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Non-Enzymatic Activation of Molecular Nitrogen.

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Non-Enzymatic Activation of Molecular Nitrogen
By G . Henrici-Olive and S . OlivC[*1
Several transition metal complexes that can absorb nitrogen from the gas phase are now
known. Some of the N2-metal complexes are stable enough to be isolated and their structure elucidated, the Nz molecule remaining chemically inert. In other cases reduction to
N3- is possible, but the structure of the reactive intermediate N2-metal complex can be
approached only by mechanistic studies. In the stable complexes the nitrogen is bound
via a lone pair of electrons in the direction of the molecular axis (“end-on”), in the
reducible complexes possibly “edge-on”.
1. Introduction
The problem of the enzymatic fixation of nitrogen in
nature is of interest to chemists, biochemists, and biologists alike. Whereas in nature the nitrogen is taken
from the air under very mild conditions with the aid
of microorganisms and incorporated, via ammonia,
into organic nitrogen compounds that are important
to life, temperatures of 300 to 600°C and nitrogen
pressures of several hundred atmospheres are required
in industry to overcomz the inertness of the nitrogen
molecule and convert N2 into NH3 (Haber-Bosch
process).
Whereas biochemists are trying to probe the secrets of
nature by extracting the nitrogen-fixing enzymes from
the biological cell environment and investigating them
separately, chemists have been trying for a number of
years to imitate nature in vitro by fixing nitrogen on
synthetic compounds and complexes. The biochemical
investigations have shown that certain transition
metals, particularly iron and molybdenum, are always
present in the natural systems, and are presumably
directly involved in the process of the fixation and
reduction of nitrogen. Transition metals, in fact, are
the deciding factors also in most synthetic systems.
The research carried out in both directions during the
past few years has been summarized by Murray and
Smith[”, and smaller areas of the field have been
reviewed by other authors L2-41.
The present article is confined to the non-enzymatic
fixation of nitrogen. In order to mark the trends and to
illustrate how far the frontiers of science have advanced in this field, a brief account on the relevant
research is given. The main part of the article, however,
is concerned specifically with mechanistic aspects of
the fixation and activation of nitrogen, and hence
particular attention is devoted to the processes occurring at the transition metal center.
[*I Dr. G. Henrici-Olive and Dr. S. Olive
Monsanto Research S.A.
CH-8050 Zurich, Eggbiihlstrasse 36 (Switzerland)
[l] R . Murray and D . C. Smith, Coordinat. Chem. Rev. 3 , 429
(1968).
[2] J . C h a f f ,Proc. Roy. SOC.(London) B 172, 327 (1969).
131 J. C h a f f ,Platinum Metals Rev. 13, 9 (1969).
[4] M . Ziegler, Chemiker-Ztg. 93, 133 (1969).
650
2. T h e Characteristics of the Nitrogen Molecule
The nitrogen molecule is well known to be particularly
resistant to chemical attack, with the result that it has
become accepted as an ideal inert gas. Its dissociation
energy is very high (225.8 kcal/mole), and no appreciable dissociation into atoms occurs at normal pressure even at 3000 “C. Apart from biological activation,
only one reaction of nitrogen at room temperature
was known until recently; this was the reduction to
Li3N by metallic lithium.
The part played by transition metals in the activation
of this inert molecule, both in enzyme systems and in
the Haber-Bosch process, almost certainly involves
the formation of a metal-nitrogen complex, leading to
weakening of the multiple bond. Processes of this type
are familiar from homogeneous catalysis on transition
metal centers, as in the polymerization of ethylene on
titanium complexes. The possibility of the existence of
such complexes of the N-N molecule had been
predicted on theoretical grounds in 1960, both by
Ruch 151 and by Orgel r61. No evidence of their occurrence was, however, found until 1965 (see Section 3);
and relatively few complexes of NZ are known even
now, in comparison with the innumerable coordination compounds of the isoelectronic CO.
What is the reason for this inertness as compared
with other compounds containing triple bonds, such
as H-C -C-H or R-C EN, and why is the tendency
to complex formation so small?
SCF-LCAO-MO [*I calculations have shown r7-91
that the nitrogen molecule has the electronic configuration
(15p)2 (lad2(2cg)2 (2%)’
(30,)’
[5] E. Ruch, Monograph: “10 Jahre Fonds der Chemischen Industrie”, Verlag der Chemischen Industrie e.V., Fond der Chemischen Industrie, Diisseldorf 1960, p. 163.
[6] L. E . Orgel: An Introduction to Transition Metal Chemistry.
Methuen, London 1960, p. 137.
[*I Self consistent field - linear combination of atomic orbitals molecular orbitals.
[7] C. W. Scherr, J. chem. Physics 23, 569 (1955).
[S] B. J. R a n d , Rev. mod. Physics 32, 239 (1960).
[91 J . W. Richardson, J. chem. Physics 35, 1829 (1961).
[lo] See, e.g., H. B. Gray: Electrons and Chemical Bonding.
Benjamin, New York 1965.
Angew. Chem. internat. Edit.
1 Vol. 8 (1969) 1 No. 9
The highest occupied orbital is thus a o orbital, and
not the degenerate x orbital as in e.g. acetylene. According to these calculations, the 3 og orbital consists
essentially of the 2p0 orbitals of the N atoms (about
25 % s-character).The first empty MO is the degenerate
antibonding x orbital lxg.These calculations are confirmed by spectroscopic observations [7,8,101.
Figure 1 shows the surface of the MO’s that are important to any complex formation or reaction (after
Chutt q.
0
-N
0
0
N.
0
N
N
la,ivacantl
[degenerate]
3u‘,iZtlJ
a
lX,ILEll
wN
ldegeneratel
N
a
carries a localized lone pair of electrons; these electron
pairs are predominantly Zs, but they have sufficiently
strong 2p, components to enable them to point
“backward” (at 180” to the bond). Between the two
atoms are three equivalent banana-shaped bonding
orbitals (which are slightly polarized toward the 0 in
CO).
m
Fig. 2.
Schematic representation of the LMO’s for Nz and CO (after
Edmisron and Ruedenberg [I 21).
