Aggregation of PMe3-stabilized molybdenum sulfides and the catalytic dehydrogenation of H2S

Article abstract of DOI:10.1021/ic026215m

The reactivity of [MoS₄]^2- (1) toward PMe₃ was explored in the presence and absence of proton donors. Whereas MeCN solutions of (Et₄N)₂[MOS₄] and PMe₃ are stable, in the presence of H₂S such solutions catalyze formation of H₂ and SPMe₃. Addition of NH₄⁺ to such solutions afforded MoS₂(PMe₃)₄ (2), which can be prepared directly from (NH₄)₂[1]. Compound 2 is reactive toward thiols via a process proposed to involve the initial dissociation of one PMe₃ ligand, a hypothesis supported by the relative inertness of trans-MoS₂(dmpe)₂. Benzene solutions of 2 react with EtSH to give Mo₂(μ-S)(μ-SH)(PMe₃)₄(SEt)₃ (3Et). Analogous reactions with thiocresol (MeC₆H₄SH) and H₂S gave Mo₂(μ-S)(μ-SH)(PMe₃)₄(SR)₃ (R = tol, H). Crystallographic analyses of 3Et, 3H, and 3tol indicate dinuclear species with seven terminal ligands and a Mo₂(μ-SR)(μ-S) core (r_Mo-Mo = 2.748(1) A). From reaction mixtures leading to 3Et from 2, we obtained the intermediate Mo₂^IV(μ-S)₂(SEt)₄ (PMe₃)₂ (4), an edge-shared bis(trigonal pyramidal) structure. Compounds 3H and 3Et react further with H₂S to give Mo₄(μ₂-S)₄(μ₃-S) ₂(PMe₃)₆(SH)₂ (5H) and Mo₄(μ₂-S)₄(μ₃-S) ₂(PMe₃)₆(SEt)₂ (5Et), respectively. Analogously, W₄(μ₂-S)₄(μ₃-S) ₂(PMe₃)₆(SH)₂ was synthesized from a methanol solution of (NH₄)₂WS₄ with H₂S and PMe₃. A highly accurate crystallographic analysis of (NH₄)₂MoS₄ (R₁ = 0.0193) indicates several weak NH···S interactions.

Full text of DOI:10.1021/ic026215m

Inorg. Chem. 2003, 42, 2410−2417
Aggregation of PMe3-Stabilized Molybdenum Sulfides and the Catalytic
Dehydrogenation of H S
2
Daniel E. Schwarz, Thomas B. Rauchfuss,* and Scott R. Wilson
Department of Chemistry, UniVersity of Illinois, Urbana, Illinois 61801
Received November 26, 2002
The reactivity of [MoS4]2- (1) toward PMe3 was explored in the presence and absence of proton donors. Whereas
MeCN solutions of (Et4N)2[MoS4] and PMe3 are stable, in the presence of H S such solutions catalyze formation
2
of H and SPMe3. Addition of NH + to such solutions afforded MoS2(PMe3)4 (2), which can be prepared directly
2
4
from (NH ) [1]. Compound 2 is reactive toward thiols via a process proposed to involve the initial dissociation of
4 2
one PMe3 ligand, a hypothesis supported by the relative inertness of trans-MoS2(dmpe)2. Benzene solutions of 2
react with EtSH to give Mo2(µ-S)(µ-SH)(PMe3)4(SEt)3 (3Et). Analogous reactions with thiocresol (MeC6H SH) and
4
H S gave Mo2(µ-S)(µ-SH)(PMe3)4(SR)3 (R ) tol, H). Crystallographic analyses of 3Et, 3H, and 3tol indicate dinuclear
2
species with seven terminal ligands and a Mo2(µ-SR)(µ-S) core (rMo-Mo ) 2.748(1) Å). From reaction mixtures
leading to 3Et from 2, we obtained the intermediate MoIV (µ-S)2(SEt)4(PMe3)2 (4), an edge-shared bis(trigonal
2
pyramidal) structure. Compounds 3H and 3Et react further with H S to give Mo4(µ2-S)4(µ3-S)2(PMe3)6(SH)2 (5H)
2
and Mo4(µ2-S)4(µ3-S)2(PMe3)6(SEt)2 (5Et), respectively. Analogously, W (µ2-S)4(µ3-S)2(PMe3)6(SH)2 was synthe-
4
sized from a methanol solution of (NH ) WS4 with H S and PMe3. A highly accurate crystallographic analysis of
4 2
2
(NH ) MoS4 (R1 ) 0.0193) indicates several weak NH‚‚‚S interactions.
4 2
Introduction
A novel feature of MoS2L4 (L ) 2e ligand) is the presence
of the rarely observed “pure” MdS double bond; in most
terminal sulfido complexes the M-S bond order is clearly
above two.8 The pure ModS functionality should exhibit
distinctive behavior relative to derivatives where the MoS
bonds order is >2. Thus, in this study our opening basic
question focused on whether MoS2(PMe3)4 is reactive at all
or is simply a thermodynamic sink. We find that it is indeed
reactive with regard to exchange of the phosphine ligands
and reactions at the sulfido center. A second issue of interest
was the possibility that MoS2(PMe3)4 could serve as a
monomer for the synthesis of larger Mo-S aggregates.
Prior work on the Mox-S-PR3 system is presented in
Scheme 1 for 1 e x e 4 together with a summary of the
preparative results in this paper and the numbering scheme
used for the compounds. Aside from MoS2(PMe3)4, other
Mo-S-PR3 species include tri-, tetra-, and hexametallic
clusters, and dodecametallic clusters have been prepared
primarily by Saito’s group. These include Mo3S5(PR3)6,9
Mo4S6(SH)2(PMe3)6,10Mo6S10(SH)2(PEt3)6,10 and the Chevrel-
like cluster Mo6S8(PR3)6 and its dimer Mo12S16(PR3)10.11,12
Molybdenum disulfide, MoS2, is important in materials
science and catalysis.1-3 In recent years, Lewis base adducts
of monomeric MoS2 have been described, but the reactivity
of these species remains largely unexplored.4 In contrast, the
behavior of cyclopentadienyl derivatives of molybdenum
disulfide has been very fruitfully examined resulting in the
discovery of several novel catalytic processes.5 The species
MoS2L4 (L ) PR3, SR2), which are formal adducts of
monomeric MoS2,6,7 are of interest because they could in
principle oligomerize to afford polymetallic aggregates,
which could exhibit new structural or catalytic properties.
