C O M M U N I C A T I O N S
yield via alkyne metathesis of substrate 8 using 1 and M under
precipitation-driven conditions.12 These preparatory scale reactions
demonstrate the high activity and durability of the POSS M catalyst.
In conclusion, POSS ligands successfully activate precatalyst 1
for alkyne polymerization and alkyne metathesis. Pentacoordinated
t
complex 5‚HN(Ar)( Bu) has activity analogous to that of the silica-
supported Mo complex. The bulky POSS M ligand stabilizes the
monomeric Mo alkylidyne complex and shows no unwanted alkyne
polymerization. In contrast, the multidentate POSS ligands D and
T with 1 were accompanied by considerable alkyne polymerization
as well as alkyne metathesis. Aniline coordination and the reactivity
of Mo(VI) complexes with multidentate POSS ligands give insight
into the active species that are involved in alkyne polymerization
and alkyne metathesis.
t
Figure 1. SHELXTL plot of 5‚HN(Ar)( Bu), showing 35% probability
ellipsoids for non-H atoms. Cyclopentyl ligands, H atoms, and dis-
ordered positions were omitted for clarity. The sum of the angles
of
N(1)-Mo-O(1),
N(1)-Mo-O(14),
N(2)-Mo-O(1),
and
N(2)-Mo-O(14) is 357(3)°.
Acknowledgment. We would like to thank Dr. Wei Zhang for
helpful discussions. This work was supported by the National
Science Foundation (Grants CHE 01-03447 and CHE 03-45254).
Supporting Information Available: Experimental details, char-
t
acterization data, and X-ray data for 5‚HN(Ar)( Bu). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Figure 2. Alkyne metathesis rate (TOF) dependence on the equivalent of
(
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-1 -1
M. Conditions: 20 µL of 2, 700 µL of toluene-d8, rt. TOF ) molp‚molc ‚s
.
4
(
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t
the role of HN(Ar)( Bu) in retarding alkyne metathesis. The most
t
likely mechanism for metathesis with 5‚HN(Ar)( Bu) involves initial
dissociation of the labile aniline ligand. Incidentally, the pentaco-
t
ordinated complex 5‚HN(Ar)( Bu) is metathesis active (entry 9) with
a rate similar to 1 + M (2 equiv) and is selective for metathesis
similar to 1 + silica (dried at 400 °C).
To further investigate the function of M as an activator for 1,
(
the reaction rates for alkyne metathesis of 2 were measured using
0.50 mol % of 1 and variable quantities of M. Interestingly, TOF
values were observed to increase sharply as the equivalent of M
increased (Figure 2). This result is contrary to the expectation that
(4) (a) Wengrovius, J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1981,
3
equiv of M would fully activate complex 1 by substituting the
1
03, 3932. (b) Strutz, H.; Dewan, J. C.; Schrock, R. R. J. Am. Chem.
2c,4c
three amide ligands.
One possibility is that association of the
Soc. 1985, 107, 5999. (c) Zhang, W.; Kraft, S.; Moore, J. S. J. Am. Chem.
Soc. 2004, 126, 329.
third silanolate ligand is competitive with amide binding. Alterna-
tively, excess M might associate with the aniline byproduct,
decreasing its interaction with the molybdenum center. For example,
(
5) (a) Dugas, V.; Chevalier, Y. J. Colloid Interface Sci. 2003, 264, 354. (b)
Zhuravlev, L. T. Colloids Surf. A 2000, 173, 1.
(6) We saw 1.6-1.7 equiv of free aniline by NMR when we added silica
1
treated at 400 °C to complex 1. The monosiloxy complex is likely to be
t
excess M could associate with HN(Ar)( Bu) through hydrogen
present, but we cannot fully account for the observed additional 0.6-0.7
equiv of free aniline without considering additional ligand displacement.
7) Why catalysts from D and T cause polymerization while M does not
remains open to speculation. The bite angle for bidentate or tridentate
ligands is certainly smaller than the coordination angle by monodentate
ligands. As a result, the space opposite of the chelating ligand, where the
alkyne substrate approaches, is expanded. In this scenario, the alkyne
substrate might insert repeatedly into the Mo-C bond to give polymers.
8) Shih, K.-Y.; Schrock, R. R.; Kempe, R. J. Am. Chem. Soc. 1994, 116,
bonding.11
(
(
(
8804.
9) Tsai and co-workers reported the X-ray structure of a tetracoordinated
molybdenum alkylidyne complex with three alkoxy ligands in which a
free aniline did not interact with the Mo center. See ref 2c.
(
10) Similarly, Schrock suggested that a two-electron donor could block
metathesis by binding strongly to the metal to give a five-coordinated
species. See: Schrock, R. R. Polyhedron 1995, 14, 3177.
11) To investigate the interaction between HN(Ar)( Bu) and M, 1H NMR
t
(
t
signals of HN(Ar)( Bu) at variable equivalents of M in the absence of 1
1
were monitored in toluene-d
8
at room temperature. The H NMR signals
t
in the aromatic region of HN(Ar)( Bu) shifted to higher frequency, while
t
those of Bu shifted to lower frequency as the amount of M increased.
However, the ∆δ is too small for a reliable determination of the association
constant (less than 0.01 ppm). The same trends were found in the mixture
of 1 and M as the amount of M increased from 1 to 6 equiv. The proton
The POSS M catalyst was applied to RCAM2e (ring closing
alkyne metathesis) and cyclooligomerization reactions. RCAM
of diyne13 6 with 1 and M gave a mixture of monomeric macrocycle
12
t
resonances of the Ar and Bu groups changed from 6.300 to 6.319 ppm
and from 1.177 to 1.170 ppm, respectively. This fact partially supports
the idea that the additional 3 equiv of M in the solution weaken the
interaction between Mo and aniline.
7
a and dimeric macrocycle 7b under vacuum-driven conditions.
We also applied 1 + M to the cyclooligomerization of arylene
ethynylenes, a reaction that requires highly active and robust
catalysts. Tetrameric macrocycle 9 was obtained in high isolated
(12) Zhang, W.; Moore, J. S. J. Am. Chem. Soc. 2004, 126, 12796.
13) Brizius, G.; Bunz, U. H. F. Org. Lett. 2002, 4, 2829.
(
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J. AM. CHEM. SOC.
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