Transfer of chirality by dithiophosphate ligands and chiral discrimination in the stereoselective formation of square-planar Ni(II) complexes

• Published in: Chirality (New York, N.Y.) Vol. 16; no. 4; pp. 251 - 262
• Main Authors: Biscarini, Paolo, Benedetti, Michele, Ferranti, Francesco, Kuroda, Reiko, Foresti, Elisabetta, Sabatino, Piera
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 2004
• Subjects: coordination compounds; crystal packing; crystal structures; induced CD; stereoselective synthesis
• ISSN: 0899-0042; 1520-636X
• DOI: 10.1002/chir.20022

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, 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 the half‐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)(λ)bdtp][(S)(S)(δ)bdtp] complex. Chirality 16:251–262, 2004. © 2004 Wiley‐Liss, Inc.