Synthesis and protonation of molybdenum-and tungsten-dinitrogen complexes bearing PNP-type pincer ligands

Article abstract of DOI:10.1021/om300011z

Novel molybdenum- and tungsten-dinitrogen complexes bearing PNP-type pincer ligands are prepared and characterized by X-ray analysis. Reactions of these molybdenum- and tungsten-dinitrogen complexes with an excess amount of sulfuric acid in THF at room temperature afford ammonia and hydrazine in good yields.

Full text of DOI:10.1021/om300011z

Article
pubs.acs.org/Organometallics
Synthesis and Protonation of Molybdenum− and Tungsten−
Dinitrogen Complexes Bearing PNP-Type Pincer Ligands
Kazuya Arashiba,† Kouitsu Sasaki,† Shogo Kuriyama,† Yoshihiro Miyake,† Haruyuki Nakanishi,‡
and Yoshiaki Nishibayashi*,†
†Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
‡Fuel Cell System Development Center, Toyota Motor Corporation, Mishuku, Susono, Shizuoka 410-1193, Japan
S
* Supporting Information
ABSTRACT: Novel molybdenum− and tungsten−dinitrogen
complexes bearing PNP-type pincer ligands are prepared and
characterized by X-ray analysis. Reactions of these molybdenum−
and tungsten−dinitrogen complexes with an excess amount of
sulfuric acid in THF at room temperature afford ammonia and
hydrazine in good yields.
PNP-type pincer ligands including 1a and their reactivity
toward protonolysis to produce ammonia and hydrazine from
the coordinated molecular dinitrogen.
INTRODUCTION
■
Since the discovery of the first transition metal−dinitrogen
complex,1 the preparation of various transition metal−
dinitrogen complexes and their stoichiometric transformation
of the coordinated dinitrogen have so far been well investigated
toward the goal of achieving a nitrogen fixation system under
mild reaction conditions.2,3 In sharp contrast to many studies
on the stoichiometric reactivity of transition metal−dinitrogen
complexes,2,3 there are only a few examples of the catalytic
transformation of molecular dinitrogen using these complexes
as catalysts under mild reaction conditions.4 In 2003, Schrock
and his co-worker reported the molybdenum-catalyzed direct
conversion of molecular dinitrogen into ammonia by using a
molybdenum−dinitrogen complex bearing triamidomonoamine
as a tetradentate ligand, where less than 8 equiv of ammonia
was produced based on the catalyst.5 After Schrock’s report,
there is no example of the catalytic conversion of molecular
dinitrogen into ammonia under mild reaction conditions.
Recently, we have found another successful example of the
molybdenum-catalyzed direct conversion of molecular dini-
trogen into ammonia by using a dinitrogen-bridged dimolyb-
denum complex bearing a tridentate PNP-type pincer ligand
RESULTS AND DISCUSSION
■
Treatment of [MoCl3(thf)3] (thf = tetrahydrofuran) with 1
t
t
equiv of Bu-PNP-type pincer ligand (2a: R = Bu) in THF
(tetrahydrofuran) at 50 °C for 18 h gave [MoCl3(2a)] (3a: R =
tBu) in 94% yield (Scheme 1). The complex 3a is paramagnetic.
Scheme 1. Preparation of Molybdenum Complexes Bearing
PNP-Type Pincer Ligand
As shown in the previous report,6 the molecular structure of 3a
was confirmed by X-ray crystallography. Similarly, reactions of
[MoCl3(thf)3] with 1 equiv of R-PNP-type pincer ligands (2b:
6
t
(1a: R = Bu) as a catalyst. In our reaction system, molecular
dinitrogen under an atmospheric pressure was catalytically
converted into ammonia in the presence of both electron
source and proton source, where 23 equiv of ammonia was
produced based on the catalyst (12 equiv of ammonia was
produced based on the molybdenum atom of the catalyst). It is
noteworthy that only 1a works as an effective catalyst, although
some related molybdenum dinitrogen complexes were
investigated toward the catalytic conversion of molecular
dinitrogen into ammonia. In this paper, we describe detailed
methods for the preparation of some derivatives of
molybdenum− and tungsten−dinitrogen complexes bearing
i
R = Pr; 2c: R = Ph) under the same reaction conditions
afforded [MoCl3(2b)] (3b: R = iPr) and [MoCl3(2c)] (3c: R =
Ph) in 92% and 71% yields, respectively. The molecular
structure of 3b was confirmed by X-ray crystallography. An
ORTEP drawing of 3b is shown in Figure 1. Selected bond
lengths and angles for 3b are listed in Table 1. The crystal
Received: January 6, 2012
Published: February 23, 2012
© 2012 American Chemical Society
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Scheme 2. Preparation of a Dinitrogen-Bridged
t
Dimolybdenum Complex Bearing a Bu-PNP Pincer Ligand
Figure 1. Molecular structure of [MoCl3(2b)]·CH2Cl2 (3b·CH2Cl2).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms and solvated molecules are omitted for clarity.
structure of 3b displays a distorted octahedral geometry around
the molybdenum center. The metrical parameters of 3b are
comparable to those of 3a.
