Synthesis and Reactivity of Iron– and Cobalt–Dinitrogen Complexes Bearing PSiP-Type Pincer Ligands toward Nitrogen Fixation

Article abstract of DOI:10.1002/ejic.201700569

Iron– and cobalt–dinitrogen complexes bearing PSiP-type pincer ligands are newly designed and prepared, on the basis of our previous proposal that iron– and cobalt–dinitrogen complexes bearing trimethylsilyl ligands are key reactive intermediates in the c

Full text of DOI:10.1002/ejic.201700569

A Journal of
Accepted Article
Title: Synthesis and Reactivity of Iron- and Cobalt-Dinitrogen
Complexes Bearing PSiP-Type Pincer Ligands toward Nitrogen
Fixation
Authors: Ryuji Imayoshi, Kazunari Nakajima, Jun Takaya, Nobuharu
Iwasawa, and Yoshiaki Nishibayashi
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700569
Link to VoR: http://dx.doi.org/10.1002/ejic.201700569

10.1002/ejic.201700569
European Journal of Inorganic Chemistry
Revised manuscript (ejic.201700569)
Synthesis and Reactivity of Iron– and Cobalt–Dinitrogen Complexes Bearing
PSiP-Type Pincer Ligands toward Nitrogen Fixation
Ryuji Imayoshi,[a] Kazunari Nakajima,[a] Jun Takaya,[b] Nobuharu Iwasawa,[b] and Yoshiaki
Nishibayashi*[a]
[a] Department of Systems Innovation, School of Engineering, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
[b]
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo
152-8551, Japan
Abstract: Iron– and cobalt–dinitrogen complexes bearing PSiP-type pincer ligands are newly
designed and prepared based on our previous finding that iron– and cobalt–dinitrogen complexes
bearing trimethylsilyl ligands are proposed to be key reactive intermediates in the catalytic
formation of silylamine from nitrogen gas under ambient reaction conditions.
These iron– and
cobalt–dinitrogen complexes are found to work as effective catalysts toward the catalytic
transformation of nitrogen gas into silylamine.
1
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INTRODUCTION
The development of catalytic reactions of nitrogen gas into ammonia in the presence of
transition metal–dinitrogen complexes is one of the most important transformations from a
viewpoint of nitrogen fixation under mild reaction conditions.[1,2]
As the first successful example
of direct conversion of dinitrogen into ammonia under ambient reaction conditions, in 2003
Schrock and Yandulov found a molybdenum–dinitrogen complex bearing a triamidoamine worked
as a catalyst.[3]
dinitrogen-bridged dimolybdenum complexes bearing PNP-type pincer ligands as catalysts.[4,5]
In 2010, we developed more effective reaction system by using
In
2013, Peters and co-workers disclosed that an iron–dinitrogen complex bearing triphosphineborane
as a tetradentate ligand worked as an effective catalyst at low reaction temperature such as –78 °C
toward the formation of ammonia from nitrogen gas.[6]
In 2016, we found the iron- and
cobalt-catalyzed reduction of nitrogen gas into ammonia at –78 °C by using iron– and cobalt–
dinitrogen complexes bearing an anionic PNP-type pincer ligands as catalysts, where a mixture of
ammonia and hydrazine was formed as nitrogenous products.[7,8]
Recently, Ashley and
co-workers have reported the iron-catalyzed selective formation of hydrazine from nitrogen gas at –
78 °C by using iron–dinitrogen complex bearing two diphosphines as ligands.[9]
Catalytic formation of silylamine from nitrogen gas is alternative nitrogen fixation under mild
reaction conditions because silylamine is readily converted into ammonia after hydrolysis.
In
1972, Shiina discovered chromium-catalyzed reduction of nitrogen gas with lithium as a reductant
and Me3SiCl as a silylating reagent under ambient reaction conditions to produce
tris(trimethylsilyl)amine (N(SiMe3)3).[10,11]
In 1989, Hidai and co-workers reported the catalytic
formation of N(SiMe3)3 using sodium as a reductant in the presence of a molybdenum–dinitrogen
complex bearing four monodentate phosphines as a catalyst.[12]
In 2011, we found that a
molybdenum–dinitrogen complex bearing ferrocenyldiphosphines worked as the most effective
catalyst, where up to 226 equiv of N(SiMe3)3 were produced based on the catalyst.[13]
Recently,
we have found the iron-, cobalt-, and vanadium-catalyzed conversion of nitrogen gas into
N(SiMe3)3 under ambient reaction conditions.[14-16]
Independently, Lu and co-workers reported
that dicobalt complexes bearing a trisphosphine(triamido)amine ligand worked as effective catalysts
2
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10.1002/ejic.201700569
European Journal of Inorganic Chemistry
Revised manuscript (ejic.201700569)
for the formation of N(SiMe3)3.[17]
by using CoH(N2)(PPh3)3 as a catalyst.[18]
After a while, Dzik investigated the silyaltion of nitrogen gas
Quite recently, Ohki and co-workers found that iron
hydride clusters showed catalytic activity for reduction of nitrogen gas into N(SiMe3)3.[19]
In the iron- and cobalt-catalyzed reactions developed by our group, simple iron and cobalt
complexes such as Fe(CO)5, ferrocene, Co2(CO)8, and cobaltocene were used as precursors (Scheme
1).
Unfortunately, we did not succeed in the isolation of iron– and cobalt–dinitrogen complexes
Previous work
cat.
rt
+
+
6 Me3SiCl
2 N(SiMe3)3
N2
(1 atm)
6 Na
cat.
