Synthesis, X-ray crystallography, spectroscopy, electrochemistry, thermal and kinetic study of uranyl Schiff base complexes

Article abstract of DOI:10.1080/00958972.2013.849343

Uranyl(VI) complexes [UO₂(L)(solvent)], where L denotes an asymmetric N₂O₂ Schiff base (salpyr, 3-MeOsalpyr, 4-MeOsalpyr, 5-MeOsalpyr, 5-Clsalpyr or 5-Brsalpyr; salpyr is N,N′-bis(salicyliden)-2,3-diaminopyridine), were sy

Full text of DOI:10.1080/00958972.2013.849343

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Synthesis, X-ray crystallography,
spectroscopy, electrochemistry,
thermal and kinetic study of uranyl
Schiff base complexes
Zahra Asadia, Fatemeh Golzarda, Vaclav Eignerbc & Michal Dusekb
a Department of Chemistry, College of Science, Shiraz University,
Shiraz, Islamic Republic of Iran
b Department of Solid State Chemistry, Institute of Chemical
Technology, Prague, Czech Republic
c Institute of Physics AS CR, v.v.i., Prague, Czech Republic
Accepted author version posted online: 01 Oct 2013.Published
online: 30 Oct 2013.
To cite this article: Zahra Asadi, Fatemeh Golzard, Vaclav Eigner & Michal Dusek (2013)
Synthesis, X-ray crystallography, spectroscopy, electrochemistry, thermal and kinetic study
of uranyl Schiff base complexes, Journal of Coordination Chemistry, 66:20, 3629-3646, DOI:
10.1080/00958972.2013.849343
To link to this article: http://dx.doi.org/10.1080/00958972.2013.849343
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 20, 3629–3646, http://dx.doi.org/10.1080/00958972.2013.849343
Synthesis, X-ray crystallography, spectroscopy,
electrochemistry, thermal and kinetic study of uranyl Schiff
base complexes
ZAHRA ASADI*†, FATEMEH GOLZARD†, VACLAV EIGNER‡§ and
MICHAL DUSEK‡
†Department of Chemistry, College of Science, Shiraz University, Shiraz, Islamic Republic of Iran
‡Department of Solid State Chemistry, Institute of Chemical Technology, Prague, Czech Republic
§Institute of Physics AS CR, v.v.i., Prague, Czech Republic
(Received 17 June 2013; accepted 10 September 2013)
Uranyl(VI) complexes [UO2(L)(solvent)], where L denotes an asymmetric N2O2 Schiff base (salpyr,
3-MeOsalpyr, 4-MeOsalpyr, 5-MeOsalpyr, 5-Clsalpyr or 5-Brsalpyr; salpyr is N,N′-bis(salicyliden)-2,3-
diaminopyridine), were synthesized and characterized in solution (UV–vis, 1H NMR, cyclic
voltammetry) and in the solid-state (X-ray crystallography, IR, TGA, C H N.). X-ray crystallography of
UO2(3-MeOsalpyr) revealed coordination of the uranyl by the tetradentate Schiff base and one
disordered solvent, resulting in seven-coordinate uranium. Another disordered solvent was not
coordinated. Cyclic voltammetry of [UVIO2(L)(solvent)] in acetonitrile was used to investigate the
effect of the substituents of the Schiff base ligands on oxidation and reduction potential. The
quasi-reversible redox reaction without any successive reactions was accelerated by groups with lesser
electron withdrawing. We also investigated the kinetics and mechanism of the exchange reaction of the
coordinated solvent with tributylphosphine using spectrophotometric method. The second-order rate
constants at four temperatures and activation parameters showed an associative mechanism for all
corresponding complexes with the following trend: UO2(5-Clsalpyr) > UO2(5-Brsalpyr) > UO2
(3-MeOsalpyr) > UO2(4-MeOsalpyr) > UO2(salpyr) > UO2(5-MeOsalpyr). It was concluded that the ste-
ric and electronic properties of the complexes were important for the reaction rate.
Keywords: X-ray crystallography; Uranyl Schiff base complex; Kinetics of thermal decomposition;
Cyclic voltammetry; Kinetics and mechanism
1. Introduction
The majority of high oxidation state complexes of the actinides contain actinyl ions
[AnO2]n+, where An = U, Np, Pu; n = 1 or 2. In contrast to transition metal dioxo com-
plexes which almost always adopt bent geometries, the actinyl ions are essentially linear
[1]. Uranium is the most widely studied actinide because of its occurrence in nature. The
chemistry of uranium is dominated by the dioxo or uranyl dication, [UO2]2+, which is found
both in aqueous solutions and in the solid state. It is the most thermodynamically stable
form of uranium, in contrast to the pentavalent uranyl cation [UO2]+, which disproportion-
ates in aqueous environment into tetravalent uranium species and [UO2]2+ [2]. Coordination
*Corresponding author. Emails: zasadi@shirazu.ac.ir; zasadi@susc.ac.ir
© 2013 Taylor & Francis

