Protonation of coordinated N2 on tungsten with H2 mediated by sulfido-bridged dinuclear molybdenum complexes

Article abstract of DOI:10.1021/ic000716v

Reaction of tungsten dinitrogen complexes [W(N₂)₂L₄](L = PM_e2Ph, PMePh₂) and 10 equiv of sulfido-bridges molybdenum complexes [Cp′Mo(μ₂-S₂CH₂)-(μ-S) (μ-SR)MoCp′]OTf (C

Full text of DOI:10.1021/ic000716v

578
Inorg. Chem. 2001, 40, 578-580
Results and Discussion
Protonation of Coordinated N2 on Tungsten with
H2 Mediated by Sulfido-Bridged Dinuclear
Molybdenum Complexes1
Treatment of trans-[W(N2)2(dppe)2] (1a; dppe ) 1,2-bis-
(diphenylphosphino)ethane) with 2 equiv of [(η5-C5H4Me)Mo-
(µ2-S2CH2)(µ-S)(µ-SMe)Mo(η5-C5H4Me)]OTf (2d) in THF at
25 °C for 1 h under 1 atm of H2 produced the hydrazido(2-)
complex trans-[W(OTf)(NNH2)(dppe)2]OTf (3a) and [(η5-C5H4-
Me)Mo(µ2-S2CH2)(µ-SMe)(µ-SH)Mo(η5-C5H4Me)] (4) in >99%
and 69% NMR yields, respectively (eq 1). Under N2, no reaction
Yoshiaki Nishibayashi,†,‡ Issei Wakiji,† Kenji Hirata,†
Mary Rakowski DuBois,*,§ and Masanobu Hidai*,†,
Department of Chemistry and Biotechnology, The University
of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and
Department of Chemistry and Biochemistry, University of
Colorado at Boulder, Boulder, Colorado 80309-0215
ReceiVed June 27, 2000
Introduction
During the course of our continuing study of the reactivities
of coordinated N2 in molybdenum and tungsten complexes of
the type [M(N2)2(L)4] (M ) Mo, W; L ) tertiaryphosphine),2
we have recently found that the coordinated N2 on tungsten can
be protonated with acidic ruthenium dihydrogen complexes to
form NH3 under mild conditions.3 Furthermore, the coordinated
N2 is transformed into NH3 by treatment with hydrosulfido-
bridged dinuclear iridium and iron complexes, although organic
thiols and H2S are not effective for the reaction.4 This indicates
that a similar type of protonation might occur in biological
nitrogen fixation, which is mediated by the bridging sulfur
ligands in the FeMo cofactor.5 An extension of our study has
now led to the investigation of the reactions of the N2 complexes
with the sulfido-bridged dimolybdenum complexes [Cp′Mo(µ2-
S2CH2)(µ-S)(µ-SR)MoCp′]OTf (2a, Cp′ ) η5-C5H5, R ) H;
2b, Cp′ ) η5-C5H5, R ) Me; 2c, Cp′ ) η5-C5H4Me, R ) H;
2d, Cp′ ) η5-C5H4Me, R ) Me; OTf ) OSO2CF3) under H2
because DuBois and co-workers previously reported that the
CtN triple bond in nitriles can be cleaved in the presence of
2a under H2 and complex 2d induces the heterolytic cleavage
of H2 by reaction with pyridine under H2.6,7 The results are
described here.
proceeded, and both 1a and 2d were recovered quantitatively.
Hydrazido(2-) complex 3a was previously obtained by proto-
nation of 1a with 2 equiv of trifluoromethanesulfonic acid
(HOTf).8 In the reaction of 1a and 2d, the heterolytic splitting
of H2 occurs on 2d, where one H atom is used for the N-H
bond formation at the coordinated N2 on the W atom and the
other H atom remains at the bridging-sulfido ligand. The formal
oxidation state of the starting and product molybdenum com-
plexes is +IV and +III, respectively, and the two electrons
required for the reduction come from the H2. Employment of
2b in place of 2d also gives rise to the formation of 3a in high
yield. On the other hand, when 1a was treated with 2a under
H2, 3a and [(η5-C5H5)Mo(µ2-S2CH2)(µ-S)2Mo(η5-C5H5)] (5)
were obtained in >99% and 84% NMR yields, respectively (eq
2). The neutral molybdenum complex 5 is presumed to be
† The University of Tokyo.
