Complexes of vanadyl and uranyl ions with a benzoxazole derivative: Synthesis, structural features and remarks on luminescence properties

Article abstract of DOI:10.1016/j.ica.2009.11.033

UO₂(NO₃)₂·6H₂O and VO(acac)₂ react with 2-(2′-hydroxylphenyl)benzoxazole (Hpbx) in methanol to give [UO₂(pbx)₂(CH₃OH)] (1) and [VO(pbx)₂] (2). Complex 1 shows a distorted pentagonal bipyrimidal geometry, characteristic for the coordination number 7. Reciprocal OH?O interactions between neighbored molecules hold 1 in a pseudo-dimeric association with an inversion center. Complex 2 achieves a distorted octahedral geometry and the molecules "stack" along the b axis through secondary interactions. The molecule heaping is wholly linear, with V^n′-O(1)^n′?V ^x′ angles of 180°. Luminescence properties of Hpbx, 1 and 2 are discussed.

Full text of DOI:10.1016/j.ica.2009.11.033

Inorganica Chimica Acta 363 (2010) 807–812
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Note
Complexes of vanadyl and uranyl ions with a benzoxazole derivative: Synthesis,
structural features and remarks on luminescence properties
a,
a
b
Davi Fernando Back a, , Gelson Manzoni de Oliveira , Marco Aurélio Ballin , Valeriano Antonio Corbellini
*
*
a Departamento de Química, Laboratório de Materiais Inorgânicos, Universidade Federal de Santa Maria, UFSM, 97115-900 Santa Maria, RS, Brazil
b Departamento de Química e Física, Universidade de Santa Cruz, UNISC, Santa Cruz do Sul, RS, Brazil
a r t i c l e i n f o
a b s t r a c t
UO2(NO3)2ꢀ6H2O and VO(acac)2 react with 2-(20-hydroxylphenyl)benzoxazole (Hpbx) in methanol to give
[UO2(pbx)2(CH3OH)] (1) and [VO(pbx)2] (2). Complex 1 shows a distorted pentagonal bipyrimidal geom-
etry, characteristic for the coordination number 7. Reciprocal OHꢀ ꢀ ꢀO interactions between neighbored
molecules hold 1 in a pseudo-dimeric association with an inversion center. Complex 2 achieves a dis-
torted octahedral geometry and the molecules ‘‘stack” along the b axis through secondary interactions.
Article history:
Received 23 October 2009
Received in revised form 19 November 2009
Accepted 23 November 2009
Available online 3 December 2009
0
0
x0
The molecule heaping is wholly linear, with Vn ꢁO(1)n ꢀ ꢀ ꢀV angles of 180°. Luminescence properties
Keywords:
of Hpbx, 1 and 2 are discussed.
Benzoxazole complexes
Uranyl complexes
Ó 2009 Elsevier B.V. All rights reserved.
VO2+ complexes
Luminescence of benzoxazole complexes
1. Introduction
organic metal complexes are potential photoluminescent or electro-
luminescent materials [5]. For example, Zinc(II) complexes of 2-(20-
It is well known that the benzene-fused oxazole ring structure
(with an odor similar to pyridine) called benzoxazole is used pri-
marily in industry and research. Being a heterocyclic compound,
benzoxazole finds use as a starting material for the synthesis of lar-
ger, usually bioactive structures, and is found within the chemical
structures of some pharmaceutical drugs. Because of its aromatic-
ity it is a relatively stable compound, although as a heterocyclic it
has reactive sites which allow for functionalization. Derived from
1,3-benzoxazole, the compound 2-(1,3-benzoxazol-2-yl)phenol
(A), commonly known as 2-(20-hydroxylphenyl)benzoxazole, has
been studied for the synthesis of metal complexes with N,O-donor
ligands.
hydroxylphenyl)benzoxazole have been recognized as noteworthy
green–blue emitters in electroluminescent devices [6,7], and fur-
ther studies have shown that the introduction of a second ligand
may also have important effects on the mechanism of emission
[2], and, consequently, on the emitting wavelength [8,9].
