Synthesis, X-ray crystallography, thermal studies, spectroscopic and electrochemistry investigations of uranyl Schiff base complexes

Article abstract of DOI:10.1016/j.saa.2012.12.024

Some tetradentate salen type Schiff bases and their uranyl complexes were synthesized and characterized by UV-Vis, NMR, IR, TG, C.H.N. and X-ray crystallographic studies. From these investigations it is confirmed that a solvent molecule occupied the fifth position of the equatorial plane of the distorted pentagonal bipyramidal structure. Also, the kinetics of complex decomposition by using thermo gravimetric methods (TG) was studied. The thermal decomposition reactions are first order for the studied complexes. To examine the properties of uranyl complexes according to the substitutional groups, we have carried out the electrochemical studies. The electrochemical reactions of uranyl Schiff base complexes in acetonitrile were reversible.

Full text of DOI:10.1016/j.saa.2012.12.024

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, X-ray crystallography, thermal studies, spectroscopic
and electrochemistry investigations of uranyl Schiff base complexes
⇑
Zahra Asadi , Mohammad Ranjkesh Shorkaei
Chemistry Department, College of Sciences, Shiraz University, Shiraz, Islamic Republic of Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ORTEP diagram and atom numbering scheme for the complex [UO2(salen)(DMF)].
"
The X-ray of uranyl Schiff base
complex shows the existence of
solvent in the fifth position of the
equatorial plane.
"
"
"
Kinetic of thermal decomposition of
the complexes have been studied
according to Coats–Redfern plots.
Thermal decomposition of the
studied complexes is first-order in
all stages.
Electrochemical studies show the
difference in redox potential of the
complexes according to the
substitutional groups.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 October 2012
Received in revised form 5 December 2012
Accepted 6 December 2012
Available online 28 December 2012
Some tetradentate salen type Schiff bases and their uranyl complexes were synthesized and character-
ized by UV–Vis, NMR, IR, TG, C.H.N. and X-ray crystallographic studies. From these investigations it is
confirmed that a solvent molecule occupied the fifth position of the equatorial plane of the distorted pen-
tagonal bipyramidal structure. Also, the kinetics of complex decomposition by using thermo gravimetric
methods (TG) was studied. The thermal decomposition reactions are first order for the studied com-
plexes. To examine the properties of uranyl complexes according to the substitutional groups, we have
carried out the electrochemical studies. The electrochemical reactions of uranyl Schiff base complexes
in acetonitrile were reversible.
Keywords:
X-ray crystallography
Schiff base
Ó 2012 Elsevier B.V. All rights reserved.
Synthesis
Uranyl complexes
Thermal studies
Electrochemical studies
Introduction
Interest in the coordination chemistry of uranium has recently
increased for several reasons. The primary reasons are the reduc-
The coordination chemistry of uranyl species is an area of
chemistry that has been largely investigated in recent times, espe-
cially when compared to the intense interest involving coordina-
tion compounds of the lanthanoids and transition metals [1,2].
tion of nuclear waste generated from reactor fuel along with the
extraction of uranium from sea water, ground water and soil [3–6].
Various ligands have been used for the selective extraction of
uranium, such as organic phosphorus oxides [7], crown ethers, aza-
crown, calixarenes [8], and Schiff base ligands [9]. Salen type Schiff
base ligands contain two oxygen and two nitrogen donor groups,
provide sufficient coordination sphere, in accordance with the
length of carbon-chain linkage, that allow metal ions to find an
⇑
Corresponding author. Tel.: +98 711 228 4822; fax: +98 711 228 6008.
E-mail addresses: zasadi@shirazu.ac.ir, zahraasadi83@yahoo.com (Z. Asadi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.12.024

Z. Asadi, M.R. Shorkaei / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
345
easy approach to bind to the Schiff base. Indeed, it is this steric
freedom that makes them an ideal class of ligands for both transi-
tion metals and the f-block elements [10].
kmax (nm),
e
(Mꢀ1 cmꢀ1) = 220(ꢁ52758), 262(ꢁ27172), 300(sh),
330(ꢁ5517).
N,N0-bis(4-methoxysalicylidene)-1,2-diaminoethan(4-OMesalen):
Uranyl tetra-dentate Schiff base complexes have the structure
as a distorted pentagonal bipyramidal with the solvent molecule
occupying the fifth coordination site. The most probable coordina-
tion number in the equatorial plane of U(VI) is five or six. On this
basis, a penta- or hexa-dentate planar ligand should be appropriate
to expel any L (mono-dentate ligand or solvent) from the equato-
rial plane.
The present work deals with the synthesis and characterization
of some new salen type Schiff base and their uranyl complexes, by
1H NMR, IR, TG, UV–Vis spectra, elemental microanalysis (C.H.N)
and X-ray crystallographic method. In these complexes the equato-
rial plane of U(VI) seems to be fully occupied by the coordinating O
and N atoms of the Schiff base ligands and one space for additional
solvent coordination remained.
Yield: 84%, Color: Yellow, m.p. = 175 °C, Anal. Found (Calc.):
C
18H22N2O4 (330.38): C, 65.89 (65.44); H, 6.84 (6.71); N, 8.77 (8.48).
