Reversible Hydride Transfer to N,N′-Diarylimidazolinium Cations from Hydrogen Catalyzed by Transition Metal Complexes Mimicking the Reaction of [Fe]-Hydrogenase

Article abstract of DOI:10.1021/acs.inorgchem.7b00806

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H₂ to N⁵,N¹⁰-methenyltetrahydromethanopterin (CH-H₄MPT⁺). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N′-diphenylimidazolinium cation (1⁺) as a structurally related alternative to CH-H₄MPT⁺. In connection with the enzymatic mechanism via heterolytic cleavage of H₂ at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF₄] to N,N′-diphenylimidazolidine (2) was achieved under 1 atm H₂ at ambient temperature in the presence of an equimolar amount of NEt₃ as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1⁺. The reverse reaction was accomplished by treatment of 2 with HNMe₂Ph⁺ as the proton source, where [(ν⁵-C₅Me₅)Ir{(p-MeC₆H₄SO₂)NCHPhCHPhNH}] was found to catalyze the formation of 1⁺ and H₂ with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using the above Ir amido catalyst, the reversible hydride transfer interconverting 1⁺/H₂ and 2/H⁺ was performed successfully. This system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1⁺ and 2 bearing one para-substituted N-phenyl group.

Full text of DOI:10.1021/acs.inorgchem.7b00806

Article
pubs.acs.org/IC
Reversible Hydride Transfer to N,N′‑Diarylimidazolinium Cations
from Hydrogen Catalyzed by Transition Metal Complexes Mimicking
the Reaction of [Fe]-Hydrogenase
Masahiro Hatazawa,† Naoko Yoshie,† and Hidetake Seino*,‡
†Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan
‡Faculty of Education and Human Studies, Akita University, Tegata-Gakuenmachi, Akita 010-8502, Japan
S
* Supporting Information
ABSTRACT: [Fe]-hydrogenase is a key enzyme involved in
methanogenesis and facilitates reversible hydride transfer from
H2 to N5,N10-methenyltetrahydromethanopterin (CH-
H4MPT+). In this study, a reaction system was developed to
model the enzymatic function of [Fe]-hydrogenase by using
N,N′-diphenylimidazolinium cation (1+) as a structurally
related alternative to CH-H4MPT+. In connection with the
enzymatic mechanism via heterolytic cleavage of H2 at the
single metal active site, several transition metal complex
catalysts capable of such activation were utilized in the model
system. Reduction of 1[BF4] to N,N′-diphenylimidazolidine (2) was achieved under 1 atm H2 at ambient temperature in the
presence of an equimolar amount of NEt3 as a proton acceptor. The proposed catalytic pathways involved the generation of
active hydride complexes and subsequent intermolecular hydride transfer to 1+. The reverse reaction was accomplished by
treatment of 2 with HNMe2Ph+ as the proton source, where [(η5-C5Me5)Ir{(p-MeC6H4SO2)NCHPhCHPhNH}] was found to
catalyze the formation of 1+ and H2 with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the
forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using
the above Ir amido catalyst, the reversible hydride transfer interconverting 1+/H2 and 2/H+ was performed successfully. This
system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with
proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1+
and 2 bearing one para-substituted N-phenyl group.
at the dinuclear catalytic sites and transfer electrons through the
iron−sulfur cluster chains.5
INTRODUCTION
■
Methanogenesis is an energy-yielding metabolic process in
methanogenic archaea, accompanied by the production of
methane.1 One of the major methanogenic pathways exploits
the reaction of CO2 with H2, which comprises of a series of
enzymatic reaction steps involving some organic cofactors. As
shown in Scheme 1, tetrahydromethanopterin (H4MPT) is one
of the essential cofactors employed in this system and functions
as a one-carbon carrier, on which the formyl carbon is bound
and reduced in two steps to the methyl level. Metabolism of H2
is mediated by different subtypes of [NiFe]-hydrogenase in
methanogenesis, but some of their roles can be substituted by
[Fe]-hydrogenase under Ni-limiting growth conditions in some
methanogens.2 [Fe]-hydrogenase catalyzes the reversible
hydride transfer from H2 to methenyl-H4MPT+ to produce
methylene-H4MPT and a proton.3 Crystallographic and
spectroscopic analyses of [Fe]-hydrogenase revealed a unique
monoiron center ligated by two CO, a guanylylpyridinol
moiety, and cysteinyl residue (FeGP cofactor, Figure 1).4 These
functional and structural features are in contrast to [NiFe]- and
[FeFe]-hydrogenases, which split H2 into protons and electrons
The iron center of the FeGP cofactor is proposed to be
divalent and has a pseudo-octahedral geometry with an open
site trans to the acyl ligand (occupied by H2O in the crystal
structure).4 This site is thought to coordinate H2 for heterolytic
splitting, and the resulting hydride is transferred to methenyl-
H4MPT+, which is located in the proximity but not directly
binding to the iron. As for the proton, computational6−8 and
experimental3c studies support the mechanism, in which the
hydroxy group of the 2-pyridinol ligand is deprotonated prior
to H2 cleavage to serve as the internal proton acceptor. As an
experimental approach to provide an insight into the enzymatic
mechanism, a number of iron complexes have been synthesized
as structural models for the FeGP cofactor since the elucidation
of its detailed structure.9,10 However, studies on reactivity of
such model complexes toward the substrates relevant to [Fe]-
hydrogenase have arisen lately. The groups of Shima and Hu
demonstrated that some of the synthetic mimics can be used to
Received: March 28, 2017
© XXXX American Chemical Society
A
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Scheme 1. Selected Reaction Steps in the Methanogenic Pathway Utilizing CO2 and H2
metal catalysts (Figure 2). The reverse conversion was also
carried out via the combination of the proton source with N,N′-
Figure 1. FeGP cofactor of [Fe]-hydrogenase.
reconstitute active [Fe]-hydrogenase from its apoenzyme.3c In
the case of the model complex alone, even the heterolytic
activation of H2 molecules was recently achieved, using the iron
complex bearing a pyridine−acyl unit and a PNP ligand.10h The
amine moiety in the PNP ligand served as a proton acceptor,
and some catalytic turnover in hydrogenation of an aldehyde
was demonstrated. Rose and co-workers developed the iron
complex having the fac-C(acyl),N,S coordination motif
supported by an anthracene scaffold, which could heterolyze
H2 (or D2) to form an iron−hydride (or deuteride) species.10i
By use of this complex, they also conducted catalytic hydride
abstraction from 1,2,3-triarylimidazolidine, a biomimetic model
for methylene-H4MPT.
