Synthesis of zinc β-diketonates

Article abstract of DOI:10.1134/S0036023607040171

Acetylacetonatozinc and trifluoroacetylacetonatozinc chelates have been prepared by electrolysis and ligand exchange. Their compositions were determined by elemental analysis, IR spectroscopy, thermogravimetry (TGA), and differential thermal analysis (DTA

Full text of DOI:10.1134/S0036023607040171

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 4, pp. 577–580. © Pleiades Publishing, Inc., 2007.
Original Russian Text © I.I. Vinokurov†, V.L. Shirokii, N.N. Kostyuk, T.A. Dik, N.V. Tereshko, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 4,
pp. 636−640.
PHYSICAL METHODS OF INVESTIGATION
Synthesis of Zinc b-Diketonates
I. I. Vinokurova†, V. L. Shirokiia, N. N. Kostyukb,
T. A. Dikb, and N. V. Tereshkob
a Institute of Physicoorganic Chemistry, National Academy of Sciences of Belarus,
ul. Surganova 13, Minsk, 220072 Belarus
e-mail: shirokii@ifoh.bas-net.by
b Research Enterprise Sevchenko Institute for Applied Physical Problems, Belarussian State University,
ul. Kurchatova 7, Minsk, 220064 Belarus
e-mail: Trebnikov@bsu.by
Received December 7, 2005
Abstract—Acetylacetonatozinc and trifluoroacetylacetonatozinc chelates have been prepared by
electrolysis and ligand exchange. Their compositions were determined by elemental analysis, IR
spectroscopy, thermogravimetry (TGA), and differential thermal analysis (DTA).
DOI: 10.1134/S0036023607040171
†
The properties of zinc(II) chelates are the subject of solution (product yields were not indicated); however,
constant scientific attention on account of their wide- it is known that because the activity of most complex-
ranging engineering, agricultural, and medical applica- forming metals is insufficient for substituting for the
tions. Zinc chelates with β-diketonates attract much hydrogen of the ligand, this method has not found wide
attention. Subliming without decomposition at moder- preparative applications.
ately high temperatures, they are widely used in the
gas-phase separation of metals, chemical vapor deposi-
tion, mass spectrometry, and other fields [1].
A promising route to metal β-diketonates is electro-
chemical synthesis. This route was exploited in works
[6–9] to prepare β-diketonates of the following 3d met-
als: iron(II), iron(III), copper(II), chromium(III),
nickel(II), and cobalt(II). Advantages of this method
are a one-stage process, high yields, and the absence of
contaminating by-products or water. The use of non-
aqueous solvents excludes the formation of hydrates
and the hydrolysis of the major product.
Ligand exchange is an ordinary synthesis method
for β-diketonates. This method underlies the prepara-
tion and characterization of 3d-metal chelates with var-
ious β-diketones [2–4]. In works [2, 3], zinc, magne-
sium, and iron β-diketonate adducts were prepared
through dissolving equimolar amounts of metal chlo-
rides or nitrates in ethanolic or acetonitrile solutions of
ligands. In work [4], the reaction between the stoichio-
metric amounts of metal chlorides and sodium acety-
lacetonate was activated mechanically to synthesize
acetylacetonates of several 3d metals. However, despite
the versatility of this method, it does not always provide
the purity of chelates required for applied purposes.
In this context, it is pertinent to prepare zinc chelates
through electrolysis and ligand exchange using zinc
carbonate as a precursor. Acetylacetone (2,4-pentanedi-
one, Hacac, C5H8O2) and trifluoroacetylacetone (1,1,1-tri-
fluoro-2,4-pentanedione, Htfa, ë5ç5é2F3) were chosen
as ligands.
The behavior of powdered zinc and zinc oxide in
nonaqueous solutions of acetylacetone was studied in
work [5]. When the powders were dissolved in ethan-
olic or acetonitrile solutions of acetylacetone, white
needle-shaped crystals were produced. In methanol,
n-propanol, dimethylformamide, or dimethyl sulfox-
ide, precipitates did not appear. Powdered zinc oxides
were better soluble than metallic zinc. X-ray diffrac-
tion, IR spectroscopy, and gravimetric analysis demon-
strated that Zn(acac) precipitated in all systems. It is
difficult to appraise the data reported in work [5] on the
interaction of the metal and metal oxide with a ligand
EXPERIMENTAL
To prepare zinc(II) β-diketones, the following
chemicals were used: Hacac, acetonitrile (both distilled
directly before use), Htfa synthesized by the process
described in work [11], recrystallized tetraethylammo-
nium chloride (ë2ç5é)4NCl (pure grade), ZnCO3, ben-
zene, hexane (all of pure for analysis grade), and etha-
nol (96%).
Electrolysis was carried out in the galvanostatic
mode in a previously described temperature-controlled
electrochemical cell [10] equipped with a zinc anode
and cathode. The surface area of each electrode was
~10 cm2. The electrolyte was a 0.1 M solution of tetra-
†
Deceased.
577

