Kinetic studies of the ligand substitution reaction of some new uranyl schiff base complexes with tri-n-butylphosphine

Article abstract of DOI:10.1002/kin.20815

Kinetics of the substitution reaction of solvent molecule in uranyl(VI) Schiff base complexes by tri-n-butylposphine as the entering nucleophile in acetonitrile at 10-40°C was studied spectrophotometrically. The second-order rate constants for the substitution reaction of the solvent molecule were found to be (8.8 ± 0.5) × 10^-3, (5.3 ± 0.2) × 10^-3, (7.5 ± 0.3) × 10^-3, (6.1 ± 0.3) × 10^-3, (13.5 ± 1.6) × 10^-3, (13.2 ± 0.9) × 10^-3, (52.9 ± 0.2) × 10 ^-3, and (88.1 ± 0.6) × 10^-3 M^-1 s^-1 at 40°C for [UO₂(Schiff base)(CH₃CN)], where Schiff base = L1-L8, respectively. In a temperature dependence study, the activation parameters ΔH^# and ΔS^# for the reaction of uranyl complexes with PBu₃ were determined. From the linear rate dependence on the concentration of PBu₃, the span of k₂ values and the large negative values of the activation entropy, an associative (A) mechanism is deduced for the solvent substitution. By comparing the second-order rate constants k_2, it was concluded that the steric and the electronic properties of the complexes were important for the rate of the reactions.

Full text of DOI:10.1002/kin.20815

Kinetic Studies of the
Ligand Substitution
Reaction of Some New
Uranyl Schiff Base
Complexes with
Tri-n-butylphosphine
ZAHRA ASADI, MOZAFFAR ASADI, FAHIMEH DEHGHANI FIRUZABADI
Chemistry Department, College of Sciences, Shiraz University, Shiraz, Islamic Republic of Iran
Received 27 October 2012; revised 15 April 2013; 16 June 2013; accepted 24 July 2013
DOI 10.1002/kin.20815
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Kinetics of the substitution reaction of solvent molecule in uranyl(VI) Schiff base
complexes by tri-n-butylposphine as the entering nucleophile in acetonitrile at 10–40◦C was
studied spectrophotometrically. The second-order rate constants for the substitution reaction
of the solvent molecule were−fo3und to be (8.8
0.5) × 10−3, (5.3
0.2) × 10−3, (7.5
0.3) × 10−3, (6.1
0.3) × 10 , (13.5
1.6) × 10−3, (13.2
0.9) × 10−3, (52.9
0.2) ×
10−3, and (88.1 0.6) × 10−3 M−1 s−1 at 40◦C for [UO2(Schiff base)(CH3CN)], where Schiff
base = L1–L8, respectively. In a temperature dependence study, the activation parameters ꢀH#
and ꢀS# for the reaction of uranyl complexes with PBu3 were determined. From the linear rate
dependence on the concentration of PBu3, the span of k2 values and the large negative values of
the activation entropy, an associative (A) mechanism is deduced for the solvent substitution.
By comparing the second-order rate constants k2, it was concluded that the steric aꢀnCd the
electronic properties of the complexes were important for the rate of the reactions.
Wiley Periodicals, Inc. Int J Chem Kinet 1–8, 2013
2013
molecular ferromagnets [1], polymerization [2], catal-
ysis [3–5], and as liquid crystals [6,7]. They also show
broad biological activity [8–13].
Unsymmetrical Schiff base ligands have clearly of-
fered many advantages over their symmetrical coun-
terparts in the elucidation of the geometry of the metal
ion binding sites in the metallo-proteins and the en-
zymes and the selectivity of the natural systems with
synthetic materials [14].
INTRODUCTION
Schiff base complexes have remained an important area
of research because of their simple synthesis, versa-
tility, and diverse range of applications in designing
Correspondence to: Zahra Asadi; e-mail: zasadi@shirazu.ac.ir.
Contract grant sponsor: Shiraz University Research Council.
C
ꢀ
2013 Wiley Periodicals, Inc.

2
ASADI, ASADI, AND FIRUZABADI
Interest in the coordination chemistry of uranium
has recently increased owing to diversity of their struc-
tures and possible applications in nuclear industry,
catalysis, anion and neutral molecule sensing, and
small molecule activation [15–19]. These applications
are causing less specialized and easier-to-appreciate
motivation for carrying out actinide-related chemical
research [20].