IR (KBr, cmꢀ1): 3417 (
OAH), 2923–3039 ( CAH), 1612 ( C@N), 1303
C@C), 1H NMR (250 MHz, CDCl3, room temperature, TMS): d
t
t
t
(t
(ppm) = 3.76 (s, 6H, OCH3), 3.89 (s, 4H, CH2), 6.35–6.42 (m, 4H, ArH),
7.75 (d, 2H(a), ArH), 8.20 (s, 2H, HC@N), 13.71 (s, 2H, OH). UV–Vis (ace-
tonitrile): kmax (nm),
e
(Mꢀ1 cmꢀ1) = 215 (ꢁ40277), 230 (sh), 274
(41666), 300 (sh), 385 (ꢁ1861).
N,N0-bis(5-methoxysalicylidene)-1,2-diaminoethan(5-OMesalen):
Yield: 75%, Color: Yellow, m.p. = 165 °C, Anal. Found (Calc.):
C
18H22N2O4 (330.38): C, 65.81 (65.44); H, 6.74 (6.71); N, 8.76 (8.48).
IR (KBr, cmꢀ1): 3450 (
OAH), 2913–3000 ( CAH), 1639 ( C@N), 1492
C@C), 1H NMR (250 MHz, DMSO-d6, room temperature):
t
t
t
(t
d (ppm) = 3.69 (s, 6H, OCH3), 3.88 (s, 4H, CH2), 6.76–7.02 (m, 6H,
ArH), 8.60 (s, 2H, HC@N), 12.71 (s, 2H, OH). UV–Vis (acetonitrile): kmax
(nm),
e
(Mꢀ1 cmꢀ1) = 214 (sh), 229 (ꢁ33793), 253 (sh), 340 (ꢁ6896).
Experimental
N,N0-bis(5-bromosalicylidene)-1,2-diaminoethan(5-Brsalen):
Yield: 88%, Color: Yellow, m.p. = 199 °C, Anal. Found (Calc.): C16
H14Br2N2O2 (426.11): C, 45.43 (45.10); H, 3.03 (3.31); N, 7.06
Reagents
(6.57). IR (KBr, cmꢀ1): 3436 (
C@N), 1475 (
ture):
t
OAH), 2912–3047 (
tCAH), 1628
All chemicals were used as obtained commercially, without fur-
ther purification.
(t
t
C@C), 1H NMR (250 MHz, DMSO-d6, room tempera-
d
(ppm) = 3.81 (s, 4H, CH2), 6.82 (d, 2H(d)
, ArH,
3JðHdÞ ꢀ HðcÞ ¼ 10 Hz, 7.44 (d, 2H(c), ArH, 3JðHcÞ ꢀ HðdÞ ¼ 10 Hz), 7.65
(s, 2H(a), ArH), 8.22(s, 2H, HC@N), 13.42 (s, 2H, OH). UV–Vis (aceto-
Instruments
nitrile): kmax (nm),
(sh), 313 (ꢁ12500).
e
(Mꢀ1 cmꢀ1) = 220 (52500), 264 (18750), 278
Light-absorption measurements in the UV–visible region were
made with a Perkin–Elmer–UV–Vis spectrophotometer-Lambda
two, equipped with a Lauda-ecoline-RE thermostat. FT-IR spectra
were run on a Shimadzu FTIR-8300 spectrophotometer. 1H NMR
spectra were recorded on a Bruker Avance DPX-250 spectrometer
in DMSO-d6 or CDCl3 solvent. Elemental micro-analyses (C.H.N.)
were obtained using a Thermo Finnigan-CHNSO Analyzer. Thermal
gravimetric analyses were recorded under Perkin–Elmer Pyris Dia-
mond model. The melting points were measured in capillary tubes
by using BUCHI 535 melting point. Electrochemistry studies were
carried out using Auto lab 302N.
N,N0-bis(salicylidene)-1,3-diaminopropane(salpn): Yield: 87%,
Color: Yellow, m.p. = 180 °C, Anal. Found (Calc.):
C17H18N2O2
(282.34): C, 72.55 (72.32); H, 6.38 (6.43); N, 9.94 (9.92). IR (KBr,
cmꢀ1): 3417(
tO–H), 2890–3031 (tCAH), 1636 (tC@N), 1498 (tC@C),
1H NMR (250 MHz, CDCl3, room temperature, TMS):
d (ppm) = 2.10 (q, 2H, CH2) 3.66 (t, 4H, CH2), 6.83–7.32 (m, 8H,
ArH), 8.34 (s, 2H, HC@N), 13.43 (s, 2H, OH). UV–Vis (acetonitrile):
kmax
(nm),
e
(Mꢀ1 cmꢀ1) = 213(ꢁ43809),
254(ꢁ21904),
315(ꢁ7936).
N,N0-bis(salicylidene)-1,2-phenylenediamine(saloph):
Yield:
72%, Color: orange, m.p. = 150 °C, Anal. Found (Calc.): C20H16N2O2
Synthesis
(316.36): C, 75.86 (75.93); H, 5.04 (5.10); N, 8.35 (8.85). IR(KBr,
cmꢀ1): 3485( C@C), 1H
tOAH), 2900–3031(tCAH), 1611(tC@N), 1481(t
Synthesis of Schiff bases
NMR (250 MHz, CDCl3, room temperature, TMS): d (ppm) = 6.88–
The Schiff bases have been prepared by mixing salicylaldehyde
or substituted salicylaldehyde (1.9 mmol) with diamine
(0.95 mmol) (2:1 ratio), in methanol (25 ml) in a round bottomed
flask equipped with a condenser. The mixture was refluxed for
about 2–4 h. The products obtained after cooling, were filtered
off and washed with diethyl ether, except for saltn that obtained
oily and was used without further purification [11].