It can be seen from Table I, however, that the calculated s-character of the lone pairs of electrons shows
a trend parallel to that assumed by Jufi?‘. The figures
show that the s-character of the bonding orbitals
steadily decreases from left to right in the periodic
system (C, N, 0 ) ,whereas that of the lone pair of
electrons increases.
Table 1 . Variation of the s character
in the bonding LMO’s and in the
L M O s for the lone pairs of electrons
on C, N, and 0 in Nz and CO 1121.
26,12EIl
2 s character
bonding [a]
lone pair
I
Atom
Fig. I . Schematic representation of the LCAO-MO’s in the nitrogen
molecule (after Chart I21).
Chatt 131 has suggested that the low reactivity of Nz is
essentially due to the energetically unfavorable position of the highest occupied orbital. The 30, orbital
of the Nz molecule, which has an energy of -15.5 eV,
is much more stable than e.g. the degenerate pair of x
orbitals of acetylene (-1 1.4 eV).
Ja@ and Orchin[IlI proposed a qualitative MO
scheme to explain the difference in the complexforming tendencies of Nz and CO. The model is
based in principle on the fact that the 2s orbital of
oxygen is very low in energy in relation to the 2s
orbital of carbon, so that 2p, of oxygen is better
suited, on energy grounds, for combination with 2s of
carbon. The lone pair of electrons on the carbon consequently has a relatively high energy, is predominantly of p-character, points in the opposite direction
to the C=O bond, and is therefore available for complex formation. In N2, on the other hand, the lone
pairs of electrons on the two N atoms, according to
this model, occupy low-lying orbitals having predomminantly s-character.
According to more recent calculations of the two
molecules by Edmiston and Ruedenberg [121, however,
this picture does not appear to be quite correct.
Localized molecular orbitals (LMO’s) were calculated
on the basis of the SCF-LCAO-MO’s for the two
molecules N2 and CO. It was found that the two isoelectronic molecules can be represented by almost the
same schematic diagram (Fig. 2), i.e. each of the atoms
1111 H. H . J a f i and M . Orchin, Tetrahedron 10, 212 (1960).
1121 C. Edmiston and K . Ruedenberg, J. chem. Physics 43, 97
(1965).
Angew. Chem. internat. Edit. J Vol. 8 (1969) / N o . 9
I
1
C
N
0
0.60
0.46
0.60
0.75
0.25
[a] In the three equivalent bonding
orbitals.
The comparison of the two schematic representationsof MO’s
in Figures 1 and 2 may at first sight seem rather confusing.
The chemist whose theoretical backgroundis not so extensive
has a strong tendency to attach an excessively concrete physical significance to a calculated set of SCF-MO’s or to the
qualitative pictures of the surfaces built up from these MO’s,
and to forget that another set can represent an equally correct
solution. Roughly speaking, it may be said that the “electron
cloud” in a given molecule can be divided into different
regions by mathematical tricks, so that different calculation
methods may give a different pattern.
The localized molecular orbitals (LMO’s) are constituted in such a way as to minimize interorbital
electronic interaction. In contrast with the SCF
SCF-LCAO-MOs ,on which they are based (cf.
Fig. l), orbitals localized on one atom are oftenfound
here for the lower shells and for lone pairs of electrons,
as well as bonding orbitals localized between the two
atoms. These calculations are thus much more closely
allied to the chemist’s concept of chemical bonding.
It can be concluded from a comparison of the electronic situations in N2 and CO that the differences between the two molecules should be quantitative rather
than qualitative (cf. [61). There is, however, a possibly
important difference in the energies of the highest
occupied orbitals of the two molecules; the difference
is about 1.4eV in favor of COr9,131. Another difference is the polarity of CO. Nevertheless, the problem of the fixation of nitrogen on transition metal
1131 R. Suhni, Trans. Faraday SOC.49,1246 (1953).
65 1
complexes looks hopeful, and several such complexes
with N2 ligands have, in fact, already become known
(see Section 3).
There still remains the question how such complexes
are to be pictured. By analogy with CO, one would
expect bonding to occur via the lone pairs of electrons
(end-on). On the other hand, comparison with other
substances containing triple bonds, such as the
alkynes, forces us to consider a complex bond normal
to the axis of the N2 molecule (edge-on), particularly
since it has been shown that the biological systems
that are to be imitated here can reduce acetylene (to
ethylene) as well as Nz.
N
111
N
M + NEN
M
Bonding in the direction of
the molecular axis (end-on)
Bonding normal to the
molecular axis (edge-on)
+-
Some indications clearly favor bonding in the direction
of the molecular axis in a number of isolable, stable
complexes (cf. Section 4.2). However, this does not tell
us anything definite about the configurations that
really activate the nitrogen molecule.
3. Present State of Research on the Fixation
of Nitrogen
In 1965, Allen and Senofl[l4J reported the first isolable
ruthenium complex containing an N2 molecule as a
ligand:
The nitrogen had not in fact been absorbed from the
gas phase in this case, but had been formed in the reaction of RuC13 with hydrazine in aqueous solution by
disproportionation of the hydrazine, with simultaneous formation of NH3. Soon afterwards, however,
Shilov et al. 1151 showed that free molecular nitrogen
can also be taken up as a ligand in RU(II)complexes.
Several other methods for the preparation of the
complex (1) and a series of similar complexes were
subsequently described. Only a few important developments will be mentioned briefly here.
Isolable complexes are so far confined to Group VIII
of the periodic system. Complexes with elements of
the first period appear to be relatively unstable, and
difficulties are often encountered in their isolation
An interesting cobalt complex has been described, in
which N2 and H2 can be reversibly exchanged 1161:
[14] A . D. Allen and C. V . Senoff; Chem. Commun. 1965, 621.
1151 A . E. Shilov, A . K . Shilov, and Yu. G . Borodko, Kinetika i
Kataliz 7, 768 (1966).
[16] A. Yamamoto, S. Kirazume, and S . Ikeda, J. Amer. chem.
SOC.90, 1089 (1968).
652
The osmium analog of (1) has been found to be
particularly stable 1171. The nitrogen is so strongly
bound in this complex that it withstands boiling in
strong hydrochloric acid.