The research with 2-(20-hydroxylphenyl)benzoxazole (A) has
been developed with the metals Zn [9–11], Re [12–14], Ru [15–
17], Pt [18,19], Pd [20–22], Cu [23], Fe [24], Mn [25,26], Ga [27],
including even Be [28] and Al [27,29]. The major goal of these
experiments is the search of new optically tuneable luminescent
materials, besides secondary interests like the catalytic oxygen
atom transfer [30] and the synthesis of metallomesogens derived
from heterocyclic benzoxazoles [31]. Benzoxazoles, nevertheless,
are important biological substances, covering an extensive spec-
trum of essential and selective antimicrobial activities, such as
amyloid fibril inhibitor [32,33], antimycobacterial [34], antitumor
[35], immunosuppressive, antiviral [36] and antiretroviral [37].
The use of biological ligands with hard base characters for the
chelation of heavy metals like uranium, thorium or lanthanides,
has been a subject of interest in our group [38–42], as these kinds
of ligands generally produce very stable chelate complexes. This
profit makes them highly appropriates for therapeutic develop-
ment of clinical chelators in safety procedures related to contami-
nation through natural or depleted thorium and uranium, and in
similar chemoprevention processes against uranium/thorium/hea-
vy metals intoxication. On the other side, in the present days the
insulin mimetic effects of vanadium complexes are well known
HO
N
O
A
These kinds of compounds have been used successfully as electrolu-
minescent emitters [1–4] since the discovery that luminescent
* Corresponding authors. Tel.: +55 55 3220 8757; fax: +55 55 3220 8031.
E-mail addresses: daviback@gmail.com (D.F. Back), manzoni@quimica.ufsm.br
(G.M. de Oliveira).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.11.033

808
D.F. Back et al. / Inorganica Chimica Acta 363 (2010) 807–812
[43–45], since the discovery that vanadate ions, low-molecular-
weight phosphate analogues, mimic most of the rapid actions of
insulin in various cell types [46]. Curiously, an important biological
compound like 2-(20-hydroxylphenyl)benzoxazole (A) has not been
used so far, neither for the chelation of heavy metals, nor to
achieve structurally well-defined vanadium complexes, potential
insulin mimetic agents.
Subsequent Fourier-difference map analyses yielded the positions
of the non-hydrogen atoms. Refinements were carried out with
the SHELXL package [52]. All refinements were made by full-matrix
least-squares on F2 with anisotropic displacement parameters for
all non-hydrogen atoms. Hydrogen atoms were included in the
refinement in calculated positions.
Because our interests lay also in the metallation of biological
ligands, we fill partially this gap reporting herein the synthesis
and the X-ray structural characterization of the new chelate
complexes of UVI and VIV, [UO2(pbx)2(CH3OH)] (1) and [VO(pbx)2]
(2) {Hpbx = 2-(20-hydroxylphenyl)benzoxazole}.
2.6. Luminescence experiments
A Cary Eclypse (Varian) spectrofluorimeter was used to obtain
fluorescence spectra at 25 °C. The spectra were obtained in the cor-
rected mode with excitation wavelength of 280 nm and the slit
opening was adjusted at 10.0 mm for the ligand (Hpbx) and
20.0 nm for 1 and 2, to compensate the high fluorescence of the li-
gand in comparison with the low intensities of the complexes. All
the measures were carried out in dimethylsulfoxide solutions.
2. Experimental
2.1. General
Elemental analyses for C, H and N were performed at a Shima-
dzu EA 112 microanalysis instrument. IR spectra were recorded on
a Tensor 27 – Bruker spectrometer with KBr pellets in the 4000–
400 cmꢁ1 region.