7.38 (m, 12H, ArH), 8.61 (s, 2H, HC@N), 12.90 (s, 2H, OH). UV–Vis
(acetonitrile): kmax (nm),
265(ꢁ82352), 328(ꢁ41746).
e
(Mꢀ1 cmꢀ1) = 206(sh), 226(sh),
Synthesis of uranyl Schiff base complexes, [UO2(Schiff base)(solvent)]
Uranyl acetate dehydrate (1 mmol) and synthesized Schiff base
(1 mmol) (1:1 ratio) were mixed in methanol (25 ml). The mixture
was refluxed for 3 h and then allowed to cool to room temperature.
An orange (red) precipitate was filtered off, washed with methanol,
and dried at room temperature (Scheme 1).
N,N0-bis(salicylidene)-1,2-diaminoethan(salen): Yield: 73%, Col-
or: Yellow, m.p. = 105 °C, Anal. Found (Calc.): C16H16N2O2 (268.31):
C, 71.68 (71.62); H, 5.84 (6.01); N, 10.53 (10.44). IR (KBr, cmꢀ1):
3440 (tOAH), 2885–3031 (
t
CAH), 1640 (
tC@N), 1496 (
t
C@C), 1H
[UO2(salen)(MeOH)] Yield: 85%, Color: orange, m.p. > 250 °C,
NMR (250 MHz, DMSO-d6, room temperature): d (ppm) = 3.90 (s,
4H, CH2), 6.82, 6.89 (dd, 2H(b), ArH), 7.26, 7.29 (dd, 2H(c), ArH),
7.38 (d, 2H(d), ArH), 7.41 (d, 2H(a), ArH), 8.67 (s, 2H, HC = N),
Anal. Found (Calc.):
C
17H18N2O5U (568.37):C,36.04(35.93);
OAH), 2550–
tCAH), 1627(tC@N), 1474(tC@C), 902(tU@O), 610(tUAN),
H,3.18(3.19); N, 4.87 (4.93). IR(KBr, cmꢀ1): 3440(
t
3090(
480(
13.35 (s, 2H, OH). UV–Vis (acetonitrile): kmax (nm),
e
(Mꢀ1
t
UAO). 1H NMR (250 MHz, DMSO-d6, room temperature):
cmꢀ1) = 212(ꢁ27769), 254(ꢁ3969), 315(ꢁ13276).
d (ppm) = 3.14(d, 3H, MeOH, JMe–OH = 2.5 Hz), 4.07(q, 1H, MeOH,
3JOH–Me = 2.5 Hz), 4.46 (s, 4H, CH2), 6.67 (dd, 2H(b), ArH), 6.93 (d,
2H(d), ArH), 7.51–7.60 (m, 4H(a,c), ArH), 9.42(s, 2H, HC@N). UV–
3
N,N0-bis(3-methoxysalicylidene)-1,2-diaminoethan(3-OMesalen):
Yield: 51%, Color: Yellow, m.p. = 147 °C, Anal. Found (Calc.):
C
18H22N2O4(330.38): C, 65.72(65.44); H, 6.84(6.71); N, 8.73(8.48).
IR (KBr, cmꢀ1): 3444(
OAH), 2885–3000 ( CAH), 1630 ( C@N), 1463
C@C), 1H NMR (250 MHz, CDCl3, room temperature, TMS): d
Vis
(acetonitrile):
kmax (nm),
e
(Mꢀ1 cmꢀ1) = 217(ꢁ51886),
t
t
t
230(sh), 262(sh), 334(ꢁ4411).
(t
[UO2(3-OMesalen)(MeOH)] Yield: 72%, Color: dark red,
(ppm) = 3.87 (s, 6H, OCH3), 3.96 (s, 4H, CH2), 6.77–7.26 (m, 6H,
ArH), 8.32 (s, 2H, HC@N), 13.57 (s, 2H, OH). UV–Vis (acetonitrile):
m.p. > 250 °C, Anal. Found (Calc.): C19H24N2O7U (630.44): C, 36.16
(36.20); H, 3.02 (3.84); N, 4.34 (4.44). IR(KBr, cmꢀ1): 3400(
tOAH),

346
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3
3
CH3), 3.14(d, 3H, MeOH, JMe–OH = 7 Hz), 4.07(q, 1H, MeOH, JOH–
Me = 7 Hz,), 4.40(m, 1H, CH), 4.76(d, 2H, CH2), 6.64–7.58(m, 8H,
ArH), 9.39(s, 1H(2), HC@N), 9.42(s, 1H(1), HC@N). UV–Vis (acetoni-
A
O
H2
H1
N
N
trile): kmax (nm), e
(Mꢀ1 cmꢀ1) = 220
a
c
a
c
U
(ꢁ32258), 256(sh), 334(ꢁ3419).
[UO2(salpn)(MeOH)] Yield: 71%, Color: red, m.p.>250 °C, Anal.