Reduction of 3a with an excess amount of Na−Hg (6 equiv)
in THF at room temperature for 12 h under an atmospheric
pressure of dinitrogen gave the diamagnetic dinitrogen-bridged
t
dimolybdenum complex [Mo(N2)2(2a)]2(μ-N2) (1a: R = Bu)
in 63% yield (Scheme 2). The molecular structure of 1a was
confirmed by NMR, IR, and Raman spectra together with an X-
ray crystallographic study, as shown in the previous paper.6 In
sharp contrast to the formation of 1a from 3a, unfortunately, no
formation of the corresponding dinitrogen-bridged dimolybde-
i
num complexes such as [Mo(N2)2(2b)]2(μ-N2) (1b: R = Pr)
and [Mo(N2)2(2c)]2(μ-N2) (1c: R = Ph) was observed by the
same method. In both cases, only unidentified products were
formed.
When reduction of 3a with an excess amount of Na−Hg (6
equiv) in THF at room temperature for 12 h under an
atmospheric pressure of dinitrogen was carried out in the
presence of 10 equiv of dimethylphenylphosphine (PMe2Ph),
the corresponding mononuclear molybdenum dinitrogen
complex bearing both 2a and the phosphine as auxiliary
ligands, trans-[Mo(N2)2(2a)(PMe2Ph)] (4a), was obtained in
48% yield (Scheme 2). The 31P{1H} NMR spectrum of 4a
shows a set of a doublet due to the PNP ligand 2a at 92.7 ppm
and a triplet at 18.5 ppm due to the PMe2Ph ligand with a
coupling constant of 5 Hz, respectively. The IR spectrum
exhibits a strong absorption for N−N stretching at 1915 cm−1,
which is comparable with that in the related mononuclear
molybdenum dinitrogen complex trans-[Mo(N2)2(NC5H5)-
(PMePh2)3] (νNN = 1914 cm−1).7 In the presence of
trimethylphosphine (PMe3) in place of PMe2Ph, a similar
reduction of 3a proceeded smoothly to afford trans-[Mo-
(N2)2(2a)(PMe3)] (4a′) in 46% yield (Scheme 2). The 1H and
31P{1H} NMR spectra of 4a′ are consistent with the
formulation. The IR spectrum of 4a′ displays one ν(NN)
band at 1915 cm−1, consistent with the trans conformation. The
molecular structure of 4a′ was confirmed by X-ray crystallog-
raphy. An ORTEP drawing of 4a′ is shown in Figure 2. Selected
bond lengths and angles for 4a′ are listed in Table 2. The PMe3
ligand occupies the position trans to the pyridine group of a
PNP ligand. The Mo−N (2.01 Å, mean) and N−N (1.13 Å,
mean) bond distances of trans-molybdenum bis(dinitrogen) in
4a′ are not unusual.
Treatment of 1a in THF at room temperature for 12 h under
an atmospheric pressure of carbon monoxide gave the
corresponding dinitrogen-bridged dimolybdenum complex
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 3b·CH2Cl2
Mo(1)−Cl(1)
Mo(1)−Cl(3)
Mo(1)−P(1)
2.4251(9)
2.4164(9)
2.531(1)
157.40(3)
103.77(3)
98.76(3)
83.62(3)
94.39(3)
87.49(7)
90.87(3)
176.26(4)
Mo(1)−Cl(2)
Mo(1)−N(1)
Mo(1)−P(2)
P(1)−Mo(1)−N(1)
P(2)−Mo(1)−N(1)
N(1)−Mo(1)−Cl(2)
P(1)−Mo(1)−Cl(3)
P(2)−Mo(1)−Cl(3)
N(1)−Mo(1)−Cl(3)
Cl(2)−Mo(1)−Cl(3)
2.409(1)
2.229(3)
2.530(1)
78.50(9)
78.92(9)
177.04(7)
94.04(3)
86.65(3)
89.19(7)
92.52(3)
P(1)−Mo(1)−P(2)
P(1)−Mo(1)−Cl(2)
P(2)−Mo(1)−Cl(2)
P(1)−Mo(1)−Cl(1)
P(2)−Mo(1)−Cl(1)
N(1)−Mo(1)−Cl(1)
Cl(1)−Mo(1)−Cl(2)
Cl(1)−Mo(1)−Cl(3)
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Scheme 3. Preparation of a Tungsten Complex Bearing a
PNP-Type Pincer Ligand
Figure 2. Molecular structure of trans-[Mo(N2)2(2a)(PMe3)] (4a′).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms and solvated molecules are omitted for clarity.
