Angewandte
Communications
Chemie
[
21]
by one-electron oxidation. Besides thermal N cleavage,
us of the participation of the alkyne. When 5-decyne or
diphenylacetylene was used in place of 3-hexyne, a similar
result was obtained (complexes 3b and 3c resp.). While with
2
photochemical processes could also favor N splitting to
2
[
22]
terminal nitride.
Among the possible above mentioned
candidates for our study, the anionic (PCP)Mo nitride (1) was
selected and its reactivity towards alkyne was examined
1-phenyl-1-hexyne, two doublets (217.4 and 221.8 ppm,
2
J
= 43.1 Hz) coupling with each other were observed in
P,P)
(
3
1
(
Scheme 2). Indeed, we postulated that the complex being
P{H} NMR, indicating that the P atoms became inequivalent
overall anionic, the iodide ligand would act as a good leaving
group, allowing alkyne coordination under mild conditions.
Disappointedly, when internal alkynes such as 3-hexyne were
added, no reaction occurred even upon heating. Nevertheless,
when 1,10-phenanthroline was added to (PCP)Mo -nitrido
iodide (1) Na(15-crown-5), a green compound was formed
in complex 3d. After many attempts, complex 3a was
crystallized from a n-pentane solution. The X-ray structure
analysis confirmed an alkyne coordinated Mo-nitrido com-
plex (Figure 2, left), with the CꢀC bond almost perpendicular
to the MoꢀN bond (81.38). The alkyne is located in the
IV
equatorial plane of the pseudo square pyramidal complex.
These complexes do not lead to nitrile formation via meta-
thesis even upon heating at 808C for 36 h. This was
rationalized by DFT calculations (vide infra).
cleanly. This diamagnetic compound showed a singlet at
3
1
2
09.0 ppm in P{H} NMR, and could be crystallized from its
concentrated diethyl ether solution. X-ray crystallography
IV
study revealed that this complex was a neutral Mo -nitrido
It has been demonstrated that alkyne metathesis is
catalyzed by high oxidation state carbyne complexes of Mo
complex with a 1,10-phen ligand (Figure 1). Satisfyingly, the
À
I anion no longer coordinated to the Mo center.
or W. The requirement for the process to occur is the
À
[23]
Thus, a stronger driving force for I elimination than
intermediacy of a planar four membered ring.
We thus
IV
simple alkyne coordination had to be found. KOTf was
anticipated that oxidation of the Mo alkyne-nitrido com-
À
therefore added to the mixture of (PCP)Mo(N)I (1) and 3-
plexes would favor the metathesis reaction, by allowing the
hexyne in THF (Scheme 3). To our delight, a very clean
coplanarity of the MoꢀN and CꢀC bonds. Accordingly, when
+
reaction happened, and a new diamagnetic complex (3a) with
1 equiv. of [FeCp ] was added to the 5-decyne coordinated
2
3
1
a singlet at 223.3 ppm in P{H} NMR was observed. In
Mo-nitrido complex (3b) at room temperature, n-pentaneni-
trile (4b) was detected and quantified by GC-MS analysis in
85% yield (compared to authentic sample). Similarly, diphe-
nylacetylene also reacted smoothly in the same condition to
give benzonitrile (4c) in 66% yield. When asymmetric
internal alkyne such as 1-phenyl-1-hexyne was used, a mixture
of n-pentanenitrile (61%) and benzonitrile (14%) was
obtained (Scheme 3). At this point, the resulting paramag-
À
contrast, if only KOTf was added to (PCP)Mo(N)I (1), no
such complex was observed by NMR. These results convinced
V
netic Mo carbyne complexes could not be isolated in pure
form, but mass spectra analyses proved their formation (see
ESI). Furthermore, the reduction of the product mixture
+
resulting from the Fc addition to complex 3b gave two major
Scheme 2. Reactivity of complex 1 towards 3-hexyne and 1,10-phen.
compounds, one of which is 3b again, and the other
3
1
compound has two doublets in P{H} NMR (see ESI).
These inequivalent phosphorus atoms strongly suggest
IV
a pseudo square pyramidal Mo -carbyne with a nitrile
coordinated in the equatorial plane, in which the CꢀN bond
is perpendicular to the MoꢀC bond (vide infra).
V
In order to further verify the participation of Mo center,
V
Mo -nitrido complex, (PCP)Mo(N)I (5), was separately
IV
À
prepared by one-electron oxidation of (PCP)Mo (N)I (1).
X-ray study of complex 5 (Figure 2, right) revealed a pseudo
Figure 1. X-ray structure of complex 2. Thermal ellipsoids shown at
5
0% probability, except for the carbon atoms of tert-butyl groups.
Hydrogen atoms have been omitted for clarity. The CCDC number can
be found in the Supporting Information.
Figure 2. X-ray structure of complex 3a (left) and 5 (right). Thermal
ellipsoids shown at 50% probability, except for the carbon atoms of
tert-butyl groups. Hydrogen atoms have been omitted for clarity. The
CCDC numbers can be found in the Supporting Information.
Scheme 3. Cross-metathesis of complex 1 with alkynes.
2
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Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!
Hong, Xin
