Selenoyl-trifluoroacetone: Synthesis, properties, and complexation ability towards trivalent rare-earth ions

Article abstract of DOI:10.1016/j.poly.2021.115383

The selenium-containing analog of thenoyl-trifluoroacetone has been synthesized and first characterized by UV–visible and NMR-spectroscopies. Synthesis of selenoyltrifluoroacetone was performed by a two-step procedure: acetylation of selenophene and interaction of 2-acetylselenophene with ethyl-trifluoroacetate. The structure of the obtained product has been confirmed using NMR, EI-MS, and elemental analysis. Spectral, acid-base, complexing, and keto-enol parameters were studied under various conditions. Received values of dissociation and protonation constants characterize selenoyltrifluroracetone as a stronger acid and stronger base than acetylacetone. The aqueous chelation with sixteen rare-earth metals has been studied in acetate and glycine buffer media at I = 0.5 M (NaCl). Values of logK non-monotonically increase from La(III) to Lu(III) and lay within the range of 4.5–11.5 logarithmic units. Most of the determined formation constants were higher than analogous values for thenoyl- and furoyl-trifluoroacetone rare-earth metal complexes. Thermodynamic and spectral parameters were simulated using both DFT and TD-DFT approaches.

Full text of DOI:10.1016/j.poly.2021.115383

Polyhedron 207 (2021) 115383
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Selenoyl-trifluoroacetone: Synthesis, properties, and complexation ability
towards trivalent rare-earth ions
,
b
,
,d
Maxim A. Lutoshkina *, Ilya V. Taydakovc
a
́
́
Universite de Lyon, Universite Claude Bernard Lyon 1, CNRS, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Villeurbanne, France
b Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center “Krasnoyarsk Science Center SB RAS”, Krasnoyarsk, Russian Federation
c P. N. Lebedev Institute of Physics of the Russian Academy of Sciences, Moscow, Russian Federation
d G.V. Plekhanov Russian University of Economics, 36 Stremyanny Per., 117997 Moscow, Russian Federation
A B S T R A C T
The selenium-containing analog of thenoyl-trifluoroacetone has been synthesized and first characterized by UV–visible and NMR-spectroscopies. Synthesis of
selenoyltrifluoroacetone was performed by a two-step procedure: acetylation of selenophene and interaction of 2-acetylselenophene with ethyl-trifluoroacetate. The
structure of the obtained product has been confirmed using NMR, EI-MS, and elemental analysis. Spectral, acid-base, complexing, and keto-enol parameters were
studied under various conditions. Received values of dissociation and protonation constants characterize selenoyltrifluroracetone as a stronger acid and stronger base
than acetylacetone. The aqueous chelation with sixteen rare-earth metals has been studied in acetate and glycine buffer media at I = 0.5 M (NaCl). Values of logK
non-monotonically increase from La(III) to Lu(III) and lay within the range of 4.5–11.5 logarithmic units. Most of the determined formation constants were higher
than analogous values for thenoyl- and furoyl-trifluoroacetone rare-earth metal complexes. Thermodynamic and spectral parameters were simulated using both DFT
and TD-DFT approaches.
1. Introduction
technology [13]. Thermodynamic parameters of selenium-containing
ligands are still poorly understood, but their coordination compounds
have exhibited considerable diversity of structures [14,15].
In this way, the goal of this work was the synthesis and character-
ization of 1,3-dicarbonyl ligand, which containing selenophene group
and analysis of its chelation capacity with rare-earth metals. To obtain
real parameters of complexation, acid-base characteristics were also
studied.
1,3-Dicarbonyl compounds constitute a wide class of bidentate O-
donor ligands, which are effective chelation agents. The enormous di-
versity of the metal–organic frameworks, chelates, and coordination
polymers with various supramolecular architectures can be formed by
β-diketonate structures [1,2]. These coordination compounds have
various potential applications in the different branches of modern
technologies: heterogeneous catalysis [3,4], optical [5] and magnetic
[6] materials, anticancer drugs [7], and others. Perfluorinated 1,3-dicar-
bonyl ligands are widely used as extraction and separation agents [8,9].
They have shown high selectivity on various groups of metal ions,
especially on Rare-Earth Metals (REMs) [10].