3630
Z. Asadi et al.
compounds of uranyl have interesting reactivity, coordination properties, and applications
[1–9].
Solvents containing oxygen can coordinate to uranyl with tetradentate Schiff base ligands
[10]. The usual coordination number in the equatorial plane of U(VI) is five or six. A penta-
or hexa-dentate planar ligand may not have solvent in the equatorial plane [5].
Our group studied the kinetics and mechanism of the addition reaction of tetraaza Schiff
base complexes of first-row transition metals with different donors [11–14].
In this paper, our interest is focused on complexes of uranyl with salpyr, 3-MeOsalpyr,
4-MeOsalpyr, 5-MeOsalpyr, 5-Clsalpyr, and 5-Brsalpyr and the solvent MeOH. X-ray
crystallography and TG reveal that one methanol coordinates weakly to the uranium center.
Thus we can study the kinetics of exchange of this solvent molecule with tributylphosphine
and parameters affecting the rate constants of the exchange reaction, such as electronic and
steric effects. The effect of substituents on the redox potential of these complexes was
studied.
2. Experimental
2.1. Materials
Salicylaldehyde, 3-methoxysalicylaldehyde, 4-methoxysalicylaldehyde, 5-methoxysalicylal
dehyde,
5-chlorosalicylaldehyde,
5-bromosalicylaldehyde,
2,3-diaminopyridine,
uranylacetatedihydrate, methanol, acetonitrile, and potassium bromide for IR spectroscopy,
tetrabutylammoniumperchlorate (TBAP), tributylphosphine, CDCl3, and DMSO-d6 were
obtained from Merck, Fluka, Aldrich, and Acros and used without purification.
2.2. Instruments
Scanning UV-vis spectra were recorded on a Perkin Elmer Lambda 2 spectrophotometer.
1H NMR spectra were recorded on a Bruker Avance DPX 250 MHZ. IR spectra were
recorded on a Shimadzu FTIR 8300 infrared spectrophotometer. Elemental microanalyses
were performed with a CHN Thermo Finnigan Flash EA1112 instrument. Melting points
were determined on a BUCHI 535. Thermogravimetric analyzes were recorded with a
Perkin-Elmer Pyris Diamond model. Electrochemistry was carried out using Auto lab
302 N. X-ray crystallography was performed by the four-cycle diffractometer Gemini of
Agilent Technologies (2012) [15].
2.3. Synthesis of Schiff base ligands
All tetradentate Schiff bases were prepared by condensation of diamine (1 mM, in 20 ml
methanol) and hydroxyaldehydes (2 mM, in 20 ml methanol), and were purified by recrys-
tallization from a dichloromethane/hexane mixed solvent through the partial evaporation of
the more volatile dichloromethane [16]. All Schiff bases were dried at 50 °C in vacuum.
2.3.1. N,N′-bis(salicyliden)-2,3-diaminopyridine (salpyr). Yield: 54%, Color: Yellow,
m.p. 145 °C. Anal. Calcd for C19H15N3O2 (%): C, 71.91; H, 4.76; N, 13.24. Found: C,

Uranyl Schiff base complexes
3631
1
71.56; H, 4.50; N, 13.31. H NMR (250 MHz, CDCl3, room temperature): δ(ppm) = 6.94–
7.54 (m, 10H, Ar–H, pyridine-H), 8.40 (d, 1H, pyridine–H10), 8.65 (s, 1H, H7’), 9.90
(s, 1H, H7), 12.82 (s, 1H, OHb), 13.35 (s, 1H, OHa). IR (KBr, cm−1): 3448 (νO–H), 2850–
3047 (νC–H), 1620 (νC=N), 1558 (νC=C), 1272 (νC–O). UV–vis. (acetonitrile): λmax (nm), ε
(M−1 cm−1) = 205(~3402), 271(~29,570), 337(~24,940).
2.3.2. N,N′-bis(3-methoxysalicylidene)-2,3-diaminopyridine (3-MeOsalpyr). Yield: 58%,
Color: orange, m.p. 222 °C. Anal. Calcd for C21H19N3O4 (%): C, 66.83; H, 5.07; N, 11.13.
Found: C, 66.45; H, 4.76; N, 11.32. 1H NMR (250 MHz, DMSO, room temperature):
δ(ppm) = 3.82 (s, 6H,OCH3), 6.86–7.91 (m, 8H, Ar–H, pyridine–H), 8.41 (s, 1H, pyridine–
H10), 8.95 (s, 1H, H7’), 9.51 (s, 1H, H7), 12.57 (s, 1H, OHb), 13.19 (s, 1H, OHa). IR (KBr,
cm−1): 3448 (νO–H), 2831–3025 (νC–H), 1612 (νC=N), 1558 (νC=C), 1242 (νC–O). UV–vis.
(acetonitrile): λmax (nm), ε (M−1 cm−1) = 224(~28,903), 284(~24,780), 308(~23,468).
2.3.3. N,N′-bis(5-methoxysalicylidene)-2,3-diaminopyridine (5-MeOsalpyr). Yield: 61%,
Color: red, m.p. 140 °C. Anal. Calcd for C21H19N3O4 (%): C, 66.83; H, 5.07; N, 11.13.
Found: C, 66.98; H, 4.88; N, 11.17. 1H NMR (250 MHz, DMSO, room temperature):
δ(ppm) = 3.82 (s, 6H, OCH3), 6.89–7.54 (m, 8H, Ar–H, pyridine–H), 8.40 (s, 1H, pyridine–
H10), 8.60 (s, 1H, H7’), 9.86 (s, 1H, H7), 12.38 (s, 1H, OHb), 12.92 (s, 1H, OHa). IR
(KBr, cm−1): 3400 (νO–H), 2831–3020 (νC–H), 1612 (νC=N), 1589 (νC=C), 1265 (νC–O).
UV–vis. (acetonitrile): λmax (nm),
ε
(M−1 cm−1) = 239(~57,835), 273(~29,502),
331(~23,033), 368(~21,174).
2.3.4. N,N′-bis(5-chlorosalicylidene)-2,3-diaminopyridine (5-Clsalpyr). Yield: 64%, Color:
yellow, m.p. 144 °C. Anal. Calcd for C19H13Cl2N3O4 (%): C, 59.09; H, 3.39; N, 10.88.
Found: C, 58.85; H, 3.19; N, 11.26. 1H NMR (250 MHz, CDCl3, room temperature,
TMS): δ(ppm) = 6.99–7.55 (m, 8H, Ar–H, pyridine–H), 8.46 (d, 1H, pyridine–H10),
8.58 (s, 1H, H7’), 9.47 (s, 1H, H7), 12.77 (s, 1H, OHb), 13.35 (s, 1H, OHa). IR
(KBr, cm−1): 3433 (νO-H), 2990–3020 (νC–H), 1620 (νC=N), 1551 (νC=C), 1265 (νC–O).
2.3.5. N,N′-bis(5-bromosalicylidene)-2,3-diaminopyridine(5-Brsalpyr). Yield: 86%, Color:
yellow, m.p. 228 °C. Anal. Calcd for C21H19N3Br2O4 (%): C, 48.04; H, 2.76; N, 8.84. Found:
1
C, 48.16; H, 2.88; N, 8.78. H NMR (250 MHz, DMSO, room temperature): δ(ppm) = 6.91–
8.04 (m, 8H, Ar-H), 8.92 (d, 1H, pyridine–H10), 9.51 (s, 1H, H7’), 10.18
(s, 1H, H7), 12.64 (s, 1H, OHb), 13.15 (s, 1H, OHa). IR (KBr, cm−1): 3463 (νO–H),
2920–3160 (νC–H), 1589 (νC=N), 1551 (νC=C), 1265 (νC–O). UV–vis. (acetonitrile): λmax (nm),
ε (M−1 cm−1) = 232(~38,978), 269(~20,593), 348(~17,934).
2.4. Synthesis of uranyl complexes
Uranyl complexes (figure 1) were prepared by addition of uranyl acetate dihydrate (5 mM
in 20 ml methanol) to a hot methanolic solution (20 ml) of the Schiff base (5 mM) (1:1 M
ratio), except for 4-methoxysalpyr complex that was synthesized with a template method.
The color of the solution changed to orange–red in a few minutes. The mixture was then
refluxed for 3 h. The solid was washed with ether, followed by drying at 50 °C in vacuum.

