Kinetics of thermal decomposition and kinetics of substitution reaction of nano uranyl schiff base complexes

Article abstract of DOI:10.1002/kin.20881

This study focuses on the synthesis, characterization, and kinetics of substitution reaction of new uranyl Schiff base complexes prepared in a crystalline state as well as in a form of nanoparticles with sizes ranging between 35 and 60 nm. Preliminary Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) measurements indicated no difference between the two forms. The compounds were characterized by UV-vis, 1H NMR, cyclic voltammetry, X-ray crystallography, FTIR, TG, and CHN analyses. X-ray crystallography revealed coordination of the uranyl by the tetradentate Schiff base ligand and one solvent molecule, resulting in seven-coordinated uranium. Cyclic voltammetry of the complexes in acetonitrile revealed the quasi-reversible redox reaction. The TG and analysis of Coats-Redfern plots revealed that the kinetics of thermal decomposition of the complexes is of the first order in all stages. The study of the kinetics and the mechanism of the exchange reaction of the coordinated solvent with tributylphosphine was performed by the spectrophotometric method. The second-order rate constants at four temperatures and the activation parameters revealed an associative mechanism for all corresponding complexes. Anticancer activity of the nano uranyl Schiff base complexes against cancer cell lines (Jurkat) was studied and determined by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazoliumbromide) assay.

Full text of DOI:10.1002/kin.20881

Kinetics of Thermal
Decomposition and Kinetics
of Substitution Reaction of
Nano Uranyl Schiff Base
Complexes
ZAHRA ASADI,1 AZADE ZEINALI,1 MICHAL DUSEK,3 VACLAV EIGNER2,3
1Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
2Department of Solid State Chemistry, Institute of Chemical Technology, 166 28, Prague, Czech Republic
3Institute of Physics ASCR, 182 21, Praha, Czech Republic
Received 2 April 2014; accepted 3 September 2014
DOI 10.1002/kin.20881
Published online 15 October 2014 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: This study focuses on the synthesis, characterization, and kinetics of substitution
reaction of new uranyl Schiff base complexes prepared in a crystalline state as well as in a
form of nanoparticles with sizes ranging between 35 and 60 nm. Preliminary Fourier transform
infrared spectroscopy (FTIR) and thermogravimetric (TG) measurements indicated no differ-
ence between the two forms. The compounds were characterized by UV–vis, 1H NMR, cyclic
voltammetry, X-ray crystallography, FTIR, TG, and CHN analyses. X-ray crystallography revealed
coordination of the uranyl by the tetradentate Schiff base ligand and one solvent molecule,
resulting in seven-coordinated uranium. Cyclic voltammetry of the complexes in acetonitrile
revealed the quasi-reversible redox reaction. The TG and analysis of Coats–Redfern plots re-
vealed that the kinetics of thermal decomposition of the complexes is of the first order in
all stages. The study of the kinetics and the mechanism of the exchange reaction of the co-
ordinated solvent with tributylphosphine was performed by the spectrophotometric method.
The second-order rate constants at four temperatures and the activation parameters revealed
an associative mechanism for all corresponding complexes. Anticancer activity of the nano
uranyl Schiff base complexes against cancer cell lines (Jurkat) was studied and dꢀeCtermined by
the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazoliumbromide) assay.
Periodicals, Inc. Int J Chem Kinet 46: 718–729, 2014
2014 Wiley
INTRODUCTION
Correspondence to: Z. Asadi; e-mail: zasadi@shirazu.ac.ir, za-
sadi@susc.ac.ir.
Contract grant sponsor: Shiraz University Research Council.
Since the beginning of this century, the interest in ma-
terials at the nanoscale has been steadily increasing
based on the fact that the quantum size of nanomaterials
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
C
ꢀ
2014 Wiley Periodicals, Inc.

