Paramagnetic Effects on the NMR Spectra of Diamagnetic Ruthenium (bis-phosphine)(bis-semiquinone) Complexes

Ligand substitution on cis-Ru(PPh3)(1,2-O2C6H4)(2) gives cis-(RuLL2)-L-1(1-2-O2C6H4)(2) (L-1 = PPh3, L-2= P(OPh)(3); L-1 = L-2 = PBu3, P(OMe)(3))- Syntheses of cis-Ru(PPh3)(2)(3,4-O2C6H2(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 cis-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.