‡ Present address: Department of Energy and Hydrocarbon Chemistry,
Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-
8501, Japan.
§ University of Colorado at Boulder.
Present address: Department of Materials Science and Technology,
Faculty of Industrial Science and Technology, Science University of Tokyo,
Noda, Chiba 278-8510, Japan.
(1) Preparation and Properties of Molybdenum and Tungsten Dinitrogen
Complexes. 68. Part 67: Seino, H.; Nonokawa, D.; Nakamura, G.;
Mizobe, Y.; Hidai, M. Organometallics 2000, 19, 2002.
(2) (a) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115. (b) Hidai, M.;
Ishii, Y. Bull. Chem. Soc. Jpn. 1996, 69, 819. (c) Hidai, M.; Mizobe,
Y. Top. Organomet. Chem. 1999, 3, 227.
(3) (a) Nishibayashi, Y.; Iwai, S.; Hidai, M. Science 1998, 279, 540. (b)
Nishibayashi, Y.; Takemoto, S.; Iwai, S.; Hidai, M. Inorg. Chem. 2000,
39, 5946.
(4) Nishibayashi, Y.; Iwai, S.; Hidai, M. J. Am. Chem. Soc. 1998, 120,
10559.
(5) (a) Dance, I. G. Aust. J. Chem. 1994, 47, 979. (b) Dance, I. G.
Transition Metal Sulfur Chemistry; Stiefel, E. I., Matsumoto, K., Eds.;
ACS Synposium Series 653; American Chemical Society: Washing-
ton, DC, 1996. (c) Dance, I. Chem. Commun. 1997, 165. (d) Dance,
I. Chem. Commun. 1998, 523. (e) Zhong, S.-J.; Liu, C.-W. Polyhedron
1997, 16, 653.
formed from dehydrogenation of the initially formed complex
[(η5-C5H5)Mo(µ2-S2CH2)(µ-SH)2Mo(η5-C5H5)].9
(6) Bernatis, P.; Laurie, J. C. V.; DuBois, M. R. Organometallics 1990,
9, 1607.
(7) Laurie, J. C. V.; Duncan, L. D.; Haltiwanger, R. C.; Weberg, R. T.;
DuBois, M. R. J. Am. Chem. Soc. 1986, 108, 6234.
(8) Field, L. D.; Jones, N. G.; Turner, P. Organometallics 1998, 17, 2394.
(9) Casewit, C. J.; Coons, D. E.; Wright, L. L.; Miller, W. K.; DuBois,
M. R. Organometallics 1986, 5, 951.
10.1021/ic000716v CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/05/2001

Notes
Inorganic Chemistry, Vol. 40, No. 3, 2001 579
Table 1. Reaction of Coordinated N2 on Tungsten and
Sulfido-Bridged Dinuclear Molybdenum Complexes under H2
reaction mechanism is not yet clear; however, it is noteworthy
that sulfido-bridged dinuclear molybdenum complexes are able
to induce heterolytic cleavage of H2, leading to the N-H bond
formation of coordinated N2. Thus, heterolytic H2 activation at
a metal-sulfur site, i.e., M-S + H2 f M(H-)-SH+, has been
proposed to realize catalytic nitrogen fixation.11
a
yield of NH3 (%)b
W-N2
Mo
condition
in
run complex complex temp (°C)/time (h) freec extractd total
1
2
3
4
5
6
7
8
1b
1b
3bf
1b
1b
1b
1b
1c
2a
2a
2a
2b
2c
5
25/24 f 55/24
25/120
3
4
5
5
3
5
2
4
35
30
94
28
43
12
15
48
38e
34
99
33
46
17
17
52e
Experimental Section
25/24 f 55/24
25/24 f 55/24
25/24 f 55/24
25/24 f 55/24
25/24 f 55/24
25/24 f 55/24
General Procedure. Preparation of complexes was performed under
1 atm of N2 or Ar dried by passage through silica gel and P2O5.
Reactions of N2 complexes with sulfido-bridged dimolybdenum
complexes were carried out under 1 atm of H2 dried by passage through
silica gel and P2O5. THF was freshly distilled over sodium benzophe-
none ketyl just before use. Unless otherwise noted, all manipulations
were done by use of Schlenk techniques. NMR spectra were run on a
JEOL JNM-LA-400 or a JEOL JNM-EX-270 spectrometer. IR spectra
were recorded on a Shimadzu FTIR-8100M spectrometer. Absorption
spectra were measured on a Shimadzu UV-2400PC. Tungsten N2
complexes12-14 (1) and sulfido-bridged dimolybdenum complexes15,16
(2, 5, 6) were prepared according to literature procedures.
