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ACS Catalysis
donor properties.39 The driving force for this ligand exchange is
presumed to be associated with the superior π-acidity of the
alkyne ligands that stabilize 2 in the formally low (+I) oxidation
state. In addition, the high steric demands of IPr may facilitate
its dissociation. We propose that an initial association of alkyne
to an open coordination site in 1 promotes rapid IPr
dissociation, followed by dimerization to form 2; however, it is
not known whether the dissociation and dimerization occur in
sequence or in a concerted fashion. Attempts to study the
kinetics of this process by UV-Vis spectroscopy were
unsuccessful due to the rapid rate of reaction.
μB and a similar connectivity was confirmed by X-ray
diffraction (Figure S5, see ESI).
1
2
3
4
5
6
7
8
Si
Et
Fe
N
N
N
N
EtCCEt
Toluene
22 °C
N
Fe
Et
N
1/2
(3)
Fe
N
Si
Si
Et
Et
1
4
57Fe Mössbauer data for 2 and 3. The zero-field 57Fe
Mössbauer spectrum of compound 2 displays a Lorentzian
doublet with an isomer shift of 0.388(2) mm/s and a quadrupole
splitting of 1.678(4) mm/s at 5 K, while that of 3 exhibits an
isomer shift of 0.483(1) mm/s and a quadrupole splitting of
2.183(3) mm/s (Figure 3). Even though, to the best of our
knowledge, no similar Fe compounds exist in the literature with
Mössbauer analysis, the isomer shifts of 2 and 3 are comparable
to those of other low-coordinate Fe complexes.8,41,42
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52
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57
58
59
60
Heating complex 2 to 60 °C resulted in coupling of the
alkynes to form
a
dimetallacyclopentadiene complex,
{[DIPP(Me3Si)N]Fe}2(μ,κ2-C4Ph4) (3, in 46% isolated yield), in
which each of the Fe atoms have been oxidized by one electron
(formally to Fe(II), eq 2). Complex 3 has a temperature-
dependent magnetic moment (approximately 2.72 μB at room
temperature) on account of weak antiferromagnetic coupling
between the two Fe(II) centers (see ESI). Magnetic
susceptibility data for 2 were fitted to the Heisenberg-Dirac-van
―2퐽 푆 ⋅ 푆
2, where
퐽
is the
Vleck exchange Hamiltonian
1
coupling constant. Fitting to the experimental data gives
퐽 = ―70 cm -1
functional theory (DFT) estimate of
, which is in reasonable agreement with a density
퐽 = ―80 cm -1
for the
simpler model system {Ph(H3Si)]Fe}2(μ,κ2-C4Me4). The X-ray
structure reveals two distinct sets of dienyl C–C bond distances:
C1–C2 = C3–C4 = 1.399(2) Å and C2–C3 = 1.497(2) Å, which
correspond to double and single bonds, respectively (Figure 2).
A similar symmetric dimetallacyclopentadiene Ta complex was
recently reported, but was not studied as a catalyst for alkyne
cyclotrimerization.40
Ph
Ph
Si
Si
Ph
Ph
FeII
N
60 °C
Toluene
FeI
N
FeII
Ph
FeI
N
(2)
N
Si
Si
Ph
Ph
Ph
2
3
Figure 3. 57Fe Mössbauer spectra of 2 (upper) and 3 (lower) at 5
K. Red lines represent the respective compounds, blue lines are
minor impurity, and black lines are the total fit.
The lower isomer shift of 2 compared to 3 suggests that the
Fe centers in the alkyne complex possess less electron density,
which may be attributed to the alkyne’s ability to accept
electron density via π-backbonding. On the other hand, the
diene’s π* orbitals in compound 3 are not accessible to the Fe
for π-backbonding due to the geometrical constraint. It should
be noted, however, that a similar trend in the change in isomer
shifts was observed for the two-coordinate Fe(I/II) system, and
may be attributed to an increase in 4s-3d mixing for Fe(I)
complexes.42 The appearance of only one doublet in the
Mössbauer spectrum of 3 is consistent with symmetric
oxidative coupling of the alkynes, with each Fe being oxidized
by one electron.
Figure 2. Solid state structure of 3 shown with thermal ellipsoids
at 50% probability. Most of the phenyls, a molecule of diethyl
ether, and all hydrogen atoms have been omitted for clarity.
Catalytic Activity and Kinetics. Both complexes 2 and 3
catalyze the cyclotrimerization of diphenylacetylene at 70 °C in
the absence of IPr. A kinetic study was performed to probe the
mechanism of alkyne cyclotrimerization with this system using
the conditions shown in Scheme 3. Complex 4 was used
because the resulting product, hexaethylbenzene, was
Presumably, the large steric profile of the phenyl groups of
the alkyne ligand stabilizes complex 2, which then requires
elevated temperatures for coupling to form 3. In contrast,
treatment of 1 with one equivalent of 3-hexyne, at ambient
significantly
more
soluble
than
theproduct
temperature,
resulted
in
the
isolation
of
{[
DIPP(Me3Si)N]Fe}2(μ,κ2-C4Et4) (4), without observation of the
(hexaphenylbenzene) from catalysis with 2 or 3. The order in
[4] was determined by initial rate measurements using varying
concentrations of 4 while the concentration of 3-hexyne was
1
bis(alkyne) intermediate (by H NMR spectroscopy) (eq 3).
Analogous to 3, this complex has a magnetic moment of 2.87
3
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