Synthesis and characterization of oxidized W6S8L6 clusters

Article abstract of DOI:10.1021/ic001099d

Cationic [W₆S₈L₆]PF₆ (L = PEt₃ (3), 4-tert-butylpyridine (4)) clusters were successfully synthesized and isolated for the first time by reacting the corresponding neutral W₆S₈L₆ (L = PEt₃ (1), 4-tert-butylpyridine (2)) clusters with [Cp₂Fe]PF₆ as the oxidant. The products 3 and 4 were characterized by NMR spectroscopy, mass spectroscopy, and X-ray crystallography (only for 3) and shown to be the desired oxidized W₆S₈ clusters with a metal electron count of 19. Magnetic property studies showed that they are paramagnetic compounds with S = 1/2. Their chemical properties and stability are also reported. Crystal data for 3·2THF: space group, R3 (No. 148); a = 13.91170(10) A; c = 32.4106(2) A; Z = 3.

Full text of DOI:10.1021/ic001099d

2660
Inorg. Chem. 2001, 40, 2660-2665
Synthesis and Characterization of Oxidized W6S8L6 Clusters
Laurie I. Hill, Song Jin, Ran Zhou, D. Venkataraman, and Francis J. DiSalvo*,†
Baker Laboratories, Department of Chemistry and Chemical Biology, Cornell University,
Ithaca, New York 14853
ReceiVed October 3, 2000
Cationic [W6S8L6]PF6 (L ) PEt3 (3), 4-tert-butylpyridine (4)) clusters were successfully synthesized and isolated
for the first time by reacting the corresponding neutral W6S8L6 (L ) PEt3 (1), 4-tert-butylpyridine (2)) clusters
with [Cp2Fe]PF6 as the oxidant. The products 3 and 4 were characterized by NMR spectroscopy, mass spectroscopy,
and X-ray crystallography (only for 3) and shown to be the desired oxidized W6S8 clusters with a metal electron
count of 19. Magnetic property studies showed that they are paramagnetic compounds with S ) 1/2. Their chemical
properties and stability are also reported. Crystal data for 3‚2THF: space group, R3h (No. 148); a ) 13.91170(10)
Å; c ) 32.4106(2) Å; Z ) 3.
Introduction
molecular Cr and W octahedral clusters as precursors for the
corresponding chromium and tungsten “Chevrel phases”, un-
known from solid-state routes, no reports of success are known
to us.14,15,18,22,23
Octahedral molybdenum clusters of the formula Mo6Q8
(Q ) S, Se, Te) are the building blocks of a class of compounds
known as Chevrel phases,1,2 which have been extensively studied
due to their interesting properties such as superconductivity,3
fast ion conductivity,4 and hydrodesulfurization catalytic
activity.5-8 Since the pioneering work by Saito and co-workers,9
many molecular octahedral clusters M6Q8 (M ) Cr, Mo, W; Q
) S, Se, Te) similar to those present in Chevrel phases have
been synthesized.10-21 Although many have tried to use the
We are interested in using the molecular compounds W6S8L6
(L is a Lewis base ligand) as precursors to cluster networks
linked by π-conjugated ditopic ligands.20,21 Extended Hu¨ckel
calculations24 performed on “bare” W6S8 clusters indicate that
neutral clusters have a metal electron count (MEC) of 20. Their
highest occupied molecular orbital (HOMO) is triply degenerate
and composed of metal d-orbitals of the appropriate symmetry
to interact with the π-orbitals of an organic ligand. Therefore,
intercluster interaction within the extended network is possible.
However, the full HOMO of the cluster at a MEC of 20 would
lead to a filled band at the Fermi level of the extended network
and result in semiconducting behavior. A network built of
clusters with a MEC of 18 or 19 would lead to a partially filled
band and a potentially conducting network. The HOMO of
clusters with higher MECs of 21-24 has δ symmetry with
respect to the M-L axis, and thus these clusters are inappropri-
ate for significant intercluster interactions through organic
ligands. In anticipation of building an extended network of
clusters with a MEC below 20, we have pursued the synthesis
of oxidized W6S8L6 (L ) PEt3, 4-tert-butylpyridine) clusters.
While reduced M6Q8L6 (M ) Mo; Q ) S, Se;11,25 and M )
W; Q ) Te18) clusters with a MEC of 21 electrons have been
synthesized and characterized, no oxidized M6Q8L6 clusters with
a MEC of 19 electrons have been reported. However, on the
basis of the electrochemical studies of W6S8L6 clusters carried
out by this20 and other groups,10 it seemed likely that stable
oxidized clusters could be prepared by chemical methods. We
report herein the synthesis and isolation of the first cationic
[W6S8L6]PF6 (L ) PEt3 (3), 4-tert-butylpyridine (4)) clusters
with a MEC of 19 using [Cp2Fe]PF6 (Cp ) C5H5, cyclopenta-
dienyl) as the oxidant. These compounds were characterized
† E-mail: fjd3@cornell.edu.
