Two versatile new routes to dinuclear molybdenum dithiolene complexes

Article abstract of DOI:10.1039/a707958c

Two routes to the dithiolene complexes [Mo₂S(μ-S)₂(SCR¹=CR ²S)(η-C₅H₅)₂] starting from the complexes [Mo₂(μ-alkyne)(CO)₄(η-C₅H ₅)₂] are described; in one case the dithiolene ligand is derived from the alkyne, and in the other from a 1,3-dithiole-2-thione reagent.

Full text of DOI:10.1039/a707958c

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Two versatile new routes to dinuclear molybdenum dithiolene complexes
Andrew Abbott,a Matthew N. Bancroft,a Michael J. Morris,*a Graeme Hogarthb and Simon P. Redmondb
a Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF
b Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ
Two routes to the dithiolene complexes [Mo2S(m-
S)2(SCR1NCR2S)(h-C5H5)2] starting from the complexes
[Mo2(m-alkyne)(CO)4(h-C5H5)2] are described; in one case
the dithiolene ligand is derived from the alkyne, and in the
other from a 1,3-dithiole-2-thione reagent.
signal for the CH group appears shifted downfield at d 8–9. This
is a characteristic feature of terminal dithiolene complexes and
can be regarded as arising either from the contribution of a
dithioglyoxal canonical form6,7 or the presence of a ring current
in the pseudo-aromatic metalladithiolene unit.8 Interestingly the
products all gave high intensity molecular ions in their electron
impact mass spectra, but most evidently disintegrated com-
pletely when subjected to fast atom bombardment (FAB).
The result of an X-ray crystal structure determination of 2a is
shown in Fig. 1.‡ The molecule is based on a dimolybdenum
centre with a metal–metal distance of 2.984(1) Å, which is
A combination of techniques have indicated that the molybde-
num atom in the molybdenum cofactor (Moco) common to a
range of oxotransferase enzymes is coordinated by a pterin
ligand which is bonded through a dithiolene linkage.1 Recent
protein crystallographic studies, however, have shown that in
some of these enzymes the metal atom is ligated by one
dithiolene ligand whereas in other cases, unexpectedly, two
such ligands were present.2 Molybdenum (and tungsten)
dithiolene complexes have been known for many years,3 and are
of continuing interest as model systems for the metal site in the
cofactor.
Here, we describe two versatile routes to dimolybdenum
complexes containing terminal dithiolene or tetrathiooxalate
ligands, both of which start with the alkyne complexes [Mo2(m-
R1C·CR2)(CO)4(h-C5H5)2] 1a–e. We have recently shown that
reactions of these compounds with thiols often proceed with
C–S bond cleavage to give sulfido-bridged species.4 In contrast,
it is notable that the reactions discussed here involve the
formation of C–S bonds as well as their scission.
In the first approach we found that heating a toluene solution
of 1 with elemental sulfur for 5 h provided the green dithiolene
complexes [Mo2S(SCR1NCR2S)(m-S)2(h-C5H5)2] 2a–e in
yields of up to 80% after isolation by column chromatography
(Scheme 1). Five representative examples are given but the
reaction works equally well with other complexes of type 1.
Complex 2c and the corresponding oxo species
[Mo2O(SCHNCPhS)(m-S)2(h-C5H5)2] 3 have been prepared
previously by a different route,5 but in contrast to this report we
find that complexes of type 2 are not particularly air sensitive
even in solution, and only slowly convert into the oxo
complexes analogous to 3; small amounts of these were isolated
from some reactions.
similar
to
that
of
2.927(1)
Å
observed
in
[Mo2O(SCHNCPhS)(m-S)2(h-C5H5)2] 3.5 The coordination
geometry in the two complexes is also very similar. Thus Mo(1)
has a pseudo-tetrahedral coordination (if the C5H5 ligand is
regarded as occupying one site) whereas Mo(2) is in a square
pyramidal environment. The Mo2(m-S)2 core is not planar: the
dihedral angle between the two intersecting Mo2S planes is
161.3°, whereas in 3 the corresponding angle is 144.6°. This
contrasts with related MoV dimers with imido ligands, e.g.
[Mo2(NPh)2(m-NPh)2(h-C5H4Me)2] which all contain strictly
planar cores.9
The bond lengths between Mo(2) and the bridging sulfur
atoms are significantly longer than the distances from Mo(1) to
the same sulfurs. Together with the rather short S(4)–C(11) and
S(5)–C(12) bonds within the dithiolene ligand itself, this could
be regarded as evidence for some contribution of the dithio-
glyoxal canonical form: if the dithiolene ligand is dianionic,
each Mo atom is formally MoV, but if it is bonded in the
dithioglyoxal form, Mo(2) would be formally MoIII. Again an
alternative interpretation is that the dithiolene ligand acts as a
delocalised 6p pseudo-aromatic system which is a better
p-donor than the sulfide ligand on Mo(1).
Our second approach is shown in Scheme 2. We recently
reported that alkyne complex 1e reacted with 1,3-dithiole-
2-thiones by cleavage of the CNS bond, ring opening of the
resulting carbene and coupling with the alkyne ligand to
The dithiolene compounds were characterised spectroscop-
ically (and by comparison with the published values for 2c, for
which our data agree exactly).† In the 1H NMR spectrum each
complex displays two rather widely separated signals for the
C5H5 rings and appropriate resonances for the substituents R1
and R2. In the complexes derived from terminal alkynes, the
C(12)
C(5)
S(5)
C(11)
C(1)
C(4)
C(2)
S(3)
S(4)
R1
Mo(2)
R1
R2
CO
C
C(3)
S
Mo(1)
S
S
C
CO
R2
S8
Mo
Mo
Mo
Mo
S
S(2)
C(8)
C(7)
heat, toluene
OC
S
C
O
C(9)
1a R1 = R2 = H
S(1)
C(6)
2a–e
C(10)
b R1 = H, R2 = Me
c R1 = H, R2 = Ph
d R1 = R2 = Et
Fig. 1 Molecular structure of complex 2a in the crystal. Selected bond
lengths (Å): Mo(1)–Mo(2) 2.984(1), Mo(1)–S(1) 2.154(1), Mo(1)–S(2)
2.263(1), Mo(1)–S(3) 2.263(1), Mo(2)–S(2) 2.406(1), Mo(2)–S(3)
2.398(1), Mo(2)–S(4) 2.387(1), Mo(2)–S(5) 2.384(1), S(4)–C(11) 1.708(5),
S(5)–C(12) 1.705(4), C(11)–C(12) 1.348(7).
e R1 = R2 = CO2Me
Scheme 1
Chem. Commun., 1998
389

