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 (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 H-1 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 (lambda(em): 680 nm; tau: 0.27 mu s), 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)(PMe3)(2)] (M = Fe, Ru) are strongly destabilized. The [Fe-II(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 ((MLCT)-M-3) excited state (tau: 14 ps) of [Fe-II(O<^>N<^>C<^>N)(PMe3)(2)] decays via a lower-lying (3)dd excited state. For the d(5) [M-III(O<^>N<^>C<^>N)(PMe3)(2)](+) (M = F; Ru), the presence of low-lying (2)dd excited states and doublet ligand-to-metal charge transfer ((LMCT)-L-2) states with short lifetimes tau: 11.7-12.6 ps) is suggested.