(1) Chevrel, R.; Sergent, M.; Prigent, J. J. Solid State Chem. 1971, 3,
515-519.
(2) Pena, O.; Sergent, M. Prog. Solid State Chem. 1989, 19, 165-281.
(3) Topics in Current Physics, Vol. 34: Superconductivity in Ternary
Compounds 2: Superconductivity and Magnetism; Maple, M. B.,
Fischer, Ø., Eds.; Springer-Verlag: Heidelberg, 1982.
(4) Mulhern, P. J.; Haering, R. R. Can. J. Phys. 1984, 62, 527-531.
(5) McCarty, K. F.; Schrader, G. L. Ind. Eng. Chem. Prod. Res. DeV.
1984, 23, 519-524.
(6) McCarty, K. F.; Anderegg, J. W.; Schrader, G. L. J. Catal. 1985, 93,
375-387.
(7) Hilsenbeck, S. J.; McCarley, R. E.; Thompson, R. K.; Flanagan, L.
C.; Schrader, G. L. J. Mol. Catal. A 1997, 122, 13-24.
(8) Hilsenbeck, S. J.; McCarley, R. E.; Goldman, A. I.; Schrader, G. L.
Chem. Mater. 1998, 10, 125-134.
(9) Saito, T.; Yamamoto, N.; Yamagata, T.; Imoto, H. J. Am. Chem. Soc.
1988, 110, 1646-1647.
(10) Saito, T.; Yoshikawa, A.; Yamagata, T. Inorg. Chem. 1989, 28, 3588-
3592.
(11) Saito, T.; Yamamoto, N.; Nagase, T.; Tsuboi, T.; Kobayashi, K.;
Yamagata, T.; Imoto, H.; Unoura, K. Inorg. Chem. 1990, 29, 764-
770.
(12) Hilsenbeck, S. J.; Young, V. G., Jr.; McCarley, R. E. Inorg. Chem.
1994, 33, 1822-1832.
(13) Ehrlich, G. M.; Warren, C. J.; Vennos, D. A.; Ho, D. M.; Haushalter,
R. C.; DiSalvo, F. J. Inorg. Chem. 1995, 34, 4454-4459.
(14) Xie, X. B.; McCarley, R. E. Inorg. Chem. 1995, 34, 6124-6129.
(15) Zhang, X.; McCarley, R. E. Inorg. Chem. 1995, 34, 2678-2683.
(16) Tsuge, K.; Imoto, H.; Saito, T. Bull. Chem. Soc. Jpn. 1996, 69, 627-
636.
(17) Xie, X. B.; McCarley, R. E. Inorg. Chem. 1996, 35, 2713-2714.
(18) Xie, X. B.; McCarley, R. E. Inorg. Chem. 1997, 36, 4665-4675.
(19) Kamiguchi, S.; Imoto, H.; Saito, T.; Chihara, T. Inorg. Chem. 1998,
37, 6852-6857.
(20) Venkataraman, D.; Rayburn, L. L.; Hill, L. I.; Jin, S.; Malik, A.-S.;
Turneau, K. J.; DiSalvo, F. J. Inorg. Chem. 1999, 38, 828-830.
(21) Jin, S.; Venkataraman, D.; DiSalvo, F. J. Inorg. Chem. 2000, 39, 2747-
2757.
(22) Hilsenbeck, S. J.; McCarley, R. E.; Goldman, A. I. Chem. Mater. 1995,
7, 499-506.
(23) McCarley, R. E.; Hilsenbeck, S. J.; Xie, X. J. Solid State Chem. 1995,
117, 269-274.
(24) Malik, A.-S. Ph.D. Thesis, Cornell University, Ithaca, NY, 1998.
(25) Mizutani, J.; Amari, S.; Imoto, H.; Saito, T. J. Chem. Soc., Dalton
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10.1021/ic001099d CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/04/2001

Oxidized W6S8L6 Clusters
Inorganic Chemistry, Vol. 40, No. 12, 2001 2661
Table 1. Crystallographic Data for [W6S8(PEt3)6]PF6‚2THF
3‚2THF
by X-ray crystallography (only for 3), NMR spectroscopy, mass
spectroscopy, and magnetic susceptibility.
chem formula
fw
space group
a, Å
C44H90F6O2P7S8W6
2341.53
R3h (No. 148)
13.91170(10)
32.4106(2)
5432.23(6)
3
-110
0.710 73
2.147
99.22
Experimental Section
General. W6S8(PEt3)6 (1) and W6S8(4-tert-butylpyridine)6 (2) were
synthesized according to previously reported procedures.20 Ferrocenium
hexafluorophosphate ([Cp2Fe]PF6) from Aldrich was recrystallized from
acetonitrile. N,N-Dimethylformamide (DMF), dichloromethane (CH2Cl2),
and acetonitrile were dried over 4 Å molecular sieves and distilled under
reduced pressure. Tetrahydrofuran (THF), benzene, and diethyl ether
were treated with sodium wire and distilled under reduced pressure.
