Subject
Chemistry
Paper No and Title
11. Inorganic Chemistry –III (Metal-Complexes and Metal
Clusters)
Module No and Title
12 and Dinitrogen complexes and their preparation
Module Tag
CHE_P11_M12
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
TABLE OF CONTENTS
1. Learning Outcomes
2. Introduction
3. Methods of preparation of dinitrogen metal complexes
4. Summary
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
1. Learning Outcomes
After studying this module, you shall be able to learn about:



The reactivity of dinitrogen as ligand in metal complexes.
The classical metal complexes of dinitrogen
Various methods of preparation of dinitrogen complexes.
2. Introduction
The discovery of N2 complexes such as [Ru(NH3)5(N2)]+2X2 (where X = Br, I, PF4,
or PF6) in 1965, and the subsequent finding that it can be prepared directly from
dinitrogen gas, led most chemists to accept that dinitrogen complexes of transition metals
are involved, in the biological "fixation" of dinitrogen. Extensive researches have been
carried out on the preparation of N2 complexes and their reactivities along with
coordinating ability with almost all transitions metals. It may eventually be found that
these complexes are an intermediate in dinitrogen "fixation." However, like the organic
diazonium salts, the dinitrogen complexes are of substantial interest and have already
showed to be valuable synthetic intermediates. Up to now, a large number of transition
metal ions have been utilized for the synthesis of dinitrogen based metal complexes
which are summarized in the table below (figure 1). The unshaded regions in the figure 1
represent the elements that have been reported to from mononuclear dinitrogen
complexes
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
The position of the metal ion in figure 1 is significant in a number of ways:

The data closely coincides with the borderline regions between “class a” and class
b” metals as defined by Ahrland and Chatt.

Most of the metal ions forming mononuclear dinitrogen complexes also
coordinate with carbon monoxide which represents a bonding similarity between
the dinitrogen and carbonyl as ligands.

The table includes iron and molybdenum which are believed to be the active sites
in nitrogenase.

