Catalytic reduction of hydrazine to ammonia by a vanadium thiolate complex

Article abstract of DOI:10.1021/ic060171q

Vanadium(III) thiolate complexes, [V(PS3″)(Cl)]^- [1a; PS3″ = P(C₆H₃-3-Me₃Si-2-S) ₃^3-] and [V(PS3′)(Cl)]^- [1b; PS3′ = P(C₆H₃-5-Me-2-S)₃^3-], were synthesized and characterized. Complex 1a serves as a precursor for the catalytic reduction of hydrazine to ammonia. The spectroscopic and electrochemical studies indicate that hydrazine is bound and activated in a V^II state.

Full text of DOI:10.1021/ic060171q

Inorg. Chem. 2006, 45, 3164−3166
Catalytic Reduction of Hydrazine to Ammonia by a Vanadium Thiolate
Complex
Wei-Cheng Chu, Chi-Chin Wu, and Hua-Fen Hsu*
Department of Chemistry, National Cheng Kung UniVersity, Tainan, Taiwan 701
Received January 30, 2006
Vanadium(III) thiolate complexes, [V(PS3′′)(Cl)]- [1a; PS3′′ )
limited.8-14 They are mononuclear complexes MCp*Me3 core
(M ) Mo or W),8,9 molybdenum thiolate complexes,10,11 and
MFe3S4 (M ) Mo or V) cubanes.12-14 The finding of
vanadium nitrogenase prompts us and others to explore the
reactivity of V compounds with substrates relevant to
nitrogenase.15,16 To continue our effort in this aspect, we have
obtained a vanadium(III) thiolate complex, [V(PS3′′)(Cl)]-
[1a; PS3′′ ) P(C6H3-3-Me3Si-2-S)33-], that servers as a
precursor for the catalytic reduction of hydrazine. Notably,
it is unprecedented that hydrazine reduction is catalyzed by
a mononuclear V center, in particular, ligating with thiolate
ligands and mimicking the sulfido environment of the V site
in vanadium nitrogenase. We detail this chemistry herein.
P(C6H3-3-Me3Si-2-S)3 -] and [V(PS3
′
)(Cl)]- [1b; PS3′ ) P(C6H3-
3
3
5-Me-2-S)3 -], were synthesized and characterized. Complex 1a
serves as a precursor for the catalytic reduction of hydrazine to
ammonia. The spectroscopic and electrochemical studies indicate
that hydrazine is bound and activated in a VII state.
Nitrogen fixation catalyzed by nitrogenases is an important
process in biology.1-3 Although the structure of molybdenum
nitrogenase has been characterized by X-ray crystallography,
how the enzyme catalyzes the reduction of dinitrogen and
other substrates is still not fully understood.4 To discern the
mechanistic steps of nitrogen fixation carried by the enzyme,
much effort has been made by researchers on synthetic metal
complexes that activate N2Hx molecules, substrates, and
intermediates of nitrogenase.5 Hydrazine is a substrate as well
as an intermediate of nitrogenase.6,7 However, reported
synthetic analogues that catalyze the reduction of hydrazine
to ammonia, the late stage of nitrogen fixation, are very
The reaction of 1 equiv of VCl3(THF)3 in THF with 1
equiv of PS3′′ in methanol generated a reddish-brown
+
solution. The addition of PPh4 in CH2Cl2 followed by
layering pentane gave a crystalline solid of 1a in 70% yield.
An analogy of compound 1a, [V(PS3′)(Cl)]- [1b; PS3′ )
P(C6H3-5-Me-2-S)33-], was obtained under a similar reaction
using the PS3′ ligand. The structures of 1a and 1b character-
ized by X-ray crystallography are similar, and their ORTEP
pictures are shown in Figure 1. Each structure embraces a
five-coordinate V center with a trigonal-bipyramidal geom-
etry in which the V center coordinates to a P atom, three
thiolate groups, and a chloride trans to the P donor. The
average V-S bond distances of 2.319(6) and 2.318(3) Å
* To whom correspondence should be addressed. E-mail: konopka@
mail.ncku.edu.tw.
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3164 Inorganic Chemistry, Vol. 45, No. 8, 2006
10.1021/ic060171q CCC: $33.50
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Published on Web 03/22/2006

