Synthesis and structural characterization of copper( II) polyhydroxolactate [610280]

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Synthesis and structural characterization of copper( II) polyhydroxolactate
obtained through oxidation of propylene glycol with copper( II) nitrate

Mircea Niculescu1*, Mihai -Cosmin Pascariu2,3*, Vasile Pode1

1 Department of Applied Chemistry and Engineering of Inorganic Compounds and Environment, Faculty of Industrial
Chemistry and Environmental Engineering, University Politehnica Timișoara, 6 Vasile Pârvan Blvd., RO -300223,
Timișoara, Romania
2 Department of Pharmaceutical Sciences, Faculty of Pha rmacy, “Vasile Goldiș” Western University of Arad, 86 Liviu
Rebreanu, RO -310414, Arad, Romania
3 Renewable Energies – Photovoltaic Laboratory, National Institute of Research and Development for Electrochemistry
and Condensed Matter – INCEMC, 144 Dr. Aurel P ăunescu -Podeanu, RO -300569, Timișoara, Romania

*[anonimizat], [anonimizat], tel. +[anonimizat]

Acknowledgements

This work was supported by the Romanian National Authority for Scientific Research (CNCS -UEFISCDI)
through project PN -II-PCCA -2011 -142. Part of this research was performed at the Cent er of Genomic Medicine of the
“Victor Babe ș” University o f Medicine and P harmacy of Timiș oara, POSCCE 185/48749, contract 677/09.04.2015.

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Abstract
The oxidation of 1,2 -propanediol (propylene glycol) with Cu(NO 3)2·3H 2O in weak acidic solutions was
investigated. The obtained copper( II) homopolynuclear coordination compound, having as ligand the oxidation product
of propylene glycol (namely, lactic acid or 2 -hydroxypropionic acid) , which is coordinated to the copper (II) complex
generator in deprotonated form ( as lactate anion, CH 3CH(O H)COO−), was studied regarding its composition an d
physical -chemical properties. This complex , having the formula [Cu 2(OH) 2L2(H2O)2]n (where L is the lactate anion), is
an emerald green solid, stable in ordinary conditions and virtually insoluble in water or the usual solvents (ethanol,
diethyl ether, benzene, acetone). It hardly dissolves in concentrated sulfuric and hydrochloric acids and in concentrated
ammonia. The copper (II) ions present a distorted octa hedral stereochemistry (the Jahn -Teller effect). It is a precursor
for a non -stoichiometric copper oxide, which is obtained through the thermolysis of the synthesized coordination
compound at relatively low temperatures. This oxide was characteriz ed by Fourier transform infrared spectroscopy and
X-ray powder diffraction.

Keywords: 1,2-propanediol, lactic acid, homopolynuclear coordination compound, copper(II) polyhydroxolactate,
copper oxide

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Introduction

Diols can be oxidized, depending on the oxidation agents and the working conditions, to aldehydes, carboxylic
acids or compounds with mixed functions. Ethylene glycol, for example, can be oxidized to glycolic aldehyde, glyoxal
and glycolic, glyoxylic or ox alic acids. Furthermore, i n strong acidic media and with powerful oxidizers, the oxidation
of ethylene glycol proceeds through degradation, with the breaking of the C -C bond, giving formaldehyde, formic acid
and carbon dioxide. Producing just one of these oxidation products is a tricky matter, requiring well established working
conditions: an adequate oxidizer, proper reagent concentrations, specific acidity, temperature and heating speed [1]. For
instance, Bencze et al. [2] h ave shown that, following the m ild oxidation of ethylene glycol with aqueous nitric acid,
glyoxylic acid is produced.
Hydroxylic compounds that contain secondary or tertiary hydroxyl groups are more resistant to oxidation.
Because diolic groups react differently, it happens very often t hat one does not obtain a unitary product, but instead
mixtures of oxidation products. Reactant concentration, oxidizer redox power, acidity of the medium, catalyzer activity
and temperature regime are all factors that have to be rigorously controlled to o btain, as major product, a specific
compound.
It is also well known that nitric acid (HNO 3), just like the nitrate (NO 3−) anion, is generally a non -selective
oxidizer, the redox reactions in which it is involved leading very often to complex mixtures of products. Standard
potential data show that the NO 3− ion is a moderate oxidizer, and from a kinetic point of view, oxidati on reactions with
NO 3− in dilute acidic solutions are slow (the kinetic barriers of redox reagents with NO 3− are high). Because protonation
of this anion determines the breaking of the N -O bond, HNO 3 in concentrated solutions (in which NO 3− is protonated,
giving HNO 3 molecules) reacts with greater speed than in dilute solution, in which HNO 3 is basically completely
ionized. Also, from a thermodynamic point of view, HNO 3 in dilute solution is a better oxidizing agent at lower pH.
Just in some specific reacti on conditions, the reduction of NO 3− ions leads to a single product; the formation of more
species with lower nitrogen oxidation states, at similar standard potentials, is also possible, and these species can
participate, in the limits of some kinetic barr iers, in interconversion reactions.
In reactions with dilute solutions of HNO 3, the II oxidation state is preferred, with the formation of NO,
according to the redox couple [3]:
NO 3− + 3e− + 4H+NO + 2H 2O E0 = 0.96 V
In our previous papers [4-9] w e have an alyzed the oxidation of several diols, such as ethylene glycol, 1,2 –
propanediol and 1,3 -propanediol, with certain metal nitrates. All the coordination compounds obtained through this
synthetic pathway contain as ligands various anions, such as glyoxylate , oxalate, lactate or 3 -hydroxypropionate. Such