The unexpected thermodynamic stability of such
nitrogen complexes is also shown by the fact that even
water can be displaced by nitrogen in some cases[187:
It should be mentioned, finally, that binuclear complexes in which the nitrogen molecule forms a bridge
between two transition metal units have also been
found. In the case of RuII, a complex of this type is
formed as follows:
again by displacement of water. The compound was
isolated from aqueous solution with [ B F 4 I e as the
anion 1181. A nitrogen-nickel(0) complex of this type is
formed 1191 on reduction of bis(2,4-pentanedionate)nickel(r1) with trimethylaluminum in the presence of
tricyclohexylphosphine and nitrogen; this complex is
formulated as follows:
I(C~H~~)~PIZN~NZN~[P(C~H~~)
These novel compounds naturally present a temptation
to hopeful comparisons with the biological activation
of nitrogen. Might this not be an analog at least of the
fixation step? Unfortunately, no evidence has so far
been found to indicate that the nitrogen, once fixed in
these stable complexes, is susceptible to chemical
attack. Early success reports, according to which the
nitrogen in (1) had been reduced to NH3 by NaBH4 1141
were found to be fallacious since C h a f f showed that
when 1 5 N z is used, the NH3 liberated from the complex contains no 1 5 N 131.
An entirely different line of investigation is based on
the observation by Volpin and S h u r [ z o l that ZieglerNatta systems and similar combinations of a transition
metal compound and an organometallic reducing
agent take up gaseous nitrogen and reduce it to We.
After hydrolysis, the nitrogen is collected as NH3. Up
to 0.4 N2 molecule per transition metal center was
fixed and reduced in these early experiments.
This approach was adopted by a number of authors
and developed in different directions [21-261.
[17] A. D . Allen and J . R . Stevens, Chem. Commun. 1967,1147.
[18] D . E. Harrison, E. Weissenberger, and H. Taube, Science
(Washington) 159, 320 (1968).
[19] P . W . Jorly and K . Jonas, Angew. Chem. 80, 705 (1968);
Angew. Chem. internat. Edit. 7, 731 (1968).
[20] M. E. Volpin and V. B. Shur, Nature (London) 209, 1236
(1966), and literature cited there.
1211 H. Brintzinger, J. Amer. chem. SOC.a) 88,4305,4307 (1966);
b) 89, 6871 (1967).
[22] G. Henrici-OlivP and S. Olive, a) Angew. Chem. 79, 898
(1967); Angew. Chem. internat. Edit. 6, 873 (1967); b) Angew.
Chem. 80,398 (1968); Angew. Chem. internat. Edit. 7,386 (1968).
1231 R . Maskill and J. M . Pratt, Chem. Commun. 1967, 950.
1241 E. E . Tamelen, G . Boche, S. W. Ela, and R . B. Fechter,
5. Amer. chem. SOC.89,5707 (1967).
Angew. Chem. internat. Edit.
1 Vol. 8 (1969) / No. 9
The reactions proceed at room temperature and at
nitrogen pressures of 1 to 150atm; because of the
presence of the organometallic component in the system (e.g. Grignard reagent, alkylaluminum, or alkali
metal naphthalide), organic solvents (usually ethers)
must be used. The nitrogen molecule is presumably
coordinately fixed to the transition metal and reduced
by the organometallic component which is always
present in great excess (cf. Sections 4.4 and 4.5). The
disadvantage of this reaction is that the nitrogen appears to adhere very strongly to the transition metal
species in the nitride form; with one exception, which
will be described later, the only method found so far
for the removal of the reduced nitrogen from the
metal is hydrolysis or alcoholysis.
The conversion in the fixation process has been increased to 100% with respect t o the metal, i.e. one N2
molecule per transition metal center. This was achieved
with VC13 and Li naphthalide in tetrahydrofuran [22aJ.
Van Tamelen et al. t25.261 developed a similar process
into a cycle of fixation, reduction, and alcoholysis.
The fixing and reducing system is a combination of
titanium tetraisopropoxide and Na naphthalide in
tetrahydrofuran. N3 8 is converted into ammonia with
an accurately measured quantity of isopropyl alcohol,
and is thus separated from the metal. The original
combination is regenerated by introduction of fresh
Na naphthalide (or simply of Na, since the naphthalene
is still available in the system). By five such cycles it
was possible t o convert up to 1.7 molecules of N2 per
titanium center into ammonia at normal pressure.
Vulpin et al. [271 recently managed to remove the nitride
nitrogen from the transition metal in a batch procedure, so making it available for further fixation.
These authors used the reducing combination of A1 and
aluminum bromide, which is well known e.g. from the
Fischer-Hafner synthesis of aromatic complexes of
transition metals [281. When this combination was used
with salts such as Tic14 or CrC13, they found that more
than 100 nitrogen molecules per transition metal
center could be reduced to N 3 e by the use of a large
excess of aluminum and Lewis acid (for example,
Ti : A1 : AlBr3 = 1: 600 :1000, 30 hours, 130 ‘C,
100 atni of N2; 142 molecules of N2 were reduced per
titanium atom). The nitrogen is presumably present
as N38attached to an aluminum species; it is liberated
as NH3 on hydrolysis.
Another development that could show promise also
comes from Volpin’s laboratory. With phenyllithium
as the reducing agent together with (CsH&TiC12 in
ether, some aniline was found as well as NH3 on hydrolysis (0.65 mole of NH3, 0.15 mole of C ~ H S N H
per
~
mole of Ti complex at 20 “C and 100 atm of N2) 1297.
A carbon-nitrogen bond was thus obtained for the
first time in this case, though only to a small extent.
[25] E. E. van Tamelen, G . Boche, and R . H . Greeley, J. Amer.
chem. SOC.90, 1677 (1968).
(261 E. E. van Tamelen, R . B . Fechter, S . W. Schneller, G. Boche,
R . H . Greeley, and B. Akermark, J. Amer. chem. SOC.91,1551
(1969).
1271 M. E. Volpin, M . A . Ilatovskaya, L. V . Kosyakova, and
V . B. Shur, Chem. Commnn. 1968, 1074.
