Carbon-carbon bond formation reactivity of a four-coordinate NHC-supported iron(II) phenyl compound

Article abstract of DOI:10.1021/om501061b

The preparation and characterization of a NHC-coordinated (NHC = N-heterocyclic carbene) ferrous phenyl complex [(IPr₂Me₂)₂FePh₂] (1; IPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) as well as its C-C bond formation reactivity have been studied. The four-coordinate iron(II) phenyl complex was prepared from the reaction of ferrous chloride with PhMgBr and IPr₂Me₂. It reacts with nonactivated primary and secondary alkyl bromides and chlorides to furnish cross-coupling products and the iron(II) monophenyl species (IPr₂Me₂)₂FePhX (X = Br (2), Cl). When it is treated with cyclooctatetraene (cot) or [Cp₂Fe][BAr^F₄] in the presence of PMe₃, it undergoes coordination or one-electron oxidation induced reductive elimination of biphenyl to form the corresponding iron(0) or iron(I) species [(IPr₂Me₂)₂Fe(?·⁴-cot)] (3) or [(IPr₂Me₂)₂Fe(PMe₃)₂][BAr^F₄] (4). All of these iron-containing products have been fully characterized by various spectroscopic methods. Complex 1 and (IPr₂Me₂)₂FeCl₂ catalyze the reaction of n-C₈H₁₇Br with (p-tolyl)MgBr to afford the cross-coupling product in moderate yields (49% and 47%), whereas the reactions employing 4 and 1/PMe₃ as catalysts give the cross-coupling product in very low yields. The results reflect the complexity of the reaction mechanism of iron-catalyzed coupling reactions.

Full text of DOI:10.1021/om501061b

Article
pubs.acs.org/Organometallics
Carbon−Carbon Bond Formation Reactivity of a Four-Coordinate
NHC-Supported Iron(II) Phenyl Compound
Yuesheng Liu,† Jie Xiao,† Lei Wang,† You Song,‡ and Liang Deng*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai, People’s Republic of China 200032
‡State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
People’s Republic of China 210093
S
* Supporting Information
ABSTRACT: The preparation and characterization of a
NHC-coordinated (NHC = N-heterocyclic carbene) ferrous
phenyl complex [(IPr2Me2)2FePh2] (1; IPr2Me2 = 1,3-
diisopropyl-4,5-dimethylimidazol-2-ylidene) as well as its C−
C bond formation reactivity have been studied. The four-
coordinate iron(II) phenyl complex was prepared from the reaction of ferrous chloride with PhMgBr and IPr2Me2. It reacts with
nonactivated primary and secondary alkyl bromides and chlorides to furnish cross-coupling products and the iron(II)
monophenyl species (IPr2Me2)2FePhX (X = Br (2), Cl). When it is treated with cyclooctatetraene (cot) or [Cp2Fe][BArF ] in
4
the presence of PMe3, it undergoes coordination or one-electron oxidation induced reductive elimination of biphenyl to form the
corresponding iron(0) or iron(I) species [(IPr2Me2)2Fe(η4-cot)] (3) or [(IPr2Me2)2Fe(PMe3)2][BArF ] (4). All of these iron-
4
containing products have been fully characterized by various spectroscopic methods. Complex 1 and (IPr2Me2)2FeCl2 catalyze
the reaction of n-C8H17Br with (p-tolyl)MgBr to afford the cross-coupling product in moderate yields (49% and 47%), whereas
the reactions employing 4 and 1/PMe3 as catalysts give the cross-coupling product in very low yields. The results reflect the
complexity of the reaction mechanism of iron-catalyzed coupling reactions.
formation of iron nanoparticles or low-valent iron species,
presumably iron(I).5
INTRODUCTION
■
The rapid development of iron-catalyzed organometallic
transformations in recent years has raised fundamental
questions about the identiies of their organoiron intermediates,
which have remained largely undisclosed.1 Iron phenyl species
are among the most popular organoiron intermediates
proposed in iron-catalyzed cross-couplings and consequently
have attracted considerable recent interests.2 In a pioneering
The aforementioned ambiguity warrants exploration of
putative iron phenyl species.6 In this context, we report herein
the isolation, characterization, and reactivity of the four-
coordinate iron(II) phenyl complex [(IPr2Me2)2FePh2]. This
NHC-supported iron(II) phenyl complex is stable at room
temperature and can readily react with nonactivated alkyl
halides to form monophenyl iron(II) complexes and cross-
coupling products in high yields. Further studies on its
tendency to undergo coordination and one-electron oxidation
induced decomposition revealed that the latter reaction can
readily take place to afford biphenyl and iron(I) species. These
results, in addition to the different catalytic performances of the
isolated iron(II) and iron(I) complexes in promoting the
reaction of n-C8H17Br with (p-tolyl)MgBr, suggest the
complexity of the reaction mechanism of iron-catalyzed
coupling reactions.
