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^III-hexa-NHC [Fe(mbmi)₃](PF₆)₃ and heteroleptic Fe^II-tetra-NHC [Fe(mbmi)₂(bpy)](PF₆)₂ (bpy = 2,2'-bipyridine) complexes. They are compared to the reported Fe^III-hexa-NHC [Fe(btz)₃](PF₆)₃ and Fe^II-tetra-NHC [Fe(btz)₂(bpy)](PF₆)₂ complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)₃](PF₆)₃ and [Fe(mbmi)₂(bpy)](PF₆)₂ are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. The Fe^II/Fe^III redox couples of [Fe(mbmi)₃](PF₆)₃ (-0.38 V) and [Fe(mbmi)₂(bpy)](PF₆)₂ (-0.057 V, both vs Fc^+/0) are less reducing than [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)₃](PF₆)₃ at 502 nm (ligand-to-metal charge transfer, ²LMCT) and [Fe(mbmi)₂(bpy)](PF₆)₂ at 410 and 616 nm (metal-to-ligand charge transfer, ³MLCT). Lifetimes of 57.3 ps (²LMCT) for [Fe(mbmi)₃](PF₆)₃ and 7.6 ps (³MLCT) for [Fe(mbmi)₂(bpy)](PF₆)₂ were probed and are somewhat shorter than those for [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. [Fe(mbmi)₃](PF₆)₃ exhibits photoluminescence at 686 nm (²LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10^-4, compared to (3 ± 0.5) × 10^-4 for [Fe(btz)₃](PF₆)₃.

Yen Tran Hoang Hai

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