3. Results and discussion
3.1. Crystal structure
The X-ray crystal data and the experimental conditions for the
analyses of the complexes [UO2(pbx)2(CH3OH)] (1) and [VO(pbx)2]
(2) are given in Table 1. Table 2 presents selected bond distances
and angles for the title compounds. Figs. 1 and 2 show, respec-
tively, the molecular structures of compounds 1 and 2. Fig. 3 dis-
plays the pseudo-dimeric association of [UO2(pbx)2(CH3OH)] (1)
2.2. Synthesis of 2-(20-hydroxylphenyl)benzoxazole [47,48]
According to literature procedures, 2-(20-hydroxylphenyl)benz-
oxazole was prepared by condensation of salicylic acid (0.65 g,
4.71 mmol) with o-aminophenol (0.5 g, 4.71 mmol) in polyphos-
phoric acid (10 ml). The mixture was heated in oil bath at 180 °C
for 6½ h under N2 atmosphere, cooled at room temperature,
poured into ice, filtered, washed with water, dried, and finally puri-
fied by column chromatography over SiO2, eluted with anhydrous
dichloromethane. Yield after the elution: 56%. The colorless crys-
tals are strongly fluorescent under ultraviolet radiation.
Table 1
Crystal data and structure refinement for [U02(pbx)2(CH3OH)] (1) and [VO(pbx)2] (2)
1
2
Empirical formula
Formula weight
T (K)
C27H20N2O7U
722.48
293(2)
C26H16N2O5V
487.35
293(2)
2.3. Synthesis of [UO2(pbx)2(CH3OH)] (1)
Radiation, X1A
0.71073
0.71073
Crystal system, space group monoclinic, P21/c
Unit cell dimensions
monoclinic, C2/c
To a solution of Hpbx (0.026 g, 0.125 mmol) in 15 mL of meth-
anol, UO2(NO3)2ꢀ6H2O (0.031 g, 0.0625 mmol) was added. After
heating for 1½ h at 70 °C the yellow solution turned bright red.
Prismatic red crystals were formed from the solution after one
day. Yield: 0.036 g, 81% based on Hpbx. Melting point: 361 °C. Anal.
Calc. for C27H20N2O7U: C, 44.87; H, 2.77; N, 3.87. Found: C, 44.93;
a (Å)
b (Å)
c (Å)
13.477(3)
19.344(4)
9.3029(19)
90
29.3399(6)
3.93430(10)
21.6908(5)
90
126.141(3)
90
2022.00(8)
4, 1.601
0.536
a
(°)
b (°)
92.306(5)
90
2423.4(9)
4, 1.980
6.749
c
(°)
H, 2.84; N, 3.89%. IR spectra (KBr): 3047 [medium,
m
(CꢁH)ar.],
V (Å3)
Z, DCalc (g cmꢁ3
)
1607 [strong,
[strong,
(CꢁO)Ph.], 1157 [medium,
as(O@U@O)].
m(C@N)], 1531, 1469, 1428 [strong,
m
(CꢁH)ar.], 1254
m
m
(H3CꢁO)], 916 cmꢁ1 [strong,
Absorption coefficient
(nmꢁ1
)
m
F (0 0 0)
1376
996
Crystal size (mm)
h Range (°)
Index ranges
0.409 ꢂ 0.384 ꢂ 0.360 0.198 ꢂ 0.143 ꢂ 0.107
2.4. Synthesis of [VO(pbx)2] (2)
1.84–30.58
1.72–28.43
ꢁ38 6 h 6 38,
ꢁ5 6 k 6 5,
ꢁ28 6 l 6 28
21 774
ꢁ19 6 h 6 19,
ꢁ27 6 k 6 25,
ꢁ13 6 l 6 12
29 601
To a solution of Hpbx (0.029 g, 0.2 mmol) in 15 mL of methanol,
VO(acac)2 (0.026 g, 0.1 mmol) was added. After heating for 1 h at
60 °C a yellow solid precipitated. The solid was redissolved in pyr-
idine and after two days brown crystals were formed from the
solution. Yield: 0.041 g, 84% based on Hpbx. Melting point:
295.4 °C. Anal. Calc. for C26H16N2O5V: C, 64.00; H, 3.28; N, 5.74.
