refluxed (ꢃ70 ^ꢁC) with I–III in a 1:2 ratio

in 40 mL of methanol for 2 h with constant stirring. A blue precipitate for 1, greenish

crystals for 2, and dark blue crystals of 3 were separated from the refluxing reaction

mixture, which was filtered, washed with methanol (3 ꢂ 10 mL), and dried in vacuum

over silica gel.

Data for [Cu(DMPz)₂Cl₂]₂(1): Yield: 2.93 g (44%); Anal. Calcd for C₂₀H₃₂Cl₄Cu₂N₈

(MW 650.01); C, 36.76%; H, 4.94%; N, 17.15%. Found: C, 36.88%; H, 5.10%; N, 17.35%.

FT-IR (ATR, cm^ꢄ1): 3147 and 3040 (m_N-H), 1583 (m_C¼N); UV-Vis [k_max(nm),

e(Lmol^ꢄ1cm^ꢄ1)]: 236 (sh), 271 (4.09 ꢂ 10⁴); ESI^þ-MS of 1: calculated m/z ¼ 256.86,

observed ¼ 256.81 ([Cu(DMPz)₂] þ H]^þ).

Data for [Cu(MPPz)₂Cl₂]₂(2): Yield: 3.15 g (35%); Anal. Calcd for C₄₀H₄₀Cl₄Cu₂N₈

(MW 901.70); C, 53.28%; H, 4.47%; N, 12.43%. Found: C, 53.48%; H, 4.71%; N, 12.55%.

FT-IR (ATR, cm^ꢄ1): 3240 and 3134 (m_N-H), 1583 (m_C¼N); UV-Vis [k_max(nm),

e(Lmol^ꢄ1cm^ꢄ1)]: 207 (2.64 ꢂ 10⁴), 256 (1.66 ꢂ 10⁴); ESI^þ-MS of 2: calculated m/

z ¼ 381.10, observed ¼ 380.95 ([Cu(MPPz)₂] þ H]^þ).

Data for [Cu(DPPz)₂Cl₂]₂(3): Yield: 6.50 g (53%); Anal. Calcd for C₅₈H₄₇Cl₄Cu₂N₈

(MW 1124.94); C, 62.67%; H, 4.21%; N, 9.74%. Found: C, 62.52%; H, 4.19%; N, 9.63%.

FT-IR (ATR, cm^ꢄ1): 3246 and 3212 (m_N-H), 1593 (m_C¼N); UV-Vis [k_max(nm),

e(Lmol^ꢄ1cm^ꢄ1)]: 252 (1.63 ꢂ 10⁴); ESI^þ-MS of 3: calculated m/z ¼ 505.22, observed ¼

505.09 ([Cu(DPPz)₂] þ H]^þ).

2.4. Oxidation of alcohols

The homogeneous catalytic oxidations of alcohols were carried out using 1, 2, and 3

in a 50 mL double-neck round bottom flask fitted with a water circulated condenser.

Conventionally benzyl alcohol (1.08 g, 10 mmol), 30% aqueous H₂O₂(9.07 g, 80 mmol),

and catalyst (7 mg) were mixed in 7.5 mL of water and heated for 6 h with constant

stirring. A small aliquot of the reaction mixture was withdrawn periodically, extracted

with ethyl acetate, and analyzed with a GC fitted with an HP-5 column. The identity of

oxidation products was confirmed by comparing with commercially available stand-

ards and GC-MS. To achieve the maximum substrate conversion(%), all the measurable

reaction parameters such as catalysts amount, oxidant amount, nature of oxidant, solv-

ent amount, types of solvent, and temperature were carefully optimized.