work, Furstner et al. reported the isolation and structural
̈
characterization of the anionic ferrous complex [Li(Et2O)2]-
[Li(1,4-dioxane)][FePh4] from the reaction of FeCl2 with PhLi
but thought it irrelevant to the catalytic process because of its
propensity to eliminate biphenyl.3 On the other hand,
Nakamura, Nagashima, et al. found that the bulky aryl species
(TMEDA)Fe(Mes)2 (TMEDA = N,N,N′,N′-tetramethylethy-
lene-1,2-diamine), which may emulate its phenyl analogue, can
react with n-C8H17Br to produce the cross-coupling product
and proposed the catalytic relevance of iron(II) phenyl
species.4a Furstner, Neidig, and their co-workers recently
̈
RESULTS AND DISCUSSION
■
found that the bis(phosphine)-supported iron(II) mesityl
species (P2)Fe(Mes)2 (P2 = 1,2-bis(diethylphosphino)ethane
(depe), 1,2-bis(bis(3′,5′-di-tert-butylphenyl)phosphino)-
benzene (SciOPP)) can also react with alkyl halides to produce
cross-coupling products.4b,c Different from Nakamura’s view,
Bedford et al. thought that iron-catalyzed reactions with smaller
aryl Grignard reagents should have a low-valent iron species as
the catalytically active species since the authors noticed that the
reactions of FeCl3 with (p-tolyl)MgBr could lead to the
Preparation and Characterization of the Iron(II)
Diphenyl Complex. The wide use of monodentate NHC
ligands in iron-catalyzed cross-coupling reactions7,8 stimulated
our interests in accessing relevant organoiron intermediates.9
To seek pertinent models of iron phenyl species with
Received: October 22, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/om501061b
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Article
monodentate NHC ligation, we examined the reactions of
(NHC)2FeCl2 (NHC = IPr2Me2; IMes = 1,3-bis(2′,4′,6′-
trimethylphenyl)imidazol-2-ylidene) with 2 equiv of PhMgBr in
THF. Both reactions could produce orange solutions, and from
the reaction with (IPr2Me2)2FeCl2 we succeeded in the
isolation of the phenyl complex [(IPr2Me2)2FePh2] (1) as
yellow crystals in 77% yield.10 Alternatively, 1 was also prepared
from the one-pot reaction of FeCl2 with IPr2Me2 (2 equiv) and
PhMgBr (2 equiv) in comparable yield (Scheme 1).
Table 1. Reactions of 1 with Primary and Secondary Alkyl
Halides
a
Scheme 1. Preparation Routes for 1
a
Conditions: (IPr2Me2)2FePh2 (0.10 mmol) and halides (0.10 mmol)
A single-crystal X-ray diffraction study has established the
molecular structure of 1 as a four-coordinate iron(II) diphenyl
species with a distorted-tetrahedral FeC4 core (Figure 1a). The
in THF (3 mL) at 30 °C with n-dodecane as the internal standard.
b
c
Time required for the full conversions of the halides. GC yields.
82% and 88% yields (Table 1). In these reactions, biphenyl,
alkanes, and alkenes have been observed as the byproducts
(Table 1).15 The occurrence of alkanes and alkenes hints at a
radical type mechanism for their carbon−halogen activation
steps, which is further supported by the observation of the ring-
opening product in the reaction of 1 with cyclopropylmethyl
bromide (Table 1).4
In addition to the organic products, monophenyl iron(II)
species in the form (IPr2Me2)2FePhX (X = Br, Cl) are the
dominant iron-containing products. Figure S2 (Supporting
Information) shows the 1H NMR spectra of the reaction
mixtures of 1 with n-C8H17Br, n-C8H17Cl, c-C7H13Br, and c-
C7H13Cl. The similar peak patterns of the paramagnetically
shifted resonances point out a common identity of the resulting
iron-containing species (IPr2Me2)2FePhX. Furthermore, a
single-crystal X-ray diffraction study on the bromide complex
[(IPr2Me2)2FePhBr] (2) that was independently prepared from
the reaction of FeBr2 with 2 equiv of IPr2Me2 and 1 equiv of
PhMgBr in THF has unambiguously confirmed its structure
Figure 1. Molecular structure of 1 (left) showing 30% probability
ellipsoids and partial atom schemes and its zero-field Mossbauer
̈
spectrum recorded at 80 K (right). Selected distances (Å) and angles
(deg): Fe1−C1 2.162(2), Fe1−C2 2.157(2), Fe1−C3 2.090(2), Fe1−
C4 2.091(2); C1−Fe1−C2 114.0(1), C3−Fe1−C4 113.3(1).
Fe−C(carbene) and Fe−C(phenyl) distances are typical of
four-coordinate high-spin iron(II) species.11 The 1H NMR
spectra of 1 recorded in THF-d8 and C6D6 are similar and
feature heavily broadened paramagnetically shifted resonances.
The measured magnetic susceptibilities by SQUID exhibit μeff
values ranging from 4.60 to 5.04 μB at 30−300 K, which is
comparable to the spin-only value of 4.90 μB for a high-spin S =
2 state (Figure S1, Supporting Information).12 The isomer shift
(δ = 0.47 mm/s) and quadrupole splitting (ΔEQ = 2.38 mm/s)
(Figure 2a). The 57Fe Mossbauer isomer shift of 2 (0.58 mm/s)
̈
analyzed from its zero-field 57Fe Mossbauer spectrum measured
̈
at 80 K (Figure 1b) are consistent with those of related four-
coordinate high-spin iron(II) species, such as [(depe)Fe-
(Mes)2] (0.39 and 1.71 mm/s)13 and the tris(NHC) iron(II)
complex [(N(CH2CH2(C3N2MesH2))3)Fe(N3)][BPh4] (0.69
and 2.27 mm/s),14 corroborating its high-spin ferrous nature.
