Synthesis and Structural Characterization of Copper(II) Polyhydroxolactate [610258]

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Synthesis and Structural Characterization of Copper(II) Polyhydroxolactate
Obtained through Oxidation of Propylene Glycol with Copper(II) Nitrate

Mircea Niculescu ,1* Mihai -Cosmin Pascariu ,2,3* and 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 Pharmacy, “Vasile Goldiș” Western University of Arad, 86 Liviu
Rebreanu, RO -310414, Arad, Romania
3 Renewable Energies – Photovoltaic Laboratory, National Institute of Research & Development for Electrochemistry
and Condensed Matter – INCEMC, 144 Dr. Aurel Păunescu -Podeanu, RO -300569, Timișoara, Romania

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

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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(OH)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. As concluded from the a nalysis of the diffuse reflectance electronic spectrum, the copper (II) ions exhibit a
distorted octahedral stereochemistry (the Jahn -Teller effect) , which was confirmed using molecular modeling . It is a
precursor f or a non -stoichiometric copper oxide, whic h is obtained through the thermolysis of the synthesized
coordination compound at relatively low temperatures. This oxide was characterized 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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1. Introduction

Diols can be oxidized, depending on the oxidation agent and the working conditions, to aldehydes, carboxylic
acids or compou nds with mixed functions. Ethylene glycol, for example, can be oxidized to glycolic aldehyde, glyoxal
and glycolic, glyoxylic or oxalic 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 reag ent concentrations and specific acidity, temperature and heating speed [1,
2]. For instance, Bencze et al. [3] have shown that, following the mild 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 that one does not obtain a unitary product, but instead
mixtures of oxidation products. Reactant concentration, ox idizer redox power, acidity of the medium, catalyzer activity
and temperature regime are all factors that have to be rigorously controlled to obtain, as major product, a specific
compound.
It is also well known that nitric acid (HNO 3), just like the nitrat e anion (NO 3−), 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, oxidation 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 reaction 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 barriers, 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 [4, 5]:
NO 3− + 3e− + 4H+NO + 2H 2O E0 = 0.96 V
In our previous papers [6–11] we have analyzed 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 gly oxylate, 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 [12–14]: they undergo thermal induced degradation to simple or m ixed metal oxides 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 consisting 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, d epending on both the structure of the coordination compound and the thermal treatment that
was applied.
The aim of the current 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 thus synthesized
coordination compound , obtained using an original method, was characterized regarding its composition, as well as its
structure and physical -chemical properties.

2. Materials and Methods

2.1. 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) and 1 M aqueous HNO 3 were used. The impurities from the se reagents
are removed in the subsequent purification 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 charact erized by Fourier transform infrared spectroscopy ( 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 reflectanc e spectrum was recorded with a
Spekol 10 spectrophotometer (Carl Zeiss Jena), using MgO as reference material.
The copper oxide , obtained by thermolysis 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 sample was grounded in order to reduce the
granulation and then pressed in the specimen holder. The 2 θ scanning range was 20–100°. The X -ray power was set at
40 KV and 50 mA. The data w as collected with the Philips X’PERT Data Collector program and processed with 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.

2.2. 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. T he reaction was considered complete when the brown -red gas (NO 2) evolution has ceased.
The obtained powdery solid was purified by refluxing in 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 consumed during the synthesis of
the coordinatio n 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 colored
[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).

2.3. Theoretical methods

All structures were initially modeled using the HyperChem 8.0.10 software [ 15]. The starting molecules,
obtained after MM+ pre -optimization, were optimized with the PM3 semi -empirical method [ 16]. The SCF
“Convergence limit” was set at 10-5, without using the “Accelerate convergence” procedure. As for “Spin Pairing”,
UHF operators were employed. For geometry optimization, the “Polak -Ribière (conjugate gradient)” algorithm was
selected , with a RMS gradient of 0.01 kcal (Å·mol)-1.
The MOPAC201 6 software [ 17] was used for the PM7 semi -empirical method [ 18]. The HyperChem structures
were converted to “.ZMT” (MOPAC Z -matrix) files and run through the MOPAC201 6 interface for geometry
optimization. The line of parameters included “CHARGE=0”, “PM7”, “GNORM=0.01”, “BONDS”, “AUX”,
“GRAPHF” and “PDBOUT” , and the keywords “SINGLET” or “TRIPLET” (with “UHF”), with or without the
keyword “OPT”. Both EF and BFGS algorithm s were used for geometry optimization , however, because BFGS gave
poor results , only the EF values wer e considered. The resulting structures were analyzed using the Jmol software [ 19].

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3. Results and D iscussion

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 [20, 21]. This
was accompanied by the appearance 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
[20].
The FTIR investigation and t he elemental analysis , given in Table 1, suggest 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−.