2. Experimental
2.1. Synthesis of selenoyl-trifuoroacetone
Thenoyl-trifluoroacetone – one of the most universal ligands for
separation of REMs(III) and can be used in combination with various
organic solvents [11,12]. The synthesis and study of its derivatives and
analogous may help to obtain new high-selectivity ligands for the
extraction and separation of REMs and other metals. Of particular in-
terest are analogous compounds with chalcogen elements – selenium
and tellurium. The presence of selenophene ring in the structure of
β-diketonate complexes could improve the spectral and luminescence
properties for application in OLEDs (organic light-emitter diodes)
All syntheses were performed under an argon atmosphere using
oven-dried (150 ◦C) glassware. Selenophene (97%) and anhydrous
magnesium perchlorate were purchased from Sigma-Aldrich, all other
chemicals – from Alfa-Aesar and Acros Organics. Tetrahydrofuran was
distilled over sodium/benzophenone in an Argon atmosphere prior to
use. The way of the synthesis is shown in Scheme 1.
2.1.1. Synthesis of 2-acetylselenophene
Borosilicate glass vial (30 mL), equipped with PTFE-lined screw cap
* Corresponding author.
E-mail address: maximsfu@yahoo.com (M.A. Lutoshkin).
https://doi.org/10.1016/j.poly.2021.115383
Received 27 May 2021; Accepted 17 July 2021
Available online 24 July 2021
0277-5387/© 2021 Elsevier Ltd. All rights reserved.

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
Scheme 1. Synthesis of 4,4,4-trifluoro-1-selenophen-2-ylbutane-1,3-dione. Reagents and conditions: (A) Ac2O, Mg(ClO4)2, 90 ◦C, 4 h; (B) NaH, THF, CF3COOEt,
5–20˚C.
and a magnetic stirring bar, was charged by 2.71 g (20.6 mmol) of
selenophene and 3 mL (3.24 g, 31.7 mmol) of Ac2O followed by addition
of 0.2 g (0.9 mmol, 4 mol %) of Mg(ClO4)2 (caution should be exercised
with regard of heating perchlorates in organic solvents [16,17]). The
closed vial then was heated at 90 ◦C (bath temperature) with continuous
stirring for 4 h and cooled to room temperature. Water (20 mL) was
carefully added and the mixture was vigorously stirred for 1 h. An oil
was separated, the aqueous phase was extracted 4 times by 15 mL
portions of CHCl3. The combined organic phase was successively washed
by 10 mL of brine, 15 mL of saturated KHCO3 solution until neutral pH
was reached and dried over magnesium sulfate. The solvent was
removed on a rotary evaporator and dark oily residue was distilled at 10
torrs. 2-Acetylselenophene was afforded as a slightly yellow oil (bp
2.2. Apparatus and procedure
NMR spectra were recorded on Bruker AM300 (1H and 19F spectra)
or on Bruker DRX500 (13C and 77Se spectra) instruments, operated at
300.13, 282.40, 125.76, and 95.36 MHz respectively. The measure-
ments were conducted using the residual signals of the deuterated sol-
vent (1H – 7.16 ppm, 13C – 77.16 ppm). The 19F and 77Se NMR chemical
shifts were referenced to external CFCl3 and Me2Se standards respec-
tively. Electronic impact ionization mass spectra (EI-MS) were recorded
on a Thermo DSQ II instrument operating in direct insert mode.
Elemental analyses were performed on a PE 2400 Series CHNS/O
Elemental Analyser (PerkinElmer). All NMR and EI-MS spectra are
contained in Electronic Supplementary Information. The UV–Vis spectra
were measured with the Leki SS2109-UV scanning spectrophotometer
(Leki Instruments, Finland) using 1 cm quartz cells. Cell thermostating
(±0.1 K) was performed with the Haake K15 thermostat connected to
the Haake DC10 controller. The absorbance of process solutions was
measured within 220–500 nm. All UV–Vis measurements were per-
formed at 298 ± 0.1 K.
◦
104–105 C /10 torr, lit. 105–106 /12 torr [18]). The yield was 81%
(2.97 g). 1H NMR (300 MHz, CDCl3): δ 2.45 (s, 3H, CH3), 7.21(t, 1H, J =
4.4 Hz, C(4)H), 7.78 (d, 1H, J = 3.3 Hz, C(3)H), 8.24 (d, 1H, J = 5.5 Hz,
C(5)H). 13C{1H} NMR (126 MHz, CDCl3): δ 26.12 (s, CH3), 130.72 (s,
–
CH), 134.97 (s, CH), 140.03 (s, CH), 151.46 (s, C(2)), 191.78 (s, C O).
–
77Se NMR (95 MHz, CDCl3): δ, ppm 830.72. Anal. calc. for C6H6OSe
(173.07): C, 41.64, H, 3.49%. Found: C, 41.27, H, 3.52%. MS (EI, 70 eV):
m/z(%) 270(100) [M]+, 201(69.4) [Mꢀ CF3]+, 159(64.7) [C4H3SeCO]+,
131(24.0) [C4H3Se]+, 69 (75.7) [CF3]+.
