Transfer of Chirality by Dithiophosphate Ligands and Chiral Discrimination in the Stereoselective Formation of Square-Planar Ni(II) Complexes

In the formation reaction of Ni2+ with the chiral racemic ligand, (R)(R)bdtp-/(S)(S)bdtp-, bdtp- = [SSPOCH(CH3)CH(CH3)O]- cyclo- O,O′-[1,2-dimethylethylene] dithiophosphato ion, the meso-complex Ni[(R)(R)(λ)bdtp][(S)(S)(δ)-bdtp] is stereoselectively produced. The meso-complex was compared with the enantiopure crystals of (+) 589Ni[(R)(R)(λ)bdtp]2 or (-) 589Ni[(S)(S)(δ)bdtp]2 or as well as racemic crystals, rac-(±)Ni[bdtp]2, which were prepared from the solution containing the two enantiomers in a 1:1 ratio. Dissociation constants in solutions indicate different stability of the meso and enantiopure complexes depending on the solvent, whereas a more efficient crystal packing, weak H-bonding, and nonbonding interactions contribute to stabilization of the meso-species over the racemic one. Molecular structures show that the outer five-membered ligand ring adopts thehalf-chair conformation C2 with either the λ or the δ chirality and the methyl groups are in equatorial (e) positions. Enantiopure ligands of (+) 589Ni[(R)(R)(λ)bdtp]2 and (-) 589Ni[(S)(S)(δ)bdtp]2 induce chirality into the symmetric SSNiSS chromophore with slightly helical distortion. Thus, their CD spectra exhibit weak negative or positive Cotton effects at 662 nm. CD spectra in L(+)- and D(-)diethyltartrate of the meso-complex and racemic crystal, rac-(±)Ni[bdtp]2, exhibit different weak Cotton effects of opposite sign. Complexes dissociate in methanol; rac-(±)Ni[bdtp] 2 in methanol undergoes a crystallization-induced second-order asymmetric transformation which finally yields crystals of the meso-Ni[(R)(R)(R)(γ)bdtp][(S)(S)(δ)bdtp] complex.