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

摘要

Ligand substitution on cis-Ru(PPh_3)_2(1,2-0_2C_6H_4)_2 gives cis-RuL~1L~2(1,2-0_2C_6H_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(3,4-0_2C_6H__2(5-0H)CO_2Me)_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(1,2-0_2C_6H_4)_2 was used to calculate the singlet—triplet free energy difference as 17.5-18.0 kJ/mol in toluene.