[281 E . 0.Fischer and W .Hafner, 2.Naturforsch. 106,665 (1955).
[291 M. E. Volpin and V . B. Shur, Bull. Acad. Sci. USSR. Div.
Chem. 1966, 1819.
[30] D . C . Owsley and G . K . Helmkamp, J. Amer. chem. SOC.89,
4558 (1967).
[31] J. Ellermann, F. Poersch, R. Kunstmann, and R . Kramolowsky, Angew. Chem. 81, 183 (1969); Angew. Chem. internat.
Edit. 8,203 (1969).
1321 P . H . Emmet: Catalysis. Reinhold, New York 1955.
[33] E. K . Rideal: Concepts in Catalysis. Academic Press, New
York 1968.
[34] R. P. Eischen and J. Jacknow, Proc. 3rd Intern. Congress on
Catalysis. North Holland Publ. Co., Amsterdam 1964, p. 627.
Angew. Chem. internat. Edit.
1 Yol. 8 (1969) / No. 9
Transition metal compounds, however, are no longer
the only synthetic nitrogen fixers. In 1967 it was
reported for the first time that the phenylsulfenium
cation (C&S’)
takes up gaseous nitrogen [3*1; the
reaction products could not be isolated. Ellermann
et al. [311, however, recently prepared an organic compound, again containing ionic sulfur, that “avidly
absorbs molecular nitrogen”. The reaction product,
which contains two nitrogen molecules per complex
unit, has been isolated. The authors assumed that the
nitrogen molecules were bound in a spirocyclic complex by the compound (2) which acts as a doubly bidentate chelating agent.
These new compounds may raise the question whether
the transition metal centers are really of decisive importance in the enzymatic fixation of nitrogen, as has been
assumed until now.
4. Attempts t o Elucidate the Mechanism of the
Activation of Nitrogen
4.1. Metal Surfaces
From the point of view of the industrial activation of
nitrogen (Haber-Bosch), great efforts were made to
establish the reaction mechanism of this industrially
important process, but these have not yet led to any
satisfactory conclusion. The active species and the
rate-determining step are still in dispute (for reviews
see 11 3 , 3 3 9 . Most of these investigations were carried
out on extremely pure metal surfaces (Group VIII).
Two studies that are related in some measure t o the
nitrogen complexes will be mentioned.
Eischen and Jacknow [341 investigated the fixation of
nitrogen by nickel on silica supports. The authors
found with the aid of IR spectroscopy that N2 is
chemisorbed on nickel at room temperature. A number
of interesting conclusions are drawn from the presence
of a strong absorption band at 2202 cm-1. The free
nitrogen molecule gives a band at 2331 cm-1, which is
due to an N = N stretching vibration; owing to the
symmetry of the N2 molecule, this band does not ap-
653
pear in the IR spectrum, but is observed only in the
Raman spectrum. The relatively high extinction coR band suggests that conefficient of the observed l
siderable polarization of the N2 molecule must have
occurred. The frequency shift, on the other hand,
indicates chemisorption with (at least partial) chemical
bonding between the metal and the N2 molecule. (No
shift would occur if the polarization was due to the
action of asymmetric surface electric fields on a purely
physically adsorbed molecule.) If the isotope l5Nz is
chemisorbed instead of 14N2, the IR band appears at
2128 cm-1. A mixture of the three isotopes 14N2
14N-15N, and 15N2 gives three bands in the correct
intensity ratio. This shows that no atomic species are
present. It is concluded from the observations as a
whole that the nitrogen molecule is attached end-on
to the nickel(0). It can be calculated from the isotopic
shift of the IR band that the force constants for this
structure are 19 x l o 5 dyne/cm for the N-N bond and
3 x 10s dyne/cm for the metal-N bond 1351. The end-on
structure would be conclusively proved if it were possible to separate the IR bands of Ni c 14N r l 5 N and
Ni t 15N =14N, for which a difference of 5-6 cm-1 is
predicted from theory [341. However, the resolution of
the instrument used was not sufficient for this.
Another interesting investigation that also points to
end-on binding of the nitrogen, in this case to iron,
was carried out by Brill et al.1361, who studied the
action of nitrogen on an iron point in a field electron
microscope. The point was exposed to a nitrogen atmosphere at 76°K. After heating for a short time at
400 OC, the micrograph shows a distinct prominence
of the 111 faces of the iron. It is thus concluded that
the nitrogen is preferentially chemisorbed on these
faces, with the result that their surface energy is reduced, and these non-equilibrium faces consequently
grow. The authors interpret the preferential deposition
of the Nz molecule on the 111 face as follows: The
iron atoms are arranged in these faces in such a way
that the atoms of the top layer surround holes having
trigonal symmetry. It is assumed that the surface
atoms make unoccupied orbitals available, and that
these can be used for the construction of molecular
orbitals having x character with respect to the axis of
the trigonal hole. These can then interact with the
occupied x orbitals of an N2 molecule arranged along
the trigonal axis ( x bonding).
4.2. Complexes with Nitrogen Ligands
The first Nz complex ( I ) isolated gave a strong IR
band at 2118 to 2167 cm-1 (depending on the anion),
and all other complexes having an N2 ligand also give
this band, the wave number of which differs by 150 to
300 cm-1 from the (Raman-active) stretching vibration
of the free nitrogen molecule. As in the case of Nz on
metal surfaces (Section 4.1), therefore, the end-on
[35] See H. Siebert: Anwendung der Schwingungs-Spektroskopie in der anorganischen Chemie. Springer, Berlin 1966.
1361 R. Brill, E. L. Richter, and E. Ruch, Angew. Chem. 79, 905
(1967); Angew. Chem. internat. Edit. 6, 882 (1967).
654
configuration is also assumed for the coordinately
bound nitrogen. This has been proved by X-ray
analysis in two cases, i.e. Allen and Senoffs ruthenium
complex (octahedron with linear Ru-N-N grouping 1371) and the complex [ C O ( N ~ ) ( P ( C ~ H ~(trig)~)~]
onal bipyramid with H at one apex and Nz at the
other; 5' deviation from linearity in the grouping
CO-N-N 1381).