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R3
C
145.6 (CH), 103.6 (C5H5), 100.1 (C5H5), 23.4 (Me). MS m/z 522 (M+), 482
(M 2 HC2Me+).
1e
R3
S
C
2d: yield 70%. mp 232–233 °C. 1H NMR d 6.00 (5 H, C5H5), 5.35 (5 H,
C5H5), 3.10 (dq, J 7 Hz, 2 H, 1 H of CH2), 2.85 (dq, J 7 Hz, 2 H, 1 H of CH2),
1.55 (t, J 7 Hz, 6 H, Me); 13C NMR d 163.2 (CEt), 103.6 (C5H5), 100.0
(C5H5), 29.8 (CH2), 16.6 (Me); MS m/z 564 (M+), 532 (M 2 S+), 482 (M
2 C2Et2+).
C
+
Mo
R3
R3
S
S
S
C
S
Mo
S
C
1
MeO2C
2e: yield 80%. mp > 250 °C. H NMR d 6.00 (5 H, C5H5), 5.50 (5 H,
CO2Me
C5H5), 3.87 (s, 6 H, Me); 13C NMR d 165.1 (CO2Me), 154.7 (CCO2Me),
103.8 (C5H5), 100.9 (C5H5), 53.0 (Me); MS m/z 624 (M+), 592 (M 2 S+),
482 [M 2 C2(CO2Me)2+].
4
S8
2f: yield 66%. mp 230 °C (decomp.). 1H NMR d 6.02 (5 H, C5H5), 5.40
(5 H, C5H5), 2.70 (6 H, Me); 13C NMR d 153.6 (CSMe), 103.7 (C5H5),
100.2 (C5H5), 19.3 (Me); MS m/z 600 (M+).
toluene, reflux
R3
S
2g: yield 58%. mp 214 °C. 1H NMR d 8.03–7.43 (m, 10 H, Ph), 6.03 (5
H, C5H5), 5.84 (5 H, C5H5); 13C NMR 189.4 (COPh), 151.4 (CSCOPh),
136.4 (Cipso), 133.8–127.7 (m, Ph), 103.7 (C5H5), 101.4 (C5H5); FAB MS
m/z 780 (M+).
S
S
R3
MeO2C
MeO2C
Mo
S
Mo
S
S
+
S
S
5: Yield 70%. mp 93–94 °C. 1H NMR d 3.97 (3 H, Me), 3.95 (3 H, Me);
13C NMR d 210.4 (CNS), 162.7, 159.4 (both CO2Me), 156.1, 146.2 (both
CCO2Me), 54.4, 53.7 (both Me); MS m/z 250 (M+).
5
2e R3 = CO2Me
f R3 = SMe
g R3 = SCOPh
‡ Crystal data for 2a: C12H12Mo2S5, M = 508.40, monoclinic, space
group C2/c (no. 15), a = 22.2528(29), b = 11.0848(21), c = 12.9379(26)
Å, b = 105.933(15)°, U = 3068.7(9) Å3, F(000) = 1984, Mo-Ka radiation
Scheme 2
(l = 0.71073 Å), m(Mo-Ka) = 22.34 cm21, Z = 8, Dc = 2.20 g cm23
.
Room temperature X-ray data were collected on a Nicolet R3mV
diffractometer. A total of 3937 reflections (3779 unique) were measured in
the range 5 < 2q < 50° by the w–2q scan technique, all of which were
corrected for Lorentz and polarisation effects and for absorption by analysis
of y-scans (minimum and maximum transmission coefficients 0.806 and
0.913); 2634 data with I ! 3.0s(I) were used in the refinement. The
structure was solved by direct methods and developed by alternating cycles
of least squares refinement and difference Fourier synthesis. All non-
hydrogen atoms were refined anisotropically while hydrogen atoms were
placed in idealised positions (C–H 0.96 Å) and assigned a common isotropic
thermal parameter (U = 0.08 Å2). The last cycle of least squares refinement
included 172 parameters for 2497 variables. The final R values were R
= 0.035 and Rw = 0.042 and the final difference Fourier map contained no
peaks > 1.00 e Å23. The structure was solved using the SHELXTL PLUS
program package12 on a Micro Vax II computer. CCDC 182/722.
produce complexes of type 4 (R3 = CO2Me, SMe, SCOPh).10
Remarkably, we have now discovered that these compounds
also react rapidly (2 h) with elemental sulfur in boiling toluene
to give dithiolene complexes 2e–g, in which the dithiolene