Reactions of the Iron(II) Diphenyl Complex with
Organic Halides. Under a dinitrogen atmosphere, 1 does
not show noticeable decomposition when its solutions (THF,
Et2O, and benzene) and the solid were kept at room
temperature for days. Its stability under ambient conditions
made a further reactivity study possible. Similarly to the bulky
aryl complexes (L2)Fe(Mes)2 (L2 = TMEDA, depe, SciOPP)
and [Fe(Mes)3]−,4,5 the four-coordinate iron(II) phenyl
compound can react with the alkyl bromides n-C8H17Br and
c-C7H13Br to produce the corresponding cross-coupling
products in high yields (Table 1). More intriguingly, 1 can
also react with the nonactivated alkyl chlorides n-C8H17Cl and
c-C7H13Cl to give the corresponding cross-coupling products in
Figure 2. Molecular structure of 2 (left) showing 30% probability
ellipsoids and partial atom schemes and its zero-field Mossbauer
̈
spectrum recorded at 80 K (right). Selected distances (Å) and angles
(deg): Fe1−C1 2.130(2), Fe1−C2 2.140(2), Fe1−C3 2.108(2), Fe1−
Br1 2.485(1); C1−Fe1−C2 100.4(1), C3−Fe1−Br1 109.4(1).
is between those of 1 (0.47 mm/s) and Meyer’s bis(NHC)-
iron(II) dibromides (0.73−0.81 mm/s).11c Along with
(IPr2Me2)2FePhX, a small amount of (IPr2Me2)2FeX2 was
1
also observed according to the H NMR spectra (Figure S2,
Supporting Information). The iron(II) dihalide complexes
might come from the further cross-coupling reactions of
(IPr2Me2)2FePhX with the alkyl halides4a and/or the ligand
B
DOI: 10.1021/om501061b
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Article
redistribution reaction of (IPr2 Me2 )2 FePhX to
(IPr2Me2)2FePh2 and (IPr2Me2)2FeX2.
be fitted with the parameters of δ = 0.57 mm/s and ΔEQ = 1.57
mm/s (Figure 3b). The increased isomer shift in comparison to
δ = 0.47 mm/s for [(IEt2Me2)2Fe(η2:η2-dvtms)]20 suggests the
enhanced metal-to-alkene π back-donation in the cot complex.
Thus, 3 could also be viewed as a formal iron(0) species. The
measured solution magnetic moment for 3 is 3.4(1) μB, which
is close to that of our previously reported formal iron(0) alkene
complexes, e.g. 3.5 μB for [(IEt2Me2)2Fe(η2:η2-dvtms)] and
[(IMes)Fe(η2:η2-dvtms)], and larger than the spin-only value
of 2.83 μB for an S = 1 system. The large magnetic moment
might be due to the presence of a large unquenched orbital
moment contribution, as noted in other low-coordinate iron
complexes.21
Olefin Coordination and Oxidation Induced Biphenyl
Reductive Elimination. Low-valent iron species were
proposed as active intermediates in many iron-catalyzed
cross-coupling reactions.1,3,5,8d,16 With the aim to identify
possible transformations of 1 to iron(0) and iron(I) species, we
studied the decomposition reactions of 1 under different
conditions.
Examining the interactions of 1 with some potential
coordinating substrates, e.g. PPh3, TMEDA, n-octene, fluo-
robenzene, chlorobenzene, and cyclooctatetraene (cot),
revealed the inertness of 1 toward most of these compounds,
except for cot. For the last case, slow formation of biphenyl was
noticed15 and the isolation of the iron(0) complex
[(IPr2Me2)2Fe(η4-cot)] (3) (Scheme 2), whose structure is
The potential of 1 to perform oxidation-induced decom-
position was probed by both electrochemical and chemical
oxidation studies. Cyclic voltammetry studies on a THF
solution of 1 revealed that its oxidation can occur at quite
negative potential with Ep = −0.53 V (vs SCE) and the process
is irreversible (Figure S3, Supporting Information). This
potential is lower than that of 0 V for the [(IMes)2FeCl2]0/+
process,11d being consistent with the stronger electron-donating
nature of phenyl anion versus chloride. The irreversibility
implies the instability of the resultant iron(III) species
[(IPr2Me2)2FePh2]+, which is different from the six-coordinate
iron dialkyl compounds [(bipyridine)2FeR2]+, which are stable
at 0 °C.22
Scheme 2. Reaction of 1 with cot
For the chemical oxidation reaction of 1 with 1 equiv of
[Cp2Fe][BArF ] in THF, GC analysis indicated the instant
4
formation of biphenyl in 80% yield. The isolation of the
resulting iron species was validated by the addition of 4 equiv of
PMe3 to the reaction mixture, from which the iron(I) complex
[(IPr2Me2)2Fe(PMe3)2][BArF ] (4) was obtained in 80% yield
4
(Scheme 3). The structure of the iron(I) cation in 4 is shown in
Figure 3. Molecular structure of 3 (left) showing 30% probability
Scheme 3. Reaction of 1 with Ferrocenium Cation in the
Presence of PMe3
ellipsoids and partial atom schemes and its zero-field Mossbauer
̈
spectrum recorded at 80 K (right). Selected distances (Å) and angles
(deg): Fe1−C1 1.995(1), Fe1−C2 2.005(1), Fe1−C3 2.161(1), Fe1−
C4 2.073(1), Fe1−C5 2.065(1), Fe1−C6 2.423(1), C3−C4 1.414(2),
C4−C5 1.424(2), C5−C6 1.416(2), C6−C7 1.426(2), C7−C8
1.370(3), C8−C9 1.419(3), C9−C10 1.355(2), C10−C3 1.446(2);
C1−Fe1−C2 98.62(5).