3632
Z. Asadi et al.
9
10
8
N
H
H
7
7'
O
N
N
6'
6
U
a
b
5'
5
R
O
O
R
O
3'
4'
4
3
S
R
R
R
R
Figure 1. Structural representation of the uranyl(VI) Schiff base complex. 5,5′: R = H (salpyr), R = OMe
(5-MeOsalpyr), R = Cl (5-Clsalpyr), R = Br (5-Clsalpyr); 4,4′; R = OMe (4-MeOsalpyr); 3,3′; R = OMe (3-MeO-
salpyr), and S = solvent.
2.4.1. [UO2(salpyr)(MeOH)]. Yield: 85%, Color: red, m.p. > 300 °C, Anal. Calcd for
C19H13N3O4U (%): C, 38.91; H, 2.78; N, 6.81. Found: C, 38.52; H, 2.54; N, 6.65.
1H NMR (250 MHz, DMSO-d6, room temperature): δ (ppm) = 3.15 (s, 3H, MeOH), 4.13
(q, 1H, MeOH), 6.70–8.32 (m, 10H, Ar–H, pyridine–H), 8.57 (d, 1H, pyridine–H10), 9.67
(s, 1H, H7’), 10.17 (s, 1H, H7). IR (KBr, cm−1): 3417 (υO–H, CH3OH), 2746–3001 (υC–H),
1604 (υC=N), 1535 (νC=C), 1203 (νC–O), 902 (υU=O), 540 (υU–N), 440 (υU–O). UV–vis. (ace-
tonitrile): λmax (nm), ε (M−1 cm−1) = 239(~34,281), 283(~25,631), 338(~16,179), 411
(~10,985). CV(V): −0.784(Epa), −1.010(Epc).
2.4.2. [UO2(3-MeOsalpyr)(MeOH)]·0.8H2O. Yield: 77%, Color: orange, m.p. > 300 °C,
Anal. Calcd for C21H17N3O6U (%): C, 38.19; H, 3.29; N, 6.07. Found: C, 37.75; H, 3.02;
1
N, 5.99. H NMR (250 MHz, DMSO, room temperature): δ (ppm) = 3.15 (d, 3H, MeOH),
4.10 (q, 1H, MeOH), 3.97 (s, 6H, OCH3), 6.67 (d, 2H, Ar–H4,4’), 7.27 (d, 2H, Ar–H6,6’),
7.44 (dd, 2H, Ar–H5,5’), 7.58 (dd, 1H, pyridine–H9), 8.30 (d, 1H, pyridine–H8), 8.56 (d,
1H, pyridine–H10), 9.65 (s, 1H, H7’), 10.15 (s, 1H, H7). IR (KBr, cm−1): 3421 (υO–H
,
MeOH), 2922–3068 (υC–H), 1602 (υC=N), 1544 (νC=C), 1207 (νC–O), 902 (υU=O). UV–vis.
(acetonitrile): λmax (nm), ε (M−1 cm−1) = 244(~36,364), 357(~20,099), 444(sh). CV(V):
−0.788(Epa), −1.001(Epc).
2.4.3. [UO2(4-MeOsalpyr)(MeOH)]. This complex was synthesized by template method.
Yield: 78%, Color: orange, m.p. > 300 °C, Anal. Calcd for C21H17N3O6U (%): C, 39.01; H,
1
3.12; N, 6.20. Found: C, 38.60; H, 3.00; N, 6.09. H NMR (250 MHz, DMSO-d6, room
temperature): δ (ppm) = 3.1 (d, 3H, MeOH), 4.1 (q, 1H, MeOH), 3.9 (s, 6H, OCH3), 6.4
(d, 2H, Ar–H5,5′), 6.5 (s, 2H, Ar–H3,3′), 7.5 (dd, 1H, pyridine H9), 7.7 (d, 1H, Ar–H6), 7.8
(d, 1H, Ar–H6′), 8.2 (d, 1H, pyridine–H8), 8.5 (d, 1H, pyridine–H10), 9.5 (s, 1H, H7’), 10.0
(s, 1H, H7). IR (KBr, cm−1): 3425 (υO–H), 2900–3070 (υC–H), 1597 (υC=N), 1519 (υC=C),
1211 (υC–O), 903 (υU=O), 532 (υU–N), 447 (υU–O). UV–vis. (acetonitrile): λmax (nm), ε
(M−1 cm−1): 217(~24,621), 257(~26,371), 313(~19,648), 357(~20,682), 432(sh). CV(V):
−0.791(Epa), −0.972(Epc).