578
SHIROKII et al.
Table 1. Parameters of the electrochemical synthesis of zinc chelates
Isolated chelate
composition m, g
CY for complex,
%
Run no.
τ, min
I, A
Q, C
Anode ∆, g
CY for Zn, %
1
2
193
241
0.25
0.5
2895
7230
1.70
1.35
Zn(acac)2
Zn(tfa)2
3.6
143.4
43.1
63.6
20.2
2.76
ethylammonium chloride in acetonitrile. The current was isolated from the mother solution in the form of
density during electrolysis was controlled with a series- pale yellow crystals (chelate 2). The CY was 20.2%.
connected milliammeter. When the synthesis was over,
For the synthesis of zinc β-diketonates through
the reaction solution was concentrated to a pasty state
ligand exchange with both Hacac and Htfa, the mea-
under heating on a magnetic stirrer and cooled to room
sure of the degree of reaction was the change of the
temperature. Then, the product was extracted with
initial color of the solution to yellow and vigorous
ether. The precipitated base electrolyte was filtered off;
carbon dioxide evolution. The weight of the acety-
the filtrate was concentrated to obtain the final product.
lacetonatozinc was 3.1 g (62.0% of the theory); that
The absence of remnant base electrolyte was verified
of trifluoroacetylacetonatozinc was 4.6 g (92.0% of
by a qualitative AgNO3 test for chlorides.
the theory).
The ligand-exchange synthesis of zinc β-diketo-
nates was carried out in acetonitrile solutions of ligands
at room temperature. Zinc carbonate and β-diketone
were taken in stoichiometric proportions to obtain 5 g
of the target product. The reaction was over when ëé2
The results of the elemental analysis of the chelates
are displayed in Table 2. The products of the exchange
between zinc carbonate and β-diketones have lower
metal proportions than required for bischelates, which
proves the adduct formation. The compositions of the che-
evolution ceased. Unreacted zinc carbonate was filtered lates are formulated as Zn(‡Ò‡Ò)2 · 2ç2é and Zn(tfa)2 ·
off. After finishing the synthesis, the solution was
purged with dry air until all acetonitrile was removed.
The precipitate was recrystallized from benzene.
H2O (chelates 3 and 4, respectively). The hydrates can
form as a result of the fact that water is a by-product:
ZnCO3 + 2HL
ZnL2 + H2O + CO2.
The composition of the acetylacetonates was deter-
mined by analysis for carbon, hydrogen, and zinc. The
trifluroacetylacetonates were gravimetrically analyzed
for the metal according to work [12]. IR spectra were
recorded as Nujol mulls and KBr pellets on a Specord
75 IR spectrophotometer in the range of 4000–400 cm–1.
Weight loss was recorded in thermogravimetric (TGA)
experiments under an argon atmosphere; the heating
rate was 3 K/min. Isothermal heating of the samples to
constant weight was carried out under argon.
Proceeding from the value of the electrochemical
equivalent for zinc(II), we calculated the metal CYs of
the clectrochemically synthesized chelates with
account for metal refinement. The CYs for electrolysis
with acetylacetonate (Table 1, run no. 1) exceed 100%,
which is far higher than for trifluoroacetylacetone and
which indicates that some amount of chelate 1 is
formed through the direct interaction of zinc with
acetylacetone. The precipitation of acetylacetonatozinc
in the course of electrolysis shifts the process equilib-
rium toward the product, which also helps to increase
the metal CY. During electrolysis with trifluoroacety-
lacetone, precipitation is not observed and chelate 2 is
piled up in the solution, which slows down complex
formation. The second negative factor for the CY is a
rise in the electrolyte temperature: this leads to some
loss of the ligand.
RESULTS AND DISCUSSION
Some characteristics of the anodic stripping of zinc
in the presence of Hacac and Htfa are listed in Table 1.
In the course of zinc electrolysis with Hacac (the
overall electrolysis time was 193 min), a white floccu-
lent deposit (chelate 1) appeared after 25–30 min. The
reduced zinc deposited on the cathode along with
hydrogen evolution; the weight of the zinc deposit was
0.30 g. The current yield (CY) of the target product was
63.6%.
Proceeding from the available data, we can suggest
that in the course of electrolysis, zinc oxidizes on the
anode to the divalent state:
Anode: Zn0 – 2e = Zn2+.
In the course of the electrosynthesis of zinc with
On the cathode, β-diketones are reduced through
Htfa, neither precipitation nor a visible change in color splitting their proton from the γposition. β-Diketones in
was observed. After 90 min of electrolysis, the reaction the deprotonated form exist as chelating acido ligands,
solution heated. The electrolysis duration was 241 min. which are capable of forming with metals stable six-
The reduced zinc deposited on the cathode, along with membered quasi-aromatic chelate rings [13]. Cathodic
hydrogen evolution. The weight of the refined zinc for zinc refining, likely, is explained by the similarity of the
electrolysis with Htfa was 0.37 g. The target product electrochemical reduction potentials of β-diketones and
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