Figure 4a. The iron center is bonded with two IPr2Me2 and two
PMe3 ligands to form a distorted-tetrahedral geometry. In
comparison to the Fe−C(carbene) distances in 1 and 3, those
in 4 (2.08 Å in average) are in the middle. The Fe−P distances
depicted in Figure 3a, confirmed the occurrence of olefin
coordination induced biphenyl reductive elimination. The
different outcomes of these reactions imply that coordination
induced biphenyl reductive elimination from the iron(II)
complex requires strong π-accepting ligands. Notably, olefin
coordination induced C−C bond-forming reductive elimination
is a well-known phenomenon for Ni(II), Pd(II), and Pt(II)17
but is unprecedented for iron(II) species.18
Complex 3 was isolated as brown crystals. Its molecular
structure established by XRD (Figure 3a) is close to that of
[(BAC)2Fe(η4-cot)] (BAC = bis(diisopropylamino)-
cyclopropenylidene) reported by Grubbs.19 The η4-bonded
cot ligand in 3 has the bond lengths of the three metal-bound
C−C bonds close to each other (1.414(2), 1.424(2), and
1.416(2) Å) and the Fe−C distances spanning a broad range
(2.073(1)−2.423(1) Å). The Fe−C(carbene) bonds (2.00 Å
on average) are longer than their congerners in [(BAC)2Fe(η4-
cot)] (1.94 Å on average)19 but shorter than those in
[(IEt2Me2)2Fe(η2:η2-dvtms)] (2.08 Å on average; IEt2Me2 =
1,3-diethyl-4,5-dimethylimidazol-2-ylidene, dvtms = divinylte-
Figure 4. Molecular structure of the cation in 4 (left) showing 30%
probability ellipsoids and partial atom schemes and the zero-field
Mossbauer spectrum of 4 recorded at 80 K (right). Selected distances
̈
(Å) and angles (deg): Fe1−C1 2.081(6), Fe1−C2 2.079(5), Fe1−P1
2.305(2), Fe1−P2 2.318(2); C1−Fe1−C2 103.9(2), P1−Fe1−P2
99.14(7), C1−Fe1−P1 104.0(2), C1−Fe1−P2 122.8(2), C2−Fe1−P1
122.9(2), C2−Fe1−P2 105.7(2).
tramethyldisiloxane).20 The 57Fe Mossbauer spectrum of 3 can
̈
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are comparable to those in [PhB(CH2PPri2)3Fe(PMe3)].23 In
addition to this, the solution magnetic moment (4.3(1) μB) and
Table 2. Iron NHC Complex Catalyzed Coupling Reactions
of p-TolylMgBr with n-Octyl Bromide
a
the Mossbauer data (δ = 0.58 mm/s and ΔE = 0.78 mm/s) of
̈
Q
4 are also similar to those of [PhB(CH2PPri2)3Fe(PMe3)].23
These resemblances are indicative of the high-spin nature (S =
3/2) of 4.
Mechanistic Considerations of the Iron NHC Complex
Mediated Cross-Coupling Reactions. The product pattern
depicted in Table 1 suggests that the reactions of 1 with the
alkyl halides are very likely proceeding in a single-electron
transfer mechanism, as in the case of the reaction between
(TMEDA)Fe(Mes)2 and alkyl halides.24 Considering the low
reduction potentials of the nonactivated alkyl halides, which are
generally lower than −1.5 V versus SCE on a glassy-carbon
electrode25 and significantly lower than the Ep value of −0.53 V
for the irreversible oxidation of 1, we thought that the reaction
between R−X and the iron(II) phenyl species (IPr2Me2)2FePh2
or (IPr2Me2)FePh2 should be an inner-sphere electron-transfer
type, rather than an outer-sphere electron-transfer type. After
the electron-transfer step (eq. 1 in Scheme 4), the resulting
yield (mmol)
entry
catalyst
A
B
C
D
1
2
3
4
1
0.49
0.47
0.09
0.08
0.13
0.16
0.18
0.18
0.22
0.19
0.35
0.32
0.22
0.27
0.28
0.27
(IPr2Me2)2FeCl2
b
4
c
1 + PMe3
a
Yields were determined by GC with n-dodecane as the internal
b
standard and were averaged from two parallel experiments. n-
c
Hexadecane (0.09 mmol) was observed. 25 mol % PMe3. n-
Hexadecane (0.09 mmol) was also observed.
tolyl)4]2−, whose anionic nature might render them highly
reducing and enhance the redox reaction with n-C8H17Br.
