Preparation of dinitrogen complexes Mo(N2)2P4 stabilised by phosphonite PPh(OEt)2 and phosphinite PPh2(OEt) ligands

Article abstract of DOI:10.1016/S0022-328X(02)01735-7

Dinitrogen complexes Mo(N₂)₂P₄ 1, 2 [P=PPh(OEt)₂ and PPh₂OEt] were prepared by allowing a MoCl₃(THF)₃ solution containing an excess of phosphine to react with magnesium under nitrogen. Substitution reactions with CO and p-tolylisocyanide were studied, and led to Mo(CO)₂P₄, Mo(CO)₃P₃, and Mo(p-tolylNC)₂P₄ derivatives. Treatment of dinitrogen compound Mo(N₂)₂[PPh(OEt)₂]₄ with an excess of HCl gave the hydrazido(2-) [MoCl(NNH₂)PPh(OEt)₂}₄]Cl derivative. Reduction reactions with zinc amalgam of complexes 1 and 2 in the presence of lutidine·HCl gave ammonia in about 8-10% yield.

Full text of DOI:10.1016/S0022-328X(02)01735-7

Journal of Organometallic Chemistry 660 (2002) 55ꢁ61
/
www.elsevier.com/locate/jorganchem
Preparation of dinitrogen complexes Mo(N2)2P4 stabilised by
phosphonite PPh(OEt)2 and phosphinite PPh2(OEt) ligands
Gabriele Albertin *, Emilio Bordignon, Federica Tonel
Dipartimento di Chimica, Universita` Ca’ Foscari di Venezia, Dorsoduro 2137, I-30123 Venice, Italy
Received 29 May 2002; accepted 19 July 2002
Abstract
Dinitrogen complexes Mo(N2)2P4 1, 2 [Pꢀ
/
PPh(OEt)2 and PPh2OEt] were prepared by allowing a MoCl3(THF)3 solution
containing an excess of phosphine to react with magnesium under nitrogen. Substitution reactions with CO and p-tolylisocyanide
were studied, and led to Mo(CO)2P4, Mo(CO)3P3, and Mo(p-tolylNC)2P4 derivatives. Treatment of dinitrogen compound
Mo(N2)2[PPh(OEt)2]4 with an excess of HCl gave the hydrazido(2-) [MoCl(NNH2){PPh(OEt)2}4]Cl derivative. Reduction reactions
with zinc amalgam of complexes 1 and 2 in the presence of lutidine×
/
HCl gave ammonia in about 8ꢁ10% yield. # 2002 Elsevier
/
Science B.V. All rights reserved.
Keywords: Molybdenum; Dinitrogen complexes; Phosphite; Reduction; Reactivity
1. Introduction
However, despite the large number of reports in this
field, no examples of molybdenum dinitrogen complexes
containing phosphites [P(OR)3] or phenyl-substituted
ones [PPh(OEt)2, PPh2OEt] have been reported,
although these ligands have shown interesting properties
The coordination chemistry of the dinitrogen mole-
cule has been studied for many decades in several metal
ligand systems, with the aim of obtaining insights into
N2 binding and activation through metal complexation
in stabilising ‘‘diazo’’ species such as diazene RNÄ
/
NH,
diazenido RN2, and hydrazine bonded to a metal
fragment [9].
[1ꢁ
/
5]. Among central metals, molybdenum is one of the
5], as classic dinitrogen
derivatives [6] Mo(N2)2P4, containing phosphines as
most frequently investigated [2ꢁ
/
Studies on the chemistry of such ‘‘diazo’’ complexes
with phosphite ligands continue to be one subject of our
research interest [10], which has involved several metal
centres such as cobalt [10a,10b] and iron triads [10e,11],
manganese [12], and rhenium [10c,10d]. We have now
extended these studies to molybdenum, and this paper
reports the synthesis and reactivity of new dinitrogen
complexes of this metal, stabilised by phosphonite and
phosphinite ligands.