Found (Calc.): C18H20N2O5U (582.40): C, 37.98 (37.12); H, 3.40
b
b
B
O
O
O
(3.46); N, 4.47 (4.81). IR(KBr, cmꢀ1): 3448(
t
OAH), 2931–3016(
UAN), 509(
UAO). 1H NMR
(250 MHz, DMSO-d6, room temperature): d (ppm) = 2.05(m, 2H,
tCAH),
B
d
d
S
1620(
t
C@N), 1465(
t
C@C), 887(
t
U@O), 586(
t
t
S = solvent
A = CH2CH2 [UO2(salen)(S)]
3
3
CH2), 3.14(d, 3H, MeOH, JMe–OH = 7 Hz), 4.07(q, 1H, MeOH, JOH–
Me = 7 Hz), 4.18 (t, 4H, CH2), 6.54–7.55 (m, 8H, ArH), 9.37(s, 2H,
A = CH2CH2, B(d) = OMe [UO2(3-MeOsalen)(S)]
A = CH2CH2 , B(c) = OMe [UO2(4-MeOsalen)(S)]
A = CH2CH2 , B(b) = OMe [UO2(5-MeOsalen)(S)]
A = CH2CH2 , B(b) = Br [UO2(5-Brsalen)(S)]
A = CH2CH2CH2 [UO2(salpn)(S)]
HC = N). UV–Vis (acetonitrile): kmax (nm),
ꢁ58823), 261(sh), 333(ꢁ6482), 382(sh).
e
(Mꢀ1 cmꢀ1) = 217(-
[UO2(saloph)(MeOH)] Yield: 79%, Color: orange, m.p. > 250 °C,
Anal. Found (Calc.): C21H18N2O5U (616.41): C, 40.68 (40.92); H,
2.82 (2.92); N, 4.67 (4.54). IR(KBr, cmꢀ1): 3456(
tOAH), 2885–
3038(
t
CAH), 1607(
t
C@N), 1445(
t
C@C), 908(
tU@O), 540(
t
UAN), 440(tU
-
AO). 1H NMR (250 MHz, DMSO-d6, room temperature):
d
,
A = CH2CH(CH3) [UO2(saltn)(S)]
(ppm) = 3.14 (d, 3H, MeOH), 4.07 (q, 1H, MeOH), 6.70 (dd, 2H(b)
A = Ph [UO2(saloph)(S)]
ArH), 7.50, 7.52 (dd, 2H, ArH), 7.50–7.80 (m, 8H, ArH), 9.59 (s, 2H,
HC = N). UV–Vis (acetonitrile): kmax (nm),
ꢁ9769), 278(sh), 341(4611), 413(sh).
e
(Mꢀ1 cmꢀ1) = 240(-
Scheme 1.
Crystal structure determination
2825–3050(tCAH), 1622(tC@N), 1436(tC@C), 894(tU@O), 620(tUAN),
529(
t
UAO). 1H NMR (250 MHz, DMSO-d6, room temperature): d
Crystals of [UO2(salen)(DMF)] were obtained in good yield from
DMF by slow evaporation at room temperature. The X-ray diffrac-
tion measurements were made on a STOE IPDS 2Tdiffractometer
3
(ppm) = 3.14(d, 3H, MeOH, JMe–OH = 5 Hz), 3.94(s, 6H, OCH3)
4.07(q, 1H, MeOH, JOH–Me = 5 Hz), 4.45 (s, 4H, CH2), 6.59 (dd,
3
2H(b), ArH), 7.15–7.22 (m, 4H(a,c), ArH), 9.40(s, 2H, HC@N). UV–
with graphite monochromated Mo-K radiation. The deep-red
a
Vis (acetonitrile): kmax (nm),
267(27368), 336(4473).
e
(Mꢀ1 cmꢀ1) = 234(ꢁ55789),
block crystal was mounted on a glass fiber and used for data collec-
tion. The data collection and reduction were performed by X-AREA
[12] program. The multi-scan absorption correction was performed
by MULABS routine [13]. The structures were solved by direct
methods using SHELXS97 [13] and subsequent difference Fourier
map and then refined by a full-matrix least-squares on F2by SHEL-
XL97 in SHELXTL package [14]. The non-hydrogen atoms were re-
[UO2(4-OMesalen)(MeOH)] Yield: 80%, Color: pale orange,
m.p. > 250 °C, Anal. Found (Calc.): C19H24N2O7U (630.44): C, 36.15
(36.20); H, 3.62 (3.84); N, 4.33 (4.44). IR(KBr, cmꢀ1): 3436(
t
OAH),
2815–3029(tCAH), 1609(tC@N), 1445(tC@C), 901(tU@O), 626(t
UAN),
578(
t
UAO). 1H NMR (250 MHz, DMSO-d6, room temperature):
3
d (PPM) = 3.14(d, 3H, MeOH, JMe–OH = 6 Hz), 3.83(s, 6H, OCH3)
fined anisotropically. All of the
H atoms were positioned
4.07(q, 1H, MeOH, JOH–Me = 6 Hz), 4.39(s, 4H, CH2), 6.30(d, 2H(b)
,
3
geometrically and constrained to ride on their parent atoms with
Uiso(H) = 1.2 or 1.5Ueq(C). All refinements were performed using
the SHELXTL crystallographic software package. All calculations
were done using PLATON [15].
ArH), 6.44(s, 2H(d), ArH), 7.47(d, 2H(a), ArH), 9.32(s, 2H, HC@N).