Despite the above example, investigation into the model
reactions using certain molecules resembling the organic
substrates of [Fe]-hydrogenase are still limited, even if
nonbiomimetic catalysts are included.11−13 Such attempts are
sometimes informative to elucidate the functions of enzy-
mes.9c,14 Corr et al. reported that a bicyclic amidinium dication
is hydrogenated with H2 (52 psi) and Pd on carbon, resulting in
the ring-opening via C−N bond cleavage.12 Kalz et al. examined
heterolytic splitting of H2 using a combination of Lewis basic
Ru complexes and 1,2,3-triarylimidazolinium salts as proton-
and hydride-acceptors, respectively.13 The imidazolinium
cations in the latter study were structurally related to
methenyl-H4MPT+, except for the substituent at the C-2
position to avoid some side reactions, and the same molecular
design was applied in the Rose’s system performing the reverse
reaction.10i Because the reaction center of methenyl-H4MPT+ is
the C-2 atom of the imidazolinium ring, C-2 unsubstituted
models should exhibit comparatively similar reactivity. In this
study, we selected N,N′-diphenylimidazolinium (1+) as the
representative model substrate and investigated hydride transfer
to 1+ from molecular H2 by using homogeneous transition
Figure 2. Reversible reaction catalyzed by [Fe]-hydrogenase and
model reactions designed in this study.
diphenylimidazolidine (2), the molecule corresponding to
methylene-H4MPT. Although a related approach has been
previously applied in a density functional theory study,15
experimental confirmation has not yet been undertaken.
While a multiscale modeling of the [Fe]-hydrogenase/
methenyl-H4MPT+ complex proposed the mechanism that
hydride is transferred to methenyl-H4MPT+ from the iron-
coordinated H2 without forming a hydride ligand,7b synthetic
functional models for the FeGP cofactor were hitherto
hypothesized to operate via the iron−hydride intermediate-
s.10h,i Formation of the metal−hydride intermediate in
heterolysis of H2 by transition metal catalysts is widely
accepted.16 Our investigations have focused on the catalyst
precursors, which split coordinated H2 heterolytically and
transfer the resulting reactive hydride to substrates in an
intermolecular manner (Figure 3). The arene−Ru amido
complex [(η6-p-cymene)Ru((R,R)-Tsdpen−H)] (3; Tsdpen−
H = (p-MeC6H4SO2)NCHPhCHPhNH) and its cyclopenta-
dienyl−Ir analogue [Cp*Ir((R,R)-Tsdpen−H)] (4; Cp* = η5-
C5Me5) are representative bifunctional catalysts, in which the
amido nitrogen atom acts as the internal basic site. These
complexes are effective for the hydrogenation of polar
unsaturated bonds such as CO and CN.17,18 Another Ir
B
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solubility of 1[BF4] in these solvents, all reaction mixtures
formed suspensions in the initial stage, while product 2
dissolved well in THF but sparingly in iPrOH.
Catalyst 3 was moderately active in iPrOH solvent at 35 °C,
but the conversion rate decreased considerably at 25 °C
(entries 1, 2). Higher activity than 3 was observed for the Ir
analogue 4 and another Ir complex 5, which achieved
satisfactory conversions at 25 °C (entries 3, 4). The fastest
catalytic rate was exhibited by 6 in THF solvent (approximately
4-fold faster than 4 and 5; entries 5, 6). Although we
anticipated that the added base would deprotonate 1+ at the 2-
C position to generate the N-heterocyclic carbene species, the
experimental results confirmed that all consumed 1+ was
exclusively converted into 2 without any such side reactions.
NMR analyses of nonvolatile materials in the reaction mixture
revealed that nearly equimolar amounts of 2 and triethylam-
monium salt were produced as a result of heterolytic H2
cleavage.
It has been previously shown that 1,3-disubstituted
imidazolinium cations are reduced to the corresponding
imidazolidines by relatively strong reducing agents such as
Zn/HCl,21 NaBH4,22 or LiAlH4.22,23 The results in this study
revealed that such conversion can be performed under very
mild conditions by the catalytic reaction using H2 as a reducing
agent.24 To gain an insight into the mechanisms of the catalysis,
the effects of proton acceptors and solvents were further
investigated for the representative catalysts 4 and 6.
Figure 3. Representative catalyst precursors examined in this study.
complex 5 possessing a C−N chelate has been reported to
catalyze the reversible reduction of NAD+.19 The Rh benzene-
dithiolato complex [TpMe2Rh(1,2-S2C6H4)(MeCN)] (6; TpMe2
= hydrotris(3,5-dimethylpyrazol-1-yl)borate) exhibits salient
chemoselectivity in the hydrogenation of CN bonds.20
Both 5 and 6 have been established to generate active hydride
intermediates under ambient conditions through proton release
from the coordinated H2.
RESULT AND DISCUSSION
■
Catalytic Hydride Addition to N,N′-Diphenylimidazo-
linium 1+. Hydrogenation experiments of 1[BF4] were
conducted under atmospheric pressure of H2. Because hydride
transfer to 1+ was conjugated with proton release, this reaction
proceeded only in the presence of an adequate proton acceptor.
Triethylamine was found to be suitable for this purpose (vide
infra), and an equimolar amount to 1+ was used in the
reactions. Activity of each catalyst complex (1 mol %) was
examined in iPrOH and THF, and the reaction medium giving
higher conversion was collected in Table 1 (see Supporting
Information Table S1 for full data). Because of the poor
Effects of Bases and Solvents on Hydride Addition
Catalyzed by Ir Amido Complex 4. Conversion of 1[BF4]
into 2 catalyzed by 4 was examined under various conditions as
listed in Table 2. For the reactions under a H2 atmosphere in
iPrOH, essentially no reaction took place in the absence of
basic additives (entry 1). It was confirmed that the amount of
base limited the conversion (entry 2), indicating a demand for a
Table 2. Examination of Reaction Conditions in Hydride
a
Addition to 1[BF4] Catalyzed by 4
b
entry
base
solvent
yield of 2 [%] (time/h)
H2 atmosphere
Table 1. Comparison of Catalysts for Hydride Addition to
1[BF4] Utilizing H2
a
1
none
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
EtOH
1 (24)
c
2
NEt3
50 (8)
3
NEt3
Lut
21 (0.5), 92 (2)
d
4
31 (2), 84 (24)
e
5
NMe2Ph
NEt3
NEt3
NEt3
NEt3
7 (24)
6
65 (0.5), 100 (2)
0 (2)
f
7
t-BuOH
THF
8
12 (2)
9
CH2Cl2
5 (2)
N2 atmosphere
10
11
12
NEt3
NEt3
Lut
iPrOH
EtOH
iPrOH
14 (0.5), 40 (2), 96 (24)
61 (0.5), 91 (2)
2 (24)
b
b
entry catalyst
solvent
yield of 2 [%]
recovery of 1[BF4] [%]
1
2
3
4
5
6
3
3
4
5
6
6
iPrOH
iPrOH
iPrOH
iPrOH
THF
32
86
68
14
8
c
a
Conditions: 1[BF4] (0.50 mmol), base (0.50 mmol), 4 (5 μmol),
92
solvent (5 mL), 1 atm, 25 °C, or otherwise stated. Lut = 2,6-
b
dimethylpyridine. Determined by 1H NMR spectroscopy. The
97
3
100
94
0
consumed amount of 1[BF4] was equivalent to the yield of 2 for
every reaction except where indicated. Data of different reaction times
d
THF
6
c
were obtained from independent reaction batches. Conducted with
a
d
Conditions: 1[BF4] (0.50 mmol), NEt3 (0.50 mmol), catalyst (5
0.50 equiv of NEt3. Recovery of 1+ was 47%. In addition to unreacted
e
μmol), solvent (5 mL), H2 (1 atm), 25 °C, 2 h, or otherwise stated.