Iron and molybdenum are being commercially used as catalysts in Haber’s
process for ammonia synthesis.
A large number of dinitrogen complexes are known till date as shown in figure below.
Most of these complexes are applied for artificial dinitrogen fixation. The first example
of N2 Complexes [Ru(NH3)5(N2)]+2X2 (X = Br, I, PF4, or PF6) was separated from the
reaction of RuCl3 with hydrazine in the aqueous solution in the presence of different
anions. While complex [Co(N2)(PPh3)3] prepared directly from gaseous N2.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
The revolutionary Haber-Bosch process was long believed to involve the adsorption of
dinitrogen as nitrogen atoms on the active Fe catalyst at high pressure and temperature. In
a nitrogen metal complex, a large number of ligands can coexist with dinitrogen on a
metal atom. Other ligands that can coexist with nitrogen in a metal complex are
phosphines, halides, hydride, ammonia, carbon monoxide, and even water. Moreover,
nitrogen fixing bacteria (metalloenzyme nitrogenase) catalytically convert nitrogen to
ammonium ions. The active sites of these bacteria contain Fe, Mo and S. Inspired by this,
a large number of Fe and Mo have been synthesized and research is under progress to
develop efficient artificial catalysts.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
3. Methods of preparation of dinitrogen metal complexes
Presently, there are many methods have been developed with different metals are
discussed.
A. The various preparations of N2 Complexes directly using N2 gas.
1. [Co(N2)(PPh3)3], [RuH2(N2)(PPh3)3], [FeH2(N2)(PR3)3] (R3 = EtPh2, n-Bu), and
[CoH(N2)(PR3)3] have been prepared by following reaction
[MHn(PR3)3] + N2 → [MHn-2(N2)(PR3)3] + H2
The reaction is exciting because it may provide a model for the first step in the "fixation"
of dinitrogen by nitrogenase. The enzymic reaction is competitively inhibited by
dihydrogen, which is consistent with a rapid equilibrium reaction between a polyhydride
and N2 to give a dinitrogen complex.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
"titanocene dimer" + N2 → "(titanocene dimer) (N2)"
(2)
2. [(π-Cp)2TiN2Ti(π-Cp)2] and fully methylated derivative have recently been synthesized
in ether at -80° (eq 2). Prolonged exposure of a benzene solution of the titanocene to N2
at 20-25° results in the formation of a complex formulated as [(π-Cp)2TiN2]2. Recently
the molybdenum analogs have been prepared under 250 atm of N2.
3. Isopropylmagnesium chloride reacts with[FeC13(PPh3)2] under N2 to produce an
incompletely characterized complex containing 1 mol of N2 per 2 mol of iron. Grignard
reagents have also been used to prepare N2 complexes of molybdenum and titanium.
[(RuL5H2O]2+ + N2 → [(RuL5(N2)]2+ + [(L5RuN2RuL5]4+ + H2O
(3)
4. These displacement reactions, which are shown in Table I, give both monomeric and
dimeric products.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
5. (a) This reaction may proceed through a solvated intermediate such as
[MHCl(depe)2]+.
[MHCl(depe)2]+ + NaBPh4 → [MH(N2)(depe)2]+(BPh4) + NaCl (M = Fe, Ru, Os) (4)
(b) For (π-Cp)Fe(dmpe), the reaction with TIBF4 in acetone under N2 gives the dimeric
species [(π-Cp)Fe(dmpe)N2Fe(dmpe)(π-Cp)](BF)2.2H2O via an acetone coordinated
intermediate.
6. A few low-valent olefin complexes will react with N2 if a tertiary phosphine is also
present. [CoH(N2)(PPh3)3], for example, has been obtained from [Co(C8H13)(C6H12)] in
this way.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
7. Many metal acetylacetonates will react with N2 in the presence of a tertiary phosphine
and an aluminum alkyl (or AIEt2(OEt)). Sometimes the product contains a hydride ligand
as
well
as
dinitrogen
and
phosphine.
[CoH(N2)(PR3)3],
[Co(N2)
(PPh3)3],
[NiH(N2)(PEt3)2], tran-[(C6H11)3P]2NiN2-Ni[P(C6H11)3]2], [Mo(π-toluene)(N2)(PPh3)2],
and trans-[Mo(N2)2(diphos)2] have been obtained by this route.
8. (a) Metal amalgams have been used as reducing agents in place of the aluminum alkyls
employed in reaction. In this manner, Chatt and his group have obtained trans trans[W(N2)2(diphos)2], cis-[W(N2)2(PMe2Ph)4], [ReCI(N2)(PMe2Ph)4] and [OsX2(N2)(PR3)3].
(b) Shilov et al have reduced a number of ruthenium and osmium halo complexes with
zinc
amalgam
and
obtained
several
dinitrogen
complexes
such
as
[RuCl2(N2)(H2O)2(THF)].
9. (a) Zinc dust, in the presence of excess phosphine and dinitrogen in THF, reduces
[MoOCI2(diphos)(THF)] to [MoCI(N2)(diphos)2] and [Mo(N2)2(diphos)2].
