Phosphine-substituted dithiolene complexes as ligands: Communication between ruthenium(II) centers through a dimolybdenum bis(dithiolene) core

Article abstract of DOI:10.1021/ic701201x

The reaction of Mo₂(SCH₂CH₂S) ₂Cp₂ (1; Cp = η-C₅H₅) with an excess of an alkyne in refluxing dichloromethane affords the bis(dithiolene) complexes Mo₂(μ-SCR¹=CR²S) ₂Cp₂ (2a, R¹ = R² = CO ₂Me; 2b, R¹ = R² = Ph; 2c, R¹ = H, R² = CO₂Me) whereas with 1 equiv of alkyne at room temperature the mixed dithiolene-dithiolate species Mo₂(μ-SCR ¹=CR²S)(μ-SCH₂CH₂S)Cp ₂ (3a, R¹ = R² = CO₂Me; 3b, R ¹ = R² = Ph) are formed. The remaining dithiolate ligand in 3 can then be converted into a different dithiolene by reaction with a second alkyne. Applying this methodology, we have used bis(diphenylphosphino)acetylene to prepare the first examples of complexes containing phosphine-substituted dithiolene ligands: Mo₂{μ-SC(CO₂Me)=C(CO ₂Me)S}{μ-SC(PPh₂)=C(PPh₂)S}Cp₂ (2g) and Mo₂{μ-SC(PPh₂)=C(PPh₂)S} ₂Cp₂ (2h). Tri- and tetrametallic complexes can then be assembled by coordination of these diphosphines to CpRuCl units by reaction with CpRu(PPh₃)₂Cl. Electrochemical studies of the Ru(II)/Ru(III) couple in Mo₂{μ-SC(PPh₂)=C(PPh ₂)S}₂Cp₂(RuClCp)₂ (4b) reveals that the two separate ruthenium centers are oxidized electrochemically at different potentials, demonstrating communication between them through the dimolybdenum bis(dithiolene) core. Density functional theory calculations were carried out to explore the electronic structures of these species and to predict and assign their electronic spectra.

Full text of DOI:10.1021/ic701201x

Inorg. Chem. 2007, 46, 9790−9807
Phosphine-Substituted Dithiolene Complexes as Ligands:
Communication between Ruthenium(II) Centers Through a
Dimolybdenum Bis(dithiolene) Core
Harry Adams,† Michael J. Morris,*,† Andrea E. Riddiough,† Lesley J. Yellowlees,‡ and A. B. P. Lever*,§
Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF, U.K., School of Chemistry,
UniVersity of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K., and Department of
Chemistry, York UniVersity, CB124, 4700 Keele Street, Toronto, Ontario M3J1P3, Canada
Received June 19, 2007
The reaction of Mo2(SCH2CH2S)2Cp2 (1; Cp ) η-C5H5) with an excess of an alkyne in refluxing dichloromethane
affords the bis(dithiolene) complexes Mo2( Ph; 2c,
-SCR1dCR2S)2Cp2 (2a, R1 R2 CO2Me; 2b, R1 R2
R1 H, R2
CO2Me) whereas with 1 equiv of alkyne at room temperature the mixed dithiolene dithiolate species
Mo2( Ph) are formed. The remaining
-SCR1dCR2S)( -SCH2CH2S)Cp2 (3a, R1 R2 CO2Me; 3b, R1 R2
µ
)
)
)
)
)
)
−
µ
µ
)
)
)
)
dithiolate ligand in 3 can then be converted into a different dithiolene by reaction with a second alkyne. Applying
this methodology, we have used bis(diphenylphosphino)acetylene to prepare the first examples of complexes
containing phosphine-substituted dithiolene ligands: Mo2{µ-SC(CO2Me)
Cp2 (2g) and Mo2{µ-SC(PPh2)dC(PPh2)S 2Cp2 (2h). Tri- and tetrametallic complexes can then be assembled by
coordination of these diphosphines to CpRuCl units by reaction with CpRu(PPh3)2Cl. Electrochemical studies of the
Ru(II)/Ru(III) couple in Mo2{µ-SC(PPh2)dC(PPh2)S 2Cp2(RuClCp)2 (4b) reveals that the two separate ruthenium
dC(CO2Me)S}{µ-SC(PPh2)dC(PPh2)S}-
}
}
centers are oxidized electrochemically at different potentials, demonstrating communication between them through
the dimolybdenum bis(dithiolene) core. Density functional theory calculations were carried out to explore the electronic
structures of these species and to predict and assign their electronic spectra.
Introduction
was considered able to bear a formal charge of between -2
and 0 due to the presence of dithiolate and dithioketone
resonance forms. Since then, dithiolene complexes have been
employed in a variety of applications such as conducting
and superconducting coordination compounds (particularly
those involving the dmit ligand, C3S52-), nonlinear optical
materials, and liquid crystals.2 More recently, nickel dithi-
olenes have received renewed attention as possible agents
for the separation of olefins from gas streams.3 In the case
of molybdenum and tungsten dithiolene complexes, added
impetus has been given to their study by their involvement
in biological systems, namely, the active sites of oxotrans-
ferase enzymes.4
Complexes containing the dithiolene (1,2-enedithiolate)
ligand have been the subject of much interest since the mid-
1960s when a major research effort, particularly by the
groups of Schrauzer, Holm, and McCleverty, led to the
discovery of synthetic routes to dithiolene complexes of most
of the transition metals.1 At the time, this interest was driven
by three important properties of the dithiolene ligand: its
redox activity, its pseudo-aromaticity, and its propensity for
stabilizing unusual coordination geometries. The ligand itself
* To whom correspondence should be addressed. E-mail: M.Morris@
sheffield.ac.uk. (M.J.M.), blever@yorku.ca (A.B.P.L.). Tel: 0114 2229363.
Fax: 0114 2229346.
Heteroleptic dithiolene complexes containing cyclopen-
tadienyl ligands constitute an important subgroup of com-
pounds, particularly for Mo and W.5 Research in our
† University of Sheffield.
‡ University of Edinburgh.
§ York University.
(1) (a) McCleverty, J. A. Prog. Inorg. Chem. 1969, 10, 49. (b) Eisenberg,
R. Prog. Inorg. Chem. 1970, 12, 295. (c) Burns, R. P.; McAuliffe, C.
A. AdV. Inorg. Chem. Radiochem. 1979, 22, 303. (d) Mueller-
Westerhoff, U. T.; Vance, B. In ComprehensiVe Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.;
Pergamon: Oxford, 1987; Vol. 2, Chapter 16.5. (e) Dithiolene
Chemistry; Stiefel, E. I., Ed.; Prog. Inorg. Chem. 2004, 52.
(2) (a) Mueller-Westerhoff, U. T.; Vance, B.; Yoon, D. I. Tetrahedron
1991, 47, 909. (b) Robertson, N.; Cronin, L. Coord. Chem. ReV. 2002,
227, 93. (c) Fourmigue´, M. Acc. Chem. Res. 2004, 37, 179.
(3) Wang, K.; Stiefel, E. I. Science 2001, 291, 106.
(4) (a) Hille, R. Chem. ReV. 1996, 96, 2757. (b) Johnson, M. K.; Rees,
D. C.; Adams, M. W. W. Chem. ReV. 1996, 96, 2817.
9790 Inorganic Chemistry, Vol. 46, No. 23, 2007
10.1021/ic701201x CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/02/2007

Dimolybdenum Bis(dithiolene) Complexes
Scheme 1. Synthesis of the Bridging Dithiolene Complexes 2 and 3
laboratory is focused on discovering new synthetic routes
to both mono- and dinuclear complexes of this type,
exemplified by the recent synthesis of CpMo(S2C2Ph2)2 and
Mo2{µ-C2(CO2Me)2}(µ-S2C2Ph2)2Cp2 (Cp ) η-C5H5) by
means of dithiolene transfer reactions6 and our development
of two complementary routes to the dinuclear Mo(V)
compounds [Mo2S(µ-S)2(SCR1dCR2S)Cp2] containing a
terminal dithiolene ligand.7
In the course of these studies, we became interested in
the related Mo(III) complexes Mo2(µ-SCR1dCR2S)2Cp2,
which contain two bridging dithiolene ligands. These have
been known since King prepared the first example (R1 ) R2
) CF3) by the reaction of Mo2(CO)6Cp2 with the dithiete
S2C2(CF3)2;8 further clarification of this reaction, together
with the X-ray structure of the product, was later provided
by Miessler and co-workers.9 Convenient access to a wider
range of compounds of this type, including the first examples
containing unsymmetrical dithiolenes (R1 * R2), became
available by exchange of the alkene portion of the dithiolate
ligands in Mo2(SCH2CHRS)2Cp2 (R ) H, Me) with the
alkynes R1CtCR2 (R1 ) R2 ) H; R1 ) H or Me, R2 )
Ph).10 The crystal structure of the parent complex (R1 ) R2
) H) was reported separately.11 More recently, Sugimori has
shown that low yields of the complex with R1 ) R2 ) CO2-
Me can be obtained (together with several other carbonyl-
containing dithiolene complexes) by the reaction of Mo2(CO)6-
Cp2 with sulfur and dimethyl acetylenedicarboxylate (DMAD,
MeO2CCtCCO2Me).12 This is reminiscent of the original
King reaction in that a dithiete may possibly be an
intermediate.13
Here, we present further developments in the chemistry
of these complexes, including (i) a general preparation of
those containing two different bridging dithiolene ligands
(whereas complexes containing two or more dithiolene
ligands are relatively common, those containing two different
dithiolene ligands are very rare); (ii) complexes containing
the first examples of phosphine-substituted dithiolene ligands;
(iii) the coordination of additional metal fragments to the
phosphine substituents; (iv) the electrochemical characteriza-
tion of the resulting species which reveals communication
between the metal centers across the dimolybdenum core
through the dithiolene ligands; and (v) density functional
theory (DFT) calculations used to explore the electronic
structures of these species and assign their electronic spectra.
Results and Discussion
Synthetic Studies. We chose to expand the methodology
of Dubois,10 as these reactions occurred under mild condi-
tions and used alkynes as the source of the dithiolene ligands.
Treatment of Mo2(SCH2CH2S)2Cp2 (1) with an excess of
DMAD or C2Ph2 in refluxing dichloromethane for 24-48 h
duly provided the symmetrical bis-dithiolene complexes 2a
and 2b as air-stable brick-red (2a) or green-brown (2b) solids
in yields of 57 and 64%, respectively (Scheme 1). Their
relative insolubility prevented the acquisition of 13C NMR
spectra, but characterization by 1H NMR spectra, mass
spectra, and elemental analysis proved straightforward. The
data obtained for 2a agreed with those recently reported by
Sugimori.12 The reaction of 1 with methyl propiolate under
similar conditions afforded the analogous complex Mo2{µ-
SCHdC(CO2Me)S}Cp2 (2c) (62%) as two separable isomers,
cis and trans, depending on the relative orientation of the
two unsymmetrical dithiolene ligands. The presence of
isomers was not observed in the previous bis-dithiolene
complexes of this type prepared from the unsymmetrical
alkynes PhCtCH and PhCtCMe.10 The 1H NMR spectrum
of the cis isomer exhibited a peak at δ 7.78 for the CH of
the dithiolene, with the corresponding peak for the trans
isomer occurring at δ 7.80. These figures are typical for
bridging dithiolene ligands, whereas protons attached to
terminal dithiolenes appear at lower field, e.g., δ 9.41 for
[Mo2S(µ-S)2{SCHdC(CO2Me)S}Cp2].6 Only the cis isomer
was sufficiently soluble to permit the recording of a 13C NMR
spectrum, in which both signals due to the dithiolene carbon
atoms appeared in the region of 160 ppm. The stereochem-
(5) Fourmigue´, M. Coord. Chem. ReV. 1998, 178-180, 823.