JOURNAL OF COORDINATION CHEMISTRY

7

3. Results and discussion

Ligands I–III and copper(II) complexes 1–3 were analyzed by FT-IR and UV-Vis spec-

troscopy, which is comparable with the data reported [17]. All the FT-IR and UV-Vis

spectra are shown in Supplementary Figures S1 –S3, and selected FT-IR data are sum-

marized in Supplementary Table S1. The pyrazole derivatives I–III were further charac-

1

terized by H and ¹³C NMR spectroscopy. All the data are in accordance with reported

1

data [26]. The H and ¹³C NMR spectra of I–III are shown in Supplementary Figures S4

and S5, and the appropriate assignment of these signals along with the detailed

chemical shifts are tabulated in Supplementary Tables S2 and S3. Prepared pyrazole

derivatives DMPz (I), MPPz (II), and DPPz (III) along with copper(II) complexes

[Cu(MPPz)₂Cl₂]₂(2) and [Cu(DPPz)₂Cl₂]₂(3) were crystallized and analyzed through sin-

gle-crystal X-ray crystallography. ORTEP plots of I–III, 2, and 3 are displayed in

Supplementary Figures S6 and S7 with 50% thermal ellipsoids. The detailed crystallo-

graphic data of I–III along with 2 and 3 are summarized in Supplementary Table S4.

All the single-crystal X-ray analysis data confirm the molecular structures of I–III, 2,

and 3.

3.1. EPR spectral analysis

Frozen DMF solution X-band EPR spectra of [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂(2)

and [Cu(DPPz)₂Cl₂]₂(3) are shown in Figure 1. All the spectra are characteristic of an

axially symmetrical paramagnetic Cu^2þcenter and display a well resolved parallel

region but unresolved perpendicular region.

In solution, 1–3 show g_║> g_┴> g (2.0023), which is characteristic of a distorted

2

square pyramidal copper(II) center where the d_x₂_–yorbital carries the unpaired elec-

tron [27]. Interaction of copper(II) nuclear spin (I ¼ 3/2) with the spin of its unpaired

electron (I ¼ 1/2) causes the hyperfine splitting in the EPR spectra of 1–3 with an aver-

age splitting of 128 G, 122 G, and 112 G, respectively. Each of the peaks in the parallel

region appears split in two, suggesting that the mutual overlapping of the hyperfine

structures of the two copper(II) centers of the dimeric complexes 1–3 causes splitting

in the peaks [27(a,b)]. Also, it indicates that both copper centers in the dimeric

copper(II) complexes 1–3 are not magnetically equivalent. Lack of forbidden DMs ¼

2 transition, at the half field, eliminates the possibility of interaction among the two

copper(II) centers in the dimeric complexes. The calculated EPR parameters for 1–3 are

listed in Table 1.

3.2. Thermal analysis

The thermal stabilities of copper(II) complexes [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂(2)

and [Cu(DPPz)₂Cl₂]₂(3) were examined by TGA/DTA under a nitrogen atmosphere

from 30 to 900 ^ꢁC at 10 ^ꢁC/min as shown in Figure 2.

Complexes 1 and 2 follow a three-step exothermic decomposition process, while 3

shows a two-step exothermic decomposition pattern. Decomposition steps are clear in

1, whereas 2 and 3 show overlapping steps during the entire temperature range.

Initial mass losses in 1–3 start around 100 ^ꢁC and continue until 180 ^ꢁC in 1 and 2,

8

A. MAURYA C. HALDAR

Figure 1. X-band EPR spectra of 1, 2, and 3 recorded in DMF at 77 K.

Table 1. Data of spin Hamiltonian parameters of 1–3 in DMF solution at 77 K.

1

2

3

Property

A

jj

g

g_┴

2.3728

2.0948

128 G

2.3949

2.0874

122 G

2.4020

2.0890

112 G

jj

and up to 240 ^ꢁC for 3. During this, 1–3 mostly eliminate adsorbed moisture and

entrapped solvent molecules. In the second step, 1 losses chloride in the temperature

range of 200 to 295 ^ꢁC. In the third and final step, 1 undergoes a massive 54.45%

mass loss from 300 to 620 ^ꢁC due to complete breakdown of the organic component,

leaving CuO as final product. However, in 2 and 3, elimination of chloride and decom-

position of organic components start in an overlapping manner. In 2, 64.94% mass

loss was observed in the second step, followed by a 10.18% mass loss in the final

step. In its second and final step, 3 displays 73.99% weight loss. From the TGA plots

complexes 1–3 are thermally stable up to 200, 181, and 180 ^ꢁC, respectively, which is

sufficient to use copper(II) complexes as catalysts under moderate temperature with-

out losing their molecular structures.

