CNC-pincer iron complexes containing a bis(N-heterocyclic carbene)Amido ligand: Synthesis and application to catalytic hydrogenation of alkenes

Article abstract of DOI:10.1016/j.jorganchem.2020.121436

This work studied preparation and catalytic application of CNC-pincer Fe complexes containing a bis(NHC)amido ligand (NHC: N-heterocyclic carbene). Deprotonation of bis(3-isopropylimidazoliumyl)amine salt [(CNC^iPr)H₃]²⁺[I^?]₂ (1a) with lithium hexamethyldisilazide (LiHMDS) afforded the corresponding bis(NHC)amido-Li complex 2a. Treatment of in-situ generated 2a with FeI₂(thf)₂ gave a CNC-pincer Fe(II) iodide complex Fe(CNC^iPr)I (3a) and a cationic homoleptic Fe(III) complex [Fe(CNC^iPr)₂]⁺I^? (4a). Reaction of in-situ generated 2a with Fe[N(SiMe₃)₂]₂ produced the corresponding amido complex Fe(CNC^iPr)[N(SiMe₃)₂] (5a). Similarly, deprotonation of a less hindered methyl analogue [(CNC^Me)H₃]₂⁺[I^?]₂ (1b) with LiHMDS followed by treatment of Fe[N(SiMe₃)₂]₂ gave an amido complex Fe(CNC^Me)[N(SiMe₃)₂] (5b). Molecular structures of 3a, 5a and 5b, which were confirmed by X-ray diffraction study, showed a distorted tetrahedral geometry. Complexes 3a and 5b were found to be active in hydrogenation of alkenes. Reaction mechanism was investigated by density functional theory (DFT) calculations.

Full text of DOI:10.1016/j.jorganchem.2020.121436

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CNC-pincer iron complexes containing a bis(N-heterocyclic carbene)Amido ligand:
Synthesis and application to catalytic hydrogenation of alkenes
Wakano Taniguchi, Jun-ichi Ito, Makoto Yamashita
PII:
S0022-328X(20)30338-7
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121436
JOM 121436
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 21 May 2020
Revised Date: 10 July 2020
Accepted Date: 10 July 2020
Please cite this article as: W. Taniguchi, J.-i. Ito, M. Yamashita, CNC-pincer iron complexes
containing a bis(N-heterocyclic carbene)Amido ligand: Synthesis and application to catalytic
hydrogenation of alkenes, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/
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CNC-pincer Iron Complexes Containing A Bis(N-Heterocyclic
Carbene)Amido Ligand: Synthesis and Application to Catalytic
Hydrogenation of Alkenes
Wakano Taniguchi, Jun-ichi Ito,* and Makoto Yamashita
Department of Molecular and Macromolecular Chemistry, Graduate School of
Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
E-mail: jito@oec.chembio.nagoya-u.ac.jp
Keywords
Pincer complexes
Iron
N-Heterocyclic carbene
Alkene hydrogenation
DFT calculations
Abstract
This work studied preparation and catalytic application of CNC-pincer Fe complexes
containing a bis(NHC)amido ligand (NHC: N-heterocyclic carbene). Deprotonation of
iPr
2+ -
bis(3-isopropylimidazoliumyl)amine salt [(CNC )H ] [I ] (1a) with lithium
3
2

hexamethyldisilazide (LiHMDS) afforded the corresponding bis(NHC)amido-Li
complex 2a. Treatment of in-situ generated 2a with FeI (thf) gave a CNC-pincer Fe(II)
2
2
iPr
iodide complex Fe(CNC )I (3a) and a cationic homoleptic Fe(III) complex
iPr
+ –
[
Fe(CNC ) ] I (4a). Reaction of in-situ generated 2a with Fe[N(SiMe ) ] produced
2 3 2 2
iPr
the corresponding amido complex Fe(CNC )[N(SiMe ) ] (5a). Similarly, deprotonation
3
2
Me
+ -
2
of a less hindered methyl analogue [(CNC )H ] [I ] (1b) with LiHMDS followed by
3
2
Me
treatment of Fe[N(SiMe ) ] gave an amido complex Fe(CNC )[N(SiMe ) ] (5b).
3
2 2
3 2
Molecular structures of 3a, 5a and 5b, which were confirmed by X-ray diffraction study,
showed a distorted tetrahedral geometry. Complexes 3a and 5b were found to be active
in hydrogenation of alkenes. Reaction mechanism was investigated by density
functional theory (DFT) calculations.
1
. Introduction
Fe complexes have been extensively studied for the development of homogeneous
catalysis, since Fe is abundant natural resource and its low toxicity.[1] Among
homogeneous catalytic reactions using Fe complexes, hydrogenation of unsaturated
hydrocarbons and carbonyl compounds has been extensively studied in terms of
replacement of precious metals, such as Ru and Ir.[2-4] In 2004, Chirik reported
seminal work for the development of highly active catalysts based on NNN-pincer Fe
complexes I having a bis(imino)pyridine ligand (Fig. 1).[5] In the same year, Peters
reported tris(phosphino)borate ligated Fe complexes II as a hydrogenation catalyst.[6]
After these reports, a variety of iron complexes having a pincer ligand such as
bis(NHC)pyridine (III, NHC: N-heterocyclic carbene),[7] bis(phosphino)carbazolide

