In the simplest case, the linear N–H molecule has its nitrogen atom sp hybridized, with two of its four non-bonded electrons as a lone pair in an sp orbital and the other two occupying a degenerate pair of p orbitals. The electron configuration is consistent with Hund's rule: the low energy form is a triplet with one electron in each of the p orbitals and the high energy form is the singlet with an electron pair filling one p orbital and the other p orbital vacant. As with carbenes, a strong correlation exists between the spin density on the nitrogen atom which can be calculated in silico and the zero-field splitting parameterD which can be derived experimentally from electron spin resonance. Small nitrenes such as NH or CF3N have D values around 1.8 cm−1 with spin densities close to a maximum value of 2. At the lower end of the scale are molecules with low D values and spin density of 1.2 to 1.4 such as 9-anthrylnitrene and 9-phenanthrylnitrene.
Formation
Because nitrenes are so reactive, they are not isolated. Instead, they are formed as reactive intermediates during a reaction. There are two common ways to generate nitrenes:
Nitrene cycloaddition. With alkenes, nitrenes react to form aziridines, very often with nitrenoid precursors such as nosyl- or tosyl-substituted phenyliodinane ) but the reaction is known to work directly with the sulfonamide in presence of a transition metal based catalyst such as copper, palladium, or gold:
Arylnitrene ring-expansion and ring-contraction: Aryl nitrenes show ring expansion to 7-membered ring cumulenes, ring opening reactions and nitrile formations many times in complex reaction paths. For instance the azide2 in the scheme below trapped in an argonmatrix at 20 K on photolysis expels nitrogen to the triplet nitrene 4 which is in equilibrium with the ring-expansion product 6.
Nitreno radicals
For several compounds containing both a nitrene group and a free radical group an ESR high-spin quartet has been recorded. One of these has an amine oxide radical group incorporated, another system has a carbon radical group. In this system one of the nitrogen unpaired electrons is delocalized in the aromatic ring making the compound a σ–σ–π triradical. A carbene nitrogen radical resonance structure makes a contribution to the total electronic picture. In 2019, an authentic triplet nitrene was isolated by Betley and Lancaster, stabilized by coordination to a copper center in a bulky ligand.
