Biologically relevant O,S-donor compounds. Synthesis, molybdenum complexation and xanthine oxidase inhibition

Article abstract of DOI:10.1039/b717172b

Two O,S-donor ligands, hydroxythiopyrone and hydroxythiopyridinone derivatives, were developed and studied, as well as the corresponding O,O-derivatives, with a view to their potential pharmacological applications as xanthine oxidase (XO) inhibitors. The biological assays revealed that the O,S-ligands present high inhibitory activity towards XO (nanomolar order, close to that of the pharmaceutical drug allopurinol), in contrast to the corresponding O,O-analogues. Due to the biomedical relevance of this molybdenum-containing enzyme, the corresponding Mo(vi) complexes were studied both in solution and in the solid state, aimed at identifying the source of the biological properties. The solution studies showed that, in comparison with the O,O-analogues, the Mo(vi) complexes with the O,S-ligands present some stabilization, which is even more pronounced for the reduced Mo(iv) species. The crystal structures of the Mo(vi) complexes with the hydroxythiopyrone revealed good flexibility of the coordination modes, with two structural isomers and two polymorphic forms for a mononuclear and a binuclear species, respectively. These results give some support to mechanistic proposals for the XO inhibition involving the interaction of the thione group with the molybdenum cofactor, thus indicating a role of the sulfur atom in the XO inhibition. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717172b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Biologically relevant O,S-donor compounds. Synthesis, molybdenum
complexation and xanthine oxidase inhibition†‡
S´ılvia Chaves,a Marco Gil,a So´nia Cana´rio,a Ratomir Jelic,b Maria Joa˜o Roma˜o,c Jose´ Trinca˜o,c
Eberhardt Herdtweck,d Joana Sousa,e Carmen Diniz,e Paula Frescoe and M. Ame´lia Santos*a
Received 8th November 2007, Accepted 10th January 2008
First published as an Advance Article on the web 20th February 2008
DOI: 10.1039/b717172b
Two O,S-donor ligands, hydroxythiopyrone and hydroxythiopyridinone derivatives, were developed
and studied, as well as the corresponding O,O-derivatives, with a view to their potential
pharmacological applications as xanthine oxidase (XO) inhibitors. The biological assays revealed that
the O,S-ligands present high inhibitory activity towards XO (nanomolar order, close to that of the
pharmaceutical drug allopurinol), in contrast to the corresponding O,O-analogues. Due to the
biomedical relevance of this molybdenum-containing enzyme, the corresponding Mo(VI) complexes
were studied both in solution and in the solid state, aimed at identifying the source of the biological
properties. The solution studies showed that, in comparison with the O,O-analogues, the Mo(VI)
complexes with the O,S-ligands present some stabilization, which is even more pronounced for the
reduced Mo(IV) species. The crystal structures of the Mo(VI) complexes with the hydroxythiopyrone
revealed good flexibility of the coordination modes, with two structural isomers and two polymorphic
forms for a mononuclear and a binuclear species, respectively. These results give some support to
mechanistic proposals for the XO inhibition involving the interaction of the thione group with the
molybdenum cofactor, thus indicating a role of the sulfur atom in the XO inhibition.
dysfunction, hypertension and heart failure;5 XO catalyses the
oxidative hydroxylation of purine substrates, at the molybdenum
centre of the pterin cofactor, with the molybdenum coordinated
to the cis-dithiolene group of one pyranopterin plus additional
Introduction
Molybdenum is an essential trace element, which plays an
important role in many biological systems.1 It is required as the
oxo, sulfido and hydroxo groups (see Scheme 1).2,4,6 That oxidative
process generates reactive oxygen species (ROS), which can cause
cell membrane disintegration, membrane protein damage and
DNA mutation, initiating or propagating the development of
diseases such as liver injury and cancer.
cofactor component of various redox enzymes involved in almost
all forms of life, for example, nitrate reductase in plants (crucial
in the metabolism of nitrogen) and the xanthine oxidase (XO)
family in humans, which is implicated in several diseases, such as
gout (hyperuricemia) and cardiovascular diseases.2–4 Clinical and
basic experiments have indicated that disorder of the XO activity
is one of the principal causes of myocardial ischemic reperfusion
injury and it is involved in the pathogenesis of vascular endothelial
aCentro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico,
Av. Rovisco Pais, 1049-001, Lisboa, Portugal. E-mail: masantos@ist.utl.pt;
Fax: +351-21-8464455; Tel: +351-21-8419273
bFaculty of Science, Institute of Chemistry, P.O. Box 60, 34000 Kragujevac,
Serbia
Scheme 1 Schematic representation of the Mo active site and the
pyranopterin (molybdopterin) cofactor of xanthine oxidase.
