Pseudo-Octahedral Iron(II) Complexes with Near-Degenerate Charge Transfer and Ligand Field States at the Franck-Condon Geometry

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⁺ (1²⁺ ) and [Fe(cpmp)(ddpd)]²⁺ (2²⁺ ) 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 1²⁺ and 2²⁺ 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.

Keywords: iron; photophysics; polypyridine ligands; time-resolved spectroscopy; tridentate ligands.

Scheme 1

Selected iron(II) complexes with tridentate N/C ligands with improved absorptivity and prolonged ³MLCT lifetimes via increasing the ^3/5MC state energies and/or lowering the ³MLCT state energies and the iron(III) complex H⁺ .

Scheme 2

Synthesis of the a) homoleptic and b) heteroleptic iron(II) complexes 1[PF₆] and 2[PF₆]₂ with the push‐pull ligand cpmp. Accepting CO and donating NMe units colored red and blue, respectively.

Figure 1

Molecular structures of a) 1²⁺ (1A²⁺ , 1B²⁺ ) and b) 2²⁺ (2A²⁺ , 2B²⁺ ) determined by single crystal XRD analyses. Thermal ellipsoids set at 50 % probability. Hydrogen atoms are omitted for clarity. Atom numbering differs from cif file numbering but fits to Table 1 for better comparability.

Figure 2

UV/Vis/NIR absorption spectra of 1[PF₆]₂ (blue) and 2[PF₆]₂ (red) in acetonitrile at 298 K.

Figure 3

a) Cyclic voltammograms of 1[PF₆]₂ (blue) and 2[PF₆]₂ (red), 1 mm in acetonitrile, 0.1 m [ ⁿ Bu₄N][PF₆], 100 mV s⁻¹ and DFT optimized geometries and spin densities (isosurface at 0.012 a. u.) of b) the iron(III) complex 1³⁺ and c) the radical ion 1⋅⁺ .

Figure 4

a) IR absorption spectra and b) UV/Vis/NIR absorption spectra of 1[PF₆]₂ in acetonitrile with 0.1 m [ ⁿ Bu₄N][PF₆] at 298 K collected during electrochemical oxidation.

Figure 5

X‐band EPR spectra at 77 K of a) 1⋅⁺ in acetonitrile and b) 1³⁺ in butyronitrile. Simulation of the spectrum with the parameters indicated shown in red. The asterisk denotes a baseline artifact.

Figure 6

a) fs‐TA spectra of 1[PF₆]₂ in acetonitrile upon excitation at 350 nm (400 nJ/pulse) and b) corresponding time traces at 500 nm (green), 610 nm (orange) and 760 nm (red‐brown).

Figure 7

a) DFT calculated potential energy diagram of 1²⁺ including the ³MLCT (green), ³MC (orange), ⁵MC (red) and ¹GS (black) along the symmetric Fe−N stretching mode as simplified reaction coordinate. The ¹MLCT minimum is estimated from the experimental absorption spectrum (blue ⊗) and this curve parallels that of the ³MLCT state for illustration. The energies of fully optimized ¹GS, ³MLCT, ³MC and ⁵MC geometries are indicated by ⊗ in black, green, orange and red, respectively. Processes assigned to the experimental time constants τ ₁–τ ₃ are indicated with dashed purple arrows. Fully DFT optimized geometries and spin densities of b) the ³MLCT, c) the ³MC and d) the ⁵MC states of 1²⁺ are displayed at an isosurface value at 0.012 a.u.