(IV),[8] bis(phosphino)amide (V),[9] and bis(phosphino)silyl (VI)[10] have been
developed for catalytic hydrogenation of alkenes. In addition to tridentate ligands,
bidentate ligands were also effective in Fe catalysts.[11] Nagashima and co-workers
reported disilaferracyclic complex VII as an effective catalyst for hydrogenation of
polysubstituted alkenes.[12] Bis(anthracene) ferrate complex VIII was also found as a
catalyst for hydrogenation.[13] Recently, Fe , Fe , and Fe nanoclusters were developed
4
6
7
as effective catalysts.[14]
Fig. 1. Reported structurally well-defined Fe complexes used as catalysts for
hydrogenation.
NHCs have been used as excellent ancillary ligands for transition metals arising from
their strong electron-donating ability and formation of a thermodynamically strong
metal-ligand bond to resist toward decomposition.[15] In this context, a number of Fe
complexes with NHC ligands were reported as highly active catalysts for cross-coupling,
hydrosilylation, and aziridination.[16] Introduction of NHC into a pincer ligand scaffold
have been studied for design of robust and versatile ancillary ligands.[17] Recently,
Danopoulos[18] and Fout[19] reported CNC and CCC-pincer Fe complexes with

bis(NHC) scaffold, in which the central pincer donors, such as phenyl and pyridyl
groups, significantly affected both coordination geometry and spin multiplicity at the Fe
center. Luo and Kunz reported bis(NHC) pincer ligands with diarylamide and
carbazolide backbones, respectively.[20,21] These CNC ligands have been used to
synthesize various transition metal complexes including groups 3 and 8–10
metals.[20-24] Although those ligands would serve as a highly strong electron donor,
introduction of Fe has not been reported. Related PNP- and NNN-pincer ligands
containing anionic nitrogen donors offered efficient Fe catalysts for polymerization,
hydrogenation, nitrogen reduction, and asymmetric reactions.[8,25] Thus, we expect
that the bis(NHC)amide ligands can construct highly active Fe catalysts arising from
their strong electron-donating ability and thermo-dynamic stability. In this work, we
report synthesis of CNC-pincer Fe complexes and their application for catalytic
hydrogenation of alkenes. A catalytic cycle was proposed based on density functional
theory (DFT) calculations.
2
. Results and discussion
R
2+
-
i
In this work, we used bis(imidazoliumyl)amine salt [(CNC )H ] [I ] {R = Pr,
3
2
bis[2-(3-isopropylimidazolium)-4-methylphenyl]amine diiodide (1a); R = Me,
bis[2-(3-isopropylimidazolium)-4-methylphenyl]amine diiodide (1b)} for ligand
precursors, which were reported by Luo.[20] According to the previously reported
deprotonation of bis(imidazoliumyl)carbazole by alkyllithium to give the corresponding
lithium bis(NHC)carbazolide,[21e] we examined deprotonation of 1a (Scheme 1).
Reaction of 1a with 3.2 equiv. of lithium hexamethyldisilazide (LiHMDS) in THF at