All reactants and solvents were stored in a glovebox filled with argon.
All glassware was flame dried before use. All manipulations were
carried out in the glovebox or using standard Schlenk techniques unless
otherwise noted.
c, Å
V, Å3
Z
temp, °C
λ, Å
F
calcd, g cm-3
µ, cm-1
R1a (I > 2σ/all)
0.0451/0.0755
0.0539/0.0774
wR2b (I > 2σ/all)
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σw(Fo - Fc2)2/Σw(Fo )2]1/2
.
2
2
1H NMR spectra were obtained in CD2Cl2 on a Varian INOVA 400
spectrometer with no 31P decoupling and were referenced to internal
residual solvent resonances. 31P NMR spectra were obtained in CD2Cl2
using a Varian VXR-400 spectrometer at 162 MHz with 85% H3PO4
hexagonal setting, a ) 13.9117(10) Å, c ) 32.4106(2) Å. The space
group (R3h, No. 148) was determined on the basis of systematic absences
and intensity statistics. A total of 11328 reflections were collected in
the θ range 1.8 to 28.9°, 2723 of which were unique. The data were
integrated using SAINT software, and an empirical absorption correction
was applied using the program SADABS.26 The structure was solved
using SHELXS and refined using full-matrix least-squares method on
Fo2 with SHELXL software packages.27 PF6- anions were modeled as
disordered such that the four F atoms on sites F(2) and F(3)
perpendicular to the three-fold axis occupy three different positions at
1/3 occupancy. The solvent THF molecules were modeled as disordered
in three different planar orientations (perpendicular to the 3-fold axis)
such that the O atom site had an occupancy of 1/3 and the two C atom
1
as an external standard and with H decoupling. High-resolution ESI-
MS data were collected with CH2Cl2 solution of the products using a
Micromass LCT mass spectrometer in the positive ion mode.
Synthesis of [W6S8(PEt3)6]PF6 (3). Method A. In a 100 mL Schlenk
flask equipped with a Teflon stir bar, 1 (0.200 g, 0.097 mmol) was
dissolved into 50 mL of CH2Cl2 to form a dark yellow-brown solution.
A blue solution of [Cp2Fe]PF6 (0.035 g, 0.106 mmol) in 20 mL of
CH2Cl2 was added to the flask. The resulting green solution was stirred
for 30 min before the solvent was removed under dynamic vacuum.
The resulting green residue was washed repeatedly with benzene and
then with diethyl ether. The dried green powder of 3 weighed 0.184 g
(85% yield).
2
sites each had an occupancy of /3. Additionally, the ethyl groups of
the PEt3 ligands were modeled as disordered and refined such that each
C atom site had an occupancy of approximately 1/2. Although it is likely
that the pseudosymmetry of the heavy cluster core could dominate the
symmetry of the full structure, attempts to refine the structure with
lower symmetry or twinning have so far been unsuccessful. Also,
possible supperlattices could not be detected in this data set. Hydrogen
atoms were added at calculated fixed positions. W, S, and P atoms
were refined anisotropically. All other non-hydrogen atoms were refined
isotropically. Final refinements converged and gave satisfactory residue
factors. The largest residual electron densities were close to W atoms.
Details of the crystallographic data are given in Table 1, selected bond
distances and angles, in comparison with those found in 1‚2C6H6,20
are given in Table 2.
Magnetic Susceptibility. Under an argon atmosphere, 80.1 mg of
3 was loaded into a gelcap. The gelcap was inserted into a straw which
was then placed into a Quantum Design SQUID magnetometer with
minimal exposure to air. Magnetic susceptibility measurements were
carried out over the temperature range 4-300 K with an applied field
of 7000 Oe. The susceptibility at 300 K as a function of field (100-
10000 Oe) was also measured to check for ferromagnetic impurities.
A measurement was performed on 4 (52.1 mg) in the same fashion
except that the applied magnetic field was 1000 Oe. The data were
corrected for small ferromagnetic impurities and also for the diamag-
netic core contributions (-1.14 × 10-3 emu/mol for 3 and -1.18 ×
10-3 emu/mol for 4) which were calculated from values for the
individual atoms.28
Method B. [Cp2Fe]PF6 (0.066 g, 0.199 mmol) was added into a
solution of W6S8(PEt)6 (0.114 g, 0.055 mmol) in 20 mL of benzene.
Upon stirring, the color of the solution changed from yellow to green
within an hour, accompanied by some precipitation. After 2 days, the
green precipitate was filtered, washed with diethyl ether, and allowed
to dry. The yield of 3 was 0.116 g (95%).
