Mono-oxo-bis-dithioveratrol-molybdate - In Solution a Model for Arsenite Oxidase and in the Solid State a Coordination Polymer with Unprecedented Binding Motifs

Article abstract of DOI:10.1002/zaac.201300076

Mono-oxo-bis-dithioveratrol-molybdate was synthesized, structurally characterized, and investigated with respect to its oxo-transfer activity. The latter was compared with the activity of the analogous tungsten complex. The title complex is a structural model for molybdopterin bearing arsenite oxidase and both complexes catalyze oxo-transfer reactions successfully to 100 % conversion. With the dithioveratrol ligand the oxidation of triphenylphosphine proved to be faster for the tungsten complex, which is untypical. The solid-state structure of the molybdenum complex exhibits an unexpected and very unusual polymeric structural motif consisting of the complex anion mono-oxo-bis-dithioveratrol-molybdate, sodium cations, and methanol. Infinite double-decker strands are formed with sodium bridges between the ether functions of two ligands in one strand and the Mo=O moiety of the second strand. Copyright

Full text of DOI:10.1002/zaac.201300076

ARTICLE
DOI: 10.1002/zaac.201300076
Mono-oxo-bis-dithioveratrol-molybdate – in Solution a Model for Arsenite
Oxidase and in the Solid State a Coordination Polymer with Unprecedented
Binding Motifs
Alexander Döring,[a] Christian Fischer,[b] and Carola Schulzke*[b]
Keywords: Cofactors; Oxidoreductases; Molybdenum; Crystal structure; Coordination polymer
Abstract. Mono-oxo-bis-dithioveratrol-molybdate was synthesized, faster for the tungsten complex, which is untypical. The solid-state
structurally characterized, and investigated with respect to its oxo- structure of the molybdenum complex exhibits an unexpected and very
transfer activity. The latter was compared with the activity of the anal-
unusual polymeric structural motif consisting of the complex anion
ogous tungsten complex. The title complex is a structural model for mono-oxo-bis-dithioveratrol-molybdate, sodium cations, and meth-
molybdopterin bearing arsenite oxidase and both complexes catalyze anol. Infinite double-decker strands are formed with sodium bridges
oxo-transfer reactions successfully to 100% conversion. With the di- between the ether functions of two ligands in one strand and the Mo=O
thioveratrol ligand the oxidation of triphenylphosphine proved to be moiety of the second strand.
to a second nucleotide as found in some bacteria, and finally
Introduction
the dithiolene moiety, which binds the active site metal.[3] The
Apart from the typical 3d transition metals with a role in
molybdopterin ligand has proven to be extremely difficult to
biology as for instance iron, copper, and zinc, there are also
synthesize with the closest model having been published just
examples, where nature resorts to the use of 4d and 5d transi-
recently, though the enzymes have been known for more than
forty years.[4] This very advanced model is still not perfectly
mimicking every aspect of MPT but it is strikingly similar.
This is, however, the result of many years of synthetic research
and a huge input of work. In order to synthesize model ligands
for MPT reliably, with good yield and with an acceptable
amount of work, typically compounds bearing the coordinating
dithiolene function and various different substituents on the
double bond are considered to model MPT sufficiently enough.
tion metals. Though molybdenum and tungsten have far less
universal roles than the metals listed above, they still perform
essential tasks and are found in almost any organism on earth
being it bacteria, plants, or human beings.[1] As an exception
to the majority of molybdenum enzymes, molybdenum is
found in the most atom economic form of the nitrogenases.[2]
It is, however, far more prevalent in the active sites of oxidore-
ductases as so-called molybdenum cofactor (Moco) and cata-
lyzes basically the same reactions as its higher congener tung-
sten, which is part of almost identical cofactors (WCo) in
closely related enzymes. Both, molybdenum and tungsten are
responsible for two-electron oxidation or reduction reactions,
which are typically accompanied by the transfer of formally
one oxygen atom (O0). In all molybdenum and tungsten de-
pendent oxidoreductases a specific ligand is present: molyb-
dopterin (MPT, see Figure 1). Molybdopterin consists of the
pterin part (the outermost two N-heterocycles), the pyrane ring,
a phosphate group, which may be even a triphosphate and bind
Figure 1. The chemical structure of molybdopterin.