SYNTHESIS OF ZINC β-DIKETONATES
579
Table 2. Chemical analysis data for acetylacetonatozinc and trifluoroacetylacetonatozinc
C, wt %
H, wt %
Zn, wt %
Chelate no.
Composition
Zn(acac)2
found
calcd.
found
calcd.
found
calcd.
1
3
2
4
45.70
40.85
45.57
40.09
–
5.91
6.01
5.35
6.05
–
24.69
21.02
17.04
16.88
24.80
21.82
17.42
6.70
Zn(acac)2 · 2H2O
Zn(tfa)2
Zn(tfa)2 · H2O
–
–
zinc in acetonitrile. Cathodic metal refining was also
observed in work [7] in the course of the electrochemi-
cal preparation of copper(II) β-diketonates.
Table 3. IR frequencies for acetylacetonatozinc and trifluo-
roacetylacetonatozinc
Cathode: çL + e = L– + 0.5H2,
Zn(acac)2 · H2O
vibration
Zn(tfa)2 · H2O
vibration
Assignment
frequencies, cm–1 frequencies, cm–1
Zn2+ + 2e = Zn0.
3550–3120 s br
1630 sh
3400–3050 s br ν(OH)
The overall reaction in the solution can be repre-
sented by the equation
1665 s
δ(H2O)
Solution: Zn2+ + 2L– = ZnL2.
1593 vs
1560 sh
1563 vs
1527 s
ν(C=O),
ν(C=C),
δ(CH3)
ν(C=O), δ(CCH3)as
δ(CCH3)s
The provisional compositions of the chelates pre-
pared by electrolysis and ligand exchange were verified
by IR spectroscopy. Table 3 displays the IR absorption
frequencies for hydrated compounds 3 and 4 with
acetylacetone and trifluoroacetylacetone in the range of
4000–400 cm–1. The main vibration frequencies of non-
hydrated acetylacetonate (chelate 1) and trifluoroacety-
lacetonate (chelate 2) are analogous to those indicated
in Table 3, except for the OH stretches.
1510 vs
1484 sh
1483 s
1478 sh
1457 m
1398 vs
1383 vs
1370 vs
δ(CH3), ν(CC)
1254 s
1182 m
1008 s
1290 vs
1230 s
ν(CC), δx, ν(CF3)
ν(C–F), δ(CF3)
ν(CO), δ(CCHγ)
ν(CF), δ(CF3)
The main difference between the spectra of chelates
3 and 4 is the appearance of strong, broad bands ν(éç)
at 3550–3120 and 3400–3050 cm–1, respectively. The
appearance of these bands confirms the existence of
coordinated neutral water molecules in the chelates.
Chelate 4 also demonstrates a strong band associated
with the bending vibrations of OH groups at 1665 cm–1.
1185 vs
1128 vs
ν(CC), δ(CH3)
1023 w
δ(CH3), ν(CF),
δ(CF3)
940 m
783 s
δ(CCH3), δx,
The absorption spectra for all test samples display
absorption bands in the range of 1600–1500 cm–1
(Table 3). These bands appear as a result of the batho-
chromic shift of the absorption stretching modes of the
carbonyl groups of β-diketonates. For acetylacetonates,
these bands appear at 1593 and 1510 cm–1; for trifluo-
roacetylacetonates, at 1563 and 1527 cm–1. They are
due to the stretching vibrations of sesquialteral CO and
CC bonds; their appearance provides evidence of the
transition of β-diketones to the acido form and the for-
mation of quasi-aromatic chelate rings [14]. On the
whole, the bathochromic shift of the absorption bands
associated with the C=O vibrations induced by the
decrease in the bond order to sesquialteral, proving the
transition of the ligands to the acido form, is more than
100 cm–1 [15]. The nonappearance of the stretching
vibrations of the carbonyl group (C=O) for acetylace-
tone and trifluoroacetylacetone proves that in the che-
lates, there no are β-diketones in the neutral form.
ν(CF), δ(CF3),
δ(CHγ)out
766 w
δ(CHγ)out
δ(CF3), ν(CF)
δxout
758 w
729 s
680 m br
654 m
685 m br
ν(ZnO), ν(CO),
δ(CF3)
δx, δ(CCH3)
ν(ZnO), δ(CCF3)
δxout, δ(CCH3)out
610 w
576 w
570 w
554 w
516 w
424 m
δ(CF3)
435 w
420 m
ν(ZnO), ν(CC), δx,
δ(CCH3)
Note: ν—stretching mode; δ—bending mode; s—strong; vs—
very strong; m br—medium broad; w—weak; s br—strong
broad; vs—very strong; sh—shoulder.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

