Monomeric bis(anilido)iron(II) complexes with N-heterocyclic carbene ligation: Synthesis, characterization, and redox reactivity toward aryl halides

Article abstract of DOI:10.1021/ic301894e

Using monodentate N-heterocyclic carbenes as the ancillary ligands, seven monomeric bis(anilido)iron(II) complexes [(IPr₂Me₂) ₂Fe(NHAr)₂] (IPr₂Me₂ = 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene; Ar = Ph, C₆H ₄-2-Prⁱ, Mes, C₆H₃-2,6-Cl ₂, Dipp) and [(IPr)Fe(NHAr)₂] (IPr = 2,5-di(2,6- diisopropylphenyl)imidazol-1-ylidene; Ar = C₆H₃-2,6- Cl₂, Dipp) have been prepared by the one-pot reactions of [Fe(Mes)₂]₂ with the corresponding N-heterocyclic carbenes, and anilines. These high-spin diamido complexes have been fully characterized by ¹H NMR, solution magnetic susceptibility, UV-vis, IR, X-ray diffraction, cyclic voltammetry, as well as elemental analysis. The strong affinity of the N-heterocyclic carbene ligands toward ferrous centers, and the steric protection exerted by the NHC ligands are the key factors to stabilize these bis(anilido)iron complexes in a monomeric manner. Reactivity studies revealed the four-coordinate complex [(IPr₂Me ₂)₂Fe(NHMes)₂] can react with 1 equiv of 1-iodo-3,5-dimethylbenzene or 1-bromo-3,5-dimethylbenzene in C₆D ₆ and THF-d₈ to furnish 1-C₆D ₅-3,5-Me₂C₆H₃, and 5-D-1,3-Me ₂C₆H₃, respectively. Under similar conditions, the three-coordinate compound [(IPr)Fe(NHDipp)₂] is inert toward these halides.

Full text of DOI:10.1021/ic301894e

Article
pubs.acs.org/IC
Monomeric Bis(anilido)iron(II) Complexes with N‑Heterocyclic
Carbene Ligation: Synthesis, Characterization, and Redox Reactivity
toward Aryl Halides
Xiaojie Wang, Zhenbo Mo, Jie Xiao, and Liang Deng*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai, P. R. China, 200032
S
* Supporting Information
ABSTRACT: Using monodentate N-heterocyclic carbenes as
the ancillary ligands, seven monomeric bis(anilido)iron(II)
complexes [(IPr2Me2)2Fe(NHAr)2] (IPr2Me2 = 2,5-diisoprop-
yl-3,4-dimethylimidazol-1-ylidene; Ar = Ph, C6H4-2-Pri, Mes,
C6H3-2,6-Cl2, Dipp) and [(IPr)Fe(NHAr)2] (IPr = 2,5-di(2,6-
diisopropylphenyl)imidazol-1-ylidene; Ar = C6H3-2,6-Cl2,
Dipp) have been prepared by the one-pot reactions of
[Fe(Mes)2]2 with the corresponding N-heterocyclic carbenes,
and anilines. These high-spin diamido complexes have been
1
fully characterized by H NMR, solution magnetic susceptibility, UV−vis, IR, X-ray diffraction, cyclic voltammetry, as well as
elemental analysis. The strong affinity of the N-heterocyclic carbene ligands toward ferrous centers, and the steric protection
exerted by the NHC ligands are the key factors to stabilize these bis(anilido)iron complexes in a monomeric manner. Reactivity
studies revealed the four-coordinate complex [(IPr2Me2)2Fe(NHMes)2] can react with 1 equiv of 1-iodo-3,5-dimethylbenzene or
1-bromo-3,5-dimethylbenzene in C6D6 and THF-d8 to furnish 1-C6D5-3,5-Me2C6H3, and 5-D-1,3-Me2C6H3, respectively. Under
similar conditions, the three-coordinate compound [(IPr)Fe(NHDipp)2] is inert toward these halides.
amido ligand.19 Besides these, the formation of iron amido
INTRODUCTION
■
c o m p l e x e s , s u c h a s [ ( N ( C H 2 C O N P r i ) -
The relevance of iron amido species to important iron-
(CH2CH2NCONHBut)2)Fe(NHC6H4-p-Me)],20 [(Si-
mediated chemical transformations, such as dinitrogen
(C6H4PPri2)3)Fe(NHC6H4-p-Me)], and [(nacnac)Fe(NHAd)-
reduction,1 C−H bond amination,2 and olefin hydroamination,3
(py-But)], from the hydrogen abstraction reactions of iron
has spurred people’s curiosities about the structure and
imido species have also been noted.20−23 Obviously, steric
reactivity properties of iron amido compounds,4 particularly,
protection exerted by either the bulky substituents on the
those of the terminal type. Studies have shown simple amido
amido ligand or the multidentate ancillary ligands plays an
anions display a strong tendency to bridge iron centers to form
iron amido,5 or even iron imido clusters,6 rendering access to
important role in stabilizing these monomeric species.
