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Transition metal dinitrogen complexes are
coordination compound A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Man ...
s that contain
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s as ion centers the dinitrogen molecules (N2) as
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s.


Historical background

Transition metal complexes of N2 have been studied since 1965 when the first complex was reported by Allen and Senoff. This
diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...
complex, u(NH3)5(N2)sup>2+, was synthesized from
hydrazine hydrate Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine h ...
and
ruthenium trichloride Ruthenium(III) chloride is the chemical compound with the formula RuCl3. "Ruthenium(III) chloride" more commonly refers to the hydrate RuCl3·''x''H2O. Both the anhydrous and hydrated species are dark brown or black solids. The hydrate, with a va ...
and consists of a u(NH3)5sup>2+ centre attached to one end of N2. The existence of N2 as a ligand in this compound was identified by IR spectrum with a strong band around 2170–2100 cm−1. In 1966, the molecular structure of u(NH3)5(N2)l2 was determined by Bottomly and Nyburg by
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. The dinitrogen complex ''trans''- rCl(N2)(PPh3)2is made by treating
Vaska's complex Vaska's complex is the trivial name for the chemical compound ''trans''-carbonylchlorobis(triphenylphosphine)iridium(I), which has the formula IrCl(CO) (C6H5)3sub>2. This square planar diamagnetic organometallic complex consists of a central iridi ...
with aromatic
acyl azide Acyl azides are carboxylic acid derivatives with the general formula RCON3. These compounds, which are a subclass of organic azides, are generally colorless. Preparation Typically acyl azides are generated under conditions where they rearrange t ...
s. It has a planar geometry. The first preparation of a metal-dinitrogen complex using dinitrogen was reported in 1967 by Yamamoto and coworkers. They obtained o(H)(N2)(PPh3)3by reduction of Co(acac)3 with AlEt2OEt under an atmosphere of N2. Containing both hydrido and N2 ligands, the complex was of potential relevance to nitrogen fixation. From the late 1960s, a variety of transition metal-dinitrogen complexes were made including those with iron, molybdenum and vanadium as metal centers. Interest in such complexes arises because N2 comprises the majority of the atmosphere and because many useful compounds contain nitrogen. Biological
nitrogen fixation Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
probably occurs via the binding of N2 to those metal centers in the enzyme
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fa ...
, followed by a series of steps that involve
electron transfer Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons. Electrochemical processes ar ...
and
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
.


Bonding modes

In terms of its bonding to transition metals, N2 is related to CO and
acetylene Acetylene ( systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
as all three species have
triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...
s. A variety of bonding modes have been characterized. Based on whether the N2 molecules are shared by two more metal centers, the complexes can be classified into mononuclear and bridging. Based on the geometric relationship between the N2 molecule and the metal center, the complexes can be classified into end-on or side-on modes. In the end-on bonding modes of transition metal-dinitrogen complexes, the N-N vector can be considered in line with the metal ion center, whereas in the side-on modes, the metal-ligand bond is known to be perpendicular to the N-N vector.


Mononuclear, end-on

As a ligand, N2 usually binds to metals as an "end-on" ligand, as illustrated by u(NH3)5N2sup>2+. Such complexes are usually analogous to related CO derivatives. This relationship is illustrated by the pair of complexes IrCl(CO)(PPh3)2 and IrCl(N2)(PPh3)2. In these mononuclear cases, N2 is both as a σ-donor and a π-acceptor. The M-N-N bond angles are close to 180°. N2 is a weaker pi-acceptor than CO, reflecting the nature of the π* orbitals on CO vs N2. For this reason, few examples exist of complexes containing ''both'' CO and N2 ligand. Transition metal-dinitrogen complexes can contain more than one N2 as "end-on" ligands, such as ''mer''- o(N2)3(PPr''n''2Ph)3 which has octahedral geometry. In another example, the dinitrogen ligand in Mo(N2)2(Ph2PCH2CH2PPh2)2 can be reduced to produce ammonia. Because many nitrogenases contain Mo, there has been particular interest in Mo-N2 complexes.