Ligand field effects in most d-transition elements
favor an arrangement where the equatorial ligands are
arranged in an almost square-planar geometry. But the
much smaller ligand field effects in the f-block ele-
ments in combination with a much larger ionic size re-
sults in a different geometry for lanthanoid and actinoid
elements [21–23]. This is further enhanced by the pres-
= =
ence of the particularly strong, trans O U O apical
=
bonds. The kinetic stabilization of the U O bonds in
3 = H, 4 = H, 5 = H
3 = OMe, 4 = H, 5 = H
3 = H, 4 = OMe, 5 = H
3 = H, 4 = H, 5 = OMe
3 = H, 4 = H, 5 = Br
3 = H, 4 = H, 5 = Cl
[UO2(L1)(CH3CN)]
[UO2(L2)(CH3CN)]
[UO2(L3)(CH3CN)]
[UO2(L4)(CH3CN)]
[UO2(L5)( CH3CN)]
[UO2(L6)( CH3CN)]
uranyl complexes promotes an additional ligand coor-
dination which typically occurs in the equatorial plane
of the linear dioxo unit [24]. This unusual coordina-
tion geometry indicates that the pathway for ligand
substitution reactions might be located in this plane, a
very different situation from those encountered in most
other coordination geometries.
The variation of the magnitudes of ꢀH= and ꢀS=,
and the sign of the latter parameter within a series of
similar ligand substitution reactions, can give a guide
to mechanistic change. Thus, in reactions with an asso-
ciative mechanism ꢀH= has smaller magnitudes com-
pared with a dissociative one and ꢀS= tends to have
negative and positive values for an associative and dis-
sociative mechanism, respectively [25].
This paper investigates the kinetics and mechanism
of solvent substitution in the equatorial plane by PBu3
and considers the electronic and the steric effects on
the rate of substitution reactions.
[UO2(L7)(CH3CN)]
[UO2(L8)(CH3CN)]
MATERIALS
All the materials were purchased commercially and
used without further purification.
Scheme 1 Structural representations of the uranyl Schiff
base complexes.
EXPERIMENTAL
All of the scanning UV–vis spectra were recorded on a
Perkin–Elmer Lambda 2 spectrophotometer equipped
with a Lauda-ecoline-RE 104 thermostat.
Schiff bases and uranyl complexes (Scheme 1) were
synthesized by the following procedure.
methanol (25 mL). The mixture was refluxed for about
5 h. The products were washed with methanol and di-
ethyl ether. All the Schiff base ligands were dried at
50◦C in vacuum.
Synthesis of the Ligands. All the tetradentate Schiff
base ligands (L1–L8) were prepared by the condensa-
tion of diamines (1 mmol) and aldehydes (2 mmol) in
Synthesis of the Uranyl Complexes. The uranyl com-
plexes were synthesized by the condensation of Schiff
base ligands (1 mmol) and uranyl acetate (1 mmol)
International Journal of Chemical Kinetics DOI 10.1002/kin.20815

KINETICS OF NEW URANYL SCHIFF BASE COMPLEXES WITH TRI-n-BUTYLPHOSPHINE
3
Scheme 2
in methanol (25 mL). The mixture was refluxed for
about 5 h in a 1:1 molar ratio. After being cooled,
the product was precipitated and filtered; then washed
with methanol (3 mL) and diethyl ether (3 mL). All the
Schiff base complexes were dried at 50◦C in a vacuum.
the absorbance in the UV–vis region was monitored
with time. The kinetics was followed at a wavelength
of maximum absorbance, where the difference in the
absorbance between the substrate and the product was
the largest (λmax). This wavelength was different for
each complex.
It was found that the uranyl complexes have
a pentagonal-bipyramidal geometry with an axial
Synthesis of the Kinetic Product: [UO2(L7)(PBu3)].
PBu3 (0.078 mL) was added to a refluxing solution
of UO2(L7)(solvent) in acetonitrile (25 mL) (1:1 mo-
lar ratio). The mixture was refluxed for 10 h under
nitrogen atmosphere. Temperature was controlled at
about 60◦C. The product was characterized by differ-
ent methods, and it was understood that PBu3 was
replaced the coordinated solvent.