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coordination compounds, which contain relatively simple organic ligands, have one main advantage over other
coordination metal complexes: they undergo thermal induced degradation to simple or mixed metal oxid es at relatively
low temperatures, with the release of gaseous species such as carbon oxides, hydrogen, hydrocarbons and water. These
studies proved that, via this new pathway, it is possible to synthesize some coordination compounds with ligands
consistin g of carboxylate or hydroxycarboxilate anions. It was also confirmed that, following the thermal treatment of
these species at considerably low temperatures, they were degraded to powders consisting of metals, alloys and/or
oxidic systems, depending on bot h the structure of the coordination compound and the thermal treatment that was
applied.
The aim of this paper consists in the investigation of the redox reaction that occurs between 1,2 -propanediol
(propylene glycol, PG) and Cu(NO 3)2·3H 2O, in a weak acidic medium (at pH ~ 2.5). The obtained coordination
compound was characterized regarding its composition, as well as its structure and physical -chemical properties.

Experimental

Reagents and equipment

For the synthesis of the complex compound, PG (Fluka AG, Buchs SG, 99% purity), Cu(NO 3)2·3H 2O
(“Reactivul” Bucharest, 99% minimum purity), 1 M aqueous HNO 3 and 32% HCl were used. The impurities from the se
reagents are removed in the subsequent purificati on step.
The copper content was determined by atomic absorption spectrometry, while carbon and hydrogen were
quantified using a Carlo Erba 1108 elemental analyzer.
The coordination compound was also characterized through Fourier transform infrared spectros copy ( FTIR ) and
electronic spectroscopy (diffuse reflectance technique). The FTIR spectrum (400 –4000 cm−1 domain) was recorded
using KBr pellets on a Nicolet FT -IR spectrophotometer, while the diffuse reflectance spectrum was recorded with a
Spekol 10 spectrophotometer (Carl Zeiss Jena), using MgO as reference material.
The copper oxide , obtained by thermolysi s of the complex compound , was characterized by FTIR and X -ray
powder diffraction (XRD) . The powder X -ray diffraction pattern was recorded on a Philips X’PERT diffractometer
using CuK α radiation (λ = 1.54056 Å). When necessary, the powder sampl e was grounded in order to reduce the
granulation and then pressed in the specimen holder. The 2θ scanning range was 20 –100°. The X -ray power was set at
40 KV and 50 mA. The data were collected with the Philips X’PERT Data Collector program and processed w ith the

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Philips X’PERT Graphics & Identify software. The less significant peaks (with an importance of less than 0.4) were
neglected.

Synthesis of the coordination compound

The coordination compound ’s synthesis is based on the PG’s oxidation by copper(II ) nitrate in a diol -water
mixture, with the simultaneous isolation of the coordination compound from the reaction system. To this end, an
aqueous solution of Cu(NO 3)2·3H 2O (4.83 g, 0.02 mol) and PG (2.28 g, 0.03 mol) was prepared. A few drops of
aqueous HNO 3 were also added to set the pH value at ~ 2.5. This mixture was gradually heated up to 95 °C in a
thermostat. The reaction was considered complete when the brown -red gas (NO 2) evolution has ceased.
The obtained powdery solid was purified by refluxing i n an acetone -water mixture (100 mL acetone and 20 mL
water). The suspension was then filtered and the emerald green solid was washed with acetone and kept in air until its
mass remained unchanged (60% yield based on PG). The metal nitrates were entirely co nsumed during the synthesis of
the coordination compound. This was verified by treating a sample with concentrated sulfuric acid, when no release of
brown vapors was observed. A negative result was also obtained for the ring reaction (no intensely brown co lored
[Fe(H 2O)5NO]SO 4 was produced with FeSO 4 and H 2SO 4). The Braccio reaction was negative as well (no NO 3− or NO 2−
anions were identified using the antipyrine test, so the nitrate’s oxidizing action did not cause its reduction to nitrite).

