Synthesis and catalytic activity of molybdenum-dinitrogen complexes bearing unsymmetric PNP-type pincer ligands

Article abstract of DOI:10.1021/om301046t

Novel dinitrogen-bridged dimolybdenum complexes bearing unsymmetric PNP-type pincer ligands are prepared and characterized by X-ray analysis. A molybdenum-dinitrogen complex bearing 2-(di-1-adamantylphosphino)methyl-6-(di- tert-butylphosphino)methylpyridine has been found to work as an effective catalyst toward the formation of ammonia from molecular dinitrogen under ambient conditions.

Full text of DOI:10.1021/om301046t

Article
pubs.acs.org/Organometallics
Synthesis and Catalytic Activity of Molybdenum−Dinitrogen
Complexes Bearing Unsymmetric PNP-Type Pincer Ligands
Eriko Kinoshita,† Kazuya Arashiba,† Shogo Kuriyama,† Yoshihiro Miyake,† Ryuji Shimazaki,‡
Haruyuki Nakanishi,§ and Yoshiaki Nishibayashi*,†
†Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
‡Future Project Division, Toyota Motor Corporation, Mishuku, Susono, Shizuoka 410-1193, Japan
§Fuel Cell System Development Center, Toyota Motor Corporation, Mishuku, Susono, Shizuoka 410-1193, Japan
S
* Supporting Information
ABSTRACT: Novel dinitrogen-bridged dimolybdenum complexes bearing unsym-
metric PNP-type pincer ligands are prepared and characterized by X-ray analysis. A
molybdenum−dinitrogen complex bearing 2-(di-1-adamantylphosphino)methyl-6-
(di-tert-butylphosphino)methylpyridine has been found to work as an effective
catalyst toward the formation of ammonia from molecular dinitrogen under ambient
conditions.
bridged dimolybdenum complex 1 works as an effective
catalyst, although some related molybdenum−dinitrogen
complexes bearing monodentate and bidentate phosphines
were investigated toward the catalytic conversion of molecular
dinitrogen into ammonia.
INTRODUCTION
■
Toward the goal of achievement of a nitrogen fixation system
under mild reaction conditions,1 we have recently found
another successful example2 of the molybdenum-catalyzed
direct conversion of molecular dinitrogen into ammonia by
using a dinitrogen-bridged dimolybdenum complex bearing
tridentate PNP-type pincer ligands (1) as a catalyst (Scheme
1).3 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 the dinitrogen-
As an extension of our study, we have already prepared some
molybdenum complexes bearing other types of PNP pincer
ligands, where both phosphorus atoms have the same
substituents such as isopropyl and phenyl groups; however,
the corresponding dinitrogen-bridged dimolybdenum com-
plexes cannot be prepared according to the previous method.4
These results prompted us to investigate the preparation of
dinitrogen-bridged dimolybdenum complexes bearing unsym-
metric PNP-type pincer ligands, where one phosphorus atom in
the pincer ligand has two tert-butyl groups as substituents and
the other has two other substituents such as 1-adamantyl,
phenyl, isopropyl, and cyclohexyl groups (Chart 1). In this
paper, we describe detailed methods for the preparation of
Scheme 1. Molybdenum-Catalyzed Reduction of Molecular
Dinitrogen into Ammonia under Ambient Conditions
Chart 1
Received: November 2, 2012
© XXXX American Chemical Society
A
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Article
unsymmetric PNP-type pincer ligands and the corresponding
dinitrogen-bridged dimolybdenum complexes. Interestingly, a
dinitrogen-bridged dimolybdenum complex bearing 2-(di-1-
adamantylphosphino)methyl-6-(di-tert-butylphosphino)-
methylpyridine has been found to work as an effective catalyst
toward the formation of ammonia from molecular dinitrogen
under ambient conditions.
RESULTS AND DISCUSSION
■
Treatment of 2,6-lutidine with 1 equiv of nBuLi in diethyl ether
t
at 0 °C for 1 h and then the addition of Bu2PCl at −78 °C to
room temperature for 2 h gave 2a (R = Bu) in 54% yield
(Scheme 2). After lithiation of 2a with 3 equiv of BuLi in
t
n
Figure 1. Molecular structure of [MoCl3(3b)] (4b). Thermal
ellipsoids are shown at the 50% probability level. Hydrogen atoms
and solvated molecules are omitted for clarity.
Scheme 2. Preparation of Unsymmetric PNP-Type Pincer
Ligands
diethyl ether at 0 °C to reflux temperature for 15 h, the
addition of Ad2PCl (Ad = 1-adamantyl) at −78 °C to room
temperature for 22 h gave 3a (R = tBu, R′ = Ad) in 72% yield as
an unsymmetric PNP-type pincer ligand. According to the same
method, other unsymmetric pincer ligands (3c and 3d) were
prepared in high yields and characterized spectroscopically.
Although the reaction of 2a with Ph2PCl gave only a complex
mixture, the reaction of 2-(diphenylphosphinomethyl)-6-
Figure 2. Molecular structure of [MoCl3(3d)] (4d). Thermal
ellipsoids are shown at the 50% probability level. Hydrogen atoms
and solvated molecules are omitted for clarity.
t
(N2)2(3b)]2(μ-N2) (5b: R = Bu, R′ = Ph) in 49% and 54%
yields, respectively (Scheme 4). The IR spectrum of 5a show a
strong absorption at 1939 cm−1 assignable to ν(NN), while the
ν(NN) value of 5b (1957 cm−1) is higher than that of 1 (1936
cm−1), suggesting the less electron-donating ability of the
pincer ligand 3b. The detailed structure of 5b has been
confirmed by X-ray crystallography. An ORTEP drawing of 5b
is shown in Figure 3. Selected bond lengths and angles for 5b
are listed in Table 3. As shown in Figure 3, complex 5b has a
dinuclear structure, where two trans-[Mo(N2)2(3b)] units are
bridged by one dinitrogen ligand in an end-on fashion. The
total coordination geometry of 5b is comparable to that of 1
except for the PNP pincer ligands. The bridging N−N bond
distance (1.131(4) Å) is close to that of the dinitrogen-bridged
Mo(0) complex [Mo(CO)(Et2PCH2CH2PEt2)2]2(μ-N2) (N−
N 1.127(5) Å).5
t
methylpyridine (2b) as starting material with Bu2PCl afforded
3b in 89% yield.
