Three-Coordinate Iron(0) Complexes with N-Heterocyclic Carbene and Vinyltrimethylsilane Ligation: Synthesis, Characterization, and Ligand Substitution Reactions

Article abstract of DOI:10.1021/acs.inorgchem.9b02009

Low-coordinate iron(0) species are implicated as intermediates in a range of iron-catalyzed organic transformations. Isolable iron(0) complexes with coordination numbers of less than four, however, are rarely known. In continuing with our interests in three-coordinate iron(0) complexes with N-heterocyclic carbene (NHC) and alkene ligation, we report herein the synthesis and ligand substitution reactivity of three-coordinate iron(0) complexes featuring monodentate alkene ligands, [(NHC)Fe(η²-vtms)₂] (vtms = vinyltrimethylsilane, NHC = 1,3-bis(2′,6′-diisopropylphenyl)-imidazol-2-ylidene (IPr), 1; 1,3-bis(2′,6′-diisopropylphenyl)-4,5-tetramethylene-imidazol-2-ylidene (cyIPr), 2; 1,3-bis(2′,6′-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7-IPr), 3). Complexes 1-3 were synthesized from the one-pot reactions of ferrous dihalides with the N-(2,6-diisopropylphenyl)-substituted NHC ligands, vtms, and KC₈. Reactivity study of 1 revealed its facile ligand substitution reactions with terminal aryl alkynes, ketones, isocyanides, and CO, by which iron(0) complexes [(IPr)Fe(η²-HCCAr)] (Ar = Ph, 5; p-CH₃C₆H₄, 6; 3,5-(CF₃)₂C₆H₃, 7), [(IPr)Fe(η²-OCPh₂)₂] (8), [(IPr)Fe(CNR)₄] (R = 2,6-Me₂C₆H₃, 9; Bu^t, 10), and (IPr)Fe(CO)₄ (11) were prepared in good yields. These iron(0) complexes have been characterized by ¹H NMR, solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, ⁵⁷Fe M?ssbauer spectroscopy, and elemental analysis. Characterization data and computational studies suggest S = 1 ground-spin states for three-coordinate iron(0) complexes 1-3 and 5-8 and S = 0 ground states for 9-11. Theoretical studies on the three-coordinate complexes 1, 6, and 8 indicated pronounced metal-to-ligand backdonation from occupied Fe 3d orbitals to the π* orbitals of the C= C, C=C, and C= O moieties of the πligands. In addition, 1 proved an effective precatalyst for the cyclotrimerization reaction of alkynes.

Full text of DOI:10.1021/acs.inorgchem.9b02009

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Three-Coordinate Iron(0) Complexes with N‑Heterocyclic Carbene
and Vinyltrimethylsilane Ligation: Synthesis, Characterization, and
Ligand Substitution Reactions
Jun Cheng,† Qi Chen,† Xuebing Leng,† Shengfa Ye,*,‡ and Liang Deng*,†
†State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai, 200032, PR China
‡Max-Planck Institut fu
̈
̈
r Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mulheim an der Ruhr D-45470, Germany
S
* Supporting Information
ABSTRACT: Low-coordinate iron(0) species are implicated as intermediates in a
range of iron-catalyzed organic transformations. Isolable iron(0) complexes with
coordination numbers of less than four, however, are rarely known. In continuing
with our interests in three-coordinate iron(0) complexes with N-heterocyclic
carbene (NHC) and alkene ligation, we report herein the synthesis and ligand
substitution reactivity of three-coordinate iron(0) complexes featuring monodentate
alkene ligands, [(NHC)Fe(η2-vtms)2] (vtms = vinyltrimethylsilane, NHC = 1,3-
bis(2′,6′-diisopropylphenyl)-imidazol-2-ylidene (IPr), 1; 1,3-bis(2′,6′-diisopropyl-
phenyl)-4,5-tetramethylene-imidazol-2-ylidene (cyIPr), 2; 1,3-bis(2′,6′-diisopropyl-
phenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7-IPr), 3). Complexes 1−3 were
synthesized from the one-pot reactions of ferrous dihalides with the N-(2,6-
diisopropylphenyl)-substituted NHC ligands, vtms, and KC8. Reactivity study of 1
revealed its facile ligand substitution reactions with terminal aryl alkynes, ketones,
isocyanides, and CO, by which iron(0) complexes [(IPr)Fe(η2-HCCAr)] (Ar = Ph, 5; p-CH3C6H4, 6; 3,5-(CF3)2C6H3, 7),
[(IPr)Fe(η2-OCPh2)2] (8), [(IPr)Fe(CNR)4] (R = 2,6-Me2C6H3, 9; But, 10), and (IPr)Fe(CO)4 (11) were prepared in good
1
yields. These iron(0) complexes have been characterized by H NMR, solution magnetic susceptibility measurement, single-
57
̈
crystal X-ray diffraction study, Fe Mossbauer spectroscopy, and elemental analysis. Characterization data and computational
studies suggest S = 1 ground-spin states for three-coordinate iron(0) complexes 1−3 and 5−8 and S = 0 ground states for 9−
11. Theoretical studies on the three-coordinate complexes 1, 6, and 8 indicated pronounced metal-to-ligand backdonation from
occupied Fe 3d orbitals to the π* orbitals of the CC, CC, and CO moieties of the π ligands. In addition, 1 proved an
effective precatalyst for the cyclotrimerization reaction of alkynes.
species, wherein iron(II) complexes bearing cyclometalated
IMes and phosphine ligands were obtained as the final
products.
INTRODUCTION
■
The renaissance of iron catalysis in the recent years has created
great interest in iron(0) species with coordination unsatura-
tion, which are considered as reactive intermediates in the
iron-catalyzed reactions of alkyne cyclotrimerization, Diels−
Alder reaction of dienes with alkynes, hydrogenation of alkenes
and alkynes, and so on.1−5 It is known from the literature that
coordinatively unsaturated iron(0) complexes are amenable to
C−H bond activation reactions. For example, the 16e−, four-
coordinate iron(0) complex [Fe(PMe3)4] in solution phase is
known to convert reversibly to the iron(II) complex [Fe-
(PMe3)3(CH2PMe2)H] upon intramolecular C(sp3)−H bond
oxidative addition reaction.6 Unsuccessful attempts to prepare
the iron(0) species with N-heterocyclic carbene (NHC)
The aforementioned C−H bond activation reactions
indicate the necessity of attenuating the strong reducing
power of low-coordinate iron(0) species for the stabilization of
low-coordinate iron(0) complex. In this regard, the combined
ligand set of phosphine with alkene proved amenable to the
task. Upon the reduction of ferrous salts in the presence of
phosphine and alkene ligands, Hoberg,9 Jolly,10 Furstner,11,12
̈
and Breuil13 achieved syntheses of four-coordinate iron(0)
complexes, e.g., [(PEt3)2Fe(η2-C2H4)2],9 [(dippp)Fe(η2:η2-
CH2CHCH2CH2CH2CHCH2)],10 [(dippp)Fe(η2-C2H4)2]11
(dippp = 1,3-bis(diisopropylphosphino)propane), and
[(dppe)Fe(η2 :η2 -dvtms)]1 3 (dppe
= 1, 2-bis-
7
ligation Fe(IMes)2 [IMes = 1,3-bis(2′,4′,6′-trimethylphenyl)-
imidazol-2-ylidene] and Fe(PMe3)2((DippC:)2CH2)8
[(DippC:)2CH2 = bis(N-(2′,6′-diisopropylphenyl)-imidazole-2-
ylidene)methylene] via the reduction of their iron(II)
precursors were also ascribed to the facile intramolecular C−
H bond oxidative addition reactions of low-coordinate iron(0)
(diphenylphosphino)ethane, dvtms = divinyltetramethyldisi-
loxane) (Chart 1). Reactivity studies showed that the alkene
Received: July 6, 2019
© XXXX American Chemical Society
A
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Chart 1. Examples of Low-Coordinate Iron(0)−Alkene
Complexes
Scheme 1. Synthetic Route for (NHC)Fe(η2-vtms)2
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [(NHC)Fe(η2-
vtms)2]. The successful preparation of [(IPr)Fe(η2-vtms)2]
(1) prompted us to investigate the accessibility of other
NHC−iron(0)−vtms complexes. Examining the one-pot
reaction of FeX2 (X = Cl or Br) with different NHC ligands
(1 equiv), vtms (5 equiv), and KC8 (2 equiv) revealed that the
reactions using the N-Dipp (Dipp = 2,6-diisopropylphenyl)-
substituted NHC ligands, cyIPr (1,3-bis(2′,6′-diisopropylphen-
yl)-4,5-tetramethylene-imidazol-2-ylidene) and 7-IPr (1,3-bis-
(2′,6′-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-yli-
dene) can furnish the desired iron(0) complexes [(cyIPr)Fe-
(η2-vtms)2] (2) and [(7-IPr)Fe(η2-vtms)2] (3) in 28 and 50%
yields, respectively (Scheme 1). The isolated yields of these
NHC−iron(0)−vtms complexes are lower than those (50−
80%) of the NHC−iron(0)−dvtms complexes, which should
be related to their decreased stability associated with the
lability of the monodentate alkene ligand. In contrast to the
accessibility of 1−3, the synthetic efforts toward (IMes)Fe(η2-
vtms)2 using the one-pot reaction protocol was unsuccessful.
