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Exploring the Noninnocent Character of Electron Rich π‑Extended 8‑Oxyquinolate Ligands in Ruthenium(II) Bipyridyl Complexes

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journal contribution
posted on 02.06.2014, 00:00 by Stephanie Bellinger-Buckley, Tse-Cing Chang, Seema Bag, David Schweinfurth, Weihong Zhou, Bela Torok, Biprajit Sarkar, Ming-Kang Tsai, Jonathan Rochford
A series of ruthenium polypyridyl complexes are presented incorporating π-extended electron rich derivatives of the 8-oxyquinolate (OQN) ligand. The π-donating property of the OQN ligand introduces covalent character to the Ru­(dπ)–OQN­(π) bonding scheme enhancing its light harvesting properties and diversifying its redox properties, relative to the classic ruthenium­(II) trisbipyridyl complex [Ru­(bpy)3]2+. Synthesis and characterization is presented for the complexes [Ru­(bpy)2­(R-OQN)]­(PF6), where bpy = 2,2′-bipyridine and R = 5-phenyl, 5,7-diphenyl, 2,4-diphenyl, 5,7-bis­(4-methoxyphenyl), 5,7-bis­(4-(diphenylamino)­phenyl). A comprehensive bonding analysis is presented for the [Ru­(bpy)2(OQN)]+ system illustrating the origin of its unique spectroscopic and redox properties relative to [Ru­(bpy)3]2+. This model is then extended to enable a consistent interpretation of spectra and redox properties for the π-extended [Ru­(bpy)2(R-OQN)]­(PF6) series. Electronic structures have been probed experimentally by a combination of electrochemical and spectroscopic techniques (UV–vis–NIR absorption, emission, EPR spectroscopy) where (metal–ligand)-to-ligand (MLLCT) charge-transfer properties are described by time dependent-density functional theory (TD-DFT) analysis, at the B3LYP/6-31g­(d,p) level of approximation. Substantial mixing, due to bonding and antibonding combinations of Ru­(dπ) and OQN­(π) orbitals, is observed at the HOMO and HOMO – 3 levels for the ruthenium–oxyanion bond in [Ru­(bpy)2(OQN)]+, which is responsible for the low-energy MLLCT based electronic transition and destabilization of the HOMO level viz. cyclic voltammetry. This noninnocent π-bonding phenomenon is consistent throughout the series which allows for controlled tuning of complex redox potentials while maintaining panchromatic absorption properties across the visible spectrum. Extensive charge delocalization is observed for the one-electron oxidized species using a combination of UV–vis–NIR, EPR spectroelectrochemistry, and Mulliken spin-density analysis, giving strong evidence for hole-delocalization across the delocalized Ru­(dπ)–OQN­(π) system, in particular for the electron rich 5,7-bis­(4-methoxyphenyl) and 5,7-bis­(4-(diphenylamino)­phenyl) systems.