4. Catalytic activity

4.1. Alcohol oxidation

Despite their well-known molecular structures [17], [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂

(2) and [Cu(DPPz)₂Cl₂]₂(3) have not been examined as catalysts. The catalytic potential

of 1–3 is investigated for the liquid phase oxidation of a series of alcohols (listed in

JOURNAL OF COORDINATION CHEMISTRY

9

Figure 2. TGA/DTA plot of [Cu(DMPz)₂Cl₂]₂(1), [Cu(MPPz)₂Cl₂]₂(2), and [Cu(DPPz)₂Cl₂]₂(3).

Table 2) in the presence of H₂O₂under moderate reaction conditions. The reaction

parameters were optimized to obtain the maximum substrate conversion (%). During

the optimization of the reaction parameters, 3 was used as a model catalyst and ben-

zyl alcohol was used as a representative substrate.

To optimize the amount of catalyst, four different amounts, i.e. 0.001 g, 0.003 g,

0.005 g, and 0.007 g were used in the presence of a fixed amount of benzyl alcohol

(1.08 g, 10 mmol), 30% H₂O₂(3.40 g, 30 mmol), and solvent (10 mL, 7 mL H₂O mixed

with 3 mL of methanol) at 100 ^ꢁC (shown in Figure 3(A)). After 6 h of reaction time,

0.001 g of catalyst shows only 12.5% of conversion. With increasing amount of catalyst,

substrate conversion increases progressively. A maximum of 28.3% conversion was

achieved by using 0.007 g of catalyst. Hence, 0.007 g of catalyst was chosen

as optimum.

The impact of the amount of oxidant in the oxidation of benzyl alcohol was studied

by employing six different substrates to oxidant ratio, i.e. 1:3, 1:4, 1:5, 1:6, 1:7, and 1:8

while keeping other reaction parameters constant; six reactions were carried out

where a fixed amount of benzyl alcohol was reacted with different oxidant amounts.

In each reaction, 1.08 g of benzyl alcohol was reacted with 3.40 g, 4.53 g, 5.66 g, 6.8 g,

7.93 g, or 9.06 g of 30% H₂O₂. It is evident from Figure 3(B) that by increasing the sub-

strate to oxidant ratio from 1:3 to 1:8, substate conversion also improves steadily.

Therefore, 1:8 substrate to oxidant ratio was set as optimum as it produces a max-

imum of 58.4% of conversion in 6 h.

10

A. MAURYA C. HALDAR

Table 2. Catalytic data for the oxidation of alcohols in the presence of 1–3 under the optimized

reaction conditions.

Selectivity (%)

TOF

S. No.

1

Cat.

Substrate

% Conv.

(h^ꢄ1

)