iPr
1
room temperature smoothly proceeded to give (CNC )Li (2a). The H NMR spectrum
of 2a in THF-d showed the disappearance of the signals of imidazolium and N–H
8
1
3
protons observed in 1a. In the C NMR spectrum, the signal of carbenes was observed
at δ 202.0 ppm. This chemical shift is comparable to that of the lithium
bis(NHC)carbazolide (δ 206.1 ppm).[21c,d]
iPr
Scheme 1. Synthesis of (CNC )Li (2a) and reaction of in-situ generated 2a or 2b with
FeI (thf) and Fe[N(SiMe ) ] .
2
2
3 2 2
Reaction of in-situ generated 2a in THF with 1 equiv. of FeI (thf) [26] resulted in the
2
2
iPr
formation of a mixture of a Fe(CNC )I complex 3a and a cationic homoleptic complex
a. Extraction of the crude mixture with toluene and recrystallization of the toluene
extract afforded red crystals of 3a in 34% yield. To the residue, THF was added to give
4

a green suspension. After the filtration of the suspension, recrystallization from the
resulting green solution yielded green crystals of 4a. (See Fig. S8 for a preliminary
result on the crystal structure of 4a). As another Fe precursor, we used Fe[N(SiMe ) ] ,
3
2 2
which has high solubility to common organic solvents.[27] Reaction of Fe[N(SiMe )2]2
3
with in-situ generated 2a at room temperature resulted in the formation of an amido Fe
iPr
complex Fe(CNC )[N(SiMe ) ] (5a). Complex 5a was isolated in 37% yield by
3
2
crystallization form toluene solution.
In contrast to the isopropyl-substituted ligand precursor 1a, the reaction of FeCl or
2
FeI (thf) with in-situ generated 2b did not allow an introduction of Fe atom to the
2
2
methyl analogue 1b. On the other hand, the reaction of Fe[N(SiMe ) ] with 2b
3
2 2
generated by a treatment of 1b with LiHMDS (3.2 equiv) at room temperature produced
Me
the desirable amido Fe complex Fe(CNC )[N(SiMe ) ] (5b). Complex 5b was
3
2
obtained in 68% yield as yellow crystals by recrystallization with toluene and n-pentane
solution.
Molecular structures of 3a, 5a and 5b were confirmed by X-ray diffraction study (Fig.
2
-4). The Fe center of 3a is described as a distorted tetrahedral geometry with geometry
factor τ = 0.77 (where τ = 1 shows a tetrahedral structure and τ = 0 shows a square
4
4
4
planar structure).[28] The molecular structures of 5a and 5b showed distorted
tetrahedral geometry [τ = 0.80 (5a) and 0.78 (5b)] with the CNC ligand and N(SiMe3)2
4
ligand. This structural feature is similar to that of the four-coordinated Fe(II) complexes
with a PNP tridentate ligand.[25b] The Fe1–N1 bond lengths [1.975(3) Å (3a),
1
.9963(14) Å (5a), 2.0168(17) Å (5b)] are close to those of anionic PNP-pincer Fe
complexes.[25a-d] The Fe–CNHC bond lengths [2.072(3), 2.059(3) Å] in 3a are slightly
shorter than that of bis(NHC) pyridine Fe(II) complex Fe(CNC)Br (2.16-2.19 Å).[18a]
2

In 5a and 5b, the Fe1–N6 bond lengths [1.9870(14) Å (5a), 1.9911(19) Å (5b)] are in
the range of those of other Fe–N(SiMe ) complexes (1.94-1.98 Å).[29]
3
2
Fig. 2. Molecular structure of 3a (thermal ellipsoid at 50% probability; hydrogen atoms
and co-crystallized toluene molecule are omitted for clarity). Selected bond distances
(Å) and angles (deg): Fe1-N1 1.975(3), Fe1-C8 2.072(3), Fe1-C21 2.059(3), Fe1-I1
2
.6468(5), C21-Fe1-C8 126.82(13), N1-Fe1-I1 124.06(8).
Fig. 3. Molecular structure of 5a (thermal ellipsoid at 50% probability; hydrogen atoms
are omitted for clarity). Selected bond distances (Å) and angles (deg): Fe1-C8

