Ligand effects on hydrogen atom transfer from hydrocarbons to three-coordinate iron imides

Article abstract of DOI:10.1021/ic300870y

A new β-diketiminate ligand with 2,4,6-tri(phenyl)phenyl N-substituents provides protective bulk around the metal without exposing any weak C-H bonds. This ligand improves the stability of reactive iron(III) imido complexes with Fe=NAd and Fe=NMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates Fe=N-C bending, a longer Fe=N bond, and greater access to the metal as potential reasons for the increase in C-H bond activation rates.

Full text of DOI:10.1021/ic300870y

Article
pubs.acs.org/IC
Ligand Effects on Hydrogen Atom Transfer from Hydrocarbons to
Three-Coordinate Iron Imides
Ryan E. Cowley and Patrick L. Holland*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: A new β-diketiminate ligand with 2,4,6-tri-
(phenyl)phenyl N-substituents provides protective bulk around
the metal without exposing any weak C−H bonds. This ligand
improves the stability of reactive iron(III) imido complexes with
FeNAd and FeNMes functional groups (Ad = 1-adamantyl;
Mes = mesityl). The new ligand gives iron(III) imido complexes
that are significantly more reactive toward 1,4-cyclohexadiene
than the previously reported 2,6-diisopropylphenyl diketiminate
variants. Analysis of X-ray crystal structures implicates FeN−C
bending, a longer FeN bond, and greater access to the metal as potential reasons for the increase in C−H bond activation
rates.
LMe,Ph3FeNAd and LMe,Ph3FeNMes, which are more stable in
INTRODUCTION
■
the absence of substrate than LMe,iPr2FeNAd, and have found
Hydrogen atom transfer (HAT) is an elementary reaction in
that their HAT rate constants correlate with imido ligand
which both a proton and an electron are moved in a single
basicity. Finally, we show that the HAT rate is modulated by
kinetic step.1 Many metal−oxo (MO) complexes are known
the supporting β-diketiminate ligand, and that the shape of the
to abstract H-atoms from organic substrates to form metal−
binding pocket is more important than simple measures of
hydroxo complexes,2 both in biological3 and model systems.4
ligand size for rationalizing HAT reactivity.
The imido ligand is isoelectronic to the oxo ligand, but HAT is
less well studied for metal−imido (MNR) complexes. HAT
reactions to imido ligands are rarely observed directly,5 despite
the strong implication of HAT in many systems.6 For
imidometal complexes, the N substituent enables electronic
and steric tuning of the imido fragment, a feature not available
in oxo complexes. Therefore, comparing HAT rates for metal−
imido complexes containing different imido N-substituents can
help elucidate electronic and steric effects on the HAT
mechanism.
RESULTS AND DISCUSSION
■
Preparation of a Novel Diketiminate and its Iron(II)
Complex. The most common synthetic route to the
diketimine proligand is the acid-catalyzed condensation
between a diketone and 2 equiv of an amine.12 Following
this precedent, heating 2 equiv of 2,4,6-triphenylaniline with
2,4-pentanedione in the presence of 1 equiv of HOTs (Ts = p-
tolylsulfonyl) affords the novel diketimine LMe,Ph3H (1)
(Scheme 1). The rate of this condensation reaction is much
slower than similar condensation reactions with bulky anilines
such as 2,6-diisopropylaniline. In the reaction to form 1,
refluxing in toluene (bp 111 °C) afforded only ∼70%
conversion after 10 days; however, full conversion can be
achieved in refluxing xylenes (bp 139−141 °C) within 24 h.
Ligand 1 was isolated as a bright yellow crystalline solid in 74%
yield after basic workup and crystallization from CH2Cl2/
MeOH. Extended heating under vacuum (120 °C for 12 h) was
required to completely remove solvent trapped in the solid.
The crystal structure of 1 (Figure 1) reveals that 1 exists in
the solid state as the enaminoimine tautomer. The downfield
Previously, the (β-diketiminato)iron imido complex
LMe,iPr2FeNAd (LMe,iPr2 = 2,4-bis(2,6-diisopropylphenylimido)-
3-pentyl; Ad = 1-adamantyl) was reported,7 and it was capable
of abstracting hydrogen atoms from 1,4-cyclohexadiene (CHD)
and other related hydrocarbons in the presence of 4-tert-
butylpyridine (tBupy).8−10 In the absence of substrate, the
imido complex decayed by intramolecular HAT by abstracting
H• from one of the flanking isopropyl groups of the β-
diketiminate ligand. This intramolecular decomposition limited
the imido complex to activating C−H bonds that were
significantly weaker than the benzylic C−H bonds of the
isopropyl groups (ca. 84 kcal/mol). As part of our efforts to
design systems that prevent unwanted attack on the supporting
ligand, we report here a modified ligand LMe,Ph3 (LMe,Ph3 = 2,4-
bis(2,4,6-triphenylphenylimido)-3-pentyl) that features phenyl
groups in place of isopropyl substituents,11 and which places no
weak C−H bonds near the metal center. Using this new ligand,
we have prepared the new iron(III) imido complexes
1
N−H resonance in the H NMR spectrum at δ 12.2 ppm is
consistent with the enamine proton that is engaged in an
intramolecular hydrogen bond, and suggests that the
Received: April 28, 2012
Published: July 16, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of the Ligand LMe,Ph3H (1) and Its Li
a
(2) and FeBr (3) Complexes
Figure 2. Absorption spectra for LMe,Ph3H (1) in CH2Cl2 and
L
Me,Ph3Li(OEt2) (2) in Et2O, and normalized emission spectrum of 2
in Et2O with excitation wavelength λex = 395 nm.
not fluoresce when excited near its maximum absorption at 377
nm.
The orange-red iron(II) bromide complex of the LMe,Ph3
ligand was prepared in 90% yield from 2 and FeBr2 in THF
(Scheme 1). The identity of the complex as the dimer
[LMe,Ph3Fe(μ-Br)]2 (3) was revealed by X-ray crystallography,
and the molecular structure of 3 is shown in Figure 3. Each iron
center is coordinated by a diketiminate ligand and two bridging
bromides in a pseudotetrahedral geometry.
a
Reagents and conditions: (a) TsOH·H2O, 24 h, reflux with Dean−
Stark condenser, xylenes. (b) nBuLi, 4 h, Et2O/hexanes. (c) FeBr2, 14
h, THF, 25 °C. (d) toluene, 100 °C.