* To whom correspondence should be addressed. E-mail: rauchfuz@
uiuc.edu.
(1) Daage, M.; Chianelli, R. R. J. Catal. 1994, 149, 414-427.
(2) Startsev, A. N. Catal. ReV.sSci. Eng. 1995, 37, 353-423.
(3) Byskov, L. S.; Norskov, J. K.; Clausen, B. S.; Topsoe, H. NATO ASI
Ser., Ser. 3 1998, 60, 155-168.
(4) Yoshida, T.; Adachi, T.; Matsumura, K.; Baba, K. Angew. Chem. 1993,
105, 1730-1732; Angew. Chem., Int. Ed. Engl. 1993, 1732 (1711),
1621-1733.
(5) Rakowski DuBois, M. In Catalysis by Di- and Polynuclear Metal
Cluster Complexes; Adams, R. D., Cotton, F. A., Eds.; Wiley-VCH:
New York, 1998; pp 127-143.
(8) Coucouvanis, D. AdV. Inorg. Chem. 1998, 45, 1-73.
(9) Tsuge, K.; Imoto, H.; Saito, T. Inorg. Chem. 1995, 34, 3404-
3409.
(6) Cotton, F. A.; Schmid, G. Inorg. Chem. 1997, 36, 2267-2278.
(7) Parkin, G. Prog. Inorg. Chem. 1998, 47, 1-166.
2410 Inorganic Chemistry, Vol. 42, No. 7, 2003
10.1021/ic026215m CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/07/2003

PMe3-Stabilized Molybdenum Sulfides
(Et4N)2MoS4
Scheme 1
PMe3 + H2S
8 SPMe3 + H2
(1)
The concentration of 1 remains unaffected by the catalytic
conversion. After 150 min at room temperature, 25.3
turnovers were observed. Solutions of (Et4N)2[1] also slowly
catalyze the desulfurization of H2S by PPh3 to give SPPh3
and H2, with the addition of heat. At 65 °C, we observed
approximately 25 turnovers in 72 h.
When MeCN solutions of (Et4N)2[1] were treated with
+
both PMe3 and NH4 , we obtained MoS2(PMe3)4 (2). This
transformation is more easily accomplished starting with
(NH4)2[1], which is a routinely available salt.14 The reaction
is initially heterogeneous but is complete within 1 h to give
the grass green product. A similar reaction of (NH4)2[WS4],
H2S, and PMe3 in MeCN gave purple WS2(PMe3)4.15
Compound 2 and its W analogue were originally pre-
pared by Parkin by the addition of H2S to M(PMe3)6 (M )
Mo, W).
Properties of MoS2(PMe3)4. The reactivity of 2 toward
H2S and thiols, the primary focus of this report, is proposed
to involve the initial dissociation of one PMe3 ligand from
1
2. Supporting this hypothesis, the H NMR spectrum of a
freshly prepared toluene solution of 2 exhibits a signal for
free PMe3 (δ 0.8) coincident with a color change from green
to brown. The ratio of free to coordinated PMe3 is 1:3. The
brown solutions remain homogeneous for hours, although
1
the H and 31P NMR spectra become increasingly complex
indicative of cluster formation. An (-)ESI mass spectrum
of the brown solution showed many signals. The most
prominent at 906 m/z corresponds approximately to Mo4S7-
(PMe3)4; however, an exact fit was not achieved. At -90
°C, green solutions of 2 are stable, exhibiting a singlet at δ
1.5 corresponding to equivalent, coordinated PMe3 ligands.
In contrast to the behavior of 2, trans-MoS2(dmpe)2
(derived from treatment of 2 with two equiv dmpe) retains
its green color in solution even at room temperature due to
the chelating nature of the diphosphine (the corresponding
dppe derivative is similar).6 trans-MoS2(dmpe)2 does not
react with organic thiols, in contrast to the behavior of 2
(see later).
Results
Reactions of PMe3 with [MoS4]2-. At the outset we
sought to develop a new synthesis of MoS2(PMe3)4, the prior
one from “Mo(PMe3)6” requiring more specialized reagents.13
To this end, we explored the reactivity of PMe3 toward
[MoS4]2- (1) in the presence and absence of proton donors
that could neutralize the negative charge on the thiometalate.
Anhydrous MeCN solutions of (Et4N)2[MoS4] containing
excess PMe3 are stable at room temperature, as confirmed
Reaction of MoS2(PMe3)4 with Thiols. Benzene solutions
of 2 react with EtSH at room temperature over the course
of hours to give Mo2(µ-S)(µ-SH)(PMe3)4(SEt)3 (3Et). The
process can be rationalized according to eq 2.
MoS2(PMe )
3 4 + 3RSH f
(2)
Mo2(S)(SH)(SR)3(PMe3)4 + H2S + SPMe3 + 3PMe3 (2)
1
by UV-vis and H NMR measurements. Addition of H2S
(3H; R ) H)
to such solutions results in the catalytic formation of H2 and
SPMe3 (eq 1). Experiments were typically performed in
sealed NMR tubes and analyzed by 1H NMR spectroscopy.
In the absence of 1, benzene solutions of PMe3 and H2S are
stable.
(3Et; R ) Et)
(3tol; R ) p-MeC6H4)
1
The H NMR spectrum of 3Et displays separate signals
for the four nonequivalent PMe3 ligands. The mutually
diaxial PMe3 ligands are nonequivalent due to the pyramidal
(10) Tsuge, K.; Imoto, H.; Saito, T. Inorg. Chem. 1992, 31, 4715-
4716.
(11) Saito, T. AdV. Inorg. Chem. 1997, 44, 45-91.
(12) Amari, S.; Imoto, H.; Saito, T. Chem. Lett. 1997, 967-968.
(13) Murphy, V. J.; Parkin, G. J. Am. Chem. Soc. 1995, 117, 3522-3528.
(14) McDonald, J. W.; Friesen, G. D.; Rosenhein, L. D.; Newton, W. E.