Treatment of [MoCl3(thf)3] (thf = tetrahydrofuran) with 1
equiv of 3a in THF (tetrahydrofuran) at 50 °C for 20 h gave
the paramagnetic molybdenum pincer complex [MoCl3(3a)]
t
(4a: R = Bu, R′ = Ad) in 97% yield (Scheme 3). Similarly,
Scheme 3. Preparation of Molybdenum Trichloride
Complexes Bearing Unsymmetric PNP-Type Pincer Ligands
On the other hand, reduction of 4c and 4d with an excess
amount of Na−Hg under the same reaction conditions gave
also the dinitrogen-bridged dimolybdenum complexes [Mo-
reactions of [MoCl3(thf)3] with 1 equiv of other unsymmetric
PNP-type pincer ligands (3b, 3c, and 3d) under the same
reaction conditions afforded [MoCl3(3)] (4b: R = tBu, R′ = Ph;
4c: R = tBu, R′ = iPr; 4d: R = tBu, R′ = Cy (Cy = cyclohexyl))
in 88%, 76%, and 92% yields, respectively. The molecular
structures of 4b and 4d were confirmed by X-ray crystallog-
raphy. ORTEP drawings of 4b and 4d are shown in Figures 1
and 2. Selected bond lengths and angles for 4b and 4d are listed
in Tables 1 and 2. The crystal structures of 4b and 4d display a
distorted octahedral geometry around the molybdenum center.
Reduction of 4a and 4b 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 corresponding
dinitrogen-bridged dimolybdenum complexes [Mo-
i
(N2)2(3c)]2(μ-N2) (5c: R = tBu, R′ = Pr) and [Mo-
t
(N2)2(3d)]2(μ-N2) (5d: R = Bu, R′ = Cy) in 43% and 61%
yields, respectively (Scheme 4). In contrast to 5a and 5b, the IR
spectrum of 5c exhibits four strong absorptions at 1968, 1949,
1897, and 1875 cm−1 attributable to ν(NN) bands. The
31P{1H} NMR spectrum of 5c shows a set of doublets at 89.5
ppm due to the tBu2P group and a doublet at 75.5 ppm due to
the iPr2P group with a coupling constant of 143 Hz, and a set of
t
doublets at 88.4 ppm due to the Bu2P group and a doublet at
i
75.7 ppm due to the Pr2P group with a coupling constant of
130 Hz, respectively. The IR and 31P{1H} NMR spectra of 5d
are similar to those of 5c. These results of spectroscopical data
for 5c and 5d indicate that the molecular structures of 5c and
t
(N2)2(3a)]2(μ-N2) (5a: R = Bu, R′ = Ad) and [Mo-
B
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 4b
Mo(1)−Cl(1)
Mo(1)−Cl(3)
Mo(1)−P(1)
2.4157(7)
2.4052(7)
2.5900(6)
155.12(2)
102.84(2)
102.02(2)
96.62(2)
84.24(2)
92.10(5)
90.46(2)
170.62(2)
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)
2.3970(6)
2.216(2)
2.5288(6)
77.73(5)
77.39(5)
177.30(5)
91.71(2)
86.39(2)
85.46(5)
91.88(2)
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)
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4d
Mo(1)−Cl(1)
Mo(1)−Cl(3)
Mo(1)−P(1)
2.4259(5)
2.4220(5)
2.6139(6)
155.90(2)
103.21(2)
100.86(2)
94.66(2)
83.92(2)
92.16(4)
90.46(2)
173.93(2)
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)
2.3997(6)
2.214(1)
2.5376(6)
77.52(4)
78.49(4)
177.22(4)
89.58(2)
90.44(2)
84.50(4)
92.81(2)
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)
Scheme 4. Preparation of Dinitrogen-Bridged
Dimolybdenum Complexes Bearing Unsymmetric PNP-
Type Pincer Ligands
5d are quite different from those of 5a and 5b. The detailed
structure of 5c has been unambiguously determined by X-ray
crystallography. An ORTEP drawing of 5c is shown in Figure 4.
Selected bond lengths and angles for 5c are listed in Table 4.
The distinct structural feature of 5c is the existence of trans-
[Mo(N2)2(3c)] and cis-[Mo(N2)2(3c)], which are bridged by
one dinitrogen ligand in an end-on fashion. The difference in
the coordination geometry around each molybdenum center is
in accord with the spectral data, suggesting that dimolybdenum
complex 5c also maintains this configuration in solution. The
bridging N−N (1.148(4) Å) and terminal N−N (1.12 Å,
mean) bond distances are comparable to those of 5b, and the
Figure 3. Molecular structure of [Mo(N2)2(3b)]2(μ-N2) (5b·3C6H6).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms and solvated molecules are omitted for clarity.
OSO2CF3) as a proton source in toluene was added a solution
of cobaltocene (72 equiv to 5a; CoCp2; Cp = η5-C5H5) as a
reductant in toluene via a syringe pump at room temperature
over a period of 1 h, followed by stirring at room temperature
for another 19 h under an atmospheric pressure of dinitrogen.
After the reaction, ammonia (14 equiv to 5a; 7 equiv to Mo
atom in 5a) was catalytically formed together with the
formation of dihydrogen (8 equiv to 5a) (Table 5, run 1).
