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 [RuIIICl(L1+)(DMSO)]3+ to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm-1
(H = −JS1S2) 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.