Based on structure homologies and reactivities, the molybd-
enum and tungsten dependent oxidoreductases have been cate-
gorized by Hille into different enzyme families in order to fa-
cilitate the scientific discussion in this field.[5] For molybd-
enum, three families were used with the xanthine oxidore-
ductase and the sulfite oxidase families comprising enzymes,
in which the cofactor bears only one MPT. These are typically
found in higher organisms. In the DMSO reductase family the
cofactor is present with two MPT ligands bound and a coordi-
nating amino acid residue from the surrounding peptide, which
constitutes the only covalent interaction between cofactor and
peptide. In the oxidized form one oxo ligand is additionally
attached to molybdenum. Arsenite oxidase is generally thought
to fall into the DMSO reductase family. It is, however, excep-
* Prof. Dr. C. Schulzke
E-Mail: carola.schulzke@uni-greifswald.de
[a] Institut für Anorganische Chemie
Georg-August-Universität Göttingen
Tammannstr. 4
37077 Göttingen, Germany
[b] Institut für Biochemie
Ernst-Moritz-Arndt-Universität Greifswald
Felix-Hausdorff-Str. 4
17487 Greifswald, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300076 or from the au-
thor.
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Mono-oxo-bis-dithioveratrol-molybdate
tional as it does not bind directly to the peptide and in place
of the amino acid residue an oxo ligand is bound, or two in
the oxidized form, respectively.[6] Such composition, i.e.
MO(dithiolene)2 (M = Mo, W) structural motives, are fre-
quently observed in synthetic inorganic chemistry and have
been used previously as models for enzymes of the DMSO
reductase family.[7]
Our efforts to understand the use of molybdenum and tung-
sten for the same tasks in very similar and clearly evolution-
arily related proteins lead us to synthesize a number of com-
plexes of this composition with both, molybdenum and tung-
sten, as coordination centers.[8] In a previous study they have
been investigated predominantly by electrochemical methods.
One such complex-couple was now studied further with re-
spect to its oxo-transfer activity. Furthermore, the synthetic
protocol for ligand and complex synthesis was refined and op-
timized and, most strikingly, the X-ray structure of the
molybdenum complex is reported, which crystallizes as an un-
expected coordination polymer exhibiting unprecedented bind-
ing motifs.
For the complex synthesis K3Na[MO2(CN)4]·6H2O (M =
Mo, W) is used as precursor, which is dissolved in aqueous
NaOH solution. The ligand dissolved in methanol is added and
the mixture is heated to 45 °C. It is important to elevate the
temperature above room temperature for the reaction to take
place but at the same time even higher temperatures tend to
destroy the desired products. Careful monitoring of the tem-
perature is therefore required. An immediate color change
from blue to brown and eventually to red can be observed.
During this reaction the cyanide ligands are replaced by the
dithiolenes. In course of this, one of the two oxo ligands,
which are in trans arrangement in the precursor, is lost. The
trans arrangement for oxo ligands is decidedly unfavorable and
only possible in the precursor due to the significant π-character
of the M–CN bonds. After the reaction mixture has cooled to
room temperature a large-cation source (here benzyl-trieth-
ylammonium chloride) is added to precipitate the anionic com-
plexes [MO(vdt)2]2–. Recrystallization from acetonitrile/ethyl
ether yields analytically pure complexes. Slow precipitation
from the mother liquor, to which only one instead of two
equivalents cation were added yields crystalline material,
which surprisingly does not contain the bulky ammonium cat-
ion but sodium ions instead plus co-crystallized methanol.