-CHO/>C ¼ O

-COOH

1

2

3

83.2

66.4

81.2

64.72

71.27

111.16

12.28

12.48

13.94

79.47

80.94

78.17

Blank

12.7

BzOH

2

3

4

5

6

7

1

2

3

41.8

30.7

37.4

32.51

32.95

51.20

20.46

25.10

22.78

61.48

58.25

59.98

1-HeptOH

Blank

5.9

8.5

7.3

4.5

5.4

6.8

1

2

3

49.2

46.4

58.4

38.27

49.80

79.95

24.34

32.99

15.44

54.34

50.03

58.78

1-HexOH

Blank

1

2

3

51.0

49.4

42.0

39.67

53.02

57.49

26.84

5.67

24.46

48.49

50.89

52.34

1-PentOH

1-ButOH

1-ProOH

Blank

1

2

3

50.3

32.7

25.6

39.12

35.10

35.04

19.81

12.65

12.30

67.89

75.46

76.57

Blank

1

2

3

74.2

58.2

48.4

57.71

62.47

66.26

14.77

9.90

6.94

81.78

87.54

90.72

Blank

1

2

3

52.6

49.4

48.2

40.91

53.02

65.98

92.78

95.49

95.88

7.22

4.51

4.12

Blank

2-ProOH

2-ButOH

CycproOH

8

9

1

2

3

88.2

80.1

77.6

68.61

85.98

106.23

98.98

99.39

100

1.02

0.61

0.00

Blank

9.8

7.2

3.6

1

2

3

57.5

53.0

56.6

44.72

56.89

77.48

98.20

98.89

98.29

1.80

1.11

1.71

Blank

10

1

2

3

37.0

47.2

60.9

28.78

50.66

83.37

97.68

95.98

93.57

2.32

4.02

6.43

Blank

CychexOH

JOURNAL OF COORDINATION CHEMISTRY

11

Figure 3. Optimization of reaction parameters for the oxidation benzyl alcohol in the presence of

3: (A) variation of the amount of catalyst, (B) variation of H₂O₂amounts, (C) variation of mixed

solvent ratio, (D) variation of the solvent amount, (E) influence of types of solvent, (F) impact of

temperature, (G) effect of type of oxidant.

12

A. MAURYA C. HALDAR

Table 3. Data for all the parameters applied to optimize the oxidation of benzyl alcohol by H₂O₂

in the presence of 3.

S. No. Cat. (mg) Oxidant Subs: oxid. Solv. (mL)

Solvent

Temp. (^ꢁC) % Conv. TON TOF (h^ꢄ1

)

1

2

3

4

5

6

7

8

1

3

5

7

H₂O₂

TBHP

1:3

1:4

1:5

1:6

1:7

1:8

10

H₂O þ MeOH

H₂O þ AcCN

H₂O

100

110

90

12.5

16.5

20.6

28.3

31.7

38.7

46.4

51.5

58.4

54.2

64.6

70.3

73.8

79.1

81.2

29.8

33.6

71.8

75.0

47.6

73.5

718.7

316.2

236.8

232.4

260.3

317.8

381.1

423.0

479.7

445.2

530.6

577.4

606.2

649.7

666.9

244.7

275.9

589.7

616.0

390.9

603.7

119.78

52.70

39.48

38.74

43.39

52.98

63.52

70.50

79.95

74.20

88.43

96.24

101.03

108.28

111.16

40.79

45.99

98.29

102.67

65.16

100.62

9

10

11

12

13

14

15

16

17

18

19

20

Solvent free

2.5

5

H₂O

MeOH

DMF

AcCN

H₂O

7.5

H₂O

100

Oxidation of benzyl alcohol was also studied in different solvent systems, water-

þ methanol (7 mL þ 3 mL), water þ acetonitrile (7 mL þ 3 mL), and pure water (10 mL).

With 64.6% substrate conversion(%), pure water appears to be the best in comparison

to the other two solvent systems (shown in Figure 3(C)).

The effect of the solvent volume was also checked by using five different amounts

of solvent, viz., 2.5 mL, 5.0 mL, 7.5 mL, 10 mL, and solvent free for the oxidation of ben-

zyl alcohol in the presence of 3. Increment in the volume of solvent from 2.5 mL to

7.5 mL causes a rise in substrate conversion(%) from 73.8% to 81.2% (shown in Figure

3(D)). Further increasing the volume of solvent from 7.5 mL to 10 mL drops the sub-

strate conversion from 73.8% to 64.6%. Moreover, the solvent-free reaction condition

does not improve the substrate conversion (70.3%) much. Thus, 7.5 mL of H₂O was

considered as optimum. Figure 3(E) displays the impact of various types of solvents,

MeOH, DMF, AcCN, and H₂O, and their polarity in the catalytic oxidation of benzyl

alcohol (reaction conditions are mentioned in Table 3). Among the four solvents,

methanol and DMF show lower substrate conversion of 29.8% and 33.6%, respectively.