Ammonia and dihydrogen were formed in 58% and 22% yields,
respectively, based on the cobaltocene. The amount of
ammonia by using 5a as a catalyst is larger than that by using
1 as a catalyst (12 equiv of ammonia to 1; Table 5, run 7).3 It is
noteworthy that 5a worked as a more effective catalyst than 1
under the present reaction conditions.
i
Mo−N−N−Mo array in 5c is almost linear. The Pr2P groups
of PNP ligands in 5c are oriented to avoid steric interactions,
where the dihedral angle between the planes defined by P(1)−
N(1)−P(2)−N(6) and P(3)−N(7)−P(4)−N(9) is 70.73(3)°.
Next, the catalytic reduction of dinitrogen into ammonia by
using 5 was investigated according to a modified procedure.3
To a suspension of 5a and 2,6-lutidinium trifluoromethanesul-
fonate (96 equiv to 5a; [LutH]OTf; Lut = 2,6-lutidine; OTf =
C
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Table 3. Selected Bond Lengths (Å) and Angles (deg) for 5b·3C6H6
Mo(1)−N(1)
Mo(1)−N(4)
Mo(1)−P(1)
N(2)−N(3)
2.195(3)
2.007(3)
2.466(1)
1.111(5)
1.131(4)
175.5(3)
175.4(3)
78.49(8)
106.40(9)
175.1(1)
95.5(1)
Mo(1)−N(2)
Mo(1)−N(6)
Mo(1)−P(2)
N(4)−N(5)
2.036(3)
2.045(3)
2.3884(9)
1.124(5)
N(6)−N(6*)
Mo(1)−N(2)−N(3)
Mo(1)−N(6)−N(6*)
P(1)−Mo(1)−N(1)
P(2)−Mo(1)−N(6)
N(1)−Mo(1)−N(6)
P(1)−Mo(1)−N(4)
P(2)−Mo(1)−N(4)
N(1)−Mo(1)−N(4)
N(4)−Mo(1)−N(6)
Mo(1)−N(4)−N(5)
P(1)−Mo(1)−P(2)
P(1)−Mo(1)−N(6)
P(2)−Mo(1)−N(1)
P(1)−Mo(1)−N(2)
P(2)−Mo(1)−N(2)
N(1)−Mo(1)−N(2)
N(2)−Mo(1)−N(6)
N(2)−Mo(1)−N(4)
175.9(4)
156.97(4)
96.64(9)
78.48(8)
92.1(1)
87.54(9)
88.2(1)
85.26(9)
92.8(1)
91.3(1)
88.3(1)
172.4(1)
However, the use of larger amounts of both cobaltocene
(216 equiv) and [LutH]OTf (288 equiv) only slightly increased
the amount of ammonia (16 equiv of ammonia to 5a; Table 5,
run 2), in contrast to the catalytic reaction by using 1 as a
catalyst under the same reaction conditions (23 equiv of
ammonia to 1; Table 5, run 8). When we reduced the time of
addition of a toluene solution of cobaltocene from 1 h to 30
min in the reaction using 5a as a catalyst, substantially a larger
amount of ammonia was produced based on the catalyst (19
equiv of ammonia to 5a; Table 5, run 3).6 These results
indicate that 5a has been found to be a less effective catalyst
than 1 toward the formation of ammonia from molecular
dinitrogen under ambient reaction conditions.
In contrast to 5a, other dinitrogen-bridged dimolybdenum
complexes such as 5b, 5c, and 5d did not work as catalysts. In
these cases, only a stoichiometric amount of ammonia was
produced based on the catalyst (Table 5, runs 4−6). We have
not yet obtained the exact reason that these dinitrogen-bridged
Figure 4. Molecular structure of [Mo(N2)2(3c)]2(μ-N2) (5c).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms and solvated molecules are omitted for clarity.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 5c
Mo(1)−N(1)
Mo(1)−N(4)
Mo(2)−N(7)
Mo(2)−N(9)
Mo(1)−P(1)
Mo(2)−P(3)
N(2)−N(3)
N(6)−N(7)
2.219(3)
2.029(3)
2.094(3)
1.977(3)
2.479(1)
2.472(1)
1.126(5)
1.148(4)
1.133(5)
175.3(3)
177.5(3)
178.9(3)
156.69(4)
78.06(9)
102.48(9)
94.27(9)
83.19(9)
93.0(1)
Mo(1)−N(2)
Mo(1)−N(6)
Mo(2)−N(8)
Mo(2)−N(11)
Mo(1)−P(2)
Mo(2)−P(4)
N(4)−N(5)
2.020(3)
1.991(3)
2.213(3)
1.999(4)
2.424(1)
2.424(1)
1.118(5)
1.122(5)
N(9)−N(10)
N(11)−N(12)
Mo(1)−N(2)−N(3)
Mo(1)−N(6)−N(7)
Mo(2)−N(9)−N(10)
P(1)−Mo(1)−P(2)
P(1)−Mo(1)−N(1)
P(2)−Mo(1)−N(6)
P(1)−Mo(1)−N(2)
P(2)−Mo(1)−N(2)
N(1)−Mo(1)−N(2)
N(2)−Mo(1)−N(6)
P(3)−Mo(2)−N(8)
P(4)−Mo(2)−N(11)
P(3)−Mo(2)−N(7)
P(4)−Mo(2)−N(7)
N(7)−Mo(2)−N(8)
N(8)−Mo(2)−N(9)
Mo(1)−N(4)−N(5)
Mo(2)−N(7)−N(6)
Mo(2)−N(11)−N(12)
P(3)−Mo(2)−P(4)
P(1)−Mo(1)−N(6)
P(2)−Mo(1)−N(1)
P(1)−Mo(1)−N(4)
P(2)−Mo(1)−N(4)
N(1)−Mo(1)−N(4)
N(4)−Mo(1)−N(6)
P(3)−Mo(2)−N(11)
P(4)−Mo(2)−N(8)
P(3)−Mo(2)−N(9)
P(4)−Mo(2)−N(9)
N(7)−Mo(2)−N(11)
N(9)−Mo(2)−N(11)
175.6(3)
173.4(3)
177.9(3)
157.26(4)
100.72(9)
78.94(9)
89.52(8)
92.06(8)
84.6(1)
90.9(1)
91.5(1)
78.59(7)
101.37(9)
94.60(8)
85.22(8)
95.7(1)
101.36(9)
78.80(8)
89.10(9)
92.24(9)
88.4(1)
87.2(1)
88.7(1)
D
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Organometallics
Article
dinitrogen complexes did not work as catalysts. These results
indicate that the modification of the PNP-type pincer ligand
dramatically affects the catalytic activity. We believe that the