Uranyl Schiff base complexes
3633
2.4.4. [UO2(5-MeOsalpyr)(MeOH)]. Yield: 90%, Color: orange, m.p. > 300 °C, Anal.
Calcd for C21H17N3O6U (%): C, 39.01; H, 3.12, N; 6.20. Found: C, 38.62; H, 2.80; N,
1
5.94. H NMR (250 MHz, DMSO−d6, room temperature): δ (ppm) = 3.13 (d, 3H, MeOH),
4.12 (q, 1H, MeOH), 3.75 (s, 6H, OCH3), 6.93 (s, 2H, Ar–H6,6′), 7.34 (m, 3H, Ar–H), 7.46
(d, 1H, pyridine–H8), 7.58 (dd, 1H, Ar–H9), 8.27 (d, 1H, Ar–H), 8.56 (d, 1H, pyridine–
H10), 9.63 (s, 1H, H7′), 10.14 (s, 1H, H7). IR (KBr, cm−1): 3417 (υO–H), 2815–3020(υC–H),
1589 (υC=N), 1535 (υC=C), 1226 (υC–O), 902(υU=O), 530(υU–N), 430(υU–O). UV–vis. (aceto-
nitrile): λmax (nm), ε (M−1 cm−1): 249(~37,206), 343(~21,069), 430(sh). CV(V): –0.796
(Epa), −1.010(Epc).
2.4.5. [UO2(5-Clsalpyr)(MeOH)]. Yield: 86%, Color: orange, m.p. > 300 °C, Anal. Calcd
for C19H11N3O4Cl2U (%): C, 34.88; H, 2.20; N, 6.12. Found: C, 34.74; H, 1.84; N, 5.96.
1H NMR (250 MHz, DMSO-d6, room temperature): δ (ppm) = 3.13 (d, 3H, MeOH), 4.11
(q, 1H, MeOH), 7.02 (s, 2H, Ar–H6), 7.64 (m, 3H, Ar–H, pyridine–H), 7.88 (d, 1H, Ar–H),
8.01 (d, 1H, Ar–H), 8.34 (dd, 1H, pyridine–H9), 8.61 (d, 1H, pyridine–H10), 9.66 (s, 1H,
H7′), 10.14 (s, 1H, H7). IR (KBr, cm−1): 3380 (υO–H, MeOH), 2995–3020 (υC–H), 1598
(υC=N), 1558 (υC=C), 1167 (υC–O), 900 (υU=O), 563 (υU–N), 455 (υU–O). UV–vis. (acetoni-
trile): λmax (nm),
ε
(M−1 cm−1): 245(~14,493), 290(sh), 338(~13,450), 390(sh).
CV(V): −0.784(Epa), −1.023(Epc).
2.4.6. [UO2(5-Brsalpyr)(MeOH)]. Yield: 86%, Color: orange, m.p. > 300 °C, Anal. Calcd
for C19H11N3O4Br2U (%): C, 30.85; H, 1.95; N, 5.42. Found: C, 30.95; H, 1.66; N, 5.36.
1H NMR (250 MHz, DMSO−d6, room temperature): δ (ppm) = 3.14 (d, 3H, MeOH), 4.10
(q, 1H, MeOH), 6.97–8.30 (d, 8H, Ar–H), 8.58 (d, 1H, pyridine–H10), 9.62 (s, 1H, H7′),
10.10 (s, 1H, H7). IR (KBr, cm−1): 3394 (υO–H, MeOH), 2920–3010 (υC–H), 1596 (υC=N),
1566 (υC=C), 1172 (υC–O), 918 (υU=O), 640 (υU–N), 446 (υU–O). UV–vis. (acetonitrile):
λmax (nm), ε (M−1 cm−1): 245(~50,274), 274(~25,772), 361(~21,994). CV(V): –0.784(Epa),
−1.003(Epc).
2.5. Kinetic studies of exchange reactions
The kinetics of exchange reactions on the uranyl center were studied. Kinetic data were
determined spectrophotometrically. In all the cases (runs from 10.0–40.0 0.1 °C), the pro-
cedure involves adding a sample of PBu3, under pseudo-first-order conditions, to a solution
containing the uranyl complex. The kinetics were followed at a predetermined wavelength,
where the difference in absorption between the substrate and product was high. After each
injection the absorbance was read with definite time intervals [11–14].
2.6. Synthesis of the kinetic product
To a refluxing solution of [UO2(salpyr)(solvent)] (0.1 g, 0.017 mM) in acetonitrile (25 ml)
was added tri-n-butylphosphine (0.042, 0.017 mM) (1 : 1 M ratio). The reaction mixture
was refluxed for 24 h under nitrogen. The resulting oil was grounded with n-hexane to
extract the impurities, and finally a powdery product was obtained [11–14]. 1H NMR
spectra of the ligand, uranyl complex, and kinetic product are shown in Supplementary
material.

3634
Z. Asadi et al.
2.6.1. [UO2(salpyr)(PBu3)]·H2O. Yield: 87%, Color: orange, m.p. < 50 °C, Anal. Calcd
1
for C31H40N3O4PU (%): C, 46.21; H, 5.25; N, 5.22. Found: C, 45.89; H, 5.02; N, 5.48. H
NMR (250 MHz, DMSO-d6, room temperature): δ (ppm) = 0.8 (t, 9H, CH3), 1.3 (m, 12H,
CH2), 1.6 (t, 6H, CH2), 6.7–8.3 (m, 10H, Ar–H, pyridine–H), 8.6 (d, 1H, pyridine–H10),
9.7 (s, 1H, H7′), 10.2 (s, 1H, H7). IR (KBr, cm−1): 2869–3056 (υC–H), 1604 (υC=N), 1535
(υC=C), 1188 (υC–O), 895 (υU=O), 540 (υU–N), 439 (υU–O). UV–vis. (acetonitrile): λmax (nm),
ε (M−1 cm−1): 281(~65,750), 353(~53,333), 435(sh).
2.7. X-ray crystallography
Red single crystals of [UO2(3-MeOsalpyr)(DMF)] were obtained in good yield, from slow
diffusion of diethyl ether into a solution of the metal complex in DMF at room temperature
during 10 days. The crystals were intensely colored. The preparation from DMF/Et2O gave
better single crystals when compared with preparation from acetonitrile, which was also
attempted. The data were collected on a Gemini diffractometer with an Atlas CCD detector
using graphite monochromated Mo Kα radiation (λ = 0.7107 Å) and corrected for absorp-
tion using CrysAlisPro software. The structure was solved by the charge-flipping method
using Superflip [17] and refined by full matrix least squares on F2 with the JANA2006
program [15]. The disordered solvent molecules were determined as rigid bodies in close
positions described by a refined translation vector and three rotations.
2.7.1. [UO2(3-MeOsalpyr)(DMF)]. Color: deep red, m.p. = 150 °C, Anal. Calcd for
C24H24N4O7U (%): C, 40.12; H, 3.37; N, 7.80. Found: C, 40.19; H, 3.32; N, 7.98.
1H NMR (250 MHz, DMSO-d6, room temperature): δ (ppm) = 2.77 (s, 3H, CH3-DMF),
2.93 (s, 3H, CH3-DMF), 3.93 (s, 6H, OCH3), 6.62–7.60 (m, 7H, Ar, pyridine), 8.01 (s, 1H,
DMF), 8.27 (d, 1H, pyridine-H8), 8.52 (d, 1H, pyridine-H10), 9.65 (s, 1H, H7′), 10.15 (s,
1H, H7). IR (KBr, cm−1): 2922–3068 (υC–H), 1643 (υC=O), 1602 (υC=N), 1544 (νC=C), 1207
(νC–O), 902 (υU=O). UV–vis. (DMF): λmax (nm), ε (M−1 cm−1) = 242(~36,364), 354
(~20,099), 442(sh).
3. Results and discussion
3.1. Characterization of the complexes
3.1.1. Crystal structure of [UO2(3-MeOsalpyr)(DMF)]. Crystallographic data and details
of the data collection are listed in table 1. The ORTEP view of this complex is shown in
figure 2, with selected bonding parameters listed in table 2.
The uranium was coordinated as a distorted pentagonal bipyramid with uranyl oxygens
located in the axial positions. Four equatorial positions were occupied by two nitrogens and
two oxygens of the tetradentate Schiff base, while the fifth position was DMF. Both the
molecules of DMF – the coordinated one as well as the free one – were disordered. The
coordination geometry around UO2 was nearly planar with the dihedral angle of 2.56(16)°
between the coordination planes of N16–U1–O24 and N9–U1–O1. C10 and C15 belonging
to the aromatic rings were out of the equatorial plane, with a distance from the plane
0.449(4) Å for C10 and 0.466(4) Å for C15. The U=O bond distances in uranyl were 1.781

