A heteroleptic ferrous complex with mesoionic bis(1,2,3-triazol-5-ylidene) ligands: taming the MLCT excited state of iron(II).

Strongly σ-donating N-heterocyclic carbenes (NHCs) have revived research interest in the catalytic chemistry of iron, and are now also starting to bring the photochemistry and photophysics of this abundant element into a new era. In this work, a heteroleptic Fe(II) complex (1) was synthesized based on sequentially furnishing the Fe(II) center with the benchmark 2,2'-bipyridine (bpy) ligand and the more strongly σ-donating mesoionic ligand, 4,4'-bis(1,2,3-triazol-5-ylidene) (btz). Complex 1 was comprehensively characterized by electrochemistry, static and ultrafast spectroscopy, and quantum chemical calculations and compared to [Fe(bpy)3](PF6)2 and (TBA)2[Fe(bpy)(CN)4]. Heteroleptic complex 1 extends the absorption spectrum towards longer wavelengths compared to a previously synthesized homoleptic Fe(II) NHC complex. The combination of the mesoionic nature of btz and the heteroleptic structure effectively destabilizes the metal-centered (MC) states relative to the triplet metal-to-ligand charge transfer ((3)MLCT) state in 1, rendering it a lifetime of 13 ps, the longest to date of a photochemically stable Fe(II) complex. Deactivation of the (3)MLCT state is proposed to proceed via the (3)MC state that strongly couples with the singlet ground state.