Proton-Coupled Oxidation of a Diarylamine: Amido and Aminyl Radical Complexes of Ruthenium(II)

Diarylamido, Q-N--Py (L-), complexes of ruthenium(II), trans-[(L-H+)Ru-II(PPh3)(2)Cl-2] (1(-)H(+)) and trans-[(L-)Ru-II(PPh3)(2)(CO)Cl] (2), using N-(5-nitropyridin-2-yl)quinolin-8-amine (HL) as a ligand are disclosed (Q and Py refer to quinoline and 5-nitropyridine fragments). 1(-)H(+) contains a zwitterionic amido ligand (Q-N--PyH+) that undergoes a concerted proton electron transfer (CPET) reaction in air, generating trans-[(L)Ru(PPh3)(2)Cl-2] (1 center dot CH2Cl2). The ground electronic state of 1 is delocalized as [(L-)Ru-III <--> (L-center dot)Ru-II] (L-center dot is an aminyl radical of type Q-N-center dot-Py). The 1(-)H(+)/1 redox potential depends on the electrolytes, and the potentials are -1.57 and -1.40 V, respectively, in the presence of [N( n-Bu)(4)]PF6 and [N( n-Bu)(4)]Cl. The rate of 1(-)H(+) -> 1 conversion depends also on the medium and follows the order k(D2O-CH2Cl2) > k(H2O-CH2Cl2) > k(CH2Cl2). In contrast, 2 containing the corresponding amido (L-) is stable and endures oxidation at 0.14 V, affording trans-[(L-center dot)Ru-II(PPh3)(2)(CO)Cl] ((2)+). The electronic structures of the complexes were authenticated by single-crystal X-ray diffraction studies of HL, 1.CH2Cl2, and 2.(toluene), EPR spectroscopy, and density functional theory (DFT) calculations. Notably, the C-Q-N (1.401(2) angstrom) and C-Py-N (1.394(2) angstrom) lengths in 1.CH2Cl2 are relatively longer than the C-Q-N-amido (1.396(4) angstrom)and C-Py-N-amido (1.372(4) angstrom) lengths in 2.(toluene). Spin density obtained from DFT calculations scatters on both N and ruthenium atoms, revealing a delocalized state of 1. The notion was further confirmed by variable-temperature EPR spectra of a powder sample and CH2Cl2 solution, where the contributions of both [(L-)Ru-III] and [(L-center dot)Ru-II] components were detected. In contrast, 2(+) is an aminyl radical complex of ruthenium(II), where the spin is dominantly localized on the ligand backbone (64%), particularly on N (27%). 2(+) exhibits a strong EPR signal at g = 2.003. 1 and 2(+) exhibit absorption bands at 560-630 and 830-840 nm, and the origins of these excitations were elucidated by TDDFT calculations on 1 and 2 in CH2Cl2.