2.3. Reagents
All chemicals were of analytical grade: LnCl3⋅6H2O (Ln = Y, Sc, La,
Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), HCl, NaCl,
CH3COOH, CH3COONa, Glycine, Citric acid, Na2HPO4⋅12H2O, Tris (tris
(hydroxymethyl) aminomethane). All lanthanide chlorides stock solu-
tions were obtained by dissolving in distilled water. Concentrations of
stock solutions of lanthanides have been determined by complexometric
titration with disodium EDTA. Buffer solutions within the pH range from
1.80 to 3.60 were prepared with glycine and HCl, from 3.60 to 5.60 with
CH3COOH and CH3COONa, and from 7.00 to 8.00 with Tris and HCl.
Citric buffer (citric acid + Na2HPO4, pH from 3.60 to 7.00) was used for
the determination of dissociation constants. The concentration of HCl
was determined by means of titration with a standardized solution of
Na2CO3. The pH meter “Mettler Toledo F20” has been applied for the
determination of pH. Interaction between components of buffers used
and studied ligand has not been detected.
2.1.2. Synthesis of 4,4,4-trifluoro-1-selenophen-2-ylbutane-1,3-dione
Operations were also performed under the Argon atmosphere. So-
dium hydride (60% dispersion in mineral oil, 1.6 g, 40 mmol) was
placed in 3-neck round bottom flask and washed 3 times with 20 mL of
dry hexane. The last portion of hexane was carefully removed by a glass
pipette, and 15 mL of dry THF was added. A solution of 2-acetylseleno-
phene (1.8 g, 10 mmol) and ethyl trifluoroacetate (2.1 g, 1.76 mL, 15
mmol) in 10 mL of THF was slowly added to a vigorously stirred sus-
pension of NaH via dropping funnel. The reaction mixture was stirred
until hydrogen evolution ceased (1–2 h), and then left at room tem-
perature for 3 h with slow stirring. Then the dark red suspension was
cooled to +5 ◦C and an excess of NaH was decomposed by slow addition
of MeOH (10 mL), followed by the addition of AcOH-Et2O mixture (2.4
mL of glacial AcOH and 10 mL of Et2O). The yellow solution was
evaporated at diminished pressure, the oily residue was mixed with 50
mL of cold water, and pH was adjusted to 4 by HCl. Diketone was
extracted by 4 portions of CHCl3 (30 mL each), combined organic ex-
tracts were washed by 10 mL of brine and dried over Na2SO4. Solvents
were removed and resulted oil was distilled in the vacuum. The fraction
with the boiling interval of 150–152 ◦C at 25 torrs was collected. The
yield was 1.46 g (54%) of yellow liquid, which solidified upon standing
Sodium chloride was used as a background electrolyte (the formation
of chloride complexes of metals can be ignored under such condition
[20]). To maintain the accurate value of ionic strength, the concentra-
tions of other ions (buffer components and metal salts) were taking into
account. The total value of ionic strength in each sample was 0.50 ±
0.03 M. The ligand was dilute in the selected sample from water-ethanol
solution (1:1 vol). The concentration of ethanol did not exceed 2% in the
final solution.
to light a yellow crystalline mass. Mp 30–31 ◦C (lit. 32–33 ◦C [19]). 1H
–
NMR (300 MHz, CDCl ): δ, ppm 6.46 (s, 1H, CH ), 7.45 (t, 1H, J = 4.2
–
3
Hz, C(4)H), 8.06 (d, 1H, J = 4.0 Hz, C(3)H), 8.52 (d, 1H, J = 5.6 Hz, C(5)
2.4. Uv–vis measurements
H), 14.5 (br. s, OH). 19F NMR (282.5 MHz, CDCl3): δ, ppm ꢀ 76.45. 13
C
1
–
–
{ H} NMR (126 MHz, CDCl ): δ, ppm 92.95 (s, CH ), 118.76 (q, J =
280.3 Hz, CF3), 131.28 (s, C3H), 134.99 (s, CH), 141.79 (t, J = 59.6 Hz,
The dissociation constants (Ka) were calculated with the following
equations [21]:
CH), 146.63 (s, C(2)), 171.07 (q, J = 36.4 Hz, HO-C(CF3) = ), 183.95 (s,
εL ⋅Ka + εHL[H+])
[Lꢀ ][H+]
C
O). 77Se NMR (95 MHz, CDCl3): δ, ppm 844.05. Anal. calc. for
ꢀ
CHL
(
–
–
Ai =
, Ka =
;
(1)
Ka + [H+]
[HL]
C8H5F3O2Se (269.08): C, 35.71, H, 1.87%. Found: C, 35.98, H, 1.92%.