In addition to this progress, we report herein that, by using
terminal type iron amide species difficult. In the case of some
steric crowded amido anions, usually secondary amido anions
monodentate N-heterocyclic carbene as the ancillary ligand, the
monomeric bis(anilido)iron(II) complexes [(IPr2Me2)2Fe-
like [N(SiMe3)2]−, [N(SiMe2Ph)2]−, [NArBMes2]−, [NBut2]−,
(NHAr)2] (Ar = Ph, C6H4-2-Pri, Mes, C6H3-2,6-Cl2, Dipp)
[NBut(C6H4-o-F)]−, and [N(C6F5)2]−, the aggregation ten-
and [(IPr)Fe(NHAr)2] (Ar = C6H3-2,6-Cl2, Dipp) can be
dency of iron amido moieties is alleviated, and monomeric iron
amido complexes can be prepared.5,7−13
readily prepared via the reaction of [Fe(Mes)2]2 with the
corresponding N-heterocyclic carbenes, and the anilines.24
The preparation of the parent and primary amido-iron
complexes is more challenging.14 There are only two parent
These compounds represent rare examples of iron complexes
amido-iron complexes [(dmpe)2Fe(NH2)H]15 and [Fe(C6H3-
featuring two terminally bonded primary amido groups, and
2,6-Dipp2)(NH2)]2,16 and a handful of primary amido-iron
their molecular structures have been unequivocally established
by single-crystal X-ray diffraction studies. Reactivity studies
complexes are known. Power et al. reported the synthesis of the
revealed the four-coordinate complex [(IPr2Me2)2Fe-
(NHMes)2] can react with aryl iodide and bromide to furnish
C(arene)-X bond cleavage products, whereas, the three-
two-coordinate complexes [Fe(NHAr*)(Ar′)] and [Fe-
(NHAr*)2] (Ar* = C6H3-2,6-Mes2, C6H3-2,6-(C6H3-2,4,6-
Pri3)2, Ar′ = C6H3-2,6-Dipp2) by using the sterically congested
terphenyl amido ligands.17 Holland et al. achieved the
coordinate compound [(IPr)Fe(NHDipp)2] is inert toward
these halides.
preparation of some heteroleptic iron(II) amido complexes
[(nacnac)Fe(NHR)] bearing bulky β-diketiminate ligands.18
Caulton et al. prepared the four-coordinate complex [(N-
Received: May 2, 2012
Published: December 18, 2012
(But2PCH2SiMe2)2)Fe(NHxyl)] featuring the bulky phosphine-
© 2012 American Chemical Society
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anilido ligands. Examining the metrical data around their
FeC2N2 units (Table 1) revealed the steric bulkiness of the
anilido ligands, as reflected by their Fe−N(amido)−C(aryl)
and N(amido)−Fe−N(amido) angles that deviate from angles
for idealized tetrahedral geometry, induces distortion of the
FeC2N2 unit. The FeC2N2 unit in [(IPr2Me2)2Fe(NHPh)2] (1)
exhibits the least distortion with the interatomic angles within
the FeC2N2 unit ranging from 107.0(1) to 114.4(1) degrees,
whereas, the FeC2N2 core in [(IPr2Me2)2Fe(NHDipp)2] (5) is
the most distorted one with the angles spanning between
93.61(8) and 128.1(1) degrees. Overall, the deviation of the
FeC2N2 units from tetrahedral geometry shows a trend of 1 < 2
< 3 ≤ 4 < 5, which accords well with the bulkiness of their
anilido ligands. In addition to the bond angles, a stepwise
elongation of the Fe−C(carbene) bonds from 2.148(1) to
2.210(2) Å is also observed in 1−5, which again could be
ascribed to the steric effect of the anilido ligands. The bond
lengths of the Fe−N(amido) moieties in 1−5 are close to each
other (1.986(2)−2.019(2) Å), but are longer than those of the
known iron(II) amido complexes such as [Fe(NHC6H3-2,6-
Mes2)2] (1.909(3) Å),17a [(nacnac)Fe(NHDipp)(py-p-But)]
RESULTS AND DISCUSSION
■
Preparation and Characterization of the Bis(anilido)
Iron Complexes. Treatment of [Fe(Mes)2]225 with 4 equiv of
IPr2Me2,26 and 4 equiv of anilines in tetrahydrofuran (THF)
gave a yellow-brown solution; after workup, the four-coordinate
bis(anilido)iron(II) complexes [(IPr2Me2)2Fe(NHAr)2] (Ar =
Ph, 1; C6H4-2-Pri, 2; Mes, 3; C6H3-2,6-Cl2, 4; Dipp, 5) were
isolated as yellow crystals in moderate yields (41%-73%). In a
similar manner, the three-coordinate complexes [(IPr)Fe-
(NHAr)2] (Ar = C6H3-2,6-Cl2, 6; Dipp, 7) have also been
prepared when using [Fe(Mes)2]2, IPr, and the corresponding
anilines in a 1:2:4 ratio (Scheme 1). The attempts to prepare
Scheme 1. Preparation of the Diamido Iron Complexes
(1.949(2) Å),18a [(N(But PCH2SiMe2)2)Fe(NHxyl)]
2
(1.951(2) Å),19 and [(PhBP3)Fe(NH-p-tolyl)] (1.913(2)
Å).21a The elongation could be due to the stronger σ-donating
property of the N-heterocyclic carbene ligand over the
phosphine and nacnac ligands.
