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

Abstract

We here report the synthesis of the homoleptic iron(II) Nheterocyclic carbene (NHC) complex [Fe(miHpbmi) 2 ](PF 6 ) 4 (miHpbmi = 4-((3-methyl-1H-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-1H-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) 2 ](PF 6 ) 2 (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. However, compared to the carboxylic acid derivative [Fe(cpbmi) 2 ](PF 6 ) 2 (cpbmi = 1,1′-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe-(miHpbmi) 2 ](PF 6 ) 4 is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi) 2 ](PF 6 ) 4 a promising pH-insensitive analogue of [Fe(cpbmi) 2 ](PF 6 ) 2 . Finally, the excited-state dynamics of the title compound [Fe(miHpbmi) 2 ](PF 6 ) 4 was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.