Sulfur-based redox reactions in Mo3S74+ and Mo3S44+ clusters bearing halide and 1,2-dithiolene ligands: A mass spectrometric and density functional theory study

Article abstract of DOI:10.1021/ic1010693

The gas phase fragmentation reactions of sulfur-rich [Mo₃S ₇Br₆]^2- (1^2-), [Mo₃S ₇(bdt)₃]^2- (2^2-), and [Mo ₃S₄(bdt)₃]^2- (3^2-) (bdt = benzenedithiolate) complexes have been investigated by electrospray ionization (ESI) tandem mass spectrometry and theoretical calculations at the density functional theory level. Upon collision induced dissociation (CID) conditions, the brominated 1^2- dianion dissociates through two sequential steps that involves a heterolytic Mo-Br cleavage to give [Mo₃S ₇Br₅]^- plus Br^- followed by a two-electron redox process that affords [Mo₃S₅Br ₅]^- and diatomic S₂ sulfur. Dianion [Mo ₃S₇(bdt)₃]^2- (2^2-) dissociates through two sequential redox processes evolving diatomic S ₂ sulfur and neutral bdt to yield [Mo₃S ₅(bdt)₃]^2- and [Mo₃S ₅(bdt)₂]^2-, respectively. Conversely, dianion [Mo₃S₄(bdt)₃]^2- (3^2-), with sulfide instead of disulfide S₂^2- bridged ligands, remains intact under identical fragmentation conditions, thus highlighting the importance of disulfide ligands (S₂^2-) as electron reservoirs to trigger redox reactions. Regioselective incorporation of ³⁴S and Se at the equatorial position of the Mo₃S ₇ cluster core in 1^2- and 2^2- have been used to identify the product ions along the fragmentation pathways. Reaction mechanisms for the gas-phase dissociation pathways have been elucidated by means of B3LYP calculations, and a comparison with the solution reactivity of Mo ₃S₇ and Mo₃S₄ clusters as well as closely related Mo/S/dithiolene systems is also discussed.

Full text of DOI:10.1021/ic1010693

Inorg. Chem. 2010, 49, 8045–8055 8045
DOI: 10.1021/ic1010693
4þ
4þ
Sulfur-Based Redox Reactions in Mo3S7 and Mo3S4 Clusters Bearing Halide and
,2-Dithiolene Ligands: a Mass Spectrometric and Density Functional Theory Study
1
†
,‡,§
†
,||
Rosa Llusar, Victor Polo,* Ederley Velez, and Cristian Vicent*
†
Departament de Quı´mica Fı ´s ica i Analı ´t ica, Universitat Jaume I, Av. Sos Baynat s/n, 12071 Castell oꢀ ,
‡
Spain, Instituto de Biocomputaci oꢀ n y Fı ´s ica de los Sistemas Complejos (BIFI), Edificio Cervantes,
Corona de Arag oꢀ n 42, Zaragoza 50009, Spain, Departamento de Quı´mica Org aꢀ nica y Quı ´m ica Fı ´s ica,
Universidad de Zaragoza, c/Pedro Cerbuna s/n, 50009 Zaragoza, Spain, and Serveis Centrals d’Instrumentacioꢀ
Cientı´fica, Universitat Jaume I, Avda. Sos Baynat s/n, E-12071, Castell oꢀ , Spain
§
||
Received May 27, 2010
2-
2-
2- 2-
2- 2-
The gas phase fragmentation reactions of sulfur-rich [Mo S Br ] (1 ), [Mo S (bdt) ] (2 ), and [Mo S (bdt) ] (3 )
3
3
7
6
3 7
3
3 4
(
theoretical calculations at the density functional theory level. Upon collision induced dissociation (CID) conditions, the
bdt = benzenedithiolate) complexes have been investigated by electrospray ionization (ESI) tandem mass spectrometry and
2-
brominated 1 dianion dissociates through two sequential steps that involves a heterolytic Mo-Br cleavage to give
-
-
-
[
[
[
Mo S Br ] plus Br followed by a two-electron redox process that affords [Mo S Br ] and diatomic S sulfur. Dianion
3 7 5 3 5 5 2
2-
2-
Mo S (bdt) ] (2 ) dissociates through two sequential redox processes evolving diatomic S sulfur and neutral bdt to yield
3
7
3
2
2-
2-
2-
2-
Mo S (bdt) ] and [Mo S (bdt) ] , respectively. Conversely, dianion [Mo S (bdt) ] (3 ), with sulfide instead of
3
3
5
3
3 5
2
3 4
2
-
disulfide S2 bridged ligands, remains intact under identical fragmentation conditions, thus highlighting the importance of
disulfide ligands (S2 ) as electron reservoirs to trigger redox reactions. Regioselective incorporation of S and Se at the
2-
34
2-
2-
equatorial position of the Mo S cluster core in 1 and 2 have been used to identify the product ions along the
3
7
fragmentation pathways. Reaction mechanisms for the gas-phase dissociation pathways have been elucidated by means of
B3LYP calculations, and a comparison with the solution reactivity of Mo S and Mo S clusters as well as closely related Mo/S/
dithiolene systems is also discussed.
3
7
3 4
4
Introduction
typically system dependent. Another type of electron transfer
processes that dominates the group 6 chemistry of mono and
dinuclear complexes is that occurring through simultaneous
metal and ligand-based electron transfer reactions promoted by
external oxidants. This represents the so-called induced internal
Group 6 transition metal sulfides are employed as catalyst
for many industrial processes. Coordination of bis(dithiolene)
1
ligands to this class of compounds has expanded the scope to
areas ranging from optical, conducting, and magnetic mate-
5,6
2
redox reactions first identified by Stiefel et al., and whose
relevance to the molybdenum and tungsten enzymes has been
envisioned. Internal induced redox processes are not exclu-
sive of sulfur-rich group 6 complexes, and they has also been
rials, as well as in biological electron transport and enzyme-
3
catalyzed reactions. It is well documented that the redox ver-
1
,7
satility associated to group 6 dithiolene complexes is essential
for many of these applications. Redox transformations involv-
ing group 6 metal, ligand-based processes, or an interplay bet-
ween them, have been observed in mono- and dinuclear group 6
sulfide compounds, the extent of such electron transfer being
8
identified in a number of V/S/dithiolene or Re/S/dithiolene
9
complexes.
Conversely to the mono- and dinuclear sulfur group 6
complexes, the redox chemistry of trinuclear Mo S cluster
3
7
*
To whom correspondence should be addressed. Fax: þ34 976 761202 (V.
P.), þ34 964 387309 (C.V.). Phone: þ34 976 761203 (V.P.), þ34 964 387344
C.V.). E-mail: vipolo@unizar.es (V.P.), barrera@sg.uji.es (C.V.).
1) Stiefel, E. I. Transition Metal Sulfur Chemistry: Biological and
(
4) Young, C. G. J. Inorg. Biochem. 2007, 101, 1562.
(
(5) Coyle, C. L.; Harmer, M. A.; George, G. N.; Daage, M.; Stiefel, E. I.
Inorg. Chem. 1990, 29, 14.
(6) Wang, K.; McConnachie, J. M.; Stiefel, E. I. Inorg. Chem. 1999, 38, 4334.
(7) Stiefel, E. I. Pure Appl. Chem. 1998, 70, 889.
(8) Halbert, T. R.; Hutchings, L. L.; Rhodes, R.; Stiefel, E. I. J. Am.
Chem. Soc. 1986, 108, 6437.
(9) (a) Murray, H. H.; Wei, L.; Sherman, S. E.; Greaney, M. A.; Eriksen,
K. A.; Carstensen, B.; Halbert, T. R.; Stiefel, E. I. Inorg. Chem. 1995, 34, 841.
(b) McConnachie, C. A.; Stiefel, E. I. Inorg. Chem. 1999, 38, 964. (c) Dessapt, R.;
Simonnet-J ꢀe gat, C.; Riedel, S.; Marrot, J.; Secheresse, F. Trans. Met. Chem. 2002,
(
Industrial Significance; Stiefel, E. I., Matsumoto, K., Eds.; American Chemical
Society: Washington, DC, 1996; Vol. 653, p 2.
(
2) (a) Allen, A. E.; Olk, R. M. Coord. Chem. Rev. 1999, 188, 211. (b)
Matsubayashi, G.; Nakano, M.; Tamura, H. Coord. Chem. Rev. 2002, 226, 143.
c) Faulmann, C.; Cassoux, P. Prog. Inorg. Chem. 2004, 52, 399. (d) Cummings,
S. D.; Eisenberg, R. Prog. Inorg. Chem. 2004, 52, 315.
3) (a) Hille, R. Chem. Rev. 1996, 96, 2757. (b) Johnson, M. K.; Rees, D. C.;
(
(
Adams, M. W. W. Chem. Rev. 1996, 96, 2817. (c) McMaster, J.; Tunney, J. M.;
Garner, C. D. Prog. Inorg. Chem. 2004, 52, 539.