Inorg. Chim. Acta 1983, 72, 205-210.
(15) Rabinovich, D.; Parkin, G. Inorg. Chem. 1995, 34, 6341-6361.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2411

Schwarz et al.
Figure 1. 1H NMR (C6D6, 500 MHz, 298 K) spectrum of the SH signals
for 3H with assignments based on selectively decoupled 1H{31P} measure-
ments. The assignments of Ha and Hb are ambiguous.
Figure 2. Molecular structure and atom numbering scheme of Mo2S(SH)-
(PMe3)4(SEt)3 (3Et).
nature of the µ-SH group, which lowers the symmetry of
the complex to C1. The SH signal appears as a complex
multiplet at δ 5.20, which is downfield relative to the
chemical shifts of related complexes with terminal MoSH
ligands (vide infra). The 31P NMR spectrum of 3Et features
three signals including an AB quartet for the diaxial
phosphine ligands.
Reaction of 2 with thiocresol (p-MeC6H4SH) gave
Mo2(µ-S)(µ-SH)(PMe3)4(Stol)3 (3tol), which is completely
analogous to 3Et, as indicated by NMR and X-ray crystal-
lographic analyses.
2.748(1) Å. The central Mo2S2 core is planar, with the sum
of the bond angles being 360°. The Mo-µ-S distances are
2.250(1) and 2.295(1) Å, much shorter than the correspond-
ing Mo-(µ-SH) distances of 2.418(1) and 2.428(1) Å. The
SH, which was located crystallographically, is pyramidal with
the sum of the ∑( Mo-S-X) ) 285° (X ) Mo, H). The
structure of 3Et is analogous to that of Mo2(µ-S)(µ-Cl)-
(PMe3)4Cl3.17-19
The structures of 3H and 3tol (not shown) are analogous
to 3Et. The protons of the four SH groups in 3H were
identified crystallographically. The M-M distance of
2.7081(6) Å for 3H is slightly shorter than the one measured
in 3Et, probably as a consequence of the smaller size of SH
versus SEt.
Solutions of 2 react with H2S in a manner analogous to
the reaction involving organic thiols to give emerald green
Mo2(µ-S)(µ-SH)(SH)3(PMe3)4 (3H). The efficiency of the
reaction is greatly enhanced when performed in one pot by
treatment of (NH4)2[1] with excess of both PMe3 and H2S.
The role of the excess PMe3 is not known, but purified
samples of 2 react more slowly with H2S than with organic
thiols. Compound 3H, being an unusual example of a
complex with several inequivalent SH ligands, exhibits a
Mo2(µ-S)2(SR)4(PMe3)2, an Intermediate in the Forma-
tion of Mo2(µ-S)(µ-SH)(PMe3)4(SR)3 from MoS2(PMe3)4.
From some reaction mixtures affording 3Et from 2, we
detected the intermediate Mo2(µ-S)2(SEt)4(PMe3)2 (4Et). We
did not observe 4tol or 4H in the syntheses of 3tol and 3H.
A small amount of 4Et was crystallized and characterized
1
structurally informative H NMR spectrum (Figure 1). SH
1
by H and 31P NMR spectroscopy as well as by X-ray
signals were assigned via a series of selective1H{31P} spectra.
crystallography (Figure 3). The crystallographic study con-
firmed the edge-shared bis(trigonal pyramidal) structure ana-
logous to that previously observed in Mo2(µ-S)2(SBu-t)4-
(PMePh2)2.20 The CH2 signals in the 1H NMR spectrum are
nonequivalent, indicating that the complex is stereochemi-
cally rigid.
On a preparative scale, treatment of 4Et with further
equivalents of EtSH and PMe3 gave 3Et, consistent with the
fact that this [Mo(IV)]2 species is an intermediate in the
conversion of 2 into 3Et (eq 3).
1
Only P-SH coupling was observed in the H NMR spec-
trum. The PMe3 signals were all doublets. The signals as-
signed to µ-SH in 3Et, 3tol, and 3H were similar in terms
of chemical shift and coupling pattern. 31P NMR signals were
assigned using 31P-31P correlation measurements (COSY).
Crystallographic analysis of 3Et (Figure 2) reveals a
dinuclear species with seven terminal ligands and two
bridging sulfur atoms. The idealized coordination geometry
at Mo(1) is trigonal bipyramidal, with P(1) and S(4)
occupying the axial positions, P(1)-Mo(1)-S(4) )
171.04(5)°. The trigonal plane is defined by S(1), S(2), and
S(3), with ∑ S-Mo-S ) 257°. The coordination geometry
at Mo(2) can be described as distorted octahedral. With an
P(2)ax-Mo(2)-P(3)ax of 166.79(5)°, the axial phosphine
ligands are bent away from the crowded binuclear core.
Overall, 3Et is an unusual example of an octahedron and a
trigonal bipyramid fused along a common edge. Usually
M2L9 dimers rearrange to form confacial bioctahedral
structures with three bridging ligands, as seen in the similar
species Mo2Cl6(C4H8S)3.16 The Mo-Mo distance is
In principle, this reaction should produce Et2S2, but
attempts to monitor this reaction by sealed-tube NMR
experiments were not successful.
(16) Moynihan, K. J.; Gao, X.; Boorman, P. M.; Fait, J. F.; Freeman, G.
K. W.; Thornton, P.; Ironmonger, D. J. Inorg. Chem. 1990, 29, 1648-
1654.
(17) Hall, K. A.; Mayer, J. M. Inorg. Chem. 1995, 34, 1145-1158.
(18) Hall, K. A.; Critchlow, S. C.; Mayer, J. M. Inorg. Chem. 1991, 30,
3593-3594.
(19) Hall, K. A.; Mayer, J. M. Inorg. Chem. 1994, 33, 3289-3298.
(20) Kamata, M.; Yoshida, T.; Otsuka, S.; Hirotsu, K.; Higushi, T. J. Am.
Chem. Soc. 1981, 103, 3572-3574.
