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

Ligand substitution on cis-Ru(PPh(3))(2)(1,2-O(2)C(6)H(4))(2) gives cis-RuL(1)L(2)(1,2-O(2)C(6)H(4))(2) (L(1) = PPh(3), L(2) = P(OPh)(3), PBu(3); L(1) = L(2) = PBu(3), P(OMe)(3)). Syntheses of cis-Ru(PPh(3))(2)(3,4-O(2)C(6)H(2)(5-OH)CO(2)Me)(2) and cis-Ru(PPh(3))(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 cis-Ru(PPh(3))(2)(1,2-O(2)C(6)H(4))(2) was used to calculate the singlet-triplet free energy difference as 17.5-18.0 kJ/mol in toluene.