Recrystallization of the crude product from methanol gives the
same product with small instead of large cations present. A
bulky ammonium halide is, however, needed to receive a pre-
cipitate in the first place. The observation of distinct composi-
tions of the product depending on the solvent used, as indi-
cated by elemental analysis, was unexpected and could only
be understood, when the crystal structure of the molybdenum
complex crystallized from methanol was solved (see below).
The solvent, used to recrystallize, apparently has a significant
influence on the composition of the product, which needs to
be taken into account for further investigations and reactions
of the product from this specific reaction. In order to reproduce
the synthesis of the sodium molybdenum complex species the
synthetic procedure was modified to abstain from the bulky
cation addition. In order to obtain sufficiently pure crystalline
material, the reaction product had to be first crystallized from
DMF (giving complex 4b as described in the Experimental
Section). From this compound a small amount of the sodium
methanol complex 4Ј could be obtained again via recrystalli-
zation from methanol. An analogous coordination polymer
with tungsten could not be characterized so far.
Results and Discussion
Synthesis
The synthesis of both, ligand and complexes, has already
been reported[8] but it was refined in the meantime and due to
potential pitfalls it is worth to go into more detail than what is
currently available. The ligand’s synthesis is based on pub-
lished procedures from the 1970s (Scheme 1).[9] Starting with
o-dimethoxybenzene a dibromination in 4,5-position is carried
out with elemental bromine in dichloromethane. The two bro-
mine substituents are replaced by thioethers via reaction with
CuI-n-butylmercaptane. This is used instead of the free n-bu-
tylthiol since the formation of HBr as side product is not suf-
ficient enough to obtain the desired product in good yield com-
pared to CuBr. CuBr is completely removed from the reaction
mixture by thorough extraction with acid, water, and finally
ammonia. The free ligand dithioveratrol (vdtH2) (3) is obtained
by a Birch reduction of the thioether.
The resulting complexes are structural models, for arsenite
oxidase in particular, among the molybdenum dependent en-
zymes and for the tungsten dependent oxidoreductases in gene-
Scheme 1. Synthesis of ligand precursor and dithiolene complexes ral, which mostly bear two molybdopterins in the active sites.
[M = Mo (4), M = W (5)].
In case of the arsenite oxidase the immediate, meaning first
and second, coordination sphere is exactly modeled. In case of
the tungsten compound the peptide coordination to the active
site was neglected. The similarity is however close enough to
expect catalytic oxo-transfer activity here as well as for the
molybdenum complex. This was investigated by the typical
model reaction for oxo transfer.[11]
Free (meaning protonated) dithiolenes are usually rather un-
stable. If the ene-function is part of a benzene ring, however,
they may be reasonably stable as is the case here. In fact the
ligand was obtained as crystalline material, of which a struc-
tural elucidation was possible, although previous attempts for
the same compound have been described as unsuccessful[10]
.
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A. Döring, C. Fischer, C. Schulzke
ARTICLE
Oxo-Transfer Catalysis
to be carried out to reliably determine Michaelis constants,
system describing equilibrium constants as well as reaction
rate constants for this molybdenum catalyst. Research in this
respect is still going on and will be communicated in a follow-
up paper.
In contrast to the molybdenum catalyzed reaction, the tung-
sten mediated reaction shows no induction period and proceeds
with zero order kinetics with respect to the substrate PPh3 until
just roughly 31% of the substrate is left. This corresponds to
one of the limiting cases in Michaelis-Menten kinetics, see
Equation (1).
