New routes to transition metal-carbido species: Synthesis and characterization of the carbon-centered trigonal prismatic clusters [W 6CCl18]n- (n = 1, 2, 3)

Article abstract of DOI:10.1021/ja035962v

Simultaneous reduction of WCl₆ and CCl₄ with bismuth metal at 400 °C affords a black solid, from which the new cluster [W₆CCl₁₈]^2- is extracted into aqueous HCl. The cluster is isolated in 34% yield as (Bu₄N)₂[W₆CCl₁₈] and features a metal-metal bonded W6 trigonal prism centered by a carbon atom and surrounded by 12 edge-bridging and 6 terminal chloride ligands. A cyclic voltammogram of [W₆CCl₁₈]^2- in DMF shows the cluster undergoes two reduction and two oxidation processes, suggesting five chemically accessible redox states. Consistent with this extensive electrochemistry, DFT calculations on the diamagnetic [W₆CCl₁₈]^2- species reveal closely spaced frontier orbitals, with an a₂″ HOMO situated 0.61 and 0.71 eV below unoccupied e″ and e′ orbitals, respectively. Oxidation of the cluster by a single equivalent of NO⁺ gives [W₆CCl₁₈]^1-); which, as expected on the basis of the [W₆CCl₁₈]^2- HOMO character, possesses a less elongated W₆ trigonal prism. Reduction of [W₆CCl₁₈]^2- with a single equivalent of cobaltocene affords [W₆CCl₁₈]^2- wherein population of a low-lying e′ orbital leads to a significant Jahn-Teller distortion. Copyright

Full text of DOI:10.1021/ja035962v

Published on Web 08/27/2003
New Routes to Transition Metal-Carbido Species: Synthesis and
Characterization of the Carbon-Centered Trigonal Prismatic Clusters
[W6CCl18]n- (n ) 1, 2, 3)
Eric J. Welch, Nathan R. M. Crawford, Robert G. Bergman,* and Jeffrey R. Long*
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460
Received May 5, 2003; E-mail: jlong@cchem.berkeley.edu
Transition metal-carbon clusters have long been of interest as
fragments of refractory metal carbides and models for intermediates
in heterogeneous catalysis.1 In these molecules, carbon typically
serves as an interstitial or capping atom within the core of a
multinuclear cluster, particularly in early transition metal halide2
and late transition metal carbonyl3 clusters. Recently, however, this
chemistry has been extended to include a few complexes featuring
a carbon atom as a terminal donor ligand.4 While exploring new
routes to such complexes, our attention was caught by a report of
the structure of W2(µ-CCl)Cl7, an unusual one-dimensional solid
containing chlorocarbyne-bridged W2 units.5 In an effort to establish
a more facile synthesis of the compound, we attempted to produce
it using bismuth metal6 to reduce WCl6 in the presence of a
stoichiometric amount of CCl4. Herein, we report that this indeed
provides an effective means of incorporating carbon into a tungsten
chloride cluster, giving rise not to the intended W2 species, but
Figure 1. Structure of the carbon-centered trigonal prismatic cluster
[W6CCl18]2- in 1. The lines connecting W atoms are intended to emphasize
the W6 trigonal prism and do not necessarily represent full single bonds
(see text). Selected mean interatomic distances (Å) and angles (deg) for
the [W6CCl18]n- (n ) 1, 2, 3) clusters in 2, 1, and 3′, respectively (∆ )
triangle edge, | ) longer rectangle edge, b ) bridging, t ) terminal):
W-W∆ 2.74(1), 2.667, 2.71(5), W-W| 2.93(2), 3.028, 2.96(8), W-C 2.157-
(8), 2.1595, 2.15(3), W-Clb∆ 2.389(4), 2.381(5), 2.412(8), W-Clb| 2.433-
(6), 2.4585(5), 2.46(1), W-Clt 2.41(1), 2.443, 2.47(1), W-C-W∆ 78.9(5),
76.28, 78(2), W-C-W| 85.7(3), 89.02, 87(4), W-Clb∆-W 70.0(3), 68.14,
68(2), W-Clb|-W 74.1(4), 76.00, 74(2).
rather to a carbon-centered trigonal prismatic cluster, [W6CCl18]2-
exhibiting extensive redox chemistry.