As shown in Figure 1, the coordination of the sterically
demanding IPr ligand in 6 and 7 allows the formation of the
three-coordinate complexes, in which the Fe center is nearly
coplanar with the C(carbene) and the two N(amido) atoms. As
listed in Table 1, the Fe−C(carbene) bond distances in 6 and 7
are 2.104(2) and 2.142(2) Å, respectively. These distances are
shorter than those of the Fe−C(carbene) bonds in
[(IPr2Me2)2Fe(NHC6H3-2,6-Cl2)2] (4) [(IPr2Me2)2Fe-
(NHDipp)2] (5), and [(IPr)Fe(N(SiMe3)2)2] (8).8 The Fe−
N(amido) separations in 6 and 7 (1.969 (2) and 1.922(2) Å on
average) are also the shorter Fe−N(amido) bonds among 1−8
(Table 1), but longer than those of the three-coordinate
iron(II) amides [(nacnac)Fe(NHR)] (1.79−1.91 Å).18a It is
noteworthy that although the molecular structures of 6 and 7
exhibit a close resemblance, 6 features weak interactions
between its iron center and the peripheral chlorine atoms
(Cl(1) and Cl(2)) with the Fe−Cl separations being 2.717(1)
and 2.732(1) Å. These weak interactions cause the near vertical
arrangement of the anilido ligands toward the coordination
plane in 6, sharply contrasting with the near coplanar aryl
groups in 7 (Figure 1).
three-coordinate complexes (IPr)Fe(NHAr)2 employing the
less bulky anilines, PhNH2, 2-Pri-C6H4NH2, gave brown, oily
materials that could not be identified.
Complexes 1−7 are air- and moisture-sensitive and soluble in
1
benzene, Et2O, and THF. They were fully characterized by H
NMR, solution magnetic susceptibility, UV−vis, IR, elemental
analysis, X-ray diffraction study, and cyclic voltammetry.
Differing from [(NHC)Fe(N(SiMe3)2)2] (NHC = IPr, IMes)
which dissociate their free carbene ligands in toluene,8 the
carbene ligands in 1−7 are more strongly bonded to the iron
centers as (1) their 1H NMR spectra in C6D6 acquired at room
temperature only feature broad 1H NMR peaks and no
1
noticeable peak arising from free NHCs, (2) the H NMR
spectra of 3 recorded in C6D6 and THF-d8 display resonances
at similar positions indicating the same species is observed, and
(3) the addition of 10 equiv of IPr2Me2 to a solution of 3 in
C6D6 did not cause the shift of the paramagnetic signals. The
magnetic moments of 2−7 measured by Evans method range
from 4.6 to 5.4 μB, suggesting their high spin electronic
configurations.27 Dissolution of the complexes in THF
develops pale yellow solutions, and only shoulder bands near
300 nm caused by LMCT were observed in their UV−vis
absorption spectra (Supporting Information, Figure s1). Since
at very low concentration these complexes might undergo
decomposition (see electrochemical studies, vide infra), these
absorptions might not directly come from 1−7 themselves.
Characteristic N−H stretching vibrations around 3350 cm−1
have been found in their IR spectra.
Reactions of the Bis(anilido) Iron Complexes with Aryl
Halides. The redox properties of 1−7 have been examined by
cyclic voltammetry and open circuit potential studies
(Supporting Information, Figures s2−s9). In general these
voltammograms are dominated by irreversible redox waves
which is in sharp contrast to the quasi-reversible one of
28
[(IMes)2FeCl2]0/1+
.
Because the peak current intensities of
the quasi-reversible waves with very negative half-wave
potentials in these voltammograms increased when more
scans were performed, we refrain from assigning chemical
events for these voltammograms.
As mentioned above, iron amido moieties are prone to
aggregation; the successful preparation of these monomeric
primary amido-iron compounds clearly indicates the strong
affinity of NHCs toward ferrous centers, which had also been
demonstrated by the isolation of the stable all-ferrous iron−
The molecular structures of 1−7 have been determined by
single-crystal X-ray diffraction studies. Figure 1 shows the
molecular structures of 1, 5, 6, and 7 as the representatives.
Table 1 lists the key bond lengths and angles. The iron centers
in 1−5 are tetrahedrally coordinated with two IPr2Me2 and two
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Figure 1. Molecular Structures of 1, 5, 6, and 7, showing 30% probability ellipsoids and the partial atom numbering scheme.
a
Table 1. Selected Bond Distances (Å) and Angles (degree) in 1−8
1
2
3
4
5
6
7
8
Fe−C
Fe−N
2.148(1)
2.150(2)
2.172(2)
2.174(2)
2.006(2)
2.175(2)
2.185(2)
2.005(1)
2.210(2)
2.104(2)
2.142(2)
2.182(2)
1.999(1)
1.986(2)
1.985(2)
1.967(2)
1.971(2)
1.921(2)
1.924(2)
1.982(2)
1.979(2)
2.019(2)
97.77(8)
126.4(1)
110.4(1)
2.007(1)
96.05(6)
128.4(1)
113.4(1)
C−Fe−C
N−Fe−N
C−Fe−N
108.5(1)
114.4(1)
107.0(1)
109.9(1)
105.0(1)
117.4(1)
112.2(1)
104.5(1)
93.61(8)
128.1(1)
97.28(6)
127.4(1)
114.4(1)
135.4(1)
112.6(1)
124.3(1)
117.1(1)
118.6(1)
103.0(1)
110.6(1)
105.1(1)
137.6(2)
142.1(2)
100.4(1)
112.4(1)
101.9(1)
139.0(1)
138.8(1)
118.2(1)
143.9(1)
118.1(1)
111.9(1)
Fe−N−C(aryl)
125.5(1)
127.0(2)
130.7(2)
128.8(2)
132.8(2)
144.3(2)
a
For [(IPr)Fe(N(SiMe3)2)2] (8), see ref 8.