Bridging, end-on

N2 also serves as a bridging ligand with "end-on" bonding to two metal centers, as illustrated by 4+. These complexes are also called multinuclear dinitrogen complexes. In contrast to their mononuclear counterpart, they can be prepared for both early and late transition metals. In 2006, a study of iron-dinitrogen complexes by Holland and coworkers showed that the N–N bond is significantly weakened upon complexation with iron atoms with a low coordination number. The complex involved bidentate chelating ligands attached to the iron atoms in the Fe–N–N–Fe core, in which N2 acts as a bridging ligand between two iron atoms. Increasing the coordination number of iron by modifying the chelating ligands and adding another ligand per iron atom showed an increase in the strength of the N–N bond in the resulting complex. It is thus suspected that Fe in a low-coordination environment is a key factor to the fixation of nitrogen by the
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fa ...
enzyme, since its Fe–Mo cofactor also features Fe with low coordination numbers. The average bond length of those bridging-end-on dinitrogen complexes are about 1.2 Å. Within some cases, the bond length can be as long as 1.4 Å, which is similar to those of N-N single bonds.


Mononuclear, side-on

In comparison with their end-on counterpart, the mononuclear side-on dinitrogen complexes are usually higher in energy and the examples of them are rare. Dinitrogen act as a π-donor in these type of complexes. Fomitchev and Coppens has reported the first crystallographic evidence for side-on coordination of N2 to a single metal center in a photoinduced metastable state. When treated with UV light, the transition metal-dinitrogen complex, s(NH3)5(N2)sup>2+ in solid states can be converted into a metastable state of s(NH3)52-N2)sup>2+, where the vibration of dinitrogen has shifted from 2025 to 1831 cm−1. Some other examples are considered to exist in the transition states of intramolecular linkage isomerizations. Armor and Taube has reported these isomerizations using 15N-labelled dinitrogen as ligands.


Bridging, side-on

In a second mode of bridging, bimetallic complexes are known wherein the N-N vector is perpendicular to the M-M vector, which can be considered as side-on fashion. One example is η5-C5Me4H)2Zrsub>2( μ2, η22-N2). The dimetallic complex can react with H2 to achieve the artificial
nitrogen fixation Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
by reducing N2. A related ditantalum tetrahydride complex could also reduce N2.


Reactivity


Cleavage to nitrides

When
metal nitrido complex Metal nitrido complexes are coordination compounds and metal clusters that contain an atom of nitrogen bound only to transition metals. These compounds are ''molecular'', i.e. discrete in contrast to the polymeric, dense nitride materials that ar ...
es are produced from N2, the intermediacy of a dinitrogen complex is assumed. Some Mo(III) complexes also cleave N2: :2Mo(NR2)3 + N2 → (R2N)3Mo-N2-Mo(NR2)3 :(R2N)3Mo-N2-Mo(NR2)3 → 2N≡Mo(NR2)3


Attack by electrophiles

Some electron-rich metal dinitrogen complexes are susceptible to attack by electrophiles on nitrogen. When the electrophile is a proton, the reaction is of interest in the context of
abiological nitrogen fixation Abiological nitrogen fixation describes chemical processes that fix (react with) N2, usually with the goal of generating ammonia. The dominant technology for abiological nitrogen fixation is the Haber process, which uses an iron-based heterogeneou ...
. Some metal-dintrogen complexes even catalyze the hydrogenation of N2 to
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
in a cycle that involves N-protonation of a reduced M-N2 complex.


See also

*
Abiological nitrogen fixation Abiological nitrogen fixation describes chemical processes that fix (react with) N2, usually with the goal of generating ammonia. The dominant technology for abiological nitrogen fixation is the Haber process, which uses an iron-based heterogeneou ...
*
Transition metal nitrido complex Metal nitrido complexes are coordination compounds and metal clusters that contain an atom of nitrogen bound only to transition metals. These compounds are ''molecular'', i.e. discrete in contrast to the polymeric, dense nitride materials that ar ...


References

{{reflist, 35em Coordination complexes Nitrogen compounds