Half-sandwich η6-benzene Ru(II) complexes of phenolate-based pyridylalkylamine/alkylamine ligands: Synthesis, structure, and stabilization of one-electron oxidized species

Four half-sandwich ruthenium(II) complexes [(η6-C6H6)Ru(L1-O)][PF6] (1), [(η6-C6H6)Ru(L2-O)][PF6] (2), [(η6-C6H6)Ru(L3-O)][PF6] (3), [(η6-C6H6)Ru(L4-O)][PF6] (4a), and [(η6-C6H6)Ru(L4-O)][BPh4] (4b) [L1-OH, 4-nitro-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L2-OH, 2,4-di-tert-butyl-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L3-OH, 2,4-di-tert-butyl-6-{[2′-((pyridin-2-yl)benzylamino)methyl}-phenol; L4-OH, 2,4-di-tert-butyl-6-{[(2′-imethylaminoethyl)methylamino]methyl}-phenol (L4-OH)], supported by a systematically varied series of tridentate phenolate-based pyridylalkylamine and alkylamine ligands are reported. The molecular structures of 1–3, 4a, and 4b have been elucidated in solution using 1H NMR spectroscopy and of 1, 3, and 4b in the solid state by X-ray crystallography. Notably, due to coordination by the ligands the Ru center assumes a chiral center and in turn the central amine nitrogen also becomes chiral. The 1H NMR spectra exhibit only one set of signals, suggesting that the reaction is completely diastereoselective [1: SRu,SN/RRu,RN; 2: RRu,RN/SRu,SN; 3: SRu,RN/RRu,SN; 4b: SRu,RN/RRu,SN]. The crystal packing in 1 and 3 is stabilized by C–H…O interactions, in 4b no meaningful secondary interactions are observed. From the standpoint of generating phenoxyl radical, as investigated by cyclic voltammetry (CV), complex 1 is redox-inactive in MeCN solution. However, 2, 3, and 4a generate a one-electron oxidized phenoxyl radical coordinated species [2]2+radical dot, [3]2+radical dot, and [4a]2+radical dot, respectively. The radical species are characterized by CV, UV–Vis, and EPR spectroscopy. The stability of the radical species has been determined by measuring the decay constant (UV–Vis spectroscopy).