cREQUIMTE-CQFB, Departamento de Qu´ımica, Faculdade de Cieˆncias e
Tecnologia, Universidade Nova de Lisboa, Portugal
dTechnische Universita¨t Mu¨nchen, Lehrstuhl fu¨r Anorganische Chemie,
Lichtenbergstrasse 4, D-85747, Garching, Germany
Inhibition of XO activity has been recognized to improve the
above pathophysiological conditions and to reduce ROS,7–9 thus
contributing to an important chemoprevention mechanism.10
Over the past years, there has been considerable effort in the
development of XO inhibitors,11 especially since allopurinol was
introduced in the clinical treatment of gout12 and was also shown to
decrease the extent of deleterious effects of tissue injury following
ischemia reperfusion. However, adverse effects associated with the
use of allopurinol prompted the need for new XO inhibitors with
increased therapeutic activity and fewer or no side effects. Most
of the reported potential XO inhibitors have been focused on
exploitation of the organic part of the compounds in order to
eLaborato´rio de Farmacologia, REQUIMTE-FARMA, Faculdade de
Farma´cia, Universidade do Porto, Rua An´ıbal Cunha, 164, 4050-123, Porto,
Portugal
† CCDC reference numbers 664272, 656368 and 656367 for complexes a-1,
b-1, and 2. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b717172b
‡ Electronic supplementary information (ESI) available: Table S1, con-
taining global formation constants for the hydrolytic species formed
2−
in the H+-MoO4 system; Fig. S2 showing a cyclic voltammogram
of MoO2(thiomaltol)2; Tables S3–S8, including more detailed data of
compound a-1; Fig. S9 illustrating compound a-1; Tables S10–S15 with
detailed data of compound b-1; Fig. S16 showing compound b-1. See DOI:
10.1039/b717172b
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improve the binding interaction with the pterin molecule and they
mostly incorporate phenolic derivatives.13
Results and discussion
Synthesis of the compounds
On the other hand, complexes of molybdenum with model
molecules have also become interest because of their potential
application as orally active drugs for the treatment of cardiac
dysfunctions associated with diabetes.14,15 Complexation of molyb-
denum(VI) with several type of O,O-donor ligands has been exten-
sively studied, namely with siderophore analogues of hydroxamate
or catecholate type.16,17 3-Hydroxy-4-pyrones and 3-hydroxy-4-
pyridinones (3,4-HP), in particular 3-hydroxy-2-methyl-4-pyrone
(maltol) and 3-hydroxy-2-methyl-4-pyridinone (DMHP or DFP),
are important bioligands in chelating therapy for the removal of
excessive metal ions from the body, such as iron (in transfusional
hemosiderosis of thalassemia major patients)18,19 or aluminium
(due to external injuries).20,21 Their Mo(VI) complexes have been
recently studied both in the solid state15 and in aqueous solution,22
aiming at their potential pharmacological application. Concerning
the corresponding thione ligands, possessing mixed O,S-donor
atoms, the study of their interaction with molybdenum(VI) seems
also of high biological interest, either as models or as inhibitors
of pterin-dependent molybdenum enzymes, the co-factors of
which have the molybdenum coordinated to S- and O-donor
atoms (see Scheme 1). However, unlike the study of molybdenum
complexation with O,O-donor ligands, identical studies with O,S-
or S,S-donor ligands are very scarce and mostly involve modelling
the reductive half reaction occurring at the molybdenum centre of
the enzymes.23,24
Following our recent interest in the development of O,O- and
O,S-donor ligands of 3,4-HP for Mo(VI) complexation22,25 and the
recognized biological interest of the O,S-donor ligands, namely
in relation to the inhibition of molybdenum-containing enzymes,
we report herein the preparation of simple O,S-donor ligands
(hydroxypyridinethione (3,4-HTP) and hydroxythiopyrone), the
physicochemical properties of their Mo complexes and results of
bioassays of these ligands as inhibitors of xanthine oxidase, in
comparison with the O,O-analogues (see Scheme 2). In particular,
the molybdenum complexes were investigated in solution, by UV-
Vis spectrophotometry and cyclic voltammetry, and also in the
solid state, by X-ray diffraction, aimed at the rationalization
of the inhibition of XO activity demonstrated by the new
O,S-compounds and also to gain some insight into potential
mechanisms.
The synthesis of the O,S-ligands, 3-hydroxy-1,2-dimethylpyridin-
4(1H)-thione (DMHTP) and 3-hydroxy-2-methyl-4H-pyran-4-
thione (thiomaltol), involved the thionation of the corre-
sponding commercially available O,O-derivatives, 1,2-dimethyl-
3-hydroxypyridin-4(1H)-one (DMHP) and 3-hydroxy-2-methyl-
4H-pyran-4-one (maltol), by refluxing with Lawesson’s reagent
in dry toluene, following a similar procedure to that previously
reported for the preparation of thiohydroxamic acids.25,26 The
molybdenum complexes in the solid state were readily prepared by
direct addition of an ammonium molybdate aqueous solution to
the thiomaltol ethanolic solution. Recrystallization from acetone
and slow evaporation of this solvent led to the appearance of two
different types of complexes at subsequent stages of the solvent
evaporation, firstly complex 1 and then complex 2.
Acid–base properties
For the equilibrium solution studies, spectrophotometric methods
were adopted because, under the usual potentiometric concentra-
tion conditions (mM), precipitation occurred in the Mo(VI) com-
plexation studies. The protonation constants of maltol, thiomaltol,
DMHP and DMHTP were determined from spectrophotometric
titrations i◦n 15% (v/v) MeOH aqueous solutions (see Fig. 1a and
2a), at 25 C and 0.1 M KCl, except for DMHTP, for which log
K2 (< 2) was determined by 1H NMR titration.
Scheme 2 Structural formulae of the synthesized compounds and
allopurinol.
Fig. 1 Spectrophotometric absorbance curves at indicated pH values for
(a) thiomaltol; (b) Mo(VI)/thiomaltol (CM = 2.52 × 10−5 M, CL/CM = 2).
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are slightly lower than those of their oxo analogues, DMHP
and maltol (9.82 and 8.67), which may be attributed to the
stabilisation enhancement of the corresponding conjugate bases,
due to the lower electronegativity and higher polarizability of
the sulfur atom, allowing greater delocalisation of the negative
charge of the phenolate. There is also a decrease in H-bond
interaction between the (thio)carbonyl group and the phenolic
proton, as compared with the corresponding oxo compound,
with concomitant decrease of the stability of the protonated
species. Identical reasons explain the fact that log K2 is lower
for DMHTP than DMHP. Furthermore, the higher acidity of
the pyrone (or thiopyrone) hydroxyl groups, as compared to that
of the corresponding pyridinone (or pyridinethione) derivatives
is attributed to the higher electronegativity of the ring O-atom
as compared to that of the N–CH3 group, allowing greater
stabilization of the phenolate negative charge. According to the
calculated protonation constants of the compounds, the neutral
HL species are predominant at physiological pH, a relevant feature
for the membrane crossing ability.
Mo(VI) complexation
Solution equilibrium studies. For the study of the Mo(VI)
complexes, spectrophotometric titrations were performed to avoid
the precipitation problems of potentiometric titrations, because
then much lower analytical concentration can be used (ca. 10−5 M).
Nevertheless, in aqueous Mo(VI) systems, the free metal ion does
Fig. 2 Spectrophotometric absorbance curves at indicated pH values for
(a) DMHTP; (b) Mo(VI)/DMHTP (CM = 2.52 × 10−5 M, CL/CM = 2).
2−
not exist (but several hydrolytic species do) and thus MoO4
was chosen as the metal (M) component in the calculations.
Therefore, the first step for the Mo(VI) complexation study was
the calculation of the overall stability constants for the Mo(VI)
hydrolytic species by spectrophotometric titration, under the
experimental conditions (I = 0.1 M KCl, 15% (v/v) CH3OH–
H2O, T = 25.0 0.1 ◦C). The calculated values are summarized
in Table S1 of ESI,‡ which also includes previously reported values
obtained under different working conditions. These hydrolytic
species absorb in the 200–250 nm range and were included in the
equilibrium models of the various Mo(VI)–ligand systems. The
spectral parameters of the different ligand species obtained from
the acid–base studies were also introduced into the computer
program to aid the fitting of the experimental data with a
complexation model. Spectra for each metal–ligand systems were
recorded at various pH values and the corresponding data for the
Mo(VI)–thiomaltol and Mo(VI)–DMHTP systems are represented
in Fig. 1b and 2b.