2412 Inorganic Chemistry, Vol. 42, No. 7, 2003

PMe3-Stabilized Molybdenum Sulfides
Figure 3. Two views showing the molecular structure and atom numbering scheme of Mo2S2(SEt)4(PMe3)2 (4Et). Selected distances (Å) and angles (deg):
Mo(1)-S(1) 2.2461(7), Mo(1)-S(2) 2.3255(7), Mo(1)-P(1) 2.5573(8), Mo(1)-Mo(1A) 2.7152(9), S(1)-Mo(1)-S(2) 117.49(1), S(1)-Mo(1)-S(2A) 117.49-
(1), S(2)-Mo(1)-S(2A) 117.12(3), S(1)-Mo(1)-P(1) 82.28(3), S(1A)-Mo(1)-P(1) 169.83(1).
Mo2(S)2(SEt)4(PMe3)2 + EtSH + 2 PMe3 f
(4Et)
Mo2(S)(SH)(SEt)3(PMe3)4 + Et2S2 (3)
(3Et)
Reactions of Mo2(µ-S)(µ-SH)(PMe3)4(SR)3 with H2S.
Compounds 3H and 3Et were found to react further with
H2S to give tetranuclear products Mo4(µ2-S)4(µ3-S)2(PMe3)6-
(SH)2 (5H) and Mo4(µ2-S)4(µ3-S)2(PMe3)6(SEt)2 (5Et) (Figure
4), respectively. These species were characterized by 1H and
31P NMR spectroscopy and X-ray crystallography. 1H NMR
spectroscopic characterization of the conversion of 3H into
5H revealed the formation of H2, PMe3, and SPMe3.
Compound 5H also catalyzes the conversion of H2S and
PMe3 to SPMe3 and H2, although at a much slower rate than
does (Et4N)2[1]. Spectroscopically, the conversion of 3H into
Figure 4. Molecular structure and atom numbering scheme of Mo4S6-
5H is quantitative; the isolation of 5H is facilitated because
of its low solubility. Saito had previously prepared compound
5H via the reaction of butylamine-THF solutions of (NH4)2-
[Mo3S(S2)6] and PMe3, although spectroscopic characteriza-
tion was not reported.21 Compounds 5H and 5Et also form
when solutions of 2 containing RSH are allowed to stand.
Compound 5tol was not detected.
(SEt)2(PMe3)6 (5Et). Selected distances (Å) and angles (deg): Mo(1)-
Mo(2) 2.8345(9), Mo(1)-Mo(1A) 2.841(1), Mo(2)-S(4) 2.393(1), Mo(1)-
Mo(2)-Mo(1A) 60.14(1), Mo(2)-Mo(1)-Mo(2A) 119.85(1).
cluster define a parallelogram, the cluster symmetry being
approximately C2h. Each of two Mo3 triangles is capped with
µ3-S ligands on opposite sides of the Mo4 plane. Each exterior
Mo-Mo bond is spanned by a µ2-S while the two “hinge”
Mo atoms have PMe3 ligands, and the “wing-tip” Mo atoms
feature both PMe3 and terminal SR ligands.
W4(µ2-S)4(µ3-S)2(PMe3)6(SH)2, which is structurally ana-
logous to 5H, was synthesized from a methanol solution of
(NH4)2WS4 with excess H2S and PMe3. The 1H and 31P NMR
signals match those observed in 5H. A derivative of this
species, W4(µ2-S)4(µ3-S)2(PMe2Ph)6(SH)2, had been previ-
ously synthesized via treatment of W(N2)2(PMe2Ph)4 with
(Me3Si)2S and MeOH,22 conditions conducive to in situ
formation of H2S. Previous workers did not detect the SH
Crystallographic Reexamination of (NH4)2MoS4. Be-
+
cause NH4 significantly influences the chemical behavior
of MoS42-, we sought some insights into the hydrogen
bonding in crystalline (NH4)2MoS4, which was characterized
at lower resolution that did not reveal hydrogen atom
positions.23 A particularly high quality refinement (R1 )
0.0193) allowed us to locate the hydrogen atoms, which
1
signals in the H NMR spectrum for W4(µ2-S)4(µ3-S)2-
(PMe2Ph)6(SH)2.
The structures of 5H and 5Et were confirmed crystallo-
graphically. The four molybdenum atoms of the tetrametallic
+
are disordered over two orientations of both NH4 cations.
(22) Kuwata, S.; Mizobe, Y.; Hidai, M. J. Chem. Soc., Dalton Trans. 1997,
1753-1758.
(21) Tsuge, K.; Mita, S.; Fujita, H.; Imoto, H.; Saito, T. J. Cluster Sci.
1996, 7, 407-421.
(23) Lapasset, J.; Chezeau, N.; Belougne, P. Acta Crystallogr., Sect. B 1976,
B32, 3087-3088.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2413

Schwarz et al.
Scheme 3
The N‚‚‚S distances for one of the cations fall in the range
3.26-3.53 Å, typical, if slightly long, for a hydrogen bond
to sulfur.24,25 The N‚‚‚S distances for the second cation are
longer, in the range 3.50-3.62 Å. The H‚‚‚S distances range
from 2.55 to 3.02 Å. The Mo-S bonds are within 0.01 Å of
those for in [Et4N]2MoS4.26
Discussion
The goal of this research was to extend our recently
-
reported reaction of PMe3, H2S, and ReS4 , which gives the
Mo-H species that undergoes protonation by H2S or
intramolecularly by a MoSH group, but insufficient informa-
tion is available to allow us to formulate a mechanism.
Protonation of [MoS4]2- is reasonable in view of the
catalytically active species ReH(SH)2(PMe3)4.27 Extension
of the PMe3 + H2S reaction to (NH4)2[MS4] (M ) Mo, W)
led to efficient syntheses of MoS2(PMe3)4 and WS2(PMe3)4.