Co2(CO)8
Fe(CO)5 Fe
Co
proposed reactive species
N
N
N
N
Me3Si
SiMe3
SiMe3
Fe
Co
N
SiMe3
SiMe3
N
O
A
B
This work
PCy2
PMe3
PCy2
R
Si Fe
N
N
Si Co
PCy2
N N
Me
H
PR3
PCy2
PR3 = PMe3, PMe2Ph, PMePh2
R = Me, Ph
Scheme 1. Iron– and cobalt–catalyzed silylation of nitrogen
gas under ambient reaction conditions.
because of their quite low stability.
However, theoretical study[14,15] suggested that iron- and
cobalt-dinitrogen complexes bearing trimethylsilyl ligands (A and B, respectively) would be
generated in situ and work as key reactive intermediates.
These findings prompted us to design
In fact,
and prepare iron– and cobalt–dinitrogen complexes bearing PSiP-type pincer ligands.
Turculet,[20] Hazari,[21] and other research groups[22] have already reported the preparation of various
transition metal complexes bearing PSiP-type pincer ligands and their stoichiometric and catalytic
reactivity.
It is noteworthy that some authors of the present paper have achieved the
palladium-catalyzed hydrocarboxylation of allenes with carbon dioxide and dehydrogenative
3
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10.1002/ejic.201700569
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Revised manuscript (ejic.201700569)
borylation of alkenes.[23]
iron– and cobalt–dinitrogen complexes bearing PSiP-type pincer ligands have not yet been reported
until now. In this paper, we report the preparation of iron– and cobalt–dinitrogen complexes
In sharp contrast to the previous study, the preparation and reactivity of
bearing PSiP-type pincer ligands and their catalytic activity toward nitrogen fixation under mild
reaction conditions.
RESULTS AND DISCUSSION
Preparation of Iron–Dinitrogen Complexes.
complexes bearing PSiP-type pincer ligands. The reaction of Fe(PMe3)4 with a PSiP-type pincer
ligand precursor (1a:
(Cy-PSiMeP)H (Cy-PSiMeP)H
At first, we prepared iron–dinitrogen
=
bis(o-(dicyclohexylphosphino)phenyl)methylsilane) in toluene at 60 ºC for 35 h under an
atmospheric pressure of nitrogen gas afforded the corresponding iron–dinitrogen complex bearing a
PCy2
R
SiH
PCy2
PCy2
PMe3
1a (R = Me)
1b (R = Ph)
R
Si Fe
N
N
N2 (1 atm)
toluene
H
PCy2
(a)
2a (R = Me): 44% yield
2b (R = Ph): 28% yield
Fe(PMe3)4
PPh2
Me
SiH
(b)
PPh2
PPh2
PMe3
(Ph-PSiP)H
Me Si Fe PMe3
toluene
rt, 24 h
H
PPh2
FeH(Ph-PSiP)(PMe3)2
64% yield
Scheme 2. Preparation of iron–dinitrogen complexes
bearing PSiP-type pincer ligands
PSiP-type pincer ligand [FeH(Cy-PSiMeP)(N2)(PMe3)] (2a) in 44% yield (Scheme 2 (a)).
The IR
4
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European Journal of Inorganic Chemistry
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spectrum of 2a in solid state (KBr) showed a strong absorption at 2041 cm–1 and a weak absorption
at 1850 cm–1, which were assigned to the terminal dinitrogen ligand and the hydride ligand,
respectively. Detailed structure of 2a was determined by X-ray analysis.
An ORTEP drawing
of 2a is shown in Figure 1 (a).
iron atom.
The complex 2a has a distorted octahedral geometry around the
Selected bond lengths and angles for 2a are listed in Table 1.
The N–N bond
distance of the dinitrogen ligand in 2a is 1.154(9) Å, which is almost the same with those of the
terminal dinitrogen ligand in reported iron(II)–dinitrogen complexes (1.028–1.147 Å).[24]
The
silicon atom in 2a is positioned trans to the dinitrogen ligand, with Fe–Si distance of 2.320(3) Å
and Fe–N distance of 1.816(6) Å.
When we conducted the reaction of Fe(PMe3)4 with a ligand precursor with phenyl group at the
Si
atom
such
as
(Cy-PSiPhP)H
(1b:
(Cy-PSiPhP)H
=
bis(o-(dicyclohexylphosphino)phenyl)phenylsilane) under the same reaction conditions, a similar
iron–dinitrogen complex [FeH(Cy-PSiPhP)(N2)(PMe3)] (2b) was obtained in 28% yield (Scheme 2
(a)).
terminal dinitrogen ligand at 2039 cm–1 and a weak absorption assignable to the hydride ligand at
1854 cm–1, which are almost the same with those of 2a.
Detailed molecular structure of 2b was
confirmed by X-ray crystallography. An ORTEP drawing of 2b is shown in Figure 1 (b).
The IR spectrum of 2b in the solid state (KBr) showed a strong absorption assignable to the
Selected bond lengths and angles for 2b are listed in Table 2. The complex 2b has a distorted
octahedral geometry around the iron atom, while P1–Fe1–P2 bite angle in 2b (146.67(6) º) is
slightly smaller than that in 2a (152.03(7) º).
The N–N bond distance of the dinitrogen ligand in
2b (1.149(6) Å) is almost the same with that in 2a (1.154(9) Å).
Previously, Sun and co-workers reported the preparation of iron–hydride complex
[FeH(Ph-PSiMeP)(PMe3)2] from the reaction of Fe(PMe3)4 with a precursor of (Ph-PSiMeP)H
5
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European Journal of Inorganic Chemistry
Revised manuscript (ejic.201700569)
((Ph-PSiMeP)H = bis(o-(diphenylphosphino)phenyl)methylsilane) (Scheme 2 (b)).[22e]
formation of [FeH(Ph-PSiMeP)(PMe3)2] proceeded via oxidative addition of Si–H bond of
(Ph-PSiMeP)H to the iron atom.