Complexes 4 and 5 were tested whether they were able to
reduce DMSO to DMS and to transfer the respective oxygen
atom to the model substrate triphenylphosphine (PPh3). The
reaction typically proceeds via oxygen atom transfer from
DMSO to the complex giving a cis-dioxo metal complex,
which consequently transfers the oxygen to PPh3. In some pre-
viously reported cases the dioxo species (obtained via oxi-
dation by DMSO, Me3NO, or other oxidizing agents) of sim-
ilar bis-dithiolene complexes have been isolated and even
structurally characterized.[7e,12] PPh3 as a tertiary phosphine is
the substrate of choice due to its good solubility in organic
solvents as well as its affinity to oxygen as has been estab-
lished previously.[11] The reaction progress can be nicely moni-
tored by 31P NMR spectroscopy due to the separate signals of
PPh3 (s, –6 ppm; in [D6]DMSO) and PPh3O (s, –26 ppm; in
[D6]DMSO). In course of the reaction the phosphorus atom is
performing a nucleophilic attack on the empty Mo=O π* or-
bital, while a free electron pair of the oxygen atom is simulta-
neously attacking the P–C σ* orbital so that the two-electron
reduction of the metal proceeds smoothly.[13] DMSO is com-
monly used as oxygen source and may be simultaneously em-
ployed as solvent and substrate with consequentially very high
excess to the catalyst.[14]
Quantification of the formation of PPH3O out of a tenfold
excess of PPh3 using 31P NMR spectroscopy results in a typi-
cal Michaelis-Menten saturation kinetic for the tungsten con-
taining complex (Figure 2, left). A plot of observed concentra-
tions vs. time for the molybdenum complex is shown in Fig-
ure 2 right. For both catalysts a 100% conversion of substrate
could be observed.
However, both reactions proceed very slowly, with the tung-
sten catalyzed reaction needing 102 h to full conversion,
whereas the molybdenum mediated reaction requires 107 h.
Although for the specific oxo transfer reaction from DMSO to
PPh3 it is more common, that molybdenum complexes are
faster catalysts than their tungsten analogues, such behavior is
not unprecedented.[15] In this specific case it may come down
to a comparably long induction period (up to the first 30 h of
reaction) for the molybdenum catalyst. This induction period is
possibly caused by the overall slow formation of the oxidized
catalyst species [MoO2(vdt)2]2– by reaction with DMSO. More
d[P] k2·[E]0·[S]
=
dt
KM + [S]
S ϾϾ KM
d[P]
= k2 [E]0 = Vsat
(1)
dt
The observed rate (Vsat) obtained from the slope of the first
70% of reaction progress equals 2.82ϫ10–5 mol·L–1 h–1. With
a catalyst concentration of 3.38ϫ10–4 mol·L–1 a first-order
rate constant k2 = 0.0834 h–1 can be identified. As discussed
by Drexler et al., the analysis of Michaelis-Menten type kinet-
ics is, in principal, not limited to the initial rate range.[16] The
fact that the reaction is running in zero order with respect to
the substrate PPh3 at these little excess amounts of substrate,
implies that the Michaelis constant (KM) has to be very small
and the equilibrium between the free catalyst and the substrate
lies on the side of the catalyst-substrate complex. The complex
pattern of molybdenum catalyzed substrate concentration
changes prevents at this point the calculation of kinetic param-
eters, which could be compared to the tungsten catalyst. What
is evident, however, is that the tungsten compound is more
efficient, i.e. faster at the investigated conditions. This is most
likely caused by molybdenum’s reluctance to be oxidized to
the MoVI species as evident from its higher redox potential in
analogous compounds, which in turn is based on the stronger
relativistic effect in tungsten atoms.[15b,17]
Crystal Structures
As mentioned above, the molybdenum complex could be
measurements with different catalyst-substrate ratios will have isolated in two forms depending on the solvent used for recrys-
Figure 2. Concentration vs. time diagram of the reaction of 3.38ϫ10–4 mol·L–1 of 5 (left) and 4 (right) with a tenfold excess of PPh3 in DMSO
at 25.0 °C.