27, 234.
r 2010 American Chemical Society
Published on Web 08/09/2010
pubs.acs.org/IC

8
046 Inorganic Chemistry, Vol. 49, No. 17, 2010
Llusar et al.
Scheme 1
complexes involves exclusively the sulfur ligands where the
Mo centers are mere spectators. Hence, reduction of Mo3S7
molecules in solution to be gently transferred to the gas-
15,16
phase.
For example, gas-phase generation and reactivity
clusters proceeds through μ-S to μ-S transformation to
and photoelectron spectroscopy studies of ESI-generated group
2
17
afford the corresponding Mo S complexes. As far as the
6 oxides or group 6/dithiolene complexes have extensively
3
4
18,19
investigated.
oxidation behavior of the Mo S clusters is concerned, we
3
7
have recently shown that coordination of bis(dithiolenes) to
Mo3S7 clusters provides oxidation activity suggesting a
dominant contribution of the dithiolene ligand to the highest
occupied molecular orbital (HOMO) of the Mo S /dithiolene
Herein, we report an experimental study on the gas-phase
production and the fragmentation reactions of group 6 sulfides
featuring Mo S and Mo S cluster cores (see Scheme 1) using
3
7
3 4
ESI and ESI tandem mass spectrometry. C -symmetrized tri-
3
7
3
10-12
cluster complex.
nuclear Mo S clusters constitute a large family of inorganic
3
7
The understanding of electron transfer reactions involving
group 6/sulfide/dithiolene complexes is crucial to anticipate the
preferred redox pathway of the target species and therefore to
control the products formed as well as their yields. For example,
compounds in which the cluster core is coordinated to a wide
spectrum of ligands with applications in multidisciplinary
20
fields. Mo
core capped by a μ
illustrated in Scheme 1. Additionally, three bridging μ-S
S
clusters present an equilateral Mo
triangular
3
7
3
-S atom that lies above the metal plane as
3
2-
the mechanism of the induced redox reaction between MoS4
2
and organic disulfides have been elucidated providing valuable
clues to the rational preparation of new lower valent group 6
groups or μ-Sax groups connect adjacent metal atoms, with
three sulfur atoms occupying equatorial positions (Seq, essen-
5,13
tially in the Mo plane), and three axial sulfur atoms (S ,
sulfur-containing species.
However, investigating electron
3
ax
located out of the metal plane) on opposite sides to that of the
μ -S capping atom. In addition, the study of trinuclear Mo S
molybdenum clusters is also motivated by their putative pre-
transfer processes at the molecular level remains a task of great
complexity, mainly because of the transient nature of the
intermediates involved, the concerted nature of the process,
orthepresenceofsidereactions. Onewaytoaddressthemecha-
nistic elucidation of a chemical process at the molecular level
consists in paralleling the chemical process observed both in
solution and solid state, in a well-defined gas-phase environ-
ment in which solvent, counteranions, aggregation processes, or
side reactions are absent making the study much simpler. In this
context, tandem mass-spectrometric methods in conjunction
with theoretical calculations have proved useful in elucidating
3
3 9
sence as intermediates during catalytic MoS -based hydro-
x
desulfurization reactions, this Mo3S unit being a widely
9
(17) (a) Waters, T.; O’Hair, R. A. J.; Wedd, A. G. J. Am. Chem. Soc. 2003,
1
2
25, 3384. (b) Feyel, S.; Waters, T.; O'Hair, R. A. J.; Wedd, A. G. Dalton Trans.
004, 4010. (c) Yang, X.; Waters, T.; Wang, X. B.; O'Hair, R. A. J.; Wedd, A. G.;
Dixon, D. A.; Wang, L. S. J. Phys. Chem. A 2004, 108, 10089. (d) Waters, T.;
O'Hair, R. A. J.; Wedd, A. G. Inorg. Chem. 2005, 44, 3356. (e) Zhai, H. J.; Huang,
X.; Waters, T.; Wang, X. B.; O'Hair, R. A. J.; Wedd, A. G.; Wang, L. S. J. Phys.
Chem. A 2005, 109, 10512. (f ) Waters, T.; Wang, X. B.; Li, S. G.; Kiran, B.;
Dixon, D. A.; Wang, L. S. J. Phys. Chem. A 2005, 109, 11771. (g) Waters, T.;
Huang, X.; Wang, X. B.; Woo, H. K.; O'Hair, R. A. J.; Wedd, A. G.; Wang, L. S. J.
Phys. Chem. A 2006, 110, 10737. (h) Vicent, C.; Feliz, M.; Llusar, R. J. Phys.
Chem. A 2008, 112, 12550. (i) Llusar, R.; Sorribes, I.; Vicent, C. J. Clust. Sci.
14,15
mechanistic aspects.
In particular, electrospray ionization
mass spectrometry (ESI-MS) and its tandem version have
become increasingly popular as an analytical tool in inorganic
and organometallic chemistry because it allows pre-existing
2
009, 20, 177. ( j) Ma, M. T.; Waters, T.; Beyer, K.; Palamarczuk, R.; Richardt,
P. J. S.; O'Hair, R. A. J.; Wedd, A. G. Inorg. Chem. 2009, 48, 598. (k) Vila-Nadal,
L.; Rodriguez-Fortea, A.; Yan, L. K.; Wilson, E. F.; Cronin, L.; Poblet, J. M.
Angew. Chem., Int. Ed. 2009, 48, 5452.
(
10) Garriga, J. M.; Llusar, R.; Uriel, S.; Vicent, C.; Usher, A. J.; Lucas,
N. T.; Humphrey, M. G.; Samoc, M. Dalton Trans. 2003, 4546.
11) (a) Llusar, R.; Uriel, S.; Vicent, C.; Coronado, E.; Gomez-Garcia,
C. J.; Clemente-Juan, J. M.; Braida, B.; Canadell, E. J. Am. Chem. Soc. 2004,
26, 12076. (b) Alberola, A.; Llusar, R.; Triguero, S.; Vicent, C.; Sokolov, M. N.;
G ꢀo mez-Garcia, C. J. Mater. Chem. 2007, 17, 3440.
12) Llusar, R.; Triguero, S.; Polo, V.; Vicent, C.; Gomez-Garcia, C.;
Jeannin, O.; Fourmigue, M. Inorg. Chem. 2008, 47, 9400.
13) Harmer, M. A.; Halbert, T. R.; Pan, W.-H.; Coyle, C. L.; Cohen,
S. A.; Stiefel, E. I. Polyhedron 1986, 5, 341.
14) (a) Alcami, M.; Mo, O.; Ya n~ ez, M. Mass Spectrom. Rev. 2001, 20,
95. (b) Armentrout, P. B. Int. J. Mass Spectrom. 2003, 227, 289. (c) Bohme,
(
18) (a) Dessapt, R.; Simonnet-Jegat, C.; Mallard, A.; Lavanant, H.;
(
Marrot, J.; Secheresse, F. Inorg. Chem. 2003, 42, 6425. (b) Waters, T.;
Blanksby, S. J.; Zhang, L. Y.; O'Hair, R. A. J. Org. Biomol. Chem. 2004, 2,
1
1
90. (c) Waters, T.; Wang, X. B.; Yang, X.; Zhang, L.; O'Hair, R. A. J.; Wang,
L. S.; Wedd, A. G. J. Am. Chem. Soc. 2004, 126, 5119. (d) Lavanant, H.;
Fressigne, C.; Simonnet-Jegat, C.; Dessapt, R.; Mallard, A.; Secheresse, F.;
Sellier, N. Int. J. Mass Spectrom. 2005, 243, 205.
(
(
(
19) Llusar, R.; Triguero, S.; Vicent, C.; Sokolov, M. N.; Domercq, B.;
Fourmigue, M. Inorg. Chem. 2005, 44, 8937.
20) (a) Sakane, G.; Shibahara, T. Transition Metal Sulfur Chemistry:
(
(
1
Biological and Industrial Significance; Stiefel, E. I., Matsumoto, K., Eds.;
American Chemical Society: Washington, DC, 1996; Vol. 653, p 225; (b) Saito,
T. Adv. Inorg. Chem. 1997, 44, 45. (c) Llusar, R.; Vicent, C. Trinuclear
Molybdenum and Tungsten Cluster Chalcogenides: From Solid State to
Molecular Materials. Inorganic Chemistry in Focus III; Meyer, G.; Naumann,
D., Wesemann, L., Eds.; Wiley-VCH Verlag: Weinheim, Germany, 2006; p 105;
(d) Fedorov, V. E.; Mironov, Y. V.; Naumov, N. G.; Sokolov, M. N.; Fedin, V. P.