Figure 1. Molecular structure of LMe,Ph3H (1) with thermal ellipsoids
shown at 50% probability. Hydrogen atoms except the amine
hydrogen are removed for clarity. Relevant bond distances (Å):
N11−C21 1.309(2); C21−C31 1.423(2); C31−C41 1.384(2); C41−
N21 1.341(2).
Figure 3. Molecular structure of [LMe,Ph3Fe(μ-Br)]2 (3) with thermal
ellipsoids shown at 50% probability. Cocrystallized solvent molecules
and hydrogen atoms are removed, and the back half of the dimer is
shown in light gray for clarity. Relevant bond distances (Å) and angles
(deg): Fe1−Br1 2.5311(7); Fe1−Br2 2.5050(6); Fe2−Br1 2.5420(6);
Fe2−Br2 2.5500(7); Fe1−N11 2.017(3); Fe1−N21 2.007(3); Fe1−
Br1−Fe2 93.05(2); Fe1−Br2−Fe2 93.48(2).
enaminoimine is the only tautomer present at room temper-
ature.
n
Deprotonation of 1 with BuLi in Et2O produces the bright
yellow complex LMe,Ph3Li(OEt2) (2). The 1H NMR spectrum of
2 shows upfield-shifted resonances for the diethyl ether
hydrogens (δ 2.68, 0.27 ppm) consistent with coordination
to the Li+ ion. Integration of the OEt2 resonances indicates a
1:1 ligand/OEt2 ratio, consistent with a single coordinated
OEt2 ligand. X-ray crystallography of Li complexes with other
β-diketiminate ligands has revealed similar etherate stoichiom-
etry, for example in the trigonal complexes LMe,iPr2Li(OEt2),13
LtBu,iPr2Li(THF),14 and LMe,F6Li(OEt2).15 The structure of 2 as
a trigonal mono(ether) complex is thus likely to be similar. In
solution, complex 2 emits an intense cyan fluorescence
(emission λmax = 475 nm, see Figure 2), presumably from a π
→ π* transition involving the conjugated orbitals. The
protonated ligand 1 is also bright yellow in color, but does
The planes defined by the diketiminate backbones are
twisted 33.9(1)° with respect to each other. This twist also
rotates the N-aryl groups on adjacent ligands to accommodate
their flanking ortho phenyl substituents. In solution, however,
the molecule displays apparent D2d symmetry as evidenced by
the 11 resonances in the 1H NMR spectrum, which implies that
the aryl substituents on opposite sides of the dimer are able to
move past each other in solution. In THF-d8 solution, the color
of 3 changes from orange-red to yellow, suggesting the
formation of monomeric LMe,Ph3Fe(Br)(THF), analogous to
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other yellow solvento species such as LMe,iPr2Fe(Br)(THF),16
LtBu,iPr2Fe(Cl)(NCMe),17 and LtBu,iPr2Fe(Cl)(3-picoline).18
Reduction of Iron(II) Affords an Iron(I) Arene
Complex. When performed under an N2 atmosphere, single-
electron reduction of other iron(II) diketiminate complexes in
ethereal solvents gives either the corresponding high-spin
intermolecular contacts, at least in the solid state (the
possibility of binding exogenous arenes in solution is addressed
below). There are other examples of diketiminate arene
moieties coordinating to metal(I) complexes, in the mono-
nuclear complex LtBu,iPr2Co,21 and in the dimers [LMe,Me2Cu]2,22
23
24
i
i
[LMe,R2Ni]2 (R = Et, Pr), and [LMe,R2V]2 (R = Et, Pr), but
the structure of 4 with N,N′ coordination and an intramolecular
metal−(η6-arene) association is unique among mononuclear
diketiminate complexes.
diiron dinitrogen complexes LR,R′Fe(μ-N2)FeLR,R′ 19 or an iron-
,
nitride as the result of N2 cleavage.20 However, addition of 2
equiv of KC8 to a slurry of 3 under N2 affords a product (4)
that does not incorporate N2 (Scheme 2).
1
The H NMR spectrum of 4 has no observable resonances,
which is reminiscent of other low-spin (S = 1/2) iron(I)
25,26
t
complexes such as LR,iPr2Fe(CNtBu)2,3 (R = Me, Bu)
and
LR,iPr2Fe(CO)2,3 (R = Me, tBu).19b,27 The S = 1/2 spin state of 4
was also established from electron paramagnetic resonance
(EPR) spectroscopy: a spectrum of 4 recorded at 50 K in a
frozen 2-methyltetrahydrofuran (2-MeTHF) solution showed a
single rhombic signal with g = [2.186, 2.012, 1.979] (Figure 5)
Scheme 2. Reduction of 3 to Afford the Iron(I) Monomer 4
The structure of 4 was revealed by X-ray crystallography, and
its molecular structure is shown in Figure 4. One of the ortho
Figure 5. X-band EPR spectrum of LMe,Ph3Fe (4) in frozen 2-
methyltetrahydrofuran solution recorded at 50 K. Acquisition
parameters: frequency = 9.3891 GHz, power = 0.1 mW, conv. time
= 35 ms, mod. amplitude = 0.7 G, mod. frequency = 30 kHz, time
const. = 41 ms. The dashed line represents a S = 1/2 simulation with g
= [2.186, 2.012, 1.979].
that resembles the spectrum of the previously characterized
iron(I)-benzene complex LMe,iPr2Fe(η6-C6H6) (g = [2.20, 2.01,
1.98]).19b The solution magnetic moment (μeff = 1.8(1) BM)
measured by the Evans method in C6D12 suggests that the low-
spin monomer formulation is maintained in solution. To test
whether toluene can displace the intramolecular arene arm in
solution, the EPR spectrum of 4 was recorded in neat toluene.
The EPR spectra in toluene and 2-MeTHF were identical
(maximum deviation in g < 0.001), suggesting that the structure
is identical in each solvent, and an intermolecular Fe-arene
interaction is disfavored. This is in concord with the
crystallographic observation of noninteracting toluene mole-
cules, described above.
Figure 4. Molecular structure of LMe,Ph3Fe (4) with thermal ellipsoids
shown at 50% probability. Cocrystallized solvent molecules and
hydrogen atoms are removed for clarity. Relevant bond distances (Å)
and angles (deg): Fe1−N11 1.936(5); Fe1−N21 1.960(5); Fe1−C72
2.034(5); Fe1−C82 2.076(6), Fe1−C92 2.128(6); Fe1−C102
2.138(6); Fe1−C112 2.160(6); Fe1−C122 2.098(6); N11−Fe1−
N21 93.6(2); Fe1−Ccentroid 1.564(6); N11−Fe1−Ccentroid 125.7(4);
N21−Fe1−Ccentroid 140.7(4).