Comparison of Fig. 1b and 2b with Fig. 1a and 2a, respectively,
reveals significant differences, especially for acidic pH values,
due to the Mo–ligand transition bands but also to concomitant
differences in the ligand spectra, namely in the UV range. The
best fit of the experimental data was obtained for the equilibrium
model for which global stability constants are reported in Table 1.
The proposed equilibrium model includes bis- and mono-chelated
complexes with the formation reactions represented by eqn (1) and
eqn (2):
Table 1 Stepwise protonation constants (log Ki) for the ligands (maltol,
thiomaltol, DMHP and DMHTP) and also global formation constants
and pM values of their Mo(VI) complexes (I = 0.1 M KCl, 15% (v/v)
CH3OH–H2O, T = 25.0 0.1 ◦C)
Maltol
Thiomaltol
DMHP
DMHTP
log K1
log K2
8.67(3)
8.67(3)a
8.513b
—
8.27(1)
8.16(2)c
8.12(2)d
—
9.82(1)
9.76e
9.70(1)
9.47c
9.749(4)f
3.74(2)
3.62e
9.44(1)d
0.95(3)g
3.655(7)f
log bMoO
log bMoO
pMo h
38.51(5)
20.71(6)
6.15
38.50(3)
21.19(3)
6.57
39.31(4)
40.22(4)f
20.10(4)
20.0(1)f
6.00
41.48(6)
21.82(5)
6.18
2 L2
3 L
a Ref. 27 (I = 0.16 M NaCl, T = 37 ◦C, in water). b Ref. 28. c Ref. 29 (I =
0.10 M KCl, T = 25.0 ◦C, in water). d Ref. 30 (I = 0.16 M NaCl, T =
25.0 ◦C, in water). e Ref. 31 (I = 0.1 M KCl, T = 25.0 ◦C, in water). f Ref.
22 (I = 0.2 M KCl, T = 25.0 ◦C, in water). g Determined by H NMR
1
titration. h pMo values (CMo = 10−6 M, CL = 10−5 M, pH = 7.4).
The calculated stepwise protonation constants obtained are
collected in Table 1, together with some literature values obtained
under different experimental conditions.22,27–31
Analysis of Table 1 shows that the fully protonated forms of
these compounds have one (maltol and thiomaltol) or two (DMHP
and DMHTP) dissociable protons, and the corresponding proto-
nation constants are in agreement with values typically attributed
to the corresponding hydroxyl and N-pyridinyl groups.32 DMHTP
and thiomaltol present log K1 values (9.70 and 8.27) which
MoO24− + 2L− + 4H+
MoO24− + L− + 2H+
MoO2L2 + 2H2O
(1)
(2)
−→
←−
−
−→
[MoO L] + H2O
←−
3
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Fig. 3 Species distribution diagrams for the Mo(VI) systems with (a) maltol; (b) thiomaltol; (c) DMHP; (d) DMHTP (CM = 2.52 × 10−5 M, CL/CM = 2).
pMo values (at physiological pH 7.4, CL/CMo = 10, CMo
=
solution of electrolyte, KNO3 0.1 M in 20% (v/v) DMSO. The
choice for this solvent was based on insolubility problems of the
reagents/products in different solvents under the CV working
conditions. The voltammetric measurements were performed at
pH ca. 4 (see cyclic voltammogram of MoO2(thiomaltol)2 in Fig.
S2 of ESI‡) because under our working conditions (concentration
and ligand-to-metal stoichiometry) species distribution simula-
tions indicated ca. 90% formation of the MoO2L2 species for all
the studied binary systems.
All the Mo complexes exhibited a two-electron irreversible
reduction peak (absence of oxidation peak) at: E = −1.08 V, for
maltol; E = −1.04 V for thiomaltol; E = −1.34 V for DMHP;
E = −1.23 V for DMHTP. The Mo(VI/IV) redox behaviour for the
Mo–DMHP complex is in agreement with the value previously
reported in DMSO (E = −1.62 V, vs. the Cp2Fe/[Cp2Fe]+ couple
at E = 0 V).33 A comparative analysis of these results indicates
that the substitution of an O-donor by a S-donor induces a
positive shift of the peak potentials, similar to what is observed in
other Mo(VI/IV) chelates.22,34 These values reflect the fact that the
reduced molybdenum(IV) complexes are more stabilized by the
softer O,S-ligands (thiomaltol and DMHTP) than by the O,O-
ligands (maltol and DMHP).
10−6 M) were determined (see Table 1), on the assumption that
no precipitation occurs for these concentration values and also
that the equilibrium model determined in this work is valid for
the imposed physiological conditions. Analysis of the calculated
pMo values shows that the softer O,S-donor ligands (thiomaltol
and DMHTP) have a slightly higher affinity for the Mo(VI) than
the harder O,O-analogues. Moreover, the formation constants
obtained for DMHP are similar to those previously determined in
water.22
The concentration distribution diagrams of the Mo complex
species at a 2 : 1 ligand-to-metal ion molar concentration ratio
(Fig. 3) show that, under the experimental methanol–aqueous
conditions, the bis-chelated species predominate below pH 4–5;
above these pH values the bis-chelate species decompose through
the formation of trioxo-molybdenum mono-chelate complexes
with maximum percentage formation in neutral media. Further-
more, while DMHP evidences some competition between the
polyoxamolybdates and the Mo–ligand complexes till pH ca. 5,
the other ligands do not show such a strong competition, and
2−
both thiomaltol and DMHTP retard the formation of MoO4 to
higher pH values (≥7) than the analogous oxo complexes.
Electrochemistry
X-Ray crystallography†
The molybdenum centre of XO participates in catalytic reactions,
undergoing an oxidation state change between Mo(VI) and Mo(IV),
although Mo(V) is also believed to be involved during the course
of the catalysis, at least in the oxidative regeneration of the active
site.2
Aimed at providing further insight into the role of the
compounds as inhibitors of XO, the electrochemical behaviour
of the corresponding molybdenum complexes was studied by
cyclic voltammetry (CV) at a glassy carbon electrode and in a
Compound 1 was found in two polymorphic forms, a-1 and b-1.
Crystal data and data collection parameters are summarized in
Table 2.