Results and Discussion

The progress of the reaction between PG and copper(II) nitrate was monitored by FTIR. As the reaction
advances, the NO 3− ion bands decrease in intensity, proving that this ion is consumed during the synthesis [10]. This
was accompanied by the appe arance of the band from 1580 –1680 cm−1, i.e. ν as(COO−), followed by an increase in its
intensity. This band is specific to ligands which possess at least two oxygen atoms as donors, like carboxylic anions [10,
11].
The elemental analysis given in Table 1, besides the FTIR investigation, suggests the following empirical
chemical formula for the complex compound: Cu(OH)L(H 2O), where L is the lactate (2 -hydroxypropionate) anion,
CH 3CH(OH)COO−.

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Table 1 Composition and elemental analysis data for the complex compound
Compound
(composition formula) % Cu % C % H
calc. exp. calc. exp. calc. exp.
Cu(OH)L(H 2O) 33.87 34.00 19.20 19.10 4.27 4.20

These results, in accordance with the ones previously presented [4, 5] , confirm the fact that, during the oxidation
of PG with copper(II) nitrate in mild reaction conditions, the secondary OH group, less reactive, will not be involved in
the oxidation process; on the contrary, the primary OH group, more expos ed to the oxidant’s attack, thus more reactive,
will be oxidized giving the carboxylate anion. As a consequence, the product of the redox reaction will be the lactate
anion [1, 4, 5] . We propose the following mechanism for the reaction between PG and coppe r(II) nitrate:

a) CH 3-CHOH -CH 2OH + H 2O → CH 3-CHOH -COO− + 4e− + 5H+
b) NO 3− + 3e− + 4H+ → NO + 2H 2O
––––––––––––––––––––––––– ––––––––––
c) 3CH 3-CHOH -CH 2OH + 4NO 3− + H+3CH 3-CHOH -COO− + 4NO + 5H 2O
d) [Cu(H 2O)4]2+ + H 2O [Cu(OH)(H 2O)3]+ + H 3O+
(1) C 3H5O3− + [Cu(OH)(H 2O)3]+ → Cu(OH)(C 3H5O3)(H 2O) + 2H 2O
(2) NO + ½O 2 → NO 2

The H 3O+ (H+) ions, produced following the hydrolysis of the copper(II) aquacation – process (d) – potentiate
the oxidant character of the NO 3− ion – process (c). It is worth mentioning that, although the oxidation reaction of PG is
slow, the in situ coordination of t he lactate anion by the CuII cation determines the shift of equilibrium (c) in the
direction of diol’s oxidation, with the generation of copper(II) polyhydroxolactate, which has the composition formula
Cu(OH)L(H 2O). It should also be stated that the same oxidation product, the lactat e anion, is also obtained for higher
copper(II) nitrate concentrations, respectively in a more acidic medium. This is because the secondary OH group,
sterically protected, is not involved in the oxidation process.
Important information regarding the struct ure of the obtained copper(II) polyhydroxolactate were extracted from
the analysis of the diffuse reflectance electronic spectrum, which is shown in Figure 1.

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Fig. 1 The diffuse reflectance electronic spectrum of the complex compound

As in the case of other complex compounds synthesized by us [6-9], in which the ligands (oxalate, glyoxylate,
lactate) are coordinated through oxygen atoms pertaining to the carboxylate group, in the electronic spectrum of
copper(II) polyhydroxolactate we can distinguish a sing le band, attributed to a d –d transition (
4 5
23 6
2 gg gg et et ):
2T2g ← 2Eg
The presence of this wide, asymmetric absorption band in the electronic spectrum is owned to the decrease of
symmetry, accomplished through the octahedron deformation, with the aim of suppressing the orbital degeneration (the
Jahn-Teller effect) . The maximum at ~760 nm and the emerald green color plead for a distorted octahedral geometry.
Elongation of the axial bonds can determine the shift from octahedral symmetry to a tetragonal or square pyramidal
symmetry [6-12].
To obtain further information regarding the type of ligand coordination to the CuII cation, and thus the
stereochemistry of the synthesized coordination compound, its FTIR vibrational spectrum was acquired (Figure 2).