Uranyl Schiff base complexes
3635
Table 1. Crystal data, data collection and
structure refinement details for [UO2(3-MeO
salpyr)(DMF)].
Complex
Formula
Formula weight
Crystal system
Hall group
Space group
T [K]
C27H31N5O8U
791.60
Triclinic
−P – 1
P −1
120
a [Å]
b [Å]
c [Å]
α [°]
9.4402(3)
10.9098(5)
14.6683(5)
106.42(3)
92.92(3)
103.56(3)
1397.53(10)
2
1.88
768.0
7722[6820]
0.043, 0.563
0.0295(5769)
0.0625(6820)
β [°]
γ [°]
V [Å3]
Z
DCalcd [Mg m−3
F (000)
Nref
]
Tmin, Tmax
R(reflections > 3σ)
wR2(all reflections)
Figure 2. ORTEP view of [UO2(3-MeOsalpyr)(DMF)].
(3) Å for U1–O1u and 1.783(3) Å for U1–O2u, typical values for uranyl compounds [1,
14]. The O1u–U1–O2u bond angle 175.83(13)° indicated that uranyl was slightly bent in
the direction opposite to coordinated DMF. The U1–O1 and U1–O24 bond distances 2.245
(3) and 2.246(3) Å, respectively, were shorter than U1–N9 and U1–N16 [2.537(3), 2.573
(3) Å]. Such a difference might imply that coordination of oxygen in the complex was
stronger than coordination of nitrogen. The bond distance between oxygen of DMF and

3636
Z. Asadi et al.
Table 2. Selected bond distances (Å) and angles (°) for [UO2(3-MeOsalpyr)(DMF)].
U1–O1u
U1–O2u
U1–O24
U1–O1
U1–O1sc
U1–O1sd
U1–N9
1.781(3)
1.783(3)
2.246(3)
2.245(3)
2.399(7)
2.310(2)
2.537(3)
2.573(3)
O1u–U1–O2u
175.83(13)
93.6(2)
94.5(5)
70.23(10)
69.89(10)
−0.7(5)
−12.0(7)
14.9(7)
O1u–U1–O1sc
O1u–U1–O1sd
O24–U1–N16
O1–U1–N9
N9–C10–C15–N16
N16–C17–C18–C23
N9–C8–C7–C2
U1–N16
uranium was 2.399(6) Å [U1–O1sc], longer than those of U1–O1 and U1–O24, suggesting
that coordination of DMF was not as strong as coordination of the Schiff base.
3.1.2. 1H NMR spectroscopy. In 1H NMR spectra of the ligands, because of the
unsymmetrical structure, two singlets for methine (HC=N) protons and two singlets for
1
hydroxyl protons were present. In H NMR spectra of the complexes a significant shift
in the imine proton was observed (9.54–10.17 ppm) as compared to the free ligand
(8.65–9.90 ppm), indicative of the imine nitrogen lone pairs coordinating. Also the
absence of hydroxyl protons at ~12 ppm showed that the Schiff base was coordinated to
the metal. 1H NMR spectra of the complexes showed a differentiation of almost all
hydrogens [18], since they were in different chemical environments due to the rigid
structure of the ligand coordinated to the metal [19]. The formation of rigid structure also
supports the visibility of aromatic protons. In spectra of complexes, two sets of reso-
nances were observed for azomethine proton, confirming their different chemical environ-
ments. The proton chemical shifts of the coordinated PBu3 in the kinetic product appear
at δ = 0.8–1.6 ppm, in agreement with previous results observed for metal complexes
with tributylphosphine as ligand [20, 21].
By considering the crystal structure of UO2(salophen)DMF [6], it is clear that one sol-
vent molecule, which was used in the crystallization, is coordinated to the uranium and
the complex has a pentagonal-bipyramidal geometry with axial O=U=O. Comparing bond
distances shows that U–O(salophen) bond distances are shorter than U–N(salophen). The
bond distance between the oxygen of DMF and uranium is the longest, suggesting that
coordination of DMF is not as strong as coordination of salophen. The solvent in the
fifth position of the equatorial plane depends on the solvent used for synthesis or crystal-
lization; changing the solvent of the synthesis or crystallization changes the coordinated
solvent [10].
1
All H NMR of the complexes were recorded in DMSO-d6 solvent, and it is expected
that DMSO-d6 replaced methanol in the coordination sphere. Peaks due to free MeOH were
1
observed in the H NMR as a quartet at 4.12 ppm related to OH and a doublet at 3.15 ppm
1
from CH3. In H NMR spectra of [UO2(3-MeOsalpyr)(DMF)] signals of methyl and formyl
of DMF were observed at 2.93, 2.77 and 8.01 ppm, respectively.
3.1.3. IR spectroscopy. IR spectra of the ligands exhibited broad medium intensity bands
at 3400–3463 cm−1 were assigned to the intramolecular hydrogen bonding vibration
(O–H⋯N) [22]. These bands should disappear due to complexation, but because of the
existence of methanol in our complexes, we could see these bands after complexation. In