THERMAL DECOMPOSITION AND SUBSTITUTION OF NANO URANYL SCHIFF BASE COMPLEXES
719
triggers new physical, electronic, and magnetic proper-
ties. Thus the synthesis and characterization of nanos-
tructures with different particle sizes and morpholo-
gies are important both from the viewpoint of ba-
sic science as well as for technological applications
[1–3]. A few studies have been done on the synthe-
sis of nanoparticles of Schiff base complexes and their
application [4].
were carried out in the potential ranging from 0.2 to
−1.4 V. TBAP was used as a supporting electrolyte.
An incubator and an ELISA reader (Bio-Tek’s ELx808,
USA) were used for anticancer studies. Transmission
electron microscopy (TEM) images were obtained on a
Zeiss EM10C transmission electron microscope using
the Acc voltage of 60 kV.
In this paper, efforts were taken on the synthesis
of three new uranyl Schiff base complexes (Fig. S1
in the Supporting Information), their characteri-
zation, and preparation in the form of nanopar-
ticles. X-ray crystallography and thermogravime-
try revealed that one-solvent molecule coordinates
weakly to the uranium center in comparison with
the Schiff base and trans oxides. We also studied
kinetics of exchange of this solvent molecule with
tributylphosphine.
Synthesis of the Schiff Base Ligands
Schiff bases were synthesized by the condensation of
2-aminobenzylamine (1 mmol) with substituted sali-
cylaldehyde (2 mmol) in methanol. The purity of the
ligands was checked by the thin-layer chromatography.
The solution was refluxed for 6–7 h (Scheme S1 in the
Supporting Information). The Schiff base ligand was
precipitated by cooling and washed with small amounts
of cold methanol (5 mL) and diethylether (5 mL).
N,Nꢁꢁ-bis(salicylidene)-2-aminobenzylamine
(H2salbz): Yield: 62.8%, color: yellow, mp =
106°C, Anal. Found (Calcd.): C21H18N2O2(330.21):
C, 76.54 (76.34); H, 5.43(5.49); N, 8.40(8.48).
IR (KBr, cm−1): 3423 (νO–H), 3042(υνC-H),
1633, 1619(νC=N), 1570–1442 (νC=C). 1H NMR
EXPERIMENTAL
Chemicals and Apparatus
2-Hydroxysalicylaldehyde,
2-hydroxysalicylaldehyde,
hydroxysalicylaldehyde,
uranylacetatedihydrate
n-butylphosphine (PBu3), chloroform (CHCl3),
methanol, DMSO-d6 for 1H NMR spectroscopy,
acetonitrile (CH3CN), and potassium bromide (KBr)
for IR spectroscopy, were purchased from Merck,
Acros, and Aldrich.
The electronic absorption spectra were recorded
using a Perkin–Elmer Lambda 2 spectrophotometer
equipped with a Lauda-ecoline-RE 104 thermostat.
Fourier transform infrared spectroscopy (FTIR) spec-
tra were recorded on a Shimadzu FTIR–8300 in-
5-chloro-
5-bromo-2-
2-aminobenzylamine,
(250 MHz, DMSO-d6, room temperature): δ ppm)
(
= 4.95 (s, 2H, CH2), 6.84–7.41 (m, 12H, ArH), 8.47
(s, 1H(a), HC=N), 8.58 (s, 1H(b), HC=N), 13.28,
13.51 (s, 2H, OH). UV–vis.(acetonitrile): λmax (nm),
ε (M−1 cm−1) = 212 (ꢀ79,599), 256 (ꢀ41,700), 404
(ꢀ24,643).
UO2(OAc)2·2H2O,
tri-
N,Nꢁꢁ-bis(5-chlorosalicylidene)-2-
aminobenzylamine (5-ClsalbzH2): Yield: 84%,
color: yellow, mp = 127°C, Anal. Found (Calcd.):
C21H16Cl2N2O2 (399.11): C, 63.23 (63.17); H, 3.86
(4.04); N, 7.18 (7.02). IR (KBr, cm−1): 3456 (νO–H),
2883 (νC–H), 1633, 1614 (νC=N), 1556–1474 (νC=C).
1H NMR (250 MHz, DMSO-d6, room temperature): δ
(ppm) = 4.93 (s, 2H, CH2), 6.85–7.52 (m, 10H, ArH),
8.37 (s, 1H(a), HC=N), 8.50 (s, 1H(b), HC=N), 13.19,
13.41 (s, 2H, OH). UV–vis.(acetonitrile): λmax (nm),
ε (M−1 cm−1) = 223 (ꢀ96178), 260 (ꢀ62,685), 332
(ꢀ21,073).
1
frared spectrophotometer. The H NMR spectra were
recorded on a Bruker Advance DPX–250 spectrom-
eter at 250 MHz. Elemental microanalyses (CHN)
were obtained using a CHN Thermo-Finingan Flash
EA1112 analyzer. A Buchi 535 instrument was
used to obtain the melting point of the compounds.
Thermal gravimetric analyses were recorded on a
Perkin–Elmer Pyris Diamond model. Electrochemi-
cal studies were carried out using an Auto lab 302N
instrument. A three-electrode system was utilized
with a glassy carbon working electrode, a reference
electrode (Ag/Ag+ in tetrabutylammoniumperchlorate
(TBAP)/acetonitrile solution), and a Pt auxiliary elec-
trode. The measurements of cyclic voltammetry (CV)
for the CH3CN solution containing uranyl complexes
(1.00 × 10−3 M, M = mol/dm3) and TBAP (0.10 M)
N,Nꢁꢁ-bis(5-boromosalicylidene)-2-
aminobenzylamine (5-BrSalbzH2): Yield: 87.9%,
color: yellow, mp = 145.3°C, Anal. Found (Calcd.):
C21H16Br2N2O2 (488.01): C, 51.65(51.67); H,
3.33(3.30); N, 5.90(5.74). IR (KBr, cm−1): 3436
(νO–H), 3058–2883(νC–H), 1633, 1604 (νC=N),
1
1556–1469 (νC=C), H NMR (250 MHz, DMSO-d6,
room temperature): δ (ppm) = 4.96 (s, 2H, CH2),
6.80–7.49 (m, 10H, ArH), 8.35 (s, 1H(a), HC=N),
8.49 (s, 1H(b), HC=N), 12.99, 13.21 (s, 2H, OH).
UV–vis. (acetonitrile): λmax (nm), ε (M−1 cm−1) =
222 (ꢀ83,803), 257 (sh), 330 (ꢀ17,538).
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