The trial produced large amount of free IMes ligand (ca. 42%
yield as quantified by 1H NMR analysis using 1,3,5-
trimethoxybenzene as the internal standard), and attempts to
isolate the desired iron(0) complex (IMes)Fe(η2-vtms)2 upon
recrystallization gave IMes as the only isolable crystalline solid.
This outcome differs from the ready accessibility of [(IMes)-
Fe(η2:η2-dvtms)].15 Similarly, the trial to prepare (IAd)Fe(η2-
vtms)2 (IAd = 1,3-bisadamantylimidazol-2-ylidene) produced
IAd as the major isolable compound, which again indicates the
important role of the steric property of NHCs in the
stabilization of (NHC)Fe(η2-vtms)2. In addition to the use
of KC8 as reductant, we also attempted to prepare 1 using the
one-pot reaction protocol with activated magnesium powder or
sodium amalgam as reductant. The trial using activated
magnesium can give the three-coordinate iron(0) complex in
35% isolated yield, whereas the reaction with sodium amalgam
merely produced a trace amount of the iron(0) complex.
ligands in these four-coordinate iron(0) complexes can be
replaced by CO, alkyne, and alkenes to form new iron(0)
complexes.10−13 These iron(0) complexes are also known to
perform reductive coupling reactions with substrates. For
instance, [(PEt3)2Fe(η2-C2H4)2] reacts with CO2 and
phosphine to give carboxylate compounds,9 and the interaction
of [(dippp)Fe(η2-C2H4)2] with cyclopropenone-1,3-propane-
diol ketal gives a ferracyclopentane derivative.12 These four-
coordinate iron(0) complexes were also applied as catalysts for
the cyclotrimerization of alkynes and heterocyclotrimerization
of alkynes with nitriles.11−13 Being analogous to bis-
(phosphine)iron(0) diene complexes, four-coordinate iron
complexes in the form of (α-diimine)Fe(diene) are also
known.14 The redox noninnocent nature of α-diimine,
however, might render these (α-diimine)−iron−alkene com-
plexes electronically much different from the phosphine−
iron(0)−alkene complexes.
As a recent advance in the stabilization of low-coordinate
iron(0) complexes, we found that the combined ligand set of
NHC with alkene can effectively stabilize three-coordinate
iron(0) complexes.15 Using the one-pot reaction of FeCl2 with
NHC, dvtms, and KC8 or Na/Hg, the three-coordinate iron(0)
complexes [(NHC)Fe(η2:η2-dvtms)] (NHC = IMes, 1,3-
bis(2′,6′-diisopropylphenyl)imidazol-2-ylidene (IPr), 3,3,5,5-
tetramethyl-1-(2′,6′-diisopropylphenyl)pyrrolidin-2-ylidene
(Me2-cAAC)) (Chart 1) were synthesized in high yields.15
More recently, we found that the three-coordinate iron(0)
complex bearing monodentate alkene ligands [(IPr)Fe(η2-
vtms)2] (1, vtms = vinyltrimethylsilane, Chart 1) is also
accessible by the one-pot reaction protocol.16 The mono-
dentate nature of the alkene ligand in 1 hints at its lability over
the bidentate dvtms ligand in [(NHC)Fe(η2:η2-dvtms)], which
led to our further study. Herein, we report the synthetic efforts
toward (NHC)Fe(η2-vtms)2 using different NHC ligands,
which led to the synthesis of two new three-coordinate iron(0)
alkene complexes [(NHC)Fe(η2-vtms)2] (NHC = cyIPr, 2; 7-
IPr, 3, Scheme 1) and more intriguingly the reactivity of the
iron(0)−vtms complexes with that of [(IPr)Fe(η2-vtms)2] (1)
as the representative. The reactivity study disclosed facile
ligand substitution reactions of 1 with terminal aryl alkynes,
ketones, isocyanides, and CO to form new three- and five-
coordinate iron(0) complexes. In the case of reactions with
alkynes, complex 1 has also proved effective in catalyzing
alkyne cyclotrimerization.
1
Complexes 2 and 3 have been characterized by H NMR
57
̈
spectroscopy, elemental analysis, and Fe Mossbauer spec-
troscopy. The molecular structure of 2 has been further
confirmed by single-crystal X-ray diffraction study (Figure 1).
The iron center in 2 displays a trigonal planar geometry, and its
Fe−C(carbene), Fe−C(alkene), and C(alkene)−C(alkene)
distances are comparable to the corresponding ones in 1.
For comparison, Table 1 compiles the key interatomic
distances and angles. As is characteristic for these three-
coordinate NHC−iron(0)−alkene complexes, the Fe−C-
(carbene) distances in 1 and 2 are shorter than those of
three-coordinate NHC−iron(II) complexes, e.g., [(IPr)Fe-
(CH2C6H5)2] (2.122(2) Å)17 and [(IPr)Fe(NHDipp)2]
B
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Figure 1. Molecular structure of 2, showing 30% probability ellipsoids
and the partial atom numbering scheme. Hydrogen atoms are omitted
for clarity.
(2.142(2) Å).18 As DFT calculation on (IPr)Fe(vtms)2
indicated the backbonding from iron to IPr is weak (vide
infra), the short Fe−C(carbene) distances might come from
strong electrostatic interaction between NHC and Fe(vtms)2
fragment in the three-coordinate iron complexes. Cavallo and
co-workers showed that electrostatic contribution plays
important role in the attractive interaction between NHC
and metal center in some transition metal NHC complexes.19
Further theoretic study is needed to disclose whether or not a
similar situation exists in the low-coordinate iron complexes.
Their C(alkene)−C(alkene) separations (1.414(4)−1.431(2)
Å) are remarkably longer than the CC double bond in free
vinylsilanes (1.32 Å).20 Similar phenomena were also observed
in low-valent iron(I)−alkene complexes [NaFe(trop2dae)-
(THF)3] (1.437(5) Å in average, trop = 5H-dibenzo[a,d]-
cyclohepten-5-yl, dae = N−CH2CH2N) and [LiFe(trop2dae)-
(Et2O)2] (1.416(9) Å in average).21 These structure features
reflect the high covalency of the Fe−C interaction and
pronounced Fe(0)-to-alkene π-backdonation in these NHC−
2
Figure 2. 57Fe Mossbauer spectra (80 K zero-filed) of [(cyIPr)Fe(η -
vtms)2] (2, top), [(7-IPr)Fe(η2-vtms)2] (3, middle), and (7-
IPr)Fe(η2-vtms) (4, bottom). The data (dots) and best fits (solid
line) are shown. The fitting data are compiled in Table 2.
̈
2.79 mm/s, respectively) are comparable to those of 1 (Table
2) and [(IMes)Fe(η2:η2-dvtms)] (δ = 0.40 mm/s, |ΔEQ| =
3.02 mm/s),15 but they are distinct from those of the low-spin
iron(0) complex Fe(CO)5 (δ = −0.10 mm/s, |ΔEQ| = 2.05
mm/s).22 As compared to those of the other three-coordinate
iron complexes, the isomer shifts of 1−3 are found between
those of the three-coordinate iron(II) complex [(IPr2Me2)Fe-
iron(0)−alkene complexes.15
57
̈
The zero-field Fe Mossbauer spectra of 2 and 3 measured
at 80 K feature single quadrupole doublets (Figure 2). Their
fitting isomer shifts (δ = 0.39 and 0.34 mm/s for 2 and 3,
respectively) and quadrupole splitting values (|ΔEQ| = 2.92 and
Table 1. Selected Distances (Å) and Angles (deg) of the Iron(0) Complexes Observed from XRD Studies
a
[(IMes)Fe(dvtms)]
1
2
6
7
8
Fe−C
Fe−X
2.030(2)
2.052(2)
2.049(2)
2.059(2)
2.052(3)
1.425(4)
1.432(3)
65.6
2.019(3)
2.033(3)
2.035(3)
2.049(3)
2.044(3)
1.418(4)
1.414(4)
65.4
2.033(2)
2.032(2)
2.039(2)
2.046(2)
2.054(2)
1.431(2)
1.422(2)
61.2
1.985(2)
1.936(2)
1.990(2)
1.932(2)
1.937(2)
1.940(2)
1.943(2)
1.274(3)
1.263(3)
85.5
2.083(2)
1.859(1)
1.859(1)
2.078(2)
2.083(2)
1.336(2)
1.336(2)
34.9
1.956(2)
Fe−Ca
1.273(3)
1.273(3)
89.2
X−Ca
b
α
a
b
Data from ref 15. Dihedral angle between the FeX2Ca2 plane and the carbene plane.
