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 ](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 .