720
ASADI ET AL.
HC=N), 9.56 (s, 1H(b), HC=N). UV–vis. (acetoni-
trile): λmax (nm), ε (M−1cm−1) = 234 (ꢀ73,360), 340
(ꢀ13,218), 404 (sh).
[UO2(5-Brsalbz)(MeOH)] Yield: 91.7%, Color:
orange, mp >250°C, Anal. Found (Calcd.):
C22H18Br2N2O5U (788.02): C, 33.34 (33.52); H, 2.10
(2.30); N, 3.63 (3.55). IR (KBr, cm−1): 3340 (νO–H),
2923–3047 (νC–H), 1585, 1612 (νC=N), 1458–1527
(νC=C), 894 (νU=O), 686 (νU–N), 640 (νU–O). 1H NMR
(250 MHz, DMSO-d6, room temperature): δ (ppm) =
3.14 (d, 3H, MeOH), 4.08 (q, 1H, MeOH), 5.32 (s,
2H, CH2), 6.87–7.99 (m, 10H, ArH), 9.27 (s, 1H(a),
HC=N), 9.56 (s, 1H(b), HC=N). UV–vis (acetoni-
trile): λmax (nm), ε (M−1 cm−1) = 235 (ꢀ71,208),
340 (ꢀ13,218), 404 (sh).
Figure
1 TEM image of nanoparticles of [UO2(5-
Kinetic Studies of the Substitution
Reactions
Brsalbz)(MeOH)] with sizes ranging between 35 and 60 nm.
The kinetics of exchange reactions on the uranyl cen-
ter was studied. Kinetic data were determined spec-
trophotometrically. In all cases (runs from 10.0 to –
40.0 0.1°C), the procedure involved adding a sam-
ple of PBu3 under the pseudo–first-order condition
to a solution containing the uranyl complex. The ki-
netics was followed at a predetermined wavelength,
where the difference in the absorption between the
substrate and the product was the largest. After each
injection, the absorbance was read with definite time
intervals.
Synthesis of Nano Uranyl Schiff Base
Complexes
Nano uranyl complexes were prepared by slow addi-
tion of uranyl acetate, dissolved in 50 mL methanol,
into a hot methanolic solution of ligand (1:1 mo-
lar ratio). The mixture was then refluxed for 24 h
(Scheme S2 in the Supporting Information). The pre-
cipitated solid was filtered and washed with methanol
and ether. TEM images showed that nanoparticles
with different sizes range between 35 and 60 nm
(Fig. 1).
[UO2(salbz)(MeOH)] Yield: 92%, Color: dark
orange, mp >250°C, Anal. Found (Calcd.):
C22H20N2O5U (630.22): C, 41.26 (41.36); H, 3.66
(3.71); N, 4.17 (4.19). IR (KBr, cm−1): 3436 (νO–H),
2932–3262 (νC–H), 1590, 1612 (νC=N), 1537 (νC=C),
906 (νU=O),756.5 (νU–N), 592.6 (νU–O). 1H NMR
(250 MHz, DMSO-d6, room temperature): δ (ppm)
= 3.14 (d, 3H, MeOH), 4.07 (q, 1H, MeOH), 5.32 (s,
2H, CH2), 6.65–7.80 (m, 12H, ArH), 9.28 (s, 1H(a),
HC=N), 9.57 (s, 1H(b), HC=N). UV–vis. (acetoni-
trile): λmax(nm), ε (M−1 cm−1) = 231 (ꢀ65,565), 267
(sh), 329 (ꢀ15,114), 397 (sh).
[UO2(5-Clsalbz)(MeOH)] Yield: 88%, Color:
orange, mp >250°C, Anal. Found (Calcd.):
C22H18Cl2N2O2U (699.12): C, 38.55 (38.79); H, 2.78
(2.59); N, 4.29 (4.01). IR (KBr, cm−1): 3294 (νO–H),
2831–3039 (νC–H), 1590,1612 (νC=N), 1458–1535
(νC=C), 902 (νU=O), 702 (νU–N), 648 (νU–O). 1H NMR
(250 MHz, DMSO-d6, room temperature): δ (ppm) =
3.14 (d, 3H, MeOH), 4.09 (q, 1H, MeOH), 5.33 (s,
2H, CH2), 6.92–7.89 (m, 10H, ArH), 9.28 (s, 1H(a),
Synthesis of the Kinetic Product,
[UO2(Schiff base)(PBu3)]
The uranyl Schiff base complex, [UO2(5-
Brsalbz)(MeOH)], was dissolved in methanol
(15 mL), and PBu3 was added gradually with constant
stirring. The stoichiometry of the complex to PBu3
was 1:1. The solution was stirred for 6–7 h. The re-
action was carried out at 50°C, under N2 atmosphere.
The precipitate was formed and washed with cold
methanol (5 mL) and cold ether (5 mL).
[UO2(5-Brsalbz)(PBu3)] Yield: 85.2%, Color:
orange, mp >250°C, Anal. Found (Calcd.):
C33H41Br2N2O4UP (958.11): C, 40.95 (41.37); H,
4.41 (4.53); N, 2.97 (2.85). IR (KBr, cm−1): 3425
(νO-H), 2869–2962 (νC-H), 1612 (νC = N), 1458–1527
(νC=C), 902 (νU=O),686 (νU–N), 632 (νU–O). 1H NMR
(250 MHz, DMSO-d6, room temperature): δ (ppm) =
0.85–1.35 (m, 27H, PBu3), 5.32 (s, 2H, CH2), 6.86–
7.99 (m, 10H, ArH), 9.27 (s, 1H(a), HC=N), 9.56 (s,
1H(b), HC=N).
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