Synthesis of Iron-Imido Complexes. The most con-
venient method for the preparation of a Mn+ imido complex is
through nitrene capture from an organoazide by a reduced
M(n−2)+ precursor, and thus we endeavored to use 4 as a source
of Fe(I) in the synthesis of Fe(III) imides. Addition of N3Ad
(Ad = 1-adamantyl) or N3Mes (Mes = mesityl; 2,4,6-
trimethylphenyl) to a solution of the iron(I) synthon 4
phenyl rings coordinates to the iron center in an η6 fashion,
which requires significant N-aryl rotation and bending at the
ipso carbon. Interestingly, toluene solvent molecules pack into
the crystal lattice of 4 in a 2:1 toluene/Fe ratio; however, the
complex lacks any Fe···toluene interactions. This demonstrates
that an intramolecular arene contact is favored over
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(Scheme 3) results in effervescence and the appearance of new
paramagnetically shifted H NMR resonances and EPR signals
important to note that the crystallographic fit gives an imido N
atom with a large thermal ellipsoid, and there is accordingly
significant uncertainty in the position of this atom that may not
be fully captured in the estimated standard deviation.
1
Scheme 3. Synthesis of Imido Complexes LMe,Ph3FeNAd (5)
and LMe,Ph3FeNMes (6)
The arylimido complex LMe,Ph3FeNMes (6) was synthesized
from 4 and N3Mes in an analogous manner to the synthesis of
5. Although 6 did not afford crystals of sufficient quality for X-
ray structure determination, the X-band EPR spectra of 5 and 6
are similar (Figure 7), suggesting a similar geometric structure.
consistent with formation of intermediate-spin iron(III) imido
products LMe,Ph3FeNAd (5) and LMe,Ph3FeNMes (6). Signifi-
cantly, the imido products featuring the LMe,Ph3 ligand are much
more stable in solution than their previously reported analogues
supported by the LMe,iPr2 ligand.8,9,26 Complexes 5 and 6 may
be handled in C6D6 for several days at ambient temperature
without significant decomposition, which contrasts to the
relatively low stability of LMe,iPr2FeNAd and of LtBu,iPr2FeNAd
(solution lifetimes ∼1 d) and apparent instability of
LMe,iPr2FeNMes (attempts at synthesis resulted in immediate
decomposition).
Figure 7. X-band EPR spectra of imido complexes 5 and 6 recorded in
frozen toluene solution at 8 K. Acquisition parameters: frequency =
9.39 GHz, power = 0.1 mW, conv. time = 35 ms, mod. amplitude = 6.1
G, mod. frequency = 30 kHz, time const. = 66 ms. The geff value for
each spectral feature is indicated.
1
In the H NMR spectrum of the crude product of the
reaction of 4 with N3Ad, several resonances appear at chemical
shifts that are very similar to those in the three-coordinate
imido complex LMe,iPr2FeNAd, which initially suggested that the
products have a similar geometry and electronic structure.
Crystallization from pentane at −45 °C afforded plate-like
crystals suitable for X-ray diffraction, and the structure of the
product LMe,Ph3FeNAd (5) is shown in Figure 6. Like
Both 5 (geff = 7.11, 1.55, 1.26) and 6 (geff = 7.42, 1.05, 0.90)
3
show rhombic EPR signals characteristic of S =
/ para-
2
magnets, and these are similar to the spectra of the imido
complexes LMe,iPr2FeNAd and LtBu,iPr2FeNAd (Supporting
Information, Table S-2 and Figure S-3).7,26 In the EPR
spectrum of 5, an additional isotropic feature at geff = 4.32 is
5
evident, which is attributed to a small amount of an S = /2
impurity. Double integration of the spectrum suggests that this
impurity accounts for <0.5% of the total signal (Supporting
Information, Figure S-4).
It is interesting to compare this synthetic method to the one
used earlier from dinuclear LMe,iPr2 and LtBu,iPr2 supported
dinitrogen complexes.7,8,26 The successful preparation of
monomeric iron(III) imido complexes from the earlier N2
complexes required the use of a coordinating solvent or
pyridine additive; otherwise the azide was reductively coupled
to give a dinuclear hexaazadienyldiiron(II) product with six
catenated nitrogen atoms.28 We have proposed that azide
coupling results from the preorganization of iron centers by the
LFeNNFeL starting material, whereas the addition of a donor
solvent led to monoiron species that prefer to eliminate N2 in
an intramolecular reaction.7 In this context, notice that
monomeric LMe,Ph3Fe (4) reacts with azides in a poor donor
solvent to give only imido products, without azide coupling.
This observation fits the proposed model,7 and suggests that
the hemilabile LMe,Ph3 ligand plays a useful role in modulating
the reactivity of the iron(I) synthon.
Figure 6. Molecular structure of LMe,Ph3FeNAd (5) shown with 50%
probability thermal ellipsoids. Hydrogen atoms are removed for clarity.
Relevant bond distances (Å) and angles (deg): Fe1−N14 1.700(5);
Fe1−N11 1.961(4); Fe1−N21 1.962(4); Fe1−N14−C14 151.2(5);
N11−Fe1−N14 139.0(2); N21−Fe1−N14 126.7(2).
L
Me,iPr2FeNAd,7 complex 5 features a planar three-coordinate
iron center, in which the sum of angles at iron is 360.1(3)°.
Compared to LMe,iPr2FeNAd, compound 5 has a longer FeN
bond (1.700(5) versus 1.670(2) Å), and a more bent FeN−
C angle (151.2(5) versus 170.4(2)°). These structural features
may explain the greater reactivity of 5 (see below), but it is
H-Atom Abstraction Reactions. We have described
details on the ability of LMe,iPr2-supported iron(III) imido
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complexes to activate the weak C−H bonds (∼77 kcal/mol) of
cyclohexadiene (CHD).9 LMe,Ph3FeNAd (5) reacts with excess
(10 equiv) of CHD at room temperature, giving a 92% yield of
the iron(II) amido product LMe,Ph3FeNHAd (5·H) within 2
Table 1. Second-Order Rate Constants for HAT to Iron(III)
Imido Complexes from 1,4-Cyclohexadiene in C6D6 Solution
a
complex
kHAT at 10 °C (M−1 s−1
)
krel
LMe,iPr2Fe(NAd)(tBupy)
LMe,Ph3FeNAd (5)
LMe,iPr2FeNAd
LMe,Ph3FeNMes (6)
LtBu,iPr2FeNAd
4.4(3) × 10−1
2.0(2) × 10−2
1.4(2) × 10−4
8.2(5) × 10−5
5400
240
1.7
1
1
min, as judged by H NMR spectroscopy. LMe,Ph3FeNMes (6)
reacts with CHD more sluggishly: 90% yield of the amidoiron
product LMe,Ph3FeNHMes (6·H) was achieved only after 12 h.