Found: C, 64.13; H, 3.31; N, 5.78%. IR spectra (KBr): 3100–3010
Reflections collected
Reflections unique
Completeness to theta max. 99.6%
7423 [Rint = 0.0321]
2534 [Rint = 0.0461]
99.5%
Absorption correction
semi-empirical from
Gaussian
equivalents
Maximum and minimum
transmission
Refinement method
1 and 0.467496
0.989 and 0.897
[m,
(CꢁH)ar.], 1264 [s,
(VO2) [51]}, 531 {
m
(CꢁH)ar.], 1614 [s,
m(C@N)], 1560, 1536, 1471, 1435 [s,
(CꢁO)Ph.], 894 {
full-matrix least-
squares on F2
7423/0/338
full-matrix least-
squares on F2
2534/0/155
1.159
m
m
mas(V@O) [49,50]}, 736 {mas
m
s(VO2) [51]}, 467, 407 cmꢁ1 {VꢁO, VꢁN [51]}.
Data/restraints/parameters
Goodness-of-fit (GOF) on F2 1.035
Final R indices [I > 2r(7)]
R1 = 0.0321,
wR2 = 0.0724
R1 = 0.0377, wR2
0.1142
=
2.5. X-ray crystallography
R indices (all data)
R
1 = 0.0499,
R
1 = 0.0564,
wR2 = 0.1402
0.623 and ꢁ0.851
wR2 = 0.0791
2.645 and ꢁ1.697
Data were collected with a Bruker APEX II CCD area-detector
Largest difference in peak
and hole (e A3)
diffractometer and graphite-monochromatized Mo K
a radiation.
The structures were solved by direct methods using SHELXS [52].

D.F. Back et al. / Inorganica Chimica Acta 363 (2010) 807–812
809
Table 2
Selected bond lengths (Å) and angles (°) for (1) and (2)
1
0(3)–U–N(2)
0(7)–U–N(2)
68.02(9)
137.90(9)
93.84(11)
85.75(11)
69.55(9)
143.89(9)
73.85(9)
147.84(9)
Bond lengths
U–O(3)
U–N(l)
U–0(4)
U–N(2)
U–0(7)
U–0(2)
U–O(l)
0(7)–H(l)
H(l)ꢀꢀꢀ0(3)0
Bond angles
0(2)–U–0(l)
0(2)–U–0(4)
0(l)–U–0(4)
0(2)–U–0(3)
0(l)–U–0(3)
0(4)–U–0(3)
0(2)–U–0(7)
0(l)–U–0(7)
0(4)–U–0(7)
0(3)–U–0(7)
0(2)–U–N(2)
0(1)–U–N(2)
0(4)–U–N(2)
2.336(2)
2.604(3)
2.226(3)
2.568(3)
2.427(3)
1.784(3)
1.788(3)
0.87(5)
0(2)–U–N(l)
0(1)–U–N(1)
0(4)–U–N(l)
0(3)–U–N(l)
0(7)–U–N(l)
N(2)–U–N(l)
2
Bond lengths
V–O(l)
V–0(2)
1.716(3)
1.612(2)
1.9421(15)
2.0832(17)
2.322(2)
178.05(12)
94.06(11)
87.59(11)
93.50(11)
85.73(11)
144.84(10)
85.47(12)
92.58(12)
143.29(9)
71.56(9)
V–N
V–O(l)00
Bond angles
0(l)–V–0(2)
0(2)0–V–0(2)
0(1)–V–N
0(2)0–V–N
0(2)–V–N
N–V–N0
0(1)–V–0(1)00
0(2)–V–0(l)00
N–V–O(l)00
V–0(1)–V000
102.41(5)
155.17(9)
96.23(4)
90.28(7)
87.05(7)
167.55(9)
180.000(1)
77.59(5)
83.77(4)
180.0
Fig. 2. Molecular structure of [VO(pbx)2] (2). Symmetry transformations used to
generate equivalent atoms: (0) 1ꢁx, y, 0.5ꢁz. For clarity the hydrogen atoms of the
pbx ligands are omitted.
85.70(12)
95.67(12)
78.39(9)
respect to nitrogen. In the case of U–O(1) and U–O(2) the notewor-
thy short distances also reflect the covalent character of these two
bonds.
Symmetry transformations used to generate equivalent atoms: 1 {(0) 1 ꢁ x, 1 ꢁ y,
1 ꢁ z}. 2 {(0) 1 ꢁ x, y, 0.5 ꢁ z; (00) x, 1 + y, z; (000) x, ꢁ1 + y, z}.
