Photoinduced Ligand-to-Metal Charge Transfer of Cobaltocene: Radical Release and Catalytic Cyclotrimerization

Abstract

Irradiation of cobalt metallocenes at the ligand-to-metal charge transfer energies results in the labilization of the cyclopentadienyl–cobalt bond and radical release. The cyclopentadienyl radical is detected by electron paramagnetic resonance (EPR) spectroscopy using a spin trap and can also be chemically trapped using hydrogen-atom-donating reagents. This reaction presents a new photochemical method of generating new cobalt complexes or of forming cyclopentadienyl cobalt(I) species that are active for catalytic [2 + 2 + 2] cyclotrimerization reactions. More importantly, these results also show that cobaltocene should not be considered as a photostable redox reagent under many conditions, including those relevant to photovoltaics or photocatalysis.

A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

“…2. 1,19,62,[94][95][96][97][98] Therefore, careful control of orbital energetics is required to preferentially enable low energy LMCT excited states.…”

“…Common applications of VLIH include actinometry and photoredox catalysis. 1,21,92,97,98,[109][110][111] Herein, we highlight two examples and discuss their mechanistic details.…”

Electronic Structure and Photophysics of Low Spin d5 Metallocenes

“…However, some reports describe photoinitiated reactivity of metallocenes. For instance, ferrocene, titanocene, and zirconocene derivatives are known to initiate photoisomerization and photopolymerization reactions, and cobaltocene was recently shown to undergo photoinduced radical chemistry to facilitate catalytic reactions, such as cyclotrimerizations. ,− However, these reactions are typically driven by high energy light to promote ligand dissociation and generate reactive radical species capable of ground state reactivity and/or catalysis. Owing to the rich excited state electronic structure of metallocenes, a wide array of excited state reactivity may be accessible from these complexes, enabling new photocatalytic scaffolds to drive challenging uphill reactions.…”