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.

Isolation and Structures of 1,2,3‐Triazole‐Derived Mesoionic Biscarb­enes with Bulky Aromatic Groups

“…[15] The room-temperature reaction of in situ generated triazolylidene 3b with iron(II) chloride in THF/ether afforded a dark purple solution at room temperature. Ammonium hexafluorophosphate was added to the solution as an anion-ex- change reagent to stabilize the compound (36 % conversion from carbene).…”

Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution

“…The characteristic excited state absorption of the 2,2′-bipyridine radical anion has proven to be a useful indicator of the MLCT excited state. 7,18,38 These X-ray and optical probes complement one another and lead to a robust interpretation of the observed dynamics. From these measurements we demonstrate that spin crossover is suppressed for [Fe(CN) 4 (bpy)] 2– which leads to more than a hundred-fold increase in the MLCT excited state lifetime compared to [Fe(bpy) 3 ] 2+ .…”

“…More recently, Sundström, Wärnmark and co-workers synthesized and characterized the excited state dynamics of a series of iron carbene complexes. 36–38 These iron carbene sensitized cells showed high charge injection yields, but low power conversion due to fast electron–hole recombination.…”

NHC-Based Iron Sensitizers for DSSCs

“…They reveal GSB as negative signals at 400 and 505 nm and positive ESA bands peaking in the near-UV (320-360 nm), the blue (420-450), and in the red part of the spectrum (>580 nm). These three ESA bands are characteristic of the 3 MLCT state in Fe(II) complexes [24,31]. A detailed analysis of the spectral dynamics allows us to establish the following photoinduced reaction scenario [30]:…”