Abiological nitrogen fixation using homogeneous catalysts
Abiological nitrogen fixation describes chemical processes that fix (react with) N2, usually with the goal of generating
Transition metals
Vol'pin and Shur
An early influential discovery of abiological nitrogen fixation was made by Vol'pin and co-workers in Russia in 1970. Aspects are described in an early review:
"using a non-protic Lewis acid, aluminium tribromide, were able to demonstrate the truly catalytic effect of titanium by treating dinitrogen with a mixture of titanium tetrachloride, metallic aluminium, and aluminium tribromide at 50 °C, either in the absence or in the presence of a solvent, e.g. benzene. As much as 200 mol of ammonia per mol of TiCl
4 was obtained after hydrolysis.…"[3]
These results led to many studies on dinitrogen complexes of titanium and zirconium.[4]
Mo- and Fe-based systems
Because Mo and Fe are found at the active site of the most common and most active form of nitrogenase, these metals have been the focus of particular attention for homogeneous catalysis. Most catalytic systems operate according to the following stoichiometry:
- N2 + 6 H+ + 6 e− → 2 NH3
The reductive protonation of
Intense effort has focussed on family of
Iron complexes of N2 are numerous. Derivatives of Fe(0) with C3-symmetric ligands catalyze nitrogen fixation.[1]
Photolytic routes
Photolytic nitrogen splitting is also considered.[6][7][8][9][10]
p-Block systems
Although nitrogen fixation is usually associated with transition metal complexes, a boron-based system has been described. One molecule of dinitrogen is bound by two transient
to a neutral compound, and reduced using water.Nitriding
In rare cases, metals react with nitrogen gas to give nitrides, a process called nitriding. For example, metallic lithium burns in an atmosphere of nitrogen, giving lithium nitride. Hydrolysis of the resulting nitride gives ammonia. In a related process, trimethylsilyl chloride, lithium and nitrogen react in the presence of a catalyst to give tris(trimethylsilyl)amine, which can be further elaborated.[12] Processes involving lithium metal are however of little practical interest since they are non-catalytic and re-reducing the Li+
ion residue is difficult.
Some Mo(III) complexes also cleave N2:[13]
- 2 Mo(NR2)3 + N2 → 2 N≡Mo(NR2)3
This and related terminal nitrido complexes have been used to make
See also
- Nitrogenase: enzymes used by organisms to fix nitrogen
- Transition metal dinitrogen complex
- Metal nitrido complex