(6) (a) Adams, H.; Gardner, H. C.; McRoy, R. A.; Morris, M. J.; Motley,
J. C.; Torker, S. Inorg. Chem. 2006, 45, 10967. (b) Adams, H.; Morris,
M. J.; Morris, S. A.; Motley, J. C. J. Organomet. Chem. 2004, 689,
522.
(7) Abbott, A.; Bancroft, M. N.; Morris, M. J.; Hogarth, G.; Redmond,
S. P. Chem. Commun. 1998, 389.
(8) King, R. B. J. Am. Chem. Soc. 1963, 85, 1587.
(9) Roesselet, K.; Doan, K. E.; Johnson, S. D.; Nicholls, P.; Miessler, G.
L.; Kroeker, R.; Wheeler, S. H. Organometallics 1987, 6, 480.
(10) Rakowski Dubois, M.; Haltiwanger, R. C.; Miller, D. J.; Glatzmeier,
G. J. Am. Chem. Soc. 1979, 101, 5245.
(11) Miller, W. K.; Haltiwanger, R. C.; VanDerVeer, M. C.; Rakowski
Dubois, M. Inorg. Chem. 1983, 22, 2973.
(12) Sugiyama, T.; Yamanaka, T.; Shibuya, M.; Nakase, R.; Kajitani, M.;
Akiyama, T.; Sugimori, A. Chem. Lett. 1998, 501.
(13) For the isolation of this dithiete by a different method, see Shimizu,
T.; Murakami, H.; Kobayashi, Y.; Iwata, K.; Kamigata, N. J. Org.
Chem. 1998, 63, 8192.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9791

Adams et al.
Scheme 2. Synthesis of the Trinuclear Mo2Ru Complex 4a
istry of the trans isomer was confirmed by an X-ray structure
determination, reported below.
In order to control the degree of functionalization of the
dithiolene ligands completely, we wanted to establish reliable
syntheses of complexes containing two different dithiolene
ligands. Dubois and co-workers had briefly reported that the
reaction of 1 with 1 equiv of phenylacetylene in CH2Cl2 at
room temperature afforded the mixed dithiolene-dithiolate
complex Mo2(µ-SCHdCPhS)(µ-SCH2CH2S)Cp2 in 25%
yield.14 This proved to be a general method: treatment of 1
with 1 equiv of DMAD or C2Ph2 under these conditions gave
the mixed complexes Mo2(µ-SCR1dCR2S)(µ-SCH2CH2S)-
Cp2 (3a, R1 ) R2 ) CO2Me; 3b, R1 ) R2 ) Ph) in moderate
yields (44 and 20%, respectively). These green compounds
are distinctly more soluble in organic solvents than the bis-
dithiolene complexes 2, so much so that recrystallization of
3a proved difficult. Their 1H NMR spectra showed a singlet
for the remaining CH2 groups, together with appropriate
signals for R1 and R2, and the 13C spectra contained peaks
due to dithiolene (ca. 160 ppm) and dithiolate (ca. 35 ppm)
carbons.
The alkene portion of the remaining dithiolate ligand in
these complexes is also readily replaced by alkynes.15 Thus,
treatment of 3a with diphenylacetylene or methyl propiolate
in refluxing dichloromethane afforded the yellow mixed bis-
dithiolene complexes Mo2{µ-SC(CO2Me)dC(CO2Me)S}(µ-
SCPhdCPhS)Cp2 (2d) and Mo2{µ-SC(CO2Me)dC(CO2-
Me)S}{µ-SCHdC(CO2Me)S}Cp2 (2e) in yields of 82 and
50%, respectively. There was no evidence for exchange of
the existing dithiolene ligands during this process (i.e., no
2a or 2b was formed during the first of these reactions).
Similarly, treatment of 3b with methyl propiolate cleanly
gave Mo2{µ-SCHdC(CO2Me)S}(µ-SCPhdCPhS)Cp2 (2f).
Presumably, this stepwise procedure could be used to gain
access to complexes of type 2 with virtually any combination
of substituents, the exact route employed depending on the
ease of adding 1 equiv of each particular alkyne.
because of its linear nature, but when π-coordinated to a
transition metal through the alkyne bond, the phosphine
substituents are sufficiently bent back to act as a bidentate
ligand to an additional metal.17 We reasoned that by
incorporation of this diphosphine into a dithiolene ligand
using the methodology described above, the resulting rehy-
bridization would afford potentially bidentate or tetradentate
ligands. So it proved: reaction of dppa with 3a afforded the
complex Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-SC(PPh2)d
C(PPh2)S}Cp2 (2g) containing one diphosphine-dithiolene
ligand, and similar treatment of 1 with an excess of the
phosphine smoothly gave the bis(dithiolene) complex Mo2-
{µ-SC(PPh2)dC(PPh2)S}2Cp2 (2h) (Scheme 1). The presence
of the diphosphine unit was clearly indicated by their 31P
NMR spectra, and also by the 13C NMR spectrum of 2g in
which the dithiolene carbon atoms of the phosphine-
substituted dithiolene ligand appear as doublets of doublets
(ABX spin system). Reaction of 1 with 1 equiv of dppa gave
the mixed dithiolene-dithiolate species Mo2{µ-SC(PPh2)d
C(PPh2)S}(µ-SCH2CH2S)Cp2 (3c), though the yield of this
reaction was disappointingly low (19%); consequently, the
chemistry of 3c has not been investigated further.
To explore the coordination of the new diphosphines, we
selected a ruthenium(II) precursor, CpRu(PPh3)2Cl, based on
the facts that (i) both PPh3 ligands are easily displaced by
diphosphines under relatively mild conditions to give good
yields of complexes such as CpRu(dppe)Cl and (ii) the Ru-
(III)/Ru(II) redox couple is readily accessible for electro-
chemistry. Treatment of 2g with 1 equiv of CpRu(PPh3)2Cl
in refluxing toluene for 5 h afforded a single orange product
Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-SC(PPh2)dC(PPh2)S}-
Cp2(RuClCp) (4a) (Scheme 2) with a 31P NMR peak at δ
60.8. Given that the corresponding peak in 2g is at δ -10.1,
this represents a coordination shift of 70.9 ppm, similar to
the 82.3 ppm shift observed on coordinating free dppe (δ
-12.2) to form CpRu(dppe)Cl (δ 70.1). Whereas in 2g the
two Cp ligands of the dimolybdenum core are equivalent,
addition of the ruthenium fragment renders them inequiva-
lent, hence the observation of three Cp signals in the 1H and
13C NMR spectra of 4a.
Phosphine-Substituted Dithiolene Complexes. Phos-
phine ligands are ubiquitous in organometallic chemistry and
catalysis. One of their strengths is the ability to tune their
roles by the systematic variation of steric and electronic
factors, along with the possibility of building in additional
useful properties, e.g., water solubility. Electrochemically
active phosphine ligands are a relatively unexplored area,
though several recent papers have examined the incorporation
of phosphine substituents into the tetrathiafulvalene (TTF)
nucleus; these can then be coordinated to metal centers in
either discrete complexes or extended arrays.16
It is well-established that the acetylenic diphosphine Ph2-
PCtCPPh2 (dppa) is incapable of acting as a chelating ligand
(14) McKenna, M.; Wright, L. L.; Miller, D. J.; Tanner, L.; Haltiwanger,
R. C.; Rakowski Dubois, M. J. Am. Chem. Soc. 1983, 105, 5329.
(15) Reference 14 mentions an experiment of this type and contains a line
drawing of one mixed bis(dithiolene) complex, Mo2(µ-SCHdCHS)-
(µ-SCHdCPhS)Cp2, but no experimental details or characterizing data
were included.
(16) (a) Fourmigue´, M.; Uzelmeier, C. E.; Boubekeur, K.; Bartley, S. L.;
Dunbar, K. R. J. Organomet. Chem. 1997, 529, 343. (b) Smucker, B.
D.; Dunbar, K. R. J. Chem. Soc., Dalton Trans. 2000, 1309. (c)
Fourmigue´, M.; Batail, P. Bull. Soc. Chim. Fr. 1992, 129, 29.
In a similar manner, treatment of 2h with 2 equiv of CpRu-
(PPh3)2Cl afforded the tetrametallic complex Mo2{µ-SC-
(17) Powell, A. K.; Went, M. J. J. Chem. Soc., Dalton Trans. 1992, 439.
9792 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
Scheme 3. Synthesis of the Tetranuclear Mo2Ru2 Complex 4b
Scheme 4. Synthesis of the Pentanuclear Mo4Ru Complex 5
bidentate phosphine 2g might react in a similar manner to
give RuCl2{Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-SC(PPh2)d
C(PPh2)S}Cp2}2 (5). In the event, an orange-beige powder
precipitated from the reaction mixture; acquisition of NMR
data was impossible due to its insolubility, but a molecular
ion peak was observed at m/z 2147 in its mass spectrum and
a correct elemental analysis was obtained. It is therefore
possible to envisage the formation of a polymeric molecular
wire from [RuCl2(PPh3)3] and 2h if the problem of insolubil-
ity could be overcome.
Crystallographic Studies. Two of the simple bis(dithi-
olene) complexes, centrosymmetric trans-2c and the mixed
dithiolene species 2d, were characterized crystallographically.
The molecular structures are shown in Figures 1 and 2,
respectively, with selected bond lengths and angles listed
in the captions. Both complexes display geometries very
similar to the previously studied Mo2{µ-SC(CF3)dC(CF3)S}2-
Cp2 and Mo2(µ-SCHdCHS)2Cp2.9,11 The two molybdenum
atoms, both formally Mo(III), are linked by a bond of
length 2.5832(6) Å in trans-2c (2.5848(8) Å in 2d), which
is typical for quadruply bridged complexes of this
oxidation state.18 This bond is bridged symmetrically by
the four sulfur atoms, forming the familiar orthogonal
arrangement of the two Mo2S2 planes with the dithiolene
CdC bonds lying essentially perpendicular to the Mo-
Mo axis. Each Mo atom thus lies in a four-legged piano
stool environment. The C(6)-C(7) distance within the
dithiolene ligands in trans-2c is 1.312(6) Å, experimentally
indistinguishable from the corresponding bond lengths
in 2d.
(PPh2)dC(PPh2)S}2Cp2(RuClCp)2 (4b) (Scheme 3). This
compound can exist as two isomers depending on the relative
orientation of the CpRuCl fragments (syn or anti). The two
isomers were readily separated by column chromatography
and are easily distinguished since in the syn isomer, the Cp
ligands of the dimolybdenum core are inequivalent, whereas
in the anti isomer they are equivalent. The Cp rings of the
ruthenium moieties are equivalent in both isomers. The anti
isomer was obtained in low yield (12%), but the yield of
the more soluble syn isomer was considerably higher (38%).
Both isomers gave appropriate mass spectra and elemental
analysis data, and in addition, the X-ray structure of the syn
isomer was determined (see below). No interconversion
between the isomers was observed.
