Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived ²LMCT Excited States

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)₂]PF₆(brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]^-, [Fe(meophtmeimb)₂]PF₆(meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]^-, and [Fe(coohphtmeimb)₂]PF₆(coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]^-. These were derived from the parent complex [Fe(phtmeimb)₂]PF₆(phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]^-by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (²LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state (²GS), completely in line with the parent complex [Fe(phtmeimb)₂]PF₆(2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.