Russ. Chem. Rev. 2007, 76, 529. (e) Llusar, R.; Vicent, C. Coord. Chem. Rev.
2010, 254, 1534.
D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336. (d) Schr €o der, D.;
Schwarz, H. Top. Organomet. Chem. 2007, 22, 1. (e) Johnson, G. E.; Tyo, E. C.;
Castleman, J., A. W. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18108. (f )
Johnson, G. E.; Mitric, R.; Bonacic-Koutecky, V.; Castleman, A. W., Jr. Chem.
Phys. Lett. 2009, 475, 1. (g) Roithova, J.; Schr €o der, D. Coord. Chem. Rev. 2009,
2
53, 666. (h) Roithova, J.; Schr €o der, D. Chem. Rev. 2010, 110, 1170.
(
(
15) O’Hair, R. A. J. Chem. Commun 2006, 1469.
16) (a) Chen, P. Angew. Chem., Int. Ed. 2003, 42, 2832. (b) Bernhardt, T. M.
Int. J. Mass Spectrom. 2005, 243, 1.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8047
investigated molecular model to reproduce the periodicity of
mixtures. (0.024 g, 52%). (Found: C, 33.03; H 4.96, S 40.97, N 1.92.
21
2- 2-
the bulk MoS solid. Forthe[Mo S Br ] (1 ) and[Mo S -
Mo
KBr) cm : 1459 (s), ν(CdC); 1054 (vs), ν(CdS); 512 (m),
ν(C-S); 469 (m), ν(Mo-Sbdt) and ν(Mo-( μ-S)); ESI-MS(-) m/z:
3 19 41 72 2
S C H N requires C 33.05, H 4.87, S 40.88, N 1.88). IR
3
3 7
6
3 7
-
1
2-
2-
(
(
bdt)3] (2 ) (bdt =1,2-benzenedithiolate) dianions investi-
gated in this work, the outer groups (Br or bdt) fill the remain-
2-
2-
2-
503 [M]
X-ray Studies. Cation exchange in compound (n-Bu
was carried out by adding an excess of PPh Br in acetonitrile that
precipitates the desired (PPh [3] compound. Suitable crystals for
X-ray studies for compound (PPh [3] were grown by slow diffu-
sion of diethylether into sample solutions in CH Cl under rigorous
.
ing two positions. In [Mo S (bdt) ] (3 ), the equatorial
3
4
3
4 2
N) [3]
atoms are missing and 1,2-benzenedithiolate ligands complete
2-
2-
4
the Mo environment. We have chosen these 1 -3 dianions
4 2
)
as models to systematically analyze the effects that modification
4 2
)
2-
2-
of the inner bridged ligand (S2 or S ) and the peripheral
ligand (Br or bdt) have on the identity of the formed product
ions upon gas-phase fragmentation conditions.
2
2
inert atmosphere. The data collection was performed on a Bruker
Smart CCD diffractometer using graphite-monochromated Mo
˚
The fragments evolved (neutral S and the dithiete bdt) upon
KR radiation (λ=0.71073 A). A hemisphere of data was collected
2
collision induced dissociation (CID) conditions formally cor-
respond to oxidation products concomitant with Mo S cluster
basedonthreeω-scans runs (starting ω=-28°) at values φ=0°, 90°,
and 180° with the detector at 2θ=28°. At each of these runs, frames
3
7
(
606, 435, and 230 respectively) were collected at 0.3° intervals and
reduction whose energetic profiles are rationalized on the basis
3
4
35 s per frame. The diffraction frames were integrated using the
SAINT package and corrected for absorption with SADABS.
of complementary isotopically S labeling experiments and
density functional theory (DFT) calculations.
26
The positions of the heavy atoms were determined by direct
methods and successive difference electron density maps using the
SHELXTL 5.10 software package were done to locate the remain-
Experimental Section
2
7
General Procedures. All reactions were carried out under a
nitrogen atmosphere using standard Schlenk techniques. Isotopi-
ing atoms. Refinement was performed by the full-matrix-least-
squares method based on F . All atoms in compound (PPh ) [3]
2
4
2
3
4
cally labeled SPPh was prepared starting from PPh and ele-
were refined anisotropically. All hydrogen atoms of the phenyl
groups were generated geometrically. Crystal data for (PPh [3]:
10, M=1515.44, monoclinic, space group Cc, a=
3
34
3
34
1
mental S (99.5% S, Sigma-Aldrich) and characterized by H,
C, and P { H}NMR. The ESI mass spectrum of CH Cl :
2 2
8
31
4 2
)
13
1
22
66 3 2
C H52Mo P S
3
4
3
CH
3
OH solutions3 SPPh
3
reveals the presence of a prominent peak
˚
13.020(2), b=23.573(4), c=21.833(3), β=106.805(4), V=6415(2) A ,
4
þ
-
1
attributed to the [ SPPh þH] (m/z = 297) adduct. Compounds
3
T=293 K, Z=4, μ(MoKR)=0.993 mm . Reflections collected/
unique =18024/8402 (Rint=0.1326). Final refinement converged
2
3
24
(
n-Bu N)
4
2
[Mo
N)
3
S
[Mo
7
Br ] ((n-Bu [Mo
6
4
N)
2
[1]), (n-Bu N)
4
2
3 4 3 6
S Se Br ],
12
and (n-Bu
4
2
3
S
7
(bdt) ] ((n-Bu N) [2]), were prepared ac-
4 2
3
with R
0
density 1.032/-1.697 e A
1 0 0 2
=0.0719 for 7305 reflections with F g 4σ(F ) and wR =
34
cording to literature methods. Regioselective S isotopic labeling at
the equatorial positions in Mo S clusters has been previously
.1866 for all reflections, GoF=1.005, max/min residual electron
˚
-3
3
7
.
23,25
reported.
3
4 2 4 2
For compounds (n-Bu N) [1] and (n-Bu N) [2] we
ESI Mass Spectrometry. A hybrid QTOF I (quadrupole-hexa-
pole-TOF) mass spectrometer with an orthogonal Z-spray-electro-
spray interface (Waters, Manchester, U.K.) was used. The desolva-
tion gas as well as nebulizing gas was nitrogen at a flow of 800 L/h
and 20 L/h respectively. The temperature of the source block was set
to 120 °C and the desolvation temperature to 150 °C. Mass calibra-
tion was performed using a solution of sodium iodide in isopropanol/
water (50:50) from m/z 100 to 1900. A capillary voltage of 3.3 KV
was used in the negative scan mode, and the cone voltage was set
to 10 V to control the extent of fragmentation of the identified ions.
Sample solutions were infused via syringe pump directly connected
to the ESI source at a flow rate of 10 μL/min. The observed isotopic
pattern of each intermediate perfectly matched the theoretical iso-
tope pattern calculated from their elemental composition using the
MassLynx 4.0 program. Tandem MS/MS spectra were obtained at
various collision energies (typically varied from Elab 0-50 eV) by
selecting the precursor ion of interest with the first quadrupole (Q1)
and scanning with the time-of-flight analyzer (TOF). The complete
envelope of each ion was mass-selected except for samples enriched
follow a modification which consists in refluxing these complexes
with a 10-fold excess of SPPh in acetonitrile for 1 h. The [Mo S -
34
3
3 4
2-
Se (bdt) ] dianion was prepared by stirring acetonitrile solutions
3
3
2-
3 4 3 6
of[Mo S Se Br ] , a 5-fold excess of the 1,2-benzenedithiol and tri-
ethylamine for 1 h and subsequently transferred to the gas-phase by
ESI. The remaining reactants were obtained from commercial
sources and used as received. Solvents for synthesis were dried
and degassed by standard methods before use. Elemental analysis
was performed on an EA 1108 CHNS microanalyzer. IR spectra
were recorded on a Perkin-Elmer System 2000 FT-IR using KBr
3
pellets. Cyclic voltammetry experiments were performed in CH CN
with an Echochemie Pgstat 20 electrochemical analyzer and a
conventional three-electrode configuration consisting of platinum
working and auxiliary electrodes and a Ag/AgCl reference electrode.