MS (EI, 70 eV): m/z(%) 174 (48.5) [M]+, 159(100) [Mꢀ CH3]+, 131
(19.3) [Mꢀ COCH3]+, 105 (19.4) [C5H3Se – C2H2]+, 43 (23.7)
[CH3CO]+.
with the Henderson-Hasselbach equation [22]:
Ai ꢀ AHL
pH = pKa + log(IR); IR =
,
(2)
ꢀ
AL ꢀ Ai
2

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
ΔΔGsolv. = ΔGgas + ΔGaq. + ΔEzpe + Ecorr
.
(9)
where IR – ionization ratio. The non-linear Cox–Yates method [23]
based on the excess acidity function
χ
[24] was applied to determine the
Ecorr. = ꢀ RTln([H2O] ) = ꢀ 9.964 kJ/mol.
(10)
protonation constant (К ) in highly acidic media (m*-solvation
H
coefficient):
here, Ecorr. is a term of free energy change associated with moving a
solvent from a standard-state solution phase concentration of 1 M to a
standard state of the pure liquid, 55.34 M [39]. Values of Ggas(H+) and Δ
Gsolv.(H+) for proton (ꢀ 26.28 and ꢀ 1108.27 kJ⋅molꢀ 1, respectively)
have been taken from the previous research [40]. The solvent effects
were evaluated using the SMD solvation model [41].
AHL ꢀ AH
[HL][H+]
[H2L+]
2L+
2L+
Ai =
+ AH , KH
=
(3)
1 + (CK )10(m*
+
H
χ
)
H
where Ai, AHL
(
εHL), AH2L+
(
ε
H2L+), and AL-
(
εL-) are the optical density
and molar extinction coefficients of the process solution, the neutral
ligands, and its conjugate acid or base, respectively.
UV–Vis absorption maximum wavelengths of complex species were
reproduced from the vertical excitation energies for the first 12 singlet
excited states by Time-Dependent Density Functional Theory (TD-DFT)
[42].
The conditional stability constants were calculated by Eqs. 4–6 [25]:
[ML]
K′ =
;
(4)
(5)
[M][HL]
3. Results and discussion
Ai = εHL(CHL ꢀ [ML] ) + εM(CM ꢀ [ML] ) + εML[ML],
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(
)
(
)
3.1. Characterization
+ CHL + CM 2 ꢀ 4CHLCM,
(6)
1
2
1
1
[ML] =
[
+ CHL + CM
ꢀ
K′
K′
In contrast with thiophene, electrophilic acylation of selenophene is
a much less studied process. Nevertheless, both heterocycles have alike
chemical properties, and consequently can be acylated under similar
conditions. Several reagents were used previously for the preparation of
2-acetylselenophene, e.g. AcCl-AlCl3 in CH2Cl2 at ꢀ 15 ◦C [43,44], Ac2O-
H3PO4 [45], Ac2O-SnCl4 [46], but the yield of the ketone in all cases was
moderate (less than 55%). Since selenophene is expensive, it would be
desirable to develop a more efficient synthetic protocol. As it was found
by Dorofeenko et al. [47], perchloric acid or magnesium perchlorate can
be used as very efficient catalysts in the acylation of thiophene by an-
hydrides of carboxylic acids. We have modified this method for the
preparation of 2-acetylselenophene. It can be used on the semi-micro
scale, as it was described in the experimental part. The target com-
pounds have been obtained in high (more than 81% after distillation)
isolated yield, and the work-up of the reaction mixture was very simple.
Both anhydrous or hydrated Mg(ClO4)2 can be used with the same
efficiency, so long as an excess of anhydride was used. The method is
safe because only 2–4 mol % of the catalyst is required. Note that mix-
tures of organic substances with perchlorates are considered to be
potentially explosive and should be treated accordingly. Diketones
bearing selenophene moiety are still poorly explored up to date.
Apparently, the simplest diketone of this type (4,4,4-trifluoro-1-sele-
nophen-2-ylbutane-1,3-dione, Se-CF3) was first obtained (without a
detailed analysis) by Yur’ev et al. by NaNH2 mediated Claisen-type
condensation of CF3COOEt and 2-acetylselenophene [19].
where Ai is an absorbance at a given wavelength, CM and CHL were
analytical concentrations of metal and ligand, respectively. K′ – condi-
tional stability constant. The εi (i = M, HL, ML) is a value of molar
extinction coefficient at single wavelength. The optimal values for K′, Ka,
KH, and extinctions were found from the least squares analysis [26] (Ki
= K′, Ka or KH):
n
∑
ꢀ
)
Ki,εi
2
Aei xp ꢀ Aci alc
minimum
(7)
̅̅̅̅̅→
i=1
For all estimations the coefficient of determination (R2) was at least
0.97. The “±” values represent confidence limits (P = 0.95) throughout
the article. Values of absorbance at single wavelength are contained in
Supporting Information. Calculations of all equilibrium constants and
molar extinction coefficients were carried out using Wolfram Mathe-
matica software package [27]. All raw spectral data are given in Elec-
tronic Supplementary Information (Tables S1-S8).