Recently, Bedford reported the observation of faster rates of
[Fe(Mes)3]− with organic halides versus that of the reaction
with [(TMEDA)Fe(Mes)2].5 The increased percentages of the
side products in the catalytic reactions (entries 1 and 2) may
also relate to the formations of the anionic aryl iron(II) species,
Scheme 4. Possible Mechanisms for the Reactions of 1 with
Alkyl Halides
as Furstner has noted that [FePh ]2− is prone to oxidation-
̈
4
induced degradation to form biphenyl.3
When the iron(I) complex 4 was used as the catalyst, the
yield of cross-coupling product dropped to 9%, and that of
octenes increased to 35% (entry 3, Table 2). The catalytic
reaction employing 1 and PMe3 gave comparable results (entry
4, Table 2). The significantly decreased yields of cross-coupling
products in entries 3 and 4 clearly indicate their distinct
catalytic mechanism. While the effect of PMe3 on the reaction
mechanism has to wait for further examination, one could
propose that the presence of PMe3 might trigger catalytic cycles
involving Fe(I) species.
iron(III) intermediate could (i) interact with the alkyl radical to
afford the cross-coupling product and the iron(II) monophenyl
species (path a), (ii) react with iron(I) species formed by the
interaction of the alkyl radical with an iron(II) diphenyl species
to yield iron(II) products (path b), or (iii) undergo
decomposition to furnish biphenyl and iron(I) species (path
c). The combination of eqs 1 and 2 corresponds to a radical
rebound mechanism,2b whereas the combination of eqs 1, 3,
and 4 lead to a bimetallic oxidative addition pathway.26 The
observation of the cross-coupling products as the major
products in Table 1 suggests that path c is not the dominant
decomposition pathway for the iron(III) intermediate.
Prompted by the fine performance of 1 in the stoichiometric
reactions with alkyl halides, we also studied its catalytic
performance in promoting the cross-coupling of n-C8H17Br
with (p-tolyl)MgBr, along with those of (IPr2Me2)2FeCl2 and 4
for comparison (Table 2). The reaction using 5 mol % of 1 can
quickly produce a red-brown solution and white precipitate.27
GC analyses indicated the formations of n-C8H17(p-tolyl), n-
octane, and octenes in 49%, 13%, and 22% yields, respectively
(entry 1, Table 2). Similar phenomena and product distribution
were observed in the reaction with (IPr2Me2)2FeCl2 as catalyst
(entry 2, Table 2). The catalytic reactions (entries 1 and 2)
differ from the stoichiometric reaction (1 with n-C8H17Br) in
their faster rates, the decreased percentages of the cross-
coupling product, and the increased percentages of the side
products. The enhanced rates might be caused by the formation
of the anionic species [(NHC)Fe(p-tolyl)3]− or [Fe(p-
CONCLUSION
■
The current study has shown that the selection of the suitable
monodenatate NHC ligand enables the preparation of stable
four-coordinate iron(II) diphenyl complex [(IPr2Me2)2FePh2].
Characterization data collectively pointed out the high-spin
nature of the iron(II) phenyl complex in the solid state.
Reactivity studies disclosed its versatile C−C bond-formation
reactions.
The iron(II) phenyl complex [(IPr2Me2)2FePh2] can react
with primary and secondary alkyl bromides and alkyl chlorides
to afford cross-coupling products and monophenyliron(II)
complexes (IPr2Me2)2FePhX. The observation of alkenes and
alkanes as the side products and the ring-opening product
PhCH2CH2CHCH2 in the reaction with cyclopropylmethyl
bromide suggests the single-electron-transfer character of the
C−X bond cleavage step of these reactions. The interaction of
[(IPr2Me2)2FePh2] with the good π-accepting ligand cot
induces the formation of the iron(0) species [(IPr2Me2)2Fe-
(η4-cot)] and biphenyl, and the one-electron oxidation of
[(IPr2Me2)2FePh2] could produce the iron(I) species and
biphenyl. The use of PMe3 as the coligand enables the isolation
of the resulting iron(I) species in the form of [(IPr2Me2)2Fe-
(PMe3)2][BArF ]. The single-electron oxidation induced trans-
4
formation suggests the propensity of iron(III) diphenyl species
[(IPr2Me2)2FePh2]+ to undergo biphenyl reductive elimination.
D
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positions by employing a riding model. Final crystal parameters and
agreement factors are reported in Table S1.
In addition to these, we also found that the iron(II) diphenyl
compound is competitive in catalyzing cross-coupling of n-
C8H17Br with (p-tolyl)MgBr to give the cross-coupling product
in moderate yield, along with substantial amounts of n-octene
and n-octane. In contrast, the reaction catalyzed by
Preparation of [(IPr2Me2)2FePh2] (1). Method A. To a
suspension of [(IPr2Me2)2FeCl2] (600 mg, 1.23 mmol) in THF/
dioxane (20 mL/4 mL) was added a THF solution of PhMgBr (1.0 M,
2.7 mL, 2.7 mmol) at −78 °C. The resulting mixture was warmed to
room temperature and stirred for 8 h, during which time the solution
turned from colorless to yellow. After removal of the solvent, the
residue was extracted with diethyl ether (15 mL × 3) and filtered. The
filtrate was concentrated to about 15 mL. Slow evaporation of diethyl
ether afforded 1 as yellow crystals (540 mg, 77%). Anal. Calcd for
C34H50FeN4: C, 71.56; H, 8.83; N, 9.82. Found: C, 71.43; H, 8.98; N,
10.15. Absorption spectrum (benzene): λmax, nm (ε, M−1 cm−1) 330
(4400), 1350 (130), 1480 (150). The 1H NMR spectrum of this
paramagnetic complex displayed two broad peaks in the range +150 to
[(IPr2Me2)2Fe(PMe3)2][BArF ] gives the cross-coupling prod-
4
uct in poor yield. The difference implies their distinct
mechanisms. The differentiated rates of the catalytic and the
stoichiometric reactions with n-C8H17Br hint that
[(IPr2Me2)2FePh2] might be irrelevant to the catalytic cycle
of the cross-coupling reaction. These results reflect the
complexity of the reaction mechanism of iron-catalyzed
coupling reactions.