In the complex [VO(pbx)2] (2) the metal centers attain a dis-
torted octahedral geometry, since the molecules stack symmetri-
0
0
cally along the
b
axis through secondary Vn ꢀ ꢀ ꢀO(1)x bonds,
represented in dashed lines in Fig. 3. These interactions have the
same distance, 2.322(2) Å, and are very longer than the VꢁO(1)
bonds, which measure 1.612(2) Å. The molecule ‘‘heaping” along
0
0
0
the b axis is rightly linear, as the Vn –O(1)n ꢀ ꢀ ꢀVx bonds have angles
of 180° (see Fig. 3 and Table 2). In the complex 2 the harder basic
character of the oxygen atoms is not so evident like in complex 1:
the VꢁO(2) and the VꢁN bonds are very similar, with distances of
1.9421(15) and 2.0832(17) Å, respectively. This is probably due to
the small size of the vanadium atoms, in comparison with the
bulky uranium centers. Finally, it would be appropriate to under-
line the impossibility to differentiate oxygen and nitrogen atoms
crystallographically. The CꢁO(N) distances, however, can be
unmistakably measured, so that we can design the molecule con-
figuration also with basis on X-ray data. In the case of complex 2,
for example, the occurrence of two VꢁN bonds is also confirmed
by the fact that the C–N distances in the ligand Hpbx are not alike
(see A). The single/double bond characters of the ligand are main-
tained in the complex: the double N–C(7) bond measures
1.3127(26) Å, while the single N–C(1) one is correspondently long-
er, with 1.3991(40) Å. The O(3)–C(7) and O(3)–C(6) distances are
1.3633(29) and 1.3833(39) Å, respectively. The O(2)ꢁC(9) bond is
the shorter of its species (1.3165(21) Å), since the ligand atom
O(2) was originally a hydroxyl group (see Fig. 2).
Fig. 1. Molecular structure (asymmetric unit) of [UO2(pbx)2(CH3OH)] (1). For
clarity the hydrogen atoms of the pbx ligands are omitted.
3.2. Optical properties
Studies on the biological activity of complex 2 are in progress,
and preliminary results allow considering this compound as a
probable insulin mimetic agent. The luminescence properties of
the 2-(20-hydroxylphenyl)-N,O-, N,N- and N,S-species seem to re-
sult from the tautomeric equilibrium shown in Scheme 1.
In the singlet ground state the tautomerization of the enol to the
keto form is a very fast process because of the existence of an intra-
molecular hydrogen bond between the hydroxyl group and the
nitrogen atom of the oxazole ring (O–Hꢀ ꢀ ꢀN). Thus, after light
absorption the compound emits fluorescence and gives a large
Stokes shift. The triplet excited state of the enol form cannot be
achieved directly via intersystem crossing due to the extremely ra-
pid tautomerization process [53,54].
and the ‘‘stacking” of the molecules of [VO(pbx)2] (2) along the b
axis.
In the uranyl complex [UO2(pbx)2(CH3OH)] (1) the uranium
atom is the center of a distorted pentagonal bipyramide, character-
istic for the coordination number 7. The seventh coordination po-
sition in [UO2(pbx)2(CH3OH)] (1) is accomplished by a methanol
molecule, which interacts through the OH group with the O(3)
atom of a neighbor molecule (see Fig. 3). These reciprocal hydrogen
bridges have a distance of 1.716(3) Å and hold the molecules in
pairs, with a pseudo-dimeric association, completed by an inver-
sion center. The U–O bonds are shorter than the U–N ones, and this
can be assigned to the harder basic character of the oxygen atom in

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D.F. Back et al. / Inorganica Chimica Acta 363 (2010) 807–812
Fig. 3. Pseudo-dimeric association of [UO2(pbx)2(CH3OH)] (1, symmetry transformations used to generate equivalent atoms: (0) 1ꢁx, 1ꢁy, 1ꢁz), and the ‘‘stacking” of the
molecules of [VO(pbx)2] (2) along the b axis. Symmetry transformations used to generate equivalent atoms: (0) 1ꢁx, y, 0.5ꢁz; (00) x, 1 + y, z; (000) x, ꢁ1 + y, z. For clarity the
hydrogen atoms of the pbx ligands are omitted.
