1H NMR study of the solvolysis of the paramagnetic tetrachloro-bis(imidazole)ruthenium(III) anion in water, methanol, and dimethyl sulfoxide

The 1 H NMR signals of the Ru(III) species present in solution are considerably broadened and shifted by paramagnetism, but they can be used to follow chloride displacement in the trans-[RuCl 4 Im 2 ] − ion. This anion remains predominant for several hours at room temperature in D 2 O, but its signals are progressively replaced by those of a monoaqua [RuCl 3 (D 2 O)Im 2 ] complex. Over a period of days, two new sets of peaks appear, corresponding to two isomers of [RuCl 2 (D 2 O) 2 Im 2 ] + . The same behaviour is observed for the 1-methyl-and 4-methylimidazole analogues. These reactions can be driven backwards by addition of KCl, but [RuCl 4 Im 2 ] − is not quantitatively regenerated in solution even for 6 M NaCl. Within several months, the [RuCl 2 (D 2 O) 2 Im 2 ] + isomers further aquate to a single species [RuCl(D 2 O) 3 Im 2 ] 2+ . In CD 3 OD, displacement of the first chloride of [RuCl 4 Im 2 ] − takes place faster, over several hours, but substitution stops at the [RuCl 3 (CD 3 OD)Im 2 ] stage. In DMSO, substitution occurs very slowly. The [RuCl 3 (DMSO)Im 2 ]:[RuCl 4 Im 2 ] − mixture (1:2) obtained after 12 days starts to show very slow reduction to two Ru(II) species, one of which precipitates as yellow crystals. From X-ray diffraction work (monoclinic, P2 1 /n, a = 9.951, b = 8.564, c = 10.527 Å, β = 92.95°, R = 0.033), the compound was identified as [RuCl 2 (DMSO-d 6 ) 2 Im 2 ], where the metal has a trans-trans-trans coordination and the DMSO ligands are S-bonded. Keywords: paramagnetic ruthenium anion, solvolysis, chloro complexes.