[UO2(L7)(PBu3)]: Yield: 54%, Color: orange,
mp >250◦C, Anal. Found (Calcd): C37H47N2O4PU.
0.5H2O (1): C, 51.62 (51.57); H, 5.42 (5.61); N, 3.41
(3.25). IR (KBr, cm−1): 3429 (υO–H) (MeOH), 2869–
3055 (υC–H), 1612 (υC=N), 1542 (υC=C), 895 (υU=O),
640 (υU–N), 463 (υU–O). 1H NMR (250 MHz, DMSO-
d6, room temperature): δ (ppm) = 0.77–0.97 (m, 9H,
CH3) (PBu3), 1.26–1.48 (m, 18H, CH2) (PBu3), 1.54–
= =
O U O moiety [26,27]. When PBu3 was added to
the solution of the complex as a nucleophile, it oc-
cupies the sixth position in the equatorial plane as a
rate-determining step, then the solvent molecule is re-
moved in a fast step. This was confirmed by synthe-
sizing [UO2(L7)(PBu3)] as one of the kinetic product,
separately.
The kinetics was followed as described in the fol-
lowing reaction:
[UO2(Schiff base)(CH3CN)] + PBu3
→ [UO2(Schiff base)(PBu3)] + CH3CN
PBu3 with the excess concentration at least (1:10
ratio) was added to the uranyl complex solution.
Therefore, the kinetics was followed under pseudo–
first-order conditions. The rate law was according
to Eq. (1):
e
c
1.61 (d, 3H, CH3 ), 4.63–4.77 (m, 2H, CH2 ), 5.14
(m, 1H, CHd), 7.25–8.36 (m, 12H, ArH), 10.25 (s, 1H,
H C N), 10.34 (s, 1H, H C N). UV–vis. (acetoni-
trile): λmax (nm), ε (M−1 cm−1) = 245 (22166), 323
(5145), 366 (sh), 404 (sh).
b
a
=
=
R = kobs[Complex]
kobs = k2[PBu3] + k1
(1)
(2)
Kinetic Studies. A solution of the uranyl complexes
with specific concentration (2.5–5) × 10−5 M in ace-
tonitrile was prepared. 2.5 mL of each complex was
poured in a cell, and a known excess concentration
of PBu3 solution in acetonitrile was added by using a
microsyringe. After rapid stirring by a microsyringe,
where k1 is the first-order rate constant for a solvent
path (Scheme 2, path 2), and k2 is the second-order rate
constant (Scheme 2, path 1).
International Journal of Chemical Kinetics DOI 10.1002/kin.20815

4
ASADI, ASADI, AND FIRUZABADI
0.1
1.00E–03
8.00E–04
6.00E–04
4.00E–04
2.00E–04
0.05
0
y = –0.000x + 0.098
= 0.997
10°C
20°C
30°C
40°C
R
–0.05
–0.1
–0.15
–0.2
–0.25
–0.3
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
[PBu3] (M)
0
100
200
300
400
500
600
700
t (s)
Figure 3 Plots of kobs versus [PBu3] for [UO2(L3)
(CH3CN)] at different temperatures.
Figure 1 Plot of ln[(A0 – A )/(At – A )] versus t(s) for
∞
∞
[UO2(L3)(CH3CN)] at 40.0◦C.
The activation parameters of the studied systems
were calculated by using the Eyring equation:
1.4
1.2
1
ln (k2/T) = −ꢀH#/RT + ꢀS#/R + 23.8 (4)
ꢀH# and ꢀS# values were obtained from plots of
ln(k2/T) versus 1/T at four different temperatures in
the range 10.0–40.0 ( 0.1)◦C. The Eyring plot for the
reaction of [UO2(L3)(CH3CN)] with PBu3 is shown in
Fig. 4.
0.8
0.6
0.4
0.2
0
ꢀG# values were calculated using Eq. (5):
200
250
300
350
400
450
500
550
ꢀG# = ꢀH# − TꢀS#
(5)
Wavelength (nm)
Figure 2 Absorption spectra of [UO2(L6)(CH3CN)] with
PBu3 at 20.0◦C in CH3CN with the isosbestic points. Time
interval: 1 min.