1+ (5%), uncharacterized byproducts were observed. In addition to
b
c
Determined by 1H NMR spectroscopy. Conducted at 35 °C.
unreacted 1+ (79%), uncharacterized byproducts were observed.
d
f
Reacted for 30 min.
Conducted at 35 °C.
C
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Scheme 2. Plausible Mechanisms for Hydride Transfer to 1+ Mediated by Ir Catalyst 4
stoichiometric amount of proton acceptor. The reaction rates
and proportions were revealed to depend on the proton-
accepting ability of bases (entries 3−5). The reaction using 2,6-
dimethylpyridine (Lut) proceeded more slowly than that with
NEt3, while the use of NMe2Ph resulted in very poor
conversion. These results are consistent with the order of pKa
units for the conjugate acids of these bases (14.9, 9.5, and 7.2,
for NEt3, Lut, and NMe2Ph, respectively, in THF).25
Furthermore, catalytic rates were considerably affected by
solvents. The reaction under H2 using NEt3 was somewhat
faster in EtOH than in iPrOH (entry 6 vs 3) but very slow in t-
BuOH, THF, and CH2Cl2 (entries 7−9). These solvent effects
indicate that EtOH and iPrOH may act as hydride donors, and
in fact, 4 is well-known to catalyze transfer-hydrogenation,
which utilizes primary and secondary alcohols as hydrogen
sources.18,26 To verify this hypothesis, the above reactions in
EtOH and iPrOH were carried out under nitrogen gas. The
conversion rates in EtOH exhibited no significant differences
between H2 and N2 atmosphere, indicating that the reactions
proceeded majorly via a transfer-hydrogenation mechanism in
this solvent (entry 6 vs 11). As for the reactions in iPrOH,
turnover frequency partly decreased under N2, while final
conversions were almost equal to each other (entry 3 vs 10). In
contrast, the reaction utilizing Lut as the proton acceptor only
minimally proceeded in the absence of H2 (entry 12). This
suggests that the combination of iPrOH with Lut exerts weaker
hydride-donating ability than the sets of H2−Lut or iPrOH−
NEt3.
solution of A resulted in the immediate disappearance of the
hydride signal and formation of 2 (equimolar amount to A).29
There are a number of possible routes regenerating A in the
catalytic cycle. Complexation of the coordinatively unsaturated
B with H2 forms the intermediate C, and the subsequent
deprotonation from the η2-H2 ligand by an external base
produces the hydride complex A. Conversion of B to A has
been reported to occur rapidly at 1 atm H2 in the presence of
an appropriate base such as NEt3.28,30,31 In contrast, the direct
action of the base on B causes proton elimination from the
amine ligand to give 4,30 which is convertible into A through
heterolysis of H2 at the Ir−amide moiety. It was previously
shown that direct addition of molecular H2 to 4 in aprotic
solvents is very slow and is accelerated by a catalytic amount of
acid because the pathway via species B and C becomes
accessible. In THF or CH2Cl2 solution, most metal species are
considered to be in the resting state of 4 via deprotonation of B
in the presence of excess base (NEt3/cat ∼ 100 in the initial
stage), as is indicated from the reddish purple color
characteristic of 4. Slow regeneration of the active species A
from 4 in these solvents may explain the low catalytic turnovers,
and thus the use of preformed A instead of 4 as the catalyst
precursor did not result in any improvement.
In contrast, some alcoholic solvents have been found to
effectively assist the addition of H2 to 4, which has been
proposed to occur via proton relay mechanisms involving the
amido ligand, H2, and an alcohol molecule (transition state
D).32 In addition, 4 can eliminate H2 from an alcohol molecule
to form A via the concerted transition state E as established for
the mechanisms of transfer hydrogenation.16,26 Competition
between H2-hydrogenation and transfer-hydrogenation in
alcoholic solvents has been observed for certain asymmetric
ketone reduction systems catalyzed by 4 and the analogue
[Cp*Ir((S,S)-Msdpen−H)] (4′; Msdpen−H = (MeSO2)NCH-
PhCHPhNH).32d When the reduction of 1+ is conducted at 1
atm H2 in iPrOH solvent, both pathways utilizing H2 or alcohol
as the hydrogen source presumably operate in parallel to
generate A. The extent of each hydrogenation route is roughly
estimated by comparing the catalytic turnovers in the presence
and the absence of hydrogen gas, and the rates of two routes do
not appear to be very different from each other under 1 atm H2
(Table 2, entry 3 vs 10); however, the rate of H2-hydrogenation
The catalytic function of 4 in the hydride transfer to 1+ may
be relevant to that in the hydrogenation of imines, which has
been previously established to proceed via ionic mechanisms.
Previous studies utilizing 4 and analogous bifunctional catalysts
showed that the imine substrate is preactivated by protonation
or coordination of Lewis-acidic metal ions and that hydride is
transferred to the resulting iminium carbon from the hydride
complex derived from 4 and H2 (structure A in Scheme 2).27,28
Analogous to this mechanism, hydride transfer to 1+ is thought
to proceed via the routes shown in Scheme 2. The key metal
species is the hydride complex A, from which the hydride is
transferred to 1+ without coordination to the metal center to
produce 2 and a cation B. This step was supported by NMR
analysis, as the addition of 1[BF4] (1.3 equiv) to a CD2Cl2
D
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is expected to increase depending on H2 pressure. When the
proton acceptor is Lut, the reaction takes place solely with the
uptake of H2 molecules, likely because of thermodynamic
disadvantages in dehydrogenation of alcohol to produce ketone
concomitantly with the relatively strong acid HLut+. In this
case, the apparent contribution of iPrOH is to assist heterolytic
cleavage of H2 by forming a hydrogen bonding network D.