result described in this paper provides useful information to
design a more effective catalyst toward the catalytic formation
of ammonia under mild reaction conditions. Further work is
currently in progress to develop a more effective reaction
system including the elucidation of the reaction pathway.9
Table 5. Molybdenum-Catalyzed Reduction of Molecular
Dinitrogen into Ammonia under Ambient Conditions
a
Cp2Co
(equiv)
[LutH]OTf
(equiv)
NH
H2
(equiv)
3 b
b
run catalyst
(equiv)
1
2
3
4
5
6
7
8
5a
5a
5a
5b
5c
5d
1
72
216
216
72
96
288
288
96
14
16
19
2
8
47
48
22
21
22
10
46
EXPERIMENTAL SECTION
■
General Methods. 1H NMR (270 MHz) and 31P{1H} NMR (109
MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in
suitable solvent, and spectra were referenced to residual solvent (1H)
or external standard (31P{1H}: 85% H3PO4). IR spectra were recorded
on a JASCO FT/IR 4100 Fourier transform infrared spectrometer.
Absorption spectra were recorded on a Shimadzu MultiSpec-1500.
Elemental analyses were performed at Microanalytical Center of The
University of Tokyo.
c
72
96
3
72
96
1
72
96
12
23
1
216
288
a
To a suspension of catalyst (5 or 1; 0.010 mmol) and 2,6-lutidinium
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. Dinitrogen-bridged dimolybdenum
complex 1,3 2-(diphenylphosphinomethyl)-6-methylpyridine (2b),10
Ad2PCl,11 and [MoCl3(thf)3]12 were prepared according to the
literature methods.
trifluoromethanesulfonate (equiv to catalyst; [LutH]OTf; Lut = 2,6-
lutidine; OTf = OSO2CF3) as a proton source in toluene was added a
solution of cobaltocene (equiv to catalyst; CoCp2; Cp = η5-C5H5) as a
reductant in toluene via a syringe pump at room temperature over a
period of 1 h, followed by stirring at room temperature for another 19
h under an atmospheric pressure of dinitrogen. Molar equiv based on
the catalyst. Addition of a solution of cobaltocene over a period of 30
min.
b
c
Preparation of 2-(Di-tert-butylphosphino)methyl-6-methyl-
pyridine (2a). To a solution of 2,6-lutidine (1.38 g, 12.8 mmol) in
n
Et2O (30 mL) at 0 °C was added BuLi (1.65 M in hexane, 7.8 mL,
12.9 mmol). The reaction mixture was stirred at 0 °C for 1 h and then
t
dimolybdenum complexes did not work as catalysts toward the
catalytic formation of ammonia under the same reaction
conditions. We consider that complex 5b has only less activated
dinitrogen ligands to exhibit a catalytic activity, based on the IR
absorbance of 5b in comparison with those of 1 and 5a.
Goldman and co-workers previously reported that iridium
complexes bearing various alkyl-substituted PCP-type pincer
ligands [Ir(CO)(RPCP)] (RPCP = κ3-C6H3-2,6-(CH2PR2)2; R
= tBu, iPr, and Ad) show almost the same electronic properties
by the measurement of the CO frequency.7 These results
indicate that the electron density of the molybdenum centers in
5c and 5d is comparable to that of 5a. Although complexes 5c
and 5d have a similar electronic effect to 5a, both complexes 5c
and 5d have different molecular structures (vide supra). At
present, we consider that the coordination environment on the
molybdenum atom may affect the thermodynamic stability of
catalytically active intermediates in the protonation and
reduction process, leading to the formation of ammonia.
Although we have not yet obtained detailed information on
the reaction pathway, dinitrogen-bridged dimolybdenum
complexes bearing PNP-type pincer ligands (5b, 5c, and 5d),
cooled to −78 °C. Bu2PCl (2.4 mL, 12.6 mmol) was added and
stirred at −78 °C for 1 h, and the reaction mixture was warmed to
room temperature and stirred at room temperature for 1 h. The
reaction was quenched with degassed water and extracted with Et2O.
The extracts were dried over anhydrous MgSO4, and the solvent was
removed under reduced pressure. Recrystallization from hexane at −40
°C afforded 2a as a white solid, which was collected by filtration and
dried in vacuo (1.72 g, 6.8 mmol, 54% yield). This sample was used for
1
subsequent reaction without further purification. H NMR (C6D6): δ
7.26 (d, J = 7.6 Hz, Ar-H, 1H), 7.09 (t, J = 7.6 Hz, Ar-H, 1H), 6.57 (d,
J = 7.6 Hz, Ar-H, 1H), 3.09 (d, J = 3.0 Hz, CH2PtBu2, 2H), 2.42 (s, Ar-
CH3, 3H), 1.11 (d, J = 10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR
(C6D6): δ 34.6 (s, PtBu2).