sulfur cubane [Fe4S4(IPr2Me2)4],29 and the dimethyl iron(II)
complex [(IEt2Me2)2Fe(CH3)2]30 in our previous studies. With
these diamido iron(II) compounds in hand, we have performed
some preliminary reactivity investigations to examine the
potential application of these iron anilido complexes in
carbon−nitrogen bond construction.31
NMR assays in C6D6 indicated [(IPr2Me2)2Fe(NHMes)2]
(3) can slowly react with 1-iodo-3,5-dimethylbenzene (1 equiv)
at room temperature (less than 5% conversion of the iodide in
3 h). When the temperature was raised to 50 °C, slow color
change from pale yellow to dark brown occurred. After 8 h, 1H
NMR spectrum analyses indicated the full consumptions of the
start materials, and the formations of 1-(C6D5)-3,5-Me2C6H3
(50% NMR yield), 1,3-Me2C6H4 (32% NMR yield), and new
paramagnetic species (Scheme 2). Interestingly, when the
reaction was conducted in THF-d8, the aryl iodide was
converted to 5-D-1,3-Me2C6H3 (93% NMR yield) exclusively.
The identities of the two deuterated arenes were confirmed by
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NHC ligand afforded four-coordinate complexes
[(IPr2Me2)2Fe(NHAr)2] (Ar = Ph, C6H4-2-Pri, Mes, C6H3-
2,6-Cl2, Dipp), whereas, the three-coordinate compounds
[(IPr)Fe(NHAr)2] (Ar = C6H3-2,6-Cl2, Dipp) were obtained
with the more steric demanding IPr. These bis(anilido)iron(II)
complexes have a high-spin electronic configuration and were
characterized by various spectroscopic methods. Reactivity
studies have revealed the four-coordinate complex
[(IPr2Me2)2Fe(NHMes)2] can react with 1-iodo-3,5-dimethyl-
benzene or 1-bromo-3,5-dimethylbenzene, possibly via single-
electron transfer mechanism, to give 1-C6D5-3,5-Me2C6H3 in
deuterated benzene, and 5-D-1,3-Me2C6H3 in THF-d8. Under
similar conditions, the three-coordinate compound [(IPr)Fe-
(NHDipp)2] is inert toward these halides.
Scheme 2. Reactions of 3 with Aryl Halides
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 in a glovebox. All organic solvents
were freshly distilled from sodium benzophenone ketyl immediately
prior to use. [Fe(Mes)2]2,25 2,5-diisopropyl-3,4-dimethylimidazol-1-
ylidene (IPr2Me2),26 2,5-di(2,6-diisopropylphenyl)imidazol-1-ylidene-
(IPr)34 were 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 MHz spectrometer. All chemical shifts were
reported in δ units with references to the residual protons of the
deuterated solvents for proton chemical shifts. Elemental analysis was
performed by the Analytical Laboratory of Shanghai Institute of
Organic Chemistry (CAS). Magnetic moments were measured at 23
°C by the method originally described by Evans with stock and
experimental solutions containing a known amount of a (CH3)3SiOSi-
(CH3)3 standard.27 GC-MS analyses were performed on a Shimadzu
GCMS-QP 2010 Spectrometer. UV−vis spectra were recorded with a
Shimadzu UV-3600 spectrophotometer. IR spectra were recorded with
a NICOLET AVATAR 330 FT-IR spectrophotometer. Electro-
chemical studies were made with a CHI 600D potentiostation in
THF solutions using a glassy carbon working electrode, 0.1 M
(Bu4N)(PF6) supporting electrolyte, and a SCE reference electrode. A
sweep rate of 50 mV/s was used for the cyclic voltammetry
measurements. Under these conditions, E1/2 = 0.55 V for the
[Cp2Fe]0,1+ couple.
NMR and GC-MS analyses. Attempts to isolate the para-
magnetic species by recrystallization were unsuccessful. Similar
products were obtained when 3 was treated with 1-bromo-3,5-
dimethylbenzene, but no reaction took place when 1-chloro-
3,5-dimethylbenzene was used. Different from the observed
reactivity of 3, the three-coordinate complex [(IPr)Fe-
(NHDipp)2] (7) was inert toward these aryl halides.
The formation of 1-(C6D5)-3,5-Me2C6H3 and 5-D-1,3-
Me2C6H3 clearly demonstrates iron complex-mediated C-
(arene)-X (X = Br, I) bond cleavage reaction has taken place.
On the basis of the observed products pattern, we propose the
reaction between 3 and the aryl halides might proceed in a
single-electron transfer mechanism to generate a transient
species [3]+[X]− and an aryl radical Ar•.30,32 In deuteroben-
zene, the aryl radical might react with a deuterobenzene
molecule to generate a σ-complex which might then be oxidized
by [3]+[X]− or other species to produce 1-(C6D5)-3,5-
Me2C6H3 (Scheme 3).33 In the case with THF-d8 as solvent,
Scheme 3. Possible Pathways for the Formation of the
Deuterated Arenes
Preparation of (IPr2Me2)2Fe(NHPh)2 (1). To a red-brown
solution of [(IPr2Me2)Fe(Mes)2] prepared by mixing [Fe(Mes)2]2
(122.9 mg, 0.20 mmol) and IPr2Me2 (155.1 mg, 0.80 mmol) in 5.0 mL
of THF was slowly added a solution of PhNH2 (85.1 mg, 0.80 mmol)
in THF (3 mL) at room temperature. After stirring for 16 h, lots of
yellow solids precipitated out from the solution. The suspension was
then filtrated to give some yellow solid and a brown mother solution.