bearing carbon monoxide as an auxiliary ligand, [Mo-
(CO)2(2a)]2(μ-N2) (5a), in 64% yield (Scheme 2). Elemental
analysis of 5a indicates that four dinitrogen ligands in 1a are
replaced by four carbonyl ligands. The Raman spectrum of a
THF solution of 5a shows one strong Raman band
corresponding to the bridging-dinitrogen ligand at 1893
cm−1, which is similar to that of the parent dinitrogen-bridged
dimolybdenum complex 1a (νNN = 1890 cm−1). As shown in
the previous report,6 the molecular structure of 5a was
confirmed by X-ray crystallography. Complex 5a is isomor-
phous with 1a and keeps the dinuclear structure during the
course of reaction of 1a with CO.
On the other hand, heating of 1a with 4 equiv of PMe2Ph
and PMe3 in THF at 50 °C for 14 h gave 4a and 4a′ in
moderate yield, respectively (Scheme 2). The result of the
ligand exchange of 1a with carbon monoxide is in sharp
contrast to that of the ligand exchange of 1a with the
phosphines. The regioselectivity of the substitution reaction of
1a is likely due to the steric repulsion with tBu groups of a PNP
ligand neighboring the terminal-dinitrogen ligands in 1a. Thus,
a small molecule such as CO or N2 can coordinate at the apical
position in 1a, whereas phosphines, which are more bulky than
CO and N2, are not substituted with terminal-dinitrogen
ligands but the bridging-dinitrogen ligands even under heating
reaction conditions.
Figure 3. Molecular structure of [WCl3(2a)]·C4H8O (6a·C4H8O).
One of the two crystallographically independent molecules is shown.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms and solvated molecules are omitted for clarity.
one of the independent molecules are listed in Table 3. The
total coordination geometry of 6a is comparable to that of 3a,
suggesting the tungsten center was reduced from W(IV) to
W(III). Unfortunately, no formation of the tungsten congener
of 1a by the reduction of 6a with an excess of Na−Hg or KC8
was observed, where only unidentified products were formed.
In the presence of PMe2Ph, a similar reaction of 6a with Na−
Hg afforded the corresponding tungsten dinitrogen complex
trans-[W(N2)2(2a)(PMe2Ph)] (7a) in 28% isolated yield
(Scheme 4). In the IR spectrum, complex 7a exhibits one
strong ν(NN) band at 1895 cm−1 indicative of the trans
conformation. The detailed structure of 7a has been
determined by X-ray crystallography (Figure 4). Selected
bond lengths and angles for 7a are listed in Table 4. The
coordination geometry of 7a is similar to that of 4a′ except for
PMe2Ph.
Treatment of [WCl4(dme)] (dme = MeOCH2CH2OMe)
with 1 equiv of 2a and 1 equiv of KC8 in THF at room
t
temperature for 24 h gave [WCl3(2a)] (6a: R = Bu) in 59%
yield (Scheme 3). The complex 6a is paramagnetic. An ORTEP
drawing of 6a is shown in Figure 3. The unit cell includes two
crystallographically independent molecules, but their structure
is essentially the same. Selected bond lengths and angles for
With the novel molybdenum− and tungsten−dinitrogen
complexes bearing PNP ligands in hand, we have investigated
the reactivity of these complexes toward protonolysis. Treat-
ment of the molybdenum− and tungsten−dinitrogen com-
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4a′
Mo(1)−N(1)
Mo(1)−N(4)
N(4)−N(5)
Mo(1)−P(2)
Mo(1)−N(2)−N(3)
P(1)−Mo(1)−P(2)
P(1)−Mo(1)−P(3)