This experimental result for the formation of
[FeH(Ph-PSiMeP)(PMe3)2] is in sharp contrast to the formation of iron–dinitrogen complexes 2
shown in Scheme 2 (a). We consider that more steric hindrance and more electron-donating
The
ability of cyclohexyl moieties on the phosphorus atom in 1a than those in (Ph-PSiMeP)H may
facilitate the ligand exchange of PMe3 with N2.
For comparison, various iron(II)–dinitrogen complexes structurally related to 2a and 2b are
shown in Figure 2 (a) together with their N–N stretching frequencies.
Peters and co-workers
have synthesized iron(II)–dinitrogen complexes bearing tetradentate silyl ligands such as
[FeH(SiPiPr )(N2)], [FeH(SiPiPr SAd)(N2)], [FeH(SiPiPrSAd )(N2)], and [Fe(SiPiPr )(N2)][BArF ] (ArF =
3
2
2
3
4
3,5-bis(trifluoromethylphenyl)).[24l–n]
The N–N stretching frequencies of 2a and 2b are slightly
These
lower than those of the iron–dinitrogen complexes bearing tetradentate silyl ligands.
results suggest that the newly prepared iron–dinitrogen complexes 2 have enough reactive
dinitrogen ligands.
Preparation of Cobalt–Dinitrogen Complexes.
corresponding cobalt–dinitrogen complex bearing a PSiP-type pincer ligand according to the same
procedure for the preparation of 2 shown in the previous section. We conducted the reaction of
Co(PMe3)4 with 1 under the same reaction conditions. However, in all cases, paramagnetic
Next, we attempted to prepare the
cobalt species were newly formed and unfortunately we cannot identify these cobalt complexes.
Elemental analysis and IR measurement indicated no formation of cobalt–dinitrogen complexes.
Based on this experimental result, we investigated another synthetic approach to prepare cobalt–
dinitrogen complexes via the reduction of the corresponding cobalt halide complexes bearing a
6
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Revised manuscript (ejic.201700569)
PSiP-type pincer ligand.
Treatment of CoI2 with (Cy-PSiMeP)H 1a in the presence of an excess
PR3
KC8
N2
(1 atm)
PCy2
Me Si Co
PCy2
Si Co
PCy2
PCy2
Et3N
CoI2
+
N
N
MeSiH
I
Me
toluene
100 ºC
20 h
THF
rt, 3 h
PR3
PCy2
PCy2
1a
3
45% yield
4a (PR3 = PMe3): 23 % yield
4b (PR3 = PMe2Ph): 34% yield
4c (PR3 = PMePh2): 40% yield
Scheme 3. Preparation of cobalt–dinitrogen complexes bearing
PSiP-type pincer ligand.
amount of triethylamine as a base in toluene at 100 °C for 20 h afforded [CoI(Cy-PSiMeP)] (3) in
45% yield (Scheme 3).[25]
An ORTEP drawing of 3 is shown in Figure 3 (a).
square-planar geometry around the cobalt atom with a geometry index τ4 = 0.29 (τ4 = 0.00 for a
The molecular structure of 3 was confirmed by X-ray crystallography.
Crystal structure of 3 has a distorted
perfect square-planar geometry and τ4 = 1.00 for a perfect tetrahedral geometry).[26]
bond lengths and angles for 3 are listed in Table 3.
Selected
Reduction of 3 with KC8 in the presence of 1.1 equiv of phosphine such as PMe3 in THF at
room temperature for 3 h under an atmospheric pressure of nitrogen gas afforded the corresponding
cobalt–dinitrogen complex bearing PMe3 as a co-ligand [Co(Cy-PSiMeP)(N2)(PMe3)] (4a) in 23%
yield (Scheme 3).[27]
PMe3, similar cobalt–dinitrogen complexes [Co(Cy-PSiMeP)(N2)(PMe3-nPhn)] (4b: n = 1; 4c: n = 2)
were formed in 34% and 40% yields, respectively (Scheme 2). The IR spectra of cobalt–
dinitrogen complexes 4 in solid state (KBr) showed a strong absorption at 2063 cm–1, 2077 cm–1,
and 2062 cm–1, respectively.
These peaks were assigned to the terminal dinitrogen ligands.
The molecular structures of 4a and 4c were determined by X-ray crystallography. ORTEP
drawings of 4a and 4c are shown in Figure 3 (b) and (c), respectively. Crystal structures of both
When other phosphines such as PMe2Ph and PMePh2 were used in place of
4a and 4c have distorted trigonal bipyramidal geometries around the cobalt atom, while P1–Co1–P2
7
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bite angle in 4a (124.51(7)º) is slightly larger than that in 4c (114.42(5)º).
Selected bond lengths
and angles for 4a and 4c are listed in Tables 4 and 5, respectively. The N–N bond distances of
the dinitrogen ligand in 4a and 4c are 1.131(8) and 1.116(5) Å, respectively. These N–N bond
distances of 4 are almost the same with those of the terminal dinitrogen ligand in reported cobalt(I)–
dinitrogen complexes (0.956–1.1246 Å).[8,28]
The silicon atom in 4 is positioned trans to the
dinitrogen ligand, with Co–Si distances of 2.2635(19) and 2.2966(13) Å for 4a and 4c, respectively,
and Co–N distances of 1.787(5) and 1.812(4) Å for 4a and 4c, respectively.
Peters and co-workers have synthesized a cobalt(I)–dinitrogen complex bearing a tetradentate
silyl ligand, which is structurally related to 4a, 4b and 4c (Figure 2 (b)).[28d]
The N–N stretching
frequencies of 4a, 4b and 4c are similar to that of the cobalt–dinitrogen complex.
