Enantiomer separation of tris(2,2'-bipyridine)ruthenium(II): Interaction of a D₃-symmetric cation with a C₂-symmetric anion

A compound widely used in the separation of the enantiomers of Δ,Λ-[Ru(bipy)₃]²⁺ (bipy = 2,2'-bipyridine) and originally described as "a curious lattice compound" with the formula Δ-[Ru(bipy)₃]₃[Sb₂(R,R-tart)₂]₂I₂·18H₂O (tart = tetradeprotonated, carboxyl and hydroxyl, tartaric acid anion) has been crystallographically characterized as this species with a slightly higher degree of hydration (19.5H₂O). The crystal lattice has a layered structure in which sheets containing Δ-[Ru(bipy)₃]²⁺ cations and iodide anions alternate with those containing [Sb₂(R,R-tart)₂]^2- anions and water. The role of the iodide ions, which lie in pseudohexagonal cavities formed by the array of three inequivalent but very similar Δ-[Ru(bipy)₃]²⁺ cations, all having their C₃ axes directed very close to perpendicular to the sheet, appears to be to favor the orientation of the maximum number of CH units toward the [Sb₂(R,R-tart)₂]^2- entities and to engender formation of (Δ-[Ru(bipy)₃]²⁺)₃([Sb₂(R,R-tart)₂]^2-)₂ groups optimizing the association of the D₃-symmetric cations with the C₂-symmetric anionic resolving agent. This may explain the known superiority of the resolution of Δ,Λ-[Ru(bipy)₃]²⁺ with the anion mixture of I^- and [Sb₂(R,R-tart)₂]^2- over that with just [Sb₂(R,R-tart)₂]^2- alone.