6
2a
a All of the reactions were carried out in THF using W-N2 complex
and 10 equiv of Mo complex under 1 atm of H2 at 25 °C for 24 h and
then at 55 °C for 24 h unless otherwise stated (see text). b Yield of
NH3 was based on the W atom. c Free NH3 was observed in the reaction
mixture. d Yield of NH4 in the water extract of the reaction mixture.
+
e Variation of (8% between experiments. f 3b was used in place of
W-N2 complex.
Reaction of trans-[W(N2)2(dppe)2] (1a) with Sulfido-Bridged
Dinuclear Molybdenum Complexes (2) under H2. A typical experi-
mental procedure for the reaction of 1a with 2 equiv of 2d is as follows.
In a 20 mL flask was placed 2d (28.6 mg, 0.0436 mmol) under 1 atm
of N2. Dry THF (15 mL) was added, and then the solution was
magnetically stirred at 25 °C for 5 min. After the N2 atmosphere was
replaced by 1 atm of H2, 1a (22.5 mg, 0.0217 mmol) was added
portionwise. The reaction mixture was stirred at 25 °C for 1 h under 1
atm of H2. The solvent was then removed under vacuum, and the residue
was dissolved in THF-d8 to measure the 1H NMR spectrum. Ferrocene
(10.1 mg, 0.0554 mmol) was added into the THF-d8 solution as an
internal reference. The NMR yield of the produced dimolybdenum
complex (4) was determined by integration of the 1H resonances against
the ferrocene standard. However, the NMR yield of the produced
hydrazido(2-) complex (3a) could not be determined in this solvent
because of overlap of the 1H resonances with those of impurities. Thus,
the solvent was again evaporated under vacuum, and the residue was
dissolved in CDCl3 to determine the NMR yield of 3a by integration
of the 1H resonances against the ferrocene standard. 4:17 1H NMR (THF-
d8) δ -1.35 (br s, 1H, SH), 1.52 (s, 3H, SMe), 2.00 (s, 6H, C5H4Me),
5.43 (br, 8H, C5H4Me), 5.79 (s, 2H, S2CH2); 69% NMR yield. 3a:8
1H NMR (CDCl3) δ 2.87 (br s, 4H, CH2), 3.00 (br s, 4H, CH2),
5.01 (br s, 2H, NNH2), 7.21-7.45 (m, 40H, Ph); >99% NMR
Interestingly, treatment of cis-[W(N2)2(PMe2Ph)4] (1b) with
10 equiv of 2a under 1 atm of H2 in THF at 25 and then 55 °C
for 24 h each resulted in the formation of NH3 in 38% total
yield based on the W atom, whereas reactions of 1a under
similar conditions did not produce NH3. Free NH3 in 3% yield
was detected in the reaction mixture, and further NH3 in 35%
+
yield was observed as NH4 in the water extract when the
reaction mixture was extracted with an excess of water. The
typical results are shown in Table 1. In the absence of H2 or
2a, no NH3 was formed. When the reaction was performed at
25 °C for a much longer time, NH3 was produced in moderate
yield even at that temperature (Table 1, run 2). However, if the
reaction was carried out at 55 °C, the formation of NH3 was
not observed. This is probably because 2a reacts much more
quickly with THF than with 1b at that temperature to give a
cationic complex with a 4-hydroxybutanethiolate ligand.10 In
all cases, only a trace amount of NH2NH2 was observed. When
1b was treated with 2 equiv of 2a at 25 °C for 1 h under H2,
the formation of the hydrazido(2-) complex trans-[W(OTf)-
(NNH2)(PMe2Ph)4]OTf3b (3b) in 39% yield was revealed by
NMR analysis of the reaction mixture. Furthermore, the reaction
of 3b with 10 equiv of 2a under H2 at 25 and then 55 °C for 24
h each produced NH3 in 99% total yield (Table 1, run 3). These
results indicate that the formation of NH3 proceeds through the
protonation of 3b as an intermediate.
