Oxo-sulfido- and oxo-selenido-molybdenum(vi) complexes possessing a dithiolene ligand related to the active sites of hydroxylases of molybdoenzymes: Low temperature preparation and characterisation

Article abstract of DOI:10.1039/c2cc35345h

Oxo-sulfido- and oxo-selenido-molybdenum(vi) complexes with an ene-1,2-dithiolate ligand are generated as models of the active sites of molybdenum hydroxylases. The sulfide and selenide groups are highly reactive toward triphenylphosphine in the order of Se > S.

Full text of DOI:10.1039/c2cc35345h

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Oxo-sulfido- and oxo-selenido-molybdenum(VI) complexes possessing
a dithiolene ligand related to the active sites of hydroxylases of
molybdoenzymes: low temperature preparation and characterisationwz
Hideki Sugimoto,*a Susumu Tatemoto,a Kazuo Toyota,*b Kenji Ashikari,a Minoru Kubo,b
Takashi Ogurab and Shinobu Itoh*a
Received 24th July 2012, Accepted 8th October 2012
DOI: 10.1039/c2cc35345h
Oxo-sulfido- and oxo-selenido-molybdenum(VI) complexes with an
ene-1,2-dithiolate ligand are generated as models of the active sites of
molybdenum hydroxylases. The sulfide and selenide groups are highly
reactive toward triphenylphosphine in the order of Se > S.
The molybdenum hydroxylase enzymes contain a labile sulphur or
selenium atom as an essential element for their catalytic reactivity.1
Scheme 1
complex possessing a dithiolene ligand, [MoVIO2L2]2ꢀ (1O, L =
The sulphur-dependent enzymes include xanthine oxidoreductase/
dehydrogenase (XOR/XDH)1,2 and quinoline 2-oxidoreductase
1,2-dicarbomethoxyethylene-1,2-dithiolate) as a more precise
model of the enzyme active sites.10 In this study, we succeeded
in generating the MoVIO(S) and MoVIO(Se) counterparts, the
physicochemical properties and reactivity of which have been
explored for the first time (Scheme 1).
(QO),1,3 and the selenium-dependent ones include nicotinate
dehydrogenase (NDH),1,4 some xanthine oxidoreductases
(XORs)1,5 and purine hydroxylase.1,6 The active site of the
molybdenum hydroxylases features a mononuclear oxo-sulfido- or
oxo-selenido-molybdenum(VI) centre (MoVIO(S) or MoVIO(Se))
coordinated with a dithiolene (ene-1,2-dithiolate) unit.1–4,7
Replacement of the sulfide and selenide groups with an oxo
group (MoVIO2) makes the enzymes inactive.1 On the other
hand, replacement of the sulfide group with selenide increases
the catalytic activity.
In the modelling studies of the molybdenum enzymes,8
non-dithiolene complexes such as Tp*MoVIO(O/S)(L0)
(Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate and its
isopropyl derivative, L0 = monodentate ligand such as alkoxide),
MoVIO(O/S)(OSiPh3)2L00 (L00 = phen and its methyl derivative)
and MoVIO(O/S/Se) (alkyloxyamido derivatives) were reported.9
However, isostructural MoVIO2, MoVIO(S) and MoVIO(Se) model
complexes having an identical molybdenum(VI)–dithiolene frame-
work have yet to be reported. Recently, we developed the MoVIO2
We have found that the oxo-silylalcholato-molybdenum(VI)
complex coordinated with L, (Et4N)[MoVIO(OSiBuPh2)L2]
(2),11 was converted to 1O in the presence of OHꢀ (Et4NOH)
in CH3CN at room temperature (see Experimental section).
Such an electrophilic reactivity of 2 was adapted to the
reaction with SHꢀ and SeHꢀ to access the target complexes.
When the deep-violet complex 2 was treated with SHꢀ
(Et4NSH) at room temperature in CH3CN, the known orange
complex, (Et4N)2[MoIVOL2] (3),12 was obtained together with
elemental sulphur quantitatively by an intramolecular redox
reaction between the molybdenum(VI) and sulfide ions. On the
other hand, when the reaction was carried out below ꢀ20 1C
in a dry argon atmosphere, we observed the formation of a
deep-red product. Fig. 1a shows spectral changes of the
titration of 2 with various amounts of SHꢀ at ꢀ40 1C in
CH3CN, where the absorption bands at 563 and 734 nm due to
2 decrease in intensity accompanied with an increase of a new
band at 485 nm, with clear isosbestic points at 456 and 527 nm.