THERMAL DECOMPOSITION AND SUBSTITUTION OF NANO URANYL SCHIFF BASE COMPLEXES
721
Table I Crystal Data, Data Collection, and Structure Refinement Details for the Complexes
[UO2(salbz)(DMF)]
[UO2(5-Clsalbz)(DMF)]
[UO2(5-Brsalbz)(DMF)]
Formula
C24H23N3O5U
671.5
Monoclinic
C-2yc
C24H21Cl2N3O5U
740.4
C24H21Br2N3O5U
829.3
Formula weight
Crystal system
Hall group
Space group
T (K)
Monoclinic
–P2ybc
P 21/c
Monoclinic
P2ybc
P 21/c
C c
120
120
120
˚
a (A)
9.9418(1)
21.7540(2)
10.9929(1)
90
107.7472(12)
90
11.5513(3)
25.4605(6)
8.4861(2)
90
94.462(2)
90
11.7171(4)
25.8147(9)
8.4127(4)
90
94.222(4)
90
˚
b (A)
˚
c (A)
α (˚)
β (˚)
γ (˚)
3
˚
V (A )
2264.34(4)
4
2488.21(11)
4
2537.71(17)
4
Z
D x (g cm−3
F(000)
N ref
)
1.970
1280.0
11633
1.976
1408.0
6823
2.171
1552.0
4507
T min, Tmax
R (reflections > 3σ)
wR2 (all reflections)
0.134, 0.512
0.0188(10739)
0.0427(11633)
0.452, 0.800
0.0273(4628)
0.0586(6076)
0.236, 0.658
0.0256(3614)
0.0304(4437)
Again, the application of this restriction had negligible
impact on the R values.
All methods of characterization have been summa-
rized in Table S1 in the Supporting Information.
X-Ray Crystallography
Red single crystals of the uranyl complexes
[UO2(Schiff base)(DMF)] were obtained in good yield
from a slow diffusion of diethyl ether into a solution
of the metal complex in dimethylformamide (DMF) at
room temperature during 10 days. The crystals were
intensely colored. The preparation from DMF/Et2O
gave better single crystals compared with the prepa-
ration from acetonitrile, which was also attempted.
The diffraction data were obtained using the Gemini
diffractometer of Agilent Technologies, with Mo Kα
radiation from a sealed X-ray tube monochromated
with a graphite monochromator and collimated with
a Mo-Enhance fiber optics collimator. The CCD At-
las was used for the detection. Data processing was
done with CrysAlis Pro [5] and the same program was
used for the absorption correction based on the crystal
shape. The structures were solved by the program Su-
perflip [6] and refined with Jana 2006 [7]. In the com-
plex 1, the DMF molecule was disordered between two
positions with occupancy 0.716(4) and 0.284(4). The
DMF has identical shape in both positions because we
used a rigid body approach in its refinement. A com-
parison with the free atomic model confirmed that the
introduction of a rigid body has negligible impact on
the R values. Moreover, we restricted the distances be-
tween the oxygen of DMF and uranium to be the same.
RESULTS AND DISCUSSION
Characterization of the Complexes
Crystal Structure of the Complexes. We determined
three single crystal structures, [UO2(salbz)(DMF)]
(1), [UO2(5-Clsalbz)(DMF)] (2), and [UO2(5-
Brsalbz)(DMF)] (3) and deposited them at the Cam-
bridge Crystallographic Data Center with CCDC codes
949282, 949281 and 949395, respectively. Crystal-
lographic data and details of the data collection are
listed in Table I. ORTEP views of the complexes are
shown in Figs. 2 and 3, and selected bond parame-
ters are listed in Table S2 in the Supporting Infor-
mation. The compounds 2 and 3 were found to be
isotypic.
The geometry around the uranyl atoms in 1–3
was close to pentagonal-bipyramidal with the axial
O=U=O moiety, two oxygen atoms, and two nitro-
gen atoms in the equatorial position and the solvent
molecule (DMF) occupying the fifth coordination site
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

722
ASADI ET AL.
Figure 2 ORTEP view of [UO2(salbz)(DMF)] complex. (A) showing disorder of the DMF molecule; (B) showing the more
occupied position (78%) of the DMF molecule.
Figure 3 ORTEP view of [UO2(5-Clsalbz)(DMF)] (C) and [UO2(5-Brsalbz)(DMF)] (D).
in the equatorial position. In (1), DMF was disordered
˚
and U1–O25 was 2.263(4) and 2.228(4) A, respec-
between two unequally occupied positions (Fig. 2).
tively. Such a difference might imply that the coor-
dination of the oxygen atoms in the complexes was
stronger than the coordination of the nitrogen atoms.
The crystal lattice of the complex contains a DMF
molecule, which was the solvent component in the re-
˚
The uranyl oxygendistance U–O1u in3 (1.747(4)A)
was slightly shorter than the corresponding distances
˚
˚
observed in 1 (1.786(2)A) and 2 (1.788(3)A). This
trend was almost reverse for U–O2u distances; 1
˚
˚
˚
(1.788(2) A), 2 (1.777(3) A), and 3 (1.793(4) A). All
these distances were in the range typically found in
other reported uranyl Schiff base complexes [8].
The fact that the Schiff bases ligands were asymmet-
ric, resulting in unequal U–N distances as well as U–O1
and U–O25 distances in all the complexes. For U–N9
and U–N17, the distances were 2.640(2) and 2.563(2)
˚
crystallization. The U–O1s distances (2.439(2) A in 1,
˚
˚
2.415(3) A in 2, and 2.414(4) A in 3) were compa-
rable to U–Osolvent distances reported for other uranyl
complexes [8]. Overall, the bond distance between the
oxygen atom of DMF and uranium was longer than
those of U–O1 and U–O25 in all complexes, suggest-
ing weaker binding of DMF when comparing with the
Schiff base.
The O=U=O angles in 2 (175.6(1)°) and 3
(175.8(2)°) indicated that the uranyl moiety was
slightly bent compared to the angle observed in 1
(177.1(8)°). These angles also indicated that the uranyl
˚
˚
A for 1, 2.666(3) and 2.559(3) A for 2, 2.656(5) and
2.561(5) A for 3. The distances U1–O1 and U1–O25
˚
were found to be shorter than U–N distances and also
the difference between them was smaller than the one
between U–N9 and U–N17. For example, in 3, U1–O1
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