The nitrogen-metal bond in these complexes is formulated by Chatt[2,31 as a sort of double bond, unlike
that discussed by Brill et al. (361 for the chemisorption
on iron. Chart's interpretation agrees with the forrnulation normally given for the carbonyl complexes [61.
An occupied IJ orbital (this refers to one of the lone
pairs of electrons according to Figure 2, and to the
orbital 3og according to Figure 1) interacts with an
unoccupied do orbital of the metal (dzz or d,z+ in
the octahedral complex). Electron back-donation (see
Fig. 3) occurs via an occupied dx orbital of the metal
(dx,, d, d,, and the empty antibonding IT, orbital
of the nitrogen ( x symmetry with respect to the
M-N-N axis).
dxz.
\
rn
yz
i
I
Fig. 3.
Double-bond-like interaction between metal and nitrogen
molecule (shaded: occupied orbitals).
The electron density passing in this way into the antibonding nitrogen orbital should weaken the bonding
between the two nitrogen atoms. The displacement of
:he stretching vibration v(N =N) by 150-300 cm-1
indicates that this indeed has happened. However, the
weakening is evidently not enough to make the coordinately bound nitrogen accessible to chemical reactions (e.g. hydrolysis or reduction). Even in complexes of the type M-Nz-M[1*,191, in which one
should expect double weakening of the N r N bond
according to these views, the nitrogen molecule still
remains in the unreactive state. It should be noted that
for these complexes, because of the symmetry of the
molecule, the N EN stretching vibration is only
Raman-active, as in the free Nz molecule.
The energy conditions for the formation of an Nz complex
seem to be extremely critical. Chatt et al. [31 have shown that
small changes in the other ligands of the metal center can
make the isolation of a stable complex impossible. For example, the iridium complex
I~C~(NZ)[P(CSHS)~~Z
1371 F. Bottomley and S . C. Nyburg, Chem. Commun. 1966,897.
[38] J . H. Enemark, B. R . Davies, J . A. McGinnety, and J. A.
Ibers, Chem. Commun. 1968, 96.
Angew. Chem. internat. Edit.
/ Vol. 8 (1969)/ NO. 9
is very easy to prepare. The corresponding iodide, on the
other hand, is too unstable to be isolated in the pure state.
The same is found when one attempts to replace the triphenylphosphine ligands with ethyldiphenylphosphine. (It
should, however, be remembered that the isolability of the
complex is not necessarily related to the chemical reactivity
of the coordinately bound Nz molecule; cf. also Section 5.)
Undoubtedly it will be interesting to follow the weakening of
the N = N bond (as indicated by the displacement of the IR
band) as a function of the other ligands in the complex[391.
4.3. The System Bis (x-cyclopentadieny1)titanium
Dichloride/Ethylmagnesium Bromide
The combination bis(ncyclopentadieny1)titanium dichloride/ethylmagnesium bromide in ether was one of
the first systems with which VoIpin and Shurr201 observed the fixation and reduction of gaseous nitrogen
in 1964. Several attempts have been made since then
to explain the mechanism of this reduction.
The first important contribution was made by Shilov
et al. [401. These authors showed by experiments with
deuterated Grignard reagent and with deuterated
ether that the nitrogen is present as nitride, and is
liberated as NH3 only on hydrolysis. VoIpin had
assumed at first that the coordinately bound nitrogen
could take up hydrogen from the solvent or the
Grignard reagent [201. On hydrolysis of the (undeuterated) reaction solution with heavy water, Shilov
e t al. also found that the isotope distribution in the
hydrogen-deuterium mixture liberated indicated the
presence of a metal hydride. This was in fact later
definitely confirmed by Brintzinger [21al by electron spin
resonance measurements.
At that time, Shilov ef al. assumed that the active species was
an unstable intermediate occurring during the reduction; this
could not be confirmed by other authors. Maskill andPraft [231
showed that the quantity of nitrogen fixed does not change
appreciably if the components of the system are first allowed
to react under argon, the nitrogen only being introduced
later (cf. also Table 2).
Further information was provided by our own investigations of the same system in benzeneC411. ESR
measurements on the nitrogen-free system gave the
same signal as had been observed by Brintzinger using
ethereal solvents 121a,421 (see Fig. 4).
This signal is a 1:2:1 triplet, in which each of the three
lines shows further multiplet splitting due to the ring
Fig. 4. ESR signal in the reaction solution CpZTiClz/CzHsMgBr in
benzene. Mg/Ti = 12; high dilution; T = 20 "C. C p = cyclopentadienyl.
1391 J . Chatt and R . L. Richards, personal communication.
[40]G. N . Nechiporenko, G. M . Tabrina, A . K . Shilova, and
A . E. Shilov, Doklady Akad. Nauk SSSR 164, 1062 (1965).
[41] G. Henrici-Olivd and S . Olivd, unpublished.
I421 H. Brintzinger, J. Amer. chem. SOC.89,6871 (1967).
Angew. Chem. internat. Edit.
/ Vol. 8 (1969)/ No. 9
protons of the cyclopentadienyl groups. The triplet is
due to the interaction of the unpaired electron of a
Ti(m) species with two equivalent protons, and SO
points to a hydride structure. The intensity of this
signal increases for 1 to 1.5 hours from the moment of
mixing of the two components, and then remains
constant. The titanium has thus been reduced to the
ESR-active state Ti111 (3dl) during this period, and then
remains in this state. The reduction is probably similar
to that in theZiegler-NattasystemCp~TiC12/C2H5AlCl2,
with alkylation of the titanium followed by decomposition of the relatively unstable alkyltitanium compound (see e.g. 1431).
Measurement of the magnetic susceptibility of such a
system after about 2 hours shows that the Ti(1Ir)
concentration is only about 50 % of the original Ti(1v)
concentration. Since all the experiments with this
system were carried out with a large excess of Grignard
reagent[20+21,40J(Mg/Ti = 12 in the experiments described here), it is hardly to be expected that only
partial reduction has occurred. It is probable, therefore, that part of the titanium has been reduced
beyond the Ti(Ir1) stage.