C
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57
as that in the three-coordinate manganese(0)−alkene complex
[(Et2-cAAC)Mn(dvtms)] (51 and 60%),28 which is consistent
with the Pauling electronegativity value of iron (1.83) being
larger than that of manganese (1.55). Notably, in contrast to
the pronounced Fe to vtms π-backdonation, the π-interaction
between the iron center and carbene carbon atom is rather
weak. These data indicated that the interaction between the Fe
center and vtms remains rather covalent; therefore, 1 should be
interpreted as an S = 1 species having a series of limiting
valence electronic structures ranging from FeIV(vtms2−)2 to
Fe0(vtms0)2, similar to [(Et2-cAAC)Mn(dvtms)].28 This
notion also holds true for complexes 2, 3, and 5−8 (vide infra).
Under a dinitrogen atmosphere, the solids of 1−3 can be
kept for months at room temperature without noticeable
decomposition. When dissolved in C6D6, the solutions of 1 and
2 also show good stability at ambient temperature. However,
when 3 was dissolved in C6D6 or toluene, dissociation of vtms
took place (Figure S14). Dissolution of 3 in toluene, followed
by removing all the volatiles by vacuum evaporation left a red
solid. Elemental analysis (C, H, and N) on the solid suggests a
composition of (7-IPr)Fe(vtms) (4). The attempts to grow
single-crystals of 4 suitable for XRD study were unsuccessful.
̈
Table 2. Zero-Field Fe Mossbauer Spectroscopic Data of
1−10 Measured at 80 K
δ (mm/s) |ΔEQ| (mm/s)
δ (mm/s) |ΔEQ| (mm/s)
1
2
3
4
5
0.39
0.39
0.34
0.45
0.29
2.86
2.92
2.79
1.06
3.15
6
7
8
9
0.22
0.27
0.54
−0.11
−0.14
1.85
2.89
1.96
1.21
0.58
10
(Mes)2] (δ = 0.30 mm/s, |ΔEQ| = 1.38 mm/s, IPr2Me2 = 1,3-
isopropyl-4,5-dimethylimidazol-2-ylidene)23 and the iron(I)
complex [(DippNC(Me))2CH)Fe(PhCCH)] (δ = 0.44 mm/s,
|ΔEQ| = 2.05 mm/s).24
To probe the electronic structure of these NHC−iron(0)−
vtms complexes, spin-unrestricted DFT calculations at the
B3LYP level25,26 has been performed on 1. Calculations based
on the molecular structure of 1 established by XRD study
indicated that the S = 1 state is lower in energy than the S = 0
state by 37.5 kcal/mol. Figure 3 shows the orbital composition
However, the 57Fe Mossbauer parameters of 4 (δ = 0.45 mm/s,
̈
|ΔEQ| = 1.06 mm/s) is comparable to those of Driess’
bis(NHC)iron(0)−arene complex [Fe((DippC:)2CH2)(η6-
C6H6)] (δ = 0.43 mm/s, |ΔEQ| = 1.37 mm/s),8 hinting that
4 might adapt a “two-legged” piano-stool geometry (Scheme
2). Arene−metal interaction was observed in a (7-IPr)Pd(0)
Scheme 2. Possible Equilibrium between 3 and 4
complex [(7-IPr)Pd(η2-maleic anhydride)].29 Interestingly,
treatment of 4 with vtms could lead to the formation 3,
suggesting the reversibility of the conversion between 3 and 4.
The vtms-dissociation behavior of 3 seems to originate from
the steric nature of the seven-membered ring NHC ligand.30
Reactions of 1 with Alkynes. Our previous study on
NHC−iron(0)−alkene complexes showed that the iron(0)−
vtms complex 1 can react with the azide bearing a bulky
organic substituent N3Ar,16 which might associate with the
lability of the monodentate alkene ligand in the three-
coordinate iron(0) complex. Further study on the reactions
of 1 with unsaturated organic molecules revealed that the vtms
ligands in 1 indeed can be replaced readily by alkynes, ketone,
and isocyanides.
The ligand substitution reactions of 1 with the terminal aryl
alkynes (two equiv) PhCCH, p-CH3C6H4CCH, and 3,5-
(CF3)2C6H3CCH completed in minutes at room temper-
ature and gave the NHC−iron(0)−alkyne complexes [(IPr)-
Fe(η2-HCCAr)2] (Ar = Ph, 5; p-CH3C6H4, 6; 3,5-(CF3)2C6H3,
7) as brown solids in 42−71% yields (Scheme 3). NMR-scale
reactions indicated that these reactions have the complete
conversion of 1 to the iron(0)−alkyne complexes with vtms as
the byproduct. In contrast, the interaction of 1 with n-
C6H13CCH (2 equiv) merely led to the formation of trace
amount of new paramagnetic species with a majority of 1
Figure 3. UNO orbitals of 1 (S = 1). The occupation number along
with the dominant atomic contribution are indicated near each
orbital.
and occupation of selected frontier molecular orbitals for the S
= 1 state. At this spin state, the low occupation numbers in the
unrestricted natural orbitals (UNOs)27 176 and 177 implies
that the electronic configuration of (dxy + πCC*)2(dx −y
+
2
2
π′CC*)2(dz2)2(dxz)1(dyz)1 is dominant among the possible
electronic structures of 1. The π-backdonation interactions
2
2
from the Fe dxy and dx −y orbitals to the CC π* orbitals can
be discerned from UNOs 172, 173, 176, and 177 in Figure 3.
When gauged by the percentage of metal’s d-orbital
contributions in the bonding orbitals, the π-backdonation in
1 (61 and 72% for UNOs 172 and 173) is not as pronounced
D
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Scheme 3. Ligand Substitution Reactions of 1 with ArCCH,
Ph2CO, RNC, and CO
remaining. No reaction was observed when 1 was treated with
the internal alkynes PhCCPh or PhCCMe in C6D6 at
ambient temperature. The ineffective ligand substitution
reaction of 1 with n-C6H13CCH might be due to the
relatively weak π-accepting ability of the alkyl chain-substituted
alkyne over vinylsilane. The inertness of 1 toward the internal
alkynes should be due to steric congestion.
Figure 4. Molecular structures of 6 (top) and 7 (bottom), showing
30% probability ellipsoids and the partial atom numbering scheme.
numbers and the different numbers of the alkynes ligands of
these iron(0) alkyne complexes. As compared to the NHC−
iron(0)−alkene complexes, the Fe−C distances in 6 and 7 are
1
Complexes 5−7 have been characterized by H NMR, 57Fe
Mossbauer spectroscopy, and elemental analysis. The solid-
shorter than those in 1 and 2. Accordingly, the 57Fe Mossbauer
̈
̈
state structures of 6 and 7 were confirmed by single-crystal X-
ray diffraction studies. As shown in Figure 4, the iron centers in
6 and 7 are coordinating with one IPr ligand and two η2-
HCCAr ligands, forming a trigonal planar geometry. Probably
due to the steric demanding nature of IPr, the two η2-HCCAr
ligands in both complexes align in a tail-to-tail fashion, rather
than a head-to-tail fashion. The distances of the C(alkyne)−
C(alkyne) bonds in 6 and 7 span a narrow range of 1.263(3)−
1.274(3) Å, longer than the CC distance in free acetylenes
(1.17 Å),31 comparable to the C(alkyne)−C(alkyne) bond in
the iron(I) complex [(DippNC(Me))2CH)Fe(η2-HCCPh)]
(1.268(3) Å),32 and slightly shorter than those of the four-
coordinate iron(0) complexes [Fe(dppe)(PMe3)(η2-EtCCEt)]
(1.303(6) Å)13 and [(dippp)Fe(η2-PhCCPh)2] (1.286(2) and
1.290(2) Å).11 The coordination of the alkyne ligands to the
iron centers also renders bent C(alkyne)−C(alkyne)−C(aryl)
alignments that have the angles in the range of 144.8−146.5°.
The Fe−C(alkyne) distances of 6 and 7 (in the range of
1.932(2)−1.956(2) Å) are comparable to the ones in
[(dippp)Fe(η2-PhCCPh)2] (1.92−2.01 Å)11 and longer than
the ones in [Fe(dppe)(PMe3)(η2-EtCCEt)] (1.874(5) and
1.860(5) Å).13 The relatively shorter C(alkyne)−C(alkyne)
and longer Fe−C(alkyne) distances observed on 6 and 7 as
compared to those in [(dippp)Fe(η2-PhCCPh)2] and [Fe-
(dppe)(PMe3)(η2-EtCCEt)] imply relatively weaker metal-to-
alkyne backdonation in the NHC−Fe(0)−alkyne complexes.
This difference should associate with the different coordination
isomer shifts (0.29, 0.22, and 0.27 mm/s for 5−7, respectively,
Table 2) of 5−7 (Figures S7−S9) are smaller than those of
NHC−iron(0)−alkene complexes 1−3 (0.34−0.39 mm/s;
Table 2).22
Three-coordinate iron(0) alkyne complexes 5−7 are para-
magnetic. Since accurate determination of their solution
magnetic susceptibilities by Evans’ method33,34 was hampered
by the ease with which these complexes to undergo
decomposition in solution phase, DFT calculations were
used to get knowledge on their electronic structures.