THERMAL DECOMPOSITION AND SUBSTITUTION OF NANO URANYL SCHIFF BASE COMPLEXES
723
moiety was slightly bent in the direction opposite to
the coordination of DMF. The deviation from lin-
earity was due to the presence of the coordinated
solvent.
The coordination geometry around UO2 was nearly
planar with the dihedral angle of 2.56(16)° between
coordination planes of N17–U1–O24 and N9–U1–O1.
Atoms C10 and C15 belonging to the aromatic rings
were out of the equatorial plane, with the distance
about 4.12 ppm related to OH and a doublet at about
3.15 ppm related to CH3.
FTIR Spectroscopy. In the IR spectra of the
nanocomplexes, [UO2(salbz)(MeOH)], [UO2(5-
Brsalbz)(MeOH)], and [UO2(5-Clsalbz)(MeOH)], a
strong peak around 1580, 1612 cm−1 (free ligands
around 1604, 1633 cm−1) indicated the coordinated
imine nitrogen. The asymmetric structure of ligands
and complexes caused vibration of imines at two
different frequencies.
˚
˚
from the plane 0.449(4) A for C10 and 0.466(4) A
for C15. The geometry of hydrogen bonds found in
the complexes is given in Table S3 in the Supporting
Information.
Vibrational bands in the region 3423–3456 cm−1
for the ligands were assigned to the vibrations of O–H
groups. It was expected that these bands should disap-
pear in the complexes due to the coordination of oxy-
gens to the uranium, but they persisted indicating the
presence of MeOH in the complexes. The strong band
close to 894–906 cm−1 was characteristic of linear
uranyl (O=U=O) in the complexes. The weak bands
at 2800–3200 cm−1 were assigned to C–H stretching
vibrations. These bands, in [UO2(5-Brsalbz)(PBu3)] as
a kinetic product, were stronger due to the coordination
of PBu3.
1H NMR Spectroscopy. In 1H NMR spectra of
the nanocomplexes, [UO2(salbz)(MeOH)], [UO2(5-
Brsalbz)(MeOH)], and [UO2(5-Clsalbz)(MeOH)], a
significant shift in the imine CH=N proton was ob-
served between the free ligands (8.35–8.58 ppm) and
nanometal complexes (9.27–9.57 ppm), indicating in-
volvement of the lone pairs on nitrogen with the metal
center. Because of the asymmetric structure of lig-
ands and complexes, two separated signals were ob-
served for imine protons. CH2 protons were observed
at about 4.92 ppm in the Schiff bases and about
5.33 ppm in the complexes; a significant shift was ob-
served. The proton chemical shifts of the coordinated
PBu3 in the kinetic product appear at δ = 0.8–1.6 ppm.
These results were in agreement with the previous re-
sults observed for metal complexes with tributylphos-
phine as a ligand [9,10]. All 1H NMR of the complexes
were recorded in the DMSO-d6 solvent, which repels
methanol from the coordination sphere, and itself was
coordinated to the uranium. Peaks due to repelled-free
UV–Vis Spectra. The electronic spectra plotted in
wavelength (nm) of ligands and their nanocomplexes
in acetonitrile were used to examine the spectral fea-
tures of the U(VI) nanocomplexes in the UV–vis region
from the viewpoint of energy (Fig. 4). The electronic
spectra of the ligands presented three intense bands.
The first band at higher energy was attributed to π →
π* transition of the phenyl ring, and the band at lower
energy arouse from π → π* transition of the azome-
thine chromophore. The farthest energy band was n →
π* transition involving the promotion of the lone pair
1
MeOH were observed in the H NMR as a quartet at
Figure 4 Electronic spectra of salbz (1.5 × 10−5 M) (dash line) and nano[UO2(salbz)(CH3CN)] (4.5 × 10−5 M) (solid line)
in acetonitrile.
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