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Mono-oxo-bis-dithioveratrol-molybdate
tallizing the material. The exact composition of the unexpected
form of the complex was determined by solving its X-ray
structure. Unusual in this case is firstly, that instead of bulky
ammonium derived cations the far smaller sodium cations are
part of the crystallized material. Secondly, the complex to-
gether with its counterion does not form a discrete molecule-
ion structure but a coordination polymer consisting of infinite
double-decker strands. Figure 3 shows the building blocks of
these infinite strands. They consist of two complex molecules,
which are arranged in opposite directions but with their apical
oxo ligands facing each other. The phenyl rings are arranged
in a perfectly parallel orientation (the angle between C1–C6
and C1A–C6A planes equals 0.00°). The oxo ligands are ac-
tively involved in holding the two molecules together. Nor-
mally, doubly bonded oxo ligands are rather strongly attached
to the central metal and are rarely observed to participate in
bridging interactions, other than the formation of dimers, by
which the double bonds become single bonds. In this case the
oxygen atom is directly bound to molybdenum (1.713 Å) and
to two sodium ions (2.344 and 2.359 Å). Crystal structures ex-
hibiting such arrangement have so far only been reported for
polyoxomolybdates but not for coordination compounds and
are generally extremely rare with only two examples among
the CCDC entries.[18] Based on the interaction with the two
sodium ions the bond length between molybdenum and oxygen
is at the longer end of the range typically observed for mono-
nuclear square pyramidal molybdenum complexes with apical
oxo ligands but the bond is not exceedingly long.[7i,12g,19]
Figure 4. The crystal lattice of (Na2[MoO(vdt)2]·4MeOH)ϱ (4Ј) from
a side view. Hydrogen bonds between methanol OH groups and
dithiolene sulfurs are indicated as dashed lines for the O8–H to S2
hydrogen bond.
software[20] are those between OH functions and sulfur in a
range of 2.28 to 2.53 Å. To the best of our knowledge such
hydrogen bonding involvement of dithiolene sulfur atoms has
not been observed before and constitutes an unprecedented
binding motif for dithiolene complexes of molybdenum and
tungsten. A participation of MPT’s dithiolene sulfur atoms in
hydrogen bonding in the active sites of the enzymes has not
been explored so far but, since these hydrogen bonds in the
vicinity of molybdenum can exist, it may be worth looking
into this from a protein crystallographers point, too.
Turning the crystal lattice around by 90° it becomes clear
that the distinct double decker strands are not arranged directly
on top of each other but rather in a staggered fashion (Fig-
ure 5). If they were arranged directly on top of each other the
methanol ligands and the sodium ions of different strands
Figure 3. The molecular structure of the smallest unit of the coordina-
tion polymer (Na2[MoO(vdt)2]·4MeOH)ϱ (4Ј) consisting of two com-
plex molecules, four sodium ions and eight methanol molecules. Ellip-
soids are drawn with 50% probability. Hydrogen atoms are omitted
for clarity reasons. Selected bond lengths /Å and angles /°: Mo1–O1:
1.713; Mo1–S(1–4): 2.367–2.383; Mo1–Na2: 3.547; Mo1–O1–Na1:
132.88; Mo1–O1–Na2: 120.36; Na1–O1–Na2: 105.53.
In the crystal lattice the double-decker zigzagging strands
are arranged parallel to each other forming, from a side view,
wavy layers (Figure 4). This is sterically by far the most favor-
able arrangement for infinite strands and this arrangement is
further stabilized by O–HǞS hydrogen bonds between one
double-decker strand and another as indicated in the Figure for
the O8–H to S2 hydrogen bond with 2.522 Å between H and
S. Except for one hydrogen bond between a methoxy methyl
group (C8) and the lone pair of a methanol oxygen atom (O7)
Figure 5. View along four of the double-decker strands in the crystal
lattice of (Na2[MoO(vdt)2]·4MeOH)ϱ (4Ј) (one double decker strand
all hydrogen bonds found with the Hbond routine of the platon left, one right, one on top, one at the bottom).