1
−150 ppm in C6D6. H NMR (300 MHz, C6D6, 302 K): δ (ppm)
61.75, −6.72. Attempts to integrate the two peaks did not give a
reasonable integration ratio. Dissolution of 1 in THF-d8 also gave a
EXPERIMENTAL SECTION
■
General Procedures. All experiments were performed under an
atmosphere of dry dinitrogen with the rigid exclusion of air and
moisture using standard Schlenk or cannula techniques or in a
glovebox. All organic solvents were freshly distilled from sodium
benzophenone ketyl immediately prior to use. [(IPr2Me2)2FeCl2]28
1
yellow solution with a H NMR spectrum similar to that observed in
1
C6D6. H NMR (300 MHz, THF-d8, 302 K): δ (ppm) 66.35, −0.25.
Method B. FeCl2 (300 mg, 2.37 mmol) and IPr2Me2 (860 mg, 4.73
mmol) were mixed in THF (25 mL), and the mixture was further
stirred for 4 h at room temperature. To the suspension was added a
THF solution of PhMgBr (1.0 M, 5.2 mL, 5.2 mmol) at −78 °C. The
resulting mixture was warmed to room temperature and stirred
overnight, during which time the solution turned from colorless to
dark orange. After removal of the solvent, the residue was extracted
with diethyl ether (25 mL × 3) and filtered. The filtrate was
concentrated to about 20 mL. Recrystallization of the solution at −30
°C afforded 1 as yellow crystals (740 mg, 55%).
and [Cp2Fe][BArF ] (ArF = 3,5-bis(trifluoromethyl)phenyl)29 were
4
prepared according to literature methods. All chemicals were
purchased from either Strem or J&K Chemical Co. and used as
received unless otherwise noted. 1H NMR spectra were recorded on a
Varian Mercury 300 or 400 MHz spectrometer. All chemical shifts
were reported in δ units with references to the residual protons of the
deuterated solvents for proton chemical shifts. GC analyses were
performed on a Shimadzu GC-2014 spectrometer. GC-MS analyses
were performed on a Shimadzu GCMS-QP2010 Plus spectrometer.
Elemental analysis was performed by the Analytical Laboratory of
Shanghai Institute of Organic Chemistry (CAS). Magnetic moments
were measured at 29 °C by the method originally described by Evans
with stock and experimental solutions containing a known amount of
(CH3)3SiOSi(CH3)3 standard.30 Cyclic voltammetry measurements
were made with a CHI 600D potentiostat in THF solutions using a
sweep rate of 100 mV/s, a glassy-carbon working electrode, 0.1 M
[Bu4N][PF6] supporting electrolyte, and an SCE reference electrode.
Under these conditions, E1/2 = 0.55 V for the [Cp2Fe]0/+ couple.
Absorption spectra were recorded with a Shimadzu UV-3600 UV−
vis−NIR spectrophotometer. Magnetic measurements on the crystal-
line sample of 1 were carried out at an applied field of 2 kOe on a
Quantum Design MPMP-XL7 superconducting quantum interference
device (SQUID) magnetometer working in the temperature range
300−1.8 K. The molar magnetic susceptibilities were corrected for
diamagnetism as estimated from Pascal’s tables and for the sample
General Procedure for Reactions of 1 with n-C8H17X, c-
C7H13X (X = Br, Cl), and Cyclopropylmethyl Bromide. A dried
Schlenk tube was charged with the alkyl halide (0.10 mmol), THF (3
mL), n-dodecane (0.10 mmol), and 1 (57 mg, 0.10 mmol) in a
glovebox at room temperature. The reaction mixture was stirred at 30
°C, followed by GC.15 During the course of the reaction, the solution
slowly turned from yellow to orange (Figure S9, Supporting
Information). When alkyl halide was consumed completely, a part of
the reaction mixture was drawn and quenched with H2O. The
quenched mixture was extracted with diethyl ether and separated. The
organic phase was then dried over MgSO4, analyzed by gas
chromatography−mass spectrometry (GC-MS) to confirm the identity
of the products, and further analyzed by GC with a flame ionization
detector (GC-FID) to quantify the yields with n-dodecane as the
internal standard. The composition of the organic products is given in
Table 1. Figure S6 (Supporting Information) shows the change in GC
yields of the cross-coupling products versus time in the reactions of 1
with n-C8H17X and c-C7H13X (X = Br, Cl). Figure S7 (Supporting
Information) depicts the organic product distribution over time in the
reaction of 1 with n-C8H17Br as the representative. On the other hand,
the unquenched organic solution that was left was dried under vacuum
holder by a previous calibration. The 57Fe Mossbauer spectra were
̈
measured with a constant-acceleration spectrometer at 80 K. Low
temperature was maintained by a CCS-850 Mossbauer Cryostat
̈
1
system (Janis Research Co.). Data were analyzed with MossWinn
4.0Pre (Beijing Shengtianjiayuan Keji Co.). Isomer shifts are relative to
iron metal at room temperature.
and redissolved in C6D6. H NMR analysis revealed the presence of
characteristic 1H NMR resonances of the monophenyl iron(II) species
(IPr2Me2)2FePhX (X = Br, Cl), which are close to those of
[(IPr2Me2)2FePhBr] (Figure S2, Supporting Information). In addition
to the monophenyl complexes, the formation of small amounts of
(IPr2Me2)2FeX2 (X = Br, Cl) was also noticed in the 1H NMR spectra.