Both amido complexes 5·H and 6·H can be prepared
independently by anion metathesis from [LMe,Ph3Fe(μ-Br)]2
(3), and the solid state structure of 6·H is shown in Figure 8.
b
b
0
0
a
b
Relative to the rate constant for LMe,Ph3FeNMes. The complex
L
tBu,iPr2FeNAd has not been observed to react with CHD, even with
excess CHD at 25 °C. See ref 26.
transfer (PT) and electron transfer (ET) contributions in a
thermodynamically constrained square (Figure 9), and more
favorable PT or ET will result in more favorable HAT.
Figure 8. Molecular structure of LMe,Ph3FeNHMes (6·H) with thermal
ellipsoids shown at 50% probability. Hydrogen atoms except H14 are
removed for clarity. Relevant bond distances (Å) and angles (deg):
Fe1−N14 1.902(1); Fe1−N11 2.0112(9); Fe1−N21 1.9819(9); Fe1−
N14−C14 139.35(8); N11−Fe1−N14 115.00(4); N21−Fe1−N14
150.62(4).
Figure 9. Thermodynamic square relating proton transfer (PT) and
electron transfer (ET) to overall HAT free energy.
In the structure of 6·H, the mesitylamido ligand is rotated by
52.8° relative to the diketiminate plane, presumably to relieve
steric pressure between the mesityl and flanking ortho-phenyl
groups. Complexes with less bulky anilido ligands such as
LtBu,iPr2FeNHPh feature coplanar anilido and diketiminate
ligands (0.8° between planes),29 supporting the notion that
the NHMes rotation in 6·H is sterically enforced in the solid
state. The 1H NMR spectrum of 6·H shows only three
resonances assigned to the mesityl group (with integrations
2:3:6), consistent with rapid mesityl rotation in solution.
The kinetics of the HAT reactions of 5 and 6 with CHD
were monitored by 1H NMR spectroscopy. Under pseudo-first-
order conditions (excess CHD) at 10 °C, the observed rates of
HAT (kobs) showed a linear dependence on [CHD]
(Supporting Information, Figures S-1 and S-2), implying the
rate laws in eqs 1 and 2, where the factors of two account for
the 2 equiv of H• that are supplied by each CHD molecule.30
Previous studies have established that strongly basic imido
and oxo ligands increase the thermodynamic driving force and
rate of HAT by making proton transfer more exergonic. For
example, heme enzymes utilize basic trans ligands to increase
the reactivity of FeO intermediates toward HAT.31 This
trend has also been observed in synthetic iron and manganese
oxo complexes, where more basic ligands trans to the oxo
correlate with faster HAT rates.32,33 In iron-imido chemistry,
the addition of pyridine donors to LMe,iPr2FeNAd increased the
HAT rate by several orders of magnitude, and pyridines with
electron-donating substituents led to even larger HAT rate
constants, suggesting that pyridine modulated both the basicity
and the reactivity of the imide toward HAT. A related effect
may be responsible for the increased reactivity of 5 relative to 6:
the electron-donating adamantyl substituent in 5 renders the
imido ligand more electron-rich, making ΔG for the PT
contribution more favorable. Although increased imido basicity
makes PT more favorable, ET will concomitantly become less
favorable, since a more electron-rich metal is harder to reduce.
Since the combined thermodynamic effect observed is that
HAT is more favorable, the free energy gain for the contribution
from PT must outweigh the free energy penalty for making ET
more difficult. Finally, we note that the N−H bond in the
alkylamido product 5·H is stronger than the corresponding N−
H in the arylamido product 6·H. The additional driving force
imparted through formation of a stronger amido bond in 5·H is
also consistent with faster HAT to 5. There is also likely a steric
contribution to the reactivity of 5 vs 6 because of the different
size and shape of their imido N-substituents, although the
d[5]
dt
−
= 2k5[5][CHD]
(1)
d[6]
dt
−
= 2k6[6][CHD]
(2)
The second-order rate constants for HAT are calculated from
the slopes of Supporting Information, Figures S-1 and S-2 to be
k5 = 2.0(2) × 10−2 M−1 s−1 and k6 = 8.2(5) × 10−5 M−1 s−1 at
10 °C (Table 1). Why is the HAT rate constant for 5 more
than 2 orders of magnitude (240 30 times) larger than the
HAT rate constant for 6? One hypothesis is that the more
electron-donating adamantyl group in 5 increases the basicity of
the imido ligand in 5 relative to 6. To evaluate this hypothesis,
the thermodynamics of HAT can be broken into proton
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magnitude of the steric effect cannot be determined in the
absence of a structure of 6.
The HAT rate constant for LMe,Ph3FeNAd (5) can also be
compared to the reaction of LMe,iPr2FeNAd with CHD (k =
1.4(2) × 10−4 M−1 s−1 at 10 °C),9 which tests the effect of the
choice of supporting β-diketiminate ligand on HAT rate. The
imido complex 5 supported by LMe,Ph3 abstracts H-atoms 2
orders of magnitude (143 25 times) faster than the imido
complex supported by LMe,iPr2. The difference is probably not
related to the change in electronic structure of the ligand, since
the isopropyl → phenyl replacement occurs on N-aryl rings that
are perpendicular to (i.e., not conjugated with) the diketiminate
π-system. Since the isopropyl/phenyl groups are substituents
on the N-aryl substituents, their inductive effect on the
electronic structure at iron, and in turn on the reactive imido
nitrogen, is expected to be negligible. If the effect on HAT rate
is not electronic in nature, a reasonable alternative explanation
is the steric properties of the two ligands affect the HAT
transition state energy. However, explaining the steric differ-
ence between LMe,Ph3 and LMe,iPr2 is not trivial because isopropyl
and phenyl groups are not the same shape. These
considerations motivated the calculation of steric parameters
that quantify the difference in overall ligand sterics between
Figure 11. Overlay of LMe,iPr2FeNAd (solid bonds) and 5 (dashed
bonds) crystal structures highlighting the rotation of the N-aryl rings
and bend of the FeN−C linkage. Only one of each N-aryl group is
shown for clarity.
diketiminate metallacycle, the 2,4,6-triphenylphenyl group in 5
is rotated 20.4° from perpendicular to the metallacycle. The
result of this 2,4,6-triphenylphenyl rotation is to increase the
amount of space directly above the imido nitrogen, which
results in a larger binding pocket for hydrocarbon substrates
(see the space-filling models in Figure 12). Therefore, we
LMe,Ph3 and LMe,iPr2
.