(b) Powdered sodium reduces cobalt(II) complexes under nitrogen to give monomeric
and dimeric dinitrogen species.
2CoCl2(PR3)2 + 2PR3 + 4Na → (PR3)3CoN2Co(PR3)3 + 4NaCl
(5)
2CoCl2(PR3)2 + PR3 + 3Na → Na[Co(N2)(PR3)3] + NaCl
(6)
R3 = Ph3, Et3Ph
10. When molybdenum(II1) acetylacetonate was reduced with triethylaluminum in
thepresence of a tenfold excess of triphenylphosphine in toluene under nitrogen, an
orange
complex
1
was
obtained,
which
was
empirically
formulated
as
Mo(N2)(PPh3).C8H5CH3. The N2 complex is moderately air stable. The yield of the
complex decreased to one-fifth when triisobutylaluminum was used in place of
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
triethylaluminum. On the other hand, the complex was not obtained by using
trimethylaluminum.
B. Preparation from Compounds Containing Chains of Nitrogen Atoms
1.
A variety of ruthenium(III) and -(lV) and osmium(III) and -(lV) chloro and
ammine complexes and even OsO4 will react with aqueous hydrazine to give complexes
of the type [M(NH3)5(N2)]X2. Unfortunately the products are often contaminated with
hydrazine or bis dinitrogen complexes. [OsCl3(PBu2Ph)3] gives a mixture of starting
material and [OsCl2(N2)(PBu2Ph)3] while [Fe(dithiocarbamate)3] with hydrazine gives
an unknown product having v(N2) at 2045 cm-1.
2. Oxidation of coordinated hydrazine (eq 7) has produced the first dinitrogen complex of
manganese.
3. Intermediates I and II (eq 8) have both been isolated and characterized.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
4. There is evidence that reaction 9, which is the best way of preparing [Ru(NH3)5N2]2+,
proceeds through a ruthenium(III) azido complex. If cis-[Ru(NH3)4X2]X used, the
product is [Ru(NH3)4(H2O)(N2)]2+ and not the bis dinitrogen complex. If reaction 9 is
done at pH 0, binuclear as well as mononuclear species are obtained. Kane-Maguire et al
have evidence that the dimers are formed by dimerization of the nitrene [Ru(NH3)5NH]+
followed by the loss of two proton.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
5. Nitrous acid reacts with coordinated azide ligands to give dinitrogen Complexes and
nitrous oxide. [RuCI(N2)(diars)2]PF6 and a mixture of [Ru(en)2(H2O)(N2)](BPh4)2 and
unstable [Ru(en)2(N2)](BPh4)2 have been prepared via this reaction.
6. [IrX(N2)](PR3)2] and [RhCl(N2)](PPh3)2]have been prepared by eq 10.
7. Another preparation is reaction 11. The osmium tetrahydride will not react with free
dinitrogen even under pressure (cf. reaction 1).
8. Many of the systems which react with N2 to give a dinitrogen complex can also react
with nitrous oxide (N2O) to give the same product. Since nitrous oxide complexes have
now been isolated, it is almost certain that these reactions proceed through an N-bonded
nitrous oxide complex which is subsequently reduced.
C. Preparations in which two nitrogen atoms are combined to give a Dinitrogen
Complex
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
1. Reaction 13 must be regarded as doubtful because the analogous reaction of
[RuCINO(diars)2]Cl2 with hydrazine has been found to give an azide and not
[RuCI2(N2)(diars)2]Cl2 as previously believed.
[Fe(CN)5NO]2- + N2H4 → [Fe(CN)5(N2)]
(13)
2. Attempts to apply reaction 15 to ammonia molecules bound to other metals, e.g.,
molybdenum and ruthenium, have not been successful.
[Os(NH3)5L]2+ + HNO2
2+
→ cis-[Os(NH3)4(N2)L]
+ 2H2O (15)
L = CO or N2
4. Reaction 16 probably proceeds via an amide complex of the
[Ru(NH3)6]3+ + NO + OH- → [Ru(NH3)5(N2)]2+ + 2HO2 (16)
5. Ammonium salts react with coordinated nitro ligands to produce dinitrogen complexes
of platinum and rhodium containing bridging N2 ligands.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation
4. Summary
1. Dinitrogen complexes are the metal complexes where dinitrogen acts as a ligand
by coordinating with a metal ion through one of the nitrogen atom.
2. Dinitrogen complexes are known with few transition metal ions such as Mo, Fe,
Ru and Os etc.
3. A large number of dinitrogen metal complexes have been synthesized and applied
for dinitrogen fixation.
4. Natural dinitrogen fixation involves reduction of dinitrogen to ammonia and other
nitrogen hydrides.
5. Dinitrogen complexes can be synthesized by a number of processes such as:

By direct reaction with N2 gas.

By reaction with compounds containing chains of Nitrogen atoms.

By methods in which two Nitrogen atoms are combined to give a
dinitrogen complex.
CHEMISTRY
Paper 11: Inorganic Chemistry –III (Metal-Complexes and
Metal Clusters)
Module 12 :Dinitrogen complexes and their preparation