To investigate the possible construction of molecular wires
from alternating Mo2 and Ru units, the synthesis of penta-
metallic complex 5 was attempted (Scheme 4). On the basis
of the fact that a convenient synthesis of trans-RuCl2(dppe)2
involves the addition of dppe to [RuCl2(PPh3)3] at room
temperature in acetone, we envisaged that 2 equiv of the
Confirmation of the incorporation of dppa to form phos-
phine-substituted dithiolene ligands was provided by the
crystal structures of both 2g and 2h, the results of which are
detailed in Figures 3 and 4, respectively. The cores of the
structures are very similar to those described above, with
the dithiolene ligands perpendicular to the Mo-Mo bond.
In each case, the phosphine substituents are clearly suitably
aligned for coordination to additional metal centers. The
(18) Pe´tillon, F. Y.; Schollhammer, P.; Talarmin, J.; Muir, K. W. Coord.
Chem. ReV. 1998, 178-180, 203.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9793

Adams et al.
Figure 1. Molecular structure of the complex 2c in the crystal (ORTEP plot, 50% probability ellipsoids). Hydrogen atoms have been omitted for clarity.
Selected bond lengths and angles: Mo(1)-Mo(1A) 2.5832(6) Å, C(6)-C(7) 1.312(6) Å, av. Mo-S 2.464 Å, av. Mo-S-Mo 63.24°.
Figure 2. Molecular structure of 2d in the crystal (ORTEP plot, 50% probability ellipsoids). Hydrogen atoms and a toluene solvent molecule have
been omitted for clarity. Selected bond lengths and angles: Mo(1)-Mo(2) 2.5848(8) Å, Mo(1)-S(1) 2.464(2) Å, Mo(2)-S(1) 2.468(2) Å, Mo(1)-S(2)
2.472(2) Å, Mo(2)-S(2) 2.466(2) Å, Mo(1)-S(3) 2.444(2) Å, Mo(2)-S(3) 2.440(2) Å, Mo(1)-S(4) 2.457(2) Å, Mo(2)-S(4) 2.442(2) Å, C(11)-C(12)
1.338(9) Å, C(13)-C(14) 1.325(9) Å, Mo(1)-S(1)-Mo(2) 63.21(4)°, Mo(2)-S(2)-Mo(1) 63.13(4)°, Mo(2)-S(3)-Mo(1) 63.91(4)°, Mo(2)-S(4)-Mo(1)
63.69(4)°.
structures of 4a and 4b (Figures 5-7, Tables 1 and 2) show
that coordination of the additional ruthenium fragments
leaves the core of the molecules relatively unchanged. For
example, in 4a, the only significant changes are that the
P(1)-C(16)-C(17) and C(16)-C(17)-P(2) angles in this
ligand contract to an average of 117.3° from 123.5° in 2g.
A similar decrease in angles is observed on going from 2h
to 4b. In 4a, the S(1)-C(16) and S(2)-C(17) bonds of the
phosphine dithiolene possibly shorten slightly to an average
of 1.794 Å from 1.820 Å in 2g, but this is at the limit of
significance and is even less clear-cut in 4b. The structure
of 4b seen from a viewpoint along the Mo-Mo bond (Figure
7) emphasizes the relative planarity of the core. The sulfur
and phosphorus atoms, together with C(21)-C(24), are all
effectively coplanar (mean deviation from this plane 0.1002
Å) with the ruthenium atoms displaced slightly to one side,
Ru(1) by 0.7899 Å and Ru(2) by 0.8038 Å. The Ru-Ru
separation is approximately 12.4 Å.
Electrochemical Studies. Cyclic voltammetry was carried
out in dichloromethane solution on selected bis(dithiolene)
complexes, with the results shown in Table 3. Complex 2a
exhibits a single reversible oxidation at 0.71 V vs AgCl/Ag,
followed by irreversible oxidation processes at 1.36 and 1.63
V; it also displays an irreversible reduction at - 1.09 V.
Previous electrochemical studies of the bis(dithiolene) com-
plex Mo2(µ-SCHdCHS)2Cp2 showed that it underwent a
reversible oxidation at 0.23 V vs SCE;9 in the C5H4Me
analogue, this wave moved to 0.15 V and a second
irreversible oxidation was observed at 0.64 V.19 In the
analogous Mo2{µ-SC(CF3)dC(CF3)S}2Cp2, the oxidation
wave was at higher potential (0.88 V vs SCE) and an
irreversible reduction (at -1.37 V) was also observed.9 As
expected, stepwise replacement of the electron-withdrawing
CO2Me substituents by PPh2 groups is accompanied by a
(19) Casewit, C. J.; Haltiwanger, R. C.; Noordik, J.; Rakowski Dubois,
M. Organometallics 1985, 4, 119.
9794 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
three reversible oxidation processes at 0.57, 0.80 and 0.98
V vs AgCl/Ag (Figures 8 and 9); each of these oxidation
processes is of the same area in the differential pulse
voltammogram (i.e., they are all one-electron couples). The
question then is which of these waves is associated with the
dimolybdenum core and which is associated with the
ruthenium centers.
In order to investigate the mixed-metal complexes in more
detail, the spectroelectrochemistry of the redox couples was
explored using the optically transparent thin-layer electrode
(OTTLE) technique. Unfortunately, the low solubility of 2a
precluded the recording of a spectrum for this complex, but
on oxidation of the more soluble η-C5H4Me analogue, peaks
grew in at 12 700 cm-1 (790 nm) and 9760 cm-1 (1025 nm).
Electrochemical oxidation of 4a was carried out at 243 K at
fixed voltages of 0.84 and 1.31 V; under these conditions,
the redox changes were fully reversible. Parts a and b of
Figure 10 show the spectroscopic changes observed for the
first and second oxidations, respectively. The first is char-
acterized by clear isosbestic points and the appearance of
two low-energy charge-transfer bands at 12 200 and 9600
cm-1, whereas the changes for the second oxidation are more
subtle, with the first of these bands shifting to slightly higher
energy (12 550 cm-1).
Oxidation of 4b was carried out at 253 K at potentials of
0.77, 0.96, and 1.39 V, under which conditions all changes
were again fully reversible. The spectrum for the first
oxidation is shown in Figure 11a and in common with that
of 4a shows the development of charge-transfer absorptions
at 11 900 and 9300 cm-1 in a similar intensity ratio.
Interestingly however, as shown in Figure 11b, the higher
energy band largely disappears on oxidation to the dication
and then reappears (at a higher energy of 12 550 cm-1) on
Figure 3. Molecular structure of 2g in the crystal (ORTEP plot, 50%
probability ellipsoids). Hydrogen atoms and a dichloromethane solvent
molecule have been omitted for clarity. Selected bond lengths and angles:
Mo(1)-Mo(2) 2.5901(9) Å, Mo(1)-S(1) 2.458(2) Å, Mo(2)-S(1) 2.438-
(2) Å, Mo(1)-S(2) 2.471(2) Å, Mo(2)-S(2) 2.443(2) Å, Mo(1)-S(3) 2.452-
(2) Å, Mo(2)-S(3) 2.461(2) Å, Mo(1)-S(4) 2.439(2) Å, Mo(2)-S(4)
2.456(2) Å, C(11)-C(12) 1.327(6) Å, C(13)-C(14) 1.295(8) Å, Mo(2)-
S(1)-Mo(1) 63.87(4)°, Mo(2)-S(2)-Mo(1) 63.61(4)°, Mo(1)-S(3)-Mo-
(2) 63.63(4)°, Mo(1)-S(4)-Mo(2) 63.89(4)°, C(12)-C(11)-P(1) 123.1(5)°,
C(11)-C(12)-P(2) 123.9(5)°.
progressive shift of the reversible oxidation to lower
potential, viz., 0.71 V in 2a, 0.59 V in 2g, and 0.47 V in 2h.
In 4a, introduction of a ruthenium(II) fragment causes the
appearance of a second reversible redox wave (E1/2 0.63 and
0.90 V). In complex 4b, the two ruthenium centers are
chemically equivalent, but the cyclic voltammogram displays
Figure 4. Molecular structure of 2h in the crystal (ORTEP plot, 50% probability ellipsoids). Hydrogen atoms have been omitted for clarity. Selected bond
lengths and angles: Mo(1)-Mo(1A) 2.586(1) Å, C(1)-C(2) 1.338(7) Å, av. Mo-S 2.447 Å, av. Mo-S-Mo 63.78°, C(1)-C(2)-P(2) 124.8(5)°, C(2)-
C(1)-P(1) 121.2(4)°.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9795

Adams et al.
Figure 5. Molecular structure of the trinuclear complex 4a in the crystal (ORTEP plot, 50% probability ellipsoids). Hydrogen atoms have been omitted for
clarity.
Figure 6. Molecular structure of the tetranuclear complex syn-4b in the crystal (ORTEP plot, 50% probability ellipsoids). Hydrogen atoms and the
dichloromethane solvent molecules have been omitted for clarity.
Figure 7. Structure of the core of syn-4b.
further oxidation to the trication, whereas the lower energy
band remains relatively unchanged throughout. We defer
discussion of these data until the DFT analysis has been
introduced.