The system Cp2TiC12/C2H5MgBr in benzene also
fixes and reduces nitrogen. The process is again independent of whether the components are brought
together under nitrogen or exposed to nitrogen only
at the time of maximum hydride concentration (after
about 1 hour). This can be seen from Table 2.
Table 2. Fixation of nitrogen on the system
CpzTiClz/CzHSMgBr in benzene.
[Ti] = 10 x 10-3 mole/]; Mg/Ti = 12;
T = 22°C; 1lOatm of Nz [411.
1
2
3
4
immediately
after 1 h
immediately
after 1 h
17
17
70
70
1.0
0.88
0.96
1.0
The ESR signal of the reaction solution was recorded
at the end of experiment 1. The hydride signal was still
present with a comparable intensity at this time,
although one nitrogen atom per titanium was already
present in the reduced form. This again shows clearly
that the hydride hydrogen is not involved in the reduction process [*I. It can also be seen from Table 2 that
the fixation of nitrogen has already reached its final
value of NH3/Ti 2 1 after 17 hours; this value cannot
be increased by the use of either a higher Mg/Ti ratio
or of higher pressures. Since not more than one N2
molecule can be fixed per two titanium centers, we
believe that the process takes place on a binuclear
complex.
Brintzinger had initially assumed the binuclear structure (3) for the hydride[zral, but later became convinced that the hydride is better described by formula
[43] G. Henrici-Olivd and S . Olivd, Angew. Chem. 79, 764 (1967);
Angew. Chem. internat. Edit. 6, 790 (1967).
[*I Erintzinger had thought at first that the ESR data pointed to
participation of the hydride hydrogens [Zlb], but he himself
disproved this by further detailed investigations [42].
655
(4) 1421. The results of our work reported here, particularly in conjunction with the data for the similar
system described in the next section, lead us to assume
that the active species is a binuclear complex containing titanium in different valence states [structure ( 5 ) ] .
independent of temperature in an interval of 100 OC,
it may be assumed that the lithium is fairly firmly
bound, presumably in the inner coordination sphere
of the titanium. (The same is true of Na 122bl.) Finally
the interaction with the ring protons of the Cp groups
again occurs, leading to splitting of each of the twelve
principal lines into a multiplet. From these data, and
taking into account the fixation of nitrogen, the most
probable formula for the ESR-active species, as in the
last section, is found to be ( 5 ) [*I. The exact location
of the lithium ion is not yet known.
It is possible, in our view, that the nitrogen is fixed to
the Ti(I1) side of the complex, and that the ESR
activity of the Ti(1rr) side is consequently not appreciably changed.
4.4. The System CpzTiClz/Alkali Metal Naphthalide
If an alkali metal naphthalide (e.g. Li+Np- or Na+Np-)
is used as the reducing agent instead of a Grignard
reagent, fixation of nitrogen occurs even at normal
pressure. In the system CpzTiCIz/LiNp in tetrahydrofuran, for example, 0.96 molecule of NH3 is obtained
per titanium center after 16 hours with Li/Ti = 6 and
under one atmosphere of nitrogen [22bl. The ratio
NH3/Ti again cannot be increased beyond about 1
under any conditions. The fixation takes place slowly;
under the same conditions, 0.5 NH3 molecule per
titanium is obtained after 2 hours, and 0.7 molecule
after 4 hours.
ESR studies again provided a number of clues as to
the mechanism in this system.
Let us first consider the system in the absence of
nitrogen. A readily resolvable ESR signal is observed
(see Fig. 5 ) , but in contrast with the Grignard system,
the ratio of reducing agent to titanium component is
critical here. The signal is obsened only at molar
ratios 3 < Li/Ti < 4. Its intensity passes through a
maximum at Li/Ti = 3.5, and is almost 0 at Li/Ti = 4.0.
At higher values of the ratio, the lithium naphthalide
signal appears. The stronger reducing agent LiNp is
evidently capable of reducing the ESR-active Ti(rr1)
species further.
Unlike in the Grignard system (preceding section), the
bridge hydrogen in this case comes from the solvent.
This is shown by the fact that an identical spectrum is
obtained with LiNp prepared from CloDg[417. The
hydride formation mechanism is thus evidently different in this case. A conductometric titration was
carried out to clarify this mechanism. NaNp was used
to avoid interference by the soluble LiCl (NaNp and
LiNp behave almost identically in these reactions [22bI).
It can be seen from Figure 6 that the conductivity
increases rapidly in the range 2
Na/Ti
3, indicating the occurrence of ionic species.
<
NalTi
<
-
Fig. 6. Conductometric titration of CpzTiClz with NaNp in THF.
[CpzTiCIZIo = 10 X 1 0 - 3 mole/l. 0,O:two sets of measurements [41].
Combination of all the data mentioned leads to the
following reaction scheme for the nitrogen-free system:
20
2 Cp2Ti"C1,
I
Fig. 5. ESR signal of the reaction solution Cp,TiClz/LiNp in tetrahydrofuran. Li/Ti = 3.5; high dilution. T = -40 "C [22bl.
I
4 LiNp
(91
I*
le
(5)
The predominant hyperfine splitting is again a 1:2 :1
triplet indicating the presence of two equivalent
protons. The coupling constant is similar to that of the
hydride in Figure 4c21a.22bl. In this case, however,
each line is further split into four components of equal
intensity; the only possible cause for this is a lithium
nucleus (7Li, nuclear spin I = 3/2). Since the Li
coupling constant is relatively large and is practically
6 56
Ize
[*] Complexes containing two Ti(rx1) centers can also be ruled
out on the basis of various ESR consideration [22b, 421.
Angew. Chem. internat. Edit.
Vol. 8 (1969) 1 No. 9
The reaction of 2 CpzTiCl2 with 4 LiNp leads to the
formation of dimeric titanocene (6), the synthesis of
which by this route is known from the literature[44].
Since titanocene is stable in tetrahydrofuran, the further reaction must be attributed to excess naphthalide.
It is assumed that the ionic Ti(r) compounds (7) are
formed as intermediates, and that these attack the
solvent. The dimeric hydride species (8) are formed,
as well as ionic reaction products of tetrahydrofuran.