Preparation of 2-(Di-1-adamantylphosphino)methyl-6-(di-
tert-butylphosphino)methylpyridine (3a). To a solution of 2a
(357 mg, 1.42 mmol) in Et2O (30 mL) at 0 °C was added nBuLi (1.62
M in hexane, 2.7 mL, 4.37 mmol). The reaction mixture was heated to
reflux temperature for 15 h and then cooled to −78 °C. Ad2PCl (457.1
mg, 1.36 mmol) was added, and the reaction mixture was warmed to
room temperature and then stirred for 22 h. The reaction was
quenched with degassed water and extracted with Et2O. The extracts
were dried over anhydrous MgSO4, and the solvent was removed
under reduced pressure. Recrystallization from Et2O at −40 °C
afforded 3a as a white solid, which was collected by filtration and dried
in vacuo (542 mg, 0.98 mmol, 72% yield). 1H NMR (C6D6): δ 7.40 (d,
J = 6.5 Hz, Ar-H, 1H), 7.23 (t, J = 6.5 Hz, Ar-H, 2H), 3.12 (br s,
CH2P, 4H), 1.97−1.58 (m, CH2PAd2, 30H), 1.15 (d, J = 10.5 Hz,
CH2PtBu2, 18H). 31P{1H} NMR (C6D6): δ 35.0 (s, PtBu2), 30.4 (s,
PAd2). Anal. Calcd for C35H55NP2: C, 76.19; H, 10.05; N, 2.54.
Found: C, 76.16; H, 9.69; N, 2.62.
Preparation of 2-(Di-tert-butylphosphino)methyl-6-
(diphenylphosphino)methylpyridine (3b). To a solution of 2b
(580 mg, 1.99 mmol) in Et2O (20 mL) at 0 °C was added nBuLi (1.59
M in hexane, 3.8 mL, 6.04 mmol). The reaction mixture was heated to
reflux temperature for 15 h and then cooled to −78 °C. tBu2PCl (380
μL, 2.00 mmol) was added, and the reaction mixture was warmed to
room temperature and then stirred for 6 h. The reaction was quenched
with degassed water and extracted with Et2O. The extracts were dried
over anhydrous MgSO4, and the solvent was removed under reduced
pressure. Recrystallization from pentane at −40 °C afforded 3b as a
i
where less sterically demanding substituents such as Ph, Pr,
and Cy groups8 are present at the phosphorus atoms, did not
work as effective catalysts toward the catalytic formation of
ammonia from molecular dinitrogen. These results indicate that
the steric hindrance of PtBu2 and PAd2 groups in the PNP-
pincer ligand is necessary to stabilize some reactive
intermediates during the catalytic reaction.
In summary, we have synthesized a series of unsymmetric
PNP-type pincer ligands and the corresponding dinitrogen-
bridged dimolybdenum complexes bearing these unsymmetric
pincer ligands. A molybdenum−dinitrogen complex bearing 2-
(di-1-adamantylphosphino)methyl-6-(di-tert-butylphosphino)-
methylpyridine has been found to work as an effective catalyst
toward the formation of ammonia from molecular dinitrogen
under ambient conditions. In contrast, other molybdenum−
E
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Article
white solid, which was collected by filtration and dried in vacuo (771
mg, 1.77 mmol, 89% yield). 1H NMR (C6D6): δ 7.50−7.46 (m,
CH2PPh2, 4H), 7.08−6.95 (m, CH2PPh2 and Ar-H, 8H), 6.64 (d, J =
7.3 Hz, Ar-H, 1H), 3.60 (s, CH2PPh2, 2H), 3.02 (d, J = 2.7 Hz,
CH2PtBu2, 2H), 1.10 (d, J = 10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR
(C6D6): δ 35.0 (s, PtBu2), −11.5 (s, PPh2). Anal. Calcd for
C27H35NP2: C, 74.46; H, 8.10; N, 3.22. Found: C, 74.11; H, 7.98;
N, 3.06.
recrystallized from CH2Cl2 (3 mL)−hexane (20 mL) to give orange-
brown needles of 4d, which were collected by filtration and dried in
vacuo (545 mg, 0.839 mmol, 92% yield). Anal. Calcd for
C27H47Cl3MoNP2: C, 49.90; H, 7.29; N, 2.16. Found: C, 49.55; H,
7.39; N, 2.00.
Preparation of [Mo(N2)2(3a)]2(μ-N2) (5a). To Na−Hg (0.5 wt %,
10.8 g, 2.36 mmol) were added THF (27 mL) and 4a (300 mg. 0.398
mmol), and then the mixture was stirred at room temperature for 12 h
under N2. The resultant solution was decanted, and then the
supernatant solution was concentrated under reduced pressure. To
the residue was added benzene (10 mL), the solution was filtered
through Celite, and the filter cake was washed with benzene (3 mL ×
6). Slow addition of hexane (15 mL) to the concentrated filtrate (ca. 5
mL) gave 5a as a dark green solid, which was collected by filtration and
dried in vacuo (140 mg, 0.097 mmol, 49% yield). Unfortunately, NMR
spectra of 5a have not been obtained because 5a is not soluble enough
in THF-d8 and C6D6 for measurement of 1H and 31P{1H} NMR. ESI-
TOF-MS of 5a shows the existence of dinitrogen-bridged
dimolybdenum species, where three or four dinitrogen ligands in 5a
are lost. ESI-TOF-MS (THF): 1354 (M − 3N2), 1326 (M − 4N2). IR
(KBr, cm−1): 1939 (s, νNN). Anal. Calcd for C70H110Mo2N12P4: C,
58.57; H, 7.72; N, 11.71. Found: C, 59.02; H, 8.04; N, 7.51.