Furthermore, the reactivity of the resulting MoS2(PMe3)4
revealed previously unnoticed relationships between various
Mo-S-PR3 compounds (Scheme 1). MoS2(PMe3)4 (as well
as WS2(PMe3)4) was originally prepared by Parkin via an
elegant but somewhat arduous route involving the addition
of H2S to Mo(PMe3)6.13 It was proposed that this conversion
proceeded via the intermediacy of the metastable MoH2-
(SH)2(PMe3)4,15 which is analogous to the robust species
ReH(SH)2(PMe3)4.27
30
existence of WS2(SH)2 and [EtSMoS3]-.31 Nucleophilic
attack of tertiary phosphine ligands at electrophilic nitrido
ligands32,33 provides a suitable precedent for the reaction
MoVIdS + PMe3. Also potentially relevant is the recent
report of phosphine adducts of d0-tungsten sulfides, e.g.,
[WS3(SR)(PR3)]-.34 Several reports describe the Pd2X2-
(dppm)2-catalyzed conversion of H2S + CH2(PPh2)2 f H2
+ CH2(PPh2)(P(S)Ph2).35-37 These reactions occur at room
temperature with 20 turnovers over a few hours.
A significant finding is that the reactivity of [MoS4]2- is
sensitive to the presence of protic reagents. The nonreactivity
of PMe3 toward (Et4N)2[MoS4] is attributed to the high
charge/atom for hypothetical desulfurized product {[MoS3]2-}n.
Despite its apparent nonreactivity toward PMe3, (Et4N)2-
[MoS4] catalyzes the reaction of PMe3 and H2S to give
SPMe3 and H2. Hoff et al. have determined the enthalpy of
the reaction PMe3 + 1/8S8 f SPMe3 to be -27.1 kcal/mol.28
The heat of formation of H2S is -4.93 kcal/mol.29 The
enthalpy of the new dehydrogenation reaction is thus
calculated to be -22.17 kcal/mol (Scheme 2). Hoff et al.
The nonreactivity of PMe3 toward (Et4N)2[MoS4] contrasts
with the efficient desulfurization of (NH4)2[MoS4] to give
+
MoS2(PMe3)4. NH4 (pKa (MeCN) ) 16.5)38 facilitates the
loss of SH- from [MoS4]2-, allowing the formation of the
neutral derivatives. Ammonium salts of [Mo3S13]2- undergo
phosphine-induced cluster building reactions; in such a case,
+
the NH4 likely also facilitates the desulfurization.9,10
A
detailed crystallographic analysis of (NH4)2MoS4 does not
reveal any striking H‚‚‚S distances, perhaps because the large
number of such interactions precludes a single strong
interaction.
The reaction of 2 with RSH reagents gives the molybde-
num(IV) species, Mo2S2(SR)4(PMe3)2, as the first isolable
adduct. Two plausible pathways for this conversion are
presented in Scheme 3; they differ with respect to the timing
of dissociation of PMe3, but in each case, the key reaction
is the addition of RSH across a ModS bond.
1
also reported the enthalpy of reaction for PPh3 + /8S8 to
form SPPh3 to be -21.5 kcal/mol which corresponds to
-16.57 kcal/mol for the analogous dehydrogenation of H2S
by PPh3.
Scheme 2
The addition of RSH across MdS has been observed for
the d2 derivatives [ReS2(S2C2R4)]-.39 For example, [ReS2-
(S2C2H4)]- adds PhSH to give square pyramidal [ReS(SPh)-
(SH)(S2C2H4)]-.39 In non-d0 systems, S-to-metal π-donation
is less important than in d0 analogues. In such cases, SdM
In view of the nonreactivity of PMe3 toward (Et4N)2-
[MoS4], it is logical that that the catalytic process is initiated
by a reaction of H2S with [MoS4]2-. Such H2S-initiated
reactions could afford [MoS3(SH)]-, resulting from proto-
nation by H2S, or [MoS3(SH)2]2-, resulting from the addition
of H2S across a ModS bond. H2 formation may indicate a
(30) Gattow, G.; Franke, A. Z. Anorg. Allg. Chem. 1967, 352, 246-251.
(31) Kruhlak, N. L.; Wang, M.; Boorman, P. M.; Parvez, M. Inorg. Chem.
2001, 40, 3141-3148.
(32) Huynh, M. H. V.; Jameson, D. L.; Meyer, T. J. Inorg. Chem. 2001,
40, 5062-5063.
(33) Crevier, T. J.; Bennett, B. K.; Soper, J. D.; Bowman, J. A.; Dehestani,
A.; Hrovat, D. A.; Lovell, S.; Kaminsky, W.; Mayer, J. M. J. Am.
Chem. Soc. 2001, 123, 1059-1071.
(24) Krepps, M. K.; Parkin, S.; Atwood, D. A. Cryst. Growth Des. 2001,
1, 291-297.
(25) McGuire, D. G.; Khan, M. A.; Ashby, M. T. Inorg. Chem. 2002, 41,
2202-2208.
(34) Arikawa, Y.; Kawaguchi, H.; Kashiwabara, K.; Tatsumi, K. Inorg.
Chem. 2002, 41, 513-520.
(26) Kanatzidis, M. G.; Coucouvanis, D. Acta Crystallogr., Sect. C: Cryst.
Struct. Commun. 1983, C39, 835-838.
(35) Wong, T. Y. H.; Rettig, S. J.; James, B. R. Inorg. Chem. 1999, 38,
2143-2149.
(27) Schwarz, D. E.; Dopke, J. A.; Rauchfuss, T. B.; Wilson, S. R. Angew.
Chem., Int. Ed. 2001, 40, 2351-2353.
(36) James, B. R. Pure Appl. Chem. 1997, 69, 2213-2220.
(37) Wong, T. Y. H.; Barnabas, A. F.; Sallin, D.; James, B. R. Inorg. Chem.
1995, 34, 2278-2286.
(28) Capps, K. B.; Wixmerten, B.; Bauer, A.; Hoff, C. D. Inorg. Chem.
1998, 37, 2861-2864.
(38) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific Publications: Oxford, 1990; Vol. 35.
(29) Weast, R. C. CRC Press: Boca Raton, FL, 1987.
2414 Inorganic Chemistry, Vol. 42, No. 7, 2003

PMe3-Stabilized Molybdenum Sulfides
Scheme 4. Schematic Description of the Condensation of 3H to 5H
were conducted on a vacuum line, which facilitated the transfer of
the volatile reagents, and worked up using Schlenk techniques.