Z. Anorg. Allg. Chem. 2013, 1552–1558
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A. Döring, C. Fischer, C. Schulzke
ARTICLE
would collide. It is also evident that the molybdenum atoms of complex being more efficient. The molybdenum complex
top and bottom single strand are slightly staggered too, pre- when recrystallized from methanol forms a coordination poly-
sumably to allow a closer contact between the two parts of the mer, in which two unusual binding motifs were observed:
double strands.
firstly the bridging interaction of the oxo ligand on molybd-
It was possible to obtain a crystal structure of the free (pro- enum with two sodium ions and secondly the participation of
tonated) ligand too (Figure S1 in the Supporting Information). dithiolene sulfur atoms in hydrogen bonding with methanol
The ligand crystallizes in the monoclinic space group C2/c as hydroxy groups.
a merohedral twin and its structure could only be refined to
a sufficient R value by applying the respective twin law. In
comparison with the complex the metric parameters involving
Experimental Section
sulfur and oxygen atoms, which are interacting with sodium
and/or molybdenum ions, are clearly influenced by coordinat- molybdenum containing species were carried out in a nitrogen atmo-
General: The Birch reduction and all manipulations dealing with
sphere using standard Schlenk techniques. Starting materials were pur-
chased and used as received.
ing the metals.
In the ligand’s crystal structure two independent molecules
are present in the asymmetric unit. The present four sulfur car-
bon distances show a significantly larger spread (1.756 to
1.777 Å) than the four sulfur carbon distances in the complex
molecule (1.772 to 1.776 Å). The average distance is slightly
shorter for the ligand than for the complex, as would be ex-
pected (1.768 versus 1.774 Å). This is an indication that by
complex formation the metal loosens the sulfur carbon bonds,
because electron density is needed for the complex bonds, but
also, that overall the electron density around the complex cen-
ter including the first and second coordination sphere is more
Ethyl ether was distilled from sodium benzophenone ketyl immedi-
ately prior to use. Methanol and ethanol were distilled from magne-
sium turnings prior to use, CDCl3 and acetonitrile from CaH2.
K3Na[MoO2(CN)4]·6H2O and complexes 4 and 5 were prepared ac-
cording to published procedures.[8,21] The ligand and ligand precursors
were prepared based on published procedures.[9]
Synthesis of 1,2-Dimethoxy-4,5-dibrombenzene (1): Bromine
(29,7 mL) was added to a solution of 1,2-dimethoxybenzene (40.0 g,
0.29 mol) in dichloromethane (10 mL) at 5.0 °C during 6 h. After leav-
evenly distributed via the metal. Regarding the ether oxygen ing the mixture warming up to room temperature overnight in an open
flask, the dichloromethane had evaporated and the red-brown precipi-
tate was dissolved in ethyl ether and washed afterwards twice with 2 m
NaOH (150 mL) and twice with distilled water (150 mL). The product
was sucked dry and recrystallized from methanol yielding 34.66 g
(40.38%) of off white needles. 1H NMR (200 MHz, CDCl3-[d1],
25 °C, TMS): δ = 3.83 (s, 6 H, OCH3), 7.03 (s, 2 H, Harom) ppm.
atoms, their oxygen carbon bond lengths are influenced only
on the aromatic side by coordinating the sodium cations
(which leads to the polymeric structure in the complex’s solid
state). The aliphatic side of the ether moiety is unperturbed by
the coordination with nearly identical average bond lengths
and distance spreads for both the complex and the free ligand.
This observation is to some extent counterintuitive as the ali-
phatic side is expected to move with far fewer restrictions than
the aromatic phenyl ring. On the aromatic side, the spread in
bond lengths is nearly identical for free ligand and complex
but the bond lengths have increased in course of the coordina-
tion event by about 0.02 Å (averages: ligand = 1.357 versus
complex = 1.376 Å). It is possible that not only the coordina-
tion of the sodium ions by the ether oxygen atoms contributes
to this elongation but also the coordination of molybdenum on
the other side of the phenyl ring. Since dithiolenes are non-
innocent ligands, this coordination of molybdenum certainly
has an influence on the delocalized π electrons throughout the
molecule, which in turn may loosen the O–Carom bonds oppo-
site to the sulfur donors even further than only the sodium
coordination would do.
