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

A series of cyclometalated Fe­(II/III) and Ru­(II/III) complexes bearing a tetradentate dianionic [O^N^C^N] ligand (H2[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 1H 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 [RuII(O^N^C^N)­(XylNC)2] displays a weak and broad emission band (λem: 680 nm; τ: 0.27 μs), while the others are nonemissive. Density functional theory (DFT)/time-dependent DFT calculations revealed that 1dd excited states of the d6 [MII(O^N^C^N)­(PMe3)2] (M = Fe, Ru) are strongly destabilized. The [FeII(O^N^C^N)­(PMe3)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 (3MLCT) excited state (τ: 14 ps) of [FeII(O^N^C^N)­(PMe3)2] decays via a lower-lying 3dd excited state. For the d5 [MIII(O^N^C^N)­(PMe3)2]+ (M = Fe, Ru), the presence of low-lying 2dd excited states and doublet ligand-to-metal charge transfer (2LMCT) states with short lifetimes (τ: 11.7–12.6 ps) is suggested.