ancillary ligands, show interesting properties in proto-
nation and reduction reactions [3ꢁ5,7,8]. Extensive
/
investigations have in fact shown the formation of
ammonia and/or hydrazine by treatment of these
Mo(N2)2P4 complexes with Brønsted acid [7,8]. Orga-
nonitrogen compounds have also been obtained from
coordinated N2, often through the hydrazido(2-)
[Mo]NNH2 intermediate [3]. However, the most popular
ligand system in molybdenum dinitrogen chemistry
involves tertiary phosphines as ancillary ligands, and
extensive studies have shown how the steric and
electronic properties of these supporting groups are
crucial in determining the stoichiometry, structure, and
2. Experimental
2.1. General considerations and physical measurements
reactivity of the N2 derivatives [3ꢁ5].
/
All synthetic work was carried out in an inert atmo-
sphere using standard Schlenk techniques or a Vacuum
Atmosphere dry-box. Once isolated, the complexes were
found to be relatively stable in air, but were stored in an
* Corresponding author. Fax: ꢂ
/
39-041-234-8917
E-mail address: albertin@unive.it (G. Albertin).
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 7 3 5 - 7

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G. Albertin et al. / Journal of Organometallic Chemistry 660 (2002) 55ꢁ61
/
inert atmosphere at ꢃ
/
25 8C. All solvents were dried
62.48; H, 5.59; N, 5.01. C56H60MoN4O4P4 requires C,
62.69; H, 5.64; N, 5.22%).
over appropriate drying agents, degassed on a vacuum
line, and distilled into vacuum-tight storage flasks.
Phosphines PPh(OEt)2 and PPh2OEt were prepared
following the method of Rabinowitz and Pellon [13].
2.2.3. Mo(CO)2[PPh(OEt)2]4 (3)
A solution of Mo(N2)2[PPh(OEt)2]4 (0.20 g, 0.21
mmol) in 20 cm3 of THF was allowed to stand under
CO atmosphere (1 atm) for 150 min. The solvent was
Phosphites P(OR)3 (RꢀMe, Et, Ph, i-Pr) (Aldrich)
/
were purified by distillation under nitrogen. Other
reagents were purchased from commercial sources in
the highest available purity and used as received.
Infrared spectra were recorded on a Nicolet Magna
750 FT-IR spectrophotometer. NMR spectra (1H, 31P)
were obtained on a Bruker AC200 spectrometer at
removed under reduced pressure giving a redꢁbrown oil,
/
which was triturated with EtOH (2 cm3). A yellow solid
slowly separated out from the resulting solution, which
was filtered and dried under vacuum; yield ]80%
/
(Found: 53.65; H, 6.33. C42H60MoO10P4 requires C,
53.40; H, 6.40%).
temperatures varying between ꢃ
/
90 and ꢂ30 8C, unless
/
otherwise noted. 1H spectra are referred to internal
Me4Si; 31P{1H} chemical shifts are reported with respect
to 85% H3PO4, with downfield shifts considered posi-
tive. The SwaN-MR software package [14] was used to
treat NMR data. The conductivity of 10ꢃ3 mol dmꢃ3
solutions of the complexes in CH3NO2 at 25 8C was
measured with a Radiometer CDM 83 instrument.
2.2.4. Mo(CO)2[PPh2OEt]4 (4) and
Mo(CO)3[PPh2OEt]3 (5)
These complexes were obtained like the related 3 by
allowing a solution of the dinitrogen compound
Mo(N2)2[PPh2OEt]4 (0.30 g, 0.28 mmol) in 20 cm3 of
THF to react with CO (1 atm). The yellow solid
obtained after evaporation of the solvent contains a
mixture of 4 and 5, which were separated by fractional
crystallisation. A typical separation involves the slow
2.2. Synthesis of complexes
cooling to ꢃ25 8C of a solution prepared by dissolving
/
The complex MoCl3(THF)3 was prepared following
the method previously reported [15].