UV–Vis (acetonitrile): kmax (nm),
245(ꢁ8787), 277(sh), 324(sh).
e
(Mꢀ1 cmꢀ1) = 222(ꢁ7651),
[UO2(5-OMesalen)(MeOH)] Yield: 69%, Color: dark red,
m.p. > 250 °C, Anal. Found (Calc.): C19H24N2O7U (630.44): C, 36.17
(36.20); H, 3.64 (3.84); N, 4.31 (4.44). IR(KBr, cmꢀ1): 3440(
tOAH),
Results and discussion
2844–3000(tCAH), 1626(tC@N), 1474(tC@C), 903(tU@O), 600(t
UAN),
508(
t
UAO). 1H NMR (250 MHz, DMSO-d6, room temperature): d
Crystal structure of [UO2(salen)(DMF)]
3
(ppm) = 3.14(d, 3H, MeOH, JMe–OH = 5 Hz), 3.32(s, 6H, OCH3)
3
4.07(q, 1H, MeOH, JOH–Me = 5 Hz), 4.44 (s, 4H, CH2), 6.87–7.47
The solid state structure of the complex was determined by sin-
gle-crystal X-ray diffraction. The ORTEP view of the complex is
shown in Fig. 1. Crystallographic data and refinement parameters
of the complex are listed in Table 1.
(m, 6H, ArH) 9.71(s, 2H, HC@N). UV–Vis (acetonitrile): kmax (nm),
e
(Mꢀ1 cmꢀ1) = 222(ꢁ117886) 241(ꢁ117880), 360(ꢁ6458).
[UO2(5-Brsalen)(MeOH)] Yield: 76%, Color: orange, m.p. >
250 °C, Anal. Found (Calc.): C17H16Br2N2O5U (726.16): C, 28.13
It was found that the complex has a pentagonal-bipyramidal
geometry with an axial O@U@O moiety. Generally the salen ligand
is known to be planar in its metal complexes. The structure revealed
that the UO2 is in a nearly planar environment. The coordination
geometry around UO2 is nearly planar with the dihedral angle of
7.2(5)° between coordination planes of N1AU1AO1 and
N2AU1AO2. Atoms C8, C9 are significantly out of plane, as can be
indicated from the torsion angle of N1AC8AC9AN2, which is
55.6(14)° and with maximum deviation from the mean plane of
0.28(2) and ꢀ0.44(2) Å for C8 and C9 atoms, respectively. The U@O
bond distances in the uranyl moiety are 1.726(9) Å[U(1)AO(3)]and
1.710(9) Å[U(1)AO(4)], which are typical values for uranyl com-
pounds [3,11]. The O(3)AU(1)AO(4) bond angle [178.1(3)°] indicate
that the uranyl moiety is slightly bent in the direction opposite to the
(28.12); H, 1.91 (2.22); N, 3.96 (3.86). IR(KBr, cmꢀ1): 3427(
tOAH),
2885–3000(tCAH), 1620(tC@N), 1459(tC@C), 902(tU@O), 636(t
UAN),
517(
t
UAO). 1H NMR (250 MHz, DMSO-d6, room temperature):
3
d (ppm) = 3.14(d, 3H, MeOH, JMe–OH = 5 Hz), 4.07(q, 1H, MeOH,,
3JOH–Me = 5 Hz), 4.47(s, 4H, CH2), 6.92(d, 1H(d), ArH), 7.64(d, 1H(c)
,
ArH), 7.78(s, 1H(a), ArH), 9.43(s, 2H, HC = N). UV–Vis (acetonitrile):
kmax (nm),
e
(Mꢀ1 cmꢀ1) = 223(ꢁ15400),
237(sh),
260(sh),
331(2000).
[UO2(saltn)(MeOH)] Yield: 66%, Color: red, m.p.>250 °C, Anal.
Found (Calc.): C18H21N2O5U (583.40): C, 36.94 (37.06); H, 3.33
(3.63); N, 4.90 (4.80). IR(KBr, cmꢀ1): 3408(
t
OAH), 2885–3050(
UAN), 460(
UAO). 1H NMR
(250 MHz, DMSO-d6, room temperature): d (ppm) = 1.30(d, 3H,
tCAH),
1628(tC@N), 1445(
t
C@C), 902(
t
U@O), 590(
t
t

Z. Asadi, M.R. Shorkaei / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
347
Fig. 1. ORTEP diagram and atom numbering scheme for the complex [UO2(salen)(DMF)].
Table 1
Crystal
[UO2(salen)(DMF)].
Table 2
data,
data-collection
and
structure-refinement
parameters
of
Selected bond length (Å) and angles (°) for [UO2(salen)(DMF)].
U(1)AO(4)
U(1)AO(3)
U(1)AO(1)
U(1)AO(2)
U(1)AO(5)
U(1)AN(2)
U(1)AN(1)
O(1)AU(1)AN(1)
O(2)AU(1)AN(2)
N(1)AC(8)AC(9)AN(2)
1.710(9)
Empirical formula
Formula weight
T (K)
C19H21N3O5U
609.42
291(2)
0.71073 Å
Monoclinic
P 2(1)/c
1.726(9)
2.248(8)
2.251(8)
2.450(7)
2.554(8)
2.586(9)
70.3(3)
k (Mo K
a)
Crystal system
Space group
Unit cell dimensions
a = 9.815(2) Å
b = 17.026(3) Å
c = 12.704(3) Å
V (Å3)
a
= 90°
b = 111.20(3)°
= 90°
69.5(3)
55.6(14)
c
1979(7)
Z
Dcalc.
l
4
2.045 Mg/m3
8.236
(mmꢀ1
)
F(000)
Crystal size (mm)
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 29.20°
Absorption correction
Max. and min. transmission
Refinement method
1152
Table 3
0.15 ꢄ 0.11 ꢄ 0.08
Hydrogen bond parameters for [UO2(salen)(DMF)]. [A° and °].