Solutions of compounds 2-5 decompose under prolonged vacuum
with formation of SPMe3.
is more nearly equivalent to the (RS)2M in an energetic
sense.7 In contrast for d0-MoIV, two thiolates are poor
substitutes for the 3-4e S2- common to d0-MoVI, hence the
nonreactivity of [MoS4]2- toward thiols.
MoS2(PMe3)4 (2).13 Onto a frozen slurry of 300 mg (1.15 mmol)
of (NH4)2MoS4 in 10 mL of MeCN was condensed 0.7 mL (6.8
mmol) of PMe3. The vessel was evacuated, sealed, and then allowed
to warm to room temperature. The solution was stirred for 2 h
during which the solution became green and a green precipitate
formed. The solvent was then removed via filter cannula leaving
an emerald green powder. The solid was then quickly dried in
vacuo. X-ray quality crystals were grown from the mother liquor
at -20 °C. Yield: 490 mg (89%). Anal. Calcd for C12H36MoP4S2
(Found): C, 31.04 (30.93); H, 7.81 (7.99); P, 26.68 (26.39); Mo,
Compounds of the type Mo2E2(ER)4L2 (E ) O, S, N; L
) neutral donor ligand) were previously generated by Otsuka
et al. via the decomposition of Mo(S-t-Bu)4 in the presence
of PPh2Me.20 In the Otsuka case, the sulfido ligands arise
by fragmentation of the tert-butylthiolate ligands.40 The
species Mo2S2(SEt)4(PMe3)2 (4Et) reacts with additional
EtSH in a formal redox process to give the Mo(III) derivative
Mo2(µ-S)(µ-SH)(SEt)3(PMe3)4 (3Et) which is related to
Mo2(µ-S)(µ-Cl)Cl3(PMe3)4.19 The similarity of these two
compounds illustrates the occurrence of many SH analogues
for early metal chlorides.41,42
Condensation of Mo2(µ-S)(µ-SR)(SR)3(PMe3)4 occurs at
room temperature in the presence of H2S or by allowing the
reaction of 2 and H2S to proceed to produce Mo4(µ2-S)4-
(µ3-S)2(SR)2(PMe3)6, a mixed valence [Mo(III)]2[Mo(IV)]2
species. This partial reoxidation is proposed to proceed via
loss of H2 (eqs 4-5).
1
20.66 (22.52). H NMR (C6D6, 500 MHz, 298 K): δ 1.52 (br s).
(NEt4)2MoS4-Catalyzed Reaction of H2S and PMe3. A sealed
NMR tube containing 0.8 mg of (Et4N)2MoS4, approximately 50×
excess of PMe3, approximately 20× excess of H2S, and 0.8 mL of
1
CD3CN was monitored by H NMR spectroscopy for 3 h at room
temperature after initial thawing. Measurements were taken every
30 min. The quantity of SPMe3 (d, δ 1.7) produced was integrated
versus the methylene signal of Et4N+ (q, δ 3.2).
WS2(PMe3)4. Onto a frozen slurry of 101 mg (0.29 mmol) of
(NH4)2WS4 in 20 mL of MeOH was condensed 0.7 mL (6.8 mmol)
of PMe3 and 310 mg (9.1 mmol) of H2S. The vessel was evacuated,
sealed, and then allowed to warm to room temperature. The solution
was stirred for 5 h during which the solution became purple.
Approximately 10 mL of the solvent was removed in vacuo, and a
purple microcrystalline solid formed. The remaining solvent was
4H2S + 2Mo2(S)(SR)(SR)3(PMe3)4 f
Mo4S6(SR)2(PMe3)6 + 4RSH + H2 + 2PMe3 (4)
2Mo2(S)(SH)4(PMe3)4 f
Mo4S6(SR)2(PMe3)6 + 2H2S + H2 + 2PMe3 (5)
1
A cartoon for the 2Mo2 f Mo4 assembly process is
presented in Scheme 4. Saito has extended the Mo-S-
SR-PR3 series to Mo6S10(SH)2(PEt3)6, where the formal Mo
oxidation states are [Mo(III)]2[Mo(IV)]4 and the Mo/S ratio
remains 1:2 as in MoS2.10 But we do not observe such further
condensations.
removed via cannula, and the solid was dried in vacuo. The H
NMR data matches what was previously reported.15 Yield: 140
1
mg (87%). H NMR (C6D6, 500 MHz, 298 K): δ 1.67 (m). 31P
NMR (C6D6, 298 K): δ -44.1 (t).
Mo2S(SH)(PMe3)4(SEt)3 (3Et). A solution of 0.2 mL (2.7 mmol)
of EtSH in 20 mL of MeCN was added to 131 mg (0.282 mmol)
of 2 via cannula. The solution was stirred for 2 h with a slow N2
purge and then filtered and stored overnight at -20 °C to give
small green needles. Yield: 42 mg (20%). X-ray quality crystals
were grown from saturated MeCN solutions of 3Et at -20 °C.
This solid was slightly contaminated with the SPMe3 byproduct.
1H NMR (C6D6, 500 MHz, 298 K): δ 5.20 (m, 1H), 4.11 (m, 1H),
3.97 (m, 1H), 3.70 (m, 1H), 3.17-3.00 (signals overlap, m, 3H),
2.16 (d, 9H), 1.88 (t, 3H), 1.68 (d, 9H), 1.50 (t, 3H), 1.40 (t, 3H),
0.61 (d, 9H), 0.49 (d, 9H). 31P NMR (C6D6, 298 K): δ 7.04 (td,
1P), -3.48 (td, 1P), -13.15 (AB quartet, 2P).
Experimental Section
The salts (NEt4)2MoS4, (NEt4)2WS4, (NH4)2WS4, and (NH4)2-
MoS4 were synthesized by previously reported methods.14 Spec-
troscopic methods have been described previously.27 Most reactions
(39) Dopke, J. A.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2000, 39,
5014-5021.
(40) Coucouvanis, D.; Hadjikyriacou, A.; Lester, R.; Kanatzidis, M. G.
Inorg. Chem. 1994, 33, 3645-3655.
(41) Peruzzini, M.; De Los Rios, I.; Romerosa, A. Prog. Inorg. Chem. 2001,
49, 169-453.