In a further attempt to examine the site of oxidation of
4a, we recorded the X-band electron paramagnetic resonance
(EPR) spectra of the radical cations of several of the
complexes reported here. However, these experiments have
so far been unhelpful, as the spectra have proved difficult
to analyze. Some complexes (e.g., the C5H4Me analogue of
2a, and 4a) give a well-defined spectrum with g ) ap-
proximately 2.01 with clearly observable Mo satellites (AMo
approximately 28 G); however, superimposed on this is a
second, much broader peak at around g ) 1.95 with no
discernible fine structure. The relative size of this second
peak varies considerably from compound to compound, and
in some cases (e.g., the cation of 4b) it is the only one
present. The size of the peak does not appear to depend on
9796 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the
Trinuclear Complex 4a
Mo(1)-S(2)
Mo(1)-S(4)
Mo(1)-Mo(2)
Mo(2)-S(4)
Mo(2)-S(1)
Ru(1)-P(1)
C(16)-C(17)
2.438(1)
2.467(1)
2.5990(8) Mo(2)-S(2)
2.458(1)
2.470(1)
2.263(1)
1.339(5)
Mo(1)-S(1)
Mo(1)-S(3)
2.463(1)
2.470(1)
2.445(1)
2.459(1)
2.257(1)
2.441(1)
1.307(6)
Mo(2)-S(3)
Ru(1)-P(2)
Ru(1)-Cl(1)
C(18)-C(19)
P(2)-Ru(1)-P(1)
P(1)-Ru(1)-Cl(1)
Mo(1)-S(2)-Mo(2)
Mo(2)-S(4)-Mo(1)
C(16)-C(17)-P(2)
84.34(4)
83.30(4)
64.32(3)
63.71(3)
117.5(3)
P(2)-Ru(1)-Cl(1)
Mo(1)-S(1)-Mo(2)
Mo(2)-S(3)-Mo(1)
C(17)-C(16)-P(1)
90.25(4)
63.60(3)
63.65(3)
117.0(3)
Figure 9. Differential pulse voltammogram of complex 4b, same
conditions as in Figure 8.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for the
Tetranuclear Complex syn-4b‚2CH2Cl2
Electronic Structure and DFT Analysis (Parent Closed-
Shell Species). The electronic structure of CpMo dimers with
four bridging sulfur atoms has been previously studied, in
relation to their UV-vis spectra, by Dubois and co-workers
for complexes in which both metal atoms are Mo(III) or Mo-
(IV) and also for the mixed-valence Mo(III)-Mo(IV)
state.20-22 According to their molecular orbital picture, for a
Mo(IV) dimer the highest-occupied molecular orbital (HOMO)
is a b1u orbital which is essentially composed of the δ*
Mo(1)-S(1)
Mo(1)-S(4)
Mo(1)-Mo(2)
Mo(2)-S(3)
Mo(2)-S(4)
Ru(1)-P(2)
Ru(2)-P(4)
Ru(2)-Cl(2)
C(23)-C(24)
2.452(2) Mo(1)-S(2)
2.475(2) Mo(1)-S(3)
2.596(1) Mo(2)-S(2)
2.461(2) Mo(2)-S(1)
2.480(2) Ru(1)-P(1)
2.263(2) Ru(1)-Cl(1)
2.266(2) Ru(2)-P(3)
2.446(2) C(21)-C(22)
1.317(9)
2.468(2)
2.480(2)
2.451(2)
2.464(2)
2.260(2)
2.434(2)
2.271(2)
1.349(8)
P(1)-Ru(1)-P(2)
P(2)-Ru(1)-Cl(1)
P(4)-Ru(2)-Cl(2)
Mo(1)-S(1)-Mo(2)
Mo(2)-S(3)-Mo(1)
C(22)-C(21)-P(1)
C(24)-C(23)-P(3)
83.76(6)
89.20(6)
85.37(6)
63.76(4)
63.40(5)
115.9(5)
118.1(5)
P(1)-Ru(1)-Cl(1)
83.94(6)
83.59(6)
91.34(6)
63.72(5)
63.21(4)
118.2(5)
117.3(5)
2
2
P(4)-Ru(2)-P(3)
P(3)-Ru(2)-Cl(2)
Mo(2)-S(2)-Mo(1)
Mo(1)-S(4)-Mo(2)
C(21)-C(22)-P(2)
C(23)-C(24)-P(4)
interaction between the metal dx -y orbitals. Lying slightly
above this, unusually, is the corresponding b2g δ bonding
2
2
interaction between the dx -y orbitals, which is destabilized
by overlap with the p orbitals of the bridging sulfur atoms,
and this forms the HOMO of the Mo(III) dimers. The UV-
vis spectra of the Mo(III) dimers were relatively featureless,
whereas the mixed-valence state was characterized by two
reasonably intense absorptions in the regions of 7300-10000
and 13700-18400 cm-1 depending on the complex (though
no bis(dithiolene) complexes were included in these earlier
experiments). The lowest energy of these two bands was
assigned to the δ* to δ transition, which also contains some
metal-to-ligand charge-transfer (MLCT) character. The pre-
vious discussion of the molecular orbital scheme for these
quadruply bridged dimolybdenum species20-22 used the same
axis system shown in Figure 12 except that we define the x
axis as bisecting the MoS2 dithiolene rings. We work with
two-fold symmetry that makes the two Mo atoms equivalent
in species 2a, whereas Dubois assumed four-fold symmetry.
The two formally Mo(III) species each have three electrons
to contribute to three molecular orbitals, which contained
the paired six electrons. Our DFT calculations use the B3LYP
functional and LanL2DZ basis set shown earlier by Cotton
and co-workers23,24 to be useful for dimolybdenum species.
Moreover, in a detailed DFT study of molybdenum dithiolene
species, both Hoffmann25 and Wakamatsu26 showed that
Table 3. Summary of Electrochemical Data for the New Complexes, V
vs AgCl/Ag
complexa
reduction 1 oxidation 1 oxidation 2 oxidation 3
2a
2g
2h
4a
4b
Epc ) -1.09 E1/2 ) 0.71 Epa ) 1.36 Epa ) 1.63
Epc ) -1.22 E1/2 ) 0.59 Epa ) 1.34 Epa ) 1.50
Epc ) -1.49 E1/2 ) 0.47 Epa ) 1.27
Epc ) -1.18 E1/2 ) 0.63 E1/2 ) 0.90
E1/2 ) 0.57 E1/2 ) 0.80 E1/2 ) 0.98
E1/2 ) 0.57 Epa ) 1.44
CpRu(PPh3)2Cl
a Recorded under an N2 atmosphere in CH2Cl2 solution with 0.3 M
[NBu4][BF4] as supporting electrolyte, at a scan rate of 100 mV s-1
Ferrocene was used as an internal standard; the ferrocenium/ferrocene couple
was measured at 0.55 V vs AgCl/Ag.
.
(20) Dubois, D. L.; Miller, W. K.; Rakowski Dubois, M. J. Am. Chem.
Soc. 1981, 103, 3429.
(21) Casewit, C. J.; Rakowski Dubois, M. Inorg. Chem. 1986, 25, 74.
(22) Casewit, C. J.; Rakowski Dubois, M.; Grieves, R. A.; Mason, J. Inorg.
Chem. 1987, 26, 1889.
(23) Cotton, F. A.; Murillo, C. A.; Villagra´n, D.; Yu, R. J. Am. Chem.
Soc. 2006, 128, 3281.
Figure 8. Cyclic voltammogram of complex 4b in CH2Cl2 solution (0.3
M [NBu4][BF4], scan rate of 100 mV s-1, ferrocene as internal standard
(not shown).
(24) Cotton, F. A.; Donahue, J. P.; Huang, P.; Murillo, C. A.; Villagra´n,
D. Z. Anorg. Allg. Chem. 2005, 631, 2606.
(25) Hofmann, M. J. Mol. Struct. THEOCHEM 2006, 773, 59.
(26) Wakamatsu, K.; Nishimoto, K.; Shibahara, T. Inorg. Chim. Acta 1999,
295, 180.
the size of the dithiolene substituents, the presence or absence
of a ruthenium atom, or the method of cation generation
(chemical or electrochemical).
Inorganic Chemistry, Vol. 46, No. 23, 2007 9797

Adams et al.
Figure 10. OTTLE spectra of complex 4a recorded at 243 K in CH2Cl2 solution. (a) First oxidation (electrogeneration at 0.84 V). (b) Second oxidation
(electrogeneration at 1.31 V).
B3LYP was one of the more useful functionals to use for
molybdenum. Note also the successful use of B3LYP by
Bitzer27 for molybdenum systems.
ruthenium-localized orbitals and in the diruthenium species
syn-4b, six filled mostly ruthenium-localized orbitals. In both
ruthenium species (4a, syn-4b), these orbitals occur at
comparable energies (Table S1, Supporting Information) to
the molybdenum-localized orbitals. There are only relatively
small contributions from the cyclopentadienyl ligands to the
filled frontier orbitals and little Cp contribution to the first
few virtual unoccupied orbitals. The dominant ligand orbitals
to these frontier levels derive mainly from the dithiolene
ligand. The LUMO in all these species contains mostly
(<xy> - <xy>), which also agrees with the Dubois analysis.
Interestingly, these researchers used extended Hu¨ckel theory,
which evidently performed well for these systems.
Before discussing each species in turn, we note some
general features illustrated in Figure 13a-d and listed in
detail in the Supporting Information (Table S1). The three
filled, predominantly molybdenum-localized orbitals can
clearly be seen in each species. In all four species the
dominant Mo-Mo bonding interaction is through overlap
of the <z2> orbitals on each Mo, and there is δ overlap via
the <x2-y2> orbitals which leads to little if any net
stabilization. The lowest-unoccupied molecular orbital
(LUMO) is approximately 30% molybdenum in all the
closed-shell species under discussion and comprises an
antibonding <xy> - <xy> composition. In the monoru-
thenium species 4a there are, in addition, three filled mostly
Species 2c, Figure 13a (also see Table S1, Supporting
Information) shows the three mainly molybdenum-localized
orbitals as the HOMO (no. 105), HOMO-1, and HOMO-3
orbitals. By combination of the two symmetry-equivalent
CpMo fragments, the dominantly metal-localized frontier
filled orbitals comprising the metal-metal bond can be
(27) Bitzer, R. S.; Pereira, R. P.; Rocco, A. M.; Lopes, J. G. S.; Santos, P.
S.; Nascimento, M. A. C.; Filgueiras, C. A. L. J. Organomet. Chem.
2006, 691, 2005.
9798 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
Figure 11. OTTLE spectra of complex 4b recorded at 253 K in CH2Cl2 solution. (a) First oxidation (electrogeneration at 0.77 V). (b) Spectra of the three
reversible oxidation products.
+ <x2-y2>) (HOMO-1) and (<x2-y2> - <x2-y2>)
(HOMO), respectively, providing a net bond order of 1.
Although there is more than 30% Mo contribution to the
LUMO, localized mostly on the dithiolene ligand, there is
no net back-donation because the Mo <xy> orbital is empty.
While orbitals 105 (HOMO), 104, and 102 contain most of
the molybdenum electron density (Figure 13a), molybdenum
also contributes in a major way to many of the deeper lying
orbitals (especially nos. 87, 88, 89, 91, 96, 97, 98). In species
2c, these contain contributions from both Mo-Mo bonding
and antibonding interactions.
In the tetraphosphine derivative 2h, the three highest
percentage molybdenum-localized orbitals (Figure 13b) are
identified as the HOMO (no. 247) and HOMO-1, which are
again the δ and δ* coupled orbitals while the σ-bonding
orbital is rather deeper at HOMO-5. Also, in this system,
there are ligand-localized orbitals with substantial bonding
Figure 12. Axis coordinate system for species 2c.
described as (<z2> + <z2>) (HOMO-3, σ-bonding) and
then a pair of bonding (δ) and antibonding (δ*) (<x2-y2>
and antibonding Mo2 combinations.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9799

Adams et al.
Figure 13. (a) Percent contribution in frontier molecular orbitals of species 2c (DFT calculation). The left-hand index shows molecular orbital numbering
with no. 105 as the HOMO and no. 106 as the LUMO. Contributions from molybdenum, sulfur ligand, and cyclopentadienyl ligands are color coded as
shown. (b) Percent contribution in frontier molecular orbitals of species 2h (DFT calculation). The left-hand index shows molecular orbital numbering with
no. 247 as the HOMO and no. 248 as the LUMO. Contributions from molybdenum, sulfur ligand, and cyclopentadienyl ligands are color coded as shown.
(c) Percent contribution in frontier molecular orbitals of species 4a (DFT calculation). The left-hand index shows molecular orbital numbering with no. 220
as the HOMO and no. 221 as the LUMO. Contributions from molybdenum, ruthenium, sulfur ligand, with and without appended ruthenium atom,
cyclopentadienyl ligands, attached to molybdenum or to ruthenium, and chloride are color coded as shown. (d) Percent contribution in frontier molecular
orbitals of species 4b (DFT calculation). The left-hand index shows molecular orbital numbering with no. 305 as the HOMO and no. 306 as the LUMO.
Contributions from molybdenum, sulfur ligands with appended ruthenium atom, cyclopentadienyl ligands, attached to molybdenum or to ruthenium, and
chloride are color coded as shown.
In the monoruthenium species 4a, the HOMO (no. 220)
is now primarily localized on ruthenium with the δ, δ*
orbitals (of somewhat distorted appearance due to the lack
of symmetry of this species) being HOMO-1 and HOMO-3
and again offering no net bonding. The bonding <z2> +
<z2> orbital is HOMO-7 (no. 213) with two ruthenium-
localized orbitals lying between it and HOMO-3 (Figure 13c).
Thus, the three Ru-localized levels hold the d6 configuration.