Catalytic Activity of Iron– and Cobalt–Dinitrogen Complexes. With these iron– and cobalt–
dinitrogen complexes in hand, we investigated their reactivity toward catalytic conversion of
molecular dinitrogen under mild reaction conditions.
molecular dinitrogen into ammonia under reaction conditions according to the procedure reported
by Peters and co-workers.[6]
When 2a was used as a catalyst in the presence of excess amounts of
At first we tried to directly reduce
KC8 (40 equiv) as a reductant and [H(OEt2)2][BArF ] (38 equiv) as a proton source at –78 ºC under
4
an atmospheric pressure of nitrogen gas, only 1.2 equiv of ammonia were obtained based on the
catalyst (Scheme 4).
Unfortunately, similar results were observed when other iron– and cobalt–
2a
(0.010 mmol)
+
+
NH3
N2
(1 atm)
KC8
(0.40 mmol)
[H(OEt2)2]BArF
(0.38 mmol)
4
Et2O
–78 ºC, 1 h
1.2 equiv
Scheme 4. Iron-mediated reduction of N2 to NH3
under mild reaction conditions.
dinitrogen complexes such as 2b and 4a–4c were used as catalysts.
More detailed informaion is
shown in Table S1 of the Supporting Information. No formation of hydrazine was observed in all
8
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10.1002/ejic.201700569
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cases.
These results indicate that these iron– and cobalt–dinitrogen complexes bearing PSiP
pincer ligands cannot promote the catalytic reduction of molecular dinitrogen into ammonia under
the present reaction conditions.
We then conducted catalytic silylation of molecular dinitrogen to N(SiMe3)3 under ambient
reaction conditions. Typical results are shown in Table 6.
The reaction of an atmospheric
pressure of nitrogen gas with Na (600 equiv) as a reductant and Me3SiCl (600 equiv) as a silylating
reagent in the presence of 2a in THF at room temperature for 20 h gave 15 equiv of N(SiMe3)3
based on the catalyst (Table 6, run 1).
N(SiMe3)3 were produced based on the catalyst (Table 6, run 2).
worked as more effective catalyst than 2a under the same reaction conditions.
the catalytic activity of iron complexes for the silylation of molecular dinitrogen is shown in
When 2b was used as a catalyst in place of 2a, 26 equiv of
These results indicate that 2b
For comparison,
Scheme 5.
As shown in Scheme 5, 2b has a moderate catalytic activity toward the formation of
catalyst
+
+
6 Me3SiCl
2 N(SiMe3)3
N2
(1 atm)
6 Na
rt
equiv/Fe or Co atom
Fe catalyst
SiMe3
SiMe3
iPr
iPr
PHCy2
PMe3
Me3Si
PtBu2
Fe
PtBu2
iPr i
Pr
N
N
Fe(CO)5
Ref 14
Fe
Fe
R
Si Fe
N
N
Fe
N
N N
Me3Si
Me3Si
SiMe3
H
PCy2
This work
Et
Et
Et
Et
Ref 14
Ref 14
Ref 7
33 equiv
Ref 6c
13 equiv 25 equiv
34 equiv
15 equiv (2a:R = Me)
26 equiv (2b:R = Ph)
24 equiv
(KC8 was used in place of Na)
P
Co catalyst
Co
P
P
N
N
N
PCy2
Co
N
N
PPh3
PPh3
N
Co2(CO)8
Co
CoCl(PPh3)3
Ph3P Co
H
Si Co
PCy2
N N
Me
PR3
P = PiPr2
Ref 17
This work
Ref 15
Ref 15
Ref 15
29 equiv
Ref 17
8 equiv 36 equiv
28 equiv (4a: PR3 = PMe3)
23 equiv (4b: PR3 = PMe2Ph)
41 equiv (4c: PR3 = PMePh2)
98 equiv
44 equiv
(KC8 was used in place of Na) (KC8 was used in place of Na)
Scheme 5. Iron- and cobalt-catalyzed transformation of dinitrogen into silylamine
under ambient reaction conditions.
N(SiMe3)3 under ambient reaction conditions.
9
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The catalytic silylation proceeded smoothly in the presence of cobalt–dinitrogen complexes 4 as
catalysts under ambient reaction conditions. Among of the cobalt–dinitrogen complexes, 4c
worked as the most effective catalyst, where 31 equiv of N(SiMe3)3 were produced based on the
catalyst (Table 6, runs 3–5).
a longer reaction time such as 40 h, 41 equiv of N(SiMe3)3 were produced based on the catalyst
(Table 6, run 6). For comparison, the catalytic activity of cobalt complexes for the silylation of
molecular dinitrogen is shown in Scheme 5. As shown in Scheme 5, 4c has a moderate catalytic
When the catalytic reaction was carried out in the presence of 4c for
activity toward the formation of N(SiMe3)3 under ambient reaction conditions.
To get more information, we monitored the formation of N(SiMe3)3 from nitrogen gas by using
4c as a catalyst.
[Co2(CO)8] as catalysts are shown in Figure 4.
smoothly than that using [Co2(CO)8]. These results indicate that the initial reaction rate of 4c is
Time profiles of formation of N(SiMe3)3 from nitrogen gas using 4c and
The catalytic reaction using 4c proceeded more
higher than that of [Co2(CO)8] although almost the same amount of N(SiMe3)3 was formed in both
cases after a longer reaction time such as 40 h. In the reaction system by using [Co2(CO)8] as a
catalyst, there was a long induction period probably due to the time for the formation of reactive
species such as cobalt–dinitrogen complex bearing trimethylsilyl ligands B shown in Scheme 1.
In sharp contrast, a completely different time profile (almost no induction period) at the initial stage
was observed in the reaction system by using 4c as a catalyst.