ligand originates from the backbone of the original thione rather
than the alkyne. Compounds 2f and 2g are rare examples of
dialkyl- and diacyl-tetrathiooxalate (or ethenetetrathiolate)
complexes respectively.
The alkyne ligand is recovered in the form of the interesting
new sulfur heterocycle 5, a hitherto inaccessible derivative of
the important 1,2-dithiole-3-thione ring system.11 Indeed, in the
case of R3 = CO2Me, the reaction represents an elaborate
isomerisation of the 1,3-dithiole-2-thione ring. The mechanism
presumably involves insertion of three sulfur atoms into the
Mo–C bonds of the bridging ligand of 4, cleavage of the m-C–S
bond to afford the dithiolene ligand, and reductive elimination
of the organic by-product 5. It is notable that this process
reforms the thione unit which is cleaved during the formation of
4.
In summary, the two synthetic routes described here are
complementary: simple dithiolene ligands are easily accessed
from the appropriate alkyne by the first method, whereas in the
second case the availability of convenient synthetic procedures
for a large number of requisite thiones, which are readily made
by alkylation or acylation of [NEt4]2[Zn(C3S5)2], should enable
the introduction of a wide variety of more complex substituents
including functionalised derivatives such as crown ethers,
macrocycles, etc. Moreover, compounds containing the 1,2-di-
thiole-3-thione ring system have been studied for their pharma-
ceutical properties, particularly as anticancer agents. We are
currently exploiting this methodology to synthesize a repre-
sentative range of dithiolene complexes and heterocycles for
further investigation.
1 D. Collison, C. D. Garner and J. A. Joule, Chem. Soc. Rev., 1996, 25,
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WethanktheEPSRCfortheawardofastudentship(M. N. B.).
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Notes and References
9 J. Fletcher, G. Hogarth and D. A. Tocher, J. Organomet. Chem., 1991,
403, 345; 405, 207; M. L. H. Green, G. Hogarth, P. C. Konidaris and
P. Mountford, J. Chem. Soc., Dalton Trans., 1990, 3781.
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12 G. M. Sheldrick, SHELXTL PLUS program package for structure
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Inc., Madison WI, 1990.
* E-mail: M.Morris@sheffield.ac.uk
† Selected spectroscopic data: (NMR in CDCl3, all signals are singlets
unless otherwise stated). Satisfactory elemental analyses were obtained for
all new compounds.
1
2a: yield 64%. mp 196–198 °C. H NMR d 8.37 (2 H, CH), 6.00 (5 H,
C5H5), 5.38 (5 H, C5H5); 13C NMR d 149.5 (CH), 103.7 (C5H5), 100.1
(C5H5); MS m/z 508 (M+), 482 (M 2 C2H2+). The dithiolene complexes
+
2a–f all show peaks at m/z 450, 418, 386, 353 and 323 (Mo2S42nCp2
n = 0–4) in the EI mass spectrum.
,
2b: yield 54%. mp 234 °C (decomp.). 1H NMR d 8.00 (1 H, CH), 6.00 (s,
5 H, C5H5), 5.40 (5 H, C5H5), 2.80 (3 H, Me); 13C NMR d 163.0 (CMe),
Received in Cambridge, UK, 5th November 1997; 7/07958C
390
Chem. Commun., 1998