The ruthenium-localized HOMO lies about 0.5 eV above the
molybdenum-localized HOMO-1. Cyclopentadienyl orbitals
from either the ruthenium- or molybdenum-bound cyclo-
pentadienyl units contribute little to the frontier orbitals.
Τhe diruthenium species (syn-4b) has a similar pattern,
but now six mainly ruthenium-localized frontier orbitals are
seen. The HOMO (no. 305) and HOMO-1 are both mainly
ruthenium-localized and lie about 0.4 eV above HOMO-2,
which is mainly molybdenum-localized (Figure 13d). In this
case, the strongly σ-bonding Mo-Mo orbital is HOMO-9
(no. 296). In both 4a and 4b, there are no low-lying empty
ruthenium-localized orbitals, but there are many which
contain significant contributions from molybdenum.
Optical Spectra of Closed-Shell Species. Time-dependent
DFT (TD-DFT) was used26,28 to predict the electronic spectra
of these species. We discuss the spectra of several of these
species as representative of them all and deal here first with
the uncharged closed-shell species. All exhibit a rather large
number of weak overlapping charge transfer, relatively low
energy, bands which generate the broad and weak unresolved
visible region absorption seen with all these species. Detailed
assignments can be found in Supporting Information (Table
S2). Species 2c is representative of the simplest species in
this group. The TD-DFT-predicted visible region spectrum
and experimental spectrum are shown in Figure 14a where
the vertical bars show the locations of the electronic
(28) Vlcek, A., Jr.; Za´lis, S. Coord. Chem. ReV. 2007, 251, 258.
9800 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
Figure 14. (a) Experimental spectrum of species 2c in dichloromethane (black line) and TD-DFT calculated spectrum (red line) based upon the X-ray
structure. The vertical bars are the locations of individual transitions scaled to the absorbance axis. In Figures 14a-d and 15a,b the upper limit of the
TD-DFT calculated spectrum is determined by the number of excited states derived. The falloff at high wavenumber is not real but reflects the termination
of the calculation. (b) Experimental spectrum of species 2h in dichloromethane (black line) and TD-DFT calculated spectrum (red line) based upon the X-ray
structure. The vertical bars are the locations of individual transitions scaled to the absorbance axis. Inset shows expansion of the short wavelength end of
the spectrum. (c) Experimental spectrum of species 4a in dichloromethane (black line), TD-DFT calculated spectrum (red line) based upon the X-ray structure,
and TD-DFT calculated spectrum (blue line) based upon the B3LYP-geometry-optimized structure. The vertical bars are the locations of individual transitions
scaled to the absorbance axis. Inset shows expansion of the short wavelength end of the spectrum. (d) Experimental spectrum of species 4b in dichloromethane
(black line) and TD-DFT calculated spectrum (red line) based upon the X-ray structure. The vertical bars are the locations of individual transitions scaled
to the absorbance axis.
transitions. Even in this simple species, there are many low
energy but weak predicted transitions. The three more intense
bands at 22 600, 25 100, and 28 900 cm-1 can be assigned,
predominantly, as MLCT Mo f S-ligand, π-π* (S-ligand)
(with some LMCT character), and mixed MLCT, d-d,
recognizing that fairly extensive mixing is occurring between
metal and ligand orbitals. The pattern of the first three
predicted transitions with two moderately intense bands and
a weak band (at ca. 24 000 cm-1) between them is mimicked
by the experimental spectrum with two obvious peaks and a
weak shoulder between them. These three experimental
features are then reasonably associated with the first three
predicted transitions.
Figure 14b compares the visible region experimental and
TD-DFT predicted spectrum of the tetraphosphine 2h. The
two broad weak shoulders tailing into the visible region are
quite well reproduced by the DFT predicted spectrum but
again there are many predicted weak transitions in this region.
The two most intense ones at 25 000 and 27 500 cm-1 are
both primarily π-π* (S-ligand) with some LMCT character,
the latter due to the presence of significant metal 4d character
in the LUMO and LUMO+1. The very weak band shown
in the inset to Figure 14b is the HOMO to LUMO transition
(no. 247 f no. 248 in Figure 13b) which is a molybdenum
d-d transition mixed with MLCT Mo f S-ligand.
Figure 14c shows the comparison of the visible region
experimental and TD-DFT predicted spectrum of the mono-
ruthenium diphosphine 4a. This is an asymmetric molecule
with a “bare” dithiolene ligand that does not have an
appended ruthenium and one that does (Table S2, Supporting
Information). The LUMO is primarily localized on the bare
dithiolene ligand without appended ruthenium. Again, there
are a plethora of fairly weak transitions below 30 000 cm-1
generating the broad-structured experimentally observed
band. Although the HOMO is ruthenium-localized, the lowest
two transitions (predicted 14500(0.0004) and 15700(0.0001)
cm-1 (oscillator strength in parenthesis)) are MLCT from
molybdenum-localized orbitals to the LUMO (Mo f π*
dithiolene) mostly on the bare dithiolene ligand but also with
molybdenum d-d character (see Figures 13c and 14c inset).
The next transition (17200(0.0002) cm-1) is mostly Ru f
π* dithiolene, followed by one at 21300(0.0012) cm-1, which
is significantly more intense and is predominantly Ru f π*
dithiolene (bare). The Mo f π* transitions lie at lower
energy than excitations from ruthenium because of the
considerable Coulombic penalty in moving charge from
ruthenium to π* dithiolene on the other side of the molecule.
As the figure shows, there follow many transitions which
are (Table S2, Supporting Information) mostly MLCT in
character; there is a predicted π-π* (bare) dithiolene
Inorganic Chemistry, Vol. 46, No. 23, 2007 9801

Adams et al.
transition at 25800(0.0049) cm-1. The fairly intense transition
at 22300(0.0086) cm-1 is a rather mixed transition but with
a major MLCT Ru f π* dithiolene (with appended Ru)
component. There is overall good agreement between the
experimental and predicted spectra, especially in regard to
the two, weak, low-energy transitions shown in Figure 14c
(inset).
Figure 14d compares the visible region experimental and
TD-DFT-predicted spectrum of the more symmetric di-
ruthenium tetraphosphine complex syn-4b. Both HOMO and
HOMO-1 are ruthenium-localized, but the first two predicted,
but weak, transitions (15100(0.0001) and 15800(0.000) cm-1)
are MLCT (but with molybdenum d-d character) from
HOMO-2 and HOMO-4, both of which are molybdenum-
localized, to the LUMO, which mostly lies on the dithiolene
ligand. The third predicted transition, substantially higher
in energy (18000(0.0005) cm-1), is the first ruthenium-to-
dithiolene MLCT transition. There follow many transitions,
almost all of which, up to 30 000 cm-1, are MLCT from
either Ru or Mo. The most intense in this grouping (23100-
(0.0095) cm-1) (HOMO f LUMO+1) is predominantly Ru
f dithiolene MLCT.
DFT Analysis (Open-Shell Oxidized Species). Oxidation
of species 2a: As a control model, a spin-unrestricted DFT
geometry optimization was carried out for the first oxidation
product of species 2a, again using the B3LYP functional
and LanL2DZ basis set. The optimized species, 2a+, has C2
symmetry, and the derived net spin density is distributed
primarily and equally over both molybdenum atoms (0.60 e
each). There is some negative spin density on the S ligating
atoms.
Figure 15. (a) Experimental spectrum of species 4a+ in dichloromethane
(black line) and TD-DFT-calculated spectrum (blue line) based upon the
B3LYP-geometry-optimized structure. The vertical bars are the locations
of individual transitions scaled to the absorbance axis. (b) Experimental
spectrum of species 4b+ in dichloromethane (black line) and TD-DFT-
calculated spectrum (blue line) based upon the B3LYP-geometry-optimized
structure. The vertical bars are the locations of individual transitions scaled
to the absorbance axis.
Oxidation of species 4a: There is uncertainty whether
oxidation of this species is localized on the dimolybdenum
center or on the ruthenium moiety. We therefore carried out
an unrestricted-spin DFT geometry optimization of the
oxidized species (4a+) and also carried out a restricted-spin
DFT geometry optimization of the ruthenium(II) precursor,
species 4a, for comparative purposes, to provide some
confidence that the 4a+ calculation was valid. It is also useful
in this context to compare the expected electronic spectrum
of the DFT-optimized species 4a with the spectrum predicted
for the X-ray geometry. It is considered preferable to use a
good DFT-optimized structure rather than the X-ray structure
for the prediction of solution electronic spectra since the
geometry in the solid may be constrained by packing
phenomena. We observe (Figure 14c) similar electronic
spectroscopic predictions for the two geometries but with
the overall band envelope displaced somewhat to lower
energy using the DFT-optimized geometry. The Mo-Mo
distance in the optimized geometry of 4a is 262 pm while
the average Mo-S bond distance is 257 pm, the Ru-P
distance is 240 pm, and the Ru-Cl distance is 253 pm. These
can be compared with distances of 260, 246, 226, and 244
pm, respectively, in the X-ray structure.
the spin density at the ruthenium atom is 0.76 e, while there
is no significant spin density at the molybdenum or sulfur
atoms. A small positive spin density is found on chloride.
Thus, the first oxidation process occurs at ruthenium in the
gas-phase-optimized structure. Oxidation at ruthenium causes
no large change in the Ru-P distance probably because of
the rigidity of the ligand, but the Ru-Cl bond shortens
considerably (relative to DFT-optimized 4a).
The predicted optical spectrum of 4a+ is compared with
the experimental spectrum in Figure 15a. Although shifted
to lower energy relative to the experimental data, the overall
band envelopes at lower energy, below 20 000 cm-1 (exp.),
are closely similar, with two peaks and a higher energy
shoulder. The lowest-energy band is mostly excitation no.
214B f no. 220B (B ) â spin), which is a composite d-d
and Cl f d LMCT transition. The next and stronger band is
highly mixed and cannot be simply designated. The dominant
component of this stronger transition (28% of the excitation)
is mostly a phosphine-ligand-to-Ru LMCT transition. The
trio of bands that create the higher energy shoulder are
S-ligand (no pendant Ru)-to-Ru LMCT, a mixed transition
not easily described, and Mo f Ru M′MCT.
In the DFT-optimized oxidation product 4a+, these dis-
tances are 263, 256-260, 244-245, and 241 pm, respec-
tively. The singly occupied molecular orbital (SOMO) is
localized at the ruthenium center. In agreement with this,
Oxidation of species 4b: In the DFT-optimized oxidation
product 4b+, the spin density is again localized on ruthenium,
equally between the two at 0.37 e each, with 0.09 e on each
chloride and no significant spin density on molybdenum,
9802 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
phosphorus or sulfur, or elsewhere. The SOMO is localized
equally at the two ruthenium centers. The key distances are
263 pm (Mo-Mo), 257-260 pm (Mo-S), 242 pm (Ru-
P), and 246 pm (Ru-Cl). Comparison of the calculated and
experimental electronic spectrum in the low-energy region
(6000-16000 cm-1) is shown in Figure 15b. Good agree-
ment is seen between the overall band envelope and energy,
though the predicted intensities in this region are much lower
than those actually observed. The weak lowest-energy
component is a d-d transition (no. 297B f no. 305B) (with
some ligand π-π* character) within what, in a mononuclear
species 4a+ and 4b+ in the characteristic long-wavelength
region (Figure 15) agree well in band shape and reasonably
well in energy with the ruthenium-localized predicted spectra.
Thus, the first oxidation processes (Table 3) in both 4a and
4b are assigned as Ru(II) oxidation to Ru(III).