,
The new cluster is extracted from a solid precursor produced at
high temperature. Under a nitrogen atmosphere, WCl6 (1.29 g, 3.26
mmol), CCl4 (0.251 g, 1.63 mmol), and Bi (0.908 g, 4.35 mmol)
were sealed in an evacuated Pyrex ampule, and then heated in a
tube furnace at 400 °C for 3 days. Colorless BiCl3 was sublimed
away at 360 °C to leave a black solid. The solid was removed from
the ampule in air and stirred in 120 mL of 1% HCl for 30 min to
give a dark green solution. Addition of (Bu4N)Cl (0.50 g, 1.8 mmol)
induced formation of a dark green solid, which was recrystallized
from acetonitrile to yield 0.388 g (34%) of (Bu4N)2[W6CCl18] (1)
as black octahedron-shaped crystals.7
Presumably, the original cluster-containing solid in the prepara-
tion arises according to the following reaction.
Figure 2. Cyclic voltammogram of compound 1 in DMF, with potentials
referenced to Cp2Fe0/1+. A scan rate of 1500 mV/s was employed to improve
the reversibility of the leftmost couple.
6WCl6 + CCl4 + 8Bi f W6CCl16 + 8BiCl3
(1)
separation of 3.028(1) Å, for which Pauling’s empirical relation
D(n) ) D(1) - 0.60 log n suggests a W-W bond order of just n
) 0.20.12 The W-C distance of 2.1595(6) Å is close to the
analogous distance of 2.197 Å in WC, the structure of which indeed
features a trigonal prismatic coordination of carbon.13 Thus, the
central W6C unit of [W6CCl18]2- can be viewed as a fragment of
WC stabilized by 18 surrounding chlorine atoms. These chlorine
atoms bridge the edgesswith one spanning each triangle edge and
two spanning each of the longer rectangle edgessand cap the
vertexes of the W6 trigonal prism in a manner that preserves its
The ensuing black powder is amorphous by X-ray diffraction, but
likely possesses an extended two-dimensional [W6CCli12]Cla2Cla-a
4/2
connectivity similar to that observed in W6Cl12.8 Possibly because
of a reaction with the surface of the Pyrex ampule at high temper-
ature, the use of 3 equiv of CCl4 in reaction 1 instead of just 1
equiv was found to result in higher yields. Reactions employing 2
equiv gave, upon workup, a mixture of [W6Cl14]2- and [W6CCl18]2-
while those employing 4 equiv afforded no soluble product.
,
X-ray analysis9 of a crystal of 1 revealed the carbon-centered
[W6CCl18]2- cluster depicted in Figure 1.10 Therein, the six tungsten
atoms are arranged in a regular trigonal prism, instead of the
octahedral geometry usually observed for centered hexanuclear
clusters of the early transition metals.2 Within the triangular faces
of the prism, the W-W distance of 2.667(1) Å is slightly longer
than the mean W-W single bond distance of 2.607(4) Å observed11
for the face-capped octahedral cluster [W6Cl14]2-. In contrast, the
longer edges of the rectangular faces of the prism exhibit a W-W
D3h symmetry. Notably, this cluster geometry has been encountered
previously only within the one-dimensional solids A3Nb6SBr17 (A
) K, Rb, Cs, Tl).10,14,15
The cyclic voltammogram of compound 1 in DMF (Figure 2)
reveals an electrochemistry that is remarkably rich for such a
compact cluster. Two reduction processes are apparent at E1/2
)
-0.649 V (∆Ep ) 88 mV) and E1/2 ) -1.360 V (∆Ep ) 142 mV)
versus Cp2Fe0/1+, corresponding to [W6CCl18]2-/3- and [W6CCl18]3-/4-
9
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J. AM. CHEM. SOC. 2003, 125, 11464-11465
10.1021/ja035962v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
dramatic, Jahn-Teller distortions have been observed previously
in octahedral Mo6 and Re6 clusters.20
Future work will attempt to extend this new method for
incorporating carbon into clusters, will probe the generality of the
centered trigonal prismatic cluster geometry, and will employ these
cluster units in the synthesis of microporous coordination solids
with noncubic symmetry.
Acknowledgment. This research was funded by NSF (CHE-
0111164 and CHE-0094349) and DOE (DE-FG03-01ER15257).
Figure 3. Left: Depiction of the central portion of one of the lowest
unoccupied e′ orbitals of [W6CCl18]2-, as calculated using DFT. Right:
Corresponding portion of the structure of the [W6CCl18]3- cluster in 3′,
displaying a Jahn-Teller distortion consistent with population of this orbital;
W-W distances are given in Å.