Experiments using deuterated 2-propanols also provided
evidence for the above H2-hydrogenation and transfer-hydro-
genation pathways under H2. When reaction was conducted in
(CD3)2CDOD under 1 atm H2, deuterium was incorporated
only at the C-2 position of the produced 2. The isotopic
distribution of the 2-methylene group was CH2:CHD:CD2 =
42:47:11, and the existence of the CH2 isotopomer proved H
transfer from H2 (see Supporting Information Figures S1 and
S3 for the spectra). The same reaction in (CH3)2CDOH also
gave the deuterated product with the ratio CH2:CHD:CD2 =
83:17:0, supporting α-D abstraction from the solvent
(Supporting Information Figure S2). Although the ratios of
H-transfer from H2 to α-D-transfer from 2-propanol are
seemingly incompatible between these solvents, it is mostly
because of the exchange with the hydroxy H/D atom via the
reverse reaction of the Ir hydride/deuteride (A) with alcohol
molecule to form 4 via D.
produce the anionic hydride [Tp*RhH(1,2-S2C6H4)]− (F) and
+
HNEt3 . As illustrated in Scheme 3, the conversion of 1+ to 2 is
Scheme 3. Plausible Mechanisms for Hydride Transfer to 1+
Mediated by Rh Catalyst 6
Solvent Effects on Hydride Addition Catalyzed by Rh
Dithiolato Complex 6. The influence of bases and solvents
on hydride transfer to 1[BF4] utilizing precatalyst 6 is
summarized in Table 3. The effects of basic additives were
Table 3. Effects of Bases and Solvents on Hydride Addition
to 1[BF4] Catalyzed by 6
thought to proceed via the same fundamental route. Hydride
transfer from F to the methenyl carbon of 1+ presumably occurs
without coordination (transition state G). Generation of the
ionic hydride F may be retarded in nonpolar or protic media to
decrease catalytic turnover. The catalytic cycle proposed here is
essentially analogous to the left cycle in Scheme 2 and may be
applicable to the reaction catalyzed by 5.19
a
b
entry
base
NEt3
solvent
conversion [%]
1
2
THF
100
c
Lut
THF
21
d
3
NMe2Ph
NEt3
NEt3
NEt3
NEt3
THF
1
66
4
4
EtOH
EtOH
CH2Cl2
toluene
Catalytic Hydrogen Evolution from N,N′-Diphenylimi-
dazolidine 2 and Proton. After evaluating hydride transfer
from H2 to 1+, efforts to model the reversible catalysis of [Fe]-
hydrogenase focused on the reverse reaction to form H2,
utilizing 2 as the hydride donor. The equilibrium of the reaction
system can be shifted toward the reverse direction by adjusting
the proton concentration, and the required acidity was
reasonably estimated from the effects of the bases in the
forward reaction. Because the above hydride transfer process
proceeded to completion when NEt3 was used and reached
equilibrium when Lut was used, the proton donors should not
be weaker than HLut+ (pKa = 9.5 (THF))25 to drive hydride
elimination from 2. This hypothesis was tested by the reactions
using 4 as the catalyst precursor (Table 4, entries 1−3). When a
mixture of 2 and an equimolar amount of [HLut][BF4] (7) in
iPrOH was placed in a closed reaction vessel with 1 mol % 4
under vacuum (entry 1), 51% yield of 1+ was obtained after 20
h at 35 °C concomitantly with the evolution of H2 (0.28 equiv
to 2 charged). This deficient conversion rate was attributable to
the equilibration of the reaction system (0.10−0.12 atm H2 in
gas phase), as the yields were not greatly affected by catalyst
loading (entry 2). In place of 7, the use of a stronger acid
[HNMe2Ph][BF4] (8) effectively improved the conversion rate,
resulting in high yield of 1+ at 25 °C after 2 h (entry 3). The
reaction also produced H2 in a yield comparable to 1+, which
originated from the hydride in 2 and proton in 8 (up to 0.40
atm partial pressure of H2 in gas phase). It was confirmed that
e
5
6
7
98
24
a
Conditions: 1[BF4] (0.50 mmol), base (0.50 mmol), 6 (5 μmol),
b
solvent (5 mL), 1 atm H2, 25 °C, 2 h, or otherwise stated. NMR yield
of 2 (confirmed to be equivalent to the consumed amount of 1[BF4]
c
d
for every reaction). Yield after 24 h was 23%. Reacted for 24 h.
e
Conducted under N2 atmosphere at 50 °C.
compared for the reactions in THF (entries 1−3). While the
reaction using NEt3 rapidly reached complete conversion, use
of Lut resulted in termination at low yield (∼23%) likely owing
to the equilibration between reactants and products. Because
the reaction utilizing Lut and catalyst 4 showed relatively high
yield (84%) in iPrOH medium (vide supra), the solvents may
significantly affect equilibrium. In contrast to 4, 6 showed
higher catalytic activity in THF than in EtOH (entries 1 and 4)
and consumed only a small amount of EtOH as a hydrogen
source (entry 5). A good conversion rate was also achieved in
CH2Cl2 solvent (entry 6), while the activity was very low in the
nonpolar toluene medium (entry 7).
In our previous study, we proposed that 6 catalyzes the
hydrogenation of imines via ionic mechanisms, in which
protonated imine undergoes hydride transfer from the metal
hydride species.20 This catalytic pathway is supported by the
observation that 6 smoothly reacts with H2 and NEt3 to
E
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a
Table 4. Reactions of 2 with Proton in the Presence of Several Hydrogenation Catalysts
yield [%]
b
c
b
entry
catalyst
H+ source
solvent
temp [°C]
time [h]
1[BF4]
H2
recovery of 2 [%]
1
2
4
7
7
8
8
8
8
8
8
iPrOH
iPrOH
iPrOH
EtOH
THF
35
35
25
25
25
25
35
35
35
35
35
20
20
2
51
54
89
33
33
45
100
58
68
8
28
26
85
18
17
19
95
21
60
3
36
25
0
d
4
3
4
4
4
3
3
5
5
6
6
4
2
57
45
48
0
5
2
6
iPrOH
iPrOH
iPrOH
iPrOH
THF
2
7
2
8
1
0
e
9
HBF4
1
0
10
11
8
8
20
49
0
iPrOH
20
48
5
a
b
Standard conditions: 2 (0.50 mmol), H+ source (0.50 mmol), catalyst (5 μmol), solvent (5 mL), under vacuum, or otherwise stated. Determined
c
d
e
1
by H NMR spectroscopy. H2 in the gas phase determined by GC analyses. Conducted with 2.5 mol % catalyst loading. Aqueous HBF4 (0.50
mmol) was used.