Preparation of [Mo(N2)2(3b)]2(μ-N2) (5b). To Na−Hg (0.5 wt %,
10.3 g, 2.26 mmol) were added THF (25 mL) and 4b (239 mg. 0.375
mmol), and then the mixture was stirred at room temperature for 12 h
under N2. The resultant dark green suspension was decanted, and then
the supernatant solution was concentrated under reduced pressure. To
the residue was added benzene (7 mL), the solution was filtered
through Celite, and the filter cake was washed with benzene (3 mL ×
5). Slow addition of hexane (18 mL) to the concentrated filtrate (ca. 5
mL) gave dark brown crystals of 5b·3C6H6, which was collected by
filtration and dried in vacuo to afford 5b as a dark brown solid (122
mg, 0.102 mmol, 54% yield). Unfortunately, NMR spectra of 5b have
not been obtained because 5b is not soluble enough in THF-d8 and
C6D6 for measurement of 1H and 31P{1H} NMR. ESI-TOF-MS of 5b
shows the existence of dinitrogen-bridged dimolybdenum species,
where four dinitrogen ligands in 5b are lost. ESI-TOF-MS (THF):
1094 (M − 4N2). IR (KBr, cm−1): 1957 (s, νNN). Anal. Calcd for
C54H70Mo2N12P4: C, 53.91; H, 5.86; N, 13.97. Found: C, 54.37; H,
6.04; N, 8.70.
Preparation of 2-(Di-tert-butylphosphino)methyl-6-
(diisopropylphosphino)methylpyridine (3c). To a solution of
n
2a (369 mg, 1.47 mmol) in Et2O (15 mL) at 0 °C was added BuLi
(1.65 M in hexane, 2.7 mL, 4.46 mmol). The reaction mixture was
heated to reflux temperature for 15 h and then cooled to −78 °C.
iPr2PCl (235 μL, 1.48 mmol) was added, and the reaction mixture was
warmed to room temperature and then stirred for 6 h. The reaction
was quenched with degassed water and extracted with Et2O. The
extracts were dried over anhydrous MgSO4, and the solvent was
removed under reduced pressure to afford 3c as a yellow oil (452 mg,
1.23 mmol, 84% yield). This sample was used for subsequent reaction
without further purification. 1H NMR (C6D6): δ 7.22−7.00 (m, Ar-H,
3H), 3.07 (d, J = 3.0 Hz, CH2P, 2H), 2.98 (d, J = 1.6 Hz, CH2P, 2H),
1.77−1.66 (m, CHMe2, 2H), 1.12 (d, J = 10.8 Hz, CH2PtBu2, 18H),
1.08−1.00 (m, CHMe2, 12H). 31P{1H} NMR (C6D6): δ 34.9 (s,
PtBu2), 10.8 (s, PiPr2).
Preparation of 2-(Di-tert-butylphosphino)methyl-6-
(dicyclohexylphosphino)methylpyridine (3d). To a solution of
n
2a (526 mg, 2.09 mmol) in Et2O (35 mL) at 0 °C was added BuLi
(1.65 M in hexane, 3.8 mL, 6.27 mmol). The reaction mixture was
heated to reflux temperature for 15 h and then cooled to −78 °C.
Cy2PCl (460 μL, 2.08 mmol) was added, and the reaction mixture was
warmed to room temperature and stirred at room temperature for 22
h. The reaction was quenched with degassed water and extracted with
Et2O. The extracts were dried over anhydrous MgSO4, and the solvent
was removed under reduced pressure. Recrystallization from hexane at
−40 °C afforded 3d as a white solid, which was collected by filtration
and dried in vacuo (545 mg, 1.22 mmol, 59% yield). 1H NMR (C6D6):
δ 7.25−7.05 (m, Ar-H, 3H), 3.09 (d, J = 2.7 Hz, CH2P, 2H), 3.04 (d, J
= 1.6 Hz, CH2P, 2H), 1.85−1.20 (m, CH2PCy2, 22H), 1.13 (d, J =
10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR (C6D6): δ 34.4 (s, PtBu2),
2.7 (s, PCy2). Anal. Calcd for C27H47NP2: C, 72.45; H, 10.58; N, 3.13.
Found: C, 72.19; H, 10.30; N, 3.14.
Preparation of [MoCl3(3a)] (4a). A mixture of 3a (298 mg, 0.539
mmol) and [MoCl3(thf)3] (207 mg, 0.495 mmol) in THF (20 mL)
was stirred at 50 °C for 20 h. The resultant red-orange solution was
concentrated under reduced pressure, and the residue was recrystal-
lized from CH2Cl2 (3 mL)−hexane (18 mL) to give orange-brown
needles of 4a, which were collected by filtration and dried in vacuo
(363 mg, 0.482 mmol, 97% yield). Anal. Calcd for C35H55Cl3MoNP2:
C, 55.75; H, 7.35; N, 1.86. Found: C, 55.75; H, 7.34; N, 1.71.
Preparation of [MoCl3(3b)] (4b). A mixture of 3b (276 mg, 0.633
mmol) and [MoCl3(thf)3] (249 mg, 0.595 mmol) in THF (11 mL)
was stirred at 50 °C for 20 h. The resultant yellow-orange suspension
was concentrated under reduced pressure, and the residue was
recrystallized from CH2Cl2 (6 mL)−Et2O (17 mL) to give orange
needles of 4b, which were collected by filtration and dried in vacuo
(334 mg, 0.523 mmol, 88% yield). Anal. Calcd for C27H35Cl3MoNP2:
C, 50.84; H, 5.53; N, 2.20. Found: C, 50.32; H, 5.59; N, 2.04.
Preparation of [MoCl3(3c)] (4c). A mixture of 3c (198 mg, 0.539
mmol) and [MoCl3(thf)3] (203 mg, 0.485 mmol) in THF (10 mL)
was stirred at 50 °C for 20 h. The resultant orange-brown solution was
concentrated under reduced pressure and washed with Et2O (2 mL ×
2), and the residue was recrystallized from CH2Cl2 (3 mL)−hexane
(18 mL) to give an orange-brown solid of 4c, which was collected by
filtration and dried in vacuo (210 mg, 0.369 mmol, 76% yield). Anal.