At present, we consider that 4c
may be involved as an active species of the catalytic reaction.
Separately, we have already tried
to isolate key reactive intermediates from 4c, however, any reactive intermediates have not yet
isolated until now.
We carried out electrochemical measurement of 4c in order to obtain an electronic property of
the newly prepared cobalt–dinitrogen complexes bearing PSiP ligands.
Cyclic voltammogram of
4c in THF using [NnBu4]PF6 as a supporting electrolyte showed a quasi-reversible reduction wave at
E1/2 = -2.66 V versus FeCp2.
This result indicates the possibility that a reduced cobalt(0)–
dinitrogen complex may be formed from 4c in the catalytic reaction conditions.[29]
In fact,
Yamamoto and co-workers previously reported the preparation of a series of anionic cobalt–
dinitrogen complexes and the transformation of the coordinated dinitrogen ligand on the cobalt
10
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atom into a mixture of ammonia and hydrazine by treatment with H2SO4 under ambient reaction
conditions although no formation of ammonia and hydrazine was confirmed from the reaction of
the corresponding neutral cobalt–dinitrogen complex under the same reaction conditions.[30]
At
present, we consider that an anionic cobalt(0)–dinitrogen complex reduced from 4c also may be
involved as an active species of the catalytic reaction.
In summary, we have newly designed and prepared iron– and cobalt–dinitrogen complexes
bearing PSiP ligands based on our previous finding that iron– and cobalt–dinitrogen complexes
bearing trimethylsilyl ligands are proposed to be key reactive intermediates in the catalytic
formation of silylamine from nitrogen gas under ambient reaction conditions.
These newly
prepared iron– and cobalt–dinitrogen complexes do not promote the direct formation of ammonia
from nitrogen gas under mild reaction conditions, but promote the catalytic silylation of nitrogen
gas into silylamine under ambient reaction conditions. We believe the finding contributes to
further development of catalytic transformation of molecular dinitrogen using transition metal–
dinitrogen complexes as catalyst under mild reaction conditions.
11
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EXPERIMENTAL SECTION
13
31
General methods.
1H NMR (270 MHz), C{1H} NMR (68 MHz), P{1H} NMR (109 M
Hz), and Si{1H} NMR (54 MHz) spectra were measured on a JEOL Excalibur 270 spectrometer
29
and spectra were referenced to residual solvent (1H and C{1H}) or external standard (31P{1H}:
13
H3PO4, 29Si{1H}: Si(CH3)4).
IR spectra were recorded on a JASCO FT/IR 4100 Fourier
Absorption spectra were recorded on a Shimadzu
Transform infrared spectrometer.
MultiSpec-1500.
Elemental analyses were performed at Microanalytical Center of The
GC analyses were carried out on a Shimadzu GC-2014 instrument
University of Tokyo.
equipped with a flame-ionization detector using a CBP 10 fused silica capillary column (25 m x
0.25 mm).
glove box.
All reactions were carried out under a dry nitrogen atmosphere or in an argon-filled
Solvents were dried over appropriate reagents and distilled prior to use. Sodium
(30 wt% dispersion in toluene, Across Organics) metal was washed with hexane and then with THF,
and were stored in an argon-filled glovebox. Me3SiCl was distilled before use.
(ortho-Bromophenyl)dicyclohexylphosphine,[31] (Cy-PSiMeP)H,[22b] Fe(PMe3)4,[32] KC8,[33] and
[H(OEt2)2]BArF [34] were prepared as described in the literature. Other reagents were purchased
4
commercially and used as received. Caution! KC8 is pyrophoric and must be used carefully under
inert atmosphere with appropriate glove-box and Schlenk techniques. The elemental analyses for
several Fe and Co complexes (2b, 4b, and 4c) were not satisfactory, probably due to the instability
toward atmospheric oxygen and moisture during the measurement operation.
measured HRMS and shows the NMR images in the Supporting Information.
We alternatively
Preparation
of
(Cy-PSiPhP)H
(1b).
To
a
Et2O
solution
of
n
(ortho-bromophenyl)dicyclohexylphosphine (1.41 g, 3.99 mmol) was added BuLi (2.45 mL, 1.63
M in hexane, 3.99 mmol) dropwise at –78 ºC.
The mixture was allowed to warm to room
temperature over 1 h.
Then, the mixture was cooled to –78 ºC and dichlorophenylsilane (355 mg,
The mixture was allowed to warm to room temperature and stirred at
2.00 mmol) was added.
room temperature for 20 h.
extracted with toluene (40 mL) and centrifuged.
vacuum to afford a white solid, which was recrystallized from Et2O at –30 ºC to give white crystals
The volatiles were removed under vacuum and the residue was
The supernatant was concentrated under
of (Cy-PSiPhP)H in 59% yield (775 mg, 1.18 mmol).
12
1H NMR (C6D6) δ 7.66–7.63 (m, 2H, ArH),
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7.49–7.41 (m, 4H, ArH), 7.19–7.13 (m, 5H, ArH), 7.05–6.99 (m, 3H, ArH, SiH), 1.97–1.03 (m,
13
1
5
2
44H, CyH). C{1H} NMR (C6D6) δ 145.8 (dd, JCP = 46.1 and JCP = 4.8 Hz, Ar), 144.0 (d, JCP =
16.7 Hz, Ar), 138.0 (d, 2JCP = 15.7 Hz, Ar), 137.1 (t, 3JCP = 4.4 Hz, Ar), 136.9 (s, Ar), 132.50 (s, Ar),
132.47 (s, Ar), 129.1 (s, Ar), 129.0 (s, Ar), 127.9 (s, Ar), 35.5 (d, 1JCP = 15.1 Hz, Cy), 35.3 (d, 1JCP =
15.1 Hz, Cy), 31.0 (d, 2JCP = 15.7 Hz, Cy), 30.9 (d, 2JCP = 16.7 Hz, Cy), 30.4 (d, 3JCP = 11.7 Hz, Cy),
29.8 (d, 3JCP = 10.0 Hz, Cy), 27.6–27.3 (m, Cy), 26.9 (s, Cy), 26.7 (s, Cy). 31P{1H} NMR (C6D6) δ –
29
3
7.1. Si{1H} NMR (C6D6) δ –26.5 (t, JSiP = 24.0 Hz). Anal. Calcd. for C42H58P2Si: C, 77.26; H,
8.95. Found: C, 76.95; H, 8.93.
Preparation of [FeH(Cy-PSiMeP)(N2)(PMe3)] (2a).