P(2)−Mo(1)−P(3)
P(1)−Mo(1)−N(2)
P(2)−Mo(1)−N(2)
P(3)−Mo(1)−N(2)
N(1)−Mo(1)−N(2)
N(2)−Mo(1)−N(4)
2.258(3)
2.013(4)
1.126(7)
2.487(1)
178.8(3)
154.52(4)
103.61(4)
101.85(4)
87.58(11)
92.16(11)
88.53(12)
89.78(15)
176.30(17)
Mo(1)−N(2)
N(2)−N(3)
Mo(1)−P(1)
Mo(1)−P(3)
Mo(1)−N(4)−N(5)
P(1)−Mo(1)−N(1)
P(2)−Mo(1)−N(1)
N(1)−Mo(1)−P(3)
P(1)−Mo(1)−N(4)
P(2)−Mo(1)−N(4)
P(3)−Mo(1)−N(4)
N(1)−Mo(1)−N(4)
2.005(4)
1.134(7)
2.500(1)
2.425(1)
177.2(4)
77.30(9)
77.22(9)
178.05(11)
91.85(11)
89.92(12)
88.05(12)
93.66(16)
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Table 3. Selected Bond Lengths (Å) and Angles (deg) for 6a·C4H8O
W(1)−Cl(1)
W(1)−Cl(3)
W(1)−P(1)
2.393(3)
2.409(3)
2.560(2)
156.85(8)
99.59(7)
103.55(6)
89.45(7)
90.13(8)
88.81(15)
91.52(8)
177.18(10)
W(1)−Cl(2)
W(1)−N(1)
W(1)−P(2)
P(1)−W(1)−N(1)
P(2)−W(1)−N(1)
N(1)−W(1)−Cl(2)
P(1)−W(1)−Cl(3)
P(2)−W(1)−Cl(3)
N(1)−W(1)−Cl(3)
Cl(2)−W(1)−Cl(3)
2.416(3)
2.176(6)
2.577(2)
78.16(14)
78.69(14)
177.73(13)
91.25(6)
88.16(7)
88.66(14)
91.05(8)
P(1)−W(1)−P(2)
P(1)−W(1)−Cl(2)
P(2)−W(1)−Cl(2)
P(1)−W(1)−Cl(1)
P(2)−W(1)−Cl(1)
N(1)−W(1)−Cl(1)
Cl(1)−W(1)−Cl(2)
Cl(1)−W(1)−Cl(3)
Scheme 4. Preparation of a Tungsten−Dinitrogen Complex
Bearing a Phosphine Ligand
small amount of hydrazine (Table 5, runs 1−3). It is
noteworthy that the yield of ammonia by the protonation of
4a was 1.38 equiv based on the molybdenum atom (Scheme 5;
Table 5, run 2). Unfortunately, the protonation of the
dinitrogen-bridged dimolybdenum carbonyl complex 5a
afforded ammonia in only a lower yield (Table 5, run 4). In
contrast to the results of the protonation of these molybdenum
dinitrogen complexes, where ammonia is mainly produced,
when the reaction of the tungsten−dinitrogen complex 7a with
an excess amount of sulfuric acid in THF at room temperature
for 24 h was performed, 0.62 equiv of hydrazine was obtained
based on the tungsten atom together with 0.17 equiv of
ammonia based on the tungsten atom (Scheme 5; Table 5, run
5). These results indicate that the nature of the metal atom in
the dinitrogen complexes bearing a PNP ligand affects the
course of the protonation reaction.
It is well known that the yields of ammonia and/or hydrazine
formed by protonolysis of [M(N2)2L4] (M = Mo, W) are
affected by not only the metal center but also the sort of
solvents, ligands, and acids.2,8,9 In fact, Chatt and co-workers
reported that the reaction of a tungsten−dinitrogen complex
bearing monodentate phosphines as auxiliary ligands, cis-
[W(N2)2(PMe2Ph)4], with an excess amount of sulfuric acid
in MeOH at room temperature produced ammonia in high
yield (1.98 equiv/W), while the molybdenum analogue
afforded only a small amount of ammonia (0.64 equiv/Mo)
due to the poorer electron-donating ability of the molybdenum
atom than that of the tungsten atom.8 Many efforts toward the
formation of ammonia from the molybdenum−dinitrogen
complex suggest a reaction pathway involving disproportiona-
tion of the hydrazido complex.8−10 We suggest that the high
yield of ammonia for 4a, which is comparable to those obtained
from conventional tungsten−dinitrogen complexes such as cis-
Figure 4. Molecular structure of trans-[W(N2)2(2a)(PMe2Ph)] (7a).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms and solvated molecules are omitted for clarity.
