Synthesis and Characterization of Low-Spin and Cation Radical Complexes of Ruthenium(III) of a Tridentate Pyridine Bis-Amide Ligand

Using a tridentate bis-amide ligand 2,6-bis(N-phenylcarbamoyl)pyridine (H2L), in its deprotonated form, a new
mononuclear ruthenium(III) complex [Et4N][RuL2]·H2O (1) has been synthesized. Structural analysis reveals that
the RuN6 coordination comprises four deprotonated amide-N species in the equatorial plane and two pyridine-N
donors in the axial positions, imparting a tetragonally compressed octahedron around Ru. To the best of our
knowledge, this is the first time that a ruthenium(III) complex coordinated solely by two tridentate deprotonated
peptide ligands has been synthesized and structurally characterized. When examined by cyclic voltammetry, complex
1 displays in MeCN/CH2Cl2 solution three chemically/electrochemically reversible redox processes:  a metal-centered
reductive RuIII−RuII couple (E1/2 = −0.84/−0.89 V vs SCE) and two ligand-centered oxidative responses (E1/2 =
0.59/0.60 and 1.05/1.05 V vs SCE). Isolation of a dark blue one-electron oxidized counterpart of 1, [RuL2]·H2O (2),
has also been readily achieved. The complexes have been characterized by analytical, solution electrical conductivity,
IR, electronic absorption and EPR spectroscopy, and temperature-dependent magnetic susceptibility measurements.
For complex 1, a weak and broad transition within the t2g level has been identified at ∼1400 nm and supported
by EPR spectral analysis (S = 1/2). Temperature-dependent magnetic susceptibility data provide unambiguous
evidence that in 2 strong antiferromagnetic coupling of the S = 1/2 ruthenium atom with the S = 1/2 ligand π-cation
radical leads to an effectively S = 0 ground state (1H NMR spectra in CDCl3 solution).