Scheme 4. Plausible Catalytic Cycle for Generation of H2
H2 elimination from iPrOH was negligible under these reaction
conditions (0.6 equivalent to 4 after 24 h at 35 °C) from the
blank test in which 8 was reacted with 1 mol % 4 in iPrOH
without adding imidazolidine 2.
Rh catalyst 6 was less reactive and formed very little H2 even
after consumption of a fair portion of 2 (entries 10, 11).
Uncharacterized side products were observed by NMR
measurement in both cases, which require further investigation.
In conclusion, among the examined catalysts, the most efficient
for H2 evolution via hydride elimination from 2 was confirmed
to be 4, which exhibited high activity even at ambient
temperature.
As illustrated in Scheme 4, the reaction pathway catalyzed by
4 was thought to follow the reverse direction of the left cycle of
Scheme 2. NMR measurement in CD2Cl2 revealed that the Ir
species existed in the protonated form B in the presence of 8
(1−2 equiv). While 2 showed no interaction with 4 in CD2Cl2
at 25 °C, addition of 8 to this mixture resulted in gradual
increases in the NMR signals of 1+. During this change, the
metal species was observed as B, and any hydride species such
as A were not detectable. Lewis-acidic B may eliminate hydride
Because HNMe2Ph+ (pKa = 7.2 (THF))25 was found to be
sufficiently acidic to completely convert 2 to 1+ and H2,
equimolar reactions containing 2 and 8 were examined under
various conditions. The catalytic turnover of catalyst 4 was
greater in iPrOH solvent than in EtOH or THF (entries 3−5).
Although the activity of the analogous Ru catalyst 3 was slightly
lower than that of 4, sufficient conversion was observed at a
relatively high temperature (entries 6, 7). The reaction utilizing
catalyst 5 was accompanied by uncharacterized side reactions
and resulted in poor yields of both 1+ and H2 despite relatively
rapid consumption of 2 (entry 8). Using aqueous HBF4 as the
proton source had only a small influence on the yield of 1+,
while the evolution of H2 appreciably increased (entry 9). The
F
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Inorganic Chemistry
Article
from 2, and the resulting hydride complex A is immediately
protonated to give B and H2 via the intermediate C. Peris and
co-workers reported that the reaction of N,N′-dibenzylimida-
zolidine with [Cp*IrCl2(L)] (L = neutral 2e− donor) in the
presence of Ag(CF3SO3) at 45 °C produces N,N′-dibenzyl-
imidazolinium and the monohydride complexes [Cp*IrHCl-
(L)].33 The dinuclear complex [(Cp*IrCl)2(μ-Cl)2] also
abstracts hydride from various N,N′-dialkylimidazolidine
derivatives at room temperature to give [(Cp*IrCl)2(μ-H)2].
The isoelectronic half-sandwich complexes of Rh and Ru react
in a similar manner. For the catalytic system in this study, the
accelerated reaction rates in iPrOH medium may be explained
by its assistance in the proton transfer step or involvement in
the hydride elimination step to form an aggregating
intermediate, as shown in Scheme 4 (bottom-right corner).
Concerted nucleophilic attack of the alcohol molecule to the 2-
C possibly decreased the activation energy of hydride transfer
to the Ir center, as related adducts between imidazolinium and
alkoxide are known.34
Scheme 5. Sequential Addition and Elimination of H2
Catalytic hydrogen evolution as described above confirms
that 2 serves as an efficient hydride donor when activated by
appropriate catalysts. Including biologically essential NADH
and NADPH, a large number of N-heterocyclic compounds
have been identified as organo-hydride donors.35 Despite their
ability to transfer hydride to various acceptor molecules,
hydride transfer to proton leading to H2 formation is rarely
observed.36 Thorn et al. demonstrated H2 evolution utilizing
N,N′-dimethylbenzimidazoline and acetic acid as the hydride
and proton donors, respectively, at room temperature in the
presence of a metallic Pd catalyst.37,38 The reaction produces
the benzimidazolium cation, and derivatives with one aryl
group at the 2-position react in a similar manner. In the same
reaction system, Peris et al. demonstrated that N,N′-dialkyl-
imidazolidine (alkyl = methyl, benzyl) and some related N-
heterocycles can act as hydride sources.33 These N-heterocycles
have relatively high hydride-donating ability, making H2
evolution strongly exergonic, while also making the reverse
reaction difficult.39 Less hydridic NADH can function as a
reversible organic hydride in the H2 release/storage cycle, as
Fukuzumi and co-workers successfully performed H2 evolution
from NADH to give NAD+ and the reverse reaction by
switching the pH of the aqueous media.19 These reactions were
conducted under ambient conditions with 5 as the catalyst
precursor. Their and our experimental results confirmed that
the homogeneous transition metal catalysts that operate
through an ionic hydrogenation mechanism were effective for
this type of hydride transfer.40 It is indicated that 2 has a
hydride-donating ability comparable to NADH and can be
replenished with hydride under moderately basic conditions
(NEt3) and atmospheric H2 pressure.
catalyst precursor in this system because it can catalyze both the
forward and the reverse reactions with reasonable activities. For
the H2-addition step, a mixture of 1[BF4], NEt3 (1.0 equiv),
and 4 (0.01 equiv) in iPrOH was treated with 1 atm H2 at 25
°C for 17 h. After freezing the resulting mixture at −196 °C and
evacuation of the gas phase, the H2-elimination step was
performed by adding proton source 8, followed by stirring
under vacuum at 25 °C, to result in the release of 70% H2 per
charged 1[BF4] after 2 h. Although the conversion rate of 1+ in
the above H2-addition step was high (92% after 2 h, vide supra),
a part of hydride originated from iPrOH via H2 transfer.
Therefore, the reaction time must be extended to hydrogenate
the resulting Me2CO. Conducting the H2-addition step for only
2 h resulted in a decrease of released H2 (56% per 1[BF4]).
Effects of Substituted Phenyl Group in Imidazolinium
Cations and Imidazolidines. While H4MPT is a specific
coenzyme in methanogenesis, another biological C1 carrier,
tetrahydrofolate (H4folate), is ubiquitous in a wide range of
organisms (Figure 4).43 The structures of H4MPT (see Scheme
Sequential Addition and Elimination of H2. Organic
molecules, which can reversibly release and add H2, have gained
attention as potential materials for chemical storage of
hydrogen gas.41 Numerous studies have been carried out to
examine molecules containing releasable hydrogen such as
cycloalkanes, saturated heterocycles, alcohols, formic acid, etc.,
by using various catalysts. Compared with heterogeneous
catalysts, the use of homogeneous catalysts is still limited, but
these reactions proceed under relatively mild conditions.32e,42
To determine the feasibility of using 1+ as a molecular hydride
storage, sequential addition and elimination of H2 was carried
out for the reaction system combined with a proton donor/
acceptor (Scheme 5). Complex 4 was selected as the single
Figure 4. Structure of N5,N10-CH+-H4folate.