Table IX shows the activation parameters ꢀG# (at
40.0◦C), ꢀH#, and ꢀS# for the reaction of the uranyl
complexes with PBu3 in CH3CN.
RESULTS AND DISCUSSION
The pseudo–first-order rate constants were calcu-
lated by fitting the data to Eq. (3):
k2 values for different complexes [UO2(L)(CH3CN)],
where L = L1–L8 with PBu3 at 40.0◦C, are as follows:
ln[(A − A )/(A − A )] = kobs
t
(3)
∞
∞
0
t
L1 L2 L3 L4 L5 L6 L7 L8
k2
8.8 5.3 7.5 6.1 13.5 13.2 52.9 88.1
(where At is absorbance at time t, A0 is absorbance at
(×103 M−1
s
−1) (0.5) (0.2) (0.3) (0.3) (1.6) (0.0) (0.2) (0.6)
t = 0, and A is absorbance at t = ∞) by means of
∞
a linear least-squares computer program (Fig. 1). As
an example, the variation of the electronic spectra for
[UO2(L3)(CH3CN)] reacted with the excess concen-
tration of PBu3 solution at 20.0◦C in CH3CN is shown
in Fig. 2.
By comparing the rate constants (k2) and the activa-
tion parameters, it can be concluded that two parame-
ters are important in the rate of substitution reactions:
The second-order rate constants k2 (M−1
s
−1) were
1. The first parameter is the electronic factor.
The electron-withdrawing groups such as Br
and Cl make the uranium center more posi-
tive; therefore, the rate of the substitution re-
action increases. The electron-releasing groups
decrease it.
obtained from the slope of the linear plots of kobs ver-
sus [PBu3]. Figure 3 shows plots of kobs versus [PBu3]
for [UO2(L3)(CH3CN)] at different temperatures in
CH3CN. kobs and k2 values for all the systems are pre-
sented in Tables I–VIII.
International Journal of Chemical Kinetics DOI 10.1002/kin.20815

KINETICS OF NEW URANYL SCHIFF BASE COMPLEXES WITH TRI-n-BUTYLPHOSPHINE
5
Table I Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2 (×103 M−1
s
−1) for the Reaction of
[UO2(L1)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
1.8
2.4
3
3.6
4.2
4.8
k1 (×104 s−1) k2 (×103 M−1 s−1
)
)
)
)
10.0
20.0
30.0
40.0
6.7 (0.3) 7.0 (0.3) 7.1 (0.3) 7.3 (0.3) 7.4 (0.3) 7.6 (0.3) 7.8 (0.3)
8.8 (0.5) 9.0 (0.6) 9.3 (0.7) 9.5 (0.6) 9.8 (0.7) 10.0 (0.1) 10.4 (0.1)
9.8 (0.1) 10.2 (0.1) 10.5 (0.1) 10.7 (0.1) 11.1 (0.8) 11.4 (0.1) 11.8 (0.1)
10.4 (0.1) 10.9 (0.2) 11.1 (0.2) 11.8 (0.2) 12.4 (0.2) 13.1 (0.2) 13.4 (0.3)
6.4 (0.1)
8.2 (0.1)
9.2 (0.1)
9.2 (0.2)
2.9 (0.1)
4.3 (0.2)
5.4 (0.2)
8.8 (0.5)
[Complex] = 5.0 × 10−5 M, λmax = 245.
aThe numbers in parentheses are the standard deviations.
Table II Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2 (×103 M−1
s
−1) for the Reaction of
[UO2(L2)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
1.8
2.4
3
3.6
4.2
4.8
k1 (×104 s−1
)
k2 (×103 M−1 s−1
10.0
20.0
30.0
40.0
3.0 (0.0) 3.2 (0.0) 3.3 (0.0) 3.5 (0.1) 3.6 (0.0) 3.8 (0.1) 3.9 (0.1)
4.4 (0.1) 4.6 (0.1) 4.7 (0.1) 4.8 (0.1) 5.0 (0.1) 5.2 (0.1) 5.5 (0.1)
5.3 (0.2) 5.5 (0.3) 5.8 (0.4) 6.0 (0.4) 6.3 (0.0) 6.5 (0.5) 6.7 (0.5)
5.8 (0.6) 6.0 (0.8) 6.4 (1.0) 6.7 (1.0) 7.1 (1.0) 7.3 (1.0) 7.7 (1.0)
2.8 (0.1)
4.0 (0.1)
4.8 (0.3)
5.1 (0.1)
2.2 (0.1)
2.9 (0.2)
3.9 (0.1)
5.3 (0.2)
[Complex] = 3.0 × 10−5 M, λmax = 244.
aThe numbers in parentheses are the standard deviations.