The second oxidation process in CpRu(PPh3)2Cl is more
than 1 V more positive than the first while the second
oxidation process in 4a is only 0.23 V more positive than
the first; it is then evidently molybdenum-localized.
The second oxidation process of 4b to produce 4b2+
evidently also takes place on ruthenium, producing the Ru-
(III)‚‚‚Ru(III) species as indicated by the DFT data above.
Comparison with the data obtained for CpRu(PPh3)2Cl shows
that the third oxidation process is too close to the second to
be ascribed to the formation of Ru(IV), and this information
and the DFT data described above show that the third
oxidation process must be molybdenum-localized, forming
the Ru(III)‚‚‚Ru(III)‚‚‚Mo(III)-Mo(IV) species 4b3+.
The above analysis reveals communication between the
two ruthenium centers in 4b. The presence of three one-
electron oxidation processes for this species requires, un-
equivocally, coupling of the two ruthenium sites across the
dinuclear molybdenum site unless one supposed that two of
the processes were molybdenum-localized, and the calcula-
tions rule that out. Thus, despite their separation by the
dinuclear molybdenum site, the two ruthenium atoms do
communicate. Assuming that the first two oxidation pro-
cesses in 4b are ruthenium-localized, then using E2 - E1 )
(RT/nF)lnKc, the comproportionation constant for the process
5
Ru(III) species, would be the t2g manifold as is typical for
Ru(III) species.29 The stronger band comprises two compo-
nents, no. 304B f no. 306B, and no. 295B f no. 305B.
The former is a mix of Ru f ligand MLCT and intermetallic
Ru f Mo transition. The latter is mostly ligand-to-Ru(III)
LMCT. There are very weak lower-lying Mo f ligand
MLCT transitions predicted in the near-infrared region.
The highest-energy-available electron in 4b+ is a â electron
on ruthenium, suggesting that the next oxidation process to
make 4b2+ is oxidation of the second ruthenium center. For
further characterization, the species 4b2+ (assumed S ) 0)
1
and 4b3+ (assumed S ) /2) were geometry-optimized and
briefly analyzed. The LUMO on 4b2+ is on ruthenium in
agreement with the supposition that it is a Ru(III)‚‚‚Ru(III)
species. In 4b3+, the spin density is primarily on both
ruthenium centers and on one molybdenum center, suggestive
then of a Ru(III)‚‚‚Ru(III)‚‚‚Mo(III)-Mo(IV) species.
Electrochemical assignments: It is now straightforward
to assign the observed electrochemical data (Table 3). The
first two oxidation processes observed with species 2a, 2g,
and 2h must be molybdenum-localized to form Mo(III)-
Mo(IV) mixed-valence and Mo(IV)-Mo(IV) species; ligand
oxidation does not occur at such a low positive potential.
The third oxidation process seen with species 2a and 2g may
be molybdenum-based, but it could also be ligand-based and
we have no information to decide which is correct.
[Ru(II)‚Ru(II)] + [Ru(III)‚Ru(III)] ) 2[Ru(II)‚Ru(III)]
is Kc ) 7.7 × 103. This, not too surprisingly, is on the low
end of a range of Kc values for a variety of bridging ligands
linking ruthenium centers, summarized by Sarkar and Kaim.30
It does, however, exceed other simple, even short, bridging
ligands such as a triazole dicarboxylic acid ligand (Kc ) 1.1
× 103).31 The Kc value is not insignificant given the
separation of the two ruthenium centers, about 124 pm, linked
via a complex network of phosphorus, carbon, sulfur, and
molybdenum atoms. Crutchley reported32 values of Kc in the
range of 10-68400, as a function of solvent, for a dicyana-
mide mixed-valence (NH3)5Ru(III)/Ru(II) species with a
similar Ru(III)‚‚‚Ru(III) intermetallic distance, but in this
case, linked by an unsaturated pathway.
The first oxidation process in species 4a and 4b could
obviously be either molybdenum- or ruthenium-localized.
The ruthenium oxidation in CpRu(PPh3)2Cl occurs at 0.57
V (Table 3), a value comparable to the first oxidation process
in species 4a and 4b. The DFT analysis above clearly shows
that the HOMO in both 4a and 4b is ruthenium-localized.
Moreover, DFT calculations of the open-shell 4a+ and 4b+
both show unpaired spin density localized on ruthenium and
not on molybdenum. In the case of 4b+, the spin density is
evenly shared between the two ruthenium atoms but this is
a characteristic of the high symmetry; the two ruthenium
atoms are essentially equivalent, and this does not, in itself,
necessarily indicate delocalization across the molecule. It is
possible that the solution species 4a and 4b differ from the
gas-phase DFT calculation and that the frontier orbitals
therein are molybdenum- and not ruthenium-localized, but
this is unlikely since there is no especial reason for this to
be the case. Moreover, the predicted electronic spectra of
Conclusions
In this work, we have demonstrated the generality of the
stepwise reaction of Mo2(SCH2CH2S)2Cp2 with two different
alkynes to give complexes with two different dithiolene
ligands. In this way, virtually complete control over the
substituents R1-R4 can be achieved. The use of the acety-
lenic phosphine dppa has allowed the incorporation of
phosphine substituents into dithiolene ligands for the first
time. In principle, it should be possible to extend this
(30) Kaim, W.; Sarkar, B. Coord. Chem. ReV. 2007, 251, 584.
(31) Baitalik, S.; Dutta, B.; Nag, K. Polyhedron 2004, 23, 913.
(32) Naklicki, M. L.; Crutchley, R. J. J. Am. Chem. Soc. 1994, 116, 6045.
(29) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier Science:
Amsterdam, 1984.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9803

Adams et al.
with m-nitrobenzyl alcohol as the matrix; the figures reported are
the highest intensity peak of each isotope envelope. Electrospray
mass spectra were obtained on an LCT instrument. UV-vis
absorption spectra were recorded on a Varian Cary 50 instrument
in the range of 200-800 nm in dichloromethane solution with a
path length of 1 cm. The X-band EPR spectra were recorded at the
University of Manchester on a Bruker EMX hybrid spectrometer
and simulated with Bruker’s Xsophe software. Elemental analyses
were carried out by the Microanalytical Service of the Department
of Chemistry at Sheffield with a Perkin-Elmer 2400 analyzer.
Electrochemical experiments were carried out in N2-purged di-
chloromethane (freshly distilled from CaH2) with 0.3 M [Bu4N]-
[BF4] as supporting electrolyte. A standard three-electrode system
was used with a Pt microdisc and a large surface area Pt wire as
the working and counter electrodes, respectively. The reference
electrode was AgCl/Ag in a solution of 0.45 M [Bu4N][BF4] and
0.05 M [Bu4N][Cl], linked to the bulk solution by a fritted tube.
At the end of every experiment, ferrocene was added as an internal
standard. The ferrocenium/ferrocene couple was observed at 0.55
V vs AgCl/Ag.37 Electrochemical reversibility was determined by
scan rate dependency, with all potentials quoted for a scan rate of
100 mV s-1. Processes are described as reversible if ∆E < 100
mV and |Ipa/Ipc| ) 1. The in situ UV-vis-near IR spectroelec-
trochemical experiments were carried out with an OTTLE cell using
a Pt-Rh gauze as the working electrode, Pt wire as the counter
electrode, and AgCl/Ag as the reference electrode in a temperature-
controlled 0.5 mm quartz cell. At the end of the experiment, the
applied voltage was returned to zero and the original spectrum was
regenerated to ensure the reversibility of the processes.
synthetic route to other complexes where dithiolene ligands
are constructed from sulfido ligands and alkynes, though two
potential problems are first, the coordination of the dppa to
the metal center through the phosphorus atoms and second,
the oxidation of the phosphine units to P(dS)Ph2 groups in
reactions that contain elemental sulfur. It is also shown that
the resulting complexes can be employed as bidentate or
tetradentate ligands to other metal fragments and that by
judicious choice of a redox-active fragment, a degree of
communication between two metal centers through the
dimolybdenum dithiolene framework can be observed.
Overall, the DFT analysis provides a greater in-depth look
at the electronic structures of these species. The electronic
spectra are shown to be extremely complicated, with many
weak low-energy transitions occurring close together and
hence responsible for the broad unresolved structure seen in
the visible-region electronic spectra. However, there is good
quantitative agreement between the observed visible-region
spectra and the predicted DFT spectra. Oxidation of both
species 4a and 4b apparently occurs at ruthenium, which is
not a conclusion readily reached from a simple consideration
of the electrochemical potentials.
Future papers will address the communication between
different types of redox-active units across the same Mo2
framework and also our attempts to discover a more general
way to produce such dithiolene ligands so that they can be
coordinated to a wider range of central linking units.
Synthesis of Mo2{µ-SC(CO2Me)dC(CO2Me)S}2Cp2 (2a). A
solution of complex 1 (360 mg, 0.712 mmol) and DMAD (0.3 mL)
in CH2Cl2 (40 mL) was heated to reflux for 24 h. The solvent was
removed in vacuo, and the residue was separated by column
chromatography. A red band, eluted with acetone/CH2Cl2 (1:99),
yielded 299 mg (57%) of 2a. The characterizing data for this
complex matched those given in the literature.12
Experimental Section
Materials. General experimental techniques were as described
in recent work from this laboratory.6 All reactions were routinely
carried out under an argon or nitrogen atmosphere, though
separation procedures were carried out without any special precau-
tions since the products described are all relatively air-stable.
Column chromatography was carried out on silica columns (Merck
Kieselgel 60 or equivalent) made up initially in light petroleum;
elution was then carried out with increasing proportions of
dichloromethane. Light petroleum refers to the fraction boiling in
the range 60-80 °C. Solvents were purified by either distillation
from appropriate drying agents or a Grubbs-type purification system
manufactured by Innovative Technology, Newburyport, MA. The
complexes Mo2(SCH2CH2S)2Cp2, CpRu(PPh3)2Cl, and RuCl2-
(PPh3)3 were prepared by literature methods.33-35 Bis(diphenylphos-
phino)acetylene, Ph2PCtCPPh2, was prepared by the reaction of
LiCtCLi with PPh2Cl.36 All other chemicals were obtained from
commercial sources and used as supplied.
Synthesis of Mo2(µ-SCPhdCPhS)2Cp2 (2b). A solution of 1
(235 mg, 0.463 mmol) and diphenylacetylene (258 mg, 1.45 mmol)
in CH2Cl2 (40 mL) was heated to reflux for 24 h. After rotary
evaporation of the solvent, chromatography of the residue gave 2b
(240 mg, 64%) as a green-brown band eluted slowly with light
petroleum/CH2Cl2 (1:1) changing progressively to pure CH2Cl2.
1
2b: mp >300 °C. H NMR: δ 7.11-6.83 (m, 20 H, Ph), 6.05
(s, 10 H, Cp). Found: C, 49.19; H, 3.47; S, 14.32. Anal. Calcd for
C38H30Mo2S4‚2CH2Cl2: C, 49.13; H, 3.48; S, 13.10. MS: m/z 807
(M+).
Synthesis of cis- and trans-Mo2{µ-SCHdC(CO2Me)S}Cp2
(2c). A solution of complex 1 (504 mg, 0.995 mmol) and methyl
propiolate (1.2 mL) in CH2Cl2 (40 mL) was refluxed for 24 h.