Mo2S(SH)4(PMe3)4 (3H). On to a frozen slurry of 200 mg (0.77
mmol) of (NH4)2MoS4 in 20 mL of MeCN was condensed 0.7 mL
(42) Kuwata, S.; Hidai, M. Coord. Chem. ReV. 2001, 213, 211-305.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2415

Schwarz et al.
Table 1. Details of Data Collection and Structure Refinement for 3H, 3Et, 3tol, 4Et, 5H, and 5Et
3H 3Et 3tol 4Et
5H
5Et
formula‚solvate
C12H46Mo2N2P4S5‚ C18H52Mo2P4S5‚ C33H58Mo2P4S5‚ C14H38Mo2P2S6
C25H64Mo4P6S8
C22H64Mo4P6S8
2CH3CN
0.5CH3CN
0.5Et2O
cryst size (mm3)
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
0.60 × 0.06 × 0.06
0.40 × 0.04 × 0.03 0.30 × 0.14 × 0.02 0.35 × 0.26 × 0.20 0.13 × 0.04 × 0.015 0.22 × 0.13 × 0.12
P3h
C2/c
P21/c
Cmca
P1h
P21/n
23.805(2)
23.805(2)
10.3094(15)
90
90
120
43.165(16)
9.645(4)
17.008(6)
90
100.198(7)
90
10.105(5)
22.454(11)
20.593(10)
90
101.225(10)
90
13.490(4)
16.068(5)
12.599(4)
90
90
90
9.887(5)
11.314(6)
11.405(6)
69.042(9)
84.116(10)
79.885(10)
1171.9(11)
1
11.605(4)
10.482(4)
19.856(7)
90
104.113(6)
90
2342.5(15)
2
1.637
5059.2(10)
6
1.462
6969(5)
8
1.459
4583(4)
4
1.403
2731.1(15)
4
1.587
Z
D
calcd (Mg m-3
)
1.687
1.625
0.9778/0.8819
8893/4124
57/223
µ (Mo KR, mm-1
)
1.251
0.9436/0.6473
1.212
0.939
1.493
1.623
max/min trans
0.9667/0.7542
20461/6170
49/325
0.9804/0.8292
28268/8474
148/504
0.883
0.7436/0.6345
10321/1742
0/101
0.8604/0.7260
15326/4281
0/191
reflns measd/indep 39717/6213
restraints/params
GOF
7/295
0.93
0.879
1.105
0.945
1.026
Rint
0.104
0.0353(0.0701)
0.0946
0.0420(0.1115)
0.2488
0.0729(0.2324)
0.0355
0.0154(0.0170)
0.0782
0.0530(0.1036)
0.0459
0.0276(0.0448)
R1 [I > 2σ]
(all data)a
wR2 [I > 2σ]
(all data)b
max peak/hole
(e-/Å3)
0.0578(0.0642)
0.0574(0.0681)
0.1109(0.1484)
0.0388(0.0394)
0.1241(0.1558)
0.0588(0.0626)
0.453/ -0.454
0.604/-0.443
1.138/-0.768
0.470/-0.273
1.422/-1.477
0.732/-0.390
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
2
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Mo2S(SH)(PMe3)4(SEt)3 (3Et)
(6.8 mmol) of PMe3 and 310 mg (9.1 mmol) of H2S. The vessel
was evacuated, sealed, and then allowed to warm to room
temperature. The mixture was stirred for 2 h during which it became
homogeneous and turned green. The solvent volume was reduced
to half in vacuo, and the remaining solvent was removed via
cannula, leaving a green microcrystalline product. Yield: 270 mg
(53%). This solid was slightly contaminated with the SPMe3
Mo(1)-S(1)
Mo(1)-S(2)
Mo(1)-S(3)
Mo(1)-S(4)
Mo(2)-S(3)
Mo(2)-S(4)
2.357(1)
2.307(1)
2.250(1)
2.418(1)
2.295(1)
2.428(1)
Mo(2)-S(5)
Mo(1)-P(1)
Mo(2)-P(2)
Mo(2)-P(3)
Mo(2)-P(4)
Mo(1)-Mo(2)
2.475(1)
2.501(1)
2.525(1)
2.535(1)
2.512(1)
2.748(1)
1
byproduct. The yield was estimated by H NMR determination of
Mo(1)-S(3)-Mo(2)
Mo(1)-S(4)-Mo(2)
S(4)-Mo(1)-P(1)
S(3)-Mo(1)-S(4)
P(2)-Mo(2)-P(3)
74.40(5)
S(1)-Mo(1)-S(2)
S(2)-Mo(1)-S(3)
S(3)-Mo(1)-S(1)
S(3)-Mo(2)-S(4)
117.44(5)
118.61(6)
120.92(5)
107.33(5)
the 3H/SPMe3 ratio. X-ray quality crystals were grown from an
MeCN solution at -20 °C. Further large needles were grown by
vapor diffusion of Et2O into MeCN, and these crystals were used
for elemental analysis. Anal. Calcd for C12H40Mo2P4S5‚0.5 Et2O
(Found): C, 24.32 (24.10); H, 6.54 (6.50); P, 17.67 (17.76); Mo,
27.36 (27.51). 1H NMR (C6D6, 500 MHz, 298 K): δ 5.21 (m, 1H),
3.27 (dt, 1H), 2.68 (dt, 1H), 2.15 (d, 1H), 1.97 (d, 9H), 1.56 (d,
9H), 0.64 (d, 9H), 0.46 (d, 9H). 31P NMR (C6D6, 298 K): δ 11.65
(td, 1P), -1.32 (td, 1P), -14.89 (AB quartet, 2P). Crystals of 3H
slowly decompose to 5H in the solid state, even in a nitrogen-
purged glovebox.
Mo2S(SH)(PMe3)4(SC6H4Me)3 (3tol). To 205 mg (0.431
mmol) of 2 was transferred a solution of 65 mg (1.6 mmol) of
4-MeC6H4SH in 20 mL of MeCN. The slurry was stirred for 2 h
with a slow N2 purge and then filtered and stored at -20 °C
overnight. Small crystals were collected by removal of the solvent
via cannula. Yield: 79 mg (19%). 1H NMR (C6D6, 500 MHz, 298
K): δ 8.22 (d, 2H), 7.73 (dd, 4H), 6.93 (m, 6H), 5.35 (m, 1H),
2.06 (s, 3H), 2.05 (s, 3H), 2.04 (s, 3H), 2.02 (d, 9H), 1.76 (d, 9H),
0.51 (d, 9H), 0.40 (d, 9H). 31P NMR (C6D6, 298 K): δ 4.15 (td,
1P), -6.20 (td, 1P), -13.59 (AB quartet, 2P).