The latter are considered to be responsible for the
electric conductivity, the occurrence of which in this
range was shown by conductometric titration (Fig. 6).
The consumption of a total of four LiNp per titanium
shows that further reduction of (8) is to be expected.
The complex ( 5 ) is the ESR-active form. The disappearance of the ESR signal in the range 3.5 Li/Ti $4
requires a further reduction to the diamagnetic complex (9). Above Li/Ti = 4, as was mentioned earlier,
the excess LiNp appears in the ESR spectrum, i.e. the
reduction process is complete.
It is of interest in connection with the nitrogen fixation
mechanism that the system fixes nitrogen both at Li/
Ti = 3.5 [maximum ESR signal, complex ( 5 ) ] and at
Li/Ti = 4.0 [no ESR signal, complex ( 9 ) ] (NH3/Ti E
0.2). In the former case, however, the signal disappears
on addition of nitrogen. Since only one of the two
species (5) and (9) is presumably responsible for the
fixation, we have assumed that an equilibrium exists
be tween them.
According to observations on polymerization catalysis
(for example, ethylene can be polymerized on TiIII/
TiIV or on TiII/TiII* complexes), the species (5), in
which the titanium atoms have different valences,
seems the more likely candidate. The difference in the
numbers of ligands means that the two titanium
centers will differ in their symmetries. A proposal that
agrees with the ESR data is shown in Figure 7. The
Ti111 has a tetrahedral environment, whereas the Ti”
is situated in a trigonal bipyramid. (The equivalence
of the two bridge hydrogens requires that the lithium
be symmetrically placed in relation to these atoms.)
the ESR signal on addition of N2.) The -N=Ngrouping is well known to be unstable in the presence
of alkali metal naphthalide, and would immediately
be reduced further to two N3-. One of the two N3evidently remains attached to the Ti center and so
blocks the path to further fixation of nitrogen.
<
Fig. 8. Suggested mechanism for the fixation of nitrogen in thesystem
Cp2TiC12/LiNp.(The tetrahedral Ti(iI1) portion has been omitted from
the bottom two complexes.)
As was mentioned, the fixation-reduction process in
this system takes several hours. Since the ESR signal
disappears rapidly on admission of N2 and the reduction to the azo compound should also be relatively
fast, the rate-determining step could be the supply of
the active species from the equilibrium.
4.5. “Electron Reservoir” Complexes of
Transition Metals
A somewhat different mechanism occurs if simple
halides of the transition metals are used with lithium
naphthalide instead of the bis(cyclopentadieny1) compound[22al. This is shown by the fact that, under
favorable conditions, two NH3 molecules can be obtained per transition metal center in this case (Table 3).
Thus one N2 molecule is fixed per transition metal
center; the active species cannot, therefore, be binuclear complexes.
The fixation-reduction process again takes a certain
time, as can be seen from Figure 9. Particularly at 1atm
of N2, the time taken appears to be of the same order
of magnitude as for the system CpzTiClz/LiNp
discussed in the last section.
[67137j
Fig. 7.
Cb
Suggested configuration of the ESR-active species (5).
On entry of a nitrogen molecule, the Ti(r1) part of the
complex could be opened out to octahedral symmetry
(Fig. 8). It is not yet possible to teI1 whether the nitrogen is end-on, as in the stable complexes (Section 4.2),
or edge-on. Again by analogy with the polymerization
of ethylene on titanium complexes, an edge-on formulation is chosen for these assumed reactive intermediates. Cis-migration of the lithium (or insertion of
the Nz at the site of the lithium) could lead to the azo
structure. (This would explain the disappearance of
[44] N. Hirota, J. Amer. chem. SOC. 89, 32 (1967).
Angew. Chem. internat. Edit. / Vol. 8 (1969) / No. 9
Table 3. Fixation of nitrogen on reduced transition
metal solutions. solvent THF I22al.
Nz
pressure
(aim)
NH,/metal
3
120
0.9
5
7
10
120
120
1
2.0
0.9
CrCl,
10
10
120
1
1.2
0.4
Tic14 [a]
10
15
120
120
1.3
1.7
VCll
1.2
[a] In toluene.
657
L
I '
I
I
10
0
I
tIh1
I
I
- 30
20
0
-
20
10
LiNpiV
+
30
Fig. 9. Time-dependence of the N2 fixation reaction in the case of the
system VCI,/LiNp [41].
Fig. 11. Evolution of hydrogen during hydrolysis in the system V C ~ J /
LiNp; reaction carried out under argon (top) and under nitrogen
(bottom).
ESR measurements show that in these systems the
metal is reduced to the zerovalent stage and beyond.
On addition of three reduction equivalents (Li/V = 3),
the signal of the bis(naphthalene)vanadium(o) sandwich complex is observed in the reaction solution, and
at higher Li/V ratios another signal, which can be
assigned to a V(-11) species, is observed[45]. In the
CrC13/LiNp system, it is found that as the Li/Cr
ratio is increased, the ESR signal of bis(naphtha1ene)chromium(1) appears first, and then that of bis(naphthalene)chromium(-1) [411. Free LiNp can be detected
in the ESR signal only above Li/M = 6 in both cases
(M = metal). The intensity of the ESR signal, however, is still low in relation to the quantity of LiNp
introduced. Evidently a further electron transfer
occurs from LiNp to M(o), with formation of negatively charged M(-x) species, which are held in solution by the naphthalene ligands.
On hydrolysis of the reduced metal solutions, considerable quantities of hydrogen are liberated, as can
be seen from Figures 10 and 11. The preparation and
hydrolysis were carried out under argon in these experiments. (LiNp does not liberate hydrogen during
the hydrolysis, but reacts to form dihydronaphthalene 1461.)
It is significant in connection with the fixation of
nitrogen that considerably less hydrogen is evolved if
0 in dimethoxyethane; 0 in tetrahydrofuran 1411.
the reduction is carried out under nitrogen (see Fig. 11;
a similar observation was reported by Shilov et al. for
the Grignard system [401). This decrease in the quantity
of hydrogen liberated does not, however, occur in systems that are not able to fix nitrogen, such as NiBr2/
LiNp (see Table 4).
