Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties.

Mohamed Darari,Antonio Monari,Bogdan Marekha,Stefan Haacke,Philippe C Gros,Antonio Francés-Monerris,Abdelatif Doudouh,Emmanuel Wenger

doi:10.3390/molecules25245991

Abstract

The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes (C1 and C2), prepared from pyridylNHC and pyridylquinoline type ligands, respectively, have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For C1, it is shown that—despite the geometrical improvement—the excited state deactivation is faster than for the parent pseudo-octahedral C0 complex. This unexpected result is due to the increased ligand flexibility in C1 that lowers the energetic barrier for the relaxation of 3MLCT into the 3MC state. For C2, the effect of the increased ligand field is not strong enough to close the prominent deactivation channel into the metal-centered quintet state, as for other Fe-polypyridine complexes.

Highlights

Ligand-field splitting is an important parameter with a deep impact on the photophysics of transition metal-based complexes
The access to CCDC 1992132 (C1) required the synthesis of the pyridylimidazolium salt precursor L1
[14],[14], where two two excited states seemseem to coco-exist, and is rather at odds with the single excited state scenario we reported for tridentate exist, and is rather at odds with the single excited state scenario we reported for tridentate Fe–N-heterocyclic carbenes (NHC)

Summary

Introduction

Ligand-field splitting is an important parameter with a deep impact on the photophysics of transition metal-based complexes. The control of the ligand-field splitting is challenging in iron (II) polyimine complexes where the MLCT states, first populated upon photo excitation, undergo an ultrafast deactivation into the low-lying metal-centered (MC) states, leading to the loss of the suitable photophysical properties [6,7]. N-heterocyclic carbenes (NHC) ligands, ideally combining a strong σ-donating and a weak-to-moderate π-accepting character, have been reported to destabilize the MC states over the MLCT manifold, impressively slowing down the deactivation pathways. In this context, tridentate pyridyldicarbene ligands with appropriate modifications of both the NHC side and the central azine. Other ligands giving rise to more rigid have been reported by us and other groups as leading to have remarkable lifetimes in the [15] or different coordination architectures [16]

Methods

Results

Conclusion