Separation of the products by chromatography gave two bands: the
orange-red trans isomer (152 mg, 25%) was eluted first with light
petroleum/CH2Cl2 (1:4), followed by a yellow band of the more
polar cis isomer (236 mg, 37%), which was eluted with acetone/
CH2Cl2 (1:99). Crystals of the trans isomer suitable for X-ray
analysis were grown by diffusion from CH2Cl2 and diethyl ether.
Physical Measurements. The H and 13C NMR spectra were
1
obtained in CDCl3 solution on a Bruker AC250 Fourier transform
machine with an automated sample-changer or on a Bruker
AMX400 spectrometer. Chemical shifts are given on the δ scale
relative to SiMe4 ) 0.0 ppm. The 13C{1H} NMR spectra were
routinely recorded using an attached proton test technique (JMOD
pulse sequence). Mass spectra were recorded on a Fisons/BG
Prospec 3000 instrument operating in fast atom bombardment mode
cis-2c: mp >300 °C. 1H NMR: δ 7.78 (s, 2 H, CH), 6.13 (s, 10
H, Cp), 3.55 (s, 6 H, Me). Found: C, 34.92; H, 2.99; S, 20.63.
(33) Cowans, B.; Rakowski Dubois, M. In Organometallic Syntheses; King,
R. B., Ed.; Elsevier: Amsterdam, 1986; Vol. 3, p 262.
(34) Bruce, M. I.; Hameister, C.; Swincer, A. G. Inorg. Synth. 1990, 28,
270.
(35) Hallman, P. S.; Stephenson, T. A.; Wilkinson, G. Inorg. Synth. 1970,
12, 237.
(37) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877. In this
review, the potential of the ferrocenium/ferrocene couple is given as
0.46 V vs SCE, which equates to 0.505 V vs AgCl/Ag, rather than
the 0.55 V which we observed. No correction to the observed potentials
has been made for this discrepancy.
(36) Morris, M. J.; Precious, G., unpublished work.
9804 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
Anal. Calcd for C18H18Mo2O4S4: C, 34.96; H, 2.93; S, 20.74. MS:
m/z 618 (M+).
H, 2.77; S, 18.93. Anal. Calcd for C20H20Mo2O6S4: C, 35.51; H,
2.98; S, 18.96. MS: m/z 676 (M+).
trans-2c: mp >300 °C. 1H NMR: δ 7.80 (s, 2 H, CH), 6.13 (s,
10 H, Cp), 3.55 (s, 6 H, Me). 13C NMR: δ 163.3 (CO2Me), 162.0
(CH), 159.2 (CCO2Me), 97.0 (Cp), 52.2 (Me). Found: C, 35.18;
H, 2.87; S, 20.68. Anal. Calcd for C18H18Mo2O4S4: C, 34.96; H,
2.93; S, 20.74%. MS: m/z 618 (M+).
Synthesis of Mo2{µ-SCHdC(CO2Me)S}(µ-SCPhdCPhS)Cp2
(2f). A solution of 3b (140 mg, 0.213 mmol) and methyl propiolate
(0.04 mL) in CH2Cl2 (30 mL) was heated to reflux for 24 h. Column
chromatography gave one orange-yellow band, consisting of 2f (106
mg, 70%), which was eluted with CH2Cl2.
2f: mp, dec above 250 °C. 1H NMR: δ 7.82 (s, 1 H, CH), 7.15-
6.76 (m, 10 H, Ph), 6.07 (s, 10 H, Cp), 3.58 (s, 3 H, Me). 13C
NMR: δ 163.6 (CO2Me), 163.2 (CH), 158.9 (CCO2Me), 154.3,
153.5 (CPh), 138.0, 137.9 (Cipso), 128.0-126.7 (m, Ph), 96.1 (Cp),
52.2 (Me). Found: C, 47.26; H, 3.39; S, 18.12. Anal. Calcd for
C28H24Mo2O2S4: C, 47.19; H, 3.39; S, 18.00. MS: m/z 712 (M+).
Synthesis of Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-SC(PPh2)d
C(PPh2)S}Cp2 (2g). A solution of 3a (385 mg, 0.637 mmol) and
dppa (376 mg, 0.95 mmol) in CH2Cl2 (30 mL) was heated to reflux
for 24 h. Column chromatography produced one orange-yellow
band, eluted with CH2Cl2, consisting of 2g (392 mg, 62%). Crystals
suitable for X-ray analysis were grown by diffusion from CH2Cl2
and diethyl ether.
Synthesis of Mo2{µ-SC(CO2Me)dC(CO2Me)S}(µ-SCH2CH2S)-
Cp2 (3a). A solution of Mo2(µ-SCH2CH2S)2Cp2 (500 mg, 0.988
mmol) and 1 equiv of DMAD (0.121 mL) in CH2Cl2 (30 mL) was
stirred for 4 days. The solvent was removed, and the residue was
chromatographed. Two bands were eluted with acetone-CH2Cl2
(1:99). The first, apple green in color, contained the monodithiolene
complex 3a (271 mg, 44%), and the second (red) consisted of bis-
dithiolene complex 2a (199 mg, 27% yield based on Mo). The
elemental analysis of 3a consistently gave a high value for carbon
but correct values for H and S (four attempts)
3a: mp, 164-168 °C. 1H NMR: δ 5.58 (s, 10 H, Cp), 3.65 (s,
6 H, Me), 1.87 (s, 4 H, CH2). 13C NMR: δ 166.2 (CO2Me), 164.2
(CCO2Me), 93.4 (Cp), 52.5 (Me), 34.6 (CH2). Found: C, 36.74;
H, 3.44; S, 20.44. Anal. Calcd for C18H20Mo2O4S4: C, 34.84; H,
3.25; S, 20.67. MS: m/z 620 (M+), 592 (M+ - C2H4), 450, 418,
386 (Mo2S4-nCp2+, n ) 0-2).
Synthesis of Mo2(µ-SCPhdCPhS)(µ-SCH2CH2S)Cp2 (3b). In
a similar manner, a solution of 1 (505 mg, 1.003 mmol) and 1
equiv of diphenylacetylene (178 mg) in CH2Cl2 (40 mL) was stirred
for 5 days. Column chromatography of the mixture gave a light
green band of 2b (30 mg, 3.7% based on Mo), eluted with CH2-
Cl2-light petroleum (1:1). Further elution with a 3:2 mixture of
the same solvents developed a darker green band due to 3b (131
mg, 20% yield).
1
2g: mp, 230-232 °C. H NMR: δ 7.42-7.16 (m, 20 H, Ph),
5.57 (s, 10 H, Cp), 3.54 (s, 6 H, Me). 13C NMR: δ 173.4 (d of d,
J ) 1.5, 3.8 Hz, CPPh2), 165.7 (CO2Me), 161.1 (CCO2Me), 135.3
(apparent t, Cipso), 133.0-128.9 (m, Ph), 96.4 (Cp), 52.4 (Me). 31
P
NMR: δ -10.1. Found: C, 48.27; H, 3.36; S, 12.82. Anal. Calcd
for C42H36Mo2O4P2S4‚CH2Cl2: C, 48.19; H, 3.57; S, 11.97. MS:
m/z 987 (M+).
Synthesis of Mo2{µ-SC(PPh2)dC(PPh2)S}2Cp2 (2h). The
phosphine Ph2PCtCPPh2 (522 mg, 1.32 mmol) was added to a
solution of 1 (295 mg, 0.583 mmol) in CH2Cl2 (40 mL), which
was then refluxed for 48 h. Column chromatography produced two
small unidentified yellow bands, followed by an orange-yellow zone
due to complex 2h. The yield was 273 mg, 38%. The compound
proved too insoluble for the recording of a 13C NMR spectrum.
Crystals suitable for X-ray analysis were grown by diffusion from
CH2Cl2 and diethyl ether.
1
3b: mp >300 °C. H NMR: δ 7.15-6.90 (m, 10 H, Ph), 5.55
(s, 10 H, Cp), 1.85 (s, 4 H, CH2). 13C NMR: δ 157.4 (CPh), 138.0
(Cipso), 127.9-127.2 (m, Ph), 92.5 (Cp), 34.7 (CH2). Found: C,
48.35; H, 3.67; S, 19.36. Anal. Calcd for C26H24Mo2S4: C, 47.60;
H, 3.69; S, 19.55. MS: m/z 628 (M+ - C2H4), 450, 418, 386
(Mo2S4-nCp2+, n ) 0-2).
1
2h: mp, dec above 280 °C. H NMR: δ 7.42-7.16 (m, 40 H,
Ph), 4.87 (s, 10 H, Cp). 31P NMR: δ -9.1. Found: C, 59.49; H,
3.96; S, 10.30. Anal. Calcd for C62H50Mo2P4S4: C, 60.09; H, 4.07;
S, 10.35. MS: m/z 1238 (M+).
Synthesis of Mo2{µ-SC(CO2Me)dC(CO2Me)S}(µ-SCPhd
CPhS)Cp2 (2d). A solution of Mo2{µ-SC(CO2Me)dC(CO2Me)S}-
(µ-SCH2CH2S)Cp2 (3a) (202 mg, 0.326 mmol) and diphenylacet-
ylene (55 mg, 0.31 mmol) in CH2Cl2 (30 mL) was heated to reflux
for 24 h, after which time the solvent was removed. Column
chromatography gave a single yellow-brown band consisting of 2d
(207 mg, 83%) eluted in CH2Cl2. Red plates suitable for X-ray
analysis were grown from toluene solution.
Preparation of Mo2{µ-SC(PPh2)dC(PPh2)S}(µ-SCH2CH2S)-
Cp2 (3c). A solution of complex 1 (529 mg, 1.045 mmol) and dppa
(412 mg, 1.045 mmol) in CH2Cl2 was stirred at room temperature
for 5 days. Column chromatography gave a small unidentified
yellow band followed by a light green band of 3c (175 mg, 19%)
eluted with CH2Cl2-light petroleum (7:3). Two further bands were
obtained on elution with CH2Cl2 but could not be identified.
1
2d: mp >300 °C. H NMR: δ 7.10-6.77 (m, 10 H, Ph), 6.23
(s, 10 H, Cp), 3.63 (s, 6 H, Me). 13C NMR: δ 166.1 (CO2Me),
161.3 (CCO2Me), 153.3 (CPh), 137.8 (Cipso), 128.1-126.8 (m, Ph),
96.6 (Cp), 52.6 (Me). Found: C, 46.66; H, 3.32; S, 16.62. Anal.
Calcd for C30H26Mo2O4S4: C, 46.67; H, 3.40; S, 16.64. MS: m/z
772 (M+).
1
3c: mp, dec above 225 °C. H NMR: δ 7.45-7.15 (m, 20 H,
Ph), 4.97 (s, 10 H, Cp), 1.67 (s, 4 H, CH2). 13C NMR: δ 177.9 (d,
J ) 2.3 Hz, CPPh2), 135.9 (apparent t, Cipso), 133.1-128.1 (m,
Ph), 92.3 (Cp), 34.4 (CH2). 31P NMR: δ -9.3. Found: C, 52.42;
H, 3.79; S, 14.49. Anal. Calcd for C38H34Mo2P2S4: C, 52.29; H,
3.93; S, 14.70. MS: m/z 873 (M+).
Synthesis of Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-SCHdC-
(CO2Me)S}Cp2 (2e). A solution of Mo2{µ-SC(CO2Me)dC(CO2-
Me)S}(µ-SCH2CH2S)Cp2 (3a) (133 mg, 0.215 mmol) and methyl
propiolate (0.038 mL) in CH2Cl2 (30 mL) was heated to reflux for
24 h. Column chromatography gave one yellow-brown band,
consisting of 2e (72 mg, 50%), which was eluted with acetone-
CH2Cl2 (1:99).
