A Tetracarbene Iron(II) Complex with a Long‐lived Triplet Metal‐to‐Ligand Charge Transfer State due to a Triplet‐Triplet Barrier

• Published in: Angewandte Chemie International Edition 2024-09, Vol.63 (39), p.e202406438-n/a
• Main Authors: Reuter, Thomas, Zorn, Dimitri, Naumann, Robert, Klett, Jan, Förster, Christoph, Heinze, Katja
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; carbene ligands; Charge transfer; Donors (electronic); Heavy metals; Iron; Isomers; Ligands; Marcus theory; Metals; pyridine ligands; Quantum chemistry; transient absorption spectroscopy; Transition metals
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.202406438

Mixed N‐heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth‐abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal‐to‐ligand charge transfer state (3MLCT), which typically decays via a triplet metal‐centered (3MC) state back to the ground state. We reveal by variable‐temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans‐ and cis‐[Fe(pdmi)2]2+ with four NHC donors show 3MLCT→3MC population transfers with very different barriers and rationalize this by computational means. While trans‐[Fe(pdmi)2]2+ possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm−1, which leads to a nanosecond 3MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi‐classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron. The 3MLCT→3MC barrier determines the excited state lifetime of iron(II) photosensitizers. In the specifically designed tetracarbene iron(II) complex with cis‐coordinated pyridines, a single Fe−N bond elongates in the excited triplet states. Variable‐temperature ultrafast transient absorption spectroscopy delivered the barrier and the electronic coupling between 3MLCT and 3MC states providing novel design criteria for iron(II) sensitizers.