The titration plots (the inset of Fig. 1a) clearly indicate the
stoichiometry of 2 : SHꢀ = 1 : 1. A CH3CN solution containing 2
and 1 equiv. of SHꢀ exhibited an ESI-mass spectrum having a peak
cluster attributable to [MoVIO(S)L2]2ꢀ at m/z = 279 together with
that attributable to [MoIVOL2]2ꢀ (m/z = 263) at ꢀ40 1C (Fig. 1b)
and a 1H NMR spectrum exhibiting a singlet for the methyl
protons of L at 3.66 ppm at ꢀ40 1C in CD3CN (Fig. S1, ESIz).
Both spectra were changed to those of 3 at room temperature.
Deep-red microcrystalline powder was isolated by addition of THF
into a deep-red CH3CN solution containing 2 and 1 equiv. of SHꢀ
a Department of Material and Life Science, Graduate School of
Engineering, Osaka University, 2-1 Yamadaoka, Suita,
Osaka 56500871, Japan. E-mail: sugimoto@mls.eng.osaka-u.ac.jp
b Department of Chemistry, Graduate School of Science, Osaka City
University, 3-3-138 Sumiyoshi-ku, Osaka 558-8585, Japan.
E-mail: toyota@sci.osaka-cu.ac.jp
c Picobiology Institute, Graduate School of Life Science, University of
Hyogo, 30201 Kouto, Ako-gun, Hyogo 678-1297, Japan
w This article is part of the ChemComm ‘Emerging Investigators 2013’
themed issue.
z Electronic supplementary information (ESI) available: Experimental
details for preparation, low temperature measurements and DFT
calculations (Tables S1–S3, Fig. S1–S10). See DOI: 10.1039/
c2cc35345h
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Fig. 2 DFT optimized structures of (a) 1S and (b) 1Se calculated at
the (U)B3LYP level of theory.
Then, the bands at 917 and 458 cmꢀ1 can be assigned to n(MoQO)
and n(Mo–Sdithiolene) stretches, respectively. In the range from
1400–1600 cmꢀ1, the rR spectrum have two bands at 1535 and
1508 cmꢀ1 assignable to n(CQC) stretches. Pink powder was
isolated from a C2H5CN solution of 1Se at ꢀ80 1C upon adding
THF. Although the pink powder immediately decomposed to 3
and grey selenium at room temperature, the elemental analysis data
were consistent with the formula (Et4N)2[MoVIO(Se)L2] (see
Experimental section). Based on the above spectroscopic and
elemental analysis data, it can be concluded that the pink complex
is the oxo-selenido-molybdenum(VI) complex supported by L,
(Et4N)2[MoVIO(Se)L2] (1Se).
Fig. 1 (a) Spectral changes observed upon addition of 1/6, 2/6, 3/6, 4/6, 5/
6, 6/6 equiv. of Et4NSH to 2 (0.30 mM) in CH3CN at ꢀ40 1C, (inset) a plot
of absorbance at 485, 563 and 734 nm against the molar ratio of [SHꢀ]/[2].
(b) ESI-mass spectrum of 1S formed by the reaction of 2 (0.50 mM)
and Et4NSH in CH3CN at ꢀ40 1C, (inset) simulation of the peak
cluster at m/z = 279 of 1S.