2
.0436(17), Fe1-C21 2.0969(17), Fe1-N1 1.9963(14), Fe1-N6 1.9870(14), Si1-N6
.7137(15), Si2-N6 1.7033(15), C8-Fe1-C21 113.86(7), N6-Fe1-N1 123.48(6).
1
Fig. 4. Molecular structure of 5b (thermal ellipsoid at 50% probability; hydrogen atoms
are omitted for clarity). Selected bond distances (Å) and angles (deg): Fe1-C8 2.109(2),
Fe1-C19 2.066(2), Fe1-N1 2.0168(17), Fe1-N6 1.9911(19), C8-Fe1-C19 123.26(8),
N1-Fe1-N6 126.31(7)
The properties in solution of 3a, 5a and 5b were studied by NMR spectroscopy and
1
electrochemistry. The H NMR spectrum of 3a showed broad signals in the range of
−62 to +73 ppm (Figure S4 in Supporting Information). The Evans method [30]
provided solution magnetic moments of 5.0 µ for 3a, 5.0 µ for 5a, and 4.7 µ for 5b
B
B
B
in THF-d at room temperature. These values are close to the spin-only magnetic
8
moment for the S = 2 spin system (4.90 µ ) [31] and are comparable to that of the
B
(
PNP)Fe(II) complex IV (4.8-4.9 µ ).[8] Cyclic voltammogram of 3a measured in THF
B
at room temperature showed an irreversible oxidation response at E = –0.56 V (vs.
+
Cp Fe/Cp Fe ) which could be assigned to the Fe(II)/Fe(III) redox couple (Fig. S5 in
2
2
Supporting Information), according to the previously reported oxidation of
pyrrole-based (PNP)Fe(II) complex (E1/2 = –0.41 V)[25c] and bis(NHC)Fe(II)

complexes (E = –0.37 - +0.42 V).[32]
1
/2
Catalytic activity of CNC-Fe complexes 3a, 5a, and 5b was evaluated in
hydrogenation of styrene 6a (Table 1). The catalytic hydrogenation of 6a quantitatively
proceeded by using 1 mol% of 3a and 2 mol% of LiBEt H at 30 ºC under 1.0 MPa of H
3
2
(
Entry 1). Decreasing the amount of LiBEt H reduced the yield of 7a (Entry 2). The
3
catalytic reaction was also completed with 0.5 mol% of 3a although a slightly longer
reaction time was required (Entry 3). No reaction was observed without addition of
LiBEt H (Entry 4). The amido complex 5a was found to be less active in hydrogenation
3
(Entry 5). In contrast, the amido complex 5b with the methyl-substituted ligand
exhibited catalytic activity without additives to give 7a quantitatively (Entry 6). The
formation of HN(SiMe ) was confirmed by GC analysis. This result suggested that 5b
3
2
underwent heterolytic splitting of H by the Fe–N(SiMe ) bond. Initial turnover
2
3 2
–
1
frequency (TOF) was estimated to be 90 h (Entry 7, 30 min.). This value was lower
-1
than that of I (TOF = 1344 h , 22 ºC, 4 atm, 16 min.).[5] Catalytic reaction proceeded
with 0.1 mol% of 5b to give 7a in 24% yield (Entry 8). To enhance the reaction rate, the
effect
4,4,5,5-tetramethyl-1,3,2-dioxaborolane) was found to be a suitable additive to increase
the yield of 7a with turnover number (TON) of 850 (Entry 9), which was lower than
of
additives
was
examined.
Consequently,
HBpin
(
[
5]
those of I (TON = 2500, 1-hexene) and VII (TON = 2000, 1-octene).[12] According
to reports about reaction of Fe[N(SiMe ) ] and [CpFe[N(SiMe ) ] with HBpin,[33]
3
2 2
3 2
reaction of 5b with HBpin might generate a hydride species as a catalytically active
intermediate.

[
a]
Table 1. Hydrogenation of styrene catalyzed by 3a, 5a, or 5b
Fe cat. (1.0 mol%)
+
H
2
Ph
Ph
THF, 30 ºC
6
a
7a
c
Entry
Fe cat.
mol%)
Additive
Time (h)
Yield (%)
(
(mol%)
1
2
3
4
5
6
7
3a (1.0)
3a (1.0)
3a (0.50)
3a (1.0)
5a (1.0)
5b (1.0)
5b (1.0)
5b (0.10)
5b (0.10)
LiBHEt3 (2.0)
1
95
77
92
0
LiBHEt3 (1.0)
3
LiBHEt3 (1.0)
3
-
3
-
3
3
-
3
99
45
24
85
-
0.5
48
12
[
[
b]
b]
8
9
-
HBpin (0.20)
[
a] Reaction condition: 6a (1.0 mmol), Fe cat. (0.010 mmol), H (1.0 MPa). [b] 6a (10.0
2
mmol), 5b (0.010 mmol), H (3.0 MPa). c Determined by GC.
2
Next, hydrogenation of other alkenes 6b-e was tested (Table 2). Catalytic reaction
was conducted with 1 mol% of 3a or 5b under 1.0 MPa of H . Hydrogenation of 1,1-
2
and cis-disubstituted alkenes, α-methylstyrene (6b) and cyclooctene (6c), proceeded at
3
0-50 ºC to give the corresponding alkane in moderate to high yield (Entries 1-4). In
these reactions, catalytic activity of 5b was higher than that of 3a. In contrast,
hydrogenation of trans-disubstituted and trisubstituted alkenes did not proceed at 50-80
ºC, probably due to the steric effect (Entries 5,6).

