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

Abstract

Diarylamido, Q-N--Py (L-), complexes of ruthenium(II), trans-[(L-H+)RuII(PPh3)2Cl2] (1-H+) and trans-[(L-)RuII(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)2Cl2] (1·CH2Cl2). The ground electronic state of 1 is delocalized as [(L-)RuIII ↔ (L•)RuII] (L• is an aminyl radical of type Q-N•-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 kD2O-CH2Cl2 > kH2O-CH2Cl2 > kCH2Cl2. In contrast, 2 containing the corresponding amido (L-) is stable and endures oxidation at 0.14 V, affording trans-[(L•)RuII(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 CQ-N (1.401(2) Å) and CPy-N (1.394(2) Å) lengths in 1·CH2Cl2 are relatively longer than the CQ-Namido (1.396(4) Å)and CPy-Namido (1.372(4) Å) 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-)RuIII] and [(L•)RuII] 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.