Diketiminate Size Comparisons. Various quantitative
descriptors such as the classic Tolman cone angle34 have been
used to describe the “effective size” of ligands, helping chemists
to understand how changes in ligand size or geometry affect
reactivity. Recently, other measures have been introduced such
as solid angles and the related G parameter,35 which take into
account the shape of the ligand in the calculation of size. The G
parameter is defined as the fraction of surface area of a sphere
of arbitrary radius that would be “shadowed” by the ligand if a
light source was placed at the metal site (Figure 10).
Figure 12. Spacefilling diagrams of (a) LMe,iPr2FeNAd and (b)
L
Me,Ph3FeNAd (5). Each diagram is oriented with the NAd group bent
directly away.
reason that an incoming substrate would more easily attack the
exposed imido nitrogen in 5 than the shielded imido nitrogen
in LMe,iPr2FeNAd, despite the overall larger shadowing of the
LMe,Ph3 ligand, because there is contiguous open space for
approach of a hydrocarbon substrate.37
In addition to the roomier attack trajectory to the imide for
incoming substrates, the increased rate of HAT for 5 may also
be attributed to the change in the FeN−R geometry.
Compared to LMe,iPr2FeNAd, complex 5 has a slightly longer
FeN bond (1.70 versus 1.67 Å) and significantly more bent
FeN−R angle (151° versus 170°). Computational studies of
LMe,iPr2FeNAd + CHD indicated that the transition state
featured a lengthened FeN bond (∼1.9 Å) and more bent
FeN−R linkage (∼140°) compared to the imido ground
state geometry.9 Thus, the faster HAT by 5 is consistent with
an imido geometry that is closer to the transition state
geometry, which would result in less structural reorganization
needed to access the HAT transition state.
Interestingly, though the imido nitrogen atom is more
exposed in 5, the iron atom is less exposed. This is shown
experimentally through the relative abilities of 5 and
LMe,iPr2FeNAd to coordinate pyridine at the iron center.
Pyridine weakly coordinates to LMe,iPr2FeNAd in the apical
position of the trigonal plane9 but not to 5, whose iron center is
Figure 10. Visual representations of the ligand G parameters in (a)
L
circumscribed sphere represents the shadow cast by the ligand when a
light source is placed at the metal site (central sphere). Note that only
one-half of each shadow is shown, with a similar shadow cast on the
back half of the circumscribed sphere.
Me,iPr2FeNAd and (b) LMe,Ph3FeNAd (5). The gray shaded area of the
The G size parameter of 5 (63.8%) is only slightly larger than
i
that for LMe,iPr2FeNAd (62.2%), and so the Pr → Ph
substitution is a minimal overall change in size of the β-
diketiminate ligand.36 However, the shape of the flanking ligand
arms (as seen by the shadows in Figure 10) offers a clearer
explanation for the difference in HAT reactivity between 5 and
LMe,iPr2FeNAd. When the X-ray structures of 5 and
LMe,iPr2FeNAd are overlaid (Figure 11), the different orientation
of the N-aryl groups with respect to the ligand backbone
becomes apparent. While the 2,6-diisopropylphenyl substituent
in LMe,iPr2FeNAd is nearly 0.4° from perpendicular to the
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water (10 mL), and the slurry was added to the reaction flask. The
flask was evacuated and backfilled with a H2 balloon, and the mixture
was heated in an 80 °C oil bath. When the reaction was complete by
TLC (ca. 14 h) the hot mixture was filtered through Celite and
allowed to cool to room temperature, at which point the product
began to crystallize from the filtrate. The mixture was cooled to −25
°C, and 14.2 g of fluffy crystalline solid was collected by filtration. An
additional crop of crystals (2.7 g) was obtained by concentrating the
supernatant and storing at −25 °C. Combined yield: 16.9 g (90%). 1H
NMR (CDCl3): δ 7.56−7.61 (m, 6H), 7.48 (t, 4H), 7.41 (s, 2H), 7.38
(m, 4H), 7.27 (d, 1H), 3.91 (s, 2H, NH2) ppm. The NMR spectrum is
identical to literature data.40
blocked by the flanking phenyl substituents. In this manner, the
LMe,Ph3 ligand shields the iron center but exposes the imido
nitrogen to incoming substrates.
CONCLUSIONS
■
Iron-imido complexes of the 2,4,6-tri(phenyl)phenyl-substi-
tuted β-diketiminate ligand LMe,Ph3H are less susceptible to
ligand C−H activation than the previously reported imido
complex LMe,iPr2FeNAd. The greater stability toward intra-
molecular decomposition led to the isolation of the first three-
coordinate arylimidoiron complex. Despite the kinetic stability
imparted by the LMe,Ph3 ligand, the iron(III) imido complexes
LMe,Ph3FeNMes and LMe,Ph3FeNAd are significantly more
reactive (a factor of ∼140) toward HAT than LMe,iPr2FeNAd.
The increase in HAT rate is explained by two observations: (1)
a ligand N-aryl group rotates away from NAd in the crystal
structure of LMe,Ph3FeNAd, allowing more facile access of the
substrate to the reactive nitrogen atom, and (2) the bent Fe
N−R linkage and lengthened FeN bond in LMe,Ph3FeNAd is
closer to the expected transition-state geometry. It is notable
that the LMe,Ph3 ligand provides a larger substrate pocket for
2,4-Bis(2,4,6-triphenylphenylimido)pentane (LMe,Ph3H) (1). A
500-mL flask was loaded with 2,4,6-triphenylaniline (16.1 g, 50.2
mmol), p-toluenesulfonic acid monohydrate (4.77 g, 25.1 mmol), 2,4-
pentanedione (2.56 mL, 25.1 mmol), and xylenes (350 mL). The
mixture was sparged with N2, and the flask was fitted with a Dean−
Stark condenser. The mixture was heated in a 160 °C oil bath for 24 h.