Calculation studies on 6 indicated that this iron−alkyne
complex possesses an S = 1 ground spin-state with an
electronic configuration of (dxy + πCC*)2(dx −y
+
2
2
π′CC*)2(dz2)2(dxz)1(dyz)1, similar to that of 1. The π-
backdonation orbitals (UNOs 178 and 179 in Figure 5) have
Fe 3d percentages of 61 and 70%, respectively, close to the
corresponding percentage values of the π-backdonation
orbitals in 1 (vide supra).
Complexes 1−3 and 5−7, in addition with the aforemen-
tioned four-coordinate iron(0) bis(alkene) and bis(alkyne)
complexes with phosphine ligation,9−13 represent an unique
class of open-shell iron(0) complexes bearing two π-ligands.
The attainment of these iron(0) bis(alkene) and bis(alkyne)
complexes rather than their further reductive coupling
products,35−38 five-membered metallacycles, indicates limited
capability of iron(0) species in mediating reductive coupling
reactions of nonactivated alkenes and alkynes. Indeed, only a
E
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Scheme 4. Cyclotrimerization of Alkynes Catalyzed by 1
1,3,5-substitued benzenes in a ratio of 3:2. The regioselectivity
of the cyclotrimerization reaction is moderate and differs from
that observed in Tilley’s system using [(IPr)Fe(N(SiMe3)-
(Dipp))] as precatalyst (7:3 ratio for cyclotrimerization of
PhCCMe).41 It is also distinct from von Wangelin’s system
employing Fe(N(SiMe3)2)2 as the precatalyst (100% and 98:2
selectivity for cyclotrimerization of p-CH3C6H4CCH and n-
C6H13CCH, respectively).42 In the latter case, the iron
particle was thought to be the genuine catalyst for the
cyclotrimerization reaction.42
The mechanism of the current iron-catalyzed alkyne
cyclotrimerization reaction is not clear at this stage. Following
the classical mechanism of low-valent transition-metal-
catalyzed [2+2+2]-cycloaddition of alkynes,43,44 we propose
that the reaction catalyzed by 1 might start from the ligand
substitution of vtms in 1 by alkynes to afford alkyne complex
A. The ligand substitution step might involve an intermediate
(IPr)Fe(vtms) that could be formed from the dissociation of
one vtms ligand from 1 and have a structure similar to that of
4. Intermediate A undergoes a reductive coupling reaction to
form ferracyclopentadidene intermediate (B). Further alkyne
insertion into one of the Fe−C(alkenyl) bond in B gives
ferracycloheptatriene C that undergoes reductive elimination
reaction to form NHC−iron(0)−arene intermediate D. The
further steps of arene extrusion and alkyne coordination from
D will then regenerate A (Scheme 5). In support of this
proposed cycle, iron(0)−alkyne complexes 5−7 that are
representatives of A have been isolated. While no intermediate
A bearing terminal alkyl alkyne or internal alkyne ligands was
synthesized yet, the relative higher temperature used for the
catalytic reaction might enable their generation via ligand
substitution reactions with terminal alkyl alkynes or internal
alkynes (1 to A and D to A). When standing the solutions of
5−7 at room temperature for hours, the bis(alkyne) complexes
undergo full decomposition to form alkyne cyclotrimerization
products as indicated by 1H and 19F NMR spectra and GC-MS
analysis. However, neither 1,3-dienes nor cyclobutadienes that
could be formed from the proposed ferracyclopentadidene
intermediates B was detected. The failure to detect
ferracyclopentadidene intermediates B might be related to
the high activity of this species.40
Figure 5. UNO orbitals of 6 (S = 1). The occupation number along
with the dominant atomic contribution are indicated near each
orbital.
few of stoichiometric reductive coupling reactions of iron
complexes with alkenes and alkynes are known. As the rare
examples, Chirik et al. showed the formal iron(0) complexes
(PDI)Fe(N2)2 (PDI denotes pyridine-2,6-diimine ligands) can
react with certain dienes, enynes, and diynes to give five-
membered metallacycles.39 Fu
̈
rstner et al. isolated a
ferracyclopentane complex from the reaction of [(dippp)Fe-
(η2-C2H4)2] with a cyclopropene compound.12 The limited
ability of those iron(0) species in mediating the reductive
coupling reactions of nonactivated alkenes and alkynes might
have its thermodynamics cause as a calculation study by Houk
et al. showed that the conversions of the nickel(0) species
(PMe3)Ni(RCCR)2 (R = Me and COOMe) to their reductive
coupling products are energetically uphill.40
It should be mentioned that while no effective ligand
substitution reaction between 1 and terminal alkyl alkynes or
internal alkynes at room temperature was observed, treatment
of these alkynes with a catalytic amount of 1 at elevated
temperature can result in alkyne cyclotrimerization. Using 5
mol % 1 as catalyst, the cyclotrimerization reactions of EtC
CEt, PhCCPh, PhCCMe, p-CH3C6H4CCH, and n-
C6H13CCH at 50 °C in 20 h gave the substituted benzenes
in 62−95% isolated yields (Scheme 4). For the reaction of
PhCCMe, the two products 1,2,4-Me3-3,5,6-Ph3C6 to 1,3,5-
Me3-2,4,6-Ph3C6 have the ratio of 8:1, whereas the reactions of
p-CH3C6H4CCH and n-C6H13CCH gave the 1,2,4- and
In addition to the mechanistic proposal having molecular
iron complexes as intermediates, an alternative mechanism for
F
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Article
ates in a way of heterogeneous or homogeneous catalysis and
further study is needed.
Scheme 5. Proposed Catalytic Cycle for the Alkyne
Cyclotrimeriztion Reaction Catalzyed by 1
Reaction of 1 with Ph2CO. The ligand substitution
reaction of 1 with the ketone Ph2CO (2 equiv) also took place
quickly at room temperature and the resultant NHC−iron−
ketone complex [(IPr)Fe(η2-OCPh2)2] (8) was isolated in
82% yield (Scheme 3).
The molecular structure of 8 established by X-ray diffraction
study shows that both ketone ligands are coordinating to the
iron center in an η2-fashion with the averaged Fe−O distance
of 1.859(1) Å and Fe−C(ketone) distance of 2.081(2) Å
(Figure 6). The C−O distance (1.336(2) Å in avarage) in 8 is
the iron-catalyzed alkyne cyclotrimerization reaction could be a
heterogeneous iron nanoparticles-catalyzed process. In accord
with this mechanism, we noticed that the addition of mercury
or PMe2Ph to the catalytic alkyne cyclotrimerization reaction
of EtCCEt using 1 as catalyst could halt the formation of
C6Et6. However, this negative effect could be inconclusive for
the heterogeneous nature of the catalytic system in as much as
Hg is hard to form an amalgam with iron,45 and its interaction
with 1 at 50 °C was found to induce the decomposition of the
iron(0) complex as indicated by the formation of black
precipitate in the NMR-scale reaction of 1 with Hg in C6D6. As
for the effect of the addition of PMe2Ph, the π-accepting ability
of PMe2Ph might make it potential competent ligand to bind
to the open coordination site of the genuine iron catalyst,
hence resulting in catalyst poisoning. Moreover, we noticed
that reaction of 6 with 1 equiv of p-CH3C6H4CCH gave the
cyclotrimerization products 1,2,4-(p-CH3C6H4)3C6H3 and
1,3,5-(p-CH3C6H4)3C6H3 in 54% yield with the ratio of the
two products being ca. 5:4 (Scheme 6). This regioselectivity is
close to that of the catalytic reaction using p-CH3C6H4C
CH, but is distinct from that of von Wangelin’s system that was
proposed to have iron nanoparticles as the genuine catalyst.42
With this limited information in hand, it is hard to determine
whether this iron-catalyzed cyclotrimerization reaction oper-
Figure 6. Molecular structure of 8, showing 30% probability ellipsoids
and the partial atom numbering scheme.
distinctly longer than that of the free ketone Ph2CO (1.23
Å),46 close to those of [(dtbpe)Ni(η2-OCPh2)] (1.331(6) Å,
dtbpe = 1,2-bis(di-tert-butylphosphino)ethane)47 and
[(PEt3)2Ni(η2-OCPh2)] (1.335(4) Å),48 and shorter than
the distances observed in the oxametallacyclopropane com-
plexes [(2,6-Ph2C6H3O)2Ti(PMe3)(OCPh2)] (1.397(8) Å)49
and [Cp2Ti(PMe3)(OCPh2)] (1.360(2) Å).50 The displace-
ment between the two carbonyl carbon atoms (C(2) and
C(3)) is 3.21 Å, precluding any bonding interaction between
them. The outcome of the reaction of 1 with OCPh2 thus
differs from the C−C coupling reactions of some Hf(II),
Zr(II), and Ti(II) complexes with ketones,51,52 The Fe−
C(carbene) bond length in 8 (2.083(2) Å) is much longer
than the Fe−C(carbene) distances in 1, 2, 6, and 7 (1.985(2)−
2.033(2) Å). Coincidently, in parallel with its long Fe−C
distance, the 57Fe Mossbauer isomer shift of 8 (δ = 0.54 mm/
̈
s) is found the highest in the series of three-coordinate iron
complexes 1−3 and 5−8.