Uranyl Schiff base complexes
3637
the IR spectrum of [UO2(3-MeOsalpyr)(DMF)], these bands were not present but a band
due to C=O of DMF was at 1643 cm−1. The vibration of azomethine of free ligand was at
1589–1620 cm−1. In the complexes, these bands shifted to lower frequencies
(1589–1602 cm−1), indicating that nitrogen of azomethine coordinated [23, 24]. Coordina-
tion of azomethine nitrogen was affirmed by the presence of new bands at 530–640 cm−1,
assignable to υU−N. A new band at 430–455 cm−1 for the complexes was assignable to υU-
O. The characteristic uranyl υU=O appeared at 902–918 cm−1. In [UO2(salpyr)(PBu3)], the
bands around 2869–3050 cm−1 were stronger due to coordination of PBu3.
3.1.4. UV–vis spectra. Electronic spectra of salpyr and its complex in acetonitrile are
shown in figure 3. The electronic spectra of the ligands have three intense bands. The first
band at higher energy is attributed to π → π* transition of the phenyl ring, the band at lower
energy from π → π* transition of the azomethine chromophore, and the lowest energy band
was n → π* transition of lone pair electrons of nitrogen to the antibonding π* orbital. Usu-
ally n → π* transition involving nitrogen occurs at low energies.
The intense absorption bands of the complexes assigned to an electric dipole allowed
transition arising from the coordinating ligand and/or from charge transfer between ligand
and uranium. Owing to the presence of phenolate, these ligands could act as electron
donors. Since U(VI) has an empty valence shell, the metal can function as acceptor for
LMCT transitions. Charge transfer (LMCT) from oxide (=O) to uranyl occurred at lower
frequencies (higher wavelengths) than the Schiff base2−→U(VI) [5].
3.1.5. Thermal analysis. During thermal analysis, heating rates were set to 20 °C min−1
under nitrogen and the weight loss was measured from ambient temperature to 1000 °C.
Thermogravimetric analysis showed that uranyl complexes had very different thermal stabil-
ities. This method also established that only one solvent molecule was coordinated to the
central uranium and this solvent did not coordinate strongly. Because these complexes were
synthesized in methanol, we expected that one methanol was coordinated and it was con-
firmed by investigating the TG and TGA curves (figure 4). All thermal data are collected in
table 3.
Figure 3. The electronic spectra of H2salpyr (dark), [UO2(salpyr)(CH3CN)](light), in acetonitrile.

3638
Z. Asadi et al.
Figure 4. TG and DTG and DTA diagram of [UO2(salpyr)(MeOH)].
Table 3. Thermal analyzes data for uranyl Schiff base complexes.
(Wt. loss %) Calcd
(found)
Temp. range in DTA (peak) Decomposition
Complex (MW)
TG (°C)
endo/exo
assignment
[UO2(salpyr)(MeOH)]
C20H17N3O5U
5.2 (6.0)
12.5 (10.3)
100–278
316–607
Endo
Exo
Loss of MeOH
Loss of C5NH3
(617.40)
[UO2(3-MeOsalpyr)
(MeOH)]·0.8H2O
C22H21N3O7U
2.1 (2.0)
4.6 (3.5)
19.1 (19.5)
44–98
100–200
200–747
Endo
Endo
Exo
Loss of H2O
Loss of MeOH
Loss of C7N3H5
(691.85)
[UO2(4-MeOsalpyr)(MeOH)]
C22H21N3O7U
(677.45)
[UO2(5-MeOsalpyr)(MeOH)]
C22H21N3O7U
(677.45)
[UO2(5-Brsalpyr)(MeOH)]
C17H16N2O5Br2U
(742.8)
4.7 (4.0)
4.7 (5.0)
4.3 (3.2)
108–190
105–292
102–288
Endo
Endo
Endo
Loss of MeOH
Loss of MeOH
Loss of MeOH
[UO2(5-Clsalpyr)(MeOH)]
C18H20N2O5U
4.6 (5.5)
11.2 (7.0)
120–289
322–423
Endo
Exo
Loss of MeOH
Loss of C5NH3
(686.29)
3.2. The electrochemical study of uranyl complexes
In order to investigate the effect of substituents of the Schiff base on the oxidation and
reduction potentials of [UO2(L)(CH3CN)], uranyl Schiff base complexes were studied by
cyclic voltammetry. Cyclic voltammetry measurements for uranyl complexes in acetonitrile
(1 × 10−3 M) and TBAP (tetrabutylammoniumperchlorate) (0.10 M) as the supporting elec-
trolyte were carried out at room temperature from −0.65 to −1.28 V at scan rates

Uranyl Schiff base complexes
3639
V = 0.1 V/s. Typical cyclic voltammograms of [UO2(L)(CH3CN)] in the potential range
from 0.0 to −1.3 V (versus Ag/AgCl) are shown in figure 5. A reduction peak observed at
ca. −1.00 V with [UO2(L)(CH3CN)], reduced to the mono anion [UO2(L)(CH3CN)]− in a
quasi-reversible one electron step (Equation 1).
½UVIO2ðLÞCH3CN] + e ! ½UVO2ðLÞCH3CN]ꢀ
(1)
Upon 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 4. 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 5-MeO < 3-MeO < 4-MeO. Methoxy para-
(5-MeO) and ortho- (3-MeO) were electron releasing, while a methoxy meta- (4-MeO) was
a weaker electron releasing group. This accelerated reduction of 4-MeO related to 5-MeO
and 3-MeO.
3.3. Kinetic study of the solvent exchange with PBu3 on uranium center
The kinetics of the interaction between the uranyl Schiff base complexes and PBu3 were
studied (Equation 2).
½UO2ðLÞðCH3CNÞꢁ þ PBu3 ! ½UO2ðLÞðPBu3Þꢁ þ CH3CN
(2)
Figure 5. Cyclic voltammograms of [UO2(L)(CH3CN)], where L = 3-MeOsalpyr(––), 4-MeOsalpyr(–––),
5-MeOsalpyr(….), in acetonitrile at room temperature. Scan rate: 0.1 V/s.
Table 4. Redox potential data of uranyl complexes in acetonitrile solution.
Compound
Epa(V→VI)
Epc(VI→V)
E1/2(V↔VI)
[UO2(salpyr)(CH3CN)]
−0.784
−0.788
−0.791
−0.769
−0.784
−0.784
−1.010
−1.001
−0.972
−1.010
−1.003
−1.023
−0.897
−0.894
−0.881
−0.889
−0.893
−0.903
[UO2(3-MeOsalpyr)(CH3CN)]
[UO2(4-MeOsalpyr)(CH3CN)]
[UO2(5-MeOsalpyr)(CH3CN)]
[UO2(5-Brsalpyr)(CH3CN)]
[UO2(5-Clsalpyr)(CH3CN)]