Most of the volatile materials were removed under vacuum, affording a
brown gooey solid. Aqueous Na2CO3 (300 mL) and CH2Cl2 (300
mL) were added to the residue, and the mixture was vigorously stirred
for 30 min. The organic layer was removed and washed with brine (2
× 300 mL) and dried over MgSO4. The solution was concentrated to
∼150 mL, and MeOH was added until the solution just became cloudy
(∼75 mL). Upon standing, dark yellow crystals deposited, which were
isolated by filtration. The crystals were ground into a fine yellow
powder and heated under vacuum at 120 °C for 12 h to ensure
complete removal of cocrystallized solvent. Yield = 10.60 g. A second
crop of crystals (2.54 g) was obtained by adding more MeOH to the
supernatant (∼100 mL), affording a total yield of LMe,Ph3H of 13.14 g
(74%). 1H NMR (CDCl3, 400 MHz): δ 12.2 (s, 1H, NH), 7.70 (d, J =
7.4 Hz, 4H), 7.58 (s, 4H), 7.44 (t, J = 7.6 Hz, 4H), 7.34 (t, J = 7.4 Hz,
2H), 7.11−7.21 (m, 20H), 4.14 (s, 1H, α-CH), 1.24 (s, 6H, CH3)
ppm. 13C{1H} NMR (CDCl3, 101 MHz): δ 160.0 (CN), 141.0
(Cipso), 140.6 (Cipso), 140.2 (Cipso), 137.3 (Cipso), 136.7 (Cipso), 129.4
(CAr−H), 128.9 (CAr−H), 128.5 (CAr−H), 128.2 (CAr−H), 127.2
(CAr−H), 127.0 (CAr−H), 126.9 (CAr−H), 126.8 (CAr−H), 96.7 (α-
CH), 21.2 (CH3) ppm. IR (thin film): 3582 (w, νN−H), 3079 (w),
3055 (m), 3027 (m), 2920 (w), 1622 (s), 1597 (m), 1544 (s), 1494
(m), 1454 (m), 1421 (s), 1397 (m), 1377 (m), 1362 (m), 1278 (m),
1179 (m), 1070 (w), 1029 (m), 887 (m), 754 (s), 733 (m) cm−1.
UV−vis (CH2Cl2) (λmax (ε in mM−1 cm−1)): 251 (77), 377 (25).
Elem. Anal. Calcd. for C53H42N2: C 90.05; H 5.99; N 3.96. Found: C
89.91; H 6.10; N 4.01.
HAT than the LMe,iPr2 ligand, despite the larger size of LMe,Ph3
.
This highlights the importance of considering the overall shape
of supporting ligands, in addition to the amount of
coordination sphere coverage. In addition to these steric
effects, the relative rates of HAT for arylimido and alkylimido
complexes is consistent with imido basicity driving the HAT
reaction. These studies illustrate the ability to modulate HAT
reactivity with both steric and electronic changes to the ligands.
EXPERIMENTAL SECTION
■
General Considerations. All air-sensitive manipulations were
performed under a nitrogen atmosphere in an MBraun glovebox
maintained below 1 ppm of O2 and H2O, or on a double-manifold
vacuum line using standard Schlenk techniques. For air-sensitive
manipulations, all glassware was dried overnight at 150 °C. 1-
Adamantyl azide was dissolved in pentane, filtered through Celite, and
crystallized from pentane prior to use. Mesityl azide was prepared as
previously described38 and distilled under vacuum. 1,4-Cyclohexadiene
was distilled from CaCl2 and stored over activated 3 Å sieves.
Anhydrous FeBr2 was prepared from Fe and conc. HBr in MeOH, and
dried at about 200 °C under vacuum for 12 h prior to use. Other
reagents were obtained commercially and used without purification.
Tetrahydrofuran (THF) was distilled from a purple sodium
benzophenone ketyl solution, and other solvents were purified were
by passage through activated alumina and “deoxygenizer” columns
from Glass Contour Co. Benzene-d6 was dried over flame-activated
alumina. Before use, an aliquot of each solvent was tested with a drop
of THF containing sodium benzophenone ketyl to qualitatively ensure
dryness. NMR data were collected on either a Bruker Avance 400 or
L
Me,Ph3Li(OEt2) (2). A Schlenk flask was loaded with LMe,Ph3H (2.15
g, 3.04 mmol) and diethyl ether (100 mL) to give a yellow mixture. A
solution of nBuLi (1.22 mL of a 2.5 M solution in hexane, 3.04 mmol)
n
was added slowly at room temperature. Upon addition of the BuLi,
the mixture developed a cyan-colored fluorescence. The mixture was
stirred for 4 h, and the volatile materials were removed under vacuum
to afford an amorphous yellow solid. The solid was slurried in pentane
(∼50 mL), and collected on a glass fritted funnel. The yellow powder
was dried under vacuum, affording 2.18 g (91%) of LMe,Ph3Li(OEt2).
1H NMR (C6D6): δ 7.68 (s, 4H), 7.52 (d, J = 7.6 Hz, 4H), 7.43 (d, J =
7.6 Hz, 8H), 7.35 (t, J = 7.6 Hz, 8H), 7.21 (t, J = 7.6 Hz, 4H), 7.15 (s,
2H), 7.14 (t, J = 7.6 Hz, 4H), 4.43 (s, 1H, α-CH), 2.68 (q, J = 6.8, 4H,
O(CH2CH3)2), 1.60 (s, 6H, CH3), 0.27 (t, J = 6.8, 6H, O(CH2CH3)2)
ppm. 13C{1H} NMR (C6D6, 101 MHz): δ 164.5 (CN), 149.3
(Cipso), 142.4 (Cipso), 141.4 (Cipso), 136.5 (Cipso), 135.4 (Cipso), 130.5
(CAr−H), 130.2 (CAr−H), 130.0 (CAr−H), 129.7 (CAr−H), 129.5
(CAr−H), 129.1 (CAr−H), 127.1 (CAr−H), 126.8 (CAr−H), 126.6
(CAr−H), 126.3 (CAr−H), 96.2 (α-CH), 66.0 (OCH2CH3), 24.7 (N
C−CH3), 13.0 (OCH2CH3) ppm. UV−vis (Et2O) (λmax (ε in mM−1
cm−1)): 255 (54), ∼290 (sh, ∼15), 418 (27). Elem. Anal. Calcd. for
C57H51LiN2O: C 86.99; H 6.53; N 3.56. Found: C 86.37; H 6.79; N
3.43.