O
H
O
H
N
X
N
X
Light absorption
Fluorescence emission
X = O,NH, S
Scheme 1.
Fig. 4. Emission spectrum of the ligand 2-(20-hydroxylphenyl)benzoxazole (A) and of the complexes [UO2(pbx)2(CH3OH)] (1, B) and [VO(pbx)2] (2, C). The emission bands of 1
and 2 are broad and weak on the contrary of the sharp and intense band of the ligand.
Although our results on the luminescence properties of Hpbx,
[UO2(pbx)2(CH3OH)] (1) and [VO(pbx)2] (2) are preliminary and
inconclusive (in particular because neither absorption nor excita-
tion spectra are presented), crude emission spectra, resumed in
Fig. 4, show that all compounds exhibit luminescence, and this find
is not surprising for 1, as uranyl complexes are well known as
notoriously good emitters under the proper circumstances. Never-
theless, the excitation at 280 nm is not adequate for complexes
with the uranyl group, so that for complex 1 some vibronic struc-
ture should be expected, particularly exciting at 400 nm. In addi-
tion, the emission spectra presented in Fig. 4 are probably
distorted by self-absorption from the colored complexes. The pre-
liminary spectra allow also concluding that the luminescence of
the ligand is much more intense than that of the complexes 1
and 2, and the vanadyl complex is approximately three times more
fluorescent than the uranyl compound. Since the fluorescence

D.F. Back et al. / Inorganica Chimica Acta 363 (2010) 807–812
811
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2009.11.033.
light absorption
V4+== O2−
V3+
O−
fluorescence
Scheme 2.
References
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the hydroxyl group of the phenyl ring – occurred on the occasion
of the formation of 1 and 2 – should restrain its (ligand) lumines-
cence, because the main luminescence mechanism has been hin-
dered. Thus, the luminescence of complexes 1 and 2 seems not
to be associated to the ‘‘antenna effect” of the ligand [55] (i.e., en-
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cited state going back to the V@O ground state, as illustrated in
Scheme 2.
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4. Conclusion
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In the refinement procedures for complex 1 we have found a
relatively significant residual electronic density, with an uncom-
mon value (2.645 e Åꢁ3) for the largest difference peak (see Table
1). This residual electronic density, however, can not be assigned
to any concrete atom, since the distance between the nearest peak
and the metal center is approximately 0.770 Å, shorter even than
the sum of the covalent radii of C and H, for example. On the other
hand, if this electronic density is attributed to any specific atom,
the discordance indices R1, wR2 (final R indices), and R1 (all data),
undergo an unsuitable increase. Also eventual positional disorders
were discarded, since the crystallographic refinement using PART
Instruction [57] did not stabilize the thermal ellipsoids calculation.
Thus, the acquired crystallographic data allow us, beyond doubt, to
present the structure of Fig. 1 as the real structure of complex 1.
Even though the theoretical predictions discussed above in the
second paragraph of the preceding subsection assign luminescence
only to the keto form of the ligand Hpbx, the luminescence of the
enol form cannot be ruled out. Although further examples of metal
complexes of Hpbx could not be found for comparison, the depro-
tonation of the hydroxyl group to form complexes probably also
leads to emission from the complexed ligand, as in our case. This
supposition should be supported by the fact that numerous
benzoxazole-substituted ligands are used as luminescence sensi-
tizers- and modulators of lanthanides and other transition metals
[58–61]. Anyway, we are yet not able to make conclusive compar-
isons between the luminescence properties of the ligand 2-(20-
hydroxylphenyl)benzoxazole, Hpbx, and that of the complexes
[UO2(pbx)2(CH3OH)] (1) and [VO(pbx)2] (2).
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Acknowledgment
This work was supported with funds from PRONEX-CNPq/FA-
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Appendix A. Supplementary material
CCDC 712123 and 712124 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge

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