the solid obtained in C2H5OH and enough CH2Cl2 to
obtain a saturated solution at 20 8C. The first solid
precipitated was the dicarbonyl Mo(CO)2[PPh2OEt]4,
2.2.1. Mo(N2)2[PPh(OEt)2]4 (1)
which was filtered and dried under vacuum; yield ]
/
To a solution of MoCl3(THF)3 (1 g, 2.39 mmol) in 16
cm3 of THF, under a N2 atmosphere (1 atm), were
sequentially added an excess of magnesium turnings (1.3
g, 53.5 mmol) and an excess of PPh(OEt)2 (2.38 cm3,
12.0 mmol). The reaction mixture was vigorously stirred
at room temperature (r.t.) for 20 h and then filtered to
separate the unreacted magnesium and the brown solid
formed. The solvent was removed by evaporation at
20%. By concentration and further cooling of the
solution, the tricarbonyl Mo(CO)3[PPh2OEt]3 separated
out as yellow microcrystals; yield ]30% (Found: 65.05;
/
H, 5.51. C58H60MoO6P4 4 requires: C, 64.93; H, 5.64.
Found: 62.22; H, 5.28. C45H45MoO6P3 5 requires C,
62.08; H, 5.21%).
2.2.5. Mo(p-tolylNC)2P4 (6, 7) [Pꢀ
/
PPh(OEt)2 (6),
reduced pressure, giving a redꢁ
/
brown oil which was
30 cm3). A yellow solid slowly
PPh2OEt (7)]
triturated with EtOH (25ꢁ
/
An excess of p-tolylisocyanide (74 ml, 0.63 mmol) was
added to a solution of the appropriate dinitrogen
complex Mo(N2)2P4 (0.25 mmol) in 10 cm3 of THF
and the reaction mixture stirred for about 3 h. The
solvent was removed under reduced pressure to give a
separated out which was filtered, washed with EtOH,
and dried under vacuum. The compound was crystal-
lised by dissolving in benzene and, after filtration and
concentration, by precipitation with EtOH; yield ]40%
/
redꢁ
brown oil, which was triturated with 2 cm3 of
ethanol. A yellow solid slowly separated out, which was
filtered and crystallised from CH2Cl2 and EtOH; yield
/
(Found: 51.10; H, 6.29; N, 5.71. C40H60MoN4O8P4
requires C, 50.85; H, 6.40; N, 5.93%).
2.2.2. Mo(N2)2[PPh2OEt]4 (2)
]70% (Found: 60.11; H, 6.59; N, 2.58. C56H74Mo-
/
This complex was prepared following the method
used for the related compound 1. In this case, however,
the dinitrogen complex is little soluble in THF and can
be obtained as a yellow solid by concentration (10 cm3)
and cooling (0 8C) of the reaction mixture. This solid
was separated by filtration, obtaining a mixture of
dinitrogen compound, magnesium and other solid
species, from which complex 2 was extracted with two
5-cm3 portions of benzene and, after concentration to
N2O8P4 6 requires: C, 59.89; H, 6.64; N, 2.49. Found:
68.92; H, 5.85; N, 2.32. C72H74MoN2O4P4 7 requires: C,
69.12; H, 5.96; N, 2.24%).
2.2.6. [MoCl(NNH2){PPh(OEt)2}4]Cl (8)
To a solution of Mo(N2)2[PPh(OEt)2]4 (0.40 g, 0.42
mmol) in 12 cm3 of THF, cooled to ꢃ
196 8C, an excess
/
of HCl (1.7 mmol, 8.5 cm3 of a 0.2 M solution in Et2O)
was added, and the reaction mixture was allowed to
reach r.t. After 2 h of stirring, the solvent was removed
2ꢁ
/
3 cm3, precipitated with THF; yield ]
30% (Found:
/

G. Albertin et al. / Journal of Organometallic Chemistry 660 (2002) 55ꢁ
/61
57
under reduced pressure to give a brown oil, which was
triturated with EtOH (2 cm3). A dark green solid slowly
separated out from the resulting solution which, after
PPh(OEt)2 and PPh2OEt phosphite ligands, giving a
reddish-brown solution from which no dinitrogen com-
plex could be isolated. We also attempted to carry out
the reaction in a two-step sequence by reacting
MoCl3(THF)3 first with phosphite, to prepare the
MoCl3P3 species, and then with magnesium, to reduce
this intermediate to Mo(0) dinitrogen derivative. How-
ever, this procedure failed, giving only traces of species 1
or 2, and only by adding magnesium immediately after
the phosphite to the MoCl3(THF)3 solution and stirring
the reaction mixture for about 20 h could solid samples
of dinitrogen complexes be obtained.