1
Samples of 3 from both methods gave the following. H NMR: δ
1.5 (t, 3, Me) and 4.0 (q, 2, CH2). 31P{1H} NMR: δ -143 (septet,
1JF-P ) 715 Hz); - 419 (broad hump). ESI-MS: m/z obsd (calcd)
2067.98 (2068.02).
Synthesis of [W6S8(4-tert-butylpyridine)6]PF6 (4). Method A. In
a 100 mL Schlenk flask equipped with a Teflon stir bar, 2 (0.200 g,
0.092 mmol) was dissolved into 75 mL of CH2Cl2 to form a dark orange
solution. A solution of [Cp2Fe]PF6 (0.034 g, 0.101 mmol) in 20 mL of
CH2Cl2 was added to the flask to result in a green-brown solution.
After stirring for 30 min, the solvent was removed by dynamic vacuum.
The resulting green solid was washed repeatedly with benzene until
the filtrate no longer became colored. After washing with diethyl ether,
the dried green powder weighed 0.150 g (70% yield).
Method B. [Cp2Fe]PF6 (0.054 g, 0.163 mmol) was added into a
solution of W6S8(4-tert-butylpyridine)6 (0.119 g, 0.055 mmol) in 20
mL of benzene. Upon stirring, the color of solution changed from dark
orange to green within an hour. After 2 days, the green precipitate was
filtered, washed with diethyl ether, and allowed to dry. The yield of 4
was 0.106 g (83%).
1
Samples of 4 from both methods gave the following. H NMR: δ
ESR Study. Solid samples of compound 3 were loaded into ESR
tubes under an argon atmosphere. The ESR spectra were obtained at a
frequency of 9.55 GHz on a Bruker ER-200 ESR spectrometer at liquid
nitrogen temperature.
Thermogravimetric Analyses (TGA). Thermogravimetric analyses
(TGA) of 3 and 4 were carried out on a Seiko TG/DTA 220 thermal
1.5 (s, 9, Me), 6.43 (broad doublet, 2, â-H), and 8.38 (broad doublet,
1
2, R-H). 31P{1H} NMR: δ -143 (septet, JF-P ) 715 Hz). ESI-MS:
m/z obsd (calcd) 2170.01 (2170.10).
X-ray Crystallographic Study of [W6S8(PEt3)6]PF6. Single crystals
suitable for X-ray studies were grown from a THF/CH2Cl2 solution of
3 by slow removal of the solvent. Most of the crystals formed as roughly
square thin green plates. A green block-shaped crystal measuring 0.15
× 0.1 × 0.1 mm was mounted on a plastic loop with polybutylene oil
and immediately placed in a cold dinitrogen stream. X-ray intensity
data were collected using Mo KR radiation at -110 °C on a Bruker
SMART diffractometer with a CCD detector. The cell constants were
determined from more than 50 centered reflections as rhombohedral
with lattice parameters a ) 13.4621(1) Å, R ) 62.222(1)° or in
(26) Sheldrick, G. M. the computer program SADABS is used by Siemens
CCD diffractometers; Institute fu¨r Anorganische Chemie der Univer-
sita¨t Go¨ttingen.
(27) Sheldrick, G. M. SHELXL, 5.03 ed.; Siemens Analytical X-ray
Instruments Inc.: Madison, WI, 1994.
(28) Selwood, P. W. Magnetochemistry, 2nd ed.; Interscience: New York,
1956; p 78.

2662 Inorganic Chemistry, Vol. 40, No. 12, 2001
Hill et al.
Table 2. Selected Bond Lengths [Å] and Angles [deg] for 3 and 1a
3
1
W(1)-W(1)A
W(1)-W(1)B
mean W-W
2.6803(4)
2.6809(3)
2.6806(4)
2.456(2)
2.449(2)
2.446(2)
2.4508(15)
2.450(2)
2.530(2)
60.006(5)
59.985(10)
60.007(5)
2.6813(6)
2.6759(6)
2.6786(6)
2.458(2)
2.4585(18)
2.4511(17)
2.4556(17)
2.456(2)
2.5230(18)
59.931(8)
60.134(17)
59.931(8)
W(1)-S(1)
W(1)-S(2)
W(1)-S(2)A
W(1)-S(2)B
mean W-S
W(1)-P(1)
W(1)A-W(1)-W(1)Bb
W(1)B-W(1)-W(1)D
W(1)C-W(1)-W(1)D
Figure 1. 1H NMR spectra (0-4.5 ppm regions) of [W6S8(PEt3)6]PF6
a Both clusters crystallized in the same R3h space group. Data for 1
were taken from ref 20. Symmetry transformations used to generate
equivalent atoms: A, -x + y, -x + 1, z; B, y - 1/3, -x + y + 1/3, -z
(top) and W6S8(PEt3)6 (bottom) in CD2Cl2.
results. Method B is simpler and consumes less solvent, but a
prolonged reaction time (>2 d) is essential for good yields. This
slow reaction is likely due to the limited solubility of [Cp2Fe]PF6
in benzene.