The solid was washed with a small amount of THF (3 mL), and dried
under vacuum, to afford 1 as a yellow powder (146.6 mg, yield: 61%).
Yellow-green crystals of 1 suitable for X-ray crystallography were
obtained by adding a small portion of toluene (1 mL) to the brown
mother solution and then evaporating THF at room temperature for 5
days. The 1H NMR spectra of the yellow powder and the yellow-green
deuterium-atom-abstraction reaction between the aryl radical
with a THF-d8 molecule might occur to yield 5-D-1,3-
Me2C6H3. The failure to achieve coupling between the anilido
ligand and the aryl halides in these trials should be due to the
outer-sphere character of the single-electron transfer step that
should produce aryl radicals surrounded by solvent molecules
and subsequently make the access of the aryl radicals toward
the anilido moiety difficulty.
1
crystal are identical. H NMR (400 MHz, C6D6): δ 41.09(2H, m-CH
on the phenyl rings), 11.46(6H, =CCH3), 8.67(12H, NCH(CH3)2),
−64.90(1H, p-CH on the phenyl rings). The signals of the ortho-CH
on the phenyl rings, the NCH(CH3)2, or NHAr was not observed.
Anal. Calcd for C34H52FeN6: C, 67.99; H, 8.73; N, 13.99. Found: C,
68.10; H, 8.59; N, 14.00. IR (KBr, cm−1): υN−H 3321.7. Because of the
low solubility of 1 in C6D6, its solution magnetic moment has not been
measured.
CONCLUSION
Preparation of (IPr2Me2)2Fe(NHC6H4-2-Pri)2 (2). To a red-
brown solution of [(IPr2Me2)Fe(Mes)2] prepared by mixing [Fe-
(Mes)2]2 (122.9 mg, 0.20 mmol) and IPr2Me2 (155.1 mg, 0.80 mmol)
in 5.0 mL of THF was slowly added a solution of 2-Pri-C6H4NH2
(109.1 mg, 0.80 mmol) in THF (3 mL) at room temperature. After
■
In summary, we have prepared a series of monomeric primary
amido iron(II) complexes bearing monodentate NHC ligation
via the one pot reactions of [Fe(Mes)2]2 with NHCs and
primary anilines. The reactions using the less steric demanding
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stirring for 18 h, the color of the mixture changed to green-brown, and
then all the volatiles were removed under vacuum to give a brown
residue. The residue was redissolved in THF (7 mL). After filtration,
the filtrate was concentrated to about 5 mL, and toluene (1 mL) was
added. Slow evaporation of THF afforded the product as yellow
crystals. Yield: 199.0 mg, 73%. 1H NMR (300 MHz, C6D6): δ
48.41(1H, m-CH on the phenyl rings), 38.15(br, m-CH on the phenyl
rings), 30.49(br, o-CH on the phenyl rings), 8.88(12H, NCH(CH3)2),
7.63(6H, =CCH3 or CCH(CH3)2), 6.41(6H, =CCH3 or CCH-
(CH3)2), −62.72(1H, p-CH on the phenyl rings). The signals of
CH(CH3)2 or NHAr were not observed. Anal. Calcd for C40H64FeN6:
C, 70.15; H, 9.42; N, 12.27. Found: C, 70.01; H, 9.58; N, 12.19.
Magnetic susceptibility: μeff = 4.6(1) μB. IR (KBr, cm−1): υN−H 3378.9.
Preparation of (IPr2Me2)2Fe(NHMes)2 (3). This complex was
obtained as yellow crystals by the reaction of [Fe(Mes)2]2 (116.5 mg,
0.20 mmol) with IPr2Me2 (146.8 mg, 0.80 mmol) and MesNH2 (109.3
mg, 0.80 mmol) in 8 mL of THF using procedures similar to that of 2.
Yield: 112.0 mg, 41%. 1H NMR (300 MHz, C6D6): δ 97.00(br, o-CH3
on the mesityl group), 80.77(3H, p-CH3 of the mesityl group),
45.47(2H, m-CH of the mesityl group), 7.38(12H, NCH(CH3)2),
3.63(6H, =CCH3). 1H NMR (400 MHz, THF-d8): δ 95.67(br, o-CH3
on the mesityl group), 79.27(3H, p-CH3 of the mesityl group),
44.33(2H, m-CH of the mesityl group), 7.97(12H, NCH(CH3)2),
4.27(6H, =CCH3). The signals of CH(CH3)2 or NHAr was not
observed. Anal. Calcd for C40H64FeN6: C, 70.15; H, 9.42; N, 12.27.
the mixture changed to yellow-brown. Removal of the volatiles under
vacuum gave a yellow solid, which was redissolved in diethyl ether (10
mL) and filtrated. The filtrate was concentrated to about 7 mL, and n-
hexane (1 mL) was added. Slow evaporation of Et2O afforded the
product as yellow crystals. Yield: 900.0 mg, 75%. 1H NMR (300 MHz,
C6D6): δ 52.33(br), 34.88(br), 1.10(br), −1.16(br), −11.01(br).