Water with 81.2% conversion exhibits the highest substrate conversion among the

examined four solvents. Hence, clean and green H₂O was chosen as a solvent for the

rest of the study.

The temperature was optimized by carrying out the catalytic reactions at 90, 100,

and 110 ^ꢁC while keeping the other reaction parameters fixed, and the result is dis-

played in Figure 3(F). With the rise in temperature from 90 to 100 ^ꢁC, substrate

increases from 47.6% to 81.2%, but at a higher temperature (110 ^ꢁC), the conversion

falls a few percent.

Entry number 14 in Table 3 represents the optimized reaction conditions for the

oxidation of benzyl alcohol in the presence of 3, which are: 0.007 g of catalyst, 7.5 mL

of H₂O, the substrate to oxidant ratio is 1:8, 100 ^ꢁC temperature and 6 h.

JOURNAL OF COORDINATION CHEMISTRY

13

Figure 4. Comparison plots of substrate conversion(%) for the oxidation of alcohols in the pres-

ence of 1–3 under optimized reaction conditions.

Figure 5. Comparison of TOF values for oxidation of alcohol in the presence of 1–3 and CuCl₂

under optimized reaction conditions.

The impact of oxidant was inspected by using TBHP in place of H₂O₂under similar

optimized reaction conditions, as mentioned above, and the result is shown in Figure

3(G). Clearly, TBHP shows comparable substrate conversion (73.5%) to H₂O₂(81.2%).

However, being an inexpensive and greener oxidant, hydrogen peroxide was the obvi-

ous choice for the oxidation of other alcohols under optimized reaction conditions.

Under the optimized reaction conditions, the blank reaction of benzyl alcohol shows

only 12.7% substrate conversion (Table 2), while 2 and 3 show 66.4% and 81.2% conver-

sions, respectively. Oxidation of benzyl alcohol in the presence of 1–3 affords benzoic

acid as a major product with nearly 80% selectivity. In terms of efficiency, with the TOF

values of 111.16 h^ꢄ1, 3 is the best among the three catalysts studied here.

14

A. MAURYA C. HALDAR

Table 4. Substrate conversion(%) and TOF values of catalytic oxidation of alcohols in the presence

of 1–3 and contemporary catalytic systems.

S. No.

1

Cat.

Substrate

% Conv. TOF (h^-1)

Ref.

1

2

3

83.2

66.4

81.2

52

64.72 Present work

71.27

111.16

–

[Cu₂(H₂O)₂(m-L²)₂] (8)

[29]

[30]

[31]

[Cu(H₂O)(L³)] (9)

22

–

BzOH

[Cu₂(L₁)] (1_Cu

)

34

75

33.4

26.7

18.9

71

98

[Cu₂(L₄)] (4_Cu

401

167

133

95

[Cu((kNN⁰O-HL)(H₂O)₂] (1)

[Fe(kNN⁰O-HL)Cl₂] (2)

[Fe(kNN⁰O-HL)Cl(m-OMe)]₂(3)

[Cu(OOC(C₆H₅)Br)(C₁₀H₉N₃)] (ClO₄)

12

[32]

2

3

1

2

3

41.8

30.7

37.4

12

17

13

32.51 Present work

32.95

51.20

1-HeptOH

[Cu(OOCC(C₆H₅)₃)(bipy)(H₂O)][ClO₄](CH₃OH) (1)

[Cu₂(OOCC₆H₄Br)(OCH₃)(bipy)₂(ClO₄)₂] (3)

[Cu(OOC(C₆H₅)Br)(C₁₀H₉N₃)](ClO₄)

(Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂)

[Cu(OOCC(C₆H₅)₃)(C₁₀H₈N₂)(H₂O)](ClO₄)(CH₃OH)

–

2

3

2

[33]

[32]

[34]

