Complexes with 6-amino-5-nitroso-2-thiouracil and violuric acid derivatives containing the fac-ReI(CO)3 core: Synthesis, XRD structural and photoluminescence characterization

Article abstract of DOI:10.1016/j.ica.2010.11.009

The fac-tricarbonylrhenium(I) complexes of the 6-amino-1,3-dimethyl-5- nitroso-2-thiouracil (DANTU) and violuric acid (VIO) and its mono- (MVIO) and dimethyl (DVIO) derivatives have been prepared. The complexes have been characterized by elemental analysis, IR, ¹H and ¹³C NMR spectral methods and luminescence spectroscopy. The structures of [ReCl(CO) ₃(DANTU)], [Re(H₂O)(CO)₃(VIOH_-1)] and [Re(H₂O)(CO)₃(DVIOH_-1)] complexes were solved from single-crystal X-ray diffraction experiments. The coordination environment around the Re(I) may be described as a distorted octahedron in which the ligand behaves in a bidentate fashion through the nitrogen atom of the nitroso group and an adjacent carbonylic oxygen, making a five-membered chelate ring. The coordination sphere is completed with three carbonyl groups in fac-arrangement and one chlorine atom (DANTU complex) or water molecule (VIO complexes). The higher acidity of violuric acids, if compared with DANTU one, may explain both synergic deprotonation and chloride substitution in the [ReCl(CO)₃]⁺ moiety to form the Re-violurato complexes.

Full text of DOI:10.1016/j.ica.2010.11.009

Inorganica Chimica Acta 366 (2011) 262–267
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Complexes with 6-amino-5-nitroso-2-thiouracil and violuric acid derivatives
containing the fac-ReI(CO)3 core: Synthesis, XRD structural and
photoluminescence characterization
⇑
Nuria A. Illán-Cabeza, Antonio R. García-García, Miguel N. Moreno-Carretero
Departamento de Química Inorgánica y Orgánica, Universidad de Jaén, 23071 Jaén, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2010
Received in revised form 2 September 2010
Accepted 6 November 2010
Available online 17 November 2010
The fac-tricarbonylrhenium(I) complexes of the 6-amino-1,3-dimethyl-5-nitroso-2-thiouracil (DANTU)
and violuric acid (VIO) and its mono- (MVIO) and dimethyl (DVIO) derivatives have been prepared.
The complexes have been characterized by elemental analysis, IR, 1H and 13C NMR spectral methods
and luminescence spectroscopy. The structures of [ReCl(CO)3(DANTU)], [Re(H2O)(CO)3(VIOHꢀ1)] and
[Re(H2O)(CO)3(DVIOHꢀ1)] complexes were solved from single-crystal X-ray diffraction experiments.
The coordination environment around the Re(I) may be described as a distorted octahedron in which
the ligand behaves in a bidentate fashion through the nitrogen atom of the nitroso group and an adjacent
carbonylic oxygen, making a five-membered chelate ring. The coordination sphere is completed with
three carbonyl groups in fac-arrangement and one chlorine atom (DANTU complex) or water molecule
(VIO complexes). The higher acidity of violuric acids, if compared with DANTU one, may explain both syn-
ergic deprotonation and chloride substitution in the [ReCl(CO)3]+ moiety to form the Re-violurato
complexes.
Keywords:
Thiouracil
Violuric acid
Rhenium complexes
X-ray crystal structure
Luminescent properties
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
photobehaviour of such complexes may be discussed in terms of
three types of excited states: (i) metal-to-ligand charge transfer
The continuing interest in the use of the radionuclide 99mTc in
diagnostic nuclear medicine and the use of 186Re and 188Re in
radiotherapy has led to the development of new radiopharmaceu-
ticals [1,2]. The chemistry of technetium and rhenium has been
(MLCT) states, (ii) ligand-to-ligand charge transfer (LLCT) states
and (iii) intraligand (IL) states. The photophysics and photochem-
istry of a great variety of diimine rhenium(I) tricarbonyl com-
plexes have been described [6–10]. They exhibit exceptionally
diverse photophysical properties which occur through their lowest
triplet excited states, due to rapid vibrational relaxation and inter-
system crossing from the upper vibrational energy levels. This
ability made these compounds very important as fluorochromes
for cell imaging using fluorescence microscopy technique [11,12].