1
2
1
1
+ /3; C, -y + 1, x - y + 1, z; D, x - y + /3, x + /3, -z + /3.b As
required by the imposed symmetry, W-W-W angles within all
equatorial squares are 90° and W-W-W angles within those triangular
faces perpendicular to the three-fold axes are 60°. The mean angles
within triangular faces are 60° by geometry.
1
1H and 31P NMR Spectroscopy of [W6S8L6]PF6. The H
NMR spectrum of 3 in CD2Cl2 is shown in Figure 1 (top) in
comparison with that of 1 (bottom). The 1H resonances for the
oxidized cluster 3 are shifted downfield from those of the neutral
cluster 1 to δ 1.50 (Me) and 4.00 (CH2),20 consistent with the
increased electron-withdrawing effect by the cationic cluster.
The signals for 3 are significantly broadened, owing to the
paramagnetic cluster core (vide infra). Interestingly, the coupling
with 31P is no longer observable in the oxidized cluster and
analyzer. The samples were loaded in air into an aluminum pan and
were heated from room temperature to 550 °C at the rate of 20 °C/min
under a flow of dinitrogen (60 mL/min).
Results and Discussion
Synthesis and Isolation of [W6S8L6]PF6 (3). A few factors
were considered when choosing the oxidizing agent(s) for the
synthesis of the oxidized clusters. First, according to our cyclic
voltammetry studies,20 the oxidizing agent needs to have a
standard reduction potential larger than 0.2 V but less than 0.6
V, beyond which possible irreversible oxidation occurs. With
the electrochemistry of ferrocenium well understood and its use
as an oxidizing agent well documented,29 [Cp2Fe]PF6 was found
to meet this criterion. Upon the addition of more than 1 equiv
of [Cp2Fe]PF6, the color of the W6S8L6 solution in CH2Cl2
instantaneously changed to green. The reaction involved is
1
the H NMR peaks appear as a triplet and a quartet, in con-
trast to the multiplets observed for 1 and free PEt3. This type
of behavior has also been found in [Co6S8(PEt3)6]+1,+2 and
[Co6Te8(PEt3)6]0,+1 30-32
.
The 31P NMR spectrum for 3 consists
-
of a septet centered at δ -143 due to PF6 (reported value δ
-14533) and a very broad signal at about δ -419 which is
tentatively assigned to the PEt3 bound to the oxidized cluster
(δ -19.2 for free PEt3 and δ -16.97 for PEt3 bound to neutral
cluster34). The discrepancy between the observed integration
ratio of PEt3 to PF6- (2.6 to 1) and the expected ratio of 6 to 1
is attributed to the paramagnetic nature of 3.
W6S8L6 + [Cp2Fe]PF6 f [W6S8L6]PF6 + Cp2Fe (1)
The 1H NMR spectrum of 4 in CD2Cl2, of which the
downfield region is shown in Figure 2 (top) along with that of
2 (bottom), consists of two equally intense broadened (due to
the paramagnetic cluster) doublets at δ 8.38 (R-H) and 6.43
(â-H). These are shifted upfield from the 1H signals of the 4-tert-
butylpyridine bound to the neutral W6S8 cluster (δ 9.20 for R-H
and 7.30 for â-H)20 and those of free 4-tert-butylpyridine in
CD2Cl2 (δ 8.46 for R-H and 7.28 for â-H). These upfield shifts
are in contrast to the downfield shifts for PEt3 in 3 and for the
methyl proton in the same cluster 4 (from δ 1.30 in 2 to 1.50
in 4). This is also contrary to the shifts of pyridine protons found
in most paramagnetic transition metal complexes, which show
an alternation of upfield and downfield shifts around the ring,
attributed to spin delocalization via the pyridine ring.35 These
It is also important that the reduction byproduct, i.e., Cp2Fe,
is inert to further reaction with the oxidized clusters and easy
to remove from the product mixture. Cp2Fe is soluble in
benzene, while the oxidized clusters are soluble in polar solvents
such as CH2Cl2, benzonitrile, acetonitrile, DMF, acetone, and
THF but only sparingly soluble in benzene and not soluble in
Et2O and heptane. Other complicating factors include the
stability of the products 3 and 4 in different solvents (vide infra).
These factors are the basis of the methods reported. In method
A, the Cp2Fe byproduct was easily removed with benzene. In
method B, the oxidized clusters precipitated out since they are
only sparingly soluble in benzene, while other byproducts or
unreacted species were left behind in the benzene solution. In
both methods, a wash with Et2O removed unreacted [Cp2Fe]-
PF6, which is especially important in method B. The advantage
of method A is the fast reaction facilitated by the good solubility
of both reactants in CH2Cl2sallowing the solution to stand for
longer periods of time (>6 h) did not have any effect on the
(30) Bencini, A.; Midollini, S.; Zanchini, C. Inorg. Chem. 1992, 31, 2132-
2137.
