How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron N-Heterocyclic Complexes: A Comparative Study

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  • Om Prakash
  • Pavel Chábera
  • Nidhi Kaul
  • Valtýr F. Hlynsson
  • Nils W. Rosemann
  • Iria Bolaño Losada
  • Yen Tran Hoang Hai
  • Ping Huang
  • Bendix, Jesper
  • Tore Ericsson
  • Lennart Häggström
  • Arvind Kumar Gupta
  • Daniel Strand
  • Arkady Yartsev
  • Reiner Lomoth
  • Petter Persson
  • Kenneth Wärnmark

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 FeIII-hexa-NHC [Fe(mbmi)3](PF6)3 and heteroleptic FeII-tetra-NHC [Fe(mbmi)2(bpy)](PF6)2 (bpy = 2,2′-bipyridine) complexes. They are compared to the reported FeIII-hexa-NHC [Fe(btz)3](PF6)3 and FeII-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 FeII/FeIII 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

Original languageEnglish
JournalInorganic Chemistry
Volume63
Issue number10
Pages (from-to)4461-4473
Number of pages13
ISSN0020-1669
DOIs
Publication statusPublished - 2024

Bibliographical note

Funding Information:
We thank the reviewers for their valuable comments. The Swedish Strategic Research Foundation (EM16-0067) and the Knut and Alice Wallenberg Foundation (2018.0074) are gratefully acknowledged for support. O.P. thanks the Carl Trygger Foundation for a postdoc fellowship. R.L. acknowledges financial support by the Swedish Research Council (VR 2020-05058). P.P. acknowledges support from the Swedish Research Council (VR 2021-05313), the e-science initiative eSSENCE, and the Swedish supercomputing facilities NSC and LUNARC through SNIC/NAIS allocations. K.W. acknowledges support from the Swedish Research Council (VR 2020-03207), the Swedish Energy Agency (Energimyndigheten, P48747-1), the LMK Foundation, and the Sten K Johnson Foundation.

Publisher Copyright:
© 2024 The Authors. Published by American Chemical Society.

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