The interactions of metal ions with pyrimidine compounds have
been extensively studied over the past years. It is known that thio-
pyrimidine derivatives act as potent inhibitors [13] and antimetab-
olites [14], and pyrimidine-2-thione shows pronounced in vitro
bacteriostatic activity [15]. Likewise, 2-thiouracil is known as a ba-
sic constituent of some t-RNAs [16], and it exhibits antitumor and
antithyroid activity because it readily incorporates into nucleic
acids [17]. Violuric acids coordinate with a large number of metal
ions [18–21] and they display analytical applications for the iden-
tification and determination of metal cations [22] being also used
as antibacterial and antifungal agents [23]. Recent applications in-
clude their use as redox mediators in enzymatic treatment of
organic pollutants of wastewater [24]. Violurato derivatives show,
well documented showing
a great diversity. The fac-M(CO)3
(M = Tc, Re) moiety recurs in many complexes having real or po-
tential utility in medical imaging. Depending on the ligand, when
bound to the fac-[Re(CO)3] core, the complexes act as luminescence
probes; when bound to fac-[99mTc(CO)3] as potential radioimaging
agents [3–5].
The [M(CO)3]+ core is an interesting low-spin d6 M(I) centre.
Compounds containing metals with closed-shell d6 exhibit a differ-
ent kinetic stability, charge, size and lipophilicity. These properties
can be tuned with an appropriate ligand, then becoming appropri-
ate for biomedical purposes.
Rhenium(I) tricarbonyl complexes containing bidentate hetero-
cyclic ligands have an important application for use in solar energy
conversion. They display intense luminescence in the visible re-
gion of spectrum and are stable to photodecomposition. The
⇑
Corresponding author. Address: Departamento de Química Inorgánica y Orgá-
nica, Facultad de Ciencias Experimentales, Universidad de Jaén, Paraje Las Lagunil-
las s/n, 23071 Jaén, Spain. Tel.: +34 953 212738; fax: +34 953 211876.
E-mail address: mmoreno@ujaen.es (M.N. Moreno-Carretero).
in coordination compounds,
strong-field ligands.
p-acceptor properties acting as
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.009

N.A. Illán-Cabeza et al. / Inorganica Chimica Acta 366 (2011) 262–267
263
Table 1
In a previous paper, we described the synthesis and character-
ization of rhenium(I) complexes with some 5-nitrosopyrimidines
[25]. Our interest in new ligand combinations with the tricarbonyl
fac-[M(CO)3]+ core for potential applications has led us to investi-
gate its interaction with violuric acids and one thiouracil
derivative.
Crystal data and structure refinement for complexes [ReCl(CO)3(DANTU)]ꢁ1/2CH3CN
(1), [Re(H2O)(CO)3(VIOHꢀ1)]ꢁ1/2H2O (2) and [Re(H2O)(CO)3(DVIOHꢀ1)] (4).
1
2
4
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
10H8C6N4.5O5ReS C7H2N3O8.5Re C9H8N3O8Re
524.92
Monoclinic
C2/c
22.410(2)
10.954(1)
12.970(1)
90
100.00(1)
90
3135.5(4)
8
2.224
8.083
1980
450.33
Triclinic
P1
470.37
Triclinic
P1
ꢀ
ꢀ
2. Experimental
6.345(4)
8.881(1)
10.383(9)
98.16(1)
91.83(1)
99.46(1)
570.4(1)
2
6.258(1)
8.500(1)
11.785(2)
83.37(2)
88.53(2)
77.14(1)
607.1(2)
2
2.573
10.057
440
2.86–27.50
ꢀ8 6 h 6 8
b (Å)
c (Å)
2.1. Instrumentation
a
(°)
b (°)
C, H, N microanalyses were performed on a Thermofinnigan
Flash 1112 Series elemental analyzer. Conductivity measurements
have been carried out using 10ꢀ3 M freshly prepared DMF solu-
tions on a Hanna HI8820 instrument. IR spectra were measured
on Bruker FT-IR Tensor-27 (4000–400 cmꢀ1, KBr pellets) and Vec-
tor-22 (600–220 cmꢀ1, polyethylene pellets) spectrophotometers.