Calcd for C21H39Cl3MoNP2: C, 44.26; H, 6.90; N, 2.46. Found: C,
44.02; H, 7.11; N, 2.30.
Preparation of [Mo(N2)2(3c)]2(μ-N2) (5c). To Na−Hg (0.50 wt
%, 8.23 g, 1.79 mmol) were added THF (20 mL) and 4c (170 mg.
0.298 mmol), and then the mixture was stirred at room temperature
for 24 h under N2. The resultant dark green suspension was decanted,
and then the supernatant solution was concentrated in vacuo. To the
residue was added benzene (8 mL), the solution was filtered through
Celite, and the filter cake was washed with benzene (3 mL × 6).
Pentane (18 mL) was added to the concentrated filtrate (ca. 2 mL)
and the solution was kept at −40 °C to give 5c as a dark green solid,
which was collected by filtration and dried in vacuo (68 mg, 0.064
1
mmol, 43% yield). H NMR (C6D6): δ 6.67−6.48 (m, Ar-H, 6H),
3.37−2.76 (m, CH2P, 8H), 2.46−1.97 (m, CHMe2, 4H), 1.79−0.79
(m, CHMe2, 24H) 1.57 (d, J = 11.6 Hz, CH2PtBu2, 9H), 1.53 (d, J =
11.6 Hz, CH2PtBu2, 9H), 1.30 (d, J = 11.6 Hz, CH2PtBu2, 9H), 1.07 (d,
J = 11.6 Hz, CH2PtBu2, 9H). 31P{1H} NMR (C6D6): δ 89.5 (d, J = 143
Hz, PtBu2), 88.4 (d, J = 130 Hz, PtBu2), 75.7 (d, J = 130 Hz, PiPr2),
75.5 (d, J = 143 Hz, PiPr2). IR (KBr, cm−1): 1968 (s, νNN), 1949 (s,
νNN), 1897 (s, νNN), 1875 (s, νNN). Anal. Calcd for C42H78Mo2N12P4:
C, 47.28; H, 7.37; N, 15.75. Found: C, 47.16; H, 7.48; N, 9.08.
Preparation of [Mo(N2)2(3d)]2(μ-N2) (5d). To Na−Hg (0.50 wt
%, 10.9 g, 2.38 mmol) were added THF (26 mL) and 4d (258 mg.
0.397 mmol), and then the mixture was stirred at room temperature
for 24 h under N2. The resultant dark green suspension was decanted,
and then the supernatant solution was concentrated under reduced
pressure. To the residue was added benzene (3 mL), the solution was
filtered through Celite, and the filter cake was washed with benzene (2
mL × 8). Pentane (15 mL) was added to the concentrated filtrate (ca.
3 mL) and the solution was kept at −40 °C to give 5d as dark green
crystals, which were collected by filtration and dried in vacuo to afford
Preparation of [MoCl3(3d)] (4d). A mixture of 3d (407 mg, 0.909
mmol) and [MoCl3(thf)3] (397 mg, 0.947 mmol) in THF (30 mL)
was stirred at 50 °C for 20 h. The resultant dark orange-brown
solution was concentrated under reduced pressure, and the residue was
1
5d as a dark green solid (149 mg, 0.121 mmol, 61% yield). H NMR
F
dx.doi.org/10.1021/om301046t | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(C6D6): δ 6.69−6.52 (m, Ar-H, 6H), 3.66−2.71 (m, CH2P, 8H),
2.22−1.20 (m, CH2PCy2, 44H), 1.68 (d, J = 12.7 Hz, CH2PtBu2, 9H),
1.59 (d, J = 11.9 Hz, CH2PtBu2, 9H), 1.27 (d, J = 11.1 Hz, CH2PtBu2,
9H), 1.00 (d, J = 11.3 Hz, CH2PtBu2, 9H). 31P{1H} NMR (C6D6): δ
89.6 (d, J = 143 Hz, PtBu2), 87.5 (d, J = 130 Hz, PtBu2), 67.7 (d, J =
130 Hz, PCy2), 66.6 (d, J = 143 Hz, PCy2). IR (KBr, cm−1): 1966 (s,
νNN), 1950 (s, νNN), 1896 (s, νNN), 1877 (s, νNN). Anal. Calcd for
C54H94Mo2N12P4: C, 52.85; H, 7.72; N, 13.70. Found: C, 52.44; H,
7.71; N, 5.04.
ACKNOWLEDGMENTS
■
This work was supported by Funding Program for Next
Generation World-Leading Researchers (GR025) and a Grant-
in-Aid for Scientific Research on Innovative Areas “Advanced
Molecular Transformations by Organocatalysts” from MEXT,
Japan. Financial support from Toyota Motor Corporation is
also gratefully acknowledged. We thank the Research Hub for
Advanced Nano Characterization at The University of Tokyo.
Catalytic Reduction of Dinitrogen to Ammonia. A typical
experimental procedure3 for the catalytic reduction of dinitrogen into
ammonia using the dinitrogen complex 5a is described below. In a 50
mL Schlenk flask were placed 5a (14.4 mg, 0.010 mmol) and 2,6-
lutidinium trifluoromethanesulfonate, [LutH]OTf (741 mg, 2.88
mmol). Toluene (1 mL) was added under N2 (1 atm), and then a
solution of CoCp2 (408 mg, 2.16 mmol) in toluene (4 mL) was slowly
added to the stirred suspension in the Schlenk flask with a syringe
pump at a rate of 4 mL per hour. After the addition of CoCp2, the
mixture was further stirred at room temperature for 19 h. The amount
of dihydrogen of the catalytic reaction was determined by gas
chromatography (GC). The reaction mixture was evaporated under
reduced pressure, and the distillate was trapped in dilute H2SO4
solution (0.5 M, 10 mL). Potassium hydroxide aqueous solution (30
wt %; 5 mL) was added to the residue, and the mixture was distilled
into another dilute H2SO4 solution (0.5 M, 10 mL). NH3 present in
each of the H2SO4 solutions was determined by the indophenol
method.13 The amount of ammonia was 0.017 mmol of NH3 collected
before base distillation of the reaction mixture and 0.143 mmol of NH3
collected after base distillation to fully liberate NH3, respectively. The
total amount of ammonia was 0.160 mmol (16.0 molar equiv based on
5a).