Synthesis of 4,5-Dimethoxy-1,2-bis(n-butylthio)benzene (2): This
procedure follows mainly the procedure given by Adams et al. in 1973:
1,2-dimethoxy-4,5-dibrombenzene (0.025 mol, 7.4 g) and freshly pre-
pared cuprous n-butylmercaptide (0.055 mol, 8.4 g) were dissolved in
pyridine (8 mL) and) quinoline (8 mL) and heated to 170 °C under
reflux for 3.5 h. The reaction mixture was poured into a mixture of
ice (150 g) with concentrated HCl (40 mL). After extraction with ethyl
ether and washing twice with 10% HCl, once with water and twice
with concentrated NH3, and drying over potassium carbonate eventu-
ally a dark orange oil was received (4.87 g, 79.10% yield). An ad-
ditional recrystallization with ethanol was working well yielding large
colorless crystals. 1H NMR (200 MHz, CDCl3-[d1], 25 °C, TMS): δ =
0.98 (t, 6 H, CH3-butyl), 1.51 (sextet, 4 H, CH2–CH3), 1.66 (quin, 4
H, CH2–CH2–CH3), 3.06 (t, 4 H, S–CH2), 3.92 (s, 6 H, OCH3), 7.32
(s, 2 H, Harom) ppm.
Synthesis of 4,5-Dimethoxybenzene-1,2-dithiol (3): As a typical
Birch reduction this reaction is carried out in liquid ammonia in a
nitrogen atmosphere. A solution of 4,5-dimethoxy-1,2-bis(n-butyl-
thio)benzene (4.87 g, 0.0154 mol) in ethyl ether was added to liquid
ammonia in a 500 mL Flask. Portionwise addition of sodium (2.26 g,
0.0985 mol) resulted in the typical color change to dark blue and stir-
ring was continued for 45 min. The reaction was quenched by cautious
addition of NH4Cl (0.77 g, 0.00504 mol) and the ammonia evaporated
in a stream of nitrogen. The dark residue was dissolved in MeOH
(5 mL) and water (10 mL). After filtration and acidification with con-
centrated HCl solution a dark green residue was obtained and (re-)
crystallized from ethyl ether to give yellow crystals (1.27 g, 25.32%
Conclusions
Two complexes (molybdenum and tungsten) with the vera-
troldithiolene ligand were synthesized based on published pro-
cedures. Since they can be considered structural models for the
immediate coordination sphere of molybdenum and tungsten
cofactors with two molybdopterin ligands, they were also
tested with respect to functional catalytic oxo transfer ability.
Both complexes catalyze the oxygen atom transfer from
DMSO to PPh3 up to a 100% conversion with the tungsten yield). The observed chemical shifts in the NMR spectra are very sim-
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Mono-oxo-bis-dithioveratrol-molybdate
ilar to those published by Garner et al. for the corresponding lithium
salt, in particular the 13C-NMR values.[10] 1H NMR (200 MHz,
CDCl3-[d1], 25 °C, TMS): δ = 2.41 (s, 2 H, SH), 4.08 (s, 6 H, OCH3),
5.62 (s, 2 H, Harom). 13C NMR (75 MHz, CDCl3-[d4], 25 °C, solvent):
δ = 56.1 (s, CH3), 114.4 (s, CH), 122.2 (s, CSH), 148.2 (s, CO) ppm.
Table 1. Structure and refinement data for compounds 3 and 4Ј.
3
4Ј
Empirical formula
T /K
C8H10O2S2
170(2)
C20H32MoNa2O9S4
133(2)
Crystal system
Space group
monoclinic
C2/c
triclinic
P1
¯
a /Å
b /Å
c /Å
α /°
26.887(5)
8.6731(17)
20.875(4)
90
130.03(3)
90
3727.5 (13)
16
1.442
0.527
1696
3.33 to 28.53
4683 / 6 / 234
8.3805(17)
9.2807(19)
20.013(4)
101.83(3)
97.32(3)
100.05(3)
1478.4(5)
2
1.542
0.798
704
2.29 to 25.83
5633 / 0 / 346
Synthesis of Na2[MoO(vdt)2] (4b): A solution of 4,5-dimethoxy-
benzene-1,2-dithol (0.3 g, 1.5 mmol) in methanol (10 mL) was added
to a blue solution of K3Na[MoO2(CN)4]·6H2O (0.34 g, 0.7 mmol) and
NaOH (0.65 g, 16.25 mmol) in degassed water (20 mL). An immediate
color change to brown and an increase in temperature was observed.