Because of the large width of the peaks, integration was not performed.
Anal. Calcd for C51H72FeN4: C, 76.86; H, 9.11; N, 7.03. Found: C,
76.10; H, 9.13; N, 7.02. Magnetic susceptibility: μeff = 5.1(2) μB. IR
(KBr, cm−1): υN−H 3337.9.
Reaction of (IPr2Me2)2Fe(NHMes)2 (3) with 1-Iodo-3,5-
dimethylbenzene in C6D6. To a solution of (IPr2Me2)2Fe-
(NHMes)2 (24.2 mg, 0.040 mmol) in C6D6 (1 mL) in a J. Young
NMR tube was added 1-iodo-3,5-dimethylbenzene (9.5 mg, 0.041
mmol), and 1,3,5-trimethoxybenzene (8.2 mg, 0.049 mmol) as internal
1
standard. The reaction was monitored by H NMR, and it proceeded
very slowly at room temperature. Then the mixture was heated to 50
°C. After 8 h, the color of the solution turned into dark brown, and 1H
NMR spectrum analyses on the mixture indicated the full
consumptions of the two start materials, and the formations of 1-
(C6D5)-3,5-Me2C6H3 and 1,3-Me2C6H4 in 50% and 32% yields,
respectively, in addition with new paramagnetic species. The identities
of these two products were confirmed by 1H NMR and GC-MS
analyses. Attempts to isolate the paramagnetic species by recrystalliza-
1
tion were unsuccessful. For 1-(C6D5)-3,5-Me2C6H3, H NMR (300
Found: C, 70.23; H, 9.55; N, 12.51. Magnetic susceptibility: μeff
=
MHz, C6D6): δ 2.14 (s, 6H, CH3), 6.78 (s, 1H, p-C6H3), 7.14 (s, 2H,
4.8(1) μB. IR (KBr, cm−1): υN−H 3349.6.
+
o-C6H3); GC-MS: calcd for C14H9D5 : 187; Found: 187. For 1,3-
1
Preparation of (IPr2Me2)2Fe(NHC6H3-2,6-Cl2)2 (4). This com-
plex was obtained as yellow crystals by the reaction of [Fe(Mes)2]2
(118.4 mg, 0.20 mmol) with IPr2Me2 (150.1 mg, 0.80 mmol) and 2,6-
Cl2−C6H3NH2 (129.2 mg, 0.80 mmol) in 8 mL of THF using
Me2C6H4, H NMR (300 MHz, C6D6): δ 2.09 (s, 6H, CH3), 6.80(s,
1H), 6.84−6.85 (d, 2H), 7.03−7.06 (m, 1H).
Reaction of (IPr2Me2)2Fe(NHMes)2 (3) with 1-Iodo-3,5-
dimethylbenzene in C6H6. In C6H6 (5.0 mL), (IPr2Me2)2Fe-
(NHMes)2 (273.7 mg, 0.40 mmol) was reacted with 1-iodo-3,5-
dimethylbenzene (92.8 mg, 0.40 mmol) at 50 °C overnight. The
mixture was then quenched with saturated aqueous NaHCO3 solution
(20 mL) and extracted with ether (3 × 10 mL). The organic phase was
separated and dried with MgSO4. Removal of the volatiles afforded a
yellow oily material, which was subjected to column chromatographic
separation (SiO2, 300−400 mesh) to give 1-phenyl-3,5-dimethylben-
1
procedures similar to that of 2. Yield: 132.0 mg, 45%. H NMR (300
MHz, C6D6): δ 9.30(6H, =CCH3), 6.77(12H, NCH(CH3)2),
−59.90(1H, p-CH on the phenyl rings). The signals of m-CH on
the phenyl rings, CH(CH3)2, or NHAr was not observed. Anal. Calcd
for C34H48Cl4FeN6: C, 55.30; H, 6.55; N, 11.38. Found: C, 55.38; H,
6.31; N, 11.42. Magnetic susceptibility: μeff = 4.7(1) μB. IR (KBr,
cm−1): υN−H 3334.7.
1
Preparation of (IPr2Me2)2Fe(NHDipp)2 (5). This complex was
obtained as yellow crystals by the reaction of [Fe(Mes)2]2 (118.2 mg,
0.20 mmol) with IPr2Me2 (151.1 mg, 0.80 mmol) and DippNH2
(149.1 mg, 0.80 mmol) in 8 mL of THF using procedures similar to
that of 2. Yield: 218.0 mg, 71%. 1H NMR (300 MHz, C6D6): δ
44.53(2H, m-CH of the phenyl rings), 9.41(12H, CH(CH3)2),
6.41(br, CH(CH3)2 or =CCH3), 5.50(br, CH(CH3)2 or =CCH3),
−62.00(1H, p-CH on the phenyl rings). The signals of CH(CH3)2 or
NHAr was not observed. Anal. Calcd for C46H76FeN6: C, 71.85; H,
9.96; N, 10.93. Found: C, 71.83; H, 9.88; N, 11.09. Magnetic
susceptibility: μeff = 4.8(1) μB. IR (KBr, cm−1): υN−H 3340.9.