1
Bruker Avance 500 spectrometer. H NMR spectra are referenced to
residual C6D5H (δ 7.16 ppm), C7D7H (δ 2.08 ppm), or TMS (δ 0.00
ppm). 13C NMR spectra are referenced to CDCl3 (δ 77.16 ppm) or
C6D6 (δ 128.06 ppm). Mass spectral data were obtained on a
Shimadzu QP2010 system with electron impact ionization. IR data
were recorded on a Shimadzu 8400S FTIR spectrometer using pressed
KBr sample pellets. UV−vis data were recorded on a Cary 50
spectrometer using Schlenk-adapted cuvettes. The emission spectrum
of 2 was recorded on a Spex Fluoromax-P fluorimeter with a
photomultiplier tube detector. Elemental analyses were determined at
the CENTC Elemental Analysis Facility using a PerkinElmer 2400
Series II Analyzer with sample capsules prepared under argon. Solution
magnetic susceptibilities were determined by the Evans method.39
2,4,6-Triphenylaniline. A 1000-mL flask was loaded with 2,4,6-
triphenylnitrobenzene (20.5 g, 58 mmol) and iPrOH (400 mL), giving
a white slurry. Dry 10% Pd/C (3.1 g, 2.9 mmol of Pd) was treated with
[LMe,Ph3Fe(μ-Br)]2 (3). A Schlenk flask was loaded with FeBr2
(0.604 g, 2.80 mmol) and THF (30 mL), and a solution of
L
Me,Ph3Li(OEt2) (2.20 g, 2.80 mmol) in THF (30 mL) was added,
giving a yellow-brown mixture. The mixture was stirred at room
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temperature for 14 h, and the volatile materials were removed under
vacuum. The solid was heated under vacuum (100 °C for 3 h) until
the solid changed color to orange-red. Toluene (100 mL) was added,
and the solution was filtered through a pad of Celite, and the volatile
materials were removed under vacuum. The orange solid was washed
with pentane (50 mL) and dried under vacuum to afford [LMe,Ph3Fe(μ-
was crystallized by vapor diffusion of pentane into a saturated toluene
solution at room temperature, affording dark red crystals of
LMe,Ph3FeNHMes (97 mg, 75%). 1H NMR (C6D6): δ 120 (6H,
backbone CH3 or Mes-CH3), 118 (4H, Ar−H), 91.7 (2H, Ar−H or
Mes-H), 63.3 (1H, backbone C−H), 16.5 (6H, backbone CH3 or
Mes-CH3), 6.5 (4H, Ar−H), 4.6 (4H, Ar−H), 4.2 (4H, Ar−H), −0.5
(2H, Ar−H or Mes-H), −8.4 (3H, Mes-CH3), −9.3 (8H, Ar−H),
−13.2 (4H, Ar−H), −38.0 (8H, Ar−H). Elem. Anal. Calcd. for
C62H53FeN3: C 83.11; H 5.96; N 4.69. Found: C 82.79; H 6.11; N
4.68.
1
Br)]2 (2.12 g, 90%). H NMR (CD2Cl2): δ 29.0, 12.1, 7.3, 0.0, −2.5,
−4.4, −9.1, −13.4, −14.2, −16.3, −25.5, −38.5 ppm. In CD2Cl2, the
peaks were very broad and overlapped, preventing accurate
1
integration. H NMR (THF-d8): δ 20.5 (4H, Ar−H), 8.6 (4H, Ar−
H), 0.0 (2H, Ar−H), −0.3 (8H, Ar−H), −0.9 (4H, Ar−H), −1.3 (4H,
Ar−H), −13.3 (8H, Ar−H), −24.4 (1H, backbone C−H), −35.4 (6H,
CH3) ppm. UV−vis (CH2Cl2) (λmax (ε in mM−1 cm−1)): 251 (160),
349 (33), ∼390 (sh, ∼20), 536 (0.82). μeff (CD2Cl2, 23 °C): 7.5(4) μB.
Elem. Anal. Calcd. for C106H82Br2Fe2N4: C 75.63; H 4.91; N 3.33.
Found: C 75.72; H 5.14; N 3.39.
1H NMR Kinetics. A general procedure for the HAT reaction
kinetics is given. A J. Young NMR tube was loaded with 0.4 mL of a 15
mM stock solution of 5 in C6D6 and an integration standard (a sealed
capillary of cobaltocene in C7D8), and the tube was sealed with a
rubber septum. The tube was chilled in an ice bath (∼0 °C), and 1,4-
cyclohexadiene was added via microsyringe. The tube was quickly
shaken and immediately inserted into the NMR probe which was
precooled to 10 °C (probe temperature calibrated using neat ethylene
glycol or methanol41). A kinetic acquisition program was started
immediately, and spectra were recorded every 50 s until the reaction
was >95% complete. The plot of integration of 5 versus time was fit to
a first-order exponential equation y = A + B·exp(−kobs·t), where A and
B are constants and kobs is the pseudo-first-order rate constant.
Crystallography. Crystals were placed onto the tip of a ∼0.1 mm
diameter glass fiber and mounted on a Bruker SMART APEX II CCD
Platform diffractometer42 for a data collection at 100.0(1) K (except
compound 4 at 103(5) K) using MoKα radiation and a graphite
monochromator. Randomly oriented regions of reciprocal space were
surveyed: four major sections of frames were collected with 0.50° steps
in ω at four different ϕ settings and a detector position of −33° in 2θ.
The intensity data were corrected for absorption.43 Final cell constants
were calculated from the xyz centroids of about 4000 strong reflections
from the actual data collection. The structures were solved using
SIR9744 and refined using SHELXL-97.45 The space groups were
determined by systematic absences and intensity statistics. All non-
hydrogen atoms were refined with anisotropic displacement
parameters, and hydrogen atoms were placed in ideal positions and
refined as riding atoms with relative isotropic displacement parameters
(except the amido hydrogen in compound 6·H described below). For
compound 1, two peaks of remaining electron density (∼0.9 e·Å−3
each) were located near the aryl ring C72 → C102. The distance
between these peaks of 2.89 Å is consistent with the Cl−Cl distance in
CH2Cl2, thus suggesting trace occupancy by a dichloromethane
molecule. When these density peaks were included in the refinement
model, occupancy of the Cl positions refined to ∼0.04 (i.e., 4%). This
did not significantly improve the overall model and its statistics; thus
the dichloromethane was omitted from the final model. Additionally,
two connected aryl rings were modeled as disordered over two
positions (88:12). For compound 4, two of the three cocrystallized
toluene solvent molecules lie on crystallographic inversion centers and
were modeled as disordered over the centers (50:50, by symmetry).