The molecular structure in the solid state of
[MoO2(thiomaltol)]2(l-O) (a-1), shown in Fig. 4, reveals an
oxo-bridged binuclear compound. Each molybdenum atom is
ligated to two terminal oxo groups, one bridging oxygen (O3) plus
one sulfur and one oxygen atoms, from the thiomaltol ligand,
forming a five-coordinated structure. The coordination sphere is
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Table 2 Crystal data and summary of data collection and refinement of [MoO2(thiomaltol)]2(l-O), complex 1, and [MoO2(thiomaltol)2], complex 2
Complex 1
a-1
b-1
Complex 2
Crystal data
Formula
C12H10Mo2O9S2
554.22
Monoclinic
C12H10Mo2O9S2
554.22
Monoclinic
C12H10MoO6S2·1/2H2O
419.27
Formula weight
Crystal system
Space group
Triclinic
¯
C2/c (no. 15)
P21/n (no. 14)
P1 (no. 2)
˚
˚
˚
Unit cell parameters
a = 31.3625(3)/A
a = 8.2509(1)/A
a = 9.8133(2)/A
˚
˚
˚
˚
˚
b = 8.8600(1)/A
b = 13.9940(2)/A
b = 10.9684(2)/A
˚
c = 12.3099(1)/A
c = 14.6426(2)/A
c = 14.5117(3)/A
b = 108.7833(6)◦
b = 105.8518(6)◦
a= 103.873(1)◦
b = 106.201(1)◦
c = 90.026(1)◦
1452.40(5)
4
3
˚
V/A
3238.40(6)
8
2.273
1.853
1626.39(4)
4
2.263
1.845
Z
D(calc.)/g cm−3
l(MoKa)/mm−1
F(000)
1.917
1.217
836
2160
1080
Crystal size/mm
Temperature/K
0.05 × 0.13 × 0.13
0.10 × 0.15 × 0.23
0.20 × 0.20 × 0.02
173
173
273
Data collection and refinement
˚
Wavelength/A
MoKa 0.71073
Rotating anode
1.4, 25.3
MoKa 0.71073
Rotating anode
2.0, 25.3
MoKa 0.71073
Sealed tube
1.51, 25
h min, max/◦
Number of reflections:
Total
37862
2966
40422
2963
81371
5108
Unique
Rint
0.044
2682
2966
0.0204
0.0476
1.06
−0.42, 0.35
0.034
2826
2963
0.0164
0.0407
1.14
−0.31, 0.30
0.0564
4450
5108
0.0316
0.0837
0.956
−0.904, 2.376
Observed data (I > 2.0r(I))
No. reflections refined
R1 (I > 2.0r(I))
wR2 (all data)
GooF
−3
˚
Residual density (min, max)/e A
The two Mo centers are essentially identical with no significant
differences in the coordination geometry or in bond distances (see
Table 3). Mo1 is coordinated to the two terminal oxo groups O1
and O2 (O4 and O5 for Mo2), to the bridging oxygen O3 and to
O18 and S1 (O28 and S2 for Mo2) from the thiopyrone ring. Both
Mo centers adopt a square-based bipyramidal geometry, with O1
and O28 (or O5 and O18 for Mo2) occupying the apical positions.
This is true for both polymorphs.
A search in the Cambridge Structural Database35 revealed
over twenty Mo(VI) oxo-bridged dioxo compounds with a similar
core geometry. The bond distances around the two Mo centers
of both polymorphs from complex 1 are comparable to the
values found for other oxo-bridged dinuclear Mo compounds.
Very similar structures are exhibited by two dianionic thio-
phenolate complexes: [PPh4]2[Mo2O5(SC6H4O)2] (CUSLIL)36 and
[NBu4]2[Mo2O5(SC6H4O)2] (NIJGES),37 which possess two diox-
omolybdenum(VI) centers bridged by one l2-oxido and two l2-O-
2-thiophenolate ligands. The long range distances for polymorph
˚
˚
a-1, Mo1–O28, 2.50 A, Mo2–O18, 2.52 A (and for b-1 Mo1–O28,
Fig.
4
ORTEP plot of the molecular structure of [MoO2-
˚
˚
2.55 A, Mo2–O18, 2.61 A) are comparable to the corresponding
distances in those complexes: CUSLIL, 2.482 A and NIJGES,
2.466 A and 2.487 A.
(thiomaltol)]2(l-O) (a-1) with ellipsoids at 30% probability, at 173 K.
Hydrogen atoms are omitted for clarity.
˚
˚
˚
extended to six with a long-range contact to the oxygen atom of
the adjacent thiomaltol ligand bound to the second molybdenum
atom. The selected bond lengths and distances are summarized in
Table 3 and a more complete list is available as ESI.‡
The molecular structure of [MoO2(thiomaltol)2] (complex 2),
the crystal data of which are given in Table 2, revealed that
the asymmetric unit contains two independent molecules of
[MoO2(thiomaltol)2] (C12H10MoO6S2) and one water molecule
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◦
˚
Table 3 Selected bond distances (A) and angles ( ) for [MoO2(thiomaltol)]2(l-O)
a-1
b-1
a-1
b-1
˚
Selected bond distances/A
Mo1–S1
2.4770(7)
2.4755(6)
1.694(2)
1.709(2)
1.914(2)
2.158(2)
2.552(1)
2.4731(6)
1.909(2)
Mo2–O4
Mo2–O5
Mo2–O18
Mo2–O28
S1–C11
S2–C21
O18–C16
O28–C26
1.702(2)
1.699(2)
2.521(2)
2.158(2)
1.711(3)
1.720(3)
1.331(3)
1.337(3)
1.704(2)
1.695(2)
2.614(2)
2.139(2)
1.712(2)
1.717(3)
1.335(3)
1.333(3)
Mo1–O1
1.699(2)
1.701(2)
1.911(2)
2.155(2)
2.500(2)
2.4725(7)
1.912(2)
Mo1–O2
Mo1–O3
Mo1–O18
Mo1–O28
Mo2–S2
Mo2–O3
Selected bond angles/◦
S1–Mo1–O1
S1–Mo1–O2
S1–Mo1–O3
S1–Mo1–O18
S1–Mo1–O28
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O18
O1–Mo1–O28
O2–Mo1–O3
O2–Mo1–O18
O2–Mo1–O28
O3–Mo1–O18
O3–Mo1–O28
O18–Mo1–O28
Mo1–O3–Mo2
100.55(7)
88.65(7)
146.38(5)
77.52(5)
83.33(5)
104.36(9)
104.37(8)
97.99(8)
169.35(7)
106.34(9)
155.56(8)
85.55(8)
76.99(7)
68.49(7)
73.01(6)
110.81(8)
100.93(6)
89.68(5)
146.77(5)
78.01(4)
83.62(4)
104.19(8)
104.75(7)
100.07(7)
169.77(7)
103.90(7)
154.49(6)
84.89(6)
77.03(6)
67.92(6)
71.74(5)
113.11(7)
S2–Mo2–O5
S2–Mo2–O4
S2–Mo2–O3
S2–Mo2–O18
S2–Mo2–O28
O3–Mo2–O4
O3–Mo2–O5
O5–Mo2–O28
O5–Mo2–O18
O4–Mo2–O5
O4–Mo2–O28
O4–Mo2–O18
O3–Mo2–O28
O3–Mo2–O18
O18–Mo1–O28
100.16(7)
88.62(7)
146.14(6)
83.45(4)
77.45(5)
106.07(9)
105.32(9)
99.73(8)
170.69(7)
104.1(1)
154.19(9)
84.49(9)
76.57(7)
68.26(7)
72.53(7)
102.15(6)
88.49(5)
145.87(5)
83.18(3)
77.89(4)
103.41(7)
105.39(7)
99.86(7)
168.21(7)
104.73(8)
153.93(7)
85.79(6)
77.88(6)
66.30(5)
70.74(5)
Table 4 Selected bond distances (A) and angles (◦) for [MoO2-
˚
of crystallization, as depicted in Fig. 5. Unexpectedly, the two
molecules are structural isomers of different geometry.
