Air-Stable Fe(II) Hexa-N-Heterocyclic Carbene Complex and Its Photoexcited State Dynamics

Elongation of the triplet metal-to-ligand charge transfer excited state (MLCT) lifetimes in Fe(II) complexes is particularly important because of their 3d⁶ electronic configuration that is analogous to that of the well-studied photoactive Ru polypyridyl complexes. Toward extending ³MLCT state lifetimes, the use of strongly σ-donating NHC ligands is known to suppress unwanted deactivations through d-d states. Hexa-NHC coordination environments seem highly promising to provide strong ligand fields; however, Fe(II) hexa-NHC complexes are unstable in air and are oxidized to Fe(III) oxidation states under ambient conditions. In this study, we sought an air-stable Fe(II) hexa-NHC complex with a slightly weaker σ-donor from a triazole-based NHC ligand, and its photoexcited state dynamics were determined by femtosecond transient absorption spectroscopy. The obtained Fe(II) state is stable under ambient conditions (E(Fe^3+/2+) = 0.33 V vs Fc^+/0), and its photoexcited state characteristics are similar to those of reported electrochemically reduced Fe(II) hexa-NHC complexes. Our findings revealed that Fe(II) oxidation states can be stabilized while keeping a strong ligand field nature of NHCs. Although the ³MLCT lifetime remains short-lived (1.4 ps), air-stable Fe(II) hexa-NHC complexes are amenable to further exploration of the long-lived ³MLCT states in Fe(II) complexes.