Synthesis. (n-Bu N) [Mo S (bdt) ] ((n-Bu N) [3]).PPh (0.03g,
4
2
3
4
3
4
2
3
0
.11 mmol.) was added to a red solution of (n-Bu
4 2 3 7 3
N) [Mo S (bdt) ]
(
nitrogen. The solution was stirred for 10 min, and the desired com-
(n-Bu N) [2]) (0.05 g, 0.03 mmol) in 10 mL of acetonitrile under
4
2
pound (n-Bu N) [3] was precipitated with diethylether. The precipi-
2
34
4
with S for which a single isotopomer was mass-selected. Argon
was used as a collision gas to produce the pressure of 3 ꢀ 10 mbar
tate was separated from the solution by filtration under inert atmo-
sphere, washed thoroughly with toluene and diethylether to
eliminate the PPh S, and recrystallized from CH Cl /diethylether
-5
as measured in the quadrupole analyzer region.
Computational Details. All calculations were carried out
3
2
2
2
using the Gaussian03 program. The commonly used B3LYP
8
(
21) (a) Jiao, H.; Li, Y.-W.; Delmon, B.; Halet, J.-F. J. Am. Chem. Soc.
29
functional is employed in combination with the 6-31G(d,p)
2001, 123, 7334. (b) Yao, X.-Q.; Li, Y.-W.; Jiao, H. J. Mol. Struct. TEOCHEM
2005, 726, 67. (c) Yao, X.-Q.; Li, Y.-W.; Jiao, H. J. Mol. Struct. TEOCHEM
2005, 726, 81. (d) Zeng, T.; Wen, X.-D.; Li, Y.-W.; Jiao, H. J. Phys. Chem. B
2005, 109, 13704. (e) Murugan, P.; Kumar, V.; Kawazoe, Y.; Ota, N. J. Phys.
30
basis set for S, Br, C, and H atoms and the effective core
(26) (a) SAINT, version 6.2; Bruker Analytical X-Ray Systems: Madison,
WI, 2001; (b) Sheldrick, G. M. SADABS, empirical absorption program;
University of G €o ttingen: G €o ttingen, Germany, 2001.
(27) Sheldrick, G. M. SHELXTL, version 5.1; Bruker Analytical X-Ray
Systems: Madison, WI, 1997.
(28) Frisch, M. J. et al. Gaussian03, revision C.02; Gaussian,Inc.: Wall-
ingford, CT, 2003.
Chem. A 2007, 111, 2778.
22) Tkachenko, S. E.; Trofimova, T. P.; Fedoseev, V. M. Chem.
Heterocycl. Compd. 1999, 35, 973.
23) Fedin, V. P.; Sokolov, M. N.; Mironov, Y. V.; Kolesov, B. A.;
Tkachev, S. V.; Fedorov, V. Y. Inorg. Chim. Acta 1990, 167, 39.
24) Fedin, V. P.; Mironov, Y. V.; Sokolov, M. N.; Kolesov, B. A.;
Fedorov, V. Y.; Yufit, D. S.; Struchkov, Y. T. Inorg. Chim. Acta 1990, 174,
75.
25) Fedin, V. P.; Kolesov, B. A.; Mironov, Y. V.; Fedorov, V. Y.
Polyhedron 1989, 8, 2419.
(
(
(
(29) (a) Becke, A. D. Phys. Rev. A. 1988, 38, 3098. (b) Lee, C. T.; Yang,
W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Becke, A. D. J. Chem. Phys.
1993, 98, 1372.
2
(
(30) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257.

8
048 Inorganic Chemistry, Vol. 49, No. 17, 2010
Llusar et al.
3
1
potentials (ECP) of Stuttgart RSC 1993 on Mo atoms. Full
geometry optimizations were performed followed by analytical
calculation of frequencies to determine the nature of the sta-
tionary point.
Results and Discussion
Compounds (n-NBu ) [1] and (n-NBu ) [2] are accessed
4
2
4 2
starting from the preassembled molecular (NH ) [Mo S ]
4
2
3 13
12,23
complex.
Complex (n-Bu N) [Mo S (bdt) ] ((n-Bu N) [3])
4 2 3 4 3 4 2
featuring a Mo S core can be easily obtained by treatment of
3
4
acetonitrile solutions of (n-Bu N) [2] with 3 equiv of triphe-
4
2
nylphosphine according to eq 1.
2
-
2 -
½
Mo
3
S
7
ðbdtÞ3ꢁ þ 3PPh
3
f ½Mo
3
S
4
ðbdtÞ3ꢁ þ 3SPPh
3
ð1Þ
Compound (n-Bu N) [3] has been characterized by IR spec-
4
2
troscopy, ESI mass spectrometry, and X-ray single crystal ana-
lysis. An Oak Ridge thermal ellipsoid plot (ORTEP) diagram
2-
of the 3 dianion is shown in the Supporting Information,
Figure S1. As pointed out above, the redox chemistry of com-
pounds (n-Bu N) [1] and (n-Bu N) [2] is dominated by reduc-
4
2
4
2
tion processes assigned to the sulfur-based reduction Mo S f
3
7
Figure 1. CID mass spectra of the 1 - (m/z = 496.6) dianion at increas-
ing Elab collision energies Elab = 10 eV (bottom), Elab = 15 eV (middle),
and Elab = 20 eV (top).
2
Mo S (formal reduction of the three disulfide-bridged ligands
3
4
10,32
to three sulfide-bridged ligands).
terminal atoms ([1] ) by the organic bdt ([2] ) results in a
Replacement of bromine
2-
2-
þ
-
36
produce [Mo S ] and [Mo S ] gas-phase ions. A com-
dramatic change in the electrochemical features, compound
3 7
3 7
parison of the identity of the ions generated using these
different ionization sources reveals that only ESI allows the
transfer of the intact transfer of dianions from the con-
densed to the gas-phase, in agreement with its inherent
softer ionization character. For example, the FAB mass
(
n-Bu N) [2] displaying two quasireversible oxidation waves
4 2
at easily accessible potentials E1/2=0.23 V (ΔE=70 meV) and
E1/2=0.41 V (ΔE=120 meV) which are assigned to dithiolene-
based oxidation processes. Compound (n-Bu N) [3] with a
Mo S also shows two oxidation waves (see Supporting In-
formation, Figure S2) at potentials E =0.47 V and E =0.65 V
versus Ag/AgCl (not observed in other Mo S clusters), thus
indicating a contribution from the dithiolene ligand to the
HOMO orbital of the Mo S cluster core. Cyclic voltammetry
experiments clearly indicate that redox chemistry of trinuclear
Mo3S7 (or Mo S )/dithiolene complexes is dominated by
4
2
3
4
2-
0
0
spectra of the dianionic [Mo S (catecholate) ] complex
3 7
3
show the presence of species of general formula [Mo S -
3 x
3
4
-
(
catecholate) ] (x=4-7, y=1-3) because of an electron-
y
detachment process together with partial release of sulfur
To examine the gas-phase fragmen-
tation reactions of dianions 1 , 2 , and 3 , the doubly
3
4
34,35
and outer ligands.
2-
2-
2-
3 4
charged species were mass selected and allowed to collide
with argon in the collision cell. CID mass spectra of the 1
dianion are shown in Figure 1. Fragmentation paths are
schematized in eqs 2 and 3.
ligand-based processes where the Mo atoms are essentially
spectators.
Gas-Phase Fragmentation Reactions of the 1 , 2 , and
2-
2
-
2-
2
-
3
Dianions. The ESI mass spectra of compounds (n-
Bu N) [1], (n-Bu N) [2], and (n-Bu N) [3] recorded un-
4
2
4
2
4
2
2
-
2 -
-
-
-
½
Mo3S7Br6ꢁ
ð1 Þ f ½Mo3S7Br5ꢁ ð1a Þ þ Br
der soft ionization conditions (that means low U cone
c
3
voltages), show the corresponding doubly charged 1
3
2-
,
ð2Þ
2
-
2-
2
, and 3 ions as base peaks. The ESI mass spectrum of
compound (n-Bu N) [1] also revealed the presence of the
4
2
-
-
-
-
-
½Mo3S7Br5ꢁ ð1a Þ f ½Mo3S5Br5ꢁ ð1b Þ þ S2 ð3Þ
The fragmentation channel depicted in eq 2 corresponds
singly charged [1 - Br] species (ca. 40% with respect to the
base peak) under our experimental conditions. The use of
mass spectrometric techniques for the characterization of
to the heterolytic cleavage of a Mo-Br bond to afford [Mo -
3
-
-
-
Mo S complexes was first reported by Hegetschweiler et al.
3
7
S Br ] (1a ) at m/z=912.1 and the Br anion. This disso-
7 5
using fast atom bombardment (FAB) and liquid secondary
ciation step was also anticipated from single-stage ESI mass
spectrum of compound (n-Bu N) [1] for which 1 and 1a
34,35
2-
-
ion mass spectrometry (L-SIMS) as ionization sources.
