Organometallics
Article
of the siloxide ligands in MoSiF9 and MoSi*F9 might be
favorable to reduce crystallographic disorder associated with
fluoroalkoxide ligands, we aimed at isolating similar MoCBD
complexes from their reaction with an excess of 3-hexyne
1.424(8) Å, respectively, a short−long−short−long alternation
of bond lengths within the MoC ring is observed. In principle,
3
these structural parameters are similar to those recently
determined for [(C
Et )Mo(OSiPh ) ]; however, with a τ
3
3 3 3 5
value of 0.37 and a slightly less pronounced bond alternation,
(Scheme 3).
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this complex is even more distorted toward TBP. This
structure is usually associated with the transition state for
MCBD interconversion, which involves inverting the SP
structure and swapping the apical positions. It is also
noteworthy that the siloxide ligand is no longer bound in a
Scheme 3. Preparation of Molybdenaclobutadienes
2
κ fashion, as found in the precursor MoSiF9, but features a
short Mo−O1 bond length of 1.869(4) Å with an almost linear
Mo−O1−Si arrangement of 173.3(2)°, revealing a significant
degree of π-donation toward the Mo atom.
Purple crystals of MoSi*F9-MCBD were isolated as
described above by storing a solution of MoSi*F9 with 10
equiv of 3-hexyne in n-pentane at −38 °C (Scheme 3). An X-
ray diffraction analysis provided another MoCBD structure;
however, a modulation is observed that affords four
independent molecules in the asymmetric unit. In addition,
disorder could only be fully resolved in one of the four
molecules, and the structural parameters should therefore be
treated with caution. Crystallographic details and ORTEP
An NMR spectroscopic characterization of both MCBD
complexes revealed different stabilities in solution. Dissolving
crystalline MoSiF9-MCBD in CD Cl or toluene-d leads to a
In the case of MoSiF9, the addition of 10 equiv of 3-hexyne
to a saturated n-pentane solution at −38 °C produced an
immediate characteristic color change from yellow to purple,
and MoSiF9-MCBD could be isolated as purple crystals by
storing this solution in the refrigerator at −38 °C for a couple
of days. An X-ray diffraction analysis confirmed the formation
of the expected metallacycle and provided a second example of
2
2
8
fast decolorization; the characteristic pink color can be
restored by cooling, which suggests the existence of an
Et
equilibrium with the alkylidyne complex MoSiF9 and 3-
1
hexyne. Accordingly, the H NMR spectra show almost
exclusively the signals for the alkylidyne complex and 3-hexyne
1
9
and one signal in the F NMR spectra at room temperature.
1
9
At lower temperatures, two F NMR signals are observed, for
example, at −72.9 and −72.0 ppm in toluene-d at −40.7 °C,
8
which can be assigned to the alkylidyne and MCBD species,
respectively. In contrast, solutions of MoSi*F9-MCBD retain
1
9
their characteristic pink color at room temperature, and the F
NMR spectrum in toluene-d shows a major signal at −72.6
8
ppm for the MCBD complex together with a minor signal at
−
73.1 ppm for the alkylidyne complex in ∼94:6 ratio,
indicating a significantly higher stability of MoSi*F9-MCBD
compared to MoSiF9-MCBD. MCBD formation could be
confirmed in both cases by an observation of characteristic
Figure 3. ORTEP diagram of MoSiF9-MCBD with thermal
displacement parameters drawn at 30% probability; hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and angles [deg]:
Mo−C1 1.877(5), Mo−C3 1.924(6), Mo−O1 1.869(4), Mo−O5
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low-field C NMR signals at 265.5 and 152.3 ppm (in CD Cl
2
2
at −39.6 °C for MoSiF9-MCBD) and at 265.1 and 151.1 ppm
(
in toluene-d at room temperature for MoSi*F9-MCBD).
8
These chemical shifts can be assigned to the α- and β-carbon
2
.058(3), Mo−O6 2.053(3), C1−C2 1.480(8), C2−C3 1.424(8),
Mo−O1−Si 173.3(2), O1−Mo−C1 131.9(2), O1−Mo−C3
44.8(2), O5−Mo−O6 162.56(14).
atoms of the MoC fragment and are in good agreement with
3
1
the NMR data reported for other MoCBD com-
7
,12,60,63,72
plexes,
MoF9-MCBD.
The latter complex, MoF9-MCBD, could also be observed
for example, 269.9 and 156.0 ppm for
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environment around the Mo atom is best described as
distorted square-pyramidal (SP) with C1 at the apex in
16
agreement with a τ value of 0.30 based on the two largest
in solution at room temperature, indicating a significant
higher stability compared to MoSiF9-MCBD and a similar
stability compared to MoSi*F9-MCBD. To analyze this
surprisingly different behavior, variable-temperature NMR
studies were performed. For this purpose, the crystalline
5
angles, specifically, O5−Mo−O6 = 162.56(14)° and O1−
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Mo−C3 = 144.8(2)°. The alternative description as trigonal-
bipyramidal (TBP) is clearly less appropriate, as the angles
O1−Mo−C1 and O1−Mo−C3 of 131.9(2)° and 144.8(2)°
differ quite significantly. In agreement with the SP assignment,
the Mo−C bond lengths of 1.877(5) Å (Mo−C1) and
MCBD complexes were dissolved in toluene-d , and their
8
reversible [2 + 2]-cycloreversion/cycloaddition reactions
involving equimolar amounts of 3-hexyne and the respective
1
.924(6) Å (Mo−C3) are markedly different, and together
1
9
with the C1−C2 and C2−C3 bond lengths of 1.480(8) Å and
propylidyne complex were monitored by F NMR spectros-
2
011
Organometallics 2021, 40, 2008−2015