How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron -Heterocyclic Complexes: A Comparative Study.

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1'-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic Fe-hexa-NHC [Fe(mbmi)](PF) and heteroleptic Fe-tetra-NHC [Fe(mbmi)(bpy)](PF) (bpy = 2,2'-bipyridine) complexes. They are compared to the reported Fe-hexa-NHC [Fe(btz)](PF) and Fe-tetra-NHC [Fe(btz)(bpy)](PF) complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis(-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)](PF) and [Fe(mbmi)(bpy)](PF) are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)](PF) and [Fe(btz)(bpy)](PF).

Tailoring the Photophysical Properties of a Homoleptic Iron(II) Tetra -Heterocyclic Carbene Complex by Attaching an Imidazolium Group to the (CNC) Pincer Ligand─A Comparative Study.

We here report the synthesis of the homoleptic iron(II) -heterocyclic carbene (NHC) complex [Fe(miHpbmi)](PF) (miHpbmi = 4-((3-methyl-1-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the π-electron-withdrawing 3-methyl-1-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)](PF) (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region.