plexes 1a, 4a, 4a′, 5a, and 7a with an excess amount of sulfuric
acid in THF at room temperature for 24 h led to the formation
of ammonia and hydrazine. Typical results are shown in Table
5. Reactions of molybdenum−dinitrogen complexes 1a, 4a, and
4a′ exclusively gave ammonia in good yields together with a
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 7a
W(1)−N(1)
W(1)−N(4)
N(4)−N(5)
W(1)−P(2)
W(1)−N(2)−N(3)
P(1)−W(1)−P(2)
P(1)−W(1)−P(3)
P(2)−W(1)−P(3)
P(1)−W(1)−N(2)
P(2)−W(1)−N(2)
P(3)−W(1)−N(2)
N(1)−W(1)−N(2)
N(2)−W(1)−N(4)
2.232(4)
1.993(5)
1.121(7)
2.483(2)
178.7(4)
153.25(5)
104.83(5)
101.80(6)
94.02(17)
87.67(15)
91.50(13)
90.33(16)
178.9(3)
W(1)−N(2)
N(2)−N(3)
W(1)−P(1)
W(1)−P(3)
W(1)−N(4)−N(5)
P(1)−W(1)−N(1)
P(2)−W(1)−N(1)
N(1)−W(1)−P(3)
P(1)−W(1)−N(4)
P(2)−W(1)−N(4)
P(3)−W(1)−N(4)
N(1)−W(1)−N(4)
1.991(4)
1.131(7)
2.492(2)
2.415(1)
179.7(5)
77.03(13)
76.27(13)
177.29(11)
86.82(16)
91.90(15)
87.59(13)
90.56(16)
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Table 5. Protonolysis of Molybdenum− and Tungsten−Dinitrogen Complexes
a
a
run
M(N2) complex
NH3 (mmol)
NH3 (equiv/M)
N2H4 (mmol)
N2H4 (equiv/M)
1
2
3
4
5
1a
4a
4a′
5a
7a
0.049
0.055
0.034
0.008
0.007
0.61
1.38
0.85
0.10
0.17
0.005
0.005
0.003
0
0.06
0.13
0.08
0
0.025
0.62
a
Equiv based on M atom (M = Mo, W).
Scheme 5. Protonolysis of Molybdenum− and Tungsten−
Dinitrogen Complexes
EXPERIMENTAL SECTION
■
General Methods. 1H NMR (270 MHz) and 31P{1H} NMR (109
MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in
a suitable solvent, and spectra were referenced to the residual solvent
(1H) or external standard (31P{1H}: 85% H3PO4). IR spectra were
recorded on a JASCO FT/IR 4100 Fourier transform infrared
spectrometer. Raman spectra were recorded on a JASCO NRS-2000.
Absorption spectra were recorded on a Shimadzu MultiSpec-1500.
Elemental analyses were performed at the Microanalytical Center of
The University of Tokyo.
All manipulations were carried out under an atmosphere of nitrogen
by using standard Schlenk techniques or glovebox techniques unless
otherwise stated. Solvents were dried by the usual methods, then
distilled and degassed before use. [Mo(N2)2(2a)]2(μ-N2) (1a),6 2,6-
bis(di-tert-butylphosphinomethyl)pyridine (2a),13 2,6-bis(di-
isopropylphosphinomethyl)pyridine (2b),1 4 2,6-bis-
(diphenylphosphinomethyl)pyridine (2c),15 [MoCl3(2a)] (3a),6 [Mo-
(N2)2(2a)(PMe2Ph)] (4a),6 [Mo(CO)2(2a)]2(μ-N2) (5a),6
[MoCl3(thf)3],16 and [WCl4(dme)]17 were prepared according to
the literature methods.
Preparation of [MoCl3(2b)] (3b). A mixture of 2b (503.6 mg,
1.48 mmol) and [MoCl3(thf)3] (516.3 mg, 1.23 mmol) in THF (20
mL) was stirred at 50 °C for 24 h. The resultant yellow suspension was
concentrated under reduced pressure, and the residue was recrystal-
lized from CH2Cl2 (12 mL)−hexane (40 mL) to give yellow needles
of 3b·CH2Cl2, which were collected by filtration, washed with Et2O (5
mL × 3), and dried in vacuo to afford 3b as a yellow crystalline solid
(617.3 mg, 1.14 mmol, 92% yield). Anal. Calcd for C19H35Cl3MoNP2:
C, 42.12; H, 6.51; N, 2.59. Found: C, 41.93; H, 6.33; N, 2.47.
Preparation of [MoCl3(2c)] (3c). A mixture of 2c (114.6 mg, 0.24
mmol) and [MoCl3(thf)3] (100.1 mg, 0.24 mmol) in THF (10 mL)
was stirred at 50 °C for 13 h. The resultant yellow suspension was
filtered, washed with Et2O (3 mL × 2), and dried in vacuo to afford 3c
as a yellow solid (115.2 mg, 0.17 mmol, 71% yield). Anal. Calcd for
C31H27Cl3MoNP2: C, 54.93; H, 4.01; N, 2.07. Found: C, 54.61; H,
4.09; N, 1.99.
