A model for protonation of dinitrogen by nitrogenase: Protonation of coordinated dinitrogen on tungsten with hydrosulfido-bridged dinuclear complexes [9]

Full text of DOI:10.1021/ja981223g

J. Am. Chem. Soc. 1998, 120, 10559-10560
10559
Chart 1
A Model for Protonation of Dinitrogen by
Nitrogenase: Protonation of Coordinated Dinitrogen
on Tungsten with Hydrosulfido-Bridged Dinuclear
Complexes1
Yoshiaki Nishibayashi, Shotaro Iwai, and Masanobu Hidai*
Department of Chemistry and Biotechnology
Graduate School of Engineering, The UniVersity of Tokyo
Hongo, Tokyo 113-8656, Japan
Scheme 1
ReceiVed April 13, 1998
The mechanism for biological nitrogen fixation remains unclear
although the X-ray structural model has recently been reported
for the FeMo-cofactor of FeMo nitrogenase.2 It has been made
clear that the site where dinitrogen (N2) is activated and reduced
is an Fe/Mo sulfido cluster.2 However, we are still uncertain about
which metal is responsible for binding N2.2-4 Several groups
claimed that protonation of the activated N2 proceeds with the
aid of the bridging hydrosulfido ligands in the cluster.2d,3c-g,4 In
Dance’s model,3c-g the bridging sulfido ligands mediate proton
transfer to the coordinated N2 bound to the Fe4 face of the Fe/
Mo sulfido cluster via µ-SH intermediates as shown in Chart 1.
Up until now, many mononuclear and polynuclear N2 com-
plexes of transition metals have been prepared,5 some of which
liberate NH3 and/or hydrazine (NH2NH2) by protonolysis with
inorganic acids such as H2SO4. Typically, molybdenum and tung-
sten N2 complexes of the type M(N2)2(PMe2Ph)4 (M ) Mo, W)
produce NH3 and/or NH2NH2 in good yields by treatment with
inorganic acids.5a,6 Previously, the reactions of organic thiols or
H2S with those N2 complexes were investigated, where H2 gas
was evolved and no N-H bond formation was observed.7,8 This
indicates that organic thiols and H2S attack the electron-rich metal
center in the N2 complexes in place of the coordinated N2. Very
recently, we have reported the formation of NH3 by ruthenium-
assisted protonation of N2 on W atom with H2 under mild condi-
tions.9 As an extension of this multimetallic approach for nitrogen
fixation, the reactivity of dinuclear complexes containing bridging
hydrosulfido ligands toward coordinated N2 was investigated and
a series of hydrosulfido-bridged dinuclear compounds of ruthe-
nium, iridium, and rhodium were prepared by our group, which
served as versatile precursors for synthesis of various polynuclear
sulfido clusters.10 Interestingly, the proton on the bridging sulfur
has been found to be transferred to the ligating N2 to form NH3.
Preliminary results about these reactions will be described here.
Treatment of cis-[W(N2)2(PMe2Ph)4] (1) with 10 equiv of
[Cp*Ir(µ-SH)3IrCp*]Cl10c (2; Cp* ) η5-C5Me5) under nitrogen
atmosphere in dichloroethane-benzene at 55 °C for 24 h afforded
NH3 in 78% total yield based on the W atom (Scheme 1). Free
NH3 in 3% yield was observed in the reaction mixture, and further
NH3 in 75% yield was released after base distillation. A longer
reaction time improved the total yield of NH3. The reaction also
proceeded at 30 °C; however, the yield of NH3 was lower. In
the absence of 2, no NH3 was obtained. In all the cases, only a
trace amount of NH2NH2 was observed. The typical results were
shown in Table 1. The 1H and 31P NMR spectra of the reaction
mixture showed the complete conversion of the N2 complex and
liberation of free PMe2Ph from the W atom; however, neither
tungsten products nor iridium products could be characterized.
Because plausible hydrazido(2-) intermediate complexes, which
might provide NH3 by base treatment, were not detected by the
NMR and IR spectra of the reaction mixture, we consider that
protonation of the coordinated N2 did not stop at the stage of the
(1) Preparation and Properties of Molybdenum and Tungsten Dinitrogen
Complexes. 56. Part 55: Ishino, H.; Ishii, Y.; Hidai, M. Chem. Lett. 1998,
677-678.