We have also performed theoretic calculations to probe the
electronic structure of the ketone complex. Single point energy
calculations on 8 revealed its S = 1 ground spin-state. As
Scheme 6. Reaction of [(IPr)Fe(η2-HCCC6H4-p-CH3)2 (6)
with p-CH3C6H4CCH
depicted in Figure 7, the qualitative frontier molecular orbital
2
diagram for the S = 1 state shows a (dz ) (dxy + πCO*)2(dx −y
2
2
2
+ π′CO*)2(dxz)1(dyz)1 electronic configuration. Compared
with 1 and 6, the Fe 3d percentage of the UNOs involving π-
backdonation in 8 (43 and 71% in UNOs 212 and 213) are
apparently lower, indicating more pronounced π-backdonation
in the ketone complex. Complex 8 has a solution magnetic
moment of 4.2(1) μB. This value locates between the spin-only
values of 2.83 and 4.90 μB for 3d metal ions with S = 1 and 2,
G
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from the trigonal bipyramidal geometry of [(NHC)Fe(CO)4]
(NHC = IPr, Me2-cAAC)54,55 and might originate from the
unique steric nature of the ligand set in 9. Despite their
different coordination geometries, the Fe−C(carbene)
(1.979(6) Å) and Fe−C(isocyanide) distances (1.794(5) Å
in average) in 9 are comparable to their counterparts in
[(IPr)Fe(CO)4]55 and [Fe(CO)(CNArMes)4]56 (ArMes = 2,6-
57
̈
bismesitylphenyl). The Fe Mossbauer data (δ = −0.11 mm/
s, |ΔEQ| = 1.21 for 9, and δ = −0.14 mm/s, |ΔEQ| = 0.58 mm/s
for 10) are typical of low-spin iron(0) complexes. Similarly, the
reaction of 1 with an excess amount of CO (1 atm.) gave the
ligand substitution product [(IPr)Fe(CO)4] (11) in 77% yield
1
(Scheme 3). The H and 13C NMR spectra of the resultant
iron carbonyl complex is consistent with those reported in
literature.57 In contrast, 1 proved unreactive with PPh3 at room
temperature.
CONCLUSION
■
In this study, we found that the combined ligand set of the
monodentate alkene ligand vtms with sterically bulky NHC
ligands, IPr, cyIPr, and 7-IPr, can stabilize three-coordinate
iron(0) complexes in the form of [(NHC)Fe(η2-vtms)2]
(NHC = IPr, 1; cyIPr, 2; 7-IPr, 3). Reactivity study with 1 as
the representative disclosed facile ligand substitution reactions
of the NHC−iron(0)−vtms complex with terminal aryl
alkynes, ketones, isocyanides, and CO, by which new iron(0)
complexes with alkyne, ketone, and isocyanide ligation
[(IPr)Fe(η2-HCCAr)] (Ar = Ph, 5; p- CH3C6H4, 6; 3,5-
(CF3)2C6H3, 7), [(IPr)Fe(η2-OCPh2)2] (8), [(IPr)Fe-
(CNR)4] (R = 2,6-Me2C6H3, 9; But, 10), and [(IPr)Fe(CO)4]
(11) were synthesized in moderate to good yields. These
iron(0) complexes have been characterized by 1H NMR
Figure 7. UNO orbitals of 8 (S = 1). The occupation number along
with the dominant atomic contribution are indicated near each
orbital.
respectively. The magnetic moment hints at the presence of
unquenched orbital magnetic moment contribution in the low-
coordinate d8 complex 8.53
spectroscopy, single-crystal X-ray diffraction studies, elemental
57
Reactions of 1 with Isocyanides and CO. Different from
the reactions with the π-ligands, the interaction of 1 with
isocyanides CNR (R = 2,6-Me2C6H3, But, 4 equiv) gave five-
coordinate diamagnetic iron(0) complexes [(IPr)Fe(CNR)4]
(R = 2,6-Me2C6H3, 9; But, 10) in high yields (Scheme 3).
These 18e−-complexes are diamagnetic. Their 13C NMR
spectra show signals at 203.77 and 209.27 ppm for 9 and
10, respectively, assignable to the signals of isocyanide carbon
atoms. A single-crystal X-ray diffraction study on 9 revealed
that its iron center adopts a distorted square-pyramidal
geometry (τ5 = 0.17) with the four isocyanide ligands sitting
on the basal positions (Figure 8). This geometry is different
̈
analysis, and Fe Mossbauer spectroscopy. Characterization
data and computational studies revealed that the three-
coordinate complexes 1−3 and 5−8 have a common S = 1
ground-spin state and that the five-coordinate complexes 9−11
are low-spin iron(0) species. Orbital composition analysis of 1,
6, and 8 revealed pronounced π-backdonation interaction
between filled Fe 3d orbitals and the empty π* orbitals of η2-
H2CCHSiMe3, η2-HCCC6H4-p-CH3, and η2-OCPh2 in these
iron(0) complexes. Hence, the effective stabilization of these
three-coordinate iron(0) species benefits from the joint effect
of the steric demanding nature of the bulky NHC ligands and
the π-accepting nature of the π-ligands.
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 techniques or in a glovebox. Organic
solvents were dried with a solvent purification system (Innovative
Technology) and bubbled with dry N2 gas prior to use. 1,3-Bis(2,6-
diisopropylphenyl)-4,5-tetramethylene-imidazol-2-ylidene (cyIPr),58
1,3-bis(2′,6′-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-yli-
dene (7-IPr),59 and potassium graphite (KC8)60 were synthesized
according to literature methods. All other chemicals were purchased
1
from either Strem or J&K Chemical Co. and used as received. H,
13C, and 19F NMR spectra were recorded on an Agilent 400, Agilent
600, or Bruker DRX400 MHz spectrometers. All chemical shifts were
reported in units of ppm with references to the residue of the
deuterated solvents for proton and carbon chemical shifts and to
CF3COOH for fluorine chemical shifts. Elemental analysis was
performed by the Analytical Laboratory of Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences. Solution magnetic
Figure 8. Molecular structure of 9, showing 30% probability ellipsoids
and the partial atom numbering scheme. Hydrogen atoms are omitted
for clarity.
H
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moments were measured by the method originally described by Evans
with stock and experimental solutions containing a known amount of
a (CH3)3SiOSi(CH3)3 standard.33,34 IR spectra of solid samples were
recorded with a NICOLET AVATAR 330 FT-IR spectrophotometer
on KBr pallets. Absorption spectra were recorded with a Shimadzu
UV-3600 UV−vis-NIR spectrophotometer.
(v1/2 = 54 Hz), 3.84 (v1/2 = 47 Hz), 3.22 (v1/2 = 57 Hz), 0.81 (very
br), 0.29 (very br), −1.35 (v1/2 = 48 Hz), −7.75 (v1/2 = 53 Hz),
−11.25 (v1/2 = 82 Hz), −17.85 (v1/2 = 117 Hz). Anal. Calcd for
C41H66FeN2Si2: C, 70.45; H, 9.52; N, 4.01. Found: C, 70.41; H, 9.38;
N, 4.24. Magnetic susceptibility (C6D6, 294 K): μeff = 3.6(1) μB.
Absorption spectrum (THF): λmax, nm (ε, M−1 cm−1) = 410 (1630),
580 (180), 720 (210). IR (KBr, cm−1): ν = 2959 (s), 2866 (m), 1701
(s), 1666 (m), 1589 (w), 1529 (w), 1470 (m), 1460 (w), 1400 (m),
1379 (w), 1362 (w), 1248 (w), 1179 (w), 1059 (w), 939 (w), 837
(w), 804 (w), 755 (s).
X-ray Structure Determination. Diffraction-quality crystals were
obtained from recrystallizations in Et2O (2, 7, 8, 9) and toluene (6).