Table III Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2 (×103 M−1
s
−1) for the Reaction of
[UO2(L3)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
2.1
3.0
3.9
4.8
5.7
6.6
k1 (×104 s−1
)
k2 (×103 M−1 s−1
10.0
20.0
30.0
40.0
2.7 (0.0) 3.1 (0.0) 3.3 (0.2) 3.4 (0.1) 3.7 (0.1) 3.9 (0.2) 4.2 (0.3)
3.7 (0.2) 4.0 (0.4) 4.3 (0.1) 4.5 (0.0) 4.8 (0.2) 5.2 (0.2) 5.5 (0.3)
4.3 (0.0) 4.8 (0.1) 5.5 (0.0) 6.0 (0.3) 6.5 (0.3) 6.8 (0.4) 7.4 (0.5)
5.8 (0.1) 6.7 (0.3) 7.3 (0.4) 7.9 (0.3) 8.7 (0.6) 9.2 (0.8) 9.9 (1.1)
2.5 (0.1)
3.2 (0.1)
3.6 (0.1)
5.0 (0.1)
2.6 (0.2)
3.6 (0.1)
5.8 (0.2)
7.5 (0.3)
[Complex] = 3.0 × 10−5 M, λmax = 258.
aThe numbers in parentheses are the standard deviations.
Table IV Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2(×103 M−1
s
−1) for the Reaction of
[UO2(L4)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
1.8
2.4
3
3.6
4.2
4.8
k1 (×104 s−1) k2 (×103 M−1s−1
10.0
20.0
30.0
40.0
5.6 (0.1) 5.7 (0.3) 5.9 (0.2) 6.1 (0.1) 6.2 (0.0) 6.3 (0.2) 6.4 (0.4)
9.9 (0.4) 10.1 (0.1) 10.4 (0.0) 10.6 (0.1) 10.7 (0.3) 10.9 (0.5) 11.0 (0.8)
11.5 (0.1) 11.9 (0.1) 12.1 (0.3) 12.4 (0.4) 12.7 (0.4) 12.9 (0.7) 13.3 (0.8) 10.9 (0.1)
17.5 (0.4) 18.0 (0.3) 18.4 (0.6) 18.7 (0.4) 18.9 (0.9) 19.4 (1.1) 19.8 (0.9) 16.8 (0.1)
5.3 (0.1)
9.6 (0.1)
2.3 (0.1)
3.0 (0.2)
4.8 (0.2)
6.1 (0.3)
[Complex] = 2.5 × 10−5 M, λmax = 254.
aThe numbers in parentheses are the standard deviations.
2. The second parameter is the steric hindrance. As
the mechanism is an associative one, the steric
hindrance plays an important role in producing
the appropriate intermediate with a higher co-
ordination number. This effect has been clearly
observed by comparing k2 values of the L2 com-
plex with L3 and L4 complexes (Tables II–IV).
Since the methoxy groups in L2 are close to the
International Journal of Chemical Kinetics DOI 10.1002/kin.20815

6
ASADI, ASADI, AND FIRUZABADI
Table V Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2 (×103 M−1
s
−1) for the Reaction of
[UO2(L5)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
1.8
2.4
3
3.6
4.2
4.8
k1 (×104 s−1) k2 (×103 M−1 s−1
)
)
)
)
10.0
20.0
30.0
40.0
1.5 (0.0) 1.7 (0.0) 1.8 (0.1) 2.1 (0.0) 2.4 (0.2) 2.6 (0.1) 2.7 (0.1)
2.5 (0.0) 3.0 (0.1) 3.3 (0.0) 3.4 (0.3) 3.8 (0.1) 4.0 (0.2) 4.5 (0.3)
4.9 (0.0) 5.4 (0.0) 5.9 (0.1) 6.2 (0.1) 6.6 (0.1) 7.2 (0.2) 8.0 (0.2)
7.8 (0.0) 8.0 (0.1) 9.2 (0.1) 10.2 (0.3) 10.9 (0.2) 11.9 (0.3) 12.2 (0.4)
1.0 (0.1)
2.0 (0.1)
3.9 (0.2)
5.5 (0.2)
3.6 (0.2)
5.0 (0.4)
8.1 (0.5)
13.5 (1.6)
[Complex] = 5.0 × 10−5 M, λmax = 420.