(2) (a) Kim, J.; Rees, D. C. Science 1992, 257, 1677-1682. (b) Kim, J.;
Rees, D. C. Nature 1992, 360, 553-560. (c) Chan, M. K.; Kim, J.; Rees, D.
J. Science 1993, 260, 792-794. (d) Rees, D. C.; Chan, M. K.; Kim, J. AdV.
Inorg. Chem. 1993, 40, 89-119. (e) Kim, J.; Rees, D. C. Biochemistry 1994,
33, 389-397. (f) Eady, R. R.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1994,
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2982. (h) Burgess, B. K.; Lowe, D. J. Chem. ReV. 1996, 96, 2983-3011.
(3) Recent theoretical studies, see: (a) Deng, H.; Hoffmann, R. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1062-1065. (b) Stavrev, K. K.; Zerner, M.
C. Chem. Eur. J. 1996, 2, 83-87. (c) Dance, I. G. Aust. J. Chem. 1994, 47,
979-990. (d) Dance, I. G. Transition Metal Sulfur Chemistry; Stiefel, E. I.,
Matsumoto, K., Eds.; ACS Symposium Series 653; American Chemical
Society: Washington, DC, 1996. (e) Dance, I. Chem. Commun. 1997, 165-
166. (f) Dance, I. Chem. Commun. 1998, 523-530. (g) Zhong, S.-J.; Liu,
C.-W. Polyhedron 1997, 16, 653-661.
+
hydradizo(2-) form, but proceeded further to form NH3 and NH4 .
Thus, base distillation of the reaction mixture was carried out to
liberate NH3. Actually, when the reaction mixture of 1 and 10
equiv of 2 at 55 °C for 24 h was extracted with an excess of
water instead of base distillation, the presence of NH3 in 50%
yield based on the W atom was observed in the water extract. As
expected, treatment of 1 with 10 equiv of thiophenol (PhSH) or
an excess of H2S under the same reaction conditions led to
evolution of H2 gas without the formation of NH3,11 whereas in
the case of a more acidic thiol (p-CF3C6F4SH), NH3 was obtained
in a low yield (9% total yield).
In contrast to the above iridium complex, the corresponding
rhodium complex, [Cp*Rh(µ-SH)3RhCp*]Cl10c (3), afforded a
small amount of NH3 (7% total yield) under the same reaction
conditions. Furthermore, the iron complex [P3Fe(µ-SH)3FeP3]-
(4) (a) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460-469 and
references therein. (b) Richards, R. L. Coord. Chem. ReV. 1996, 154, 83-97
and references therein.
(5) (a) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115-1133 and
references therein. (b) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom,
A. L.; Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
Soc. 1996, 118, 8623-8638. (c) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.;
Young, V. G. Science 1997, 275, 1445-1447. (d) Shan, H.; Yang, Y.; James,
A. J.; Sharp, P. R. Science 1997, 275, 1460-1462. (e) Zanotti-Gerosa, A.;
Solari, E.; Giannini, L.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1998, 120, 437-438.
(6) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc., Dalton
Trans. 1977, 1852-1860. (b) Takahashi, T.; Mizobe, Y.; Sato, M.; Uchida,
Y.; Hidai, M. J. Am. Chem. Soc. 1980, 102, 7461-7467.
(7) Organic thiol: (a) Dilworth, J. R.; Richards, R. L.; Dahlstorm, P.;
Hutchinson, J.; Kumar, S.; Zubieta, J. J. Chem. Soc., Dalton Trans. 1983,
1489-1493. (b) Povey, D. C.; Richards, R. L. J. Chem. Soc., Dalton Trans.
1984, 2585-2587. (c) Hughes, D. L.; Lazarowych, N. J.; Maguire, M. J.;
Morris, R. H.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1995, 5-15 and
references therein.
(9) Nishibayashi, Y.; Iwai, S.; Hidai, M. Science 1998, 279, 540-542.
(10) (a) Hashizume, K.; Mizobe, Y.; Hidai, M. Organometallics 1996, 15,
3303-3309. (b) Tang, Z.; Nomura, Y.; Ishii, Y.; Mizobe, Y.; Hidai, M.