A mixture of 1a (294 mg, 0.498 mmol)
and Fe(PMe3)4 (201 mg, 0.557 mmol) in toluene (20 mL) was stirred at 60 ºC for 35 h under an
atmospheric pressure of molecular dinitrogen. After the reaction, formation of 2a together with
unreacted Fe(PMe3)4 and 1a in about 30% recovery was observed by NMR analysis. The volatiles
were removed under vacuum.
The residue was washed with hexane and extracted with Et2O.
The solvent was removed under vacuum to afford yellow solid of 2a in 44% yield (166 mg, 0.221
1
mmol). H NMR (C6D6) δ 8.20 (br s, 2H, ArH), 7.48–7.16 (m, 6H, ArH), 2.85 (br s, 2H, CyH),
2
2
2.42–0.82 (m, 54H, CyH, PMe3, SiMe), –14.84 (td, JHP = 68.9 and JHP = 23.0 Hz, 1H, FeH).
31P{1H} NMR (C6D6) δ 84.8 (d, 2JPP = 16.5 Hz, PCy2), 11.5 (t, 2JPP = 16.5 Hz, PMe3). 29Si{1H} NMR
(THF-d8) δ 57.7 (td, 2JSiP = 35.9 Hz and 2JSiP = 25.6 Hz). IR (KBr, cm–1) 2041 (s, νNN), 1850 (s, νFeH).
Anal. Calcd. for C40H65FeN2P3Si: C, 63.99; H, 8.73; N, 3.73. Found: C, 63.95; H, 8.48; N, 3.06.
Preparation of [FeH(Cy-PSiPhP)(N2)(PMe3)] (2b).
A mixture of 1b (228 mg, 0.349 mmol)
and Fe(PMe3)4 (188 mg, 0.522 mmol) in toluene (10 mL) was stirred at 50 ºC for 48 h under an
atmospheric pressure of molecular dinitrogen. After the reaction, formation of 2b together with
unreacted Fe(PMe3)4 and 1b in about 30% recovery was observed by NMR analysis. The volatiles
were removed under vacuum.
The residue was washed with hexane and extracted with toluene.
The solvent was removed under vacuum to afford yellow solid of 2b in 28% yield (80.2 mg, 0.0987
mmol). 1H NMR (THF-d8) δ 7.67 (br s, 2H, ArH), 7.56 (br s, 2H, ArH), 7.19–6.96 (m, 9H, ArH),
2
2.88 (br s, 2H, CyH), 2.23–0.62 (m, 51H, CyH, PMe3), –14.63 (td, JHP = 73.2 and JHP = 20.3 Hz,
2
1H, FeH). 31P{1H} NMR (THF-d8) δ 87.4 (d, 2JPP = 15.6 Hz, PCy2), 11.0 (t, 2JPP = 15.6 Hz, PMe3).
IR (KBr, cm–1) 2039 (s, νNN), 1854 (s, νFeH). Anal. Calcd. for C45H67FeN2P3Si: C, 66.49; H, 8.73; N,
13
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3.45. Found: C, 63.84; H, 8.41; N, 2.86. HRMS (FAB) Calcd. for C45H67FeN2P3Si [M]: 812.3636.
Found: 812.3660.
Preparation of [CoI(Cy-PSiMeP)] (3).
mg, 0.201 mmol), and Et3N (0.278 mL, 2.01 mmol) in toluene (10 mL) was stirred at 100 ºC for 20
h. After the reaction, the volatiles were removed under vacuum. The residue was washed with
A mixture of CoI2 (75.3 mg, 0.241 mmol), 1a (118.8
hexane (15 mL) and extracted with toluene (25 mL). The solvent was removed under vacuum.
Recrystallization from toluene/hexane afforded dark red crystals of 3 in 45% yield (70.2 mg, 0.0905
mmol).
Anal. Calcd. for C37H55CoIP2Si: C, 57.29; H, 7.15. Found: C, 57.55; H, 7.05.
Preparation of [Co(Cy-PSiMeP)(N2)(PMe3)] (4a).
A mixture of 3 (197 mg, 0.254 mmol),
PMe3 (1.0 M in THF, 0.279 mL, 0.279 mmol), and KC8 (37.6 mg, 0.278 mmol) in THF (8 mL) was
stirred at room temperature for 3 h under an atmospheric pressure of molecular dinitrogen. After
the reaction, formation of 4a together with unreacted PMe3 was observed by NMR analysis. The
volatiles were removed under vacuum.