Supporting Information Available: Full experimental details on
the preparations of 1-3, and results from the DFT calculations
performed on [W6CCl18]2- (PDF). X-ray crystallographic files (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
couples, respectively. In addition, two oxidation processes occur
at E1/2 ) 0.407 V (∆Ep ) 64 mV) and E1/2 ) 0.974 V (∆Ep ) 58
mV), corresponding to [W6CCl18]2-/1- and [W6CCl18]1-/0 couples,
respectively. The reversibility of the latter couple is improved
significantly at higher scan rates, suggesting a reduced solubility
for [W6CCl18]0. Potentially, all five redox states of the cluster will
be chemically accessible.
Consistent with its electrochemistry, DFT calculations16 per-
formed on the diamagnetic [W6CCl18]2- cluster revealed many
closely spaced energy levels in the frontier orbital region.17 The
seven highest-energy occupied orbitals are primarily W-W bonding
in character. Situated 0.86 eV above an a1′ orbital, the a2′′ HOMO
exhibits W-W bonding character within the W3 triangles, but
W-W antibonding character between opposing W3 triangles.
Unoccupied e′′ and e′ orbitals lie 0.61 and 0.71 eV above the
HOMO, respectively, and also feature a mixture of W-W bonding
and antibonding character. Given their comparable energies, it is
difficult to predict just which of these orbitals would be occupied
upon reduction of the cluster.
References
(1) Muetterties, E. L.; Stein, J. Chem. ReV. 1979, 79, 479.
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Corbett, J. D. J. Alloys Compd. 1995, 229, 40 and references therein. (c)
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Chem. Soc. 2002, 124, 1580.
(5) Beck, J.; Wolf, F. Z. Anorg. Allg. Chem. 2002, 628, 1453.
(6) Bismuth metal had previously been shown to be a particularly effective
reductant in the synthesis of clusters such as [W6Cl14]2- and [W6O6Cl12]2-
:
(a) Kolesnichenko, V.; Messerle, L. Inorg. Chem. 1998, 37, 3660. (b)
Crawford, N. R. M.; Long, J. R. Inorg. Chem. 2001, 40, 3456.
(7) Characterization of 1: UV-vis spectrum (MeCN) λmax (ꢀM) 217 (56 100),
247 (24 200), 289 (19 900), 313 (15 900), 415 (3670), 481 (1970), 608
(1400), 776 (655) nm. 13C NMR (MeCN-d3): δ 280.12 (t, JWC ) 25 Hz)
ppm. ES--MS (MeCN): m/z 876 ([W6CCl18]2-), 1718 ([W6CCl17]1-), 1995
({(Bu4N)[W6CCl18]}-). Anal. Calcd for C33H72Cl18N2W6: C, 17.71; H,
3.24; N, 1.25. Found: C, 18.08; H, 3.36; N, 1.07.
(8) Scha¨fer, H.; von Schnering, H.-G. Z. Anorg. Allg. Chem. 1967, 353, 281.
(9) Crystal and structure refinement parameters for 1: C33H72Cl18N2W6, T )
129 K, Fd3hc, Z ) 32, a ) 36.0040(5) Å, V ) 46671.6(11) Å3, R1
)
Addition of a single equivalent of (NO)(BF4) to an acetonitrile
solution of 1 prompts a color change from green to purple, signaling
formation of the one-electron oxidized species [W6CCl18]1-. Cooling
the solution to -40 °C permitted isolation of (Bu4N)[W6CCl18] (2)
in 87% yield as a black precipitate.18 X-ray analysis9 of a black,
irregularly shaped crystal of 2 obtained directly from the reaction
solution confirmed retention of the trigonal prismatic cluster
geometry. Showing no significant deviations from D3h symmetry,
the monoanion differs from the dianion most notably in its W-W
distances, which are longer along the triangular edges and shorter
along the other three edges of the trigonal prism (see Figure 1
legend). These differences are consistent with removal of an electron
0.0428, wR2 ) 0.1142. 2: C17H36Cl18NW6, T ) 113 K, P1h, Z ) 2, a )
12.4658(5), b ) 13.2168(5), c ) 13.6449(5) Å, R ) 99.757(2)°, â )
113.240(1)°, γ ) 92.073(2)°, V ) 2022.59(13) Å3, R1 ) 0.0492, wR2 )
0.1193. 3′: C79H73Cl18NOP3W6, T ) 120 K, P21/n, Z ) 4, a ) 14.069-
(3), b ) 39.39(1), c ) 15.887(3) Å, â ) 93.93(1)°, V ) 8782(3) Å3, R1
) 0.0563, wR2 ) 0.1117.