Organometallics 1997, 16, 151-154. (c) Tang, Z.; Nomura, Y.; Ishii, Y.;
Mizobe, Y.; Hidai, M. Inorg. Chim. Acta 1998, 267, 73-79. (d) Tang, Z.;
Nomura, Y.; Kuwata, S.; Ishii, Y.; Mizobe, Y.; Hidai, M. Inorg. Chem. in
press.
(11) An excess of methanol (pKa 15.5) reacts with 1 at 50 °C to form NH3
in good yield.20 Both PhSH (pKa 6.6) and H2S (pKa 7.0) are assumed to have
enough acidity to protonate the coordinated N2 in the complex 7; however,
protonation did not occur.
(8) H2S: Kuwata, S.; Mizobe, Y.; Hidai, M. J. Chem. Soc., Dalton Trans.
1997, 1753-1758.
S0002-7863(98)01223-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/25/1998

10560 J. Am. Chem. Soc., Vol. 120, No. 40, 1998
Communications to the Editor
Table 1. Protonation of Coordinated Dinitrogen on Tungsten with
Scheme 3
Sulfido-Bridged Dinuclear Complexes to Produce Ammoniaa
yield of
yield of
NH3 (%)b
NH3 (%)b
com- temp time
com- temp time
plex (°C) (h) freec basicd total plex (°C) (h) freec basicd total
2
2
2
2
55 24
55 60
30 24
55 24
3
0
2
75 78
91 91
26 28
50e
3
4
5
6
55 24
55 24
55 24
55 24
3
2
0
2
4
36
6
7
38
6
Table 2. Protonation of Coordinated Dinitrogen on Tungsten with
Sulfido-Bridged Dinuclear Complexes to Produce Acetone Azinea
7
9
temp time
acetone
temp time
acetone
a All of the reactions were carried out in dichloroethane-benzene
using 0.10 mmol of 1 and 1.00 mmol of complex. b Yield of NH3 was
based on the W atom. c Free yield was before base distillation of the
reaction mixture. d Basic yield was after base distillation to fully liberate
NH3. e This yield of NH3 was observed in the water extract of the
reaction mixture (see text).
complex (°C) (h) azine (%)b complex (°C) (h) azine (%)b
2
2
2
3
55
55
30
55
24
48
48
24
48
74
67
8
4
5
55
55
55
55
48
24
24
24
95
14
15
15
6
10
a All of the reactions were carried out in acetone-benzene using
0.10 mmol of 1 and 1.00 mmol of complex. b Yield of acetone azine
was based on the W atom.
Scheme 2
hydrosulfido-bridged complexes is also considered to proceed
through hydrazido(2-) intermediates. These findings indicate that
the µ-SH ligand17 especially in the cationic dinuclear complexes
is able to protonate the coordinated N2 on W atom, in sharp
contrast to RSH and H2S.
Interestingly, treatment of 1 in the presence of acetone with
10 equiv of 2 under nitrogen atmosphere in benzene at 55 °C for
24 h afforded acetone azine in 48% yield based on the W atom
(Scheme 3). The typical results were shown in Table 2. In
contrast to the formation of NH3, 4 produced acetone azine in a
yield higher than 2. Other hydrosulfido-bridged dinuclear
complexes such as 5, 6, and [Cp*RuCl(µ-SH)2RuClCp*]10a (10)
afforded acetone azine in 14-15% yield under the same condi-
tions. In the absence of the hydrosulfido-bridged dinuclear
complexes, no acetone azine was formed. On the other hand,
employment of dirhodium sulfido-bridged complexes such as
[Cp*RhCl(µ-SH)2RhClCp*],10b [(PPh3)2RhHCl(µ-SH)2RhHCl-
(PPh3)2],18 and [(triphos)RhH(µ-SH)2RhH(triphos)](PF6)219 (triph-
os ) 1,1,1-tris(diphenylphosphinomethyl)ethane) did not give rise
to the formation of acetone azine. The formation of acetone azine
in these reactions is considered to proceed through diazoalkane
intermediates containing the WdN-NdCMe2 moiety, which is
formed by the condensation of a hydrazido(2-) intermediate with
acetone. The mechanism is essentially the same as that proposed
previously for the reaction of 1 with a methanol/acetone mixture.20
In summary, we have found that coordinated N2 on the W atom
can be protonated with the µ-SH ligand in dinuclear complexes
to produce NH3 and acetone azine under mild conditions. The
cationic µ-SH dinuclear complexes 2 and 4 are the most effective
for protonation of coordinated N2. To our knowledge, this is the
first example of proton transfer from metal SH complexes to
coordinated N2. Whether such proton transfer occurs in nitro-
genase is still completely open to conjecture; however, this type
of model system will provide valuable information about the
mechanism of biological nitrogen fixation by nitrogenase.