3640
Z. Asadi et al.
By dissolving the complexes in acetonitrile, methanol was exchanged with acetonitrile, thus
acetonitrile occupied the fifth position in the equatorial plane. Plots of kobs versus [PBu3]
exhibited a nonzero intercept. Thus the rate-law for the reaction was (Equations 3–5):
R ¼ fk1 þ k2½PBu3ꢁg½complexꢁ
under pseudo-first-order conditions:
(3)
(4)
(5)
R ¼ kobs½complexꢁ
where
kobs ¼ k1 þ k2½PBu3ꢁ
k2 is the second-order rate constant (scheme 1, Path 1) and k1 is the first-order rate constant
(scheme 1, Path 2).
The rate constants and the activation parameters are collected in tables 5–11. The mecha-
nism shown in scheme 1 was suggested for the reaction of uranyl complex with PBu3.
Two paths were suggested for the reaction. In Path 1, PBu3 was added to the uranium
center by an associative mechanism with a rate constant k2, with the suggested structures
shown in scheme 1. An eight-coordinate intermediate was formed and then dissociation of
the solvent gave a seven-coordinate product. In Path 2 (a dissociative path) with k1 rate con-
stant a six-coordinate intermediate was formed and PBu3 coordinated to uranium in the fast
step.
Variation of the electronic spectra for [UO2(salpyr)(CH3CN)] reacting with an excess of
PBu3 at 10 °C is shown in figure 6.
The k2 values were obtained from the slope of linear plots of kobs versus the donor con-
centration [PBu3] and the k1 values were obtained from their intercept (figure 7).
The activation parameters of the studied systems were calculated by using the Eyring
Equation (6):
lnðk2=TÞ ¼ ꢀDH#=RT þ DS#=R þ 23:8
(6)
O
U
O
O
U
O
O
U
O
O
O
O
O
O
O
CH3CN
fast
Path 1
N
N
N
N
N
N
k2
CH3CN
PBu3
+PBu3
-CH3CN
PBu3
k1
t
s
a
f
-
C
O
U
O
H
O
O
u 3
3 C
B
P
N
N
N
+
Path 2
Scheme 1. Suggested paths for the reaction mechanism.

Uranyl Schiff base complexes
3641

3642
Z. Asadi et al.

Uranyl Schiff base complexes
3643
Table 7. Pseudo-first-order rate constants, 103kobs (s−1), k1 (s−1), k2 (M−1 s−1), for the reaction of
a
a
a
[UO2(5-Brsalpyr)(CH3CN)] with PBu3 in acetonitrile at different temperatures. [Complex] = 2.3 × 10−5 M.
103[PBu3]/M (°C) 7.48
14.96
22.44
29.92
37.40
44.88
59.84 103k1/s−1 101k2/M−1 s−1
10
20
30
40
8.0(0.1) 8.9(0.1) 13.0(0.1) 14.1(0.2) 14.7(0.2) 20.0(0.3) 25.0(0.7) 4.7(1.0)
3.3(0.3)
3.6(0.4)
4.0(0.0)
4.3(0.4)
8.1(0.0)
8.3(0.2)
13.5(0.2) 14.3(0.8) 18.3(0.3) 22.1(0.0)
13.7(0.3) 19.5(0.2) 23.0(0.8) 29.0(1.2) 5.0(0.3)
4.9(1.1)
8.4(0.3) 9.0(0.3) 16.8(0.5) 18.4(0.7) 22.2(0.6) 24.3(1.0) 30.0(1.1) 5.1(1.3)
Note: The numbers in parentheses are the standard deviations of kobs
.
Table 8. Pseudo-first-order rate constants, 103kobs (s−1), k1 (s−1), k2 (M−1 s−1), for the reaction of
a
a
a
[UO2(5-MeOsalpyr)(CH3CN)] with PBu3 in acetonitrile at different temperatures. [Complex] = 3.2 × 10−5 M.
103[PBu3]/M (°C) 7.5
15.0
22.5
30.0
37.5
45.0
52.5
60. 0 103k1/s−1 102k2/M−1 s−1
10
20
30
40
1.3(0.0) 1.4(0.1) 1.5(0.2) 1.6(0.2) 1.7(0.4) 1.8(0.5) 1.8(0.7) 1.9(1.0) 1.2(0.0)
3.2(0.1) 3.2(0.0) 3.3(0.2) 3.4(0.8) 3.6(0.3) 3.8(0.0) 3.9(0.8) 3.9(0.8) 3.0(0.0)
5.1(0.2) 5.2(0.2) 5.5(0.3) 5.6(0.3) 5.8(0.2) 5.9(0.8) 6.1(1.0) 6.1(1.2) 4.9(0.0)
8.0(0.3) 8.3(0.3) 8.6(0.5) 8.8(0.7) 9.1(0.6) 9.4(1.0) 9.4(1.0) 9.8(1.1) 7.7(0.0)
1.2(0.3)
1.6(0.0)
2.1(0.1)
3.3(0.1)
Note: The numbers in parentheses are the standard deviations of kobs
.
Table 9. Pseudo-first-order rate constants, 103kobsa (s−1), k1a (s−1), k2 (M−1 s−1), for the reaction of [UO2(4-MeO-
a
salpyr)(CH3CN)] with PBu3 in acetonitrile at different temperatures. [Complex] = 2.9 × 10−5 M.
103[PBu3]/M (°C) 7.50
11.25 15.00 18.75 22.50 26.25 30.00 33.75 103k1/s−1 102k2/M−1 s−1
10
20
30
40
0.8(0.1) 0.8(0.2) 0.8(0.1) 0.9(0.2) 0.9(0.2) 0.9(0.4) 1.5(0.6) 1.0(0.8) 0.7(0.0)
1.3(0.0) 5.1(0.3) 1.3(0.2) 1.4(0.8) 1.4(0.3) 5.7(1.0) 3.2(0.9) 3.3(1.2) 1.1(0.0)
0.8(0.5)
1.1(0.0)
2.4(0.3)
3.8(0.1)
2.7(0.2)
2.9(0.3) 3.0(0.4) 3.2(0.2)
5.5(0.6) 5.7(0.9)
5.8(0.9)
2.5(0.0)
4.7(0.0)
5.0(0.3) 5.1(0.6) 5.2(0.5)
Note: The numbers in parentheses are the standard deviations of kobs
.
Table 10. Pseudo-first-order rate constants, 103kobsa (s−1), k1a (s−1), k2a (M−1 s−1), for the reaction of [UO2(salpyr)
(CH3CN)] with PBu3 in acetonitrile at different temperatures. [Complex] = 2.7 × 10−5 M.
103[PBu3]/M (°C) 7.5
15.0
22.5
30.0
37.5
45.0
52.5
60. 0 103k1/s−1 102k2/M−1 s−1
10
20
30
40
0.9(0.1) 1.0(0.2) 1.0(0.2) 1.1(0.4) 1.1(0.3)
1.5(0.1) 1.5(0.3) 1.6(0.2) 1.7(0.8)
2.5(0.2) 2.5(0.1) 2.7(0.3) 2.8(0.5) 2.9(0.2) 3.1(0.7) 3.4(1.0) 3.5(1.2) 2.2(0.0)
4.0(0.3) 4.7(0.3) 4.8(0.5) 5.2(0.9) 5.1(0.6) 5.5(1.0) 5.8(0.9) 6.0(0.7) 4.0(1.0)
1.3(1.2) 0.9(0.2)
1.9(0.4) 1.5(0.6) 2.1(0.5) 1.3(0.0)
0.8(0.5)
1.4(0.0)
2.1(0.3)
3.5(0.1)
Note: The numbers in parentheses are the standard deviations of kobs
.
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(salpyr)(CH3CN)] is shown in figure 8. Also the ΔG#
values were calculated using Equation (7) at T = 313 K.