Laser vaporization techniques have also been used to
4
2
are the dominant species observed at relatively low ioniza-
tion conditions (typically U = 10 V). The 1a anion further
-
c
-
(
31) Dolg, M.; Stoll, H.; Preuss, H.; Pitzer, R. M. J. Phys. Chem. 1993, 97,
852.
32) Zimmermann, H.; Hegetschweiler, K.; Keller, T.; Gramlich, V.;
dissociates by loss of neutral S leading to the [Mo S Br ]
2
3 5
5
5
(
(36) (a) Lightstone, J. M.; Mann, H. A.; Wu, M.; Johnson, P. M.; White,
Schmalle, H. W.; Petter, W.; Schneider, W. Inorg. Chem. 1991, 30, 4336.
M. G. J. Phys. Chem. B 2003, 107, 10359. (b) Lightstone, J. M.; Patterson, M. J.;
White, M. G. Chem. Phys. Lett. 2005, 413, 429. (c) Gemming, S.; Tamuliene, J.;
Seifert, G.; Bertram, N.; Kim, Y. D.; Gantef €o r, G. Appl. Phys. A: Mater. Sci.
Process. 2006, 82, 161. (d) Patterson, M. J.; Lightstone, J. M.; White, M. G.
J. Phys. Chem. A 2008, 112, 12011.
(
(
33) Cech, N. B.; Enke, C. G. Mass Spectrom. Rev. 2001, 20, 362.
34) Hegetschweiler, K.; Keller, T.; Amrein, W.; Schneider, W. Inorg.
Chem. 1991, 30, 873.
35) Hegetschweiler, K.; Caravatti, P.; Fedin, V. P.; Sokolov, M. N. Helv.
Chim. Acta 1992, 75, 1659.
(

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8049
number of isomers can be envisioned depending on the S
atoms involved in the dissociations. For example, the
expulsion of diatomic S might come from rearrange-
2
ment processes within the Mo S cluster core to actually
3
7
evolve two equatorial, two axial or mixed equatorial-axial
sulfur atoms (the possibility of evolving the capping
μ -Satom isnot considered because ofits well-documented
3
3
7
inertness). To account for the type of sulfur evolved,
3
4
CID spectra of isotopically S labeled and Se analo-
gues have been investigated in combination with reliable
quantum chemical calculations as detailed in the next
section.
3
4
Regioselective S Isotopic Labeled Mo ( μ -S)(S ) -
3
3
ax 3
3
4
(
S ) and Se Homologues Mo ( μ -S)(S ) (Se ) . After
eq 3
2 34
eq 3
3
3
-
ax 3
2-
treating acetonitrile solutions of 1 and 2 with SPPh3
for one hour (see Experimental Section), the ESI mass
spectra reveal prominent doubly charged species centered
at m/z values slight shifted to higher values with respect to
2-
2-
the 1 and 2 dianions in agreement with the occurrence of
the S enrichment on the Mo S core at the equatorial posi-
tions. Figure 3 a) shows the ESI mass spectrum of the
reaction mixture of [Mo S Br ] and SPPh in the m/z =
34
3
7
23
2-
34
3
7
6
3
9
00 to 935 region.
Figure 2. CID mass spectra of the 22- (m/z = 466.7) dianion at increas-
ing Elab collision energies Elab = 10 eV (bottom), Elab = 20 eV (middle),
and Elab = 30 eV (top).
3
As can be inferred from Figure 3a, partial S enrich-
4
2-
ment at the equatorial sites of 1 has occurred as mani-
fested by the mass gain of about 4 Da (note that complete
S incorporation at the Seq would give rise to a cluster
2-
34
species at m/z = 848.2. CID mass spectra of the 2 dianion
are shown in Figure 2. Reactions 4, 5, and 6 depict its
fragmentation paths.
-
34
peak centered at m/z=918 for [1 - Br] while S incorpo-
ration at both equatorial and axial positions would afford
a peak centered at m/z=924). According to this, liberation
2
-
2 -
2 -
2 -
of a mixture of isotopically labeled S neutral molecules
2
½
Mo3S7ðbdtÞ3ꢁ
ð2 Þ f ½Mo3S5ðbdtÞ3ꢁ
ð2a Þ þ S2
34 32
is expected upon CID conditions where neutral S S
3
should predominate over S2 and S2. CID mass spectra
4
32
ð4Þ
(
6
Figure 3 b) of mass-selected 916 reveallossesof 64, 66, and
3
8 associated to S , S S, and S2, where liberation of
2
32 34
34
2
2
-
2 -
2 -
2 -
3
2 34
½
Mo3S5ðbdtÞ3ꢁ
ð2a Þ f ½Mo3S5ðbdtÞ2ꢁ ð2b Þ þ bdt
neutral S S is dominant, thus indicating that equatorial
sulfur atoms are released in the fragmentation reactions
depicted in eq 3 and 4. We also faced the study of chemical
analogues to further validate the preferred involvement of
the equatorial chalcogen atoms upon CID conditions.
ð5Þ
2
-
2 -
-
-
½
Mo3S5ðbdtÞ2ꢁ
ð2b Þ f ½Mo3S5ðbdtÞ2ꢁ ð2b Þ þ e -
ð6Þ
For the 2 dianion, the loss of S (eq 4) leads to the dia-
2-
2-
Hence, the [Mo S Se Br ] and [Mo S Se (bdt) ] spe-
3
3
4
3
6
3
4
3
cies, where the equatorial positions are occupied by sele-
nium, were also gas-phase generatedand subjectedto CID.
2-
2
2-
2-
2-
2-
CID spectra of [Mo
3
S
4
Se Br
3
6
]
and [Mo S Se (bdt) ]
3 4 3 3
nion of formula [Mo S (bdt) ] (2a ) at m/z = 434.8 in a
3
2-
2-
3
5
-
were identical to that of 1 and 2 (see Supporting
Information, Figure S3 for the [Mo S Se Br ] dianion)
similar way to that observed for the 1a anion (see eq 3), ex-
cept that lower E collision energies are required to liberate
2-
3
4
3
6
lab
2
-
-
except that diatomic Se molecules are exclusively released,
2
S2 from the 2 dianion in comparison with 1a . Increasing
the collision energy up to E =30 eV results in the liberation
thus supporting that equatorial chalcogen atoms are in-
variably evolved in the CID spectra of Mo Q (Q = S, Se)
3 7
lab
2-
of the neutral dithiete bdt (eq 5) to give the [Mo S (bdt) ]
3
specie at m/z=364.8 together with a minor fragmentation
channel that consists in a one-electron detachment step to
34,35
clusters.
5
2
Determination of the Elementary Steps along the Dis-
sociation Pathways Using Quantum Chemical Calcula-
tions. Full geometry optimizations using the B3LYP
approach and a 6-31G(d,p) basis set for S, Br, C, and H
atoms and Stuttgart pseudopotentials for Mo atoms have
been carried out for all species involved in the fragmenta-
tion processes depicted in eqs 1-6 described above. The
-
afford the [Mo S (bdt) ] anion at m/z = 727.6 (eq 6). The
5
3
2
2-
CID spectra of the 3 dianion recorded under identical
conditions only reveal a minor fragmentation channel at high
collision energies (typically Elab 60 eV), which consist in a
-
one-electron detachment process to afford the 3 anion.
-
2-
dissociation processes of S from 1a and 2 clusters are
2
2
-
2 -
-
-
½
Mo S ðbdtÞ ꢁ ð3 Þ f ½Mo S ðbdtÞ ꢁ ð3 Þ þ e -
3
4
3
3
4
3
ð7Þ
As far as the identity of the product ions formed
through processes depicted in eqs 1-6 is concerned, a
(
37) (a) Meienberger, M. D.; Hegetschweiler, K.; R u€ egger, H. Inorg.
Chim. Acta 1993, 213, 157. (b) Sokolov, M. N.; Abramov, P. A.; Gushchin, A. L.;
Kalinina, I. V.; Naumov, D. Y.; Virovets, A. V.; Peresypkina, E. V.; Vicent, C.;
Llusar, R.; Fedin, V. P. Inorg. Chem. 2005, 44, 8116.

8
050 Inorganic Chemistry, Vol. 49, No. 17, 2010
Llusar et al.
-
34
(top) and the observed peak for the S enriched [Mo
-
Figure 3. (a) Simulated isotopic pattern for [Mo
3 7 5
S Br ]
3 7 5
S Br ]
anion (bottom) generated by
2-
34
and SPPh
34
; (b) CID mass spectra of the S enriched [Mo S Br ] anion (m/z = 916.0) at Elab = 3 and 15 eV.