(31) Cecconi, F.; Ghilardi, C. A.; Midollini, S.; Orlandini, A.; Zanello, P.;
Cinquantini, A.; Bencini, A.; Uytterhoeven, M. G.; Giorgi, G. J. Chem.
Soc., Dalton Trans. 1995, 3881-3890.
(32) Cecconi, F.; Ghilardi, C. A.; Midollini, S.; Orlandini, A.; Bencini, A.
J. Chem. Soc., Dalton Trans. 1996, 3991-3994.
(29) See for example: Page, J. A.; Wilkinson, G. J. Am. Chem. Soc. 1952,
74, 6149. Kuwana, T.; Bublitz, D. E.; Hoh, G. J. Am. Chem. Soc.
1960, 82, 5811. Ge, Y.-W.; Ye, Y.; Sharp, P. R. J. Am. Chem. Soc.
1994, 116, 8384-8385. Westerberg, D. E.; Pladziewicz, J. R.;
Abrahamson, A. J.; Davis, R. A.; Likar, M. D. Inorg. Chem. 1987,
26, 2058.
(33) Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987.
(34) Jin, S.; DiSalvo, F. J., unpublished result.
(35) See for example: Pearson, C.; Beauchamp, A. L. Can. J. Chem. 1997,
75, 220. Miller, R. G.; Stauffer, R. D.; Fahey, D. R.; Parnell. D. R. J.
Am. Chem. Soc. 1970, 92, 1511. Happe, J. A.; Ward, R. L. J. Chem.
Phys. 1963, 39, 1211.

Oxidized W6S8L6 Clusters
Inorganic Chemistry, Vol. 40, No. 12, 2001 2663
W-W bond length in the oxidized clusters is to be expected,
since one electron is removed from a slightly bonding orbital24
leading to weaker bonding. However, this effect is expected to
be small, as observed here, since only one electron out of 20 is
removed. This change is quite small compared with the change
made by adding one electron to neutral clusters, as observed in
[Mo6Q8(PEt3)6]- (Q ) S, Se) (0.010 Å elongation)11 and
[W6Te8(py)6]- (0.017 Å shortening).18 The regular octahedron
of W atoms found in 3 is in contrast to the tetragonally distorted
octahedra found in the group VI clusters with a MEC of 21,
[Mo6Q8(PEt3)6]- (Q ) S, Se)11 and [W6Te8(py)6]-,18 which has
been attributed to the Jahn-Teller effect resulting from the
partially filled degenerate eg HOMO orbitals in those clusters.
Similar distortions have also been found in the clusters with a
MEC of 19, Nb6I11 (Nb6I8 I6/2a-a) (Nb-Nb ) 2.72-2.96 Å)
i
and Nb6I9S (Nb6I6 S2/2
I
a-a) (Nb-Nb ) 2.80-3.16 Å),37
i
i-i
Figure 2. 1H NMR spectra (6-10 ppm regions) of [W6S8(4-tert-
butylpyridine)6]PF6 (top) and W6S8(4-tert-butylpyridine)6 (bottom) in
CD2Cl2.
6/2
which have been attributed to a Jahn-Teller effect resulting
from the partially filled degenerate t2u HOMO orbitals. However,
the “strain” caused by the connectivity between the clusters can
also lead to the distortions,38 an explanation supported by the
fact that similar distortions were observed in the clusters HNb6I11
(Nb-Nb ) 2.806-2.950 Å), CsNb6I11(Nb-Nb ) 2.771-2.940
Å), and HNb6I9S (Nb-Nb ) 2.888-3.014 Å)39 which have a
MEC of 20 but the same connectivity as the 19 electron clusters.
The W-P bond length in 3 compared to that in the neutral
cluster 1 is of particular interest to us. A decrease in bond length
on oxidation would likely lead to greater overlap between the
orbitals which is important to an extended, conducting network
because it would lead to a more disperse band at the Fermi
level and increase the probability of obtaining a conducting
system. In the case at hand, the average W-P bond length
seemingly opposite shifts in 4 can be explained by the
combination of two competing electron density effects on the
chemical shifts of the protons on the aromatic ligand.36 One
effect is the electron-withdrawing effect, which shifts resonances
downfield when electron density is withdrawn. The other is the
ring current effect, which shifts resonances upfield when electron
density is withdrawn from the ring. When 4-tert-butylpyridine
is bound to the neutral cluster, the electron-withdrawing effect
must be the dominant effect and results in downfield shifts
compared to those of the free ligand. However, the main effect
incurred on oxidation is the ring current effect, evidenced by
the similar shifts of both aromatic protons from the positions
in the neutral cluster, since all ring protons are expected to be
affected in the same way. The electron-withdrawing effect is
still present, affecting mostly the R-H. The methyl protons are
primarily affected by the greater electron-withdrawing effect
from the pyridine ring as a result of lower ring current and thus
are shifted downfield.
