Abstract
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)](PF(6))(3) and heteroleptic Fe(II)-tetra-NHC [Fe(mbmi)(2)(bpy)](PF(6))(2) (bpy = 2,2'-bipyridine) complexes. They are compared to the reported Fe(III)-hexa-NHC [Fe(btz)(3)](PF(6))(3) and Fe(II)-tetra-NHC [Fe(btz)(2)(bpy)](PF(6))(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)](PF(6))(3) and [Fe(mbmi)(2)(bpy)](PF(6))(2) are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)(3)](PF(6))(3) and [Fe(btz)(2)(bpy)](PF(6))(2). The Fe(II)/Fe(III) redox couples of [Fe(mbmi)(3)](PF(6))(3) (-0.38 V) and [Fe(mbmi)(2)(bpy)](PF(6))(2) (-0.057 V, both vs Fc(+/0)) are less reducing than [Fe(btz)(3)](PF(6))(3) and [Fe(btz)(2)(bpy)](PF(6))(2). The two complexes show intense absorption bands in the visible region: [Fe(mbmi)(3)](PF(6))(3) at 502 nm (ligand-to-metal charge transfer, (2)LMCT) and [Fe(mbmi)(2)(bpy)](PF(6))(2) at 410 and 616 nm (metal-to-ligand charge transfer, (3)MLCT). Lifetimes of 57.3 ps ((2)LMCT) for [Fe(mbmi)(3)](PF(6))(3) and 7.6 ps ((3)MLCT) for [Fe(mbmi)(2)(bpy)](PF(6))(2) were probed and are somewhat shorter than those for [Fe(btz)(3)](PF(6))(3) and [Fe(btz)(2)(bpy)](PF(6))(2). [Fe(mbmi)(3)](PF(6))(3) exhibits photoluminescence at 686 nm ((2)LMCT) 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)](PF(6))(3).