yield; 31P{1H} NMR (CDCl3) δ 37.8 (s with 183W satellites, JPW
)
320 Hz).
Formation of NH3 in the Reactions of Dinitrogen Tungsten
Complexes (1b, 1c) with Sulfido-Bridged Dinuclear Molybdenum
Complexes (2) under H2. A typical procedure for the reaction of 1b
with 10 equiv of 2a under 1 atm of H2 is as follows. In a 500 mL flask
was placed 2a (307 mg, 0.500 mmol) under 1 atm of N2. Dry THF (20
mL) was added, and then the mixture was magnetically stirred at 25
°C for 5 min. After the N2 atmosphere was replaced by 1 atm of H2,
Reactions of 1b with 10 equiv of other cationic sulfido-
bridged dinuclear molybdenum complexes (2b and 2c) under
H2 led to the formation of NH3 in similar yields (Table 1, runs
4 and 5). However, in the case of neutral dinuclear molybdenum
complexes such as 5 and [(η5-C5H5)Mo(µ-SH)(µ-S)]2 (6), NH3
was obtained in lower yields (Table 1, runs 6 and 7).
(11) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460 and references
therein.
The reaction of trans-[W(N2)2(PMePh2)4] (1c) with 10 equiv
of 2a under the same conditions gave NH3 in 52% yield (Table
1, run 8), which is almost the same as that from the reaction of
1b with 2a (vide supra). This is in sharp contrast to our previous
observation that treatment of 1b with 10 equiv of trans-[RuCl-
(η2-H2)(dppe)2]OTf at 55 °C under H2 gives NH3 in moderate
yield while the similar reaction of 1c affords NH3 in very low
yield.3b
(12) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. Nature (London) 1975,
253, 39. (b) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc.,
Dalton Trans. 1977, 1852. (c) Anderson, S. N.; Fakley, M. E.;
Richards, R. L.; Chatt, J. J. Chem. Soc., Dalton Trans. 1981, 1973.
(13) Hussain, W.; Leigh, G. J.; Ali, H. M.; Pickett, C. J.; Rankin, D. A. J.
Chem. Soc., Dalton Trans. 1984, 1703.
(14) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc., Dalton Trans.
1977, 2139.
(15) (a) Casewit, C. J.; Haltiwanger, R. C.; Noordik, J.; DuBois, M. R.
Organometallics 1985, 4, 119. (b) Birnbaum, J.; Godziela, G.;
Maciejewski, M.; Tonker, T. L.; Haltiwanger, R. C.; DuBois, M. R.
Organometallics 1990, 9, 394. (c) Birnbaum, J.; Haltiwanger, R. C.;
Bernatis, P.; Teachout, C.; Parker, K.; DuBois, M. R. Organometallics
1991, 10, 1779.
In summary, we have found that the reactions of tungsten
N2 complexes and sulfido-bridged dinuclear molybdenum
complexes under H2 produce NH3 under mild conditions. The
(16) DuBois, M. R.; VanDerveer, M. C.; DuBois, D. L.; Haltiwanger, R.
C.; Miller, W. K. J. Am. Chem. Soc. 1980, 102, 7456.
(17) Casewit, C. J.; DuBois, M. R. J. Am. Chem. Soc. 1986, 108, 5482.
(10) Godziela, G.; Tonker, T.; DuBois, M. R. Organometallics 1989, 8,
2220.

580 Inorganic Chemistry, Vol. 40, No. 3, 2001
Notes
1b (39.4 mg, 0.0497 mmol) was added portionwise. The reaction
mixture was stirred at 25 and then 55 °C for 24 h each under 1 atm of
H2. The reaction mixture was evaporated under reduced pressure, and
the distillate was trapped in dilute H2SO4 solution (1 N; 10 mL). The
residue was extracted with H2O (150 mL), and the aqueous extract
was treated with activated charcoal and filtered through Celite. NH3
and NH2NH2 present in the H2SO4 and the aqueous solution were
quantitatively analyzed by using indophenol and p-(dimethylamino)-
benzaldehyde reagents, respectively.3b,18,19
Acknowledgment. This work was supported by a Grant-
in-Aid for Specially Promoted Research (09102004) from the
Ministry of Education, Science, Sports, and Culture of Japan.
(18) Takahashi, T.; Mizobe, Y.; Sato, M.; Uchida, Y.; Hidai, M. J. Am.
Chem. Soc. 1980, 102, 7461.
(19) Weatherburn, M. W. Anal. Chem. 1967, 39, 971.
IC000716V