REFERENCES
■
(1) For recent reviews, see: (a) MacKay, B. A.; Fryzuk, M. D. Chem.
Rev. 2004, 104, 385. (b) Gambarotta, S.; Scott, J. Angew. Chem., Int. Ed.
2004, 43, 5298. (c) Hidai, M.; Mizobe, Y. Can. J. Chem. 2005, 83, 358.
(d) Himmel, H.-J.; Reiher, M. Angew. Chem., Int. Ed. 2006, 45, 6264.
(e) Chirik, P. J. Dalton Trans. 2007, 16. (f) Fryzuk, M. D. Acc. Chem.
́
Res. 2009, 42, 127. (g) Ballmann, J.; Munha, R, F.; Fryzuk, M. D.
Chem. Commun. 2010, 46, 1013. (h) Hazari, N. Chem. Soc. Rev. 2010,
39, 4044. (i) Shibata, Y.; Seino, H.; Mizobe, Y.; Hidai, M. Encyclopedia
of Catalysis; Wiley VCH: New York, 2010 (DOI: 10.1002/
0471227617.eoc016.pub2). (j) Nishibayashi, Y. Dalton Trans. 2012,
41, 7447. (k) Hinrichsen, S.; Broda, H.; Gradert, C.; Soncksen, L.;
̈
Tuczek, F. Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem. 2012, 108, 17.
(2) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76.
(b) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock,
A. S.; Davis, W. M. J. Am. Chem. Soc. 2004, 126, 6150. (c) Schrock, R.
R. Acc. Chem. Res. 2005, 38, 955. (d) Weare, W. W.; Dai, X.; Byrnes,
M. J.; Chin, J. M.; Schrock, R. R.; Muller, P. Proc. Natl. Acad. Sci. U. S.
A. 2006, 103, 17099. (e) Schrock, R. R. Angew. Chem., Int. Ed. 2008,
47, 5512.
̈
(3) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2011, 3,
X-ray Crystallography. Crystallographic data of 4b, 4d,
5b·3C6H6, and 5c are summarized in Table S1. Selected bond lengths
and angles are summarized in Tables 1−4, and their ORTEP drawings
are shown in Figures 1−4. Diffraction data for 4b were collected for
the 2θ range of 6° to 55° at −180 °C on a Rigaku Saturn 70 CCD
diffractometer with graphite-monochromated Mo Kα (λ = 0.71075 Å)
radiation with VariMax optics. Diffraction data for 4d, 5b·3C6H6, and
5c were collected for the 2θ range of 5° to 55° at −90 °C (for 4d and
5b·3C6H6) or −75 °C (for 5c) 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 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.14 The positions of
the non-hydrogen atoms were determined by direct methods (SIR
9715 for 4b, SIR 200216 for 4d, SHELX-9717 for 5b·3C6H6 and 5c)
and subsequent Fourier syntheses (DIRDIF-9918) and were refined on
120.
(4) Arashiba, K.; Sasaki, K.; Kuriyama, S.; Miyake, Y.; Nakanishi, H.;
Nishibayashi, Y. Organometallics 2012, 31, 2035.
(5) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Butcher, R. J.; Bryan, J. C.
Inorg. Chem. 1995, 34, 6538.
(6) In the reaction using 5a as a catalyst, addition of cobaltocene in
toluene for a shorter time such as 15 min dramatically decreased the
formation of ammonia (15 equiv based on 5a).
(7) Punji, B.; Emge, T. J.; Goldman, A. S. Organometallics 2010, 29,
2702.
(8) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(9) As an extensive study, we have quite recently found the first
successful example of iron-catalyzed reduction of molecular dinitrogen
under ambient reaction conditions; see: Yuki, M.; Tanaka, H.; Sasaki,
K.; Miyake, Y.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2012,
DOI: 10.1038/ncomms2264.
2
(10) Mothes, E.; Sentets, S.; Luquin, M. A.; Mathieu, R.; Lugan, N.;
Lavigne, G. Organometallics 2008, 27, 1193.
Fo using all unique reflections by full-matrix least-squares with
anisotropic thermal parameters except for a few solvated molecules
(C28−C39 for 5b·3C6H6), which were refined isotropically. All the
other hydrogen atoms were placed at the calculated positions with
fixed isotropic parameters.
(11) Kollhofer, A.; Plenio, H. Chem.Eur. J. 2003, 9, 1416.
̈
(12) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001,
2699.
(13) Weatherburn, M. W. Anal. Chem. 1967, 39, 971.
(14) CrystalStructure 4.0, Single Crystal Structure Analysis Software;
Rigaku Corp: Tokyo, Japan, and MSC: The Woodlands, TX, 2010.
(15) SIR-97: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.
L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115.
(16) SIR-2002: Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano,
G. L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr.
2003, 36, 1103.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for 4b, 4d, 5b·3C6H6, and 5c are available
in CIF format, including a table of crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(17) SHELX-97: Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64,
112.
AUTHOR INFORMATION
■
(18) Beurskens, P. T.; Beurskens, G.; de Gelder, R., García-Granda,
S.; Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF-99 Program
System; Crystallography Laboratory, University of Nijmegen: Nijme-
gen, The Netherlands, 1999.
̈
Corresponding Author
*E-mail: ynishiba@sogo.t.u-tokyo.ac.jp.
Notes
The authors declare no competing financial interest.
G
dx.doi.org/10.1021/om301046t | Organometallics XXXX, XXX, XXX−XXX