How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron <i>N</i>-Heterocyclic Complexes: A Comparative Study
Om Prakash, Pavel Chábera, Nidhi Kaul, Valtýr Freyr Hlynsson, Nils W. Rosemann, Iria Bolaño Losada, Yen Tran Hoang Hai, Ping Huang, Jesper Bendix, Tore Ericsson, Lennart Häggström, Arvind Kumar Gupta, Daniel Strand, Arkady Yartsev, Reiner Lomoth, Petter Persson, Kenneth Wärnmark
Abstract
High Resolution Image Download MS PowerPoint Slide 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 .