Table 2. Hydrogenation of alkenes 6b-e catalyzed by 3a or 5b
[
d]
Entry
Alkenes
Cat
3a
Temp (ºC) / Time (h)
40 / 24
Yield (%)
1
2
3
4
[a]
[b]
[a]
[b]
47
71
38
85
5b
3a
30 / 6
30 / 24
5b
50 / 6
5
[c]
[c]
5b
5b
50 / 24
80 / 24
0
0
6
[
a] Reaction condition: 6 (1.0 mmol), 3a (0.010 mmol), LiBHEt (0.020 mmol), H (1.0
3 2
MPa). [b] 6 (1.0 mmol), 5b (0.010 mmol), H (1.0 MPa). [c] 6 (1.0 mmol), 5b (0.010
2
mmol), HBpin (0.20 mmol), H (1.0 MPa). [d] Determined by GC.
2
In the hydrogenation of alkenes, we assume two reaction mechanisms based on
monohydride and dihydride intermediates (Scheme 2).[34] In the monohydride
mechanism, insertion of an alkene into the Fe-H bond of the monohydride intermediate
gives a corresponding alkyl intermediate, which undergoes hydrogenolysis to give a
product (Scheme 2a). In the dihydride mechanism, the reaction of H with the
2
monohydride intermediate produces the dihydride intermediate with a concomitant
formation of protiated ligand (Scheme 2b). Subsequent insertion of an alkene followed
by reductive elimination gives a product. Another issue in the catalytic reaction is spin
multiplicity. Recently, DFT calculation for hydrogenation of styrene with complex II

shown in Figure 1 reveled that the reaction proceeds via the triplet.[35a] In contrast,
hydrogenation of 1-butene by complex I was proposed to involve the open-shell singlet
state.[35c] To distinguish two mechanisms and spin multiplicity, we performed
stoichiometric reactions and DFT calculations.
Scheme 2. Proposed mechanism of hydrogenation of alkene.
First, we conducted stoichiometric reactions to generate hydride and alkyl species.
However, reaction of 3a with LiBEt H (1.0 equive.) at room temperature gave
3
unidentified products and reaction of 5b with 1.0 MPa of H at room temperature
2
resulted in no reaction. Although, reaction of 3a with MeLi (1.8 equiv.) or reaction 3a
with LiBEt H in the presence of styrene (1.0 equiv.) was performed to synthesize an
3
alkyl complex, the products were not currently identified.
To obtain insights into the reaction mechanism of the present hydrogenation
catalyzed by 5b, DFT calculations were conducted by using a model complex 9
containing a slightly simplified CNC ligand (methyl groups on the benzene rings were
removed) on quintet and triplet states (Figure 5 and see Supporting Information for
5
details). First, we calculated the reaction of H with the amido complexes 9 on quintet
2
3
state and 9 on triplet state (where the superscript number denotes their spin multiplicity,

5
Figure 5a).[36] Hydrogenolysis of the quintet 9, which corresponds to the
experimentally observed structure, with H occurs in a concerted fashion without an
2
2
5
5
η -H intermediate to give hydridoiron-amine complex 10 via TS1. This step is
2
endergonic by 20.1 kcal/mol with relatively high energy barrier of 33.4 kcal/mol. The
5
subsequent dissociation of the amine from 10 leads to formation of tetracoordinate
5
3
hydride intermediate 11. In contrast, the triplet amido complex 9 is higher in energy
5
than the quintet 9 by 19.8 kcal/mol. The subsequent hydrogenolysis of the triplet state
3
2
3
9
proceeds as a stepwise process involving an η -H intermediate 12. The energy
2
3
barrier (19.7 kcal/mol) through TS1 in the triplet state is smaller than that in the quintet
3
5
state. The resulting similar hydride intermediate 11 is more stable than 11 in the
quintet state by 3.4 kcal/mol. These results suggest a spin-crossover process between
triplet and quintet states should take place during the hydrogenolysis.[35a,37]