(thiomaltol)2]
Molecule A
Molecule B
˚
Selected bond distances/A
Mo1–O12
1.714(3)
Mo2–O21
Mo2–O22
Mo2–O48
Mo2–O38
Mo2–S3
1.708(3)
1.714(3)
1.975(3)
2.130(2)
2.4630(10)
2.7376(9)
Mo1–O11
Mo1–O28
1.718(3)
2.095(3)
Mo1–O18
Mo1–S1
Mo1–S2
2.127(2)
2.4832(10)
2.4993(10)
Mo2–S4
Selected bond angles/◦
O12–Mo1–O11
O12–Mo1–O28
O11–Mo1–O28
O12–Mo1–O18
O11–Mo1–O18
O28–Mo1–O18
O12–Mo1–S1
O11–Mo1–S1
O28–Mo1–S1
O18–Mo1–S1
O12–Mo1–S2
O11–Mo1–S2
O28–Mo1–S2
O18–Mo1–S2
S1–Mo1–S2
102.62(14)
91.60(12)
158.55(12)
159.57(12)
90.65(12)
80.58(10)
84.00(10)
107.61(9)
89.59(7)
77.14(7)
109.08(9)
83.15(9)
77.01(7)
87.66(7)
O21–Mo2–O22
O21–Mo2–O48
O22–Mo2–O48
O21–Mo2–O38
O22–Mo2–O38
O48–Mo2–O38
O21–Mo2–S3
O22–Mo2–S3
O48–Mo2–S3
O38–Mo2–S3
O21–Mo2–S4
O22–Mo2–S4
O48–Mo2–S4
O38–Mo2–S4
S3–Mo2–S4
103.82(12)
96.88(12)
106.95(12)
90.59(11)
159.87(12)
84.75(10)
102.12(9)
85.28(10)
154.26(7)
77.90(7)
165.34(9)
90.36(9)
75.05(7)
76.65(7)
82.47(3)
Fig. 5 ORTEP plot of the two independent structural isomers of
[MoO2(thiomaltol)2] in the asymmetric unit and one water molecule of
crystallization, drawn with ellipsoids at 30% probability.
161.26(3)
In both structures the Mo coordination sphere adopts a dis-
torted octahedral geometry, in which the Mo atom is coordinated
by two terminal oxo groups and two O,S-thiomaltol ligands. The
sulfur and oxygen atoms from the thiomaltol moiety form a five-
membered chelate ring with the Mo atom.
A search in the Cambridge Structural Database35 revealed
only five Mo(VI) dioxo compounds with two additional bidentate
O,S-ligands and a similar core geometry (KELXII, LIMSEF,
MIJCEN, MPTHMO, XAWDUV).37–41 With the exception of
the long Mo2–S4 distance (see Table 4), all other bond distances
around the Mo centers of both molecules A and B are comparable
to values found for the related MoO2 compounds.37–41
Analysis of selected bond lengths and angles given in Table 4
(a more complete list is given in ESI‡) shows that in molecule A,
the two sulfur atoms are trans to each other, with an S1–Mo1–
S2 angle of 161.26(3)◦, while in molecule B they are cis-oriented
and the corresponding angle, S3–Mo2–S4, is 82.47(3)◦. This cis
orientation of the two thiolates in molecule B has implications
for the relative Mo–S distances. While the bond distance of
1778 | Dalton Trans., 2008, 1773–1782
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Table 5 Xanthine oxidase (XO) inhibition data of the studied compounds
and of standard XO inhibitor (allopurinol), for the oxidation of xanthine
to uric acid and superoxide (results are mean SD (n))
˚
Mo2–S3, 2.4630(10) A, is of the same order of magnitude as
those reported for other thio complexes,37–42 the Mo2–S4 bond
distance, 2.7376(9) A, is significantly longer. This bond elongation
˚
can be attributed to the strong trans effect of a terminal oxygen
Compound
IC50/lM SD
n
=
trans to a thiolate sulfur atom, as found for Mo O24 and
Mo–S4 (O21–Mo–S4, 165.34(9)◦). Similar trans effects of the
Maltol
Thiomaltol
DMHP
DMHTP
Allopurinol
1418 330
104 26
159 52a
60 35
6
20
3
4
12
=
Mo O bond have been reported for other dioxo Mo compounds,
with distances of Mo–S of 2.769 A and 2.65 A, respectively.
Noticeable is the fact that a similar Mo–S bond lengthening (ca.
0.3 A) was also found by XAS studies on the reduced forms
40
41
˚
˚
41 19
˚
a p < 0.05, from values obtained in the presence of allopurinol.
of xanthine oxidase and xanthine dehydrogenase,43,44 which was
interpreted as resulting of the protonation to give a MoIV–SH
group. Similar evidence for lengthening of this bond was also
found in the crystal structure data of the reduced form of the
aldehyde oxidoreductase.45 However, in the present unit, excluding
any H-bond interaction between that sulfur atom and the water
molecule, as well as the existence of a reduced state for Mo2,
the reason for the weakening of the Mo–S interaction remains
unclear, although it can be probably attributed to electronic effects
associated with the above-mentioned trans effect.
compounds maltol and thiomaltol, the sulfur compound exhibits
better inhibition than the oxygen analogue. However, some sulfur
compounds have been previously related with toxic properties (4-
amino-6-mercaptopyrazolo[3,4-d]pyrimidine has toxic properties
as stated in the Lancaster catalogue). This reinforces the need
for more biological studies to confirm/discard any eventual toxic
effect of the herein studied O,S-donor compounds.