The residue was washed with hexane and dried in vacuo
to afford a red solid of 4a in 23% yield (44.1 mg, 0.059 mmol). 1H NMR (C6D6) δ 7.67 (d, 3JHH =
3
7.3 Hz, 2H, ArH), 7.48 (d, JHH = 7.0 Hz, 2H, ArH), 7.19–7.06 (m, 4H, ArH), 2.47–2.01 (m, 8H,
31
2
CyH), 1.71–0.70 (m, 48H, CyH , PMe3, SiMe3). P{1H} NMR (C6D6) δ 67.0 (d, JPP = 86.2 Hz,
2
29
2
2
PCy2), –14.2 (d, JPP = 86.2 Hz, PMe3). Si{1H} NMR (THF-d8) δ 56.2 (td, JSiP = 44.9 and JSiP
=
25.0 Hz). IR (KBr, cm–1) 2063 (s, νNN). Anal. Calcd. for C40H64CoN2P3Si: C, 63.81; H, 8.57; N, 3.72.
Found: C, 64.00; H, 8.51; N, 3.61.
Preparation of [Co(Cy-PSiMeP)(N2)(PMe2Ph)] (4b). A mixture of 3 (367 mg, 0.473 mmol),
PMe2Ph (70.4 mg, 0.521 mmol), and KC8 (70.5 mg, 0.521 mmol) in THF (12 mL) was stirred at
room temperature for 3 h under an atmospheric pressure of molecular dinitrogen. After the
reaction, formation of 4b together with unreacted PMe2Ph was observed on NMR analysis. The
volatiles were removed under vacuum.
The residue was washed with hexane and dried in vacuo
1H NMR (C6D6): δ 7.76 (d, 3JHH
to afford a red solid of 4b in 34% yield (130 mg, 0.160 mmol).
= 7.0 Hz, 2H, ArH), 7.62–7.49 (m, 5H, ArH, PhH), 7.26–7.05 (m, 6H, ArH, PhH), 2.38–2.30 (m,
31
2
4H, CyH), 2.06–0.85 (m, 49H, CyH, PMe2Ph, SiMe). P{1H} NMR (C6D6): δ 67.1 (d, JPP = 75.3
2
Hz, PCy2), 1.5 (t, JPP = 75.3 Hz, PMe2Ph). IR (KBr, cm–1): 2077 (s, νNN). Anal. Calcd. for
14
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C45H66CoN2P3Si: C, 66.32; H, 8.16; N, 3.44. Found: C, 65.31; H, 8.15; N, 3.25. HRMS (FAB)
Calcd. for C45H66CoN2P3Si [M]: 814.3540. Found: 814.3536.
Preparation of [Co(Cy-PSiMeP)(N2)(PMePh2)] (4c).
A mixture of 3 (145 mg, 0.187 mmol),
PMePh2 (41.1 mg, 0.205 mmol), and KC8 (27.7 mg, 0.204 mmol) in THF (15 mL) was stirred at
room temperature for 3 h under an atmospheric pressure of molecular dinitrogen. After the
reaction, formation of 4c together with unreacted PMePh2 was observed by NMR analysis. The
reaction, the volatiles were removed under vacuum. The residue was washed with hexane and
dried in vacuo to afford a red solid of 4c in 40% yield (66.4 mg, 0.0757 mmol).
1H NMR (C6D6):
δ 7.82 (d, 3JHH = 6.8 Hz, 2H, ArH), 7.57–7.49 (m, 6H, ArH, PhH), 7.28–7.02 (m, 10H, ArH, PhH),
2.41–2.31 (m, 4H, CyH), 2.16–0.90 (m, 43H, CyH, SiMe), 0.69 (s, 3H, PMePh2). P{1H} NMR
31
(C6D6): δ 67.4 (d, 2JPP = 78.8 Hz, PCy2), 22.0 (t, 2JPP = 78.8 Hz, PMePh2). 29Si{1H} NMR (THF-d8)
δ 55.1 (pseud q, 2JSiP = 29.0 Hz). IR (KBr, cm–1): 2062 (s, νNN). Anal. Calcd. for C50H68CoN2P3Si: C,
68.47; H, 7.82; N, 3.19. Found: C, 67.67; H, 7.54; N, 2.88. HRMS (FAB) Calcd. for
C50H68CoN2P3Si [M]: 876.3697. Found: 876.3693.
Reduction of Dinitrogen to Ammonia.
A typical procedure is as follows.
In a 50 mL
Schlenk flask were placed 2a (7.5 mg, 0.010 mmol), KC8 (54.2 mg, 0.401 mmol), and
[H(OEt2)2]BArF (385 mg, 0.380 mmol).
After the mixture was cooled to –196 ºC, Et2O (5 mL)
4
was added to the mixture by trap-to-trap distillation. The Schlenk flask was warmed to –78 ºC
and then was filled with nitrogen gas (1 atm). After stirring at –78 ºC for 1 h, the mixture was
warmed to room temperature and further stirred at room temperature for 20 min. The reaction
mixture was evaporated under reduced pressure, and the distillate was trapped in an aqueous
solution of H2SO4 (0.5 M, 10 mL). An aqueous solution of KOH (30 wt %, 10 mL) was added to
the residue, and the mixture was distilled into another aqueous solution of H2SO4 (0.5 M, 10 mL).
The amount of ammonia in each of the aqueous solutions was determined by the indophenol
method.[35]
The amount of hydrazine in each of the aqueous solutions was determined by the
p-(dimethylamino)benzaldehyde method.[36]
Catalytic Reduction of Molecular Dinitrogen to N(SiMe3)3.
A typical procedure is as
follows.
A mixture of 2a (15.0 mg, 0.0200 mmol), Na (275 mg, 12.0 mmol), and Me3SiCl (1.52
mL, 12.0 mmol) in THF (8 mL) was stirred at room temperature (22–24 ºC) for 20 h under a gentle
15
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stream of molecular dinitrogen. To the reaction mixture was added cyclododecane (81.3 mg) as
an internal standard for GC analysis. After stirring, the mixture was centrifuged to remove
insoluble materials. The resultant clear supernatant was subjected to GC analysis.
