title: A heteroleptic ferrous complex with mesoionic bis(1,2,3-triazol-5-ylidene) ligands: taming the MLCT excited state of iron(II).
abstract: 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.
authors.affiliation: Department of Chemical Physics, Lund University, P.O. Box 124, 22100 Lund (Sweden); Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, 22100 Lund (Sweden).
authors.forename: Yizhu
authors.initials: Y
authors.lastname: Liu
authors.forename: Kasper S
authors.initials: KS
authors.lastname: Kjaer
authors.forename: Lisa A
authors.initials: LA
authors.lastname: Fredin
authors.forename: Pavel
authors.initials: P
authors.lastname: Chábera
authors.forename: Tobias
authors.initials: T
authors.lastname: Harlang
authors.forename: Sophie E
authors.initials: SE
authors.lastname: Canton
authors.forename: Sven
authors.initials: S
authors.lastname: Lidin
authors.forename: Jianxin
authors.initials: J
authors.lastname: Zhang
authors.forename: Reiner
authors.initials: R
authors.lastname: Lomoth
authors.forename: Karl-Erik
authors.initials: KE
authors.lastname: Bergquist
authors.forename: Petter
authors.initials: P
authors.lastname: Persson
authors.forename: Kenneth
authors.initials: K
authors.lastname: Wärnmark
authors.forename: Villy
authors.initials: V
authors.lastname: Sundström
keywords: N ligands; N-heterocyclic carbenes; femtosecond spectroscopy; iron; metal-to-ligand charge transfer