1H and 13C NMR spectra were recorded using a Bruker DPX-300
apparatus (DMSO-d6 solutions). Fluorescence excitation and emis-
sion spectra were measured with a Cary Elipse fluorescence spec-
trophotometer in CH3CN at room temperature.
c
(°)
Volume (Å3)
Z
Dcalc (g cm-3
)
2.622
10.701
416
l
(mmꢀ1
)
F(0 0 0)
h (°)
Limiting indices
5.04–27.50
ꢀ29 6 h 6 29
ꢀ12 6 k 6 14
ꢀ16 6 l 6 16
14375/0/3574
3.91–27.49
ꢀ8 6 h 6 8
ꢀ11 6 k 6 11 ꢀ11 6 k 6 11
ꢀ13 6 l 6 13
ꢀ15 6 l 6 15
Data/restrains/
parameters
2611/0/172
2786/0/190
Goodness-of-fit on F2
1.058
1.078
1.108
R1/wR2 [I > 2
r
(I)]
0.0364/0.0642
0.0451/
0.1152
0.0513/
0.1207
0.0330/
0.0819
0.0379/
0.0842
2.2. Crystallography
R1/wR2 (all data)
0.0693/0.0754
The measurements were performed on a Bruker-Nonius Kappa
CCD diffractometer with graphite monochromated Mo
Ka
(k = 0.71073 Å) radiation. The temperature (120 K) was controlled
employing an Oxford Cryosystem low-temperature device. Lorentz,
polarization, and multiscan absorption corrections were applied
with SADABS [26]. The structure was solved by the direct method
and subsequently completed by the difference Fourier recycling
[27,28]. All the non-hydrogen atoms were refined anisotropically
using full-matrix, least-squares technique and the hydrogen atoms of
the ligands were introduced at calculated ideal positions following
riding models. In the complex [Re(H2O)(CO)3(VIOHꢀ1)], the poor
data quality has limited the accuracy of the refinement. After aniso-
tropic refinement, two main unassigned electron density peaks were
observed in the final difference Fourier map, the highest being
9.2 e Å3 at 1.25 Å from hydrogen atom H1 and the lowest being
ꢀ2.40 e Å3 at 0.82 Å from Re; despite this fact and the presence of
disordered solvent molecules, the main moiety was unambiguously
determined. Other experimental details for the crystallographic
study are given in Table 1.
Fig. 1. More stable tautomeric structure and nomenclature of the ligands: (left)
R@R0@CH3, 6-amino-1,3-dimethyl-5-nitroso-2-thiouracil (DANTU, 6-amino-1,3-
dimethyl-5-nitroso-2-thioxo-2,3-dihydropyrimidin-4(1H)-one) (right) R@R0@H,
violuric acid (VIO, 5-hydroxyimino-pyrimidine-2,4,6(1H,3H,5H)-trione);
R or
R0@CH3, methylvioluric acid (MVIO, 5-hydroxyimino-1-methylpyrimidine-
2,4,6(1H,3H,5H)-trione); R@R0@CH3, dimethylvioluric acid (DVIO, 5-hydroxyimi-
no-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione).
2.3. Ligands
1073s m
(N–O). 1H NMR (d, ppm): 3.05, 3.11 (N–CH3), 11.51, 11.61
(N–H), 14.71 (C5–N5OH); 13C NMR (d, ppm): 26.71, 27.36 (N–
Commercial grade chemicals were used without further purifica-
tion. The compound 6-amino-1,3-dimethyl-5-nitroso-2-thiouracil
(DANTU) was synthesized by cyclocondensation of N,N0-dim-
ethylthiourea and cyanacetic acid in acetic anhydride, followed by
the nitrosation with NaNO2 and acetic acid. Violuric acid derivatives
were obtained by means of acid hydrolysis with HCl from the corre-
sponding 6-amino-5-nitrosopyrimidines. Analytical and characteri-
zational data are in agreement with the structures depicted in Fig. 1.
NMR assignments were corroborated from 1H, 13C, DEPT, HMQC and
HMBC (13C–1H) spectra. Data are as follows: DANTUꢁH2O: IR (cmꢀ1):
CH3), 135.66, 136.22 (C5), 150.04 (C2), 158.18, 159.05 (C4),
153.78, 154.52 (C6). DVIOꢁH2O: IR (cmꢀ1): 1748m, 1676b
m(C@O);
1632sh m(C@C); 937m m
(N–O). 1H NMR (d, ppm): 3.11 (N1–CH3),
3.17 (N3–CH3), 14.78 (C5–N5OH); 13C NMR (d, ppm): 27.52 (C1),
28.21 (C3), 135.66 (C5), 150.62 (C2), 153.54 (C6), 158.19 (C4).