Preparation of [(IPr2Me2)2FePhBr] (2). FeBr2 (400 mg, 1.86
mmol) and IPr2Me2 (690 mg, 3.82 mmol) were mixed for 4 h at room
temperature in THF/dioxane (15 mL/3 mL). To the suspension was
added a THF solution of PhMgBr (1.0 M, 1.9 mL, 1.9 mmol) at −78
°C. The resulting mixture was warmed to room temperature and
stirred overnight, during which time the solution turned from colorless
to light yellow. After removal of the solvent, the residue was extracted
with THF (5 mL × 3) and filtered. The filtrate was concentrated to
about 5 mL. Slow evaporation of THF afforded 2 as colorless crystals
(850 mg, 80%). The coexistence of a small amount of (IPr2Me2)2FeBr2
in the products made the attempts to collect a satisfactory elemental
analysis data unsuccessful. Anal. Calcd for C34H50FeN4: C, 58.65; H,
7.91; N, 9.77. Found: C, 57.71; H, 7.90; N, 10.16. Absorption
X-ray Structure Determinations. The structures of the five
compounds in Table S1 (Supporting Information) were determined.
Crystallizations were performed at room temperature. Crystals were
coated with Paratone-N oil and mounted on a Bruker APEX CCD-
based diffractometer equipped with an Oxford low-temperature
apparatus. Cell parameters were retrieved with SMART software and
refined using SAINT software on all reflections. Data integration was
performed with SAINT, which corrects for Lorentz−polarization and
decay. Absorption corrections were applied using SADABS.31 Space
groups were assigned unambiguously by analysis of symmetry and
systematic absences determined by XPREP. All structures were solved
and refined using SHELXTL.32 The metal and first coordination
sphere atoms were located from direct-methods E maps; other non-
hydrogen atoms were found in alternating difference Fourier synthesis
and least-squares refinement cycles and during the final cycles were
refined anisotropically. Hydrogen atoms were placed in calculated
E
DOI: 10.1021/om501061b
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
spectrum (benzene): λmax, nm (ε, M−1 cm−1) 330 (1660), 1464 (135).
Reaction of (p-tolyl)MgBr with n-C8H17Br Catalyzed by 4.
The reaction was carried out with 4 (72.0 mg, 0.05 mmol), n-dodecane
(0.50 mmol), n-C8H17Br (1.0 mmol, 1.0 mL), and (p-tolyl)MgBr (1.0
M, 1.5 mL, 1.5 mmol) in diethyl ether (1 mL) using procedures and
reaction time similar to those described above. The entire volume of
the reaction mixture was ca. 2.5 mL. A similar color change from
yellow to red-brown was observed (Figure S10, Supporting
Information). Yields: n-C8H17(tolyl-p), 0.09 mmol (9%); n-octane,
0.18 mmol (18%); octenes, 0.35 mmol (35%); n-hexadecane, 0.09
mmol (18%); p-tolyl−tolyl-p, 0.28 mmol.
Reaction of (p-tolyl)MgBr with n-C8H17Br Catalyzed by 1 in
the Presence of PMe3. The reaction was carried out with 1 (72.0
mg, 0.05 mmol), PMe3 (19 mg, 0.25 mmol), n-dodecane (0.50 mmol),
n-C8H17Br (1.0 mmol), and (p-tolyl)MgBr (1.0 M, 1.5 mL, 1.5 mmol)
in diethyl ether (1 mL) using procedures and reaction time similar to
those described above. The entire volume of the reaction mixture was
ca. 2.5 mL. A similar color change from yellow to red-brown was
observed. Yields: n-C8H17(tolyl-p), 0.08 mmol (8%); n-octane, 0.18
mmol (18%); octenes, 0.32 mmol (32%); n-hexadecane, 0.09 mmol
(18%); p-tolyl−tolyl-p, 0.27 mmol.
1
The H NMR spectrum of this paramagnetic complex displayed four
characteristic peaks in the range +150 to −150 ppm in C6D6. 1H NMR
(300 MHz, C6D6, 302 K): δ (ppm) 100.84 (2H), 18.65 (12H), 9.67
(24H), −34.91 (1H). Magnetic susceptibility (THF-d8, 302 K): μeff
5.0(1) μB.