Synthesis of Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-SC(PPh2)d
C(PPh2)S}Cp2(RuClCp) (4a). Complex 2g (116.2 mg, 0.118
mmol) and CpRu(PPh3)2Cl (95.1 mg, 0.131 mmol) were dissolved
in toluene (50 cm3). The solution was heated to reflux for 5 h,
after which the solvent was removed and the residue absorbed on
silica. Column chromatography produced one orange-brown band
of 4a (104.2 mg, 74% yield based on Mo), which was eluted with
CH2Cl2-acetone (99:1).
1
2e: mp, 244 °C. H NMR: δ 7.78 (s, 1 H, CH), 6.17 (s, 10 H,
Cp), 3.59 (s, 6 H, co-incident Me), 3.56 (s, 3H, Me). 13C NMR: δ
166.0, 165.4, 163.2 (all CO2Me), 161.5 (CH), 161.5, 159.7, 158.1
(all CCO2Me), 97.3 (Cp), 52.5 (2 Me), 52.3 (Me). Found: C, 35.48;
1
4a: mp >300 °C. H NMR: δ 7.65-6.94 (m, 20H, Ph), 6.03,
5.58 (s, 5H, MoCp), 4.42 (s, 5H, RuCp), 3.55 (s, 6H, CO2Me). 13
C
Inorganic Chemistry, Vol. 46, No. 23, 2007 9805

Adams et al.
Table 4. Summary of Crystallographic Data for the Six Structures
trans-2c
C18H18Mo2O4S4
618.44
2d‚C6H5Me
C37H34Mo2O4S4
empirical formula
fw
862.76
cryst syst
monoclinic
triclinic
space group
a/Å
P21/c
9.2537(7)
P1h
8.141(1)
15.020(2)
15.855(2)
111.491(3)
91.397(3)
101.413(3)
1758.3(4)
2
b/Å
c/Å
9.4933(7)
11.8024(9)
90
98.249(1)
90
1026.1(1)
2
R/deg
â/deg
γ/deg
V/Å3
Z
θ range for data collection/deg
reflns collected
independent reflns
data/restraints/params
GOF on F2
2.22-28.32
1.49-28.29
6260
10 889
2461 [R(int) ) 0.0889]
2461/0/127
1.157
7947 [R(int) ) 0.0868]
7947/30/424
0.926
final R1, wR2 [I > 2σ(I)]
R1 ) 0.0525, wR2 ) 0.1230
R1 ) 0.0602, wR2 ) 0.1288
1.177 and -3.820
R1 ) 0.0642, wR2 ) 0.1489
R1 ) 0.1126, wR2 ) 0.1701
2.579 and -2.342
(all data)
largest diff peak and hole/e‚Å-3
2g‚CH2Cl2
2h
empirical formula
fw
C43H38Cl2Mo2O4P2S4
1071.69
C62H50Mo2P4S4
1239.02
cryst syst
space group
orthorhombic
P212121
monoclinic
P21/n
a/Å
10.484(3)
14.475(5)
b/Å
17.491(4)
10.517(4)
c/Å
23.751(6)
18.194(6)
R/deg
90.000(6)
90.000(6)
90.000(6)
4355(2)
90
â/deg
105.430(7)
90
2670(2)
γ/deg
V/Å3
Z
4
2
θ range for data collection/deg
reflns collected
independent reflns
data/restraints/params
GOF on F2
1.45-28.32
1.61-25.00
27 108
13 021
10 491 [R(int) ) 0.0861]
10491/311/524
0.881
4690 [R(int) ) 0.0857]
4690/239/325
0.969
final R1, wR2 [I > 2σ(I)]
R1 ) 0.0491, wR2 ) 0.0742
R1 ) 0.0880, wR2 ) 0.0827
0.905 and -1.020
R1 ) 0.0501, wR2 ) 0.0895
R1 ) 0.1019, wR2 ) 0.1022
0.859 and -1.081
(all data)
largest diff peak and hole/e‚Å-3
4a
syn-4b‚2CH2Cl2
empirical formula
C47H41ClMo2O4P2RuS4
C74H64Cl6Mo2P4Ru2S4
fw
1188.38
1812.09
cryst syst
orthorhombic
monoclinic
space group
P212121
P21/c
a/Å
10.525(3)
19.551(5)
b/Å
14.839(4)
19.742(5)
c/Å
29.328(8)
21.079(6)
R/deg
90
90
90
4580(2)
90
â/deg
116.572(5)
90
7277(3)
γ/deg
V/Å3
Z
4
4
θ range for data collection/deg
reflns collected
independent reflns
data/restraints/params
GOF on F2
1.54-28.32
1.16-28.29
23 503
45 612
10 562 [R(int) ) 0.0806]
10562/342/550
1.058
17 552 [R(int) ) 0.0856]
17552/570/836
0.928
final R1, wR2 [I > 2σ(I)]
R1 ) 0.0359, wR2 ) 0.0849
R1 ) 0.0406, wR2 ) 0.0872
1.366 and -0.934
R1 ) 0.0633, wR2 ) 0.1516
R1 ) 0.1143, wR2 ) 0.1694
2.164 and -1.753
(all data)
largest diff peak and hole/e‚Å-3
NMR: δ 181.8 (d of d, J ) 34.3 and 34.3 Hz, CPPh2), 165.2 (CO2-
Me), 160.2 (CCO2Me), 136.5, 132.2 (apparent d of t, Cipso, Ph),
133.9, 132.1, 127.5, 126.7 (apparent t, Ph), 129.7, 129.3 (Ph), 97.4,
96.6 (MoCp), 79.1 (RuCp), 52.2 (Me). 31P NMR: δ 60.8. Found:
C, 47.62; H, 3.27; Cl, 3.06; S, 10.62. Anal. Calcd for C47H41-
ClMo2O4P2RuS4: C, 47.46; H, 3.47; Cl, 2.98; S, 10.78. MS: m/z
1190 (M+).
Synthesis of Mo2{µ-SC(PPh2)dC(PPh2)S}2Cp2(RuClCp)2 (4b).
A solution of 2h (251 mg, 0.203 mmol) and CpRu(PPh3)2Cl (320
mg, 0.441 mmol) in toluene (50 cm3) was refluxed for 6 h, then
absorbed onto silica and chromatographed. Elution with CH2Cl2-
acetone (99:1) developed two minor pale yellow bands which were
discarded, followed by an orange band and a red band. The orange
band consisted of the anti isomer of 4b (40.8 mg, 12.3% based on
9806 Inorganic Chemistry, Vol. 46, No. 23, 2007

Dimolybdenum Bis(dithiolene) Complexes
Mo), and the red band consisted of the syn isomer (128.1 mg, 38.5%
based on Mo); the latter was washed with diethyl ether to remove
a small amount of a green impurity. The former complex proved
too insoluble to obtain a 13C NMR spectrum.
Molecular orbital compositions and the overlap populations were
calculated using the AOMix program51 using the Mulliken
scheme.52-54 Atomic charges were calculated using the Mulliken
procedure as implemented in Gaussian 03.
1
anti-4b: mp >300 °C. H NMR: δ 7.64-6.85 (m, 40H, Ph),
5.38 (s, 10H, MoCp), 4.35 (s, 10H, RuCp). 31P NMR: δ 61.3.
Found: C, 52.73; H, 3.60; Cl, 4.52; S, 8.03. Anal. Calcd for C72H60-
Cl2Mo2P4Ru2S4: C, 52.66; H, 3.68; Cl, 4.32; S, 7.81. MS: m/z
1643 (M + H+).
Acknowledgment. This work was supported by the
EPSRC (studentship to A.E.R.), the University of Sheffield,
and the Natural Sciences and Engineering Research Council
(Ottawa) (ABPL). We also thank Johnson Matthey for a
generous loan of ruthenium chloride, Prof. M.D. Ward for
useful discussions, and Drs. D. Collison and J. Wolowska
(University of Manchester) for recording and simulating the
EPR spectra.
1
syn-4b: mp >300 °C. H NMR: δ 7.62-6.90 (m, 40H, Ph),
5.77, 4.87 (both s, 5H, MoCp), 4.36 (s, 10H, RuCp). 13C NMR: δ
182.5 (d of d, J ) 34.7 and 34.7 Hz, CPPh2), 137.1-127.0 (m,
Ph), 96.9, 95.7 (MoCp), 79.5 (RuCp). 31P NMR: δ 61.7. Found:
C, 52.51; H, 3.51; Cl, 4.38; S, 7.78. Anal. Calcd for C72H60Cl2-
Mo2P4Ru2S4: C, 52.66; H, 3.68; Cl, 4.32; S, 7.81. MS: m/z 1642
(M+).
Supporting Information Available: CIF files for the six crystal
structures, observed and calculated isotope patterns for the molecular
ion in the mass spectrum of complex 5, and more detailed DFT
results in Tables S1-S2. This material is available free of charge
via the Internet at http://pubs.acs.org.
Synthesis of RuCl2{Mo2{µ-SC(CO2Me)dC(CO2Me)S}{µ-
SC(PPh2)dC(PPh2)S}Cp2}2 (5). A solution containing RuCl2-
(PPh3)3 (100 mg, 0.104 mmol) in acetone (40 mL) was treated with
solid 2g (206 mg, 0.208 mmol) and then stirred for 2 h. The
insoluble orange-beige product 5 (80 mg, 36%) was then filtered
off and washed with acetone.
IC701201X
5: mp >300 °C. Found: C, 46.77; H, 3.50; Cl, 3.37; S, 11.62.
Anal. Calcd for C84H72Cl2Mo4O8P4RuS8: C, 47.02; H, 3.38; Cl,
3.30; S, 11.96. Electrospray MS: m/z 2147 (M+).
(39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
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structures (trans-2c, 2d, 2g, 2h, 4a, and syn-4b) are collected in
Table 4. General procedures were as described in previous
publications. A Bruker Smart CCD area detector with Oxford
Cryosystems low-temperature system was used for data collection
at 150(2) K. Complex scattering factors were taken from the
program package SHELXTL38 as implemented on a Viglen Pentium
computer. Hydrogen atoms were placed geometrically and refined
in riding mode with Uiso constrained to be 1.2 times Ueq of the
carrier atom.
Calculations. DFT calculations utilized the Gaussian 03W
program (revision C.02).39 The spin-restricted method modeled the
closed-shell species, and the spin-unrestricted method modeled the
open-shell species. The geometries of the crystal structures, where
available, were used to derive percent composition of the frontier
molecular orbitals using the B3LYP exchange-correlation func-
tional40,41 with the LanL2DZ basis set.42-45 Tight SCF convergence
(10-8 a.u.) was used for all calculations. TD-DFT46-48 was used
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electronic transitions of each complex (15 for the large oxidized
4a and 4b species). These were transformed with the SWizard
program49 into simulated spectra as described before,50 using
Gaussian functions with half-widths of 3000 cm-1. For comparison
purposes and to derive data for the oxidized species, the DFT-
geometry-optimized structures were obtained for species 4a and
its oxidized product 4a+ (S ) 1/2) and also for 4b+ (S ) 1/2), 4b2+
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1
(S ) 0) and 4b3+ (S ) /2).
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Inorganic Chemistry, Vol. 46, No. 23, 2007 9807