2.4. Preparation of the complexes
[ReCl(CO)3(DANTU)]ꢁ1/2CH3CN (1): A suspension of the ligand
(0.25 mmol) in acetonitrile (30 ml) was reacted with the [Re-
Cl(CO)5] compound (0.25 mmol). The mixture was stirred and
heated for three hours. Then, it was left to cool at room tempera-
ture and red crystals suitable for X-ray diffraction were obtained.
ReCl(CO)3(DANTU)ꢁ1/2CH3CN: Yield ca. 70%; IR (cmꢀ1): 3203b
3146br m(N–H); 1692s m(C@O); 1638m m(C@C); 1499m m(N@O);
1030w d(N–H); 879m
m (C@S); 802,761w x
(NH2). 1H NMR (d,
ppm): 3.72 (N1–CH3), 3.70 (N3–CH3), 9.11, 12.85 (C6–NH2); 13C
NMR (d, ppm): 35.43 (C1), 36.18 (C3), 178.53 (C2), 158.35 (C4),
140.12 (C5), 144.01 (C6). VIOꢁH2O: IR (cmꢀ1): 3110br
m (N–H);
1753sh, 1721s
m
(C@O); 1638m
m
(C@C); 1060s
m
(N–O). 1H NMR (d,
m
m
(N–H); 2039s, 1939s, 1892s
(C@C); 1518m (N@O); 1041m d(N–H); 879m
(NH2); 451w (Re–C); 273w
(Re–Cl).1H NMR data
(d, ppm): 3.80 (N1–CH3), 3.78 (N3–CH3), 11.18, 12.18 (C6–NH2);
m
(C„O); 1636s
m
(C@O); 1596m
ppm): 11.39 (N1–H), 11.29 (N3–H), 14.72 (C5–N5OH); 13C NMR (d,
m
m
(C@S); 782w,
ppm): 140.12 (C5), 149.76 (C2), 158.35 (C4), 144.01 (C6). MVIO: IR
720w
x
m
m
(cmꢀ1): 3114b
m (N–H); 1742m, 1665s m(C@O); 1583m m(C@C);

264
N.A. Illán-Cabeza et al. / Inorganica Chimica Acta 366 (2011) 262–267
13C NMR (d, ppm): 36.15 (C1), 37.91 (C3), 137.77 (C5), 148.11 (C6),
170.12 (C4), 176.12 (C2), 198.15, 195.31, 188.88 (CO).
Table 2
Bond lengths (Å) and angles (°) around the rhenium atom for complexes
[ReCl(CO)3(DANTU)] (1), [Re(H2O)(CO)3(VIOHꢀ1)] (2) and [Re(H2O)(CO)3(DVIOHꢀ1)]
(4).
[Re(H2O)(CO)3(VIOHꢀ1)]ꢁ1/2H2O (2) and Re(H2O)(CO)3(MVI-
OHꢀ1)ꢁ1/2H2O (3): The synthesis of the complexes were carried
out by reacting 0.25 mmol of the ligand with 0.25 mmol of [Re-
Cl(CO)5] in methanol. A solution of the reaction product was ob-
tained after heating at 50–60 °C and stirring for ca. 3 h. After this
time, it was added hexane appearing a red precipitate. The solids
were filtered, washed with EtOH and Et2O and air dried. Recrystal-
lization using acetone:H2O (1:1) afforded crystals suitable for
X-ray diffraction of compound 2 (but not very good). [Re(H2O)
1
2
4
(X@Cl; Y@N;
Z@S)
(X@Ow; Y@O;
Z@O)
(X@Ow; Y@O;
Z@O)
Re–C(9)
Re–C(8)
Re–C(7)
Re–N(5)
Re–O(4)
Re–X
C(2)–Z(2)
O(4)–C(4)
1.901(8)
1.948(8)
1.899(7)
2.132(5)
2.166(4)
2.471(2)
1.627(7)
1.256(7)
1.345(7)
1.267(6)
1.295(7)
88.6(3)
88.8(3)
89.1(3)
