Cyclometalated Iron and Ruthenium Complexes Supported by a Tetradentate Ligand Scaffold with Mixed O, N, and C Donor Atoms: Synthesis, Structures, and Excited-State Properties

Abstract

A series of cyclometalated Fe(II/III) and Ru(II/III) complexes bearing a tetradentate dianionic [O^N^C^N] ligand (H 2 [O^N^C^N] = 2-(6-(3-(pyridin-2-yl)phenyl)pyridin-2-yl)phenol) was synthesized and structurally characterized. The strong-field dianionic [O^N^C^N] ligand enforces all of these complexes in a low-spin state at 298 K as revealed by 1 H NMR, magnetic susceptibility, and electron paramagnetic resonance (EPR) measurements. A 77 K 2-MeTHF (THF = tetrahydrofuran) glassy solution of the bis(2,6-dimethylphenyl isocyanide) Ru(II) complex [Ru II (O^N^C^N)(XylNC) 2 ] displays a weak and broad emission band (λ em : 680 nm; τ: 0.27 μs), while the others are nonemissive. Density functional theory (DFT)/time-dependent DFT calculations revealed that 1 dd excited states of the d 6 [M II (O^N^C^N)(PMe 3 ) 2 ] (M = Fe, Ru) are strongly destabilized. The [Fe II (O^N^C^N)(PMe 3 ) 2 ] exhibits panchromatic absorption up to 850 nm. A combined study of femtosecond transient absorption (fs-TA), spectro-electrochemistry, and theoretical calculation revealed that the possible triplet metal-to-ligand charge transfer ( 3 MLCT) excited state (τ: 14 ps) of [Fe II (O^N^C^N)(PMe 3 ) 2 ] decays via a lower-lying 3 dd excited state. For the d 5 [M III (O^N^C^N)(PMe 3 ) 2 ] + (M = Fe, Ru), the presence of low-lying 2 dd excited states and doublet ligand-to-metal charge transfer ( 2 LMCT) states with short lifetimes (τ: 11.7− 12.6 ps) is suggested.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…This illustrates that well-established principles for precious metal compounds are not necessarily applicable to the first-row of transition metals, here in this case mainly because the respective metal orbital energies differ strongly . Computational studies forecasted this effect, and furthermore predicted that cyclometalating ligands could lead to long-lived MLCT states. ,− Synthetic advances now provide access to such cyclometalated Fe II complexes, ,, and in one case led to the claim of an emissive MLCT state in solution at room temperature (Figure d) . The energy gap between this MLCT state and the electronic ground state is only 1.1 eV, whereas Ru II and Os II polypyridines with comparable energy gaps are usually nonemissive. , Thus, the results reported for this Fe II compound seem surprising, particularly for an excited state that is substantially distorted, as the apparent Stokes shift of ∼5000 cm –1 and the emission bandwidth of ∼1300 cm –1 imply …”

“…96 Computational studies forecasted this effect, 119 and furthermore predicted that cyclometalating ligands could lead to long-lived MLCT states. 61,64−68 Synthetic advances now provide access to such cyclometalated Fe II complexes, 6,72,120 and in one case led to the claim of an emissive MLCT state in solution at room temperature (Figure 6d). 74 The energy gap between this MLCT state and the electronic ground state is only 1.1 eV, whereas Ru II and Os II polypyridines with comparable energy gaps are usually nonemissive.…”

“…Site-selective deuteration experiments showed that the C−H/C−D oscillators in immediate proximity to the metal core have the biggest influence, and furthermore the energetic match between vibrational overtones and the photoactive excited state is a key factor (Table 5, entry 5). 114,120 The isocyanide ligands needed to obtain emissive Cr 0 and Mn I complexes feature high-energy CN stretching vibrations (∼1950 cm −1 ) in immediate proximity to the metal core, 83,85,89,90,92,94,145 which could represent an inherent disadvantage of this class of compounds.…”

“…This strategy has long been applied to lanthanide complexes as well as 4d 6 and 5d 6 polypyridines. , Recent work demonstrated that it can also be very relevant for first-row transition metal compounds, ,− though 3d 6 complexes played no important role until now. Site-selective deuteration experiments showed that the C–H/C–D oscillators in immediate proximity to the metal core have the biggest influence, and furthermore the energetic match between vibrational overtones and the photoactive excited state is a key factor (Table , entry 5). , The isocyanide ligands needed to obtain emissive Cr 0 and Mn I complexes feature high-energy CN stretching vibrations (∼1950 cm –1 ) in immediate proximity to the metal core, ,,,,,, which could represent an inherent disadvantage of this class of compounds.…”

Electronic Structure and Geometry of the Lowest 2LMCT State of Fe(III) Potential Fluorescent Emitters

“…In the same years, their potential as dyes for dye-sensitized solar cells was also reported, in which they would advantageously stand in for ruthenium . In case facile or even spontaneous Fe­(II) → Fe­(III) oxidation should be avoided, the use of electron-withdrawing substituents could moderate this tendency ,, and stabilize the Fe­(II) form, but in the absence of electroattracting groups, both Fe­(II) complexes − and Fe­(III) complexes − have been isolated.…”