[35]

17

12.2

1

2

3

49.2

46.4

58.4

56

72

63.5

38.27 Present work

49.80

79.95

1-HexOH

Cu0.1@Tannin-OMP

Eu(NO₃)₃

V₂O₅-Al₁₃nanohybrid

–

[36]

[37]

[38]

4

5

1

2

3

51.0

49.4

42.0

38

39.67 Present work

53.02

57.49

1-PentOH

1-ButOH

K₈[BW₁₁O₃₉H]₁₃H₂O

V₂O₅-Al₁₃nanohybrid

–

[39]

[38]

62

1

2

50.3

32.7

25.6

64

1

37

39.12 Present work

35.10

35.04

3

Cu0.1@Tannin–OMP

PC-700

–

[36]

[40]

[41]

[42]

TBAP

[Cu₃(nph)(m-Cl)₂(H₂O)₆]ꢀ2H₂O

40

6

7

1

2

3

74.2

58.2

48.4

15

57.71 Present work

62.47

66.26

1-ProOH

2-ProOH

[Cu₃(nph)(m-Cl)₂(H₂O)₆]ꢀ2H₂O

–

[42]

[43]

Fe₃O₄@Ni–Co-BTC NPs (II)

30

1

2

3

52.6

49.4

48.2

25

40.91 Present work

53.02

65.98

Fe₃O₄@Ni–Co-BTC NPs (II)

[Mo^VIO₂fHdfmp(bhz)₂g

(MeOH)] (1)

–

[43]

[44]

94

[Mo^VIO₂fHdfmp(inh)₂g

(MeOH)] (2)

92

90

–

[Mo^VIO₂fHdfmp(nah)₂g

(MeOH)] (3)

(continued)

JOURNAL OF COORDINATION CHEMISTRY

15

Table 4. Continued.

S. No.

8

Cat.

Substrate

% Conv. TOF (h^-1)

Ref.

1

2

3

88.2

80.1

77.6

100

41

68.61 Present work

85.98

106.23

17

(Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂)

Mn_xO_y/CeO₂

[34]

[45]

–

2-ButOH

Mn_xO_y/ZrO₂

51

–

Mn_xO_y/Al₂O₃

51

–

Mn_xO_y/SiO₂

55

86

–

[Mo^VIO₂fHdfmp(bhz)₂g

(MeOH)] (1)

[44]

[Mo^VIO₂fHdfmp(inh)₂g

(MeOH)] (2)

87

–

[Mo^VIO₂fHdfmp(nah)₂g

(MeOH)] (3)

Fe₃O₄

82.5

[46]

9

1

2

3

57.5

53.0

56.6

71

50

44.72 Present work

56.89

77.48

(Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂)

HENU-1

[Pr₄(H₂O)₆(pzdc)₂As₆W₅₈O₂₀₆

12

–

[34]

[47]

[48]

CycproOH

CychexOH

38ꢄ

]

(1^a)

10

1

2

3

37.0

47.2

60.9

65.6

35.8

36.7

28

28.78 Present work

50.66

83.37

[Cu((kNN⁰O-HL)(H₂O)₂] (1)

[Fe(kNN⁰O-HL)Cl₂] (2)

328

179

184

–

[31]

[33]

[Fe(kNN⁰O-HL)Cl(m-OMe)]₂(3)

[Cu(OOCC(C₆H₅)₃)(bipy)(H₂O)][ClO₄](CH₃OH) (1)

[Cu(OOC(C₆H₅)Br)(C₁₀H₉N₃)][ClO₄] (2)

29

–

[Cu₂(OOCC₆H₄Br)(OCH₃)(bipy)₂(ClO₄)₂] (3)

53

38

29

53

–

157

5

9

4

[Cu₂(L₁)] (1_Cu

)

[30]

[32]

[34]

[35]

[Cu(OOC(C₆H₅)Br)(C₁₀H₉N₃)](ClO₄)

(Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂)

[Cu(OOCC(C₆H₅)₃)(C₁₀H₈N₂)(H₂O)](ClO₄) (CH₃OH)

28

Carefully optimized reaction conditions were used to study oxidation of several

other alcohols. In order to extend the scope of the catalysts, a total of 10 alcohols

(five primary aliphatics, four secondary aliphatic, and one aromatic alcohol) were

tested. Among the four aliphatic secondary alcohols, two are cyclic and two are acyclic

or straight-chain alcohols. The detailed catalytic data of the alcohols are tabulated in

Table 2.