aThe numbers in parentheses are the standard deviations.
Table VI Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2 (×103 M−1
s
−1) for the Reaction of
[UO2(L6)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
1.8
2.4
3
3.6
4.2
4.8
k1 (×104 s−1
)
k2 (×103 M−1 s−1
10.0
20.0
30.0
40.0
2.6 (0.1) 2.9 (0.4) 3.1 (0.0) 3.2 (0.4) 3.4 (0.3) 3.6 (0.0) 3.9 (0.0)
4.1 (0.2) 4.5 (0.1) 4.9 (0.2) 5.2 (0.3) 5.4 (0.1) 5.6 (0.4) 6.1 (0.1)
5.4 (0.4) 5.7 (0.4) 6.4 (0.3) 6.9 (0.1) 7.5 (0.2) 7.8 (0.5) 8.1 (0.5)
5.3 (0.8) 6.7 (0.4) 7.3 (0.2) 7.8 (0.1) 8.6 (0.0) 9.0 (0.1) 10.5 (0.8)
2.2 (0.1)
3.5 (0.1)
4.4 (0.2)
4.5 (0.3)
3.3 (0.2)
5.2 (0.3)
8.0 (0.4)
13.2 (0.0)
[Complex] = 3.0 × 10−5 M, λmax = 249.
aThe numbers in parentheses are the standard deviations.
Table VII Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2(×103 M−1
s
−1) for the Reaction of
[UO2(L7)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
1.2
1.8
2.4
3.0
3.6
4.2
4.8
k1 (×104 s−1) k2 (×103 M−1 s−1
10.0
20.0
30.0
40.0
9.4 (0.5) 9.9 (0.4) 10.3 (0.5) 10.8 (0.2) 11.1 (0.1) 12.5 (0.1) 12.7 (0.1)
11.5 (0.1) 12.0 (0.1) 12.9 (0.0) 13.5 (0.1) 14.2 (0.0) 15.0 (0.1) 16.1 (0.2)
16.8 (0.1) 17.0 (0.1) 19.7 (0.1) 21.3 (0.1) 23.5 (0.1) 24.7 (0.1) 28.0 (0.1) 10.9 (0.1)
17.8 (0.1) 21.0 (0.1) 23.3 (0.1) 25.9 (0.1) 29.6 (0.1) 33.5 (0.0) 37.0 (0.0) 12.1 (0.3)
8.1 (0.1)
9.7 (0.2)
9.5 (0.2)
12.9 (0.0)
31.4 (0.2)
52.9 (0.2)
[Complex] = 5.0 × 10−5 M, λmax = 422.
aThe numbers in parentheses are the standard deviations.
Table VIII Pseudo–First-Order Rate Constants 104kobsa, k1 (×104 s−1), and k2 (×103 M−1
s
−1) for the Reaction of
[UO2(L8)(CH3CN)] with PBu3 at Different Temperatures
[P] (×102 M)
Temperature
(◦C)
0.6
1.2
1.8
2.4
3.0
3.6
4.2
k1 (×104 s−1) k2 (×103 M−1 s−1
10.0
20.0
30.0
40.0
11.6 (0.1) 12.5 (0.0) 13.1 (0.3) 14.7 (0.2) 15.2 (0.2) 17.1 (0.4) 17.8 (0.1) 10.3 (0.0)
21.0 (0.0) 25.5 (0.1) 26.6 (0.0) 29.0 (0.2) 32.0 (0.2) 34.2 (0.3) 35.1 (0.4) 19.7 (0.1)
34.6 (0.4) 37.1 (0.7) 39.9 (0.3) 43.0 (0.5) 51.6 (0.6) 57.7 (0.5) 59.7 (0.6) 27.9 (0.2)
38.1 (0.2) 40.4 (0.0) 45.7 (0.5) 49.3 (0.7) 55.7 (0.3) 62.8 (0.3) 69.2 (0.7) 30.4 (0.1)
17.8 (0.4)
38.7 (0.3)
76.3 (0.9)
88.1 (0.6)
[Complex] = 5.0 × 10−5 M, λmax = 419.
aThe numbers in parentheses are the standard deviations.