Crystals were coated with Paratone-N oil and mounted on a Bruker
APEX-II CCD-based diffractometer or a Bruker D8 Venture
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.61 Space
groups were assigned unambiguously by analysis of symmetry and
systematic absences determined by XPREP. All structures were solved
and refined using SHELXTL.62 Metal and first coordination sphere
atoms were located from direct-methods E-maps. Non-hydrogen
atoms were found in alternating difference Fourier synthesis and least-
squares refinement cycles and during final cycles were refined
anisotropically. Tables S1 summarizes the crystal data and summary
of data collection and refinement for the complexes. CCDC
1937521−1937525 contain the supplementary crystallographic data
for complexes 2, 6·toluene, and 7−9. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
Preparation of [(7-IPr)Fe(η2-vtms)2] (3). To a THF (50 mL)
solution of 7-IPr (1.95 g, 4.64 mmol) was added FeBr2 (1.00 g, 4.64
mmol) at room temperature. The mixture was stirred overnight at
room temperature and then vtms (2.6 mL, 18.2 mmol, 4 equiv) was
added. After stirring for 10 min, the mixture was cooled to −30 °C
and then a THF (40 mL) suspension of KC8 (1.25 g, 9.28 mmol, 2
equiv) was added. The mixture was further stirred for 18 h at −30 °C,
during which the color of the mixture changed gradually to tan. The
resultant mixture was then filtered through diatomaceous earth, and
the filtrate was subjected to vacuum to remove all the volatiles. The
resultant green solid was washed with n-hexane (3 × 3 mL) and Et2O
(3 × 2 mL) and then dried under vacuum to leave [(7-IPr)Fe(η2-
1
vtms)2] (3) as a green powder (1.58 g, 50%). H NMR (400 MHz,
C6D6, 298 K): δ (ppm) 22.88 (ν1/2 = 89 Hz), 14.31 (ν1/2 = 36 Hz),
10.88 (ν1/2 = 36 Hz), 9.32 (ν1/2 = 41 Hz), 8.23 (ν1/2 = 94 Hz), 4.68
(very br), 4.56 (very br), −3.06 (ν1/2 = 70 Hz), −5.05 (ν1/2 = 39 Hz),
−13.22 (ν1/2 = 62 Hz), −28.59 (ν1/2 = 460 Hz). Anal. Calcd for
C39H66FeN2Si2: C, 69.40; H, 9.86; N, 4.15. Found: C, 67.73; H, 9.68;
N, 4.22. The deviation of carbon content from the theoretic value
could be caused by the ease with which 3 to release vtms. IR (KBr,
cm−1): ν = 3068 (w), 2963 (s), 2930 (m), 2867 (m), 1629 (s), 1584
(w), 1470 (s), 1420 (m), 1382 (w), 1361 (m), 1323 (w), 1254 (m),
1182 (w), 1098 (w), 1062 (w), 997 (m), 937 (w), 838 (m), 804 (w),
756 (m), 523 (w), 456 (w).
www.ccdc.cam.ac.uk/data_request/cif.
57
Fe Mossbauer Spectroscopy. All solid samples for 57Fe
̈
̈
Mossbauer spectroscopy were run on nonenriched samples of the as-
̈
isolated complexes. Each sample was loaded into a Delrin Mossbauer
sample cup for measurements and loaded under liquid nitrogen. Low-
temperature Fe Mossbauer measurements on 1−3 and 5−9 were
performed using a See Co. MS4Mossbauer spectrometer integrated
57
̈
̈
with a Janis SVT-400T He/N2 cryostat for measurements at 80 K.
The zero-field Mossbauer spectrum of 4 was recorded on a
Preparation of (7-IPr)Fe(η2-vtms) (4). A toluene solution (20
mL) of 3 (300 mg, 0.44 mmol) was stirred at room temperature for 4
h, during which the mixture was periodically concentrated under
vacuum to remove the released alkene ligand vtms. The reaction
mixture was then subjected to vacuum to remove the volatiles, which
̈
conventional spectrometer with alternating constant acceleration of
the γ-source (57Co/Rh, 1.8 GBq), which was kept at room
temperature. The sample temperature was maintained constant (80
K) in an Oxford Instruments Variox cryostat. Isomer shifts were
1
determined relative to α-Fe at 298 K. All Mossbauer spectra were fit
using the program WMoss (SeeCo).
leaves (7-IPr)Fe(η2-vtms) (4) as a red powder (206 mg, 81%). H
̈
NMR (400 MHz, C6D6, 298 K): δ (ppm) 6.86 (very br), 6.26 (very
br), 5.61 (very br), 3.24 (ν1/2 = 39 Hz), 1.99 (very br), 1.74 (very br),
1.71 (very br), 1.57 (very br), 1.24 (very br), 0.08 (ν1/2 = 6 Hz),
−0.33 (very br). Anal. Calcd for C34H54FeN2Si: C, 71.05; H, 9.47; N,
4.87. Found: C, 70.48; H, 9.36; N, 4.80. Absorption spectrum (THF):
λmax, nm (ε, M−1 cm−1) = 390 (3010). IR (KBr, cm−1): ν = 2962 (s),
2935 (m), 2867 (m), 1654 (s), 1628 (s), 1586 (w), 1487 (w), 1465
(s), 1419 (w), 1360 (w), 1323 (w), 1260 (s), 1184 (w), 1100 (s),
1055 (s), 1021 (s), 939 (w), 805 (s), 755 (m), 450 (w).
Reaction of 1 with PhCCH. To a green solution of 1 (315 mg,
0.49 mmol) in n-hexane (10 mL) was added PhCCH (95 mg, 0.93
mmol) at room temperature. The color of the solution changed to
brown immediately with precipitation of a brown solid. After stirring
for 20 min, the solid was collected by filtration, washed with Et2O (3
× 2 mL), and dried under vacuum, which afforded (IPr)Fe(η2-
HCCPh)2 (5) as a brown solid (160 mg, 50%). 1H NMR (400 MHz,
C6D6, 292 K): δ (ppm) 11.98 (ν1/2 = 78 Hz), 8.91 (very br), 1.42
(very br), 0.36 (very br), 0.31 (very br), −0.79 (ν1/2 = 34 Hz), −1.91
(ν1/2 = 33 Hz), −2.38 (ν1/2 = 33 Hz), −10.63 (ν1/2 = 113 Hz). Anal.
Calcd for C43H48FeN2: C, 79.61; H, 7.46; N, 4.32. Found: C, 79.36;
H, 7.20; N, 4.18. Magnetic susceptibility (C6D6, 294 K): μeff = 3.6(1)
μB. Absorption spectrum (THF): λmax, nm (ε, M−1 cm−1) = 317
(11 980), 331 (13 380), 348 (9490). IR (KBr, cm−1): ν = 3085 (w),
2960 (s), 2927 (w), 2868 (w), 1679 (s), 1590 (w), 1532 (w), 1471
(s), 1422 (w), 1383 (w), 1330 (w), 1258 (w), 1231 (w), 1131 (w),
1061 (w), 923 (w), 810 (w), 751 (w), 691 (w).
Preparation of [(IPr)Fe(η2-vtms)2] (1). Method A: Complex 1
was synthesized according to the literature by using KC8 as the
reducing reagent.16 Method B: To a THF (30 mL) solution of IPr
(1.46 g, 3.70 mmol) was added FeCl2 (480 mg, 3.80 mmol). The
mixture was stirred overnight at room temperature, and then
vinyltrimethylsilane (vtms, 2.2 mL, 15.1 mmol, 4 equiv) and Mg
powder (110 mg, 4.6 mmol, 1.2 equiv) were added. The resultant
mixture was further stirred for 48 h at room temperature. During the
course of the stirring, the color of the mixture changed gradually to
green. The reaction mixture was then filtered through diatomaceous
earth, and the filtrate was concentrated under vacuum to leave a green
residue. The residue was extracted with n-hexane (60 mL) and
filtered. The filtrate was subjected to vacuum to remove all the
volatiles. The resultant green solid was washed with n-hexane (3 mL)
and dried under vacuum to afford [(IPr)Fe(vtms)2] (1) as a green
powder (854 mg, 35%). 1H NMR (400 MHz, C6D6, 298 K): δ (ppm)
25.01 (ν1/2 = 17 Hz), 22.95 (ν1/2 = 54 Hz), 10.29 (ν1/2 = 17 Hz), 4.65
(ν1/2 = 11 Hz), 0.25 (very br), −0.90 (ν1/2 = 13 Hz), −8.44 (ν1/2 = 21
Hz), −12.08 (ν1/2 = 41 Hz), −15.52 (ν1/2 = 57 Hz). The NMR data
of 1 is identical to those reported previously.16
Preparation of [(cyIPr)Fe(η2-vtms)2] (2). This complex was
prepared as a green powder (0.6 g, 28%) by the reaction of FeCl2
(0.40 g, 3.12 mmol) with cyIPr (1.38 g, 3.12 mmol), vtms (2.4 mL,
16.4 mmol, 5 equiv), and KC8 (0.93 g, 6.88 mmol, 2.2 equiv) using
procedures similar to that of [(IPr)Fe(η2-vtms)2] (Method A). Green
crystals of 2 suitable for single-crystal X-ray diffraction study were
obtained by standing its Et2O solution at room temperature via slow
Reaction of 1 with p-CH3C6H4CCH. To a green solution of 1
(454 mg, 0.71 mmol) in n-hexane (10 mL) was added p-
CH3C6H4CCH (167 mg, 1.44 mmol) at room temperature. The
color of the solution changed to brown immediately with precipitation
1
evaporation of Et2O. H NMR (400 MHz, C6D6, 292 K): δ (ppm)
22.75 (v1/2 = 81 Hz), 10.62 (v1/2 = 50 Hz), 5.83 (v1/2 = 44 Hz), 4.89
I
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Article
of a brown solid. After stirring for 20 min, the precipitate was
collected by filtration, washed with Et2O (3 × 2 mL), and dried under
vacuum, which afforded [(IPr)Fe(η2-HCCC6H4CH3-p)2] (6) as a
brown solid (340 mg, 71%). Single-crystals of 6 suitable for X-ray
crystallography were obtained by cooling its saturated toluene
139.78, 133.20, 129.35, 128.45, 126.44, 19.44. These NMR data are in
accordance with the literature.63
For the reaction of p-CH3C6H4CCH (58.7 mg, 0.50 mmol), n-
hexane/EA (50:1) was used as the eluent for separation, and a
mixture of the substituted benzenes 1,2,4-(p-CH3C6H4)3C6H3 and
1,3,5-(p-CH3C6H4)3C6H3 were isolated in 62% yield as a white solid.