12
BF4 (4; P3 ) bis(2-diphenylphosphinoethyl)phenylphosphine)
was employed as a closer model component of nitrogenase be-
cause µ-SH moieties bound to the iron atoms in nitrogenase are
considered to mediate proton transfer to coordinated N2 (Vide
supra). In this reaction, NH3 was formed in 38% total yield.
However, metal products could not be characterized. In the
present model reactions, we consider that proton was transferred
through the intermolecular interaction between the µ-SH ligand
in the dinuclear complexes and the coordinated N2 on W atom
(Vide infra).
On the other hand, the neutral hydrosulfido-bridged diiridium
complex, [Cp*IrCl(µ-SH)2IrClCp*]10b,c (5), afforded NH3 in a
lower yield than the above cationic hydrosulfido-bridged complex
2.13 This shows that the µ-SH ligand in the cationic complex is
more acidic than that in the neutral complex. A heterodinuclear
complex, [Cp*RuCl(µ-SH)2TiCp2]14 (6), also gave a low yield
of NH3.
The reaction of trans-[W(N2)2(dppe)2] (7) with 2 equiv of 2 or
4 under a nitrogen atmosphere in dichloroethane-benzene at 55
°C for 24 h gave the hydradizo(2-) complexes trans-[WCl(NNH2)-
(dppe)2]Cl (8) and trans-[WF(NNH2)(dppe)2]BF4 (9) in 60% and
80% NMR yields, respectively (Scheme 2).15 The same hy-
drazido(2-) complexes were previously obtained from 7 by
protonation with HCl or HBF4.5a,6a Furthermore, the direct transfer
of a proton from the µ-SH ligand in the dinuclear complex 4 to
the coordinated N2 in complex 1 was preliminarily confirmed by
the experiment using the deuterated hydrosulfido complex [P3-
Fe(µ-SD)3FeP3]BF4 (4′); the deuterated hydrazido(2-) complex
[WF(NND2)(dppe)2]+ 9′ (νND ) 2399 cm-1) and nondeuterated
hydrazido(2-) complex 9 were obtained in 35 and 45% NMR
yield, respectively, under the same reaction conditions.16 These
results provide direct evidence for protonation of the coordinated
N2 with the µ-SH ligand in these dinuclear complexes. Thus,
the formation of NH3 by the reaction of 1 with the above
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.
JA981223G
(16) The relatively low yield of 9′ is considered to be due to the reaction
of 7 with the nondeuterated hydrosulfido complex 4, which was probably
formed by the reaction of the deuterated hydrosulfido complex 4′ with
adventitious water in the solvent.
(17) Since the µ-SH dinuclear complexes (2-6) are readily deprotonated
by Et3N, the pKa values of these complexes are estimated to be far below the
pKa of Et3NH+ (10.8).
(18) Mueting, A. M.; Boyle, P.; Pignolet, L. H. Inorg. Chem. 1984, 23,
44-48.
(19) (a) Bianchini, C.; Mealli, C.; Meli, A.; Sabat, M. Inorg. Chem. 1986,
25, 4617-4618. (b) Bianchini, C.; Meli, A. Inorg. Chem. 1987, 26, 4268-
4271.
(12) Vaira, M. D.; Midollini, S.; Sacconi, L. Inorg. Chem. 1979, 18, 3466-
3469.
(13) The 1H NMR spectrum of the reaction mixture showed the formation
of the cubane cluster [(Cp*Ir)4(µ-S)4], which was previously obtained by the
reaction of 5 and Et3N.10b,c
(14) Kuwata, S.; Hidai, M. Chem. Lett. 1998, 885-886.
(15) The anion (chloride or fluoride) in the hydradizo(2-) complexes 8 and
9 came from the counteranion of the hydrosulfido complexes 2 and 4.
(20) Wakatabe, A.; Takahashi, T.; Jin, D.-M.; Yokotake, I.; Uchida, Y.;
Hidai, M. J. Organomet. Chem. 1983, 254, 75-82.