724
ASADI ET AL.
Table II Results of Thermal Decomposition of Uranyl Schiff Base Complexes
(Weight Loss%) Temperature
Calcd. (Found) Range in TG (°C)
DTA (Peak)
Endo/Exo
Decomposition
Assignment
Complex (F.W.)
[UO2(salbz)(MeOH)]·H2O (630.22)
2.85 (3)
5.1 (5)
Under 160
160–220
320–420
T > 420
Under 80
80–270
270–400
T > 400
Under 320
320–380
T > 380
Endo
Endo
Exo
Loss of H2O
Loss of MeOH
Loss of C6H7N
16.5(16)
79.9(76)
2.3 (2.3)
4.2 (3.8)
15.3(16)
78.2(78)
5.26(4.2)
15.6(15)
81(80.8)
Loss of UO2C15H9O4N
Loss of H2O
[UO2(5-Brsalbz)(MeOH)]·H2O (788.02)
Endo
Endo
Exo
Loss of MeOH
Loss of C7H6N2
Loss of UO2C14H8O4
Loss of CH3CN
Loss of C7H6N2
Loss of C12H6N2O4Br2U
Exo
[UO2(5-Brsalbz)(CH3CN)] (797.02)
Endo
Endo
Exo
electrons of the nitrogen atom to the antibonding π*
orbital. Usually n → π* transition involving nitrogen
atoms occurred at lower energies.
thesized in acetonitrile, in [UO2(5-Brsalbz)(CH3CN)]
the first stage corresponds to the loss of acetoni-
trile under 320°C. All thermal data are collected in
Table II.
Because of the strong intensity of the characteris-
tic absorption bands of the complexes, these absorp-
tion bands could be assigned to an electronic dipole–
allowed transition arising from the coordinating lig-
and and/or from charge transfer between the ligand
and uranium atom. Owing to the presence of the
phenolate group, these ligands could act as electron
donors. Since uranyl complexes contained U(VI) with
an empty valence shell, the metal center was only ca-
pable of functioning as an acceptor moiety for LMCT
(ligand to metal charge transfer) transition. It seemed
that charge transfer band (LMCT) from oxide (=O)
to uranyl occurred at lower frequencies (higher wave-
The Kinetic Aspects of Thermal Decomposition.
All the well-defined stages of DTG curves were
selected to study the kinetics of decomposition of
the complexes. The kinetic parameters (the activa-
tion energy E* and the preexponential factor A*)
were calculated using the Coats–Redfern equation (1)
[12]:
ꢀ
ꢁ
ꢀ
ꢁ
− log(1 − a)
AR
βE
2RT
log
= log
1 −
T 2
E
lengths) than the one for the transfer Schiff base2−
U(VI) [11].
→
E
−
(1)
2.303RT
(w0−wt )
Thermal Analysis. TG data of the nanocomplexes,
[UO2(salbz)(MeOH)] and [UO2(5-Brsalbz)(MeOH)],
have been studied to proof the existence of an as-
sociated water or solvent molecules either coordi-
nated to the metal center or located freely in the
crystal lattice. In thermal analysis, heating rates
were suitably controlled at 20°C min−1 under ni-
trogen atmosphere, and the weight loss was mea-
sured from the ambient temperature up to 1000°C.
TG analysis for [UO2(salbz)(MeOH)] and [UO2(5-
Brsalbz)(MeOH)] (Fig. S2 in the Supporting Informa-
tion) showed two stages in the range of 30–270°C.
The first stage at about 30–160°C corresponded to
the loss of one hydrated water molecule. The sec-
ond stage at about 100–270°C occurred due to the
loss of coordinated methanol. For the complex syn-
where a =
, w0 the initial mass of the sample,
f
w −w
(
)
0
wt is the mass of the sample at temperature T, wf is the
final mass at a temperature at which the mass loss is
approximately unchanged, β is the heating rate, and R
is the gas constant. Plots of left-hand side of Eq. (1)
against 1/T gave a straight line, in which slope and
intercept were used to calculate the kinetic parameters.
The goodness of fit was checked by calculating the
correlation coefficient. Other systems and their steps
showed the same trend.
The entropy of activation S* was calculated using
Eq. (2):
KTs
h
∗
A∗ =
eS /T
(2)
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

THERMAL DECOMPOSITION AND SUBSTITUTION OF NANO URANYL SCHIFF BASE COMPLEXES
725
Table III Thermal and Kinetic Parameters for the Uranyl Complexes
Compound
ꢀT (°C)a
E* (kJmol−1
)
A* (s−1
)
S* (J mol−1 K−1
)
H* (kJ mol−1
)
G* (kJ mol−1
)
[UO2(salbz)(MeOH)]
under 160
320–420
320–380
30.18
18.70
61.97
9,592
119.97
581,050.2
−171.26
−211.28
−140.77
26.79
13.49
56.74
23.03
74.54
50.13
[UO2(5-Brsalbz)(MeOH)]
Figure 5 Cyclic voltammograms of uranyl complexes (4.5 × 10−5 M) in acetonitrile at room temperature. Scan rate:
100 mV/s. 1: Cl, 2: Br, 3: H.
where k, h, and Ts are the Boltzman constant,
the Planck constant, and the peak temperature, re-
spectively. The enthalpy and free energy of acti-
vation were calculated using Eqs. (3) and (4) [13,
14]
to Coats–Redfern plots (Fig. S3 in the Support-
ing Information), the kinetics of thermal decompo-
sition of the studied complexes was first order in all
stages.
The Electrochemical Study of Uranyl Complexes.
To investigate the effect of substitutional groups of
the Schiff base ligands on the oxidation and reduc-
tion potential of [UO2(Schiff base)(CH3CN)], uranyl
Schiff base complexes in the nanoform were dis-
solved and studied by the CV method. CV measure-
ments for uranyl complex solutions in acetonitrile (1
× 10−3M) and TBAP (0.10 M) as the supporting elec-
trolyte were carried out at room temperature and in
the potential range from −0.65 to −1.28 V at the scan
rate V = 0.1 V/s. A typical cyclic voltammograms
of [UO2(Schiff base)(CH3CN)] in the potential range
from 0.0 to −1.3 V (vs. Ag/AgCl) is shown in Fig. 5.
[UO2(Schiff base)(CH3CN)] was reduced to the mono
anion [UO2(L)(CH3CN)]− in a quasi-reversible one
E∗= H ∗+RT
(3)
(4)
G∗= H ∗ − TS∗
Kinetic parameters for all complexes are collected
in Table III.
A typical linear plot of left-hand side of the Coats–
Redforn equation versus 1/T is shown in Fig. S3 in the
Supporting Information.
The negative values of entropy of activation in-
dicated that the activated complex had a more or-
dered structure than the reactants [14]. According
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

