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

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.

Ultrafast Spectroscopy of Fe(II) Complexes Designed for Solar‐Energy Conversion: Current Status and Open Questions

“…Combined with the π deficiency and MLCT stabilization due to improved ligand conjugation, ESLs in the ns range were achieved, as required for photochemical applications. In addition to these design principles relevant for the ground state geometry of the complexes, it is clear that the ESL of MLCT states is also subject to the ligand structural flexibility [40,41,57] since the MLCT-MC relaxation implies a lengthening of the Fe-ligand distances. This parameter is more difficult to control by design but will most probably be addressed in future work.…”

“…Some recent papers reporting on ultrafast ES branching or the simultaneous population of two excited states used global fitting and target analysis (GF-TA) to determine the branching ratio. [29,57,86] GF-TA is a method for testing excited state "connectivity schemes". [87] Indeed, global fitting provides decayassociated difference spectra (DADS), meaning the spectral distribution of the amplitudes associated with an observed lifetime, when the data can be fitted by the following expression, assuming wavelength-independent lifetimes τ i :…”

Bidentate Pyridyl‐NHC Ligands: Synthesis, Ground and Excited State Properties of Their Iron(II) Complexes and the Role of the fac/mer Isomerism

“…All these exciting results have thus set up a new paradigm for Earth‐abundant transition‐metal complexes, gathering an increasing interest of the scientific community due to the new possibilities for the development of truly sustainable photoactive materials. In the case of iron complexes, while different design strategies have been reported, more systematic studies are missing so as to provide a clearer picture of the photophysics and photochemistry of this class of organometallic compounds, where the inherent excited state electronic structure and the structural relaxation/flexibility are intimately linked [47,48] . In this microreview we will present our recent investigations on a series of Fe II ‐NHC complexes with bidentate ligands featuring facial and meridional isomers.…”

“…In the case of iron complexes, while different design strategies have been reported, more systematic studies are missing so as to provide a clearer picture of the photophysics and photochemistry of this class of organometallic compounds, where the inherent excited state electronic structure and the structural relaxation/flexibility are intimately linked. [47,48] In this microreview we will present our recent investigations on a series of Fe II -NHC complexes with bidentate ligands featuring facial and meridional isomers. Their synthesis and characterization will be described, including ultrafast transient absorption spectroscopy and Time Dependent-Density Functional Theory (TD-DFT) modelling.…”

Higher MLCT lifetime of carbene iron(ii) complexes by chelate ring expansion

“…The combination of four carbenes with six-membered chelate rings in iron(II) complexes with tridentate meridionally coordinating CNC ligands only provided low MLCT lifetimes of around 1 ps of the majority of the excited state population which has been ascribed to a higher flexibility of this CNC ligand. 31 Here, we combine high octahedricity with p-accepting pyridine and s-donating carbene ligands in a more rigid coordination environment to increase the 3 MLCT 3 MLCT lifetimes below 0.1 ps. 17 We demonstrate that the more octahedral and rigid complex geometry increases the lifetime even beyond that of a [C 4 N 2 ] donor set in distorted carbene pyridine iron(II) complexes (t = 9, 8.1, o0.3 ps for [Fe(CNC) 2 ] 2+ with Me, i Pr, t Bu N-substituents).…”