Fig. 5. Gibbs free energy profiles (kcal/mol) for (a) hydrogenolysis of model amido
5
3
complexes 9 and 9 with H , (b) migratory insertion of styrene to hydride intermediates
2
5
3
5
3
1
1 and 11, and (c) hydrogenolysis of 1-phenylethyl intermediates 15 and 15 with H
2

5
3
(
Superscript and in the compound number denote their spin multiplicity. The model
CNC ligand structure is shown in the top).
3
5
Next, migratory insertion reactions of styrene into hydride intermediates 11 and 11
were investigated (Fig. 5b).[35] In this process, 2,1-insertion to a hydride intermediate
is more favorable than 1,2-insertion (Fig. S15 and S17 in Supporting Information).
3
2
3
Coordination of styrene to 11 gives a η -styrene intermediate 13. Subsequent
3
migratory insertion of styrene into the Fe-H bond in 13 leads to a 1-phenylethyl
3
intermediate 14, which has a β-agnostic interaction between the 1-phenylethyl ligand
3
and the Fe atom.[35a,e] The energy barrier of transition state TS2 is 8.4 kcal/mol and
3
3
the formation of 14 is endergonic by 6.6 kcal/mol. Isomerization of 14 breaks the
3
3
β-agnostic interaction via TS3, leading to a 1-phenylethyl intermediate 15 with an
3
energy gain of 5.1 kcal/mol in comparison with 11. In contrast to the triplet state, the
5
5
coordination of styrene to the quintet 11 giving a styrene-adduct 13 is an endergonic
5
reaction. Subsequent migratory insertion through TS2 with the energy barrier of 13.3
5
kcal/mol gives 1-phenylethyl intermediate 15, which is more stable than the triplet sate
3
1
5 by 12.7 kcal/mol. From these observations, this process might proceed via triplet
state.
The subsequent hydrogenolysis of the triplet 1-phenylethyl complex 15 contains a
3
2
3
stepwise process, consisting of the formation of a η -H intermediate 16 through a
2
coordination of H and σ-bond metathesis leading to the product 7a, to reproduce the
2
3
3
hydride complex 11 via TS4 with an energy barrier of 15.9 kcal/mol (Fig. 5c). This
3
activation energy is higher than that for TS2. In contrast, hydrogenolysis of the quintet
5
5
1
5 proceeds via TS4 with the energy barrier of 27.5 kcal/mol, which is higher than

that of the triplet state. Currently, we propose that the catalytic reaction proceeds via a
3
triplet state and hydrogenolysis of 15 is the rate limiting step.
For the dihydride mechanism, we calculated the heterolytic cleavage of H with the
2
3
5
3
Fe-N bond of hydride intermediates 11 and 11 to afford dihydride intermediates 17
5
3
5
and 17, respectively (Fig. 6a). However, the formation of 17 and 17 is a highly
3
5
endergonic process and energy barriers of transition states TS5 and TS5 are very
large.

3
5
Fig. 6. Gibbs free energy profiles (kcal/mol) for (a) reaction of H with 11 and 11, (b)
2
subsequent hydrogenation of styrene, and (c) oxidaitive addition of H2.

Next, migratory insertion of styrene into the Fe-H bond and subsequent reductive
5
elimination of ethylbenzene were calculated (Fig. 6b). In the quintet 17, coordination
5
of styrene affords a styrene-adduct 18. The following migratory insertion proceeds via
5
5
TS6 with the energy barrier of 9.7 kcal/mol to form intermediate 19. However,
5
5
transition state TS7 for reductive elimination giving an ethylbenzene-adduct 20 has a
relatively high energy barrier of 40.0 kcal/mol. Then, dissociation of ethylbenzene
5
3
produces 21. In the triplet 17, migratory insertion of styrene occurs with a lower
3
3
energy barrier to afford 19. The subsequent reductive elimination proceeds via TS6
5
with lower energy barrier compared to that of the quintet TS6.
3
5
Finally, generation of dihydride intermediates 17 and 17 via oxidative addition of
H were investigated (Fig 6c). Although we attempted to optimize a reaction pathway
2
on the triplet surface, we could not find transition state for oxidative addition of H . In
2
5
2
5
contrast, coordination of H to the quintet 21 affords an η -H intermediate 22, which
2
2
5
5
undergoes oxidative addition through TS8 to give 17.
To compare with monohydride and dihydride mechanisms, the dihydride
3
5
intermediates 17 and 17 are much higher in energy than the monohydride
3
5
5
3
3
intermediates 11 and 11. Energy barriers of TS5 and TS5 for the formation of 17
5
3
and 17 are also higher than that of TS4, which is the rate limiting step in the
monohydride mechanism. Those results suggest that the monohydride mechanism
3
5
involving 11 and 11 is favored.
3
. Conclusion

