Chromatographic Resolution of a Diastereomeric Mixture of Tris(trans-1,2-diaminocyclohexane)cobalt(III) by Bis(μ-tartrato)diantimonate(III) Anion and Association Modes Found in Their Diastereomeric Salts

The chromatographic resolution of a diastereomeric mixture of Λ-ob3-, Δ-ob3-, Λ-lel3-, and Δ-lel3-[Co(chxn)3]3+ (chxn = trans-1,2-diaminocyclohexane) was carried out on an ion-exchange column using bis(μ-tartrato)diantimonate(III), [Sb2(d-tart)2]2− as a resolving reagent. The resulting elution order of Λ-ob3 > Λ-lel3 > Δ-lel3 > Δ-ob3 was interpreted on the basis of an association model in which lone-pairs on three oxygen atoms of [Sb2(d-tart)2]2− form triple hydrogen bonds with three N–H groups directed along the C3-axis of [Co(chxn)3]3+. Since the three N–H groups of the Λ-ob3 isomer have a right-handed helicity, they form the strongest linear hydrogen bonds with the three lone-pairs of [Sb2(d-tart)2]2−, which are also spirally oriented in the same sense, whereas those of the two lel3 isomers having practically no such helicity form weaker hydrogen bonds, and those of the Δ-ob3 isomer having the opposite helicity have the hydrogen bonds much damaged. To obtain structural evidence for the above association model, crystal structures of diastereomeric salts, Λ-ob3-[Co(chxn)3]X[Sb2(d-tart)2]·nH2O, X− = Cl− (1) and ClO4− (2) have been determined by single-crystal analyses. The crystal data are as follows: 1 is orthorhombic, with a space group P212121, Z = 4, a = 10.823(2), b = 17.901(7), c = 21.477(6) Å, V = 4161(2) Å3, and R = 0.057; 2 is monoclinic with a space group P21, Z = 4, a = 13.338(2), b = 17.119(3), c = 18.060(3) Å, β = 98.31(1)°, V = 4080(1) Å3, and R = 0.049. In both crystals, [Sb2(d-tart)2]2− approaches Λ-ob3-[Co(chxn)3]3+ along the C3-axis of the complex to form multiple hydrogen bonds, as expected, and one of the contact modes found in 2 is close to our proposed association model.