[W(N2)2(PMe2Ph)4], is due to the higher reducing ability of
the molybdenum center in 4a bearing both the PNP ligand 2a
and PMe2Ph.11 A related example of the protonation of 4a is
the reaction of a molybdenum−dinitrogen complex bearing
mixed tridentate and monodentate ligands, trans-[Mo-
(N2)2(triphos)(PPh3)] (triphos = PhP(CH2CH2PPh2)2), with
an excess amount of HBr in THF at room temperature, giving
0.87 equiv of ammonia.9a
On the other hand, the protonation reaction of the
tungsten−dinitrogen complex 7a preferentially produced
hydrazine. Previously, Hidai and co-workers reported that the
reaction of cis-[M(N2)2(PMe2Ph)4] (M = Mo, W) with an
excess amount of HCl in DME at room temperature gave
hydrazine as a main product in moderate yield.12 The reaction
pathway involving the hydrido−hydrazido complex formed by
protonation of the corresponding hydrazido complex at the
metal center is proposed for the formation of hydrazine. Thus,
it might be possible that the more electron-rich nature of the
tungsten center in 7a than that of the molybdenum analogue 4a
causes the proton attack on the tungsten center of the
hydrazido intermediate (WN−NH2) to give hydrazine.
In conclusion, we have synthesized a series of molybdenum−
and tungsten−dinitrogen complexes bearing a PNP-type pincer
ligand. Protonation of the molybdenum−dinitrogen complexes
with an excess of sulfuric acid in THF afforded ammonia in
good yields. In contrast, hydrazine was obtained upon the
reaction of the more electron-rich tungsten complex with
sulfuric acid under the same reaction conditions. We believe
that the presence of the pincer ligand in the molybdenum− and
tungsten−dinitrogen complexes realizes this unique reactivity
toward the protonolysis. We will report the detailed catalytic
activity of molybdenum−dinitrogen complexes bearing a pincer
ligand in due course. Further studies on the preparation and
reactivity of related early transition metal (Re, V, Nb, Ta, Cr,
Mo, and W)−dinitrogen complexes bearing pincer ligands
including other PCP-type pincer ligands are currently under
way.
Preparation of trans-[Mo(N2)2(2a)(PMe2R)] (4a, R = Ph; 4a′, R
= Me). From 3a: To Na−Hg (0.5 wt %, 4.2309 g, 0.91 mmol) was
added THF (10 mL). After the sequential addition of PMe3 (1 M
solution in toluene, 1.5 mL, 1.5 mmol) and 3a (89.7 mg. 0.15 mmol),
the mixture was stirred at room temperature for 12 h under a N2
atmosphere. The resultant dark purple solution was decanted, and
then the supernatant solution was concentrated in vacuo. To the
residue was added Et2O (6 mL), the solution was filtered through
Celite, and the filter cake was washed with Et2O (2 mL × 5). After the
combined filtrate was concentrated in vacuo to about 1 mL, the
obtained solution was stood at −40 °C to afford 4a′ as purple crystals,
which were collected by filtration and dried in vacuo (43.3 mg, 0.07
2
mmol, 46% yield). 31P{1H} NMR (C6D6): δ 94.6 (d, JPP = 5 Hz,
2
1
PNP), 6.6 (t, JPP = 5 Hz, PMe3). H NMR (C6D6): δ 6.74−6.68 (m,
PNP, 3H), 3.25 (br s, CH2PtBu2, 4H), 1.70 (d, JPH = 5.1 Hz, PMe3,
2
9H), 1.26 (pseudo t, CH2PtBu2, 36H). IR (KBr, cm−1): 1915 (s, νNN).
Calcd for C26H52MoN5P3: C, 50.08; H, 8.41; N, 11.23. Found: C,
50.10; H, 8.33; N, 11.21. The synthetic method for 4a from 3a was
previously reported.6 From 1a: To a solution of 1a (73.1 mg. 0.065
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Table 6. X-ray Crystallographic Data for [MoCl3(2b)]·CH2Cl2 (3b·CH2Cl2), trans-[Mo(N2)2(2a)(PMe3)] (4a′),