2.09 to 29.20°
ꢀ10 6 h 6 13, ꢀ23 6 k 6 20, ꢀ17 6 l 6 17
10447
5162 [R(int) = 0.0846]
96.1%
DAH...A
d(D-H)
d(H...A)
d(D...A)
<(DHA)
C(17)AH(17A)...O(1)
C(18)AH(18C)...O(3)#1
0.93
0.96
2.47
2.51
3.023(13)
3.343(16)
119
145
Semi-empirical from equivalents
1.000 and 0.793
Symmetry transformations used to generate equivalent atoms: #1x, ꢀy ꢀ 3/2, z ꢀ 1/2.
Full-matrix least-squares on F2
5162/0/255
Data/restraints/parameters
Goodness-of-fit on F2
0.855
UV–Vis spectra
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r
(I)]
R1 = 0.0547, wR2 = 0.1131
R1 = 0.1180, wR2 = 0.1275
2.018 and ꢀ4.162 e Åꢀ3
The electronic spectra data of the ligands and their complexes
in acetonitrile are mentioned in the experimental section. The elec-
tronic spectra of the ligands feature almost three intense bands
system. The first band at higher energy is attributed to
sitions associated with the phenyl ring and the band at lower en-
p?
pꢃ tran-
coordination of DMF. The U(1)AO(1) and U(1)AO(2) bond distances
[2.248(8), 2.251(8) Å] are shorter than U(1)AN(1) and U(1)AN(2)
[2.586(9), 2.554(8) Å]. Such a difference might imply that the coor-
dination of the oxygen atoms in salen is stronger than the coordina-
tion of the nitrogen atoms. The bond distance between the oxygen
atom of DMF and uranium is 2.450(7) Å [U(1)AO(5)] which is longer
than those of U(1)AO(1) and U(1)AO(2) (Table 2).
The geometry of hydrogen bond is given in Table 3. The crystal
packing of the complex shows a one dimensional infinite chain
through C–Hꢂ ꢂ ꢂO interactions along the c-axis (Fig. 2).
ergy arise from
p?
pꢃ transitions associated with the azomethine
chromophore. The furthest energy band is n?pꢃ transitions involv-
ing the promotion of one of the lone pair electrons of the nitrogen
atom to the antibonding
group. Usually n?pꢃ transitions involving nitrogen atoms occur at
lower energies and are less intense than the
pꢃ transitions. Sub-
stitution on the ring, by auxochromes and chromophores causes
red shift of these bands.
p orbital associated with the azomethine
p
?

348
Z. Asadi, M.R. Shorkaei / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
In the spectra of complexes because of the strong intensity of
the characteristic absorption bands, this absorption band can be
assigned to an electronic-dipole allowed transition arising from
the coordinating ligand and/or from charge transfer between li-
gand and uranium atom. Owing to the presence of phenolate
groups these ligands can act as electron donors. Since uranyl com-
plexes contain U(VI) with an empty valence shell the metal center
is only capable to function as acceptor site for LMCT transitions. It
seems that charge transfer band (LMCT) from oxide (=O) to uranyl
occurred at lower frequencies (higher wavelengths) than Schiff
base2- ? U(VI).
NMR spectra
In 1H NMR spectra, peaks due to Schiff base have down field
shift when coordinated to uranyl; peak due to OH of the Schiff base
is not observed after formation of the uranyl complex. A significant
shift in imine HC@N proton is observed between the free ligands
(8.2–8.4 ppm) and metal complexes (9.2–9.5 ppm), indicating
involvement of the lone pairs on nitrogen with the metal center.
All 1HNMR of the complexes monitored in DMSO-d6 solvent,
thus DMSO is coordinated to the central uranium and was replaced
the MeOH from the coordination sphere. Peaks due to MeOH are
observed in 1H NMR as a quarted at 4.07 ppm (OH) and a doublet
at 3.14 ppm (CH3).
IR spectra
The IR spectra of ligands and complexes are also obtained. Com-
plexes exhibits a vibration at about 902 cmꢀ1 which we have as-
signed to
masym(U@O). Peaks for the carbon nitrogen imine
stretching in the spectra of the free Schiff base seen to shift to low-
er frequencies in the uranyl complexes and it indicates the coordi-
nation of imine nitrogens to the metal center.
Thermal studies (TG)
Fig. 2. Packing diagram for complex [UO2(salen)(DMF)], shows a one-dimensional
infinite chain through CAHꢂ ꢂ ꢂO interactions along the c-axis. Intermolecular
interactions are shown as dashed line.
Thermogravimetric analysis shows that uranyl complexes have
very different thermal stability. This method was used also to
establish that: (i) Only one solvent molecule is coordinated to
100.0
2.200
600.0
Sh
2
[UO2(salen)(MeOH)]
95.0
90.0
85.0
80.0
75.0
70.0
65.0
60.0
55.0
50.0
1.000
0.500
0.000
-0.500
-1.000
-1.500
-2.000
500.0
400.0
300.0
200.0
100.0
0.0
2.000
1.800
1.600
1.400
1.200
1.000
100.0
200.0
300.0
400.0
500.0
600.0
700.0
Temp Cel
Fig. 3. TG, DTA and DTG of [UO2(salen)(MeOH)].

Z. Asadi, M.R. Shorkaei / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
349
100.0
90.0
80.0
70.0
60.0
50.0
40.0
80.0
60.0
[UO2(salen)(DMF)]
1
200.0
150.0
100.0
50.0
180.0
160.0
140.0
120.0
100.0
80.0
40.0
20.0
0.0
-20.0
-40.0
-60.0
-80.0
-100.0
-120.0
0.0
200.0
400.0
Temp Cel
600.0
800.0
Fig. 4. TG, DTA and DTG of [UO2(salen)(DMF)].