1H NMR analysis indicated a ratio of 3:2 for the 1,2,4- and 1,3,5-
isomers. 1,2,4-(p-CH3C6H4)3C6H3: 1H NMR (400 MHz, CDCl3, 298
K): δ (ppm) 7.47 (d, J = 7.8 Hz, 1H), 7.25 (d, J = 6.2 Hz, 2H), 7.12−
7.04 (m, 8H), 2.41 (s, 3H), 2.33 (s, 3H), 2.32 (s, 3H). The 1H NMR
signals of other hydrogen atoms on aromatic rings overlap with those
of its isomer. 13C NMR (101 MHz, CDCl3, 298 K): δ (ppm) 140.78,
140.03, 139.15, 138.75, 138.40, 137.80, 137.14, 136.12, 136.02,
131.11, 129.74, 129.70, 129.54, 129.28, 128.68, 128.66, 126.95,
125.72, 21.15. 1,3,5-(p-CH3C6H4)3C6H3: 1H NMR (400 MHz,
CDCl3, 298 K): δ (ppm) 7.73 (s, 3H), 7.29 (d, J = 8.4 Hz, 6H),
2.43 (s, 9H). The 1H NMR signals of other hydrogen atoms on
aromatic rings overlap with those of its isomer. 13C NMR (101 MHz,
CDCl3, 298 K): δ (ppm) 142.17, 138.40, 137.27, 129.54, 127.19,
124.58, 21.17. These NMR data are in accordance with the
literature.64,65
1
solution at −30 °C. H NMR (400 MHz, C6D6, 294 K): δ (ppm)
10.93 (ν1/2 = 86 Hz), 9.26 (ν1/2 = 29 Hz), 8.43 (very br), 4.10 (ν1/2
=
39 Hz), 0.88 (very br), 0.01 (very br), −0.59 (ν1/2 = 34 Hz), −1.48
(ν1/2 = 29 Hz), −11.19 (ν1/2 = 122 Hz). Anal. Calcd for C45H52FeN2:
C, 79.86; H, 7.74; N, 4.14. Found: C, 79.55; H, 7.92; N, 3.72.
Magnetic susceptibility (C6D6, 294 K): μeff = 3.5(1) μB. Absorption
spectrum (THF): λmax, nm (ε, M−1 cm−1) = 291 (17 960), 336
(11 680), 355 (8250). IR (KBr, cm−1): ν = 3290 (w), 3085 (w), 2960
(s), 2926 (w), 2868 (w), 1679 (s), 1590 (w), 1533 (w), 1507 (w),
1471 (m), 1422 (w), 1383 (w), 1361 (w), 1331 (w), 1131 (w), 1060
(w), 923 (w), 817 (m), 811 (m), 750 (w), 689 (w).
Reaction of 1 with 3,5-(CF3)2C6H3CCH. To a green solution
of 1 (193 mg, 0.30 mmol) in n-hexane (10 mL) was added 3,5-
(CF3)2C6H3CCH (148 mg, 0.62 mmol) at room temperature. The
color of the solution changed to brown immediately with precipitation
of a brown solid. After stirring for 10 min, the precipitate was
collected by filtration, washed with n-hexane (3 × 2 mL), and dried
under vacuum, which furnished [(IPr)Fe(η2-HCCC6H3-(CF3)2-3,5)2]
(7) as a brown solid (117 mg, 42%). Single-crystals of 7 suitable for
X-ray crystallography were obtained by cooling its saturated Et2O
For the reaction of n-C6H13CCH (60 mg, 0.55 mmol), n-hexane
was used as the eluent for separation, and a mixture of the substituted
benzenes 1,2,4-(n-C6H13)3C6H3 and 1,3,5-(n-C6H13)3C6H3 were
isolated in 68% yield as a pale yellow oil. 1H NMR analysis indicated
a ratio of 3:2 for the 1,2,4- and 1,3,5-isomers. 1,2,4-(n-C6H13)3C6H3:
1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) 7.05 (d, J = 7.6 Hz,
1
solution at −30 °C. H NMR (400 MHz, C6D6, 292 K): δ (ppm)
12.18 (ν1/2 = 52 Hz), 8.25 (very br), 3.40 (ν1/2 = 7 Hz), 0.43 (ν1/2
=
1
1H), 6.96−6.93 (m, 2H). The H NMR signals of other hydrogen
16 Hz), −1.05 (ν1/2 = 11 Hz), −1.32 (ν1/2 = 15 Hz), −2.68 (ν1/2 = 14
Hz), −11.03 (ν1/2 = 88 Hz). 19F NMR (376 MHz, C6D6, 292 K): δ
(ppm) −67.88 (ν1/2 = 94 Hz). Anal. Calcd for C47H44F12FeN2: C,
61.31; H, 4.82; N, 3.04. Found: C, 61.18; H, 4.81; N, 2.81. Magnetic
susceptibility (C6D6, 294 K): μeff = 3.2(1) μB. Absorption spectrum
(THF): λmax, nm (ε, M−1 cm−1) = 319 (15 190), 332 (15 530), 349
(10 710). IR (KBr, cm−1): ν = 3310 (w), 3085 (w), 2964 (s), 2929
(w), 2869 (w), 1679 (s), 1591 (w), 1534 (w), 1470 (m), 1422 (w),
1379 (s), 1331 (w), 1280 (s), 1179 (s), 1135 (s), 1107 (w), 922 (w),
898 (m), 848 (w), 750 (w), 700 (w), 684 (w), 663 (w).
atoms on alkyl chains overlap with those of its isomer. 13C NMR (101
MHz, CDCl3, 298 K): δ (ppm) 140.30, 140.13, 137.66, 129.19,
128.91, 125.66, 35.98, 35.61, 32.78, 32.34, 31.76, 31.58, 31.39, 29.52,
29.14, 22.65, 22.62, 14.16. 1,3,5-(n-C6H13)3C6H3: 1H NMR (400
MHz, CDCl3, 298 K): δ (ppm) 6.82 (s, 3H). The 1H NMR signals of
other hydrogen atoms on alkyl chains overlap with those of its isomer.
13C NMR (101 MHz, CDCl3, 298 K): δ (ppm) 142.68, 125.79, 31.76,
31.56, 31.39, 29.14, 22.62, 14.10. These NMR data are in accordance
with the literature.66,67
Reaction of 1 with Ph2CO. To a green solution of 1 (159 mg,
0.25 mmol) in toluene (10 mL) was added Ph2CO (93 mg, 0.51
mmol) at room temperature. The color of the solution changed to
brown immediately. The reaction mixture was stirred at room
temperature for 2 h and then was subjeted to vacuum to remove all
the volatiles. The residue was washed with n-hexane (3 × 2 mL) and
dried under vacuum to afford [(IPr)Fe(η2-OCPh2)2] (8) as a brown
solid (164 mg, 82%). Single-crystals of 8 suitable for X-ray
crystallography were obtained by standing its Et2O solution at room
temperature via slow evaporation of Et2O. 1H NMR (400 MHz, C6D6,
General Procedure for the Alkynes Cyclotrimerization
Reactions Catalyzed by 1. A toluene (1 mL) solution of an alkyne
with the catalyst 1 (5 mol %) was heated at 50 °C for 20 h. The
mixture was then subjected to vacuum to remove the volatiles. The
residue was subjected to fast column chromatography (silica gel) that
gave substituted benzene derivative(s) as colorless solids.
For the reaction of PhCCPh (46 mg, 0.25 mmol), n-hexane/EA
(20:1) was used as the eluent for separation, and the substituted
benzene C6Ph6 was isolated in 84% yield. 1H NMR (400 MHz,
CDCl3, 293 K): δ (ppm) 6.84 (m). 13C NMR (101 MHz, CDCl3, 293
K): δ (ppm) 140.56, 140.25, 131.37, 126.53, 125.14. These NMR
data are in accordance with the literature.63
295 K): δ (ppm) 54.04 (very br), 13.85 (ν1/2 = 174 Hz), 8.99 (ν1/2
=
24 Hz), 8.68 (ν1/2 = 44 Hz), −5.51 (ν1/2 = 30 Hz), −17.91 (very br),
−20.27 (ν1/2 = 41 Hz), −28.74 (ν1/2 = 47 Hz). Anal. Calcd for
C53H56FeN2O2: C, 78.70; H, 6.98; N, 3.46. Found: C, 78.33; H, 7.22;
N, 3.40. Magnetic susceptibility (C6D6, 294 K): μeff = 4.2(1) μB.
