Structures and Properties of Ruthenium(II) Complexes of Pyridylamine Ligands with Oxygen-Bound Amide Moieties: Regulation of Structures and Proton-Coupled Electron Transfer

Tris(2-pyridylemthyl)amine (TPA) derivatives having two amide moieties at the 6-positions of the two pyridine rings
of TPA and their Ru(II) complexes were synthesized and characterized by spectroscopic methods, X-ray
crystallography, and electrochemical measurements. The complexes prepared were [RuCl(L)]PF6 (L = N,N-bis(6-(1-naphthoylamide)-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (1), N,N-bis(6-(2-naphthoylamide)-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (2), N,N-bis(6-(isobutyrylamide)-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (3)); the crystal
structures of the three compounds were established by X-ray crystallography. In variable-temperature 1H NMR
spectra of 1 and 2 in CD3CN solutions, the π−π stacking in 1 was too rigid to exhibit any fluxional motions in NMR
measurements; however, the π−π stacking of 2 was weaker and showed fluxional behavior in nearly T-shaped
π−π interaction for the 2-naphthly groups (ΔH° = −2.3 kJ mol-1; ΔG° = −0.9 kJ mol-1 and ΔS° = −7.7 J
mol-1 K-1 at 233 K in CD3CN). For each of these three complexes, one of the amide moieties coordinated to the
Ru(II) center through an amide oxygen. The other uncoordinated amide N−H formed intramolecular hydrogen
bonding which remained intact even in aqueous media, indicating the intramolecular hydrogen bonding was
geometrically compelled to form. The amide coordination is also stabilized and strengthened by the hydrogen
bonding, so that the structure of each compound is maintained in solution. It is suggested that this hydrogen
bonding lowers the redox potentials of the Ru(II) centers due to polarization of the coordinated amide CO bond,
in which the oxygen atom becomes more electrostatically negative and its electron-donating ability is strengthened.
The N−H protons in the coordinated amide moieties were found to undergo a reversible deprotonation−protonation
process, and the redox potentials of the Ru(II) centers could be regulated in the range of 500 mV in CH3CN
solutions. The Pourbaix diagram for 1 clearly showed that this proton-coupled redox behavior is a one-electron/one-proton process, and the pKa value was estimated to be ∼6.