We synthesized a series of CNC pincer-Fe complexes with bis(NHC) amido ligands.
Those Fe complexes exhibited catalytic activity for hydrogenation of alkenes. DFT
calculations suggested that the monohydride mechanism involving the migratory
insertion step and the hydrogenolysis through triplet-states is more favorable than that
the dihydride mechanism.
4
. Experimental Section
4
.1. General Information.
All manipulations of air- and moisture-sensitive compounds were carried out under Ar
atmosphere by using Schlenk and glove box technique. Et O, THF, toluene and
2
n-hexane were purified by passing through a solvent purification system (Grass Contou).
THF-d was purified by distillation over sodium benzophenone. Compounds 1a and
8
1
b,[20] and FeI (thf) [26] were prepared according to the literature methods. NMR
2 2
1
13
spectra were recorded on JEOL ECA (400 MHz for H, 100 MHz for C, and 155 MHz
7
for Li) spectrometer. Chemical shifts were reported in δ (ppm) relative to the residual
signals at δ 1.72 and δ 25.3 ppm for THF-d and LiCl in D O at δ 0 ppm. Elemental
H
C
8
2
Li
analysis were recorded on a Perkin Elmer 2400II. Melting points (mp) were determined
with an MPA100 OptiMelt instrument (Tokyo Instruments, Inc.) and are uncorrected.
GC analysis were recorded on SHIMAZU GC-2014 with GL Sciences InertCap
5
MS/Sil capillary column (0.25 mm LD. × 30 m, df = 0.25 µm).
4
4
.2. Preparation of iron complexes
iPr
.2.1. Preparation of Fe(CNC )I (3a)

To a solution of LiHMDS (267.8 mg, 1.6 mmol) in THF (10 mL) was added 1a (334.5
mg, 0.5 mmol). After being stirred at room temperature for 1 h, a solution of FeI (thf)
2
2
(226.9 mg, 0.5 mmol, 1.0 eq.) in THF (15 mL) was added. The brown solution was
stirred at room temperature for 12 h. After removal of the solvent in vacuo, the residue
was washed with n-hexane. The crude product was extracted with toluene and the
extract was filtered through a pad of Celite. Crystallization of the concentrated solution
at room temperature gave red crystals of 3a (102.9 mg, 0.172 mmol, 34%). To the
residue obtained by extraction with toluene, THF was added to give a green suspension.
After the filtration of the suspension, recrystallization from the resulting green solution
1
yielded 4a. 3a: Mp. 129 º (dec.); H MMR (400 MHz, THF-d ): δ = 73.88, 53.37, 42.49,
8
3
0.66, −1.63, −34.39, −62.38 ppm; µeff = 5.0 µ (Evans method, THF-d ); elemental
B 8
analysis calcd for C H FeIN ·C H : C 57.66, H 5.57, N 10.19; found: C 57.73, H 5.41,
2
6
30
5
7
8
N 10.12.
iPr
4
.2.2. Preparation of Fe(CNC )[N(SiMe ) ] (5a)
3 2
To a solution of LiHMDS (160.6 mg, 0.96 mmol) in Et O (6 mL) was added 1a (200.7
2
mg, 0.30 mmol). After being stirred at room temperature for 17 h, the solvent was
removed in vacuo. The residue was suspended in toluene and a solution of
Fe[N(SiMe ) ] (113.0 mg, 0.30 mmol) in toluene was added. The orange suspension
3
2 2
was stirred at room temperature for 34 h. Insoluble materials was removed by
centrifugation and the solvent was removed in vacuo. The residue was washed with
n-hexane and was extracted with toluene. The extract was filtered through a pad of
Celite and the solvent was removed in vacuo. Crystallization of the crude product with a
mixture of toluene and n-pentane gave yellow crystals of 5a (58.6 mg, 0.093 mmol).