2.5. Ab initio study
Quantum-chemical computations were performed using the
GAMESS US [28] program package on the cluster MVS-1000 M of the
Institute of computational modeling SB RAS. All theoretical calculations
were performed with a temperature of 298 K. Geometry optimization
was performed by density functional theory (DFT) with PBE0 [29]
functional under Grimme’s empirical correction [30], B3LYP [31], BLYP
[32], M06, M06-2X, M06-HF, M11-L [33], and revTPSS [34]. The full-
electron cc-pVDZ [35] and Def2-TZVP [36] basis set functions were
applied for C, O, H, F, and Sc atoms. For Y, La, and Lu atoms CRENBL
ECP pseudopotential [37] has been used. The free Gibbs energy of re-
actions (Δ Δ Gsolv.) has been calculated using gas-phase energy (Δ Ggas),
Although the yield of the desired diketone was around 70 %, sodium
amide is a hazardous, non-stable, and heavy-handled reagent. We have
investigated previously [48] condensation of 2-acetylthiophenes with
esters of perfluorinated carboxylic acids in the presence of different
bases (NaH, NaOEt, or NaOMe). Here we have adapted this method for
preparation of 4,4,4-trifluoro-1-selenophen-2-ylbutane-1,3-dione. The
3–4-fold molar excess of NaH is mandatory to achieve a high and
reproducible yield of diketone. Although the yield of diketone by the
described method is lower, the utilization of NaH instead of NaNH2 is
much more convenient⋅THF is the solvent of choice for this synthesis
because sodium diketonate is soluble in it and the reaction proceeds in a
homogeneous solution. The isolation of diketone is simple – vacuum
distillation is sufficient and purification via precipitation of copper
chelate is not needed. Analytically pure samples can be obtained by
additional recrystallization of the distilled compound from MeOH but
this procedure leads to significant yield losses.
solvation free energy (Δ Gaq.), and zero-point energy correction (Δ EZPE
)
[38]:
logKcalc
=
ꢀ ΔΔGsolv./(2.303RT)
(8)
Table 1
Extinction coefficients of the maximum absorption wavelengths for various
forms of selenoyl-trifluroracetone and acid-base parameters.
I, M (NaCl)
0.25
0.50
1.00
pKa
6.32 ± 0.03
4.04
6.40 ± 0.05
4.03
6.50 ± 0.05
4.04
logε279(neutral) ± 0.01
logε343 (anion) ± 0.01
logε346(protonated) ± 0.01
-pKH ± 0.03
3.2. Acid-base properties
4.26
4.27
4.27
3.53
ꢀ 1.85
0.45
1,3-Diketones exhibit acid-base properties[49,50]. To determine
m* (solvation coefficient) ± 0.05
parameters
of
these
properties,
Se-CF3
was
studied
3

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
Fig. 1. UV–visible spectra (left) and absorption at single wavelength (right) of selenoyl-trifluoroacetone in pH media (top, C(Se-CF3) = 4.83⋅10-5 M, I = 0.25 M, λ =
343 nm) and in concentrated HCl (bottom, C(Se-CF3) = 1.74⋅10-4 M, λ = 346 nm).
spectrophotometrically in various media. Dissociation processes were
studied using glycine-HCl, citric-phosphate, and TRIS-HCl buffers in the
pH 3.0–8.4 region. Protonation of selenoyl-trifluoroacetone was con-
ducted in hydrochloric acid solutions, at concentrations of HCl more
than 3 M.
The value of pKa for thenoyl-trifluoroacetone is 6.20 (I = 0.50 M) [51],
which lines up almost precisely with the value obtained for selenoyl-
trifluoroacetone at I = 0.25 M. Both of these ligands are stronger acid
than acetylacetone (pKa = 8.83 [52]). The similarity of the acidic
properties of thenoyl- and selenoyl-trifluoroacetone and their difference
from acetylacetone demonstrates the pronounced induction effects of
the -CF3 group.
Optical and acid-base characteristics of the studied ligand are given
in Table 1. Spectra of selenoyl-trifluoroacetone at different pH values are
depicted in Fig. 1. Relationships logIR-pH (eq. (2)) are contained in
supporting information (Fig. S1) and demonstrate that one proton dis-
sociates in the studied region of pH.
Base properties of Se-CF3 characterized by the value of the proton-
ation constant -pKH = 1.85 ± 0.03. The value obtained suggests that the
protonation of Se-CF3 takes place at HCl concentration of more than 7 M.