-
treatment of [Mo
3
7 6
S Br ]
3
3 7 5
discussed jointly for a better comparison while the loss of
2
-
the bdt molecule observed for the 2a cluster is analyzed
separately. These fragmentations (eq 3-5) cannot be
described by a single step, and reaction pathways leading
to a structure in which the observed fragments are ready
to be dissociated have to be elucidated. The stationary
points along these pathways are labeled by the corre-
-
2-
sponding parent compound, 1a (eq 3), 2 (eq 4), and
2
2
-
a
(eq 5), followed by a number (for minima) or by the
symbol TS (for the transition structures connecting mini-
ma), and the total charge of the structure.
Loss of Br from 1 . The heterolytic rupture of one
Figure 4. Geometrical representation of the DFT optimized structures
for the fragmentation process of 1 into 1a plus Br . Mo-Mo bonds
are not displayed for clarity.
2-
-
-
-
2-
2-
Mo-Br bond in the 1 dianion (eq 2) renders the unsatu-
-
2-
-
-
for the formation of S
. Therefore, the species 1b and 2a
2
rated anion [Mo S Br ] (1a ) whose structure is repre-
sented in Figure 4. Although there are two non-equivalent
3
7
5
are proposed by removal of two S (see Figure 5).
eq
-
Since the 1a product ion possesses C symmetry, there
s
bromine atoms (those located trans and cis to the μ Satom),
3
are two types of Seq atoms: (i) S7 and S8, attached to the
unsaturated Mo1, and (ii) S9 (see Figure 5 for number-
ing of involved atoms). Consequently, two possible reac-
-
the same 1a cluster is obtained upon full geometry opti-
mization irrespective of the removed bromine. The sum
-
-
of the energy of the dissociated fragments (1a and Br ) is
9
-
tion paths starting from the 1a intermediate should be
considered to account for the loss of S from equatorial
-
1
.6 kcal mol below the 1 dianion pointing out the strong
2-
2
tendency of bromine anion to be lost, as evidenced in the
ESI mass spectrum with the presence of significant abun-
dances (ca. 40%) of the 1a . Coulomb repulsion in this
sulfur atoms. Similar energy differences were obtained
when S7 and S8 or S7 and S9 were removed. The cluster
-
2
-
2
equivalent, and there is only one possible pathway leading
possess C3v symmetry; consequently all Seq atoms are
2-
doubly charged 1 species may also contribute to the ob-
3
8
2-
served charge splitting process. Removal of this bromine is
not associated to large variations in the main geometric
features within the Mo S cluster core as compared with
to structure 2a (see Supporting Information, Figure S4).
The DFT calculated fragmentation energies for eq 3 and
-
1
4 are 19.9 and 6.3 kcal mol , respectively. The ground
3
7
2-
2-
3
-
g 2
1
. The only significant geometric difference between 1
and 1a clusters is related to the relative disposition of the
state ( Σ ) for the S molecule has been considered. The
-
main geometrical parameters within the Mo3 cluster
core do not change significantly upon removal of the two
Seq atoms.
terminal bromine group attached to the unsaturated metal
site, which in 1a appears located trans to the μ -S capping
-
3
sulfur atom leading to a rare pseudooctahedral unsaturated
molybdenum site (see Figure 4).
The characterization of the fragmentation pathways eqs 3
and 4 are not straightforward, and a stepwise molecular
-
2-
Loss of S from 1a and 2 Clusters. There are three
2
mechanism allowing the formation of the S molecule inter-
2
-
2-
-
2-
different types of sulfur atoms in 1a and 2a clusters
capping, equatorial and axial), which could be released
acting weakly with the clusters 1b and 2a has to be deter-
-
2-
(
mined for 1a and 2 structures. Previous studies suggested
a concerted mechanism in which the reaction coordinate
is defined by opening of the angle Seq-Sax-Scapping for
upon fragmentation, opening a very large number of possible
fragmentation pathways to be explored. However, on the
3
4
35
basis of S isotopically labeling experiments detailed in the
previous section, only equatorial sulfur atoms are released
two Seq atoms moving simultaneously. All attempts to find
theoretically a concerted pathway were unsuccessful, and
a stepwise mechanism based on the sequential opening of
(
38) Schr o€ der, D. Angew. Chem., Int. Ed. 2004, 43, 1329.
the S -S -S
angles for two Seq atoms has been
eq
ax
capping

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8051
Figure 5. Geometrical representation of the DFT optimized structures for the fragmentation process of 1a- into 1b plus S
displayed for clarity.
-
2
. Mo-Mo bonds are not
-
2-
-
2-
Figure 6. Energetic profile calculated at the DFT level for the fragmentation mechanism from1a and 2 clusters to 1b þ S
2
(dashed line) and 2a þ S
2
-1
(
dotted line). Energies relative to the corresponding parent compound (in kcal mol ).
Figure 7. Geometrical representation of the DFT optimized structures for all stationary points along the fragmentation process of 1a- into 1a-4-.
Mo-Mo bonds are not displayed for clarity.
determined. Because of the large computational cost, the bdt
ligands have been replaced by the model S C H group. The
computed energetic profiles of the fragmentation reac-
tion evolving S for the 1a and 2 dianions are shown
2
in Figure 6 while the optimized geometries for all species
along the reaction paths are represented in Figures 7 and 8.
Thermochemical data for all species are reported in the
Supporting Information.
2
2
2
The ligands (Br and bdt) coordinated to the Mo
centers outside the Mo S cluster core do not partici-
pate in the mechanism, therefore both clusters (1a and
-
2-
3 7
-
2
-
2
) share the same elementary steps for the loss of two

8
052 Inorganic Chemistry, Vol. 49, No. 17, 2010
Llusar et al.
Figure 8. Geometricalrepresentationofthe DFToptimized structuresforallstationarypointsalong thefragmentation processof22- into 2-32-. Mo-Mo
bonds and dithiolene ligands on Mo atoms are not displayed for clarity.
Seq into S . The only difference is that the unsaturated
2
Comparison of the energetic profiles (see Figure 6)
for the transformation of equatorial sulfur atoms
into terminal ones reveals that the process for
-
Mo of cluster 1a allows the formation of an inter-
mediate which is not present in the reaction mechanism
2-
-
of cluster 2 . The angle S7-S5-S4 (see Figures 7 and 8)
at the starting structure equals 57.5° and 60.0°. The
process starts with the rotation of S7 atom around the
S5 atom, breaking both Mo1-S7 and Mo2-S7 bonds
via a transition state with an energy barrier of 48.4 and
1a cluster is more energetically demanding than for
2
2
-
cluster. This can be explained by the total charge of
the cluster; the excess of negative charge stabilizes the
formation of terminal sulfur atoms and facilitates the
process.
The following step is the bond formation between
terminal sulfur atoms (S7 and S8) and the concomitant
rupture of two S -S bonds. Because of the vacancy at
-
1
-
2-
1
9.8 kcal mol , 1a-TS0
and 2-TS
, being the
-1
0-1
angle S4-S5-S7 of 125.2° and 139.2° and character-
ized by one imaginary frequency of 387.2i and 178.7i
axial
eq
-
1
-
-
cm , respectively. The mode corresponding to the
imaginary frequencies can be described as to Mo1-S7
and Mo2-S7 bond stretching and also umbrella inver-
sion at the S5 atom.
Mo1 in the 1a cluster, a new intermediate (1a-3 ) appears
via a low energy barrier (1a-TS , 57.0 kcal mol ,
106.1i cm ) in which the S7 atom is bonded to Mo1 and
-
-1
2-3
-
1
-
1
-
S5, lying 44.6 kcal mol above 1a . In a second step,
a transition structure corresponding to the S7-S8 bond
-
2-
Intermediates 1a-1 and 2-1 are located 27.0 and
.7 kcal mol above the parent compound respectively,
-
1
-
-1
3
and S7 is now a terminal atom with considerable partial
formation is found, 1a-TS
at 45.6 kcal mol , being
characterized by one imaginary frequency 115.5i cm and
3-4
-
1
-
2-
2-
˚
charges of -0.186e (1a-1 ) and -0.335e (2-1 ). The se-
a S7-S8 distance of 2.777 A. For the 2
cluster,
2-
cond S atom follows a similar process by breaking the
Mo2-S8 and Mo3-S8 bonds via 1a-TS
the formation of the S7-S8 bond from 2-2 occurs via
eq
-
2-
and 2-
located 70.5 and 31.9 kcal mol above the res-
a single step, the 2-TS
2-3
structure which is found
1
-2
2
-
-1
-1
TS
at 38.6 kcal mol for a S7-S8 intranuclear distance
of 2.679 A, displaying one imaginary frequency of
1
-2
˚
pective parent compounds and presenting similar geo-
metrical and vibrational features to the previous transi-
tion structures. An intermediate, labeled 1a-2 and 2-
-
1
-246.3i cm associated mainly to the S7-S8 bond stretch-
ing. The resulting structures, 1a-4 and 2-3 correspond to
-
-
2-
2
-
-
2-
2
, is found with relative energies of 54.8 and 27.6 kcal
the 1b and 2a clusters, respectively, forming intramole-
cular adducts with the S molecule which are dissociated.