COMMUNICATION
Table 1. Catalytic Reduction of Hydrazine to Ammoniaa
complex
added N2H4,b equiv
NH3 yield, equiv
convn,c %
1a
1 (2)
2.0
5.5
8.1
8.2
8.5
NRf
NRf
100(0)
92(2)
81(1)
59(2)
42(1)
NR
3 (3)
5 (3)d
7 (3)
10 (3)
10 (2)e
3 (2)
1b
NR
a The reaction time is 24 h. See the Supporting Information for reaction
details and a complete table (Table S2). The indophenol method was used
for ammonia analysis (ref 18). b The number of experiments is in paren-
theses. c The range (() is shown in parentheses. d To ensure a N-atom
balance, analyses for N2H4 were carried out after the reactions. A total of
0.91 and 0.85 equiv of N2H4 were detected for two experiments. The results
gave a N-atom balance of approximately 95%. The PDMAB method was
used for N2H4 analysis (ref 19). e Without Co(Cp)2 and Lut‚HCl present.
f No ammonia is detected above the background level.
Figure 1. Thermal ellipsoid plots (35% probability) of the anions of 1a
(left) and 1b (right). The H atoms and countercations have been omitted
for clarity. Selected bond lengths [Å] and angles [deg] for 1a: V1-S1
2.314(2), V1-S2 2.318(2),V1-S3 2.324(2), V1-Cl1 2.325(2), V1-P1
2.367(2), S2-V1-S1 116.46(8), S2-V1-S3 115.75(8), S1-V1-S3
122.48(8), S2-V1-Cl1 102.74(8), S1-V1-Cl1 92.55(7), S3-V1-Cl1
98.10(7), S2-V1-P1 82.57(7), S1-V1-P1 82.19(7), S3-V1-P1
82.18(7), Cl1-V1-P1 173.84(8). For 1b: V1-S1 2.315(1), V1-S2
2.317(1), V1-S3 2.322(1), V1-Cl1 2.375(1), V1-P1 2.379(1), S1-V1-
S2 118.31(5), S1-V1-S3 118.05(5), S2-V1-S3 119.09(5), S1-V1-Cl1
96.71(4), S2-V1-Cl1 96.50(4), S3-V1-Cl1 98.19(4), S1-V1-P1
82.87(4), S2-V1-P1 82.82(4), S3-V1-P1 82.90(4), Cl1-V1-P1
178.90(4).
found in 1a and 1b, respectively, are similar to those found
in five-coordinate vanadium(III) thiolate complexes.16a,b
Importantly, compound 1a catalyzes the reduction of N2H4
to NH3 in CH3CN with the presence of a reducing agent
and a proton source, cobaltocene [Co(Cp)2] and 2,6-luti-
dinium chloride (2,6-Lut‚HCl), respectively (eq 1).17 A total
N2H4 + 2e- + 2H+ f 2NH3
(1)
Figure 2. Variation in UV/vis/NIR spectra of 1a in CH2Cl2 (7.68 × 10-4
M) with the addition of various equivalents of N2H4. Inset: UV/vis/NIR
spectra of 1a (4.61 × 10-4 M) in CH2Cl2 (blue line) and 2 (6.50 × 10-4
M) in CH2Cl2 (red line).
of 5 equiv of N2H4 can be reduced catalytically with a nearly
quantitative yield (90%) after 48 h (Table S1 in the
Supporting Information). In the absence of an external proton
source and a reducing agent, the reaction of 1a and N2H4
does not produce ammonia, indicating that 1a does not assist
the disproportionation of N2H4 (eq 2).
voltammogram (CV) of 2 in CH3CN does not show any
reduction wave in the range of -1.0 to -2.2 V [vs Fe(Cp)2/
+
Fe(Cp)2 ]. The CV of cobaltocene measured at the same
condition displays a redox potential corresponding to a Co-
+
+
3N2H4 f 4NH3 + N2
(2)
(Cp)2/Co(Cp)2 couple at -1.33 V [vs Fe(Cp)2/Fe(Cp)2 ],
indicating that Co(Cp)2 is not a sufficient reducing agent to
convert 2 to its reduced form.
The reactions with various ratios of hydrazine to 1a in
CH3CN at ambient temperature and pressure were investi-
gated for the reaction time of 24 h, as shown in Table 1.
Accordingly, a high yield of ammonia was generated when
less than 5 equiv of N2H4 was attempted to be reduced.
However, the yield was decreased in the reactions that
attempted to reduce more than 5 equiv of N2H4.
The low conversion yield for reducing more equivalents
of N2H4 is mainly attributed to the formation of [V(PS3′′)-
(N2H4)3] (2), a catalytically inactive compound. Compound
2, reported previously, was obtained in 65% yield from the
reaction of 1a with 10 equiv of N2H4.16a However, 2 does
not produce ammonia with the addition of Co(Cp)2 and 2,6-
Lut‚HCl, implying that 2 is not a substrate-bound intermedi-
ate in the catalytic cycle of hydrazine reduction. This result
is consistent with the electrochemical studies of 2. The cyclic
During speculation about hydrazine-bound intermediates
in the catalytic cycle, titration of 1a with N2H4 in CH2Cl2 at
ambient temperature was monitored by UV/vis/NIR absorp-
tion spectroscopy, as shown in Figure 2.20 Upon the addition
of increasing amounts of N2H4, the characteristic band of
1a at 992 nm gradually decreased while absorption increased
at 537 nm, a feature of 2. The conversion was marked by
two isosbestic points, 515 and 860 nm, which strongly
suggests that 2 is produced directly from 1a and no other
hydrazine-bound adducts such as [V(PS3′′)(N2H4)] and
[V(PS3′′)(N2H4)2] are formed in the reaction mixture. Thus,
in the catalytic cycle of hydrazine reduction by 1a, a
substrate-binding species is at a VII state rather than a VIII
state. This leads to a possible reaction pathway in which 1a
is reduced to a VII species prior to hydrazine binding.
(17) The following experiments without the addition of 1a in CH3CN at
ambient temperature were carried out by Coucouvanis’ laboratory:
(a) the reaction of N2H4, Co(Cp)2, and Lut‚HCl and (b) the reaction
of Co(Cp)2 and Lut‚HCl (ref 14). None of these reactions generated
ammonia, ruling out possible ammonia production without the presence
of 1a.
(18) Chaney, A. L.; Marbach, E. P. Clin. Chem. (Winston-Salem, N.C.)
1962, 8, 130.
(19) Watt, G. W. Anal. Chem. 1952, 24, 2006.
(20) The titration was performed in CH2Cl2 instead of CH3CN because of
the insolubility of 2 in CH3CN.
Inorganic Chemistry, Vol. 45, No. 8, 2006 3165

