Mixed-valence di-ruthenium(ii,iii) complexes of redox non-innocent N-aryl-o-phenylenediamine derivatives

Abstract

A mononuclear ruthenium(II), [(L₁^IQ)Ru²⁺(PPh₃)₂Cl₂]·CHCl₃ (1·CHCl₃), a di-ruthenium(II,II), [(L₂^IQ)₂Ru₂⁴⁺Cl₄(PPh₃)₂] (2) and a mixed-valence di-ruthenium(II,III), [(L₃^IQ)Ru₂⁵⁺Cl₅(PPh₃)₂]·MeOH (3·MeOH) complex, where L₁^IQ, L₂^IQ and L₃^IQ are, respectively, o-diiminobenzoquinone forms of redox non-innocent N-(5-nitropyridyl)-o-phenylenediamine (L₁H₂), N-(2,4-dinitrophenyl)-o-phenylenediamine (L₂H₂) and N-(3-nitropyridyl)-o-phenylenediamine (L₃H₂) derivatives, were successfully isolated. The molecular and electronic structures of 1·CHCl₃, 2 and 3·MeOH were confirmed by single-crystal X-ray crystallography, EPR, UV-Vis-NIR spectroscopies and density functional theory (DFT) calculations. Both 1·CHCl₃ and 2 exhibited reversible anodic waves due to the Ru(III)/Ru(II) redox couple, while the cyclic voltammogram of 3·MeOH displayed two successive cathodic waves due to ruthenium(III)/ruthenium(II) and (L₃^IQ/L₃^ISQ) redox couples. EPR spectroscopy and DFT calculations inferred that 1⁺ is a ruthenium(III) complex of L₁^IQ, [(L₁^IQ)Ru³⁺(PPh₃)₂Cl₂]⁺, 2⁺ is a mixed-valence di-ruthenium(II,III) complex of L₂^IQ, [(L₂^IQ)₂Ru₂⁵⁺Cl₄(PPh₃)₂]⁺ and 3⁻ is a di-ruthenium(II,II) complex, [(L₃^IQ)Ru₂⁴⁺Cl₅(PPh₃)₂]⁻, while 3²⁻ is a di-ruthenium(II,II) complex of L₃^ISQ, [(L₃^ISQ)Ru₂⁴⁺Cl₅(PPh₃)₂]²⁻, where L₃^ISQ is the o-diiminobenzosemiquinonate anion radical form of the L₃H₂ with a significant contribution of the mixed-valence di-ruthenium(III,II) form, [(L₃^AM)Ru₂⁵⁺Cl₅(PPh₃)₂]²⁻ (L₃^AM is the di-amido form of the L₃H₂ ligand). The spin density obtained from the Mulliken spin population analyses of the broken-symmetry (BS) DFT calculations was localized on Ru(2) in 3, while it was delocalized over both Ru(1) and the o-phenylenediamine fragment in 3²⁻. The inter-valence charge-transfer (IVCT) transitions of 2⁺ and 3 were relatively weaker and occurred at 1680 and 1685 nm, respectively, while 3²⁻ exhibited a broader NIR absorption band at 1500–2000 nm. The calculated electronic-coupling parameters (H_ab) using the Mulliken–Hush equation for 3 and 2⁺ ion, respectively, were 62 and 103 cm⁻¹, defining these as Robin–Day class II mixed-valence systems.