97.2(2)
91.7(2)
174.0(2)
170.5(2)
96.8(2)
99.1(2)
74.9(2)
91.0(2)
176.7(2)
94.2(2)
85.1(1)
83.2(1)
1.91(1)
1.956(9)
1.914(9)
2.153(7)
2.159(6)
2.202(6)
1.23(1)
1.26(1)
1.34(1)
1.26(1)
1.22(1)
88.0(4)
89.9(4)
86.6(4)
96.2(4)
95.8(3)
173.5(3)
170.8(3)
95.5(3)
98.7(3)
75.1(3)
94.4(3)
177.5(3)
93.3(3)
84.0(2)
82.1(2)
1.884(7)
1.931(7)
1.890(7)
2.130(5)
2.141(4)
2.199(5)
1.215(8)
1.253(8)
1.352(8)
1.248(7)
1.219(8)
88.6(3)
88.1(3)
88.5(3)
94.6(2)
95.0(2)
175.6(2)
170.1(2)
94.2(2)
101.4(2)
75.7(2)
97.1(2)
173.3(2)
95.2(2)
(CO)3(VIOHꢀ1)]ꢁ1/2H2O: Yield ca. 60%; IR (cmꢀ1): 3424b
m(O–H);
3169w
1660m
m(N–H); 2047s, 1946s, 1919s m(C„O); 1742m, 1725m,
m
(C@O); 1494m m(N@O); 437w m
(Re–C). 1H NMR data
N(5)–C(5)
N(5)–O(5)
C(6)–Y(6)
(d, ppm): 12.44 (N1–H), 11.09 (N3–H); 13C NMR (d, ppm): 142.88
(C5), 148.72 (C2), 153.08 (C6), 178.28 (C4), 197.58, 195.03,
192.00 (CO). [Re(H2O)(CO)3(MVIOHꢀ1)]ꢁ1/2H2O: Yield ca. 70%; IR
C(9)–Re–C(7)
C(9)–Re–C(8)
C(7)–Re–C(8)
C(9)–Re–N(5)
C(7)–Re–N(5)
C(8)–Re–N(5)
C(9)–Re–O(4)
C(7)–Re–O(4)
C(8)–Re–O(4)
N(5)–Re–O(4)
C(9)–Re–X
C(7)–Re–X
C(8)–Re–X
N(5)–Re–X
O(4)–Re–X
(cmꢀ1): 3385b
m(O–H); 3208sh
m
(N–H); 2044s, 1927s, 1905s
(C@C); 1505m (N@O);
m
(C„O); 1721s, 1668m (C@O); 1629m
m
m
m
447w
m
(Re–C). 1H NMR data (d, ppm): 3.29, 3.31 (N–CH3), 11.36,
12.69 (N–H); 13C NMR (d, ppm): 26.30, 27.75 (N–CH3), 142.54,
143.27 (C5), 148.88 (C2), 151.94, 152.75 (C6), 176.82, 177.43
(C4), 197.58, 194.93, 191.95 (CO).
[Re(H2O)(CO)3(DVIOHꢀ1)] (4): To a stirred solution of DVIO
(0.25 mmol) in a H2O:acetone (1:1) mixture was added [ReCl(CO)5]
(0.25 mmol). The solution was heated for three hours and left to
cool. Some days later, X-ray-quality red crystals were obtained.
81.2(2)
79.5(2)
[Re(H2O)(CO)3(DVIOHꢀ1)]: Yield ca. 60%; IR (cmꢀ1): 3447b
m
(O–
(C„O); 1721m, 1709m,
(N@O); 449w
(Re–C). 1H
H)+
1654m
m
(N–H); 2038s, 1944s, 1904s
m
m
(C@O); 1573 (C@C); 1508m
m
m
m
NMR (d, ppm): 3.10 (N1–CH3), 3.37 (N3–CH3); 13C NMR (d, ppm):
27.19 (C1), 28.82 (C3), 142.87 (C5), 149.39 (C6), 151.77 (C2),
176.19 (C4), 197.51, 194.85, 191.87 (CO).
angle formed by O4–Re–N5 is significantly narrowed from the
expected octahedral 90° value to 74.9° (1), 75.1° (2) and 75.7° (4),
respectively. This value is consistent with those obtained for other
tricarbonyl rhenium(I) compounds with -diimine, lumazines and
hydrazones ligands that form five-membered metallocyclic rings
[29–32].
