How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron N-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-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, (LMCT)-L-2) and [Fe(mbmi)(2)(bpy)](PF6)(2) at 410 and 616 nm (metal-to-ligand charge transfer, (MLCT)-M-3). Lifetimes of 57.3 ps ((LMCT)-L-2) for [Fe(mbmi)(3)](PF6)(3) and 7.6 ps ((MLCT)-M-3) 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 ((LMCT)-L-2) in acetonitrile at room temperature with a quantum yield of (1.2 +/- 0.1) x 10(-4), compared to (3 +/- 0.5) x 10(-4) for [Fe(btz)(3)](PF6)(3).