Warming up to 45.0 °C and stirring for 15 min resulted in a further
color change to red, while the solution became inhomogeneous. After
cooling to room temperature the light blue solution was taken off with
a syringe and the remaining red precipitate was washed once with
methanol. Dissolving in DMF (4 mL) gave a red solution, which was
layered with diethyl ether to form a red microcrystalline material,
which was used for the NMR-measurements (see below). Yield:
0.39 g, 23.6%.
β /°
γ /°
V /Å3
Z
Dcalcd. /g·cm–3
μ /mm–1
F(000)
θ range /°
Data / restraints / param-
eters
Reflections collected /
unique
9330 / 4683
(Rint = 0.0369)
0.0459, 0.1098
0.0649, 0.1236
1.022
11820 / 5633
(Rint = 0.0322)
0.0258, 0.0706
0.0281, 0.0707
1.079
R1, wR2 [I Ͼ 2σ(I)]a)
R1, wR2 (all data)a)
GoF
Recrystallization of 4 or 4b from methanol results in the coordination
polymer (Na2[MoO(vdt)2]·4MeOH)ϱ (4Ј), the structure of which could
Δρ(max), Δρ(min) /e·Å–3
0.383, –0.478
0.650, –0.720
2 2 0.5
1
be determined crystallographically. H NMR (300 MHz, MeOD-[d4],
2
2 2
a) R1 = Σ||Fo|–|Fc||/Σ|Fo|. wR2 = [Σw(Fo –Fc ) /Σw(Fo ) ]
.
25 °C, referenced versus solvent): δ = 3.82 (s, 6 H, CH3), 7.29 (s,
2 H, CH). 13C NMR (75 MHz, MeOD-[d4], 25 °C, referenced versus
solvent): δ = 56.9 (OCH3), 85.26 (CH), 113.73 (C–S), 117.61
(C–O) ppm.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ,
UK. Copies of the data can be obtained free of charge on quoting the
depository numbers CCDC-922482 (3) and CCDC-922481 (4Ј)
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
Other Physical Measurements: NMR spectra were recorded with
either a Bruker Avance 500 spectrometer (500.13 MHz for 1H and
125.77 MHz for 13C) or a Bruker Avance 200 (200.13 MHz for 1H and
50.32 MHz for 13C). 1H-NMR spectroscopic shifts are reported in parts
per million (ppm) and referenced to the residual proton resonance of
the deuterated solvent (CDCl3 1H: δ = 7.24 ppm, 13C: 77.0 ppm) re-
lated to external tetramethylsilane (δ = 0 ppm). For the catalytic stud-
ies, complexes 4 or 5, respectively, and PPh3 were mixed in NMR
tubes in degassed, dry DMSO (0.6 mL) as solvent. The reactions were
monitored by 31P NMR spectroscopy. The presence of dimethyl
sulfide in the reaction mixtures was detected by isolating Me2S as
(Me2S)(HgCl2)3.[22] Elemental analyses were performed with
an Elementar 4.1 vario EL 3 elemental analyzer. Infrared spectra were
recorded as KBr pellets with a Digilab FTS 1000 from 4000 to
400 cm–1. ESI-MS samples were disolved and recorded with an Ap-
plied Biosystems API 2000.
Supporting Information (see footnote on the first page of this article):
Molecular structure of 4,5-dimethoxybenzene-1,2-dithiolene (3).
Acknowledgements
Generous financial support from the ERC (project MocoModels) is
gratefully acknowledged. COST Action CM1003 is acknowledged for
providing networking and exchange opportunities with respect to the
research topic discussed above.
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Received: February 7, 2013
Published Online: May 6, 2013
1558
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Z. Anorg. Allg. Chem. 2013, 1552–1558