cooling to ꢃ25 8C to complete the precipitation, was
/
filtered and dried under vacuum; yield ]
/
25%; LMꢀ
/
82.4 S cm2 molꢃ1 (Found: 48.46; H, 6.25; N, 2.71; Cl,
7.01. C40H62Cl2MoN2O8P4 requires C, 48.55; H, 6.31;
N, 2.83; Cl, 7.16%).
2.2.7. Reduction reactions
Reduction of dinitrogen complexes was carried out in
THF at r.t. under N2 using an excess of Zn/Hg as
reducing agent in the presence of lutidine hydrochloride
We also attempted to prepare dinitrogen derivatives
with phosphites different from PPh(OEt)2 and PPh2OEt,
(Lu×/HCl). A typical experiment involved the addition of
a 20-fold excess of Zn (2.0 mmol), as 2% zinc amalgam
(6.5 g), to a solution of the appropriate dinitrogen
complex (0.1 mmol) in 10 cm3 of THF. A 30-fold excess
of lutidine hydrochloride (3.0 mmol, 0.43 g) in 10 cm3 of
THF was then added to the reaction mixture, which was
stirred for a reaction time from 24 to 72 h. An excess of
hydrochloric acid (2.0 mmol, 2 cm3 of 1 M solution in
Et2O) was added to the reaction mixture, which was
stirred for 1 h, and then the solvent was removed under
reduced pressure. The resulting solid was treated with
five 5-cm3 portions of H2O and, after filtration,
ammonia from the solution was quantified by the
indophenol method [16], while hydrazine was deter-
mined by the p-dimethylaminobenzaldehyde test [17].
of the type P(OR)3 (Rꢀ
/
Me, Et, Ph, i-Pr) or PPh(OR)2
(Rꢀi-Pr, n-Bu), but only traces of dinitrogen species
/
were obtained. It therefore seems that only with
phosphonite PPh(OEt)2 and phosphinite PPh2OEt as
ancillary ligands can stable dinitrogen complexes of
molybdenum be obtained. Furthermore, when reduction
of MoCl3(THF)3 gives rise to dinitrogen species, exclu-
sively bis(derivatives) Mo(N2)2P4 were obtained, and the
infrared data on raw samples excluded the presence of
other dinitrogen derivatives such as mono-species
MoCl(N2)P4, observed in previous cases [6c].
Bis(dinitrogen) complexes 1 and 2 were isolated in
relatively good yields (30ꢁ40%) as yellow solids stable in
/
air and soluble in most common organic solvents.
However, although these solutions are quite stable
under N2, they decompose upon exposure to air, giving
rise to a change in colour and separation of a solid
substance.
We also attempted to obtain crystals suitable for X-
ray analysis from the solutions of complexes 1, 2, but
only yellow powders were obtained. However, analytical
and spectroscopic data (Table 1) strongly support the
proposed formulation and suggest the mutually trans
position of the two dinitrogen ligands, as shown in Fig.
1.
3. Results and discussion
3.1. Synthesis of dinitrogen complexes
Phosphite-containing dinitrogen complexes of molyb-
denum Mo(N2)2P4 1, 2 were prepared by reducing
MoCl3(THF)3 solutions with magnesium turnings in a
N2 atmosphere and in the presence of an excess of
phosphite, as shown in Scheme 1.
Crucial for the successful preparation of the dinitro-
gen complexes was the rigorous 1:5 ratio between
MoCl3(THF)3 and phosphite, and carrying out the
reaction at room temperature for 20 h. Otherwise, a
low yield in dinitrogen complex and a large number of
decomposition products, which prevented purification
of the solid, were found in the final reaction products.
The nature of the reducing agent was also crucial for
success of the synthesis, and only the use of magnesium
turnings allowed dinitrogen complexes to be obtained.