3644
Z. Asadi et al.
Table 11. Activation parameters ΔH#, ΔS# and ΔG# for the reaction of [UO2(Ln)(CH3CN)]
with PBu3 in acetonitrile. The standard deviations are in parentheses.
ΔH#/kJ M−1
ΔS#/JK−1 M−1
aΔG#/kJ M−1
Salpyr
3.4 (0.1) × 10−4
5.0 (0.5) × 10−5
3.8 (0.4) × 10−4
2.1 (0.2) × 10−4
2.0 (0.2) × 10−5
4.4 (0.1) × 10−5
−163.5(4.3)
−243.4(1.7)
−151.5(13.8)
−206.2(8.0)
−254.7(0.5)
−238.7(0.4)
8.7(0.0)
4.2(0.1)
8.5(0.0)
8.9(0.0)
2.1(0.0)
2.2(0.1)
3-MeOsalpyr
4-MeOsalpyr
5-MeOsalpyr
5-Clsalpyr
5-Brsalpyr
Note: Calculated from ΔG# = −RT ln k2 at T = 40 °C.
Figure 6. The absorption spectral changes with the isosbestic points during the reaction of [UO2(salpyr)
(CH3CN)], with an excess of PBu3 at 10 °C in acetonitrile.
Figure 7. The plots of kobs vs. [PBu3] for [UO2(3-MeOsalpyr)(CH3CN)] at different temperatures.
DG# ¼ ꢀRT ln k2
(7)
The k2 values for the ligands showed high span, suggesting the dependence of the rate
constant on the nature of the complex. The low ΔH# values and the large negative ΔS#
values were compatible with an associative (A) mechanism. For studying the effect of
substituents, we compared three complexes – 5-Br, 5-Cl, and 5-MeO so that the position
of the group was the same but complexes differ in electronic properties; the obtained
trend showed that k2 for [UO2(5-MeOsalpyr)(CH3CN)] was the smallest because of the

Uranyl Schiff base complexes
3645
Figure 8. Eyring plot for the reaction of [UO2(salpyr)(CH3CN)] with excess PBu3 in acetonitrile.
electron releasing group on the aromatic ring, while [UO2(5-Clsalpyr)(CH3CN)] showed
the highest k2 because of the electron withdrawing group. Existence of the chloro made
the Schiff base complexes better acceptors. In [UO2(5-Brsalpyr)(CH3CN)], there was
again an electron withdrawing group that was not very different from Cl and k2 values
confirmed this.
Then three other complexes – 3-MeO, 4-MeO, and 5-MeO were compared, but in this
case the positions of the substituent differed. Methoxy is an electron releasing group, but
this effect becomes strongest when methoxy was located para- to the oxygen of the phenyl
ring. Thus k2 values for 5-MeO were smaller than 4-MeO. When MeO was located in the
ortho- position (3-MeO), the steric hindrance became more important than the electronic
effect. According to the proposed mechanism, solvent in the 3-MeO complex was coordi-
nated weakly, thus PBu3 could interact with the complex faster than for 4-MeO and 5-MeO
complexes.
Both electronic and steric effects took part during the exchange reaction, and the follow-
ing trend was obtained: UO2(5-Clsalpyr) > UO2(5-Brsalpyr) > UO2(3-MeOsalpyr) >
UO2(4-MeOsalpyr) > UO2(salpyr) > UO2(5-MeOsalpyr).
4. Conclusion
We synthesized uranyl asymmetric Schiff base complexes. By evaluating the X-ray
structure of [UO2(3-MeOsalpyr)(DMF)], one DMF coordinates weakly to uranium, while
the other was not coordinated. Both DMF molecules were disordered. The presence of
coordinated solvent was confirmed by thermal gravimetric studies. The kinetics and mecha-
nism of solvent exchange reactions with tributylphosphine were investigated in acetonitrile.
PBu3 acted as a donor and replaced the weakly coordinated solvent. High span of k2 as well
as low ΔH# and large negative ΔS# suggested an associative (A) mechanism. Both
electronic and steric effects were important for the rate of the exchange reaction. The trend
of k2 values showed that electron withdrawing groups like Cl, Br, and MeO in the meta
position to the phenolic oxygen of the Schiff base accelerated the exchange reaction. On
the other hand, electron releasing groups like MeO in the para-position to the phenolic
oxygen of the Schiff base reduced the rate of the exchange reaction, because the uranium
center was not very electron deficient. The steric effect was important in 3-MeO complex.

3646
Z. Asadi et al.
Cyclic voltammetry was used to investigate the effect of substituents of ligands on
reduction and oxidation of uranium U(VI) ↔ U(V). Again, electron withdrawing groups
accelerated the reduction of uranium.
Supplementary material
CCDC 926,790 contains the supplementary crystallographic data for this article. These data
can be obtained free of charge from The Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif. Supplemental data for this article can be accessed
http://dx.doi.org/10.1080/00958972.2013.849343.
Acknowledgments
We are grateful to the Shiraz University Research Council for its financial support. The project
P204/11/0809 of the Grant agency of the Czech Republic supported the crystallographic part of
the work.
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