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Fig. 2 The FTIR spectrum of the coordination compound

Table 2 presents the FTIR characteristi c bands (in cm−1) for the complex compound, together with the
corresponding assignments.

Table 2 The FTIR characteristic bands (in cm−1) for the coordination compound and the corresponding assignments
ν
(OH) νas
(COO) νs
(COO) νs(CO)
+δ(OCO) ν
(C-OH) ν
(bridge OH) δ(OCO)
+ν(Cu -O) ρ
(H2O) ν
(Cu-O)
3450 1650 1420 -1370 1330 1130 1050 810 670 500

The intense and wide band, having the maximum at 3450 cm−1, is assigned to the existence of hydrogen bonds
generated between the water molecules and the OH groups [10]. The intense band from 1650 cm−1 is attributed to the
carboxylate’s asymmetric vibration, ν as(COO), while the lower intensity band, having the maximum at 1420 cm−1, is
assigned to its symmetric vibration, ν s(COO), values that confirm the coordination of the carboxylate anion to CuII
cation. The band from 1330 cm−1 confirms that the carboxylate group functions as a bidentate ligand. The band from
1050 cm−1 is assigned to the bridge OH group vibration, while the band from 500 cm−1 is attribu ted to the ν(Cu -O)
vibration, with the oxygen atom belonging to the COO− group.
The synthesized compound is an emerald green solid, stable in ordinary conditions, practically insoluble in water
or the usual solvents (ethanol, diethyl ether, benzene, and ac etone). It hardly dissolves in concentrated sulfuric and
hydrochloric acids, being decomposed through ligand protonation. It also dissolves in concentrated ammonia, with the
formation of the CuII ammine complex. These results, as well as the spectral data given above, suggest a polynuclear
structure that corresponds to the following formula:
[Cu 2(OH) 2L2(H2O)2]n

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In accordance with the elemental analysis data, the (distorted) octahedral stereochemis try of CuII ion, the
bridging OH groups and the bidentate lactate anion, the homopolynuclear coordination compound should possess the
following structure (Figure 3):

Fig. 3 Proposed structure of the studied coordination compound

The polynuclear structure is due to the coordination of water molecules to two metal cations from adjacent
layers. Also, because of the strong hydrogen bonds [13-16], the complex compound is basica lly insoluble in water and
organic solvents and can only be destroyed in drastic conditions.
Through the thermal decomposition at 550°C of the coordination compound [17], a black -grayish powder is
obtained. The residue was characterized by FTIR and XRD.
The FTIR spectrum (Figure 4) of the thermal conversion product obtained at 550°C exhibits the bands
characteristic for copper(II) oxide (535 and 421 cm−1), in agreement with the literature values [18].

Fig. 4 The FTIR spectrum of the product obtained by thermal conversion of [Cu 2(OH) 2L2(H2O)2]n

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The X-ray diffractogram of the obtained residue is given in Figure 5.

Fig. 5 The X-ray diffractogram of the product obtained by thermal conversion of [Cu 2(OH) 2L2(H2O)2]n

The analysis of the X-ray diffractogram reveals the presence of the characteristic CuO peaks in the 30 -80° 2θ
domain (the most intense lines being: d 1 = 2.52887 Å; d 2 = 2.32840 Å; d 3 = 1.86563 Å), as found in the JCPDS 02 –
1040 standard file. Also, some characteristic Cu 2O lines appear (d 1 = 2.46721 Å; d 2 = 1.50859 Å; d 3 = 2.13678 Å),
according to the JCPDS 02 – 1067 file.
This proves that the obtained copper oxide is a non -stoichiometric oxide, more precisely copper(II) oxide
impurified with copper(I) oxide, or CuO 1-x.

Conclusions

Propylene glycol is oxidized to lactic acid during its reaction with copper(II) nitrate. The oxidation reaction is
slow, but the in situ coordination of the oxidation product (in deprotonated form, as lactate anion) to the CuII complex
generator determines the equilibrium shift towards the diol’s oxidation, with the formation of the [Cu 2(OH) 2L2(H2O)2]n
homopolynuclear coordination compound , which was confirmed by both chemical analysis and physical -chemical
characterization.
The complex compound studied in this paper shows a high chemical stability, its composition remaining
unchanged with time. Due to the strong hydrogen bonds it is basica lly insoluble in water and organic solvents and can
only be destroyed in drastic conditions.

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Through the aerobic thermal conversion of the synthesized coordination compound, a non -stoichiometric copper
oxide is obtained.

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