The studied ligand contains two potential centers of protonation: sele-
nophen- and β-dicarbonyl-group. The solvation coefficient (m* from eq.
The dissociation constant (pKa) lies within the range 6.32–6.50
logarithmic units and slightly decreases with increases in ionic strength.
Fig. 2. Spectra of selenoyl-trifluoroacetone under various Praseodymium (III) concentration (top) and absorption at single wavelength as a function of C(Pr3+) –
bottom; C(Se-CF3) = 6.19⋅10-5 M, pH = 5.4, I = 0.50 M (NaCl).
4

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
concentrations and pH, indicating a lack of polynuclear complexes. The
value of Δ A depends only on metal concentrations and a set of spectral
data can be effectively modeling only by the 1:1 complex formation
model (eq. 4–6).
Table 2
Conditional (K’) and “true” (K) stability constants for selenoyl-trifluoroacetone-
REMs(III) systems.
Metal
pH ± 0.01
logK’±0.005
logε
(356 nm) ± 0.01
logK ± 0.05
Some systems (Sm3+- and Gd3+-selenoyltrifluoroacetone) were
studied under several values of pH (5.0–5.4). Both systems demonstrate
the linear dependency between conditional stability constants and pH,
but the slope coefficient deviates from 1 (0.65–0.70). This is related to
the side reactions: interaction of REMs(III) ions with acetate ions, hy-
drolysis, and dissociation of the ligand. To account for background re-
actions, following equations were used [21]:
Sc
Y
3.60
5.00
5.40
5.40
5.40
5.40
5.00
5.20
5.40
5.40
5.00
5.20
5.40
5.40
5.40
5.20
5.20
5.00
5.20
5.00
2.240
2.492
2.302
2.397
2.364
2.304
2.148
2.334
2.407
2.493
2.211
2.361
2.490
2.642
2.718
2.669
2.741
2.647
2.819
2.681
4.55
4.34
4.22
4.26
4.27
4.29
4.29
4.30
4.29
4.27
4.31
4.33
4.30
4.31
4.32
4.32
4.34
4.33
4.34
4.31
11.49 ± 0.07
5.61
4.87
4.78
4.99
4.73
5.55
5.55
5.44
5.04
4.95
4.92
4.86
4.54
5.50
5.42
5.49
5.40
5.64
5.50
La
Ce
Pr
Nd
Sm
K = αMαLK’
(11)
Eu
Gd
∑
n
αM = 1 +
βn[L] ,
(12)
∑
Tb
Dy
Ho
Er
αL = 1 +
KH[H+].
(13)
In equations (11)–(13), KH is 1/Ka of a ligand (determined above), K’
and βn are conditional and cumulative stability constants of the side
reactions, respectively. The stability constants of glycine-[53], acetate-
[54–56], and hydroxo-complexes [57] were used for calculating αM and
αL coefficients (Table S9). K is the “true” formation constant, which does
not depend on the effects of media (pH, side reactions) and depends only
on temperature and ionic strength. “True” stability constants are set out
in Table 2.
Tm
Yb
Lu
(3)) – an important indicator of the mechanism of protonation – is equal
to 0.45, which corresponds to coordination through the β-diketonate
group [51]. The received value of pKH is close to the analogous pa-
rameters for thenoyltrifluoroacetone derivatives with -C2F5 and -C3F7
groups (-pKH = 2.15–2.40 [49]).
Values of logK lay within a range of 4.5–11.5. The maximum and
minimum formation constant has been observed for Sc3+ and Tb3+
,
respectively. It is useful to compare obtained parameters with analogous
complexes of thenoyl- and furoyl-trifluoroacetone (Fig. 3-A [48]): for-
mation constants for complexes of Y(III), La(III), and Sm(III) are
maximum for Se-CF3. In contrast, the stability of Tb(III), Tm(III), and Lu
(III) complexes are markedly lower than for O- and S-containing
trifluoroacetones.
3.3. Study of complexation with REMs
The molar series method was used for the study of chelation pro-
cesses between Se-CF3 and rare-earth metals. Small ligand concentration
(4.8–8.7 × 10-5 M), pH, and ionic strength (I = 0.5 M, NaCl) were
invariant for each series. Metal concentration was the variable param-
eter (CM≫CL). Study of complexation was performed in the pH 5.0–5.4
region (selenoyl-trifluoroacetone exists in the neutral form at this
acidity). All these factors have contributed to the formation of 1:1
monocomplex species. Typical spectra of the suited ligand under various
REMs3+ concentrations are depicted in Fig. 2.
Overall, the change of chalcogen atoms in the C4H4X-group (X = O,
S, or Se) has a limited effect on the chelation capacity of the ligand.