Sodium amalgam or finely-divided sodium metal also
reacted with MoCl3(THF)3 solutions containing
The infrared spectra, in fact, show only one strong nN2
band at 2004 cmꢃ1 for 1 and at 1990 cmꢃ1 for 2, fitting
the proposed geometry. This was confirmed by the
31P{1H}-NMR spectra which show, in the temperature
range between ꢂ
/
30 and ꢃ80 8C, only one sharp singlet,
/
indicating the magnetic equivalence of the four phos-
phite ligands. It may also be noted that the nN2 value of
our compounds 1, 2 is higher than the values found for
related Mo(N2)2(PÃ
/
P)2 and Mo(N2)2P4 compounds [3ꢁ
/
5], containing phosphine instead of phosphite as sup-
porting ligands, and this datum fits the better p-acceptor
properties of phosphites as compared with tertiary
phosphines. However, phosphites PPh(OEt)2 and
PPh2OEt are able to stabilise dinitrogen complexes of
molybdenum, the synthesis of which is now accessible
relatively easily.
Scheme 1. PꢀPPh(OEt)2 1, PPh2OEt 2.
/

58
G. Albertin et al. / Journal of Organometallic Chemistry 660 (2002) 55ꢁ61
/
Table 1
IR and NMR data for the molybdenum complexes
a
b,c
Compound
IR
1H-NMRb
Spin system
31P{1H}-NMR
d (J Hz
/
n¯ (cmꢃ1
)
Assignment
d (J Hz)
Assignment
Ph
d
d
1
Mo(N2)2[PPh(OEt)2]4
2085w
2004s
n(NÅN)
7.60 (m)
A4
183.5 s
7.15 (m)
4.05 (m)
3.85 (m)
1.28 (t)
CH2
CH3
Ph
d
d
2
3
Mo(N2)2[PPh2OEt]4
1990s
1844s
n(NÅN)
n(CO)
7.60ꢁ/6.90 (m)
A4
152.6 s
186.4 s
3.08 (m)
0.67 (t)
CH2
CH3
Ph
Mo(CO)2[PPh(OEt)2]4
7.40ꢁ/7.10 (m)
A4
3.71 (m)
3.50 (m)
1.19 (t)
CH2
CH3
Ph
4
5
Mo(CO)2[PPh2OEt]4
Mo(CO)3[PPh2OEt]3
1832s
n(CO)
n(CO)
7.60ꢁ
/
7.00 (m)
A4
155.7 s
3.68 (qnt)
1.19 (t)
CH2
CH3
Ph
1973m
1882s
1859s
7.60ꢁ/7.00 (m)
A2B
dA 149.0
dB 142.8
JABꢀ30
3.40ꢁ/3.20 (m)
CH2
CH3
0.94 (t)
0.82 (t)
6
7
8
Mo(p-tolylNC)2[PPh(OEt)2]4
Mo(tolylNC)2[PPh2OEt]4
1925s
n(CN)
n(CN)
7.50ꢁ
/
6.40 (m)
Ph
CH2
A4
A4
A4
188.0 s
155.5 s
150.1 s
4.10ꢁ
/
3.40 (m)
2.30 (s)
1.13 (t)
CH3 p-tol
CH3 phos
Ph
1940s
7.87ꢁ6.60 (m)
/
4.01 (m)
2.25 (s)
1.18 (t)
CH2
CH3 p-tol
CH3 phos
Ph
[MoCl(NNH2){PPh(OEt)2}4]Cl
3385w
3255sh
1616
n(NH)
7.60ꢁ6.70 (m)
/
3.65 (m)
1.81 (qnt, br)
1.23 (t)
CH2
NNH2
CH3
d(NH2)
a
In KBr pellets.
b
c
In CD2Cl2 at 25 8C, unless otherwise noted.
Positive shift downfield from 85% H3PO4.
In C6D6.
d
Fig. 1.
Scheme 2. PꢀPPh(OEt)2 3, 6, PPh2OEt 4, 5, 7.