In molecule A, where the thiolate sulfurs are cis to the oxo
groups and trans to each other, both Mo–S bond distances (Mo1–
S1 2.4832(10) A and Mo1–S2 2.4993(10) A) are within the typical
Mechanistic proposals for the XO inhibition
˚
˚
In the present study, the inhibitory activity against XO has been
found to be higher for the thio than for the oxo compounds,
thus pointing towards an important role of the sulfur atom in
the inhibitory cycle, probably in the reductive half-reaction of
the Mo center. Moreover, as stated above, the phenolic hydroxyl
groups are slightly more acidic for the studied thio compounds
(pKa = 8.27; 9.70) than the corresponding oxo derivatives, (pKa =
8.51; 9.82) and also than the C-8 proton of xanthine (pKa = 14).2
Therefore, upon reduction of the Mo center, a proton can probably
range for most related compounds, specifically for the XO pterin
2
˚
cofactor in which MO–S (thiolate) distance is ca. 2.4 A. This is
the geometry found for most related compounds.
In the asymmetric unit, one water molecule is bridging both
molecules through hydrogen bonding interactions. The oxygen
˚
atom is placed at 2.797 A (O5–O13) from the terminal oxo of
˚
molecule A and 3.011 A (O5–O38) from the oxygen atom of one
of the thiomaltol ligands of molecule B. The corresponding D–H–
A angles are 170.0◦ and 160.9◦.
=
be transferred to the Mo S group, followed by the nucleophilic
attack of the thiolate (or phenolate) to the Mo center, while the
Mo-coordinated hydroxide can exchange with the solvent. The
attack of the molybdenum by the thiolate group would provide
a rationale for the inhibition requirement of a thione deriva-
tive. According to this mechanistic proposal, the O,S-ligands
(3-hydroxy-4-thiopyrone and 3-hydroxy-4-thiopyridinone) may
compete with xanthine or purines, for the molybdenum center,
inactivating and disabling the cofactor for the usual catalytic
oxidative hydroxylation of these substrates. The formation of such
a stable intermediate could be rationalized by the existence of a
high affinity between the sulfur atom of the ligand and Mo(IV)
(soft base and acid respectively), according to Pearson’s HSAB
principle,49 as well as by the electron buffering effect of the ligand
benzenoid ring. Actually, the electrochemical studies provided
confirmation of the formation of stable Mo(IV) complexes with
the O,S-ligands.
The polymorphism exhibited by complex 1, as well as the
presence of the two isomers co-crystallized in complex 2, reflect
the high flexibility of both complexes, allowing changes of the
coordination mode of thiomaltol.
Biological studies
Xanthine oxidase produces uric acid and superoxide and its
activity has been reported to increase during oxidative stress and
in radical-mediated diseases.46 In the present work, the inhibitory
activity of XO by the studied compounds was evaluated and
compared with that of allopurinol. Allopurinol has a structure
analogous to that of hypoxanthine and represents a competitive
inhibitor of XO.47
The compounds in this study showed an inhibitory effect on
the XO-catalyzed conversion of xanthine to uric acid. It was
monitored and evaluated in the presence of various concentrations
of the compounds. Thiomaltol, DMHP and DMHTP were found
to be good inhibitors of XO, but DMHTP was revealed to be
the most promising compound, although its potency is slightly
lower than that of allopurinol (see Table 5). As previously
demonstrated, XO inhibitory activity can differ depending on the
type of substituting groups present in the inhibitor as well as
on their positions.48 The obtained results revealed that DMHTP
is a stronger XO inhibitor than DMHP, which seems to result
from the presence of a sulfur donor atom in the compound
backbone instead of an oxygen atom. Likewise, for the pair of
Several crystallographic studies on xanthine oxidase,50–52 as well
as on the related enzyme (aldehyde oxido-reductase, AOR) from
Desulfovibrio gigas,45,53 have led to proposals of the XO reaction
mechanism. They have reported different modes of inhibition,
which can be either mechanism, based on interaction of the
inhibitor/substrate with the Mo of the reduced enzyme or on
shape complementarities of the substrate-binding pocket.
In analogy with the XO reaction mechanism, the partial one-
electron oxidation and sulfur proton dissociation may also be
admitted to lead to intermediate species possessing molybdenum
in the paramagnetic V valence state.
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This work represents a first approach to the evaluation of these
types of O,S-ligands as inhibitors of XO, but further studies will
be performed to clarify the mechanism involved, including the
identification of eventual Mo(V) intermediates, examination of
kinetic parameters and eventual crystallographic studies of the
XO–inhibitor adduct.
(d, 1 H, H-Py), 7.31 (d, 1 H, H-Py), 2.45 (s, 3 H, CH3). m/z (FAB):
143 (M + 1). Anal. calc. for C6H6O2S: C, 50.69; H, 4.25; S, 22.55%.
Found: C, 50.67; H, 4.02; S, 22.42%.
3-Hydroxy-1,2-dimethylpyridin-4(1H)-thione (DMHTP). To a
solution of 1,2-dimethyl-3-hydroxypyridin-4(1H)-one (1.0 g,
7.2 mmol) in dry toluene (150 mL), Lawesson’s reagent (1.45,
3.60 mmol) was added and the mixture was left refluxing under
nitrogen for 4 h. The reaction mixture was left to reach room
temperature under nitrogen and then the solvent was removed
under vacuum. The product was purified by flash chromatography,
eluent CH2Cl2–MeOH 12 : 1. After evaporation of the solvent, the
product was obtained as a pure solid. Yield: 55% (0.61 g); mp:
165–167 ◦C. IR (KBr) 1622 cm−1 (mC=S). 1H-NMR (CDCl3/TMS):
8.21 (s, 1 H, OH), 7.55 (d, 1 H, H-Py), 7.26 (d, 1 H, H-Py), 3.80
(s, 3 H, CH3), 2.44 (s, 3 H, CH3). m/z (FAB): 156 (M + 1). Anal.
calc. for C7H9NOS: C, 54.17; H, 5.84; S, 20.66%. Found: C, 53.99;
H, 5.80; S, 20.51%.