International Journal of Chemical Kinetics DOI 10.1002/kin.20815

KINETICS OF NEW URANYL SCHIFF BASE COMPLEXES WITH TRI-n-BUTYLPHOSPHINE
7
UO2 moiety in the axial positions. In the uranyl
tetradentate Schiff base complexes, the fifth
position of the equatorial plane is occupied by
the solvent molecule [28], which is weakly co-
ordinated to the UO2 center. In a substitution
reaction, PBu3 can easily replace the solvent
molecule (Scheme 2). The k2 values obtained
in this study were compared with our previous
work [29]. It is clear that although the complexes
were different in structure but the overall results
were similar, i.e., the mechanism was an asso-
ciative one.
−10
−11
−12
y = –0.029x–1.088
R = 0.982
310
320
330
105(1/T) (K–1)
340
350
360
Figure 4 The Eyring plot for the reaction of [UO2(L3)
(CH3CN)] with PBu3.
CONCLUSIONS
reaction center, therefore k2 values are smaller
compared with L3 and L4.
The kinetics of some uranyl Schiff base complexes
with PBu3 was investigated spectrophotometrically in
the UV–vis region. The span of k2 values, the low
ꢀH# values, and the large negative ꢀS# values revealed
that the mechanism of the substitution reaction is an
associative one.
By considering the electronic and the steric ef-
fects of the uranyl complexes, the following trends are
observed:
This effect has also been observed by comparing k2
values for the L8 complex with the L7 complex (Tables
VII and VIII). The methyl group in the L7 complex acts
as an electron-releasing group; therefore, the k2 values
for L7 complex are smaller.
By comparing k2 values for the L6 complex with
the L5 one (Tables V and VI), it is clear that the two
complexes approximately have the same k2 values, be-
cause of the similarity of the steric hindrance and the
electronic property.
1. Electronic effect:
◦ [UO2(L5)(CH3CN)] > [UO2(L1)(CH3CN)]
> [UO2(L4)(CH3CN)].
By comparing the rate constants and the activation
parameters, the following conclusions were reached:
2. Steric hindrance:
◦ [UO2(L3)(CH3CN)] > [UO2(L4)(CH3CN)]
> [UO2(L2) (CH3CN)].
1. When the temperature is increased, the second-
order rate constant is increased too.
◦ [UO2(L8)(CH3CN)] > [UO2(L7)(CH3CN)].
3. Electronic and steric effect:
2. The low ꢀH# values and the large negative ꢀS#
values are compatible with an associative mech-
anism, i.e., PBu3 as a nucleophile entered in a
rate-determining step with the k2 rate constant
then the solvent left the intermediate complex
in a fast step. The uranyl complexes have a
pentagonal bipyramidal structure with the trans-
◦ [UO2(L5)(CH3CN)] ≈ [UO2(L6)(CH3CN)]
BIBLIOGRAPHY
1. Miyasaka, H.; Ikeda, H.; Mastumoto, N.; Crescenzi, R.;
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Table IX Activation Parametersa ꢀG#, 104ꢀH#, and ꢀS# for the Reaction of Uranyl Complexes with PBu3 in CH3CN
ꢀG#b (kJ mol−1
)
ꢀS# (J K−1 mol−1
)
ꢀH# (×104 kJ mol−1
)
Complexes
65.6 (3.0)
71.0 (1.0)
64.8 (2.5)
67.6 (2.1)
57.9 (2.8)
57.0 (1.4)
42.7 (6.4)
44.6 (0.9)
−209.7 (9.6)
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aThe numbers in parentheses are the standard deviations.
bCalculated at 40.0◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin.20815

8
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