3. Results and discussion
The Re–carbonyl bond distances (1.886–1.957 Å) are consistent
with those found in other Re–tricarbonyl complexes and show
3.1. Crystallographic studies
multiple-bond character as consequence of the
p-acceptor ability
The molecular unit of [ReCl(CO)3(DANTU)] (1) is depicted in
Fig. 2; the XRD structures of [Re(H2O)(CO)3(VIOHꢀ1)] (2) and
[Re(H2O)(CO)3(DVIOHꢀ1)] (4) are similar but the 6-amino group
(N6) and the Cl atom are substituted by an 6-oxo group (O6) and
a water molecule (Ow), respectively. Selected interatomic dis-
tances and angles useful to describe the metal coordination sphere
as well as the tautomeric form (keto-oxime, keto-nitroso or nitro-
so-enol) of the ligands are included in Table 2. The coordination
geometry at the rhenium(I) centre is distorted octahedral, with
the three carbonyl groups arranged in a facial fashion. The bite
of the CO. The C–Re–C angles have an average value of 89.5°. Also,
the distances Re–N5, Re–O4, Re–Cl and Re–Ow are longer than
those expected due to the trans influence of the carbon monoxide
ligands. As it is previously reported [25], in every structure, the car-
bon monoxide in trans with the N5 atom seems to be weaker
bound to the Re, since the Re–C8 distances are always 0.04–
0.05 Å longer than Re–C7 and Re–C9 ones, a concomitant shorten-
ing by ca. 0.02 Å of C8–O8 bonds taking place.
In complex 1, the ligand DANTU is almost planar with small
deviations from the mean plane. The angle between the ligand
and the chelate ring (Re–O4–C4–C5–N5) is 5.5(3)°. The geometrical
data suggest a significant electronic delocalization and the pres-
ence of the ligand in its more stable ketone-nitroso tautomeric
form [33], as it has been previously reported for [ReCl(CO)3(DA-
NU)]ꢁCH3CN [25]. Although the sulphur atom displays important
coordinative properties, its involvement is hindered at all by the
presence of methyl-blocked pyrimidine nitrogens, since usually it
needs an available adjacent deprotonated nitrogen atom to make
a four-membered chelate ring [34].
Despite violuric acid ligands contain one acidic proton involved
in
a ketoneoxime¡nitrosoenol tautomeric equilibrium, XRD
experiments clearly confirmed the existence of the metal-free tau-
tomeric ketone-oxime form [35]. However, crystallographic data
suggest the coordination to the rhenium(I) centre favours the
ligand deprotonation and the change to the nitrosoenolato form.
Thus, the N5–O5 and C5–N5 distances are shorter and longer than
those found in the free ligand (0.092 Å and 0.052 Å, respectively),
Fig. 2. ORTEP drawing (50% probability ellipsoids) and atom-labelling scheme for
[ReCl(CO)3(DANTU)]. The reported XRD structures of [Re(H2O)(CO)3(VIOHꢀ1)] (2)
and [Re(H2O)(CO)3(DVIOHꢀ1)] (4) are similar but the 6-amino group (N6) and the Cl
atom are substituted by an 6-oxo group (O6) and
a water molecule (Ow),
respectively.

N.A. Illán-Cabeza et al. / Inorganica Chimica Acta 366 (2011) 262–267
265
Fig. 3. View, down the a axis, of the 2D-hydrogen bonded network in the [ReCl(CO)3(DANTU)]ꢁ1/2CH3CN structure. Disordered acetonitrile molecules are omitted for clarity.
H-bonds parameters (Dꢁ ꢁ ꢁA (Å); <D–Hꢁ ꢁ ꢁA (°)): N6–Hꢁ ꢁ ꢁO5, 2.604(6), 130; N6–Hꢁ ꢁ ꢁO7 (x, ꢀy, 1/2 + z), 3.024(7), 142; N6–Hꢁ ꢁ ꢁCl (1/2 ꢀ x, 1/2 + y, 1/2 ꢀ z), 3.306(5), 148.
this fact strongly supporting the presence of a 5-nitroso group
(–N@O) over a 5-oxime one (@N–O). The C2–O2 and C6–O6 distances
correspond to a double bond (ca. 1.22 Å) whereas C4–O4 distance
is longer (ca. 1.25 Å) as consequence of both deprotonation and
coordination effects. A similar behaviour of the Re-coordinated
are slightly displaced to a upper wavenumber by coordination.