Table 4
Thermal analyses data for uranyl Schiff base complexes.
a
Complex (M.W.)
TGA (Wt. loss%) calc. (found)
Temp range in TG (°C)
DTA (peak) Endo/Exo
Decomposition assignment
[UO2(salen)(MeOH)]
17H18N2O5U (568.37)
[UO2(salen)(DMF)]
19H21N3O5U (609.42)
[UO2(3-MeOsalen)(MeOH)]H2O
19H22N2O7U
5.6 (6.0)
150–210
420–460
110–200
390–520
30–240
320–380
405–550
80–170
360–400
140–230
375–410
450–550
185–235
360–392
420–550
135–230
380–420
460–550
Endo
Exo
Endo
Exo
Endo
Endo
Exo
Endo
Exo
Endo
Exo
Exo
Endo
Exo
Exo
Loss of MeOH
Loss of C16H14N2O
Loss of DMF
Loss of C16H14N2O4
Loss of MeOH + H2O
Loss of C2H4N2
Loss of C2H6O4
Loss of MeOH
Loss of C18H18O5N2
Loss of MeOH
Loss of C2H4N2
Loss of C16H14O2
Loss of MeOH
Loss of C2H4N
Loss of C14H8Br2N
Loss of MeOH
C
44.0 (44.0)
12.0 (12.0)
48.9 (44.1)
7.6 (7.0)
8.9 (9.1)
15.0 (15.1)
5.0 (5.8)
54.4(54.0)
5.1 (5.0)
8.9 (9.0)
37.9 (36.0)
4.4 (4.1)
5.8 (6.0)
C
C
(628.42)
[UO2(4-MeOsalen)(MeOH)]
C
19H22N2O7U (628.42)
[UO2(5-MeOsalen)(MeOH)]
19H22N2O7U
C
(628.42)
[UO2(5-Brsalen)(MeOH)]
C
17H16N2O5Br2U
(726.16)
[UO2(saltn)(MeOH)]
48.2 (48.0)
5.5 (5.5)
7.2 (9.5) 36.7 (37.0)
Endo
Exo
Exo
C
18H20N2O5U
Loss of C3H6
Loss of C12H10O2N2
(582.40)
a
Percentage of weight lose found (calculated).
the central uranium and (ii) This solvent molecule did not coordi-
nate strongly and removed more easily than the tetra-dentate
ligand and also trans oxides. Because these complexes were syn-
thesized in methanol we expected that one methanol molecule is
coordinated to U and when the complex was synthesized in
DMF, DMF has replaced the coordinated methanol (Figs. 3 and 4).
A general decomposition pattern is concluded, whereby the
complexes decomposed in three stages. Besides these three
decomposition stages, those complexes which have lattice water,
exhibited additional decomposition step. The first decomposition
stage is the loss of the coordinated methanol or DMF molecule,
after that the deligation process started (Table 4).
Kinetic aspects of thermal studies
Fig. 5. Coats–Redfern plots of [UO2(saltn)(MeOH)], step 1.
In the kinetic study of decomposition Coats–Redfern equation
was used [16,17]. Kinetic parameters (activation energy E and
pre-exponential factor A) values were calculated from this Eq. (1).
ꢀ
ꢁ
ꢀ
ꢁ
ꢀlogð1 ꢀ aÞ
AR
bE
2RT
E
E
Log
¼ log
1 ꢀ
ꢀ
ð1Þ
T2
2:303RT

350
Z. Asadi, M.R. Shorkaei / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
Table 5
Thermal and kinetic parameters for uranyl complexes.
Compounds
D
T (°C)a
Eꢃ (kJ molꢀ1
)
Aꢃ (sꢀ1
)
Sꢃ (kJ molꢀ1 Kꢀ1
)
Hꢃ (kJ molꢀ1
)
Gꢃ (kJ molꢀ1
)
[UO2(salen)(MeOH)]
150–210
420–460
170–200
390– 520
30–240
320–380
405–550
80–170
360–400
140–230
375–410
450–550
185–235
360–392
420–560
120–230
370–420
59.61
23.59
78.03
50.94
42.69
33.84
24.15
54.35
33.64
70.64
50.21
45.03
104.49
84.85
63.43
60.97
49.49
7.36 ꢄ 106
1.85 ꢄ 102
2.43 ꢄ 109
2.24 ꢄ 104
3.45 ꢄ 106
1.84 ꢄ 103
2.77 ꢄ 102
4.20 ꢄ 107
8.21 ꢄ 102
6.42 ꢄ 107
1.89 ꢄ 104
6.31 ꢄ 103
1.72 ꢄ 1011
1.40 ꢄ 107
1.30 ꢄ 105
4.73 ꢄ 106
1.45 ꢄ 104
ꢀ117.1
ꢀ208.7
ꢀ68.91
ꢀ169
55.76
17.71
74.18
44.94
39.86
28.59
18.9
50.72
28.21
66.65
44.74
38.49
100.47
79.45
56.64
57.14
44.02
22.35
90.88
13.19
75.81
8.16
67.67
73.22
19.8
74.37
20.53
65.32
92.56
7.22
43.22
84.55
22.55
110.14
[UO2(salen)(DMF)]
[UO2(3-OMesalen)(MeOH)]
ꢀ120.8
ꢀ188.7
ꢀ204.4
ꢀ121.3
ꢀ195.6
ꢀ99.38
ꢀ169.6
ꢀ180.2
ꢀ33.83
ꢀ114.6
ꢀ155.4
ꢀ120.3
ꢀ171.8
[UO2(4-OMesalen)(MeOH)]
[UO2(5-OMesalen)(MeOH)]
[UO2(5-Brsalen)(MeOH)]
[UO2(saltn)(MeOH)]
a
The temperature range of decomposition pathways.
