Synthesis and characterization of low-spin and cation radical complexes of ruthenium(III) of a tridentate pyridine bis-amide ligand

Abstract

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 Ru^III-Ru^II couple (E_½ = -0.84/-0.89 V vs SCE) and two ligand-centered oxidative responses (E_½ = 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 t_2g level has been identified at ~1400 nm and supported by EPR spectral analysis (S =½). Temperature-dependent magnetic susceptibility data provide unambiguous evidence that in 2 strong antiferromagnetic coupling of the S =½ ruthenium atom with the S =½ ligand π-cation radical leads to an effectively S = 0 ground state (¹H NMR spectra in CDCl₃ solution).