The DFT calculations at the (U)B3LYP level of theory were
performed to give structural characterisations of 1S and 1Se,
using the X-ray crystal structure of 1O10 as a starting point for
optimisation. The DFT optimised structure of 1O is nearly
identical to its crystal structure. The optimised structures of 1S
and 1Se are given in Fig. 2. The two molybdenum(VI) centres of
1S and 1Se can be described as distorted octahedrons as in the
case of 1O.11 The MoQS bond length of 2.21 A is in the range of
those of the six-coordinate MoVIO(S) complexes with phen,
Me4phen, or Tp* ligand (2.132(2)–2.257(3) A)9 but slightly
longer than those of the five-coordinate MoVIO(S) centres of
XOR (2.13 A)2 and QO (2.13 A),3 reflecting differences of the
coordination numbers. The MoQSe bond distance of 2.36 A is
also slightly longer than that of the five-coordinate MoVIO(Se)
centre of NDH (2.29 A). The calculated n(CQC) values (1S: 1520
and 1491 cmꢀ1, 1Se: 1526 and 1486 cmꢀ1) are in reasonable
agreement with the experimental frequencies for the modes. The
experimentally and theoretically obtained frequencies of 1Se are
close to those of 1S, suggesting that the n(CQC) stretches are not
sensitive to the exchange of the terminal sulfide and selenide
groups. The bond dissociation energy (kcal molꢀ1) of MoVIQE
decreases as the E atom goes from O (88 k cal molꢀ1) to S (47) and
to Se (35). The net charge of the chalcogen atom increases in the
order of O (ꢀ1.07 for 1O) o S (ꢀ0.85 for 1S) o Se (ꢀ0.69 for 1Se),
reflecting their electronegativity. The absorption band at around
485 (1S)–490 nm (1Se) with molar coefficient >3500 Mꢀ1 cmꢀ1 is
absent in 1O, suggesting that the band involves some contributions
of S or Se - Mo(VI) charge transfer (the overlaid absorption
spectra are given in Fig. S5, ESIz).
at ꢀ40 1C. Data of the elemental analysis were consistent with
the formula (Et4N)2[MoVIO(S)L2], although the deep-red powder
gradually changed to the known compound 3 and elemental
sulphur at room temperature (see Experimental section). Thus,
the deep-red species generated in situ is assigned to a desired oxo-
sulfido-molybdenum(VI) complex, (Et4N)2[MoVIO(S)L2] (1S).y A
resonance Raman (rR) spectrum was measured at ꢀ80 1C with
laser excitation at 363.8 nm in C2H5CN, which exhibited bands
at 919, 461 and 442 cmꢀ1 (Fig. S2a–c, ESIz). By comparing the
bands with those of 2, 1O and 1Se (vide infra),10,11 these bands are
assignable to the n(MoQO), n(Mo–S) and n(MoQS) stretches,
respectively. In addition, two bands were observed at 1536 and
1505 cmꢀ1 in the range from 1400–1600 cmꢀ1, which are
assignable to n(CQC) stretches.
The reaction of 2 with SeHꢀ (Et4NSeH) was also examined
to observe the formation of (Et4N)2[MoVIO(Se)L2] (1Se) as a
deep-pink species below ꢀ80 1C in C2H5CN under strictly dry
conditions. At higher temperature, however, 3 was formed by
an intramolecular redox reaction as in the case of the sulphur
system. Thus, the addition of SeHꢀ to a C2H5CN solution of 2
at ꢀ80 1C resulted in the spectral changes shown in Fig. S3a
(ESIz), where the absorption bands at 563 and 734 nm due to 2
decreased with a concomitant increase of the absorption band
at 490 nm due to 1Se with clear isosbestic points at 430, 471
and 533 nm.z The titration plot indicates that 2 reacts
with 1 equivalent of SeHꢀ as in the reaction of 2 and SHꢀ
(Fig. S3b, ESIz). The shape of the UV-vis spectrum of the deep-
pink complex is similar to that of the deep-red complex 1S, where
the lmax values are slightly red shifted. The rR spectrum of a
deep-pink complex generated in C2H5CN at ꢀ80 1C with
363.8 nm laser excitation showed bands centered at 917, 458
and 367 cmꢀ1 (Fig. S4a–c, ESIz). The band at 367 cmꢀ1 can be
assigned to n(MoQSe) stretch since the rR spectrum of 1S did
An atom transfer reaction from 1S/Se to triphenylphosphine
was investigated. In a preparative scale reaction, 1S generated
in situ reacted with Ph3P in CH3CN at ꢀ40 1C to give Ph3PS in
a 96% yield. Also, upon addition of Ph3P to 1Se in C2H5CN at
ꢀ80 1C, Ph3PSe was obtained in 82% yield. The results
indicate that selective sulphur and selenium atom transfer
occurred (see Experimental section). Then, the atom transfer
reactions were analysed kinetically. The spectral change for
the reaction of 1S with Ph3P in CH3CN at ꢀ20 1C is shown in
Fig. S6 (ESIz), where the intensity of the absorption band at
485 nm decreases with concomitant appearance of a new band
not exhibit any bands in the region from 300 to 400 cmꢀ1
.
c
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transfer reactivity. Our finding will demonstrate the merit of
using a sulfido or a selenido group as an electrophile instead
of an oxo group. Further studies are now in progress
using dithiolene ligands with less electron-withdrawing sub-
stituents to prevent an intramolecular redox reaction within
the MoVIO(S/Se) core.