Since aliphatic alcohols are hard to oxidize, 1–3 show excellent catalytic perform-

ance for oxidation of the studied alcohols. In the presence of 1, all 10 alcohols, except

1-hexanol and cyclohexanol, show higher substrate conversion(%) compared to 2 and

3 (shown in Figure 4). Catalyst 3 exhibits higher substrate conversion(%) for the oxida-

tion of 1-hexanol and cyclohexanol in comparison to the other two catalysts.

Irrespective of the catalytic system, all the primary aliphatic alcohols produce the cor-

responding carboxylic acid as a major product. Selectivity(%) of carboxylic acid

increases with decreasing carbon chain length of the aliphatic alcohols. Secondary ali-

phatic alcohols show preference toward corresponding aldehyde/ketone rather than

the carboxylic acid. In all the cases, regardless of the catalytic system, nearly 100%

product selectivity was observed during the oxidation of secondary aliphatic alcohols.

16

A. MAURYA C. HALDAR

Scheme 2. Suggested reactive species generated in situ in the solution by the reaction of H₂O₂

with 3.

Solubility of aliphatic alcohols in water decreases with an increase in carbon chain

length. Thus the order of solubility of primary alcohols in water is 1-propanol > 1-butanol

> 1-pentanol > 1-hexanol > 1-heptanol and the order of solubility for secondary alcohols

is 2-propanol > 2-butanol > cyclopentanol > cyclohexanol. Due to partial solubility, alco-

hols maintain a constant concentration in water, which primarily decides the rate of the

reaction [28]. Hence, it can be predicted that benzyl alcohol, 1-propanol, and 2-propanol

will exhibit higher substrate conversion because they are miscible in water. Catalytic data

also support this except for 2-propanol. The same trend of catalytic activity (based on the

solubility of alcohols) can be observed for oxidation of primary and secondary alcohols in

the presence of 1. However, catalytic data of 2 and 3 do not obey the order of solubility

consistently. Probably a second factor, other than solubility of the alcohol in water, influ-

ences the oxidation reaction. Nonetheless, alcohols with lower solubility display very good

substrate conversion(%) in the presence of 1–3.

Under optimized reaction conditions, oxidation of benzyl alcohol in the presence of

1–3 shows 83.2%, 66.4%, and 81.2% substrate conversion, respectively. Several

reported homogeneous and heterogeneous catalytic systems [29–48] are far behind in

terms of substrate conversion(%) as well as TOF values in comparison to 1–3 (Table 4).

All three catalysts show excellent substrate conversion(%) for 2-butanol and display

88.2%, 80.1%, and 77.6% substrate conversion, respectively. The performance toward

other aliphatic primary and secondary alcohols is not as good as for benzylic or 2-

butanol. Still, the substrate conversion achieved for oxidation of aliphatic alcohols by

1–3 is significantly higher than contemporary catalytic systems [29–48]. A detailed

comparison is shown in Table 4 with relevant catalytic systems.

All the catalysts show excellent TOF values for oxidation of listed alcohols. For com-

parison, catalytic oxidations of alcohols were also performed using CuCl₂as catalyst.

The TOF value comparison reveals (shown in Figure 5) that the efficiency of 1–3 is far

superior to CuCl₂, suggesting the active role of the ligands in 1–3 for catalytic

JOURNAL OF COORDINATION CHEMISTRY

17

oxidation of alcohols. The control reaction of individual substrate exhibits less than

10% substrate conversion (shown in Table 2) under similar optimized reac-

tion conditions.

