Enantiomeric Complexes Based on Ruthenium(III) and 2,2'-Biimidazole: X-ray Structure and Magnetic Properties.

We have prepared and characterized two Ru(III) compounds based on the 2,2'-biimidazole (H 2 biim) ligand, namely, a single complex of formula cis-[RuCl 2 (H 2 biim) 2 ]Cl·4H 2 O ( 1 ) and a racemic mixture of formula {cis-[RuCl 2 (H 2 biim) 2 ]Cl} 2 ·4H 2 O ( 2 ), which contains 50% of Ru(III) complex 1 . Both compounds crystallize in the monoclinic system with space groups C 2 and P 2 1 for 1 and 2 , respectively. These complexes exhibit the metal ion bonded to four nitrogen atoms from two H 2 biim molecules and two chloride ions, which balance part of the positive charges in a distorted octahedral geometry. Significant differences are observed in their crystal packing, which leads to the observation of differences in their respective magnetic behaviors. Despite having imidazole rings in both compounds, π-π stacking interactions occur only in the crystal structure of 2 , and the shortest intermolecular Ru···Ru separation in 2 is consequently shorter than that in 1 . Variable-temperature dc magnetic susceptibility measurements performed on polycrystalline samples of 1 and 2 reveal different magnetic behaviors at low temperatures: while 1 behaves pretty much as a magnetically isolated mononuclear Ru(III) complex with S = 1/2, 2 exhibits the behavior of an antiferromagnetically coupled system with S = 0 and a maximum in the magnetic susceptibility curve at approximately 3.0 K.