Angewandte
Chemie
alkylidyne carbon atom is observed in the expected downfield
range at d = 285.6 ppm with
coupling constant 1J-
a
(13C,183W) = 272 Hz. The formation of a Cs-symmetric com-
plex is indicated by the observation of one set of resonances
for the alkoxide ligands together with two quartets for the
diastereotopic CF3 groups in the 13C and 19F NMR spectra.
Single crystals of 1 suitable for X-ray crystal structure
determination[20] were obtained from a saturated diisopropyl
ether solution at À358C, and the structure of one of the two
independent molecules in the asymmetric unit is shown in
Figure 1; the crystal structure confirms the formation of a
monomeric tungsten alkylidyne complex with a slightly
distorted tetrahedral geometry.[12]
Figure 2. ORTEP diagram of 2 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths [] and angles [8]: W-
C1 1.992(4), W-C2 2.209(4), W-C3 1.879(4), W-N1 1.853(3), W-O1
2.034(3), W-O2 2.043(3), C1-C2 1.387(6), C2-C3 1.533(6), N1-C10
1.309(5); W-N1-C10 162.6(3), C1-W-N1 152.40(16), C3-W-N1
126.28(16), O1-W-O2 157.93(11), W-C1-C2 79.4(3), W-C3-C2 80.0(2),
C1-C2-C3 119.3(3).
À
À
ment, the W C bond lengths are markedly different (W C3
À
À
1.879(4), W C1 1.992(4) ), and together with the C1 C2
À
and C2 C3 bond lengths (1.387(6) and 1.533(6) , repec-
tively), a clear short-long-short-long alternation of bond
lengths within the WC3 ring is observed. This effect is much
more pronounced than those previously reported for the
related complexes [W(C3Et3){OCH(CF3)2}3][11] and [W-
(C3Et3){O-2,6-C6H3(iPr)2}3].[13]
Figure 1. ORTEP diagram of one of the two independent molecules 1
with thermal displacement parameters drawn at 50% probability.
Selected bond lengths [] and angles [8] in molecule 1/molecule 2:
W1-C1 1.768(3)/1.764(3) , W1-N1 1.852(2)/1.844(2), W1-O1
1.929(2)/1.936(2), W1-O2 1.927(2)/1.923(2), N1-C6 1.315(4)/1.328(4);
W1-C1-C2 171.8(2)/173.2(2), W1-N1-C6 164.2(2)/162.1(2).
To test complex 1 as a catalyst for preparative alkyne
metathesis, we investigated the homodimerization of 1-
phenylpropyne (3a) at room temperature under low pressure.
In
a typical experiment, a solution of 3a (260.0 mg,
2.24 mmol) and 1 (18.1 mg, 22 mmol) in hexane (15 mL) was
stirred for 30 min at 350 mbar to remove 2-butyne continu-
ously (Scheme 3). Filtration through alumina and elution with
The reactivity of 1 towards alkynes was investigated by
treatment of a hexane solution with a tenfold excess of 3-
ꢀ
hexyne (EtC CEt), leading to an instantaneous color change
from yellow-orange to deep red. Cooling of this reaction
mixture to À358C afforded crystalline red plates, which were
subjected to X-ray diffraction analysis.[20] The resulting
molecular structure is shown in Figure 2, and it reveals that
the metallacyclobutadiene complex 2 has formed. Its forma-
tion can be rationalized by exchange of the neopentylidyne
Scheme 3. Cross-metathesis of 1-phenylpropynes.
ꢀ
tungsten moiety {Me3CC W} in 2 for a propylidyne moiety
ꢀ
ꢀ
{EtC W} with concomitant formation of the alkyne Me3CC
CEt and subsequent [2+2] cycloaddition of the intermediate
alkylidyne complex with a second equivalent of 3-hexyne. The
coordination geometry around tungsten is best described as
square-pyramidal (SP) with C3 at the apex; the basal atoms
hexane afforded diphenylacetylene (tolane, 4a) in greater
than 90% yield after evaporation of the solvent. To compare
the catalytic performance of 1 with the most widely used
À
ꢀ
are coplanar to within Æ 0.04 , but the W C3 vector makes
homogeneous alkyne metathesis catalyst [Me3CC W-
an angle of 228 to the normal to this plane. The alternative
description as trigonal bipyramidal with axial alkoxide and
equatorial imido and C3Et3 ligands is less appropriate, as the
angles N1-W-C1 (152.40(16)8) and N1-W-C3 (126.28(16)8)
(OCMe3)3],[7a] the homodimerization of 3a was monitored
by GC with catalyst loadings of 1 mol% (Figure 3); this
experiment revealed that the room-temperature catalytic
performance of 1 is superior. The same trend is observed in
the homodimerization of the more sterically hindered 1-(2-
methylphenyl)propyne (3b), which is completed within seven
À
À
differ significantly and as the W O1 and W O2 bonds are
considerably tipped away from the WC3 ring, with an O1-W-
O2 angle of 157.93(11)8. In agreement with the SP assign-
ꢀ
hours under the above conditions. In contrast, [Me3CC
Angew. Chem. Int. Ed. 2007, 46, 8890 –8894
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8891
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