Experimental
General information
1H-NMR spectra were recorded on a Varian Unity 300 MHz
spectrometer at probe temperature. The following abbreviations
are used: s, singlet; d, doublet. IR spectra were recorded with a
Perkin-Elmer System 2000 FTIR instrument. The pD values were
adjusted with DCl or CO2 free KOD, by using a Crison 2001 instru-
ment fitted with a combined Mettler Toledo U402-M3-S7/200 mi-
croelectrode. Melting points were determined with a Leica Galen
III hot stage apparatus and are uncorrected. Elemental analyses
were performed on a Fisons EA1108 CHNF/O instrument. Mass
spectra were obtained on a VG TRIO-2000 GC/MS instrument.
Electronic spectra were recorded with a Perkin-Elmer Lambda
9 spectrophotometer, using 1 cm path length cells, at 25.0
Mo–thiomaltol complex. To a solution of 3-hydroxy-2-methyl-
4H-pyran-4-thione (30 mg, 0.21 mmol) in ethanol (2 mL), an
aqueous solution (3 mL) of ammonium molybdate (0.126 g,
0.105 mmol) was added. After 2 h 30 m, the orange precipitate
was filtered off and washed three times with water, ethanol and
◦
0.1 C. Cyclic voltammetry was performed at room temperature
ether. The pure complex was obtained as an orange solid. Yield:
◦
43.8% (19 mg); mp >230 C. IR (KBr) 1572, 1497 cm−1 (mC=
,
using an AUTOLAB potentiostat, and a Metrohm 744 pH-meter
with a Metrohm combined electrode 6.0228.000 PT1000 was used
for the respective pH measurements. X-Ray diffraction studies
were developed with a Nonius Kappa CCD and a Bruker X8
APEXII diffractometers. An Amplex Red Xanthine/Xanthine
Oxidase Assay Kit (Invitrogen Detection Technologies, Carlsbad,
CA) was used for monitoring xanthine oxidase activity and the
readings were made in a BioRad Benchmark Microplate Reader
(Hercules, CA).
S
mC=C ring). Anal. calc. for [MoO2(C12H10O4S2)]: C, 35.13; H, 2.46; S,
15.63%. Found: C, 34.68; H, 2.22; S, 15.95%.
A solution of molybdenum complex (20 mg) in acetone (2 mL)
was left at room temperature. Two types of yellow crystals were
formed during the slow evaporation of the solvent: firstly complex
1 and then complex 2, which were studied by X-ray diffraction.
Spectroscopic solution studies
Reagents and solutions. The molybdenum(VI) stock solution
(0.1007 M) was prepared with Na2MoO4 and the exact metal
ion concentration was determined gravimetrically via precipitation
of the quinolin-8-olate.22 The titrant solution (0.1 M KOH) was
prepared from a carbonate-free commercial concentrate (Titrisol)
and standardized by potentiometric titration with potassium
hydrogen phthalate; it was discarded whenever the percentage of
carbonate (Gran’s method)55 was about 0.5% of the total amount
of base.
Materials
All the chemicals were of analytical reagent grade and used as
supplied without further purification. Whenever necessary, the
organic solvents were purified by standard methods.54 Where
noted, compounds were purified by flash chromatography column
with Merck 40–70 mesh silica gel. DMHP (1,2-dimethyl-3-
hydroxypyridin-4(1H)-one) and maltol (3-hydroxy-2-methyl-4H-
pyran-4-one) were purchased from Aldrich. Tetramethylsilane
(TMS) and sodium 3-trimethylsilyl-d4-propionate (DSS) were
used as 1H-NMR internal references in CDCl3 and D2O, re-
spectively. For the electrochemical measurements the solvent was
DMSO (Aldrich) and the supporting electrolyte KNO3 (Merck).
Measurements. The 1H-NMR titration of a DMHTP solution
(≈0.02 M) in 15% (v/v) MeOD–D2O with DSS was performed in
NMR tubes (0.6 < pD < 7), the combined microelectrode being
calibrated with standard buffered aqueous solutions. The final pD
values were determined from the equation pD = pH* + 0.40,56 in
which pH* corresponds to the reading of the pH meter previously
calibrated with aqueous buffers at pH 4 and 7. Spectrophotometric
titrations of the ligands and their molybdenum complexes in 15%
(v/v) MeOH–H2O solution were performed at ionic strength (I)
0.1 M KCl. Electronic spectra were recorded in the range 200–
450 nm and the working temperature was maintained at 25.0
0.1 ◦C by using a Grant W6 thermostat. For all the samples
prepared, the total volume was 20 mL, the ligand concentration
was about 5.0 × 10−5 M and the Mo(VI) concentration was ca.
2.5 × 10−5 M. Spectrophotometric titrations of Mo(VI) solutions
(CMo ca. 4 × 10−5 M) under the same working conditions were
Synthesis of ligands and molybdenum complexes
3-Hydroxy-2-methyl-4H-pyran-4-thione (thiomaltol). To a so-
lution of 3-hydroxy-2-methyl-4H-pyran-4-one (0.40 g, 3.2 mmol)
in dry toluene (25 mL), Lawesson’s reagent (0.64, 1.58 mmol) was
added and the mixture was left at 80 ◦C under nitrogen for 1 h
30 m. The reaction mixture was left to reach room temperature
under nitrogen and then the solvent was removed under vacuum.
The product was purified by flash chromatography, eluent CH2Cl2.
After evaporation of the solvent, the pure product was obtained
as a yellow solid. Yield: 78.3% (0.35 g); mp: 74–77 ◦C. IR (KBr)
1622 cm−1 (mC=S). 1H-NMR (CDCl3/TMS): 7.77 (s, 1 H, OH), 7.59
1780 | Dalton Trans., 2008, 1773–1782
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also carried out to determine the Mo(VI) hydrolytic species. Under
the experimental conditions used, the value determined for the
ionisation constant (pKw) was 13.84.
were refined accordingly. Small extinction effects were corrected
with the SHELXL-97 procedure with e = 0.00071(11).