For compound 5, highly disordered solvent was found around a
crystallographic inversion center that could not be identified or
refined. Reflection contributions from this solvent were removed using
program PLATON, function SQUEEZE,46 which determined there to
be 144 electrons in 432 Å3 removed per unit cell. Since the exact
identity and quantity of the solvent was unknown, it was not included
in the molecular formula. Thus all fields that derive from the molecular
formula (e.g., F(000), calculated density) are known to be incorrect.
For compound 6·H, the amido hydrogen atom H14 was found from
the difference Fourier map, and its positional and isotropic
displacement parameters were refined independently from those of
its bonded nitrogen atom. Full crystallographic refinement details are
given in the Supporting Information.
L
Me,Ph3Fe (4). A 20-mL scintillation vial was loaded with
[LMe,Ph3Fe(μ-Br)]2 (578 mg, 0.34 mmol) and diethyl ether (∼12
mL). KC8 (93 mg, 0.69 mmol) was added in portions over 1 min, and
the mixture immediately turned dark red-brown. The mixture was
stirred for 4 h and filtered through Celite. The filter cake was washed
with Et2O until the washes were colorless (∼200 mL), and the
resulting solution was dried under vacuum to afford a dark red-brown
1
solid (464 mg, 89%). H NMR (C6D12): no peaks. EPR (2-MeTHF,
50 K): g = 2.186, 2.102, 1.980. IR (KBr): 3055 (w), 3028 (w), 2960
(w), 2923 (w), 1597 (w), 1523 (m), 1493 (w), 1423 (m), 1379 (vs),
1361 (s), 1342 (w), 1277 (w), 1258 (w), 1185 (m), 1072 (w), 1022
(w), 887 (w), 798 (w), 758 (s), 698 (s), 624 (m) cm−1. UV−vis
(THF) (λmax (ε in mM−1 cm−1)): 209 (92), 252 (76), 364 (17), 451
(5.1), 541 (2.4), ∼610(sh) (∼1), 878 (0.17). μeff (C6D12, 25 °C):
1.8(1) μB. Elem. Anal. Calcd. for C53H41FeN2: C 83.57; H 5.43; N
3.68. Found: C 84.22; H 5.79; N 3.54.
L
Me,Ph3FeNAd (5). A 20-mL scintillation vial was loaded with
Me,Ph3Fe (232 mg, 0.304 mmol) and benzene (∼8 mL). With
L
vigorous stirring, a solution of N3Ad (54 mg, 0.31 mmol) in benzene
(∼1 mL) was added, causing effervescence and a subtle color change
to lighter brown. The mixture was stirred 30 min, and the volatile
materials were removed under vacuum. The resulting brown solid was
triturated with a small amount of Et2O until solid began to crystallize.
The mixture was stored overnight at −45 °C, affording LMe,Ph3FeNAd
1
(115 mg, 42%). H NMR (C6D6): δ 73.8 (6H, CH3 or Ad-H), 43.2
(1H, backbone C−H), 32.5 (3H, Ad-H), 30.4 (Ad-H), 27.3 (3H, Ad-
H), 4.8 (4H, Ar−H), 2.0 (2H, Ar−H), −0.8 (4H, Ar−H), −3.0 (4H,
Ar−H), −5.7 (8H, Ar−H), −10.0 (4H, Ar−H), −22.5 (6H, CH3 or
Ad-H), −35.5 (8H, Ar−H) ppm. EPR (toluene, 8 K): geff = 7.11, 1.55,
1.26. μeff (C6D6, 25 °C): 4.0(2) μB. Small impurities were evident by
1H NMR spectroscopy, which prevented elemental analysis.
L
Me,Ph3FeNMes (6). A 20-mL scintillation vial was loaded with
Me,Ph3Fe (51 mg, 67 μmol) and benzene (∼3 mL). With vigorous
L
stirring, a solution of N3Mes (10 μL, 67 μmol) in benzene (∼1 mL)
was added dropwise, causing effervescence and a subtle color change
to reddish brown. The mixture was stirred 30 min, and the volatile
materials were removed under vacuum. The resulting brown solid was
1
crystallized from toluene, affording LMe,Ph3FeNMes (37 mg, 61%). H
NMR (C6D6): δ 50.1 (1H, backbone C−H), 42.9 (2H, Ar−H or Mes-
H), 6.6 (2H, Ar−H or Mes-H), 4.4 (4H, Ar−H), 2.0 (8H, Ar−H),
−4.8 (8H + 3H, Ar−H + Mes-CH3), −6.1 (4H, Ar−H), −7.1 (4H,
Ar−H), −21.4 (6H, backbone CH3 or Mes-CH3), −28.3 (6H,
backbone CH3 or Mes-CH3), −59.3 (4H, Ar−H) ppm. EPR (toluene,
8 K): geff = 7.42, 1.03, 0.89. μeff (C6D6, 25 °C): 4.1(3) μB. Small
1
impurities were evident by H NMR spectroscopy, which prevented
elemental analysis.
L
Me,Ph3FeNHMes (6·H). A 20-mL scintillation vial was loaded with
a solution of freshly distilled MesNH2 (19.6 mg, 0.145 mmol) and
THF (2 mL). A solution of nBuLi in hexane (58 μL of a 2.5 M
solution, 0.150 mmol) was added, and the mixture was stirred for 10
min. This solution was added to a solution of [LMe,Ph3Fe(μ-Br)]2 (122
mg, 0.072 mmol) in THF (5 mL), which caused a color change from
yellow to orange-red. The mixture was stirred 10 min, and the volatile
materials were removed under vacuum. The resulting orange-red solid
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Inorganic Chemistry
Article
Bominaar, E. L.; Munck, E.; Que, L. Angew. Chem., Int. Ed. 2009, 48,
̈
ASSOCIATED CONTENT
* Supporting Information
Additional kinetic, crystallographic, and spectral data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: holland@chem.rochester.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the National Science Foundation
(CHE-0911314 to P.L.H.) and the University of Rochester
(Elon Hooker Fellowship to R.E.C.) for financial support. We
thank Prof. Richard Eisenberg for access to the fluorimeter, Dr.
William McNamara and Dr. William Brennessel for exper-
imental assistance, and Dr. Eckhard Bill for spectroscopic
advice.
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