Table 4. Hydrolysis of transition
metal solutions reduced under argon
or under nitrogen LiNp/metal = 10;
T = 20 "C; 1 a t a of N2 122aI.
under Ar under N2 found
V
2.0
lnformation about the source of the gas evolved can
be obtained by the use of heavy water. Figure 12 shows
the results of the mass-spectroscopic analysis of the
deuterolysis gases for the CrCls/LiNp system.
-
,,o-y-
,
1
O
5
10
15
,
20
I
25
Fig. 12. Results of deuterolysis for CrCIdLiNp. 0 : reduction under
argon; e: reduction under nitrogen 1411.
1
0
10
LiNpiM
I
I
20
I
I
30
Fig. 10. Evolution of hydrogen during hydrolysis in the systems CrCId
LiNp (top) and NiBrz/LiNp (bottom) 1451.
1451 G. Henrici-Olive and S. Olive, J. organometallic Chem. 9,
325 (1967).
1461 G. E. Cuufes: Organometallic Compounds. Methuen, London 1960.
It can be seen that by far most of the gas evolved is
deuterium; it must therefore have been formed by decomposition of the heavy water.
The fact that some hydrogen is liberated points to the
presence of metal-hydrogen bonds. The number of
hydrogen atoms per metal atom is in the neighborhood of 1 up to Li/M = 10, but increases slightly and
appears to tend toward a limiting value of two.
Figure 12 also shows the results of a comparison experiment in which the reduction was carried out under
Angew. Chem. internat. Edit.
I
Yol. 8 (I9691 1 No. 9
nitrogen, i.e. under conditions such that the system
fixed nitrogen. It is found that the quantity of deuterium liberated decreases, but not the quantity of
hydrogen.
From all these observations, the formulation (10)can
be deduced for the nitrogen-fixing species.
The presence of naphthalene is shown by the ESR experiments. The presence of metal-hydrogen bonds is
interpreted as resulting from the removal of H from
x-bonded naphthalene and simultaneous transition
into a D-naphthyl-metal bond (cf. similar processes on
an Ru(o)-Np complex studied by Chatt et al. 1471).
Solvent molecules (LM) must also be introduced, their
number m being such as to satisfy the symmetry requirements of the metal in question. The hydrolysis
experiments show that the complex can accept further
electrons supplied by LiNp molecules These extra
charges are presumably delocalized, particularly in the
antibonding x orbitals of the aromatic ligands, and
compensated for by Lif cations. The maximum number of these extra charges appears to be 6 . This is
shown by the deuterolysis experiments (Fig. 12),
taking into account the well known fact that finely
divided zerovalent Cr or V decomposes heavy water
in accordance with
The fixation of nitrogen may be assumed to involve
coordination of the Nz molecule to the metal center,
with displacement of a solvent molecule.
The extra charges are used to reduce the nitrogen in
accordance with
NZ
6e
+
2N3-
Several complexes with different charges x are presumably present in equilibrium, but only one species
(e.g. that with x = 6 ) is capable of attacking the nitrogen molecule. This would explain why the ratio NH3/M
can reach a value of 2 only when LiNp is present in a
large excess. The supply of the active species from the
equilibrium could again be the rate-determining step.
Since the ratio NH3/M cannot be increased above 2, it
must be assumed that the reduced N3e again remains
in the complex, and is liberated only on hydrolysis.
5. Outlook
Nitrogen has evidently lost some of its eminence as an
inert gas in recent years. On the one hand it is to be
expected that the number of new stable complexes
~
[47] J . Chatt and J. M . Davidson, J. chem. SOC.(London) 1965,
843.
,'
Angew. Chem. internat. Edit. Vol. 8 (1969) No. 9
with N2 ligands will grow steadily. On the other hand
nitrogen has been reduced to N3Q under mild conditions (with regard to temperature and pressure) with
strong reducing agents in aprotic solvents.
One might ask whether this can be expected to mean
competition for the industrial processes for the production of ammonia. In view of the present price of
NH3, the answer to this must be no. However, the
question as asked is wrongly phrased, since ammonia is
only the industrial starting point for the chemistry of
nitrogen. A more sensible question would be whether
any competition is to be expected with the current
processes for the production of amines, amides, nitriles, etc. In view of the outstanding progress that has
been made in homogeneous catalysis on transition
metal complexes in the course of the last 10-20 years,
this possibility must be taken very seriously.
The N3e stage attached to the transition metal is
probably of no more value in this respect than the
isolable stable Nz complexes. If we may be allowed a
(possibly somewhat daring) cornparsion with olefin
chemistry, a large number of stable complexes of
olefins with transition metals are known, but these are
not generally catalysts. The catalytically active intermediates formulated for reactions such as the 0x0
process, the Wacker process, cyclooligomerization of
butadiene, Ziegler-Natta polymerization, etc., on the
other hand, in which the olefin is supposed to be coordinately bound t o the transition metal center, have
so far never been isolated.
Let us turn again briefly to the question of end-on or
edge-on bonding. In all the stable complexes studied so
far, the nitrogen is most probably attached end-on.
However, edge-on attachment of N2 molecules cannot
be detected as easily as end-on attachment, which is
indicated by the I R band at _N 2100cm-1. The only
relatively simple method of recognizing complexes of
the edge-on type, apart from elemental analysis, would
be from their Raman spectra. Who knows whether
edge-on complexes have already passed through the
hands of chemists without being recognized? Concerning the active species in the reduction process N2 +
2 N3-, the question of end-on or edge-on must necessarily remain open, since these intermediates cannot
be isolated.
Concluding this report, it may be said that the investigation of the systems known at present has yielded
some information about the interrelations between
transition metal species and the nitrogen molecule.
Above all, however, it has shown which directions
lead to dead ends and which should be followed in
further work.
We are grateful to Professor H. H . Zeiss, Monsanto
Research S.A., who brought the interesting subject of
the .fixation of nitrogen to our notice, and to Professor
J. Chatt, University of Sussex, for stimulating discussions.
Received: June 16, 1969
[A 713 IE]
German version: Angew. Chem. 81, 619 (1969)
Translated by Express Translation Service, London
659
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