580
Table 4. Results of isothermal heating of chelates 3 and 4
Maximal Calculated Calculated
SHIROKII et al.
The weight losses upon isothermal heating are close
to calculated values for the first stage of the process,
which is described by Scheme 2 (Table 4).
weight loss weight loss weight loss for
upon heat- for removal ZnL1.5 · OH0.5
From the data on the thermolysis of acetylaceto-
natozinc and trifluoroacetylacetonatozinc, we may
infer that acetylacetonatozinc has a higher thermal sta-
bility. The comparison of the stability of hydrated and
unhydrated chelates, as expected, proves the higher sta-
bility of the latter.
Chelate
ing, %
of nH2O, % formation,%
Zn(acac)2 · 2H2O
Zn(tfa)2 · H2O
24.7
19.4
12.0
4.6
25.8
22.1
In summary, acetylacetonatozinc and trifluoroacety-
lacetonatozinc have been prepared through electrolysis
and ligand exchange. The composition of the chelates
formulated as Zn(‡Ò‡Ò)2, Zn(tfa)2, Zn(‡Ò‡Ò)2 · 2H2O, and
Zn(tfa)2 · ç2é, was confirmed by elemental analysis, IR
spectroscopy, TGA, DTA, and isothermal heating in
combination with the IR spectroscopy of products.
The composition of the chelates was refined using
TGA, DTA, and isothermal heating data.
The TG curves for Zn(acac)2 and Zn(tfa)2 have no
intermediate weight-stabilization plateaus. This con-
firms the absence of neutral ligand molecules in the
compounds.
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the TGA experiment is 68.1 and 75.5%, respectively;
these values are close to the calculated values (69.2 and
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ç2é. The calculated weight losses on the assumption
of quantitative removal of neutral ligands demonstrate
a significant divergence from the experimental results
(Table 4).
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Τ0
Τ0
ZnL2 · nç2é
ZnL1,5OH0.5
ZnO.
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Scheme 2.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007