Fig. 6. Cyclic voltammogram of uranyl complexes in acetonitrile at room temperature. Scan rate: 100 mV/s.
ðw0
where a ¼ ꢀw Þ. w0 initial mass at the sample, wt is mass of the
t
Table 6
Redox potential data of uranyl complexes in acetonitrile solution.
ðw0ꢀwf Þ
sample at temperature T, wf is the final mass at the temperature
at which the mass loss is approximately unchanged, b is the heating
rate and R is the gas constant. A plot of left hand side of Eq. (1)
against 1/T gives straight line (see for example Fig. 5). The slope
of this plot is: ꢀE/2.303R and A values can be calculated from the
intercept of the equation.
Compound
Epc(Vl ? V)
Epa(V ? Vl)
E1/2(V M Vl)
[UO2(salen)(MeOH)]
ꢀ0.952
ꢀ0.950
ꢀ0.927
ꢀ0.958
ꢀ0.982
ꢀ0.772
ꢀ0.775
ꢀ0.746
ꢀ0.789
ꢀ0.738
ꢀ0.862
ꢀ0.862
ꢀ0.836
ꢀ0.873
ꢀ0.860
[UO2(3-OMesalen)(MeOH)]
[UO2(4-OMesalen)(MeOH)]
[UO2(5-OMesalen)(MeOH)]
[UO2(5-Brsalen)(MeOH)]
The entropy of activation
D
S# can be obtained using this Eq. (2):
#=T
KTs
h
A ¼
eS
ð2Þ
energy and enthalpy of activation for each following stage is smal-
ler than previous stage. This is probably due to the unstable inter-
mediate of the proceeding stages and according to Coats–Redfern
plots (Fig. 5) the kinetic of thermal decomposition of studied com-
plexes is first-order in all stages.
where h is the Planck constant, K is the Boltzman constant and Ts is
the peak temperature from DTG. The enthalpy and free energy of
activation were calculated using Eqs. (3) and (4):
The Activation energy (E) for the first stage (missing methanol)
for 3-OMe, 4-OMe and 5-OMe complexes indicate that 3-OMe com-
plex misses methanol easier (with lower activation energy) than 4-
OMe and 5-OMe complexes. It is probably due to more steric repul-
sion causes by 3-OMe in the vicinity of the coordinated methanol.
From comparing [UO2(salen)(MeOH)] and [UO2(salen)(DMF)] it is
concluded that DMF removing is more difficult (with higher activa-
tion energy) than MeOH removing.
E ¼ H# þ RT
ð3Þ
G# ¼ H# ꢀ TS#
ð4Þ
The other complexes were studied using the same equations.
Kinetic parameters for all complexes were presented in Table 5.
By comparing the activation energy (E) for all complexes, the
following result can be obtained: for all complexes, the activation

Z. Asadi, M.R. Shorkaei / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 344–351
351
Fig. 7. Cyclic voltammogram of uranyl complexes in acetonitrile at room temperature. Scan rate : 100 m V/s.
Electrochemical studies
tional group on the meta position (4-OMe) acts as an electron accep-
tor by inductive effect.
The cyclic voltammetry of synthesized complexes were carried
out in acetonitrile at room temperature. A typical cyclic voltammo-
gram of [UO2(salen)(MeOH)] complex in the potential range from
0.0 to 1.3 V (vs. Ag/AgCl) is shown in Fig. 6 (H).
Acknowledgement
We are grateful to Shiraz University Research Council for its
financial support.
An oxidation peak is observed at about Ca. ꢀ0.77 V. [UO2
(salen)(MeOH)]ꢀ is oxidized to the [UO2(salen)(MeOH)] in a fully
reversible one electron step [18]. The electron is removed from
the nonbonding orbitals and the U(VI) complex is formed. Upon
reversal of the scan direction, the U(VI) complex is reduced to
U(V) at lower potentials. Multiple scans resulted in nearly super-
posable cyclic voltammograms, thereby showing that the seven
coordinated geometry is stable in both oxidation states, at least
on the cyclic voltammetry time scale. These results revealed that
the redox process of all uranyl Schiff base complexes is the one-
electron transfer reaction. The oxidation potentials for the different
complexes are collected in Table 6. The formal potentials
(E1/2 (VI M V)) for the U(VI/V) redox couple were calculated as
the average of the cathodic (Epc) and anodic (Epa) peak potentials
of this process.
In order to investigate the effect of Schiff base ligands’ substitu-
tional groups on oxidation potential of uranyl complexes, cyclic
voltammetry method was used. The results show that the anodic
peak potential (Epa) varies as it can be expected from the electronic
effects of the substituents at position 5. Thus, Epa becomes less neg-
ative showing the following trend; MeO < H < Br and have been
interpreted by assuming that the strong electron-withdrawing
group stabilizes the lower oxidation state while the electron donat-
ing groups have a reverse effect [19,20].
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.12.024.
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