/
3.2. Reactivity
The chemical reactivity of dinitrogen complexes 1 and
2 is summarised in Schemes 2 and 3. Carbon monoxide
and p-tolylisocyanide can substitute the dinitrogen
ligands in compounds 1 and 2 to give the related
Mo(CO)2P4 compound 3 was isolated. Instead, the
reaction of the PPh2OEt derivative Mo(N2)2(PPh2OEt)4
2 with CO gave a mixture of bis (4) and tris(carbonyl)
(5) compounds, which were separated by fractional
crystallisation. The reactions were also monitored by
infrared spectra, which showed the disappearance of the
nN2 bands of dinitrogen precursors 1, 2, and the
complexes 3
Treatment of Mo(N2)2[PPh(OEt)2]4 1 with CO gave a
/
7, which were isolated and characterised.
ꢁ
pale yellow solution from which the bis(carbonyl)

G. Albertin et al. / Journal of Organometallic Chemistry 660 (2002) 55ꢁ
/61
59
In the nCO region, the IR spectra of dicarbonyls
Mo(CO)2P4 3, 4 showed only one strong band at 1844ꢁ
/
1832 cmꢃ1, suggesting a trans arrangement of the two
carbonyl ligands (II). This geometry was confirmed by
the 31P{1H}-NMR spectra which, in the temperature
range between ꢂ
/
30 and ꢃ80 8C, showed only one sharp
/
singlet, fitting the magnetic equivalence of the four
phosphite ligands.
The infrared spectrum of tricarbonyl compound
Mo(CO)3P3 5 shows one medium-intensity band at
1973 cmꢃ1 and two strong ones at 1882 and 1859
cmꢃ1, suggesting a mer arrangement of the three
carbonyl ligands (III). In the temperature range between
Scheme 3.
appearance of those due to the nCO of carbonyl species
3ꢁ5. The spectra also reveal the absence of other
/
ꢂ
/
30 and ꢃ
80 8C, the 31P-NMR spectra display an A2B
/
absorptions attributable to nN2 or nCO, thus excluding
the presence of any appreciable amounts of intermedi-
ates of the type Mo(N2)(CO)P4. In the case of PPh2OEt,
the spectra also indicate that CO replaces not only
dinitrogen, but also one phosphite ligand, affording
tricarbonyl species Mo(CO)3P3 (5). Both complexes 4
and 5, however, were always present in the reaction
mixture and were separated by fractional crystallisation.
p-Tolylisocyanide also replaces N2 ligand in 1 and 2,
giving bis(isocyanide) complexes Mo(p-tolylNC)2P4 6,
7.
The lability of the N2 group in molybdenum deriva-
tives 1, 2 prompted us to study the substitution reaction
with ligands such as hydrazine and diazonium cations,
in an attempt to prepare new ‘‘diazo’’ derivatives. The
reaction proceeded with a colour change of the solution
and evolution of N2, but no stable compound was
isolated.
multiplet, thus confirming mer geometry III for the
complex. The mutually trans position of the two
isocyanide ligands occurs in Mo(p-tolylNC)2P4 com-
plexes 6, 7, whose IR spectra show only one nCN band at
1925 cmꢃ1 (6) and 1940 cmꢃ1 (7), whereas only one
singlet is present in the 31P{1H}-NMR spectra, suggest-
ing the magnetic equivalence of the four phosphite
ligands, as expected for geometry IV.
The protonation reactions of the dinitrogen com-
plexes with Brønsted acids were also studied, and the
results are shown in Scheme 3. Treatment with an excess
of HCl in THF of Mo(N2)2[PPh(OEt)2]4 1 gives a
reddish-brown solution from which, after work-up, a
dark-green solid was isolated and characterised as the
hydrazido(2-) [MoCl(NNH2)P4]Cl 8 derivative. The
related Mo(N2)2(PPh2OEt)4 2 also reacts with HCl in
THF, giving a green solution, but no stable solid was
separated.