Absolute values of formation constant (K) for considered ligands differ
by 1.1–3.7 times from each other, but their logarithmic values have
similar magnitudes. For each of C4H4X-(C(=O)–CH2-C(=O))-CF3 dike-
tones, we can see the characteristic trend: logK-M3+ curves have the
zigzag shape. Fig. 4 (left) demonstrate the logK-M3+ curve for studied
systems. The stability of complexes decreases in the series Yb < Sm = Dy
= Er = Lu < Ho = Tm < Eu = Pr < Gd < Ce < Nd < Tb for lanthanide,
and Sc > Y > La for other rare-earth metals. Among the lanthanides, two
triads (Nd-Sm-Eu and Gd-Tb-Dy) form local maximum and minimum,
Values of conditional stability constants are given in Table 1. Several
aspects demonstrate the formation of the ML complex only. Isosbestic
points show the presence of two absorption species: initial ligand and
formed complex. Absorption maximum wavelength on the Δ A-λ curves
(Fig. S2, example for Gd3+) remains invariant at various metal
Fig. 3. logK values for thenoyl-, furoyl-, and selenoyl-trifluoroacetones.
5

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
Fig. 4. Relationships logK-REMs (A), logK-Z/R (B), and ΔG- Z2/R (C) distribution.
6

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
Fig. 5. Structures and parameters of the keto-enol equilibrium.
respectively. Values for stability constants for heavy lanthanides (from
Dy to Lu) are almost equal.
contribution of Coulomb interaction.
For the first group of metals (La3+, Ce3+, Pr3+, and Nd3+), a weak
correlation is observed: formation constants non-monotonically increase
with ionic potential. The second group demonstrates the lack of rela-
tionship between logK and z/r. Formation constants for this group are
decreased from Sm3+ to Tb3+. A similar pattern was found for other CF3-
diketones [51]. These four metals have the highest spin among rare-
Due to the observed behavior is quite hard to explain, we present
here the empirical data with the discussion within the first-order
approximation. Analysis of the link between thermodynamic and elec-
trostatic parameters can help to understand the nature of formed com-
plexes (Fig. 4, B) [58]. The relationship between formation constants
and ionic potential (z/r, where z-charge of metal ion (+3), r-ionic radius
[59]) demonstrates a linear correlation and a clear division between
three groups: La-Nd, Sm-Tb, and Dy-Lu. For each of these groups, the
character of Z/R-logK correlation is different. The gradual increase in
the stability constants with an increase in the ionic potential corre-
sponds to the electrostatic model of interaction [60]. The ΔG-Z2/R graph
(Fig. 4, C) shows that the free Gibbs energy of the chelation process is
correlated with electrostatic dissipative (Z2/R). Since Z2/R can be
assigned to a molar Gibbs function of solvation (the change in energy of
the electric field [61]), this correlation also demonstrates the
earth metals (Sm3+ (6H5/2; spin 5/2), Eu3+ (7F0; spin 3), Gd3+ (8S7/2
;
spin 7/2), Tb3+ (7F6; spin 3)). Probably, the effect of the lack of elec-
trostatic dependency may be related with spin-orbital interaction i.e.
with relativistic effects.
The group of heavy lanthanides (Dy-Lu) show the linear dependency,
and for these metals (except Lu3+) stability constants to slowly increase
with z/r. As it can be seen above, the considerable variation of the
Coulomb interaction contributes exists in the various groups of lantha-
nides complexes with selenoyl-trifluoroacetone. Similar patterns are
presented to a lesser degree for other β-diketones [51].
Fig. 6. Experimental (a, b, c) and calculated (a1, b1, c1) absorption spectra; Neutral (a1, a), anionic (b, b1), and protonated (c, c1) form; Bandwidth on ½ weight
– 25.
7

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
However, the first peak of this form (284 nm) can be reproduced within
±10–15 nm by the use of most of the tested functionals. Visualized
spectra of various forms are shown in Fig. 6 (revTPSS functional). Acid-
base properties were computed using a thermodynamic cycle (Scheme 3
[51]).
In the previous works [46], was shown the effectivity of cc-pVDZ and
Def2-TZVP basis set functions for modeling of dissociation and proton-
ation processes, respectively. Here, we use the same computational
Scheme 2. Tautomerism for protonated form of Se-CF3.
protocol. Table
3 demonstrates that this approach is justified.
3.4. DFT calculations
Disagreement between theoretical and experimental values of pKa for
M06, M06-2X, and BLYP density functionals does not exceed 0.2 loga-
rithmic units. For the protonation process, local M11-L gives the best
results. All of the other functionals show significant deviations.