COMMUNICATION
than that of Co(Cp)2 but similar to that of 1a in CH2Cl2
(Figure S1 in the Supporting Information). This implies that
the CH3CN molecule does not replace the bound chloride
ion in 1b while dissolving 1b in CH3CN. This finding is
also supported by the UV/vis/NIR spectra of 1b taken in
CH2Cl2 and CH3CN that display almost identical absorption
bands (Figure 3). The different reactivity of 1a and 1b toward
the CH3CN molecule is likely related to more electron-
donating substituents in the PS′′ ligand than in the PS′ ligand;
electron donation to the V center would then facilitate the
replacement of a bound anionic chloride by a neutral CH3-
CN ligand. A reported VIIIN2H4 compound, [V(NS3)(N2H4)]
[NS3 ) N(CH2CH2S)33-], also does not carry the reduction
of hydrazine with the external electron and proton sources.16b
Failure to access a VII oxidation state for 1b and [V(NS3)-
(N2H4)] appears to be an explanation why these two
compounds are catalytically inactive for hydrazine reduction.
In summary, we have isolated and characterized a vana-
dium(III) thiolate complex, 1a, that servers as a precursor
for the reduction of hydrazine catalytically. On the basis of
spectroscopic and electrochemical studies, we propose that
the initial step of the reaction involves the replacement of
the bound Cl- ion in 1a by the solvent molecule, CH3CN.
Subsequently, [V(PS3′′)(CH3CN)] is reduced to a VII species
that binds and activates N2H4. Importantly, the vanadium
thiolate complex shows reactivity in the N-N bond cleavage
of hydrazine, an intermediate of nitrogen fixation, emphasiz-
ing the role of the V site in vanadium nitrogenase, which
might be involved in the catalytic pathway of the late stage
of nitrogen fixation. A further investigation of the reaction
mechanism is in progress.
Figure 3. UV/vis/NIR spectra of 1a (top) and 1b (bottom) in CH2Cl2
(solid line) and CH3CN (dashed line).
However, the CV of 1a measured in CH2Cl2 displays an
irreversible reduction wave at -2.48 V [vs Fe(Cp)2/
+
Fe(Cp)2 ], much lower than the redox potential of Co(Cp)2
measured at the same condition, which indicates that the
reducing power of Co(Cp)2 is not strong enough to reduce
1a to a VII state (Figure S1 in the Supporting Information).
In contrast, when the CV of 1a is performed in CH3CN, the
reduction wave shifted dramatically to -1.34 V. Likely, this
is caused by the replacement of a bound chloride ion by a
CH3CN molecule, leading to the formation of [V(PS3′′)(CH3-
CN)]. Therefore, the reducing power of Co(Cp)2 is strong
enough to reduce [V(PS3′′)(CH3CN)] to a VII species. The
replacement of a bound chloride in 1a by CH3CN while
dissolving 1a in CH3CN was confirmed by the UV/vis/NIR
spectra, as shown in Figure 3. The spectra of 1a taken in
CH2Cl2 and CH3CN display similar patterns with very
different extinction coefficients. Indeed, the catalytic reaction
carried out in CH2Cl2 rather than in CH3CN produced no
ammonia, consistent with the findings above.
Acknowledgment. This work is supported by a grant
from the National Science Council in Taiwan (Grant NSC
94-2113-M-006-013). We thank Prof. Ju-Hsiou Liao at
National Chung Cheng University and Ting-Shen Kuo at
National Taiwan Normal University for crystallographic
structural determination.
Supporting Information Available: General consideration,
physical methods, syntheses for complexes 1a and 1b, crystal-
lographic determination and data for 1a and 1b, experiments for
catalytic reactions, complete tables for catalytic reactions (Tables
S1 and S2), cyclic voltammograms (Figure S1), and crystallographic
data in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
The catalytic reductions of hydrazine by 1b were also
investigated in parallel (Table 1). Interestingly, no reaction
yielded ammonia above the background level. This finding
can be rationalized by the reduction wave of 1b in CH3CN
+
appearing at -2.18 V [vs Fe(Cp)2/Fe(Cp)2 ], much lower
IC060171Q
3166 Inorganic Chemistry, Vol. 45, No. 8, 2006