20
slightly increases on oxidation from 2.523(2) Å in 1‚2C6H6
to 2.530(2) Å in 3‚2THF, indicating perhaps a slight decrease
in the bond strength.
Despite many attempts, single crystals of 4 suitable for X-ray
study have not been acquired. It would be interesting to see
whether oxidation has the same effect on W-L bond length
when a π-conjugated ligand, such as 4-tert-butylpyridine, is
bonded to the cluster.
The 31P NMR spectrum of 4 only consists of one septet
corresponding to PF6-, which, combined with the absence of
[Cp2Fe]+ by 1H NMR and microprobe analysis, further supports
the fact that the W6S8(4-tbp)6 cluster is indeed oxidized. The
purity of both 3 and 4 is evident in the absence of any extraneous
Magnetic Properties. Magnetic susceptibility data were
collected for 3 and 4 with the expectation of paramagnetic
behavior due to the one unpaired electron in the HOMO. Plots
of corrected ø vs 1/T for 3 and 4 are available in the Supporting
Information. Indeed, the data follow a Curie-Weiss law (ø(T)
) C/(T - θ) + ø0) and indicate paramagnetic behavior for both
compounds over the temperature range of 4-300 K. The Curie
constants calculated for compounds 3 and 4 from these plots
are 0.368 and 0.318 mol-1 K-1, respectively. From these Curie
constants, the effective moments per cluster were calculated to
be µeff ) 1.71 µB for 3 and µ ) 1.60 µB for 4, respectively,
which are close to the calculated spin-only magnetic moment,
1
peaks in the H NMR spectra.
Crystal Structure of [W6S8(PEt3)6]PF6‚2THF. X-ray struc-
ture analysis confirmed that cluster 3 was the oxidized cationic
cluster as proposed. 3‚2THF crystallizes in the space group R3h
with Z ) 3. Figure 3 shows the hexagonal arrangement of the
[W6S8(PEt3)6]+ ions located in planes perpendicular to the c-axis.
Both the [W6S8(PEt3)6]+ clusters and the PF6 anions are
-
centered on special positions with 3h symmetry (3b and 3a
positions, respectively). The disordered THF molecules are
centered on positions with 3 symmetry (6c position). The
packing of these moieties can be understood in terms of the
“sphere” packing of clusters. The [W6S8(PEt3)6]+ clusters
approximately form ABCABC... close packed layers perpen-
dicular to the c-axis; all octahedral holes in this array are
occupied by PF6- anions and all tetrahedral holes are occupied
by the disordered THF molecules.
The [W6S8(PEt3)6]+ cationic cluster has an imposed 3h
symmetry. The average W-W distance in 3 is 2.6806(4) Å,
slightly longer than that in the neutral cluster 1, 2.6786(6) Å,20
but within 2σ. This difference is insignificant. A slightly longer
1
µ ) 1.73 µB (if g ) 2 and S ) /2). We attribute the slightly
lower observed moment for 4 to the partial degradation of the
sample by water or other species during handling. Also from
the Curie-Weiss plots, the temperature-independent suscepti-
bilities (ø0) after diamagnetic corrections were found to be 5.11
× 10-4 emu/mol for 3 and 9.32 × 10-4 emu/mol for 4. The
Weiss constants (θ) were very close to zero (<0.1 K) for both
compounds, which reflects a weak intercluster exchange as
expected.
(37) Meyer, H. J.; Corbett, J. D. Inorg. Chem. 1991, 30, 963-967.
(38) Simon, A.; Schnering, H. G.; Schaefer, H. Z. Anorg. Allg. Chem. 1967,
355, 295-310.
(36) Friebolin, H.; Basic One- and Two-Dimensional NMR Spectroscopy;
VCH Publishers: New York, 1993; Chapter 7.
(39) Imoto, H.; Corbett, J. D. Inorg. Chem. 1980, 19, 1241-1245.

2664 Inorganic Chemistry, Vol. 40, No. 12, 2001
Hill et al.
Figure 3. View of one layer of W6S8(PEt3)6+ ions along the plane z ) 1/6 showing the hexagonal arrangement in the crystal structure. The disordered
-
1
PF6 ions and THF molecules are centered along the planes z ) 0 and z ) /12, respectively. Additional THF molecules (omitted for clarity) lie
-
1
directly above the PF6 ions at z ) /4.