Currently, there are no suitable basis sets, which provide SCF-
convergence for large lanthanide complexes with open-shell configura-
tion. Assessments of chelation processes were performed for closed-shell
metals only. Theoretically calculated values of formation constants and
wavelengths of maximum absorbance are given in Table 4 (at level
PBE0/SMD).
Keto-enol tautomerism is the characteristic process for β-diketones in
aqueous solutions. These processes are not always an equilibrium be-
tween keto- and enol-form. Thenoyl-trifluoroacetone exists in 1,3-dihy-
droxy form in ethanol-aqueous media [57]. For other diketones,
tautomerism has also apparently included several complicated forms.
The analysis of such systems is a difficult case for NMR since per-
fluorinated β-diketones have low solubility in water. Study in the water-
organic solvent leads to the shift of the keto-enol equilibrium due to a
reduced dielectric constant of the media.
Aquacomplexes of REMs(III) exist in the form of [M(H2O)n]3+, n =
5–10. In this work, aquacomplex species of Sc3+, Y3+, La3+, and Lu3+
were approximated by [M(H2O)8]3+ structures (Fig. S3). This approxi-
mation has proved its effectiveness for similar perfluorinated diketones,
even for light lanthanides [51]. Structures of formed chelates were
Quantum-chemical simulations may be effective in this case. Fig. 5
demonstrates possible tautomeric forms of Se-CF3 and their calculated
relative energies. For most diketones, analogous estimates suggest that
the enol-form is the most stable isomer [49,50]. In the case of selenoyl-
trifluoroacetone, ketone has the lowest energy.
approximated by
a
six-membered chelate cycle: selenoyl-
Theoretical spectra (Fig. 6 - a and a1) and positions of wavelength of
maximum absorbance for keto-form are close to experimental parame-
ters. Thus, the structure of selenoyl-trifluoroacetone can be simulated by
keto-form (Fig. 5, above). The protonated form of Se-CF3 can also exist
in several forms (Scheme 2). For the same reason, the structure of the
cation can be approximated by protonated enol-from. In addition, used
method (PBE0/SMD) also demonstrate the applicability for calculating
of spectral parameters for other β-diketones [51].
Table 4
Theoretical parameters of complexation processes.
Complex
m
Δ Δ G, kJ/
logKcalc
logKexp
λcalc
(nm)
360
,
λexp
(nm)
360
,
mol
Sc(III)-
FBTA
1
ꢀ 68.77
12.05
11.49
Table S10 contains wavelengths of maximum absorbance for various
forms estimated by using nine density functionals. M06-L and PBE0
demonstrate a good correlation with experimental parameters for
neutral and anionic forms, respectively. Theoretical values for proton-
ated form have a significant difference with observed parameters.
Y(III)-FBTA
La(III)-
FBTA
2
2
ꢀ 38.55
ꢀ 21.70
6.75
3.80
5.61
4.87
353
353
355
356
Lu(III)-
FBTA
3
ꢀ 27.47
4.81
5.50
357
355
Scheme 3. Thermodynamic cycle for DFT estimations.
Table 3
Theoretical values of dissociation and protonation constants.
Dissociation process (cc-pVDZ)
Protonation process (Def2-TZVP)
Density functional
ΔΔG, kJ/mol
logKcalc
logKexp
6.40
Density functional
ΔΔG, kJ/mol
ꢀ pKcHalc
ꢀ pKeHxp
PBE0
43.30
35.10
37.36
35.86
42.67
37.43
26.13
7.58
6.15
6.54
6.28
7.47
6.56
4.58
revTPSS
PBE0
ꢀ 5.57
0.98
ꢀ 2.96
5.17
1.85
revTPSS
M06
ꢀ 16.89
ꢀ 29.51
ꢀ 25.60
ꢀ 61.28
ꢀ 44.08
ꢀ 11.90
B3LYP
BLYP
M06-2X
B3LYP
BLYP
4.48
M06-HF
M06-2X
M11-L
10.74
7.72
M06-HF
2.08
8

M.A. Lutoshkin and I.V. Taydakov
Polyhedron 207 (2021) 115383
trifluoroacetone as a bidentate ligand and six water molecules (Fig. S3).
It’s easy to see that for this model PBE0 functional provides the best
results - predicted wavelengths are almost equal to the experimental
data (Table 4). To achieve a good agreement between theory and
experiment, the solvation forms of the ligand were used (m = 1–3,
Scheme 3). Calculated values of stability constants (logK) differ from
experimental on 0.5–1.2 logarithmic units. The best value is observed
for scandium(III), probably due to the use of the full-electron basis set.
Theoretical and experimental values of stability constants are increased
Numerical computations were performed on the MVS-1000M cluster of
the Institute of computational modeling SB RAS.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.poly.2021.115383.
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