1) and H4folate are very similar; one of the differences in their
active sites is that two extra methyl groups are attached to the
carbon skeleton in H4MPT (C7 and C9 sites44). Another
significant difference is the para-substituent on the benzene
ring bound to the N10 atom, which is an electron-donating
alkyl chain in H4MPT and an electron-withdrawing amide
carbonyl in H4folate, giving H4MPT derivatives more negative
redox potentials than the corresponding H4folate derivatives
G
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Article
(e.g., −70 mV difference for CH+/CH2 couples). In connection
with the structural and functional divergences between these
natural coenzymes, reactivity was investigated for the series of
1+ and 2 analogues, one of whose phenyl groups had various
substituents (X) at the para-position (represented as 1X and
2X).
throughout the reaction course. Therefore, it is not appropriate
to compare the tendency of reactivity with method I conducted
in THF, which effectively dissolves the generated 2X. The
lowest conversion of 1CN[BF4] among the series of substrates
may be explained in part by binding of the cyano group to the
catalyst.28,30,45
+
Hydride addition to a series of imidazolinium salts 1X[BF4]
was examined under 1 atm H2 in the presence of 1 equiv of
NEt3 by using two catalytic systems (Table 5). The reactions
Hydride elimination from substituted imidazolidines 2X was
performed by utilizing 1 equiv of 8 and 1 mol % 4 at 25 °C
(Table 6). Substrates with strongly electron-withdrawing
Table 5. Catalytic Hydride Addition to Asymmetrically
Substituted Diphenylimidazolinium Salts Utilizing H2
Table 6. Hydride Elimination from Asymmetrically
Substituted Diphenylimidazolidines Catalyzed by 4
a
a
b
yield of 2X [%]
yield [%]
c
d
b
c
entry
substrate
1F[BF4]
X
method I
method II
entry
substrate
X
1X[BF4]
H2
1
2
3
4
5
6
7
8
9
F
100
98
60
57
34
68
54
22
92
45
40
1
2
3
4
5
6
7
8
2F
F
92
67
11
0
85
43
7
1Cl[BF4]
1Br[BF4]
Cl
Br
2Cl
Cl
100
100
79
2CF3
2COOMe
2CN
2
CF3
1CF3[BF4]
1COOMe[BF4]
1CN[BF4]
CF3
COOMe
CN
1
COOMe
CN
0
0
e
87
H
89
63
53
85
58
49
1[BF4]
H
94
97
2Me
2OMe
Me
1Me[BF4]
Me
OMe
ef
,
1OMe[BF4]
OMe
62
a
Common reaction conditions: 2X (0.50 mmol), 8 (0.50 mmol), 4 (5
a
b
μmol), iPrOH (5 mL), 25 °C, 2 h, under vacuum. Determined by 1H
Common reaction conditions: 1X[BF4] (0.50 mmol), NEt3 (0.50
c
mmol), catalyst (5 μmol), solvent (5 mL), H2 (1 atm), 25 °C, or
NMR spectroscopy. H2 in the gas phase determined by GC analyses.
b
c
otherwise stated. Determined by 1H NMR spectroscopy. Catalyst 6,
THF solvent, 0.5 h, or otherwise stated. Yield reached 100% after 2 h
d
groups were poorly converted (entries 3−5), which was
consistent with the prediction that these substituents electroni-
cally destabilize the cationic forms. While the reactivity of F-
substituted 2F was similar to that of 2, reactions of other
substrates (entries 2, 7, 8) showed relatively poor conversion,
regardless of the electronic nature of the substituents.
Therefore, additional factors may determine reactivity such as
solubility, proton acceptability, and tolerance of the catalyst to
functional groups. Nevertheless, a number of imidazolinium
derivatives were confirmed to allow reversible hydride addition
and elimination associated with the uptake and release of H2 by
altering the proton concentration.
for every entry except 6 and 9. Catalyst 4, iPrOH solvent, 2 h.
e
f
Conducted for 2 h. 67% yield after 24 h.
with catalyst 6 in THF (method I) were effective for converting
a wide range of substituted derivatives. Halo- and trifluor-
omethyl-substituted imidazoliniums were converted more
rapidly than the original 1[BF4] (entries 1−4 vs 7). Although
electron-withdrawing COOMe and CN groups were expected
to accelerate the incorporation of hydride, the conversion rates
of these derivatives were rather slow (entries 5, 6). The
presence of excess nitrile has been known to retard the catalysis
of 6 due to coordination to the site of hydrogen activation.20b
Using method I with S/C = 100, conversion into the
corresponding imidazolidine 2X was completed within 2 h for
all examined substrates except for the cyano and methoxy
CONCLUSION
■
In this study, we used N,N′-diphenylimidazolinium cation 1+ as
a mimic of methenyltetrahydromethanopterin (CH-H4MPT+),
the substrate for [Fe]-hydrogenase, and investigated its
reversible reduction with H2 into N,N′-diphenylimidazolidine
2 and a proton. Nonbiological noble metal complexes that
activate H2 heterolytically were examined in this model system.
Hydride addition to 1+ utilizing H2 is practicable by conjugation
with proton trapping, and accordingly, the conversion ratio is
influenced by basic additives, whose basicity must not be less
than lutidine to advance the reaction. We confirmed that NEt3
is a suitable base, as it led to complete conversion into 2 with a
derivatives: reaction of the latter converged at 1OMe+/2OMe
=
33/67.
In contrast, transformation of any substituted 1X[BF4] using
4 as the catalyst precursor in iPrOH (method II) resulted in
lower conversion than that of nonsubstituted 1[BF4]. The
remaining substrate was unchanged, and no appreciable side
reactions such as hydrogenation at the functional groups were
observed under the applied conditions (1 atm H2, 25 °C).
Because a series of 1X[BF4] and 2X are both severely insoluble
in iPrOH at 25 °C, all reactions formed heterogeneous mixtures
H
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Smart iTR with a ZnSe crystal and were recorded over a wavenumber
range of 600−3800 cm−1 with 2 cm−1 resolution. Elemental analyses
were performed at The Microanalytical Laboratory, Department of
Chemistry, Graduate School of Science, The University of Tokyo. GC
analyses were carried out on a Shimadzu GC-14B gas chromatograph
equipped with a TCD and stainless columns filled with molecular
sieves 5A (mesh 60−80, 3.0 mm × 2.0 m) by using argon as the carrier
gas.
stoichiometric amount and caused no appreciable deprotona-
tion of 1+ to form the N-heterocyclic carbene. The examined
catalysts exhibited satisfactory turnovers under ambient
temperature and pressure. It is likely that all of these catalysts
transfer the metal-bound hydride intermolecularly to the C-2
atom of the imidazolinium ring; this key step parallels one of
the proposed mechanisms of [Fe]-hydrogenase reaction. Our
bioinspired catalytic system supports that CH-H4MPT+ is a
good acceptor of hydride, which can be generated from H2 on
transition metals under mild conditions. However, it should be
kept in mind that the mechanisms of H2 activation by these
catalysts are not compatible with what occurs on the protein-
bound FeGP cofactor.