We thank Dr Graham N. George of University of Saskatchewan
for S/Se/Mo K-edge XANES measurement and Mr Takashi
Nomura of University of Hyogo for rR measurements. This work
was partly supported by grants (No. 24108725 and 2410915 to
H. S.) for Scientific Research on Priority Areas from MEXT of
Japan and a grant (No. 23350027 to H. S.) for Scientific Research
(B) from the Japan Society for Promotion of Science. S. I. and T. O.
acknowledge financial support by grants (No. 22105007 (S. I.)
and No. 22018026 (T. O.)) for Scientific Research on Priority
Areas from MEXT of Japan.
Fig. 3 (a) The plot of pseudo-first order rate constant (kobs) vs. [Ph3P].
(b) The double reciprocal plots of kobs against [Ph3P]ꢀ1. (c) Proposed
ꢀ1
mechanism for the sulphur/selenium atom transfer of 1S/Se to Ph3P.
at 354 nm, keeping clear isosbestic points at 331 and 363 nm.
The lmax and e values of the final spectrum are identical to
those of 3. The reaction obeys pseudo-first-order kinetics (the
inset of Fig. S6, ESIz), and the plot of the observed rate
constant, kobs (sꢀ1), against [Ph3P] gave a saturation curve
(Fig. 3a), suggesting a complex formation between 1S and
Ph3P prior to the S atom transfer (Fig. 3c). In such a case, the
Notes and references
y Our several trials to measure the S/Se/Mo K-edge XANES to get
strong evidence of the formation of the MoVIQS and MoVIQSe bonds
were unsuccessful due to sample preparation problems.
z Among a series of spectroscopic measurements we tried, only the
UV-vis and rR spectra could be obtained but other trials such as
1H NMR, ESI-MS and Se/Mo K-edge XANES were unsuccessful
owing to instability of the deep-pink complex.
reaction rate, v, can be expressed by eqn (1), where kobs
=
kb[Ph3P]/(K + [Ph3P]) and K = (kꢀa + kb)/ka. Thus, the double
reciprocal plot of kobsꢀ1 against [Ph3P]ꢀ1 gave a linear correlation
with a slope of K/kb and an intercept of kbꢀ1 on the y-axis (Fig. 3b),
from which K and kb have been determined to be 4.9 ꢁ 10ꢀ3 Mꢀ1
and 1.7 ꢁ 10ꢀ2 sꢀ1, respectively. The selenium atom transfer from
1Se to Ph3P also proceeded smoothly in C2H5CN at ꢀ80 1C
(Fig. S7a, ESIz), where a saturation dependence was also observed
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ꢀ1
at high concentrations of Ph3P. The double reciprocal plot of kobs
vs. [Ph3P]ꢀ1 gave the kinetic parameters K (9.7 ꢁ 10ꢀ3 Mꢀ1) and kb
(2.4 ꢁ 10ꢀ2 sꢀ1) (Fig. S7b and c, ESIz). The kb value at ꢀ80 1C for
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1Se) indicates that the selenium atom transfer is significantly faster
than the sulphur atom transfer by ca. 104 times. On the other hand,
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enhanced atom transfer reactivity of 1S and 1Se when compared
with 1O can be explained from the LUMO’s character, where the
LUMO orbitals of 1S and 1Se have 34.42% and 35.33% contribu-
tion of the sulfido and selenido groups, respectively (Fig. S9, ESIz).
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of the oxo group character.
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v = kobs[1S/Se
]
= kb[1S/Se]T[Ph3P]/(K + [Ph3P]) (1)
T
where K = (k + kb)/ka.
ꢀa
In summary, low temperature spectroscopic techniques
enabled us to characterise metastable oxo-sulfido- and oxo-
selenido-molybdenum(VI) complexes with an identical
molybdenum(VI)–dithiolene framework. The complexes were
also characterised by using DFT calculations. The new
complexes, when coupled with the known MoVIO2 complex
of the same supporting ligand, have enabled a systematic study
of the effects on the terminal chalogenide group in the atom
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Chem. Commun.