[WCl3(2a)]·C4H8O (6a·C4H8O), and trans-[W(N2)2(2a)(PMe2Ph)] (7a)
3b·CH2Cl2
4a′
6a·C4H8O
7a
chemical formula
fw
C20H37Cl5MoNP2
626.67
C26H52MoN5P3
623.59
C27H51Cl3NOP2W
757.86
C31H54N5P3W
773.57
dimens of cryst
cryst syst
space group
a, Å
0.50 × 0.35 × 0.20
monoclinic
C2/c
0.25 × 0.10 × 0.10
monoclinic
P21/c
0.40 × 0.30 × 0.15
monoclinic
C2
0.45 × 0.25 × 0.03
triclinic
P1
̅
19.9052(5)
10.7519(3)
27.3135(8)
90
8.8589(8)
20.731(2)
17.840(2)
90
29.2304(8)
14.9824(4)
19.7839(5)
90
10.6999(6)
13.0508(6)
13.9197(7)
73.199(1)
79.697(2)
69.329(2)
1734.6(2)
2
b, Å
c, Å
α, deg
β, deg
107.3880(9)
90
106.435(3)
90
126.2874(7)
90
γ, deg
V, Å3
5578.5(3)
8
3142.5(5)
4
6983.9(3)
8
Z
ρcalcd, g cm−3
1.492
1.318
1.441
1.481
F(000)
μ, cm−1
2568
1320
3064
788
10.715
5.924
36.517
34.996
transmn factors range
no. reflns measd
no. unique reflns
no. params refined
0.665−0.807
26 830
0.480−0.942
19 707
0.461−0.578
55 741
0.444−0.900
16 833
6224 (Rint = 0.021)
299
5487 (Rint = 0.090)
368
15 123 (Rint = 0.041)
676
7818 (Rint = 0.046)
415
a
R1 (I > 2 σ(I))
0.0508
0.0330
0.0316
0.0406
b
wR2 (all data)
0.1259
0.1090
0.0785
0.0925
c
GOF (all data)
1.022
1.002
1.009
1.017
Flack param
0.007(5)
max. diff peak/hole, e Å−3
2.28/−1.92
0.47/−0.30
3.98/−0.95
2.15/−1.98
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo )2]1/2, w = 4Fo /[pFo2 + qσ(Fo )] [p = 0, q = 12.0 (3b·CH2Cl2); p = 0.0006, q = 0.16
2
2
2
c
(4a′); p = 0, q = 3.3 (6a·C4H8O); p = 0, q = 1.45 (7a)]. GOF = [∑w(Fo − Fc2)2/(No − Nparams)]1/2
.
2
mmol) in THF (5 mL) was added PMe2Ph (40 μL, 0.281 mmol), and
then the mixture was stirred at 50 °C for 14 h. The resultant dark
purple solution was concentrated, and the residue was extracted with
Et2O (10 mL). After the extract was concentrated to about 3 mL, the
obtained solution was stood at −40 °C to afford 4a as purple crystals
(45.5 mg, 0.066 mmol, 51% yield). The spectroscopic data are in
agreement with those previously reported. The synthetic method for
4a′ from 1a is similar to that of 4a. 4a′: purple crystals, 49% yield.
Preparation of [WCl3(2a)] (6a). A mixture of 2a (198.9 mg, 0.50
Anal. Calcd for C31H54N5P3W: C, 48.13; H, 7.04; N, 9.05. Found: C,
48.11; H, 7.17; N, 8.85.
Protonolysis of Molybdenum− and Tungsten−Dinitrogen
Complexes. To a solution of molybdenum− or tungsten−dinitrogen
complex (0.040 mmol) in THF (5 mL) was added concentrated
sulfuric acid (50 μL). The mixture was stirred at room temperature for
24 h. The reaction mixture was evaporated under reduced pressure.
Potassium hydroxide aqueous solution (30 wt %; 5 mL) was added to
the residue, and the mixture was distilled into dilute H2SO4 solution
(0.5 M, 10 mL) under reduced pressure. The amount of NH3 or N2H4
was determined by the indophenol18 or p-(dimethylamino)-
benzaldehyde method,19 respectively. Detailed results are shown in
Table 5.
mmol), [WCl4(dme)] (208.0 mg, 0.50 mmol; dme
=
MeOCH2CH2OMe), and KC8 (67.8 mg, 0.50 mmol) in THF (20
mL) was stirred at room temperature for 24 h. The resultant dark
purple solution was filtered. Slow addition of hexane (40 mL) to the
filtrate afforded 6a·C4H8O as dark brown crystals, which were
collected by filtration, washed with Et2O (5 mL), and dried in vacuo
to afford 6a as a brown crystalline solid (202.4 mg, 0.30 mmol, 59%
yield). Anal. Calcd for C23H43Cl3WNP2: C, 40.28; H, 6.32; N, 2.04.
Found: C, 40.31; H, 6.25; N, 1.82.
