Pseudo‐Octahedral Iron(II) Complexes with Near‐Degenerate Charge Transfer and Ligand Field States at the Franck‐Condon Geometry. Issue 57 (10th August 2022)

• Record Type: Journal Article
• Authors: Moll, Johannes; Naumann, Robert; Sorge, Lukas; Förster, Christoph; Gessner, Niklas; Burkhardt, Lukas; Ugur, Naz; Nuernberger, Patrick; Seidel, Wolfram; Ramanan, Charusheela; Bauer, Matthias; Heinze, Katja
• Is Part Of: Chemistry. Volume 28:Issue 57(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-10
• Subjects: iron -- photophysics -- polypyridine ligands -- time-resolved spectroscopy -- tridentate ligands; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202201858
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Increasing the metal‐to‐ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo‐ and heteroleptic complexes [Fe(cpmp)2 ] 2+ (1 2+ ) and [Fe(cpmp)(ddpd)] 2+ (2 2+ ) with the tridentate ligands 6, 2''‐carboxypyridyl‐2, 2'‐methylamine‐pyridyl‐pyridine (cpmp) and N, N '‐dimethyl‐ N, N '‐di‐pyridin‐2‐ylpyridine‐2, 6‐diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal‐ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six‐membered chelate rings. The push‐pull ligand cpmp provides intra‐ligand and ligand‐to‐ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 1 2+ and 2 2+ were accessed by X‐ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X‐ray emission spectroscopy, static and time‐resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations. Abstract : Iron(II) complexes with 6, 2''‐carboxypyridyl‐2, 2'‐methylamine‐pyridyl‐pyridine ligands possess low‐energy 1 MLCT states. 3 MLCT and 3/5 MC excited states are nearly degenerate at the Franck‐Condon geometry. Expansion of the coordination sphere lowers the 3/5 MC energies shortening the 3 MLCT lifetime. Cooling and solvent reorganization occurs within 33 ps to give the relaxed 5 MC state with ca. 0.5 ns lifetime.