Both carbonyl (3ꢁ
/
5) and isocyanide (6, 7) molybde-
orange solids,
num derivatives are yellow or yellowꢁ
/
Protonation reactions were also studied with other
Et2O and H2SO4, but no solids
stable in air, diamagnetic and soluble in most common
organic solvents, in which they behave as non-electro-
lytes. Analytical and spectroscopic data (Table 1)
support the proposed formulation for the complexes,
and a geometry in solution of the type shown in Fig. 2
may also be established.
acids, such as HBF4×
/
were isolated and no evidence of the formation of
nitrogen-containing products was detected, although,
in the case of H2SO4, traces of NH3 were found in the
reaction mixture.
Hydrazido(2-) [MoCl(NNH2)P4]Cl 8 is a dark green,
diamagnetic solid, soluble in polar organic solvents in
which it behaves as 1:1 electrolyte [18]. It is stable as a
solid, but its solution slowly decomposes at room
temperature even in an inert atmosphere. Microanalysis
and spectroscopic data support the proposed formula-
tion, and trans geometry V (Fig. 3) in solution was also
established.
Fig. 2.
Fig. 3.

60
G. Albertin et al. / Journal of Organometallic Chemistry 660 (2002) 55ꢁ61
/
The presence of the hydrazido NNH2 ligand in
complex 8 was confirmed by the infrared spectra, which
show two weak bands at 3385 and 3255 cmꢃ1
Acknowledgements
The financial support of MURST (Rome)*
mi di Ricerca Scientifica di Rilevante Interesse Nazio-
nale, Cofinanziamento 2000ꢁ2001*is gratefully
acknowledged. We thank Daniela Baldan for technical
assistance.
/Program-
,
attributed to the nNH of the hydrazido ligand. The
spectra also shows a medium-intensity band at 1616
/
/
cmꢃ1, attributed to the dNH of the NNH2 group.
Further support for the presence of hydrazido(2-) in the
2
1
complex comes from H-NMR spectra, which show a
broad quintet at 1.81 ppm, attributed to the NNH2
protons. The 31P{1H}-NMR spectrum consists of a
sharp singlet at 150.1 ppm, indicating the magnetic
equivalence of the four phosphite ligands, as in trans
geometry V.
References
[1] M.D. Fryzuk, S.A. Johnson, Coord. Chem. Rev. 200 (2000) 379.
[2] M. Hidai, Coord. Chem. Rev. 185 (1999) 99.
[3] M. Hidai, Y. Mizobe, Chem. Rev. 95 (1995) 1115.
[4] J.R. Dilworth, R.L. Richards, in: G. Wilkinson, F.G.A. Stone,
E.W. Abel (Eds.), Comprehensive Organometallic Chemistry, vol.
8, Pergamon, Oxford, 1982, p. 1073.
The protonation reactions of dinitrogen complexes of
molybdenum have been reported to give various species,
depending upon the nature of acid, solvent, and
supporting ligands [3,8]. In some cases, hydrazido(2-)
complexes were obtained, and these precedents
[6f,8,19,20] support our formulation for species 8
obtained by protonation with HCl. The reaction prob-
ably proceeds first through protonation of one dinitro-
gen ligand, with concurrent substitution of the other by
Clꢃ to give a ‘‘diazenido’’ MoCl(NNH)P4 intermediate,
which further reacts with Hꢂ to give the final
[MoCl(NNH2)P4]ꢂ cation. Unfortunately, no inter-
mediate was detected following the reaction in an
NMR tube, since the spectrum only showed the forma-
tion of the final hydrazido(2-) derivative. Its separation,
however, indicates that monodentate phosphites are
able to stabilise the nitrogen-containing ligand obtained
by protonation of the coordinated N2 group. It can also
be noted that the previously reported dinitrogen com-
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phosphine PPhMe2 give, by protonation with HCl, the
neutral triphosphine species MoCl2(NNH2)P3. Instead,
cationic complexes with four phosphorus ligands like 8
were obtained with bidentate ligands or with tungsten as
the central metal [3,21].
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/
HCl (in THF at room temperature),
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case rather low. However, although these are only
preliminary results which need further investigation,
reduction with zinc amalgam of our phosphite-
containing dinitrogen complexes to give ammonia is
an interesting result in the field of dinitrogen fixation
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