Crystals of [MoO2(thiomaltol)2] (complex 2) were grown from
acetone, mounted on a glass fiber and diffraction data were
measured (see Table 2). The unit cell parameters were obtained
by full-matrix least-squares refinement of 9209 reflections. Data
collection and cell refinement were performed with APEX265 and
data reduction was performed with SAINT-Plus.66 The structure
was solved using the SHELXTL package,67 and refined against all
data. All non-hydrogen atoms were refined anisotropically. Full-
matrix least-squares refinements with 400 parameters were carried
Calculation of equilibrium constants. The stepwise protonation
constants, Ki = [HiL]/[Hi−1L][H], (including the log KD value)
and the overall metal-complex stability constants (bM
=
H
L
l
m
h
[MmHhLl]/[M]m[H]h[L]l) were calculated by the fitting analysis of
the spectroscopic titration data for the ligand and the ligand–
Mo(VI), respectively, with the PSEQUAD program.57 The herein
determined Mo(VI) hydrolytic species were included in the equi-
librium model. The species distribution curves were plotted with
the HYSS program.58
2
2
out by minimizing Rw(Fo − Fc )2 with a SHELXTL weighting
scheme.
The log KD value determined in a 15% (v/v) MeOD–D2O
solution was subsequently converted into an approximate value in
H2O by using the equation pKD = 0.32 + 1.044 pKH.59 Although
this correlation was obtained for D2O solutions, the authors
admitted that it can also give an approximate value of proton
dissociation constant in the 15% (v/v) MeOD–D2O medium.
Biological studies
In the bioassay, xanthine oxidase catalyses the oxidation of purine
bases (hypoxanthine or xanthine) to uric acid and superoxide. In
the reaction mixture, the superoxide spontaneously degrades to
hydrogen peroxide (H2O2) which, in the presence of horseradish
peroxidase (HRP), reacts with Amplex Red Reagent to generate
the red fluorescent oxidation product, resorufin. Resorufin has an
absorption maximum of approximately 560 nm (and a fluorescence
emission maximum of approximately 585 nm) and because the
extinction coefficient is high (54000 cm−1 M−1), the assay can be
performed either fluorimetrically or spectrophotometrically.
The inhibitor under study was added to the working solution
containing xanthine (50 lM), Amplex Red Reagent (0.0101 M),
HRP (20 U), xanthine oxidase (XO: 1 U), and Reaction Buffer
(0.125 M). Reactions were incubated at 37 ◦C for 30 min.
Readings were made in triplicate in 96-well plates at 560 nm in
a spectrophotometer. The XO inhibitory activity was expressed as
the percentage of inhibition of XO, calculated as 100 − (A/B ×
100) where A and B are the absorbances with and without
inhibitor, respectively. After fitting the data of each experiment
into sigmoidal dose-response curves, IC50 values were calculated
and presented as mean SD of n experiments, and compared by
one-way ANOVA followed by Dunnet’s t test. P values lower than
0.05 were considered to indicate statistically significant differences.
Cyclic voltammetry measurements
Voltammetric measurements were performed in 20% (v/v) DMSO
and 0.1 M KNO3 with a three-electrode cell. A glassy carbon was
used as the working electrode, a platinum electrode as auxiliary
and an Ag/AgCl reference electrode. The glassy carbon electrode
surface was hand polished with 0.05 lm alumina to a mirror-like
finish. In a typical experiment, the cell volume was 12 mL and the
concentrations of ligand and Mo(VI) were ca. 6.25 × 10−4 M and
1.25 × 10−4 M, respectively. Complexes were generated in situ by
using molybdic acid in 1 : 5 stoichiometric relation; the working
pH was 4 and scans were performed at 50 mV s−1. All the solutions
were degassed with type U nitrogen previously bubbled through
anhydrous DMSO.
X-Ray crystallographic analysis†
Crystals of [MoO2(thiomaltol)]2(l-O) (compound 1) were grown
from acetone and two polymorphs were isolated: a-1 and b-1.
Crystals were fixed on the tip of a glass fiber and diffraction
data were measured (see Table 2). The unit cell parameters were
obtained by full-matrix least-squares refinement of 3170 (3102)
reflections. Data collection, cell refinement and data reduction
were performed using COLLECT.60,61 The structures were solved
using SIR9262 and SHELXL-97,63 and refined against all data.
All non-hydrogen atoms were refined anisotropically. Full-matrix
least-squares refinements with 266 (229) parameters were carried
Conclusions
The set of a-hydroxythione derivatives studied in this work were
revealed to be good XO inhibitors and the most promising
compound (DMTHP) can be considered a potential drug can-
didate. The biological results have been supported not only by
the solution equilibrium studies, which show that these O,S-
compounds present a slightly higher affinity for the Mo(VI) than
the corresponding O,O-derivatives, but also by the crystal struc-
ture results, which suggest a high flexibility for the coordination of
the O,S-ligands to molybdenum. Moreover, the electrochemical
studies also reflected the fact that the reduced Mo(IV) complexes
are more stabilised by the O,S- than the O,O-ligands, which gives
support to a mechanistic proposal for the XO inhibition involving
the formation of a stable intermediate resulting from the attack
on the molybdenum by the thiolate group. Therefore, from the
physicochemical properties studied herein, it can be inferred that
the sulfur atom must play an important role in the XO inhibition.
2
2
out by minimizing Rw(Fo − Fc )2 with a SHELXL-97 weighting
scheme. Neutral atom scattering factors for all atoms were taken
from the International Tables for X-Ray Crystallography.64
Specials for a-1: At later stages of the refinement, all hydrogen
atoms could be found and were allowed to refine with individual
isotropic displacement parameters. Specials for b-1: Methyl hydro-
gen atoms were calculated as a part of rigid rotating groups, with
˚
dC–H = 0.98 A and Uiso(H) = 1.5Ueq(C). All other hydrogen atoms
were placed in ideal positions and refined using a riding model,
˚
aromatic dC–H distances of 0.95 A and Uiso(H) = 1.2Ueq(C). For one
of the methyl groups (C17), disorder of the three hydrogen atoms
could be assigned to partially occupied positions (50 : 50), which
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Acknowledgements
The authors thank the Portuguese Fundac¸a˜o para a Cieˆncia e
Tecnologia (FCT) (project PCDT/QUI/56985/04) for financial
support. We also thank Dr Vitor Felix (Aveiro University) and Dr
Leo Straver (Bruker-AXS BV, Delft) for help with data collection
and processing of [MoO2(thiomaltol)2], which was measured at
Bruker-AXS, Delft, The Netherlands. Dr Abhik Mukhopaadhyay
is also acknowledged for helpful discussions. Ratomir Jelic thanks
the Ministry of Science of Serbia for financial support.
36 W. Xiu-Jian, C. Zhen-Feng, L. Hong and Y. Kai-Bei, Jiegou Huaxue,
1997, 16, 403–407.
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