Synthesis of Materials. The following compounds were prepared
by reported procedures with some modifications.
N,N′-Diphenylimidazolinium Tetrafluoroborate (1[BF4]).47 N,N′-
Diphenylethylenediamine (4.26 g, 20.1 mmol) and ammonium
tetrafluoroborate (2.46 g, 23.1 mmol) were placed in a dried 50 mL
Schlenk flask equipped with a magnetic stirring bar. The flask was
evacuated and refilled with nitrogen gas three times. Under a stream of
nitrogen, triethyl orthoformate (30 mL, 180 mmol) was added, and
this mixture was refluxed for 5 h. After cooling to room temperature,
the pale yellow precipitate was filtered off and repeatedly recrystallized
three times from acetonitrile/EtOH to give colorless needles (2.85 g,
46% yield).
The reverse conversion (2 to 1+) is promoted under opposite
conditions: a stoichiometric amount of HNMe2Ph+ was added
as a proton source in the representative experiments in this
study. In contrast to the forward reaction (1+ to 2), significant
differences in activity and selectivity were observed between the
catalysts. The most efficient catalyst was the Ir complex 4,
which produced 1+ and H2 with high selectivity and moderate
turnover frequencies at room temperature. Its Ru analogue 3
exhibited the same selectivity but relatively slow reaction rates,
similar to the forward reaction. Unexpectedly, the most active
catalyst for the forward reaction in this study, 6, promoted
hydride elimination from 2 but evolved minimal H2. A similar
tendency was observed for 5, and the existence of side reactions
consuming the hydride was suspected. Additionally, reactions
were accelerated in alcoholic media regardless of catalysts. This
is in contrast with the catalyst-dependent solvent effects on the
hydride transfer to 1+, which is ascribed to the H2 activation
mechanism. The assistance of alcohol molecules in the hydride
elimination step is postulated (Scheme 4), and these problems
will be investigated in future studies. Finally, by using a complex
catalyst 4, a catalytic system utilizing H2 to reversibly reduce 1+
into 2 was successfully developed. This reaction model focusing
on the substrates of [Fe]-hydrogenase (CH+/CH2-H4MPT and
H2) may provide information regarding the roles of the
H4MPT cofactors.
2,6-Lutidinium Tetrafluoroborate (7).48 2,6-Lutidine (2.00 mL,
17.2 mmol) was added dropwise to a mixture of HBF4·Et2O (2.00 mL,
14.5 mmol) and diethyl ether (20 mL) with vigorous stirring. A white
precipitate was immediately formed, which was filtered and repeatedly
washed with diethyl ether. Recrystallization of the remaining solid
from dichloromethane (30 mL) and diethyl ether (100 mL) resulted
in moderately hygroscopic colorless crystals (2.64 g, 93% yield).
N,N-Dimethylanilinium Tetrafluoroborate (8).48,49 A dried
Schlenk flask equipped with a magnetic stirring bar was evacuated
and refilled with nitrogen gas three times. Under a stream of nitrogen,
25 mL of diethyl ether and N,N-dimethylaniline (2.8 mL, 22 mmol)
were added to the flask. This solution was stirred with cooling at 0 °C,
and HBF4·Et2O (3.0 mL, 22 mmol) was added dropwise. The
colorless solid that had immediately deposited was crushed with a
spatula, and the resulting mixture was gradually warmed to ambient
temperature with stirring. After 1 h, the white precipitate was collected
by filtration and repeatedly washed with diethyl ether. The solid was
dissolved in dichloromethane (20 mL), and diethyl ether (60 mL) was
layered on the filtered solution. After standing for 5 days, deposited
colorless crystals were separated by filtration and dried in vacuo (2.64
g, 58% yield).
Hydride Addition Reaction. In a dry Schlenk flask (internal
volume of 33−38 mL) equipped with a magnetic stirring bar,
imidazolinium tetrafluoroborate (1X[BF4], 0.500 mmol) was placed.
The flask was repeatedly evacuated and refilled with nitrogen gas three
times. Under a counterflow of nitrogen gas, solvent (5 mL) and
organic base (0.50 mmol) were added through a syringe. The flask was
sealed with a glass stopcock and degassed by two freeze−pump−thaw
cycles using liquid nitrogen. As the mixture was kept frozen in a liquid
nitrogen bath, the catalyst precursor was added to the flask under a
counterflow of nitrogen. The flask was evacuated again at liquid
nitrogen temperature and connected to a balloon filled with pure
hydrogen gas. The reaction mixture was allowed to stir at a specific
temperature and time. If the reaction was conducted in THF or
dichloromethane, the gas phase was replaced with N2 and triphenyl-
methane (ca. 0.5 mmol, accurately weighed) was added to the flask as
an internal standard, followed by the addition of acetonitrile (2 mL) to
form a homogeneous solution. If the solvent was alcohol or toluene,
hydrogen gas and solvent were removed under high vacuum before
adding triphenylmethane, and the mixture was dissolved completely by
using dichloromethane and acetonitrile (total ca. 5 mL) under a
nitrogen atmosphere. Approximately 0.5 mL of the solution was
sampled, dried by rotary evaporation, and analyzed by 1H NMR
spectroscopy in acetonitrile-d3.
EXPERIMENTAL SECTION
■
General Procedures. All the manipulations were performed using
standard Schlenk techniques. Toluene, THF, and diethyl ether were
purified by passing over columns of activated alumina and a supported
copper catalyst supplied by Hansen & Co. Ltd., or they were dried
using common procedures (CaH2 for toluene and NaH for others)
and distilled under nitrogen prior to use. Dichloromethane was dried
and distilled over P2O5 and degassed under nitrogen before use.