The shift to a lower wavenumber of the
m(N@O) vibration (ca.
20 cmꢀ1) strongly suggests, in accordance with X-ray structural re-
sults, the coordination through the N5 nitrogen.
The infrared spectra of 2, 3 and 4 show as main feature the
DVIOH anion has been found, in a similar way than the analo-
disappearance of
oxime group and the concomitant appearance of the
m
(N–O) band (ca. 930–1080 cmꢀ1) from the
ꢀ1
gous VIOHꢀ1 complex and other related coordination compounds
[18]. The remaining distances and angles are similar to those of
violuric acid. Also, the higher acidity of violuric acids, if compared
with DANTU one, promotes both the deprotonation of violuric
acids to bind to Re(I) and the substitution of the chloride ligand
by water in the tricarbonyl-Re-core.
In every complex, the organic moiety and the five-membered
chelate ring are quasi-coplanar because the pyrimidine skeleton
least-squares plane and the chelate rings (Re–O4–C4–C5–N5)
define acute angles of 1.5(4)° (2) and 2.2(3)° (4). The pyrimidine
cycles show small deviations from planarity: N1 (0.011(7) Å)
for Re/VIO and C2 (0.024(7) Å) and N3 (ꢀ0.022(6) Å) for Re/DVIO.
In the crystal structure of [ReCl(CO)3(DANTU)]ꢁ1/2CH3CN, the
hydrogen bonding network is formed by sheets which are placed
parallel to the bc plane, as shown in Fig. 3. In the [Re(H2O)(CO)3
(DVIOHꢀ1)] structure, a 2D supramolecular layer parallel to the ab
plane is formed by means of the cooperative effect of O(carbonyl)
ꢁ ꢁ ꢁO(carbonyl) van der Waals interactions with three hydrogen
bonds involving the coordinated water molecules (see Fig. 4).
m(N@O)
band (ca. 1490–1510 cmꢀ1) from the nitroso group indicating
that, upon complexation, the ketone-oxime form of the free-
ligands becomes completely in the nitroso-enolato one; also, the
bands assignable to C–O stretching vibrations are also shifted
[18–20] due to both deprotonation and coordination cooperative
effects.
In the 600–200 cmꢀ1 range, one band, at around 451 cmꢀ1
including the stretching vibrations of Re–C and Re–O, and another,
at around 270 cmꢀ1, due to
observed.
m(Re–Cl) [38] for complex 1, can be
The coordination mode of the ligands in the rhenium(I) com-
plexes deduced from IR data is confirmed by a general deshielding
of some signals in NMR. In the 1H NMR spectra of the complexes
the signals are downfield shifted due to the coordination process.
The ligand DANTU forms a strong intramolecular H-bond between
the oxygen atom from the 5-nitroso group and one hydrogen atom
from the 6-amino group. This explains the appearance of two
signals in 1H NMR spectra at around 12 and 11 ppm, the proton in-
volved in the H-bond being downfield shifted.
As it was before mentioned, the violurato ligands in the rhe-
nium(I) complexes are found in the nitroso-enolato tautomeric
form. Due to the pseudo-symmetry of the uracil moiety through
the pseudo-two-fold axis connecting O2–C2–C5–N5 atoms, the
free-MVIO could establish an intramolecular H-bond through
either @N–OHꢁ ꢁ ꢁO4 or @N–OHꢁ ꢁ ꢁO6, an identical duality for the
coordination (N5,O4 or N5,O6) can take place as well. Both reasons
can explain the couple of signals assignable to both the N–CH3 and
N–H groups in the 1H NMR spectra of free-MVIO and the Re-MVIO
compounds; accordingly, the 13C NMR spectra of free-MVIO and
Re-MVIO display two signals for methyl, C4, C5 and C6 carbon
atoms due to the existence of two potential quasi-equivalent me-
tal-binding arrangements, thus each carbon atom lying into two
slightly different electronic surroundings.
3.2. Conductivity and spectral studies
The molar conductivity measurements indicate all rhenium(I)
compounds behave as non-electrolyte [36].
In the infrared spectra of metal complexes, tentative assign-
ments for the more significant bands have been made by compar-
ison with the ligands ones [37]. The spectra of the complexes
exhibit a sharp, strong band over 2040 cmꢀ1 and two broad bands
around 1940 and 1900 cmꢀ1 attributed to
m(C„O) of the fac-
[Re(CO)3]+ unit.