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 [6, 7], 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 exposed 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, 2, 6, 7]. We propose the following mechanism for the reaction between PG and copper(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) 2NO + O 2 → 2NO 2

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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 la ctate anion by the CuII cation determines the shift of equ ilibrium c in the direction
corresponding to the 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 lactate 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.
Some important information regarding the structure of the obtained copper(II) polyhydroxolactate w as extracted
from the analysis of the diffuse reflectance electronic spectrum, shown in Figure 1.

FIGURE 1: The diffuse reflectance electronic spectrum of the complex compound .

As in the case of other complex compounds synthesized by us [8–11], 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 single 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.

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Elongation of the axial bonds can determine the shift from octahedral symmetry to a tetragonal or square pyramidal
symmetry [8, 22].
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).

FIGURE 2: The FTIR spectrum of the coordination compound .

Table 2 presents the FTIR characteristic 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 [20]. 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 ca rboxylate anion to the 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 attributed to the ν(Cu –O)
vibration, with the oxygen atom belonging to the COO− group.

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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 acetone). It hardly d issolves 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, sugges t a polynuclear
structure that corresponds to the following formula:
[Cu 2(OH) 2L2(H2O)2]n
In accordance with the elemental analysis data, the (distorted) octahedral stereochemistry of the CuII ion, the
bridging OH groups and the bidentate lactate anion, the homopolynuclear coordination compound should possess the
following structure (Figure 3):

FIGURE 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 [23–26], the complex compound is basically insoluble in water and
organic solvents and can only be destroyed in drastic conditions.
The complex compound was modeled using the PM3 and PM7 s emi-empirical methods (Figure 4 ). The isomer
with the central hydrogen atoms (those belonging to the hydroxide groups) in cis position is the most stable, according
to the heat of formation values (Table 3). The configuration of the lactate ligands was randomly chosen as being R.

FIGURE 4: Molecular model of t he coordination compound (cis isomer, PM3 , singlet) .

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TABLE 3: Heat of formation values for the complex compound (the cis isomer is shown in Figure 4) .
Isomer Heat of formation (kcal mol-1)
PM3 PM7
Singlet Triplet Singlet Triplet
cis -838.808 -815.706 -537.323
-525.064* -551.943
-540.501*
trans -834.213 -828.793 – –
* OPT keyword was used in MOPAC201 6.

The bond lengths, corresponding to the optimized molecules whose heat s of formation values are marked in bold
in Table 3, are given in Table 4. The H 2O ligands from two different molecules come close (Figure 5), indicating the
formation of hydrogen bonds (1.79 –1.82 Å PM3, 1.70 -1.78 Å PM7 ); these can also form between H 2O and OH (1.58 Å)
or COO (1.62 Å) groups, as predicted by the PM7 method , allowing for a more closer packing of the two units .

TABLE 4: Bond lengths for the cis isomer of the complex compound (Figure 4) .
Bond Length (Å)
PM3 (singlet) PM7 (triplet)
Cu-Cu 3.30 2.97
Cu–OH 2 1.97–2.01 2.44–2.65
Cu–OH 1.93–1.94 1.83–2.09
Cu–OC 1.89–2.19 2.02–2.33
O–C–O 1.26–1.30 1.25–1.28
C–C 1.53 1.47–1.53
C–OH 1.41 1.41, 1.52

FIGURE 5: Two units of the coordination compound, interacting through hydrogen bonds (PM3 , singlet) .

Through the thermal decomposition at 550°C of the coordination compound [27], a black -grayish powder is
obtained. The residue was characterized by FTIR and XRD.

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The FTIR spectrum (Figure 6) 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 [28].

FIGURE 6: The FTIR spectrum of the product obtained by thermal conversion of [Cu 2(OH) 2L2(H2O)2]n.

The X-ray diffractogram of the obtained residue [29] is given in Figure 7.

FIGURE 7: 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° 2 θ
domain (the most intense lines being: d1 = 2.52887 Å; d2 = 2.32840 Å; d3 = 1.86563 Å), as found in the JCPDS 02 -1040
standard file. Also, some characteristic Cu 2O lines appear ( d1 = 2.46721 Å; d2 = 1.50859 Å; d3 = 2.13678 Å), according
to the JCPDS 02 -1067 file.

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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.

4. 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 lactat e 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 basically insoluble in water and organic solvents
and can only be destroyed in drastic conditions. Through the aerobic thermal conversion of the synthesized coordination
compound, a non -stoichiometric copper oxide is obtaine d.

Competing Interests
The authors declare that there is no conflict of interest regarding the publication of this paper.

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 Center of Genomic Medicine of the
“Victor Babeș” University of Medicine and Pharmacy of Timișoara, POSCCE 185/48749, contract 677/09.04.2015.

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