The ESR spectrum was collected for a powdered sample of
3 at liquid nitrogen temperature. The ESR spectrum, which is
available in the Supporting Information, shows a broad signal
centered at g ) 2.04. This indicates that 3 contains units with
one unpaired electron. The 21 e- reduced cluster compound
NaW6Te8(pyridine)6 was reported to have a g value of 2.26,
measured by ESR.18
Chemical Properties and (In)Stability of the Oxidized
Clusters. The compounds 3 and 4 are fairly stable in the solid
state as evidenced by observing the same NMR spectra in
solution after the solids were heated under argon at 100 °C for
7 h. However, these compounds display various degrees of
“instability” in solution, which are the complicating factors in
the syntheses and workup of the oxidized clusters. While the
mechanisms and the products of these instabilities are not yet
fully understood, some preliminary observations are reported
here.
neutral cluster 2 is formed, but in most cases, whatever is formed
from the decomposition is currently beyond our full understand-
ing. This behavior supports the results obtained by cyclic
voltammetry which showed [W6S8(4-tert-butylpyridine)6]+ to
be extremely sensitive to reduction and other reactions in
solution, possibly due to impurities such as water or oxygen.20
In summary, extreme care must be taken in dealing with
solutions of clusters 3 and 4, especially 4. The fact that they
are generally unstable in DMF and acetonitrile might be
attributed to the trace amounts of water and amine impurities,
which are common in these solvents and very difficult to
eliminate.40
Thermogravimetric Analysis (TGA). Thermogravimetric
analyses were performed on compounds 3 and 4 as indicators
of bond strength or lability of the ligands. The TGA traces for
3 and 4 are available in the Supporting Information. The
decomposition temperature of 3 (indicated by the onset of the
weight loss) is 150 °C, about 100 °C lower than that of the
corresponding neutral cluster 1,20 but perhaps consistent with a
slightly increased W-P distance in 3. The total weight loss
corresponds to the loss of all six ligands. In contrast, the oxidized
cluster 4 begins its thermal decomposition at 200 °C, even
slightly higher than the neutral W6S8(4-tert-butylpyridine)6 (190
°C).20 This may indicate a slightly stronger W-N bond, though
a definite conclusion cannot be drawn without a crystal structure.
The amount of weight loss corresponds to the loss of ap-
proximately five ligands. It is interesting to note that while 4
seems to be significantly more sensitive to decomposition in
solution than 3, the opposite appears to be true with respect to
thermal decomposition of the solids.
[W6S8(PEt3)6]PF6 (3) is stable in CH2Cl2 and THF solutions
at room temperature for at least a week as indicated by no
change in the appearance of the solutions and no change in the
NMR spectra. 3 is stable at 100 °C in CH2Cl2 solution for at
least 8 h by the same NMR criterion. When 3 is dissolved in
acetonitrile or DMF at room temperature, it degrades slowly as
indicated by some color change. In some cases, neutral
1
W6S8(PEt3)6 is formed as evidenced by the H NMR spectra.
Unlike 3, 4 is quite sensitive. Although in a solution of
carefully purified CH2Cl2, 4 appears to be stable for days at
room temperature, it undergoes degradation rapidly at a slightly
higher temperature (50 °C). Even at room temperature, decom-
position occurs in a solution of THF within a few days. The
instability of the cluster cation in acetonitrile and DMF is even
more dramatic for 4. After dissolution of 4 in acetonitrile or
DMF, the original green solutions change color and some
amorphous solid precipitates within minutes. In some cases, the
(40) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, 1988.

Oxidized W6S8L6 Clusters
Inorganic Chemistry, Vol. 40, No. 12, 2001 2665
Conclusions
Research Science and Engineering Centers Program (DMR-
0079992). We thank Dr. Mingtao Ge and Prof. Jack Freed for
their assistance in collecting the ESR data. We also thank Prof.
J. M. J. Fre´chet and Mr. Frank Ling for the convenience of the
mass spectrometer.
We have successfully oxidized the W6S8L6 clusters with
[Cp2Fe]PF6. The products were studied with NMR spectroscopy,
mass spectroscopy, and X-ray crystallography (only for 3) and
are shown to be the desired oxidized [W6S8L6]PF6 clusters with
a MEC of 19. Magnetic property studies showed that they are
paramagnetic compounds.
Supporting Information Available: ESR spectrum of [W6S8-
(PEt3)6]PF6, 31P{1H} NMR spectra, ESI-MS, plots of ø vs 1/T, and
thermogravimetric traces of [W6S8(PEt3)6]PF6 and [W6S8(4-tert-
butylpyridine)6]PF6 and X-ray crystallographic file in CIF format for
the structure determination of [W6S8(PEt3)6]PF6‚2THF. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the Depart-
ment of Energy (Grant DE-FG02-87ER45298). This work made
use of the Polymer Characterization Facility and Electron and
Optical Microscopy Facility of the Cornell Center for Materials
Research through the National Science Foundation Materials
IC001099D