=
Reaction of 1 with cot. To a solution of [(IPr2Me2)2FePh2] (120
mg, 0.20 mmol) with n-dodecane (0.10 mmol) in THF (6 mL) was
added cyclooctatetraene (cot; 21 mg, 0.20 mmol) at room
temperature. The mixture was stirred at room temperature and
followed by GC. Figure S8 (Supporting Information) shows the
change in the GC yield of biphenyl versus time. After 36 h, the
reaction mixture turned dark brown and the yield of biphenyl reached
70%. All volatiles were removed under vacuum. The brown residue
was washed with n-hexane (2 mL), extracted with diethyl ether (2 mL
× 3), and filtered. The filtrate was concentrated to about 3 mL. Slow
evaporation of diethyl ether afforded [(IPr2Me2)2Fe(η4-cot)] (3) as
brown crystals (69 mg, 66%). Anal. Calcd for C30H48FeN4: C, 69.22;
H, 9.29; N, 10.76. Found: C, 69.03; H, 9.15; N, 10.77. Absorption
spectrum (benzene): λmax, nm (ε, M−1 cm−1) 360 (8800), 822 (250),
1
1340 (216). The H NMR spectrum of this paramagnetic complex
ASSOCIATED CONTENT
displayed four characteristic peaks in the range +150 to −150 ppm in
C6D6. 1H NMR (300 MHz, C6D6, 302 K): δ (ppm) 56.70, 9.39, 6.84,
−95.29. Magnetic susceptibility (C6D6, 302 K): μeff = 3.4(1) μB.
Reaction of 1 with [Cp2Fe][BArF4]. To a solution of
[(IPr2Me2)2FePh2] (120 mg, 0.20 mmol) and n-dodecane (0.10
■
S
* Supporting Information
Figures, tables, and CIF files giving X-ray crystallographic data,
absorption spectra, NMR spectra, a cyclic voltammogram of 1,
photographs of the cross-coupling reactions, and GC graphs of
selected reactions. This material is available free of charge via
the Internet at http://pubs.acs.org.
mmol) in THF (5 mL) was added a THF solution of [Cp2Fe][BArF ]
4
(220 mg, 0.20 mmol) at −80 °C. Immediately, the solution turned
from yellow to orange. A THF solution of PMe3 (61 mg, 0.80 mmol)
was added to the mixture at the same temperature. The resulting
mixture was warmed to room temperature. During the course of the
reaction the color changed gradually from orange to yellowish green.
GC analysis revealed the formation of Cp2Fe in 90% yield and
biphenyl in 80% yield. After removal of the volatiles, the residue was
washed with toluene (2 mL × 3) and n-hexane (2 mL × 3), extracted
with diethyl ether (3 mL), and filtered. Slow evaporation of diethyl
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for L.D.: deng@sioc.ac.cn.
Notes
The authors declare no competing financial interest.
ether afforded [(IPr2Me2)2Fe(PMe3)2][BArF ] (4) as yellowish green
4
crystals (230 mg, 81%). Probably due to the lability of the coordinated
phosphine ligand, satisfactory elemental analysis data could not be
obtained after several attempts. Anal. Calcd for C60H70BF24FeN4P2: C,
50.33; H, 4.93; N, 3.91. Found: C, 49.64; H, 4.72; N, 4.03. Absorption
spectrum (THF): λmax, nm (ε, M−1 cm−1) 315 (6800), 580 (350), 895
(570), 1131 (830), 1537 (270). The 1H NMR spectrum of this
paramagnetic complex displayed six characteristic peaks in the range
+150 to −150 ppm in C6D6 and THF-d8. 1H NMR (300 MHz, C6D6/
THF-d8 5/1, 302 K): δ (ppm) 78.41, 24.56, 17.70, 8.07, 7.59, 5.07.
Magnetic susceptibility (THF-d8, 302 K): μeff = 4.3(1) μB.
Reaction of (p-tolyl)MgBr with n-C8H17Br Catalyzed by 1. 1
(28.5 mg, 0.05 mmol), n-dodecane (0.50 mmol), and n-C8H17Br (1.0
mmol) were mixed at room temperature in diethyl ether (1 mL). A
diethyl ether solution of p-tolylMgBr (1.0 M, 1.5 mL, 1.5 mmol) was
then added in one portion. The entire volume of the reaction mixture
was ca. 2.5 mL. Immediately, the mixture turned from yellow to red-
brown (Figure S10, Supporting Information). After it was stirred for 5
min at room temperature, the reaction mixture was quenched with
water, extracted with diethyl ether (1.0 mL), dried over MgSO4, and
analyzed by GC. Yields: n-C8H17(tolyl-p), 0.49 mmol (49%); n-octane,
0.12 mmol (12%); octenes, 0.22 mmol (22%); p-tolyl−tolyl-p, 0.22
mmol.
Reaction of (p-tolyl)MgBr with n-C8H17Br Catalyzed by
[(IPr2Me2)2FeCl2]. The reaction was carried out with
[(IPr2Me2)2FeCl2] (24.5 mg, 0.05 mmol), n-dodecane (0.50 mmol),
n-C8H17Br (1.0 mmol), and (p-tolyl)MgBr (1.0 M, 1.5 mL, 1.5 mmol)
in diethyl ether (1 mL) using procedures and reaction time similar to
those described above. The entire volume of the reaction mixture was
ca. 2.5 mL. A similar color change from yellow to red-brown was
observed (Figure S10, Supporting Information). Yields: n-C8H17(tolyl-
p), 0.47 mmol (47%); n-octane, 0.16 mmol (16%); octenes, 0.19
mmol (19%); p-tolyl−tolyl-p, 0.27 mmol.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Basic
Research Program of China (973 Program, No.
2011CB808705), the National Natural Science Foundation of
China (Nos. 21222208, 21421091, and 21432001), and the
Chinese Academy of Sciences.
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G
DOI: 10.1021/om501061b
Organometallics XXXX, XXX, XXX−XXX