A Ferrocene Metal–Ligand Triplet Diradical with a Terminal Iminyl Group Discovered by Time-Resolved Mid-Infrared Spectroscopy

Ferrocene-based radicals and diradicals with ligand-centered spin density are highly promising building blocks for extended magnetic materials. In addition, the unpaired electrons may dress such systems with intriguing ligand-centered chemical reactivity that can be utilized for structural diversification of the functional material. Just like their inorganic analogues, metallonitridyls and metallonitrenes, nitrogen-centered organic radicals and diradicals, like iminyls and nitrenes, are particularly appealing spin centers because they offer an elegant entry into a wide spectrum of chemical reactivities ranging from remote CC-functionalization and radical relay to CH-amination and amidation. Here, we report on the photochemical generation of an exceptional metal-ligand ferrocene diradical from a photolabile diamagnetic azidoferrocene precursor. The diradical product has a triplet electronic ground state and contains the organic cyclopentadiene-iminyl neutral radical ligand (S = 1/2) that is ferromagnetically coupled to a low-spin (S = 1/2) iron(I) center. This unique species appears within 13 ps after optical excitation of the precursor and is formed with a primary quantum yield of 50%. Within less than 1 ps after impulsive electronic excitation in its metal-to-ligand charge-transfer region, the azidoferrocene finds itself in an excited quintet azide-ππ* state, which is electronically preconfigured to adiabatically release dinitrogen and to form a quintet iminyl intermediate. The latter then undergoes a rapid intersystem crossing to the triplet iminyl final product. The presence of the terminal iminyl moiety was verified unambiguously by conducting the optical excitation in the presence of an isonitrile quencher, thereby trapping the photochemical diradical product as a ferrocenyl carbodiimide.