In the current work, the detailed reaction mechanism for alcohol oxidation was not

studied. However, the reaction mechanism for transition metal-catalyzed alcohol oxida-

tion is well discussed in the literature [49]. By considering the reaction mechanism of

alcohol oxidation of similar systems, it is suggested that initially the metal complex

converts into the peroxo species by reacting with hydrogen peroxide. Thus, in situ

generated metal peroxo complex regains its native form through oxidation of alcohols.

A methanolic solution of H₂O₂mixed with [Cu(DPPz)₂Cl₂]₂(3) was analyzed through

ESI-MS, and related data are displayed in Supplementary Figures S14 and S15. Scheme

2 shows various intermediate species identified in situ during the alcohol oxidation.

The m/z values 641.04, 603.09, 567.11, and 503.16 are due to the formation of inter-

mediate species A, B, C, and D, respectively. First, the dimeric copper(II) complex

reacts with H₂O₂in methanolic solution and converts into two units of monomeric

copper(II) dihydroperoxido species (A), which readily converts into peroxido hydroper-

oxido species (B) by losing a labile chloride. Upon losing the second chloride from the

molecule, (B) transforms into copper(II) diperoxido species (C). As expected, copper(II)

diperoxido species (C) further undergoes decomposition, which leads to the formation

of D as the base peak in the ESI-MS plot. Thus, three types of intermediate species

form by adding H₂O₂to solution of 3. During the oxidation of alcohol, hydroperoxido

species is considered solely responsible for oxidation of alcohol [7(c), 50]. However,

current MS data indicate that along with the dihydroperoxido species (A), alcohol oxi-

dation may also be influenced by intermediate species (B) and (C).

5. Conclusion

Pyrazole derivatives DMPz (I), MPPz (II), and DPPz (III) were prepared by an easy, clean,

and straightforward pathway. Copper(II) complexes 1–3 oxidized various alcohols

selectively with high substrate conversion and efficiency in comparison to most of the

currently available homogeneous copper-based catalysts. Relatively more reactive, sec-

ondary aliphatic alcohols are highly selective (more than 92%) toward the formation

of ketones, whereas both aromatic and aliphatic primary alcohols produce carboxylic

acids as major products during the oxidation of alcohols in the presence of H₂O₂.

Copper dihydroperoxido species (A) is suggested for the oxidation; however, influence

from the other intermediate species (B and C) cannot be ignored. Catalysts 1–3 show

excellent performance compared to contemporary catalysts. By considering the eco-

nomic and environmental factors, the use of water as a green solvent and H₂O₂as a

green oxidant in the current work is an added advantage.

Acknowledgements

C.H. is very thankful to SAIF Mumbai for recording the TGA/DTA and EPR spectra. The authors

also acknowledge CRF, IIT Ropar for ESI-MS data. The authors would like to thank CIF, IISER

Bhopal, for the 400 MHz NMR facility. C.H. acknowledges CFR IIT(ISM) Dhanbad for providing sin-

gle-crystal XRD analysis. A.M. is thankful to IIT (ISM) for fellowship. The GC and GC-MS used in

18

A. MAURYA C. HALDAR

this study were procured from the grant given by the Science and Engineering Research Board

(SERB), Department of Science and Technology (DST), grant no. SB/FT/CS-027/2014 and grant

no. SB-EMEQ-055/2014, respectively, Government of India, New Delhi, India to C.H.

Disclosure statement

There are no conflicts of interest to declare.

Funding

C.H. thanks the Science and Engineering Research Board (SERB), Department of Science and

Technology (DST), the Government of India, New Delhi for financial support [grant No. SB-EMQ-

055/2014].

ORCID

Abhishek Maurya

Chanchal Haldar

http://orcid.org/0000-0001-6977-9532

http://orcid.org/0000-0003-4642-7918

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Article Doi

Green, homogeneous oxidation of alcohols by dimeric copper(II) complexes

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