726
ASADI ET AL.
Table IV Redox Potential Data of Uranyl Schiff Base Complexes in the Acetonitrile Solution
Compound
E pa (V → VI)
E pc (VI → V)
E 1/2 (VI ↔ V)
[UO2(salbz)(MeOH)]
[UO2(5-Brsalbz)(MeOH)]
[UO2(5-Clsalbz)(MeOH)]
−0.7864
−0.7938
−0.7913
−0.9583
−0.9485
−0.9173
−0.8724
−0.8711
−0.8543
Path 1
O
O
O
O
O
O
O
O
CH3CN
Fast
N
U
N
N
U
N
O
k2
+PBu3
N
U
N
PBu3
CH3CN
–CH3CN
PBu3
O
O
O
t
s
k1
3
a
u
F
B
N
P
C
+
3
H
C
–
O
U
O
O
O
N
N
CH3CN
CH3CN
Path 2
Scheme 1
electron step (Eq. (5)).
By dissolving the complexes in acetonitrile,
methanol was exchanged with acetonitrile, thus ace-
tonitrile occupied the fifth position in the equatorial
plane.
[UVIO2(Schiff base)CH3CN] + e
→ [UVO2(Schiff base)CH3CN]−
(5)
Plots of kobs versus [PBu3] exhibited a nonzero in-
tercept. Thus the rate law for the reaction is as follows
(Eqs. (7) and (9))
Upon the reversal of the scan direction, the U(V)
complex was oxidized to U(VI) at over potentials. The
oxidation potentials for the different complexes are
collected in Table IV. The formal potentials (E1/2(VI
↔ V)) for the U(V/VI) redox couple were calculated
as the average of the cathodic (Epc) and anodic (Epa)
peak potentials of this process.
The cathodic peak potentials increased in the order
of H < Br < Cl. Chloride and Bromide groups on
the para position (5-Br and 5-Cl) acted as an electron-
withdrawing groups. This factor accelerated reduction
of 5-Br and 5-Cl related to 5-H.
R = {k1+k2[PBu3]}[complex]
under the pseudo–first-order condition:
R = kobs[complex]
(7)
(8)
(9)
where
Kinetic Study of the Exchange of Solvent with PBu3
on the Uranyl Complexes. The kinetics of the interac-
tion between the nano uranyl Schiff base complexes as
an acceptor and PBu3 as a donor was studied (Eq. (6)).
kobs= k1+k2[PBu3]
k
is the second-order rate constant (Scheme 1, Path
2
1) and k1 is the first-order rate constant (Scheme 1,
Path 2).
The rate constants and the activation parameters are
collected in Tables V, VI and S4–S6 in the Supporting
[UO2(Schiff base)(CH3CN)] + PBu3
→ [UO2(Schiff base)(PBu3)] + CH3CN
(6)
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

THERMAL DECOMPOSITION AND SUBSTITUTION OF NANO URANYL SCHIFF BASE COMPLEXES
727
Table V 101 k1 (s−1), 10−2 k2 (M−1
s
−1) for the Reaction of Complexes with PBu3 at Different Temperatures
[UO2(5-Clsalbz)(CH3CN)] [UO2(5-Brsalbz)(CH3CN)]
a
a
[UO2(salbz)(CH3CN)]
Temperature (°C)
k1 (s−1
)
k2 (M−1 s−1
)
k1 (s−1
)
k2 (M−1 s−1
)
k1 (s−1
)
k2 (M−1 s−1
)
10
20
30
40
5.1 (0.1)
2.2 (0.1)
1.7 (0.2)
2.6 (0.3)
0.4 (0.1)
0.7 (0.1)
0.8 (0.2)
0.9 (0.2)
6.6 (0.2)
0.4 (0.1)
2.9 (0.1)
5.6 (0.1)
5.8 (0.1)
8.6 (0.2)
10.7 (0.1)
13.1 (0.2)
5.1 (0.1)
9.7 (0.1)
2.9 (0.1)
3.8 (0.1)
3.6 (0.1)
4.2 (0.1)
4.5 (0.2)
5.5 (0.3)
aThe numbers in parentheses are the standard deviations of k.
Table VI Activation Parameters ꢀH#a, ꢀS#a, and ꢀG#b for the Interaction of the Complexes with PBu3
Complex
ꢀH# (kJ mol−1
)
ꢀS# (J K−1 mol−1
)
ꢀG# (kJ mol−1
)
Nano-[UO2(5-Clsalbz)(MeOH)]
Nano-[UO2(5-Brsalbz)(MeOH)]
Nano-[UO2(salbz)(MeOH)]
17.1 (2.0)
7.1 (1.4)
17.1 (5.2)
−130.9 (6.6)
−171.2 (4.8)
−152.9 (17.5)
58.1 (2.9)
60.7 (2.1)
64.9 (7.5)
aThe numbers in parentheses are the standard deviations.
bꢀG# was calculated at T = 40 °C.
Figure 6 Plots of kobs versus [PBu3] for [UO2(5-Clsalbz)(CH3CN)] at different temperatures (10–40°C).
Information. The mechanism as shown in Scheme 1
was suggested for the reaction of uranyl complexes
with PBu3.
The k2 values were obtained from the slope of
the linear plots of kobs versus the donor concentra-
tion [PBu3], and the k1 values were obtained from their
intercept (Fig. 6).
The activation parameters of the studied systems
were calculated by using the Eyring equation (10):
Two paths were suggested for the reaction. In Path
1, PBu3 was added to the uranium center by an as-
sociative mechanism with a rate constant k2, with the
suggested structures shown in Scheme 1. An octacoor-
dinate intermediate was formed and then by repelling
the solvent molecule a heptacoordinate product reap-
peared. In Path 2 (a dissociative path) with the k1 rate
constant, a hexacoordinate intermediate was formed
and PBu3 was coordinated to the uranium center in a
fast step.
ln(k2/T) = − ꢀH #/RT +ꢀS#/R + 23.8 (10)
A typical linear Eyring plot of ln(k2/T) versus 1/T
with a good correlation of 0.97–0.99 at four different
temperatures for [UO2(5-Clsalbz)(CH3CN)] is shown
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