Absorption spectrum (THF): λmax, nm (ε, M−1 cm−1) = 348 (2310),
527 (280), 789 (140). IR (KBr, cm−1): ν = 3085 (w), 2960 (s), 2927
(w), 2868 (w), 1680 (s), 1659 (s), 1598 (m), 1578 (w), 1534 (w),
1475 (m), 1450 (m), 1422 (w), 1383 (w), 1360 (w), 1318 (m), 1278
(s), 1177 (w), 1150 (w), 1060 (w), 941 (m), 922 (m), 810 (m), 764
(w), 703 (s), 639 (m).
Reaction of 1 with 2,6-Me2C6H3NC. To a green solution of 1
(141 mg, 0.22 mmol) in Et2O (10 mL) was added 2,6-Me2C6H3NC
(114 mg, 0.87 mmol) at room temperature. The color of the solution
changed to red immediately. The mixture was stirred for 2 h and was
then subjected to vacuum to remove all the volatiles. The residue was
washed with n-hexane (2 × 2 mL) and dried under vacuum to afford
[(IPr)Fe(CNC6H3-2,6-Me2)4] (9) as a red solid (170 mg, 80%).
Single-crystals of 9 suitable for X-ray crystallography were obtained by
standing its Et2O solution at room temperature via slow evaporation
of Et2O. 1H NMR (400 MHz, C6D6, 292 K): δ (ppm) 7.09−7.07 (m,
4H), 7.02−6.99 (m, 2H), 6.76−6.69 (m, 12H), 6.67 (s, 2H), 3.40−
3.33 (m, 4H), 2.23 (s, 24H), 1.58 (d, J = 6.8 Hz, 12H), 1.17 (d, J =
For the reaction of EtCCEt (43 mg, 0.50 mmol), n-hexane/EA
(20:1) was used as the eluent for separation, and the substituted
benzene C6Et6 was isolated in 80% yield. 1H NMR (400 MHz,
CDCl3, 292 K): δ (ppm) 2.64 (q, J = 7.5 Hz, 12H), 1.20 (t, J = 7.5
Hz, 18H). 13C NMR (101 MHz, CDCl3, 292 K): δ (ppm) 137.84,
22.16, 15.72. These NMR data are in accordance with the literature.63
For the reaction of PhCCMe (126 mg, 1.08 mmol), n-hexane/
EA (30:1) was used as the eluent for separation, and a mixture of
1,2,4-Me3-3,5,6-Ph3C6 and 1,3,5-Me3-2,4,6-Ph3C6 were obtained in
1
95% yield. H NMR analysis indicated a ratio of 8:1 for 1,2,4-Me3-
3,5,6-Ph3C6/1,3,5-Me3-2,4,6-Ph3C6. For 1,2,4-Me3-3,5,6-Ph3C6: 1H
NMR (400 MHz, C6D6, 292 K): δ (ppm) 2.17 (s, 3H), 2.14 (s, 3H),
1
2.11 (s, 3H). The H NMR signals of hydrogen atoms on aromatic
rings overlap with those of its isomer. 13C NMR (101 MHz, CDCl3,
292 K): δ (ppm) 142.40, 141.58, 141.56, 141.38, 140.59, 139.18,
133.91, 131.86, 131.23, 130.27, 129.35, 128.40, 127.30, 127.27,
126.45, 125.70, 125.64, 19.44, 18.29, 18.11. For 1,3,5-Me3-2,4,6-
Ph3C6: 1H NMR (400 MHz, C6D6, 292 K): δ (ppm) 2.07 (s). The 1H
NMR signals of hydrogen atoms on aromatic rings overlap with those
of its isomer. 13C NMR (101 MHz, CDCl3, 292 K): δ (ppm) 142.09,
J
DOI: 10.1021/acs.inorgchem.9b02009
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
6.8 Hz, 12H). 13C NMR (101 MHz, C6D6, 292 K): δ (ppm) 206.32
(Ccarbene), 203.77, 145.87, 139.35, 133.13, 132.35, 128.64, 123.67,
123.53, 123.40, 28.52, 25.49, 23.25, 19.11. Anal. Calcd for
C63H72FeN6: C, 78.08; H, 7.49; N, 8.67. Found: C, 78.07; H, 7.53;
N, 8.76. Absorption spectrum (THF): λmax, nm (ε, M−1 cm−1) = 422
(21 580). IR spectrum (KBr, cm−1): ν = 2962 (s), 2926 (m), 2868
(w), 2094 (s), 1679 (s), 1624 (w), 1588 (w), 1386 (w), 1328 (w),
1286 (w), 1091 (w), 1061 (w), 800 (w), 766 (m), 755 (m), 670 (w),
481 (w).
Cartesian coordinates for the calculated structures of 1,
6, and 8 (PDF)
Accession Codes
CCDC 1937521−1937525 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Reaction of 1 with ButNC. To a green solution of 1 (178 mg,
0.28 mmol) in Et2O (10 mL) was added ButNC (96 mg, 1.16 mmol)
at room temperature. The color of the solution changed to yellow
immediately. The mixture was stirred for 2 h and was then subjeted to
vacuum to remove all the volatiles. The residue was washed with n-
hexane (2 × 2 mL) and dried under vacuum to afford (IPr)Fe-
(CNBut)4 (10) as a yellow solid (152 mg, 72%). 1H NMR (400 MHz,
C6D6, 294 K): δ (ppm) 7.31−7.27 (m, 2H), 7.24−7.22 (m, 4H), 6.48
(s, 2H), 3.25 (hept, J = 6.9 Hz, 4H), 1.59 (d, J = 6.7 Hz, 12H), 1.26
(s, 36H), 1.20 (d, J = 6.9 Hz, 12H). 13C NMR (101 MHz, C6D6, 294
K): δ (ppm) 209.27, 206.95 (Ccarbene), 146.87, 140.06, 128.25, 123.20,
123.03, 53.91, 31.40, 28.19, 25.33, 24.17. Anal. Calcd for C47H72FeN6:
C, 72.66; H, 9.34; N, 10.82. Found: C, 72.82; H, 9.55; N, 10.83.
Absorption spectrum (THF): λmax, nm (ε, M−1 cm−1) = 307
(13 380). IR spectrum (KBr, cm−1): ν = 3068 (w), 2962 (s), 2924
(m), 2868 (m), 2107 (s), 1678 (s), 1614 (m), 1557 (w), 1474 (s),
1386 (w), 1362 (w), 1284 (w), 1065 (w), 939 (m), 810 (w), 754
(m), 688 (w), 447 (w).
Reaction of 1 with CO. A green solution of 1 (102 mg, 0.16
mmol) in THF (10 mL) was cooled with liquid N2, and CO was
introduced to the solution mixture via a CO balloon (1 atm.). The
solution was then warmed to room temperature and stirred for 5 h,
during which the color of the solution changed to yellow gradually.
The mixture was then subjected to vacuum to remove solvent. The
yellow residue was washed with n-hexane (2 mL) and then dried
under vacuum to give [(IPr)Fe(CO)4] (11) as a yellow solid (68 mg,
77%).1H NMR (400 MHz, C6D6, 298 K): δ (ppm) 7.30−7.26 (m,
2H), 7.13 (d, J = 7.8 Hz, 4H), 6.57 (s, 2H), 2.72 (hept, J = 6.8 Hz,
4H), 1.42 (d, J = 6.8 Hz, 12H), 1.00 (d, J = 6.9 Hz, 12H). 13C NMR
(101 MHz, C6D6, 298 K): δ (ppm) 215.53 (CO), 191.05 (Ccarbene),
146.34, 136.72, 130.62, 125.47, 124.32, 28.67, 25.60, 22.50. These
NMR data are identical to those reported in the literature.57
Computational Details. To have a better understanding of the
electronic structures of 1, 6, and 8, DFT68,69 study was performed
with the ORCA 4.1.0 program70 using the B3LYP methods.25,26 The
SVP71,72 basis set was used for the C, N, and H atoms, and the
TZVP73 basis set was used for the other atoms. The RIJCOSX74
approximation with matching auxiliary basis sets71,72 was employed to
accelerate the calculations. The calculation utilizes the atom-pairwise
dispersion correction with the Becke−Johnson damping scheme
(D3BJ).75,76 The atomic coordinates of 1, 6, and 8 used for
calculation were obtained from X-ray diffraction studies, and only the
positions of hydrogen atoms were optimized. Table S2 summarizes
the corresponding single point energies relative to the S = 0 and 1
states. Mulliken spin populations of 1, 6, and 8 are listed in Figures
S38−S40. The UNO orbitals of 1, 6, and 8 are listed in Figures 3, 5,
and 7. The natural orbitals of 1, 6, and 8 are listed in Figures S41−
S43.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: shengfa.ye@kofo.mpg.de (S.Y.).
*E-mail: deng@sioc.ac.cn (L.D.).
ORCID
Shengfa Ye: 0000-0001-9747-1412
Liang Deng: 0000-0002-0964-9426
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the National Natural Science
Foundation of China (Nos. 21725104, 21690062, 21432001,
and 21821002), the National Key Research and Development
Program (2016YFA0202900), and the Strategic Priority
Research Program of the Chinese Academy of Sciences (No.
XDB20000000). S.Y. gratefully acknowledges the financial
support from the Max-Planck Society, in particular the joint
work space of MPI-CEC and MPI-KOFO.
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