Dehydrated grade alcohols and acetonitrile were purchased from
Wako Pure Chemical Industries. The catalyst precursor [(η6-p-
cymene)Ru((R,R)-Tsdpen−H)] (3; Tsdpen−H = (p-MeC6H4SO2)-
NCHPhCHPhNH) was commercially available from Kanto Chemical
Co. and used as received. Other complexes [Cp*Ir((R,R)-Tsdpen−
H)] (4; Cp* = η5-C5Me5), [Cp*IrH((R,R)-Tsdpen)] (Tsdpen = (p-
MeC6H4SO2)NCHPhCHPhNH2),18b [Cp*Ir(4-(1H-pyrazol-1-yl-
κN2)benzoic acid-κC3)(H2O)]2SO4 (5),19 and [TpMe2Rh(1,2-S2-
C6H4)(MeCN)] (6; TpMe2 = hydrotris(3,5-dimethylpyrazol-1-yl)-
borate),20 were synthesized as described previously. N,N′-Diphenyl-
imidazolidine (2) was prepared by a previously described procedure46
and recrystallized from hot ethanol. Unless otherwise indicated, all
other reagents were obtained from commercial sources and were used
without further purification. NMR spectra (1H at 400 MHz and 13C at
100.5 MHz) were recorded on a JEOL ECP-400 spectrometer at 20
°C, and chemical shifts were referenced using those of residual solvent
resonances. IR spectra were obtained using a Thermo Scientific
Nicolet iS10 FTIR Spectrometer equipped with a Thermo Scientific
Hydride Elimination Reaction. A dry Schlenk flask of a known
internal volume (33−38 mL) was equipped with a magnetic stirring
bar, and imidazolidine (2X, 0.500 mmol) and solvent (5 mL) were
added under nitrogen. The flask was sealed with a glass stopcock, and
freeze−pump−thaw degassing manipulation was carried out twice.
While keeping the flask frozen at liquid nitrogen temperature, proton
I
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Article
reagent (0.500 mmol) and catalyst precursor were added under a
counterflow of nitrogen gas. The flask was evacuated at −196 °C, and
the reaction mixture was allowed to stir under the indicated
conditions. After the reaction, the flask was backfilled with pure
nitrogen gas to atmospheric pressure, and the gas phase was analyzed
by gas chromatography (three measurements using 100 μL sample gas
for each experiment). The partial pressure of H2 in the apparatus was
no more than 0.40 atm for each reaction. The amount of dissolved H2
estimated using the Henry’s constant was approximately 7 μmol at the
maximum, which is much lower than that in the gas phase.50
Thereafter, the organic materials were determined according to the
procedure similar to that used in the above hydride addition reaction.
Sequential Hydride Addition and Elimination Reactions. A
magnetic stirring bar and N,N′-diphenylimidazolinium tetrafluoro-
borate (1[BF4]; 155 mg, 0.500 mmol) were placed in a Schlenk flask
(33−38 mL internal volume accurately measured), and iPrOH (5 mL)
and triethylamine (70 μL, 0.50 mmol) were added under a nitrogen
atmosphere. After degassing the mixture by freeze−pump−thaw
cycles, [Cp*Ir((R,R)-Tsdpen−H)] (4; 3.5 mg, 5.0 μmol, S/C = 100)
was added to the frozen mixture under a nitrogen stream and the flask
was evacuated at liquid nitrogen temperature. A balloon filled with
pure hydrogen gas was connected to the flask, and the reaction mixture
was stirred for 17 h at 25 °C. The resulting mixture was frozen in a
liquid nitrogen bath, the gas phase was evacuated, and N,N-
dimethylanilinium tetrafluoroborate (8; 104 mg, 0.50 mmol) was
added under a counterflow of nitrogen. After evacuating the gas phase
at −196 °C, the contents of the flask were allowed to stir at 25 °C for
2 h. Quantitative analysis of evolved H2 and organic substances was
carried out as described above.
Synthesis of N-Aryl-N′-phenylimidazolidines. These com-
pounds were synthesized as described in the literature53 with some
modifications. Crude N-aryl-N′-phenyl-1,2-ethylenediamine prepared
as described above (up to 10 mmol) was dissolved in 50 mL of
methanol. Excess 37% aqueous formaldehyde (10 mL, 130 mmol) and
formic acid (0.5 mL, 13 mmol) were added to this solution. The
colorless solid precipitated immediately, and the resulting mixture was
stirred for 3 h at room temperature. The precipitate was filtered off
and recrystallized from hot ethanol, followed by vacuum drying.
Characterization data for these new compounds are included in the
Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00806.
Additional experimental data for hydride addition to
1[BF4] under H2 utilizing various catalysts and
deuterated solvents, and spectroscopic and microana-
lytical data of imidazolinium salts and imidazolidines
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: seino@gipc.akita-u.ac.jp. Fax: +81-18-889-2589.
■
ORCID
Synthesis of N-Aryl-N′-phenyl-1,2-ethylenediamines. These
compounds were prepared as described by Shafir et al.51 (Method A: X
= F, Cl, Me, OMe) or Wolfe et al.52 (Method B: X = Br, CF3,
COOMe, CN) with some modifications.
Masahiro Hatazawa: 0000-0002-4260-2830
Hidetake Seino: 0000-0002-8792-1974
Notes
Method A. To a Schlenk flask equipped with a magnetic stirring bar,
p-substituted iodobenzene (10 mmol), CuI (95 mg, 0.5 mmol), and
Cs2CO3 (6.5 g, 20 mmol) were added. When fluoroiodobenzene was
reacted, it was added later with other liquid reagents. The flask was
evacuated and refilled with nitrogen gas three times, and 2-
isobutyrylcyclohexanone (0.3 mL, 2 mmol), N-phenylethylenediamine
(1.6 mL, 12 mmol), and DMF (20 mL) were added through a syringe.
The mixture was stirred at ambient temperature, and the reaction was
monitored by using TLC until the iodobenzene derivative was
consumed. The resulting mixture was filtered, and the filtrate was
evaporated to dryness in a rotary evaporator. The products were
purified by short column chromatography on silica gel eluted with
dichloromethane. The obtained N-aryl-N′-phenylethylenediamine was
used in the next step without further purification.
Method B. To a Schlenk flask equipped with a magnetic stirring bar,
p-substituted bromobenzene (10 mmol) and N-phenylethylene-
diamine (1.6 mL, 12 mmol) were added. After the flask was evacuated
and refilled with nitrogen gas three times, toluene (20 mL) was added.
To this solution, Pd(dibenzalacetone)2 (0.1 mmol), 2,2′-bis(diphenyl-
phosphino)-1,1′-binaphthyl (0.1 mmol), and t-BuONa (14 mmol)
were added, and the mixture was heated at 100 °C. After full
conversion of the bromobenzene derivative (checked by TLC), the
reaction mixture was filtered, and the filtrate was evaporated to dryness
in a rotary evaporator. The products were purified by short column
chromatography on silica gel eluted with dichloromethane. The
obtained N-aryl-N′-phenylethylenediamine was used in the next step
without further purification.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Numbers
21350033 (B) and 25410108 (C). M.H. acknowledges financial
support from JSPS through the Program for Leading Graduate
Schools (MERIT).
REFERENCES
■
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DOI: 10.1021/acs.inorgchem.7b00806
Inorg. Chem. XXXX, XXX, XXX−XXX