Cyclic Voltammetry. Cyclic voltammetry was performed on an ALS/Chi model 610C
electrochemical analyzer with platinum working electrode. The measurement was carried out
with a solution of 4c (1 mM) and [NnBu4]PF6 (0.1 M) in THF under N2 atmosphere.
X-ray Crystallography.
Crystallographic data of 2a•C4H10O, 2b, 3, 4a, and 4c are
summarized in Tables S2–S6 in the Supporting Information.[37]
Diffraction data were collected
for the 2θ range of 6 to 55° at –100 ºC on a Rigaku RAXIS RAPID imaging plate area detector with
graphite-monochromated Mo-Kα radiation (λ = 0.71075 Å) with Varimax optics. Intensity data
were corrected for Lorentz and polarization effects, and for empirical absorption (ABSCOR).
The structure solution and refinements were carried out by using the CrystalStructure
crystallographic software package.[38]
The positions of non-hydrogen atoms were determined by
direct methods (SIR97[39] for 2a•C4H10O, 2b, 4a, and 4c; SHELXS97[40] for 3) and subsequent
Fourier syntheses (DIRDIF-99[41] for 2a•C4H10O, 2b, 3, and 4c; SHELX 2016/6[40] for 4a), and were
refined against Fo2 using all unique reflections by full-matrix least squares methods with anisotropic
thermal parameters except for four carbon atoms of trimethylphosphine ligands (C(38) and C(39),
C(78) and C(80)) of 2a and two ethereal molecules (C(82)–C(81)–O(1)–C(83)–C(84), C(86)–
C(85)–O(2)–C(87)–C(88)) solved as rigid groups in 2a•C4H10O.
All the hydrogen atoms were
placed at the calculated positions with fixed isotropic parameters, except for the positions of
hydride atoms (H(1) and H(66) in 2a•C4H10O, H(67) in 2b) which were determined on peaks in the
difference Fourier maps and further refined isotropically, while the positions of ethereal hydrogen
atoms solved as rigid groups in 2a•C4H10O were not determined.
Thus, the calculated and
reported molecular weights are different for 2a•C4H10O. The maximum residual peak (2.69 e Å–3)
in 2a•C4H10O appears at the position very near to the Fe(2) atom, presumably due to an unsolved
minor disorder.
Disorders among a cyclohexyl group and a methyl group have led to a large
Hirshfeld difference between C(14) and C(19) atoms and a deviation of a methyl group (C(38)),
respectively. R1 values of 2b and 4c are larger than 0.10, because minor disorders of these
16
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compounds have not been further solved.
This contributes to a void of 106 Å3 surrounded by
trimethylphosphine ligands and cyclohexyl groups but without any large residual peaks in 2b.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Experimental details and CIF data (2a, 2b, 3, 4a, and 4c).
AUTHOR INFORMATION
Corresponding Author
ynishiba@sys.t.u-tokyo.ac.jp
ORCID
Yoshiaki Nishibayashi
0000-0001-9739-9588
Homepage URL(s) for the Corresponding Author
http://park.itc.u-tokyo.ac.jp/nishiba/
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The present project is supported by CREST, JST (JPMJCR1541), and Mitsubishi Foundation.
We thank JSPS KAKENHI Grant Numbers JP17H01201 and JP15H05798 in Precisely Designed
Catalysts with Customized Scaffolding to Y.N. and 15H05800 in Precisely Designed Catalysts with
Customized Scaffolding to N.I. from JSPS and MEXT.
Fellowships for Young Scientists.
R.I. is a recipient of the JSPS Predoctoral
17
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Keywords: Dinitrogen complex; Iron; Cobalt; Ammonia; Silylaimine
18
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(b)
(a)
P1
P1
H1
P3
N1
N2
Si1
Si1
N2
N1
P3
Fe1
Fe1
P2
H1
P2
Figure 1.
ORTEP drawings of (a) 2a·C4H10O and (b) 2b.
Hydrogen atoms and
solvated molecules are omitted for clarity except for the hydride ligand.
26
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European Journal of Inorganic Chemistry
Revised manuscript (ejic.201700569)
(a)
BArF
N
N
4
N
N
N
H
N
H
PCy2
PMe3
P
E
Fe
P
Fe
P
P
Fe
P
P
S
R
Si Fe
N
N
P
Si
Si
Si
H
PCy2
P = PiPr2
S = SAd
P = PiPr2
Ref 24l
2143
P = PiPr2
Ref 24n
This work
Ref 24m
νNN (cm–1) 2041 (2a:R = Me)
2039 (2b:R = Ph)
2061
2055 (E = P)
2060 (E = S)
N
(b)
N
PCy2
P
Co
P
P
Si Co
PCy2
N N
Si
Me
PR3
P = PiPr2
This work
Ref 28d
2063
νNN (cm–1) 2063 (4a: PR3 = PMe3)
2077 (4b: PR3 = PMe2Ph)
2062 (4c: PR3 = PMePh2)
Figure 2.
Iron(II)– and cobalt(I)–dinitrogen complexes bearing various silyl ligands.
27
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Revised manuscript (ejic.201700569)
(a)
P1
Si1
Co1
I1
P2
(b)
(c)
P1
P1
Si1
Si1
P2
N2
Co1
N1
N1
N2
Co1
P3
P3
P2
Figure 3.
ORTEP drawings of (a) 3, (b) 4a, and (c) 4c.
Hydrogen atoms are omitted
for clarity.
28
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Revised manuscript (ejic.201700569)
50
40
30
N(SiMe3)3
(equiv)
20
4c
Co2(CO)8
10
0
0
10
20
30
t (h)
40
50
Figure 4.
Time profiles of catalytic formation of N(SiMe3)3 from N2 using 4c and
[Co2(CO)8] as catalysts.
29
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