User:Jiahaow/Transition metal dinitrogen complex

Transition metal-dinitrogen complexes are coordination compounds that contain transition metals as ion centers the dinitrogen molecules (N2) as ligand s. In the area of coordination chemistry, the atomic and diatomic forms of nitrogen are distinguished, although otherwise "nitrogen" refers to N2.

Historical Background
Transition metal complexes of N2 have been studied since 1965 when the first complex was reported by Allen and Senoff. This diamagnetic complex, [Ru(NH3)5(N2 )] 2+, was synthesized from hydrazine hydrate and ruthenium trichloride and consists of a 16e−[Ru(NH3)5]2+ centre attached to one end of N2. N2 as a ligand in this compounds was identified by IR spectrum with a strong band around 2170-2100 cm-1. In 1966, the molecular structure of [Ru(NH3)5(N2)]Cl2 was determined by Bottomly and Nyburg using single-crystal X-ray analysis.

In 1966, Collman and Kang reported the second example of transition metal-dinitrogen complexes, trans-[IrCl(N2)(PPh3)2]. This complex was made by treating Vaska's iridium complex with aromatic acyl azides in chloroform and has a planar geometry.

The first example of directly reacting with dinitrogen gas to obtain a transition metal-dinitrogen complex was reported by Yamamoto and coworkers in 1967. In their report, the salt Co(acac)3 reacting with AlEt2OEt under atmospheric pressure of N2 gives out the product Co[H(N2)(PPh3)3] with both hydrido and N2 as ligands.

From the late 1960s, a variety of transition metal-dinitrogen complexes have been made and explored including those with iron, molybdenum and vanadiumas metal centers. Interest in such complexes arises because N2 comprises the majority of the atmosphere and because many useful compounds contain nitrogen atoms. Biological nitrogen fixation probably occurs via the binding of N2 to those metal centers in the enzyme nitrogenase, followed by a series of steps that involve electron transfer and protonation.

Bonding modes
In terms of its bonding to transition metals, N2 is related to CO and acetylene as all three species have triple bonds. 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 [Ru(NH3)5N2]2+. Such complexes are usually analogous to related CO derivatives. A good example of this relationship are the complexes IrCl(CO)(PPh3)2 and IrCl(N2)(PPh3)2. In these mononuclear cases, N2 molecule act both as a σ-donor and a π-acceptor, making the bond angles of M-N-N close to 180°. The most well-investgated mononuclear dinitrogen complexes with end-on fashion bonding modes are those who have mid to late transition metal centers and low oxidation states enough to support backbonding with nitrogen. Transition metal-dinitrogen complexes can contain more than one N2 as "end-on" ligands, such as mer-[Mo(N2)3(PPrn2Ph)3], which has octahedral geometry. In another example, the dinitrogen ligand in W(N2)2(Ph2PCH2CH2PPh2)2 can be reduced to produce ammonia.

An end-on complex also containing a hydride ligand is [FeH(N2)(dmpe)2)]+. This cation undergoes deprotonation to form a Fe(N2)(dmpe)2 which reacts with hydrogen chloride to give ammonium (dmpe = bis(dimethylphosphino)ethane).

Bridging, End-on
N2 also serves as a bridging ligand with "end-on" bonding to two metal centers, as illustrated by {[Ru(NH3)5]2(μ-N2)}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 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 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, [Os(NH3)5(N2)]2+ in solid states can be converted into a metastable state of [Os(NH3)5(η2-N2)]2+, where the vibration of dinitrogen has shifted from 2025 to 1831 cm-1.

Some other exmamples 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)2Zr]2(μ2,η2,η2-N2). The dimetallic complex can react with H2 to achieve the artificial nitrogen fixation by reducing N2. A related ditantalum tetrahydride complex could also reduce N2.