The spectrum of complex 1 exhibits one broad band around
3200 cmꢀ1 corresponding to the antisymmetric stretching vibra-
tion of 6-amino group which clearly supports the presence of the
thiopyrimidine ligand in ketone-nitroso form. The ligand bands

266
N.A. Illán-Cabeza et al. / Inorganica Chimica Acta 366 (2011) 262–267
Fig. 4. Crystal packing diagram of [Re(H2O)(CO)3(DVIOHꢀ1)] from the a direction showing the arrangement of the bidimensional H-bonded sheets. H-bond parameters (Dꢁ ꢁ ꢁA
(Å)): Owꢁ ꢁ ꢁO2 (x, 1 ꢀ y, z), 2.651; Owꢁ ꢁ ꢁO5 (1 ꢀ x, y, z), 2.796; Owꢁ ꢁ ꢁO6 (1 ꢀ x, y, z), 2.758. Oꢁ ꢁ ꢁO van der Waals’ interactions (Å): O8ꢁ ꢁ ꢁO7 (ꢀ1 + x, y, z) 2.993; O9ꢁ ꢁ ꢁO7 (1 ꢀ x,
ꢀy, ꢀz) 3.021.
Fig. 5. Emission spectra of DANTU (1), VIO (2), MVIO (3), DVIO (4), [ReCl(CO)3(DANTU)]ꢁ1/2CH3CN (5), [Re(H2O)(CO)3(VIOHꢀ1)]ꢁ1/2H2O (6), Re(H2O)(CO)3(MVIOHꢀ1)ꢁ1/2H2O
(7), [Re(H2O)(CO)3(DVIOHꢀ1)] (8).
On comparing the 13C NMR spectra of complexes with those of
free ligands, a downfield shift of the signal of the C4 atom (ꢂ12–
18 ppm) can be observed. The tricarbonylrhenium(I) core has a
ligand shows a luminescence band at 290 nm assigned to a
* transition. In the rhenium(I) complex, a shift to a lower en-
ergy is observed (10 nm) with increase of the fluorescence inten-
p–p
dual function as
conclude that donor character of O4 atom predominates over the
-backbonding of Re(I) to the exocyclic carbonyl group. The signal
r acceptor and p donor but, in our case, we can
sity. Also, it can be observed a shoulder over 360 nm which is
assigned to a metal-to-ligand charge-transfer (MLCT) dp(Re)–p*
(ligand) transition.
p
of the carbon C5 is upfield shifted (ꢂ3 ppm) in Re(I)–DANTU com-
plex because of the electronic effects in the heterocyclic ring,
whereas in the rhenium(I) complexes of violuric acids this signal
is deshielded around 8 ppm, as consequence of the different tauto-
meric form present.
A weak emission is observed for VIO and Re–VIO complex when
excited to 225 nm. The ligand VIO shows a band around 350 nm
(p–p
* transition) which is blue-shifted in the rhenium(I) complex
(65 nm).
The emission spectrum recorded at 225 nm for MVIO shows
two bands at 294 and 350 nm which are assigned to * transi-
p–p
tions. In the Re/MVIO complex, the band to 350 nm is displaced
to 325 nm as consequence of the coordination of the ligand to
the metal ion. The absence of emission for complex 4 at 242 nm,
3.3. Luminescent properties
The photochemistry of diimine rhenium(I) tricarbonyl com-
plexes has been extensively studied and they play and important
role in understanding the photophysical and light-induced elec-
tron-transfer and electronic energy-transfer processes [7–10,31].
Fig. 5 displays the emission data of the complexes in CH3CN
solution at room temperature (10ꢀ5 M). The emission spectrum
of DANTU and Re/DANTU complex were measured from 250 nm
to 450 nm with an excitation wavelength of 226 nm. The free
suggests that the complex formation quenches the
of the ligand DVIO.
p–p* emission
Acknowledgements
Thanks are due to the ‘‘Plan de Apoyo a la Investigación, al
Desarrollo Tecnológico y a la Innovación’’ of the Universidad de

N.A. Illán-Cabeza et al. / Inorganica Chimica Acta 366 (2011) 262–267
267
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crystallographic data for compounds 1, 2 and 4. These data can be
obtained free of charge 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,
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