-
1
mol , respectively. This structure presents two terminal
sulfur atoms (S7 and S8) carrying partial negative
charges of -0.147e (S7) and -0.147e (S8) for cluster
2
-
2-
Formation of the 1b and 2a species represents a formal
reduction of two bridging disulfide ligands through an in-
ternal two-electron redox process to give two sulfide ligands
-
2-
1
a-2 and -0.303 (S7) and -0.298 (S8) for cluster 2-2 .

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8053
and a molecule of S according to eqs 8 and 9:
2
-
-
½
Mo3ðμ -SÞðμ-S2Þðμ-S2Þ Br5ꢁ ð1a Þ
3
2
-
-
f ½Mo ðμ -SÞðμ-S Þðμ-SÞ Br ꢁ ð1b Þ þ S
ð8Þ
3
3
2
2
5
2
2
-
2 -
overall reaction
2S2
f 2S þ S2
2
-
2 -
½
Mo3ðμ -SÞðμ-S2Þðμ-S2Þ ðbdtÞ ꢁ ð2
Þ
3
2
3
2
-
2 -
f ½Mo3ðμ -SÞðμ-S2Þðμ-SÞ ðbdtÞ ꢁ ð2a Þ þ S2
3
2
3
ð9Þ
Figure 9. Geometrical representation of the DFT optimized structures
2-
2-
2-
2
-
2 -
for the fragmentation process of 2a into 2b plus bdt or into 2c plus
bdt. Mo-Mo bonds are not displayed for clarity.
overall reaction
2S2
f 2S þ S2
For comparative purposes, we also investigate theore-
tically the abstraction of S from two equatorial positions
2
with respect to a hypothetical disulfide S -S abstrac-
ax
eq
tion (see Supporting Information, Figure S5). The former
process is energetically favored to give the product 1b
-
-
1
lying only 19.9 kcal mol higher in energy than 1a ,
-
whereas the loss of one bridging disulfide yields the cluster
1
-
-1
-
c which is 62.2 kcal mol above 1a . Hence, the pro-
posed fragmentation channel agrees with the isotopic
labeling experiments as well as the selective chalcogen
exchange experiments described above where the equa-
torial-chalcogen (Q ) atoms are more prone to be re-
eq
-
leased. On the basis of the molecular structure of the 1c
anions, its formation would correspond to the internally
induced disulfide to S oxidation and the concomitant
2
metal reduction. Analogous reactions triggered by exter-
nal oxidants are ubiquitous in mono- and dinuclear group
Figure 10. Energetic profile calculated at DFT level for the fragmenta-
2-
2-
2 2 2
tion mechanism from 2a cluster to 2b plus S C H . Energies relative
5
,6,39
-1
6
been observed for their trinuclear counterparts.
chalcogenide chemistry,
although they have not
to the corresponding parent compound (in kcal mol ).
2
-
2-
explain how the disulfide ligand assists the departure of
the bdt ligand, a low energy possible fragmentation path-
way from 2a to 2b has been theoretically determined
Loss of bdt from 2a Cluster. The cluster 2a presents
two non-equivalent molybdenum atoms, those bridged
by the disulfide ligand or that located far from disulfide-
bridged ligand. Consequently, two different dithiolene
ligands are prone to be dissociated. Fragmentation of bdt
without participation of the disulfide bridge would repre-
2-
2-
(Figure 10).
The process taking place can be related to the proposed
redox isomerism where a trithiolene structure has been
4,7,40
2
-
postulated on the basis of experimental findings. The
corresponding trithiolene specie is 2.8 kcal mol more
stable than the starting cluster 2a . Therefore, a stepwise
sent an internally induced bdt to bdt oxidation and the
concomitant metal reduction. However, theoretical cal-
culations on the possible species upon release of the bdt
-1
2-
2
-
reaction mechanism is proposed and determined theore-
tically (see Figure 11 for atom numbering) considering the
trithiolene intermediate. The basic steps can be described
as follows: (i) migration of the equatorial sulfur atom
(S10) taking part of a bidentate disulfide ligand to a
metal-dithiolene bond (Mo2-S11), (ii) insertion of S10
into the Mo2-S11 bond leading to the trithiolene inter-
mediate, and (iii) breaking of the S10-S11 and Mo2-S12
bonds and formation of a terminal sulfide ligand accom-
panied by release of a neutral dithiolene ligand. This
ligand, 2b (that evolving the bdt ligand close to the
2
-
disulfide bridge) and 2c (that evolving the bdt ligand far
from the disulfide bridge), reveal very different stability
and participation of the disulfide ligand. Hence, the
2
-
2-
fragmentation reaction of species 2a to yield 2b plus
bdt (32.2 kcal mol ) involves chemical transformation of
-
1
2
-
a S2 ligand to one sulfide and one terminal sulfur ligand
see Figure 9) and is thermodynamically favored over that
fragmentation reaction affording 2c in which the disul-
(
2-
-
1
fide ligand remains intact (102.8 kcal mol ). Experimental
evidence supporting that 2b is the resulting specie of the
2-
mechanism can be regarded as a formal two-electron reduc-
ligand to give one bridged and one
2-
2-
tion of one bridging S
terminal sulfide (S ) ligands and the concomitant oxida-
tion of the dithiolene ligand, thus corresponding to the
2
fragmentation of 2a comes from the absence of fragmen-
tation of 3 because of the lack of equatorial S atoms
which facilitate somehow the release of bdt. Therefore, to
2-
2-
(40) Pilato, R. S.; Eriksen, K. A.; Greaney, M. A.; Stiefel, E. A.;
(
39) Sugimoto, H.; Tajima, R.; Sakurai, T.; Ohi, H.; Miyake, H.; Itoh, S.;
Goswami, S.; Kilpatrick, L.; Spiro, T. G.; Taylor, E. C.; Rheingold, A. L.
Tsukube, H. Angew. Chem., Int. Ed. 2006, 45, 3520.
J. Am. Chem. Soc. 1991, 113, 9372.

8
054 Inorganic Chemistry, Vol. 49, No. 17, 2010
Llusar et al.
2-
2-
Figure 11. Geometrical representation of the DFT optimized structures for all stationary points along the fragmentation process of 2a into 2a-4
Mo-Mo bonds and dithiolene ligands on Mo1 and Mo3 atoms are not displayed for clarity.
.
following ligand-based redox reaction 10.
group and the cluster core, explaining the lack of the frag-
mentation processes observed for the 3 cluster accord-
2
-
2
-
2 -
ing to eq 11. Hence, reaction 11 is strongly endothermic
(
of an equatorial sulfur atom, the fragmentation of the
oxidized bdt ligand is unaffordable (see Supporting In-
formation, Figure S6).
½
Mo ðμ -SÞðμ-S Þðμ-SÞ ðbdtÞ ðbdtÞꢁ ð2a
Þ
3
3
2
2
2
-
1
122.5 kcal mol ) indicating that without the assistance
2
-
2 -
f ½Mo3ðμ -SÞðμ-SÞðSÞðμ-SÞ ðbdtÞ ꢁ ð2b Þ þ bdt
3
2
2
ð10Þ
2
-
2 -
2
-
2 -
2 -
½
Mo3ðμ -SÞðμ-SÞ ðbdtÞ ꢁ ð3
Þ
overall reaction
bdt þ S2
At the 2a structure the equatorial sulfur (S10) takes part
f bdt þ 2S
3
3
3
2
-
2 -
2-
f ½Mo ðμ -SÞðμ-SÞ ðbdtÞ ꢁ ð3a Þ þ bdt ð11Þ
3
3
3
2
˚
in a disulfide bridge (distance S10-S9 of 2.103 A) between
Mo2 and Mo3 atoms (distances S10-Mo2 and S10-Mo3
of 2.582 A), and it is located far from the dithiolene ligand
Conclusions
˚
˚
The combined use of ESI, ESI tandem mass spectrometry,
and DFT calculations provides detailed mechanistic insights
into the gas-phase generation and fragmentation reaction
studies of the 1 , 2 , and 3 dianions. In general, the domi-
nant fragmentation paths involve liberation of neutral mole-
attached to Mo2 (S10-S11 distance 3.943 A). However, the
dithiolene is not totally rigid, and it can be rotated to a mode-
rate extent. A transition structure is found for the migration
of the S10 atom from the equatorial position to the S11 atom
2-
2-
2-
-
1
with an energy barrier of 25.3 kcal mol and one imaginary
frequency of 217.0i cm . The geometry of the 2a-TS
-
1
2-
-1
cules (S
or bdt) associated to two-electron redox processes.