1
Mp. 127 º (dec.); H MMR (400 MHz, THF-d ) δ = 74.00, 69.81, 61.36, 55.47, 44.34,
8
3
2.02, 29.66, 23.26, 20.14, 18.55, 12.55, -4.19, -10.87, -13.99, -82.75 ppm; µ = 5.0 µ
eff B
(
Evans method, THF-d ); elemental analysis calcd for C H FeN Si : C 61.13, H 7.69,
8 32 48 6 2
N 13.37; found: C 61.14, H 7.74, N 13.40.
Me
4
.2.3. Preparation of Fe(CNC )[N(SiMe ) ] (5b)
3 2
To the solution of LiHMDS (80.3 mg, 0.48 mmol) in Et O (6 mL) was added 1b (92.0
2
mg, 0.15 mmol). After being stirred at room temperature for 17 h, the solvent was
removed in vacuo. The residue was suspended in toluene and a solution of
Fe[N(SiMe ) ] (56.5 mg, 0.15 mmol) in toluene was added. The orange suspension was
3
2 2
stirred at room temperature for 20 h. Insoluble materials in toluene was removed by
centrifugation and the solvent was removed in vacuo. The residue was washed with
n-hexane and was extracted with toluene. The extract was filtered through a pad of
Celite and the solvent was removed in vacuo. Crystallization of the crude product with a
mixture of toluene and n-pentane gave yellow crystals of 5b (70.6 mg, 0.12 mmol). Mp.
1
1
39 º (dec.); H MMR (400 MHz, THF-d ) δ = 62.39, 45.23, 30.32, 23.38, 17.11, -26.27
8
ppm; µeff = 4.7 µB (Evans method, THF-d ); elemental analysis calcd for
8
C H FeN Si : C 58.72, H 7.04, N 14.68; found: C 58.74, H 6.69, N 14.61.
28
40
6
2
4
.3. General procedure of catalytic hydrogenation of alkenes.
All alkenes were dried by CaH and purified by distillation. Alkene (1.0 mmol) was
2
added to the solution of iron complex (0.10 mmol) and additive in THF (1.0 mL). The
mixture was transferred to a 5 mL autoclave. Then, H gas was introduced to the
2
autoclave. After the reaction, internal standard (n-dodecane) was added to the reaction

mixture, which was analyzed by GC.
4
.4. X-ray diffraction analysis.
Details of the crystal data and collection parameters are listed in Table S4. The crystals
were coated with immersion oil and put on a MicroMountTM (MiTeGen, LLC), and
then mounted on diffractometer. Diffraction data were collected on a Rigaku XtaLAB
Synergy-R diffractometer equipped with a HyPix-6000HE HPC detector using MoKα
[
38]
radiation. The Bragg spots were integrated using CrysAlisPro program package.
Absorption corrections were applied. All the following procedure for analysis were
[
39]
performed using Yadokari-XG 2009
as a graphical interface. The structure was
[
40]
solved by a direct method with programs of SHELEXT and refined by a full-matrix
[
40]
least squares method with the program of SHELXL-2018.
Anisotropic temperature
factors were applied to all non-hydrogen atoms. The hydrogen atoms were put at
calculated positions and refined applying riding models. Deposition Number CCDC
1
982151 (3a), 1982152 (4a), 1982153 (5a) and 1982154 (5b) contain the
supplementary crystallographic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre.
4
.5. DFT calculations.
[
41]
Calculations were performed with the Gaussian 16 program package. Geometries of
[42]
the ground state and the transition state were optimized by using B3PW91-D3
[
43]
functional with D3 dispersion correction in gas phase. Lanl2dz
with the effective
core potentials was used for Fe and the 6-31G(d) basis set was used for C, H, N and Si
atoms (BS1). The vibrational frequencies and zero-point energy were calculated at the

same level of theory. Transition states was confirmed by intrinsic reaction coordinate
IRC) computation to connect to relevant minima. Single-point energies for each
(
[
44]
optimized geometry were performed with the SDD (Fe) and 6-311+G(d,p) (C, H, N,
and Si) basis sets (BS2). The solvent effect was evaluated using the conductor-like
[45]
polarizable continuum model (CPCM) for all calculations of single-point energies.
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research from JSPS
(KAKENHI 2717H01191). Financial sup-port was also received from JST CREST
1
4529307. Theoretical calculations were carried out using resources of the Re-search
Center for Computational Science (Okazaki, Japan) and the Information Technology
Center of Nagoya University (Nagoya, Japan).
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