Mo2S2(SEt)4(PMe3)2 (4Et). Onto 100 mg (0.215 mmol) of 2
was transferred a solution of 0.4 mL (5.4 mmol) of EtSH in 20
mL of MeCN. The solution was stirred for 1 h with a brisk N2
purge and then stored at -20 °C. Small dark green crystals grew
over 24 h and were collected by removal of the solvent via cannula.
1H NMR (C6D6, 500 MHz, 298 K): δ 3.06 (dq, 4H), 2.85 (dq,
4H), 2.20 (m, 18H), 0.90 (t, 12H). 31P NMR (C6D6, 298 K): δ
0.457 (s, 2P). These samples were invariably contaminated with
3Et; further attempts at purification were unsuccessful. Addition
69.08(4)
171.04(5)
109.19(5)
166.79(5)
of >10 equiv of EtSH and PMe3 to MeCN solutions of 4Et produce
3Et, as observed by a sealed tube NMR experiment.
Mo4S6(SH)2(PMe3)6 (5H). In analogy to the preparation of 3H,
onto a frozen slurry of 300 mg (1.15 mmol) of (NH4)2MoS4 in 10
mL of MeCN was condensed 0.7 mL (6.8 mmol) of PMe3 and 310
mg (9.1 mmol) of H2S. The vessel was evacuated, sealed, and then
allowed to warm to room temperature. The slurry was stirred for 3
h during which it turned green and eventually a brown microcrys-
talline precipitate began to form. The flask was then left undisturbed
for 2 h further during which the solution turned brown and brown
microcrystals formed, which were isolated by removal of the solvent
via cannula. Yield: 250 mg (80%). This material was invariably
contaminated with SPMe3, and the yield was estimated by 1H NMR
1
determination of the 5H/SPMe3 ratio. H NMR (C6D6, 500 MHz,
298 K): δ 2.74 (d, 2H), 1.77 (d, 18H), 1.45 (m, 36). 31P NMR
(C6D6, 298 K): δ 2.24 (q, 2P), -23.98 (t, 4P). Alternatively, 5H
can be made from 3H by simply allowing a MeCN solution of 3H
at room temperature to stand overnight. The solution turns brown,
and brown microcrystals form. This process is greatly accelerated
through the addition of excess H2S (>10 equiv).
Mo4S6(SEt)2(PMe3)6 (5Et). A solution of 10 mL of MeCN and
0.2 mL of EtSH (2.7 mmol) was added to 200 mg (0.43 mmol) of
2. The mixture was stirred for 3 h with a brisk N2 purge and then
left undisturbed overnight during which the solution turned brown
and brown microcrystals formed, which were isolated by removal
2416 Inorganic Chemistry, Vol. 42, No. 7, 2003

PMe3-Stabilized Molybdenum Sulfides
of the solvent via cannula. Yield: 198 mg (40%). This material
was invariably contaminated with SPMe3, and the yield was
estimated by H NMR determination of the 5Et/SPMe3 ratio. H
NMR (C6D6, 500 MHz, 298 K): δ 3.57 (q, 4H), 1.94 (d, 18H),
1.46 (t, 6H), 1.43 (m, 36H). 31P NMR (C6D6, 298 K): δ -0.34 (q,
2P), -24.01 (t, 4P).
W4S6(SH)2(PMe3)6. Onto a frozen slurry of 300 mg (0.84 mmol)
of (NH4)2WS4 in 10 mL of MeOH and 10 mL of MeCN was
condensed 0.7 mL (6.8 mmol) of PMe3 and 310 mg (9.1 mmol) of
H2S. The vessel was evacuated, sealed, and then allowed to warm
to room temperature. Over the course of 12 h, the solution assumed
a dark green coloration. This solution was placed in the freezer for
4 days to allow crystals to grow. The resulting green crystals were
isolated by decanting the solution and drying in vacuo. Yield: 20
mg (9%). 1H NMR (C6D6, 500 MHz, 298 K): δ 3.66 (d, 2H), 1.85
(d, 9H), 1.53 (m, 18H). 31P NMR (C6D6, 298 K): δ -26.80 (m,
2P), -59.54 (t, 4P).
Patterson maps and refined using full-matrix least-squares on
SHELXTL V6 (Bruker, 2001). Methyl H atom positions for PMe3
were optimized by rotation about P-C bonds with idealized C-H
distances. S-H atoms were all located. H atom U values were
assigned on the basis of the Ueq of adjacent non-H atoms. The data
were corrected for absorption on the basis of face centered scans.
Specific details for each crystal are given in Table 1, and bond
lengths and angles for 3Et are provided in Table 2. Full crystal-
lographic data for 3Et, 3H, 3tol, 4Et, 5Et, and 5H have been
deposited with the Cambridge Crystallographic Data Center as
supplementary publication numbers 197576 (3H), 197577 (4Et),
197578 (3Et), 197579 (3tol), 197580 (5Et), and 197581 (5H).
1
1
Acknowledgment. This research was supported by the
NSF. We thank Steve Smith for assistance with the synthesis
of MoS2(dmpe)2.
Supporting Information Available: X-ray data for (NH4)21,
3H, 3Et, 3tol, 4Et, 5H, and 5toland the 1H{31P} NMR spectra for
3H. This material is available free of charge via the Internet at
http://pubs.acs.org.
Crystallography. Crystals of (NH4)21, 3Et, 3H, 3tol, 4Et, 5Et,
and 5H were mounted on a thin glass fiber by using oil (paratone-
N, Exxon) before being transferred to the diffractometer. Data were
collected on a Siemens CCD automated diffractometer at 193 K.
Data processing was performed with the integrated program package
SHELXTL. All structures were solved using direct methods or
IC026215M
Inorganic Chemistry, Vol. 42, No. 7, 2003 2417