Preparation of (IPr)Fe(NHC6H3-2,6-Cl2)2 (6). To a solution of
[Fe(Mes)2]2 (117.8 mg, 0.20 mmol) in THF (7 mL) was added IPr
(156.1 mg, 0.40 mmol) at room temperature. After stirring for 1 h, a
yellow suspension was formed. Then a solution of 2,6-Cl2−C6H3NH2
(131.2 mg, 0.8 mmol) in THF (3 mL) was slowly added to the
suspension, and the mixture was further stirred for 8 h. Removal of the
volatiles under vacuum gave a yellow solid, which was redissolved in
diethyl ether (10 mL) and filtrated. The filtrate was concentrated to
about 5 mL and n-hexane (1 mL) was added. Slow evaporation of
Et2O afforded the product as yellow crystals. Yield: 110.0 mg, 36%. 1H
NMR (300 MHz, C6D6): δ 60.15(2H), 27.91(1H), 12.20(2H),
9.25(6H), 1.20(1H), −3.20(6H), −10.11(2H), −62.72(1H). Anal.
Calcd for C39H45Cl4FeN4: C, 61.03; H, 5.91; N, 7.30. Found: C, 61.23;
H, 5.95; N, 7.48. Magnetic susceptibility: μeff = 5.4(2) μB. IR (KBr,
cm−1): υN−H 3390.7.
zene (49.7 mg, 66% yield) as a colorless solid. H NMR (300 MHz,
C6D6): δ: 2.42 (s, 6H, CH3), 7.03 (s, 1H, p-C6H3), 7.25 (s, 2H, o-
C6H3), 7.34−7.38 (m, 1H, p-C6H5), 7.43−7.47 (m, 2H, C6H5), 7.60−
7.62 (m, 2H, C6H5). 13C NMR (300 MHz, C6D6): δ 21.42, 125.09,
127.06, 127.17, 128.62, 128.87, 138.24, 141.23, 141.43. The NMR
spectra are idential to those reported in literature.35
Reaction of (IPr2Me2)2Fe(NHMes)2 (3) with 1-Iodo-3,5-
dimethylbenzene in THF-d8. The reaction between (IPr2Me2)2Fe-
(NHMes)2 (25.0 mg, 0.036 mmol) and 1-iodo-3,5-dimethylbenzene
(8.5 mg, 0.036 mmol) with 1,3,5-trimethoxybenzene (6.0 mg, 0.032
mmol) as the internal standard was performed in THF-d8 (1 mL).
After being heated at 50 °C for 8 h, the color of the solution turned to
dark brown, and 1H NMR spectrum analyses indicated the full
conversion of 1-iodo-3,5-dimethylbenzene to 5-D-1,3-Me2C6H3 in
1
93% NMR yield. For 5-D-1,3-Me2C6H3, H NMR (300 MHz, C6D6):
δ 2.22(s, 6H), 6.90(s, 2H), 6.95(s, 1H); GC-MS: calcd for C8H9D+:
107; Found: 107. Paramagnetically shifted 1H NMR signals with
similar chemical shifts as those observed in the reaction conducted in
1
C6D6 were also observed in the H NMR spectrum.
Reaction of (IPr2Me2)2Fe(NHMes)2 (3) with 1-Bromo-3,5-
dimethylbenzene in C6D6. In a J. Young NMR tube, the reaction
mixture of (IPr2Me2)2Fe(NHMes)2 (18.0 mg, 0.026 mmol), 1-bromo-
3,5-dimethylbenzene (4.7 mg, 0.025 mmol), and 1,3,5-trimethox-
ybenzene (4.4 mg, 0.026 mmol) in C6D6 (1 mL) was heated at 50 °C.
1
After 20 h, H NMR spectrum analyses indicated the retention of 1-
bromo-3,5-dimethylbenzene (6%), and the formations of 1-(C6D5)-
3,5-Me2C6H3, and 1,3-Me2C6H4 in 50% and 21% yields, respectively,
in addition with unknown paramagnetic species. Attempts to isolate
the paramagnetic species by recrystallization were unsuccessful.