728
ASADI ET AL.
ture plates (Greiner) and kept at 37°C in a humidi-
fied atmosphere of 5% CO2 in a CO2 incubator. All
the experiments were performed using cancer cell line
(Jurkat) of 10–15 passage. The growth inhibitory ef-
fect of uranyl complexes toward the cancer cells was
measured using the 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide (MTT) assay. Briefly, the
cleavage and conversion of the soluble yellowish MTT
to the insoluble purple formazan by active mitochon-
drial dehydrogenase of living cells has been used to
develop an assay system alternative to other assays for
the measurement of cell proliferation. The drug treat-
ment performed as the harvested cells was seeded in
to a 96-well plate (2.5 × 104 cell/well) with varying
concentrations of the sterilized uranyl complexes (0–
100 μM) and incubated for 24 and 48 h. Four hours
to the end of incubations, 25 μL of the MTT solution
(5 mg/ml in PBS) was added to each well containing
fresh and cultured medium. At the end, the insolu-
ble formazan was produced and it was dissolved in
a solution containing 10% SDS and 50% DMF (left
for 1 h at 37°C in dark conditions) and optical density
(OD) was read against a reagent blank with a multiwell
scanning spectrophotometer (ELISA reader; Bio-Tek’s
ELx808, Winooski, VT) at a wavelength of 570 nm.
The absorbance was a function of the concentration of
a converted dye. The OD value of study groups was
divided by the OD value of untreated control and pre-
sented as a percentage of control (as 100%). Also the
values of IC50 (the concentrations required for 50%
growth inhibition), after 24 h of incubation with the
complexes, were calculated.
Figure 7 Linear plot for ln(k2/T) against 1/T for [UO2(5-
Clsalbz)(CH3CN)].
in Fig. 7. Also, the ꢀG# values were calculated using
the equation ꢀG# = ꢀH# – TꢀS# at T = 313 K.
The k2 values for the ligands entry showed high
span, suggesting the dependence of the rate constant
on the nature of the complex. The low ꢀH# values
and the large negative ꢀS# values were compatible
with an associative (A) mechanism. For studying the
effect of substitutional groups, we compared the com-
plexes, 5-Br, 5-Cl, and salbz, so that complexes differ
in the electronic properties; the smallest k2 values were
found for [UO2(salbz)(CH3CN)], whereas the highest
values existed for [UO2(5-Clsalpyr)(CH3CN)] because
of the electron-withdrawing groups on the aromatic
ring. It was clear that the existence of the bromo and
chloro groups made the Schiff base complexes better
acceptors.
According to Table VII and Fig. 8,
[UO2(salbz)(MeOH)] did not show anticancer
activity, whereas [UO2(5-Clsalbz)(MeOH)] in 50–100
μM was able to kill about 60–70% of the cancer cell.
It was concluded that the electronic effect took part
during the substitution reaction. The following trend of
rate constant was obtained: UO2(5-Clsalpyr) > UO2(5-
Brsalpyr) > UO2(salpyr).
CONCLUSIONS
Anticancer
Activity. Anticancer
activity
of
This study involved the synthesis of crystalline and
nanoforms of uranyl unsymmetrical Schiff base com-
plexes. The preliminary TG and IR measurements con-
firmed the same behavior of both forms. The kinetics
and mechanism of solvent substitution reactions with
[UO2(salbz)(MeOH)] and [UO2(5-Clsalbz)(MeOH)]
complexes was tested on the Aspergillus fungus.
Details of the experiment are presented below.
Cell Culture and MTT Assay for Analysis of An-
ticancer Properties of the Complexes. The cancer
cell lines were cultured in RPMI 1640 medium
(HiMedia, Mumbai, India) supplemented with 10%
fetal calf serum (Biochrom, Berlin, Germany).
100 IU/mL of penicillin and 100 mg/mL of strepto-
mycin were also added to the media as antibiotics
to control the growth of contaminating microorgan-
isms. The cells were cultured in 96-well tissue cul-
Table VII IC50 Values (Mm/mL) of the Complexes
against Jurkat Cell Line
Complex
IC50
[UO2(salbz)(MeOH)]
–
[UO2(5-Clsalbz)(MeOH)]
45.8
International Journal of Chemical Kinetics DOI 10.1002/kin.20881

THERMAL DECOMPOSITION AND SUBSTITUTION OF NANO URANYL SCHIFF BASE COMPLEXES
729
Figure 8 Viability (%) versus the complex concentration. All the experiments were performed using cancer cell line (Jurkat)
of 10–15 passage.
tributylphosphine was investigated in acetonitrile spec-
trophotometrically. The uranium in UVIO22+ has no 4f
electron and acted as an acceptor, whereas PBu3 acted
as a donor and replaced the weakly coordinated sol-
vent. The high span of the rate constant (k2) as well as
low ꢀH# and large negative ꢀS# value suggested an
associative (A) mechanism. The electronic effect was
important for the rate of the substitution reaction. The
trend of k2 values showed that electron-withdrawing
substitutional groups such as Cl and Br to the phenolic
oxygen of the Schiff base accelerated the substitution
reaction by attracting PBu3 as an electron donor toward
the uranium center.
CV was used to investigate the effect of the substitu-
tional group of ligands on the reduction and oxidation
of uranium U(VI) ↔ U(V). Again, electron-withdrawing
substitutional groups accelerated the reduction of ura-
nium.
The kinetics of the thermal decomposition was in-
vestigated. Kinetic parameters (the activation energy
E* and the preexponential factor A*) were reported,
and the negative values of the entropy of activation
showed that the activated complex had a more or-
dered structure than the reactants. According to Coats–
Redfern plots, the kinetics of the thermal decomposi-
tion of the studied complexes was of the first order in
all stages.
The project P204/11/0809 of the grant agency of the Czech
Republic supported the crystallographic part of the work.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20881