Synthesis and characterization of novel ferrocene-containing pyridylamine ligands and their ruthenium(II) complexes: Electronic communication through hydrogen-bonded amide linkage

Tris(2-pyridylmethyl)amine (TPA) derivatives with one or two ferrocenoylamide moieties at the 6-position of one or two pyridine rings of TPA were synthesized. The compounds, N-(6-ferrocenoylamide-2-pyridylmethyl)-N,N- bis(2-pyridylmethyl)amine (Fc-TPA; L1) and N,N-bis(6-ferrocenoylamide-2- pyridylmethyl)-N-(2-pyridylmethyl)amine (Fc₂-TPA; L2), were characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. Their Ru(II) complexes were also prepared and characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. [RuCl(L1)(DMSO)]PF₆ (1) that contains S-bound dimethyl sulfoxide (DMSO) as a ligand and an uncoordinated ferrocenoylamide moiety exhibited two redox waves at 0.23 and 0.77 V (vs ferrocene/ferrocenium ion as 0 V), due to Fc/Fc⁺ and Ru(II)/Ru(III) redox couples, respectively. [RuCl(L2)]PF₆ (2) that contains both coordinated and uncoordinated amide moieties showed two redox waves that were observed at 0.27 V (two electrons) and 0.46 V (one electron), assignable to Ru(II)/Ru(III) redox couples overlapped with the uncoordinated Fc/Fc⁺ redox couple and the coordinated Fc/Fc⁺, respectively. In contrast to 2, an acetonitrile complex, [Ru(L2)(CH₃CN)](PF₆)₂ (3), exhibited three redox couples at 0.26 and 0.37 V for two kinds of Fc/Fc⁺ couples, and 0.83 V for the Ru(II)/Ru(III) couple (vs ferrocene/ferrocenium ion as 0 V). In this complex, the redox potentials of the coordinated and the uncoordinated Fc-amide moieties were discriminated in the range of 0.11 V. Chemical two-electron oxidation of 1 gave [Ru^IIICl(L1 ⁺)(DMSO)]³⁺ to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm^-1 (H = -JS₁S ₂) which was estimated by its variable-temperature electron spin resonance (ESR) spectra in CH₃CN. The electron spins at the Ru(III) center and the Fe(III) center are ferromagnetically coupled via an amide linkage. In the case of 2, its two-electron oxidation gave the same ESR spectrum, which indicates formation of a similar triplet state. Such electronic communication may occur via the amide linkage forming the intramolecular hydrogen bonding.