X-ray Crystallography. Crystallographic data of 3b·CH2Cl2, 4a′,
6a·C4H8O, and 7a are summarized in Table 6. Selected bond lengths
and angles are summarized in Tables 1−4, and their ORTEP drawings
are shown in Figures 1−4. Diffraction data were collected at −100 °C
on a Rigaku RAXIS RAPID imaging plate area detector with graphite-
monochromated Mo Kα radiation (λ = 0.71075 Å). Reflections were
collected for the 2θ range of 5° to 55°. Intensity data were collected
for Lorentz−polarization effects and for empirical absorption
(REQAB). The structure solution and refinements were carried out
by using the CrystalStructure crystallographic software package.20 The
positions of the non-hydrogen atoms were determined by heavy atom
Patterson methods (PATTY21 for 3b·CH2Cl2, SHELX-9722 for
6a·C4H8O) or direct methods (SHELX-9722 for 4′, SIR-2004 for 7a)
and subsequent Fourier syntheses (DIRDIF-9923) and were refined on
Preparation of trans-[W(N2)2(2a)(PMe2Ph)] (7a). To Na−Hg
(0.43 wt %, 7.96 g, 1.51 mmol) were added THF (15 mL), PMe2Ph
(75 μL, 0.53 mmol), and 6a (178 mg. 0.26 mmol), and then the
mixture was stirred at room temperature for 13 h under N2. The
resultant dark reddish-purple solution was decanted, and then the
supernatant solution was concentrated in vacuo. To the residue was
added Et2O (10 mL), the solution was filtered through Celite, and the
filter cake was washed with Et2O (5 mL × 3). After the combined
filtrate was concentrated in vacuo to about 1 mL, the obtained solution
was stood at −40 °C to afford 7a as purple crystals, which were
collected by filtration and dried in vacuo (55.6 mg, 0.07 mmol, 28%
2
yield). 31P{1H} NMR (C6D6): δ 73.1 (d with W satellites, JPP = 5.5
2
2
2
Fo using all unique reflections by full-matrix least-squares with
Hz, JPW = 328.2 Hz, PNP), −17.4 (t with W satellites, JPP = 5.5 Hz,
2JPW = 405.2 Hz, PMe2Ph). H NMR (C6D6): δ 7.65 (m, PMe2Ph,
1
anisotropic thermal parameters except for a few solvated molecules
(O1−O4 and C47−C61 for 6a·C4H8O), which were refined
isotropically. All other hydrogen atoms were placed at calculated
positions with fixed isotropic parameters.
2H), 7.27 (m, PMe2Ph, 2H), 7.02 (m, PMe2Ph, 1H), 6.81−6.70 (m,
PNP, 3H), 3.35 (br s, CH2PtBu2, 4H), 2.13 (d, 2JPH = 5.9 Hz, PMe2Ph,
6H), 1.26 (pseudo t, CH2PtBu2, 36 H). IR (KBr, cm−1): 1889 (s, νNN).
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(10) Molybdenum− and tungsten−hydrazido complexes (MN−
NH2) are well known as one of the intermediates in the transformation
of dinitrogen into ammonia. See ref 2.
ASSOCIATED CONTENT
■
S
* Supporting Information
(11) (a) We have confirmed in the previous report that the reaction
of the molybdenum hydrazido complex [MoF(2a)(NNH2)(pyridine)]
BF4, which is prepared by the reaction of 1a with HBF4 followed by
addition of pyridine, with an excess amount of 2,6-lutidinium
trifluoromethanesulfonate produced ammonia (see ref 6). On the
basis of the experimental results, it is generally believed that tungsten
complexes have a stronger reducing ability via π back-donation than
molybdenum complexes do. (b) Hidai, M.; Mizobe, Y.; Sato, M.;
Kodama, T.; Uchida, Y. J. Am. Chem. Soc. 1978, 100, 5740. (c) Tuczek,
F.; Horn, K. H.; Lehnert, N. Coord. Chem. Rev. 2003, 245, 107. (d)
See ref 2.
(12) Takahashi, T.; Mizobe, Y.; Sato, M.; Uchida, Y.; Hidai, M. J. Am.
Chem. Soc. 1980, 102, 7461.
(13) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960.
(14) Leung, W.-P.; Ip, Q. W.-Y.; Wong, S.-Y.; Mak, T. C. W.
Organometallics 2003, 22, 4604.
Crystallographic data for 3b·CH2Cl2, 4a′, 6a·C4H8O, and 7a are
available in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ynishiba@sogo.t.u-tokyo.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for Next
Generation World-Leading Researchers (GR025). Financial
support from Toyota Motor Corporation is also gratefully
acknowledged. We thank the Research Hub for Advanced
Nano Characterization at The University of Tokyo.
(15) Cochran, B. M.; Michael, F. E. J. Am. Chem. Soc. 2008, 130,
2786.
(16) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001,
2699.
(17) Persson, C.; Andersson, C. Inorg. Chim. Acta 1993, 203, 235.
(18) Weatherburn, M. W. Anal. Chem. 1967, 39, 971.
(19) Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.
(20) CrystalStructure 3.80: Single Crystal Structure Analysis Software;
Rigaku Corp: Tokyo, Japan, and MSC: The Woodlands, TX, 2000−
2007.
(21) Beurskens, P. T., Admiraal, G., Beurskens, G., Bosman, W. P.,
Garcia-Granda, S., Gould, R. O., Smits, J. M. M., Smykalla, C. PATTY:
The DIRDIF Program System; Technical Report of the Crystallography
Laboratory, University of Nijmegen: Nijmegen, The Netherlands,
1992.
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