Reaction of (IPr2Me2)2Fe(NHMes)2 (3) with 1-Bromo-3,5-
dimethylbenzene in THF-d8. In a J. Young NMR tube, the reaction
mixture of (IPr2Me2)2Fe(NHMes)2 (30.9 mg, 0.045 mmol), 1-bromo-
Preparation of (IPr)Fe(NHDipp)2 (7). To a solution of
[Fe(Mes)2]2 (441.3 mg, 0.75 mmol) in THF (15 mL) was added
IPr (582.9 mg, 1.5 mmol) at room temperature. After stirring for 1 h, a
yellow suspension was formed. Then a solution of DippNH2 (531.8
mg, 3.0 mmol) in THF (5 mL) was slowly added to the suspension,
and the mixture was further stirred for 19 h, during which the color of
63
dx.doi.org/10.1021/ic301894e | Inorg. Chem. 2013, 52, 59−65

Inorganic Chemistry
Article
Table 2. Crystal Data and Summary of Data Collection and Refinement for 1−7
1
2
3
4
5
6
7
formula
C34H52FeN6
C40H64FeN6
C40H64FeN6
C34H48Cl4FeN6
C46H76FeN6
C39H44Cl4FeN4
C51H72FeN4
crystal size (mm) 0.341 × 0.211 ×
0.25 × 0.23 ×
0.18
0.25 × 0.22 ×
0.14
0.30 × 0.18 ×
0.11
0.35 × 0.30 ×
0.20
0.35 × 0.25 ×
0.14
0.370 × 0.350 ×
0.321
0.169
fw
600.67
684.82
monoclinic
P2/n
684.82
738.43
768.98
monoclinic
C2/c
766.43
monoclinic
Cc
796.98
crystal system
space group
a, Å
monoclinic
C2/c
monoclinic
P2(1)/c
11.097(2)
22.560(2)
16.012(1)
90
triclinic
triclinic
P1
̅
P1
̅
17.095(2)
9.450(1)
20.360(2)
90
21.079(2)
9.236(1)
21.123(2)
90
10.7210(1)
12.184(1)
15.355(2)
96.194(1)
102.035(2)
106.672(1)
1848.8(3)
2
25.435(3)
11.582(1)
18.907(4)
90
10.281(1)
22,888(3)
16.865(2)
90
11.141(7)
13.530(1)
18.027(0)
101.373(1)
91.985(1)
114.128(1)
2411.0(3)
2
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
100.219(2)
90
107.531(2)
90
103.916(1)
90
126.605(1)
90
99.463(3)
90
3236.9(7)
4
3921.4(7)
4
3891.1(4)
4
4471.1(1)
4
3914.7(9)
4
Z
D
calcd, Mg/m3
1.233
1.160
1.169
1.326
1.142
1.300
1.098
radiation (λ), Å
Mo Kα (0.71073)
Mo Kα
(0.71073)
Mo Kα
(0.71073)
Mo Kα
(0.71073)
Mo Kα
(0.71073)
Mo Kα
(0.71073)
Mo Kα (0.71073)
2θ range, deg
μ, mm−1
4.06 to 52.00
0.498
3.26 to 53.98
0.419
3.18 to 51.00
0.422
3.54 to 54.00
0.729
3.98 to 60.00
0.374
4.32 to 59.04
0.690
3.40 to 52.00
0.348
F(000)
1296
1488
1488
776
1680
1600
864
no. of obsd reflns 11156
27589
441
26529
442
14177
418
20930
250
17743
441
13266
550
no. of params
refnd
197
goodness of fit
1.068
0.974
0.992
1.026
1.032
0.999
1.001
R1
0.0269
0.0733
0.0451
0.1048
0.0396
0.0952
0.0312
0.0837
0.0469
0.1256
0.0434
0.0903
0.0492
0.1113
wR2
3,5-dimethylbenzene (7.5 mg, 0.041 mmol), and 1,3,5-trimethox-
ybenzene (5.7 mg, 0.034 mmol) in THF-d8 (1 mL) was heated at 50
°C. After 20 h, 1H NMR spectrum analyses indicated the retention of
1-bromo-3,5-dimethylbenzene (30%), and the formation of 5-D-1,3-
Me2C6H3 in 70% NMR yield. Paramagnetically shifted 1H NMR
signals with similar chemical shifts as those observed in the reaction
AUTHOR INFORMATION
Corresponding Author
*E-mail: deng@sioc.ac.cn.
Notes
■
The authors declare no competing financial interest.
1
conducted in C6D6 were also observed in the H NMR spectrum.
ACKNOWLEDGMENTS
X-ray Structure Determinations. The structures of the seven
compounds in Supporting Information, Table S1 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. Data were collected with scans of 0.3 s/frame for 30 s.
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.36 Space
groups were assigned unambiguously by analysis of symmetry and
systematic absences determined by XPREP. All structures were solved
and refined using SHELXTL.37 Metal and first coordination sphere
atoms were located from direct-method E-maps; other non-hydrogen
atoms were found in alternating difference Fourier synthesis and least-
squares refinement cycles, and during final cycles were refined
anisotropically. All the hydrogen atoms were placed in calculated
positions employing a riding model. Final crystal parameters and
agreement factors are reported in Table 2.
■
We are thankful for the financial support from the National
Basic Research Program of China (973 Program, No.
2011CB808705) and the National Natural Science Foundation
of China (Nos. 20872168, 21002114, and 21121062).
ABBREVIATIONS
■
NHC, N-heterocyclic carbene; IPr2Me2, 2,5-diisopropyl-3,4-
dimethylimidazol-1-ylidene; IEt2Me2, 2,5-diethyl-3,4-dimethyli-
midazol-1-ylidene; IPr, 2,5-di(2,6-diisopropylphenyl)imidazol-
1-ylidene; IMes, 2,5-di(1,3,5-trimethylphenyl)imidazol-1-yli-
dene; sIPr, 2,5-di(2,6-diisopropylphenyl)-3,4-dihydroimidazol-
1-ylidene; Mes, 2,4,6-trimethylphenyl; Dipp, 2,6-diisopropyl-
phenyl; Cp, cyclopentadienyl
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* Supporting Information
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dx.doi.org/10.1021/ic301894e | Inorg. Chem. 2013, 52, 59−65

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(24) During our study, the preparation of the three-coordinate
complexes [(sIPr)Fe(NHDipp)Cl], [(sIPr)Fe(NHDipp)2], and [(IPr)
Fe(NHDipp)2] via a different route has been mentioned in a recent
1
report. These amido complexes were characterized by H NMR, but
their molecular structures remain unknown. For reference, see:
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