2
0
reveals that the S10 is moving toward S11 (S9-S10 distance
Because of the intrinsic electrochemical and structural proper-
2-
2-
2-
˚
˚
of 2.500 A and S10-S11 distance of 2.386 A), while the
ties of the 1 , 2 , and 3 dianions, these two-electron pro-
2-
2-
˚
distance S10-Mo2 has been shortened to 2.364 A. The
intermediate 2a-1 is found 2.1 kcal mol above 2a ,
and the Mo2 atom is coordinated to a disulfide group formed
cesses are based on the sulfur ligands, thus making 1 , 2 ,
2-
-1
2-
2-
and 3 dianions prototypical models to investigate ligand-
based redox transformations. Ligand-based redox chemistry
has been rarely observed at the active site of a molybdenum
2-
-1
by S10 and S11. The 2a-TS
7
structure (9.1 kcal mol ,
5.6i cm ) shifts the coordination mode of Mo2 from the
disulfide group to the recently inserted S10 atom, leading to
1-2
-
1
41
enzime. Although the data presented here have been gathered
from gas-phase experiments, the results might reveal general
trends that should be applicable for either enhancing or deter-
ring the appearance of redox reactions in solution as follows:
2-
-1
the trithiolene structure 2a-2 situated at -2.8 kcal mol .
The departure of the dithiolene ligand is obtained by stepwise
breaking of the S10-S11 and Mo2-S12 bonds. The 2a-
(i) We have observed that the elimination of neutral S
2
2
-3
-
-1
-1
2-
2-
TS
(20.3 kcal mol , 149.4i cm ) presents a S10-S11
from 1 and 2 dianions proceeds through simul-
taneous abstraction of equatorial sulfur atoms, even
though the net charge and the identity of the outer
ligands in the 1a and 2 dianions are different. In
addition, they both involve ligand-based redox reac-
tions sharing identical reaction mechanisms (see the
energetic profiles in Figure 6). Identical gas-phase
2
2-
-1
˚
distance of 3.093 A and leads to 2a-3 (1.6 kcal mol ). The
second step is the decoordination of S12 from Mo2 which
2
-4
-
-1
-
2-
occurs via the 2a-TS
structure (23.5 kcal mol , 31.6i
cm ) leading to the formation of a charge-transfer complex,
3
-
1
2-
-1
2a-4 , at 18.0 kcal mol previous to the dissociation into
the 2b cluster and the dithiolene ligand.
2-
2
Loss of bdt from 3 Cluster. Although the considered
-
fragmentation has been experimentally observed for
þ
path involving the expulsion of neutral bdt is not experi-
the cationic [Mo
3
Q
7
(dtc) ] (Q=S, Se; dtc=dithio-
3
2
-
35
carbamate) complex, thus suggesting an inherent
mentally observed for the 3 dianion, it is also included
with the purpose of providing a deeper understanding of
the fragmentation process. Theoretical calculations allow
calculating the bond interaction energy between the bdt
(41) George, G. N.; Costa, C.; Moura, J. J. G.; Moura, I. J. Am. Chem.
Soc. 1999, 121, 2625.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8055
loss of equatorial diatomic S characteristic for the
2
protonated by adventious water in the reaction
media.
6
Mo S clusters in the gas-phase. Analogous selective
7
3
abstraction is observed for Mo S complexes in the
3
7
condensed phase using reducing agents such as
cyanide, H PO , or phosphines.
2 -
2 -
½
MoS4ꢁ þ 3ðdithieteÞ f ½MoðdithiolateÞ ꢁ
19,42
3
3
2
þ 43 3 S3 3
ð12Þ
(
ii)
A parallelism between the gas-phase behavior
and the electrochemical properties of dithiolene-
2-
2-
containing 2 and 3 complexes is observed. As
can be inferred from CID experiments, electron
detachment process is a common fragmentation
2
-
2 -
½
MoS2O2ꢁ
þ 2ðdithieteÞ f ½MoðOÞðdithiolateÞ2ꢁ
2
-
2-
path to 2
corresponds to the oxidation of the cluster core
eqs 6 and 7)). This behavior is not observed for
and 3 dianions (that formally
þ 3 3 S3 3 þ 3 3 O3 3
ð13Þ
(
2-
The overall reactions 12 and 13 are complex since
they involve simultaneous metal and dithiete re-
duction along with sulfur oxidation, and the
identification of reaction intermediates remains
unknown. However, it can be reasonably assumed
that reactions 12 and 13 are triggered by initial
dithiete coordination to the Mo site containing
the ModS functional group. If one adopts the
the Br-containing 1 dianion in which charge
reduction proceeds via heterolytic Mo-Br cleav-
age. This experimental evidence correlates with the
electrochemical behavior described above for these
Mo S and Mo S complexes, clearly indicating
3
7
3 4
that dithiolene ligands are crucial to provide oxi-
dation activity in the Mo S and Mo S clusters
3
7
3 4
while Mo centers do not participate in the electron
transfer process.
2-
2-
2-
generalized reaction bdt þ S
f bdt þ2S
2
2-
(
eq 10) described in detail above, we hypothesize
(
iii) The importance of disulfide ligands (S2 ) as elec-
tron reservoirs to trigger intracomplex redox reac-
tions is also manifested by two distinctive fragmen-
that the first step of the reaction showed in
reactions 12 and 13 corresponds to the oxidation
of two sulfide ligands accompanied by the reduc-
tion and coordination of the dithiolene ligand.
-
tations, expulsion of S , observed for [Mo S Br ]
3 7
2
5
-
2-
2-
( (2 ) clusters, and
dissociation of a neutral bdt ligand from a Mo
atom in [Mo S (bdt) ] (2a ) which requires the
3
1a ) and [Mo S (bdt) ]
3
7
3
2-
2-
Acknowledgment. The financial support of the Spanish
Ministerio de Ciencia e Innovaci oꢀ n (Grants CTQ2008-
02670 and CTQ2009-14629-C02-02), Fundaci oꢀ Bancaixa-
UJI (research project P1.1B2007-12), Generalitat
Valenciana (ACOMP/2010/276 and Prometeo/2009/053)
is gratefully acknowledged. Prof. V. Fedin is acknowl-
5
3
assistance of a neighboring disulfide ligand. Our
results suggest the plausible involvement of trithio-
lene ligands as intermediates in the release of the
dithiete bdt molecule.
(
iv) We have observed that the presence of disulfide
2-
34
ligands in 2 is crucial to promote bdt dissocia-
tion leading to the [Mo S (bdt) ] species with
edged for providing us the S-enriched sample of ele-
2-
mental sulfur. The authors also thank the Servei Central
d’Instrumentaci oꢀ Cientifica (SCIC) of the Universitat
Jaume I for providing us with the mass spectrometry and
X-ray facilities. The authors also are grateful to the Servei
d’Informatica, Universitat Jaume I for generous allotment
of computer time.
3
5
2
terminal ModS groups. This process closely re-
sembles the reverse step of the induced internal
redox reaction between the tetrathiometalate
2
-
2-
MoS4 or MoS O
1
with bis(trifluoromethyl)-
,2-dithiete in which, remarkably, the starting
materials are the product ions observed in the
2
2
2-
Supporting Information Available: Complete reference for
Gaussian 03; Cyclic voltammogram and ORTEP representation
of the 3 dianion, CID spectra of the [Mo dianion;
geometrical representation of the DFT optimized structures for
the fragmentation process of 2 to 2a plus S
plus S , geometrical representation of the DFT optimized
2
structures for the fragmentation process of 3 into 3a plus
bdt and thermochemical data calculated at B3LYP level for all
species. Crystallographic data (excluding structure factors) for
the structures reported in this paper. This material is available
free of charge via the Internet at http://pubs.acs.org.
CID spectra of 2 . Equation 12 is quantitative
and clean and affords the mononuclear dithiolene
complexes [Mo(1,2-dithiolene) ] together with
3
elemental sulfur (denoted as 4 S ); however, the
yield for eq 13 to afford compound [Mo(O)-
2-
2-
2-
3
S Se
3
Br
3
]
6
6
3
3
3 3
2-
2-
-
-
2
, 1a into 1c
2-
2-
2-
(dithiolate) ]
as S ), is low and presumably the remaining
plus elemental sulfur (depicted
2
3
3
3 3
3
oxo ligand (depicted as O ) is hydrolyzed or
3
3 3
(
42) Fedin, V. P.; Sykes, A. G. Inorg. Synth. 2001, 33, 162.