Paramagnetic effects on the NMR spectra of "diamagnetic" ruthenium (bis-phosphine)(bis-semiquinone) complexes

Article abstract of DOI:10.1021/ic802302v

Ligand substitution on cis-Ru(PPh₃)2(1,2-O₂C ₆H₄)₂ gives cis-RuL¹L ²(1,2-O₂C₆H₄)2 (L¹ = PPh₃, L² = P(OPh)₃, PBu₃; L ¹ = L² = PBu₃, P(OMe)₃). Syntheses of cis-Ru(PPh₃)₂(3,4-O₂C₆H ₂(5-OH)CO₂Me)₂ and cis-Ru(PPh₃) ₂(AGSQ)₂ (AGSQ = the semiquinone derived from 1,2,3-trihydroxyanthracene-9, 10-dione) are also reported. The upfield chemical shifts and line broadening of the semiquinone 4,5-proton resonances in the NMR spectra indicate that these complexes, while having no detectable magnetic moments, have a weak, temperature-, ligand- and solvent-variable residual paramagnetism, not previously recognized in this series. Density functional theory (DFT) calculations predict a low-lying triplet state, about 14 kJ/mol (0.15 eV) above the singlet ground state. The paramagnetic effects on the NMR spectra are attributed to singlet-triplet equilibria. Temperature dependence of the proton resonances of the semiquinone rings of c/s-Ru(PPh₃) ₂(1,2-O₂C₆H₄)₂ was used to calculate the singlet-triplet free energy difference as 17.5-18.0 kJ/mol in toluene.