(10) While this manuscript was in press, a report describing two solids
containing clusters of this type appeared: Zheng, Y.-Q.; von Schnering,
H. G.; Chang, J.-H.; Grin, Y.; Engelhardt, G.; Heckmann, G. Z. Anorg.
Allg. Chem. 2003, 629, 1256.
(11) Zietlow, T. C.; Schaefer, W. P.; Sadeghi, B.; Hua, N.; Gray, H. B. Inorg.
Chem. 1986, 25, 2195.
(12) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University
Press: Ithaca, NY, 1960; Chapter 7.
(13) Leciejewicz, J. Acta Crystallogr. 1961, 14, 200.
(14) (a) Womelsdorf, H.; Meyer, H.-J. Angew. Chem., Int. Ed. Engl. 1994, 33,
1943. (b) Womelsdorf, H.; Meyer H.-J. Z. Anorg. Allg. Chem. 1996, 622,
2083.
from the a2′′ HOMO calculated for [W6CCl18]2-
.
The one-electron reduced cluster [W6CCl18]3- was generated by
adding 1 equivalent of cobaltocene to an acetonitrile solution of 1.
Addition of ether to the ensuing green-brown solution precipitated
(Cp2Co)(Bu4N)2[W6CCl18] (3) in 95% yield.19 Crystals of 3
exhibited extensive disorder; however, metathesis with (Ph4P)Cl
led to black rectangular plate-shaped crystals of (Ph4P)3[W6CCl18]‚
MeCN‚Et2O (3′) suitable for X-ray analysis.9 Therein, the trianionic
cluster displays a significant distortion away from D3h symmetry,
as shown at the right in Figure 3. Relative to the structure of the
dianion in Figure 1, the distortion mainly involves pinching the
two foremost W atoms together to give a shorter W-W| distance
of 2.8497(9) Å, while simultaneously pulling these same two atoms
forward (away from the rearmost W4 rectangle) to give four longer
W-W∆ distances in the range 2.7405(10)-2.7601(10) Å. These
changes are fully consistent with a Jahn-Teller distortion arising
from population of one of the low-lying unoccupied e′ orbitals (see
Figure 3, left) calculated for [W6CCl18]2-. Similar, albeit much less
(15) A trigonal prismatic arrangement of metal atoms also occurs in the
noncentered cluster [Tc6Cl12]
2-; however, with fewer bridging chlorides
and Tc-Tc multiple-bonding, this species exhibits a very different
electronic structure: Wheeler, R. A.; Hoffmann, R. J. Am. Chem. Soc.
1986, 108, 6605 and references therein.
(16) DFT calculations (HCTH407/LANL2DZ) utilized: HPCCG’s NWChem,
A Computational Chemistry Package for Parallel Computers, version 4.5;
PNNL: Richland, WA, 2003. See Supporting Information for details.
(17) See ref 14b for another analysis of the bonding in this structure type.
(18) Characterization of 2: UV-vis spectrum (MeCN) λmax (ꢀM) 309 (19 500),
425 (2110), 506 (2340), 730 (2580) nm. ES--MS (MeCN): m/z 876
([W6CCl18] ] )
2-), 1718 ([W6CCl17 1-), 1995 ({(Bu4N)[W6CCl18]}-). µeff
2.31 µB at 295 K. Anal. Calcd for C17H36Cl18NW6: C, 10.23; H, 1.82; N,
0.70. Found: C, 10.35; H, 1.74; N, 0.73.
(19) Characterization of 3: UV-vis spectrum (MeCN) λmax (ꢀM) 262 (81 200),
299 (16 700), 442 (4180), 645 (2020), 801 (1290), 1002 (1130) nm. ES--
MS (MeCN): m/z 876 ([W6CCl18]2-), 1718 ([W6CCl17]1-), 1995 ({(Bu4N)-
[W6CCl18]}-). µeff ) 3.05 µB at 295 K. Anal. Calcd for C43H82Cl18
-
CoN2W6: C, 21.28; H, 3.41; N, 1.15. Found: C, 21.08; H, 3.53; N, 1.11.
(20) (a) Saito, T.; Yamamoto, N.; Nagase, T.; Tsuboi, T.; Kobayashi, K.;
Yamagata, T.; Imoto, H.; Unoura, K. Inorg. Chem. 1990, 29, 764. (b)
Baudron, S. A.; Deluzet, A.; Boubekeur, K.; Batail, P. Chem. Commun.
2002, 2124.
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